Principles of Microsurgery

Microsurgery uses the operating room microscope or high-powered loupe magnification to facilitate the microvascular surgical techniques employed in anastomosing small vessels and nerves. ^[1]  Microsurgical reconstruction is applied to complex reconstructive surgery problems when other options (eg, primary closure, healing by secondary intention, skin grafting, and local or regional flap transfer) are not adequate.

The field of microsurgery began with the introduction of the operating microscope, when Jacobson and Suarez described the anastomosis of blood vessels. In the 1960s, as microsurgical techniques were perfected, increasing success was seen with digital artery repairs and finger replantation. This laid the foundation for microsurgical composite tissue transfer, which became popular in the 1970s. ^[2]

In the 1980s, an emphasis was placed on improved function with autologous tissue transplantation, which is exemplified by mandibular reconstructions for cancer. Composite grafts consisting of soft tissue and bone aided in stabilizing the mandible, assisted with mastication, and allowed reliable coverage during the postoperative period, when radiation usually was required. Today, microsurgical techniques have become an integral part of the armamentarium for plastic surgeons, allowing for soft-tissue coverage and function after trauma or oncologic resections.

Microsurgery may not be the best solution for all reconstructive dilemmas and usually is not the first choice in the reconstructive ladder. However, it can offer the reconstructive surgeon an important tool for achieving complex reconstruction by proceeding with free tissue transfer from distant sites. ^[3] Free tissue transfer includes flaps such as the following:

• Isolated transfers
• Composite tissue transfers
• Functioning free muscle transfers
• Vascularized  bone grafts
• Toe transplantation

In addition, specific tissue transfers such as neural grafts or vein grafts are also considered examples of free tissue transfer. In specific cases, such as large defects of the face after tumor resection, free tissue transfer may be the best option for closure of the defect.

Reconstructive microsurgery has entered a stage where, because of continued developments in technology and a better understanding of the anatomy, anastomosis of very small vessels (0.3 mm) is possible. These highly challenging procedures are referred to in the literature as supermicrosurgery. ^[4] They allow anastomosis of perforator flaps such as the medial plantar flap to perforator recipient vessels. ^[5]  Additional applications include complex digit reimplantation and lymphatic anastomosis.

Newer optical and robotic systems are being developed for microsurgery and supermicrosurgery; further study is required. ^{[6, 7]}

Although microsurgery continues to develop, the basic principles of microsurgery remain the same:

• Select patients carefully
• Develop a careful preoperative plan and a backup plan
• Use a well-defined workhorse flap
• Obtain full patient consent
• Pay attention to intraoperative details
• Employ meticulous microsurgical technique
• Remain vigilant during postoperative care

This article outlines the basics of microsurgery, preoperative planning, specific operative techniques, and postoperative care. In addition, it describes some of the flaps most commonly used for microsurgical reconstruction.

Indications for tissue transfer utilizing microsurgical techniques include the following:

• Need to cover exposed vital structures, such as joint surfaces, tendons, vessels, and bone denuded of periosteum
• Need to restore shape, as in the breast after mastectomy
• Need to restore function, as in the muscles of the face

Finger reimplantation or transfer may represent another aspect of this technique. ^[8] Microsurgery may also be used as a new approach to achieve lymphatic drainage in cases of lymphedema. ^[9]

The specific indications for microsurgical reconstruction and the particular type of flap used depend on the type of tissue required and the size and location of the defect. Defects can be an isolated tissue type (eg, soft-tissue defects on the dorsum of the hand) or some combination of skin, subcutaneous tissue, nerves, muscle, tendons, cartilage, bone, and mucosa.

Free flaps can be categorized into two different types of transplants: isolated and composite. Isolated tissue transplants include skin, fascia, muscle, nerve, or bone individually. The more common composite tissue transplant represents a more complex flap and provides more than one type of tissue. Such flaps include myocutaneous, osteocutaneous, or innervated myocutaneous flaps. ^[10]

Historically, reconstruction of a defect was based on a reconstructive ladder, with local and simple procedures being performed before more extensive procedures or distant tissue transfers. Today, the use of free tissue transfer is no longer seen as the apex of the reconstructive ladder. Instead, it is a generalized tool for carrying out complex or composite tissue transfers, for treating wounds with poor healing or inflow, and for addressing situations in which postoperative radiation may play a factor in wound healing. (See Table 1 below.)

Table 1. Examples of Free Tissue Transfer (Open Table in a new window)

Patient issues

Contraindications associated with the patient include any condition that may place his or her life in danger or significantly increase the probability of postoperative flap loss. The time required to harvest and insert a flap is relatively long. Therefore, any medical condition that inhibits the patient’s ability to withstand prolonged anesthesia (eg, severe respiratory disease) is an absolute contraindication. Microsurgical free tissue transfer is absolutely contraindicated in patients who have the following:

• Critical illness
• Ongoing sepsis
• Uncontrolled coagulopathy

Age alone is not a risk factor in the success or failure of free flaps when preexisting medical conditions are not taken into account. ^[11] However, peripheral vascular disease and renal disease are strong predictors of reconstructive failure and patient morbidity and mortality. ^{[12, 13]}

Relative contraindications include any condition that increases the risk of intraoperative or postoperative complications. Common conditions that are not contraindications but can increase the risk of complications include the following:

• Cardiovascular disease
• Diabetes mellitus
• Raynaud syndrome
• Scleroderma
• Other collagen vascular diseases
• Smoking
• Radiation
• Ongoing infections

In general, a thorough review of the patient’s medical history and current conditions is critical in formulating a treatment algorithm and determining optimal timing of surgery.

Tobacco use has been shown to affect cutaneous blood flow, wound healing, and survival of pedicled flaps. The overall effect of cigarette smoke is to promote a thrombogenic state through vasoconstriction of the microvasculature. Surprisingly, the current literature has failed to show any damaging effects of cigarette smoke on free tissue transfer. ^{[14, 15]}

Surgical issues

Surgical issues include lack of a properly trained surgeon or surgical team. In current practice, this usually is not an issue, because microsurgery is now common and forms a major part of most plastic surgery training programs.

Other surgical issues include limited resources that might inhibit the staff from properly caring for the patient intraoperatively or postoperatively and lack of access to specialized microsurgical instruments.

Vessel injury and regeneration

Vessel injury and regeneration occur through the following steps:

• Formation of a platelet plug
• Pseudointima formation
• Endothelial regeneration

The first step in healing of a fresh arterial or venous anastomosis is the formation of a platelet plug. With intimal injury, exposed collagen triggers platelet adhesion. Platelets aggregate and activate fibrinogen, which adheres to platelets and links them together to form a plug. Fibrinogen is converted to fibrin, strengthening the platelet plug. If the vessel walls are not damaged and the anastomosis is secure, the platelet plug gradually disappears over the first 3-5 days, with the pseudointima forming by day 5. New endothelium covers the anastomotic site 1-2 weeks later.

The critical period of thrombus formation in the anastomosis is the first 3-5 days of healing. ^[16]  The underlying theme of microvascular free flap failures is a result of endothelial disruption with exposure of subendothelial collagen and formation of a platelet plug. If platelet aggregation reaches a critical mass, it will trigger a cascade of events leading to eventual thrombus formation in the vessel.

Skin, subcutaneous tissue, muscle, and bone have different ischemic tolerances. Skin and subcutaneous tissue are relatively resistant to anoxia and can tolerate warm ischemia for 4-6 hours and cold ischemia for as long as 12 hours. ^{[16, 17]}  Skeletal muscle is less tolerant to ischemia than skin is. Muscle can tolerate warm ischemia for as long as 2 hours; irreversible damage to the microcirculation begins at 6 hours, even under cold ischemia. ^{[18, 19]}  Bone is more resistant to anoxia and can tolerate up to 24 hours of cold ischemia. ^[20]

Classification of flaps

Mathes and Nahai ^[21] classified flaps as either random or axial on the basis of blood supply. A random flap is perfused by random small blood vessels without a proper name (eg, local bilobed flap). An axial flap is based on a known, named blood vessel or set of blood vessels. Mathes and Nahai classified these flaps as follows:

• One vascular pedicle (eg, tensor fasciae latae [TFL])
• Dominant pedicle(s) and minor pedicle(s) (eg, gracilis)
• Two dominant pedicles (eg, gluteus maximus)
• Segmental vascular pedicles (eg, sartorius)
• One dominant pedicle and secondary segmental pedicles (eg, latissimus dorsi)