The Summary
Posterior shoulder instability is being increasingly recognized as an important clinical problem - both in terms of the resulting compromise in the patient's shoulder comfort and function as well as a prelude to shoulder arthritis. In the Walch glenoid classification system, B0 is a pre-arthritic glenoid with posterior humeral instability that precedes the biconcave morphology and bone loss that defines the B2 glenoid. It is not known whether surgical intervention (and what type of surgical intervention) can mitigate the progression to arthritis.
Acromial morphology — specifically a high, flat posterior acromion with PAH >23mm — is consistently associated with posterior shoulder instability across multiple independent cohorts and imaging modalities. A high, flat acromion predicts posterior shoulder instability better than almost any other imaging finding.
Extraordinary association is not causation, and strong association does not automatically justify operative intervention. The acromion shares its embryonic developmental origins with every other tissue involved in posterior shoulder stability. The most parsimonious explanation for its remarkable predictive value is that a high, flat acromion is the most visible and measurable expression of a multi-tissue developmental syndrome — a marker, not the mechanism.
Evidence supporting acromial correction in posterior shoulder instability is lacking: it includes no prospective clinical trials. The only clinical data rests on a single unreplicated 10-patient case series. The biomechanical studies, while mechanistically interesting, were performed under conditions (e.g. normal glenoid anatomy) that do not reflect typical clinical patients.
The clinical problem
Posterior shoulder instability is defined as dynamic, recurrent, and symptomatic partial or total loss of posterior glenohumeral joint contact. Multiple anatomic factors — labral tears, glenoid morphology, capsular laxity, bone loss, and acromial morphology — may contribute in varying combinations.
Relation of acromial morphology to posterior instability
The morphology of the acromion has long been implicated in shoulder pathology, particularly in relation to subacromial impingement and rotator cuff disease. More recently, interest has shifted toward the posterior acromion, with studies examining its potential role in posterior instability
Reproducibility and Relevance of Acromial Morphology Measurements in Shoulder Pathologies: A Critical Review of the Literature reviewed nine studies assessing sagittal acromial tilt, posterior coverage, and acromial height. In posterior instability the acromion is generally described as more horizontally oriented, less covering posteriorly, and positioned higher. Although these trends suggest a possible biomechanical role for the acromion, reported values vary widely between studies, and significant overlap exists between pathological and control groups. Such variability is compounded by differences in imaging modality and definitions of anatomical landmarks.These methodological inconsistencies undermine reproducibility and limit the clinical applicability of posterior acromial parameters. The reduction of a complex three-dimensional structure like the acromion into two-dimensional projections inevitably loses information about the spatial relationship between the acromion and the posterior humeral head. Acromial parameters identify groups at elevated risk, but do not provide the individual-level diagnostic precision needed to justify surgical targeting of the acromion in a specific patient.
Posterior Acromial Morphology Is Significantly Associated With Posterior Shoulder Instability carried out a study of 41 patients with unidirectional posterior instability and a control group of 53 patients with no instability in which the authors compared measurements of acromial morphology. Significantly, concurrent measures of glenoid morphology were not reported. (The article also presented an analysis of patients with anterior instability, but those are not relevant to our interest in comparing patients with posterior instability to controls).
Radiographic acromial characteristics included posterior acromial tilt, posterior acromial coverage (PAC), and posterior acromial height (PAH).
Patients with posterior instability had greater mean posterior acromial height (30.9 versus 20.4 mm) and posterior acromial tilt (63.6° versus 55.9°) compared with the control group. Posterior acromial coverage (48.8° versus 61.6°) was lower in the posterior instability group than in the control group. The authors concluded that in shoulders with posterior instability, the acromion is situated higher and is oriented more horizontally in the sagittal plane than in normal shoulders; and suggested that this acromial position may provide less osseous restraint against posterior humeral head translation.
Consideration of posterior instability requires consideration of the critical role the glenoid plays in glenohumeral stability through the concavity compression mechanism. Glenohumeral stability from concavity-compression: A quantitative analysis found the effective glenoid depth and stability ratios were lowest with loading of the humeral head in the posterior direction, meaning that the glenohumeral joint is constitutively most vulnerable posteriorly, especially since the great majority of shoulder functions are in forward flexion, a position that challenges posterior stability.
Glenoid retroversion is a well-established contributor to posterior instability. Glenoid Retroversion Is an Important Factor for Humeral Head Centration and the Biomechanics of Posterior Shoulder Stability found that every 5° increment produces approximately 2.0mm of additional humeral decentralization; retroversion >10-15° significantly affects joint centralization.
Posterior Shoulder Instability but Not Anterior Shoulder Instability Is Related to Glenoid Version found PSI patients averaged vault retroversion of −21° versus controls (−17.8°). Using chondrolabral version, the retroversion of PSI patients averaged −16.6° versus controls ( −9.2°).The Influence of Glenoid Retroversion on Posterior Shoulder Instability: A Cadaveric Study each 1° increase in retroversion correlated with a 3.5% decrease in resistance to posterior translation; spontaneous dislocation with an intact labrum occurred at a mean of 22.7° of retroversion. A 4–7° difference in retroversion translates to roughly a 14–25% reduction in posterior restraint from osseous anatomy alone, before any soft tissue or acromial contribution is considered.
Acromion Morphology Is Associated With Glenoid Bone Loss in Posterior Glenohumeral Instability demonstrated that acromial morphology is associated with the severity of glenoid bone loss in posterior instability. Patients without glenoid bone loss had a steeper acromial tilt (58.5°) versus those with ≥13.5% bone loss (67.7°). A flatter acromion tracks with more severe glenoid pathology — consistent with both being expressions of the same underlying developmental abnormality of the scapula.
Thinking outside the glenohumeral box: Hierarchical shape variation of the periarticular anatomy of the scapula using statistical shape modeling demonstrated that glenoid inclination and acromial anatomy represented distinct but related components of overall scapular morphology.
Association of the Posterior Acromion Extension with Glenoid Retroversion: A CT Study in Normal and Osteoarthritic Shoulders In this 3D morphometric study of 31 normal scapulae, glenoid retroversion correlated significantly with posterior acromial extension, characterized by the acromion posterior angle (APA). Combining the APA with the acromion length angle (ALA) and acromion tilt angle (ATA) helped improve the correlation.
I found one preliminary study that combined acromial osteotomy with glenoid osteotomy. Presumably the two procedues were combined because the authors were not satisfied with the acromial osteotomy alone. Scapular (glenoid and acromion) osteotomies for the treatment of posterior shoulder instability: technique and preliminary results reported the outcome after a “scapular (acromion and glenoid) corrective osteotomy" for posterior escape in 9 consecutive patients. One case had persistent subluxation, and osteoarthritis progression. The other 8 patients had improved clinical scores. In 5 patients the humeral head was recentered.
The strongest clinical application of acromial morphology data may lie in diagnosis and risk stratification, not operative planning. A high, flat posterior acromion on imaging should prompt the clinician to think more broadly about the shoulder's developmental anatomy — to look carefully at glenoid version, labral quality on MRI, posterior capsular volume, and rotator cuff muscle belly cross-section. It is a signal that the whole posterior stabilizing complex may be developmentally underprogrammed, not just the acromion.
The recognition of the B0 glenoid as a precursor to shoulder arthritis raises the question of whether identifying this developmental syndrome early — high acromion, glenoid retroversion, and posterior subluxation together — should inform decisions about some type of surgical intervention before the shoulder progresses to the arthritic B2. Answering that question will require thoughtful clinical research.
Trying to figure this out.
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Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link)
The total shoulder arthroplasty (see this link)
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link)
