A work colleague of mine had a 3cm cyst in his knee, caused him to cut rides short all the time. Had it removed after 3 years of discomfort (aged 29) and within 6 months he was riding again, (fixed) much further and in more comfort than before. Felt like a new knee apparently.
Before he had it removed, the doctors were telling him it was muscle damage. Took 4 doctors to find one who would x-ray it to finally find the cyst.