Poster 298: Establishment of a Novel Distance and Angle and Determining Their Relationship with the ACL

OBJECTIVES: Sagittal plane tibial deformity has been implicated in anterior cruciate ligament ruptures, as most commonly quantified by measurement of the posterior tibial slope (PTS) using lateral knee radiographs as well as full-length lateral tibia radiographs. We propose the measurement of a novel distance between the superior aspect of the tibial tubercle and the tibial plateau (Lateral SupraTubercle Distance (LSTD)) on the lateral knee radiographs. This distance seems to have large variability based on tibial anatomy and no prior correlation has been reported in an ACL or non-ACL cohort. It is possible that this distance may be related to the PTS and supratubercle variances could account for the tibial deformity causing increased PTS. Therefore, this could be used to predict the risk of ACL reconstruction failure. This distance also has implications for tibial osteotomy technique: if this distance is too small, supra-tubercle posterior slope-reducing osteotomy may not be feasible. We also propose the measurement of a novel angle (Lateral SupraTubercle Angle (LSTA)) on the lateral knee radiographs. It is possible that this angle may be related to the PTS and could be used to predict the risk of ACL reconstruction failure as well. Our primary objective was to determine normative data for the LSTD as measured as the distance between the superior aspect of the tibial tubercle and the tibial plateau on a lateral knee radiograph, as well as the LSTA as measured as the angle formed between a line drawn from the distal aspect of the posterior tibial flare of the PCL insertion to the proximal point of the tibial tubercle and a line drawn parallel to the tibial plateau. Our secondary objective was to evaluate the relationship between the LSTD and/or LSTA and PTS on a lateral knee radiograph. Ultimately, we sought to evaluate whether LSTD and/or LSTA may be another tool that could be used pre-operatively to determine the risk of rupture after ACL reconstruction. The LSTD can also be used to help plan supra-tubercle versus infra-tubercle tibial osteotomy during slope-correcting surgery. METHODS: This was a retrospective cohort study using 2 disparate cohorts of skeletally mature patients. Other criteria included absence of osteoarthritis on radiographs and no history of prior knee surgery. Radiographic requirements included xrays with a centered knee and posterior femoral condyle overlap of less than 5 mm as measured between the medial and lateral femoral condyles. The first cohort included patients with a knee complaint not related to ligament injury who underwent lateral knee radiographs. This cohort was used to establish a normal value for the LSTD and LSTA. Measurements were taken using both the medial and lateral plateaus. The LSTD was defined as the distance along the anterior tibial cortex between a point at the superior aspect of the tibial tubercle and a point tangential to a line drawn parallel to the tibial plateau. The LSTA was defined as the angle between a line drawn from the superior tibial tubercle to the posterior tibial flare of the PCL insertion and a line drawn parallel to the tibial plateau. The second cohort included patients with an ACL tear proven on MRI who underwent lateral knee radiographs. This cohort was used to compare the LSTD and LSTA to the previously established normal value or range of values and establish an association, if any, between the LSTD or LSTA and ACL tear. RESULTS: Measurements were obtained on 100 lateral knee xrays that met inclusion criteria. This included 65 xrays in the control cohort and 35 xrays in the ACL cohort. The mean measurements for LSTA-medial and LSTA-lateral in the control group were 12.2 ± 3.3 and 13.12 ± 3.75, respectively. The mean for PTS-medial and PTS-lateral in the cohort group were 8.14 ± 3.57 and 9.1 ± 3.0, respectively. In the ACL group, the mean measurements for LSTA-medial and LSTA-lateral were 13.7 ± 4.3 and 14.37 ± 4.04, respectively. The means for PTS-medial and PTS-lateral in the ACL group were 8.77 ± 3.74 and 9.23 ± 3.40, respectively. The mean measurements for LSTD-medial and LSTD-lateral in the control group were 26.34 ± 4.27 and 30.15 ± 4.23, respectively. In the ACL group, the mean measurements for LSTD- medial and LSTD-lateral were 27.4 ± 4.05 and 30.71 ± 4.1, respectively. Pearson correlation calculations were performed to evaluate each measurement’s ability to predict ACL injury. The Pearson correlations for LSTA (r=0.19) were higher than that of posterior tibial slope (r=0.083). CONCLUSIONS: We have established mean values for LSTA-medial and LSTA-lateral as well as mean values for LSTD-medial and LSTA-lateral. Based on our data, we cannot discern for sure that LSTA or posterior tibial slope had significant correlations with ACL injury. However, according to our correlation calculations, the LSTA has stronger correlations than posterior tibial slope. Thus, LTSA could be a better predictor for ACL injury than posterior tibial slope. Further studies with larger cohorts should be performed to further clarify the implications that LTSA has on risk of ACL injury. This novel measurement could be utilized in pre-operative planning to determine whether high-tibial ostoetomy would be beneficial when performed concurrently with ACL reconstruction. Furthermore, future studies could also be performed to evaluate LTSD as it relates to the choice of supra or infra-tubercle osteotomy.