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Abstract

Background: Clinical understanding of normal glenohumeral ligament function is largely derived from in-vitro experimental observations, as little in-vivo data on glenohumeral joint kinematics and ligament function exists. Such information is relevant to understanding normal static restraints of the shoulder joint. Objective: To investigate humeral head translation and glenohumeral ligament function during in-vivo shoulder abduction with neutral, external and internal rotations. Study Design: In-vivo study using dual-orthogonal fluoroscopic system and magnetic resonance imaging (MRI). Methods: Five young and healthy living shoulders (2 left, 3 right; average age: 26 years) were investigated. Each subject underwent MRI scanning of the shoulder for the purpose of constructing a 3D anatomic shoulder model. In-vivo kinematics during active abduction from 0° to 90° and then combined with external and internal rotation of the abducted shoulder was captured using a dual-orthogonal fluoroscopy system. The in-vivo shoulder positions were then reproduced by matching the 3D shoulder model to dual-fluoroscopic images placed in a virtual fluoroscopic system built in a 3D solid modeling software. The origin and insertion of the superior, middle, anterior band and posterior band of the inferior glenohumeral ligaments (SGHL, MGHL, AB-IGHL, and PB-IGHL) were determined based on prior anatomic studies, and then used as a basis for the analysis of ligament length changes. Results: During abduction and rotation the humeral head translated more in the anterior-posterior direction (range: 6 mm) than the superior-inferior direction (range: 2.5 mm). Maximal elongation of the SGHL was observed at low abduction while the MGHL elongated the most above 45° of abduction and in a much larger range than the SGHL. The maximal elongation of the AB-IGHL was observed at high degrees of abduction with maximal external rotation, while the PB-IGHL was also maximally elongated at high degree of abduction, but with maximal internal rotation. Conclusion: This study shows that there is a large range of anterior-posterior translation during shoulder rotation, and thus not pure ball-in-socket motion suggested by prior experimental studies. Our observations show that the SGHL may only function to restrain the joint in low abduction angles while the MGHL likely functions at higher abduction angles, as having been reported in previous in-vitro studies. The AB-IGHL lengthens most with the arm in extreme abduction and external rotation, thus it is an important static restraint during the cocking phase of the throwing motion. The PB-IGHL plays a reciprocal functional role as it maximally lengthens when the shoulder is in maximum abduction and internal rotation. Clinical Relevance: These data further clarify the reciprocal relationship of glenohumeral ligaments as static restraints to excessive translations and rotations during in-vivo normal shoulder motion. The data is clinically relevant as restoration of normal motion is the goal of surgery for the unstable shoulder, and overconstraint during capsular shift procedures is possible if shoulder rotation is not taken into consideration when tightening each region of the capsule. If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use). If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use). If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use).