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BIEN 167 Module 4 - Biomechanical Interfaces

Module 4: Biomedical Devices and Interfaces in Rehab

 

 

Rehab Biomechanics of Physical Contact Interfaces

Basic Biomechanical Principles Underlying of Physical Contact Interfaces

  • Statics of Human-Device Interfaces
  • Solids at Interfaces
  • Dynamics of Interface, or Human-Device System
  • Bi-causal Contact Interfaces

Overview of Interfaces for Whole-Body Mobility Aids

Seating/Positioning as Designed Physical Interfaces  

  • Sites:
  • Categories for seating
  • Aims / Desired Outcomes
  • Evaluation (see figures in Hobson)
  • Biomechanical Principles (see also 4-page handout from Cooper's book)
  • Seating Technologies
  • Pressure Sore Development
    • Pressure measurement (see figures in Hobson)

Physical Interfaces in Prosthetics and Orthotics

  • Intro, see also (e.g., OandP Product Review, EMedicine Overviews for Orthotics, Prosthetics)
  • Basic Biomechanical Principles
  • Interfaces Considerations for Prosthetics
    • Terminology, general stump interface mechanics and strategies
    • Lower extremity (under mostly compression/shear)
    • Upper Extremity (under variety of loading, often smaller in magnitude)
    Interface Considerations for Orthotics
    • Terminology
    • Lower extremity orthoses (often gait is key task)
    • Spinal orthoses, halo orthoses
    • Arm orthoses, hand-finger orthoses
    • Orthotic Fabrication Procedures (classic steps for thermoplastic orthoses, "on the spot", CAD/CAM)

Basic Biomechanical Principles Underlying Physical Contact Interfaces

  • Statics of Human-Device Interfaces
    • Braces often employ 3-point force systems across joints
    • Interface at hand: normally both: force and moments can be transferred
    • Joint moment: sum of individual tissue moments, each with M = r x F
  • Solids at Interfaces
    • Average stress = force/area (rarely however is stress uniform)
    • Stress ("pressure") distribution strategy: lower peak stress by seeking uniform distribution over larger surface area
  • Dynamics of Interface, or Human-Device System
    • Inertial dynamics of linkage systems, dynamic coupling
    • Momentum and interfaces
  • Bi-causal Contact Interfaces
    • Power transmission: force*velocity
      • Power transmission and gearing strategies
    • Contact Impedance: dynamic force/velocity coupling
      • Human can vary their contact impedance, within limits
    • Uni-causal information flow only if impedance mismatch (e.g., air, wall)
    • Concept of Extended Physiological Proprioception (concept originally from upper extremity prosthetics field)
      • mechanical requirement: transfer of both force and velocity (and thus length) across interface
      • extension of self with practice (e.g., prostheses, tennis racquet, pencil, hand tool)

Overview of Interfaces for Whole-Body Mobility Aids

  • General concepts
    • Human-Device interfaces (# contact DOF, etc)
    • Device-environment interface (# contact DOF, etc.)
    • (Seating-positioning issues is topic of next section)
  • Overview of Contact Interfaces for Aids for "Biped" Locomotion
    • Biomechanics of use of canes (e.g., older adult)
    • Biomechanics of use of crutches (e.g., lower-limb injury, neurological impairment)
    • Biomechanics of use of walkers (e.g., older adult, SCI, cerebral palsy)
      • 0-wheel, 2-wheel, 4-wheel (less common now)
      • posterior, anterior
    • Biomechanics of use of wheelchairs
      • Manual
        • self-propelled (by rims)
        • attendant-propelled
        • recreation & sport design features
      • Battery Powered
        • need to operate controller (more variety)
    • Scooters
      • targets: persons with stability yet too weak for manual
        • typically able to walk, but not long distances
        • children with MD, arthritis, MS, stroke, ...
        • typical max speed: 5 MPH
        • 3-wheel (FWD, RWD), auto/parking brakes
    • Carts
      • targets: similar to scooters, but often great than 1 person
    • Lifts (Transfer Devices)
      • types of transfers (pivot, slide; self/dependent)
        • types of assistive interfaces (sling, chair, ...)
  • Wheelchair Technology and Biomechanics
    • Reading materials for this section and next on seating/positioning (from University of Pittsburgh): Wheelchair University (http://www.wheelchairnet.org/wcn_wcu/SlideLectures/Lectures/lectures.html), See especially:
    • Biomechanical/Ergonomic Considerations in Design
      • Positioning: spinal biomechanics
      • Task identification and analysis
      • Propulsion biomechanics (manual)
        • kinematics of arm/hand rim
        • kinetics - power of overall system
        • kinetics - inverse dynamic analysis
          • contact » wrist » elbow » shoulder » torso
      • Dynamic Stability & ANSI/RESNA safety standards
    • Manual Wheelchair Technology Considerations
      • attendant / foot / hand driven
      • frame types
        • folding  (cross-brace, parallel-brace, forward-folding)
        • rigid
      • frame styles: box, cantilever, T or I frame
      • wheels (rubber, pneumatic, combo)
      • casters (help with base of support while being mechanically passive for steering)
        • 5-20 cm dia - "ride" vs foot clearance
        • caster flutter, trail, float
      • overall size considerations: wheelbase, width, camber
      • push rims (hand-rim contact is physical interface)  
        • metal or plastic rings (~ 1 cm dia), often with coating
        • several cm smaller in dia than wheel
      • seat & backrest height and orientation 
        • tradeoffs: comfort, propulsion, control/stability
    • Powered Wheelchair Technology
      • General classification - indoor, indoor/outdoor, active indoor/outdoor
      • Motors - permanent magnet d.c. motors (brushes)
        • model: L & R in series, like low-pass filter (in: voltage; out: speed), back emf ~ speed
      • Drive trains - reduce speed, increase force
        • either belts or gear (worm, spur)  boxes
        • wheels 20-50 cm (8-20 in)
        • efficiencies can be over 80% (with pulse-width modulated servos)
          • spur, belts even better
          • lower-speed lubrication (e.g., PTFE)
      • Control interfaces 
        • joysticks, sip-&-puff, switches, etc. 
        • toggle vs proportional controllers
      • Batteries
        • 24-volt d.c. (two 12-volt in series)
        • power draw: max about 60 amps, typical about 10 amps
        • wet & gel cell batteries, charging
      • Adjustments for Posture
        • Seat height, width, tilt
        • Seatback tilt, highback option, neck tilt, armrest height & angle, footrest location
      • Advances
        • stand-up and stair-climbing wheelchairs
        • microcontrollers to facilitate user-centered powered wheelchair needs
          • intelligent system/control parameter tuning algorithms
        • Service - remote diagnosis and trouble-shooting

Seating/Positioning as Designed Physical Interface(s)  

  • See especially the following reading material:
  • Some basics follow, in outline form (based on Cook & Hussey AT book, Chapter 5):
    • Categories for seating
      • for postural control and deformity management (e.g., cerebral palsy)
      • for pressure and postural management (e.g., spinal cord injury)
      • for comfort and postural accommodation (e.g., older adult)   
    • Aims / Desired Outcomes
      • maintenance of neutral skeletal alignment, and prevention of deformity
      • prevention of tissue breakdown
      • increased comfort and tolerance of position
      • decreased fatigue
      • minimization of influence of abnormal tone/reflexes
      • facilitation of components of desired movements
      • enhanced respiratory, oral-motor, and digestive function
      • maximized stability to enhance function
      • facilitation of care provision
    • Evaluation (see figures in Hobson)
    • Biomechanical Principles
      • postural management & control
        • statics (3-point, center of gravity with activities, positioning belts, ...)
        • kinematics (geometric alignment, workspace, ...)
        • solids (contact surfaces - pressure, friction)
        • proximal stabilization (e.g., hand for stabilization?
      • pressure management
        • application of solids (contact stresses (normal and shear)) 
    • Seating Technologies
      • factors
        • density, stiffness, resilience (creep), dampening, envelopment
      • interface (cushion) materials:
        • foams - linear, standard-contoured, custom-contoured
        • air-filled (small vs large cell size)
        • gel-filled (high viscous flow)
        • water-filled (lower viscosity)
        • alternating pressure
        base: rigid, sling
    • Pressure Sore Development
      • Steps
        • local ischemia (reversible?) & reactive hyperemia (temporary reddening)
        • non-blanching hyperemia, beginnings of swelling, blistering, ulceration
        • ulceration progresses beyond dermis to subcutaneous tissue
        • ulceration progresses to subcutaneous fat, deep fascia, muscle
        • deep necrosis, enlarged area 
      • Factors that Contribute to Pressure Sore Development
        • external forces (compressive, shear)
        • many others (see figure in Hobson) 
        • theories
          • interface pressure theory - interface and deep pressures similar
          • shape preservation theory - best if both near-constant hydrostatic pressure and "normal" body shape maintained (predicts semi-rigid shells)
        • importance of pressure relief 
          • e.g., min 3 sec every 20 min
    • Pressure measurement (see figures in Hobson)
      • Bladder sensors (switch-activated)
      • Conductive polymer force-sensing resistors
        • mylar with conductive ink (R drops with pressure, nonlinear)
      • Capacitive pressure mats

Physical Interfaces in Prosthetics and Orthotics

  • Focus here is on the physical interfaces, as Module 6 will cover Prosthetics and Orthotics in more depth
  • For a general overviews, see also:
  • Basic Biomechanical Principles
    • 3-point force systems
    • Joints (DOFs, ROM, spring-loading, ...)
    • Pressure and contact stress distributions
    • 3D interface geometry and analysis
    • Roles for devices crossing an interface, such as harnesses
    • materials science
  • Interface considerations for Prosthetics
    • Terminology (e.g., BE, AE, BK, AK, trans----).
    • Interface considerations, general:
      • Key start is to understand the nature of the residual limb that is available
      • Fitting is by prosthetist
      • Relief - design of socket to lower stresses concentrations in areas that are sensitive to high pressure (e.g., bony prominences), often by forming a concave surface that helps distribute stresses to other areas (and over a greater area)
      • Buildup - design of socket so as to increase surface contact in an area, typically for areas that are tolerant to high pressure (such as a bulge), often by forming a convex surface
    • Interface considerations, Lower Extremity:
      • Key issue is designing for pressure distribution - for most levels (especially all but for toes), the human-device interface typically involves high compression forces and often bending moments, and some shear and torsion, but rarely much tension
      • amputations are often planned with a certain class of prosthesis in mind (e.g., to distribute load-sharing along the stump of the residual limb)
      • Socket: designed to protect the residual limb and transmit the forces during standing and ambulation.
        • Typically start with a preparatory (temporary) socket, adjusted as the volume of the residual limb stabilizes, that uses a plaster mold of the residual limb as a template or CAD
        • Ex: Transtibial Patellar-Bearing total contact socket: emphasizes increased weight-bearing near area of patellar tendon, but 3D contact (with selective relief) throughout so as to distribute loading and minimize skin breakdown, etc.
    • Interface considerations, Upper Extremity:
      • Socket typically has a dual-wall design fabricated from lightweight plastic or graphite composite materials
        • an inner socket that is fabricated to fit the patient's residual limb
          • typically fabricated with flexible plastic materials, soft liner fabric
          • aims are comfort, support for function (e.g., high bending moments and shear, significant tension and torsion, occasional compression)
        • an outer wall that is designed to be the same length and contour as the opposite limb.
          • typically a rigid frame that provides structural support, and attachment sites for cables and joints as needed.
          • the open areas, or windows in the outer socket allow movement, permit relief over bony prominences, and enhance comfort.
      • Suspension system
        • Used to hold the prosthesis securely to the residual limb, as well as accommodate and distribute the forces associated with the weight of the prosthesis and any superimposed lifting loads.
        • Harnessed-based systems (most common)
          • Many approaches, typically cross torso and use it as a base
          • Ex: figure-8 strap, where a harness loops around the axilla to anchor the harness and provides the counterforce for suspension and control-cable forces
            • anterior (superior) strap carries the major suspending forces to the prosthesis by attaching directly to the socket in a transhumeral prosthesis or indirectly to a transradial socket through an intermediate Y-strap and triceps cuff.
            • posterior (inferior) strap on the prosthetic side attaches to the control cable.
        • Approaches without harnesses (or with harness as an augment)
          • Self-suspending are largely limited to wrist or elbow disarticulations and to transradial amputations (e.g., with myoelectrically controlled transradial prosthesis).
          • Suction suspending, most common for a transhumeral amputation, are similar to lower extremity options, with use of an external, elastic suspension sleeve, a one-way air valve or roll-on gel suspension liner with a pin-locking mechanism.
      • Terminal device (device-environment interface)
        • This most distal part of an arm prosthesis that substitutes for the human hand and interacts with the environment
        • It may be a prosthetic hand (often just 1 DOF, sometimes more), a hook (1 DOF), or another device.
  • Interface Considerations for Orthotics
    • Terminology (e.g., AFO, KAFO)
    • Interface considerations, Lower Extremity Orthoses (often gait is key task)
      • foot/shoe orthotics
      • ankle: "plastic joints" vs hinge joints (with springs, mechanical stop?)
      • specialized knee controllers (polycentric mechanisms; passive or active)
      • reciprocating gait orthoses
    • Interface considerations for Spinal orthoses
      • often one more more 3-point for systems
      • cervical, head cervical, cervical thoracic
      • thoracolumbar (T10-L2), lumbrosacral
      • bracing for scoliosis (3-point but also often axial "traction")
    • Halo orthoses (with interface of skull pins, coupling bars, then interface of vest)
    • Arm orthoses
      • many functional aims (e.g., from increasing to restricting ROM), and this can affect interface
    • Hand-finger orthoses
    • Classic Orthotic Fabrication Procedures (for now, think about interface aspects)
      • Classic steps for thermoplastic orthoses
        • Client visit #1: presentation and evaluation, prescription, measurements & negative impression (mold) by orthotist
        • Fabrication facility (normally all by trained lab technician): fabricating/modifying positive cast, fabrication of orthotic material (under heat), adjustments in certain regions, addition of off-the-shelf components (e.g., straps)
        • Client visit #2 (days after first visit): Fitting on client, any mild adjustments by orthotist
      • "On the spot"
        • Client visit: low-temperature thermoplastics (purchased sheets, etc), gross formation by heat gun (even blow dryer) or hot placement in water bath (or bought with a general pre-fab shape for a body region), placement on person (with lining material between, mild fitting adjustment, addition of any other components (e.g., velcro), client leaves (often after doffing the orthosis, to let it fully cool before donning it so that it has a better chance of holding its shape)
        • Advantages over traditional: one visit, could be another type of health professional such as a PT or OT
        • Disadvantages over traditional : material is not as stiff or strong, and quality control
        • Most common application: upper extremity, or other lower loading applications
      • CAD/CAM
        • Client visit #1: Digitizing device, interactive computer program (with 3D shape-manipulation)
        • Fabrication: CAM milling machines and production thermoforming (local in-house facility or remote outsourcing)
        • Client visit #2 (in some case 1 visit, with a wait): fitting on client, mild adjustments by orthotist
        • Advantages: automation may save labor costs, and potentially quite reliable and faster turn-around time
        • Disadvantages: Maintenance of and imperfections of automation technology

 

 

 

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