Ppod 1: biomechanics

Question 0

Ideal socket shape (5)

Answer 0

• not the same shape as the shape of the stump for which it is made
• its design must:
• carry loading which varies throughout the walking cycle
• give stable support under dynamic loading
• take account of the variation in tissue properties and their respective sensitivity to loading

Question 1

Prosthetic socket biomech (3)

Answer 1

• the forces are transferred from the socket to the skeletal structure through the skin and underlying soft tissues
• socket shape and volume is critically important
• if the socket is uncomfortable, the patient may not wear the device or may suffer local tissue damage

Question 2

Total contact socket advantages (3)

Answer 2

• P=F/A
• proprioception
• aids venous return

Question 3

Total contact socket P=F/A (2)

Answer 3

• provides maximum area over which the stump socket interface forces may be distributed
• even though the loads supported by some areas of the stump may be small, they do decrease to some extent the load which must be borne by the other areas of the stump

Question 4

Total contact socket: proprioception (1)

Answer 4

-provides good sensory feedback due to the overall close contact with the skin

Question 5

Total contact socket: aids venous return (1)

Answer 5

-helps to reduce oedema and aid venous return due to the pumping action of the muscles within the socket during use

Question 6

Socket comfort (2)

Answer 6

• comfort is relate to the pressure at the tissue/device interface
• localised high pressure can cause severe discomfort such as that resulting from a stone inside a shoe

Question 7

Comfort equation (3)

Answer 7

-pressure=force/area (same as definition of stress)
so to increase comfort we must:
-increase area over which forces are applied and/or
-reduce forces

Question 8

Maximise area of socket (4)

Answer 8

• total contact socket makes use of as much tissue as possible
• but the stump is not comprised of uniform tissues
• there are areas of relative firmness or softness and
• areas which are relatively tolerant of pressure and others sensitive to pressure

Question 9

Objectives of socket design (3)

Answer 9

• to distribute pressure over those areas which are pressure tolerant
• to relieve those areas which are pressure sensitive
• take account in the variation in firmness of the tissues

Question 10

For good socket pressure distribution (3)

Answer 10

Modifications of the socket shape so that it differs from that of the stump is necessary through

• reliefs in the socket over the firm or pressure sensitive areas
• inward bulging the socket wall over soft or pressure tolerant areas

Question 11

Socket pressure distribution may be improved by (4)

Answer 11

• cast modification
• casting in such a way that modification is not necessary (or reduced)
• using a lining material within the socket
• manufacturing the socket using flexible materials

Question 12

Hydro casting (3)

Answer 12

• casting techniques have been developed which try to ensure a uniform pressure distribution at the time of taking the cast
• pressure casting considerably reduces the amount of modification required. Murdock (1968)
• literature indicates that hand casting is less repeatable than pressure casting (Buis et al 2003) and modifications are subject to discrepancies between prosthetists. Convert et al. 2003

Question 13

Literature suggests that hydrostatic sockets… (3)

Answer 13

• produce considerably fewer pressure peaks than ptb during the gait cycle (hachisuka et al 1998)
• promote elongation of the tissue, promoting stiffness of the residuum, whilst protecting the distal tibia (convery et buis 1999-case study)
• improves weight acceptance on the amputated side to a more normal level (Yigiter et al. 2000)

Question 14

Flexible socket materials (1)

Answer 14

-the socket wall can deflect in areas of high pressure and allow the load to be spread over a larger area and thereby reduce the pressure

Question 15

Using a lining material for a socket (3)

Answer 15

• the soft lining material will distribute any localised spots of high force and reduce the pressure
• best used in conjunction with good rectification
• the orthogel liner is made of highly elastic polyurethane

Question 16

Cast modification (3)

Answer 16

• selective removal of material from the positive model in areas tolerant to pressure
• addition of material to areas less tolerant or sensitive to pressure
• the shape of the final socket reflects these changes

Question 17

Rotational stability in a socket (2)

Answer 17

• rectification can help increase the resistance of the socket to rotate during use
• the socket can key into anatomical sites such as the tibial flares or the femoral condyles

Question 18

Ptb cast modification (7)

Answer 18

• patella tendon
• distal end of tibia/fibula
• medial and lateral tibial flares
• fibular head
• popliteal area
• creation of posterior shelf
• supra condylar suspension

Question 19

Difficulties with modification (3)

Answer 19

• casting and modification procedures have been described as subjective, inconsistent, and performed on the basis of skill and judgement of the prosthetist (kristinsson 1983)
• plaster of paris modifications present inherent difficulties in the quantification and recording of results used to produce comfortable sockets (Saunders et al 1989)
• errors association with modification are not well understood or quantified (convery et al 2003)

Question 20

Repeatability of modification (3)

Answer 20

• in modified areas:
• the mean difference between prosthetists was quantifies as 2mm and the standard deviation about the mean was +/- 1mm for each prosthetist
• the maximum variation by one prosthetist was recorded over the fibular head where a sd of 4.3mm was recorded (convery et al 2003)

Question 21

Socket comfort (4)

Answer 21

• pressure=force/area (same as definition of stress)
• so to increase comfort we must:
• increase area over which forces are applied and/or
• reduce forces to a minimum

Question 22

Socket forces (3)

Answer 22

• socket forces are associated with
• donning the prosthesis/orthosis
• using the prosthesis/orthosis

Question 23

Socket Forces for donning (2)

Answer 23

• during rectification we can ensure that certain areas are loaded
• this gives deformation of tissue as the device is donned

Question 24

Reducing loading during use of socket to a minimum (4)

Answer 24

• we must consider:
• the effect of the inclination of the supporting surface
• alignment of the prosthesis/orthosis
• length of the limb

Question 25

Alignment (4)

Answer 25

• spatial relationship between prosthetic socket knee and foot
• the purpose of alignment:
• position the prosthetic socket with respect to the knee and foot so that undesirable patterns of force applied to the residual limb are avoided
• to produce normal pattern of gait

Question 26

Effect of alignment (2)

Answer 26

• the couple generated by the socket weight bearing force and the grf Rb
• must be balanced by the couple which is generated within the socket and acting in the opposite direction, Fa

Question 27

Effect of pros socket alignment (2)

Answer 27

• when alignment is optimised, there will normally be some stabilising forces throughout the gait cycle
• poor alignment is likely to result in larger stabilising forces as well as a less normal and more tiring gait pattern

Question 28

Effect of stump length (5)

Answer 28

• for stability Fa=Rb, if a increases F decreases
• as the length of stump increases the stump/socket forces reduce
• also there is more area over which to distribute forces
• these effects both lead to a reduction in pressure
• thus a short stump is more likely to experience pressure problems (all other things being equal)