Unicompartment Knee Arthroplasty is Superior to Total Knee Arthroplasty for Functional Outcome

Geoffrey F Dervin, M.D., MSC, FRCSC
Head, Division of Orthopaedic Surgery, The Ottawa Hospital
Chairman, Division of Orthopaedic Surgery, University of Ottawa
Ottawa, ON

Unicompartmental knee arthroplasty (UKA) is now being reconsidered by a new generation of orthopaedic surgeons for the treatment of unicompartmental osteoarthritis (OA). Minimally invasive surgery techniques and comparable survivorships with fixed1 and mobile-bearing designs2,3 have permitted decreased short-term morbidity and faster recuperation4,5 allowing even outpatient procedures for healthy patients.


Patient expectations now extend beyond pain control, and are more rooted in functional preservation or restoration, not often realized with total knee arthroplasty (TKA). A survey with validated self-administered questionnaire confirmed increased limitation of functional activities involving the knee in 52% of patients who had TKA compared to 22% of the age- and gender-matched patients with no previous knee disorders6. Further, satisfaction with the outcome of TKA was highly variable and 14% were "dissatisfied" or "very dissatisfied"7. It was further asserted that satisfaction with TKA is primarily determined by meeting patient expectations, and not the absolute level of function. For younger patients, the treating surgeon has the double challenge of restoring the desired activity and performance while being mindful of the increased revision rate8, so why chose UKA?

Clinical Outcome
There is paucity of data for any direct prospective comparisons of UKA and TKA but Newman et al9 randomized 102 patients (mean age 69) suitable for either a UKA or TKA after arthrotomy. Patients in the UKA group showed less perioperative morbidity, regained knee movement more rapidly and were discharged from hospital sooner. At five years, two UKAs and one TKA had been revised; another TKA was radiologically loose. Pain relief was good in both groups but the number of knees able to flex ≥ 120º was significantly higher in the UKA group (p < 0.001) and there were more excellent results in this group. Ackroyd et al showed comparable survivorship in a non-randomized study between fixed-bearing UKA St Georg Sled and Kinemax knees at ten years with better flexion in the UKA group10. Amin at al11 also found higher postoperative range of motion in mobile-bearing UKA vs. matched cases of TKA. Walton et al12 also confirmed in a retrospective comparative study that patients with UKA had better activity and Oxford knee scores with increased return to sport than age, gender and preop activity matched controls undergoing TKA. Willis-Owen et al13 have recently shown in a case control study that both medial and lateral UKA patients scores were indistinguishable from normal age - gender specific controls and clearly superior to TKA using the validated TKQ14.

Fixed vs. Mobile
A major consideration for the surgeon is whether to consider fixed- or mobile-bearing UKA.

In randomized studies of medial UKA, Li et al15, Gleeson, et al16 and Confalonieri et al17 were unable to show any difference in functional outcomes of fixed-bearing vs. mobile-bearing UKA. Gleeson did find that the pain component of the Bristol Knee Score was significantly better for the fixed-bearing group (St Georg Sled) and there was a higher early revision rate in the Oxford group. Conversely, the improved mobile-bearing kinematics may favour the latter in longer term survivorship based on retrieval studies18,19. Further, Li15 described improved kinematics of mobile-bearing (Oxford) UKA compared with patients treated with fixed-bearing (Miller-Galante) with a lower incidence of radiolucency at the bone implant interface (8% vs. 37%, p < 0.05).

The functional results and outcomes for younger patients with fixed and mobile-bearing UKA would appear to be acceptable. Pennington et al20 reported on 41 consecutive patients 60 years of age or younger with Miller-Galante fixed-bearing UKR. At mean follow-up of 11 years, the Hospital for Special Surgery (HSS) score was excellent in 93% of cases. Though nine knees had progression of arthritis in the unresurfaced compartment, none of these knees were revised, and none of the patients had deterioration in the HSS Score yielding 11-year survivorship of 92%. Price et al21 reported the experience for the Oxford knee related to patient's age and showed the ten-year all-cause survival for patients in the under 60 years of age group was 91% (95% CI 12) with mean HSS score at ten-year follow-up of 94 out of 100. Indeed it is this author's observation that some of the greatest benefits lie with active patients in their 50s who can benefit from the permissive activities of a well functioning UKA without sacrificing future conversion to TKA (Figures 1a,b).


Figure 1: Postoperative AP (a) and lateral (b) radiographs in a 57-yr-old male seven years after mobile-bearing UKA for medial osteoarthritis of the knee.

What about Revision?
Of particular interest for the young active patient is the potential need for future revision. Several authors have confirmed that the majority of revisions of well performed UKA can be to primary components22-24 with comparable results to primary TKA. The evidence would also suggest that contrary to proximal tibia osteotomy survivorship, UKA shows greater longevity in a relatively undercorrected coronal alignment so as to minimize contralateral compartment wear (Figure 2). The benefits of smaller surgical exposures with UKA seem to cause less scarring in the joint and pretibial region with significantly less postoperative patella baja25 easing exposure for later revisions. Robertsson et al26 surveyed Swedish patients operated on between 1981 and 1995 and found no difference in proportions of satisfied patients whether they had primarily been operated on with a TKA or a medial UKA; although those patients revised from medial UKA were more satisfied than those patients revised from primary TKA.


Figure 2: Standing AP radiographs two years post left medial mobile-bearing UKA in a 53-year-old man showing preservation of the patient's natural coronal tibiofemoral alignment.

Registry data show the usage of UKA to be at 7-8 % of all knee arthroplasty - significantly less than the 25% suggested by high volume and designing surgical centres. For the surgeon treating medial or less commonly lateral unicompartment OA, there is clearly wide variation in the adoption and utilization of this technique. Nonetheless, the reported experience supports Scott27 who claimed "Unicompartmental knee arthroplasty is the right operation for the right patient when performed by the right surgeon using the right surgical technique."


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