Why is a traumatic hip dislocation typically associated with more clinical morbidity than a shoulder dislocation?
A traumatic hip dislocation is typically
associated with more clinical morbidity than a shoulder dislocation for two
- More force is needed to dislocate the hip and greater force implies more associated injuries.
- The neurovascular structures near the shoulder are less tethered and accordingly more tolerant of some displacement. By contrast, a dislocation of the hip is more likely to damage the nerves and arteries nearby.
Let’s review: the shoulder joint consists of the humeral head (ball) and the glenoid fossa (socket) of the scapula. The humeral head is much larger than the glenoid fossa and held in place mainly by soft tissues. Unlike the hip or the elbow, the glenohumeral joint has little “static stability,” that is, the bony anatomy is less constraining. Essentially, the humeral head is like a golf ball sitting on a tee formed by the glenoid.
Figure 1: A CT scan of the shoulder, showing the mismatch between the size of the glenoid (yellow) and the size of the humeral head (red). (https://radiopaedia.org/cases/normal-ct-shoulder-2)
By contrast, the femoral head is held tighter and more securely in the acetabulum by both soft tissue and bone. The hip is more similar to a ball in a socket than a golf ball on a tee.
Figure 2: A sagittal section of a pelvic CT scan shows that the femoral head fits in the acetabulum of the hip much like a ball in a socket. (image from https://radiopaedia.org/cases/normal-ct-hip-2)
Because the acetabulum constrains the femoral head to a far greater extent than the glenoid holds the humerus, more force is required to dislocate the femoral head. In turn, with higher energy trauma, there will be more structural damage about the hip (for example, a fracture). In addition, it is more likely that we will see concomitant injuries such as urethral injury or abdominal injury.
There is yet another consideration: because the shoulder is less stable than the hip, the surrounding structures are more tolerant of movement. Specifically, the humeral head normally subluxates (slides out of joint just a bit) with routine shoulder movement. Given this normally-occurring subluxation, the axillary artery and nerve cannot be tightly constrained. (If they were, routine shoulder movement would damage them, and that is not seen.)
On the other hand, because the hip normally does not move out of the socket at all, the nearby neurovascular structures are more firmly ensconced in their resting position and are not as accommodating of displacement. Thus, a hip dislocation is more apt to stretch the blood vessels that supply the femoral head when there is a hip dislocation. This stretching can cause damage to the inner lining of the vessel, causing thrombosis and blockage to blood flow. With that, the femoral head is at risk for osteonecrosis (as described in Link to question).
In short, the less constrained anatomy of the shoulder means that less force is needed to dislocate the shoulder, meaning there is usually less skeletal damage seen. Additionally, the blood vessels that supply the humeral head are less tethered, meaning there is a lower chance of osteonecrosis of the humeral head.
Additional Points to Consider
- If you are thinking that constrained anatomy makes hip dislocations rarer than shoulder dislocations and that it would be harder to insert an arthroscope into the hip as compared to the shoulder - you would be correct on both counts.
- The word “traumatic” in the question was chosen advisedly. There is, of course, a condition of developmental dysplasia of the hip in which the joint may be dislocated. This is a gradual (“developmental”!) process and thus not associated with secondary pathology seen with traumatic dislocation. (Rather, it has its own problems (as described in Link to question)).