bio-materials project on an acl attachment

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ACL AttachmentReplacing Conventional Interference Screws with a Clamp inAnterior Cruciate Ligament (ACL) Reconstruction to Increase

the Creep Resistance of Synthetic Ligaments

March 11, 2011

Meagan Higgins

Amy LautenbachStacy PatchAlex Stanley



Outline

Background: Current Technology

Material Science and Mechanics

Proposed Solution Test Methods and Procedure

Results

Discussion Recommendations



Background: CurrentTechnology

Most frequently studied limb in orthopaedic research

ACL provides stability and function of the knee

3 types of ACL Reconstructions

Autografts Allografts

Synthetic



Background: CurrentTechnology

Interference screws

Endo-button and spiked washer (Smith & Nephew)

Femoral cross pins



Material Science andMechanics

Ligaments under physiologic loading exhibit nonlinear, viscoelasticbehavior

Further loading recruits more ligament fiber bundles

Viscoelastic Polymers When subjected to applied stress, entropy decreases (less

“disordered”) permanent elongation due to unwinding of the polymer chains




Stress relaxation is directly related tostrain

Stress Relaxation in ACL graftsreduces graft forces from the initiallevel set at surgery

Creep represents the non-linearincrease in deformation over timewhen the stress is constant orsubjected to cyclically repetitivestress

Non-linear viscoelasticity indicates time dependent mechanical behavior Principles relate creep and stress relaxation of biological materials




Hysteresis: Energy dissipation

When a viscoelastic materialis loaded and unloaded, theunloading curve will notfollow the loading curve

The difference between thetwo curves => amount ofenergy that is dissipated




Kevlar

(Aramid Fiber)

Carbon Fiber

TensileStrength (KSI)

464 348

FlexuralStrength (KSI)

363 319

Elongation (%Strain)

1-1.3% 1.8-2%



Problem Statement/ ProposedSolution

Problem Statement: The elongation due to creepfound in synthetic ACLs prevents the ligamentfrom providing sufficient long-term stability and

makes the ligament more likely to fail. Proposed Solution: If we use an attachment that

better distributes the force exerted by the body

onto the ligament across the entire area of theligament, there will be less stress and wear oncertain fibers which will make it able to lastlonger.



Proposed Solution

Screw Clamp

C u r r e n

t

: P r o p o s e d :



Test Method/Procedure

Perform a stress relaxation test on siliconesamples embedded with Kevlar or Carbonfiber braids

DoE: 2 Factors, 2 Levels

Factors: Attachment method, Fiber Material

Levels: Clamp/screw, Kevlar/carbon fiber



Performing the Test



Graphing the Results

1. Kevlar; Clamp2. Kevlar; Screw3. Carbon Fiber; Screw4. Carbon Fiber; Clamp



More Graphs

1. Carbon Fiber; Clamp2. Carbon Fiber; Screw3. Kevlar; Clamp4. Kevlar; Screw



Discussion/Conclusions

Unable to calculate stress relaxation

Observations show clamp holds higher loadsfor more time

No consistency of kevlar versus carbon fiber



Recommendations and FutureDirections

Different strain

More consistent measurements of silicone

More trials

bio-materials project on an acl attachment

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