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Knee injuries: What are they?
There are four main ligaments in the knee - one on either side and two across the middle.
The ligament injured in the case of van Nistelrooy is the medial ligament, which runs up and down the inside of the knee.
Former England international Paul Gascoigne's famous knee injury was of one of the cruciate ligaments, which are cross-shaped within the knee-joint.
Such injuries are common in sportsmen and women.
Top flight footballers
Specialist centres, such as the one at Robert Jones and Agnes Hunt Orthopaedic and District Hospital in Shropshire, handles many cases of top flight footballers with damaged knee ligaments each year.
A senior consultant said: "You can have a minor pull which heals within a few weeks, or a major pull which takes longer. Or it can be completely torn which takes even longer.
"The vast majority of such cases do not require operations, but occasionally people run into chronic trouble."
The medial ligament is linked to the cartilage so further problems can result.
There is some variation in opinion among orthopaedic surgeons about the best way to treat medial ligament injuries, but the vast majority favour allowing them to heal without surgery.
Physiotherapy is used instead, as the medial ligament, being outside the joint, heals more easily than the internal cruciate ligaments.
The vast majority of isolated medial ligament injuries heal without an operation.
Cruciate injuries usually require surgery to repair the damaged ligament.
There are four main ligaments in the knee - one on either side and two across the middle.
The ligament injured in the case of van Nistelrooy is the medial ligament, which runs up and down the inside of the knee.
Former England international Paul Gascoigne's famous knee injury was of one of the cruciate ligaments, which are cross-shaped within the knee-joint.
Such injuries are common in sportsmen and women.
Top flight footballers
Specialist centres, such as the one at Robert Jones and Agnes Hunt Orthopaedic and District Hospital in Shropshire, handles many cases of top flight footballers with damaged knee ligaments each year.
A senior consultant said: "You can have a minor pull which heals within a few weeks, or a major pull which takes longer. Or it can be completely torn which takes even longer.
"The vast majority of such cases do not require operations, but occasionally people run into chronic trouble."
The medial ligament is linked to the cartilage so further problems can result.
There is some variation in opinion among orthopaedic surgeons about the best way to treat medial ligament injuries, but the vast majority favour allowing them to heal without surgery.
Physiotherapy is used instead, as the medial ligament, being outside the joint, heals more easily than the internal cruciate ligaments.
The vast majority of isolated medial ligament injuries heal without an operation.
Cruciate injuries usually require surgery to repair the damaged ligament.