The elbow is where the two bones of the forearm – the radius on the thumb side of the arm and the ulna on the pinky finger side – meet the bone of the upper arm -- the humerus.
The lower end of the humerus flares out into two rounded protrusions called epicondyles, where muscles attach. The upper end of the ulna also has two protrusions – the olecranon, which forms the point of the elbow, and the caronoid process. These protrusions fit into two corresponding depressions (the olecranon fossa and coronoid fossa) at the lower end of the humerus to form the hinge-like humeroulnar joint, which allows you to bend and straighten your arms. It is what we typically think of as the elbow joint. However, there are two lesser-known, but equally important joints that make up the elbow:
- humeroradial joint – the joint formed where the radius and humerus meet. The joint not only allows you to bend and straighten your arms, it also is involved in supination and pronation, the motion of turning the hand over so the palm faces up or down.
- proximal radioulnar joint – the joint where the radius and ulna meet. While the joint is not involved in bending the arm, it allows you to rotate the lower arm.
The ends of the bones, where they meet to form the joints, are covered in a layer of thick, shiny articular cartilage that absorbs shock and allows the bones to glide smoothly against one another. The cartilage of the elbow is thinner than that of weight-bearing joints, such as the knee or hip.
Other major structures of the elbow include:
Tough bands of connective tissue called ligaments hold the bones of the elbow together. The two main ligaments connecting the humerus and ulna are:
- ulnar-collateral ligament, also called the medial collateral ligament, which runs along the inside of the elbow
- lateral collateral ligament, also called the radial collateral ligament, which runs along the outside of the elbow.
A third major ligament of the elbow is the annular ligament, which circles the top, or head, of the radius, holding it against the humerus.
The ligaments form a capsule around the joint that is lined with a smooth membrane called the synovium. The synovium produces a viscous liquid, called synovial fluid, which lubricates the joint.
Several muscles surround the elbow, making it possible for you to move your arm. The two main muscles responsible for bending and straightening the arm at the elbow are:
- triceps, located at the back of the arm
- biceps, located at the front of the arm
The main muscles of the elbow involved in wrist and finger movement are:
- flexors, which attach to the inside of the elbow and enable you to bend your wrists and fingers
- extensors, which attach to the outside of the elbow and enable you to extend or straighten your wrists and fingers.
Several tendons connect the muscles and bones of the elbow. The primary tendons are:
- biceps tendon, which attaches the biceps on the front of the arm to the radius, enabling you to forcefully bend your bend your elbow
- triceps tendon, which attaches the triceps to the ulna, enabling you to forcefully straighten your elbow .