Applied Radiology

Findings

A semicircular radiopacity lies anterior to the distal humerus on the lateral radiograph (Figure 1). This density is smoothly corticated on about 75% of its margin. An ovoid density projects over the olecranon fossa on the anterioposterior (AP) view (Figure 2).

Computed tomography (CT) reveals a normal radial head (Figure 3). A semicircular bone fragment lies anterior to the distal humerus. A defect in the distal humerus is present at the expected location of the capitulum. The donor site also involves a small part of the trochlea.

Discussion

Capitulum fractures are rare, particularly in children,¹ and involve only the articular surface of the lateral condyle. This injury may lead to significant disability by limiting motion of the elbow if it remains undiagnosed. The mechanism of injury is a direct force, applied through the radial head as when a patient falls on the outstretched hand with the elbow in extension.² As the capitulum is situated on the anterior portion of the lateral condyle, the fragment displaces anteriorly and superiorly³ (Figure 4).

The clinical presentation is nonspecific. The patient most commonly complains of elbow pain, swelling, and restricted range of motion after a fall on the outstretched hands. However, AP radiographs may appear nearly normal.⁴ Diagnosis depends on the lateral radiograph. A semicircular radiopaque density anterior and superior to the distal humerus represents the capitulum lying within the radial fossa. Since the fragment may have only a small piece of radiolucent subchondral bone, the diagnosis may be difficult. CT scanning is helpful to indicate the size of the donor site and its extension into the trochlea.

The primary differential diagnostic consideration is myositis ossificans. Myositis ossificans, heterotopic ossification in soft tissues following trauma and hemorrhage, commonly involves the elbow and the muscles of the thigh. Myositis ossificans may follow any local injury sufficient to cause bruising of the muscle or a frank hemorrhage within the muscle. It progresses over a few weeks from hematoma, to ill-defined calcification, to well-organized cortical and trabecular bone. Characteristically, this lesion develops in a centrifugal pattern, more densely calcified peripherally than centrally. The calcification is initially ill-defined, becomes flocculent, then ossifies, and, ultimately, may decrease in size or completely disappear.

CONCLUSION

Capitulum fractures are easily missed on AP radiographic views. However, lateral radiographs and thin-section CT scans help to prevent a misdiagnosis that could lead to significant functional debility.

1. Letts M, Rumball K, Bauermeister, S, et al. Fractures of the capitellum in adolescents. J Pediatr Orthop. 1997;17:315-320.
2. Resnick D.Bone and Joint Imaging.Philadelphia, PA: W.B. Saunders. 1989:840-843.
3. Resnick D, Niwayama G. Diagnosis of Bone and Joint Disorders, 2nd ed, Vol. 5. Philadelphia, PA: W.B. Saunders. 1988:2860-2861.
4. Greenspan A, Norman A. The radial head, capitellum view: Useful technique in elbow trauma. AJR Am J Roentgenol. 1982;138:1186-1188.