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

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 • Medial Tibia • Lateral Tibia • Popliteal Depression • Anterodistal Area

Data Acquision Circuit design: current to voltage converter

Circuit Design

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

References