Detailed University Study Guide: Dynamic Ankle-Foot Orthosis Support and Design

Mechanics and Design of the Posterior Leaf Spring (PLS) AFO

• General Definition: The posterior leaf spring, or PLS, ankle-foot orthosis (AFO) represents a category of bracing specifically designed with a thinner rear trim line.

• Trim Line Placement: The design of the PLS involves trim lines that fall behind the medial and lateral malleoli.

• Structural Components:

  • The bracing features a thinner posterior heel support.

  • This support widens as it moves superiorly into a calf band.

  • The calf band provides the necessary support for flexion.

• Flexibility and Control:

  • The PLS works to control plantar flexion, clinically relevant for managing foot drop.

  • It can assist with dorsiflexion because the bracing possesses inherent flexibility.

  • The thickness of the thermoplastic material is the determining factor for the degrees of resisted plantar flexion and dorsiflexion.

  • A typical design allows for a $\text{few degrees of motion}$.

Functional Biomechanics of the PLS and Carbon Fiber Composites

• Simulated Eccentric Control: The flexibility of the thermoplastic material allows the orthosis to simulate the eccentric contraction of the pretibial muscles.

  • This function assists with the controlled lowering of the foot during the loading response phase of gait.

  • This mechanical simulation is critical to prevent foot slap.

• Swing Phase Dynamics:

  • In the swing phase, the PLS prepositions the foot to prepare for initial contact.

  • It assists the limb with swing clearance to prevent toe drag.

• Stability Limitations:

  • Due to the narrow posterior upright and a relatively shallow heel cup, the PLS is not effective at stabilizing the calcaneus or the talus during the stance phase.

  • The trim lines are both posterior and inferior to the malleoli, which contributes to this lack of stabilization.

• Supplemental Stability: To address medial and lateral foot and ankle instability, the PLS can be supported with a custom UCBL foot plate.

• Primary Indications: Indications for the use of a PLS include:

  • Foot drop.

  • Peroneal nerve palsy.

• Carbon Fiber Composite AFO:

  • Works similarly to the PLS/AFO for patients presenting with foot drop due to limited activity in the anterior compartment muscles of the lower leg.

  • It assists with foot clearance through the swing phase.

  • Limitations: Carbon fiber bracing is limited in its ability to be modified. It also fails to provide adequate medial and lateral ankle support or maintain foot alignment for individuals with significant medial-lateral instability.

Articulating Ankle-Foot Orthosis with Dorsiflexion Assist

• Joint Mechanism: This type of orthosis features a joint (hinge double joint) designed to provide functional range of motion (ROM) into both dorsiflexion and plantarflexion.

• Stability and Alignment: It maintains the alignment of the joint while providing strong transverse and medial-lateral stability for the foot and ankle complex.

• Adjustability: The ankle joint can be adjusted using springs or pins to increase support or restrict motion.

• Gait Mechanics and Tibial Translation:

  • The joint is pre-positioned into dorsiflexion to assist with the forward translation of motion.

  • This supports tibial translation starting from the second rocker, moving through mid-stance, and continuing to terminal stance.

• Initial Contact and Swing:

  • Stabilization at the joint improves foot pre-positioning for initial contact.

  • The inclination toward dorsiflexion assists with limb clearance during the swing phase.

• Clinical Indications:

  • Impaired motor control at the ankle musculature.

  • Limited or absent dorsiflexion at initial contact.

  • Presence of a dynamic equinus deformity.

  • Need for tibial translation during sit-to-stand movements and stair navigation.

  • Presence of excessive hyperextension.

Double Upright Metal and Hybrid Orthosis Designs

• Design Variations: Variations include double upright metal supports or thermoplastic-metal hybrid ankle supports.

• Motion Control: Springs and set screws are utilized to assist or limit motion, allowing clinicians to change the alignment or the $\text{degrees of motion}$ the patient can access.

• Accommodating Edema: The traditional double upright design is often more accommodating for patients experiencing fluctuations in limb size due to edema.

• Disadvantages:

  • This design is generally less effective at controlling abnormal foot positions.

  • It increases the patient's risk for skin irritation caused by the metal joint surfaces.

• Supramalleolar Support: These designs can be supplemented with a flexible, super malleolar orthotic liner.

  • The liner maintains intimacy and congruence with the foot structure and external bracing.

  • It provides circumferential support, which is particularly beneficial for the pediatric foot.

Articulating AFO with Plantar Flexion Stop

• Indication for Spasticity: This design is indicated for patients with significant plantar flexor spasticity who might push through the support provided by a standard hour articulating ankle-foot orthosis.

• Fabrication of the Stop: The plantar flexion stop is created during the casting and fabrication process. It occurs when the posterior edge of the foot section and the calf section come into contact to physically prevent excessive plantar flexion.

• Mechanical Function:

  • Encourages a knee flexion moment from initial contact through mid-stance.

  • Provides transverse and medial-lateral stability for the foot and ankle complex.

• Gait Progression: Moving from mid-stance to terminal stance, the orthosis allows for ankle dorsiflexion to facilitate advancement onto the contralateral limb.

• Swing Support: Pre-positioning in slight dorsiflexion facilitates foot clearance during swing.

Rehabilitation Progression and Strapping

• Adjustable Posterior Strapping: Both standard articulating AFOs and those with plantar flexion stops can be supplemented with an additional strap on the posterior shell of the autopsies.

• Limiting Range of Motion: This strapping is used to limit the $\text{degrees of dorsiflexion}$ allowed.

• Role in Rehab:

  • The strapping is essential throughout the rehabilitation process.

  • It allows for incremental adjustments as the patient gains motor control and strength.

  • It supports the transition through dynamic surfaces, such as stairs, which require higher levels of stability.