Mandibular Rim Excision in Seven Dogs

BOAZ ARZI, DVM and FRANK J. M. VERSTRAETE, DrMedVet, MMedVet, Diplomate AVDC & ECVS

Objective—To describe a surgical technique for excision of minimally invasive mandibular tumorsat the level of the premolar and molar teeth, and report outcome in 7 dogs that had mandibular rimexcision.Study Design—Case series.Animals—Dogs (n¼ 7) with a mandibular tumor at the level of the premolar and molar teeth.Methods—Using an intraoral approach to the mandible, buccal, and lingual mucosal incisions aremade to obtain a 10mm clean margin beyond neoplastic tissue. After subperiosteal soft tissueelevation, a curvilinear rim mandibulectomy is performed, leaving the mandibular canal and ventralcortex intact, followed by osteoplasty. The remaining attached gingiva and alveolar mucosa aresutured over the bony defect.Results—Seven dogs were treated (1997–2008) for odontogenic and early malignant neoplasmsinvolving the mandible by mandibular rim excision. All dogs had healed, healthy gingival coveringover the surgical defect, very good postoperative function, and good quality of life.Conclusion—Mandibular rim excision, with preservation of the ventral cortex and mandibularcanal content, can be a good option for treatment of early odontogenic and malignant lesions of themandible in medium to large breed dogs.Clinical Relevance—In medium to large dogs with minimally invasive mandibular neoplasia, man-dibular rim excision should be considered as a viable surgical option.r Copyright 2010 by The American College of Veterinary Surgeons

INTRODUCTION

CANINE ORAL cavity neoplastic lesions (odonto-genic and nonodontogenic tumor types) represent

� 6% of canine cancer and are the 4th most commoncancer overall.1 Understanding the biologic behavior ofthe tumor enables the surgeon to select the method oftreatment. For malignant oral tumors and benign butlocally invasive lesions of the mandible, surgical treat-ment by mandibulectomy is most commonly indicated.2

In 1987, a rim mandibulectomy procedure that removedthe occlusal aspect of the mandible but left a strong ven-tral aspect was proposed to treat mandibular tumors withearly bone invasion in people.3 In people, this techniqueis an effective oncologic procedure that may provide localcancer control comparable with segmental mandibulec-

tomy.4 This procedure has the main advantages of supe-rior postoperative function, absence of mandibular drift,esthetics, as well as improved capacity for reconstructionand rehabilitation.5 Postoperative iatrogenic fracturesoccur in up to 15% in people.3 Fracture occurrence ismost likely caused by sharp-angled excision techniqueand diminished blood supply secondary to site irradia-tion, excessive periosteal stripping, and wound dehis-cence.5 There are strong indications in people thatcurvilinear excision resists higher occlusal forces, withsmaller residual ventral segments, than do right-angledexcisions.4,5 Additionally, in people neoplastic growthgenerally follows an elliptiform pattern, and therefore acurvilinear excision is more bone sparing, because it fol-lows the form of lesion more closely than right angledexcision (Fig 1).4,5

Corresponding Author: Dr. Frank J. M. Verstraete, DrMedVet, MMedVet Diplomate AVDC & ECVS, Department of Surgical and

Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616. E-mail: fjverstraete@ucdavis.edu.

Submitted January 2009; Accepted March 2009

From the William R. Pritchard Veterinary Medical Teaching Hospital and Department of Surgical and Radiological Sciences, School

of Veterinary Medicine, University of California, Davis, CA.

r Copyright 2010 by The American College of Veterinary Surgeons0161-3499/09doi:10.1111/j.1532-950X.2009.00630.x

226

Veterinary Surgery

39:226–231, 2010

When examining the architecture of the mandible, itis obvious that most of the thick cortical bone is at theventral border and provides much of the strength ofthe mandible. In addition, there is a need to spare themandibular canal content: inferior alveolar nerve, veinand artery. Inclusion of the mandibular canal contentin the excised tissue is likely to jeopardize the stabilityof the residual rim without any benefit to tumorcontrol6,7; however, the surgeon needs to be awarethat any sign of tumor invasion of the mandibularcanal precludes rim excision. Small dogs have propor-tionally larger mandibular 1st molar teeth relative tomandibular height compared with larger dogs.8 Thisanatomic consideration precludes small dogs from be-ing candidates for mandibular rim excision as the rootsof the 1st mandibular molar teeth are in close prox-imity to the ventral cortex of the mandible. Studies inpeople demonstrate that preserving the ventral cortexand periosteum will eventually result in spontaneousbone healing with bone filling the osseous defect,further adding to the strength and integrity of themandible.9

Although this technique has been generally describedin dogs,10 we report this technique with additionalconsideration and modification as well as our experiencewith 7 dogs treated by means of mandibular rim excisionfor treatment of odontogenic and small malignanttumors.

MATERIALS AND METHODS

Preoperative Considerations

We routinely perform periodontal treatment during theanesthetic episode of the clinical staging, diagnostic imaging,and biopsy, especially if there is large amount of dental cal-culus and plaque present.11

Routine preoperative investigations include dental radiog-raphy and computerized tomography (CT) preferably in 1mmsections, with and without contrast. We use both investiga-tions together before the procedure to try and predict thelikely presence and extent of mandibular invasion. In addition,measurement of predictive length of the associated mandib-ular teeth roots and the distance required to achieve 10mm ofclean margins is obtained from the combination of imagingtechniques (Fig 2A and B). Considerable hemorrhage is pos-sible and therefore we assess hemostasis by means of mucosalbleeding test; however, other tests may be required.12 We alsoroutinely perform cross-matching before this surgery.

The oral cavity is rinsed with chlorhexidine gluconate in anaqueous nonalcohol-containing solution to decrease bacterialburden during surgery.13,14 We prefer positioning the dog insternal recumbency, with the head elevated and the maxillassuspended between intravenous poles or secured to the anes-thesia screen.10 We use a cuffed endotracheal tube and securedpharyngeal pack to prevent aspiration.

Inferior alveolar nerve block using a long-acting local an-esthetic facilitates analgesia during the procedure. Currently,we administer ampicillin (20mg/kg intravenously [IV]) at thetime of induction in the presence of systemic disease, hemato-logic abnormalities, presence of implants, immunodeficiency,animals undergoing chemotherapy, or if the surgical site hasbeen previously irradiated.15–19 The planned surgical marginsand incision line are outlined using a sterile surgical skinmarker (Secureline surgical skin marker; Precision DynamicCorp., San Fernando, CA).

Surgical Technique

A full-thickness mucoperiosteal incision using a 15 scalpelblade is made 10mm from the tumor margins at the inter-proximal space of the teeth distal and mesial to the tumor.The soft tissues are elevated away from the planned ostectomysite using a 24G periosteal elevator (Hu-Friedy, Chicago, IL).The attached gingiva at the margin of the incision is preservedto not compromise the teeth on either side of the ostectomysite and to be able to cover the bone with soft tissues withouttension. With the soft tissues protected, ostectomy is per-formed from 1 interdental space to the other in a curvilinearfashion including the teeth distal and mesial to the tumor inthe removed bone fragment (Fig 3A and B). Care should betaken to avoid the mandibular canal. If the mandibular canalcontent is accidentally damaged, hemostasis is achieved bymeans of ligature or Hemoclips

s

(Teleflex Medical, NC). Wecurrently use a surgical hand piece (INTRAsurge 300; KaVoAmerica Corp., Lake Zurich, IL) designed for major oralsurgery (no air insufflation, built-in sterile fluid irrigation)

Fig 1. Diagram of rim excision of increasing size, completed

in right-angled, and curvilinear configuration.

227ARZI AND VERSTRAETE

combined with an ostectomy bur (Lindemann bur; Hu Friedy,Chicago, IL). After removal of the tumor-containing bonefragment, a careful inspection is performed to make sure thatroot tips have not been left behind as they may cause focus ofinfection and pain. Osteoplasty to smooth the cut bone mar-gins is performed using a round diamond bur. After copiousflushing with sterile saline (0.9% NaCl) solution, the remain-ing attached gingiva on either side of the defect and the al-veolar mucosa are sutured over the bony defect with 4-0monofilament absorbable suture on a small reverse cuttingneedle avoiding tension (Fig 4).

The removed tumor-containing bony segment is color-coded for determination of surgical margins (Fig 5) using theDavidson Marking System

s

(Bradley Products Inc., Bloom-ington, MN).20

Postoperative Considerations

IV fluid therapy is continued until normal oral intake isresumed.2 Water is offered after 12 hours and soft food after24 hours.11 Nutritional support by means of enteral feeding

tube is very rarely, if at all, needed. Soft food is continueduntil soft tissues are healed. Pain management is achieved by adual approach that combines nonsteroidal antiinflammatoryagents and opioids. A fentanyl patch is applied at the end ofthe surgery and the dog is maintained on either morphine,oxymorphone, or hydromorphone for the first 12–24 hours.Nonsteroidal antiinflammatory agents are administered par-enterally for the first 24 hours and continued orally for 7–10days.

Chlorhexidine gluconate 0.12% oral hygiene rinse (C.E.Ts

,Virbac Animal Health, Fort Worth, TX) is prescribed for aperiod of 10–14 days to keep the surgical area clean and todecrease the bacterial burden on the wound. Wound healing isassessed in 14 days after the surgery. The surgical site is mon-itored every 3–6 months for signs of recurrence.

RESULTS

All dogs (Table 1) had good physical condition andresults of hematological, serum biochemical analysis, and

Fig 2. (A) Dental radiograph of the left caudal mandible used for the measurement of the predictive length of the mandibular teeth

roots (arrows) and the planned rim excision site (dotted line). (B) Dog 7. Computerized tomography 1-mm section demonstrating

the soft tissue mass (white arrow), the distance of the mass from the mandibular canal (opposite direction arrow), and a mild bony

involvement (single black arrow).

Fig 3. (A) Diagram of rim excision from the mesial aspect of the left mandibular 4th premolar to the mesial aspect of the left

mandibular 2nd molar. (B) Dog cadaver. Rim excision in a curvilinear fashion was performed from the mesial aspect of the right

mandibular 3rd premolar to the mesial aspect of the right mandibular 2nd molar. Note the preservation of the mandibular canal

content.

228 MANDIBULAR RIM EXCISION

urinalysis were considered normal. Thoracic radiographsand abdominal ultrasonography performed during tumorstaging revealed no abnormalities. The mandibular massdid not involve the ventral cortex and the mandibularcanal in any dog. No surgical complications occurred. Noneoplastic cells were identified in the surgical margins ofthe submitted specimens.

In all dogs, the 3–4-week, 6-month, and yearly re-checks of the surgical site demonstrated intact gingivalcovering of the surgical defect and on digital palpation,the ventral mandibular cortex was intact. Dental radio-graphs obtained at 4 weeks (dog 3; Fig 7), 6 months (dogs1, 5), and 9 months (dog 4) demonstrated that bone wasfilling in the defect, without any radiographic signs oftumor recurrence. At each recheck examination, ownersreported no apparent medical problems.

DISCUSSION

Mandibular rim excision is a promising technique fortreatment of minimally invasive odontogenic and malig-nant tumors of the canine mandible. Segmental and totalmandibulectomy have the inherent complications ofmandibular drift, traumatic malocclusion, and long-termpatient discomfort. The main advantage of mandibularrim excision over segmental or total mandibulectomy isthe comparative simplicity of the reconstruction, thequick return to normal function, and absence of maloc-clusion (e.g., mandibular drift). The decision to proceedto a rim resection was taken after an assessment of thepreoperative imaging, clinical examination, intraopera-tive observation, and the absence of distant metastasis.CT with and without contrast is highly recommended forpreoperative planning. Tumors may grossly invade themandibular canal or extend down the periodontal liga-ment in the vicinity of tumor. Extension of the tumor intothe canal would preclude rim excision. Gross periodontalligament involvement may or may not allow rim excisiondepending upon the margins and tooth root–bone rela-tionship. The information obtained from CT should beinterpreted in light of the biological behavior of the tu-mor and the recommended surgical margins.

The use of a surgical hand piece designed for majororal surgery without air insufflation and built-in sterilefluid irrigation combined with an ostectomy bur is highlyrecommended. Piezoelectric osteotomy is a relatively newtechnique in craniofacial surgery that can also be advan-tageous for mandibular rim excision. Piezoelectric osteo-tomy permits micrometric selective cutting and clearsurgical site because of the cavitation effect created by thecooling solution and the oscillating tip, and has minimalto no damage to adjacent soft tissues.21,22 However, the

Fig 4. Dog 7. Closure over the bony defect.

Fig 5. Dog 6. The excised tumor-containing segment color-

coded for determination of surgical margins.

229ARZI AND VERSTRAETE

overall operative time may increase as compared with useof a conventional bur. The use of air driven, high-speedrotary instrument is not recommended as its use nearopen wounds may cause subcutaneous emphysema, or-bital emphysema, and involvement of vital structures.23,24

Also, the use of an orthopedic cutting saw is discouragedbecause it may be technically impossible to create a del-icate curvilinear ostectomy at the mandible.

All specimens were color-coded for evaluation of sur-gical margins using the Davidson Marking System

s

(Bradley Products Inc., Bloomington, MN), and all were

analyzed as having clean surgical margins. The currentveterinary literature does not provide a uniform opinionregarding the extent of surgery needed to obtain cleanmargins.1,25 Most of the reports are anecdotal and sug-gest obtaining 10mm margins for all oral tumor types,25

whereas others suggest obtaining 20mm margins for oralsquamous cell carcinoma, malignant melanoma, and fib-rosarcoma.1 One undifferentiated sarcoma and 1 fibro-sarcoma were treated by means of mandibular rim

Fig 7. Dog 3. New bone formation filling the defect 4 weeks

after the surgery.

Fig 6. Dog 2. Postoperative radiograph demonstrating right

mandibular rim excision with intact mandibular canal and

ventral cortex.

Table 1. Summary Data for 7 Dogs that had Mandibular Rim Excision for Oral Cavity Neoplasia

Dog Age Sex Breed

Weight

(kg)

Tumor

Excision Site

Follow-Up (No

Gross Signs

Tumor

Recurrence)Type Size

Location

(Mandible,

Teeth)

1 13 m FS Australian

shepherd

19.4 Complex

odontoma

2 coalescing

masses 2–20mm

diameter

Right P3–M3 Mesial aspect,

RmandP3 to distal

aspect RmandM3

12 y

2 3 y M Shetland 11.4 Undifferentiated

sarcoma

20mm diameter Interproximal

space Right

P4–M1

Mesial aspect,

RmandP4 to mesial

aspect RmandM2

(Fig 6)

5 y

3 6 m FS Golden

retriever

17.7 Compound

odontoma

Multilobulated

20mm diameter

Left M1–M2 Mesial aspect,

LmandM1to rostral

aspect of ramus

8 y

4 6.5 y MC Weimaraner 42 Fibrosarcoma Gross tumor not

visible, previous

excision biopsy

Interproximal

space Right

P3–P4

Mesial aspect,

RmandP3 to mesial

aspect RmandM1

2 y

5 1 y MC Labrador

retriever

36 Peripheral

odontogenic

fibroma

Gross tumor not

visible, previous

excision biopsy

Lingual aspect

Left M1

Mesial aspect,

LmandM3 up to

mesial aspect

LmandM2

1.5 y

6 8 y MC Springer

spaniel

23 Acanthomatous

ameloblastoma

10mm diameter Interproximal

space, Left

P4–M1

Mesial aspect,

LmandP4 to mesial

aspect LmandM2

1.5 y

7 12 y FS Golden

retriever

33.4 Plasmacytoma 10mm diameter Buccal aspect

Left M1

Mesial aspect,

LmandP4 to mesial

aspect LmandM3

1 y

m, month; y, year; M, male; MC, Male castrate; FS, Female spayed.

230 MANDIBULAR RIM EXCISION

excision attempting to obtain 10mm margins. In these 2dogs, we observed the presence of intact gingival coveringat the surgical defect and the absence of gross tumorrecurrence for 12 years (dog 1) and 2 years (dog 4). Thesefindings are anecdotal and more clinical studies areneeded to evaluate less invasive resection of early malig-nant tumors of the mandible in certain instances. Becauseof mandibular anatomy, attempting to obtain 20mmclean margins may technically preclude most dogs frombenefiting from mandibular rim excision. Follow-up ra-diographs of the surgical area obtained for 4 dogs indi-cated that there was new bone formation that was fillingthe surgically created bony defect. These findings are inagreement with previous studies in people.9

We concluded that mandibular rim excision is a soundoncologic operation to treat small odontogenic and non-odontogenic tumors with limited ventral mandibular in-volvement. Preoperative examination, appropriateimaging studies, and proper surgical judgment are essen-tial when selecting dogs for mandibular rim excision.Preserving the integrity of the mandible contributesgreatly to the postoperative function and rehabilitation.

ACKNOWLEDGMENTS

We thank Mr. John Doval from the Media Laboratory,

Department of Surgical and Radiological Sciences, Uni-

versity of California, Davis, for the artwork.

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