Functional analysis of the effect of the posterior stabilising cam in two total knee replacements. A comparison of the Insall/Burstein and Bisurface prostheses

The Bisurface knee prosthesis (BP) has a posterior stabilising cam (ball-and-socket joint) in the mid-posterior region of the femorotibial joint in an attempt to improve the range of movement. Based on an in vitro weight-bearing study contact areas of the Insall/Burstein 2 (IB2) and the BP knee were compared using pressure-sensitive films. The stability afforded by the cam was evaluated by means of dislocation distances in the vertical and horizontal planes. Significant adverse anterior translation in mid-flexion was not observed with the BP knee since the cam was effective above 60 degrees of flexion. At flexion of 60 degrees or more, the total contact areas were larger, as the cam represented a weight-bearing surface. The dislocation distances for the BP knee compared favourably with those for the IB2 knee. We conclude that the cam of the BP knee allows good movement, stability and wear.     

The Bisurface total knee replacement: a unique design for flexion. Four-to-nine-year follow-up study

BACKGROUND: The Bisurface knee prosthesis was designed in 1989 to improve knee flexion without affecting the durability of the prosthesis. The prosthesis has a unique ball-and-socket joint in the midposterior portion of the femoral and tibial components, which functions as a posterior stabilizing cam mechanism and causes femoral rollback. The femoral component was made of alumina ceramic. The purpose of this study was to review the clinical results of the first 223 arthroplasties performed with this prosthesis in order to assess whether this new implant had achieved its design objectives. METHODS: From December 1989 to May 1994, all patients who were scheduled for primary total knee arthroplasty were enrolled in a prospective study of the Bisurface knee. The patients were evaluated clinically according to The Hospital for Special Surgery knee-rating system and with a self-administered questionnaire, and they were evaluated radiographically according to the system of the Knee Society. Kaplan-Meier survivorship analysis was performed with revision of the knee or recommendation for revision as the end point. RESULTS: One hundred and sixty-six patients treated with a total of 223 consecutive primary total knee arthroplasties were enrolled in the study, and 182 knees were followed for 3.9 to 9.0 years (mean, 5.8 years). Preoperatively, the mean Hospital for Special Surgery knee score was 44.5 points. At the time of latest follow-up, the mean knee score was 86.3 points. The mean preoperative and postoperative ranges of flexion were 119 and 124 degrees, respectively. The patients, even those with a good preoperative range of motion, rarely lost deep flexion of the knee after the procedure. A revision operation was performed in eight knees (because of infection in five, instability in two, and breakage of the peg of the patellar component in one). Two knees had recurrent medial-lateral subluxations of the femorotibial articulation, which were treated nonoperatively. No prosthesis had loosened aseptically and no alumina ceramic femoral component had broken by the time of latest follow-up. The rate of survival of the implant was 94 percent (95 percent confidence interval, 90 to 98 percent) at six years. According to the patient questionnaires, 20 percent of the knees sometimes felt loose in daily living activities, which prompted us to improve the intrinsic stability of the prosthesis by improving the congruity of the ball-and-socket joint. CONCLUSIONS: Total knee arthroplasty with the Bisurface prosthesis resulted in an excellent range of motion and a high level of satisfaction with the operation; the durability of the prosthesis is promising.     

In vivo kinematics of high-flex mobile-bearing total knee arthroplasty, with a new post-cam design, in deep knee bending motion

Purpose

The objective of this study was to evaluate the in vivo knee kinematics to assess the available functional motion of the characteristic mobile-bearing prosthesis design and to examine whether the artificial joint would work in vivo according to its design concept.

Methods

We studied 14 knees (11 patients) implanted with the Vanguard RP Hi-Flex prosthesis. This prosthesis has a highly original form of post-cam called a PS saddle design with high compatibility, and with a rotating plate mobile-bearing mechanism. The cylinder-type post-cam is designed to enable contact in early flexion ranges, and to prevent paradoxical anterior femoral component movement. Each patient performed weight-bearing deep knee bending under fluoroscopic surveillance. Motion between each component including the polyethylene insert was analyzed using the 2D/3D registration technique.

Results

The mean range of motion was 122.0°. The mean femoral component rotation for the tibial tray was 5.0°. No paradoxical anterior movement of the nearest point was confirmed between the femoral component and the tibial tray in the early flexion ranges. Initial contact of the post-cam was confirmed at a knee flexion angle of 33.8°. Subsequently, the wide contact of the post-cam was maintained until flexion reached 120° in all knees, but disengagement of the post-cam was observed in two knees when flexion was ≥130°.

Conclusions

The results of this study demonstrated that the prosthesis design generally works in vivo as intended by its design concept. The present kinematic data may provide useful information for improvement of high-flex type prostheses.     

Computational analysis of customized cruciate retaining total knee arthroplasty restoration of native knee joint biomechanics

The purpose of this study was to verify if customized prosthesis better preserves the native knee joint kinematics and provides lower contact stress on the polyethylene (PE) insert owing to the wider bone preservation than that of standard off‐the‐shelf prosthesis in posterior cruciate‐retaining type total knee arthroplasty (TKA). Validated finite element (FE) models for were developed to evaluate the knee joint kinematics and contact stress on the PE insert after TKA with customized and standard off‐the‐shelf (OTS) prostheses as well as in normal healthy knee through FE analysis under dynamic loading conditions. The contact stresses on the customized prosthesis decreased by 18% and 8% under gait cycle loading conditions, and 24% and 9% under deep‐knee‐bend loading conditions, in the medial and lateral sides of the PE insert, respectively, compared with the standard OTS prosthesis. The anterior‐posterior translation and internal‐external (IE) rotation in customized TKA were more similar to native knee joint behaviors compared with standard OTS TKA under gait loading conditions. The difference from normal knee kinematics was lower for femoral rollback and IE rotation in customized TKA than in standard OTS TKA in the deep‐knee‐bend condition. In general, customized prostheses achieve kinematics that are close to those of the native healthy knee joint and have better contact stresses than standard OTS prostheses in gait and deep‐knee‐bend loading conditions.     

The use of functional analysis in evaluating knee kinematics

The importance of understanding the six-degrees-of-freedom kinematics of the knee during ambulatory activities was examined in the context of the function of total knee arthroplasty. Studies of knee kinematics during walking, stair climbing, and a deep flexion squat indicate that knee kinematics is activity-dependent. A comparative study of patients and healthy subjects during stair climbing indicates the importance of maintaining the function of the posterior cruciate ligament. A second study used walking kinematics derived from patient testing as input to a wear simulator. There was increased wear relative to standard simulator input that was related to the slip velocity at the contact surface. Finally, results from a study of deep flexion indicate that substantial femoral rotation is required during deep flexion activities. The current study shows the importance of studying in vivo knee kinematics for future enhancement in the treatment of the arthritic knee.     

In vitro investigation of knee joint kinematics following cruciate retaining versus cruciate sacrificing total knee arthroplasty

The aim of this biomechanical study was to investigate knee joint kinematics following total knee arthroplasty. We compared eight congruent posterior cruciate ligament retaining and four ultracongruent cruciate sacrificing Natural Knee prostheses to the untreated human cadaveric knee joint. A six-degree-of-freedom testing device was used to evaluate knee joint kinematics with a load of 300 Newton and without load application (0 Newton). Statistical analysis was performed using the Wilcoxon rank sum test. A significant increase in antero-posterior translation and tibial rotation was seen in both types of total knee arthroplasty. Implantation of the ultracongruent prosthesis was followed by distinctly more kinematic changes in comparison to the congruent prosthesis. Load application of 300 Newton leads to an anterior dislocation of the femoral component of the ultracongruent prosthesis at 60 degrees of flexion in vitro, indicating an increased demand of compensatory muscular activity in vivo.     

Anterior tibial post impingement in a posterior stabilized total knee arthroplasty.

Despite the numerous long-term success reports of posterior stabilized (PS) total knee arthroplasty (TKA), recent retrieval studies of various PS TKA designs revealed wear and deformation on the anterior side of the tibial post. This study investigated the mechanisms of anterior impingement of the post with the femoral component. Seven cadaveric knees were tested to study kinematics and tibial post biomechanics during simulated heel strike using an in vitro robotic testing system. Intact knee kinematics and in situ anterior cruciate ligament (ACL) forces were determined at hyperextension (0 degree to -9 degrees) and low flexion angles (0 degrees to 30 degrees) under the applied loads. The same knee was reconstructed using a PS TKA. The kinematics and the tibial post contact forces of the TKA were measured under the same loading condition. The ACL in the intact knee carried load and contributed to knee stability at low flexion angles and hyperextension. After TKA, substantial in situ contact forces (252.4 +/- 173 N at 9 degrees of hyperextension) occurred in the tibial post, indicating anterior impingement with the femoral component. Consequently, the TKA showed less posterior femoral translation compared to the intact knee after the impingement. At 9 degrees of hyperextension, the medial condyle of the intact knee translated 0.1 +/- 1.1 mm whereas the medial condyle of the TKA knee translated 5.6 +/- 6.9 mm anteriorly. The lateral condyle of the intact knee translated 1.5 +/- 1.0 mm anteriorly whereas the lateral condyle of the TKA knee translated 2.1 +/- 5.8 mm anteriorly. The data demonstrated that anterior tibial post impingement functions as a substitute for the ACL during hyperextension, contributing to anterior stability. However, anterior post impingement may result in additional polyethylene wear and tibial post failure. Transmitted impingement forces might cause backside wear and component loosening. Understanding the advantages and disadvantages of the tibial post function at low flexion angles may help to further improve component design and surgical techniques and thus enhance knee stability and component longevity after TKA.     

垫片改良设计对膝关节置换术后临床功能和膝关节运动的影响

目的 探讨膝关节假体垫片设计对膝关节置换术后临床功能和膝关节运动学的影响.方法 分析2007年7月至2009年6月使用GENESIS Ⅱ假体行全膝关节置换术的28例膝关节骨关节炎患者的临床资料,共42膝,平均随访27.7个月.按照使用垫片的种类分为高屈曲垫片组(男性1例,女性14例,23膝;年龄54～74岁,平均62.8岁)和标准组(男性1例,女性12例,19膝;年龄54～74岁,平均64.3岁).通过膝关节评分比较两组的临床功能.采用循环透视和软件分析的方法比较两组病例在膝关节屈伸活动时股骨后滚、伸膝装置力臂等参数,评价垫片设计对膝关节运动的影响.结果 标准组和高屈曲垫片组的术后活动度分别为120°和123°,差异无统计学意义.两组在膝关节学会评分上没有差异.高屈曲垫片组术后的Feller评分高于标准组,差异有统计学意义(P=0.012).在影像学测量上,两组病例在0～120°范围活动时股骨后滚和伸膝装置力臂的差异均无统计学意义;活动范围达到130°时,两组的伸膝装置力臂差异有统计学意义(P=0.034).结论 高屈曲垫片的改良设计在减少膝关节置换术后膝前痛的发生、改善膝关节功能方面有一定的作用;但可能会减小伸膝装置的力臂,从而影响股四头肌的做功.     

Differences in the stress distribution in the distal femur between patellofemoral joint replacement and total knee replacement: a finite element study

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

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.     

A three-dimensional MRI analysis of knee kinematics.

PURPOSE: To quantify normal, in vivo tibio-femoral knee joint kinematics in multiple weight bearing positions using non-invasive, high-resolution MRI and discuss the potential of developing future kinematic methods to assess patients with abnormal joint pathologies. METHODS: Ten volunteers with clinically normal knees pushed inferiorly on the footplate of a weight bearing apparatus inside the MR scanner. The volunteers held the weight (133 N) for five scans as the knee motion was evaluated from 0 degrees to 60 degrees of flexion. Full extension was set as the zero point for all measured parameters. Using 3D reconstructions, tibia motion relative to the femur and flexion angle was measured as varus-valgus angle, axial rotation, anterior-posterior translation, and medial-lateral translation. Medial and lateral compartment tibio-femoral contact areas were examined and centroids of the contract areas were calculated. RESULTS: Tibial internal rotation averaged 4.8 degrees at 40 degrees of flexion and then decreased. Tibial valgus increased by 8 degrees at 60 degrees of flexion. Femoral roll back also increased to 18.5 mm average at 60 degrees of flexion, while the tibia translated medially 2.5 mm. Medial compartment femoro-tibial contact area started at 374 mm2 and decreased to 308 mm2 with flexion of 60 degrees, while lateral compartment contact area did not change significantly from 276 mm2. CONCLUSIONS: Results correlate with previous studies of knee kinematics while providing greater three-dimensional detail. MR imaging allows excellent non-invasive evaluation of knee joint kinematics with weight bearing. This tool may potentially be used for assessing knee kinematics in patients with knee pathology.     

Endoprostheses of the knee joint

Due to the advances in prosthesis design and standardisation of implant techniques, the maintenance of implants in arthroplasty of the knee joint has enormously increased during the past few years. Various arthroplastic designs enable an adjustment towards the patient's individual indiction and anatomy. For one-compartmental arthrosis, monocondylous prosthesis can be implanted using microinvasive operation techniques. Prerequisite for mere joint resurfacing is an intact lateral ligament apparatus, whereas the the symmetrical ligament balancing is essential for long-term analgesia, joint stability and good flexion. Aseptic loosening, discreet infections, instabilities and patellar problems are common reasons for painful knee arthroplastics.     

In vivo contact stresses during activities of daily living after knee arthroplasty

We implanted an electronic knee prosthesis to measure tibial forces in vivo during activities of daily living after total knee arthroplasty. We used tibial forces and knee kinematic data collected in vivo to calculate contact stresses using finite element analysis. The polyethylene insert was modeled as an elastoplastic material, and predicted contact stresses were validated using pressure sensitive sensors. Peak contact stresses generated during walking were similar but about 18% lower than those calculated for International Standards Organization (ISO)‐recommended wear simulation conditions. Stair climbing generated higher contact stresses (32 MPa) than walking (26 MPa). However, both high flexion activities (lunge and kneel) generated even higher contact stresses, with the lunge activity generating the highest stresses (56 MPa). The activities that generated high contact stresses also resulted in high equivalent plastic strain. However, the lunge activity generated dramatically higher plastic equivalent strain than the other activities. In vivo measurement of kinematics, forces, and contact stresses may be used to develop more clinically relevant wear simulator protocols. Contact stresses generated during high flexion activities were substantially higher and were largely due to the reduced contact area in deep flexion rather than due to an increase in contact forces. Our results support the use of “high flexion” designs that improve contact conditions and preserve contact area at high flexion angles. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res     

Posterior cruciate ligament rupture alters in vitro knee kinematics

Isolated posterior cruciate ligament injuries usually are treated nonoperatively, although some patients remain symptomatic, and degenerative changes within the patellofemoral joint and the medial compartment of the tibiofemoral joint have been seen in followup studies. In vitro simulation of knee squatting was done to quantify the influence of the posterior cruciate ligament on tibiofemoral and patellofemoral kinematics. For five knee specimens, knee kinematics were measured before and after sectioning the posterior cruciate ligament, and compared using a Wilcoxon signed rank test. The only kinematic parameters that changed significantly after sectioning the posterior cruciate ligament were the tibial posterior translation and patellar flexion. The posterior translation of the tibia increased significantly between 25 degrees and 90 degrees flexion. The average increase in the posterior translation exceeded 10 mm at 90 degrees flexion. The patellar flexion increased significantly from 30 degrees to 90 degrees flexion. The average patellar flexion increase peaked at 4.4 degrees at 45 degrees flexion. Increased tibial translation could adversely influence joint stability. Increased patellar flexion could increase the patellofemoral joint pressure, especially at the inferior pole, leading to degenerative changes within the patellofemoral joint.     

Dislocation of a posterior stabilized total knee prosthesis. A report of two cases

Posterior stabilized knee prostheses have been recommended for knees with posterior cruciate deficiency, as well as for knees with prior patellectomy. Two cases are presented in which a complete dislocation of a Kinematic II Stabilizer prosthesis occurred after primary knee arthroplasty. The mechanism of dislocation was a varus or valgus stress while the knee was flexed. This previously unreported complication of dislocation after primary knee arthroplasty with a posterior stabilized knee prosthesis was, we believe, due, in part, to the design of this prosthesis, which provides little mediolateral stability in flexion, in combination with a mild degree of laxity of the collateral ligaments. This complication could be prevented by use of a prosthesis with greater inherent mediolateral stability.     

Knee motion in total knee arthroplasty. A roentgen stereophotogrammetric analysis of the kinematics of the Tricon-M knee prosthesis

The three-dimensional kinematics of the Tricon-M knee prosthesis during active knee flexion and extension were recorded in 11 patients with arthrosis or rheumatoid arthritis using roentgen stereophotogrammetric analysis. Twenty-three normal knees constituted the control group. The prosthetic knees displayed the same degrees of freedom regarding rotational and translational movements as the normal knees, although the kinematics were different. A combination of internal rotation, abduction, and lateral translation of the tibia was recorded during flexion, and the reversed movements were recorded during extension. During the first 25 degrees of flexion, these movements were small, reflecting the high congruency between the articular surfaces, while beyond 25 degrees they increased. The normal knees displayed a combination of internal rotation, adduction, and medial translation of the tibia during flexion and the reversed movements during extension. The prosthetic knees also exhibited an increased posterior displacement during increasing flexion when compared with the normal knees. There was a correlation between the positioning of the femoral component in the sagittal plane and the recorded anterior/posterior translations. In conclusion, the kinematics of the Tricon-M knee prosthesis significantly differ from the normal knee, probably because of the design of the prosthesis and the absence of the cruciate ligaments.     

全膝关节置换术中股骨假体屈曲对髌股关节生物力学影响的实验研究

[目的]探讨全膝置换术中股骨假体矢状位上屈曲对髌股关节生物力学的影响,为临床指导人工膝关节置换的手术技术提供实验依据,以减少术后髌股关节的并发症。[方法]取正常国人新鲜冷冻尸体的5个膝关节作为研究对象,模拟膝关节自站立位屈膝下蹲的动作,设计制作膝关节实验架,与生物力学测试仪共同搭建实验平台。人工膝关节采用DePuy PFC假体全膝系统,手术由同一位有经验的术者实施以控制实验误差,置换髌骨。比较股骨假体相对于按下肢力线位、前屈5°、10°及后屈5°、10°、15°位置时的髌股关节的生物力学指标。选择屈膝30°、60°、90°、120°为观察角度,采用美国Tecscan公司生产的感测片测定髌股关节接触压峰值,最后软件处理得到数字化的结果。[结果]除了在膝关节屈曲30°、60°、90°,股骨假体前屈5°时,髌股关节内侧间隙接触压峰值与下肢力线位相比较无显著性差异(P>0.05),其余各种屈膝角度下,股骨假体不同屈曲角度所致髌股关节内外侧间隙接触压峰值与下肢力线位比较有显著性差异(P<0.05)。股骨假体后屈角度越大,峰值的升高越明显。[结论]全膝关节置换术中,股骨假体在矢状位上争取按下肢力线位置入,以降低术后并发症的发生。     

The Mark Coventry Award Articular: Contact Estimation in TKA Using In Vivo Kinematics and Finite Element Analysis

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

The intact, injured and repaired knee: in-vitro experimental biomechanics and level walking]

In vitro simulation of knee joint movement during walking on flat ground was developed in this experimental study. Such movements are of interest for surgical and rehabilitation medicine in order to improve knowledge in the field of dynamic behavior of the knee joint during the entire range of motion: spatial kinematics and strains and stresses on the different components, in the case of intact knee, as well as of the operated one. As internal strains and stresses could not be measured directly, the aim of this experimental study was also to simulate such factors in a model using experimental input data. A simulator of joint flexion was built: it was composed of a hydraulic universal testing machine which allowed the main flexion-extension of the knee joint and an additional hydraulic device to impose the quadriceps extension force which represented the muscular action, synchronized with the imposed flexion-extension. The anatomical automatic passive rotation and valgus-varus motion were freely allowed, and these were measured during joint motion. In addition the lateral ligaments were fitted with strain sensors in order to measure their time-dependent behavior. The imposed flexion motion and quadriceps force were also measured to verify that they were accurately synchronized. The time-dependent values of force and flexion were taken from the literature. The analysis gave the mean result of eight reliable knee joint determinations: first of all intact, then after removing the anteroexternal cruciate ligaments, and finally after joint replacement by total knee prosthesis. One main conclusion was the comparison between automatic rotations, which decreased significantly after prosthesis surgery.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of tibial component posterior slope on in vivo knee kinematics in fixed-bearing total knee arthroplasty.

The relation between prosthesis component kinematics and posterior slope of the tibial component in total knee arthroplasty is much debated. Three-dimensional kinematics of the replaced knee was obtained by video fluoroscopy in 23 knees treated by cruciate-retaining or cruciate-substituting arthroplasty. Relative position and orientation of the metal components were calculated in stair ascending, getting up from and sitting down on a chair, and single step up-and-down. Significant correlations were found between tibial component posterior slope and anteroposterior position of tibiofemoral lateral contact and between this slope and maximum knee flexion. These correlations were task and design specific. However, the average of the tibiofemoral contact positions over all three motor tasks was slightly posterior to the midline of the tibial base plate, reaching at most 84% of its anteroposterior dimension. Performing a posterior slope of the tibial cut does not put total knee arthroplasty with high conforming designs at higher risk of failure, even when large posterior inclinations need to be achieved.