B. David Horn
DEFINITION
Adolescent bunion is a complex deformity consisting of medial deviation of the first metatarsal (metatarsus primus varus), lateral deviation of the great toe through the first metatarsophalangeal joint (hallux valgus), and enlarged medial eminence of the distal first metatarsal.
Other findings include contracted lateral and lax medial soft tissues of the first metatarsophalangeal joint, lateral subluxation of the sesamoids, pronation of the great toe, and plantar subluxation of the abductor hallucis muscle.
ANATOMY
Metatarsus primus varus
Obliquity of the medial cuneiform–first metatarsal joint
Medial prominence of the first metatarsal head
Valgus angulation through the first metatarsophalangeal joint
Minimal or no deformity through the first interphalangeal joint
Lateral translation of sesamoids
Plantar-lateral positioning of the abductor hallucis with unopposed pull of the adductor hallucis muscle
Lateral subluxation of the extensor hallucis longus and flexor hallucis longus tendons
Pronation (internal rotation) of the first toe
Differs from an adult bunion:
Physis of the first metatarsal and proximal phalanx are located proximally (this limits ability to perform proximal osteotomies in skeletally immature patients).
The first metatarsophalangeal joint does not have osteoarthrosis.
The medial eminence is less prominent in adolescent bunions than in adult bunions.
PATHOGENESIS
Multiple theories exist; it is difficult to differentiate primary findings from secondary ones.
Extrinsic and intrinsic factors contribute to formation of adolescent bunions.
Intrinsic
Metatarsus primus varus
Obliquity of the medial cuneiform–first metatarsal joint
Long first metatarsal
Ligamentous laxity
Heelcord contracture causes foot pronation, which in turn places a valgus force on the hallux while walking
Extrinsic
Shoe wear, particularly those with a narrow toe and elevated heel
NATURAL HISTORY
Natural history is believed to be favorable. Most patients with adolescent bunions can be treated nonoperatively.
PATIENT HISTORY AND PHYSICAL FINDINGS
Patients typically present in late childhood or adolescence.
Complaints about appearance of foot.
Complaints of pain over the medial exostosis or about the first metatarsophalangeal joint.
Pain is exacerbated by shoe wear.
Complaints about finding shoes that are comfortable.
Physical examination
Areas of tenderness: first metatarsophalangeal joint, medial prominence
Alignment when standing and walking
Mobility of first metatarsophalangeal joint
Skin condition: the clinician should search for calluses, areas of irritation
Foot and ankle range of motion
Careful neurologic examination
IMAGING AND OTHER DIAGNOSTIC STUDIES
Standing anteroposterior (AP), lateral, and oblique radiographs should be obtained if surgical correction is being contemplated.
Measurements on the AP radiograph (FIG 1)
Intermetatarsal (IM) angle: normal is 9 degrees or less
IM angle usually is 12 to 18 degrees in adolescent bunion.
FIG 1 • Measurements made on the AP radiograph.
Hallux valgus angle: normal is 16 degrees or less
Less than 25 degrees indicates mild deformity.
25 to 40 degrees is moderate deformity.
More than 40 degrees is severe.
Most adolescent bunions are mild to moderate.
Distal metatarsal articular angle: normal is 15 degrees or less
Proximal phalangeal articular angle: normal is 5 degrees of valgus
DIFFERENTIAL DIAGNOSIS
Hallux valgus interphalangeus
NONOPERATIVE MANAGEMENT
Initial treatment is nonoperative.
Proper-fitting shoes
Achilles tendon stretching if there is a heel cord contracture
Orthotics may be useful when there is also ligamentous laxity and pes planus.
SURGICAL MANAGEMENT
Surgery should be reserved for patients with persistent symptoms despite adequate nonoperative care.
Goals are to decrease pain and to restore the alignment of the first ray, with respect to both the second ray and the joints of the ray itself.
If feasible, surgery is delayed until early adolescence, as recurrence rates are higher in younger children.
For “typical” adolescent bunion (IM angle 12 to 18 degrees, hallux valgus angle less than 40 degrees), surgery generally consists of a distal soft tissue procedure, excision of the medial prominence, and corrective osteotomy.
Multiple procedures have been described.
In older adolescents, where the physis of the first metatarsal is closed, the corrective osteotomy can be performed proximally.
If the physis of the first metatarsal is open, a first metatarsal neck osteotomy has been described (Mitchell procedure).
Preoperative Planning
The surgeon should review the patient's radiographs: IM angle less than 15 to 18 degrees, hallux valgus angle less than 40 degrees.
Positioning
The patient is positioned supine.
A tourniquet is used.
Approach
A dorsomedial incision is made starting just distal to the first metatarsophalangeal joint and extending proximally for 5 to 6 cm.
The surgeon should avoid injury to the dorsal median sensory nerve.
TECHNIQUES
MITCHELL BUNIONECTOMY (STEVENS MODIFICATION)
Expose the medial first metatarsophalangeal joint.
Make a distally based Y-shaped incision in the capsule and periosteum. The stem of the Y is over the metatarsal, while the upper portion of the Y is formed distally.
The joint and medial eminence are then exposed.
A medial release of the first metatarsophalangeal joint is performed. Leave the lateral portion of the joint intact to avoid disrupting the blood supply to the head of the first metatarsal.
The first cut involves removing the prominent medial eminence with an osteotome, starting distally at the sagittal groove (groove of Clark).
The second cut is made at the distal metaphyseal–diaphyseal junction of the first metatarsal. This should be perpendicular to the shaft of the first metatarsal and extend two-thirds the width of the shaft of the first metatarsal (TECH FIG 1A).
The third, proximal cut is made about 2 to 3 mm proximal to the first cut, and is created completely across the first metatarsal. The cut is oriented perpendicular to the shaft of the second metatarsal when viewed from the dorsum of the foot and is angled (when viewed from the medial aspect of the first metatarsal) to create a small plantar-based wedge (TECH FIG 1B). This ensures that the distal fragment does not dorsiflex during reduction of the osteotomy.
The interposed bone is removed.
The osteotomy is reduced and pinned with two smooth 0.062-inch Kirschner wires (TECH FIG 1C).
The prominence of the distal portion of the metatarsal shaft is smoothed off with a rongeur, and a capsulorrhaphy is performed with absorbable sutures.
Sterile dressings are applied and the toe is splinted in neutral to slight plantarflexion. A short-leg cast is usually applied over the dressing for additional protection.
TECH FIG 1 • A. Bone cuts required to perform a modified Mitchell osteotomy. The medial prominence is excised first (line 1). The first cut of the osteotomy is performed two thirds of the way through the first metatarsal at the junction of the metaphysis and diaphysis and is oriented perpendicular to the long axis of the first metatarsal (line 2). The second bone cut (line 3) is made completely through the bone and completes the osteotomy. It should be made 2 to 4 mm proximal to the first cut and is perpendicular to the long axis of the second metatarsal. B. When seen from the medial side, the osteotomy should be oriented so that a small plantar-based wedge is produced. This helps avoid dorsiflexion of the distal fragment when the osteotomy is reduced. C. The osteotomy is reduced and stabilized with a 0.062-inch smooth Kirschner wire.
POSTOPERATIVE CARE
The toe is splinted in slight flexion.
The dressing is covered with a cast.
Weight bearing is allowed as tolerated.
Pins are removed in 6 weeks.
OUTCOMES
Most studies report 65% to 85% good to excellent results with the Mitchell osteotomy.
The modified Mitchell osteotomy (described above) produces 81% satisfactory results, with no cases of malunion, nonunion, avascular necrosis of the first metatarsal head, infection, or transfer metatarsalgia.
Sixty percent good to excellent results are reported in younger patients.
COMPLICATIONS
Infection
Neurovascular injury
Inadequate fixation of the osteotomy
Malunion or nonunion of the osteotomy
Avascular necrosis of the first metatarsal head
Transfer metatarsalgia
Recurrence
Stiffness of the first metatarsophalangeal joint
Hallux varus (overcorrection)
Pronation
Pain
REFERENCES
· Kuo CH, Huang PJ, Cheng YM, et al. Modified Mitchell osteotomy for hallux valgus. Foot Ankle Int 1998;19:585–589.
· McDonald MG, Stevens DB. Modified Mitchell bunionectomy for management of adolescent hallux valgus. Clin Orthop Relat Res 1996;332:163–169.
· Mitchell CL, Fleming JL, Allen R, et al. Osteotomy-bunionectomy for hallux valgus. J Bone Joint Surg Am 1958;40A:41–58.
· Weiner BK, Weiner DS, Mirkopulos N. Mitchell osteotomy for adolescent hallux valgus. J Pediatr Orthop 1997;17:781–784.