CV joints are protected from contamination by air tight rubber bots secured with metal retaining clips. Failure of these boots is the primary reason for CV joint failure. A split or broken boot will eventually cause the failure of the CV joint in all cases.
Maintenance is usually limited to checking that the rubber boot that covers them is secure and not split or broken. If the rubber boot is damaged, the lubricating grease that the joint is packed with, will be thrown out. The joint will then pick up dirt and water that will cause the joint to overheat and wear, and the grease can also contaminate the brakes. In the worst cases, the CV joint may separate causing the vehicle to loose the ability to drive. Damaged CV joint boots will usually cause a car to fail a safety inspection.