LONG-TERM OSSEOUS MODIFICATION OF THE POS TERIOR ARCH AFTER DECOMPRESSIVE SURGERY FOR LUMBAR SPINAL STENOSIS

Purpose of the study Many studies have indicated favorable results of decompressive surgery for symptomatic lumbar spinal stenosis. However, little is known about the osseous changes that occur at the operative sites. Postacchini in 1992 and Chen in 1994 have studied, only from p lain radiographs, osseous changes at the operative sites, and have sug gested that bone regrowth possibly affects the neurologic result. The aims of this study were: to assess bone regrowth at the operative site , to compare the bone regrowth rate calculated from plain radiographs and CT-Scan examinations, to determine the effects of bone regrowth on clinical outcome, to investigate the factors promoting the bone regro wth. Material and method 28 patients who underwent decompressive surge ry for lumbar spinal stenosis were retrospectively studied with an ave rage follow-up of 8.4 years. In order to evaluate the degree of bone r egrowth at the posterior arch, early postoperative radiographs and CT images of the operative sites were compared with those obtained at fin al follow-up. Bone regrowth at the sites operated upon was evaluated a s a percentage of regrowth of the original laminectomy site based upon plain radiographs and CT images. Results Decompressive lumbar spinal stenosis is responsible for bone regrowth at the operative site in mos t patients. However, this regrowth was mild, the mean bone regrowth ra te evaluated from plain radiographs was 12 per cent In average and the obtained from CT images was 8,2 per Gent in average. Changes were fou nd to be predominant at the facet joint level compared to the pedicle level. The evaluation of regrowth obtained from plain films and CT ima ge examinations were compared. Radiographs seem to overestimate bone r egrowth. Postoperative spinal instability was statistically significan tly associated with new bone development. This variable was the only f actor that affected the degree of bone regrowth. No relationship betwe en bone regrowth and clinical outcome was found. Discussion and conclu sion Natural course of laminectomy defect includes probably new bone f ormation in most patients. New bone results from gradual regrowth of t he laminae and articular processes partially resected at surgery and f rom coalescence of islets of bone tissue within the tissue filling the laminectomy defect. In the present study bone regrowth rate was moder ate but in other ones it was marked. If some factors (like postoperati ve destabilization) promoting bone regrowth were identified many remai n unknown. Factors influencing rapidity of regrowth progression remain also unknown. Patient's intrinsic features such as spinal stenosis ch aracteristics are probably closely related to quantitiative and kineti c characteristics of regrowth. Consequences of bone regrowth are also variable: in some cases regrowth may reproduce pathological conditions identical previous ones, in other ones new bone spreads around the du ra a mater without any nerves roots compression. Study of bone regrowt h requires further research including prospective studies and using a more precise method for the regrowth evaluation.