Repeated episodes, though, are a type of migraine.

Equally, repeated episodes such as this are unlikely to be due to injury, toxins or poisoning, and normal blood tests should have ruled out most metabolic causes.

Such as locking, recurrent pain, repeated episodes of swelling or your knee just feeling 'dodgy' - typically worse whenever you try to get back on the pitch.

In sickle-cell disease, low-oxygen tension promotes red blood cell sickling and repeated episodes of sickling damage the cell membrane and decrease the cell's elasticity.

With repeated episodes the joints become damaged with multiple joint replacements required.