Anterior Vertebral Body Screw Pullout Testing With The Hollow Modular Anchorage System - A Comparative in vitro Study. Hohltonnenschrauben als neues Verankerungskonzept an der Wirbelsäule - Lastauszugsversuche als biomechanische Vergleichsstudie

Pullout of implants at the proximal and distal ends of multilevel constructs represents a common spinal surgery problem. One goal concerning the development of new spinal implants is to achieve stable fixation together with the least invasive approach to the spinal column. This biomechanical study measures the influence of different modes of implantation and different screw designs, including a new monocortical system, on the maximum pullout strength of screws inserted ventrolaterally into calf vertebrae. The force pullout of eight different groups were tested and compared. Included were three bicortical used single screws (USS, Zielke-VDS, single KASS). To further increase pullout strength either a second screw (KASS) or a pullout-resistant nut can be added (USS with pullout nut). A completely new concept of anchorage represents the Hollow Modular Anchorage System (MACS-HMA). This hollow titanium implant has an increased outside diameter and is designed for monocortical use. Additionally two screw systems suitable for bicortical use were tested in monocortical mode of anchorage (USS, single KASS). We selected seven vertebrae equal in mean size and bone mineral density for each of the eight groups. The vertebral body and implant were connected to both ends of a servohydraulic testing machine. Displacement controlled distraction was applied until failure at the metal-bone-interface occurred. The maximum axial pullout force was recorded. Mean BMD was 312 +/- 55 mg CaHA/ml in cancellous bone and 498 +/- 98 mg CaHA/ml in cortical bone. The highest resistance to pullout found, measured 4.2 kN (KASS) and 4.0 kN (USS with pullout nut). The mean pullout strength of Zielke-VDS was 2.1 kN, of single KASS 2.5 kN, of MACS-HMA 2.6 kN and of USS 3.2 kN. There was no statistically significant difference (t-test, p > 0.05) between bicortical screws and the new monocortical implant. For the strongest fixation at the proximal or distal end of long spinal constructs the addition of a second screw or a pullout-resistant nut behind the opposite cortex offers even stronger fixation.

[1]  L. Claes,et al.  Biomechanical comparison of calf and human spines. , 1996, Journal of orthopaedic research : official publication of the Orthopaedic Research Society.

[2]  W. Edwards,et al.  In vitro spinal arthrodesis implant mechanical testing protocols. , 1989, Journal of spinal disorders.

[3]  E. Transfeldt,et al.  Experimental Pullout Testing and Comparison of Variables in Transpedicular Screw Fixation: A Biomechanical Study , 1990, Spine.

[4]  T C Hearn,et al.  Insertional Torque and Pull‐out Strengths of Conical and Cylindrical Pedicle Screws in Cadaveric Bone , 1996, Spine.

[5]  Friedrich-Alexander-Universität Erlangen-Nürnberg,et al.  Die MACS-HMA-Hohltonne Eine alternative Möglichkeit der stabilen Implantatverankerung im Wirbelkörper auch für langstreckige Fusionen , 2002 .

[6]  P. Eysel,et al.  Operative Treatment of Scoliosis With Cotrel‐Dubousset‐Hopf Instrumentation: New Anterior Spinal Device , 1997, Spine.

[7]  J. Schatzker,et al.  The holding power of orthopedic screws in vivo. , 1975, Clinical orthopaedics and related research.

[8]  I. Lieberman,et al.  Anterior Vertebral Body Screw Pullout Testing: A Comparison of Zielke, Kaneda, Universal Spine System, and Universal Spine System With Pullout‐Resistant Nut , 1998, Spine.

[9]  R. Putz,et al.  Changes in Cadaveric Cancellous Vertebral Bone Strength in Relation to Time: A Biomechanical Investigation , 1998, Spine.

[10]  C. E. Bowman,et al.  Holding power of orthopedic screws in bone. , 1970, Clinical orthopaedics and related research.

[11]  B. J. Doherty,et al.  A Biomechanical Study of Anterior Thoracolumbar Screw Fixation , 1998, Spine.

[12]  M Arand,et al.  Combined anteroposterior spinal fixation provides superior stabilisation to a single anterior or posterior procedure. , 2001, The Journal of bone and joint surgery. British volume.

[13]  W. Hutton,et al.  Strength of Fixation of Anterior Vertebral Body Screws , 1996, Spine.

[14]  H. Uhthoff,et al.  Mechanical factors influencing the holding power of screws in compact bone. , 1973, The Journal of bone and joint surgery. British volume.

[15]  D N Kunz,et al.  Pedicle Screw Pullout Strength: Correlation with Insertional Torque , 1993, Spine.

[16]  L. Claes,et al.  Testing criteria for spinal implants: recommendations for the standardization of in vitro stability testing of spinal implants , 1998, European Spine Journal.

[17]  J. Roush,et al.  Holding Power of Orthopaedic Screws in Metacarpal and Metatarsal Bones of Young Calves , 1992, Veterinary and Comparative Orthopaedics and Traumatology.

[18]  Peter‐Michael Zink,et al.  Performance of Ventral Spondylodesis Screws in Cervical Vertebrae of Varying Bone Mineral Density , 1996, Spine.

[19]  K. Kaneda,et al.  A Biomechanical Analysis of Zielke, Kaneda, and Cotrel‐Dubousset Instrumentations in Thoracolumbar Scoliosis A Calf Spine Model , 1991, Spine.

[20]  Kuniyoshi Abumi,et al.  New Anterior Instrumentation for the Management of Thoracolumbar and Lumbar Scoliosis: Application of the Kaneda Two‐Rod System , 1996, Spine.