A modified open annular ligament desmotomy followed by passive open drainage was used to treat 12 horses with chronic digital septic tenosynovitis due to trauma in the form of open wounds or self-sealing punctures. The surgical approach included complete transection of the palmar/plantar annular ligament of the fetlock and proximal digital annular ligament, which facilitated the removal of fibrin, selective debridement and synovectomy, followed by lavage of the digital sheath. The incision was partially closed leaving a 2 cm gap open distally to allow for passive open drainage. Gentamicin was administered intrathecally during surgery. Postoperatively the horses received ceftiofur and non-steroidal antiinflammatory drugs. A sterile bandage was used to protect the wound until the gap had healed completely by second intention. Infection was controlled in 11 of the horses which all returned to their previous levels of work, but one horse was euthanased as a result of the persistence of the infection and financial considerations.