Critical size of a glenoid defect causing anterior shoulder instability post Bankart repair under physiological joint loading

Aims Patients with recurrent anterior dislocation of the shoulder commonly have an anterior osseous defect of the glenoid. Once the defect reaches a critical size, stability may be restored by bone grafting. The critical size of this defect under non-physiological loading conditions has previously been identified as 20% of the length of the glenoid. As the stability of the shoulder is load-dependent, with higher joint forces leading to a loss of stability, the aim of this study was to determine the critical size of an osseous defect that leads to further anterior instability of the shoulder under physiological loading despite a Bankart repair. Patients and Methods Two finite element (FE) models were used to determine the risk of dislocation of the shoulder during 30 activities of daily living (ADLs) for the intact glenoid and after creating anterior osseous defects of increasing magnitudes. A Bankart repair was simulated for each size of defect, and the shoulder was tested under loading conditions that replicate in vivo forces during these ADLs. The critical size of a defect was defined as the smallest osseous defect that leads to dislocation. Results The FE models showed a high risk of dislocation during ADLs after a Bankart repair for anterior defects corresponding to 16% of the length of the glenoid. Conclusion This computational study suggests that bone grafting should be undertaken for an anterior osseous defect in the glenoid of more than 16% of its length rather than a solely soft-tissue procedure, in order to optimize stability by restoring the concavity of the glenoid. Patients with recurrent anterior dislocation of the shoulder commonly have an anterior osseous defect in the glenoid. The prevalence of fracture or erosion of the rim of the glenoid ranges from 8% (18/226) to 73% (116/158) in these patients.1,2 As osseous defects may result in loss of concavity of the glenoid, the shoulder becomes unstable in the mid-range of movement when stability is achieved by compression of the humeral head into the glenoid labrum through contraction of the rotator cuff.3 Biomechanical studies have shown an inverse relationship between the size of the defect and stability: the larger the defect, the less stable the shoulder.3–5 The rate of recurrent instability after arthroscopic Bankart repair in patients with a large osseous defect of the glenoid is higher than after bone grafting.6,7 The rationale for not performing bone grafting in all patients with a glenoid defect include concerns about the rate of complications, the long term sequelae, and surgical learning curve.8 Thus, defining the critical size of an osseous defect that leads to persistent anterior instability under loading conditions expected by the patient during functional activities will assist clinical decision making.9 Currently, only three biomechanical studies from one group have quantified the critical size of an osseous defect that necessitates bone grafting.3–5 These studies found that, for defects that correspond to 19% to 21% of the length of the glenoid, stability cannot be maintained by a Bankart repair and that stability is only restored after reconstruction of the concavity of the glenoid by bone grafting. Clinical guidelines have taken these data as recommendations for bone grafting.10 These biochemical studies involve cadaveric experiments and have limitations, including non-physiological loading with a maximum of 50 N. In vivo measurements from instrumented shoulder implants have reported loads of 151% of body weight during activities of daily living (ADLs).11 As the stability of the shoulder is load-dependent, with higher forces leading to a loss of stability,12,13 the use of physiological loading is essential for the assessment of stability. Finite element (FE) modelling may be used to overcome this limitation of cadaveric studies and to simulate testing under in vivo conditions. The aim of this study was to determine the critical size of an anterior osseous defect in the glenoid that leads to persistent anterior instability of the shoulder under physiological loading despite a Bankart repair. This may assist in choosing the appropriate surgical procedure for patients with recurrent anterior dislocation