Microsurgery is defined as surgery with significant magnification. Typically obtained with an operative microscope, magnification is critical for adequate visualization in microsurgery where coaptation and anastomosis of structures less than a millimeter are often performed. Magnification with adequate lighting permits an accurate and detailed view of the microscopic anatomy leading to greater accuracy for the surgeon.

Modern day microscopes include 2 heads for collaborative surgery in a small visual field, screen projection for the entire operative team to keep informed about the microsurgical progress, perfusion analysis of tissues from intravenous dye injections, and recording capabilities for secondary analytics and education. Focus, zoom, and magnification of the microscope can be adjusted either with foot pedals or hand controls. Surgical microscopes today have high-precision optics and high-power illumination, which prevent shadows from obstructing parts of the operating field. Three-dimensional visualization through stereopsis is also available to provide further depth for anatomy navigation.

Variable magnification microscopes allow surgeons greater flexibility to switch the field of view.

It is essential for a microsurgeon to become familiar with their equipment as any surgeon would with instruments they will use for each surgery.

The main microscope companies available include Mitaka, Zeiss, and Leica, all of which create several different microscopes. The Zeiss Pentero 800 and Kinevo 900 allow for an 11x magnification ( Fig. 1 ), as do the Leica M530 and M525, with the Leica M720 allowing for 12x magnification. ^, Resolution enhancers are available for these models, which can increase magnification up to 40x or even 60x and a resolution up to 120lp/mm for the increasing demands of fine microsurgery and supermicrosurgery ^,. The Mitaka MM51 has the highest resolution optics available for supermicrosurgery and reconstructive microsurgery with a 42x magnification, without the need for resolution enhancers, which lower the quality of the image by lowering the light, depth of focus, and contrast ( Fig. 2 ). This is particularly important for supermicrosurgical procedures such as lymphovascular anastomoses. Its large lens, combined with an 8:1 zoom system, allows for twice the resolution and magnification of other surgical microscopes. Most microscopes are additionally capable of multispectral indocyanine green (ICG) and fluorescein imaging. The working distance from the corpus to the surgical field of about 200 to 600 mm. The ocular-corpus length varies with the Mitaka having the smallest at 270 mm and the Zeiss having the longest at about 340 mm. Research comparing particular microscopes has not been robust thus far; however, there are hypotheses that the shorter ocular-corpus length is associated with improved ergonomics and can reduce fatigue of the shoulder muscles.

Zeiss microscope.

Mitaka microscope.

Microsurgery instruments are designed for ultrafine dexterity offering microscopic precision with the movements of the diminutive distal tips and pinch closure. Essential elements of such instruments are fine unbent distal tips, nonreflective material, and easy manipulation. Many instruments are spring loaded and have pen-type precision grip, which significantly improves precision and avoids fatigue. Instruments are typically made of stainless steel or titanium to achieve a balance of substantial feel between the fingertips without excessive weight.

The instruments have a variety of handle lengths, which help to maintain a comfortable working distance for the surgeon. It is usually necessary for the microsurgeon to be able to stabilize their ulnar side of their hands on the working surface table to permit finger dexterity to manipulate the fine instruments without tremors.

Essential instruments for a microsurgery tray include: straight and curved scissors, forceps, vessel dilators, needle-drivers, and vessel clamps. Scissors must have sharp, narrow tips, and often do not require a significant length of handle. Excellent forceps are critical; gradually tapering and precise tips are key to precisely and atraumatically grasping tissue and holding sutures. The needle drivers should have optimal spring tension and jaws that close in parallel over their entire length for maximal precision while suturing. Vessel clamps are also an essential instruments that have evolved from the original bulldog clamps. Commonly seen clamps today are atraumatic and have a fine balance of applying sufficient pressure to prevent blood flow into the anastomotic field while avoiding compressive damage to the vessel wall.

Spending time to ensure body position is optimal ultimately helps the surgeon take full advantage of the equipment while preventing fatigue or musculoskeletal compromise. Most surgeons have some degree of tremor. A minor tremor to the naked eye can easily become problematic with attempts at manipulation of the small structures in the magnification of microsurgery. Modifying external factors such as sleep deprivation, digestive stimulants, and other stressors plays a role in minimizing tremors, but comfortable positioning during surgery and bracing of the hand remains a key for maximizing precision. There is growing evidence that a surgeon’s ill posture while operating contributes to musculoskeletal strain and chronic pain. When appropriately used, the cervical unloading allowed by microscopes can contribute to extending a surgeon’s pain free career by decreasing cervical spine stress and injury.

Microsurgical proficiency begins with foundational concepts. The surgeon holds and manipulates the instruments almost entirely with the fingertips, like one would a pen. Forceps are primarily used for the nondominant hand, while needle drivers and scissors are primarily instruments for the dominant hand. Creating multiple points of contact between the wrist and ulnar hand and the operating table permits improved stability and tremor avoidance. Occasionally forceps are used as needle drivers for miniscule suturing.

Maintaining appropriate hemostasis and a relatively bloodless field is the goal for optimum visualization of the lumen or fascicles being sutured. In general, the operative field should have gross hemostasis before the coaptation or anastomosis is initiated. Clamped vessels should be devoid of continuous blood flow. If adequate hemostasis is difficult, a very gentle continuous suction can be used but the coaptation or anastomotic site is protected from direct suction. Care is also taken, however, to avoid desiccation of the vessels, lymphatics, and nerves. Saline or heparinized saline irrigation is often used to dampen the field to prevent desiccation.

The microsurgical anastomoses are governed by principles similar to other vascular, neural, and lymphatic anastomoses. The smaller scale of these structures, which can be 0.1 mm to 10 mm is what makes these anastomoses different and particularly challenging. Microsurgical anastomosis technique includes using very fine needles and sutures of up to 12–0. The coaptive and anastomotic techniques are surgeon specific but may involve suture triangulation of the vessel followed by suturing the gaps between each initial suture, suturing the anterior wall first and then flipping the vessel by 180° to suture the posterior wall, or simply starting on the posterior wall and running circumferentially around the vessel or nerve. Irrespective of the suturing technique, the sutures should be placed with precision evenly in distance from the edge of the lumen or nerve edge, and evenly between one suture and the next. Adventitial excess is removed around the edge of the vessels but does not need to be excessive more than a millimeter or 2 from the end of the vessel. Excess adventitia may be inadvertently flipped into the lumen, which would be a nidus for platelet aggregation and clot formation. ^,,, The microsurgeon should have constant surveillance of the lumen to remove any minor debris or clot before the final suture is placed. It is common to irrigate the vessel lumen with a heparinized saline solution to make sure foreign material and clot are removed.

Patient selection is a critical element of microsurgery. Given the resources, time and effort required as well as the delicate nature of the structures involved, it is essential to carefully pre-operative plan the surgery to minimize modifiable risk factors that could lead to failure. Certain patient characteristics such as age which were once thought to be a contraindication to microsurgery, have been shown since to be less of an exclusion criteria than other risk factors such as medical comorbidities. Diabetes, obesity, and other medical and vascular comorbidities can have more of an effect on outcomes and wound healing. The presence of multiple chronic illnesses can be a significant risk to undergoing any extensive procedure, as they are more prone to worsening of medical comorbidities with the stress of surgery, overall functional decline, and impaired cognition while admitted. ^, The presence of multiple medical comorbidities is associated with increased morbidity and mortality related to a variety of complications, including pulmonary, cardiac, thromboembolism, stroke, and sepsis. Inadequate glucose control is a known risk factor for delayed wound healing and infection. Research has demonstrated that high glucose levels result in a slower rate of intimal repair after vascular anastomosis and stiffened red blood cell membranes, leading to increased blood viscosity. While some studies have shown concerns regarding the adverse outcomes of diabetic patients undergoing microsurgery, risks of complications are minimized with aggressive preoperative and postoperative control.

Another modifiable risk factor for delayed healing and microvascular complications is smoking. ^, Smoking compromises the vascular system through vasodilation, increasing thrombogenesis, and endothelial, platelet, and macrophage cellular dysfunction, and can therefore increase the risk of failure for a variety of flap types. ^,, Despite this impairment in healing associated with smoking, there is evidence in the literature that smoking does not affect overall flap survival. ^,,, While there are different opinions on the timing of smoking cessation, it is clear that active smoking can increase the risk of hematoma formation, wound dehiscence, and infection.

Preoperative imaging of perforator vasculature is common for planned microvascular reconstruction. The gold standard for perforator mapping remains computed tomography angiography scan (CTA), although magnetic resonance angiography scan (MRA), Doppler signals, ultrasound duplex, and angiography are also possible alternatives. Good planning and awareness of the variety of options available are important for many surgeries and critical in terms of understanding the vascular anatomy in free tissue transfers. It is important to consider the available tissues and the goals of the procedure before beginning. While it is important to be adaptable and adjust according to intraoperative findings, it is also critical to have an initial plan. For complex wound closures, for example, where free flaps may be required, it is critical to consider the thickness, tissue type, and size of tissue required as well as the available tissues around the wound and elsewhere on the body that would fit the specific requirements. A good flap design must account for the 3-dimensional configuration of the defect, flap, and donor site. Complex flaps for defects with composite tissue require special consideration where the different components of a chimeric flap may have an independent or codependent blood supply. Good flap design achieves the restoration of form and function to the recipient site while minimizing the donor morbidity. The flap chosen should try to mirror the tissue at the recipient site as much as possible to avoid unsightly or dysfunctional recipient sites. Similarly, the donor site defect should be considered to have the same considerations. Primary closure in an inconspicuous location is preferred for donor sites.

Staging of the procedure is also a consideration in microsurgery. The microsurgeon may consider using a 1-staged procedure or a 2-staged procedure, or even a multistaged procedure to achieve the final optimal results. Closure of a defect that only needs soft tissue is different from a defect that requires multiple tissue types and the addition of a functional component. The microsurgeon should take into consideration the characteristics of the needed flap and therefore the number of flaps needed to achieve appropriate form and function as defined by preoperative goals.

Sometimes, the resources available arise from unexpected situations. For example, it is important to consider spare parts reconstruction for mutilated wounds. Although preoperative planning is limited in trauma situations, it is important to take the time to understand the tissues available especially if nonreplantable amputations are involved. All tissues available and their vascular supply should be inspected and considered as an available flap to potentially avoid the need to harvest flaps from other areas of the body if coverage is required. Spare parts reconstruction has the potential to use a mutilating injury to the patient’s advantage and provide tissue for reconstruction while limiting additional morbidity from a donor site.

While tissue from multiple locations may be used for free flaps, some have the advantage of allowing a 2-team approach and maximizing the chances of a good outcome while minimizing donor site morbidity. This has led to increased popularity of lower extremity tissue transfers, breast reconstruction, and head and neck reconstructions.

Intraoperative decision-making in microsurgery involves a variety of considerations. End-to-end versus end-to-side anastomoses may be a preference of the microsurgeon or a necessity because of anatomic variances. The end-to-end anastomosis sacrifices a vessel, which may cause compromise down-stream. For many however, end-to-end suturing is technically easier. End-to side anastomosis may be technically more difficult for some but does offer advantages of minimizing vessel sacrifice and dilating the anastomosis site with restoration of vascular flow. Many variations of anastomosis techniques exist as discussed earlier, and the literature regarding the decision on whether to proceed with end-to-end or end-to-side anastomosis is mixed, although there is evidence that both are safe options. ^,

The position of the vessels following the anastomosis is very important as well. Several studies have indicated that free flap failure may be related to venous and arterial compression through kinking, leading to thrombosis. Vein walls are significantly thinner than arterial walls making veins easier to compress. Vessel placement in the recipient site and overlying closure of the flap over the vessels may result in vessel malrotation and kinking. Gentle curves to a pedicle tolerate well but sharp turns especially at the anastomosis site are prone to thrombosis. The surgeon should verify good vessel positioning before closure of the recipient site. Occasionally a small fat graft or muscle graft can be placed around the vessel to ensure an optimum course under the flap and at the anastomosis site.

It is necessary to acknowledge that despite good planning, there may be a length discrepancy between the recipient bed vessel and the pedicle of the flap, requiring the use of vein grafts. The increased flap failure rates with the use of vein grafts are controversial, however, there are times when the vein grafts are necessary. Preoperative awareness of the need for vein grafts is preferred so that donor site prepping can be appropriate and harvesting of the vein optimized. Occasionally, due to anatomic variations, a vein graft was not anticipated and the microsurgeon will need to prep a new area of the patient’s body to harvest the vein, unless a vein graft site exists in the surgical field.

An intraoperative vascular compromise should be handled with haste so that the thrombosis does not negatively affect the flap. Intraoperative venous compromise may be related to several causes including vein kinking, vein compression, different venous outflow channels for the flap, systemic vasculopathy, suture techniques of the anastomosis, poor arterial inflow, or vein injury during flap harvest. Mechanical compromise is corrected by eliminating the compression and restoring appropriate unopposed directional flow. Technical aspects of the anastomosis may require redoing the anastomosis. Insufficient venous outflow from other areas of the flap may require distant venous anastomosis to other recipient sites.

Adjuncts such as thromboelastometry can be used perioperatively to inform surgical decision-making. While it can be difficult to predict adverse events, monitoring changes in hemostasis during cases such as free-flaps allows for the real-time assessment of coagulation and intervention before flap compromise.

Intraoperative arterial compromise may have similar causes as intraoperative venous compromise. Usually, however, redoing the anastomosis is required. Mechanical compression is managed as per venous mechanical compression.

Salvage procedures for vascular insufficiency are discussed later.

Postoperative care should be focused on close monitoring of the patient. This is best performed in a dedicated microsurgery unit by staff familiar with flap monitoring. Patient monitoring does not necessarily require ICU-level care, which is typically seen for one on one nursing care should the situation be required. Patients are also typically kept nothing by mouth (NPO) for the first 24 hours since vascular compromise of the flaps are most often seen in the first 24 to 48 hours postoperatively, and this is also the time when intervention, such as return to the operating room for exploration, is most often successful. ^,

Preventing microvascular thrombosis is a key component of postoperative care for microsurgical cases. To do this, it is important to monitor patients for dehydration and possibly anemia. As mentioned earlier, compression over the pedicle is to be avoided to prevent thrombosis. Gravity and swelling of the flap may compress or provide tension on a new anastomosis and therefore progressive activity allowances are usually followed. ^, Anticoagulation may be used to try to prevent thrombosis and venous congestion or arterial insufficiency. ^, The risks of hematoma formation with the use of anticoagulation is an important consideration. Hematoma formation may compromise the flap through external compression of the pedicle. Current literature provides conflicting evidence regarding the use of aspirin and heparin in particular. Oral anticoagulation prophylaxis has not been shown to be effective for flap thrombosis specifically, although there is evidence in the literature that their use is safe for venous thromboembolism prophylaxis, as it did not increase the incidence of postoperative major bleeding when compared to enoxaparin. ^,

There is limited research on the topic of perioperative anticoagulation. A survey of practicing US surgeons demonstrated that the majority use postoperative anticoagulation with the agent of choice being aspirin followed by lovenox as the second preferred. The most common duration of anticoagulation according to the surveyed microsurgeons was greater than 7 days.

Should venous congestion occur, surgeons can attempt reoperation or, in some cases, use adjuncts to salvage the flap. Therapies such as leeches and heparin can aid in decreasing venous congestion and increasing venous outflow. The literature also includes descriptions of venocutaneous fistula creation for free flap salvage, particularly for cases when revision of the anastomosis is not possible. In this procedure, after evacuation of a thrombus, the surgeon may exteriorize the vein through the skin and allow for direct venous drainage and relief of venous congestion.

Ultimately, it is critical to closely monitor the flap to detect arterial or venous compromise early. ^, Clinical changes in the appearance of a skin paddle can provide important information to nursing. Doppler signals can be used to monitor perfusion to free flaps, as can more advanced technologies such as infrared thermography. Different means of perfusion monitoring have different effectiveness and reliability, there is also some consideration regarding invasive and noninvasive means of monitoring. Overall, in terms of reliability, there is evidence that adjuncts to clinical assessment have higher reliability. Cook-Swartz Doppler has high sensitivity and specificity as well as a true positive rate estimated at 80% and a salvage rate of 83%. Tissue oximetry is also a very reliable method with true positive rates of about 75% and salvage rates of about 88%. pH monitoring is also a physiologically relevant and accessible method for detecting vascular compromise in free tissue flaps, but its accuracy in clinical practice is not as well established. ^, Temperature monitoring is also a useful, low-cost adjunct for free flap assessment but is less sensitive and specific than other monitoring such as tissue oximetry or implantable Doppler.

Summary of methods for free flap perfusion monitoring. ^,,,,,,,

	Monitoring Technique	Description	Advantages	Disadvantages
Noninvasive	Clinical assessment	Physical examination of the flap	Versatile, does not require specific equipment	Subjective, delayed detection of compromise
	Doppler	Sound allows for the detection of blood flow and velocity	Ease of use, particularly when the perforator site is marked	User-dependent, limited data about partial or progressing compromise
	Laser doppler	Measuring Doppler shift of red blood cells	Quantifiable measurements, noninvasive	Limited by tissue motion or macroscopic blood vessels, less effective buried flaps
	Tissue oximetry	Continuous tissue oxygen saturation measurement	More sensitive and specific than Doppler and clinical examination	Less sensitive than more invasive methods
	Duplex echography	Combination of ultrasound and Doppler	Allows early detection of vascular compromise	Requires staff knowledge, limited by frequent monitoring
	MRI	MRI of free flap tissue	Detailed anatomic information	Costly and not always accessible, no real-time monitoring
	Fluorescence imaging	Injection of ICG and visualization under near-infrared light	Dynamic assessment of perfusion, sensitive and specific	Requires specialized equipment and training; repeated ICG injections are needed for continuous monitoring. Most effective for superficial tissues
	Temperature monitoring	Free flap temperature	Low-cost and reliable adjunct	Less sensitive and specific than other means of monitoring
Invasive	Cook-Swartz Doppler	Implantable Doppler probe	High true positive and salvage rate, prompt identification of compromise. Useful for buried tissue	Requires implantation during surgery
	Oxygen tension monitor	Implanted sensors measuring the partial pressure of oxygen	Early monitoring for vascular compromise, objective and quantifiable	Accuracy depends on probe placement
	Microdialysis	Measuring interstitial biological markers (glucose and lactate)	High sensitivity and specificity, objective and quantifiable. Prove viability earlier than other measurements	Requires insertion of a catheter into tissue, which can risk infection and tissue damage. Technically complex
	pH monitoring	Capillary blood pH	Tissue free flap pH responds quickly to perfusion changes	Accuracy in clinical scenario not well-established

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