PURPOSE: To present a method of posterior arch and lateral mass screw (PALMS) insertion and to prove its feasibility. METHODS: Four formalin-fixed specimens and 40 macerated atlas vertebras were used to describe the relevant anatomy. The height of the posterior arch was measured on 42 consecutive patients using standard CT of the cervical spine. The operative technique and the special CT reconstructions used for preoperative planning are described. Eight patients underwent posterior fixation using this technique. RESULTS: We described the relevant anatomy and important anatomical landmarks of the posterior arch of the atlas. PALMS placement was modified according to these anatomical findings. Fifteen PALMSs were placed in eight patients using this technique without vascular or neural injury. CONCLUSION: It is feasible to place PALMS using the described technique. CT angiography is of crucial importance for preoperative planning using the described special reconstructions. The arch posterior to the lateral mass (APLM) is defined as the bone stock situated posterior to the lateral mass, respecting its convergence. The ideal entry point for a PALMS is on the APLM above the center of the converging lateral mass. A complete or incomplete ponticulus posticus and a retrotransverse foramen or groove can be used as an accessory landmark to refine the entry point.

Clinical Anatomy Tübingen University of Tübingen Elfriede Aulhorn Str 8 72076 Tübingen Germany

Department of Neurosurgery Neurocenter Regional Hospital of Liberec Husova Str 10 46063 Liberec Czech Republic

Department of Orthopaedic Surgery Klinikum Esslingen Hirschlandstrasse 97 73730 Esslingen a N Germany

Department of Radiology Klinikum Esslingen Hirschlandstrasse 97 73730 Esslingen a N Germany

Laboratory for Applied and Clinical Anatomy Department of Anatomy Histology and Embriology Semmelweis University Budapest Tuzolto u 58 Budapest Hungary

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