Glenoid & Bipolar Bone Loss Assessment in Shoulder Instability (Glenoid Track)

The plain films still have named jobs

Radiographs do not quantify bone loss, but "don't decide from a plain film" is not the same as "don't take one", and a viva will ask which views:

• AP in external rotation and the Stryker notch view profile the Hill-Sachs lesion.
• The West Point axillary is the plain view for the anteroinferior glenoid rim - it projects the anteroinferior quadrant free of overlap and shows rim erosion or a bony fragment.
• The Bernageau (glenoid profile) view shows the anterior glenoid rim in profile and demonstrates the "cliff" or blunting of erosive loss.
• A standard AP may show loss of the normal glenoid contour, which is one of the ISIS points above.

Two further recognitions belong here. At arthroscopy, the "inverted pear" glenoid - where the normally pear-shaped glenoid loses its wider inferior half and looks like an upside-down pear - corresponds to substantial loss, conventionally quoted in the region of 25 to 30%, and is the intra-operative equivalent of the critical threshold. And the contralateral-template method has a limit worth stating: it assumes side-to-side symmetry, which is generally but not universally true, and it is unusable in the patient with bilateral instability - so a best-fit circle on the affected side is the fallback.

The subcritical band around 13.5-20% and the critical one around 20-25% are not two guesses at the same number - they measure two different failures, and knowing which is which is the examinable point.

The distinction comes from the quartile analysis that coined the term. Recurrent dislocation rose sharply only in the highest quartile: failure was 7.3% below about 20% bone loss versus 27.8% above it, so the classical 20-25% "critical" figure is a re-dislocation threshold. But patient-reported function deteriorated much earlier. Above about 13.5%, WOSI scores fell to a level consistent with an unacceptable outcome - and they did so even in the patients whose repair never re-dislocated.

That last clause is the whole point, and it changes how the operation is judged. A shoulder that is quantified as a success because it has not dislocated again can still be a functional failure, so counting recurrences alone systematically flatters isolated Bankart repair in the subcritical range. It is why "the repair held" is not a sufficient answer for a young, high-demand patient with 15% bone loss, and why augmentation is discussed well below the classical critical figure.

Two caveats keep this honest. The cohort was 72 patients at a single military institution with a high level of mandatory physical activity, which is close to the worst case, so 13.5% is a warning line rather than a universal mandate - a sedentary, older, first-time dislocator with 15% loss is a different proposition from a 22-year-old infantry soldier or collision athlete. And the study was a Level III retrospective cohort, not a randomised comparison of repair against augmentation.

Bone loss decides the operation only in combination with the patient, and the page's instruction to "integrate clinical risk" needs the actual instrument.

The Instability Severity Index Score (ISIS) is the scoring system to name, built from six preoperative variables scored out of 10:

A score above 6 predicts an unacceptable recurrence rate after arthroscopic Bankart repair - of the order of 70% in the original series - and is the conventional trigger to offer an open or bony procedure instead. Note what the last two items imply: two of the ten points come from plain radiographs, which is a reminder that the score was designed as a rapid clinical screen, not as a substitute for the CT quantification described above. Use both - the score to flag the high-risk patient, the imaging to decide what the bone actually needs. The score is developed further in Bankart Lesions and Anterior Shoulder Instability.

The other clinical modifiers worth stating explicitly are a first dislocation before 20, a high number of prior dislocations, seizures (which drive very high recurrence and bilateral disease), generalised laxity on the Beighton score, and occupational demand - the soldier, the firefighter and the collision athlete all sit at the aggressive end of any threshold.