The deep intrinsic muscles of the foot are composed of:
- plantar quadratus muscle
- lumbrical muscles
- flexor hallucis brevis muscle
- flexor muscle of the little toe
- interosseous muscles.
1- Lumbar muscles
2- Plantar square muscle
3- Tendon of the flexor digitorum longus
The quadratus plantaris and lumbricals attach distally to tendinous expansions of the flexor digitorum longus. The plantar square has 2 heads, the medial head (larger) inserts on the medial aspect of the calcaneus and the lateral head (thinner and tendinous) inserts on the lateral aspect of the calcaneus and on the long plantar ligament. The long plantar ligament is between the 2 heads of the plantar square. Note that the lateral plantar nerve passes between the long plantar ligament and the first more superficial muscle layer.
The lumbrical muscles attach proximally to the flexor digitorum longus tendon and distally to the expansion of the flexor digitorum longus tendon on each toe.
1- Sesamoid bones of the hallux
2- oblique head hallux adductor muscle
3- flexor hallucis brevis
4- 1st metatarsal bone
5- cuboid bone
6- flexor digitorum brevis muscle of the little toe
7- adductor muscle of the transverse head hallux
The flexor digitorum brevis attaches proximally to the base of the fifth metatarsal and distally to the lateral edge of the base of the proximal phalanx of the little toe.
The hallux adductor muscle has 2 heads (oblique and transverse). The oblique head takes its proximal insertions at the base of the 2nd, 3rd, and 4th metatarsals and its distal insertion at the lateral part of the base of the proximal phalanx of the hallux. The transverse head takes its lateral insertions from the metatarsophalangeal ligaments of the 5th, 4th, and 3rd toes and its medial insertions from the lateral part of the first phalanx of the hallux.
The two heads of the flexor hallucis brevis attach proximally to the cuboid and lateral cuneiform and distally to the medial and lateral parts of the proximal phalanx of the hallux.
Plantar view of the interosseous muscles of the foot
1- 3rd, 2nd and 1st plantar interossei
The 3 plantar interossei attach, proximally, to the base of the corresponding metatarsal and, distally, to the medial edge of the proximal phalanx of the corresponding toe.
Dorsal view of the dorsal interossei
2- 4th, 3rd, 2nd and 1st Dorsal Interosseous
3- Dorsal aponeuroses and toe extensors (sectioned)
The 4 dorsal interossei are bifid muscles located between two metatarsals. They take their proximal insertions on the two adjacent metatarsals and their distal insertions on the aponeurosis of the corresponding extensor digitorum longus tendon.
When standing, the muscles of the foot provide the flexibility needed to absorb shocks on the ground and the rigidity needed to propel the foot when walking. They work as a whole.
The plantar quadratus helps the flexor digitorum longus to flex the distal phalanges of the last 4 toes. They also give stability to the lumbrical muscles.
The lumbrical muscles flex the proximal phalanx and extend the 2 distal phalanges.
The flexor digitorum brevis muscle flexes the proximal phalanx of the little toe.
The hallux adductor muscle adducts the first toe and helps flex the proximal phalanx. It contributes to the transverse stability of the forefoot.
The flexor hallucis brevis flexes the proximal phalanx of the hallux. The medial head leads to abduction and the lateral head to adduction.
The plantar and dorsal interossei play a role in the longitudinal stability of the foot. They also have a role in the abduction and adduction of the toes. Their role in the flexion and extension of the toes depends on their distal anatomical insertions which vary according to the subjects.
Myofascial syndrome of the deep intrinsic muscles of the foot
Referred pain related to the plantar quadratus muscle projects to the plantar aspect of the heel.
The adductor muscle of the hallux gives pain referred to the level of the plantar surface of the forefoot.
The pain associated with the myofascial syndrome of the flexor hallucis brevis projects at the base of the hallux in plantar and dorsal.
The interossei give pain referred to the plantar and dorsal surfaces of the corresponding toe but also of the corresponding metatarsal.
The factors contributing to this pain are:
- shoes that are too tight or too small
- a fracture of the ankle or foot bones
- any direct or indirect foot trauma may be responsible
- Attention immobilization by plaster of the ankle and the foot aggravating these problems
- structural anomalies of the foot (Morton, flat foot, etc.) may also be responsible
The treatment is done through massage and stretching techniques. Injections with 1% lidocaine in the responsible areas followed by stretching can also work. We will learn self-massage techniques by golf ball or roller at the plantar level to be done daily
Of course, the correction of a structural anomaly is essential.