Abstract
Objectives:
It is well established that the posterior tibial slope has been identified as a risk factor for ACL ruptures. Slope correcting osteotomies have become an important aspect of the orthopedic surgeon’s armamentarium when faced with revision ACL reconstruction (ACLR) in the setting of increased tibial slope. However, these current procedures produce the same amount of slope correction for the medial and lateral tibial slope and do not account for difference in tibial slope between the compartments. These differences have been shown to affect both risk factors for ACL tears with concomitant meniscal pathology, poorer clinical outcomes of ACLRs with medial or lateral meniscal repairs, and risk of the pivot shift phenomena (anterior tibial translation and internal knee rotation) after ACLR. It is unclear, however, how changes to the tibial slope and the asymmetry between medial and lateral tibial slopes affects ACL graft force, anterior tibial translation, or knee rotation. The aim of this study is to determine how the difference in medial and lateral tibial slope affect anterior tibial translation, knee rotation, and ACL graft force in the setting of an ACL reconstruction. We hypothesize that greater asymmetry between the medial and lateral slopes will result in greater tibial translation, knee rotation, and ACL graft forces.
Methods:
Twelve fresh frozen human cadaveric knees were acquired, dissected, and imaged via a computed tomography (CT) scanning. Native medial and lateral tibial slopes were measured from the CT scans by an orthopaedic surgeon. Anterior-wedge, slope-reducing osteotomies were performed on the medial and lateral compartments independently utilizing cuts similar to those used for unicompartmental knee arthroplasty such that the slope could be reduced to zero then increased incrementally back to the native slope. A custom external fixture was installed to stabilize the osteotomy, and a soft-tissue ACLR was performed with the proximal end of the graft secured via an interference screw. Custom 3D printed wedges were created to stabilize the osteotomies such that six testing states could be tested (Table 1). Four specimens underwent biomechanical testing at time of submission. Each specimen was first tested with both the medial and lateral compartments in their native slopes, then each testing state was tested in random order (Table 1). Each specimen was mounted in full extension to an axial-torsion testing machine via a custom jig that permitted anterior-posterior tibial translation, and the distal end of the ACLR graft was secured to a force transducer. A 500 N compressive load and 0-N torque was applied to the specimen. Tibial translation was measured using a linear displacement sensor and knee rotation was measured with the testing machine for all testing states. ACL graft forces were measured via the force transducer clamped to the graft. Differences in these outcomes between the native state and the testing states were calculated.
Results:
Graphical displays of the means and standard deviations of the outcomes for the four specimens can be found in Figure 1. Testing states where the medial compartment was greater than the lateral compartment (Testing states 1, 5, and 6) experienced greater differences in knee rotation, anterior tibial translation, and ACL graft force compared to the native state. These effects appear to be exacerbated when the medial-lateral asymmetry is greatest (Testing States 1 and 6). Testing states where the lateral compartment was greater than the medial compartment (Testing states 2, 3, and 4) experienced lesser differences in knee rotation, anterior tibial translation, and ACL graft force compared to the native state.
Conclusions:
This study revealed that a greater tibial slope of the medial compartment compared to the lateral compartment may affect knee rotation, anterior tibial translation, and ACL graft force, and that these effects may be exacerbated as the medial-lateral tibial slope asymmetry increases. This biomechanical evidence supports clinical outcomes which report that increased medial-lateral tibial slope asymmetry increases the likelihood of ACL graft failure. This study supports the need to develop a new technique that would allow for independent medial and lateral slope reducing osteotomies. These results indicate that independently reducing the tibial slope of the medial compartment may reduce the likelihood of ACL graft force failure.
