Two Reasons: Hip dislocation (Figure 1) can potentially disrupt the blood supply of the femoral head leading to osteonecrosis. Also, the articular surface can be damaged by the force of dislocation. Both osteonecrosis and articular surface damage can cause arthritis by independent mechanisms.
Recall that the blood supply to the femoral head is indirect: the vessels ascend the femoral neck to reach the head. (In adults there is no significant direct blood supply that reaches the head directly from the pelvis.) Therefore, hip dislocation, which can stretch and thereby damage these blood vessels, especially the medial circumflex femoral artery (see Figure 2), can lead to osteonecrosis.
Dead bone cannot remodel and if osteoclasts cannot break down old bone and osteoblasts synthesize new bone, microscopic damage accumulates in the dead bone. Over time, the bone loses compliance (i.e. becomes stiffer) and breaks rather than bends.
In the femoral head, the loss of compliance induced by osteonecrosis can lead to collapse of the region of bone just below the cartilage surface in the so-called subchondral bone (see Figure 3). This collapse produces an irregular joint surface that in time becomes clinically significant arthritis.
The second issue is that the articular surface can be damaged by the force of dislocation. The hip is well contained, such that the femoral head cannot escape without colliding with the pelvis. This impact can cause degradation of the cartilage matrix and chondrocyte death even in the absence of grossly apparent structural damage.
(Similar impaction injuries can be seen on the surface of the lateral femoral condyle after tears of the anterior cruciate ligament and on the talus after ankle sprains.)
Needless to say, if the dislocation is accompanied by a fracture there is an even greater (theoretical) risk of post-traumatic arthritis.
Because of the concern about fractures, it is often critical to obtain a CT scan to confidently rule out these injuries (Figure 4).
Additional Points to Consider
- University of Alabama quarterback Tua Tagovailoa dislocated his hip on 11/16/19 in a game against Mississippi State. The dislocation was “immediately reduced at the stadium," a statement released by the school said. How does urgent reduction affect the risk of complications from dislocation?
- According to a systematic review by Kellam and Ostrum [doi: 10.1097/BOT.0000000000000419], there were four cases of osteonecrosis in 239 patients whose dislocation was reduced within 12 hours, whereas there were 37 cases osteonecrosis among the 85 patients whose dislocation was reduced more than 12 hours after injury. The authors concluded that this finding can help “accurately informing patients of their prognosis after traumatic dislocations of the hip.” Why is prognosis (and not, say, a treatment guideline) the correct inference?
- For a skeletally immature patient with a traumatic hip dislocation, an MRI might be a more appropriate test than a CT. Why?