Management of Mechanical Metatarsalgia

By Andrew Horwood MSK Podiatrist M.Pod.A, D.Pod.M.

Metatarsalgia

Metatarsalgia is one of those great terms that is a symptom, but is commonly used to "hood wink" patients to believe they have been given a diagnosis. Metatarsalgias can be immediately put into three categories:

Non‐mechanical Metatarsalgia

Partial Mechanical Metatarsalgia

Mechanical Metatarsalgia

Non‐mechanical Metatarsalgia

These include vascular, neuropathic and neoplastic causes of pain in the forefoot. Such conditions tend not to have a relationship with activity levels or weight bearing. Conditions that tend to cause metatarsalgia symptoms in this category include:

• Peripheral vascular disease
• Peripheral Neuropathy
• Osseous neoplasm (osteomas, osteosacromas etc)
• Soft tissue neoplasm (non‐pigmented villonodular synovitis,)
• Verrucae pedis
• Sero‐positive arthropathies (Rheumatoid Arthritis, SLE)
• Sero‐negative arthropathies (Psoriatic Arthritis)

It is essential that these conditions are diagnosed and treated appropriately. Even though most are a relatively rare aetiology of metatarsalgia, every time you see metatarsalgia these conditions must be in you're mind. Please refer to Jill Halstead's BritPod Conference notes for clinical clerking.

A special note on VPs because I get quite a few every year sadly, occasionally referred by podiatrists for gait assessment as to the cause of the plantar lesion. A recent case underlines what to look for if the appearance is a little indistinct from a corn. This male patient in his forties developed metatarsalgia over a twoweek period in January 2009. He thought his new shoes were the cause. The pain got worse despite changing his shoes and he noted a hard lump on the bottom of his foot. He saw a podiatrist who told him that he had a corn, caused by the way he walked, and he would need treatment ever few weeks for the rest of his life. After four treatments, feeling no benefit except for the first two weeks after treatment he came to me for a second opinion. I guessed it was a VP before he took his shoe off. Why? Time scale!

Partial Mechanical

Some conditions that cause metatarsalgia are a bit of a crossover and that really includes all the non‐mechanical causes. A VP is unlikely to cause pain unless it sits nicely under a metatarsal head during terminal stance phase of gait. A diabetic patients suffering from allodynia, whose peripheral neuropathy has also caused classic digital plantarflexor atrophy will have retracted toes and exposed metatarsal heads. But often these patients experience rest and night pain as well as weight bearing pain. A prominent metatarsal head produced as a result of RA synovitis, or an unstable dactylitis from psoriatic arthritis, is going to be mechanically overloaded. Therefore, although the initial cause is not mechanical, some of the tricks used in dealing with these types of metatarsalgia means that altering the mechanics can still give some symptom relief, if not resolve the precipitating factor. That factor must of course be addressed.

There is one particular condition that should be viewed as partial mechanical. That is plantar digital neurofibroma, also known as Morton's Neuroma or neuralgia when it is located in the third metatarsal inter‐space. These common neuralgias give an indistinct origin, but apart from a genetic/congenital or developmental element, there is a mechanical factor or two. Instability of the third inter‐space, and compressive footwear certainly are involved. Footwear choice, manipulation, mobilization and foot rehab may have a larger part to play conservatively than silicone orthoses. Interestingly there is little good evidence to support any intervention in Morton's neuralgia, and that includes surgery (Thomson et al, 2004).

Mechanical Metatarsalgia

Mechanically induced metatarsalgia is the most common cause of metatarsalgia you will see in your clinic. Other terms used are predislocation syndrome, metatarsophalangeal joint synovitis, and lesser metatarsal over load, which more describes pathomechanical aetiology. Terms like "dropped metatarsal" are misleading and should be avoided. It is associated with dysfunction of the digits during late mid‐stance and terminal stance phases of gait. Much of the problem is due to dysfunction of the tissues that make up the flexor plate mechanism, including the plantar extrinsic and intrinsic muscles. Like flexor digitorum profundus in the hand, flexor digitorum longus (FDL) runs up against a fibro‐cartilaginous plate on the plantar surface under the metatarsal head. The FDL tendon then perforates the tendon of flexor digitorum brevis (FDB) to attach to the base of the distal phalanx, just as flexor digitorum sublimis (superficialis) is perforated in the hand. Some authors have tried to relate to one structure such as the flexor plate/plantar plate or the flexor tendon sheath (Perez & Roberts, 2009). I feel this is difficult to establish clinically or on diagnostic image, and it is best considered as dysfunction of the unit with soft tissue injury within the mechanism, probably in multi‐anatomical structures.

A brief run through of the mechanical function of the metatarsal phalangeal joints (MTPJ's) will help in understanding the problem and the management.

FDL starts to fire around the end of loading response, but its activity is low, building up by late midstance. This is important, as too much early activity will tend to cause the distal phalanx to buckle on the distal interphalangeal joint (DIPJ). What the FDL is trying to do is create proximal force down the digit into the MTPJ, thereby stabilizing the MTPJ and preventing the metatarsal head from shearing anteriorly. The eccentric contraction (contracting while lengthening) of the FDL helps to reduce arch drop by restricting this anterior drift of the met heads.

As the heel lifts and we enter terminal stance, greater proximal force is required to stabilize the metatarsal head. The force vector coming down the foot is trying to push the metatarsal head anteriorly, while the bending moments generated through the shaft of the metatarsal by ground reaction forces are trying to torque the metatarsal head dorsally, and the metatarsal base plantarly. The FDL rapidly increases its contraction concentrically (shortening), which not only provides greater proximal stability, but also help raise the arch with tibialis posterior and flexor hallucis longus (FHL).

The long flexors (FHL shown) eccentrically contract in late midstance, to allow the arch to reduce in height, and the tibia to move over the talus producing ankle dorsiflexion. This creates a proximal force on the metatarsal head helping to limit arch drop. This proximal force increases during the terminal stance with concentric contraction of the long flexors, stabilizing the metatarsal head from shearing anteriorly.

However, because the FDL is attached to the plantar aspect of the base of the distal phalanx, as the metatarsals take on more of a plantargrade declination angle, the increasing tension in the FHL tendons are at risk of buckling the digits. But fortunately the FHL is not alone. From just prior to heel lift the FDB starts to contact, flexing the Proximal interphalangeal joint (PIPJ), which prevents digital buckling, thereby generating an efficient proximal force passing down the digit to its corresponding metatarsal head.

The best way to visualize this action as previously mentioned is the pole vault. The action of the proximal force generated by FDL and FDB is to create the hole that the athlete must put the pole in to stabilize the end of the pole so he can rise up and over the bar using the pole.

So why with such an efficient system do humans develop so many problems? This is can be put down to three factors:

  1. Evolutionary history. The foot has developed from a branch grasping structure, where extension of the toes is not the main action. That is why so much foot anatomy is analogous to the anatomy of the hand. Essentially we have botched the structures under the plantar foot to allow and restrict the extension at the MTPJ's that is necessary for our bipedal gait.
  2. Mismatch to our environment. The foot finds it self in a digital extension exaggerated/digital plantarflexion restricted environment. Hard flat surfaces, plus the presence of a rigid shoe sole under our feet increases the amount of dorsiflexion our toes do, while restricting the amount of plantarflexion our toes do. The higher the heel of the shoe the greater the dorsiflexed position of the digits. The choice of many people to compound the problem with limited digit space in the shoe leads to atrophy of plantarflexors and often weakness and tightness of digital dorsiflexors.
  3. Foot morphology. Abnormalities in metatarsal declination angle can affect the angle of the moment arm producing the proximal stabilization force. High declination angles of cavoid foot types tend to exaggerate dosiflexion moments on the digits, and plantarflexion moments on metatarsal heads, producing retracted digits. Hallux valgus destabilizes the 2nd MTPJ causing hammertoes and first metatarsal instability, increasing the loading rate on the 2nd MTPJ. Forefoot instability, often as part of the "pronated foot", cause muscle imbalance across the foot which can lead to early and abnormal plantarflexor activity in an attempt to stabile the arch height, often producing clawed toes.

FDL tendon enters the foot medially running, and winding under the sustemtalculum tali then passes obliquely to divide around the level of the navicular to the cuneiforms into four slips. It then sends a tendon slip to each digit, although this is variable with sometimes an extra slip to the hallux and in about 1%, no slip to the 5th digit. This variation may come as a surprise, but it is nothing compared to the variation in the attachment of tibialis posterior, to the tendon slip variation in extensor digitorum longus. If our toes depended on just the FDL, then in plantarflexion our digits would adduct. They do not as long as we have the full action of flexor digitorum accessorius (FDA) otherwise known as Quadratus plantae. This muscle attaches from two points separated by the long plantar ligament, from the plantar surface of the calcaneaus, distal to the plantar tubercle. The muscle runs distal to attach to the FDL tendon just before and two the division of the digital tendon slips. It effectively straightens the pull of FDL.

FDB is particularly variable in regards to absence in its lateral tendon slips, in about 25% of cases. Medial tendon slip variation is rare. As mentioned already FDB is essential in stabilizing the PIPJ. However, there are more muscles that help to keep the digits fairly straight during gait. These are the lumbricles that stabilize the MTPJ medially and plantarly, the plantar interossei, that stabilize the lateral 3 MTP joints medially, and the dorsal interossei that stabilize the lesser MTP joints laterally except the 1st dorsal interossei that stabilizes the medial side of the 2nd MTPJ.

All these muscles become particularly interesting when they are related to the clever stabilization mechanism of the MTPJ. This is the extensor hood, flexor (plantar) plate mechanism. Lesser MTPJs are enveloped by this structure and the tendons of the digits are bound into this structure. The extensor hood is made up of two distinct areas the sling and the wing, each having different fiber arrangements. The sling, which envelops the extensor digitorum longus tendon, has fibers that run from dorsal to plantar and attach to the fibrocartilagenous flexor plate. This helps pull the flexor plate tight to the metatarsal head to add metatarsal head lift just prior to toe off. The wing has oblique running fibers that run to the extensor tendon just short of the PIPJ, and it is attached to the lumbricles. The sling helps to keep the proximal phalanx from dorsiflexing during terminal stance. Underneath the flexor plate and bound by fibrous tissues lies FDL and FDB. Failure of any part of this mechanism can set off metatarsalgia.

Added to this complex situation is the effect of obliquely orientated medial muscles of the 1st MTP joint. These oblique muscles such as flexor hallucis brevis (FHB) and the transverse and oblique heads of adductor hallucis make perfect sense if you are using an opposable hallux. However, to try and stop a metatarsal from being unstable from ground reaction forces necessary for bipedal gait, it would be good to have a strong plantarflexor attached to the metatarsal head. The only attachments are weakly attached to the metatarsal base. Those oblique muscles attach to the sesamoids and via them to the base of the proximal phalanx of the hallux. As the first metatarsal drifts laterally in Hallux Valgus, the first dorsal interossei proximal attachments are pulled apart, putting a dorsiflexion/adduction moment on the second digit. This is why hammertoes that ride over the hallux are so common. The muscular anatomy of the forefoot sets us up for an easy fall.

An adductovarus hammertoe over riding the first.

Mismatch

Consent walking on flat surfaces does not exercise our long and short digital plantarflexors. If you don't use it you loose it. Shoes tend to exaggerate this problem, because most have heel lifts of some degree, and hard shoes tend not to allow digital plantarflexion. The rather boring flatness of the modern city environment keeps the range of motion in our foot joints small. Lack of motion reduces muscle strength, particularly for our plantar intrinsics.

High‐heeled dress shoes are particularly problematic. They hold the digits in permanent extension, stretching the plantar structures and due to lack of use, cause atrophy of the plantar musculature. The way, in which the plantar plate works with the extensor sling, the fibrocartilaginous plate becomes shifted anteriorly, no longer acting as a protector of plantar pressure on the metatarsal head. Now with digital dorsiflexion the lumbricles can no longer use the extensor wing to stabilize the proximal phalanx against the dorsiflexion moments, which makes it hard for the FDB to stabilize the PIPJ.

The foot evolved in an uneven surface environment around 3 million years ago, and has essential remained unchanged for at least 2 million years. Although we have used shoes since the middle Paleolithic (Trinkaus, 2005), flat hard floors and fashion shoes are really a feature of the last 500 years. We are now in a state of mismatch.

Foot Morphology

Certain foot morphologies will predispose to metatarsalgia. Any foot type with excessive metatarsal declination angles, be it forefoot equinus, pes cavus , a short or a plantarflexed metatarsal. The reason for this is the change in action of the plantar interossei and lumbricles from weak pl flexors to weak dorsiflexors.

Diagnostic Tests

• Painful heel pain, with pain in forefoot.
• Dorsal stress test positive for pain with/without MTPJ subluxation
• Diagnostic injection
• Dorsal and planter compression
• Distal metatarsal shaft bending moment

For MTPJ synovitis pain can be provoked on dorsal and plantar compression. Dorsal (vertical) stress test.

Originally by Thompson & Hamilton (1987), this test is graded 0 for no motion, grade 1 for subluxation, grade 2 for dislocation, and 3 if the dislocation is fixed. Many metatarsalgia patients have pain when the test is grade 0. This probably represents a strain.

The Forces

Mechanical Intervention

Dealing with metatarsalgia can be quite a challenge. Not least because of poor patient compliance with treatment. Trying to take away the precipitating factors, especially certain shoes must be handled sympathetically but firmly. Always ask patients when they get the metatarsalgia. If it is only in footwear, or one particular pain of shoes, concentrate on dealing with that issue.

  • FOOTWEAR: High heel, narrow toe box, and thin soles are all commonly associated with mechanical metatarsalgia, and are a big factor in dealing with plantar digital neuralgia.
  • FUNCTIONAL FOOT ORTHOSES: These must be designed to control forefoot instability not hindfoot motion! If the FFO is just going to fill up the shoe and reduce forefoot space, it won't help. Try and off load the metatarsal head with support behind. Don't make an aperture, as given time, the plantarly unstable metatarsal will be able to plantarflex further as you have increased the digital dorsiflexion moment.
  • SILICONE TOE SPLINTS: These simple orthoses are a very effective treatment particularly if any digital deformity is mobile and reducible, or at least semi reducible.
  • TAPING: A number of taping techniques can be employed to provide fierce resistance to digital dorsiflexion if silicones fail. This is a short term treatment, that can be reapplied by the patient, to give instant symptom relief and start to alter muscle imbalance.
  • REHABILITATION: Metatarsalgia is a essentially a muscle imbalance problem brought about by environmental mismatch. To not attempt to rehabilitee the muscle imbalance should be thought as negligent, though sadly at the present time forefoot rehabilitation is not performed well by any profession.
  • SURGERY: When digital deformity is non reducible, or the flexor plate mechanism cannot be re‐engaged; surgery may be the only option. Surgery must always be considered the last option. Do not kid patients that surgery will allow them to wear any shoe they like after. Remember if surgery in the forefoot goes wrong, if can be one of the most challenging podiatric problems, beyond almost any forefoot pathology possible without surgery. Best procedures are Weil osteotomy, with digital correction (IPJ arthrodesis), extensor tendon releases, flexor plate mechanism repairs and plantar flexor tendon transfers. If significant HAV present, this should also be addressed.

Footwear

If your patient wears lace ups, try and adjust the laces to increase toe box space. Great for HAV patients! Try and get patients to wear trainers at least as a utilitarian shoe. Restrict narrow toe box slip on shoes and heels as much as the patient will tolerate. If the patient is not compliant or concordant on this point out that they have made effective treatment much harder.

  • Shoe style advise
  • Correct shoe size and width
  • Lace adjustments
  • Cushioning soles (trainers very good)
  • Don't over pack the shoe with treatments.

Functional Foot Orthoses

A simple study by Kilmartin & Wallace (1994) showed that controlling the hindfoot didn't help metatarsalgia. Hardly surprising as the structures you are trying to off load are in the forefoot. Any prescription for metatarsalgia must reduce loading rate on the effected metatarsal head. A well place metatarsal pad, or distal shell extension often proves successful, as done a little cushioning raise under the effected metatarsals. Try and narrow the width of the forefoot if there is significant forefoot splay.

  • Try and reduce forefoot splay (met pads, distal shell extensions)
  • Offload affected met heads (met pads, distal shell extensions)
  • Don't make apertures under metatarsal heads. Good in sort term, bad in long term. Don't go it.
  • Add a little cushioning under met heads
  • Don't get side track in the hindfoot

Mechanics of Silicones

Silicone can be used in a variety of ways to counter act the forces causing the metatarsal head to increase its plantar pressure, by trying to increase the proximally directed force from the digit. With a material as flexible to mold as silicone putty, and shape can be corrected. However, it is best to manipulate and mobilize the MTPJ and digit prior to molding, as you want to capture the digit in its least deformed position.

Common approaches are:

  1. For a hammertoe (mobile & fixed)
  2. For retracted toes
  3. For clawed toes
  4. You Y‐Sign and deformity
  5. For concurrent inter‐digital neuralgia

Taping Techniques

If a silicone fails to resolve the metatarsalgia, taping is a great option.

  • Dorsally subluxing hammertoe.
  • Y‐Sign and deformity
  • For early dorsal drift and under burrowing

Manipulations

Simple MTP releases can be very effective in helping to relieve metatarsalgias, including Morton's neuralgia, but be careful when learning to introduce this technique.

Lesser MTP manipulation technique. Drawing taken from Michaud, T.C. (1993) Foot Orthoses and other forms of conservative foot care. p135,. Baltimore, Williams & Wilkins.

Simple Rehabilitation for Metatarsalgia

Trying to strengthen plantarflexors and stretch dorsiflexors is a very important part of metatarsalgia rehabilitation.

  • Exercise ball roll &pick up
  • Anterior muscle group stretch
  • Heel to first MTP calf strengthening



References

Kilmartin T.E, Wallace W.A,(1994). Effect of pronation and supination orthosis on Morton's neuroma and lower extremity function. Foot and Ankle International. 15(5): 256‐262.
Perez, H.R, Roberts, J. (2009). Flexor tendon sheath as a source of pain in lesser metatarsal overload. Joural of the American Podiatric Medical Association. 99 (2) 129‐134.
Thomson C.E, Gibson J.N.A, Martin D, (2004). Interventions for the treatment of Morton's neuroma. The Cochrane Datadase of Systematic Reviews. Issue 1. Art. No.: CD003118.pub2. DOI:10.10002/14651858.CD003118.pub2.
Thomson C.E, Gibson J.N.A, Martin D, (2004). Interventions for the treatment of Morton's neuroma. The Cochrane Datadase of Systematic Reviews. Issue 1. Art. No.: CD003118.pub2. DOI:10.10002/14651858.CD003118.pub2.
Trinkaus, E. (2005) Anatomical evidence for the antiquity of human footwear use. Journal of Archaeological Science. 32: 1515‐1526.
Yu, G.V, Judge, M.S, Hudson, J.R, Seidelmann. (2002). Predislocation syndrome: progressive subluxation/dislocation of the lesser metatarsophalangeal joint. Journal of the American Podiatric Medical Association. 92 (4) 182‐199.