The exact risk percentage may be found for both the Allergan, and the Mentor studies in related questions. The problem most often is the result of inadequate soft tissue (breast and skin) overlying the breast implants. It is unusual in cases of primary augmentation, and is more commonly seen in the compromised soft tissue of the reconstructed breast.
If the extrusion if only threatened, and no implant is actually showing at the time of detection, a salvage procedure may be done to improve the soft tissue cover atop the implant. If the implant is exposed, many plastic surgeons will not attempt salvage, as by definition, and exposed implant is a contaminated implant. The breast implant will be removed, patient allowed to heal, and implant replaced at a later time.