Anatomical Safety of the U.S.S. Pedicular Hook-Screw in the Adult Thoracic Spine

Introduction Pedicle hooks have been extensively used in spine surgery. One of the major concerns of such instrumentation is the pullout of the implants. Recently, a screw that anchors the pedicle was added to secure the hook, and to improve both pullout strength as well as purchase. An anatomical study was performed on human cadavers to assess the anatomical safety of these implants. From this study, we can conclude that the use of screws to hold the pedicle hooks is generally safe. Further studies should be done to assess this in deformed spines. Materials & Methods: Eleven embalmed human cadavers with a mean age of 76 years (6 male bodies and 5 female bodies) were used for this study. The same staff spine surgeon performed all surgical procedures. The thoracic pedicles were instrumented bilaterally starting from T1 down to T12 with an AO Universal Spine System Pedicular Hook Screw. Abandoning the procedures was permitted whenever the safety of any neural structures was jeopardized or the implant could not be placed correctly. The surgeon's comments were documented. For subsequent analyses after instrumentation, the spines were harvested en-bloc and later dissected into individual vertebrae. Two independent blinded observers did a visual inspection to grade the hook and screw positions. All specimens were then radiographed individually in three dimensions (antero-posterior, medio-lateral & cranio-caudal). Two blinded observers independently evaluated the hook and screw positions following the visual classification system. Only the antero-posterior and lateral radiographs were provided, because only these would be available in a clinical situation. Results Thirty-five implant placements (ten in T11, eighteen in T12 and seven in all other levels) were abandoned prior to the surgical procedure. Six hooks were requested to be repositioned. Four implant placements were abandoned after drilling the screw hole (three screws with a medial deviation, and one screw with a lateral deviation). Of the remaining 225 implants, 29 were lost during harvesting and preparation, leaving 196 implants for further evaluation. One hundred and eighty three implants were considered safe by the surgeon at the end of the surgical procedure, 13 were believed not to be safe and were removed. No screws were found to be disrupting the integrity of the neural structures in the canal or in the foramen. A comparison between the visual and radiological evaluations of both the screw and hook positions was performed. The visual evaluation showed an inter-observer agreement of 85% for the hooks and 78% for the screws. The radiographic evaluation showed an agreement of 75% for the hooks, and of 43% for the screw between observers, a value considerably lower than expected. When comparing each of the two radiological evaluations to the “gold standard,” only 62% to 67% of the hooks and 42% to 48% of the screws had an agreement. Conclusion Overall, the use of U.S.S. pedicle hook-screws is suggested to be safer than either the conventional pedicle hook, which is often malpositioned inside the canal, or the classical pedicle screw, which is too large for the thoracic pedicles.

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