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Optimizing Flexion After Total Knee Arthroplasty: Advances in Prosthetic Design

Sultan, Peter, G.*; Most, Ephrat***; Schule, Steven*; Li, Guoan*; Rubash, Harry, E.*

Clinical Orthopaedics and Related Research: November 2003 - Volume 416 - Issue - p 167-173
doi: 10.1097/01.blo.0000081937.75404.ee
SECTION I SYMPOSIUM: Papers Presented at the Knee Society 2003
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The clinical results with most modern total knee arthroplasty (TKA) designs are highly satisfactory regarding pain relief and improving walking ability. However, one problem that has not been addressed fully by most current designs is the ability to consistently achieve flexion greater than 120°. Although the human knee is capable of flexion of more than 150°, an analysis of the results of contemporary TKA reveals that on average, patients rarely flex beyond 120°. Key factors influencing range of flexion after TKA include preoperative knee motion, surgical technique, prosthetic design, and rehabilitation. The success of any total knee system may in part be linked to its ability to optimally restore normal kinematic function. Some arthroplasty designs currently are available that incorporate modifications aimed at improving range of flexion, but limited data currently are available on their function and potential advantages. Currently, an in vitro experimental model incorporating robotics is being used to investigate the kinematics of the native knee and various TKA designs at flexion angles beyond 120°. This robotic model in conjunction with clinical studies may provide an understanding of the limitations of contemporary knee designs regarding achieving higher degrees of knee flexion. This may lead to the refinement of existing designs and development of newer prostheses that may enhance the range of flexion that is achievable after TKA.

From the *Harvard Medical School, MGH/BIDMC Boston, MA;

**Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA.

Funding for EM was provided by The Hugh Hampton Young Memorial Fund Fellowship, Massachusetts Institute of Technology, Cambridge, MA.

Reprint requests to Harry Rubash, MD, Orthopaedic Surgery, Massachusetts General Hospital, Harvard Medical School, 55 Fruit Street, Suite GRB 624, Boston, MA 02114. Phone: 617-724-9904; Fax: 617-726-2351; E-mail: hrubash@partners.org.

Total knee arthroplasty is a reliable and widely used surgical procedure. Initially developed to relieve pain in patients with severe arthritis of the knee, the procedure has evolved such that current designs and modern surgical techniques are capable of providing long-term success rates in excess of 85% at 10 to 15 years followup. 9,11,12,26,29,34,37,38 In general, the clinical results with most modern TKA designs are satisfactory regarding pain relief and improving walking ability. However, one major problem that has not been addressed fully is that patients do not gain high degrees of flexion after TKA. Perhaps even more startling is that even patients with good preoperative ROM often lose deep flexion (defined as flexion beyond 120°) after TKA. 3, 38

Knee flexion is integral to function in many situations of every day life and the amount of flexion has been linked to functional outcome and activities of daily living. 21 In many situations, patients require flexion beyond 90°. For example, to squat and kneel, an individual would require as much as 160° flexion. 41 Bathtub use requires 135° flexion. 35 To check the line for a putt on the green in golf requires that the individual be able to squat, which requires knee flexion greater than 120°. Overall, sufficient knee flexion is essential to the lifestyle of individuals who participate in recreational activities as part of their daily life. In addition, some individuals require knee flexion greater than 120° for their work. 40

Unfortunately, the flexion achieved after contemporary TKA rarely exceeds 120°. 1,7,10,13,15,16,22,24,30–32,34 Initial experiences with the cruciate-sacrificing total condylar prostheses produced results with flexion limited in the range of 90° to 95°. This degree of flexion approaches the theoretical limit for this prosthetic design. 17 During the 30-year evolutionary process that has brought the TKA to its modern form, improved flexion has been achieved with posterior-stabilized and PCL-retaining prostheses. Data pooled from multiple studies reveal a mean flexion of 100° to 115° with both types of prostheses. 5,27 Current prosthetic designs and surgical techniques may not be meeting the needs of patients who require deep knee flexion for their daily activities.

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FLEXION OF THE NATIVE KNEE

The mechanisms that allow for deep flexion of the native knee first must be understood to have basis for understanding the limitation of motion after arthroplasty. Native knee flexion is guided by the geometry of the articulating surfaces and the soft tissues about and within the knee, which includes the ligaments and menisci. Limitation on native knee flexion may be attributable in part to the presence of posterior osteophytes and quadriceps contracture.

It has been reported that the femoral condyles are offset posteriorly relative to the posterior femoral cortex and that the medial and lateral condyles have a lesser radius posteriorly than distally 18,42 (Fig 1). Deep flexion is made possible by the posterior space, which is in essence formed proximal to the posterior femoral condyles. Without this offset, the posterior edge of the tibial plateau and soft tissues would impinge on the posterior cortex of the distal femur and limit flexion. Similarly, posterior femoral translation (femoral rollback) also is essential for knee flexion because it helps to create a space for the tibia and intervening soft tissues posterior to the femur 42 (Fig 2).

Fig 1

Fig 1

Fig 2

Fig 2

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

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RANGE OF FLEXION AFTER TKA

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.

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PREOPERATIVE FACTORS

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

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INTRAOPERATIVE FACTORS

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.

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POSTOPERATIVE FACTORS

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.

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HIGH FLEXION ARTHROPLASTY DESIGNS

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.

Fig 3

Fig 3

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°.

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DISCUSSION

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.

Fig 4

Fig 4

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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