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Test Your Knowledge About The Pathomechanics Of Hallux Abducto Valgus Deformity

I recently listened to a panel of expert foot and ankle surgeons at a large virtual scientific conference discuss their approach to hallux abducto valgus (HAV) surgery. It became very clear to me that even among recognized experts, there appears to be a lack of understanding about the true pathomechanics of HAV (bunion) deformity. As a result, there is lack of agreement about how to fix this problem.

Having just completed the writing of my book entitled Pathomechanics of Common Foot and Ankle Disorders (to be published by Springer/Nature in November 2020), I thoroughly reviewed published research verifying the pathomechanics of HAV deformity. Based upon current levels of understanding, I developed a quick quiz to unveil potential misconceptions about how bunions develop. Knowledge of this subject area is of critical importance to the podiatric physician as a bunion deformity is among the most common reasons which patients seek consultation and treatment from foot and ankle specialists.   

1.  True or false: The first ray moves independently in the direction of  pronation as hallux abducto valgus deformity progresses.

Answer: False 

Pronation is a triplane motion of dorsiflexion, abduction and eversion of a skeletal segment relative to the midline of the body. When there is HAV deformity, the first metatarsal rotates into excessive dorsiflexion accompanied by inversion (not eversion).1 Relative to the midline, the first metatarsal moves into adduction (not abduction) as the first intermetatarsal angle increases. None of these motions are consistent with pronation of the first ray.

2.  How many tendons and ligaments stabilize the hallux at the first MPJ?

A. seven

B. 10

C. 12

D. 15

Answer: D 

A remarkable number of ligaments surround the first MPJ and many tendons cross this joint to stabilize the hallux and first ray for weightbearing.2 This includes seven muscles and eight ligaments.

The seven muscles include …

  1. abductor hallucis 
  2. medial head of the flexor hallucis brevis
  3. lateral head of the flexor hallucis brevis 
  4. flexor hallucis longus 
  5. adductor hallucis 
  6. extensor hallucis brevis 
  7. extensor hallucis longus 

The eight ligaments include … 

  1. medial sagittal hood 
  2. lateral sagittal hood 
  3. medial sesamoid
  4. lateral sesamoid 
  5. intersesamoid 
  6. medial collateral 
  7. lateral collateral 
  8. transverse metatarsal 

3.  Of all these tendons and ligaments, how many directly attach to the first metatarsal head?

A. 10

B. 12

C. Seven

D. Four

Answer: D

The medial and lateral tubercles, located on the head of the first metatarsal, serve as the attachment of the medial and lateral sesamoid ligaments, and the medial and lateral collateral ligaments. Out of the eight total ligaments that stabilize the first MPJ, these four ligaments are the only structures anchored to the first metatarsal head. Loss of this attachment medially is an essential event in the progression of HAV deformity.2

4. True or false: Lateral and valgus displacement of the sesamoid apparatus relative to the first metatarsal head is caused by pronation of the first ray.

Answer: False  

With HAV deformity, the sesamoids do not shift medial or lateral relative to the rest of the foot as they firmly anchor to the lesser metatarsals via the strong attachment to the deep transverse metatarsal ligament. Instead, the first metatarsal rotates relative to the fixed sesamoids.3-5 This rotation is in the direction of dorsiflexion and inversion, which leaves the sesamoids in a valgus position relative to the first metatarsal.6 As the first metatarsal migrates medially, the lateral sesamoid will displace dorsally due to a redirection of moment and resultant imbalance of the muscles that insert upon the sesamoid apparatus.2,4-6 The sesamoid apparatus, not the first metatarsal, may thus appear everted or “pronated” relative to the first metatarsal and the supportive surface.

5. How much sagittal plane motion is contributed by the first metatarsocuneiform joint (first tarsometatarsal joint) in comparison to the medial naviculocuneiform joint?

A. Twice the amount

B. Half the amount

C. The same amount

D. Three times the amount

Answer: B 

In healthy feet, the medial naviculocuneiform joint is the primary contributor to sagittal plane motion of the first ray.7

6. True or false: In cases of HAV deformity, hypermobility of the first ray is characterized by not only excessive dorsiflexion but excessive inversion of the first metatarsal as well.

Answer: True   

Hypermobility in HAV deformity appears to be accentuated at the first metatarsocuneiform joint in the direction of dorsiflexion, inversion and adduction.8 Some authors have speculated that this increased motion demonstrates a change in orientation of the axis of the first ray as a result of pes planus or greater magnitude of pronation of the rearfoot.9,10 Notwithstanding, this pronation motion occurs in the rearfoot, while the first metatarsal independently rotates into inversion, not eversion in HAV deformity.

7. True or false: Reducing first ray hypermobility in HAV deformity requires arthrodesis of the first metatarsocuneiform joint.

Answer: False  

Several studies show that excessive dorsal mobility of the first ray will significantly reduce with osteotomy of the first metatarsal, which corrects the intermetatarsal angle deviation.11-14

8. True or false: The alignment of the sesamoid apparatus and the insertion of the soft tissue structures at the first MPJ provide more restraint to dorsal mobility of the first ray than the ligaments of the first metatarsocuneiform joint.

Answer: True   

Studies show that the plantar aponeurosis and sesamoid apparatus are the primary stabilizers of the first ray in the sagittal plane.11,15-17

9. True or false: Recent research shows that the first ray rotates around a new axis in HAV deformity, allowing dorsiflexion with eversion causing pronation of the first metatarsal.

Answer: False   

There are no valid kinematic studies disputing the dorsiflexion-inversion axis of the first ray as originally described by Hicks and then verified by ten other high-quality studies.6-8,18-25

10. The appearance of a “pronated” position of the first metatarsal on a plantar axial sesamoid radiographic view could be due to:

A. Plantarflexion of the first ray induced by the axial positioning device

B. Eversion of the talonavicular joint

C. Pronation of the entire foot independent of isolated first ray motion

D. All of the above

Answer: D   

The plantar axial positioning device dorsiflexes the hallux and allows retrograde plantarflexion with eversion of the first metatarsal.26 This radiographic view is not a reliable predictor of frontal plane first metatarsal position in patients with HAV deformity.27 A weightbearing computed tomography (CT) imaging study of patients with HAV deformity showed that any significant contribution of eversion of the entire medial column comes from the talonavicular joint.6 Eversion or pronation of the talonavicular joint can be induced by pronation of the entire foot, which is an intrinsic risk factor for HAV deformity.6 Pes planus or a pronated foot posture is strongly associated with the development of HAV deformity.28,29 

Therefore, a “pronation deformity” of the first metatarsal, as one may see in some patients with HAV deformity, originates proximal to the first metatarsocuneiform joint. In reality, the first metatarsocuneiform joint is actually inverted in cases of HAV deformity.6,8

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. 


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Additional Reference

30. Shibuya N, Jupiter DC, Ciliberti LJ, VanBuren V, La Fontaine J. Characteristics of adult flatfoot in the United States. J Foot Ankle Surg. 2010;49(4):363–368.

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