Lower Limb Prosthetic Introduction

Original Editor - Sarah Evans as part of the WCPT Network for Amputee Rehabilitation Project

Top Contributors - Admin, Tarina van der Stockt, Tony Lowe, Naomi O'Reilly, Kim Jackson, Amanda Ager, Rachael Lowe, Simisola Ajeyalemi and Amrita Patro  

Sockets - Transtibial

The prosthetic socket is the primary interface between the amputee’s residual limb and the ground and therefore a good, comfortable fit is required to ensure a positive outcome is reached in an amputee’s rehabilitation.

Patella Tendon Bearing Sockets (PTB)

• Total contact design
• Created in 1959 by Radcliffe and Foort, the socket design identified pressure sensitive and pressure tolerant areas of the residual limb.
• Pressure tolerant areas of the residuum are the patella tendon, popliteal fossa, medial tibial flare, paratibial musculature, the gastrocnemius muscle belly and the fibula shaft.
• Pressure sensitive areas of the residuum are the tibial crest, tibial tubercle, lateral proximal tibia, distal fibula, fibula head, patella and the hamstring tendons
  • These areas are pressure sensitive as they are bony prominences, areas of poor blood supply or areas which are near to prominent nerves, such as the fibula head and the common peroneal nerve.

The above picture shows the pressure sensitive areas (red) and the pressure tolerant areas (green)

The above diagram shows the triangular profile of a PTB socket which aids in weightbearing and preventing rotation of the socket on the limb. Area 1 is the anterior muscle compartment, Area 2 is the medial tibial flare and Area 3 is the popliteal region

Indications

Primary Amputees

• The PTB socket is good for primary amputees as the socket can be modified to accommodate the changes in the residual limb that occur for 12-18 months after the amputation.

Sensitive Residual Limbs

• If the amputee has a particular area of sensitivity on their residuum it is possible in a PTB socket to relieve these areas more easily than in a total surface bearing style socket.

Bulbous residual limbs

• The construction of a PTB socket, an inner liner and outer hard socket, allows for build-ups to be applied to the inner liner allowing easier donning and doffing for an amputee with a bulbous limb

Poor Hand Dexterity/ Poor Eyesight/ Hemiparesis

• PTB sockets are much easier to don/doff than total surface bearing sockets

Contraindications

• Active amputees can find the PTB trimlines and suspension methods too restrictive, especially with regards to knee flexion.
• Some amputees can find the PTB prosthesis pistons
• Some amputees cannot tolerate the pressure on their patella tendon required for a PTB prosthesis to work effectively.
  

Total Surface Bearing Sockets

• TSB sockets “uniformly distribute weight over the entire residual limb” which therefore “delivers a minimal skin pressure”.
• TSB sockets are volume matched to the residual limb with 100% surface contact during the gait cycle.
• Successful fitting of a TSB socket requires good control of the soft tissues, minimised pressure peaks and distribution of load over the maximum surface area available.

Advantages and Indications

• Active amputees benefit from the lower trimlines possible with the TSB style design .
• TSB sockets reduce pistoning of the socket on the residual limb by providing total contact throughout the gait cycle
• Proprioception is increased due to weightbearing over the entire residual limb and good pressure distribution by the socket walls, which in turn enables the amputee to have better balance with eyes open or closed
• Suspension of the TSB socket is also noted to be better than the PTB design as it is integrated in the socket using locking pins or suction.
• Due to the entire surface of the residual limb accepting weight in the TSB socket it is believed that these sockets are more comfortable because overall socket pressure is reduced.

Disadvantages and Contraindications

• TSB sockets are not suitable for primary amputees due to volume changes in the first 12-18 months post-amputation
  • For the same reason TSB sockets are also not suitable for amputees undergoing treatments such as dialysis due to volume fluctuation
• Unsuitable for patients with short residual limbs, less then 10cm long, which require higher trimlines for stability around the knee.
• Some amputees may experience pain at the distal end of their residual limb due to the way a TSB/HST socket weight bears over the entire limb
  • Also patients with excessive soft tissue may drop down into a TSB/HST socket too much which will cause pain at the distal end
  • Amputees with bony spurs are also not suitable for TSB/HST sockets.
• Discomfort during knee flexion may result due to the silicon liner bunching up in the popliteal region
• Increased perspiration may result due to the silicon liner, which can lead to irritation of the residual limb
• TSB/HST sockets are not indicated for amputees with visual/sensory disturbances or Hemiparesis as they are more difficult to don/doff than a PTB design.
• Amputees with excessive soft tissue may find they get discomfort upon knee flexion due to creasing of the silicon liner
  

Prosthetic Knees

Single Axis Knees

• The single axis knee works like a door hinge
  • The distal part of the prosthesis rotates about a single centre of rotation at the knee.
• Basic single axis knees without additional stance or swing controls are rarely prescribed .

Stance Control Knees

• Stance control knees are commonly prescribed to amputees
• Typically these types of knee have a weight activated friction brake which locks the knee as the amputee applies weight to the prosthesis in early stance.
  • Some of these types of knee will lock the knee in a small amount of flexion which can help the amputee control the prosthesis should they stumble.
• In order to flex the knee the amputee must fully lift weight off the prosthesis to release the brake
• This type of knee is most appropriate for limited ambulators who only walk at slower speeds.
• The stance control knee should not be used bilaterally as the amputee would not be able to control a fall if they should lose balance
  • If using this type of knee for a bilateral amputee, it should only be used on one side.

Polycentric Knees

• Polycentric knee have multiple centres of rotation
  • This is due to the geometry of the linkage bars of the knee
• As a polycentric knee is flexed the instantaneous centre of rotation moves more distally and anteriorly reflecting the action of an anatomic knee significantly more than single axis knees
• The movement of the ICOR results in a knee joint that is very stable in early stance but much more flexible in late stance, even under partial weight bearing
• Due to the function of the links of the polycentric knee there is increased toe clearance in swing phase due to effective shortening of the prosthesis
  • This is beneficial in long trans-femoral amputations and knee disarticulations in sitting
• Polycentric knees are very useful to bilateral amputees as they offer increased stability without preventing knee flexion in partial weight bearing

Manual Knee Lock Knees (Hand Operated Knee Lock and Semi-Automatic Knee Lock)

• Manual knee lock knees offer maximum stability in stance as the knee is locked in full extension.
• This type of prosthetic knee can be beneficial to elderly amputees who benefit from the stability.
• A manual knee lock knee, specifically the semi-automatic locked knee, should not be used for bilateral amputees as they would be unable to flex the knees if they were to fall.
• The locked knee is frequently used for children to enable them to develop better balance before moving onto a free knee.

Fluid-Controlled Knees

• Fluid controlled knees contain a pneumatic or hydraulic unit which assist in enabling the amputee to achieve differing cadences.
• The unit can be pneumatic or hydraulic
• Pneumatic tends to be used for swing phase control
  • These units are relatively large and heavy
• Hydraulic tends to control stance phase
  • This is important as liquids are not compressible after a point and therefore are more secure for stance phase than pneumatic units
  • These units are smaller and lighter but more expensive and require more maintenance.

Microprocessor Units

• These types of knees use a microprocessor to control the fluid-controlled knee units more effectively to producing a more energy efficient and normal gait.
• These knees are however much more expensive and therefore not widely available.

References

• Michael J.W (2004) Prosthetic Suspensions and Components Chapter 33 (pages 420-423)

[Cited In: Smith D.G, Michael J.W and Bowker J.H (2004) Atlas of Amputations and Limb Deficiencies – Surgical, Prosthetic and Rehabilitation Principles – Third Edition American Academy of Orthopaedic Surgeons USA]

Prosthetic Feet

• Prosthetic feet can be made from wood, rubber, urethane, titanium, graphite and carbon fibre.
• Prosthetic feet can be lightweight, energy-storing or dynamic and some can allow adjustability of heel height.
• All prosthetic feet should provide passive plantarflexion in early stance, neutral position in mid stance and toe hyperextension in late stance

SACH feet

• Low mobility users
• Good shock absorption at heel strike
• Durable
• Inexpensive
• Requires minimal maintenance.

Multi-axial feet

• Increase stability and comfort on uneven surfaces
• Provide good shock absorption
• Allow transverse rotation, inversion and eversion and dorsiflexion and plantarflexion
• Less stable on level surfaces than SACH feet
• Increased maintenance

Adjustable Heel Height feet

• Allow adjustment of the angle of the shank of the prosthesis in relation to the foot to enable use of differing heel heights

Dynamic response feet

• Enable the user to be more active for longer periods of time
• Usually composed of carbon fibre

Sports feet

• Non-articulating foot and ankle design made of carbon fibre
• Used for high impact sports
• Keel prevents inversion and eversion providing a foot which is stable medial-laterally and resists torsion
• The keel of the flex foot extends from the MTP joint of the foot to the bottom of the socket/knee unit
  • The keel unit is therefore the shank of the prosthesis
  • This creates a long leaf spring which bends when loaded and straightens forcibly when the load is reduced

References

• Berger N and Edelstein J.E (1990) Lower Limb Prosthetics - Chapter Six Prosthetic Orthotic Publications New York (Pages 76-81)
• Carroll K and Edelstein J.E (2006) Prosthetics and Patient Management – A Comprehensive Clinical Approach Slack Incorporated USA (Pages 90-91)
• Kapp S.L and Fergason J.R (2004) Transtibial Amputation: Prosthetic Management Chapter 39 (pages 509-510)

[Cited In: Smith D.G, Michael J.W and Bowker J.H (2004) Atlas of Amputations and Limb Deficiencies – Surgical, Prosthetic and Rehabilitation Principles – Third Edition American Academy of Orthopaedic Surgeons USA]