ABSTRACT
As population shifts occur in age and demographics, our goals for restoration of function have grown in knee arthroplasty.With this focus has come the advent of high-flexion components whose designs have sought to advance the recreation of normal knee kinematics. A review of the theory, mechanics, and documented performance is fundamental in the determination of their worth. High-flexion designs aim at the restoration of axial femoral-tibial rotations and posterior femoral translation for efficacy. In cadaver studies, these systems have reproduced 80–90% of normal posterior femoral translation and improved kinematic curves. In addition, kinematic improvement has been documented when studied in vivo. When the results between standard and high-flexion systems are compared, three retrospective studies have documented significant increases in flexion while two prospective studies have found no significant difference in flexion. Functional scores have not shown statistically significant improvement. Multiple systems have been developed using combinations of design changes to improve the biomechanics of arthroplasty. Laboratory and clinical studies have evaluated these systems. Kinematic improvement is documented in the laboratory setting and in the clinical setting; however, the results of clinical studies are equivocal in regards to functional improvement.
INTRODUCTION
The goal of arthroplasty historically has been to relieve pain and restore function in patients with osteoarthritis; however, as population shifts occur, particularly in the Far East, our goals have changed. Now we seek to restore patients with osteoarthritis to near normal. In an effort to achieve this, advances in total knee arthroplasty (TKA) systems have been made in an effort to achieve greater flexion.
There are multiple factors that affect the extent of flexion that a patient will obtain after full rehabilitation from TKA. These factors include operative technique, prosthesis design, preoperative motion, soft-tissue rearrangement or release and patient motivation. Patient outcomes and goals differ in terms of activities that involve moderate to deep flexion and knee stability. Approximately 50% of patients report that kneeling and gardening are important activities while 40% report that squatting is important. In addition, 70% of patients reported limitations with squatting and kneeling, and 55% reported limitations with gardening (Figures 1 and 2). 1 Assimilation of these results with other studies have created the recent goals of increasing flexion, as mid to deep flexion is important interms of leisure andmany activities of daily living (Table 1). 2–4 Historically, this is one area of knee arthroplasty that we have not perfected, as studies reveal that average flexion postoperatively is less than 120 degrees (Table 2).5–10
To establish and critique recent advances in design, it is first necessary to review the kinematics of the normal knee and what deficiencies have existed with previous arthroplasty design. It is at this point that one can understand the goals set forth for improvement and how these goals have translated into component innovation. The literature has documented the kinematic performance of new design in cadaver models and when implemented in vivo. In addition, retrospective and prospective reviews have been performed with function in mind.
NORMAL KNEE KINEMATICS
The kinematics of the natural knee have been studied extensively. As a result, there is a plethora of models and descriptive terminology describing ideal knee motion. Asano et al. 11 employed a biplanar image-matching technique to create three-dimensional knee models for the study of natural knee kinematics. Their results provide a model for ideal knee kinematics and a goal for arthroplasty designs. The natural knee can obtain a maximal flexion value of 150–160°. 2,12,13 The axial rotation of the tibia in relation to the femur and translational movement in the sagittal plane are two elements of knee motion that are of particular importance.
Department of Orthopaedic Surgery, Wake Forest University Baptist
Medical Center, Medical Center Boulevard, Winston Salem, North
Carolina, USA
Correspondence to Riyaz Jinnah, MD, Department of Orthopaedic Surgery,
Medical Center Boulevard, Winston Salem, NC 27157-1070, USA
Tel: +1 336 716 9657;
fax: +1 336 716 6286;
e-mail: rjinnah@wfubmc.edu
Wake Forest Orthopaedics and one of the authors receive research and consultancy support from Zimmer and Wright Medical.
1940-7041 © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Keywords: arthroplasty, high-flexion, knee
Through the flexion arc of 0–120°, the normal knee averages 29.1° of internal rotation of the tibia in relation to the femur. In addition, as flexion occurs there is a shift of the center of rotation from the distal condyles to the posterior condyles. In the lateral compartment, this represents a transition from the larger radius of the distal condyle to the smaller radius of the posterior condyle. This transition couples with the geometry of the tibial plateau in the creation of natural motion. The medial plateau’s concave shape in relation to the convex shape of the lateral plateau creates more rotation and less translation in the medial compartment with the opposite in the lateral compartment.
As a result, the medial condyle translates anteriorly 3.8mm while the lateral condyle translates posteriorly 17.8mm. 11 This complex scenario has been given different names, including the screw-home phenomenon and medial pivot. Two elements of this motion have been the focus of recent development in knee arthroplasty design: internal rotation in the axial plane and posterior translation in the sagittal plane. These two elements are important for avoiding impingement of the femur on the tibia when maximizing flexion.
FIGURE 1. Results of a questionnaire sent to 176 TKA patients regarding the importance of activities in which they participate. Reproduced with permission.
doing activities because of their knee replacement
FIGURE 2. Results of a questionnaire sent to 176 TKA patients regarding the limitations of activities in which they participate. Reproduced with permission. 1
Table 1. A collection of activities and the estimated flexion necessary to participate2–4
| Activity | Flexion Necessary (in degrees) |
|---|---|
| Squat/sit cross leg | 111–165 |
| Kneeling for prayer | >150 |
| Ascending and descending stairs | 90–120 |
| Stepping into a bathtub | 135 |
LIMITATIONS OF PREVIOUS DESIGNS
No consensus exists on what components of a natural knee allowfor deep flexion and what important alterations occur in TKA. Knee arthroplasty requires resection of menisci and disruption of the posterior capsule. After TKA, the articulation andinteractions of the femoralcomponent, tibialcomponent, and polyethylene liner determine stability, perceived comfort, and ultimately function. Previous designs often were not formulated with high flexion in mind. As a result, when motion exceeds 1208 with standard systems, the femur and the polyethylene insert have little articulation.Table 2. Average weight-bearing flexion obtained in retrospective review of TKA5–10
| Author | System Type | Degrees of Flexion |
|---|---|---|
| Dennis et al.6 | CR PS | 103 113 |
| Banks et al.9 | CR PS MB | 109 121 102 |
| Kanekasu et al.10 | PS | 139* |
| Bellemans et al.8 | CR | 117 |
CR, cruciate retaining; MB, mobile bearing; PS, posterior stabilized.
*Non-weight bearing flexion in a population with average preoperative flexion 145 degrees.
In addition, this articulation involves the posterior tibial plateau and the most proximal aspect of posterior femoral condyles. As a consequence, a great force is focused on a small area at the posterior aspect of the tibial tray creating a considerable stress. This phenomenon is referred to as edge loading. High-edge loading is thought to be accompanied by instability and discomfort. These two factors cause reluctance in patients to perform high-flexion activities after arthroplasty.
HIGH-FLEXION DESIGN
Theories abound about what exact anatomic components, their relations, and forces are the most important in the production of a stable artificial construct that provides stability and high flexion. Five goals have been set to recreate natural knee flexion: restoring axial femoral-tibial rotations, restoring posterior femoral translation, increasing femoral offset in relation to the tibia, decreasing the tension forces on the extensor mechanism and increasing the femoralpolyethylene articulation area. The greatest amount of energy has been focused on increasing articulation area, posterior translation and femoral offset at high flexion because it is theorized that edge loading and impingement are most responsible for instability and discomfort.8The NexGen Flex System (NexGen-F; Zimmer, Inc, Warsaw, IN, USA) was developed to increase the amount of posterior femoral translation and increase the articulation area at higher flexion angles by increasing the thickness of the posterior wall of the femoral component by 2mm. This, in turn, extends the posterior condylar surface, which theoretically increases articular contact area at higher flexion angles and moves the vector of articulation anteriorly. In addition, the cam and spine engage at 80° and increase the amount of posterior femoral displacement. The polyethylene includes a deeper anterior cut out, which provides more clearance of the patellar tendon at high flexion (Figure 4).15The Scorpio Non-Restrictive Geometry system (Scorpio NRG; Stryker Orthopedics, Mahwah, NJ, USA) is a highflexion, posterior-stabilized fixed bearing design. This design employs a decrease in the radius of curvature involving the posterior femoral component, i.e. the radius of femoral curvature is greater at 0–95° than from 95–155°. In addition, the post-cam interaction was altered such that posterior translation continues after 120° of flexion. A rounded post is theorized to add stability by increasing surface area and allowing for increased axial rotation. The anterior flange underwent a reduction in size to decrease the tension placed on the extensor mechanism, and the intercondylar notch was enlarged to allow greater axial rotation (Figure 5).16 •The Genesis II High-Flex, Posterior Stabilized System (Gen II PS-F; Smith and Nephew, Memphis, TN, USA) incorporates changes in the polyethylene to produce high-flexion angles. By beveling the insert, the patellar mechanism is allowed more room for flexion. In addition, the post is cut in such a way as to prevent impingement with the patella.17 •HIGH-FLEXION KINEMATICS IN VITRO
Multiple studies have sought to determine if the high-flexion systems show a kinematic improvement in the laboratory setting. Li et al.14 compared the kinematics of a cadaveric knee before and after implantation of a NexGen-F (Zimmer, Inc, Warsaw, IN, USA) a high-flexion posterior stabilized system. The high-flexion design produced a similar kinematic curve when compared to the natural knee through range of motion 0–150°. At 150°, specifically, it recreated 90% of the natural posterior femoral translation. The posterior translation was best recreated at angles from 90– 150°. Cam spine engagement occurred from 80 to 135°. Disengagement occurred with flexion greater than 130° secondary to soft-tissue, thigh-calf contact; however, further posterior femoral translation was observed past 130°. The authors theorized that the compression of the posterior soft tissues, including the posterior capsule, muscle, skin and pericapsular fat, pushes the tibia anteriorly and helps to recreate posterior femoral translation.In a similarly designed study, Most et al.18 sought to compare the conventional NexGen Cruciate Retaining TKA (NexGen CR; Zimmer Inc, Warsaw, IN, USA) with the Nex- Gen Cruciate Retaining High-Flexion system (NexGen CR-F; Zimmer Inc, Warsaw, IN). Both systems had similar kinematics, retaining 80% of posterior femoral translation when compared to the natural knee. The only difference between the two systems was found in femoral translation of the lateral condyle at 150°. The NexGen CR reached a peak of 27.2mm of posterior translation compared to 24.8mm in the NexGen CR-F. In addition to kinematic analysis, posterior cruciate ligament (PCL) tensioning was studied. PCL tension peaked at approximately 90° of flexion, and posterior femoral translation continued to rise in the natural knee and TKA models despite a corresponding decrease in PCL forces past 90°.
FIGURE 3. MRI sagittal view of the knee at extremes of flexion, illustrating the loss of articulation of the femur and tibia with compression of the posterior
menisci. Reproduced with permission.14
FIGURE 4. Zimmer Nex Gen System. Reproduced with permission.15
HIGH-FLEXION KINEMATICS IN VIVO
In addition to comparisons in the laboratory study, there has been kinematic review of high-flexion systems in the clinical setting. Tamaki et al.16 evaluated the kinematics of 20 patients who received the Scorpio NRG. Deep-bending abilities of patients were elicited at an average of 8 months after implantation. In their cohort, the average total range of motion was 126°, and the mean tibiofemoral internal rotation at 140° was 15.6°. Kinematic analysis of normal knee patterns has revealed that average internal rotation of the tibia is near 20°. In addition, they recorded posterior translation of the lateral femoral condyle to be on average 12.8mm and of the medial femoral condyle to be 7.2mm. This design produced a medial pivot from -10 to 60° and a bicondylar roll back event from 60 to 140°. Fifteen of the 20 patients exhibited a medial pivot knee pattern.16 •
Cates et al.,21 in a similar way, compared deep knee bend kinematics of the NexGen CR-F with the Legacy LPS-Flex High-Flexion system (Legacy LPS-F; Zimmer Inc, Warsaw, IN, USA) in 30 individuals. On average, the cruciate retaining group averaged 117° of flexion with weight bearing, 4.9mm of femoral rollback, and 4.8° of axial rotation. The cruciate sacrificing group had an average of 112° of flexion, 6.4mm of femoral roll back, and 2.9° of axial rotation. The two systems produced similar magnitudes in terms of kinematics; however, this was achieved with unique patterns of movements.
Similar to the in vitro studies, the recreation of femoraltibial articulation also has been studied in vivo. Sharma et al.22 compared four systems in terms of contact areas, forces and stresses. These systems included the Legacy LPS-F, the NexGen CR-F, the Sigma Rotating Platform (Depuy Orthopaedics, Inc, Warsaw, IN, USA), and the Sigma Fixed Bearing Posterior Stabilized (Depuy Orthopaedics, Inc, Warsaw, IN, USA). Although all four systems exhibited similar kinematics, the high-flexion systems experienced higher contact forces, higher contact areas, and lower contact stresses in the deep-flexion range.
HIGH-FLEXION CLINICAL OUTCOMES
Multiple studies have sought to elicit the clinical outcomes of high-flexion systems. Laskin 17 • performed a retrospective cohort comparison of patients undergoing standard posterior stabilized TKA with Genesis II Posterior Stabilized System (Gen II PS; Smith and Nephew, Memphis, TN, USA) and a second cohort undergoing TKA with Gen II PS-F. The standard system was implanted in 40 consecutive patients over a 7-month period followed by implantation of the high-flex design in 40 consecutive patients over a 7-month period. Both groups improved postoperatively. At 2 years, the highflexion group had a mean flexion of 133° in comparison to 118° in the standard group. This difference was statistically significant. There was no significant difference in the mean Knee Society Knee (KS) score between the two groups. Using a similar protocol, Bin and Nam<sup15 compared 97 reconstructions with the standard NexGen LPS (Zimmer, Inc,Warsaw, IN, USA) in a first year followed by 96 reconstructions with the NexGen LPS-F (Zimmer, Inc, Warsaw, IN, USA) in the following year. Outcomes were measured with the Hospital of Special Surgery (HSS) score and range of motion at 1 year postoperatively. Maximal flexion in the standard group was found to be 124.3° while it was found to be 129.8° in the high-flexion group. This difference also was statistically significant. The functional scores determined by the HSS score were not found to not be significant; however, in patients with preoperative range of motion less than 90°, the NexGen LP-F implants did yield significantly greater results of flexion. The authors postulated that the 5-10° difference, although not significant in the HSS score, is important in terms of comfort with daily social activities.15
Huang et al.23 also performed a retrospective review. Over 5 months, 93 primary reconstructions were performed, including 25 using the NexGen LPS-F system. The results of the 25 high-flexion reconstructions were compared with 25 standard reconstructions. Patients were matched in terms of body weight, age, sex, diagnosis, and preoperative range of motion. At 2 years, they found a significant increase in range of motion for the NexGen LPS-flex patients. A mean of 138° was found in the high-flexion cohort in comparison to 126° in the standard cohort. This significance was not seen at discharge or 2 weeks follow-up. They found that the difference began from the 6th postoperative week and progressed to a plateau at 1 year. In addition, they found that 80% of their NexGen LPS-F patients had the ability to squat upon request in comparison to 32% of patients with the standard implant. The authors noted, however, that none of their patients who were able to squat reported that they did so in their daily activities. There was no significant difference in KS scores between the two cohorts.
Two prospective randomized studies currently are documented in the literature. Kim et al. 24 performed the first in the context of bilateral TKA. It compared the NexGen LPS and NexGen LPS-F in 50 patients. The study was arranged such that patients underwent bilateral TKA in one sitting. The standard NexGen LPS prosthesis was placed in one knee and the NexGen LPS-F was placed in the contralateral knee. At 2 years, there was no significant difference in range of motion or in HSS and KS scores. The NexGen LPS produced a mean range of motion, measuring 135.8° while the NexGen LPS-F produced a mean of 138.6°. When posterior femoral condylar offset was measured, there was a statistical increase in measurement in the high-flexion group. The authors also reported a subjective advantage when evaluating lateral radiographs obtained with maximal flexion in terms of articular contact.24
Nutton et al.••25 orchestrated a randomized, controlled, double-blinded study comparing the NexGen PS and NexGen PS-F (Zimmer, Inc, Warsaw, IN, USA) in 56 patients. Activities were measured at 1 year with an electrogoniometer aligned with the longitudinal axis of the tibia and femur. Low-flexion activities (sitting in a high chair, rising from a high chair, ascending stairs, descending stairs, walking on flat ground, walking up an incline and walking down an incline), as well as high-flexion activities (sitting in a low chair, rising from a low chair, stepping into a bath, stepping out of a bath, squatting and flexing while standing), were observed. No significant difference was found when comparing maximal flexion in low-flexion and high-flexion activities. In both cohorts, patient’s perceived improvement in knee function according to response testing; however, an increase in motion was only found to be significant in a few specific functional activities in both cohorts. This led the authors to postulate that the perceived improvement related to TKA is more dependent on alleviation of pain in comparison to increase in motion.
CONCLUSION
The orthopaedic community is always in pursuit of improvement. In the setting of TKA, this pursuit is for the restoration of natural knee kinematics and mechanical forces, with the idea that recreating these will reconcile osteoarthritic patients with the norm. The theoretical advantage of high flexion and near normal kinematics has produced a new generation of components that have been shown to better recreate knee kinematics in the laboratory setting. In vivo, these components have performed when graded on kinematics alone. These theoretical advances, however, have translated into marginal gains in knee flexion and function when graded by set standards. The gravitas of these gains has yet to be proven. It is important as a community to determine what goals we should set, considering the transition of our patient population in terms of age and activity. New standards for success must be set to determine if we are keeping pace with the changing demands of our patient population.
• of special interest
•• of outstanding interest
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The above publication appeared in: Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 24, No 5 (May), 2008: pp 604-611
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