Poster 245: Supra-Tubercle Angle May Provide Better Assessment of Proximal Tibia Deformity and Predict Risk of ACL Graft Failure
Authors: Helm James M. , Mansour Alfred A. , Gattu Nikhil , Alford Travis , Anz Hayden , Aboulafia Alexis H. , Siahaan Jacob , Lowe Walter R. 
Published: August 2024

Objectives:

Posterior tibial slope (PTS) has been validated across the literature to assess proximal tibial deformity and has been demonstrated to impact anterior cruciate ligament (ACL) failure. Due to the lack of consensus regarding the most accurate method of PTS measurement, attention has recently shifted to the proximal tibia superior to the tibial tubercle as a potential source of sagittal deformity. We recently proposed novel measurements of the proximal tibia including the lateral supra-tubercle angle (LSTA) and lateral supra-tubercle distance (LSTD). Our initial study established a normative range for these values and demonstrated increased values in ACL deficient knees. The aim of this study is to build on the prior study by reporting these values in ACL revision knees in which original grafts have failed. We seek to further explore the relationship between these novel measurements and the risk of ACL graft failure, and how they relate to PTS. Study in this area has the capacity to better predict risk of ACL failure, as well as to impact preoperative planning and osteotomy selection in this patient population.

Methods:

The original study population included a control group of 65 knees with patent native ACLs, as well as 35 knees with MRI-confirmed primary ACL tears. We additionally included 50 patients with MRI-confirmed ACL graft failures presenting for revision ACL reconstruction. Inclusion criteria consisted of patients who were skeletally mature, were absent of osteoarthritis, had no lower-extremity fractures, and had a lateral radiograph with a posterior femoral condylar overlap of <5 mm. Lateral radiographs were reviewed by 2 independent observers. For each x-ray, LSTA, LSTD, and PTS were all measured for both the medial and lateral tibial plateaus. Mean values were calculated for all measurements, including 1 standard deviation from the mean. To establish reliability across measurements, a 2-way mixed effects model was used to provide an interclass correlation coefficient (ICC). Measurements were also analyzed and compared between knees with PTS <12° and those with PTS ³ >12°.

Results:

The average LTSA-medial (LSTA-M) in the ACL revision cohort was 16.3 ± 6.3, compared to the values of 14.2 ± 4 and 12.7 ± 4.2 in the primary ACL and control group respectively from the initial study (p < 0.001). The average LTSA-lateral (LSTA-L) in the ACL revision cohort was 17.3 ± 6.6 compared to 14.9 ± 4 and 13.6 ± 4 in the primary ACL and control groups respectively from the initial study (p < 0.001). The average LTSD-medial (LSTD-M) and LTSD-lateral (LSTD-L) in the ACL revision cohort was 28.5 ± 6.4 and 32.9 ± 6.7 respectively. These values were not significantly different from those in the primary ACL or control cohorts. The average PTS-medial (PTS-M) and PTS-lateral (PTS-L) in the ACL revision cohort was 8.5 ± 3 and 10 ± 3.3 respectively. These values were also not significantly different from the initial primary ACL and control cohorts. When comparing all 3 cohorts, LSTD-M and LSTA-M showed the greatest reliability and similarity between observers with independent intraclass correlation coefficients (ICCs) of 0.72 and 0.58, and average ICCs of 0.83 and 0.73 respectively. LSTD-L, LSTA-L, PTS-M, and PTS-L all showed absolute agreement between both observers. When comparing all 3 cohorts, LSTA-M, LSTD-M, and LSTA-L were each significantly higher in knees with PTS ≥12 than knees with PTS <12 (16.2 ± 4.4 vs 13.8 ± 5.2, p < 0.001; 30.7 ± 4.4 vs 27.7 ± 5.6, p < 0.001; 16.6 ± 5 vs 14.6 ± 5.3, p < 0.001).

Conclusions:

The novel supra-tubercle angle measurement was significantly greater in patients with ACL graft failures while PTS remained the same. This suggests that supra-tubercle angle may be a more sensitive measurement for predicting ACL graft failure than PTS. Elevated supra-tubercle angles also appear to correlate with increased PTS. With the high variability in PTS measurement methodology, we propose the use of the novel supra-tubercle angle to focus deformity evaluation on the proximal tibia superior to the tubercle and demonstrate its efficacy in predicting ACL graft failure. Future study should continue to validate this measurement and evaluate its relationship with ACL failure and PTS.