Weight Drop Closed Head Injury

In this model, injury is produced using the gravitational forces of a free-falling, guided weight after exposing the skull of the anesthetized animal and attaching it to the impactor or bottom end of the trauma device. The majority of investigations have been performed with the head of the animal placed unrestrained on an adjustable platform; however, in several studies the head has been restrained before the impact is delivered. Due to the height and mass of the weight, injury severity can be varied from mild to severe TBI. Because the head is freely moveable in the majority of studies and no trephination is performed, the exact injury site can be selected on the basis of the special objectives of the particular study. This aspect, together with the relatively short duration involved in the preparation of the animal for the induction of TBI (no trephination of the skull or fixing of protective plates to the skull), makes this model easy and fast to use. However, because the vertex of the skull in rodents is extremely thin, a variable and somewhat uncontrolled number of skull fractures causes variability in the results when animals are subjected to higher magnitudes of injury severity.

With low injury severity, this model is able to produce concussive-like TBI without overt contusion or focal lesion. However, in a rigorous histological evaluation of the brains of mildly injured animals, cortical cell loss directly beneath the impact site and bilateral damage to selectively vulnerable regions remote from the direct injury site have been described. If animals are submitted to an injury of higher magnitude, this model is designed to produce mostly focal damage, thus replicating contusions found in the clinical situation. This characteristic feature is confirmed by the presence of acute hemorrhagic lesions in the acute and hemosiderin-laden macrophages when traumatized brains are evaluated in the more chronic posttraumatic period. Additionally, a necrotic cavity surrounded by a rim of rarefied tissue evolving over time from the hemorrhagic contusion has been reported. Robust impairments in neurological motor function and cognitive deficits, closely correlated with injury severity, have been shown to occur after experimental TBI employing this method. However, no long-term study has evaluated behavioral dysfunction in the more chronic period after TBI using this injury paradigm.

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