Defining sagittal knee phenotypes via monopedal static anterior tibial translation. Part 1: Translating weight-bearing sagittal position into clinical risk profiles.

Static anterior tibial translation (sATT) is a reproducible monopodal weight-bearing radiographic parameter that reflects the resting sagittal position of the tibiofemoral joint. Distinct from manual laxity tests that quantify passive displacement limits, sATT captures the functional equilibrium of the tibia under physiological load, representing the cumulative interplay of bony morphology, neuromuscular control, meniscal containment and time-dependent adaptations. Current literature frequently conflates passive laxity with weight-bearing sagittal position, leading to an incomplete understanding of why isolated soft-tissue reconstructions fail in biomechanically hostile environments. Elevated sATT manifests in three clinically distinct sagittal phenotypes with specific pathomechanisms: osseous anteriorization primarily related to increased posterior tibial slope and loss of the anterior cruciate ligament; acquired soft-tissue anteriorization resulting from sagittal decompensation due to chronic injury and secondary stabilizer, particularly meniscal deficiency; and inherent soft-tissue anteriorization characteristic of generalized hyperlaxity. This paper, Part 1 of a two-part series, establishes the biomechanical 'set-point' theory, highlights the necessity of strict monopedal imaging to unmask instability and defines how distinct sagittal phenotypes may converge, creating cumulative or synergistic risk patterns that increase the mechanical burden on the central pivot. LEVEL OF EVIDENCE: Level V.