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Walch defined the pathologic characteristics of glenohumeral osteoarthritis on the basis of patterns of glenoid morphology and humeral head subluxation. However, it is unclear how pathologic changes evolve over time. The purpose of this study was to determine whether there are common patterns of pathologic progression based on the Walch classification in primary glenohumeral osteoarthritis and if glenoid bone-loss patterns correlate with rotator cuff fatty infiltration.A retrospective chart review identified 65 shoulders with glenohumeral osteoarthritis for which at least 2 computed tomography (CT) scans had been performed at least 24 months apart. The CT scans were classified using a modification of the Walch classification. The amount and location of glenoid bone loss were measured using a vault model, and rotator cuff fatty infiltration was calculated as a percentage of cross-sectional muscle area.The initial CT scans showed 42 A-type glenoids and 23 B-type glenoids. CT scans made at an average (and standard deviation) of 74 ± 32 months after the initial scans showed that only 8 of the 42 A1 glenoids had evidence of pathologic progression (5 to A2 type and 3 to B type) whereas 17 of 19 B1 glenoids had progressed (15 to B2 and 2 to B3); this difference was significant on univariate and multivariate analysis (p < 0.001). The odds of joint line medialization occurring were 8.1 times higher (95% confidence interval [CI]: 2.1 to 31.4) for B-type glenoids than for A-type glenoids. Among the glenoids that underwent medialization, those classified as B-type showed more medialization over time (estimated change, 0.70 mm/year; p = 0.036), whereas no significant relationship between medialization and time was observed for A-type glenoids (estimated change, 0.013 mm/year; p = 0.95). The median percent fatty infiltration in the infraspinatus muscle was higher in association with B-type glenoids than in association with A-type glenoids on both the initial (14% versus 7%; p < 0.001) and the final follow-up (16% versus 10%; p = 0.003) CT scans.Asymmetric bone loss rarely develops in A1 glenoids, whereas initial posterior translation of the humeral head (B1 glenoids) may be associated with subsequent development and progression of posterior glenoid bone loss over time. Differences in fatty infiltration of the posterior aspect of the rotator cuff were seen between A-type and B-type glenoids, but the clinical relevance of this finding is currently unknown.Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.