Posterior Glenoid Osteotomy With Capsulolabral Repair Improves Resistance Forces in a Critical Glenoid Bone Loss Model

Posterior shoulder stability depends on acromial anatomy: a cadaveric, biomechanical study

BACKGROUND

Failure rates in the management of recurrent posterior shoulder instability remain a concern. Cadaveric studies have established that posterior capsulolabral tears, glenoid retroversion, and posterior glenoid bone loss result in increased posterior humeral head translation in the setting of a posteriorly directed force. A high and flat acromion has recently been associated with posterior instability. Therefore, the purpose of this study was to evaluate a potential stabilizing effect of the acromion against posterior humeral head displacement.

METHODS

Eight fresh-frozen human cadaveric shoulders were biomechanically tested in a shoulder simulator in the load-and-shift and Jerk test positions. Prior to testing, computed tomography scans were performed to measure native glenoid width, glenoid retroversion, posterior acromial coverage (PAC), sagittal acromial tilt (SAT), and posterior acromial height (PAH). Each specimen underwent 4 testing conditions using preplanned and 3D printed cutting and reduction guides: (1) Intact joint, native acromion; (2) Intact joint, severe acromial malalignment (SAT 69°, PAC 47°, PAH 26 mm); (3) Intact joint, moderate acromial malalignment (SAT 59°, PAC 57°, PAH 20 mm); (4) Intact joint, corrected acromial alignment (SAT 48°, PAC 70°, PAH 11 mm). The degree of acromial malalignment and acromial reorientation was chosen based on a previous study that defined acromial anatomy in patients with posterior instability. The humeral head was translated posteriorly until reaching either (1) a peak force of 150N or (2) a maximum posterior displacement of 50% of the glenoid width. Forces (N), displacement (mm), and acromiohumeral contact pressures (kPA) were simultaneously recorded.

RESULTS

The force needed to displace the humeral head by 50% of the glenoid width decreased between 23% and 60% in moderate to severe acromial malalignment (high and flat acromion) and increased up to 122% following surgical correction of acromial alignment (low and steep acromion) when compared to the native condition. Correction of acromial alignment significantly increased stability compared to all other scenarios after ≥5% of displacement (P < .05 for all comparisons). Furthermore, it increased acromiohumeral contact pressures compared with severe malalignment in 30° flexion and with moderate and severe acromial malalignment in 60° flexion (P < .05 for all comparisons).

CONCLUSION

The acromion acts as a mechanical buttress to posterior humeral head displacement. Surgical correction of acromial malalignment cannot only effectively restore but increase glenohumeral joint stability. Future studies are needed to define the quantitative relevance of the different factors contributing to posterior shoulder instability and assist in defining the optimal amount of correction needed in an individual situation.

Biomechanics and Pathoanatomy of Posterior Shoulder Instability

Posterior glenohumeral instability represents a wide spectrum of pathoanatomic processes. A key consideration is the interplay between the posterior capsulolabral complex and the osseous anatomy of the glenoid and humeral head. Stability is dependent upon both the presence of soft tissue pathology (eg, tears to the posteroinferior labrum or posterior band of the inferior glenohumeral ligament, glenoid bone loss, reverse Hill Sachs lesions, and pathologic glenoid retroversion or dysplasia) and dynamic stabilizing forces. This review highlights unique pathoanatomic features of posterior shoulder instability and associated biomechanics that may exist in patients with posterior glenohumeral instability.

Arthroscopic Posterior Glenoid Osteotomy

Management of posterior shoulder instability in patients with excessive glenoid retroversion can be challenging. However, a corrective posterior glenoid osteotomy is an option. Although various open techniques are available, minimally invasive and arthroscopy surgery are the most advantageous. This study describes the feasibility and safety of an arthroscopic posterior open wedge glenoid osteotomy using an autologous scapular spine graft along with additional posterior capsulolabral complex reattachment. This procedure is a viable option for patients with symptomatic posterior shoulder instability.

Open treatment of posterior glenoid bone loss and bipolar bone loss

Posterior glenohumeral instability is an increasingly common and challenging orthopaedic problem. While an arthroscopic soft tissue stabilization procedure (i.e., reverse Bankart repair) is effective in treating most cases of posterior instability, this procedure may be inadequate in shoulders with critical posterior glenoid bone loss (GBL), or in cases of an engaging reverse Hill-Sachs lesion. Thus, the purpose of the present manuscript was to report contemporary surgical approaches, techniques, and outcomes for the open treatment of glenoid or humeral head bone loss in posterior instability to help guide clinical decision making. Open osteoarticular augmentation procedures have emerged as a popular option to treat posterior bone loss, with bony auto- and allografts utilized from a variety of donor sites including iliac crest, scapular spine, acromion, distal clavicle, and distal tibia. The combination of glenoid retroversion and bone loss can be addressed with a posterior glenoid opening wedge osteotomy. Bipolar bone loss may be treated with a combination of the aforementioned techniques, in addition to a reverse remplissage, a modified McLaughlin procedure, or various arthroplasty-related options. Although short and mid-term outcomes are dependable, studies reporting long-term outcomes are sparse. Moreover, there is no current consensus regarding the most effective treatment of posterior shoulder instability in the setting of bone loss, and open surgical techniques continue to evolve. Further research is necessary to determine long-term effectiveness of these surgical options.

Biomechanical Evaluation of Posterior Shoulder Instability With a Clinically Relevant Posterior Glenoid Bone Loss Model

BACKGROUND

Existing biomechanical studies of posterior glenoid bone loss and labral pathology are limited by their use of anterior instability models, which differ in both orientation and morphology and have been performed in only a single, neutral arm position.

PURPOSE

To evaluate the biomechanical effectiveness of a posterior labral repair in the setting of a clinically relevant posterior bone loss model in various at-risk arm positions.

STUDY DESIGN

Controlled laboratory study.

METHODS

Ten fresh-frozen cadaveric shoulders were tested in 7 consecutive states using a 6 degrees of freedom robotic arm: (1) native, (2) posterior labral tear (6-9 o'clock), (3) posterior labral repair, (4) mean posterior glenoid bone loss (7%) with labral tear, (5) mean posterior glenoid bone loss with labral repair, (6) large posterior glenoid bone loss (28%) with labral tear, and (7) large posterior glenoid bone loss with labral repair. Bone loss was created using 3-dimensional printed computed tomography model templates. Biomechanical testing consisted of 75 N of posterior-inferior force and 75 N of compression at 60° and 90° of flexion and scaption. Posterior-inferior translation, lateral translation, and peak dislocation force were measured for each condition.

RESULTS

Labral repair significantly increased dislocation force independent of bone loss state between 10.1 and 14.8 N depending on arm position. Dislocation force significantly decreased between no bone loss and small bone loss (11.9-13.5 N), small bone loss and large bone loss (9.4-14.3 N), and no bone loss and large bone loss (21.2-26.5 N). Labral repair significantly decreased posterior-inferior translation compared with labral tear states by a range of 1.0 to 2.3 mm. In the native state, the shoulder was most unstable in 60° of scaption, with 29.9 ± 6.1-mm posterior-inferior translation.

CONCLUSION

Posterior labral repair improved stability of the glenohumeral joint, and even in smaller to medium amounts of posterior glenoid bone loss the glenohumeral stability was maintained with labral repair in this cadaveric model. However, a labral repair with large bone loss could not improve stability to the native state.

CLINICAL RELEVANCE

This study shows that larger amounts of posterior glenoid bone loss (>25%) may require bony augmentation for adequate stability.