Unicompartmental knee arthroplasty (UKA) is a valid treatment for end-stage knee osteoarthritis (OA), affecting a single femoro-tibial compartment. The popularity of unicompartmental knee replacement (UKR) has increased as excellent functional outcomes and survival have been reported in long-term follow-up studies. UKA has important advantages compared to total knee arthroplasty (TKA), including lower intraoperative blood loss and risk of transfusionas well as accelerated recovery.
Unicompartmental knee arthroplasty (UKA) is a valid treatment for end-stage knee osteoarthritis (OA), affecting a single femoro-tibial compartment [1]. The popularity of unicompartmental knee replacement (UKR) has increased as excellent functional outcomes and survival have been reported in long-term follow-up studies. UKA has important advantages compared to total knee arthroplasty (TKA), including lower intraoperative blood loss and risk of transfusion [2,3] as well as accelerated recovery [4]. In addition, UKA is associated with a decreased length of stay in hospital, lower readmission rates [5], lower infection rates and fewer major medical complications, such as thromboembolism, stroke and myocardial infarction, compared to patients undergoing TKA [6]. Some authors have reported that UKA produces more natural knee biomechanics and healing of physiological gait pattern [7], with superior patient-reported clinical and functional outcomes [8,9,10,11]. Despite these advantages, data from national joint registries showed a higher risk or revision in patients undergoing UKA [12]. Correct patient selection is paramount to achieve good outcomes for UKA, reducing the risk of UKA failure and revision surgery. According to the Kozinn and Scott criteria proposed in 1989, body weight over 82 kg is a contraindication to UKA [13]. More recently, it has been demonstrated that many of the “traditional” contraindications to UKA are not necessary, including a high BMI [14,15,16]. However, the role of BMI and its influence on the results of UKA and TKA is still being debated. Over the last few decades, the number of obese patients needing treatment for end-stage knee arthritis has significantly increased. Body weight has been shown as a modifiable risk factor for knee osteoarthritis and disease progression [17,18,19]. In addition, adverse events such as dislocation, aseptic loosening, superficial and deep infection and revision surgery are more common in obese patients undergoing TKA [20,21,22]. In contrast, the impact of obesity on the results of UKA is still unclear, with some surgeons offering UKA to both obese and non-obese patients, while others consider a high BMI as a contraindication and a reason of concern for potential early failure. The aim of this systematic review and meta-analysis is to compare the results of obese and non-obese patients in terms of clinical and functional scores and risk of revision. Our hypothesis is that a higher BMI would be associated with lower functional outcome scores and higher risk of septic and aseptic failures.
The PRISMA flowchart for study selection is shown in Figure 1 . The literature search resulted in a total of 916 references. After abstract evaluation, 875 papers were excluded due to duplication (26) or being off-topic (849). After full-text evaluation, 16 further papers were excluded because they did not meet the inclusion criteria or reported incomplete data. Three studies [15,24,25] reported the results of the same cohort (or similar cohorts) of patients. When present in the same analysis, only the study with the longest follow-up was considered. Therefore, 22 papers were included in the final systematic review: 12 of these studies were retrospective studies [26,27,28,29,30,31,32,33,34,35,36,37], 5 were prospective studies [38,39,40,41,42] and 5 were case series [25,43,44,45,46]. Thirteen studies showed adequate information on revisions and functional outcomes to be included in the meta-analysis.
Patient demographics for each study are summarized in Table 1 . Eleven studies [25,26,27,28,29,30,33,35,38,39,42] provided mean age, mean BMI and mean follow-up time for all BMI subgroups. Other studies reported mean age, BMI or follow-up time for the whole study population and not for each BMI subgroup. Two studies [43,45] only considered patients with BMI > 40 and BMI > 30, respectively. One study [28] divided patients according BMI but did not report the mean BMI of each subgroup. One study [31] did not report the number of procedures, mean age or mean BMI but only the division of patients according to BMI and rate of revision. The reported follow-up periods ranged from a minimum of 2 years to a maximum of 12 years.
Survival rate, revision rate and cause of revision are described in Table 3 . Not all studies reported survival rate or distinguished revision causes by patient BMI subgroup.
The meta-analysis performed a comparison between patients with a BMI < 30 and with a BMI > 30 for functional outcomes and revision rates. Eight studies [25,27,29,37,38,39,40,42] analyzed the clinical outcomes after UKR between obese and non-obese patients ( Figure 2 ). OKS was significantly higher in patients with a BMI < 30 compared to those with a BMI > 30 (MD 3.81, 95% CI, 2.06 to 5.56, p < 0.0001). The KSS knee showed better improvements in non-obese patients, but no significant differences (MD 2.15, 95% CI, −0.60 to 4.89, p = 0.13). KKS function increased significantly after UKA in non-obese group (MD 6.61, 95% CI, 1.50 to 11.72, p = 0.01). Finally, evaluating all the reported clinical outcomes, a significant difference was shown in favor of patients with a BMI < 30 compared to patients with BMI > 30 (MD 4.38, 95% CI, 2.28 to 6.48, p < 0.0001). Moreover, 11 studies [27,28,29,31,33,38,39,40,41,42,44] analyzed the revisions after UKA and showed a significantly increased likelihood for revision in patients with a BMI > 30 (OR 1.42, 95% CI, 1.05 to 1.92, p = 0.02) ( Figure 3 ). Instead, the rate of septic revisions did not show significant differences between the two groups (OR 0.90, 95% CI, 0.41 to 1.97, p = 0.79) ( Figure 4 ).
Our systematic review and meta-analysis demonstrated that the risk of revision was greater in obese patients (BMI > 30). However, the difference was lower than reported by similar studies on TKA. The risk of revision for infection in patients with a BMI > 30 was not significantly higher than that of non-obese patients. Although the improvements in OKS and KSS function were statistically significant for patients with a BMI < 30, obese and non-obese patients experienced similar improvements after UKA. Therefore, this meta-analysis suggests that all patients undergoing UKA benefit from the intervention, regardless of BMI. Accordingly, BMI should not be considered as a contraindication for UKA. However, obese patients should be informed about the increased risk of failure and inferior functional outcome of joint replacement surgery and should lose weight prior to undergoing surgery.
This entry is adapted from the peer-reviewed paper 10.3390/jcm10163594