A New Arthroscopic Technique to Determine Anterior-Inferior Glenoid Bone Loss: Validation of the Secant Chord Theory in a Cadaveric Model
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Anterior Shoulder Instability Arthroscopic Treatment
Arthroscopic treatment of anterior shoulder instability has grown in popularity in recent years, with results comparable to open stabilization. However, satisfactory outcomes are dependent on appropriate patient selection. It has been well documented that patients with soft-tissue incompetence, humeral head deficiencies, or glenoid bone loss have higher failure rates than patients without these abnormalities. Specifically, much attention has been given to glenoid bone loss as a reason for failure of arthroscopic anterior stabilization techniques. Burkhart and De Beer emphasized this point when they showed a markedly increased failure rate (67% v 4%) in arthroscopic stabilization when significant bone loss was present on the glenoid, causing an “inverted-pear” appearance.
Various techniques have been used to identify the presence of glenoid bone loss both preoperatively and intraoperatively. Techniques using plain radiographs such as the apical oblique and West Point views have been described. Sugaya have popularized the use of computed tomography (CT) reconstructions to calculate the area of glenoid bone loss. Burkhart have advocated an arthroscopic technique that uses the bare spot of the glenoid as a reference point. In this technique the arthroscope is placed in the anterosuperior portal, and a graduated arthroscopic probe is placed in the posterior portal. By use of the glenoid bare spot (GBS) as the center reference point (B), the distance from the bare spot to the anterior rim of the glenoid (AB) is compared with the distance from the bare spot to the posterior rim (BC). In an intact glenoid this measurement should be equal. If glenoid bone loss is present, AB will be less than BC; the percent bone loss can be calculated as follows: (BC AB)/(BC 2).
Glenoid Bone Loss Treatment
Other authors have called into question the validity of using the bare spot as a central reference point. Kralinger have shown that the GBS is actually not the center of the inferior portion of the glenoid, but instead, it was found to be located approximately 1.4 mm anterior to the true center of the glenoid. If the reference point for this technique is not exactly in the center of the glenoid, an incorrect assessment of glenoid bone loss will result. Furthermore, this arthroscopic technique was described with the use of 1 posterior portal. With many surgeons varying the location of the posterior arthroscopic portal, it remains unclear whether the GBS can reliably be reached with different posterior portal positions by use of the arthroscopic probe and whether the change in portal position results in a change in measured percentage loss.
Although the accuracy of the GBS has been brought into question in the literature, there have been numerous reports that show the inferior portion of the glenoid to be reliably circular. With this information, it would seem logical that an arthroscopic technique that uses the circular geometry of the inferior glenoid without using the bare spot as a reference might be an alternative way of measuring glenoid bone loss arthroscopically. To our knowledge, no technique using these principles has been described in the literature.
Measure the Amount of Anterior-Inferior Glenoid Bone Loss
The purpose of this study was to validate a technique to measure the amount of anterior-inferior glenoid bone loss using the secant chord method. The hypothesis was that the secant chord theory (SCT) technique is more accurate than using the bare spot as a central reference point in measuring the amount of glenoid bone loss present by use of an arthroscopic model of anterior shoulder instability.