Dysplastic hips share a common pattern of anatomic abnormalities. In hip dysplasia the deficiency is mainly on the acetabular side. Occasionally the whole pelvis is underdeveloped.
The acetabulum (socket of hip joint) is usually shallow; deficient anteriorly and superiorly with a decreased antero-posterior diameter and antiverted.
The Femoral side (thigh bone) shows persistent femoral anteversion, valgus neck, hypoplastic, narrow canal, small and posteriorly placed greater trochanter.
The combination of these abnormalities leads to a significant reduction in the contact area between the femoral head and acetabulum, the 'ball' doesn't fit snugly in the 'socket' of the hip joint. As a result, high forces are transmitted through a limited surface area and the increased and uneven loading of the joint results in articular degenerative changes and eventually osteoarthritis.
These are usually associated with proximal migration of the femur. These include hypoplastic, short muscles acting on the hip joint, thickened joint capsule, femoral nerve displacement proximally and laterally as well as sciatic nerve shortening.
Patients with dysplasia typically experience groin pain that increases with exercise. Other features such as catching, locking and giving way of the hip are symptoms that may indicate associated labral tear or articular cartilage pathology.
Diagnosis is made by plain radiograph (x-ray). Different classification systems are used to assess the severity of the condition and help to choose the best method of treatment.
Hartofilikadis classification. This classification is reasonably simple:
Crowe’s classification: Measures proximal migration as a proportion of the height of the femoral head compared to the distance from inter-tear-drop line.
Hip arthritis is the end result of hip dysplasia for which total hip arthroplasty is the procedure of choice for most patients.
However, the anatomic abnormalities associated with the dysplastic hip increase the complexity of hip arthroplasty. In principle it is desirable to restore normal anatomy and use an uncemented implant. In order to restore normal anatomy the metal shell needs to be inserted near the normal anatomical position of the acetabular to restore hip centre of rotation. The uncemented acetabular components allow biologic fixation with potentially improved results compared with cemented cups, especially in young patients.
Due to soft tissue shortening, femoral shortening is required in some cases. This includes the sciatic nerve which it is at most risk if the leg is lengthened by more than four centimetres. Shortening can be achieved by subtrochanteric osteotomy.
The results of total hip arthroplasty for hip dysplasia demonstrates a high rate of pain relief and functional improvement for patients.
Acetabular reconstruction placement of hip centre:
Femoral reconstruction has two considerations:
Type 1 hips can be reached through a standard posterior approach. For more exposure a trochanteric slide, Chevron or flat osteotomy at the base of the greater trochanter can be used to retract and expose more of the ilium and then reattrached via wires. To assess how deep to ream the approach is to drill through the floor and measure its thickness and then ream to within 0.5-1.cm and insert trial if <70% is covered. Then use the femoral head to create a shelf.
Acetabular component: Has better results with uncemented cups, more native bone coverage and positioning the component as near as to the anatomical hip centre.
Femoral component: Shows cemented femurs perform better than cemented cups. Uncemented femurs show promise in younger patients since uncemented are more biological implants. Subtrochanteric osteotomy has 80% satisfactory results and are better with uncemented implants.