Patients and design
All patients admitted to the Physical Therapy and Rehabilitation Outpatient Clinic, Antalya Training and Research Hospital, between January 2018–January 2019, with the complaint of knee pain, were screened for the eligibility for the study. The inclusion criteria were (a) being between 50 and 70 years of age (b) knee pain in more than 25 days of during the last month (c) crepitation during movement (d) morning stiffness less than 30 min or radiographic osteophytes (e) Kellgren–Lawrence class I–IV knee osteoarthritis and (f) willingness to participate in the study. The exclusion criteria were (a) previous knee operation (b) rheumatic or metabolic disease (c) malignant disease (d) central or peripheral neurologic disease that may cause neuropathic pain (e) cardiopulmonary disease that may interfere with the functional tests (f) pregnancy (g) active infections (h) a standardized mini-mental status examination score of less than 26 (i) intra-articular steroid injection in the last 3 months (j) hip or ankle pathology (k) treatment for neuropathic pain in the last 6 months.
Age, sex, education, working, smoking, and marital status, symptom duration, number of comorbid diseases and drugs used, and painful areas other than knee were recorded for each patient along with the presence of postural deviations (thoracic hyperkyphosis, lombar hyperlordosis, scoliosis, genu varum and recurvatum, and pes planus).
Tests and measurements: anthropometric measurements and body composition
Standing height was measured using a wall-mounted stature meter. Waist and hip circumferences were measured using a tape measure. Weight, body mass index and composition were measured using a professional digital scale (Tanita®MC-180MA, Tanita Corp., Tokyo, Japan).
Western Ontario and McMaster Osteoarthritis Index (WOMAC)
It is a health status instrument with pain, stiffness and physical function domains designed for patients with osteoarthritis and validated in Turkish population [14, 15].
Short form-36 (SF-36)
It is a 36-item questionnaire for general health and quality of life and divided into eight domains: physical function, social function, physical role limitations, emotional role limitations, bodily pain, general mental health, vitality, and general health perceptions [16, 17].
Geriatric depression scale
It is a 30-item scale first created by Yesavage et al.  and validated in Turkish population . A score of 0–10 was considered as no depression, 11–13 as possible depression, and 14 or higher as depression.
A 30-s chair-stand, stair-climbing, and 6-min walk tests were performed for each patient as described previously [20,21,22].
Computerized tetra-ataxiometric posturography, Tetrax®(Sunlight Medical Ltd., Tel Aviv, Israel), was used for the assessment of static posturographic stability in eight positions: stable surface/eyes open/face forward, stable surface/eyes closed/face forward, unstable surface/eyes open/face forward, unstable surface/eyes closed/face forward, stable surface/eyes closed/head rotated 45° to the right, stable surface/eyes closed/head rotated 45° to the left, stable surface/eyes closed/head tilted back 30°, and stable surface/eyes closed/head tilted 30° forward. Fall risk index (low: 0–36, intermediate: 37–58, high: 59 or higher), stability index (the higher the index the lower the stability), and Fourier harmony index (< 0.9: abnormal stability; normal: 0.9–1) were calculated for each patient [23, 24].
Douleur neuropathique-4 (DN-4)
It is a scale composed of four questions. A score of 4 or higher indicates neuropathic pain [25, 26]. Additionally, a ten centimeter visual analogue scale (VAS) was used for the severity of the knee pain at rest and while walking (0 = no pain, 10 = extreme pain).
A standard standing anteroposterior knee X-ray with the knees in extension were obtained for each patient and severity of osteoarthritis was graded according to the Kellgren–Lawrence classification .
Hemoglobin concentration, erythrocyte sedimentation rate and serum C-reactive protein, fasting plasma glucose and lipid profile (total cholesterol, LDL cholesterol, and triglyceride), serum uric acid, creatinine, and alanine aminotransferase, plasma 25-hydroxy-vitamin-D3 and thyroid stimulating hormone levels of each patient were measured.
The patients were divided into two groups according to the DN-4 score: Group 1 with neuropathic pain (a DN-4 score of 4 or higher) and Group 2 with no significant neuropatic pain (a DN-4 score less than 4). The two groups were compared in terms of the demographic and clinical features and the tests and measurements. Since the sex was found to have a influencer effect, the study groups were further divided into the sex subgroups in the analyses.
SPSS Statistics v.18 (IBM Corp, Chicago, IL, USA) was used for the statistical analyses. Data were expressed as numbers with percentages for the categorical and means or medians with standard deviations or minimum and maximum values for the continuous variables. Categorical variables were compared by using chi-square of Fisher’s exact tests. Distributions of the continuous data were analyzed by histograms and tested for normality by the Shapiro–Wilk test. Mann–Whitney U or a t-test was used for comparison of unrelated samples according to the distribution. Multiple logistic regression analysis was used to control for the factors associated with neuropathic pain. p < 0.05 was considered statistically significant.