The humerus, the single bone of the upper arm, connects with the ulna and the radius, the two bones of the forearm, at the elbow joint. This complex joint connects the upper limb and the forearm, facilitating a wide range of motion. The ulna, located on the medial side of the forearm, forms a direct connection with the humerus. This interaction is responsible for much of the elbow’s mechanical function.
Identifying the Elbow Joint Articulation
The meeting point between the humerus and the ulna is specifically termed the humeroulnar joint. This articulation is a component of the larger elbow joint, which also includes the humeroradial joint and the proximal radioulnar joint. The humeroulnar joint is structurally classified as a synovial joint and functionally as a hinge joint. This means the joint permits movement primarily along a single plane, similar to a door hinge. Although the radius also meets the humerus at the elbow, the humeroulnar articulation establishes the primary mechanical axis for bending and straightening the arm.
Specific Bony Features That Connect
The interaction between the humerus and ulna relies on complementary shapes on each bone that interlock tightly. On the distal end of the humerus is a spool-shaped structure called the trochlea. This medial portion articulates directly with the ulna bone. The ulna presents a large, C-shaped concavity at its proximal end, known as the trochlear notch. This notch fits over the humeral trochlea, creating a highly congruent joint that resembles a wrench fitting around a spool.
The precise adaptation of the trochlea to the trochlear notch prevents excessive side-to-side or lateral movement in the elbow. The inferior margin of the trochlear notch is defined by a lip of bone called the coronoid process. The back of the ulna features a large bony prominence known as the olecranon process. When the arm is straightened, the olecranon process moves into a corresponding depression on the back of the humerus called the olecranon fossa. This bony engagement acts as a check, limiting the extent to which the arm can extend backward.
Function and Range of Motion
The primary movements permitted by the humeroulnar joint are flexion and extension. Flexion involves decreasing the angle between the forearm and the upper arm, while extension involves straightening the arm to increase that angle. The normal range of motion for flexion is approximately 140 degrees, with full extension being considered zero degrees. Some individuals may exhibit slight hyperextension, reaching up to 5 to 10 degrees beyond the zero point.
The stability of this hinge joint is not solely dependent on the shape of the bones; it is significantly reinforced by surrounding soft tissues. Strong bands of connective tissue, known as collateral ligaments, are positioned on both sides of the articulation. The ulnar collateral ligament extends from the humerus to the ulna, acting as a fixed-length stay. These ligaments resist forces that would otherwise cause the forearm to shift sideways, ensuring that movement remains restricted to the flexion and extension plane.