FOREFOOT DERANGEMENT: ETIOLOGY OF HAMMERTOES AND BT]NIONS

Tbomas J. Merrill, DPAI Ibn A. Imani, DPM

The most common cause of deformity motion of each metatarsal attempt to compensate and associated digital deformities is the long term For the everted position of the forefoot in relation progressively destabilizing effects of excessive to the weightbearing surface. The first metatarsal pronation. Abnormal pronation produces a lnlls- has the largest dorsiflexory range of motion, fol- cle tendon imbalance across the joints of the lowed by the fifth and fourth metatarsals. The forefoot. Over time, these asymmetrical forces, if seconcl and third metatarsals have a limited dorsi- not controlled, can eventually alter normal align- flexory range of motion. Palpation of the neutral ments of the metatarsals and digits. Inappropriate subtalar joint position and locking of the mid- surgical intervention can produce this derange- tarsal joint allows evaluation of each of the ment much more rapidly. metatarsal's range of motion. Long-term excess subtalar joint pronation The peroneus longus muscle is a major fac- changes the normal function of the joints and tor in the stabilizatton of the first metatarsal muscles of the foot during the gait cycle. During against the ground, and abnormal pronation the contact phase of gait, the normal subtalar alters its function. The fulcrum of the tendon, the ioint goes through the motion of pronation to aid cuboid, loses stability and the muscle pull in shock absorption. At the end of the contact decreases as its functional length increases. phase the subtalar joint stops pronating at a few Because of the everted position of the forefoot, degrees past neutral and then begins the motion the insertion of the peroneus longus into the base of supination. This motion of resupination during of the first metatarsal is lower in relationship to midstance produces a rigid stable forefoot to the fulcrum of the cuboid. The plantarflexory accept the weight of the body and to al1ow force vector of the tendon on the first metatarsal propulsion. is decreased or lost. The first metatarsal becomes During forefoot Ioading, both axes of hypermobile and develops an increase in dorsi- motion at the midtarsal joint pronate to a locked flexion, adduction, and inversion around its nor- position. The locked position of the midtarsal mal axis of motion. joint is dependent on the position of the subtalar Abnormal subtalar ioint pronation with cal- joint. Every degree of subtalar joint pronation caneal eversion and talar plantarflexion will cause produces an exponential increase in midtarsal the forefoot to abduct on the rearfoot. Abduction joint pronation. Therefore, in abnormal subtalar of the forefoot contributes to digital deformities by joint pron:rtion, the midtarsal joint cannot reach changing the normal relationship between the the locked position and the forefoot continues to intrinsic muscles, the long flexors and extensors, evefi and abdr-rct during midstance. The longitu- and their digital insefiions. Nthough abnonnal sub- dinal axis of the midtarsal joint and the range of talar joint pronation may occur for many reasons,

157 the derangement of the forefoot that results devel- fibular sesamoid. These stfttctures are held stable ops fiom this same partern of loint instability. as the first metatarsal heacl mo\res auray from the second metatarsal head. The lateral collateral, IATROGENIC ETIOLOGY OF fibular sesamoidal and intersesamoidal ligaments ABNORMAL PRONATION remain intact. The first metatarsal head effectiveiy leaves the fibular sesamoicl behind as it moves Surgical intervention can produce dramatic along its increased axis of motion. The flexor hal- realignment of muscle tenclon function, but can lucis longus and brevis tenclons are rotated from also produce devastating muscle tendon imbal- uncler the metatarsal head gradually into the ance. There are several reasons why the forefoot interspace. A11 of the periarticular structures that derangement that is iatrogenically produced is cross the first metatarsophalangeal joint become much more progressive than the forefoot rotated or clisplaced. The abductor hallucis ten- derangement secondary to abnormal pronation. don is displaced plantarly and the extensor hallu- The attempted surgical procedure did not realign cis longus and brevis tendons are clisplaced later- the muscle tendon imbalance and that imbalance al to the joint. continues. Normal anatomy in the are2t that had As the lateral joint structures rotate into the previously provided stability may have been interspace, the proximal phalanx of the hallux clestroyed. The underlying etiology of abnormal assumes a valgus position. The dorsomedial pronation is probably sti11 present and these aspect of the joint capsule stretches and the forces will continue to influence the postopera- medial aspect of the first metatarsal head tive result. becomes prominent. The soft tissue ovedying the The approach to an iatrogenic deformiry is dorsiflexed, addr-rcted metatarsal head becomes the same as to other biomechanical deformities. inflamed from shoegear and a bursa may form A current inventory of the functional anatomic l',etarrse ol the irrita(ion. structllres is the first step. Identification of the The abducted hallux position and the periar- individual defbrming forces influencing the mus- ticular structural displacement combine to pro- cle tendon imbalance is the second step. The duce the retrograde buckling force that increases selection of surgical procedures to redirect or the metatarsus primus adductus and progressively neutralize these forces is the third step. The accentuates the deformity. The adaptive soft tis- immediate and long term postoperative manage- slle of the flexor hallucis brevis ancl ment is the fourth step. the adductor tendon acting in conjunction with the displaced fibuiar sesamoid can produce a Hallux Abducto Valgus tension-band like effect that will resist relocation of the proximal phalanx on the metatarsal head. Patients often develop a hallux abducto valgus The adductor tendon may also be responsible for deformity secondary to the hypermobility result- lateral tracking of the proximal phalanx on the ing from excess pronation. \fith the hypermobile metatarsal head during dorsiflexion. As the joint motion of the first metatarsal, the first metatarsal gradtrally dislocates, the abnormal lateral forces head clorsiflexes, inverts, and adducts. As prona- on the articular cartilage surfaces can produce increases tion forefoot abduction and first adaptive or degenerative changes. metatarsal motion, the tension on the transverse The lateral collateral ligament, fibular head adductor of the hallucis tendon increases. sesamoidal ligament, adductor hallucis tendon The anatomy the first metatarsophalangeal of and flexor hallr-rcis brevis undergo shortening and joint effects the development of a hallux abducto contracture over time. Each of these anatomic because no muscles attach into structures must be identified and addressed indi- the first metatarsal head. The intrinsic muscles vidually in the surgical repair of the hallux insert into the capsule, the sesamoicls and the abducto valgus deformity. base of the proximal phalanx. The adductor ha1- A pinch callus at the medial plantar aspect lucis tendon and the deep transverse inter- of the medial condyle of the head of the proxi- metatarsal ligament have strong attachments into mal phalanx may develop because of the changes the lateral aspect of the joint capsuie and the in hallux function with excess pronation. As the

158 hypermobile first metatarsal dorsiflexes and the or contracts. As the subtalar joint resupinates the hallux moves into a valgus position, the hallux dorsiflexory force on the forefoot locks the mid- deviates from the line of progression. The body tarsal joint and a1lows the peroneus longus to act weight falls more medially than in a normal foot over a stable cuboid to plantarflex the first ray and the patient subsequently rol1s over the medi- and stabllize the forefoot. This allows the intrinsic al side of the abducted hallux. muscles to stabilize the against the ground without buckling. Both the interossei and lumbri- Iatrogenic Hallux Abducto Valgus cales help to stabilize the toes, although the interossei may be the primary stabilizers during An unsuccessful surgical attempt can produce a stance, whereas the lumbricales function primari- more progressive deformity. A hyperkeratotic ly during the swing phase. A partial resection of lesion plantar to the second metatarsal head the base of the proximal phalanx also has no would be expected with a hypermobile first biomechanical basis. The resultant muscle tendon metatarsal and a hammertoe of the second digit. imbalance will deviate and eventually dislocate The plantar tyloma is not due to a structural the metatarsophalangeal joint. The total resection deformity of the second metatarsal, but rather the of the base of the proximal phalanx has no muscle tendon imbalance of the first ray and the biomechanical basis. The loss of the intrinsic digits. Realizing the etiology of the plantar lesion, muscle stablhzation reproduces the effects of a procedure that realigns the intrinsic muscle bal- abnormal pronation immediately. The resultant ance across the first metatarsophalangeal joint is floating digit has no sagittal plane stability and indicated. Procedures that appear to address the does not purchase the ground. immediate symptoms, in reality can actuaily \7hen the subtalar joint fails to resupinate, increase the forefoot derangement. The bunion- the distal joints cannot be stabilized. The intrinsic ectomy procedure alone will reduce the medial muscles lack the strength to stabilize the foot and capsular stability and al1ow for a more raprd they fatigue in the attempt to do so. An additional development of the hallux abducto valgus defor- attempt to regain digital stability is initiated with mity. The elevating osteotomy of the second excessive firing of the flexor digitorum longus metatarsal will dramatically increase the abnormal muscle. The pull of the long flexor tendons pro- pronation of the subtalar joint when associated duces a hammer deformity because the proxi- with a neglected hypermobile first metatarsal. mal and middle phalanges are no longer stabi- lized. This grasping of the toes, called flexor Contracted Toes stabilizationi occllrs during late midstance and propulsion. Flexor stabilization can also cause an of the most common deformities caused by One adducto varus deformity of the digits to develop. muscle tendon imbalance is the contracted toe. As the foot pronates and the forefoot abducts, the Digital deformities usually occur in conjunction pull of the long flexor shifts medially and increas- other forefoot changes such as hallux with es the amollnt of adduction that occurs with flex- valgus, lesser metatarsalgia and planlar abducto ion. The fifth toe contracts most frequently, with tylomas. These deformities are closely related and the fourth and third toes also being affected in the interaction between them is the key to the more serious cases. The digital effects of evaltiation and treatment. Digital abnormai pronation can be obserued as a sepa- affect the function of the metatarsophalangeal rate entity. joint and metatarsals. As the toes contract, the joints are dorsiflexed and a metatarsophalangeal Flexor Cap Dislocation retrograde plantarflexory force is placed on the metatarsals. A decision must be made as to the one of the problems that occurs after long term etiology of a plantar hyperkeratotic lesion: strlrc- pronation with digital contracture is dislocation of tural metatarsal etiology or functional digital con- the metatarsophalangeal joint flexor cap. This tracture etiology. occurs most commonly at the second metatar- A normal foot will resupinate as the leg sophalangeal joint and is aggravated by an begins to rotate externally and the tibialis posteri- abducted hallux that forces the second toe dorsal-

759 ly and medially. As the toe displaces, the long mobile first metatarsal is much more common flexor and extensor tendons are pulled to the and a true structurally plantarflexed second medial side of the joint, and the flexor cap may melalarsal is rare. Failure to differentiate between ride up around the medial side of the metatarsal the lwo is a common problem and can result in a head. As this occurs, contraction of the long flex- very difficult iatrogenic postoperative situation. or no ionger just plantarflexes the toe, but also The resection of the second metatarsal head adducts it. has no biomechanicai basis and exaggerates the A hallux abducto valgus deformity often hammertoe deformity and the hallux abduction contributes to toe deformities, especially those of deformity. The weightbearing distribution of the the second toe. As the hallux drifts laterally it remaining metatarsals is destroyed and stress frac- tends to push the lesser toes laterally, then it may tures or plantar lesions result. The tension distri- under-ride or override them. As the hallux moves bution between the division of the flexor digito- under the second toe, a develops rum longus and flexor digitorum longus is also and the entire second metatarsophalangeal joint disrupted, A loading phenomenon muscle tendon wili eventually dislocate. A parient with this prob- imbalance develops and creates progressive ham- lem may present only with a chief complaint of a mefioe deformities of the remaining digits. painful second toe, but the underlying hallux abducto valgus deformity needs to be addressed. Bibliography

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