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Ligament injuries are one of the most common injuries to the knee and very few players fully recover from the problem.
Footballers Robert Pires, Paul Gascoigne, Alan Shearer, Ronaldo, and Ruud van Nistelrooy have all suffered from knee ligament damage.
Cricketers Simon Jones, Darren Gough and Michael Vaughan have also experienced knee ligament problems.
Too much strain put on the knee.
It is usually a twisting or turning action which damages the ligaments.
Direct contact of the knee from opponents can also cause damage to the ligaments.
Jones ruptured his knee when his studs got caught in the turf when he was sliding to field a ball.
How many ligaments are there in the knee?
Jones ruptured his anterior cruciate ligament, one of the cross-shaped ligaments within the knee-joint.
Gazza famously also famously injured the same part of the knee in the 1991 FA Cup final at Wembley.
If it is completely torn you could be out for a year.
The medial ligament is linked to the cartilage so further problems can result.
Physiotherapy is used as the medial ligament, being outside the joint, heals more easily than the internal cruciate ligaments.
Cruciate injuries, however, require surgery to repair the damaged ligament and several hundred such operations are carried out on sports stars each year.