Chapter 3 Instability

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CHAPTER 3 | Instability

The only thing that remains to be determined is whether or not the medial aspect of the Hill-Sachs lesion extends medial to the glenoid track, and if it does, then the Hill- Sachs lesion is off-track since the anterior glenoid rim will engage the Hill-Sachs lesion (see Fig. 3-34). In determining the medial extent of the Hill-Sachs lesion, it is useful to keep in mind the concept of the Hill-Sachs interval (HSI), which is the width of the Hill- Sachs lesion (HS) plus the width of the bone bridge (BB) between the rotator cuff attachments and the lateral aspect of the Hill-Sachs lesion: HSI HS BB = + If HSI > GT, the Hill-Sachs lesion is off-track and will engage the anterior glenoid. If HSI < GT, the Hill-Sachs lesion is on track and will not engage the anterior glenoid (see Fig. 3-35). The problem with using the 3D CT scan to measure HSI is that the bony ridge where the posterior cuff attaches is often indistinct and very difficult if not impossible to accu- rately locate on the CT scan. Therefore, we prefer to obtain all our measurements arthroscopically. In obtaining arthroscopic measurements, we first look at the glenoid through an anterosuperolateral portal. Through a posterior working portal, we insert a calibrated

The 3D CT scan can be used for obtaining these mea- surements. In terms of the glenoid, there are two important measurements: 1. The diameter (D) of the intact inferior glenoid (i.e., the diameter before any bone loss, either traumatic or attritional) and 2. The width (d) (Fig. 3-38) of the bone loss from the anterior glenoid (either attritional from compression or erosion due to repetitive dislocations, or traumatic with a bony Bankart fragment). Yamamoto et al. 7 have shown that the glenoid track width (GT) can be calculated as follows: GT D d = 0 83. − The inferior glenoid diameter (D) can easily be mea- sured from the en face 3D view of the normal glenoid (see Fig. 3-36A). The width of glenoid bone loss (d) can be calculated by subtracting the width of the inferior glenoid diameter on the involved side (D 1 ) from the inferior gle- noid diameter on the normal side (D): d D D = − 1 Now that we know both D and d, we can calculate the width of the glenoid track (GT = 0.83D − d).

FIGURE 3-38  Case with bony defect of glenoid (A) and large Hill-Sachs lesion (B) . By use of the contralateral glenoid as a reference (100%), 83% width is determined ( black double-headed arrow ). Then, the defect width (d) is subtracted from this 83% length to obtain the glenoid track width for this case ( white double-headed arrow ). Dotted line R represents the medial margin of the rotator cuff attachments. It should be noted that there is normally an intact “bone bridge” between the cuff attachments and the lateral border of the Hill-Sachs lesion. Dotted line G1 indicates the location of the medial margin of the glenoid track. If there had been no glenoid bony defect, the medial margin of the glenoid track would have been dotted line G2 . In this case, the Hill-Sachs lesion extends medially beyond the medial margin of the glenoid track ( dotted line G1 ), so this is an off- track lesion.