ACL INTERFERENCE SCREW K. J. Davis, A. J. Huser, C. R. Kreofsky, D. C. Nadler, J. R. Poblocki Department of Biomedical Engineering, University of Wisconsin – Madison Client: Professor W. L. Murphy Ph.D. Advisor: Professor K. S. Masters Ph.D. Abstract Final Design Maximum Axial Load Testing and Results The objective of this project was to develop a novel ACL interference screw that not only secures a graft in place, but incorporates a material intended to promote bone tissue growth. This material is composed of a mineralized alginate scaffold that mimics a natural bone matrix. Using this material along with the selected growth factors in an interference screw may greatly improve recovery and longevity of the graft. A potential solution has been developed that utilizes a structurally sound thermoplastic while optimizing the amount of mineralized alginate scaffold present in the screw. Preliminary work has been done testing the feasibility of the fabrication process for this type of biphasic screw using model materials. Comparative mechanical testing was completed to ensure that the structural integrity of the screw had not been compromised by the addition of alginate. A controlled study was performed with screws containing 0%, 2%, or 5% of cross-sectional area alginate pocket cutouts. Experimental data suggests the 2% cross-sectional area alginate pockets can be incorporated; however, more testing is required to determine the maximum pocket size. Composite of mineralized alginate and plastic polymer Mineralized alginate Provides 3-D scaffold for bone cell proliferation and tissue growth Dope with growth factors and nutrients to augment bone growth Challenge: Mineralized alginate provides negligible mechanical strength Placed on outer perimeter in “pockets” for direct contact with tissue Possibility of in situ addition of alginate to driver shaft cavity Biodegradable thermoplastic (PLGA) Provides primary mechanical structure and threads of screw. Completely surrounds driver shaft to withstand and distribute insertion forces. Growth holes allow tissue growth into driver shaft cavity Tissue surrounds plastic before degradation for support Increases osteo-conductive environment Driver cavity Tests performed Simple Axial Compression Simple Radial Compression Insertion Torque Each test had a sample size of three (n = 3) Results The Axial Compression Curves show material and structural behavior There was a significant difference between 0% and 2% as well as 0% and 5% for axial compression and maximum insertion torque There is no significant difference for radial compression between the various cross-sectional areas of the samples * * * Plastic Maximum Forces Tolerated by Screw Amount of Cross-sectional Area Removed Maximum Axial Load (ft.lb) Maximum Radial Load (ft.lb) Maximum Insertion Torque (in.lb) 0% 146.9 ± 5.9 201.3 ± 13.6 65.3 ± 3.8 2% 142.6 ± 4.8 149.6 ± 17.6 37.8 ± 2.6 5% 109.1 ± 8.5 173.4 ± 12.2 30.3 ± 16.8 Alginate Background 90,000 annual ACL surgeries occur worldwide Patellar or hamstring tendon grafts are implanted in the femur and tibia Grafts secured with interference screws Interference Screw Titanium Extremely Strong Biocompatible Interferes with tissue re-growth Mismatch of mechanical properties Degradable Plastics Multiple polymers available Poly(L-Lactic) Acid [PLLA] Poly(Lactic-co-Glycolic) Acid [PLGA] Uneven degradation leads to scar tissue Anterior Cruciate Ligament (ACL) www.jnjgateway.com www.arthrotek.com http://miranda.ingentaselect.com Conclusions Design Calculations r a M The maximum insertion torque the screw needs to withstand is 17.7 in.lb; therefore, the PCL with 2% cross-sectional area removed would be able to tolerate the insertion torque with a safety factor of 2 Taking into consideration this safety factor and the fact that PLGA can withstand twice the shear stress of PCL, we believe that we can extend these findings to a PLGA screw The same cannot be said about the 5% cross-sectional area removed because the standard deviation of the insertion torque for the 5% data makes the value inconclusive The 0% screw can tolerate larger loads in the axial plane, and all three screws can tolerate the same loads in the radial plane; in terms of threshold values for axial and radial compression, we were unable to find any values in the literature Maximize mineralized alginate incorporation Roark’s Formulas for Stress and Strain Based on known surgical insertion torque (17.7 in.lb Nyland, J et al.) Approximately 2% of cross-sectional area can be removed Screw Fabrication Model Material PCL: to mimic PLGA Mold Tapped aluminum rod At surgical scale Driver Shaft Triangle: displaces melted plastic Removes screw from mold once set Alginate pockets drilled on perimeter Future Considerations Cast mold from Rapid Prototype Test desired material: PLGA Possibility of mineralizing thermoplastic In vitro testing Degradation Tissue growth Objective The primary goal of this project is to design an interference screw for ACL reconstruction that will simultaneously promote bone tissue growth while the screw degrades in an effort to reduce failures and the need for second surgeries. Acknowledgements Professor Murphy, Professor Masters, John Dreger of Structures & Materials Testing Lab, and BME graduate students of Murphy/Masters Lab. Plastic model made through rapid prototyping. This shows one half of the scaled-up model.