Device Principles — Organized into Categories
1. What Is the Device Trying to Do? — Device Purpose
Start with the clinical problem and intended effect.
Protect — protect healing or vulnerable tissue
Prevent — prevent deformity or unwanted movement
Correct — move a flexible deformity toward a desired position
Accommodate — accept a fixed deformity while redistributing load
Support/Stabilize — supplement inadequate structural or muscular control
Assist movement — supplement deficient muscle action
Restrict/stop movement — limit harmful or unwanted motion
Redistribute/unload — alter where forces are transmitted
Improve function — walking, transfers, ADLs, work, recreation
This is the “WHY?” of the orthosis. Orthoses are intended to modify structural and functional characteristics of the neuromusculoskeletal system through externally applied forces (Morris, 2007).
2. How Does the Device Produce the Effect? — Force Production
This becomes the biomechanics section.
Force
Magnitude
Direction
Location of application
Three-point force system
Primary corrective force
Two counterforces
Moment / torque
Moment = Force x Moment Arm
Longer lever arm → greater moment for the same force
Or less force required to produce the same moment
Pressure
Pressure = Force/Area
Same force over smaller area → greater pressure
Same force over larger area → lower pressure
These concepts explain why orthotic shape, height, contact area, straps, shells, pads, and uprights matter rather than simply being components to memorize (Edelstein & Moroz, 2011; Morris, 2007).
This is the “HOW?”
3. How Is the Force Controlled? — Device Design
Now students examine the physical characteristics that determine how those forces are delivered.
Material
Rigid ↔ flexible
Stiffness
Elasticity/deformation
Durability
Weight
Geometry
Thickness
Contours
Trimlines
Shape
Lever-arm length
Longer versus shorter device
Location relative to joint axis
Articulation
Solid
Hinged
Free motion
Limited motion
Stops
Assists/resists
Alignment
Position relative to anatomical joint axes
Position relative to GRF
Ankle/foot position
Shoe-device interaction
Therefore:
Same material ≠ same mechanical behavior.
Changing thickness, geometry, trimlines, alignment, or lever-arm length can change what the device does. Orthotic design depends on both material characteristics and biomechanical design (Morris, 2007).
This is the “WHAT CONTROLS THE HOW?”
4. How Does the Device Interact With the Patient? — Human–Device Interface
This deserves its own category because this is where PT examination becomes particularly important.
Pressure distribution
Total contact
Pressure-tolerant areas
Pressure-sensitive areas
Bony prominences
Suspension
Does the device stay where it is supposed to?
Migration
Pistoning
Rotational control
Shear and friction
Skin movement within device
Socks/liners
Moisture
Repetitive loading
Fit
Too tight
Too loose
Improper strap tension
Changing edema/volume
Growth in pediatrics
Skin integrity
Redness
Blistering
Breakdown
Pain
Sensory impairment
This makes an important distinction for students:
A device can be mechanically correct but clinically unsuccessful because the interface is poor.
Fit, alignment, pressure distribution, and device evaluation are recurring clinical responsibilities in orthotic and prosthetic management (Edelstein & Moroz, 2011).
The Overall Device-Principle Framework
I think this could become a recurring graphic throughout the course:
1. PURPOSE
What are we trying to change?
↓
2. FORCE PRODUCTION
What force/moment is necessary?
↓
3. DEVICE DESIGN
How will the device generate/control that force?
↓
4. HUMAN–DEVICE INTERFACE
Can the patient safely tolerate and use it?
↓
5. FUNCTIONAL EFFECT
Did it improve activity or participation?
That gives students a much stronger clinical reasoning pathway than teaching “AFO → KAFO → knee brace → TLSO…” as separate topics.
Then every device lecture can begin with the same five questions:
Why does this device exist? → How does it work? → What design features make it work? → Where can the device-patient interface fail? → Did it improve what matters?
References
Edelstein, J. E., & Moroz, A. (2011). Lower-limb prosthetics and orthotics: Clinical concepts. SLACK Incorporated.
Morris, C., & Dias, L. S. (Eds.). (2007). Paediatric orthotics. Mac Keith Press.
World Health Organization. (2003). ICF checklist: Version 2.1a, clinician form. World Health Organization.
Device Principles: A Clinical Reasoning Framework for Orthotic Selection
Orthotic management should begin with the patient's problem, not with the name of the device. A student who memorizes that a particular diagnosis receives a particular orthosis may recognize familiar cases but struggle when the presentation changes. Instead, students should be able to determine why external support is needed, what needs to change, how the device can produce that change, what design features are required, and whether the change improves meaningful function.
This produces a repeatable clinical reasoning sequence:
WHY? → WHAT? → HOW? → WITH WHAT? → SO WHAT?
The framework shifts the emphasis from “Which brace goes with this diagnosis?” to “What problem am I trying to solve?” This approach is consistent with orthotic management as a biomechanical intervention intended to alter alignment, movement, loading, stability, and function (Chui et al., 2020; Edelstein & Moroz, 2011).
1. WHY? — Purpose of the Device
The first question is:
Why does this patient need external support?
The purpose should be established before selecting the device.
These purposes overlap clinically. A single orthosis may simultaneously protect tissue, stabilize a joint, and redistribute load. The key is identifying the primary therapeutic purpose for the individual patient rather than assuming the purpose from the diagnosis alone (Edelstein & Moroz, 2011).
Think in tissues and systems
When determining why a patient might need an orthosis, students can screen several major sources:
Muscle — weakness, paralysis, imbalance, injury
Bone — fracture, deformity, malalignment, prominence
Ligament — injury, laxity, instability
Tendon — injury, repair, rupture, insufficient function
Joint/cartilage — arthritis, instability, contracture, painful movement
Nerve/neurologic system — peripheral nerve injury, paralysis, spasticity, impaired motor control
Skin/soft tissue — wound, scar, pressure vulnerability
Bursa — compression or movement sensitivity
This prevents the student from thinking only in terms of diagnoses.
2. WHAT? — What Needs to Change?
Once the purpose has been identified, the next question is:
What specifically needs to change?
This is where the examination findings become a mechanical target.
The orthosis may need to change:
Position/alignment
Motion
Stability
Load/weight bearing
Pressure distribution
Muscular demand
Pain-provoking mechanical stress
Tissue exposure to stress
Gait mechanics
For example:
Muscle weakness alone doesn't tell us what orthosis is needed.
The PT must determine what the weakness is doing:
Dorsiflexor weakness
↓
Inadequate ankle control during swing
↓
Foot clearance problem
Now there is something mechanically specific for the orthosis to address.
Orthotic evaluation therefore requires translating impairments into their effects on alignment, movement, stability, gait, and functional performance (Chui et al., 2020).
3. HOW? — What Mechanical Action Is Required?
Now ask:
How must the device change the patient's mechanics?
This is where restrict belongs.
Restriction is not necessarily the purpose of an orthosis. It is often the mechanism used to accomplish the purpose.
The device can:
Restrict unwanted motion
Stop/immobilize motion
Allow desired motion
Assist motion
Guide motion
Redirect motion
Maintain a position
Apply a corrective force or moment
Stabilize a body segment
Redistribute/offload forces
For example:
Healing ligament → WHY: Protect → WHAT: Excessive joint motion → HOW: Restrict that motion
Compare that with:
Ligamentous laxity → WHY: Stabilize → WHAT: Excessive joint motion → HOW: Restrict that motion
The HOW is identical, but the WHY is different.
This distinction becomes important when students begin comparing devices.
Protect
Protection is appropriate when a healing or vulnerable structure needs to be shielded from excessive mechanical stress. This may include a muscle following strain or repair, bone following fracture or surgery, ligament following sprain or repair, or tendon following a tear or surgical repair. Protection may also be required following an acute or postoperative joint injury or when skin and other soft tissues are healing or vulnerable.
Prevent
Prevention is appropriate when the goal is to stop an anticipated deformity, contracture, injury, or tissue problem before it develops or progresses. Examples include muscle weakness or imbalance that places a joint at risk, bone at risk for progressive deformity, recurrent ligamentous instability, or a joint at risk for contracture. Neurologic conditions involving paralysis, spasticity, or abnormal motor control may create a risk for progressive positioning problems. Prevention can also involve protecting skin that is at risk for recurrent pressure injury.
Correct
Correction is appropriate when an existing deformity or malalignment remains flexible and can be moved toward a more desirable position. The underlying problem may originate from muscle imbalance, developing bony malalignment, a flexible joint deformity, or abnormal positioning associated with neurologic tone or motor-control dysfunction. Developing soft-tissue contracture may also create an opportunity for corrective positioning before the deformity becomes fixed.
Accommodate
Accommodation becomes appropriate when a deformity cannot or should not be corrected. Instead, the device accepts the patient's existing anatomy while attempting to improve comfort, contact, pressure distribution, or function. Examples include a fixed bony deformity or prominence, a fixed joint contracture, vulnerable skin, or an established fixed deformity associated with a chronic neurologic condition.
A useful distinction for students is:
Flexible deformity → consider correction
Fixed deformity → consider accommodation
Support / Stabilize
Support or stabilization is required when the patient's own musculoskeletal or neuromotor system provides insufficient control of a body segment or joint. This may result from muscle weakness or paralysis, bony structural insufficiency, ligamentous laxity, joint instability, impaired neurologic motor control, or tendon insufficiency. The device supplements the stability that the patient's intrinsic structures cannot adequately provide.
Assist
Assistance is appropriate when the patient has insufficient active movement but movement remains functionally desirable. The problem may arise from muscle weakness or paralysis, loss or impairment of musculotendinous function, peripheral nerve injury or denervation, or a neurologic disorder affecting selective motor control. Rather than simply preventing movement, the device supplements the patient's ability to produce the desired movement.
Unload / Redistribute
Unloading or redistribution is appropriate when the primary problem is excessive, concentrated, or poorly distributed mechanical loading. The target may be a bony prominence or stress injury, excessive loading of joint or cartilage, an overloaded tendon, or a focal area of vulnerable skin or an existing wound. Unloading may also be appropriate when a nerve or bursa is sensitive to localized compression. The objective is not necessarily to eliminate loading, but to change where and how mechanical forces are distributed across the involved tissues.
4. SO WHAT? — Did the Device Improve Function?
The final question should not be:
“Did the brace correct the ankle?”
It should be:
“Did changing the ankle matter to the patient?”
This is where the ICF model becomes particularly valuable.
Body Structure and Function
Did the device change:
Alignment?
ROM?
Stability?
Pain?
Muscle demand?
Tone?
Pressure/loading?
↓
Activity
Did that allow the patient to:
Walk farther?
Walk faster?
Clear the foot?
Transfer safely?
Negotiate stairs?
Stand longer?
Perform ADLs?
↓
Participation
Did that change the person's ability to participate in:
Work?
School?
Family roles?
Community mobility?
Recreation?
Sports?
↓
Contextual Factors
And can the person realistically use the device considering:
Comfort
Shoes
Environment
Appearance
Patient preference
Caregiver assistance
Ability to don/doff
Cost/access
Lifestyle
The ICF framework distinguishes body functions and structures from activity, participation, and contextual influences, making it particularly useful for determining whether an orthotic intervention has produced a meaningful functional outcome rather than merely changing an impairment.
The Complete Device-Principle Algorithm
The entire orthotics section can therefore repeatedly return to one clinical reasoning pathway:
QuestionClinical reasoningExample:
Foot Drop
WHY? What is the purpose?
Assist deficient function
WHAT? What needs to change?Improve swing-phase foot clearance
HOW? What mechanical action is required?Assist/maintain dorsiflexion during swing
WITH WHAT? What device characteristics produce it? Appropriate AFO stiffness, geometry, trimlines, lever arm, alignment
INTERFACE? Can the patient safely use it? Check fit, pressure, suspension, skin, footwear
SO WHAT? Did it improve meaningful function? Safer walking → community mobility/participation
The same framework works for a completely different problem:
Knee OA
WHY? Unload
WHAT? Excessive compartment loading
HOW? Alter the external knee moment/load distribution
WITH WHAT? Appropriate force system, lever arms, straps, alignment and brace rigidity
INTERFACE? Is it comfortable enough to wear during meaningful activity?
SO WHAT? Does the patient experience improved walking/activity?
And again:
Healing ligament injury
WHY? Protect
WHAT? Excessive stress/motion
HOW? Restrict the provocative motion
WITH WHAT? Appropriate rigidity, articulation, stops, straps and lever arms
INTERFACE? Does it remain properly positioned without creating excessive pressure?
SO WHAT? Can the patient safely perform the desired activity while the tissue heals?
That gives students one framework that can be applied to essentially every orthosis:
WHY → WHAT → HOW → WITH WHAT → INTERFACE → SO WHAT
The device name comes after the clinical reasoning, not before it.
References
Chui, K. K., Jorge, M., Yen, S.-C., & Lusardi, M. M. (Eds.). (2020). Orthotics and prosthetics in rehabilitation (4th ed.). Elsevier.
Edelstein, J. E., & Moroz, A. (2011). Lower-limb prosthetics and orthotics: Clinical concepts. SLACK Incorporated.
World Health Organization. (2003). ICF checklist: Version 2.1a, clinician form. World Health Organization.