Hip dysplasia commonly refers to Acetabular dysplasia, particularly acetabular deficiency in covering the femoral head. Acetabular deficiency can be global or focal to lateral coverage by the sourcil, or anterior or posterior wall coverage of the femoral head. Instability of the hip joint suggests a hip’s tendency to subluxate or dislocate. Acetabular dysplasia alone or in combination with proximal femoral morphology facilitates hip instability. Risk factors for lateral instability are acetabular under coverage laterally, up sloping sourcil, and coxa valga. Additional anterior deficiency combined with femoral anteversion causes anterosuperior instability while posterior acetabular deficiency along with femoral retroversion causes posterosuperior instability. On the other hand, Coxa vara can compensate for lateral dysplasia and femoral version in the opposite direction can compensate for acetabular wall deficiency.
Left hip dysplasia with femoral head subluxation.
Chronic progressive hip instability commonly occurs with simple weight bearing inducing lateral subluxation of the head. This subluxation is resisted initially with hypertrophy of the labrum, capsule, ligamentum teres and the muscles. Subluxation becomes symptomatic in older children, when the hypertrophied labrum degenerates and tears. These labral tears are unique inside out avulsions which can tear some articular cartilage and acetabular rim bone with them. The fractured lateral rim is called ‘Rim fracture’ or ‘Os Acetabuli’. The edge loading on the lateral acetabulum produces degeneration of both the femoral head and acetabular cartilage called ‘Acetabular Rim Syndrome’
A natural progression of dysplasia with subluxation to severe arthritis
Dysplasia in children is usually not painful. A limp and limb shortening can be seen with subluxation or dislocation. Pain after prolonged standing and walking is usually from abductor fatigue. The hip range of motion is increased, especially increased FIR with anterior impingement sign. Apprehension sign is positive for anterior and posterior instability.
Lateral subluxation is seen on the AP view as a break in Shenton’s line, decreased LCE, narrowed lateral joint space, and widened medial space. Anterior subluxation is seen on the Faux Profil view as decreased ACE, narrowed anterior joint space, and posterior space widening. Reducibility of the femoral head can be confirmed with abduction-internal rotation x-ray (AIR view). CT scan can show femoral head and acetabular shape well. MRI can show labral tear, hypertrophy, labral cysts, and cartilage damage.
Subluxation is a sign of instability and poor prognosis and a strong indication to intervene at any age. Dysplasia in young children can be treated with abduction bracing or femoral osteotomy expecting acetabular remodeling. After 4 years of age, acetabular remodeling does not correct dysplasia well and acetabular osteotomy is ideal. Untreated subluxation in a child worsens the acetabular dysplasia. Femoral varus osteotomy can compensate for mild acetabular dysplasia, but causes shortening, abductor weakness, and lateral hip prominence. Severe varus of < 100 degrees stops compensating for acetabular dysplasia and starts contributing to lateral subluxation because the weight bearing loads pass lateral to calcar of the neck and head in addition to causing abductor dysfunction.
Left hip severe dysplasia with coxa vara and magna treated with PAO and RNL
Indications for additional femoral osteotomy are
- High subluxation and soft tissue tension may need femoral shortening to reduce the head and rotate the acetabulum.
- Inadequate acetabular correction
- Persistent Foveal contact with the sourcil.
- Irreducible femoral head (Aspherical or too large)
- Valgus osteotomy to match a flat head to a flat sourcil
Good hip reconstruction aims at reducing a round femoral head deep into a flat acetabulum with good anterior and posterior wall coverage. This may require femoral shortening to decrease the soft tissue tension in high subluxation or dislocation, varus osteotomy to correct coxa valga, valgus osteotomy if hip congruity requires it, open reduction if there are any soft tissue obstacles, cam osteoplasty for impingement, and head reduction osteotomy if the head is too large or misshapen to fit well into the acetabulum. Labral repair is rarely required.
Once the head is round and placed well in the acetabulum, the acetabulum needs to be modified to prevent subluxation of the head. This is accomplished by bending the acetabular roof (Pemberton, Dega) if it is enlarged and steep, redirecting the acetabulum (Salter, Triple, PAO) without changing its shape, and enlarge the acetabulum with bone support over the capsule (Chiari, Shelf) when the head is not reducible or the acetabulum is already flat. Occasionally there is poor acetabulum development to reconstruct requiring capsular interposition arthroplasty in a high dislocated position called Tectoplasty or reduction into a reamed acetabulum in the native hip center called Colonna arthroplasty. If the head is too small or absent, varus osteotomy to place the greater trochanter into the acetabulum (Trochanteric arthroplasty) and valgus osteotomy for Pelvic support are options. Advanced cartilage damage causing disabling pain can be treated with arthrodesis, excision arthroplasty with or without pelvic support, and total hip arthroplasty.
Medial dysplasia is a rare entity of medial deficiency of the head by the shorter sourcil and causes medial arthritis from foveal impingement rather than true instability. Medial coverage is measured with medial center edge angle (MCE). Ideal coverage is 20 degrees. Medial dysplasia makes the acetabular fossa high. When medial dysplasia is coupled with a negative sourcil angle, there will be medial subluxatory stress on the femoral head and more weight bearing on the acetabular fossa. This also loads the fovea and crushes the ligamentum teres. Medial dysplasia can be coupled with lateral overcoverage and impingement. This combination requires a reverse acetabular osteotomy because rim trimming to relieve the lateral impingement will make the overall sourcil width small and does not address the acetabular fossa loading. Valgus osteotomy can compensate for mildly negative sourcil angle to an extent.
Medial rotation of the PAO to get the sourcil flat lateralizes the inferior half of the acetabulum and adduction and rotations in extension will be decreased from impingement. So the inferior parts of the anterior and posterior walls need to be resected. This type of correction also requires shortening of the pubis and femoral nerve decompression.
The posterior wall may cross close to the center of the head and still be deficient superiorly called posterosuperior dysplasia. LCE of the posterior wall can be measured to the point where the posterior wall touches the sourcil and take less than 20 as an indication to consider an anteverting PAO in a young person who will not have anterolateral arthritis yet.
Negative sourcil angle
The sourcil angle is less important than the mechanics of the sourcil. If the sourcil has adequate medial coverage and excessive lateral coverage and the head does not show tendency for medial subluxation or medial loading, excision of the lateral over coverage makes the sourcil of normal size, shape, and tilt. So rim trimming and labral repair or reconstruction is a good option if the hip does not have too much arthritis.
Lateral dysplasia – LCE is only one objective measure of lateral coverage. Put all three sourcil parameters together. Total size with LCE and MCE or a total arc of 50-60 degrees is good. Concave sourcil on the convex head contains it well. Most important parameter is the sourcil angle. Ideally it should be flat or perpendicular to the primary compression trabeculae. Basically, with these measures, we are looking to see if the sourcil is going to allow lateral subluxation, rim loading, labral tear and arthritis.