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Non Accidental Head Injury Cases (NAHI, formerly referred to as Shaken Baby Syndrome [SBS]) - Prosecution Approach

Published: 6 January 2011

Annex B: Bio Mechanics in Non Accidental Head Injuries (NAHI)

The study of bio mechanics is the practice of applying the principles of mechanics to biological systems.

Some specialists, medical and non-medical, in Child Abuse and NAHI cases believe that evidence of bio mechanical studies should not be part of the complex investigation and prosecution of such cases.

One straightforward problem is that the dummies do not replicate the particular human structures that are central to the 'triad of injuries'; they have no brain, eyes or venous architecture (network of veins). It is also worth noting that the neck mechanism in such dummies only replicates movement in one direction (backwards and forwards) and does not reflect the range of movement possible in a baby or child.

These dummies are designed to replicate a forward and backward movement, with no capacity for side to side or angular movements. The skull of a child is flexible due to immaturity and, dependent upon age and birth circumstances, the sutures (areas of the skull) are not yet fused. By contrast, the fibreglass skull of the dummy is solid; it is made of several layers of fibreglass matting, coated with resin and allowed to react with a catalyst which causes the fibreglass to set vary hard.

A human brain is of course set within fluid, with a complex bridging vein structure; the CRABI skull (see below) contains a metal box set within the fibreglass skull with data cables and sensors attached.

Depending of the age of a child, no response values are considered if, for example, a child is old enough to break their fall by placing hands or arms out before impact. The dummy arms and legs are bolted on and set to a certain G-force of response in an attempt to replicate immature knee joint responses.

Outlined below is a basic description of a widely used model in bio mechanical studies, the CRABI.

CRABI (Child Restraint Air Bag Interaction dummy)

The CRABI is used to evaluate air bag exposure to infants restrained in child safety seats that are placed in the front seat. CRABI dummies come in three sizes: 6-month-old, 12-month-old and 18-month-old.

The CRABI was only intended to be used in this context. It was never intended to be used in experiments to replicate paediatric internal head injury as seen at hospital and at post mortem.

Child restraint airbag interaction dummy

The CRABI 12-Month Old was developed in 1993 by a Society of Automotive Engineers (SAE) Task Force for the purpose of testing forward and rearward facing child restraints.


The skull is manufactured from fibreglass with a steel weldment insert, and the head skin is moulded urethane. An upper neck load cell simulator is assembled with the head to allow for installation of three uniaxial accelerometers in a triaxial configuration at the centre of gravity (CG). A separate uniaxial accelerometer can be installed at the rear of the accelerometer mount to measure angular acceleration in the sagittal plane.

Crash test dummy head


A flexible moulded rubber neck is used to give the head assembly human-like flexion and extension characteristics. A six-axis load cell can be mounted at both the upper and lower ends of the neck assembly.

Crash test dummy neck


Chest foam and a chest foam support assembly mounted on a thoracic spine box make up the upper torso. The shoulders are made of flexible rubber joints. A two-axis load cell can be mounted in each shoulder. The thoracic spine is a welded aluminium structure that provides a mounting location for the triaxial configuration accelerometers at the T1 vertebrae.

Crash test dummy upper torso exploded view

The pelvis/lumbar assembly includes a welded aluminium pelvic structure and a flexible moulded rubber lumbar spine. A six-axis load cell may be mounted between the base of the lumbar spine and the top of the pelvis. The pelvic aluminium structure also provides space for triaxial configuration accelerometers which are mounted at the bottom of the lumbar. The pelvis also can accommodate an optional two-axis pubic load cell. The abdominal insert is made of an open cell urethane foam. The torso flesh is moulded urethane with nylon netting reinforcement.

Crash test dummy torso


The arms and legs are moulded urethane around an aluminium structure. The hip joint is a flexible rubber joint like the shoulder. The elbow and knee joints have stops to provide the correct range of motion.

Crash test dummy arms exploded view

Crash test dummy legs exploded view

To date, there appears to be no crash test dummy that has been developed and is available on the market that can show an exact reconstruction of a short fall, not just the G forces related to the skull, bearing in mind the triad of injuries, as well as, from time to time, haemorrhages within the spinal cord.

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