Subtalar joint (STJ) range of motion:
It is not possible to measure the true range of motion of the subtalar joint due to its triplane axis of motion – but it is common to determine the frontal plane excursion of the posterior aspect of the calcaneus as a representation of STJ motion. The amount of frontal plane motion will depend on position of the STJ axis.
• patient prone, contralateral hip may need to be elevated by a pillow to as it is vital the posterior aspect of the calcaneus must be place in the frontal plane
• lateral side of the forefoot is grasped and the ankle joint is dorsiflexed to resistance
• the subtalar joint is maximally inverted and the angle between the bisections of the posterior aspect of the calcaneus and the leg is measured or observed
• the subtalar joint is maximally everted and the angle between the bisections of the posterior aspect of the calcaneus and the leg is measured or observed
• the range of motion is determined from this
Can be measured or observed as being adequate or inadequate.
Due to movement of the skin during inversion/eversion, it has been suggested that the bisection be redrawn with the calcaneus maximally everted and again with the calcaneus maximally inverted.
• must be a certain minimum range of motion for normal activity (less is needed for most normal daily activities, but higher amounts may be needed in some sporting activities)
• 20 degrees of inversion and 10 degrees of inversion is considered the normal range (will depend on sagittal plane orientation of STJ axis)
• however that assumed 2:1 ratio of inversion to eversion is no longer considered valid
• in the presence of a structural or functional variation that could cause excessive pronation at the STJ and a limited range of eversion compensatory motion will occur at the midtarsal joint
However – it is now considered that the non-weightbearing range of the STJ has no relationship to the dynamic weightbearing range showed that the amount of eversion is 30% greater during dynamic function). It may be more appropriate to assess the range of subtalar joint motion during maximum inversion and eversion during stance.
Reliability of clinicians measuring STJ range of motion is poor .
Sources of error:
• bisections not drawn accurately
• excessive skin movement during inversion and eversion (may need to redraw bisections with calcaneus maximally inverted and maximally everted)
• plantarflexing the ankle during subtalar joint pronation and supination (more motion occurs if the foot is allowed to plantarflex – Milgrom et al , 1985)
• internally or externally rotating the leg relative to the tractograph at the end ranges of motion