Bilateral phalangeal fillet technique for metacarpal pad reconstruction in a dog

Abstract

Loss of the metacarpal or metatarsal pad requires reconstruction with other pad tissue to allow functional weight-bearing on the limb. This report describes the use of a bilateral phalangeal fillet technique to reconstruct a weight-bearing surface in a dog following complete excision of its right metacarpal pad for malignant melanoma. This resulted in a functional weight-bearing limb.

Loss of a metacarpal or metatarsal pad can have a debilitating effect on ambulation (1–3). Foot pads are specialized cutaneous structures that are comprised of a thick, heavily keratinized epidermis and a modified hypodermis that consists of abundant layers of adipose tissue (4–7). They are the toughest region of skin, designed to withstand and absorb the shock of standing and ambulation, and resist the abrasive forces and shear stress of ambulation (1,3,4,6–9). Foot pads are subject to forces that are not usually applied to haired skin, particularly cyclic tension and splaying caused by weight-bearing (4).

When metacarpal or metatarsal pads are excised for surgical resection of a neoplastic lesion, or are irreversibly traumatized, it is necessary to reconstruct the defect with similar pad tissue to re-create a weight-bearing surface (1,3,5,6,9). Haired skin is not an appropriate substitute for pad tissue because it does not undergo metaplastic change to become thick, keratinized “pad-like” tissue, and thus it cannot provide adequate strength and durability to withstand the stresses of ambulation (1–3,5,6,9).

This report describes the successful use of a bilateral phalangeal fillet technique to reconstruct a weight-bearing surface following excision of the metacarpal pad for resection of malignant melanoma in an Australian cattle dog.

Case description

A 10-year-old neutered male Australian cattle dog weighing 27.5 kg was presented to our institution for a malignant melanoma in its right metacarpal pad. The dog had a 2-month history of right foreleg lameness, which was initially thought to be due to a small laceration to its metacarpal pad. However, the lesion changed in appearance and became darker and proliferative. The lesion was biopsied and a histologic diagnosis of malignant melanoma was made.

On initial presentation, the dog had a 10-mm lesion on its right metacarpal pad which was irregular, poorly demarcated and ulcerative, with adjacent multifocal to coalescing circular regions of depigmentation. There was a central, full-thickness defect with external sutures in place at the previous biopsy site. A mild ipsilateral prescapular lymphadenomegaly was also noted on physical examination. The remainder of its physical examination was unremarkable.

Oncologic staging was based on hematology, serum biochemistry, urinalysis, thoracic radiography, abdominal ultrasonography, and fine-needle aspiration (FNA) of the ipsilateral prescapular lymph node. There were no notable changes on hematology, biochemistry, and urinalysis. Three-view thoracic radiography revealed normal pulmonary parenchyma, heart, and vessels with no evidence of metastatic pulmonary disease. Ultrasonographic examination of the abdomen showed small hypoechoic nodules in the liver, consistent with regenerative nodules, although inflammation and neoplasia could not be ruled out without further investigation. Microscopic examination of FNA samples from the prescapular lymph node showed a mixed lymphocyte population (mainly small lymphocytes) as well as frequent scattered macrophages containing small amounts of melanin pigment, eosinophils, non-degenerate neutrophils, and low numbers of plasma cells, along with occasional melanocytes containing abundant melanin pigment and melanin granules in the pale basophilic background. We concluded that the dog had regional metastasis to the prescapular lymph node. Liver metastasis could not be ruled out.

Surgical excision of diseased tissue, including the ipsilateral prescapular lymph node, with adjuvant immunotherapy was recommended. Complete excision of the melanoma with a clean margin required excision of the entire metacarpal pad. Options for closure of the defect and provision of a functional weight-bearing surface were considered and bilateral phalangeal fillet of digits II and V was recommended to reconstruct the metacarpal pad. A staged closure was planned to ensure that the excision was complete, based on histologic verification of tumor-free cut margins, before proceeding with reconstruction.

The dog returned to our institution 2 wk later for excision of the right metacarpal pad and ipsilateral prescapular lymph node. A 3-cm skin incision was made overlying the right prescapular lymph node using electrosurgery and the underlying soft tissues were blunt dissected to expose the right prescapular lymph node. The lymph node was excised and placed in 10% buffered formalin for later microscopic examination. The subcutaneous tissues were closed with 3-0 polydioxanone (PDS II, Ethicon, Bridgewater, New Jersey, USA) simple continuous sutures and the skin was closed with 3-0 nylon (Ethilon, Ethicon) cruciate sutures. Cephazolin (Kefzol, Aspen Pharmacare, Durban, South Africa), 22 mg/kg body weight (BW), IV was administered q2h during surgery.

A Penrose drain was tightened around the right antebrachium to reduce blood flow to the distal limb. An encircling incision was made around the edge of the right metacarpal pad and the pad was completely excised using a combination of electrosurgery, sharp dissection, and blunt dissection (Figure 1). The metacarpal pad was placed in 10% buffered formalin and submitted for histologic examination. The metacarpal defect was left open while awaiting histologic confirmation of tumor-free margins of the excised tissue. The defect was covered with hydrogel (IntraSite, Smith & Nephew, North Ryde, Australia) and hydrocellular polyurethane (Allevyn, Smith & Nephew) dressing followed by a secondary layer of synthetic padding (Soffban Synthetic; BSN Medical, Mount Waverley, Australia) and a permeable elastic self-adhesive tertiary layer (Rip-Rap Lite; ZebraVet, Brisbane, Australia) incorporating a splint made of fibreglass substrate and water-activated polyurethane resin (Delta-Lite Plus; BSN Medical). Perioperative analgesia was provided with an intraoperative fentanyl continuous rate infusion (CRI) at a rate of 3 to 20 μg/kg BW per hour that was weaned down and discontinued after surgery, tramadol hydrochloride, 2 mg/kg BW, IV immediately after surgery, and a 75 μg/h fentanyl transdermal patch (Durogesic; Janssen-Cilag, North Ryde, Australia) applied after surgery. The dog was discharged from hospital on the same day with tramadol hydrochloride (Tramal; Grunenthal GmbH, Aachen, Germany), 1.8 mg/kg BW, PO 2 to 3 times daily and carprofen (Rimadyl; Pfizer Animal Health, West Ryde, Australia), 1.8 mg/kg BW, PO, q12h for 5 d then 1.8 mg/kg BW, PO q24h for another 10 d. The dog’s owner was instructed to keep the dog cage-confined at home with short lead walks for elimination purposes only.

Figure 1.

Metacarpal defect following complete excision of the right metacarpal pad of a 10-year-old neutered male Australian cattle dog for malignant melanoma.

Microscopic examination of the excised metacarpal pad confirmed the diagnosis of malignant melanoma in the metacarpal pad with > 10 mm tumor-free margins. Histologically, there was a focally extensive, unencapsulated, infiltrative, and poorly demarcated proliferation of neoplastic melanocytes within the dermis with epithelioid differentiation accompanied by intercalated streams of spindle cells. There were also numerous melanomacrophages present in the superficial dermis admixed with an inflammatory cell infiltrate indicating pigmentary incontinence. S100 and Melan A immunohistochemical staining revealed that almost 100% of the neoplastic melanocytes were positive for S100, whilst approximately 30% to 40% of the cells were positive for Melan A, which was consistent with the typical immunoreactivity pattern for melanoma due to the lower sensitivity of Melan A (10). Histologic examination of 7 sections of the excised prescapular lymph node revealed individual and small aggregates of macrophages containing melanin and hemosiderin, with no cytologically atypical cells. Following S100 and Melan A staining, there was mild non-specific background staining in the S100 and no Melan A positive cells. The lack of atypical individual or aggregates of S100 was interpreted as no histologic evidence of melanoma metastasis in the sections examined.

Figure 2A shows the appearance of the paw before the original surgery. Nine days after the original surgery, the dog returned to our institution for reconstruction of the defect on the right foreleg. The right antebrachium was clipped and aseptically prepared for surgery. A Penrose drain was tightened at the level of the mid-antebrachium to reduce blood flow to the distal limb. This tourniquet was removed after 40 min, and replaced 20 min later for an additional 20 min. The defect was covered by granulation tissue which was debrided. A sample of the granulation tissue was obtained for bacterial culture and sensitivity testing. An elliptical incision was made around the nail bed of digit II and the digit was disarticulated at the level of the second and third phalanges (P2 and P3, respectively) following sharp dissection around P3 (Figure 2B). Two palmar incisions along each side of P1 and P2 with blunt dissection as close to the phalanges as possible allowed removal of P1 and P2 (Figures 2C, D). The first incision around the nail bed was sutured with a single 4-0 nylon cruciate suture. The skin flap with intact digital pad was then wrapped in saline-soaked swabs while the same procedure was repeated on digit V. Once both digits had been filleted (Figure 3), the two skin flaps with digital pads subsequently created were transposed into the region of the granulation bed where the metacarpal pad was previously excised (Figure 2E). Excess skin was excised. The digital pads were sutured in place with 3-0 polydioxanone buried interrupted sutures in the subcutaneous tissue (Figure 2F). The skin edges were apposed with 4-0 nylon simple continuous and cruciate sutures. A non-adherent, sterile dressing (Melolin; Smith & Nephew) was placed over the paw and a sterile carpal flexion bandage (non-weight bearing) was applied.

Figure 2.

Phalangeal fillet technique of the second and fifth digits to harvest 2 digital pads to reconstruct a weight-bearing surface following complete excision of the right metacarpal pad in a 10-year-old neutered male Australian cattle dog. A — Malignant melanoma lesion on the metacarpal pad with a central full-thickness defect and superficial sutures in a biopsy site. B — An elliptical incision was made around the nail bed of digits II and V, and the digit was disarticulated between the second and third phalanges (P2 and P3) following sharp dissection around P3. C — Two palmar incisions were made along each side of P1 and P2 of digits II and V to allow removal of P1 and P2. D — Both P1 and P2 were removed by blunt dissection as close to the phalanges as possible. E — Once both digits had been filleted, the digital pads were transposed into the granulation tissue wound bed where the metacarpal pad had been previously excised. F — The digital pads were sutured in place with 3-0 polydioxanone in the subcutaneous tissue and 4-0 nylon in the skin.

Figure 3.

Bilateral phalangeal fillet of digits II and V to reconstruct a metacarpal defect following complete excision of the right metacarpal pad in a 10-year-old neutered male Australian cattle dog for malignant melanoma.

The following day, the bandage was changed under sedation with medetomidine (Domitor; Pfizer Animal Health), 5 μg/kg BW, IV. The skin flaps were intact; however, the transposed pads appeared warm but dark. A hydrocellular polyurethane dressing (Allevyn; Smith & Nephew) was applied and the distal right foreleg was re-bandaged in carpal flexion. Sedation was reversed with atipamezole (Antisedan; Pfizer Animal Health), 0.03 mg/kg BW, IM. The dog was discharged from hospital with carprofen (Rimadyl; Pfizer Animal Health), 1.8 mg/kg BW, PO, q24h for 2 wk, tramadol hydrochloride (Tramal; Grunenthal GmbH), 1.8 mg/kg BW, PO 2 to 3 times daily, and cephalexin (Rilexine; Virbac Corporation, Regents Park, Australia), 22 mg/kg BW, PO, q12h for 1 wk.

Four days after surgery, the bandage was again changed under sedation with medetomidine (Domitor; Pfizer Animal Health), 5 μg/kg BW, IV. The transposed pads were dark reddish-brown and there was exudative discharge from all edges of the skin flaps. The surgical site also appeared moderately hyperemic and edematous. The right foreleg was re-bandaged in a modified spica sling and the sedation was reversed with atipamezole (Antisedan; Pfizer Animal Health), 0.03 mg/kg BW, IM. Enrofloxacin (Baytril; Bayer, Pymble, Australia), 5.5 mg/kg BW, PO, q24h for 10 d was prescribed empirically.

The bandage was changed every 2 to 3 d for the next week, and there was still a small amount of exudative discharge from the grafts. However, the grafts were intact and there was no evidence of necrosis.

Eleven days after surgery, the bandage and the skin sutures were removed. The wound edges were apposed and intact. There was no necrosis or discharge. The dog was weight-bearing on the bandaged leg.

Over the next 6 wk, there were minor complications associated with self-trauma, wound infection, and minor wound dehiscence. Bacterial culture and sensitivity testing of tissue taken from the edge of the dehisced margin revealed a heavy growth of Staphylococcus pseudintermedius (11) and Pseudomonas aeruginosa, sensitive to only gentamicin. Enrofloxacin was discontinued and gentamicin was administered at 6.5 mg/kg BW, IM, q24h for 5 d. Intermittent bandage changes were performed over this period, and the transposed pads continued to heal. By day 41, the dog had only a grade 1/5 lameness in the right foreleg and was allowed short lead walks.

Forty-six days after surgery, the dog’s wounds had healed (Figure 4). A xenogeneic DNA vaccine encoding for human tyrosinase (ONCEPT Canine Melanoma Vaccine; Merial, North Ryde, Australia) was administered. The delay was associated with the logistics in importing the vaccine with permission. The dog received its initial series of the vaccine (4 doses, 1 every 2 wk), transdermally in the right caudomedial thigh in an aseptic manner using the VET JET transdermal vaccination device (Merial). Subsequently, the dog’s owner relocated and the dog was referred to a veterinary practice for ongoing management and biannual ONCEPT (Merial) booster vaccines. At a follow-up visit 13 wk after surgery, the dog was ambulating well on the right foreleg (Figure 5) and there was no evidence of local tumor recurrence. In a final follow-up with the owner 6 mo after surgery, the dog was reported to be doing well and ambulatory on the right foreleg.

Figure 4.

Appearance of a successful reconstruction of a weight-bearing surface following complete excision of the right metacarpal pad for malignant melanoma in a 10-year-old neutered male Australian cattle dog, using the phalangeal fillet technique in both the second and fifth digits.

Figure 5.

Functional weight-bearing following a bilateral phalangeal fillet technique to reconstruct a completely excised right metacarpal pad for malignant melanoma in a 10-year-old neutered male Australian cattle dog.

Discussion

Use of the phalangeal fillet technique bilaterally in this dog allowed harvesting of 2 digital pads to reconstruct the defect created by excision of the metacarpal pad for a malignant melanoma lesion. This was successful in creating a robust weight-bearing surface and hence a functional limb.

The phalangeal fillet technique is described for the repair of minor defects in the metacarpal or metatarsal pad, which involves removing the phalanges of the second and or fifth digits on the affected limb and advancing the remaining flap of skin and associated digital pad into the metacarpal or metatarsal defect (1–4). There are few reports on the clinical use of the phalangeal fillet technique, and most reported cases have used either the second or fifth digit. In this case, we used both the second and the fifth digits to create a larger weight-bearing surface. The phalangeal fillet technique described in the literature recommends careful blunt dissection as close to the bone as possible when removing the phalanges to avoid inadvertently lacerating the common digital arteries or proper digital arteries and maintain innervation to the skin and pad (5,12). The palmar or plantar approach makes it more challenging to fillet the digits compared to a dorsal approach due to a greater risk of vascular compromise to the pad; however, it has the advantage of being a single step procedure compared to a staged 2-step procedure with the dorsal approach (12). Maintaining vascularity and preventing self-trauma are vital for a favorable outcome (5).

Sacrificing the second and fifth digit is less likely to interfere with normal weight-bearing capacity of the limb since the major weight-bearing footpads in dogs are the digital pads of the third and fourth digits (2,13–15). Failing to provide some weight-bearing surface to replace the excised metacarpal pad is unlikely to have a satisfactory outcome as load is more evenly distributed than previously suggested in other studies, with the digital pad of the fifth digit and the metacarpal or metatarsal pad bearing a substantial amount of load as well (6,13,15).

Interestingly, in a recent study on partial foot amputations in 11 dogs for local management of malignant foot tumors, 9 dogs had the third and fourth digits amputated, while 2 dogs had the fourth and fifth digits amputated (16). Eight of the dogs returned to normal function, with a median time to resolution of lameness of 37 d, but a mild lameness persisted in 3 dogs. This study showed that limb function was good to excellent despite the removal of 1 or both weight-bearing digits (16).

The successful use of microneurovascular digital pad transfer has been reported in dogs, in which the digital pad of the fifth digit on a hindleg was transferred to the region of an excised metacarpal pad on a foreleg to reconstruct a functional weight-bearing surface (14). There is also a report of the successful use of autogenous free pad grafts to reconstruct a metacarpal pad in a dog, and a report of the successful use of autogenous free pad grafts to reconstruct a functional weight-bearing surface in a cat (1,17). These are potentially viable alternatives for this case; however, microneurovascular digital pad transfer is technically demanding and associated with longer surgical times. Also, these procedures require another distal limb to be surgically prepared and operated, which contributes to additional anesthetic duration and creates some morbidity. Longer anesthetic and surgical durations increase the risk of post-operative infection (18–20). Partial limb amputation with a coaptive prosthesis or an intraosseous transcutaneous amputation prosthesis, or forequarter amputation, are the alternatives to local excision and reconstruction of a metacarpal pad, and these would be considered salvage procedures.

Neoplasia affecting the metacarpal or metatarsal pad requires complete resection of the epidermal surface and underlying adipose tissue, with the goal of achieving clean surgical margins (5). The decision to resect the affected foot pad versus limb amputation depends on the tumor type, stage, prognosis and availability of adjunctive therapies such as chemotherapy or radiation, and in the case of melanoma, immunotherapy (5). Prognosis is affected by the specific tumor biology and stage (5).

In a study of 384 cases of canine melanocytic tumors, 32 cases had involvement of the feet, 7 of which involved footpads (21). In that study, the cases involving the feet and lips had intermediate mortality characteristics, indicated by the reduced number of cases demonstrating malignant behaviour (38%, compared to 59% of oral melanoma cases), and the reduced number of dogs that died from causes related to the melanoma (30%, compared to 68% of oral melanoma cases). The median percentages of dogs surviving the total follow-up time period of 4 years, calculated using the product-limit method of survival function estimation, were 88% for dogs with cutaneous melanocytic tumors, 30% for dogs with oral tumors, and 67% for dogs with tumors of the feet and lips (21).

The current recommendations for adjunctive treatment of canine malignant melanoma following surgical excision may include one or a combination of the following: radiotherapy, platinum-based chemotherapy, immunotherapy, metronomic chemotherapy, and piroxicam (22–31). The expression of vascular endothelial growth factor and c-kit in canine melanomas also makes the use of small molecule inhibitors, including toceranib, a potential treatment option (32,33).

There are few published studies assessing the benefit of chemotherapy following surgical excision of canine malignant melanoma; however, according to one report, the response rates to chemotherapy are low and short-lived in dogs (34). The ONCEPT^® Canine Melanoma Vaccine (Merial) is licensed in the USA for use in dogs with stage II and III oral malignant melanoma and its use in this case was off-label with the owner’s permission. This DNA vaccine’s safety is well-established, and based on the current literature, there is a reasonable expectation of efficacy (27,35).

In conclusion, the phalangeal fillet technique using both the second and fifth digits was an effective alternative to limb amputation in this dog for reconstructing a functional, robust weight-bearing surface following complete excision of the metacarpal pad. CVJ