Measurement of Glenoid Bone Loss: a Comparison of Measurement Error Between 45 Degrees and 0 Degrees Bone Loss Models and with Different Posterior Arthroscopy Portal Locations
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The successful arthroscopic treatment of recurrent anterior shoulder instability is predicated on optimal patient selection, as postoperative failure is higher in patients with soft tissue incompetence, humeral head deficiency, and glenoid bone loss. Considerable attention has been directed to the preoperative evaluation and optimal treatment of patients with bone loss, especially of the glenoid. Glenoid bone loss remains a challenging issue with increased failure rates, ever since Burkhart and DeBeer highlighted the dramatic increase in surgical failure (up to 61%) after arthroscopic stabilization of rugby players with anteroinferior glenoid bone loss. Thus, it is important to determine the extent of glenoid bone with radiographic studies or via intraoperative measurement during arthroscopy.
The measurement of the amount of anteroinferior glenoid bone loss has been described with both arthroscopic methods and radiographic methods, such as the apical oblique and West Point radiographs. The arthroscopic technique is performed with the arthroscope in the anterosuperior portal while several measurements of the glenoid are taken with a graduated probe. The measurements are referenced off the bare spot and are converted into a percentage anteroinferior bone loss based on the differential loss of bone from the bare spot anteriorly versus the bare spot posteriorly and presented as a ratio or overall percentage bone loss. If the bone loss is significant (>20%), then the term “inverted-pear” glenoid has been applied, which describes the altered shape of the anteroinferior glenoid in the setting of bone loss.
Although the intra-articular measurement of anteroinferior glenoid bone loss is commonly performed, the original work14 that documented the measurements and mathematics in a bone loss setting used a simulated cadaveric osteotomy that was 45° relative to the long axis of the glenoid. The osteotomy essentially connected the 3-o’clock and 6-o’clock points on the glenoid, and bone loss measurements were validated with this type of simulated bone osteotomy. However, we now know from 3-dimensional CT studies in the setting of anterior shoulder instability that glenoid bone loss occurs nearly parallel to the long axis of the glenoid, that is, parallel to the 12-o’clock and 6-o’clock line. Furthermore, the accuracy of glenoid bone loss measurements based on the bare spot as a reference point has been questioned.
The intra-articular measurement of bone loss was originally determined based on 1 posterior portal position. If the trajectory or initial starting position of the posterior arthroscopic portal is altered (for example, from 2 to 3 o’clock), this could potentially result in differing and erroneous determinations of the amount of glenoid bone loss. As such, little is known regarding the initial posterior arthroscopic portal starting position on determination of glenoid bone loss.
Because the original measurements of glenoid bone loss were validated in a cadaveric specimen with a simulated bone loss angle that is approximately 45° different from what generally happens in a clinical scenario, we sought to better define the measurements of glenoid bone loss in a clinically relevant bone loss model. Accordingly, the purposes of our study were (1) to determine the differences in glenoid bone loss measurements between a 45° inverted- pear osteotomy and a 0° clinically relevant osteotomy bone loss model, (2) to determine if the initial arthroscopic posterior portal position has any effect on the determination of glenoid bone loss, and (3) to determine if the location of measurement along the osteotomy has any effect on the determination of glenoid bone loss.