Cerebral palsy (CP) is the most common cause of physical disability affecting children in developed countries.1 CP describes a group of permanent disorders of the development of movement and posture, causing activity limitation, that are attributed to nonprogressive disturbances that occurred in the developing fetal or infant brain. CP is defined by the presence of a static neurological injury, with associated progressive musculoskeletal pathology.2,3 Hip displacement in children with CP is the second most common deformity after spastic equinus of the foot.3–5
The classification of CP can be made according to topographical distribution, function severity and motor type. Five motor types of CP include: spastic, hypotonic, ataxic, dystonic, or mixed, and the definitions for each continue to be updated. The most common topographical distributions are spastic diplegia, spastic quadriplegia, and spastic hemiplegia, with asymmetrical diplegia and triplegia also recognized.2 Yet one of the challenges in CP is that neither the definition nor the descriptors of motor type and topographic distribution have been proven to be prognostic or reliable in terms of assessing functional or disease severity.
In the late 1990s the Gross Motor Function Classification System (GMFCS) was developed and over the last 20 years it has developed into the gold standard communication tool used for clinicians caring for children with CP.6 The GMFCS is a 5-level ordinal classification in which different descriptors are used according to the age of the child. The GMFCS has been shown to be valid, reliable, stable, and predictive of long-term gross motor function in children with cerebral palsy between the ages of 2 and 18 years.7 A series of population-based studies have demonstrated that a child with CP’s risk of lateral hip displacement is related to severity of neurological involvement,8 limitations in walking ability,9,10 and is directly related to gross motor function as classified by GMFCS.4
Children with CP are born with anatomically normal hips without evidence of hip displacement, dislocation or dysplasia.10,11 Progressive lateral hip displacement is common in nonambulant children with CP and can progress silently.2,3,10 Approximately one quarter of children with CP are not able to communicate verbally and almost half suffer from intellectual disability,12 further challenging our ability to detect pain and dysfunction in this at risk population. Clinical examination has been shown to be poorly correlated with radiographic measures of hip displacement and this measure alone is not accurate to detect early hip displacement.2,10,13
Given the heterogeneity of the CP population and the variability in treatment protocols it can sometimes be difficult to draw firm conclusions from individual studies surrounding the natural history of spastic hip displacement. Weaknesses in the evidence base including lack of clinical trials, heterogeneity of the patient populations studied, variability in the surgical treatments performed, and short-term clinical follow up further complicate this issue.14 Despite these challenges, the purpose of this paper is to outline the natural history of untreated spastic hip displacement in children with CP and identify which interventions are most efficacious according to the literature.
A search of the English-written medical literature was carried out for papers discussing the natural history of spastic hip displacement in children with CP. We began with a PubMed search using multiple terms related to CP, hip displacement, dislocation, and subluxation. We then cross referenced selected articles bibliographies to ensure thoroughness of our search. We excluded case reports and expert opinions. We selected papers which focused on the natural history and outcome of no treatment, rather than focus on results of treatment interventions. After review of the literature we classified articles into four main categories: epidemiology, pathophysiology, prevention, and outcome.
The initial searches included three sets of search terms: “cerebral palsy” and “hip dislocation” or “hip subluxation” or “hip displacement.” The results were filtered to include only those written in English and related to humans. From our searches in PubMed we found a total of 605 articles which were reduced to 467 after the removal of duplicates. These 467 articles were screened by title and abstract narrowing down to 151 articles. The remaining articles underwent further screening and those with full text were read and references were evaluated, yielding the inclusion of 46 studies. An additional 33 studies were identified and selected to bolster understanding of the epidemiology and classification of CP, radiographic measures, surveillance guidelines, and quality of life measures that were not included in the initial searches, yielding a total of 79 studies included in this review.
Historically, the incidence of CP has been reported ranging from 1.5 to 2.5 per 1000 live births in several well-designed population-based studies.15 However, with recent improved resuscitation and care of the premature infant, a modest increase (3 per 1000 live births) has been reported in North America.16 A series of authors have reported that the incidence of hip displacement in CP is related to the degree of body involvement; ranging from very low risk in children with spastic hemiplegia (1%) to very high risk in children with spastic quadriplegia (75%).5,9 Three large population-based studies have calculated that the overall incidence of hip displacement is approximately 35% across a population of children with CP.4,13,17 Soo et al4 demonstrated that the population-based incidence of hip displacement in nonambulant (GMFCS IV/V) children ranged between 69% and 90% and the relative risk of hip displacement in this cohort was between 4.6 to 5.9 compared with ambulant (GMFCS II) children. Hagglund et al13 found that the CP subtype as well as GMFCS level contributed to the risk of hip displacement, 79% of patients with spastic quadriplegia and 64% of those at the GMFCS V level had hip displacement compared with 0% of those with pure ataxia or GMFCS I. Connelly et al17 confirmed the linear association of GMFCS level and hip displacement. The risk of dislocation in children with CP has been reported to be between 15% and 20%.18 Progressive lateral hip displacement in nonambulant children with CP is commonly not painful until substantial femoral and acetabular deformities are present.9 Furthermore, the associated communication difficulties and concomitant medical comorbidities in this subset of children with CP make it easy to forget and overlook progressive lateral hip displacement.9,10
Traditionally, great emphasis has been placed on adductor spasticity or contracture “pulling” hips out of the socket by increased forces. However, hypotonic GMFCS IV and V children have the same risk of hip displacement as similar children with hypertonia.4 This suggests that the limitations in walking and weight bearing, as demonstrated by GMFCS predict hip displacement, rather than the type of motor disorder, for example, hypotonia, dystonia, or spasticity. Progressive lateral hip displacement is common in nonambulant children with CP and can progress silently.2,3,10 Hip displacement has been demonstrated to contribute to high rates of pain and impaired health-related quality of life (HRQOL) in nonambulant children with CP.3,19
Clinical examination alone is insufficient to evaluate hip displacement in children with CP.20 Decisions for treatment and surveillance must be made in conjunction with a well-taken anteroposterior (AP) radiograph of the pelvis and hip joint with the child in the supine position. For hip surveillance to be successful, a standardized radiographic technique must be followed, ensuring reliability between interval radiographs and between patients. In children with CP, excessive femoral anteversion, hip flexion, and adduction contractures are often present.21 Recognition and correct position in these circumstances is necessary to generate consistent radiographs. Unrecognized hip flexion contracture, will create a lordotic pelvis (excessive anterior pelvic tilt), which can be corrected by raising the legs on pillows to flatten the lumbar spine.22 Positioning the legs parallel to each other will address adduction or abduction contractures, and pointing the patellae upwards will correct for excessive femoral anteversion.23 A film-focus distance of 115 cm has been suggested to standardize film magnification between patients.8
The most accepted and reproducible measurement for hip displacement is Reimer’s migration percentage (MP),24,25 which is a measure of the femoral head’s containment within the acetabulum in the coronal plane.22 This measurement is obtained by identifying Hilgenreiner’s line and Perkin’s line, and then measuring as a percentage the amount of ossified femoral head that has migrated beyond Perkin’s line laterally (Fig. 1). The MP is a linear measure of hip displacement and is the most, valid, reliable, and useful measure of hip displacement in children with CP.20,24–27 In the setting of hip surveillance, reliability of this measure can be improved with special training of the recording health care providers.27 In cases of moderate to severe hip displacement, there is often a “gothic-arch” deformity of the lateral acetabular margin and in these cases, the midpoint of the lateral acetabular margin instead of Perkins line is used.25
The acetabular index (AI)28 is one of the most commonly used acetabular measures for spastic hip disease. This index measures in degrees the angle between the slope of the acetabulum and Hilgenreiner’s line. However, when measuring the AI in children with CP, observers must be cognizant of increased acetabular anteversion and posterior acetabular insufficiency, which are common and result in underestimation of the degree of acetabular deficiency. In this scenario, the apex of the acetabulum may not be the most lateral point of the acetabulum.20 Although some studies have demonstrated utility of the AI in predicting hip instability,20 others have concluded that it has a low interobserver reliability with increased variability based on patient positioning.13,22,29,30
The neurological lesion associated with CP manifests as a nonprogressive or static encephalopathy; however, the associated musculoskeletal pathology is progressive, resulting in contractures of muscle tendon units, bony torsional deformity and ultimately joint instability.3 Children with CP are born with anatomically normal hips without evidence of hip displacement, dislocation or dysplasia;10,11 however, the natural history of nonambulant children with CP is one of progressive lateral displacement of the hip.9,10 Asymmetric muscle spasticity has long been felt to be a major contributor to hip instability in children with CP. Sharrard et al31 demonstrated progressive limitation of abduction, often associated with flexion deformity, was an indicator of early instability of the hip. In their study, they found that no hip with radiologic evidence of subluxation had abduction >45 degrees, suggesting limited abduction and possibly spasticity plays some role in the development of hip instability. However, more recently Wynter et al32 found in their systematic review of hip surveillance that range of hip motion and movement disorder to be poor indicators for hip displacement with hypotonic children experiencing similar rates of hip displacement.
In early childhood, biomechanical forces associated with walking and normal activity across the hip are responsible for remodeling of femoral neck anteversion and neck-shaft angle.21 There is consensus that femoral neck anteversion decreases from a mean of 35 degrees to 40 degrees at birth to approximately 10 degrees to 15 degrees at skeletal maturity.33 Robin et al21 demonstrated that children with CP experience increased femoral anteversion which is proportional to their ambulatory status, where minimally to nonambulant children (GMFCS IV/V) do not experience remodeling of their infant anteversion with recorded values approximately 40 degrees. Perhaps the lack of femoral remodeling is related to delayed or nonexistent weight bearing in the early years of development of nonambulant children with CP.2,13,21
Abnormal proximal femoral anatomy in children with CP revolves around excessive femoral neck anteversion in the transverse plane and increased femoral neck-shaft angle in the coronal plane.21,33 Robin et al21 demonstrated that neck-shaft angle is directly related to a child’s GMFCS level.21 Perhaps increased neck-shaft angle is related to spasticity, tone and dose of neuromuscular disease as demonstrated by GMFCS level.
The definition of a hip “at-risk” of displacement is defined as having an MP of >30%.24 Miller and Bagg,26 found that children (2 to 18 y) whose MP was >60% progressed to dislocation (MP>90%) during childhood, provided there were >2 years of growth remaining. Furthermore, those children (2 to 18 y) with a MP between 30% and 60% had equal rates of hip displacement (25%) and recommended close surveillance and operative intervention to prevent hip dislocation. Settecerri and Karol34 noted that a preoperative MP of >50% was associated with poor results and a higher revision rate. Oh et al35 reported similar results using a MP of >50%, regardless of whether children were treated with femoral and pelvic osteotomy or femoral osteotomy in isolation.
Through a population-based study in Norway, Terjesen et al36 found that hip displacement, defined as an MP>33%, occurred in 26% of a population of children with CP. The authors attempted to quantify the rate of hip displacement according to GMFCS level. Of those with hip displacement, 63% were GMFCS levels IV or V. Dislocation occurred in 14 children at a mean age of 4 years 5 months (range, 1 y 10 mo to 9 y 7 mo). Mean yearly progression of MP increased with decreasing functional level, from 0.2% per year at GMFCS level I to 9.5% at level V.36 In addition, age plays a factor as the average annual increase in MP for children with quadriplegic CP, with 13% increase before 5 years of age and 7% after age 5 years old.8
Significant association between hip displacement and pain has been previously reported in children with CP.37,38 Progressive hip displacement has been shown to affect daily care resulting in seating discomfort, windswept deformity, challenges in perineal care, adduction contracture, and progressive scoliosis.18,30,38–40 Variable rates of pain have been reported in association with hip dislocation in children with CP.39,40
Historically the variable rate of pain presentation associated with hip dislocation has been previously attributed to inconsistent caregivers in institutionalized adult patients and a lack of standard HRQOL instruments. Recently in 2006, the Caregivers Priorities and Child Health Index of Life Disabilities (CPCHILD) questionnaire was developed and validated for severely involved children with CP to evaluate HRQOL.41 A systematic review of quality of life (QOL) outcome measures for children with CP identified the CPCHILD as 1 of 2 outcome measures with the strongest psychometric properties and clinical utility.42 Hip displacement (MP>40%) was significantly associated with lower scores in items in the comfort and emotions; communication and social interaction and health domains in large population-based studies.43 Current evidence and outcomes assessment demonstrates that progressive lateral hip displacement is associated with lower CPCHILD scores and a lower health-related quality of life.18,19,37,38
The surgical goals of treating hip displacement in children with CP are to maintain flexible, well-located and painless hips with a symmetrical range of motion. Previous studies have illustrated the importance of establishing a concentric femoral head within the acetabulum before age 5 for normal hip joint development to occur.44 The surgical treatment of spastic hip disease is guided by the degree of displacement of the femoral head and acetabular dysplasia. Treatment is stratified according to MP, acetabular dysplasia and severity of muscle spasticity into three broad categories: preventive, reconstructive, and salvage surgeries. Accepted indications for preventive surgery include MP>40%, or an increase in MP>10% over the last year and hip abduction <30 degrees.10,23 Reconstructive surgery is indicated when the MP is >50% and there is evidence of hip subluxation or early dislocation, without evidence of degenerative changes to the femoral head. Finally salvage surgery is indicated for those who present with painful and degenerative dislocated hips or where reconstruction has previously failed or now is not an option because of degree of degeneration.
The primary goal of hip surveillance programs is to identify children “at risk” of hip displacement, to monitor their hip development over time, and offer early appropriate intervention.23 Surveillance is achieved by systematic and periodic clinical and radiographic assessment of the hips to detect early displacement, prompt surgeon referral and timely treatment of said displacement.18 Early identification of these “at-risk” children is critical; however, timely referral to and triage by an orthopedic surgeon is just as important. Identification of progressive hip displacement has limited value unless effective intervention is available.45 Early identification and orthopedic intervention has been shown to alter treatment outcomes, reduce the number of reconstructive surgeries required, and avoid the need for salvage surgery.4,23,46 As a result, hip surveillance has become an integral part of evidence-based care for children with CP in many developed countries based on growing evidence supporting surveillance programs and their outcomes.17,22,23,29,30,46,47
The frequency of radiographic surveillance should be directly related to the risk of hip displacement which is in turn related to the child’s GMFCS level. At GMFCS level I, there is little risk of hip displacement and radiographs are only required if there are findings on clinical examination. More regular radiographs are required at GMFCS levels II and III. Radiographs every 6 to 12 months may be required at GMFCS levels IV and V to monitor progressive hip displacement.45 Current recommendations for the first screening radiographs in minimal to nonambulant children should occur before the age of 2,23 and continue every 6 to 12 months until skeletal maturity.32,45,46 Surveillance may need to be continued after skeletal maturity if the hips remain unstable, especially in the presence of symptomatic displacement, pelvic obliquity and progressive scoliosis. Available surveillance guidelines on the internet include the Australian Hip Surveillance Guidelines for Children with Cerebral Palsy (2014),48 The Swedish CPUP Radiographic Follow-up to Prevent Hip Dislocation49 and the British Columbia Hip Surveillance Program.50
Four population-based studies have reported a significant decrease in the incidence of hip dislocation after implementation of formal hip surveillance programs.17,18,30,36,46 A statistically significant reduction in the incidence of dislocation was found in children who were under surveillance compared to children who were not under surveillance.30 There is now evidence of progressive hip displacement after skeletal maturity, particularly in the presence of concurrent risk factors such as progressive scoliosis, pelvic obliquity, leg length discrepancy, or deteriorating gait, and/or if the child functions in GMFCS III-V.32
Progressive lateral hip displacement in children with spastic CP is common and can progress silently due to combination of delayed motor milestones, increased femoral anteversion and coxa valga. Untreated lateral hip displacement leads to pain, positioning challenges and decreases in HRQOL. Clinical examination alone is not a sensitive enough measure to detect early hip displacement. Hip surveillance programs identify critical early indicators of progressive hip displacement, and early detection coupled with appropriate surgical intervention of spastic hip displacement can prevent hip dislocation and the need for more invasive surgery.
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