SECOND METATARSOPHALANGEAL JOINT INSTABILITY
January 4th, 1995
Paul T. Fortin, MD and Mark S. Myerson, MD
Introduction
The second toe, due to its juxtaposition with the hallux, relative length, and muscular imbalance, is a common location for problems in the forefoot. The second metatarsophalangeal (MP) joint is the most common chronically dislocated joint in the foot.1 Instability of this joint, which presents as a spectrum of disorders ranging from idiopathic synovitis to fixed dislocation, often leads to deformity such as clawtoe, hammertoe, or crossover toe. Recent investigations have advanced our knowledge of the pathologic anatomy and biomechanics of this joint, but controversy regarding treatment still exists.
The purpose of this article is to review the etiology of second MP joint instability, its pathologic anatomy, and the treatment rationale for the various stages of deformity.
Anatomy and Biomechanics
For 40% of the stance phase of gait, the forefoot functions in weight transfer.2 The toes are exposed to high compressive and shear forces. The main function of the toes is to increase the weight-bearing area of the forefoot so all force is not taken by the metatarsal heads alone during toe-off.3,4 Based on clinical observation, Lambrinudi4 postulated this mechanism was effective only if the toes were maintained flat on the ground with synergistic contraction of the long toe flexors and intrinsics. Other work with force plate analysis has shown that the contribution of the toes to load bearing during standing is minimal.5 Hughes et al3, however, have recently demonstrated the importance of the toes as load bearing during walking. Using a dynamic pedobarograph and normal unshod subjects, they demonstrated that the toes were in contact with the ground for three-quarters of the stance phase of gait and exerted peak pressures similar to those of the metatarsal head. Nearly 25% of the total mean peak pressure under the toes was born by the second toe. As a consequence of the relative length of the second metatarsal, two axes exist at the MP joint level.2,6 Bojsen-Moller2 described a transverse axis passing through the heads of the first and second metatarsals and an oblique axis from the second through the fifth metatarsal heads. The two axes are supposedly used for different mechanical purposes based on the demands for power and balance during push-off. The second MP joint is, therefore, common to both axes and, in theory, continually exposed to high forces during push-off.
Normal toe position is a combination of intrinsic and extrinsic muscle balance and competent static joint restraints. The extensor digitorum longus functions to dorsiflex the proximal phalanx by its attachment to a fibroaponeurotic sling that suspends the phalanx. It is only able to extend the proximal interphalangeal (PIP) joint when the proximal phalanx is in a flexed or neutral position. The extensor digitorum longus, therefore, becomes an ineffective extensor of the PIP joint if the MP joint is fixed in hyperextension.7 The work of Coughlin and Mann shows that the extensor digitorum longus may become the single strongest deforming force leading to subluxation and dislocation of the MP joint.7-9 Flexion of the MP joint is primarily the function of the intrinsic musculature10 (Fig. 1). The flexor digitorum longus and brevis flex the PIP and distal interphalangeal joints but are weak flexors of the MP joint. The intrinsic muscle control of the second toe is unique in that it is has two dorsal interossei and no plantar interossei.10,11 The lumbrical tendon insertion into the medial extensor hood adds an unopposed adduction force to the second toe. This may become an important deforming force and potentially lead to a crossover toe deformity when lateral restraints become lax or torn.7 Both of the interossei as well as the lumbrical tendon pass plantar to the axis of rotation of the MP joint. With hyperextension of the proximal phalanx, the interossei sublux dorsal to the axis of the MP joint and are no longer efficient flexors of that joint.2,7,10,12 The lumbrical tendon is tethered by the deep transverse metatarsal ligament and so remains plantar to the axis of rotation but an inefficient flexor due to the acute angular line of pull. Myerson and Shereff13 found in cadaveric dissections that this lumbrical axis averaged 35o in normal second toes and 90o in claw- and hammertoes. Thus, as the proximal phalanx hyperextends, there are no major muscular antagonists to further extension and progressive subluxation of the MP joint. Static restraints (capsule, collateral ligaments, plantar plate) also play a major role in maintaining joint stability.2,7,10,12,14 The plantar plate has previously been thought to be the primary static MP joint stabilizer.7,10,14 A recent biomechanical study of the static restraints to dorsal dislocation of the second MP joint demonstrated a 48% reduction in the force required to dislocate the joint when both collateral ligaments were sectioned, compared to a mean reduction in force of only 29% when the plantar plate alone was sectioned.15
Incidence and Etiology
Although synovitis of the second MP joint may occur associated with hallux valgus, it is far more commonly associated with attritional changes in the volar plate due to a long second metatarsal. The length of the second metatarsal may also be associated with, or cause, attritional rupture of the lateral collateral ligament and lateral joint capsule, leading to the crossover toe deformity.
DuVries16 stated that dislocation of the second MP joint is the most common dislocation of the forefoot. The type of shoe worn is believed to be the major etiologic factor.17 Females with a long second metatarsal who wear high-fashion footwear are said to be the population at risk.4,14,18 The elevated heel and narrow toe box characteristic of women's footwear in conjunction with a long second toe can lead to a buckling effect on the toe with subsequent hyperextension of the MP joint.8 Scheck14 measured the dorsiflexion angles of the second MP joint of men and women of different age groups and found a statistically significant increase in the dorsiflexion angle with age in both men and women. He attributed the development of hammertoe deformity to a age-related attrition in the plantar capsule, plantar aponeurosis, and intrinsic flexors. Idiopathic intrinsic muscle atrophy has been proposed as the etiology of the hyperextended position of the proximal phalanx.4 Taylor,19 however, found no gross abnormality of the intrinsic muscles during surgery for hammertoe deformity in 68 patients. Biopsy of the intrinsic musculature failed to demonstrate any evidence of degeneration.
Studies attempting to quantify the effect of heel height on pedal pressures have focused on loading of the metatarsal heads.20,21 None of these studies assessed the pressures under the toes with increasing heel height and, therefore, its effect with respect to progressive MP joint instability must be inferred.20,21
Subluxation or dislocation of the second MP joint is often associated with hallux valgus deformity.11,22,23 Presumably, due to extrinsic pressure from the hallux, the proximal phalanx is pushed laterally and hyperextends, and eventually subluxation occurs. Jahss23 reported isolated MP synovitis in association with a developing hallux valgus, which he felt to be the incipient stage of instability. Mann and Mizel24 reported on six patients with nonspecific synovitis of the second MP joint and suggested most synovitis occurred without hallux valgus.
Subluxation and dislocation of the MP joint typically occurs in the sagittal plane; however, the second toe may also deviate in the horizontal plane, dorsomedially over the hallux. Coughlin22 has described a second MP joint instability pattern characterized by insidious medial deviation and subsequent crossover deformity of the second toe. Only one patient in this series had associated hallux valgus; 90% of the patients had a long second metatarsal and 80% were females. He postulated that a restrictive toe box could lead to impingement between the second and third MP joints and attrition of the lateral capsule and collateral ligament. This type of idiopathic MP joint instability can also occur in the young athletic patient. Coughlin25 recently reported a series of seven athletes with second MP subluxation presenting as an overuse syndrome. In contrast to the first series, more than half of the patients were male. Garth and Miller26 evaluated second toe range of motion and intrinsic muscle strength in 17 running athletes with posteromedial one-third tibial pain and tenderness along the flexor digitorum longus muscle origin. The results were compared to a matched group of asymptomatic athletes. They found a statistically significant displacement of the arc of motion of the second toe in the symptomatic athletes toward extension of the MP joint and a decrease in flexion. Weakness or dysfunction of the intrinsic muscles in the symptomatic athletes was also found to be significant. The authors believed that weak foot intrinsics lead to a substitution pattern with overuse of the flexor digitorum longus in an attempt to prevent hyperextension of the MP joint.
Other etiologies of subluxation and dislocation that need to be considered include trauma with acute disruption of the plantar capsule and collateral ligaments, capsular insufficiency from chronic synovitis seen with systemic arthritides, and neuromuscular diseases causing muscle imbalance.9
Diagnosis
The initial stages of MP joint instability often go unrecognized. It is important to realize that MP joint instability can occur without interphalangeal joint deformity. Thompson and Hamilton27 have described a digital Lachman or toe translation test which can be graded in terms of how much proximal phalanx can be uncovered vertically (Fig. 2). In Stage 0, there is no laxity to dorsal translation. In Stage 1, the proximal phalangeal base can be subluxated but not dislocated. In Stage 2, the phalangeal base can be dislocated but also manually reduced. In Stage 3, the phalangeal base is fixed in a dislocated position. The patient's pain typically is reproduced with dorsoplantar stress.
Idiopathic synovitis of the second MP joint is often characterized by localized swelling of the joint but may involve the entire second toe.9,24 There may be warmth and tenderness to palpation with diminished range of motion, particularly flexion.24 Development of joint instability is insidious and may not be present initially. Pain emanating from the joint may be difficult to differentiate from the neuritic pain of an interdigital neuroma.7 Acute pain on the lateral aspect of the joint in the second intermetatarsal space may be accompanied by progressive medial deviation of the toe from degeneration of the lateral capsule and collateral ligamentous complex. Coughlin22 reported ill-defined pain in the second intermetatarsal space in all patients with crossover second toe deformity. Inflammation with capsular distension may also cause irritation of the adjacent interdigital nerve with subsequent neuroma formation.24
Although definitions vary,28 hammertoe in the current report indicates an extended MP joint, a flexed PIP joint, and an extended distal interphalangeal joint; clawtoe posture indicates an extended MP joint, a flexed PIP joint, and a flexed distal interphalangeal joint. The crossover toe deformity introduced by Coughlin involves subluxation that may occur in both the mediolateral and dorsoplantar direction. Typically the second toe overlaps the hallux.8
The extent to which any deformity can be passively corrected should be assessed, and the stability of the MP joint should be tested with the patient both sitting and standing. Flexible or dynamic clawtoe and hammertoe deformities are present when standing and correct when the foot is relaxed in equinus.9 The contribution of the long flexor to any PIP joint deformity is determined by examining the change in PIP flexion with ankle dorsiflexion. A tight long flexor tendon leads to increased PIP deformity with ankle dorsiflexion. It is important to consider the extent to which hallux valgus deformity may be contributing to MP subluxation due to extrinsic pressure. Often there will be insufficient space available for a second toe in an anatomically correct position if a significant hallux valgus exists.
Radiographs are important to assess joint congruity, the relative metatarsal lengths, the presence of any arthritic changes, and the degree of deformity. Incipient subluxation with nonspecific synovitis may demonstrate widening of the MP joint space.24 Typically, however, the MP joint clear space diminishes as the base of the proximal phalanx subluxates dorsally over the metatarsal head.9 Radiographs can also be used to help rule out other causes for symptoms, such as systemic arthritides, Freiberg's disease, and osteochondral fractures.
Management
Nonoperative Treatment
Nonsurgical treatment of fixed deformity is generally unsuccessful.29 All of the patients with nontraumatic monarticular synovitis reported by Mann and Mizel24 were treated initially with antiinflammatory medication, accommodative shoes, and a metatarsal support. Only one of the six patients had resolution of symptoms with this regimen. In the early stages of subluxation, stabilizing the toe with either a toe retainer or taping can eliminate symptoms and prevent progression of deformity. In a more recent study, Mizel et al.30 reported on the conservative management of second MP synovitis. In their series of 30 patients, 70% achieved good or excellent results with a single intraarticular injection of steroid and a stiff-soled shoe. The results were worse if more than one joint was involved. In Coughlin's series,22 three patients with mild to moderate crossover deformity treated with taping had no progression of their deformity. Two of these three patients, however, continued to have symptoms of intermetatarsal space pain. Taping often is necessary for several months and may lead to problems with chronic edema or ulceration of the toe.7 Padding to relieve metatarsal head pressure and toe crests to facilitate function of shortened cocked-up toes can be effective temporizing measures.31 Corticosteroid injections can provide temporary relief of symptoms from synovitis, but must be done judiciously when impending subluxation exists.9
The relief of pain and swelling from corticosteroid injections is dramatic and almost immediate. When combined with 0.5 cc of lidocaine, careful intraarticular injection can also be used for diagnostic purposes to differentiate pain from other sources. It has been our experience that relief of pain from synovitis lasts between 3 and 6 months in most patients, some of whom obtain permanent relief.
Surgical Treatment
Results of surgical treatment have not been universally successful.32 Factors that must be considered when contemplating surgery include:
1. Degree of MP instability
2. Presence of hallux valgus
3. Flexibility of deformity
4. Magnitude of deformity, and
5. Presence of a crossover deformity.
Specific Situations
Idiopathic synovitis/instability/no deformity. The activity level and individual demands of the patient should be considered. Stability at the expense of mobility may be an unacceptable solution in certain situations.33 Dancers, for example, require full dorsiflexion of the MP joint and may be impaired by a flexor to extensor transfer.27
Chronic swelling and pain at the second MP joint despite nonoperative treatment is an indication for synovectomy. Other systemic causes for the synovitis must be ruled out with the appropriate serologic studies and pathologic examination of the synovium. The metatarsal head should be inspected for any osteochondral injury, particularly in the athlete.33
It is also important to assess the stability of the joint. If unstable to dorsoplantar stress, transfer of the flexor digitorum longus to the extensor surface of the proximal phalanx is often used to stabilize the MP joint.9,27,34-36 Since Taylor19 originally reported transferring both the long and short flexors to the extensor expansion for clawtoe deformities, several modifications have been proposed.37,38 The effectiveness of the long flexor in maintaining reduction of the MP joint has recently been substantiated in a biomechanical study by Bhatia et al.15 The authors created an unstable MP joint by sectioning the volar plate and collateral ligaments; this unstable MP joint was then subjected to a forced rate of loading on a materials tester. They demonstrated that transfer of the flexor digitorum longus (FDL) to the extensor expansion restored the load/displacement curves of the unstable second MP joint to that of the normal toe. Thompson and Deland1 clinically evaluated a group of patients who had flexor tendon transfers for instability of the second MP joint. All patients had relief of symptoms, despite the fact that radiographs demonstrated joint reduction in only 54% of the patients. Stiffness with diminished dorsiflexion was noted in a significant number of patients. The transfer has the theoretical advantage of being a dynamic transfer but may, in fact, ultimately result in a static tenodesis, which may account for some of the postoperative restriction of dorsiflexion. The reason for the high incidence of imperfectly reduced MP joints in the study by Thompson and Deland is perhaps related to limitation of this technique of FDL transfers. With an unstable or subluxated MP joint, transfer of the FDL may not be able to restore stability since the vector of force is distal to and not adjacent to the MP joint. The further away from the MP joint, the less likely is the FDL transfer to reduce and stabilize the MP joint. Theoretically, therefore, transferring the tendon immediately adjacent to the joint should provide more stability. This has been reported by Kuwada and Dockery37, who transferred the FDL through a drill hole in the metaphysis of the proximal phalanx. Another theoretical consideration in dorsal transfer of the FDL is the manner in which the tendon is exposed and transferred. If the flexor pulleys are all preserved, then the vector of force may still be diverted closer to the MP joint even if the tendon is transferred more distally along the proximal phalanx.
Coughlin25 performed flexor tendon transfers in six of seven athletes with subluxation of the MP joint. All patients returned to athletic activities (running, aerobics) and were asymptomatic. The FDL tendon should be transferred with the foot in neutral dorsiflexion, which then presumably functions as a tenodesis during push-off when the toes are passively dorsiflexed.
Conditions coexistent with early MP instability, such as hallux valgus and interdigital neuroma, may need to be treated surgically. A moderate to severe symptomatic hallux valgus should be appropriately corrected to help avoid recurrence of the second MP instability from extrinsic pressure.39 Although seemingly difficult to differentiate, some authors believe concomitant interdigital neuroma can occur and should be excised if neuritic symptoms are present.24,40 In our practice, we have found it very difficult to accurately diagnose neuritis associated with synovitis, the latter of which is far easier to identify clinically. No guidelines exist as to simultaneous surgical treatment of the neuritis and synovitis. In runners, Lillich and Baxter40 believed dorsal subluxation could result in traction of the interdigital nerve as it is tethered by the transverse metatarsal ligament. In addition to resecting the interdigital nerve, they recommended removing the plantar portion of the metatarsal head to allow the volar plate to re-adhere and thereby prevent subluxation.
Subluxation/dislocation/deformity. Numerous surgical procedures have been described for correction of the chronically subluxed or dislocated toe with secondary deformity.7,34,35,41-43
Common to all of the procedures is a sequential release of the pathologically shortened soft tissues. Based on the motion gained after stepwise release of pathologically shortened soft tissues in the cadaver, Myerson and Shereff13 recommended tenotomy of the extensor digitorum longus and brevis, dorsal capsulotomy or partial capsulectomy, followed by release of the accessory portion of the collateral ligaments and possibly the interossei, if required to restore normal alignment. Many other authors support a similar approach, but suggest step-cut lengthening of the long extensor tendon and blunt stripping of adhesions between the plantar plate and metatarsal head that may block reduction.7,27,29,44 In the presence of painful callosity under the second metatarsal head, some authors recommend an arthroplasty using the technique of DuVries to shave the prominent metatarsal condyle.7,12 In cases of mild to moderate subluxation, this may be all that is needed to effect a stable reduction. Based on the stability and position of the joint with the ankle at 90o, a flexor to extensor transfer has been used to stabilize the base of the proximal phalanx when it is able to be relocated but remains unstable.1,7,12
When complete sequential release of pathologically shortened soft tissues fails to reduce the base of the proximal phalanx, bone shortening at some level is required to avoid potential ischemia from neurovascular stretch.12,45 Scheck46 reported degenerative MP joint changes after soft tissue injury without decompression in patients with severe deformity. Metatarsal head specimens examined histologically demonstrated subchondral collapse similar to avascular necrosis. Scheck46 postulated that this was due to excessive articular compressive forces from tension in the surrounding soft tissue. He noted no MP degenerative changes in patients with similar deformities treated by resection of the base of the proximal phalanx. Bone decompression can be in the form of a PIP joint resection arthroplasty,9,27,45 partial basal phalangectomy,41-43,46,47 diaphyseal waist resection of the proximal phalanx,48 Duvries metatarsal head arthroplasty9,27,45 (Fig. 3), or shortening osteotomy of the second metatarsal.45 Correction of chronic deformity at both the MP and PIP joints in conjunction with a flexor to extensor transfer requires special attention to the neurovascular status of the toe.1,19
Total proximal phalangectomy often results in a flail toe, and basal hemiphalangectomy can lead to a crossover toe deformity.13,23,38 Failure rates of 6 to 77% have been reported when basal hemiphalangectomy alone was performed for second toe deformity.14,32,41 Smith and Conklin49 recently reported that 27% of their patients were dissatisfied with the outcome after partial proximal hemiphalangectomy. Most of the poor results were secondary to persistent PIP flexion deformity in patients with hammertoes or residual pain under the metatarsal head in those with preoperative metatarsalgia. When this procedure was performed for transverse deviation, MP synovitis, or rheumatoid deformity, patient satisfaction was high. Kelikian23 suggested syndactilization of the second and third toes in an attempt to control toe position after proximal phalanx base resection. This has been criticized, as it reportedly fails to stabilize the MP joint and can lead to deformity of the otherwise normal third toe.9,18,42 Daly and Johnson42 treated a group of patients with second MP subluxation or dislocation by basilar phalangectomy of both the second and third toes in combination with subtotal webbing. Overall, 75% of the patients had good or excellent results when assessed for pain, cosmoses, and shoe wear. Of these three parameters, pain was most predictably improved. Forty-three percent of patients continued to complain of significant shoe wear problems and 28% were notably displeased with the appearance.
"Sculpting" of the metatarsal head to reduce the proximal phalanx in severe deformities has gained much support in the literature1,7,8,12 (Fig. 4). These authors recommend sequential MP soft-tissue release and shaving of the metatarsal head to allow sufficient decompression for reduction of the joint. A flexor to extensor transfer may be needed based on the stability of the MP joint. Proponents claim that preservation of the base of the proximal phalanx is important for stability, cosmetically more acceptable, and associated with less recurrence.8,12,50 One can only infer from the results of these two differing approaches that basilar hemiphalangectomy or metatarsal head arthroplasty works successfully in most, but certainly not all, patients. Stiffness of the MP joint is more a problem after shaving of the metatarsal head, whereas instability, weakness, and poor cosmesis are problems after partial hemiphalangectomy. Although proponents of each procedure laud its merits, no long-term clinical results on either class of procedure are available.
For the patient with a fixed dislocation of the MP joint, shortening of the second metatarsal, combined with dorsal soft-tissue release and flexor to extensor transfer, is an important consideration. Although no long-term results are available, proponents of this procedure have identified a relatively normal MP joint flexibility of the toe and resolution of second metatarsalgia12 (Hanson, ST: personal communication).
Early in the history of the surgical management of lesser toe deformities, amputation was the procedure of choice.51 Although its indications are now much more limited, it remains an alternative for the frail elderly patient with severe deformity.12,19 Vanderwilde and Campbell50 recently reported results of second toe amputations performed for chronic painful deformity: 91% of the patients were satisfied with their result. Progressive hallux valgus did occur but did not affect the clinical result. Medial drift of the third and fourth toes always occurs, but is not always symptomatic. Amputation may be also considered in the younger patient with severe intractable deformity, but it should be combined with arthrodesis of the hallux MP joint. Thompson39 cautioned that patients should understand amputation will not eliminate pain from metatarsalgia and a similar problem may occur in the third toe if the hallux drifts laterally.
Crossover toe deformity. The crossover toe deformity should be approached sequentially in a manner similar to mild to moderate hyperextension deformity at the MP joint.12 After an extensor tendon lengthening and transverse dorsal capsulotomy, the medial collateral ligament and medial capsule are incised. Both the lumbrical and first dorsal interosseous tendon may need to be released and the lateral capsule plicated to allow reduction.12,45,52 If dorsomedial instability persists, Coughlin recommended a flexor to extensor tendon transfer performed in a manner such that the lateral limb of the flexor digitorum longus tendon is preferentially tightened.22 This, however, rotates (pronates) the toe, and although the transfer stabilizes the MP joint, horizontal deviation is not fully corrected. Using the extensor digitorum brevis as a tenodesis or an active tendon transfer to correct the deformity is under investigation.
References
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52. Goldner JL, Ward WG. Traumatic horizontal deviation of the second toe: mechanism of deformity, diagnosis, and treatment. Bull Hosp Joint Dis Orthop Inst 1987; 47(2):123-135.
Figure Legends
Fig. 1. The anatomy of the extrinsic and intrinsic musculature of the MP joint. (From Myerson M: Claw toes, crossover toe deformity, and instability of the second metatarsophalangeal joint. In Myerson M (ed): Current Therapy in Foot and Ankle Surgery, pp 19-26. St. Louis, Mosby-Year Book, Inc, 1993; with permission.)
Fig. 2. Manual vertical stress of the MP joint is demonstrated. In the presence of instability and/or synovitis, this maneuver reproduces the patient's pain. (From Myerson M: Claw toes, crossover toe deformity, and instability of the second metatarsophalangeal joint. In Myerson M (ed): Current Therapy in Foot and Ankle Surgery, pp 19-26. St. Louis, Mosby-Year Book, Inc, 1993; with permission.)
Fig. 3. The DuVries' condylectomy is demonstrated, with resection of the plantar aspect of the condyles only (A). The condyle is retrieved with adequate soft-tissue retraction (B). (From Myerson M: Metatarsal osteotomy. In Myerson M (ed): Current Therapy in Foot and Ankle Surgery, pp 153-158. St. Louis, Mosby-Year Book, Inc, 1993; with permission.)
Fig. 4. Sculpting of the metatarsal head is demonstrated (A), secured with a Kirschner wire (B). (From Myerson M: Claw toes, crossover toe deformity, and instability of the second metatarsophalangeal joint. In Myerson M (ed): Current Therapy in Foot and Ankle Surgery, pp 19-26. St. Louis, Mosby-Year Book, Inc, 1993; with permission.)
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