Paper 29: The Contribution of Posteroinferior Capsule Tightness to Subacromial Impingement During Pitching Motion

Abstract

Loss of internal rotation as well as excessive external rotation has been recognized in baseball pitchers. Tightness of the posteroinferior capsule including the posterior band of the inferior glenohumeral ligament is assumed to be the cause of this loss (Burkhart, 2003Burkhart S.S. et al.Arthroscopy. 2003; 19: 404-420Abstract Full Text Full Text PDF PubMed Scopus (855) Google Scholar). Previously, the posterior capsule tightness was speculated to lead to subacromial impingement during shoulder flexion and horizontal adduction (Harryman, 1990Harryman D.T. et al.J Bone Joint Surg. 1990; : 1334-1343PubMed Google Scholar). Although intra-articular (internal) impingement during the pitching motion is well accepted, the relationship between the posterior capsule tightness and subacromial (external) impingement is unclear. The purpose of this study was to determine the effect of posteroinferior capsule tightness on contact pressure and area of the coracoacromial arch and humeral head during the pitching motion. Seven fresh cadaveric glenohumeral joints were harvested and used for the study. Specimens with rotator cuff tears or radiographic evidence of acromial spurs and glenohumeral osteoarthritis were excluded. All tissues except the rotator cuff muscles and coracoacromial ligament were removed. The specimen was secured in a custom-designed shoulder experimental device which allows 6 degree-of-freedom motion of the glenohumeral joint. Flexible tactile force sensors (K-SCAN model 4000; Tekscan Inc, South Boston, Mass) were utilized to measure the contact pressure and area on the undersurface of the acromion and coracoacromial ligament. Simultaneously, an electromagnetic tracking device (FASTRAK, Colchester, VT) and motion capture software (MotionMonitor, Innovative Sports Training, Chicago, IL) were used to digitize and animate the surfaces of the scapula and humeral head and observe the contact area on them from various angles. The posteroinferior capsule tightness was simulated by clamping the capsule in the region from 6 to 8 o'clock. The static testing positions in the study correlated to the early cocking, late cocking, acceleration, deceleration, and follow-through phases of the pitching motion. In each phase, the forces applied through the rotator cuff muscles were based on the muscle's cross-sectional areas and muscle activations during pitching. A paired t-test was used to compare all the parameters between the intact and posterior capsule tightness conditions. In the posteroinferior capsule tightness condition, the peak contact pressure during the deceleration (0.58±0.26MPa) and follow-through (0.85±0.43MPa) phases were almost twice that of the intact condition (0.30±0.26 and 0.37±0.21MPa, respectively) (p<0.05), whereas there was no difference in the other phases. After clamping the posteroinferior capsule, the contact area on the coracoacromial arch shifted posteriorly and was located on the undersurface of the acromion from early cocking to the acceleration phases, however, the area shifted anteriorly and was located underneath the coracoacromial ligament during the deceleration and follow-through phases. In almost all specimens, the contact area on the humeral head was at the greater tuberosity during each phase, but, in a specimen, lesser tuberosity during the follow-throw phase. Unlike the coracoacromial arch, the contact areas did not shift after clamping. Previously, posteroinferior capsule tightness has been recognized as a factor which causes SLAP lesion in the late cocking phase (Burkhart, 2003Burkhart S.S. et al.Arthroscopy. 2003; 19: 404-420Abstract Full Text Full Text PDF PubMed Scopus (855) Google Scholar) and subacromial impingement during the elevation motion (Meister, 2000Meister K. et al.Am J Sports Med. 2000; 28: 265-275PubMed Google Scholar). However, our findings demonstrate an increase in contact pressure on the subacromial arch during the deceleration and follow-through phases. An anterior shift of the contact area and high contact pressure on the coracoacromial arch during the follow-through phases could be explained by anterior translation of the humeral head with horizontal adduction due to posterior capsule tightness (Harryman, 1990Harryman D.T. et al.J Bone Joint Surg. 1990; : 1334-1343PubMed Google Scholar). Although internal impingement during the late cocking phase has been recognized to be a cause of injury of the rotator cuff and its surrounding tissues, posteroinferior capsule tightness could also lead to higher pressure on the coracoacromial arch and increase risk of injury.