In situ calibration of an oxygen sensor in intervertebral discs

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Abstract
Intradiscal hypoxia has been suspected of playing a role in the pathogenesis of degenerative back diseases. Investigating this hypothesis requires quantitative methods for measuring Po, in the intervertebral disc. Tissue oxygen tensions are usually measured with a membrane-covered oxygen-consuming sensor. Calibrating this sensor is complicated by the existence of an oxygen-diffusion gradient in joint of the membrane. This article describes a method based on mass spectrometry to measure the oxygen tension in the intervertebral disc and an ‘in situ’ calibration technique, which is able to compensate for errors caused by the oxygen-diffusion gradient. The principle of the calibration technique is to measure the signal of an inert reference gas, whose partial pressure in the intervertebral disc is known, and to utilize the measured signal to estimate the magnitude of the oxygen-diffusion gradient. The oxygen-diffusion gradient is shown to cause a mean error of -53.8% in the measured oxygen partial pressure; this error falls to +07% after correction by the ‘in situ’ calibration technique described.