Approach to Pes Cavus – CMT
The pes cavus associated with CMT is caused by progressive weakness of intrinsics, peroneus brevis, and tib ant. The first ray is pulled down by peroneus longus; the hindfoot swings into varus by the force of tib post to stabilize the weightbearing foot (tripod), and the long toe extensors try to work as ankle dorsiflexors, eventually causing clawing. So the typical deformities in CMT eventually are: hindfoot varus, cavus deformity (mainly through the forefoot), and forefoot valgus.
– get a neurologist and physiotherapist involved
– need strengthening, aerobic conditioning, and stretching exercises
– WATCH OUT FOR HIP DYSPLASIA IN THE CMT PATIENT
– AFO to prevent equinus contracture
– orthotics to accommodate for the plantarflexed first ray and to support the hindfoot
Treatment Principles – Get the foot stable, plantigrade, and painless. This is the goal.
Do what needs to be done – if there are soft tissue problems that are flexible/correctable – do soft tissue procedures
– if there are inflexible deformities/bony deformities – do bony procedures
– basically, start with soft tissue procedures, then do osteotomies if you canï¿½t get it fully get it corrected with the soft tissue procedures.
– then do tendon transfers to balance the foot
– it used to be felt that you just let them go and plan for triple arthrodesis; now it is felt that you should get to them early and try to improve function, hopefully preventing the development of the painful foot that requires the triple.
– because CMT is an evolving disease, the younger you do them, the more risk of recurrence
Soft Tissue Procedures – in general, when the deformities are still supple and correctable
– release plantar fascia and medial structures: abductor hallucis, long and short plantar ligaments, quadratus plantae
– transfer peroneus longus (strong – pulls down on 1st ray) to peroneus brevis (weak – lose inversion)
– Jones IP fusion and transfer of EHL to metatarsal neck for the clawing of the hallux. (This is key to preventing the EHL from continuing to plantarflex the first metatarsal.)
– Girdleston-Taylor flexor to extensor transfers for the lesser toe clawing; or EDL tenotomies + IP fusions
– lengthen heel cord after youï¿½ve done the other soft tissue procedures
Bony Procedures – in general, for rigid, uncorrectable deformities
– rigid first ray plantarflexion requires dorsal closing wedge osteotomy
– may require multiple dorsal closing wedge osteotomies if other metatarsals are flexed down
– hindfoot varus that is rigid can be corrected by a lateral wedge osteotomy (Dwyer)
– finally, triple arthrodesis that addresses both the hindfoot and forefoot deformities
– transfer tib post to dorsum of foot through interosseous membrane or around the ankle to the cuboid (tib post contributes to the varus hindfoot because its inversion moment is not countered by the eversion moment of the p.brevis.
– consider transferring tib ant to the lateral side of the foot – BEWARE – tib ant is usually pretty weak and may not be eligible for transfer.