KNEE OSTEOARTHRITIS AND AGING: EVALUATION OF THE DIFFERENT MUSCLES OF THIGH

Introduction: Osteoarthritis (OA) is a chronic inflammatory degenerative disease, characterized by progressive degeneration of the articular cartilage, affecting mostly the knee joint. The elderly population is the most affected, intensifying the aging process. The concentration of intramuscular adipose tissue in the thigh muscles and knee OA at different ages remains under investigation. Objective: Investigate the cross-sectional area of thigh muscles at different ages with the relationship of the presence of intramuscular fat and knee OA. Methods: 80 participants were paired into 4 groups: Young Osteoarthritis Group and Old Osteoarthritis Group, both sedentary with knee OA diagnosis II or III; Healthy Young Group and Healthy Old Group, both healthy and sedentary. The groups were paired according to gender and body mass index, submitted to physiotherapeutic evaluation, WOMAC questionnaire, knee X-ray and thigh CT scans. Morphometric analysis was performed manually with ITK-SNAP software (version 3.6), by a single evaluator. Statistical analysis used the one-way ANOVA test followed by Bonferroni post-hoc for dependent variables (p≤0.05). Results: Comparison of the cross-sectional area of the rectus femoris, vastus lateralis, semitendinosus, sartorius and gracilis muscles between the groups indicated no significant differences (p>0.05). The study of attenuation for muscle adipose tissue indicated no significant difference in the values of between the OA-Young and Aged-S groups for all the muscles evaluated. Conclusion: Young individuals with knee OA present muscle morphological characteristics similar to those found during the aging process, characterizing early aging of the thigh muscles. Level of Evidence III; Retrospective cross-sectional and observational study


INTRODUCTION
Osteoarthritis (OA) is a chronic degenerative inflammatory joint disease characterized by the degeneration of articular cartilage and impairment of adjacente tissues .1Among the body's joints, the knee stands out as one of the most affected, 2,3 resulting in several functional, social, and economic consequences. 4uscle tissue is one of the most affected by the presence of OA. 5 Moreover, the increased presence of adipose tissue in the muscle belly (intramuscular fat) and between muscle bundles (intermuscular fat) has been related to reduced ability to produce muscle strength and physical function. 6tudies have shown, through tomography images, that the amount of intramuscular fat infiltration may affect the mass of musculoskeletal structure and, consequently, the muscular performance and mobility of the muscle group involved. 7Therefore, as a way of analyzing the relationship between muscle weakness and aging, it is necessary to investigate which components of the muscles lead to this functional loss 8 and what other factors related to this process can be similar to the development of knee OA. 9 Currently, there are no studies in the literature that investigate these components.Santos et al. (2011), 10 in a study comprising 80 elderly women diagnosed with knee OA, evaluated strength, endurance, and muscle balance using an isokinetic dynamometer.The authors concluded that in the aging process, important changes occur in the cross-sectional area, such as a decrease in muscle mass and an increase in the concentration of intramuscular adipose tissue.Therefore, although the changes that occur in the composition of muscles during the aging process are known, it is not yet clear how knee OA can intensify them.
The similarity between the aging process and the risk factors related to knee OA may be a hypothesis for the relationship between the disease and the observed changes over time in adults and elderly individuals.Thus, the decrease in muscle mass and the increase in intramuscular fat infiltration are conditions related to both the presence of knee OA and the aging process. 11Recent studies have listed several predisposing factors to knee OA and aging-related changes in the cross-sectional area of the thigh muscles.However, it is also necessary to investigate the concentration of intramuscular adipose tissue in different thigh muscles.
Therefore, the aim of this study was to evaluate and compare the cross--sectional area and muscle attenuation of different thigh muscles in individuals with and without knee OA, distributed across different age groups.

METHODS
The evaluations were performed at the Joint Function Analysis Laboratory (LAFAr) of the Physiotherapy Department of the Federal University of São Carlos (UFSCar), and imaging tests were performed at the University Hospital (HU) in São Carlos, São Paulo.
A total of 80 participants of both sexes were included in the study, divided into four groups matched by gender and body mass index.Participants aged between 40 and 50 years old, sedentary, and clinically diagnosed with knee OA grade II or III by radiography were allocated to the Young Osteoarthritis Group.Participants aged 70 years or older, sedentary, and clinically diagnosed with knee OA grade II or III by radiography were allocated to the Elderly Osteoarthritis Group.Individuals aged between 40 and 50 years old, sedentary, and healthy were allocated to the Young Healthy Group, and individuals aged 70 or over, sedentary, and healthy were allocated to the Elderly Healthy Group.The research project was publicized through local media, social networks, the University' s website, and radio in the city of São Carlos.Participants were contacted by phone calls, email, or face-to-face visits.
This project was submitted and approved by the Research Ethics Committee on Human Beings of the Federal University of São Carlos-SP (CAAE: 64171617.9.0000.5504).The participants were informed about the procedures to which they would be submitted and, after agreeing to participate, signed a consent form free and clear.The work was carried out in accordance with the norms of Resolution 466/2012 of the National Health Council on research involving human beings.
The study included participants between 40 and 50 years of age and ≥70 years of age in good health, and participants in the same age group who had a radiographic diagnosis of unilateral or bilateral knee OA, grade II or III, according to the criteria of Kellgren and Lawrence (1956)  (12) and the American College of Rheumatology (2000).For participants with bilateral knee OA, the most symptomatic side was used for assessment.
The exclusion criteria were the same as those previously described by Aily et al. (2019). 13

Experimental procedure
On the first visit, held at LAFAr/UFSCar, participants underwent a physiotherapeutic assessment consisting of inspection, palpation, strength tests, flexibility, and range of motion (ROM) of the knee and hip, in addition to tests for assessing ligament integrity.Subsequently, the participants were instructed and familiarized with the experimental procedures to be performed.They completed an evaluation form and the informed consent form.On this same visit, the participant received a referral to perform an X-ray examination of the knees.The second visit was scheduled after the diagnosis and classification of OA through radiography.In this visit, the participants underwent a body composition exam (Dual-Energy X-Ray Absorptiometry) and the application of the questionnaire.
The radiographic examination was performed bilaterally in posteroanterior views with the knee semiflexed (45°) with weight bearing (orthostatic position), axial to the patellofemoral joint 14 and profile or lateral, in the supine position at 45° of flexion of the knee.Knee OA classification was performed by a radiologist who was a collaborator of this study according to the criteria of Kellgren & Lawrence (1956) and from the American College of Rheumatology (2000).
To assess pain, joint stiffness, and physical function, a questionnaire called WOMAC (Western Ontario and McMaster Universities) was used.The questionnaire consisted of 24 self-reported questions based on information from the 48 hours prior to the application.The questionnaire is divided into three domains: pain, stiffness, and physical function, and the score is determined using a Likert-type scale. 15The final score was obtained by taking the maximum score in each domain, with a higher score indicating worse outcomes.
Thigh images were obtained using a Multislice CT scanner (Brilliance CT 16-slice, Philips) at the University Hospital of UFSCar.The exam was performed by a specialized professional under the supervision of a radiologist.
To acquire the images, criteria from the Eastwood study were used. 16he suggested protocol provided an image of the mid-thigh of the most affected leg or one chosen at random.The images of thigh muscles were obtained using a 5 mm thick cut, halfway between the greater trochanter and the intercondylar notch of the femur.
Muscle areas were analyzed manually by a single evaluator who had been trained using the ITK-SNAP software (version 3.6).The area of interest was selected based on the desired tissue attenuation rate (in mm2).The attenuation rate for intramuscular adipose tissue (IMAT) corresponded to the range from -190 to -30 Hounsfield units (HU). 17nter-rater and test-retest reliabilities were calculated with 10 participants on two different days.The intraclass correlation coefficients for the rectus femoris, vastus lateralis, semimembranosus, semitendinosus, gracilis, long head biceps, sartorius, and adductor magnus and longus were 0.971, 0.865, 0.794, 0.675, 0.986, 0.992, 0.916, and 0.909, respectively.

Statistical Analyses
The data were analyzed using the Statistical Package for the Social Sciences software version 20.0 (SPSS Inc, Chicago, IL, USA).Descriptive analyses of the studied variables were initially performed using the mean, standard deviation, and coefficient of variation.Subsequently, normality and homogeneity of variances were assessed using the Kolmogorov-Smirnov and Levene tests, respectively.
If a normal distribution was not observed, the data were transformed to enable the application of parametric tests.One-way ANOVA was used to compare the dependent variables between groups, followed by Bonferroni's post-hoc test.A significance level of 5% (p≤0.05) was used for all analyses.

RESULTS
Table 1 summarizes the sample characteristics and anthropometric variables of the osteoarthritis groups and the control group.No significant difference was found between groups regarding BMI and total WOMAC score (p>0.05).The cross-sectional area and muscle attenuation of the thigh muscles are described in Table 2.

Rectus Femoris
The cross-sectional area of the rectus femoris muscle did not differ between the groups (p>0.05).However, the average muscle attenuation of the Young-H group showed the highest values of muscle density compared to the other three groups, while the Elderly-OA group had the smallest.Finally, the Young-OA and Elderly-H groups did not present significant differences.

Vastus Lateralis
There was no difference between groups for the cross-sectional area of the vastus lateralis muscle (p>0.05).However, the mean muscle attenuation of the Young-H group showed the highest values of muscle density compared to the other groups, while the Elderly-OA group had the smallest.However, the Young-OA and Elderly-H groups showed no significant differences.

Semitendinosus
The cross-sectional area of the semitendinosus muscle also showed no difference between the groups (p>0.05).However, the analysis of the mean muscle attenuation showed that the Young-H group obtained the highest muscle density values among the groups, and the Elderly--OA group, the smallest.However, the Young-OA and Elderly-H groups showed no significant differences.

Biceps Femoris (Long Head)
The cross-sectional area of the long head of the biceps muscle of the Young-OA group showed significant differences when compared to the Elderly-OA group (p<0.05).The average muscle attenuation of the Young-H group was the highest for muscle density among the groups, while the Elderly-OA group had the lowest.The Young-OA and Elderly-H groups did not show significant differences between them.

Sartorius
The cross-sectional area of the sartorius muscle did not differ between the groups (p>0.05).On the other hand, the average muscle attenuation of the Young-H group showed the highest values for muscle density in relation to the other groups, while the Elderly-OA group had the lowest.The Young-OA group obtained significantly different results when compared to the Elderly-OA group (p<0.05), and the Elderly-OA group also showed different results when compared to the Young-H group.

Gracilis
For the cross-sectional area of the gracilis muscle, there was no difference between the groups (p>0.05).However, the mean muscle attenuation of the Elderly-OA group was significantly lower in relation to the Young-H group.In addition, the highest average of muscle attenuation found was that of the Young-H group, and the lowest was from the Elderly-OA group.

Adductor Magnus and Longus
The cross-sectional area of the adductor magnus and longus muscles of the Young-OA group were significantly different when compared to the Elderly-OA group.The mean muscle attenuation of the Young-H group was the highest, while the Elderly-OA group was the lowest.

Semimembranosus
For the cross-sectional area of the semimembranosus muscle, it was found that the Elderly-OA group had significantly different results

Table 2 .
Cross-sectional area and muscle attenuation of thigh muscles.: HU, Hounsfield.All values were expressed as mean (SD); a p<0.05, significantly different compared to the Elderly-OA group; b p<0.05, significantly different compared to the Young-H group; c p<0.05, significantly different compared to the Elderly-H group. Abbreviations