Reoperations after total hip replacement (THR) are not uncommon. The most common reasons for reoperation are loosening of the implant, repeated dislocation, or bone fracture around the implant.
Most patients report minor trauma before bone fracture. Spontaneous fracture (no known cause) is more likely after revision surgery. Revisions are done to repair or replace the primary (first) THR.
Other reasons for reoperation can include nonunion of the fracture or refracture. Sometimes infection or fracture of the implant can occur.
Researchers are collecting data to help sort out who is at risk for implant failure or reoperation. One way to do this is to create a national patient registry. For example in Sweden, anytime someone has a total hip replacement, the surgeon must report information about the case to the Swedish National Hip Arthroplasty Register.
This registry has three separate databases. Each one collects slightly different bits of information. This allows researchers to group data together for easier analysis. Information can be used to identify who has a reoperation, fracture or refracture, or other complications.
Patient selection, implant choice, and experience of the surgeon all seem to be important factors in THR surgery. Your parents' situation could be the result of one of these factors -- or it could indeed be just a coincidence. We still don't always know how to tell exactly what caused the problem in order to prevent it.