STN position can be manually estimated by palpation or by using a mathematical model developed by Root. If using palpation, the examiner identifies the anteromedial andanterolateral aspects of the talar head with the thumb and index fingers of the hand closest to the patient’s midline, placing the thumb just proximal to the navicular tuberosity, approximately 1 inch below and 1 inch distal to the medial malleolus (Figure 8-7). In STJ pronation, the anteromedial talar head is most prominent beneath the thumb, and an anterolateral sulcus (the sinus tarsi) is apparent. The index finger is placed in this sulcus, where the talar head is found to protrude when the foot is fully supinated. The thumb and index finger of the other hand grasp the fourth and fifth metatarsal heads, moving the foot in an arc of adduction and inversion (supination) and abduction and eversion (pronation). STN is the point where the talar head is equally prominent anteromedially and anterolaterally.^41,43,44 The examiner then “loads” the foot by applying a dorsally directed pressure against the fourth and fifth metatarsal heads until slight resistance is felt. The loading procedure locks the MTJ against the rearfoot, mimicking GRFs of MSt.^4,45 The angular relation between the bisection of the calcaneus and the bisection of the lower third of the leg is measured with a goniometer (see Figure 8-6), recorded on the evaluation form as rearfoot STN position.

Root’s mathematical model for determining STN position uses a quantitative goniometric formula.4,⁴¹ First, end ROM calcaneal inversion and eversion are determined by goniometric measurement. Total calcaneal ROM is the sum of the inversion and eversion values. The STN position is determined as the calcaneus is moved into inversion at one third of the total calcaneal ROM. If end-range calcaneal inversion is 25 degrees and end-range calcaneal eversion is +5 degrees, total calcaneal ROM would be 30 degrees. STN position is calculated to be at 5 degrees calcaneal inversion, one third of the distance from its fully everted position.

Reliability and clinical validity of both models have been debated.^{40,41,43,46–48} Although acceptable reliability is possible, it is influenced by examiner experience. Palpation to determine STN position is efficient in terms of time but requires more advanced manual skills and experience than the mathematical model. The mathematical model may be more reliable for the entry-level practitioner.

Calcaneal inversion and eversion occur primarily at the STJ, with lesser contributions from the TCJ. Calcaneal ROM is assessed with the patient in the prone position with the same anatomical landmarks and lines of bisection as for STN assessment. The examiner grasps the calcaneus in one hand, fully inverts it in the frontal plane until end ROM is achieved, and then takes a goniometric measurement.48 The procedure is repeated for calcaneal eversion. The TCJ must be maintained in a neutral to slightly dorsiflexed position while measuring to lock it in a closed-packed position and better isolate STJ motion.⁴⁹ Normative values of 20 degrees for calcaneal inversion and 10 degrees beyond vertical for eversion have been reported.⁴

Because values of calcaneal eversion are larger when assessed in a full weight-bearing position, some clinicians suggest that this position is more clinically valid.^38,48 Assessment of calcaneal eversion in the weight-bearing position represents a total “functional” pronation and eversion that is the summation of motion occurring at the STJ and compensatory motion occurring extrinsic to the STJ (e.g., TCJ, MTJ). Passive assessment of calcaneal eversion in non–weight bearing remains the most accurate method to determine the degree of composite pronation acquired from the STJ itself.

In normal walking, the TCJ is maximally dorsiflexed just before heel rise when the knee is fully extended and the STJ is in a nearly neutral position.4,⁵⁰ When and whether an actual STN position ever occurs during gait is disputed.³⁸ The use of a standard position of knee extension and STN, however, offers a consistent point of reference when assessing TCJ dorsiflexion. According to most sources, a minimum of 10 degrees TCJ dorsiflexion is required for normal gait; anything less is classified as an equinus deformity.^4,50–53 A minimum of 20 degrees of plantarflex is also required for normal gait.⁴

Ankle dorsiflexion is measured in a non–weight-bearing position with the STJ held in the neutral position and the knee extended. The examiner forcefully dorsiflexes the ankle with active assistance from the patient. Active assistance encourages reciprocal inhibition of the calf muscle group and is essential for accurate measurement.⁵⁰ The proximal arm of the goniometer is positioned along the lateral aspect of the fibula, the distal arm along the lateral border of the fifth metatarsal, and the axis distal to the lateral malleolus.⁵⁴ An alternative placement of the distal arm of the goniometer along the inferolateral border of the calcaneus may more effectively isolate true TCJ dorsiflexion (Figure 8-8). This value is recorded as rearfoot dorsiflexion. Forefoot dorsiflexion is measured by repositioning the distal arm along the lateral aspect of the fifth metatarsal. This method allows the examiner to identify contributions or restrictions in sagittal plane motion from the oblique axis of the MTJ.

If ankle dorsiflexion is less than 10 degrees when measured with the knee extended, remeasurement with the knee flexed may rule out soft tissue restriction of the gastrocnemius-soleus complex.4,⁵⁰ If dorsiflexion values are consistent in both positions, the limitation is likely a result of osseous equinus formation of the ankle.

During gait, ankle dorsiflexion occurs in a closed kinetic chain as the tibia and fibula rotate forward over a fixed foot. On the basis of this, some have suggested that assessing ankle dorsiflexion may be more accurate with the patient in a weight-bearing position.^38,52 The weight-bearing technique measures the angle between the tibia and the floor as the patient leans forward with the foot flat on the floor. Unwanted compensations are often difficult to control during weight bearing and may mask true TCJ limitations. The non–eight-bearing technique allows the examiner to assess end feel and joint play, as well as mechanical blocks or joint laxity, providing additional information not accessible in the weight-bearing examination.

Normal rearfoot position is one in which STN is 1 to 4 degrees of varus.39,^40,47,48 Values of more than 4 degrees are described as a rearfoot varus deformity. This deformity may be the result of ontogenetic failure of the calcaneus to derotate sufficiently during early childhood development.^4,45,55 Because this deformity is a torsional structural malalignment of the calcaneus, not a joint-related problem, the TCJ and STJ lines remain congruent when observed in the non–weight-bearing STN position. As a structural deformity, it cannot be corrected or reduced by joint mobilization or a strengthening program. Instead, it is managed with a functional foot orthosis that partially supports the calcaneus in its inverted alignment while preventing excessive STJ pronation.

Assessment of calcaneal ROM predicts quality of motion and the integrity of the STJ. Measuring calcaneal motion into eversion allows the examiner to determine whether a rearfoot deformity is compensated or uncompensated (Figure 8-9).

An equinus deformity occurs when fewer than 10 degrees of ankle dorsiflexion are available as a result of osseous or muscular problems.51,^53,56,57 Clubfoot (talipes equinovarus) is a congenital osseous deformity that includes varus deformity of both rearfoot and forefoot, rearfoot equinus, and an inverted and adducted forefoot.^58,59 The angular relation between the body and the head and neck of the talus is decreased, and the navicular is shifted medially. Muscular forms of equinus include congenital or acquired soft tissue shortening or muscle spasm.⁵³ Tissue contracture or shortening occurs in both contractile tissues (gastrocnemius, soleus, and plantaris) and noncontractile tissues (teno-Achilles and plantar fascia).^51,53

Compensation for an equinus deformity occurs at the foot through pronation, perpetuating soft tissue contractures. The STJ is forced to pronate maximally to gain as much sagittal plane dorsiflexion as possible. Although foot pronation allows some dorsiflexion from the STJ, the amount is often inadequate. Pronation of the STJ unlocks the MTJ, creating an unstable midfoot while allowing further dorsiflexion and forefoot abduction from the oblique axis of the MTJ.⁵³ Other compensatory strategies for equinus deformity include knee flexion (especially in individuals with cerebral palsy), early heel rise, toe walking, shortened stride length of the contralateral lower limb, and toe-out walking.^31,41,53 Clinical consequences of long-term ankle equinus include many conditions normally associated with the excessively pronated foot: plantar fasciitis, heel spurs, bunions, and capsulitis.⁵³

If the forefoot is properly balanced, the plane of the three central metatarsals is perpendicular to the bisection of the calcaneus when in STN (Figure 8-10). In a forefoot varus deformity, the forefoot is excessively supinated or inverted, whereas in a forefoot valgus the forefoot is excessively pronated or everted.

To isolate STN position in the non–weight-bearing examination, the examiner attempts to lock the MTJ by applying a dorsally directed loading pressure with the thumb and index fingers over the fourth and fifth metatarsal heads of the patient’s foot (see Figures 8-6 and 8-7). Loading force must be gently applied over the fourth and fifth metatarsal heads until tissue slack is taken up from the normally plantar flexed resting position of the ankle.^4,55 Overload of the forefoot leads to dorsiflexion and abduction of the foot, placing the forefoot in an excessively pronated position and giving a false valgus orientation.

Although many forefoot measurement devices are available, forefoot orientation can be accurately assessed with a standard goniometer.⁶⁰ To assess the forefoot to rearfoot relation, the proximal arm of the goniometer is aligned along the bisection of the calcaneus, with the axis just below its distal border. The distal arm is positioned in the plane of the three central metatarsal heads (Figure 8-11). The angular displacement is recorded on the evaluation form under STN position for the forefoot assessment (see Figure 8-5).

Although the prevalence of forefoot deformity, with or without symptoms, is well documented, less agreement exists regarding which types of deformity are most common.47,⁶¹ If an individual has bilateral forefoot deformity, the deformities may not be of the same severity or type. MTJ deformities change the location of the lock of the forefoot against the rearfoot.⁴ Although these osseous frontal plane deformities alter the direction of motion, they do not limit the total ROM of the MTJ.⁴ In forefoot varus, locking of the forefoot occurs in an inverted position relative to the rearfoot.⁴ Forefoot varus results from ontogenetic failure of the normal valgus rotation of the head and neck of the talus in relation to its body during early childhood development.^4,45,55

Compensations for foot deformities are viewed in a relaxed weight-bearing position referred to as relaxed calcaneal stance (RCS). When excessive pronation of the STJ compensates for the deformity on weight bearing, the condition is called a compensated forefoot varus (Figure 8-12A). When the STJ cannot adequately pronate to accommodate an inverted forefoot, an uncompensated forefoot varus is present. The medial forefoot does not make contact with the ground, and the lateral forefoot is subjected to excessive pressure. Thick callus develops beneath the head of the fifth metatarsal, and the risk of stress fracture is increased. Plantarflex of the first ray and pronation at the MTJ are common compensations that allow the medial forefoot to make contact with the ground (see Figure 8-12B). Persistent MTJ stress may lead to joint damage and excessive forefoot abduction and eversion.

Forefoot valgus occurs in the frontal plane deformity, locking the forefoot in eversion relative to the rearfoot.4 Root suggests that this deformity results from ontogenetic overrotation of the talar head and neck in relation to its body during early childhood development.^4,45,55 Forefoot valgus can be a rigid or flexible deformity. In rigid forefoot valgus, the compensatory weight-bearing mechanism occurs at the STJ as excessive supination or calcaneal inversion. It is a result of excessive premature GRFs at the first metatarsal head, causing rapid STJ inversion and increasing loading forces beneath the fifth metatarsal head. Thick callosities are often present beneath the first and fifth metatarsal heads. In contrast, flexible forefoot valgus is usually an acquired soft tissue condition. It most often occurs as a consequence of uncompensated rearfoot varus as an attempt to increase weight bearing along the medial foot. Because this deformity is flexible, no compensatory mechanism is necessary. Contact force beneath the first metatarsal head simply pushes it up out of the way, and the foot functions as if this condition were not present.

Assessment of first ray position is also carried out in STN position. Ideally, the first ray lies within the common transverse plane of the lesser metatarsal heads. To examine mobility of the first ray, the examiner holds the first metatarsal head between the thumb and index finger and performs a dorsal and plantar glide while stabilizing the lesser metatarsal heads with the other hand. Normally, first ray movement is at least one thumb width above and below the plane of the other metatarsal heads.4

As stance phase is completed, activity of the peroneus longus creates a pronatory twist of the forefoot, stabilizing the medial column of the foot on the ground, locking the MTJ about its longitudinal axis, and converting the foot to a rigid lever for propulsion. Adequate plantarflex of the first ray must be present for conversion from flexible forefoot to a rigid lever. Three factors determine how much first ray plantarflex must occur: amount of inversion of the foot at propulsion, width of the foot, and length of the second metatarsal.⁴ The more the foot inverts during propulsion, the further the first ray must plantarflex to make ground contact. Elevation of the medial forefoot is related to foot width; wide feet require more first ray plantarflex. An excessively long second metatarsal also increases the distance that the first ray must plantarflex to make ground contact.

In some instances, the first ray is inappropriately dorsiflexed above the plane of the other metatarsals, resulting in restriction of plantarflex and impeding normal propulsion. The first ray may also be plantar flexed below the plane of the other metatarsals. In uncompensated rearfoot varus, for example, the eversion motion of the calcaneus is insufficient, the medial condyle fails to make contact with the ground, and excessive weight bearing is present on the lateral border of the foot. The peroneus longus contracts to pull the first metatarsal head toward the ground in an attempt to load the medial side of the foot. This action is possible because of the cuboid pulley system (Figure 8-13).⁴ When the STJ remains abnormally pronated in late stance phase, orientation of the cuboid tunnel is altered and the mechanical advantage of the peroneus longus is lost. The MTJ cannot lock, and the foot is unstable throughout propulsion.

The presence of a rigid plantar flexed first ray sometimes results in a functional forefoot valgus (Figure 8-14). The compensatory mechanism for this condition is similar to that for rigid forefoot valgus: STJ supination or calcaneal inversion on weight bearing to lower the lateral aspect of the foot to the ground.

In normal gait, dorsiflexion of the hallux occurs during the late propulsive phase as the body moves forward over the foot. Sagittal plane motion of the hallux is assessed as passive ROM. The stationary arm of the goniometer is positioned along the medial first metatarsal and the mobile arm along the medial proximal phalanx of the hallux. The axis is medial to the first MTP joint.54 Sufficient force is applied to bring the hallux to its end ROM. Normal range of hallux dorsiflexion is between 70 and 90 degrees.^4,54

In hallux limitus deformity, pathomechanical functioning of the first MTP joint prevents the hallux from moving through its full range of dorsiflexion during propulsion. Repetitive trauma to the first MTP joint can lead to ankylosis, or hallux rigidus. Functional hallux limitus is a condition in which full first MTP ROM is present when non–weight bearing, but a functional restriction of hallux dorsiflexion occurs during gait. Functional hallux limitus disrupts the normal windlass mechanism previously described.^62,63

Limitation of hallux dorsiflexion prohibits the normal progression of the foot and interferes with propulsion of the body over the hallux. Several gait compensations can overcome this limitation.^62,63 An abducted or toe-out gait pattern shifts propulsion to the medial border of the hallux. A pinched callus then develops from friction between the hallux and shoe during propulsion. Alternatively, the IP joint of the hallux may hyperextend, causing a callus in the sulcus of the IP joint.

Hallux abductovalgus (HAV) is a progressive, acquired deformity of the first MTP joint that eventually results in a valgus subluxation of the hallux.^4,45 This deformity is caused by abnormal STJ pronation with hypermobility of the first ray.⁴ A common misconception is that HAV is hereditary. Although the congenital osseous abnormalities that lead to aberrant STJ pronation are hereditary, HAV occurs to compensate for these other deformities. Another misconception is that HAV is caused by restrictive footwear. Although inappropriate or restrictive footwear can accentuate or speed the progression of HAV deformity when present, the deformity is frequently observed in populations that do not typically wear shoes.^4,45

With the patient in double-limb stance posture, a line bisecting the posterior surface of the calcaneus is visualized and the angular relation between the line and the floor taken. Because the infracalcaneal fat pad often migrates (related to prolonged weight bearing), care must be taken to avoid errors in visual assessment (Figure 8-15). Palpation of the osseous medial, lateral, and inferior borders of the calcaneus helps factor out fat pad migration and improve measurement accuracy. Calcaneal alignment can also be quantified with a protractor to measure the degree of calcaneal tilt relative to vertical.⁶⁴

In contrast, forefoot varus deformity requires excessive compensatory STJ pronation; calcaneal eversion occurs in weight bearing. The position of the calcaneus with respect to the floor provides insight into the type of STJ compensation present and can be correlated with the biomechanical findings of the non–weight-bearing examination. If the STJ is unable to pronate enough to compensate completely for a deformity, additional pronatory motion occurs at the MTJ or by eversion tilting of the talus within the ankle mortise.4,^41,48 The functional rearfoot unit (calcaneus and talus) may assume a valgus (everted) position relative to the floor, even if calcaneal eversion is restricted.

Proximal structural malalignments, such as tibial varum or valgum, contribute to abnormal foot pronation and overuse injuries. In osseous congenital tibial varum, the distal third of the tibia is angled medially in the frontal plane, whereas in tibial valgum, the distal tibia inclines away from the midline.41

Tibial alignment can be measured with either a standard goniometer (Figure 8-16) or a bubble inclinometer; both assess the angular relation between the bisection of the distal third of the lower leg relative to the supporting surface.^41,65 Radiographic measurement of lower leg position is better correlated with clinically assessed tibiofibular position values than with isolated tibial position. Radiographic measurement may be the most accurate method to isolate true tibial varum.⁶⁶

Test position is critical because variation in STJ alignment greatly influences tibiofibular varum measurement values. Tibiofibular varum values are larger in RCS than in the STN position because of the combined effects of osseous malalignment and varus leg alignment associated with compensatory STJ pronation in stance.65–67 The incidence of tibiofibular varum appears to be high, although no clear normative values have been established.

The alignment of the distal third of the leg relative to the floor more accurately represents tibiofibular position than tibial position. Values assessed in STN reflect neutral tibiofibular alignment, whereas values assessed in RCS represent compensatory tibiofibular repositioning in response to STJ and MTJ pronation. High tibiofibular varum values measured in STN elevate the medial foot from the supporting surface, requiring excessive compensatory foot pronation during gait. High tibiofibular varum values in RCS suggest excessive foot pronation, although the source of that pronation cannot be isolated.

The position of the navicular is examined by using the navicular drop test, a composite measure of foot pronation focusing on displacement of the navicular tuberosity as a patient moves from closed kinetic chain STN to RCS position.68 Excessive navicular displacement is associated with collapse of the MLA and may be correlated with midfoot pain or other symptoms of excessive foot pronation.⁴¹ Subotnick⁶⁹ established an interdependency between MTJ and STJ function based on articulation of the navicular and cuboid with the talus and calcaneus. Brody⁶⁸ suggested that the navicular drop test is a valid assessment of STJ function in the closed kinetic chain. Although navicular drop occurs with STJ pronation that stems from intrinsic foot deformity, it can also be the result of muscle insufficiency or ligamentous laxity.⁴¹

To measure navicular drop, an index card is held perpendicular to the medial foot, the level of the navicular tuberosity is marked in STN position and RCS, and the distance between the marks is calculated.³⁸ Normative studies report a mean navicular drop of between 7.3 and 9 mm.^39,70 A navicular drop of more than 10 mm is considered abnormal.⁷⁰

When excessive STJ pronation is present in stance, the talus moves into adduction and plantarflex. Displacement of the talar head causes an observable medial bulge in the region of the talonavicular joint.71 The height of the MLA normally decreases moderately in weight bearing as a result of normal STJ pronation. Pes planus deformity (flatfootedness) is characterized by excessive collapse of the MLA. In hereditary rigid flatfoot, the MLA is low or absent in non–weight-bearing and weight-bearing positions. In flexible flatfoot, the height of the MLA is normal in non–weight bearing but drops excessively in weight bearing because of abnormal STJ pronation. In normal foot alignment, the medial malleolus, navicular tuberosity, and first metatarsal head fall along the Feiss line.⁷¹ In a severely pronated foot, the navicular tuberosity lies below the Feiss line. In extreme cases the tuberosity may even rest on the floor.^71,72

A positive toe sign indicates excessive pronation or abduction of the foot in the transverse plane (Figure 8-17). The sign is determined by the number of toes that can be seen, in a posterior view, when the patient is standing in RCS with a neutral foot placement angle.⁶³ Normally no more than 1.5 toes are visible beyond the lateral border of the foot. If more toes can be seen, abnormal pronation may be present, causing excessive transverse plane motion or abduction of the foot. A false-positive toe sign can occur in the presence of a relative toe-out foot placement angle associated with lateral rotational deformities (e.g., femoral retroversion) or muscle imbalances that limit internal rotation of the hip (e.g., tight piriformis). Ensuring that the patient’s patellae are oriented in the frontal plane before assessing toe sign reduces the risk of false-positive findings.