Development of a stick-on hip protector: A multiple methods study to improve hip protector design for older adults in the acute care environment

Introduction

Over 90% of hip fractures in older adults result from falls, and hospital patients are at especially high risk. Specific types of wearable hip protectors have been shown to reduce hip fracture risk during a fall by up to 80%, but user compliance has averaged less than 50%. We describe the development and evaluation of a "stick-on" hip protector (secured over the hip with a skin-friendly adhesive) for older patients in acute care.

Methods

An initial version of the product was evaluated with six female patients (aged 76-91) in a hospital ward, who were asked to wear it for one week. We subsequently refined the product through biomechanical testing and solicited feedback from 43 health professionals on a second prototype.

Results

The first prototype was worn by five of six patients for the full week or duration of their hospital stay. The second prototype (20 mm thick, surface area 19 × 15.5 cm) provided 36% force attenuation, more than common garment-based models (20-21%). Feedback from patients and health professionals highlighted usability, comfort, cost, and appearance.

Conclusions

Our results from biomechanical and user testing support the need for further work to determine the value of stick-on hip protectors in acute care.

Effect of neck flexor muscle activation on impact velocity of the head during backward falls in young adults

Falls are a common cause of traumatic brain injuries (TBI) across the lifespan. A proposed but untested hypothesis is that neck muscle activation influences impact severity and risk for TBI during a fall. We conducted backward falling experiments to test whether activation of the neck flexor muscles facilitates the avoidance of head impact, and reduces impact velocity if the head contacts the ground. Young adults (n=8) fell from standing onto a 30cm thick gymnastics mat while wearing a helmet. Participants were instructed to fall backward and (a) prevent their head from impacting the mat ("no head impact" trials); (b) allow their head to impact the mat, but with minimal impact severity ("soft impact" trials); and (c) allow their head to impact the mat, while inhibiting efforts to reduce impact severity ("hard impact" trials). Trial type associated with peak magnitude of electromyographic activity of the sternocleidomastoid (SCM) muscles (p<0.017), and with the vertical and horizontal velocity of the head at impact (p<0.001). Peak SCM activations, expressed as percent maximal voluntary isometric contraction (%MVIC), averaged 75.3, 67.5, and 44.5%MVIC in "no head impact", "soft impact", and "hard impact" trials, respectively. When compared to "soft impact" trials, vertical impact velocities in "hard impact" trials averaged 87% greater (3.23 versus 1.73m/s) and horizontal velocities averaged 83% greater (2.74 versus 1.50m/s). For every 10% increase in SCM %MVIC, vertical impact velocity decreased 0.24m/s and horizontal velocity decreased 0.22m/s. We conclude that SCM activation contributes to the prevention and modulation of head impact severity during backward falls.

Risk factors for hip impact during real-life falls captured on video in long-term care

Hip fracture risk is increased by landing on the hip. We examined factors that contribute to hip impact during real-life falls in long-term care facilities. Our results indicate that hip impact is equally likely in falls initially directed forward as sideways and more common among individuals with dependent Activities of Daily Living (ADL) performance.

INTRODUCTION

The risk for hip fracture in older adults increases 30-fold by impacting the hip during a fall. This study examined biomechanical and health status factors that contribute to hip impact through the analysis of real-life falls captured on video in long-term care (LTC) facilities.

METHODS

Over a 7-year period, we captured 520 falls experienced by 160 residents who provided consent for releasing their health records. Each video was analyzed by a three-member team using a validated questionnaire to determine whether impact occurred to the hip or hand, the initial fall direction and landing configuration, attempts of stepping responses, and use of mobility aids. We also collected information related to resident physical and cognitive function, disease diagnoses, and use of medications from the Minimum Data Set.

RESULTS

Hip impact occurred in 40 % of falls. Falling forward or sideways was significantly associated with higher odds of hip impact, compared to falling backward (OR 4.2, 95 % CI 2.4-7.1) and straight down (7.9, 4.1-15.6). In 32 % of sideways falls, individuals rotated to land backward. This substantially reduced the odds for hip impact (0.1, 0.03-0.4). Tendency for body rotation was decreased for individuals with dependent ADL performance (0.43, 0.2-1.0).

CONCLUSIONS

Hip impact was equally likely in falls initially directed forward as sideways, due to the tendency for axial body rotation during descent. A rotation from sideways to backward decreased the odds of hip impact 10-fold. Our results may contribute to improvements in risk assessment and strategies to reduce risk for hip fracture in older adults.

Effects of hip abductor muscle forces and knee boundary conditions on femoral neck stresses during simulated falls

Through experiments that simulated sideways falls with a mechanical hip impact simulator, we demonstrated the protective effect of hip abductor muscle forces in reducing peak stresses at the femoral neck and the corresponding risk for hip fracture.

INTRODUCTION

Over 90% of hip fractures are due to falls, and an improved understanding the factors that separate injurious and non-injurious falls (via their influence on the peak stress generated at the femoral neck) may lead to improved risk assessment and prevention strategies. The purpose of this study was to measure the effect of muscle forces spanning the hip, and knee boundary conditions, on peak forces and estimated stresses at the femoral neck during simulated falls with a mechanical system.

METHODS

We simulated hip abductor muscle forces and knee boundary conditions with a mechanical hip impact simulator and measured forces and stresses at the femoral neck during sideways falls.

RESULTS

Peak compressive and tensile stresses, shear force, bending moment, and axial force are each associated with hip abductor muscle forces and knee boundary conditions (p < 0.0005). When muscle force increased from 400 to 1,200 N, peak compressive and tensile stresses decreased 24 and 56%, respectively. These effects were similar to the magnitude of decline in fracture strength associated with osteoporosis and arose from the tension-band effect of the muscle in reducing the bending moment by 37%. Furthermore, peak compressive and tensile stresses averaged 40 and 51% lower, respectively, in the free knee than fixed knee condition.

CONCLUSIONS

Contraction of the hip abductor muscles at the moment of impact during a fall, and landing with the knee free of constraints, substantially reduced peak compressive and tensile stresses at the femoral neck and risk for femoral fracture in a sideways fall.

Age-related changes in dynamic compressive properties of trochanteric soft tissues over the hip

Hip fracture risk increases dramatically with age, and 90% of fractures are due to falls. During a fall on the hip, the soft tissues overlying the hip region (skin, fat, and muscle) act as shock absorbers to absorb energy and reduce the peak force applied to the underlying bone. We conducted dynamic indentation experiments with young women (aged 19-30; n=17) and older women (aged 65-81; n=17) to test the hypothesis that changes occur with age in the stiffness and damping properties of these tissues. Tissue stiffness and damping were derived from experiments where subjects lay sideways on a bed with the greater trochanter contacting a 3.8cm diameter indenter, which applied sinusoidal compression between 5 to 30Hz with a peak-to-peak amplitude of 1mm. Soft tissue thickness was measured using ultrasound. On average, stiffness was 2.9-fold smaller in older than young women (5.7 versus 16.8kN/m, p=0.0005) and damping was 3.5-fold smaller in older than young women (81 versus 282Ns/m, p=0.001). Neither parameter associated with soft tissue thickness. Our results indicate substantial age-related reductions in the stiffness and damping of soft tissues over the hip region, which likely reduce their capacity to absorb and dissipate energy (before "bottoming out") during a fall. Strategies such as wearable hip protectors or compliant flooringmay compensate for age-related reductions in the shock-absorbing properties of soft tissues and decrease the injury potential of falls.

Perception of postural limits during reaching

The relationship between perceived and actual postural limits in reaching by healthy young and middle-aged participants was assessed. Subjects (N = 51) first estimated their expected performance and then executed maximum reaches along a tape measure mounted at shoulder height. Measures of standing and bending reaches were obtained. Subjects estimated their reach limits reasonably accurately but significantly underestimated bending reach and overestimated standing reach. That finding suggests that individuals scale perceived abilities with perceived risk in attempting a given action. The accuracy of a participant's perceived bending reach was unrelated to his or her height, weight, age, and gender, and was only weakly correlated with actual reach excursion (bending - standing reach). The accuracy was strongly correlated with the accuracy of subjects' perceived standing reach; individuals who underestimated standing reach underestimated bending reach much more. That result and the observed lack of correlation between the magnitudes of estimated bending reach and estimated standing reach suggest a serial strategy of estimating bending reach by considering and summing perceived arm's length and perceived reach excursion.

Pressure distribution over the palm region during forward falls on the outstretched hands

Falls on the outstretched hands are the cause of over 90% of wrist fractures, yet little is known about bone loading during this event. We tested how the magnitude and distribution of pressure over the palm region during a forward fall is affected by foam padding (simulating a glove) and arm configuration, and by the faller's body mass index (BMI) and thickness of soft tissues over the palm region. Thirteen young women with high (n=7) or low (n=6) BMI participated in a "torso release experiment" that simulated falling on both outstretched hands with the arm inclined either at 20° or 40° from the vertical. Trials were acquired with and without a 5 mm thick foam pad secured to the palm. Outcome variables were the magnitude and location of peak pressure (d, θ) with respect to the scaphoid, total impact force, and integrated force applied to three concentric areas, including "danger zone" of 2.5 cm radius centered at the scaphoid. Soft tissue thickness over the palm was measured by ultrasound. The 5mm foam pad reduced peak pressure, and peak force to the danger zone, by 83% and 13%, respectively. Peak pressure was 77% higher in high BMI when compared with low BMI participants. Soft tissue thickness over the palm correlated positively with distance (d) (R=0.79, p=0.001) and force applied outside the danger zone (R=0.76, p=0.002), but did not correlate with BMI (R=0.43, p=0.14). The location of peak pressure was shunted 4 mm further from the scaphoid at 20° than that of 40° falls (d=25 mm (SD 8), θ=-9° (SD 17) in the 20° falls versus d=21 mm (SD 8), θ=-5° (SD 24) in the 40° falls). Peak force to the entire palm was 11% greater in 20° compared with 40° falls. These results indicate that even a 5 mm thick foam layer protects against wrist injury, by attenuating peak pressure over the palm during forward falls. Increased soft tissue thickness shunts force away from the scaphoid. However, soft tissue thickness is not predicted by BMI, and peak pressures are greater in high individuals than that of low BMI individuals. These results contribute to our understanding of the mechanics and prevention of wrist and hand injuries during falls.

Effect of hip protectors, falling angle and body mass index on pressure distribution over the hip during simulated falls

BACKGROUND

We examined how a soft shell hip protector affects the magnitude and distribution of force to the hip during simulated falls, and how the protective effect depends on the fall direction and the amount of soft tissue padding over the hip.

METHODS

Fourteen young women with either high or low body mass index participated in a "pelvis release experiment" that simulated falls resulting in either lateral, anterolateral or posterolateral impact to the pelvis with/without a soft shell hip protector. Outcome variables were the magnitude and location of peak pressure (d, theta) with respect to the greater trochanter, total impact force, and percent force applied to four defined hip regions.

FINDINGS

The soft shell hip protector reduced peak pressure by 70%. The effect was two times greater in low than high body mass index individuals. The protector shunted the peak pressure distally along the shaft of the femur (d=52 mm (SD 22), theta=-21 degrees (SD 49) in the unpadded trials versus d=81 mm (SD 23), theta=-10 degrees (SD 35) in the padded trials). Peak force averaged 12% greater in posterolateral and 17% lower in anterolateral than lateral falls.

INTERPRETATION

Our results indicate that the hip protector we tested had a much stronger protective benefit for low than high body mass index individuals. Next generation protectors might be developed for improved shunting of pressure away from the femur, improved protection during posterolateral falls, and greater force attenuation for low body mass index individuals.

Hip protectors: recommendations for biomechanical testing--an international consensus statement (part I)

INTRODUCTION

Hip protectors represent a promising strategy for preventing fall-related hip fractures. However, clinical trials have yielded conflicting results due, in part, to lack of agreement on techniques for measuring and optimizing the biomechanical performance of hip protectors as a prerequisite to clinical trials.

METHODS

In November 2007, the International Hip Protector Research Group met in Copenhagen to address barriers to the clinical effectiveness of hip protectors. This paper represents an evidence-based consensus statement from the group on recommended methods for evaluating the biomechanical performance of hip protectors.

RESULTS AND CONCLUSIONS

The primary outcome of testing should be the percent reduction (compared with the unpadded condition) in peak value of the axial compressive force applied to the femoral neck during a simulated fall on the greater trochanter. To provide reasonable results, the test system should accurately simulate the pelvic anatomy, and the impact velocity (3.4 m/s), pelvic stiffness (acceptable range: 39-55 kN/m), and effective mass of the body (acceptable range: 22-33 kg) during impact. Given the current lack of clear evidence regarding the clinical efficacy of specific hip protectors, the primary value of biomechanical testing at present is to compare the protective value of different products, as opposed to rejecting or accepting specific devices for market use.

Effect of soft shell hip protectors on pressure distribution to the hip during sideways falls

INTRODUCTION

While hip protectors represent a promising strategy for preventing hip fractures, clinical efficacy has been limited by poor user compliance. Soft shell protectors may be more acceptable to users than traditional hard shell designs. However, before embarking on clinical trials to assess efficacy, laboratory experiments are required to determine how soft shell protectors affect the force applied during impact to the hip. This was the goal of the current study.

METHODS

Fifteen women participated in "pelvis release experiments," which safely simulate the impact stage of a sideways fall. During the trials, we measured total impact force and mean pressure over the greater trochanter with the participant unpadded, and while wearing two commercially available soft shell protectors.

RESULTS

Mean pressure over the greater trochanter was reduced by 76% by a 14-mm thick horseshoe-shaped protector and by 73% by a 16-mm thick continuous protector. Total force was reduced by 9% by the horseshoe and by 19% by the continuous protector.

CONCLUSIONS

Soft shell hip protectors substantially reduce the pressure over the greater trochanter, while only modestly reducing total impact force during simulated sideways falls. These data support the need for clinical trials to determine whether soft shell protectors reduce hip fracture risk in vulnerable populations.

Elderly subjects' ability to recover balance with a single backward step associates with body configuration at step contact

BACKGROUND

In the event of a slip or trip, one's ability to recover a stable upright stance by stepping should depend on (a) the configuration of the body at the instant of step contact and (b) the forces generated between the foot and ground during step contact. In this study, we tested whether these two variables associate with elderly subjects' ability to recover balance by taking a single backward step after sudden release from an inclined position.

METHODS

Twenty-six community-dwelling subjects (12 women, 14 men) of mean age 75+/-4 (SD) years each underwent five trials in which they were suddenly released from a backward inclination of 7 degrees and instructed to "recover balance with a single step." Body segment motions and foot contact forces were analyzed to determine step contact times, stepping angles, body lean angles at step contact, and the magnitudes and times (after step contact) of peak foot-floor contact forces and peak sagittal-plane torques at the ankle, knee, and hip of the stepping leg.

RESULTS

Fifty percent of subjects were predominantly single steppers (successful at recovering with a single step in greater than three of five trials), 27% were multiple steppers (successful in less than two of five trials), and 23% were mixed response steppers (successful in two of five or three of five trials). Recovery style associated with the ratio of stepping angle divided by body lean angle at step contact (p = .003), which averaged 1.4+/-0.5 for single steppers and 0.6+/-0.5 for multiple steppers, but not with step contact time, stepping angle, or contact forces and joint torques during step contact.

CONCLUSIONS

These results suggest that elderly subjects' ability to recover balance with a single backward step depends primarily on the configuration of the body (in particular, the ratio of stepping angle to body lean angle) at step contact.

Impact severity in self-initiated sits and falls associates with center-of-gravity excursion during descent

Although the energy available during a fall from standing greatly exceeds that required to produce hip fracture, this occurs in only about 2% of falls in the elderly. This is thought to be due in part to one's ability to reduce the vertical impact velocity (nu(nu)) and kinetic energy (KE(nu)) of the body through energy absorption in the lower extremity muscles during descent. The present study tested the hypothesis that the magnitude and percent attenuation in nu(nu) and KE(nu) associate with the horizontal and vertical excursion of the body's center-of-gravity during descent. Measures were acquired of whole-body kinematics and lower extremity kinetics as young subjects underwent backward descents involving vertical drops of either thigh length (SIT) or lower extremity length (FALL), and horizontal pelvis excursions of either 33 or 66% of lower extremity length. In all trials, subjects attempted to "land as softly as possible." While attenuation in nu(nu) and KE(nu) (which averaged 62 and 92% respectively), did not associate with trial type, raw magnitudes of these parameters did, with nu(nu) averaging 2-fold greater, and KE(nu) averaging 6-fold greater, in 66% FALL than in 33% SIT or 66% SIT trials. This was due to a rapid increase in downward velocity accompanying the final stage of descent in 66% SIT and 66% FALL trials, which coincided with the knee moving posterior to the ankle. Accordingly, severe impacts likely accompany not only large fall heights, but also falls where the feet are thrown rapidly forward, as during a backward slip.

Prevention of falls and fall-related fractures through biomechanics

Falls and fall-related injuries are a major health problem for elderly people. Biomechanical studies provide important insight into the cause of such events and reveal new techniques for preventing them. The topics reviewed in this article include balance recovery, safe landing responses, impact forces during falls, and fracture prevention through exercise programs, hip pads, and energy-absorbing floors.

Biomechanical influences on balance recovery by stepping

Stepping represents a common means for balance recovery after a perturbation to upright posture. Yet little is known regarding the biomechanical factors which determine whether a step succeeds in preventing a fall. In the present study, we developed a simple pendulum-spring model of balance recovery by stepping, and used this to assess how step length and step contact time influence the effort (leg contact force) and feasibility of balance recovery by stepping. We then compared model predictions of step characteristics which minimize leg contact force to experimentally observed values over a range of perturbation strengths. At all perturbation levels, experimentally observed step execution times were higher than optimal, and step lengths were smaller than optimal. However, the predicted increase in leg contact force associated with these deviations was substantial only for large perturbations. Furthermore, increases in the strength of the perturbation caused subjects to take larger, quicker steps, which reduced their predicted leg contact force. We interpret these data to reflect young subjects' desire to minimize recovery effort, subject to neuromuscular constraints on step execution time and step length. Finally, our model predicts that successful balance recovery by stepping is governed by a coupling between step length, step execution time, and leg strength, so that the feasibility of balance recovery decreases unless declines in one capacity are offset by enhancements in the others. This suggests that one's risk for falls may be affected more by small but diffuse neuromuscular impairments than by larger impairment in a single motor capacity.

Perception of postural limits in elderly nursing home and day care participants

This study explored whether, when compared to young community-dwelling individuals, elderly nursing home and day care participants have less accurate perceptions of their postural stability borders (postural limits). Subjects estimated their performance before executing maximum forward reaches while maintaining the feet stationary. Whereas young subjects tended to underestimate their reaching limits, elderly subjects displayed no significant difference between estimated and actual values. Furthermore, errors in estimated reach limits associated with reaching ability, with less-able reachers tending to more greatly overestimate their abilities. This suggests that elderly nursing home and day care participants, and especially those with impaired postural limits, lack the potential "safety factor" observed in young subjects of underestimating their stability borders. Therefore, the link between decreased postural limits and falls in older persons may in part be due to lack of awareness of such declines, and the resulting tendency to plan movements which create loss of balance.

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 (Fmax2) than the high-frequency component (Fmax1). However, increases in fall height more strongly influenced Fmax1, which exceeded Fmax2 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 Fmax1 is not transmitted proximally), it undergoes considerably greater deflection, and thereby absorbs the majority of impact energy during a fall.

Surface stiffness affects impact force during a fall on the outstretched hand

Falls on the outstretched hand are among the most common causes of traumatic bone fracture. However, little is known regarding the biomechanical factors that affect the risk for injury during these events. In the present study, we explored how upper-extremity impact forces during forward falls are affected by modification of surface stiffness, an intervention applicable to high-risk environments such as nursing homes, playgrounds, and gymnasiums. Results from both experimental and linear biomechanical models suggest that during a fall onto an infinitely stiff surface, hand contact force is governed by a high-frequency transient (having an associated peak force Fmax1), followed by a low-frequency oscillation (having an associated lower magnitude peak force Fmax2). Practical decreases in surface stiffness attenuate Fmax1 but not Fmax2 or the magnitude of force transmitted to the shoulder. Model simulations reveal that this arises from the compliant surface's ability to decrease the velocity across the wrist damping elements at the moment of impact (which governs Fmax1) but inability to substantially reduce the peak deflection of the shoulder spring (which governs Fmax2). Comparison between model predictions and previous data on fracture force suggests that feasible compliant surface designs may prevent wrist injuries during falls from standing height or lower, because Fmax1 will be attenuated and Fmax2 will remain below injurious levels. However, such surfaces cannot prevent Fmax2 from exceeding injurious levels during falls from greater heights and therefore likely provide little protection against upper-extremity injuries in these cases.

Common protective movements govern unexpected falls from standing height

Simple energy considerations suggest that any fall from standing height has the potential to cause hip fracture. However, only 1-2% of falls among the elderly actually result in hip fracture, and less than 10% cause serious injury. This suggests that highly effective movement strategies exist for preventing injury during a fall. To determine the nature of these, we measured body segment movements as subjects (aged 22-35 yr) stood upon a gymnasium mattress and attempted to prevent themselves from falling after the mattress was made to translate abruptly. Subjects were more than twice as likely to fall after anterior translations of the feet, when compared to posterior or lateral translations. In falls which resulted in impact to the pelvis, a complex sequence of upper extremity movements allowed subjects to impact their wrist at nearly the same instant as the pelvis (average time interval between contacts = 38 ms), suggesting a sharing of contact energy between the two body parts. Finally, marked trunk rotation was exhibited in falls due to lateral (but not anterior or posterior) perturbations, resulting in the avoidance of impact to the lateral aspect of the hip. These results suggest that body segment movements during falls, rather than being random and unpredictable, involve a repeatable series of responses which facilitate safe landing.

Predicting the impact response of a nonlinear single-degree-of-freedom shock-absorbing system from the measured step response

We measured the step response of a surrogate human pelvis/impact pendulum system at force levels between 50 and 350 N. We then fit measured response curves with four different single-degree-of-freedom models, each possessing a single mass, and supports of the following types: standard linear solid, Voigt, Maxwell, and spring. We then compared model predictions of impact force during high-energy collisions (pendulum impact velocity ranging from 1.16 to 2.58 m/s) to force traces from actual impacts to the surrogate pelvis. We found that measured peak impact forces, which ranged from 1700 to 5600 N, were best predicted by the mass-spring, Maxwell, and standard linear solid models, each of which had average errors less than 3 percent. Reduced accuracy was observed for the commonly used Voigt model, which exhibited an average error of 10 percent. Considering that the surrogate pelvis system used in this study exhibited nonlinear stiffness and damping similar to that observed in simulated fall impact experiments with human volunteers, our results suggest that these simple models allow-impact forces in potentially traumatic falls to be predicted to within reasonable accuracy from the measured response of the body in safe, simulated collisions.

Distribution of contact force during impact to the hip

Hip fracture is a common, costly, and debilitating injury occurring primarily in the elderly. Commonly viewed as a consequence of osteoporosis, it is less often appreciated that > 90% of hip fractures are caused by falls, and that fracture risk is governed not only by bone fragility, but also by the mechanics of the fall. Our goal is to develop experimental and mathematical models that describe the dynamics of impact to the hip during a fall, and explain the factors that influence hip contact force and fracture risk during a fall. In the current study, we used "pelvis release experiments" to test the hypothesis that, during a fall on the hip, two pathways exist for energy absorption and force generation at contact: a compressive load path directly in line with the hip, and a flexural load path due to deformation of muscles and ligaments peripheral to the hip. We also explored whether trunk position or muscle contraction influence the body's impact response and the magnitude of force applied to the hip during a fall. Our results suggest that only 15% of total impact force is distributed to structures peripheral to the hip and that peak forces directly applied to the hip are well within the fracture range of the elderly femur. We also found that impacting with the trunk upright significantly increases peak force applied to the hip, whereas muscle contraction has little effect. These results should have application in the development of fracture risk indices that incorporate both fall severity and bone fragility, and the design of interventions such as hip pads and energy-absorbing floors that attempt to reduce fracture risk by decreasing in-line stiffness and hip contact force during a fall.

Etiology and prevention of age-related hip fractures

Falls and fall-related injuries are among the most serious and common medical problems experienced by the elderly. Hip fracture, one of the most severe consequences of falling in the elderly, occurs in only about 1% of falls. Despite this, hip fracture accounts for a large share of the disability, death, and medical costs associated with falls. As measured by their frequency, influence on quality of life, and economic cost, hip fractures are a public health problem of crisis proportions. Without successful international initiatives aimed at reducing the incidence of falls and hip fractures, the implications for allocations of health resources in this and the next century are staggering. Identifying those at risk for harmful falls requires an understanding of what kinds of falls result in injury and fracture. In elderly persons who fall, in most of whom hip bone mineral density is already several standard deviations below peak values, fall severity (as reflected in falling to the side and impacting the hip) and body habitus are important risk factors for hip fracture and touch on a domain of risk entirely missed by knowledge of bone mineral density. These findings clearly suggest that factors related to both loading and bone fragility play important roles in the etiology of hip fracture. We provide a strategy, based on engineering approaches to fracture risk prediction, for determining the relative etiologic importance of loading and bone fragility and to summarize some of what is known about both sets of factors. We define a factor of risk, phi, as the ratio of the loads applied to the hip divided by the loads necessary to cause fracture and summarize available data on the numerator and the denominator of phi. We then provide an overview of the complex interplay between the risks associated with the initiation, descent, and impact phases of a fall, thereby suggesting an organized approach for evaluating intervention efforts being used to prevent hip fractures. The findings emphasize the continuing need for combined intervention strategies that focus on fall prevention, reductions in fall severity, and maintaining or increasing femoral bone mass and strength, either through targeted exercise programs, optimal nutrition (Ca, Vitamin D), and/or in the use of osteodynamic agents. By developing and refining the factor of risk, a property that captures both the contributions of bone density and the confounding influences of body habitus and fall severity, we believe these intervention strategies can be targeted more appropriately.

Energy-shunting hip padding system attenuates femoral impact force in a simulated fall

Recent studies suggest that hip padding systems reduce the incidence of hip fractures during falls. However, no data exist on the force attenuating capacity of hip pads under realistic fall impact conditions, and thus it is difficult to compare the protective merit of various pad designs. Our goal is to design a comfortable hip padding system which reduces femoral impact force in a fall below the mean force required to fracture the elderly cadaveric femur. In pursuit of this objective, we designed and constructed a hip pad testing system consisting of an impact pendulum and surrogate human pelvis. We then developed a hip pad containing a shear-thickening material which allows for shunting of the impact energy away from the femur and into the surrounding soft tissue. Finally, we conducted experiments to assess whether the surrogate pelvis accurately represents the impact behavior of the human female pelvis in a fall, and to determine whether our energy-shunting pad attenuates femoral impact force in a fall more effectively than seven available padding systems. We found the surrogate pelvis accurately represented the human female pelvis in regional variation in soft tissue stiffness, total effective stiffness and damping, and impact force attenuation provided by trochanteric soft tissues. We also found that our padding system attenuated femoral impact force by 65 percent, thereby providing two times the force attenuation of the next best system. Moreover, the energy-shunting pad was the only system capable of lowering femoral impact force well below the mean force required to fracture the elderly femur in a fall loading configuration. These results suggest that the force attenuating potential of hip pads which focus on shunting energy away from the femur is superior to those which rely on absorbing energy in the pad material. While these in-vitro results are encouraging, carefully designed prospective clinical trials will be necessary to determine the efficacy of these approaches to hip fracture prevention.

Force attenuation in trochanteric soft tissues during impact from a fall

The risk for hip fracture from a fall is known to decrease with increased body mass index (weight/height2), a relative measure of obesity. To explore whether this reduced risk is due to the protective effect of increased soft-tissue cushioning in obese individuals, we used an impact pendulum and surrogate human pelvis to conduct simulated fall impact experiments on trochanteric soft tissues harvested from the cadavers of nine elderly individuals. For each impact, the total applied energy was 140 J. Peak forces ranged from 4,050 to 6,420 N, and tissue energy absorption ranged from 8.4 to 81.6 J. Increased tissue thickness correlated strongly with both decreased peak force (r2 = 0.91) and increased tissue energy absorption (r2 = 0.76). However, peak forces in all cases were within 1 SD of previously reported average fracture forces for elderly cadaveric femora. This suggests that force attenuation in trochanteric soft tissues alone is insufficient to prevent hip fracture in falls in which an elderly person lands directly on the hip. In such falls, additional energy-absorbing mechanisms, such as breaking the fall with an outstretched hand and eccentric contraction of the quadriceps during descent, are likely to be involved if fracture does not occur.

Prediction of femoral impact forces in falls on the hip

A major determinant of the risk of hip fracture in a fall from standing height is the force applied to the femur at impact. This force is determined by the impact velocity of the hip and the effective mass, stiffness, and damping of the body at the moment of contact. We have developed a simple experiment (the pelvis release experiment) to measure the effective stiffness and damping of the body when a step change in force is applied to the lateral aspect of the hip. Results from pelvis release experiments with 14 human subjects suggest that both increased soft tissue thickness over the hip and impacting the ground in a relaxed state can decrease the effective stiffness of the body, and subsequently reduce peak impact forces. Comparison between our fall impact force predictions and in-vitro measures of femoral fracture strength suggest that any fall from standing height producing direct, lateral impact on the greater trochanter can fracture the elderly hip.

A tongue force measurement system for the assessment of oral-phase swallowing disorders

A computer-aided measuring system using a highly sensitive beam transducer has been developed to provide a quantitative, reliable measure of tongue strength. This tool has application in both the diagnosis and treatment of dysphagic patients with oral-stage dysfunction. The device is customized to comfortably adapt to each individual. Audiovisual feedback is used to enhance subject interest and motivation. The device has proven reliable in measurements of upward and side tongue thrust in six able-bodied subjects measured during five separate sessions. It has also been used with two dysphagic patients.