Introduction

A 2-year-old 5.1-kg (11.2-lb) castrated male Siberian cat was examined because of a history of an abnormal right pelvic limb gait and a 4- to 5-month history of progressive constipation. Radiographs obtained by the referring veterinarian showed an osteo-productive and osteolytic bony lesion involving the right ischium that filled the ipsilateral obturator foramen. Results of a CBC and serum biochemical profile performed by the referring veterinarian were within reference limits.

On physical examination, a hard mass was palpable in the right inguinal area (Figure 1). No appreciable right pelvic limb lameness was noted during the examination. Rectal examination performed under general anesthesia revealed a smooth bony mass on the ventral aspect of the right pelvic floor with marked reduction in the pelvic canal space.

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Photograph of a mass in the right inguinal region of a 2-year-old 5.1-kg (11.2-lb) castrated male Siberian cat that was examined because of a history of an abnormal right pelvic limb gait and a 4- to 5-month history of progressive constipation.

Citation: Journal of the American Veterinary Medical Association 259, 4; 10.2460/javma.259.4.401

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Computed tomography of the abdomen, thorax, and proximal aspects of the pelvic limbs was performed, with images obtained before and after contrast administration. A 3.9 X 3 X 4.6-cm, markedly expansile, mineralized, multilobular mass bridging the right obturator foramen was present. The ventral component of the mass was slightly larger than its dorsal component (Figures 2 and 3). Lysis of the right pubic bone was present. The mass was closely associated with the remaining pelvic bones in this area without distinct evidence of disruption of these bones. There was no obvious soft tissue involvement. The mass was also occupying most of the pelvic canal, resulting in marked leftward and dorsal deviation of the terminal portion of the large intestine and rectum. No evidence of pulmonary metastasis was present. Incidental findings of duplicated caudal vena cava and bilateral circumcaval ureters were noted. Also, the transverse and descending portions of the large intestine were distended with dense feces.

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Postcontrast axial-plane reconstructed CT image of the right pelvic mass. Image is displayed in a soft tissue window; slice thickness was 1 mm. A = Anterior (ventral). L = Left. P = Posterior (dorsal). R = Right.

Citation: Journal of the American Veterinary Medical Association 259, 4; 10.2460/javma.259.4.401

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Postcontrast transverse-plane reconstructed CT image of the right pelvic mass (circle). Image is displayed in a soft tissue window; slice thickness was 1 mm. F = Tail. H = Head. L = Left. R = Right.

Citation: Journal of the American Veterinary Medical Association 259, 4; 10.2460/javma.259.4.401

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Fine-needle aspirates of the pelvic mass were obtained for cytologic examination, which revealed osteoblast proliferation. The cytologic findings were indicative of reactive or hyperplastic bone with periosteal fibrosis. A definitive diagnosis was not reached.

The patient was premedicated with methadone (0.5 mg/kg [0.23 mg/lb], IV) and dexmedetomidine (5 µg/kg [2.3 µg/lb], IV), and anesthesia was induced with propofol (4 mg/kg [1.8 mg/lb], IV, to effect) and maintained with isoflurane. A lumbosacral epidural block was performed with 1 mL of 0.5% bupivacaine, which resulted in complete anesthesia of the pelvic region and pelvic limbs. An indwelling urinary catheter was placed for intraoperative identification of the urethra and for postoperative management of the patient.

The right pelvic limb to the dorsal midline, ventral aspect of the abdomen, and caudal pelvic region were prepared for surgery. The cat was positioned in dorsal recumbency with the pelvic limb free draped. The pelvic limb was abducted, and a skin incision was made from the medial aspect of the midthigh region, extending over the ischial mass and across the midline. The sartorius muscle was isolated with blunt dissection, freed from its distal insertion, and preserved in a moistened laparotomy sponge for reconstruction, as previously described.¹ The ventral aspect of the pelvis was exposed with a periosteal elevator. Osteotomies of the pubis and ischium were performed with an oscillating saw just to the left of the midline and approximately 1 cm from the tumor. The underlying tissue was protected with Hohmann retractors. The left obturator nerve and urethra were identified and protected.

The limb was abducted, and the hip joint capsule was located and incised. The round ligament was then located and incised, and the hip joint was disarticulated. The origin of the vastus lateralis muscle was elevated from the femoral neck. The limb was externally rotated, and a femoral head and neck excision was performed with an oscillating saw. The acetabulum was exposed, and the underlying soft tissue, including the right sciatic nerve, was protected with Hohmann retractors. An osteotomy was performed through the central part of the acetabulum approximately 1 cm from the tumor to release the ilium from the ischium. The semimembranosus and semitendinosus muscles were incised just distal to their origins. The quadratus femoris and adductor muscles were incised over the lesser ischiatic notch. The internal obturator muscle was not dissected from the tumor. The ischium and tumor within the obturator foramen were now free except for soft tissue attachments. These were gently dissected from the bone with a combination of a periosteal elevator and electrocautery. The adductor, gracilis, and pectineus muscles were dissected from the pubic midline with a combination of sharp dissection and periosteal elevation.

The bone segment and tumor were removed en bloc. The sartorius muscle flap was rotated into the resultant ventral defect, and the deep muscles were attached to surrounding tissues in their previous direction of action, when possible. The semimembranosus and semitendinosus muscles were sutured to the edge of the sartorius muscle. Muscles were sutured with 3-0 polydioxanone in a cruciate pattern. The subcutaneous tissues were closed with 4-0 polydioxanone in a simple continuous pattern. The skin was closed with 4-0 polypropylene in a cruciate pattern.

A wound diffusion catheter^a was placed in the subcutaneous tissue to allow for postoperative administration of bupivacaine (1.5 mg/kg [0.68 mg/lb], q 6 h). Postoperative radiography was performed to document the extent of excision (Figure 4). The excised tissues were inked for margin evaluation (Figure 5) and were then placed in neutral-buffered 10% formalin and submitted for histologic examination.

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Ventrodorsal radiographic image obtained immediately after a limb salvage procedure involving internal hemipelvectomy with ipsilateral ischiectomy, contralateral partial ischiectomy, ipsilateral partial acetabulectomy, and femoral head and neck excision was performed.

Citation: Journal of the American Veterinary Medical Association 259, 4; 10.2460/javma.259.4.401

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Photograph of the excised mass.

Citation: Journal of the American Veterinary Medical Association 259, 4; 10.2460/javma.259.4.401

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After surgery, the cat received crystalloid fluids (2.2 to 4.4 mL/kg/h [1.0 to 2.0 mL/lb/h], IV), cefazolin (22 mg/kg [10 mg/lb], IV, q 8 h), maropitant (1 mg/ kg [0.45 mg/lb], IV, q 24 h), lactulose (2 to 4 mL, PO, q 8 h), bethanechol (2.5 mg, PO, q 12 h), robenacoxib (2 mg/kg [0.9 mg/lb], SC, q 24 h), and a continuous rate infusion of fentanyl (3 µg/kg/h [1.4 µg/lb/h], IV). The cat had substantial postoperative swelling and decreased mobility of the right pelvic limb for the first 48 hours after surgery but was able to ambulate well by the third day of hospitalization. The urinary catheter and wound diffusion catheter were removed 72 hours after surgery; the cat was urinating well on its own after the urinary catheter was removed. The cat had a bowel movement while still hospitalized 4 days after surgery and was subsequently discharged that day with prescriptions for robenacoxib (6 mg, PO, q 24 h for 3 to 5 days), buprenorphine (0.09 mg, PO, q 8 h for 5 to 7 days), amoxicillin–clavulanate potassium (62.5 mg, PO, q 12 h for 7 days), and polyethylene glycol 3350 (PO, q 24 h, as needed for stool softening).

Histologic examination of the submitted excised tissue revealed a benign osteoma arising from an osteochondroma with complete excision. The cat was tested for FeLV antigen after surgery, and results were negative. At a recheck examination 9 days after surgery, the owners reported that the cat was doing well at home and ambulating on all 4 limbs, but that a large seroma had developed at the incision site.

The owners had noted signs of pollakiuria at home, and prazosin (1 mg/kg, PO, q 8 h) was prescribed because of possible postoperative urethral spasm. Three days later, the cat was examined by an emergency service because of stranguria and hematuria. On evaluation, the urinary bladder was soft and moderately distended. The cat was sedated with alfaxalone (2 mg/kg, IM) and buprenorphine (0.02 mg/ kg [0.009 mg/lb], IM) and immediately afterwards urinated a large amount in the litter. The emergency service added gabapentin (25 mg, PO, q 8 h) and buprenorphine (0.02 mg/kg, PO, q 8 h) to the cat's treatment regimen.

The cat was returned for suture removal 2 weeks after surgery. At this time, the seroma had resolved, and the sutures were removed from the incision made for the femoral head and neck excision. However, the sutures were not removed from the ventral abdominal incision because of signs of mild superficial infection, and the cat was prescribed amoxicillin–clavula-nate potassium (62.5 mg, PO, q 12 h).

Although the cat was ambulating well on the right pelvic limb, the owners were encouraged to enroll the cat in a rehabilitation program consisting of treadmill hydrotherapy and passive range-of-motion exercises to increase right pelvic limb muscle strength. The cat began the rehabilitation program 2 weeks after surgery and was reexamined 3 weeks after surgery. At the time of reexamination, the mild super-ficial incision infection had resolved, and the skin sutures were removed. On evaluation by the rehabilitation service, excellent limb use was noted with external rotation of the pelvic limb when the cat was standing. Passive range of motion of the tarsal, stifle, and hip joints was normal. Partial flexion of the tarsal and stifle joints was present. Therefore, the rehabilitation program was aimed at increasing the strength of the biceps femoris, semitendinosus, and semimembranosus muscles with the goal of improving stifle joint flexion. The cat underwent regular rehabilitation therapy consisting of treadmill hydrotherapy and home exercises for 3 months (Supplementary Videos S1 and S2, available at: avmajournals.avma.org/doi/suppl/10.2460/javma.259.4.401).

Ten months after surgery, contact was made with the primary care veterinarian, who reported that the cat was doing well using the right pelvic limb and that there had been no further episodes of urologic disorder. Follow-up with the owners 1 year after surgery revealed excellent limb usage (Supplementary Video S3, available at: avmajournals.avma.org/doi/suppl/10.2460/javma.259.4.401).

Discussion

The present report describes the surgical technique and functional outcome in a cat with a pelvic osteochondroma that underwent internal hemipelvectomy with ipsilateral ischiectomy, contralateral partial ischiectomy, ipsilateral partial acetabulectomy, and femoral head and neck excision for limb preservation. This surgical technique allowed for limb preservation, even with removal of the ischium and a salvage procedure of the hip joint. To the authors' knowledge, this represented the first case report in the veterinary literature that describes internal hemipelvectomy with ischiectomy, acetabulectomy, and femoral head and neck excision for treatment of a primary bone tumor.

Hemipelvectomy procedures can be subdivided into external hemipelvectomy and internal hemipelvectomy.² External hemipelvectomy involves ipsilateral limb amputation,³ whereas internal hemipelvectomy spares the ipsilateral limb.⁴ In dogs and cats, external hemipelvectomy is more commonly performed for treatment of neoplastic conditions,^5,6 but in humans, in whom limb preservation is preferred, internal hemipelvectomy is elected when possible.⁷

Although external hemipelvectomy has been well described in the veterinary literature, only isolated case reports^8–10 describing internal hemipelvectomy have been published. Oramas et al,¹⁰ for example, reported using iliectomy for treatment of a primary bone tumor in a dog with preservation of the ipsilateral hip joint and limb, and Oblak and Boston⁸ described using ischiectomy to remove a primary bone tumor in a dog with preservation of the ipsilateral acetabulum and hip joint. A similar procedure was used in 3 cats in which portions of the pelvis were removed after pelvic fracture and development of malunion and obstipation.⁹

If an ischial tumor extends cranially to include the acetabulum, ischiectomy and acetabulectomy must be performed to achieve appropriate surgical margins. In these cases, it is not possible to preserve the hip joint, and either the limb must be amputated or a salvage procedure for the hip joint must be considered. In humans, reconstruction with a prosthesis is typically performed after excision of the acetabulum.⁷

In humans with pelvic tumors, internal hemipelvectomy is a standard treatment because it spares the limb and preserves the patient's ability to walk.¹¹ Following internal hemipelvectomy in humans, the pelvis is left unreconstructed¹² or is reconstructed with a prosthesis.^13,14 The major disadvantage of internal hemipelvectomy without pelvic reconstruction is a greater leg-length discrepancy.¹² Usually these patients need long-term rehabilitation. Although it is challenging to compare outcomes between bipeds and quadrupeds, in the present case, the cat had excellent limb use even before initiating rehabilitation. The rehabilitation program was largely directed at strengthening muscles required for adduction of the limb and for stifle joint flexion and extension.

The case report⁸ describing ischiectomy in a dog noted that the patient had a grade 4/5 lameness 14 days after surgery. Aggressive rehabilitation was recommended but not pursued, and the dog remained mildly lame on the affected limb. In contrast, the cat described in the present report had excellent limb use immediately after surgery. Following femoral head and neck excision, the postoperative functional outcome is typically better in dogs weighing < 18 to 20 kg (40 to 44 lb).¹⁵ Thus, it is possible that cats may have a better functional outcome after internal hemipelvectomy than dogs, owing to their lower body weight.

The major differential diagnoses for a mass arising from the pelvis in a young cat are benign lesions such as osteochondroma and malignant lesions such as osteosarcoma, chondrosarcoma, and injection-site sarcoma. Computed tomography is warranted for staging and surgical planning, and thoracic and abdominal CT was performed in the cat described in the present report. Results of cytologic examination of a fine-needle aspirate of the bony lesion were inconclusive in the present case. However, a recent study¹⁶ reported that overall accuracy in diagnosing bone lesions was similar for cytologic and histologic examination. Ideally, a cytologic or histologic diagnosis would be obtained before performing surgery. In this case, biopsy was not performed before surgery because the cat was obstipated, and the final diagnosis would not have changed the surgical approach. Still, surgical oncologic principles were followed, with approximately 1-cm gross surgical margins around the tumor, which necessitated contralateral pubectomy and contralateral partial ischiectomy.

Osteochondromas are benign growths typically seen in young cats. They arise from the bone surface to form a projection covered by cartilage.¹⁷ The pathogenesis is not completely understood, but a causal relationship is suspected between FeLV and osteochondroma in cats.^17–19 In the present case, the cat tested negative for FeLV, and the underlying cause of the osteochondroma was unclear.

Hemorrhage is the most commonly reported complication of hemipelvectomy,^5,6,20 and some patients will require blood transfusions during or after surgery. The cat described in the present report did not require perioperative blood transfusions, and the major complication was the episode of postoperative urethral spasm. The reason for this episode was unclear, but the most likely causes were feline urologic syndrome and iatrogenic trauma. In humans and dogs, genitourinary complications have been reported after hemipelvectomy.^21,22 Placement of a uri-nary catheter during the procedure may aid in identification of the urethra intraoperatively, preventing iatrogenic damage.²³ In the present case, a urinary catheter was placed immediately before the surgery and maintained for 72 hours. The cat was urinating well in the hospital after the removal of the urinary catheter but developed temporary signs of feline uro-logic syndrome at home.

The cat described in the present report had a good functional outcome following surgery. To maintain mobility of the limb, preservation of the sciatic and femoral nerves is imperative; however, the obturator nerve is not necessary for mobility of the limb and can be sacrificed. In humans who have undergone acetabulectomy without prosthesis reconstruction, lifelong use of crutches is required,² but our findings suggested that acetabulectomy with femoral head and neck excision can result in good limb use in cats.

In conclusion, long-term results for the cat described in the present report suggested that internal hemipelvectomy involving ischiectomy, partial acetabulectomy, and femoral head and neck excision can result in a good functional outcome in cats if the procedure is planned appropriately with a full understanding of the regional anatomy and adherence to surgical oncologic principles.