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

Two bones, one problem. Bone loss in anterior shoulder instability is bipolar. On the glenoid side it is a bony Bankart fragment or attritional, erosive loss of the anterior rim; on the humeral side it is the Hill-Sachs impaction of the posterosuperior head. The two lesions interact during instability, so both must be measured and judged together rather than one at a time.

What the measurements are for. Glenoid loss and Hill-Sachs status, combined with the patient's clinical risk, drive the choice between an arthroscopic Bankart repair, a remplissage and a bony procedure such as the Latarjet.

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The reference view. The inferior two-thirds of the glenoid is normally a near-perfect circle. On the en-face (sagittal-oblique) view a best-fit circle is fitted to the intact inferior glenoid and the defect is expressed as a percentage of that circle. 3D CT is the gold standard for this static assessment; 3D MRI has been validated as a reliable alternative, with the advantage of also showing the soft tissues.

The contralateral template. The most accurate reference is the contralateral normal glenoid, used as a one-to-one template. It assumes side-to-side symmetry, which is generally but not universally true, and it is unusable in a patient with bilateral instability, where a best-fit circle on the affected side is the fallback.

Two methods, two numbers. Quantifying glenoid loss needs a defined technique, not just a percentage, because the two main methods do not give the same number for the same defect:

• Width (linear) method - the width of the defect as a fraction of the circle's diameter. Quick, but it tends to slightly over-read the percentage compared with the area method.
• Surface-area (Pico) method - the area of the missing segment as a fraction of the circle's total area, often referenced against the contralateral glenoid surface.

Always state which method was used when a value is quoted against the subcritical and critical thresholds.

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Amount and morphology. The type of loss matters as much as the amount, so describe both. A discrete bony-Bankart fragment may be repairable or incorporated if it is of adequate size and unites, whereas purely attritional loss has no fragment to fix and generally behaves worse. The bony Bankart itself is covered in Bankart Lesions and Glenoid Fractures; the point here is the measurement.

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
• West Point axillary - the plain view for the anteroinferior glenoid rim; it projects the anteroinferior quadrant free of overlap and shows rim erosion or a bony fragment
• Bernageau (glenoid profile) view - shows the anterior rim in profile, with the "cliff" or blunting of erosive loss
• Standard AP - may show loss of the normal glenoid contour, one of the ISIS points (see The Clinical Half of the Decision)

The inverted pear. At arthroscopy the normally pear-shaped glenoid can lose its wider inferior half and look like an upside-down pear. This corresponds to substantial loss, conventionally quoted in the region of 25 to 30%, and is the intra-operative equivalent of the critical threshold.

The concept. The glenoid track (Yamamoto, Di Giacomo) is the zone of contact of the glenoid on the humeral head in the provocative abduction-external rotation position. A Hill-Sachs lesion that stays within the track is on-track. One that extends medial to the track's medial margin is off-track: it will engage the glenoid rim, with a high risk of recurrence. It is the key dynamic, bipolar tool in the assessment.

Two measured numbers. The calculation compares the track width with the Hill-Sachs interval (HSI): the width of the Hill-Sachs lesion plus the bone bridge, the intact bone between the medial margin of the Hill-Sachs and the edge of the rotator-cuff footprint. An interval greater than the track width is off-track; an interval less than the track width is on-track. That comparison is precisely what "medial to the medial margin of the track" means.

Why both bones are measured together. Glenoid bone loss is subtracted from the track width, so more glenoid loss shrinks the track and can flip an otherwise on-track Hill-Sachs to off-track. The bipolar interaction is captured in a single calculation. The Hill-Sachs lesion itself is covered in Hill-Sachs Lesions, and the filling procedure in Remplissage.

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Two failures, two numbers. Glenoid loss is judged against a subcritical band of about 13.5-20% and a critical one of about 20-25%. Critical is the level above which an isolated soft-tissue Bankart repair has unacceptably high failure; subcritical is where its outcomes are already compromised and augmentation should be considered. They are not two guesses at the same number: they measure two different failures, and knowing which is which is the examinable point.

Recurrence. The distinction comes from the quartile analysis by Shaha and colleagues that coined the term. Recurrent dislocation rose sharply only in the highest quartile, with failure at 7.3% below about 20% bone loss against 27.8% above it, so the classical 20-25% critical figure is a re-dislocation threshold.

Function. 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.

Why that clause matters. A shoulder counted 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. That 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. The cohort was 72 patients (73 shoulders) at a single military institution with a high level of mandatory physical activity, 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. The study was also a Level III retrospective cohort, not a randomised comparison of repair against augmentation.

The score to name. Bone loss decides the operation only in combination with the patient, and "integrate clinical risk" needs an actual instrument. The Instability Severity Index Score (ISIS) is built from six preoperative variables and scored out of 10.

Reading the score. 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. Four of the ten points, from two radiographic items, come from plain radiographs, a reminder that the score was designed as a rapid clinical screen and not as a substitute for CT quantification. 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.

Beyond the score. The other clinical modifiers worth stating explicitly are these:

• 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
• Occupational demand - the soldier, the firefighter and the collision athlete all sit at the aggressive end of any threshold

From measurement to operation. Glenoid loss, Hill-Sachs status and the patient together select the procedure.

Where the rows overlap. Subcritical glenoid loss with an off-track Hill-Sachs appears in two rows: it may be treated with Bankart plus remplissage or with a bony procedure. Clinical risk (age, contact or collision sport, hyperlaxity and the instability score) modifies the thresholds and bears on which.

Remplissage. Filling the Hill-Sachs with posterior capsule and infraspinatus makes an off-track lesion functionally on-track.

Bony procedures. The Latarjet (coracoid transfer) restores the glenoid arc and adds the sling effect; a glenoid bone block is the alternative. Recurrence after a soft-tissue repair is itself an indication for a bony procedure rather than another isolated Bankart.

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