Total knee arthroplasty (TKA) is a widely accepted surgical treatment for patients with moderate or severe joint degeneration. Good clinical results have been reported for cruciate-retaining (CR) TKA in the flexion range of up to 125°.3,4,7 However, many daily activities involve high degrees of knee flexion,17 such as getting in and out of the bathtub (135°),20 kneeling (160°),23 squatting (145°- 165°),24 or praying (150°-165°),17 especially for Asian and Mid-eastern populations. Therefore, surgical techniques and component designs should allow patients to flex beyond 120° postoperatively.
Few kinematic analyses report the amount of femoral translation required to flex the knee beyond 120°.3,9 Hefzy et al9 reported the tibia articulates with the most proximal points of the femoral condyles in deep knee flexion. Bellemans et al3 suggest the main factor limiting reconstructed knees from achieving high flexion may be direct posterior impingement of the tibial insert against the back of the femur. Our previous studies suggest posterior femoral translation is necessary, but not sufficient, to allow patients to achieve higher flexion angles.11,15,16
A recent literature review notes several recent arthroplasty designs incorporate modifications of various geometric features of existing TKAs to improve the range of motion after TKA.22 Some of the motivation for these modifications was preventing edge loading on the posterior tibial articular surface and increasing the tibiofemoral contact area at high degrees of flexion.2 In one in vitro kinematic analysis, a high-flexion TKA performed similar to the conventional TKAs in 6 degrees-of-freedom (DOF) kinematics from full extension to 150° of flexion.15 However, the influence of these designs on tibiofemoral articular contact behavior is unknown.
We hypothesized that even though the kinematics of the high-flexion TKA may not differ from those of the conventional TKA, a high-flexion TKA designed to reduce posterior edge loading would alter articular contact area and contact location at the tibiofemoral interface at high flexion of the knee.
MATERIALS AND METHODS
The study is designed for a within-subject analysis of the tibiofemoral contact areas and contact locations after two consecutive total knee arthroplasties. Because both TKAs were performed on the same knee, a direct comparison between the two designs could be performed and interspecimen variations minimized. Five cadaveric human knee specimens (age, 79 ± 6 years; four female knees and one male knee) were used in this study. A power analysis indicated the five specimens did not have sufficient power (< 20% power) to detect what we presumed were meaningful differences (10 mm2) in contact areas of the two TKA designs.
Each knee was thawed overnight at room temperature before testing. Anteroposterior and mediolateral radiographic images of each specimen were taken to exclude specimens with previous injuries or surgeries. Each knee included approximately 25 cm of bone above and below the joint, leaving all soft tissues (muscles, ligament, capsule, skin) around the knee intact. All knees were manually flexed from full extension to 150° of flexion to assure full range of motion (ROM). The fibula was secured to the tibial shaft in its anatomic position using a cortical screw. The ends of the tibial and femoral shafts were exposed (stripped of all soft tissue), potted in bone cement (Fastray™, Harry J. Bosworth, Skokie, IL), and rigidly secured on the robotic testing system. A pulley system and weights were used to simulate quadriceps and hamstring muscles loading. The tendons of the quadriceps and hamstring (semitendinosus/semimembranosus and biceps femoris) muscles were identified and isolated. The end of each tendon was sutured with a number 5 polyester suture (Ethibond Excel, Ethicon, Inc, Johnson & Johnson, Piscataway, NJ) to a rope for the application of weights.
Each specimen was preconditioned by manually flexing 10 times before installation on the robotic testing system described previously.11,14 The overall testing protocol has been described (Fig 1). The native knee was tested first. Its passive path was recorded and served as the reference for the application of loads. Next, the kinematics (femoral translation and tibial rotation) under combined muscle loads (quadriceps = 400 N; hamstrings = 200 N) were determined at selected flexion angles (0°, 30°, 60°, 90°, 120°, 135°, and 150°).11,14,16
A conventional CR TKA (NexGen® CR, Zimmer Inc, Warsaw, IN) was then performed by the senior author (HER). The step-by-step surgical procedure has been described previously.16 The anterior cruciate ligament was excised. The posterior cruciate ligament (PCL) was manually palpated and inspected to assess tension when the components were inserted. The necessity for the release of the PCL did not occur in any of the specimens. The femoral component was inserted in a press-fit manner. The tibial component was press fit and four screws were used to secure the component to the bone. The patella was not resurfaced in any of these experiments. Following the same protocol as for the native knee, the robot determined a new passive path and new kinematics under combined muscle loads for the conventional CR TKA knee.
Next, the femoral and polyethylene components were removed. The posterior femoral recut guide was used to remove an additional 2 mm of the posterior femoral condyles to facilitate the implantation of high-flexion CR TKA components (Nex-Gen® CR-Flex, Zimmer Inc) (Fig 2). The femoral component of this TKA is 2 mm thicker at posterior femoral condyle as compared to the conventional CR design. This feature is thought to allow increased femoral translation, an additional larger contact area at high knee flexion, and, potentially, increased knee flexion. The femoral component was inserted in a press-fit manner.
The tibial metal tray remained unchanged. A new polyethylene insert, which included a deeper anterior cutout, was inserted. It is believed this cutout (ie, removal of material from the anterior face of the conventional articular surface component) provides greater clearance for the patellar tendon at high knee flexion. The testing protocol was repeated for the CR-Flex TKA, including a new passive path and new kinematics under the same loading conditions.
After determining the kinematics for the conventional and high-flexion TKAs, all soft tissue around the knee was removed, and a thin film (0.1 mm thickness) electronic pressure sensor (K-scan 4000, Tekscan, Boston) was placed on the articular surface. The posterior edge of the sensor was aligned with the posterior edge of the polyethylene (Fig 3). The prerecorded TKA kinematics were then replayed in series using the appropriate components, and the corresponding contact areas and peak contact points were recorded at selected flexion angles (0°, 30°, 60°, 90°, 120°, 135°, and 150°) for both designs. The peak contact point was located within the contact area and was defined as the point where maximum pressure was observed. In this study, the contact point was measured from the posterior edge of the polyethylene liner.
A two-way repeated measure analysis of variance was performed to detect whether knee state and flexion angles had an effect on the contact area and the peak contact point. A Student-Newman-Keuls test was used to detect statistical significance (p < 0.05) between knee states.
The medial and lateral peak contact points translated posteriorly with increasing flexion in conventional and high-flexion TKA (Figs 4 and 5). At full extension, the medial contact points were at 14.3 ± 6.3 mm and 15.5 ± 4.5 mm anterior to the posterior edge of the tibial polyethylene for the conventional and high-flexion TKAs, respectively. The lateral contact points were at 20.6 ± 4.2 mm and 17.8 ± 5.2 mm anterior to the posterior edge of the tibial polyethylene in conventional and high-flexion CR TKA, respectively. The peak contact point locations of the TKAs at full extension were similar in both implants.
At 135° of flexion, the medial and lateral contact points of the high-flexion TKA were more anterior (p < 0.05) to the conventional TKA. The medial contact point of the conventional CR TKA was 5.7 ± 3.8 mm from the posterior edge, whereas the high-flexion TKA was 6.6 ± 4 mm from the posterior edge of the polyethylene liner. The lateral contact point of the conventional CR TKA was 2.5 ± 5.1 mm from the posterior edge, whereas the high-flexion TKA was 4.6 ± 5.3 mm from the posterior edge of the polyethylene liner. At 150° of flexion, the medial contact points of the conventional and high-flexion CR TKAs were similarly positioned at 3.5 ± 2.6 mm and 3.8 ± 3.3 mm anterior to the posterior edge of the tibial polyethylene, respectively (Fig 6). The lateral contact point of the conventional and high-flexion CR TKAs were similarly positioned at 1.3 ± 2.5 mm and 1.3 ± 2.2 mm anterior to the posterior edge of the tibial polyethylene, respectively.
At 150°, the contact area of the conventional CR TKA was 13.9 ± 6.9 mm2 and 6.5 ± 8.8 mm2 on the medial and lateral condyles, respectively (Table 1). At the same flexion angle, the contact area of the high-flexion design was 24.9 ± 33.2 mm2 for the medial condyle and 7.3 ± 9.6 mm2 on the lateral condyle. The high-flexion component showed, on average, a larger contact area than observed for the conventional component on both the medial and lateral sides in high flexion (Fig 5).
Independent of component design, surgical technique, or patient selection, most patients cannot flex beyond 125° after surgery.1,3,4,7,13,18 In vitro kinematic analysis revealed conventional and high-flexion TKAs restored the posterior femoral translation to approximately 80% of the intact knee, implying the restoration of knee kinematics may not be the sole obstacle preventing patients from achieving high knee flexion.12,15 Instead, the tibiofemoral articular contact at high flexion angles was believed a factor affecting knee stability.12
There are several limitations to our study. Based upon the power analysis for sample size, five specimens were likely inadequate to draw conclusions of a clinically meaningful effect size although some statistical differences were observed. Knee function was only examined under simulated muscle loads. Weightbearing effects were not included. The simulated muscle loads were also lower in magnitude than in vivo muscle loads, which can reach several times body weight. Future investigations should focus on investigation of the knee biomechanics during in vivo activities. This study only investigated tibiofemoral contact kinematics. Patellofemoral tracking and contact were not included in this study, which might also be a critical aspect for achieving high flexion. Due to the in vitro nature of the study, it might be difficult to draw a direct clinical relevance from these data. One should be cautious when extending the findings of this study to explain in vivo patient function. Despite these limitations, this study provided a quantitative comparison of the conventional and the high-flexion TKAs under the same loading conditions and may be used as a starting point for future in vivo investigations of the TKA biomechanics.
We compared the contact areas and the peak contact point locations between conventional and high-flexion CR TKA from full extension to 150° of flexion. The high-flexion TKA incorporates thicker posterior femoral condyles compared to the conventional CR TKA, a feature intended to prevent edge loading on the posterior tibial articular surface and facilitate larger contact areas at high knee flexion. Although both TKAs showed similar kinematic behavior,15 the high-flexion CR TKA demonstrated different tibiofemoral articular contact at high flexion of the knee. The high-flexion TKA reached the posterior edge of the polyethylene component in later flexion angles than the conventional TKA.
Several recent studies have reported tibiofemoral contact points along anteroposterior directions,6,8,10 but it is difficult to directly compare our results with those reported in literature because of different loading conditions and components. A qualitative comparison demonstrated similarities in contact kinematics, such as the medial and lateral contact points posterior with respect to the midcoronal plane for a sitting position in patients having TKA.10 A similar trend was found in the current study (Fig 4).
The data demonstrate the high-flexion TKA reached the polyethylene posterior edge at higher flexion angles as compared to the conventional design. From a mechanical point of view, this might improve tibiofemoral contact mechanism at higher flexion angles, thus potentially enhancing the flexion capabilities of the knee. At 150° of flexion, both TKAs reached the polyethylene posterior edge (Fig 6). At this flexion angle, the polyethylene liner surface of the conventional CR TKA implant not only articulated with the femoral condyle, but also impinged on the femoral condylar bone. This extreme posterior impingement of the tibial polyethylene could lead to instability of the knee at high flexion. The tibial and femoral components should provide enough posterior articular coverage to reduce the potential for edge-loading, thereby improving tibiofemoral articulation at high flexion.
Polyethylene wear directly correlates with contact stress.5,19,21 From a biomechanical point of view, increased contact area may reduce polyethylene peak contact pressure and increase tibiofemoral stability at high knee flexion. Various TKA designs have been developed to improve contact area in the full range of motion in total knee arthroplasty. Our study did not show the high-flexion CR TKA provided a larger contact area on the medial and lateral condyles at high knee flexion as compared to the conventional CR TKA. Therefore, we could make no conclusion on the advantage of the high-flexion TKA with respect to the tibiofemoral articular contact areas under the simulated muscle loads.
The high-flexion TKA reached the polyethylene posterior edge later in flexion than the conventional TKA, which may indicate an advantage of the high-flexion TKA in tibiofemoral contact biomechanics over the conventional design. An in vivo comparison of patients after conventional TKAs with those after high-flexion TKAs is necessary to better understand the limitations preventing patients from achieving high knee flexion after TKA and to validate if the high-flexion TKA has improved tibiofemoral contact behavior under physiological loading conditions.
The authors would like to acknowledge the technical assistance of Dr. Harry E. Rubash and Jeremy F. Suggs.
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