1. 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)
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.