1 ACL Interference Screw Katherine Davis - BSAC Aaron Huser - BWIG Cole Kreofsky – Team Leader Dana Nadler - Communicator Joe Poblocki - BSAC Advisor: Professor Masters Client: Professor Bill Murphy

2 Motivation  Interference screw secures tendon graft implanted in femur and tibia  Issues with current ACL screws Incomplete degradation Tissue/material property mismatch Second surgery may be required Naturally slow tissue re-growth

3 Project Goals  Create novel interference screw: Mimic bone matrix using mineralized alginate  Promote tissue growth Maximize alginate volume (bioactive) into mechanically sound thermoplastic  Design reproducible method for screw fabrication Scale to surgical size

4 Design Overview  Composite of mineralized alginate and degradable plastic polymer  Growth holes allow in-growth of tissue into driver shaft cavity Increases osteo-conductive environment Tissue surrounds plastic before degradation for support

5 Current Status  Scaled up Model (2.5X diameter) Polypropylene (PP) beads Crude mold from large metal coupling

6 Current Status  Lab Procedure Hot oil bath Machined inserts Drilled growth holes  Mechanical Testing Simple compression and torsion Alginate pocket design vs. control  Rapid prototype of scaled up mold

7 Required Modifications  Rapid prototype of scale model  Modify shaft shape Optimize force distribution e.g. triangle vs. square driver  Use Poly-Caprolactone (PCL) as model material Similar to PLGA  Machine smaller inserts

8 Experimental Methods- Mechanical Testing  Torsion on PCL Model Material Solid PCL cylinders needed for material properties testing (larger size) Stress vs. angle of twist (Φ)  Testing will provide: Quantification of max stress (not reported in literature) Useful PCL screw torsion behavior for calculations

9 Experimental Methods- Mechanical Testing  Simple Compression Test Shaft of screw, minus head & tip Stress vs. strain relationship Alginate pocket designs vs. control  Testing will provide Determination of max stresses tolerable during fixation of graft Identification of high stress areas Empirically sound design

10 Experimental Methods- Driver Shaft Testing  Evaluate different driver shapes  Compare max insertion torque values with torque wrench  Hexagon, Triangle, Square, etc.

11 Data Analysis  Theoretical Calculations: 2% max of cross sectional area for alginate  From Testing Compare theoretical to experimental data Evaluate structural integrity Maximize amount of alginate

12 Next Steps  Receive rapid prototype of model Cast metal mold  Evaluate model PCL material  Testing  K-12 outreach

13 Acknowledgements Professor Kristyn Masters Professor William Murphy Todd Kile ME Dept. Professor Tim Osswald Polymer Engr. Center