Subtalar joint axis alignments in pathological feet of children with cerebral palsy

Children suffering cerebral palsy (CP) often develop foot deformities [1]. These manifest as pathological postures including equinovarus, planovalgus non-midfoot break (PNMFB) and midfoot break (MFB) [2]. Although the mechanism for the development of foot deformity is poorly understood, recent research has highlighted how sensitive muscle moment arms [3] and joint moments are to the orientation of the subtalar joint (STJ) axis. Both are contributors to foot deformity. Studies have demonstrated a large variability in STJ axis orientations in healthy populations [4] and it is hypothesised that the variability in deformed feet will be even higher and correlate with specific deformities. How do STJ axis orientations in CP children with equinovarus, PNMFB and MFB deformities compare with typically developing children? Weight bearing (WB) and non-weight bearing (NWB) cone beam CT images of 21 feet from 17 CP patients (8 equinovarus, 7 PNMFB, 6 MFB, aged 12-17) and 7 feet from 7 typically-developing controls (aged 7-16) were acquired using a Verity (Planmed Oy) and Multitom Rax (Siemens) CBCT systems. Foot bones were semi-automatically segmented using Mimics 24.0, Materialize or Disior Bonelogic and remeshed to 1.0 mm isotropic edge length (OpenFlipper 4.1). Using the STAPLE pipeline [5], spheres were fitted to the talar head and talocalcaneal articulating surfaces and a cylinder to the talocrural articulating surface. STJ axis was approximated by the line joining the two fitted spheres [6]. The talocrural joint axis was approximated by the cylinder fitted to the talocrural articulating surface. An anterior-posterior (AP) line was calculated as the cross product of the ground normal and the talocrural joint axis. For each participant, STJ axis medial deviation and inclination from the AP line was calculated. A 2-sample t-test was used to test for statistically significant differences between groups. Mean STJ axis orientation in healthy participants was 23.2±5.7° (inclination) and 22.0±4.3° (medial deviation, Fig. 1). Inclinations varied from 31.4±6.3° for equinovarus feet to 20.2±4.2 for PNMFB and 4.0±10.6° for MFB patients. Mean medial deviations were 32.7±10.5° (equinovarus), 25.4±6.5° (PNMFB), and 28.8±4.5° (MFB). Both MFB and equinovarus groups exhibited STJ axis medial deviation angles greater than healthy controls. However, where the equinovarus group demonstrated 8.2° (p<0.05) greater inclination angle than the healthy controls, the MFB feet exhibited inclination angles 19.2° lower (p<0.05).Download : Download high-res image (99KB)Download : Download full-size image Although the analysis shows clear groupings in STJ axis orientations, further analyses of a greater range of CP pathological feet are needed to confirm these differences between groups. The abnormal STJ axis orientations of the deformed feet imply that abnormal moments are present during gait, further contributing to deformity. In conclusion, there is a measurable difference between the STJ orientations between pathological CP and healthy feet. Understanding how these differences contribute to deformity will inform the development of effective interventions.