ACL Interference Screw

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 (F)
• 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