The Patella in Total Knee Replacement:
The Case Against Resurfacing
Jeffrey Gollish, BASc.,
M.D., F.R.C.S.(C) Lecturer,
University of Toronto Orthopaedic
Surgeon Sunnybrook
& Women’s College Health Science Centre Orthopaedic
& Arthritic Campus Toronto,
Ontario Patellar resurfacing in total knee replacement remains a controversial
area. There are those who would
suggest that the patella should be resurfaced in every case, those who
suggest that it should never be resurfaced and those who suggest resurfacing
in certain circumstances. In considering the case against patellar resurfacing, there are
several areas which require review.
These include knee function after replacement, mechanical
considerations with respect to the patellofemoral joint, and complications. With respect to function, various authors have reported no advantage
to patellar arthroplasty in modern designs of total knee prostheses. In these designs the trochlea of the
femoral component has been shaped to accept the natural patella. In older designs of knee arthroplasty,
where the femoral trochlea was not patellar friendly, there were advantages
to replacing the patella, using the specific prosthesis designed to track in
the femoral trochlea. With respect to mechanical considerations, it is necessary to consider
patellar tracking and congruence as well as patellofemoral pressure. Patellar tracking is not a function of
patellar arthroplasty, but rather a function of restoration of the alignment
of the knee and appropriate rotation of both the tibial and femoral
components to allow for central patellar tracking. Congruence of the natural patella in the femoral trochlea has
been significantly enhanced through modern prosthesis design and congruence
no longer relies on matching a patellar component to the specific trochlear
design. It is in the area of pressures in the patellofemoral joint where the
strongest arguments against patellar resurfacing arise. The important concept is that of compressive
yield strength which is the compressive pressure above which material failure
occurs. The yield strengths for the
materials under discussion are as follows:
Studies of the pressures produced in the patellofemoral joint with
various activities of day-to-day living have demonstrated that at flexion
angles of greater than 60 degrees, resisted knee extension produces contact
pressures in the range of 20 to 50 Mpa, far exceeding the yield strength of
UHMWPE. With respect to complications, the re-operation rate after patellar
arthroplasty has been reported to be higher than that associated with not
resurfacing the patella. Patellar
fracture is more common after resurfacing. In summary, if you were needing knee replacement and you considered the function achieved in knees without a resurfaced patella, the mechanical concerns associated with patellar arthroplasty and the potential complications, it is likely that you would say to your surgeon “please do not resurface my patella”. |
