Multiquadrant digital analysis of shoulder capsular thickness.

PURPOSE Nonablative thermal capsular shrinkage has been developed in an attempt to address the plastic capsule deformation thought to cause increased rates of recurrent instability following arthroscopic stabilization procedures. Although the temperature required to optimize collagen shrinkage is known, a safe depth of thermal penetration, in various locations about the shoulder capsule, has not been defined. The purpose of this study was to measure shoulder capsule thickness by quadrant and circumferentially from the glenoid to the humerus so that thermal energy in shoulder procedures can be more precisely applied to limit possible injury to pericapsular structures. TYPE OF STUDY This is an anatomic study using a cadaveric shoulder specimens. MATERIALS AND METHODS Soft tissue was dissected from 8 fresh cadaveric shoulders to isolate intact glenohumeral joint capsules. The humeral insertion was released and the capsule was cut into 6 longitudinal quadrants around the glenoid. The capsule specimens were then flash frozen and stored at -80 degrees C. Quadrant tissue was cut into longitudinal sections 14 to 16 microm wide and stained with hematoxylin and eosin. The specimens were then digitized under a dissecting microscope and measured using computer imaging software at approximately 4-mm intervals. Two-way analysis of variance (ANOVA) was performed on the measurements of the intact capsule specimens 2.5 cm off the glenoid. Humeral insertion data were recorded separately. RESULTS A total of 248 separate measurements were made throughout the capsule in 8 specimens. Capsular thickness increased from an average of 2.42 mm anteriorly to 2.80 mm in the inferior capsular pouch and again thinned to 2.22 mm posteriorly. Global shoulder capsule thickness ranged from 1.32 to 4.47 mm. When analyzed by position, from glenoid to humerus, a general thinning was noted with a mean thickness of 3. 03 mm at the glenoid to 2.17 mm at the humeral insertion. Two-way ANOVA showed a significant thickness variation along the specimen (P <.05), a nearly significant thickness variation with regard to quadrant (P <.03), and no significant interaction (P >.07) when applied to specimen measurements approximately 2.5 cm off the glenoid. CONCLUSIONS The thickness of the shoulder capsule ranges from 1.32 to 4.47 mm, with a significant thinning laterally from the glenoid to the humerus. Further, capsule thickness ranges from 2.76 to 3.18 mm in the regions in closest proximity to the axillary nerve. These data may help determine the proper amount of thermal penetration necessary when performing shrinkage procedures and provide safety guidelines to limit the depth of thermal penetration to avoid possible injury to pericapsular structures.