To Retain or Sacrifice the PCL…That is the Question
In the normal knee, the ligaments provide stability during movement of the joint. There are four major ligaments in the knee: the medial and lateral collateral ligaments (MCL and LCL), which provide “side to side” stability and the anterior and posterior cruciate ligaments (ACL and PCL) which provide “front to back” stability.
In total knee replacement surgery, the ACL is routinely removed, the MCL and LCL are preserved, but the PCL is either retained or sacrificed. This brings up the obvious question-is it better to retain or sacrifice the PCL in total knee replacement?
There are many reasons why the posterior cruciate ligament may be removed or retained. Both scenarios have been associated with certain advantages and disadvantages.1 In either case, the implants used have specific geometry to account for the presence or absence of the ligament.
Retaining the PCL is believed to aid in proprioception (the ability to sense where parts of the body are in relation to each other) and could make activities like climbing stairs feel more “stable” or “natural”. Also, when the ligament is maintained, it can promote more normal front to back knee motion, possible aiding in deep flexion.
The implant used in these cases (referred to as “cruciate retaining or ‘CR’) is specially designed to allow for the presence of the PCL. However, in order for the PCL to perform it’s intended function, it must be in relatively good, “healthy” condition. Also, it must be properly “balanced” after the knee replacement have been oriented and implanted by the surgeon. This can sometimes be challenging and can result in less predictable results from patient to patient.
Also, just as the surgeon must balance the PCL if retained, the MCL and LCL must be properly balanced as well. This can be more difficult with the PCL intact. One can imagine that the tibia (shine bone) is like a swing rotating underneath the femur (thigh bone). The MCL and LCL are analogous to the chains or ropes holding the swing. The PCL is like a third chain coming down in the middle. It is easy to appreciate given this model, why the PCL makes creating the proper tension in the MCL and LCL more difficult since the tension in the PCL itself affects the tension in the other structures.
When the PCL is sacrificed, special geometry in the implant components substitutes for the function of the ligament. That is why these “PS” implants are sometimes referred to as PCL Substituting. When the surgeon removes the PCL, the joint space (the space between the prepared femur and tibia) becomes larger, making exposure easier. Also, the MCL and LCL tension can be more easily assessed.
Finally, since the ligament (whose function can be variable from patient to patient), is now replaced by very consistent implant geometry, results tend to be more repeatable (repeatable meaning less variation, not necessarily better overall).
So given all of this, how does long-term performance of these different types of implants and surgical approaches compare? In a recent study by Kolisek et. al., a group of patients receiving a CR implant was compared to a group with the posterior cruciate ligament removed.2 Two surgeons each did about half of the knee replacements in each group. Here is a summary of how the groups compared:
So as can be seen, and as the surgeons concluded, the study did not demonstrate a distinct advantage for one version over the other. Simply stated, PCL retention versus sacrifice is often a matter of surgeon preference and depends upon the “existing pathology of the posterior cruciate ligament” at the time of surgery. In short, the answer to the question on whether it is better to retain or sacrifice the PCL cannot be answered definitively.
1 Wheeles Textbook of Orthopaedics. “TKR – Posterior Cruciate Ligament Retaining Prosthesis.” http://www.wheelessonline.com/ortho/tkr_posterior_cruciate_ligament_retaining_prosthesis. Mar 2009
2. Kolisek FR, McGrath MS, Marker D, Jessup N, Seyler TM, Mont MA, Barnes CL. “Posterior-Stabilized vs. Posterior Cruciate Ligament-Retaining Total Knee Arthroplasty” The Iowa Orthopaedic Journal. Vol. 29. p 23-27.