Bipolar bone loss and distance to dislocation

Studies have shown that glenoid- and humeral-sided bone loss may be present in up to 73-93% of individuals with recurrent anterior shoulder instability. As such, bone loss must be addressed appropriately, as the amount of bone loss drives surgical decision making and influences outcomes. Methods to describe and measure bone loss have changed over time. Originally, glenoid and humeral bone loss were viewed separately. However, the concepts of bipolar bone loss, the glenoid track (GT), and "on/off-track" lesions arose, highlighting the interplay between the two entities in contributing to recurrent instability. Classically, "off-track" lesions have been described as those Hill-Sachs interval (HSI) greater than the GT, and have been shown to result in higher rates of re-instability when addressed nonoperatively or with Bankart repair alone. More recently, further attention has been given to "on-track" lesions (HSI < GT). The new concept of "distance to dislocation" (DTD) has gained popularity. DTD is calculated as the difference between the GT and HSI, and literature evaluating DTD suggests that not all "on-track" lesions should be treated in the same manner. The purpose of this concept review article is twofold: (I) describe glenoid, humeral, and bipolar bone loss in the setting of anterior shoulder instability; and (II) elaborate on the new concept of "DTD" and its use in guidance of management.