p2.ch3.4
2. Development of the foot and ankle, and its
characteristics
Development of the Antigravity Plantar Flexors
Soleus:
The most characteristic finding in the foot and ankle is a
well-developed antigravity soleus muscle. The proximal long
muscle fibers on the soleus arise from a broad proximal area while
its shorter muscle fibers come from its distal origin. It is our
clinical observation that this well-developed soleus muscle can
provide antigravity stability.
Electromyographically, the soleus acts continuously in the
stance phase where the body is in antigravity position, whereas,
the gastrocnemius acts only in the accelerating phase (terminal
stance and heel-off) where the body propels without antigravity
activities (Fig. 10, 11). Clinically also, stability of the foot is
preserved only when the release of the soleus is minimized, and
only hypertonic muscles such as the gastrocnemius are selectively
released (Fig.3AB, 6AB, 25AB, 107AB, 108AB, 117AB, 120AB).
In treatment of equinus deformities including equinovarus
or equinovalgus, activity of the soleus should be preserved as much
as possible. It can be recognized that simple heel cord lengthening
reduces antigravity activities of the soleus resulting in a powerless
couched posture, especially in the diplegics (Fig. 106AB). In order
to achieve sufficient correction and at the same time preserve the
soleus, other hypertonic muscular factors, which cause equinus
deformity, need to be identified.
Intrinsic Muscles:
Another characteristic finding regarding the antigravity
activities is development of the short intrinsic antigravity muscles.
Abductor hallucis, flexor hallucis brevis and adductor hallucis are
well developed to support the medial column of the foot, while the
flexor digitorum brevis, interossei, flexor digiti minimi, quadratus
plantae and abductor digiti minimi are well developed to support
the lateral column of the foot. These intrinsic muscles flex the
midtarsal, metatarso-phalangeal, and proximal and middle
interphalangeal joints, when supporting the body.
Development of the Antigravity Dorsiflexor
Development of the antigravity dorsiflexors is yet another
characteristic finding in the foot and ankle of the humans. With
well-developed dorsiflexors of the foot, the ankle can be
dorsiflexed to the plantigrade position during standing and gait.
Dorsiflexion of the ankle is mostly achieved by activities of the
tibialis anterior, peroneus tertius, and peroneus brevis.
In cerebral palsy, activities of these antigravity dorsiflexors
are depressed by hyperactive plantar flexors. Therefore, facilitation
of these antigravity dorsiflexors by release of hyperactive plantar
flexors is essential for achieving a stable gait.
Transfer and lengthening of the tibialis anterior or peroneus
brevis have been recommended for correction of varus or valgus
deformity of the foot. However, it is obvious that by these
procedures the antigravity activities of these dorsiflexors will be
somewhat lost. Antigravity activities of these dorsiflexors is
important for achieving stability of the foot in cerebral palsy in
which weakness of the dorsiflexors already exists from the
beginning. Even a minor loss of antigravity power of the
dorsiflexors by transfer or release will aggravate the equinus
deformity, and therefore should be avoided.
Balance Between Inversion and Eversion
Another interesting characteristic of the human foot is that it
has two opposite movements in the horizontal plane: Inversion and
Eversion.
The human body can keep its center of gravity within the
sole by preventing excessive lateral and medial shift of the body
weight out of the sole, with alternate movements of inversion and
eversion. In the humans, muscles for inversion and eversion of the
foot are well developed. On the basis of this development of
invertors and evertors, the humans can stand even on s single foot
without any external support. If the body begins to tilt laterally
when standing on one foot, and its center of gravity also begins to
shift laterally, the invertors begin to act to bring back the center of
gravity to center of the sole to prevent the lateral fall. Similarly if
the body tilts medially when standing on one foot and its center of
gravity begins to shift medially, the evertors begin to act to bring
back the center of gravity to center of the sole to prevent the medial
fall. In cerebral palsy, this elaborate mechanism is disturbed with
occurrence of inversion and eversion deformities. So in order to
achieve stable weight bearing on the soles in cerebral palsy,
acquisition of well-balanced activities of the inverting and everting
muscles are desirable. Profound understanding of inverting and
everting activities of the muscles would be essential for correction
of the varus or valgus deformity.
Skeletal maturity in the Foot and ankle
Development of the Calcaneus and Lateral Column
One of the characteristics in the human skeletal development
is the development of the lateral column and posterior growth of
the calcaneal tuberosity. The lever arm of the calcaneus from the
central axis of the leg to the posterior tip of the calcaneus is
lengthened with the development of the calcaneal tuberosity, and
therefore, the soleus can effectively act to plantar flex the ankle.
Phylogenetically, calcaneus is not present in reptiles where the
gastrocnemius tendon is directly attached to the plantar fascia and
consequently there is little antigravity activity in their feet. The
calcaneus has developed posteriorly, during the period of mammals
and primates, providing a long lever arm for effective plantar
flexion of the foot. This skeletal development of the calcaneus in
combination with the development of the antigravity soleus muscle
allows upright bipedal gait in the humans.
Development of the Lateral Column and Subtalar Joint
The foot was originally a propelling apparatus, by kicking
the ground with flexion of the toes. The plantar flexion movements
of the toes are directly used for propulsion. However, during the
developmental process to four-point crawling and bipedal walking,
the lateral column consisting of the calcaneus, cuboid and
metatarso-phalangeal complex of the lateral toes gets formed. At
the same time, the subtalar joint, between talus and calcaneus is
also formed. The lateral column flexes and extends separately from
the medial column, as a lateral unit of the foot, at the subtalar
joint. This lateral column is considered to be one of the
well-developed skeletal structures in the human body.
On the other hand, the medial column is originally formed
with the talus, naviculum, 1-3 cuneiforms, 1-3 metatarsals, 1-3
proximal phalanges, 2-3 middle phalanges, and 1-3 distal
phalanges. This medial column is considered to be a more
primitive skeletal structure. In the human body, dorsal and plantar
flexion of the lateral column at the subtalar joint, and of the medial
column at the ankle joint can be done simultaneously or
alternatively.
In the human foot, when the invertors and plantar flexors
act, the lateral column plantar flexes, and rolls in under the talus
and medial column. This condition can be called as inversion or
varus foot.
On the other hand, when the evertors and plantar flexors act,
the medial column plantar flexes, and the lateral column is forced
to roll out of the talus at the subtalar joint. This condition can be
called as eversion or valgus foot. Thus, when the body weight
excessively shifts laterally and the body also tends to fall laterally,
the invertors act to flex the lateral column predominantly at the
subtalar joint and prevent lateral fall. On the other hand, when the
body weight excessively shift medially and the body tends to fall
medially, the evertors act to flex the medial column predominantly
at the ankle joint and prevent medial fall. So then with those fine
alternate mechanisms, the human body has accomplished
independent standing ability without any support, even on one foot.
In the foot, the anterior subtalar joint has developed, and so
the anterior part of the calcaneus can rotate around the head of the
talus, when foot is dorsi flexed or plantar flexed. This rotational
movement of the calcaneal head in plantar flexion is called "roll-in
movement", while the rotational movement in dorsiflexion is called
as "roll-out movement". With combined calcaneus movements of
dorsiflexion and plantar flexion at the posterior talocalcaneal joint
and that of "roll in and roll out" movements of the calcaneus at the
anterior talocalcaneal joint, the foot has achieved a highly
elaborate weight bearing mechanism called inversion and eversion.
Dorsiflexion of the Talus at the Ankle Joint
Dorsiflexion of the talus and calcaneus is another
characteristic finding observed in the human body.
Phylogenetically, in quadrupedal mammals, the talus and
calcaneus are considered to be in equinus position, and weights
bearing on the feet are made through the toes.
During the developmental process, the dorsiflexors of the
foot developed in the primates, forcing gradually the lateral
column with the calcaneus into dorsiflexion and inducing valgus
foot with vertical talus. This is the skeletal situation of the foot in
the primates. The body weight is still mainly borne by the medial
column of the feet, and not much inversion of the foot is possible.
Strong antigravity activity against the lateral shift of body gravity
cannot be achieved at this level. Further development of the
dorsiflexors such as the tibialis anterior promotes full dorsiflexion
of the talus against the tibia and fibula. Then, the body weight can
be borne even by the lateral column and inversion of the foot can
also be made possible.
For achieving bipedal upright posture and locomotion,
skeletal changes which make dorsiflexion of the talus and
calcaneus possible are necessary.
Muscular and Skeletal Changes in Cerebral Palsy
-Pes planovalgus or equinovalgus deformities
If the central nervous system is injured or underdeveloped,
the well developed dorsiflexors especially the tibialis anterior are
easily weakened. At the same time, the plantar flexors, especially
the multiarticular plantar flexors such as the peroneus longus
become hypertonic and consequently cause the valgus deformity.
Anatomically, the talus is fixed in plantar flexion, while the
calcaneus is relatively dorsiflexed. This condition is called as the
plano-valgus feet, observed in feet with mild hypertonicity.
-Equinus deformity
When injury of the central nervous system is more severe, all
the dorsiflexors can be weakened while the plantar flexors both on
medial and lateral sides can be hypertonic causing an equinus
deformity. Anatomically, both the talus and calcaneus are fixed in
plantar flexion position. This is the typical condition observed in
the equinus deformity. When hypertonicity of the invertors are
predominant, equinovarus deformity will occur, whereas when
hypertonicity of the evertors are predominant, equinovalgus
deformity will be caused.
-Equinovarus deformity in hemiplegic feet
Another foot problem in CP is that of equinovarus deformity.
In severe deformity, cavus is also associated. In this group,
weakness of the everting dorsiflexors such as peroneus brevis and
tertius are most characteristic. When weakness of all the
dorsiflexors is predominant, recurrence is often seen even after
operation.
-Calcaneus Deformity in Severe Quadriplegic Patients
In severe quadriplegics, the triceps surae muscle is often
severely weakened resulting in pes calcaneus deformity.
Facilitation of antigravity plantar flexion can be achieved by
selective releases of the dorsiflexors. However, achieving
antigravity stability is difficult, because of the weak plantar
flexors. Therefore, occasionally in adolescent and adult patients,
pantalar arthrodesis is considered to attain antigravity stability in
this deformity.
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