Influence of the shape of the acromion on joint reaction force and humeral head translation during abduction in the scapular plane

The effect of critical shoulder angle on functional compensation in the setting of cuff tear arthropathy

Introduction

Critical shoulder angle (CSA) has been shown to influence rates of rotator cuff tears and glenohumeral arthritis with a larger CSA associated with rotator cuff tears and a smaller CSA associated with glenohumeral arthritis. There has been no study to determine whether such radiographic measurement influences the function of patients with demonstrated cuff tear arthropathy (CTA). The purpose of this study was to examine whether smaller CSAs were associated with greater range of motion (ROM) in patients diagnosed with CTA.

Materials and methods

Ninety-three patients with a diagnosis of CTA with adequate anteroposterior shoulder radiographs were included in the study. Patient demographics were recorded. The presence of a rotator cuff tear was confirmed via advanced imaging or when applicable via the operative report. Patients' ROM was evaluated through the physician's office note. Shoulder radiographs were used to measure CSA, glenoid inclination, acromial index (AI), and acromiohumeral interval. Patient ROM was measured and grouped into 2 different tiered cohorts: cohort 1 had 4 subgroups of forward elevation (FE) (ie, ≤45°, 45°-90°, 91°-135°, and 136°-180°) and cohort 2 had 2 subgroups of FE (ie, ≤90° and >90°). We then analyzed FE between these groups in the context of their radiographic measurements.

Results

The average patient age was 73.8 ± 8.0 years. There was no significant difference in acromiohumeral interval. AI was found to be significantly different between patients presenting with ≤90° in FE compared with those >90° (P = .02). Average CSA was significantly lower in patients with FE greater than 90° at 33.7° ± 3.9° compared with patients with FE less than 90° at 37.1° ± 6.3° (P = .002). There was also a significant difference with regard to CSAs, with those patients with FE ≤ 45° having a mean CSA of 38.2° ± 8.3° compared with those patients with FE ≥ 135° having a mean CSA of 33.3° ± 4.3° (P = .02).

Conclusion

Patients diagnosed with CTA can significantly vary in their shoulder function and ability to forward elevate. Lower CSA was found to be associated with higher FE in patients with CTA preoperatively. In addition, patients with a smaller AI were also found to have better overhead function. Analyzing CSA on plain radiographs may help manage functional expectations in patients with CTA.

Acromion and glenoid shape: Why are they important predictive factors for the future of our shoulders?

The shape of the acromion differs between patients with degenerative rotator cuff tears and individuals without rotator cuff pathology.It can be assessed in the sagittal plane (acromion type, acromion slope) and in the coronal plane (lateral acromion angle, acromion index, critical shoulder angle).The inter-observer reliability is better for the measurements in the coronal plane.A large lateral extension (high acromion index or high critical shoulder angle) and a lateral down-sloping of the acromion (low lateral acromion angle) are associated with full-thickness supraspinatus tears.The significance of glenoid inclination for rotator cuff disease is less clear.The postulated patho-mechanism is the compression of the supraspinatus tendon between the humeral head and the acromion. Bursal side tears might be caused by friction and abrasion of the tendon. Articular side tears could be due to impairment of the gliding mechanism between tendon fibrils leading to local stress concentration. Further research is needed to understand the exact pathomechanism of tendon degeneration and tear. Cite this article: EFORT Open Rev 2017;2. DOI: 10.1302/2058-5241.2.160076. Originally published online at www.efortopenreviews.org.