Introduction 1 4 5 6 4 7 8 9 Because of the favourable published clinical results, surgeons at the Martini Hospital in Groningen, the Netherlands, began using the Oxford knee prosthesis in 1998. The goal of this independent prospective study for patients 60 years of age and above was to compare and evaluate the clinical midterm results of the Oxford phase-3 unicompartmental knee replacement using the minimally invasive technique in a community hospital. Materials and methods 1 Table 1 Oxford phase-3 unicompartmental knee replacement Criteria Results Number of patients 132 Number of knees 154 Left/right knee (%) 53.8/46.2 Age (mean/range, in years) 69.2 (60–93) Gender (M/W) 57 (40%)/86 (60%) BMI 30.7 ± 4.9 Follow-up range 2–7 years 10 2 11 Table 2 The Alhback radiological scoring system for estimating the severity of OA Grade 0  Normal  Grade 1  Joint narrowing  Grade 2  Joint obliteration  Grade 3  Bone destruction <5 mm  Grade 4  Bone destruction >5 mm Grade 5  Subluxation 3 Table 3 Scoring results of the non-revised patients Scoring Results Knee society score Knee score  Preoperative 39.2 (SD 18.2)  Postoperative 89.4 (SD 14.0) Function  Preoperative 55.8 (SD 14.3)  Postoperative 77.1 (SD 24.7) Total score  Preoperative 47.6 (SD 12.3)  Postoperative 83.4 (SD 16.8) WOMAC score Pain  Preoperative 50.3 (SD 18.7)  Postoperative 78.6 (SD 21.5) Stiffness  Preoperative 51.2 (SD 22.6)  Postoperative 71.2 (SD 20.8) Function  Preoperative 50.6 (SD 20.7)  Postoperative 76.2 (SD 20.4) SF-36 questionnaire Function  Preoperative 35.7 (SD 17.6)  Postoperative 56.1 (SD 24.5) Physical  Preoperative 28.2 (SD 37.2)  Postoperative 57.2 (SD 44.3) Pain  Preoperative 32.7 (SD 19.2)  Postoperative 59.8 (SD 26.5) Health  Preoperative 63.7 (SD 22.2)  Postoperative 61.4 (SD 21.7) Social function  Preoperative 52.6 (SD 17.1)  Postoperative 64.5 (SD 17.6) Emotional  Preoperative 64.5 (SD 44.6)  Postoperative 70.5 (SD 40.7) Mental health  Preoperative 73.7 (SD 17.9)  Postoperative 75.1 (SD 18.8) SD 11 All medial compartment arthroplasties were performed using the minimally invasive technique and under tourniquet control. The discharge criteria were control of immediate postoperative pain and the ability to flex the operated knee to a minimum of 90° with no lack of extension. All complications and revisions were reported, and a revision was defined as any surgical procedure resulting in removal or exchange of any of the prosthetic components. Results 4 Table 4 Revisions of Oxford phase-3 knee replacement surgery  Incidence Revision of a component of UKA 3  Revision of the mobile bearing 1  Revision of the femoral component and the bearing 1  Revision of the tibial component and the bearing 1 Conversion to a TKR 14 Reason for revision to a TKA   Inappropriate indication 1  Misalignment and loosening 5  Infection 1  Progression of osteoarthritis in lateral compartment 4  Persisting anteromedial pain >1 year 3 One bearing was replaced because of luxation after a hyperflexion trauma. A new bearing of the same size was inserted, and no recurrence of luxation was seen at follow-up. In another case of luxation of the bearing, the femoral component, and the bearing were changed 9 months after the primary surgery. The fixation of the femoral component in this case was insufficient. The multiple small drill holes were not made, and there was no cement in the large drill hole. With flexion, the loose femoral component moved distally, causing luxation of the bearing. The tibial component and bearing revision was performed seven months after the primary surgery because of misalignment of this tibial component. With flexion, there was impingement of the bearing with the tibial component, causing a clicking sensation and rotation of the bearing. 11 Staphylococcus aureus Three patients with persisting anteromedial pain underwent revision. In two cases, no cause was found, and in both pain persisted after TKR. In the third case, the synovial biopsy showed synovitis villonodularis pigmentosa, and after the TKR this patient was pain-free. Except for the two patients with persisting anteromedial pain, all patients with a conversion to TKA were pain-free. No special augmentations or revision prosthetic components were necessary in these procedures; there were no bone defects that required the use of particulate autograft or allograft, and primary cruciate-retaining TKA was used in the revisions. Postoperative complications occurred after the primary unicompartmental knee replacements. One patient had a traumatic medial tibia plateau fracture 4 weeks postoperatively, which was treated conservatively. Another patient developed hemarthrosis that required extended hospitalisation; this was resolved with conservative treatment. There was one deep infection, and no deep venous thrombosis was reported. At the time of the most recent follow-up, average flexion was 125.8 ± 13.8°, with two patients achieving <90° flexion. The average flexion deformity/extension was 0.3 ± 2.2°. 3 The final follow-up radiographs showed an average anatomical axis, femorotibial alignment of 8.8° of valgus (range 4°–22° of valgus). The knees were corrected by an average of 6.4° (range 2°–14°). This relative overcorrection gives increased stress on the lateral compartment. Signs of osteoarthritis progression in the uninvolved tibiofemoral compartment on the radiograph at the last follow-up were noted in 43 knees (grade-1 Ahlback osteoarthritis in 39 knees and grade-2 Ahlback osteoarthritis in four knees). No grade-3 or -4 changes were noted. At final radiographic evaluation, no component showed evidence of loosening. No knees had >2 mm of tibial cement-bone radiolucency. There were no radiolucent lines seen at the posterior aspect of the femoral components. Seventeen knees were revised, resulting in a survival rate of 89% in these 2–7 years of follow-up interval. Discussion 7 12 5 6 The primary need for revision surgery could be attributed to indication and technical failures. Thirteen of the 17 revisions were probably related to human error, the remaining four are in one case a hyperflexion trauma and luxation of the bearing, one case with deep infection, and two cases with unexplained persisting anteromedial pain. Misalignment of the components was the primary cause of technical failure. With the minimally invasive technique, the visual field is restricted, making mobile-bearing unicompartmental knee replacement surgery a demanding procedure. Introduction of the minimally invasive option makes the terms surgical technique and pitfalls actual again. For the remaining 113 patients (140 knees) who did not undergo revision, the Knee Society score, WOMAC and SF-36 questionnaires showed an improvement in the outcome. All three scores indicated less pain and improvement in function, as confirmed by an average clinical average flexion of 126° at the latest follow-up. The Knee Society score total of 83.4 indicates a successful outcome. Over the 7-year period of our study, eight senior surgeons performed the operation with an average of <10 procedures a year per surgeon. All surgeons attended the instructional course organized by the designer group. There is no evidence for a learning curve in our study. The outcome should be attributed to the number of operations performed. As a result of the relatively low survival rate of this study, the number of senior surgeons performing the procedure in this hospital is now reduced to two. Conclusion 13 14 14