Introduction

Lipomas are benign tumors composed of mature adipocytes that can develop as solitary masses in any part of the body where fatty tissue is found.^1,2 Lipomas have been subclassified in humans according to their site of origin. These include subcutaneous, intermuscular and intramuscular, intrathoracic, intraperitoneal and retroperitoneal, intraoral, arising in various organs or nervous systems, synovial, and bone lipomas.² Intra-articular lipomas are masses of fatty tissue contained within a thin fibrous capsule.^3,4 These lipomas arise within the joint either by penetrating the synovial membrane or as a result of fat overgrowth from within the intra-articular synovial tissue.² When the tumor has rich fibrous tissue, it is referred to as a fibrolipoma. Although lipomas are the most common soft tissue tumors, intra-articular lipomas are extremely rare in humans.⁵ In the human literature, intra-articular lipomas are only reported as case reports. The majority of intra-articular lipomas in humans arise from the knee joint, but they have also been described in the ankle, hip, shoulder, elbow, and facet joint of the lumbar spine.^5–10 For the knee, they tend to occur in the fat pad or in the layer of fat deep to the suprapatellar pouch.^1,3

People report an acute onset of pain that primarily occurs with increasing activity and shows no improvement with nonsteroidal anti-inflammatories or physical therapy. Locked knee has been documented due to impingement of the lipoma within the patellofemoral joint, intercondylar notch, or with the meniscus. In these instances, an interruption of normal motion occurs intermittently. Additionally, many of these lipomas have a narrow stalk that it can twist around and become strangulated.¹¹ Surgical removal in these cases has been reported to provide immediate resolution of discomfort.^12,13

Intra-articular lipomas are further classified in the human literature. These consist of lipoma arborescens and the true lipoma.¹⁴ Lipoma arborescens is a proliferation of the adipose tissue from the synovial membrane. This is related to a degenerative process from chronic synovial irritation, rather than a true neoplasm. Lipoma arborescens is encountered more frequently in the knee joint compared with a lipoma.⁸ This has been reported in a dog once, but no surgical intervention was required.¹⁵ This can be differentiated from a true lipoma by its villous appearance and associated joint effusion or synovial cysts.¹⁴ Additional methods for diagnosis are through diagnostic imaging. Matsumoto et al. were the first to report a true intra-articular lipoma on MRI. They described that the intra-articular lipoma shows a high signal intensity with linear structures of low signal intensity on T1-weighted and T2-weighted images. The difference in the signal intensity on T1-weighted images is explained by the histologic extent of mucoid degeneration in the tumor. Differentiation between true lipomas and lipoma arborescens is essential because the treatment differs in that lipoma arborescens is typically treated by synovectomy and true lipomas are locally excised.⁸

An intra-articular lipoma has been documented only once prior in a dog. This was an abstract presentation of a 2 yr old male intact pit bull terrier who presented for evaluation of a pelvic limb lameness. On examination, a swelling was present from the mid-diaphyseal region of the femur to the proximal aspect of the stifle joint. Radiographs and MRI revealed cranioproximal translation of the patella and a heterogeneously hyperintense T1-weighted and T2-weighted signal intensity mass lesion, respectively. The mass was surgically resected and thought to have arisen from the infrapatellar fat pad. Histopathology in this case was consistent with a fibrolipoma, but an infiltrative lipoma was not excluded. The dog had a full recovery following surgical excision with 3 mo of follow up.¹⁶

The purpose of this case report was to describe the clinical presentation and treatment of an intra-articular fibrolipoma in the stifle of a dog and provide the short- and medium-term outcome.

Case Report

A 12 yr old male castrated miniature Australian shepherd presented to the Colorado State University Veterinary Teaching Hospital for consultation of bilateral medial patellar luxations (MPLs). The patient had a history of known bilateral MPLs but became acutely left pelvic limb lame 3 mo before presentation. Gabapentin and carprofen^a were started because of the suspicion of exacerbation of chronic osteoarthritis and MPLs. The dog’s other medical history included suspected tracheal collapse, which was managed with theophylline and hydrocodone. On presentation, the patient was 11 kg with an overconditioned body condition score of 7/9.¹⁷ A new grade II/VI left apical systolic heart murmur was ausculted, which revealed degenerative mitral valve disease, stage B1, on echocardiogram. On orthopedic examination, the patient had a thoracic limb orthopneic posture, which was reportedly present since being young, with an associated bilateral grade 2/5 lameness. Additionally, the dog had a stilted gait with a hunched posture and grade 3/5 left and grade 2/5 right pelvic limb lameness. The patient was hesitant to sit and slow to rise but was able to do so. There was bilateral elbow varus and decreased carpal flexion. The right stifle had a grade 3/4 MPL with mild periarticular thickening but no palpable effusion or instability. The left stifle had a grade 2/4 MPL with a mass-like effect (approximately 2 × 3 cm) medially. No stifle instability or effusion was palpated on the left side.

Orthogonal radiographs were performed of the stifle joints bilaterally. The right stifle had a medially luxated patella, hypoplastic femoral trochlear ridges, patellar ligament enthesophytosis, calcaneal tendinopathy with enthesophytosis, and mild stifle osteoarthrosis. The left stifle had a medially luxated patella with moderate patellar desmopathy and enthesophytosis, mild osteoarthrosis and increased intra-articular soft tissue opacity consistent with synovial hyperplasia/effusion, and mild calcaneal tendinopathy, as well as significant medial distention of the joint capsule by a soft tissue opacity (Figure 1). Joint taps and a fine-needle aspirate of the suspected medial stifle joint mass were performed. Joint taps were performed of the carpi and stifle joints bilaterally to assess for a polyarthropathy. Cytology of the right carpal joint fluid was consistent with a normal joint, and the left carpal joint was acellular and only had a small amount of joint fluid that did not allow for cytologic interpretation. The right stifle cytology was consistent with mild mononuclear inflammation, and the left stifle cytology contained a low cellularity of polygonal to spindle-shaped cells with mild anisocytosis and anisokaryosis associated with large mononuclear cells and mesenchymal cells. A reactive or neoplastic process was prioritized for the left stifle joint.

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A complete blood count and chemistry profile revealed no abnormalities. Three-view chest radiographs had been previously performed at the primary veterinarian, which revealed no abnormalities. Abdominal ultrasound was elected for metastatic screening before surgical intervention and revealed a nodule on the cranial pole of the left adrenal gland suspected to be most consistent with a benign lesion such as adenomatous hyperplasia or adrenocortical adenoma, and less likely adenocarcinoma. No other abnormalities were identified.

An exploratory stifle arthrotomy was offered to assess for the presence of a mass and the severity of the MPL and evaluate the cruciate ligaments and menisci. If a mass was found, then either an incisional or excisional biopsy could be performed. However, if no mass or cruciate ligament pathology was identified, then MPL correction would be performed. The contemporary standard of care, as described in AAHA policy statements, was provided. A left medial parapatellar stifle arthrotomy was performed. Upon incision of the joint, an ∼2 × 4 × 1 cm fat-like mass was visualized attached to a fibrous stalk at the craniolateral aspect of the joint. It was suspected to be arising from the infrapatellar fat pad (Figure 2). The lesion was easily removed after the stalk was transected with monopolar electrosurgery. The mass and a section of joint capsule were submitted for histopathology. Following removal, a standard stifle joint explore was performed. The cruciate ligaments and menisci were intact without pathologic changes. However, the trochlear groove was shallow and moderate osteophytosis was present along the trochlear ridges. Owing to patient desaturation under general anesthesia and suspicion that the large intra-articular mass could be the sole cause for their lameness, no MPL correction was performed. The left popliteal lymph node was extirpated to screen for metastasis. Routine closure was performed with incisional infiltration of bupivacaine liposome injectable suspension^b. Following anesthetic recovery, the patient was discharged on the same day with instructions for 2 wk of activity restriction. It was recommended to continue carprofen and gabapentin as was previously prescribed by the referring veterinarian.

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Histopathology results were consistent with a benign fibrolipoma with necrosis of the joint mass and fibroadipose of the joint capsule. The popliteal lymph node found sinus hemorrhage, histiocytosis, and neutrophilia. The current level of excision was expected to be curative. Four days after surgery, the patient’s owner perceived the left pelvic limb lameness to have greatly improved. At the 14-day postoperative recheck, the family reported a return to normal gait with marked improvement in comfort. The patient’s orthopedic examination was similar to that previously described other than the absence of the left stifle mass effect. Stifle range of motion was comfortable and MPL grading was unchanged. The incision site was healed and it was instructed that the patient could return to normal activity.

The patient was reassessed at 24 wk following surgery. Since surgery, the left pelvic limb lameness resolved. The family elected to continue him on gabapentin for generalized osteoarthritis management. The patient’s activity was maintained at 10 min walks twice daily. Further activity was limited because of the patient’s collapsing trachea. Recheck examination revealed similar bilateral grade 2/5 thoracic limb lameness and grade 2/5 right pelvic limb lameness. The previous grade 3/5 left pelvic limb lameness had decreased to a grade 1/5 lameness. The grade 3/4 right MPL was unchanged and previous grade 2/4 left MPL was reduced to a grade 1/4 MPL. Repeat radiographs were performed, which revealed similar left medial patellar luxation and enthesophytes with mildly progressive periarticular osteophytosis. No residual soft tissue within or displacement of the joint capsule was visualized. No further surgical interventions were recommended. Informed consent was obtained from the patient’s owners before all aforementioned diagnostics, anesthesia, and surgery.

Discussion

This case report documents the presence of a rare intra-articular lipoma in a dog, which has only been documented once previously in an abstract presentation.¹⁵ Our patient presented for an acute worsening of a unilateral pelvic limb lameness that was suspected to be associated with historical MPLs. Examination revealed a mass effect at the medial aspect of the left stifle that was nonpainful on palpation. The previously diagnosed MPL was noted without the presence of stifle instability. Considering this was an older patient with a historical MPL diagnosis and who had an acute onset of a worsening unilateral pelvic limb lameness, further workup was performed to investigate for other causes of lameness. Radiographs and fine-needle aspirate of the left stifle mass identified expansion of the stifle joint capsule medially and a mesenchymal cell population, respectively. The intra-articular mass was suspected to be a soft tissue instead of a fat opacity on radiographs owing to the fibrous component of the mass. Stifle joint exploration revealed a large fat mass encompassing the majority of the stifle joint. Histopathology was consistent with a fibrolipoma as opposed to lipoma arborescens. Resection of the tumor resulted in immediate improvement in the patient’s comfort as perceived by the owners at dismissal, as well as at 2 and 24 wk rechecks.

The majority of the information for intra-articular lipomas exists within the human literature; however, even that is sparse and consists of individual case reports. For people, MRI is the gold standard tool for diagnosis and preoperative planning of intra-articular masses. True lipomas are seen as well-circumscribed solitary lesions with a thin capsule and a signal intensity analogous to that of subcutaneous adipose tissue on all sequences.^18,19 The diagnosis and excision of an intra-articular lipoma has also been described with arthroscopy using a standard arthroscopic shaver.¹³

Based on this patient’s age, known orthopedic disease, and osteoarthritic changes, lipoma arborescens was considered as a differential but was not consistent on histopathologic review. A case report by Min et al. described an intra-articular synovial lipoma of the knee joint causing patellar dislocation.²⁰ We suspect this is not the case in this patient, however, as the MPL was bilateral and is a common developmental disorder predisposed by skeletal abnormalities in small-breed dogs. However, the mass may have been the cause of the increased MPL grade as the grade reduced following surgery.

The limitations of this report are inherent to the nature of being a single case with short-term clinical follow-up and objective outcome measures. Subjective outcome could have been considered with gait analysis.

Conclusion

This report provides additional information for a rarely documented intra-articular lipoma in a dog. Fibrolipomas of the stifle, although rare, should be considered as a differential for a smooth intra-articular mass. For better characterization before surgical intervention, MRI could be considered in future cases. This study is limited in long-term follow up; however, excision is expected to be curative.