Localization of pressure along the plantar aspect of the foot illustrates the relative length of time that the center of pressure dwells beneath the metatarsal heads during normal walking (see Chapter 1). After heel strike, the center of pressure moves very rapidly to the metatarsal head area, where it dwells for more than 50% of the stance phase, after which it moves toward the toes (Fig. 8-1). Because of this extended period of weight bearing in the metatarsal area, abnormal alignment of the forefoot, either localized or generalized, can result in the formation of a keratotic lesion.

Figure 8-1 Movement of the center of pressure along the plantar aspect of the foot during walking. Note the way the center of pressure rapdly moves from the heel and dwells in the metatarsal region before moving distally to the great toe. (From Hutton WC, Stott JRR, Stokes JAF: In Klenerman L, editor: The foot and its disorders, Oxford, 1982, Blackwell Scientific, p 42.)

Plantar Keratoses

A broad differential diagnosis (Box 8-1) must be considered in the evaluation of the patient presenting with a plantar callus. The diagnosis is usually not difficult to make, but at times the diagnosis may not be clear, and one needs to consider these various causes.

Anatomy

The predominant pattern of the metatarsal cascade demonstrates that the first metatarsal is shorter than the second approximately 60% of the time.²⁴ The mobility of the first metatarsocuneiform (MTC) joint or the first metatarsophalangeal (MTP) joint determines the degree of weight bearing of the first ray. The plantar aponeurosis mechanism brings about plantar flexion of the first metatarsal during the last half of stance phase and may become less functional as a hallux valgus deformity develops. As a result, plantar flexion of the first ray might not occur, which can lead to increased pressure under the second metatarsal head⁵⁷ and diffuse callus formation secondary to increased weight bearing. Hypermobility of the first MTC joint, which may be pathologic in no more than 5% of patients, can also lead to increased weight bearing by the first metatarsal. Pressure is then transferred to the lesser metatarsals, where a diffuse callus can develop beneath the second and possibly third metatarsal (Fig. 8-2). The base of the second and third metatarsals is quite stable because of the rigidity of their tarsometatarsal articulations; therefore, if the first metatarsal is hypermobile or elevated from any cause, callus formation can occur beneath these metatarsal heads. The fourth and fifth metatarsals are more mobile, and therefore calluses rarely develop beneath them unless there is an abnormal foot posture or an osseous deformity.

Figure 8-2 Harris mat print of a patient with hypermobility of the first metatarsal. Because the first metatarsal does not carry much weight, pressure is transferred to the second and third metatarsals as well as to the tip of the hallux.

The posture of the foot must always be considered in evaluating the patient with an intractable plantar keratosis (IPK). Abnormal callus formation can be due to an equinus deformity of the ankle joint, a cavus foot, pes planus, a varus forefoot deformity, a pes planovalgus deformity (Fig. 8-3), or abnormal alignment of the MTP joints. Unless the underlying deformity is addressed, the IPK will persist.

Figure 8-3 A and B, Radiographs of a patient with a severe pes planovalgus deformity. C, Clinical photograph of same patient demonstrating the deformity when viewed from the posterior aspect. D, Callus formation under first metatarsophalangeal (MTP) joint from inadequate weight bearing of medial column. E, Callus formation under first metatarsocuneiform (MTC) joint from collapse of medial longitudinal arch.

An equinus deformity of the ankle joint that results in increased weight bearing on the metatarsal heads can result in a diffuse callus beneath the first, second, and third metatarsal heads. This can be painful, particularly in the older patient with atrophy of the fat pad. In the patient with an insensate foot, such as the patient with diabetes or a peripheral neuropathy, ulceration of the plantar aspect of the foot can occur.

In the cavus foot, the calcaneus is usually in a dorsiflexed position and the forefoot in equinus, leading to an abnormally diminished weight-bearing area and resultant diffuse callus formation beneath the metatarsal head area. This condition often becomes more symptomatic with increasing patient age and fat pad atrophy.

The foot with pes planus does not usually develop significant callus formation beneath the metatarsal heads, unless it is associated with a hallux valgus deformity. This combination can lead to callus formation along the plantar medial aspect of the great toe at the level of the interphalangeal (IP) joint.

A varus forefoot deformity, in which the lateral border of the foot is more plantar flexed than the medial border, often results in a diffuse callus beneath the fifth metatarsal head (Fig. 8-4).

Figure 8-4 Harris mat print of patient with varus forefoot deformity. Note that pressure is borne on the lateral aspect of the foot, with significantly decreased weight bearing beneath the medial aspect of the foot.

A valgus forefoot deformity, in which the first metatarsal is more plantar flexed in relation to the lesser metatarsals, often results in a diffuse callus beneath the first metatarsal head region (Fig. 8-5). This is often associated with a cavus foot deformity.

Figure 8-5 Harris mat print of patient with valgus forefoot deformity in which increased weight is borne along the medial side of the foot and, in particular, beneath the first metatarsal head.

Abnormal alignment of the metatarsophalangeal joints, secondary to either subluxation or dorsal dislocation, results in a plantarward force on the metatarsal head and development of a diffuse callus beneath the involved metatarsal head. An extreme example is a patient with advanced rheumatoid arthritis (Fig. 8-6).

Figure 8-6 A, Foot in a patient with rheumatoid arthritis with large diffuse plantar calluses beneath the metatarsal heads and nonfunctional toes. B, Harris mat print demonstrating concentration of pressure beneath metatarsal heads and lack of weight bearing by lesser toes.

Diagnosis

The evaluation of a patient with a plantar keratotic lesion begins with a careful history of the condition. This should include pertinent medical conditions, history of trauma, prior surgeries, what type of footwear aggravates and relieves the pain, the wear pattern of footwear, as well as what type of treatment has been attempted in the past.

The physical examination begins with the patient standing. Careful observation should be made of the posture of the toes, characteristics of the MTP joints and longitudinal arch, and the position and posture of the hindfoot in relation to the forefoot. Strength testing of all muscle groups, a thorough neurovascular examination, and range of motion of the ankle, subtalar, transverse tarsal, and MTP joints should be noted. Hypermobility of the first ray, the presence of hallux rigidus,²² as well as swelling, thickening, synovitis, and instability of the lesser MTP joints⁵⁷ are important diagnostic findings.

The plantar aspect of the foot is carefully evaluated. The location of the lesion and its characteristics are observed. The callus may consist of a localized seed corn, a discrete plantar keratosis beneath the fibular condyle of a metatarsal head or beneath the tibial sesamoid, a diffuse callus beneath a metatarsal head, or a diffuse callus beneath several metatarsal heads.

Weight-bearing radiographs should be obtained along with sesamoid views, if indicated. A small marker can be placed over the lesion to identify the offending structure if needed.

It is important to differentiate a wart, or plantar verruca, from a callus. Plantar verrucae are common, affecting 7% to 10% of the population.10,37 A plantar verruca is usually localized but occasionally demonstrates a mosaic pattern and is usually not discretely located beneath a metatarsal head. Papillary lines, or rete ridges, diverge around the lesion, whereas in a callus, the rete run unchanged across the lesion.²¹ To facilitate identification, the lesion is often trimmed with a No. 17 blade. Trimming enables the clinician to identify the margins of the callus, because in a well-localized plantar keratosis, the callus itself has definite circumscribed edges, as opposed to a diffuse callus, which is a generalized thickening of the plantar skin without a defined margin. A wart, on the other hand, may have a small amount of hyperkeratotic skin overlying it, but very quickly one enters the warty material, which consists of multiple end arteries that bleed vigorously, as opposed to a true plantar keratosis, which has no blood supply (Fig. 8-7). Warts are commonly caused by the human papilloma virus (HPV),⁴⁷ with the dark and moist environment within a shoe facilitating survival. Numerous treatments have been described for this often recalcitrant lesion, including chemical ablation, immunologic modifying treatments, and surgery. Common chemical ablation techniques include phenol, 10% fermaldehyde, 10% formaldehyde with monochloroacetic acid,³⁰ fluorouracil, and bleomycin sulfate.⁵³ Topical treatments include salicylic acid, fluorouracil,⁶⁴ canthadrin and podophyllotoxin,⁶ formaldehyde, silver nitrate, corticosteroids, and garlic extract.⁷ Cryotherapy and electrocautery have been used with varying results. Surgical excision is often required, with recurrence not uncommon.

Figure 8-7 A, Wart on the plantar aspect of the foot does not usually occur on a weight-bearing area. B, Histologic features of the wart demonstrate considerable vascularity within the lesion. C, Mosaic wart has similar histology but is more widespread.

Other rare dermatologic lesions that present as plantar callosities include keratosis palmoplantaris nummularis and epithelioma cuniculatum. If these lesions are suspected, dermatology consultation is recommended (Fig. 8-8). Keratosis palmoplantaris nummularis is an autosomal dominant skin disorder characterized by painful callus formation over areas of mechanical stress on the palms and plantar surface of the foot. It often presents upon the onset of walking in a child, with pain being the primary complaint. Physical examination reveals essentially normal motor and sensory function, with multiple areas of large hyperkeratotic callus formation over weight-bearing areas of the feet that are confluent with the surrounding skin.⁵² Histologic evaluation is consistent with epidermolytic hyperkeratosis.⁶¹ Operative treatment has been reported to be unsuccessful.⁶¹ The mainstay of treatment is shaving of the callosities, orthotics, and footwear modification. The natural history and successful treatment of the disorder remain elusive.

Figure 8-8 A and B, Examples of feet with hyperkeratotic skin. C and D, Plantar hyperkeratotic lesions consistent with keratosis palmoplantaris nummularis. E, The same patient in C and D also had painful keratotic lesions on her hands, such as this lesion on the volar interphalangeal (IP) flexion crease of the thumb.

Epithelioma cuniculatum is a form of verrucous carcinoma that affects the plantar surface of the feet.⁴⁵ It is an uncommon low-grade squamous cell carcinoma that is typically slow growing and has a tendency to local recurrence. Although metastatic disease is rare, lack of timely diagnosis can lead to its spread.⁵⁵ It can present as a painless, nonhealing ulceration on the plantar surface of the foot that slowly increases in size and is refractory to standard callus treatment.³³ Diagnosis of this rare lesion can be challenging, and deep biopsy is required. The human papilloma virus has been implicated as a causative factor, but both HPV-positive and -negative lesions have been reported.³⁷ Histologic analysis reveals a well-differentiated tumor composed of keratinocytes that extend the dermis and deeper structures, forming sinuses and keratin-filled cysts. Tumor cells in the granular layer may show vacuoles.⁵⁵ There is often marked hyperkeratosis, with perakeratosis of a prominent granular layer occasionally seen.³³ Treatment options include imiquimod (a topical immune-modulating drug), carbon dioxide (CO₂) laser ablation, and surgical excision.³³

Treatment

Conservative Treatment

The plantar callus can be trimmed with a sharp knife. A No. 17 podiatric blade is especially useful for trimming of hyperkeratotic skin. When trimming a callus, one should attempt to reduce the hyperkeratotic tissue, and if it is invaginated, this too should be trimmed. With a deep-seated lesion, however, it may not be possible to remove all of the keratotic lesion at the first trimming, and several trimmings may be necessary to permit the deep-seated portion of the callus to surface. Occasionally, a seed corn, which is a well-localized keratotic lesion, usually 2 to 3 mm in diameter, can be removed at the first sitting, although it might require a second trimming (Fig. 8-9).

Figure 8-9 A, Debridement of callus with No. 17 blade. B, A seed corn is a small keratotic lesion on the plantar aspect of the foot that at times becomes symptomatic. C, Appearance of seed corn after debridement. Keratotic lesion with seed corn on dorsum of fifth proximal interphalangeal (PIP) joint. D, Predebridement. E, Debridement of callus with seed corn remaining. F, After debridement of seed corn.

After debridement of the callus, a soft metatarsal support is used to relieve pressure on the involved area (Fig. 8-10). The soft support can be used, provided that the patient’s shoe is of adequate size. If the patient is wearing stylish women’s dress shoes, there may not be sufficient volume for the foot and the metatarsal support. The patient with a significant keratotic lesion should be encouraged to wear a broad, soft, preferably low-heeled shoe to provide more cushioning for the plantar aspect of the foot. The metatarsal support should be placed into the shoe just proximal to the area of the lesion (Fig. 8-11). It is important to instruct the patient that this support may initially feel uncomfortable and may take a period of breaking in of a week to 10 days.

Figure 8-10 Various types of soft metatarsal supports that may be used to relieve pressure in the metatarsal area of the foot.

Figure 8-11 Placement of a soft metatarsal support in the shoe. A, Support is placed just proximal to the “smudge” on the insole. This is usually around the base of the tongue. B and C, The support can be moved medially or laterally, depending on the location of the plantar lesion.

The patient is then seen periodically in the office for trimming of the lesion, adjustment of the metatarsal support, and possibly even placement of a larger one, as necessary. If the patient has a postural abnormality of the foot, such as forefoot varus or valgus, or possibly a pes planovalgus deformity or a cavus foot, a custom-made, well-molded orthotic may be of benefit.

Other footwear modification, such as a wide or tall toe box or the addition of a rocker-bottom sole, can potentially provide relief. If an equinus contracture is a contributing cause, physical therapy to stretch the gastrocsoleus complex can increase the relative length of the Achilles tendon unit, thereby increasing ankle dorsiflexion.¹⁸

If the callus persists and is symptomatic, surgical intervention can be considered.

Surgical Treatment: Types of Plantar Callosities

Surgical management of a plantar keratosis is based on the characteristics of the callus.

A discrete callus with a central keratotic core is observed beneath the fibular condyle of the metatarsal head and beneath the tibial sesamoid (Fig. 8-12). When the patient with a discrete, localized lesion walks over a Harris mat, the imprint created is well localized beneath the prominence that has brought about the callus (Fig. 8-13). Histologically, this lesion is a dense, keratinized lesion with a central core (Fig. 8-14).

Figure 8-12 Examples of discrete intractable plantar keratoses (IPKs). A, Lesion beneath the second metatarsal head. B, Discrete keratosis beneath the third metatarsal head. C, Discrete keratosis beneath the fourth metatarsal head. D, Discrete IPK under the second metatarsal head. E, Same patient as in D after shaving of the lesion with a podiatric blade. (B from Mann RA: Intractable plantar keratosis Instr Course Lect 33:289, 1984.)

Figure 8-13 Harris mat print demonstrating well-localized area of pressure—a discrete plantar keratosis.

Figure 8-14 A, Location of a discrete plantar keratosis beneath a prominent fibular condyle. B, Histologic features of the keratotic lesion demonstrate layers of keratin with no blood vessels. Compare with Figure 8-7B.

This should be differentiated from a diffuse callus, which is often caused by deformity at the MTP joints because of instability, and usually associated with abnormal weight bearing during the third rocker portion of the gait cycle, in which only the forefoot is in contact with the ground (Fig. 8-15).¹⁵ A diffuse callus is observed beneath a metatarsal head that does not have a prominent fibular condyle and is noted most often beneath the second metatarsal head. Occasionally, a diffuse callus is noted beneath multiple metatarsal heads. When this type of callus is present and is trimmed, although the material consists of hyperkeratotic skin, there is no central core as observed in a discrete callus. When this patient walks over a Harris mat, the print that is observed is diffuse beneath the entire metatarsal head or beneath multiple metatarsal heads (Fig. 8-16). In general, a callus underneath several metatarsals is often due to forefoot overload, such as seen in ankle equinus; plantar flexion of the metatarsals, as in a cavus foot; or first-ray insufficiency. This indicates overload during the foot flat portion of the gait cycle during the second rocker portion of swing phase.¹⁵ This type of diffuse lesion is occasionally observed beneath the first metatarsal head if it is plantar flexed. It can be observed beneath the second metatarsal head when the first metatarsal is short, hypermobile, or due to first-ray instability brought about by an advanced hallux valgus deformity or possibly insufficiency after bunion surgery, if the joint has been destabilized. A diffuse callus beneath the second, third, and fourth metatarsal heads is usually observed in a patient with an extremely short first metatarsal, which results in increased weight bearing beneath the three middle metatarsals (Fig. 8-17). Occasionally, a diffuse callus is present beneath the fifth metatarsal head in patients with a varus configuration of the forefoot, or with a plantar-flexed fifth metatarsal associated with a tailor’s bunion.

Figure 8-15 A and B, Examples of diffuse keratotic lesions. When these lesions are debrided, there is no central core, which distinguishes them from discrete keratotic lesions. C, Diffuse callus formation under second and third metatarsophalangeal (MTP) joints resulting from second crossover toe deformity. D and E, Clinical photographs of patient in C with second crossover toe deformity.

Figure 8-16 Harris mat print demonstrates diffuse keratotic lesion beneath second metatarsal head. Compare with Figure 8-15. (From Mann RA: Intractable plantar keratosis Instr Course Lect 33:293, 1984.)

Figure 8-17 Typical findings with Morton foot (toe). A, Plantar aspect demonstrates diffuse callus beneath second and third metatarsal heads. B, Harris mat print demonstrates increased weight bearing beneath the second and third metatarsals, with little or no weight bearing beneath the first metatarsal head. C, Radiograph demonstrates typical Morton foot with short first metatarsal and relatively long second and third metatarsals.

A diffuse type of callus can also result after trauma. A metatarsal malunion can result in a plantar-flexion deformity and resultant callus underneath the fractured metatarsal. A dorsiflexion malunion can lead to callus formation beneath an adjacent metatarsal, with the normal metatarsal left to bear increased weight.

Differentiating the type of callus is essential because the nature of the callus determines the type of surgical procedure that will give the best result. In general, a localized callus will require an elevating procedure, whereas a diffuse callus requires a shortening procedure.¹⁵

Localized Intractable Plantar Keratosis

A localized IPK, which is usually caused by the prominence of the fibular condyle, can be treated by a DuVries metatarsal condylectomy. DuVries 12–14 initially described the procedure in which he carried out an arthroplasty of the MTP joint by removing a portion of the distal articular surface and the plantar condyle. Coughlin³ and Mann⁴¹ modified the procedure by removing only the plantar condyle. Both procedures seem to lead to satisfactory clinical results.

DuVries Metatarsal Condylectomy

Surgical Technique

1. A hockey stick–shaped incision begins in the second web space and is carried across the metatarsal head proximally to about the distal third of the metatarsal shaft (Fig. 8-18A).

Figure 8-18 DuVries plantar metatarsal condylectomy. A, Hockey stick–shaped incision beginning in the web space is carried obliquely across the joint to about the middle of the metatarsal shaft. Skin and extensor tendons are retracted, and the capsule is incised longitudinally. B, Capsule and collateral ligaments on both sides of the metatarsal head are sectioned. C, The involved toe is plantar flexed with the left thumb while pressure is applied on the plantar aspect of the metatarsal shaft with the index finger. D, About 2 mm of articular cartilage is removed. E, The plantar 30% of the condyle is removed with an osteotome. Note angulation to facilitate removal of more of the fibular aspect. F, Edges of the metatarsal are smoothed; then the capsule and skin are closed.

2. Passing medially and laterally to the extensor hood, the transverse metatarsal ligament is identified and released.

3. The interval between the long and short extensor tendons is opened and continued through the joint capsule to expose the MTP joint.

4. The collateral ligaments are transected, and the MTP joint is sharply plantar flexed while pressure is applied to the plantar aspect of the foot with the index finger of the same hand (Fig. 8-18B and C).

5. The distal 2 mm of articular cartilage is removed from the metatarsal head in a plane perpendicular to the metatarsal shaft. As this cut is made, the osteotome is angled slightly proximally; otherwise, it will skid off the cartilage and make an oblique cut (Fig. 8-18D).

6. The MTP joint is now sharply plantar flexed to bring into view the plantar condyle. The plantar 20% to 30% of the metatarsal head is removed with a 10- to 12-mm thin osteotome. The osteotome is angled slightly plantarward to avoid inadvertent splitting of the metatarsal shaft (Fig. 8-18E).

7. On completion of the osteotomy in the metatarsal head, the plantar condyle is delivered by use of a Freer elevator or thin rongeur in the adjacent interspace, and the condyle is removed. It is difficult to pull this fragment of bone out directly over the proximal phalanx, so it is pushed into the interspace and removed (Fig. 8-19).

Figure 8-19 Excised plantar condyle. A, Note marked prominence of the fibular portion of the condyle in this specimen. B, Plantar view of the same specimen illustrating the large condyle. (A from Mann RA: Intractable plantar keratosis Instruct Course Lect 33:292, 1984.)

8. The edges are carefully rongeured, and the joint is reduced (see Fig. 8-18F). The skin is closed in a routine manner.

9. A compression dressing is applied for 12 to 18 hours, and the patient is permitted to ambulate in a postoperative shoe.

Postoperative Care

The bulky surgical dressing is changed after 18 to 24 hours, and the patient is placed into a snug compression dressing consisting of 2-inch conforming gauze (Kling) and adhesive tape. The patient is permitted to ambulate in the postoperative shoe, which is worn for 3 weeks. The shoe is removed, and the patient is encouraged to work on range-of-motion exercises.

Results

In a review of 100 patients surgically treated by Mann and DuVries,⁴² there was 93% patient satisfaction. It was observed that 42% of the lesions were beneath the second metatarsal head, 31% beneath the third, 19% beneath the fourth, and 8% beneath the fifth. A transfer lesion occurred in 13% of the patients, and the original lesion failed to resolve in 5%.

The 5% complication rate included fracture of a metatarsal head, avascular necrosis of the metatarsal head, and clawing of the toe. There were no cases of dislocation of the MTP joint after this procedure. Postoperative range of motion of the MTP joint rarely demonstrates more than 25% loss of motion. It has always been somewhat surprising that so little motion is lost after this type of arthroplasty, whereas when a similar procedure is carried out for a dislocated MTP joint, significantly more motion is lost.

Coughlin Modified Metatarsal Condylectomy

The modification of the condylectomy by Coughlin is carried out in the same way as the DuVries metatarsal condylectomy, except that the distal portion of the metatarsal is not removed (Fig. 8-20 and Video Clip 69).^3,41 This makes removing the plantar condyle a little more difficult. Until now, there has been no published report regarding the results of this modification of the DuVries procedure, although preliminary data indicate about a 5% incidence of transfer lesions. The use of a power rasp with which to shave down the prominent fibular condyle⁴⁶ has been described. This avoids the potential difficulty of retrieving the osteotomized plantar condyle from the interspace. Thermal necrosis and bone debris can be avoided by use of irrigation.