There were no major systemic complications following 209 consecutive Oxford UKAs (179 patients). This study revealed no significant differences in the perioperative complication rates in patients between the different groups. All patients obtained satisfactory clinical outcomes, but compared to patients over 80 years old, patients between the ages of 60-79 had a higher function score in both HSS and WOMAC scores.
Clinical Outcome of UKA
Iacono et al.[10] evaluated results obtained in patients older than 75 years treated with UKA. All clinical scores improved significantly at follow-up, and the outcome was considered good or excellent in 92.6% of the patients, but the prosthesis 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[11]. A meta-analysis reported that the functional outcome of UKA in the elderly is good, with low rates of perioperative morbidity and mortality[12]. Inale et al.[13] 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 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 scores 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. However, Group 3 had lower scores in the functional dimension in HSS and WOMAC scores, and there was no statistical difference in the pain aspects in HSS and WOMAC scores. This is considered small and still within the 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. A proportion of patients, whose knees flex more than 120 degrees, are required to perform daily activities that include squatting and sitting on the floor, which may lead to different clinical outcomes from Western populations. Lim et al. reported that Oxford UKA can yield satisfactory clinical and functional results and has a ten-year survival rate of 94% in Korean patients[7]. Yoshida et al. report similarly good medium-term results with a 10-year survival rate of 95% in Japanese patients[14]. 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.
Surgery-related Complications
One systematic review assessed over 8,000 Oxford UKA patients and found the 10-year survival to be 93%, 15-year survival to be 89%, and a medical complication incidence of 0.8%. Very good outcomes were achieved by both designing and non-designing surgeons[15]. 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[15-19]. 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. This is different from the results reported by other studies. Previous literature shows that the incidence of RLL ranged from 62-96%, which was not clinically related to inferior functional outcomes[20-23]. 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, short follow-up time, and lack of standard X-rays (we could usually not get standard X-rays for outpatient follow-ups). Goodfellow et al. describe pathological RLL being >2 mm thickness, poorly defined, and often related to aseptic loosening. On the contrary, physiological RLL are 1-2 mm thick, and well-defined[24]. The presence of RLL in our patients was not related to their symptoms or indicative or predictive of loosening and, according to the X-rays, we confirmed them to be physiological RLL (Figure 1). We still need to assess the clinical outcomes through mid- and long-term follow-ups.
One patient suffered a bearing dislocation four months after surgery. Figure 2 shows the imaging before and after surgery. The possible reason for this may be that the abnormal morphology of the patient’s femur lead to a deviation in the intramedullary rod positioning and the femoral prosthesis was placed medially. The poor prosthesis position caused rotation of the bearing during 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 is higher in Asian populations than in Western populations[5, 25, 26]. It can occur in the presence of an unbalanced flexion-extension gap, impingement of the bearing with adjacent bone or the tibial/femoral component, instability of the medial compartment due to MCL injury, or secondary to femoral/tibial component loosening[5].
One patient (81 years old) developed a periprosthetic fracture 2 months after surgery due to a fall, 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 fracture in knee arthroplasties are reported to be from 0.2-2.5% in clinical studies and 0.02-0.17% in worldwide arthroplasty registers[27]. Risk factors associated with unicompartmental component periprosthetic fracture include malalignment with increased local stresses due to malpositioning, progressive osteoarthritis, and cruciate ligament deficiency. Patients with a BMI greater than 30 are also at greater risk[28].
Perioperative Complications
The major perioperative complications in our study were calf muscular vein thrombosis (CMVT) and superficial infection. There were no deaths during the perioperative period, pulmonary embolisms, or symptomatic deep vein thrombosis (DVT).
Chan et al. compared one-stage and two-stage bilateral unicompartmental knee replacements 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[29]. If the patient was diagnosed with a DVT or CMVT, they should receive low molecular weight heparin (nadroparin 0.4 mL, twice per day) for 2 weeks. Two weeks after surgery, patients were treated with Rivaroxaban for anticoagulant therapy. The deep vein ultrasound of the lower extremity should then be checked and the drug stopped 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, previous infection associated with a prosthetic joint at the same site, tobacco use, obesity, rheumatoid arthritis, a neoplasm, immunosuppression, and diabetes mellitus[30]. 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[30, 31].
We conclude that Oxford UKA is a safe procedure with a low rate of perioperative complications, similar to previous studies[32]. Previous studies also showed that increased patient age and history of cardiovascular disease were identified as risk factors for perioperative death in TKA[33]. However, in our study, patients older than 80 who underwent Oxford UKA also showed good clinical outcomes with a low rate of perioperative and other complications.
Limitations
There are several limitations in our study. The study sample was relatively small, and the follow-up 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. Moreover, we did not consider the potential influence of sex, and there was no control group in this study.