2. Biomechanics of Transtibial Prosthesis
The comfort, stability and function of a prosthesis are achieved
primarily by the application of certain biomechanical principles.
3. A. Pressure as a Determinant of Comfort
The magnitude of pressure between the stump and socket is one
of the major determinants of comfort in a prosthesis.
P = F/A
P =average pressure, and F and A represent, respectively, the
applied force and the area over which the force is applied.
To minimize discomfort, it is important to avoid excessive
pressure on the stump. One way to reduce pressure is to increase
the area over which force is applied.
4. In principle, it would seem to be a simple matter to reduce pressure
on the stump by increasing the contact areas between stump and
socket.
In practice it is not quite so simple because the relative firmness or
softness of the tissues of the stump are not uniform and some areas of
the stump tolerate pressure quite well, while other areas are relatively
sensitive to pressure.
5. B. Socket Contours and Shape Related to
Pressure Distribution
To start with a general concept, consider a heavy bar resting on a
cube. The pressure on top of the cube is equal to the weight of the
bar divided by the area of the cube's upper surface.
In another figure, the same bar being supported, not only on the
original cube, but by two additional cubes identical to the original
and at equal distances from it. In this case the pressure on the
middle cube will be only one-third.
These two examples illustrate the application of a basic principle
in prosthetic fitting, that is, to utilize as much area of the stump as
possible to distribute the force applied by the socket by the stump.
6. Both the figures show top views of contours of the socket
walls of two different types of below-knee prostheses.
Figure 1 is derived from a PTB socket while Figure 2 is
derived from a type of socket that was fitted before the PTB
socket came into general use.
Although the socket periphery of Figure 2 conforms more
closely to that of the below-knee stump than is the case with
the PTB socket, the PTB socket is generally the more
effective of the two from the standpoint of comfort and
function.
7. C. Accommodating for Differences in Relative
Firmness
In most cases, various areas of the stump will differ in relative
firmness. If an objective in fitting is to distribute pressure evenly
over these areas in spite of different degrees of firmness of the
tissues, then modifications of socket contour with respect to the
stump become necessary.
If the cubes in figure had the same dimensions but differed in
their relative firmness, the pressure would not be distributed
evenly. For example, if the middle cube were made of steel and
the other two were made of soft rubber, the steel cube would
support most of the load.
8. By cutting a slot, or relief, in the steel bar, as shown
in the figure , a more even distribution of pressure
would be obtained, in spite of the differences in
relative firmness of the steel and rubber cubes.
However, if the rubber cubes were very soft, the relief
alone might not be enough to produce the desired
distribution.
9. Suppose the steel bar had a mass of 200 kg and the rubber cubes
were so soft that a mass of 15 kg compressed them so that the
steel bar came to rest firmly on a steel cube in the middle. Then
the steel cube would have to support a mass of 170 kg as opposed
to the 15 kg supported by each of the rubber cubes.
The load distribution could be improved by cutting a deeper relief
in the steel bar. Since this might weaken the bar, an alternate
solution might be used.
10. Contd.
Instead of making the relief deeper, two "build-
ups" could be added to the bar. If the relief were
cut to the proper depth and "build-ups" of the
proper thickness were applied, an even
distribution of pressure would be obtained.
11. The diagram is a schematic representation
of a stump that is essentially circular in
cross-section, encased in a socket that
accurately matches the periphery of the
stump.
If the stump were of uniform firmness, the
stump-socket pressure would also be
uniform.
12. The diagram is similar to previous
figure , except that the stump is not of
uniform firmness. The areas indicated
by the letter "F" are relatively firm,
while softer areas are indicated by the
letter "S".
13. Contd.
If the socket were shaped to match the stump accurately,
the pressure on the stump would not be evenly
distributed. The firm areas, designated by "F" would
take relatively more of the load, while the soft areas,
designated by "S", would take relatively less.
A more even distribution of pressure could be obtained
by purposely modifying the socket by making reliefs in
the socket over the firm areas and bulging the socket
inward over the soft areas.
14. D. Accommodating for Different Tolerance to
Pressure
In this case the purpose is different. Instead of producing an even
distribution of pressure, the objective is to produce a selective
loading of the tissues so that more of the weight will be supported
by the pressure-tolerant areas and less weight will remain on the
pressure-sensitive areas.
15. It is a schematic diagram of a theoretical stump of uniform
firmness, encased in a socket. The socket has reliefs over the
pressure-sensitive areas, which are designated by "S", and
inward contours over the pressure-tolerant areas, which are
designated by “T".
The inward bulges over the pressure tolerant areas will cause
the tissues in these locations to sustain a relatively large
portion of the load, leaving a smaller part of the load to be
borne by the sensitive areas. The reliefs in the socket will
assist further in reducing the pressure on the sensitive areas.
16. By incorporating appropriate reliefs and contours, sockets can be
shaped to accommodate differing degrees of firmness of the stump and
different tolerances to pressure.
The typical patellar-tendon-bearing prosthesis for the below-knee
amputee has an inward protuberance that fits under the patellar tendon
and an inward contour that fits under the medial flare of the tibia.
The posterior wall bulges inward slightly, and the lateral wall is
shaped to provide firm pressure on the lateral distal half of the stump.
Reliefs in the socket are usually provided for pressure-sensitive areas
at the anterior distal aspect of the stump, over the crest of the tibia and
over the head of the fibula.
17. E. Effect of Relative inclination of Supporting Surface
Pressures on the stump are greatly influenced by the
relative movement of the supporting surfaces that
are in contact with the stump.
In this figure, it shows two men supporting a box
weighing 200 lbs. If we assume that the weight of
the box is evenly distributed, each man will have to
exert an upward force of 100 lbs., so that the
combined upward thrust equal the weight of the
box.
18. In this figure, the weight of the box is the
same Since the men are not thrusting
directly upward, only the vertical
components of the forces they exert will
be effective in supporting the box.
Assume that the supporting surface is
inclined 30 degree.
19. Contd.
Since the value of the vertical components of VB and V'B' must each
be equal to 100 lbs., and since angles BAV and B'A'V' are equal to 60
deg., it is possible to calculate the magnitude of the forces that must
be exerted, as follows:
VB/AB= Sin 60°
AB= VB/Sin 60°
AB= 100/0.866
AB= 115.5 = A′B′
From the preceding calculation, it can be seen that the two men
together must exert a total of over 230 lbs. to support the 200-lb. box.
20. Now consider the supporting surface is inclined 70
degree.
The vertical components of the forces exerted by
the men must be equal to the weight of the 200-lb.
box. As before, each man must exert a force great
enough that the vertical components, represented
by VB and V'B', will be equal to 100 lbs. each.
Following the same procedure used in the
preceding section, it is possible to calculate that
the total force the two' men must exert to support
the box is over 584 lbs.
21. Contd.
VB/AB = Sin 20°
AB = 100/Sin 20° = 100/0.3420
AB = 292.4 = A′B′
To summarize, with a supporting surface that is inclined 30 deg.,
the total force applied is 15 percent greater than the weight that is
supported. With the supporting surface inclined to 70 deg., the
force applied will be almost three times as great as the weight that
is being supported.
In conclusion, the more closely the supporting surface approaches
the vertical, the greater will be the counterforces that must be
applied to support a given weight.
22. In the case of a socket for a below-knee prosthesis, except for the
Syme’s amputation, a horizontal surface on which a major part of the
load can be supported is not readily available.
While the bottom of the socket for a midleg amputation may be
approximately horizontal, the end of the stump cannot tolerate heavy
loading.
If the below-knee stump is fitted in a socket in which the steeply
inclined walls provide the major supporting surface, it is usually
necessary to support part of the weight by means of a thigh corset.
23. Contd.
By fitting the stump in slight initial flexion and by purposely
modifying the contour of the socket by making an inward bulge to fit
under the area of the patellar tendon, an effective weight-bearing area
is made available, While this area is not horizontal in the sagittal
plane, it is much less steeply inclined than the other supporting areas
of the socket wall.
24. Although the supporting area of the patellar tendon is less steeply
inclined than the other supporting areas, it still has a downward and
backward slope.
The stump, supported on this area, tends to slide downward and
backward. To prevent this, an anteriorly directed counterforce is
required.
The anteriorly directed counterforce is applied by the posterior wall of
the prosthesis. The posterior wall should be relatively high to provide
as much area as possible over which to distribute the counterforce, but
it must not be so high that it interferes with sitting comfort.
25. The posterior wall of the socket must be flattened, or even have
an inward bulge, so that the tissues with which it is in contact are
under some initial compression to provide a counterforce to arrest
the motion(downward and backward movement of stump).
An inward contour of the socket in the area of the medial flare of
the tibia can provide an effective weight-bearing area.
To maintain the stump on the inclined surface, a counterforce
must be supplied by the lateral wall of the socket to the lateral
aspect of the stump.
The major part of this counterforce will be applied to the distal
half or distal third of the stump.
26. F.Total Contact
From a biomechanical standpoint, the total-contact design is generally
preferable because it offers the following advantages:
1. It helps to prevent edema and aids venous return. This is the most
important advantage. In the intact limb, the pumping action of the
muscles is an important factor in moving venous blood back toward
the heart. In the amputated limb this important pumping action is
reduced. There is a tendency for edema to develop in the dependent
stump unless pressure is applied to the entire stump.
27. 2. The total-contact socket provides greater area over which to
distribute the load. Even though the load supported by the end of
a midleg stump is not great, it does decrease the load that must
be borne by the other areas of the stump to some extent.
3. Because it is in contact with a greater area of the stump, the total
contact socket probably provides better sensory feedback to the
wearer.
28. G. Alignment and Pressure
Distribution
Alignment refers to the relative position of the various parts of
the prosthesis with respect to each other, particularly the socket
and foot in the case of the below-knee prosthesis.
The alignment of a prosthesis influences the magnitude and
distribution of forces applied to the stump by the socket.
By means of alignment, the forces acting on the stump can be
controlled so that relatively higher pressures will be applied to the
areas of the stump where they are best accommodated and most
effective, and relatively lover pressures will be applied to
sensitive areas.
29. The prosthesis is subjected to forces applied by the stump from
above and the counterforces applied by the floor or ground from
below.
At any given instant, if the resultant of the downward forces
applied by the stump to the prosthesis, and the opposing
counterforces applied by the floor were acting along the same
straight line, there would be no tendency for the socket to change
its angular relationship with respect to the stump.
30. If, however, the resultants of the opposing forces are not collinear,
there is a tendency for the socket to change its angular
relationship with respect to the stump.
This tendency is resisted because of the intimate fit of the stump
in the socket.
With an intimate fit the tissues on opposite aspects of the stump
are compressed as the angular change begins to take place.
The counter forces developed by the compression of the tissues
establish equilibrium and arrest the incipient motion.
31. H. Stump Length Related to Pressure on Stump
The length of the stump has a marked bearing on the magnitude of the
pressures that are applied whenever the socket tends to change its angular
relationship with respect to the stump.
This figure delineates the resultant of noncollinear forces applied to the
prosthesis by the stump from above and the floor from below. These opposing
forces constitute force couple that tends to rotate the prosthesis in a counter
clockwise direction, as indicated by the curved arrows.
At a given instant assume that the rotation is taking place around the axis
designated by “O”. The moment of force generated by the forces represented
by AB and CD is indicated by the formula: M1= (AB*d1) + (CD*d2)
M1= moment of force due to AB and CD.
AB= the force applied by the body to the prosthesis from above.
CD= the counterforce applied by the floor to the prosthesis.
d1=the perpendicular distance from “O” to the line of action of force AB.
d2= the perpendicular distance from “O” to the line of action of force CD.
32. This figure represents the counter forces applied to the
proximomedial and distolateral aspects of the socket as the tissues
in the corresponding areas are compressed because of the
incipient motion of the socket. The incipient motion arrested
when the resisting clockwise moment developed by the forces
represented by LL and are equal to the counter clockwise moment
developed by forces AB and CD. The clockwise moment due to
LL and MM is represented by the formula:
M2= (LL*dL)+ (MM.dM)
M2= the resulting clockwise moment
LL - counterforce on the distolateral socket wall that is developed as tissues are compressed
MM= counterforce on the proximomedial socket wall as tissues are compressed.
O= the instantaneous center of rotation of the socket as it tends to change its angular relationship
dL- perpendicular distance from O to the line of action of LL
dM - perpendicular distance from O to the line of action of MM
33. At any given instant, if no change in angular relationship is taking
place between socket and stump, It indicates that the resultants AB
and CD are collinear, or that there is equilibrium between the
moments developed by the forces represented by AB, CD, LL, MM.
in the latter case, the relationship between these forces and moments
is the by the formulas
(AB*d1)+(CD*d2 )-(LL*dL)-(MM*dM)=0
34. I. Mediolateral Alignment
In aligning the prosthesis, the prosthetist may displace the foot
toward the midline, producing a narrow base, or the may displace
the foot laterally, which will result in a relatively wider walking
base.
The term "Insert" is used to refer to relative displacement of the
foot toward the midline; the term "out set" refers to displacement
of the foot away from the midline.
These displacements are relatively small in magnitude, but they
can have a significant effect on the pressure applied to the stump
as well as on the gait pattern.
35. Medial displacement of the foot tends to make the socket change
its relationship to the stump in the direction shown by the curved
arrow, with resulting increased compression of tissues on the
distolateral and proximo medial aspects of the stump.
When a PTB prosthesis is aligned with a relatively narrow
walking base, it is essential that the socket be shaped and fitted
properly so that the greater part of the mediolateral forces will be
distributed over lateral distal half or distal third of the stump, and
on the medial flare of the tibia and the area in contact with the
superomedial aspect of the socket.
36. This figure shows exaggerated lateral displacement of the foot to
illustrate the effect on pressure distribution. With this alignment
the socket tends to rotate so that relatively greater pressure is
applied to the proximo lateral and distomedial aspects of the
stump.
From the two illustrations, it can be seen that as the foot is
displaced medially from an initial lateral position, pressure on the
distolateral and proximomedial aspects of the stump is increased,
with relative decrease of pressure on the proximo lateral and
distomedial aspects of the stump.
This is good procedure to follow, since the proximolateral area is
pressure sensitive (fibula head) while the proximomedial area
(medial flare) is pressure tolerant.
37. In this figure, S indicates the support point on the foot, which is a
theoretical point at which the forces acting on the support area are
assumed to be concentrated. When, during stance phase, the
center of gravity is medial to the support point, gravity will tend
to use the body to fall toward the unsupported side.
Immediately after heel strike, as the toe off the opposite foot
leaves the ground, the tendency of the body to fall to the
unsupported side is opposed by the inertial effect that is generated
by the side-to-side movement of the body in walking.
The center of gravity follows a sinusoidal path is following this
path the center of gravity changes with respect to the line of
progressions. At toe-off the center of gravity is moving toward
the supported side.
38. With a wide walking base, the distance between the weight line (the
vertical projection of the center of gravity) and the support point of
the foot would be greater than normal, and the tendency of the body
to fall to the unsupported side proportionately increased, unless the
amputee adopted compensatory body movements.
For this reason, an individual who was with a base significantly
wider than normal will walk with an increased side-to-side
movement of the body, or he will resort to lateral bending of the
trunk to displace the center of gravity toward the support point, or he
may take quick steps on the sound side.
39. J. Anteroposterior Alignment
When an amputee wearing a PTB prosthesis walks with a pattern
that closely approaches normal gait, his knee flexes shortly after
heel strike. The action of the knee extensors controls the rate and
amount of flexion.
As the prosthetic forefoot approaches the ground, the downward
and forward forces applied by the stump to the prosthesis produce
a tendency for the socket to change its angular relationship with
the stump in the direction indicated by the curved arrow in this
figure.
This, tendency is resisted by the counterforces that are developed
as the socket increases its pressure on the anterodistal and
posteroproximal aspects of the stump as shown by the short
arrows.
40. If the amputee is to control the rate and amount of knee flexion
by action of his knee extensors, the contours of the socket must
accommodate the stump socket pressures that are built up in these
areas.
This indicates the need for a high posterior socket wall that
presses firmly against the tissues on the posterior aspect of the
stump, and for suitable relief as the anterodistal aspect of the
stump to distribute the pressure away from the pressure-sensitive
tip of the femur.
41. At this time the counterforce applied to the prosthetic foot by the floor restrains the
forward inclination of the shank, and the socket tends to change its angular
relationship with respect to the stump, as shown by the curved arrow in this figure.
This relative change in angular relationship tends to increase pressure on the
anterosuperior and posterodistal aspects of the stump at this time. In the PTB
prosthesis a well fitted supracondylar strap helps to reduce pressure on the stump,
since incipient relative motion in the direction indicated by the curved arrow will be
resisted to the counter forces developed as the knee strap compresses the tissues
above the patella.
In aligning the prosthesis the prosthetist must try to achieve optimum
anteroposterior alignment of the socket with respect to the foot. If the socket is
displaced to far forward, the amputee’s stump may be subjected to excessive forces
in the anterodistal and posteroproximal areas following heel strike and during the
end of push-off phase. If the socket is too far back, the knee may be subjected to
forces tending to produce hyperextension just before the heel-off position is reached.
42. K. Suspension Methods and Stump socket
Pressures
Since the major supporting forces in a PTB socket are applied to the
patellar tendon and the area of the medial flare of the tibia, relatively
short stump would not be at a marked disadvantage as far as support
alone is concerned. But the forces applied to the stump are not due to
body weight alone.
Additional forces are applied to the stump by the socket whenever the
prosthesis tends to change its angular relationship with respect to the
stump.
43. Suspension systems are designed to hold the prosthesis on the stump.
In addition, the suspension system can reduce stump-socket pressure.
This is particularly useful in dealing with short stumps.
Two suspension variants of the basic PTB prosthesis introduced
previously are the supracondylar system and the supracondylar/
suprapatellar system. Both of these variants employ high medial and
lateral socket walls that encompass the condyles.
In addition to the high medial and lateral walls, the
supracondylar/suprapatellar system also has a high anterior wall. The
high walls serve not only to hold the prosthesis on, but they also
constitute extended liver arms.
