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Abstract Background Patellofemoral joint replacement is a successful treatment option for isolated patellofemoral osteoarthritis. However, results of later conversion to total knee replacement may be compromised by periprosthetic bone loss. Previous clinical studies have demonstrated a decrease in distal femoral bone mineral density after patellofemoral joint replacement. It is unclear whether this is due to periprosthetic stress shielding. The main objective of the current study was to evaluate the stress shielding effect of prosthetic replacement with 2 different patellofemoral prosthetic designs and with a total knee prosthesis. Methods We developed a finite element model of an intact patellofemoral joint, and finite element models of patellofemoral joint replacement with a Journey PFJ prosthesis, a Richards II prosthesis, and a Genesis II total knee prosthesis. For each of these 4 finite element models, the average Von Mises stress in 2 clinically relevant regions of interest were evaluated during a simulated squatting movement until 120 degrees of flexion. Results During deep knee flexion, in the anterior region of interest, the average Von Mises stress with the Journey PFJ design was comparable to the physiological knee, while reduced by almost 25% for both the Richards II design and the Genesis II total knee joint replacement design. The average Von Mises stress in the supracondylar region of interest was similar for both patellofemoral prosthetic designs and the physiological model, with slightly lower stress for the Genesis II design. Conclusions Patellofemoral joint replacement results in periprosthetic stress-shielding, although to a smaller degree than in total knee replacement. Specific patellofemoral prosthetic design properties may result in differences in femoral stress shielding.

Abstract Background The bone mineral density (BMD) of the distal femur decreases by 16-36% within one year after total knee arthroplasty (TKA) because of the femoral component's stress-shielding effect. The aim of this prospective study was to determine the quantitative change from the baseline BMD in the distal femur 1 year after patellofemoral arthroplasty using dual-energy X-ray absorptiometry (DXA). Methods Between December 2007 and December 2008, 14 patients had patellofemoral arthroplasty for isolated patellofemoral osteoarthritis. Distal femoral BMD was assessed using DXA in 2 regions of interest (ROI) on the lateral view 2 weeks before and 12 months after patellofemoral arthroplasty. The contra-lateral knee was used as a control, with BMD measurements performed in identical ROIs. Results The mean change from baseline BMD in the operated knees after 1 year was -0.169 g/cm2 (95% CI: -0.293 to -0.046 g/cm2) behind the anterior flange (-15%), and -0.076 g/cm2 (95% CI: -0.177 to 0.024 g/cm2) in the supracondylar area 1 cm above the prosthesis (-8%) (p = 0.01 and p = 0.13, respectively). The mean change from baseline BMD in the non-operated knees after 1 year was 0.016 g/cm2 (95% CI: -0.152 to 0.185 g/cm2) behind the anterior flange (2%), and 0.023 g/cm2 (95% CI: -0.135 to 0.180 g/cm2) in the supracondylar area 1 cm above the prosthesis (2%) (p = 0.83, and p = 0.76, respectively). Conclusions Our findings suggest that patellofemoral arthroplasty results in a statistically significant decrease in BMD behind the anterior flange.

Abstract Background Asymptomatic local bone resorption of the tibia under the baseplate can occasionally be observed after total knee arthroplasty (TKA). Its occurrence is not well documented, and so far no explanation is available. We report the incidence of this finding in our practice, and investigate whether it can be attributed to specific mechanical factors. Methods The postoperative radiographs of 500 consecutive TKA patients were analyzed to determine the occurrence of local medial bone resorption under the baseplate. Based on these cases, a 3D FE model was developed. Cemented and cementless technique, seven positions of the baseplate and eleven load sharing conditions were considered. The average VonMises stress was evaluated in the bone-baseplate interface, and the medial and lateral periprosthetic region. Results Sixteen cases with local bone resorption were identified. In each, bone loss became apparent at 3 months post-op and did not increase after one year. None of these cases were symptomatic and infection screening was negative for all. The FE analysis demonstrated an influence of baseplate positioning, and also of load sharing, on stresses. The average stress in the medial periprosthetic region showed a non linear decrease when the prosthetic baseplate was shifted laterally. Shifting the component medially increased the stress on the medial periprosthetic region, but did not significantly unload the lateral side. The presence of a cement layer decreases the stresses. Conclusion Local bone resorption of the proximal tibia can occur after TKA and might be attributed to a stress shielding effect. This FE study shows that the medial periprosthetic region of the tibia is more sensitive than the lateral region to mediolateral positioning of the baseplate. Medial cortical support of the tibial baseplate is important for normal stress transfer to the underlying bone. The absence of medial cortical support of the tibial baseplate may lead to local bone resorption at the proximal tibia, as a result of the stress shielding effect. The presence of a complete layer of cement can reduce stress shielding, though. Despite the fact that the local bone resorption is asymptomatic and non-progressive, surgeons should be aware of this phenomenon in their interpretation of follow-up radiographs.

Abstract Background Sufficient primary stability is a prerequisite for the clinical success of cementless implants. Therefore, it is important to have an estimation of the primary stability that can be achieved with new stem designs in a pre-clinical trial. Fast assessment of the primary stability is also useful in the preoperative planning of total hip replacements, and to an even larger extent in intraoperatively custom-made prosthesis systems, which result in a wide variety of stem geometries. Methods An analytical model is proposed to numerically predict the relative primary stability of cementless hip stems. This analytical approach is based upon the principle of virtual work and a straightforward mechanical model. For five custom-made implant designs, the resistance against axial rotation was assessed through the analytical model as well as through finite element modelling (FEM). Results The analytical approach can be considered as a first attempt to theoretically evaluate the primary stability of hip stems without using FEM, which makes it fast and inexpensive compared to other methods. A reasonable agreement was found in the stability ranking of the stems obtained with both methods. However, due to the simplifying assumptions underlying the analytical model it predicts very rigid stability behaviour: estimated stem rotation was two to three orders of magnitude smaller, compared with the FEM results. Conclusion Based on the results of this study, the analytical model might be useful as a comparative tool for the assessment of the primary stability of cementless hip stems.

Three-dimensional gait analysis is the 'gold standard' for measurement and description of gait. Gait variability can arise from intrinsic and extrinsic factors and may vary between walking conditions. This study aimed to define the inter-trial and inter-session repeatability in gait analysis data of children with cerebral palsy (CP) who were walking in four conditions, namely barefoot or with ankle-foot orthosis (AFO), and overground or treadmill. Ten children with spastic CP (7♀; 9.9y ± 3.5y; GMFCS-level I-III) were included in this study. Overall, we found good to excellent intra-class correlation (ICC)-values and favourable standard error of measurement (SEM)-values for the inter-session Gait Profile Score (ICC = 0.85-0.98, SEM = 0.45-0.91°) and Gait Variable Scores (ICC = 0.85-0.99, SEM = 0.22-1.11°) for the lower-limb joints. Taking the total joint-range-of-motion into account, the knee joint showed the most repeatable motion (%SEM = 0.5-1.8 %), while ankle motions showed the lowest repeatability (%SEM = 0.8 %-3.0 %). For the continuous waveform data, only the ankle joint showed repeatability differences between walking conditions, namely, smaller SEM-values for the AFO-condition (mean inter-trial = 0.14°; mean inter-session = 1.121°) in comparison to the barefoot-condition (mean inter-trial = 0.55°; mean inter-session = 2.22°). For all the kinetic parameters, the treadmill conditions showed smaller SEM-values in comparison to the overground condition. In conclusion three-dimensional gait analysis was found to be reliable in all four walking conditions for children with CP. The resulting measurement errors can be used as a reference during clinical interpretations of gait analyses. Clinical trial registration number: Trial ID from an internationally recognized trial registry (ClinicalTrials.gov): NCT06355869., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Ankle-foot orthoses (AFOs) are commonly prescribed to children with cerebral palsy (CP). The conventional AFO successfully controls the first and second ankle rocker, but it fails to correct the third ankle rocker, which negatively effects push-off power. The current study evaluated a new powered AFO (PAFO) design, developed to address the shortcomings of the conventional AFO. Eight children with spastic CP (12.4 ± 3.4 years; GMFCS I-III; 4/4-♂/♀; 3/5-bi/unilateral) were included. Sagittal kinematic and kinetic data were collected from 20 steps during barefoot walking, with conventional AFOs and PAFOs. In the PAFO-condition, an actuation unit was attached to a hinged AFO and through push-pull cables to a backpack that was carried by the child and provided patient-specific assistance-as-needed. SnPM-analysis indicated gait cycle sections that differed significantly between conditions. For the total group, differences between the three conditions were found in ankle kinematics (49.6-66.1%, p = 0.006; 88.0-100%, p = 0.011) and angular velocity (0.0-6.0%, p = 0.001; 45.1-51.1%, p = 0.006; 62.2-73.0%, p = 0.001; 81.2-93.0%, p = 0.001). Individual SnPM-analysis revealed a greater number of significant gait cycle sections for kinematics and kinetics of the ankle, knee, and hip. These individual results were heterogeneous and specific per gait pattern. In conclusion, the new PAFO improved the ankle range-of-motion, angular velocity, and power during push-off in comparison to the conventional AFO.

Background: Ankle-foot orthoses (AFOs) are frequently prescribed in children with cerebral palsy (CP) to improve their gait. Due to the heterogeneous nature of CP and contradictions among previous studies, it is important to evaluate the AFO-specific effects, as well as explore their effects on different gait patterns., Research Questions: a) What are the prevalence and specific features of AFOs in children with CP? b) How do AFOs affect gait pathology in children with CP? c) What are the pattern-specific effects of AFOs in children with CP?, Methods: A group of 170 patients with CP underwent a three-dimensional gait analysis with and without AFOs (either carbon fiber, rigid, flexible or hinged). The gait profile score, the gait variable scores of the hip, knee and ankle joints, non-dimensional step length and walking speed were used as outcome measures. The AFO-specific effects on the kinematic and kinetic waveforms were investigated using statistical non-parametric mapping (SnPM). Effects were considered relevant when the minimal clinically important difference (MCID) or the standard errors of measurement, for the parameters or the waveforms respectively, were exceeded., Results: Rigid AFOs were prescribed for more than 80 % of the children. Significant beneficial effects were observed for non-dimensional step length and walking speed. Most changes in gait indices were not considered relevant. The SnPM-analyses on the total group and specific gait patterns revealed that walking with AFOs improved the kinematic and kinetic waveforms. These effects were relevant, and were most obvious for crouch, apparent equinus and the total group., Significance: The use of AFOs improves gait, whether we inspect a total -and thus heterogeneous- group or focus on specific gait patterns. However, focussing on specific parameters (i.e. general gait indices) does not provide a full picture of the AFO-effects., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Purpose: The present study assessed the effect of insert articular surface geometry (anatomical versus conventional insert design) on anteroposterior (AP) translation and varus-valgus (VV) laxity in balanced posterior cruciate ligament (PCL) retaining total knee arthroplasty (TKA). Secondly, we evaluated if the AP translation and VV laxity in the reconstructed knee resembled the stability of the native knee., Methods: Nine fresh-frozen full-leg cadaver specimens were used in this study. After testing the native knee, anatomical components of a PCL-retaining implant were implanted. The knee joints were subjected to anteriorly and posteriorly directed forces (at 20° and 90° flexion) and varus-valgus stresses (at 20°, 45° and 90° flexion) in both non-weightbearing and weightbearing situations in a knee kinematics simulator. Measurements were performed in the native knee, TKA with anatomical insert geometry (3° built-in varus, medial concave, lateral convex), and TKA with symmetrical insert geometry., Results: In weightbearing conditions, anterior translations ranged between 2.6 and 3.9 mm at 20° flexion and were < 1 mm at 90° flexion. Posterior translation at 20° flexion was 2.7 mm for the native knee versus 4.0 mm (p = 0.047) and 7.0 mm (p = 0.02) for the symmetrical insert and the anatomical insert, respectively. Posterior translation at 90° flexion was < 1.1 mm and not significantly different between the native knee and insert types. In non-weightbearing conditions, the anterior translation at 20° flexion was 5.9 mm for the symmetrical and 4.6 mm for the anatomical insert (n.s.), compared with 3.0 mm for the native knee (p = 0.02). The anterior translation at 90° flexion was significantly higher for the reconstructed knees (anatomical insert 7.0 mm; symmetrical insert 9.2 mm), compared with 1.6 mm for the native knee (both p = 0.02). Varus-valgus laxity at different flexion angles was independent of insert geometry. A valgus force in weightbearing conditions led to significantly more medial laxity (1°-3° opening) in the native knee at 45° and 90° flexion compared with the reconstructed knee for all flexion angles., Conclusions: Insert geometry seems to have a limited effect with respect to AP translation and VV laxity, in the well-balanced PCL-retaining TKA with an anatomical femoral component. Secondly, AP translation and VV laxity in the reconstructed knee approximated the laxity of the native knee., (© 2021. European Society of Sports Traumatology, Knee Surgery, Arthroscopy (ESSKA).)

Introduction: Unicompartmental knee arthroplasty (UKA) and total knee arthroplasty (TKA) are standard procedures for treating knee joint arthritis. Neither UKA nor TKA seems to be optimally suited for patients with bicompartmental osteoarthritis that affects only the medial and patellofemoral compartments. A bicompartmental knee arthroplasty (BKA) was designed for this patient group. This study aimed to compare the effectiveness of a BKA and TKA in restoring the kinematics of the knee joint., Materials and Methods: In this in vitro study, three types of knee arthroplasties (BKA, posterior cruciate ligament-retaining, and posterior cruciate ligament-resecting TKA) were biomechanically tested in six freshly frozen human cadaveric specimens. Complete three-dimensional kinematics was analyzed for each knee arthroplasty during both passive and loaded conditions in a validated knee kinematics rig. Infrared motion capture cameras and retroreflective markers were used for recording data., Results: No significant differences could be found between the three types of arthroplasties. However, similar kinematic changes between BKA and a native knee joint were documented under passive conditions. However, in a weight-bearing mode, a significant decrease in femoral rotation during the range of motion was found in arthroplasties compared to the native knee, probably caused by contraction of the quadriceps femoris muscle, which leads to a decrease in the anterior translation of the tibia., Conclusions: Kinematics similar to that of the natural knee can be achieved by BKA under passive conditions. However, no functional advantage of BKA over TKA was detected, which suggests that natural knee kinematics cannot be fully imitated by an arthroplasty yet. Further prospective studies are required to determine the anatomic and design factors that might affect the physiologic kinematics., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Purpose: Femoral component malrotation in total knee arthroplasty (TKA) is clinically proven to cause dissatisfaction and impaired function. This study is an attempt to characterize the tibiofemoral kinematics following femoral malrotation in posterior stabilized (PS) TKA. It was hypothesized that internal malrotation would introduce the most pronounced changes., Methods: Six fresh-frozen cadaver specimens were mounted in a kinematic rig. Three motion patterns were applied with the native knee and following PS TKA (passive motion, open chain extension, and squatting) while infrared cameras recorded the trajectories of markers attached to femur and tibia. Three different femoral implants were tested: a conventional posterior stabilized component, and adapted components of the same implant with 5° of intrinsic external and internal rotation, respectively., Results: The implantation of the PS TKA resulted in less tibial internal rotation (squat 33-70°, p < 0.05) and the medial femoral condyle shifted posteriorly especially in deep flexion (squat 84-111°, p < 0.05). Internal component malrotation caused internal rotation and abduction of the tibia in flexion (squat 33-111°, p < 0.05), an elevated (squat 43-111°, p < 0.05) and more anterior (passive 61-126°, p < 0.05) located medial femoral condyle and a lateral femoral condyle located more posterior and inferior (squat 73-111°, p < 0.05) than in the neutrally aligned TKA. External component malrotation caused only little changes under passive motion. Under a squat there was less internal rotation and more adduction to the tibia (33-111°, p < 0.05). The medial femoral condyle was moved more posterior (squat 59-97°, p < 0.05), the lateral femoral condyle more superior (squat 54-105°, p < 0.05) than in the neutrally aligned TKA., Conclusion: The greatest differences to the native tibiofemoral kinematics were introduced by internal rotation of the femoral component. Also neutrally and externally rotated femoral components introduce kinematic changes, but to a lesser extent. With respect to the alterations introduced to kinematics internal malrotation should be avoided when performing PS TKA.

Background: Oxidized zirconium (OxZr) is produced by thermally driven oxidization creating an oxidized surface with the properties of a ceramic at the top of the Zr metal substrate. OxZr is much harder and has a lower coefficient of friction than cobalt-chrome (CoCr), both leading to better wear characteristics. We evaluated and compared damage to the cartilage of porcine patella plugs, articulating against OxZr vs CoCr. Our hypothesis was that, owing to its better wear properties, OxZr would damage cartilage less than CoCr. If this is true, OxZr might be a better material for the femoral component during total knee arthroplasty if the patella is not resurfaced., Methods: Twenty-one plugs from porcine patellae were prepared and tested in a reciprocating pin-on-disk machine while lubricated with bovine serum and under a constant load. Three different configurations were tested: cartilage-cartilage as the control group, cartilage-OxZr, and cartilage-CoCr. Macroscopic appearance, cartilage thickness, and the modified Mankin score were evaluated after 400,000 wear cycles., Results: The control group showed statistically significant less damage than plugs articulating against both other materials. Cartilage plugs articulating against OxZr were statistically significantly less damaged than those articulating against CoCr., Conclusion: Although replacing cartilage by an implant always leads to deterioration of the cartilage counterface, OxZr results in less damage than CoCr. The use of OxZr might thus be preferable to CoCr in case of total knee arthroplasty without patella resurfacing., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Introduction: No evidence-based guidelines are available to determine the appropriate stem length, and whether or not to cement stems in revision total knee arthroplasty (TKA). Therefore, the objective of this study was to compare stresses and relative movement of cemented and uncemented stems of different lengths using a finite element analysis., Materials and Methods: A finite element model was created for a synthetic tibia. Two stem lengths (95 and 160 mm) and two types of fixation (cemented or press fit) of a hinged TKA were examined. The average compressive stress distribution in different regions of interest, as well as implant micromotions, was determined and compared during lunge and squat motor tasks., Results: Both long and short stems in revision TKA lead to high stresses, primarily in the region around the stem tip. The presence of cement reduces the stresses in the bone in every region along the stem. Short stem configurations are less affected by the presence of cement than the long stem configuration. Press-fit stems showed higher micromotions compared to cemented stems., Conclusions: Lowest stresses and micromotion were found for long cemented stems. Cementless stems showed more micromotion and increased stress levels especially at the level of the stem tip, which may explain the clinical phenomenon of stem-end pain following revision knee arthroplasty. These findings will help the surgeon with optimal individual implant choice.

Purpose: The objectives of this in vitro project were to compare the dynamic three-dimensional patellofemoral kinematics, contact forces, contact areas and contact pressures of a contemporary patellofemoral prosthetic implant with those of the native knee and to measure the influence of patellar resurfacing and patellar thickness. The hypothesis was that these designs are capable to reproduce the natural kinematics but result in higher contact pressures., Methods: Six fresh-frozen specimens were tested on a custom-made mechanical knee rig before and after prosthetic trochlear resurfacing, without and with patellar resurfacing in three different patellar thicknesses. Full three-dimensional kinematics were analysed during three different motor tasks, using infrared motion capture cameras and retroflective markers. Patellar contact characteristics were registered using a pressure measuring device., Results: The patellofemoral kinematic behaviour of the patellofemoral arthroplasty was similar to that of the normal knee when the patella was resurfaced, showing only significant (p < 0.0001) changes in patellar flexion. Without patellar resurfacing, significant more patellar flexion, lateral tilt and lateral rotation was noticed. Compared to the normal knee, contact pressures were significantly elevated after isolated trochlear resurfacing. However, the values were more than doubled after patellar resurfacing. Changes in patellar thickness only influenced the antero-posterior patellar position. There was no other influence on the kinematics, and only a limited influence on the contact pressures in the low flexion angles., Conclusion: The investigated design reproduced the normal patellofemoral kinematics acceptable well when the patella was resurfaced. From a kinematic point of view, patellar resurfacing may be advisable. However, the substantially elevated patellar contact pressures remain a point of concern in the decision whether or not to resurface the patella. This study therefore not only adds a new point in the discussion whether or not to resurface the patella, but also supports the claimed advantage that a patellofemoral arthroplasty is capable to reproduce the natural knee kinematics.

Purpose: Balancing unicondylar knee arthroplasty (UKA) is challenging. If not performed properly, it may lead to implant loosening or progression of osteoarthritis in the preserved compartment. This study was aimed to document the biomechanical effects of improper balancing. We hypothesised that overstuffing would lead to more valgus, higher strain in the medial collateral ligament (sMCL), and higher lateral contact force., Methods: Six fresh-frozen cadaver specimens were mounted in a kinematic rig. Three motion patterns were applied with the native knee and following medial UKA (passive motion, open-chain extension, and squatting), while infrared cameras recorded the trajectories of markers attached to femur and tibia. Three inlay thicknesses were tested (8, 9, 10 mm)., Results: Overstuffed knees were in more valgus and showed less tibial rotation and higher strains in the sMCL (p < 0.05). Lateral contact forces were higher in some specimens and lower in others. Stiffening of the medial compartment by UKA, even well balanced, already leads to a knee more in valgus with a more stressed sMCL. Overstuffing increases these effects. Knees with a tight sMCL may even see lower lateral contact force. Biomechanics were closest to the native knee with understuffing., Conclusion: The first two hypotheses were confirmed, but not the latter. This underlines the importance of optimal balancing. Overstuffing should certainly be avoided. Although kinematics is only slightly affected, contact forces and ligament strains are considerably changed and this might be of more clinical importance. It is advisable to use thinner inlays, if stability is not compromised.

Purpose: Most total knee arthroplasty tibial components are metal-backed, but an alternative tibial component made entirely of polyethylene (all-polyethylene design) exists. While several clinical studies have shown that all-poly design performs similarly to the metal-backed, the objective of this study is to perform a biomechanical comparison., Methods: Loads, constraints and geometries during a squat activity at 120° of flexion were obtained from a validated musculoskeletal model and applied to a finite element model. Stresses in the tibia and micromotions at the bone-implant interface were evaluated for several implant configurations: (1) three different thicknesses of the cement penetration under the baseplate (2, 3 and 4 mm), (2) the presence or absence of a cement layer around the stem of the tibial tray and (3) three different bone conditions (physiological, osteopenic and osteoporotic bone)., Results: All-polyethylene tibial components resulted in significantly higher (p < 0.001) and more uneven stress distributions in the cancellous bone under the baseplate (peak difference: +128.4 %) and fivefold increased micromotions (p < 0.001). Performance of both implant designs worsened with poorer bone quality with peaks in stress and micromotion variations of +40.8 and +54.0 %, respectively (p < 0.001). Performance improvements when the stem was cemented were not statistically significant (n.s.)., Conclusion: The metal-backed design showed better biomechanical performance during a squat activity at 120° of flexion compared to the all-polyethylene design. These results should be considered when selecting the appropriate tibial component for a patient, especially in the presence of osteoporotic bone or if intense physical activity is foreseen.

Purpose: Total knee arthroplasty (TKA) is a very successful surgical procedure. However, implant failures and patient dissatisfaction still persist. Sometimes surgeons are not able to understand and explain these negative performances because the patient's medical images "look good", but the patient "feels bad". Apart from radiograph imaging and clinical outcome scores, conventionally used follow-up methods are mainly based on the analysis of knee kinematics. However, even if kinematics remains close to the "normal" range of motion, the patient may still complain about pain and functional limitations. To provide more insight into this paradox, a better quantitative understanding of TKA mechanics must be developed. For this purpose, improved techniques for clinical follow-up, combining kinetics and kinematics analysis, should be introduced to help surgeons to assess and understand TKA performance., Methods: An analysis on four TKA designs was performed, and the changes in kinematics and in kinetics induced by several implant configurations (simulating implant malalignment and different knee anatomy) were compared. More specifically, analysed tibio-femoral and patello-femoral contact forces and tibio-femoral kinematics were analysed during a squat task up to 120°., Results: The results from this study show that contact forces (with changes up to 67 %) are more heavily affected by malconfigurations than kinematics, for which maximum deviations are of the order of 5 mm or 5°, similar to the simulated surgical errors. The results present a similar trend for the different designs., Conclusions: The results confirm the hypothesis that kinematics is not the only and also not the most relevant parameter to predict or explain knee function after TKA. In the future, techniques to analyse knee kinetics should be integrated in the clinical follow-up.

Background: When researchers describe data from their studies, there is no rule defining the best way to represent results. Therefore, collecting and explaining results from personal research or understanding data from publications is not always straightforward. These issues are even worse in fields such as biomedical engineering, where researchers from different backgrounds, usually engineers and surgeons, need to interact and exchange information. For these reasons, the purpose of this study is to introduce and illustrate an innovative method to represent, concisely and intuitively, biomechanical knee behavior, called KneePrints., Methods: To test the KneePrints method, a huge amount of data from previously published sensitivity analyses were used and represented both with conventional techniques and with this new graphical method. Then, a survey has been distributed among different international specialists in the orthopedic field, such as surgeons and researchers. In the survey, interviewees were asked to select the favorite method that addressed to be the most effective to show the same results., Results: Collecting the outcomes from the survey, the KneePrints method resulted to be more effective than standard graphs, such as tables and histograms. KneePrints method has been selected to be clearer in representing outputs and more immediate in results understanding independently from the occupation of the interviewees by the survey. The general preference for the KneePrints is 63 %, up to 74 % being surgeons' choice., Conclusions: The innovative KneePrints method has been endorsed to be effective in representing and making more understandable knee joint outputs. This method can be extended also to other topics.

Purpose: Currently, controversy exists whether restoration of neutral mechanical alignment should be attempted in all patients undergoing TKA. Our hypothesis was that restoration of constitutional rather than neutral mechanical alignment may in theory lead to a more physiological strain pattern in the collateral ligaments; therefore, it could potentially be beneficial to patients. Thus, the aim of this study was to measure collateral ligament strains during three motor tasks in the native knee and compare them with the strains noted after TKA in different post-operative alignment conditions., Methods: Six cadaver specimens (approval number ML4190 from the Research Ethics Committee of University of Leuven, Belgium) were examined using a validated knee kinematics rig under physiological loading conditions. The effect of coronal malalignment was evaluated by using custom-made tibial implant inserts that induced different alignment conditions. The study of six specimens allows us to show that a difference in the mean strains in MCL and LCL of 3.6 and 5.8 %, respectively, was statistically significant with a probability (power) of 0.8., Results: The results indicated that after TKA insertion, the strains in the collateral ligaments closely resembled the pre-operative pattern of the native knee specimens when constitutional alignment was restored. Restoration to neutral mechanical alignment was associated with greater collateral strain deviations from the native knee., Conclusion: Based upon this study, it was concluded that restoration of constitutional alignment within a "safe zone" of ±2° during TKA leads to more physiological peri-articular soft tissue strains during loaded as well as unloaded motor tasks.

Background: Trochlear dysplasia appears in different geometrical variations. The Dejour classification is widely used to grade the severity of trochlear dysplasia and to decide on treatment., Purpose: To investigate the effect of trochlear dysplasia on patellofemoral biomechanics and to determine if different types of trochlear dysplasia have different effects on patellofemoral biomechanics., Study Design: Controlled laboratory study., Methods: Trochlear dysplasia was simulated in 4 cadaveric knees by replacing the native cadaveric trochlea with different types of custom-made trochlear implants, manufactured with 3-dimensional printing. For each knee, 5 trochlear implants were designed: 1 implant simulated the native trochlea (control condition), and 4 implants simulated 4 types of trochlear dysplasia. The knees were subjected to 3 biomechanical tests: a squat simulation, an open chain extension simulation, and a patellar stability test. The patellofemoral kinematics, contact area, contact pressure, and stability were compared between the control condition (replica implants) and the trochlear dysplastic condition and among the subgroups of trochlear dysplasia., Results: The patellofemoral joint in the trochlear dysplastic group showed increased internal rotation, lateral tilt, and lateral translation; increased contact pressures; decreased contact areas; and decreased stability when compared with the control group. Within the trochlear dysplastic group, the implants graded as Dejour type D showed the largest deviations for the kinematical parameters, and the implants graded as Dejour types B and D showed the largest deviations for the patellofemoral contact areas and pressures., Conclusion: Patellofemoral kinematics, contact area, contact pressure, and stability are significantly affected by trochlear dysplasia. Of all types of trochlear dysplasia, the models characterized with a pronounced trochlear bump showed the largest deviations in patellofemoral biomechanics., Clinical Relevance: Investigating the relationship between the shape of the trochlea and patellofemoral biomechanics can provide insight into the short-term effects (maltracking, increased pressures, and instability) and long-term effects (osteoarthritis) of different types of trochlear dysplasia. Furthermore, this investigation provides an empirical explanation for better treatment outcomes of trochleoplasty for Dejour types B and D dysplasia., (© 2015 The Author(s).)

Introduction: Correct restoration of the joint line is generally considered as crucial when performing total knee arthroplasty (TKA). During revision knee arthroplasty however, elevation of the joint line occurs frequently. The general belief is that this negatively affects the clinical outcome, but the reasons are still not well understood., Material and Methods: In this cadaveric in vitro study the biomechanical consequences of joint line elevation were investigated using a previously validated cadaver model simulating active deep knee squats and passive flexion-extension cycles. Knee specimens were sequentially tested after total knee arthroplasty with joint line restoration and after 4 mm joint line elevation., Results: The tibia rotated internally with increasing knee flexion during both passive and squatting motion (range: 17° and 7° respectively). Joint line elevation of 4 mm did not make a statistically significant difference. During passive motion, the tibia tended to become slightly more adducted with increasing knee flexion (range: 2°), while it went into slighlty less adduction during squatting (range: -2°). Neither of both trends was influenced by joint line elevation. Also anteroposterior translation of the femoral condyle centres was not affected by joint line elevation, although there was a tendency for a small posterior shift (of about 3 mm) during squatting after joint line elevation. In terms of kinetics, ligaments lengths and length changes, tibiofemoral contact pressures and quadriceps forces all showed the same patterns before and joint line elevation. No statistically significant changes could be detected., Conclusions: Our study suggests that joint line elevation by 4 mm in revision total knee arthroplasty does not cause significant kinematic and kinetic differences during passive flexion/extension movement and squatting in the tibio-femoral joint, nor does it affect the elongation patterns of collateral ligaments. Therefore, clinical problems after joint line elevation are probably situated in the patello-femoral joint or caused by joint line elevation of more than 4 mm.

Purpose: The aim of this study was to measure the tibia tubercle trochlea groove distance (TT-TG) as a function of knee flexion. Our hypothesis was that there is a different pattern in healthy volunteers and patients with patella instability (PFI)., Methods: Thirty-six knees of 30 patients with at least one dislocation of the patella and 30 knees of 30 healthy volunteers as control group were analysed with magnetic resonance imaging by three different observers. The TT-TG was measured in steps of 15° between 0° and 90° of knee flexion. Furthermore, the alignment of the leg (MA), the femur torsion (FTor) and the tibia torsion (TTor) was calculated., Results: The TT-TG was higher in patients compared to volunteers and in extension compared to flexion. This difference was statistically significant (p<0.05). Most of the patients with a TT-TG above 20 mm in extension showed a high decrease in flexion to normal values. In some patients, this compensating mechanism fails. MA, FTor and TTor were not different in patients and control group (n.s.)., Conclusion: The TT-TG distance is dynamic and decreased significantly during flexion in knees with PFI and healthy volunteers. However, there were a small number of patients in the PFI group where this compensation mechanism did not work. Therefore, the decision to perform a tibia tubercle osteotomy should not be based on one single measurement in extension or 30° of knee flexion., Level of Evidence: II.

Purpose: Although controversy still remains, isolated patellofemoral arthroplasty recently gained in popularity as a treatment option for patellofemoral osteoarthritis. It has compared to total knee arthroplasty the advantage of preserving the tibiofemoral articulation, which in theory would allow the preservation of natural tibiofemoral kinematics. Today, however, no data exist to support this assumption. This study was therefore performed in order to investigate the effect of isolated patellofemoral arthroplasty on the native three-dimensional tibiofemoral kinematic behaviour and whether a change in patellar thickness would have an influence., Methods: Six fresh-frozen cadavers were fixed on a custom-made mechanical knee rig. Full 3D kinematics was analysed during passive flexion-extension cycles, open chain extension, with and without mechanical resistance, as well as deep knee squats, using infrared motion capture cameras and retroflective markers. Measurements were taken for the native knee and after prosthetic trochlear resurfacing with and without patellar resurfacing in three different patellar thicknesses., Results: Compared to the natural knee, patellofemoral arthroplasty resulted in significant changes in tibiofemoral kinematics, which were most pronounced in the most loaded motor tasks. Increased internal tibial rotation was noted in the mid- and high flexion ranges, reaching at 120° of flexion a mean difference of 4.5°±4.3° (p<0.0001) during squat motion, over the whole flexion range during open chain motion and in deeper flexion beyond 50° (mean at 70°, 1.9°±3.7°) during resisted open chain. During squats, also, a more posterior translation of the lateral femoral condyle was observed. The effect was accentuated in case of patella overstuffing, whereas kinematics was closer to normal with patellar thinning., Conclusion: Isolated patellofemoral arthroplasty alters natural tibiofemoral kinematics, and the effects become more pronounced in case of increased patellar thickness. Therefore, it might be recommended to aim for a slight over-resection of patellar bone if sufficient bone stock is available.

To investigate the biomechanical effect of skeletal knee joint abnormalities, the authors propose to implant pathologically shaped rapid prototyped implants in cadaver knee specimens. This new method was validated by replacing the native trochlea by a replica implant on four cadaver knees with the aid of cadaver-specific guiding instruments. The accuracy of the guiding instruments was assessed by measuring the rotational errors of the cutting planes (on average 3.01° in extension and 1.18° in external/internal rotation). During a squat and open chain simulation, the patella showed small differences in its articulation with the native trochlea and the replica trochlea, which could partially be explained by the rotational errors of the implants. This study concludes that this method is valid to investigate the effect of knee joint abnormalities with a replica implant as a control condition to account for the influence of material properties and rotational errors of the implant., (© IMechE 2014.)

Purpose: It is assumed that unicondylar knee arthroplasty (UKA) features kinematics close to the natural knee. Clinical studies have also shown functional benefits for UKA. There is to date only little biomechanical data to support or explain these findings. The purpose of this study was to investigate whether UKA is able to preserve natural knee kinematics or not., Methods: Six fresh frozen full leg cadaver specimens were prepared to be mounted in a kinematic rig with six degrees of freedom for the knee joint. Three motion patterns were applied before and after medial UKA: passive flexion-extension, open chain extension, and squatting. During the loaded motions, quadriceps and hamstrings muscle forces were applied. Infrared cameras continuously recorded the trajectories of marker frames rigidly attached to femur, tibia, and patella. Prior computer tomography allowed identification of coordinate frames of the bones and calculations of anatomical rotations and translations., Results: Native kinematics was reproduced after UKA in all the specimens. In the unloaded knee and during open chain extension, femoral rollback patterns after UKA were very close to those in the native knee. During squatting, the medial femoral condyle after UKA tended to be more posterior and superior with flexion and there was less tibial internal rotation. The tibia was found to be more in valgus after UKA during all motion patterns., Conclusion: As ligaments, lateral compartment and patellofemoral anatomy are preserved with UKA; the unloaded knee closely resembles native kinematics. The slight kinematic changes that were found under load are probably due to loss of the conforming medial meniscus and to the mismatch in geometry and stiffness introduced by UKA. These patterns resemble those found in knees with significant loss of function of the medial meniscus.

The aim of this study was to quantify the effects of understuffing and overstuffing UKA on bone stresses, load distribution and ligament strains. For that purpose, a numerical knee model of a cadaveric knee was developed and was validated against experimental measurements on that same knee. Good agreement was found among the numerical and experimental results. This study showed that, even if a medial UKA is well-aligned with normal soft tissue tension and with correct thickness of the tibia component, it induces a stiffness modification in the joint that alters the load distribution between the medial and lateral compartments, the bone stress and the ligament strain potentially leading to an osteoarthritic progression., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Purpose: Modular bicompartmental knee arthroplasty (BKA) for treatment of medio-patellofemoral osteoarthritis (OA) should allow for close to normal kinematics in comparison with unicondylar knee arthroplasty (UKA) and the native knee. There is so far no data to support this., Scope: Six fresh frozen full leg cadaver specimens were prepared and mounted in a kinematic rig with six degrees of freedom for the knee joint. Three motion patterns were applied with the native knee and after sequential implantation of medial UKA and patellofemoral joint replacement (PFJ): passive flexion-extension, open chain extension, and squatting. During the loaded motions, quadriceps and hamstrings muscle forces were applied. Infrared cameras continuously recorded the trajectories of marker frames rigidly attached to femur, tibia and patella. Prior computer tomography allowed identification of coordinate frames of the bones. Strains in the collateral ligaments were calculated from insertion site distances., Results: UKA led to a less adducted and internally rotated tibia and a more strained medial collateral ligament (MCL). Addition of a patellofemoral replacement led to a more posterior position of both femoral condyles, a more dorsally located tibiofemoral contact point and higher MCL strain with squatting., Conclusion: In comparison to UKA modular BKA leads to a more dorsal tibial contact point, a medial femoral condyle being located more posteriorly, and more MCL strain. Mainly the changes to the trochlear anatomy as introduced by PFJ may account for these differences., (© 2014 Elsevier B.V. All rights reserved.)

When a stemmed TKA is needed in a femur in which a THA is already present, choosing an appropriate length for the TKA stem is crucial. Many surgeons intuitively fear that the distance between the stem tips correlates with the femur risk for fracture (RF). However, to date, no biomechanical data to support this intuition are available. Therefore, in this study, the RF in such a configuration was determined and compared for several activities, using a finite element modeling technique. During gait and sideways falling no difference in RF among different stem lengths was shown. However, a clear threshold appears during four-point bending. Stem tip distances shorter than 110 mm dramatically increased RF and, in osteoporotic bone, will certainly lead to fracture (RF>1) and thus should be avoided., (Copyright © 2013 Elsevier Inc. All rights reserved.)

We investigated the influence of freezing on the tensile strength of fresh frozen tendon grafts. The biomechanical characteristics of tendons that are less commonly used in knee surgery (tibialis anterior, tibialis posterior, peroneus longus and medial and lateral half of Achilles tendons) were compared to those of a semitendinosus and gracilis graft harvested from the same 10 multi-organ donors. All right side tendons constituted the study group and were frozen at -80 degrees C and thawed at room temperature 5 times. All left side tendons were frozen at -80 degrees C and thawed at room temperature once. There were 59 tendons in the control group and 56 in the study group. The looped grafts were clamped at one side using a custom-made freeze clamp and loaded until failure on an Instron 4505 testing machine. The average ultimate failure load was not significantly different between the control and the study group (p > 0.05). The failure load of the medial tendon Achilles was the lowest in both study and control group (p < 0.001). There was no significant difference in maximum stress, maximum displacement, maximum strain and stiffness between the control and study group (p > 0.05). From our study, we conclude that freezing tendons at -80 degrees C and thawing several times does not influence the maximum load, maximum stress, maximum displacement, maximum strain and stiffness. The medial half of the Achilles tendon is clearly the weakest tendon (p < 0.001). These findings show that tendon grafts can be frozen at -80 degrees C and thawed at room temperature several times without altering their biomechanical properties.

Background: Passive knee stability is provided by the soft tissue envelope. There is consensus among orthopedic surgeons that good outcome in Total Knee Arthroplasty requires equal tension in the medial/lateral compartment of the knee joint, as well as equal tension in the flexion/extension gap. The purpose of this study was to quantify the ligament laxity in the normal non-arthritic knee before and after Posterior-Stabilized Total Knee Arthroplasty. We hypothesized that the Medial/Lateral Collateral Ligament shows minimal changes in length when measured directly by extensometers in the native human knee during varus/valgus laxity testing. We also hypothesized that due to differences in material properties and surface geometry, native laxity is difficult to reconstruct using a Posterior-Stabilized Total Knee., Methods: Six specimens were used to perform this in vitro cadaver test using extensometers to provide numerical values for laxity and varus-valgus tilting in the frontal plane., Findings: This study enabled a precise measurement of varus-valgus laxity as compared with the clinical assessment. The strains in both ligaments in the replaced knee were different from those in the native knee. Both ligaments were stretched in extension, in flexion the Medial Collateral Ligament tends to relax and the Lateral Collateral Ligament remains tight., Interpretation: As material properties and surface geometry of the replaced knee add stiffness to the joint, we recommend to avoid overstuffing the joint, when using this type of Posterior-Stabilized Total Knee Arthroplasty, in order to obtain varus/valgus laxity close to the native joint., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Total Knee Arthroplasty (TKA) is a very successful surgical procedure but clinical outcomes were reported to be affected by implant design, ligament balancing, alignment or patient-related anatomical factors. It was recently demonstrated that malpositioning of the TKA components and patient related anatomical factors can considerably alter tibio-femoral (TF) and patellofemoral maximum contact forces. However, up to now, how a component malpositioning and different soft-tissue anatomy changes TF knee kinematics was not yet fully investigated. The goal of this study was to evaluate how sensitive TF kinematics are to these factors during a simulated loaded squat for different TKA designs. Four TKA types (a fixed bearing, posterior stabilized prosthesis; a high flexion fixed bearing guided motion prosthesis; a mobile bearing prosthesis and a hinge prosthesis) were virtually implanted on the same virtual cadaver leg model which underwent a loaded squat between 0° and 120°. The reference models were then modified to simulate either component malpositioning in several directions or changes in ligaments geometry by change in the collateral ligament insertions. The results showed that, for all implant designs, TF kinematics were affected by changes in implant positioning and anatomical factors. While the ranges of motion predicted for all tested configurations were generally similar to the reference configuration for each type of TKA, the modifications resulted in shifts in the maximum and minimum values for the TF rotations and translations., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Background: In patients with patellar instability, a pathological tibial tubercle-trochlear groove (TT-TG) distance is a risk factor. However, the TT-TG distance gives no information about the location of the malformation., Hypothesis: Not all patients with a pathological TT-TG distance (≥20 mm) had lateralization of the tibial tubercle., Study Design: Cohort study (diagnosis); Level of evidence, 2., Methods: Fifty-eight knees in 49 patients with 2 or more patellar dislocations and 60 knees in 30 volunteers with no history of dislocation were analyzed using magnetic resonance imaging (MRI). The tibial tubercle-posterior cruciate ligament (TT-PCL) distance was defined as the mediolateral distance between the tibial tubercle midpoint and the medial border of the posterior cruciate ligament. The distance was measured parallel to the dorsal aspect of the proximal tibia (dorsal tibia condylar line). Three observers performed the measurements. Significant differences in the TT-PCL distance between the patient and the control group were estimated using an unpaired t test. The inter- and intraobserver variability of the measurement was performed., Results: The intraclass correlation coefficients for inter- and intraobserver variability of the TT-PCL distance were higher than 0.74 and 0.93, respectively. A statistically significant difference (P < .05) was found between the TT-PCL distance in the control group (mean [SD], 18.4 [3.35] mm) and in patients (21.9 [4.30] mm). The mean (SD) TT-TG was 18.9 (5.16) mm in the patient group and 11.9 (4.67) mm in the control group (P < .05). In the control group, 95% had a TT-PCL distance <24 mm. In the patient group, 22 of 58 knee joints (38%) had a TT-PCL distance ≥24 mm. Seventeen of 40 knee joints (43%) with a TT-TG distance ≥20 mm had a TT-PCL distance <24 mm., Conclusion: Only 57% of the patients with a pathological TT-TG distance (≥20 mm) had lateralization of the tibial tubercle in relation to the posterior cruciate ligament. The TT-PCL distance is an alternative method for determining the position of the tibial tubercle.

Total knee arthroplasty (TKA) is a very successful procedure, but pain or difficulties during activities still persist in patients. Patient outcomes in TKA surgery can be affected by implant design, alignment or patient-related anatomical factors. This paper presents a numerical sensitivity analysis of several TKA types: a fixed bearing, posterior stabilized prosthesis, a high flexion fixed bearing guided motion prosthesis, a mobile bearing prosthesis and a hinge prosthesis. Each prosthesis was virtually implanted on the same cadaver leg model and it underwent a loaded squat, in 10s, between 0° and 120°, similar to several previous experimental tests performed on knee kinematics simulators. The aim of this examination was to investigate the sensitivity of the patello-femoral (PF) and tibio-femoral (TF) contact forces to patient-related anatomical factors, and component position in the different implant types. The following parameters were used for the sensitivity study: the proximo-distal patellar position, the patellar component tilting, the tibial component position and orientation, the locations of the medial and lateral collateral ligaments with respect to femur and tibia and the patellar tendon length. The sensitivity analysis showed that PF contact forces are mostly affected by patella height (increases up to 67% for one TKA type in patella-alta configuration), by an anterior tibial component translation (increases up to 30%), and by patellar component tilting (increases up to 29%); TF contact forces are mostly affected by the anterior displacement of the insertion points of the medial collateral ligament with respect to the reference position (increases up to 48%)., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Aseptic loosening of the tibial component remains a major cause of failure in total knee arthroplasty and may be related, directly or indirectly, to micromotion. Therefore, good fixation of the tibial component is a prerequisite to achieve long-term success of the implant. Cementing technique is one of the factors that play a role in this respect. We investigated the effect of different cementing techniques on the cement penetration in the proximal tibia. We compared 5 different cementing techniques in an anatomical open pore sawbone model (n = 25), using a contemporary total knee arthroplasty design and standard polymethylmetacrylate cement. We demonstrated that applying cement to both the undersurface of the tibial baseplate and as well as onto the tibial bone, either by a spatula or fingerpacking technique, leads to an optimal cement penetration of 3 to 5 mm. When cement is applied only onto the tibial component, penetration is insufficient. When a cement gun is used, cement penetration is too excessive., (Copyright © 2011 Elsevier Inc. All rights reserved.)

A comparative kinematics study was conducted on six cadaver limbs, comparing tibiofemoral kinematics in five conditions: unloaded, under a constant 130 N ankle load with a variable quadriceps load, with and without a simultaneous constant 50 N medial and lateral hamstrings load. Kinematics were described as translation of the projected centers of the medial (MFT) and lateral femoral condyles (LFT) in the horizontal plane of the tibia, and tibial axial rotation (TR) as a function of flexion angle. In passive conditions, the tibia rotated internally with increasing flexion to an average of -16 degrees (range: -12/-20 degrees , SD = 3.0 degrees ). Between 0 and 40 degrees flexion, the medial condyle translated forwards 4 mm (range: 0.8/5.5 mm, SD = 2.5 mm), followed by a gradual posterior translation, totaling -9 mm (range: -5.8/-18.5 mm, SD = 4.9 mm) between 40-140 degrees flexion. The lateral femoral condyle translated posteriorly with increasing flexion completing -25 mm (range: -22.6 to -28.2 mm, SD = 2.5 mm). Dynamic, loaded measurements simulating a deep knee bend were carried out in a knee rig. Under a fixed ankle load of 130 N and variable quadriceps loading, tibial rotation was inverted, mean TR = 4.7 degrees (range: -3.3 degrees /11.8 degrees SD = 5.4 degrees ), MFT = -0.5 mm (range: = -4.3/2.4 mm, SD = 2.4 mm), LFT = 3.3 mm (range: = -3.6/10.6 mm, SD = 5.1 mm). Compared to the passive condition, all these excursions were significantly different (p < or = 0.015). Adding medial and lateral hamstrings force of 50 N each reduced TR, MFT, and LFT significantly compared to the passive condition. In general, loading the knee with hamstrings and quadriceps reduces rotation and translation compared to the passive condition. Lateral hamstring action is more influential on knee kinematics than medial hamstrings action.

In vivo fluoroscopy is a well-known technique to analyze joint kinematics of the replaced knee. With this method, however, the contact areas between femoral and tibial components, fundamental for monitoring wear and validating design concepts, are hard to identify. We developed and tested a novel technique to assess condylar and post-cam contacts in TKA. The technique uses in vivo motion data of the replaced knee from standard fluoroscopy as input for finite element models of the prosthesis components. In these models, tibiofemoral contact patterns at the condyles and post-cam articulations were calculated during various activities. To test for feasibility, the technique was applied to a bicruciate posterior-stabilized prosthesis. Sensitivity of the finite element analysis, validation of the technique, and in vivo tests were performed. To test for potential in the clinical setting, five patients were preliminarily analyzed during chair rising-sitting, stair climbing, and step up-down. For each task and patient, the condylar contact points and contact line rotation were calculated. The results were repeatable and consistent with corresponding calculations from traditional fluoroscopic analysis. Specifically, natural knee kinematics, which shows rolling back and screw home, seemed replicated in all motor tasks. Post-cam contact was observed on both the anterior and posterior faces. Anterior contact is limited to flexion angle close to extension; posterior contact occurs in deeper flexion but is dependent on the motor task. The data suggest the proposed technique provides reliable information to analyze post-cam contacts.

Although several techniques exist for cementing the femoral component in TKA, no data are available on which is the best one to use. We therefore compared four cementing techniques in an anatomical open pore sawbone model (n=20), in order to investigate the influence of cementation technique on overall cement penetration as well as length of the cement mantle over the different cuts. The technique which included cement application onto the anterior and distal bone surfaces, as well as the posterior flanges of the prosthesis, was statistically superior to the other techniques. We therefore advocate this technique as the standard for cementing the femoral component.

Background: The tibio-talar contact area has been widely investigated to monitor biomechanical changes due to articular incongruities or an altered loading. This study aims to investigate for the first time in a systematic way the extent of the inter-specimen variability of the tibio-talar contact area, and its repercussions when analyzing data concerning this parameter., Methods: Ten specimens were loaded to record the tibio-talar contact characteristics by use of pressure sensitive film. The size of the talar dome area, the size of the (normalized) tibio-talar contact area, the position of the tibio-talar contact area, and the shape of the latter were determined and analyzed. Inter-specimen variability was expressed as the coefficient of variation and was calculated for the datasets of previous studies as well., Findings: The size of the tibio-talar contact area showed a very high inter-specimen variability, as is the case in previous studies. This high variability persisted when a normalized tibio-talar contact area was calculated. The shape of the tibio-talar contact area showed some basic characteristics, but a high variation in details could be observed., Interpretation: Every specimen can be considered to have its own "ankle print". By this variability, articular incongruities are expected to have a different effect on local biomechanical characteristics in every single individual. Therefore, every single case has to be evaluated and reported for significant changes. In case of modeling, this also underscores the need to use subject specific models fed by sets of parameters derived from a series of single specimens.

In a robot-assisted procedure for preparing the tibia in total knee arthroplasty, developed in the authors' laboratory, an intramedullary rod is used to register the tibia. In 18 formalin-fixed tibias, the difference in orientation was calculated between the intramedullary rod and several longitudinal tibial axes used in clinical practice. This was done using roentgenstereophotogrammetric analysis. Three tibial axes and two insertion techniques were considered. In three-dimensional space, small differences between the axes are observed. The results showed a high standard deviation, indicating the importance of anatomic differences. In the frontal plane, the difference in orientation between rod and tibial axes never exceeded +/- 2 degrees. In the sagittal plane, the observed differences were larger. Significant differences between the considered axes appeared. The results of the two insertion techniques were not significantly different. Because an intramedullary rod frequently is used for alignment of the tibia in conventional surgery, these results also are valuable for conventional surgery. In the current study, the accuracy of the intramedullary alignment is examined, without influences of the sawing procedure. Moreover, the study is not limited to the frontal plane; the total accuracy in three-dimensional space, and the accuracy in the frontal and the sagittal planes were studied.

Design and materials selection are equally important in the development of orthopaedic implants. Two case studies are presented to illustrate this: the development of a femoral component of a total hip prosthesis and the study of alternative designs of a tibial component of a total knee prosthesis. Bioactive surface coatings may be applied to enhance the stability of fixation of the implant, even in difficult clinical cases. It is argued that an improved long-term performance of an implant can only be achieved by considering the biomechanics and biomaterials aspects of joint replacement together, and at the same time guaranteeing the quality of surgery by providing the surgeon with better pre-surgical planning systems and advanced surgical tools.

We introduce a new method with a motion-analysis system (MAS) to study the vertebral model in vitro. Compared with the currently most accurate technique, roentgen stereophotogrammetric analysis (RSA), the difference between the RSA and the MAS is 0.12 degree +/- 1.64 degrees. An accuracy with an error of 0.08 degree +/- 1.15 degrees is determined by means of an angle gauge. Although a significant difference between the MAS and the goniometer (p = 0.04) is found around the X-axis (theta; transverse plane), it is limited to < 1 degree. The MAS provides an in-depth insight into the mechanism of the three-dimensional rotation at each vertebra in vivo. The backward inclination of the apical vertebra (AV) and forward inclination of the upper-end vertebra (UEV) around the Y-axis (phi) results in a correction of the hypokyphosis shown by the Cobb angle in the sagittal plane. The clockwise rotation of the UEV in the Z-axis (psi) leads to a reduction of the Cobb angle in the frontal plane. Additionally, the MAS as an intraoperative alternative shows different results of the derotation maneuver by the Cotrel-Dubousset instrumentation (CDI) compared with the computed tomography (CT) scan. Our method gives more direct details of the derotation not influenced by patient posture, as observed in the CT scan.