# Shoulder joint angles in supine and upright imaging of the preoperative reverse total shoulder arthroplasty patient

## TL;DR

This study shows that shoulder joint angles differ significantly between supine and upright positions, which could affect preoperative planning for shoulder surgery.

## Contribution

The study quantifies patient-specific variations in joint angles between supine and upright imaging, highlighting implications for preoperative planning.

## Key findings

- The scapula was more downward rotated in upright pose compared to supine (9.3° ± 12.9°, P = .004).
- The humerus was more elevated in upright pose compared to supine (13.2° ± 17.6°, P = .020).
- Patient-specific variations in joint angles exceeded common thresholds for implant selection and placement.

## Abstract

Reverse total shoulder arthroplasty is a common procedure for end-stage glenohumeral arthritis, rotator cuff disease, fracture, or failed arthroplasty. Preoperative planning software allows surgeons to assess implant placement and relies on imported bone models that come from imaging, typically a supine computed tomography (CT) scan. However, daily activities are performed upright. This study quantified the differences in scapulothoracic and glenohumeral joint angles between preoperative supine and upright imaging.

Seven patients underwent preoperative supine CT and upright biplane fluoroscopy imaging. Scapulothoracic and glenohumeral joint angles in supine and upright poses were calculated using Euler angles and compared using 2-sided paired t-tests.

The scapula was 9.3 ± 12.9° more downward rotated in the upright pose than supine (P = .004), while the humerus was 13.2 ± 17.6° more elevated (P = .020). Patient-specific changes varied widely for scapulothoracic upward/downward rotation (+0.5° to −17.6°), pro/retraction (+10.4° to −22.4°), and posterior/anterior tilt (+17.4° to −9.3°), as well as glenohumeral elevation/depression (+26.7° to −4.0°), anterior/posterior plane of elevation (+19.3° to −5.6°), and internal/external axial rotation (+5.5° to −44.6°).

Patient-specific variation in joint angles between supine and upright poses demonstrates gross changes in both magnitude and direction that exceed common thresholds in implant selection and placement. Thus, preoperative planning using supine CT may inaccurately pose bones, with consequent effects on the surgical plan, the resultant shoulder biomechanics, and clinical outcomes. Preoperative planning software should consider the influence of supine posture when orienting bones for surgical planning.