MR study of the intrinsic acromial angle in 74 symptomatic patients

The study was undertaken to define some measurements on sagittal MR images that could provide quantitative data on the subacromial space in patients affected by nonspecific impingement syndrome and to correlate these measurements with the Bigliani classification of acromial morphology.

We retrospectively reviewed 74 shoulders in 74 patients (47 males, 27 females, age range 35-62 years) who presented with mechanical disability and pain. The patients underwent MRI using T1-weighted sagittal oblique and coronal SE sequences (TR 500 ms ,TE 12 ms) and T2-weighted FSE sequences (TR 3500, TE 25 ms) with dedicated coil. Two parameters were evaluated: the intrinsec acromial angle and the acromio-humeral distance. The measurements were made on the sagittal images at the acromion-clavicular junction and correlated to the Bigliani classification of acromial morphology. The images were evaluated by two radiologists blinded to the patients' identity, history and arthroscopic results.

The acromial angle was measured in 72 shoulders. The values were found to correlate with the Bigliani classification. In our series, the intrinsic acromial angle (resulting from the intersection between the longitudinal acromial axis and the tangent to the acromial undersurface) ranged from 0 degrees to 17 degrees. In shoulders with rotator cuff tears the acromial angle varied between 8 degrees and 17 degrees. There were no tears for acromial angles below 8 degrees, whereas in patients with full-thickness tears the angles ranged from a minimum of 12 degrees to a maximum of 15 degrees. Acromio-humeral distance ranged from 2 mm to 10 mm; in patients with cuff tears it could not be assessed as it was close to zero in nearly all cases, whereas in cases of impingement without cuff tear it ranged from 5 mm to 10 mm. Impingement without cuff tear was detected in 31 cases, with angles ranging from 10 degrees to 17 degrees.

Our findings demonstrate the effectiveness of the two quantitative parameters (intrinsic acromial angle and acromio-humeral distance) which are important causative factors in impingement syndrome. These parameters were found to be consistent and comparable with the Bigliani classification of acromial morphology which, although providing useful qualitative data, does not fully respond to our quantitative needs. Our study suggests that impingement syndrome is negatively correlated to acromio-humeral distance and positively correlated to intrinsic acromial angle. These parameters, which are readily detected with MRI, may be used by the radiologist and orthopaedic surgeon to screen patients with nonspecific impingement syndrome.