Posterior Cruciate Ligament Retaining Total Knee Arthroplasty

Chapter Preview

CHAPTER SYNOPSIS

A brief history of the development of modern total knee design is reviewed with an emphasis on the early split between cruciate retaining and cruciate substituting philosophies. This chapter explores the similarities and differences between the two designs and provides technical tips on balancing the posterior cruciate ligament (PCL).

IMPORTANT POINTS

1

Most of the differences between posterior cruciate substituting and retaining implants are of historical interest only. Both systems have been modified to improve longevity and range of motion.
2

Long-term follow-up has not shown differences in survivorship or outcome scores between the two designs.
3

A meta-analysis of eight randomized studies showed only an 8-degree difference in range of motion between the two designs; this is of doubtful clinical significance.
4

Benefits of PCL retention include conservation of bone, improved balancing of the flexion space, and avoidance of problems such as post wear and patella clunk.
5

Contrary to popular belief, the PCL retaining knee is not difficult to balance.

CLINICAL/SURGICAL PEARLS

1

During exposure, the fibers of the PCL must be clearly exposed.
2

A measured resection technique is preferred in the PCL retaining knee.
3

The PCL acts as the lateral ligament of the medial side of the knee, mollifying the difference between the tight medial structures and the looser lateral structures.

CLINICAL/SURGICAL PITFALLS

1

It is important to assess the tightness of the PCL with the trial components in place and the knee flexed. If it is too tight, the PCL can be selectively recessed from either the tibia (if the entire PCL is tight) or the femur (if only the anterior lateral band is tight).
2

Use a curved or lipped polyethylene insert.
3

In the cruciate retaining, rotating bearing implant, a tight PCL can contribute to spinout.
4

In our experience, late PCL instability or rupture has not been a significant clinical problem.

VIDEO AVAILABLE

Not available.

HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM

The posterior cruciate ligament (PCL) of the knee has been the subject of controversy since the beginning of total knee implant design. Much of the debate and controversy are now of historical interest only as improvements in modern implant design have ensured similar outcomes in regard to clinical function and survival. Many surgeons now selectively decide to retain or substitute the PCL depending on patient characteristics, while other surgeons remain adamant PCL substituters or retainers. The goal of this chapter is to explain the similarities and differences between the two designs and to provide technical tips on balancing the PCL.

In the early 1970s, there were two metal and plastic total knee designs that were developed and would become the precursors to modern total knee devices. One of these designs was the total condylar knee, which was a cruciate sacrificing knee. It was initially performed on selected relatively low functioning patients in specialized centers that were dedicated to the development of the total knee design. The initial results were promising, but the early design of this prosthesis did not provide adequate flexion, due in part to its cylinder and trough design. Later modifications added a central eminence to improve both rollback in flexion, and the cruciate sacrificing knee was transformed into a posterior cruciate substituting (PS) design. The prototype for this knee was the Insall Burnstein total knee. In contrast, in the duocondylar knee, which eventually became the duopatellar knee, the PCL was retained.

In the early 1980s, cruciate retaining (CR) knees dominated the market; approximately 85% of knees that were implanted were of this design. The development of universal instruments allowed surgeons outside of specialized joint centers to implant these knees. This knee design provided patients with better motion than did the early total condylar knee. However, there were problems with the early CR designs of this era. This was a period where problems with polyethylene manufacturing had yet to be recognized. The polyethylene in this implant was thin with a flat-on-flat design. Metal-backed patellae were also commonly used in this era. Not surprisingly, these characteristics led to early osteolysis, loosening, and polyethylene failure. Furthermore, a design feature in one of the early CR knees predisposed to flexion instability in one leading system. With more CR knees being implanted than PS during this time period, the relative number of CR knees that failed was much higher than the PS knees. Consequentially, the PS knee began to increase in surgeon preference. In the meantime, design changes addressing the issues that lead to early failure of the early CR implants were made.

The implants we now use today are direct descendents of the original duocondylar and total condylar knee systems. Both the PS and CR knees have been modified to provide improved flexion and longevity of the implants. Today, most of the differences between the two knee designs are of historical interest. Long-term follow-up has shown no difference in survivorship of the two designs. Rasquinha et al. has published 12-year follow-up data on 150 consecutive PS knees with a 94.6% survivorship. This compares to Dixon et al., who reported 92.6% 15-year survivorship on 139 CR knees, as well as Rodricks et al., who reported 92.9% overall survival in a report of a 17-year follow-up on 160 CR knees. Outcomes are similar between implant designs even when the surgery is performed at nonacademic institutions and in younger patients. In Gioe et al.’s review of outcomes of 1047 patients aged 55 and younger in a community registry database, there was no difference in revision rates between the PS and CR knees. They report an overall 84.5% 14-year survival for cemented total knee replacement in this relatively young patient population.

Multiple comparative studies have shown no difference in functional outcome scores between the two implant designs. However, there is still some debate as to whether there are real differences in range of motion between CR and PS knees. A meta-analysis in 2005 of eight well-designed randomized studies found a statistically significant 8-degree increase in range of motion for PS knees with a cam and post design. This finding has not been universally true in all studies. For example, Tanzer et al. compared 40 knees of PS and CR design and found no statistical difference in range of motion, with average motion of 112 ± 13 degrees for the CR and 111 ± 17 degrees for the PS. In contrast, Maruyama et al. reviewed 20 bilateral knees and found an average range of motion in CR knees of 112 ± 15 degrees compared with 131 ± 13 degrees in PS knees, a statistically significant difference. However, with such a wide range of measured motion in both designs, the clinical significance that an average 8-degree difference in flexion has is debatable, as both designs allow for more than 105 degrees, which is what is needed to successfully climb stairs and arise from a seated position. Flexion can be affected by factors other than implant design, including but not limited to preoperative motion, tibial slope, removal of osteophytes from the distal femur, posterior condylar offset of the implant, and intraoperative flexion. Furthermore, as the body mass index of our patients increases, flexion is often limited by the contact of the patient’s calf on the thigh, more so than by the design of an implant.

The effect of the PCL on the ability to climb stairs is debatable. In a small gait analysis study of 14 patients, Bolanos et al. found no differences between PS and CR knees in level gait and stair climbing. In contrast, in a similar study, Dorr et al. found that the PS knee required increased quadriceps and biceps femoris contraction on level ground and increased soleus activity during stair climbing. The clinical relevance of these small studies is debatable. Patients with bilateral total knees, with one CR and the other PS, have not been shown to consistently favor one leg over the other on stair climbing. Furthermore, there are no differences in which knee they feel performs better.

Fluoroscopic studies have also been used to determine differences in tibial femoral kinetics between PS and CR implant designs. Femoral lift-off in knee flexion and the mechanism of posterior rollback are the two kinematic variables that have been most frequently examined in these studies. From these studies, it is clear that no total knee design currently available accurately reproduces “normal” knee kinematics. The optimal kinematics for a total knee remains subject to debate.

In the nonarthritic knee with an intact anterior cruciate ligament (ACL) and PCL, knee flexion occurs by a combination of rolling and sliding. In the much-discussed “screw home mechanism,” the femur pivots medially, directing the lateral femoral condyle posteriorly on the tibia with flexion. Dennis et al. and Yoshiya et al. compared knee motion with flexion, gait, and stair climbing in well-functioning PS and CR knees using a video fluoroscopy technique. In several studies, they have found that in flexion, the PS knee results in a consistent rollback of the lateral femoral condyle, whereas the motion in the CR knee is more variable and even exhibits a paradoxical anterior slide. In normal gait, they have not found differences between the PS and CR knees, with both exhibiting paradoxical axial sliding motion. Fantozzi et al. studied activities of daily living with fluoroscopic analysis and similarly found that in stair climbing and standing from a seated position, the PS knees exhibited a more consistent femoral rollback than did the CR knees. Although femoral rollback may be more reproducible in PS knees, this design also seems to be associated with an increase in femoral condylar lift-off during flexion. In a prospective, randomized studied of bilateral total knees, only 28% of CR knees had lift-off of the femoral condyle compared to 67% of PS knees.

What do these kinematic studies imply? The kinematics of a total knee replacement are different than a “normal” knee, regardless of whether the PCL is spared or substituted. An increase in femoral lift-off in the PS knee could increase contract stresses, thereby increasing polyethylene wear. Likewise, less consistent posterior rollback in the CR knee could also have a negative impact on polyethylene wear. Long-term year survival data on both types of implants have not yet shown clinical differences in revision rates, but the impact of these kinematic differences may be more pronounced in the younger, more active patient with knee replacements. Ultimately, the clinical relevance of this kinematic data is yet to be determined.

What are the effects of proprioception with retention or substitution of the PCL? Much has been written about the role of proprioception in the unicompartmental knee replacement attributed to retention of both the ACL and PCL, with patients reporting that their knee feels “more normal” compared to a traditional total knee. Does saving the PCL affect proprioception? The PCL has been shown by immunohistochemistry to contain mechanoreceptors. But, is the disruption of the pathway to these receptors clinically relevant? Although proprioception was an early argument in favor of PCL retention, more recent studies have shown this not to be the case. For example, Swanik et al. compared proprioception and balance before and after total knee arthroplasty (TKA) randomized to a PS or CR knee. Although both proprioception and balance improved following knee replacement, there was no significant difference between the two implant designs.

JUSTIFICATION FOR POSTERIOR CRUCIATE LIGAMENT RETENTION

The original presumption that PCL retention aids proprioception rollback and knee kinematics has not been borne out by either in vivo or in vitro testing. The major argument for cruciate retention in primary TKA is that it is easier to balance the flexion space. Moreover, the fact that PCL retention is bone sparing is based not only on the reality of femoral bone resection for the cam and post mechanism but also on that cutting the PCL increases the flexion space, requiring an obligatory increase in distal femoral resection to balance flexion and extension gaps.

PS advocates contend that the PCL is difficult to balance. If one looks at the components of the flexion space, this argument seems flawed. The medial component of the flexion space is the medial collateral ligament, which is broad and strong and goes from femur to tibia. The lateral component of the flexion space is the fibular collateral ligament going from femur to the fibula and the popliteus muscle and tendon, which traverses a more oblique pathway. Moreover, the lateral flexion space is normally more lax than the medial side to allow increased lateral rollback. The PCL serves to mollify the difference between the tight medial structures and the less tight lateral structures. In essence, it is the lateral ligament of the medial side of the knee. This is important as the medial compartment serves as the main pivot point during flexion.

If one uses a flat-on-flat articulation, it requires a precise tensioning of the PCL. With a more conforming insert, the tibiofemoral geometry is initially driven by the articulation until a recessed PCL is allowed to heal. This is the major reason for posterior cruciate recession from the tibia where there is a broad decussation of fibers, which can heal during the perioperative period. Selective release of tight anterolateral PCL fibers can also be performed from the femoral side.

There has also been concern about excessive, early wear of the post in PCL sacrificing knees. The cam and post mechanism adds constraint to the knee, the amount of constraint varying by implant design. If not perfectly rotationally aligned, the post may impinge on the metal component during flexion. This impingement may theoretically lead to increased polyethylene wear, especially if the polyethylene is cross linked, which decreases its mechanical strength. Puloski et al. published a report of 23 retrieved polyethylene implants, all of which showed significant post wear after an average 35 months in vivo, with a third of the inserts showing evidence of delamination and severe damage. The presence of a post, therefore, may contribute to increasing polyethylene wear, especially if the knee is not perfectly balanced and is relying on the post for constraint ( Fig. 10-1 A ). The added constraint of the cam and post design may also lead to increased backside wear ( Fig. 10-1 B ).