Documentation:FIB book/Factors Affecting Hip Injuries from Falls

Introduction

Hip fractures are among the most severe consequences of falls in older adults. As populations age, the burden of hip fractures is projected to increase, increasing the need for improved prevention, diagnosis, and treatment strategies. Factors such as hip rotation, BMI, age, sex, and duration of the fall play a central role in determining the severity of a hip injury. Understanding these factors and the biomechanics leading to hip injury is critical for interpreting injury risk and understanding fracture mechanisms. Although hip fractures can occur in various fall directions, this review focuses primarily on sideways falls, as lateral hip impacts account for the majority of fracture-producing events in older adults.

Experimental and computational efforts have advanced knowledge of hip fracture mechanics. Cadaveric studies have generated insights into bone strength and fracture patterns. However, these approaches face notable limitations: cadaveric testing is constrained by variability in donor anatomy and tissue quality, and computational models require material and geometric inputs to generate clinically relevant predictions.

Despite this progress, the translation of biomechanical findings into clinical practice remains limited. Variability in experimental methods, lack of real-world fall data, and incomplete characterization of population-specific differences restrict the applicability of current models. To address these gaps, ongoing research must combine improved experimental models with broader epidemiological data and enhanced fall-measurement tools. This paper will explore the factors that impact the chances and severity of hip injuries from falls, focusing on hip orientation, differences in age, sex, BMI, and the effect of muscle activation. Advancing this knowledge base is essential for refining biomechanical models and prioritizing future research directions.

Background

Epidemiology

Hip fractures are one of the most severe and costly consequences of falls, with falls responsible for nearly 95% of all hip fractures in older adults. Globally, more than 1.6 million hip fractures occur each year, and this number is projected to rise due to the aging population.

The outcomes of hip fractures are often devastating, as nearly one in four patients die within a year of their injury, while up to half experience long-term disability or loss of independence. The risk of hip fracture is further amplified in institutionalized populations. Although only about 6% of older adults in Canada reside in long-term care (LTC), this group accounts for approximately 30% of all hip fractures. In LTC residents, early mortality is high, with roughly 25% dying within three months of a hip fracture, and long-term survival is significantly lower than in community-living older adults.

Age-related changes in bone and soft tissue significantly increase vulnerability to hip fractures. As age increases, osteoporosis leads to reduced bone mineral density and deterioration of the trabecular structure, making the proximal femur and pelvis less capable of withstanding impact forces. At the same time, sarcopenia decreases the protective muscle mass around the hip, reducing the body’s ability to absorb energy during a fall. These are two contributing factors that lead to the increased significance and complications of hip fractures in the elderly population.

The economic and healthcare burden of hip fractures are also quite significant. In particular, the one-year costs of hip fracture patients who were transferred to LTC facilities was almost $50,000 CAD. While the average cost of a patient's care depends on factors such as place of residence, age, and survival to 1 year, annual economic implications of hip fracture in Canada are $650 million and are expected to rise to $2.4 billion by 2041.

Long Term Complications

Beyond mortality, hip fracture patients can experience serious complications during recovery, including deep vein thrombosis, pneumonia, and heart failure. While typical recovery of a hip fracture can take up to 1 year, half of people who break a hip never fully regain the same level of mobility they had before the injury. Postoperative infection is also a risk, as rates range from 0.6% to 3.6%, varying by the type of surgical procedure. Additional complications can include pain, bleeding, neurovascular injury, and wound issues. In severe cases, these problems may lead to prolonged immobility, increased risk of pressure ulcers, or even paralysis resulting from nerve compression or surgical complications. These complications contribute to a staggering 30% one-year post hip fracture mortality rate.

Injury Mechanism

The hip joint is a high load-bearing joint that connects the femoral head to the acetabulum of the pelvis. Its primary functions are to bear weight and transmit forces from the axial skeleton to the lower extremities. The hip also allows movement in the three anatomical planes, permitting flexion, extension, abduction, adduction, and rotation of the femoral region. Muscles surrounding the hip joint, such as the iliopsoas, gluteus maximus, and adductor brevis, drive the movements of the hip. The activation of these muscles during a fall can affect the severity of an injury during a fall, as will be discussed in a later section.

Hip fractures occur when energy absorbed by the proximal femur exceeds a critical point. The biomechanics and severity of hip injuries depend on many factors such as age, sex, BMI, and muscle activation.

The injury mechanism can be influenced by the impact location. Kelly et al. investigated two hip fracture mechanisms in elderly (65 & 92 years old) in ground-level falls. In both cases, an elderly woman fell forward and landed on their flexed knee. Their study strongly suggested that a knee impact can transmit energy to the femur, resulting in a hip fracture. The mechanism can also depend on velocity at contact, and the time available during the fall to generate protective responses. During a fall, a person instinctively activates muscles in the hip region and extend their upper extremities to protect themselves. This can cause a fall on an outstretched hand (FOOSH) injury, rather than an injury to the lower extremities.

Research Methods

Pelvis Release Experiments

Figure 1: Generalized Schematic of a Pelvis Release Experiment

Research focusing closely on the influence of soft tissues or the stiffness of the entire hip and pelvis can be difficult to perform without human subjects. One way to safely test hip impacts on living humans is by using pelvis release experiments. In this setup, the participant's hip is raised a short distance of 5 cm and then released onto a force transducer, which can measure pressure distribution and peak force. Additional sensors, such as motion tracking cameras or ultrasound probes, can be used to extract further variables.

Cadaveric Femur Impact Experiments

Figure 2: Setup For a Femur Impact Experiment

Another method used to evaluate the mechanical factors contributing to hip injuries during sideways falls is the cadaveric femur impact experiment. This approach provides direct measurement of the femur’s structural response under realistic fall-loading conditions, allowing quantification of fracture strength, deformation behavior, and energy absorption. In this setup, fresh or frozen human femora are mounted in a custom fall simulator that replicates the posture and loading direction of a lateral fall. Each specimen rests on an instrumented impact plate equipped with a load cell to record impact forces, and a guided drop mass is released from a calibrated height to deliver controlled impact energy equivalent to a fall from standing height.

Current Research

Effect of Hip Rotation During a Sideways Fall

A study on the effect of hip rotation was performed by Choi et. al. on twenty individuals using pelvis release experiments, which simulated sideways falls safely. The results found that the Trochanteric force experienced when the hip is rotated externally 15 degrees is up to 15% lower than a 15-degree internal rotation. This means hip fracture risk in the worst-case sideways fall could actually be reduced if the femur is externally rotated at impact. Contrary to expectations, this reduction in force was not due to thicker or more protective trochanteric soft tissues with rotation. Trochanteric soft tissues only absorb a tiny fraction (~3 J) of the total energy of a fall from standing height (600 ~ 800J). Instead, this study suggests that external rotation makes the effective stiffness of the pelvis lower at impact, which reduces the force transmitted into the hip.

While rotating the hip and femur can change how impact force is transmitted during a fall, the direction of the force hitting the femur also plays a major role. Pinilla et al. showed through cadaver testing that even small changes in loading direction can noticeably reduce the femur’s strength. In their experiment, thirty-three elderly femora were split into three groups with a mean age of 79.2 ± 10.9±, 81.1 ± 6.7, and 73.9 ± 11.0 years. Each group corresponds to the impact angle of 0°, 15°, and 30° to simulate the hip being rotated slightly forward, directly to the side, or slightly backward during a fall. The mean maximum failure loads were 4050 N, 3820 N, and 3060 N respectively, showing about a 24 percent decrease as the impact angle moved away from pure lateral. Importantly, this magnitude of decline aligns with what has been observed for approximately 16% loss in femoral BMD, which typically accumulates over two to three decades of age-related bone loss in older adults. In other words, simply changing the loading direction can diminish fracture tolerance to a degree comparable to nearly 25 years of aging. Even though the fracture patterns stayed consistent across angles, the overall force capacity was lower, meaning the main effect of rotation came from how the load was transmitted rather than the fracture type. The study found that the direction of loading alone can significantly affect fracture risk, even when bone density stays the same.

Effect of Tissue Distribution Due to BMI and Sex

Soft tissue composition and distribution, classified through metrics like BMI, sex, and muscle activation, affect how force and pressure are distributed over the hip region during sideways falls. A study by Pretty et. al. focused on these metrics to determine their influence on hip injury. Low BMI participants and males (which generally have less hip soft tissue than women) did not have higher total impact forces, but they experienced much more localized pressure over the greater trochanter. This suggests that people with less soft tissue over the hip absorb less energy in surrounding areas and instead concentrate impact loads right at the fracture-prone region. These findings offer a mechanistic explanation for why low BMI is a major hip fracture risk factor, and show that body composition differences between males and females meaningfully change how fall forces are distributed, even if total force is similar.

Effect of Age

Figure 3: Fracture Regions in the Upper Femur

Beyond body composition, aging itself has a major effect on femoral impact strength. Courtney et al. tested cadaveric femurs from young and elderly adults with mean ages of 33 and 74 years old under the same fall-loading setup and found that older specimens failed at roughly half the load of younger ones (3.4 kN vs 7.2 kN) and absorbed only one-third as much energy before fracture. Despite these differences, both age groups fractured at similar locations along the inferior basicervical region of the femoral neck (Figure 3), suggesting that the mode of failure remained consistent while the bone material itself became less capable of withstanding stress. The study also showed that bone mineral density at the femoral neck explained nearly all of the variation in failure strength (r² = 0.92), confirming that the material quality of bone deteriorates with age even when geometry stays constant. These results support epidemiological findings that link aging and reduced BMD to a sharp rise in hip-fracture risk after falls.

Effect of Hip Muscle Activation

The earlier study referenced by Pretty et al. showed that muscle activation (tensing the hip region) increased overall impact force by 10%, yet peak pressure did not rise. This is because activated muscles alter the shape, tension, and stiffness of the soft tissues around the greater trochanter, leading to a 17% increase in contact area. This suggests muscle activation changes how force is transmitted through surrounding tissues, but may not directly raise peak pressure at the trochanter.

Another study by Kim et. al. also examined how hip muscle activation changes the mechanical behavior of the soft tissue over the greater trochanter during sideways falls. Though also performing a pelvis release experiment, this study focused on the stiffness and energy of each impact using an ultrasound probe to find soft tissue compression. The researchers found that when the hip muscles were activated, the trochanteric soft tissue became up to about 59% stiffer than when relaxed. However, the relaxed condition absorbed more energy than the contracted condition. This suggests that while activation makes the tissue harder, relaxing muscles during impact may allow the tissue to deform more and take up more impact energy.

Although the total amount of energy absorbed by the trochanteric soft tissue is small compared to the total energy available in a fall, even small changes may matter when the energy reaching the bone is near fracture thresholds.

More research is needed on muscle activation to get a clearer picture of its influence on fracture. These studies show that there is an influence on fall variables, but it is not immediately clear which is more important in determining hip fracture rates.

Discussion

Strengths and Limitations of Existing Research

Studies examining the effects of soft tissues are often limited to human participants, since cadaveric specimens do not capture realistic soft-tissue behavior or the changes in shape and stiffness that occur with muscle activation. Human studies must involve much lower forces and cannot induce injury in the participants. A method used by some studies for human testing is pelvis release experiments. This method releases the pelvis from a small height against a force transducer. While the influence of soft tissues, body orientation, and participant build can be seen on some metrics, such as force output, it may not correlate well to the much larger forces seen during a full fall. Furthermore, these experiments often involve young participants due to safety considerations, yet their data are frequently extrapolated to understand injury mechanisms in older adults.

In contrast, cadaveric experiments provide important strengths that address some of these limitations. They allow researchers to test femora under realistic fall loading with full impact energies, making it possible to directly measure fracture strength, deformation patterns, and energy absorption. These experiments also preserve natural human bone geometry and material properties, which makes them valuable for studying structural failure. However, cadaveric specimens cannot reproduce physiologic muscle tone, soft-tissue responses, or reflexive protective behaviors. Their properties also vary with age, sex, health status, and tissue quality, which can introduce variability into experimental results.

Controversies

A significant controversy when studying hip fractures is determining the appropriate and best model to represent the real-life scenario. Typically, the decision boils down to whether cadaveric pelvis models or computational/surrogate models provide the most accurate representation of real-life hip fracture mechanics. Generally, cadaveric experiments are considered to be the gold standard approach as they are real human tissue which preserves both the natural geometry and composition of the hip. However, they cannot replicate certain physiological characteristics such as muscle tone, reflexive protective responses, or age-related changes in living tissue. Additionally, cadaveric specimens also present significant difficulties to researchers in terms of cost, availability, storage, transport, biosafety, and disposal. Additionally, these cadaveric specimens can vary in their properties and geometry due to age, sex, build and general health of the deceased which can produce results not accurate for a given research question.

Girardi et. al. compared a 4th-generation composite pelvis to cadavers and found that the composite specimens had significantly higher overall stiffness than cadavers and slightly lower variability between specimens. Strain distributions measured at specific sites were similar only when applied loads were scaled to account for differences in overall stiffness. This indicates that while composites can reasonably replicate local strain patterns, they overestimate global stiffness compared to real human bones. Interestingly, the within-group inter-specimen variability for stiffness and strain was significantly lower in the composite models compared to the cadaveric models. This precision demonstrates that composite models increase the repeatability of experiments, a key advantage for controlled biomechanical research.

The controversy of the choice of model is important as the model directly influences conclusions about fracture risk factors and the design of protective interventions. Cadaver studies may overestimate fracture forces because soft tissues are inactive and cannot absorb energy as living tissue does. Computational models may underestimate localized stress concentrations or fail to account for anatomical variability across populations. These discrepancies between cadaveric and computational/surrogate studies can create conflicting evidence regarding hip protector efficacy, optimal fall-mitigation strategies, and the mechanical thresholds for femoral fracture. For example, a hip protector design validated in a surrogate model may perform differently in older adults with low BMI or frail bones. Resolving this controversy is critical for improving the accuracy and reliability of biomechanical research and ensuring that interventions are safe and effective for the populations at greatest risk.

Future Research

Despite substantial progress in understanding the biomechanics of hip injury during falls, several knowledge gaps continue to limit the consistency and completeness of current biomechanical evidence. A major priority is the development of high-fidelity experimental models. Current evidence is largely derived from cadaveric tests, simplified mechanical rigs, and low-resolution fall reconstructions, all of which fail to fully capture the complexity of real-world falls. Emerging work using composite surrogate pelvises shows promise, as these models offer reproducible and ethically accessible testing platforms for studying hip fracture mechanics and implant performance. Refinement of their material properties, geometries, and loading configurations will continue to improve their biomechanical fidelity, supporting more accurate prediction of injury risk during sideways falls. Future work should focus on advanced computational models, more realistic surrogate systems, and dynamic fall simulations that incorporate variability in posture, impact velocity, and fall direction.

Another critical area is improving population-specific data. Most existing studies rely on narrow demographic samples and do not account for differences in age, sex, bone density, or comorbidities that influence fall mechanics. Expanding research to include broader and more diverse populations, alongside the creation of robust longitudinal datasets, will enhance external validity and help identify subgroups most at risk.

Additionally, there is a need for better real-time measurement and monitoring of fall events. Emerging technologies such as wearable sensors and high-speed motion capture may enable more accurate quantification of fall dynamics outside controlled laboratory settings. Integration of these tools with machine learning could further advance the ability to predict high-risk movement patterns,  and have the potential to strengthen biomechanical understanding.

Finally, accurate and consistent biomechanical research remains essential for informing future designs such as hip protectors or balance-training programs. Future work should focus on improving experimental rigor and methodological consistency so that clinicians and designers have reliable evidence to draw upon. Stronger collaboration among engineers, clinicians, and public health experts will be essential for advancing evidence-based strategies to reduce the global burden of hip fractures.