TOTAL KNEE ARTHROPLASTY AND PATELLAR STABILITY ACCORDING TO ROTATORY CONSTRAINT ON THE TIBIAL PLATEAU

Abstract

Purpose of the study: The stability of the patella over the femur depends on several factors, one being the rotatory freedom of the tibia. Femorotibial rotatory laxity of a total knee arthroplasty (TKA) can be:

1. dictated by the congruence of the polyethylene in an ultracongruent plateau;

2. completely free, depending solely on the ligament structures in a self-aligning prosthesis;

3. the consequence of a compromise between the two, using a semi-constrained prosthesis. With the OMNIA system, the unique femoral piece can be combined with a Wallaby ultracongruent plateau (WUC), a self-aligning mobile plateau (SAL), or a semiconstrained plateau with preservation of the posterior cruciate ligament (Wallaby 1, W1). A comparative study of these three prostheses was performed to evaluate the influence of femorotibial rotatory constraint on patellar stability.

Material and methods: The series included 157 TKA: 68 SAL,44 WUC and 45 W1. The same surgeon operated all patients in three successive series. The operative technique was the same using the same instrument set. The femoral piece was inserted first with an automatic rotation systematically set at 5° with the posterior condyles. The knees were varus (71%), valgus (20%) and perfectly aligned (9%). A patellar prosthesis was used in only 10.8% due to excessive wear as assessed intraoperatively. A medial approach was used for the varus knees and a lateral approach for the valgus knees greater than 10°, independently of preoperative patellar stability. Radiological outcome was assessed on the 30° femoro-patellar views. Patellae tilted more than 3° and/or offset more than 5 mm were considered to be excentered.

Results: In the SAL group, 55 patellae were centered and 13 off-centered. The ratio was 36 for 6 in the WUC group and 39 for 6 in the W1 group. The percentages of centered patellae were respectively 80, 85, and 86%. The difference was not significant (p=0.66. For knees with preoperative genu varum, the percentages of centered patella were 83, 84, and 86%; and for genu varum, 72, 0 and 75% (sample too small for statistical significance).

Discussion: The use of rotatory constraint for TKA does not affect patellar stability. This was confirmed in a subpopulation of genu varum knees and not in genu valgum.