Introduction

Glomus tumors are benign neoplasms that are a form of arteriovenous anastomoses arising from the neuromyoarterial glomus [1]. They are rare and account for 1.6 % of soft tissue tumors in the extremities [2]. Approximately 75 % of all glomus tumors occur in the hand, especially in the subungual region of the fingertips [3]. They are less commonly found in extradigital locations, where they have been reported to occur 27.6 % of the time [4]. Classic subungual tumors occur more frequently in women, while extradigital ones are more common in men [5].

In regards to the musculoskeletal system, extradigital glomus tumors have been described around the shoulder, elbow, hip, ankle, and foot [6–10]. Awareness of the entity is important as delays to diagnosis of between 20 to 30 years have been reported [11, 12]. A PubMed search for “glomus” and “knee” yielded approximately 53 reported cases of extradigital glomus tumors around the knee, the majority of which were isolated case reports. Of these, where specific locations were mentioned, the majority were in the overlying subcutaneous tissues [13–15]; a couple were in Hoffa’s fat pad [16, 17]; and one was within the joint itself [18].

We report a rare case of an extradigital glomus tumor that experienced a delay in diagnosis and ultimately required ultrasound guided needle localization for excision. Our patient exhibited classic symptoms and had supporting imaging findings of a glomus tumor, but went undiagnosed despite a decade-long workup. We believe that knowledge of glomus tumors occurring in extradigital locations would have raised higher suspicion and could have enabled an earlier diagnosis. We also underscore the importance of the radiologist’s role in management once the tumor is identified as they are small in size and can be difficult to excise without preoperative localization.

Case report

A 51-year-old male presented to us with chronic right knee pain for approximately 10 years in duration. It was reported that the symptoms initially began following a single episode of low level trauma. Since that time, the patient experienced chronic pain that interfered with activities of daily living.

On physical examination, the patient walked with an altered gait and had decreased range of motion in the right knee. There was exquisite tenderness localized to the medial aspect of the right quadriceps tendon insertion. The area was hypersensitive to touch and pain was excruciating when any pressure was applied. No systemic symptoms were reported.

The patient had previously undergone an extensive workup with multiple physicians prior to his visit. This included radiographs and magnetic resonance imaging (MRI) as well as diagnostic arthroscopy at outside institutions, which by report were all unrevealing. Over the past decade, several diagnoses had been rendered including medial plica syndrome, quadriceps tendinosis, chondromalacia patella, saphenous neuritis, and complex regional pain syndrome. This prompted several unsuccessful treatments including multiple steroid injections, non-steroidal medications, viscosupplementation, and numerous rehabilitation regimens.

Due to the unremitting nature of his pain, a new non-contrast MRI was obtained when he presented to our institution. At this time, a well-circumscribed 7-mm-mass was identified within the medial aspect of the suprapatellar fat pad (Fig. 1). No surrounding edema was present. There was no invasion into the adjacent bone or quadriceps tendon. There was no correlate finding on radiographs and an ultrasound was not initially obtained.

Fig. 1
figure 1

MRI. Sagittal T1 (a), sagittal proton density fat saturated (b), and axial proton density fat saturated (c) images demonstrating a small round mass within the suprapatellar fat pad (arrows)

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Since this mass corresponded to the site of the patient’s pain, an open surgical excision was performed. The presumed diagnosis was that of a ganglion cyst or peripheral nerve sheath tumor. Preoperative localization was not pursued because both access to and identification of the mass were not anticipated to be problematic as it was relatively superficial in location and well marginated by fat. However, intraoperative visualization proved to be difficult and the resultant histopathology yielded a diagnosis of fibroadipose tissue with fat necrosis and a foreign body giant-cell reaction.

Following excision, the patient’s pain maintained the same level of intensity as it had been previously. A repeat contrast enhanced MRI showed persistence of the mass, which was uniformly enhancing, but otherwise unchanged compared to the prior study (Fig. 2). A subsequent diagnostic ultrasound revealed the correlate of a rounded hypoechoic mass without associated vascularity (Fig. 3).

Fig. 2
figure 2

Contrast enhanced MRI. Coronal (a) and sagittal (b) images demonstrating a uniformly enhancing mass in suprapatellar fat pad (arrows)

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Fig. 3
figure 3

Sagittal ultrasound. Round hypoechoic nodule (arrow) in suprapatellar fat pad beneath the quadriceps tendon (arrowheads) (a–b). No associated increase in vascularity (b)

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Given the difficulty of intraoperative localization during the first excision, a needle localization procedure was planned prior to re-excision. Although the imaging appearance and chronic nature of the patient’s symptoms made risk for malignancy unlikely, the decision was made to bracket the mass to avoid any possibility of intra-articular tumor spill. Under ultrasound guidance, two 20-gauge-needles (Kopans Breast Lesion Localization Needle; Cook Medical, Bloomington, IN) were advanced to the superior and inferior margins of the mass through the patient’s pre-existing scar. Two 15-cm-spring-hookwires were then deployed so that they would converge and bracket the mass.

Intraoperatively, visual confirmation of the mass again proved to be difficult and a targeted resection of the tissue between the localizing needles was performed (Fig. 4). Parts of the adjacent synovium were resected to ensure clear margins and to decrease the risk of recurrence. Histopathology confirmed the excised mass to be a glomus tumor (Fig. 5). At postoperative follow-up, the chronic knee pain had dissipated and the patient was able to resume normal activity.

Fig. 4
figure 4

Intraoperative photographs. a Needle localization wires (arrowheads) bracketing the mass and guiding surgical resection. b Gross specimen following resection

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Fig. 5
figure 5

Glomus tumor histopathology. a Well-circumscribed homogenous and vascular nodule (arrows) located in adipose tissue (star) (2×). b Characteristic round cells (arrow), with eosinophilic cytoplasm, round and mostly central nuclei, and the accompanying blood vessels (arrowhead) in a myxoid/hyaline stroma (20×). Immunohistochemical stains were positive for c caldesmon (arrow) and d smooth muscle actin (arrow), as is typically seen in glomus tumors, confirming the diagnosis

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Discussion

The first description of a glomus tumor was by Wood in 1872 [19]. Masson later characterized it as a benign tumor arising from the glomus body [20]. Clinically, they are described as having a classic triad of hypersensitivity to cold, paroxysmal pain, and pinpoint pain [21]. They are most frequently subungual in location [3], but do infrequently occur in a variety of extradigital locations [6–10].

When they occur outside the finger, glomus tumors are found in a different demographic with a higher male to female ratio of 4:1 compared to 1:2 in the finger [22, 23]. Clinical symptoms are similar except for the fact that these patients frequently lack the temperature sensitivity component [22].

Although their small size may make them easy to overlook on imaging, glomus tumors are usually accurately identified on MRI. The imaging appearance is that of a small well-circumscribed mass that is iso- to slightly hypointense on T1-weighted sequences and hyperintense on proton density or T2-weighted sequences with homogeneous enhancement [24]. On radiographs, when they are subungual in location, there may be chronic extrinsic erosion of the underlying bone, which can simulate a lytic lesion with smooth sclerotic margins [25]. On ultrasound, glomus tumors are seen as round or ovoid hypoechoic masses [26]. They have been reported to be hypervascular [27], although in our case there was no associated vascularity.

The differential diagnosis for an extradigital glomus tumor includes a neuroma, hemangiopericytoma, hemangioma, angiomyoma, and a peripheral nerve sheath tumor [5]. A site-specific differential for the suprapatellar fat pad includes a ganglion or synovial cyst, lipoma, hemangioma, focal fibrosis, and peripheral nerve sheath tumor. Resection allows for a definitive diagnosis. At histopathological analysis, glomus tumors are comprised of vascular, smooth muscle, and neural components, measuring less than 1 cm in largest dimension [1].

The clinical symptoms and characteristic imaging findings of a subungual mass often provide adequate information for diagnosis of a glomus tumor. However, even in these cases that can be considered more typical, patients may see an average of 2.5 physicians prior to diagnosis [28]. The delay in diagnosis can be even longer when they occur in atypical locations.

This case illustrates the common characteristics associated with extradigital glomus tumors. Our patient was male and experienced the usual symptoms of intense pain without temperature sensitivity. He had nearly a 10-year lapse in time from the beginning of his symptoms until a diagnosis was made. We believe that a lack of awareness of extradigital glomus tumors occurring around the knee led to the failure of diagnosis. Indeed, upon retrospective review, an MRI performed at an outside institution 4 years prior to diagnosis showed the small mass.

The management of this case is noteworthy. Most glomus tumors are treated with curative surgical excision [17]. The first attempt at surgical excision in our case was unsuccessful as the small size of the tumor made intraoperative identification difficult. Ultrasound-guided needle localization is commonly performed preoperatively by mammographers to localize calcifications and masses. Although this is not the first role of needle localization outside of the breast, it reinforces the usefulness of targeting small lesions for excision.

Conclusions

This case details the unique clinical presentation and imaging findings of an extradigital glomus tumor in the suprapatellar fat pad. Radiologists and clinicians should be aware that glomus tumors can occur in atypical locations. A delay in diagnosis can be avoided if a high clinical suspicion is maintained. Due to the small size of these tumors, the radiologist may play an important role not only in imaging diagnosis, but also for localization prior to surgical excision.