EFFECT OF STATIC LOAD ON MATRIX SYNTHESIS RATES IN THE INTERVERTEBRAL DISC MEASURED IN-VITRO BY A NEW PERFUSION TECHNIQUE

The effect of static loading on matrix synthesis was measured in intact bovine coccygeal discs. The discs were maintained at 37 degrees C in a humid atmosphere and were perfused across their upper and lower surfaces for as long as 8 hours with media containing radioisotopes (H-3-proline and S-35-sulphate) via porous filter disks embedded in Perspex (Plexiglas) holders. Static loads were applied to the disc with use of weights. The activity of free, nonincorporated isotope in the centre of the disc was monitored continuously with a microdialysis probe. Incorporated tracer in different regions of the disc was determined at the end of each experiment from the activity of the nondialysable tracer. Free tracer activity rose steeply over the first 3-4 hours of perfusion, as tracer diffused into the disc, and reached a steady value after 5-6 hours, as expected from diffusion theory. The rate of tracer incorporation estimated from the integrated value of free tracer concentration was constant for as long as 8 hours. Incorporation rates varied across the disc: the highest rates were in the inner layer of the annulus fibrosus, and the lowest rates were in the outer layer of the annulus. The rates for both S-35-sulphate and H-3-proline varied with load. The rates were lowest in the nucleus pulposus and the inner layer of the annulus fibrosus with a 0.5 kg load, and they almost doubled as the load was increased to 5-10 kg. Heavier loads (15 kg) led to a decrease in incorporation. Hydration of the disc also varied with load; hydration decreased as load increased and was maintained near in vivo values with 5-10 kg of load (0.13-0.26 MN/m(2)). The effect of load on incorporation rates in the disc is compatible with the suggestion that rates are at a maximum at in vivo hydrations and decrease if the hydration decreases or if the disc swells.