Unicompartmental Knee Arthritis: Unicompartmental Knee Arthroplasty

Mitchell J. Winemaker, M.D., FRCSC
Hamilton, ON

Unicompartmental knee arthroplasty (UKA) is one of several options in the treatment of unicompartmental knee osteoarthritis. Initial interest in UKA has been curtailed by the inferior long-term survivorship in comparison to total knee arthroplasty (TKA)1-4. Some interest in UKA persisted because of the purported functional superiority5 to TKA in terms of range of motion and ease of rehabilitation and revision6-8. A reduced risk of serious complications and infections when compared to TKA has also supported UKA as an attractive alternative9. The improved cost effectiveness of UKA relative to TKA has further been claimed in recent literature10,11.

Minimally invasive surgery has been linked to UKA, promoting an easier recovery, but the results of UKA with MIS surgery may be less favourable than through a standard approach12 and could risk compromise of implant position or fracture13. Computer navigation may; however, improve accuracy when combined with MIS UKA. More recent literature has shown >90% survivorship beyond ten years in carefully selected patients by experienced and appropriately trained surgeons14. These results have been reproduced with both fixed and mobile-bearing designs15-18.

The ideal candidate for UKA is a thin patient with isolated medial compartment disease, mild varus deformity that is passively correctable, well preserved ROM, an intact ACL and normal patellar and lateral compartments (Figure 1a and 1b). Certainly these indications have been expanded by some, including combined ACL reconstruction and UKA in young patients19,20 and osteonecrosis21. Long-term outcomes in the less than ideal patients are unknown and should be pursued with caution22,23. Lateral UKA has shown less favourable outcomes in part due to implant positioning24,25 and with the use of mobile-bearing implants that are more prone to instability in this compartment relative to medial UKA (Figure 2a and 2b)26. Others have shown fixed-bearing lateral UKA to be a viable alternative to TKA in the appropriate patient27.

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Figure 1a: Preop AP and lateral radiograph of left medial unicompartmental osteoarthritis of the knee.

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Figure 1b: Postoperative AP and lateral medial UKA.

Inferior outcomes have been associated with low surgical volumes in the Swedish Knee Registry28. The irony is that careful patient selection improves outcomes but reduces individual surgeon volumes. Only 10% of patients who are arthroplasty candidates meet the inclusion criteria for UKA. The solution to achieving excellent clinical outcomes, by maintaining high surgical volume, and yet careful patient selection, has not been entirely resolved.

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Figure 2a: AP and Lateral radiograph of a lateral UKA with dislocated bearing insert.

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Figure 2b: AP and lateral radiographs of the failed UKA converted to a primary TKA.

The notion that a UKA is half a TKA has been largely dispelled in the last decade29. The techniques differ and separate training in UKA and TKA has been more aggressively advocated30. Better understanding of surgical technique including maintaining a slight undercorrection of the deformity, achieving adequate polyethylene thickness and soft tissue balance have contributed to improved outcomes31-35. Introduction of newer implant designs and instrumentation such as computer navigation may improve future outcomes but more evidence is needed36-38.

For the most part, excellent long-term outcomes of UKA have been reported in elderly, low demand patients9. More recently, with increasing demands for arthroplasty in younger patients, UKA has been advocated as a bone-conserving, time-buying procedure prior to TKA39. One must be cautioned that the trade off of bone conservation may be thinner polyethylene, which has been associated with earlier failure of UKA40. The recent literature is conflicting as to whether the long-term outcomes of UKA are inferior in younger patients1,41-43. The benefit of a more normal feeling knee that can later be revised with outcomes similar to a primary TKA is no doubt attractive. This notion is supported by data from the Swedish Registry44 but still remains controversial45,46. Advocates of high tibial osteotomy (HTO) for unicompartmental disease would argue that UKA is not as durable as HTO and may be just as difficult to revise47. Certainly most would agree that the mode of failure and the duration that failure is undetected or left untreated, determines the ease of converting either an HTO or UKA to a TKA48. Polyethylene wear and tibial loosening and disease progression in other compartments of the knee remain a limiting factor for success in the long-term49-51.

We can look forward to seeing continued use of UKA in the appropriately selected younger patient linked with MIS surgery and computer navigation. The expanded indications will likely shift to more conservative indications. Regional arthroplasty centres will specialize in UKA to combine surgeon experience and volume to achieve optimum care.

References

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