R. Avery Bennett, DVM, MS, DACVS
Department of Veterinary Clinical Medicine, College of Veterinary Medicine, University of Illinois, Urbana, IL, USA
The avian beak is a unique anatomic and physiologic structure. It is subject to trauma and abnormal growth. Traumatic beak injuries can be closed primarily or managed open to heal by second intention. Because the beak and its bone in small birds is so thin, it is difficult to achieve primary healing. Birds adapt well, even if part of or the entire upper or lower beak is lost, and are generally able to eat and drink without assistance. Beak deformities occur secondary to trauma or, in baby birds, are developmental abnormalities. If caught early, the prognosis for young birds with deformities is good. Currently there are no reports of long-term success using any type of implants to affix prosthesis for birds that have lost a section of beak.
Anatomy and Physiology
The upper and lower beaks are composed of bone covered by a horny sheath called the rhamphotheca. The rhamphotheca is further differentiated into the rhinotheca (keratin over upper beak) and gnathotheca (keratin over lower beak). The tomia are the cutting edges of the beak. The anatomy is typical of dermal bone, as the dermis is attached to the periosteum and is covered by stratified squamous epithelium. The stratum corneum is particularly thick and very hard. The hardness is caused by an increase in free calcium phosphate and hydroxyapatite. Just like skin, there is a constant turnover of the keratin on the surface of the beak. The stratum germinativum produces daughter cells that migrate to the surface. Combined with the effects of transitional cells, a specific pattern of cell migration develops in columns at various angles toward the tomia. In addition, the growth pattern is generally from the base of the beak to the tip.
The bones of the upper beak include the premaxilla and nasal bones. The upper beak is hollow in most species containing the rostral diverticulum of the infraorbital sinus. The bone itself is quite thin, with the strength derived from the special keratin produced on the surface. In psittacines, the upper beak is attached to the skull by the craniofacial hinge, which is a synovial joint allowing the upper beak to move independent of the skull. The base of both the upper and lower jaw bones is covered by normal skin with or without feathers depending on the species. Birds may generate very high pressures when they bite, potentially exceeding 200 PSI in macaws.
Most traumatic injury to the beak can be classified into four types: simple fracture, depressed fractures, fractures with bone defects, and avulsion fractures. Beak injury can be difficult to treat because of some inherent problems. The bone is quite thin so bone purchase for fixation devices is limited. The blood supply to the dermal bone of the beak is tenuous. There is very little soft tissue coverage to protect the bone. Fractures are difficult to stabilize, especially considering the extreme forces they are subjected to, and they are often contaminated with a high potential for infection, which will affect healing.
Crush injury to the beak is most commonly associated with bite injury from another bird or another household pet. They are often associated with missing fragments and depression fractures. They are not only contaminated by environmental and endogenous flora, but also the bacterial flora from the animal that caused the injury. The bite can also be associated with vascular compromise from the crushing effects of the bite. This type of injury is best treated using temporary stabilization until the tissues are healthy. Treat the site as an open wound and apply stabilization after the tissues are healthy.
Simple clean fractures can be closed primarily if the tissues appear healthy and minimally contaminated. Successful healing depends on the ability to stabilize the fragments against the forces applied to them. Small patients are particularly difficult to stabilize adequately. Basic principles of fracture stabilization apply to fractures of the beak (rigid fixation, anatomic alignment, and early return to function). Methods of stabilization that have been applied successfully to bird beaks include pins and orthopedic wires, modified external skeletal fixation devices, and bone plates and screws.
Bone plates and screws have not been used much historically because the bone of the beak is so thin and has virtually no soft tissue coverage. The thin bone of the beak in most birds does not provide adequate screw purchase. Plates may be used in large birds with fractures of the caudal mandibular beak where the bone is thicker and covered by muscle and normal skin. Another potential application would be on top of the keratin surface of the beak. Used in this manner the plate acts more like an external fixator because the device is outside the body tissues, while the screws into the bone maintain reduction. Necrosis of the epidermis under the plate is likely but if the fracture heals quickly the bone will potentially reepithelialize.
Intramedullary pins and orthopedic wires are also difficult to apply to the avian beak because there is no distinct medullary canal within the flat bones of the beak and the paucity of soft tissue over the bone would result in wires being exposed. Interfragmentary wires alone are generally inadequate to stabilize and counter the forces acting on the fracture; however, they will hold fragments in opposition. One technique using pins and wires that works well in some beak fractures involves placement of a pin across the fracture and a figure-eight wire around the pins. Applied to the tension side of the fracture, this type of stabilization can result in fracture compression. This construct can be made even stronger by applying polymethylmethacrylate bone cement over the surface to hold the components rigidly in place.
External skeletal fixation is readily applicable to beak fracture stabilization. Because of the thin bone of the avian beak, fixation pin purchase is not good. It is best to place fixation pins through both sides of the beak using the unaffected side as an anchor for improved pin purchase and better fracture stabilization. This is generally not possible for caudal mandibular fractures because of the tongue. Threaded pins also offer improved purchase. Fixation pins are connected with acrylic cement on either the external and/or lingual surface of the beak. The Beak Repair Kit by Ellman International (Ellman International Inc, Oceanside, NJ) contains various splints and meshes along with acrylic cements useful for avian beak repair.
Beak bonding is analogous to dental bonding as done in mammals. The upper and lower beak are wired or cemented together using the unaffected beak to support the fractures. The beaks can be wired together or the Ellman splints and acrylic can be used to bridge between the beaks. An esophagostomy tube must be placed first and vomiting or regurgitation can result in life-threatening aspiration pneumonia.
Some fractures have devitalized or missing pieces of beak. The unhealthy tissue must be removed. With contaminated injuries, closure is delayed until the tissue is healthy. The area surrounding the defect is cleaned and roughed. A motorized rotary woodworking tool is especially useful for this. The acrylic is applied over the defect onto the clean, roughened rhamphotheca. The tissue will granulate and reepithelialize under the acrylic. Eventually, the acrylic patch will slough off due to the normal turnover of keratin, leaving a healthy beak below. If pieces are missing, the plastic mesh splint material works well to provide scaffolding over which the acrylic is applied. The rhamphotheca is cleaned and roughened as described above. The cement is used to secure strips of plastic splint over the defect onto healthy beak. Once the defect is covered with plastic mesh splint, acrylic is used to cover over the mesh and onto healthy beak.
A split mandible is a common beak injury occurring most often in small psittacine birds. The mandible splits at or near the rostral midline. This type of injury is very difficult to get to heal primarily. Most commonly, the two halves heal separately and never unite. The epithelium migrates over the fracture ends, resulting in two separate halves of the mandible. Unfortunately, the author has not been able to develop a technique that consistently results in fracture healing. Because of the poor prognosis for fracture healing, it is difficult to recommend orthopedic intervention. Fortunately, these small birds recover from this injury quickly and learn to eat with a permanently split mandibular beak. Some birds may need nutritional support until they are able to eat on their own. Pain medications are also indicated and may speed recovery.
Beak avulsion is another common traumatic injury seen with some degree of frequency. It usually occurs when one bird tears the beak off another bird at its base. Attempts to reattach the beak are uniformly unsuccessful. Most birds adapt and are able to function while missing either the upper or lower beak. They may require nutritional support, which may include an esophagostomy feeding tube, until they learn to eat with the defect. Though some birds will fail to adapt, it is worth giving them a chance. Many seem to do better when missing the mandibular beak.
Luxation of the palatine bone occurs primarily in macaws and is characterized by persistent hyperextension of the premaxilla. It appears that when the bird is startled and suddenly hangs from the upper beak, the palatine bone locks on the vomer bone preventing the bird from closing its mouth. Reduction is accomplished by inserting a Steinmann pin through the base of the premaxilla to use as a handle. The beak is hyperextended even more, freeing the palatine bone from the vomer bone. Downward pressure is applied to the pin and the upper beak gently closed down with respect to the lower beak. To prevent recurrence, the jugal bone is sutured to the infraorbital rim.
Scissor beak (lateral deviation of the premaxilla to the right or left) and mandibular prognathism (mandible longer than the premaxilla) are the two common types of beak deformity encountered in birds. Proposed etiologies include malposition in the egg, syringe feeding on only one side, genetics, and nutritional imbalances. Scissor beak tends to occur primarily in macaws, while mandibular prognathism occurs primarily in cockatoos. Physical therapy can be successful in correcting the problem if instituted early enough; when the bone is still pliable before it is well mineralized. It involves repeatedly and frequently applying pressure to oppose the abnormal growth. For example, if the beak is deviating to the right, pressure is applied on the right side toward the left side. For scissor beak, it is recommended the bird be fed consistently on the opposite side until the deviation is corrected.
If the bird is too old or physical therapy fails, intervention is required. There are two basic methods used to correct scissor beak, the acrylic ramp and the pin and rubber band techniques. With the acrylic ramp technique, a ramp is applied to the surface of the beak to apply force to the beak every time it closes, forcing it in the opposite direction, thereby straightening the beak. The gnathotheca is scarified to improve the bonding of the cement to the beak. Circular wires are applied to the gnathotheca to serve as anchors. The wire mesh splint is secured to the gnathotheca with cement. Acrylic is added to create a ramp high enough so the maxilla cannot slide over to the affected side. A rotary tool is used to smooth and contour the ramp, as well as to create a notch into which the premaxilla will rest. As an example, if the premaxilla deviates to the right, the ramp is created on the right side as well. Every time the beak closes the ramp forces the premaxilla to the left, toward the midline.
With the pin and rubber band technique, a Steinmann pin is placed through the base of the premaxilla. A hook is bent on the end of the pin on the side opposite to which the beak is deviated. A notch is created in the tip of the premaxilla and a rubber band is placed from the hook on the pin to the notch on the beak. The rubber band is secured with cement to prevent it from slipping off. The rubber band applies constant tension to the tip of the premaxilla correcting the deviation. This method usually corrects the deviation more quickly but must be monitored carefully as over-correction can occur.
Both methods used for scissor beak are applicable to prognathism as well. With mandibular prognathism, the acrylic ramp is built on the rostral end of the premaxilla so that it extends beyond the rostral aspect of the mandible. The same procedure is followed for application of the ramp. With the ramp on the rostral aspect of the premaxilla, when the beak closes, the premaxilla is forced more rostrally. As the beak grows, the premaxilla will grow out so that ultimately, it properly opposes the mandibular beak. Using the pin and rubber band technique, the pin is again placed at the base of the beak. The ends of the pin are bent rostrally and dorsally with hooks added. The rubber band is connected to the hooks and to the tip of the beak to apply tension to the tip of the premaxilla, correcting the malformation.
The younger the bird is the more rapidly the defect will correct. As the bird matures and the beak mineralizes it takes more time to correct the deviation. These devices must be closely monitored. Owners should also be educated to check the device to assure it is secure, functioning properly, and that the bird is developing properly. Once the beak is properly aligned the device must be removed to prevent over-correction. A rotary tool is used to bur away the acrylic cement to allow it to be removed from the bone. If wires were placed through the beak for added security, bur the cement to expose the wires so they can be cut and removed. The holes quickly granulate in and reepithelialize.
Prosthetic beak replacements are often considered for birds missing part of the beak. There are various reports of methods for creating beak prostheses. It would be ideal to be able to create a permanent prosthesis that would function normally for the life of the bird. Unfortunately, none of the supposed permanent beak prostheses can claim long-term success. The strong forces acting on any type of beak prosthesis will eventually result in it falling off. Prostheses applied to the surface of the beak will eventually be lost due to the natural turnover of the keratin surface. Prostheses implanted into bone will eventually fail due to cycling resulting in bone resorption and/or implant failure. If there is bone resorption, subsequent attempts to apply a prosthesis will generally fail more quickly because there is less bone stock. At the present time, there is no known permanent beak prosthesis applicable to birds.
Temporary prostheses may be used if needed. The best results are achieved using devices applied to the surface keratin, as they do not affect the remaining tissues. Unfortunately, they do not last long as they fall off when the keratin they are applied to sloughs. These prostheses also require that there be sufficient beak left to which the prosthesis can be applied. If the beak was avulsed and there is no base, there is nothing to which the prosthesis can be secured.
Prostheses secured with implants such as screws or threaded pins will generally last longer. Unfortunately, they result in bone resorption and are predisposed to infection because the implants are placed through a contaminated surface (the surface of the beak) into viable bone tissue.