The Stereotactical Phenomenon in TBI Biomechanics could explain the correlation between the Traumatic Brain Injury (TBI) and Alzheimers disease (AD)
Posted 31 July 2001
By Catalin Obreja
N.T.C.P. - not-for-profit research association
Different epidemiological studies showed that the traumatic brain injuries (TBI) are
correlated with higher incidence (1, 2, 3, 4) and earlier onset (3)
of Alzheimer's disease (AD), particularly when associated with APOE epsilon 4 allele (2, 4) and when the loss
of consiousness exceeds five minutes (3). Thus, the
TBI is probably one of the main environmental factors in the AD pathogenesis.
In AD, the earliest loss of neurons occurs in the nucleus basalis and the
entorhinal cortex (5). Post traumatic diffuse axonal
injury (DAI) is concentrated in the deep cerebral regions (6),
near the entorhinal cortex and including the nucleus basalis.
TBI biomechanics explores the mechanical phenomenon that causes the initial
cranio-cerebral lesions and thus represents the starting point for the overall
understanding of the TBI pathophysiology.
TBI is the consequence of the spatio-temporal pressure variations occurring inside the brain during head traumas. The spatial distribution of the pressure gradient (PG) is responsible for the tissue strains (compression, tensile, shear), the cerebral lesions' localization and the consequent neurological signs (7). Beside the skull's deformation -- caused by the contact loading and determining skull vibrations
and/or fractures -- classical biomechanical theories concern two inertial phenomena: the linear acceleration and the rotational head movements (8, 9). The first theory explains the superficial brain lesions. The second theory seems to better explain the deep cerebral lesions and the concussion mechanism but is still a controversial topic (10).
The stereotactical theory presented here mainly considers the approximately spherical shape of the interface skull-brain. The skull-brain relative movements, caused by the acceleration phenomena - linear or rotational - and by the skull vibrations, generate secondary pressure waves with approximately spherical wave fronts that concentrically propagate toward the deep cerebral structures. The wave front's spoke and its surface progressively decreases. According to the energy conservation law, the amplitude of the pressure waves progressively increases. Thus, the PG will be maximal in areas close from the geometrical center of the implied skull vault segment (11,
The stereotactical phenomenon can explain many common post-traumatic neurological signs (the initial loss of consciousness, the post-traumatic amnesia) and is compatible with previously reported experimental results (11, 12, 13). Its complementarity with the classical biomechanical theories could allow us to integrate the TBI biomechanics in a common concept in order to better understand the TBI pathophysiology and its relationship with Alzheimer's disease.
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