Ulnar nerve elbow entrapment is the second most common nerve entrapment in the upper extremity after carpal tunnel syndrome. The ulnar nerve is compressed in the ulnar nerve groove at the elbow, etiology; elbow trauma, osteoarthritic lesions, compression of the two heads of the ulnar carpal flexors, elbow flab, ulnar nerve slipped, elbow canal composed of: the ulnar nerve groove + the two heads of the ulnar carpal flexors, it is a bony fibrous canal in the elbow, and there is ulnar nerve passing through the canal. Any decrease in the volume of the elbow canal can cause the ulnar nerve to be compressed or stretched in the elbow canal, thus causing atrophy and weakness of the hand muscles and numbness and pain on the ulnar side of the hand, i.e. elbow canal syndrome. Common causes include repeated elbow flexion and extension activities, post-traumatic elbow deformity, congenital deformity, osteoarthritis, tuberculosis, and rheumatoid arthritis. The course of the ulnar nerve in the elbow, from the proximal Stuthers’ arch to the distal penetration of the ulnar flexor carpi ulnaris, can be impinged at a total of 5 places. The two most likely sites for entrapment are the medial epicondylar groove and the two heads of the ulnar flexor carpi ulnaris. From 10 cm above the elbow to 5 cm below the elbow, a total of 5 places can occur jamming. 1.Struthers arch (proximal) to the medial epicondyle (distal) Struthers arch is a myofascial band located 8 cm proximal to the medial epicondyle, with a width of 1.5~2 cm, and passes obliquely from the surface of the ulnar nerve. Its anterior border is the medial interosseous interval and its outer border is the deep fibers coming from the medial head of the triceps. When Struthers’ bow is not present, the medial interosseous can cause entrapment (common in ulnar nerve forward dislocation and surgical anterior transposition without adequate resection of the medial interosseous) The medial head of the triceps muscle can also cause entrapment (e.g., in bodybuilders, muscle hypertrophy, resulting in friction neuritis). 2. Near the medial epicondyle: elbow valgus deformity secondary to supracondylar humerus fractures and epicondylar fractures. 3.Medial epicondylar groove/hawksbill groove The medial epicondylar groove is a bony fibrous groove with the medial epicondyle at the anterior border, the hawksbill and ulnohumeral ligaments at the outer border, and fibrous tendinous structures at the inner border. Entrapment in this area can be caused by many factors and can be subdivided into 3 categories. Intra-groove lesions include fracture blocks, arthritic bone spurs, bone hypertrophy, soft tissue tumors, cysts, osteochondromas, rheumatoid synovitis, tuberculous synovitis, post-traumatic hematomas, hemophilic hematomas, and others. Extra-groove factors refer to the long time, often repeated use of the elbow joint, especially the flexion of the joint. Examples include truck drivers driving with the elbow joint resting on the window, chronic illnesses with prolonged bed rest, surgery with a restraining band pressing the medial elbow directly against the side of the surgical bed, and forearms bound in a brace in the rotated forward position. Factors that predispose the nerve to subluxation These factors predispose the nerve to slip forward during elbow flexion and return during elbow extension. The nerve can slip over or in front of the medial epicondyle. Examples include congenital laxity or traumatic tearing of the fibrotendinous structures on the surface of the nerve groove, congenital hypoplasia of the pulley, and traumatic deformity of the medial epicondyle. Note the differentiation from asymptomatic increased nerve mobility, which can occur in 20% of the normal population. Easily compressed, such as repeated friction, plaster splint, medial epicondylitis injection therapy. 4. The canal between the humeral head and ulnar head of the ulnar carpal flexor muscle: the base is the medial collateral ligament of the elbow, and the top is a fibrous band that continues with the fibro-tendinous structures of the medial epicondylar groove (known as Osborne’s ligament/triangular ligament/arch ligament/humeral ulnar arch). Although elbow tunnel syndrome is currently referred to as a compression anywhere in the elbow, the narrower, more accurate term “elbow tunnel” should be used to refer to this specific area. When the elbow is flexed, the Osborne’s ligament pulls and tightens, while the basal medial collateral ligament relaxes and folds, resulting in a narrowing of the elbow canal and entrapment of the nerve. The cross-sectional shape of the elbow canal is oval in extension and flattens in flexion. The internal pressure in the elbow canal increases 7-fold when the elbow is flexed, and increases more than 20-fold if there is a concomitant contraction of the ulnar carpal flexor muscle. These factors cause mechanical deformation of the nerve and changes in intraneural perfusion. 5. Penetration of the ulnar flexor carpi ulnaris muscle: The nerve enters the ulnar flexor carpi ulnaris muscle from the elbow canal, travels about 5 cm within the muscle, penetrates the fascial layer, and is located between the deep and superficial muscles of the flexor digits. The point of exit can be compressed by fascial tissue. These fascia are called “flexor pronator teres”. Under normal conditions, the ulnar nerve has a range of motion of about 10 mm proximal to the medial epicondyle and about 6 mm distal to the medial epicondyle during elbow movement. When the elbow is flexed, the nerve itself can be tugged 4.7mm, and can be tugged even longer when the shoulder is abducted, externally rotated and the wrist is extended. Any scarring throughout the nerve travel that restricts the normal sliding of the nerve can cause a traction injury.