At higher degrees of flexion, the posterior surfaces of the femoral condyles articulate with the posterior tibia. The normal tibia has a natural posterior slope of approximately 10°, which is vital to femoral rollback. In reconstruction of the knee, failure to appreciate this posterior slope could result in a tighter posterior capsule and flexion space. In a three-dimensional computer model, the posterior tibial slope has been shown to be the most important surgical variable in optimizing flexion. 42
The factors influencing range of flexion after TKA can be classified broadly into three major groups: preoperative, intraoperative, and postoperative factors. Regarding preoperative influences, the factors considered most relevant in the literature include ROM, diagnosis, deformity, age, gender, and patient weight. 3,14,19,23,28,33,38 Intraoperative factors that may influence TKA flexion include balancing of the flexion and extension gap, patella resurfacing and tracking, PCL management, wound closure, and component sizing and prosthetic design. Postoperative rehabilitation also plays a role in knee flexion and covers such issues as the use of continuous passive motion devices and the specifics of the chosen physical therapy protocol.
Although diagnosis, deformity, age, gender, and patient weight all are considered important preoperative factors influencing TKA ROM, it is widely agreed that the most important influence on range of flexion after arthroplasty is preoperative ROM. 19,25 Poor long-standing preoperative ROM may result in several changes that can occur in and around the knee leading to limitations to motion. For example, bony structural changes, periarticular soft tissue fibrosis, and extensor mechanism stiffness may result from prolonged limitations on flexion. These changes may be irreversible and so ROM after TKA may be compromised. 20
A structure that deserves attention in a discussion of high-flexion of a TKA is the PCL. As the knee flexes, this structure tightens and this may effectively constrain the dimensions of the flexion gap. Range of motion may be compromised by an insufficient flexion gap. Therefore, in instances where the PCL is retained, recession of this structure has been suggested to obtain a sufficient flexion gap and to avoid limiting flexion. 4 Use of a PCL-substituting design may allow the surgeon to consistently attain a larger and more predictable flexion gap as the flexion gap increases after PCL removal. A larger flexion gap may translate into improved flexion. 8,39
Another structure affecting knee flexion is the extensor mechanism. During flexion, the extensor mechanism is stretched across the anterior aspect of the knee. As this structure tightens at higher degrees of flexion, it can limit motion. Attempts at decreasing the forces seen by this structure in the reconstructed knee may result in increases in the range of flexion. The extensor mechanism may effectively be loosened by resection of more than the standard amount of patella during reconstruction. Unfortunately, patella fracture may result from excessive patella resection during the resurfacing process. 6
Overhanging osteophytes of the femur and tibia are an additional factor requiring attention in knee reconstruction with respect to ROM. Overhanging posterior osteophytes cause early posterior impingement and should be removed because they may inhibit full flexion.
Finally, component positioning also may influence knee flexion. Achieving adequate posterior tilt of the tibial component may result in an enhanced range of flexion.
Regarding postoperative factors, it has been reported that rehabilitation plays a significant role in achieving deep knee flexion. 36 Relevant issues include the use of continuous passive motion devices and the specifics of the chosen physical therapy protocol. Despite excellent surgical technique, soft tissue contracture can occur, which may limit the range of flexion. Therefore, aggressive rehabilitation accompanied by adequate pain control may be necessary to optimize postoperative results.
The success of any total knee system may in part be linked to its ability to optimally restore normal kinematic function. Unfortunately, few arthroplasty components incorporate design-specific features aimed at improving knee kinematics at high flexion angles. Three examples of total knee systems that incorporate design features intended to improve knee kinematics in high flexion include the Zimmer Legacy Knee LPS-Flex (Zimmer Inc, Warsaw, IN), the Hy-flex II total knee system (Depuy International Inc, Leeds, United Kingdom), and the Bisurface knee prosthesis (Kyocera, Kyoto, Japan).
The LPS-Flex is a posterior-stabilized TKA designed to accommodate flexion to 155°. By extending the area of the posterior femoral condyles, this component is designed to provide a greater arc of flexion by attempting to prevent “digging in” of the posterior femur into the tibial articular surface (Fig 3). Second, the tibial PE insert incorporates a deep anterior patellar cut-out to reduce tension on the extensor mechanism by providing greater clearance for the patellar tendon during deep flexion. Finally, the prosthesis incorporates a modified posterior-stabilized cam-spine mechanism to attempt to increase subluxation resistance and enhance posterior femoral translation at deep flexion angles.
The Hy-flex II total knee system incorporates several design paradigms that are aimed at providing an increase in the amount of flexion achievable after TKA. These include: (1) a small posterior femoral condylar radius; (2) 4° of posterior slope for the tibial joint surface; and (3) equal tension of the soft tissues obtained by using a ligament tensor. In one study of 84 patients with RA this prosthesis was implanted into 114 knees. At 1 year followup, the average flexion was reported to be 122.1° ± 15° with 71.9% of knees obtaining 120° flexion or greater. 43
The Bisurface knee prosthesis incorporates one design-specific feature aimed at improving knee flexion. At the midposterior portion of the femoral and tibial components, there is a secondary articulation similar to a ball and socket joint that acts as a posterior-stabilizing cam mechanism promoting femoral rollback. This additional articulartion also serves as a load-bearing surface in high flexion, which is intended to prevent tibiofemoral impingement. In a series of 223 consecutive primary TKAs using the Bisurface design, the mean postoperative range of flexion was 124°. 2 However, in this series, the mean preoperative flexion was 119°.
Although the human knee may be capable of flexion up to 160°, an analysis of the results of contemporary TKA reveals that on average, patients rarely flex beyond 120°. Unfortunately, the precise biomechanical mechanisms that inhibit higher knee flexion after TKA still are unknown. Until now, most in vivo and in vitro biomechanical studies related to knee arthroplasty have focused on knee function below 120° flexion. As a result, the biomechanical mechanisms that limit higher knee flexion remain unclear. To our knowledge, limited data have been reported on the capability of current TKA systems to reproduce native knee kinematics beyond 120° flexion.
Currently, an in vitro experimental model incorporating robotics is being used to investigate the capability of various TKA designs to restore intact, native knee kinematics at flexion angles of as much as 150° (Fig 4). Until now, we have investigated the capabilities of fixed and mobile-bearing posterior cruciate-stabilized TKA to restore native knee kinematics using an in vitro robotic experimental set-up at high flexion angles (> 120°) under simulated muscle loads. This study directly compared the intact knee, fixed-bearing, and mobile-bearing TKAs on the same knee. The results from this study show that fixed-bearing, and mobile-bearing arthroplasties restored approximately 90% of the native knee at high flexion angles. No statistically significant difference (p > 0.05) was detected regarding tibial rotation between fixed-bearing, and mobile-bearing TKAs or between the TKAs and the native knee. The results from this study suggest that the knee is highly constrained at highflexion. These data provide important kinematic information regarding the behavior of differing arthroplasty designs at high flexion angles.
Ultimately this robotic model in conjunction with clinical studies may provide an understanding of the limitations of contemporary knee designs regarding achieving deep flexion, leading to the development of prostheses that may enhance kinematics and result in enhancement of range of flexion that is achievable after TKA.
In general, the clinical results of TKA are satisfactory regarding pain relief and overall function. However, patients almost uniformly do not achieve high degrees of flexion after knee replacement. The influences on range of flexion after TKA are multifactorial. Influential concepts include preoperative knee motion, surgical technique, prosthetic design, and rehabilitation issues. Although some arthroplasty designs currently are available that incorporate modifications aimed at improving range of flexion, limited data currently are available on their function and potential advantages. Through additional investigation into the motion of the native knee and a deeper understanding of the limitations of contemporary total knee designs, newer implants may be created that accommodate for improved flexion.
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