Summary

In this in-vivo biomechanics analysis of posterior tibial slope, an anterior tibial closing wedge osteotomy result in an anterior translation in the weighted center of contact in both the medial and lateral compartments.

Abstract

Objectives: Increased posterior tibial slope (PTS) is a well-established risk factor for graft failure after anterior cruciate ligament (ACL) reconstruction and primary ACL insufficiency. Biomechanics studies have demonstrated that decreased PTS decreases force across the ACL in reconstructed knees and decreases anterior translation of the tibia relative to steeper PTS. No studies, however, have assessed the consequences of a tibial ACWO on tibiofemoral joint contact forces. The objective of this pilot study is to quantify the effects of high and low PTS on joint biomechanics across the tibial plateau of human knees during simulated standing. We hypothesize that a lower PTS following tibial ACWO will result in anterior translation of the weighted center of contact (WCoC) and increase peak contact stresses.

Methods

Three knees were denuded of skin, subcutaneous fat, muscle, and patella, while preserving the cruciate ligaments, collateral ligaments, and capsule. Knees were cemented into place using a custom fixture and mounted on a robotic test system [VIVO, AMTI] programmed to apply forces and moments to mimic level standing. An electronic, intra-articular pressure sensor [Model 4011, Tekscan] was calibrated and attached across the tibial plateau of each knee. Osteotomy procedures were performed using a standard supra-tubercle approach. Specimen specific half bodyweight was applied to each knee for each of the following conditions: (1) native knee, (2) AOWO – ‘increased PTS’ condition, and (3) ACWO – ‘decreased PTS’ condition. The AOWO was performed with a 5-degree polymethyl methacrylate (PMMA) cement wedge fixed with bone staples. Similarly, a 5-degree wedge of bone was removed for the ACWO and secured using bone staples. WCoC, peak contact stress, and contact area were recorded for the medial and lateral compartments.

Results

AOWO resulted in posterior translation of the WCoC for both compartments, whereas an ACWO resulted in an anterior shift in the WCoC. The average posterior translation in the WCoC after AOWO was 2.16mm in the medial compartment and 1.89mm in the lateral compartment. Average anterior translation in the WCoC after ACWO was 4.36mm in the medial compartment and 2.84mm in the lateral compartment. Average peak contact stress decreased after ACWO in both the medial and lateral compartments and increased after ACWO for one specimen in the lateral compartment. Peak contact stress decreased marginally in the medial compartment following AOWO, whereas peak contact stresses increased uniformly in the lateral compartment following AOWO.

Conclusions

The findings of this pilot study support the hypothesis that there is an anterior translation in WCoC following ACWO in both medial and lateral compartments. Peak contact stress decreases marginally following ACWO, however, preliminary results are mixed and require further investigation. The overall high degree of variability of the results, particularly with regards to changes in WCoC in the setting of increased PTS, will expectantly be reduced with further testing and increased statistical power. In the setting of ACL insufficiency and the associated risk for post-traumatic osteoarthritis (PTOA), the findings of this study may improve our collective understanding of additional risk or mitigation of risk related to PTOA as a result of ACWO.