In an effort to obtain more information about the mechanics of the lumbar discs and about the production of low-back pain, direct in vivo measurements of lumbar intradiscal pressures were made under various conditions. A needle with a pressure-sensitive polyethylene membrane at its tip was constructed for this purpose.
Pressure values were obtained in eighteen normal third and fourth lumbar discs of sixteen subjects. The sitting position was used in all cases; other positions and factors studied included standing, reclining, holding total weights of 9.1 and 22.7 kilograms, performing the Valsalva maneuver, and wearing an inflatable corset.
High intradiscal pressures (ten to fifteen kilograms per square centimeter) were found in the normal discs with the subject in the sitting position. In all cases, pressures were highest in the sitting position, about 30 per cent less in the standing position, and about 50 per cent less in the reclining than in the sitting position.
It was found that the lower lumbar discs have to support total loads of as much as 100 to 175 kilograms when the subject is seated. For the standing position total loads of between ninety and 120 kilograms were calculated from the pressure values obtained.
It was calculated that, in the normal discs tensile forces of considerable magnitude (sixty to eighty kilograms per square centimeter) exist in the posterior part of the annulus; this fact gives strong support to the mechanical theory of the production of ruptures of the posterior annulus.
The Valsalva maneuver generally increased the intradiscal pressure, but the results were variable. The inflatable corset decreased the pressure in the examined discs by about 25 per cent.
The loads on discs spanned by a posterior fusion in two patients were approximately 30 per cent less than would have been anticipated. In the one with a pseudarthrosis, further loading increased the intradiscal pressure to a value similar to that in subjects without fusion. In the one with a successful fusion demonstrated by surgical exploration, the force on the disc was proportionately reduced with further loading.
The data support and help to explain such observations as increased pain in sitting as compared with standing and reclining in patients with low-back pain, pain during the Valsalva maneuver, and the codfish appearance of osteoporotic spines with relatively normal discs.
Lumbar discometry is suggested as a means not only of determining the basic mechanics of the disc in different positions and under different conditions of stress but also of evaluating therapeutic measures such as posterior fusion and the wearing of corsets and braces.
