Head Model for Protection - Computational Biomechanics of the Musculoskeletal System - page 250

19.3 StreSS dIStrIbutIon oF the Skull and
the StraIn rate oF the braIn
It is assumed that the woodpecker's head could be protected against acceleration-deceleration
impact-related head injury, although no studies have been carried out to prove it comparatively.
Simple reasoning would indicate that if woodpeckers got headaches, they would stop pecking.
To clarify why woodpeckers have no head injuries, three-dimensional kinematics, mechanical
properties, and macro/micro morphological structures were observed in our previous study
(Wang et al., 2011a; Wang et al., 2011b; Wang et al., 2013). The dynamic responses of human
and woodpecker heads were analyzed quantitatively in view of biomechanics. It was shown that
the woodpecker's beak plays an important role in resisting head injuries. In addition, there was
some evidence that sudden changes of relevant mechanical parameters in terms of effective
stress, shear strain and stress, and relative motion between the brain and skull do indeed cause
surface contusions, concussion, and diffuse axonal injury (DAI), as well as acute subdural hema-
toma. Shear deformation of the brain due to head rotation has long been postulated as a major
cause of brain injury, since brain tissue has low shear stiffness (Ruan et al., 1993; Bandak and
Eppinger, 1994).
Unfortunately, the measurement of stress on the skull or strain rate on the brain was almost
impossible during an impact, particularly in vivo. Alternatively, the FE method can be adopted.
Previous studies had developed a two-dimensional FE model of the woodpecker's head using the
relevant mechanical parameters of the human head (Oda et al., 2006). The model in this chapter
has the exact three-dimensional geometry obtained from micro-CT images, and the measured
elastic modulus of the woodpecker's skull and beak may make the results closer to biological
reality. The correlation of predicted responses obtained in the FE model and experiment during
impact was good (Wang et al., 2011b).
19.3.1 S treSS d iStriBution of tHe S kull at tHe S elected p ointS
Parametric analysis was done by changing the impact location, such as the forehead and nose,
mainly for the developed human head model, to evaluate the biomechanical effects during peck-
ing. It was expected that variation of the impact location would influence the impact mechan-
ics and load transmission. Two points at the forehead and occiput on the skull and brain were
selected to study the time history of the effective stress and strain rate, respectively. As shown
in Figure 19.5, the effective stress-time and stress rate-time histories at all of the selected points
(forehead and occiput on the skull and brain, respectively) were described when the human head
collided with a rigid wall at the location of the forehead and nose respectively, woodpecker's
pecking.
19.3.2 S train r ate of tHe B rain at tHe S elected p ointS
Figure 19.6 shows the stress distribution of human and woodpecker heads in the process of collid-
ing with a rigid wall. Maximum effective stress and shear stress concentration of the woodpecker's
skull always occurred near the point of collision for all three simulations. It was shown that brain
injury is correlated with strain and strain rate (Orsini and Longhi, 2013). Strain rates at the fore-
head and occiput of the brain were analyzed using the present models. It was found that upper and
lower beaks with equal lengths consistently induced higher strains at all three locations on the
woodpecker's brain after the comparisons of FE predicted stress on the skull and brain strain rate
on the anterior and posterior of the skull and brain during impact, respectively. In addition, the
occurrence time of maximum stress was later than that of the beak and skull. The hyroid bone did