John Finkenberg MD, Laurie Sanders BS,MS, Scot Miller DO Variation in axial force and distribution at the vertebra-graft interface resulting in increased multilevel anterior cervical fusions with semi-constrained plates John Finkenberg MD, Laurie Sanders BS,MS, Scot Miller DO San Diego, California 2010

Study Design Biomechanical study demonstrating a variation in axial pressure when using constrained or semi-constrained plates Prospective, multi-center, patient outcome and radiographic study evaluating fusion, SF-36, ODI and VAS.

METHOD Four anterior cervical plates: Bone blocks polyurethane foam Cyprus (fixed) constrained screws Cyprus (fixed) semi-constrained screws VueLock (fixed) semi-constrained screws C-Tek Slotted (dynamic) constrained screws Bone blocks polyurethane foam 15 pcf / .24 gm/cc density pre-formed PCF-15 intervertebral bone graft DynaMight 8841 apparatus Ingstron force application 25 N – 150 N (loading posterior half of graft) PressureX pressure sensitive paper Ultralow 28-85 psi Data collection Topaq system software

COMPUTER EVALUATION Pseudocolor Analysis Paper Exposure Topaq® Pseudocolor Analysis Pressurex® Paper Exposure Anterior graft Posterior graft Topaq® Pressure Histogram Topaq® Pressure Statistics

50-N 100-N 150-N Fixed/ Constrained Screws Fixed/ Anterior Cervical Plating System Fixed/ Semi-Constrained Screws Anterior Cervical Plating System Dynamic/ Semi-Constrained Screws Anterior Cervical Plating System

CONCLUSION Fixed Anterior Cervical Plates with Constrained Screws shift the intervertebral axial forces anteriorly toward the plate. Anterior Cervical Plates with Semi-Constrained screws shifted the axial forces more posteriorly but the majority of the forces were still in the anterior half of the graft. Dynamic cervical plates noted increased axial force over the posterior portion of the graft where the load was placed.

Prospective Multicenter Study 186 Patients (72 men, 114 women) 78 one level, 87 two level and 21 three level 56 active smokers 102 workers compensation patients 131 allograft and 55 autograft Two year follow-up

Modified Oswestry (ODI) Score p<0.001

Visual Analog (VAS) Pain Scales

SF-36 Change from Pre-Op (+ = Improvement, - =Worsening)

Conclusion 96% fusion rate with no differences noted for graft type, levels fused, smoking history or workers compensation coverage. Increased axial forces over a larger surface area promotes bone fusion at the vertebra-graft interface.

Disclosures Biomet (Consultant) K2M, Satori World Medical (Stocks) Biomet (Royalties)