ASTM F2665

Purpose of the specification
The aim of ASTM F2665 is to define important design, material and performance characteristics for total ankle replacement prostheses.
The specification applies to talar and tibial components that together provide an articulating ankle replacement. It encompasses fixed-bearing and mobile-bearing systems and recognizes different levels of joint constraint.
Mechanical performance cannot be assessed solely from the properties of the raw materials. Implant geometry, manufacturing processes, fixation concept and component interaction can all influence the behaviour of the final device. ASTM F2665 therefore combines material requirements with functional and mechanical performance evaluations.

Mechanical performance
Components of a total ankle replacement are expected to withstand relevant static and dynamic physiological loads without compromising their intended function.
Fatigue testing is particularly important for components that may be exposed to repeated loading and partial loss of bony support. The specification recommends evaluating individual tibial, bearing and talar components under relevant loading conditions, including appropriate worst-case scenarios.
For tibial tray components, ASTM F2665 specifies fatigue testing of five specimens to 10 million cycles without failure. The exact loading conditions must be selected and justified for the implant design being evaluated.

Range of motion and constraint
The functional movement of the ankle replacement is another central aspect of ASTM F2665.
The prosthesis should provide sufficient dorsiflexion and plantar flexion for its intended function. The standard requires a range of motion of at least 15° in both dorsiflexion and plantar flexion under the defined non-clinical test conditions.
Testing should consider both neutral alignment and relevant worst-case misalignment conditions, since implant positioning can influence the available range of motion.
The degree of constraint of the prosthesis may also be evaluated. This includes the resistance of the implant to:

• internal-external rotation
• anterior-posterior translation
• medial-lateral translation
  These measurements help characterize how strongly the implant geometry restricts or permits movement between the tibial and talar components.

Contact area and contact pressure
Contact conditions between the bearing surfaces can provide useful information about the mechanical behaviour of the implant.
ASTM F2665 recommends determining contact area and contact pressure at different flexion angles. These measurements can be used to assess how loads are distributed across the articulating surfaces and to compare different implant geometries under consistent test conditions.
The results are particularly relevant when evaluating bearing design and potential regions of increased contact stress.

Modular connections
If the ankle replacement contains modular components, the integrity of the connecting mechanisms must also be evaluated.
Depending on the design, appropriate testing may include static or dynamic shear testing, bending testing, tensile testing, or a combination of these methods.
The aim is to demonstrate that the modular connection maintains sufficient mechanical integrity for the loads expected during use.

Wear performance
The wear behaviour of the articulating surfaces is an important part of the evaluation of a total ankle replacement.
ASTM F2665 requires functional wear testing of the tibiotalar articulation according to ISO 22622 unless a different approach or omission of wear testing is appropriately justified.
Wear is evaluated under simulated physiological conditions. Depending on the implant design, additional factors such as mobile bearing articulation, third-body abrasion or the characteristics of generated wear particles may also be relevant.
For new or different material combinations, the wear performance should be compared with an appropriate reference material combination or otherwise technically justified.

Coatings and bearing surfaces
For implants with metallic, hydroxyapatite or other calcium phosphate coatings, ASTM F2665 also addresses coating characterization.
Depending on the coating type, this may include evaluation of:

• coating composition and morphology
• mechanical strength
• adhesion-related properties
• fatigue behaviour
• other material-specific characteristics
  The specification also defines requirements for the surface quality of metallic and polymeric bearing surfaces. These requirements help ensure that the articulating surfaces are suitable for their intended function.

Evaluation
ASTM F2665 does not result in one single test value. Instead, the overall evaluation is based on the relevant mechanical and functional characteristics of the specific ankle replacement system.
The required testing depends on factors such as implant geometry, materials, bearing design, fixation concept and modularity. Worst-case configurations should be considered where appropriate so that the selected test specimens represent mechanically critical conditions.
Together, the different evaluations provide a comprehensive picture of the mechanical performance and functional behaviour of the total ankle replacement under controlled laboratory conditions.