What is an ulnar nerve injury and what causes it?

Ulnar Nerve: injury can occur from repeated application pressure to or traction of the ulnar nerve usually at a point where the nerve lies close to the body surface overlying bone (inside of elbow or base of hand on the pinky side. It results in parasthesias in the 5th and outside half of the 4th finger and if further up the arm can result in loss of function of some muscles in the hand (& thumb adductor)

Ulnar nerve injury: An ulnar nerve injury occurs due to trauma or compression, often at the inner elbow. It may cause numbness of the forearm and hand and weakness, especially in the hand. It can be treated with splinting, but often surgery is needed for severe cases. http://www.neurocuro.com/peripheral-nerve-entrapment/