The effect of the transverse location of the subtalar joint axis on foot kinetics and kinematics

Abstract

Background: Locating the subtalar joint axis (STJA) as per Kirby (1987), is a routine test used by clinicians during biomechanical assessment of patients. However, its validity in relation to peak plantar pressure (PPP), pressure time integral (PTI) and foot kinematics is poorly evidenced in literature. Aim: To investigate a possible relationship between the location of the STJA using the palpation technique as described by Kirby (1987) with foot kinetics, mainly PPP and PTI and foot multisegment kinematics. Methods: A non-experimental quantitative study, using a correlation research design and a deductive approach was employed in this research study. Twenty six healthy participants were recruited through convenience sampling and categorized into four groups according to the Foot Posture Index by an independent researcher. Participants were assessed for the location of the STJA using the palpation technique as described by Kirby (1987) and categorized into two groups ie. those having their STJA intersecting the longitudinal bisection of the foot (Group A), and those whose STJA did not intersect the longitudinal bisection of the foot (Group B). Foot kinetics, mainly PPP and PTI, were collected using a Tekscan HRMat™ and kinematic data using a 16 camera Vicon® Nexus Optoelectronic Motion Capture System through the Istituto Ortopedico Rizzoli multisegment foot model for both groups. Results: Participants with a medially deviated STJA exhibited a significant increase in PPP and PTI at the hallux (p=<0.0005), 2-5th metatarsal heads (p=0.032), medial calcaneus (p=0.001) and lateral calcaneus (p=0.004). Higher PPP values were also noted in the total forefoot area (p=0.018), and lateral midfoot area in Group B (p=0.029). As for kinematics, an increase in the total range of motion of the 1st metatarsal to ground motion was observed as the transverse location of the STJA was more medially deviated (p=0.024). Significant relationship was noted between the STJA angle and calcaneus to metatarsal foot segmental motion; where a significant increased dorsiflexion, increased inversion and decreased adduction motion was noted in individuals with a more medially deviated STJA. Moreover the calcaneus in relation to the shank was more adducted and remained longer in an everted position during stance phase in Group A. The forefoot was also more abducted in relation to the shank during stance phase in Group A than Group B.

Conclusion: This research study has revealed that individuals with varying location of the STJA have distinct pattern of foot motion and kinetics. Moreover these results demonstrate that the foot is a flexible structure where movements can occur independently and adjust according to foot postural changes and adaptations generated during locomotion. These results are congruent with previous literature relating foot posture characteristics with foot kinetics and kinematics especially with regards to results relating forefoot kinetics; kinematics of calcaneus to metatarsal segment; 1st metatarsal to ground kinematics and shank to calcaneus kinematics.