FRACTURES OF THE RADIUS AND ULNA SECONDARY TO POSSIBLE VITAMIN D DEFICIENCY IN CAPTIVE POLAR BEARS (Ursus Maritimus)
Emmanuel Engeli, Med. Vet.; Rebecca C. Lin;
Laurie R. Goodrich DVM, MS, DACVS; Allan W. Prowton DVM*
Cornell University Hospital for Animals, College
of Veterinary Medicine,
Cornell University, Ithaca, New York
*Zoological Society of Buffalo, Buffalo, New York
Fractures in captive polar bears are not uncommon. A recent informal survey revealed a total of eighteen fractures in ten different captive polar bears in the United Statesi. The survey results, however, are not exhaustive; thus, the true prevalence of fractures may be even greater. Of the eighteen fractures, ten fractures involved the radius or ulna; four animals had concurrent radial and ulnar fractures and the remaining two were singular radial fractures in two animals. Six of the ten bears with fractures were male, two were female, and in two cases no information on sex was available. The majority (60%) of cases occurred in cubs (bears less than three years of age). Causes of fractures included falling into moats, injury while playing in the exhibit, and slipping on ice.
To our knowledge, there has been no case report published on fracture repair in bears of any species. The purpose of this study was to compare cases of antebrachial (radius and ulna) fractures in captive polar bears and to identify any common etiologic factors among the cases. Of the six polar bears with antebrachial fractures, medical records for only four bears were available.
Case 1: An eight-month-old female cub (86 kg) at Seneca Park Zoological Society (SPZS) was observed to slip and fall between two rocks while playing with her twin brother in the exhibit. Thereafter she was non-weight-bearing lame in the right forelimb. Physical examination revealed crepitus and abnormal motion in the right mid-antebrachium. Radiographs of the right forelimb revealed a closed, complete, comminuted and displaced short-obliqued mid-diaphyseal fracture of the radius with a butterfly fragment at the cranial aspect of the bone and a distal diaphyseal fracture of the ulna with a similar configuration as the radial fracture. The fractures were reduced and stabilized by internal fixation. Separate surgical approaches were necessary for each bone. The radius was repaired with a broad dynamic compression plate (DCP) and 2 cerclage wires; the ulna was stabilized with a narrow DCP. Post-operatively, the cub was confined to a squeeze cage for approximately 9 days, followed by a small stall, where she paced considerably. Radiographs taken one month post-operatively revealed that the ulnar plate had broken and the radial plate had shifted. Bony remodeling and callus formation were evident. The bear was again confined to a squeeze cage for another month, followed by 1½ months in a small pen, before slowly being reintroduced to her family in the exhibit (3 ½ months post-operatively). Synostosis (bony bridging) between the radius and ulna developed, and the bear was observed to be intermittently lame in the repaired limb. The bear retained good range of motion in both the elbow and carpus with mild limitation of pronation and supination.
Case 2: A one-year-old male cub (88 kg) was found non-weight-bearing lame in the right forelimb at the Zoological Society of Buffalo (ZSB). Physical examination revealed crepitus and abnormal motion in the right mid-antebrachium. Radiographs of the right forelimb revealed a closed, complete, comminuted and moderately displaced medial to lateral short-obliqued proximal diaphyseal fracture of the radius with a large cranial and medial butterfly fragment and a closed, comminuted, cranial to caudal obliqued mid-diaphyseal fracture of the ulna with a cranial butterfly fragment. The fractures were reduced and stabilized by internal fixation through two separate surgical approaches. The radius was repaired with a broad DCP and 2 cerclage wires; the ulna was stabilized with a narrow DCP. A full-limb fiberglass cast was applied. The cub recovered in a squeeze cage and remained there until cast removal 5 days post-operatively. The bear was then confined to a small pen with a pool for 3 months. Post-operative radiographs confirmed good alignment and adequate healing of both bones with synostosis formation proximally. Both bone plates were removed 4 months after the initial surgery to free the bridged distal radial physis, but the synostosis could not be removed surgically. The bear made a full recovery with good range of motion in the affected limb and mild limitation of pronation and supination. Five months after the fracture, the cub returned to the exhibit. The bear died at 13.5 years due to cardiac disease (myocardial fibrosis).
Case 3: An eighteen-year-old sow (155 kg) also kept at the ZSB was pushed into a moat by a male bear. She was subsequently non-weight-bearing lame in the left forelimb. Physical examination revealed moderate swelling with mild crepitus over the left mid-antebrachium with minimal instability. Radiographs revealed a non-displaced greenstick fracture of the left distal radial diaphysis. A full-limb Plaster of Paris cast reinforced with wire mesh was applied. This cast was completely destroyed by the bear within the first 48 hours. Thereafter, the bear was confined to a small cage without any external coaptation for 3 months before gradual reintroduction to the exhibit. She made a full recovery with full range of motion in the left forelimb. At age 23, the bear was shot and killed by police attempting to rescue a trespassing teenager in the exhibit.
Case 4: An eleven-year-old male polar bear (340 kg) at Baltimore Zoological Society (BZS), jumped into a partially filled pool and emerged lame on the right forelimb. Physical examination revealed swelling, crepitus and abnormal motion in the right mid-antebrachium. Radiographs of the right forelimb revealed a closed, comminuted short-obliqued mid-diaphyseal fracture of the radius and a comminuted short-obliqued fracture of the distal ulnar metaphysis. Two separate surgical approaches were necessary to reduce and stabilize the bones by internal fixation. The radius was repaired with a human tubular bone plate and a femoral bone plate; the ulna was stabilized with a human T-shaped bone plate. The limb remained casted for 11 weeks (full-limb casts, 3 cast changes) and the bear was confined to a small pen for this period. After cast removal, the bear returned to the exhibit and developed a chronic, low-grade lameness in the affected limb with transient carpal swelling and moderately decreased range of motion in that carpus. Radiographs revealed carpal (antebrachiocarpal joint) osteoarthritis with a small distal ulnar subchondral bone defect. The lameness was managed with non-steroidal anti-inflammatory medication and chondroprotective supplements.
Confirmed or suspected trauma was the cause of all the fractures reported in this case series. It was hypothesized that a certain percentage of these fractures could be due to decreased mineral density in the affected bones. Because these are captive animals on strictly formulated and regulated diets, the possibility of vitamin or mineral deficits, in particular Vitamin D, calcium and phosphorus, should be considered. Vitamin D promotes absorption of calcium and phosphorus from the intestine and mobilization of calcium from the boneii. In many animals, Vitamin D is obtained via cutaneous production pathways as well as through the dietiii. It is thought that polar bears, however, obtain Vitamin D solely through oral intake and that synthesis of this vitamin in the skin is minimaliv. Decreased Vitamin D intake or availability combined with lack of exercise (captivity) could lead to decreased bone density and strength and predispose to fracture (osteomalacia).
Serum 25-OH-Vitamin D, a metabolite of Vitamin D, can be measured to evaluate body stores of Vitamin D.v,vi Serum 25-OH-Vitamin D levels were measured in Cases 1, 2 and 4 and in five other polar bears (three additional from SPZS and two additional from the ZSB). No serum was available for Case 3 (lived from 1956 – 1979). Case 4 from BZS was the only animal to show normal levels (463 – 600 nmol/L in 3 different samples). The remaining tested animals (seven) had subnormal (85 – 125 nmol/L) or low-normal levels (one bear, 136 nmol/L) of 25-OH-Vit D compared to the published range in free-ranging polar bears (135 – 870 nmol/L)4. Sub-normal levels of 25-OH-Vit D were therefore identified in 2 of the 3 bears (Cases 1 and 2) with fractures that were tested.
Although only two of the four cases described in this case series involved cubs, over half (60 %) of the total number of animals with fractures (10 animals) were cubs1. It was hypothesized that cubs may be more frequently affected by fractures because of their immature bones and youthful play. Like all growing animals, polar bear cubs must have high Vitamin D requirements in order to provide the large amounts of calcium and phosphorus needed for rapid skeletal growth. The major sources of Vitamin D for free-ranging polar bear cubs are maternal milk and seal blubber. Initially, milk is the most important source of Vitamin D with gradual transition to seal blubber toward 2 ½ years of agevii.
The likely reason for the frequency of fractures of the radius and ulna (6 out of 10 bears, 10 out of 18 fractures) is anatomic. In the ursids as in other quadripeds, a greater percentage of total body weight is borne by the forelimbs (due to the weight of the head and neck) than the hindlimbs. Thus, while falling or stumbling, a bear is more likely to injure a forelimb. Furthermore, the antebrachium is the most vulnerable region of the forelimb skeleton due to the thinness of the radius and ulna and the relative lack of protective soft tissue surrounding these bones. Their mobility (pronation and supination) also renders them more vulnerable. Of note is the fact that three out of the four cases in our study involved the right forelimb. This finding raises the possibility of handedness in polar bears, as it has been shown in other speciesviii,ix,x.
Concurrent and complete fractures of the radius and ulna should be repaired by internal fixation (bone plates). Synostosis formation is a possible sequel to callus formation in growing bears and can result in elbow subluxation and lameness. Immobilization of the forelimb with a cast can provide adequate support for a single, non-displaced radial or ulnar fracture because of the splinting provided by the remaining intact antebrachial bone. Some bears, however, may not tolerate a cast and can heal with prolonged confinement to a small pen.
In conclusion, low Vitamin D levels possibly secondary to inadequate intake may predispose captive polar bears, and cubs in particular, to fractures. Vitamin D supplementation may be beneficial in contributing to prevention of fractures in captive polar bears of all ages. Additional research is warranted to investigate the relationship between fracture disease and nutrition in captive polar bears.
Acknowledgments:
Mary Denver, DVM, Baltimore Zoological Society
Norm G. Ducharme DVM, MS, DACVS, Cornell University Hospital for
Animals;
Peter A. Graham BVMS, PhD, MRCVS, Animal Health Diagnostic
Laboratory, Michigan State University
David E. Kenny VMD, Denver Zoological Gardens
Roy R. Pool, DVM, PhD, DACVP, College of Veterinary Medicine,
Texas A&M University
Tim Reichard, DVM, MS, Toledo Zoological Society
Jeff Wyatt, DVM, MPH, Seneca Park Zoological Society