To The Editor:

I am delighted that our article sparked discussion of the importance of re-establishing the joint line in knee replacements after tumor resection.(1) Dr. Springfield presents a clear tutorial on how to accomplish this: match the femoral resection and replacement lengths, and then match the tibial resection and replacement lengths. This tautology is everyone's goal. For various reasons, we were not always able to achieve it.

Nevertheless, discussion of the average technical adequacy of the procedure misses the point. Citing the high joint line in our cases as the cause ("their mean LT/HI ratio of 1.3 is clearly abnormal and indicates that on average the joint lines are abnormally high," ) Dr. Springfield overlooks the paradox that the LT/HI ratio was low in patients with impingement (0.9). This vitiates his argument. Curiously the high ratios that he understandably criticizes (1.4) were found in patients without any impingement. When the reader looks at the data, it is clear that the ratios aren't so important and another cause for patellar impingement should be sought. Even when the joint line is reproduced accurately, (in our case, with a minimum 15 - 17 mm tibial cut, 3 mm metal backed tray and either a 12 or 14 mm polyethylene tibial bearing), the problem still occurs. Why?

The vascular hypothesis is just that: an hypothesis. These cases are much different from regular joint resurfacing procedures. All synovium, capsule, and fat pad are removed, and the geniculates interrupted. Kayler's "abundant blood supply" doesn't exist in these reconstructed joints. The open question is if this dysvascularity is sufficient to contribute to patella contracture and impingement.

Finally, the two appended figures document the changes that occur to the patella tendon length, the inferior pole of the patella, and in children, the anterior tibial plateau. There is little doubt that this phenomenon is real.

Fig. 1 Lateral views of the knee show the changing relationship of the patella tendon length over 3 years 5 months. Heavy dark lines define the tendon length and its relationship to the patellar length. It was well beyond the patellar button marker initially, and well short of it as last follow-up. The dashed line highlights the top of the polyethylene tibial bearing.

Fig. 2 During 4 years 3 months of follow-up bone has grown from the tibia and from the inferior pole of the patella (arrows) shortening the functional patellar tendon.

Reference:

(1) Dempsey S. Springfield, MD Tibial measurement critical. Letter J Bone Joint Surg (Am) (16 October 2006)

To The Editor:

The authors 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…”(1). 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 might have fibrous bands as described by Thorpe(2). Thorpe did not have patients with patella baja caused by fibrous bands and these patients were cured with an arthroscopic release of the bands. The authors mention scarring of the patella tendon possibly related to relative devascularization. The reference given discusses only the vascularity of the patella not the patellar tendon and actually reports that the vascular supply is abundant(3). To support a theory that the patella 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 femur resected. The tibial component should be equal to the thickness of tibia resected. If the reconstruction is short the femur should be lengthened. The thickness of the tibial polyethylene should not be increased. Putting in a thicker tibial polyethylene tray raises the joint line producing patella baja and risk patella impingement. Their mean LT/HI ratio of 1.3 is clearly abnormal and indicates that on average the joint lines are abnormally high.

The author(s) of this letter to the editor did not receive payment or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author(s) are affiliated or associated.

References:

1. Schwab JH, Agarwal P, Boland PJ, Kennedy JG, Healey JH. Patellar complications following distal femoral replacement after bone tumor resection. J Bone Joint Surg Am. 2006;88:2229.

2. Thorpe CD, Bocell JR, Tullos HS. Intra-articular fibrous bands. Patellar complications after total knee replacement. J Bone Joint Surg Am. 1990; 72: 811.4.

3. Kayler DE, Lyttle D. Surgical interruption of patellar blood supply by total knee arthroplasty. Clin Orthop Relat Res. 1988; 229: 221-7.