Posteroinferior glenoplasty can change glenoid shape and increase the mechanical stability of the shoulder

The treatment of recurrent posterior glenohumeral instability remains an un solved clinical problem. Although various types of capsulorraphy have been advocated, outcome studies indicate that if is difficult to achieve a balan ce between stability and mobility. Alterations of the bony glenoid for post erior instability have been proposed, but are not well understood from a me chanical perspective. This investigation had 2 purposes: (1) to determine i n a cadaver model if posteroinferior glenoplasty can change the shape of th e glenoid, and (2) to determine if altering the shape of the glenoid can in crease the mechanical stability of the glenohumeral joint. We determined th e effective glenoid shape in 7 normal cadaver glenoids by cracking the path of the center of the humeral head as it was translated across the glenoid face in 8 different directions. These determinations enabled us to calculat e the maximum effect;ve slope of the glenoid in each direction. We then det ermined the mechanical stability of the glenoids in each of the 8 direction s by measuring the tangential force required to dislocate the shoulder unde r a 50-N compressive load. The ratio of the dislocating force to the compre ssive load was defined as the stability ratio. All measurements were repeat ed after a standardized posteroinferior glenoplasty was performed. Posteroi nferior glenoplasty increased the posteroinferior glenoid depth from 3.8 +/ - 0.6 mm to 7.0 +/- 1.8 mm and shifted the center of the humeral head an av erage of 2.2 mm anteriorly and 7.8 mm superiorly These changes in dimension could be directly visualized as an immediate mechanical consequence of the glenoplasty procedure, particularly because of the insertion of the bone w edge. Glenoplasty increased the posteroinferior glenoid slope from 0.55 +/- 0.07 to 0.83 +/- 0.12 and increased the posteroinferior stability ratio fr om 0.47 +/- 0.10 to 0.81 +/- 0.17. This is a more than 70% increase in the tangential force that can be resisted before dislocation. The increase can be quantitatively understood as a direct mechanical consequence of the alte red shape of the glenoid concavity. These numbers indicate that, in this ca daveric model, posteroinferior glenoplasty results in defined changes in th e effective glenoid shape and in the mechanical stability of the glenohumer al joint. However, this study does not establish the role of this procedure in the clinical management of posterior glenohumeral instability.