Biomedical Engineering Theory And Practice/Biomechanics III

Head injury mechanism
There are three major types of head injury by direct impact or by direct high accelerations.
 * 1) Brain injury:Brain injury can be classified into diffuse injuries and focal injury.Diffuse injuries are because of high accelerations to the entire brain and can produce injuries from mild concussion to diffuse axonal injury often related to impacts with rigid flat or semi blunt objects. Focal injuries produced by a direct impact to a local area of the brain from minor contusions (bruising) to direct penetration of the brain often related to blunt or sharp object impacts.
 * 2) Skull fracture:Skull fracture can be produced by direct impact. Cranial fractures can be produced by two different impact-loading mechanisms.
 * 3) Impact with a flat surface producing linear type fractures
 * 4) Impact with a blunt object producing localized depressed fractures
 * 5) Facial lacerations:Both the skeleton and soft tissue structure in the facial area is very complicated. The main bone groups are the zygomas, which make up the skeletal structure around the eye obits and nasal cavity, maxilla, which forms the upper jaw and mandible, which forms the lower jaw. Facial fractures are produced from direct impacts from both flat plate and blunt objects, and often occur in distinct locations along lines of weakness.  Simple fracture to the mandible has an AIS score 1, fractures to the maxilla and zygoma a score 2, and complicated fractures 3.

The most common parameter for head injury is acceleration. The head can withstand higher accelerations for shorter durations and any exposure above the injury becomes serious. Gadd proposed severity index in 1961,now known as known as the Gadd Severity Index (GSI) : $$\mathit{S}\mathit{I} = \int_{0}^{t}a(t)^{2.5} dt $$

where a is the instantaneous acceleration of the head, and T is the duration of the pulse.If the integrated value is over 1000, a severe injury will result. A modified form of the GSI, now known as the Head Injury Criterion (HIC), was proposed by Versace .It is defined as:

$$\mathit{H}\mathit{I}\mathit{C} = \bigg\{ \Big[ \frac{1}{t_{2}-t_{1}} \int_{t_{1}}^{t_{2}} a(t) dt\Big]^{2.5}\left ( t_{2}-t_{1}\right ) \bigg\}_{max} $$

where t1 and t2 are the initial and final times (in seconds) of the interval during which HIC attains a maximum value, and acceleration a is measured in gs (standard gravity acceleration). Note also the maximum time duration of HIC, t2 – t1, is limited to a specific value between 3 and 36 ms, usually 15 ms.

'''Table. Tolerance Estimates for MTBI''' Source: King, A.I., Yang, K.H., Zhang, L. et al. 2003. Is head injury caused by linear or angular acceleration? In Bertil Aldman Lecture, Proceedings of the 2003 International IRCOBI Conference on the Biomechanics of Impact, pp. 1–12,

Neck injury mechanism
The neck is from the base of the skull(altas) to seven vertebra referred to as C-1 to C-7 and connects to the thoracic spine (the upper back). The spinal column contains about two dozen interconnected, oddly shaped bony segments and some hyaline cartilage called vertebrae. The spinal column protects spinal cord(a bundle of nerve tissue. The prominence of the thyroid cartilage called "Adam's apple" is a noticeable external neck feature.. The Adam’s apple is better marked in men as the cartilage meets at a 90-degree angle; in women, the angle is typically 120-degrees. With complicated structure of the neck, the neck injury mechanism is various. For flexion-extension modeling, the neck injury criterion modeling is developed in 1986 by Alderman . A Neck Injury Criterion (NIC) based on a model of the pressure effects was developed in 1996 .The Intervertebral Neck Injury Criterion (IV-NIC) is based on the hypothesis that intervertebral motion beyond the physiological limit may injure cervical soft tissues . Based on the hypothesis that a neck protection criterion for rear-end collisions should consider a linear combination of loads and moments, a new criterion called Nkm is proposed in 2002.
 * 1) Tension-flexion injuries
 * 2) Tension–Extension Injuries
 * 3) Compression–Flexion Injuries
 * 4) Compression–Extension Injuries
 * 5) Injuries Involving Lateral Bending

Compression Injury
Acceleration is not related to the two main thorax injuries, rib fracture and injury to internal organs. The ribs themselves have some elastic compliance causing compression of the rib before fractures. Therefore, Rib fractures are related to both force and chest compression as both are related to the biomechanical stiffness of the thorax. Compression Criterion (CC), in other words, maximum chest compression has been adopted as the better criterion for predicting rib fracture than acceleration or force, as it is potentially easier to measure.

Acceleration Injury
The Thoracic Trauma Index (TTI) is an injury criterion for the thorax in the case of the side effect. It assumes that the appearance of injury is in connection with the mean of the maximum lateral acceleration experienced by the stuck side rib cage and the lower thoracic spine. In addition, the TTI consider the weight and the age of the test subject. The TTI(dimension(g)) is defined as follows: $$TTI = 1.4 * AGE + 0.5 * (RIB_{Y} + T12_{Y}) * MASS / M_{std}$$

where

TTI =Thoracic Trauma Index (dimension: g)

AGE =age of the subject in years

RIBY =maximum absolute value of lateral acceleration in g’s of the 4th and 8th rib on struck side after signal filtering

T12Y = maximum absolute value of lateral acceleration in g’s of 12th thoracic vertebra after signal filtering

MASS = test subject mass in kg

Mstd =standard reference mass of 75 kg

When using a 50th percentile HybridIII dummy for crash tests, the different TTI called TTI(d) can be used:

$$TTI(d) = 0.5 * (RIB_{Y} + T12_{Y})$$

Where TTI(d) is the definition of the TTI used for 50 th percentile dummies.

Viscous Injury
Injury to the internal organs like heart, lungs, liver, spleen and blood vessels is better related to the rate of intrusion and not the chest compression. But, a compression and rate dependent criterion seems to be the most applicable. Therefore, the velocity/compression based, or viscous criterion, has been adopted as the best indicator of injury level before maximum deflection. This criterion accommodates impacts of different velocities, 3 to 30 m/s, and mass or energy stored in the thorax and not the energy dissipated. It is used in both frontal and lateral impacts. It also assumes that varied loading conditions by different restraints need a more restraint specific injury criterion The viscous criterion, or V*C, is a time function by the product of the velocity of deformation: V(t) and the instantaneous compression function: C(t).V(t) is calculated by differentiation of the deformation, and C(t) is calculated in relation to initial torso thickness (D).

$$VC =V(t)*C(t)=\frac{d[D(t)]}{dt}*\frac{D(t)}{D}$$

The tolerance levels for both frontal and lateral impacts are used with the chest compression criterion, which predicts rib fractures.

Injury of the ThoracoLumbar Spine
If the injury is related to the spinal cord, paraplegia can be happened.This injury for these wedge fractures is by a combined compressive and bending load. Two load paths down the spine to transmit vertical (axial) compression is generated by inertial loading, relying on the orientation of the lumbar spine. The two load paths are the discs and the articular facets that transmit compressive load by bottoming the tips the inferior facets onto the laminar of vertebra below. Disc rupture happens very slowly. An extremely violent single loading can cause the nucleus pulposus to extrude from the side of the disc.

Combined Thoracic Index
Combined Thoracic Index(CTI) contains both peak chest acceleration and maximum chest deflection. It was found to show superior predictive capability compared to the others. The equation of the CTI is:

$$CTI= \frac{A_{max}}{A_{int}}+ \frac{D_{max}}{D_{int}}$$

where $$A_{max}$$ and $$D_{max}$$ are the maximum observed acceleration and deflection, and $$A_{int}$$ and $$D_{int}$$ are the corresponding maximum allowable intercept values.

Injury Risk Assessment
Until now, tolerances have been done for most responses of the chest and abdomen. The following table shows tolerance levels from reviews by Cavanaugh, Rouhana and Viano et al..

'''Table. Human Tolerance for Chest and Abdomen Impact''' Source: (Adapted from Cavanaugh J.M., The Biomechanics of Thoracic Trauma,In Accidental Injury: Biomechanics and Prevention, Nahum A.M. and Melvin J.W.,(Eds.), pp. 362–391, Springer-Verlag, New York, 1993 and Rouhana S.W., Biomechanics of Abdominal Trauma, In Accidental Injury: Biomechanics and Prevention,Nahum A.M. and Melvin J.W., (Eds.), pp. 391–428, Springer-Verlag, New York, 1993.)

The following equation shows the relationship between injury probability p to a biomechanical response x:

$$p(x)=1+[\alpha-\beta x]^{-1}$$

where α and β are parameters derived from statistical analysis of biomechanical data.The following table summarizes available parameters for chest and abdominal injury risk assessment.

Vestibular Mechanics
Vestibular system is a part of the auditory system together with the cochlea. It constitutes the labyrinth of the inner ear in most mammals and situated in the vestibulum in the inner ear.The vestibular system is composed of the otolith and saccule (collectively by called the otolithic organs), which are the linear motion sensors, and the three semicircular canals (SCCs), which sense rotational motion.