A Single Axial Impact Load Causes Articular Damage That Is Not Visible with Micro-Computed Tomography: An Ex Vivo Study on Caprine Tibiotalar Joints

Objective: Excessive articular loading, for example, an ankle sprain, may result in focal osteochondral damage, initiating a vicious degenerative process resulting in posttraumatic osteoarthritis (PTOA). Better understanding of this degenerative process would allow improving posttraumatic care with the aim to prevent PTOA. The primary objective of this study was to establish a drop-weight impact testing model with controllable, reproducible and quantitative axial impact loads to induce osteochondral damage in caprine tibiotalar joints. We aimed to induce osteochondral damage on microscale level of the tibiotalar joint without gross intra-articular fractures of the tibial plafond. Design: Fresh-frozen tibiotalar joints of mature goats were used as ex vivo articulating joint models. Specimens were axially impacted by a mass of 10.5 kg dropped from a height of 0.3 m, resulting in a speed of 2.4 m/s, an impact energy of 31.1 J and an impact impulse of 25.6 N·s. Potential osteochondral damage of the caprine tibiotalar joints was assessed using contrast-enhanced high-resolution micro-computed tomography (micro-CT). Subsequently, we performed quasi-static loading experiments to determine postimpact mechanical behavior of the tibiotalar joints. Results: Single axial impact loads with a mass of 15.5 kg dropped from 0.3 m, resulted in intra-articular fractures of the tibial plafond, where a mass of 10.55 kg dropped from 0.3 m did not result in any macroscopic damage. In addition, contrast-enhanced high-resolution micro-CT imaging neither reveal any acute microdamage (i.e., microcracks) of the subchondral bone nor any (micro)structural changes in articular cartilage. The Hexabrix content or voxel density (i.e., proteoglycan content of the articular cartilage) on micro-CT did not show any differences between intact and impacted specimens. However, quasi-static whole-tibiotalar-joint loading showed an altered biomechanical behavior after application of a single axial impact (i.e., increased hysteresis when compared with the intact or nonimpacted specimens). Conclusions: Single axial impact loads did not induce osteochondral damage visible with high-resolution contrast-enhanced micro-CT. However, despite the lack of damage on macro- and even microscale, the single axial impact loads resulted in “invisible injuries” because of the observed changes in the whole-joint biomechanics of the caprine tibiotalar joints. Future research must focus on diagnostic tools for the detection of early changes in articular cartilage after a traumatic impact (i.e., ankle sprains or ankle fractures), as it is well known that this could result in PTOA.