A novel MRI measurement for analysing coracoacromial morphology.

Abstract

Aims: Extrinsic factors of subacromial impingement of the shoulder can be potentially characterised by the angle between the acromion and coracoacromial ligament.  A retrospective study was designed to explore it.

Patients and Methods: The landmarks and outcome measures were determined on sagittal cuts of default 1.5 T MRI scans.  Two independent observers made the measurements on 50 annotated images of a cohort of healthy and young individuals.

Results: The mean acromial-coracoacromial ligament (ACAL) angle was 129.8 degrees, (SD 10.0).  The two independent and the repeated measurements were compared for testing inter- and intra-observer reliability.  Measurement of ACAL showed high internal consistency and strong measurement agreement between observers (Cronbach’s alpha values: 0.88, 0.86 and 0.94; Intraclass correlation coefficient values: 0.78, 0.75, 0.88).  However, a statistically significant difference (p=0.035) was found on the retest measurement (1^st to 2^nd) of the same observer, despite the little difference (1^st to 3^rd p=0.78; 2^nd to 3^rd p=0.75) in inter-observer relation on paired sample t-test.

Conclusions: The measuring error came from poor definition of the landmarks on standard 3 mm slice thickness MRI scans.  Altered scanning sequences with thinner slices might provide more details of the structures, improve reliability, and prove to be a clinically useful measure in a future prospective study.