Aims
Sagittal plane alignment influences stability and kinematics in total knee arthroplasty (TKA) yet lacks a pragmatic phenotypic framework. This study introduces the Sagittal Plane Alignment of the Knee (SPAK) classification, integrating medial contact point (MCP) and medial tibial slope (MTS) to characterize sagittal tibiofemoral morphology.
Methods
Sagittal alignment was prospectively analyzed in 309 knees (262 patients) undergoing robotic-assisted TKA. Robotic-assisted MCP was measured at 90°, 45°, and 0°, and MTS was measured using computed tomography. MCP at 90° (mean 62.1%; SD 5.3%) and MTS (median 6.1°; IQR 4°-8.5°) were used to define nine SPAK phenotypes. Validation analyses assessed changes in SPAK type following functionally-aligned (FA) TKA, differences in MCP at 0°, 45° and 90° according to SPAK type, and differences in sagittal laxity at 90° according to SPAK type.
Results
SPAK boundaries for MCP were anterior <55%, central 55-65%, posterior >65%; and for MTS, low <4°, neutral 4°-8°, and high >8°. Type V (central MCP, neutral MTS) was the most common phenotype (28.8%). Following FA TKA, SPAK distribution converged towards five phenotypes with Type V predominating (44.6%). FA anteriorized MCP at 90° (mean -6.5%; p<0.001) and 45° (mean -5.4%; p<0.001) and posteriorized MCP at 0° across all phenotypes (mean 8.1%, p<0.001). Posterior MCP phenotypes (III, VI, IX) demonstrated the greatest anterior MCP translation at 45° at 90° following FA TKA. Type IV demonstrated the greatest sagittal laxity at 90° whereas posterior phenotypes (III, VI) demonstrated the least. Following FA TKA, native SPAK type no longer predictive sagittal laxity, indicating normalization of sagittal kinematics.
Conclusion
SPAK provides the first sagittal phenotypic framework, integrating medial tibiofemoral position and tibial alignment. SPAK phenotypes demonstrated constitutional variability associated with MCP changes and sagittal laxity, whilst FA TKA normalized these patterns. As individualized surgical strategies evolve, SPAK provides a new platform to study sagittal knee morphology and phenotype-specific surgical strategies.
