Patrick R. Walsh

Human head-neck biomechanics under axial tension.

A significant majority of cervical spine biomechanics studies has applied the external loading in the form of compressive force vectors. In contrast, there is a paucity of data on the tensile loading of the neck structure. These data are important as the human neck not only resists compression but also has to withstand distraction due to factors such as the anatomical characteristics and loading asymmetry. Furthermore, evidence exists implicating tensile stresses to be a mechanism of cervical spinal cord injury. Recent advancements in vehicular restraint systems such as air bags may induce tension to the neck in adverse circumstances. Consequently, this study was designed to develop experimental methodologies to determine the biomechanics of the human cervical spinal structures under distractive forces. A part-to-whole approach was used in the study. Four experimental models from 15 unembalmed human cadavers were used to demonstrate the feasibility of the methodology. Structures included isolated cervical spinal cords, intervertebral disc units, skull to T3 preparations, and intact unembalmed human cadavers. Axial tensile forces were applied, and the failure load and distraction were recorded. Stiffness and energy absorbing characteristics were computed. Maximum forces for the spinal cord specimens were the lowest (278 N +/- 90). The forces increased for the intervertebral disc (569 N +/- 54). skull to T3 (1555 N +/- 459), and intact human cadaver (3373 N +/- 464) preparations, indicating the load-carrying capacities when additional components are included to the experimental model. The experimental methodologies outlined in the present study provide a basis for further investigation into the mechanism of injury and the clinical applicability of biomechanical parameters.

Adjunctive Use of Endoscopy During Acoustic Neuroma Surgery

Objective/Hypothesis: In specific clinical situations, endoscopes offer better visualization than the microscope during acoustic neuroma (vestibular schwannoma) surgery and can therefore decrease the incidence of the postoperative complications of cerebrospinal fluid (CSF) leakage and recurrence of tumor. This study was undertaken to determine if the use of adjunctive endoscopy provides complementary information to the operating surgeon during surgery for acoustic neuromas.

Early somatosensory evoked potentials

The early somatosensory evoked potential secondary to median nerve stimulation in the human had an onset latency of 9--12 msec when recorded from scalp electrodes at vertex-to-mastoid, vertex-to-inion or at the base of the skull. Similar latencies were observed from responses recorded over the cervical dorsal columns during neurologic surgery. A latency difference of 1.5 msec was observed between the early response and the responses recorded from the junction of medial lemniscus and nucleus ventralis posterior lateralis of the thalamus during human stereotaxic surgery. Cervical cord transections and transection at the midpontine levels of the monkey showed that the evoked potential was due to generators between these levels. Depth recording of the monkey indicate that the early evoked potential originates in the region of dorsal column nuclei, while the later components are secondary to generators in cerebral cortex.

Evoked Potentials in Experimental Myelopathy

Evoked potentials recorded from the cerebral cortex and spinal cord secondary to peripheral stimulation are reversibly reduced in amplitude by both pathologic distraction and pathologic flexion of the vertebral column. While the cerebral responses are lost within two minutes after complete occlusion of the ascending aorta, the responses recorded from the spinal cord persist without change for approximately ten minutes and then gradually disappear. During the first few minutes after aortic occlusion, changes produced by spinal distraction and spinal flexion are indistinguishable from those produced when the same maneuvers are made with the aorta patent. The responses mediated by dorsal columns and corticospinal tracts are affected in the same way by flexion and distraction, suggesting that somatosensory evoked potential recordings should be a reliable means of detecting spinal cord dysfunction during surgical procedures affecting the spinal cord. It may also be possible to differentiate a mechanical from a vascular insult by the time required for the evoked potential to become abnormal following a particular surgical maneuver.

Experimental Studies of Brain and Neck Injury

Static and dynamic axial tension loads were applied to the intact and isolated cervical column of the monkey and human cadaver. Radioactive microspheres were used to evaluate brain and spinal cord perfusion in the monkey. To determine neural pathway damage, somatosensory evoked potentials were recorded with stimulation of sensorimotor cortex, and in spinal cord with stimulation of cauda equina. The evoked potential amplitude decreased prior to heart rate and blood pressure changes presumably due to brainstem distention. The preliminary studies show, 1) the brain and spinal cord were well perfused as measured with the microspheres when the evoked potentials decreased, 2) the cervical isolated cadaveric monkey spinal column ligaments failed statically at approximately 1/2 to 1/3 the force required for dynamic disruption, 3) in the intact monkey, the cervical ligaments failed statically at approximately 1/2 the dynamic failure force, 4) the isolated human cervical ligaments failed at loads approximately three times those observed in the isolated monkey cervical column.

Stereotactic Aspiration of Deep Cerebral Abscesses after CT-Directed Labeling

Multiple, deep cerebral abscesses were treated in 2 patients by stereotactic aspiration after CT-guided needle labeling of single cavities. Separation of labeling from stereotactic manipulations avoids CT artifact generated by standard frame components; the technique will, therefore, accommodate most stereotactic instruments in current use. Alignment of CT and stereotactic coordinates hinged upon coidentification of the Pantopaque-labeled cavity on CT scans and on routine skull films with the stereotactic frame in place. Proximity of the label to targets eliminated the need for special head fixation or complex baseline correction.

Physiological and Histological Effects of Cerebellar Stimulation

Cerebellar implants have been placed in 62 patients with postoperative follow-up of 4 months to 3 years. Initially currents were applied through electrodes of alternate polarity on the superior surface of the cerebellar hemispheres and subsequently through negative electrodes on the superior surface to positive electrodes on the posterior surface. The amount of current required for clinical improvement was approximately the same as that required to significantly reduce the amplitude of the somatosensory evoked potential. The clinical and electrophysiological effects were proportional to the intensity of current and to the number of electrodes through which the currents were applied. Currents applied through the cerebellum were more effective than those confined near the cerebellar surface. Histological examination of the cerebellum from the chimpanzees and from 1 patient who died of causes unrelated to stimulation failed to demonstrate any evidence of neuronal damage related to application of current.

A clinical evaluation of xenon enhancement for computed tomography.

Xenon increases the attenuation coefficients of tissues in which it dissolves and, therefore, enhances lipid-rich material. The authors evaluated xenon enhancement for computed tomography. Patients and normal subjects were given xenon in subanesthetic concentrations and were scanned in a large aperture CT scanner. Patients tolerated 30-50% inhaled xenon well. Cerebral tissues and lung were enhanced by this concentration of xenon. Xenon inhalation in subanesthetic concentrations will be a useful technique for characterizing abnormal lipid tissues and for estimating cerebral blood flow.

THE EVOKED POTENTIAL: AN EXPERIMENTAL METHOD FOR BIOMECHANICAL ANALYSIS OF BRAIN AND SPINAL INJURY

Abstract : Axial forces were applied between the shoulders and skull of eight male Macaca mulatta monkeys. Forces from 556 to 1444 Newtons produced marked changes in blood pressure, heart rate and distraction of the cervical spinal column with minimal ligamentous disruption. Somatosensory evoked potentials recorded at the cortical and thalamic levels following dorsal column or peripheral nerve stimulation were altered prior to or during changes in heart rate or blood pressure. Similar findings were observed in the efferent responses recorded from electrodes placed on the thoracic spinal cord following stimulation of sensorimotor cortex. Studies in four monkey cadaveric isolated cervical column preparations indicate that disruption occurs with axial loads which are approximately one-third of the maximum used in the in vivo studies.