© Borgis - Postępy Nauk Medycznych 1/2017, s. 11-15
*Agata Kusz-Rynkun, Iga Żarnowska
Sarcopenia and osteoporosis – cofactors of increased risk of falls and bone fractures
Sarkopenia i osteoporoza – czynniki zwiększonego ryzyka upadków i złamań kostnych
Department of Geriatrics, Internal Medicine and Metabolic Bone Diseases, Centre of Postgraduate Medical Education, Warsaw
Head of Department: Associate Professor Marek Tałałaj, MD, PhD
Wraz ze starzeniem się wzrasta ryzyko upadków, a w konsekwencji także ryzyko złamań kości. Skutkiem złamań u starszych osób może być nie tylko ból, ale także okresowe lub stałe unieruchomienie, pogorszenie funkcjonalne oraz wzrost śmiertelności. Po złamaniu biodra około połowa osób, które poruszały się samodzielnie przed urazem, nie wraca do pełnej sprawności, a 1/3 z nich pozostaje uzależniona od pomocy otoczenia lub wymaga instytucjonalizacji. Redukcja masy i wytrzymałości kości w osteoporozie oraz masy i siły mięśniowej w sarkopenii odgrywają główną rolę w złamaniach niskoenergetycznych związanych z wiekiem. W ostatnich latach zwraca się uwagę na związek osteoporozy i sarkopenii z zespołem lokomocyjnym wśród starszych pacjentów. Chociaż redukcję masy mięśniowej u seniorów można częściowo tłumaczyć zmniejszeniem aktywności fizycznej, to należy pamiętać, że w przebiegu sarkopenii obserwowane są także zaburzenia metaboliczne. Należy podkreślić, że siła mięśniowa odgrywa ważną rolę w redukcji ryzyka upadków i w konsekwencji ryzyka złamań kości.
With aging, there is an increase in incidence of falls and, in consequence, of bone fractures. Fractures in elderly persons may cause not only pain and temporal immobilization but also permanent mobility deficiency, functional deterioration and increased mortality. Following hip fracture approximately half of the people who had managed to move independently prior to the injury do not regain their full mobility and 1/3 remain dependent of environment with the high risk of institutionalization. Reduced mass and strength of bone in osteoporosis and skeletal muscles in sarcopenia play a main role in the age-related incidence of fragility fractures. Both sarcopenia and osteoporosis have recently been noted for their relationship with locomotive syndrome and are increasingly frequent among older people. Although the reduction of muscle tissue can be partially explained by reduced physical activity with age, sarcopenia also involves metabolic abnormalities. Muscle strength plays an important role in reducing the risk of falling and thus the risk of fractures.
Ageing results in the progressive and parallel loss of bone tissue (osteopenia) and skeletal muscle (sarcopenia) with profound consequences for quality of life. Age-associated sarcopenia results in reduced endurance, poor balance, and reduced mobility that predispose elderly individuals to falls, which frequently result in skeletal fractures because of concomitant osteoporosis. A better understanding of the mechanisms underlying the parallel involution of these tissues is critical to developing new and more effective means to prevent both osteoporosis and sarcopenia in our growing older population (1).
RISK OF FALLS AND BONE FRACTURES
With aging, an increased incidence of falls and, in consequence, of bone fractures is observed. It is caused by two main determinants. The first is the weakness of bones which are unable to resist mechanical loading due to loss of tissue contained within the periosteal envelope and deterioration of the cortical and trabecular bone microstructure – specified in the definition of osteoporosis (2, 3). The second is reduced muscle mass and strength (i.e. sarcopenia) that result in postural instability, increased risk of falling, and higher incidence of fractures. Fracture risk is additionally intensified by hip subcutaneous fat pad thinness which increases mechanical impact exerted on already fragile bone structure (3).
Falls among elderly people are known as one of the great geriatric problems that have major individual and social consequences. According to United States Department of Health and Human Services falls of elderly are the main reason of injuries and the second main reason of death caused by unintended injuries in this age group (4). Falls are undesirable consequences of sarcopenia and the frailty syndrome that includes sarcopenia together with reduced muscle strength, lower physical activity and functional limitations. Approximately 5-6% of falls result in bone fractures followed by dysfunction, forcible immobilization, and complications such as infections, thromboembolism, exacerbating of chronic diseases especially of cardiovascular and pulmonary systems, decubitus, ulcers and disturbances of urinary system (5).
The elements connecting the higher risk of falling and skeletal fractures in elderly are contained in the term “sarcoosteoporosis”. The fractures caused by falls are low-energy fractures. The results of the Study of Osteoporotic Fractures that included over 9,000 white women aged 65 years or more, that have been observed for 4 years, confirmed positive correlation between the frequency of falls and the occurrence of fractures. Among women who experienced at least one fall the nonvertebral fractures as well as fractures of the proximal femur and distal forearm occurred nearly twice as often compared to women who did not fall during the observation (6).
Fractures in elderly persons cause not only pain and temporal immobilization but may also result in permanent mobility deficiency, functional deterioration and increased mortality. That refers primarily to the fractures of proximal femur, spine and distal forearm. It was estimated that following hip fracture approximately half of the people who had managed to move independently prior to the injury do not regain their full mobility and 1/3 remain dependent of environment with the high risk of institutionalization (5).
Many studies have focused on sarcopenia since Irwin Rosenberg first identified the condition in 1989 but a precise definition, measurement methods and index of sarcopenia have yet to be established (7, 8). In recent studies, sarcopenia is defined as a systemic continuous decrease in skeletal muscle strength and mass so that the diagnosis of sarcopenia has to be confirmed by measurements of muscle mass and assessment of functional muscle strength (3).
Sarcopenia is diagnosed in 30% of individuals aged 60 years or older and in more than 50% of those aged at least 80 years (9). It is caused by imbalance between protein synthesis and degradation. Increased, with age, fatty infiltration of skeletal muscles (myosteatosis) reduces muscle strength and increases the propensity to falls as well as the risk of fractures (3, 10).
Sarcopenia is considered to be one of the major factors responsible for functional limitations and motor dependency in elderly persons (11). This relationship was originally thought to be explained by an increased risk of falling in sarcopenic patients (12). Since a definition of sarcopenia encompasses muscle size, strength and physical performance, the relationships between each of these parameters and bone size, density and strength were investigated to interrogate this hypothesis further in participants from the Hertfordshire Cohort Study (HCS). In years 2004-2005, 437 men and 447 women from the geographical area of East Hertfordshire were invited for a follow up study. Of these, 322 men (65%) and 320 women (68%) agreed to participate. A detailed questionnaire was administered to obtain information on lifestyle, medical history, cigarette smoking and alcohol consumption. Details regarding physical activity, dietary calcium intake, socioeconomic status and, in women, years since menopause and use of estrogen replacement therapy had already been obtained from a questionnaire which was administered by trained nurses when the participants were initially recruited into the HCS (1998-2003). Physical activity was calculated as a standardized score ranging from 0 to 100 derived from frequency of gardening, housework, climbing stairs and carrying loads in a typical week. Higher scores indicated greater levels of activity. Dietary calcium intake was assessed using a food frequency questionnaire. Socioeconomic status was determined using own current or most recent occupation of the participant in men and single women, and of the husband in ever-married women based on the OPCS Standard Occupational Classification scheme for occupation and social class. The Hertfordshire Cohort Study has shown that muscle size is strongly associated with bone size and bone strength in both men and women and that these relationships remain robust after rigorous adjustment (12).
It was shown that muscle mass decreased by 3-8% per decade just after the age of 30 years, and after the age of 60 years the rate of muscle loss has only accelerated (13). The loss of muscle mass and strength was found to be the result of progressive atrophy, loss of muscle fibers, reduced motor neuron input and impaired function of the contractile apparatus within each fiber. Reduction in quality of muscle units and motor neuron number was reported to contribute to the significant progressive decline in physical capacity with aging (14).
Although the reduction of muscle tissue can be partially explained by reduced physical activity in elderly persons, sarcopenia also involves metabolic abnormalities, including reduced insulin sensitivity, fat and connective tissue infiltration, impaired resistance against oxidative stress, reduced hormone levels, and decreased mitochondrial activity that further confound muscle function (9, 10, 15, 16).
Genetic, developmental, endocrine and lifestyle factors, such as physical activity, exert significant effects on both muscles and bones. Several mechanisms were also determined to play a role in the age-related decrease in muscle mass and strength, including altered hormonal status, inflammatory processes, reduced physical activity and malnutrition, most often resulting from low dietary intake of energy and protein (3, 9).
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