Congenital Dislocation of the Patella

Abstract

Congenital dislocation of the patella may occur as a persistent lateral dislocation of the patella that presents with a knee flexion contracture and the patella tethered lateral to the femoral condyles or as an intermittent dislocation of the patella. In the latter syndrome, the patella dislocates completely with each flexion and extension cycle of the knee and is best termed obligatory dislocation of the patella, because the patient has no control over the patella dislocating as he or she moves the knee. The first type of congenital dislocation, which is fixed, often is associated with syndromes such as arthrogryposis and should be corrected surgically by lateral release and realignment of the patella. Obligatory dislocation of the patella tends to be an isolated dysplastic anomaly and may be relatively well tolerated. Rebalancing of the patella usually is done at a later age because of less interference with function. The current author describes the natural history of patella femoral dysplasia, detailing the pathologic changes that are present, and recommends surgical techniques for correcting both types of congenital dislocation of the patella.

Congenital dislocation of the patella is a rare deformity that presents as dysfunction and disability during childhood. Two clinical syndromes have been described in the literature. One syndrome is referred to as congenital dislocation of the patella ^{5,6,13,15–18,25,30,36,40} or fixed lateral dislocation of the patella, and the second syndrome is described as habitual dislocation of the patella. ^{4,10,32,34,39}

The term obligatory dislocation of the patella better describes the second syndrome and will be used in the current study. These two clinical variations of congenital dislocation of the patella are the basis for the discussion of the pathologic features and treatment of patellofemoral joint dysplasia in children that follows.

Clinical Presentation

Persistent lateral dislocation of the patella and obligatory dislocation of the patella have two different clinical presentations as shown in Table 1.

TABLE 1: Congenital Dislocation of the Patella

The infant with persistent lateral dislocation of the patella presents with a knee flexion contracture and no patella palpable anterior to the femoral condyles. Palpation of the hypoplastic patella in its lateral position is difficult. A magnetic resonance imaging (MRI) examination is necessary to be certain of the location of the cartilaginous patella.

The child with fixed lateral dislocation of the patella may present with a delay in the ability to walk independently or with an abnormal gait related to a knee flexion contracture. Frequently, lateral tibial rotation is associated with the knee flexion contracture. ^33,37

Affected patients may have an associated musculoskeletal syndrome, of which the congenital dislocation of the patella is a feature. Congenital dislocation of the patella may occur bilaterally but more commonly is unilateral. Congenital dislocation of the patella has been recorded as part of the following syndromes: arthrogryposis, Larsen’s syndrome, ¹⁹ dyschondrosteosis ²⁴ Rubinstein-Taybi syndrome, ^26,29 Down Syndrome, ^7,20,27 and nail patella syndrome. ²³ The author also has seen congenital dislocation of the patella in Ellis-Van Crevald syndrome ⁸ and in chondroosteodystrophy.

The patella is not palpable in the anterior knee. By careful palpation, one may feel the patella lateral to the lateral femoral condyle. The patella is hypoplastic and laterally placed, and defies detection by clinical examination alone in most infants.

Obligatory dislocation of the patella usually presents after a child begins to walk and is a striking abnormality in that the patella dislocates during each flexion and extension cycle of the knee. Classically, the patella dislocates laterally in full extension and relocates in flexion, but the opposite type of cycle can occur. In these instances, the patella is dislocated in flexion and relocates in extension. Patients with obligatory dislocation usually do not have a flexion contracture, but they have a feeling of instability in the knee that may be surprisingly well tolerated. Pain is not the presenting complaint in childhood but rather dysfunction and deformity.

Classification

The term congenital dislocation of the patella is appropriate for persistent or fixed lateral dislocation of the patella, which can be detected at or near the time of birth. In this syndrome, the patella is never in the trochlear groove and rests laterally at all times because of contracture of the lateral capsule and thick adhesion to the iliotibial band.

In patients with obligatory dislocation of the patella, the abnormal movements of the patella are neither a matter of the patient’s will nor the development of a voluntary pattern or habit. Rather, the abnormal dislocation and reduction of the patella with each movement cycle is imposed on the patient by the pathologic anatomy. For this reason, the term obligatory dislocation is preferred because it more accurately describes the fact that patients are obliged to have the patella move in and out of its normal position in the trochlear groove whenever they flex or extend their knee.

A third type of patellar dysplasia is much more common and seen in the adolescent. This type of dysplasia usually is referred to as malalignment. ²⁸ The malalignment is a result of abnormal growth and development, which causes the quadriceps mechanism to be deviated laterally. This type of dysplasia of the patellofemoral joint is common, but will not be discussed in the current study. Of interest, the entire spectrum from fixed lateral dislocation, to obligatory dislocation, to intermittent or recurrent dislocation occurs in patients with Down syndrome when they are examined as a group.

Natural History

Dysplasia of the patellofemoral joint is a common malformation that may cause patellar instability and may be a factor in anterior knee pain. The natural history of this abnormal growth and development is not documented. Patients with clinical symptoms usually present in their teenage years, and the point at which the abnormal development occurs seldom is clear.

Patients with congenital dislocation of the patella are limited in their walking by a flexion contracture and genu valgum of the knee. Cases of degenerative arthrosis in adult patients with persistent fixed displacement of the patella have been documented in the literature. ^22,35,38,39 Degenerative arthrosis of the patellofemoral joint and the lateral compartment of the tibiofemoral joint develops in these patients. The natural history is that the flexion contracture persists and valgus deformity of the knee also develops, which causes unequal and inefficient loading of the knee, leading to degenerative arthrosis. In some instances, the dislocation is tolerable if there is no flexion contracture and the main problem is weakness of the extensor. This tolerance of persistent lateral dislocation is especially true in Down syndrome, where the patient may be unaware of these conditions.

In patients with obligatory dislocation of the patella, the problem is relatively well tolerated with some degree of instability, but without the contractures and angular deformities seen in complete lateral dislocation. The trochlear groove in the distal femur is hypoplastic and the instability of the patellofemoral joint becomes more of a functional problem as the patient grows. With time, a degree of degenerative arthrosis develops, much as is seen in patients with recurrent dislocation of other joints.

Pathologic Anatomy

All of the structures derived from embryonic mesoderm are abnormal in dysplasia of the patellofemoral joint including bone, muscle, and ligament. The pathologic change in each of these tissue elements must be appreciated to restore normal alignment and function of the extensor mechanism. The term dysplasia may be applied to an individual cell or to an organ. Dysplasia of the patellofemoral joint refers to abnormal growth and development of the anterior portion of the knee and related structures. ¹¹

Characteristically, abnormal deviation of the patella and the associated quadriceps mechanism is lateral. The documented cases of medial dislocation have followed overenthusiastic surgical manipulation of the quadriceps mechanism. The most consistent pathologic finding in patellofemoral joint dysplasia is the lateral soft tissue contracture, which causes deviation and malalignment of the mechanism of the quadriceps muscle, the patella, and the quadriceps tendon. This contracture is most severe in persistent lateral dislocation of the patella where there is a thick fibrous band that tethers the patella to the lateral intermuscular septum.

In obligatory dislocation of the patella, lateral fibrous tethering of the capsule and retinaculum occurs, but to a lesser degree.

In the common cases of recurrent patellar subluxation or dislocation, the contracture limits the mobility of the patella medially away from the margin of the lateral femoral condyle, but the patella remains more or less within the trochlear groove, although it may be tilted or shifted laterally from true central alignment.

The degree of abnormality of the medial capsule and retinaculum correlates directly with that of the lateral. In congenital dislocation of the patella, the medial retinaculum is thin and drawn across the anterior portion of the joint as a hood over the distal femur. In most of these cases, the medial capsule and retinaculum is adherent to the articular surface of the distal femur and must be dissected free to recenter the patella. In less severe degrees of dysplasia, the medial muscles are underdeveloped or weak, requiring some type of imbrication or augmentation to support centering of the patella in the trochlear groove.

In cases of severe dysplasia such as seen in congenital dislocation of the patella, the pathologic features of the capsule are similar to those of arthrogryposis in that the joint capsule is contracted and constricted, so that the volume of the joint is decreased.

The quadriceps muscle usually is shortened. In severe degrees of dysplasia, the quadriceps mechanism is of insufficient length to allow the patella to be centered in its normal position. With severe lateral displacement, as seen in congenital dislocation, the quadriceps muscle acts as a flexor and external rotator of the tibia, which contributes to the flexion and valgus deformity seen in older individuals when the malalignment is not corrected.

In terms of osseous anomalies, the patella is small and hypoplastic. The distal end of the femur may have a complete absence of the trochlear groove in cases of congenital dislocation. Exposing the distal femur, it looks smooth and round. In lesser degrees of dysplasia, lack of development of the trochlear groove is characterized by a low lateral femoral condyle. The lateral femoral condyle usually is the highest part of the distal femur viewed in cross section and resists lateral displacement of the patella. When the lateral femoral condyle is hypoplastic, there is less bony resistance to lateral displacement of the patella. However, development of the trochlea seems to be in response to a normal functioning quadriceps mechanism. When the patella is realigned in the young child, development of the trochlear groove occurs during additional growth and development. This reciprocal relationship between the patella and the trochlear groove seems to be analogous to that seen between the femoral head and the acetabulum in that the hip develops normally when the femoral head is centered within the acetabulum.

The upper tibia is abnormal in dysplasia of the patellofemoral joint in that the tibial tubercle tends to be displaced laterally. This displacement may be associated with external tibial torsion, which can be a clinical clue to diagnosis. The lateral displacement of the tibial tubercle may be an isolated feature with normal tibial torsion of the leg.

Anatomists consider the patellar ligament to be an extension of the quadriceps tendon that is continued distal to the patella to insert into the tibial tubercle. ^14,21 The lateral placement of the tibial tubercle is associated with an abnormal lateral insertion of the patellar ligament. This malalignment generates a lateral vector that must be considered and often surgically corrected in cases of patellofemoral dysplasia.

The quadriceps muscle is weak and often is shortened. The hamstring muscles may be shortened and the posterior capsule knee may be contracted. A flexion force also comes from the displaced lateral quadriceps mechanism that acts posterior to the center of rotation of the joint. Recognizing that this pathologic feature contributes to the knee flexion contracture is an important part of correcting the deformity in congenital dislocation of the patella and allowing full extension and normal gait.

In cases of bone dysplasia such as spondyloepiphyseal dysplasia or Ellis-van Creveld syndrome, the degree of genu valgum that develops because of abnormal bone growth may create such a strong lateral vector that dislocation of the patella occurs either full-time or intermittently (Fig 1). The major component of correction in these cases is osteotomy, which realigns the vectors of action of the quadriceps muscle and is the most important element in establishing normal patellofemoral joint function.

Fig 1.: Radiograph of an 8-year-old child with untreated congenital dislocation of the patella. The child has severe genu valgum secondary to lateral contracture. The growth arrest lines in the femur and tibia are incidental findings unrelated to the patella dislocation.

Diagnostic Imaging

The patella ossifies when individuals are between 3 or 4 years of age and therefore cannot be seen on plain radiographs in young children. Magnetic resonance imaging can show the cartilaginous patella lying lateral to the femur in suspected cases of congenital lateral dislocation of the patella and is valuable in imaging the pathologic features in young children.

Several authors ^2,31 have used ultrasound to define the position of the cartilaginous patella. In older individuals, computed tomography (CT) is useful to image the alignment of the patellofemoral joint. ⁹ Computed tomography scans can document the alignment in the last 15° degrees extension, which is the most critical. The most impressive imaging is with the use of a video loop sequence of CT to show the pattern of patella movement, but is an academic application for analyzing congenital dislocation of the patella.

Surgical Treatment

The major indication for surgical correction of congenital dislocation of the patella is absence or loss of function. The child with persistent dislocation of the patella has difficulty beginning to walk. Because of the flexion contracture and subsequent valgus deformity, gait is inefficient and clumsy even when the child can walk. The child with obligatory dislocation complains of the knee giving way or feeling weak and unreliable. Pain is usually a later symptom and may herald degenerative arthrosis. Premature degenerative changes in the patellofemoral joint occur consistently in adults with untreated persistent lateral dislocation. ³

Wearing a brace is a temporizing measure for minimally symptomatic obligatory dislocation of the patella but wearing a brace is ineffective for definitive treatment.

Operative Procedure

A midline surgical incision over the patella tends to heal with more proliferative scarring in children than in older adults. The author advocates making the surgical incision over the anterior lateral knee so that the scar is not under direct pressure against the patella in the postoperative period. The incision should be long enough to expose a sufficient portion of the quadriceps muscle so that it can be realigned. ^8,12 For good exposure in infants with congenital dislocation of the patella, the incision may extend halfway up the thigh. The lateral tether is released by dividing the fibrous adhesions that may be extensive along the lateral intermuscular septum. The medial capsule and retinaculum are stretched and elongated. Once the lateral retinaculum has been released, the medical retinaculum is mobilized from the anterior surface of the femur by careful sharp dissection, separating it from the articular cartilage. The medial retinaculum is adherent to the anticular cartilage because it lies across the medial condyle similar to a hood.

The patellar ligament often inserts abnormally to the lateral side. In infants, the insertion can be shifted medially by sharply dissecting the insertion off the cartilaginous tibial tubercle and resuturing it in the midline of the upper tibia. The quadriceps musculotendinous unit is shortened. The tendon is lengthened by a Z-plasty or Z-Y plasty. The lengthening is done proximal to the patella through the tendinous fibers of the muscle as they insert into the patella.

The soft tissue attachments lateral and medial to the patella require balancing. The capsule is left open laterally and is plicated medially. The medial flap is elevated and sutured anterior to the patella. The suture line usually is along the lateral edge of the patella, as the flap extends all the way across the patella centering the bone in the midline beneath the medial retinacular flap.

In older patients, augmentation of the medial repair vector is needed in addition to plication. Transferring the semitendinosus tendon into the patella by the method of Galeazzi strengthens the medial force vector and moves the distal attachment rector to the midline. The Krogius type plication is insufficient to maintain the reduction in these patients. ³ The semitendinosus tendon is detached through a small incision posterior at its myotendinous junction and left attached distally to the pes anserinus. The proximal portion of the tendon then is rerouted across the patella through a bony channel to provide a distal and medial vector that is effective in medial augmentation of the vectors acting on the patella. ¹

If the flexion contracture is not corrected completely by these maneuvers, a posterior release is indicated. Posterior exposure usually is necessary in congenital dislocation, but not in obligatory dislocation. The hamstrings are Z-lengthened, and the posterior capsule is divided. To lengthen the hamstring tendons, two vertical incisions may be used, one across the posteromedial comer of the knee and the other across the posterolateral comer of the knee. Working through the posterior lateral incision and protecting the peroneal nerve, the capsular release is completed after mobilizing the lateral head of the gastrocnemius from the femur. The popliteal nerve and artery are protected by dissecting in a plane directly on the posterior capsule.

Patient Examples

Case 1

A 2-month-old infant was referred for evaluation of severe external tibial torsion and knee flexion contractures recognized at birth. Her general appearance and examination otherwise appeared normal with no features indicative of any syndrome. Initially her contractures were thought to be similar to a localized arthrogryposis. Stretching was recommended, as was reevaluation in several months.

The patient did not return until 20 months of age at which time she walked in a kneeling fashion because she was unable to walk on her feet. The patient still had striking external tibial torsion and a knee flexion contracture approximately 25°. The patella was absent on careful palpation of the anterior knee. The patella was dislocated laterally and fixed in a position lateral to the lateral femoral condyle in each knee. Surgical correction was recommended.

The left knee was operated on first followed by the right knee 3 weeks later. Surgical treatment consisted of lateral release and realignment of the quadriceps mechanism associated with Z-lengthening of the quadriceps tendon and medial plication of the quadriceps mechanism. Hamstring lengthening also was done, and full knee extension was obtained.

Postoperatively, the patient’s extremity was immobilized in full extension in a cast for 3 weeks. She was able to walk independently by 3 months after both patellas were realigned. At 5 years of age, a rotational osteotomy of the tibia on the left was done because of persistent external tibial torsion that was functionally limiting in terms of an efficient gait.

At 10 years of age, the patient has knee motion from 0° to 120° bilaterally. Both patellas are located in the trochlear groove. The patient still has some decreased bulk in her quadriceps although she has strong active knee extension and a rhythmic gait. Radiographically, the trochlear groove has developed to a normal appearance despite its absence at the time of the initial surgical realignment.

Case 2

A 6-year-old girl presented with obligatory dislocation of the patella such that her patella dislocated completely from the trochlear groove in full extension but tended to relocate at approximately 30° flexion. This abnormal movement occurred with each flexion and extension cycle of the patella. Other than the obvious abnormal appearance of the patellas, the patient had few symptoms, and the parents were not interested in surgical treatment.

During the next 4 years, the patient became somewhat more symptomatic and wore a neoprene knee sleeve with a patellar cutout for athletic activities. Because of increased insecurity in the knees and mild pain, surgery was done when the patient was 13 years. A lateral release was done accompanied by a Galeazzi type of semitendinosus transfer to the patella.

After 5 years of followup, the patient has normal patellar tracking and only occasional patellar subluxation in the left knee. Her quadriceps bulk is less than would be expected for a girl of her size, but she is able to participate in normal activities.

Outcome

A good outcome in the treatment of severe patellofemoral joint dysplasia of childhood is to have a patella that remains located in the trochlear groove and a range of motion of at least 0° to 90°. Some weakness or atrophy can be expected long-term despite realignment of the patellar mechanism. The trochlea groove develops if the patellar mechanism is recentered, and this response is seen more strongly in the younger patient. In cases of associated syndromes, the result may not be as good because functional limitation often is imposed by other features of the syndrome, for example bone dysplasia.