Total shoulder arthroplasty preoperative planning: the effect of patient's humeral position on the lateralization and distalization measurements.

backgroundThe aim of this study was to define the variability of the scapulohumeral position during preoperative Computed Tomography (CT) acquisition and to evaluate its influence on angular lateralization and distalization measurements. We hypothesized that the preoperative resting arm position, in terms of humeral abduction, flexion and internal rotation, would vary significantly between patients and that this variability would influence the lateralization (LSA) and distalization shoulder angle (DSA).

methodsWe analyzed a prospectively collected database of preoperative de-identified CT scans from a single Total Shoulder Arthroplasty (TSA) planning system (Equinoxe, Exactech GPS, Blue-Ortho), including all cases with scapular and humeral segmentation. Validated three-dimensional (3D) bone models were used to compute and automatically position scapular and humeral landmarks. These included: the superior glenoid tubercle; the most lateral border of the acromion and the most lateral border of the greater tuberosity. The position of the humerus relative to the scapula was automatically calculated, allowing the angles of abduction, flexion and internal rotation of the scapulohumeral joint to be assessed. Additionally, the potential relationship between the body mass index (BMI) and the resting arm position was assessed. Finally, LSA and DSA were calculated. A multiple linear regression analysis was performed to assess the relationship between the humeral position and the LSA and DSA.

resultsA total of 21,863 patients were included. Preoperative humeral positioning relative to the scapula showed a mean abduction of 10.3°±12.4 (-14.0°; 36.6°), mean flexion of 3.9°±8.9 (-16.0°; 26.1°) and mean internal rotation of 6.5°±18.9 (-41.4°; 48.9°). The preoperative median of LSA and DSA were 87.4°±14.3° and 43°±12.4, respectively. Among the independent variables, abduction showed the strongest negative correlation with LSA (β = -0.2998, p < 0.0001), followed by flexion (β = -0.04342, p < 0.0001). Internal rotation was positively correlated with LSA (β = 0.1229, p < 0.0001). For DSA, abduction had a weak positive influence (β = 0.04321, p < 0.0001), while flexion (β = -0.04302, p < 0.0001) and internal rotation (β = -0.04654, p < 0.0001) were negatively associated. Notably, a 10° variation in abduction, flexion or internal rotation led to a -3°, -0.4° and + 1.2° change in LSA, respectively, whereas DSA was minimally affected, with variations limited to + 0.4°, -0.4° and - 0.5°, respectively.

conclusionThe resting arm position during preoperative CT scans varies significantly, potentially affecting the preoperative planning of TSA. The main findings of this study suggest that there exists a weak correlation between the initial scapulohumeral position and the LSA/DSA measurements. LEVEL OF EVIDENCE: Level IV. Case series with no comparison group.