1. The infection may originate in the synovium, the proximal femur (epiphysis, metaphysis, femoral neck, or trochanteric apophysis), the acetabulum, or the gluteal/ iliopsoas bursae.
2. Cold abscesses may be palpable in the femoral triangle, the ischiorectal fossa, or the thigh. Sinuses may occur in any of these locations.
3. The radiographic findings vary considerably depending on the primary location and degree of involvement.
4. A lesion in the acetabular roof (“wandering acetabulum”) may result in subluxation, and clinically there will be limb shortening without positioning. True pathologic dislocation may occur as well, which will be associated with both limb shortening and positioning.
5. Protrusio may be associated with lesions in the acetabular floor.
6. Coxa magna may be confused with Perthes' disease in pediatric patients.
7. Significant joint space narrowing without an osseous focus (“atrophic”) may be difficult to differentiate from rheumatoid arthritis.
8. Destruction on both sides of the joint may result in irregularity of the femoral head and incongruity (“mortar and pestle”).
9. Disease passes into 4 stages:
(A) Stage of synovitis:
- X-RAY – soft tissue swelling , haziness of articular margins & rarefaction
- USG – soft tissue swelling
- MRI – synovial effusion
(B) Stage of early arthritis:
- X-RAY – osteopenia , erosion of articular margins , ↓ joint space.
- MRI – synovial effusion , edema , minimal bone destruction
(C) Stage of advanced arthritis:
- X-RAY – further decrease in joint space.
(D) Advanced arthritis with subluxation / dislocation :
- X-RAY– Furhter destruction of acetabulum , head , capsule and ligaments. Head – upwards and posteriorly. Wandering / migrating acetabulum. Mortle & pestle appearance. Reduced joint space.