Chapter 3 Instability

147

CHAPTER 3 | Instability

FIGURE 3-32  (A) AP and (B) axillary x-rays show good position of the coracoid bone graft, which is beginning to unite to the anterior glenoid.

This is in contradistinction to an “on-track” Hill-Sachs lesion in which the medial margin of the Hill-Sachs lesion is lateral to the glenoid rim and the humeral articular sur- face is well supported by the anterior glenoid rim through- out the range of motion (Fig. 3-35). The importance of the “off-track” Hill-Sachs lesion is that it may exist in association with a glenoid that does not have significant bone loss (i.e., <25% glenoid bone loss). In such a case, if an arthroscopic Bankart repair alone is done to address the instability, the Hill-Sachs lesion will still be able to engage the anterior glenoid rim and cause recurring insta- bility, as we have demonstrated in the biomechanics lab. 8 We have also demonstrated in the biomechanics lab that remplissage will eliminate engagement of “off-track” Hill- Sachs lesions at the same time that it significantly increases the biomechanical stiffness of the repair construct. 8 Therefore, remplissage is our preferred technique for arthroscopically addressing the Hill-Sachs defect in patients with anterior instability who have an “off-track” Hill-Sachs in association with <25% glenoid bone loss. But in order to implement this arm of the treatment paradigm, the surgeon must be able to calculate whether or not the Hill-Sachs is “off-track.” So how is that done? As we have stated, in patients who have <25% glenoid bone loss in association with an off-track Hill-Sachs lesion, we prefer to treat them with a combination of arthroscopic Bankart repair and arthroscopic remplissage. We have pre- viously explained how glenoid bone loss can be measured arthroscopically or from a 3D CT scan (Fig. 3-36). 7 Similarly, one can measure the Hill-Sachs interval (HSI) and calculate the glenoid track either arthroscopically or from a 3D CT scan (Fig. 3-37). One can then deduce whether the Hill-Sachs lesion is off-track (engaging) or on-track (nonengaging). The Hill-Sachs Lesion: Is It OnTrack or Off Track?

Hill-Sachs defect to make it an extra-articular defect that can no longer engage). Determining whether a Hill-Sachs lesion is “on-track” (nonengaging) or “off-track” (engaging) requires a bit of simple math, but it is not complicated and can be eas- ily computed in the operating room from measurements taken at the time of surgery. The “glenoid track” is a concept that must be understood if one is to fully understand how to calculate whether a Hill- Sachs lesion is “off track.” 7 The glenoid track can be thought of as the imprint or “track” that the glenoid would make on the humerus as the armis abducted inmaximumexternal rotation. Think of the “footprint” that the surface of the glenoid makes on the humerus where the two bones come into contact. Then, imagine the continuous trail of consecutive “footprints” that combine as amoving contact surface between the two bones as the arm is abducted in full external rotation (Fig. 3-33A). This continuous trail of footprints is the “glenoid track.” If there is a glenoid defect, the glenoid track narrows. When there is not any glenoid bone loss, the glenoid track is typically 83% of the diameter of the inferior glenoid (because the posterior glenoid pushes the cuff attachments 17% of the glenoid width posteriorly as the arm goes into forward abduction in external rotation). If there is a glenoid defect, then the glenoid track is narrowed by the width of the defect, so that the glenoid track (GT) = 0.83D − d where D = diameter of glenoid and d = width of glenoid defect (Fig. 3-33B, C). Hill-Sachs lesions typically occupy the part of the humerus that is in contact with the glenoid track. The larger the Hill- Sachs becomes, the closer it gets to the medial margin of the glenoid track. Once the Hill-Sachs lesion reaches the point where it extends medial to the medial margin of the glenoid track, the humeral articular surface is no longer supported by the anterior glenoid margin and the humerus “falls off” the edge of the glenoid, which then engages the Hill-Sachs. At that point, we call it an “off-track” Hill-Sachs lesion (Fig. 3-34).