DOI: 10.1148/radiol.2242010352
(Radiology 2002;224:485-486.)
© RSNA, 2002
The Trough Line Sign1
Devang M. Gor, MD
1 From the Department of Radiology, University of Medicine and Dentistry of New Jersey, University Hospital, 150 Bergen St, Rm C-320, Newark, NJ 07103. Received January 23, 2001; revision requested March 6; revision received April 18; accepted May 2. Address correspondence to the author (e-mail: roentgen1895@yahoo.com).
Index terms: Humerus, fractures, 415.422 • Shoulder, dislocation, 414.422 • Shoulder, injuries, 414.422 • Signs in Imaging
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APPEARANCE
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The trough line sign refers to a vertical or archlike line of cortical bone projecting parallel and lateral to the articular cortical surface of the humeral head. It is seen on conventional frontal radiographs of the shoulder.
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EXPLANATION
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The trough line sign indicates posterior dislocation of the shoulder. In posterior dislocation, the humeral head is forced posteriorly in internal rotation. The anterior aspect of the humeral head becomes impacted against the posterior glenoid rim. With sufficient force, this causes a compression fracture on the anterior aspect of the humeral head. Frontal radiographs reveal two nearly parallel lines in the superomedial aspect of the humeral head. The more medial, slightly convex line is the subchondral bone of the humeral head, and the other more laterally located line represents the margin of a troughlike compression fracture—the trough line (Fig 1) (1). The axillary view (Fig 2) aids in better understanding the mechanism of injury by accurately depicting the compression fracture itself and the posterior glenoid rim. This compression fracture is analogous to the Hill-Sachs compression fracture seen with anterior dislocation of the glenohumeral joint.
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Figure 1. Frontal radiograph of the left shoulder in a patient with posterior dislocation shows the medial cortex of the humeral head (large arrows) and the trough line (small arrows) on the superomedial aspect of the humeral head.
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Figure 2. Axillary view of the left shoulder in a patient with posterior dislocation demonstrates a compression fracture (arrows) in the anterior aspect of the humeral head resulting from impaction on the posterior glenoid rim.
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DISCUSSION
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The shoulder is the most frequently dislocated joint in the body, accounting for 50% of joint dislocations (2). Posterior dislocations account for 2%–4% of all shoulder dislocations (3,4). In adults, convulsive disorder is the most common cause. Bilateral dislocations are not infrequent, and when present, they almost always result from convulsions. In some instances, this may be the initial manifestation of patients with a seizure disorder (5). Direct forceful impact to the shoulder and arm while in internal rotation and extension are other causes. In the pediatric population, posterior dislocations are rare and atraumatic. They occur secondary to birth-related paralysis or congenital anomalies (6).
The average age of those with traumatic posterior dislocation is 50 years (6). Patients present with history of trauma, pain, and inability to externally rotate the arm. Physical signs may be less noticeable in instances of other clinically important injuries or development of a hematoma around the shoulder. Posterior dislocation may be missed initially on frontal radiographs in 50% of cases, and persistent pain with limitation of motion may clinically mimic adhesive capsulitis (7). Signs on frontal radiographs are subtle, including the trough line sign, the loss of normal "half-moon overlap" (6), and the rim sign (8). Axillary, scapular Y, or posterior oblique projections are needed for confirmation.
The absence of external rotation on images in a standard shoulder series is a clue to posterior dislocation. Normally, on a true anteroposterior image, there is a half-moon overlap between the humeral head and the glenoid (6). In a posteriorly dislocated shoulder, there is lateral displacement of the humeral head with respect to the glenoid, thus losing the half-moon overlap (6). In a study by Arndt and Sears (8), the distance from the medial border of the humeral head to the anterior glenoid rim was measured in 100 normal shoulders on frontal radiographs. This distance was determined to be from a negative value up to 6 mm. The authors postulated that a distance of more than 6 mm is abnormal, being present in 31 of 48 frontal radiographs of posterior dislocation, and they called it the "rim sign." This sign is also seen in lymphedema of the extremity and hemarthrosis without dislocation, and, thus, should be regarded with caution.
In a series by Cisternino et al (1), the trough line sign was seen in 75% of patients with posterior dislocation of the shoulder and was present in seven of nine cases that were negative for the rim sign. In the neutral position, the greater tuberosity is seen in profile, forming the lateral margin of the humeral head. The lesser tuberosity is seen en face as a line inferior and medial to the greater tuberosity. With increasing internal rotation, the lesser tuberosity is projected medially until it forms the medial contour of the humeral head. In any degree of internal rotation, the superomedial aspect of the humeral head is always featureless. The presence of a line in this area nearly parallel to the articular surface of the humeral head—the trough line—is indicative of posterior dislocation of the shoulder. In many instances, this may be the only sign of posterior dislocation on frontal radiographs.
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ACKNOWLEDGMENTS
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The author thanks Marsia Blacksin, MD, and Cornelia Wenokor, MD.
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FOOTNOTES
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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REFERENCES
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- Cisternino SJ, Rogers LF, Stufflebam BC, Kruglik GD. The trough line: a radiographic sign of posterior shoulder dislocation. AJR Am J Roentgenol 1978; 130:951-954.[Abstract]
- Harris JH, Jr. Shoulder, including clavicle and scapula. In: Harris JH, Jr, Harris WH, eds. The radiology of emergency medicine. 4th ed. Philadelphia, Pa: Lippincott Williams & Wilkins, 2000; 299-339.
- Neviaser JS. Posterior dislocations of the shoulder: diagnosis and treatment. Surg Clin North Am 1963; 43:1623-1630.[Medline]
- Nobel W. Posterior traumatic dislocation of the shoulder. J Bone Joint Surg Am 1962; 44:523-538.[Abstract/Free Full Text]
- Pearl BL. Bilateral posterior fracture dislocation of the shoulder: an uncommon complication of convulsive seizure. N Engl J Med 1970; 283:135-136.
- Rogers LF, ed. The shoulder and humeral shaft In: Radiology of skeletal trauma. 2nd ed. New York, NY: Churchill Livingstone, 1992; 732-740.
- Hill NA, McLaughlin HL. Locked posterior dislocation simulating a frozen shoulder. J Trauma 1963; 3:225-234.[Medline]
- Arndt JH, Sears AD. Posterior dislocation of the shoulder. Am J Roentgenol Radium Ther Nucl Med 1965; 94:639-645.[Medline]
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