Introduction: Interest in surgical decompressive craniectomy is increasing as a treatment for uncontrolled intracranial hypertension following severe head injury. However, the understanding of the biomechanical changes which occur in the brain tissue after this procedure is limited. The ability to measure the brain tissue stiffness may be able to indicate how the brain responds after surgery, inform clinical management and also determine when the bone might be replaced.
Methods: A novel handheld indentation device has been developed for the measurement of brain tissue stiffness. The probe consists of an indenting tip (depth 2mm) of 10mm diameter surrounded by an annular ring connected to the probe body. These are independently connected to two load cells and via an interface to a laptop computer for data collection and display. The device has been assessed with bench top studies and a pilot clinical study is being undertaken to ascertain values for human brain stiffness. In the first instance, clinical readings are being taken intra-operatively after bone flap removal and dural opening in patients undergoing routine neurosurgical procedures.
Results: Laboratory testing showed a good linearity of the probe over a range from 0 to 10N (r>0.999). Intra-operator variability was assessed from 6 readings taken on successive days and inter-operator variability by readings taken by 6 operators. The coefficients of variance were 7.57% and 8.75% respectively. The device was applied to sections of foam of differing density (40, 65 and 70kg/m3); the values obtained showed clear differences between each of the foam types. Three clinical measurements have been taken and indicate that values of brain tissue stiffness appear to be towards the low end of the device's measurable range.
Conclusion: Laboratory results show that the indentation probe is linear and has low inter and intra operator variability. To our knowledge this is the first time that brain tissue stiffness has been measured in-vivo. The very early clinical results from intra-operative readings demonstrate that this device is capable of measuring tissue stiffness. They are at the lower end of the range of measurement for the indentation probe. Although more work needs to be done to calibrate the device and to standardize its application, these are promising results and the device may be of value in assessing patients after decompressive craniectomy.
Martin D, Clark A, Mitchell P, Mendelow AD, Chambers I. Measurement of brain tissue stiffness in-vivo. British Neurosurgery Research Group meeting, Manchester. 1st-2nd February 2007.
Dr. Iain R. Chambers, Cleveland Unit, Middlesbrough. Tel: +44 (0)1642-854-778.
Regional Medical Physics Department, Freeman Hospital, Newcastle upon Tyne, NE7 7DN, UK.
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