Determination of pre-arthropathy scapular anatomy with a statistical shape model - Part I: Rotator Cuff Tear arthropathy.

Abstract Hypothesis and Background Rotator Cuff Tear Arthropathy (RCTA) is a pathology characterized by a massive rotator cuff tear combined with acromiohumeral and/or glenohumeral arthritis. Severity of RCTA can be staged according to Hamada. It is unknown why some patients develop RCTA. Furthermore, RCTA patients can develop distinctly different articular damage patterns on the glenoid side as categorized by Sirveaux (glenoid erosion). The goal of this study was to determine if there exists an association between scapular anatomy and RCTA, different severity stages of RCTA and their associated glenoid erosion types. Methodology A statistical shape model (SSM) of the scapula was constructed from a dataset of 110 CT scans using principal component analysis. Sixty-six patients with degenerative rotator cuff pathology formed the control group. The CT scan images of eighty-nine patients with RCTA were included and grouped according to Hamada and Sirveaux. A complete 3D scapular bone model was created and SSM reconstruction was performed. Next, automated 3D measurements were performed for glenoid version and inclination, scapular offset, critical shoulder angle (CSA), posterior acromial slope (PAS) and lateral acromion angle (LAA). All measurements were then compared between controls and RCTA patients. Results The control group had a median of 7° retroversion (variance 16°), 8° superior inclination (variance 19°) and 106mm scapular offset (variance of 58mm). The median CSA, PAS and LAA was 30° (variance 14°), 65° (variance 60°) and 90° (variance 17°) respectively. In terms of inclination, version, scapular offset and PAS we found no statistically significant difference between the RCTA and the control group. For RCTA patients, median CSA and LAA was 32° (p≤ 0.01) and 86° (p≤ 0.01) respectively. For all investigated parameters, we did not find any significant difference between the different stages of RCTA. Patients with an E3 type of erosion had a different pre-arthropathy anatomy with increased retroversion (12°, p= 0.006), CSA (40°, p≤ 0.001), and reduced LAA of 79° (p≤ 0.001). Discussion Our results seems to indicate that a 4° more inferiorly tilted and 2° more lateral extended acromion is associated with RCTA. RCTA patients who develop an E3 type of erosion have a distinct pre-arthropathy scapular anatomy with a more laterally extended and more inferiorly tilted acromion and a more retroverted glenoid in comparison with RCTA patients with no erosion. There does not seem to be a different pre-arthropathy scapular anatomy between patients with different stages of RCTA.