Correction of Static Posterior Shoulder Subluxation by Restoring Normal Scapular Anatomy Using Acromion and Glenoid Osteotomies: A Case Report

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.

Good functional outcome but high rates of instability recurrence after posterior open-wedge glenoid osteotomy for the treatment of posterior shoulder instability with increased glenoid retroversion at mid-term follow-up

PURPOSE

To evaluate clinical, functional and radiological mid-term outcomes following posterior open-wedge glenoid osteotomy (POWGO) for the treatment of posterior shoulder instability (PSI) associated with increased glenoid retroversion.

METHODS

Patients who underwent POWGO for the treatment of symptomatic PSI with glenoid retroversion >10° and participated in a previous study assessing short-term outcomes were included after a minimum follow-up of 5 years. Clinical (Rowe score and physical examination) and functional outcomes (Oxford Shoulder Instability Score [OSIS] and visual analogue scale [VAS] for pain) were assessed. Preoperative versus follow-up magnetic resonance imaging (MRI) assessments were compared for changes in posterior humeral head subluxation (PHHS) and progression of osteoarthritis (shoulder osteoarthritis severity [SOAS] score).

RESULTS

Eight patients (nine shoulders) were included 92.0 months (88.0-109.5 months) post-operatively, of which seven patients (eight shoulders) underwent MRI. Shoulder function was good (Rowe score: 80.0 [76.3-91.3], OSIS: 41.0 [31.0-41.5]) and pain levels were low (VAS for pain: 3.0 [1.0-3.0]) at follow-up. Overall, the degree of PHHS did not change between preoperatively and follow-up (p > 0.05). Four shoulders demonstrated PHHS preoperatively, of which two had a centred humeral head at follow-up. Shoulder osteoarthritis progressed significantly (SOAS score: 17.0 [11.0-24.5] to 33.0 [31.0-45.0], p = 0.018). Residual PSI was evident in 75.0% of shoulders.

CONCLUSION

At mid-term follow-up, POWGO for PSI associated with increased glenoid retroversion led to good functional outcomes but failed to reliably restore posterior shoulder stability and prevent osteoarthritis progression.

LEVEL OF EVIDENCE

Level IV.

Causal associations between scapular morphology and shoulder condition estimated with Bayesian statistics

BACKGROUND AND OBJECTIVE

While there is a reported correlation between shoulder condition and scapular morphology, the precise impact of typical anatomical variables remains a subject of ongoing debate. This study aimed to evaluate this causal association, by emphasizing the importance of scientific modeling before statistical analysis.

METHODS

We examined the effect of scapular anatomy on shoulder condition, and conditioning on sex, age, height, and weight. We considered the two most common pathologies: primary osteoarthritis (OA) and cuff tear arthropathy (CTA). We combined the other pathologies into a single category (OTH) and included a control category (CTRL) of adult subjects without pathology. We represented acromion and glenoid morphology by acromion angle (AA), acromion posterior angle (APA), acromion tilt angle (ATA), glenoid inclination angle (GIA), and glenoid version angle (GVA). GVA was negative for posterior orientation. These variables were automatically calculated from CT scans of 396 subjects in the 4 shoulder condition groups by a deep learning model. We applied do-calculus to assess the identifiability of the causal associations and used a multinomial logistic regression Bayesian model to estimate them. To isolate the effect of each anatomical variable on each shoulder condition, we increased it from -2 to 2 z-score while constraining all other variables to their average value, and reported the effect on shoulder condition probability as percentage points (pp) for females and males.

RESULTS

Increasing AA reduced the probability of OA by 44 pp for females and 17 pp for males while increasing the probability of CTA by 36 pp for females and 33 pp for males. Increasing APA raised the probability of OA by 15 pp for females and 4 pp for males and increased the probability of CTA by 12 pp for females and 4 pp for males. Increasing ATA increased the probability of OA by 15 pp for females but decreased it by 25 pp for males, while also raising the probability of CTA by 11 pp for females and 21 pp for males. Increasing GIA decreased the probability of OA by 55 pp for females and 23 pp for males while increasing the probability of CTA by 45 pp for females and 31 pp for males. GVA (more anterior), decreased the probability of OA by 33 pp for females and 63 pp for males. The effects of APA and ATA were less important compared to the other variables. Overall, morphological effects were more pronounced for females than for males, except for GVA's impact on OA.

CONCLUSIONS

We developed a Bayesian causal model to answer interventional questions about the scapular anatomy's effect on shoulder condition. Our results, consistent with clinical knowledge, hold promise for aiding in early pathology detection and optimizing surgical planning within clinical settings.

Posterior shoulder instability

Posterior shoulder instability (PSI) is defined by dynamic, recurrent and symptomatic partial or total loss of posterior joint contact. Anatomic risk factors comprise ligament hyperlaxity, glenoid retroversion or dysplasia, and high horizontal acromial morphology. Associated anatomic lesions comprise labrum lesions, posterior glenoid erosion and/or fracture, and anterior humeral head notching. We distinguish two subcategories of PSI: functional and structural, respectively without and with anatomic lesions. In both categories, there may be anatomic risk factors. Clinically, functional PSI involves reproducible asymptomatic voluntary subluxation or sometimes reproducible involuntary subluxation. Functional PSI implicates impaired external-rotation rotator cuff and scapular stabilizer muscle activity. Treatment is non-operative, by rehabilitation and shoulder pace maker. Structural shoulder instability involves anatomic lesions, often due to iterative microtrauma; pain is the most frequent symptom. It usually concerns young athletic subjects, but the clinical forms of structural and of anterior shoulder instability are not superimposable. Treatment may be surgical; arthroscopic labrum repair is effective in the absence of significant bone lesions, whereas otherwise posterior bone block is the treatment of choice. Surgical treatment of scapular features underlying structural PSI is improved by 3D preoperative planning, cutting guides and dedicated internal fixation systems. LEVEL OF EVIDENCE: expert opinion.

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.

Influence of Critical Shoulder Angle and Rotator Cuff Tear Type on Load-Induced Glenohumeral Biomechanics: A Sawbone Simulator Study

Glenohumeral (GH) biomechanics after rotator cuff (RC) tears are not fully understood. The purpose of our study was to determine if the critical shoulder angle (CSA), type of RC tears, and level of weight bearing increase GH translation, instability based on the instability ratio, muscle forces and joint reaction force (JRF), and shifts the center of force (CoF) superiorly. A GH simulator with muscle-mimicking cable systems was used to simulate 30° abduction in the scapular plane. A Sawbone humerus and five specimen-specific scapular anthropometries were used to test six types of RC tears, three weight-bearing loads, and the native and adjusted (to different CSAs) deltoid origin sites. Linear mixed effects models (CSA, RC tear type, and weight bearing) with random effects (specimen and sex) were used to assess differences in GH biomechanics. With increasing CSA, GH translation increased, JRF decreased, and the CoF position was more inferior. RC tears did not significantly alter GH translation but shifted the CoF position superiorly, close to where glenoid erosion occurs in patients with RC tears with secondary osteoarthritis. Weight bearing significantly increased GH translation and JRF. RC and deltoid muscle forces increased with the presence of RC tears and increased weight bearing. The remaining RC muscles of intact tendons compensated for the torn RC tendons but not for the altered CoF position. GH translation remained comparable to shoulders with intact RC. These findings highlight the importance of early detection, clinical management, and targeted rehabilitation strategies for patients with RC tears.

A Posterior Acromial Bone Block Augmentation Is Biomechanically Effective at Restoring the Force Required To Translate the Humeral Head Posteriorly in a Cadaveric, Posterior Glenohumeral Instability Model

PURPOSE

To assess the biomechanical utility of a posterior acromial bone block (PABB) for the treatment of posterior glenohumeral instability.

METHODS

Ten fresh-frozen cadaveric specimens were obtained based upon an a priori power analysis. A 2.5-cm scapular spine autograft was harvested from all shoulders. A custom robot device was used to apply a 50-N compressive force to the glenohumeral joint. The humeral head was translated 10 mm posteroinferiorly at 30 degrees from the center of the glenoid at a rate of 1.0 mm/s in 6 consecutive conditions: (1) intact specimen, (2) intact with PABB, (3) posterior capsulolabral tear, (4) addition of the PABB, (5) removal of the PABB and repair of the capsulolabral tear (LR), and (6) addition of the PABB with LR. The maximum force required to obtain this translation was recorded. Paired t tests were performed to compare relevant testing conditions.

RESULTS

Ten cadavers with a mean ± SD age of 54.4 ± 13.1 years and mean ± SD glenoid retroversion of 6.5 ± 1.0 degrees were studied. The PABB provided greater resistance force to humeral head translation compared to the instability state (instability, 29.3 ± 15.3 N vs PABB, 47.6 ± 21.0 N; P = .001; 95% confidence interval [CI], -27.6 to -10.0). When comparing PABB to LR, the PABB produced higher resistance force than LR alone (PABB, 47.6 ± 21.0 N; LR, 34.2 ± 20.5 N; P = .012; 95% CI, -23.4 to -4.1). An instability lesion treated with the PABB, with LR (P = .056; 95% CI, -0.30 to 20.4) or without LR (P = .351; 95% CI, -6.8 to 15.7), produced resistance forces similar to the intact specimen.

CONCLUSIONS

A PABB is biomechanically effective at restoring the force required to translate the humeral head posteriorly in a cadaveric, posterior glenohumeral instability model. A posterior acromial bone block is a biomechanically feasible option to consider in patients with recurrent posterior instability.

CLINICAL RELEVANCE

Augmentation of the posterior acromion may be a biomechanically feasible option to treat posterior shoulder instability.

Are glenoid retroversion, humeral subluxation, and Walch classification associated with a muscle imbalance?

BACKGROUND

The etiology of humeral posterior subluxation remains unknown, and it has been hypothesized that horizontal muscle imbalance could cause this condition. The objective of this study was to compare the ratio of anterior-to-posterior rotator cuff and deltoid muscle volume as a function of humeral subluxation and glenoid morphology when analyzed as a continuous variable in arthritic shoulders.

METHODS

In total, 333 computed tomography scans of shoulders (273 arthritic shoulders and 60 healthy controls) were included in this study and were segmented automatically. For each muscle, the volume of muscle fibers without intramuscular fat was measured. The ratio between the volume of the subscapularis and the volume of the infraspinatus plus teres minor (AP ratio) and the ratio between the anterior and posterior deltoids (APdeltoid) were calculated. Statistical analyses were performed to determine whether a correlation could be found between these ratios and glenoid version, humeral subluxation, and/or glenoid type per the Walch classification.

RESULTS

Within the arthritic cohort, no statistically significant difference in the AP ratio was found between type A glenoids (1.09 ± 0.22) and type B glenoids (1.03 ± 0.16, P = .09), type D glenoids (1.12 ± 0.27, P = .77), or type C glenoids (1.10 ± 0.19, P > .999). No correlation was found between the AP ratio and glenoid version (ρ = -0.0360, P = .55) or humeral subluxation (ρ = 0.076, P = .21). The APdeltoid ratio of type A glenoids (0.48 ± 0.15) was significantly greater than that of type B glenoids (0.35 ± 0.16, P < .01) and type C glenoids (0.21 ± 0.10, P < .01) but was not significantly different from that of type D glenoids (0.64 ± 0.34, P > .999). When evaluating both healthy control and arthritic shoulders, moderate correlations were found between the APdeltoid ratio and both glenoid version (ρ = 0.55, P < .01) and humeral subluxation (ρ = -0.61, P < .01).

CONCLUSION

This in vitro study supports the use of software for fully automated 3-dimensional reconstruction of the 4 rotator cuff muscles and the deltoid. Compared with previous 2-dimensional computed tomography scan studies, our study did not find any correlation between the anteroposterior muscle volume ratio and glenoid parameters in arthritic shoulders. However, once deformity occurred, the observed APdeltoid ratio was lower with type B and C glenoids. These findings suggest that rotator cuff muscle imbalance may not be the precipitating etiology for the posterior humeral subluxation and secondary posterior glenoid erosion characteristic of Walch type B glenoids.