Biomedical subjects

Brian D Stemper

Publications and source records attributed to Brian D Stemper.

At least 19 recordsLinked to original sources

PubMed2006

Biomechanical characterization of internal layer subfailure in blunt arterial injury.

Blunt carotid artery injuries occur in 0.3% of blunt injured patients and may lead to devastating neurological consequences. However, arterial mechanics leading to internal layer subfailure have not been quantified. Twenty-two human carotid artery segments and 18 porcine thoracic aorta segments were opened to expose the intimal side and longitudinally distracted to failure. Porcine aortas were a geometrically accurate model of human carotid arteries. Internal layer subfailures were identified using videography and correlated with mechanical data. Ninety-three percent (93%) of vessels demonstrated subfailure prior to catastrophic failure. All subfailures occurred on the intimal surface. Initial subfailure occurred at 79% of the stress and 85% of the strain to catastrophic failure in younger porcine specimens, compared to 44% and 60%, respectively, in older human specimens. In most cases, multiple subfailures occurred prior to catastrophic failure. Due to limitations in human specimen quality (age, prior storage), young and fresh porcine aorta specimens are likely a more accurate model of clinical blunt carotid artery injuries. Present results indicate that vessels are acutely capable of maintaining physiologic function following initial subfailure. Delayed symptomatology commonly associated with blunt arterial injuries is explained by this mechanics-based and experimentally quantified onset of subcatastrophic failure.

Animals↗

PubMed2006

Mechanics of arterial subfailure with increasing loading rate.

Arterial subfailure leads to delayed symptomatology and high morbidity and mortality rates, particularly for the thoracic aorta and carotid arteries. Although arterial injuries occur during high-velocity automotive collisions, previous studies of arterial subfailure focused on quasi-static loading. This investigation subjected aortic segments to increasing loading rates to quantify effects on elastic, subfailure, and ultimate vessel mechanics. Sixty-two specimens were axially distracted, and 92% demonstrated subfailure before ultimate failure. With increasing loading rate, stress at initial subfailure and ultimate failure significantly increased, and strain at initial subfailure and ultimate failure significantly decreased. Present results indicate increased susceptibility for arterial subfailure and/or dissection under higher-rate extension. According to the present results, automotive occupants are at greater risk of arterial injury under higher velocity impacts due to greater body segment motions in addition to decreased strain tolerance to subfailure and catastrophic failure.

Animals↗

PubMed2006

Stabilizing effect of precontracted neck musculature in whiplash.

STUDY DESIGN: This study investigated the effect of neck muscle precontraction in aware occupants in whiplash. Head angulation relative to T1 and facet joint capsular ligament distractions were compared between aware and unaware occupants. OBJECTIVE: To quantify changes in facet joint capsular ligament distractions between aware occupants with precontracted neck muscles and unaware occupants with reflex muscle contraction. SUMMARY OF BACKGROUND DATA: Clinical studies have reported that patients aware of the impending impact had decreased symptom intensity and faster recovery after whiplash. To date, no study has investigated the effects of precontracted neck musculature on localized spinal soft tissue distortions in whiplash. METHODS: Aware occupants with precontracted neck muscles and unaware occupants with reflex muscle contraction in whiplash were simulated using a validated computational model. Muscle contraction attained maximum levels before impact in the aware occupant and implemented reflex delay, electromechanical delay, and finite muscle rise time in the unaware occupant. RESULTS: Precontraction of neck muscles in aware occupants resulted in 63% decreased maximum head angles, elimination of cervical S-curvature, and up to 75% decrease in maximum facet joint capsular ligament distractions. CONCLUSIONS: Occupants aware of an impending whiplash impact with precontracted neck muscles can markedly reduce overall head-neck and spinal motions. It is our theory that this would reduce whiplash injury likelihood.

Accidents, Traffic↗

PubMed2006

Trabecular bone density of male human cervical and lumbar vertebrae.

The objective of this study was to determine the bone mineral density (BMD) of cervical vertebrae and correlate with the lumbar spine. Fifty-seven young adult healthy male volunteers, ranging from 18 to 41 years of age, underwent quantitative computed tomography (QCT) scanning of C2-T1 and L2-L4 vertebrae. To account for correlations, repeated measures techniques were used to compare data as a function of spinal level and region. Linear regression methods were used (+/-95% CI) to compare data as a function of spinal level and region. The mean age and body height were 25.0 +/- 5.8 years and 181.0 +/- 7.6 cm. BMD decreased from the rostral to caudal direction along the spinal column. Grouped data indicated that the neck is the densest followed by the first thoracic vertebra and low back with mean BMD of 256.0 +/- 48.1, 194.3 +/- 44.2, and 172.2 +/- 28.4 mg/cm(3), respectively; differences were statistically significant. While BMD did not vary significantly between the three lumbar bodies, neck vertebrae demonstrated significant trends. The matrix of correlation coefficients between BMD and spinal level indicated that the relationship is strong in the lumbar (r = 0.92-0.96) and cervical (r = 0.73-0.92) spines. Data from the present study show that the trabecular bony architecture of the neck is significantly different from the low back. These quantitative BMD data from a controlled young adult healthy human male volunteer population may be valuable in establishing normative data specifically for the neck. From a trabecular bone density perspective, these results indicate that lumbar vertebrae cannot act as the best surrogates for neck vertebrae. Significant variations in densities among neck vertebrae, unlike the low back counterpart, may underscore the need to treat these bones as different structures.

Adolescent↗

PubMed2006

Biomechanics of side impact: injury criteria, aging occupants, and airbag technology.

This paper presents a survey of side impact trauma-related biomedical investigations with specific reference to certain aspects of epidemiology relating to the growing elderly population, improvements in technology such as side airbags geared toward occupant safety, and development of injury criteria. The first part is devoted to the involvement of the elderly by identifying variables contributing to injury including impact severity, human factors, and national and international field data. This is followed by a survey of various experimental models used in the development of injury criteria and tolerance limits. The effects of fragility of the elderly coupled with physiological changes (e.g., visual, musculoskeletal) that may lead to an abnormal seating position (termed out-of-position) especially for the driving population are discussed. Fundamental biomechanical parameters such as thoracic, abdominal and pelvic forces; upper and lower spinal and sacrum accelerations; and upper, middle and lower chest deflections under various initial impacting conditions are evaluated. Secondary variables such as the thoracic trauma index and pelvic acceleration (currently adopted in the United States Federal Motor Vehicle Safety Standards), peak chest deflection, and viscous criteria are also included in the survey. The importance of performing research studies with specific focus on out-of-position scenarios of the elderly and using the most commonly available torso side airbag as the initial contacting condition in lateral impacts for occupant injury assessment is emphasized.

Accidents, Traffic↗

PubMed2006

Bone mineral density of human female cervical and lumbar spines from quantitative computed tomography.

STUDY DESIGN: This study determined bone mineral density (BMD) of cervical, thoracic, and lumbar vertebrae in healthy asymptomatic human subjects. OBJECTIVES: To test the hypothesis that BMD of neck vertebrae (C2-C7) is equivalent to BMD of lumbar vertebrae (L2-L4). SUMMARY OF BACKGROUND DATA: BMD of lumbar vertebrae is correlated to their strength. Although numerous studies exist quantifying BMD of the human lumbar spine, such information for the cervical spine is extremely limited. In addition, BMD correlations are not established between the two regions of the spinal column. METHODS: Adult healthy human female volunteers with ages ranging from 18 to 40 years underwent quantitative computed tomography (CT) scanning of the neck and back. All BMD data were statistically analyzed using paired nonrepeating measures ANOVA techniques. Significance was assigned at a P < 0.05. Linear regression analyses were used to compare BMD as a function of level and region; +/-95% confidence intervals were determined. RESULTS: When data were grouped by cervical (C2-C7), thoracic (T1), and lumbar (L2-L4) spines, mean BMD was 260.8 +/- 42.5, 206.9 +/- 33.5, and 179.7 +/- 23.4 mg/mL. Average BMD of cervical vertebrae was higher than (P < 0.0001) thoracic and lumbar spines. Correlations between BMD and level indicated the lowest r value for T1 (0.42); in general, the association was the strongest in the lumbar spine (r = 0.89-0.95). The cervical spine also responded with good correlations among cervical vertebrae (r ranging from 0.66 to 0.87). CONCLUSIONS: The present study failed to support the hypothesis that BMD of lumbar spine vertebrae is equivalent to its cranial counterparts. The lack of differences in BMD among the three lumbar vertebral bodies confirms the appropriateness of using L2, L3, or L4 in clinical or biomechanical situations. However, significant differences were found among different regions of the vertebral column, with the cervical spine demonstrating higher trabecular densities than the thoracic and lumbar spines. In addition, the present study found statistically significant variations in densities even among neck vertebrae.

Adolescent↗

PubMed2006

Biomechanics of the aging spine.

Experimental studies indicate age and degeneration affect spinal biomechanics. In vitro biomechanical experimentation is used to validate finite element cervical spine models. A high percentage of experimental studies have utilized older specimens. Computer models based on these experimental studies may not accurately represent the normal population. Younger full-column and C5-C6 motion segments were tested under pure sagittal plane moments. A review of literature was conducted, and results from previous studies were compared to present data to determine whether age was an influencing factor in spinal biomechanics. Findings indicate younger specimens under equivalent pure moment loading magnitudes underwent greater ranges of motion between 0.5 and 2.5 Nm. Based on these preliminary findings, validation of finite element modeling to ensure biofidelity should consider age as a factor that may affect biomechanics.

Aging↗

PubMed2006

Interface parameters of impact-induced mild traumatic brain injury.

Commonly considered a continuum of injuries, diffuse brain injury (DBI) ranges from mild concussion to severe diffuse axonal injury. The lower end of the spectrum is generally referred to as mild traumatic brain injury (MTBI). More severe forms of DBI have garnered extensive experimentation while these milder cases are considerably less explored. Recently, a new device was designed to generate DBI in the rodent using impact-induced angular acceleration. This device is modifiable so the entire spectrum of DBI can be investigated. Severity of DBI is critically dependent on magnitude of angular acceleration. A small animal surrogate like a rodent has a relatively small brain mass. This constraint poses a unique problem because the angular acceleration necessary for DBI is inversely related to brain mass. Prior experimentation estimated an angular acceleration of approximately 350 krad/s2 is necessary for the induction of mild traumatic brain injury (MTBI) in the rodent. To induce these magnitudes of angular acceleration in a repeatable manner, the impacting interface must be critically analyzed. This investigation uses a mathematical model based on parameters of a previously developed experimental model to assess the impacting interface such that angular accelerations are sufficient to produce MTBI in the rodent.

Acceleration↗

PubMed2006

Worldsid assessment of far side impact countermeasures.

Far side impact trauma has been demonstrated as a significant portion of the total trauma in side impacts. The objective of the study was to assess the potential usefulness of countermeasures and assess the trade-offs associated with generic countermeasure design. Because the WorldSID dummy has demonstrated promise as a potential far side impact dummy, it was chosen to assess countermeasures in this mode. A unique far side impact buck was designed for a sled test system that included, as a standard configuration, a center console and outboard three-point belt system. This configuration assumed a left side driver with a right side impact. The buck allowed for additional options of generic restraints including shoulder or thorax plates or an inboard shoulder belt. The entire buck could be mounted on the sled in either a 90-degree (3-o'clock PDOF) or a 60-degree (2-o'clock PDOF) orientation. A total of 19 WorldSID tests were completed. The inboard shoulder belt configuration produced high shear forces in the lower neck (2430 N) when the belt position was placed over the mid portion of the neck. Shear forces were reduced and of opposite sign when the inboard belt position was horizontal and over the shoulder; forces were similar to the standard outboard belt configuration (830 - 1100 N). A shoulder or thorax restraint was effective in limiting the head excursion, but each caused significant displacement at the corresponding region on the dummy. A shoulder restraint resulted in shoulder displacements of 30 - 43 mm. A thorax restraint caused thorax deflections of 39 - 64 mm. Inboard restraints for far side impacts can be effective in reducing head excursion but the specific design and placement of these restraints determine their overall injury mitigating characteristics.

Accidents, Traffic↗

PubMed2005

Reflex muscle contraction in the unaware occupant in whiplash injury.

STUDY DESIGN: Computer modeling and parametric analysis were used to determine the effect of reflex contraction of the neck muscles in the unaware occupant in whiplash. OBJECTIVE: To delineate effects of reflex contraction on spinal segmental kinematics during the retraction phase. SUMMARY OF BACKGROUND DATA: The ability of reflex neck muscle contraction to mitigate whiplash injury in the unaware occupant remains unclear. Analyzing relative timing between electromyographic and head-neck kinematics, previous investigators theorized that muscle contraction alters spinal kinematics, decreasing injury likelihood. Other investigators suggested that injury occurs during the initial (retraction) phase of head-neck kinematics, before significant muscle force generation. METHODS: Computer modeling was used to determine reflex contraction effects on segmental angulations, implementing parametric analysis techniques to vary reflex delay and impact severity. RESULTS: Shorter reflex delays had a greater effect on segmental angulations later in the event and at lower impact severities. However, the magnitude of this effect, particularly at higher impact severities and during maximum cervical S-curvature (factors implicated in the whiplash injury mechanism) was minimal, altering segmental angulations by a maximum of 19%. CONCLUSIONS: Because reflex contraction did not substantially alter spinal kinematics, muscle contraction likely does not initiate in sufficient time to mitigate whiplash injuries that may occur during the retraction phase.

Cervical Vertebrae↗

PubMed2005

Effect of head restraint backset on head-neck kinematics in whiplash.

Although head restraints were introduced in the 1960s as a countermeasure for whiplash, their limited effectiveness has been attributed to incorrect positioning. The effect of backset on cervical segmental angulations, which were previously correlated with spinal injury, has not been delineated. Therefore, the practical restraint position to minimize injury remains unclear. A parametric study of increasing head restraint backset between 0 and 140mm was conducted using a comprehensively validated computational model. Head retraction values increased with increasing backset, reaching a maximum value of 53.5mm for backsets greater than 60mm. Segmental angulation magnitudes, greatest at levels C5-C6 and C6-C7, reached maximum values during the retraction phase and increased with increasing backset. Results were compared to a previously published head restraint rating system, wherein lower cervical extension magnitudes from this study exceeded mean physiologic limits for restraint positions rated good, acceptable, marginal, and poor. As head restraint contact was the limiting factor in head retraction and segmental angulations, the present study indicates that minimizing whiplash injury may be accomplished by limiting head restraint backset to less than 60mm either passively or actively after impact.

Biomechanical Phenomena↗

PubMed2005

Anterior longitudinal ligament injuries in whiplash may lead to cervical instability.

Although whiplash injuries account for a significant annual cost to society, the exact mechanism of injury and affected tissues remain unknown. Previous investigations documented injuries to the cervical anterior longitudinal ligament in whiplash. The present investigation implemented a comprehensively validated computational model to quantify level-dependent distraction magnitudes of this structure in whiplash. Maximum ligament distractions approached failure levels, particularly in middle to lower cervical levels, and occurred during the initial phase of head-neck kinematics. In particular, the C5-C6 anterior longitudinal ligament sustained distraction magnitudes as high as 2.6mm during the retraction phase, corresponding to 56% of distraction necessary to result in ligament failure. Present results demonstrated that anterior structures in the lower cervical spine may be susceptible to injury through excess distraction during the retraction phase of whiplash, which likely occurs prior to head restraint contact. Susceptibility of these structures is likely due to non-physiologic loading placed on the cervical spinal column as the head translates posteriorly relative to the thorax. Injury to anterior spinal structures can result in clinical indications including cervical instability in extension, axial rotation, and lateral bending modes. Mitigation of whiplash injury may be achieved by minimizing head retraction during initial stages of whiplash.

Biomechanical Phenomena↗

PubMed2005

Influence of thoracic ramping on whiplash kinematics.

BACKGROUND: Some experimental whiplash investigations using human volunteers and full-body cadavers reported thoracic ramping, characterized by superior translation and extension rotation of the cervico-thoracic junction. The effect of this phenomenon on cervical spinal kinematics has not been quantitatively determined. METHODS: A comprehensively validated computational model exercised in 2.7 m/s rear impact was used to determine effects of superior translation and extension rotation of T1 on cervical segmental kinematics during the retraction phase. FINDINGS: In general, thoracic ramping had a minimal effect on cervical intervertebral kinematics during retraction. INTERPRETATION: Results of the present study demonstrated that magnitude of thoracic ramping plays a minimal role in the whiplash injury mechanism due to decreased effect on cervical segmental kinematics.

Acceleration↗

PubMed2005

Effects of abnormal posture on capsular ligament elongations in a computational model subjected to whiplash loading.

Although considerable biomechanical investigations have been conducted to understand the response of the cervical spine under whiplash (rear impact-induced postero-anterior loading to the thorax), studies delineating the effects of initial spinal curvature are limited. This study advanced the hypothesis that abnormal curvatures (straight or kyphotic) of the cervical column affect spinal kinematics during whiplash loading. Specifically, compared to the normal lordotic curvature, abnormal curvatures altered facet joint ligament elongations. The quantifications of these elongations were accomplished using a validated mathematical model of the human head-neck complex that simulated three curvatures. The model was validated using companion experiments conducted in our laboratory that provided facet joint kinematics as a function of cervical spinal level. Regional facet joint ligament elongations were investigated as a function of whiplash loading in the four local anatomic regions of each joint. Under the normal posture, greatest elongations occurred in the dorsal anatomic region at the C2-C3 level and in the lateral anatomic region from C3-C4 to C6-C7 levels. Abnormal postures increased elongation magnitudes in these regions by up to 70%. Excessive ligament elongations induce laxity to the facet joint, particularly at the local regions of the anatomy in the abnormal kyphotic posture. Increased laxity may predispose the cervical spine to accelerated degenerative changes over time and lead to instability. Results from the present study, while providing quantified level- and region-specific kinematic data, concur with clinical findings that abnormal spinal curvatures enhance the likelihood of whiplash injury and may have long-term clinical and biomechanical implications.

Cervical Vertebrae↗

PubMed2005

Localized cervical facet joint kinematics under physiological and whiplash loading.

OBJECT: Although facet joints have been implicated in the whiplash injury mechanism, no investigators have determined the degree to which joint motions in whiplash are nonphysiological. The purpose of this investigation was to quantify the correlation between facet joint and segmental motions under physiological and whiplash loading. METHODS: Human cadaveric cervical spine specimens were exercise tested under physiological extension loading, and intact human head-neck complexes were exercise tested under whiplash loading to correlate the localized component motions of the C4-5 facet joint with segmental extension. Facet joint shear and distraction kinematics demonstrated a linear correlation with segmental extension under both loading modes. Facet joints responded differently to whiplash and physiological loading, with significantly increased kinematics for the same-segmental angulation. The limitations of this study include removal of superficial musculature and the limited sample size for physiological testing. CONCLUSIONS: The presence of increased facet joint motions indicated that synovial joint soft-tissue components (that is, synovial membrane and capsular ligament) sustain increased distortion that may subject these tissues to a greater likelihood of injury. This finding is supported by clinical investigations in which lower cervical facet joint injury resulted in similar pain patterns due to the most commonly reported whiplash symptoms.

Biomechanical Phenomena↗

PubMed2005

Subcatastrophic failure characteristics of the porcine descending aorta.

Failure of internal arterial layers prior to overall catastrophic vessel failure has been clinically documented and has significant clinical implications. For example, intimal failure of the internal carotid artery was associated with extremely high rates of morbidity (40-80%) and mortality (up to 40%). However, failure of internal vessel layers prior to catastrophic vessel failure has been experimentally documented in a limited number of studies and the mechanical relationship between initial and ultimate vessel failure has not been systematically explored. In the present investigation, seven porcine descending aorta specimens were mechanically tested using a novel protocol to quantify failure of internal layers relative to overall vessel failure. The protocol consisted of opening the vessel at the mid-diameter level, mechanically distracting the vessel to failure under quasi-static loading, and imaging the event from intimal and adventitial sides using a high-speed, high-resolution digital imaging system. The internal layer failed prior to catastrophic vessel failure in all specimens. Adventitial (outer) layers remained intact until ultimate failure. Initial internal layer subfailures occurred at 88% of the strain to ultimate failure. The present results demonstrate internal layer susceptibility to subcatastrophic failure without overall failure of the vessel. Although the porcine descending aorta may not be particularly susceptible to distraction failure, this vessel may be a suitable model for human arterial tissue due to similarities in geometric size and vessel structure.

Animals↗

PubMed2004

Methodology to study intimal failure mechanics in human internal carotid arteries.

While the incidence of blunt carotid artery injuries is low, the mortality rate is extremely high (40%). Clinical evidence indicates that the intimal region of the artery often sustains failure, while maintaining the integrity of the outer layers. This condition may lead to delayed ischemic symptoms, commonly reported in clinical literature. To date, the mechanical properties of the intima relative to the outer vessel layers have not been quantified in the human carotid artery. The purpose of the present study was to develop a methodology to determine the longitudinal mechanical properties of the human internal carotid artery in tension, with an emphasis on intimal failure. This was accomplished by opening the vessel at the mid-diameter level, creating an 'I'-shaped testing specimen, subjecting the specimen to failure loading, documenting the stretch characteristics of the intimal and adventitial sides in the temporal domain, and correlating the synchronized videography with mechanical loading. Intimal failure data were quantified using stress and strain parameters in conjunction with digital videography of the intimal and adventitial sides. The present methodology can be used to determine the mechanical properties of the intima relative to ultimate carotid artery failure. These data will assist in the understanding of blunt carotid artery injuries, its diagnosis and treatment.

Biomechanical Phenomena↗

PubMed2004

Gender- and region-dependent local facet joint kinematics in rear impact: implications in whiplash injury.

STUDY DESIGN: Localized facet joint kinematics resulting from whiplash acceleration were analyzed in the dynamic domain during the time of cervical S-curvature using intact head and neck specimens and a pendulum mini-sled loading apparatus. OBJECTIVES: To determine the effects of gender, impact severity, cervical level, and anatomic joint region on shear and distraction motion of lower cervical facet joints. SUMMARY OF BACKGROUND DATA: Clinical and experimental studies identify cervical facet joints to be a likely location of whiplash injury. Epidemiologic studies report that female occupants sustain a greater percentage of whiplash injuries. Previous experimental studies have not analyzed facet joint motion as a function of variables such as gender. METHODS: Intact head and neck complexes were subjected to whiplash acceleration using a pendulum mini-sled apparatus at four impact severities. Facet joint kinematics were analyzed using digital high-resolution video at 1000 frames per second during the time of maximum cervical S-curvature. Shear and distraction motions were analyzed in the ventral and dorsal joint regions from C4-C5 to C6-C7 levels. Analysis of variance techniques were used to analyze biomechanical data. RESULTS: Intact head and neck complexes sustained cervical S-curvature during whiplash loading. Lower cervical facet joints demonstrated dorsally directed shear motion with distraction in the ventral and compression in the dorsal regions of the joint. Magnitudes of distraction and compression were significantly lower than shear motion (P < 0.05). Facet joint shear and distraction motion increased with impact severity. Lower cervical facet joint shear and distraction motions in female specimens were greater than in male specimens. This difference reached statistical significance at C4-C5 (P < 0.05). CONCLUSIONS: Secondary to whiplash loading, lower cervical facet joints responded with a shear plus distraction mechanism in the anatomic ventral and shear plus compression mechanisms in the dorsal region. Injury to the ventral region stems from tensile failure of the joint capsule. Injury to the dorsal region stems from pinching of the joint capsule or synovial fold and contact between subchondral bone of superior and inferior facet processes. Because excess spinal motion is biomechanically related to abnormalities and because lower cervical facet joints sustain greater motion in female specimens, this population is more likely to be injured under whiplash loading. Potential contributors for the susceptibility of females to injury, including genotypic (apolipoprotein APOE-epsilon4), hormonal, structural, and tolerance factors, are discussed.

Biomechanical Phenomena↗