Bursitis is the inflammation of one or more bursae (fluid filled sacs) of synovial fluid in the body. They are lined with a synovial membrane that secretes a lubricating synovial fluid. There are more than 150 bursae in the human body. The bursae rest at the points where internal functionaries, such as muscles and tendons, slide across bone. Healthy bursae create a smooth, almost frictionless functional gliding surface making normal movement painless. When bursitis occurs, however, movement relying on the inflamed bursa becomes difficult and painful. Moreover, movement of tendons and muscles over the inflamed bursa aggravates its inflammation, perpetuating the problem. Muscle can also be stiffened. Signs and symptoms
Bursitis commonly affects superficial bursae. These include the subacromial, prepatellar, retrocalcaneal, and pes anserinus bursae of the shoulder, knee, heel and shin, etc. (see below). Symptoms vary from localized warmth and erythema to joint pain and stiffness, to stinging pain that surrounds the joint around the inflamed bursa. In this condition, the pain usually is worse during and after activity, and then the bursa and the surrounding joint becomes stiff the next morning.Bursitis could possibly also cause a snapping, grinding or popping sound – known as snapping scapula syndrome – when it occurs in the shoulder joint. This is not necessarily painful. Cause
There can be several concurrent causes. Trauma, auto-immune disorders, infection and iatrogenic (medicine-related) factors can all cause bursitis. Bursitis is commonly caused by repetitive movement and excessive pressure. Shoulders, elbows and knees are the most commonly affected.
The role of surgery in AIN syndrome remains controversial. Indications for considering surgery include a known space-occupying lesion that is compressing the nerve (a mass) or fascial compression, and persistent symptoms beyond 1 year of conservative treatment. Symptoms and signs
Patients may experience poorly localised, transient pain in the wrist, i.e. where the sensory branch of the AIN is mapped in the brain. The pain is sometimes referred into the cubital fossa and elbow pain has been reported as being a primary complaint.⁠
The characteristic severe impairment of the pincer movement of the thumb and index finger is most striking. Clinical signs
In a pure lesion of the anterior interosseous nerve there is weakness of the long flexor muscle of the thumb (Flexor pollicis longus), the deep flexor muscles of the index  and middle fingers (Flexor digitorum profundus I & II), and the pronator quadratus muscle.There is no sensory deficit since the anterior interosseous nerve has no cutaneous branch to skin, but there is a large sensory branch to the volar carpus, and transient wrist pain may be experienced.