Lecture 10: AFO Design Principles

Patient data collected during patient evaluation

• MMT (UE and LE)

• ROM

  • Contractures

  • Ligamentous laxity

  • With fracture -No ROM unless directed by physician

• Tone (Can it be broken or utilized?)

• Ambulatory Requirements

  • Balance

  • Proprioception

  • Foot Placement

• Shoes (heel height important with joint stops

Levels of Ambulation Goals

• Transfers

• Assisted Ambulation

  • Human

  • Walker

  • Cane

• Terrain

  • Indoors, smooth, flat, watch for carpets

  • Outdoors, uneven terrain, cracks, barriers

AFO Biomechanics

• M/L – Coronal

• A/P – Sagittal

• Rotation – Transverse

• Gait

When to Accommodate vs. Correct

• Rigid deformity → Accommodate

• Flexible deformity → Correct

AFO Footplate Trim Lines Options

1. Proximal to metatarsal heads (3/4 length)

2. To toe sulcus

3. Full length

When is Proximal to metatarsal head trimline appropriate?

• Low Tone

• No need to control forefoot

• Easier to don into shoes

• Not dysvascular / Has protective sensation

When is toe sulcus trimline appropriate?

• Moderate to high tone, no toe clawing

When is full length trimline appropriate?

• High tone with toe clawing

• Better choice for dysvascular and insensate patients

• Control all foot segments

• More difficult to fit in shoe

AA-AFO

• Angle of the Ankle in the Ankle Foot Orthosis

• angle between the line of the shank relative to the line of the foot

  • The line of the foot is defined as the line between the base of the heel and the most inferior point of the foot under the fifth metatarsal head. It is described in degrees of plantarflexion, dorsiflexion or as plantigrade

• The physiologic angle at which the AFO holds the ankle in the sagittal plane with no forces applied.

SVA

• Shank-to-Vertical Angle

• the angle between the shank and the foot that is formed when the AFO is loaded while the patient is standing

Optimal SVA for Normal Gait

An SVA of 10–12 degrees inclined is considered the optimum position of the shank in midstance for normal gait, with a range of 7-15 degrees considered normal.

Casting considerations

• Traditional tibial angle to floor is equal to about half proper SVA.

  • If SVA goal is 10-12 degrees, then roughly 5-7 degrees tibial inclination should be the casting goal

• Clinically, a vertical tibia during casting is usually closer to 7+ degrees of tibial inclination/SVA

  
  

Bent knee position/crouch gait

• Strong quads

• Plantar flexors too weak

  • Too much tibia advancement

  • Too much dorsiflexion

  • Test mmt single leg heel raise

    • Trans met amputee…low grade, missing leverage

    • Number they have to hit varies (6 to 20)

      
      

3 Point Force to Create Knee Extension / Resist Knee Flexion

AFO must stop dorsiflexion at late stance to create knee extension moment

1. posterior force by strap or anterior shell

2. downward force by shoe

3. GRF upward into heel

3 Point Force to Create Knee Flexion / Resist Knee Extension

• AFO must stop plantarflexion at loading response to create knee flexion moment

• AFO only effective until midstance

• Must have adequate quadriceps strength

• A tall posterior trimline is crucial

  • Cant go into popliteal but can get close

• Mainly at IC, loading response

  • Need good quads to control

• Doesn’t work once GRL ant to knee

• Weak plantarflexors, resist tibia falling by locking out knee keeping GRL ant

1. anterior force at back of knee

2. posterior force at ankle

3. upward force at metheads

Indications for Conventional AFO

1. Unusually Large or Heavy Individual

2. Minimal Contact of the Leg is Desired

3. Fluctuating Edema

4. Heat Sensitive Individual

5. Change of Size Anticipated

Conventional AFO Metal Components

1. Sidebars

2. Joints

3. Stirrup

4. Pins/Springs

Types of Stirrups

1. Standard

   1. permanently mounted to the shoe

2. Long Tongue

   1. permanently mounted to the shoe

3. Split Stirrup with Caliper Plate

   1. caliper plate needs to be permanently attached below the sole of each shoe the patient wishes to wear with the AFO

   2. Stirrup then inserted into caliper plate

4. Insert Stirrup

   1. a rigid foot insert will be used as the base of the AFO

   2. attached to the lateral and medial borders of the foot insert

   3. no shoe modifications

Conventional Ankle Joints

1. Free motion

2. Klenzak

3. Single Action/Single Channel

4. Double Action/Double Channel

Pins vs. Springs (where you put them, what they do)

• Springs

  • In Posterior Channel

    • Plantarflexion resist (Pf. Res.)

    • Dorsiflexion assist (Df. Asst.)

• Pins

  • In Posterior Channel

    • Plantarflexion stop (Pf. Stop)

  • In Anterior Channel

    • Dorsiflexion stop (Df. Stop)

Ankle Joint Selection

1. Free Motion

2. Dorsiflexion Assist

3. Plantarflexion Stop

4. Dorsiflexion Stop

5. Rigid Ankle

Thermoplastic AFO Indications

1. Accommodates Anomalies

2. Maximum Contact for Control of the Leg/Foot is Desired

3. Lightweight

4. Allows Patient to Easily Use Multiple Shoes

5. Aesthetically More Acceptable Than Conventional AFO

Solid Ankle AFO

• Ankle rigid in sagittal plane

• Ankle will not plantarflex during swing phase or loading response (Generates moderate knee flexion moment)

• Will not dorsiflex under loading from mid to late stance phase (Generates knee extension moment, depending on the length of the foot plate.)

• Good coronal stability

Solid Ankle Indications

• high tone

  • Trimline to sulcus or full toe extension

Solid Ankle Analogous to what pin and spring configuration

to pins in anterior and posterior channels of conventional double action ankle joints

AFO – PLS

• Dorsiflexion-Assist

• Ankle flexible in the sagittal plane

• Rigid enough prevent plantarflexion during swing phase (Prevent foot drop)

• Flexible enough to plantarflex at loading response (Generates minimal knee flexion moment)

• Flexible enough to dorsiflex under loading from mid to late stance phase (Smoother gait)

AFO – PLS Indications

• dropfoot

• Not indicated for high tone

• Minimal coronal stability

PLS Analogous to what pin and spring configuration

spring in posterior channel of conventional double action ankle joints

AFO Articulated (with posterior stop)

• Ankle freely dorsiflexes

• Ankle will not plantarflex during swing phase or loading response (Generates knee flexion moment)

• Maintains neutral ankle position during swing phase

• Good coronal stability

• Will freely dorsiflex during late stance phase (Generates no knee extension moment)

  • Increases ease of standing from sitting

AFO Articulated (with posterior stop) Indications

• High tone

AFO Articulated (with posterior stop) PLS Analogous to what pin and spring configuration

• to nothing in anterior channel and pin in posterior channel of conventional double action ankle joint

Thermoplastic AFO Joints

• Oklahoma

• Tamarack (Straight or Dorsi Assist)

• Gillette (Straight or Dorsi Assist)

• Gaffney - Steel

Thermoplastic Motion Stops

1. Adjustable plantar flexion stop

2. Combination dorsiflexion and plantar flexion stop

3. Polypropylene plantar flexion stop

4. Posterior strap limits dorsiflexion

Ground Reaction AFO

• Resists forward progression of the tibia over the foot through ground reaction forces. 

• typically used to substitute for weak plantarflexors and provide a knee extension moment at late stance by stopping dorsiflexion

• The rigid anterior shell and tall anterior trimlines provide more resistance to knee flexion and a more comfortable distribution of forces on the anterior of the leg as compared to a strap

Ground Reaction AFO 3 Point System

• patient bears weight on the footplate of the GRAFO, the GRAFO resists rolling forward

• As the patient moves past midstance, the AFO resists the forward progression of the tibia over the foot and the knee extension moment is created

• The effectiveness of the GRAFO’s knee extension moment will be affected by the length of the footplate – a longer footplate will increase the knee extension moment.

Ground Reaction AFO - Indications

• Mild Knee instability (Minimum Muscle Grade of 4)

• Weakness of Plantar Flexors

  • Stops tibial progression at late stance

• Weakness of Dorsi and Plantar Flexors

  • Stops tibial progression at late stance

  • Resists plantar flexion (assists dorsiflexion) during swing to clear foot

  • Holds ankle in neutral for heel first initial contact

Axial Resist AFO

• is designed to reduce weightbearing on the plantar surface

• to achieve this reduction, a constrictive calf section or Patella Tendon Bearing (PTB) modification is incorporated to suspend some of the patient’s weight

• The PTB design is borrowed from the PTB prosthetic socket design

Axial Resist AFO Indications

• Instability of the Ankle in all Planes

• Axial Unloading??

• Fracture Management

• Charcot Joint

Supramalleolar AFO

• variety of different AFOs that have a reduced height calf section

• may or may not have ankle joints, and can be fabricated for various levels of rigidity

• often indicated for patients with pes planusI

  • children - often due to neuromuscular disorders

  • adults - often a result of posterior tibial dysfunction or rupture

• They are best at controlling coronal motion and are not an effective lever arm for sagittal control of the ankle

  • For medial lateral not sagittal plane

    
    

Varus / Valgus Control Additions

• Trim lines

  • Ex: Sabolich trim line

• Straps

  • T-straps (conventional)

• Pads

Neurological Foot Plates

• tone-reducing, tone-inhibiting AFOs

• Characteristics

  • Full length footplate

  • Elevated toe bed

  • Metatarsal relief

  • Loading of transverse arch (shafts of metatarsals)

  • Calcaneal relief

Composite AFOs

• can be custom fit (off the shelf) or custom fabricated

• involve a varying layup of carbon, fiberglass, and other fibers to achieve a strong and lightweight orthosis

• Some include a posterior strut that can be changed based on patient presentation to be more flexible or more stiff, such as the PDE strut or Coyote Dynamic Strut.

Custom Fit

• typically are useful when attempting to provide a lightweight device to assist with weak dorsiflexors

• Most manufacturers have varying sizes (ie: small, medium, large) and provide either posterior or anterior calf section

• Some manufacturers provide the strut on the medial side of the foot or lateral side of the foot

• These orthoses are limited in the modifications that can be made to them, and are typically limited to trimming the footplate to the proper length.

Custom Carbon AFOs

• sometimes called dynamic AFOs at times

  • ‘‘dynamic’ can = AFOs with tone-reducing modifications, AFOs with articulated motion, and is often used in the naming of prefabricated AFOs

• The designs depicted above are custom fabricated and require specialized casting, modification, and fabrication techniques

• These AFOs are based on the same concepts as prosthetic dynamic response feet and are intended to provide a dynamic push off and energy return

• These orthoses provide added stability during stance phase, as well as, during swing phase.

AFO Clearance Guidelines

1. Proximal trimline

• 20 mm below distal border of head of fibula

• 25 mm distal to apex of fib head

2. Ankle joint – conventional skin to metal

• Medial: 6 mm

• Lateral: 5 mm

3. Thermo plastic buildup not a textbook answer

• 3mm minimum