Take a look at your palm. Now make a fist as tight as you can. Chances are you can now see two seperate tendons running over the middle of your wrist. One of these tendons is the tendon of palmaris longus. 10% of the population will NOT see this tendon when they make a fist because they were born without it. It's not a big deal, the tendon is pretty useless functionally. Thus, when a patient tears an elbow ligament, surgeons often scavenge the palmaris longus tendon and reattach it to the elbow in place of the torn ligament.
We were told an unfortunate story about a surgeon who rushed a surgery and cut into a patient's wrist, hoping to scavenge the palmaris longus tendon. Turns out that this patient was in the 10%, he did not have the tendon. Guess what is usually underneath the tendon? The median nerve. This surgeon cut clean through the nerve controlling the muscles of the patient's thumb, index, and middle finger. These fingers are now non-functional for this patient. A really tragic case and a reminder to all physicians not to rush their surgeries.