In our study, there was no major systemic complication following 209 consecutive Oxford UKAs (179 patients). This study revealed no significant difference in the perioperative complication rate in patients between the different group All patients obtained satisfactory clinical outcome, but compared to the patients over 80 years old, the patients between the ages of 60 and 79 had higher function scores in both HSS and WOMAC scores.
Clinical outcome of UKA
Iacono F et al. evaluated the results obtained in patients older than 75 years treated with UKA. All clinical scores improved significantly at this follow-up, and the outcome was considered good or excellent in 92.6% of the patients, but the prosthesis they used was different from ours.
Concerning the clinical outcomes in very old patients who underwent OXFORD UKA, a recent study from the Oxford center analyzed 1000 OXFORD UKAs and found that at 10-year follow-up, patients younger than 60 at the time of the operation had significantly better American Knee Society Score Function (AKSS-F) score, Oxford Knee Score (OKS), and Tegner Activity Score than patients older than 60, but no difference in functional outcomes was seen between the groups. A meta-analysis reported that the functional outcome of UKA in the elderly is good, with low rates of perioperative morbidity and mortality. Inale P A et al. reviewed the short-term results of mobile bearing medial UKA in elderly patients and compared the results with younger patients. The differences between the knee scores from the elderly patients and from the younger patients were not statistically significant. Revision rate and survival of the implant were not different among the different groups.
In our study, there was a clear improvement in HSS and WOMAC scores in both groups after surgery. WOMAC scores evaluate efficacy through three aspects: function, pain and stiffness. HSS score evaluate efficacy through two aspects: function and pain. There was no statistical difference between the three groups in the total HSS score and WOMAC score. But the Group 3 had lower score in the function dimension in HSS and WOMAC score, and there was no statistical difference in the pain aspect in HSS and WOMAC score. This is considered small and still within the minimum clinically important difference (MCID) of the function outcome measurement. Thus, even though the difference is statistically significant, it might not be clinically important. The lack of exercise and the decline of activity by patients older than 80 might also have led to this finding. In Asian populations, body size, BMI, lifestyle, and knee morphology of Asian populations differ from those in western countries. And a proportion of patients, whose knees flex more than 120°, is required to perform daily activities that include squatting and sitting on the floor. And that may lead to different clinical outcome from western populations. But in our study the clinical outcome of Chinese patients is similar to western patients’ clinical outcome, and the ROM also changes significantly in both groups before and after surgery. Lim et al. reported that MIS-UKA can yield satisfactory clinical and functional results and has a ten-year survival rate of 94% in Korean patients. Yoshida et al. reported similar good medium-term results with a ten-year survival rate of 95% in Japanese patients.
One systematic review assessed over 8,000 OXFORD UKA patients and found the 10-year survival to be 93% and 15-year survival to be 89%, and a medical complication incidence of 0.8%. Very good outcomes were achieved by both designer and non-designer surgeons. The literature shows that the main reasons that led to failures of OXFORD UKA were bearing dislocation, aseptic loosening, lateral compartment arthritis progression, and persistent unexplained pain[14-18]. Of the 209 OXFORD UKAs in our study, 19 (9.1%) patients were found to have radiolucent lines (RLL) under the tibial component on radiographs at final follow-up. That is different from the result reported by other studies. Previous literature showed that the incidence of RLL ranged from 62% to 96%, which was not clinically related to inferior functional outcomes[19-22]. The etiology of radiolucency remains unknown.
The incidence of RLL in the current study was lower than in the previous literature; several reasons may lead to this phenomenon, such as the small sample size and short follow-up time and cannot get standard X-rays. About the X-ray, that it is our limitation, because we usually cannot get standard X-ray when outpatient follow-up. As Goodfellow et al. described, pathological RLL is >2 mm, poorly defined, and often related to aseptic loosening. On the contrary, physiological RLL is 1–2 mm and well-defined. The presence of these radiolucent lines in patients was not related to symptoms or indicative or predictive of loosening, and according to the X-rays, we confirmed it was physiological RLL (Figure 1). We still need to assess clinical outcomes through mid- and long-term follow-up. One patient developed a bearing dislocation four months after surgery. (Figure 2 shows the data of imaging before and after surgery.) The possible reason for this may be that the abnormal morphology of the patient's femur leads to a deviation in the intramedullary positioning, and the femoral prosthesis is placed close to the inside. Poor prosthesis position caused the rotation of the bearing during the knee flexion, resulting in dislocation of the bearing. Bearing dislocation is a major complication of OXFORD UKA, as previous literature has reported, and the rate of bearing dislocation was higher in the Asian population than that in the Western population[5, 24, 25]. It can occur in the presence of an unbalanced flexion–extension gap, impingement of the bearing with adjacent bone or tibial/femoral component, instability of the medial compartment due to medial collateral ligament (MCL) injury, or secondary to femoral/tibial component loosening.
One patient developed periprosthetic fracture two months after surgery due to a fall (81 years old), and we performed an open reduction of the tibial fracture with internal fixation. (Figure 3 shows the X-ray before and after surgery.) The literature shows that the rates of fractures of knee arthroplasties were reported to be from 0.2% to 2.5% in clinical studies and 0.02% to 0.17% in worldwide arthroplasty registers. Risk factors associated with unicompartmental component fracture included malalignment with increased local stresses due to malpositioning, progressive osteoarthritis, and cruciate ligament deficiency. Patients with a BMI greater than 30 were also at greater risk.
The major perioperative complications in our study were CMVT and superficial infection. There were no deaths during the perioperative period nor pulmonary embolisms or symptomatic DVTs in this study.
Chan et al. compared one-stage and two-stage bilateral unicompartmental knee replacement during the first 30 days postoperatively and found that the rates of proximal DVT, pulmonary embolus, and death secondary to pulmonary embolus to be 0.9%, 1.9%, and 0.3%, respectively. If the patient was diagnosed with DVT or CMVT, the patient should receive low molecular weight heparin (nadroparin 0.4 mL, twice per day) for two weeks. After two weeks after surgery, patients were treated with Rivaroxaban for anticoagulant therapy. Check the deep vein ultrasound of the lower extremity and stop the drug if the thrombus disappears or dissolves. Other patients received low molecular weight heparin (nadroparin 0.4 mL once per day) after surgery. In total, 13 patients developed superficial infection after surgery (11 patients older than 80). Patient-related risk factors included previous revision arthroplasty or previous infection associated with a prosthetic joint at the same site, tobacco use, obesity, rheumatoid arthritis, a neoplasm, immunosuppression, and diabetes mellitus. Postoperative risk factors included incision healing complications (e.g., superficial infection, hematoma, delayed healing, incision necrosis, and dehiscence), atrial fibrillation, myocardial infarction, urinary tract infection, and prolonged hospital stay[29, 30].
We conclude that OXFORD UKA is a safe procedure with a low rate of perioperative complications, similar to previous studies. Previous studies also showed that increasing patient age and history of cardiovascular disease were identified as risk factors for perioperative death in TKA. However, in our study, patients older than 80 who underwent OXFORD UKA also showed good clinical outcomes with a low rate of perioperative complications and other complications.
There are several limitations in our study. The study sample was relatively small, and the follow-up was relatively short. Further research, large samples, and long-term follow-up are required to evaluate function. The mean follow-up time of the study group in the present study was 21.76 months, which is comparatively long-term if the entry age of 80 years is considered. And we did not consider the potential influence of gender. At last, there is no control group in this study.