Miniplate fixation for the repair of segmental mandibular defects filled with autogenous bone in cats

Purpose: To evaluate the use of maxillofacial miniplate 1.5 in the repair of segmental mandibular defects filled with autogenous bone in cats. Methods: Twelve adult cats were divided into two groups. A segmental defect of 4mm was created in one of the hemimandibles and filled with autogenous iliac crest bone graft. The operated hemimandible was fixed with a 1.5mm titanium miniplate. In group 1 (n=6), the defect was performed in the body of the mandible, behind the 1st molar. In group 2 (n=6), the defect was performed between the 4nd premolar and 1st molar, with extraction of the 1st molar. Oral alimentation was reinitiated 24 hours after surgery. Cats were euthanized at 20 weeks postoperative. Results: Incorporation of the graft was suggested by the radiographs taken 20 weeks after surgery. Macroscopic examination confirmed alignment and bone union of operated hemimandibles. Histological examination showed formation of woven bone in rostral and caudal mandible/graft interfaces. The percentage of bone tissue at these areas was measured by the histometry. There was no statistically significant difference between the values of group 1(64.48 ± 4.51) and group 2 (71.69 ± 14.47) (Mann-Whitney’s test p= 0.294). Conclusion: The use of miniplate 1.5 for the fixation of mandibular defects filled with autogenous bone in cats provided the main goals in the treatment of mandibular fractures: bone union, normal dental occlusion and immediate return to oral alimentation.


Introduction
The treatment of mandibular and maxillofacial fractures in humans was revolutionized with the advent of miniplates.
Miniaturized implants minimize tissue trauma and preserve periosteal blood supply.Besides that, smaller plate systems are desirable because of the limited surgical field observed in these cases.The crescent clinical application of this method is also observed in small animal surgery [1][2][3][4][5] .
The stabilization of mandibular defects with miniplates has also been performed in humans and animals for mandibular reconstruction with bone grafts or synthetic composite bone substitutes 4,[6][7][8][9] .
The objective of this study was to evaluate the use of maxillofacial miniplate 1.5 in the repair of segmental mandibular defects filled with autogenous bone in cats.We hypothesized that these miniaturized implants could offer enough stability to provide incorporation of the cortico-cancellous autograft.

Methods
This study was approved by the Ethics Committee for Animal Research of UNESP, Araçatuba-SP, Brazil.

Animals and experimental design
Twelve healthy young adult (aging between 12 and 18 months) mixed breed cats (weighing 2.9 0.2kg) were utilized.
Six spayed females and 6 neutered males were used and evenly distributed between 2 groups.A segmental defect of 4mm was created at random in either the right or the left hemimandible and filled with autogenous iliac crest bone graft.The operated hemimandible was fixed with a 1.5mm titanium miniplate.In group 1 (n=6, 2.8±0.2kg), the defect was performed in the body of the mandible, behind the 1 st molar.In group 2 (n=6, 3.0±0.2kg), the defect was performed between the 4 nd premolar and 1 st molar, with extraction of the 1 st molar.The cats were euthanized at 20 weeks postoperative.

Surgical procedure
Food was withheld for 12 hours prior to anesthesia.Thereafter the cats were positioned in dorsal recumbency.
Local anesthesia of the inferior alveolar nerve was done with mepivacain (7mg/kg) and epinephrine.The ventral cranial and cervical regions were clipped, prepared, and draped under sterile technique.A ventral surgical approach to the mandible was performed.The hemimandible was transected transversely.
Initially a gap was performed in the cortical bone of the lingual and buccal surfaces and the ventral border with a burr on a lowspeed hand-piece under watering.Another gap was done distally at a distance of 4mm.Next, the two osteotomies were completed with an osteotome and mallet.In group 1, the defect was performed in the body of the mandible, behind the 1 st molar.In group 2, the defect was performed between the 4 nd premolar and 1 st molar, with extraction of the 1 st molar.The gingival mucosa was sutured with 4-0 polyglactin 910.The hemimandible was fixed with a 6-hole titanium miniplate 1.5.The miniplate was applied with 4 bicortical screw placement on the ventrobuccal aspect of the hemimandible.
The segmental defect was filled with autogenous iliac crest bone graft.Subcutaneous layer was closed with 4-0 polyglactin 910.
Skin closure was with 4-0 nylon.Skin sutures were removed after 7 days.

Postoperative observations and radiographs
Oral alimentation was reinitiated 24 hours after surgery with canned food.This soft food was offered during 6 weeks.After this period, the animals were feed with commercial dry feline diet.
Cats were weighted again at the end of the study (3.1±0.4kg).
Radiographs were performed one week after surgery and at the end of the study, i.e., after 20 weeks postoperative.The oblique lateral and ventrodorsal views of the mandible were taken.
Intraoral radiographs were also performed.

Macroscopic analysis, histolopathology and histometry
Cats were euthanized at 20 weeks postoperative by For the histometric analysis, the images of sections were acquired using a digital camera (Olympus DP71) coupled to a microscope (Olympus BX61).The percentage of newly formed bone in rostral and caudal mandible/graft interfaces was measured using the Image Pro Plus 6.1 software (Figures 1 and 2).The percentage of bone tissue of unoperated hemimandibles were also measured.

Statistical analysis
Non-parametric tests were utilized for the data analysis, taking into consideration the nature of the variables.Mann-Whitney's test was used to compare histometric data between the two groups.Significance was set at p<0.05.The percentage of bone tissue in the areas of graft incorporation was measured by the histo metry.There was no statistically significant difference between the values of group 1 (64.48 ± 4.51) and group 2 (71.69 ± 14.47) (Mann-Whitney's test p= 0.294) (Table 1).These values were smaller from those observed in unoperated hemimandibles (Group 1: 90.08 ± 5.18;

Discussion
In the present study we used the same technique described by Boudrieau and Kudisch 4 .The miniplate was applied on the ventrobuccal aspect of the mandible in order to avoid damage to the teeth roots.
The use of a single miniplate provided enough stability to provide incorporation of the graft.In the present study a block of cortico-cancellous autograft was inserted in the segmental defect.The same procedure was adopted by Sverzut et al. 10 , that used a iliac bone graft to fill a 10mm long segmental defect in the mandibles of dogs and by Schliephake et al. 11 that used a calcium phosphate cylinder to fill a segmental defect of 35mm long in the mandibles of sheep.
Special feed management and body weight monitoring must be considered in animals submitted to maxillofacial surgeries [4][5][6]12,13 . Our rsults showed that the use of a single miniplate provided a fixation stable enough to allow immediate oral alimentation.Besides that, the cats maintained their initial weight or even gained weight at the end of the present study.
Postoperative radiography demonstrated a good reduction throughout the observation period.Incorporation of the graft was suggested by the radiographs taken 20 weeks after surgery.
Bone healing, as manifested by radiopacity at the original osteotomy sites was indicated in the radiographs taken at 6 and 12 week postoperatively in an experimental study with goats which evaluated a segmental defect filled with an autogenous bone graft from the iliac crest mixed with platelet-rich plasma 13 .
Similar results were obtained by Huh et al. 8 , as the radiographs taken at 6 week postoperatively showed incorporation of the original particulate bone mixed with platelet-enriched fibrin glue used to reconstruct a segmental defect of 15mm length in the dog mandible.
In the experiment conducted by Yuan et al.The areas of graft incorporation were clearly located in histological sections of demineralised bone embedded in paraffin because this technique allowed preparation of 2 cm length samples.
For this reason, these sections were chosen for the histometric examinations.These sections showed no significant disruption of the trabecular structure as bone trabeculae were thicker after an observation period of 20 weeks.In contrast, histological sections of undecalcified bone embedded in glycolmethacrylate could be obtained only from small samples (5mm diameter), as reported by several authors [14][15][16][17] .There was some difficulty in seccioning and most histological sections showed disruption of bone trabeculae.
Nevertheless, sections of undecalcified bone can be stained with specific histological methods to differentiate mineralised from unmineralised matrix.In the present study, the Masson-Goldner stain provided clear differentiation between mineralised bone (green) and osteoid (orange), as described by Yang et al. 17 As the operated hemimandibles showed an advanced stage of bone union, the presence of osteoid in histological sections was discrete.
intravenous thiopental overdosage.The operated and unoperated hemimandibles of each cat obtained at necropsy were fixed in 10% formaldehyde solution.Then, each of these hemimandibles was divided longitudinally into two segments.The superior half was demineralised in an aqueous solution of 5.66 formalin 10% to 1 of nitric acid (v/v).The demineralised specimens were dehydrated in ethanol, clarified in xylol and embedded in paraffin.Thin sections were made and stained with hematoxilin-eosin for microscopic evaluation.The inferior half was processed undecalcified.The undecalcified specimens were dehydrated in ethanol, clarified in xylol and embedded in glycol methacrylate.The sections were stained with Masson-Goldner for microscopic evaluation.

FIGURE 1 -
FIGURE 1 -Histologic section of the operated hemimandible of cat 23 of group 2 shows the mandible/graft interface at 20 weeks postoperative bridged by woven bone.Demineralised bone specimen.(HE -100X).

FIGURE 2 -
FIGURE 2 -The same histologic section prepared for histometry.The bone tissue was marked in blue and the soft tissue in yellow.Then the percentage of bone tissue was measured using the Image Pro Plus 6.1 software.
were no clinical complications and the operated hemimandibles healed uneventfully and with appropriate occlusion.During the experimental period the cats showed normal activity and mastication and maintained their initial weight or even gained weight.Postoperative radiography (intraoral, oblique lateral and ventrodorsal views) was useful in evaluating mandibular alignment and bone healing.Good reduction was obtained and maintained throughout the observation period of 20 weeks and no implant failure occurred.Incorporation of the graft was suggested by the radiographs taken 20 weeks after surgery (Figures 3 to 8).

FIGURE 3 -
FIGURE 3 -Radiograph in oblique lateral projection taken 1 week after surgery shows the segmental defect filled with autogoneous bone graft.Cat 5 of Group 1.

FIGURE 4 -
FIGURE 4 -Radiograph in ventrodorsal projection taken 1 week after surgery shows the segmental defect filled with autogenous bone graft.Cat 20 of Group 2.

FIGURE 5 -
FIGURE 5 -Intraoral radiograph taken 1 week after surgery shows the segmental defect filled with autogenous bone graft.Cat 14 of Group 1.

FIGURE 6 -
FIGURE 6 -Intraoral radiograph taken 20 weeks after surgery shows incorporation of the graft into the segmental defect.Cat 14 of Group 1.

FIGURE 7 -
FIGURE 7 -Intraoral radiograph taken 1 week after surgery shows the segmental defect filled with autogenous bone graft.Cat 20 of Group 2.

FIGURE 8 -
FIGURE 8 -Intraoral radiograph taken 20 weeks after surgery shows incorporation of the graft into the segmental defect.Cat 10 of Group 2.

FIGURE 9 -
FIGURE 9 -Histologic section of the operated hemimandible shows that the mandible/graft interface was bridged by woven bone.Cat 22 of Group 2 specimen.(HE -100X).

TABLE 1 -
Bone fill percentage in rostral and caudal mandible/graft interfaces of operated hemimandibles of group 1 and group 2.