Measuring temperature rise during orthopaedic surgical procedures

A reliable means for measuring temperatures generated during surgical procedures is needed to recommend best practices for inserting fixation devices and minimizing the risk of osteonecrosis. Twenty four screw tests for three surgical procedures were conducted using the four thermocouples in the bone and one thermocouple in the screw. The maximum temperature rise recorded from the thermocouple in the screw (92.7±8.9°C, 158.7±20.9°C, 204.4±35.2°C) was consistently higher than the average temperature rise recorded in the bone (31.8±9.3°C, 44.9±12.4°C, 77.3±12.7°C). The same overall trend between the temperatures that resulted from three screw insertion procedures was recorded with significant statistical analyses using either the thermocouple in the screw or the average of several in-bone thermocouples. Placing a single thermocouple in the bone was determined to have limitations in accurately comparing temperatures from different external fixation screw insertion procedures. Using the preferred measurement techniques, a standard screw with a predrilled hole was found to have the lowest maximum temperatures for the shortest duration compared to the other two insertion procedures. Future studies evaluating bone temperature increase need to use reliable temperature measurements for recommending best practices to surgeons.

Comparison of pregnant and non-pregnant occupant crash and injury characteristics based on national crash data

The objective of this study was to provide specific characteristics of injuries and crash characteristics for pregnant occupants from the National Automotive Sampling System/Crashworthiness Data System (NASS/CDS) database for pregnant women as a group, broken down by trimester, and compared to non-pregnant women. Using all NASS/CDS cases collected between the years 2000 and 2012 with at least one pregnant occupant, the entire pregnant data set included 321,820 vehicles, 324,535 occupants, and 640,804 injuries. The pregnant occupant data were compared to the characteristics of NASS/CDS cases for 14,719,533 non-pregnant females 13-44 years old in vehicle crashes from 2000 to 2012. Sixty five percent of pregnant women were located in the front left seat position and roughly the same percentage of pregnant women was wearing a lap and shoulder belt. The average change in velocity was 11.6 mph for pregnant women and over 50% of crashes for pregnant women were frontal collisions. From these collisions, less than seven percent of pregnant women sustained MAIS 2+ injuries. Minor differences between the pregnant and non-pregnant occupants were identified in the body region and source of injuries sustained. However, the data indicated no large differences in injury or crash characteristics based on trimester of pregnancy. Moreover, the risk of an MAIS 2+ level injury for pregnant occupants is similar to the risk of injury for non-pregnant occupants based on the total vehicle change in velocity. Overall this study provides useful data for researchers to focus future efforts in pregnant occupant research. Additionally, this study reinforces that more detailed and complete data on pregnant crashes needs to be collected to understand the risk for pregnant occupants.

The Tolerance of the Maxilla to Blunt Impact

This study reports the results of 38 infraorbital maxilla impacts performed on male cadavers. Impacts were performed using an unpadded, cylindrical impactor (3.2 kg) at velocities between 1 and 5 m/s. The peak force and acoustic emission data were used to develop a statistical relationship of fracture risk as a function of impact force. Acoustic emission sensors were used to provide a noncensored measure of the maxilla tolerance and were essential due to the increase in impactor force after fracture onset. Parametric and nonparametric techniques were used to estimate the risk of fracture tolerance. The nonparametric technique produced an estimated 50% risk of fracture between 970 and 1223 N. The results obtained from the parametric and nonparametric techniques were in good agreement. Peak force values achieved in this study were similar to those of previous work and were unaffected by impactor velocity. The results of this study suggest that an impact to the infraorbital maxilla is a load-limited event due to compromise of structural integrity.

The Tolerance of the Frontal Bone to Blunt Impact

The current understanding of the tolerance of the frontal bone to blunt impact is limited. Previous studies have utilized vastly different methods, which limits the use of statistical analyses to determine the tolerance of the frontal bone. The purpose of this study is to determine the tolerance of the frontal bone to blunt impact. Acoustic emission sensors were used to provide a noncensored measure of the frontal bone tolerance and were essential due to the increase in impactor force after fracture onset. In this study, risk functions for fracture were developed using parametric and nonparametric techniques. The results of the statistical analyses suggest that a 50% risk of frontal bone fracture occurs at a force between 1885 N and 2405 N. Subjects that were found to have a frontal sinus present within the impacted region had a significantly higher risk of sustaining a fracture. There was no association between subject age and fracture force. The results of the current study suggest that utilizing peak force as an estimate of fracture tolerance will overestimate the force necessary to create a frontal bone fracture.

The tolerance of the nasal bone to blunt impact

The nasal bone is among the most frequently broken facial bone due to all types of trauma and is the most frequently fractured facial bone due to motor vehicle collisions. This study reports the results of anterior-posterior impacts performed on male cadavers using a free-falling impactor with a flat impacting surface. The force at fracture onset was determined using an acoustic emission sensor. These non-censored data were utilized in parametric and non-parametric techniques to determine a relationship between applied force and fracture risk. Based on these analyses a 50% risk of fracture corresponded to an applied force of approximately 450 to 850 N. There was no correlation between fracture force and anthropometric measures of the nasal bone. Interestingly, age had a statistically significant relationship with the risk of nasal bone fracture. This study demonstrates the need for a non-censored measure of fracture occurrence when evaluating structures that can continue to support load after fracture onset.

Finite element modeling of human placental tissue

Motor vehicle crashes account for a large portion of placental abruption and fetal losses. To better understand the material properties of the human placenta, a Finite Element (FE) model of human placenta tissue was created and verified using data from uniaxial tension tests. Sixty-four tensile tests at three different strain rates of 7% strain/s, 70% strain/s, and 700% strain/s from six whole human placentas were used for model development. Nominal stresses were calculated by dividing forces at the grips by the original cross-sectional area. Nominal strains were calculated by dividing cross-head displacement by the original gauge length. A detailed methodology for interpreting experimental data for application to material model development is presented. A model of the tension coupon was created in LS-DYNA and stretched in the same manner as the uniaxial tension tests. The behavior of the material was optimized to the uniaxial tension test using a multi-island genetic algorithm. The results demonstrate good correlation between experiments and the model, with an average difference of 2% between the optimized FE and experimental first principal stress at the termination state. The material parameters found in this study can be utilized in FE models of placental tissues for behavior under dynamic loading.

The use of acoustic emission in facial fracture detection

This study describes a method for utilizing Acoustic Emission (AE) in facial fracture detection and demonstrates the association between fracture and acoustic emission magnitude. AE sensors were mounted to the frontal bone and mandible of cadaver skulls (n=14) exposed to impacts to the frontal bone, nasal bone, maxilla and mandible. The presence of AE during fracture and non-fracture tests necessitated the development of a threshold to distinguish AE associated with fracture. Based on the obtained data, a threshold of 9 volts was established. The association between above-threshold AE and fracture was demonstrated by performing low-severity impacts on pre-fractured frontal bones. These tests demonstrated that above threshold AE is produced as a result of high-severity impacts, resulting in fracture as well as low-severity impacts on prefracture bone. The results of these tests indicate that above threshold AE was associated with the fracture process and not a function of the force applied to the facial bones.

Methodology for dynamic biaxial tension testing of pregnant uterine tissue

Placental abruption accounts for 50% to 70% of fetal losses in motor vehicle crashes. Since automobile crashes are the leading cause of traumatic fetal injury mortality in the United States, research of this injury mechanism is important. Before research can adequately evaluate current and future restraint designs, a detailed model of the pregnant uterine tissues is necessary. The purpose of this study is to develop a methodology for testing the pregnant uterus in biaxial tension at a rate normally seen in a motor vehicle crash. Since the majority of previous biaxial work has established methods for quasi-static testing, this paper combines previous research and new methods to develop a custom designed system to strain the tissue at a dynamic rate. Load cells and optical markers are used for calculating stress strain curves of the perpendicular loading axes. Results for this methodology show images of a tissue specimen loaded and a finite verification of the optical strain measurement. The biaxial test system dynamically pulls the tissue to failure with synchronous motion of four tissue grips that are rigidly coupled to the tissue specimen. The test device models in situ loading conditions of the pregnant uterus and overcomes previous limitations of biaxial testing. A non-contact method of measuring strains combined with data reduction to resolve the stresses in two directions provides the information necessary to develop a three dimensional constitutive model of the material. Moreover, future research can apply this method to other soft tissues with similar in situ loading conditions.

Utilizing cryogenic grips for dynamic tension testing of human placenta tissue

Computational models are used to investigate placental abruption in motor vehicle crashes, which is the leading cause of traumatic fetal injury mortality in the United States. Material parameters for computational modeling of pregnant occupant kinematics come from early research on placenta tissue at quasi-static loading rates. The purpose of this research is to develop a methodology for using cryogenic grips to test placenta specimens in uniaxial tension at a rate normally seen in a motor vehicle crash. For dynamic testing of placental tissue, implementing and adapting a cryogenic grip mechanism provides the ability to grip the tissue throughout the thickness and eliminates potential slipping of the tissue in the grip during the dynamic test. The validation for using the cryogenic grips is presented with video images of a typical test event showing the tissue failing in the active area. Additionally, local and global strain measures are compared to confirm the tissue strain is similar throughout the specimen. The cryogenic grips provide a low-cost and effective method of gripping and pulling a thick soft tissue in uniaxial tension. As a result, these methods can be used to acquire the material properties of placenta tissue loaded at a dynamic rate to apply in a computational pregnant model.

Analysis of linear head accelerations from collegiate football impacts

Sports-related concussions result in 300,000 brain injuries in the United States each year. We conducted a study utilizing an in-helmet system that measures and records linear head accelerations to analyze head impacts in collegiate football. The Head Impact Telemetry (HIT) System is an in-helmet system with six spring-mounted accelerometers and an antenna that transmits data via radio frequency to a sideline receiver and laptop computer system. A total of 11,604 head impacts were recorded from the Virginia Tech football team throughout the 2003 and 2004 football seasons during 22 games and 62 practices from a total of 52 players. Although the incidence of injury data are limited, this study presents an extremely large data set from human head impacts that provides valuable insight into the lower limits of head acceleration that cause mild traumatic brain injuries.

Head acceleration is less than 10 percent of helmet acceleration in football impacts

Sports-related concussions constitute 20 percent of brain injuries each year in the United States. Concussion research has included a variety of instrumentation and techniques to measure head accelerations. Most recently, the Head Impact Telemetry (HIT) System (Simbex, Lebanon, NH), a wireless system that provides real-time data from impacts, is used to measure in-situ head accelerations in collegiate football. The purpose of this study is to compare helmet shell acceleration to head center of gravity acceleration using two measures of linear head acceleration. A study of 50 helmet to helmet impact tests using a pendulum provided a range of head accelerations from 5 g to 50 g. The primary measure of head acceleration is accelerometers mounted at the center of gravity of the Hybrid III head. A secondary measure is the in-helmet HIT System. The series of 50 pendulum impacts included three impact velocities of 2.0 m/s, 3.5 m/s and 5.0 m/s at four different impact locations. The impact locations were on the side, back, top and just above the facemask on the front. By comparing these two measured head accelerations and the helmet acceleration during a pendulum impact, it is shown that the response of the head and the helmet vary greatly and the in-helmet system matches the head and not helmet acceleration. Specifically, head acceleration is less than 10 percent of helmet acceleration in football impacts; moreover, the HIT System is able to accurately measure the head acceleration.

Prevention of facial fractures from night vision goggle impact

Facial bone fractures in the military can result from direct loading of night vision goggles on the orbital region. Facial fracture research has shown that increasing the area over which the load is applied increases the load tolerance. The purpose of this study is to apply this concept to reducing the risk of facial bone fracture from night vision goggle impacts. The effectiveness of countermeasures in prevention of orbital fracture was evaluated using a vertical drop tower with two impact velocities of 2.6 m/s and 3.6 m/s. The countermeasure used was a rigid plastic custom face shield made from a plaster impression of each head. In addition to two human cadaver subjects, one male and one female, tests were completed on a Hybrid III 50th percentile dummy head. Three impacts to the dummy headform included no countermeasure, safety glasses, and a custom face shield. These tests yielded peak loads of 8700 N, 7500 N, and 5640 N respectively. Using the female subject, impacts were preformed successively until injury occurred. These two impacts to the subject wearing a custom face shield resulted in peak loads of 4025 N and 5158 N. The highest load corresponds to an impact velocity of 3.6 m/s and a nasal bone fracture. Two impacts to the male subject with a custom face shield resulted in peak loads of 4554 N and 5101 N with no injury. The final impact to the male subject had a peak load of 2010 N with complete orbital fracture due to the absence of a countermeasure. From these tests it is shown that facial fracture risk from night vision goggle impact can be reduced using a contoured rigid face shield.