Valgus Subsidence of the Tibial Component Caused by Tibial Component Malpositioning in Cementless Oxford Mobile-Bearing Unicompartmental Knee Arthroplasty

BackgroundValgus subsidence (VS) of the tibial component is a rare complication of unicompartmental knee arthroplasty (UKA), and surgeons might consider revision surgery. The present study aimed to identify the factors related to VS of the tibial component after cementless Oxford mobile-bearing UKA.MethodsThe study included 120 patients who underwent Oxford mobile-bearing UKA using a cementless tibial component in our center between September 2015 and September 2016. Six showed VS of >2° after surgery. Patients were stratified into 2 groups according to the occurrence of VS of the tibial component (VS group, n = 6; no-subsidence group, n = 114). Postoperative radiographic evaluations were conducted to assess the varus/valgus alignment, rotation, and mediolateral position of the tibial and femoral components. The Oxford Knee Score (OKS) was assessed at 3, 6, and 12 months postoperatively. Positional parameters and sequential change in OKS were compared between the two groups using unpaired t-test (P < .05, statistically significant).ResultsAt 3 months postoperatively, an average VS of 3.4° in the VS group was observed, with a significant decrease in OKS. VS was associated with a significantly more medial position and external rotation of the tibial component. After 3 months, VS stopped, and the OKS gradually improved without revision surgery.ConclusionsVS might be caused by the malpositioning of the tibial component. VS of the tibial component after UKA appears to stop, with simultaneous pain relief, even without revision after 3 months postoperatively.     

全膝关节置换术后股胫关节运动学轨迹的数字化模型研究

 目的 通过透视技术结合数字化模型注册技术分析全膝关节置换术后股骨假体与胫骨垫片之间的相对运动和接触位置。方法 2007年7月至2008年6月,接受GENESISⅡ假体全膝关节置换术患者16例,均为女性;年龄56～76岁,平均66.4岁。随访48～60个月,平均(56±3)个月。采用膝关节学会评分(Knee Society Score,KSS)评价膝关节功能;采用循环透视方法获取影像学数据,对假体逆向数字建模,进行数字模型和影像学数据的匹配,重建膝关节的三维运动;测量股骨内、外髁接触位置的移动,计算胫骨内旋角度,测量股骨凸轮和胫骨立柱的接触时相和范围。结果 末次随访时KSS膝评分(93±5)分,功能评分(88±13)分,与术前比较差异有统计学意义。股骨内髁的移动范围(8.5±2.5) mm,外髁的移动范围(9.5±4.8) mm,胫骨内旋角度2.5°±8.4°。屈膝约30°～40°时凸轮和立柱发生接触,立柱后方的接触范围(8.0±1.8) mm。胫骨平台后倾角度越大,凸轮和立柱的接触越晚。结论 全膝关节置换术后股胫关节的运动学特征与正常膝关节不同,膝关节屈曲10°~30°时股骨内髁前移,屈曲大于40°后股骨内、外髁后移,胫骨平台后倾与凸轮和立柱的接触时相有相关性。     

Posterior Bearing Overhang Following Medial and Lateral Mobile Bearing Unicompartmental Knee Replacements

This study explores the extent of bearing overhang following mobile bearing Oxford unicompartmental knee replacement (OUKR) (Oxford Phase 3, Zimmer Biomet). The Oxford components are designed to be fully congruent, however knee movements involve femoral rollback, which may result in bearing overhang at the posterior margin of the tibial implant, with potential implications for; pain, wear, and dislocation. Movement is known to be greater, and therefore posterior overhang more likely to occur, with; lateral compared to medial implants, anterior cruciate ligament (ACL) deficiency (ACLD) compared to ACL intact (ALCI), and at extremes of movement. Twenty‐four medial, and 20 domed lateral, OUKRs underwent sagittal plane knee fluoroscopy during step‐up and forward lunge exercises. The bearing position was inferred from the relative position of the femoral and tibial components. On the basis of the individual component sizes and geometry the extent the posterior part of the bearing which overhung the posterior part of the tibial component was calculated. There was no significant posterior overhang in knees with medial implants. Knees with lateral domed implants exhibited overhang at flexion angles beyond 60°, the magnitude of which increased with increasing flexion angle, reaching a maximum of 50% of the bearing length at 140° (range 0–140°). This demonstrates a clear difference between the kinematics, and prevalence and extent of posterior bearing overhang between medial and lateral OUKRs. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1938–1945, 2019     

The Effect of Posterior Tibial Slope on Joint Gap and Range of Knee Motion in Mobile-Bearing Unicompartmental Knee Arthroplasty

BackgroundIt is widely known that the posterior tibial slope (PTS) has an influence on the clinical outcome of arthroplasty. However, the influence of PTS on unicompartmental knee arthroplasty (UKA) is still not fully clear. The objective of this study is to reveal the effect PTS has on knee flexion and extension joint gap and the postoperative range of motion in mobile-bearing UKA. Moreover, we investigated an adequate PTS angle in mobile-bearing UKA.MethodsOxford UKA was performed so that the flexion gap would be equal to the extension gap. Correlation between the gap value difference from 90° to 120° of the knee flexion and the PTS was evaluated. Correlation between postoperative range of motion and the PTS was also evaluated to find whether a small degree of PTS would cause knee flexion restriction.ResultsThe PTS had a moderate positive correlation with the flexion gap difference. However, the PTS had no correlation with the knee flexion angle both postoperative and 1 year after surgery.ConclusionIt was suggested that the degree of the PTS should not be so large to avoid joint looseness throughout every knee angle. Increasing the degree of the PTS had the potential to dislocate the bearing. Since a small degree of the PTS does not have an influence on the clinical outcome, surgeons should aim to cut the tibia with a posterior slope of less than 7°.     

Central Implantation of the Femoral Component Relative to the Tibial Insert Improves Clinical Outcomes in Fixed-Bearing Unicompartmental Knee Arthroplasty

BackgroundThe direct relationship between clinical outcomes and femoral component positioning relative to a tibial insert remains unknown. We determined whether the femoral component position relative to the tibial insert could affect clinical outcomes after fixed-bearing unicompartmental knee arthroplasty (UKA).MethodsThe femoral component position relative to the tibial insert of 66 patients with anteromedial osteoarthritis and osteonecrosis of the knee who underwent fixed-bearing UKA was assessed at 2 weeks postoperatively. We classified patients according to the contact point of the femoral component with the tibial component: group M (medial), 18 knees; group C (central), 30 knees; and group L (lateral), 18 knees. Patient-derived clinical scores using the 2011 Knee Society Score were also assessed preoperatively and at 2 years postoperatively and compared among the 3 groups using the analysis of variance.ResultsThe average 2-year postoperative “symptom” and “patient satisfaction” scores based on the 2011 Knee Society Score were significantly higher in group C than in group M or group L.ConclusionCentral implantation of the femoral component relative to the tibial insert plays an important role in decreasing pain and could result in better patient satisfaction after fixed-bearing UKA at 2 years postoperatively. Surgeons should set the femoral component at the center relative to the tibial insert for better patient satisfaction and higher active knee flexion after fixed-bearing UKA.     

Fixed‐bearing medial unicompartmental knee arthroplasty restores neither the medial pivoting behavior nor the ligament forces of the intact knee in passive flexion

Medial unicompartmental knee arthroplasty (UKA) is an accepted treatment for isolated medial osteoarthritis. However, using an improper thickness for the tibial component may contribute to early failure of the prosthesis or disease progression in the unreplaced lateral compartment. Little is known of the effect of insert thickness on both knee kinematics and ligament forces. Therefore, a computational model of the tibiofemoral joint was used to determine how non‐conforming, fixed bearing medial UKA affects tibiofemoral kinematics, and tension in the medial collateral ligament (MCL) and the anterior cruciate ligament (ACL) during passive knee flexion. Fixed bearing medial UKA could not maintain the medial pivoting that occurred in the intact knee from 0° to 30° of passive flexion. Abnormal anterior–posterior (AP) translations of the femoral condyles relative to the tibia delayed coupled internal tibial rotation, which occurred in the intact knee from 0° to 30° of flexion, but occurred from 30° to 90° of flexion following UKA. Increasing or decreasing tibial insert thickness following medial UKA also failed to restore the medial pivoting behavior of the intact knee despite modulating MCL and ACL forces. Reduced AP constraint in non‐conforming medial UKA relative to the intact knee leads to abnormal condylar translations regardless of insert thickness even with intact cruciate and collateral ligaments. This finding suggests that the conformity of the medial compartment as driven by the medial meniscus and articular morphology plays an important role in controlling AP condylar translations in the intact tibiofemoral joint during passive flexion. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1868–1875, 2018.

     

Kinematics of medial osteoarthritic knees before and after posterior cruciate ligament retaining total knee arthroplasty

Total knee arthroplasty (TKA) is a widely accepted surgical procedure for the treatment of patients with end‐stage osteoarthritis (OA). However, the function of the knee is not always fully recovered after TKA. We used a dual fluoroscopic imaging system to evaluate the in vivo kinematics of the knee with medial compartment OA before and after a posterior cruciate ligament‐retaining TKA (PCR‐TKA) during weight‐bearing knee flexion, and compared the results to those of normal knees. The OA knees displayed similar internal/external tibial rotation to normal knees. However, the OA knees had less overall posterior femoral translation relative to the tibia between 0° and 105° flexion and more varus knee rotation between 0° and 45° flexion, than in the normal knees. Additionally, in the OA knees the femur was located more medially than in the normal knees, particularly between 30° and 60° flexion. After PCR‐TKA, the knee kinematics were not restored to normal. The overall internal tibial rotation and posterior femoral translation between 0° and 105° knee flexion were dramatically reduced. Additionally, PCR‐TKA introduced an abnormal anterior femoral translation during early knee flexion, and the femur was located lateral to the tibia throughout weight‐bearing flexion. The data help understand the biomechanical functions of the knee with medial compartment OA before and after contemporary PCR‐TKA. They may also be useful for improvement of future prostheses designs and surgical techniques in treatment of knees with end‐stage OA. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:40–46, 2011     

Sagittal Alignment of the Femoral Component and Patient Height Are Associated With Persisting Flexion Contracture After Primary Total Knee Arthroplasty

BackgroundThe aim of our retrospective case-control study is to identify risk factors associated with a persisting flexion contracture after total knee arthroplasty (TKA). This is an important clinical issue as a flexion contraction can lead to poor long-term clinical outcomes and patient satisfaction after TKA.MethodsThe study group included 120 knees treated for a varus osteoarthritic deformity of the knee using a posterior cruciate-retaining TKA. We evaluated the association between a flexion contracture >10°, 2 years after surgery, and the following potential risk factors, using logistic regression analysis: age, body height, body mass index, preoperative knee extension and hip-knee-ankle angle, and radiological parameters of component alignment, namely the femoral component medial angle, the femoral component flexion angle (FFA), the tibial component medial angle, and the posterior tibial slope.ResultsOf the 120 knees, a persisting flexion contracture >10° was identified in 33 (28%). The mean FFA in these cases was 7.3° (standard deviation, 1.4) compared to 4.2° (standard deviation, 1.2) for cases with a contracture of ≤10° (P = .034). On multivariate analysis, the FFA (odds ratio, 3.73; 95% confidence interval, 1.16-17.81; P = .034) and body height (odds ratio, 0.43; 95% confidence interval, 0.29-0.57; P = .041) were independent predictive risk factors for a residual flexion contracture >10°.ConclusionClinicians should be aware that flexed position of the femoral component, particularly in patients of short stature, is associated with increased occurrence of persistent flexion contracture.     

ACL forces and knee kinematics produced by axial tibial compression during a passive flexion–extension cycle

Application of axial tibial force to the knee at a fixed flexion angle has been shown to generate ACL force. However, direct measurements of ACL force under an applied axial tibial force have not been reported during a passive flexion–extension cycle. We hypothesized that ACL forces and knee kinematics during knee extension would be significantly different than those during knee flexion, and that ACL removal would significantly increase all kinematic measurements. A 500 N axial tibial force was applied to intact knees during knee flexion–extension between 0° and 50°. Contact force on the sloping lateral tibial plateau produced a coupled internal + valgus rotation of the tibia, anterior tibial displacement, and elevated ACL forces. ACL forces during knee extension were significantly greater than those during knee flexion between 5° and 50°. During knee extension, ACL removal significantly increased anterior tibial displacement between 0° and 50°, valgus rotation between 5° and 50°, and internal tibial rotation between 5° and 15°. With the ACL removed, kinematic measurements during knee extension were significantly greater than those during knee flexion between 5° and 45°. The direction of knee flexion–extension movement is an important variable in determining ACL forces and knee kinematics produced by axial tibial force. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:89–95, 2014.     

In vitro kinematic patterns are similar for a fixed platform and a mobile bearing prosthesis

In vitro dynamic simulation of knee flexion was performed to quantify knee kinematics for a mobile bearing prosthesis that allows the tibial insert to translate and rotate with respect to the baseplate. Six cadaver knees were tested in the intact state, after implanting a fixed platform prosthesis, and after implanting a mobile bearing prosthesis. The mobile bearing prosthesis significantly increased the tibial internal rotation and medial shift compared with the intact knee, near 90 degrees of flexion. Both prostheses increased the patellar medial shift near 90 degrees of flexion. The patellar flexion was significantly larger for the mobile bearing prosthesis than for the fixed platform prosthesis for most of flexion. Motion of the insert with respect to the baseplate may have contributed to the variations in tibiofemoral kinematics, whereas tibiofemoral kinematic changes influenced the patellofemoral kinematics. Although the kinematics were similar for the 2 types of prosthesis, the possibility of complications related to increased patellar flexion and backside wear of the tibial insert should be considered.     

Knee biomechanics after UKA and its relation to the ACL--a robotic investigation.

Unicompartmental knee arthroplasty (UKA) has regained popularity in recent years. However, limited data exist on how UKA affects knee biomechanics. The role of the anterior cruciate ligament (ACL) after fixed bearing UKA remains controversial. In this study, a robotic testing system was used to apply a quadriceps/hamstrings load to cadaveric knee specimens in the intact state, after medial UKA, and after transection of the ACL in UKA. The load was applied to the knee from full extension to 120 degrees of flexion in 30 degrees increments. UKA generally did not affect anterior-posterior (AP) femoral position, but did cause external tibial rotation and variations in varus-valgus rotation compared to the intact knee. ACL transection after UKA shifted the femur posteriorly compared to the intact and UKA knees and increased internal tibial rotation compared to the UKA knee at low flexion. The AP motion of the articular contact position in the implant was increased after ACL transection. These data might help explain the mechanism of tibial component loosening and provide insight into further investigations of polyethylene wear in UKA. Based on the kinematic data, the ACL should be functional to provide patients the greatest opportunity for long-term success after medial UKA.     

Bearing Separation From the Lateral Wall of the Tibial Component Is a Risk of Anterior Dislocation of the Mobile Bearing in Oxford Unicompartmental Knee Arthroplasty

BackgroundBearing dislocation is a serious complication after Oxford unicompartmental knee arthroplasty. Bearing separation from the lateral wall can cause it to spin (90° horizontal rotation) and eventually dislocate because there is just a 2 mm difference in height in both the lateral and medial sides from the bottom of the bearing, compared with the anterior (5 mm) and posterior (3 mm) sides. The details of this problem have not been previously examined.MethodsTwenty-one dislocations in 12 patients were retrospectively analyzed. Bearing separation was defined as the bearing position being sufficiently distant from the lateral wall of the tibial component to allow spinning. We analyzed the incidence of separation, the direction and the recurrence of the dislocations, and their causes and treatments.ResultsFive of the 12 patients had separation. Of the total of 21 dislocations, 11 occurred in cases of separation (52%). Seven of 11 anterior dislocations were found to have separation, whereas nine of 13 posterior dislocations occurred without separation (P = .0237). Three of 5 patients with separation had recurrence of dislocation, and eventually 2 underwent revision to fixed-bearing unicompartmental knee arthroplasty.ConclusionBearing separation from the lateral wall of the tibial component can cause bearing dislocation, especially in an anterior direction. To prevent separation, the wall-bearing distance should be evaluated before the keel slot preparation, with manipulation as necessary. Conversely, posterior dislocation was predominant in our nonseparation cases.     

Contact stress analysis in meniscal bearing total knee arthroplasty

The effect of a mobile meniscal bearing on tibiofemoral contact stress was tested with a standard fixed tibial component and with movable tibial components (anteroposterior sliding, rotationally sliding, and anteroposterior and rotationally sliding). A digital electronic sensor was used to detect tibiofemoral contact location in five cadaver knees, then the location was reproduced while peak and mean stresses were measured under compressive load at 0°, 30°, 60°, and 90° of flexion. Stresses were measured when the tibial component was normally aligned and at 15° internal and 15° external rotation. To evaluate the effect of excessive overhang of the polyethylene articular surface, undersurface stress of the rotationally sliding component was also measured with a 30° and a 45° malrotated tibial tray. Uppersurface stresses of the fixed-bearing components were significantly higher at full extension than those recorded in components with rotational mobility. Undersurface stresses were always lower than uppersurface stresses, but correlated with uppersurface stresses. Undersurface stresses of the rotationally sliding component gradually increased as the malrotation angle of the tray increased. A mobile meniscal bearing surface appears to offer an advantage over a standard fixed component when rotational malalignment of the tibial component occurs. However, with severe rotational malalignment, edge contact markedly increases undersurface stresses, which could cause deformity and subluxation.     

In vivo kinematics of a low contact stress rotating platform total knee arthroplasty system under weight bearing and non-weight bearing condition

Background

The patterns and magnitudes of axial femorotibial rotation are variable due to the prosthesis design, ligamentous balancing, and surgical procedures. We aimed to investigate the effects of the weight bearing (WB) condition on the kinematics of mobile-bearing total knee arthroplasty (TKA).

Methods

We examined 12 patients (19 knees) implanted with a low contact stress (LCS) mobile-bearing TKA system using a two-dimensional to three-dimensional registration technique. The in vivo kinematics of dynamic deep knee flexion under WB and non-WB (NWB) conditions were compared. We evaluated the knee range of motion, femoral axial rotation relative to the tibial component, anteroposterior translation, and kinematic pathway of the femorotibial contact point for both the medial and lateral sides.

Results

Under the WB condition, the mean range of motion was 117.8° ± 16.7°. Under the NWB condition, the mean range of motion was 111.0° ± 4.4°. The mean range of axial rotation from full extension to maximum flexion was 3.0° ± 1.5° under the WB condition and 2.2° ± 1.0° under the NWB condition. With regard to the anteroposterior translation, the LCS mobile-bearing TKA system showed the same kinematic patterns under both conditions, except for axial rotation at 0°, 10°, and 110°. From hyperextension to maximum flexion, the kinematic pattern reflected a central pivot under both conditions.

Conclusions

In conclusion, this study demonstrated that, in an LCS mobile-bearing TKA system, knee kinematics showed the same patterns under NWB and WB conditions, except for axial rotation at the early phase. Further understanding of knee kinematics could provide us with useful information for future design concepts of TKA implants.     

Activity Level Does Not Affect Survivorship of Unicondylar Knee Arthroplasty at 5-Year Minimum Follow-Up

BackgroundPatients are often cautioned against a high level of activity after knee arthroplasty. The purpose of this study was to report on implant survivorship and outcomes of high-activity patients compared with low-activity patients after unicondylar knee arthroplasty (UKA).MethodsWe identified 487 patients (576 knees) who underwent UKA with the Oxford mobile bearing knee (Zimmer Biomet, Warsaw, IN) with a 5-year minimum follow-up. Patients were divided into 2 groups: low activity (LA) (University of California Los Angeles ≤6) and high activity (HA) (University of California Los Angeles ≥7). Preoperative and postoperative range of motion, Knee Society scores, complications, and reoperations were evaluated.ResultsMean follow-up was 9 years (range, 4 to 13.1). The HA group had significantly more male patients, were younger, and had higher knee society clinical scores and knee society functional scores preoperatively. The HA group had significantly higher improvements in Knee Society scores and pain postoperatively. Revisions were performed in 8.4% of the LA group and 6.2% of the HA group (P = .43). At our mean of 9-year follow-up, there was 94.0% survival for the HA group and 92.1% for the LA group (P = .6085).ConclusionsHighly active patients had no increased risk of revision after UKA with the Oxford mobile bearing knee at 5-year minimum follow-up.     

Biomechanical comparison of fixed‐ and mobile‐bearing for unicomparmental knee arthroplasty using finite element analysis

Unicomparmental knee arthroplasty (UKA) is a popular alternative to total knee arthroplasty (TKA) and high tibial osteotomy for unicompartmental knee conditions, especially in young patients. However, failure of UKA occurs due to either progressive osteoarthritis (OA) in the other compartment or wear on the polyethylene (PE) insert. This study used finite element (FE) analysis to investigate the effects of PE insert contact pressure and stress in opposite compartments for fixed‐ and mobile‐bearing UKA. Analysis was performed using high kinematics displacement and rotation inputs, which were based on the kinematics of the natural knee. ISO standards were used for axial load and flexion. The mobile‐bearing PE insert had lower contact pressure than the fixed‐bearing PE insert. With the mobile‐bearing UKA, lower stress on the opposite compartment reduces the overall risk of progressive OA in the knee. The fixed‐bearing UKA increases the overall risk of progressive OA in the knee due to higher stress on the opposite compartment. However, the PE insert of mobile‐bearing showed pronounced backside stress at the inferior surface. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:338–345, 2014.     

Factors affecting the postoperative limb alignment and clinical outcome after Oxford unicompartmental knee arthroplasty

We evaluated the postoperative mechanical axis deviation and clinical outcome according to bearing size, femoral component position, and tibial resection angle after unicompartmental knee arthroplasty (UKA). A total of 104 patients with 124 knees underwent Oxford phase 3 UKA. The overall changes in mechanical axis deviation and tibiofemoral angle were significantly different according to bearing size (P = .001 and < .001), but they were not significantly different according to the tibial resection angle and femoral component position. The postoperative mechanical axis fell into the zone C or zone 2 in 108 knees (87%) and into the zone 3 or zone 4 in 16 cases (13%). One hundred eight cases, which had the mechanical axis passing the zone C or zone 2, did not show any progression of arthritis. Limb alignment is a function of the thickness of the bearing rather than alignments of femoral and tibial implant.     

How to Perform Better on Oxford UKA? A Technical Note from over 500 Surgical Experiences

Oxford unicompartmental knee arthroplasty (UKA) has been particularly prevalent because the concept of knee preservation is deeply rooted in people's minds. Mobile bearing UKA is a surgical type of UKA with considerable advantages. This note describes some surgical techniques, including patient position, surgical field exposure, selection of the size of the prosthesis, sagittal tibial osteotomy, placement of the femoral prosthesis and gap balance, to assist surgeons with less experience in performing these operations successfully. The techniques described in this note have been used in over 500 Oxford UKA cases, and nearly 95% patients achieved good prosthesis position and satisfactory postoperative outcome. We hope that the empirical summaries from numerous cases will help surgeons to learn Oxford UKA quickly and effectively, driving the spread of the technique and benefiting more patients.     

Two Year Follow-up of the Preservation Unicompartmental Knee Implant

Reported results of unicondylar knee arthroplasty (UKA) have mixed reviews in comparison with results of tri-compartmental knee arthroplasty (TKA). We prospectively evaluated the short-term results (2 years) of a newer design of a UKA implant (Preservation UKA) with a cobalt–chromium femoral component and an all polyethylene tibial component. Seventy-two patients with intact ligaments and loss of only medial articular cartilage received the Preservation prosthesis. Data were obtained using WOMAC, Knee Society score (KSS), and standard radiographs. WOMAC scores improved by 24 points and KSS improved by 33 points at 2-year follow-up. Mean flexion increased by 4° to126° at 2 years. On X-ray, only one patient had a radiolucency. No fractures occurred. Two knees were revised due to clinical symptoms of medial compartment pain. This 2-year follow-up study of the Preservation UKA shows promising early results. Long-term data would be necessary to compare results with TKA or other unicompartmental replacements.     

Die AMC-Unikondylarprothese

Since its introduction in the 1970s, unicompartmental knee arthroplasty (UKA) has become a standardized routine technique. Increasing experience in determining the indication for UKA and improvements in design and materials of the prosthesis have led to better results.The AMC UKA has an unconstrained mobile bearing with congruent area contact from 0 degrees to 30 degrees of flexion. The principle is that a polyethylene mobile bearing, concavely spherical above and flat below, can maintain perfect congruity between the spherical metal femoral condyle and the flat metal tibial plateau. This ensures complete freedom to rotate and slide upon one other with physiologic kinematic and low intrinsic stability. The low contact stress results in reduced polyethylene wear.The 361 AMC UKA replacements of the current study showed safe anchorage of the prosthesis and good durability of the mobile bearings. The clinical results of the investigated patients demonstrate that the AMC UKA is a successful concept. The comparison of 30 minimally invasive UKA with 30 conventional UKA and 30 total knee replacements shows an advantage of the minimally invasive technique with regard to reduced rehabilitation time. The accuracy of implantation was comparable between the conventional and the minimally invasive techniques.