Analyzing the forces within unilateral transtibial prosthetic sockets

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 Socket 1. Transtibial Patient Evaluation a. Limb measurements b. Skin type and integrity c. Range of motion d. Hand dexterity e. Fine and gross motor skills f. Cognition 2. Gel Liner Interface Material Selection a. Most common: Urethane, thermoplastic elastomer, silicone 3. Fit Gel Liner to Patient

Current Process for Constructing a Transtibial Socket (cont. ) 4. Cast and measure over gel liner 5. Modify negative model a. Computer modeling b. Hand modification 6. Fabricate positive check socket 7. Fit positive check socket – static and dynamic assessments 8. Fit final laminated socket

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 forces 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 (combination of foot, liner, and socket) in which forces are optimally distributed throughout the residual limb-socket interface

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

Circuit Design Circuit design: current to voltage converter

Design/Safety Considerations • Wire thickness ▫ Thin enough to prevent interference with force data ▫ Thick enough to remain durable during movement • FSR-wire connection: 2 types of epoxy used ▫ Needs to withstand dynamic forces within socket ▫ Must be safe to place in contact with skin • Power Supply • ± 12 V limited to 5 V by voltage regulator

Placement of FSRs • Impractical to cover every area of the residual limb with sensors • Used one FSR in each area of clinical interest Pressure Tolerant • (1) Tibialis Anterior • (2) Distal End of Limb • (3) Medial Tibial Flare • (4)Mid Patella Tendon • (5) Gastrocnemius Pressure Sensitive • (6) Fibula Head • (7) Distal Tibia • (8) Tibial Tubercle

Testing Combinations • 3 SocketsLiners • Urethane (Otto Bock) • Silicone (Evolution) • Thermoplastic Elastomer (Ohio Willow Wood) • 3 Feet Each • Flex-Foot & Multi-axial Ankle • SACH • Vertical Shank ESF • Several Conditions • • • Liner Only Standing on Normal Limb Only Standing on Both Limbs Standing on Prosthetic Limb Only Walking 2 mph

Recent Work • Created new Lab. View program to record data • Calibrated each FSR • Created Excel template to convert voltage readings into pressure data • Tested 9 combinations of feet and liners

Lab. View Program

FSR Calibration

Excel Template File

Current Status Analyzing Results With Our Advisor…

Future Work • Analyze results, determine regions containing peak forces • Based on results, test several new sockets with Cody • Make specific regions of each trial socket more flexible or rigid as needed to optimally distribute forces • Design and develop final socket: provide more cushioning in areas of greatest force • Determine success from patient feedback and peak force reduction in critical regions

References