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The influence of sex on preoperative expectations and their fulfillment following total hip and knee arthroplasty (THA/TKA) remains unexplored. We investigated differences between men and women in perceived preoperative expectations on the outcome of THA/TKA and their fulfillment 1 year postoperatively. We performed a cohort study. Expectations were collected preoperatively and 1 year postoperatively using the Hospital for Special Surgery Hip/Knee Replacement Expectations Surveys (HSS-HRES/KRES; not applicable = 0, applicable: back to normal = 1, much = 2/moderate = 3/slight improvement = 4). Fulfillment of expectations was calculated by subtracting preoperative from postoperative scores (score < 0:unfulfilled; score ≥ 0:fulfilled). We included patients with "applicable" expectations. Chi-square and ordinal regression were used to compare expectations and fulfillment regarding sex. 2333 THA (62% women) and 2398 TKA (65% women) patients were included. 77% of THA and 76% of TKA patients completed the HSS-HRES/HSS-KRES both preoperatively and 1 year postoperatively. Men more often perceived items as "applicable", with differences in 9/20 (HSS-HRES) and 9/19 (HSS-KRES) preoperative items and, respectively, 12/20 (HSS-HRES) and 10/19 (HSS-KRES) postoperative items. The largest differences (> 10%) were found in sexual activity and working ability. 16/20 (HSS-HRES) and 14/19 (HSS-KRES) items showed an increased probability of having higher preoperative expectations of ≥ 10%, in favor of men. In all items, 60% of the respondents indicated that their expectation was fulfilled. Differences were observed in 16/20 (HSS-HRES) and 6/19 (HSS-KRES) items in favor of men. Sex differences were present in expectations and fulfillment, with higher applicability of items, preoperative expectations and fulfillment in men, especially on items related to functional activities. Trial registration: Trial-ID NTR3348. [ABSTRACT FROM AUTHOR]

Background: Total hip or knee arthroplasties (THA/TKA) show favorable long-term effects, yet the recovery process may take weeks to months. Physical therapy (PT) following discharge from hospital is an effective intervention to enhance this recovery process. To investigate the relation between recovery and postoperative PT usage, including the presence of comorbidities, 6 months after THA/TKA.Methods: Multicenter, observational study in primary THA/TKA patients who completed preoperative and 6 months postoperative assessments. The assessments included questions on PT use (yes/no and duration; long term use defined as ≥ 12 weeks), comorbidities (musculoskeletal, non-musculoskeletal, sensory comorbidities and frequency of comorbidities). Recovery was assessed with the HOOS/KOOS on all 5 subdomains. Logistic regression with long term PT as outcome was performed adjusted for confounding including an interaction term (comorbidity*HOOS/KOOS-subdomain).Results: In total, 1289 THA and 1333 TKA patients were included, of whom 95% received postoperative PT, 56% and 67% received postoperative PT ≥ 12 weeks respectively. In both THA and TKA group, less improvement on all HOOS/KOOS domain scores was associated with ≥ 12 weeks of postoperative PT (range Odds Ratios 0.97-0.99). In the THA group the impact of recovery was smaller in patient with comorbidities as non- musculoskeletal comorbidities modified all associations between recovery and postoperative PT duration (Odds Ratios range 1.01-1.05). Musculoskeletal comorbidities modified the associations between Function-in-Daily-Living-and Sport-and-recreation recovery and postoperative PT. Sensory comorbidities only had an effect on Sport-and-recreation recovery and postoperative PT. Also the frequency of comorbidities modified the relation between Function-in-Daily-Living, pain and symptoms recovery and postoperative PT. In the TKA group comorbidity did not modify the associations.Conclusion: Worse recovery was associated with longer duration of postoperative PT suggesting that PT provision is in line with patients' needs. The impact of physical recovery on the use of long-term postoperative PT was smaller in THA patients with comorbidities.Trial Registration: Registered in the Dutch Trial Registry on March 13, 2012. TRIAL ID NTR3348; registration number: P12.047. https://www.trialregister.nl/trial/3197 . [ABSTRACT FROM AUTHOR]

Chronic pain is frequently reported after total hip and knee arthroplasties (THA/TKA) in osteoarthritis (OA) patients. We investigated if severity of acute postoperative pain following THA/TKA in OA patients was associated with pain during the first postoperative year. From an observational study, OA patients scheduled for primary THA/TKA (June 2012–December 2017) were included from two hospitals in the Netherlands. Acute postoperative pain scores were collected within 72 h postoperatively and categorized as no/mild (NRS ≤ 4) or moderate/severe (NRS > 4). Pain was assessed preoperatively, 3, 6 and 12 months postoperatively using the HOOS/KOOS subscale pain. With Multilevel Mixed-effects-analyses, we estimated associations between acute and chronic pain until one year postoperative, adjusted for confounders and including an interaction term (Time*Acute pain). 193 THA and 196 TKA patients were included, 29% of THA and 51% of TKA patients reported moderate/severe pain acutely after surgery. In the THA group, the difference in pain at 3 months between the no/mild and moderate/severe groups, was approximately six points, in favor of the no/mild group (95% CI [−12.4 to 0.9]) this difference became smaller over time. In the TKA group we found similar differences, with approximately four points (95% CI [−9.6 to 1.3]) difference between the no/mild and moderate/severe group at 6 months, this difference attenuated at 12 months. No association between severity of acute postoperative pain and pain during the first postoperative year was found. These findings suggest that measures to limit acute postoperative pain will likely not impact development of chronic pain. [ABSTRACT FROM AUTHOR]

Knee contact mechanics play an important role in knee implant failure and wear mechanics. Femoral condylar contact loss in total knee arthroplasty has been reported in some studies and it is considered to potentially induce excessive wear of the polyethylene insert. Measuring in vivo forces applied to the tibial plateau with an instrumented prosthesis is a possible approach to assess contact loss in vivo' but this approach is not very practical. Alternatively, single-plane fluoroscopy and pose estimation can be used to derive the relative pose of the femoral component with respect to the tibial plateau and estimate the distance from the medial and lateral parts of the femoral component towards the insert. Two measures are reported in the literature: lift-off is commonly defined as the difference in distance between the medial and lateral condyles of the femoral component with respect to the tibial plateau; separation is determined by the closest distance of each condyle towards the polyethylene insert instead of the tibia plateau. In this validation study, lift-off and separation as measured with single-plane fluoroscopy are compared to in vivo contact forces measured with an instrumented knee implant. In a phantom study, lift-off and separation were compared to measurements with a high quality bi-plane measurement. The results of the in vivo contact-force experiment demonstrate a large discrepancy between single-plane fluoroscopy and the in vivo force data: single-plane fluoroscopy measured up to 5.1 mm of lift-off or separation, whereas the force data never showed actual loss of contact. The phantom study demon-strated that the single-plane setup could introduce an overestimation of 0.22 mm ± ± 0.36 mm. Correcting the out-of-plane position resulted in an underestimation of medial separation by - 0.20 mm ± ± 0.29 mm. In conclusion, there is a discrepancy between the in vivo force data and single-plane fluoroscopic measurements. Therefore contact loss may not always be determined reliably by single plane fluoroscopy analysis. [ABSTRACT FROM AUTHOR]

It has been recently suggested that mechanical loads applied at frequencies close to the natural frequencies of bone could enhance bone apposition due to the resonance phenomenon. Other applications of bone modal analysis are also suggested. For the above-mentioned applications, it is important to understand how patient-specific bone shape and density distribution influence the natural frequencies of bones. We used finite element models to study the effects of bone shape and density distribution on the natural frequencies of the femur in free boundary conditions. A statistical shape and appearance model that describes shape and density distribution independently was created, based on a training set of 27 femora. The natural frequencies were then calculated for different shape modes varied around the mean shape while keeping the mean density distribution, for different appearance modes around the mean density distribution while keeping the mean bone shape, and for the 27 training femora. Single shape or appearance modes could cause up to 15% variations in the natural frequencies with certain modes having the greatest impact. For the actual femora, shape and density distribution changed the natural frequencies by up to 38%. First appearance mode that describes the general cortical bone thickness and trabecular bone density had one of the strongest impacts. The first appearance mode could therefore provide a sensitive measure of general bone health and disease progression. Since shape and density could cause large variations in the calculated natural frequencies, patient-specific FE models are needed for accurate estimation of bone natural frequencies. [ABSTRACT FROM AUTHOR]

Implant failure related to polyethylene wear remains an important issue in total knee arthroplasty. Polyethylene wear is usually assessed in vivo by measuring the remaining insert thickness on X-ray images of the knee. To reflect the amount of wear debris more accurately, a 3-dimensional overlap measurement has been suggested, which is based on implant component models which are matched on calibrated stereo X-ray images using model-based roentgen stereophotogrammatic analysis. The goal of this study was to determine the influence of pose estimation, insert thickness deviation and variation in the femoral-tibial contact location on the accuracy and precision of the measurement using simulations and a phantom experiment. We found that the pose estimation was the largest source of variation. The 95% prediction interval varied between 111 and 283 mm³, which is approximately 100-200% of the detected volumetric wear. Insert thickness variation resulted in prediction intervals of 74-174 mm³. Variation of the femoral-tibial contact location in the phantom experiment gave a prediction interval of 40mm³. Large differences in the detected wear volume were found for different flexion angles. At most 56% of the true wear volume was detected (129 of 230 mm³, 30° of flexion). In summary, both the accuracy and precision of the volumetric wear measurement were low. The prediction interval of the volumetric wear measurement is at least as large as the measurement outcome itself. This is an important limitation to the applicability of the volumetric wear measurement in clinical practice and further clinical validation is required. [ABSTRACT FROM AUTHOR]

Model-based Roentgen Stereophotogrammetric Analysis (RSA) is able to measure the migration of metallic prostheses with submillimeter accuracy through contour-detection and 3D surface model matching techniques. However, contour-detection is only possible if the prosthesis is clearly visible in the radiograph; consequently Model-based RSA cannot be directly used for polymeric materials due to their limited X-ray attenuation; this is especially clinically relevant for all-polyethylene implants. In this study the radiopacity of unicompartmental Ultra-High Molecular Weight Polyethylene (UHMWPE) knee bearings was increased by diffusing an oil-based contrast agent into the surface to create three different levels of surface radiopacity. Model-based RSA was performed on the bearings alone, the bearings alongside a metallic component held in position using a phantom, the bearings cemented into a Sawbone tibia, and the bearings at different distances from the femoral component. For each condition the precision and accuracy of zero motion of Model-based RSA were assessed. The radiopaque bearings could be located in the stereo-radiographs using Model-based RSA an accuracy comparable to metallic parts for translational movements (0.03 mm to 0.50 mm). For rotational movements, the accuracy was lower (0.1 ∘ to 3.0 ∘ ). The measurement accuracy was compared for all the radiopacity levels and no significant difference was found (p=0.08). This study demonstrates that contrast enhanced radiopaque polyethylene can be used for Model-based RSA studies and has equivalent translational measurement precision to metallic parts in the superior-inferior direction., Competing Interests: Declaration of Competing Interest Funding: The University of Bath funded Dr Zaribaf's PhD project through the University Research Studentship Award funding scheme. Santander funded Dr Zaribaf's placement at Leiden through the Postgraduate Mobility Award Scheme. Zimmer-Biomet provided the Oxford Partial knee components used for the analysis. Ethical approval: Not required. Competing interests: None., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Background: Cementless fixation is an alternative to cemented unicompartmental knee replacement (UKR). The aim of this study was to determine if cementless UKR fixation is as good as cemented by comparing the five-year migration measured radiostereometric analysis (RSA) in a randomised controlled trial., Methods: Thirty-nine patients were randomised to receive either a cemented or a cementless Oxford UKR and were studied at intervals up to five years to assess migration with RSA and radiolucencies with radiographs., Results: During the first year there was a small and significant amount of migration, predominantly in an anterior direction, of both the cemented (0.24 mm, SD 0.32, p = 0.01) and cementless (0.26 mm, SD 0.31, p = 0.00) femoral components. Thereafter there was no significant migration in any direction. At no stage was there any significant difference between the migrations of the cemented or cementless femoral components. During the first year, particularly the first three months, the cementless tibial components subsided 0.28 mm (SD 0.19, p = 0.00). This was significantly (p = 0.00) greater than the subsidence of the cemented tibial component (0.09, SD 0.19, p = 0.28). Between the second and fifth years there was no significant migration of either cemented or cementless tibial components. At five years radiolucent lines occurred significantly less with cementless (one partial) compared to cemented (six partial and one complete) tibial components., Conclusions: As, between two and five years, there was no significant migration of cemented or cementless components, and no significant difference between them, we conclude that cementless fixation is as reliable as cemented. It may be better as there are fewer radiolucent lines., Competing Interests: Declaration of competing interest The authors declare that personal and institutional funding was received from Zimmer Biomet. Zimmer Biomet did not play any role in the study design, the collection, analysis and interpretation of data, in the writing of the manuscript, or in the decision to submit the manuscript for publication., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Fluoroscopy-derived joint kinematics plays an important role in the evaluation of knee prostheses. Fluoroscopic analysis requires estimation of the 3D prosthesis pose from its 2D silhouette in the fluoroscopic image, by optimizing a dissimilarity measure. Currently, extensive user-interaction is needed, which makes analysis labor-intensive and operator-dependent. The aim of this study was to review five optimization methods for 3D pose estimation and to assess their performance in finding the correct solution. Two derivative-free optimizers (DHSAnn and IIPM) and three gradient-based optimizers (LevMar, DoNLP2 and IpOpt) were evaluated. For the latter three optimizers two different implementations were evaluated: one with a numerically approximated gradient and one with an analytically derived gradient for computational efficiency. On phantom data, all methods were able to find the 3D pose within 1mm and 1° in more than 85% of cases. IpOpt had the highest success-rate: 97%. On clinical data, the success rates were higher than 85% for the in-plane positions, but not for the rotations. IpOpt was the most expensive method and the application of an analytically derived gradients accelerated the gradient-based methods by a factor 3-4 without any differences in success rate. In conclusion, 85% of the frames can be analyzed automatically in clinical data and only 15% of the frames require manual supervision. The optimal success-rate on phantom data (97% with IpOpt) on phantom data indicates that even less supervision may become feasible., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Excessive wear in total knee arthroplasty is detected by measuring the minimum joint space width (mJSW) in anterioposterior radiographs. The accuracy of conventional measurement methods is limited and can be improved using model-based techniques. In this study, the model-based wear measurement (MBWM) is introduced. Its accuracy and reproducibility are assessed and compared to the conventional measurement. Forty anterioposterior radiographs were obtained of a knee prosthesis using a phantom setup. Both measurement methods were applied and the accuracy and precision were compared. The reproducibility was calculated with inter- and intra-observer experiments. Three observers measured the mJSW in 30 clinical radiographs with both the conventional measurement and the MBWM and repeated this after 6 weeks. The experiments were conducted with a NexGen mobile bearing and fixed bearing prostheses. In the phantom experiment, the accuracy (mean of the absolute error) was significantly higher (t-test, p<0.01) for the MBWM as for the conventional measurement (0.15 mm versus 0.43 mm, 0.14 mm versus 0.35 mm for the mobile and fixed bearing, respectively). The standard deviation of the measurements is the smallest for the MBWM measurement for both prosthesis types (0.16 mm versus 0.47 mm, Levene's test, p<0.01). In the reproducibility experiment, both the intra- and inter-observer agreements were higher for the MBWM than for the conventional method. The results show that the MBWM is superior to the conventional measurement in both accuracy and reproducibility. Although the use of a phantom experiment poses some limitations in conveying the findings to clinical practice, this improved mJSW measurement can lead to better wear detection for surgery decisions and research purposes., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Accurate in vivo measurements methods of wear in total knee arthroplasty are required for a timely detection of excessive wear and to assess new implant designs. Component separation measurements based on model-based Roentgen stereophotogrammetric analysis (RSA), in which 3-dimensional reconstruction methods are used, have shown promising results, yet the robustness of these measurements is unknown. In this study, the accuracy and robustness of this measurement for clinical usage was assessed. The validation experiments were conducted in an RSA setup with a phantom setup of a knee in a vertical orientation. 72 RSA images were created using different variables for knee orientations, two prosthesis types (fixed-bearing Duracon knee and fixed-bearing Triathlon knee) and accuracies of the reconstruction models. The measurement error was determined for absolute and relative measurements and the effect of knee positioning and true seperation distance was determined. The measurement method overestimated the separation distance with 0.1mm on average. The precision of the method was 0.10mm (2*SD) for the Duracon prosthesis and 0.20mm for the Triathlon prosthesis. A slight difference in error was found between the measurements with 0° and 10° anterior tilt. (difference=0.08mm, p=0.04). The accuracy of 0.1mm and precision of 0.2mm can be achieved for linear wear measurements based on model-based RSA, which is more than adequate for clinical applications. The measurement is robust in clinical settings. Although anterior tilt seems to influence the measurement, the size of this influence is low and clinically irrelevant., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Fluoroscopic analysis is an important tool for assessing in vivo kinematics of knee prostheses. Most commonly, a single-plane fluoroscopic setup is used to capture the motion of prostheses during a particular task. Unfortunately, single-plane fluoroscopic analysis is imprecise in the out-of-plane direction. This can result in reconstructing physically impossible poses, in which--for example--the femoral component intersects with the insert, as the normal pose estimation process does not take into account the relation between the components. In the proposed method, the poses of both components are estimated simultaneously, while preventing femur-insert collisions. In a phantom study, the accuracy and precision of the new method in estimating the relative pose of the femoral component were compared to those of the original method. With reverse engineered models, the errors in estimating the out-of-plane position decreased from 2.0+/-0.7 to 0.1+/-0.1 mm, without effects on the errors in rotations and the in-plane positions. With CAD models, the errors in estimating the out-of-plane position decreased from 5.3+/-0.7 mm (mean+/-SD) to 0.0+/-0.4 mm, at the expense of a decreased precision for the other position or orientation parameters. In conclusion, collision detection can prevent reconstructing impossible poses and it improves the position and motion estimation in the out-of-plane direction., (Copyright 2009 Elsevier Ltd. All rights reserved.)

Migration measurements of hip prostheses using marker-based Roentgen stereophotogrammetric analysis (RSA) require the attachment of markers to the prostheses. The model-based approach, which does not require these markers, is, however, less precise. One of the reasons may be the fact that the spherical head has not been modelled. Therefore, we added a 3D surface model of the spherical head and estimated the position and orientation of the combined stem-head model. The new method using a combined stem-head model was compared in a phantom study on five prostheses (of different types) and in a clinical study using double examinations of implanted hip prostheses, with two existing methods: a standard model-based approach and one using elementary geometrical shapes. The combined model showed the highest precision for the rotation about the longitudinal axis in the phantom experiments. With a standard deviation of 0.69 degrees it showed a significant improvement (p=0.02) over the model-based approach (0.96 degrees ) on the phantom data, but no improvement on the clinical data. Overall, the use of elementary geometrical shapes was worse with respect to the model-based approach, with a standard deviation of 1.02 degrees on the phantom data and 0.79 degrees on the clinical data. This decrease in precision was significant (p<0.01) on the clinical data. With relatively small differences in the other migration directions, these results demonstrate that the new method with a combined stem-head model can be a useful alternative to the standard model-based approach.

Model-based Roentgen Stereophotogrammetric Analysis (RSA) was recently developed for the measurement of prosthesis micromotion. Its main advantage is that markers do not need to be attached to the implants as traditional marker-based RSA requires. Model-based RSA has only been tested in uniplanar radiographic set-ups. A biplanar set-up would theoretically facilitate the pose estimation algorithm, since radiographic projections would show more different shape features of the implants than in uniplanar images. We tested the precision of model-based RSA and compared it with that of the traditional marker-based method in a biplanar set-up. Micromotions of both tibial and femoral components were measured with both the techniques from double examinations of patients participating in a clinical study. The results showed that in the biplanar set-up model-based RSA presents a homogeneous distribution of precision for all the translation directions, but an inhomogeneous error for rotations, especially internal-external rotation presented higher errors than rotations about the transverse and sagittal axes. Model-based RSA was less precise than the marker-based method, although the differences were not significant for the translations and rotations of the tibial component, with the exception of the internal-external rotations. For both prosthesis components the precisions of model-based RSA were below 0.2 mm for all the translations, and below 0.3 degrees for rotations about transverse and sagittal axes. These values are still acceptable for clinical studies aimed at evaluating total knee prosthesis micromotion. In a biplanar set-up model-based RSA is a valid alternative to traditional marker-based RSA where marking of the prosthesis is an enormous disadvantage.

Image-based Roentgen stereophotogrammetric analysis (IBRSA) integrates 2D-3D image registration and conventional RSA. Instead of radiopaque RSA bone markers, IBRSA uses 3D CT data, from which digitally reconstructed radiographs (DRRs) are generated. Using 2D-3D image registration, the 3D pose of the CT is iteratively adjusted such that the generated DRRs resemble the 2D RSA images as closely as possible, according to an image matching metric. Effectively, by registering all 2D follow-up moments to the same 3D CT, the CT volume functions as common ground. In two experiments, using RSA and using a micromanipulator as gold standard, IBRSA has been validated on cadaveric and sawbone scapula radiographs, and good matching results have been achieved. The accuracy was: |mu |< 0.083 mm for translations and |mu| < 0.023 degrees for rotations. The precision sigma in x-, y-, and z-direction was 0.090, 0.077, and 0.220 mm for translations and 0.155 degrees , 0.243 degrees , and 0.074 degrees for rotations. Our results show that the accuracy and precision of in vitro IBRSA, performed under ideal laboratory conditions, are lower than in vitro standard RSA but higher than in vivo standard RSA. Because IBRSA does not require radiopaque markers, it adds functionality to the RSA method by opening new directions and possibilities for research, such as dynamic analyses using fluoroscopy on subjects without markers and computer navigation applications.

When measuring knee kinematics with skin-mounted markers, soft tissue and structures surrounding the knee hide the actual underlying segment kinematics. Soft-tissue artefacts can be reduced when plate-mounted markers or marker trees are used instead of individual unconstrained mounted markers. The purpose of this study was to accurately quantify the soft-tissue artefacts and to compare two marker cluster fixation methods by using fluoroscopy of knee motion after total knee arthroplasty during a step-up task. Ten subjects participated 6 months after their total knee arthroplasty. The patients were randomised into (1) a plate-mounted marker group and (2) a strap-mounted marker group. Fluoroscopic data were collected during a step-up motion. A three-dimensional model fitting technique was used to reconstruct the in vivo 3-D positions of the markers and the implants representing the bones. The measurement errors associated with the thigh were generally larger (maximum translational error: 17mm; maximum rotational error 12 degrees ) than the measurement errors for the lower leg (maximum translational error: 11mm; maximum rotational error 10 degrees ). The strap-mounted group showed significant more translational errors than the plate-mounted group for both the shank (respectively, 3+/-2.2 and 0+/-2.0mm, p = 0.025) and the thigh (2+/-2.0 and 0+/-5.9mm, p = 0.031). The qualitative conclusions based on interpretation of the calculated estimates of effects within the longitudinal mixed-effects modelling evaluation of the data for the two groups (separately) were effectively identical. The soft-tissue artefacts across knee flexion angle could not be distinguished from zero for both groups. For all cases, recorded soft-tissue artefacts were less variable within subjects than between subjects. The large soft-tissue artefacts, when using clustered skin markers, irrespective of the fixation method, question the usefulness of parameters found with external movement registration and clinical interpretation of stair data in small patient groups.

The hypothesis of this study was that the polyethylene bearing in a rotating platform total knee prosthesis shows axial rotation during a step-up motion, thereby facilitating the theoretical advantages of mobile-bearing knee prostheses. We examined 10 patients with rheumatoid arthritis who had a rotating platform total knee arthroplasty (NexGen LPS mobile, Zimmer Inc. Warsaw, USA). Fluoroscopic data was collected during a step-up motion six months postoperatively. A 3D-2D model fitting technique was used to reconstruct the in vivo 3D kinematics. The femoral component showed more axial rotation than the polyethylene mobile-bearing insert compared to the tibia during extension. In eight knees, the femoral component rotated internally with respect to the tibia during extension. In the other two knees the femoral component rotated externally with respect to the tibia. In all 10 patients, the femur showed more axial rotation than the mobile-bearing insert indicating the femoral component was sliding on the polyethylene of the rotating platform during the step-up motion. Possible explanations are a too limited conformity between femoral component and insert, the anterior located pivot location of the investigated rotating platform design, polyethylene on metal impingement and fibrous tissue formation between the mobile-bearing insert and the tibial plateau.

Roentgen stereophotogrammetric analysis (RSA) measures micromotion of an orthopaedic implant with respect to its surrounding bone. A problem in RSA is that the markers are sometimes overprojected by the implant itself. This study describes the so-called Marker Configuration Model-based RSA (MCM-based RSA) that is able to measure the pose of a rigid body in situations where less than three markers could be detected in both images of an RSA radiograph. MCM-based RSA is based on fitting a Marker Configuration model (MC-model) to the projection lines from the marker projection positions in the image to their corresponding Roentgen foci. An MC-model describes the positions of markers relative to each other and is obtained using conventional RSA. We used data from 15 double examinations of a clinical study of total knee prostheses and removed projections of the three tibial component markers, simulating occlusion of markers. The migration of the tibial component with respect to the bone, which should be zero, for the double examination is a measure of the accuracy of algorithm. With the new algorithm, it is possible to estimate the pose of a rigid body of which one or two markers are occluded in one of the images of the RSA radiograph with high accuracy as long as a proper MC-model of the markers in the rigid body is available. The new algorithm makes RSA more robust for occlusion of markers. This improves the results of clinical RSA studies because the number of lost RSA follow-up moments is reduced.

For individualization of a biomechanical model, it is necessary to estimate the muscle attachments of the person to whom it is to be adapted. One of the methods to estimate muscle attachments is to use model transformations to transform a model with known muscle attachments to the bones of a person. We hypothesize that the location and shape of muscle attachment sites correlate with the shape of the bones they are attached to. If this hypothesis holds, it is possible to predict the location of muscle attachments when the shape of the bones is known. To validate this hypothesis, geometric models of three sets of shoulder bones were built. These models consist of 3-D surface models of the scapula, clavicle, and humerus, with the muscle attachment contours connected to them. By means of geometric transformations, the models were transformed, so the muscle attachments of the different data sets could be compared. Using these techniques, 50 per cent of the muscle attachment contours could be predicted with high accuracy. The muscle attachment contours that could not be predicted were all influenced by measurement errors. For 30 per cent of the muscle attachment contours, it was not possible to distinguish the interindividual differences from the inaccuracies of the method used. From this study, we concluded that most muscle attachment contours can be predicted by means of geometric models of the bones.

Roentgen stereophotogrammetric analysis (RSA) was developed to measure micromotion of an orthopaedic implant with respect to its surrounding bone. A disadvantage of conventional RSA is that it requires the implant to be marked with tantalum beads. This disadvantage can potentially be resolved with model-based RSA, whereby a 3D model of the implant is used for matching with the actual images and the assessment of position and rotation of the implant. In this study, a model-based RSA algorithm is presented and validated in phantom experiments. To investigate the influence of the accuracy of the implant models that were used for model-based RSA, we studied both computer aided design (CAD) models as well as models obtained by means of reversed engineering (RE) of the actual implant. The results demonstrate that the RE models provide more accurate results than the CAD models. If these RE models are derived from the very same implant, it is possible to achieve a maximum standard deviation of the error in the migration calculation of 0.06 mm for translations in x- and y-direction and 0.14 mm for the out of plane z-direction, respectively. For rotations about the y-axis, the standard deviation was about 0.1 degrees and for rotations about the x- and z-axis 0.05 degrees. Studies with clinical RSA-radiographs must prove that these results can also be reached in a clinical setting, making model-based RSA a possible alternative for marker-based RSA.