Acetabular dysplasia

Acetabular dysplasia is referred to as a shallow acetabulum, not being able to provide sufficient coverage for the femoral head and thus leading to instability of the hip joint.

Terminology

Acetabular dysplasia is a form of developmental dysplasia of the hip (DDH) often referred to in the adolescent and adult population ¹.

Epidemiology

Adult hip dysplasia has an estimated prevalence of 0.1% and is more common in women ^2,3. Male sex is apparently associated with posterosuperior deficiency ³.

Associations

Acetabular dysplasia is associated with the following conditions ^2,3:

• infantile developmental dysplasia of the hip 

• excessive femoral anteversion

• acetabular retroversion

• coxa valga

• femoroacetabular impingement morphology in ~50%: more commonly cam than pincer morphology ^12,13

Risk factors

Risk factors for acetabular dysplasia include the following ^2,3:

• developmental dysplasia of the hip

• previous septic arthritis

• previous trauma

Clinical presentation

The clinical presentation of acetabular dysplasia has been described as variable ¹¹. Patients can present with hip pain or groin pain especially with extreme positions e.g. high hip flexion and hyperextension. In addition, there may be limping or signs and symptoms of hip instability ¹¹. Clinical signs include a positive hip impingement test ¹¹  and/or anterior apprehension test.

Complications

25-50% of patients with acetabular dysplasia will develop early hip osteoarthritis if left untreated ^1-6.

Other complications include^2-4:

• chondral and labral damage

• subchondral fractures

• hip instability

• hip subluxation or dislocation

Pathology

Acetabular dysplasia is characterized by a smaller weight-bearing surface than the normal acetabulum, which ultimately leads to increased contact stress and static overload due to under-coverage of the femoral head and structural instability. This adds to cartilage and/or labral injury as well as damage to the joint capsule ^2-4.

Etiology

Acetabular dysplasia might be the result of abnormal growth after treatment or missed developmental dysplasia of the hip during childhood, the etiology of which is multi-factorial in nature. It may be also due to other hip pathologies that have occurred during childhood as septic arthritis, trauma or Legg-Calve-Perthes disease ^ref.

Subtypes

Acetabular dysplasia can be divided into different patterns concerning 3-D morphology. One grading scheme subdivides the acetabular dysplasia into the following patterns with lateral acetabular deficiency being constantly present ³:

• global acetabular deficiency

• anterosuperior acetabular deficiency

• posterosuperior acetabular deficiency

Other schemes differentiate anterior, posterior and global or lateral deficiencies ².

Radiographic features

Radiographic assessment of acetabular dysplasia or adult hip dysplasia includes plain radiographs of the pelvis and additional planes as the false profile view of Lequesne, cross-table lateral or frog-leg lateral views.  

Cross-sectional imaging is advised for better three-dimensional characterization, preoperative planning ³ and the detection of chondral and labral lesions.

Plain radiograph

The most common measurement in acetabular dysplasia is the lateral center-edge angle on a plain anterior-posterior radiograph of the pelvis ^2,4.  Patients should be in the supine position with both legs in 15° of internal rotation to maximize femoral neck length ².

A value of ≤25° is considered abnormal or borderline and a value of ≤20° is an indicator for dysplasia ^2,4,5. The cut-off should be also adapted to the measurement and whether the lateral bony acetabular rim or the lateral edge of the acetabular sourcil was taken to measure the lateral center-edge angle ⁷. 

Further measurements to confirm insufficient acetabular coverage are the following ^2,4,5:

• acetabular index or Tönnis angle

• extrusion index

• medial center edge angle

• acetabular arc

• the anterior center-edge angle of Lequesne (assessed on false profile view – moderate reproducibility)

• femoral head-neck-shaft angle (to assess for coxa valga and surgical planning)

CT

The main role of CT is improved characterization of the three-dimensional acetabular morphology in a setting of preoperative planning. It should also comprise the assessment for excessive acetabular anteversion or acetabular retroversion. Other than the crossover sign on the anterior-posterior view of the pelvis, CT can also differentiate posterior under-coverage in the setting of acetabular dysplasia from anterior overcoverage in the setting of pincer morphology or plain acetabular retroversion and it can better assess the grade of posterior deficiency than the posterior wall sign ³.

CT measurements for the assessment of acetabular dysplasia include the following and are conducted one cut above the greater trochanters ²:

• anterior acetabular sector angle: a value of ≤50° indicates dysplasia

• posterior acetabular sector angle: a value of ≤90° indicates dysplasia

• horizontal acetabular sector angle: a value of ≤140° indicates dysplasia

MRI

In addition to the three-dimensional assessment of the acetabular and femoral morphology, which highly benefits from 3D imaging in this situation, MRI allows for assessment of concomitant labral, chondral or ligamentum teres injury as well as the evaluation of the joint capsule ¹.

Radiology report

The radiological report should include the description of the following ^2,3:

• acetabular morphology

• measurements: e.g. lateral center-edge angle, acetabular index and/or extrusion index

• signs of early osteoarthritis: subchondral sclerosis, cysts, osteophytes and Tonnis grade

• labral and/or chondral injury or ligamentum teres injury

• fovea alta (if present)

Treatment and prognosis

Treatment options of acetabular dysplasia include in particular periacetabular osteotomy ^1,6 and other acetabular osteotomies (e.g. Salter) which reorient the acetabulum in a fashion that provides better coverage for the femoral head. Another surgical option is acetabuloplasty, which alters the morphology of the acetabulum (e.g. Pemberton) ^6,8. Periacetabular osteotomy is in particular indicated in patients with a preserved range of motion.

Salvage osteotomies (e.g. Chiari or shelf osteotomy) do not preserve articular cartilage and can be considered if the hip needs to be stabilized in incongruous joints ⁸.

In the case of a severely arthritic hip, joint replacement surgery might be considered and include total hip replacement and resurfacing arthroplasty ².

The role of conservative management is limited due to the early progression to osteoarthritis and should decrease pain and can be chosen in very mild dysplasia with mild symptoms.  Non-operative treatment includes weight loss, activity and lifestyle modifications as well as nonsteroidal anti-inflammatory drugs, specialized physical therapy intra-articular injections ^ref.

Treatment complications

Operative complications include the following:

• femoroacetabular impingement in case of overcorrection ^2,9

Differential diagnosis

• hip osteoarthritis

• femoroacetabular impingement

• other forms of hip impingement

• transient osteoporosis of the hip

• osteonecrosis of the hip

• subchondral insufficiency fractures

See also

• developmental dysplasia of the hip