Pathology—now for the science bit
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
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.