Short answer: 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.
Figure 1: A dislocated hip is shown (modified from https://commons.wikimedia.org/wiki/File:Hueftluxation_links.png)
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.
Osteonecrosis can cause collapse of the femoral head; and a collapsed femoral head will have a distorted and irregular articular (joint) surface. This irregular surface is not only “arthritic” per se on the femoral side, but will induce arthritis on the acetabular (socket) side as well.)
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 structural properties deteriorate and the bone 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.
Figure 2: The blood supply to the femoral head reaches its destination circuitously: branches of the profunda femoris must ascend the femoral neck, as shown in the figure, (modified from https://ars.els-cdn.com/content/image/3-s2.0-B9780323316965001030-f103-001-9780323316965.jpg?_)
Figure 3: Femoral head with collapse of subchondral bone (Image courtesy of C.P. Beauchamp, MD, Orthopaedia.com)
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. Interestingly, chondrocyte viability can be reduced significantly even in the absence of grossly apparent structural damage.
(Similar impaction injuries can be seen with tears of the anterior cruciate ligament and possibly with the shoulder dislocations.)
Needless to say, if the dislocation is accompanied by a fracture there is an even greater (theoretical) risk of post-traumatic arthritis.
Additional Points to ConsiderUniversity 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?