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.
B0 Glenoid
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).
Posterior acromial tilt is determined by measuring the angle formed by the reference line (connecting the inferior angle of the scapula with the center of the intersection of the small arms of the “Y”) and a line connecting the most posterior point of the inferior aspect of the acromion to the most anterior point of the inferior aspect (white area).
The posterior acromial coverage (PAC) refers to an angle formed by the reference line (connecting the inferior angle of the scapula with the center of the intersection of the small arms of the “Y”) and a line drawn from the intersection of the small arms of the “Y” to the most posterior point of the inferior aspect of the acromion (red area).
To measure the posterior acromial height (PAH), a perpendicular line is drawn from the reference line (connecting the inferior angle of the scapula with the center of the intersection of the small arms of the “Y”) to the most posterior point of the inferior aspect of the acromion. The PAH (green bracket) is then measured as the distance from the center of the intersection of the small arms of the “Y” to the perpendicular line.
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.
Correlation of Acromial Morphology With Risk and Direction of Shoulder Instability: An MRI Study posterior instability patients had significantly less posterior acromial coverage than in the control group (68.3° vs. 81.7°) and greater
posterior acromial height (11.0mm vs. 0.7 mm). Crucially, this study excluded patients with multidirectional instability, glenoid bone loss >13.5%, or retroversion >10° — thereby studying the "cleanest" possible posterior instability cases, without the most severe osseous co-pathology. This exclusion criterion is worth noting: by removing patients with the most significant glenoid abnormalities, the study actually enhanced the apparent signal from acromial parameters in isolation. Real clinical populations are rarely this clean.
The glenoid co-develops embryonically with the acromion and other parts of scapular anatomy. What role does it plan in posterior stability?
It is worth noting that the acromion, glenoid, scapular body, humerus, posterior labrum, posterior capsule, and rotator cuff all arise from the same embryonic mesenchymal condensation during upper limb development. A single disruption to this shared developmental program could plausibly produce co-occurring abnormalities across all of these structures simultaneously. Perhaps acromial morphology is not the primary cause of posterior instability but instead is the most radiographically accessible expression of the scapular morphotype that is visible on a scapular Y-view.
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.
Anatomical description of the scapular coordinate system (OXYZ), acromion landmarks (AA, AC), trigonum spinae (TS), angulus inferior (AI), posterior extension of the acromion (AAx), acromion posterior angle (APA), acromion tilt angle (ATA), acromion length angle (ALA), and glenoid retroversion angle (GRA). The three axes (x, y, and z) correspond to postero-anterior, infero-superior, and medio-lateral, respectively.
Can modification of the acromion improve posterior stability in cadavers?
The biomechanical evidence that posterior acromial bone grafting can restore stability was generated in cadavers with normal glenoid anatomy — a condition that does not describe typical patients.
Eight fresh-frozen human cadaveric shoulders were biomechanically tested in a shoulder simulator in the load-and-shift
and Jerk test positions.
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.
At 30° of flexion, the force needed to displace the humeral head 50% increased by 659% when a PABG was added to a moderately malaligned acromion and by 1249% when a PABG was added to a severely malaligned acromion. Experimentally, a PABG provided comparable or superior stability to that achieved with surgical acromial reorientation while representing a technically simpler and potentially less invasive approach. While these biomechanical results for acromial correction in cadavers with normal glenoid anatomy and without soft tissue contribution are striking, they may not translate to clinical application where the glenoid and soft tissues are likely to be abnormal.
Can modification of the acromion improve posterior stability in patients?
The entire published clinical evidence base for isolated acromial correction in posterior shoulder instability consists of a single case series by Roberto Scapinelli, an Italian surgeon who first performed the procedure in 1970 and published results in 2006.
This single series has never been independently replicated in over 50 years. Posterior addition acromioplasty in the treatment of recurrent posterior instability of the shoulder reported an extraarticular surgical procedure for the treatment of recurrent posterior instability of the shoulder by grafting an inverted segment of the scapular spine to the posterior border of the acromion so that it exerts a slight pressure over the infraspinatus muscle pressing the humerus forward.
The Scapinelli series in plain terms: n=10 patients. One surgeon. 1970-2006. No controls. No validated outcome instruments. No independent replication. No imaging follow-up for glenoid version or labral integrity. This is the weakest possible form of clinical evidence — and it is all we have. I could find no other reports of isolated acromial surgeries in patients.
Current thinking regarding the role of acromial surgery in the management of posterior instability may be reflected in Acromion Morphology Is Associated With Failure of Arthroscopic Posterior Capsulolabral Repair finding that while failure of arthroscopic posterior shoulder capsulolabral repair was associated with a higher acromion the authors did not recommend surgical intervention (posterior acromial bone graft or posterior acromial osteotomy) to change acromial anatomy in the treatment of posterior shoulder instability. This direct statement from proponents of the acromial association with posterior instability should help inform clinical decision-making.
Going forward
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.
Acorn Woodpecker
Tucson
2020
Here are some videos that are of shoulder interest
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) Shoulder rehabilitation exercises (see this link).
