The two most commonly described types of Structural Pathology are Hip Dysplasia and Femoroacetabular Impingement. They share the same foundation of abnormal hip architecture, yet they are fully distinct conditions . There is always natural variation to the shape of our bones and there is a wide spectrum that falls within "normal". However, its becoming increasingly apparent that certain shapes of the hip can increase the risk of acute pain and future osteoarthritis. The correlation between anatomy and symptoms, anatomy and osteoarthritis, is not clear in most cases due to confounding variables. Yet a properly identified patient can avoid debilitating pain or the high probability of early hip arthritis by undergoing targeted procedures that re-balance the forces within the hip joint.
Background. Adolescent/Adult dysplasia, like Infantile DDH, has a higher female predominance and is commonly bilateral.
Increased acetabular obliquity leads to anterior-lateral undercoverage of the femoral head. A smaller surface area of contact between the femoral head and acetabulum increases peak forces on the cartilage and promotes subtle hip instability that increases shear stresses on the cartilage and promotes fatigue failure of the labrum. Chronic mechanical overload may lead to osteoarthritis. Uncorrected hip dysplasia is the most common etiology of hip OA in young patients requiring THA, with underlying dysplasia in 50% of patients under 50 yo [1].
Exam. Patients may present with pain in groin or lateral hip (giving the classic “C” sign when asked to locate their pain). They may also report instability, mechanical symptoms, and activity related pain.
Imaging. Hips that have less coverage, defined by many of the measurements discussed below, have been shown to have an increase in OA [2].
The amount of acetabular deficiency is characterized by the Lateral Center-Edge Angle and the Acetabular Inclination. A dysplastic hip with a lateral center-edge angle < 16, a center-edge angle < 25, and an acetabular index > 15 is associated with high risk for early arthritis if treated without surgery. The prognosis for mild dysplasia is less clearly understood.
Surgical Treatment. "Hip Preservation" is a surgical intervention that alters the position of the acetabulum to increase coverage of the femoral head and return radiographic measurements to within a normal range. The goals of hip preservation are to correct the primary deformity, eliminate instability, prevent articular damage, and delay or prevent the onset of OA.
There are multiple preservation techniques which all utilize acetabular osteotomies to re-orienting the hip joint. Bernese Peri-Acetabular Osteotomy (PAO) has emerged as the leading technique. Its main advantages are that it preserves the pelvic ring, while allowing for acetabular correction in multiple planes (anterior coverage, lateral coverage, and medialization).
Outcomes. The outcomes of the Bernese PAO are promising. 20 year survival is about 60-80%, with the end point being hip replacement. Importantly, a PAO does not appear to negatively impact hip arthroplasty in cases of progressive OA. However, there is a concern that overcorrection of the acetabulum (excessive anterior coverage, the acetabulum becomes retroverted) leading to iatrogenic impingement.
Overtime, OA can still occur. Predictors of conversion to THA include surgery at older age (>35) and preoperative XRs that show poor joint congruency, or early signs of OA. Postoperative radiographs showing improved congruency and coverage have been associated with better long term outcomes.
More recently, delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) has played a role in analyzing hips concerning for dysplasia and the likelihood of success of PAO vs. conversion to THA. Glycosaminoglycans (GAGs) in articular cartilage are lost with OA. Gadopentetate (Gd-DTPA) distributes in cartilage inversely to the concentration of GAGs. So, it’s low in normal cartilage and high in degraded cartilage. It’s also been found to correlate with pain and other radiographic measurements, like the L-CEA, and may show signs of dysplasia and OA before evidence on XRs, when hip preservation may be too late anyway. It can help predict which hips are more likely to get converted to a THA.
However, if/when conversion to a THA occurs after PAO, it is important to consider changes in the hip from the original surgery. Correction from the PAO can often lead to a more retroverted acetabulum making exposure more difficult and requiring trimming of the anterior wall or bone grafting, and a conscious effort to achieve proper acetabular version. However, compared to patients who underwent a THA for dysplasia, there was no difference in blood loss, OR time, or complications.
THA in the dysplasic population without undergoing PAO is also more technically changing than a standard primary THA due to the shallow, more vertical acetabulum, with poor anterior bone stock. The severity of dysplasia can be described by the Crowe Classification, which reports worsening hip subluxation due to the dysplasia. A Crowe 4 is complete hip dislocation, which can cause a pseudoacetabulum above the normal hip center. This can make proper cup placement more difficult. When evaluating the soft tissues, there can be muscle contracture or elongation of the capsule.
References.
1. Clohisy JC, Dobson MA, Robison JF, Warth LC, Zheng J, Liu SS, et al. Radiographic structural abnormalities associated with premature, natural hip-joint failure. J Bone Joint Surg Am. 2011;93 Suppl 2:3-9.
2. Murphy SB, Ganz R, Muller ME. The prognosis in untreated dysplasia of the hip. A study of radiographic factors that predict the outcome. J Bone Joint Surg Am. 1995;77(7):985-9.