PATHOMECHANICS OF ELBOW INJURIES

Near the end of arm cocking, the shoulder is in extreme external rotation, the elbow is flexed approximately 90°, and the forearm is in a valgus position. To resist valgus stress, a large varus torque is produced at the elbow (Figure 2). Tension in the ulnar collateral ligament (UCL) provides nearly half of this varus torque. This high tension is near the ultimate tensile strength of the UCL, leaving the UCL susceptible to injury. The flexor and pronator muscle mass of the forearm also contribute varus torque; thus when the flexor-pronator mass is weak or fatigued, the load on the UCL may be even greater, further amplifying the ligament's risk of injury.

Repetitive tension that the UCL and joint capsule exert on the ulna can cause spur formation, which can compress the ulnar nerve. Mechanical compromise of the ulnar nerve medially from tension, friction, or compression can lead to ulnar neuritis. Furthermore, ulnar neuropathy has been documented due to repetitively throwing breaking balls.

The lateral elbow is also susceptible to injury near the end of arm cocking (Figure 2). Compressive forces between the radial head and humeral capitellum contribute approximately one-third of the elbow's varus torque. This compression can lead to avascular necrosis, osteochondritis dissecans, or osteochondral chip fractures.

The posterior elbow is susceptible to injury during the arm acceleration phase. Significant varus torque is produced throughout this phase. Some of this varus torque is caused by wedging of the olecranon into the olecranon fossa. This impingement, in combination with the rapid elbow extension during arm acceleration, can lead to osteophytes at the posteromedial olecranon tip, chondromalacia, or the formation of loose bodies.