Three-dimensional tibiofemoral kinematics during deep flexion kneeling in a mobile-bearing total knee arthroplasty

Achieving very deep flexion after total knee arthroplasty is an important goal of most patients in Japan, Asia, and the Middle East because of floor-sitting lifestyles. Numerous knee arthroplasty designs have been introduced to permit high flexion. We performed an in vivo radiographic analysis of tibiofemoral motions during weight-bearing kneeling in one high-flexion knee arthroplasty design. Twenty knees implanted with a posterior-stabilized rotating-platform knee arthroplasty flexed an average of 126°. The femoral condyles translated posteriorly from extension to maximum flexion. Total posterior condylar translations averaged 11.6 and 4.7 mm for the lateral and medial condyles, respectively. Tibial internal rotation in 19 knees averaged 9° from extension to maximum flexion. Knees implanted with a posterior-stabilized, rotating-platform knee arthroplasty show deep flexion knee kinematics consistent with the implant design intent.     

How the knee moves

The arc of flexion used in almost all the activities of everyday life extends from about 20°±10° to 110°/120°. During this arc, the human knee corresponds to the quadrupedal mammalian knee. Both the femoral surfaces are circular with a similar radius and rotate around their geometrical centres as the knee flexes. The medial femoral condyle does not move antero-posteriorly with flexion, i.e. stability depends on the medial side of the knee. In contrast, the lateral femoral condyle is antero-posteriorly mobile and as it moves it carries the meniscus with it. This AP movement results in longitudinal tibial rotation which is facultative rather than obligatory: if posterior motion occurs, the femur rotates externally around a medial axis with flexion whereas if no AP motion occurs, the knee can flex as would a uniaxial hinge. Rotation first appears in arboreal quadrupeds (apes) and may be becoming vestigial in Man. The axis of longitudinal rotation during flexion, parallel to the tibia and perpendicular to the flexion axis, approximately intersects the latter in the centre of the medial femoral condylar sphere. Varus/valgus rotation, around an AP axis which also passes through the centre of the femoral sphere, permits the lateral femoral condyle to lift away from the tibia because the lateral collateral ligament (LCL) is slack at 90° in mid external/internal rotation. Thus, in the arc ‘20’–120° the medial femoral condyle resembles the femoral head: it is spherical, it does not translate during flexion and all three axes of rotation intersect at its centre. At 90°, forced longitudinal rotation does result in AP movement of the medial condyle and on the lateral side in a reciprocal translation which is almost sufficient to abolish the translation accompanying flexion. This movement occurs around a vertical axis which is slightly lateral to that representing longitudinal rotation with flexion.The arc from 10° to full extension is accompanied by the so-called ‘locking’ and ‘screw-home’. It appears to be a feature of bipedal terrestrial gait with an erect stance, i.e. human gait. Although the arc exists, it is rarely used fully in everyday life. The motion is complex and involves asymmetrical articular surfaces other than those used from 20° to 120°. On the medial side, the femur ‘rocks’ forward onto the upward-sloping anterior surface of the tibia and then rotates into extension around an anterior, larger radiused circular surface. On the lateral side, the femur rolls down onto the anterior horn. The result is ‘lift-off’ of the posterior facets used in the arc ‘20’–120° and progressive tightening of the structures attached posteriorly to the femur, in particular the ACL. This ligament, as it tightens, may move the lateral femoral condyle anteriorly so that extension is accompanied by about 5° of obligatory femoral internal rotation. Flexion and longitudinal rotation occur by rotation around, and translation along, a 20° oblique screw axis penetrating medially the epicondyle and, laterally, the region of the tibio-femoral contact surface.From 120° to full flexion, the motion is passive rather than active. Both femoral condyles move backwards and both lose contact with the tibia. Thus, the tibio-femoral joint is strictly speaking subluxed. Medially, the femoral condyle rolls up onto the posterior horn. Laterally, the femoral condyle rolls backwards and downwards, finally to lie posterior to the tibia, resting on the posterior horn.Although the motion of the knee is complex, it can be (and has been) imaged by MRI in the unloaded cadaveric knee, the unloaded living knee and the loaded living knee. The keys to its understanding are to divide flexion into three arcs and to appreciate that in the functional active arc (‘20’–120°) the medial femoral condyle, like the femoral head, is spherical, that it does not translate and that it rotates around three axes which intersect at its centre. By contrast, the lateral femoral condyle rolls and slides antero-posteriorly on the tibia to result in longitudinal rotation (a possibly vestigial movement in Man) around a medial axis.     

Knee kinematics with a high-flexion posterior stabilized total knee prosthesis: an in vitro robotic experimental investigation

BACKGROUND: An analysis of contemporary total knee arthroplasty reveals that, on the average, patients rarely flex the knee beyond 120 degrees. The biomechanical mechanisms that inhibit further flexion after total knee arthroplasty are unknown. The objective of the present study was to investigate the capability of a single design of a fixed-bearing, high-flexion posterior stabilized total knee arthroplasty system (LPS-Flex) to restore the range of flexion to that of the intact knee. METHODS: Thirteen cadaveric human knees were tested, with use of a robotic testing system, before and after total knee arthroplasty with the LPS-Flex prosthesis. The passive path and the kinematics under an isolated quadriceps force of 400 N, under an isolated hamstring force of 200 N, and with these forces combined were determined. Posterior femoral translation of the lateral and medial femoral condyles and tibial rotation were recorded from 0 degrees to 150 degrees of flexion. RESULTS: The medial and lateral condyles of the intact knee translated posteriorly from full extension to 150 degrees, reaching a mean peak (and standard deviation) of 22.9 +/- 11.3 mm and 31.9 +/- 12.5 mm, respectively, under the combined muscle forces. Following total knee arthroplasty, the amount of posterior femoral translation was lower than that observed in the intact knee. At 150 degrees, approximately 90% of the intact posterior femoral translation was recovered by the total knee replacement. Internal tibial rotation was observed for all knees throughout the range of motion. The cam-spine mechanism engaged at approximately 80 degrees and disengaged at 135 degrees. Despite the absence of cam-spine engagement, further posterior femoral translation occurred from 135 degrees to 150 degrees. CONCLUSIONS: The tibiofemoral articular geometry of the intact knee and the knee after total knee arthroplasty with use of the LPS-Flex design demonstrated similar kinematics at high flexion angles. The cam-spine mechanism enhanced posterior femoral translation only at the mid-range of flexion. The femoral component geometry of the LPS-Flex total knee prosthesis may improve posterior tibiofemoral articulation contact in high flexion angles.     

In vivo kinematic analysis of a high-flexion posterior stabilized fixed-bearing knee prosthesis in deep knee-bending motion

The objective of this study was to evaluate in vivo kinematics of a high-flexion, posterior-stabilized fixed-bearing, total knee arthroplasty in weight-bearing deep knee-bending motion. A total of 20 knees implanted with the Scorpio Non-Restrictive Geometry knee system in 17 patients were assessed in this study. The Scorpio Non-Restrictive Geometry is a recent implant design with modifications made to accommodate a higher flexion range of motion and greater axial rotation, particularly during more functionally demanding activities. Patients were examined during a deep knee-bending motion using fluoroscopy, and femorotibial motion was determined using a 2-dimensional to 3-dimensional registration technique. The average flexion angle was 126.5° (110°-149°). The femoral component demonstrated a mean of 13.5° (5.2°-21°) external rotation. The external rotation increased up to maximum flexion. The pivot pattern was a medial pivot pattern similar to that reported in normal knee kinematics.     

In vivo determination of posterior femoral rollback for subjects having a NexGen posterior cruciate–retaining total knee arthroplasty

This study determines the in vivo kinematics during a deep-knee bend activity for subjects implanted with a posterior cruciate–retaining total knee arthroplasty having asymmetric geometries. Of 20 subjects, 19 experienced posterior femoral rollback (PFR) of the lateral condyle (average −3.9 mm), and 13 subjects experienced PFR of the medial condyle (average −3.1 mm). As a result of the lateral condyle rolling further posterior than the medial condyle, on average, subjects experienced 1.4° of normal axial rotation. Of 20 subjects, 10 experienced normal axial rotation, whereas 10 experienced an opposite rotation pattern. Condylar lift-off occurred predominantly with the lateral condyle. Contrary to previous in vivo studies, the subjects in this study experienced consistent PFR of the posterior cruciate–retaining total knee arthroplasty. It can be hypothesized that having asymmetric femoral condyles may lead to PFR with increasing knee flexion.     

Femoral rollback after cruciate-retaining and stabilizing total knee arthroplasty

Limited data comparing the kinematics of posterior cruciate ligament-retaining or substituting total knee arthroplasty with its own intact knee under identical loadings is available. In the current study, posterior femoral translation of the lateral and medial femoral condyles under unloaded conditions was examined for intact, cruciate-retaining, cruciate ligament-deficient cruciate-retaining and posterior-substituting knee arthroplasties. Cruciate-retaining and substituting total knee arthroplasties behaved similarly to the cruciate-deficient cruciate-retaining total knee arthroplasty between 0 degrees and 30 degrees flexion. Beyond 30 degrees, the posterior cruciate-retaining arthroplasty showed a significant increase in posterior translation of both femoral condyles. The posterior cruciate-substituting arthroplasty only showed a significant increase in posterior femoral translation after 90 degrees. At 120 degrees, both arthroplasties restored approximately 80% of that of the native knee. Posterior translation of the lateral femoral condyle was greater than that observed in the medial condyle for all knees, indicating the presence of internal tibial rotation during knee flexion. The data showed that the posterior cruciate ligament is an important structure in posterior cruciate-retaining total knee arthroplasty and proper balancing is imperative to the success of the implant. The cam-spine engagement is valuable in restoring posterior femoral translation in posterior cruciate-substituting total knee arthroplasty.     

In Vivo Motion of Femoral Condyles During Weight-Bearing Flexion After Anterior Cruciate Ligament Rupture Using Biplane Radiography

The purpose of this study was to investigate in vivo three- dimensional tibiofemoral kinematics and femoral condylar motion in knees with anterior cruciate ligament (ACL) deficiency during a knee bend activity. Ten patients with unilateral ACL rupture were enrolled. Both the injured and contralateral normal knees were imaged using biplane radiography at extension and at 15°, 30°, 60°, 90°, and 120° of flexion. Bilateral knees were next scanned by computed tomography, from which bilateral three-dimensional knee models were created. The in vivo tibiofemoral motion at each flexion position was reproduced through image registration using the knee models and biplane radiographs. A joint coordinate system containing the geometric center axis of the femur was used to measure the tibiofemoral motion. In ACL deficiency, the lateral femoral condyle was located significantly more posteriorly at extension and at 15° (p < 0.05), whereas the medial condylar position was changed only slightly. This constituted greater posterior translation and external rotation of the femur relative to the tibia at extension and at 15° (p < 0.05). Furthermore, ACL deficiency led to a significantly reduced extent of posterior movement of the lateral condyle during flexion from 15° to 60° (p < 0.05). Coupled with an insignificant change in the motion of the medial condyle, the femur moved less posteriorly with reduced extent of external rotation during flexion from 15° to 60° in ACL deficiency (p < 0.05). The medial- lateral and proximal-distal translations of the medial and lateral condyles and the femoral adduction-abduction rotation were insignificantly changed after ACL deficiency. The results demonstrated that ACL deficiency primarily changed the anterior-posterior motion of the lateral condyle, producing not only posterior subluxation at low flexion positions but also reduced extent of posterior movement during flexion from 15° to 60°.

Key Points

• Three-dimensional tibiofemoral kinematics and femoral condylar motion in ACL-deficient knees during upright weight-bearing flexion were measured using biplane radiography with the geometric center axis.
• ACL deficiency caused posterior subluxation of the lateral condyle with excess external femoral rotation at early flexion positions.
• On flexion from 15° to 60°, the lateral condyle moved slightly posteriorly in ACL deficiency leading to reduced extent of external femoral rotation.

Key words: anterior cruciate ligament, injury, kinematics, tibiofemoral, femoral condyle, radiography     

Biomechanics of posterior-substituting total knee arthroplasty: an in vitro study

The cam-spine system in posterior-substituting total knee arthroplasty was designed to improve posterior stability and to increase posterior femoral translation (rollback). Little is known on its effectiveness in the restoration of femoral rollback under functional loads. In the current study, the effect of cam-spine engagement on knee motion under simulated muscle loads was investigated using knees from cadavers. The translations of the lateral and medial femoral condyles of the knee before and after total knee arthroplasty were compared from 0 degrees to 120 degrees flexion. Cam-spine contact forces were measured under the same muscle loads. The posterior translations of both femoral condyles in the total knee arthroplasty were significantly lower than that of the native knee beyond full extension. Cam-spine engagement occurred between 60 degrees and 90 degrees flexion followed by an increase in posterior translation of both femoral condyles. However, the resultant femoral translation of the total knee arthroplasty was still lower than that of the native knee from 90 degrees to 120 degrees flexion. Knee motion after cam-spine engagement was independent of muscle loads, indicating the importance of the cam-spine mechanism at high flexion angles. Decreased posterior translation of both femoral condyles after total knee arthroplasty may be a limiting factor at high flexion.     

In-vivo kinematics of high-flex posterior-stabilized total knee prosthesis designed for Asian populations

Purpose

The purpose of this study was to determine in-vivo kinematics of our developed posterior-stabilized (PS) total knee prosthesis for Asian populations in comparison with a popular high-flexion PS prosthesis.

Methods

We analyzed 62 osteoarthritic knees: 31 knees with the new PS prosthesis (group A) and 31 knees with a popular high-flexion PS prosthesis (group B). Radiographic knee images were taken during standing, lunge, and kneeling activities. The three-dimensional position and orientation of the implant components were determined using model-based shape matching techniques.

Results

Group A showed slightly greater implant flexion angles compared with knees with conventional prosthesis at maximum lunge (average: 119 vs. 110°, p?=?0.001), and at maximum kneeling (121 vs. 114°, p?=?0.004), although the range of motion was not significantly different. The femoral centre positions were more posterior in group A at standing, at 90° lunge, at maximum lunge (-9 and -7 mm, p?=?0.004), at 90° kneeling, and at maximum kneeling (-9 vs. -7 mm, p?=?0.016), and posterior translations of the femoral center were greater at 90° knee flexion postures. The femoral centre positions had a strong negative correlation with implant flexion angles at maximum lunge in group B (r?=?-0.893, p?<?0.001), but not in group A (p?=?0.242).

Conclusions

The new PS prosthesis designed for Asian knee morphology achieved flexion angles and range of motion at least comparable to that of conventional high-flexion PS prosthesis. The femoral roll-back pattern, however, is different from a conventional knee, reflecting the post/cam design.

     

Videofluoroscopic Evaluation of the Influence of a Gradually Reducing Femoral Radius on Joint Kinematics During Daily Activities in Total Knee Arthroplasty

BackgroundParadoxical anterior translation in midflexion is reduced in total knee arthroplasties (TKAs) with a gradually reducing femoral radius, when compared to a 2-radii design. This reduction has been shown in finite element model simulations, in vitro tests, intraoperatively, and recently also in vivo during a lunge and unloaded flexion-extension. However, TKA kinematics are task dependent and this reduction has not been tested for gait activities.MethodsThirty good outcome subjects (≥1 year postoperatively) with a unilateral cruciate-retaining TKA with a gradually reducing (n = 15) or dual (n = 15) femoral radius design were assessed during 5 complete cycles of level walking, stair descent (0.18-m steps), deep knee bend, and sitting down onto and standing up from a chair, using a moving fluoroscope (25 Hz, 1 ms shutter time). Kinematic data were extracted by 2D/3D image registration.ResultsTibiofemoral ranges of motion for flexion-extension, abduction-adduction, internal-external rotation, and anteroposterior (AP) translation were similar for both groups, whereas the pattern of AP translation-flexion-coupling differed. The subjects with the dual-radii design showed a sudden change in direction of AP translation around 30° of flexion, which was not present in the subjects with the gradually reducing femoral radius design.ConclusionThrough the unique ability of moving fluoroscopy, the present study confirmed that the gradually reducing femoral radii eliminated the paradoxical sudden anterior translation at 30° present in the dual-radii design in vivo during daily activities, including gait and stair descent.     

In Vivo Kinematic Analysis of a High-Flexion, Posterior-Stabilized, Mobile-Bearing Knee Prosthesis in Deep Knee Bending Motion

The objective of this study was to evaluate the kinematics of a high-flexion, posterior-stabilized, mobile-bearing total knee arthroplasty (TKA) in weight-bearing, deep knee bending motion. Thirteen patients implanted with the Legacy Posterior Stabilized Flex (Zimmer, Warsaw, IN) mobile-bearing TKA were examined during a deep knee bending motion using fluoroscopy. Femorotibial motion was determined using a 2-dimensional to 3-dimensional registration technique, which used computer-assisted design models to reproduce the position of metallic implants from single-view fluoroscopic images. The average flexion range of motion between the metallic implants was 116°. The average rotation of the femoral component was 9.3° external rotation. The mean kinematic pathway was early rollback, lateral pivot with external rotation, and bicondylar rollback. We found that the kinematic pattern of the Legacy Posterior Stabilized Flex mobile-bearing TKA was different than normal knee kinematics.     

Posterior cruciate ligament removal contributes to abnormal knee motion during posterior stabilized total knee arthroplasty

Abnormal anterior translation of the femur on the tibia has been observed in mid flexion (20–60°) following posterior stabilized total knee arthroplasty. The underlying biomechanical causes of this abnormal motion remain unknown. The purpose of this study was to isolate the effects of posterior cruciate ligament removal on knee motion after total knee arthroplasty. We posed two questions: Does removing the posterior cruciate ligament introduce abnormal anterior femoral translation? Does implanting a posterior stabilized prosthesis change the kinematics from the cruciate deficient case? Using a navigation system, we measured passive knee kinematics of ten male osteoarthritic patients during surgery after initial exposure, after removing the anterior cruciate ligament, after removing the posterior cruciate ligament, and after implanting the prosthesis. Passively flexing and extending the knee, we calculated anterior femoral translation and the flexion angle at which femoral rollback began. Removing the posterior cruciate ligament doubled anterior translation (from 5.1 ± 4.3 mm to 10.4 ± 5.1 mm) and increased the flexion angle at which femoral rollback began (from 31.2 ± 9.6° to 49.3 ± 7.3°). Implanting the prosthesis increased the amount of anterior translation (to 16.1 ± 4.4 mm), and did not change the flexion angle at which femoral rollback began. Abnormal anterior translation was observed in low and mid flexion (0–60°) after removing the posterior cruciate ligament, and normal motion was not restored by the posterior stabilized prosthesis. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:1494–1499, 2008     

固定平台后稳定型假体全膝关节置换术后的运动学研究

 目的 探讨固定平台后稳定型假体全膝关节置换(total knee arthroplasty,TKA)术后膝关节在负重屈膝下蹲时的运动学特征。方法 选取10名健康志愿者和10例固定平台后稳定型假体TKA术后患者。制作骨骼及膝关节假体三维模型,在持续X线透视下完成负重下蹲动作,膝关节屈曲度每增加15°截取一幅图像。通过荧光透视分析技术完成三维模型与二维图像的匹配,再现股骨与胫骨在屈膝过程中的空间位置,通过连续的图像分析比较正常与固定平台后稳定型假体TKA术后膝关节在负重下蹲时股骨内、外髁前后移动及胫骨内外旋转幅度。结果 负重下蹲时,正常膝关节平均屈曲136°,股骨内、外髁分别后移(7.3±1.2) mm和(19.3±3.1) mm,胫骨平均内旋23.8°±3.4°;TKA术后膝关节平均屈曲125°,股骨内、外髁分别后移(1.4±1.6) mm和(6.4±1.7) mm,胫骨平均内旋8.5°±3.4°。结论 固定平台后稳定型假体TKA术后膝关节运动与正常膝关节相似,均表现出股骨内、外髁后移及胫骨内旋运动,但幅度小于正常膝关节,且在屈膝过程中存在股骨矛盾性前移及胫骨外旋现象。     

In vivo three-dimensional patellar tracking on the femur

In vivo three-dimensional patellar tracking under weightbearing conditions was investigated with the principal reference axes of the femur in the coronal and axial planes, using a biplanar image-matching technique. Three-dimensional knee models of eight healthy volunteers were constructed using computed tomography scanning. Projection images of the models were fitted onto anteroposterior and lateral radiographs of the knees at hyperextension and at every 15 degrees from 0 degrees to 120 degrees flexion. Knee motion then was reconstructed on a computer. Patellar tracking during knee flexion was described simply with a medial jerk shift of 8 mm in early flexion until 30 degrees and a linear tracking with minimal mediolateral translation. The linear tracking portion was located laterally 5 mm from the mediolateral center of the femoral condyles. On average, the direction of this linear tracking was almost perpendicular to the distal condylar line in the coronal plane and perpendicular to the posterior condylar axis in the axial plane. These results help improve the understanding of patellofemoral kinematics and provide useful information for the design and positioning of the prostheses used in total knee arthroplasty.     

In Vivo Comparison of Knee Kinematics Before and After High-Flexion Posterior Cruciate-Retaining Total Knee Arthroplasty

The objectives of this study were to compare preoperative and postoperative knee kinematics for subjects implanted with flexion-enhanced posterior cruciate-retaining total knee arthroplasty during deep flexion and to examine flexion performance of the prosthesis design. Three-dimensional kinematics was analyzed by fluoroscopic examinations of subjects using a single-plane model-image registration technique. Preoperatively, knee kinematics demonstrated small posterior femoral translation and limited axial rotation. These motions differed significantly from patterns previously reported for normal knees. Postoperatively, flexion performance was maintained, averaging 130°, and kinematic patterns were similar to preoperative patterns. Although total knee arthroplasty can reduce pain and maintain functional performance, it appears that the characteristics of varus arthritic knee mechanics persist after arthroplasty.     

Radiographic analysis of low contact stress meniscal bearing total knee replacements

BACKGROUND: Meniscal bearing total knee replacements were developed to decrease the contact stresses on polyethylene and to reduce polyethylene wear. The kinematics of meniscal bearing knee replacements is poorly understood. The present study was designed to evaluate, with radiographic analyses, the motion of the meniscal bearings and the femoral rollback of the Low Contact Stress meniscal bearing knee replacement during knee flexion. METHODS: Eighty-one Low Contact Stress meniscal bearing total knee replacements in seventy-six male patients were assessed on fluoroscopically centered lateral radiographs made with the knee in full extension and in full flexion at an average of six years (range, twenty-four to 147 months) after the operation. The distance and direction of motion of the meniscal bearings and the center contact position of the femoral condyles were measured. Knee evaluations were performed with use of the Knee Society rating system. RESULTS: The average range of motion of the knees, measured on lateral radiographs, was 90 degrees (range, 45 degrees to 136 degrees). As they moved from terminal extension to terminal flexion, thirty-nine knees (48%) exhibited anterior motion of both bearings and sixteen (20%) demonstrated posterior motion of both bearings. Ten knees (12%) had reciprocal motion of the two bearings (one bearing moving anteriorly and one bearing moving posteriorly) with flexion, nine knees (11%) had motion of only one bearing, and seven knees (9%) had no motion of either bearing. When moving from full extension to full flexion, eighteen knees (22%) demonstrated femoral rollback, six knees (7%) showed no change in the position of femoral contact, and fifty-seven knees (70%) exhibited anterior sliding of the femoral condyles. Flexion of the knees demonstrating femoral rollback averaged 104 degrees (range, 76 degrees to 128 degrees), and flexion of the knees demonstrating anterior sliding averaged 94 degrees (range, 45 degrees to 125 degrees). The difference was significant (p = 0.03). According to the Knee Society rating system, the average clinical score for the entire group was 76 points (range, 27 to 100 points) and the average functional score for the entire group was 72 points (range, 30 to 100 points). The average clinical score was 79 points (range, 27 to 98 points) for the knees that exhibited anterior sliding of the femoral condyles and 87 points (range, 52 to 100 points) for those exhibiting femoral rollback (p = 0.09). The average functional scores were 64 points (range, 30 to 100 points) and 72 points (range, 45 to 100 points), respectively (p = 0.15). CONCLUSIONS: Radiographic analysis of meniscal bearing total knee replacements demonstrated an average anterior motion of both the medial and the lateral meniscal bearing of 4.7 mm (range, 1 to 14 mm) in thirty-nine knees (48%) as they moved from terminal extension to terminal flexion. Sixty-three knees (78%) demonstrated no femoral rollback as they were flexed. Knees with anterior sliding of the condyles had a significantly smaller average range of flexion (p = 0.03) and a lower average Knee Society score than did knees demonstrating femoral rollback. We believe that lack of rollback indicates a functional insufficiency of the posterior cruciate ligament.     

Observations of femoral rollback in cruciate-retaining knee arthroplasty

The current study evaluated kinematics of the knee during a stair-climbing activity after total knee arthroplasty. All patients received a posterior cruciate ligament retaining prosthesis of the same design, having an anatomically shaped femoral component and an unconstrained tibial insert. All patients had the same surgical technique done by two experienced surgeons. Patients had some posterior femoral rollback and screw-home type axial rotations during weightbearing activities. However, patients treated by different surgeons had different patterns of tibiofemoral motions. In Group 1, rollback occurred early in the flexion range and was maintained until 80 degrees flexion. In Group 2, the lateral condyle had rollback in early flexion, but both condyles translated forward as flexion increased to 80 degrees. An anatomic femoral component seems to be necessary to produce consistent early femoral rollback. However, soft tissue balance can have a significant effect on the kinematics of sagittally unconstrained posterior cruciate retaining total knee arthroplasty, because it may vary among surgeons.     

A high flexion total knee arthroplasty design replicates healthy knee motion

Deep flexion affects both femorotibial contact pattern and the patellofemoral articulation. The purpose of this study was to compare the patellofemoral motion of nonimplanted and implanted knees and to analyze femorotibial kinematics after total knee replacement designed for deep flexion. Three-dimensional patellofemoral kinematics were evaluated during a deep knee bend using fluoroscopy for five control patients with a healthy knee, five patients with an anterior-cruciate-ligament-deficient knee, and 20 patients who had a high flexion total knee arthroplasty. Less translation of patellofemoral contact position was seen in patients who had knee replacements than in patients with healthy knees, but the average motion and the patella tilt angles were similar to the healthy knees. On average, patients who had a total knee arthroplasty had 4.9 degrees normal axial rotation, and all patients had at least -4.4 mm of posterior femoral rollback. The average weightbearing range of motion of the patients in the total knee arthroplasty group was 125 degrees . In this study, patients implanted with a high-flexion knee replacement design had kinematic patterns that were similar to the healthy knee. It can be hypothesized that forces acting on the patella were not substantially increased for patients who had a total knee arthroplasty compared with the control patients.     

Fluoroscopic analyses of cruciate-retaining and medial pivot knee implants

Contemporary posterior cruciate-retaining total knee designs have provided pain relief and improved knee function but have failed to reproduce the kinematics and stability of the normal nonarthritic knee. The Medial Pivot total knee design features a near constant radius of curvature of the femoral component. The tibial surface is highly congruent and asymmetric, permitting a medial pivot motion during knee flexion. The purpose of the current study was to analyze and compare the gait kinematics of the Sigma posterior cruciate-retaining total knee implant, the Advance Traditional posterior cruciate-retaining total knee implant, and the Advance Medial Pivot knee implant using fluoroscopic analysis. In vivo kinematics were determined for 15 clinically successful total knee arthroplasties. Five knee implants were evaluated from each group. The authors analyzed the kinematics of knee motion during the stance phase of gait for each patient. On average, subjects with the Medial Pivot knee implant had a medial pivot motion. Both posterior cruciateretaining designs had a paradoxical roll forward of the tibia on femur during knee flexion and had greater excursion of both condyles during knee flexion than the medial pivot design. Nine of 10 of the posterior cruciate-retaining designs had condylar lift-off averaging 1.7 mm whereas only one Medial Pivot knee implant had condylar lift-off measuring 1.1 mm.     

Morphology of the transepicondylar axis and its application in primary and revision total knee arthroplasty

A morphologic anatomic study was done of the lower extremity to investigate various relationships of the transepicondylar axis (TEA). Thirteen cadaver specimens were dissected and mounted to a metal frame with a pin passing through the TEA. The center of the knee was determined as the depth of the anterior intercondylar groove. The ratio of the upper leg to lower leg measured from femoral head center and ankle center to TEA was 1.02. The mean distance of the TEA to the joint line was 3.08 cm medial and 2.53 cm lateral. The mean femoral angle comparing the TEA to mechanical axis was 0.61° varus. The mean tibial angle comparing the TEA to the mechanical axis was 0.4° varus in extension and 0.43° in flexion, with no significant difference in the lower extremity angle with flexion (P < .01). The TEA is an important landmark that, from this study, is virtually perpendicular to the mechanical axis of the lower extremity and parallels the knee flexion axis. Femoral component rotation and joint line positioning in total knee arthroplasty can be determined using the TEA.