Adult dysplasia of the hip is the result of defective development of the hip joint from birth and continuing on to adulthood, resulting in a shallow, hypoplastic acetabulum and a defective proximal femur. In fact, developmental dysplasia of the hip is the most common orthopaedic disorder in newborns, its incidence reaching 1:100, while for hip dislocation it is 1:1000. The disease evolves in a spectrum involving dysplasia, which refers to a shallow and underdeveloped acetabulum, subluxation, which refers to the partial loss of contact between the joint surface of the femoral head and the acetabulum, and, finally, dislocation, when complete loss of contact and proximal migration of the femur has occured. The incidence of adult hip dysplasia in the USA is estimated at 3-5%, while 10% of all total hip arthroplasties are performed due to hip arthritis secondary to dysplasia. Female sex carries a two- to four- fold greater risk of developing hip dysplasia, while other risk factors include breech presenation, oligohydramnios, primiparity, improper swaddling and positive family history.
While adult hip dysplasia almost exclusively requires some form of surgical treatment to improve function and alleviate symptoms, developmental dysplasia of the hip, which consists its primary form in newborns and infants, can be diagnosed as early as the second month of life, at which time it can be succesfully treated in almost 90% of patients, initially, with special casts or orthoses. As such, the screening of newborns with clinical examination and, depending on the regional guidelines, hip ultrasonography cannot be overemphasised.
The bony as well as soft tissue abnormalities associated with hip dysplasia result in abnormal movement of the femoral head within the acetabulum, i.e. the hip socket. Thus, the rim of the acetabulum is overloaded leading to cartilage damage and the gradual development of secondary osteoarthritis. The spectrum of anatomic abnormalities in hip dysplasia includes a shallow acetabulum with various degrees of lateral and superior wall deficiency, with or without a more proximal false acetabulum, while also associated with femoral side deformtites, such as excessive femoral anteversion, head-neck junction abnormalities, coxa valga, femoral intramedullary canal hypoplasia and posterior displacement of the greater trochanter.
There are two classification systems for hip dysplasia. The Hartofilakidis classification identifies three categories: type A or dysplasia, where the femoral head is still contained within the acetabulum albeit in a subluxated position·type B or low dislocation, wherte the femoral head is dislocated and articulates to a false acetabulum directly superior to the true one·and, finally, type C or high dislocation, where the femoral head is dislocated completely and has migrated far superior and posterior to the true acetabulum. The Crowe classification, on the other hand, categorises dysplasia in four grades based on the proximal migration or the sublaxation of the femoral head expressed as a percentage of the vertical height of the pelvis or the vertical diameter of the femoral head, respectively.
Patients with hip dysplasia most commonly present with activity – related hip and groin pain of insidious onset which can be exacerbated by hip flexion and internal rotation. Lateral hip pain due to trocahnteric bursitis, or a limp, also known as Trendelenburg gait, may be present as a result of abductor fatigue or deficiency. Upon physical examination, the patient’s stance and gait are evaluated, as well as the presence of overall ligametous laxity (Beighton score). Patients frequently exhibit increased hip external rotation due to increased femoral anteversion, pain with active or passive hip flexion and rotation and positive provocative tests indicating instability of the joint, such the anterior apprehension or the prone external rotation test.
Regarding imaging for hip dysplasia, plain radiographs are the commonest initial modality involving anteroposterior, lateral and false profile views. Various radiographic parameters can be measured and are useful both for diagnosing as well as classifying hip dysplasia. Computed tomography of the hip is usually reserved for preoperative planning and assessement of acetabular and proximal femoral morphology and femoral anteversion.
Regarding imaging for hip dysplasia, plain radiographs are the commonest initial modality involving anteroposterior, lateral and false profile views. Various radiographic parameters can be measured and are useful both for diagnosing as well as classifying hip dysplasia. Computed tomography of the hip is usually reserved for preoperative planning and assessement of acetabular and proximal femoral morphology and femoral anteversion.
Pelvic osteotomies can also be accompanied by femoral osteotomies when less than 90° of flexion and 15° of internal rotation can not be achieved intraoperatively so as to avoid femoro-acetabular impingement, i.e. abuttment of the femoral head-neck junction on the acetabular walls preventing full hip range of motion and leading to labral injuries and pain. Finally, joint preserving techniques such as the aforementioned can nowadays be complemented, in select cases, by hip arthroscopy to address chondral, labral and cam-type lesions of the femoral head-neck junction. However, this technique should only be offered in cases of minimally dysplastic hips or post-periacetabular osteotomy, as it can be accompanied by increased failure and reoperation rates as well as accelerated progression to arthritis when performed in moderate to severe dysplasia.
In patients with unreduced or incongruous hips and inadequate femoral head coverage, salvage pelvic osteotomies, such as the Chiari or the Shelf osteotmy, have been somewhat successfully performed in order to delay the development of end-stage arthtrits. When, however, the latter has set, the treatment of choice remains total hip arthroplasty, which can reliably alleviate the patients’ pain and improve functional outcomes, although arthroplasty for dysplasia has been associated with an increased complication profile compared to arthropasty for primary hip osteoarthritis. Finally, hip resurfacing, a more bone – preserving type of hip arthroplasty, has been described in dysplastic hips as well, but given the frequency of proximal femoral deformities in such patients it must be implemented after strict patient selection, so as to avoid unacceptable complication rates.
In conclusion, adult hip dysplasia is an uncommon yet debilitating disorder of the hip which represents the natural course of developmental hip dysplasia in adolescents and young adults. Prevention of its development through newborn screening is of utmost importance both for the patients themselves as well as for healthcare systems. However, adult dysplasia can be treated with a variety of surgical modalities ranging from hip preservation to hip arthroplasty leading to improved functional outcomes.
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