Subdermal Plexus Flaps

Subdermal plexus flaps are a category of pedicle grafts used in reconstructive surgery to repair skin defects, particularly in the distal extremities of dogs and cats. These flaps rely on the subdermal vascular plexus, a network of vessels located in the superficial fascia just beneath the dermis, for their blood supply. Unlike axial pattern flaps, which are based on a single, named direct cutaneous artery and vein, subdermal plexus flaps are random pattern flaps, meaning they do not have a specific, named vascular pedicle. Instead, they are perfused by the dermal and subdermal plexuses, which are fed by perforating vessels from the underlying musculature. The subdermal plexus is a rich anastomotic network that allows for the survival of skin flaps with a length-to-width ratio of approximately 1.5:1 to 2:1 in dogs and cats, depending on the location and vascularity of the donor site. These flaps are commonly used for wound closure when primary closure is not possible due to excessive tension, when there is a lack of local skin, or when the wound bed is not suitable for a free skin graft. They are particularly useful for covering defects on the distal limbs, where skin is limited and tension is high. The surgical technique involves elevating a flap of skin and subcutaneous tissue, preserving the subdermal plexus, and then rotating or advancing the flap into the defect. The flap is then sutured in place, and the donor site is closed primarily or left to heal by second intention. Subdermal plexus flaps are classified based on their movement: advancement flaps, rotation flaps, transposition flaps, and interpolation flaps. Each type has specific indications and limitations. The success of these flaps depends on meticulous surgical technique, proper patient selection, and postoperative care to ensure adequate perfusion and prevent complications such as flap necrosis, infection, and seroma formation.

Subdermal plexus flaps are not a disease but a surgical technique; however, the conditions that necessitate their use are the underlying etiologies. These include traumatic wounds (e.g., degloving injuries, lacerations, avulsions, burns, and bite wounds) that result in skin loss, particularly on the distal extremities where skin is scarce. Neoplastic conditions, such as mast cell tumors, soft tissue sarcomas, and squamous cell carcinomas, may require wide surgical excision, leaving large defects that cannot be closed primarily. Congenital defects, such as dermoid sinuses or skin aplasia, may also require reconstructive surgery. Iatrogenic causes include previous surgical procedures that have compromised local skin viability or created excessive tension. Additionally, chronic non-healing wounds, such as those associated with radiation therapy, pressure sores, or vascular insufficiency, may necessitate flap reconstruction. The anatomical vulnerability of the distal limbs, where skin is thin and tightly adhered to underlying structures, makes these areas particularly prone to traumatic skin loss and challenging for primary closure. Biomechanical triggers, such as shearing forces and tension, can compromise wound healing and necessitate flap coverage. Cellular mechanisms involve the inflammatory response, angiogenesis, and tissue remodeling, which are critical for flap survival and integration.

Subdermal plexus flaps are used in both dogs and cats, with no specific breed or sex predisposition. However, certain breeds with loose skin, such as the Basset Hound, Bloodhound, and Shar-Pei, may have more available skin for flap creation, while breeds with tight skin, such as the Greyhound or Whippet, may have limited donor sites. Age can influence skin elasticity and vascularity, with younger animals generally having better healing potential. Working dogs, such as hunting or herding breeds, are at higher risk for traumatic wounds that require flap reconstruction. The incidence of conditions requiring subdermal plexus flaps is not well-documented, but traumatic wounds are common in veterinary practice, particularly in outdoor or active animals. Neoplastic conditions requiring wide excision are also common, with mast cell tumors being one of the most frequently diagnosed skin tumors in dogs. The use of subdermal plexus flaps is a standard technique in veterinary reconstructive surgery, and the success rate is generally high when performed correctly. Breed-specific anatomical factors, such as the presence of excessive skin folds, can affect flap design and survival. Additionally, the location of the defect influences the choice of flap type; for example, defects on the distal limb may require a transposition flap from the adjacent skin, while defects on the trunk may be closed with advancement or rotation flaps.

The pathophysiology of subdermal plexus flaps revolves around the vascular supply and the physiological response to surgical elevation and transfer. The subdermal plexus is a network of vessels located in the superficial fascia, just beneath the dermis. This plexus is supplied by perforating vessels that arise from the underlying musculature and penetrate the deep fascia. When a flap is elevated, these perforating vessels are severed, and the flap becomes dependent on the subdermal plexus for its blood supply. The viability of the flap depends on the adequacy of this plexus, which is influenced by the length-to-width ratio, the location of the flap, and the presence of any underlying vascular disease. After elevation, the flap undergoes a series of physiological changes, including vasoconstriction due to sympathetic stimulation, followed by vasodilation as the flap becomes ischemic. This is followed by a period of neovascularization, where new vessels grow from the wound bed into the flap. The flap is most vulnerable to ischemia in the first 24-48 hours after surgery, and any factors that compromise perfusion, such as tension, kinking, or hematoma formation, can lead to flap necrosis. The subdermal plexus also plays a role in the inflammatory response, with the release of cytokines and growth factors that promote healing. The biomechanical properties of the skin, such as elasticity and tension, are also important, as excessive tension can compromise blood flow and lead to dehiscence. Systemic factors, such as hypovolemia, hypotension, and hypothermia, can further compromise flap perfusion. Understanding these pathophysiological mechanisms is essential for successful flap surgery.

Several factors predispose animals to the need for subdermal plexus flaps. Intrinsic factors include age, with older animals having reduced skin elasticity and vascularity, making flap survival less predictable. Body condition score is also important, as obese animals have thicker subcutaneous fat, which can compromise the subdermal plexus and increase the risk of flap necrosis. Genetic factors, such as breed-specific skin characteristics, can influence the availability and quality of donor skin. Extrinsic factors include trauma, which is the most common indication for flap reconstruction. The severity and location of the wound, as well as the presence of contamination or infection, can affect the success of the flap. Prior surgeries in the area can compromise the local blood supply, making flap elevation more difficult. Nutritional status is critical, as protein and vitamin deficiencies can impair wound healing and flap survival. Management factors, such as the use of corticosteroids or other immunosuppressive drugs, can also negatively impact healing. Excessive activity or movement of the affected area can increase tension on the flap and lead to dehiscence. Additionally, the presence of concurrent diseases, such as diabetes mellitus or hyperadrenocorticism, can impair healing and increase the risk of complications. Careful patient selection and preoperative optimization are essential to minimize these risks.

The clinical signs associated with conditions requiring subdermal plexus flaps are primarily related to the underlying wound or tumor. Traumatic wounds may present with skin loss, hemorrhage, swelling, and pain. The wound may be contaminated with debris, hair, or foreign material, and there may be evidence of infection, such as purulent discharge or a foul odor. Degloving injuries, where the skin is stripped away from the underlying tissue, are particularly common on the distal limbs and can result in exposed bone, tendons, or joints. Neoplastic conditions may present as a mass or ulcerated lesion, which may be painful or pruritic. The mass may be fixed to underlying tissues or freely movable, and there may be regional lymphadenopathy if metastasis is present. In cases of chronic non-healing wounds, there may be a history of prolonged wound care without improvement. The clinical signs are often graded based on the severity of the wound, the degree of tissue loss, and the presence of complications such as infection or necrosis. A thorough physical examination is essential to assess the extent of the wound, the viability of the surrounding skin, and the overall health of the patient. Orthopedic and neurological examinations may be necessary to rule out underlying fractures or nerve damage. Systemic signs, such as fever, lethargy, and anorexia, may be present if there is significant infection or inflammation.

When considering the need for a subdermal plexus flap, the primary differential diagnoses are other methods of wound closure, including primary closure, second intention healing, free skin grafts, and axial pattern flaps. Primary closure is indicated for small, clean wounds with minimal tension, but is not possible for large defects or those with excessive skin loss. Second intention healing may be appropriate for small wounds in areas with good blood supply, but can result in significant scarring and contracture, particularly on the distal limbs. Free skin grafts, such as full-thickness or split-thickness grafts, are an alternative for covering defects, but they require a healthy wound bed and have a higher risk of failure compared to flaps. Axial pattern flaps, such as the caudal superficial epigastric flap or the thoracodorsal artery flap, have a more reliable blood supply and can cover larger defects, but they are limited by the availability of donor sites and the need for a named vessel. Other differentials include the use of skin stretchers or tissue expanders, which can be used to gradually increase the amount of available skin. The choice of technique depends on the size and location of the defect, the condition of the wound bed, and the overall health of the patient. A thorough assessment of the wound, including the viability of the surrounding skin and the presence of infection, is essential to determine the most appropriate closure method.

The diagnostic algorithm for a patient requiring a subdermal plexus flap begins with a thorough history and physical examination. The wound should be assessed for size, depth, location, and the presence of contamination or infection. The viability of the surrounding skin should be evaluated by assessing capillary refill time, color, and temperature. If there is a mass, a fine-needle aspiration or biopsy may be performed to determine the nature of the lesion. Preoperative laboratory tests, including a complete blood count, serum biochemistry profile, and urinalysis, are recommended to assess the overall health of the patient and identify any underlying conditions that may affect healing. Coagulation testing may be indicated if there is a history of bleeding disorders or if the patient is to undergo extensive surgery. Imaging studies, such as radiography or ultrasonography, may be performed to evaluate the underlying bone, joints, or soft tissues for concurrent injuries. In cases of suspected neoplasia, staging tests, such as thoracic radiographs or abdominal ultrasound, may be recommended to rule out metastasis. Once the wound has been assessed and the patient is deemed stable, the decision to proceed with a subdermal plexus flap is made. The flap design is planned based on the size and location of the defect, the availability of donor skin, and the expected tension on closure. Intraoperative assessment of flap viability, such as fluorescein dye or Doppler ultrasound, may be used to confirm adequate perfusion. Postoperative monitoring is essential to detect any signs of flap compromise, such as discoloration, swelling, or necrosis.

Laboratory findings in patients undergoing subdermal plexus flaps are typically nonspecific but are important for preoperative assessment. A complete blood count may reveal leukocytosis with a left shift if there is infection or inflammation, or anemia if there has been significant blood loss. Serum biochemistry may show elevations in liver enzymes or creatinine if there is concurrent organ dysfunction. Total protein and albumin levels are important for assessing nutritional status, as hypoalbuminemia can impair wound healing. Coagulation parameters, including prothrombin time (PT), activated partial thromboplastin time (aPTT), and platelet count, should be evaluated to rule out coagulopathies that could increase the risk of bleeding and hematoma formation. In patients with chronic wounds, a urinalysis may be indicated to rule out urinary tract infection, which could be a source of bacteremia. Inflammatory biomarkers, such as C-reactive protein (CRP) and serum amyloid A (SAA), may be elevated in the presence of infection or inflammation, but are not specific. Synovial fluid analysis may be performed if there is a concurrent joint effusion, but is not routinely indicated. Blood gas analysis may be useful in critically ill patients to assess acid-base status and oxygenation. Overall, laboratory findings are used to identify any underlying conditions that could affect the success of the surgery and to guide perioperative management.

Imaging plays a crucial role in the preoperative planning of subdermal plexus flaps. Radiography is commonly used to evaluate the underlying bone and joints for fractures, luxations, or osteomyelitis, particularly in cases of trauma. Stress radiography may be performed to assess joint stability. Ultrasonography can be used to evaluate the soft tissues, including the skin, subcutaneous tissue, and underlying muscles, to assess the extent of the wound and the viability of the tissues. Color Doppler ultrasonography can be used to assess the blood flow to the area and to identify the location of perforating vessels, which can aid in flap design. Computed tomography (CT) with 3D reconstructions may be useful for complex wounds, particularly those involving the head or distal limbs, to better understand the anatomy and plan the surgical approach. Magnetic resonance imaging (MRI) is rarely indicated for skin wounds but may be useful if there is suspicion of underlying neoplasia or deep tissue involvement. Angiography or fluoroscopy can be used to visualize the vascular supply to the area, particularly if an axial pattern flap is being considered. However, for subdermal plexus flaps, which are random pattern flaps, advanced imaging is not typically necessary. Intraoperative imaging, such as fluorescein dye injection, can be used to assess flap perfusion, but is not a standard imaging modality. Overall, imaging is used to rule out concurrent injuries and to plan the surgical approach, but the primary assessment of the wound is based on physical examination.

Cytology and histopathology are important in the evaluation of wounds and masses that may require subdermal plexus flaps. Fine-needle aspiration of a mass can provide a preliminary diagnosis, such as mast cell tumor, sarcoma, or carcinoma. Cytological evaluation of wound exudate can help identify bacterial or fungal infection and guide antimicrobial therapy. Histopathology of a biopsy specimen is essential for definitive diagnosis and grading of tumors, which influences the surgical margins and the need for adjuvant therapy. For example, mast cell tumors are graded based on the Patnaik or Kiupel system, and high-grade tumors require wider margins. Soft tissue sarcomas are graded based on mitotic index, necrosis, and differentiation, and may require radiation therapy if margins are incomplete. Histopathology is also important for evaluating the margins of the excised tissue to ensure complete removal. In cases of chronic wounds, a biopsy may be taken to rule out neoplasia or to assess the viability of the tissue. Special stains, such as immunohistochemistry, may be used to characterize the tumor type. The results of cytology and histopathology are critical for determining the appropriate surgical plan and for predicting the prognosis.

The treatment for conditions requiring subdermal plexus flaps is primarily surgical. The goal is to achieve primary closure of the wound with a tension-free flap that has adequate blood supply. The surgical technique involves several steps. First, the wound is thoroughly debrided to remove all necrotic tissue, foreign material, and bacteria. The wound bed is then assessed for viability, and any exposed bone, tendon, or joint is covered with healthy tissue if possible. The flap is designed based on the size and location of the defect, with a length-to-width ratio of 1.5:1 to 2:1. The flap is elevated in the subdermal plane, preserving the subdermal plexus. The flap is then moved into the defect using one of several techniques: advancement, rotation, transposition, or interpolation. Advancement flaps are moved directly forward into the defect, while rotation flaps are rotated around a pivot point. Transposition flaps are moved laterally into the defect, and interpolation flaps are moved over a bridge of skin. The flap is sutured in place using a two-layer closure: a deep layer of absorbable sutures (e.g., 3-0 polydioxanone or polyglactin 910) to appose the subcutaneous tissue, and a skin layer of non-absorbable sutures (e.g., 3-0 nylon or polypropylene) using a simple interrupted or continuous pattern. The donor site is closed primarily if possible, or left to heal by second intention. Postoperative care includes pain management, antimicrobial therapy, and wound protection. The flap should be monitored closely for signs of ischemia, such as discoloration, swelling, or necrosis. If the flap fails, additional surgery may be required, such as a free skin graft or a second flap. The choice of surgical technique depends on the individual case, and the surgeon must have a thorough understanding of the vascular anatomy and the principles of flap surgery.

The prognosis for subdermal plexus flaps is generally good, with success rates reported in the literature ranging from 80% to 95% in dogs and cats. The success of the flap depends on several factors, including the size and location of the defect, the condition of the wound bed, the surgical technique, and the postoperative care. Flaps on the distal limbs have a higher risk of failure due to the limited blood supply and the high tension on closure. The presence of infection, contamination, or concurrent disease can also negatively affect the outcome. Complications such as flap necrosis, dehiscence, seroma formation, and infection can occur, but are often manageable with appropriate treatment. The long-term functional and cosmetic outcome is usually excellent, with the flap providing durable coverage of the defect. Negative prognostic indicators include a length-to-width ratio greater than 2:1, the use of a flap in a previously irradiated area, and the presence of systemic disease such as diabetes mellitus or hyperadrenocorticism. The overall prognosis is also influenced by the underlying condition, such as the grade of a tumor, which may require additional treatment. With careful patient selection and meticulous surgical technique, the prognosis for subdermal plexus flaps is favorable.

Postoperative follow-up for subdermal plexus flaps is crucial to ensure successful healing. The patient should be hospitalized for at least 24-48 hours after surgery to monitor the flap for signs of ischemia, such as changes in color, temperature, or capillary refill time. The flap should be kept clean and dry, and an Elizabethan collar should be used to prevent self-trauma. Sutures are typically removed 10-14 days after surgery, but the timing may vary depending on the location and tension on the wound. Serial radiographic evaluation may be indicated if there was an underlying fracture or if there are concerns about bone healing. The patient should be restricted from excessive activity for at least 2-3 weeks to allow the flap to heal and to prevent tension on the wound. Physical therapy, such as passive range of motion exercises, may be recommended to prevent joint stiffness, particularly if the flap is on a limb. The wound should be monitored for signs of infection, such as redness, swelling, discharge, or dehiscence. Long-term follow-up is recommended to assess the cosmetic and functional outcome, and to monitor for any late complications, such as contracture or alopecia. In cases of neoplasia, regular rechecks are necessary to monitor for recurrence or metastasis. The owner should be instructed to monitor the wound at home and to contact the veterinarian if any concerns arise.

Clinical pearls for subdermal plexus flaps include: 1) Always design the flap with a length-to-width ratio of 1.5:1 to 2:1 to ensure adequate perfusion. 2) Elevate the flap in the subdermal plane, preserving the subdermal plexus, and avoid excessive thinning of the flap. 3) Handle the flap gently with skin hooks or stay sutures to avoid crushing the tissue. 4) Ensure the flap is tension-free by using a walking suture technique or by undermining the surrounding skin. 5) Use a two-layer closure to reduce tension on the skin sutures. 6) Consider using a drain to prevent seroma formation. 7) Monitor the flap closely in the postoperative period, and if there are signs of venous congestion, consider applying leeches or using medical leech therapy. 8) In areas with poor blood supply, such as the distal limb, consider using a delayed flap technique to improve survival. Pitfalls to avoid include: 1) Making the flap too long or too narrow, which can lead to necrosis. 2) Elevating the flap too thick or too thin, which can compromise the blood supply. 3) Creating excessive tension on the flap, which can lead to dehiscence. 4) Failing to debride the wound adequately, which can lead to infection. 5) Using electrocautery excessively, which can damage the subdermal plexus. 6) Placing sutures too tightly, which can compromise blood flow. 7) Ignoring signs of flap compromise, such as discoloration or swelling, which can lead to complete flap loss. 8) Not providing adequate postoperative analgesia, which can lead to self-trauma. By following these pearls and avoiding these pitfalls, the surgeon can maximize the chances of a successful outcome.

Perioperative drug protocols for subdermal plexus flaps are based on Plumb's Veterinary Drug Handbook. Prophylactic antimicrobials are indicated to prevent surgical site infection, particularly in contaminated wounds. A common protocol is cefazolin (22 mg/kg IV) administered 30 minutes before incision and repeated every 90 minutes during surgery. Postoperative oral antibiotics, such as amoxicillin-clavulanate (13.75 mg/kg PO q12h) or cephalexin (22 mg/kg PO q8h), may be continued for 7-10 days if there is significant contamination or infection. Analgesia is essential for pain management. Opioids, such as hydromorphone (0.05-0.1 mg/kg IV or IM q4-6h) or methadone (0.1-0.5 mg/kg IV or IM q4-6h), are used in the immediate postoperative period. Nonsteroidal anti-inflammatory drugs (NSAIDs), such as carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h), are often used for their analgesic and anti-inflammatory effects, but should be used with caution in patients with renal or hepatic disease. Local anesthetic blocks, such as a lidocaine or bupivacaine line block, can provide additional analgesia. A constant rate infusion (CRI) of lidocaine (25-50 mcg/kg/min IV) or ketamine (0.1-0.5 mg/kg/h IV) may be used for multimodal analgesia. Muscle relaxants, such as methocarbamol (22 mg/kg IV or PO q8h), may be indicated if there is muscle spasms. Chondroprotectants, such as glucosamine and chondroitin sulfate, are not directly relevant to flap surgery but may be used if there is concurrent joint disease. The dosages and routes should be adjusted based on the patient's condition and the duration of surgery. It is important to monitor for adverse effects, such as gastrointestinal upset, renal toxicity, or respiratory depression, and to adjust the protocol accordingly.

The evidence base for subdermal plexus flaps in veterinary surgery is derived from clinical studies, case series, and expert opinion. Key studies have evaluated the survival of random pattern flaps in dogs and cats, with reported success rates of 80-95%. A landmark study by Pavletic (1990) described the principles of subdermal plexus flaps and emphasized the importance of the length-to-width ratio. Studies have also compared the survival of flaps in different locations, with distal limb flaps having a higher risk of necrosis. The use of delayed flaps has been shown to improve survival in areas with poor blood supply. The role of postoperative care, including the use of drains and the management of seromas, has been investigated. The use of fluorescein dye to assess flap viability has been described, but its routine use is limited. The ACVS and ECVS have published consensus guidelines on reconstructive surgery, which include recommendations for flap selection and surgical technique. The AO Vet guidelines also provide principles for wound management and flap surgery. Overall, the literature supports the use of subdermal plexus flaps as a reliable method for wound closure, but emphasizes the importance of patient selection, surgical technique, and postoperative care. Further research is needed to evaluate the long-term outcomes and to compare different flap techniques.