Presuming you mean the CV axle joint boots. Typically when an axle boot is torn, the grease is flung out leaving the joint to dry out. Dirt & moisture can also enter and cause excessive wear. If you catch it in time when the hole is still very small, yes, you can replace just the boot and repack it with clean grease. There are many aftermarket kits for this, find the appropriate one for your vehicle. Otherwise, if it has been this way for a while, the hole in the boot is big (no more grease), or the joint is making noise... then yes, you should replace the entire axle unit.