Current Concepts in Hallux Valgus Biomechanics

The description of over 150 different surgical procedures to correct hallux abductovalgus (HAV) suggests a lack of agreement about the disorder’s underlying cause.¹ In the past decade, many new surgical technologies emerged, targeting both proximal and distal locations along the first ray based upon seemingly divergent notions of the primary etiology of HAV. For instance, some promote a strategy of triplane surgical correction despite a lack of agreement about the origin of the frontal plane component of HAV. This article will provide an update on current knowledge of the biomechanics of HAV to provide a useful knowledge base for the treating clinician to implement appropriate treatment options.

The Human Foot: A Biomechanical Risk for HAV Deformity

In the healthy adult foot, the first metatarsal deviates medially from the second metatarsal by 7 to 8 degrees, and the hallux deviates laterally from the long axis of the shaft of the first metatarsal by 12 to 15 degrees.² The normal angle and base of gait during human ambulation is 17 degrees abducted from the midline.³ With the first metatarsal adducted 8 degrees and the hallux abducted 15 degrees, the net alignment of the hallux from the midline or the line of progression during gait in the healthy normal foot is 24 degrees abducted (see figure 1). Thus, ground reaction forces will automatically create a lateral-directed abduction moment on the first MTPJ during walking.⁴

Further abduction and valgus torque is exerted across the healthy first MTPJ in gait due to the kinematics of the foot during heel rise and push-off. In the terminal stance phase of the gait cycle, the rearfoot inverts or supinates while the forefoot pronates.⁵ Valgus moment is produced on the hallux during push-off by pronation of the entire forefoot as well as plantarflexion and eversion of the first ray.⁶ Active contraction of the abductor hallucis muscle resists this valgus moment, along with the static restraint provided by key ligaments stabilizing the hallux against the first metatarsal head.⁷

Hallux abductovalgus begins when there is transverse plane subluxation of the first MTPJ.¹ This can result from excessive transverse plane motion of the first metatarsal, the hallux, or both. A review of published research on the pathomechanics of HAV reveals evidence supporting that both the abnormal direction of hallux motion and the abnormal direction of first metatarsal motion can be the primary event initiating the development of this deformity.¹

The Distal Mechanism of HAV Deformity

Several studies show that lateral deviation of the hallux precedes medial deviation of the first metatarsal in the development of HAV.^8-11 The primary contributor to lateral torque at the first MTPJ resulting in HAV is improper footwear.^12-15 As the normal foot already undergoes valgus torque on the hallux, any increased force on the medial aspect of the hallux, whether from constrictive footwear or pronation of the foot, will set forth a progressive cascade of events culminating in the HAV deformity.⁷

Snijders and coworkers demonstrated the profound effects of the flexor hallucis longus tendon (FHL) in creating retrograde force from the hallux to the first metatarsal.¹⁶ With slight lateral deviation of the hallux, the FHL generates a force couple which produces an abduction moment on the hallux and a reciprocal adduction moment on the first metatarsal head, resulting in medial deviation of the first metatarsal (see figure 2). The magnitude of this medial-directed adduction force from the FHL on the first metatarsal exponentially increases as the severity of the hallux abductus angle increases.¹⁷

The head of the first metatarsal has no direct muscular attachments. Forces applied distally to the first metatarsal head face resistance from structures that span the first MTPJ and attach to the first metatarsal via ligamentous connections. If there is one anatomic structure critical to the formation of HAV deformity, it is the combined medial collateral and medial sesamoid ligaments of the first MTPJ and their solitary attachment to the medial epicondyle of the first metatarsal. These ligaments resist abduction and valgus rotation of the hallux as well as medial drift of the first metatarsal. One article deemed failure of these structures as “the early and essential lesion of hallux abductovalgus.”¹⁸

As HAV deformity progresses, the sesamoid complex remains firmly attached to the second metatarsal via the deep transverse metatarsal ligament.^19,20 The sesamoids do not move relative to the second metatarsal in the transverse plane as HAV deformity progresses.^21,22 In reality, it is the first metatarsal that escapes medially, leaving the sesamoids behind.²² Hence, the sesamoids do not “sublux away from the first metatarsal” in the transverse plane as is often described in reports of HAV deformity (see figure 3).^9-11

What Causes Frontal Plane Rotation of the Sesamoids?

As HAV progresses, the hallux now laterally displaces the FHL, found embedded between the sesamoids. With lateral rotation of the hallux, the two heads of the flexor hallucis brevis and the adductor hallucis also displace laterally relative to the first metatarsal, which rotates medially. The sesamoids also assume a valgus position relative to the first metatarsal, which independently rotates medially and into inversion relative to the second metatarsal.^23,24 A shift of alignment of the abductor hallucis from a medial to a plantar direction weakens its abduction torque at the first MTPJ.¹ Frontal plane eversion or valgus rotation of the sesamoid apparatus relative to the supporting surface is due to muscle imbalance from hallux rotation and loss of dorsal contact of the fibular sesamoid to the overlying head of the first metatarsal.¹ One can visualize this change of sesamoid/alignment in the frontal plane relative to the supporting surface on axial radiographs as the sesamoid rotation angle (see figure 4).¹

Shibuya and coworkers showed that sesamoid rotation directly relates to the transverse plane position of these bones on the standard dorsoplantar radiograph.²⁵ Furthermore, this study points out that surgeons can reduce sesamoid rotation with a traditional lateral displacement osteotomy of the first metatarsal without incorporating frontal plane rotation and without a Lapidus procedure. They cite two studies by Lamo-Espinoza and team²⁶ as well as Ramdass and Meyr,²⁷ who document significant reduction of sesamoid rotation by Scarf osteotomy or distal metaphyseal osteotomy of the first metatarsal without the need for any frontal plane rotation. It appears from their investigations that restoration of alignment of the first metatarsal over the sesamoid complex with simple transverse plane translation will correct the soft tissue imbalance, which causes frontal plane rotation of the sesamoids in HAV deformity.

Does Hallux Abductovalgus Actually Cause First Ray Hypermobility?

The plantar fascia, the FHL tendon, and the soft tissue attachments to the sesamoids are critical for sagittal plane stability to the first ray, particularly during propulsion with the engagement of the windlass mechanism.^28,29 The lateral shift of these structures relative to the first metatarsal head is the primary event leading to “first ray hypermobility,” detected by static exam.³⁰ Several authors feel the condition of first ray hypermobility in the sagittal plane is the result of, rather than the cause of, HAV deformity.^31,32

The Proximal Mechanism of HAV Deformity                   

Several authors implicate instability of the first metatarsal-medial cuneiform joint as the primary driver of HAV deformity.^33-36 However, only recently have studies measured and verified significant instability or excessive motion of this joint in patients with HAV. Geng and colleagues³⁷ and Kimura and team,³⁸ using simulated weight-bearing computed tomography (CT) scans, found statistically significant increased dorsiflexion, inversion, and adduction of the first metatarsal at the first metatarsal-medial cuneiform joint in those with HAV compared to healthy controls. The study by Kimura and team also showed greater mobility of other joints of the medial column (naviculocuneiform joint and talonavicular joint) in patients with HAV compared to healthy subjects.³⁸ The authors of both studies, which showed increased mobility of the first metatarsal-medial cuneiform joint in patients with HAV, were unable to determine whether the instability at that joint was the cause of, or perhaps the result of, HAV deformity.

A review of published research concluded that the cause of sagittal plane hypermobility of the first ray in HAV deformity is not from instability of the first metatarsal-medial cuneiform joint, but secondary to medial displacement of the head of the first metatarsal from the sesamoid envelope.³⁹ Also, transverse plane displacement of the first metatarsal at the first metatarsal-medial cuneiform joint in HAV deformity appears to be driven by retrograde force from the hallux.^8-10,16 Therefore, there is no clear evidence that instability or hypermobility of the first metatarsal-medial cuneiform joint precedes or causes the development of HAV deformity. Thus far, studies have only found increased motion of this joint in patients who already have HAV deformity.^37,38,81 However, there is evidence that the direction, rather than the magnitude, of motion of this joint may be altered by foot posture, which by itself could contribute to the development of HAV deformity.

Speculation has long existed that instability of the first ray secondary to rearfoot pronation is a mechanism for the development of HAV deformity.^40-42 Glasoe and coworkers published two studies demonstrating how the axis of rotation of the first ray becomes oriented more vertically as the foot’s arch lowers.^43,44 This vertical orientation of the first ray axis theoretically allows more abduction/adduction of the first metatarsal and increases the intermetatarsal (IM) angle with loading.

Contradicting the theory proposed by Glasoe’s team are studies showing that a flatter medial arch alignment measured with plain radiographs is not a consistent finding in patients with HAV deformity.^21,45-47 A meta-analysis of radiographic studies of those with HAV compared to healthy controls showed no differences in measures of arch height, including navicular height, talocalcaneal angle, talar declination angle, and first metatarsal declination angle.¹²

Even without clear radiographic evidence of flatfoot deformity, certain gait parameters causing excessive medial loading of the foot show a connection to HAV deformity.^48-50 Kinematic studies support the findings of these kinetic studies, whereby HAV patients have shown flattening of the medial longitudinal arch and delayed push off due to loss of power of the FHL.^51-54

Examining the Triplane Component of HAV Deformity

Kinetic and kinematic studies verify that pronation and excessive medial loading of the rearfoot and forefoot is associated with HAV.^48-50 Therefore, it is not surprising that studies of HAV patients published over the past 50 years observe a pronated position of the first metatarsal.^55-65

Reports of axial rotation of the first metatarsal in HAV often use the term “pronation” to describe an everted position of the head of the first metatarsal relative to the second metatarsal or the supporting surface.^61,62 Further confusion arises when one considers that the axis of the first ray itself does not produce the triplane motion of pronation but instead provides eversion combined with plantarflexion of the first metatarsal.^63,64 Notwithstanding, studies have measured first metatarsal pronation (eversion) rotation in patients with HAV deformity using standard radiography, 61 simulated weight-bearing CT imaging,^62,65 and true weight-bearing CT multiplanar imaging.^66,67 When comparing patients with HAV to healthy control subjects, these studies report various ranges of pronation of the first metatarsal relative to the ground and relative to the second metatarsal.

Some authors have identified a pronated position of the first metatarsal as a causative factor for HAV deformity and a risk factor for recurrence after surgical correction.^68,69 Hence, multiple surgical procedures have emerged to address the pronation rotation component of the “triplane” HAV deformity.^70-74

However, the literature has not clearly identified a mechanism explaining how an everted first metatarsal leads to HAV deformity. The work of Snijders, cited previously, can offer a plausible explanation.¹⁶ Positioning the first metatarsal into eversion will presumably carry the hallux to a similar everted position relative to the supportive surface. This everted position will redirect the ground reaction force (GRF) vector from a plantar direction to a medial direction on the hallux. This medial-to-lateral direction of GRF against the hallux will generate abduction and a valgus moment at the first MTPJ. Also, the FHL tendon will realign to pull the hallux towards the second digit rather than against the ground. Reduced friction force of the hallux against the ground will allow increased transverse plane instability of the first MTPJ.¹⁶

Saltzman and colleagues used the plantar axial radiograph to measure axial rotation of the head of the first metatarsal, introducing the metatarsal pronation angle (MPA).⁶¹ (see figure 4) These researchers found that patients with HAV deformity had a mean pronation rotation of 5.7 degrees while healthy control subjects had 1.6 degrees; however, the wide range of rotation measured in both groups negated the statistical significance of the differences.

In 2015, Kim and coworkers challenged the accuracy of measuring the frontal plane position of the first metatarsal using weight-bearing radiographs and introduced a measurement known as the “alpha angle” using simulated weight-bearing CT imaging (see figure 5B).⁶² The subjects in this study with HAV deformity demonstrated 21.9 degrees of pronation of the first metatarsal while the control subjects showed 13.8 degrees, a difference of 8.1 degrees which was statistically significant.

Another study by Campbell and team using simulated weight-bearing CT imaging recognized the shortcomings of using the ground as a reference point for measuring first metatarsal pronation as this fails to take into account the effect of whole-foot pronation compared to the independent motion of the first ray.⁶⁵ The authors developed a three-dimensional computer-aided model to measure axial positioning of the first metatarsal relative to the second metatarsal. Patients with HAV deformity, when compared to healthy controls, had a statistically significant increase in pronation by an average of 8.2 degrees (P = .048).

What Do Recent Studies Say About First Metatarsal Pronation in HAV?

Weight-bearing cone-beam CT imaging provides better visualization of bone and joint position of the feet while the subject is standing upright in static stance compared to standard radiographs and simulated weight bearing CT imaging.^66,67 This technology allows direct measurement of the first metatarsal axial position without dorsiflexing the hallux as required with standing radiographs.⁷⁵

Using this technology, two studies showed no significant difference in first metatarsal pronation when comparing patients with HAV deformity to healthy controls.^66,75 Mahmoud and coworkers found that other measures of HAV deformity, including hallux valgus (HV) angle, IM angle, and tibial sesamoid position, were significantly different between the two groups but not the alpha angle of first metatarsal pronation.⁷⁵ The authors concluded that the alpha angle of first metatarsal pronation viewed with any imaging has poor diagnostic ability as it can be abnormally high in patients without HAV deformity.

Two studies of healthy patients without HAV deformity using cone-beam weight-bearing CT imaging show wide ranges of axial position of the first metatarsal, ranging from 7.5 degrees supinated to 24 degrees pronated.^76,77 It appears from studies using true weight-bearing CT imaging that normal healthy individuals can have a pronated position of the first metatarsal which can have equal or greater magnitude than that seen in individuals with HAV deformity.^66,75-77 This contradicts previous studies using simulated weight-bearing CT imaging that the magnitude of first metatarsal pronation is higher in patients with HAV deformity.^64,65 The conflicting findings of these studies raise questions about the role that first metatarsal pronation rotation has in the etiology of HAV deformity. The magnitude of first metatarsal pronation does not correlate with the severity of hallux abductus angle or IM angle in HAV deformity, contradicting a direct cause-effect relationship.^62,65,75,78

Adding further to this conundrum, Conti and coworkers noted that studies of first metatarsal pronation used the supporting surface as the plane of reference for which one measures rotation angles.⁷⁸ The authors appropriately note that these measurements reflect the total of first metatarsal pronation and whole foot pronation. These authors developed a methodology to measure the axial position of the first metatarsal relative to the second metatarsal called the triplanar angle of pronation (TAP), which included references to both the floor (floor TAP) and base of the second metatarsal (second TAP) (see figure 5C). Indeed, the values of first metatarsal pronation did not correlate when using the ground as a reference compared to the second metatarsal as the plane of reference. The authors concluded that the first metatarsal position relative to the floor represented an aggregate of whole foot pronation. They observed some patients with HAV deformity who had significant whole foot pronation yielding a high first metatarsal pronation angle to the floor but a normal rotation position relative to the second metatarsal.

Where Does the Pronation Deformity of the First Metatarsal Originate?

Given the fact that the first ray moves into the direction of dorsiflexion and inversion with loading of the foot,^63,64 it is clear that motion at other joints or an alteration of bone anatomy would have to account for a final position of eversion of the first metatarsal head relative to the supportive surface. In their comprehensive review of first metatarsal rotation and HAV deformity, Steadman and coworkers cite three mechanisms that can create first metatarsal pronation:⁷⁹ lower medial arch, hypermobility of joints of the medial column, and torsion within the first metatarsal. They cite a study by Eustace and colleagues which showed a correlation between a flatter medial arch angle and increased first metatarsal pronation in patients with HAV deformity.⁸⁰ This study suggested that instability along the entire medial column of the foot could cause first metatarsal pronation, but the measurements only came from standing radiographs. Since that time, several important studies using simulated and true weight-bearing CT imaging have revealed which joints specifically move in the direction of pronation to ultimately place the head of the first metatarsal everted to the ground in HAV deformity.^37,38,81

Using simulated weight-bearing CT imaging, three studies tracked bony displacement along the medial column of the foot of patients with HAV deformity and healthy controls as the foot moved from an unloaded to a fully loaded position. Geng and coworkers studied 20 patients with HAV deformity and 20 control patients, measuring rotation at the first metatarsocuneiform joint in unloaded and simulated weight-bearing positions.³⁷ The first metatarsal moved in the direction of inversion, dorsiflexion, and adduction relative to the medial cuneiform, and the magnitude of this motion was greater in all three planes in the patients with HAV deformity.

Wantanabe and colleagues also used CT images to study rotation of the bones of the medial column of the foot in 11 patients with HAV deformity and 11 controls.⁸¹  With simulated loading of the foot (one-third body weight), the navicular and the medial cuneiform bones moved into the direction of eversion. At the first metatarsocuneiform joint, the first metatarsal moved in the direction of inversion. There was no difference in magnitude of motion at these joints when comparing the HAV patients with the control group.

Kimura and coworkers conducted a similar study as Wantanabe’s team but applied greater axial load to the feet to simulate full body weight.³⁸ Ten patients with HAV deformity demonstrated significant instability of all joints of the medial column of the foot compared to ten healthy control patients. Specifically, at the talonavicular joint, they observed significantly greater dorsiflexion of the navicular relative to the talus in the hallux valgus group compared with the control group. At the navicular-medial cuneiform joint, the hallux valgus group showed significantly greater eversion and abduction of the medial cuneiform relative to the navicular. At the first metatarsocuneiform joint, the hallux valgus group showed substantially greater dorsiflexion, inversion, and adduction of the first metatarsal relative to the medial cuneiform. In addition, they noted dorsal hypermobility of the first ray in patients with HAV deformity who exhibited more dorsiflexion at the first metatarsocuneiform joint than healthy controls. The medial column’s overall net frontal plane motion was in the direction of eversion, primarily due to the motion of the navicular on the talus. Pronation motion of the navicular measured 9.6 degrees which counteracts 4.9 degrees of supination measured in the first metatarsal (see figure 6). Therefore, when the entire foot pronates, the net pronation or eversion of the joints proximal to the first metatarsocuneiform articulation can ultimately place the first metatarsal everted to the ground. This analysis is limited to joint motion alone. Structural deformity of the first metatarsal must also become a consideration.

Is First Metatarsal Torsion the Hidden Answer to the Pronation Mystery?

Along with studies of medial column joint motion, one must examine important studies on the shape of the first metatarsal bone, which can influence the position of the head of the first metatarsal on the ground. It has been long recognized that the first metatarsal in non-human primates has an intrinsic “twist” or torsion from the proximal margin to the distal margin in the direction of valgus or eversion.⁸²

Two studies have shown that humans with HAV deformity have valgus torsion in the first metatarsal, while healthy humans without HAV deformity have minimal torsion in the first metatarsal.^83,84 Both Ota and colleagues and Cruz and team used slightly different methodologies to measure the intrinsic torsion of the first metatarsal but found remarkably similar findings when comparing the magnitude of torsion in HAV patients compared to control subjects without HAV deformity. Ota and colleagues measured 17.6 degrees of valgus torsion in HAV subjects and 4.7 degrees valgus torsion in controls, with a difference between the groups of 12.9 degrees which was significant.⁸³ Cruz and team measured a mean metatarsal bone rotation of 15.36 degrees in the direction of valgus in the HAV group and 3.45 degrees in the control group with a difference of 11.9 degrees which again was significant.⁸⁴ Both sets of authors proposed that torsion in the first metatarsal could be an inherited trait causing HAV or could be a developmental, acquired deformity from the HAV condition itself.^83,84

However, Cruz and team points out that several studies verify that the magnitude of first metatarsal pronation does not increase or correlate with the severity of HAV deformity when measuring hallux valgus and intermetatarsal angles.^64,65,71,78 This supports the notion that first metatarsal pronation likely precedes HAV deformity and is not the result of the condition. However, one must also note that studies have shown that significant first metatarsal pronation is possible in patients without HAV, and this may originate more from joint rotation than torsional changes.^75-77

The only study published thus far measuring first metatarsal torsion and joint position of the first ray relative to the supportive surface recently came from Randich and coworkers.⁸⁵ This study of 10 patients with HAV and 36 control patients used weight-bearing CT imaging to measure the sesamoid rotation angle, the alpha angle of the first metatarsal head, the first metatarsal base angle, and the medial cuneiform angle. Compared to controls,  patients with HAV deformity showed a significantly more pronated sesamoid position (23.4 degrees versus 6.6 degrees), significantly more pronated first metatarsal head position (17.7 degrees versus 9.8 degrees), and significantly more valgus torsion in the first metatarsal (22.2 degrees vs 13.5 degrees). However, a critical finding of this study was the observation that the position of the first metatarsocuneiform joint was essentially the same in both HAV patients and controls. In fact, there was a trend for this joint to be more supinated in HAV patients than in controls. This finding is in agreement with the studies published by Geng,³⁷ Watanabe,⁸¹ and Kimura.³⁸ Randich and team concluded that their findings support the notion that the pronation deformity in HAV is due primarily to torsion of the first metatarsal bone itself rather than rotation at the first metatarsocuneiform joint.⁸⁵ However, their study did not evaluate the position of the proximal joints of the medial column, specifically the talonavicular joint, which has demonstrated significant pronation rotation in HAV patients.³⁸

In Summary

The following are key concepts regarding the current literature on hallux valgus biomechanics that readers should know:

1. Lateral positioning of the hallux causing medial displacement of the first metatarsal are the key components of the distal mechanism of HAV deformity.
2. Patients with HAV deformity have increased transverse plane and sagittal plane motion of the first metatarsocuneiform joint. There is debate as to whether this is a cause or a result of HAV deformity.
3. Kinetic and kinematic studies of patients with HAV deformity demonstrate a pronated hindfoot with excessive medial arch loading.
4. A pronated position of the first metatarsal has been identified in both HAV patients and healthy individuals without deformity.
5. Patients with HAV deformity show more frontal plane motion in the first metatarsocuneiform joint than healthy subjects, but this motion is in the direction of supination, not pronation with loading of the foot.
6. Isolated rotation of the first metatarsal in HAV deformity is in the direction of supination, while total rotation of the entire medial column of the foot is in the direction of pronation.
7. The talonavicular joint is the major contributor to pronation motion of the entire first ray in HAV deformity.
8. Torsion within the first metatarsal bone in the direction of eversion is more significant in patients with HAV deformity compared to healthy controls.  

Dr. Richie is an Adjunct Associate Professor within the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt University in Oakland, Calif. He is a Fellow and Past President of the American Academy of Podiatric Sports Medicine. Dr. Richie is a Fellow of the American College of Foot and Ankle Surgeons, and the American Academy of Podiatric Sports Medicine. He discloses that he is Founder and President of Richie Technologies.

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