The effect of antipronation orthotics on ankle and subtalar joint kinematics

This is a short summary of a recent study Liu and co-workers published (2012) a study the effect of antipronation orthotics on ankle and subtalar joint kinematics. Some of our experience and knowledge is also added to the summary of this article.

Orthosis are fabricated to control rearfoot, ankle and subtalar joint, pronation. It has been found that the amount of subtalar joint eversion and abduction is reduced with 1-2°. This is only describes the amount of motion, but not the rate at which this is happening. Eversion and abduction velocity is therefore also studied, since this reflects the rate of loading of muscles and tendons. Traditionally these motions are considered to take place at the subtalar joint, however we know this not to be true. Studies have shown these motions are also taking place in the ankle, talocrural joint, and sometimes to a greater extent than at the subtalar joint. This might be due to individual anatomical differences, tibia/fibula geometry or stiffness of ligaments and capsules. Understanding what joints are controlled with orthotics is important since this can help us understand why orthosis are effective in some individuals while ineffective in others. The purpose of the study was to see if antipronation orthotics would impact the amount and rate of pronation and how the subtalar and ankle joint were affected.

Five subjects were recruited. A 4° medial rearfoot wedge were used for all subjects Intracortical pins were used in this study. This is the most accurate form of motion analysis, in that pins are drilled into the bones, rather than placed on the surface of the skin. A 12-camera motion capture system (Qualisys) was used to obtain the kinematic data at 240 Hz. The data was analyzed in Visual 3D with a standard definition of joint axes. Cardan angles were then calculated (sequence x, y and z). The data was recorded for five walking trials, however no mention of speed of the different subjects was made.

The results show that there are individual responses to anti-pronation orthosis. There was a reduced peak and range of rearfoot eversion and, to a lesser degree, abduction relative to the leg.

From the discussion we would like to quote the authors; there are many interrelated variables affecting subtalar and ankle kinematics, and therefore, a systematic kinematic response to a foot orthosis seems unlikely. The role of the orthosis cannot be to adjust foot position or motion to achieve a single optimum kinematic pathway or foot position as current orthotic paradigms suggest (based on concepts de- scribed in Root et al. [11]). Rather, the orthosis influences the external loads that determine the person-specific foot kinematics, and as a result, the orthosis causes the joints of the foot to pass through adjusted versions of a person’s underlying kinematic pathways.

Then authors conclude that; The antipronation foot orthosis produced small and un- systematic reductions in eversion and abduction of the heel relative to the leg at various times during stance. This was achieved via complex changes at the ankle and subtalar joints that were specific to each subject tested. These changes contradict existing orthotic paradigms and are indicative of a strong interaction between the ankle and subtalar joints.

We feel it is necessary to comment on this study and raise some important questions. In many scientific studies interventions or treatments are given to a group of subjects regardless of need. This is also the case here. Did all five subjects really need a 4° medial rearfoot wedge? We know this might be difficult to determine who needs what, but there is a need to have tests that can identify who needs what and why. Furthermore it would nice to have more information of how many steps were recorded, if only the right foot was recorded, and if so what was worn on the left foot, what was the average walking speed and range of walking speeds and why only five trials were recorded. We know that there is great variability between steps and the number of steps analyzed is important information in this regard.

Regardless of these comments, it is an interesting study pointing to important aspects of foot function and the use of orthosis