DISCUSSION
In this multidimensional study which investigated the presence of sarcopenia in patients with osteoarthritis, the rate of sarcopenia in knee OA patients was found to be approximately 12%. Sarcopenic OA patients were found to be older, weaker, thinner, malnourished, and had limited physical activity levels. Unlike other studies, the presence of sarcopenia in a multidimensional way, including body composition parameters, ultrasonographic muscle structure evaluations, isometric muscle strength evaluations, handgrip strength and walking speed evaluations, biochemical evaluations, anthropometric measurements and comprehensive functional evaluation tests measured by DEXA were examined. As far as we know, this is the first study in the literature to use all of these parameters at the same time. In this study, all parameters except biochemical parameters were found to have predictive value in diagnosing sarcopenia.
Kemmler et al. in their study with elderly individuals over the age of 70, found the prevalence of sarcopenia to be 4.5% and reported that patients with hip and lower extremity OA were more likely to have sarcopenia than those without osteoarthritis (9.1% with OA; 3.5% without OA) (16). The rate of sarcopenia found in our study appears to be similar to Kemmler et al. Clinical evidence suggests that the link between OA and sarcopenia occurs through functional and cellular pathways. It has been reported that patients with OA and rheumatoid arthritis are more prone to the development of sarcopenia due to the increased cytokines (5).
Baumgartner et al., for the diagnosis of sarcopenia, used the appendicular muscle mass (ASM) and skeletal muscle mass index (SMI) calculated by DEXA and reported the prevalence of sarcopenia as 13-24% under the age of 70 and > 50% in those aged 80 and over. Studies have found a strong correlation between DEXA and body composition measurements in elderly individuals with MR and CT measurements, which are gold standard methods (17).
In recent years, it has been suggested that muscle ultrasonography can be used as a practical, easily accessible and inexpensive method in the evaluation of sarcopenia. In the literature reviews, 4 studies were found in which DEXA and US were used together to diagnose sarcopenia. In all of these studies, ultrasonographic measurements of muscle thickness from many regions and thighs were found to be consistent with muscle mass values measured by DEXA.(18-21). In our study, the relationship between left gastrocnemius pennation angle and isokinetic muscle strength (peak torque extensor at an isokinetic 180 ° angular velocity) and appendicular lean body mass measured by DEXA was shown. The relationship between the angle of pennation of the right gastrocnemius muscle were found to be correlated with muscle strength (peak torque at an isokinetic 180-degree angular velocity and peak torque/body weight extensor ). A strong positive correlation was found between SMI and bilateral gastrocnemius medialis muscle thickness also.
The advantage of using DEXA in the diagnosis of sarcopenia is the better evaluation of SMI and skeletal muscle mass which was found to be associated with physical disability (22). Handgrip strength and quadriceps muscle strength are associated independently with predicting disability (23). Isometric handgrip strength, lower extremity muscle strength was found to be strongly correlated with knee extension torque and thigh cross-sectional muscle area (24). In our study, bilateral extensor isokinetic 60 degrees angular velocities of the knee joint were found to be statistically lower in the sarcopenic group compared to the other 2 groups. In functional evaluations, bilateral handgrip strength was found to be decreased in the sarcopenic group. Bilateral handgrip strength and bilateral knee extension strengths were positively correlated as moderate and with respect to several health outcomes.
Additionally, in this study, sarcopenia was found to be significantly associated with physical dependence, regardless of age, ethnic difference, comorbidities, health-related behaviours and fat mass (25). Gait speed plays an important role in the evaluation of sarcopenia. Gait speed is the first functional assessment to be considered in the evaluation of sarcopenia (26).
Active muscle strength test in the elderly is difficult to perform due to the reasons such as immobilization, pain, injury, surgical procedure, cognitive impairment, and technical complexity. The cross-sectional area of the muscles shows muscle mass and is related to muscle strength. Therefore, the cross-sectional area can be evaluated in order to have an idea about muscle strength in clinical applications. It has been stated that in intensive care patients, muscle loss can be monitored with rectus femoris and vastus intermedius muscle thickness (27). Although the quadriceps femoris pennation angle was not found to be significantly correlated with functional muscle parameters, in the study conducted by Strasser et al. with 26 healthy elderly individuals, the pennation angle was found to be lower than the control group consisting of 26 healthy young individuals (27). In our study, bilateral measurements were including in 3 regions by ultrasonograph. Fascicular length and pennation angle of the gastrocnemius muscle were evaluated as well as muscle thickness, and non-sarcopenic patients and healthy volunteers were included in the study as well as sarcopenic patients. In our study, it was found that bilateral gastrocnemius fascicle lengths and right-sided gastrocnemius pennate angle were lower in sarcopenic patients than those without and in the control group, and there was no significant difference between bilateral rectus femoris subcutaneous tissue and muscle thickness.
Decreased muscle quality and muscle mass can be considered as the most important factors in OA pathogenesis (28). At the same time, the painful joint can cause a decrease in muscle tone and consequently a decrease in muscle mass. When the relationship between OA and sarcopenia was examined, it could not be understood whether OA is a risk factor or a direct result of sarcopenia. For this, it may be recommended to conduct a systematic study investigating the development of sarcopenia in the ipsilateral extremity with OA occurring in a single joint and its effect on physical functions (29). In our study, the WOMAC function and total score evaluated in the sarcopenia group were found to be statistically significantly higher than the other two groups. The decrease in muscle mass and muscle quality caused by sarcopenia in patients with OA may explain the increasingng in WOMAC scores. In addition, a negative correlation between WOMAC function and total score and left-hand grip strength was found as an indicator of upper extremity muscle function in our function studies. This suggests that sarcopenia may affect not only the lower extremity but the whole body in these patients.
In our study, a relationship between rectus abdominis subcutaneous tissue thickness and right-hand grip strength, and between rectus abdominis muscle thickness and FMI was found. Although Abe et al. did not find a correlation between BMI and body mass with abdominal muscle thickness (2-3 cm from the side of the umbilicus, lower part of the rectus abdominis) measured by ultrasound, a correlation with handgrip strength and standing up test performance was found. The body mass with the appendicular lean soft tissue mass (ALM) parameter measured by DEXA was evaluated (30). In another study using BIA in patients with metabolic syndrome, a positive correlation was found between rectus abdominis muscle thickness and skeletal muscle mass index (SMI) (31). In a cross-sectional study conducted by Kara et al. with 30 healthy volunteers, low vitamin D levels were found to be associated with decreased muscle strength (especially knee and ankle extensors), but no relationship was found with muscle architecture (32). In our study, a positive correlation was found between bilateral gastrocnemius medius muscle thickness and SMI and vitamin D levels. Therefore, it can be thought that Vitamin D supplementation may increase muscle mass and decrease sarcopenia in these patients.
In the study conducted by Kositsawat et al. where 1826 people were studied; a relationship was found between low vitamin D levels and the decreased walking speed (33). In our study, a negative relationship was found between gait speed, and FM measured by DEXA, and a positive relationship with vitamin D level. Although there was no significant difference between vitamin D levels in our study, the average vitamin D levels in the sarcopenic group were determined to be 19 ng/mL. These levels can be considered as a deficiency in the literature and may explain the relationship between vitamin D levels and walking speed in this study. Additionally, in our study, a cross-sectional relationship between increased BMI index and decreased walking speed and a positive correlation between increased leptin levels and BMI was found. In another study, although no relationship was found between the thigh muscle cross-sectional area and the decrease in walking speed, a relationship was found between the change in the thigh muscle area and the change in walking speed. The total amount of fat and muscle fat infiltration has been reported to be important predictors for a reduction in walking speed (34). This report may likewise explain our findings.
As shown in our study, it is an expected result that sarcopenic patients are weaker (low body weight and BMI) and their bilateral hand grip strength, walking speed, waist and hip circumference, upper-middle arm circumference, calf and thigh circumferences are lower compared to non-sarcopenic patients. Similar to our findings, in a study conducted by Marini et al. with 207 sarcopenic people between the ages of 65-93, it was found that waist circumference, upper middle arm and calf circumference, weight and BMIs were significantly lower in both genders compared to those without sarcopenia (35).
The relation of sarcopenia and malnutrition has been shown in many studies (36, 37). In their study Velazquez et al. which involved 90 elderly women with an average age of 78, it was found that the prevalence of sarcopenia was 41%, when the nutritional status was evaluated according to MNA founded that 7.1% of those with normal, 45.9% of those with malnutrition risk, and 77.2% of those with nutritional deficiency were sarcopenic (38). In our study, MNA was used for malnutrition assessment and it was found that sarcopenic patients were significantly malnourished compared to non-sarcopenic and control groups. Therefore, it is our understanding that improving the nutritional status of OA patients may be beneficial in increasing muscle mass and preventing sarcopenia in these patients.
Leptin has been found to be increased in the cartilages of patients with OA, and although it is mainly synthesized from adipocytes, it is also synthesized from chondrocytes (18). A positive correlation has been found between the leptin level measured in synovial fluid and BMI, and it has been stated that this may be one of the metabolic factors explaining the role of obesity in the pathogenesis of OA (5). The function of leptin in OA is not fully determined but it has a biphasic effect which facilitates cartilage synthesis at low levels and causes inflammation and degeneration of cartilage at high levels (39). In our study, the relationship between sarcopenia, plasma leptin and adiponectin levels were evaluated and the relationship between leptin and adiponectin levels and muscle ultrasound and isokinetic evaluations of the patients were studied. A weak and low positive correlation was found between leptin levels and subcutaneous tissue thickness of the gastrocnemius and rectus femoris muscle in the muscle ultrasound evaluation performed in the non-sarcopenic knee OA group. This link may be due to the correlation between leptin and obesity.
In our study, although the serum leptin level was found to be lower in the sarcopenia group compared to the group without sarcopenia, there was no statistically significant difference between the two groups. Moreover, there were significantly different BMIs between these two groups. The BMI values in the sarcopenic group were significantly lower than the group 2 and 3 The lower serum leptin levels in the sarcopenia group compared to the group without sarcopenia may be related to lower BMI. The studies of Toussirot et al. and Sarı et al. stated that the low leptin levels found in patients with ankylosing spondylitis could be attributed to the low BMI values of the patient group in furtherance to our study (40, 41). In our study, the FM value measured by DEXA in sarcopenic patients was found to be lower than the other two groups. The low level of leptin in the sarcopenic group compared to the other two groups may be related to the low FM value.
Adiponectin plays a role in many processes such as energy metabolism, insulin resistance, and vascular physiology. High molecular weight adiponectin is the most active form of adiponectin which is the most sensitive marker reflecting insulin resistance and metabolic rate (42). High adiponectin levels have been recognized as a predictor for bone mineral density loss (43). In addition, high adiponectin levels have been shown to contribute to physical disability in a prospective cohort in which the effect of adiponectin on physical functions was investigated (44). In our study, a negative correlation was found between adiponectin levels and the VAS score in motion. This relationship was found in the sarcopenic group. The fact that adiponectin is seen as more determinant in terms of pain intensity in patients with low BMI, especially biomechanical factors, may be related to its role in inflammatory pathology. In our study, the relationship between adiponectin level and quality of life were also studied. In our study, the quality of life of the patients was evaluated using the SF-36 scoring system. When SF-36 sub-scores were compared with adiponectin levels in the OA group, a moderate negative correlation was observed between the pain sub-score. Quality of life can be affected by conditions such as disease activation, pain status, and the presence of comorbidities. It is a predictable situation that a more serious disease state will progress when quality of life worsens.
One of the limitations of our study is that the limit cut-off value for DEXA measurement was used according to the values calculated in previous studies. Since there is no muscle mass limit value determined for Turkish society, our evaluations was based on the studies with the highest number of patients and the values suggested by EWSGOP. In our study, only female patients, thinking that they would create a more homogeneous group, were included. Most previous studies investigating sarcopenia in OA patients also included only women in literature.