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Background: Noncruciate total knee arthroplasty designs, including ultracongruent, medially congruent, and medial pivot, are gaining increasing attention in total knee arthroplasty surgery. However, there is no consensus for the bearing surface design, whether there should be different medial, lateral, anterior, and posterior laxities, or whether the medial side should be a medial pivot. This study proposes the criterion of reproducing the laxity of the anatomic knee, defined as the displacements and rotations of the femur on the tibia in the loaded knee when shear and torque are applied. The purpose of this study was to determine the ideal tibial radii to achieve that goal., Methods: The femoral component was based on the average knee from 100 mild arthritic knee scans. There were 8 tibial components that were designed with different sagittal radii: antero-medial, antero-lateral, postero-medial, and postero-lateral. Radii were defined as the percent height reduction from full conformity with the femoral profile. Components were 3-dimensional-printed. A test rig was constructed where the tibial component was fixed and shear and torque were applied to the femoral component. Displacements and rotations of the femoral component were measured at 0 and 45° of flexion, the latter representing any flexion angle due to the constant femoral sagittal radius., Results: Displacements ranged from 0 to 11 mm, and rotations ranged from 1 to 11°. Anterior femoral displacements were higher than posterior due to the shallow distal-anterior femoral profile. The final femoral and tibial components with the most closely matched anatomic laxity values were designed and tested., Conclusions: A steeper distal-anterior femoral radius was an advantage. High medial-anterior tibial conformity was important. However, on the lateral side, the posterior sagittal tibial radius had to be shallower than ideal to allow femoral rollback in high flexion. This meant that the posterior laxity displacements on the lateral side were higher than anatomic, and there was no guidance for lateral femoral rollback., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Background: Spacer blocks, tensors, or instrumented tibial trials are current methods of balancing the knee during surgery but there are no current techniques for measuring ligament forces. Our goal was to study the relationship between the collateral ligament forces and the condylar contact forces to determine whether there was equivalence., Methods: A test rig was constructed modeling an artificial knee joint with collateral ligaments. The ligament forces as well as the lateral and medial tibial contact forces were measured during flexion for different positions of the femoral component on the femur, producing a set of forces for the simulated conditions. A regression analysis was used to study the correlation between the ligament and contact forces., Results: The combined medial and lateral ligament and contact forces showed a linear relation with a correlation coefficient of 0.98. For the medial and lateral sides separately, the correlations were 0.85 and 0.88, respectively, with more than 80% of points within a ±25% deviation from the linear relations. This deviation from the linear correlation is linked to differences in medial-lateral femoral-tibial contact point locations at different flexion angles., Conclusion: Within balancing accuracies generally achieved at surgery, the collateral ligament forces were linearly correlated to the condylar contact forces. These forces can also be equally correlated to the distraction forces as well as the moments at which condylar liftoff would occur from varus-valgus moments. This indicated a unification of the different balancing parameters, and hence such quantitative methods can be used interchangeably., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Background: Balancing is an important part of a total knee procedure, and in recent years, more emphasis has been given to quantifying the process., Methods: During 101 total knee surgeries, initial bone cuts were made using navigation. Lateral and medial contact forces were determined throughout flexion using an instrumented tibial trial. Balancing was defined as a ratio of the medial and total force, the target being 0.5 (equal lateral and medial forces). Based on the initial values, surgical corrections were selected to achieve balancing. The most common corrections were soft tissue releases (63 incidences), including MCL, posterolateral corner, posteromedial corner, and changing tibial insert thicknesses (34 incidences)., Results: After final balancing, the mean ratio was 0.52 ± 0.14, between 0.35 and 0.65 being achieved in 80% of cases. In 84% of cases, only 0-2 corrections were required. The average total force on the condyles was 215 ± 86 N., Conclusion: Our study provides data to surgeons on the results to expect when balancing a knee, which can enhance both accuracy and consistency of the procedure., (Copyright © 2016 Elsevier Inc. All rights reserved.)

An accelerometer attached to the anterior proximal tibia was investigated as an evaluation of knee stability of Total Knee Arthroplasty (TKA) patients while performing daily activities. Acceleration data of 38 TKA knees with a minimum follow up of 6months were compared with 34 control knees. The activities performed were: walking three steps forward and coming to a sudden stop; turning in the direction of non-tested knee; sit-to-stand; and stepping up and down from a 7 inch step. The acceleration results showed significant differences between TKA and controls while stepping down and while turning in the non-tested knee direction. The higher accelerations with the TKA group may have represented an objective measure of stability, even if this was not directly discernible to the patient., (Copyright © 2013 Elsevier Inc. All rights reserved.)

The objective was to develop a simple, rapid, and low-cost method for evaluating proposed new total knee arthroplasty (TKA) models and then to evaluate 3 different TKA models with different kinematic characteristics. A "desktop" knee testing rig was used to apply forces and moments over a full flexion range, representing a spectrum of positions and activities; and the positions of the femur on the tibia were measured. The average neutral path of motion (for compressive force only) and the laxities about the neutral path (for superimposed shear and torque) were determined from 8 knee specimens to be used as a benchmark for the TKA evaluations. A typical posterior-stabilized TKA did not display the normal external femoral rotation with flexion and also showed abnormal anterior sliding on the medial side. A medial-pivot type of guided-motion design showed medial stability comparable to anatomical but still did not produce external femoral rotation and posterior lateral displacement with flexion. The addition of a central cam-post produced the rotation and displacement but only after 75° of flexion. It was concluded that the test method satisfied the objective and could be used as a design tool for evaluating new and existing designs, as well as for formulating a TKA with anatomical characteristics., (Copyright © 2011. Published by Elsevier Inc.)

The goal of the study was to achieve a normal neutral anatomical path of motion with a total knee arthroplasty (TKA) using specific motion-guiding design features. Two reference TKA models were used, consisting of a partially conforming double-dished geometry and the same with a central cam-post for femoral rollback. Four experimental TKA models included features to produce femoral rollback with and without guidance for tibial rotation, and a feature to prevent paradoxical anterior femoral sliding. The femur was loaded down the tibial axis, and the femoral-tibial positions were recorded at a sequence of flexion angles. Subsequently, the positions were recorded with an anterior shear force superimposed. Software was used to reconstruct the paths of the transverse femoral axis on the tibia, during a full flexion range. The reference knees did not reproduce a normal neutral path of motion. However, this was achieved with an experimental design incorporating all of the motion-guiding features.

Maximum flexion-or impingement angle-is defined as the angle of flexion when the posterior femoral cortex impacts the posterior edge of the tibial insert. We examined the effects of femoral component placement on the femur, the slope angle of the tibial component, the location of the femoral-tibial contact point, and the amount of internal or external rotation. Posterior and proximal femoral placement, a more posterior femoral-tibial contact point, and a more tibial slope all increased maximum flexion, whereas rotation reduced it. A mobile-bearing knee gave results similar to those of the fixed-bearing knee, but there was no loss of flexion in internal or external rotation if the mobile bearing moved with the femur. In the absence of negative factors, a flexion angle of 150 degrees can be reached before impingement.

The kinematics of a mobile bearing knee, which allowed +/-20 degrees of rotation and 4.5 mm of anteroposterior translation, was measured for ascending and descending a step, deep-knee bend, normal walking, and twisting. A fluoroscopic technique was used, analyzed by 2 different methods. The rotations and displacements during the activities were similar to those of moderate-to-high constrained fixed bearing knees. The motion patterns were variable among test subjects and in general did not reproduce normal knee motion. Because of the freedom of anteroposterior translation and rotation in the design, however, each knee could determine its own neutral position and its own axis of internal-external rotation, depending on the activity.

When the normal knee is flexed from 0 to maximum, the lateral femoral condyle displaces posteriorly, but the medial condyle remains almost in the same position. There is anteroposterior and rotational laxity about this neutral path. The hypothesis of this study was that this motion could be provided in a total knee arthroplasty by specially shaped bearing surfaces, in particular, where the lateral femoral condyle converged inward around the periphery from extension to full flexion. Tibial surfaces were generated with such femoral condyles moving through a neutral motion path. Displacing the femur in anteroposterior or rotation about the neutral position at a given angle of flexion intersected a volume of the tibial surface in both directions, showing that the femur was "at the bottom of the tibial dish." The volumes were comparable with those for conventional condylar replacement total knee arthroplasties. These data supported the general concept of converging femoral condyles, which may be present in the natural knee itself to guide motion.

Cases in which there is a total hip arthroplasty and a stemmed total knee arthroplasty in the same femur, with loosening of 1 or both components, with serious endosteal bone loss or even a fracture between the stems present a difficult reconstruction problem. We describe a reconstruction using a combined hip and stemmed knee, designed so that they could be rigidly connected during the surgical procedure. The advantages of this implant design are that the entire femur with its muscle attachments is preserved, and the inherent stability allows for immediate weight bearing. To determine the viability of the connection between the hip and the knee, a stress analysis was carried out using finite element analysis. Guidelines were thus provided for the required metal and cement thicknesses. Three case examples are presented with an average follow-up of 3 years. It was shown that the combined hip-knee implant could provide successful results for these difficult reconstructive problems in appropriately selected cases.

Total hip joint arthroplasty (THA) in the presence of severe femoral anteversion (>50 degrees) is technically demanding. This problem is often encountered in patients with osteoarthritis secondary to hip joint dysplasia or congenital dislocation. We describe a method of THA in which an uncemented femoral prosthesis is used in conjunction with subtrochanteric derotational osteotomy. This technique allows the restoration of the normal proximal femoral anatomy, including the abductor muscle lever arm without resorting to greater trochanteric transfer. Correction of the excessive femoral anteversion avoids the tendency for postoperative anterior instability. The osteotomy site may also serve as the site for femoral shortening or angular correction when required, which preserves the normal femoral flare. The prostheses used were custom CAD/CAM (computer-assisted design/computer-assisted manufacturer) in design and had the following features: a close intramedullary proximal fit, with collar, lateral flare, and hydroxyapatite coating to achieve early proximal fixation, and longitudinally cutting fluted stem to provide immediate rotational stability across the osteotomy site. Although we used CAD/CAM custom prostheses, off-the-shelf uncemented hip prostheses with similar design features have become available. We report on 7 patients who underwent THA using this technique. The average patient age was 49 years (range, 34-61 years) with a mean follow-up period of 31 months (range, 16-60 months). To date, all cases have had a satisfactory outcome with evidence of union at the osteotomy site. Harris hip scores improved from an average of 44 preoperatively to 91 by the end of follow-up period. One case was complicated by delayed union at the osteotomy site, which was successfully corrected with bone grafting and temporary plate stabilization.

The forces and moments in the shaft of a distal femoral replacement were measured by telemetry for a subject during different activities, and calculations were then made of the forces at the knee. The axial force showed a small peak at heel-strike followed by two main peaks during stance. In level walking, the peak axial force was between 1,487 and 1,718 N (2.2-2.5 BW), the peak shear force was 269-368 N (0.4-0.54 BW) directed anteriorly on the tibia, the peak axial torque was 7 Nm internal, while the patellofemoral force was 466-571 N. The highest axial force was recorded for descending stairs (2.8 BW). Standing on one leg produced 2.4 BW, while lying supine and raising the leg produced 1.7 BW. The data produced may resemble that of a normal subject, and has application to basic joint mechanics, to joint reconstruction, and to total knee replacement design and evaluation.

Whether oxidation of ultrahigh-molecular-weight polyethylene (UHMWPE) causes delamination of the plastic in total knee arthroplasties (TKAs) was investigated. Examination of retrieved TKAs has shown that oxidation of UHMWPE can be caused by postirradiation damage leading to a subsurface band or, to a lesser extent, by mechanical forces during use leading to surface oxidation. Delamination cracks propagated through the subsurface oxidized band. In wear tests, delamination occurred in artificially aged UHMWPE where only subsurface oxidized bands had formed. Increased surface wear predominated where oxidation was associated with the surface of the plastic. Similarly, in tensile and fatigue tests of oxidized UHMWPE, there was a significant reduction in the ultimate tensile strength and in the fatigue resistance of specimens that had developed a subsurface band. Oxidation increased fatigue crack growth rate. It was observed that UHMWPE from different manufacturers varied in its resistance to oxidation. This study demonstrates that the effect of oxidation, which results in the development of a subsurface white band, combined with high subsurface shear forces observed in TKAs, is to enhance delamination wear.

The plastic components of 280 retrieved unicondylar and total knee arthroplasties were studied. Wear was visually scored using a relative ranked data method. Although wear on the components was highly variable, several conclusions could be drawn regarding the nature and causes. Wear was associated more with the medial than the lateral condyle. Delamination was the most severe type of wear and occurred in short (< 5 year)-, medium (5-10 years)-, and long (> 10 years)-term retrievals. In the short term, delamination wear was associated with hot pressing of the tibial plastic or with fracture of the tibial baseplate. For a single design, a significant difference in the amount of delamination on hot-pressed and non-hot-pressed tibial components was observed. In medium- and long-term retrieved specimens of the designs with moderately high conformity, delamination wear was associated with restriction of rotational movement of the femoral component or with abrupt changes in the radius of the tibial component. In flatter, less conforming designs, wear was associated with laxity, such that the polyethylene delaminated toward the edges of the tibial component. Wear attributed to cement abrasion or entrapment occurred on the more conforming designs. Delamination was associated with the presence of fusion defects in the polyethylene but could also occur in the absence of such defects. That delamination was the principal were type and that this is caused by a fatigue mechanism mean that the incidence of failure could accelerate considerably over follow-up periods beyond 10 years. Designs of moderate conformity without abrupt changes in radii may prolong the duration of plastic tibial components before serious delamination occurs.

If the shape of the femoral canal could be predicted with sufficient accuracy for the design of custom femoral stems, this would reduce the cost and provide a simpler design method than using computed tomography scans. Five groups of femurs were used for the study. The first two groups were used to determine an average femur shape, described by 25 transverse sections. In the next group of femurs, the shape was predicted from radiographs by distorting the shape of the average femur to conform to the radiographic outlines. The profiles were accurate to within 1 mm on average, the larger errors being in the trochanteric regions. In the proximomedial region, and in the distal canal, the accuracy was better than 0.3 +/- 0.8 mm. When femoral stems were designed from the actual and predicted canals, there was minimal difference in the sections of the two stems. The stems had a particular geometric form with a straight distal end, and flares at the proximomedial and proximoanterior locations; however, when stems designed by either method were inserted into canals prepared by distal reaming and proximal rasping, there was an error in fit in certain regions of approximately 1 mm. It was concluded that for femurs without serious abnormality of shape, a biplane radiographic method was sufficiently accurate for the prediction of canal shape and for the design of a standard type of custom uncemented stem.

The first 218 cases of limb salvage using cemented intramedullary Stanmore massive distal femoral arthroplasties with a single-axis hinged knee, followed for an average of 58 months with a maximum of 251 months, were reviewed. Loosening (5%), local recurrence (4%), intramedullary stem fracture (3%), and infection (2%) were the major complications requiring surgical intervention. Loosening and implant fracture were treated by the insertion of a second massive replacement in all but one case. All recurrences and all but one infected case required amputation. Survivorship analysis showed that the overall probability of one of these early prostheses surviving for 10 years was 65%, and for 20 years it was 53%. For cases of bone tumor, the probability of surviving for 10 years was 68% (not including recurrence), for primary knee replacement cases it was 32%, for other bony diseases plus trauma it was 86%, and for cases of revised massive prostheses it was 53%. It was concluded that this type of distal femoral arthroplasty provided good medium-term results, but showed some deficiencies in the long term, particularly in younger patients.

Nineteen total knee arthroplasties in 16 patients were performed using a new prosthesis designed specifically for uncemented, press-fit fixation without any provision for tissue ingrowth. This design may be suited for use in patients with juvenile rheumatoid arthritis, postseptic knees, failed total knee arthroplasties with large bone stock deficiencies, and young, active, overweight individuals. Preservation of bone stock is possible because the fixation stems are nails rather than larger cement pegs, removal for revision is easier without cement, sequestration of microbes within cement cannot occur, and bone graft may be continually loaded with this type of prosthesis. Pain relief, discarding crutch support, operative blood loss, and corrective alignment were equivalent to results obtained with cemented arthroplasty. Seventeen of the 18 knees available for review (average follow-up period, 3 years, 7 months; range of follow-up period, 2 years, 10 months to 4 years, 11 months) were pain-free. Range of motion averaged -2 degrees of extension to 101 degrees of flexion. Ambulation was unrestricted and unsupported in the majority of patients. Radiographic evaluation showed the development of increased bone density under the tibial plateau as well as a sclerotic line at the ends of the femoral and tibial stems. This observation gives support to the theory that multiple microtrabecular fractures with healing form a new supportive "subchondral bone plate." There was one incidence of tibial subsidence. Prosthesis migration and loosening were not observed.

The effect of tibial component stem and post design on load distribution and on displacements at the component-bone interface was investigated. Four different configurations were tested with a flat component used as a control. Loading conditions included vertical central, vertical offset (eg, varus/valgus), shear and torque. Both artificial and cadaveric bones were used in the study. Pressure-sensitive film was used to obtain pressure patterns at the interface. Image processing was then used to quantify the load distribution. For shear and torque, relative motion was seen as a smeared pressure pattern. This was calibrated in terms of microns of displacement as a function of image density. The central stemmed and bladed designs performed better than short-pegged designs, in resistance to offset loading. In shear and torque, short pegs close to the component periphery, or a central stem with blades, produced the least interface displacement. The application of this work to the design of components for both press-fit and cemented application is discussed.

A photoelastic coating method was used to study the strain patterns on the surface of the femur, before and after insertion of femoral stems. The anatomically shaped stems were press-fit without a collar, press-fit with a collar, and press-fit with proximal cementing to approximate a bone ingrowth situation. The strain patterns of the intact femurs were consistent with bending. The pattern changed considerably after stem insertion, probably due to the stiffening effect of the stems. Nevertheless, the collarless press-fit stem preserved 64% of the magnitude of the proximal medial strain. The collar restored the average to normal, but the surface strains varied because of the localized regions of contact between the collar and the bone. Proximal cementing gave results similar to those of the collarless press-fit. The press-fit stems often showed local patches of high strain, probably reflecting local endosteal contact points. Local high stresses at the level of the distal tip were seen in only a few instances.

Ten fresh knee specimens with prosthetic components inserted were tested in a loading rig. Compressive and shear force were applied to the femur with the tibia held fixed. The location of the femoral-tibial contact points was measured. The contact reaction forces, the shear forces, and the rocking moments transmitted to the tibial component were calculated. The variations in the test conditions were: high and low compressive force, flexion angles of 0 degree, 45 degrees, and 90 degrees, three curvatures of tibial plastic inserts, and the posterior cruciate retained or resected. When the posterior cruciate was retained, the contact points were close to the center of the component; for cruciate resection, the contacts were close to the anterior of the component. The shear forces and rocking moments were higher for cruciate resection, but the contact reaction forces were lower. There is a wide variety of knee prosthesis designs, but the amount of inherent stability between the femoral and tibial surfaces, and whether the posterior cruciate ligament is retained or sacrificed, are two of the most important design variables. This study shows that cruciate resection increases the shear forces and the rocking moments to the tibial components and that additional fixation means may be necessary to compensate. On the other hand, cruciate retention with low conformity gives higher contact forces, which may lead to more wear in the long term. Cruciate sacrificing designs with intercondylar guiding surfaces are a separate category of design and were not considered in this study.

The anterior-posterior and rotatory laxities of 14 total knee prosthesis designs were measured in a loading rig with compressive, shear, and torque loads representative of physiologic loads. The measured laxities covered a wide range, both greater and smaller than that of the anatomic knee. This range was mainly due to the curvature or flatness of the plastic tibial surface and conformity with the femoral component. Pressure patterns showed the corresponding contact track and area on the tibial surfaces. It is proposed that for normal function, the laxity of the device should complement the remaining anatomic structures to produce a combined laxity resembling that of the normal knee. Excessive prosthetic laxity will lead to the risk of instability, soft tissue attenuation, edge-loading on components, and high contact stresses on the plastic. Inadequate prosthetic laxity may lead to altered kinematics and excessive stresses at the interface, running the risk of long-term loosening. The authors show the laxities of many currently used devices, providing important background information for assessing the role intrinsic prosthetic constraint might play in total joint performance in clinical analyses.

One thousand sixty-nine consecutive cemented Kinematic Condylar total knee arthroplasties performed by one group of orthopaedists were studied. The maximum follow-up period was 7 years. Most patients had rheumatoid arthritis or osteoarthritis, and the average patient age was 67 years (range, 12-90 years). Aseptic revisions for loosening were required for only one tibial component and six patella components. Average postoperative flexion was 2.5-107 degrees. The preoperative to postoperative change in range of flexion was not affected by the tilt angle of the tibial component in the sagittal plane. With the use of external alignment guides, the average postoperative alignment was ideal but the standard deviation was high; the standard deviation and the extremes were lower when intramedullary guides were used. There was a 14% incidence of femoral radiolucency and a 30% incidence of tibial radiolucency, which increased only slightly with time. Most radiolucencies on the tibial side were small and restricted to the extreme edges; rarely did radiolucency occur around the central peg. More than one half of the thicker radiolucencies occurred adjacent to wedge-shaped bone defects that were filled with cement.

Wear of the ultra-high-molecular-weight polyethylene (UHMWPE) components in total knee arthroplasties is a potential long-term problem. Ninety total knees of various designs with implant times up to 10 years were retrieved. The wear noted in the majority of components was much greater than that noted in wear studies of acetabular components in total hip prostheses. Abrasion from cement or bone and delamination wear were particularly pronounced in the knee. Delamination, consisting of complete breakup of material in flakes and particles, appeared to be initiated by intergranular material defects and propagated by the excessive subsurface stresses beneath the contact zone. Material that was free of defects did not show delamination wear even after long time periods in a highly stressed, low-conformity design. Wear particles of UHMWPE can result in adverse tissue reaction with cellulitis, giant cell reaction, and necrotic tissue, and these effects could be cumulative with time. There is some evidence that particles can lead to bone resorption, including at the implant-bone interface, which could accelerate loosening. There is cause for concern as to the long-term effects of UHMWPE in total knee arthroplasty. This suggests the need for improved processing methods or more wear-resistant materials.