Orthoses — Lower Limb (KAFO/AFO)

Orthoses are external devices applied to the body to support, align, prevent deformity, assist function, or offload specific anatomical structures. Lower limb orthoses range from simple insoles and ankle supports to complex knee-ankle-foot orthoses (KAFOs) used for paralytic conditions. A thorough understanding of the biomechanical principles, terminology, indications, and limitations of the most clinically relevant lower limb orthoses — the ankle-foot orthosis (AFO) and the knee-ankle-foot orthosis (KAFO) — is essential for orthopaedic surgeons involved in the rehabilitation of patients with neurological, musculoskeletal, and congenital conditions.

• Orthosis nomenclature: orthoses are named by the joints they span; the standard naming convention uses the body parts covered from proximal to distal: FO (foot orthosis — insole/arch support); AFO (ankle-foot orthosis — extends from the foot to below the knee); KAFO (knee-ankle-foot orthosis — extends from the thigh to the foot, spanning the knee); HKAFO (hip-knee-ankle-foot orthosis — extends to the pelvis, used for complete paraplegia); TLSO (thoracolumbar sacral orthosis — spinal); the suffix `O` = orthosis; `P` = prosthesis
• Biomechanical principles: orthoses work by applying three-point force systems (two forces in one direction, one force in the opposite direction) to correct or control limb alignment; the ground reaction force (GRF) is the upward force from the floor that passes through the foot and limb during walking; a ground reaction AFO (GRAFO) exploits this force to extend the knee during stance; understanding the GRF vector in relation to joint centres is fundamental to orthotic prescription

• KAFO — design and components: a KAFO extends from the thigh to the foot and includes: (1) a foot-ankle component (typically a solid AFO or articulated ankle); (2) metal uprights on the medial and lateral side of the leg running from the ankle to the thigh; (3) a knee joint mechanism (see below); (4) a thigh band/cuff for proximal fixation; (5) a thigh shell (for total-contact plastic KAFOs); the KAFO prevents knee collapse during stance and controls ankle position; it is bulkier and heavier than an AFO but necessary when quadriceps weakness prevents the patient from stabilising the knee during walking
• Knee joint mechanisms in KAFO: (1) Drop-lock knee joint: a simple free-swinging knee joint with a drop-lock ring that can be dropped down to lock the knee in full extension for standing and walking; the ring is raised to allow free knee flexion for sitting; the most common knee joint used in KAFOs for complete quadriceps paralysis; (2) Bail lock (posterior bail/Scottish roll lock): a posterior lever that automatically locks the knee in extension when the patient stands; unlocks when the patient sits back (the chair pushes the bail forward releasing the lock); more convenient than the drop-lock as it does not require manual operation; (3) Offset knee joint: the hinge is placed posteriorly to the knee joint centre; this positioning means the GRF passes anterior to the offset hinge during stance, generating a passive knee extension moment; allows the knee to swing freely in flexion without locking; used for patients with sufficient knee extension power to stabilise in terminal stance but who need assistive extension at initial contact
• Stance control (stance phase control) KAFO: the most technologically advanced KAFO knee joint; the knee is automatically locked during the stance phase of gait (preventing buckling) and unlocked during the swing phase (allowing knee flexion for toe clearance and a more natural gait pattern); examples: C-Brace (Ottobock — microprocessor-controlled; sensors detect stance vs swing phase and adjust knee joint stiffness in real time); Levitation (mechanical stance control); significantly improves gait pattern, reduces energy cost, and increases walking speed compared to locked-knee KAFOs; appropriate for active patients with incomplete paralysis or significant quadriceps weakness who can initiate swing phase knee flexion
• Indications for KAFO: complete quadriceps paralysis (spinal cord injury, post-polio syndrome); knee hyperextension (genu recurvatum — prevents the knee from buckling backwards); combined ankle + knee instability (Charcot neuropathic joints); muscular dystrophies (Duchenne — KAFOs prolong ambulation by several years); spina bifida (lumbar level lesions L3–L4 — quadriceps present but hamstrings/hip extensors absent); severe genu valgum or varum awaiting corrective surgery

• Total contact insole (TCI): a custom-moulded insole that conforms to the entire plantar surface of the foot; distributes weight-bearing pressure evenly across the foot; the key orthosis for diabetic neuropathic foot and Charcot foot; `off-loading` insoles with cavities or areas of reduced contact beneath pressure points (1st metatarsal head, heel) prevent plantar ulceration; in diabetes, peak plantar pressure beneath the metatarsal heads is the primary driver of neuropathic ulceration — TCI with metatarsal pad reduces peak plantar pressures by 30–50%
• UCBL (University of California Biomechanics Laboratory) insole: a deep heel cup insert that controls hindfoot valgus; the rigid heel cup forces the calcaneus into neutral position and prevents medial arch collapse; used for pes planus (flatfoot), posterior tibial tendon dysfunction (early stages), paediatric flexible flatfoot, and rigid-heel support post-calcaneal fracture
• Metatarsal pad / bar: a transverse pad placed behind the metatarsal heads in the insole; transfers weight posteriorly from the metatarsal heads; reduces pressure on the 2nd–4th metatarsal heads; used for metatarsalgia, Morton`s neuroma, and sesamoiditis

• AFO nomenclature: FO (foot only); AFO (ankle-foot — below knee); KAFO (knee-ankle-foot — full leg); HKAFO (hip-knee-ankle-foot — trunk to foot); named by joints spanned proximal to distal
• Posterior leaf spring AFO: mild-moderate foot drop (CPN palsy, mild spastic drop foot); flexible polypropylene; assists dorsiflexion in swing; allows plantarflexion at initial contact (normal heel strike); NOT for fixed contractures or severe spasticity
• Solid AFO ankle position: set in 5–10° dorsiflexion; pushes GRF anterior to the knee → knee extension moment → prevents knee buckling; plantarflexion (equinus) AFO pushes GRF posterior to knee → knee flexion moment → crouch gait; ankle position is the critical prescription variable
• GRAFO (ground reaction AFO): anterior tibial shell redirects GRF anterior to knee; generates passive knee extension moment; reduces quadriceps demand; for crouch gait in CP and quadriceps weakness; requires plantigrade foot and no fixed knee flexion contracture
• KAFO knee joints: drop-lock (manual lock/unlock; simplest; complete paralysis); bail lock (automatic lock on standing, unlocks with sitting); offset knee joint (posterior to joint centre = passive extension moment = hands-free); stance control KAFO (C-Brace — microprocessor; locks in stance, free in swing; most advanced; most natural gait)
• Charcot-Marie-Tooth (CMT): hereditary motor and sensory neuropathy; pes cavus + intrinsic muscle wasting + foot drop; AFO (PLS or solid) for foot drop; UCBL for cavovarus heel correction; may require surgical correction (plantar fascia release, calcaneal osteotomy, tendon transfer)
• Diabetic foot orthosis: total contact insole (TCI) redistributes plantar pressure; metatarsal pad reduces peak pressure at metatarsal heads; Charcot foot = total contact cast (TCC) in acute phase; CROW boot (Charcot Restraint Orthotic Walker) for chronic Charcot
• 3-point force system: all orthoses work via a 3-point force system; two forces in one direction, one opposing force; understanding this principle explains how each orthosis achieves its biomechanical effect on joint moments