Distal Humerus

Ossification of the distal humerus proceeds at a predictable rate. In general, the rate of ossification in girls exceeds that of boys.^23,26,30 In some areas, such as the olecranon and lateral epicondyle, the difference between boys and girls in ossification age may be as great as 2 years.²⁶ During the first 6 months, the ossification border of the distal humerus is symmetric (Fig. 15-1).

Lateral Condyle

On average, the ossification center of the lateral condyle appears just before 1 year of age but may be delayed as late as 18 to 24 months.¹⁴ When the nucleus of the lateral condyle first appears, the distal humeral metaphyseal border becomes asymmetric. The lateral border slants and becomes straight to conform with the ossification center of the lateral condyle (Fig. 15-2). By the end of the second year, this border becomes well defined, possibly even slightly concave. The capitellar ossification center is usually spherical when it first appears. It becomes more hemispherical as the distal humerus matures,¹³ and the ossific nucleus extends into the lateral ridge of the trochlea (Fig. 15-3). On the lateral view, the physis of the capitellum is wider posteriorly. This is a normal variation and should not be confused with a fracture.¹³

Medial Epicondyle

At about 5 to 6 years of age, a small concavity develops on the medial aspect of the metaphyseal ossification border. In this area, a medial epicondyle begins to ossify (Fig. 15-4).

Trochlea

At about 9 to 10 years of age, the trochlea begins to ossify. Initially, it may be irregular with multiple centers (Fig. 15-5).

Lateral Epicondyle

The lateral epicondyle is last to ossify and is not always visible (Fig. 15-6). At about 10 years of age, it may begin as a small, separate oblong center, rapidly fusing with the lateral condyle.¹³

The Fusion Process in Pediatric Distal Humeral Fractures

Just before completion of growth, the capitellum, lateral epicondyle, and trochlea fuse to form one epiphyseal center. Metaphyseal bone separates the extra-articular medial epicondyle from this common humeral epiphyseal center (Fig. 15-7). The common epiphyseal center ultimately fuses with the distal humeral metaphysis. The medial epicondyle may not fuse with the metaphysis until the late teens.

Proximal Radius

The head of the radius begins to ossify at about the same time as the medial epicondyle (Fig. 15-4). The ossification center is present in at least 50% of girls by 3.8 years of age but may not be present in the same proportion of boys until around 4.5 years.²³ Initially, the ossification center is elliptical, and the physis is widened laterally because of the obliquity of the proximal metaphysis. The ossification center flattens as it matures. At about age 12, it develops a concavity opposite the capitellum.¹³

Ossification of the radial head may be bipartite or may produce an irregularity of the second center. These secondary or irregular ossification centers should not be interpreted as fracture fragments.

Olecranon

There is a gradual proximal progression of the proximal ulnar metaphysis. At birth, the ossification margin lies halfway between the coronoid process and the tip of the olecranon. By about 6 or 7 years of age, it appears to envelop about 66% to 75% of the capitellar surface. The final portion of the olecranon ossifies from a secondary ossification center that appears around 6.8 years of age in girls and 8.8 years in boys (Fig. 15-8A). Peterson and Peterson⁴⁹ described two separate centers: one articular and the other a traction type (Fig. 15-8B). This secondary ossification center of the olecranon may persist late into adult life.⁴⁷

Fusion of the Ossification Centers

The epiphyseal ossification centers of the distal humerus fuse as one unit and then fuse later to the metaphysis. The medial epicondyle is the last to fuse to the metaphysis. The ranges of onset of the ossification of various centers and their fusion to other centers or the metaphysis are summarized in Figure 15-9. Each center contributes to the overall architecture of the distal humerus (Fig. 15-9C).

Fusion of the proximal radial and olecranon epiphyseal centers with their respective metaphyses occurs at around the same time that the common distal humeral epiphysis fuses with its metaphysis (i.e., between 14 and 16 years of age).^9,12,53

Noting that the pattern and ossification sequence of the six secondary ossification centers around the elbow were mainly derived from studies conducted more than 30 years ago, Cheng et al.¹⁷ evaluated elbow radiographs of 1,577 Chinese children. They found that the sequence of ossification was the same in boys and girls—capitellum, radial head, medial epicondyle, olecranon, trochlea, and lateral epicondyle—but ossification was delayed by about 2 years in boys in all ossification centers except the capitellum (Table 15-1).

Blood Supply to Pediatric Distal Humerus

Extraosseous

There is a rich arterial network around the elbow (Fig. 15-10).⁶⁵ The major arterial trunk, the brachial artery, lies anteriorly in the antecubital fossa. Most of the intraosseous blood supply of the distal humerus comes from the anastomotic vessels that course posteriorly.

Three structural components govern the location of the entrance of the vessels into the developing epiphysis. First, there is no communication between the intraosseous metaphyseal vasculature and the ossification centers. Second, vessels do not penetrate the articular surfaces. The lateral condyle is nonarticular only at the origin of the muscles and collateral ligaments. Third, the vessels do not penetrate the articular capsule except at the interface with the surface of the bone. Thus, only a small portion of the lateral pondyle posteriorly is both nonarticular and extracapsular (Fig. 15-11).³¹

Intraosseous

The most extensive study of the intraosseous blood supply of the developing distal humerus was conducted by Haraldsson^30,31 (Fig. 15-12A), who demonstrated that there are two types of vessels in the developing lateral condyle. These vessels enter the posterior portion of the condyle just lateral to the origin of the capsule and proximal to the articular cartilage near the origin of the anconeus muscle. They penetrate the nonossified cartilage and traverse it to the developing ossific nucleus. In a young child, this is a relatively long course (Fig. 15-12A). These vessels communicate with one another within the ossific nucleus but do not communicate with vessels in either the metaphysis or nonossified chondroepiphysis. Thus, for practical purposes, they are end vessels.

The ossification center of the lateral condyle extends into the lateral portion of the trochlea. Thus, the lateral crista or ridge of the trochlea derives its blood supply from these condylar vessels. The medial ridge or crista remains unossified for a longer period of time. The trochlea is covered entirely by articular cartilage and lies totally within the confines of the articular capsule. The vessels that supply the nucleus of the ossific centers of the trochlea must therefore traverse the periphery of the physis to enter the epiphysis.

Haraldsson’s³¹ studies have shown two sources of blood supply to the ossific nucleus of the medial portion of the trochlea (Fig. 15-12B). The lateral vessel, on the posterior surface of the distal humeral metaphysis, penetrates the periphery of the physis and terminates in the trochlear nucleus. Because this vessel supplying the trochlea is an end vessel, it is especially vulnerable to injury by a fracture that courses through either the physis or the very distal portion of the humeral metaphysis. Injury to this vessel can markedly decrease the nourishment to the developing lateral ossific nucleus of the trochlea. The medial vessel penetrates the nonarticulating portion of the medial crista of the trochlea. This multiple vascular source may account for the development of multiple ossification centers in the maturing trochlea, giving it a fragmented appearance (Fig. 15-5). When growth is complete, metaphyseal and epiphyseal vessels anastomose freely. The blood supply from the central nutrient vessel of the shaft reaches the epicondylar regions in the skeletally mature distal humerus.³⁹

Intra-Articular Structures of the Pediatric Distal Humerus

The articular surface lies within the confines of the capsule, but nonarticulating areas involving the coronoid and radial fossae anteriorly and the olecranon fossa posteriorly are also within the confines of the articular cavity.⁶⁴ The capsule attaches just distal to the coronoid and olecranon processes. Thus, these processes are intra-articular.³⁶ The entire radial head is intra-articular, with a recess or diverticulum of the elbow’s articular cavity extending distally under the margin of the orbicular ligament. The medial and lateral epicondyles are extra-articular.

The anterior capsule is thickened anteriorly. These longitudinally directed fibers are very strong and become taut with the elbow in extension. In hyperextension, the tight anterior bands of the capsule force the ulna firmly into contact with the humerus. Thus, the fulcrum of rotation becomes transmitted proximally into the tip of the olecranon in the supracondylar area. This is an important factor in the etiology of supracondylar fractures.

Fat Pads

At the proximal portion of the capsule, between it and the synovial layer, are two large fat pads (Fig. 15-13). The posterior fat pad lies totally within the depths of the olecranon fossa when the elbow is flexed. The anterior fat pad extends anteriorly out of the margins of the coronoid fossa. The significance of these fat pads in the interpretation of radiographs of the elbow is discussed later.

Ligaments

The pertinent ligamentous anatomy involving the orbicular and collateral ligaments is discussed in the sections on the specific injuries involving the radial neck, medial epicondyle, and elbow dislocations.

RADIOGRAPHIC FINDINGS IN PEDIATRIC DISTAL HUMERAL FRACTURES

Because of the ever-changing ossification pattern, identification and delineation of fractures about the elbow in the immature skeleton may be subject to misinterpretation. The variables of ossification of the epiphyses should be well known to the orthopedic surgeon who treats these injuries.

Several studies have suggested that children with a normal range of elbow motion after elbow trauma do not require immediate radiographic evaluation. In a large multicenter prospective study, Appelboam et al.³ found that of 780 children evaluated for elbow trauma, 289 were able to fully extend their elbow; among these, 12 (4%) fractures were identified, all at their first evaluation. Among the 491 children who could not fully extend their injured elbow, 210 (43%) had confirmed fractures. These authors suggested that an elbow extension test can be used to rule out the need for radiographs, provided the physician is confident that an olecranon fracture is not present and that the patient can return for reevaluation if symptoms have not resolved in 7 to 10 days. Lennon et al.,⁴² in a study involving 407 patients ranging in age from 2 to 96 years, proposed that patients aged no more than 16 years with a range of motion equal to the unaffected side do not require radiographic evaluation. Darracq et al.¹⁹ found that limitation of active range of motion was 100% sensitive for fracture or effusion, whereas preservation of active range of motion was 97% specific for the absence of fracture. Other studies^20,40 have confirmed a high sensitivity (91% to 97%) of an inability to extend the elbow as a predictor of elbow fracture in both children and adults. More recently, however, Baker and Borland warned that in children with blunt trauma a normal range of motion does not rule out significant injury and should not be used as a screening tool. In their 177 patients, an abnormal range of motion had a negative predictive value of only 77%.⁵

Waters et al. described a subset of serious injuries to the pediatric elbow that they termed TRASH (The Radiographic Appearance Seemed Harmless) lesions (Table 15-2). These lesions represent predominantly osteochondral injuries in children younger than 10 years of age who have sustained high-energy trauma; the lesions are often associated with unrecognized, spontaneously reduced elbow dislocations (Fig. 15-14). Any elbow dislocation in a child younger than 10 years of age should raise concern about a displaced, intra-articular osteochondral fracture, especially with a high-energy mechanism of injury and more swelling than the seemingly benign radiograph demonstrates. A high index of suspicion and early additional imaging (ultrasound, arthrogram, or magnetic resonance imaging [MRI]) usually contribute to a more accurate diagnosis of these injuries.⁶²

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The Immature Skeleton Management of the Multiply Injured Child The Orthopedic Recognition of Child Maltreatment Proximal Tibial Physeal Fractures Clavicle and Scapula Fractures: Acromioclavicular and Sternoclavicular Injuries Humeral Shaft and Proximal Humerus, Shoulder Dislocation

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