2. Anatomy and injury criteria for near-side thorax and pelvis

Near-side impact load path design begins with a precise picture of what the load passes through and what failure looks like at each station along that path. The struck-side occupant in a lateral collision loads the door trim, armrest, and side bolster first; the force then enters the rib cage through the lateral chest wall, distributes across the ribs and costal cartilages, and — depending on height and magnitude — continues into the pelvis through the greater trochanter, iliac wing, and pelvic ring. Each of these structures has a distinct geometry, a distinct failure mode, and a distinct injury criterion. Designing a load path without knowing which structure you are trying to spare, and by how much, is guesswork. This section establishes the anatomical reference frame, the injury mechanisms, and the AIS/ISS scoring logic that converts physical damage into the design targets used later in the course.

Reference frame and anatomical stations

All directions in this course follow the occupant-centric SAE convention: +X forward, +Y to the occupant's right, +Z upward, with the origin at the vehicle reference point. A near-side impact therefore arrives from the struck side along −Y (for a left-side strike on a left-seated occupant, the loading direction on the body is +Y). Keep this convention fixed; every force, deflection, and criterion value in later chapters inherits it.

The thorax and pelvis form two structural regions coupled by the lumbar spine and the abdominal cavity. Vertically, the relevant stations are: the shoulder/upper chest (ribs 1–4, clavicle, scapula), the mid-thorax (ribs 5–8, the region most exposed to door trim and armrest), the lower thorax (ribs 9–12, floating ribs, costal margin), the abdomen (liver, spleen, kidneys — no skeletal protection), and the pelvis (iliac crest down to the ischial tuberosities). In a typical sedan side impact, the armrest and door bolster align with the lower thorax and upper abdomen, which is precisely why that band produces the highest rib and solid-organ injury rates.

Rib cage: geometry, mechanism, and tolerance

The rib cage is a compliant, curved shell. Each rib articulates posteriorly with the thoracic vertebra, curves laterally and anteriorly, and terminates at the sternum via costal cartilage (ribs 1–7) or at the costal margin (ribs 8–10); ribs 11–12 are floating. When a lateral load is applied over a small contact patch, the rib acts initially as a curved beam in bending, then as a membrane in tension as deflection grows. Fractures typically initiate at the lateral or posterolateral arc — the point of maximum bending moment — and propagate. The clinically important consequences are not the fractures themselves but what they do: flail segments, hemothorax, pneumothorax, and, most critically, aortic and solid-organ injury driven by the rate and magnitude of chest compression.

Two quantities dominate rib-cage tolerance in side impact:

• Maximum lateral chest deflection — the peak inward displacement of the struck-side rib cage relative to the spine, usually measured at mid-sternum or at the struck-side rib. This is the single strongest predictor of thoracic AIS 3+ in side impact and is the primary injury criterion in regulated and consumer lateral test protocols.
• Deflection rate and viscous response — VC = (compression rate) × (compression), capturing the rate-dependent softening of soft tissue. High-rate loading produces injury at lower peak deflection than quasi-static loading, which matters for near-side impacts where door intrusion velocity can exceed 6–8 m/s.

As an illustrative design target (not a measured result), a mid-size sedan side structure is often developed so that a struck-side occupant's peak chest deflection stays below roughly 40–45 mm in a 50 km/h MDB (moving deformable barrier) side impact, with the exact threshold set by the chosen criterion and dummy. The number is a starting point for load-path budgeting, not a universal limit.

Pelvis ring and sacroiliac joint

The pelvis is a closed ring: two innominate bones (ilium, ischium, pubis) joined anteriorly at the pubic symphysis and posteriorly to the sacrum at the two sacroiliac (SI) joints. This ring is stiff in compression but vulnerable in lateral loading because the load enters through the greater trochanter and iliac wing, well off the ring's neutral axis. The dominant near-side pelvic injury mechanisms are:

• Lateral compression of the ring — the struck hemipelvis rotates inward, loading the pubic rami in bending and the SI joint in shear/rotation. Pubic ramus fractures and SI joint disruption are the classic lateral-compression pattern (Young-Burgess LC-I/LC-II).
• Trochanteric loading and acetabular fracture — a concentrated load at the greater trochanter can drive the femoral head into the acetabulum, producing acetabular fractures and central dislocation.
• Iliac wing fracture — direct loading of the iliac crest, common when the door armrest or a protruding object contacts the pelvis above the trochanter.

The practical tolerance metric is peak lateral force at the greater trochanter (or the pubic symphysis force), because the pelvis is a force-limited structure rather than a deflection-limited one. In side-impact dummy and post-mortem human subject (PMHS) testing, pelvic force on the order of 3–5 kN is commonly cited as the threshold band for AIS 2+ pelvic injury, with the exact value depending on loading rate, padding, and load application area. Designers use this to size the door trim and armrest stiffness: the trim must absorb energy without transmitting a sharp force spike into the trochanter.

From physical injury to AIS and ISS

The Abbreviated Injury Scale (AIS) grades individual injuries from 1 (minor) to 6 (currently untreatable), by body region. For load-path work, the relevant AIS anchors are:

The Injury Severity Score (ISS) combines the three most severely injured ISS body regions: ISS = A² + B² + C², where A, B, C are the highest AIS values in three different regions. Because the score is squared, a single AIS 4 injury (16 points) dominates a cluster of AIS 2 injuries. This is why near-side load-path design targets AIS 3+ outcomes: keeping the worst thoracic and pelvic injuries at AIS 2 or below keeps the ISS in a survivable range, whereas a single AIS 4 flail chest or unstable pelvis pushes the score into major-trauma territory. Note that AIS 6 in any region assigns ISS = 75 by convention.

For design work, the criterion chain is: measurable physical quantity (chest deflection, pelvic force, VC) → injury probability curve (e.g., 25% risk of AIS 3+ at a given deflection) → design target (the deflection or force the structure must not exceed). The curves are population-based; a design target at 25% risk is a policy choice, not a physical constant, and should be stated explicitly whenever a load-path budget is set.

Working example: setting a load-path budget from criteria

Suppose a program targets a 50 km/h MDB side impact and wants to keep the struck-side occupant's worst thoracic and pelvic injuries at AIS 2 or below. The engineering sequence is:

1. Select criteria and risk level. Choose peak chest deflection and peak trochanter force as the two tracked quantities, and adopt a 25% risk-of-AIS-3+ design point from the applicable injury-probability curves. State this policy explicitly in the requirements document.
2. Allocate the budget vertically. Decide how much of the total lateral load the door structure, the armrest/bolster, and the seat side bolster each absorb before the remainder reaches the body. A common split is door beam and trim carrying the majority of the intrusion energy, with the armrest tuned soft enough to limit trochanter force and the upper bolster tuned to spread chest load over a larger contact patch.
3. Check the coupling. Verify that stiffening the lower door to protect the pelvis does not raise lower-rib deflection beyond the thoracic target. If it does, redistribute stiffness upward or add a compliant transition zone at the costal margin.
4. Verify against the criterion chain. In simulation or sled test, extract peak deflection and peak force at the defined measurement locations, map them to injury probability, and confirm the worst AIS stays at 2 or below. If the thoracic criterion is exceeded first, the upper load path is too stiff or too concentrated; if the pelvic criterion is exceeded first, the armrest or door bolster is transmitting a force spike.

The anatomy and criteria established here become the fixed reference for everything that follows: the rib cage defines the deflection-limited upper path, the pelvic ring defines the force-limited lower path, and the AIS/ISS framework defines what "good" means when the two paths are tuned against each other.