Posterior Stabilization in Total Knee Arthroplasty

Giles R. Scuderi, MD, FACS

Douglas Vanderbrook, MD

HISTORICAL PERSPECTIVE

The present day posterior-stabilized (PS) total knee arthroplasty (TKA) design is the product of over six decades of innovation. During the late 1960s at Imperial College London Hospital, the Freeman Swanson prosthesis was designed which necessitated resection of both cruciate ligaments.¹ This design innovation occurred concurrently with the design of prosthesis such as the polycentric and geometric designs.² These designs proved to have a high incidence of early component loosening, breakage, subsidence, in addition to high incidence of early infections.³ Early failures did not deter other innovators from proceeding and two distinct schools of thought emerged: the anatomical approach versus the functional approach.

The anatomical approach involved an implant that preserved one or both cruciate ligaments. The Duocondylar and subsequent Duo-Patellar implants were the first of their kind within the United States, developed at the Hospital for Special Surgery (HSS) by Peter Walker with contributions from John Insall. The Duo-Patellar design had excellent results at HSS, but the popularization of cruciate-retaining (CR) TKA would occur at the Robert Breck Brigham Hospital in Boston.⁴,⁵

The functional approach aimed at simplifying the biomechanics of the arthroplasty design by removing, and ultimately substituting, the cruciate ligaments. The first functional design was the total condylar prosthesis (TCP) developed in 1973 at HSS.⁶ The total condylar design was successful; however, the lack of a femoral cam and tibial post mechanism in this early design led to anterior femoral translation on the tibia, which occasionally led to tibial component loosening and flexion instability. It is thought that these complications were the result of errors in surgical technique rather than implant design.⁷ In addition, the early TCP design had limited flexion, averaging only 90°.⁸ Insall and his colleagues redesigned the TCP and incorporated a high tibial post to prevent femoral translation in the TCP II design. The TCP II was implanted between 1976 and 77 and discontinued due to early tibial component loosening. In 1978 the Insall-Burstein (IB-I) prosthesis was introduced to address the issues experienced with the TC design.⁹ The design changes included substituting the PCL with a tibial post and femoral cam mechanism along with a dished conforming articular surface. The femoral cam engaged the tibial post at approximately 70° of flexion and produced reliable femoral roll-back and facilitated greater knee flexion, averaging 115°. While the original IB-I tibial monoblock design had an all-polyethylene tibial component, studies showed that metal-backed tibial components had a more uniform load dispersal and the original design was soon changed to incorporate a metal-backed tibial component.¹⁰

The IB-I had an excellent history of clinical performance and survivorship, with recent reports at 15- to 19-year follow-up showing survival of 92.4% when using revision as the end point (Fig. 36B-1).¹¹,¹²,¹³,¹⁴ The design was modified in 1988 and the IB-II, providing a modular tibial component (Fig. 36B-2) and the ability to add augments and stem extensions to the prosthesis, was introduced to the market. These modifications allowed the surgeon more flexibility in obtaining optimal knee alignment and stability while accommodating bone deficiencies. The IB-II design performed exceptionally well for over a decade before further advancements were made.

In the mid-1990s the Insall legacy of PS design continued with NexGen Legacy Posterior Stabilized (LPS) (Zimmer, Warsaw, IN) and subsequently the NexGen Legacy Posterior Stabilized Flex (LPS-flex) designs (Zimmer, Warsaw, IN). Major advancements in the LPS design included laterality with anatomic-specific femoral components with an enhanced lateral phalange and an elongated and deepened trochlear recess. These design modifications had the goal of improving patellofemoral kinematics and avoiding patellar clunk, which had been reported in the IB-II design. Elongation of the trochlear groove moved the femoral cam more posterior on the femoral component. This posterior position of the femoral cam mechanism had a beneficial effect on post/cam kinematics in that the cam continued to engage the post at 70° of flexion as it had with the predecessor IB-II prosthesis, but in the LPS implant the cam would ride down the tibial post, thus increasing jump distance and providing stability in flexion. Fuchs et al¹⁵ reported a Knee Society Score of 96 in the LPS TKA at 2- to 6-year follow-up, and multiple survivorship studies report highly favorable outcomes with this design.

A desire by Insall to expand TKA to the Middle East and Asia, both cultures which require higher degrees of knee flexion for social and religious activities, led to the development of the LPS-flex total knee. Augmentation of the posterior femoral condyles allowed for greater clearance before impingement in deep knee flexion as well as enhanced posterior condylar geometry which provided
greater contact surface area. Modifications made to the anterior lip of the tibial polyethylene articular surface permitted increased flexion without patellar tendon impingement. The LPS-flex was designed to obtain 140° to 150° of flexion compared with the 120° permitted by traditional PS implants. Later these implants were modified to include narrow versions to better accommodate anatomic differences between the genders. Since this time, numerous modifications from other vendors have expanded the implant inventory for higher flexion and gender-friendly designed femoral components.

FIGURE 36B-1 AP radiograph of IB I prosthesis at 30 years.

FIGURE 36B-2 Frontal view of the IB II modular prosthesis.

KINEMATICS

Retention of the posterior cruciate ligament (PCL) in CR designs was intended to preserve near-normal knee kinematics and facilitate femoral rollback. Literature, however, did not support this claim and suggested that the in vivo kinematics of the PCL-retaining knee are unpredictable. CR implants often led to paradoxical motion with anterior translation of the femur on the tibia, believed to be due in large part to incorrect PCL balancing.¹⁶,¹⁷ Due to difficulty appropriately balancing the PCL as well as reports of late PCL failure and subsequent instability, implant design shifted to replacing, rather than retaining, the PCL. Biomechanical studies have shown PS designs to produce a femoral rollback more closely replicating that of the normal knee than the CR design.¹⁶,¹⁸ This has further evolved into a relatively new category of ultracongruent or guided-rollback designs that sacrifice the PCL but do not substitute with the cam-post mechanism.

Specific Design Features

Post/Cam Mechanism

The IB-I was the first design to incorporate a tibial spine and femoral cam mechanism to facilitate femoral rollback and enhanced range of motion. This mechanism substitutes the function of the PCL and provides a mechanism for reliable femoral rollback. The tibial post engages the femoral cam at approximately 70° of flexion and allows for a controlled femoral rollback.¹⁸ Fluoroscopic studies have shown PS femoral rollback to more closely represent that of a native knee than previous CR designs.¹⁹,²⁰ It bears noting that post-cam mechanisms are highly variable in design features and all PS knee designs do not function in the same way. Arnout et al²¹ demonstrated large variations in the flexion angle at which the post and cam engage, maximal contact force, contact pressure, and contact area. They found that post-cam mechanisms that engage at lower flexion angles provide more normal rollback and tibial rotation.

The femoral cam of the IBPS initially engaged the lowest part of the tibial spine and as flexion increased the cam incrementally climbed the posterior tibial spine. This was not an issue when flexion was not expected to exceed 115° to 120°, but as patient demands for greater knee flexion increased the issue of stability in higher degrees of flexion arose. The sensitivity of the spine cam mechanism to instability became evident when the IB-I was changed to the IB-II. Following the initial introduction of the IB-II, there was a series of knee dislocations.²² To address this problem, the tibial spine was moved 2 mm anterior and 2 mm higher. This design change improved the resistance of the femoral cam riding over the top of the tibial spine. Further modification of the spine cam mechanism occurred with later designs. In the mid 1990s Insall improved upon his IB-II design with the NexGen LPS prosthesis. The femoral cam was moved to a more posterior position, which had a beneficial effect
of engaging the tibial post at 70° of flexion and riding down, rather than climbing up, the tibial spine. This feature increased the jump distance and produced increased stability with deep knee flexion.²³ Modern implants including the Smith & Nephew Genesis II as well as the Depuy Attune also utilize similar designs which rely upon implant surface geometry and soft-tissue balancing for stability through the first 60° of flexion, with cam-post engagement occurring at 60° to 75° of flexion. Additional advancements have been made in post geometry. For example, the patented “S-shape” of the Attune cam-post mechanism was developed to provide a large contact area as the cam engages the post and then smoothly translate the contact force distal down the spine as the knee moves into deeper flexion. In deeper degrees of flexion this design produces a compressive vector of force through the insert into the proximal tibia rather than a shear force that has the potential to ultimately lead to post wear and fracture.

Articular Conformity

A more conforming articular surface with a PS design is advantageous, since it increases the contact area and hence decreases polyethylene contact stress. This increased articular congruity evident in PS knees offers the advantage of lower shear forces that has been demonstrated in CR knees with paradoxical anterior femoral translation, with implications on polyethylene wear.²⁴

Increased articular congruity also proves beneficial in the event of femoral condylar liftoff. Femoral condylar liftoff has been demonstrated via fluoroscopic motion analysis studies.²⁵,²⁶ In the event of condylar liftoff, the conforming femoral-tibial articulation reduces the degree of tibial edge loading. In a prospective study of patients with bilateral paired posterior CR and PS TKAs performed by Lee et al,²⁶ they demonstrated condylar liftoff in 28% and 67% of knees, respectively. They postulated that the lack of PCL constraint in flexion contributes to this liftoff. Insall et al²⁷ examined the correlation between condylar liftoff and femoral component alignment in an LPS TKA, and reported that placement of the femoral component parallel to the transepicondylar axis could lessen the incidence of liftoff. Using computed tomography, it was determined that 69.2% of the subjects had a correlation between condylar liftoff and malalignment of the femoral component relative to the epicondylar axis. Therefore, it is thought that this phenomenon is multifactorial and variables such as ligament balance, component position, and static and dynamic limb alignment should be considered and the benefits of the PS design is advantageous.

High Flexion Designs

Consideration of patients’ activities, lifestyle, and cultural practices drove innovation of high flexion designs and continue to be a factor that surgeons must consider. Activities that require high flexion include squatting, sitting cross-legged, and kneeling with the knee fully flexed. These activities require up to 165° of flexion.²⁸ Additionally, everyday activities such as climbing stairs, sitting in a chair, and stepping in and out of a bathtub require between 90° and 135° of flexion.²⁹

PS knees have consistently shown greater degrees of flexion attributed to unique implant design and surgical techniques intended to prevent impingement of the posterior tibial articulating surface and the posterior femoral metaphysis. High flexion is traditionally defined as greater than 125° of flexion after TKA. However, current high flexion implants are designed to accommodate 135° to 155° of flexion. Design modifications to the implant aimed at attaining deeper degrees of flexion include:

Thickening and extension of the posterior condylar surface of the femoral component proximally (increases posterior femoral condylar offset and continued radius of curvature in an attempt to prevent posterior impingement).³⁰,³¹,³²
An elongated trochlear groove is required as the contact region of the patella moves distally in deeper degrees of flexion in order to prevent the patella from being caught in the intercondylar box.³³
Recession of the anterior aspect of the tibial polyethylene (prevents patellar-polyethylene impingement in deep flexion).³³
Posterior placement of tibial post (cam engages the post earlier and allows for greater femoral rollback).³³

In addition to implant design, a number of surgical considerations are important when attempting to maintain a high degree of knee flexion. Above all, soft-tissue balancing must remain of paramount importance to attain a stable and balanced articulation in both flexion and extension. Restoration of posterior condylar offset is critical for balance, and meticulous surgical technique cannot be understated. Additionally, attention must be paid to restoration of the posterior recess through removal of impinging posterior osteophytes.³¹ Placement of appropriately sized and positioned components also impacts the final clinical outcome. Current contemporary designs with more anatomically shaped femoral and tibial components provide a more precise fit of the components without intraoperative compromise (Fig. 36B-3). Component malalignment must be avoided as deviation from optimal alignment has detrimental effects on patellar tracking, condylar liftoff, increased tibiofemoral wear, and an association with arthrofibrosis.²⁷,³⁴ When preparing the patella, care must be taken to avoid overstuffing the patellofemoral joint. Reconstruction with too thick a patellar component or a femoral component with increased trochlear height have both been shown to limit flexion.³⁵,³⁶

Appropriate soft-tissue balancing and implant positioning allows for maintenance of preoperative joint line, which is vital to obtaining high flexion. Elevation of the joint line from a preoperative level creates a relative patella infera, which leads to early impingement and decreased motion.³⁷

FIGURE 36B-3 Intraoperative view of the Persona PS prosthesis.

Several studies have demonstrated increased contact stress during deep knee flexion in some high flexion designs with the potential for adverse wear characteristics.³⁸,³⁹ However, modern biomechanical research comparing PS high flexion designs with CR high flexion designs has demonstrated equivalent prosthetic load in deep flexion, though the amount of femoral rollback produced by the CR implants was inferior to that of PS implants.⁴⁰ This limited femoral rollback is likely due to inappropriate PCL balancing which may be a limitation when considering high flexion implants in a CR design.

INDICATIONS

Specific indications for PS TKA remain a topic of controversy. CR loyalists claim that correction of nearly all deformities except for severe flexion contractures can be addressed through PCL retention. With the more recent introduction of the ultracongruent polyethylene, characterized by an elevated anterior lip and a deeper trough, surgeons have expanded their use of CR implants on patients who previously may not have been candidates for PCL retention.⁴¹,⁴² However, traditional indications for PCL substitution remain popular among arthroplasty surgeons.

First, severe flexion contracture is best treated with PCL substitution. Severe flexion contracture is an ill-defined term, but most comparative studies exclude patients from receiving CR implants with contractures greater than 15°.⁴³ PCL retention in cases of severe flexion contracture requires extensive release which may compromise function and predispose to early failure and/or attritional rupture. PS implants are more easily balanced in such cases.

PCL substitution is more appropriate in several additional preoperative conditions including inflammatory arthritis, postpatellectomy, periarticular osteotomy, and PCL deficiency.⁴⁴,⁴⁵,⁴⁶,⁴⁷,⁴⁸ Inflammatory arthritis, most commonly rheumatoid arthritis, is associated with extensive synovitis. It is this inflammatory synovitis that contributes to tissue laxity and ultimate ligament incompetence.⁴⁴ Reports exist of an increased incidence of late instability and recurvatum with CR knees in patients with rheumatoid arthritis.⁴⁵ Modern advances in antirheumatologic treatments have drastically decreased the severity of this disease process, but the pathologic condition does still exist and needs to be considered when TKA is indicated.⁴⁶

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