Isolated dislocation of the pisiform bone is a relatively rare injury. Pisiform is a sesamoid bone that is formed in the flexor carpi ulnaris (FCU) tendon, which continues distally as pisohamate and pisotriquetral ligaments ^[1]. A forceful pull of the FCU or sudden hyperextension of the wrist can result in pisiform dislocation ^[2]. Here, we present a case of isolated dislocation of pisiform following an assault with a knife. There was an injury to the ulnar neurovascular bundle. The mechanism of injury causing a fracture and neurovascular injury makes this case report unique in the literature.

A 34-year-old male was assaulted with a knife which he tried to avoid with his left hand. In the process, he sustained a cut lacerated wound over his left wrist which was extending into the palm. There was profuse bleeding from the site of injury; however, there were no other injuries. The patient was rushed to the orthopedic casualty where the wound was washed and packed, and later, radiographs were ordered. The lateral view of the left wrist showed a complete dislocation of the pisiform bone (Fig. 1). The patient was taken up for surgery after a few hours. The surgical team comprised of a microvascular surgeon and an orthopedic surgeon. Surgery was performed under general anesthesia after applying a tourniquet. Wound exploration revealed that there was injury to both the ulnar vessels. The injury to the ulnar nerve was in zone 1 of the Guyon’s canal. The cut ends of both the ulnar nerve and ulnar artery were identified. There was an associated fracture dislocation of the pisiform bone; however, there was no injury to the FCU tendon (Fig. 2). After performing a thorough debridement of the wound, the pisiform bone was fixed back into its position using a 1.8 mm k wire. Later, repair of the ulnar nerve and artery was performed by the microvascular surgeon. Tension-less repair of both the ulnar artery and the nerve was done using 8-0 ethilon. Skin edges were sutured back and the wrist was immobilized in full volar flexion using a below elbow slab.

The slab was removed after 2 weeks and full wrist movements started. K wire was removed after 4 weeks (Fig. 3). The patient regained full wrist and finger movements. There was no intrinsic claw deformity. The patient reported no sensory deficits in the ulnar nerve distribution. Power in the interossei was more than M4 (Fig. 4). Grip strength was 60% when compared to the other side and the DASH score at the end of 1 year was 4.42.

Pisiform is a sesamoid bone formed in the FCU tendon. FCU is the strongest muscle in the wrist whose tendon’s function is enhanced by the pisiform ^[3]. FCU tendon continues distally as pisotriquetral and pisohamate ligaments ^[4]. The ulnar border of the Guyon’s canal is formed by the pisiform bone which transmits the ulnar neuro-vascular bundle ^[5]. There are two mechanisms of pisiform dislocation described in literature. One is the direct forceful pull of the FCU resulting in the tear of pisohamate and pisotriquetral ligaments and causing the pisiform dislocation. The other mechanism is a sudden dorsiflexion of the wrist resulting in the stretching of FCU tendon and thus causing pisiform dislocation ^[2]. We present a rare mechanism of pisiform dislocation in our case. A 34-year-old male suffered an assault injury with a knife over his right wrist. The injury sustained was along the ulnar border of the wrist and palm. The injury was deep and it resulted in the transection of ulnar vessels. Ulnar nerve was also cut. Considering the sharp nature of the injury and minimal contamination, a primary repair of the neurovascular structures was planned. There was associated fracture dislocation of the pisiform bone. No other carpal bones were injured. The avulsion of pisiform had to be fixed before repairing the neuro-vascular structures. The pisiform bone was reduced and held in its position with a 1.8 mm k wire. Various treatment options have been described in the management of pisiform dislocation. Some authors prefer conservative management ^[6] while others have tried excision of the pisiform bone with FCU repair. Petrou et al. described excision of pisiform bone without FCU ligamentoplasty ^[7]. K wire fixation is also an option to keep the pisiform bone in place until the soft tissues have healed [8, 9]. In our case, the pisiform was dislocated with associated neurovascular injury. Once the bone was held in place with a k wire, the neurovascular structures could be easily repaired. The wound healed without any complications and the k wire was removed after 4 weeks. Ulnar nerve divides into sensory and motor branches in the Guyon’s canal. Based upon this division, Gross and Gilberman divided the Guyon’s canal into three zones ^[10]. Zone 1 comprises the nerve before division, Zone 2 involves the deep motor branch, and Zone 3 surrounds the superficial branch. Kokkalis et al. reported a series of 32 cases of post-traumatic ulnar nerve transaction in the Guyon’s canal ^[11]. In their series, they reported excellent result following the repair in 96% of the cases. In our case, the nerve was injured in zone 1. Good tension-free repair was achieved using 9-0 nylon sutures. Post–operatively, the patient had no intrinsic claw deformity. Power in the interossei was more than M4 and there were no sensory deficits. ^[12].

Isolated pisiform fracture dislocation associated with neurovascular injury is a rare presentation. Treating the fracture dislocation conservatively while repairing the neurovascular injury would not be sufficient to aid in improving the functionality post-trauma. Fixation of the fracture along with repositioning of the bone is of paramount importance in such injuries.

Pisiform fracture dislocations are a rare entity of trauma which are usually missed due to their uncommon presentation. Pisiform though is a sesamoid bone, it is embedded within the FCU tendon providing a smooth surface for it to glide over and also attachment for the adductor digiti minimi. Thus, if left untreated, it may cause impaired functioning at the wrist though it does not directly take part in formation of the wrist joint. Therefore, proper clinical examination is of paramount importance. Obtaining an additional oblique radiograph along with the standard radiograph may help in diagnosis.