The pathogenesis of the clinical picture in sciatica.

h is no,ir widely accepted that a largo proportion of cases of sciatica are associated with a prolapsed lumbar intervertebral disc. There is, however, vel',Y little agreement 0 11 how a prolapsed dise gIves rise to symptoms and sign~. I thought it w011 ld be useful to discuss some of the ex perimenta l \\~ork we have done on tIllS problem. First, of allwliy do these poople get b,,,,kache? In this respect it is relevant to discuss spondylolysis. In this conditi,on a defe<:t occurs in the pars intcmrt icularis on one or both sides (fig. 1). There is no agreement as to the retiology of this lesion. It may be ,Iue to a lack of fusion between two centres of ossification on eaeh side of t.he n CUl',11 arch. ~o one, however, ha~'i convincingl.): shown t.hat the~e double centres occur. It ha.s been suggesteel by Galluceio, Robcl'ts and others t hat tliege dcfccts OCCUT' as a T'esult of stross fractul'Cs, sueh as are soon in t.JJe loot, the t ibia , t,he ne"k of tl16 femur and the inferior ramus of t he pubis. • 'In the short time that we have been looking for this lesion , we havo seen several examples and agree with Robert~ that the lesion OCf!Ul'!:i lllore D.'equcntly than \va cOlnmolll y suppose.