Abstract
The debate regarding the superiority of retaining (CR) or substituting (PS) for the PCL in total knee arthroplasty (TKA) has continued for a generation, without a clear consensus as to which is superior. That debate continues today.
Many studies on this subject have been published, including recent meta-analyses, which demonstrate similar outcomes between CR and PS TKA. Pain scores and functional outcomes appear quite similar between the groups. One outcome measure that appears superior in PS knees, although by a small margin, is in final range of motion, with higher final flexion observed in PS knees. Another study demonstrated superior extensor mechanism efficiency in PS designs compared to CR.
The primary explanation for improved motion is the rollback induced by the cam and post mechanism of the PS knee. By insuring rollback of the femur on the tibia, the chance of impingement between the back of the femur and polyethylene is diminished. This cam and post induced rollback has been shown to be more consistent with native knee kinematics compared to the CR knee, which can be subject to “paradoxical motion”, the forward sliding of the femur on the tibia during knee flexion.
The clear downsides of the PS knee include an increased rate of patellar clunk and crepitus, intercondylar femur fracture, post failure, and bone loss due to the box cut. The gaps must be carefully balanced to prevent a loose flexion gap which can allow the cam to “jump the post”. With improved implant design and materials, each of these complications has been reduced, but surgeons need to recognise these possibilities to reduce the incidence and identify them when they occur. Improvements in the trochlear geometry have significantly reduced the patellofemoral complications, a significant historical problem for patients with PS devices.
Indications for using a PS knee include patients with a prior patellectomy, patients with PCL deficiency, or those with a higher risk of late PCL rupture such as patients with poor neuromuscular control or rheumatoid arthritis, although there are reports successful use of CR TKA in rheumatoid arthritis.
