A Customized Novel Halo with Displacement Based Pin Tightening for Pre-Operative Gravity Traction to Treat Severe Scoliosis

Abstract

Severe scoliosis is a deformity of the curvature of the spine, mostly occurring in children. It is currently treated by performing Halo Gravity Traction (HGT) to reduce the curvature prior to surgery. In this procedure, a Bremer halo ring is fitted around the child's head and connected to the skull with a numberof pins. During the 3-month treatment period, step-wise increased weight is applied to pull the ring upwards so as to elongate the spine. With the current design of the Bremer halo ring, the pins are tightened while measuring the amount of torque applied, influences each other during tightening, loosenover time and leave visible scars on the forehead. Furthermore, the Bremer halo ring is not customized.In the present project, a novel halo with a different tightening technique was designed, manufactured and evaluated with the aim of solving the above mentioned shortcomings of the Bremer halo.A Surface Tessellation Language (STL) file was generated from Computed Tomography (CT) images of a male cadaver head. Based on the desired ring stiffness and the geometry of a custom-designed halo for the cadaver head, a model was created in Solid Edge ST 10. The model was analyzed and adjustedby using the Finite Element Method (FEM). The customized halo ring was then produced by means of Selective Laser Sintering (SLS) and then equipped with strain gauges in order to derive the forces acting on the skull by the pins during tightening. The customized novel halo and the Bremer halo were compared with respect to moment arm and pin orientation, both influencing the local pin site behavior. The moment arms, pin positions and pin orientations of the 3D model of the novel halo were validated in reality by consecutive application of the halo to the cadaver head. The halo was analyzed by measuring the pin force degradation over a period of 24 h, which was hypothesized to be caused by the visco-elastic behavior of the skull of the cadaver head. The tightening procedure was analyzed on a block of steel to determine the influence on the axial pin reaction forces.The novel halo showed smaller and more consistent lengths of moment arm than the Bremer halo. Furthermore, one pin of the Bremer halo showed a difference of >15° from 90°, which has been regarded as contributor to pin loosening. The novel halo was predicted to be able to keep sufficient axial pin force without pin re-tightening during the traction period. The intended axial pin forces of the novel halo were achieved with an accuracy of 94.5%.With the customized novel halo, pins were tightened based on the displacement of the C-contours, resulting in an increased accuracy of pin force during tightening. The anterior pins were designed to be located in the musculus temporalis region, thereby leaving no visible scars and to be less prone to loosening due to a lower anterior ring stiffness. In conclusion, customizing the halo ring brings opportunities toengineer and control important parameters which contribute to better wearing comfort, higher pin force accuracy and less pin loosening, although it is yet a costly and time-consuming procedure.