Stochastic-rheological Simulation of Free-fall Arm Impact in Children: Application to Playground Injuries

The aim of this study was to develop and pilot a stochastic-rheological biomechanical model to investigate the mechanics of impact fractures in the upper limbs of children who fall in everyday situations, such as when playing on playground equipment. The rheological aspect of the model characterises musculo-skeletal tissues in terms of inertial, elastic and viscous parameters. The stochastic aspect of the model allows natural variation of children's musculo-skeletal mechanical properties to be accounted for in the analysis. The relationship of risk factors, such as fall height, impact surface, child mass and bone density, to the probability of sustaining an injury in playground equipment falls were examined and found to closely match findings in epidemiological, clinical and biomechanical literature. These results suggest that the stochastic-rheological model is a useful tool for the evaluation of arm fracture risk in children. Once fully developed, information from this model will provide the basis for recommendations for modifications to playground equipment and surface standards.     

Prediction of distal radius fracture in children, using a biomechanical impact model and case-control data on playground free falls

OBJECTIVES: To assess the ability of a biomechanical impact model to predict the likelihood of distal radius fracture in children using data gathered for a previous epidemiological case-control study of falls from playground equipment. METHODOLOGY: Factor of Risk (FR) values were generated for each of selected subjects from the case-control study using a biomechanical model. Logistic regression curves were fitted to examine the relationship between the FR values and the probability of radius fracture. RESULTS: Forty-five cases and thirty-one controls were selected. The logistic regression analyses showed a significant association between the probability of fracture and FR. CONCLUSIONS: The biomechanical model distinguished between children who fractured their distal radius and those who did not. The model can be used to test how risk factors, such as fall height and ground surface type, affect physical stresses transmitted through the arm and their relation to the fracture tolerance of the distal radius.     

200 injuries caused by playground equipment.

Two-hundred children with injuries caused by playground equipment were studied. Whereas only about 9% of the total casualty attendances are for fractures, 26-5% (53) of these children had fractures. The climbing frame and the slide seemed to be associated with more severe injuries than the swing or other equipment, but more cases need to be studied to confirm this. The youngest children were at particular risk on equipment such as the wooden rocking horse or roundabout, when the speed of operation could be controlled by older children. Many of the injuries to the very youngest children occurred when they were walking behind a moving swing. Faulty equipment did not seem to be a major factor in causing accidents, but the use by older children of apparatus designed for young ones led to accidents. There was supervision, either at home, in a school playground, or in a park, in 62% of the cases. Many of the accidents were the result of the normal desire of children for experimentation and adventure.     

School Playground Surfacing and Arm Fractures in Children: A Cluster Randomized Trial Comparing Sand to Wood Chip Surfaces

Background

The risk of playground injuries, especially fractures, is prevalent in children, and can result in emergency room treatment and hospital admissions. Fall height and surface area are major determinants of playground fall injury risk. The primary objective was to determine if there was a difference in playground upper extremity fracture rates in school playgrounds with wood fibre surfacing versus granite sand surfacing. Secondary objectives were to determine if there were differences in overall playground injury rates or in head injury rates in school playgrounds with wood fibre surfacing compared to school playgrounds with granite sand surfacing.

Methods and Findings

The cluster randomized trial comprised 37 elementary schools in the Toronto District School Board in Toronto, Canada with a total of 15,074 students. Each school received qualified funding for installation of new playground equipment and surfacing. The risk of arm fracture from playground falls onto granitic sand versus onto engineered wood fibre surfaces was compared, with an outcome measure of estimated arm fracture rate per 100,000 student-months. Schools were randomly assigned by computer generated list to receive either a granitic sand or an engineered wood fibre playground surface (Fibar), and were not blinded. Schools were visited to ascertain details of the playground and surface actually installed and to observe the exposure to play and to periodically monitor the depth of the surfacing material. Injury data, including details of circumstance and diagnosis, were collected at each school by a prospective surveillance system with confirmation of injury details through a validated telephone interview with parents and also through collection (with consent) of medical reports regarding treated injuries. All schools were recruited together at the beginning of the trial, which is now closed after 2.5 years of injury data collection. Compliant schools included 12 schools randomized to Fibar that installed Fibar and seven schools randomized to sand that installed sand. Noncompliant schools were added to the analysis to complete a cohort type analysis by treatment received (two schools that were randomized to Fibar but installed sand and seven schools that were randomized to sand but installed Fibar). Among compliant schools, an arm fracture rate of 1.9 (95% confidence interval [CI] 0.04–6.9) per 100,000 student-months was observed for falls into sand, compared with an arm fracture rate of 9.4 (95% CI 3.7–21.4) for falls onto Fibar surfaces (p≤0.04905). Among all schools, the arm fracture rate was 4.5 (95% CI 0.26–15.9) per 100,000 student-months for falls into sand compared with 12.9 (95% CI 5.1–30.1) for falls onto Fibar surfaces. No serious head injuries and no fatalities were observed in either group.

Conclusions

Granitic sand playground surfaces reduce the risk of arm fractures from playground falls when compared with engineered wood fibre surfaces. Upgrading playground surfacing standards to reflect this information will prevent arm fractures.

Trial Registration

Current Controlled Trials ISRCTN02647424 Please see later in the article for the Editors'' Summary     

Biomechanical simulations of forward fall arrests: effects of upper extremity arrest strategy,gender and aging-related declines in muscle strength

Computer simulation was used to predict the extent to which age-related muscle atrophy may adversely affect the safe arrest of a forward fall onto the arms. The biomechanical factors affecting the separate risks for wrist fracture or head impact were examined using a two-dimensional, 5-link, forward dynamic model. The hypothesis was tested in older females that age-related loss in muscular strength renders the use of the arms ineffective in arresting a forward fall without either a torso impact exceeding 0.5m/s or distal forearm loads sufficient to fracture the wrist. The results demonstrate that typical age-related decline in arm muscle strength substantially reduces the ability to arrest a forward fall without the elbows buckling and, therefore, a risk of torso and/or head impact. The model predicted that older women with below-average bone strength risk a Colles fracture when arresting typical falls, particularly with an extended arm.     

Current playground surface test standards underestimate brain injury risk for children

Playgrounds surface test standards have been introduced to reduce the number of fatal and severe injuries. However, these test standards have several simplifications to make it practical, robust and cost-effective, such as the head is represented with a hemisphere, only the linear kinematics is evaluated and the body is excluded. Little is known about how these simplifications may influence the test results. The objective of this study was to evaluate the effect of these simplifications on global head kinematics and head injury prediction for different age groups. The finite element human body model PIPER was used and scaled to seven different age groups from 1.5 up to 18 years old, and each model was impacted at three different playground surface stiffness and three head impact locations. All simulations were performed in pairs, including and excluding the body. Linear kinematics and skull bone stress showed small influence if excluding the body while head angular kinematics and brain tissue strain were underestimated by the same simplification. The predicted performance of the three different playground surface materials, in terms of head angular kinematics and brain tissue strain, was also altered when including the body. A body and biofidelic neck need to be included, together with suitable head angular kinematics based injury thresholds, in future physical or virtual playground surface test standards to better prevent brain injuries.     

Sensitivity of subject-specific models to errors in musculo-skeletal geometry

Subject-specific musculo-skeletal models of the lower extremity are an important tool for investigating various biomechanical problems, for instance the results of surgery such as joint replacements and tendon transfers. The aim of this study was to assess the potential effects of errors in musculo-skeletal geometry on subject-specific model results. We performed an extensive sensitivity analysis to quantify the effect of the perturbation of origin, insertion and via points of each of the 56 musculo-tendon parts contained in the model. We used two metrics, namely a Local Sensitivity Index (LSI) and an Overall Sensitivity Index (OSI), to distinguish the effect of the perturbation on the predicted force produced by only the perturbed musculo-tendon parts and by all the remaining musculo-tendon parts, respectively, during a simulated gait cycle. Results indicated that, for each musculo-tendon part, only two points show a significant sensitivity: its origin, or pseudo-origin, point and its insertion, or pseudo-insertion, point. The most sensitive points belong to those musculo-tendon parts that act as prime movers in the walking movement (insertion point of the Achilles Tendon: LSI=15.56%, OSI=7.17%; origin points of the Rectus Femoris: LSI=13.89%, OSI=2.44%) and as hip stabilizers (insertion points of the Gluteus Medius Anterior: LSI=17.92%, OSI=2.79%; insertion point of the Gluteus Minimus: LSI=21.71%, OSI=2.41%). The proposed priority list provides quantitative information to improve the predictive accuracy of subject-specific musculo-skeletal models.     

Effects of body configuration on pelvic injury in backward fall simulation using 3D finite element models of pelvis–femur–soft tissue complex

Injuries due to backward fall apart from sideways fall are a major health problem, particularly among the aged populations. The objectives of this study was to evaluate the responses to changing body configurations (angle between the trunk and impacting floor as 0°, 15°, 45° and 80°) during backward fall, based on a previously developed CT-scan-derived 3D non-linear and non-homogeneous finite element (FE) model of pelvis–femur–soft tissue complex with simplified biomechanical representation of the whole body. Under constant impact energy, these FE models evaluated the pelvic injury situations on the basis of peak impact force (7.64–16.74 kN) and peak principal compressive strain (more than 1.5%), consistent with the clinically observed injuries (sacral insufficiency, coccydynia). Also the change in location of peak strain and increase in peak impact force for changing configurations from 0° to 80° indicated the effect of whole body inertia during backward fall. It was also concluded that the inclusion of sacro-iliac and acetabular cartilages in the above FE models will further reduce above findings marginally (9.2% for 15° fall). These quantifications would also be helpful for a better design and development of safety structures such as safety floor for the nursing home or home for the aged persons.     

A computational study of the EN 1078 impact test for bicycle helmets using a realistic subject-specific finite element head model

Abstract

In the present study, the free fall impact test in accordance with the EN1078 standard for certification of bicycle helmets is replicated using numerical simulations. The impact scenario is simulated using an experimentally validated, patient-specific head model equipped with and without a bicycle helmet. Head accelerations and intracranial biomechanical injury metrics during the impacts are recorded. It is demonstrated that wearing the bicycle helmet during the impact reduces biomechanical injury metrics, with the biggest reduction seen in the metric for skull fracture.     

Effect of pre-impact movement strategies on the impact forces resulting from a lateral fall

Approximately 90% of hip fractures in older adults result from falls, mostly from landing on or near the hip. A three-dimensional, 11-segment, forward dynamic biomechanical model was developed to investigate whether segment movement strategies prior to impact can affect the impact forces resulting from a lateral fall. Four different pre-impact movement strategies, with and without using the ipsilateral arm to break the fall, were implemented using paired actuators representing the agonist and antagonist muscles acting about each joint. Proportional-derivative feedback controller controlled joint angles and velocities so as to minimize risk of fracture at any of the impact sites. It was hypothesized that (a) the use of active knee, hip and arm joint torques during the pre-contact phase affects neither the whole body kinetic energy at impact nor the peak impact forces on the knee, hip or shoulder and (b) muscle strength and reaction time do not substantially affect peak impact forces. The results demonstrate that, compared with falling laterally as a rigid body, an arrest strategy that combines flexion of the lower extremities, ground contact with the side of the lower leg along with an axial rotation to progressively present the posterolateral aspects of the thigh, pelvis and then torso, can reduce the peak hip impact force by up to 56%. A 30% decline in muscle strength did not markedly affect the effectiveness of that fall strategy. However, a 300-ms delay in implementing the movement strategy inevitably caused hip impact forces consistent with fracture unless the arm was used to break the fall prior to the hip impact.     

Fall-related upper body injuries in the older adult: a review of the biomechanical issues

Although the epidemiology of fall-related injuries is well established for the elderly population over 65 years of age, the biomechanics of how, when and why injuries do and do not occur when arresting a fall have received relatively little attention. This paper reviews the epidemiological literature in the MEDLINE data base pertinent to the biomechanics of fall-related injuries, including data on fall rates, fall-related injury rates, fall directions and types of injuries available. It also covers primary sources not listed on MEDLINE, along with the pertinent biomechanics literature. Many falls in older adults are in a forward direction, and as a result the upper extremities are one of the most commonly injured structures, presumably in protecting the head and torso. In this review emphasis is placed on what is, and what is not, known of the biomechanical factors that determine the impact forces and injury risk associated with upper extremity injuries in forward falls. While decreased bone mineral density may be contributory, it is not a reliable predictor of fracture risk. Evidence is presented that fall-related impact forces can be reduced by appropriate volitional arrest strategies. Further theoretical and experimental research is needed to identify appropriate fall-arrest strategies for the elderly, as well as the physical capacities and skills required to do so. Inexpensive interventions might then be developed to teach safe fall-arrest techniques to older individuals.     

A complete set of control equations for the human musculo-skeletal system

A mathematical model of the total human musculo-skeletal system is presented. The model comprises a link-mechanical and a musculo-mechanical set of ordinary first-order differential equations which describe the dynamics of the segment model and muscle model respectively. The interdependence of the two sets of equations is demonstrated. The set of musculo-mechanical equations contains the two neuromuscular control parameters motor unit recruitment and stimulation rate, and the significance of such a representation for a control-theoretical treatment of musculo-skeletal systems is discussed. Finally, after a short discussion of the successful application of the present model in the prediction of an optimal human motion, further possibilities are indicated of the use of the model for investigations into the control behaviour of musculo-skeletal systems.     

The effect of safer play equipment on playground injury rates among school children

Background

Changes to Canadian Standards Association (CSA) standards for playground equipment prompted the removal of hazardous equipment from 136 elementary schools in Toronto. We conducted a study to determine whether applying these new standards and replacing unsafe playground equipment with safe equipment reduced the number of school playground injuries.

Methods

A total of 86 of the 136 schools with hazardous play equipment had the equipment removed and replaced with safer equipment within the study period (intervention schools). Playground injury rates before and after equipment replacement were compared in intervention schools. A database of incident reports from the Ontario School Board Insurance Exchange was used to identify injury events. There were 225 schools whose equipment did not require replacement (nonintervention schools); these schools served as a natural control group for background injury rates during the study period. Injury rates per 1000 students per month, relative risks (RRs) and 95% confidence intervals (CIs) were calculated, adjusting for clustering within schools.

Results

The rate of injury in intervention schools decreased from 2.61 (95% CI 1.93–3.29) per 1000 students per month before unsafe equipment was removed to 1.68 (95% CI 1.31–2.05) after it was replaced (RR 0.70, 95% CI 0.62–0.78). This translated into 550 injuries avoided in the post-intervention period. In nonintervention schools, the rate of injury increased from 1.44 (95% CI 1.07–1.81) to 1.81 (95% CI 1.07–2.53) during the study period (RR 1.40, 95% CI 1.29–1.52).

Interpretation

The CSA standards were an effective tool in identifying hazardous playground equipment. Removing and replacing unsafe equipment is an effective strategy for preventing playground injuries.Playgrounds provide a recreational refuge for children, away from traffic and other outdoor hazards. In addition, playground activities can enhance children''s cognitive, physical and psychosocial skills. Playground safety is of concern to physicians, parents and injury prevention advocates. Of all playground injuries that result in a visit to a hospital emergency department, 27%–40% are fractures and 17% require hospital admission — a greater frequency of admission than that associated with any other cause of pediatric injury except traffic.^1,2,3,4 The results of an observational study in Wales showed that 90% of all playground injuries resulting in a visit to an emergency department were related to the playground equipment.¹ As might be expected, playgrounds are the location within elementary schools with the highest injury rates and the most severe injuries.⁵ In a study conducted in Kingston, Ont., children were 12 times more likely to be injured in school playgrounds than in municipal playgrounds.³Standards for playgrounds have been developed both in Canada⁶ and internationally.^{7,8,9,10,11,12} The Canadian Standards Association (CSA) standards for the design, installation and maintenance of playgrounds and equipment were most recently revised in 1998.⁶ No published data exist on the relation between equipment standards and injury rates. If applying standards can identify unsafe playgrounds and, more importantly, reduce the rate of child injury, such standards would be a useful tool for school and municipal authorities responsible for playgrounds.We sought to determine the effect of replacing unsafe playground equipment (as determined using the new CSA standards) on injury rates among school children.     

Biomechanical and age-related differences in balance recovery using the tether-release method.

This paper reviews experimental studies that have used the tether-release method to examine the biomechanical and age-related differences in the stepping response used after a simulated fall. The tether-release method has been used to create a repeatable perturbation that simulates the initial unbalanced configuration of the body during a trip or slip. In this technique, the test subject is held in an initial forward or backward inclined position by means of a horizontal tether or cable. To initiate a fall, the subject is released from this position after a short time delay. This review focuses on studies that have explored various biomechanical parameters in an effort to understand what attributes allow for successful balance recovery by stepping or stumbling. Strong associations between recovery ability and biomechanical parameters such as step length, step timing, and joint torques point to the importance of neuromuscular capacities that relate to lower extremity flexibility, reaction time, and strength. Therefore, the maintenance or enhancement of these necessary attributes should be considered when developing exercise-based fall intervention programs for older adults.     

Effect of upper and lower extremity control strategies on predicted injury risk during simulated forward falls: a study in healthy young adults

Fall-related wrist fractures are common at any age. We used a seven-link, sagittally symmetric, biomechanical model to test the hypothesis that systematically alterations in the configuration of the body during a forward fall from standing height can significantly influence the impact force on the wrists. Movement of each joint was accomplished by a pair of agonist and antagonist joint muscle torque actuators with assigned torque-angle, torque-velocity, and neuromuscular latency properties. Proportional-derivative joint controllers were used to achieve desired target body segment configurations in the pre- andor postground contact phases of the fall. Outcome measures included wrist impact forces and whole-body kinetic energy at impact in the best, and worst, case impact injury risk scenarios. The results showed that peak wrist impact force ranged from less than 1 kN to more than 2.5 kN, reflecting a fourfold difference in whole-body kinetic energy at impact (from less than 40 J to more than 160 J) over the range of precontact hip and knee joint angles used at impact. A reduction in the whole-body kinetic energy at impact was primarily associated with increasing negative work associated with hip flexion. Altering upper extremity configuration prior to impact significantly reduced the peak wrist impact force by up to 58% (from 919 N to 2212 N). Increased peak wrist impact forces associated greater shoulder flexion and less elbow flexion. Increasing postcontact arm retraction can reduce the peak wrist impact force by 28% (from 1491 N to 1078 N), but postcontact hip and knee rotations had a relatively small effect on the peak wrist impact force (8% reduction; from 1411 N to 1303 N). In summary, the choice of the joint control strategy during a forward fall can significantly affect the risk of wrist injury. The most effective strategy was to increase the negative work during hip flexion in order to dissipate kinetic energy thereby reducing the loss in potential energy prior to first impact. Extended hip or elbow configurations should be avoided in order to reduce forearm impact forces.     

Experimental production of extra- and intra-articular fractures of the os calcis

Although studies have been conducted in the past to duplicate traumatic fractures of the os calcis, biomechanical force data as a function of extra- and intra-articular fractures are not available. Consequently, in this study, a dynamic single impact model was used to provide such information. Using intact human cadaver lower extremities, impact loading was applied to the plantar surface of the foot using a mini-sled pendulum equipment. The proximal tibia was fixed in polymethylmethacrylate. Following impact, pathology to the os calcis was classified into intact (no injury; 14 cases), and extra-articular (6 cases) and intra-articular (6 cases) fractures. Peak dynamic forces were used to conduct statistical analysis. Mean forces for the intact and (both) fracture groups were 4144 N (standard error, SE: 689) and 7802 N (SE: 597). Mean forces for the extra- and intra-articular fracture groups were 7445 N (SE: 711) and 8159 N (SE: 1006). The peak force influenced injury outcome (ANOVA, p<0.005). Differences in the forces were found between intact and injured specimens (p<0.01); intact specimens and specimens with extra-articular pathology (p<0.001); intact specimens and specimens with intra-articular pathology (p<0.005). The present experimental protocol, which successfully reproduced clinically relevant os calcis pathology, can be extended to accommodate other variables such as the simulation of Achilles tendon force, the inclusion of other angles of force application, and the application of the impact force to limited regions of the plantar force of the foot in order to study other injury mechanisms.     

Surface friction in near-vertex head and neck impact increases risk of injury

A computational head-neck model was developed to test the hypothesis that increases in friction between the head and impact surface will increase head and neck injury risk during near-axial impact. The model consisted of rigid vertebrae interconnected by assemblies of nonlinear springs and dashpots, and a finite element shell model of the skull. For frictionless impact surfaces, the model reproduced the kinematics and kinetics observed in near-axial impacts to cadaveric head-neck specimens. Increases in the coefficient of friction between the head and impact surface over a range from 0.0 to 1.0 resulted in increases of up to 40, 113, 9.8, and 43% in peak post-buckled resultant neck forces, peak moment at the occiput-C1 joint, peak resultant head accelerations, and HIC values, respectively. The most dramatic increases in injury-predicting quantities occurred for COF increases from 0.0 to 0.2, while further COF increases above 0.5 generally produced only nominal changes. These data suggest that safety equipment and impact environments which minimize the friction between the head and impact surface may reduce the risk of head and neck injury in near-vertex head impact.     

Mineral heterogeneity affects predictions of intratrabecular stress and strain

Knowledge of the influence of mineral variations (i.e., mineral heterogeneity) on biomechanical bone behavior at the trabecular level is limited. The aim of this study is to investigate how this material property affects the intratrabecular distributions of stress and strain in human adult trabecular bone. Two different sets of finite element (FE) models of trabecular samples were constructed; tissue stiffness was either scaled to the local degree of mineralization of bone as measured with microCT (heterogeneous) or tissue stiffness was assumed to be homogeneous. The influence of intratrabecular mineral heterogeneity was analyzed by comparing both models. Interesting effects were seen regarding intratrabecular stress and strain distributions. In the homogeneous model, the highest stresses were found at the surface with a significant decrease towards the core. Higher superficial stresses could indicate a higher predicted fracture risk in the trabeculae. In the heterogeneous model this pattern was different. A significant increase in stress with increasing distance from the trabecular surface was found followed by a significant decrease towards the core. This suggests trabecular bending during a compression. In both models a decrease in strain values from surface to core was predicted, which is consistent with trabecular bending. When mineral heterogeneity was taken into account, the predicted intratrabecular patterns of stress and strain are more consistent with the expected biomechanical behavior as based on mineral variations in trabeculae. Our findings indicate that mineral heterogeneity should not be neglected when performing biomechanical studies on topics such as the (long-term or dose dependent) effects of antiresorptive treatments.     

Prediction of upper extremity impact forces during falls on the outstretched hand

Among the most common causes of upper extremity fracture is a fall on the outstretched hand. Yet few data exist on the biomechanical factors which affect injury risk during this event. In this study, we measured impact forces during low-height (0–5 cm), forward falls onto the outstretched hand, and found that these are governed by an initial high-frequency peak and a subsequent, lower-frequency oscillation. This behavior was well-simulated by a two-degree-of-freedom, lumped-parameter mathematical model. Increases in body mass caused greater increases in the peak magnitude of the low-frequency component (F_max2) than the high-frequency component (F_max1). However, increases in fall height more strongly influenced F_max1, which exceeded F_max2 for all but very low fall heights. Model predictions suggest that fall heights greater than 0.6 m carry significant risk for wrist fracture, since above this height, peak forces surpass the average fracture force of the distal radius. Finally, while the shoulder experiences lower peak force than the wrist (since F_max1 is not transmitted proximally), it undergoes considerably greater deflection, and thereby absorbs the majority of impact energy during a fall.