Measurement of the Balance Stability Angle to Predict the Stability Ratio in Patients With Recurrent Anterior Shoulder Dislocation: A Novel Computed Tomography–Based Protocol

BACKGROUND: The stability ratio (SR) is an important biomechanical parameter for evaluating glenoid stability in patients with recurrent anterior shoulder dislocation (RASD), and it cannot be practically and conveniently measured in clinical scenarios. PURPOSE: To investigate a novel computed tomography (CT)–based protocol to estimate the SR efficiently. STUDY DESIGN: Descriptive laboratory study. METHODS: A total of 102 patients with RASD were included. Demographic information, CT scans, and bone defect area (BDA) were collected. The new protocol, based on balance stability angle (BSA) measurements on CT, was conducted to estimate the SR (SR(CT)) by 2 surgeons independently. Biomechanical testing was then performed on patient-specific 3-dimensional (3D)–printed glenoid models to calculate the SR (SR(3Dprint)), which was used to (1) analyze the reliability of SR(CT) and (2) examine if the BDA could predict SR(3Dprint). To validate whether the 3D-printed glenoid could reflect the actual biomechanical properties of the shoulder, the SR from 5 cadaveric glenoid specimens (SR(cadaver)) was also calculated and compared with that from the 3D-printed glenoid (SR(3Dprint)) under 6 osteotomy conditions. Linear regression and intraclass correlation coefficients (ICCs) were used for statistical analysis. RESULTS: The interrater reliability of SR(CT) measurements was high (ICC = 0.95). SR(CT) was highly correlated with SR(3Dprint) (R (2) = 0.86; ICC = 0.92). The mean BDA was 11.44% ± 6.72% by the linear ratio method, with a weak correlation with SR(3Dprint) (R (2) = 0.31; ICC = -0.46). The cadaveric validation experiment indicated that SR(cadaver) was highly correlated with SR(3Dprint) (R (2) = 0.86; ICC = 0.77). CONCLUSION: Results indicated that (1) the proposed CT-based protocol of obtaining BSA measurements is promising for the SR estimation in patients with RASD, (2) the BDA was not an effective parameter to predict the biomechanical SR, and (3) the 3D-printed glenoid could reflect the biomechanical properties of cadaveric shoulders regarding the SR estimation. CLINICAL RELEVANCE: Traditional BDA measurements cannot accurately reflect the biomechanical stability of the glenoid. The newly proposed CT-based protocol is practical for surgeons to estimate the SR.