Unicompartmental Knee Arthroplasty - An Option for Treating Unicompartmental Osteoarthritis

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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 its purported functional superiority5 to TKA in terms of range of motion (ROM) and ease of rehabilitation and revision.6-8 A reduced risk of serious complications and infections when compared to TKA has also supported UKA as an attractive alternative.9 The improved cost effectiveness of UKA relative to TKA has further been claimed in recent literature.10,11

Minimally invasive surgery (MIS) 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 fracture.13 Computer navigation may, however, improve accuracy when combined with MIS UKA. More recent literature has shown >90% survivorship beyond 10 years in carefully selected patients by experienced and appropriately trained surgeons.14 These results have been reproduced with both fixed and mobile-bearing designs.15-18

Indications

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 (Figures 1a-b). Certainly these indications have been expanded by some, including combined ACL reconstruction and UKA in young patients19,20 and osteonecrosis.21 Long-term outcomes in less-than-ideal patients are unknown and should be pursued with caution.22,23

  
Figure 1a. Preop AP (left) and lateral radiographs of left medial unicompartmental osteoarthritis of the knee

  
Figure 1b. Postoperative AP (left) and lateral medial UKA


Lateral UKA has shown less favourable outcomes in part due to implant positioning,24,25 as well as with the use of mobile-bearing implants that are more prone to instability in this compartment relative to medial UKA 26 (Figures 2a-b). Others have shown fixed-bearing lateral UKA to be a viable alternative to TKA in the appropriate patient.27

  
Figure 2a. AP (left) and lateral radiographs of a lateral UKA with dislocated bearing insert

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


Inferior outcomes have been associated with low surgical volumes in the Swedish Knee Registry.28 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 carefully selecting patients – has not been entirely resolved.

Technique

The notion that a UKA is half a TKA has been largely dispelled in the last decade.29 The techniques differ, and separate training in UKA and TKA has been more aggressively advocated.30

Better understanding of surgical technique – including maintaining a slight undercorrection of the deformity and achieving adequate polyethylene thickness and soft tissue balance – have contributed to improved outcomes.31-35 Introduction of newer implant designs and instrumentation such as computer navigation may improve future outcomes, but more evidence is needed.36-38

Outcome

For the most part, excellent long-term outcomes of UKA have been reported in older, low-demand patients.9 More recently, with increasing demands for arthroplasty in younger patients, UKA has been advocated as a bone-conserving, time-buying procedure prior to TKA.39 One must be cautioned that the trade-off of bone conservation may be thinner polyethylene, which has been associated with earlier failure of UKA.40

The recent literature is conflicting as to whether the long-term outcomes of UKA are inferior in younger patients.1,41-43 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 controversial.45,46

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 revise.47 Certainly most would agree that the mode of failure and the length of time that failure is undetected or left untreated determine the ease of converting either an HTO or UKA to a TKA.48 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.

Conclusion

We can look forward to seeing continued use of UKA in appropriately selected younger patients 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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Reprinted with permission from the Summer 2008 issue of COA Bulletin

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