Three-dimensional biometrics using weight-bearing imaging shows relationship between knee and hindfoot axial alignment.

Background Existence of a relationship between knee and hindfoot alignments is commonly accepted, but not clearly proven. While studied in the coronal plane using 2D imaging, axial alignment has not been studied yet, likely requiring 3D measurements. We aimed to investigate how knee and hindfoot rotational alignments are related using 3D biometrics and modern 3D weight-bearing technologies. Hypothesis Hindfoot Alignment is correlated with femoral and tibial torsions. Patients and Methods All patients who underwent both weight-bearing CT (WBCT) and low dose biplanar radiographs (LDBR) were selected in this retrospective observational study, resulting in a cohort of 157 lower limbs from 99 patients. Patients' pathologies were stratified in subgroups and those with a history of trauma or surgery affecting lower limb alignment were excluded. Foot Ankle Offset was calculated from WBCT; femoral and tibial torsions and coronal alignment were calculated from LDBR, respectively. Results Overall, mean Foot Ankle Offset was 1.56% (SD 7.4), mean femoral anteversion was 15.6° (SD 9.5), and mean external tibial torsion was 32.6° (SD 7.6). Moderate negative correlation between Tibial Torsion and Foot Ankle Offset was found in the whole series (rho=-0.23, P=0.003) and for non-pathologic patients (rho=-0.27, P=0.01). Linear models to estimate Tibial Torsion with Foot Ankle Offset and conversely were found, with a low adjusted R² (3%<R²<7%). No relationship was found between FAO and femoral torsion. Discussion External tibial rotation was associated with varus hindfoot configuration in the group without pathologies, suggesting that compensatory mechanisms may occur between knee and hindfoot alignments. In pathological cases, however, the same relationship wasn't found, raising concerns about compensatory failure in spite of the numbers available. We didn't find similar correlations with the femur possibly because the hip has a degree of liberty in the axial plane. Level of evidence: III, retrospective comparative study.