GROWTH PLATES, APOPHYSIS, AND SECONDARY CENTERS OF OSSIFICATION

To a casual observer, the radiograph of a child’s elbow is an enigma—no two ever seem alike. The reason for this, of course, is that because the child is constantly growing, ossification centers are appearing and fusing, and cartilage is calcifying progressively until skeletal maturity is attained.

It is important to emphasize that there is usually a normal contralateral control that can be radiographed and compared with the radiograph of the injured elbow. This is not recommended as a routine practice, but sometimes it is necessary and useful, especially for those who treat elbow injuries in children only occasionally.

In general, the younger the child at the time of injury, the more difficult it is to assess the elbow, owing to the larger percentage of cartilage that is present about the elbow joint. Fortunately, new imaging modalities allow more accurate assessment.^7,8 Yet, in the newborn or infant, it may be very difficult to diagnose an elbow injury or to determine whether it is a transcondylar fracture or a dislocation of the elbow (the former being much more common at this age). The ossific nuclei about the elbow joint are helpful in radiologic interpretation of elbow dislocation (see Chapter 12). The capitellum, whose center of ossification should be present by 6 months of age, facilitates the interpretation of radial head alignment, because a line drawn through the radial head should always intersect the capitellum no matter what view is taken (Fig. 20-1). This interpretation is improved even further with the appearance of the radial head secondary center of ossification, at around 5 years of age. The secondary center of ossification of the olecranon, which appears at about 9 years of age, allows a more accurate assessment of the position of the proximal ulna in relation to the distal humerus, an important consideration in the management of dislocations of the elbow in young children.

FIGURE 20-1 A, A line drawn through the middle of the neck and head of the radius must always pass through the capitellum in every view. B, If it does not, dislocation of the radial head is present.

Both the medial and the lateral apophyses of the distal humerus may be injured in dislocations of the elbow in a child. Although many mnemonics have been devised by residents trying to remember the timing of ossification of the various centers about the elbow, the most important center to remember from a practical standpoint is the medial epicondylar apophysis of the distal humerus. This center is usually present by the age of 5 to 6 years, and because it is frequently entrapped within the joint following a dislocation of the elbow, it should always be searched for and identified after this age. Hence, if the center cannot be identified, it should be assumed that it is within the joint itself. In children younger than the age of 5 years, the diagnosis of entrapment must be clinical or by arthrography, because the apophysis is entirely cartilaginous.

The lateral epicondylar apophysis is injured less frequently. In posteromedial dislocations, it may suffer avulsion, owing to a severe varus strain on the elbow and may need to be repaired or fixed surgically.⁵

ELBOW FLEXIBILITY

In children younger than 10 years of age, elbow stability is provided almost entirely by cartilage. Because of this, there is considerable flexibility in the elbow joint in children. It is not unusual for a child to be able to hyperextend the elbow joint by 10 or 15 degrees and to have a much greater degree of laxity than is seen in an adult or even an adolescent. It is this combination of hyperflexibility and a lack of osseous stability in a joint subjected to considerable trauma that predisposes the elbow joint to dislocation. The major stabilizing ligaments on the medial and lateral sides are attached to the distal humerus through apophyses—structurally weak areas that are prone to avulsion with subsequent loss of joint integrity. In the many syndromes and conditions such as Ehlers-Danlos and cutis laxa syndromes,^111,119 ligamentous laxity is enhanced and stability is accentuated.

OLECRANON

The ulnohumeral joint provides the greatest articular stability in the elbow joint. In children, the coronoid process, which provides anterior stability resisting posterior dislocation of the ulna, is not well developed until about 12 years of age. Even at this age, the coronoid process is largely cartilaginous and, when stressed, is yielding and resilient. Similarly, the olecranon itself is a largely cartilaginous structure until the early teenage years. Neither the coronoid nor the olecranon fossa of the distal humerus is well developed until later in childhood, and they do not contribute as effectively to the “locking in” phenomenon in flexion and extension that occurs with well-ossified coronoid and olecranon processes.

RADIAL HEAD AND NECK

The radial head and neck in children are cartilaginous but have the same relative diameters as the radial head and neck in adults. Dislocation of the radial head, either as an isolated event or in association with a Monteggia fracture, or with dislocation of the elbow joint itself, is facilitated by the resiliency of the cartilaginous component. Children’s bones have plasticity and can be bent like the proverbial greenstick without fracturing. In the type A Monteggia lesion, for instance, it is conceivable that the ulna bends to the point of fracture, whereas the radius only bends to the point at which the radial head slips under the annular ligament and dislocates anteriorly (Fig. 20-2).

FIGURE 20-2 Annular ligament reconstruction using forearm fascia (Boyd technique).

It is of interest to note that in most cases requiring open reduction of the radial head, the annular ligament is actually intact. A similar situation may be found with the traumatic isolated dislocation of the radial head that occurs in very young children in which the radius bends just enough for the head and neck to slip under the annular ligament (called nursemaid’s elbow). When trauma is less severe, as in a pulled elbow, the head of the radius has simply slipped into the annular ligament, and there is no actual dislocation. A supination maneuver “screws” the radial head out of the annular ligament, usually with no actual damage to the ligament itself.

This combination of generalized laxity, the large cartilaginous component, the lack of osseous stability, and the presence of osseous plasticity as well as numerous secondary centers of ossification and apophyses all contribute to the anachronism of a greater tendency of dislocation of the pediatric elbow joint than seen with other joints.

CONGENITAL DISLOCATION

Congenital dislocation of the radial head is a controversial lesion, because some maintain this lesion does not exist at all and all such appearances are simply traumatic or developmental dislocations. This subject is discussed in more detail in Chapter 13. Here, I reserve the diagnosis of congenital dislocation for that entity in which congenital malformation of the extremity is obvious (Fig. 20-3). When isolated dislocation of the radial head is not accompanied by other congenital lesions, the congenital basis for the lesion cannot be substantiated. The long-standing nature of the dislocation can be inferred from the marked convexity of the radial head associated with elongation of the radial neck (Fig. 20-4).^27,28 Congenital dislocation of the radial head may be associated with radioulnar synostosis, the synostosis almost always occurring between the proximal radius and the ulna.^32–36

FIGURE 20-3 A and B, Congenital dislocation of the radial head associated with other congenital malformations of the forearm. Note the rounded convexity of the radial head.

FIGURE 20-4 Developmental or long-standing dislocation of the radial head. A, Note the rounded appearance of the head, convexity of the articular surface, and narrow neck typical of this deformity. B, Opposite normal elbow. C, Convexity of head develops if the radius is not in contact with the capitellum.

Hypoplasia of the capitellum associated with dislocation of the radial head strongly suggests that the dislocation is congenital. The radiologic appearance of congenital dislocations of the radial head has been emphasized by Miura.³⁷ In congenital dislocations, the posterior border of the ulna is usually concave rather than slightly convex, with the radial head being dome-shaped with no central depression (see Fig. 20-3). Posterior congenital dislocation, which constitutes about 40% of congenital dislocations of the radial head, is associated with an accentuation of the normal convexity of the posterior border of the ulna. In fact, because we have not been able to diagnose this pathology, at best, we consider this a developmental problem.²⁸

DEVELOPMENTAL DISLOCATION

Many instances of developmental or secondary dislocation of the radial head are misinterpreted as being congenital in origin.²⁸ Developmental dislocation is defined as any dislocation of the radial head that results from maldevelopment of the forearm. There are many inherited and acquired disease processes affecting the growth plate of the forearm bones that result in asymmetric growth between the radius and the ulna and subsequent dislocation of the radial head. These include the nail patella syndrome, Silver syndrome, arthrogryposis, Cornelia de Lange syndrome, and cleidocranial dysostosis. Asymmetric growth also occurs in multiple exostoses or diaphyseal aclasis. The ulna is most frequently affected at the distal ulnar growth plate; the radius then overgrows relative to the ulna (Fig. 20-5). Paralysis of the muscles innervated by the C5-6 nerve root, as in a nerve root palsy, also predisposes to a gradual dislocation of the radial head that occurs over a number of years of growth or occasionally in infancy.¹⁷ Cerebral palsy also may produce isolated dislocation of the radial head through marked spasticity of the muscles attached to the radius (Fig. 20-6).²¹ Trauma to the radius or the ulna, resulting in asymmetric growth, may also produce dislocation of the radial head. Fracture of the neck of the radius that has not been corrected adequately may result in the proximal radial epiphysis growing laterally instead of toward the capitellum (Fig. 20-7).^20–26

FIGURE 20-5 A to D, Dislocation of the radial head posterolaterally in a patient with multiple exostoses.

FIGURE 20-6 A and B, Long-standing dislocation of the radial head in a child with cerebral palsy. The elongation of the neck and convexity of the head indicate the presence of a prolonged dislocation. C, Spasticity or contraction of the biceps tendon may contribute to isolated dislocation of the radial head in children.

FIGURE 20-7 A and B, A child sustained a fracture of the ulna and neck of the radius that healed in malunion. Four years later, the radial head is dislocating laterally owing to malposition of the proximal radial epiphyseal plate.

A detailed developmental posterior study at the Mayo Clinic describes several grades, or types, of radial head dislocation with characteristic radiographic appearance (Fig. 20-8). Types II and III are complete dislocations and are more obvious cosmetically but have relatively little functional loss except forearm rotation.²⁸ Type I dislocations commonly are associated with late degenerative arthrosis and consist more of a subluxation than a frank dislocation. However, consistent with the definition of forearm maldevelopment, all types have a previous proximal ulnar bow.

FIGURE 20-8 The Mayo classification of posterior radial head instability. A, Type I is subluxation with characteristic radial head elongation and is associated with a poor functional result. B, Type II is complete dislocation but without subluxation. These patients typically have minimal pain but moderate loss of forearm rotation. Forearm prominence may be noticed C, Dislocation with posterior subluxation, type III, causes a marked cosmetic deformity but little functional impairment. Surgery is performed only for cosmetic reasons.

There are few indications for operative treatment of developmental dislocation of the radial head. For example, a malunion of the radius and the ulna that is obviously directing the head of the radius laterally, posteriorly, or anteriorly should be corrected with an osteotomy to redirect the proximal radius or the deformed ulna¹⁹; otherwise, excision of the radial head can be effective to improve motion, lessen pain, or to improve cosmesis.

In patients with cerebral palsy, if the bicipital tendon appears to be subluxating the radial head anteriorly, lengthening the biceps may prevent future dislocation. Once the dislocation is well established, attempts to relocate the radial head probably should not be made, and the dislocation should be accepted. Future resection of the radial head at skeletal maturity can be performed if the head is cosmetically or functionally a problem. The gradual nature of the dislocation and adjacent changes in the surrounding tissues and bone make this type of relocation of the radial head much more difficult than the acute traumatic injury.²⁶

Relocation of the radial head by shortening the radius and reconstitution of the annular ligament is ineffective.

TRAUMATIC DISLOCATION

Solitary dislocation of the radial head is uncommon but occurs much more frequently in younger children. It is essential to differentiate this entity from a developmental dislocation of the radial head that typically is diagnosed incidentally when assessing a minor elbow injury.

The history is of limited value in these cases because these children are often young, prone to frequent elbow injuries, and unable to make a reliable contribution to the history. The radiograph, however, is usually diagnostic because it shows the rounded concave appearance of the radial head in the congenital or developmental dislocation (see Fig. 20-4).^1–16

CLOSED REDUCTION

If the child is seen within 3 weeks of the injury, a closed reduction may be achieved. Direct pressure over the radial head with gradual flexion of the arm and immobilization in a flexed position of more than 100 degrees is usually successful.

If the radial head has been dislocated for several weeks and if the annular ligament has become entrapped, preventing adequate reduction, an open reduction may have to be performed.

If the radial head can be reduced but is not stable, a Kirschner wire fixation to the ulna may be effective. Driving a pin across the elbow joint through the capitellum and into the radial head is not recommended because these wires often break, making removal difficult. Breakage occurs because it is virtually impossible to immobilize the child’s elbow completely, and minor motion, even when in a cast, may result in a fatigue fracture of the wire. The elbow joint will then have to be opened unnecessarily to remove the fractured pin.

OPEN REDUCTION

Triceps Fascial Reconstruction

The technique of open reduction of an anterior dislocation of the radial head in children described by Lloyd-Roberts and Bucknill²⁰ is one I have used with success. This consists of using the lateral portion of the tendon of the triceps for reconstruction of the annular ligament (Fig. 20-9A).

FIGURE 20-9 A, Repair of radial head dislocation by reconstruction of annular ligament using triceps fascia (Lloyd Roberts-Bucknill technique). B, A Kirschner wire passed through the capitellum into the radial head (left) is not recommended owing to danger of pin fatigue and breakage. The radial head can be safely held in the reduced position by a pin across the radius and ulna (right).

A posterolateral incision is preferred rather than a posterior incision, which may disorient the surgeon to the position of the radial head. The triceps tendon is identified, and a long (10-cm) strip is removed from the lateral margin, ensuring attachment at the distal ulnar insertion. The tendon is increased in length by continuing the dissection through the periosteum to a point opposite the neck of the radius, where it is then passed around the neck and sutured to itself and the ulnar periosteum with enough tension to hold the radial head in place. A Kirschner wire is then passed through the ulna into the radius to ensure solid fixation until the tendon has healed (see Fig. 20-9B).¹⁸

The extremity is kept immobilized in an above-elbow plaster cast for 6 weeks; gradual mobilization is begun at 6 weeks after the Kirschner wire has been removed. If there is any difficulty in reducing the radial head, careful inspection of the joint capsule may reveal some infolding or tissue interposition, which may have to be excised.

Specific care must be exercised when exposing the neck of the radius in a child. Unlike the adult, the radial nerve may be only a fingerbreadth below the head of the radius rather than the classic two fingerbreadths that is often referenced.

UNTREATED ANTERIOR DISLOCATION OF THE RADIAL HEAD

A child with a long-standing anterior dislocation of the radial head actually may have good elbow function. The range of motion is usually functional, although the extremes of flexion and extension are usually limited by 20 or 30 degrees. In our experience, pain may develop at a later date, usually the fourth decade. The cosmetic deformity is also a concern, and the elbow may appear somewhat deformed, especially in a small, thin arm. Excision of the radial head at skeletal maturity is generally successful, and the radius does not migrate proximally.

ETIOLOGY

The most common cause of dislocation of the radial head associated with an ulnar fracture in childhood is a hyperextension injury,^44,62 followed by a hyperpronation injury.⁴⁵ In hyperpronation, Bado³⁹ pointed out that the bicipital tuberosity is posterior, thus predisposing the proximal radius to the greatest force during violent contraction of the bicipital tendon. In young children, the force generated by the biceps is less than that in the adult, and this mechanism probably is significant only in older children.

A direct blow over the posterior proximal ulna will produce a Monteggia lesion with anterior dislocation of the radial head, but this is an uncommon mechanism in children.

In our experience, this lesion is most frequently produced by a hyperextension injury. Further support for this theory is the observation that, in open type C injuries, the proximal ulnar fragment pierces the skin on the volar ulnar aspect of the forearm. This would not be possible if the arm were in full pronation because of imposition of the radius.

Because of the plasticity of the forearm bones, the radial head and neck may slip under the annular ligament and dislocate as the shaft of the radius bends. Indeed, many of the isolated traumatic dislocations of the radial head are undoubtedly variations of the Monteggia^46–48 (Monteggia equivalent), in which the ulna has simply bent but not fractured. The radial shaft is bent to the extent that the head and neck are slipped from within the annular ligament, resulting in an apparent isolated dislocation of the radial head.^30,43

CLASSIFICATIONS

Classifications of the Monteggia lesion are based largely on the injury in adults³⁹ (see Chapter 27). Because of differences in the configuration of the injury in childhood, the following pediatric classification is suggested to include dislocation of the radial head associated with the plasticity of the forearm bones in childhood (Fig. 20-10).

FIGURE 20-10 Classification of the pediatric Monteggia fracture dislocation: types A through E.

CLASSIFICATION OF PEDIATRIC MONTEGGIA LESIONS

Type A: Anterior dislocation of the radial head with anterior bowing of the ulna (Fig. 20-11).

FIGURE 20-11 A and B, Ulnar bend with lateral dislocation of the radial head, a type A Monteggia fracture dislocation.

Type B: Anterior dislocation of the radial head with greenstick fracture of the ulna (Fig. 20-12).

FIGURE 20-12 A and B, Type B Monteggia fracture associated with a greenstick fracture of the ulna and anterior dislocation of the radial head.

Type C: Anterior dislocation of the radial head with transverse fracture of the ulna (Fig. 20-13).