Design of a Rotational Stability Measurement Device

For Analysis of ACL Reconstruction

University of PittsburghSenior Design – BioE 1160/1161

Stephanie BechtoldKatie Dillon

Kara Wagner

Mentor: Thore Zantop, M.D.

April 18, 2005

Goal

To develop and test a novel device to measure the rotational stability of the knee after an Anterior Cruciate Ligament (ACL) reconstruction

ACL anatomy

• Two distinct fiber bundles

• Anteromedial (AM)

• Translational stability

• Posterolateral (PL)

• Rotational stability

www.aclsolutions.com/ images

Current Reconstruction Methods

• Single bundle

• Restore only the AM bundle

• Susceptible to reinjuries by pivoting

• Double bundle

• Restores both AM and PL bundles

• More anatomically correct

Devices to Evaluate ACL Reconstruction

• Current devices only measure translational stability of the knee

• Double bundle technique creates need for device to measure rotational stability

• Evaluate effectiveness of PL bundle reconstruction

http://www.medmetric.com/kt1.htm http://www.aircast.com

Problem Statement

• A device is needed to measure the rotational stability of the knee

• Comprehensive analysis of reconstruction techniques

• An immobilization device is needed to comfortably restrict movement in the hip and ankle joints

• Ensure pure rotation of the knee is measured

Market Considerations

• Market size

• 16,000 Orthopedic Surgeons (AAOS)

• Predicate Devices• KT1000: $3900

• Rolimeter: $850

• Adjust boot to keep ankle immobile

• Fix knee at various flexion angles (0, 30, 45, 60, 90 degrees)

• Comfortably immobilize hip

• Consistently and safely apply known moment

• Collect repeatable data

Initial Design Considerations

• Universal force-moment sensor (JR3, Woodland, California)

Prototype Development- Boot• Aircast® Pneumatic Walker Brace• Nest of Birds Ascension Technology Corporation, Burlington, VT/USA

Photos courtesy of Ferguson Lab

Adjust boot to keep ankle immobile

• Fix knee at various flexion angles (0, 30, 45, 60, 90 degrees)

• Comfortably immobilize hip

Consistently apply known moment

• Collect repeatable data

Initial Design Considerations

Prototype Development- Hip Brace

• Lateral Decubitus position

• Wheelchair leg rest

Adjust boot to keep ankle immobile

Fix knee at various flexion angles (0, 30, 45, 60, 90 degrees)

• Comfortably immobilize hip

Consistently and safely apply known moment

• Collect repeatable data

Initial Design Considerations

Prototype development

• Redesign of initial prototype • Patient placed in supine position

• Lower leg horizontal

• Adjustable leg rest

• Knee flexion angle

• Length of femur

• Minimize metal components

Prototype Fabrication

Materials Selection

• Constructed of acrylic

• Eliminates metal components

• Minimizes interference with magnetic sensors

• Durable

• Comfortable for patient

Evaluation of Device

• 4 subjects - at 3 flexion angles

• 5 trials each

• Ensure repeatability

• Goal: Range of motion within ±1°

• Subject reports stability of ankle and hip joints

Experimental Methods

• Subjective knee evaluation performed by a clinician to determine health of knee

• Subject fitted with boot and brace to comfort

• Nest of Birds (NOB) sensors placed on

• Proximal Tibia

• Distal Femur

• Front of Boot

Experimental Methods

• Lower leg leveled at horizontal

• Creates neutral start position

• Moment applied by clinician (10Nm)

• Range of motion recorded by Nest of Birds

Experimental Methods

Sample Results

Preliminary Results

• Range of Motion, in degrees

• 4 subjects, 5 trials each

 FlexionAngle Subject 1 Subject 2 Subject 3 Subject 4

0 degrees 40.75 ± 1.7 62.20 ± 1.4 43.40 ± 1.5 64.40 ± 1.9

45 degrees 49.60 ± 1.8 52.00 ± 1.0 31.60 ± 0.9 57.00 ± 0.7

90 degrees 37.00 ± 1.6 37.00 ± 0.7 27.20 ± 1.5 54.20 ± 1.8

Adjust boot to keep ankle immobile

Fix knee at various flexion angles (0, 30, 45, 60, 90 degrees)

Comfortably immobilize hip

Consistently and safely apply known moment

Collect repeatable data

Initial Design Considerations

Discussion

• Range of motion was repeatable within ±2° for all subjects

• Subjects reported ease of use and comfort of the boot and brace

• Overall apparatus is heavy for operator

Competitive Analysis

Our Device• Strengths

• More comprehensive analysis of ACL reconstruction

• Weaknesses• Complex design• Higher cost

Predicate Devices• Strengths

• Lightweight• Simple design

• Weaknesses• Limited analysis of

reconstruction techniques

• Race for “first to market”• Competition to develop similar device

Constraints limiting Phase I testing

• IRB approval

• Create baseline data for normal subjects

• Overhead cost of measurement devices

Quality System Considerations

• Class I device: non-invasive

• Human factors• Biocompatibility

• Ease of use for clinician

• Patient’s Comfort

• Ethical issues• Broad range of normal subjects

• Reduce cost

Future• Streamline design

• Minimize cost• Lower weight of apparatus

• Testing on reconstruction patients• Evaluate effectiveness of technique

• Possible combination device based on KT1000 or Rolimeter

• To measure both translational and rotational

stability in the same device

Acknowledgements

• Kevin Bell M.S.

• Volker Musahl M.D.

• Ryan Costic M.S.

• Larry Herman

• Department of Bioengineering

• Drs. Hal Wrigley and Linda Baker

IKDC form

• Standard, subjective knee evaluation

• Patient• History of injury

• Symptoms

• Activity Level

• Clinician• Translational stability evaluated using

KT1000 or rolimeter

• Subjective evaluation of rotational stability

Background

• 100,000 ACL reconstructions per year

• 6th most common orthopedic procedure in the US

• Debate over best method• Single vs. double bundle

• Complex anatomy• AM bundle - translation• PL bundle - rotation

Courtesy of Ferguson Lab

Work Breakdown

• Kara Wagner• Background research, contact with

machine shop

• Katie Dillon• Testing

• Stephanie Bechtold• Solidworks prototype

• Everyone• Design History File, SBIR

Milestones

• Initial meeting with mentor – October 2004

• Initial draft of design history file – December 8, 2004

• Initial draft of SBIR – December 17, 2004

• Ordered materials – First week of March 2005

• Completed Solidworks – Third week of March

• Prototype Completed – Last week of March

• Preliminary testing – First week of April