Impingement in one region of the hip can produce a lever effect on the femoral neck and subluxatory stress on the femoral head in the region opposite to the site of impingement.
Anterior Impingement — Posterior Instability
Premature contact between the femoral head-neck junction and the acetabulum anteriorly — from decreased internal rotation in 90° of flexion (FIR) caused by anterior cam, femoral retroversion, or acetabular retroversion — causes anterior impingement. Attempts to gain more internal rotation produce subluxatory stress on the head posteriorly. The normal hip capsule prevents such subluxation, but the posterior forces can produce shear stresses on the posterior acetabular cartilage and labrum from the suprafoveal part of the femoral head — called contrecoup damage. Suprafoveal damage in SCFE and cysts in the head in that region are probably from this mechanism.
Significant trauma can tear the posterior hip capsule and produce acute posterior dislocation. Most posteriorly dislocated hips have risk factors for anterior impingement and decreased FIR. Though recurrent dislocation is not common, chronic discomfort in these hips can be from persistent anterior impingement and posterior instability. Assessment of FIR and correction of anterior impingement is recommended. Posterior dysplasia may also need correction and may indicate open reduction and internal fixation of even a small posterior wall fracture.
Total Hip Arthroplasty
Recurrent dislocation of total hips is almost always from anterior impingement of a malpositioned femoral or acetabular component. The definitive treatment is revision of component position.
Hinged Abduction
Lateral impingement from an irreducible lateral part of the head in hip abduction causes medial space widening or subluxation seen on radiographs taken in abduction. Bilobed heads develop from pressure of the acetabular rim on the subluxated femoral head.
| Treatment Option | Mechanism |
|---|---|
| Valgus osteotomy | Moves the lateral impingement point laterally to increase abduction before the hip impinges; can bring the fovea up against the weight-bearing sourcil |
| Open reduction of lateral head | Expecting remodelling of the bump and acetabulum against each other; most likely to work in children with open triradiate cartilage |
| Shelf augmentation | Covers the lateral head and reduces the lateral bump against the new acetabulum |
| Head reduction osteotomy | Makes the head spherical; instability of the smaller head requires concomitant acetabular osteotomy |