Posterior Cruciate Ligament Retention in Total Knee Replacement

J. Joseph Gholson, MD

Brett R. Levine, MD, MS

Posterior cruciate ligament (PCL) retention in a well-balanced total knee arthroplasty (TKA) construct continues to be the standard by which other designs are compared. There is ongoing debate as to whether posterior cruciate-substituting or posterior cruciate-sacrificing designs provide improved kinematics and increased patient satisfaction. Proponents of PCL retention point to superior range of motion, strength, stability, and durability as reasons to spare the ligament in total knee replacement (TKR).¹ With similar enthusiasm, advocates of PCL substitution argue that resection eases exposure and correction of deformity, decreases polyethylene wear, and provides reliable sagittal plane stability.² Studies of both systems have demonstrated the long-term durability of PCL substitution and retention at 10 years (Table 36A-1), with many cohorts demonstrating 20-year survival of approximately 90%.⁵,⁶,⁷,⁸,⁹,¹⁰,¹¹,¹²,¹³,¹⁴,¹⁵ Furthermore, posterior cruciate retention in TKA allows for improved bone preservation, a smaller flexion gap secondary to maintaining the PCL, and the lowest long-term revision rates in registries for a multitude of implant designs. It is for these reasons that cruciate-retaining (CR) TKA continues to be a mainstay of TKA designs throughout the world. In this chapter, we review pertinent anatomy to CR TKA, discuss the kinematics of cruciate retention, address controversies in CR TKA, provide rationale for continued use of CR TKA over other designs, and describe our surgical technique for PCL-retaining TKA.

POSTERIOR CRUCIATE LIGAMENT ANATOMY

The PCL runs in a slightly oblique direction from the lateral aspect of the medial femoral condyle to the lateral aspect of the posterior intercondylar area of the tibia (Fig. 36A-1). The tibial insertion flares out for a distance of 2 cm below the articular surface. The broad, distal insertion makes PCL retention and balance during TKR feasible. Two anatomically inseparable bands comprise the PCL. A large anterolateral band tightens in flexion, and a smaller posteromedial band tightens in extension.¹⁶ The anterior cruciate ligament (ACL) crosses in front of the PCL, running from the medial aspect of the lateral femoral condyle to the medial aspect of the anterior intercondylar area of the tibia. The synovium covers the anterior surface of the PCL and then fans out laterally onto the surface of the capsule.¹⁷ Inflammatory arthritides such as rheumatoid arthritis often spare the PCL because the synovium does not surround the ligament, making it an extrasynovial structure. In contrast, patients with rheumatoid arthritis often lack an ACL, as it is entirely surrounded by synovium.

KNEE KINEMATICS PERTAINING TO THE POSTERIOR CRUCIATE LIGAMENT

Bony contours provide little inherent stability to the knee. The lateral tibial plateau is convex in the sagittal and coronal planes. The medial tibial plateau is slightly larger than the lateral plateau and concave in both planes. Muscles, ligaments, and capsule combine to provide knee stability. Studies of sequential sectioning of the PCL reveal that in isolation the ligament has a limited role in providing varus/valgus and rotational stability.¹⁸ The PCL plays a large role in preventing anterior/posterior translation of the tibia relative to the femur. Prosthetic designs must account for this stability through prosthetic geometry, a post-and-cam mechanism, or PCL retention.

A combination of rolling, sliding, and rotation occurs in normal knee motion. The synchrony of these motions depends on the articular contours of the tibia and femur, menisci, capsular structures, and an intact ACL/PCL in the nondiseased native knee. During the first 30° of knee flexion, motion occurs predominantly from rolling of the tibial and femoral surfaces relative to one another. As the knee further flexes, tightening of the PCL leads to sliding at the articular interface. This sliding, also referred to as femoral rollback, prevents impingement of the posterior surface of the tibia and femur during maximal flexion and allows flexion of approximately 140° in the nondiseased knee. The convex lateral tibial plateau allows more sliding than the more conforming, concave surface of the medial plateau. This creates an obligatory internal rotation of the tibia relative to the femur with knee flexion. Femoral rollback also lengthens the moment arm and improves the direction of pull of the quadriceps, which increases the strength of the quadriceps as the knee flexes.¹⁹

EVOLUTION OF THE CRUCIATE-RETAINING TOTAL KNEE REPLACEMENT

In the early 1970s, many different prosthetic knee designs were being used. Early hinge designs suffered many early and late complications, including infection

and loosening, which made their routine use unacceptable. Early resurfacing knee replacements included the polycentric (1970), modular (1972), UCI (1972), McKeever (1960), Geometric (1971), and Duocondylar (1973). These surface replacement designs relied on intact native ligaments and capsule to provide knee stability. These prostheses consisted of medial and/or lateral polyethylene tibial components that were separate or connected by a small bar. Early techniques preserved both cruciate ligaments and made no provision for the patellofemoral joint. The conformity of the articulation of early TKAs varied based on the designer’s philosophy. Less conforming articulations attempted to simulate normal knee rollback and motion (polycentric and Duocondylar). Other designs focused on stability and provided more congruent femoral and tibial articular surfaces (Geometric). Problems of tibial subsidence and loosening, patellofemoral pain, and lateral subluxation of the tibia on the femur complicated these early designs (Fig. 36A-2).²⁰,²¹,²²,²³

TABLE 36A-1 Long-Term Follow-Up to Cruciate-Retaining Total Knee Replacement

Study	Prosthesis	Manufacturer	No. of Patients/Knees	Average Follow-Up (y)	Mean Age (y)	Osteoarthritis (%)	Rheumatoid Arthritis (%)	10-y Survivorship	Infection (%)	Radiographic Lucent Lines (%)	Instability (%)	Reoperation/Revision Other than Infection (%)
Dennis et al³⁰	Posterior cruciate condylar	Howmedica	35/42	11	62.8	50	50	N/A	0	75	2.3	4.7
Schai et al³¹	Press-Fit Condylar	Johnson & Johnson	122/155	10.5	68	62	33	90	1.2	Tibia 16 Femur 3 Patella 3	0	13.5
Berger et al⁵	Miller-Galante II	Zimmer	92/109	9	72	94.4	4.6	100	2.8	Tibia 13 Femur 11 Patella 1.4	0	1.8
Parker et al⁶	Miller-Galante I	Zimmer	67/67	12.8	66	100	0	90	6	N/A	0	52
Gill and Joshi⁷	Kinematic condylar	Howmedica	177/216	10.1	68	88	11	98.2	1	Femur 4.5 Tibia 8.3 Patella 4	0.5	3
Buechel et al⁸	Low contact stress (LCS) meniscal bearing	DePuy Orthopaedics	116/140	12.3	65	89	6.5	100	0.7	Femur 0 Tibia 6.6 Patella 0	0.7	5.7
Ritter et al⁹	Anatomic graduated components	Biomet	3054/4583	N/A	70.4	87	N/A	98	1.3	N/A	N/A	N/A
Sextro et al¹⁰	Kinematic condylar	Howmedica	118/168	15.7	65.2	64.9	31	96.5	1.2	N/A	0.6	7.7
N/A, not available.

FIGURE 36A-1 Dissection of the posterior cruciate ligament during total knee replacement. Note the robust nature of the posterior cruciate ligament passing from the lateral aspect of the intercondylar notch to the posterior tibia.

Modern total knee designs reflect the lessons learned from the shortcomings of early resurfacing knee replacements. These include a one-piece metal condylar femoral component with a trochlear flange, a one-piece metal-backed or all-polyethylene tibial component, and resurfacing of the patella with an all-polyethylene component. From these early condylar designs emerged two schools of thought, namely, preservation or sacrifice of the PCL. The latter school, recognizing limited flexion in early cruciate-sacrificing designs and the advantage of femoral rollback and improved patellofemoral mechanics, evolved into cruciate substitution by the addition of a central polyethylene eminence on the tibial component. This eminence engaged in mid-flexion (60° to 80°) to achieve these goals.

Advocates of cruciate sacrifice/substitution developed the total condylar prosthesis in 1973 in response to their experience with the Duocondylar TKR (Fig. 36A-3).²⁴ This design provided a femoral component with an anterior femoral flange and allowed patellar resurfacing. The tibial component was a one-piece all-polyethylene component that necessitated sacrifice of the PCL. The total condylar prosthesis proved durable and gave reliable results.²⁵,²⁶ Limitations of this prosthesis related to range of motion and posterior subluxation of the tibia relative to the femur. To overcome these shortcomings, the posterior-stabilized (PS) total knee was introduced in 1978. These devices possess built-in posterior constraint and achieved femoral rollback by a post-and-cam mechanism. Numerous authors have subsequently reported excellent results with PS designs.²⁷

FIGURE 36A-2 Early resurfacing total knee replacement (Marmor) demonstrating lateral subluxation of the tibia on the femur.

Over the same time frame, advocates of CR designs of TKR grew as a result of the limitations these surgeons experienced with the Duocondylar, McKeever, Modular, and other early knee replacements.²⁰,²⁸ One
example of an early PCL-retaining design, the Duo-Patellar, evolved from experience with the Duocondylar TKR. Implantation of the Duo-Patellar prosthesis at the Robert Breck Brigham Hospital began in 1974.³ The femoral component provided an anterior flange to facilitate patellar tracking. The initial tibial component consisted of separate medial and lateral tibial pieces and allowed for ACL and PCL retention. The tibial contour changed from flat in the sagittal plane to a curved surface to increase articular constraint. The Duo-Patellar was redesigned in 1978 to a one-piece tibial component with a stem to better distribute weight-bearing forces (Fig. 36A-4).⁴ This tibial component mandated sacrifice of the ACL but maintained a cutout for preservation of the PCL.

FIGURE 36A-3 Duocondylar total knee replacement.

Over the next 7 years, gradual refinement of the femoral, tibial, and patellar components occurred and the Duo-Patellar evolved into Robert Breck Brigham Hospital and Kinematic total knee systems.²⁹ The goals of these prostheses were to increase range of motion, improve fixation of the tibial component, and maintain the overall excellent clinical results of the previous designs. The trochlear groove of the femoral component was deepened and aligned in 7° of valgus to improve patellar tracking. In 1980, the tibial component changed to a metal-backed design and reverted to a flattened surface in the sagittal plane to allow rollback. Femoral and tibial intramedullary stems were made available for situations of bone deficiency. Experience with the Kinematic total knee continued the clinical success of previous designs and taught valuable lessons on axial alignment, component position, and cementing technique.²³,³⁰

FIGURE 36A-4 A: Duo-Patellar total knee replacement. B: Design change with one-piece, stemmed tibial component.

Advocates of cruciate retention remained concerned about a kinematic conflict if a conforming polyethylene surface directed motion antagonistic to the strong intact PCL. Moreover, cruciate tension was variable and difficult to match with tibiofemoral conformity. This concern led to the development of flat-on-flat cruciate-sparing designs. Unfortunately, these designs did not compensate for the abduction/adduction moments during gait and caused edge loading and polyethylene wear. Additionally, if the PCL was too tight, there was abnormal rollback and posterior polyethylene wear; if the PCL was too loose, random contact leading to abnormal shear stresses and even paradoxical roll forward occurred during flexion.

Recognizing the concerns of kinematic conflict and the problems of flat-on-flat designs, a new direction to cruciate retention was introduced. The Press-Fit Condylar (PFC) design incorporated different tibial contact patterns (posterior-lipped and curved) to accommodate for different cruciate tensions (Fig. 36A-5). Balancing the PCL by recession from either the tibia or femur allowed cruciate retention while accommodating sufficient conformity to allow low contact stress. In a series of patients from our institution who had posterior CR TKR with the PFC system, Schai et al found no tibial or femoral loosening at a minimum of 10-year follow-up.³¹ Similarly, Buehler et al had 98.7% survivorship with posterior cruciate retention with the PFC system at 9-year follow-up.³² The PFC modular cruciate-substituting design was introduced, thus coupling the PCL-retention and PCL-substituting philosophies. In 1996, the PFC Sigma design was introduced as a merged philosophy to allow smooth transition
between cruciate retention and substitution, expand revision capabilities, and eliminate gamma-radiated polyethylene sterilized in air. Since that time, multiple CR designs have incorporated a more conforming polyethylene with improved kinematics, range of motion, and excellent survivorship.

FIGURE 36A-5 A: Posterior-lipped polyethylene insert. B: Curved polyethylene insert.

CURRENT CONTROVERSIES IN POSTERIOR CRUCIATE LIGAMENT-RETAINING AND POSTERIOR CRUCIATE LIGAMENT-SUBSTITUTING TOTAL KNEE REPLACEMENT

Rollback and Kinematics

Neither the cruciate-retaining nor cruciate-substituting knee can reproduce the kinematics of the normal knee. In fact, the unique aspects of the knee with the material properties of the articular cartilage, the cruciate ligaments, and the medial and lateral menisci confer very different properties than seen in metal-to-plastic condylar designs. Meniscal-bearing and rotating-platform knees attempt to more closely mimic normal knees but fall short of this goal. Most meniscal-bearing designs demonstrate paradoxical motion, whereas rotating-platform designs rotate about a fixed central axis rather than accomplishing the complex motion that occurs in the normal knee. It was originally believed that with proper conformity or mobile-bearing designs, normal kinematics could be achieved.

Recent fluoroscopic and gait laboratory data have contrasted with these early beliefs.³³,³⁴,³⁵ Fluoroscopic studies demonstrate that cruciate-retaining and cruciate-substituting designs fail to reproduce normal rollback. Strain-gauge studies also demonstrate the inability of TKR to reproduce normal ligament strain behavior.³⁶ As these studies would predict, clinical comparisons show little difference in range of motion achieved in either design.³⁷,³⁸,³⁹,⁴⁰,⁴¹ Similarly, difference in quadriceps efficiency and stair-climbing ability also appears equivocal in cruciate-retaining and cruciate-substituting devices.⁴²,⁴³ Excellent clinical results and range of motion can be achieved with the use of recession to balance the PCL in conjunction with more conforming polyethylene inserts.⁴⁴,⁴⁵,⁴⁶,⁴⁷

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