BACKGROUND:
The aim of this study was to radiographically analyze the long-term glenoid migration patterns following total shoulder arthroplasty to better understand the factors responsible for loosening.

METHODS:
Complete radiographic follow-up of more than five years was available for 518 total shoulder arthroplasties performed for primary glenohumeral osteoarthritis with use of an anatomically designed prosthesis with a cemented, all-polyethylene, keeled glenoid component. Radiographs were assessed for humeral head subluxation, periprosthetic radiolucent lines, and shifting of the position of the glenoid component. The type of migration of the glenoid was defined according to the direction of tilt, or as subsidence in the case of medial migration.

RESULTS:
Definite radiographic evidence of glenoid loosening was observed in 166 shoulders (32%) and was characterized by radiolucency of ≥2 mm over the entire bone-cement interface in thirty shoulders and by a migration of the glenoid component (shift or subsidence) in 136 shoulders. Three predominant patterns of migration of the glenoid component were observed: superior tilting in fifty-two shoulders (10%), subsidence in forty-one shoulders (7.9%), and posterior tilting in thirty-three shoulders (6.4%). Superior tilting of the glenoid was associated with three risk factors: low positioning of the glenoid component, superior tilt of the glenoid component on the immediate postoperative coronal plane radiographs, and superior subluxation of the humeral head (p < 0.05 for all). Subsidence of the glenoid component was associated with the use of reaming to optimize the seating and positioning of the glenoid component (p < 0.001). Posterior tilting of the glenoid component was associated with preoperative posterior subluxation (i.e., a Walch type-B glenoid) and with excessive reaming (p < 0.01 for both).

CONCLUSIONS:
The three patterns of migration observed in this study underscore the potential importance of the supporting bone beneath the glenoid component. In some shoulders, use of a keel or pegs to provide fixation of a polyethylene component in the absence of good support from subchondral bone may not be sufficient to resist compressive and eccentric forces, resulting in loosening. Preserving subchondral bone may be important for long-term longevity of the glenoid component.





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