Chapter 5: The Elbow and Forearm

STRUCTURE AND FUNCTION

Humeroulnar and Humeroradial Joints

Anatomy

The humeroulnar and humeroradial joints between the upper arm and the forearm are considered to be a hinged compound synovial joint (Figs. 5.1 and 5.2). The proximal joint surface of the humeroulnar joint consists of the convex trochlea located on the anterior medial surface of the distal humerus. The distal joint surface is the concave trochlear notch on the proximal ulna.

The proximal joint surface of the humeroradial joint is the convex capitulum located on the anterior lateral surface of the distal humerus. The concave radial head on the proximal end of the radius is the opposing joint surface.

The joints are enclosed in a large, loose, weak joint capsule that also encloses the superior radioulnar joint. Medial and lateral collateral ligaments reinforce the sides of the capsule and help to provide medial-lateral stability (Figs. 5.3 and 5.4).¹

When the arm is in the anatomical position of full elbow extension and supination, the long axes of the humerus and the forearm form an acute angle at the elbow. This angle is called the "carrying angle" (Fig. 5.5) and is approximately 10 to 12 degrees in men and 13 to 17 degrees in women.^2,3 The carrying angle of the dominant arm is reported to be slightly greater (1.5 degrees) than the nondominant arm and slightly greater (2 degrees) in adults than in children.⁴ An angle that is greater (more acute) than average is called "cubitus valgus."⁵ An angle that is less than average is called "cubitus varus."

Osteokinematics

The humeroulnar and humeroradial joints have 1 degree of freedom; flexion–extension occurs in the sagittal plane around a medial–lateral (coronal) axis. In elbow flexion and extension, the axis of rotation lies approximately through the center of the trochlea.³