The mid-term outcomes of the Oxford Domed Lateral unicompartmental knee replacement J. S. Weston-Simons, H. Pandit, B. J. L. Kendrick, C. Jenkins, K. Barker, C. A. F. Dodd, D. W. Murray From Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, Botnar Research Centre, University of Oxford, Windmill Road, Headington, Oxford, OX3 7LD, United Kingdom  J. S. Weston-Simons, MRCS (Eng), Orthopaedic ST4  H. Pandit, FRCS (Orth) DPhil, Orthopaedic Surgeon, Honorary Senior Clinical Lecturer  B. J. L. Kendrick, FRCS (Orth) DPhil, Arthroplasty Fellow  D. W. Murray, MD, FRCS(Orth), Professor of Orthopaedic Surgery Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences,, Botnar Research Centre, University of Oxford, Windmill Road, Headington, Oxford, OX3 7LD, UK.  C. Jenkins, MCSP MPhil, Research Physiotherapist  K. Barker, PhD MCSP, Clinical Director, Orthopaedics  C. A. F. Dodd, FRCS, Consultant Orthopaedic Surgeon Nuffield Orthopaedic Centre, Oxford University Hospitals Trust, Headington, Oxford, OX3 7LD, UK. Correspondence should be sent to Professor D. W. Murray; e-mail: [email protected]

©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B1. 31630 $2.00 Bone Joint J 2014;96-B:59–64. Received 15 January 2013; Accepted after revision 28 August 2013

Mobile-bearing unicompartmental knee replacements (UKRs) with a flat tibial plateau have not performed well in the lateral compartment, owing to a high dislocation rate. This led to the development of the Domed Lateral Oxford UKR (Domed OUKR) with a biconcave bearing. The aim of this study was to assess the survival and clinical outcomes of the Domed OUKR in a large patient cohort in the medium term. We prospectively evaluated 265 consecutive knees with isolated disease of the lateral compartment and a mean age at surgery of 64 years (32 to 90). At a mean follow-up of four years (SD 2.2, (0.5 to 8.3)) the mean Oxford knee score was 40 out of 48 (SD 7.4). A total of 13 knees (4.9%) had re-operations, of which four (1.5%) were for dislocation. All dislocations occurred in the first two years. Two (0.8%) were secondary to significant trauma that resulted in ruptured ligaments, and two (0.8%) were spontaneous. In four patients (1.5%) the UKR was converted to a primary TKR. Survival at eight years, with failure defined as any revision, was 92.1% (95% confidence interval 81.3 to 100). The Domed Lateral OUKR gives good clinical outcomes, low re-operation and revision rates and a low dislocation rate in patients with isolated lateral compartmental disease, in the hands of the designer surgeons. Cite this article: Bone Joint J 2014;96-B:59–64.

Unicompartmental knee replacement (UKR) provides good clinical outcome for the treatment of isolated compartment osteoarthritis (OA) of the knee.1 The available designs can generally be classified as either fixed or mobile bearings. The latter have the advantage of lower linear wear rates compared with their fixed-bearing counterparts.2 This is important in the context of isolated lateral compartment OA, as during knee flexion and rotation there is a large amount of movement of the lateral femoral condyle on the tibia.3,4 The initial experience with the Oxford UKR (OUKR) (Biomet UK Ltd, Swindon, United Kingdom) in the lateral compartment was not a success. A review by Gunther et al5 of Phase 1 and Phase 2 OUKRs reported an eight-year survival rate of 76% and recommended that mobile bearings should not be used in the lateral compartment. The primary cause of failure was the high rate of bearing dislocation in the first year (11%). A subsequent radiological review of the causes of bearing dislocation in the OUKR showed that the joint line was significantly more elevated in cases with dislocation than in the rest.6 In order to overcome this problem, the surgical technique was modified by taking

VOL. 96-B, No. 1, JANUARY 2014

care not to remove too much bone from the distal femur, nor to overtighten the knee by using too thick a bearing, as both serve to raise the joint line. Further modifications included internally rotating the vertical tibial cut, and using a short lateral parapatellar incision without dislocating the patella.2 However, dislocations continued to occur, albeit at a significantly reduced rate.2 This is principally because the lateral compartment in flexion distracts by about 7 mm (compared with 2 mm on the medial side).7 Additionally, in high knee flexion the lateral femoral condyle subluxes posteriorly and inferiorly off the back of the convex lateral tibial plateau.8 The Phase 1, 2 and 3 OUKRs had lateral bearings that were 5 mm higher at the front and back than at the centre, and therefore have 5 mm of entrapment meaning that for anterior or posterior dislocation 5 mm of distraction of the knee was necessary, which could readily occur. In order to reduce the likelihood of dislocation, a new Domed Lateral OUKR (Biomet UK Ltd) was developed. It has a spherically convex, domed tibial plateau, and a biconcave bearing that has fully congruent contact with both femur and tibia in all positions of flexion of the knee. The biconcave 59



bearings have 7 mm entrapment anteriorly and posteriorly with the specific aim of reducing the dislocation rate. An additional advantage of the domed tibia is that it more closely mimics the anatomical morphology, and so should help retain normal kinematics. We have previously reported a comparison of dislocation rates and clinical outcome of the Domed OUKR with the previous Phase 1, 2 and 3 OUKRs implanted in the lateral compartment.2 This provided evidence that the new domed bearing and the new surgical technique have led to a lower dislocation rate and had good clinical outcomes in the short term. A more recent publication from an independent series demonstrated a survival at three years of 94% (95% CI: 82 to 98), with a cumulative incidence of dislocation of 6.2% (95% CI 2.0 to 17.9).9 A further study reported excellent clinical outcomes with no dislocations.10 However, these studies reported on a relatively small number of cases with a short follow-up. Our study was undertaken to evaluate the longer-term survival and clinical outcome of a larger series of Domed Lateral OUKRs in the designers’ hands.

Anteroposterior radiograph demonstrating the progression of medial compartmental osteoarthritis in a knee that required revision at six years and six months.

Patients and Methods Between September 2004 and July 2012 we performed a consecutive series of 265 Domed OUKRs in 258 patients. There were 91 men and 167 women with a mean age of all patients at surgery of 63.6 years (32 to 90). All operations were carried out by, or in the presence of, the designer surgeons (DWM and CAFD). The primary indication was OA of the lateral compartment with bone articulating on bone, although there were a small number of knees that had other indications, including osteonecrosis of the femoral condyle (four cases), pigmented villonodular synovitis (one) and a patient who had had a previous femoral osteotomy. The anterior cruciate ligament (ACL) was functionally intact in all cases and there appeared to be full-thickness cartilage in the medial compartment. Any intra-articular valgus deformity was correctable. The state of the patellofemoral joint was not considered to be a contraindication unless there was bone loss and patellar subluxation. All procedures were carried out using the modified minimally invasive surgical (MIS) technique for the Phase 3 Domed OUKR.2 This involved a lateral parapatellar approach, internal rotation of the tibial component, anatomical positioning of the femoral component, and selection of the bearing thickness in full extension. All components were secured with polymethylmethacrylate cement. Patients were assessed clinically annually by an independent physiotherapist (CJ), including the Oxford knee score11 (from 0 (worst outcome) to 48 (best outcome)), the American Knee Society score functional (AKSS-F) and objective (AKSS-O)12 and the Tegner activity score.13 Radiological assessment. Post-operative radiographs were taken with either a fluoroscope or digital radiography system. The anteroposterior (AP) radiographs were aligned with the tibial component and the lateral radiographs were

aligned so that the femoral condyles were mutually superimposed.14 Serial radiographs were assessed for evidence of component loosening, the presence, type and extent of radiolucencies, and for progression of OA in the medial compartment. Radiographs were reviewed independently by two observers who were blinded to the clinical outcomes (JSWS and BJLK). The inter-observer and intra-observer repeatability for radiological assessment was assessed by the unweighted κ statistic. Radiolucencies were analysed and classified as pathological or physiological, based on descriptions by Goodfellow et al.15,16 A physiological radiolucency is defined as being a non-progressive radiolucency < 2 mm (usually 1 mm) thick, with a sclerotic margin, in contrast to a pathological radiolucency, which is progressive, poorly defined and > 2 mm thick. As all radiographs were taken in a standardised fashion, subsidence of the tibial component (diagnostic of implant loosening) could be detected by comparing the immediate post-operative film with subsequent images. Progression of arthritis in the retained medial compartment was assessed using the Ahlbäck scoring system.17 Statistical analysis. We used SPSS v18.0 software (SPSS Inc., Chicago, Illinois). The data were found to be not normally distributed and were therefore analysed using Wilcoxon’s signed ranks test for discrete data and the Mann–Whitney U test for continuous data. Survival, with failure defined in various different ways was assessed using the life table method, with 95% confidence intervals (CI) calculated using the method described by Peto et al.18 Revision was defined as any further operative intervention

Fig. 1




Table I. Summary of patient data for all complications requiring a revision (ACL, anterior cruciate ligament; UKR, unicompartmental replacement; TKR, total knee replacement; OA, osteoarthritis) Patient/Gender/Age (yrs)

Time to failure (mths)

Reason for revision

1 / M / 68 2 / M / 57 3 / F / 59 4 / F / 65 5 / F / 68

1 5 5 12 19

6 / F / 42 7 / M / 55

22 34

8 / F / 69 9 / M / 45 10 / F / 75

42 49 51

11 / F / 49 12 / F / 58

51 78

Early infection requiring bearing exchange and washout Secondary bearing dislocation managed with bearing exchange Spontaneous bearing dislocation managed with bearing exchange and screws Delayed infection requiring revision Traumatic bearing dislocation managed with bearing exchange and screws with associated ACL injury Spontaneous bearing dislocation managed with bearing exchange and screws Bearing exchanged when explored for continuing pain. Previous meniscectomy suture removed and posterolateral corner debridement after which symptoms improved Medial UKR performed for progression of OA Revised to TKR at another centre for continuing pain Post-traumatic infection, patellar fracture managed with ORIF, progression of medial OA leading to TKR Femoral component revision due potential overhang as the cause of pain Progression of medial OA requiring TKR

that included the change or addition of any knee replacement component or bearing exchange as part of the procedure. A p-value < 0.05 was considered significant.

Results The mean follow-up was 4.1 years (0.5 to 8.3). Of the cohort, eight patients (eight knees) died from unrelated medical conditions. There were two recorded postoperative deep-vein thromboses (two knees) one of which was associated with a pulmonary embolus, confirmed by CT pulmonary angiogram. These were appropriately medically managed without further complication. One knee required a manipulation under anaesthesia (MUA) and an arthroscopy for clearance of synovitis three months after the initial surgery, after which there were no further complications (with an OKS of 45 at last follow-up one year post-operatively). His range of movement improved from 60° before MUA to 125° at last follow-up. One knee was revised to a TKR because of symptomatic progression of OA in the medial compartment (Fig. 1), and another patient had a revision to TKR performed at another centre for continuing pain, although previous investigations had not illustrated a mechanical or pathological cause for his pain. This patient was unfortunately lost to further follow-up. A further patient had a medial UKR performed because of progression of OA in that compartment (Table I). Post-operative infection occurred in three knees. One occurred within four weeks of the operation and was treated with washout, bearing exchange and intravenous antibiotics. A further infection occurred at one year and was treated with washout, debridement, bearing exchange and intravenous antibiotics. Both of these cases required no further interventions that we are aware of, with one making a good recovery with an OKS of 38 at one year post intervention, and the other lost to follow-up. One patient underwent a two-stage revision for an infection that VOL. 96-B, No. 1, JANUARY 2014

Fig. 2 Post-operative anteroposterior radiograph showing the technique for treating bearing dislocation of the polyethylene bearing.

occurred four years after the original procedure. This probably was related to the treatment of an unrelated ipsilateral patellar fracture that was managed by tension band wiring and subsequently became infected. Four patients sustained a dislocation of the mobile bearing (an overall dislocation rate of 1.5%). These were managed by the method described by Weston-Simons et al,19 in which a new bearing is inserted and two or three screws are inserted with their heads above the medial wall of the tibial plateau to prevent recurrent dislocation (Fig. 2). Two of the patients (0.8%) had dislocations secondary to trauma. One patient fell down stairs and the other stepped into a rabbit hole and fell. At exploration in both there was evidence of damage to the ACL and to the



Table II. Life table demonstrating overall survival for the cohort, with failure defined as any revision. Years since operation

Number at start


At risk

Cumulative survival (%, 95% CI)

0 to 1 1 to 2 2 to 3 3 to 4 4 to 5 5 to 6 6 to 7 7 to 8 8 to 9

265 243 203 174 138 98 65 35 8

4 2 1 1 3 0 1 0 0

256 224 189 157 120 82 51 22 4

98 (96.9 to 100) 98 (95.6 to 99.5) 97 (94.7 to 99.4) 96 (93.6 to 99.3) 94 (89.9 to 98.1) 94 (89.0 to 99.0) 92 (85.1 to 99.2) 92 (81.3 to 100) 92 (67.0 to 100)


Cumulative survival (%)

90 80 70 60 50 40 30 20 10 0











Years since operation

Fig. 3 Graph demonstrating cumulative survival for the cohort with 95% confidence intervals.

posterolateral corner of the tibial plateau. In one of the other two cases the dislocation was asymptomatic and was identified on a routine radiograph at one year. Despite having screws inserted, this patient continued to dislocate. The tibial plateau was removed and a fixed-bearing plateau inserted. This had a good outcome at two months with knee flexion to 110 degrees but subsequently the patient was lost to follow-up. A further two patients required bearing exchange as part of explorations for continuing mechanical pain. One patient’s symptoms resolved after the knee was explored for continuing pain, when retained suture material from a previous lateral meniscectomy was removed and an area of the posterolateral corner that might have been causing impingement was trimmed. In the other patient the femoral component was revised; it was felt that the initial component was overhanging as it was one size too large. However, she has subsequently been diagnosed with chronic regional pain syndrome and is being managed medically. The complications are summarised in Table I.

Survival. Overall survival at eight years, considering all revisions as failures, was 92.1% (95% CI 81.3 to 100) (Table II) (Fig. 3). If revisions and reoperations are considered to be failures, the survival is still 92%. If the case of the revision for the infection secondary to the infected patellar wiring is excluded, survival at eight years improves to 92.9% (95% CI 82.2 to 100) and if septic revisions are excluded eight-year survival is 93.5% (95% CI 82.6 to 100). If conversion to primary TKR is taken as the endpoint,1 survival at eight years is 96.4% (95% CI 88.7 to 100). If dislocation is the endpoint the survival at eight years is 98.3% (95% CI 92.3 to 100). It was notable that the overall survival of the first 50 knees was 90.0% (95% CI 81.7 to 98.3) at six years, at which time the remainder of the cohort’s survival was 96.1% (95% CI 91.8 to 100). Patient-reported outcome measures. There was a significant improvement in all clinical scores (Wilcoxon’s signed ranks test, p < 0.001) (Table III), but there was no significant difference, (Mann-Whitney U, OKS: p = 0.863; AKSS-O p = 0.558, AKSS-F: p = 0.254, Tegner: p = 0.567), between THE BONE & JOINT JOURNAL



Table III. Summary of outcomes for the cohort (CI, confidence interval) Pre-operative *






95% CI




95% CI


24.1 47.8 68.2 2.2

8.6 18.3 18.9 1.2

5 to 46 2 to 95 10 to 100 0 to 7

23.1 to 25.9 43.1 to 48.7 66.2 to 72.3 2.1 to 2.5

40.3 85.6 83.0 2.9

7.8 16.4 21.0 1.2

9 to 48 27 to 100 -10 to 100 0 to 8

39.4 to 42.1 84.5 to 90.1 82.6 to 88.9 2.8 to 3.2

* OKS, Oxford knee score; AKSS-O/-F, American Knee Society score – objective/function

Fig. 4 An antero-posterior radiograph showing a knee eight years post-operatively with an OKS of 47.

the most recent clinical scores for the first 50 patients (OKS: 42.2, SD: 4.8; AKSS-O: 89.1 SD: 14.1; AKSS-F: 85.9, SD: 19.1; Tegner: 3.3, SD: 1.3) when they were compared with the remainder of the cohort (OKS: 40.5, SD: 7.6; AKSS-0: 87.0, SD: 15.2; AKSS-F: 85.7, SD: 16.7; Tegner: 3.0 SD: 1.1) (Fig. 4). Radiological analysis. The inter-observer agreement for the radiological analysis was 0.679 and intra-observer reliability was 0.742. There was no evidence of component loosening in the cohort. In all six patients had evidence of complete radiolucency (2.3%), and 13 (4.9%) had partial radiolucency. Of the partial radiolucencies, 12 involved the far lateral zones (1 and 2) with a single radiolucency involving the keel. All radiolucencies were < 1 mm thick. None were progressive or associated with implant loosening.

Discussion This study has demonstrated that patients with isolated OA of the lateral compartment treated with the Domed OUKR can expect a successful clinical outcome and survival in the mid-term. Overall survival with revision as end point of 92.1% (95% CI: 81.3 to 100) at eight years is comparable with VOL. 96-B, No. 1, JANUARY 2014

that of other forms of UKR,20,21 and better than that reported for the previous version of our design.5 The advantages of UKR over TKR include the maintenance of normal knee kinematics, a shorter hospital stay and faster recovery.22 In addition, the associated morbidity and mortality are significantly less than with TKR.23 Although it appears that the survival is less than that of TKR,23 a direct comparison cannot be made as the types of revision are different. The survival improved or the surgeon became more experienced. In this cohort of Domed Lateral UKRs the mean OKS was 40 (9 to 48). This is similar to what has been achieved in other series of medial and lateral UKRs.1,24 There are no comparable series of TKR performed for isolated lateral UKR with reported OKS, but in general the OKS following TKR tends to be substantially less than 40.25,26 The Domed OUKR was introduced in a response to the high dislocation rate seen with previous designs and the unmodified surgical techniques.2 This study has confirmed that the biconcave bearing and convex tibial component have substantially reduced the dislocation rate. In the hands of the designer surgeons the primary dislocation rate is less than 1% at eight years. However, in an independent study of 50 Domed lateral UKRs there were three dislocations.9 Further work therefore needs to be done to ensure that all surgeons achieve similarly low rates of dislocation to that of the designer surgeons. All the dislocations occurred within the first two years and all occurred medially, which is in line with previous reports.9,27 The dislocation occurs in a standard pattern, with the bearing subluxing so that its medial side is on top of the tibial component wall in the intercondylar notch, and with the remainder in the lateral compartment. The observation that putting screws into the tibial eminence so their heads are just above the wall tends to prevent recurrent dislocation suggests that further modifications to the initial implantation procedure could reduce the dislocation rate further. In this designer study our cumulative mid-term primary dislocation rate was less than 1%. This large series demonstrates that good mid-term survival and clinical outcomes can be achieved with the Domed OUKR when used to treat isolated lateral compartmental OA. However, some concern still exists over the dislocation rate in nondesigner series. Further work is required to address this.



The authors wish to thank B. E. Marks and J. Brown for their assistance with this study. The study was supported by the NIHR Biomedical Research Unit into Musculoskeletal Disease, Nuffield Orthopaedic Centre, and the University of Oxford. Financial support has been received from Biomet. The author or one or more of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund, foundation, educational institution, or other non-profit organisation with which one or more of the authors are associated. This article was primary edited by D. Rowley and first-proof edited by G. Scott.

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The mid-term outcomes of the Oxford Domed Lateral unicompartmental knee replacement.

Mobile-bearing unicompartmental knee replacements (UKRs) with a flat tibial plateau have not performed well in the lateral compartment, owing to a hig...
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