Technical noteBi-directional mechanical properties of the posterior region of the glenohumeral capsule
Introduction
The glenohumeral joint allows a large range of motion which is primarily stabilized by the joint capsule and the rotator cuff muscles. Instability of the glenohumeral joint can occur in the anterior, posterior, and inferior directions, and can be classified as unidirectional or multidirectional. The clinical manifestations of posterior instability can result in repetitive microtrauma, which can be particularly disabling (Lamar et al., 2001). Clinically, capsular shift procedures that translate the posterior capsule in the medial-to-lateral and superior-to-inferior directions have reduced the symptoms of instability. However, recurrent instability following a capsular shift procedure for posterior instability has been reported to be as high as 72% (Hurley et al., 1992).
In addition to recurrent instability, patients also suffer from osteoarthritis, pain, and loss of joint motion following repair procedures. Recent research from Gerber et al. (2003) has demonstrated that shifting the capsule in the posterosuperior direction had a dramatic impact on the joint function by limiting internal rotation of the abducted arm by . Furthermore, capsular shift procedures used to treat anterior instability have also affected the posterior capsule as the strain was increased by 2.9–6.6% during humeral elevation (Hurley et al., 1992).
This interaction of the capsular regions has also been demonstrated by Debski et al. (1999b) who found that significant forces are transmitted between the regions. Furthermore, recent work has demonstrated the importance of the bi-directional mechanical properties of the axillary pouch of the glenohumeral capsule (Moore et al., 2004). Thus, as the capsule is a continuous structure, injury or repair to one region of the capsule may have an adverse effect on the properties of the neighboring regions and on joint function. However, the mechanical properties of the capsule have only been determined uniaxially with the exception of the axillary pouch (Bigliani et al., 1992, Itoi et al., 1993, McMahon et al., 1998, Ticker et al., 1996).
Therefore, the objective of this study was to determine the mechanical properties of the posterior capsule in the directions perpendicular (transverse) and parallel (longitudinal) to the posterior band (PB) of the inferior glenohumeral ligament (IGHL). This objective investigated the general hypothesis that the posterior capsule functions in a multi-axial manner. These data will advance the current understanding of the role of the posterior capsule in the glenohumeral joint stability. Furthermore, it will serve as input into a finite element model that treats the entire glenohumeral capsule as a continuous structure allowing for the computation of the stress and strain fields throughout the capsule during complex joint motions.
Section snippets
Materials and methods
Tissue samples from 11 fresh-frozen cadaveric shoulders (age: years) were used in this study. The PB-IGHL is one of the most identifiable landmarks in the posterior capsule and marks a visible transition from the IGHL to the posterior capsule as the tissue thickness drastically decreases. Therefore, the superior margin of the PB-IGHL was chosen as the reference for the transverse (perpendicular to the longitudinal axis of the PB-IGHL) and longitudinal (parallel to the
Results
The cross-sectional area for the transverse and longitudinal posterior capsule tissue samples was and , respectively (2.5 mm width due to hardened steel punch dimensions). All tissue samples failed in the midsubstance. One longitudinal tissue sample did not fail due to the safety limits of the load cell. Thus, the tangent modulus of this tissue samples was reported whereas the ultimate stress, ultimate strain, and strain energy density are not reported.
Representative
Discussion
The mechanical properties of the posterior region of the glenohumeral capsule were determined in the transverse and longitudinal directions with respect to the longitudinal axis of the PB-IGHL. Significant differences were detected between the ultimate stress and tangent modulus of the transverse and longitudinal tissue samples. No significant differences were observed between the ultimate strain and strain energy density of the transverse and longitudinal tissue samples.
The data presented in
Acknowledgements
The support of the Whitaker Foundation is gratefully acknowledged.
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