2. Human anatomy and physiology for crash injury assessment

Injury biomechanics is applied anatomy under extreme loading. To predict how a body region will fail in a crash, you must first know what it is made of, how the load travels through it, and which structure reaches its tolerance limit first. This chapter builds a region-by-region map of the skeletal, soft tissue, and organ systems that dominate real-world injury outcomes in the head, neck, chest, abdomen, and lower extremity. The goal is not clinical diagnosis but engineering literacy: knowing which anatomical element governs the injury mechanism, which physical parameter drives it, and which measurable quantity in a crash test corresponds to it.

2.1 Tissue Types and Their Mechanical Roles

Crash injury reduces to a small set of tissue behaviors. Cortical bone is stiff and strong in compression but comparatively weak in tension and shear; it fails at roughly 1–2% strain. Trabecular (cancellous) bone absorbs energy through progressive pore collapse and is the primary crush-absorbing material in vertebrae, the calcaneus, and the femoral head. Ligaments and tendons are collagenous, rate-sensitive, and carry load along a defined axis; they rupture when strain exceeds roughly 10–15%, with tolerance falling as loading rate rises. Muscle is viscoelastic and actively resists elongation, which is why an aware, braced occupant often tolerates higher acceleration than a relaxed one. Organs are nearly incompressible fluid-filled or air-filled structures; they injure mainly through differential motion, compression against rigid boundaries, or pressure gradients rather than through direct tensile failure.

Two concepts recur throughout this course. First, tolerance is rate-dependent: the same deflection that is harmless in a slow push can rupture tissue in a 30 ms impact. Second, injury is a chain: an external load produces local deformation, which produces a tissue-level strain, which produces a specific structural failure. Identifying the weakest link in that chain is the core analytical skill.

2.2 Head: Skull, Brain, and Facial Structures

The head is the most injury-critical region because it combines high mass, a long lever arm above the neck, and low tolerance of neural tissue. The skull is a layered sandwich: a stiff outer table, a porous diploë, and a stiff inner table. It tolerates focal impact reasonably well but fractures when contact force concentrates over a small area, and a depressed fracture can drive bone fragments into the underlying cortex.

The brain is the governing structure. It is a gelatinous tissue of very low shear modulus, suspended in cerebrospinal fluid and tethered to the skull by bridging veins and the falx and tentorium membranes. Three mechanisms matter:

• Translational acceleration produces linear motion and pressure gradients; high linear acceleration is associated with focal contusion and skull fracture.
• Rotational (angular) acceleration produces shear strain in the brain substance, especially at the gray–white matter interface and the corpus callosum. This is the dominant mechanism of diffuse axonal injury (DAI) and acute subdural hematoma.
• Focal contact produces coup and contrecoup contusions and epidural hematoma from middle meningeal artery rupture.

For engineering purposes, the head is assessed with linear acceleration (the Head Injury Criterion, HIC) and, increasingly, with rotational velocity and the Brain Injury Criterion (BrIC). A useful reference point: HIC = 1000 corresponds to roughly a 15% risk of severe head injury for a 50th-percentile adult male, and is the classic FMVSS 208 limit. Facial structures — nasal bones, zygomatic arches, maxilla, mandible — are weaker than the cranial vault and fracture at lower force, but they are rarely life-threatening; they matter mainly for occupant–airbag interaction and for facial fracture patterns in pedestrian impacts.

2.3 Neck: Cervical Spine and Soft Tissue

The cervical spine has seven vertebrae (C1–C7), with the most mobile and least protected segments at C4–C6. Each motion segment consists of a vertebral body, an intervertebral disc, facet joints, and a complex of ligaments (anterior and posterior longitudinal, ligamentum flavum, interspinous). The neck is a load path and a lever: it transmits head inertial load to the torso and amplifies it through bending.

Two injury families dominate. Compression–flexion loading, typical of head-first contact with a roof or A-pillar, drives the vertebral body into the disc and can produce burst fractures and spinal cord compression. Tension–extension loading, typical of rear impacts, stretches the anterior longitudinal ligament and can produce facet fracture, disc disruption, and, in severe cases, bilateral facet dislocation. In rear-impact testing, the relevant metric is NIC (Neck Injury Criterion) and the associated N_ij interaction criterion, which combines axial force and bending moment to predict AIS 3+ cervical injury.

2.4 Chest: Rib Cage, Lungs, Heart, and Great Vessels

The thorax is a semi-rigid shell protecting two very different contents. The rib cage (12 pairs of ribs, sternum, costal cartilage, thoracic spine) is the primary load-bearing structure. Ribs are curved beams that fail in bending; the weakest point is typically the lateral rib shaft, and fracture tolerance decreases with age as bone mineral density falls. Flail chest — three or more adjacent ribs fractured in two or more places — is a marker of severe thoracic load and is associated with pulmonary contusion.

Inside the cage, the lungs are air-filled and compressible; they injure through compression against the chest wall (pulmonary contusion) or through rapid pressure change (pneumothorax). The heart sits between the sternum and the spine and is vulnerable to direct sternal compression, which can produce myocardial contusion or, in extreme cases, cardiac rupture. The aorta is the most lethal thoracic structure: it is tethered at the aortic root and at the ligamentum arteriosum, so rapid deceleration produces a shear and bending load at the isthmus just distal to the left subclavian artery, causing traumatic aortic rupture.

Thoracic injury is assessed with chest deflection (the classic criterion is 63 mm for a 50th-percentile male, scaled by size), viscous criterion VC (rate-dependent, VC_max = 1.0 m/s is a common AIS 3+ threshold), and 3 ms clip acceleration (60 g for the chest). Deflection governs skeletal and lung injury; VC governs soft-tissue and rate-sensitive injury.

2.5 Abdomen: Solid Organs, Hollow Organs, and the Pelvic Ring

The abdomen has no protective bony shell, so it is injured by direct compression and by belt or airbag loading. Solid organs — liver, spleen, kidneys — are dense, vascular, and fracture or lacerate under compression, producing internal hemorrhage. The liver is the most commonly injured abdominal organ in frontal crashes, typically from belt loading across the right upper quadrant. Hollow organs — stomach, intestines, bladder — injure through compression and burst when internal pressure exceeds wall strength, often with delayed symptom onset.

The pelvic ring is a closed loop of ilium, ischium, pubis, and sacrum. It fails when the ring is loaded in compression (lateral impact, "open book" or "closed book" fracture patterns) or in shear. Pelvic fracture is a major source of hemorrhage and is strongly associated with lateral impact and with submarining in frontal crashes, where the occupant slides under the lap belt and the belt loads the abdomen instead of the pelvis.

2.6 Lower Extremity: Femur, Knee, Tibia, and Ankle

The lower extremity is the most frequently injured body region in frontal crashes, though usually at lower AIS than the head or chest. The femur is the strongest long bone in the body and fails in bending under axial compression, typically at the mid-shaft or distal metaphysis. The knee is a hinge with limited tolerance to shear and to posterior displacement of the tibia relative to the femur; dashboard contact drives the femur posteriorly and can rupture the posterior cruciate ligament and the popliteal artery. The tibia and fibula fail in bending and torsion, and the ankle — particularly the calcaneus and talus — is injured by axial load through the foot, as in toe-pan intrusion. The relevant criteria are femur force (typically 10 kN for AIS 3+ in the 50th-percentile male) and tibia index (TI), which combines axial force and bending moment.