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Gait

An Introduction To Biomechanics

The foot is a highly complex system capable of producing balance and propulsion of the body by these two opposing mechanisms. The foot is the foundation for the whole body and its actions will effect other parts of the body, and indeed, its function will be affected by numerous forces acting upon it. The two principle components of foot function are the motions of pronation and supination:

Mobile adaptor (Pronation).   During this function it is capable of absorbing shock and adapting to any terrain: flat, angled, rough, soft etc. whilst maintaining the equilibrium of the body.
Rigid lever (Supination).   During this function it is locked into a structure capable of stabilising, lifting and propelling the body.

Definitions

Frontal Plane Motion:   Inversion and eversion with axis in sagittal and transverse planes.
Sagittal Plane Motion:   Plantar and dorsiflexion with axis in frontal and transverse planes.
Transverse Plane Motion:   Abduction and adduction with axis in frontal and sagittal planes.
Triplane axis:   Ankle, subtalar, midtarsal, 1st & 5th rays have triplane axis with motion perpendicular to the axis. Axis of motion is determined by the shape of the articular surface and the position of the entire joint (axis of motion for the midtarsal joint is dependent upon the relative position of the subtalar joint.)
Compensation:   A change in structure or function of one part of the foot to neutralise or accommodate the effect of an abnormal force, resulting from a deviation in structure or function of another part.
Plane Dominance:   When the foot compensates for a deformity or abnormal force then the compensation will occur in the plane that can produce the most motion in the same plane as the deformity or abnormal force. This is the dominant plane for that deformity.
    For example, when a sagittal plane deformity exists (Ankle Equinus deformity) then to compensate the most effectively, the body needs to compensate within the sagittal plane. This can be achieved by a combination of actions: flexion of the knee, early heel lift, wearing a high heeled shoe, or by pronation of the subtalar joint to unlock the midtarsal joint to allow motion around the oblique axis of the midtarsal joint. All of these actions are compensation in the sagittal plane.

The Gait Cycle

gait cycle

S = Heel Strike FFL = Forefoot loading HL = Heel Lift
TO = Toe Off HSO = Heel Strike opposite foot.

Overview Of The Adaptive And Rigid Process Of The Foot During The Gait Cycle.

At the contact phase of gait (heel strike) the motion of the foot is that of pronation. This is seen by the eversion, abduction and dorsiflexion of the calcaneus. It is a complex motion between the talus and the calcaneus (subtalar joint). Since the talus is locked in the ankle mortise, the leg will internally rotate and can be seen by the movement of the tibia backwards (best observed by the tibial malleolus). The result of this pronation is the unlocking of the Midtarsal Joint and a very mobile foot. This continues for approx 25% of the gait cycle at which point the foot must start to become a rigid lever. The foot will then start to move in the direction of supination (inversion, adduction and plantarflexion) but will remain in a pronated position.

The foot should reach its neutral position by 50% of the cycle (Midstance). This continues to allow the midtarsal joint to become locked with all the metatarsals in ground contact. The first ray will plantarflex to allow the hallux to provide the range of motion required for propulsion. The plantarflexion of the first ray is vital to allow the hallux to dorsiflex to 60. Failure of this mechanism will cause the hallux to impinge and jam against the head of the first metatarsal - a condition known as functional hallux limitus.

KEY LANDMARKS - During The Contact Phase

  1. Subtalar joint pronation throughout. At end of the contact phase the foot starts to supinate and continues throughout stance phase. Pronation only normally occurs during contact phase (27% of cycle). Its function is to allow the foot to adapt to forces from the ground (uneven ground etc). Pronation is stopped mainly by the Posterior Tibialis muscle with Tibialis Anterior helping to control the extent and rate of pronation.
  2. The leg internally rotates during contact phase and externally rotates from the end of the contact phase. This is linked to the motion of the subtalar joint and can be seen by the movement of the tibia in a forwards direction (Tibial malleolus becomes prominent from the front).
  3. Metatarsus becomes fully loaded during contact period.
  4. Vertical ground reaction forces peak for the first time by the end of contact. Shared between the heel and metatarsal heads.

KEY LANDMARKS - During Midstance Phase

  1. Midstance commences at the end of the contact phase and shortly after toe off of the opposite foot. It ends with heel lift and makes up the intermediate 40% of the cycle.
  2. The subtalar joint continues to supinate and converts the foot into a rigid lever. It moves out of a pronated position at the end of the contact phase and into a supinated position before heel lift. The motion is a combined effect of the calf muscles and leg rotation.
  3. The leg externally rotates.
  4. Vertical ground reaction forces decrease by up to 75% body weight it increases again before heel lift. (The swinging of the leg is a main factor).
  5. The opposite foot is in swing phase and should pass the weight bearing foot before heel lift occurs.

KEY LANDMARKS - During Propulsive Phase

  1. The propulsive phase begins with heel lift and ends at toe off. It makes up the final 33% of the gait cycle.
  2. The subtalar joint supination continues to increase skeletal stability and enhance the lever effects of the foot actions. The foot will pronate slightly just prior to toe off. External leg rotation also continues.
  3. Vertical ground reaction forces peak again by up to 25% of body weight upon the metatarsals and toes.
  4. Body weight is transferred from the lateral side of the foot to the medial side and onto the opposite foot as it loads. The 5th metatarsal is free of weight at heel lift.
  5. The toes are generally not loaded during the other phases but may be active during the toe off period.

Major Pathological Gait Defects

Variance from the normal smooth locomotory function of gait can be associated with a deformity in:

  1. Osseous - congenital, developmental, metabolic, neoplastic.
  2. Neurological - sensory, motor, spastic, paralytic.
  3. Muscular, Soft Tissue - contractures, fibrosis, laxity, metabolic etc.
  4. Functional - lack of coordination, neuromuscular.

The subtalar joint axis

The commonly accepted angular values for the subtalar joint are 42 deg to the transverse plane, 16 deg to the sagittal plane and 48 deg to frontal plane. It must be stressed that these are average values and variances do occur. However, assuming that the axis is approximately 45 deg then every 1 deg of calcaneal motion (inversion/eversion) should equal 1 deg of transverse leg rotation (internal & external leg rotation) in closed chain motion. Therefore, clinically a high inclination angle of the axis (60 deg) will produce more leg rotation than calcaneal motion and, therefore, more postural problems. A low axis of motion will lead to more calcaneal motion and more foot problems than postural ones.

Furthermore, the axis can be relatively abducted or adducted to the longitudinal axis of the foot. The average position of the axis is from the posterior lateral aspect of the calcaneus to the first metatarsal medial cuneiform joint. Therefore, a shift in the axis medially (adducted) will allow forces across the forefoot and ground reaction force to produce a strong pronatory force at the Subtalar joint. A shift laterally (abducted) will produce a supinatory effect. Kirby has defined non-weight bearing techniques to assess the direction of the axis, but the DYNASTAT allows the assessment of the axis in the weight bearing position.

DYNASTAT - Interpretation of Results

When the axis of motion shows a medial shift, then the orthosis will require modification. This can be achieved by medial and lateral additions to the positive cast to increase the weight bearing area and increase the supinatory effect of the varus post. The supinatory effect can also be increased by the medial heel skive technique or lateral calcaneal tubercle addition. The principle of the Heel Skive technique is to remove Plaster of Paris from the medial side of the positive to increase the amount of varus wedging, when the plate material is pressed.

Lateral Calcaneal Tubercle Addition can be achieved when Plaster of Paris is added to the lateral tubercle of the calacneus to create the same affect as the medial heel skive technique. It is smoothed to blend with the cast and creates the necessary supinatory force to control the Subtalar joint pronation.

Assessment of the inclination angulation of the calcaneus will provide information used in the modification of the orthosis post to correct for the amount of "normal pronation". When there is a high inclination angle, more normal pronation is required and can be achieved by increasing in the pronatory grind off on the post. Conversely, when there is a low inclination angle, the pronatory grind off can be decreased. This inclination angle of the calcaneus can be assessed by the DYNASTAT. Sagittal Plane Scale showing 8 degrees inclination angle

Sagittal Plane Scale showing 8 degrees inclination angle

Common Foot Deformities

The following lists of conditions are not generally seen as single entities but as a combination of pathologies.

Subtalar Varus (Calcaneal Varus)

An inverted position of the posterior aspect of the calcaneus relative to the lower third of the tibia due to an incomplete derotation from its infantile position. Note that the forefoot will be in the same alignment as the rearfoot, if there is no forefoot to rearfoot deformity.

Compensation
Pronation of the subtalar joint to evert the calcaneus until the medial side of the rearfoot and forefoot are in contact with the ground. The amount of pronation may be sufficient to fully compensate or only partially compensate for the amount of subtalar varus.

Clinical Observations

  • Signs
    • Dorso medial bunion.
    • Lesion of 5th toe, Digiti Quiniti Varus (Tailor's bunion).
    • Hammer toes.
    • Plantar callus under 2nd MTPJ but possible under 3rd & 4th MTPJ depending on extent of compensation.
    • Haglund's deformity - Heel bump due to movement of heel against shoe. In relaxed calcaneal stance position the calcaneus may appear:
      1. inverted if limited subtalar joint motion,
      2. vertical if subtalar varus only deformity or
      3. everted if other pronatory deformity present.
  • Symptoms
    • Leg fatigue and nocturnal leg cramp.
    • Low back pain or fatigue.
    • Lateral ankle sprains.
  • Treatment.
    Cast the foot in the subtalar joint neutral position and post the rear of the orthosis with the degrees of subtalar varus. Allow for normal pronation by medial grind off of the post. The degree of grind off should be related to the extent of sagittal plane motion of the calcaneus.
    Posts of over 6 are uncommon and not well tolerated without modifications and/or extensions to the cast or orthosis i.e. medial and lateral cast expansion.
    When the rearfoot pathology is excessive, then other posting techniques will be required. This can be achieved by the Kirby medial heel grind technique or by lateral calcaneal tubercle additions.
Measuring Rearfoot Varus with the DYNASTAT

Measuring Rearfoot Varus with the DYNASTAT

Forefoot Varus

An osseous deformity of the forefoot as a result of failure of the head and neck of the talus to derotate from the infantile position. The plane of the lesser metatarsals is inverted to a bisection of the posterior aspect of the calcaneus when the subtalar joint is in its neutral position and the midtarsal joint is fully pronated.

Compensation.
Pronation occurs at the subtalar joint to allow the medial side of the forefoot to reach the ground. This will cause the rearfoot to evert and unlock the midtarsal joint. Pronation will occur during the stance phase of gait and often into the swing phase. The extent of pronation is limited by the range of motion at the subtalar joint.

Clinical Observations

  • Signs
    • Hallux abducto valgus or hallux limitus/rigidus depending upon the forefoot angulation, relative length of first metatarsal and shape of the metatarsal head.
    • Overlapping toes especially 2nd.
    • Lesion 5th toe.
    • Plantar keratoma and callus under 2nd MTPJ but also possibly 3rd & 4th.
    • Genu valgum from internal leg rotation (knock Knees).
    • Calcaneal spurs, Heel pain syndrome, Plantar fascia pain.
    • Everted calcaneus with medial bulging of the talar head. Lateral cuboid break and abducted forefoot relative to the rearfoot with midtarsal joint involvement.
  • Symptoms
    • Hallux bursitis.
    • Generalised forms of metatarsalgia.
    • Chronic low back pain.
    • Inferior calcaneal bursitis or plantar fasciitis.
    • Severe fatigue.
  • Treatment
    Cast the foot with the subtalar joint in its neutral position and the midtarsal joint fully pronated. Make an orthosis to provide a post to support the medial side of the foot and produce a heel bisection that is vertical. This can be achieved intrinsically or extrinsically. Intrinsic posting requires the addition of plaster of Paris to the medial side of the positive cast and smoothed and reduced laterally to create the medial wedge. When the plate is moulded the post will be created within the plate. The traditional extrinsic post is achieved by adding the posting material to the plate to the required angle.
Measuring forefoot varus

Measuring forefoot varus

Forefoot Valgus and Plantarflexed 1st Ray (rigid)

 

An osseous deformity of the forefoot in which the plane of the lesser metatarsals is everted relative to the bisection of the posterior aspect of the calcaneus, when the subtalar joint is in its neutral position and the midtarsal joint is fully pronated. The everted appearance of the forefoot may be due to the plantarflexed position of the 1st ray (2 - 5 in same plane) or the complete rotation of all metatarsals (2 - 5 everted). The 1st ray will not dorsiflex to the level of the other metatarsals when the subtalar joint is in neutral.

A plantarflexed 1st ray may be caused by:

  1. Congenital torsional deformity of the head of the talus resulting in the everted forefoot.
  2. Neurological conditions.
  3. Congenital plantarflexion of the 1st ray.
  4. Trauma or surgery.

Compensation.
The parallel plane to the ground of the metatarsals is achieved by ankle joint dorsiflexion and subtalar joint supination (to invert the forefoot).

Clinical Observations

  • Signs
    • Hammer toes.
    • Large keratoma under the 1st MTPJ and/ or 5th MTPJ due to supination.
    • Relaxed calcaneal stance position will show inverted calcaneal bisection with high arch medially.
  • Symptoms
    • Chronic lateral ankle sprains.
    • Haglaund's deformity.
    • Sesamoiditis.
    • Morton's neuroma.
    • Leg and thigh fatigue.
    • Stress fractures.
    • Lateral knee pain.
    • General shock absorption symptoms.
  • Treatment.
    Neutral subtalar joint cast with midtarsal joint fully pronated to produce orthosis with forefoot post on lateral side to prevent subtalar joint supination. This can also be achieved intrinsically or extrinsically.
Measuring Forefoot Valgus with the DYNASTAT

Measuring Forefoot Valgus with the DYNASTAT

Plantarflexed 1st Ray (flexible)

A first metatarsal whose neutral position is below the plane of the other lesser metatarsals and can be dorsiflexed to the same plane, by a force applied to its plantar surface. This foot type is often confused with a forefoot valgus deformity because of its position off the ground. It also masks other conditions of forefoot varus and subtalar varus by plantarflexing to provide stability to the foot through a tripod stance (rearfoot, 5th MTPJ and 1st MTPJ).

Compensation.
Compensation involves pronation of the subtalar joint to unlock the midtarsal joint and allow the 1st ray to dorsiflex to the level of the lesser metatarsals. When there is a forefoot varus deformity the midtarsal joint will also supinate to bring the plane of the lesser metatarsals to the ground and dorsiflex the 1st ray.

Clinical Observations

With Subtalar Varus

  • Signs
    • Dorso medial bunion.
    • Lesion 5th toe.
    • Hammer toes.
    • Tailor's bunion.
    • Plantar callus under 2 MTPJ and or 3 & 4 MTPJ with possible keratoma.
    • Haglund's deformity.
  • Symptoms
    • Leg fatigue and nocturnal leg cramp.
    • Low back pain or fatigue.
    • Lateral ankle sprains.

With Forefoot Varus

  • Signs
    • Hallux abducto valgus / limitus.
    • Lesion 5th toe.
    • Overlapping toes commonly 2nd.
    • Plantar callus of 2 MTPJ and or 3 &4 MTPJ with deep keratoma.
    • Genu Valgum.
    • Calcaneal spurs.
  • Symptoms
    • Hallux bursitis.
    • Generalised metatarsalgia.
    • Chronic low back pain.
    • Inferior calcaneal bursitis.
    • Plantar fasciitis.
    • Severe fatigue.
  • Treatment.
    The main aim is to control the foot around its neutral position and establish a 1st MTPJ against the ground. A neutral cast of the foot to produce an orthosis with rearfoot at neutral and the forefoot with either a forefoot bar post (2 - 5) or a forefoot varus post with 1st ray cut out.
Forefoot Valgus Post

Forefoot Valgus Post
This shows how the lateral border of the foot is brought into ground contact at the same time as the medial border. The effects of the valgus deformity have been corrected.

Ankle Equinus

A failure of the ankle joint to achieve a dorsiflexed position of at least 10 degrees past the vertical when the subtalar joint is in neutral.

Compensation.
The subtalar joint pronates to allow the oblique axis of the midtarsal joint to produce dorsiflexion within its range of motion. Often the midtarsal joint will be subluxed and an early heel lift during gait may produce a bouncing gait.

Clinical Observations.

These are dependent on the extent of compensation occurring in the foot. It is possible for the compensation to occur during gait by an early heel lift seen by the bouncing type of gait. It can also be compensated for by abduction of the feet or flexed or hyperextended knee deformities.

Therapy Complications of Orthosis

Complaint

Causes

Considerations
Discomfort at lateral heel or along border of plate
  1. Seat too narrow
  2. Restricted STJ motion
    • Rearfoot over posted
    • Valgus post required at forefoot
    • Forefoot posted with excessive varus post.
    • Equinus deformity not controlled.
    • Poor grinding of plate along lateral edge.
  • New plate with lateral heel expansion on cast.
  • Remove rearfoot post or check pronatory grind off.
  • Post forefoot.
  • Reduce density or rigidity of post / plate.
  • Recast with pronated Subtalar Joint.
  • Remake device to "root plate" and ensure appropriate grinding.
Pain under metatarsals
  • Device too long or not finished (bevelled).
  • Forefoot incorrectly posted.
  • Shorten device.
  • Reduce / remove forefoot post to allow first metatarsal to plantarflex.
  • Apply forefoot extension of shock absorbing material.
Medial arch pain
  • Equinus deformity.
  • Internal rotation still excessive.
  • Forefoot or rearfoot post incorrect.
  • No pronatory grind off.
  • Raise heel and increase stretch programme.
  • Change to less corrective devices.
  • Extend rearfoot post medially.
  • Consider change in posting technique at rearfoot.
Pain in distal heel
  • Equinus deformity.
  • Casts taken in supinated position.
 As above.

Check casts have straight lateral border.
Knee pain

- Medial
  • Check forefoot & rearfoot post (excessive).
  • Genu recurvatum.
    •
    • 1Change posts.
    • Use Flexible device.
    • Check inclination angle of calcaneus and pronatory grind off.
    - Lateral
    • Check rearfoot post - limiting normal pronation.
    • Forefoot Valgus present.
    • Change rearfoot post to allow pronation. Use softer post material or increase grind off.
    • Post forefoot.
    - Posterior
    • Equinus deformity.
    • Reduce control or add heel lift.
    • Consider stretching programme.
    - Anterior
    • Excess post or no normal pronation.
    • Equinus deformity.
    • Supinated cast.
    • Change posts to correct angle, softer or increase grind off.
    • Heel raise.
    • Recast.
    • Consider Chondromalasia patella and treat accordingly.
    Tendon Achilles pain
    • Supinated device.
    • Rearfoot over posted.
    • Equinus deformity
    • Remake and check neutral STJ position.
    • Heel raise.
    Plantar heel pain
    • Supinated cast.
    • Equinus deformity.
    • Forefoot post incorrect.
  • Remake device and check posting and flexibility.
  • Raise heel.
    • Pain around 1st MTPJ
    • Reduced ROM at joint - Functional hallux Limitus.
    • Tibial sesamoid overload (Sesamoiditis)
    • Check length of device and posting.
    • 1st ray cut out or rocker bar extension.
    General leg muscle pain
    • Anterior shin splints
    • Posterior shin splints
  • Check post angle for correctness.
  • May require forefoot post.
  • Post modification techniques may be required.
    • Secondary back pain Over control or no shock absorption. Reduce posts or change to flexible device.
    Shoe conditions
    • Heel slippage.
    • Heel counter break down.
    • Change footwear to deeper heel seat.
    • Medial wear consider neuromotor defect or internal rotation.
    • Lateral wear - Over posted rearfoot or Forefoot Valgus deformity present.

     

     

    Source: information and pictures provided courtesy of Mark Price at  Dynastat Biomechanics