Caudal thigh intermuscular lipomas in dogs: Anatomic review and approach to surgical excision

Abstract

The surgical approach for excision of caudal thigh intermuscular lipomas (IML) in dogs is described with relevant anatomy and short-term outcomes reported. Medical records were reviewed to identify dogs that underwent IML excision between 2015 to 2019. Signalment, location of the lipoma, pre-operative diagnostic tests, histopathology results, use of a closed-suction drain, and follow-up information including drain and suture removal were recorded. Mean age of patients in this study was 8.7 years. Multiple breeds were affected and there was no predilection for either left or right hind limb. Pre-operative diagnostic tests included fine-needle aspirate, radiography, peripheral ultrasonography, and/or computed tomography scan. In 45% (5/11) of the cases, a closed suction drain was placed. All masses removed were deemed grossly consistent with a lipoma by the attending clinician and 5 were confirmed by histopathology. No complications were noted in any case. Removal of caudal thigh IML requires careful identification of and dissection around the sciatic nerve, which is easily achieved with appropriate knowledge of the relevant anatomy and surgical approach.

Introduction

Lipomas are mesenchymal tumors of adipose tissue that may occur in any location of the body (1). They are most often identified in middle-aged to older dogs (1,2). Intermuscular lipomas (IML) in dogs have previously been described, and typically arise in the caudal thigh, most often between the semimembranosus and semitendinosus muscles (1). Dogs with a caudal thigh IML may present with a large, firm mass that can cause discomfort, gait alterations, and lameness (1,2). Affected patients are often referred to a specialist following a presumptive diagnosis of malignancy with a suspected poor prognosis (2). However, these masses are often easily removed and surgical excision frequently carries an excellent prognosis (1). Limited information exists in the veterinary literature regarding IMLs, and the surgical approach to the caudal thigh for their removal has not been described previously. The purpose of this study is to describe the surgical approach for the excision of a caudal thigh IML; including a detailed description of the relevant anatomy and to report the short-term outcome of dogs undergoing this procedure.

Materials and methods

Medical records were searched for dogs diagnosed with a caudal thigh intermuscular lipoma at Veterinary Specialty and Emergency Center in Pennsylvania evaluated between the years 2015 to 2019. All dogs underwent surgical excision with the same procedure. Data retrieved included age, breed, weight, sex, sterilization status, location of the lipoma, pre-operative diagnostic tests performed, histopathology results, use of a closed-suction drain, and follow-up information including drain removal and suture removal.

Relevant anatomy

Musculature

The muscles of the caudal thigh include 2 distinct groups that originate from the ischiatic tuberosity; one of which travels lateral to the stifle joint (biceps femoris and abductor cruris caudalis) and one that courses medially (semimembranosus and semitendinosus) (3). Together, these 2 muscles groups are referred to as the hamstring muscles (3). The biceps femoris originates from the sacrotuberous ligament and ischiatic tuberosity and extends distally to its insertion points on the patella, patellar ligament, cranial border of the tibia, and tuber calcanei (3). This muscle extends the hip and contributes to both flexion and extension of the stifle as well as tarsal joint extension (3). The abductor cruris caudalis lies deep to the biceps femoris and originates from the aponeurosis of the biceps femoris on the caudoventral edge of the sacrotuberous ligament near the ischiatic tuberosity and inserts on the digital extensors. This muscle allows for both abduction of the limb and flexion of the stifle (3). The semimembranosus muscle originates from the ischiatic tuberosity and inserts at the distal femur and proximal tibia (3). It lies between the biceps femoris and semitendinosus laterally and the adductor and gracilis medially (3). This muscle flexes the stifle and allows extension of the hip when the paw is in contact with the ground. The semitendinosus muscle arises from the ischiatic tuberosity and inserts on the medial tibia and tuber calcanei (3). It allows for extension of the hip and tarsal joints as well as flexion of the stifle in the non-weight-bearing limb. Intermuscular lipomas typically lie between the bellies of the semimembranosus and semitendinosus muscles (1,2).

Vasculature

The femoral artery is an extra-abdominal continuation of the external iliac artery that serves as the primary supply of arterial blood to the hind limb (3). Also arising from the external iliac artery is the deep femoral artery. This artery arises from the caudomedial surface of the external iliac artery, runs over the medial surface of the external iliac vein, and exits the abdominal cavity over the caudal aspect of the vascular lacuna. The deep femoral artery then becomes the medial circumflex femoral artery (3). This artery courses caudally distal to the femoral triangle and before ascending toward the obturator foramen and anastomosing with the obturator artery (Figure 1) (3).

Figure 1.

Vascular anatomy of the canine pelvic limb [courtesy of Hermanson et al (3)].

Innervation

Innervation to the previously described muscles is supplied by the sciatic nerve. It arises from the roots of the 6th and 7th lumbar nerves as well as the 1st sacral nerve root and is a continuation of the lumbosacral trunk (3). As it courses distally in the thigh, the sciatic nerve lies along the quadratus femoris, adductor, and semimembranosus; it then travels deep to the superficial gluteal muscle, and biceps femoris muscle (3). Along its course, the sciatic nerve branches into the tibial and common fibular nerves, though the level at which this occurs may be variable. This division is typically located between the biceps femoris and adductor muscles (Figure 2) (3).

Figure 2.

Cross-section of the proximal thigh. The red X marks the location of the sciatic nerve at this level [adapted from Hermanson et al (3)].

Surgical technique

As a general description of surgical technique, the patient is placed in dorsal recumbency, and the affected limb is positioned using a hanging limb technique (Figure 3). The limb is aseptically prepared from the level of the tarsocrural joint to the dorsal midline. The clipped area should extend cranially to the level of the 4th lumbar vertebra and caudally to the base of the tail. An aseptic covering is wrapped around the limb at the level of the tarsocrural joint and is continued distally to cover the entire paw. Four corner drapes are applied and held in place with Backhaus towel clamps at each corner.

Figure 3.

A — Patient preparation prior to surgical procedure with hanging limb technique prior to draping. B — The intended skin incision is marked with a dashed line. (Photographs courtesy of Catherine Popovitch).

A longitudinal skin incision is made over the mass on the caudal aspect of the thigh and is extended through the subcutaneous tissues. The muscle bellies (most commonly the semimembranosus and semitendinosus) are identified, bluntly separated by digital dissection, and retracted. The caudal approach minimizes the risk of encountering large blood vessels during the procedure, and hemostasis may be easily achieved with electrocautery. Occasionally, the lipoma may be identified between different muscle groups; including, the quadriceps and gracilis, semimembranosus and biceps femoris, semimembranosus and gracilis or semitendinosus and biceps femoris (1,2). However, all these locations may be accessed by the same initial surgical approach. The lipoma is identified and bluntly dissected from the surrounding musculature using hemostats and digital dissection. When the specific muscles cannot be identified, the mass should be palpated to guide dissection through the muscle bellies. If the mass cannot be palpated or visualized, it is recommended to dissect between the semitendinosus and semimembranosus muscles, since this is where the mass is most commonly located. Lipoma growth may displace the sciatic nerve, causing it to lie superficial to the tumor, and care must be taken to identify and avoid the nerve during dissection (Figure 4). In some cases, particularly when the mass is large, the sciatic nerve may be noted to travel within the lipoma itself; therefore, the importance of careful, blunt dissection cannot be overemphasized. Sharp dissection should be avoided to minimize the risk of iatrogenic trauma to the sciatic nerve. Small blood vessels often supply the lipoma and should be cauterized or ligated as required throughout the procedure. Following lipoma excision, the surgical site is flushed with warm sterile saline. The muscle fascia is closed, and effort is made to minimize dead space with tacking sutures, using care to avoid inadvertent penetration of the sciatic nerve. The subcutaneous tissue and skin are closed routinely. If dead space is present and of concern following tumor removal, a closed-suction drain may be placed before closure.

Figure 4.

Identification of sciatic nerve after dissection and removal of an intermuscular lipoma. The semimembranosus muscle is marked by the X. (Photographs courtesy of Catherine Popovitch).

Results

Signalment and history

Eleven cases were available for review (Table 1). Mean age of dogs at the time of surgery was 8.7 y (range: 6 to 13 y). Affected breeds included mixed (n = 3), Weimaraner (n = 1), beagle (n = 1), Cairn terrier (n = 1), Labrador retriever (n = 1), Jack Russell terrier (n = 1), goldendoodle (n = 1), corgi (n = 1), and Australian shepherd (n = 1). Mean weight was 26.8 kg (range: 7.4 to 47 kg). Fifty-five percent (6/11) were spayed females and 45% (5/11) were neutered males. All patients were sterilized. The right limb was affected in 55% (6/11) of cases and the left limb was affected in 45% (5/11) of cases. In 6 cases, the presenting complaint was a chronic swelling of the hind limb observed by the owners. In 2 of these cases, the swelling was noted to result in pain; however, in the remaining 4, the swelling was not painful. Four cases presented for evaluation of an acute onset of rapid swelling in the hind limb. In one case, the swelling was identified by the groomer.

Table 1.

Description of cases.

Case	Age (y)	Breed	Weight (kg)	Sex	Laterality	Reason for visit	Location	Biopsy	Diagnostics performed?	Follow-up (d)	Other	Complications
1	9	Coonhound mix	39.2	SF	Left	Swelling	Between biceps femoris and semitendinosus	Infiltrative lipoma	US	12		No
2	11	Weimaraner	37.4	MN	Right	Swelling	Between semimembranosus and gracilis	Not performed	FNA	13		No
3	9	Beagle	17.2	MN	Right	Mass present for 2 y and then started to grow faster	Between semimembranosus and semitendinosus	Lipoma	US	12	JP drain placed	No
4	7	Cairn terrier	12.6	SF	Left	Noted by groomer	Between semimembranosus and semitendinosus	Infiltrative lipoma	CT	13	JP drain placed	No
5	6	Mixed breed	10.9	SF	Right	Swelling	Between semimembranosus and biceps femoris	Not performed	FNA	11		No
6	8	Labrador retriever	45.2	SF	Left	Mass growing	Between semimembranosus and semitendinosus	Lipoma	US, CT	120	JP drain	No
7	12	Jack Russell terrier	7.4	MN	Right	Swelling in inguinal area	Between semimembranosus and semitendinosus	Not performed	FNA, Radiographs	21	Bandage placed	No
8	9	Goldendoodle	40.4	MN	Right	Mass growing	Between quadriceps and gracilis	Lipoma	CT	11	JP drain placed	No
9	13	Corgi	10.3	SF	Left	Swollen and painful hind limb	Between semimembranosus and semitendinosus	Not performed	US	240		No
10	12	Australian shepherd	27.4	SF	Right	Mass growing and owners concerned	Between semimembranosus and semitendinosus	Not performed	None	15		No
11	9	Mixed breed	47	MN	Left	Mass growing, discomfort	Between semimembranosus and semitendinosus	Not performed	CT	180	JP drain placed	No

SF — Spayed female; MN — Neutered male; US — Ultrasound; FNA — Fine-needle aspirate; CT — Computed tomography; JP drain — Jackson-Pratt drain.

Physical examination

In each case, a mass effect in the caudal thigh region was noted on physical examination. The presence or absence of lameness and detailed measurements of the mass were not consistently recorded in the medical records.

Preoperative diagnostic tests

In all cases, complete blood (cell) count and serum biochemistry panels were completed before general anesthesia and surgery. Fine-needle aspirate was done in 3 cases, 2 of which were consistent with a lipoma and 1 of which was inconclusive. Nine percent (1/11) of cases had radiographs performed in which a large, fat-opaque mass in the stifle region of the pelvic limb was identified. Focal ultrasound alone was performed in 27% (3/11) of cases. Ultrasound typically revealed a hyperechoic to echogenic coarse encapsulated mass between muscle bellies, most frequently the semimembranosus and semitendinosus muscles. Nine percent (1/11) had both an ultrasound and a CT scan performed. Twenty-seven percent (3/11) cases had a CT scan performed alone. No diagnostic imaging was performed in 9% (1/11) of cases.

Diagnoses

In 54% of the cases (6/11), the excised mass was not submitted for histopathology. Each of these masses were assumed to be a lipoma based on gross appearance as determined by the attending clinician. Forty-five percent (5/11) of the excised masses were submitted for histopathology. Twenty-seven percent (3/11) of cases were lipomatous. These masses were described as being composed of well-differentiated adipocytes arranged in sheets divided by thin fibrous bands. Nuclei were small and compressed to the periphery of the cells by large, clear cytoplasms. Less than 1 mitotic figure per high power field (hpf ) was identified. Eighteen percent (2/11) of the cases in which biopsies were performed were diagnosed as an infiltrative lipoma. These neoplasms were described as composed of sheets of well-differentiated, mature adipocytes dissecting between collagen bundles and skeletal muscle bundles. Nuclei were small and compressed to the periphery of the cell by abundant cytoplasmic lipid. Mitoses were less than 1 per hpf.

Anatomic location of the lipoma

The mass was located between the semitendinosus and semimembranosus in 64% (7/11) of cases in this study. The remaining masses were identified between the quadriceps and gracilis (9%, 1/11), semimembranosus and biceps femoris (9%, 1/11), semimembranosus and gracilis (9%, 1/11), and between semitendinosus and biceps femoris (9%, 1/11).

Follow-up

Mean follow-up was 58.9 d (range: 11 to 240 d). In the 45% (5/11) of cases in which a Jackson-Pratt closed suctioned drain was placed, it was removed a mean of 5.4 d following surgery (range: 3 to 7 d). Each patient returned for incision evaluation a mean of 13.8 d after surgery (range: 11 to 22 d). Upon re-evaluation, all dogs were considered to have recovered well with no evidence of lameness or seroma formation. No complications were noted in any of these cases.

Discussion

To the authors’ knowledge, the surgical technique for excision of a caudal thigh IML has not been previously described. An understanding of the normal anatomy of the canine hind limb as well as appropriate and careful utilization of the surgical approach as described may aid in surgical planning and minimize the risk of iatrogenic trauma to local vessels and nerves. Although surgical removal of large masses such as IMLs can initially appear difficult, these tumors may be removed successfully with minimal risk for complications through the use of blunt dissection and by careful avoidance of the sciatic nerve.

In the present study, all dogs were geriatric, which is consistent with reported data (1). Previously, Labrador retrievers were reported to be an overrepresented breed; however, in our study, only 1 Labrador retriever was identified (1,2). It is possible that this is due to our small sample size.

Intermuscular lipomas manifest as large, firm masses associated with the musculature of the caudal thigh. Soft tissue sarcomas and other invasive masses can also develop in this region, which may be why these cases are often referred to a Board-certified surgeon for further evaluation (2). Useful preoperative diagnostic testing may include fine needle aspirates, radiography, peripheral ultrasound, and/or computed tomography. Although fine-needle aspiration is the least sensitive of these diagnostic tests, it may be used for increasing the clinician’s level of suspicion and for ruling out other potential malignancies. Similarly, radiographs are neither specific nor sensitive for the diagnosis of lipomatous masses, although they too may help increase the level of clinical suspicion based on the presence of a large mass with a fat opacity located caudal to the stifle joint. Peripheral ultrasound can be used to further characterize caudal thigh IML. These masses have been described to have a striped appearance with a thin hyperechoic capsule (4). If the mass does not have this appearance, it is less likely to be a lipoma and further diagnostic tests are recommended (4). Another modality to identify this type of mass is computed tomography (CT). This typically reveals a round- to oval-shaped, well-defined mass of strongly hypoattenuating tissue that is tomographically similar to subcutaneous fat and displaces surrounding muscle bellies, causing them to conform to the outer surface of the mass (5). All masses evaluated by CT were well-defined and infiltration of muscle and/or bone was not appreciated in any case. Currently, no published data comparing the utility of ultrasound compared to CT for the identification of IMLs exist in the veterinary literature and therefore it remains unknown whether one diagnostic modality is more sensitive or specific than the other when used for this purpose.

All cases in which samples were submitted for histopathology were diagnosed as a lipoma or infiltrative lipoma, which was consistent with findings on preoperative diagnostic tests. Typically, tumor definition and shape are the most useful parameters for differentiation between lipomas and infiltrative lipomas on peripheral ultrasound and CT scan.

Jackson-Pratt closed suctioned drains were placed in 5/11 cases following removal of the mass. The intended function of the drain was to minimize dead space and to reduce potential seroma formation. None of the cases in this study developed a seroma; however, it is difficult to draw conclusions about the efficacy of closed-suction drainage to reduce seroma occurrence due to the small sample size. In addition, due to the location of the defect created by removing the mass, it is possible that a deep seroma may form and go unnoticed during the postoperative evaluation. Future studies to determine the efficacy of seroma reduction by use of closed-suction drains following IML removal may prove beneficial. Although the establishment of passive drainage (i.e., placement of a Penrose drain) was not performed in any of the patients evaluated in this study, this may also be an option if dead space is present and concerns regarding the risk of seroma development exist.

Although our report offers a practical guide and comprehensive description of lipoma removal from the caudal thigh, limitations inherent to retrospective nature of the study exist. In addition, as the presence of a caudal thigh IML is a relatively infrequent presenting complaint, the sample size of affected dogs in this study is small. It is possible that both diagnostic findings and short-term outcomes may have been different if a larger number of cases had been assessed. Certain cases had closed-suction drains placed according to surgeon preference and therefore this was not consistent through all the cases. Furthermore, as long-term follow-up evaluation was not performed, the rate of recurrence or subsequent development of new lipomatous masses in the affected limb (or distant locations) cannot be determined.

In conclusion, removal of IML from the caudal thigh can be performed easily following incision and blunt separation of the thigh musculature. This requires careful identification of and dissection around the sciatic nerve, which is possible to perform with minimal risk of iatrogenic damage provided the anatomy and surgical approach are well-understood. CVJ