How is the elbow designed, and what is its function?

The elbow is the joint where three long bones meet in the middle portion of the arm. The bone of the upper arm (humerus) meets the inner bone of the forearm (ulna) and the outer bone of the forearm (radius) to form a hinge joint. The radius and ulna also meet in the elbow to allow for rotation of the forearm. The elbow functions to move the arm like a hinge (forward and backward) and in rotation (twisting outward and inward). The biceps muscle is the major muscle that flexes the elbow hinge. The triceps muscle is the major muscle that extends the elbow hinge. The outer bone of the elbow is referred to as the lateral epicondyle and is a part of the humerus bone. Tendons are attached to this area which can be injured, causing inflammation or tendinitis (lateral epicondylitis, or "tennis elbow"). The inner portion of the elbow is a bony prominence called the medial epicondyle.

Additional tendons from the muscles attach here and can be injured, causing medial epicondylitis, "golfer's elbow." A fluid-filled sac (bursa), which serves to reduce friction, overlies the tip of the elbow (olecranon bursa). The elbow can be affected by inflammation of the tendons or the bursae (plural for bursa) or conditions that affect the bones and joints, such as fractures, arthritis, or nerve irritation. Joint pain in the elbow can result from injury or disease involving any of these structures.

What injuries can cause elbow pain?

Lateral epicondylitis (tennis elbow)

The lateral epicondyle is the outside bony portion of the elbow where large tendons attach to the elbow from the muscles of the forearm. These tendons can be injured, especially with repetitive motions of the forearm, such as using a manual screwdriver, washing windows, or hitting a backhand in tennis play. Tennis elbow then leads to inflammation of the tendons, causing pain over the outside of the elbow, occasionally with warmth and swelling but always with local tenderness. The elbow maintains its full range of motion, as the inner joint is not affected, and the pain can be particularly noticed toward the end of the day. Repeated twisting motions or activities that strain the tendon typically elicit increased pain. X-rays are usually normal, but if chronic tendinitis has occurred, X-rays can reveal calcium deposits in the tendon or reveal other unforeseen abnormalities of the elbow joint. 

Medial epicondylitis (golfer's elbow)

Medial epicondylitis is inflammation at the point where the tendons of the forearm attach to the bony prominence of the inner elbow. As an example, this tendon can become strained in a golf swing, but many other repetitive motions can injure the tendon. Golfer's elbow is characterized by local pain and tenderness over the inner elbow. The range of motion of the elbow is preserved because the inner joint of the elbow is not affected. Those activities which require twisting or straining the forearm tendon can elicit pain and worsen the condition. X-rays for epicondylitis are usually normal but can indicate calcifications of the tendons if the tendinitis has persisted for extended periods of time. 

Olecranon bursitis
Olecranon bursitis (inflammation of the bursa at the tip of the elbow) can occur from injury or minor trauma as a result of systemic diseases such as gout or rheumatoid arthritis, or it can be due to a local infection. Olecranon bursitis is typically associated with swelling over the tip of the elbow, while range of motion of the inner elbow joint is maintained.

Sprain
A sprain is a stretching or tearing injury to a ligament. One or more ligaments can be injured during a sprain. This might occur when the elbow is hyperextended or simply jammed, such as in a "stiff arm" collision. The severity of the injury will depend on the extent of injury to a single ligament (whether the tear is partial or complete) and the number of ligaments involved.

Conditions that can cause elbow pain

Arthritis of the elbow
Inflammation of the elbow joint (arthritis) can occur as a result of many systemic forms of arthritis, including rheumatoid arthritis, osteoarthritis, gouty arthritis, psoriatic arthritis, ankylosing spondylitis, and reactive arthritis (formerly called Reiter's disease). Generally, they are associated with signs of inflammation of the elbow joint, including heat, warmth, swelling, pain, tenderness, and decreased range of motion. Range of motion of the elbow is decreased with arthritis of the elbow because the swollen joint impedes the range of motion.