Distal Humerus Fractures: Isolated Fracture of the Capitellum

Introduction

The capitellar fracture of the humerus is a relatively uncommon injury but can result in significant functional impairment if not appropriately treated. Since its first description in 1841 by Cooper, the fracture has been recognized as an articular fracture of the lateral part of the humerus without metaphyseal extension to the column above the olecranon fossa. Our current understanding of this fracture has recognized a more complex injury pattern commonly involving the trochlea. McKee et al. used the term coronal shear fractures to describe the fracture with trochlear involvement (see Chapter 47 ).

Anatomy

The capitellum is directed 30 degrees anteriorly and distally with respect to the long axis of the humerus, and its center of rotation is located 12 to 15 mm anteriorly to the humeral shaft axis ( Fig. 46.1A ) (see Chapter 2 ). The hemispherical anterior portion is covered by hyaline cartilage of about 2-mm thickness. Medially, the capitellum is separated from the trochlea by the trochleocapitellar groove ( Fig. 46.1B ). The distal articulation with the radial head forms the radiocapitellar joint. The recent analysis of 50 cadavers by Sabo et al. reveals that the capitellum is closer to an ellipsoidal shape rather than a spherical shape, with a greater radius of curvature in a medial–lateral direction.

The capitellum and the lateral aspect of the trochlea receive extraosseous blood supply through posterior perforating vessels arising from the lateral arcades which are formed by the radial recurrent, radial collateral, and interosseous recurrent arteries ( Fig. 46.1C ). Intraosseous blood supply is through posterior condylar perforating vessels.

Epidemiology

The capitellar fracture is truly rare, accounting for only 1% of all elbow fractures and 3% to 6% of all distal humerus fractures. It is also reported to have female predominance, possibly due to an increased carrying angle, a greater prevalence of cubitus recurvatum, or osteoporosis. A concomitant injury of the radial head occurs in 25% of all capitellar fractures, and an associated collateral ligament injury has been reported. The incidence in patients under the age of 12 is extremely rare as the capitellum is not fully ossified, which results in a lateral condylar fracture or supracondylar fracture in the younger age group. A review of the Mayo Clinic trauma database for patients aged 16 and older reveals the incidence of isolated capitellar fracture to be 1.6% of all elbow fractures and comprises 40% of all distal humerus coronal shear fractures.

Injury Mechanism

The capitellar fracture occurs as a result of vertical shear stress transmitted from the radial head. Clinical reports indicate that the fracture commonly occurs after a fall from a standing height on an extended or semiflexed arm similar to that of a radial head fracture. The fracture can also occur as a result of a high-energy injury, such as a fall from a greater height or a motor vehicle accident in young patients, and associated injuries are more common in these cases.

Capitellar and radial head fractures have been produced experimentally from an indirect impact along the radius in full extension and another fracture from direct impact to the end of the distal humerus with the elbow in 90 degrees of flexion.

Function of the Capitellum

The radiocapitellar joint provides valgus and external rotatory stability primarily when the medial collateral ligament is dysfunctional. Experimentally, minimal valgus instability occurs with about 10 degrees of flexion when the capitellum is excised but the collateral is intact. However, increased instability occurs with involvement of lateral trochlea, and a slight 3-degree increase in valgus laxity occurs with forearm pronation. It has also been suggested that absence of the lateral joint may increase external rotation, but we have found this to be a minimal effect if the ligaments are intact.

Finally, increased loads at the ulnohumeral joint have been demonstrated when the load-sharing function of the radiocapitellar joint is absent.

Classification

Historically, the original descriptions identified two distinct fractures and provided a basis for a classification. The type I fracture, also known as a Hahn–Steinthal fracture , is defined as a single large fragment comprising the entire anterior capitellum and the underlying cancellous bone. The type II appears as a thin subchondral shell of the anterior cartilage and has been referred to as a Kocher–Lorenz or Mouchet fracture . The classification described by Bryan and Morrey is probably most widely used. A type III fracture involving comminution of the articular surface and subchondral bone is often found in association with a radial head fracture ( Fig. 46.2 ). A type IV fracture, later suggested by McKee et al., involves part or all of the trochlea and is discussed in Chapter 47 .

Watts et al. reviewed 79 shearing fractures and observed the type I fracture was the most common (47%) followed by type IV (41%). Type II and III fractures occurred in 8% and 4%, respectively.

Diagnosis

This can be very difficult for all but the type I injury. A clear and detailed history is suggestive and also helps to identify associated injuries. Physical exam often shows crepitus on flexion or extension and also with pronation/supination. Mechanical block can be presented on flexion and extension but not with supination and pronation. Localized pain and swelling on the lateral side of the elbow is a common presentation, but generalized pain and swelling of the entire elbow can also variably occur. The senior author has found painful pronation and supination as the most sensitive physical finding in patients with the type II and III fractures.

The type I fracture may appear as a semilunar fragment. The fragment displaced proximally into the radial fossa suggests the capitellum since the radial head fragment rarely displaces into this quadrant ( Fig. 46.3 ). When the fractured fragment is small or thin—type II—the anteroposterior (AP) plane film often appears near normal ( Fig. 46.4 ). The lateral radiograph may also appear normal if there is minimal bone involved but typically shows a positive fat pad sign. Type III fractures are commonly misinterpreted as type I in simple radiograph. A double-arc sign can be present when the lateral portion of the trochlea is involved and represents the subchondral bone of the capitellum (larger arc) and the ridge of the trochlea (smaller arc) ( Fig. 46.5 ).

The radial–capitellar view is helpful in diagnosing nondisplaced fractures or identifying small fracture fragments. However, a computed tomography (CT) scan with three dimensional reconstruction is the clear gold standard for diagnosing the capitellar or any subtle elbow fracture. What appears to be a simple fracture in the simple radiograph is often found to be a complex injury ( Fig. 46.6 ).