Tweet
In short, and in my opinion, one of the major causes of plantar plate injury is likely weakness in the lumbrical and interosseus muscles. Reducing the risk of this type of injury would include strengthening these muscles.
Explanation
The lumbricals and interossei act to straighten the lesser toes at the interproximal joints . If these joints remain straight during gait then the flexor digitorum brevis( FDB) and flexor digitorum longus (FDL)will produce a plantar flexion moment at the MTPJ. This will support the joint and share load with the plantar plate, which is considered a continuation of the plantar fascia. If the toes flex at the interproximal joints then the situation is very different and these muscles, FDB and FDL , will act not to plantarflex the proximal phalanx at the MTPJ but to dorsiflex it . This puts the MTPJ in a pre-dorsiflexed position and makes it more susceptible to hyperextension and overload.
Below is an image of a hammer toe.This configuration can be achieved by contracting the FDB in the absence of effective contributions from the lumbricals and interossei. In the image, the 2nd phalanx is being plantar flexed at the proximal interphalangeal joint . This causes the proximal phalanx to dorsiflex .This dorsiflexion is opposed by tension in the plantar plate( a continuation of the plantar fascia ) when the foot is under load, and so ground reaction forces are generated under the distal part of the toe . Remove the load on the foot, and therefore the PF and plantar plate, and very little force is generated under the distal part of the toe.
Looking at the image ,the longest bone is being plantarflexed by the plantar plate/fascia . However, activity in the FDB , which is inserted into the middle bone , flexes this bone down to the ground causing the long bone not to plantarflex, but to dorsiflex as is seen in the image . Contraction of the Flexor digitorum brevis will generate ground reaction forces only as long as tension in plantar plate and hence the long bone gives it something to work against to work against . Rupture of the plantar late would give a floating toe.
If the lumbricals and interossei are strong enough to keep the interproximal toe joints extend during foot loading, then the FDB will produce a plantarflexing moment at the MTPJ ,reducing tension in the plantar plate. However, if they are weak and collapse ,(" functional lesser toe deformity" ),then the FDB will produce a dorsiflexing moment at the MTPJ ,increasing tension in the plantar plate.
image.png
Extract from podiatry arena
The linked to video, below ,from Eric Fuller does a good job of illustrating the action of the flexor hallucis brevis (FDB)) when this is the only structure applying a force to the bones of a lesser toe other than ground reaction forces, that is to say when plantarflexing forces from the plantar fascia are removed from the equation along with the straightening effect of the interossei and the lumbricals. All that happens in the model is the middle phalanx is plantarflexed at the proximal IP joint and this causes the proximal phalanx to dorsiflex at the MTPJ. No significant ground reaction forces would be produced beneath the "bones "of a subject toe at all .
So you could say that lack of tension in the plantar fascia produces a floating toe, and no amount of FDB strengthening will change that since , without the lumbricals or interossei, the FDB does not produce a plantarflexing moment around the MTPJs ( see video)
If we added a plantar fascia to Erics model then the situation would change . The plantar fascia would apply a plantarflexing moment ( force) to the proximal phalanx . The FDB, when active, would plantarflex the middle phalanx at the proximal IP joint and this would cause the proximal phalanx to tend to dorsiflex at the MTPJ. The proximal phalanx is being "held down to the ground "( plantarflexed) by tension in the plantar fascia so when the middle phalanx is plantarflexed by the action of the FDB , ground reaction forces will now be developed ( I should probably provide a drawing here ).
In a healthy foot with strong active lumbricals and interossei, things change yet again .Toes which remain straight (extended) at the proximal IP joint when the FDB is activated will not causes dorsiflexion at the MTPJ but plantarflexion . The forces being generated by the FDB will compliment and add to those being generated by tension in the plantar fascia . They will not be opposing forces.
Much the same sort of mechanics applies to the forces generated by the flexor digitorum longus . If we have a plantarflexing moment of the proximal phalanx due to tension in the PF, then flexion at the IP joints caused by FDL activity will see a net dorsiflexing moment at the MTPJs with forces from the fascia being overpowered by the dorsiflexing action at the MTPJs of FDL activity . Keep the IP joints extended and the FDL will act to plantarflex the entire toe at the MTPJ.
With straight IP joints all of ,(1) tension in the plantar fascia ,(2) force from the FDB , (3) force from FDL
(4 )Forces from the lumbricals and interossei , will act together to plantarflex the toe at the MTPJ.
Likely the four named forces acting together would make injury to the forefoot area much less likely than would be found in "functional lesser toe deformity" where some forces are opposing and not acting together.
Healthy lumbricals and interossei are absolutely vital to foot health. Key!
The ideas contained in the above text are my own and, as far as I can tell, are not in the literature . Any input would be most welcome!
Video link https://youtu.be/vW7CBl4k3ac
Explanation
The lumbricals and interossei act to straighten the lesser toes at the interproximal joints . If these joints remain straight during gait then the flexor digitorum brevis( FDB) and flexor digitorum longus (FDL)will produce a plantar flexion moment at the MTPJ. This will support the joint and share load with the plantar plate, which is considered a continuation of the plantar fascia. If the toes flex at the interproximal joints then the situation is very different and these muscles, FDB and FDL , will act not to plantarflex the proximal phalanx at the MTPJ but to dorsiflex it . This puts the MTPJ in a pre-dorsiflexed position and makes it more susceptible to hyperextension and overload.
Below is an image of a hammer toe.This configuration can be achieved by contracting the FDB in the absence of effective contributions from the lumbricals and interossei. In the image, the 2nd phalanx is being plantar flexed at the proximal interphalangeal joint . This causes the proximal phalanx to dorsiflex .This dorsiflexion is opposed by tension in the plantar plate( a continuation of the plantar fascia ) when the foot is under load, and so ground reaction forces are generated under the distal part of the toe . Remove the load on the foot, and therefore the PF and plantar plate, and very little force is generated under the distal part of the toe.
Looking at the image ,the longest bone is being plantarflexed by the plantar plate/fascia . However, activity in the FDB , which is inserted into the middle bone , flexes this bone down to the ground causing the long bone not to plantarflex, but to dorsiflex as is seen in the image . Contraction of the Flexor digitorum brevis will generate ground reaction forces only as long as tension in plantar plate and hence the long bone gives it something to work against to work against . Rupture of the plantar late would give a floating toe.
If the lumbricals and interossei are strong enough to keep the interproximal toe joints extend during foot loading, then the FDB will produce a plantarflexing moment at the MTPJ ,reducing tension in the plantar plate. However, if they are weak and collapse ,(" functional lesser toe deformity" ),then the FDB will produce a dorsiflexing moment at the MTPJ ,increasing tension in the plantar plate.
image.png
Extract from podiatry arena
The linked to video, below ,from Eric Fuller does a good job of illustrating the action of the flexor hallucis brevis (FDB)) when this is the only structure applying a force to the bones of a lesser toe other than ground reaction forces, that is to say when plantarflexing forces from the plantar fascia are removed from the equation along with the straightening effect of the interossei and the lumbricals. All that happens in the model is the middle phalanx is plantarflexed at the proximal IP joint and this causes the proximal phalanx to dorsiflex at the MTPJ. No significant ground reaction forces would be produced beneath the "bones "of a subject toe at all .
So you could say that lack of tension in the plantar fascia produces a floating toe, and no amount of FDB strengthening will change that since , without the lumbricals or interossei, the FDB does not produce a plantarflexing moment around the MTPJs ( see video)
If we added a plantar fascia to Erics model then the situation would change . The plantar fascia would apply a plantarflexing moment ( force) to the proximal phalanx . The FDB, when active, would plantarflex the middle phalanx at the proximal IP joint and this would cause the proximal phalanx to tend to dorsiflex at the MTPJ. The proximal phalanx is being "held down to the ground "( plantarflexed) by tension in the plantar fascia so when the middle phalanx is plantarflexed by the action of the FDB , ground reaction forces will now be developed ( I should probably provide a drawing here ).
In a healthy foot with strong active lumbricals and interossei, things change yet again .Toes which remain straight (extended) at the proximal IP joint when the FDB is activated will not causes dorsiflexion at the MTPJ but plantarflexion . The forces being generated by the FDB will compliment and add to those being generated by tension in the plantar fascia . They will not be opposing forces.
Much the same sort of mechanics applies to the forces generated by the flexor digitorum longus . If we have a plantarflexing moment of the proximal phalanx due to tension in the PF, then flexion at the IP joints caused by FDL activity will see a net dorsiflexing moment at the MTPJs with forces from the fascia being overpowered by the dorsiflexing action at the MTPJs of FDL activity . Keep the IP joints extended and the FDL will act to plantarflex the entire toe at the MTPJ.
With straight IP joints all of ,(1) tension in the plantar fascia ,(2) force from the FDB , (3) force from FDL
(4 )Forces from the lumbricals and interossei , will act together to plantarflex the toe at the MTPJ.
Likely the four named forces acting together would make injury to the forefoot area much less likely than would be found in "functional lesser toe deformity" where some forces are opposing and not acting together.
Healthy lumbricals and interossei are absolutely vital to foot health. Key!
The ideas contained in the above text are my own and, as far as I can tell, are not in the literature . Any input would be most welcome!
Video link https://youtu.be/vW7CBl4k3ac
Comment