IT happened in a split second - one moment Ben Doltis was playing football with friends, the next he was prostrate, clutching his right knee in agony. The pitch was frozen and slippery, and his leg had twisted awkwardly while he ran. As he fell, he had heard a sickening - and familiar - crack. 'A year before, I tore a ligament in my left knee and had to take a month off work after surgery to repair it, so I knew it was serious, says Ben, a managing director from London in his 30s. A trip to A&E;confirmed his fears - he had torn another ligament. The knee is the largest joint in the body, where three leg bones meet. The thigh bone and shin bone form a hinge joint and the patella, or knee cap, sits over them and slides as the leg moves. Crucial to this structure are the ligaments, the strong fibrous cables of tissue connecting the bones. The anterior cruciate ligament is one of the four main ligaments, which acts as a stabiliser to the leg. This is what Ben had damaged. It is one of the most frequent ligament injuries and is usually caused by a sudden stop and twisting movement or from a blow to the front of the knee. Damage often occurs while playing sports such as football and rugby, while running for the bus or simply stepping out of the bath. The reconstructive process has historically been lengthy and more painful than the injury itself. Daily Mail