The modified occipital condyle screw: A quantitative anatomic study investigating the feasibility of a novel instrumented fixation technique for craniocervical fusion

Abstract

Study Design: Prospective human anatomical study. Objective: Occipitocervical fusion with occipital plate or condyle screws has shown higher failure rates in those with skeletal dysplasia. The modified occipital condyle screw connects the occipital condyle to the pars basilaris of the occipital bone that may achieve fortified bony purchase and serve as a more rigid fixation point. We evaluate anatomical feasibility of a novel cranial fixation technique designed to decrease risk of pseudarthrosis. Materials and Methods: Occipital condyles were analyzed morphologically using multiplanar three-dimensional reconstructed, ultra-thin section computed tomography. The following parameters were obtained: occipital condyle length, maximal cross section, location of hypoglossal canal, axial and sagittal orientation of the long axis, occipital condyle pedicle (OCP) diameter, maximal length of OCP screw, and entry point. Results: Forty patients with total of 80 occipital condyles were analyzed and the following measurements were obtained: occipital condyle length 24.1 mm (20.5–27.7, standard deviation [SD]: 2.2); condyle maximum axial cross-section 12.6 mm (9–15.8, SD: 1.9); length of OCP screw 38.9 mm (29.3–44, SD: 5.7); diameter of OCP 3.4 mm (3.2–3.6, SD: 0.2); clearance below hypoglossal canal 4.5 mm (3.4–7, SD: 1.1); and distance of screw entry point from condylar foramen 2 mm (range 0-4, SD 1.6). Conclusion: The modified occipital condyle screw connects the condyle with the clivus through the pars basilaris and represents a safe and technically feasible approach to achieve craniocervical fusion in skeletally mature individuals. This cephalad anchor point serves as an alternate fixation point of the occipitocervical junction with increased strength of construct and decreased risk of hardware failure or pseudarthrosis given cortical bone purchase and longer screw instrumentation.