BACKGROUND: Patellofemoral replacements (PFRs) have a higher rate of revision than unicompartmental knee arthroplasty or TKA. However, there is little information regarding why PFRs are revised, the components used for these revisions, or the outcome of the revision procedure. Some contend that PFR is a bridging procedure that can easily be revised to a TKA with similar results as a primary TKA; however, others dispute this suggestion.
QUESTIONS/PURPOSES: (1) In the setting of a large national registry, what were the reasons for revision of PFR to TKA and was the level of TKA constraint used in the revision associated with a subsequent risk of rerevision? (2) Is the risk of revision of the TKA used to revise a PFR greater than the risk of revision after a primary TKA and greater than the risk of rerevision after revision TKA?
METHODS: Data were obtained from the Australian Orthopaedic Association Joint Replacement Registry through December 31, 2016, for TKA revision procedures after PFR. Because revisions for infection may be staged procedures resulting in further planned operations, for the revision analyses, these were excluded. There were 3251 PFRs, 482 of which were revised to TKA during the 17-year study period. The risk of second revision was calculated using Kaplan-Meier estimates of survivorship for PFRs revised to TKAs, and that risk was compared with the risk of first revision after TKA and also with the risk of a second revision after revision TKA. Hazard ratios (HRs) from Cox proportional hazards models were used to compare second revision rates among the different levels of prosthesis constraint used in the index revision after PFR (specifically, cruciate-retaining versus cruciate-substituting).
RESULTS: The main reasons for revising a PFR to TKA were progression of disease (56%), loosening (17%), and pain (12%). With the numbers available for analysis, there was no difference in the risk of a second revision when a PFR was revised to a cruciate-retaining TKA than when it was revised to a cruciate-substituting TKA (HR, 1.24 [0.65-2.36]; p = 0.512). A total of 204 (42%) of the PFR revisions had the patella component revised when the PFR was converted to a TKA. There was no difference in rates of second revision when the patella component was revised or not revised (HR, 1.01 [0.55-1.85]; p = 0.964). When we eliminated the devices that ceased to be used before 2005 (older devices), we found no change in the overall risk of repeat revision. The risk of a PFR that was revised to a TKA undergoing a second revision was greater than the risk of TKA undergoing a first revision (HR, 2.39 [1.77-3.24]; p < 0.001), but it was less than the risk of a revision TKA undergoing a second revision (HR, 0.60 [0.43-0.81]; p = 0.001).
CONCLUSIONS: The risk of second revision when a PFR is revised is not altered if cruciate-retaining or posterior-stabilized TKA is used for the revision nor if the patella component is revised or not revised. The risk of repeat revision after revision of a PFR to a TKA was much higher than the risk of revision after a primary TKA, and these findings did not change when we analyzed only devices in use since 2005. When PFR is used for the management of isolated patellofemoral osteoarthritis, patients should be counselled not only about the high revision rate of the primary procedure, but also the revision rate after TKA. Further studies regarding the functional outcomes of these procedures may help clarify the value of PFRs and subsequent revisions.
LEVEL OF EVIDENCE: Level III, therapeutic study.