Articular congruence, not neutrality: a systematic review of how native knee geometry shapes load-dependent alignment.

BACKGROUND: Native knee alignment under load is not a neutral, static state. Medio-lateral (M/L) asymmetry of articular congruence and compartmental containment necessarily creates a load-dependent functional joint-line orientation; in parallel, antero-posterior (A/P) contact-centroid position governs sagittal tilt and translation. This review maps the native-knee literature describing these coronal and sagittal mechanisms.

METHODS: A systematic search of a large academic corpus was performed and followed by manual bibliographic verification against PubMed, publisher pages, DOI records, and original metadata when available. Adult native-knee in-vivo imaging and intact cadaveric studies were examined. Measures included joint-line obliquity (JLO), frontal alignment, M/L contact behavior, contact area/pressure, meniscal extrusion, sagittal geometry, posterior tibial slope (PTS), tibial translation, and A/P contact behavior. Studies were mapped by mechanistic domain rather than ranked as evidence classes.

RESULTS: Thirty-nine candidate records were examined and all were reported in a study-by-study evidence map. The literature consistently supports the principle that native-knee geometry shapes load-dependent alignment. Weight-bearing imaging showed that the medial proximal tibial articular surface becomes more parallel to the ground under load. Axial compression produced varus orientation and medial translation in early flexion. Varus/valgus moments and imposed JLO amplified the same coronal mechanism into degree-level tilt, posterior medial condylar shift, contact redistribution, and meniscal loading changes. Sagittal studies showed that PTS, medial/lateral slope asymmetry, and A/P tibial position are linked to loading, rotation, and subluxation patterns.

CONCLUSIONS: The phenomenon is best interpreted as a geometric load-response of the native knee: unequal M/L contact heights produce coronal inclination, and A/P contact-centroid position produces sagittal behavior. The current literature provides multiple convergent manifestations of this mechanism across WB imaging, axial compression, cadaveric stress testing, JLO manipulation, sagittal geometry, meniscal mechanics, and dynamic in-vivo studies. Clinically, this framework is most directly applicable to native knees with preserved meniscal containment and intact or mildly degenerative compartmental congruence, where static alignment phenotypes should be interpreted together with the load paths that produce them. Future work should directly co-measure ΔJLO, Δh, A/P contact position, and early-flexion tilt in the same standardized protocol.