To The Editor:
In "Patellar Complications Following Distal Femoral Replacement After
Bone Tumor Resection" (2006;88:2225-30), Schwab et al. draw our
attention to an aspect of prosthetic reconstruction after a distal femoral
resection that is important and often overlooked. They, correctly in my view,
state, "the position of the joint line deserves special
attention..." Unfortunately, I believe they do not appreciate the cause
of their most common complication, which is elevation of the joint line. This
causes so-called patella baja and patellar impingement. The authors reference
an article by Thorpe et al. and suggest that some of their patients may have
had fibrous bands as described in that study1. The patients in the
study by Thorpe et al. did not have patella baja caused by fibrous bands, and
they were cured with an arthroscopic release of the bands. Schwab et al.
mention scarring of the patellar tendon that was possibly related to relative
devascularization. The reference they cite describes only the vascularity of
the patella, not of the patellar tendon, and actually notes that the vascular
supply is abundant2. To support a theory that the patellar tendon
shortens in the postoperative period, the authors should show serial lateral
radiographs demonstrating this
shortening.
The surgeon controls the position of the joint line. The amount of bone
removed from the tibia must be equal to the thickness of the tibial component,
including the polyethylene, if the joint line is to be maintained. If
insufficient bone is removed, or if a thicker polyethylene component is added
to lengthen the reconstruction, the joint line is raised. The authors
recognize this but then indicate that they measure the femoral resection with
the tibial resection and combine these to decide the length of the
reconstruction. The femur should be measured separately from the tibia, and
the femoral component should equal the length of the femur that was resected.
The tibial component should be equal to the thickness of the tibia that was
resected. If the reconstruction is short, the femur should be lengthened. The
thickness of the tibial polyethylene should not be increased. Implanting a
thicker tibial polyethylene tray raises the joint line, producing patella baja
and risking patellar impingement. Their mean LT/HI (patellar tendon
length/height of patellar tendon insertion) ratio of 1.3 is clearly abnormal
and indicates that, on the average, the joint lines are abnormally high.