Rim Sign

1. High-Yield Overview & Pathognomonic Essence

• Radiographic Hallmark: Widening of the radio-humeral joint space exceeding 6 mm between the anterior glenoid rim and the medial articular cortex of the humeral head on a standard anteroposterior (AP) radiograph of the shoulder.
• Pathognomonic Association: Pathognomonic radiographic indicator of posterior glenohumeral dislocation, classically accompanying fixed internal rotation of the humerus (light-bulb sign), loss of the physiological half-moon overlap, and anteromedial humeral head impaction (reverse Hill-Sachs or trough line sign).
• Diagnostic Utility & Surgical Relevance: Serves as a vital primary screening checkpoint to prevent missed or delayed diagnosis of posterior shoulder dislocations, which are overlooked in 50% to 80% of initial clinical presentations; recognition mandates immediate orthogonal axillary or scapular Y views, urgent closed reduction, and cross-sectional CT staging to evaluate reverse Hill-Sachs defect sizing and posterior glenoid bone loss.

2. Diagnostic Sign Summary Matrix

3. Clinical Case Imaging & Standardized Figures

Standard plain radiography represents the initial diagnostic checkpoint in any patient presenting with acute shoulder trauma, seizures, or unexplained severe loss of external rotation. In posterior shoulder dislocations, anteroposterior radiographs can appear deceptively normal to the untrained eye because the humeral head remains at the level of the glenoid fossa without inferior subglenoid displacement. Recognition of the positive Rim Sign provides immediate diagnostic evidence of posterior subluxation.

As demonstrated in Figure 1, the classic positive Rim Sign manifests as an abnormally widened clear space between the sharp cortical line of the anterior glenoid rim and the medial articular cortex of the humeral head. The normal anatomical overlap between the humeral head and the glenoid cavity is completely abolished. In addition, the proximal humerus displays the characteristic globular, symmetric contour of the light-bulb sign resulting from fixed internal rotation, alongside a distinct vertical cortical condensation representing the trough line of an anteromedial impaction fracture.

4. Pathophysiology & Biomechanical Mechanism

The biomechanical origin of the Rim Sign is directly rooted in the spatial orientation of the glenohumeral joint and the translational kinematics of posterior humeral displacement. Anatomically, the scapula does not lie in the coronal plane of the thorax; rather, it rests at approximately 30 to 45 degrees of anterior rotation along the curved thoracic wall (the plane of the scapula). Furthermore, the glenoid fossa possesses physiological anteversion or slight retroversion relative to the scapular blade, facing anterolaterally and slightly superiorly. Consequently, a standard anteroposterior radiograph directed perpendicular to the coronal plane of the thorax does not project parallel to the glenohumeral articular surface, but rather obliquely across the joint.

Under normal anatomical conditions, this oblique thoracic projection causes the medial third of the spherical humeral head to superimpose upon the anterior and posterior cortical margins of the glenoid fossa. This creates an elliptical zone of physiological overlap termed the “half-moon overlap sign.” The true radiological joint space, representing the combined thickness of the articular cartilage of the glenoid and humeral head, measures between 2 and 4 mm.

Posterior glenohumeral dislocations typically result from two distinct clinical mechanisms: violent uncoordinated muscular contractions (associated with generalized epileptic seizures, electrocution, or lightning strike) and high-energy traumatic impaction (axial loading on an adducted, flexed, and internally rotated arm, such as during a motor vehicle collision dashboard injury). During violent muscular contraction, the internal rotators of the shoulder (subscapularis, pectoralis major, latissimus dorsi, and teres major) vastly overpower the weaker external rotators (infraspinatus and teres minor). The resulting forceful internal rotation and posterior translation propel the humeral head posteriorly over the cartilaginous posterior glenoid labrum.

As the humeral head translates posteriorly and settles behind the posterior glenoid rim, its spatial relationship with the anterior glenoid margin changes fundamentally. Because the glenoid fossa faces anteriorly and laterally, posterior displacement shifts the humeral head further away from the anterior rim. On an anteroposterior radiograph, the anterior glenoid rim projects as a distinct vertical cortical line, while the dislocated humeral head rests several millimeters posterior and lateral to this margin. This posterior displacement abolishes the physiological elliptical overlap and creates an apparent widening of the radio-humeral space. In 1965, Arndt and Sears demonstrated that a distance exceeding 6 mm between the anterior glenoid margin and the medial articular border of the humeral head definitively signifies posterior dislocation.

Simultaneously, as the humeral head is driven against the rigid, cortical posterior glenoid rim, the softer cancellous bone of the anteromedial humeral articular margin undergoes impaction. This produces a vertical, wedge-shaped compression fracture known as the reverse Hill-Sachs lesion (McLaughlin defect). The posterior glenoid rim locks into this osteochondral defect, mechanically anchoring the humerus in internal rotation and preventing passive external rotation. This persistent internal rotation and locked impaction permanently sustain the widened radiographic distance defining the positive Rim Sign.

5. Modality-Specific Imaging Findings

Plain Radiography

Plain radiography remains the initial frontline imaging investigation, although diagnostic accuracy relies heavily on obtaining a complete trauma series rather than relying solely on a single AP view:

• Standard Anteroposterior (AP) View: Demonstrates the positive Rim Sign, characterized by a clear space exceeding 6 mm between the anterior glenoid rim and the medial humeral head margin. The normal elliptical half-moon articular overlap is absent (“vacant glenoid sign”). Due to fixed internal rotation, the proximal humerus assumes a rounded, symmetrical contour resembling an electric light bulb (“light-bulb sign”), as the greater tuberosity rotates anteriorly and medially out of its standard lateral profile. A dense vertical line parallel to the medial humeral border represents the trough line sign, reflecting the compacted cortical margin of a reverse Hill-Sachs lesion. Avulsion fractures of the lesser tuberosity may also be identified.
• True AP View (Grashey View): Directed parallel to the scapular blade (35 to 45 degrees of patient rotation toward the cassette). Confirms complete loss of joint congruency and documents that the humeral head no longer aligns within the glenoid fossa.
• Axillary Lateral View: The definitive plain radiographic projection. By directing the x-ray beam superior-to-inferior through the axilla with the arm in abduction, this view unequivocally demonstrates the posterior relationship of the humeral head relative to the glenoid fossa, reveals the depth of the reverse Hill-Sachs defect, and identifies posterior glenoid rim fractures (reverse Bankart lesions).
• Velpeau Axillary View: Critical modified projection for patients who cannot tolerate abduction due to acute pain, muscular spasm, or locked internal rotation. The patient sits or reclines 30 degrees backward over the cassette, and the vertical beam passes through the shoulder from superior to inferior, confirming posterior displacement without requiring arm manipulation.
• Scapular Y (Transscapular) View: The humeral head is projected posteriorly relative to the central junction of the scapular spine, acromion, and coracoid process, confirming posterior subluxation when an axillary projection is technically compromised.

Computed Tomography

Computed tomography (CT) with multiplanar 2D axial reformats and 3D volume rendering represents the gold standard for definitive anatomical assessment, surgical planning, and quantitative defect staging:

• Defect Quantification: Accurately measures the size, depth, and articular percentage of the reverse Hill-Sachs lesion. Impaction defects are categorized into less than 20%, 20% to 40%, and greater than 40% of the articular surface, which dictates operative reconstruction strategy.
• Glenoid Rim Integrity: Identifies posterior glenoid rim fractures (reverse Bankart lesions), assesses fragment size, displacement, and comminution, and quantifies critical posterior glenoid bone loss.
• Glenoid Version Assessment: Evaluates native glenoid retroversion and dysplastic morphology, which may predispose to recurrent posterior instability.
• Chronic Locked Dislocation Staging: In neglected cases, CT visualizes osteopenic changes, head deformity, pseudo-glenoid articulation on the posterior scapular neck, and secondary degenerative changes.

Magnetic Resonance Imaging & Ultrasound

Advanced soft-tissue modalities provide critical complementary diagnostic information regarding capsuloligamentous integrity, rotator cuff status, and bedside reduction confirmation:

• Magnetic Resonance Imaging (MRI): Accurately characterizes tears of the posterior labrum (reverse Bankart lesions, posterior labrocapsular periosteal sleeve avulsions [POLPSA], and Kim lesions), posterior capsular stripping, and patulous posterior pseudo-pouches. T1-weighted sequences assess muscle fatty infiltration and atrophy (Goutallier classification) of the subscapularis and infraspinatus in chronic presentations. T2-weighted and fluid-sensitive sequences highlight prominent trabecular bone marrow edema within the anteromedial humeral head and posterior glenoid rim, while evaluating for concomitant full-thickness subscapularis tendon ruptures.
• Point-of-Care Ultrasound (POCUS): Rapid, non-invasive bedside evaluation using a high-frequency linear transducer placed transversely over the posterior infraspinatus fossa. Demonstrates posterior offset of the curvilinear humeral head relative to the bony posterior glenoid margin, dynamic lack of rotation, and hemarthrosis. As established in systematic reviews, bedside ultrasound provides greater than 98% sensitivity and 99% specificity for detecting shoulder dislocations and confirming successful closed reduction.

6. Diagnostic Utility, Sensitivity & Specificity

The clinical and radiographic recognition of posterior shoulder dislocation has historically presented one of the most notorious challenges in acute musculoskeletal trauma. In their classic series, McLaughlin (1952) and Hawkins et al. (1987) reported that 60% to 80% of posterior shoulder dislocations were missed at the initial clinical encounter, resulting in diagnostic delays ranging from several weeks to months. Contemporary analyses confirm that missed diagnoses remain frequent, with approximately 50% to 79% of acute injuries overlooked upon initial presentation when relying solely on non-orthogonal plain radiographs.

In 1965, Arndt and Sears established the diagnostic parameter of the Rim Sign: on non-magnified standard AP radiographs, the distance between the anterior glenoid rim and the medial margin of the humeral head rarely exceeds 4 mm in a normal joint. A measured distance greater than 6 mm demonstrates high specificity (exceeding 90% to 95%) for posterior dislocation, provided true anatomical joint space widening is differentiated from inferior pseudo-subluxation.

However, the standalone sensitivity of the AP radiograph remains suboptimal, averaging between 50% and 60% when evaluated by non-specialist clinicians. Kirilmaz et al. (2026) demonstrated in a comparative diagnostic performance analysis that emergency physicians identified posterior shoulder dislocations on AP views with 58.3% sensitivity, whereas orthopaedic surgeons achieved 76.7% sensitivity, with detection closely tied to deliberate recognition of the rim sign and light-bulb sign. Combining multiple secondary AP signs (Rim Sign, light-bulb sign, trough line sign, and loss of half-moon overlap) raises the overall diagnostic sensitivity to approximately 80%. When orthogonal axillary lateral views or multiplanar CT scans are obtained, diagnostic accuracy reaches 100%.

7. Differential Diagnosis & Radiographic Mimics

When an apparent widening or abnormality of the radio-humeral interface is identified on plain radiography, clinicians must systematically differentiate the true positive Rim Sign of posterior dislocation from anatomical and pathological mimics:

8. Clinical Significance & Management Implications

The identification of a positive Rim Sign represents an orthopaedic urgency that mandates immediate clinical staging and tailored therapeutic intervention. Management depends primarily on chronicity (acute versus neglected chronic locked dislocation), patient physiological demands, and the anatomical size of the reverse Hill-Sachs defect:

Acute Closed Reduction Technique & Pitfalls

In acute presentations (within 3 weeks of injury), prompt closed reduction under procedural sedation or general anesthesia with complete muscle relaxation should be performed. The reduction maneuver must follow precise biomechanical principles:

• Traction and Translation: In-line axial traction is applied to the adducted arm while an assistant applies gentle anterior translational pressure to the posterior aspect of the humeral head to disengage the reverse Hill-Sachs defect from the posterior glenoid rim.
• The Fulcrum Danger Zone: Clinicians must strictly avoid forceful external rotation while the humeral head remains engaged on the rigid posterior glenoid rim. Applying external rotation against the locked defect converts the posterior glenoid into a rigid fulcrum, producing catastrophic spiral or transverse fractures of the humeral surgical neck. Gentle neutral rotation should only be attempted after anterior translational disengagement.

Defect-Based Surgical Algorithm

Following successful reduction, joint stability is directly governed by the dimensions of the reverse Hill-Sachs impaction defect and the integrity of the posterior glenoid rim. Cross-sectional CT multiplanar staging guides the operative pathway:

• Defect Less than 20% of Articular Surface: The joint is typically stable post-reduction. Management consists of immobilization in a neutral or slight external rotation (10 to 15 degrees) sling for 4 to 6 weeks to avoid tension on the posterior capsule, followed by supervised rotator cuff and periscapular rehabilitation.
• Defect Between 20% and 40% of Articular Surface: The defect engages the posterior glenoid rim during functional internal rotation, resulting in recurrent instability. Operative reconstruction is indicated:
  • McLaughlin Procedure: Open tenodesis of the subscapularis tendon into the anteromedial humeral head defect, obliterating the notch.
  • Neer Modification: Osteotomy of the lesser tuberosity with its attached subscapularis tendon, transferring and rigidly fixing the osseous block into the defect with cancellous screws, providing superior bone-to-bone healing.
  • Anatomic Reconstruction (Humeroplasty or Allograft): Elevation and percutaneous balloon/tamp humeroplasty with subchondral bone grafting, or segmental structural fresh-frozen osteochondral allograft (femoral head or humeral head) contoured to restore spherical articular geometry.
• Defect Greater than 40% of Articular Surface or Chronic Locked Dislocation: Severe articular destruction, osteonecrosis risk, and persistent instability preclude joint-preserving procedures. Hemiarthroplasty (with humeral component retroversion adjusted to 0 to 10 degrees to prevent posterior subluxation) or Reverse Total Shoulder Arthroplasty (RTSA) is indicated, particularly in elderly patients, chronic presentations exceeding 6 months, or cases with extensive posterior glenoid bone loss.

9. Master Class Pro-Tip

Master Class Pro-Tip: Never dismiss an apparently unremarkable anteroposterior shoulder radiograph in a patient presenting with locked internal rotation and severe loss of external rotation following a seizure, electrocution, or high-energy trauma. Systematically interrogate the radio-humeral joint space for the positive Rim Sign (>6 mm distance between the anterior glenoid rim and medial humeral articular cortex) alongside the globular symmetry of the light-bulb sign and loss of the physiological half-moon articular overlap. If the patient cannot tolerate abduction for a standard axillary projection due to severe pain and spasm, immediately obtain a Velpeau axillary view by having the patient recline 30 degrees backward beneath a vertical radiographic beam, or proceed directly to non-contrast axial CT. During closed reduction, never force external rotation against a locked reverse Hill-Sachs impaction, as the rigid posterior glenoid rim acts as a fulcrum that can induce an iatrogenic humeral neck fracture; always apply gentle in-line axial traction with anterior translational pressure on the posterior humeral head to disengage the defect prior to rotation.

Master Class Radiological Pearl: Avoiding Diagnostic Pitfalls and Reduction Fractures in Posterior Shoulder Dislocation

10. Evidence-Based Synthesis

11. References

1. Rouleau DM, Hebert-Davies J. Incidence of associated injury in posterior shoulder dislocation: systematic review of the literature. J Orthop Trauma. 2012;26(4):246-251. DOI [Level I: Systematic Review / Meta-Analysis]
2. Gottlieb M, Patel D, Marks A, et al. Ultrasound for the diagnosis of shoulder dislocation and reduction: A systematic review and meta-analysis. Acad Emerg Med. 2022;29(8):998-1007. DOI [Level I: Systematic Review / Meta-Analysis]
3. Kirilmaz A, Erdem TE, Yaka H, et al. Diagnosis of shoulder dislocation on AP radiographs: A comparative analysis of diagnostic performance between orthopedic surgeons, emergency physicians, and ChatGPT models. Injury. 2026;57(2):112957. DOI [Level II: Comparative Study / Decision Analysis]
4. Rouleau DM, Hebert-Davies J, Robinson CM. Acute traumatic posterior shoulder dislocation. J Am Acad Orthop Surg. 2014;22(3):145-152. DOI [Level IV: Clinical Case Series / Cohort Study]
5. Vaishya R, Saiyed A, Vaish A, et al. Chronic posterior shoulder dislocation: Current evidence, treatment options, and a decision-making algorithm. J Orthop. 2026;55:181-187. DOI [Level IV: Clinical Case Series / Cohort Study]
6. Cicak N. Posterior dislocation of the shoulder. J Bone Joint Surg Br. 2004;86(3):324-332. DOI [Level IV: Clinical Case Series / Cohort Study]
7. Koutserimpas C, Piagkou M, Karaiskos I, et al. Posterior Dislocation of the Shoulder: The Light-Bulb Sign. Cureus. 2023;15(10):e47800. DOI [Level IV: Clinical Case Series / Cohort Study]
8. Arndt JH, Sears AD. Posterior dislocation of the shoulder. Am J Roentgenol Radium Ther Nucl Med. 1965;94:639-645. PubMed [Level IV: Clinical Case Series / Cohort Study]
9. Hawkins RJ, Neer CS 2nd, Pianta RM, et al. Locked posterior dislocation of the shoulder. J Bone Joint Surg Am. 1987;69(1):9-18. PubMed [Level IV: Clinical Case Series / Cohort Study]
10. McLaughlin HL. Posterior dislocation of the shoulder. J Bone Joint Surg Am. 1952;34-A(3):584-590. PubMed [Level IV: Clinical Case Series / Cohort Study]