Abstract
Introduction
The Walch Type B2 glenoid has the hallmark features of posteroinferior glenoid erosion, retroversion, and posterior humeral head subluxation. Although our understanding of the pathoanatomy of bone loss and its evolution in Type B's has improved, the etiology remains unclear. Furthermore, the morphology of the humerus in Walch B types has not been studied. The purpose of this imaging based anthropometric study was to examine the humeral torsion in Walch Type B2 shoulders. We hypothesized that there would be a compensatory decrease in humeral retroversion in Walch B2 glenoids.
Methods
Three-dimensional models of the full length humerus were generated from computed tomography data of normal cadaveric (n = 59) and Walch Type B shoulders (n = 59). An anatomical coordinate system referencing the medial and lateral epicondyles was created for each model. A simulated humeral head osteotomy plane was created and used to determine humeral version relative to the epicondylar axis and the head-neck angle. Measurements were repeated by two experienced fellowship-trained shoulder surgeons to determine inter-rater reliability. Glenoid parameters (version, inclination and 2D critical shoulder angle) and posterior humeral head subluxation were calculated in the Type B group to determine the pathologic glenohumeral relationship. Two-way ANOVAs compared group and sex within humeral version and head-neck angle, and intra-class correlation coefficients (ICCs) with a 2-way random effects model and absolute agreement were used for inter-rater reliability.
Results
There were statistically significant differences in humeral version between normal and Type B shoulders (p < .001) and between males and females within the normal group (p = .043). Normal shoulders had a humeral retroversion of 36±12°, while the Walch Type B group had a humeral retroversion of 14±9° relative to the epicondylar axis. For head-neck angle, there were no significant differences between sexes (p = .854), or between normal and Type B shoulders when grouped by sex (p = .433). In the Type B group, the mean glenoid version was 22±7°, glenoid inclination was 8±6°, 2D critical shoulder angle was 30±5° and humeral head subluxation was 80±9%. Inter-rater reliability showed fair agreement between the two experienced observers for head-neck angle (ICC = .562; 95% CI: -.28 to .809) and excellent agreement for humeral version (ICC = .962;.913 to .983). Although only fair agreement was found between observers in head-neck angle ICC, the difference in mean angle was only 2°.
Discussion
Although much time and effort has been spent understanding and managing Type B2 glenoids, little attention has been paid to investigating associated humeral contributions to the Type B shoulder. Our results indicate that the humeral retroversion in Type B shoulders is significantly lower than in normals. These findings have several implications, including, helping to understanding the etiology of the B2, the unknown effects of arbitrarily selecting higher version angles for the humeral component, and the unknown effects of altered version on glenohumeral joint stability, loading and implant survivorship post-arthroplasty. Our results also raise an important question, whether it is best to reconstruct Type B humeral component version to pathologic version or to non-pathologic population means.
