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Nutrient transport measurement

Biological repair of degenerate discs involves regeneration of the disc matrix through stimulation of resident disc cells or by implanting new cells. However in many degenerate discs, the nutrient supply may be inadequate to support the requirements of cells involved in such treatments. Here we describe a method for assessing nutrient supply rapidly during routine surgical procedures. The method is based on measurement of diffusion of a low molecular weight tracer into the disc. Nitrous oxide (N2O), routinely used as an anaesthetic gas, is the tracer of choice as its concentration in the disc can be measured electrochemically in seconds. Silver needle electrodes and Ag/AgCl references electrodes were made in house and calibrated against known concentrations of nitrous oxide using a custom-built potentiostat adapted to fulfil requirements for medical use.

The method for assessing transport into the disc was validated by inserting needle microelectrodes into an intact bovine disc in vitro and measuring N2O diffusion into the discs in an in-vitro, perfusion system. Measured profiles were in agreement with those calculated from diffusion theory, thus validating the reliability of the electrochemical measurements. The equipment was then used in vivo on patients undergoing surgery for treatment of back pain. Measurements of intra-discal N2O concentrations were made prior to discectomy in discs of 25 consenting patients (31-70 yrs) operated on for disc prolapse. The needle microelectrode was inserted into the disc to a calibrated distance during surgery after the disc was exposed but before excision. Measurements in patients found that N2O transport efficiency was low (below 40%) in 22/25 patients. All discs examined were degenerate (Grades 3 - 5); transport efficiency showed no correlation with grade assessed by MRI or with age showing that these parameters are not predictive of sufficiency of nutrient supply. Results show that transport efficiency into the disc can be measured rapidly during surgical procedures.

The equipment is relatively inexpensive but requires development for routine use. It is of interest not only for the disc but also for other tissues where nutrient transport could be restricted and has proved of interest in investigation of rotator cuff tears or femoral heads. Methods could be adapted for outpatient procedures.

Reported by

Oxford University, Physiology Laboratory
Parks Road
OX1 3PT Oxford
United Kingdom
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