“Fall apart”, do you mean loosen?
When the prosthesis becomes loose, you may experience pain, change in alignment, or instability. Aseptic loosening is the most common mode of failure of knee implants. If you are feeling pain, a repeated surgery may be an option. Usually 1 percent of people need a repeated surgery in 1 year, and about 10 percent of people need a repeated surgery in 10 years. If you're active and over weighted, the chance of loosening is higher.
Although in the joint replacement surgery, implants are firmly fixed, they may become loose over time. Friction caused by the joint surfaces rubbing against each other wears away the surfaces of the implant, creating tiny particles that accumulate around the joint. In a process called aseptic (non-infected) loosening, the bond of the implant to the bone is destroyed by the body's attempt to digest the wear particles. During this process, normal bone is also digested (a condition called osteolysis), which can weaken or even fracture the bone.