*2.1.1 Preoperative period*

Preoperative ROM limitation is the most important risk factor for postoperative stiffness [4, 9]. Patients with a greater degree of preoperative ROM had superior postoperative ROM and functional scores, with less complications. Only 71.4 percent of patients with preoperative ROM less than 90 degrees would achieve postoperative ROM at least 90 degrees, while more than 90 percent of patients with preoperative ROM greater than 90 degrees could perform ROM more than 90 degrees postoperatively [10]. Lee et al. demonstrated that 33 percent of patients with preoperative ROM less than 50 degrees developed either superficial or deep infection, as well as skin necrosis after the operation, whereas only 13 percent of patients with preoperative ROM between 50 and 90 degrees suffered from these complications [11]. The cause of stiffness before surgery is also one of the considerable factors for postoperative stiffness. The same study by Lee et al. showed patients with osteoarthritis or rheumatoid arthritis had greater postoperative ROM than patients with prior infectious arthritis or traumatic arthritis significantly [11]. Patients with younger age, absence of diabetes mellitus, and lower preoperative walking limitations were found to be the additional predictors with better postoperative ROM [12]. Moreover, obesity might be another factor influencing postoperative ROM. Järvenpää et al. proposed patients with body mass index (BMI) greater than 30 kg/m<sup>2</sup> had poorer postoperative ROM at 1-year follow-up approximately 6 degrees than patients with less BMI [13].
