TRICOMPARTMENT KNEE PROSTHESIS AND LIGAMENT TENSOR: IMPLANT POSITION AND EFFECTS ON LAXITY AND LOWER LIMB AXIS

Abstract

Purpose: Independent cuts are generally used for tri-compartment knee prostheses but interdependent cuts may be needed. This can modify the height of the articular space or induce alignment errors. The purpose of this study was to examine the position of the implants, and the effects on laxity and lower limb alignment after implantation of a tricompartment knee prosthesis with a ligament tensor.

Material and methods: Between January 1998 and October 2000, 109 total knee prostheses (posterior stabilised Legacy®) were implanted in 94 patients. Three patients died, 3 who lived far from the centre were questioned by phone, 88 patients (103 prostheses) were retained for analysis at mean follow-up of 22.5 months. None of the patients were lost to follow-up. All of the prostheses in this series were implanted with a V-STAT® ligament tensor used to guide medial and lateral capsuloligamentary balance in flexion and extension under constant tension.

Results: At review, the IKS radiological scores were mean alpha 95.9° (90–108°) with 76.7% of the implants between 93° and 99°. The mean gamma angle was 1° (−8° to +8°) with 73.8% of the implants between −3° and +3°. The mean beta angle was 89.8° (86–98°) with 81.5% of the implants between 87° and 93°. Mean tibial slope measured from the mechanical tibial axis was 8.4° (2–15.5°) with 67% of the implants between 4° and 10° (desired slope 7°). The mean HKA at last follow-up was 178.8° (172.5–191°) with 75.7% of the knees between 175° and 185°. Correction was more significant for more pronounced preoperative deviation. The height of the articular space was significantly increased compared with the preoperative value. Mean radiological laxity in varus at last follow-up was 3.1° for a preoperative value of 2.8°. Mean radiological laxity in valgus at last follow-up was 3.2° for a preoperative value of 4°, a significant decrease. Mean sum of the radiological frontal laxities at last follow-up was 6.4° for a preoperative value of 6.8°, a non-significant decrease. Mean radiological sagittal laxity at last follow-up was 4.2 mm. There was no significant difference between preoperative and last follow-up sagittal laxity.

Discussion: The mean values obtained in this series are in agreement with data reported in the literature. While the height of the articular space was significantly greater at last follow-up, it was not correlated with a decrease in the height of the patella at last follow-up. Decreased patellar height at last follow-up was correlated with increased patellar joint surface (AP distance of the Blackburne and Peel index) and with shortened patellar tendons. Use of the V-STAT® ligament tensor allowed homogeneous mediolateral distribution of the frontal laxity while controlling sagittal laxity and preserving a normal axis of the lower limb.