Knee pain as a result of a solitary subcutaneous mass can be a common clinical finding. Those localized over the anterior aspect of the knee can prove to be more symptomatic as they see direct pressure when kneeled on. Common knee masses are aseptic bursitis, ganglia cysts, and lipomas [1, 2, 3]. They are often treated conservatively but for those that provoke significant pain, surgical excision can be considered ^[4].

A 40-year-old was seen for a yearly physical, at one of our institution’s primary care facilities. He was found to have prepatellar fluid collection on the anterior aspect of the left knee with pain when kneeled on and with direct palpation. His primary care provider noted it as possible bursitis and referred the patient to our orthopedics center for further evaluation. During the orthopedic evaluation, the patient presented with a small bump on the anterior of his left knee that had recently increased in size. The patient stated extreme sensitivity to touch not allowing him to put direct pressure on this area. A thorough physical examination showed that there was no evidence of infection, warmth to touch, and seemed fluid filled. He exhibited a painless arc of motion, no pain with weight bearing, and no signs of mechanical instability.
X-ray imaging revealed no foreign body and no joint or bone pathology. Considering the size, location, and benign features of the mass, further imaging was not felt indicated. The patient was diagnosed with a 1 cm × 1 cm fluid filled subcutaneous mass over the anterior aspect of the inferior pole patella. Conservative and operative treatment options to remove the mass were suggested. Considering the amount of sensitivity, the patient was experiencing, the decision to proceed with surgical excision was made.
An open excision under local anesthesia was performed to remove a small palpable soft-tissue mass of the patients left knee. Gross inspection revealed a small, bluish-tinged, and fluid filled cystic structure. Histopathologic examination revealed a vascular neoplasm in cross-section dominated by bland-appearing oval cells. Immunochemical stains were positive for smooth muscle actin and synaptophysin and negative for chromogranin, S-100 protein, and melanoma antigens. This is consistent with a glomangioma.

At their most recent post-operative follow-up, the wound was healed with no evidence of infection or reoccurrence. The patient denied any residual tenderness or pain and was symptom free.

Glomus tumors were first depicted by Masson in 1924 as a benign tumor with morphologically distinct features ^[5]. They are rare and only constitute for <1.6% of all benign soft-tissue extremity tumors ^[6]. Histologically, there are three subtypes of glomus tumor: Solid glomus tumor, glomangioma, and glomangiomyoma ^[7]. Roughly 73% of all these tumors occur in the upper extremities, with most found in the subungual regions ^[8]. Extradigital glomus tumors are rarely found in atypical regions such as the kidney, liver, lung, and oral cavity [8, 9, 10, 11]. They can also be found in the lower extremity [12, 13]. Lower extremity presentations of glomangioma tend to present in older adults, thereby mimicking osteoarthritis symptoms which tends to be a common complaint in this demographic. However, our patient was diagnosed with a glomangioma at 40 years old, with complaints since the age of 33. Three common symptoms of sensitivity to cold, pain on palpation, and paroxysmal help identify a glomus tumor ^[14]. ^[14]. In a case study of 51 patients, 29% presented a blue discoloration and 33% presented a pulp nodule ^[15]. Knee pain caused by a glomus tumor is a rare occurrence and is often miss diagnosed [14, 15]. A case study at the University of Florida reported that a patient attempted to have a cyst in the superficial soft tissue of their patella removed. Prolonged bleeding led them to being admitted into the emergency department. Further imaging and a biopsy determined that the mass was a glomangioma and a surgical excision was performed ^[6]. Another case study in Greece treated a patient for 2 years for chondromalacia of the patella before the patient exhibited walking pain. An ultrasound and MRI confirmed a mass and a surgical excision was performed. Following the operation, histopathologic examination revealed a glomangioma ^[16]. Out of the three common symptoms of a glomus tumor, our patient only presented pain on palpation. In this case, the mass was felt to be subcutaneous and fluid filled, such as a small benign cyst or ganglion [Fig. 1-3], and considering the sensitivity to touch the patient was experiencing, it was felt safe to exercise. The histopathologic report showed the presence of round appearing oval cells and smooth muscle actin, which are consistent with a glomangioma [Fig. 9]. On immunochemical staining, synaptophysin was present [Fig. 10]. Other possible diagnoses’ were ruled out by examining the remaining stains [Fig. 4-8]. There are case reports presenting synaptophysin in atypical visceral locations such as the pancreas and liver, but very few reporting its presence in the lower and upper extremities [17, 18]. A glomus tumor is a rare cause of knee pain, but if a subcutaneous mass possesses the characteristics as described above, especially extreme sensitivity to touch, it should be considered in the differential diagnosis.

Solitary subcutaneous masses are a common clinical finding, but often require further examination to determine the underlying pathology. Glomus tumors are often found in patients with pain on palpation and are mainly found in the digital and subungual regions. An extradigital occurrence of a glomangioma, a subtype of a glomus tumor, is rarely found, especially on younger patients. Therefore, a histopathological examination must be done after the removal of any subcutaneous mass.

Diagnosing the pathology of a small subcutaneous mass can prove challenging. There are many possible differential diagnoses; which is why a histopathological examination following the surgical removal must be done.