Clinical application and customization of a 3D-printed static ankle-foot orthosis

Krista Asoluka
October 9, 2023


Spentys offers a range of orthotic and prosthetic products, which we categorize in English alphabets from A1 – W2 and include both upper and lower limb orthotics and prosthetics. We launched our B series parts in 2022, which included the "static(B2) and dynamic ankle foot orthosis(B8)."

Ankle foot orthoses are used quite routinely in clinics and thus have a lot of ambiguous terminology. According to ​(Eddison et al., 2022)​, an AFO that blocks movement in all three planes is often termed a solid AFO (SAFO), a fixed AFO, or a rigid AFO. While our B2 AFO aligns with the criteria of a solid AFO, we have chosen to denominate it as a static AFO characterized by its trim lines through the malleoli. It's important to underline that our nomenclature distinguishes AFOs without articulations as static, in contrast to their dynamic counterparts, which incorporate articulations.

On the Spentys app, there are options for standard designs and custom designs, both of which give the clinician the autonomy to decide how they would like to vary the design of the static AFO from including loops, rivets, varying thickness, adding alveoli (holes) and even including supra malleolar boot or as we refer to as "TPU Boot."  

This application note goes beyond the basics, offering an in-depth exploration of advanced techniques and nuanced customization strategies. By delving into the intricacies of this AFO variant, orthotists can refine their skills and expand their understanding, ultimately leading to more tailored and effective treatments. This application note is a valuable resource, equipping experts with the insights needed to elevate their practice further and provide top-notch patient care.

Product Overview

B2 Options with TPU boot

Clinicians commonly prescribe an AFO for patients experiencing weakness in the ankle dorsiflexor or plantar flexor muscles. This is typically due to neurological or musculoskeletal disorders like stroke, cerebral palsy, spinal cord injury, and peripheral nerve injury. For simplicity, we shall refer to static AFOs as B2 models for the remaining part of this application note.  

The B2 model is predominantly applied to completely limit ankle joint movement in patients with foot drop, weak plantarflexion, and other foot and ankle biomechanical disorders that clinicians may find appropriate.

Standard Specifications: Our standard specification for a typical B2 model has varied thicknesses, with the global thickness being 3mm and the toe plate thickness being 1.5mm. The ankle trim lines go through the malleoli, and the foot trim lines on the medial border are just above the apex of the navicular and contain the 5th ray on the lateral border. At this point, it is essential to note that this model is designed primarily for pediatric use and patients weighing 45 Kg or below. The standard closing systems are an external belt loop style on the proximal calf and an integrated loop at the instep.

To enhance ankle and midfoot control, you can incorporate a thermoplastic polyurethane inlay with laces or without laces. This inlay can be either attached or detached from the B2 model.

Fact: Thermoplastic polyurethane (TPU) is an additively manufacturable polymer material with high flexibility, good hydrolysis resistance, excellent biocompatibility, and outstanding abrasion resistance. It can be used in structures that require increased flexibility, such as energy-absorbing structures  and wearable devices

Custom Specification: This allows you to specify all the parameters from height to trim lines, thickness, and global offset. It also allows you to carry out specific scan corrections that will give an overview in a later section. As with the standard specification, you can include a TPU inlay.  

However, you have the option to choose from either a full boot (encompasses the plantar surface of the foot) or a half boot that attaches along the medial and lateral border of the B2 model (does not contain the plantar surface of the foot)

Comparative Analysis: B2 Model vs. Traditional Polypropylene AFOs

While we advocate for a hybrid fabrication approach, it is essential to consider the traditional polypropylene SOLID AFO alongside the B2 model. The B2 model boasts distinct advantages over conventional counterparts, including:

  • Weight: Crafted using multi-jet fusion technology with PA11/PA12 material, the B2 model offers the necessary rigidity while weighing only half as much as conventionally oven-heated polypropylene (PP). This reduced weight is especially beneficial for pediatric patients with mobility concerns.
  • Thickness: Unlike uniform thickness in traditional PP solid AFOs, the varied thickness of the B2 models permits accommodation within the footwear and controlled deformation throughout the gait phases.
  • Design Flexibility: The realm of 3D-printed AFOs enables clinicians to incorporate intricate design elements like alveoli, a feat challenging to achieve with heat-moulded AFOs.

Material Properties: The table below gives you an overview of the properties of the materials used in traditional heat-moulded AFOs and 3D-printed AFOs

Table 1: Material properties

Case study

A 16-year-old female with left hemiplegic dystonia and choreoathetosis required an ankle-foot brace to assist in foot clearance during the swing phase and provide a stable base of support. The patient had previously used an off-the-shelf Posterior leaf splint AFO, which helped proprioception and balance because of decreased bulk weight and prevented excessive drop foot during the swing phase. However, this was ineffective in controlling ankle instability and was not rigid enough to account for the tone present in the patient.  

Gait assessment of patient

B2 AFO as a solution: Design reasoning

  1. Foot and ankle immobilization: We chose the B2 model with a fully flexible inlay to address weak dorsiflexion and subtalar joint instability.  
  1. Hindfoot correction: A lateral heelpost was added to the design to correct the hindfoot varus.  
  1. Reduced bulk: Integrated straps were added to minimize bulkiness and allow easy shoe insertion.  
  1. Breathability: Alveoli (perforations) were added for breathability and weight reduction.  
  1. Mediolateral stability: The moulded inner boot, composed of TPU material, was designed to provide mediolateral stability and flexibility while covering the malleoli.
  1. Material: The AFO was printed with PA12, which is nylon known for having good impact strength and allowing flexibility without fracture.  

Fabrication: The printed AFO and TPU inner boot was post-processed with vapor smoothing, further improving strength and flexibility. It also seals the surface sufficiently to prevent bacteria from getting trapped in the orthosis and proliferating.  


B2 Workflow
  1. Product Selection: The B2 (static AFO) with customized TPU inlay boot is chosen.
  1. Scanning: Using an iPad, a structured sensor scanner captures a positive cast scan.
  1. Correction: The Spentys app offers four correction options: no correction, form correction, standard correction, and custom corrections. Custom correction allows adjustments in heel height, malleoli offset, forefoot and hindfoot angle alignment, and ankle range.
  1. Modeling: Specifications are transferred to the Spentys designer for AFO design.
  1. Fabrication Specification: Local 3D printing employs MJF technology with PA12.


B2 with half TPU boot
  1. Autonomy of the orthotist: The orthotist could decide how exactly the AFO would be designed and could vary parameters as much as possible.
  1. Lightweight: The device is lightweight and easy to don with footwear.  
  1. Breathability: The device will not cause as much sweating because of the nature of the material and the addition of the alveoli, which helps prevent bacteria buildup

Commonly asked questions

  1. Are Spentys B2 Models clinically validated?

Yes, Spentys B2 Models are clinically validated. Our current validation process involves HP Multi Jet Fusion (MJF) printers. However, we are actively working on extending this validation to other 3D printing technologies. These validations ensure that our AFOs meet stringent clinical standards for safety, effectiveness, and patient satisfaction.

  1. How strong are the AFOs, and have they been mechanically tested?

Spentys AFOs, including the B2 Models, are engineered to be robust and durable. They undergo comprehensive mechanical testing to assess their strength and load-bearing capabilities. This testing ensures that our AFOs are built to withstand daily use and provide reliable patient support.

  1. How long does it take to order a B2 model on the Spentys App?

Placing an order for a B2 model on the Spentys App is a quick and efficient process. Once the required specifications are entered, the app generates an order summary and an estimated delivery timeline. This process typically takes just a few minutes, allowing healthcare professionals to obtain customized AFO solutions for their patients.

  1. Can you scan the patient directly rather than a mold?

Spentys offers the capability to scan patients using advanced scanning technology directly. This digital scan is the foundation for creating tailored B2 AFOs, enhancing precision and patient comfort. The process minimizes the need for traditional molds, streamlining the customization process.

  1. Are AFOs only 3D printed with HP MJF printers?

Spentys AFOs, including the B2 Models, have been validated for 3D printing using HP Multi Jet Fusion (MJF) technology. However, we are actively extending our validation efforts to include other 3D printing technologies. This commitment reflects our dedication to innovation and providing clinicians and patients with a broader range of options.

  1. What colors do they come in?

In Europe, HP MJF printers offer AFOs in black and white. However, it's important to note that practitioners in the US have access to a wider variety of colors due to the availability of different printer models. These color options can be tailored to individual preferences while maintaining the clinical efficacy of the AFOs.


​​Eddison, N., Healy, A., Buchanan, D., & Chockalingam, N. (2022). Standardized classification system for bespoke thermoplastic ankle foot orthoses. The Foot, 53, 101924.

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