1B. Non-modifiable risk factors

Age represents a non-modifiable risk factor associated with the development of blisters after TKA. Wound blisters can be classified as hemorrhagic or serous (Figure 1). Hemorrhagic blisters (HB) typically have a longer re-epithelialization time and are associated with severe trauma to skin cells. HB is associated with a sub dermic blood accumulation creating a cleavage plane between the epidermis and dermis. Differently, serous blisters (SB) are characteristically present with serous exudate accumulates in the epidermis layer, so these often have a shorter re-epithelialization time^9,13. SB have an average epithelialization time of 12 days, while HB require 16 days⁹. Aging is associated with loss of collagen and elastin in connective tissue, as well as vascular atrophy, compromising the competence of the dermis in cases of increased interstitial pressure. Thus, elderly patients undergoing TKA surgery have an increased risk of developing blisters following the procedure¹³.

2. Risk factors related to the surgical procedure

Tourniquet deflation at the end of the TKA is a critical moment for the development of postoperative blisters. In cases where the dressing is performed before releasing the tourniquet, blood flow return associated with dressing mechanical pressure might increase shear forces between the dermis and epidermis increasing the risk for postoperative wound blisters. Tourniquet release before applying the dressing was associated with a lower incidence of blisters after TKA^14,15. Antimicrobial adhesive (Ioban, 3M^®) is used to reduce the risk of infection after arthroplasty, but this benefit has as yet not been validated in the literature¹⁶. Improper removal of antimicrobial adhesive at the end of the surgical procedure can result in shear forces between the dermis and epidermis, which in turn may favor the development of post-operative wound blisters ¹⁷.

When planning the location of the surgical incision for TKA, surgeons must be aware that the anterior vascular supply of the knee runs from medial to lateral. A more lateral incision tends to preserve a better arterial blood flow to the anterior knee subdermal tissue, decreasing the risk of post-operative wound necrosis ¹¹. Subcutaneous dissection above the fascia preserves the subdermal vascularization of the anterior region of the knee, thus reducing the risk of superficial wound necrosis (Figure 2)¹¹. In patients with previous incisions of the knee, a distance of 5 to 7cm between them must be respected to avoid vascular compromise of the epidermis. Oblique incisions can be included in a vertical incision¹⁸. Surgical incision vertices should be V-shaped (V sign). In the case of incisions with the U-shaped vertices (U sign), the incision must be elongated avoiding excessive skin tension¹⁹. An appropriated preoperative plan of surgical incision can reduce the risk of superficial and deep surgical wound necrosis.

3. Management of acute wound complications

The presence of serosanguineous drainage 72 hours after the TKA is defined as a persistent wound drainage²⁰. In this scenario, the patient must not be discharged, and some additional postoperative actions should be taken. Restricted range of motion, compressive dressing, suspension of anticoagulation medication and negative pressure therapy (NPT) are alternatives in order to control persistent wound drainage. If drainage persists between 5-7 days postoperatively acute infection should be considered. Therefore, the patient must return to the operating room for irrigation, debridement, and procurement of samples for culture and biopsy ²¹. The presence of purulent discharge from the knee capsule or any communication with superficial layers implies the necessity for deep joint debridement and exchange of modular components. In this scenario the patient should not be discharged and associated intravenous antimicrobial therapy is mandatory until results of samples culture and biopsy are available²¹.

The management of a postoperative blister adjacent to a surgical wound remains controversial. When intact, the blister fluid is considered sterile. However, following rupture a local polymicrobial contamination was described during the epithelialization period^9,22. After blister rupture, an intravenous antimicrobial prophylaxis can prevent superficial surgical wound infection. The use of systemic antibiotics with topic silver sulfadiazine is suggested associated to a low-compressive dressing²³.

The treatment of patients with soft tissue necrosis adjacent to the surgical wound depends on the size of tissue damaged and any possible sign of local infection. Patients with superficial necrosis of the surgical wound must be immobilized and their range of motion restricted in order to reduce wound tension. Edges debridement with necrotic tissue removal must be performed using an aseptic technique. After this step, re- epithelialization can occur by secondary intention or surgical tension-free suture can be perfomed²⁴. NPT represents an adjuvant option to remove recurrent fluid, stimulating microcirculation and improving a local oxygen flow. Therefore, NPT is a useful additional option in patients who a complete wound suture was not achieved²⁵.

Local skin grafts are another reasonable surgical means for patients where tension-free suture was not feasible. To perform this technique, it is necessary to have an intact fascia and a soft tissue infection must be excluded²⁴. The ipsilateral superolateral thigh is a common source for harvesting a skin graft often providing one with an appropriate graft length. The donor site usually heals by second intention without

side effects. NPT can be used as an adjuvant method improving local granulation and reducing fluid accumulation²⁴.

The use of a fasciocutaneous flaps represents an alternative with less morbidity than muscle flaps. Cases of superficial wound necrosis where there has not been sufficient granulation tissue can be addressed using fasciocutaneous flaps. Post- operative success depends on the blood flow remaining after superficial and deep tissue layers after fasciocutaneous graft transplants. Fasciocutaneous flap originates from the posterior calf are based on the popliteal artery descending cutaneous branch, which may cover an area greater than 8 x 17 cm. A Doppler preoperative assessment should be performed considering anatomical variations described in up to one third of patients²⁷. Knee medial superficial necrosis can be addressed with the saphenous fasciocutaneous flap, supplied by the saphenous artery, a branch of the descending genicular artery. This technique can be performed to cover an area of approximately 6 x 10 x 15cm²⁸.

Deep wound necrosis, bone or prosthetic implant exposure are often associated with local bacterial contamination. In this scenario, the gastrocnemius muscle flap as a well vascularized graft provides local antimicrobial input in lesions limited to the proximal third of the tibia²⁹. The medial portion of the gastrocnemius muscle has an independent vascular supply (middle sural artery) and is longer and more mobile than the lateral portion. The rotational mobility of the medial gastrocnemius flap allows to cover wound necrosis located in the proximal third of the tibia, medial articular interline, medial tubercle and the patella, covering an area of up to 49 cm². A free skin flap is then transferred after granulation tissue been achieved^29,30.

The gracilis muscle flap is an option for reducing donor side morbidity and can be performed through the same surgical incision as the previous TKA, with an estimated length of 6 x 20 cm. Its vascularization comes from the medial branch of the medial femoral circumflex artery³¹. In cases of deep wound necrosis including the joint capsule, or extensor mechanism, muscle flaps from the medial vastus, lateral and gastrocnemius muscles can be considered. Musculocutaneous flaps to manage wound complications after TKA are described in Table 1.

Table 1. Musculocutaneous flaps options to manage wound complications after TKA. *Saphenous artery.

**Direct and independent vascularization.

Vascularized free flap transfer techniques are an option for cases where muscle or fasciocutaneous flaps are not appropriate or when other treatment techniques for surgical wound complications have previously failed. In this clinical scenario, the preoperative planning must consider skin thickness, size, contour of the region to be grafted, and the graft vascular supply³³. The latissimus dorsi free flap provides excellent coverage for the anterior region of the knee and can be used for extensive skin necrosis. Its vascular pedicle originates from the thoracodorsal artery, a branch of the subscapular artery. Thus, the vascular pedicle longer than 12 cm must be anastomosed proximally to the descending genicular artery, a branch of the superficial femoral artery³⁴.

The free miocutaneous rectus abdominals flap muscle is based on the pedicle of the deep inferior and superior epigastric arteries. Its flat shape represents a reasonable option based on the anterior knee anatomy. Possible donor side effects include abdominal muscle weakness or abdominal herniation.

For obese patients, other vascularized free flap options should be considered^35,36. Finally, a free vascularized flap from the lateral proximal region of the thigh with vascular pedicle from the lateral femoral circumflex artery, can also be used.³⁷ In cases with a small area to be addressed, the radial fasciocutaneous flap from the volar region of the forearm, vascularized by the radial artery, represents an additional option³⁸.

Conclusion

Early surgical wound complications after TKA should have a multidisciplinary approach. A microsurgery or a plastic surgery team is helpful to assist in the decision- making process. Careful preoperative assessment and intraoperative management potentially decreased risks of postoperative wound and potential side effects. A prolonged hospital length of stay leads to patient psychological and social issues and increases complications public and private health system costs.

Conflict of interests

The authors declare no conflict of interest.

Our acknowledgements and tribute to the late Dr. Victor Favilla who was a member of our staff and had suggested this manuscript.