A ligament is a rope-like structure in the body that connects two bones, thereby controlling the movement and stability across the joint. In the elbow, one of the most important ligaments is the ulnar collateral ligament. This ligament provides considerable and important strength on the inner aspect of the elbow while throwing. The ulnar collateral ligament (UCL) traverses the inner aspect of the elbow from the ulna to the humerus.
Figure 1, at left, shows the three bands in the UCL. The anterior band of the UCL is the arm's primary restraint from valgus stress to the elbow. During throwing, a tremendous valgus stress is placed across the elbow joint. Studies have shown that pitching a baseball generates a valgus force at the elbow estimated at 290 Newtons, resulting in an angular velocity in excess of 3000 degrees per second.
These high forces across the inner aspect of the elbow occur in throwing during the late cocking and early acceleration phase of throwing (Figure 2, below). The tremendous force and rotational velocity generated by pitching and throwing place stress on the ulnar collateral ligament. The forces generated at the elbow during throwing can exceed the tensile strength of the UCL.
This force can cause failure of the ligament all at once (acute rupture of the UCL) or more commonly a slower, attritional rupture of the ligament due to microtrauma and repetitive stress. The repetitive stress on the ligament leads to lengthening, tearing and attrition of the UCL which in turn causes medial (inner) elbow pain, decrease in velocity of throwing, and an inability to throw effectively.
SYMPTOMS
Most commonly, an UCL tear presents with a
gradual onset of medial elbow pain due to repetitive stresses on the ligament; this is termed a chronic UCL tear. Occasionally, throwing athletes may experience a sharp "pop" or develop sharp pain along the inside of the elbow joint on one particular throw; this is termed an acute UCL tear. Either way, the tear in the ligament leads to a decrease in velocity, pain while pitching, and lack of control. Occasionally, the ulnar nerve, which runs very close to the UCL, can be stretched, leading to numbness and tingling in the small and ring fingers.
guess it can be sudden or gradual.............
http://www.gamradtortho.com/conditionsUlnarCollateralLigament.php
Albert Breer 
