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Adaptable Retractor for Total Hip Replacement Surgery

• Total hip replacement surgery (THR) surgery involves removing a diseased hip joint and replacing it with an artificial joint [1]

• More than 193,000 total THRs are performed each year in the United States [2]

• 34% of the U.S. population is obese [3]• Multiple retractors (3-5) are required in obese

patients, thus decreasing the amount of work room

• A retractor is a surgical instrument available in multiple shapes and sizes that functions to:

• Hold back adipose tissue• Increase the incision window

• Surgical Techs needed to hold retractors• Costs $20/hr * 1-2hr/surgery[4,5]

To design a hip retractor that eliminates one retractor during surgery, while increasing the viewing capabilities for the surgeon and that meets the following criteria:• Able and strong enough to retract adipose

tissue• Fit multiple patient sizes and provide a clear

view of the surgical site• Must be cost efficient

• Simple manufacturing• Inexpensive material

• Ready for use in surgery• Easily attached• Easily sterilized

• Computer Modeling Protocol• Use PDE Toolbox in MATLAB to analyze

pressure exerted by tissue • Obtain force load on the retractor from

tissue pressure • Model stress on the retractor using

obtained force loads using Finite Element Method Analysis

• MATLAB Results• 300 Pa of pressure applied to adaptor

piece• 1.5 N of force applied to retractor handle

Finite Element Method Analysis

Mechanics of Device Safety and Industry Standards• FDA Standards:

• Device Classification: Class I• 510(K) – Exempt, GMP – Not exempt

[7]• Safety Considerations

• Material must be biocompatible• No sharp edges or small or loose pieces

• Sterilization • Autoclave at 273°C for two minutes [8] • 17-4 PH retrains strength up to 315.5°C

[6]Cost Benefit Analysis

References[1]Staff, Mayo Clinic. "Hip Replacement - MayoClinic.com." 2011[2]"Total Hip Replacement - Your Orthopaedic Connection - AAOS.” [3] Belluck, Pam.” Obesity Rates Hit Plateau in U.S.” NY Times, 2010[4] NIH, “Hip Replacement,”2010.[5]“Average Wages”. National Bureau of Labor Statistics [6] “Product Data Bulletin: 17-4 PH Stainless Steel.”.2007[7] FDA,”Product Classification,” 2011. [8] WVR Lead Free Autoclave Instruction Sheets

• Previous team designed a single adjustable retractor

• Complex and impractical for manufacturing

• Prototype could not be used in surgery• Mechanism causing problem:

• Physical properties of adipose tissue allow it to wrap around narrow retractors which decreases the range of view of the surgeon

• Consequences of problem:• Poor surgical field of vision increases cost

for additional materials and personnel

Department of Biomedical Engineering, Vanderbilt University

Trey DeLong, Lacey Gorochow, Brian Rappa, Adam Vandergriff, Sandra WadeerAdvisors: Dave Martinez (Zimmer) and Dr. Michael Christie (Southern Joint Replacement Institute)

Problem Statement

Final Design

• Dimensions of Adaptor Piece• Height: 6 cm • Length: 9 cm • Thickness: 0.3 cm • Top curvature: 4.7 cm • Side curvature: 4.2 cm

• Design of Adaptor Piece• Increasing curvature along top to retract

and envelop fat and skin tissue• Dimensions based on incision size of

overweight patient• Curvature based on angle of retraction• Sliding mechanism for simple

attachment• Clip holds it in place

• Material• 17-4 Precipitation Hardening Stainless

Steel• Corrosion resistant, easily sterilized [6]• High strength maintained up to 600°F

[6]

Figure 6. Maximum Von Misses stress is 20.93 MPa.

Figure 7. Maximum displacement is 1.226 mm .

• Both retractor and adaptor assigned to stainless steel material properties

• Assembly constrained at bottom surface of retractor

• Maximum stress of retractor retractor significantly less than ultimate tensile strength of 17-4 PH stainless steel: 1000-1340 MPa [6]

• Displacement not significant in adaptor piece or retractor

Figure 4. Adaptor piece. Top: Fully RoundedBottom: Lofted

Figure 3. Assembly of adaptor piece and retractor

Figure 1. Surgeon’s work room obstructed by crowding surgical technicians holding retractors in place

Figure 2. Incision window of THR. Surgical forceps point to obstructing fat that wraps around retractor. Length of incision:12-16 cm, Width: 7-8 cm, Depth: 4-15 cm

Figure 5. Stress based on the tissue modeling .

Dr. Christie’s Opinion • “Best one yet”• System is flexible enough to be used in different

types of surgeries• Customized retractor set for specific surgeries

will save hospitals money • Instead of having 5,000 instruments on hand,

move towards customizing instruments and have only five or six ready for use

Purpose & Design RequirementsBackground

•Current Surgery Cost • Hip Retractor: $800-$1500 (avg. of $1150)• Surgical Techs: $20/hr * 2 hr surgery time • Three retractors needed in each surgery• Retractors used 50 times a year • Retractor used for 5 years=250

uses/lifetime• Retractor cost per use: $1150/250

uses=$4.60• Cost/surg.:$4.6*3+2 techs*20/hr*2

hrs=$93.80•Cost with Adaptable Retractor

• Hip Retractor: $1400• 2 retractors,1 surg tech needed in each

surgery • Retractor cost per use:$1400/250=$5.60• Cost/surg.: $5.6*2+1 tech*$20/hr*2

hrs=$51.20•Hospital Savings

• $93.80-$51.20=$42.60 savings/surgery• Surgeries with obese patients: 193,000 * 34% =65,620 surgeries/year• Total savings/yr:$42.60*65,620=2.8

mil/year