Introduction Spinal implants are used to treat a wide variety of painful and disabling spinal disorders. Most modular spinal in- strumentation systems are based on the pedicle screw as a primary anchor. Segmental posterior fixation of the spine by means of pedicle screws, bars and bone grafting normally provide a Abstract Several types of pedicle screw systems have been utilized to augment lumbar spine fusion. The majority of these systems are made of stainless steel (Ss), but titanium- alloy (Ti-alloy) devices have recently been available on the market. Ti-alloy implants have several potential ad- vantages over Ss ones. High bioac- tivity and more flexibility may im- prove bone ingrowth and mechanical fixation, and the material also offers superior magnetic resonance imaging (MRI) and computed tomography (CT) resolution and significantly less signal interference. However, no data are available from loaded spinal con- structs regarding bony ingrowth and mechanical fixation. The aim of this study was to analyse the effect of Ti- alloy versus Ss pedicle screws on mechanical fixation and bone in- growth in a loaded mini-pig model. Eighteen adult mini-pigs underwent total laminectomy and posterolateral spinal fusion at L3-L4, and were ran- domly selected to receive either Ss (n = 9) or Ti (n = 9) pedicle screw devices. In both groups, the device used was compact Cotrel-Dubousset instrumentation (Sofamore Danek) of an identical size and shape. The postoperative observation time was 3 months. Screws from L3 were used for histomorphometric studies. Me- chanical testing (torsional tests and pull-out tests) was performed on the screws from L4. The Ti screws had a higher maximum torque (P < 0.05) and angular stiffness (P < 0.07), measured by torsional testing. In the pull-out tests, no differences were found between the two groups with respect to the maximum load, stiff- ness and energy to failure. No corre- lation between removal torque and the pull-out strength was found (r = 0.1). Bone ongrowth on Ti was in- creased by 33% compared with Ss (P < 0.04), whereas no differences in bone volume around the screws were shown. Mechanical binding at the bone-screw interface was signifi- cantly greater for Ti pedicle screws than for Ss, which was explained by the fact that Ti screws had a superior bone ongrowth. There was no corre- lation between the screw removal torque and the pull-out strength, which indicates that the peripheral bone structure around the screw was unaffected by the choice of metal. Key words Pedicle screw · Titanium · Stainless steel · Pull-out test · Torsion · Histomorphometry ORIGINAL ARTICLE Eur Spine J (2000) 9 : 97–103 © Springer-Verlag 2000 Finn Bjarke Christensen Michel Dalstra Flemming Sejling Søren Overgaard Cody Bünger Titanium-alloy enhances bone-pedicle screw fixation: mechanical and histomorphometrical results of titanium-alloy versus stainless steel Received: 9 August 1999 Revised: 8 December 1999 Accepted: 22 December 1999 F. B. Christensen () · M. Dalstra · F. Sejling · S. Overgaard · C. Bünger Orthopaedic Research Laboratory, University Hospital of Aarhus, Nørrebrogade 44, 8000Aarhus C, Denmark e-mail: fbc@dadlnet.dk, Tel.: +45-89494134, Fax: +45-89494150 F. B. Christensen · F. Sejling · S. Overgaard · C. Bünger Institute of Experimental Clinical Research, University Hospital of Aarhus, Denmark