Pad Corns

The following is an article from Veterinary Practice News Sept 26, 2018

A canine corn is a focal circular area of hyperkeratinization found in the digital paw pads of sighthound breeds (e.g. greyhound, whippet, and lurcher). A prevalence of around five to six per cent in retired greyhounds has been reported in the literature (Lord et al, 2007), making it the most common dermatologic condition among this group of dogs. Approximately 80 per cent of corns occur in digits three and four, while 80 per cent are found in the digits of the thoracic limbs (Guilliard et al, 2010). Their occurrence has not been reported in the metacarpal and metatarsal paw pads. Corns commonly cause lameness, which can be severe. The lesion is analogous to heloma durum (corn/clavus), which usually occurs on the foot in humans (Roven, 1968).

A number of etiologies have been proposed—these include viral infection, foreign body penetration, and repeated mechanical trauma (Swaim et al, 2014). In humans, corns are thought to have a mechanical etiology, developing due to chronic repeated trauma from a variety of extrinsic (e.g. ill-fitting shoes) and intrinsic (e.g. bony prominences, faulty foot function) factors (Yale, 1987).

Many forms of treatment have been suggested, however, none offer consistent, long-term results. Further, recurrence is common. Currently, there is little to no evidence base on which to make a treatment decision. The following review is a summary of the current evidence regarding greyhound corns, as well as the authors’ experience and opinions.

What’s causing the problem?

While a viral aetiology is commonly thought to be the cause of corns, studies have been unable to show any relationship. For example, a study of six greyhounds (Balara et al, 2009) failed to identify papillomavirus in any surgically excised corns using polymerase chain reaction (PCR), histology, and immunohistochemistry. This is further supported by the failure to identify any positive result for canine papillomavirus in more than 100 corns submitted for testing (unpublished data, Doughty and Guilliard). Ultrastructural studies using electron microscopy to identify virus particles have been negative (Swaim et al, 2004). Recently, Anis et al, 2016 identified novel canine papillomavirus using PCR in four corns from two unrelated adult greyhounds. However, this study did not show the identified virus was localized in the lesion, and therefore, the possibility of it being a non-pathogenic commensal could not be ruled out. A viral/infectious etiology is unlikely to be the cause of the majority of corns in greyhounds.

The theory that foreign body penetration can result in corn formation through either the presence of a foreign body, the introduction of pad tissue into the subdermal tissues, or from the formation of scar tissue is unlikely. In the author’s (Guilliard’s) clinical experience, no foreign body has been found macroscopically during excision surgery. Surgical removal of glass from the subdermal pad tissue has never resulted in subsequent corn formation in cases seen by the author (Guilliard). Further, histological examination of more than 800 surgically excised corns has failed to identify a foreign body as the primary etiology. Stone-like material, along with a small amount of fluid after removal of the corn core, is occasionally seen. Histological analysis of this material typically shows fragments of vital and/or necrotic bone that are likely due to the calcareous degeneration of an underlying mechanically induced (adventitious) bursa. While foreign bodies have been reported to cause heloma durum in humans, its occurrence is thought to be rare.

Figure 1: A contrast-enhanced microCT image of an amputated toe with corn and tendon changes. Degeneration, calcification, and splitting of the deep digital flexor tendon can be seen, as well as a mechanically induced fluid-filled bursa (B); P2: middle phalanx; P3: distal phalanx.

A mechanical etiology, specifically repetitive mechanical trauma, has often been suggested as the most common cause of both human and greyhound corns. This is supported by observations made in veterinary practice, as well as translating evidence from data and experience of heloma durum in human medical practice.

Concomitant anatomical deformity has been seen in 40 per cent of cases (Guilliard et al, 2010), with damage to the deep digital flexor tendon being the most common. It is rarely a complete rupture, but rather an elongation of the tendon presenting as an elevated nail when the limb is held in extension, altering the pad’s weight-bearing area.

Supporting these clinical findings, microCT imaging and histological examination of amputated toes with corns have shown a large number with tendinopathy-like changes, including tendinosis, tendinitis/partial rupture, paratenonitis, and paratenonitis with tendinosis (Figure 1). Histologically, these appear as collagen disorientation, disorganization and fibre separation, focal necrosis and calcification, and neovascularization.

Altered weight-bearing is one example of how mechanical factors can play a part in the formation of a new corn. Consider a greyhound that presented with a recurrent corn on digit 3—the veterinarian performed surgery to amputate the digit through the metatarsal/phalangeal joint. Subsequently, the superficial digital flexor tendon of digit 4 ruptured and the digit became flat. Corns then developed in digits 2 and 5, rapidly recurring after surgical excision (Figure 2). However, it has been reported that if a pad’s weight-bearing aspect is altered surgically by a distal ostectomy (i.e. amputation), the corn naturally regresses (Guilliard et al, 2010).

While the reason for the exclusivity of corn formation to sighthounds is still not fully understood, one proposal is it may be due to this breed having less subdermal fibro-adipose tissue compared to others, thereby increasing the risk of mechanical trauma (Swaim et al, 2010). The significance of dog racing is uncertain, as corns have been reported in both racing and non-racing greyhounds and whippets. Some have suggested hereditary factors may be involved in the susceptibility of this lesion, as it has been noted some dogs from the same dam or sire have developed corns. Currently, there is no evidence to support or refute this.

Pathology

Figure 2: A greyhound with corns on digits 2 and 5. These corns appeared after amputation of digit 3 through the metatarsal/phalangeal joint to treat a recurrent corn. Both corns recurred rapidly after surgical excision.

Digital pad corns have similar histological features to mechanically induced focal keratoses of the soles of the feet in humans, typically characterized by a large central focal plug of para- and orthokeratotic keratin and loss of the underlying stratum granulosum (Figure 3). In a corn that has been present for a short time, often the underlying epidermis is hyperplastic, and mitoses can usually be identified. In contrast, the underlying epidermal layers in chronic corns are atrophied due to the focal pressure. Interestingly, surgical excision of corns that have had the corn core removed a few days before surgery typically show a hyperproliferative state in the underlying epidermis.

Although mechanical load is the most likely cause of corn formation, it is unclear what types of mechanical loads (e.g. compression, shear, or torsion) or how much is required to trigger the hyperkeratosis process. What is known is that trauma of the skin leads to the release of local growth factors (McKay and Leigh, 1991). It is thought mechanical stress triggers the rapid transit time of keratinocytes, coupled with delayed differentiation of the cells. The cells, therefore, progress to the surface of the skin without having fully differentiated. This process leads to structures that are immature and, therefore, compromised. Changes in these differentiating and adhesion factors lead to altered corneocyte cohesion and desquamation, causing focal thickening of the stratum corneum. At this point, the corn becomes clinically apparent.

It is important for the clinician to appreciate that a corn is not just a lesion limited to the epidermis. Changes in the underlying dermis are typically seen and include inflammation, collagen degeneration, formation of an epidermal inclusion cyst, nerve hypertrophy, vascular changes, bone atrophy, or hypertrophy (Figure 4).

Diagnosis – A lame Greyhound? Think Corns

Typically, a greyhound with a corn presents with lameness, which requires a thorough clinical examination, including the digital paw pads. One of the first challenges for a veterinarian presented with a lame greyhound is to think of the possibility of a corn in the list of differential diagnosis. However, many veterinarians have not been taught about corns at veterinary school and may have limited experience of greyhounds and their idiosyncrasies. When a greyhound presents with lameness, a corn should always be ruled out before more advanced diagnostic procedures (e.g. CT scans, magnetic resonance imaging [MRI]) are performed. The authors have heard numerous histories from owners where their greyhounds were referred to specialists and had extensive and expensive diagnostic procedures performed, yet none had examined the digital paw pads. Corns are by far the most common cause of lameness in retired racing greyhounds. The main differential diagnosis is foreign body penetration usually showing an entry wound with a serous discharge.

Lameness can vary in its intensity and is often worse on hard ground. The nail of the affected digit may be elevated and long compared to the others. A visual inspection of the pads usually shows a circular area of keratin that may be obviously thickened, but often does not protrude from the pad. Figure 5 illustrates the many appearances of digital pad corns. They can vary from virtually nothing at all to large lesions dominating and deforming the pad. Wetting the pad makes observation easier.

Focal digital pressure applied both mediolaterally and dorsoventrally produces a repeatable pain response. Thickening of the pad may be apparent. A mediolateral radiograph can rule out most foreign body penetrations.

Treatment: many options, many failures

Figure 3: This image shows the structure of a corn (surgically excised). In the centre (C) is a plug of compact keratin, with an underlying hyperplastic epidermis (E). Note a small amount of dermis (D) has also been removed. In the author’s (Doughty’s) experience, this is associated with a reduced chance of recurrence.

There is no definite treatment for digital pad corns, though many have been reported with various regimes, including conservative management, topical medications, and surgical intervention (Swaim et al, 2014). It is fair to say that when a condition has a wide variety of treatments available, none have outstanding efficacy. Currently, there is limited scientific evidence for the efficacy of treatments, so owners are often guided by anecdote and trial and error.

Often, the first-line treatment is the enucleation of the corn using manual manipulation, dental root elevator, or surgical scalpel. Regular removal of the central core of the corn through hulling or parring does alleviate pain and discomfort, and is commonly practiced as a palliative measure; however, recurrence is normal and often rapid.

Topical treatments can be classified into three groups:

1) Keratolytics (e.g. salicylic acid), which break down built-up keratin and cause sloughing off of the cornified epithelium.

2) Keratoplastics (e.g. urea and various emollients), which soften the hardened keratin and alter its biomechanical properties.

3) Caustic agents (e.g. silver nitrate) that burn or destroy the tissue.

Protective boots are also often used to provide symptomatic relief, especially when dogs are exercised on hard surfaces. Digital denervation has been attempted, but proved unsuccessful and is not recommended.

One novel treatment that has been proposed is the injection of silicone gel under the corn to provide a protective cushion (Swaim et al, 2004). Despite encouraging results showing an injection of this type alters ground contact pad pressures in greyhounds without corns, to the authors’ knowledge, this treatment has not been trialed in greyhounds with this lesion. While this treatment may hold promise, migration of the silicone from the injection site may limit the duration of the treatment effect.

A variety of surgical methods have been tried, including excision with a scalpel blade, laser or punch biopsy, and amputation of the affected toe. All surgical methods, apart from amputation, are performed to remove the corn, as well as a portion of the underlying dermis. The use of a biopsy punch is not recommended, as it can damage the underlying deep digital flexor tendon, which often lies less than a millimetre under the epidermis. The advantages of laser over scalpel blade excision are unknown.

Figure 4: Note the changes in the structures beneath the corn. Directly under the corn (C), there is a mechanically induced (adventitious) bursa, as well as compression atrophy of the bone (line). P2: middle phalanx; P3: distal phalanx; A: adipose tissue.

Surgical excision requires either a general anesthetic or heavy sedation with medetomidine and butorphanol together with a digital nerve block, for instance. Hemostasis is achieved by an assistant pinching the caudal aspect of the pad between the index finger and thumb. A full thickness elliptical incision is then made around the corn to include a small margin of normal pad tissue, allowing the corn to be teased out from the underlying fibro-adipose tissues. The pad margins are closed with simple absorbable sutures, the author’s (Guilliard’s) preference being polyglactin. The paw needs to be covered in a protective dressing for up to three weeks and changed frequently. The sutures soon wear through and disappear.

The role of toe amputation in the management of corns is typically considered to be the treatment of last resort, since it may alter weight distribution between the remaining toes, resulting in a new corn developing on one of them. However, in the authors’ experience, this rarely occurs. This consideration should be weighed against the possibility toe amputation can render a greyhound corn- and pain-free.

A distal digital amputation through either the distal interphalangeal joint or the distal second phalanx has been advocated. This technique is more likely to be successful in digits 2 and 5, as they are not the main weight-bearing digits. However, in the author’s (Guilliard’s) experience, this has shown mixed results.

The author (Guilliard) is currently conducting trials of a novel surgical treatment for corns that involves cutting the flexor tendons. He has postulated that if the affected toe is unloaded by this method, the mechanical trauma will cease, the dog will not be lame, and the corn will grow out and not recur. The corn is not excised, but pared down only if prominent.

Short-term results to date on a small number of cases are very encouraging, with all owners reporting great improvement after only two or three days. The longest follow-up period is six months with no lameness reported.

Outcome of treatment

Currently, the only treatment with any published outcome data is surgical excision (Guilliard et al, 2010) where approximately 50 per cent of the dogs had no recurrence of lameness at one year post-treatment.

Figure 5: This group of images illustrates the many appearances of digital pad corns. They can vary from virtually nothing at all to large lesions dominating and deforming the pad.

Conclusion

While being a relatively simple lesion, the greyhound corn is both debilitating for the patient and frustrating for the veterinarian and owner. The first challenge for a veterinarian presented with a lame greyhound is to think of the possibility of a corn. While surgery offers a chance for successful resolution of the lesion, recurrence is still common. Palliative management, including removing the lesion’s core, can offer symptomatic relief.

Currently, there is a lack of information regarding the biology and epidemiology of greyhound corns and the efficacy of different treatment options. With this in mind, the Greyhound Health Initiative (GHI)—a U.S. non-profit organization dedicated to improving the health and well-being of sighthounds on a global scale—is currently raising funds that will go toward researching this topic. As a first step, an Internet-based greyhound corn study discussion group has been started. Any veterinary professional interested in participating in this or in any of GHI’s corn research projects can contact them at info@greyhoundhealthinitiative.org[1].

References
Anis, E. A., Frank, L. A., Francisco, R., & Kania, S. A. (2016). Identification of canine papillomavirus by PCR in Greyhound dogs. PeerJ, 4, e2744.
Balara, J. M., McCarthy, R. J., Kiupel, M., Buote, M. A., Wise, A. G., & Maes, R. K. (2009). Clinical, histologic, and immunohistochemical characterization of wart-like lesions on the paw pads of dogs: 24 cases (2000–2007). Journal of the American Veterinary Medical Association, 234(12), 1555-1558.

Guilliard, M. J., Segboer, I., & Shearer, D. H. (2010). Corns in dogs; signalment, possible aetiology and response to surgical treatment. Journal of Small Animal Practice, 51(3), 162-168.
Kim, S.H., Kim, S., Choi, H.I., Choi, Y.J., Lee, Y.S., Sohn, K.C., Lee, Y., Kim, C.D., Yoon, T.J., Lee, J.H. and Lee, Y.H. (2010). Callus formation is associated with hyperproliferation and incomplete differentiation of keratinocytes, and increased expression of adhesion molecules. British Journal of Dermatology, 163(3), 495-501.
Lord, L. K., Yaissle, J. E., Marin, L., & Couto, C. G. (2007). Results of a web‐based health survey of retired racing Greyhounds. Journal of Veterinary Internal Medicine, 21(6), 1243-1250.
McKay, I. A., & Leigh, I. M. (1991). Epidermal cytokines and their roles in cutaneous wound healing. British Journal of Dermatology, 124(6), 513-518.
Roven, M. D. (1968). Helomata, A review of some of the literature. Journal of the American Podiatry Association, 58(5), 209.
Swaim, S. F., Amalsadvala, T., Marghitu, D. B., Sartin, E. A., Hudson, J. A., & Stoenescu, E. D. (2004). Pressure reduction effects of subdermal silicone block gel particle implantation: a preliminary study. Wounds—A compendium of clinical research and practice, 16(10), 299-312.
Swaim, S. F., Bohling M. W., Wright, J. C., et.al., (2014) Paw pad pain: A review of corns in dogs. Veterinary Medicine, 109, 61.
Yale, I. (1987). Yale’s podiatric medicine. Baltimore: Williams & Wilkins.

Richard W. Doughty, M.Sc., MB ChB (Hons), BVSc, is an attending (medical) pathologist at a university hospital near Oslo, Norway. He holds a bachelor of science degree in physiology and master of science in toxicology. In 2002, Dr. Doughty completed his bachelor of veterinary science at the University of Bristol Veterinary School. He then went on to complete his human medical degree at University of Liverpool Medical School in 2011 and residency in anatomic pathology in 2018. Doughty has previously worked in clinical veterinary practice involving both racing and retired greyhounds and has a long-standing interest in greyhound corns. He can be reached at dr.rwdoughty@yahoo.co.uk[2].

Michael Guilliard, MA, VetMB, CertSAO, FRCVS graduated from the University of Cambridge Veterinary School in 1972. Working in private practice for his entire career, he developed an interest in the racing greyhound. This led to several publications on greyhound injuries in peer-reviewed veterinary journals for which Dr. Guilliard twice received the British Small Animal Veterinary Association (BSAVA) Dunkin Award for the most valuable article published in the Journal of Small Animal Practice by a small animal practitioner. In 2012, he became a fellow of the Royal College of Veterinary Surgeons (RCVS) for his thesis entitled, “The nature, incidence and response to treatment of injuries to the distal limbs in the racing greyhound,” and in 2014 received BSAVA’s Simon Award for outstanding contributions in the field of veterinary surgery. Guilliard now runs his own orthopedic veterinary practice in the United Kingdom, with running dogs contributing to 50 per cent of his workload. He can be contacted at guilliard@talk21.com[3].

Endnotes:

  1. info@greyhoundhealthinitiative.org: mailto:info@greyhoundhealthinitiative.org
  2. dr.rwdoughty@yahoo.co.uk: mailto:dr.rwdoughty@yahoo.co.uk
  3. guilliard@talk21.com: mailto:guilliard@talk21.com