Prosthesis Study Guide

📖 Core Concepts Prosthetic implant – an artificial device that replaces a missing body part (trauma, disease, congenital). Amputee – a person who has undergone an amputation. Interdisciplinary rehab team – led by a physiatrist and includes prosthetists, nurses, PTs, OTs. Socket – the interface that fits over the residual limb; quality of fit dictates comfort, skin health, and control. Myoelectric vs. Body‑powered – Myoelectric devices read electrical signals from residual muscles; body‑powered devices transmit motion through a harness and cable. Microprocessor‑controlled knee – uses sensors (knee angle, joint moment) and a processor to modulate hydraulic resistance for a more natural gait. Osseointegration – direct skeletal attachment of a titanium implant, eliminating the socket. Terminal device – the end‑effector (e.g., hook, prehensor, hand) that performs grasp or weight‑bearing tasks. --- 📌 Must Remember Energy cost – Transfemoral (above‑knee) amputees expend 80 % more energy walking than intact individuals. Grip force limits – Voluntary‑opening split hooks < 20 lb; voluntary‑closing prehensors ≥ 100 lb. Replacement cycles – Components ≈ 3–4 yr; sockets may need replacement within months if fit changes. Weight impact – Lower weight improves balance, comfort, and speed for all users. Fit extremes – Loose socket → pressure points, sweat, skin irritation; tight socket → high interface pressure → skin breakdown. Suspension methods – Determine how securely the socket stays on the limb (suction, straps, liners, osseointegration). --- 🔄 Key Processes Shape Capture – Obtain residual‑limb geometry via plaster, laser/structured‑light scan, or 3‑D photography. Rectification – Add volume over bony prominences/pressure points; remove volume from load‑bearing zones. Fabrication – Wrap semi‑molten plastic sheet or carbon‑fiber/epoxy laminate around the model, or 3‑D‑print the digital socket. Fit & Alignment – Clinical prosthetist aligns components; PT trains gait with verbal/tactile cues and treadmill work. Follow‑up – Periodic adjustments as residual‑limb volume changes; re‑evaluate alignment, suspension, and skin health. --- 🔍 Key Comparisons Myoelectric vs. Body‑powered Control: EMG signals vs. mechanical harness/cable. Durability: Body‑powered more robust; myoelectric sensitive to sweat/moisture. Training: Myoelectric requires longer motor‑learning period. Voluntary‑opening Hook vs. Voluntary‑closing Prehensor Grip: Opening hook passive, limited force (< 20 lb); prehensor active, high force (≥ 100 lb). Feedback: Prehensor can provide proportional biofeedback; hook provides none. Microprocessor Knee vs. Mechanical Lock Knee Adaptability: MP‑knee adjusts resistance in real‑time; mechanical lock is fixed. Maintenance: MP‑knee needs battery, water protection; mechanical lock is low‑maintenance. Socket vs. Osseointegration Interface: Socket → soft‑tissue contact; osseointegration → direct bone‑implant bond. Pros: Osseointegration → better proprioception, no skin issues. Cons: Osseointegration → risk of fracture during high‑impact activity. --- ⚠️ Common Misunderstandings “Myoelectric = more natural” – True for cosmetic appearance, but they are less durable, need more training, and lack sensory feedback. “A tighter socket is always better” – Over‑tightening causes pressure necrosis; optimal fit balances secure suspension with tissue health. “All above‑knee prostheses need microprocessor knees” – Many users can function well with mechanical knees; cost and maintenance are major considerations. “Osseointegration eliminates all socket problems” – It removes skin‑related issues but introduces surgical risks and activity limitations. --- 🧠 Mental Models / Intuition “Fit is the foundation” – Imagine the socket as a shoe; if it’s too loose or tight, walking becomes painful regardless of the shoe’s technology. “Signal → Intent → Action” – For myoelectric & robotic limbs: muscle/nerve signal (input) → controller interprets intent → actuator produces movement (output). “Energy spring” – Energy‑storage‑and‑return feet act like a spring: they store impact energy during stance and release it to aid propulsion. --- 🚩 Exceptions & Edge Cases High‑impact activities – Even with advanced materials, high‑impact sports may exceed design limits of lightweight carbon‑fiber or osseointegrated implants. Children – Rapid growth (≈ 1.9 cm/yr) requires modular components and frequent socket adjustments; durability of feet is lower due to activity level. Low‑resource settings – Simple, manually locking knees and split hooks may be the only affordable, maintainable options. --- 📍 When to Use Which Myoelectric upper‑extremity – Choose when cosmetic appearance and fine motor control (multiple grip patterns) are priority and user can tolerate maintenance. Body‑powered (harness) device – Ideal for heavy‑duty tasks, low cost, or environments with moisture/sweat. Microprocessor knee – Best for active transfemoral users needing adaptable gait on varied terrain and stairs. Mechanical lock knee – Sufficient for sedentary users, low‑budget contexts, or when water exposure is high. Osseointegration – Consider for users with chronic socket problems, high functional demands, and willingness for surgery. --- 👀 Patterns to Recognize Energy‑return foot + lightweight carbon socket → reduced metabolic cost (common in active transtibial users). High‑force grip + proportional feedback → voluntary‑closing prehensor (look for “≥ 100 lb” and biofeedback mentions). Sensor lag + EMG noise → myoelectric lag issues (often noted in humid or sweaty conditions). Frequent socket revisions → residual‑limb volume fluctuation (common in the first 6–12 months post‑amputation). --- 🗂️ Exam Traps “Myoelectric prostheses are more durable than body‑powered” – Reverse; body‑powered is actually the more durable option. “Osseointegration eliminates the need for any maintenance” – Wrong; implants require hygiene, periodic checks, and are vulnerable to high‑impact loads. “All prosthetic feet provide the same ground reaction force” – Incorrect; stiffness and shape dramatically affect COP trajectory and GRF. “Microprocessor knees work underwater” – False; they are susceptible to water damage and need careful protection. “Higher energy cost is only a problem for transfemoral amputees” – While most pronounced in transfemoral, any suboptimal socket or foot can increase energy expenditure.