52 - Biomechanics of Tailor's Bunion Deformities Flashcards
1
Q
KNOW FOR EXAM
A
- Know normal function of the 5th ray (acts independently, tri-planar motion)
- Describe the deformity known as a “tailor’s” bunion
- Identify the structural and biomechanical causes of a “tailor’s” bunion – etiology
2
Q
Description of a Tailor’s bunion
A
- An abnormally prominent 5th met head, can be a dorsal, dorsal-lateral, or plantar-lateral bunion
- Usually associated with an adductovarus deformity of the 5th toe
- Shearing force of the shoe often causes an adventitious bursa overlying the bone
3
Q
Definition of a Tailor’s bunion
A
- Symptomatic Prominence, Hypertrophy, or Irritation Involving Bone or Soft Tissue About the Lateral or Dorsolateral Fifth Metatarsophalangeal Joint
- The Deformity is a Mirror Image of HAV
- The Fifth Toe is Usually in a Varus Angulation
4
Q
Axis of the 5th metatarsal
A
- Axis of motion of 5th ray lies 20° from transverse plane and 35° from sagittal plane
- It runs from proximal-plantar-lateral to distal-dorsal-medial
- Motion (supination and pronation) results in a small amount of abduction and adduction
- The 5th ray axis is parallel to the MTJ oblique axis
- NOTE: best way to know the axis of the 5th met… it is parallel to the base of the 5th met
5
Q
Triplane motion
A
- There is about 10 degrees of tri-plane motion within the axis of the 5th metatarsal
- Most important thing to know about the axis of the 5th metatarsal is that the majority of the motion is frontal plane motion – inversion and eversion
- The least motion occurs in the transverse plane – adduction and abduction
6
Q
Etiologies
A
NEED TO KNOW which etiology creates which type of Tailor’s bunion
- Structural
- Functional
7
Q
Structural etiologies
A
- Increased IM angle
- Bowing of the metatarsal (more than the normal bowing)
- Dumbbell shaped metatarsal head
- Accessory ossicle
- Soft tissue hypertrophy
8
Q
Functional etiologies
A
- Uncompensated rearfoot varus
- Uncompensated forefoot varus
- Forefoot valgus foot types
- Abnormal STJ pronation is important in the FUNCTIONAL ETIOLOGIES
- Uncompensated or partially compensated RF varus
- Uncompensated, partially compensated, FF varus
- Congenitally dorsiflexed or plantarflexed 5th ray deformity
- Flexible FF valgus
- Compensated gastroc-soleus equinus
9
Q
Abnormal STJ pronation
A
- Abnormal STJ pronation by itself will not cause a tailor’s bunion; it must be present along with one of the other etiological factors
- THIS LEADS TO…
- Hypermobility of the 5th ray against fixed shoe pressure leads to a dorsiflexed, abducted, everted position
10
Q
Radiographic analysis (went back to this twice… probably a good idea to know)
A
- Due to the everted position, when viewed on an AP x-ray, the plantar concavity of the shaft becomes laterally positioned, making the shaft appear to be curved
- The plantar condyles become laterally positioned and often are mistaken for an exostosis
- You can look at the sesamoids to see how much the foot is pronating (everting) on radiograph
- If the foot is pronating during radiograph, you may have this view, mistaking the 5th metatarsal to be bowing and containing an exostosis – this is why removing exostosis alone is not effective
- If the etiology is due to pronation, you will need to implement orthotics or other compensation
- Remember, pronation in conjunction with hypermobility will cause that Tailor’s bunion to form
11
Q
IM angle ***
A
- IM angle of Fallat and Buckholz: 8.71° associated with the Tailor’s bunion***
- Normal IM angle is 6.22°-6.47°
- NOTE: when measuring the IM angle, don’t use the bisection of the base of the 5th metatarsal, because that will alter your measurement, use the medial surface of the 5th metatarsal
12
Q
Lateral deviation angle **
A
- Lateral deviation angle of Fallat and Buckholz: 8° is associated with a Tailor’s bunion***
- Normal lateral deviation angle is 2.64°-7.5°
- Distal 1/3 to 1/2 of the 5th metatarsal is measured for any lateral deviation in the bone
itself, not the joint angle
13
Q
Splayfoot
A
- Tailor’s bunion can be associated with a splayfoot deformity
- An IM angle > 12° between the 1st and 2nd mets, and an IM angle > 8° between 4th and 5th mets
- As the shaft everts, the abductor digiti quinti is placed more plantarly, so it loses its abductory force on the 5th toe, the toe adducts and moves into varus (loses its MECHANICAL advantage)
- Adductovarus deformity of 5th toe may produce joint changes at the 5th MPJ
- Almost EVERY Tailor’s bunion will have a VARUS deformity of the 5th metatarsal
14
Q
Uncompensated and partially compensated varus deformities
A
- Must occur in a fully pronated foot to cause a hypermobility of the 5th ray
- 5th met is forced into a dorsiflexed, abducted, and everted position by ground reaction forces
15
Q
Example
A
- A varus foot will exist in a fully pronated foot when the total amount of degrees in varus exceed the amount of calcaneal eversion
Example:
o 3 degrees of tibial varum, 10 degrees of calcaneal varus
o STJ ROM is 24 degrees ( 8 eversion)
o Total rearfoot varus 13 when STJ is in neutral
o STJ can maximally evert 8 degrees
- Resulting in a 5 degrees of rearfoot varus when maximally pronated
- This means that in this example we have an individual who is partially compensated and the underlying varus deformity in the pronated foot is going to cause the individual to have a Tailor’s bunion
