Slope-reducing tibial osteotomy decreases ACL-graft forces and anterior tibial translation under axial load

Purpose Posterior tibial slope (PTS) represents an important risk factor for anterior cruciate ligament (ACL) graft failure, as seen in clinical studies. An anterior closing wedge osteotomy for slope reduction was performed to investigate the effect on ACL-graft forces and femoro-tibial kinematics in an ACL-deficient and ACL-reconstructed knee in a biomechanical setup. Methods Ten cadaveric knees with a relatively high native slope (mean +/- SD): (slope 10 degrees +/- 1.4 degrees, age 48.2 years +/- 5.8) were selected based on prior CT measurements. A 10 degrees anterior closing-wedge osteotomy was fixed with an external fixator in the ACL-deficient and ACL-reconstructed knee (quadruple Semi-T/Gracilis-allograft). Each condition was randomly tested with both the native tibial slope and the post-osteotomy reduced slope. Axial loads (200 N, 400 N), anterior tibial draw (134 N), and combined loads were applied to the tibia while mounted on a free moving and rotating X-Y table. Throughout testing, 3D motion tracking captured anterior tibial translation (ATT) and internal tibial rotation (ITR). Change of forces on the reconstructed ACL-graft (via an attached load-cell) were recorded, as well. Results ATT was significantly decreased after slope reduction in the ACL-deficient knee by 4.3 mm +/- 3.6 (p < 0.001) at 200 N and 6.2 mm +/- 4.3 (p < 0.001) at 400N of axial load. An increase of ITR of 2.3 degrees +/- 2.8 (p < 0.001) at 200 N and by 4.0 degrees +/- 4.1 (p < 0.001) at 400 N was observed after the osteotomy. In the ACL-reconstructed knee, ACL-graft forces decreased after slope reduction osteotomy by a mean of 14.7 N +/- 9.8 (p < 0.001) at 200 N and 33.8 N +/- 16.3 (p < 0.001) at 400N axial load, which equaled a relative decrease by a mean of 17.0% (SD +/- 9.8%), and 33.1% (SD +/- 18.1%), respectively. ATT and ITR were not significantly changed in the ACL-reconstructed knee. Testing of a tibial anterior drawing force in the ACL-deficient knee led to a significantly increased ATT by 2.7 mm +/- 3.6 (p < 0.001) after the osteotomy. The ACL-reconstructed knee did not show a significant change (n.s.) in ATT after the osteotomy. However, ACL-graft forces detected a significant increase by 13.0 N +/- 8.3 (p < 0.001) after the osteotomy with a tibial anterior drawer force, whereas the additional axial loading reduced this difference due to the osteotomy (5.3 N +/- 12.6 (n.s.)). Conclusions Slope-reducing osteotomy decreased anterior tibial translation in the ACL-deficient and ACL-reconstructed knee under axial load, while internal rotation of the tibia increased in the ACL-deficient status after osteotomy. Especially in ACL revision surgery, the osteotomy protects the reconstructed ACL with significantly lower forces on the graft under axial load.