In vivo kinematic analysis of posterior-stabilized total knee arthroplasty for the valgus knee operated by the gap-balancing technique

BackgroundMost in vivo kinematic studies of total knee arthroplasty (TKA) report on the varus knee. The objective of the present study was to evaluate in vivo kinematics of a posterior-stabilized fixed-bearing TKA operated on a valgus knee during knee bending in weight-bearing (WB) and non-weight-bearing (NWB).MethodsA total of sixteen valgus knees in 12 cases that underwent TKA with Scorpio NRG PS knee prosthesis and that were operated on using the gap balancing technique were evaluated. We evaluated the in vivo kinematics of the knee using fluoroscopy and femorotibial translation relative to the tibial tray using a 2-dimensional to 3-dimensional registration technique.ResultsThe average flexion angle was 111.3° ± 7.5° in WB and 114.9° ± 8.4° in NWB. The femoral component demonstrated a mean external rotation of 5.9° ± 5.8° in WB and 7.4° ± 5.2° in NWB. In WB and NWB, the femoral component showed a medial pivot pattern from 0° to midflexion and a bicondylar rollback pattern from midflexion to full flexion. The medial condyle moved similarly in the WB condition and in the NWB condition. The lateral condyle moved posteriorly at a slightly earlier angle during the WB condition than during the NWB condition.ConclusionsWe conclude that similar kinematics after TKA can be obtained with the gap balancing technique for the preoperative valgus deformity when compared to the kinematics of a normal knee, even though the magnitude of external rotation was small. Level of evidence: IV.     

Intraoperative kinematics of bicruciate-stabilized total knee arthroplasty during high-flexion motion of the knee

BackgroundIt is unknown whether intraoperative kinematics of bicruciate-stabilized total knee arthroplasty (BCS-TKA) are different for different activities. It has also not been established whether intraoperative high-flexion motions correlate with postoperative patient-reported outcome measures (PROMs). We aimed to clarify the intraoperative kinematics of BCS-TKA during high-flexion activities and describe the relationship between intraoperative and postoperative patient-reported outcomes.MethodsWe examined 33 knees from 31 patients who underwent BCS-TKA and measured intraoperative knee kinematics, passive knee flexion, and cross-legged flexion using a navigation system. We also calculated knee flexion, varus-valgus, and rotation angles. As a secondary evaluation, we divided the patients into two clusters based on the PROMs and compared the kinematics between them.ResultsThe valgus moved by 1.3 ± 1.3° beyond 90° knee flexion during passive flexion. In contrast, during cross-legged flexion, the varus moved by 4.6 ± 5.1° beyond 30° flexion. This indicated significantly increased varus alignment in the cross-legged flexion as compared with passive flexion. Beyond 60° of flexion, the femur displayed 8.8 ± 4.8° of external rotation relative to the tibia. In cross-legged flexion, the femur displayed 9.2 ± 6.5° of external rotation relative to the tibia beyond 45° of flexion. At 90° of flexion, the cross-legged knees rotated more externally. There were no significant postoperative differences between the high- and low-score clusters.ConclusionThe intraoperative knee kinematics after BCS-TKA during high-flexion motions differed depending on the performance of an individual. This will be useful for physicians who might recommend BCS-TKA to new patients.     

Rotational kinematics differ between mild and severe valgus knees in total knee arthroplasty

BackgroundThere is no consensus regarding femorotibial rotational kinematics in total knee arthroplasty (TKA) for valgus knee deformity. Additionally, whether the degree of valgus deformity influences intraoperative rotational kinematics and postoperative clinical scores remains unclear. The objectives of this study were to investigate whether the valgus angle is associated with intraoperative rotational kinematics in TKA for valgus knee deformity and to examine the relationship between rotational kinematics and postoperative clinical results.Materials and methodsA total of 24 knees with valgus deformity for TKA were included in this study and were divided into two groups depending on the femorotibial angle (FTA); there were 11 knees in the severe valgus group (FTA < 160°) and 13 knees in the mild valgus group (FTA ≥ 160°). Intraoperative femorotibial rotational kinematics from knee extension to flexion were evaluated using an image-free navigation system and postoperative clinical results (range of motion and subjective outcomes) were evaluated 1 year postoperatively. All parameters were compared between the two groups.ResultMild valgus knee showed tibial internal rotation during knee flexion before implantation, whereas severe valgus knee showed tibial external rotation during knee flexion before implantation. The postoperative flexion angle was positively correlated with the tibial internal rotation angle after implantation in the mild valgus group only.ConclusionIntraoperative rotational kinematics before implantation differed between mild and severe valgus knee deformity in TKA. Intraoperative tibial rotation influenced the postoperative knee flexion angle in mild, but not severe, valgus knee deformity. Ideal postoperative rotational kinematics may be different between the two groups and the difference may be taken into consideration in implant selections and surgical techniques.     

Modern femoral component design in total knee arthroplasty shows a lower patellar contact force during knee flexion compared with its predecessor

BackgroundThe relationship between the femoral component design in total knee arthroplasty (TKA) and the patellofemoral contact force, as well as the soft tissue balance, has not been well reported thus far.MethodsTwenty-eight mobile-bearing posterior-stabilized (PS) TKAs using the traditional model (PFC Sigma) and 27 mobile-bearing PS TKAs using the latest model (Attune) were included. Surgeries were performed using the measured resection technique assisted with the computed tomography (CT)-based free-hand navigation system. After all the trial components were placed, patellar contact forces on the medial and lateral sides were measured using two uniaxial ultrathin force transducers with the knee at 0°, 10°, 30°, 60°, 90°, 120°, and 135° of flexion. The joint component gap and the varus ligament balance of the femorotibial joint were also measured. The non-paired Student’s t-test was conducted to compare the values of the two groups.ResultsThe medial patellar contact force was significantly lower for Attune group than for PFC Sigma group at 120° of knee flexion (P = 0.0058). The lateral patellar contact force was also significantly lower for Attune group than PFC Sigma group at 120° and 135° of knee flexion (P = 0.0068 and P = 0.036). The joint component gap, as well as the varus ligament balance, showed no statistically significant difference between the two groups.ConclusionsReduced thickness and width of the anterior flange of the femoral component in the Attune may play a role in low patellar contact force.     

In vivo kinematic comparison before and after mobile-bearing unicompartmental knee arthroplasty during high-flexion activities

BackgroundMany patients who undergo unicompartmental knee arthroplasty (UKA) have an expectation that their knee flexion would increase following its replacement. Additionally, the survival rate of mobile-bearing UKA (MB-UKA) is high. However, the effect on the patient's kinematics remains unknown. This study aimed to clarify the kinematic effect of MB-UKA knees during high-flexion activities by comparing the in vivo kinematics before and after surgery.MethodsA squatting motion was performed under fluoroscopic surveillance in the sagittal plane before and after MB-UKA. To estimate the spatial position and orientation of the knee, a two-dimensional/three-dimensional registration technique was used. The femoral rotation and varus–valgus angle relative to the tibia and anteroposterior (AP) translation of the medial and lateral side of the femur on the plane perpendicular to the tibial mechanical axis in each flexion angle were evaluated.ResultsRegarding the varus–valgus angle, the preoperative knees indicated a significant varus alignment compared with the postoperative knees from 10° to 60° of flexion. There were no significant differences in the femoral rotation angle, AP translation, and kinematic pathway before and after MB-UKA in the mid-flexion of the range of motion.ConclusionThere were differences between the varus–valgus knee kinematics before and after MB-UKA, from 10 to 60° of flexion, but no difference from midrange of flexion to deep flexion. In addition, the rotational knee kinematics before and after MB-UKA was not significantly different.     

Wiberg Type III patellae and J-sign during extension compromise outcomes of total knee arthroplasty without patellar resurfacing

BackgroundUnderstanding the risk factors associated with postoperative pain and worse outcome can guide surgeons on whether primary patellar resurfacing is warranted during total knee arthroplasty (TKA). The purpose of this study was to determine whether clinical scores and pain after TKA without patellar resurfacing are correlated with patellar shape and postoperative patellar position and kinematics.MethodsRadiographs as well as anterior knee pain according to the Visual Analogue Scale (pVAS) were collected pre- and postoperatively for 100 knees aged 68 ± 7.7 years that received uncemented TKA without patellar resurfacing. At a minimum follow-up of 12 months the Forgotten Joint Score (FJS), the Oxford Knee Score (OKS) as well as the flexion range of motion and the presence of J-sign during active extension were recorded. Uni- and multivariable linear regression analyses were performed to determine associations between the collected clinical scores and patient demographic and radiographic data.ResultsPostoperative OKS (79 ± 14.4) was worse for Wiberg Type III patellae (β = − 11.4, P = .020, compared with Type II). Anterior pVAS (2 ± 2) was greater in knees with J-sign during extension (β = 2.8, P < .001). None of the other radiographic measurements (patellar tilt, congruence angle and lateral patellar displacement) were correlated with postoperative OKS or anterior pVAS.ConclusionIncongruent patellar shape (Wiberg Type III) is associated with worse clinical scores, and abnormal kinematics (J-sign) with increased pain after TKA without patellar resurfacing. The authors therefore recommend routine resurfacing for Wiberg Type III patellae, though further studies are required to confirm whether resurfacing truly improves clinical scores and pain in this subgroup.     

Characteristic kinematics of floor-sitting activities after posterior-stabilized total knee arthroplasty determined using model-based shape-matching techniques

BackgroundDetailed kinematics of floor-sitting activities after total knee arthroplasty (TKA) have not been well explored. Knee kinematics of cross-legged sitting, seiza-sitting, and side-sitting after TKA were examined to clarify the differences in tibiofemoral kinematics of each activity.MethodsSubjects were 40 knees in 20 osteoarthritic patients who underwent bilateral TKA with a high-flexion fixed-bearing posterior-stabilized prosthesis. Dynamic radiographs of floor-sitting activities were taken, and the knee kinematics were compared among the three activities. The patients were also divided into two groups (possible/easy group and impossible/no-try group) for each activity, and group comparisons were conducted.ResultsThe maximum implant flexion angle was significantly greater in seiza-sitting. In valgus/varus rotation, seiza-sitting demonstrated neutral rotation, while cross-legged sitting showed varus of about 10°, and side-sitting exhibited valgus. In tibial internal/external rotation, seiza-sitting demonstrated a constant rotational angle, while cross-legged sitting showed tibial internal rotation with flexion, and side-sitting exhibited tibial external rotation with flexion. The kinematic pathway during deep flexion illustrated the medial pivot pattern in cross-legged sitting, a small amount of bicondylar rollback in seiza-sitting, and the weak lateral pivot pattern in side-sitting. A greater flexion angle was the important factor for the performance of each floor-sitting activity followed by varus laxity at 10° knee flexion.ConclusionsThis study successfully revealed characteristic kinematic patterns of TKA knees in three floor-sitting activities. Obtaining a greater knee flexion with adequate lateral laxity is the key to enhancing postoperative floor-sitting activities.     

The effect of component alignment on clinical outcomes in fixed bearing unicompartmental knee arthroplasty

BackgroundThe aim of this study is to report component alignment in a series of ZUK fixed bearing unicompartmental knee arthroplasty (UKA) implants and compare this to clinical outcomes.MethodsThe radiographs, Knee Society Scores (KSS) and knee flexion of 223 medial UKAs were evaluated. The following alignment parameters were assessed; coronal and sagittal femoral component angle (c-FCA and s-FCA), coronal and sagittal tibia component angle (c-TCA and s-TCA) and the coronal tibiofemoral angle (c-TFA). Each alignment parameter was grouped at consecutive 2.5° intervals, mean KSS and knee flexion was then compared between the interval groups.Results96.4% of femoral components were between 7.5° of varus and valgus and 95.1% between 7.5° extension and 5° flexion. 89.6% of tibial components were between 7.5° of varus and 2.5° valgus and 97.3% between 2.5° and 15° flexion. There was no significant difference between the KSS or knee flexion between any of the incremental groups of component alignment. Mean c-TFA was 0.2 ± 3.0°, 92.4% were between −5° (varus) and 5° (valgus). KSS were significantly greater for two of the increments with slightly more varus. Linear regression analysis showed there was very weak correlation (R² = 0.1933) between c-TFA and c-TCA.ConclusionsThe results of this study show that fixed bearing UKA components are forgiving to accommodate some variation in tibial and femoral component position without effecting clinical outcome scores or knee flexion. Limb alignment matters more than component position and knees with slight varus tibiofemoral alignment have better clinical scores than those with valgus.     

Prospective sequential comparison of femoral roll-back between cruciate-retaining and posterior-stabilized total knee arthroplasty using an intra-operative sensor

BackgroundImplant design and surgical techniques affect postoperative knee kinematics in total knee arthroplasty (TKA). This study aimed to compare femoral roll-back between cruciate-retaining (CR) and posterior-stabilized (PS) TKA in the same knee by objectively quantifying the contact point kinematics of the tibiofemoral joint using a sensor.MethodsIn the present prospective study, we used an intraoperative sensor to compare medial and lateral roll-back during 0-120° knee flexion in 33 knees that underwent CR and PS TKA. We also examined the relationship between mediolateral balance and the lateral-to-medial roll-back ratio. We defined the contact percentage position as the vertical length to the contact point divided by the anteroposterior length of the tibial plate.ResultsThe roll-back percentage following PS TKA (19.8 ± 5.1%) was significantly higher than that after CR TKA in both the medial (19.8 ± 5.1% versus 7.1 ± 2.5%, P < 0.001) and lateral (26.8% ± 3.8% versus 18.7 ± 3.8%, P < 0.001) compartments. The medial contact pressure at 90° was significantly correlated with the increased lateral-to-medial roll-back ratio in both CR and PS TKA (both P < 0.001).ConclusionPS TKA resulted in a higher percentage of femoral roll-back in the medial and lateral compartments than CR TKA. CR TKA caused a higher lateral-to-medial roll-back ratio compared to PS TKA. To reproduce medial pivot knee motion similar to that of a normal knee, the medial soft tissue needed to be balanced more tightly than the lateral soft tissue during TKA. These findings provide some clinical evidence of TKA design selection and proper mediolateral balancing for successful TKA.     

Insufficient lateral joint laxity after bicruciate-retaining total knee arthroplasty potentially influences kinematics during flexion: A biomechanical cadaveric study

BackgroundSoft tissue balancing in bicruciate-retaining (BCR) total knee arthroplasty (TKA) is a challenge that must be overcome to achieve excellent clinical outcomes. However, the optimal degree of joint laxity has yet to be clarified. This cadaveric study sought to examine joint laxity after BCR TKA using a navigation system.MethodsKnee joint laxity was quantified using an image-free navigation system in 8 intact fresh frozen cadavers under three conditions: the native knee, BCR TKA knee, and BCR TKA knee after anterior cruciate ligament resection. Rotational kinematics in the BCR TKA knee during flexion were compared according to whether joint laxity was increased or decreased.ResultsKnee joint laxity after BCR TKA under varus-valgus movement, anterior translation, and internal-external rotation loadings was similar to that of the native knee. However, lateral joint laxity was decreased during flexion in some cases. BCR TKA-treated knees with decreased lateral joint laxity at 90° of flexion demonstrated more limited tibial internal rotation in deep flexion than the native knee (p < 0.05). The loss of internal rotation in deep flexion was partly recovered by using a lateral insert with a posterior slope of +3°.ConclusionsRestoring optimal joint laxity was not always straightforward in BCR TKA if the 4 ligaments were preserved. Lateral joint laxity was potentially decreased in BCR TKA and may result in kinematic conflict during flexion. Surgeons should be aware of the need to achieve sufficient lateral joint laxity in this type of BCR TKA.     

Femorotibial rotational mismatch of the Oxford unicompartmental knee in the flexion position is a risk for poor outcomes

BackgroundFemorotibial rotational mismatch has been reported to cause unsatisfactory outcomes after total knee arthroplasty. However, to our knowledge, no previous reports have described the relationship between the femorotibial rotational mismatch and clinical outcomes of Oxford unicompartmental knee arthroplasty (UKA).MethodsIn total, we studied 52 knees with primary varus knee osteoarthritis that underwent Oxford UKA with a navigation system. Tibial component internal rotation angles relative to the femoral component at extension and flexion angle of 90° were measured using a navigation system. We evaluated the relationship between the clinical outcomes and femorotibial rotational mismatch angles. Additionally, we evaluated the relationships between the outcomes and rotational alignments of the femur and tibial components measured by computed tomography (CT).ResultsThe tibial component internal rotational angle relative to the femoral component was significantly larger at a flexion angle of 90° than on extension (P < .001) and showed negative correlations with the Knee Society Functional Score and the pain and sports subscales of the Knee Injury and Osteoarthritis Outcome Score. Rotational alignment of the femur and tibial components on computed tomography was not associated with clinical outcomes.ConclusionThe tibial component internal rotational angle relative to the femoral component in the flexion position was negatively correlated with clinical outcome. Surgeons should pay attention to a rotational mismatch between components in the flexion position during the Oxford UKA procedure. Navigation systems will be effective in reducing the femorotibial rotational mismatch and improving clinical outcomes.     

In vivo weight-bearing kinematics with medial rotation knee arthroplasty

Knee arthroplasties are designed to accommodate flexion, axial rotation and anteroposterior (AP) translation. Axial rotation during extension varies, with some rotating platform devices allowing unrestricted rotation while some conforming fixed-bearing designs almost none. The purpose of this study was to examine in vivo kinematics of a fixed-bearing medial rotation-type arthroplasty (MRK) during weight-bearing activities. Fifteen knees with a medial pivot TKA design were studied during step and pivot activities using lateral fluoroscopy and model-image registration. Average knee kinematics during the step activity showed little AP translation or rotation from 0°–100° flexion. During the pivot activity, the mean tibial internal rotation in individual knees was 7° (3°–19°). Mean condylar translations for individual knees were 3 mm medially and 5 mm laterally. The medial pivot prosthesis design provides anteroposterior stability during demanding activities, and exhibits a medial pivot motion pattern when subjected to twisting.     

Kinematics of a cementless mobile bearing posterior cruciate ligament-retaining total knee arthroplasty

Mobile bearing (MB) total knee arthroplasty (TKA) was developed to provide low contact stress and unconstrained joint motion. We studied a consecutive series of 41 knees with mobile-bearing, posterior cruciate-retaining (CR) TKAs to determine if kinematics resembled normal knees or if kinematics changed over time. Patients were studied at 3 and 21 months average follow-up with weight-bearing radiographs at full extension, 30° flexion and maximum flexion. Shape-matching techniques were used to measure TKA kinematics. Implant hyperextension, maximum flexion and total ROM increased with follow-up. Tibial rotation and condylar translations did not change with time. The medial condyle did not translate from extension to 30°, but translated 5 mm anteriorly from 30° to maximum flexion. Lateral condylar translation was 3 mm posterior from extension to 30°, with no translation from 30° to maximum flexion. Tibiofemoral kinematics in CR-MB-TKAs were stable over time, but did not replicate motions observed in healthy knees. The mobile tibial insert showed rotation and translation at both follow-up examinations, but the patterns of translation were not predictable.     

Effects of patellar resurfacing on contact area and contact stress in total knee arthroplasty

The objective of this study was to examine the effects of patellar resurfacing on patellofemoral joint contact pressure and contact area in total knee arthroplasty. We tested seven fresh-frozen cadaveric knees using a custom knee jig which permits the simulation of physiologic quadriceps loading. Before patellar resurfacing, the mean peak contact pressure of medial and lateral patellofemoral joints was less than 10 MPa at knee flexion angles of 30 degrees, 60 degrees and 90 degrees, that of medial and lateral patellofemoral joints were 11.63 MPa and 11.42 MPa at a knee flexion angle of 120 degrees respectively, and the mean contact area of patellofemoral joint ranged from 70 to 150 mm2. After patellar resurfacing, the mean peak contact pressure of medial and lateral patellofemoral joints ranged from 50 to 100 Mpa (P<0.05), which exceeds the yield strength of ultrahigh molecular weight polyethylene, and the mean contact area of patellofemoral joint reduced to 10-15 mm2 (P<0.05). The contact pressure of patellofemoral joint was lower than the yield strength of articular cartilage before patellar resurfacing. Our results indicate that the yield stress of UHMWPE is exceeded after patellar resurfacing.     

全膝关节置换个体化患者右转步态的骨肌多体动力学仿真

目的构建个体化患者全膝关节置换(total knee replacement,TKR)的骨肌多体力学模型,模拟患者下肢右转步态时体内膝关节的生物力学行为。方法以1位具体患者的相关数据为材料,基于骨肌动力学仿真软件Any Body及其依赖于力的运动学建模方法,建立与患者相对应的TKR下肢骨肌多体动力学模型,并对患者的右转步态进行模拟。通过逆动力学分析右转步态,同时预测患者膝关节接触力、关节运动、肌肉活性和韧带力。结果模型预测的胫骨-股骨关节内、外侧接触力的均值均方根误差分别为285、164 N,相关系数分别为0.95和0.61,预测的髌骨接触力均值最大值为250 N。模型预测的接触力和肌肉活性与患者实验测量结果基本一致。此外,模型预测的胫骨-股骨的伸展弯曲、内外旋和内外翻运动的均值分布范围分别为3°~47°、-3.4°~1.5°、0.2°~-1.5°,胫骨-股骨的前后、上下和内外侧平移的运动范围分别为2.6~9.0 mm、1.6~3.2 mm、4.2~5.2 mm。模型还预测了内、外侧旁系韧带力和后交叉韧带力,其最大值分别为190、108、108 N。结论所开发的模型能够预测人工膝关节体内生物力学行为,为后续研究膝关节假体临床失效问题提供强有力的计算平台。     

Intraoperative assessment of midflexion laxity in total knee prosthesis

BackgroundSoft-tissue balancing of the knee is fundamental to the success of a total knee arthroplasty (TKA). In posterior-stabilized TKA, there is no stabilizer of the anterior–posterior translation in the midflexion range in which the cam-post mechanism does not engage yet. Therefore, instability in the midflexion range is suspected to occur in posterior-stabilized TKA. The purpose of this study was to measure the joint gap throughout a full range of motion and to analyze the joint gap laxity in the midflexion range after implantation of a mobile-bearing posterior-stabilized total knee prosthesis.MethodsJoint gap kinematics in 259 knees with varus osteoarthritis were measured during TKAs using a tensor device with the same shape of a total knee prosthesis of the same design was used. After the implantation of a mobile-bearing posterior-stabilized prosthesis and the reduction of the patellofemoral joint, the joint gap was measured at 0°, 30°, 60°, 90°, 120°, and 145° of flexion.ResultsThe center size of the joint gap was tight in extension and deep flexion and loose at midflexion ranges, especially at 30° of flexion (p < 0.001). The symmetry of the joint gap was varus at 0° and 145° of flexion (p < 0.001).ConclusionsOur results showed the joint gap laxity in the midflexion range after the implantation of a mobile-bearing posterior-stabilized prosthesis. Our new tensor device, which can attach the polyethylene insert trial, will provide the important information about the joint gap kinematics after implantation of total knee prostheses.Level of evidenceIV.     

Contact kinematics of patient-specific instrumentation versus conventional instrumentation for total knee arthroplasty

BackgroundThe goal was to evaluate the joint contact kinematics of total knee arthroplasties implanted using patient-specific instrumentation (PSI) compared to conventional instrumentation (CI). We hypothesized that use of PSI would not significantly alter contact kinematics.MethodsThe study was a prospective randomized controlled trial, with equal allocation of fifty patients to PSI and CI groups. At two years post-operation, patients underwent weight-bearing stereo X-ray examinations at 0°, 20°, 40°, 60°, 80°, and 100° of flexion. The shortest tibiofemoral distance on each condyle determined the contact location. Magnitude of the shortest distance was measured and condylar separation was analyzed using thresholds of 0.5 and 0.75 mm. Kinematic measurements derived from the shortest distance included anteroposterior (AP) translation, excursion, axial rotation, and paradoxical anterior motion. Pivot position and cam/post contact were also investigated.ResultsThere were no differences (p > 0.05) in medial and lateral AP contact locations, excursions, and magnitude of anterior motion, or in axial rotation, pivot patterns, frequency of cam/post engagement, frequency of medial anterior motion, and condylar separation at a 0.75 mm threshold. Significant differences were found in frequency of lateral anterior motion (p = 0.048) and condylar separation at a 0.5 mm threshold (p = 0.010). Both groups displayed typical kinematics for a fixed-bearing posterior-stabilized implant.ConclusionsWe found no major differences in knee kinematics between PSI and CI groups, which suggest that PSI does not provide a significant kinematic advantage over conventional instruments.     

The role of the patellar tendon angle and patellar flexion angle in the interpretation of sagittal plane kinematics of the knee after knee arthroplasty: A modelling analysis

Background

Many different measures have been used to describe knee kinematics. This study investigated the changes of two measures, the patellar tendon angle and the patellar flexion angle, in response to variations in the geometry of the knee due to surgical technique or implant design.

Does malrotation of components correlate with patient dissatisfaction following secondary patellar resurfacing?

BackgroundThe aim of our study was to identify whether there was any correlation between the outcome of secondary patellar resurfacing and malrotation of either the femoral or tibial component.MethodsWe identified patients that underwent secondary patellar resurfacing following previous primary total knee arthroplasty (TKA) at a single, large orthopaedic department. Patients were reviewed for range of movement, satisfaction, health status and knee function. CT scanning was performed, assessing rotational alignment of the components.ResultsTwenty-one patients (23 knees) were reviewed. Nine out of 21 (39%) were satisfied while 14 (61%) remained dissatisfied after the secondary patellar resurfacing. There were no complications after the secondary procedure. All knees were internally rotated. The mean femoral internal rotation in the satisfied group was 0.92°, and in the dissatisfied group was 2.88° of internal rotation. In the dissatisfied group eight out of 14 TKAs were in > 3° femoral internal rotation compared with only one in nine TKAs in the satisfied group (p < 0.05).ConclusionsInvestigation for malrotation should be considered in patients with post-operative pain, especially anteriorly, causing significant dissatisfaction amongst patients following TKA. This is especially true if the patella has not been primarily resurfaced and secondary resurfacing is being considered. Patients with more than 3^° of femoral internal rotation undergoing secondary patella resurfacing should be warned of the possibility of a poor outcome. It may well be that if the underlying problem is component malrotation, revision knee replacement may lead to a more satisfactory outcome than secondary resurfacing alone.Level of EvidenceLevel of Evidence III.     

How does asymmetric tibial insert affect tibiofemoral kinematics and contact stresses in total knee Arthroplasty?

BackgroundAsymmetric tibial insert design is expected to restore normal knee kinematics better than symmetric design. A tri-condylar implant has asymmetric and symmetric tibial inserts with a ball-and-socket joint to replace the post-cam mechanism. The purpose of this study was to compare the knee kinematics of the two designs and to measure tibiofemoral contact stresses, including that of the ball-and-socket joint.MethodsUsing a computer simulation, the anteroposterior position and axial rotation of the femoral component were simulated during a weight-bearing deep knee bend for six validated models. Contact forces were simultaneously simulated in the medial, lateral, and ball-and-socket compartments. The relative position and the magnitude and direction of each contact force were applied to aforce/displacement control knee simulator. The contact stresses were measured individually using a pressure sensor.ResultsThe asymmetric tibial insert demonstrated a more posterior position of the femoral component in the lateral compartment during the entire range of motion and greater external rotation of the femoral component, compared to the symmetrical tibial insert. The mean peak contact stress of the medial and lateral compartments was < 9 Mpa, with no significant differences between the two designs except at 0°. The contact stress of the ball-and-socket joint was < 5 MPa.ConclusionsAsymmetry of the tibial insert shows significant kinematic difference and has little influence on the peak contact stress, which is considerably lower than the yield strength of polyethylene. The asymmetric tibial insert can lead to clinical benefits owing to its kinematic and kinetic properties.