Management of Hip Instability in Trisomy 21

Abstract

The unstable hip in Trisomy 21 presents with a spectrum of hip instability with different problems at different ages. What links this multiphase problem, in many patients, is the final common pathway of untreated instability, that of a stiff, dislocated, and often-painful hip, leading to significant functional disability. Historically, the results of treating hip instability in Trisomy 21 were variable with a notable frequency of poor results. With an improved understanding of the Trisomy 21 hip in terms of its pathoanatomy and a more contemporary surgical approach to hip reconstruction, much improved results can be expected and indeed have recently been shown. The mainstay of treatment for the habitual dislocation group presenting before 8 years of age is the femoral varus derotation osteotomy. The older group presenting with painful subluxation often show signs of secondary acetabular dysplasia and thus are best treated with redirectional acetabular osteotomy with or without the use of femoral varus derotation osteotomy. The presence of radiographic features of degenerative arthritis in the fixed dislocation group precludes the use of joint-preserving techniques for hip reconstruction, and these patients can achieve excellent results with total joint arthroplasty. The natural history, historical results, assessment, treatment, and management of complications of hip instability in Trisomy 21 are addressed in this paper.

The patient with Trisomy 21 may present to the orthopaedic surgeon for the management of a number of musculoskeletal problems such as occipitocervical and cervical spine instability, scoliosis, hip disease, patellar instability, and pes planus.^1,2 Some of the more common childhood hip diseases have an increased incidence in Trisomy 21, such as slipped capital femoral epiphysis (1.3%) and Perthes disease (2%)³; however, this paper will emphasize the diagnosis, assessment, and management of hip instability, which is arguably the most challenging of the musculoskeletal issues in Trisomy 21 to treat.

The unstable hip in Trisomy 21 presents with a spectrum of hip instability with different problems at different ages. What links this multiphase problem in many patients is the final common pathway of untreated instability, that of a stiff, dislocated, and often-painful hip, leading to significant functional disability.^4,5 The different presentations at different ages may represent different severity of disease. It is conceivable that the children exhibiting more laxity and lower muscle tone present with symptoms and signs of habitual dislocation at an earlier age. The children presenting later, in adolescence, may have globally less severe soft tissue laxity avoiding the more dramatic habitual dislocation phase but presenting later with progressive, and initially asymptomatic, subluxation and radiographically advanced acetabular dysplasia.

NATURAL HISTORY

The natural history of the hip in Trisomy 21 can be classified into one of 4 overlapping groups, that of initial phase (0 to 2 y), dislocation phase (2 to 8 y), subluxation phase (>8 y), and fixed phase (>15 y).⁴ These groups are ordered by both age and pathology and as such they each tend to have specific modes of presentation and particular radiographic findings, which warrant different approaches to management (Fig. 1).

FIGURE 1: Three phases of instability. A, Habitual—dislocated right hip and subluxated left hip with increased femoral anteversion and relatively normal acetabular morphology. B, Subluxation—in addition to bilateral subluxation of the hips note the progressive appearance of bilateral secondary acetabular dysplasia with widened teardrop, and increased acetabular inclination. C, Fixed dislocation—bilaterally dislocated hips with both proximal femoral and severe secondary acetabular dysplasia.

Of adults with Trisomy 21, 28% have radiographic signs of hip pathology, and at best 60% of these patients are wheelchair or household ambulators.⁵ Patients with Trisomy 21 without radiographic signs of hip subluxation or dislocation are twice as likely to be community ambulators as those with signs of radiographic abnormality. It is clear that deterioration in hip status may preclude community-based living and many patients with Trisomy 21 cannot function as community ambulators after 30 years of age.⁵ This disability coupled with aging parents who struggle to care for their often overweight disabled, now adult, child leads to a very unhappy situation for both the patient and their carers.

POOR HISTORICAL RESULTS

Historically, the results of treating hip instability in Trisomy 21 were variable with a notable frequency of poor results.^4,6,7 In 1982, Bennet describes a variety of interventions including 5 patients who underwent closed reduction and cast immobilization between the ages of 7 and 12 years. Only one of these patients had an initially successful result, with 4 patients remaining subluxated, dislocated, or requiring osteotomies to achieve stabilization. Their experience with capsular plication was equally disappointing with a 50% failure rate whether combined with bony surgery or not. Another 4 patients received a Chiari osteotomy, all resubluxated, or redislocated. Finally, those who underwent femoral osteotomy alone fared little better, with 5 redislocations in 9 patients.⁴ In 1985, Aprin published the results of surgical intervention in 6 patients (10 hips) and divided them into 2 groups based on the presence of acetabular dysplasia. The group of 3 patients with a normal acetabulum all received femoral varus derotation osteotomies (with or without capsular plication), and all had a good or excellent result with no redislocations. The group with acetabular dysplasia underwent innominate osteotomies (with or without femoral osteotomies), and of these 3 patients, only 1 patient achieved a stable hip at final follow-up.⁶ Beguiristain et al⁷ shows a similar pattern of interventions and poor results.

Experience in treating hip instability in Trisomy 21 is limited due to the small numbers encountered in practice and, therefore, equally small case series are often all that is available in guiding management. These poor results were due to a number of contributing factors, not least a limited understanding of the unique pathoanatomy of the hip in Trisomy 21, particularly the failure to understand the importance of acetabular retroversion leading to the arbitrary use of a variety of acetabular reconstructive procedures including the Chiari, Shelf, Innominate, and Pemberton procedures, some of which are likely to worsen acetabular retroversion and hence stability.^4,6,7 In some instances, there was a reliance on isolated soft tissue procedures, which are prone to failure in a condition characterized by soft tissue laxity, as the tissues tend to stretch out over time particularly on a background of bony structural deficiency.^4,7 Furthermore, this pessimism was compounded by the high complication rate of surgical intervention, and that life expectancy was significantly reduced due to heart disease in this group of patients. Life expectancy at the time of writing of many of the key historical articles on the hip in Trisomy 21 was merely 25 years of age. Life expectancy has since increased to more than 50 years of age by 2002, so that neurodegenerative disease is now a more likely cause of death than cardiovascular disease.⁸ Accordingly, orthopaedic issues have now become more important, and with an improved understanding of the Trisomy 21 hip in terms of its pathoanatomy^9–11 and a more contemporary surgical approach to hip reconstruction, much improved results can be expected and indeed have recently been shown.^12,13

ASSESSMENT

Because of the range of anatomic abnormalities of the unstable hip that occur in Trisomy 21, a detailed clinical and radiographic assessment is necessary. A good quality anteroposterior radiograph of the hip joints, with appropriate pelvic tilt, is necessary to identify features of femoral and acetabular dysplasia, subluxation, or dislocation. Relevant parameters include an assessment of Shenton line, femoral neck shaft angle (NSA), the status of the triradiate cartilage, the width and shape of the acetabular teardrop, the centre-edge angle, the presence of crossover or posterior wall sign.^9,14 A false-profile view may be used to measure the anterior centre-edge angle. Computed tomography is very helpful in determining the degree of femoral anteversion and acetabular retroversion.^9,15 Only with this extensive assessment can a detailed surgical plan be constructed to correct the abnormalities pertaining to the instability.

In general, the deformities commonly encountered in Trisomy 21 are as follows: femoral anteversion is on average moderately increased with mean of 33.6 degrees (range, 0 to 59 degrees). The reported values for NSAs vary with 1 report suggesting little abnormality with a mean value of 134 degrees (range, 115 to 148 degrees),³ whereas another report suggests an increase in NSA of 167 degrees (range, 162 to 171 degrees).¹² Routine radiographs will tend to show an apparent increase in NSA due to the position of the leg when the radiograph is taken, a common source of error in hypermobility syndromes. Any final decision on the true NSA can be made intraoperatively under fluoroscopy when position of the leg can be more accurately controlled.

The primary bony abnormality of the acetabulum in Trisomy 21 is that of retroversion. Sankar studied a cohort of patients with a mean age 14.1 years (range, 8 to 20 y); the acetabular version showed a mean of 2.1±11.0 degrees compared with an age-matched control group with a mean of 13.4±5.5 degrees. Furthermore, any differences were not shown to be related to age in this study, although only 2 patients were aged below 12 years.⁹

Secondary acetabular changes occur over time and may include a reduced centre-edge angle, an increased Sharp acetabular angle, an increased Tönnis angle, and widening of the acetabular tear drop. Widening of the acetabular teardrop reflects ossification of the cotyloid cavity in the medial wall of the acetabulum and may reflect long-standing lateralization of the femoral head or premature closure of the triradiate cartilage.

All of this detailed radiographic information does not demonstrate a causal relationship between abnormal bony parameters and hip instability. It is conceivable that many patients with Trisomy 21 will have similar skeletal abnormalities yet never demonstrate frank hip instability nor come to the attention of an orthopaedic surgeon. Consistent abnormalities are seen, however, in symptomatic patients and form the basis for surgical hip reconstruction.

GOALS OF TREATMENT

Once hip instability in the child with Trisomy 21 has become symptomatic, it is likely to become more disabling over time. The secondary acetabular pathology is likely to complicate the primary bony structural abnormalities leading to a more complex reconstruction; therefore, it is prudent to be aggressive in ones surgical management to correct hip instability before such secondary acetabular changes appear, typically at 8 to 9 years of age. The goals of management are therefore dependent on the phase of the disease; in the initial phase to reduce a dislocated hip; in the habitual dislocation phase the goal is to stabilize the hip and prevent the development of secondary acetabular dysplasia. In the subluxation phase, the goal is to reduce the hip and correct acetabular dysplasia to prevent resubluxation and degenerative joint disease. In the fixed phase, the goal is to relieve pain and optimize ambulatory status.

INITIAL PHASE

Some babies with Trisomy 21 present with a reducible dislocation of the hip. This Ortolani-positive dislocation initially responds well to bracing techniques used in developmental dysplasia of the hip; however, bracing may need to be applied for a prolonged period to achieve stability. It is rarely necessary to proceed to closed reduction and spica application, which is unlikely to be the definitive treatment due to the underlying pathology. Many of these hips will progress to the habitual dislocation phase after walking age and are best managed expectantly until they are large enough to be treated as described below.

HABITUAL DISLOCATION

This group usually presents before 8 years of age and has particularly marked soft tissue laxity. Although primary acetabular retroversion may be present, features of secondary acetabular dysplasia are not usually present. Femoral anteversion is variably increased, as is the NSA. The mainstay of treatment in the author’s practice is the femoral varus osteotomy with or without the judicious use of derotation, based on good results recently published from our institution using this protocol. Nine children with 16 hips were reviewed. All children were aged 5 to 7 years at the time of surgery and had a femoral varus derotation osteotomy. Two hips required a concurrent periacetabular osteotomy. There were no recurrent subluxations or redislocations.¹² An alternative approach to the habitually dislocating hip has been proposed by Sankar et al,¹³ whose recommendation is that of an isolated anteverting triple osteotomy of the acetabulum. The rationale for both treatments differs according to the philosophies of the authors. Knight argues that the major structural abnormality in this group is the presence of significant proximal femoral dysplasia with hyperlax soft tissues and therefore recommends the use of the femoral varus osteotomy. Sankar, in contrast, argues that primary acetabular retroversion of the acetabulum exists even at this young age and therefore directs management to the correction of this aspect of the hip deformity. The use of the triple osteotomy for the habitually dislocating hip has also been published demonstrating good results and is reviewed elsewhere in this supplement.

In performing a femoral varus osteotomy, the coronal plane correction aims to decrease the NSA to approximately 105 degrees, restore Shenton line (Fig. 2). Leaving the NSA significantly >105 degrees and hip stability cannot be assured. Reducing the NSA to significantly <105 degrees and the biomechanical disadvantage of a markedly varus proximal femur outweighs the benefits of the increased stability. The surgeon needs to pay particular attention to the use of derotation so as not to cause posterior instability in the face of a retroverted acetabulum. A minimum of 20 degrees of femoral neck anteversion should remain, and this can be assessed intraoperatively to ensure maximum stability. A standard blade plate is the hardware of choice due to its ability to be appropriately positioned in a narrow femoral neck and achieve the necessary corrections. Contemporary pediatric locking hip plates, although user friendly, may not be as suitable for use in the Trisomy 21 hip due to the narrow femoral neck. Preoperative planning is, of course, essential to preempt such issues. The use of capsulorrhaphy is controversial and not usually necessary. Correcting an unstable hip with capsulorrhaphy alone is prone to failure as the underlying genetically lax soft tissues will tend to stretch out in the postoperative course. Bony surgery is always required in such circumstances. In the unlikely event that the hip remains unstable after an optimally positioned femoral osteotomy then the addition of an acetabular reorienting procedure can be considered. A triple osteotomy is the procedure of choice due to its ability to correct the acetabular retroversion. Postoperative immobilization is recommended with the use of Petrie casts for 6 to 8 weeks to allow the soft tissues to stabilize. It is notoriously difficult to maintain protected weight bearing status in this group of patients without the use of supplementary casting, and thus, it is recommended in every case

FIGURE 2: Patient with bilateral habitual dislocation of the hip preoperatively (A) and after bilateral femoral varus derotation osteotomies (B).

SUBLUXATION

Patients who present later in childhood, after 8 years of age often fit the criteria for the subluxation group where secondary acetabular pathology has developed. The menu of appropriate surgical procedures reflects these changes. Acetabular retroversion is often accompanied by coronal plane deformities noted by a reduced centre-edge angle of Wiberg, increased angle of Tönnis, and widened teardrop. In the face of significant acetabular dysplasia proximal femoral osteotomy alone is insufficient to prevent progressive subluxation and dislocation. The procedure of choice for this type of hip deformity adheres to the same principles, as the habitual dislocation group in that one should aim to correct the morphology of the hip joint to normal radiographic parameters to ensure maximum stability. This will include the use of a Bernese periacetabular osteotomy in those with a closed acetabular triradiate cartilage or a triple osteotomy in the skeletally immature where the triradiate cartilage is open. Sankar has published improved results with complete redirectional acetabular osteotomy for hip instability. It is notable that the mean age of patients undergoing surgery in his study is 11.8 years; hence, many will be of the subluxation type with secondary acetabular dysplasia rather than a pure habitual dislocation. Of the 12 patients who received acetabular osteotomy alone, all but 1 (92%) remained stable at the latest follow-up, which makes this the first study to report success in this most difficult of phases.¹³ The correction afforded by these redirectional osteotomies will allow increased acetabular anteversion as well as lateral and anterior femoral head coverage.

The addition of a proximal femoral varus osteotomy may be required depending on the radiographic measurements of femoral morphology and intraoperative assessment of hip stability. Historically, procedures such as the Salter innominate osteotomy and Pemberton acetabuloplasty have been utilized in this phase, but neither corrects the inherent bony pathology, and in fact are both likely to exacerbate the posterior acetabular deficiency thus increasing instability. Occasionally postoperative immobilization is required in this group but is not as essential as the habitual dislocation group due to a tendency to less soft tissue laxity, more inherent stability from the bony procedures, and the older age of the patient.

FIXED DISLOCATION

The presence of radiographic features of degenerative arthritis precludes the use of joint-preserving techniques for hip reconstruction. Degenerative arthritis may occur secondary to hip instability with progressive subluxation or dislocation but may also occur secondary to the other hip pathologies associated with Trisomy 21, namely slipped capital femoral epiphysis and Perthes disease. In patients with debilitating symptoms of pain and immobility secondary to arthritis, total hip arthroplasty (THA) is indicated. Trisomy 21 per se should not be considered a contraindication to the use of THA despite concerns about postoperative dislocation related to intellectual disability or soft tissue laxity, nor from technical concerns regarding abnormal bony anatomy including severe acetabular dysplasia or the small dysplastic morphology of the proximal femur and acetabulum. A number of published series highlight good and excellent results of THA in Trisomy 21 and offer practical steps to ensure the best possible result.^16–19

COMPLICATIONS

As with any surgical procedure, complications occur, and in many cases the complication rate is not significantly different from that which would be expected with other etiologies. What is consistent, however, is the increased incidence of 3 particular postsurgical complications.^4,6,11,12,20 The first is resubluxation or redislocation. With the contemporary approach to hip reconstruction outlined above, the rate of residual subluxation and redislocation is now significantly lower than previously described.^12,13 This is mainly due to a more complete understanding of the pathoanatomy of hip dysplasia in Trisomy 21 and its progression with age combined with a surgical algorithm that aims to systematically correct each of the important elements without exacerbating known deficiencies.

The second complication of note is postoperative infection. Infection rates of up to 20% have been quoted after hip reconstruction.⁴ This is likely due to defective cellular and humoral responses as many of these children are immunodeficient, and although inherent to this patient population, one must bear in mind the increased risk and take appropriate perioperative precautions.²¹

The third complication of note is peri-implant fracture¹² (Fig. 3). The etiology of this complication may be multifactorial as although patients with Trisomy 21 have low areal bone mineral density (areal BMD), their values for volumetric bone mineral density (volBMD) are equivalent to normal subjects.^22,23 Areal BMD is a function of both true density of the bone tissue and of bone size. Smaller proximal femoral dimensions in Down syndrome therefore explain this discrepancy. VolBMD is a more accurate measure of bone density, accounts for discrepancy in bone size, and is more closely associated with fracture risk. To date, no evidence exists to highlight an increased fracture risk in Trisomy 21, although a number of features of Trisomy 21, namely institutionalization, low activity level, poor muscle tone, and endocrine abnormalities are considered risk factors for pathologic fracture.^22,24 Nevertheless, because of the apparent increase in peri-implant fractures, it is recommended to remove proximal femoral hardware after surgery after bony union has taken place.¹²

FIGURE 3: A, Postoperative periprosthetic fracture at the proximal aspect of the blade plate, fixed with a fixed-angle locking hip plate (B).

SUMMARY

Hip instability in Trisomy 21 is a challenging problem that has enjoyed a renewed interest in the literature of late and for good reason. Historic surgical results have been generally poor, but now that we have acquired a better understanding of the pathoanatomy and with the emergence of new techniques we have a clearer understanding of how to successfully reconstruct symptomatically unstable hips. In the past 30 years, patients with Trisomy 21 have enjoyed a significant increase in their life expectancy and, therefore, these recent advances in the orthopaedic management of hip instability could not have come at a better time.