In revision carpal tunnel surgery, incomplete release of the flexor retinaculum in the previous surgery and scarring of the median nerve to overlying structures are common findings [4]. Perineural fibrosis of the median nerve, which involves dense scar tissue surrounding the median nerve trunk, makes the median nerve adhere to the wall of the carpal tunnel. The synovial flap is indicated in cases in which relatively small perineural fibrosis in the carpal tunnel is identified at the time of revision surgery. A particularly good indication is perineural fibrosis at the palmar side of the median nerve, which makes the median nerve affix to overlying structures. After complete release of the median nerve, synovial flap coverage of the palmar side of the median nerve can be easily performed. These procedures provide reliable symptomatic relief with minimal additional dissection.

When the area of the perineural fibrosis of the median nerve is large or circumferential (affecting the dorsal and palmar side of the median nerve), the synovial flap is not large enough to cover the area of the median nerve adhesion. In these cases, larger flaps such as radial artery perforator adipofascial flaps should be considered. Other reasons for revision carpal tunnel surgery such as intraneural fibrosis and neuroma of the palmar cutaneous branch of the median nerve are relative indications for the synovial flap technique. Synovial flaps can be used in these cases to restore a good gliding tissue cover, to reduce the risk of adhesions of the median nerve, and to ensure neovascularization to improve nerve regeneration.

Contraindications of the synovial flap technique are cases that involve rheumatoid arthritis or synovial tumors in which pathological tenosynovitis causes carpal tunnel syndrome. In these cases, surgeons should perform a tenosynovectomy to prevent median nerve compression. If perineural fibrosis of the median nerve is identified, other flap techniques such as that involving the hypothenar fat pad flap should be considered to restore protective coverage of the median nerve.

In revision carpal tunnel surgery, the skin incision is made longer than the previous surgical scar, starting more proximally and distally, working toward the zone of previous surgery. For safe dissection, the median nerve should be identified in the areas without scar, i.e., proximal and distal to the prior surgical scar zone. After identifying the median nerve proximally and distally, dissection is performed into the carpal tunnel. Opening the carpal tunnel at the ulnar palmar side facilitates the identification of the median nerve without injuring the median nerve branches [21]. Careful dissection is important when moving from normal nerve to adhered nerve. Safe dissection through the scar tissue that has caused recurrent compression or traction neuropathy can be very challenging. External neurolysis of the median nerve is usually required in order to free the nerve from both compressive and traction forces. The complete visualization and release of all potential sites of residual compression is critical (Fig. 22.3). It is sometimes necessary to include other procedures such as neuroma excision and nerve grafting.

After careful dissection and mobilization of the median nerve from the scar tissue, coverage of the median nerve is performed with a synovial flap that can provide interposition and neovascularization to the scarred median nerve. A vascularized synovial flap can keep the median nerve within scar-free tissue and protect it against new scar compression. A wide flap of synovial tissue deep to the median nerve is elevated from the radial to the ulnar direction, utilizing the ulnar aspect as the vascularized pedicle because the synovial flap is consistently supplied by a vascular pedicle from the ulnar artery (Figs. 22.4 and 22.5). Because the vascular pedicle runs between the superficial flexor tendons of the ring and small fingers, the flap elevation has to be stopped over the tendon of the ring finger. The flap is then placed superficial to the median nerve, turned radially, and fixed to the radial wall of the carpal tunnel with absorbable stitches (Fig. 22.6). The advantage of this flap is that it provides vascularized interposing tissue to the median nerve through the same operation field without the need for extensive further dissection. The flexibility of the tenosynovial tissue can provide a wide flap to wrap the median nerve without excessive tension. Frequent intraoperative checks must be performed to ensure that the fingers are passively mobilized in flexion through extension. This motion must not cause any tension to the synovial flap because tension of the flap over the median nerve can cause new nerve compression. If tension is identified, further dissection of the synovial flap is necessary for flap mobilization and tension reduction.