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Performance status of the patients was reported as Karnofsky score [18] and was clinically addressed by a preoperative evaluation by one of the participating doctors (M.S.S., P.F.H. or T.C.). Dissemination status of the cancer disease was evaluated by patient interview combined with evaluation of preoperative imaging (computer tomography (CT), positron emissions tomography (PET), magnetic scans or bone scintigraphy).

Comorbidity: American Society of Anesthesiologists classification (ASA score) [19] was evaluated by the attending anesthesiologist preoperatively. A patient was considered as having ischemic heart disease (IHD) in case of verified echocardiography with cardiac output <45%. Comorbidity with diabetes mellitus (DM) was considered in cases where the patient suffered from this regardless of medical treatment or HbA1C level.

Complete fracture of the treated lesion was evaluated preoperatively by CT or X-ray.

Biochemical variables were obtained prior to surgery. Hemoglobin, alkaline phosphatase, CRP and absolute leukocyte and neutrophile counts were obtained from routine laboratory roundups and were considered missing if they were obtained more than 7 days prior to surgery. Blood samples (plasma) for IL-6 and YKL-40 analysis were obtained separately from the routine blood samples within 3 days prior to surgery. Blood was drawn into a 6 mL ethylene-diamine-tetra-acetic acid tube and stored between 1/2–2 h at room temperature and then centrifuged for 10 min at 3000 rounds per minute. Thereafter plasma was isolated and stored at −80 °C.

Plasma IL-6 was determined by ELISA (Quantikine HS600B, R&D Systems, Abingdon, UK), and plasma YKL-40 was determined by ELISA (Quidel, California, CA, USA) according to the instructions by the manufacturer. The analyses were performed blinded to the clinical data. Elevated plasma IL-6 level was defined as the cut-off for the 95th percentile in healthy blood donors, i.e., >4.50 pg/mL [20]. Since plasma YKL-40 increases exponentially with increasing age, a formula (Percentile = 100/1 + (YKL-40−3) * (1.062age) * 5000) was used to calculate the age-corrected YKL-40 percentile of the patients [21].

Primary cancer was evaluated by history (as concluded by prior biopsy of primary lesion or imaging) or preoperative biopsy of the bone metastases. To ensure correct diagnosis, histopathology of the lesion was obtained intra-operatively. In case of unknown origin preoperatively (despite preoperative imaging) the primary cause of lesion as shown by intra operative histopathology arbitrated the primary cause for the metastatic lesion. If no primary cancer could be found in histopathology analysis, the primary cancer was considered unknown.

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