On-track and off-track shoulder lesions

Bipolar shoulder lesions of anterior glenohumeral dislocations (i.e. combined bony Bankart and Hill-Sachs lesions) put patients at increased risk of Hill-Sachs engagement or recurrent instability. Determining if the Hill-Sachs lesions are on-track or off-track can help guide management, although arthroscopy remains gold-standard for diagnosis.

Pathology

The glenoid track refers to the area of contact between the humeral head and glenoid and is defined as ~83% of glenoid width . A Hill-Sachs lesion that is smaller than the track ("on-track") will maintain contact and is at lower risk of engagement and instability. Conversely, a Hill-Sachs lesion that is larger than the glenoid track ("off-track") will be at increased risk of engagement and instability (i.e. is an "engaging Hill-Sachs lesion").

Thus there are two factors that contribute to determining if a bipolar bone lesion is on-track or off-track:

• anteroinferior glenoid bone loss
• Hill-Sachs interval size

Off-track lesions can result from either a large bony Bankart lesion or Hill-Sachs lesion or from both Hill-Sachs and bony Bankart lesions being moderate-sized.

Radiographic features

Determining on-track or off-track lesions was initially described on CT but can also be calculated on MRI . Two measurements are required :

• glenoid track: calculated on a coronal oblique plane of the glenoid using the best-fit circle method
  • a best-fit circle is placed on the glenoid, matching the posterior and inferior borders
  • a horizontal line is drawn through the center of the best-fit circle reaching both anterior and posterior aspects (D)
  • a second horizontal line is drawn along the same plane from the anterior aspect of the circle to the anterior glenoid (d), i.e. measuring the width of anterior glenoid bone loss
  • glenoid track = (0.83 x D) - d
• Hill-Sachs interval = Hill-Sachs lesion + bone bridge between the rotator cuff attachment and lateral aspect of the Hill-Sachs lesion
  • measured in the axial plane

A bipolar lesion is said to be engaging if the Hill-Sachs interval is larger than the glenoid track.

Treatment and prognosis

Off-track bipolar lesions may be treated surgically with reconstitution of the anteroinferior glenoid bone stock (commonly via a Latarjet procedure) +/- remplissage depending on the size of the Hill-Sachs lesion .