Prosthetics
Definition
- Prosthesis is a device designed to replace the function and appearance of a body part as much as possible.
- Prosthesis technically embraces all substitutes for bodily defects
Successful prosthetic use depends on patient and prosthesis factors.
Patient Factors
- Pre-morbid activity level
- State of contra-lateral limb
- Degree of trunk control and upper limb strength
- Static and dynamic balance
- Motivation to learn new skills
- Other co-morbidities
Prosthetic Factors
- Ease of use
- Aesthetic appearance
- Durability and reliability
- Comfort
Upper Limb
- Limb salvage is preferable, as a partially sensate limb is more functional than an insensate prosthesis.
Amputations
Pre-op Assessment of Achieving a Successful Wound
Nutrition
- Poor indicators include:
- Albumin < 3.5 g/dL
- Total lymphocyte count < 1500
- Serum transferrin decreased
Vascularity
- ABPI: > 0.45
- Transcutaneous Oxygen tension: > 40 is a positive predictor (< 20 negative)
Techniques
- Myoplastic: Suture of muscles to each other.
- Myodesis:
- Preferred option.
- Suture of muscle to the bone directly (periosteum or drill holes).
- Better fixation and muscle power.
- Prevents contracture formation.
General Technical Points
- Nerves sectioned cleanly under tension to prevent neuroma formation.
- Bone ends beveled and smooth.
- Avoid non-absorbable sutures except for skin.
- Scar away from bony edges and well healed before fitting.
- Stump shape should be conical or cylindrical.
- Avoid excessive soft tissue distal to the bone.
Prosthetic Fitting
- Should be done when wounds have healed, scar is mature, and edema and neuropathy controlled.
- Excessive delay can also affect the outcome.
- Skin desensitization programs are useful before fitting.
- Early fitting (<30 days) has a better outcome in the upper limb but must be balanced against having a stable wound.
Optimal Levels of Common Amputations
| Type | Optimum | Shortest | Longest |
|---|---|---|---|
| Forearm (trans radial) | Junction of proximal 2/3 and distal 1/3 forearm | 3 cm below biceps insertion | 5 cm above wrist to allow space for rotary prosthesis |
| Humerus (trans humeral) | Middle 1/3 arm | 4 cm below axillary fold | 10 cm above olecranon to allow space for elbow prosthesis |
| Trans Femoral | Middle 1/3 thigh | 8 cm below pubic ramus | 15 cm above knee joint to allow space for knee mechanism |
| Trans Tibial | 8 cm per 1 meter of height | 8 cm below knee joint | At any level, a myoplasty can be performed for cosmesis |
Disarticulation
Advantages
- Amputation retains the weight-bearing surface.
- End-bearing stump may be harder wearing.
- Bulbous shape assists suspension of the prosthesis.
Disadvantages
- May compromise the choice of prosthetic available.
- Prosthesis may appear bulky due to the bulbous shape of the stump.
Classification of Prosthesis
| Category | Description |
|---|---|
| Level of amputation | Trans femoral, transradial, etc. |
| Structure | Endo-skeletal (modular) or Exo-skeletal |
| Function | Cosmetic or functional |
Common Elements of Prostheses
| Element | Description |
|---|---|
| Socket/Interface | The connection between the prosthesis and residual limb, manufactured using computers or plaster mould. |
| Suspension mechanism | Attaches prosthesis to the residual limb. Can be a standard suction cup or elastomeric suction for an airtight seal. |
| Struts/Pylons | Restore limb length and connect socket to terminal device. |
| Articulating Joints | If needed, replace joint function. |
| Terminal Device | The most distal part of the prosthesis; either passive (better cosmesis) or active (more function). Active devices can be controlled by cables, struts, etc. or may be myoelectric, relying on action potentials from muscles. |
| Active Devices | Can be voluntary opening (closed at rest) or voluntary closing (open at rest). |
Upper Limb Prostheses
Upper limb prostheses aim to restore the 5 types of grip:
- End Pinch: Thumb against index finger.
- Chuck Pinch (tripod): Thumb against index and middle finger.
- Lateral Pinch: Thumb against lateral aspect of the index finger.
- Hook Power Grip: E.g., carrying a suitcase.
- Spherical Grip: Turning a doorknob or light bulb.
Common Prostheses
Prosthetic Feet
The terminal device for all lower limb prostheses - 5 types:
- Single Axis Foot
- SACH Foot
- Dynamic Response Foot
- Articulated
- Non-articulated
Single Axis Foot (non-energy storing)
- Simple hinge allowing dorsiflexion/plantar flexion of the ankle.
- Poor durability and cosmesis.
SACH Foot (Solid Ankle Cushioned Heel)
- Non-energy conserving.
- Was standard for a long time but now going out of vogue.
- For low-demand patients and causes overloading on the other foot.
Dynamic Response Foot (energy storing)
- Allows more normal activity and tailored to patients’ needs.
- Energy conserving.
- Split into:
- Articulated Dynamic Response Foot: Best for general use; has a keel that deforms with loading and recoils to give a spring-like push-off; allows for inversion and eversion.
- Non-Articulated Dynamic Response Foot: High-demand prosthesis for running, etc.
Trans-Tibial Prostheses
Components: - Socket: Traditionally patellar tendon bearing; now more often total contact or total surface bearing, distributing load more evenly over the residual limb. - Suspension: Different methods including suction, usually from the supracondylar area. - Articulating Joint: Ankle may be articulating or fixed in one position; terminal device as above for feet.
Trans Femoral
Socket: - Shape is the key factor. - Plug Fit: Conventional socket onto end of stump. - Ischial Tuberosity Bearing: Q and H sockets (traditional); failed to prevent abduction. - Ischial Ramal Socket (ischial containment): Attaches to ischial tuberosity, greater trochanter, and inferior pubic ramus; modern socket that aims to keep hip adducted and provide tension in the gluteus medius, aiding stance.
Suspension: Various devices attaching to pelvis or residual stump.
Terminal Device: Foot as above.
Articulating Joint: The knee; needs to restore knee motion during gait to allow stance and swing. A combination of pneumatic devices, hydraulic devices, microprocessors, and locks/gravity assistance.
Types of Knee Articulation
| Type | Description |
|---|---|
| Polycentric (4 bar linkage) | Moves COR allowing more flexion for sitting; good for transfemoral, bilateral, and knee disarticulations. |
| Hydraulic/Pneumatic Knee | Heavier but provides a more fluid gait; alters knee flexion resistance during the gait cycle; good for younger, more demanding patients. |
| Constant Friction Knee | A hinge with a screw or rubber pad that dampens flexion; common in children but gives poor stance control; relies on knee COR alignment for stability. |
| Variable Friction (cadence control) Knee | Has staggered pads; allows varied friction and varied walking speed; poor durability. |
| Stance Phase Control (safety knee) | Knee freezes to provide stability in stance by high friction; good for older patients or heavier patients. |
| Manual Locking Knee | Allows locking in extension to aid stance; used for very weak, unstable patients. |
Knee and Ankle Disarticulations
- Socket needs to accommodate the bulbous nature of the stump.
- Sockets may be differential:
- Two-part with a changeable plaster molded insert.
- Windowed (window cut out of socket to accommodate stump).
Hip Disarticulations and Trans Pelvic Amputations
- These prostheses are bulky.
- Hip Disarticulation: Can rely on the ischial tuberosity for socket support.
- Trans Pelvic: Contralateral ischial tuberosity is required for support.
- Knee articulation can be 4 bar, allowing it to fold anteriorly to allow the patient to sit.
Trans Humeral and Elbow Disarticulations
- Munster Socket: Supracondylar suspension.
- Function is
achieved with voluntary opening and closing devices. - Loss of elbow function and shoulder complexity necessitates a thorough analysis of the residual limb and contra-lateral limb function to decide the best method for socket fabrication.
Proximal Transhumeral or Shoulder Disarticulations
Overview
- Functionality is very poor due to the nature of the amputation.
- A prosthetic universal shoulder joint is typically used and is controlled with the opposite hand.
Complications of Prostheses
Energy Consumption by Amputation Level
| Amputation Type | Energy Consumption (%) |
|---|---|
| Long Trans Tibial | 10% |
| Trans Tibial | 25% |
| Bilateral Trans Tibial | 30% |
| Short Trans Tibial | 40% |
| Trans Femoral | 65% |
| Hip Disarticulation | 100% |
| Bilateral Transfemoral | 200% |
- Note: Longer stumps generally result in reduced energy consumption, while shorter stumps lead to increased energy needs.
Transfemoral Prosthetic Gait Abnormalities
| Gait Abnormality | Prosthetic Problem |
|---|---|
| Lateral trunk bending | Short prosthesis, weak abductors, poor fit |
| Abducted gait | Poor socket fit medially |
| Circumducted gait | Prosthesis too long, excess knee friction |
| Vaulted gait | Prosthesis too long, poor suspension |
| Foot rotation at heel-strike | Heel too stiff, loose socket |
| Short stance phase | Painful stump, knee too loose |
| Knee instability | Knee too anterior, foot too stiff |
| Medio-lateral whip | Excessive knee rotation, tight socket |
| Terminal snap | Quadriceps weakness, insecure patient |
| Foot slap | Heel too soft |
| Knee hyperextension | Heel too hard |
| Knee flexion | Heel too hard |
| Excessive lordosis | Hip flexion contracture, socket problems |
Prosthetic Training
- Acquisition of the device: Ideally, patients should be involved in the selection of the prosthesis.
- Training: Should begin with basic activities and progress to more complex activities like walking on uneven terrain, stairs, or curbs.
- Psychological support: Assists in adjusting to the new prosthetic limb.