This is a biomechanical study measuring the maximum pull-out strength of implants inserted into vertebral bodies of the calf spine. The objective is to investigate the influence of different anchoring systems.

The following implants were used: Zielke USIS (Ulrich, Ulm), Kaneda KASS (DePuy, Sulzbach). Universal Spine System (USS, Synthes, Umkirch) and Hollow Modular Anchorage (HMA) system (Aesculap, Tuttlingen). We selected nine groups with seven vertebrae equal in mean sizes and Bone Mineral Density (BMD) for each system. Vertebral body and implant were connected to both ends of a servohydraulic testing machine. Distraction was applied until failure and the maximum axial pullout force was recorded.

No significant correlation of BMD and pullout strength appeared. The student t-test showed significant higher stability for USS with pullout resistant nut (4.0 kN) and KASS (two-screws, 4.2 kN) compared to all other systems (p < 0.025). The mode of failure was a burst fracture in these vertebrae and shearing in all other systems. Bicortical screws of USS (3.2 kN) showed stronger hold than single bicortical KASS (2.5 kN) and HMA 12 mm (2.6 kN). Zielke (2.1 kN) was equal to monocortical KASS (one screw 2.1 kN) and superior to monocortical USS (1.6 kN). All those provided less stability than HMA 14 mm (2.4 kN).

For in-vitro testing with calf spines the influence of BMD seems to be less important than that of implant design. Maximum strength of Kaneda KASS depends on angulation of screws. Stability of USS implants can be increased by use of pullout resistant nuts. Of all monocortical implants only HMA presents pullout resistant strength comparable to bicortical screws. In-vivo use of monocortical anchorage bears the lowest risk of vascular injury, because the far cortex remains intact.