ABSTRACT
Se define como estrés (stress) tanto la fuerza que una carga externa ejerce sobre un cuerpo sólido como la fuerza reactiva que acompaña a la primera (Ley de Newton), por unidad de área imaginaria transversal a su dirección. Las cargas internas reactivas inducen deformaciones proporcionales del cuerpo. La resistencia del cuerpo a deformarse se llama rigidez. La deformación puede resquebrajar el cuerpo y, eventualmente, producir una fractura por confluencia de trazos. La resistencia del cuerpo a separarse en fragmentos por esa causa se llama tenacidad. La resistencia del cuerpo a la fractura es proporcional al stress que puede soportar sin separarse en fragmentos por deformación (no hay fractura sin deformación y sin stress previo). El stress máximo que un cuerpo puede soportar sin fracturarse resulta de una combinación de ambas propiedades: rigidez y tenacidad, cada una con distintos determinantes biológicos. Una o varias deformaciones del cuerpo pueden provocarle resquebrajaduras sin fracturarlo. La acumulación de resquebrajaduras determina la "fatiga" del material constitutivo del cuerpo, que reduce su rigidez, tenacidad y resistencia a la fractura para la próxima ocasión ("fragilidad por fatiga"). En el caso de los huesos, en general, los términos stress y fatiga tienen las connotaciones amplias referidas, respecto de todas las fracturas posibles. La fatiga predispone a fracturas a cargas bajas, que se denominan (correctamente) "fracturas por fatiga" y también (incorrectamente) "fracturas por stress", para distinguirlas de las que ocurren corrientemente, sin resquebrajaduras previas al trauma, que se denominan (incorrectamente) "fracturas por fragilidad, o por insuficiencia". En realidad, todas las fracturas se producen por stress y por fragilidad o insuficiencia (en conjunto); pero la distinción grosera entre fracturas "por fatiga, o por stress", por un lado, y "por fragilidad" o "por insuficiencia", por otro, aceptando las amplias connotaciones referidas antes, tiene valor en la práctica clínica. Este artículo intenta explicar esas particularidades biomecánicas y describir las distintas condiciones que predisponen a las fracturas "por fatiga o por stress" en la clínica, distinguiéndolas de las fracturas "por fragilidad o por insuficiencia" (manteniendo estas denominaciones) y detallando las características de interés directo para su diagnóstico y tratamiento. (AU)
The term "stress" expresses the force exerted by an external load on a solid body and the accompanying, opposed force (Newton's Law), expressed per unit of an imaginary area perpendicular to the loading direction. The internal loads generated this way deform (strain) proportionally the body's structure. The resistance of the body to strain expresses its stiffness. Critical strain magnitudes may induce micro-fractures (microdamage), the confluence of which may fracture the body. The body's resistance to separation into fragments determines its toughness. Hence, the body's resistance to fracture is proportional to the stress the body can support (or give back) while it is not fractured by the loadinduced strain (no stress, no strain -> no fracture). Therefore, the maximal stress the body can stand prior to fracture is determined by a combination of both, its stiffness and its toughness; and each of those properties is differently determined biologically. One or more deformations of the body may induce some microdamage but not a fracture. Microdamage accumulation determines the fatigue of the material constitutive of the body and reduces body's toughness, leading to a "fatigue-induced fragility". In case of bones, in general, both stress and fatigue have the referred, wide connotations, regarding any kind of fractures. In particular, bone fatigue predisposes to low-stress fractures, which are named (correctly) "fatigue fractures" and also misnamed "stress fractures", to distinguish them from the current fractures that occur without any excess of microdamage, that are named (wrongly) "fragility" or "insufficiency" fractures. In fact, all fractures result from all stress and fragility or insufficiency as a whole; however, the gross distinction between "fatigue or stress fractures", on one side, and "fragility or insufficiency fractures", on the other, accepting the wide connotations of the corresponding terminology, is relevant to clinical practice. This article aims to explain the above biomechanical features and describe the different instances that predispose to "fatigue or stress fractures" in clinical practice, as a different entity from "insufficiency or fragility fractures" (maintaining this nomenclature), and describe their relevant features to their diagnosis and therapy. (AU)
Subject(s)
Humans , Biomechanical Phenomena/physiology , Fractures, Stress/physiopathology , Osteogenesis Imperfecta/etiology , Bone and Bones/physiology , Bone and Bones/chemistry , Frailty/physiopathology , Flexural Strength/physiologyABSTRACT
We investigated the importance of daily free activity in the cage and body weight gain during the recovering of bone structural and mechanical properties in growing rats after hindlimb unloading. Eight-week-old male Wistar rats were randomly divided into control (CG, n=24) and suspended (SG, n=24) groups. Animals from SG underwent a four-week hindlimb unloading period by tail-suspension. Animals from CG and those from SG after release were kept in collective cages and sacrificed at the age of 12, 16 and 20 weeks. Both femurs were removed and its area, bone mineral density (BMD), resistance to failure and stiffness were determined. Four-week hindlimb unloading decreased (p<0.05) body weight (CG, 373.00 ± 9.47 vs. SG, 295.86 ± 9.19 g), BMD (CG, 0.19 ± 0.01 vs. SG, 0.15 ± 0.01 g/cm2 ), bone resistance to failure (CG, 147.75 ± 5.05 vs. SG, 96.40 ± 5.95 N) and stiffness (CG, 0.38 ± 0.01 vs. SG, 0.23 ± 0.02 N/m). Eight weeks of free activity in cage recovered (p>0.05) the body weight (CG, 472.75 ± 14.11 vs. SG, 444.75 ± 18.91 g), BMD (CG, 0.24 ± 0.01 vs. SG, 0.22 ± 0.01 g/cm2 ), bone resistance to failure (CG, 195.73 ± 10.06 vs. SG, 178.45 ± 8.48 N) and stiffness (CG, 0.56 ± 0.02 vs. SG, 0.47± 0.03 N/m) of SG animals. Body weight correlated strongly with bone structural and mechanical properties (p<0.0001). In conclusion, free activity in the cage associated with body weight gain restored bone structural and mechanical properties in growing rats after hindlimb unloading.
Investigou-se a importância das atividades diárias na caixa e o ganho de peso durante a recuperação das propriedades ósseas estruturais e mecânicas em ratos jovens após hiposinesia de membros pélvicos. Ratos Wistar com oito semanas de idade foram divididos em grupos controle (CG, n=24) e suspensos (SG, n=24). Animais do SG permaneceram quatro semanas suspensos pela cauda. Animais do CG e aqueles do SG após a liberação foram alojados em caixas coletivas e sacrificados com 12, 16 e 20 semanas de idade. Foram mensuradas área, densidade mineral (DMO), resistência de fratura e rigidez do fêmur. Quatro semanas de hiposinesia reduziu (p<0.05) o peso corporal (CG: 373,00 ± 9,47 vs. SG: 295,86 ± 9,19 g), DMO (CG: 0,19 ± 0,01 vs. SG: 0,15 ± 0,01 g/cm2 ), resistência de fratura (CG: 147,75 ± 5,05 vs. SG: 96,40 ± 5,95 N) e rigidez óssea (CG: 0,38 ± 0,01 vs. SG: 0,23 ± 0,02 N/m). Oito semanas de atividade na caixa recuperou (p>0.05) o peso corporal (CG: 472,75 ± 14,11 vs. SG: 444,75 ± 18,91 g), BMO (CG: 0,24 ± 0,01 vs. SG: 0,22 ± 0,01 g/cm2 ), resistência de fratura (CG: 195,73 ± 10,06 vs. SG: 178,45 ± 8,48 N) e rigidez óssea (CG: 0,56 ± 0,02 vs. SG: 0,47± 0,03 N/m) do SG. Peso corporal correlacionou fortemente com as propriedades ósseas, estrutural e mecânica, (p<0.0001). Concluiu-se que a atividade livre na caixa associada ao ganho de peso restaurou as propriedades ósseas estruturais e mecânicas em ratos jovens após hiposinesia dos membros pélvicos.
Subject(s)
Osteoporosis , Bone Diseases, Metabolic , Bone DensityABSTRACT
Objective To investigate the effects of external fixator with dynamic device under low frequency and controlled micromovement on the callus calcification and mechanical property of healing bone.MethodsForty-five sheep were performed transverse osteotomy with a gap of 2 mm on the mid-shafts of both tibias,and the hind limbs were fixed with unilateral external fixators connected to a controlled micromovement device.Ten days after osteotomy,one hind limb of each sheep was randomly selected for micromovement(30 min/d).According to different micromovement frequencies,the sheep were randomly divided into 3 groups: 0.5 Hz group,1 Hz group and 5 Hz group(n=15).The amplitude of micromovement was 0.25 mm and the micromovement stopped by the end of the fourth week postoperation.The other hind limb of each sheep was served as control group without micromovement.Morphometry of callus was done at the end of 4,6 and 9 weeks after osteotomy.Bone formation velocity,bone mineral density and biomechanical properties were compared at the end of 9 weeks.Results The areas of mineralized bone and osteoid in different miromovement groups were larger than that of control group at the end of 4,6 weeks postoperation(P
ABSTRACT
The purpose of this study was to determine the influence of a 6-month unsupervised, flexible and fairly light intensity walking program on endurance fitness, strength, lipids and lipoproteins and bone health in a group of middle-aged sedentary women. Six pre-menopausal and 8 post-menopausal women, aged 54 yr, served as the walk training group (W) and 9 women (2 post-menopausal), aged 49yr, served as controls (C) . W walked an average of 10, 000 steps per day for 6 months, which included an average of 5, 000 steps of brisk walking for 30 min, 4 to 5 days per week. Workloads, heart rates and double-product break point (during incremental maximal ergometer exercise), body weight and %fat, serum lipids, leg strength and bone density (by ultrasound) and induces of bone metabolism were measured at baseline and after 3 and 6 months. Walk training in W resulted in a significant improvement in maximal workload during the exercise test compared with C. Double product break point in W during exercise significantly shifted towards higher workloads and resting heart rate was reduced. Isokinetic muscular strength of leg extensors and abdominal muscular endurance measured by situps were also significantly increased in W. Estimated calcaneal bone density showed a tendency to increase after 6 months of training in W. Indicators of bone resorption and growth remained unchanged. Changes in serum lipids and lipoproteins were also favorable, but not significant. In conclusion, these results show that a flexible and self-regulated walking program is sufficient to elicit improvements in cardiovascular endurance, aerobic capacity measured by DPBP and strength of leg and abdominal muscles. Bone strength and serum lipids were not clearly improved after 6 months with this walking program. If training time were extended to 12 months, significant improvements in these measures can be expected because tendencies toward improvements were observed.
ABSTRACT
The purpose of this study was to examine the bone stiffness of healthy girls during their growth period and factors affecting on such stiffness. The subjects were 302 secondary school girls in Tokyo. Parameters examined included the body characteristics (standing height, body weight, bone stiffness measured by ultrasound, muscle thickness measured with the ultrasound B-mode system), extension power of the lower limb (containing the knee and hip joints), and a questionnaire about the daily intake of milk, kinds of meals and number of years from menophania. These examinations were carried out in June 1997.<BR>Results were summarized as follows : 1) From the observation of bone stiffness in secondary school girls, it appears that bone stiffness increases during the junior high school period. In other words, the bone stiffness of the subjects had almost reached on adult level by high school. 2) A significant positive correlation was recognized between chronological age and bone stiffness (r=0.365, p<0.05) . A positive correlation also existed between the years from menophania and bone stiffness (r=0.477, p<0.05) . These coefficients showed that the years from menophania correlate with bone stiffness more closely in comparison with chronological age. 3) In the period when short comparatively years from menophania, body characteristics, which were the index of maturity, correlated to bone stiffness. However, muscle thickness/power, which was related to exercise habit, became the major parameter correlating with bone stiffness. These results suggest that factors affecting bone stiffness should differ according to the growth period. 4) Comparison of different athletic clubs showed that the bone stiffness of volleyball players was higher than that of control. These data suggest that physical education class, which was given 3 times a week, is not enough for total body development in both quality and quantity. In other words, physical education should be better matched with total physical development, including the growth of healthy bones.
ABSTRACT
The gastrocnemius muscles are composed predominantly of type II B and II A fibers while the soleus muscle is composed of type I fibers. However, the relationships between the calcaneal bone stiffness and the triceps surae muscles consisting of the different types of skeletal muscle fibers are unknown. The purpose of this study was to investigate the relationships between the calcaneal bone stiffness and the gastrocnemius or soleus muscle thickness in 73 postmenopausal women. We measured the speed of sound (SOS) and the broad band ultrasound attenuation (BUA), using a ultrasonic measurement of the calcaneal and the gastrocnemius or soleus muscle thickness used to measure the B-mode ultrasound anatomy. There was a significant differences in age between the high-and low-SOS groups. In the high-BUA group, the body mass index, the lower leg girth and the triceps surae muscle thickness were significantly higher than those in the low-BUA group. This data indicated that SOS is related to ageing and BUA is related to the body mass or the muscle thickness. The other side, stiffness and SOS were significantly correlated with the gastrocnemius muscle thickness, but not with the soleus muscle mass. Therefore, this study suggests that the calcaneal bone stiffness is closely related to the muscle thickness, which may be related especialy to the fast twitch muscle.