Obesity significantly affects outcomes in both total knee arthroplasty (TKA) and unicompartmental knee arthroplasty (UKA). Studies targeting Caucasian populations have shown that obesity, particularly morbid obesity (body mass index [BMI] ≥ 40 kg/m2), is associated with an increased risk of complications following TKA and UKA. Obese patients undergoing these procedures have a higher incidence of deep vein thrombosis (DVT) and pulmonary embolism (PE) [[1], [2], [3], [4]]. Surgical site infections are also more common in obese individuals, with risk increasing in parallel with higher BMI levels [2,5,6]. However, several studies have reported a lower overall incidence of venous thromboembolism (VTE) in Asian populations, raising questions about the necessity of routine chemoprophylaxis in these groups [7].

The incidence of DVT and PE after knee arthroplasty in Asian populations varies across studies. Without antithrombotic prophylaxis, DVT rates range from 8.6 % to 41.0 %, with proximal DVT rates of 0.4 %–10.2 % [[8], [9], [10]]. PE rates are generally low, ranging from 0.01 % to 0.6 % [11,12]. Risk factors for DVT include female gender, bilateral surgery, and high BMI [8]. UKA and revision knee arthroplasties have lower DVT and PE risks compared to primary total knee arthroplasty [13]. While some studies suggest prophylaxis may be beneficial for high-risk patients [8], others report low symptomatic DVT rates without antithrombotic prophylaxis [11]. Previous studies have not clarified the incidence of DVT and PE in Asian populations under the current widespread use of antithrombotic therapy. In addition, the risk of postoperative complications after knee arthroplasty in patients with severe obesity (BMI ≥35 kg/m2), as defined by the Japanese Guidelines for the Management of Obesity Disease [14], has not been previously examined.

The Japanese Diagnosis Procedure Combination (DPC) database is a beneficial resource for investigating postoperative complications in large-scale cohort studies in orthopedic surgery [[15], [16], [17], [18], [19], [20]]. However, the risk of complications in severely obese Japanese patients undergoing TKA or UKA has not been thoroughly investigated. This study aims to utilize a nationwide database of severely obese Japanese patients who underwent TKA or UKA to compare the incidence of in-hospital complications between severely obese and non-severely obese patients. To evaluate the risk of complications in severely obese patients undergoing knee arthroplasty, we conducted a large-scale nationwide cohort study using a Japanese medical claims database. This study compared the incidence of complications, including DVT, PE, pneumonia, cerebrovascular events, surgical site infections, and in-hospital mortality in cohorts matched for age, sex, surgical method (TKA, UKA, or simultaneous bilateral surgery), Charlson comorbidity index, and comorbidities. We hypothesized that, despite the widespread use of antithrombotic therapy, severely obese Japanese patients (BMI ≥35 kg/m2) undergoing knee arthroplasty may have a significantly higher risk of postoperative VTE, including DVT and PE, compared to non-severely obese patients.