The effect of articular malposition after total shoulder arthroplasty on glenohumeral translations, range of motion, and subacromial impingement

The articular surface of the normal humeral head has a variable posterior a nd medial offset with respect to the central axis of the humeral shaft. Rec reation of the normal humeral head shaft offset is postulated to be an impo rtant consideration during shoulder arthroplasty. However, the effect of hu meral head malposition is unknown. The purpose of this study was to determi ne the effect of articular malposition after total shoulder arthroplasty on glenohumeral translation, range of motion, and subacromial impingement. Tw enty-one human cadavers were dissected and tested with the use of an active or passive shoulder model. Range of motion and translation were recorded b y means of an electromagnetic tracking device. The experiment was performed in 2 phases. For kinematics study, 11 cadaver shoulders were positioned bo th passively and actively from maximum internal rotation to maximum externa l rotation at 90 degrees of total elevation in the scapular plane, Three ro tator cuff and 3 deltoid muscle lines of action were simulated for active j oint positioning. Passive joint positioning was accomplished with the use o f a torque wrench and a nominal centering force. The testing protocol was u sed for the natural joint as well as for 9 prosthetic head locations: cente red and 2- and 4-mm offsets in the anterior, posterior, inferior, and super ior directions. Repeated-measures analysis of variance was used to test for significant differences in the range of motion and translation between act ive and passive positioning of the natural joint as well as all prosthetic head positions. (2) For impingement study, 10 cadaver shoulders were used i n a passive model, loading the tendons of the rotator cuff with a 30-N cent ering force. The humerus was passively rotated from maximum internal rotati on (1500 Nmm) to maximum external rotation (7500 Nmm) by means of a continu ous-recording digital torque wrench. Trials were performed with the use of centered, 4-, 6-, and 8-mm offset heads in the anterior, posterior, superio r, and inferior positions before and after removal of the acromion and cora coacromial ligament. The relation between change in mean peak torque (with and without acromion), passive range of motion, and humeral head offset was analyzed by means of repeated-measures analysis of variance. In the kinema tics study, total range of motion and all humeral translations were greater with passive joint positioning than with active positioning (P =. 01) exce pt for total superior-inferior translation and superior-inferior translatio n in external rotation. Anterior to posterior humeral head offset was assoc iated with statistically significant changes in total range of motion (P =. 02), range of internal rotation (P =.02), range of external rotation (P =. 0001), and total anterior-posterior translation (P =. 01). Superior to infe rior humeral head offset resulted in statistically significant changes in t otal range of motion (P=.02), range of internal rotation (P =. 0001), anter ior-posterior translation during external rotation (P =. 01), and total sup erior-inferior translation (P =.03). In the impingement study, there was a significant increase in torque from centered to 4-mm inferior offset (P =.0 06), 6-mm inferior offset (P <,001), and 8-mm inferior offset (P <.001). Th ere was no significant increase in torque with superior, anterior, and post erior offsets. Glenohumeral motion significantly decreased from 129 degrees for centered head to 119 degrees for 8-mm superior (P =.002), 119 degrees for 8-mm anterior (P =.014), 118 degrees for 8-mm inferior (P<.001), and 11 4<degrees> for 8-mm posterior (P =.001). Humeral articular malposition of 4 mm or less during prosthetic arthroplast y of the glenohumeral joint may lead to small alterations in humeral transl ations and range of motion. inferior malposition of greater than 4 mm can l ead to increased subacromial contact; offset of 8 mm in any direction resul ts in significant decreases in passive range of motion. Therefore if subacr omial contact is to be minimized and glenohumeral motion maximized after sh oulder replacement, anatomic reconstruction of the humeral head-humeral sha h offset to within 4 mm is desirable.