Bioactive Interference Screws for ACL Reconstruction Dana Nadler – Communicator Cole Kreofsky – BSAC Katherine Davis – BSAC Aaron Huser – BWIG Joe Poblocki – Team Leader Client – Professor William Murphy Advisor – Professor Kristyn Masters

Problem Statement The primary objective of this project is to design an interference screw for ACL reconstruction that will simultaneously promote the growth of tissue while the screw degrades.

Background Information - ACL 90,000 annual ACL reconstruction surgeries worldwide Reconstructions use patellar or hamstring tendon grafts Grafts are implanted in the femur and tibia Grafts secured with interference screws http://miranda.ingentaselect.com/vl=2967323/cl=12/nw=1/fm=docpdf/rpsv/cw/pep/09544119/v217n1/s9/p59

Background Information - Screws Titanium: Extremely strong Biocompatible Reliable Second surgery may be necessary Degradable Plastics: Strength and shape Degradable Multiple polymers used Poly(L-Lactic) Acid (PLLA) Poly(Lactic-co-Glycolic) Acid (PLGA) http://www.arthrotek.com/products/aclpcl_interference.cfm http://www.jnjgateway.com/home.jhtml?loc=USENG&page=viewContent&contentId=09008b9880a6f43e&parentId=09008b9880a6f43e

Problem Motivation Problems: A new interference screw is needed that: Current screw may cause unwanted debris in the knee Current material does not promote tissue growth A new interference screw is needed that: Mimics bone scaffold structure Promotes tissue growth

Design Constraints The screw must be: Bioactive Biocompatible Biphasic Easily sterilized or autoclaved Structurally sound

Design Materials Alginate Hydrogel: Can be doped with growth factors, nutrients, metabolites, etc. Provides three dimensional scaffolding Promotes cell proliferation and tissue growth

Mold Investigations Plastic Transition temperature must be larger then melting point of thermoplastic Data: Plastic Tm(oC) Tg(oC) PLGA 210 37 PLA 185 57 PGA 225 40 Paper Will burn when thermoplastic is added Metal May be only alternative that can handle the temperature of thermoplastic

Design Evolution Limitations with mechanical properties of mineralized hydrogel Change geometry of thermoplastic to increase structural strength Minimize stress on mineralized hydrogel portions of screw Alginate PLGA

Hexagon Core Design

Triangle Core Design

Alginate Calculations Shear stress at point M: Where B varies with a/r over the range r a M For four grooves: For two grooves: Maximum Shear Stress of PLGA: ~14 MPa Insertion Torque: 1.5 N*m Maximum Radius of Screw: 5mm

Alginate Radius = 0.5mm 2% of total area can be alginate

Torque Analysis

Torque Analysis T r F Combining the two equations gives:

Future Work Find thermoplastic with: Adequate melting point Structural integrity FDA approval Investigate further alginate addition Quantitatively analyze designs Mold Development Prototype Development Documentation