- MED is a common skeletal dysplasia, inherited in an autosomal dominant fashion, with the most common mutations affecting COMP or Collagen IX
- Presentation is usually due to joint pain or contractures, and the spine is not affected (differentiating from spondyloepiphyseal dysplasia)
- Radiographs in the growing child show fragmented epiphyses, commonly in the hip
- Mature radiographs show ovoid femoral heads and squared distal femoral condyles
- Early treatment usually focuses on appropriate femoral head coverage, while arthroplasty is more appropriate in adults
Description:MED is a common skeletal dysplasiausually inherited by autosomal dominant transmission. Recently, recessive cases have been described as being due to mutations in a transport protein. This type of dysplasia affects the epiphysis, producing symptoms mainly in those bones with significant load bearing, and causes few changes in the physis or metaphysis. Historically, it was described as occurring in two separate forms, with eponyms that are still used today: Ribbing dysplasia, having mild involvement, and Fairbank dysplasia, a more severe type.
Clinical Findings:Patients typically present later in childhood with joint pain in the lower extremities, decreased range of motion, gait disturbance, or angular deformities of the knees, such as genu varum or valgum. There may be a delay in walking or flexion contractures of the knees or elbows. However, symptoms may also develop as late as adulthood. These patients have somewhat short stature, with height ranging from 145 to 170 cm (57 to 67 in.). The face and spine are normal, and there is no visceral involvement.
Imaging Studies:Most changes in MED involve the epiphyses and any of the ossification centers may be delayed in appearance. There are occasional irregularities of streaking in the metaphyses, but they are minor. The appearances of the epiphyses in the immature and in the mature patient are different and characteristic[r1].
In the growing patient, the epiphyses are fragmented and small in size. The epiphyseal ossification centers eventually coalesce, but the overall shape of the epiphysis is smaller. An arthrogram may be helpful when it is necessary to assess the shape of the joint surface. The more fragmentation there is in the capital femoral epiphysis, the earlier is the onset of osteoarthritis. Coxa vara occurs in some patients.
After maturity, there is some degree of flattening of the major load-bearing epiphyses —flattening of the femoral condyles, an ovoid femoral head, decreased sphericity of the humeral head, and squaring of the talus. In adulthood, major joints develop premature osteoarthritis, which is most common and most severe in the hips. Avascular necrosis may be superimposed on MED; this occurs in about one half of the femoral heads.
An orthopaedic surgeon should be able to differentiate MED from Perthes disease. Several radiographic clues may be helpful. In MED, abnormalities in the acetabulum are primary, and are more pronounced. The radiographic changes of the proximal femur are symmetric and fairly synchronous. It is also helpful to obtain radiographs of the knees, ankles, shoulders, and wrists to detect epiphyseal irregularities characteristic of MED.
Radiographs of the knees show that the femoral condyles are flattened, and may be in valgus. There may be irregular ossification, just as in the hip. The condyles appear somewhat squared in the lateral view. Osteochondritis dissecans may be superimposed. Some patients with MED also show a double-layered patella in the lateral view. This is a complete or partial double radiodensity, which is rarely seen in other conditions.
In MED, the ankles are also in valgus; changes occur more in the talus than in the distal tibia. Upper extremity involvement is less severe; there may be irregularities in the proximal and distal humerus and radius. The humeral head involvement in adulthood, termed hatchet head, results from diminished growth of the head and neck. The hatchet head occurs in those children who are the most severely affected with MED. Radial ray hypoplasia may occur sporadically. The carpal ossification centers are delayed in appearing. The hand and wrist involvement may predict future stature.
The absence of vertebral changes differentiates MED from spondyloepiphyseal dysplasia. Hypothyroidism may produce similar radiographic findings as MED and can be screened for with thyroid function tests.
Treatment:The orthopaedic surgeon may become involved in the care of the patient with MED in either of two periods. There is a small role for realignment procedures in the early period during which the hip is deformed, provided there is progressive subluxation or pain, which utilize the principle of coverage that is the same as that used in Perthes disease. Acetabular shelf augmentation may be a consideration in these instances.
Hemiepiphysiodesis or osteotomies may be useful for angular deformities of the lower extremities. Patients having a double-layered patella may have symptoms because of the relative movement of one over the other. This may be treated by excision of one or fusion of the two segments, as appropriate.
Degenerative joint disease is the biggest problem, and it occurs in the second or third decade of life. The disease results not so much from malalignment of the joints as from an intrinsic defect in cartilage, and it produces stiffness from an early age and pain, eventually leading to a total joint arthroplasty of the hip and/or knees. The shoulder is also commonly affected by degeneration and shoulder arthroplasty may be necessary.
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