Introduction

A 5-year-old 4.0-kg (8.8-lb) castrated male domestic shorthair cat was referred to the authors' clinic because of grade 4/5 lameness of the left hind limb. Physical examination of the affected limb revealed a painful swelling of the second metatarsal bone. Orthogonal radiographic views showed the swelling to be attributable to cystic enlargement of the bone associated with cortical thinning (Figure 1). Cytologic evaluation of fine-needle aspirates of the swelling revealed malignant mesenchymal cells, and a primary bone tumor was diagnosed on the basis of cytologic findings. Computerized tomography of the entire body was done to screen for other possible tumors, but results were negative for other tumors. Multiplanar reconstruction of the second metatarsal bone showed an expansile osteolytic lesion associated with a periosteal reaction on the third metatarsal bone (Figure 2). On the basis of results of radiography and CT, a bone biopsy was recommended. However, the owner declined the procedure because of its associated risks, including the possibility of iatrogenic bone fracture. The offer was made to completely excise the lesion using 1 of the following 4 surgical techniques: amputation of the distal portion of the limb, amputation of the second and third metatarsal bones in combination with partial tarsal arthrodesis,¹ amputation of only the second metatarsal bone with monitoring of the periosteal reaction on the third metatarsal bone, and amputation of the second and third metatarsal bones with stabilization of the medial compartment of the tarsometatarsal joint without permanent joint immobilization. The first 3 options were declined by the owner because of the associated high risk of complications and the potential that revision surgery would be needed. The fourth option was considered the most attractive option because of the lower likelihood of complications. In addition, surgical failure related to this option could be managed through revision surgery involving tarsal arthrodesis or limb amputation.

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Radiographic images of a 5-year-old domestic shorthair cat referred because of grade 4/5 left hind limb lameness and swelling in the region of the second metatarsal bone. Dorsoplantar (A) and mediolateral (B) radiographic views of the left tarsus showed that the swelling was a result of cystic enlargement of the bone associated with cortical thinning. The lesion was later confirmed to be an ABC. The second and third metatarsal bones and associated phalanges were amputated, and the tarsometatarsal joint was stabilized with a figure-eight suture running from a screw placed in the distal row of the tarsal bones to a hole in the proximal metaphysis of the fourth metatarsal bone. Follow-up radiographs were obtained immediately (C and D) and 90 (E and F) and 210 (G and H) days after surgery.

Citation: Journal of the American Veterinary Medical Association 259, 3; 10.2460/javma.259.3.294

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Preoperative multiplanar reconstructed (A) and 3D reconstructed (B) CT images of the left metatarsal region. Notice the periosteal reaction involving the third metatarsal bone.

Citation: Journal of the American Veterinary Medical Association 259, 3; 10.2460/javma.259.3.294

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The owner was made aware of the novel aspects of the procedure and provided informed consent. Surgery was carried out 1 week after CT was performed. The cat was premedicated with methadone (0.2 mg/kg [0.09 mg/lb], IM), dexmedetomidine (5 µg/kg [2.3 µg/lb], IM), and ketamine (1 mg/kg [0.45 mg/lb], IM). Anesthesia was induced with propofol (4 mg/kg [1.8 mg/lb], IV) and maintained with isoflurane administered via an endotracheal tube. Intraoperative analgesia was supplemented with a peripheral nerve block. Cefazoline (30 mg/kg [13.6 mg/lb], IV) and lactated Ringer solution (5 mL/ kg/h, IV) were administered.

The patient was positioned in dorsal recumbency, and a dorsal approach was used for disarticulation of the second and third metatarsal bones and their associated phalanges. The remaining metatarsal bones were determined intraoperatively to be unstable with respect to the tarsal bones. Therefore, a 2.0-mm cortical screw (14-mm length) was inserted from the medial to the lateral aspect of the distal row of tarsal bones, and a 1.5-mm hole was drilled from the dorsal to the plan-tar surface of the proximal metaphysis of the fourth metatarsal bone. A single strand of 2-0 monofilament nonabsorbable suture material (polypropylene) was passed through the bone tunnel and around the head of the screw in a figure-eight fashion (Figure 3) to stabilize the tarsometatarsal joint. The subcutaneous tissues and skin were closed routinely, and the excised meta-tarsal bones were submitted for histo-logic examination.

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Intraoperative photograph obtained after amputation of the second and third metatarsal bones showing the head of the screw in the distal tarsal bones (asterisk) and the hole at the base of the fourth metatarsal bone (cross).

Citation: Journal of the American Veterinary Medical Association 259, 3; 10.2460/javma.259.3.294

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Standard (Figure 1) and stressed (Figure 4) orthogonal radiographic projections were obtained immediately after surgery. Joint stability was evaluated and confirmed to be present both clinically and radiographically. No external coaptation was applied. Pain during the hospitalization period was managed with IM administration of methadone every 4 hours. The cat was discharged 3 days after surgery with prescriptions for cefalexin (30 mg/kg, PO, q 12 h) for 10 days and meloxicam (0.05 mg/kg [0.023 mg/lb], PO, q 24 h) for 5 days. Cage rest was recommended for 60 days.

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Dorsal (A), plantar (B), medial (C), and lateral (D) stress radiographic views of the left tarsus obtained immediately after amputation of the second and third metatarsal bones and stabilization of the tarsometatarsal joint.

Citation: Journal of the American Veterinary Medical Association 259, 3; 10.2460/javma.259.3.294

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Clinical reevaluations were performed 7, 15, 30, 60, 90, and 210 days after surgery, and radiographic reevaluations were performed 90 and 210 days after surgery (Figure 1). Lameness was no longer observed 30 days after surgery, and signs of pain were not evident during palpation of the foot. For this reason and given the aggressive temperament of the cat, a decision was made with the owner to omit radiographic reevaluations 30 and 60 days after surgery.

At the 90-day reevaluation, the cat was sedated for radiography, and stress radiographic images indicated that stability of the tarsometatarsal joint was intact. Seven months after surgery, the cat was presented for evaluation of a skin lesion on the medial aspect of the tarsus caused by self-trauma. The subcutaneous polypropylene knot was determined to be the source of irritation, and the cat was anesthetized for removal of all implants. Bacterio-logic culture of a swab specimen obtained intraoperatively did not yield any bacterial growth. Joint stability was confirmed clinically and radiographically immediately after implant removal (Figure 5). The cat was reevaluated 18 months after implant removal, and clinical evaluation revealed that the treated joint was stable. The owner declined to allow the patient to be sedated for radiographic reevaluation.

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Medial (A) and lateral (B) stress radiographic views of the left tarsus obtained after removal of the implants, 7 months after the initial surgery.

Citation: Journal of the American Veterinary Medical Association 259, 3; 10.2460/javma.259.3.294

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Histologic examination of the second metatarsal bone revealed an ABC characterized as an isolated bone lesion with expansile and lytic features.

Discussion

Aneurysmal bone cysts are composed of large vascular sinusoids separated into compartments by thin-walled connective tissue stroma^1,2 and have been described in human beings and a variety of animal species.^1–21 Although different hypotheses have been proposed, the pathogenesis remains unknown. Possible causes include bone dystrophy,^2–7,19 bone hematoma attributable to trauma,^2,3,5,7–11 local circulatory disturbances and coagulopathy,^2,4,5,7–9 and benign bone tumor.^2,7–11 Aneurysmal bone cysts are generally characterized by locally aggressive growth but do not metastasize.^1,2 They could potentially become malignant as a result of surgical manipulation, but the relationship between ABCs and malignant tumors is unknown.³ The diagnosis of an ABC is made on the basis of clinical signs and the results of radiographic, cytologic, and histologic examinations.⁴ In the cat described in the present report, histologic examination was necessary to make a definitive diagnosis of ABC, because there was a discrepancy between cytologic and histologic findings, with results of cytologic examination suggestive of a malignant lesion or, more precisely, a sarcoma. These findings highlight the importance of considering cytologic findings in conjunction with diagnostic imaging findings and the clinical course of the disease. Although, in our case, radiographic findings were not necessarily suggestive of a malignant bone tumor, we decided to amputate the affected bone on the basis of the assumption that biopsy could have weakened or fractured the bone, leading to a worsening of clinical signs. If an ABC had been diagnosed cytologically, amputation would not have been performed and a less invasive technique would have been proposed.

Treatment options for ABCs include amputation,^13,16,19 saucerization,^2,22 radiation therapy, curettage, curettage with bone grafting, filling of the lesion with polymethylmethacrylate, alcohol injections, arterial embolization, injections of steroids and calcitonin, and cyst excision.^{2,4,13,16,19,21–24} Surgical treatment is usually recommended if the cyst is causing lameness, swelling, or pain as a result of compression of the surrounding soft tissues and stretching of the periosteum or if there is joint involvement, bone lysis, or pathological fractures.^13,14,17,25 The prognosis is excellent in human patients and animals when complete excision is feasible.^2–4,26

In the present case, treatment options were based on the results of cytologic examination, which suggested the presence of malignant mesenchymal cells. The treatment for malignant bone tumors such as osteosarcoma is complete excision of the affected bone and chemotherapy.²⁷ Although complete excision is one of the reported treatments for ABCs, it would not be the first surgical choice of the authors. If the cytologic examination had revealed an ABC, our first surgical choice would have been curettage with bone grafting rather than amputation.

Limb-sparing surgery was considered in the present case because of the patient's age and the possibility of contralateral limb disorders later in life with amputation and because the lesion was limited to the second metatarsal bone with CT evidence of a periosteal reaction involving the third metatarsal bone. Mesenchymal bone tumors do not usually invade surrounding tissues and do not typically cross the joint surface,²⁸ although they can behave atypically in some cases.²⁷ For this reason, limb-sparing surgery is recommended to preserve limb function.²⁹

During preoperative planning for the cat described in the present report, medial tarsometatarsal joint instability was expected because of resection of the ligaments attached to the medial surface of the proximal metaphysis of the second metatarsal bone and disarticulation of the second and third metatarsal bones. Medial tarsometatarsal joint instability is a concerning condition because the medial aspect of the tarsus is the tension side, and with instability, normal weight bearing would be expected to result in valgus deviation of the foot.³⁰ Treatments aimed at preserving tarsometatarsal joint movement, such as temporary immobilization with a cast or external fixator, have been associated with poor outcomes.^30–33 Thus, partial tarsal arthrodesis is considered the treatment of choice because it is associated with a better prognosis than temporary immobilization.^30,31,34

Intraoperative evaluation of tarsometatarsal joint stability in our patient revealed medial subluxation alone. For this reason, joint stability was restored with a polypropylene suture running from the head of a screw inserted in the distal tarsal bones to a tunnel in the base of the fourth metatarsal bone. This procedure was more conservative than arthrodesis and allowed preservation of anatomic structures and function. It would have been impossible to perform arthrodesis of the tarsometatarsal joint alone without including the other tarsal bones and joints.

External coaptation, such as a splint or cast, has been suggested after arthrodesis to avoid or manage postoperative swelling and reduce loading of the implants.^29,30,35,36 Other authors have recently reported tarsal arthrodesis in dogs and cats without postoperative coaptation.^37,38 Splint management in our case would have been challenging because of the temperament of the cat, and the outcome was good without the use of external coaptation. The condition of the cat improved during the postoperative period, and there was no apparent lameness 30 days after surgery.

The polypropylene suture used in the present case allowed immediate stabilization of the joint. It is possible that the suture stimulated periarticular fibrosis, which supported the articular load, because joint stability was present 7 months after implant removal. Moreover, this surgical technique allowed the removal of 2 metatarsal bones with preservation of joint movement. This eliminated the need for arthrodesis with its associated complications and reduced the cost of the operation.

Complications associated with tarsal arthrodesis can be major or minor and include failure of joint fusion, premature implant loosening or breakage, angular and rotational deformities due to incorrect positioning of the joint, and infection.³² In addition, the tarsal region has sparse soft tissue coverage, and use of a plate for tarsal arthrodesis is associated with a risk of postoperative wound dehiscence.³⁸ Because the tarsometatarsal stabilization technique used in the present case was novel, potential complications were uncertain. Potential major complications could include implant infection, implant failure, iatrogenic fracture of the proximal metatarsal metaphysis or the distal row of the tarsal bones, and recurrence of tarsometatarsal joint instability. Revision surgery is challenging after arthrodesis, whereas complications associated with our stabilization technique could be resolved by arthrodesis as a revision surgery.

To the authors' knowledge, there are no other case reports describing amputation of > 1 metatarsal bone in cats or techniques less invasive than arthrodesis for stabilization of the tarsometatarsal joint after amputation of 2 metatarsal bones. Moreover, the occurrence of an ABC in a metatarsal bone in a cat has not been reported previously. On the basis of the outcome for our case, we believe that this technique could be a feasible surgical option when treating various pathological conditions that necessitate metatarsal bone amputation.