The Journal of Arthroplasty xxx (2014) xxx–xxx

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Oxford Unicompartmental Knee Arthroplasty Versus Age and Gender Matched Total Knee Arthroplasty – Functional Outcome and Survivorship Analysis Jun W. Lim, BMSc, Gerard R. Cousins, MBChB BSc(MedSci) MRCS, Benedict A. Clift, BMSc FRCSOrtho, David Ridley, BSc MSc, Linda R. Johnston, RGN BN MSc Department of Orthopaedics and Trauma, Ninewells Hospital, DD1 9SY Dundee, United Kingdom

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Article history: Received 10 December 2013 Accepted 31 March 2014 Available online xxxx Keywords: unicompartmental knee arthroplasty total knee arthroplasty revision functional outcome survivorship

a b s t r a c t We compared the medium-term outcomes of age and gender matched patients with unicompartmental knee arthroplasty (UKA) and total knee arthroplasty (TKA). We retrospectively reviewed the pain, function and total knee society scores (KSS) for 602 UKAs and age and gender matched TKAs between 2001 and 2013. Function scores remained significantly better in UKAs from preoperative until 3 years follow up. The change of function scores was not significantly different. TKAs performed better than UKAs for pain scores. Total KSS for both groups were not significantly different in the study. Fewer medical complications were reported in UKA group. 6.30% of UKAs and 2.99% of TKAs were revised. The theoretical advantages of UKA were not borne out, other than in immediate postoperative complications. © 2014 Elsevier Inc. All rights reserved.

The surgical treatment of unicompartmental osteoarthritis of the knee remains controversial. In the United Kingdom, unicompartmental knee arthroplasty (UKA) has increased in popularity with The National Joint Registry for England and Wales reporting that UKA comprises 8% of all knee arthroplasties performed [1]. There have been impressive survivorship studies for the Oxford UKA (Biomet Ltd, Bridgend, UK), with a claimed implant survival rate of 95% at ten years [2,3]. UKA offers several potential advantages over total knee arthroplasty (TKA) including the preservation of bone stock and a minimally invasive technique. In principle, the normal biomechanics of the knee are recreated. Weale et al. reported no progression of osteoarthritis in the lateral compartment with medial UKA, which has been considered a significant potential risk [4]. Newman et al. conducted an industry sponsored prospective randomised controlled trials to establish whether UKA outcomes are comparable with TKA. They reported comparable pain at 5 years but also significantly improved function at that stage in the UKA group [5]. The same research group followed up the same cohort of patients at 15 years and suggested that there was an increased revision rate in the TKA group and improved function in the UKA group [6]. A randomised controlled trial – Total or Partial Knee Arthroplasty Trial (TOPKAT) – has been designed to assess the clinical

The Conflict of Interest statement associated with this article can be found at http:// Conflict of Interest: No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Reprint requests: Jun Wei Lim, BMSc, Department of Orthopaedics and Trauma, Level 5, Ninewells Hospital, Dundee, DD1 9SY.

and cost-effectiveness of UKA and TKA [7]. The results of this trial are not available yet, but it is hoped they may lead to evidence based guidance for the treatment of unicompartmental knee osteoarthritis. Contrary evidence regarding the role of UKA exists in the literature. A 2-year follow up patient-reported questionnaire-based study with a knee-specific Knee Injury and Osteoarthritis Outcome Score demonstrated better activities performance with UKA, particularly with regard to the ability to bend the knee, possibly due to the preservation of cruciate ligaments [8]. However, there was a little difference in pain or function between the TKA and UKA groups and there was a higher risk of revision with UKA [8]. A recent comparison of patient recorded outcome measures (PROMS) from the National Joint Registry (NJR) further suggested that there was no difference in knee specific or general health outcomes between TKA and UKA [9]. In addition, Lyons et al. demonstrated that although UKA is associated with better clinical and functional outcomes, both preoperatively and postoperatively, the change of outcome scores is similar in both groups. The survivorship at 5 and 10 years were significantly better in TKA group [10]. The conversion of failed UKA to TKA is believed to be safe, reliable and repeatable [11]. Multi-centre studies have suggested that the revision of UKA to TKA is not technically difficult and the results are comparable to the results of primary TKA [12]. Willis-Owen et al. produced evidence demonstrating that UKA is functionally superior and offers a substantial cost saving over TKA, accounting for both primary UKA and revision of failed UKA to TKA [13]. There is opposing evidence however, such as the study by Padgett et al. that suggested the conversion of a failed UKA carries a significant risk of poor outcomes [14]. The New Zealand National Joint Registry reviewed the rate of 0883-5403/© 2014 Elsevier Inc. All rights reserved.

Please cite this article as: Lim JW, et al, Oxford Unicompartmental Knee Arthroplasty Versus Age and Gender Matched Total Knee Arthroplasty – Functional Outcome and Survivors..., J Arthroplasty (2014),


J.W. Lim et al. / The Journal of Arthroplasty xxx (2014) xxx–xxx

Table 1 Patient Demographics.

Age (years) Gender (F:M) Side (left:right) BMI (kg/m) Hospital stay (days)



70 ± 10 283 : 319 319 : 283 30.19 ± 5.03 4±2

70 ± 10 283 : 319 291 : 311 30.33 ± 5.79 7±4

revision of UKA and reported a relatively poor outcome of revision of UKA to TKA [15]. Chou et al. further demonstrated that the clinical results of revised failed UKAs are inferior to primary TKA [16]. These studies questioned the increasing use of UKA, suggesting that it does not

confer a significant benefit and should not therefore be used as a more ‘conservative’ procedure in the younger population [15,16]. An extensive literature review reveals little further evidence directly comparing UKA to TKA. The relevant practical question is: do the reported improvements in function and recovery offset the possible negative aspects of reduced implant longevity and disappointing outcomes with revision? This study aims to compare the medium-term outcomes of age and gender matched patients treated with UKA and TKA. We also further evaluated the outcomes of any subsequent revision. Methods We retrospectively reviewed all UKAs with an age and sex matched groups of primary TKAs entered into the Tayside Arthroplasty Audit Group (TAAG) prospective database since 2001 [17]. The TKA group were selected from a possible 5146 cases where: • Age was between the minimum and maximum ages in the UKA group; • Date of operation was between the earliest and latest dates of operation in the UKA group; • Surgery was performed at the same hospitals as those in the UKA group.



An individual match for each UKA case was randomly selected from a subset of TKA cases of the same gender and age. Where none existed, age matching was relaxed in increments of 1 year either way until a match or matches was found. When more than one possible match existed, the TKA case was randomly chosen. The functional outcomes for both UKA and TKA groups were obtained (Appendix 1) [18]. The knee society score (KSS) for pain, function and total score were recorded preoperatively and postoperatively for up to 10 years. For this study, the data was compared at 1, 3 and 5 years. Subsequently, we identified all joint arthroplasties, both UKAs and TKAs that required revisions. The functional outcomes were analysed and compared across groups. Medical complications were recorded and compared across the groups. Arrhythmias, ischaemic incidents and heart failure were considered as cardiac complications. Anaemia was classified as a haematology complication. Ileus, vomiting, bowel obstruction, jaundice, hiatus hernia and gastrointestinal bleeding were classified as gastrointestinal complications. Confusion, dizziness, cerebrovascular accident and unresponsive episode were recorded as neurological complications. Urinary tract infection and retention were classified as urinary complications. Respiratory complication signified a lower respiratory tract infection. Any thrombotic or embolic incidents were classified as thromboembolism. Patients with more than 1 complication reported were placed into ‘N1 complications’ category. Wound complications were subdivided into superficial infection, deep infection or “wound problem”, which included a leaking wound, delayed wound healing or formation of haematoma. The mean, standard deviation and range were used for descriptive purposes. Statistical analysis was performed using Statistical Package for the Social Sciences software (SPSS for Windows, Version 20.0). Data was tested for normal distribution using the Kolmogorov–Smirnov test. The Mann–Whitney U test was used to assess the statistical significance between UKA and TKA groups. The Kaplan–Meier test was used for survival analysis, up to 12 years; with 95% confidence intervals (CI) calculated. The comparison of revision-free survival was made using logrank tests. P values less than 0.05 were regarded as statistically significant. Results

C Fig. 1. A: Knee Society Score (Pain) over 5-year. B: Knee Society Score (Function) over 5year. C: Knee Society Score (Total) over 5-year.

602 Oxford UKAs and 5146 primary TKAs were performed in NHS Tayside between 2001 and 2013. A randomised matched group of 602 unconstrained primary TKAs were collected from the TAAG database. All UKAs had a femoral component with a single fixation peg. The TKA implants were: Kinemax (Stryker), Columbus (Braun), and LPS NexGen (Zimmer). Patient demographics are shown in Table 1. The average

Please cite this article as: Lim JW, et al, Oxford Unicompartmental Knee Arthroplasty Versus Age and Gender Matched Total Knee Arthroplasty – Functional Outcome and Survivors..., J Arthroplasty (2014),

J.W. Lim et al. / The Journal of Arthroplasty xxx (2014) xxx–xxx


Fig. 2. Medical complications for both groups.

hospital stay for UKA was 4 days compared to 7 days for the TKA group (P = 0.000). 11.7% (141 of 1204) of patients were lost to follow-up over the 10 years. Of these, 52.5% (74 of 141) had died from unrelated events. There was a 12.0% (72 of 602) rate of loss from the database in the TKA cohort and 11.5% (69 of 602) rate of loss in the UKA group. This rate of loss was not significantly different between the groups. The comparison of KSS for pain, function and total are shown in Fig. 1 (A, B and C). The pre-operative KSS for pain and total scores were not significantly different between UKA and TKA (6.61 vs 6.05, P = 0.219 and 37.58 vs 36.43, P = 0.328 respectively) whereas the preoperative function score was significantly better for the UKA group (55.65 vs 51.10, P = 0.000). There was a general trend for TKA to perform better than UKA for pain scores (Fig. 1A). At one year, the KSS for pain was significantly better in the TKA group (41.08 vs 44.14, p = 0.009). However, it was not significantly different at 3 and 5 years follow up (p = 0.314 and p = 0.064 respectively). Fig. 1B demonstrates that function in the UKA group was superior to TKA preoperatively. The KSS (function) remained significantly better with UKA until 3 years follow up. However, further analysis revealed no statistically significant difference in the change of function scores over

Fig. 3. Kaplan Meier cumulative survival analysis of unicompartmental knee arthroplasty and total knee arthroplasty.

time for both UKA and TKA (p = 0.487, p = 0.319, p = 0.453). The total KSS for UKA and TKA were not significantly different at any point of the 5-year study (Fig. 1C). The medical complications are shown in Fig. 2. In the majority of complication categories, there were fewer complications in the UKA group. There was one intra-operative vascular injury where the popliteal artery was damaged during UKA. There was one intraoperative neurological injury where the patient had a neuropraxia of the common peroneal nerve after TKA. All patients were given chemical thromboprophylaxis postoperatively. However, there were 5 thromboembolic events in each group. There were no immediate postoperative or intraoperative deaths. There were fewer wound complications in the UKA group. In patients who had undergone UKA, there were 1 deep infection, 7 superficial infections and 5 wound problems. The TKA group had 4 patients with deep infection, 10 patients with superficial infection and 6 patients with wound problems. Despite the higher wound complication rate in TKA group, all superficial infections resolved within 6 weeks in both groups. All the deep infections required washout but none in TKA group required revision whereas 1 in UKA group required revision surgery. No revision surgery was reported for patients with wound problems. At 12 years, of the 602 UKAs recorded, 38 (6.30%) had required revision (95% CI 10.47–11.13) whereas of the 602 TKAs recorded, only 18 (2.99%) had required revision (95% CI 11.08–11.44). There was a statistical difference between UKA and TKA in the Kaplan-Meier survival analysis (P = 0.012) (Fig. 3). The reasons for revision surgery are shown in Fig. 4. The main cause for revision surgery in the UKA group was aseptic loosening, followed by osteoarthritis of the contralateral compartment. Further analysis by the senior author (GC, BC) revealed that tibial component loosening accounted for most of the aseptic loosening (Table 2). Further analysis was not possible in 4 cases recorded as ‘aseptic loosening’ due to unavailability of preoperative x-rays. Interestingly, more TKAs (4 TKAs vs 1 UKA) were revised due to technical error such as rotational or other malalignment of the implants. The outcome scores after revision at 1-year follow-up (Table 3) suggested that UKAs have a worse function score, but a better total KSS after the revision surgery. The preoperative and postoperative pain, function and total KSS were not statistically different. Furthermore, the low number of cases (n = 11 for TKA) suggested that the statistical analysis should be interpreted with caution.

Please cite this article as: Lim JW, et al, Oxford Unicompartmental Knee Arthroplasty Versus Age and Gender Matched Total Knee Arthroplasty – Functional Outcome and Survivors..., J Arthroplasty (2014),


J.W. Lim et al. / The Journal of Arthroplasty xxx (2014) xxx–xxx

Fig. 4. Reasons of revision for unicompartmental knee arthroplasty.

Discussion To our knowledge, this is the first paper to compare the outcome of age and gender matched UKA and TKA groups. We found no significant difference in pain or total KSS for both UKA and TKA groups. The function scores were significantly different preoperatively between the groups, and this continued throughout the period of analysis, presumably due to more widespread arthritis being present in the knees of the TKA group. There is inevitably a preoperative selection bias. Bremner-Smith et al. have suggested that the function score was in general more likely to be influenced by demographic variables and tended to fall with advancing age in an elderly population [19], a factor accounted for by age-matching the two groups in this study. When change in function over time was analysed there was no difference between the groups. This result is contrary to reports of improved function with UKA [6]. Our data suggested that there was peak improvement in all pain, function and total KSS at 1 year with a gradual decline in function score after 1 year, irrespective of which implant was used. On the other hand, the pain and total KSS remained relatively constant postoperatively. Although not statistically significant, the pain score tended to improve more and remain better in the TKA group. TKA was associated with more medical complications than the UKA group, especially gastrointestinal, respiratory and ‘N1’ complications. Furthermore, TKA was associated with a higher rate of wound problems, although none of these TKAs required revision surgery. Interestingly, the UKA group was associated with a higher rate of cardiac complications. The incidence of thromboembolism was similar in both groups. This finding is contrary to existing literature, which suggests that TKA carries a higher risk of thromboembolic events than UKA [20,21]. The hospital stay recorded in this study was before the widespread introduction of enhanced recovery protocols. Therefore the difference may now be less marked and of less relevance. The lower rate of medical complications in UKA may have implications when deciding on which procedure to

Table 3 Revision Score.

Time (Years)

Table 2 Breakdown of Aseptic Loosening Group.

Both components Tibial component Femoral component Without preoperative revision X-ray

recommend in a physiologically frail individual. With respect to revision, our data demonstrated that UKA did not necessarily prevent the progression of osteoarthritis in other compartments as it was a relatively common reason for revision [4]. The UKA group had a two-fold increase in the rate of revision compared to the age and gender matched TKA group. The reasons for UKA revision are consistent with the literature [22]. It has been suggested that the higher incidence of revision of UKA is often due to a generally lower threshold for revision in UKA and poor patient selection. We found no difference in preoperative revision scores in both groups, suggesting that the revision thresholds were the same. Furthermore, the most common reason for revision was aseptic loosening which was unlikely to be a direct result of poor patient selection. We believe that the high aseptic loosening rates could be secondary to technical difficulty. As a non-specialist institute, our UKA revision rate is considerably higher than the originating centre, which is consistent with the literature [16]. This may prompt further evaluation of the role of UKA in non-specialist centre. The study is however a ‘pragmatic’ trial which reflects actual clinical practice by a mixed group of surgeons in the National Health Service. All the surgeons had undergone comprehensive specific training in the use of the UKA implant, as well as the generic United Kingdom orthopaedic training. Current literature suggests that if revision is required for UKA, a good outcome can be achieved by conversion to TKA [11]. The numbers were too small in this study to provide meaningful statistics. However, the trend was for UKAs that required revision to have worse function scores than the TKAs that required revision. This finding is consistent with previous studies that have shown poorer outcomes than expected for UKAs that required revision [15]. The authors acknowledge that the current study has certain limitations. Firstly, due to the decreasing numbers of patients as time



3 6 3 4

1 2 1 0



Knee Society Score Pain Function Total Pain Function Total









P Value

32 32 29 25 25 25

4.06 49.84 38.79 30.60 63.20 73.84

0–30 0–100 0–73 0–50 0–100 32–100

11 10 10 8 8 8

4.55 57.00 34.36 30.63 65.00 63.00

0–10 30–90 18–55 10–50 0–100 9–98

0.483 0.475 0.308 0.853 0.757 0.665

Please cite this article as: Lim JW, et al, Oxford Unicompartmental Knee Arthroplasty Versus Age and Gender Matched Total Knee Arthroplasty – Functional Outcome and Survivors..., J Arthroplasty (2014),

J.W. Lim et al. / The Journal of Arthroplasty xxx (2014) xxx–xxx

progressed, partly due to the loss of patients and despite the cohort size, a relatively small number who have reached the relevant stage, long-term deductions about implant survival need to be interpreted with caution. Despite that, the performance for UKAs was still inferior to TKAs in the Kaplan Meier cumulative survival analysis at 5 years. Secondly, due to the low number of TKAs that required revision in the current study, no conclusion can be made regarding the comparative outcomes of revisions. Furthermore, TAAG does not record preoperative co-morbidity and we are unable to comment about the postoperative complications profile in each group. We therefore made the assumptions that both cohorts are in similar fitness level with the matching of patients. In conclusion, our study suggests that UKA is associated with fewer postoperative complications, however TKA provides better initial pain relief and is less likely to require revision. The revision rate for UKA was twice as much as TKA. The theoretical advantages of UKA are not borne out by the findings in this study other than immediate postoperative complications. Acknowledgments The authors would like to thank the Tayside Arthroplasty Audit Group team who have contributed data to this study. The authors are grateful to Dr. Weijie Wang for statistical support and Mr. Ian Christie for image illustrations. Appendix 1. Knee Society Score Category

Knee Society Score

Excellent Good Fair Poor

80-100 70-79 60-69 b60

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Please cite this article as: Lim JW, et al, Oxford Unicompartmental Knee Arthroplasty Versus Age and Gender Matched Total Knee Arthroplasty – Functional Outcome and Survivors..., J Arthroplasty (2014),

Oxford unicompartmental knee arthroplasty versus age and gender matched total knee arthroplasty - functional outcome and survivorship analysis.

We compared the medium-term outcomes of age and gender matched patients with unicompartmental knee arthroplasty (UKA) and total knee arthroplasty (TKA...
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