Keywords
locking plates - radius - ulna - rabbit - fracture
Introduction
Bone fractures are an important and common problem in the domestic rabbit (Oryctolagus cuniculus); however, there is little research available on methods of treatment.[1]
Causes of fractures in the rabbit occur from a variety of reasons including handling
errors, neoplasia and cage injuries; however, the majority occurs with no known cause.[1]
[2] Radial and ulnar fractures are recognized as being one of the more prevalent long
bone fractures making up 17% of all long bone fractures in the rabbit.[2] Fractures in the radius and ulna are frequently present in the distal and diaphyseal
regions of the bone.[1]
[2]
Fractures in the radius and ulna have been treated by external skeletal fixators,
transfixation pin splinting, external coaptation and amputation.[3]
[4]
[5] External skeletal fixator has been the most common treatment option in rabbit long
bones due to the brittle nature of these bones making them historically difficult
to surgically repair otherwise.[6]
[7] Moreover, the absence of plates and screws necessary for internal fixation of thin,
short bones such as the radius and ulna renders treatment of these fractures difficult.[1]
[3]
[5]
Previously, clinical reports in the use of internal fixation with plating have been
limited to one tibia and four femurs where only one minor complication was reported.[3]
[8] Furthermore, tibial fracture models repaired with bone plating have shown greater
strength and improved bone healing speed in relation to external skeletal fixator.[9] The recent availability of 1.5 mm double-threaded locking adaptation plates (LAPs)
has allowed for treatment of radial and ulna fractures in toy and miniature breed
dogs with encouraging outcomes.[10] These plates offer several advantages to conventional compression plates.11 These advantages include reduced need for plate contouring and secondary loss of
reduction is mitigated by axial and angular stability.[11]
[12] Locking plates were initially indicated for use in osteoporotic femurs in female
humans as the brittle bone made the use of traditional compression plates problematic.[11]
[12]
[13] A recent publication of a laboratory fracture model described the repair of the
radius and ulna in the domestic rabbit using locking plate systems and concluded that
locking plates are sufficient for fracture stabilization.[14]
Ensuring structural rigidity of a repair is imperative in mitigating implant failure.
One technique which has been widely used within dogs and cats to reduce plate strain
as well as increase overall structural strength of a repair is orthogonal plating.[15]
[16]
[17] A laboratory fracture model of the radius and ulna of cats showed that when two
locking compression plates were fixated orthogonally, the construct had a significantly
higher failure load than other radial or dual bone fixation methods.[15] To our knowledge, orthogonal plating by means of locking plate fixation of both
the radius and ulna in a clinical case of the domestic rabbit has not been previously
reported.
Case Report
A 9-month-old, 2 kg, neutered male Mini Lop Cross was presented to its local general
veterinary practice with acute lameness of unknown cause. Clinical examination showed
crepitation and deviation of the left forelimb. Diagnostic lateral and craniocaudal
radiographs of the radius and ulna were obtained under general anaesthesia using glycopyrrolate
(Glycopyrrolate; West-ward, New Jersey, United States; 0.02 mg/kg subcutaneously [SC])
for premedication and fentanyl citrate (Fentanyl; Hospira Inc., Illinois, United States;
0.15 mg/kg intramuscular [IM]) and ketamine (Ketamil; Ilium, New South Wales, Australia;
7 mg/kg IM) for induction. Using a dental radiography unit (CR7 Vet Image Plate X-ray
Scanner; iM3, Sydney, Australia), X-rays depicted a mildly comminuted complete mid-diaphyseal
radius and ulna fracture with caudoproximal and lateral displacement of the distal
fragment ([Fig. 1]). The left forelimb was then temporary stabilized using a splinted modified Robert
Jones dressing, and then given meloxicam (Metacam; Boehringer Ingelheim, Missouri,
United States; 0.3 mg/kg SC) and buprenorphine (Temvet; Troy Laboratory, New South
Wales, Australia; 0.03 mg/kg SC) for pain relief by the primary care veterinarian.
The case was referred to a board-certified small animal veterinary surgeon (A.S.L.),
who in-turn recommended surgery to repair the fractures. Due to financial constraints,
there was a delay in repair of the fracture for 2 weeks.
Fig. 1 (A) Preoperative craniocaudal radiograph of the left forelimb. (B) Preoperative mediolateral radiograph of the left forelimb.
Methods
The rabbit was premeditated using glycopyrrolate (Glycopyrrolate; 0.02 mg/kg SC) and
induced with ketamine (Ketamil; 7 mg/kg IM) and fentanyl citrate (Fentanyl; 0.15 mg/kg
IM). Intraoperative procaine penicillin (Propercillin; Ilium, New South Wales, Australia;
40 mg/kg SC) and cephazolin sodium (Cefazolin; Hospira Inc., Illinois, United States;
22 mg/kg intravenously) were administered. The rabbit was then intubated and maintained
at 2.5% isoflurane (Isoflo, Zoetis Australia Pty Ltd, Rhodes NSW, Australia) in oxygen
throughout the procedure and was monitored through a Surgivet anaesthetic monitor
(Advisor 3 Parameter Vital Signs Monitor; Smiths-Medical, Ohio, United States) using
capnography, pulse oximetry, blood pressure. The rabbit was positioned in dorsal recumbency
and prepped for surgery with a hanging limb. A craniomedial incision was made in the
skin with a no 15 blade extending from 2 cm distal to the elbow joint to 0.5 cm proximal
to the carpal joint. Dissection was continued through the subcutaneous tissue to expose
the musculature superficial to the cranial surface of the radius. The extensor carpi
radialis and other extensor muscles were retracted laterally. Soft tissue was removed
from the cranial surface of the radial diaphysis via combination sharp and blunt dissection.
Haemorrhage was controlled with electrocautery. A comminuted fracture of the radial
diaphysis with caudal–proximal displacement of the distal fragment was identified;
significant fibrous tissue was noted. The fractured radial and ulna segments were
identified and ends were ostectomized 1 to 3 mm each side with the sagittal saw to
aid in precise reduction in a transverse bone. No gross evidence of bone pathology
was appreciated. The bone segments were held in reduction, while a 1.5 mm 12 holes
LAP (DT AP 1.512; Veterinary Orthopedic Implants Inc., Florida, United States, Veterinary
Orthopaedic Implants. 1.5 mm double threaded locking adaptation plates-12 hole [https://vetimplants.com/1-5mm-double-threaded-locking-adaption-plate-12-hole/]) was cut to size (7 holes) and applied to the cranial surface of the radius. A locking
drill guide and a brand new 1.1 mm drill bit were used to drill into the radius. Six
screws were inserted into the radius to fixate the plate. The most distal cortical
screw hole was filled first to compress the plate to the bone; then most proximal
cortical screw, followed by the cortical screw hole distal to the fracture site then
proximal to the fracture site and finally the locking screws. The middle hole was
left open as it was too close to the fracture site. Another 1.5 mm 12 holes LAP (DT
AP 1.512; Veterinary Orthopedic Implants, Inc., Florida, United States) was cut to
size (five holes). A locking drill guide and a 1.1 mm drill bit were used to drill
into the ulna. The plate was placed at an angle to the first plate to utilize the
concept of orthogonal plate fixation. Four screws were inserted into the ulna (two
cortical and two locking) to fixate the plate in a like-wise manner. The surgical
site was lavaged with sterile saline. The joints were palpated through a complete
range of motion and the elbow and carpus were free of crepitus. Limb and joint alignment
appeared appropriate. Closure of the skin was done using 4/0 nylon ford interlocking.[5]
Results
Postoperative Evaluation
A splinted bandage was applied from the most distal aspect of the left forelimb to
above the elbow postoperatively. Buprenorphine (Temvet; 0.03 mg/kg SC) and meloxicam
(Metacam Solution for Injection; 0.3 mg/kg SC) were administered as postoperative
pain relief. The rabbit recovered from anaesthesia uneventfully, was given hay, water
and monitored overnight until normal eating habits returned. The rabbit was kept on
soft, clean bedding and kept caged confined. Weight bearing on the splinted limb resumed
the following day the patient was discharged on meloxicam (Metacam Oral Suspension;
Boehringer Ingelheim, Missouri, United States; 1 mg/kg per os) for 7 days and procaine
penicillin (Propercillin; 40 mg/kg SC) for 5 days.[3]
Follow-Up Evaluation
At follow-up evaluation with the general practice 7 days postoperatively, the splint
was removed and the limb was rebandaged in a modified Robert Jones dressing. The bandage
was changed at weekly intervals for a total of 3 weeks before removal, during which
time the owners reported that limping had ceased. During the follow-up period, the
rabbit was under strict cage confinement for a total of 6 weeks with follow-up X-rays
performed at week 8 postoperatively.
Radiographic Evaluation
Postoperative lateral and craniocaudal radiographs of the left radius and ulna confirmed
appropriate fracture reduction, alignment, apposition and implant placement ([Fig. 2]).
Fig. 2 (A) Immediate postoperative craniocaudal radiograph of the left forelimb. (B) Immediate postoperative mediolateral radiograph of the left forelimb.
Follow-up craniocaudal and mediolateral radiographs of the left radius and ulna of
the rabbit were obtained at 8 weeks postoperatively. Radiographs depicted complete
osseous union of the fracture and no implant complications on both views ([Fig. 3]). Subjectively, based on the scoring system previously described by Pozzi et al,
where grade 1 is complete healing of the fracture line with no callus formation and
grade 5 is nonunion with no signs of healing, the fracture was graded 1.[18]
Fig. 3 (A) Craniocaudal radiograph of the left forelimb 8 weeks postoperatively. (B) Mediolateral radiograph of the left forelimb 8 weeks postoperatively.
Discussion
Successful fixation of plates to the radius and ulna of a domestic rabbit was demonstrated
using 1.5 mm LAPs.
In this report both the radius and ulna were plated that may be an important step
in the outcome of rabbit radius–ulna fracture repair. The reasons for this are twofold.
First, the diameter of the rabbit ulna appears equal to or larger than the radius
in size. Failure to plate the ulna may lead to overload of the radial plate if the
ulna forms a non-union. Additionally, plating the ulna protects the radial implant
from overload and vice versa. Second, having two plates at angles to one another protects
either plate from deleterious bending forces similar to orthogonal plating.[19]
[20] Orthogonal plating is fixating a plate such that the bending forces are aligned
against the width of the plate rather than the depth as is seen in parallel plating.
This method uses the principle of area moment of inertia to determine the optimal
angle at which the plate is fixated to increase overall structural strength of the
repair.[16]
An open screw hole was left on both the radial and ulnar fracture repairs near the
fracture site. Although this is not ideal, this arose due to the small size of the
bones being repaired. Open screw holes can cause a stress-riser at the plate and can
lead to plate breakage. We speculate that this did not occur for three reasons. First,
the radial and ulna plates were orthogonal and protective of one-another, second a
degree of load sharing was achieved through bone contact by ostectomy and reduction
and third good patient aftercare.
The bone ends of both the radius and ulna were ostectomized in this present report.
This likely aided in both bone healing and bone remodeling as it allowed for complete
and perfect reduction without undue force. Furthermore, allowing the bone cortices
to meet and thus enabling a degree of load-sharing are vital in early progression
of bone healing and protecting the implant from over-loading.[1] An additional advantage of osteotomizing the bone ends was that it opened the medullary
cavity (blood supply) of the bones which had been sealed with fibrous tissue due to
the delay in time between injury and repair. We felt that the limb shortening caused
by doing this was not clinically significant. An investigation into what length of
the radius can be ostectomized before locomotion is affected should be evaluated.
Two locking and four cortical screws were used to fixate the radial plate, and two
locking and two cortical screws were used to fixate the ulna plate. The cortical screws
were placed first the reasons for this are threefold[21]: (1) to compress the plate to the bone thereby optimizing construct strength, (2)
to maintain compression of the fracture site and (3) to maintain positioning of the
plate on the bone before application of the locking screws. Additionally, this sequence
avoids stripping of the cortical screws. Given that the radius and ulna are effectively
operating as one functional unit, we therefore had three cortical and two locking
screws on each side of the fracture which was sufficient for stability.[22]
In this report, the authors applied a splinted bandage to the repaired limb. Currently,
the literature is unclear on the use of bandaging after internal fixation of distal
limb fractures and thus bandaging is regularly left to surgeons' preference. The authors
have found anecdotally that postoperative caudal splinting, using fiberglass or similar,
in radius–ulna fractures in toy breed dogs leads to low rate of complications in our
hands. However, we submit that there are publications showing the potential deleterious
effects of bandaging. In one study, bandage use in toy and miniature-breed dogs, after
bone plate fixation of radius–ulna fractures, led to a large proportion of the minor
complications encountered postoperatively.[23] Furthermore, when casts were used in distal limb orthopaedic conditions in dogs
and cats, it was found, in another study, that 63% developed soft tissue injuries
with a higher prevalence in sighthounds.[24] Although this may be the case, it is not known whether bandaging, splinted or otherwise,
contributed to the success in this report.
A few limitations are apparent in this report. First, as this was a referral case,
which was managed postoperatively by the primary care veterinarian, frequent follow-up
examinations to thoroughly assess fracture healing radiographically were not possible.
Second, the relatively higher cost of 1.5 mm LAPs and screws may be prohibitive to
widespread use in clinical cases. Third, the applicability of this technique to inexperienced
veterinarians may be low due to the technical skill and equipment required to implant
such plates without complications. Finally, this report only reports success in a
single rabbit.
In conclusion, this is the first clinical report where 1.5 mm LAPs have successfully
been used to repair a fracture of the radius and ulna in a rabbit. Future studies
should investigate the complication rate of placing 1.5 mm LAPs in a larger rabbit
sample size, and whether plating both the radius and ulna or only radius is necessary
for success.
