Abstract

Congenital bifid sternum (cleft sternum) is an uncommon defect previously reported only in humans.^1,2,4,5 Superior, inferior, and complete cleft sterna have been described with inferior cleft considered the most rare.¹ Approximately half of the patients presenting with bifid sternum have clinical signs.⁵ Clinical signs occur as a result of respiratory compromise secondary to the paradoxical movements of the skin and viscera (usually the heart) through the defect during respiration and include cyanosis, dyspnea, and recurrent respiratory infections. This can be a life-threatening condition in human patients. In patients without clinical signs, indications for surgical repair include providing protection of the subcutaneously located heart, and cosmetics as the view of the protruding, pulsating heart is rather disconcerting.^2,5,6 In humans, surgical management within the first 6 weeks of life is recommended in order to close the defect while the chest and lungs are still relatively compliant. In older patients, the use of costal chondrotomies, osteotomies, and artificial implants with Marlex or Teflon mesh, or acrylic plates has been described. This report describes the surgical repair of inferior bifid sternum in two unrelated African grey parrots (Psittacus erithacus) using bilateral myocutaneous pectoralis muscle transfer.

Case 1

A 7-month-old African grey parrot of unknown gender weighing 370 g was referred from Salem, OR to the Veterinary Medical Teaching Hospital at the University of Florida for surgical treatment of inferior bifid sternum. The bird was sent air cargo and arrived without consequence. On physical examination the bird appeared healthy in all respects except for the presence of an inferior bifid sternum. The keel had fused cranially but was separated about 1 cm caudal to the cranial apex of the sternum. The distance between the halves of the sternum was 2 cm at midsternum and 3 cm at the caudal extent of the sternum. The cardiac movements were clearly visible subcutaneously and the skin over the heart was eroded. Pre-operative complete blood count (CBC) and plasma biochemistry values were within reference ranges.

The bird was anesthetized with isoflurane in oxygen delivered by mask. Once anesthesia was induced the patient was intubated and maintained on isoflurane in oxygen. An IV catheter was placed in the medial metatarsal vein and LRS was administered at 10 ml/kg/h. A single dose of cefazolin (20 mg/kg) was administered IV.

The bird was prepared for aseptic surgery and an incision was made from 1 cm caudal to the cranial apex of the sternum to 2 cm caudal to the defect along the midline. The coelomic viscera (heart and liver) were directly dorsal to (beneath) the skin overlying the defect and were visualized through the incision. The skin was carefully elevated from the pericardium. The insertion of the pectoral muscles was incised along the keel and the muscles were elevated off the keel on each side using a periosteal elevator. Where the sternum was cleft a portion of the keel (perpendicular to the sternum) was present on each side. Since this vertical section of bone would inhibit the muscle transposition it was removed using bone cutters. The pectoral muscles were then elevated off the sternum progressing laterally continuing the dissection over the ribs. The superficial and deep pectoral muscles were not separated but, rather, elevated together as a unit with the skin. The thickness of the combined pectoral muscles was 1.5 cm.

Once the pectoral muscles were adequately mobilized on both sides they were sutured together along the midline using 3-0 polyglactin 910 (Vicryl) in a simple interrupted pattern. Tension on the sutures was minimal. The skin was closed with 3-0 nylon in a simple interrupted pattern.

Recovery was uneventful and the bird was given butorphanol at 1 mg/kg IM q 4 h as a postoperative analgesic for 24 hours. The bird was housed in a small cage to minimize wing activity which might have stressed the pectoral muscle sutures. Skin sutures were removed 2 weeks postoperatively and at that time cardiac movements were not visible.

Case 2

A 5-year-old African grey parrot of unknown gender weighing 340 g presented to Sonora Veterinary Surgery and Oncology for surgical repair of an inferior bifid sternum. Several attempts had previously been made to close the sternal cleft; the most recent effort used Marlex mesh in an attempt to cover the defect. Unfortunately, the skin over the mesh became devitalized and the mesh was grossly visible protruding through the skin. The cardiac movements were visible beneath the mesh.

The bird appeared healthy in other respects. Preoperative CBC and plasma biochemistry panel values were within reference ranges. The anesthetic and surgical procedure used in this bird were analogous to that described above. The Marlex mesh was removed following skin incision and the soft tissues surrounding the mesh were debrided.

Recovery was uneventful and the bird’s wings were bandaged in an effort to prevent wing-flapping which would stress the suture line by increasing tension in the pectoral muscles. The bird returned for suture and wing bandage removal 2 weeks postoperatively. The incision had healed and the cardiac pulses could no longer be visualized.

Discussion

Congenital bifid sternum is an uncommon defect reported in humans with superior (cranial) cleft being the most common and inferior (caudal) cleft sternum being the most uncommon.^1,2,6 Embryologically, avian sternal development parallels mammalian development.^1,3 In humans, the sternal bars develop at the sixth week of gestation. The sternal bars fuse from cranial to caudal with fusion being complete at the ninth week.^1,6 The manubrium develops differently which may account for the higher incidence of superior cleft sternum in humans.^1,2

Respiratory compromise as a result of the paradoxical chest wall movement in the area of the defect accounts for the cyanosis, dyspnea, and recurrent pneumonia in clinically affected individuals.⁶ Birds do not rely on a negative intrapleural pressure for movement of air through the respiratory tract and are physiologically unaffected by bifid sternum. Other indications for surgery include cosmetics and to provide protection for the heart.^2,5,6 It is unnerving to visualize the bird’s heart beating immediately beneath the skin. The bird in case 1 had skin erosions over the heart creating concern that, left unattended, the heart might eventually migrate through the skin with fatal consequences.

Early repair of bifid sternum is recommended in humans as the chest and lungs are more compliant in neonates. The surgical procedure is much more complicated when the patient is greater than 2 months of age requiring extensive orthopedic reconstruction and synthetic implants.^2,4,6 In both cases reported here, the sternum was ossified and not mobile enough to allow the halves of the keel to be approximated. Additionally, because of the air sac system, if the halves had been approximated, respiratory compromise may have occurred by decreasing the air sac volume. Because the pectoral muscles in birds are large in comparison to mammals, they provided a thick muscle pad to protect the heart from exterior trauma. For this reason, it was determined that a bony plate was not necessary. The keel (vertical portion) had to be removed in the area of the defect to allow the muscles to slide together over the defect to the midline. This section of bone was easily removed without complication.

Meshes have been used to repair bifid sternum in older human patients where it was not possible to reconstruct the sternum due to patient age. Repairs using mesh are more prone to complications and avoidance of the use of implants is a major reason early surgical intervention is recommended in humans.² Birds have relatively little subcutaneous tissue to support a synthetic material. This is likely the cause of its failure in case 2.

This report describes a previously unreported congenital defect in birds-inferior bifid sternum-and its successful surgical management. In birds it appears that the defect is not associated with clinical signs and surgery is recommended in order to protect the heart from exterior trauma.

Note

Dr. Bennett has recently been contacted about a grey cheeked parakeet (Brotogeris pyrrhoptera) with inferior bifid sternum.

Literature Cited

1.  Eijgelaar A, Bijtel JH. Congenital cleft sternum. Thorax. 1970;25:490–498.

2.  Greenberg BM, Becker JM, Pletcher BA. Congenital bifid sternum: repair in early infancy and literature review. Plast Reconst Surg. 1991;88(5):886–889.

3.  Hanson FB. The ontogeny and phylogeny of the sternum. Am J Anat. 1919;26:41–115.

4.  Knox L, Tuggle D, Knott-Craig CJ. Repair of congenital sternal clefts in adolescence and infancy. J Ped Surg. 1994;29(12):1513–1516.

5.  Krontiris A, Tsironis A. Bifid sternum - successful repair by use of an acrylic plaque: report of a case. Thor Surg. 1964;41(3):301–307.

6.  Jewett TC, Butsch WL, Hug HR. Congenital bifid sternum. Ped Surg. 1962;52(6):932–936.