Revisions of total knee arthroplasty (TKA), or knee replacement, can be necessary for several reasons. If the problem is with the mechanics of the actual implant, perhaps some loosening, this surgery is likely possible just to be a repair job, rather than a full replacement.
However, if the implant is grossly damaged or defective, or if the patient has experienced trauma or deterioration of the bone, a full replacement may be needed. If the bone has deteriorated, the patient may require a different type of implant, allowing the knee to function again. For this reason, a repair wouldn't be possible.