DEVELOPMENTAL DISLOCATION OF THE HIP
Methods for the early detection of developmental dislocation of the hip (DDH) have been reported for nearly 100 years. The first screening program in the United States was described and initiated in the 1930s. After World War II, extensive screening programs in the United States, Sweden, and England resulted in the early identification and, ultimately, the simple, effective, and safe treatment protocols.
In the United States, approximately 10 in 1000 infants are born with DDH. As a result of screening programs, 96% of these children have normal hip function. The longer the dislocation goes untreated, the more difficult it is to obtain a satisfactory result. Routine screening for this entity should be an integral part of newborn well-child care.
The etiology of DDH remains multifactorial. Mechanical and physiologic properties of both the mother and infant have been implicated, combining to produce instability and, ultimately, dislocation.
The time at onset of instability affects the severity of the condition. The typical developmental dislocation develops just before delivery in an otherwise normal infant. At birth, the clinical findings are often subtle and radiographs are usually normal. In contrast, if the dislocation occurs early in gestation, the clinical and radiographic findings are more severe with advanced adaptive changes in the femoral head and pelvis. This is often termed teratologic dislocation and is typically found in patients with underlying conditions such as arthrogryposis, chromosomal abnormalities, and other severe congenital anomalies such as spina bifida and lumbosacral agenesis. Teratologic dislocations occur in less than 2% of patients with DDH.
Mechanical factors most likely occur in the last trimester of pregnancy. All have the effect of restricting intra-uterine space for the fetus, also termed packaging. Roughly 60% of infants with DDH are firstborns, suggesting that the tight unstretched uterus and abdominal wall inhibit fetal movement. It is believed that the fetal pelvis becomes trapped within the maternal pelvis, not allowing the normal flexion of the fetal hip and knee.
Breech presentation also plays a significant role in DDH. Thirty to 50 percent of affected children are delivered in breech presentation. When the knees are extended in the frank breech position, excessive hamstring stretch contributes further to hip laxity and instability. DDH is seen more frequently in children with congenital knee dislocation or recurvatum, as well as other packaging phenomena such as metatarsus adductus and congenital muscular torticollis.
The left hip is more commonly involved than the right, in theory because the left hip is trapped against the maternal sacrum in the most frequent presenting position left occiput anterior. The maternal sacrum forces the left hip into flexion and adduction, contributing to instability and, ultimately, DDH. In this position, the femoral head is covered more by the joint capsule than the acetabulum proper.
Maternal estrogens and other hormones that affect relaxation of the pelvis right before delivery also affect the fetal hip. Their presence may lead to temporary laxity in the hip joint in the fetus and newborn. Studies suggest that the female infant is more affected by these hormones, which may explain the higher incidence of DDH (6 : 1) in females.
Up to 20% of cases are deemed familial. This may be due to an inborn or possibly an inherited error of estrogen metabolism and may explain the higher rates of familial disease in Northern Italians, Scandinavians, and some Navajo tribes.
POSTNATAL ENVIRONMENTAL FACTORS
In the first months of life, normal hip position is that of abduction and flexion. In cultures in which swaddling of infants places the hips in positions of extension and adduction, there is a 10-fold increase in DDH rates. The practice of holding the child by the feet in the newborn period places the hips in extension and should be avoided.
The pathogenesis of a typical developmentally dislocated hip is probably quite simple. Near the time of delivery, the joint capsule is likely distended and elastic. After delivery, the femoral head is free to move about the distended lax capsule and can come in and out of the acetabulum freely. In newborns, the femoral head can easily be returned to the normal position. At this stage, the soft tissue structures and joint surfaces are essentially normal and therefore the capacity to develop a normal hip exists. Therefore, for a stable hip to develop, the femoral head needs to stay in contact with the acetabulum as the joint capsule loses its laxity and returns to a normal configuration. However, if the dislocation is allowed to persist, the capsule, joint, and other soft tissue go through a series of adaptive changes. This makes the dislocation more difficult to reduce, and the chance of obtaining a satisfactory long-term result diminishes significantly.
It has been demonstrated that the stimulus for development of a normal acetabulum is the presence of a normal femoral head within the acetabulum. Conversely, a normal femoral head will develop if it is contained within a normal acetabulum. Because the infant grows so rapidly in the first year of life (tripling in size in the first 12 months), there is tremendous remodeling potential in this period to convert pathologic changes to normal anatomy.
If the dislocation is not treated, the initially simple problem becomes more complex. Persistent dislocation is met with normal muscle forces causing proximal and lateral migration of the femoral head. The iliopsoas, adductors, and hamstrings do not exist at their normal resting length and become contracted. The acetabulum becomes dysplastic (more shallow) without the stimulus of the femoral head. Fibrofatty tissue termed pulvinar develops within the joint. The ligamentum teres becomes thickened, and the transverse acetabular ligament becomes taught and migrates superiorly into the joint. The joint capsule balloons anteriorly and becomes redundant. The tight iliopsoas tendon compresses the joint capsule, trapping the femoral head outside the acetabulum blocking reduction of the joint. This creates an hourglass configuration of the joint space. Undue pressure on the labrum causes it to enlarge and some- times invert into the joint, preventing reduction and thus it is termed an inverted limbus.
The femoral head becomes misshapen and flattened as it articulates with the outer table of the pelvis. The normal femoral rotation is blocked, and the femoral head and neck stay in a position of relative anteversion and valgus.
EXAMINATION OF THE NEONATE AND INFANT
The examination procedures described by Ortolani and Barlow are the most reliable methods of making the diagnosis of DDH in the neonate (see Plate 2-25). The Ortolani test is a test of hip reduction. If the infant’s hip is dislocated, then it can be reduced with this maneuver, which is also termed an Ortolani positive hip. The Barlow test demonstrates the reverse. Gentle flexion and adduction of the infant’s hip produces a painless clunk of dislocation, indicating an unstable joint. Over time, if instability is allowed to persist, these findings go away and the only reliable finding is limited abduction.
Based on the variability of the clinical findings, there exists a spectrum of disease in DDH. The least severe form is simple dysplasia with stable hip joints, followed by a Barlow positive hip (reduced but dislocatable), then an Ortolani positive hip (dislocated but reducible), and finally teratologic or unreducible hips. This spectrum is best illustrated during the neonatal period.
CLINICAL MANIFESTATIONS IN OLDER CHILDREN
The findings of DDH become more obvious as children grow. The surrounding soft tissue and bone gradually adapt to the abnormal position of the femoral head. With time, it becomes increasingly difficult to reduce the femoral head into the acetabulum, and the Ortolani test ultimately becomes negative, with the femoral head remaining trapped outside the acetabulum. All major muscle groups about the hip become contracted. Most apparent is the adductors, which manifests clinically as decreased abduction of the affected hip. The thigh is relatively shortened and the skin and subcutaneous tissue bunch up, producing asymmetry in the thigh skin folds on occasion. With the patient supine and the hips and knees flexed, the knees are not at the same level (positive Allis or Galeazzi sign). The femur can be moved freely up and down, which is described as pistoning or telescoping.
The child walks with a limp, owing to relative limb shortening and pelvic tilt due to abductor weakness. The Trendelenburg test can be used to assess abductor weakness. When both hips are dislocated, the perineal space is widened and the trochanters appear more prominent than normal. There is hyperlordosis of the lumbar spine, and the child walks with a waddling gait.
Selective screening imaging in children with suspected disease is the most reliable and cost-effective measure to diagnose and treat DDH in an efficient manner.
Ultrasound examination can provide an accurate assessment of the femoral and acetabular anatomy and is superior to radiographs in the first 3 months of life. Higher false-positive results are seen in ultrasound examinations earlier than at 4 weeks of life due to immaturity. Ultrasound can also provide a dynamic assessment of hip stability because the Ortolani and Barlow tests can be applied while visualized with ultrasound. The true percentage of acetbular coverage of the femoral head can be quantified.
Plain radiographs (anteroposterior and frog-leg views) become more useful after 3 months of life. Char-acteristic findings of DDH include (1) proximal and lateral migration of the femoral head/neck on the ilium, (2) a shallow, incompletely developed acetabulum, (3) development of a false acetabulum, and (4) delayed ossification of the proximal femoral ossific nucleus. A useful method of assessing infantile hips involves a system of lines on the anteroposterior pelvis radiograph. Accurate positioning of the child for the radiograph is critical. The legs must be extended and the hips in neutral rotation. These are most helpful when unilateral disease is present because there is a normal comparison on the same film.
In the older child, contrast arthrography may be helpful to visualize articular changes. It is seldom indicated as a sole imaging modality but rather used in conjunction with treatments such as closed reduction and assessing the adequacy of reduction.
The goal of treatment remains to return the femoral head to its normal position within the acetabulum and to keep it there to allow further development. In the infant younger than the age of 6 months, closed reduction can usually be accomplished and is almost always accompanied by an adductor tenotomy to maintain reduction in a cast. In older children, gentle closed reduction becomes more difficult, and more invasive measures are needed to achieve the goal of a stable reduction.
TREATMENT OF CLINICALLY REDUCIBLE HIP
Reduction of the hip is typically achieved with the patient under general anesthesia. Gentle flexion and abduction of the hip is applied. The reduced hip must be maintained in a comfortable and normal physiologic position of flexion and abduction. This position is critical. It must avoid excessive stress to the joint yet also keep the femoral head from redislocating. Ninety degrees of flexion and moderate abduction is the ideal position, referred to by Salter as the “human position.” The femoral head and neck should point toward the triradiate cartilage on a plain radiograph. Tension on the hip can be lessened with a percutaneous adductor tenotomy, which is utilized in a majority of such cases. In a large number of children, hip instability noted at birth spontaneously resolves in a few weeks. Simple positioning and close clinical and radiographic follow-up are all that is typically needed in these cases. The newborn with documented more severe instability (Ortolani-positive hips) should be placed in some type of restraint device, typically a Pavlik harness. The reduction should be maintained for several weeks.
Although there are many devices that have historically been employed to maintain reduction of the un-stable newborn hip (e.g., double diapers, Ilfeld splint, Von Rosen splint, Frejka pillow), the current choice remains the Pavlik harness. It provides the proper re-straints with a shoulder harness, foot cuffs, and straps that tether the limbs at customizable degrees of flexion and abduction. Velcro closures make the harness simple to apply and remove for bathing.
The harness is applied loosely with the straps maintaining reduction of the hip in a position of around (no greater than) 100 degrees of flexion and limiting adduction such that the knees cannot touch when brought to midline. This zone can be adjusted based on the stability of the hip. Care must be taken to avoid excessive flexion or abduction, because they can result in femoral nerve neurapraxia and avascular necrosis, respectively. Once the device is properly applied, ultrasonography is used to confirm reduction. Follow-up ultrasound examinations in the harness can also be used for monitoring as well as radiographs after 3 months of age.
The advantages of the Pavlik harness are several. It allows for spontaneous hip motion within the limits of stability. It also prevents extension of the hip and knee, which predisposes hips to instability. The infant can remain in the harness for essentially all care except bathing, and the risk of avascular necrosis remains very low because there is no forced abduction. The duration of treatment is typically related to patient age at onset of treatment.
Treatment of the Clinically Unreducible Hip
The clinically unreducible hip is typically seen in a child older than 6 months of age. The soft tissue contractures and adaptive changes make it such that the hip cannot spontaneously reduce. If more than the simple force needed for an Ortolani examination is required to reduce the hip, then closed reduction under anesthesia is indicated. Preoperative traction has been used historically to stretch the soft tissues before closed reduction. This has largely been replaced with percutaneous adductor tenotomy at the time of closed reduction. This lessens the tension on the reduction, allowing the femoral head to rest within the acetabulum without undue tension or force.
Once a closed reduction under anesthesia is accomplished, the hip is immobilized in a spica cast. Radiographs with or without an arthrogram should demonstrate the femoral head and neck directed toward the triradiate cartilage. Before cast immobilization, the safe zone of Ramsey is determined. This is defined as the range of motion within which the hip remains reduced. The safe zone can, and in most cases should, be increased with a percutaneous adductor tenotomy.
If an adequate closed reduction is unable to be achieved and maintained, then open reduction should be considered.
Medial (Adductor) Approach
This approach is typically used in patients younger than the age of 12 months. Although age is an important consideration for this approach, perhaps more important is patient size. The larger the child, the farther away the femoral head is from the medial-based incision and hence the more difficult it becomes to accomplish reduction through this approach.
The medial approach allows direct access to the contracted portion of the capsule but does not permit visualization or access to the redundant capsule for plication to prevent redislocation. The medial femoral circumflex artery is at risk in this approach and lies in close proximity to the psoas tendon.
Once the capsule is encountered, it is opened and the femoral head is reduced. Anatomic obstacles to reduction such as the pulvinar may need to be excised to permit reduction. An inverted limbus can be mobilized if necessary, and a taught transverse acetabular ligament can be incised if necessary to permit reduction. Once the hip is reduced, capsular closure is not necessary.
After wound closure, the child is immobilized in a bilateral hip spica cast. The cast is carefully molded about the trochanter to maintain reduction and prevent redislocation into the redundant capsule. Excessive abduction force should not be used.
In older and larger children, the femoral head cannot be adequately accessed via a medial approach. The anterolateral approach is utilized for these cases. An oblique incision just distal to and along the course of the iliac crest allows access and provides a better cosmetic result. As with the medial approach, the intra-articular obstacles to reduction are removed and the hip is reduced. The redundant portion of the capsule is excised and plicated to add stability to the reduction. A postoperative case is applied to support the reduction. If the position of the hip alone does not provide enough stability, then the quality of the reduction/capsulorrhaphy should be reassessed.
After open reduction, children younger than the age of 2 can wear a postoperative hip abduction orthosis until acetabular remodeling has occurred. In older children with less remodeling potential, innominate osteotomy can be performed to improve the acetabular anatomy. The Salter innominate osteotomy provides additional anterior and lateral coverage, which is deficient in DDH patients. Derotational osteotomies of the proximal femur can also be performed to better position the femoral head in the acetabulum. Femoral shortening osteotomy is frequently used to decrease tension on the reduction. These bony procedures can be used at the time of open reduction if deemed necessary by the surgeon.
COMPLICATIONS OF TREATMENT
Avascular Necrosis of the Femoral Head Forcing the hip into severe and unusual positions can have severe consequences, the most devastating of which is avascular necrosis of the femoral head. Compromise of the blood supply to the femoral head for even a short time can produce complete death of the femoral head. Redislocation of the hip can be problematic but can ultimately be corrected. Although it can be tempting to keep the hip in certain positions to maintain reduction, extreme positions must be avoided.
Several points regarding the femoral head blood supply and reduction of the femoral head have been described by Ogden. In the newborn, both the lateral and medial circumflex femoral arteries supply the femoral head. Contributions from the lateral circumflex regress by age 5 to 6 months. The medial femoral circumflex artery residing on the posterior femoral neck becomes the predominant blood supply to the femoral head. Interruption of the medial circumflex in the newborn may have little effect on the femoral head, but in the child older than 6 months, it can produce devastating necrosis of the entire femoral head. This has a profound effect on the developing proximal femur.
Earlier rates of avascular necrosis after closed reduction of the hip were unacceptably high. It is postulated that forced wide abduction places pressure on the medial femoral circumflex artery with contact from the limbus and other posterior structures, occluding the primary vessel to the femoral head. The vessel can also be compressed along its tortuous course by the iliopsoas tendon against the inferior pubic ramus as well as the pectineus-adductor group. Surgical release of these muscles lessens the tension on the reduction as well as tension on the critical blood supply to the femoral head, reducing the rate of avascular necrosis.