Current Concepts In Medial Column Hypermobility

   The impact of medial column hypermobility on foot function and deformity development has gained significant attention in the past few decades.1 It has been associated with pes planus, metatarsus primus adductus, hallux valgus, midfoot arthritis, metatarsalgia, plantar plate injury and lesser metatarsal stress fractures. Dudley Morton, an anatomist, introduced the concept of hypermobility.2 The so-called “Morton’s foot” includes hypermobility, equinus and a short first metatarsal.3    While the existence of hypermobility is generally accepted, its definition remains an area of controversy. In simple terms, most would agree hypermobility is an excess of first ray motion in the sagittal plane. Although hypermobility is often used to describe the sagittal plane, it may affect any plane. Morton’s theory suggests hypermobility may functionally translate into a loss of medial column stability and the first ray may become ineffective in resisting the vertical ground reactive force.3 Accordingly, the weightbearing loads are transferred laterally to the lesser metatarsals.    The term first ray insufficiency better describes the lesser metatarsal overload due to the first ray incompetence. Early clinical signs of first ray insufficiency include the absence of callus under the first metatarsal head and/or the presence of callusing under the lesser metatarsal heads. The second metatarsal typically takes the brunt of this force and a discrete callus may be present. Later stages may include metatarsalgia, plantar plate injury, hammertoe formation and lesser metatarsal stress fracture. Advanced first ray insufficiency may result in midfoot collapse and arthrosis.    Medial column motion is the combined motion of the talonavicular joint, naviculocuneiform joint, first intercuneiform joint and the first metatarsocuneiform joint (MCJ). Therefore, hypermobility does not pertain only to the first MCJ. Moreover, hypermobility is often inappropriately used synonymously with instability. While the first ray may be hypermobile, it may be a functionally stable unit in the propulsive phase of gait due to engagement of the windlass mechanism. Palladino believes a nonhypermobile first ray can be dynamically unstable with pronation.4 Nonetheless, the overall stability of the medial column is dependent on a complex interaction between the skeletal structure and dynamic function of the ligaments and musculature.    First ray motion is triplanar.5 The first ray is composed of the first metatarsal and medial cuneiform. The axis is deviated 45 degrees from the frontal and sagittal plane, and is almost parallel with the transverse plane. This orientation results in triplanar motion in which sagittal plane motion is associated with equal frontal and transverse plane contributions. However, with pronation, it has been postulated that the first ray axis shifts and favors adduction, and the dominant planes of deformity accordingly become the transverse and sagittal planes.    Morton stated that “the first metatarsal segment is the most important element of medial stability in the foot.”3 During gait, the foot functions as a mobile adapter and as a rigid lever. The mobile adapter function is needed at heel contact to absorb the force of impact. Throughout stance, the foot must gain stability to act as a rigid lever for propulsion. One achieves this through external rotation of the leg, supination of the subtalar joint and locking of the midfoot. As body weight passes over the foot, the arch must resist the vertical load and maintain its stability for the propulsive phase of gait.

How To Assess For Medial Column Hypermobility

   When identifying medial column hypermobility, one should measure first ray mobility in terms of the total excursion and its relationship to the lesser metatarsals. It is a non-weightbearing measurement in which the examiner isolates and stabilizes the lesser metatarsals with one hand and tests first ray excursion with the other hand. One would do this measurement with the ankle and subtalar joints in neutral position. Normal first ray range of motion, consisting of equal dorsiflexion and plantarflexion, is 10 mm.5    Researchers have questioned the reliability and validity of manually measuring first ray motion.6,7 This is partly due to the confusion on the varied definitions of hypermobility. Measurement techniques, experience and subjectivity also contribute to the problem. Several investigators advocate using a special device to better quantify the motion.6,8-10 Klaue measured motion with a micrometer and modified ankle-foot orthoses.9 Lee clinically measured first ray motion with simple handheld rulers.6 Other devices have been used as well.6,7 A study specifically evaluating intra-rater measurements and inter-rater agreement demonstrated poor reliability with manual methods when authors compared them to measuring first ray motion with a device.6    Roukis and Landsman advocate testing first ray mobility with the windlass mechanism engaged via the “dynamic Hicks test.”11 One would do this by dorsiflexing the hallux while evaluating first ray mobility. The goal of this testing is to gain insight into how the first ray would function during propulsion with the windlass engaged. Roukis and Landsman believe there is no instability if the excessive medial column motion returns to normal with the dynamic Hicks test.    While some might cite overall sagittal excursion, it is actually the ability of the first ray to rise above the level of the lesser metatarsals that is responsible for the pathology associated with first ray insufficiency. Morton used the phrase “dorsal hypermobility of the first metatarsal segment” to distinguish this entity further. Glasoe, Yack and Saltzman classify a first ray as hypermobile when the inferior aspect of the first metatarsal head rises above the plane of the lesser metatarsals.12 Using a first ray measuring device, Klaue and Glasoe separately defined hypermobility as a dorsal excursion >9 mm.6,7 Voellmicke and Deland use the term “dorsal instability” and proposed that >8 to 10 mm of dorsal displacement with manual testing is abnormal.13    McInnes and Bouche have recognized the importance of first ray position and distinguished among three groups: parallel, elevated and plantarflexed.14 The parallel group demonstrates equal dorsiflexion and plantarflexion. The elevated group demonstrates only dorsal excursion as the first ray is unable to plantarflex below the level of the lesser metatarsals. The plantarflexed group is unable to dorsiflex above the level of the lesser metatarsals. In theory, a plantarflexed first ray should not demonstrate the clinical stigma of first ray insufficiency.    While medial column hypermobility is a clinical finding, it does not necessarily describe how the foot will function throughout gait because a hypermobile segment may be stabilized during gait with the windlass engaged and through the action of peroneus longus. Although the dynamic Hicks test is an attempt to differentiate hypermobility from instability, it does not encompass the total clinical effect.    Can one consider hypermobility pathologic in asymptomatic individuals without foot deformity? When hypermobility is present along with the stigma of first ray insufficiency and/or a moderate to large hallux valgus deformity in symptomatic individuals, I have found the term “pathologic hypermobility” to be a better representation of the clinical picture.

Key Insights Into Transverse Plane Instability

   Transverse plane motion should be evaluated in addition to sagittal plane motion. Transverse plane motion is difficult to measure without hallux valgus and is usually measured by the amount of first metatarsal adduction — the intermetatarsal angle. Of course, we know that transverse plane motion exists because patients would not develop metatarsus primus adductus in the first place. One would treat most transverse plane motion by restoring the intermetatarsal angle with an osteotomy or fusion.    Additionally, we know that transverse plane motion exists because it is often the cause for recurrent metatarsus primus adductus after correcting the intermetatarsal angle with an osteotomy or isolated first MCJ fusion. Recurrence after an isolated first MCJ fusion further illustrates that proximal and/or concurrent instability, other than the first MCJ, also exists. This may have pathologic implications if it is not appreciated at the index operation.    Excess transverse plane motion (hypermobility) is difficult to identify preoperatively. According to Palladino, the patient has transverse plane instability of the first MCJ if the intermetatarsal angle increases when one manually applies a hallux retrograde force preoperatively.4 While this maneuver may be helpful, it is difficult to measure small or subtle increases, and it does not determine which midfoot joint is responsible for the instability. McInnes and Bouche attempt to distinguish a rigid from flexible deformity by manually distracting the hallux to relieve the retrograde force at the first metatarsophalangeal joint while manually applying a medial stress to the first metatarsal head.14    Transverse plan hypermobility is usually synonymous with instability. With regard to an isolated first MCJ arthrodesis, I define persistent transverse plane instability as persistent transverse plane motion that occurs after one restores the intermetatarsal angle. The same concept could be applied for a metatarsal osteotomy but one should take into account the first MCJ motion.

A Guide To Radiographic Findings Of First Ray Insufficiency

   Several radiographic findings have been thought to be associated with hypermobility. These include first MCJ obliquity, cortical thickening of the second metatarsal shaft, diastasis of the first intercuneiform joint articulation and a short first metatarsal.    According to Morton’s theory, a short metatarsal in relation to the second metatarsal transfers weight laterally. The obliquity of the first MCJ is thought to be an atavistic trait. Obliquity greater than 30 degrees has been considered abnormal and an indication for a first MCJ arthrodesis.15 However, radiographic studies have illustrated that this obliquity may be radiographically exacerbated.6,17    Diastasis between medial cuneiform and second metatarsal base and/or diastasis of the first intercuneiform joint may suggest the presence of hypermobility in a location other than the first MCJ. One should be careful to notice radiographically exacerbated widening as well.    Morton considered hypertrophy of the second metatarsal shaft the physiologic response to increased load secondary to first ray hypermobility. Some surgeons have also supported this association while others dispute it.18-22 Coughlin and Grebing found no association between first ray mobility and second metatarsal cortical hypertrophy in 43 patients with hallux valgus compared to a control group.22 Although patients with hypermobility were not specifically compared to the controls, the mean mobility of the first ray was 6.9 mm (ranging from 3 mm to 12 mm).

Which Medial Column Joint Is Responsible For Hypermobility?

   Two separate cadaveric studies have attempted to determine the contribution of medial column joints to total mobility. Roling and colleagues performed selective arthrodeses in a cadaveric open kinetic chain study on six specimens.23 The cited contributions are as follows: talonavicular joint (9 percent), naviculocuneiform joint (50 percent) and first MCJ (41 percent).    Faber, et. al., performed a similar cadaveric study in hallux valgus feet and demonstrated that the first MCJ contributed 57 percent, the talonavicular joint contributed 8 percent and the naviculocuneiform joint contributed 35 percent of medial column motion.24 Both cadaveric studies suggest the first MCJ is a large contributor to medial column mobility.    In the presence of hypermobility, it may be unclear which of the medial joints are specifically responsible for the excessive motion from the clinical exam alone. In some cases, one may see the gross motion on the dorsal aspect of the first MCJ when testing sagittal first ray motion. McInnes and Bouche perform an isolated dorsal drawer (translation only) of the first MCJ and consider excessive motion indicative of the first MCJ instability.14 The radiographs often offer clues to which medial column joint is involved. Clinicians can discern obvious sagittal plane collapse of the first MCJ or naviculocuneiform joint. Drawing Meary’s angle may identify subtle collapse. In some cases, one may identify slight widening of the plantar aspect of the first MCJ.    The presence of medial cuneiform obliquity and metatarsus primus adductus suggests the first MCJ as a prime contributor to medial column mobility. Diastasis of the first intermetatarsocuneiform area suggests more proximal hypermobility. I believe this widening may represent attenuation of Lisfranc’s ligament and may indicate transverse plane instability.    In some cases, more proximal instability from the first intercuneiform or naviculocuneiform joint may contribute to the hypermobile segment. This may result in persistent transfer metatarsalgia or lesser metatarsal stress fractures after a Lapidus. Unappreciated hypermobility, especially in the transverse plane, is the cause of bunion recurrence after an isolated first MCJ arthrodesis for hallux valgus.

What You Should Know About The Intraoperative Hypermobility Test

   Today, most surgeons perform a modified Lapidus procedure with an isolated first MCJ arthrodesis. One should accordingly perform an intraoperative evaluation after stabilizing the first MCJ and also assess the sagittal motion and transverse plane stability. While hypermobility is considered to predominately affect sagittal plane stability, some patients exhibit multiplanar instability. Splaying of the intermetatarsal angle indicates concomitant transverse plane instability and excessive sagittal plane motion indicates associated proximal hypermobility.    The purpose of the intraoperative exam is to identify hypermobility in areas other than the first MCJ and to identify persistent transverse plane instability. One would perform the evaluation after correcting the intermetatarsal angle and fixating the first MCJ with screws. Evaluate overall sagittal excursion and metatarsal position in the method described above. One should compare the amount of motion to preoperative levels and correlate it with the clinical scenario. In some situations, additional joint fusions may be indicated.    It should also be mentioned that overdissecting the lateral aspect of the medial cuneiform may temporarily destabilize the first intercuneiform joint and may be mistaken for persistent proximal hypermobility and/or transverse plane instability. Nonetheless, the sagittal plane motion should not increase after isolated fixation of the first MCJ. One must carefully assess persistent sagittal plane motion and correlate it with the clinical scenario. In some situations, additional joint fusions may be indicated.    Clinicians can test transverse plane stability by placing an index finger between the metatarsal heads in the first interspace after performing the first MCJ fusion. The metatarsal heads should be situated close together.14 If the metatarsals splay open, then proximal transverse plane instability exists. Of course, the intermetatarsal angle must be reduced for this test to be valid. Excessive persistent transverse plane instability is less forgiving since additional fusions are often necessary to prevent the intermetatarsal angle from splaying. This is especially important when treating hallux valgus because the bunion will likely return and the patient may consider it a failed surgery. McInnes and Bouche attributed intercuneiform diastasis to one case (of 32 feet) of recurrent metatarsus primus adductus after an isolated first MCJ fusion, which the patient rated as “ineffective.”14

Hypermobility: How Does It Affect Bunion Surgery Selection?

   Several investigators have associated hypermobility with hallux valgus.8,9,25 It is unclear if hypermobility is an etiologic factor in hallux valgus or is simply another morphological factor in its development. Klaue and Hansen believe the latter theory because they observed hallux valgus in patients without hypermobility.9 Lee, et. al., found 38 percent of 60 patients with hallux valgus had hypermobility.8 Faber identified hypermobility in 68 of 101 feet.26 Shimizu suggested that “increased mobility in the sagittal plane may play a considerable role in the development of pain” in patients with hallux valgus.27    Although the Lapidus procedure is indicated for treating hypermobility, there is evidence that metatarsal osteotomies are similarly effective and an alternative to the Lapidus.    A recent prospective, randomized study by Faber and colleagues demonstrated similar outcomes in comparing a Lapidus to a Hohmann procedure in patients with hypermobility.26 This study included mild to moderate bunion deformities. The authors suggested that surgeons can better determine the indication for Lapidus by the degree of deformity (IM >15 percent) rather than the presence of hypermobility. A prospective study evaluating the role of first ray hypermobility in the outcome of the Lapidus and proximal metatarsal osteotomy has yet to be performed.    Coughlin has speculated that a proximal periarticular (crescentic) osteotomy with temporary screw fixation across the first MCJ may lead to MCJ stiffness.28 Coughlin demonstrated that a proximal metatarsal osteotomy with a distal soft tissue release reduces first ray motion in cadavers with hallux valgus.29 In this study, the average sagittal mobility was 11 mm. Rush and Christensen demonstrated in cadavers that reducing the intermetatarsal angle through a distal metatarsal osteotomy restores the windlass mechanism.30    Hypermobility with hallux valgus is an indication for the Lapidus procedure.18,20,21,31,32 The decision to treat the hypermobility through selective fusions depends on the following: degree of deformity (intermetatarsal angle), overall sagittal mobility, first ray position, ability to restore the windlass mechanism, transverse plane instability, first ray length, stigma of first ray insufficiency and surgeon experience.    In the absence of hypermobility, surgeons have utilized the Lapidus arthrodesis for moderate to large bunions because of the procedure’s ability to treat the deformity at the apex. In some severe cases of metatarsus primus adductus, it would be extremely difficult to correct the intermetatarsal angle with a metatarsal osteotomy. Selective fusions address the hypermobility at the source by providing structural stability to the first ray.    When treating hypermobility, one should consider the degree of sagittal mobility and metatarsal position because this is often the determining factor if the foot will be dynamically stable. Extreme amounts of hypermobility, especially with a midfoot fault, are likely difficult to stabilize dynamically even when the intermetatarsal angle is corrected. In my opinion, it is better to treat these cases with a fusion because surgeons can address concomitant proximal hypermobility at the same time. McInnes and Bouche identified that “... the Lapidus failed to eliminate the majority of first ray motion” in patients who had severe hypermobility preoperatively.14    In most situations, the first MCJ is responsible for the majority of the hypermobility. In unpublished data, Palladino has identified a 40 percent reduction in first ray mobility after an isolated first MCJ fusion.33 Interestingly, this number correlates with the first MCJ contribution to the total first ray mobility identified in the cadaveric study by Roling and Christensen.23 Typically, an isolated first MCJ fusion will return the first ray motion to a nonhypermobile or “normal” amount. Therefore, the positioning of the fusion is of utmost importance to prevent the first ray from rising well above the plane of the lesser metatarsals. One can achieve this through plantarflexion of the fusion or translation of the metatarsal inferiorly.    When it comes to gross hypermobility after the intraoperative hypermobility test (after first MCJ fusion), one may address this via a concomitant first intercuneiform or naviculocuneiform joint arthrodesis. If the surgeon finds equivocal persistent hypermobility with lesser metatarsal overload, he or she may want to consider additional fusions in these cases. This can be determined on a case by case basis. If first ray excursion does not rise above the level of the lesser metatarsals (plantarflexed), additional fusions may not be necessary. Similarly, if the first ray demonstrates equal dorsal and plantar excursion (parallel), additional fusion may not be needed because the windlass mechanism may dynamically stabilize the segment.

Other Essential Considerations

   After performing a first MCJ fusion, if one identifies persistent transverse plane instability via the intraoperative hypermobility test, surgeons must treat this appropriately. If one does not correct the transverse plane instability, the intermetatarsal angle will not be maintained postoperatively.    As per the original article by Lapidus, surgeons should fuse the bases of the first and second metatarsals together in conjunction with the first MCJ.34-36 It is obvious this concomitant fusion addresses proximal hypermobility as well as transverse plane instability. Coetzee also includes the second metatarsal base into the fusion.37-39 When incorporating the second metatarsal base into the first MCJ fusion, one must pay close attention to the final sagittal position of the first ray because the metatarsal will be rigidly fixed in the position obtained in the operating room.    I have found incorporating the second metatarsal base is unnecessary in most patients since it provides significant midfoot rigidity. In my isolated first MCJ fusions, I often incorporate a temporary screw into the intermediate cuneiform during the healing process. I have found patients seem to prefer the added mobility when one removes that screw. After one performs an isolated first MCJ fusion, it is important to understand the first ray still maintains some motion and the pathological motion will have been eliminated, significantly reduced and/or dynamically stabilized.    After performing a first MCJ fusion, if one identifies persistent transverse plane instability via the intraoperative hypermobility test, surgeons should consider treating this because the intermetatarsal angle will likely not be maintained postoperatively. When one identifies persistent sagittal plane hypermobility or transverse plane instability, the options for correction include incorporating the base of the second metatarsal into the fusion (original Lapidus technique), fusing the first intercuneiform and/or the naviculocuneiform joint.    In mild cases of intraoperative metatarsal splaying, I have found that the temporary screw into the medial cuneiform (as described above) stiffens up the midfoot complex to limit this splaying. In some instances, the screw is left permanently in place. One may address significant intraoperative metatarsal splaying with a concomitant intercuneiform fusion.    Courriades also recommended fusion of the first intercuneiform joint for first ray hypermobility.40 In most situations, I prefer addressing the sagittal hypermobility and transverse instability at the first intercuneiform joint, as opposed to the original Lapidus technique, because it is difficult to access and prepare the second metatarsal base for incorporation into the fusion after fixation of the first MCJ. The first intermetatarsal artery may be at increased risk for iatrogenic injury. One can easily access the first intercuneiform joint by extending the dorsal incision. Surgeons can incorporate the intercuneiform fusion within the first MCJ fusion by utilizing one or two screws from the medial cuneiform and/or a screw from the base of the first metatarsal. When performing the intercuneiform fusion, one should use bone graft to help ensure a successful fusion.    Theoretically and in my experience, the concomitant intercuneiform joint fusion provides adequate stability to combat persistent hypermobility of transverse plane instability. Additionally, I have not seen persistent hypermobility or instability in the operating room or postoperatively following the concomitant intercuneiform fusion. I would estimate that 5 percent of moderate to severe bunions have persistent transverse plane instability. Do not take the intercuneiform fusion lightly. It is a powerful procedure and provides significant midfoot rigidity similar to that of a naviculocuneiform joint fusion or original Lapidus technique. One should inform patients of a potential need for concomitant fusions prior to surgery.

In Conclusion

   The purpose of this article is not to promote the widespread use of the Lapidus or midfoot fusions but rather to further illustrate concepts that are important when surgically treating pathologic hypermobility. One must carefully consider selective fusions based on the preoperative exam, intraoperative evaluation and clinical scenario. Studies that specifically compare the different options for selective fusions with persistent hypermobility and instability would be useful. Similarly, studies focused on the relationship between hypermobility and transverse plane instability with recurrent metatarsus primus adductus are needed. Dr. Blitz is an attending podiatric surgeon within the Department of Orthopedics at the Kaiser Permanente Medical Center in Santa Rosa, Ca. He is a Fellow of the American College of Foot and Ankle Surgeons.
 

References:

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