THE ETIOLOGY AND PATHOGENESIS OF HALLUX VALGUS
May 20th, 1997
Mark S. Myerson, MD, and William H. B. Edwards, MB, MS, FRACS
Hallux valgus, ie valgus angulation of the great toe on the first metatarsal, is a common deformity associated with many other abnormalities, not only those of the first ray but also those of the rest of the forefoot. Although some of these associated abnormalities (eg those of the lesser toes) are secondary to the great toe deformity, some may cause or exacerbate the deformity of the hallux.
In this article, we present a perspective on the natural history of hallux valgus and examine some of the extrinsic and intrinsic factors in the pathogenesis of this deformity. The structural and pathologic changes in the forefoot as the deformity progresses are discussed.
Unfortunately, few published studies document the progression of hallux valgus without treatment. If it were available, such data (including patient age, radiographic appearance of the foot, intrinsic factors such as foot shape, and extrinsic factors such as shoewear) would be particularly helpful when treating a patient with hallux valgus because, currently, the likelihood of progression of deformity is assumed but not really known or understood. If one assumes that there is an inevitable progression of the deformity, then one may be more likely to consider surgery at an earlier time, with its inherent advantages.
Although somewhat arbitrary, the best information on the natural history of hallux valgus is Piggot's23 well-defined study on the radiologic patterns of the metatarsophalangeal (MP) joint of the great toe and its influence on the clinical development of hallux valgus. He identified three radiologic patterns (congruous, deviated, and subluxated) (Fig. 1) and found that if the MP joint was congruent, hallux valgus did not progress, malrotation of the hallux was not observed, the sesamoids remained well aligned, and the intermetatarsal (IM) angle remained constant. However, the deviated and the subluxated MP joint demonstrated a progressive increase in deformity over time, although the magnitude and progression of deformity was more pronounced in the subluxated group. As MP joint instability increased, so did the degree and progression of deformity, which is quite consistent with current clinical observation. This finding was further substantiated by Hardy and Clapham,11 who reviewed 3,642 pediatric feet and 417 adult feet with and without hallux valgus. By comparing childhood and adult indices, they found that the greatest increase in the hallux valgus angle occurred before the increase in the IM angle. These data could be interpreted to support the view that MP joint instability occurs earlier in the disease process, before the development of worsening metatarsus primus varus. Indeed, several studies have shown that, for 50% of adults with hallux valgus, deformity began in adolescence11,23 and that, for 40% of juvenile patients, the deformity develops before the age of 10.5
One of the more interesting philosophical arguments with regard to the pathogenesis of hallux valgus is the causative role of valgus deviation of the hallux on the first metatarsal versus the effect of metatarsus primus varus on the development of hallux valgus. In 1951, Hardy and Clapham11 reported a strong correlation between the hallux valgus angle and the IM angle, which became greater as the valgus deformity increased. However, they also identified a weaker correlation between the hallux valgus angle and the orientation of the first metatarsal in relation to the midfoot. This argument for hallux valgus being the primary deforming force was further substantiated by Piggott,23 who recognized the relationship between hallux valgus and an increase in the first-second IM angle in the adult. However, Piggott23 noted that some adolescents with hallux valgus showed no abnormality of the IM angle, a condition frequently identified clinically. Although encountering a patient with profound hallux valgus and a normal IM angle is unlikely, lesser hallux valgus and a normal IM angle is more common.
The primary role of metatarsus primus varus in the development of hallux valgus also had its proponents. Truslow30 was the first to introduce the term metatarsus primus varus, defining it as an abnormal medial inclination of the first metatarsal relative to the midfoot.30 Although proportionately similar to the IM angle, the latter is influenced by the orientation of the lesser metatarsals. For example, a substantial metatarsus primus varus may be present with a minor IM angle if generalized metatarsus adductus is also present (Fig. 2).
Lapidus18 believed that varus of the first metatarsal and internal rotation of the first ray represented a degree of atavism. To understand the concept of atavism, the effect of an unstable first metatarsal associated with either hypermobility or an oblique first MC joint must first be understood. The normal motion at the MC joint occurs through a range from dorsomedial to plantar-lateral and, although some clinicians believe hypermobility occurs only in the sagittal plane, this is not entirely accurate. The presence of instability is detected as increased motion, not only in the sagittal, but also in the transverse, plane.21 If one assumes that metatarsus primus varus is pathologic, particularly when associated with a medially inclined MC joint, does hypermobility affect the development of hallux valgus (Fig. 3)? In a study of adults, Hardy and Clapham11 found a strong correlation between clinically limited tarsometatarsal joint mobility and hallux valgus. In contrast, Klaue et al17 recently demonstrated increased medial column sagittal instability in patients with hallux valgus. The medial column motion was not at the metatarsocuneiform joint, but centered just anterior to the naviculocuneiform joint both in normal patients and those with hallux valgus. However, no evidence was presented as to whether there was a consequent, coincident, or causal relationship to hallux valgus; therefore, one cannot draw conclusions based on their findings. A far better study was conducted by Wanivenhaus and Pretterklieber,31 who anatomically demonstrated abduction and adduction movement at the MC joint in only 11% of specimens, whereas excessive dorsiflexion was present in 9% of feet. Movement in all planes increased dramatically with dorsal subluxation of the MC joint, as seen in either degenerative disease or conditions associated with ligamentous laxity31 (Fig. 4). In a study of 20 hallux valgus patients and 20 controls, Carl et al4 similarly demonstrated a correlation between generalized ligamentous laxity and the presence of symptomatic hallux valgus. What, therefore, is the role of hypermobility in the pathogenesis of hallux valgus? From these data, one may conclude that although hypermobility is probably relevant to the pathogenesis of hallux valgus and metatarsus primus varus in a minority of patients, it has more relevance to the selection of the appropriate surgical procedure.
There is clearly a reciprocal relationship between the deformity of the hallux and the IM angle. This is evident in patients after correction of severe hallux valgus with an arthrodesis of the hallux MP joint in whom the IM angle deformity decreases postoperatively, proportionate to the preoperative deformity of the hallux.20 This concept was further substantiated by Antrobus,2 who noted that correction of the distal deformity may produce correction of both the IM angle and the metatarsus primus varus angulation (Fig. 5).
The site of deformity of the first metatarsal is most likely to be at the metatarsocuneiform (MC) joint.14,29 Using a computer-assisted analysis of dorsoplantar radiographs of feet with hallux valgus, Tanaka et al29 confirmed that the angulation of metatarsus primus varus occurs at the first MC joint. The medial inclination of this joint is normally 10o, and some authors have therefore determined that, if the inclination is >25o, an arthrodesis of the MC joint should be performed.9,21 However, this inclination should not be the only variable used to determine the need for an arthrodesis at this articulation. Although the IM angle does increase with worsening hallux valgus, the orientation of the first metatarsal in relationship to the first cuneiform may remain unaffected.14 Although anatomic dissections have confirmed that minor variation in the positioning of the foot for radiography will affect the apparent position of the MC joint,3,10 the IM angle is less dependent on radiographic technique.3 What, therefore, does an abnormally increased angulation at the MC joint imply with respect to treatment? One should not assume that an oblique MC joint is the cause of instability or hypermobility of that joint because the radiographic appearance is highly variable and depends on the plane in which the radiograph is obtained. Nevertheless, in the presence of a steeply inclined MC joint, particularly if associated with hypermobility of the first metatarsal and generalized ligamentous laxity, one should assume that this joint is pathologic, and should determine surgical correction accordingly.
The medial eminence is not an intraarticular structure, rather, it is the prominent point of attachment for the medial collateral and sesamoidal ligaments. The exostosis plays a minor role in its formation because, as the proximal phalanx deviates laterally, the medial part of the metatarsal head takes progressively less part in load transmission and, hence, the cartilage degenerates. The sagittal groove seen on the medial edge of the articular surface of the head represents the site of maximal cartilage degeneration. At this location, the head contacts neither the proximal phalanx nor the medial collateral ligament.
The medial eminence can also be thought of as a dorsal extension of the original groove for the medial sesamoid on the plantar aspect of the head. This is frequently made more prominent by minor osteophyte formation on the medial margin of this groove.1,10 In patients with very prominent exostosis, the latter is caused by subcutaneous soft-tissue hypertrophy and the formation of an adventitious bursa. In more advanced cases, there may be considerable osteophyte formation on the lateral as well as the medial aspects of the head.1,10
Intrinsic and Extrinsic Factors
In evaluating the causative role of intrinsic and extrinsic factors, there are clearly many factors to be taken into consideration. The literature has been summarized in terms of the roles/effect of shoewear, hereditary factors, the shape of the MP joint, pes planus, Achilles contracture, and a short first metatarsal on the development of hallux valgus.
Shoewear. Regardless of the type and style, the current widespread use of shoes in most societies has been implicated in many historical studies as exacerbating, if not causing, the deformity of hallux valgus. In 1958, Sim-Fook and Hodgson28 compared the feet of 225 Chinese in Hong Kong, half of whom wore shoes. Although no individual in either group complained of foot problems, the differences in foot pathology were notable. In the barefoot group, 24% had metatarsus primus varus, 13% had hypermobility of the first ray, and 1.9% had hallux valgus. The shod feet were noted to be markedly stiffer and to have a dramatically higher incidence of callosities. In the shod group, 6% had metatarsus primus varus, 0.9% had hypermobility of the first ray, and 33% had hallux valgus. In a similar study, Shrine27 estimated the hallux valgus angle from the footprints of 3,515 people (88% of the population of the island of St. Helena) and found that it correlated strongly with the time spent wearing shoes. The incidence of hallux valgus was approximately 2, 16, and 48% in barefoot individuals, shoe-wearing mean, and shoe-wearing women, respectively. In 1964, Maclennan19 reviewed 1,256 New Guinean natives, none of whom wore shoes, and found that the overall incidence of hallux valgus was 2%.
Hereditary factors. In a review of 60 feet with juvenile hallux valgus, Coughlin5 noted a maternal transmission of the disorder in 72% of patients and a variable penetrance of the trait. Hardy and Clapham11 noted that 77% of patients reported that their mothers had had a bunion deformity. It is quite clear that the deformity is considerably more common among females, both in bare foot communities and those who wear shoes.27,28 The reason for the increased incidence of hallux valgus in females in the absence of extrinsic factors, eg shoewear, is not clear.
The MP joint. Clearly, the shape of the metatarsal head should affect the development of hallux valgus. A flat first metatarsal head is unlikely to subluxate and, therefore, is less likely to develop the deformity of hallux valgus. Intuitively, a round metatarsal head, particularly if the articular surface points laterally, should be associated with increasing deformity (Figs. 6 and 7). On radiographs, Richardson et al24 measured the angle made by the articular surface of the first metatarsal head and its shaft (known as the distal metatarsal articular angle, or DMAA) in 100 normal anatomic specimens. They found it to be quite reproducible, measuring approximately 6o of valgus. An increase in the DMAA is not always associated with hallux valgus; however, Coughlin5 noted that, with juvenile hallux valgus, the DMAA was generally increased. As previously discussed, Piggott23 noted the association between instability of the joint and progressive hallux valgus deformity. He supported the view of Peabody,22 who believed that, in most cases, hallux valgus is caused primarily by great toe deformity and that is caused only occasionally by metatarsus primus varus.
Pes Planus. Inman15 observed that hallux valgus was rare in the cavus foot. Furthermore, he supported but never substantiated the view that hallux valgus is always combined with pes planus and that pes planus is the predisposing factor in hallux valgus. Using foot prints to estimate arch height, Kilmartin and Wallace16 reviewed 64 children, half with and half without hallux valgus, and found that there was no association between the hallux deformity and arch height. These and subsequent studies have not documented any association between hallux valgus and pes planus.5,26
Forefoot pronation. Pronation, particularly of the forefoot, causes rotation the first ray, forcing the hallux MP joint to lie more obliquely with respect to the floor. As a result of this malalignment, the physiologic arc of movement of the hallux changes and is less able to withstand plantar pressure. In a anatomic study of 20 feet, Alvarez et al1 demonstrated that, in the presence of forefoot pronation, the normal forces of ambulation stretch the medial collateral ligament and capsular structures and push the hallux into a valgus position. Eustace et al6 developed a radiologic measure of first metatarsal pronation and attempted to quantify arch height by measuring the inclination of the first metatarsal; they found significant metatarsal pronation in 84% of 50 cases of hallux valgus.6
Great toe pronation. As the hallux assumes a valgus position, it begins to pronate, which is seen clinically as a prominent callosity developing on the plantar medial aspect of the hallux, particularly under the interphalangeal joint. With increasing pronation of the hallux, the abductor hallucis displaces from its medial position to one more on the plantar surface of the distal metatarsal, further perpetuating this pronation. In addition to the reorientation of the intrinsic tendons about the hallux MP joint, the long flexor and extensor tendons shift laterally. Sesamoid displacement, particularly the fibular sesamoid around the metatarsal head, further exerts a torque to the toe, increasing pronation of the hallux. This rotation of the sesamoids is not apparent in all patients, but certainly increases with toe pronation and hallux valgus severity.
Sesamoid displacement. According to Hardy and Clapham,11 lateral sesamoid displacement is seen in less than 10% of normal feet and in more than 88% of feet with hallux valgus. With increasing hallux valgus, more lateral shift of the sesamoids occur, progressively increasing pronation of the toe occurs, and the medial sesamoid encroaches on the crista on the undersurface of the first metatarsal head. The crista undergoes a slow erosion, eventually flattening, and is no longer visible with advanced cases of hallux valgus (Figs. 8 and 9). Subluxation or dislocation of the sesamoids will not be seen until this ridge is flattened. Eventually, the tibial sesamoid lies in the lateral sulcus, while the fibular sesamoid shifts laterally, dorsally, and proximally. In addition, the fibular sesamoid rotates so that the articular surface becomes applied to the lateral neck of the first metatarsal.
The position of the sesamoids with respect to the second metatarsal does not change, suggesting that there is no contracture of the deep transverse metatarsal ligament. In an anatomic study, Haines and McDougall10 noted that patients with hallux valgus also have stretched medial sesamoid ligaments but unaffected deep transverse metatarsal ligaments. This finding was confirmed by Saragas and Becker26 in a radiologic review of 118 feet; they noted that the distance between both sesamoids and the second metatarsal did not differ between hallux valgus and normal feet. From this finding, one may conclude that the first metatarsal head moves medially away from the sesamoids, and that the sesamoids and proximal phalanx do not move away from the head. This was well expressed by Girdlestone and Spooner,7 who noted that "the fore part of the foot is splayed, not through stretching of the adductor structures, but because the first metatarsal head escapes from the control exercised by the base of the proximal phalanx into which the muscles are inserted. The phalanx and the sesamoids remain held by the adductors, while the first metatarsal head drifts away out of control."
Extrinsic tendon contracture. Sanders et al25 demonstrated that flexion of the hallux against resistance in patients with hallux valgus caused an increase in valgus and a widening of the forefoot. The greater the hallux deformity, the greater the increase in hallux valgus angle seen.25 The reverse effect was found in normal feet. This correlates well with clinical experience, because extensor tendon contracture worsens as the hallux valgus deformity increases. With this contracture, a bowstring effect of the extrinsic tendons occurs, and their force is redirected from the sagittal to the transverse plane. This imbalance is more marked in patients with spasticity, as in patients with cerebral palsy in whom flexor and extensor imbalance may be the primary deforming forces.8
In patients with pes planovalgus, the Achilles tendon gradually tightens as the heel assumes a more valgus position. The contracted Achilles increases the forces on the medial foot and, with increasing pronation, the valgus thrust on the hallux during toe off is accentuated. As noted above, however, there is no clinical correlation between pes planus and the pathogenesis of hallux valgus. The pathologic role of the Achilles contracture with respect to the forefoot remains unclear. Although this contracture may rarely have an adverse effect on outcome after bunionectomy, lengthening of the Achilles tendon performed in conjunction with bunionectomy is not recommended.
First metatarsal length. There is little agreement on the association of first metatarsal length with hallux valgus, and if this relationship is in fact present, it is not marked. Hardy and Clapham11 noted that the mean length of the first metatarsal compared to the mean length of the second metatarsal was 2 mm longer in patients with hallux valgus than in controls. Using computer-assisted analysis of dorsoplantar radiographs, Tanaka et al29 found the first metatarsal to be slightly longer in patients with hallux valgus than in controls. However, in a survey of 7,167 feet, Harris and Beath12 found that the length of the first metatarsal was not in any way related to foot pathology, although they did not specifically compare the length of the metatarsal with the incidence of hallux valgus.12 Saragas and Becker's26 review of 118 feet supported this latter view.
Pathogenesis of Hallux Valgus Deformity
It is likely that there are at least two major subgroups of hallux valgus. Approximately 4% of the population develop the deformity regardless of footwear. Another, larger group is susceptible to the deformity and will develop it given the right stimulus, eg shoes. The third group is comprised of individuals who develop the deformity only with a major insult to the stability of the hallux, eg as with the disruption of ligaments by severe trauma or inflammatory disease.
In the first group, the primary problem is malalignment of the first ray. These patients have metatarsus primus varus as a prominent and early component of their deformity and, as described above, this may be the primary source of the pathology. In these patients, hallux valgus is present, but the MP joint remains congruent until late in the disease process. This joint is malaligned, that is, there is an increased DMAA. Rotational deformity and joint instability are late features, if they occur at all.
In the more common second group, hallux valgus is associated with progressive instability of the hallux MP joint. Initially there is valgus deviation of the toe on the metatarsal, associated with a decreased abductor hallucis function and increasing contracture of the adductor hallucis due the forces of shoewear. This deviation causes the extrinsic flexor and extensor tendons to displace laterally, shifting the sagittal plane forces to a valgus thrust on the toe, thereby causing increasing varus force on the first metatarsal head. This realignment of the extrinsic flexor and extensor forces directs the further progression of the hallux valgus deformity.
Worsening deformity is initially limited by the medial anatomic restraints of the MP joint, including the collateral ligaments and joint capsule. The medial capsuloligamentous structures eventually become attenuated because of persistent valgus forces. The sesamoids remain firmly attached both to each other by the intersesamoidal ligament and to the proximal phalanx, and they behave as one structural unit. Progression of the MP joint subluxation is initially limited by impingement of the tibial sesamoid on the crista of the metatarsal head, which ultimately undergoes a slow attrition. Once the crista is eroded, the MP joint becomes less stable. The ligament of the lateral sesamoid and the lateral collateral ligament remain intact and provide a final tether to the dorsolateral metatarsal head about which the deformity then hinges. Similarly, the anatomy of the transverse metatarsal ligament is unchanged and the relationship of the sesamoids to the second metatarsal remains relatively constant. The lateral sesamoid is therefore held to the first metatarsal neck.
With progressive displacement of the metatarsal head medially, the medial sesamoid comes to lie under the lateral aspect of the metatarsal head whilst the lateral sesamoid is displaced dorsally by the intact lateral sesamoid ligament. The lateral collateral ligament similarly tethers the proximal phalanx. This movement imparts a rotational torque to the hallux, causing the internal rotation or pronation that is clinically associated with callosity on the plantar medial aspect of the hallux. This rotational deformity moves the abductor hallucis from its medial position to a plantar one, removing yet another stabilizing force. Only when the lateral collateral and sesamoid ligaments are disrupted and the whole joint capsule becomes attenuated does dislocation of the MP joint occur. The metatarsal head escapes control and moves medially, while the proximal phalanx and its associated sesamoids come to lie lateral to the first metatarsal, head held by the deep transverse metatarsal ligament to the second metatarsal.
It is apparent from multiple studies of populations that the incidence of hallux valgus is approximately 4% even in those who remain unshod.13 A large proportion of the population is susceptible to the deformity as a result of shoewear, yet others, despite wearing shoes, develop hallux valgus only under unusual circumstances. In adolescents, the hallux valgus deformity varies from that in adults because, in the former, the MP joint is stable but malaligned. The radiographic appearance and therefore the pathogenesis of juvenile hallux valgus may be different because the deformity tends to be one of joint angulation manifested by an increase in the DMAA as opposed to true joint instability. Furthermore, parameters other than articular congruity, including metatarsus adductus, hypermobility, and generalized ligamentous laxity, need to be considered in adolescents. It is likely that the hallux valgus which develops in non-shoe-wearing societies arises due to these anatomic factors, commonly identified in the adolescent. The studies of both Piggott23 and Hardy and Clapham11 reflect their experience with individuals susceptible to developing the deformity. In these cases, MP joint instability precedes increasing valgus and pronation of the toe, followed by an increase in either the IM angle or metatarsus primus varus. One may conclude, therefore, that in most patients the primary deformity is at the MP joint, and that in a smaller percentage, the etiology of the deformity results from MC instability associated with metatarsus primus varus and possibly hypermobility.
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Fig. 1. The shapes of the articulation of the MP joint are illustrated as originally described by Piggot23: A, congruent; B, deviated; and C, subluxated.
Fig. 2. This radiograph indicates a moderate hallux valgus (HV) deformity (50°) but only a mild IM angle (10°). However, note the marked generalized metatarsus adductus, which is associated with a first metatarsus primus varus (MPV) angle of 34°.
Fig. 3. Hypermobility may be detected as increased motion in the sagittal plane in both a plantar (A) and dorsal (B) direction. However, one should be aware of additional planes of motion in the first metatarsal that may be associated with hypermobility.
Fig. 4. This patient has marked generalized ligamentous laxity and severe hallux valgus deformity associated with instability of the MP joint. Note the hallux valgus and widely splayed forefoot on standing (A). The hallux was easily subluxated and could almost be dislocated with passive manipulation (B)..
Fig. 5. The reciprocal relationship between hallux valgus (HV) and first metatarsus varus is demonstrated in this patient, in whom hallux valgus was associated with a painful MP joint. Comparison of the preoperative (A) and postoperative (B) radiographs demonstrates the substantial correction of the IM angle after arthrodesis of the hallux MP joint.
Fig. 6. This radiograph demonstrates an increased DMAA.
Fig. 7. This patient had numerous anatomic problems, including hallux valgus (HV) associated with a round metatarsal head and a marked increase in the DMAA, metatarsus primus varus (MPV), and mild generalized metatarsus adductus.
Fig. 8. The sesamoids gradually move under the metatarsal head, although they do not actually undergo any true lateral shift because the metatarsal moves off the sesamoids n=medially. In the normal joint (A), note the relationship between the sesamoids and the balance of the abductor and adductor tendons. With hallux valgus (B), the crista of the metatarsal head is eroded, the abductor and adductor tendons are not balanced, and the fibular sesamoid rotates in the first web space.
Fig. 9. Note the change in the sesamoid position and the undersurface of the metatarsal head. A, normal MP joint and sesamoid complex. B, MP joint and sesamoid complex associated with progressive hallux valgus deformity.