Posterior Cruciate Ligament Retention Versus Substitution

Historical Perspective

The controversy over whether to retain or substitute for the posterior cruciate ligament (PCL) has been ongoing since the advent of condylar total knee arthroplasty (TKA) in the early 1970s, and three schools exist. One preserves the PCL and uses a tibial insert without much conformity. The second sacrifices or partially releases the PCL and uses an insert with increased sagittal conformity (i.e. “ultra-congruent” or “medial pivot”). I like to refer to this technique as “PCL supplementation.” The third school sacrifices the PCL and substitutes it with an insert that restores its stabilizing function via a polyethylene post on the tibial insert that articulates with a constraint on the intercondylar portion of the femoral component.

Historically, Boston has been considered the school of PCL retention, whereas New York has been the school of PCL sacrifice and substitution. In 1974, when I was chief resident in orthopedic surgery at Massachusetts General Hospital, a surgeon and an engineer from New York (Drs. Chit Ranawat and Peter Walker) came to Boston to share their latest designs for condylar knee prostheses. Both were at the Hospital for Special Surgery at that time and collaborated with Dr. John Insall on early knee prosthetic designs. A meeting was held in the Smith-Petersen room and notable attendees included Dr. William Jones, Chief of the Knee Service, and Dr. William Harris, Chief of the Hip Service. We were shown two options: total condylar prosthesis, which sacrificed the PCL, and the duopatellar prosthesis, which preserved it. The total condylar option had a dished tibial sagittal topography; whereas the duopatellar tibial topography was flat. With the PCL retained and functioning, this flat tibial topography allowed the femur to roll back on the tibia and enhance potential flexion ( Fig. 1.1 ). Users of the total condylar technique were reporting an average flexion of approximately 85 degrees, and duopatellar users were achieving more than 100 degrees of flexion. These findings were shared with surgeons at The Robert B. Brigham Hospital, where 85% of patients undergoing TKA at that time had rheumatoid arthritis. Because patients with rheumatoid arthritis often had significant involvement of the upper extremities, the potential for enhanced flexion by retention of the PCL was very attractive. Unless patients achieved well over 100 degrees of knee flexion, they would have difficulty rising from a chair and negotiating stairs and would depend on their upper extremities for these activities. The cruciate-preserving technique, therefore, was adopted in Boston to better serve the rheumatoid population. Nearly all Boston orthopedic residents and fellows were trained in this technique, whereas in New York the practice of sacrificing the PCL predominated. This led to a friendly rivalry between the Boston and New York camps and spawned many formal and informal debates that have continued for decades. In the United States in the 1980s and 1990s, the ratio of retaining to sacrificing the PCL was 60:40 in favor of retention. By 2010, this ratio had slowly reversed and has remained constant at 60:40 in favor of PCL sacrifice.

Advantages of Posterior Cruciate Ligament Retention

There are many potential advantages of preserving the PCL. Because stability is imparted by a biologic structure, the prosthesis can be less constrained; therefore less force is imparted on the insert–tray interface and the prosthesis–cement (or bone) interface.

With PCL retention, it is also possible to preserve the joint line at a near-normal location. When the PCL is cut, the flexion gap increases and requires a thicker polyethylene insert for any given amount of bone resection. This thicker polyethylene in turn requires a greater than anatomic distal femoral resection to allow full extension of the knee. Thus the joint line is elevated in both flexion and extension by several millimeters with cruciate-sacrificing designs. This means there is a mandatory distortion of the collateral ligament kinematics. Although it is possible to equalize the 90-degree flexion gap with the full extension gap, midflexion laxity is bound to occur to some extent when the joint line is elevated. There are some designs of cruciate-sacrificing knees whose femoral components provide increased thickness of the posterior condyle relative to the posterior condylar bone resection. These, in theory, should lead to less distortion of the joint line.

Finally, cruciate-retaining knees allow for preservation of intercondylar bone stock for future revision, if it becomes necessary.