Distal Femoral Fracture Repair with a Modified Cobra Head Plate in a Roan Antelope (Hippotragus equinus)
A 6-month-old, 78.72-kg, intact female roan antelope was examined for an acute nonweight bearing lameness of the left rear leg after colliding with a wall. Prior to the injury, the antelope was maintained with two other conspecifics in an isolation area for preshipment testing. The diet consisted of a high-quality pelleted feed with 17% protein, ADF 15.5% (Browser®, Mazuri, PMI Nutrition International, PO Box 66812, St. Louis, MO 63166-6812 USA), and water and timothy grass hay ad lib.
To facilitate radiography, physical examination and diagnostic sample collection, 0.01 mg/kg carfentanil citrate (Wildnil®, Wildlife Laboratories, Fort Collins, CO 80524 USA) and 0.25 mg/kg xylazine hydrochloride (Rompun®, Bayer Corp Agriculture Division, Animal Health, Shawnee Mission, KS 66201 USA) were administered using a remote drug delivery dart. Once the animal was in sternal recumbency, 1.25 mg/kg ketamine hydrochloride was administered IV (Ketaset®, Ft. Dodge Laboratories, Overland Park, KS 66210 USA) for additional chemical restraint. The animal was transported to a small, high walled box stall for continued observation and treatment. Following examination and diagnostic sample collection, immobilization drug reversal was accomplished with naltrexone, 0.66 mg/kg IV and 2.0 mg/kg SC (Trexonil®, Wildlife Pharmaceuticals Inc, 1401 Duff Drive, Suite 600, Fort Collins, CO 80524 USA), and yohimbine (Yobine®, Lloyd Inc. 604 West Thomas Avenue, PO Box 86, Shenandoah, IA 51601 USA) 0.75 mg/kg IV. A single lateral radiograph revealed a Salter-Harris type 2 fracture of the distal femur. Complete blood count evaluation revealed a leukocytosis (11,500 cells/µl), heterophilia (63%, 7200 cells/µl), and monocytosis (5%, 600 cells/µl) consistent with a stress leukogram.
Due to the value of a female roan antelope with reproductive potential, surgical fixation with a reverse cobra head plate was elected. Presurgical medications administered included flunixin meglumine, 0.5 mg/kg SID IM (Banamine®, Schering-Plough Animal Health Corp. Union, NJ 07083 USA) and ceftiofur sodium, 1.1 mg/kg IM SID (Naxcel®, Pharmacia & Upjohn Co. Animal Health Division, 7000 Portage Road, Kalamazoo, MI 49001 USA) for the 2 days prior to surgery. Anesthetic induction was accomplished using the same carfetanyl/xylazine combination administered by dart as previously described. Once laterally recumbent the antelope was intubated and maintained on isoflurane (IsoFlo®, Abbott Labs, North Chicago, IL 60064 USA) in oxygen.
A caudal epidural consisting of 9.4 mg/kg of bupivicaine hydrochloride (Marcaine®, Abbott Labs, North Chicago, IL 60064 USA) and 0.1 mg/kg of morphine sulfate (Astra USA, Inc. Westborough MA 01581 USA) was administered for intra and postoperative regional analgesia. The animal was placed in right lateral recumbency on a padded table. Lactated Ringer’s solution (Abbott Labs, North Chicago, IL 60064 USA) was administered IV during surgery and the heart rate was monitored via ECG and auscultation, pulse quality was monitored via palpation and SpO2 was monitored via pulse oximetry. Body temperature was measured per rectum and respirations were visually observed.
The affected limb was suspended and traction applied while the area of approach was prepared for aseptic surgery. A curved lateral skin incision centered over the distal half of the femur was made similar to that described for femoral fractures in foals.3 A fascial incision was made and of the insertions of part of superficial gluteal and biceps femoris muscles were incised. These muscles were elevated to provide exposure to the fracture sufficient to accommodate a cobra head plate (Synthes USA, PO Box 1766, 1690 Russel Road, Paoli, PA 19301 USA). The overriding fracture was reduced using Association for the Study of Internal Fixation (ASIF) reduction forceps and Hohman retractors with some difficulty. The plate was positioned on the lateral aspect of the femur with the cobra head of the plate over the epiphysis. Cortical screws, 4.5 mm, (Synthes USA) were used to secure the plate to the bone. Two proximal screw holes and one screw hole on the flared head were not utilized due to their proximity to the bone edge, fracture site and joint. After placement, the screws were loosened and the area under the plate was filled (luted) with polymethylmethacrylate (PMMA) (Surgical Simplexâ P, Radiopaque Bone Cement, Howmedica Incorporated, Rutherford, NJ 08540 USA) impregnated with 2 g of cefazolin (Bristol-Myers Squibb, Princeton, NJ 08540 USA). Prior to the PMMA curing, all screws were secured tightly to the bone.
The surgical approach was closed routinely. A stent bandage was sutured in place over the surgical site to provide protection of the incision from contamination. After an uneventful recovery, the antelope was treated for an additional 2 days with flunixin meglumine (0.5 mg/kg IM SID). The animal was maintained on antibiotics for an additional 34 days postoperatively. Six days postoperatively the animal was restrained with carfentanil/xylazine administered via dart and the stent bandage was removed. The incision appeared to be healing well and postoperative radiographs revealed adequate bone apposition with no apparent implant loosening. Eight weeks postoperatively the animal was released to the herd enclosure with other roan antelope at which time no lameness could be observed at any gait. Sixteen weeks postoperatively the animal still had no apparent lameness when transferred to the Oregon Wildlife Foundation in Sheridan Oregon. No radiographs were made after those made 6 days postoperatively; however, 9 months following the surgery, no apparent lameness, angular deformity, draining tracts or other problems associated with the affected leg have been observed in this animal.
Classically, compression plating is the preferred method of repair for long bone fractures in bovids and equids.1 Unfortunately, this method of fixation generally does not provide acceptable results in the repair of fractures with a short fragment as the number of screws that can provide purchase to hold the plate in place is usually inadequate. The cancellous nature of metaphyseal and epiphyseal bone fragments in such fractures provides minimal purchase for fixation devices.
Purchase can be further limited by periarticular soft tissues such as joint capsular, ligamentous, tendinous, and musculotendinous attachments as well as by the active cartilaginous growth plate.1 The cobra head plate (Synthes USA) is designed for treatment proximal fractures of the human femur. The plate has an expanded end resembling the shape of a cobra’s head. This flared end allows for placement of six screws within a relatively small area; however, in this antelope, only five screws were used. The original configuration for humans is not applicable to animal fractures. Modification of the plate including reversal and machining of the dynamic compression holes from the opposite side makes this plate suited for use in distal radial and humeral fractures of hoofstock. Additional benefits of the plate include a strong profile and no need for instrumentation other than that used for applying a dynamic compression plate.2 This plate has been used successfully in distal radial and humeral fractures in both equids and bovids.1 As a general rule, it is not recommended that a dynamic compression plate be applied across an active growth plate. In equine and bovine fracture management it has been shown that even if the plate causes physeal closure, the animal compensates for the discrepancy in length between the femora.2 This case represents the first known apparently successful use of this fracture fixation method in a nondomestic species.
The authors would like to thank Dr. Julie Smith for anesthetic consultation regarding this case and the staff of the Baton Rouge Zoo for their support and cooperation in this project.
1. Kirker-Head, CA. Use of the cobra head plate for distal long bone fractures in large animals. Vet Surg. 1989;18:227–234.
2. Auer J, Stick JA. Equine Surgery. 2nd ed. W.B. Philadelphia, PA: Saunders, Co.; 1999;658–659.
3. Milne D, Turner AS. An Atlas of Surgical Approaches to the Bones of the Horse. Philadelphia, PA: W.B. Saunders, Co.; 1979;154–159.