LIGAMENT BALANCE IN EXTENSION IN TOTAL KNEE PROSTHESIS FOR GENU VARUM: WHAT IS EXCESSIVE LAXITY?

Abstract

Purpose of the study: It is currently accepted that ligament balance should be one of the goals for total knee arthroplasty (TKA) and that this balance should be obtained by correct bone cuts or appropriate ligament procedures. There is however no standard way of assessing this balance. The purpose of this study was to define limit values for knee laxity observed in a series of normal knees and in a series of 54 TKA reviewed at more than ten years.

Material and methods: Laxity in extension of normal knees was measured on forced varus and valgus films using the contralateral knees of patients who had undergone knee surgery for osteotomy or prosthesis implantation. Laxity in extension of TKA knees was measured the first postoperative year and at last follow-up by measuring the decoaptation between the tibial and femoral pieces on single-leg stance films. The change in decoaptation over time was compared with the postoperative and last follow-up goniometry figures, the IKS knee score, the number of loosenings and the number of lucent lines. Multifactorial analysis was considered significant at p< 0.05.

Results: For the normal knees in extension, the medial compartment gap was 2 mm on average (range 1.5–3.5 mm) on the forced valgus images and the lateral compartment gap was 3 mm on average (range 2–4 mm) on the forced varus images. The corresponding angular value was 1° decoaptation on the forced valgus images and 1.5° on the forced varus images. Among the 54 knees with a TKA, the first postperative single-leg stance image revealed a lateral decoaptation _ 3° for 12 knees considered to present laxity, and was _ 2° for 42 knees considered not to present laxity. At last follow-up (13 years on average, range 11–14 years) the 42 knees without laxity remained unchanged without decoaptation, including the 34 normocorrected knees (±3°) and the eight undercorrected knees presenting more than 3° varus (mean undercorection 5°, range 3–7°). The 12 knees presenting postoperative radiographic decoaptation _ 3° showed at last follow-up a significant increase in laxity (p< 0.05) and 2.5° further increase in decoaptation. The increase in decoaptation occurred on normocorrected (n=7) or undercorrected (n=5) knees. This increase in decoaptation was greater with greater residual genu varum. Four groups of knees could be distinguished: normocorrected and stable; normocorrected and unstable; undercorrected and stable; overcorrected and unstable. The number of loosenings requiring revision and the number of progressive lucent lines were significantly greater among unstable knees (two loosenings, and five progressive lucent lines) than among stable knees (no loosening or lucent lines). They were also greater in the group of normocorrected and unstable knees (one loosening and two lucent lines) than in the group of undercorrected and stable knees (no loosening or lucent line). The IKS knee score of stable knees was higher than that of unstable knees irrespective of the correction (p< 0.05).

Discussion: Postoperative laxity in varus with angular decoaptation greater than 3° corresponds to a lateral compartment gap and should be avoided even if the knee is properly aligned postoperatively. If the knee is stable, moderate undercorrection (3–5° varus) does not appear to have an unfavorable long-term effect on knee laxity or on the femoral and tibial pieces.

Conclusion: For knees with constitutional genu varum, moderate undercorrection with a stable knee is preferable to normocorrection at the cost of lost stability.