Since its introduction, UKA has been an alternative to TKA or high tibial osteotomy for management of isolated unicompartmental knee arthritis. The proposed advantages of UKA include preservation of femoral and tibial bone stock, a less invasive surgery, improved knee range of motion [2] and kinematics, and shorter postoperative lengths of hospitalization [22]. Although early midterm studies cited a high rate of satisfactory results after UKA [12], long-term survivorship and outcome continue to be a concern [9, 13, 20]. Although these concerns are not without foundation, they have been addressed with advancements in prosthetic component design, instrumentation and surgical technique, and appropriate patient selection [5, 7, 27]. Some reports have cited reliable UKA survivorship of greater than 90% for more than 10 years after implantation [1, 11, 19, 21, 26]. When compared with TKA as an index arthroplasty procedure, however, the cumulative revision rate for UKA tends to be higher [8, 9, 15, 17, 20].

Early investigations of UKA revision (rev-UKA) reported technical difficulties associated with the procedure, including substantial bone loss with grafting [20], the need for stemmed revision components, or even the need for custom implants [3]. More recent reports concerning contemporary rev-UKA have been more favorable, noting a technically straightforward approach in terms of revision joint replacement surgery [4, 6, 14, 16, 18, 23, 24].

Although the typical recommendation for UKA failure is revision to TKA, little information is available regarding rev-UKA survival or the survival of rev-UKA compared with rev-TKA. Also, numerous reports of rev-UKA and rev-TKA mostly have been limited to one surgeon, academic, or institutional experience. Infrequently have such reports been gathered from series reflective of the community experience, which may be more representative of the overall efficacy of a surgical procedure.

We hypothesized, in comparison with rev-TKA, rev-UKA would be (1) less complex as reflected by lower use of modular stems and augments and shorter operative time, (2) less expensive as reflected in implant costs, total hospital charges, and higher percentage of patients discharged home, and (3) more durable as reflected in improved implant survival.

Data maintained in the registry allowed for identification and subsequent analysis of several variables, including patient factors (date of primary and revision surgery, gender, age, diagnosis at the time of the index procedure, discharge disposition), indication for revision surgery, operative time, components necessary for revision (patellar resurfacing, tibial polyethylene insert thickness, number of wedges, number of stems), and implant costs and hospital charges.

Univariate analyses were performed using Pearson’s chi square for categorical variables (patellar resurfacing, metal augmentation, stems, diagnosis at index procedure, indication for revision surgery, discharge disposition, and gender) and Student’s t test for continuous variables (polyethylene insert thickness, operative time, time to revision, implant costs and hospital charges, and age). Cumulative revision rates (CRRs) were calculated using the Kaplan-Meier method. The log-rank test was used to compare survival between rev-UKA and rev-TKA groups. Hazard ratios were calculated using the Cox proportional-hazards model.

When a UKA fails, revision to TKA generally is recommended [17]. Numerous studies have cited consistent modes of UKA failure, including progressive adjacent compartment arthritis, aseptic femoral or tibial component loosening, and polyethylene wear [3, 4, 6, 9, 16, 17, 24, 25]. We sought to determine how rev-UKAs perform in a community registry population in comparison to rev-TKAs. In particular, we were interested in the relative complexity, cost, and survival of these two revision procedures.

Limitations of this study include the relatively small population and those inherent to any registry population, where a small percentage of patients (estimated at 6% in the HealthEast Joint Registry [10]) may seek revision surgery elsewhere and where revision itself is the end point measured. We cannot comment, therefore, on impending revisions, poor radiographic or clinical results in either group, or patients who may have deferred additional surgery secondary to medical comorbidities. The use of metal augments and stems and operative time were used as proxies for bone loss and complexity of the surgical procedure, although many other factors that add to surgical complexity are not part of the registry database. Similarly, because numerous surgeons performed the revision surgeries, indications for the procedure and surgical techniques undoubtedly varied. However, such results may be more generalizable to the community population and to the general orthopaedist than those seen in specialized centers.

As anticipated, we found differences between the rev-UKA and rev-TKA groups in terms of implant costs and hospital charges. More patients who had rev-TKAs had implant costs greater than $5200 and hospital charges greater than $33,000 compared with patients who had rev-UKAs. As there was no difference found in length of hospitalization between the groups or in discharge disposition, the difference in hospital charges undoubtedly reflects the higher surgical/perioperative costs associated with the costlier implants and longer operating time.

In our registry, the CRR is higher for primary UKA compared with TKA, indicating a decreased overall survivorship for UKA compared with TKA, a finding consistent with those reported previously [8, 9, 15, 17, 20]. In contrast, no difference was found in the secondary CRR between the rev-UKA and rev-TKA groups. Despite the fact that rev-TKA was technically more challenging compared with rev-UKA in terms of the requirement for stems and metal augmentation at the time of revision surgery and the greater operative time, the secondary survival of rev-UKAs and rev-TKAs was similar. This may reflect the morbidity associated with the rev-TKA and a tendency for the patient to autoprotect the multiply operated prosthetic knee or defer additional surgery, the limitations inherent to a registry if additional revision operations were performed outside the registry capture area, or the relatively small numbers of rerevisions performed in both groups.

In our community registry, rev-UKA is a reliable and relatively economical procedure when performed by the community orthopaedic surgeon. A substantial proportion may be revised to primary TKA designs without the need for stems or metal augmentation, but survival in this population was no better than in the rev-TKA population.