For patients who underwent two-stage replacement of both knee joints (one knee surgery - one week term- the other knee surgery), postoperative pain and analgesic usage with patient-controlled analgesia (PCA) increased at the second stage, which suggests that central sensitization occurs within a short period (one week) in patients who undergo total knee arthroplasty (TKA). Neurosteroids act on NMDA and AMPA receptors, GABAa receptors, and voltage-dependent Ca2+ or K+ channels of sensory neurons to increase invasive or neuropathic pain and, conversely, to exhibit analgesic and anticonvulsant effects. These actions mean that the neurosteroid acts as an endogenous regulator of pain control and central sensitization. The purpose of this study is to confirm that the hypothalamus-pituitary-adrenal (HPA) axis, which is the main body of endocrine neurosteroid, is associated with increased pain sensitivity after TKA. The concentrations of cortisol and dehydroepiandrosterone (DHEA) in the saliva of patients who undergo two-staged bilateral knee replacement surgery (one knee surgery - one week term- the other knee surgery) will be measured at each stage and analyzed for correlation between concentration-related changes of HPA and postoperative knee pain variations.

Inclusion Criteria:

• Two-staged bilateral total knee arthroplasty (one knee - one week - the other knee)
• American Society of Anesthesiologists (ASA) class I, II
• Postmenopausal women aged under 71

Exclusion Criteria:

• Patients who have used analgesics before surgery, such as chronic pain patients
• ≥ ASA class III, including patients with renal impairment or symptomatic cardiovascular disease
• Patients who refuse to participate in the study or from whom receive informed consent cannot be received.

Patients who visit Seoul National University Bundang Hospital for two-staged bilateral total knee arthroplasty.