37 Development of a model for occipital xation— validation of an analogue bone material H Mullett1*, T O’Donnell 1, P Felle2, K O’Rourke1 and D FitzPatrick3 1 Department of Orthopaedic Surgery, St Vincent’s University Hospital, Dublin, Republic of Ireland 2 Department of Anatomy, University College Dublin, Republic of Ireland 3 Department of Mechanical Engineering, University College Dublin, Republic of Ireland Abstract: Several implant systems may be used to fuse the skull to the upper cervical spine (occipito- cervical fusion). Current biomechanical evaluation is restricted by the limitations of human cadaveric specimens. This paper describes the design and validation of a synthetic testing model of the occipital bone. Data from thickness measurement and pull-out strength testing of a series of human cadaveric skulls was used in the design of a high-density rigid polyurethane foam model. The synthetic occipital model demonstrated repeatable and consistent morphological and biomechanical properties. The model provides a standardized enviroment for evaluation of occipital implants. Keywords: synthetic model, occipital bone, biomechanical evaluation, implant 1 INTRODUCTION In the current regulatory enviroment in the European Union for cadaveric testing there is a need for stan- dardized analogue material and models for testing and validation of medical implants. Mechanical instability of the upper cervical spine and the junction between cervi- cal spine and skull represents a challenging clinical prob- lem. Operative treatment requires fusion of the occiput to the upper cervical spine (occipito-cervical fusion). This is a technically demanding procedure with signi- cant morbidity and mortality [ 1 –5 ]. Inadequate pur- chase in the occipital bone (skull ) may lead to screw backout or development of non-union with subsequent failure of xation. Occipito-cervical fusion is most com- monly performed for cervical rheumatoid arthritis where osteoporotic bone, poor soft tissues and the patient’s generalized debilitated condition increase the risk of fail- ure. Failure of occipital xation may lead to non-union rates of approximately 30 per cent [ 6 ]. Occipito-cervical fusion has evolved from the use of simple onlay bone grafts and wiring to sophisticated modular titanium implants (see Fig. 1) [ 5, 7, 8 ]. Most recent authors agree that internal xation is necessary with occipito-cervical Fig. 1 Occipito-cervical fusion using modular implant system fusion because union is di Ycult to obtain as a result of the abundance of synovial tissue in the upper cervical region. Concerns about occipital bone variability have prompted alternative methods, techniques and place- The MS was received on 21 May 2001 and was accepted after revision ment of standard screw-plate designs. Occipital thickness for publication on 9 October 2001. has received only limited study and there is no consensus * Corresponding author: Department of Orthopaedic Surgery, St Vincent’s University Hospital, Elm Park, Dublin 4, Republic of Ireland. as to which screw position is best. Heywood et al.[ 9 ] H02901 © IMechE 2002 Proc Instn Mech Engrs Vol 216 Part H: J Engineering in Medicine