50: Biomechanics of HAV - Mahoney Flashcards
PAD DAB
plantar interossei adduct (toward the 2nd)
dorsal interossei abduct (away from 2nd)
do lumbricals adduct or abduct?
adduct
Adduction of the lesser toes caused by pull of the lumbricales utilizes the midline of the body as the reference point
abduction and adduction of the hallux
- ABduction of the great toe means that the hallux moves away from the midline of the body
- [towards the 2nd toe (caused by pull of the adductor hallucis)]
- ADduction means towards the midline of the body [away from the 2nd toe
- (caused by pull of the abductor hallucis)]
- The names of abductor hallucis and adductor hallucis utilize the 2nd toe as the midline
define HAV
Prominence of the medial aspect of the 1st metatarsal head secondary to medial deviation of the 1st metatarsal shaft and lateral deviation of the great toe
etiology of HAV
***
- Primary etiology is STJ pronation
- Calcaneocuboid and talonavicular joints assume a more parallel relationship which allows more movement in the forefoot so it can dorsiflex and invert on the rearfoot
- If pronation prolonged, resupination is delayed during midstance and propulsion (resupination normally starts at 25% of the stance phase)
- Foot is unstable and becomes hypermobile
- As the arch lowers and the cuboid dorsiflexes, the cuboid is on the same plane as the 1st metatarsal
- Peroneus longus loses the pulley action of the cuboid to plantarflex the 1st metatarsal
where does hypermobility really exist?
traditional = first metcuneiform jt
but also significant at naviculocuneiform jt
define lapidus procedure
fusion of 1st metatarsal-cuneiform joint
(does this really correct all the hypermobility?)
what is hypermobility?
Technical definition: “Increased range of motion of joints, joint laxity occurring normally in young children or as a result of disease”
Biomechanical definition: “Abnormal motion of bone caused by unresisted forces at a time when the joint should remain stable”
how do you measure hypermobility of 1st ray biomechanically?
Quasi-scientific by using 10 mm as normal range with 5mm dorsal and 5mm plantar
accept that 65 degrees dorsiflexion great teoneeded for propulsion in gait, where does the ROM come from?
- The first 30° of hallux dorsiflexion comes purely from the MPJ
- To obtain 65°, the 1st ray plantarflexes which shifts the transverse axis of the 1st MPJ more proximally over the sesamoids
rate of bunion development depends upon (6) …
- Extent of abnormal STJ pronation
- Degree of calcaneal eversion
- Amount of forefoot adductus
- Extent of 1st MPJ inflammation
- Angle and base of gait, stride length
- Type of footgear (tight shoes)
Stage 1 bunion development
- Characterized by apparent lateral subluxation of base of proximal phalanx away from the 1st metahead
- Hallux is everting and plantarflexing in relationship to the inversion (eversion?) and dorsiflexion of the 1st metatarsal, as well as, abducting (subluxing laterally) due to adduction of the 1st metatarsal
- EHB pulls laterally
- Correcting the eversion (by inverting) of the 1st metatarsal, realigns the sesamoids
- Tension develops on medial side of 1st MPJ, and compression on lateral side (medial sesamoidal ligaments stretch, lateral ligaments tighten)
stage 2 bunion development
- Characterized by a true abduction deformity of hallux, with hallux pressing against second toe
- Long axis of 1st metatarsal moves more medially which causes bowstringing of EHL and FHL tendons, as they move further away from the axis of motion
- Leads to further lateral displacement of sesamoids
- Or do they appear laterally displaced due to the valgus rotation of the 1st metatarsal?
- Abductor hallucis loses its medial stabilizing force as the 1st metatarsal dorsiflexes
- This more plantar orientation prevents it from balancing the abductory force of adductor hallucis
- FHB now contributes to lateral abductory force, as the sesamoids “drift” more laterally
- This drift is actually being accomplished in this stage by some active pull of the tendons
- Joint space widens and first signs of lateral joint deviation may be noted (however, most joint changes occur in Stage 3)
- If the bunion progresses at a steady rate, bony adaptation maintains symmetric joint margins
- If it progress rapidly, unequal joint margins are noted
stage 3 bunion developement
- Characterized by increase in the intermetatarsal angle and widening of the foot
- Longitudinal axis of 1st metatarsal:
- moves more medially and becomes oriented from proximal/lateral/plantar to distal/medial/dorsal
- 1st metatarsal is dorsiflexed and adducted
- Transverse axis of motion of 1st MPJ:
- no longer level with the transverse plane and orients from proximal/lateral/dorsal to distal/medial/plantar
- Sesamoids appear to move more laterally-actually, are held stable by the adductor hallucis, and once again, it is the 1st metatarsal that moves
- Crista becomes eroded as the tibial sesamoid moves laterally beneath it
- Extrinsic and intrinsic muscles increase their deforming forces from Stage 2
- Intrinsic muscles tend to only stabilize the medial aspect of hallux during propulsion, thereby increasing its frontal plane valgus rotation
stage 4 bunion development
- Characterized by subluxation or dislocation of the 1st MPJ
- 2nd toe can no longer act as a buttress for the great toe, and the hallux either underrides or overrides the 2nd toe
