L. M. Thomas

Studies on mechanical impedance of the human skull: Preliminary report

The steady state vibration response of the human cadaver head across a frequency range including its first three modes of vibration has been studied. Important first and third modes were found near 300 Hz (antiresonance) and 900 Hz (resonance), respectively. Below 200 Hz the skull moved relatively as a rigid body. In the antiresonant mode, maximum mechanical impedance (force/velocity) was found to occur, with acceleration amplification on the occiput greater than frontal input acceleration by a factor of 3. In the resonance mode minimal mechanical impedance occurs in which only that part of the head adjacent to the driven point is moving under the action of a vibratory force. It is hypothesised that long duration impacts (t > 0·005 sec) produce predominantly rigid body motion because the frequency spectrum of the pulse is too low to excite the lowest natural frequency of the skull. Consequently the driving force produces primarily acceleration of the head. Shorter duration pulses, particularly those with short rise time, have a broader frequency spectrum which can excite skull modes, thereby augmenting or modifying the skull flexure patterns produced by the force. It is not yet understood in what proportions these factors influence head injury, but impact head accelerations have been recorded opposite the blow for short duration impacts (t < 0·004 sec) that do not correlate with rigid body acceleration (a = force/head wt.) sometimes being greater by a factor of 2.

Results of Endarterectomy in the Treatment of Cerebrovascular Diseasea

During the past 6 years, 94 patients have been explored in the neck for endarterectomy of the carotid bifurcation and/or the first portion of the vertebral artery., Of these, 16 had a complete occlusion of the internal carotid artery at the time of the endarterectomy, 72 had varying degrees of stenosis of the carotid bifurcation, and 6 were examples of involvement of the first portion of the vertebral artery in which 4 were treated by endarterectomy; endarterectomy was deemed inadvisable at the time of exploration in the other 2 because of the small size of the

EVALUATION OF TORTUOSITY OF EXTRACRANIAL VESSELS.

four major vessels supplying the brain. In addition, there were many of these patients who had what appeared to be an abnormally wide carotid bifurcation angle. It was our general impression that arterial tortuosity and a widened carotid bifurcation angle were more common in those individuals with clinically determined cerebrovascular disease. Metz et al.,’ however, in their review of 1000 carotid angiograms concluded that there was no relationship between the severity of kinking, clinical symptoms of cerebrovascular disease, hypertension, or increasing age. It should be noted that their review was limited to angiograms which were otherwise considered normal, and, therefore, may well have excluded those subjects with both a clinical course suggestive of cerebrovascular disease and a significant abnormality on the carotid angiogram ,Uch as vessel occlusion. To determine the relationship between tortuosity of the carotid