It has been well documented that the ankle joint can be subjected to forces up to five times body weight during normal walking, escalating to ten times body weight during running (Brockett and Chapman, 2016). The health and functionality of the ankle joint are fundamentally predicated by the distribution and magnitude of contact pressures exerted on its tibiotalar articulating surfaces (Anderson et al., 2011). These pressure patterns play a pivotal role in both the immediate mechanical performance and long-term integrity of the cartilage, even minor alterations, such as those in the knee meniscus, can have profound implications on the peak stresses experienced by the cartilage in the knee (Delco et al., 2017, Nicolella et al., 2013). Abnormal articular ankle contact mechanics have been implicated as a determinant factor for joint degeneration in several ankle conditions, such as osteochondral lesions, malalignment, constitutional deformities, and ankle trauma (Dahmen et al., 2021, Lacorda et al., 2020, Lloyd et al., 2006, Ramsey and Hamilton, 1976, van Dijk et al., 2010). Research indicates that the combination of incongruity and instability in the ankle joints results in disturbances in the distribution of contact stress (McKinley et al., 2008). Specifically, studies have reported that an incongruent articular surface resulting from ankle trauma notably elevates contact stress levels, making the patient more susceptible to ankle issues and post-traumatic osteoarthritis (PTOA) (Anderson et al., 2011, Demetriades et al., 1998, Lloyd et al., 2006, Ramsey and Hamilton, 1976, Saltzman et al., 2005). It is therefore critical to better understand contact pressure variation and alteration throughout the articular surfaces of the ankle in order to better understand how to address and potentially prevent such pathologies. Moreover, differentiating normal variation from pathologic contact pressures requires a more thorough understanding of the normal articular ankle stress thresholds. Given the wide range of differences in ankle biomechanics among individuals, it is difficult to define a single standard for normality, as one arbitrary cut-off criterion may not apply to a population. (Brockett and Chapman, 2016). Several authors, therefore, have emphasized using the contralateral healthy side as an internal control and thereby the most ideal reference for comparison (Baumbach et al., 2022, Bhimani et al., 2020, Patel et al., 2019).

To the best of our knowledge, no studies to date have explored the normal range of contact stress and the side-to-side differences in the ankle joint. Finite element analysis (FEA) is currently regarded as the gold standard for assessing in-vivo articular contact mechanics (Anderson et al., 2008, Donahue et al., 2002, Henak et al., 2013, Kern and Anderson, 2015; Wendy Li et al., 2008, Li et al., 2008). However, its computationally demanding and time-consuming nature makes it impractical for clinical practice or for analysis of large cohorts (Abraham et al., 2013). Discrete element analysis (DEA) was recently validated as an accurate alternative to FEA for predicting articular ankle contact mechanics, mitigating the aforementioned flaws encountered in FEA (Anderson et al., 2010; Ivan Benemerito et al., 2020, Benemerito et al., 2020, Peiffer et al., 2023, Peiffer et al., 2023). Moreover, the three-dimensional (3D) geometry of the ankle joint required for patient-specific computational contact simulations has been traditionally derived from supine imaging (Ivan Benemerito et al., 2020, Benemerito et al., 2020, Li et al., 2022; Wendy Li et al., 2008, Li et al., 2008). As this does not represent the physiological condition of the ankle bones, previous studies had to assume the position of the loaded ankle bones in their simulations (Li et al., 2020; Wendy Li et al., 2008, Li et al., 2008, Verim et al., 2014). The emerging use of weightbearing cone-beam computed tomography (WBCT) overcomes these drawbacks by enabling imaging of both ankles during bipedal stance (Ashkani Esfahani et al., 2022, Burssens et al., 2018a). This advancement allows for an accurate 3D analysis of both ankle joints under loaded conditions (Peiffer et al., 2023, Peiffer et al., 2023). Additionally, given the aforementioned interindividual differences in ankle biomechanics, bilateral WBCT facilitates a direct comparison between left and right. These recent technologies open the door for establishing reference values for the contact mechanics of a normative ankle.

The aims of our study were two-fold: 1) To determine the presence of significant side-to-side articular contact differences in healthy ankles and 2) establish normative values for articular ankle contact parameters in the ankle. More specifically, our goal was to determine a reference range of the normal side-to-side differences.