Biomechanical evaluation of fetal calf skull as a model for testing halo-pin designs for use in children

Rigid immobilization of the cervical spine in children is normally accomplished with a halo ring attached to the skull with pins. Concern exists about the risk of halo pin complications in small children due to their diminished skull thickness. More data are needed on biomechanical properties of the immature skull and on safe levels for halo pin penetration forces. The study included halo pin penetration tests on 43 skull samples obtained from eight fetal calves, radial compression tests of 11 skull samples, and histology. Compressive composite elastic modulus (15–139 MPa), yield stress (1–5 MPa) and composite consolidation modulus (188–479 MPa) were measured in the skull's radial direction. Pin penetration force (F) in Newtons at a pin-penetration depth equal to the original skull thickness (T) in mm, was related to T as: F=100+4.3eT (R2=0.76, p<0.0001). However, the 95% confidence limits on individual predictions were wide, e.g., 0–475 MPa for T=1.5 mm and 0–700 MPa for T=4 mm. These results suggest that skull thickness cannot be reliably used to predict halo pin penetration loads in a skull with similar structural and mechanical properties to that of the fetal calf. Due to the lack of available human data for comparison, the relevance of using the fetal calf skull as a model for human infants and young children remains inconclusive. Clinical recommendations regarding pediatric halo pin penetration loads cannot be made without further study of children's skull structure and biomechanical properties.