UKR--time for caution?

SAGITTAL PLANE KNEE LAXITY AFTER LIGAMENT RETAINING UNCONSTRAINED ARTHROPLASTY: A MATHEMATICAL ANALYSIS

Passive knee laxity is an important clinical measure to assess function after joint replacement. Clinical observations suggest that the use of minimally invasive surgical techniques in knee arthroplasty may affect the surgeon's ability to orient and position the prosthetic components accurately. Further, recent studies suggest that malplaced prosthetic components in ligament retaining unconstrained unicompartmental knee arthroplasty (UKA) can affect the ligament forces and, hence, the knee laxity. In the present study, a sagittal plane mathematical model of the knee with intact ligaments and unconstrained prosthetic components is used to analyze antero-posterior (A–P) knee laxity during passive flexion at different force levels. Also, the effects of errors in component placement are evaluated. The model calculations show a reasonable agreement with the experimental observations reported in literature. The results show that the A–P laxity during 0°–120° flexion first increases from 0° to about 30°, remains nearly constant for another 10° and then decreases somewhat linearly for higher flexion angles. Some errors in the placement of femoral component of the order of 1 mm can affect the knee laxity by nearly 3 mm in some flexion positions. The analysis has clinical relevance and suggests that the UKA requires close attention to component placement.

Minimally invasive unicompartmental knee replacement: retrospective clinical and radiographic evaluation of 83 patients

We performed a retrospective clinical and radiographic evaluation of 83 nonconsecutive patients operated in our institute between February 1996 and March 2003 with a mean follow-up of 60 months to assess the efficiency of unicompartmental knee replacement (UKR) performed with a minimally invasive technique. The aim of this study was to correlate the clinical outcome with the pre- and post-operative alignment and with implant positioning on coronal and sagittal plane. Eighty-three nonconsecutive patients (60 males, 23 females) underwent cemented UKR (De Puy Preservation Uni with all-poly tibial component), for both medial OA (80 patients) and AVN of the medial femoral condyle (3 patients). All patients were available at final follow-up evaluation, and they all presented an evident varus alignment at pre-operative clinical and radiographic evaluation. At radiographic measurement, we considered a knee with femoro-tibial angle (FTA) > 175 degrees as varus knee, 170 degrees < FTA < 175 degrees as normal knee and a knee with a FTA < 170 degrees as a valgus knee. Moreover, we considered a tibial plateau angle (TPA) > 90 degrees for valgus knee and a TPA < 90 degrees for varus knee. According to Hospital for Special Surgery (HSS) scoring system, at a mean follow-up of 60 months, 61 (74%) cases were excellent (100-85 points), 15 (18%) cases were good (84-70 points) and 7 cases (8%) had fair results (<70 points). In our series, patients with an excellent clinical result presented a mean varus deformity of 7.2 degrees (3.6 degrees-10.8 degrees) pre-operatively. According to literature, we demonstrated that a small amount of undercorrection with a small amount of residual varus deformity of 3 degrees-5 degrees is the goal to be reached in order to avoid both rapid degeneration of the nonreplaced compartment and the premature loosening of the replaced compartment. We performed a mean axial correction of 5 degrees (SD 3.9 degrees), leaving a mean axial varus deformity of 2.2 degrees in the excellent group. In our series, the group with excellent results also showed a post-operative PTS of 7 degrees (2.4 degrees-11.6 degrees), while mean pre-operative PTS was 6.5 degrees (2.7 degrees-10.3 degrees). In this study, results have shown that minimally invasive UKR producing a small amount of varus undercorrection in selected patients with medial tibio-femoral osteoarthritis or moderate avascular necrosis of the medial femoral condyle provides excellent clinical and functional results. Overcorrection of varus malalignment with a UKR may produce both rapid degeneration of the lateral tibio-femoral compartment and the early failure of the replaced compartment.

Fixed versus mobile bearing unicompartmental knee replacement: a meta-analysis

This systematic review compares the clinical, radiological and kinematic outcomes of fixed compared to mobile bearing unicompartmental knee replacements (UKRs). A meta-analysis of pooled mean difference and relative risk data was undertaken following a review of electronic databases. Five studies were identified. Analysis suggested that there was no significant difference in clinical outcome or complication rate between mobile and fixed bearing UKR. However, the evidence reviewed presented with a number of methodological limitations. Areas for further study are recommended.

Early complications after minimally invasive mobile-bearing medial unicompartmental knee arthroplasty

The purpose of this article is to review the early complications of minimally invasive mobile-bearing unicompartmental knee arthroplasty by analyzing the first 100 consecutive cases. All cases were medial unicompartmental arthroplasties with a follow-up of at least 24 months (range, 24-51 months). Seven revision procedures were performed due to 2 fractures of the medial tibial plateau, 1 loosening of the femoral component, and 4 dislocations of the meniscal bearing. The early results as determined by The Knee Society scores, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores and postoperative range of motion were satisfactory. However, there exists a high rate of early failures, which occurred during the initial learning curve. Therefore, surgeons must be aware that in order to minimize these complications, attention to detail is mandatory.

Influence of component alignment on outcome for unicompartmental knee replacement

This study's aim was to assess the effect of component mal-alignment on outcome of Oxford Unicompartmental Knee Replacement (UKR). Two hundred and eleven knees implanted with a medial UKR, using a minimally invasive approach, were followed up at a minimum of 4 years. Sagittal and frontal plane femoral and tibial component alignments were determined from antero-posterior and lateral radiographs. The cohort was divided into alignment groups which represented consecutive 2.5 degrees intervals over the range of measured values for femoral varus/valgus, femoral flexion/extension, tibial varus/valgus and tibial tilt. The Oxford Knee Score (OKS) and incidence of radiolucency (RL) were compared between alignment groups for each alignment parameter. In 98% of cases the femoral components were positioned between 10 degrees varus and 10 degrees valgus; all femoral components were within +/-10 degrees flexion. In 92% of cases the tibial components were positioned between +/-5 degrees varus/valgus and superior/inferior tilt (neutral tilt being 7 degrees). Within these ranges there were no significant differences in OKS or RL between the alignment groups; nor were there any differences between those at the extremes of component alignment compared to those in the inner ranges of alignment. We conclude that, because of the spherical femoral component, the Oxford UKR is tolerant to femoral mal-alignment of 10 degrees and tibial mal-alignment of 5 degrees.

Development of a patient-specific femoral component for unicompartmental knee replacement

This study describes the development of a novel, patient-specific unicompartmental knee prosthesis. The geometries of the lateral and medial condyles are approximated by polynomials, instead of single radius circles that are commonly used. Furthermore, a database containing the geometries of healthy knees is used to generate appropriate knee geometries according to certain measurements of the unhealthy knee. This new method enables a customized design of a unicompartmental knee replacement that will closely resemble the articulating surfaces of the normal, healthy knee joint.

Accuracy of implantation of components in the Oxford knee using the minimally invasive approach

Screened postoperative X-rays of 224 Oxford knees implanted through the minimally invasive approach were analysed using 16 criteria. The technique was as recommended by the Oxford Group except that the femoral intramedullary rod was used only as a guide to flexion/extension of the femoral component. All femoral components were within the recommended range for varus/valgus and mediolateral position. Eighteen femoral components were either too flexed or extended, but by a maximum of only 10 degrees . Tibial components were inserted in slight varus (mean 1.8 degrees , S.D. 8.8, range 10 to -10). All were within the range for posterior slope in spite of difficulty in establishing objective landmarks. Tibial coverage was imprecise with the phase 3 implants which were short and broad relative to the cut surface. In 36% of knees the tibial tray was implanted too anterior resulting in posterior under hang. There have been no clinical signs to indicate tipping of the meniscus in deep flexion. The depth of cement penetration under the keel was excessive in three cases, resulting from porotic bone rather than technical error in the depth of the cut. There has been one case of femoral component loosening following a fall and no tibial loosening in spite of imprecise alignment of some components.