Orthosis — Definition & Types Overview

Definition

An orthosis (pl. orthoses) is an externally applied device designed to modify the structural or functional characteristics of the neuromusculoskeletal system. The four core purposes are:

Immobilization — restrict motion to protect injured/healing structures
Mobilization — assist or enable movement (dynamic orthoses)
Restriction — limit a specific range of motion
Torque transmission / load redistribution — alter forces acting on a joint or segment

Orthoses are named by the body segments they span (e.g., AFO = ankle-foot orthosis).

Classification by Body Region

Lower Limb

Abbreviation	Full Name	Key Uses
FO	Foot Orthosis	Align/support foot; three subtypes: rigid (limit motion), semirigid (support + shock absorption), soft (accommodate fixed deformities, neuropathic feet)
AFO	Ankle-Foot Orthosis	Most commonly prescribed lower-limb orthosis; footdrop, plantar spasticity, spinal cord injury; can be rigid or articulating; fabricated with metal bars or thermoplastic elastomer (TPE)
KAFO	Knee-Ankle-Foot Orthosis	Upper thigh to foot; unstable knee joint; mediolateral stability with flexion/extension control; various knee lock designs
HKAFO	Hip-Knee-Ankle-Foot Orthosis	Hip and pelvic stability; rarely used in adult paraplegics due to cumbersome nature; used in children with upper lumbar myelomeningocele (reciprocating gait orthosis)

Upper Limb

Abbreviation	Full Name	Key Uses
EO	Elbow Orthosis	Hinged (ligament instability), dynamic spring-loaded (flexion/extension contractures), elbow strap (lateral epicondylitis)
WHO	Wrist-Hand Orthosis	Post-injury/reconstructive care; static, static-progressive, or dynamic; wrist-driven devices for lower cervical quadriplegia (tenodesis-powered or motor-driven)

Spine

Region	Device	Details
Cervical	Soft collar → rigid collar → cervicothoracic orthosis (CTO) → halo vest	Range from basic motion reduction (C2–C5) to near-complete immobilization (halo vest); CTOs control C5–C7 and restrict flexion better than extension
Thoracolumbar	TLSO (thoracolumbosacral orthosis), corsets, rigid braces	Reduce back pain via three-point pressure mechanism and increased body cavity pressure; three-point orthoses use lever arm length to limit motion

Fracture Braces

Prefabricated fracture orthoses: used for simple foot/ankle fractures, ankle sprains, simple hand injuries
Tibial/fibular fracture bracing: remains a valuable non-operative option

Pediatric Orthoses

Pavlik harness — mainstay for developmental dislocation of the hip (DDH)
Dynamic orthoses for Perthes disease containment
Reciprocating gait orthoses (modified HKAFOs) for children with myelomeningocele

Material Types

Material	Properties	Use Case
Rigid (metal, hard plastic)	Maximum control, least accommodation	Fracture immobilization, severe instability
Semirigid (semi-flexible plastics)	Balanced support and shock absorption	AFOs, functional ankle bracing
Soft (foam, leather, neoprene)	Best shock absorption, accommodating	Neuropathic/dysvascular feet, mild support

Key Biomechanical Principles

Three-point pressure system: two forces in one direction, one corrective force in the opposite; fundamental to spinal orthoses and many limb braces
Ground reaction force (GRF) management: AFOs used post-hindfoot fusion to absorb and redirect GRF
Trimline design in TPE AFOs: full/anterior, intermediate, or posterior trimlines determine degree of mediolateral control
Tenodesis action: wrist-driven hand orthoses harness natural tenodesis to allow functional grasp in cervical SCI patients

— Miller's Review of Orthopaedics, 9th Edition, p. 887

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