In dogs, bone cysts reportedly develop in the metaphyseal or diaphyseal regions of the distal portion of the radius, ulna, femur, tibia, or humerus (Schrader and others 1983).
Benign/Simple/Unicameral cysts : the proposed mechanism of cyst development involves local trauma, hematoma formation, osteoclast hyperplasia, and venous obstruction caused by local remodeling of bone, which causes accumulation of fluid (Resnick and others 1995).
Aneurysmal bone cysts : possibly secondary to an insult to the bone that disrupts bone marrow vasculature leading to arteriovenous shunting. Bony reaction and blood accumulation results in an expansile blood-filled cyst with cortical destruction resulting from increased local blood supply. A thin shell of bone is deposited by the periosteum as it is displaced (Barnhart 2002). There are reports of aneurysmal cysts associated with underlying pre-existing neoplasia in the dog and of a chondrosarcoma that occurred following surgical manipulation of a cyst (Barnhart 2002).
Odontogenic cysts : rare in animals, and classification of the lesions tends to vary between authors. Odontogenic cysts are derived from cell rests of Malassez, cell rests of dental laminae, reduced enamel epithelium, or malformed enamel organs. Types of odontogenic cysts reported in dogs include radicular cysts, dentigerous cysts, and keratocysts. Radicular cysts contain an inflammatory infiltrate and can be associated with devitalized teeth. Dentigerous cysts are defined as cysts that contain part or all of a tooth, which is often malformed and often remains impacted or unerrupted (Doran and others 2008).
Predisposing factors General
Trauma to the metaphyseal regions of bones of young animals.
Devitalized or malformed teeth.
Pathophysiology
Cysts can cause expansion of the bone and result in localized enlargement of the region (eg distal radius or mandible).
Lameness can be due to the expansile nature of the disease (periosteal discomfort, soft tissue compression) or secondary to pathological fracture either of the cyst wall or the bone itself.
Infection of cysts is a theoretical possibility.
Hemorrhage from aneurysmal bone cysts can be brisk.
Neoplastic transformation following surgical manipulation of a cyst has been suggested (Barnhart 2002).
Timecourse (incubation, duration)
The potential for pain free growth makes it impossible to define how rapidly these lesions develop or progress.
Epidemiology (population dynamics)
Long bone cysts are reported to occur typically in dogs <18 months old (Schrader and others 1983) however there are reports of bone cysts affecting older animals (Duval and others 1995).
Aneurysmal and mandibular and maxillary odontogenic cysts have been reported in mature dogs.
Signs associated with interference with normal function of a localized area, eg tenesmus with pelvic cysts or paresis/paralysis associated with spinal cysts.
Client history
Varies from gradual progressive swelling to acute painful lameness. The lesions have also been reported as an incidental finding (MacInnes 2005).
Clinical signs
Swelling unless situated in an area surrounded by a large soft tissue envelope.
Most are pain free or cause low grade lameness.
Pain.
Lameness in a single limb.
Diagnostic investigation
Hematology
Unremarkable.
Biochemistry
Usually unremarkable but serum alkaline phosphatase levels may be elevated .
Imaging Radiography
Well circumscribed, radiolucent area often with a finely trabeculated appearance (soap bubbles) (Barnhart 2002). The expansile nature of the lesion is usually marked and characterized by endosteal erosion of the cortex to the extent that the cortex may become thin or absent. Mandibular cysts are reported to have a similar appearance and often result in marked loss of the bone support of the teeth (Doran and others 2008).
The intraosseus part of the lesion is limited by a well demarcated, smooth and in places rather sclerotic layer of bone. There is no zone of transition between the lesion and normal surrounding bone suggesting it is a relatively quiescent lesion. Some extremely aggressive and rapidly growing bone tumors can have a similarly short zone of transition but most tumors show an intermediate zone between the lesion and normal bone pattern.
In the spine, the dorsal spinous processes are more commonly involved than the vertebral bodies. In the long bones the lesion is usually positioned towards the end of the bone, involving the ends of the diaphysis and the metaphysis but not the epiphysis.
Variations from this presentation can occur.
Three-view thoracic radiographs are indicated to check for metastases when neoplasia is a differential.
Ultrasonography
Limited use in diagnosis of the primary lesion.
Other
Computed Tomography (CT) : can be useful in some circumstances to gain additional information regarding the three dimensional size and location of the lesion and more thoroughly evaluate the surrounding bone. CT measurements can be obtained in order to identify the content of the lesion as fluid and therefore support a pre-operative diagnosis of a cyst (MacInnes and others 2005).
Confirmation of diagnosis Discriminatory diagnostic features
Definitive diagnosis is reached by histopathology.
The radiographic appearance and presentation characteristics are usually all that is required to reach a diagnosis prior to treatment; advanced imaging (CT) may provide greater spatial detail and more clearly define the extent of the cyst.
The potential for underlying neoplasia or infection should always be considered and a biopsy for histopathology and culture should be obtained at the time of treatment.
Definitive diagnostic features
Histopathological appearance.
Gross autopsy findings
Swelling of the bone results in a mass effect.
The cortices associated with the lesions are usually thin.
Aneurysmal cysts are blood filled, sponge-like lesions (Barnhart 2002).
The soft tissues surrounding the bones are usually unaffected unless the lesion has been damaged, infected or leaked.
The lesions contain fluid of varying type depending on the underlying pathology (unicameral cystic fluid is usually straw colored (Halliwell 1993)).
Histopathology findings
Aneurysmal bone cysts : well-differentiated fibro-osseous proliferation with formation of woven bone.
Odontogenic cysts : cystic spaces lined by a rim of fibrous tissue and mixed, predominantly mononuclear, inflammatory cells. The inner lining of the space comprised attenuated stratified squamous epithelium. Radicular cysts can appear very similar (see below) (Doran and others 2008).
Differential diagnosis
Well demarcated margins and lack of evidence of active surrounding bone pathology, benign differentials are usually considered most appropriate (MacInnes and others 2005):
Benign unicameral bone cyst.
Epidermoid inclusion.
Intraosseous ganglion.
Subchondral cyst secondary to osteoarthritis .
Osteolytic neoplasia , such as multiple myeloma , lymphoma .
In humans, bone cysts have been treated by curettage, grafting, amputation, irradiation or en-bloc local excision depending on the location. Complete excision is associated with an excellent prognosis. The veterinary literature is lacking in this area and treatment information is therefore limited. Most are either excised or treated by curettage and grafting.
Samples should be collected at the time of treatment for histopathology, fungal and bacterial culture.
Curettage and bone graft : most cysts are treated by thorough curettage of the lesion followed by packing with autogenous cancellous or corticocancellous allograft (available commercially). In the case of mandibular or maxillary cysts, loose or damaged teeth are removed.
Synthetic bone grafts and polymethylmethacrylate bone cement have been used as alternatives to fill the void of the cyst. Synthetic bone grafts theoretically result in filling of the void with host bone with time whereas bone cement may result in thermal destruction of the cyst lining during cement polymerization, lessens the risk of pathological fracture, and serves as a permanent space filler.
En-bloc resection of lesions has been reported without recurrence.
If the potential for pathological fracture is considered to be high then some form of support or stabilization should be provided while the lesion resolves. The specific type of support of stabilization will depend on the nature and location of the cyst, nature of the patient, and the degree of owner compliance.
The reported prognosis for odontogenic cysts treated by curettage and grafting is good, with recurrence of the cyst reported as uncommon (Poulet and others 1992). Treatment via curettage was reported in 2 previous cases of radicular cysts in dogs. This treatment was judged to be effective, with no disease recurrence found at 6 and 14 months after surgery (Beckman 2003; French & Anthony 1996).
Resection of cystic lesions is reported to have a good prognosis.
Benign unicameral / simple cysts have a relatively favorable prognosis for resolution following curettage and grafting (Schrader 1983) and some have resolved without surgical therapy.
Recurrence or failure to resolve has been associated with treatment of aneurysmal bone cysts by curettage and grafting (Duval and others 1995). In humans, the high recurrence rate was reduced by the use of mechanical burring rather than hand curettage. The use of radiotherapy also reduced the incidence of recurrence in humans but malignant transformation following radiotherapy has been reported.
Expected response to treatment
Excision of a benign lesion should result in cure.
Curettage and cancellous bone graft placement for simple cysts has the potential to resolve the cyst and presenting signs.
Aneurysmal cysts seem to be associated with a less certain prognosis. Recurrence or failure to resolve has been reported. However, lameness resolved in one dog diagnosed with a distal tibial lesion despite failure to resolve the lesion following two surgical curettage and grafting procedures (Duval 1995).
Underlying neoplasia remains a concern, particularly with aneurysmal cysts and has the potential to be missed at the first biopsy. Repeat biopsy is therefore indicated in cases with progression, recurrence or failure to improve following treatment.
Reasons for treatment failure
Incomplete curettage of the lining (hand curettage vs. mechanical burr).
Pathological fracture due to insufficient support during the healing phase.
Infection.
Poor graft quality.
Underlying neoplastic process.
Aneurysmal bone cysts appear more challenging to treat - however, numbers are lacking in the literature for this to be certain.
Doran I, Pearson G, Barr F & Hotson-Moore A (2008) Extensive bilateral odontogenic cysts in the mandible of a dog.Vet Pathol45 , 58-60 PubMed.
MacInnes T J, Thompson M S & Lewis D D (2005) What's your diagnosis?JAVMA227 , 1561-1562 PubMed.
Sarierler M, Cullu E, Yurekli Y et al(2004) Bone cement treatment for aneurysmal bone cyst in a dog.J Vet Med Sci66 ,1137-1142 PubMed.
Beckman B W (2003) Radicular cyst of the premaxilla in a dog.J Vet Dent20 , 213-217 PubMed.
Barnhart M D (2002) Malignant transformation of an aneurysmal bone cyst in a dog.Vet Surg31 , 519-524.
Stickle R, Flo G, Render J (1999) Radiographic diagnosis - benign bone cyst.Vet Rad and Ultrasound40 , 365-366 PubMed.
Nomura K, Sato K (1997) Pelvic aneurysmal bone cyst in a dog.J Vet Med Sci 59 ,1027-1030 PubMed.
French S L, Anthony J M G (1996) Surgical removal of a radicular odontogenic cyst in a four year old Dalmatian dog.J Vet Dent13 , 149-151.
Duval J M, Chambers J N & Newell S M (1995) Surgical treatment of an aneurysmal bone cyst in a dog.Vet Comp Ortho Traumatol8 , 213-217.
Poulet F M, Valentine B A, Summers B A (1992) A survey of epithelial odontogenic tumours and cysts in dogs and cats.Vet Pathol29, 369-380.
Schrader S C, Burk R L, Lin S (1983) Bone cysts in two dogs and a review of similar cystic bone lesions in the dog. JAVMA182 , 490-495.
Other sources of information
Kelly C et al(2001) The Use of a Surgical Grade Calcium Sulfate as a Bone Graft Substitute: Results of a Multicenter Trial. SECTION I SYMPOSIUM Clinical Orthopaedics & Related Research382 , 42-50, January 2001.
Resnick D, Kyriakos M, Guerdon D et al (1995) Tumors and tumorlike lesions of bone: imaging and pathology of specific lesions. In: Resnick D, ed. Diagnosis of bone and joint disorders. 3rd ed. Philadelphia: WB Saunders Co. pp 3559–3576.
Halliwell W H (1993) Tumourlike lesions of bone. In: Disease mechanisms in Small Animal SurgeryBojrab MJ. Ed Philadelphia: Lea and Febiger. pp 932-943.
Vetstream contributor(s)
Mark C Rochat DVMMS DipACVS, Department of Veterinary Clinical Sciences Center for Veterinary Health Sciences, Oklahoma State University, 01 Farm Road, Stillwater, OK 74078, USA.
Chris Shales MA VetMB CertSAS DipECVS MRCVS European Specialist in Small Animal Surgery, Department of Clinical Veterinary Medicine, University of Bristol, UK.
Some extremely aggressive and rapidly growing bone tumors can have a similarly short zone of transition but most tumors show an intermediate zone between the lesion and normal bone pattern.