Hattie C. Cutcliffe

In vivo measurement of ACL length and relative strain during walking

Although numerous studies have addressed the effects of ACL injury and reconstruction on knee joint motion, there is currently little data available describing in vivo ACL strain during activities of daily living. Data describing in vivo ACL strain during activities such as gait is critical to understanding the biomechanical function of the ligament, and ultimately, to improving the surgical treatment of patients with ACL rupture. Thus, our objective was to characterize the relative strain in the ACL during both the stance and swing phases of normal level walking. Eight normal subjects were recruited for this study. Through a combination of magnetic resonance imaging, biplanar fluoroscopy, and motion capture, we created in vivo models of each subjects normal walking movements to measure knee flexion, ACL length, and relative ACL strain during gait. Regression analysis demonstrated an inverse relationship between knee flexion and ACL length (R(2)=0.61, p<0.001). Furthermore, relative strain in the ACL peaked at 13±2% (mean±95%CI) during mid-stance when the knee was near full extension. Additionally, there was a second local maximum of 10±7% near the end of swing phase, just prior to heel strike. These data are a vital step in further comprehending the normal in vivo biomechanics experienced by the ACL. In the future, this information could prove critical to improving ACL reconstruction and provide useful validation to future computational models investigating ACL function.

Effect of the mandible on mouthguard measurements of head kinematics.

Wearable sensors are becoming increasingly popular for measuring head motions and detecting head impacts. Many sensors are worn on the skin or in headgear and can suffer from motion artifacts introduced by the compliance of soft tissue or decoupling of headgear from the skull. The instrumented mouthguard is designed to couple directly to the upper dentition, which is made of hard enamel and anchored in a bony socket by stiff ligaments. This gives the mouthguard superior coupling to the skull compared with other systems. However, multiple validation studies have yielded conflicting results with respect to the mouthguard׳s head kinematics measurement accuracy. Here, we demonstrate that imposing different constraints on the mandible (lower jaw) can alter mouthguard kinematic accuracy in dummy headform testing. In addition, post mortem human surrogate tests utilizing the worst-case unconstrained mandible condition yield 40% and 80% normalized root mean square error in angular velocity and angular acceleration respectively. These errors can be modeled using a simple spring-mass system in which the soft mouthguard material near the sensors acts as a spring and the mandible as a mass. However, the mouthguard can be designed to mitigate these disturbances by isolating sensors from mandible loads, improving accuracy to below 15% normalized root mean square error in all kinematic measures. Thus, while current mouthguards would suffer from measurement errors in the worst-case unconstrained mandible condition, future mouthguards should be designed to account for these disturbances and future validation testing should include unconstrained mandibles to ensure proper accuracy.

Injuries of the head from backface deformation of ballistic protective helmets under ballistic impact.

Modern ballistic helmets defeat penetrating bullets by energy transfer from the projectile to the helmet, producing helmet deformation. This deformation may cause severe injuries without completely perforating the helmet, termed “behind armor blunt trauma” (BABT). As helmets become lighter, the likelihood of larger helmet backface deformation under ballistic impact increases. To characterize the potential for BABT, seven postmortem human head/neck specimens wearing a ballistic protective helmet were exposed to nonperforating impact, using a 9 mm, full metal jacket, 124 grain bullet with velocities of 400–460 m/s. An increasing trend of injury severity was observed, ranging from simple linear fractures to combinations of linear and depressed fractures. Overall, the ability to identify skull fractures resulting from BABT can be used in forensic investigations. Our results demonstrate a high risk of skull fracture due to BABT and necessitate the prevention of BABT as a design factor in future generations of protective gear.

Pediatric Head and Neck Dynamics in Frontal Impact: Analysis of Important Mechanical Factors and Proposed Neck Performance Corridors for 6- and 10-Year-Old ATDs

Objective: Traumatic injuries are the leading cause of death of children aged 1–19 in the United States and are principally caused by motor vehicle collisions, with the head being the primary region injured. The neck, though not commonly injured, governs head kinematics and thus influences head injury. Vehicle improvements necessary to reduce these injuries are evaluated using anthropomorphic testing devices (ATDs). Current pediatric ATD head and neck properties were established by scaling adult properties using the size differences between adults and children. Due to the limitations of pediatric biomechanical research, computational models are the only available methods that combine all existing data to produce injury-relevant biofidelity specifications for ATDs. The purpose of this study is to provide the first frontal impact biofidelity corridors for neck flexion response of 6- and 10-year-olds using validated computational models, which are compared to the Hybrid III (HIII) ATD neck responses and the Mertz flexion corridors. Methods: Our virtual 6- and 10-year-old head and neck multibody models incorporate pediatric biomechanical properties obtained from pediatric cadaveric and radiological studies, include the effect of passive and active musculature, and are validated with data including pediatric volunteer 3 g dynamic frontal impact responses. We simulate ATD pendulum tests—used to calibrate HIII neck bending stiffness—to compare the pediatric model and HIII ATD neck bending stiffness and to compare the model flexion bending responses with the Mertz scaled neck flexion corridors. Additionally, pediatric response corridors for pendulum calibration tests and high-speed (15 g) frontal impacts are estimated through uncertainty analyses on primary model variables, with response corridors calculated from the average ± SD response over 650 simulations. Results and Conclusions: The models are less stiff in dynamic anterioposterior bending than the ATDs; the secant stiffness of the 6- and 10-year-old models is 53 and 67 percent less than that of the HIII ATDs. The ATDs exhibit nonlinear stiffening and the models demonstrate nonlinear softening. Consequently, the models do not remain within the Mertz scaled flexion bending corridors. The more compliant model necks suggest an increased potential for head impact via larger head excursions. The pediatric anterioposterior bending corridors developed in this study are extensible to any frontal loading condition through calculation and sensitivity analysis. The corridors presented in this study are the first based on pediatric cadaveric data and provide the basis for future, more biofidelic, designs of 6- and 10-year-old ATD necks.

Impact responses of the cervical spine: A computational study of the effects of muscle activity, torso constraint, and pre-flexion

Cervical spine injuries continue to be a costly societal problem. Future advancements in injury prevention depend on improved physical and computational models, which are predicated on a better understanding of the neck response during dynamic loading. Previous studies have shown that the tolerance of the neck is dependent on its initial position and its buckling behavior. This study uses a computational model to examine three important factors hypothesized to influence the loads experienced by vertebrae in the neck under compressive impact: muscle activation, torso constraints, and pre-flexion angle of the cervical spine. Since cadaver testing is not practical for large scale parametric analyses, these factors were studied using a previously validated computational model. On average, simulations with active muscles had 32% larger compressive forces and 25% larger shear forces-well in excess of what was expected from the muscle forces alone. In the short period of time required for neck injury, constraints on torso motion increased the average neck compression by less than 250N. The pre-flexion hypothesis was tested by examining pre-flexion angles from neutral (0°) to 64°. Increases in pre-flexion resulted in the largest increases in peak loads and the expression of higher-order buckling modes. Peak force and buckling modality were both very sensitive to pre-flexion angle. These results validate the relevance of prior cadaver models for neck injury and help explain the wide variety of cervical spine fractures that can result from ostensibly similar compressive loadings. They also give insight into the mechanistic differences between burst fractures and lower cervical spine dislocations.

The compressive stiffness of human pediatric heads

Head injury is a persistent and costly problem for both children and adults. Globally, approximately 10 million people are hospitalized each year for head injuries. Knowing the structural properties of the head is important for modeling the response of the head in impact, and for providing insights into mechanisms of head injury. Hence, the goal of this study was to measure the sub-injurious structural stiffness of whole pediatric heads. 12 cadaveric pediatric (20-week-gestation to 16 years old) heads were tested in a battery of viscoelastic compression tests. The heads were compressed in both the lateral and anterior-posterior directions to 5% of gauge length at normalized deformation rates of 0.0005/s, 0.01/s, 0.1/s, and 0.3/s. Because of the non-linear nature of the response, linear regression models were used to calculate toe region (<2.5%) and elastic region (>2.5%) stiffness separately so that meaningful comparisons could be made across rate, age, and direction. The results showed that age was the dominant factor in predicting the structural stiffness of the human head. A large and statistically significant increase in the stiffness of both the toe region and the elastic region was observed with increasing age (p<0.0001), but no significant difference was seen across direction or normalized deformation rate. The stiffness of the elastic region increased from as low as 5 N/mm in the neonate to >4500 N/mm in the 16 year old. The changes in stiffness with age may be attributed to the disappearance of soft sutures and the thickening of skull bones with age.

A comparison of patellofemoral cartilage morphology and deformation in anterior cruciate ligament deficient versus uninjured knees

Anterior cruciate ligament (ACL) deficient patients have an increased rate of patellofemoral joint (PFJ) osteoarthritis (OA) as compared to the general population. Although the cause of post-injury OA is multi-factorial, alterations in joint biomechanics may predispose patients to cartilage degeneration. This study aimed to compare in vivo PFJ morphology and mechanics between ACL deficient and intact knees in subjects with unilateral ACL ruptures. Eight male subjects underwent baseline MRI scans of both knees. They then performed a series of 60 single-legged hops, followed by a post-exercise MRI scan. This process was repeated for the contralateral knee. The MR images were converted into three-dimensional surface models of cartilage and bone in order to assess cartilage thickness distributions and strain following exercise. Prior to exercise, patellar cartilage was significantly thicker in intact knees as compared to ACL deficient knees by 1.8%. In response to exercise, we observed average patellar cartilage strains of 5.4 ± 1.1% and 2.5 ± 1.4% in the ACL deficient and intact knees, respectively. Importantly, the magnitude of patellar cartilage strain in the ACL deficient knees was significantly higher than in the intact knees. However, while trochlear cartilage experienced a mean strain of 2.4 ± 1.6%, there was no difference in trochlear cartilage strain between the ACL deficient and uninjured knees. In summary, we found that ACL deficiency was associated with decreased patellar cartilage thickness and increased exercise-induced patellar cartilage strain when compared to the uninjured contralateral knees.

The response of the adult and ATD heads to impacts onto a rigid surface

Given the high incidence of TBI, head injury has been studied extensively using both cadavers and anthropomorphic test devices (ATDs). However, few studies have benchmarked the response of ATD heads against human data. Hence, the objective of this study is to investigate the response of adult and ATD heads in impact, and to compare adult Hybrid III head responses to the adult head responses. In this study, six adult human heads and seven ATD heads were used to obtain impact properties. The heads were dropped from both 15cm and 30cm onto five impact locations: right and left parietal, forehead, occiput and vertex. One set of drops were performed on the human heads and up to four sets were carried out on the ATD heads. For each drop, the head was placed into a fine net and positioned to achieve the desired drop height and impact location. The head was then released to allow free fall without rotation onto a flat aluminum 34 -inch thick platen. The platen was attached to a three-axis piezoelectric load cell to measure the impact force. The peak resultant acceleration, head impact criterion (HIC) and impact stiffness were calculated using the force/time curve and drop mass. No statistical differences were found between the adult human heads and the adult Hybrid III head for 15cm and 30cm impacts (p>0.05). For the human heads, the mid-sagittal impact locations produced the highest HIC and peak acceleration values. The parietal impacts produced HICs and peak accelerations that were 26-48% lower than those from the mid-sagittal impacts. For the ATD heads, the acceleration and HIC values generally increased with represented age, except for the Q3, which produced HIC values up to higher than the other ATD heads. The impact responses of the adult Hybrid III onto different impact locations were found to adequately represent the impact stiffness of human adult head impacts from 30cm and below onto a rigid surface. The Q3 dummy consistently produced the highest HIC values of the ATD heads, and produced higher acceleration and HIC values than the adult human heads as well, which is contrary to neonatal data demonstrating that the head acceleration increases with age.

Foul tip impact attenuation of baseball catcher masks using head impact metrics

Currently, no scientific consensus exists on the relative safety of catcher mask styles and materials. Due to differences in mass and material properties, the style and material of a catcher mask influences the impact metrics observed during simulated foul ball impacts. The catcher surrogate was a Hybrid III head and neck equipped with a six degree of freedom sensor package to obtain linear accelerations and angular rates. Four mask styles were impacted using an air cannon for six 30 m/s and six 35 m/s impacts to the nasion. To quantify impact severity, the metrics peak linear acceleration, peak angular acceleration, Head Injury Criterion, Head Impact Power, and Gadd Severity Index were used. An Analysis of Covariance and a Tukey’s HSD Test were conducted to compare the least squares mean between masks for each head injury metric. For each injury metric a P-Value less than 0.05 was found indicating a significant difference in mask performance. Tukey’s HSD test found for each metric, the traditional style titanium mask fell in the lowest performance category while the hockey style mask was in the highest performance category. Limitations of this study prevented a direct correlation from mask testing performance to mild traumatic brain injury.

Obesity alters the in vivo mechanical response and biochemical properties of cartilage as measured by MRI

BackgroundObesity is a primary risk factor for the development of knee osteoarthritis (OA). However, there remains a lack of in vivo data on the influence of obesity on knee cartilage mechanics and composition. The purpose of this study was to determine the relationship between obesity and tibiofemoral cartilage properties.MethodsMagnetic resonance images (3T) of cartilage geometry (double-echo steady-state) and T1rho relaxation of the knee were obtained in healthy subjects with a normal (n = 8) or high (n = 7) body mass index (BMI) before and immediately after treadmill walking. Subjects had no history of lower limb injury or surgery. Bone and cartilage surfaces were segmented and three-dimensional models were created to measure cartilage thickness and strain. T1rho relaxation times were measured before exercise in both the tibial and femoral cartilage in order to characterize biochemical composition. Body fat composition was also measured.ResultsSubjects with a high BMI exhibited significantly increased tibiofemoral cartilage strain and T1rho relaxation times (P <0.05). Tibial pre-exercise cartilage thickness was also affected by BMI (P <0.05). Correlational analyses revealed that pre-exercise tibial cartilage thickness decreased with increasing BMI (R2 = 0.43, P <0.01) and body fat percentage (R2 = 0.58, P <0.01). Tibial and femoral cartilage strain increased with increasing BMI (R2 = 0.45, P <0.01; R2 = 0.51, P <0.01, respectively) and increasing body fat percentage (R2 = 0.40, P <0.05; R2 = 0.38, P <0.05, respectively). Additionally, tibial T1rho was positively correlated with BMI (R2 = 0.39, P <0.05) and body fat percentage (R2 = 0.47, P <0.01).ConclusionsStrains and T1rho relaxation times in the tibiofemoral cartilage were increased in high BMI subjects compared with normal BMI subjects. Additionally, pre-exercise tibial cartilage thickness decreased with obesity. Reduced proteoglycan content may be indicative of pre-symptomatic osteoarthritic degeneration, resulting in reduced cartilage thickness and increased deformation of cartilage in response to loading.