Analyzing the forces within unilateral transtibial prosthetic sockets and design of an improved force minimizing socket

Christine Bronikowski, Amanda Chen, Jared Mulford, Amy Ostrowski

Advisor: Aaron Fitzsimmons, The Surgical Clinic

Problem Statement

•Lack of research in the socket interface between the artificial limb and the residual limb, specifically force profiles▫Majority of research based on models with historically

proven success and qualitative assessments

Current Process for Constructing a Transtibial Socket1. Transtibial Patient Evaluation

a. Limb measurementsb. Skin type and integrityc. Range of motiond. Hand dexteritye. Fine and gross motor skillsf. Cognition

2. Gel Liner Interface Material Selectiona. Most common: Urethane, thermoplastic

elastomer, silicone3. Fit Gel Liner to Patient

Current Process for Constructing a Transtibial Socket (cont.)4. Cast and measure over gel liner5. Modify negative model

a. Computer modelingb. Hand modification

6. Fabricate positive check socket7. Fit positive check socket – static and dynamic

assessments8. Fit final laminated socket

Current Socket Designs

Designed on a case-by-case basis for individual patients

Problems with Current Models▫ Skin abrasion▫ Pain or discomfort▫ Tissue breakdown at the skin surface and

within deep tissues▫ Pressure ulcerations and resultant infections

at the socket interface

Many of these problems arise from stresses at prosthetic interfaces

Project Goals

•Acquire accurate measurements of perpendicular forces acting on the residual limb of transtibial amputee during various movements

•Pinpoint regions with highest forces•Design a socket system in which forces are optimally

distributed throughout the residual limb-socket interface

• Increase overall patient comfort

Forces Acting on the Limb

•Shear– resulting from frictional forces between skin and socket▫Can be

minimized using socket liners

•Perpendicular

Method of Force Analysis• Force Sensing Resistor (FSR) placed between liner and

socket• Very thin– will not cause variation in force determination• Decrease in resistance with increasing force, which leads

to increasing output voltage

Placement of FSRs

•Impractical to cover every area of the residual limb with sensors

•One FSR used in each area of clinical interest (i.e. areas expected to face larger pressures and cause patient discomfort)

• Patellar Tendon • Anterodistal Area

• Medial Tibia

• Lateral Tibia

• Popliteal Depression

Data Acquision

Circuit design: current to voltage converter

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Circuit Design

Peak force expected to be around 4000 g feedback resistor selected to be around 500 Ω to avoid saturation of op-amp

Current Status

•Compact RIO (analog-to-digital converter) connection with computer set up

•FSRs connected to measuring circuit•1/21/2011 – First trial at The Surgical

Clinic with Cody, a transtibial amputee patient▫Test if circuit reaches saturation▫Check sensor sensitivity – changes in

resistance that are too rapid with changes in force undesirable

Design/Safety Considerations

•Wire thickness▫Thin enough to prevent interference with force

data▫Thick enough to remain durable during

movement •FSR-wire connection

▫Cannot break during movement

Future Work

• Successful first trial construct more systems for more patients (~10)▫Rotate FSRs within socket to cover entire

area▫Test multiple surfaces (incline, flat, stairs)

• Analyze results, determine regions containing peak forces

• Use different types of sockets on Cody• Design and develop new socket: provide more

cushioning in areas of greatest force

Determination of Success

•Design is patient-driven•Measure forces before and after fitting of

new socket and compare values

ReferencesEngsberg, J.R., Springer, M.J.N., and J.A. Harder. (1992). Quantifying interface

pressures in below-knee-amputee sockets. J Assoc Child Prosthet Orthot Clin 27(3), 81-88.

Houston, V. L., Mason, C.P., LaBlanc, K.P., Beattie, A.C., Garbarini, M.A., and E.J.

Lorenze. Prelimary results with the DVA-Tekscan BK prosthetics socket: residual limb stress measurement system. In: Proceedings fo the 20th Annual Meeting American Academy of Orthotist and Prosthetist, Nashville TN. P 8-9

Jendrzejczyk, D. J. (1985). Flexible Socket Systems. Clin. Prosthet. Orthot. 9 (4), 27-31. Lee, W.C., and M. Zhang. Using computational simulation to aid in the prediction of

socket fit: a preliminary study. Med Eng Phys. 2007 Oct;29(8):923-9. Polliack, A.A., Sieh, R.C., Craig, D.D., Landsberger, S., Mcneil, D.R., and E. Ayyappa.

Scientific validation of two commercial pressure sensor systems for prosthetic socket fit. Prosthetics and Orthotics International, 2000, 24, 63-73.

Sanders, J.E., Daly, C.H., and E.M. Burgess (1993). Clinical measurement of normal

shear stresses on a transtibial stump: Characteristics of wave-form shapes during walking. Prosthet Orthot Int 17, 38-48.