Ariel Simkin

In vivo measurement of human tibial strains during vigorous activity

Our understanding of mechanical controls on bone remodeling comes from studies of animals with surgically implanted strain gages, but in vivo strain measurements have been made in a single human only once. That study showed that strains in the human tibia during walking and running are well below the fracture threshold. However, strains have never been monitored in vivo during vigorous activity in people, even though prolonged strenuous activity may be responsible for the occurrence of stress fractures. We hypothesized that strains > 3000 microstrain could be produced on the human tibial midshaft during vigorous activity. Strains were measured on the tibiae of two subjects via implanted strain gauges under conditions similar to those experienced by Israeli infantry recruits. Principal compressive and shear strains were greatest for uphill and downhill zigzag running, reaching nearly 2000 microstrain in some cases, about three times higher than recorded during walking. Strain rates were highest during sprinting and downhill running, reaching 0.050/sec. These results show that strain is maintained below 2000 microstrain even under conditions of strenuous activity. Strain rates are higher than previously recorded in human studies, but well within the range reported for running animals.

Combined Effect of Foot Arch Structure and an Orthotic Device on Stress Fractures

In a prospective study, quantitative measures of the structure of the longitudinal arch of the foot were established and related to the incidence of stress fractures in the bones of the lower limbs of military recruits. In addition, the role of a semirigid orthotic device (Langer military stress orthotic) in preventing stress fractures was evaluated as a function of the structure of the longitudinal arch. Femoral and tibial stress fractures were found to be more prevalent in the presence of feet with high arches, whereas the incidence of metatarsal fractures was higher in feet with low arches. The use of an orthotic device reduced the incidence of femoral stress fractures only in the presence of feet with high arches and the incidence of metatarsal fractures only among feet with low arches. The findings suggest that the normal foot with a low arch acts as a better shock absorber than the normal foot with a high arch, and that an orthotic device may improve the shock absorbing capacity of the arch.

Increased trabecular bone density due to bone-loading exercises in postmenopausal osteoporotic women

SummaryA program of diverse and dynamic loading exercises of the distal forearm, a common site of osteoporotic fractures, was applied three times a week for 5 months to 14 postmenopausal osteoporotic women. Two parameters were used to assess the effect of the exercises on bone mass. The mass density of bone (g/cm3) was measured by the Compton scattering technique. The bone mineral content (g/cm2) was measured by single photon absorptiometry. Both measurements were taken at the same location in the distal radius 1 year prior to the exercise period, at its beginning, and at its end, in both the exercised group and a matched control group of 25 osteoporotic women. During the exercise period the mean bone density decreased by 1.9% in the control group whereas it increased by 3.8% in the exercise group. The bone mineral content results did not demonstrate any significant trend over the whole period of the study. It is concluded that the trabecular bone tissue in the distal radius of postmenopausal osteoporotic women responds favorably to dynamic and diverse bone stressing exercises even in the seventh decade of life.

Stress fractures Identifiable risk factors

To answer the question why such large differences in stress fracture morbidity rates (2% to 64%) exist in different countries, we prospectively evaluated 312 re cruits for possible risk factors for stress fractures. Prior to training, each recruit underwent an evaluation includ ing the following: orthopaedic examination, foot and tibial radiographs, measurements of tibial bone width, bone mineral content, bone density, aerobic physical fitness and leg power, assessments of somatotype and smoking habits, and evaluation of sociological and psy chological factors. Using a multivariate analysis, two risk factors were identified: recruits with stress frac tures had significantly narrower tibiae (P < 0.001), and a higher degree of external rotation of the hip (P = 0.016). These two variables were independent and cumulative. Stress fracture morbidity was 17%, 29%, and 45% when neither, one, or both risk factors were present, respectively (P < 0.001). Identification of these risk factors might explain the susceptibility of some people to stress fractures.

Risk Factors for Lateral Ankle Sprain: A Prospective Study Among Military Recruits

In a prospective study of risk factors for lateral ankle sprain among 390 male Israeli infantry recruits, a 18% incidence of lateral ankle sprains was found in basic training. There was no statistically significant difference in the incidence of lateral ankle sprains between recruits who trained in modified basketball shoes or standard lightweight infantry boots. By multivariate stepwise logistic regression a statistically significant relationship was found between body weight × height 2 (a magnitude which is proportional to the mass moment of inertia of the body around a horizontal axis through the ankle), a previous history of ankle sprain, and the incidence of lateral ankle sprains. Recruits who were taller and heavier and thus had larger mass moments of inertia (P = 0.004), and those with a prior history of ankle sprain (P = 0.01) had higher lateral ankle sprain morbidity in basic training.

FRACAS: A system for computer‐aided image‐guided long bone fracture surgery

This article describes FRACAS, a computer-integrated orthopedic system for assisting surgeons in performing closed medullary nailing of long bone fractures. FRACASs goal is to reduce the surgeons cumulative exposure to radiation and surgical complications associated with alignment and positioning errors of bone fragments, nail insertion, and distal screw locking. It replaces uncorrelated, static fluoroscopic images with a virtual reality display of three-dimensional bone models created from preoperative computed tomography and tracked intraoperatively in real time. Fluoroscopic images are used to register the bone models to the intraoperative situation and to verify that the registration is maintained. This article describes the system concept, software prototypes of preoperative modules (modeling, nail selection, and visualization), intraoperative modules (fluoroscopic image processing and tracking), and preliminary in vitro experimental results to date. Our experiments suggest that the modeling, nail selection, and visualization modules yield adequate results and that fluoroscopic image processing with submillimetric accuracy is practically feasible on clinical images.

Effect of intense physical activity on the bone-mineral content in the lower limbs of young adults.

The effect of intense physical training on the bone-mineral content of young adults who are unaccustomed to physical activity has not yet been established. In this study, 268 male recruits, eighteen to twenty-one years old, were evaluated before and after fourteen weeks of strenuous physical training. The bone-mineral content of both legs at the level of the distal third of the tibia in each individual was measured using single-beam photon absorptiometry. The subjects started their training with equal values for bone-mineral content in both legs. During the training period, the average bone-mineral content of the left leg increased by 11.1 per cent and that of the right leg, by 5.2 per cent. In the group of subjects who did not complete the training course, mainly because they incurred stress fractures, the increase in bone-mineral content was significantly less than the increase in those who completed the program. This study indicated that in young adults a high level of loading of bone results in either a stress fracture or a rapid increase in bone-mineral content.

Stress fractures and tibial bone width. A risk factor

A prospective study of 295 infantry recruits has shown that the mediolateral width of the tibia measured radiographically at each of three different levels in the bone had a statistically significant correlation with the total incidence of stress fractures as well as with those in the tibia alone or the femur alone. A narrow tibial width was shown to be a risk factor, but cortical thickness was not found to be significant.

A Prospective Study of the Effect of a Shock-Absorbing Orthotic Device on the Incidence of Stress Fractures in Military Recruits

In a prospective study of stress fractures the hypothesis that a shock-absorbing orthotic device worn within military boots could lessen the incidence of stress fractures was tested. The incidence of metatarsal, tibial, and femoral stress fractures was lower in the orthotic group, but only the latter difference was statistically significant. The time of onset and the location of stress fractures between orthotic and nonorthotic users did not differ. These findings suggest that the incidence of femoral stress fractures, which are the most dangerous type of stress fracture because of their high risk of developing into displaced fractures, can be reduced by an orthotic device.

In vivo strain measurements to evaluate the strengthening potential of exercises on the tibial bone

Mechanical loading during physical activity produces strains within bones. It is thought that these forces provide the stimulus for the adaptation of bone. Tibial strains and rates of strain were measured in vivo in six subjects during running, stationary bicycling, leg presses and stepping and were compared with those of walking, an activity which has been found to have only a minimal effect on bone mass. Running had a statistically significant higher principal tension, compression and shear strain and strain rates than walking. Stationary bicycling had significantly lower tension and shear strains than walking. If bone strains and/or strain rates higher than walking are needed for tibial bone strengthening, then running is an effective strengthening exercise for tibial bone.