Robotic-assisted knee arthroplasty methods provide insight into Homo neanderthalensis gait and the evolutionary origins of modern human knee variation

Lower-limb alignment is central to human bipedal locomotion and a key determinant of joint kinematics and osteoarthritis risk in modern clinical practice. However, its significance in hominins, and therefore its evolutionary implications, remain poorly understood. Advances in robotic-assisted total knee arthroplasty (TKA) provide highly precise measurements of lower-limb alignment, joint morphology and soft-tissue balance that can elucidate these understudied aspects of hominin functional morphology. Here, we perform a proof-of-concept robotic-assisted TKA on an anatomically accurate model of the Spy II Neandertal. Using a robotic-assistance system, we registered bony morphology and replicated soft-tissue laxity extrapolated from a modern patient with a comparable alignment phenotype. We confirmed a pronounced constitutional varus alignment, classifying Spy II as a CPAK type I knee. This phenotype resembles that of modern athletes engaged in significant weight-bearing sports or intense physical activity during growth. The demonstration of constitutional varus in a hominin fossil thus suggests a deep evolutionary origin for this alignment pattern in modern humans, contextualizing normal knee variation and supporting an individualized alignment approach to modern TKA.