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IV.    SURGICAL ANATOMY OF THE LUMBAR SPINE (click here for related Pathological conditions
Figure 10 shows the surgical hemi-laminotomy (enlarging the intralaminar space on one side only) approach on the right side for exposure of an L4-5 disc herniation to decompress the L5 nerve root to relieve sciatica.  On the left side an L5-S1 hemi-laminotomy approach is illustrated to decompress the S1 nerve root.  Both approaches allow subtotal foraminotomy as needed to increase the nerve root space for exit from the canal.  The nerve root is gently retracted medially to expose the herniated disc fragment.  Only part of the lamina is removed, assuring movement over the shoulder of the nerve root and retracting it medially from the sidewall. This approach avoids injury when completing the lateral exposure for the removal of the ligamentum flavum and exploring the nerve root foramen. 
Figure 10. The L4-5 hemi-laminotomy exposures for both right L5 nerve root and L5-S1 approach for left S1 nerve root compromise. 
Figure 11. Total laminectomy exposure for spinal stenosis to decompress the central canal and approach needed to do partial L4, L5 and S1 foraminotomies bi-laterally. 
Laminectomy (removal of the lamina), can be performed on one side for a unilateral exploration in cases where hemi-laminotomy (enlarging the interlaminar space demonstrated in Figure 10), would not provide sufficient exposure or in rare cases of one level unilateral neuroclaudication from spinal stenosis.  Usually spinal stenosis is bilateral requiring extensive exposure through total laminectomy as demonstrated in Figure 11.  Laminectomy thus allows a more complete decompression of nerve roots in both the central canal and the nerve root foramen.  A general rule for decompression laminectomy is to either leave one of the facets intact or remove no more than half of both facets to avoid risking future anterior slippage.  Removal of no more than half of either motion segment's facet joint should neither render an aged motion segment unstable nor suggest a need for fusion.  With either laminotomy or laminectomy, the goal is to provide sufficient nerve roots decompression.  This can be grossly gauged by being able to gently retract the nerve root to the midline or easily introduce a Penfield 3 retractor into the nerve root foramen after partial foraminotomy through the Laminectomy exposure. 
Figure 12 demonstrates the postero-lateral fusion between the transverse processes at L4-5 or L5 to sacrum when confronted with neuroclaudication and slip (spondylolisthesis) with motion (see fig. 13 and 14) or removal of more than half of each facet joint.  The remainder of the facet and posterior elements is included in the fusion.  More intricate instrumental techniques have been introduced but not yet proven to improve clinical outcomes, only complications (see Fusion Caveat). 
Figure 12 Posterolateral fusion is performed by placing grafts of bone from elsewhere onto denuded facet joints, transverse processes, posterior vertebral elements. Fusion would be performed on both sides to stabilize a slip with continued motion at either L4-5 or at L5-S1.  The use of implants with fusion is more useful with in treatment of fracture, dislocation, tumor, infection, severe deformity of scoliosis or severe spondylolisthesis (>50%).  Otherwise, use of spinal fusion hardware is difficult to justify their related high serious complication rate
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