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Current Concepts Review

The Pathogenesis of Hallux ValgusA.M. Perera, FRCS(Orth), Lyndon Mason, MRCS(Eng), and M.M. Stephens, FRCSI

Investigation performed at the University Hospital of Wales, Cardiff, United Kingdom

� The first ray is an inherently unstable axial array that relies on a fine balance between its static (capsule, ligaments,and plantar fascia) and dynamic stabilizers (peroneus longus and small muscles of the foot) to maintain itsalignment.

� In some feet, there is a genetic predisposition for a nonlinear osseous alignment or a laxity of the static stabilizersthat disrupts this muscle balance. Poor footwear plays an important role in accelerating the process, but occu-pation and excessive walking and weight-bearing are unlikely to be notable factors.

� Many inherent or acquired biomechanical abnormalities are identified in feet with hallux valgus. However, theseassociations are incomplete and nonlinear.

� In any patient, a number of factors have come together to cause the hallux valgus. Once this complex pathogenesisis unraveled, a more scientific approach to hallux valgus management will be possible, thereby enabling treatment(conservative or surgical) to be tailored to the individual.

In the nineteenth century, hallux valgus was thought to be dueto an enlargement of the metatarsophalangeal joint of the greattoe1. It was not until Carl Hueter (1838-1882), a German-bornsurgeon, coined the term hallux abducto valgus 2 that the de-formity was more correctly described as a lateral deviation ofthe great toe at the metatarsophalangeal joint. A century ofdebate has failed to settle the importance of intrinsic versusextrinsic causes in the etiology of hallux valgus. In the 1950s,Sim-Fook and Hodgson compared shoe-wearing and non-shoe-wearing groups and showed a dramatic increase in theprevalence of hallux valgus among the shoe-wearing group3.Unfortunately, it did not explain the prevalence of hallux valgusin the community of people who had never worn shoes, nor didit account for the many individuals who wear high-fashionfootwear and never become affected. Clearly, the issue is morecomplex than simply a problem of footwear. Although muchresearch has been done to define the multifactorial origin of

hallux valgus and the effect of those factors on surgical out-comes, the quality and strength of this evidence have beenvariable.

Pathoanatomy of Hallux ValgusDevelopment of Hallux Valgus (Figs. 1 through 4)It is generally accepted4,5 that hallux valgus occurs in steps,frequently on a background of several predisposing factors(Table I). These steps do not necessarily occur in series but maytranspire in parallel. These steps are as follows:

1. As the only medial supporting structures of the firstmetatarsophalangeal joint are the medial sesamoid and medialcollateral ligaments, their failure is the ‘‘early and essentiallesion.’’6

2. The metatarsal head can then drift medially, slippingoff the sesamoid apparatus. An oblique or an unstable tarso-metatarsal joint may encourage this movement.

Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support ofany aspect of this work. One or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission ofthis work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. Noauthor has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence whatis written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of thearticle.

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COPYRIGHT � 2011 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED

J Bone Joint Surg Am. 2011;93:1650-61 d http://dx.doi.org/10.2106/JBJS.H.01630

3. The proximal phalanx moves into a valgus position asit is tethered at its base to the sesamoids, the deep transverseligament (via the plantar plate), and the adductor hallucistendon.

4. The metatarsal head sits on the medial sesamoid andcan erode the cartilage and the crista. The lateral sesamoid canappear to sit in the intermetatarsal space although it does notactually move.

Fig. 1 Fig. 2

Fig. 1 Illustration of the medial view of the hallux, showing the medial structures whose failure is essential for hallux valgus deformity to occur. Fig. 2

Illustration of the metatarsal head of the hallux in the anteroposterior plane, showing the medial shift of the metatarsal head (step 2 in the development of

hallux valgus), with the valgus displacement of the proximal phalanx due to its attachment to the sesamoids, the deep transverse ligament (via the plantar

plate), and the adductor hallucis tendon (step 3). The extensor hallucis longus bowstrings laterally (step 6).

Fig. 3 Fig. 4

Fig. 3 Illustration showing the medial shift of the metatarsal head in the axial plane (step 2 in the development of hallux valgus) and the pronation of the

metatarsal head that results from the muscle forces acting on it (step 7). The figure also illustrates the position of the sesamoids, abductor hallucis (AbH),

adductor hallucis (AdH), flexor hallucis longus (FHL), and extensor hallucis longus (EHL). Fig. 4 Illustration of the deformity of hallux valgus in the axial plane.

The metatarsal is pronated and shifted medially, resulting in the lateral shift of the abductor hallucis (AbH), adductor hallucis (AdH), flexor hallucis longus

(FHL), and extensor hallucis longus (EHL) (steps 6 and 8 in the development of hallux valgus). The bursa overlying the medial eminence thickens because of

the pressure effect of footwear on a prominent medial eminence (step 5). Because of the pressure of the medial sesamoid on the crista, the cartilage is

eroded and the crista flattened (step 4).

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TH E J O U R N A L O F B O N E & JO I N T SU R G E RY d J B J S . O R G

VO LU M E 93-A d NU M B E R 17 d S E P T E M B E R 7, 2011TH E PAT H O G E N E S I S O F HA L LU X VA L G U S

5. The bursa overlying the medial eminence can thickenbecause of the pressure effect of footwear on a prominentmedial eminence.

6. The extensor and flexor hallucis longus tendons appearto bowstring laterally7, increasing the valgus displacement andoccasionally acting as dorsiflexors of the proximal phalanx.

7. As the metatarsal head drops off the sesamoidapparatus, it pronates because of the muscle forces actingacross it.

8. Normally, the abductor hallucis strongly resists valgusof the proximal phalanx, but it becomes dysfunctional as itsmedial and plantar attachment rotates inferiorly8. The adduc-tor hallucis is attached on the plantar surface laterally so ittends to pull the phalanx into pronation as well as tetheringits base.

9. The weaker dorsal metatarsophalangeal joint capsule isnot reinforced by any tendons and rotates medially withpronation and provides poor stability 9.

10. The metatarsal head elevation with medial motioncan transfer plantar pressure laterally. The relatively mobilefifth metatarsal may also splay.

First Ray BiomechanicsThe first ray plays a key role in maintaining the structure of themedial arch9, and as the main load-bearing structure10, it issubject to substantial forces during gait. Failure anywhere alongthe first ray, from the distal phalanx to the talonavicular joint,can result in hallux valgus. It is therefore worth considering thefirst ray biomechanics as a common factor to many of the keytheories. There are no tendon attachments on the metatarsalhead, and maintenance of this inherently unstable axial arrayrequires (1) a congruent and stable metatarsophalangeal jointduring push-off, (2) a distal metatarsal articulation angle thatencourages stability, (3) balanced static and dynamic restraints,and (4) a stable tarsometatarsal joint11.

SesamoidsThe functions of the sesamoid bones are, first, to absorbweight-bearing forces and enhance the load-bearing capacity

of the first ray. The sesamoids increase the moment arm ofthe flexor hallucis brevis, which powers plantar flexion of thehallux, and, finally, they function to elevate the first metatarsalhead, which dissipates the forces on the metatarsal head12-16.The first metatarsal head is elevated on the sesamoids duringstance, but the sesamoids move during hallux dorsiflexion,coming to lie anterior to the metatarsal head rather than in-ferior. The sesamoid sling thus facilitates the first metatarsalplantar flexion that is essential for hallux dorsiflexion.

The sesamoids can appear to subluxate with first meta-tarsal pronation alone17,18, but true subluxation requires anumber of events to occur first. The joint reaction force ofthe metatarsosesamoid joint is normally sufficient to preventsubluxation19. Thus, the metatarsosesamoid joint must becomeunloaded either by elevation of the first metatarsal head or bythe transfer of plantar pressure laterally. Finally, the soft-tissuerestraints and the cristae need to fail.

First Ray MotionMorton believed that dorsal hypermobility of the first meta-tarsal segment was responsible for the widest array of footdeformity 20. However, several studies have questioned whethermotion at the tarsometatarsal joint even exists21-24. The studiesthat described motion of the first tarsometatarsal joint hadno consensus with regard to either the axis of movement orthe magnitude. In so-called normal feet, the small amount ofmovement permitted at the first tarsometatarsal joint is am-plified by the long metatarsal shaft, resulting in an averagedorsoplantar motion of 6 mm25. However, the mean range ofmotion of the entire first ray in the foot with hallux valgus issignificantly greater in both the sagittal and frontal planes.

First Metatarsophalangeal Joint MotionThe first metatarsophalangeal joint is a partial ball-and-socketjoint rather than a simple hinge. When the hallux is held stable(as in push-off), the kinematic coupling of the first ray and theankle joint motion results in a frontal plane rotation, pronatingthe great toe and causing a medial transverse plane motion. Thesemotions both increase the loading on the medial aspect of thetoe26, creating a valgus moment at the metatarsophalangealjoint.

With the foot flat on the ground and loaded evenly (i.e.,midstance), the dorsiflexion of the metatarsophalangeal joint islimited to approximately 20� as further motion requires plantarflexion of the first metatarsal27. The phalanx and metatarsal arecoupled such that 1� of phalangeal dorsiflexion requires 3� ofmetatarsal plantar flexion28. The spiral-shaped nature of themetatarsal head29 forces a translational sliding motion as theproximal phalanx rotates30,31. Thus, the locus of the axis ofrotation has to move in an arc32 that necessitates metatarsalmotion proximally and plantarward in order to avoid com-pression at the metatarsophalangeal joint. Prevention of thisplantar flexion hinders dorsiflexion of the first metatarsopha-langeal joint even when non-weight-bearing33. Plantar flexionof the first ray also maintains ground contact during heel risewhen the obliquity of the metatarsophalangeal break, which is

TABLE I Potential Intrinsic and Extrinsic Factors

Extrinsic Intrinsic

High-heeled narrow shoes Genetics

Excessive weight-bearing Ligamentous laxity

Metatarsus primus varus

Pes planus

Functional hallux limitus

Sexual dimorphism

Age

Metatarsal morphology

First-ray hypermobility

Tight Achilles tendon

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the axis of the four lateral metatarsophalangeal joints, is en-hanced during late stance, causing lower-extremity externalrotation and inversion of the subtalar joint34.

Normal gait uses up to 65� of first metatarsophalangealdorsiflexion, and first ray elevation substantially compromisesthis range of motion35. Furthermore, the normal hallux has atendency toward valgus (and any pronation further encouragesthis tendency). This combination means that, when dorsiflexionis restricted, the toe is forced to ‘‘escape’’ laterally in the directionof least resistance. As the transverse sphericity of the first meta-tarsal head29 permits multiplanar motion36, the collaterals,sesamoids, and the so-called rein effect of the first metatarso-phalangeal joint rotator cuff are all required for stability.

Etiology of Hallux ValgusThe different factors can be divided into extrinsic and intrinsicrisks (Table I).

Extrinsic FactorsFootwearEven prior to the understanding of hallux valgus pathology, theuse of footwear has been implicated as an etiology1,37,38. In 1909,Porter39 advised against performing corrective surgery on pa-tients unwilling to wear appropriate footwear because of agreater risk of recurrence of the deformity. There is a lowprevalence of hallux valgus in unshod populations40,41, and theprevalence increases with changes in shoe fashion42. However,the association is not complete (Table II), and footwear is not atall important in juvenile hallux valgus43.

High heels are commonly blamed for hallux valgus, andthere is a direct association between increased first metatarsalloading and a valgus moment44. This forefoot loading is exac-erbated by the forefoot sliding forward into the toe-box, pro-nating as it does so. A third of the population naturally favorsloading the lateral part of the forefoot and is at greatest risk ofthis deformity. However, this forefoot loading is probably moreimportant in deformity progression than initiation45. There issome limited evidence that altering the position of the heelcould counteract this tendency28.

The prevalence of hallux valgus in women who wear shoeswith a narrow toe-box or a high heel is certainly not 100%. Thismay be due, in part, to the buttressing effect of the second toe, inthe association between hallux valgus and a hammer toe of thesecond digit46 or between the length of the great toe and risk ofhallux valgus47. So even in women, footwear is probably moreimportant in progression than causation. It is intuitive to pre-sume that this risk is greater in those with a wider foot.

Excessive LoadingHallux valgus develops slowly, suggesting a process of repetitivetrauma; however, despite a perception that occupation48 orexcessive walking and weight-bearing49 is important, there is noproven link50. The only exception is a weak association withballet dancing51,52. Mann and Coughlin53 reviewed the literatureon cumulative industrial trauma and dismissed any occupa-tional link.

No clear link has been established between hallux valgusand obesity 54 and other factors that affect loading such as footprogression angle55 or foot dominance56. There are differencesin metatarsal loading in hallux valgus, but the relevance isunknown56.

Intrinsic FactorsGenetic FactorsA genetic predisposition has long been suspected57. Amongthe inheritable factors that may be relevant are metatarsalformula, arch height, and hypermobility 58,59. The best evi-dence showed that 90% of 350 white patients had at least oneaffected relative60, with the most common pattern of inher-itance being autosomal dominant with incomplete penetrance.The role of genetics in juvenile43 and young adult49 hallux valgusis much more established, with maternal transmission found in94% (twenty-nine) of thirty-one patients with a family history.There is weak evidence of a racial difference. The prevalence ofhallux valgus in whites has been reported to be two times greaterthan that in black Africans40,61.

Sexual DimorphismThe true sex ratio is unknown, although the male-to-femaleratio of 1:1560,62,63 among those who have corrective surgery iswell established. The higher prevalence among women may bedue to footwear that is either poor (up to 90% of shoes in asurvey of 365 women were too small64) or less forgiving, re-sulting in earlier and more frequent presentation. Nevertheless,the prevalence is still greater in women, but there is no qualityevidence to support the finding65. Even less is known of theprevalence between the sexes with regard to juvenile halluxvalgus.

There are fundamental differences in osseous anatomy 66;for instance, the metatarsal head articular surface in femalepatients is more rounded and smaller, providing a less stablejoint67. Women also tend to have a more adducted first meta-tarsal68, which in turn may be due to differences in the tarso-metatarsal articulation69. Other differences include metatarsaldimension and distal and proximal articulation shape and

TABLE II Prevalence of Deformity in Shod and Unshod Feet*

DeformityShod

Feet (%)UnshodFeet (%)

Hallux valgus 33 2

Flatfoot 11 5

Atavistic forefoot 7 17

Metatarsus elevatus 7 39

Metatarsus primus varus 6 25

Hypermobility of the metatarsus 1 13

*The data are from the study by Sim-Fook and Hodgson3. Therewere 118 subjects in the group that wore shoes and 107 subjects inthe group that did not wear shoes.

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angle68,70. Differences in foot pressures have been noted, but thesignificance of these findings is unknown71.

Ligamentous laxity72 and first ray hypermobility73 aremore common in women, although several major series on theoutcomes of hallux valgus surgery have failed to provide male-to-female ratios9,74-77. No link has been made with pregnancy.

Systemic ConditionsLigamentous LaxityMild ligamentous laxity is common in women with halluxvalgus78 and has been reported in 70% of twenty patients withjuvenile hallux valgus74. Therefore, in conditions with generalizedligamentous laxity79, such as Marfan syndrome, Ehlers-Danlossyndrome10, and rheumatoid arthritis80, hallux valgus is morecommon and more difficult to treat. It is interesting, althoughcounterintuitive, that the only study assessing laxity withuse of the Beighton scoring system81 failed to find any as-sociation between generalized ligamentous laxity and hal-lux valgus78. Despite the fact that patients with laxity have amajor risk for recurrence, little work has been done on thiscondition82-84.

AgeA biomechanical study in elderly patients85,86 showed thatchanges in posture, joint kinematics, and plantar pressure areassociated with a greater risk of hallux valgus. However, age is apoor predictor of hallux valgus angle87. Although the peak onsetis from thirty to sixty years of age, it is more likely that theinitial changes occur during adolescence43,88 or even earlier forjuvenile hallux valgus.

Metatarsus Primus VarusThe association between metatarsus primus varus and halluxvalgus is well known43,89-93, but it is not clear whether it is acause or an effect94. Humbert et al.95 argued that metatarsusprimus varus comes first, stating that, as the abductor hallucistendon subluxates inferiorly, it becomes dysfunctional, re-sulting in hallux valgus. Metatarsus primus varus is importantin juvenile hallux valgus92 and has been reported to be foundin up to 75% of such patients (forty-nine of sixty-five)96,97.This rate is greater than the prevalence in patients with adulthallux valgus (57% of 122 subjects)49 and in the general popu-lation (<1% of 635 subjects)98, but the appearance of meta-tarsus primus varus lags behind the development of the halluxvalgus49,91.

A weak association has been found between the magni-tude of metatarsus primus varus and hallux valgus in women49,99,but the evidence shows that it is related to the choice of footwear.No direct association is found until the deformity becomes se-vere and self-propagating100,101; by that stage, muscle compressionforces push the metatarsal head medially with a force equal to theposterior shear force of the hallux on the ground times the halluxvalgus angle102.

Snijders et al. concluded that the metatarsus primusvarus was secondary to the toe deformity, on the basis of bio-mechanical investigations38. This finding supports the obser-

vation that when hallux valgus is corrected, the metatarsusprimus varus can improve without an attempted correction ofthe first metatarsal itself. This has been shown for basal oste-otomy49, first metatarsal phalangeal joint fusion103,104, and evenKeller osteotomy 105, suggesting that metatarsus primus varus isa secondary phenomenon. Cronin et al. believed that the ad-ductor hallucis was a deforming force that could be used afterfusion to adduct the entire ray without a basal first metatarsalosteotomy 103.

It appears that some people have an innate propensitytoward metatarsus primus varus and are at risk of juvenile halluxvalgus. If they wear high-heeled or small toe-box footwear, theyhave an increased risk of developing adult hallux valgus. In se-vere hallux valgus, a self-propagating cycle of worsening halluxvalgus and metatarsus primus varus can develop.

Metatarsal AnatomyMetatarsal dimension: Metatarsal formula refers to the rela-tive lengths of the metatarsals. The normal order in terms ofdecreasing length is second, first, third, fourth, and thenfifth, but the first and third are commonly equal in length.Morton20 described the short first metatarsal of Morton footthat he believed led to pronation and hypermobility of thefirst ray and therefore hallux valgus. There is no clinicalevidence for this relationship, and more reliable measure-ment techniques have found the true association to be as lowas 4%49,106-108.

Mancuso et al. found that 80% of 110 patients withhallux valgus had a so-called zero-plus first metatarsal (i.e., itwas equal to or greater in length than the second metatarsal),whereas 80% of 100 control subjects had shorter first meta-tarsals109. According to Root et al., the long first metatarsal actsas a ‘‘functional metatarsus primus elevatus’’27 as it cannotplantar flex below the transverse plane. This additional lengthinhibits first metatarsophalangeal joint dorsiflexion and cancause subluxation110.

A long first metatarsal creates a so-called buckle point111,112,resulting in a hallux valgus with a high intermetatarsal angle,and there is a strong association between protrusion distanceand intermetatarsal angle108. On this basis, Mancuso et al. ad-vocated that the definition of so-called normal metatarsalprotrusion should be reduced to –2 to 0 mm from the acceptedstandard of –2 to 12 mm109. It is important to remember thatpronation of the foot causes metatarsal dorsiflexion, making itappear longer than it actually is113. However, the new definitiondoes not necessarily refute the theories regarding the long firstmetatarsal, as the function of the foot when weight-bearing isthe important factor.

Metatarsal articular morphology: Heden and Sorto112

observed that a round first metatarsal head is common in halluxvalgus (occurring in 90% of 100 affected subjects compared with20% of 210 control subjects). A round first metatarsal headcreates a more unstable articulation than other shapes and isassociated with a higher rate of recurrence of hallux valgus114.The round-shaped head is unlikely to be due to remodeling as itis not associated with any degenerative change36. The flattened,

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so-called square or chevron-shaped head is more stable49,115.Phillips116 noted that, as the vector of the extensor and flexortendons runs through the vertical axis of motion, the articulationbehaves somewhat like a hinge. However, the more rounded it is,the closer the vertical axis lies to the surface. Thus, even smalldisplacements medially or laterally can produce greater angularchanges than in a flattened head36,49,53,82,91,117,118.

The biggest objection to this theory is that it is not clearwhether the described head shapes are truly discrete anatomicalentities, and magnetic resonance imaging studies support thisobservation115. These appearances may be spurious, as apparentshape varies with metatarsal pronation and inclination67,114.Unfortunately, there is still no consistent or accurate method ofdescribing metatarsal head shape or of taking into account theconcept of traveling distance of the head119.

Measurement of the distal metatarsal articular angle isnotoriously unreliable120, and there is a very wide variation (–14�to 130�) of normal. However, a congruent metatarsophalangealjoint in hallux valgus requires an altered distal metatarsal artic-ular angle, and the two are directly related38. This relationship isstrongest for juvenile hallux valgus43, suggesting a congenitalorigin especially as degeneration of the joint is rare121. But there isevidence of a 1� to 3� increase in the distal metatarsal articularangle with every decade of life, suggesting that it may be ac-quired122. It is interesting that the congruent metatarsophalan-geal joint appears to be more stable and less likely to progress87.There is no association with metatarsal length, adduction, mo-bility, range of motion, or inheritance49.

The proximal metatarsal articulation shows individualvariation and an association between obliquity and halluxvalgus123,124. This association appears well established; however,these studies are all based on radiographic appearances, andapparent angulation varies considerably with foot posture7,125.Intermetatarsal facets occur in approximately one-third ofhumans126 and are associated with metatarsus primus varus andincreased obliquity of the first tarsometatarsal joint127 but notwith hallux valgus49,122.

Recent unpublished work presented at a British Ortho-paedic Foot & Ankle Society meeting, held in Nottingham in2010, indicated that the proximal articular morphology varies.The authors found that an articular surface with a single facetwas associated with hallux valgus, and an articular surface withthree facets only occurred in subjects with normal feet. Theyhypothesized that the increasing number of articular facetsevoked stability128.

Metatarsal bunion: The bunion is not an osteophyte7, newbone formation129, or ossification of inflamed tissues. There isactually no increase in the size of the medial eminence49,130. In-stead, the metatarsal head is increasingly exposed by cartilageloss because of the lack of contact from the phalanx131. Thesagittal groove, which is a thinning of the articular cartilage thatdevelops laterally on the metatarsal head, is thought to be causedby pressure from the phalangeal margin131. It is an area ofminimal pressure (or fossa nudata)132, and robust histologicaldata have shown that it is due to a lack of stimulation rather thanerosion116. As the sagittal groove moves laterally with increasing

hallux valgus deformity, it is not considered an indication forbunionectomy in severe hallux valgus7.

Metatarsal BiomechanicsStatic stabilizers around the first metatarsophalangeal joint: Nomusculotendinous structures attach to the metatarsal head.The only structures on the medial side are the capsule, collat-eral ligament, and medial sesamoid ligament. These structuresare the most important joint stabilizers, and their insufficiencyis essential for the development of deformity. Sectioning themalone results in a valgus angulation of >20�132. These structuresare mechanically abnormal in hallux valgus, with altered or-ganization of the type-I and type-III collagen, leaving the firstmetatarsophalangeal joint vulnerable to continuous and cy-clical distraction during gait133. The insufficiency of thesestructures is more likely effect than cause unless it is part of ageneralized ligamentous laxity.

McBride9 advocated transverse metatarsal ligament tran-section for correction, but there is no radiographic evidence tosupport the use of this procedure86. This finding is not surprisingas the deep transverse ligament joins the five plantar pads to-gether and not the metatarsal heads134. Sectioning the transverseligament hardly changes the valgus deformity and does not alterthe relationship between the first and second metatarsals135.Thelateral sesamoid is held by the transverse ligament and the ad-ductor hallucis via the conjoined tendon and does not move. It isthe medial sesamoid ligament that fails6.

Dynamic stabilizers around the first metatarsophalangealjoint: The abductor hallucis abducts, plantar flexes, and invertsthe great toe, while the adductor hallucis adducts, plantarflexes, and everts the toe, providing a balanced so-called plantarrotator cuff. When these moment arms are altered, the im-balance plays an important role in deformity progression.Suggestions of a primary muscle imbalance based on histo-logical and electromyographic studies136,137 probably reflect changessecondary to the deformity 9,138.

Normal variations in the attachment of the abductorhallucis have been described, but no association with halluxvalgus has been found139. The abductor hallucis also has asecondary role as a medial arch support and, when the tendonbecomes dysfunctional in hallux valgus7, it may be responsiblefor some of the tibialis posterior dysfunction140. There is noevidence of shortening7 or overactivity of the adductor tendon,although botulinum toxin injection into the muscle has suc-cessfully treated hallux valgus141.

Snijders et al.38 and Sanders et al.142,143 studied the role offlexion forces in the etiology of hallux valgus. Downward pullof the hallux onto the ground creates a force couple with avalgus moment on the hallux and a varus moment on the firstmetatarsal head, producing medial deviation and widening ofthe foot. In the normal subjects studied, the foot narrowed. Inaddition, the further the flexor hallucis longus is from the firstmetatarsal head, the weaker the moment arm of the flexor andthe greater all three deformities become144. The moment arm ofthe flexors moves from an inferior to a lateral direction as thegreat toe pronates or moves into valgus145.

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Migration of the sesamoids over the crista is important indeformity progression146,147. When the medial sesamoid liga-ment is attenuated15 and the loss of the restraint provided by thecrista148 occurs, deterioration can be rapid. The twofold in-crease in the prevalence of bipartite tibial sesamoids in feet withhallux valgus149 is unexplained150.

Metatarsal KinematicsThe first-ray hypermobility theory states that the plane ofmotion of the first ray described by Hicks151 is exaggerated26

because of tarsometatarsal joint instability. There are no liga-mentous structures binding the distal first and second meta-tarsals so the first tarsometatarsal joint can be affected by anumber of factors, including pes planus, a long hallux, or afunctional equinus of the foot152. The elevation causes the pres-sures under the first metatarsal head to reduce. However, thepronation and varus moments cause a relative increase in theload on the medial side of the great toe, resulting in a valgusmoment on the hallux35.

The reported increased recurrence of hallux valgus aftersurgical correction when the first tarsometatarsal joint is notfused79 is disputed153. Furthermore, it has been shown that rayrealignment alone can stabilize sagittal motion without tarso-metatarsal joint fusion73,78,153-155, probably because of a realignmentof the plantar fascia improving the windlass mechanism156,157.This raises the question of whether the corollary is correct, i.e.,is the instability due to a reduction in soft-tissue stability re-sulting from the malalignment158 or is the malalignment a re-sult of reduction in soft-tissue stability?

Hypermobility is still not well understood159, and data onthe effect of first tarsometatarsal joint fusion are lacking. In-terestingly, hypermobility usually refers to sagittal plane mo-tion, but transverse plane motion (i.e., metatarsus primusvarus) may be in fact more important160,161.

Pes PlanusMuch has been written18,162 about the role of pes planus in theetiology of hallux valgus163. The mechanism appears obvious,i.e., pronation increases loading on the plantar medial borderof the hallux during heel rise, but there are several otherchanges116.

1. Pes planus produces an elevation and thus a functionallengthening of the first metatarsal164, which can limit firstmetatarsal phalangeal joint movement.

2. The peroneus longus is less able to stabilize the firstray 165. If this insufficiency is prolonged, hypermobility of thefirst ray can result166.

3. In the planovalgus foot, hindfoot and midfoot eversionreduce the load on the first metatarsophalangeal joint, althoughweight-bearing through the medial arch increases. This changeis due to the relative mobility of the first tarsometatarsal jointcompared with the second tarsometatarsal joint and the loss ofthe pull of the peroneus longus116.

4. As the hindfoot everts, the foot becomes abducted tothe line of progression, increasing the abduction force indorsiflexion on heel rise.

5. There is early and excessive firing of the abductor andadductor hallucis in the pronated foot167,168. Their line of pullalters as the sesamoids rotate, resulting in an overall valgusmoment169.

Coughlin et al.17 showed that, as the foot pronates, thefirst ray also rotates on its longitudinal axis. The first metatar-sophalangeal joint collaterals are somewhat loose, allowing up to2 mm of translation in the transverse plane on dorsiflexion30,which can result in a repetitive injury to the medial restraints.With pronation comes axial rotation of the so-called plantarrotator cuff30 that further exacerbates the deformity131.

Despite the commonly held belief that pes planus plays animportant role in hallux valgus, there is cogent pedobarographicand radiographic evidence to the contrary49,73,170,171, especially injuvenile hallux valgus43,172. Coughlin and Jones49 assessed severaldifferent measures of pes planus in hallux valgus and found noneto be significant. A link was detected between the prevalence ofplantar gapping of the first tarsometatarsal joint and severity ofhallux valgus, but this may be effect rather than cause. No studyhas looked at the prevalence of hallux valgus in pes planus. Theassociation is likely to be far from 100%, given the differencebetween the relative prevalence of the two (i.e., a 20% rate of pesplanus173 versus a 2% to 4% rate of hallux valgus174). It is importantto note that even studies implicating pes planus found rates closeto this background rate105. Mann and Coughlin53 believed pesplanus to be only clinically relevant in patients with a backgroundof neuromuscular deficit.

Given the proposed mechanism by which pes planus causeshallux valgus, correction of the hallux valgus in isolation should beassociated with a higher recurrence rate. However, this has notbeen the case43,53,77,172,175, although only one study53 looked at olderpatients in whom the biomechanical abnormalities had a longertime to produce an acquired deformity.

Eustace et al. showed that first metatarsal pronation isassociated with hallux valgus and increases as the intermetatarsalangle increases, such that pronation and varus are intimatelyrelated176. Furthermore, they showed that medial longitudinalarch collapse is also associated with first metatarsal prona-tion176 (a medial arch of <20� is associated with a first metatarsalpronation of >10�). They were unable to demonstrate whichcomes first, but it is logical when one considers the forcesinvolved that arch collapse drives the pronation rather than viceversa177.

At present, the most that can be said is that any individualwith pes planus and hallux valgus is at risk for a more rapidprogression because of the forces that encourage furtherdeformity17.

Functional Hallux LimitusStructural hallux limitus is a limitation of dorsiflexion onboth weight-bearing (<12�) and non-weight-bearing (<50�).It can predispose to hallux rigidus, which is not relevant tothis review. Functional hallux limitus, on the other hand,describes limitation of motion on weight-bearing only. Thispoorly understood condition was first described, as far as weknow, in 1972178, but there continues to be little information

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on this subject in the orthopaedic literature to either confirmor refute it. In essence, functional hallux limitus refers to therestriction of hallux dorsiflexion that occurs when the first rayis dorsiflexed. It can be observed to an extent even in normalfeet and is due to the axis of rotation of the joint shifting plan-tarward as the first ray elevates179. Functional hallux limitus ispurported to predispose to either hallux rigidus or hallux val-gus180, depending on the coexisting biomechanics of the foot.

In hallux valgus, structural hallux limitus appears to beexaggerated and the passive range of motion at the meta-tarsophalangeal joint commonly reduces on weight-bearing49.The proposed link is that certain foot types (i.e., an evertedhindfoot, a flexible forefoot valgus, or a plantar-flexed firstray)181 increase ground reaction force under the first metatarsalhead (Fig. 5). The predisposed foot types also increase theground reaction force earlier and for longer in the gait cycle182.Feet with good midtarsal movement can accommodate thisfirst ray elevation. As the first ray dorsiflexes, the axis of the firstmetatarsophalangeal joint inverts, so the greater the elevationthe greater the inversion. As the metatarsophalangeal jointdorsiflexion is limited at heel rise, the hallux is forced in thedirection of least resistance. So feet with a great deal of mobilityin the first ray are at risk of hallux valgus. This theory seems tofit in well with some of the other theories of the pathogenesisof hallux valgus, and there is evidence that a dorsiflexed firstmetatarsal is important180. This biomechanical theory still isnot proven. The actual prevalence of functional hallux limitusand subsequent hallux valgus is low.

Windlass ModelThe windlass model explains these findings more simply183. Anincrease in hallux valgus deformity on weight-bearing184 is dueto tightening of the plantar fascia and the pronation effect ofweight-bearing157. When this motion is excessive, the plantaraponeurosis is further tightened157. On heel rise, the first

metatarsophalangeal joint has to dorsiflex to the same extent,activating the windlass mechanism. This tension can preventthe hallux from dorsiflexing. When the heel is lifted off theground, the metatarsophalangeal joint should dorsiflex to anequal degree, but a tight plantar fascia causes a plantar flexionmoment on the hallux. This plantar flexion moment opposesthe dorsiflexion moment of the ground reaction forces on thehallux, causing it to veer along the path of least resistance. If thedorsiflexion forces are transmitted by a relatively rigid firstmetatarsophalangeal joint through to the first metatarsal, thenthis could explain the hypermobility often seen in the firstray156.

Tight Achilles TendonMann and Coughlin53 and Hansen185 postulated that a tightAchilles tendon can predispose to hallux valgus. This is be-cause of early and increased forefoot loading186. The naturaltendency is to externally rotate the foot, rolling over themedial border rather than going forward through the thirdrocker (i.e., toe off, which is dorsiflexion at the metatar-sophalangeal joint with concentric contraction of the gastroc-nemius), increasing the valgus force. The evidence comesmainly from work done with diabetic ulcers187, but there isevidence of an association with hallux valgus if there is <5� ofdorsiflexion at the ankle188. Clinical studies of hallux valgushave echoed this finding, but only if Achilles tendon tightness isdefined as being <10� of ankle dorsiflexion49. Others havefound no association35,189, and there is no evidence that failureto address Achilles tendon tightness results in a higher recur-rence of hallux valgus107.

OverviewHallux valgus is a complex condition with a range of defor-mities varying in severity, suggesting that several factors areresponsible. Inheritance and sex are important, but other

Fig. 5

Proposed mechanism of hallux valgus development as a consequence of functional hallux limitus. MT = metatarsal and MTP = metatarsophalangeal.

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anatomical and biomechanical factors, such as anatomicalmetatarsal variants, including a long first metatarsal(probably most important in men190), a rounded articulation,and metatarsus primus varus, play an important role. Thesevariants increase the vulnerability to first-ray hypermobility,pes planus, and ligamentous laxity.

The toe is at risk if loading is increased on the medialside. If the forefoot is in a narrow toe-box or pronated becauseof a hypermobile first ray or pes planus, the altered muscle pullcan combine with the ground reaction forces and be sufficientto result in repetitive injury to the medial tissues. There is a lackof evidence of a sufficient scientific level to support or disprovethe role of pes planus, first-ray instability 11, or functional halluxlimitus, although all have some intellectual appeal and possiblysome basis in terms of treatment response.

In the normal foot, there is a tendency for the great toe tobe pulled into valgus, but the static restraints (the ligamentsand the sesamoid apparatus) act like reins, preventing thistendency 135. The muscles attached to the base of the phalanxhelp to control the metatarsal head. Once the metatarsal headstarts to escape, this control diminishes and the muscles maybecome deforming forces instead191. The deep transverse lig-ament (and to an extent the adductor) holds the phalanx in

place, while the incompetent medial sesamoid ligament andmedial collateral ligament allow the metatarsal head to driftinto varus.

We know that poor footwear is a risk, and yet few peoplewho wear high-fashion ladies’ shoes develop hallux valgus. Thesame can be said of the other potential causes. The true answerlies in the interplay of the various intrinsic and extrinsic factorsthat come together in any one particular foot. Without large-scale population studies or longitudinal studies, there will al-ways be some unanswered questions on the true pathogenesisand optimal treatment of hallux valgus. n

A.M. Perera, FRCS(Orth)Lyndon Mason, MRCS(Eng)University Hospital of Wales,Cardiff, CF14 4XB, UK.E-mail address for A.M. Perera: [email protected]

M.M. Stephens, FRCSICappagh National Orthopaedic Hospital,Finglas, Dublin 11, Ireland

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