ANTERIOR-SUPERIOR GLENOHUMERAL ESCAPE: A CADAVERIC ANALYSIS

+*Blum, G T; *Weldon, E J +*University of Hawaii, Honolulu, HI

INTRODUCTION Anterior-superior glenohumeral instability is a condition in which the humeral head attains a position anterior and superior to the glenoid. Although relatively uncommon, this condition can lead to marked dysfunction and often proves difficult to treat. Processes that challenge the structural integrity of the coracoacromial arch have been implicated in the causation of this condition. Acromioplasty, in which part of the anterior acromion and often the coracoacromial ligament is removed, is one process that modifies the arch and has been associated with anterior-superior glenohumeral instability. Despite the popularity of this procedure, how much anterior acromion can safely be removed before the structural integrity of the arch has been compromised is unknown. The purpose of this study is to quantify the minimal amount of anterior acromion that must be removed (along with the coracoacromial ligament) before the humeral head can pass between the remaining acromion and corocoid process. METHODS Eight fresh-frozen cadaveric shoulders with intact rotator cuffs were obtained. Following removal of the skin, subcutaneous tissue, and deltoid muscle, supraspinatus outlet views were obtained. The rotator cuff muscles were then divided transverse to the orientation of their fibers approximately 2 cm from the musculotendinous junction. Each scapula was immobilized in a custom mount, allowing free movement of the humerus. The rotator cuff was reflected, a cable attached to the deltoid insertion on each humerus, and 250N weight was applied without acceleration with the humerus in 20 degrees of abduction and extension. Acromion was removed in 1 mm increments until anterior and superior dislocation of the humeral head occurred. The experiment was then repeated with the rotator cuff in place. Cadaveric and radiographic measurements were made throughout the course of the experiment. Measurements included (Fig 1): Humeral head diameter with and without a rotator cuff, acromial length (curved and straight), and the interosseous length (distance between the posterior aspect of the coracoid and anterior aspect of the acromion).

Fig. 1: Cadaveric and radiographic measurements including interosseous distance (A), curved acromial length (B), straight acromial length (C), and humeral head diameter with (D) and without (E) a rotator cuff.

RESULTS Anterior-superior escape did not occur in any of the specimens with an intact coracoacromial arch. In rotator cuff intact speciments, an average of 15.0 mm (range: 7.7-23.7 mm) of anterior acromion was removed prior to glenohumeral escape when measured cadaverically. By contrast, in rotator cuff deficient specimens, an average of 9.5 mm (range: 4.7-14.0 mm) of anterior acromion was removed prior to escape. The rotator cuff represents an approximately 50mm spacer. Anterior-superior escape occurs when the radiographic interosseus distance increases to an average of 83% of the humeral head diameter. There is a strong correlation between the humeral head diameter and interosseus distance at escape (Fig. 2) measured both radiographically (R2=0.94) and cadaverically (R2=0.91).

Fig. 2: Correlation between interosseous distance and humeral head diameter measured radiographically (left) and cadaverically (right). DISCUSSION Anterior-superior glenohumeral instability is less likely to occur in the setting of an intact rotator cuff and coracoacromial arch. The amount of acromion removed prior to escape in the rotator cuff deficient shoulders was within the described limits of open acromioplasty literature. This study suggests that the mechanism of escape may not be direct passage of the entire humeral head through the arch; instead, a component of combined superior and lateral subluxation of the humeral head around the arch may occur. Simple pre-operative radiographic measurements (i.e. humeral head diameter and interosseous distance on supraspinatus outlet view) may help identify the amount of acromion that can safely be removed. Given the 4-55% rate of subsequent rotator cuff rupture after acromioplasty and 40-90% failure rate of arthroscopic and open repairs of massive rotator cuff tears, routine acromioplasty, especially in the setting of a large rotator cuff tears, needs to be reconsidered. Although the pull of the rotator cuff was not simulated, to our knowledge this is the first study of the mechanical effects of acromioplasty on coracoacromial arch stability. Future direction includes simulation of the rotator cuff, evaluation of different sized tears and acromioplasty techniques, and measurement of glenohumeral contact pressure changes in the setting of acromioplasty. REFERENCES 1. Wiley AM. CORR 263: 135-138, 1991. 2. Neer CS. JBJS 54-A: 41-50, 1972. 3. Lee SB et al. Arthroscopy 17(4): 365-372, 2001. 4. Rockwood CA et al. JBJS 77-A: 857-866, 1995. 5. Sanchez-Sotel J et al. JBJS 83-A: 1814-1822, 2001. AFFILIATED INSTITUTIONS FOR CO-AUTHORS *Straub Clinic and Hospital, Honolulu, HI ACKNOWLEDGEMENT Funding was provided by a research grant from the Straub Foundation.

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Paper No: 194952nd Annual Meeting of the Orthopaedic Research Society