POSTERIOR STABILIZATION IN L5-S1 ISTHMIC SPONDYLOLISTHESIS WITH PARALAMINAR SCREW FIXATION - ANATOMICAL AND CLINICAL-RESULTS

Combined anterior and posterior fusion with posterior instrumentation may be indicated in the treatment of select cases of L5-S1 spondylolis thesis. The instrumentation, however, is expensive and usually bulky, occasionally requiring removal. In an effort to avoid these problems, an L5-S1 paralaminar screw technique was developed for posterior stabi lization after an L5-S1 anterior interbody fusion. The technique invol ves the placement of cortical screws from the base of the articular pr ocess of S1 to the pedicle of L5. This study evaluates the anatomic ap plications and clinical results of this technique. The relationship be tween the screw and L5 nerve root was examined using five cadaveric sp ecimens with olisthesis of 0, 25, 50, and 75%. This work demonstrates that the screws can only be inserted safely if an L5-S1 olisthesis of at least 25% is present. If <25%, the screws will either impinge on or directly injure the L5 nerve root. In the clinical study, the outcome s of 20 patients who had an isthmic spondylolisthesis of 25-81% and we re treated with partial reduction, L5-S1 anterior interbody fusion, an d L5-S1 posterior paralaminar screw fixation were reviewed. Nineteen p atients had adequate posterior stabilization to completely heal an L5- S1 anterior interbody fusion without loss of the correction. In one pa tient, a pseudarthrosis occurred secondary to poor surgical technique of both anterior and posterior fusions. This patient required an addit ional L4-S1 posterior fusion 9 months later and had a good clinical ou tcome. No other complications due to screw placement occurred We concl ude that this procedure can be used safely and reliably for the poster ior stabilization of L5-S1 after stable anterior L5-S1 interbody fusio n in residual slips of at least 25%. Prerequisites are proper patient compliance and low weight. Compared with other posterior instrumentati on systems, this screw fixation is inexpensive and does not require im plant removal. The disadvantages of the method are the degree of diffi culty of the procedure and the limited clinical application to cases o f L5-S1 spondylolisthesis with corrected residual slips of 25 to 50-60 %. The procedure is technically demanding and should be limited to tho se surgeons who are comfortable with the method.