TF SOCKET BIOMECHANICS

Top view of the above knee socket known as a "plug fit socket".

If the tissues of the stump were equally firm and pressure tolerant, a uniform distribution of stump socket pressures would be desirable.

The amputee's stump is inserted into the socket as a plug into a cylinder.

The section is round it is essentially and has an inner configuration approximating the cylindrical shape of above knee stump.

General consideration

In this presentation, we will study the quadrilateral socket principles.

RELIEF FOR THE GLUTEUS MAXIMUS

ISCHIUM

RELIEF FOR THE ADDUCTOR

TENDON

BULGE FOR THE SCARPA TRIANGLE

RELIEF FOR THE RECTUS

FEMORIS

The quadrilateral socket contours can achieve a better pressure distribution.

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On the picture , the forces applied to the stump by the walls of the socket are represented by F1,F2,F3,F4,F5 and F6.

SOCKET CHARACTERISITICS

The vertical components of the forces applies to the stump by the socket ( F1,F2,F3.F4,F5.F6,F6)

must be equal to:

the body weight W supported by the prosthesis

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But only the vertical components A1,A2,A3,A4,A5,A5,A6 of these forces

will be effective in opposing the downward force W exerted by the stump

The sum of F1 + F2 + F3 + F4 + F5 + F6 is equal and opposite to the weight W.

Comments:

A knee disarticulation and some supra condylar amputations, can tolerate high weight bearing forces on the distal end of the stump.

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F1,F2,F3,F4,F5,F6

The forces F1 + F2 + F3 + F4 + F5 + F6 are applied by an essentially horizontal surface.

The ischial seat and gluteus shelf are horizontal surfaces : they are supporting the major portion of the body weight

CHARACTERISTICS OF THE WALLS OF A QUADRILATERAL SOCKET

A) POSTERIOR WALL

The posterior wall can act:

• to stabilize the trunk "1" ( action of the hip extensors).

• to control the knee flexion "2"

• to stabilize the prosthetic knee "3".

Comments:

as the major part of the weight can be borne by the posterior brim, the support which must be supplied by the remaining walls of the socket is correspondingly reduced.

Anterior wall height : 5 to 6 cm higher than the posterior wall.

B) ANTERIOR WALL

The anterior wall of the socket be relatively high in order to maintain the ischial tuberosity in its proper place on the ischial seat.

The anterior wall also provides more area over which to distribute the forces applied to the stump.

Comments :

-The height of the anterior wall should not restricted the hip flexion ( 90 °).

- the sitting position has to be comfortable

The anterior wall is applying a counter force to resist to the anterior pelvis rotation and sliding forward, as the weight line is anterior to the ischial seat) .

Bulge for Scarpa triangle

Relief for Rectus Femoris tendonRelief for Adductor

Longus tendon

The inward bulge in the area of Scarpa's triangle helps to provide a more effective stabilizing counterforce

Comments:

Because of the relative softness of the tissues of the Sarpa's triangle area, the inward bulge provides for the initial compression of the tissues.

C) LATERAL WALL

Normal gait

During the swing phase, the pelvis tends to drop slightly on the unsupported side. This tendency is opposed primarily by the action of the gluteus medius.

This can be explained as follows:

A counter moment of force M2 which resists to M1

W = patient weight

Dw = distance between the weight line and the centre of the hip joint

One moment M1 tends to make the pelvis drop to the unsupported side.

To keep the pelvis stabilized during the gait: M1 = M2

M1 = W x Dw

Fg= the force developed by the gluteus MediusDf= distance between the hip joint to the line of action of Fg.

M2 = Fg x Df

For an amputee:

The magnitude of the counter forces necessary to stabilize the pelvis is indicated by the formula:

W x Dw = F1 x Df

S

W : Patient Body weight

W

Dw: perpendicular distance from the support point " S" ( ischial tuberosity) to the line of action of the weight.

Dw

Df: perpendicular distance from the support point to the line of action of F1.

Df

F1: counter force applied by the wall.

F1

The lateral wall of the sockets plays an important role of stabilizing the pelvis during sound limb swing phase.

The pelvis tends to drop to the unsupported side due to patient weight W causing a corresponding lateral movement of the stump.

The lateral wall is exerting a resisting counter force F1 on the lateral aspect of the stump.

The counterforce is distributed over the area of lateral aspect of the stump "F1".

The femur in a relatively fixed position with respect to the pelvis.acting at a perpendicular distance Df from the support point "S".

The line of action of the weight passes medial "Dw" the support point "S" (ischial tuberosity) which is located on the ischial seat.

The gluteus medius is stabilizing the hip joint and maintaining femur in relative fixed position

Conclusion

- For long stump, stabilizing counter force is smaller than for short stump ( df is longer).

- For short stump. a greater counterforce are distributed over smaller area to stabilize the pelvis.

-To reduce the moment created par patient weight , patient can make a lateral trunk bending, (it will reduce dw).

- When the pelvis tends to drop, tissues of the lateral aspect of the stump are compressed until they generate the necessary counter force.

- To stabilize the hip , the gluteus medius should exert enough force.

- Lateral wall has to be adducted in order to place the Gluteus Medius in a right position to stabilize the pelvis.

- Relief has to be made on the lateral wall at the lateral distal end extremity of femur level to avoid extra pressure on the stump during stance phase.

D) MEDIAL WALL

PROVIDE EVEN PRESSURE ON THE ADDUCTOR MUSCLES

o THE MEDIAL WALL SHOULD:

CONTAIN MEDIAL TISSUES AND PREVENT ANY ADUCTOR ROLL

o MEDIAL WALL IS:

PARALLELE TO THE LIGNE OF PROGRESSION

NOT MAKE ANY PRESSURE INTO THE PERINEUM

HEIGTH IS SLIGHTLY LOWER THAN ISCHIAL SEAT ( IT CAN BE AT THE SAME HEIGHT.