MINIMALLY INVASIVE UNICOMPARTMENTAL KNEE PROSTHESIS: EVALUATION OF 100 CASES AT A MEAN FOLLOW-UP OF 60 MONTHS

Abstract

We performed a retrospective clinical and radiographic evaluation of 100 cases operated in our institute between February 1996 and March 2003 with a mean follow-up of 60 months to assess the efficiency of UKR performed with a new minimally invasive technique. The aim of this study is to correlate the clinical outcome of the patients with the pre- and post-op alignment, and with implant positioning on coronal and sagittal plane.

100 patients (23 ♂, 64 ♀) underwent cemented UKR (De Puy Preservation Uni with all poly tibial component), both for arthritis and osteonecrosis. At the pre-op clinical and radiographic evaluation, 82 patients presented a varus deformity, 5 patients a valgus deformity. The Hospital for Special Surgery Score (HSS) was used to determine the subjective and objective clinical status of the patients before and after the intervention.

Pre-op antero-posterior (AP) x-rays of the knee were executed to establish the femoro-tibial angle (FTA) and the angle between the affected tibial plateau and the tibial anatomical axis (PTA), while latero-lateral (LL) x-rays were performed to determine the posterior tibial slope (PS). To analyze ligamentous balancing, x-rays were performed both in supine and in plain weight bearing stance. Post-op, we performed supine AP e LL X-rays and at a mean follow-up of sixty months (12–84 months) we performed AP and LL plain weight bearing x-rays.

We considered a knee with FTA > 175° as varus knee, 170°< FTA< 175° as normal knee and an FTA < 170° as valgus knee. Moreover, we assumed a TPA > 90° for valgus knee and a TPA< 90° for varus knee.

According with HSS scoring system, at a mean follow-up of 60 months, 63 (76%) cases were excellent (100-85 points), 15 (18%) cases were good (84-70 points), 5 (6%) bad results (< 60 points). Our results demonstrate that patients with a pre-operative varus alignment of 7 degrees are slightly more likely to be selected for UKR. In our series, patients with an excellent clinical result presented pre-operatively a mean varus deformity of 7,9°. According to literature, we demonstrated that a small amount of undercorrection with a residual varus deformity of 3–5° is the goal to be reached in order to avoid both rapid degeneration of the non-replaced compartment as well as the premature loosening of the replaced compartment. We performed a mean axial correction of 5,1° leaving a mean axial varus deformity of 2,8° in the excellent group. In our series the group with excellent results also showed a post-operative PTS of 7,1°, while mean pre-operative PTS was 6,6°. Moreover, the further our radiographic findings were from the optimal position suggested, the worst were the results : a decrease was evident comparing excellent group with good group and this was even more marked comparing excellent group with bad results group.