1. Between PT & DT (close to PT)
rEDUCING WEAR DEBRIS AND INCREASING LOWER-LIMB AMPUTEES’ COMFORT BY OPTIMIZING PROSTHETIC SOCKET DESIGN USING LOCAL CONTACT PRESSURE RELIEF AND IMPLEMENTING APPROPRIATE HOLES
Gabi Nehme, Youssif Khalife, Antoun Chagoury Saeed Ghalambor
Department of Mechanical Engineering Department of Mechanical Engineering,
University of Balamand University of Texas at Arlington
El-koura, Lebanon Arlington, Texas
Table 1: ANOVA Analysis where stresses and displacement are analyzed statistically to check the significantly
affected areas to be relieved
Looking at the results, it is noticeable that as the speed increases the inertia force increase from N
at T=1.95 sec to 156 N at T=0.9 sec Unlike the Inertia force the lateral force which decrease with increasing the speed from 633N at T=1.95 sec and N at 0.9 sec. Concluding that at high speeds the lateral force is reduced, and at low speeds the lateral force is increased causing more pain at the FH, therefore magnifying the importance of implementing a hole at the FH. The Fibular Head (FH) hole on different sockets types were calculated using FEM and DOE data then optimized. Selected plots and Figures show the diameter of the holes that should be implemented to enhance patient comfort and flexibilities.
Introduction
A well-fitting socket is a necessity for trans-tibial amputees (TTA). Finite element model (FEM) and design of experiment (DOE) are necessary to implement relief areas and holes in socket structure to help ensure better pressure distributions and successful rehabilitation. Reducing thickness at designated areas and hole implementation between the Patellar Tendon (PT) and Distal Tibia (DT) would decrease stiffness and increase prosthesis range of motion where flexibility is needed without compromising mechanical efficiency and the structural integrity of the socket. The analysis takes into consideration the lateral force when using FEM model.
The lateral force L is calculated according to the figure:
Where:
“W” is the weight (N)
“a” is the distance between the center of gravity and the mid-knee (m),
“I” is the horizontal inertia force (N),
“c” is the distance between the center of gravity and the mid-knee (m),
“b” is the length of the residual limb (m).
Considering that the body is in motion and therefore the center
of gravity is in motion with each step. The horizontal forces are
only taken into account, and noting the center of gravity varying
with a sin ωt function with each step.
The two points vary with each location of the foot.
Another assumption lies within the line of action where the
CG moves is located 4 cm below the belly button. The location of Figure1: FBD
the center of gravity (CG) is assumed to be moving according to this equation: diagram for TTA
amputees
X = A sin t Eq 1
Results
Four sockets were modeled after real TTA patients of the same age group (56-60 year), were amputated due to diabetes, retired and have very similar lifestyle: two 3 mm overall thickness sockets with Duraform and UHMWPE materials without any relief, two 3 mm overall thickness sockets with Duraform and UHMWPE materials with relief using the value calculated by the DOE analysis. During 24 months period, patients were asked consistently about the level of comfort and flexibility during movement. During the early days, results of these trials indicated no major difference in patients feeling. After six months several complaints were reported from patients using unrelieved sockets. Toward the end of the period, it was clear that patients with relieved sockets with holes felt more comfortable than their colleagues.
Even though the sample is small; SEM images of six sockets and optical images of patient skins ( Figures 1-4) show great differences between relieved and unrelieved ones especially when holes are inserted at the specific locations calculated by FEM and DOE analysis. Wear debris and redness spots on patients skins can be easily noticed. This might be due to higher stress concentrations and higher pressure on the residual limb and hence increases irritation of the skins
Looking deeper into the PT and FH of the ANOVA tables; we observe that the P-Values at the max stress and max displacement in both the model and the residual limb closely match the required values of significance.
In order to verify the results concluded above, some live data were collected from patients in “Singer Center for Prosthesis” in
Besides the difficulty of eliminating the long Lasting pain of the socket, patients from different groups noted that they are suffering from pain on the FH side and this was shown in Table 2
Table 2: Real Patients data experiencing pain when wearing sockets
Objectives
Calculate the period of oscillation between old and young amputees
CANFIT digital representation and FEM-DOE analysis
Determine points of high stresses on amputees using different socket types and materials
Comparison between theory and actual amputees wearing sockets
Wear data using Scanning Electron Microscopy (SEM) for relieved and unrelieved sockets for similar age groups and different medically graded materials (Ultra High Molecular Weight Polyethylene: UHMWPE , and Polyamide 6: DURAFORM)
Patient Name
Cause of Amputation
Duration of Amputation
1st Point of Pain
2nd point of Pain
Patient “A”
Assassination Trail (Bomb)
11 Years FH DT
Patient “B”
Landmine
33 Years
Between PT & DT (close to PT)
Patient “C”
Work
Accident
Few months PT
Patient “D”
Biological
Illness
3 Years
Patient “E”
War
4 Years
Patient “F”
Patient “G”
2 Years PD
Patient “H”
Diabetes
Methods, Data and Analysis
Socket Enhancement using CANFIT /CAD Software FEM mesh
Figure 1
SEM images of 3 mm overall thickness Duraform (Polyamide 6) sockets worn for 24 month with sleeves of the same brand by BK amputees of approximately the same age groups and the same weight of approximately 80 Kg (movement is almost the same since they are retired and they do not do much physical work): a) is a relived socket without holes, b) is a relieved socket with holes at the PT and FH, c) is an unrelieved socket.
Figure 2
SEM images of 3 mm overall thickness UHMWPE sockets worn for 24 month with sleeves of the same brand by BK amputees of approximately the same age groups and the same weight of approximately 80 Kg (movement is almost the same since they are retired and they do not do much physical work): a) is a relived socket without holes, b) is a relieved socket with holes at the PT and FH, c) is an unrelieved socket
Summary of socket types used in DOE analysis Summary of FEM data
By manipulating the lateral force equation and Eq 1 for displacement we can find the acceleration of patient movement as follow:
The double derivative of the displacement in Eq 1 yields the acceleration
 α=-ω2X
 And the inertia of the horizontal force is approximately:
I=mα=-ω2Xm
ω=2 π f; f = 1/T; where T is the period of oscillation and I represent the inertia
The horizontal force that acts on the center of gravity as it varies by location, and due to the stresses lying at the extremities, the calculations are focused to attain the maximum force, by assigning X by its maximum value. Therefore we assume the center is gravity is moving between -40 cm < X < +40 cm.
X is assumed to be at maximum position: X=A=40cm; where it is the max position of the center of gravity before the leg move.
After all these calculations, it is concluded that the period to move increases as you grow old and therefore their level of pain increases.
Figure 3
below knee amputee patients’ frontal skin images of the same spots, socket is Duraform (Polyamide 6) that was worn for 24 months. 3 mm overall thickness. a) is a relived socket without holes, b) is a relieved socket with holes at the PT and FH, and c) is an unrelieved socket.
Figure 4
below knee amputee patients’ frontal skin images of the same spots, socket is UHMWPE that was worn for 24 months. 3 mm overall thickness. a) is a relived socket without holes, b) is a relieved socket with holes at the PT and FH, and c) is an unrelieved socket
Conclusions
Results, validation through trials and optimization revealed that holes with relief areas are appropriates for lower extremities patients.
SEM images show direct relations between relieved sockets with holes at FH and patient lifestyle and activity.
A patient that moves rather slowly, as a result of old age or sedentary level of activity
would greatly benefit from the FH socket hole implimentation.