0196-601 1/67/0901-001 1$02.00/0 THEJOURNAL OF ORTHOPAEDIC AND SPORTS PHYSICAL THERAPY Copyright (O 1987 by The Orthopaedic and Sports Physical Therapy Sections of the American Physical Therapy Association

Abnormal Biomechanics of the Foot and Ankle

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ROBERT DONATELLI, MA, PT*

The biomechanics of the foot and ankle is important to the normal function of the lower extremity. The foot is the terminal joint in the lower kinetic chain that opposes external resistance. Proper arthrokinematic movement within the foot and ankle influences the ability of the lower limb to attenuate the forces of weightbearing. It is important for the lower extremity to distribute and dissipate compressive, tensile, shearing, and rotatory forces during the stance phase of gait. Inadequate distribution of these forces could lead to abnormal stress and eventual breakdown of connective tissue and muscle. Pathologies such as heel spurs, hallux valgus, neuromas, hallux limitus, shin splints, and nonspecific knee pain result from abnormal mechanics of the foot and ankle. The use of orthotics to re-establish the normal biomechanics of the foot and ankle have profound clinical applications. The combined effect of muscle, bone, ligaments, and normal biomechanics will result in the most efficient force attenuation in the lower limb.

Abnormal mechanics of the foot and ankle is the breakdown of the mechanisms designed to distribute and dissipate the normal forces of weightbearing. Pronation and supination are the arthrokinematic movements within the foot and ankle that are essential for proper attenuation of compressive, tensile, shearing, and rotatory forces during the stance phase of gait. For example, Mann8describes a 150 Ib individual walking at a stride length of two 21/2 ft, for 1 mile, would apply 127 tons of force to his feet. If that same individual ran a mile with a stride length of q1/2 ft, he would increase the amount of force to 220 tons.8 A long distance runner's feet contact the ground approximately 5000 times in an hour's run, with 2-3 times the body weight at every heel strike.',' These excessive forces are normally attenuated by the dynamic action of muscle, periarticular tissue strength, flexibility, and proper arthrokinematics. Abnormal pronation and supination are nothing more than hyper or hypomobilities, respectively, within the joints of the foot and ankle. Excessive motion or restricted motion reduces the ability of the foot to act as a shock absorber, torque convertor, mobile adaptor to the terrain, and a rigid 'Assistant Professor. Master Degree Program Orthopaedic and Sports Physical Therapy, Georgia State University, Atlanta, GA 30303.

lever to push off from. As a result of changes in the joint mobility, connective tissue changes occur, in addition to alterations in muscle function. This could result in pathological conditions such as neuromas, hallux valgus, tailors bunions, keratosis, shin splints, nonspecific knee pain, plantar fascitis, heel spurs, metatarsalgia, and achilles tendinitis.',l5

ABNORMAL PRONATION Abnormal pronation is a compensation for a soft tissue or osseous deformity.15The deformity can be intrinsic or extrinsic to the foot. In the majority of cases, excessive pronation occurs at the subtalar joint to compensate for the abnormality.3z'5 The compensation is persistent and results in pathology.15 Maximum pronation is achieved at footflat and supination begins at midstance. Excessive pronation means that the foot is pronating beyond 25% of the stance phase.138s14915 Consequently, at midstance the foot does not begin to resupinate, it remains in maximum pronation. The foot may resupinate late in the stance phase or never resupinates, as seen in a rigid flatfoot. This results in the inability of the foot to effectively absorb the forces of weightbearing. The etiology of abnormal pronation can result from many different factors. Congenital, develop-

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DONATELLI

mental, intrinsic, (within the foot) and/or extrinsic, (outside the foot), are the categories we will discuss. The most common intrinsic congenital deformities resulting in abnormal pronation in the adult foot include convex pes valgus (vertical talus), tarsal coalitions, and congenital metatarsus ~ a r u s . ~ ~The ' , ' ~ developmental deformities intrinsic to the foot include talipes calcaneovalgus, talipes calcaneovarus, postural metatarsus adductus, and forefoot v a r ~ s . ~The ~ ~ congenital .'~ deformities result from genetic factors, which are beyond the scope of this article. The developmental or postural deformities may result from a poor position in the uterus.17 Additional factors causing developmental flatfeet involve ligament laxity and/or a tight achilles tendon.17 CONGENITAL DEFORMITIES

Convex pes valgus is a primary dorsal and lateral dislocation of the talocalcaneonavicular j ~ i n t . ~The . ' ~ navicular articulates with the dorsal aspect of the talus, locking it in the vertical position. This deformity has been described as a "rocker bottom foot." Tarsal coalitions include complete or incomplete fusion of the major tarsal The coalitions producing a flstfoot are, calcaneonavicular or talocalcaneal at the middle, anterior, or posterior f a ~ e t . ~Clinical , ' ~ signs may exist, such as limitation and/or rigidity of the major tarsal articulations, valgus deformities of the rearfoot, and soft tissue contractures or spasm of the peroneus longus and brevis7,l7 Congenital metatarsus varus is medial subluxation of the tarsometatarsal joints with adduction and inversion deformity of the metatarsal^.^^'^ One important clinical finding is the inability of the forefoot to be passively abducted to the neutral position. This differs from the postural metatarsus adductus deformities, which can be reduced by passively moving the forefoot into abduction. In the newborn, the prognosis for 10O0/0 correction of the postural metatarsus adductus deformities is excellent within the first few month^.^.'^

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and eversion. The most important clinical observation is the limitation of plantarflexion at the ankle joint. The plantarflexion range is usually limited to 90' or less. Another clinical finding is dorsiflexion and eversion of the foot with stroking of the plantar surface of the foot. This sign is lost by the age of 4-6 months.17 Calcaneovarus can best be described as inversion of the entire foot, with no limitation in dorsiflexion at the ankle j ~ i n t . ~ , ' ~ Root et a1.15and Tax1' report the most common intrinsic deformity resulting in abnormal pronation is forefoot varus. The definition of forefoot varus is an inversion of the forefoot on the rearfoot with the subtalar joint in neutral (Fig. l).15 Root et aI.l5 describe it as a frontal plane deformity, that is compensated at the subtalar joint by eversion or a valgus position of the calcaneus in weightbearing (Fig. 1). The compensation is nothing more than excessive talar adduction, plantarflexion, and calcaneal eversion (abnormal pr~nation).~.'~ This author has observed forefoot varus deformity as the single most common intrinsic cause of mechanical pain and dysfunction within the foot, lower one-third of the leg, and knee. McCreag defines forefoot varus as a sagittal plane deformity of the first ray. The first ray is dorsiflexed and hypermobile. This' hypermobility results from the inability of the peroneus longus to stabilize the first ray. The cuboid pulley allows the peroneus longus to plantarflex and abduct the first ray producing stability (Fig. 2).15 This pulley system is reinforced by a locking up of the midtarsal joint during supination of the subtalar j ~ i n t . ' ~The ~ ' ~cuboid ~ ' ~ becomes a rigid structure, producing a stable pulley for the peroneus longus. Abnormal or excessive pronation reduces the abil-

POSTURAL DEFORMITIES

Calcaneovalgus, the most common postural deformity, occurs in approximately 1 in 1000 births. There is a high correlation of calcaneovalgus in the newborn and the development of a flexible flatfoot in the older ~ h i l d . ~ . ' ~ The appearance of the foot is one of dorsiflexion

Fig. 1 . Uncompensated (A) and Compensated (B) forefoot varus. 1, First ray; 2, articular surface of talus and navicular; 3, Articular surface of calcaneus and cuboid; 4, calcaneus; 5, talus; 6, tibia; 7, fibula; 8, keratosis under second ray.

JOSPT July 1987

ABNORMAL BIOMECHANICS OF THE FOOT AND ANKLE

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A common x-ray finding in a flatfoot is the change in the alignment of the midtarsal joint line. The lateral weightbearing x-rays demonstrate a line forming an S, called the Cyma line, which passes between the talus and navicular and cuboid and calcaneus (midtarsal joint) (Fig. 3).' With excessive pronation there is an anterior break in the Cyma line. The talus has moved anterior relative to the cuboid and calcaneus. The anterior movement of the talus compensates for the lack of anterior movement of the tibia.

Fig. 2. Cuboidpulley.A, Normal pulley system for the peroneus longus, subtalar neutral; B, abnormal pronation and less efficient pulley system. A and F represent the vectors of the peroneus longus action; C, abduction vector; F, plantarflexion vector; P, peroneus longus. 1, Cuboid; 2, cuneiforms; 3, talus; 4, fibula; 5, tibia; 6, calcaneus.

ity of the foot to return to supina:ion. Thus, the midtarsal joint, specifically the cuboid, is in a poor position. This creates an inefficient pulley for the peroneus longus, causing instability of the first ray.15 The dorsiflexed and hypermobile first ray can produce a hallux valgus, subluxation of the metatarsophalangeql joint of the big toe in the sagittal and transverse planes.15 The instability of the first ray during the stance phase of gait causes the weight to be shifted from the first to the second metatarsal. The first ray is twice as large and is four times as strong as any of the other rays6 It has stronger muscles attached to it than any other ray.6 It has progressed in evolution from an adducted position to a midline position to enhance it's weight-bearing ability." The second ray is not designed to deal with the excessive forces of weightbearing. As a result of first ray insufficiency, a callus or keratosis develops under the head of the second metatarsal and stress fractures are not uncommon in this area.6 This author and others have observed ankle jaint equinus to be a frequent factor in the development of abnormal pronati~n.~,'~ Root et aI,l5 define ankle joint equinus as the lack of dorsiflexion of the ankle joint with the subtalar joint in neutral. This inability of the tibia to move anterior to the talus from footflat to midstance can result from a tight achilles tendon or a flattened dome of the talus. The compensation for this deformity can occur at the subtalar joint by excessive pronation.14 If the tibia cannot move anterior to the talus, the talus will move anterior.

EXTRINSIC CONGENITAL AND DEVELOPMENTAL DEFORMITIES

The most common extrinsic congenital deformities causing abnormal pronation include hip dysplasia, femoral antitorsion, tibia1 torsions, and genu varum or valgus.' The most common developmental deformities this author has observed in the clinic include femoral antiversion secondary to tight internal rotators or anterior hip capsule, pelvic imbalances, and muscle imbalances within the pelvis and lower extremity. The above deformities produce a rotation of the lower limb that could be compensated by pronation of the subtalar joint. For example, excessive external rotation of the lower limb will shift the center of gravity in weightbearing to the medial aspect. This increase in medial stress to the foot causes the tibia and talus to rotate medially (plantarflexes and adducts), while the calcaneus rolls laterally (into v a l g ~ s ) . ~ ~ ~ ~ ' ~ Internal rotation of the lower limb produces excessive forces to the lateral aspect of the foot. The center of gravity in weightbearing should pass through 'the center of the foot. In an attempt to shift the center of gravity more medially the fore-

Fig. 3. Medial arch view on lateral weightbearing x-ray. A, Abnormal-anterior break in the Cyma line, with reduced height of the calcaneal inclination angle, cuboid, and navicular. Normal inclination angle of the calcaneus 75-30°. B, Normal alignment of the medial arch. "S"shaped Cyma line. 1, cuboid; 2, navicular; 3, calcaneus; 4, inclination angle of the calcaneus; 5, cyma line; 6, talus.

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DONATELLI

JOSPT Vol. 9, No. 1

foot abducts on the rearfoot or the foot abducts weightbearing x-rays, the inclination angle of the on the leg, producing excessive pronation of the calcaneus is reduced, reducing the height of the subtalar j ~ i n t . ' ~ .The calcaneus to the ground. The normal angle of ' ~ toe-in gait abnormality in inclination is 15-30°.9317The height of the cuboid children is a good example of an internally rotated and navicular is also reduced (Fig. 3).',l7 Patients lower limb. Subtalar joint pronation is one possible with flatfeet (excessive pronators) demonstrate a compensation for this gait abnormality. wide foot, secondary to the spreading apart of As a result of the deformities listed, the alignthe tarsal bones. ment of the calcaneus, talus, cuboid and navicular The change in the mechanics of the rearfoot change. This in turn produces poor articular congruity and changes in the arthrokinematics of the and midfoot produces certain anatomical changes ankle joint, subtalar joint, and midtarsal. The extypical to a flatfoot, that can be observed in the cessive arthrokinematic movements occur beweightbearing position. These changes include a tween four bones, calcaneus, talus, navicular, and valgus (eversion) position of the calcaneus, bulgcuboid. The tibia, talus, and calcaneus move siing of the navicular tuberosity medially, abduction multaneously. As the talus and tibia rotated meof the forefoot on the rearfoot, and a reduction in dially, the calcaneus rolls laterally ( ~ a l g u s ) . ~ ~ . ~the ~ .height '~ of the medial In abnormal pronation the calcaneus is described ABNORMAL SUPINATION as subluxing under the talus.' The talus during weightbearing is fixed in the ankle joint.mortise, There are three basic classifications for abnorrestricting movement in the frontal plane. The tibia mal supination.'s16First is a pes cavus foot. This moves anterior and medial producing internal rofoot demonstrates a fixed plantarflexed forefoot, tation of the talus and tibia. The talar movements or an equinus forefoot. The rearfoot in the weightare described as adduction and plantarflexion bearing position is in neutral (no eversion or in(Fig. 4). The weightbearing stress is more medial version is visible). The second type is pes cavocausing a medial force to the heel, resulting in a varus. This foot demonstrates a fixed plantarvalgus heel. flexed medial column or first ray. In the weightIt is important to note that the above arthrokibearing position the calcaneus is in varus or innematic movements are abnormal because they verted. are excessive and persistent. Normal pronation Root et aI.l5 defines forefoot valgus as eversion compensation is a temporary condition of the of the forefoot on the rearfoot with the subtalar subtalar joint.14 The normal compensation might joint in neutral. The compensation for a forefoot occur in response to a change in the terrain. valgus is inversion of the calcaneus in the weightThe navicular and cuboid are influenced by the bearing position (Fig. 5).14,16Forefoot valgus and/ movements of the talus and calcaneus. Excessive or a fixed plantarflexed first ray are the most pronation causes the navicular and cuboid to common intrinsic deformities resulting in abnormal move away from each other. As noted on lateral supination of the subtalar joint. This author has observed a combined fixed plantarflexed first ray

Fig. 4. Closed kinetic chain pronation. A, Anterior view of the subtalar joint and the talocrual joint; B, posterior view of the subtalar and talocrual joint. 1, ~alcaneal/cuboidarticulation; 2, talo/navicular articulation; 3, sustentaculum tali; 4, calcaneus anterior aspect; 5, talus; 6, tibia; 7, fibula.

Fig. 5. Uncompensated (A) and compensated (B) forefoot valgus. 1, First ray; 2, talo/navicular articulation; 3, calcaneal/ cuboid articulation;4, calcaneus; 5, talus; 6, tibia; 7, fibula.

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JOSPT July 1987

ABNORMAL BIOMECHANICS OF THE FOOT AND ANKLE

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and a forefoot valgus present with rearfoot varus possible to neutral. For example, to correct a compensation at the subtalar joint. forefoot varus a medial post or support is used to Pes equinovarus is the third clas~ification.~~'~ bring the ground up to the first ray (Fig. 6). In This foot demonstrates a fixed plantarflexed foreaddition, a medial post is also used to prevent foot and rearfoot. No compensation is needed in and/or control the calcaneus from rolling into valthe weightbearing position. gus. The etiology of a pes cavus foot falls under two A general rule to follow is "post what you see." basic classifications, neuromuscular and idiIf the forefoot varus deformity was measured at o p a t h i ~ . ~Several ,'~ neuromuscular disorders can 5O, a 5O medial post should be prescribed in cause a pes cavus foot such as cerebral palsy, fabrication of the permanent orthotic. The post is Charot-Marie-Tooth disease, Frederick's ataxia, designed to support the first ray and/or the foreand muscular d y s t r ~ p h y . ~ . ' ~ foot foot deformity. The idiopathic type of cavus foot is thought to develop secondary to causative factors such as SUMMARY measles, scarlet fever, and diptheria causing a discrepancy in the growth of bone and m ~ s c l e . ~ * ' ~ Abnormal pronation and supination indicates Functionally, abnormal supination is the inability excessive or restricted motion within the foot and of the foot to pronate. At heel strike the foot ankle, secondary to soft tissue and/or bony abshould be in neutral and begin to pronate immenormalities. A compensation for the deformity is diately. An excessive supinator remains supinated necessary to allow the foot to function correctly throughout the stance phase or pronates late in during the gait cycle. A large majority of the time stance, (from heel-off to push-off). Root et aI.l5 the compensation occurs at the subtalar joint. state that more trauma occurs to the foot when Consequently, the normal amount of pronation pronation occurs during the push-off phase of and supination required during the stance phase gait. This can be observed during the gait cycle of gait occurs in addition to the amount needed by a whipping or unstable heel in push-off. Patients that excessively supinate develop irrito overcome the deformity. This is considered tations within the foot such as plantar fascitis, excessive pronation or supination. heel spurs, achilles tendinitis, metatarsalgia, and There are two basic types of abnormal pronacalcaneal bursitis. tors or supinators. The first type of foot remains pronated or supinated throughout the stance FUNCTIONAL BIOMECHANICAL ORTHOTIC phase of gait. This type of foot never resupinates or repronates. The second type resupinates or The functional biomechanical orthotic is depronates at the wrong time in the stance phase. signed to restore normal alignment of the subtalar For example, pronation might begin at heel lift or and midtarsal joints, controlling excessive pronapush-off. The foot needs to be supinating during tion and supination, reducing the abnormal forces the final phase of stance, to establish a rigid lever through the kinetic chain. The general concept of to push off from. If the foot is pronating, stability a functional biomechanical orthotic is to support the forefoot varus or valgus deformity and repois lost producing trauma to the foot and a less sition the rearfoot (subtalar joint) as close as efficient propulsive phase of gait. The use of orthotics to control the duration and the amount of pronation and supination during the stance phase of gait have profound effects on pain and dysfunction of the lower extremity. It is important to evaluate muscle imbalances extrinsic and intrinsic to the foot, in addition to evaluating forefoot and rearfoot deformities. The treatment must include correction of abnormalities within the foot and throughout the lower extremity. Fig. 6. Biomechanical orthotic posting. 1, First ray; 2, calcaneus in neutral; 3, medial forefoot post and rearfoot post.

Acknodedgments: My wife Joni for her support and enwuragernent. My partners in practice and in Clinical Education Associates, S w t Irwin, Steve Kraus, Micheal Wooden, and Marty Kaput.

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DONATELLI

REFERENCES 1. Buchbinder RM, Napora NJ, Biggs EW: The relationshipof abnor-

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3. O U C ~ FW f~i Wbdjarkfttsrdsm& r i a f H M m & of the forefoot. Clin Orthop 177:39-48, 1983 4. Gary E, Basmaijian J: Electromyography and cinematography of leg and foot (normal and flat) during walking. Anat Rec 161:16, 1968 5. Hanis R, Beath T: Hypermobile flat-foot with short tendoachilles. J Bone Joint Surg (Am) 30:116-140,1948 6. Hutton WC, Dhanedran M: The mechanics of normal and hallux valgus feet-a quanitative study. Clin Orthop 157:7-13, 1981 7. Jahss MH: Disordersof the Foot. Vol 1. Philadelphia:WB Saunders CO, 1982 8. Mann RA: Biomechanics of running. In: Mack (ed), Symposium on the Foot and Leg in Running Sports, pp 1-29. St Louis: CV Mosby, 1982 9. McCrea, JD: Pediatric Orthopaedics of the Lower Extremity. Mt Kisco, NY: Futura Publishing Co, 1985

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10. Mittleman G: Transverse plane abnormalities of the lower extremities, intoe and outtoe gait. J Am Podiatry Assoc 61:l-7, 1971 11. Olsen TR, Seidel M: The evolutionary basis of some clinical disorders of the humanfoot. A comparativesurvey of the living primates.

Foot Ankle 3322-341,1983 12. Peny J: Anatomy and biomechanics of the hindfoot. Clin Orthop 77:9-15, 1983 13. Phillips RD, Phillips RL: Quantitativeanalysis of the locking position of& ~ f a r s aJAm / ~Pod/ayAssclc 73578-522,19617 14. Ramig D, Shadle J, Watkins A, Cavolo D: The foot and sports medicine-biomechanicalfoot faults as related to chondromalacia patellae. J Orthop Sports Phys Ther 248-50, 1980 15. Root ML, Orien WP, Weed JH: Clinical Biomechanics.Vol II:Normal and Abnonnal Function of the Foot. Los Angles: Clinical Biomechanics Corp, 1977 16. Subotnick S: Biomechanics of the subtalar and mid-tarsaljoints. J Am Podiatry Assoc 65:756-764,1975 17. Tachdjian MO: The Child's Foot. Philadelphia: Saunders WB Co, 1985. 18. Tax H: Flexible flatfoot in children. J Am Podiatry Assoc 67:616619,1977