Flexion deformity and laxity as a function of knee position at the time of tensioning of rigid anatomic hamstring ACL grafts

BACKGROUND: Anatomic ACL grafts routinely display the anisometric length-tension behaviour seen in the native ligament with maximum length in full knee extension. Recent improvements in hamstring graft preparation and fixation have improved graft rigidity to the point where total graft lengthening after implantation may be less than 1 mm. Despite this it remains common practice to fix these grafts in a knee flexed position. METHODS: Nineteen participants underwent all-inside ACL reconstruction with optimally preconditioned 4 strand semitendinosus grafts using bi-cortical adjustable suspensory loop fixation. Using a computer navigation system, baseline measures of anisometricity, extension range, and tibial rotation were made. The graft was tensioned and provisionally fixed with the knee flexed 5° beyond its anisometric point and extension range recorded. The graft was then definitively fixed with the knee fully extended and extension range and tibial rotation recorded again. Anterior laxity measurements were made pre-operatively and postoperatively using a manual arthrometer and compared to those from the contralateral limb. RESULTS: Fixing the graft with the knee flexed produced a mean FD of 10.9° (p < 0.0001) and fixing in extension restored full extension (p = 0.661). Fixing in extension restored anterior laxity at 30° (p = 0.224) and at 90° (p = 0.668). There were very strong correlations between post-operative and control extension range (r = 0.931, p < 0.0001) and anterior laxity and 30° (r = 0.830, p < 0.0001) measures. Constraint of tibial internal rotation increased by 2.9° during the pivot-shift (p < 0.001) and increased with pivot shift grade (r = 0.474, p = 0.040). CONCLUSION: Fixing rigid anatomic hamstring grafts in a knee flexed position routinely produces a flexion deformity. Tensioning and fixing grafts with the knee fully extended restores full extension and anterior laxity at 30° and 90°. Rotational constraint is significantly improved and correlates with the pivot-shift grade. CLINICAL RELEVANCE: Rigid anatomic grafts should be tensioned and fixed with the knee fully extended.