Tensile properties of the superior glenohumeral and coracohumeral ligaments

The collagen fibers of the anteroinferior capsulolabrum have multiaxial orientation to resist shoulder dislocation

Instability of the glenohumeral joint can be associated with anteroinferior capsulolabral rupture. To understand its static stabilizing effect better, the collagen fiber orientation of the inferior glenohumeral ligament (IGHL), a component of the anteroinferior capsulolabrum, was studied with a small angle light scattering technique. Three rectangular samples (approximately 11 x 6 mm) were excised from the axillary pouch, one from the anterior band (AB) of the IGHL and one control sample from the long head of the biceps tendon of 7 cadaveric shoulders. The small angle light scattering technique scans the tissue with a helium-neon laser beam and quantifies the fiber alignment based on the resultant scattering pattern. The fiber orientation was quantified by an orientation index, defined as the angle within which 50% of the fibers lie. The axillary pouch had a random orientation, whereas the AB-IGHL was random with some regions of localized alignment. The percentage of tissue with an orientation index range of 25 degrees to 45 degrees was 23.2% +/- 8.5% and 29.0% +/- 13.1% for the axillary pouch and the AB-IGHL, respectively, whereas that for the long head of the biceps tendon was 61.6% +/- 15.2%. This suggests that the collagen fibers in the IGHL are not highly aligned and the anteroinferior capsulolabrum can be modeled as a continuous sheet. Moreover, a biomechanical evaluation of the anteroinferior capsulolabrum that investigates the possibility that the mechanical properties may be directionally independent should be conducted.

Anatomy and function of the glenohumeral ligaments in anterior shoulder instability

The anatomy of the glenohumeral ligaments has been shown to be complex and variable and their function is highly dependent on the position of the humerus with respect to the glenoid. The superior glenohumeral ligament with the coracohumeral ligament was shown to be an important stabilizer in the inferior direction, even though the coracohumeral ligament is much more robust than the superior glenohumeral ligament. The middle glenohumeral ligament provides anterior stability at 45 degrees and 60 degrees abduction whereas the inferior glenohumeral ligament complex is the most important stabilizer against anteroinferior shoulder dislocation. Therefore, this component of the capsule is the most frequently injured structure. An appropriate surgical procedure to repair the inferior glenohumeral ligament complex after shoulder dislocation must be considered. In addition, a detached labrum can lead to recurrent anterior instability and a compromised inferior glenohumeral ligament complex. However, additional capsular injury usually is necessary to allow anterior dislocation.

The pathophysiology of shoulder instability

Over the last several decades there has been an improved understanding of the intricate anatomy that provides stability to the glenohumeral joint. In addition, significant advances in identifying the pathologic etiology of the unstable shoulder have occurred because of basic science glenohumeral ligament cutting studies, clinical evaluation, and the advent of arthroscopic evaluation and treatment of the unstable shoulder. This article will review the pertinent anatomy of the normal glenohumeral joint and will carefully review the pathoanatomy found in the unstable shoulder. Sports medicine specialists who treat athletes with unstable shoulders should have a firm understanding of both the normal and pathologic shoulder conditions to be able to provide the best care for these athletes.

Rotator cuff interval: evaluation with MR imaging and MR arthrography of the shoulder in 32 cadavers

PURPOSE

The purpose of this work was to establish the optimal means of evaluation of the rotator cuff interval (RCI) and rotator interval capsule and demonstrate normal anatomy of the RCI using MR imaging and MR arthrography.

METHOD

MR arthrography was performed in 32 cadaveric shoulders. In 20 cases, MR imaging was completed prior to arthrography. Pre- and postarthrography studies included standard imaging planes. Images were evaluated by the consensus of two musculoskeletal radiologists with attention to the RCI, rotator interval capsule (measurements on postarthrographic studies), and crossing structures. In five cases, specialized imaging planes were performed after arthrography.

RESULTS

The RCI, rotator interval capsule, and crossing structures were best evaluated by MR arthrography. The anteroposterior dimension of the rotator interval capsule could be best depicted on postarthrogram images.

CONCLUSION

MR arthrography, with both standard and specialized imaging planes, is a useful way to evaluate the RCI, the rotator interval capsule, and its crossing structures.

Anatomy and functional aspects of the rotator interval

The anatomy of the region between the supraspinatus and subscapularis tendons, called the rotator interval, was studied in 22 shoulders of 12 cadavers. Its function was then examined by sequential cutting of tendon or rotator interval structures. The rotator interval was found to be composed of parts of the supraspinatus, subscapularis, coracohumeral ligament, superior glenohumeral ligament, and glenohumeral joint capsule. A medial part composed of 2 layers was defined and distinguished from a more lateral part composed of 4 layers. The most superior 3 layers of the lateral part formed a fibrous plate. The medial part was found to primarily limit inferior translation and, to a lesser extent, external rotation. The fibrous plate of the lateral part of the rotator interval mainly limited external rotation of the adducted arm. The coracohumeral ligament played a key role in external rotation as well as in inferior translation.

The long head of the triceps: a detailed analysis of its capsular origin

Whether the triceps brachii muscle provides any significant contribution to stability about the shoulder girdle is unknown. This study seeks to document the anatomy of the origin of the long head of the triceps tendon and to resolve discrepancies in the anatomic literature. Fifteen fresh frozen cadaveric shoulder girdle specimens were dissected, the long head of the triceps and the posterior capsule being exposed. The long head origin averaged 29.5 mm (range, 26-34 mm) in width and 5.7 mm (range, 4-7 mm) in thickness. The average cross-sectional area of origin of the tendon was 168 mm2, and the fibrous contribution from the glenohumeral capsule averaged 2.3% (range, 1.4% to 3.1%) of the tendon's area. A capsular contribution was found in each specimen. The documentation of this capsular contribution may warrant further biomechanical and electromyographic study.

Addressing glenohumeral stiffness while treating the painful and stiff shoulder arthroscopically

The shoulder can be primarily or secondarily stiff. Cadaveric cutting studies have shown increases in passive range of glenohumeral motion when certain portions of the capsule are released. This study has recorded the intraoperative gains made in passive range of motion for external rotation, flexion, abduction, and internal rotation with sequential release of the rotator interval, inferior capsule, and posterosuperior capsule, regardless of initial etiology, and followed-up over time. Thirty one of 60 shoulders, found clinically to have a loss of passive range of motion and having failed a nonoperative approach, underwent a capsular release. Eighteen patients underwent a partial capsular release (group 1) and 13 patients (group 2) underwent a complete capsular release. Thirty of 31 shoulders had statistically significant gains in passive range of motion with sequential release. In general, resection of the rotator interval contributed to gains in external rotation; resection of the inferior capsule (anteroinferior and posteroinferior) contributed gains to external rotation, forward flexion, and internal rotation; and resection of the posterosuperior capsule contributed to gains only in internal rotation. At a minimum of 18 months follow-up, 30 of 31 shoulders retained their intraoperative gains. There was no difference in the results between primarily and secondarily stiff shoulders for motion gains (P >.05). Arthroscopically addressing capsular tightness is beneficial in returning shoulders with a loss of passive glenohumeral motion to normal regardless of the etiology.

Modeling the stability of the human glenohumeral joint during external rotation

An analytical model of the human glenohumeral joint was developed to predict glenohumeral kinematics and investigate how the glenohumeral capsule and articular contact between the humeral head and the glenoid stabilize the joint. This was performed during a simulation of an apprehension clinical exam or the cocked phase of throwing, when the humerus is susceptible to anterior instability or dislocation. Contact between the joint surfaces was modeled using a deformable articular contact method and the capsule was modeled as five elements with the ability to wrap around the surface of the humeral head. Experimental measurements (Novotny et al., Journal of Shoulder and Elbow surgery, 1998, 7, 629-639) provided geometric data from four in vitro specimens and kinematic results to validate model predictions. Material properties were taken from the literature. An equilibrium approach was used with the forces and moments produced by the ligaments and surface contact balanced against those applied externally to the humerus during external rotation of the abducted and extended humerus. The six equilibrium equations were solved for the position and orientation of the humerus. The center of the humeral head translated posteriorly and superiorly with external rotation. Model predictions for translational and rotational ranges of motion were not significantly different from experimental findings; however, at individual moment increments, the model underestimated the external rotation and overestimated the superior-inferior position of the humerus relative to the glenoid. The anterior band of the inferior glenohumeral ligament increased in tension with external rotation, while the axillary pouch and posterior band decreased in tension. Contact area, stress and force increased with external rotation and the contact area moved posteriorly and inferiorly around the rim of the glenoid. The model results provide information on how the relationship between the ligament element tensions and contact forces may act to avoid glenohumeral instability.

An analytical approach to determine the in situ forces in the glenohumeral ligaments

The purpose of this study was to use an analytical approach to determine the forces in the glenohumeral ligaments during joint motion. Predictions from the analytical approach were validated by comparing them to experimental data. Using a geometric model, the lengths of the four glenohumeral ligaments were determined during anterior-posterior loading simulations and forward flexion-extension. The corresponding force in each structure was subsequently calculated based on length data via load-elongation curves obtained experimentally. During the anterior loading simulation at 0 deg of abduction, the superior glenohumeral ligament carried up to 71 N at the maximally translated position. At 90 deg of abduction, the anterior band of the inferior glenohumeral ligament had the highest force of 45 N during anterior loading. These results correlated well with those found in previous experimental studies. We believe that this validated analytical approach can be used to predict the forces in the glenohumeral ligaments during more complex joint motion as well as assist surgeons during shoulder repair procedures.

A dynamic analysis of glenohumeral motion after simulated capsulolabral injury. A cadaver model

We used a dynamic shoulder-testing apparatus and nine fresh-frozen, entire upper extremities from cadavera to evaluate the effects of varying degrees of capsulolabral injury on the kinematics of the glenohumeral joint during abduction in the scapular plane and external rotation. Joint kinematics were recorded with use of a six-degrees-of-freedom magnetic tracking device before and after the creation of each capsulolabral lesion in a progressive manner. Dislocation did not occur after simulation of a large Bankart lesion or even after sectioning of the anterior aspect of the joint capsule. However, division of the entire joint capsule (that is, both the anterior aspect and the posterior aspect) resulted in a significant increase (p < 0.05) in posterior translation during abduction in the scapular plane, and two of the nine shoulders dislocated posteriorly. External rotation of the abducted extremity produced no increase in anterior or posterior translation.