Mastering Surgery of the Undersurface of the Acromion

Comprehensive Introduction and Patho-Epidemiology

Since the publication of the first edition of foundational orthopedic texts, arthroscopic shoulder surgery has seen a significant expansion, and consequentially, open procedures have become increasingly less common. Thankfully, the principles behind the various procedures have stayed the same. The undersurface of the acromion is a critical anatomical watershed zone, serving as the superior boundary of the subacromial space and a frequent site of pathologic mechanical abutment. Mastering surgery of this region requires a profound understanding of the dynamic interplay between the rotator cuff, the periscapular musculature, and the rigid coracoacromial arch. The transition from Neer’s classic open anterior acromioplasty to modern arthroscopic subacromial decompression (ASAD) represents one of the most significant evolutionary leaps in shoulder surgery, offering reduced morbidity, accelerated rehabilitation, and superior visualization of concomitant intra-articular pathology.

Pathophysiologically, subacromial impingement syndrome (SAIS) is characterized by the mechanical compression of the rotator cuff tendons and the subacromial bursa between the humeral head and the undersurface of the anterior acromion, coracoacromial (CA) ligament, and occasionally the acromioclavicular (AC) joint. This phenomenon was initially described by Charles Neer in 1972 as an extrinsic mechanical process. However, contemporary understanding acknowledges a multifactorial etiology, blending extrinsic compression with intrinsic tendon degeneration, microvascular insufficiency, and altered scapulothoracic kinematics. Chronic impingement initiates a cascade of bursal hypertrophy, tendinosis, and ultimately, partial or full-thickness rotator cuff tears. Understanding this continuum is paramount; surgery of the undersurface of the acromion is rarely a standalone solution for intrinsic tendinopathy but is highly effective for alleviating extrinsic structural abutment.

Epidemiologically, shoulder pain is the third most common musculoskeletal complaint presenting to primary care, with subacromial impingement accounting for up to 65% of these cases. The incidence of acromial undersurface pathology increases linearly with age, peaking in the fifth and sixth decades of life. This demographic trend parallels the natural history of rotator cuff degeneration and the progressive ossification of the CA ligament insertion, which forms the classic anterior acromial enthesophyte. Furthermore, overhead athletes and manual laborers experience a disproportionately high burden of this disease due to repetitive microtrauma and obligate forward elevation, which dynamically narrows the subacromial space.

Similarly, several large and well-published randomized controlled trials have recently questioned the efficacy of commonly used procedures, particularly isolated ASAD, and the practice of many surgeons is changing as a result. Studies such as the CSAW (Can Shoulder Arthroscopy Work?) and FIMPACT trials have demonstrated that the clinical benefit of subacromial decompression may not significantly exceed that of structured rehabilitation or placebo surgery in select populations with isolated impingement. Consequently, the modern orthopedic surgeon must exercise stringent patient selection, reserving surgery of the acromial undersurface for patients with refractory symptoms, distinct morphological abnormalities (e.g., Type III hooked acromion), or as an adjunct to rotator cuff repair. We describe the process of a diagnostic shoulder arthroscopy and then expand on how therapeutic procedures are performed, describing open alternatives and the treatment of degenerative joint disease with arthroplasty.

Detailed Surgical Anatomy and Biomechanics

Osteology and Ligamentous Anatomy of the Acromion

The acromion process is the lateral extension of the scapular spine, projecting anteriorly and superiorly to overhang the glenohumeral joint. Its morphological variance is a critical determinant of subacromial space volume. Bigliani and Morrison famously classified acromial morphology into three distinct types based on supraspinatus outlet radiographs: Type I (flat), Type II (curved), and Type III (hooked). A Type IV (convex) was later added. The Type III hooked acromion has a statistically significant correlation with full-thickness rotator cuff tears due to its aggressive anterior-inferior projection, which dynamically impinges on the supraspinatus insertion during forward elevation and internal rotation.

Embryologically, the acromion develops from multiple ossification centers: the basi-, meta-, meso-, and pre-acromion. Failure of these centers to fuse by age 22 results in an os acromiale, most commonly occurring between the meso- and meta-acromion. This anatomical variant is present in up to 8% of the population and is often bilateral. An unstable os acromiale can cause dynamic impingement by tilting inferiorly during deltoid contraction. Surgical management of the undersurface of the acromion in the presence of an os acromiale requires extreme caution; aggressive acromioplasty can destabilize the synchondrosis, leading to catastrophic deltoid dysfunction and persistent pain.

The coracoacromial (CA) ligament is a robust, triangular band extending from the lateral border of the coracoid process to the anterior undersurface of the acromion. Biomechanically, it functions as a critical tension band, preventing superior migration of the humeral head, particularly in the setting of massive, irreparable rotator cuff tears. The CA ligament also serves as the primary restraint against anterosuperior translation. During an arthroscopic subacromial decompression, the surgeon must carefully balance the need to release the thickened, impinging lateral band of the CA ligament against the risk of destabilizing the coracoacromial arch. In patients with massive cuff deficiency, complete release of the CA ligament is strictly contraindicated, as it precipitates rapid anterosuperior escape of the humeral head and accelerated cuff tear arthropathy.

The Coracoacromial Arch and Subacromial Space

The coracoacromial arch is an unyielding osseoligamentous vault formed by the acromion, the coracoid process, and the spanning CA ligament. The subacromial space, situated beneath this arch and above the humeral head, contains the subacromial-subdeltoid bursa, the rotator cuff tendons (primarily supraspinatus), and the long head of the biceps tendon. In a healthy shoulder, the height of this space ranges from 7 to 14 mm on an anteroposterior radiograph. A reduction in this interval to less than 7 mm is highly predictive of rotator cuff pathology and structural impingement.

The subacromial bursa is a highly innervated, synovial-lined sac that facilitates frictionless gliding of the rotator cuff beneath the arch. In pathologic states, the bursa undergoes florid hypertrophy, angiogenesis, and fibrosis, transforming from a potential space into a thick, pain-generating tissue mass. This bursitis is often the primary source of nociception in impingement syndrome. Effective surgery of the undersurface of the acromion mandates a meticulous, near-total bursectomy to eliminate this inflammatory tissue, expose the underlying rotator cuff footprint, and clearly visualize the osseous landmarks of the acromion before any bone resection is attempted.

Biomechanically, the subacromial space is dynamically modulated by the force couples of the shoulder. The deltoid exerts a superiorly directed shear force on the humerus, which is counteracted by the inferiorly directed compressive forces of the intact rotator cuff. When the rotator cuff is fatigued or torn, this force couple is uncoupled, allowing the deltoid to drive the humeral head superiorly into the undersurface of the acromion. Therefore, surgical decompression must always be paired with aggressive postoperative rehabilitation aimed at restoring these dynamic periscapular and glenohumeral stabilizers.

Neurovascular Considerations

A profound understanding of the regional neurovascular anatomy is essential to prevent iatrogenic injury during portal placement and subacromial instrumentation. Diagnostic shoulder arthroscopy and subacromial surgery carry relatively low complication rates, but the proximity of major nerves demands respect. The musculocutaneous nerve is at risk during the establishment of the anterior portal. It courses medially to the coracoid process, penetrating the coracobrachialis muscle. The anterior portal must be created lateral to the coracoid process to maintain a safe distance from this nerve and the adjacent cephalic vein.

The axillary nerve, which provides critical motor innervation to the deltoid and teres minor, courses transversely across the deep surface of the deltoid muscle, approximately 5 cm distal to the lateral edge of the acromion. When establishing the lateral subacromial working portal, the incision and subsequent instrument insertion must remain strictly superior to this 5 cm safe zone. Plunging with a trocar or extending the portal too distally can result in devastating axillary neuropraxia or complete transection, leading to catastrophic deltoid paralysis.

The suprascapular nerve can be damaged by the inexperienced arthroscopist, particularly during extensive posterior capsular releases or when addressing pathology at the spinoglenoid notch. It courses through the suprascapular notch (beneath the transverse scapular ligament) and the spinoglenoid notch, supplying the supraspinatus and infraspinatus. Furthermore, the acromial branch of the thoracoacromial artery provides the primary blood supply to the anterior acromion and CA ligament. Brisk bleeding from this vessel is frequently encountered during CA ligament release and requires prompt electrocautery to maintain a clear visual field.

Exhaustive Indications and Contraindications

Patient Selection Criteria

The decision to proceed with surgery of the undersurface of the acromion must be grounded in a rigorous clinical evaluation and a documented failure of comprehensive non-operative management. Diagnostic shoulder arthroscopy can be used as part of the arthroscopic treatment of various pathologies, but also purely for diagnostic purposes given the relatively low complication rates. However, therapeutic acromioplasty is primarily indicated for patients with classic, primary subacromial impingement syndrome who have experienced persistent pain and functional limitation despite a minimum of 3 to 6 months of conservative therapy. This therapy should include targeted physical therapy focusing on rotator cuff strengthening and scapular stabilization, oral NSAIDs, and subacromial corticosteroid injections.

Secondary indications include the presence of a morphologically aggressive Type III hooked acromion in a symptomatic patient, calcific tendonitis refractory to barbotage, and as an obligate adjunct during the repair of full-thickness rotator cuff tears to protect the repair construct from postoperative mechanical abrasion. In cases of partial articular-sided tendon avulsion (PASTA) lesions or bursal-sided partial tears, clearance of the subacromial space is often necessary to accurately assess tear depth and facilitate either debridement or completion and repair.

Conversely, the procedure is strictly contraindicated in specific clinical scenarios. As previously noted, massive, irreparable rotator cuff tears rely on the intact coracoacromial arch for superior stability; resecting the undersurface of the acromion and the CA ligament in these patients will inevitably lead to anterosuperior escape and severe pseudoparalysis. Furthermore, patients with secondary impingement due to multidirectional instability or isolated scapular dyskinesia should not undergo acromioplasty, as their pathology is kinematic rather than structural. Operating on these patients will fail to resolve their symptoms and may exacerbate underlying instability.

Markdown Table of Indications and Contraindications

Pre-Operative Planning, Templating, and Patient Positioning

Diagnostic Imaging and Templating

Comprehensive preoperative imaging is the cornerstone of successful subacromial surgery. A standard radiographic series must include a true anteroposterior (Grashey) view, an axillary lateral view, and a supraspinatus outlet (Y-scapula) view. The outlet view is critical for assessing the Bigliani acromial morphology, identifying anterior acromial enthesophytes, and evaluating the acromiohumeral interval. An interval of less than 7 mm on the AP radiograph strongly suggests significant rotator cuff disease. The axillary view is essential for ruling out glenohumeral osteoarthritis, subtle dislocations, and identifying an os acromiale.

Magnetic Resonance Imaging (MRI) without contrast is the gold standard for evaluating the soft tissue envelope of the subacromial space. MRI provides exquisite detail regarding the integrity of the rotator cuff tendons, the presence of bursal fluid or hypertrophy, and the status of the long head of the biceps tendon. Furthermore, sagittal oblique T2-weighted sequences allow for precise measurement of acromial thickness and the extent of the CA ligament footprint. This enables the surgeon to preoperatively "template" the amount of bone resection required, ensuring an adequate decompression without violating the structural integrity of the acromion.

In cases where an os acromiale is suspected or complex bony deformity is present, a computed tomography (CT) scan with 3D reconstruction can be invaluable. CT provides unparalleled resolution of osseous anatomy, allowing the surgeon to precisely map the synchondrosis of an os acromiale or evaluate the extent of acromioclavicular joint hypertrophy (the "kissing lesion") that may be contributing to the impingement process.

Anesthesia and Patient Positioning

The choice of anesthesia and patient positioning significantly impacts the surgical workflow and intraoperative hemodynamics. Surgery of the undersurface of the acromion is typically performed under general anesthesia supplemented with a regional interscalene nerve block. The interscalene block provides profound postoperative analgesia, reducing the need for systemic opioids and facilitating early mobilization. However, the anesthesia team must be vigilant regarding the potential for phrenic nerve palsy and subsequent hemidiaphragmatic paresis, which is a common but usually transient side effect of the block.

Patient positioning is broadly divided into the beach chair (BC) and lateral decubitus (LD) positions. The beach chair position, with the patient seated at approximately 45 to 60 degrees, offers the distinct advantage of an upright anatomical orientation, making conversion to an open procedure straightforward if necessary. It also allows for dynamic assessment of the shoulder through a full range of motion. However, the BC position is associated with the risk of cerebral hypoperfusion and the Bezold-Jarisch reflex—a triad of profound bradycardia, hypotension, and peripheral vasodilation caused by venous pooling in the lower extremities.

Alternatively, the lateral decubitus position provides excellent visualization of the subacromial space by utilizing longitudinal and lateral traction to distract the glenohumeral joint and open the subacromial interval. The patient is positioned on their side with the operative arm suspended in 45 degrees of abduction and 15 degrees of forward flexion using a dedicated traction device. While this position mitigates the risk of cerebral hypoperfusion, it carries a higher risk of traction-induced neuropraxia to the brachial plexus if the traction weight exceeds 10-15 pounds or if the surgical duration is prolonged.

Portal Placement Strategy

Precise portal placement is the linchpin of arthroscopic subacromial surgery. A poorly placed portal will result in an ongoing struggle with instrument crowding, poor visualization, and incomplete decompression. The standard posterior viewing portal is established first, located approximately 2 cm inferior and 1 cm medial to the posterolateral corner of the acromion. This portal provides the primary panoramic view of both the glenohumeral joint and the subacromial space.

The anterior working portal is created under direct intra-articular visualization using an outside-in spinal needle technique. It is typically located in the rotator interval, bounded by the long head of the biceps superiorly, the subscapularis inferiorly, and the glenoid medially. As previously mentioned, this portal must be kept lateral to the coracoid process to avoid the musculocutaneous nerve.

For surgery of the undersurface of the acromion, the lateral portal is the primary working conduit. It is established 2 to 3 cm lateral to the lateral edge of the acromion, directly in line with the posterior aspect of the clavicle. This portal allows for the introduction of shavers, radiofrequency wands, and motorized burrs parallel to the undersurface of the acromion. The surgeon must strictly adhere to the 5 cm safe zone to protect the axillary nerve during the creation and utilization of this lateral portal.

Step-by-Step Surgical Approach and Fixation Technique

Diagnostic Glenohumeral Arthroscopy

Every procedure targeting the undersurface of the acromion must begin with a comprehensive diagnostic glenohumeral arthroscopy. Diagnostic shoulder arthroscopy can be used as part of arthroscopic treatment of various pathologies, but also purely for diagnostic purposes given the relatively low complication rates. The arthroscope is introduced through the posterior portal, and a systematic 15-point diagnostic tour is executed. The surgeon must evaluate the articular cartilage of the humeral head and glenoid, the integrity of the superior, anterior, and posterior labrum, and the capsular ligaments.

Particular attention is directed to the intra-articular portion of the long head of the biceps tendon and the articular undersurface of the rotator cuff. The biceps tendon should be pulled into the joint using a probe to assess for hidden tears or tenosynovitis within the bicipital groove. The undersurface of the supraspinatus and infraspinatus must be meticulously probed to identify partial articular-sided tendon avulsion (PASTA) lesions, which may not be visible from the subacromial space. Once the intra-articular evaluation is complete, the arthroscope is withdrawn into the subcutaneous tissue and redirected superiorly into the subacromial bursa.

Subacromial Bursectomy and Space Preparation

Upon entering the subacromial space, the initial view is often obscured by hypertrophic bursal tissue—the so-called "snowstorm" appearance. The first objective is to create a "room with a view." A blunt trocar is used to sweep the bursa off the undersurface of the acromion. A motorized shaver is then introduced through the lateral portal to perform a systematic, near-total bursectomy. The shaver is operated with the cutting window facing superiorly to avoid inadvertent damage to the underlying rotator cuff.

The bursectomy begins laterally and progresses medially toward the AC joint, and then anteriorly toward the coracoacromial ligament. Meticulous hemostasis is critical during this phase; a radiofrequency ablation wand is frequently utilized to coagulate the highly vascular bursal tissue and the acromial branch of the thoracoacromial artery. The goal of the bursectomy is to completely expose the osseous undersurface of the acromion, the CA ligament, the distal clavicle, and the bursal surface of the rotator cuff footprint. Without this pristine visualization, an accurate and safe acromioplasty is impossible.

Arthroscopic Acromioplasty Technique

With the subacromial space prepared, the arthroscopic acromioplasty (subacromial decompression) is initiated. The radiofrequency wand is used to delineate the anterior and lateral margins of the acromion and to release the superficial fibers of the CA ligament from their bony attachment. This release should be precise; completely detaching the CA ligament is unnecessary and potentially destabilizing. The wand is then used to score the undersurface of the acromion, creating a visual template for the planned bone resection.

A 4.0 mm or 5.5 mm barrel burr is introduced through the lateral portal. The classic "cutting block" technique is employed. The surgeon begins at the anterolateral corner of the acromion, burring from anterior to posterior to create a flat channel that serves as a depth gauge. The resection typically removes 4 to 6 mm of bone anteriorly, tapering to zero posteriorly. The burr is then swept medially to resect the remaining anterior hook, converting a Type II or III acromion into a flat Type I morphology.

Throughout the resection, the surgeon must frequently rotate the arthroscope to view the acromion from both the posterior and lateral portals, ensuring a planar resection without creating iatrogenic ridges or "divots." If the patient has symptomatic AC joint arthrosis with inferior osteophytes contributing to impingement, an arthroscopic coplaning of the distal clavicle or a formal distal clavicle excision (Mumford procedure) is performed concurrently. The final step involves a dynamic examination; the arm is taken through a full range of forward elevation and internal rotation while visualizing the subacromial space to confirm that all mechanical abutment has been eliminated.

Open Shoulder Procedures and Arthroplasty Considerations

While arthroscopic techniques dominate modern practice, open shoulder procedures remain a vital component of the orthopedic armamentarium. As noted, since the publication of the first edition of this book, arthroscopic shoulder surgery has seen a significant expansion and consequentially open procedures have become increasingly less common. However, the principles behind the various procedures have stayed the same. Open anterior acromioplasty is still indicated in cases of massive, retracted rotator cuff tears requiring complex open mobilization and repair, or when arthroscopic fluid management fails catastrophically.

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