The subacromial space is important for shoulder joint function. It is confined by the acromial arch superiorly and by the humeral articular surface and greater tuberosity inferiorly. When the subacromial space is narrowed, the distance between the inferior border of the acromion and the proximal humerus, defined as the acromiohumeral distance (AHD), is shortened. The AHD has been reported to be a quantitative measurement that can be used to assess changes in the height of the subacromial space. Narrowing of the subacromial space has been linked to several shoulder pathologies, including subacromial impingement syndrome and rotator cuff disease. Abduction in the scapular plane and flexion of the shoulder joint have been reported to reduce the AHD, possibly resulting in subacromial impingement. Several anatomic variations of the acromial arch have been associated with impingement and rotator cuff disease. These include a larger lateral extension of the acromion and hooked morphology (type III) of the anterior acromion. Clinical examination of the subacromial space is important for the diagnosis of subacromial space pathologies and produces pain during forced upward elevation against the acromion. However, the structures being impinged, assuming this is causing the pain, as a function of arm elevation angle during clinical testing have not been well defined.
To better understand and treat shoulder pathology, a precise measurement of shoulder joint kinematics is required. The AHD is certainly influenced by arm position as the various structures, such as the supraspinatus tendon and footprint, rotate beneath the acromial arch. In addition to understanding how arm position affects AHD, normative values are necessary to serve as a reference when clinicians are trying to restore normal kinematics in patients with shoulder pathology, such as rotator cuff repair in patients with rotator cuff tears and subsequent superior migration of the humeral head.
Accurate and precise measurement of glenohumeral kinematics is difficult because the bones of the shoulder complex, especially the scapula, move significantly underneath the skin. Therefore, skin-based measurement techniques cannot measure shoulder joint motion with sufficient accuracy to measure AHD. A highly accurate, emerging tool to measure the in vivo 3-dimensional (3D) kinematics of a joint is biplane fluoroscopy, which eliminates this skin motion artifact by imaging the bones directly. The most accurate measurement method is roentgen stereophotogrammetry analysis, which requires the insertion of small (approximately 1.6-mm) markers inside the bone. Reported accuracies of better than 0.1 mm are possible, which make it a suitable reference method for the validation of less invasive techniques. Model- based techniques, which use bone models reconstructed from computed tomography (CT) or magnetic resonance imaging (MRI), have also been reported to have submillimeter accuracy in measurements of joint kinematics relative to marker-based techniques.
The purpose of this study was to measure the AHD in vivo during abduction in the scapular plane (scaption) and forward elevation in the sagittal plane to study how arm elevation influenced AHD by use of a biplane fluoroscopy system. Our hypothesis was that the AHD would narrow with increasing arm elevation. In addition, we hypothesized that the location of the minimum distance point on the proximal humerus from which the AHD would be measured would vary with arm position.