Trough Line

1. High-Yield Overview & Pathognomonic Essence

• Morphological Hallmark: The trough line manifests on anteroposterior (AP) shoulder radiographs as an anomalous vertical, dense cortical line running parallel to the medial border of the humeral head, producing the characteristic appearance of two parallel vertical lines of cortical bone.
• Pathognomonic Significance: The inner (more lateral) line represents the edge of an anteromedial humeral head impaction fracture (reverse Hill-Sachs or McLaughlin lesion) sustained when the internally rotated head impacts violently against the rigid posterior glenoid rim during a posterior glenohumeral dislocation.
• Critical Diagnostic Mandate: Posterior shoulder dislocations are notoriously misdiagnosed on initial emergency presentation in over 50% of cases; recognizing the trough line on an AP radiograph alerts the clinician to obtain mandatory orthogonal axillary lateral or Velpeau views and definitive computed tomography (CT) before attempted reduction.

2. Diagnostic Sign Summary Matrix

3. Clinical Case Imaging & Standardized Figures

4. Pathophysiology & Biomechanical Mechanism

Posterior glenohumeral dislocation accounts for approximately 2% to 5% of all shoulder dislocations and characteristically occurs through violent muscular contractions—classically during grand mal epileptic seizures, accidental high-voltage electrical shock, or electroconvulsive therapy—or severe high-energy blunt trauma involving axial loading on an adducted, internally rotated, and flexed upper extremity. In convulsive episodes, the massive internal rotator muscle group (latissimus dorsi, pectoralis major, and subscapularis) mechanically overpowers the comparatively weaker external rotators (infraspinatus and teres minor), violently driving the humeral head backward out of the shallow glenoid fossa.

As the humeral head translates posteriorly over the posterior glenoid labrum and osseous rim, its relatively softer, cancellous anteromedial articular surface is driven forcefully against the dense, hard cortical margin of the posterior glenoid rim. Under sustained muscular spasm or traumatic momentum, the anterior humeral articular surface sustains an osteochondral compression fracture. This indentation crater—originally described by Harrison L. McLaughlin in 1952—is termed a reverse Hill-Sachs lesion, McLaughlin lesion, or trough defect.

On anteroposterior projection, the radiographic x-ray beam passes tangentially across this vertical impaction crater. Two distinct cortical boundaries are projected simultaneously: the normal curved medial cortex of the humeral head (the true anatomical articular margin) and an anomalous vertical, linear cortical density representing the lateral boundary or floor of the compressed trough. This diagnostic cortical duplication forms the pathognomonic trough line described by Cisternino and colleagues in 1978.

5. Modality-Specific Imaging Findings

Plain Radiography

Standard AP views in posterior dislocation can appear deceptively benign to the unsuspecting clinician because the humeral head remains approximately at the level of the glenoid fossa. However, meticulous inspection reveals several key pathognomonic signs:

• The Trough Line: A sharp, dense vertical cortical line traversing the medial aspect of the humeral head, oriented parallel to the medial humeral articular contour. This represents the impaction margin projected tangentially.
• Lightbulb Sign: Fixed internal rotation of the humerus brings the greater and lesser tuberosities into profile directly anteriorly and posteriorly, causing the humeral head to lose its anatomical asymmetric neck-shaft angle and project with symmetrical, rounded bulbous contours mimicking an incandescent lightbulb.
• Rim Sign: Widening of the apparent glenohumeral joint space greater than 6 mm between the anterior glenoid rim and the medial humeral cortex, caused by posterior displacement of the humeral head away from the glenoid articular surface.
• Vacant Glenoid Sign & Loss of Elliptical Overlap: In normal AP radiographs, the medial humeral head overlaps the posterior glenoid rim creating a classic half-moon elliptical overlap; in posterior dislocation, this overlap disappears, leaving the anterior half of the glenoid fossa radiographically vacant.

Because standard AP projections miss up to 50% of posterior dislocations, dedicated orthogonal views are non-negotiable. An axillary lateral radiograph definitively demonstrates posterior subluxation of the humeral head and reveals the engagement of the anteromedial trough defect against the posterior glenoid margin. In acute trauma where pain prevents 90-degree abduction, a Velpeau view (angled 30 degrees caudally with the patient leaning backward 30 degrees) or a true scapular Y (transscapular) view confirms whether the humeral head is centered within the intersection of the coracoid, spine, and body of the scapula.

Computed Tomography (CT)

Computed tomography with two-dimensional multiplanar (axial, coronal, sagittal) and three-dimensional surface reconstructions represents the ultimate gold standard for evaluation. Axial CT images unequivocally display the position of the humeral head relative to the glenoid, depict the exact location and arc depth of the reverse Hill-Sachs defect, and identify associated posterior glenoid rim fractures (reverse Bankart fracture) or lesser tuberosity avulsions. CT is essential for quantitative defect measurement, calculating the percentage of humeral head articular involvement to drive surgical decision-making.

Magnetic Resonance Imaging & Ultrasound

Magnetic resonance imaging (MRI) is indicated primarily in younger athletic patients or following successful closed reduction to assess capsulolabral and rotator cuff integrity. On T1-weighted sequences, the trough lesion produces a prominent cortical contour defect with low signal intensity. T2-weighted and STIR sequences reveal extensive surrounding bone marrow edema within the anteromedial humeral head and posterior glenoid neck. MRI demonstrates associated soft-tissue injuries, including detachment of the posterior labrum (reverse Bankart lesion), posterior capsular stripping, posterior capsular sleeve tears, and partial or complete subscapularis or infraspinatus tendon disruptions.

6. Diagnostic Utility, Sensitivity & Specificity

The diagnostic utility of the trough line lies in its ability to rescue the clinician from a missed posterior dislocation on routine AP trauma series. In the landmark retrospective investigation by Cisternino et al. (1978), the trough line was clearly identifiable in 75% of confirmed posterior shoulder dislocations. Modern multicenter series (Rouleau and Hebert-Davies, 2012) emphasize that while individual AP signs such as the lightbulb sign possess sensitivities ranging from 60% to 70%, the combination of the trough line with fixed internal rotation yields a diagnostic specificity exceeding 95% for posterior glenohumeral dislocation.

Axillary lateral radiography and axial CT demonstrate 100% sensitivity and 100% specificity for confirming posterior dislocation and mapping the trough defect. When an apparent trough line is identified on AP films, subsequent cross-sectional CT imaging has a positive predictive value (PPV) approaching 98% for demonstrating a structural osteochondral defect requiring surgical stabilization or guarded closed reduction protocols.

7. Differential Diagnosis & Radiographic Mimics

8. Clinical Significance & Management Implications

The clinical presence of a trough line directly dictates urgent management and surgical reconstructive algorithms, categorized by the duration of dislocation and the structural size of the anteromedial humeral defect:

• Acute Presentation (< 3 to 6 Weeks): For acute posterior dislocations with a minimal trough defect involving less than 20% of the articular surface, closed reduction under conscious sedation or general anesthesia with muscle relaxation is attempted using gentle in-line traction, forward flexion, and lateral translation. Following reduction, stability is tested: if stable, the shoulder is immobilized in a neutral to 10-15 degree external rotation brace for 4 to 6 weeks.
• Intermediate Defects (20% to 45% Articular Involvement): Defects in this range are inherently unstable following reduction and inevitably re-dislocate into the trough defect upon internal rotation. Surgical reconstruction is mandatory, utilizing the McLaughlin Procedure (subscapularis tendon transfer into the reverse Hill-Sachs defect), the Neer Modification (lesser tuberosity osteotomy with subscapularis transfer fixed into the defect with screws), or Anatomic Restoration (structural osteochondral allograft or balloon humeroplasty).
• Massive Defects (> 45% to 50% Articular Involvement) or Chronic Locked Dislocation: Severe defects or long-standing locked dislocations (> 6 months) complicated by osteonecrosis or extensive cartilage erosion cannot maintain stability or function with soft-tissue or localized bone grafting. These cases necessitate prosthetic reconstruction, utilizing hemiarthroplasty (with retroversion adjusted to 0-10 degrees to prevent posterior subluxation) or reverse total shoulder arthroplasty in elderly patients with deficient posterior rotator cuff musculature.

9. Master Class Pro-Tip

Master Class Pro-Tip: In any patient presenting to the emergency department or trauma bay following an epileptic convulsion or high-voltage shock who complains of shoulder pain and exhibits inability to passively externally rotate the arm past neutral, posterior shoulder dislocation must be assumed until proven otherwise. Never accept a solitary AP radiograph as diagnostic reassurance. If the patient cannot tolerate 90-degree abduction for a standard axillary view, immediately obtain a Velpeau view or emergency non-contrast CT. If an AP radiograph shows a vertical trough line parallel to the medial humeral border, do not attempt aggressive closed external rotation reduction on the ward: forceful twisting of an engaged reverse Hill-Sachs defect against the posterior glenoid rim can shear off the entire humeral head, transforming an impaction dislocation into a catastrophic spiral surgical neck fracture.

Master Class Radiological Pearl: Clinical Index of Suspicion & Safe Reduction Rules

10. Evidence-Based Synthesis

11. References

1. Cisternino SJ, Rogers LF, Stufflebam BC, Kruglik GD. The trough line: a radiographic sign of posterior shoulder dislocation. AJR Am J Roentgenol. 1978;130(5):951-954. DOI [Level IV: Retrospective Radiographic Analysis]
2. McLaughlin HL. Posterior dislocation of the shoulder. J Bone Joint Surg Am. 1952;34(3):584-590. DOI [Level IV: Case Series]
3. Hawkins RJ, Neer CS 2nd, Pianta RM, Mendoza FX. Locked posterior dislocation of the shoulder. J Bone Joint Surg Am. 1987;69(1):9-18. DOI [Level IV: Retrospective Cohort Study]
4. Robinson CM, Aderinto J. Posterior shoulder dislocations and fracture-dislocations. J Bone Joint Surg Am. 2005;87(3):639-650. DOI [Level III: Systematic Review]
5. Cicak N. Posterior dislocation of the shoulder. J Bone Joint Surg Br. 2004;86(3):324-332. DOI [Level IV: Instructional Review]
6. Saupe N, White LM, Bleakney R, Schweitzer ME, Recht MP, Jovane F, Robinson CM. Acute traumatic posterior shoulder dislocation: MR findings. Radiology. 2008;248(1):185-193. DOI [Level II: Prospective Cross-Sectional Study]
7. Rouleau DM, Hebert-Davies J. Incidence of associated injury in posterior shoulder dislocation: problems of presentation and imaging. J Orthop Trauma. 2012;26(4):246-251. DOI [Level III: Multicenter Retrospective Cohort Study]