An Opening in the Abdomen, An Opening in the Market: Hernia Tensiometers

GROUP 2: Martha Ingram, Megan Johnston, Chelsea SamsonMENTORS: Dr. Michael Holzman, Dr. William Beck, Dr. Benjamin Poulose

AN OPENING IN THE ABDOMEN,AN OPENING IN THE MARKET:HERNIA TENSIOMETERS

Hernias and their repair

Background

What is a hernia?Protrusion of the abdominal contents through a hole in the musculature Affects 5 million

Americans every year Must surgically realign

the musculature and create a barrier, or circulatory and digestive complications may result

Repairing Hernias

Minimally-invasive laparoscopic surgery

Open hernia surgery performed for hernias greater than 25cm wide

Current solutions… …and complications

Tension from the abdominal muscles and fascia pulls sutures apart

Failure in over 40% of patients, despite “tension-free” repair method

Acceptable recurrence rate is 15%

“Making the Case for Hernia Research”Benjamin Poulose et al., 2011(Vanderbilt Medical Center)

Financial Impact of Decreasing Hernia Recurrence

Cost/Benefit Analysis

“Currently, there is a lack of standardization in…hernia repair procedures, with widespread variation in delivery”

Lack of standardization Increased post-surgical

complications Failures and increased costs

Ventral hernia repairs in 2006: 154,278 inpatient + 193,543 outpatient =

348,000 operations

Inpatient operation = $15,374

Outpatient operation = $3,745

Background Results

Total Annual Expenditures on Ventral Hernia Procedures:

$3.1 billion

40% recurrence 15% recurrence

= $775 million savedThe application of an intra-operative tension-

measuring device could increase understanding of and prevent hernia recurrence, significantly

decreasing costs.

Device requirements

Using Tension Measurements to Determine Recurrence Rates

Data Collection

Record tension measurements (in Newtons) for all hernia repair proceduresUp to one year post-surgery, monitor if sutures failCreate curve that correlates tension values and probability of repair failureDetermine tension at 15% recurrence

Tension (N)

100 %

90 %80 %70 %60 %50 %40 %30 %20 %10 %0 %

Probability of Recurrence

User Response

Close hernia with sutures

Relaxing incisions

Mesh patch (inelastic polypropylene or Gore-Tex)

Tension correlates to ≤15% recurrence probability

Tension correlates to >15% recurrence probability

Evolution of the device

Designing the Tensiometer

Device Functions and Qualities

Measure tension resisting closure at the central suture line, longitudinal to muscle alignment, after: dissection of the injury area separating fascia from muscle

Sterilizable, reusable, strong

Literature Search: Bassini et al., 1988

Disadvantages:

Multiple parts

Invasive

Strain gauge exposed

For use on a limited range of hernia sizes

Hernia edges clamped between

metal platesSerrations hold tissue

in place

Lash strain gauges across

opening10

– 25 c

m

Design 1: Close and Measure

Advantages: Self-contained Less invasive Adapts to hernia sizes

Disadvantages: Not appropriate for small surgical area, thick muscle Would not withstand large muscle forces

FRONT

SIDE

TOP

Static arm

Mobile arm

Digital display

Gear to wind hernia edges

together

Force Sensor

Design 2: Indentation Testing

Advantages: Point measurements Non-invasive

Disadvantages: Complex design Measures transverse, instead of longitudinal, tensions

Tonomoter

“Air puff” glaucoma testing

10-20 mmHg pressure applied

Measure deformation with laser Intraocular pressure can be determined to ±0.5 mmHg Not very accurate; dependent on thickness of cornea and point of application

Design 3: Modified Surgical Clamp

Advantages: Small Tool familiar to user Use on all hernia sizes Sterilizable and strong

Disadvantages: Surgeon may introduce some variability

Angle of pull Clamp location High-frequency

noise

Strain gauges

Fascia Fiber Directions

Transversalis fascia:

Collagen fibers are oriented perpendicular to the muscle fibers Parallel to the direction of the tensiometer pull Fibers elongate due to tensile forces and can rupture

Surgical Clamp

Proof of Concept

Strain gauge in Wheatstone Bridge Instrumentation

Amplifier [Low Pass Filter]Output Voltage

Resistance changes when clamp is pulled

Voltage changes when clamp is pulled

Clamp end

Handle end

Design 4: Surgical Table Arm

Advantages: Does not touch tissue Attaches to any clamp Use on all hernia sizes Use a retractor to create sliding height levels

Disadvantages: Hangs over patient Need to stiffen joints (epoxy)

Thompson Retractor

Surgical tableTurnbuckle

Surgical

clampForce scale

Future Proof of Concept• Testing on porcine model the week of April 4th

• Clamp on one side of hernia; force scale on other• Determine tension values and standard deviations