How To Address Key Biomechanical Issues With Second MPJ Injuries

Injuries to the second metatarsophalangeal joint (MPJ) can be challenging to treat. Our expert panelists discuss predisposing factors to injury and review pertinent biomechanical considerations. They also discuss conservative treatment options, including variations of orthotic therapy and modifications that they have employed in clinical practice.

Q: What are the predisposing factors (gender, foot type, activity, etc.) that are associated with injuries to the second MPJ?
A: Second MPJ injuries may have a variety of etiological causes, according to Kevin Kirby, DPM. He notes the most common causes are increased second metatarsal length, decreased dorsiflexion, first ray stiffness (such as increased dorsiflexion compliance of the first ray), obesity, thin-soled, high-heeled shoes, excessive subtalar joint pronation and plantar metatarsal fat pad atrophy. In addition, Dr. Kirby says athletic activities, such as running and jumping activities, which increase the loading forces on the forefoot, may lead to an increased risk of second MPJ injuries.
As James Clough, DPM, says, the second MPJ is prone to injury whenever the foot structure faces increased medialization of weightbearing forces. He says this could be caused by a myriad of biomechanical abnormalities including an increased Q angle and pronation of the rearfoot past the vertical position. A Morton’s foot type is also a significant factor that predisposes one to pronation, according to Dr. Clough. He also cites second MPJ overload due to the length discrepancy that exists between the first metatarsal and the second metatarsal.
Dr. Clough says second MPJ injuries are also common among people who walk only short distances during the day as they develop an apropulsive gait pattern due to a lack of active engagement of the windlass mechanism. He adds that second MPJ injuries can also affect people who stand in one spot for long periods of time and never use the foot dynamically. Dr. Clough says both types of patients are predisposed to second MPJ injuries as the first metatarsal does not plantarflex into the ground to accept normal weight distribution. He notes that older people who have a shuffling gait because of poor proprioceptive issues are significantly predisposed to second MPJ pain.
Richard Bouché, DPM, sees an equal distribution of males and females with second MPJ instability in all patient groups, including sedentary, active and athletic patients. Patients over age 60 seem to experience these injuries more commonly due to attrition, according to Dr. Bouché. He occasionally sees traumatic or iatrogenic causes. Dr. Bouché says the most common iatrogenic cause is steroid injections that use triamcinolone (Kenalog, Bristol-Myers-Squibb). Patients who seem to be prone to this problem include those with a moderate to severely pronated foot, those with flexible forefoot equinus and/or patients with rheumatic disease, according to Dr. Bouché.
Dr. Clough says other predisposing factors include high heels that increase forefoot overload and shoes that are too rigid in the forefoot as they do not allow for proper bending of the first MPJ.

Q: What biomechanical factors are associated with the pathomechanics of injury to the second MPJ?
A: Dr. Clough maintains that injury to the second MPJ is first and foremost a problem with insufficiency of the first ray. If the windlass mechanism is not engaging properly and the first metatarsal is not plantarflexing into the ground to accept adequate weightbearing, Dr. Clough says then the second metatarsal will be overloaded.
Not only is the first ray capable of excessive dorsal displacement but Dr. Clough says it is also capable of significant plantar displacement with proper foot function. If the first metatarsal head remains in a dorsally displaced position due to inadequate function of the first MPJ, he says this results in the inevitable overload of the second MPJ.
“This of course is the case with functional hallux limitus, which is present in a great number of feet in clinical practice,” says Dr. Clough. He says research has shown that hallux limitus is present in 62 percent of asymptomatic feet and adds that the condition is present in close to 80 percent of the feet he sees in clinical practice.1
“It is a pervasive problem that needs to be dealt with for effective therapeutic outcomes,” emphasizes Dr. Clough.
Dr. Bouché says one must consider biomechanical factors such as ankle equinus, a hypermobile medial column with or without bunion deformity, and/or a prominent second metatarsal that is either long or plantarflexed.
Dr. Kirby says the vast majority of second MPJ injuries are actually injuries to the plantar plate, which is a fibrocartilaginous structure plantar to the lesser MPJs and which is continuous with the plantar fascia. He explains that the plantar plate is subject to large magnitudes of tensile forces from the plantar fascia and is also subject to large magnitudes of compressive forces from ground reaction force (GRF) on the forefoot during weightbearing activities. This greatly increases the risk of the plantar plate developing partial or complete tears within its structure, according to Dr. Kirby.

Q: A common dilemma is differentiating a neuroma in the second interspace from pathology in the second MPJ. In terms of conservative treatment, is it important to nail down the exact pathology? How do you do this?
A: One will commonly see localized edema within plantar plate injuries of the second MPJ. This may also cause irritation to the plantar digital nerves that are adjacent to the plantar plate area, according to Dr. Kirby. He says careful manual examination of the plantar MPJ area will reveal that the majority of plantar plate injuries are the most tender just proximal to the central aspect of the proximal phalanx base. Dr. Kirby adds that plantar digital nerve irritation, or neuritis, will be most tender either medially or laterally, off-center, from the second MPJ.
Furthermore, Dr. Kirby says plantar plate injuries will often be painful with plantarflexion testing of the digit at the MPJ and this finding is uncommon with neuromas. In addition, a neuroma by itself will not likely be associated with plantar MPJ edema, which Dr. Kirby says is the case in plantar plate injuries.
“Therefore, if you see a swollen second MPJ with classic plantar plate tenderness that also has burning and numbness in the second intermetatarsal space, think secondary interdigital neuritis caused by plantar plate injury, not neuroma,” advises Dr. Kirby. Dr. Bouché confirms plantar plate pathology with clinical use of the Lachman (dorsal drawer or vertical stress) test. He says one can confirm a plantar plate tear with an arthrogram using X-ray or MRI.
Dr. Bouché says there may be plantar plate attenuation if one administers a local anesthetic injection and the amount of local anesthetic injected exceeds 2 cc. He notes that the usual capacity for a local injection in the second MPJ is 1 to 1.5 cc.
Whether one is dealing with pathology in the second MPJ or an interspace neuroma, Dr. Clough says the basic problem is that the second and third MPJs, and the intrametatarsal space are being overloaded. Inevitably, the nerve in the second interspace is often involved to some degree in the inflammatory process, according to Dr. Clough. In most situations, he says it is not critical to differentiate between a neuroma and pathology at the joint level.
“In either situation, your basic task is trying to offload that portion of the foot and re-establish proper mechanics of the first MPJ in order to get the first ray to plantarflex into the ground and accept more weightbearing,” notes Dr. Clough.
Dr. Clough has seldom seen a large neuroma in the second intermetatarsal space. If this condition is present, he notes one will see diminished sensation in the distribution of the digital proper nerve branches in the second and third digits. In these cases, Dr. Clough says simple offloading of the second and third MPJs will fail. He believes such a diagnosis is mainly one of exclusion when symptoms persist despite objective evidence that one has offloaded the MPJs. In his experience, Dr. Clough has rarely found surgical resection necessary for a neuroma of the second intermetatarsal space. He says proper conservative treatment should be sufficient.
Nailing down the exact pathology can be a difficult task, acknowledges Dr. Bouché. He emphasizes the importance of a thorough history and physical exam, diagnostic injections and MRI studies to help validate the clinical impression of an interspace neuroma. To confirm the diagnosis, Dr. Bouché says there should be “congruency” of all of these evaluation methods.

Q: What is your protocol for conservative treatment of suspected injury to the plantar plate of the second MPJ?
A: All three panelists cite the use of icing. Dr. Kirby suggests icing 20 minutes twice a day directly plantar to the second MPJ. He says this can significantly reduce plantar edema in the MPJ, which in turn can help reduce the compression forces on the plantar plate during weightbearing activities.
Dr. Kirby also cites plantarflexion taping of the digit to relieve the tensile forces on the plantar plate. This allows the injury to heal faster and leads to reduced pain with weightbearing, according to Dr. Kirby. Dr. Bouché concurs. He also suggests using digital spacers if there is any transverse plane component to the deformity. Dr. Bouché also recommends using a metatarsal binder or corset to stabilize the medial column by decreasing the first and second intermetatarsal angle.
In order to allow normal healing, Dr. Kirby says it is essential to use modified over-the-counter foot orthoses or prescription foot orthoses that are designed to reduce the ground reaction forces plantar to the second MPJ.
For Dr. Bouché, orthotic treatment options also include OTC or custom orthoses to control excessive pronation, forefoot extensions with the second metatarsal cutout and/or a metatarsal “cookie,” and a rigid rocker-soled shoe to offload the forefoot. Depending on the stage of the problem, Dr. Bouché says NSAIDs or a walking boot can be helpful in the acute/subacute stages. Dr. Kirby also recommends four to six weeks of a boot walker brace or a below-knee immobilization cast, which may be necessary to rest the injured plantar plate and to permit healing if other methods have not prevailed.
Conservative treatment for offloading of the second MPJ must be the first and foremost objective, emphasizes Dr. Clough. He says one can only accomplish that by restoring normal motion of the first MPJ and properly engaging the windlass mechanism of the foot structure.
If a functional hallux limitus is not properly engaging the windlass mechanism, he says the first metatarsal will not displace into the ground as part of dorsiflexion of the first MPJ and therefore, the first metatarsal will not accept adequate weightbearing into propulsion. Dr. Clough explains that when the foot goes into propulsion with a very unstable foot structure and an unstable first ray, the second MPJ will overload since the anatomy of the foot dictates that the second metatarsal will not be able to displace dorsally under weightbearing forces as well as the first metatarsal is capable of doing.
Dr. Clough notes the Cluffy Wedge® is often a very effective addition to a shoe insole or OTC device before one considers custom orthotic therapy. As he explains, the wedge will offload the second MPJ by increasing dorsiflexion of the first MPJ and improving first metatarsal plantarflexion. After offloading the second MPJ, he says physical therapeutic modalities may be of some benefit. Dr. Clough warns that under no circumstances should one inject corticosteroids since it will further weaken the soft tissues and aggravate the condition.

Q: In terms of orthotic therapy, what are the specific orthotic requirements and prescription criteria for offloading the second MPJ?
A: One should use a balanced/negative impression cast and cast out supinatus deformity if it is present, according to Dr. Bouché. In addition, he advises using a deep heel seat, a moderate medial arch fill, no lateral arch fill (to capture the lateral arch fully), a forefoot extension with a sub-second metatarsal cutout and/or metatarsal cookie. One may use additional strategies depending on the severity of the pronated foot deformity. Dr. Bouché says options may include a medial skive, inverted orthoses or a medial extended rearfoot post.
For the past 15 years, Dr. Kirby has used the following orthosis modifications to achieve very good therapeutic results in treating second MPJ pathology. He uses a 3/16-inch polypropylene plate with a standard rearfoot post, minimal medial arch fill and a 2 to 3 mm medial heel skive in the orthosis to increase the supination forces on the foot and redirect ground reaction force (GRF) toward the lateral forefoot. He says one should also make the anterior orthosis edge with an abrupt 3/16 inch drop-off or an “internal metatarsal bar” to reduce the GRF on the metatarsal heads.
In addition, Dr. Kirby notes that clinicians should also order the anterior orthosis edge so the orthosis shell parabola extends distally to all of the metatarsal necks and the orthosis is much longer under the distal second metatarsal shaft. He calls this a “capsulitis modification.” He uses a full length, 1/8-inch neoprene topcover along with a 1/8-inch korex forefoot extension plantar to the first, third, fourth and fifth metatarsal heads. Sometimes he combines this with a metatarsal pad sandwiched between the top cover and orthosis shell to facilitate further reduction of the GRF on the second MPJ.
During the casting of the orthotic, Dr. Clough advocates maximum dorsiflexion of the first MPJ to plantarflex the first ray and allow the first ray to bear weight when one is dispensing the orthotic. As he explains, this maneuver will reduce forefoot supination and eliminate the need to correct an inverted forefoot deformity. Intrinsic or extrinsic balancing of a forefoot supination will always result in jamming of the first MPJ and he says one should avoid this in all situations in which this deformity is reducible. Minimal arch fill is necessary to slow down any eversion velocity over the foot structure and provide pressure to the base of the first metatarsal, according to Dr. Clough.
Re-establishing the first ray function is critical for an orthotic to be effective and Dr. Clough feels applying a Cluffy wedge is a good solution as it pre-stresses the hallux in dorsiflexion and allows proper first MPJ motion to occur. He says this reliably overcomes a functional hallux limitus. As the first MPJ dorsiflexes, he says the first metatarsal plantarflexes and helps offload the second MPJ.
Other orthotic modifications like a reverse Morton’s extension or a kinetic wedge rely on increased pressure underneath the lesser metatarsals and decreasing weightbearing underneath the first metatarsal to improve the range of joint motion for the second MPJ, says Dr. Clough. He notes this is counterintuitive if one is trying to decrease weightbearing on the second MPJ. Further, he says it is critical to establish normal first metatarsal weightbearing pressure to overcome second MPJ forefoot pathology but still enable the coupling mechanism of rearfoot supination to occur in a timely fashion.

Dr. Bouché is a Staff Podiatrist at The Sports Medicine Clinic in Seattle. He is a Fellow of the American Academy of Podiatric Sports Medicine, a Diplomate of the American Board of Podiatric Surgery and a Fellow of the American College of Foot and Ankle Surgeons.

Dr. Clough in practice at the Foot and Ankle Clinic in Great Falls, MT. He is the inventor of the Cluffy Wedge. He is a Diplomate of the American Board of Podiatric Surgery. He can be reached at jclough@sofast.net

Dr. Kirby is an Adjunct Associate Professor in the Department of Biomechanics at the California School of Podiatric Medicine at Samuel Merritt College. He is the Director of Clinical Biomechanics at Precision Intricast Inc.

Dr. Richie is an Adjunct Associate Clinical Professor in the Department of Applied Biomechanics at the California School of Podiatric Medicine. He is in private practice in Seal Beach, California. He can be reached at drichiejr@aol.com

References:

1. Payne C, Chuter VC, Miller K. Sensitivity and specificity of the functional hallux limitus test to predict foot function. JAPMA 92:269, 2002.