UPMSP Artificial Limb & Organ Tech cheat sheet

Bone is 60-70% mineral (hydroxyapatite) and 20-25% organic matrix (mostly type I collagen); cortical bone porosity is 5-10%, cancellous bone 50-90%.

Long bone regions: diaphysis (shaft, cortical), metaphysis (flared, trabecular), epiphysis (articular end); the physis (growth plate) lies between epiphysis and metaphysis in growing bone.

Osteon (Haversian system): concentric lamellae around a central canal carrying vessels; Volkmann canals run transversely connecting central canals to periosteum and medullary cavity.

Wolff's law: bone remodels along lines of applied stress; disuse or stress shielding (e.g., by a stiff stem) causes resorption, while increased load increases density.

Three bone cells: osteoblasts (form matrix), osteocytes (mechanosensors in lacunae), osteoclasts (resorb, derived from hematopoietic precursors).

Skeletal muscle contraction: actin-myosin cross-bridge cycling driven by Ca2+ release from sarcoplasmic reticulum; the H zone and I band shorten while the A band stays constant.

Muscle fiber types: type I slow oxidative (fatigue-resistant, posture), type IIa fast oxidative-glycolytic, type IIx fast glycolytic (rapid force, fatigable).

Knee ligaments: ACL resists anterior tibial translation; PCL resists posterior translation; MCL and LCL resist valgus and varus respectively; menisci distribute load and increase congruence.

Stress = force/area (MPa); strain = change in length/original length (dimensionless); elastic modulus = stress/strain in the linear region, measured in GPa for metals and MPa for polymers.

Yield point marks the onset of permanent (plastic) deformation; ultimate tensile strength is the maximum stress before necking; fatigue failure occurs below yield under repeated cyclic loading.

Common prosthetic metals: stainless steel 316L (E ≈ 190-200 GPa), Ti-6Al-4V (E ≈ 110 GPa, density 4.4 g/cm3), aluminium alloys (E ≈ 70 GPa, low density); lower modulus reduces stress shielding.

Thermoplastics for sockets: PP (polypropylene, flexible, tough), PE (polyethylene), acrylic (PMMA, rigid, thermoformable at ~160-180 °C), PETG and copolymer for check sockets.

Lamination resins: acrylic (PMMA), epoxy, and polyester; reinforcement fibers include fiberglass, carbon fiber (high stiffness-to-weight), and aramid (Kevlar, impact-tough).

Silicone elastomers: addition-cure (platinum) is skin-safe and dimensionally stable; tin-cure is cheaper but shrinks more and may irritate skin.

Liner materials: silicone gel, mineral-oil gel, and polyurethane; higher durometer = firmer; distal attachment (pin, suction, seal-in) must match liner choice.

Corrosion and biocompatibility: 316L is susceptible to pitting/crevice corrosion in chloride environments; titanium forms a passive TiO2 layer; ISO 10993 governs biological evaluation of medical devices.

Transfemoral measurement landmarks: ischial tuberosity to floor (ITF), medial tibial plateau to floor (MTP), ischial tuberosity to adductor longus tendon and to greater trochanter for socket flexion/abduction.

Transtibial landmarks: patellar tendon bar, tibial crest, fibular head, popliteal fossa; measure MTP to floor, tibial tubercle to floor, and ML (medial-lateral) width at the tibial flares.

Cast rectification rules: relieve pressure-sensitive areas (tibial crest, fibular head, distal tibia, patella), build up pressure-tolerant areas (patellar tendon, tibial flares, popliteal area, gastroc bulge).

Transtibial socket biomechanics: patellar tendon-bearing (PTB) design uses a bar to load the patellar tendon, with relief over the fibular head and distal tibia; total surface bearing (TSB) distributes load across the whole residual limb.

Transfemoral socket designs: quadrilateral (ischial seat) and ischial containment (narrow ML, ischium inside socket); MAS (Marquette) socket uses a ramus strut and medial wall.

Check socket (test socket) is made from the rectified positive model, usually clear thermoplastic, to verify fit and alignment before definitive lamination.

Volume management: residual limb volume changes throughout the day and over months; measure circumference at defined levels and record date/time; adjust sock ply as needed.

Suspension options: suction, pin/lock, seal-in liner, TES belt, Silesian belt, and anatomical (condylar) suspension; each has distinct indications and contraindications.

Transtibial alignment: bench alignment sets socket flexion ~5°, adduction ~5-7°, foot in neutral rotation; dynamic alignment adjusts for comfort, gait symmetry, and knee stability.

Transfemoral alignment: socket flexion 5-10°, adduction 5-7°, knee axis over the foot; stability depends on the trochanter-knee-ankle (TKA) line passing anterior to the knee axis.

Knee units: single-axis (constant friction), polycentric (four-bar, more stability in stance), and microprocessor-controlled (hydraulic/pneumatic damping, stumble recovery).

Feet: SACH (solid ankle cushion heel) for low activity; single-axis and multiaxis for uneven terrain; energy-storing (ESR) feet for higher activity (K-level 3-4).

Upper limb levels: transradial, transhumeral, shoulder disarticulation, forequarter; terminal devices include split hooks (voluntary opening/closing) and myoelectric hands.

Body-powered upper limb: harness and Bowden cable; voluntary opening (VO) is standard for hooks; cable excursion ~5 cm for full terminal device opening.

Myoelectric control: surface electrodes over agonist/antagonist muscles; signal processing (threshold, proportional control); prosthetic hand grip force typically 60-100 N.

Fabrication sequence: measurement → cast → rectification → check socket → definitive lamination → assembly/alignment → dynamic fitting → delivery and follow-up.

Orthosis naming: joint(s) crossed and direction (e.g., KAFO = knee-ankle-foot orthosis; AFO = ankle-foot; TLSO = thoraco-lumbo-sacral).

Three-point pressure principle: a force couple applied at three points controls angulation; used for scoliosis TLSO and genu varum/valgum knee orthoses.

AFO indications: foot drop (peroneal nerve palsy), ankle instability, and tibialis posterior dysfunction; plantarflexion stop angle and dorsiflexion assist are key prescription parameters.

KAFO: knee joint with lock or drop-lock; used for quadriceps weakness or knee instability; alignment must match the mechanical knee axis to the anatomical knee axis.

Scoliosis orthosis: Boston brace (TLSO, underarm) for curves with apex below T8; Milwaukee brace (cervico-thoraco-lumbo-sacral) for high thoracic/cervical curves.

Ground reaction AFO (e.g., carbon fiber) uses a stiff foot plate and posterior strut to resist plantarflexion and assist push-off; contraindicated in severe spasticity without assessment.

Biomechanics: normal walking speed ~1.2-1.4 m/s; cadence ~110-120 steps/min; stance ~60% and swing ~40% of gait cycle; step length ~0.7 m.

Orthotic forces: pressure = force/area; padding increases contact area and reduces pressure; shear forces are reduced by low-friction interfaces and proper strap orientation.

Gait cycle phases: initial contact, loading response, mid-stance, terminal stance, pre-swing (stance ~60%); initial swing, mid-swing, terminal swing (swing ~40%).

K-levels (Medicare functional classification): K0 no potential, K1 household ambulator, K2 limited community, K3 community with variable cadence, K4 high activity/high impact.

Amputee assessment: residual limb length, shape, volume, skin integrity, sensation, pain (phantom and residual), strength, ROM, and contralateral limb status.

Gait deviations: excessive knee flexion (prosthetic foot too plantarflexed or knee too flexed), vaulting (prosthesis too long), lateral trunk lean (hip abductor weakness or socket too abducted).

Prosthetic training progression: pre-prosthetic (wrapping, desensitization, strengthening), static standing balance, weight shifting, gait with parallel bars, then progression to cane and free ambulation.

Outcome measures: Timed Up and Go (TUG), 6-minute walk test (6MWT), L-test, AMP (Amputee Mobility Predictor), and Prosthetic Evaluation Questionnaire (PEQ).

Rehabilitation team: physiatrist, prosthetist/orthotist, physical therapist, occupational therapist, nurse, social worker, and psychologist; goal setting is patient-centered and SMART.

Contracture prevention: proper positioning, daily ROM exercises, and early prosthetic fitting; knee flexion contracture >15° complicates transfemoral fitting.

PPE for workshop: safety glasses, dust mask/respirator (N95 or P2 for plaster and resin dust), hearing protection, nitrile gloves for resin, and closed-toe shoes.

Plaster work: water-to-plaster ratio ~1:1 by weight (or per manufacturer); set time ~5-10 min; exothermic reaction; never immerse hands in setting plaster (burns).

Casting saws and band saws: keep hands >10 cm from the blade, use push sticks, secure work, and never remove guards; blade tension and tooth direction must be correct.

Resin lamination: work in ventilated area or fume hood; acrylic monomer is flammable and a respiratory irritant; store in a cool, dark, closed container away from ignition sources.

Router and grinder safety: clamp work, use both hands, wear eye protection, and select the correct bit/abrasive for the material (wood, plaster, plastic, or metal).

Oven/thermoforming: acrylic softens ~160-180 °C; use heat-resistant gloves, allow cooling, and avoid overheating (bubbles, discoloration, fume release).

Hand tools: use the correct size screwdriver, sharp blades, and cut away from the body; chisels and knives must be sharp to reduce force and slipping.

Waste disposal: plaster in general waste (never down drains), resin and solvent waste in labeled hazardous containers, and sharps in rigid containers.

Hemodialysis: blood flows through a dialyzer with a semipermeable membrane; countercurrent dialysate removes urea, creatinine, and excess fluid; typical session 3-4 h, 3 times/week.

Dialysis membrane materials: cellulose triacetate, polysulfone, and polyethersulfone; flux and surface area determine clearance; biocompatibility affects complement activation.

Heart-lung machine: roller or centrifugal pump plus oxygenator (membrane or bubble); used in cardiopulmonary bypass; requires systemic anticoagulation (heparin).

Ventricular assist device (VAD): continuous-flow centrifugal/axial pump; LVAD for end-stage heart failure; risks include thrombosis, bleeding, and driveline infection.

Cochlear implant: microphone, speech processor, transmitter, and electrode array in the cochlea; bypasses damaged hair cells to stimulate the auditory nerve.

Assistive devices: wheelchair (manual vs powered), walker, crutch, cane; cane used on the stronger side; axillary crutches require 2-3 finger-breadth axillary clearance to avoid nerve compression.

Prosthetic heart valves: mechanical (bileaflet, tilting disc; requires lifelong anticoagulation) vs bioprosthetic (porcine/pericardial; lower thrombogenicity, limited durability).

Artificial limb interface: socket-liner interface pressure must remain below capillary closing pressure (~30 mmHg) for prolonged periods to prevent skin breakdown.

Newton's laws: F = ma; for static equilibrium, sum of forces = 0 and sum of moments about any point = 0.

Moment (torque) = force × perpendicular distance; SI unit N·m; a longer lever arm increases moment for the same force.

Pressure = force/area (Pa = N/m2); 1 kPa = 7.5 mmHg; 1 atm = 101.3 kPa = 760 mmHg.

Stress = F/A; strain = ΔL/L; Hooke's law σ = Eε in the elastic region; Poisson's ratio = lateral strain/axial strain (≈0.3 for metals).

Beam bending: maximum bending moment for a simply supported beam with central point load = FL/4; for a cantilever with end load = FL.

Technical drawing: first-angle vs third-angle projection; orthographic views (front, top, side); hidden lines dashed; center lines chain-dotted; dimensions in mm unless stated.

Tolerances and fits: clearance, interference, and transition fits; ISO tolerance grades (IT01-IT18); surface roughness Ra in micrometers.

Gait physics: work = force × distance; power = work/time; energy cost of walking increases with prosthetic mass and misalignment; ground reaction force ~1.0-1.2× body weight in walking.

Informed consent: explain procedure, risks, benefits, alternatives, and the right to refuse; document consent in the patient record.

Confidentiality: patient information is protected (e.g., HIPAA in the US, DPDP Act in India); share only on a need-to-know basis and secure records.

Scope of practice: work within your training and licensure; refer out when the case exceeds competence; never diagnose or prescribe outside your role.

Infection control: hand hygiene before/after patient contact; clean vs sterile technique; disinfect tools and equipment between patients.

Conflict of interest: disclose financial or personal interests; do not accept gifts that could influence clinical decisions.

Communication: use plain language, confirm understanding (teach-back), involve family/caregivers with consent, and provide written instructions.

Incident reporting: report workplace injuries, near-misses, and equipment failures promptly; document objectively without blame.

Professional boundaries: maintain respectful, non-exploitative relationships; avoid dual relationships that could impair judgment.