POSTERIOR EPIDURAL FAT PAD AND LUMBAR CAN AL STENOSIS - FILLING TISSUE OR CONFLICT FACTOR

The role played by the epidural fat has been reported in lipomatosis i nduced by exogenous glucocorticoids and in severe obesity with lipomat osis. The role played by the ''normal'' posterior epidural fat (PEF) i n lumbar canal stenosis (LCS) is less well known. The purpose of this study was to determine the part taken by PEF in LCS patients without e ndocrine disease, corticosteroid therapy or obesity. For this, we trie d to specify the amount and distribution of PEF among the soft tissues in the vertebral canal, to demonstrate the involvement of PEF in dura l sac compression, to describe the radiological features observed in c ases of LCS and to look for associated morphological factors.The recor ds of 30 LCS patients without exogenous or endogenous lipomatosis and in whom the essential pathogenic factor in 40 levels was PEF were revi ewed retrospectively. At disc level, PEF was evaluated in the lower pa rt of the mobile segment by means of CT or MRI axial sections cut thro ugh one or two spaces between L2-L3 and L4-L5. Measurements were made in 25 men (80 %) and 6 women (20 %) aged from 33 to 83 years (mean: 58 years). Most patients were suffering from lumbar pain, radiculopathy and/or neurogenic intermittent claudication. The data measured were: a ntero-posterior (AP) diameter of the dural sac, AP diameter of the bon y lumbar canal (BLC), interligamentous distance (LLD) opposite the art icular facets, and surface of PEF. The soft elements present on the mi dline - anterior epidural space (AES) and posterior epidural fat (PEF) - were expressed as percentage of the AP diameter of the bony lumbar canal. Pathological levels were L2-L3 in 10 % (n = 4), L3-L4 in 35 % ( n = 14) and L4-L5 in 55 % (n = 22). Ten patients (33 %) had two pathol ogical levels. In all cases the percentage of PEF, expressed as percen tage of the AP diameter of BLC, was increased by 44 % on average. The 40 levels presented with congenital or acquired diminution of the tran sverse diameter and, in particular, of the interligamentous diameter ( ILD) opposite the articular facets : 9.3 +/- 1.6 mm at L4-L5 level, 9. 7 +/- 1.4 mm at L3-L4 level and 10.5 +/- 0.5 mm at L2-L3 level. Under these conditions the laterally compressed PEF moves forward and compre sses the dural sac. This compression is responsible for a morphologica l change in the dural sac which is no longer convex posteriorly but be comes concave posteriorly (85 %, n = 34) or rectilinear (15 %, n = 6). The role played by PEF, as well as by other soft tisues in the canal, must be taken into consideration in the mechanical pathology of the s pinal canal. It appears that PEF becomes pathogenic when the surface o f this canal is reduced, and especially when the stenosis is transvers e. The posterior epidural fat pad, therefore, must be regarded as one in the actor in the conflict between soft tissues, bony structures and nervous elements.