Infant Hip Dysplasia: Residual Acetabular Dysplasia During Childhood

FUNDAMENTALS

The goals for treatment of a dislocated hip secondary to developmental dysplasia of the hip (DDH) are concentric reduction of the hip and maintenance of the reduction, allowing for acetabular and femoral remodeling. Although the hip may be reduced either by closed or open means, the remodeling of the acetabulum is not always complete, and residual acetabular dysplasia may persist. The management of residual acetabular dysplasia after closed or open reduction is highly controversial because there are limited data about the natural history of acetabular remodeling throughout infancy. Moreover, specific information about limits of acetabular morphology to accept during remodeling and the most appropriate age to intervene is lacking.

Therefore, the dilemma of whether and when to correct residual acetabular dysplasia during childhood remains. On one side of the debate are those who favor early intervention because it allows for correction of the hip mechanics and it may give the best chance of the hip to further develop as close to normal as possible. On the other hand, others may argue that early surgery will lead to some unnecessary surgery because of the capacity of remodeling with further growth and the authors’ inability to precisely predict which patient will not sufficiently remodel. These surgeons favor waiting until an age close to skeletal maturity to determine whether or not reorientation of the acetabulum should be performed by a more powerful osteotomy of the pelvis, such as Bernese periacetabular osteotomy (PAO).

To plan better for the treatment of residual acetabular dysplasia, it is essential that one understands the natural development of a normal acetabulum during childhood, the remodeling of the acetabulum after closed or open reduction, and the available techniques to correct the dysplastic acetabulum. Classically, most surgeons use a reference of acetabular development based on the acetabular index (AI) measurements performed by Tönnis in the 1970s.¹ Recently, one study² evaluated 1152 normal anteroposterior pelvic radiographs to measure the AI and the acetabular depth-width ratio in children from 1 month to 14 years of age and reported a graph with the Z-score distribution of the AI for age (Figure 4.1). Therefore, assessment of acetabular remodeling should not be done by following only the raw values of AI. Instead, the authors recommend using Z scores and the percentile curves to evaluate whether the acetabulum is indeed remodeling over time. The authors consider true acetabular remodeling if the Z scores improve over time. In other words, at each visit, the values of AI are plotted against the normal graph, and true remodeling presents when the values are getting closer to the mean and changing categories for the percentile distribution. Simple decrease in the values of AI remaining in the same zone is not considered true remodeling and warrants close follow-up.

The appropriate age and AI thresholds to perform a corrective pelvic osteotomy after closed or open reduction of the hip remain controversial. Kim and colleagues recommended considering a pelvic osteotomy in hips with an upward acetabular sourcil and lateral displacement of the femoral head greater than 6% around 4 to 5 years of age.³ Shin and colleagues showed that among hips with AI greater than or equal to 32° at the age of 3 years and those who have undergone an osteotomy, there was a higher proportion of satisfactory outcomes compared to those not treated.⁴ Albinana and colleagues published the most valuable paper about acetabular remodeling after closed or open reduction.⁵ These authors showed that the major improvement of AI after reduction occurred after the first year, but remodeling would continue up to 6 years of age if concentric reduction was present by 1 year postreduction. They confirmed that the severity of acetabular dysplasia at skeletal maturity was a predictor of the risk of a total hip arthroplasty because of joint arthritis later in life with a mean follow-up of 30 years. Moreover, age at reduction was highly correlated with residual acetabular dysplasia. Finally, the authors established thresholds for AI values and the sensitivity and specificity for residual acetabular dysplasia.

FIGURE 4.1. Graph describing the acetabular index according to age on a normal population of children and adolescents. The dark gray area represents values above the 90th percentile, and the light gray area represents values between the 50th and 90th percentile. A, Females. B, Males. Adapted from Novais et al.²

Another useful tool for the assessment of the quality of the acetabular cartilage during the remodeling phase is magnetic resonance imaging (MRI). Wakabayashi and colleagues showed that the presence of a high-signal-intensity area within the weight-bearing portion of the acetabular cartilage on T2-weighted MR coronal section images was associated with poor acetabular growth.⁶ Therefore, MRI is currently used to assess the morphology of the acetabulum and femur and, most importantly, the quality of the cartilage in the acetabular weight-bearing area. The presence of high-signal-intensity area within the weight-bearing portion of the acetabular cartilage is not normal and is another relative indication for surgical correction.

The authors’ preference is to follow patients after closed or open reduction yearly with an anteroposterior (AP) pelvis radiograph to assess the morphology of the acetabulum including measurement of the AI and depth-width ratio and assessment of the teardrop figure. The values of AI are plotted against the normal percentile curves, and the lack of significant improvement is a relative indication for a pelvic osteotomy. At around 4 years postreduction, the AI should be less than 22°. A pelvic osteotomy may be considered if the AI is higher than 22°, especially if MRI shows the presence of high-signal-intensity area within the weight-bearing portion of the acetabular cartilage. Hip subluxation with a broken Shenton line during follow-up after reduction is an absolute indication for surgical intervention because those hips do not remodel and are associated with poor Severin classification at skeletally mature age and development of osteoarthritis later in life.

Clinical Scenario

A 5-year-old girl presents to clinic for evaluation following treatment of a dislocated left hip performed elsewhere. When she was 20 months old, she underwent a closed reduction of the left hip at another hospital followed by immobilization in a spica cast for about 4 months (Figure 4.2). She was then subsequently seen at the authors’ clinic at age 5 years (Figure 4.3). Although she had a large improvement in the AI from preoperative to 1 year after reduction, the values of AI remained high over the years following the closed reduction (Figure 4.4). Lack of improvement of the AI after closed reduction is a relative indication that the hip is not going to completely remodel with additional growth. MRI was obtained to search for changes in the weight-bearing portion of the acetabular cartilage in the affected left side. Notably, there was a high-signal-intensity area within the weight-bearing portion of the acetabular cartilage on T2-weighted coronal section images (Figure 4.5). The patient was then subsequently treated with a Dega pelvic osteotomy (see description later).

FIGURE 4.2. A 20-month-old female presented to an outside clinic with a left dislocated hip. A, Preoperative anteroposterior pelvic radiograph shows a complete dislocated left hip. B, Intraoperative arthrogram after closed reduction shows complete coverage of the femoral head by the acetabular labrum. The patient was subsequently placed in a spica cast.

FIGURE 4.3. Postoperative anteroposterior pelvic radiograph obtained at (A) age 3 years and at (B) 5 years following closed reduction shows persistent acetabular dysplasia of the left hip.

FIGURE 4.4. Patient-specific graph with the acetabular index (AI) measured over time since the closed reduction. Blue dots represent the values of AI measured for the right uninvolved hip, whereas red dots represent the left involved hip. The values of the left hip are consistently higher in the abnormal range without significant improvement at 3 years postreduction.

FIGURE 4.5. T2-weighted magnetic resonance coronal section image shows the presence of a high-signal-intensity area within the weight-bearing portion of the acetabular cartilage on the left hip (white arrow) with normal cartilage signal on the right hip (black arrow).

SURGICAL PROCEDURES TO TREAT RESIDUAL ACETABULAR DYSPLASIA DURING CHILDHOOD

Once the diagnosis of residual acetabular dysplasia with or without subluxation is made and a decision is in place for a corrective pelvic osteotomy, the next step is to determine the type of osteotomy to be performed. Osteotomies that reshape the acetabulum with a fulcrum of rotation at the triradiate cartilage include the Pemberton,⁷ the Dega, and the San Diego osteotomies. Osteotomies that reorient the acetabulum include the Salter⁸ and the double and triple pelvic osteotomies (Figure 4.6). Once the triradiate cartilage is closing, a complete redirectional osteotomy of the
pelvis can be performed and the authors’ preferred technique is the Bernese PAO,⁹ which is described completely in Chapter 5.

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