596 THE JOURNAL OF BONE AND JOINT SURGERY NERVE ROOT COMPRESSION IN SPONDYLOLYSIS AND SPONDYLOLISTHESIS

Thirty-four bony specimens of isthmic spondylolysis were examined and, in a significant number (32%), stenosis of the intervertebral foramen was noted. Although not emphasised in previous reports, this finding may be an important factor in the aetiology of nerve root compression when this is associated with spondylolysis and spondylolisthesis. Anatomical guidelines for adequate surgical decompression are suggested. We have examined 34 skeletal specimens ofisthmic spon-dylolysis, to our knowledge the largest group of speci-mens reported in the orthopaedic literature. The patho-anatomy is surprisingly consistent and helps to explain the radicular involvement sometimes seen in this dis-order. It confirms our clinical impression, shared by some (Newman 1973; Helfet and Gruebel Lee 1978, pp. 40-51) but disputed by others (Rothman and Simeone 1975, p. 233; Hensinger 1983) that most radicular patho-logy in the typical L5/Sl slip involves the L5 nerve root. We suggest that this is the result of a particular form of nerve root canal stenosis, mentioned but not emphasise