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The Effect of Frontal Plane Position on First Ray Motion: Forefoot Locking Mechanism.
Perez HR, Reber LK, Christensen JC. Foot Ankle Int. 2008 Jan;29(1):72-76
Quote:
BACKGROUND: Several joints in the foot have a locking mechanism that allows the foot to function as a rigid lever. The transverse tarsal joint (talonavicular and calcaneocuboid joints) has a locking mechanism that is well understood. The purpose of the study is to determine if the first ray also has such a locking mechanism.
METHOD: Five cadaver limbs were loaded onto a custom frame. The first metatarsal was attached to a jig that placed a force of 50 N in plantarflexion and dorsiflexion. The motion of the jig was measured with the first ray in three positions: maximally everted, neutral, and maximally inverted. No tendons were loaded to ensure that any change in motion was solely due to osseous position.
RESULTS: The average motion of the first ray for the three testing position was as follows: 7 mm in the everted position, 14 mm in the neutral position, and 18 mm in the inverted position. There was a statistically significant increase in range of motion from an everted position to a neutral position (p = 0.003). This increase in range of motion continued when the first ray was inverted compared to neutral, but not statistically significance (p = 0.07).
CONCLUSION: This study demonstrates that the frontal plane position of the first ray affects the sagittal plane motion. An everted position has the least mobility, and we hypothesize that this represents a closed-packed or locked position.
I caught this a couple of days ago and have been waiting to see if anyone took the bait: Well Understood !!
Jeff Christensen is one of my classmates from the CCPM, Class of 1983, and is a very bright fellow. However, you can't really assess whether a joint "locks" or "doesn't lock" with only placing a single magnitude of moment across the joint. In order to see if the joint truly does "lock" or not, a variable magnitude of force would need to be used to see if the joint continued to deform under increasing load. I'll tell you the results before the experiment is performed: no joint in the human body truly "locks" since all joints will deform further with increasing magnitudes of external moment applied across them.
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
no joint in the human body truly "locks" since all joints will deform further with increasing magnitudes of external moment applied across them.
Which is why we need to change to increased and decreased stiffness.
__________________ Craig Payne
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Terminology not withstanding, do you think that, as the PL plantarflexes and everts the 1st ray, this may be increased evidence that a 1st ray c/o increases the potential for the 1st ray to be allowed to become less complian to GRF.
It would be interesting to figure out the mechanism by which eversion of the metatarsal leads to a decrease in the range of motion. Perhaps, a twist in the ligaments.
I really detest the terminology of Locking of joints and close packed position of joints. Both of these terms treat the joint as black box whose inner workings can't be seen. If there is truly a reduction in range of motion, there must be an anatomical structure responsible for it.
I'm trying to figure out how they measured range of motion while maintaining frontal plane position of the metatarsal. The device that prevents frontal plane motion could easily limit sagittal plane motion.
It would be interesting to figure out the mechanism by which eversion of the metatarsal leads to a decrease in the range of motion. Perhaps, a twist in the ligaments.
I really detest the terminology of Locking of joints and close packed position of joints. Both of these terms treat the joint as black box whose inner workings can't be seen. If there is truly a reduction in range of motion, there must be an anatomical structure responsible for it.
I'm trying to figure out how they measured range of motion while maintaining frontal plane position of the metatarsal. The device that prevents frontal plane motion could easily limit sagittal plane motion.
Cheers,
Eric
Eric
I would suggest that the majority of the range of motion is about the NCJ. The MCJ is a more or less a plane joint and so tends to produce restisting moments in all planes. The TNJ is a section of a ball joint but its saggital plane RoM restricted by the spring ligament complex. The NCJ is a saddle joint and has a 'preffered' or more compliant axis of motion. That is it tends not to produce resisting moments in one direction and does produce resistive moments in another. Looking at a foot model the more mobile or more compliant position appears to be when the 1st ray is inverted, where inversion corresponds to the same motion as STJ / calcaneal inversion. That is when looking at the 1st ray from the rear view then inversion = clockwise rotation.
I would suggest that the majority of the range of motion is about the NCJ. The MCJ is a more or less a plane joint and so tends to produce restisting moments in all planes. The TNJ is a section of a ball joint but its saggital plane RoM restricted by the spring ligament complex. The NCJ is a saddle joint and has a 'preffered' or more compliant axis of motion. That is it tends not to produce resisting moments in one direction and does produce resistive moments in another. Looking at a foot model the more mobile or more compliant position appears to be when the 1st ray is inverted, where inversion corresponds to the same motion as STJ / calcaneal inversion. That is when looking at the 1st ray from the rear view then inversion = clockwise rotation.
Dave
Dave,
From my dissections, I would say that the amount of motion of the met-cun is about the same as the cun-nav joint. Both joints have ball and socket surfaces with a very large radius of curvature. There is no bony resistance to motion except when there is a force couple produced with the ligaments. What I mean by that is that the joint will move on the ball and socket surface until a ligament becomes taught. They become taught sooner when moving medial to lateral as opposed to moving dorsal to plantar. (Also medial-lateral motion is limited by the neighboring 2nd metatarsal.
It is possible that twisting the first ray could increase the tightness of the ligaments and that would produce the results seen in the study. However, I question whether there could be enough twist in the ray to limit motion in response to ground reaction force acting on the metatarsal head in a weight bearing foot. I think, the load from ground reaction force would tend to untwist the ligaments. Again, an anatomical explanation of the phenomenon must be found. That would help determine if this "locking" is clinically relevant.
First ray locking mechanism
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