New insights on the Neanderthal knee-joint loading environment from the endostructural configuration of the patellae from Krapina, Croatia

The Neanderthal patella is anteroposteriorly thicker and shows more symmetric articular facets associated with higher angles than in recent humans. While it does not appear to be a close relationship between the degree of patellar facet asymmetry and the configuration of the distal femur, which is similar among Neanderthals, fossil and recent humans, the functional implications of such differences are still debated. By using micro-focus X-ray tomography (isotropic voxel size range: 24-50 μm), we characterized the endostructural pattern of six Neanderthal patellae representing five individuals from the OIS 5e site of Krapina, Croatia, and compared the results to those from a sample of 31 patellae representing 22 adult recent humans of various origins and socio-economic patterns. For the cortico-trabecular complex and trabecular thickness, the Neanderthal values fall within the human range of variation. Conversely, in Krapina cancellous bone density (BV/TV) in the medial aspect is higher, and the degree of anisotropy in the inferior aspect is lower than measured in humans. Also, while a bone reinforcement in the lateral and superior aspects is present in humans, in Krapina it is only found laterally. Interestingly, as commonly observed in humans, in Krapina the BV/TV is higher in the left patella. The only other evidence available from an adult Neanderthal patella, Regourdou 1 (France, OIS 4), does not fully overlap the average signal recorded in Krapina. Endostructural differences of the patella thus reveal differing knee-joint loading environments between Neanderthals and humans, as well as inter-individual variation likely age-, sex- and/or physical activity-related.