Intra-operative Laxity Following Total Knee Arthroplasty is Highly Variable and Different Than Osteoarthritic and Normal Knees

Abstract Background Achieving a stable joint is an important yet challenging part of total knee arthroplasty (TKA). Neither manual manipulation of the knee nor instrumented sensors biomechanically characterize knee laxity or objectively characterize how TKA changes the laxity of an osteoarthritic (OA) knee. Therefore, the purposes of this study were: 1) objectively characterize changes in knee laxity due to TKA, 2) objectively determine whether TKA resulted in equal amounts of varus-valgus motion under a given load (i.e., balance) and 3) determine how TKA knee laxity and balance differ from values seen in non-osteoarthritic knees. Methods Two surgeons used a custom navigation system and intra-operative device to record varus-valgus motion under quantified loads in a cohort of 31 patients (34 knees) undergoing primary TKA. Similar data previously were collected from a cohort of 42 native cadaveric knees. Results Performing a TKA resulted in a “looser knee” on average, but great variability existed within and between surgeons. Under the maximum applied moment, 20 knees were “looser” in the varus-valgus direction, while 14 were “tighter”. Surgeon 1 generally “loosened” knees (OA laxity 6.1°±2.3°, TKA laxity 10.1°±3.6°), while Surgeon 2 did not substantially alter knee laxity (OA laxity 8.2°±2.4°, TKA laxity 7.5°±3.3°). TKA resulted in balanced knees, and, while several differences in laxity were observed between OA, TKA, and cadaveric knees, balance was only different under the maximum load between OA and cadaveric knees. Conclusions Large variability exists within and between surgeons suggests in what is considered acceptable laxity and balance of the TKA knee when it is assessed by only manual manipulation of the leg. Knees were “balanced” yet displayed different amounts of motion under applied load. Clinical Relevance Our results suggest that current assessments of knee laxity may leave different patients with biomechanically different knees. Objective intra-operative measurements should inform surgical technique to ensure consistency across different patients. Level of Evidence Level II prospective observational study