Posterior Shoulder Dislocations
A posterior shoulder dislocation occurs when the upper arm bone moves backward out of its shoulder socket. While this injury represents only 2-5% of all shoulder dislocations, it poses unique diagnostic challenges that make imaging important for proper identification and treatment. This article will discuss posterior shoulder dislocation so that you better understand your injury and recovery process.
What Is a Posterior Shoulder Dislocation
Your shoulder joint consists of a ball-and-socket connection where the round top of your upper arm bone (humerus) fits into a shallow cup in your shoulder blade (glenoid). In a posterior dislocation, the ball moves backward behind the socket rather than staying in its normal position. This backward displacement creates distinct patterns that radiologists look for on imaging studies.
Unlike the more common anterior shoulder dislocations where the arm bone moves forward, posterior dislocations often result from seizures, electrical shock, or sports injuries. The injury can be subtle and can be missed during initial emergency room visits, making proper imaging interpretation essential.
How Posterior Shoulder Dislocation Appears on X-rays
Standard shoulder X-rays provide the first line of imaging for suspected shoulder injuries. However, posterior dislocations can be challenging to identify on routine views. On an anteroposterior (front-to-back) X-ray, several key signs may indicate a posterior dislocation.
The lightbulb sign represents one of the most recognizable findings. When your arm bone dislocates posteriorly, it becomes locked in internal rotation, creating a rounded appearance that resembles a lightbulb on the X-ray image. The normal bony landmarks that radiologists typically see become obscured or altered.
Another important finding is the widening of the glenohumeral joint space. The gap between your arm bone and shoulder socket appears larger than normal because the bones are no longer properly aligned. This increased distance serves as a red flag for radiologists reviewing your images.
Advanced Imaging for Posterior Shoulder Dislocation Diagnosis
When X-rays are unclear or when doctors suspect additional injuries, advanced imaging techniques provide more detailed information. CT scans are good at showing bone damage and can reveal fractures that might not be visible on regular X-rays. These scans help doctors assess whether you have sustained bone chips or compression fractures along with your dislocation.
MRI imaging offers superior visualization of soft tissue structures around your shoulder joint. This type of scan can identify tears in your labrum (the cartilage rim around the socket), damage to your rotator cuff muscles, and injuries to the joint capsule and ligaments. MRI arthrography, where contrast material is injected into the joint before scanning, provides even more detailed images of these soft tissue structures.
Reverse Hill-Sachs Lesions on Imaging
A reverse Hill-Sachs lesion represents a specific type of bone damage commonly seen with posterior shoulder dislocations. Unlike the typical Hill-Sachs lesion that occurs with anterior dislocations, the reverse version affects the front part of your arm bone’s head. When your arm bone dislocates backward, it can compress against the back edge of your socket, creating an indentation or depression fracture on the front surface of the bone. This finding appears as a flattened or dented area on imaging studies and helps radiologists confirm that a posterior dislocation has occurred.
Imaging Signs That May Be Missed
The trough sign represents another imaging finding that can be overlooked. This appears as a vertical line or depression on the X-ray where the compressed bone creates a groove-like appearance.
Posterior glenoid rim fractures may accompany the dislocation and appear as small bone chips or irregular edges on the back part of your socket. These fractures can contribute to ongoing shoulder instability if not properly identified and treated.
Why Imaging Timing Matters
The timing of your imaging studies can significantly impact what radiologists can identify. Immediately after injury, swelling and muscle spasm may obscure certain findings or make the dislocation appear less severe than it actually is. Follow-up imaging after the acute phase often reveals additional details about soft tissue damage.
Some posterior dislocations reduce spontaneously before you reach the hospital, meaning the bones slip back into place on their own. In these cases, the dislocation itself may not be visible on X-rays, but associated injuries like reverse Hill-Sachs lesions or labral tears remain detectable on advanced imaging.
Treatment Implications Based on Imaging Findings
Your imaging results directly influence treatment recommendations. Small reverse Hill-Sachs lesions affecting less than 25% of the bone surface may heal well with conservative treatment and physical therapy. Larger lesions often require surgical intervention to prevent recurrent dislocations.
When imaging reveals significant soft tissue damage, such as extensive labral tears or rotator cuff injuries, early surgical repair may be recommended to restore normal shoulder function. The presence of bone fragments or loose bodies visible on CT scans typically necessitates surgical removal.
Understanding Your Radiology Report
When reviewing your radiology report, look for terms like “posterior glenohumeral dislocation,” “reverse Hill-Sachs deformity,” or “posterior labral tear.” These findings indicate specific aspects of your injury that will guide your treatment plan.
Conclusion
Proper imaging interpretation ensures that posterior shoulder dislocations receive appropriate treatment. Understanding what your imaging studies show allows you to participate actively in treatment decisions and set realistic expectations for recovery.
While posterior shoulder dislocations are less common than other types, modern imaging techniques allow for accurate diagnosis and complete assessment of associated injuries. This detailed evaluation helps develop an effective treatment plan tailored to your specific injury pattern and functional needs.
References
- Radiopaedia.org – Posterior shoulder dislocation. Reference article. https://radiopaedia.org/articles/posterior-shoulder-dislocation?lang=us
- StatPearls – Posterior Shoulder Dislocations. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK441919/
- Imaging of shoulder instability. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC5594016/