One of the most challenging disorders of the foot is the unstable second metatarsophalangeal joint (MPJ). Due to the specific anatomical and biomechanical conditions, the surgeon has to define the plane of deformity as he or she will encounter a wide range of mild and passively reducible to severe and fixed deformities.

Indeed, there are special considerations regarding second MPJ biomechanics. The seven interossei pass plantar to the axis of the metatarsal head and insert into the base of the proximal phalanx and the plantar plate respectively. They act as strong flexors of the MPJ and, according to their position lateral or medial to the central axis of the MPJ, as abductor or adductor in the transverse plane for the MPJ simultaneously.

The four lumbricales originate from the tendon of the flexor digitorum longus (FDL) and insert into the medial aspect of the proximal phalanx and extensor hood, passing the deep transverse metatarsal ligament plantarly. They function as strong flexors of the MPJ with extension of the proximal interphalangeal joint (PIPJ) and distal interphalangeal joint (DIPJ). It is important keep in mind that the second toe has two dorsal interossei and no plantar interossei. The medial insertion of the lumbricalis tendon exerts an unopposed adduction force to the second toe.
The plantar fascia, plantar plate and the collateral ligaments provide the static stabilization of the lesser toes. The plantar fascia centers the plantar plate under the metatarsal head. This trapezoidal plate has multiple attachments to the deep transverse and collateral ligaments, the tendon sheath of the flexors and the extensor hood. It serves as a strong deterrent against dorsal dislocation of the MPJ. The collateral ligaments stabilize the MPJ in the transverse plane and also contribute to the restraint against dorsal dislocation.

A Pertinent Primer On Possible Causes Of Second Toe Deformities

There are several disorders that lead to a dislocation of the second toe, whether it is isolated in the sagittal or transverse plane, or is part of a so-called multiplanar deformity.

Monoarticular synovitis of the second MPJ reveals a thickening of the synovia with a distended joint space. This results in a diminished range of motion with a deviation in the transverse plane. The reasons for this monoarticular synovitis still remain unclear. Some authors associate a long second metatarsal with consecutive changes in the stability of the plantar plate with increasing dislocating forces.1 Others describe some relation to first metatarsal instability in hallux valgus, which causes persistent inflammation in a constantly overloaded second MPJ.1

Sagittal instability may range from subluxation to complete dislocation.

Persistent inflammation causes the attenuation of the plantar plate. The effect of the ground reactive forces with a dorsally directed force vector also contributes to the subluxation in the sagittal plane and the rupture of the plantar plate.

Aside from the aforementioned factors, some have advocated localized trauma as one of the most frequent causes for this condition.2 The literature describes seronegative and seropositive rheumatoid arthritis as well as hypermobility (e.g. Ehlers-Danlos syndrome) as etiologic factors.3,4 High-heeled shoes in combination with a narrowed toe box can create a situation of persistent hyperextension at the MPJ, especially in the case of a long second metatarsal.

With the rupture of the lateral structures (e.g., the lateral collateral ligament and/or the tendon of the first dorsal interosseus muscle), a multiplanar instability (or crossover toe) will develop.

The differential diagnosis includes interdigital neuroma, stress fracture of the metatarsal neck, trauma, synovial cysts or Freiberg’s disease.

Essential Keys To The Clinical Examination

The typical patient is middle-aged and presents with painful swelling and tenderness of the second MPJ. The pain increases with passive flexion and the range of motion decreases, particularly in flexion. In the case of attenuation of the lateral ligamentous structures, the palpation of the lateral aspect of the MPJ is even more painful.

Sometimes, the swelling can involve the entire second toe. Distraction will cause pain, especially at the endpoints of dorsiflexion and plantarflexion. Depending on the amount of swelling, the degree of flexibility varies accordingly.

The role of the push-up test is to define the degree of instability in both the sagittal and transverse planes. Some patients show a remarkable difference in sagittal and transverse stability in non-weightbearing and weightbearing. One will see increasing elevation and medial deviation of the second toe with weightbearing. For that reason, the proper examination should include the standing position of the patient.

Thompson and Hamilton have described another useful test called the vertical instability test.5 The physician stabilizes the metatarsal shaft and forces the digit in a dorsal direction. Similar to the Lachman test for the anterior cruciate ligament, there are four stages of vertical instability. In stage 0, the patient shows no dorsal translation but complains about pain. In stage 1, the patient can have a subluxation. In stage 2, the patient has a total dislocation that is flexible (manually reducible). In stage 3, the patient has a fixed dislocation and no reduction is possible.

This vertical instability test is very useful to differentiate between instability and a second web space interdigital neuroma. Moreover, the transverse arch compression will not cause pain in patients with instability of the second MPJ.

The normal clear space in the second MPJ is 2 to 3 mm anteroposterior.3 Dorsal subluxation or complete dislocations lead to a loss of this clear space. One would measure the lateral or medial delineation in relation to the middle axis of the third metatarsal. Physicians can easily measure the parabola of the forefoot. A difference of more than 4 mm in length to the adjacent metatarsals is defined as a long second metatarsal. A bone scan or MRI can be helpful in excluding a stress fracture or tumorous conditions. When it comes to subtle dislocations, one should obtain magnetic resonance imaging (MRI), which should easily reveal a plantar plate disorder.

How To Address Mild Deformities

Conservative treatment of mild deformities (e.g., stage 0 or 1) includes taping of the second toe in a rectus position for at least four weeks. In case of a painful swelling due to synovial inflammation of the MPJ, a corticosteroid injection may be helpful in the acute stage. However, it will increase the risk of plantar plate rupture if one applies it more frequently.
Additionally, physicians should counsel patients on considering a change of footwear.

Treating Sagittal Plane Instability: What You Should Know

The plane of instability determines the choice of the surgical procedure.

The surgical treatment of sagittal plane instability is dependent on the etiology of the deformity and the degree of dislocation. With stage 1 and 2 deformities, in which the Lachman’s test shows a reducible dislocation of the MPJ, the revision and stable reduction of the MPJ joint is indicated. Fusion of the PIPJ is an advisable concomitant procedure to create a stronger lever arm for the flexors.

Carry out the fusion as an end-to-end or peg-in-hole arthrodesis. Both types of arthrodesis require K-wire fixation for about four to six weeks. Drive the pin into the metatarsal head to stabilize the MPJ after a repair.

In some cases, the PIPJ fusion causes a toe that is too cosmetically straight for the patient. Although it is the correct procedure, it may also be too straight from the biomechanical standpoint.

Alternatively, one could perform the flexor tendon transfer with less predictable results. Surgeons can easily access the long flexor tendon through the PIPJ after resecting the head of the proximal phalanx. Preserve the short flexors and detach the long flexor tendon as far distally as possible. Then split the tendon in half. Suture together both slips over the dorsal aspect of the proximal phalanx. Take care to avoid dislocation of that loop as that would lead to a dislocation of the MPJ.

One can cut an additional groove into the dorsal aspect of the proximal shaft area of the proximal phalanx, which will ensure the position of the flexor tendon transfer. The radiograph is important in evaluating the length pattern of the metatarsals as the shortening of a long second ray may be necessary to get a longstanding result.

The surgeon should perform the osseous procedure before the soft tissue repair. Perform the shortening osteotomy of the metatarsal as an oblique osteotomy from distal dorsal to proximal plantar. Be sure to excise an adequate slice of bone.

The popular shortening osteotomy of the metatarsal without the excision of bone often leads to an inadvertent increased plantar positioning of the metatarsal head. This would result in increased pressure for the plantar plate. One can achieve the fixation by using a 2 mm cannulated screw or a snap-off screw, which is more difficult to remove if necessary. Expose the MPJ through the standard dorsal incision and inspect it. Before entering the joint, perform the intraoperative drawer test to evaluate the stability of the plantar plate.

If the surgeon can identify the flexor tendons at the plantar aspect of the joint, the plantar plate is ruptured and needs to be repaired. One can achieve this through the dorsal incision or through a separate plantar approach. One should repair the defect in the plate with non-absorbable suture material.6 With a large defect, surgeons can achieve the repair by inserting a soft tissue anchor into the base of the proximal phalanx. The patient then undergoes stabilization with a K-wire for four weeks.

In regard to stage 3 deformities with a fixed dislocation of the MPJ, the Stainsby operation seems to be the procedure of choice.7 Bear in mind that the primary indication for the Stainsby procedure is the correction of a rheumatoid forefoot deformity with a dislocated MPJ.

Resect the base of the proximal phalanx at its junction to the shaft. Transect the extensor digitorum longus (EDL) tendon about 1.5 inches proximal to the metatarsal head, interpose it into the MPJ and suture it to the plantar plate. Stabilize the result with a K-wire that runs through the interposed extensor tendon into the metatarsal head for four weeks.

A Guide To Treating Transverse Plane Dislocation

The isolated transverse plane dislocation is rare. The reconstruction must deal both with the repair of the suspensory ligament and the collateral ligament. One must also perform a shortening and/or transpositional osteotomy of the metatarsal. Surgeons can perform the transposition of the extensor digitorum brevis (EDB) into the base of the proximal phalanx as an adjunctive procedure. The osseous procedure is particularly important when it comes to reestablishing the correct axial relationship of the joint.

It is common for surgeons to perform the medial transpositional osteotomy with or without shortening. Carry out the oblique osteotomy in the neck area of the respective metatarsal from distal dorsal to proximal plantar. Then transpose the head segment of the metatarsal medially until the toe is in a rectus position. The transposition should not be more than half of the width of the metatarsal in order to maintain stability. One can achieve fixation with a 2 mm cannulated screw or any adequate device.

One should perform the soft tissue repair after the osseous procedure. This may require a soft tissue anchor to ensure proper fixation of the soft tissue to the proximal phalanx.

What About A Combined Dislocation?

The combined dislocation pattern is commonly associated with a crossover toe deformity. The repair consists of a combination of the aforementioned procedures. Carry out the repair as a stepwise approach and evaluate the result after each step. Thoroughly inspect the PIPJ and the MPJ. Bear in mind that even the severe crossover toes do not necessarily have a ruptured plantar plate.

The surgeon should carry out osseous procedures like the shortening and/or medial transpositional osteotomy before the soft tissue repair. It is advisable to perform the plantar plate repair through a plantar incision. One can correct the PIPJ via an arthrodesis or an arthroplasty with flexor tendon transfer.

In Conclusion

The treatment of second MPJ instability remains a challenging issue. Conservative treatment is applicable for subtle dislocations and in elderly patients. In order to choose the appropriate surgical procedure, it is critical to understand the underlying biomechanical issues.

In cases with an isolated sagittal plane deformity, we recommend a stepwise approach. The surgeon has to pay particular attention to the repair of the plantar plate. The PIPJ fusion is a good adjunctive procedure to create a better lever arm for the flexors.

One should consider the isolated transverse instability as a “lesser MPJ bunion” and treat it with a transpositional osteotomy in addition to the soft tissue repair.

Multiplanar instability requires a complex procedure. This encompasses both soft tissue and osseous techniques to restore the disturbed balance of the joint.

Dr. Olms practices at Chirurgie Bad Schwartau in Germany. He is a faculty member of the Podiatry Institute.

Dr. Randt practices at Chirurgie Bad Schwartau in Germany.