Practical considerations for determination of scapular internal rotation and its value in reverse total shoulder arthroplasty

Abstract

Abstract Background Scapulothoracic orientation, especially scapular internal rotation (SIR) may influence range of motion in reverse total shoulder arthroplasty (RTSA) and is subjected to body posture. Clinical measurements of SIR rely on apical bony landmarks which are subjected to changes in scapulothoracic orientation, while radiographic measurements are often limited by the restricted field of view (FOV) in CT scans. Therefore, the goal of this study was (1) to determine whether the use of CT scans with a limited FOV to measure SIR is reliable and (2) if a clinical measurement could be a valuable alternative. Methods This anatomical study analyzed the whole-body CT-scans of 100 shoulders in 50 patients (32 male and 18 female) with a mean age of 61.2 ± 20.1 years (range 18; 91). (1) CT-scans were rendered into 3D models and SIR was determined as previously described. Results were compared to measurements taken in 2D CT-scans with a limited FOV. (2) Three apical bony landmarks were defined: (the angulus acromii (AA), the midpoint between the AA and the coracoid process tip (C) and the acromioclavicular (AC) joint. The scapular axis was determined connecting the trigonum scapulae with these landmarks and referenced to the glenoid center. The measurements were repeated with 0°, 10°, 20°, 30° and 40° anterior scapular tilt. Results Mean SIR was 44.8° ± 5.9° and 45.6° ± 6.6° in the 3D and 2D model, respectively (p < 0.371). Mean difference between the measurements was 0.8°± 2.5° with a maximum of 10.5°. A Bland-Altman-Plot showed 5% outliers. Midpoint AA/C showed no significant difference to the scapular axis at 0° (p = 0.203) as did the AC joint at 10° anterior scapular tilt (p = 0.949). All other points showed a significant difference from the scapular axis at all degrees of tilt. Conclusion 2D CT-scans are reliable to determine SIR, even if the spine is not depicted. Position of the patient in the CT scanner needs to be considered and supine positioning presents a limitation. Clinical measurements using apical superficial scapula landmarks are a possible alternative, however anterior tilt influenced by posture alters measured SIR.