Participants and sampling
This study employed a cross-sectional design. Using convenience sampling, participants were recruited from outpatient and inpatient departments of two hospitals in Guangzhou and Shenzhen, Guangdong Province, China, from October 2023 to August 2024. The inclusion criteria for participants are given below:
(1) clinically diagnosed with KOA according to the Chinese guideline for diagnosis and management of osteoarthritis(2018 edition): (a) Recurrent knee pain symptoms occurring within the last one month; (b) X-ray (standing or weight-bearing position) showing narrowed joint space, subchondral sclerosis and/or cystic changes, and osteophyte formation at the joint margins (The level of knee degeneration was determined using the Kellgren-Lawrence Grade [10], all severity grades were considered); (c) Age ≥ 50 years; (d) Morning stiffness lasting ≤ 30 min; (e) A grinding sound or sensation during activity. KOA can be diagnosed if a and any two of b, c, d and e are met; (2) aged ≥ 60 years.(3) who were aware of and agreed to participate in this study. The exclusion criteria for participants are given below: (1) had severe neurological diseases or joint structural damage and were unable to complete bioelectrical impedance and grip strength measurements. (2) had undergone any inpatient surgery within the past six months. (3) participants with metal implants, such as pacemakers, arterial stents, and joint prostheses; (4) participants with ascites,limb oedema, and other conditions that affect the results of the examination.
We performed a sample size calculation based on the reported sarcopenia prevalences in patients with KOA [11], Using a CI of 95%, and an error probability of 5%, we determined a sample size of 262 participants. Anticipating a dropout rate of 10%, a sample size of participants 292 was considered sufficient for this study.
Ethics approval and consent to participate
After this study was approved ([2023]No. 89), we obtained the informed consent of all participants. We ensure that the work has been carried out following The Code of Ethics of the World Medical Association (Declaration of Helsinki).
Measure
Handgrip strength (HGS)
HGS was measured using an electronic handgrip dynamometer (CAMEY-EH101, China). Both the right and left hands of the participants were measured twice, and the maximum values were then recorded for analysis.
Calf circumference (CC)
Participants were instructed to bend their knees at a 90-degree angle while sitting. The measurement was taken on the side with the most severe symptoms of knee osteoarthritis. Each measurement was repeated twice, and the average values were subsequently calculated.
Ishii test
The sarcopenia scoring system, developed by Ishii et al. was used to assess sarcopenia risk. The male score was calculated as follows: 0.62×(age−64)−3.09×(grip strength−50)−4.64×(CC−42). The female score was calculated as follows: 0.8×(age−64)−5.09×(grip strength-34)−3.28×(CC−42). Sarcopenia is considered when the total score is ≥ 105 or higher for males, or ≥ 120 for females.
Body composition detection
The measurement of skeletal muscle mass was conducted using bioelectrical impedance analysis (Inbody120, Biospace, Korea). Participants were instructed to fast, refrain from drinking fluids, and avoid intense physical activity for at least 2 hours before testing. During the test, participants were asked to remain calm, wear light clothing, and avoid carrying any metal items. They were then instructed to stand barefoot on the device’s platform electrodes while holding the hand electrodes, maintaining this position for 10-15 seconds. Appendicular skeletal muscle mass (ASM) was also recorded as part of the assessment. The skeletal muscle index was calculated as the ratio of appendicular skeletal muscle mass to height squared.
Other variables
Age is calculated according to the date of birth on the patient’s ID card; Height, weight, and BMI (Body Mass Index) were measured by stadiometer (SH-200). Among them, BMI=weight (kg)/height (m)²; This instrument is often used in clinical practice for physical examination. These indicators were measured twice and then analyzed for the average value.
Diagnostic criteria for sarcopenia
Sarcopenia was defined as age-related muscle mass loss accompanied by low muscle strength and/or reduced physical function, based on the 2019 consensus report of the Asian Working Group on Sarcopenia (AWGS). Adjusted for height, the AWGS 2019 cut-off for diminished muscle mass was ASM < 7.0 kg/m2 for males and ASM < 5.7 kg/m2 for females. Meanwhile, muscle strength was estimated by HGS, with a diagnostic cut-off of < 28 kg for men and < 18 kg for women. The above two conditions can be defined as confirmed Sarcopenia.
Statistical analysis
Statistical analyses were performed stratified by sex, using SPSS version 26.0 for Windows (IBM Corp). Based on the 2-tailed test, P < 0.05 is considered statistically significant. Descriptive statistics were summarized using the mean ± standard deviation for normally distributed continuous variables, and the median (interquartile range, IQR) for non-normally distributed continuous variables. Categorical variables were presented as frequencies and percentages. The Area Under the Curve (AUC) of the Receiver Operating Characteristic (ROC) was used to assess the accuracy of the Ishii score test in predicting sarcopenia in older adults with KOA. The closer the AUC value is to 1, the greater the predictive accuracy of the tool; The optimal cut-off value was determined using the Youden index [12]. Additionally, we calculated the sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) for the best cut-off value. The Kappa value was used to evaluate the agreement between the Ishii score test and the reference diagnostic standard for sarcopenia. A Kappa value between 0.40 and 0.74 was considered to indicate moderate to high consistency, while a Kappa value below 0.40 indicated poor 189 consistency [13].
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