Keywords
lipoma - intra-articular - wrist - pisotriquetral - musculoskeletal - ultrasonography - CT
Introduction
Lipomas are benign tumors that arise from mesenchymal cells. Although they are the most common soft-tissue tumor of the body, they are relatively rare in the hand and make up only 8% of all benign hand tumours.[1] Lipomas in the hand can occur in different locations, such as subcutaneous, subfascial, intramuscular, and even intraosseous.[2] This particular case is unique because it involves an intra-articular lipoma of the hand, which affects the pisotriquetral (PT) joint.
Case Report
A 24-year-old male patient with severe pain in their right leg and foot, as well as pain in the right wrist, presented to the emergency department following a motor vehicular accident. A routine trauma screening radiograph of the affected bone was performed, which revealed fractures of the distal radius and triquetrum. A “pooping duck sign” indicating a dorsal triquetral fracture could also be demonstrated on the lateral wrist radiograph ([Fig. 1]). Subsequently, a computed tomography (CT) of the hand was performed to assess the triquetral and distal radius fracture. In addition to confirming the distal radius and triquetral fractures, the CT scan revealed an incidental soft-tissue mass in the PT joint ([Fig. 2]). The soft-tissue mass was well-circumscribed and demonstrated intrinsic Hounsfield attenuation identical to subcutaneous fat in keeping with a simple lipoma. Ultrasonography revealed a well-defined hypoechoic lesion arising from the PT joint ([Fig. 3]). Unfortunately, a magnetic resonance imaging (MRI) could not be performed as the patient had undergone metallic fixation for his bony injuries. The imaging diagnosis was that of a benign intra-articular synovial lipoma. No sinister features were seen within the lesion. Since the lesion was incidentally detected with no lesion-related symptoms, a surgical excision biopsy was deemed inappropriate and was not performed. The patient was reassured and told to keep an eye on the lesion and report back to the doctors in case development of any lesion-related symptoms.
Fig. 1 Anteroposterior (A) and lateral (B, C) radiographs of the right wrist. Fractures of the distal radius and triquetrum seen (arrow). The arrow points to an avulsed fragment from the dorsal cortex of the triquetrum. “Poop” in the pooping duck sign refers to the avulsed fragment (C).
Fig. 2 Sagittal computed tomography images (A–C) with schematic (C) show a well-defined hypodense lesion (−126 Hounsfield unit) (arrow) in the pisotriquetral joint (A).
Fig. 3 Ultrasound short-axis (A–E) and longitudinal (C, F) images show a well-defined hypoechoic lesion arising from the pisotriquetral joint space (yellow). L, lesion, PISI, pisiform.
Discussion
Understanding the PT joint's intricate anatomy and potential pathologies is essential for musculoskeletal radiologists, as lesions within or adjacent to the PT joint are an important but underdiagnosed cause of wrist pain. The pisiform is a small, pea-shaped carpal bone situated on the wrist's volar (palmar) aspect, proximal to the volar surface of the triquetrum. These two bones articulate to form the PT joint. The PT joint is a synovial joint with a loose capsule that provides little or no stability ([Fig. 4]). The pisiform is stabilized by the flexor carpi ulnaris and three ligaments, namely the pisohamate ligament and pisometacarpal ligament. PT ulnar ligament with secondary stabilization provided by flexor and extensor retinaculum, as well as the abductor digiti minimi muscle insertion.[3] Damage to the structures surrounding the pisiform can lead to instability and dysfunction of the PT joint.
Fig. 4 Schematic showing the close proximity of the pisotriquetral joint to the Guyon's canal.
Pisiform forms the ulnar boundary of Guyon's canal, and its hypermobility due to PT joint instability can potentially lead to ulnar nerve irritation or compression by distorting the anatomy of Guyon's canal.
Clinicians and musculoskeletal radiologists must consider PT joint disorders when evaluating patients presenting with pain and tenderness at the palmar and ulnar aspects of the wrist, especially when this pain is exacerbated by wrist flexion and/or ulnar deviation. Besides trauma and degenerative arthritis, a few other pathologies affecting the PT joint include ganglion cysts, synovial osteochondromatosis, and tenosynovial giant cell tumours.[4]
After reviewing the current literature, this appears to be the first documented case of an intra-articular lipoma in the PT joint. Although the lesion was not well-appreciated on the initial radiograph, a well-defined radiolucency can often be seen if the lesion is large in size.[2] Ultrasound is a valuable modality for evaluating pathologies in and around the PT joint due to its high image resolution and the superficial location of the PT joint. On ultrasonography, lipomas appear as well-circumscribed lesions and predominantly hyperechoic or isoechoic to subcutaneous fat, with some lesions appearing hypoechoic (as in our case). The variations in the echotexture of the lipomas can be attributed to the acoustic impedance mismatch at the fat–water interphase. Pure fat lipomas have few interfaces and less acoustic impedance mismatch, so they appear echo-free. In contrast, echogenicity increases when there is mixed cellularity or fibrous septae because it means more interfaces and acoustic impedance mismatch.[5]
MRI is the best modality for imaging soft-tissue lesions, including lipomas, because of its excellent soft-tissue resolution and multiplanar capability. However, we could not perform an MRI in our case because the patient had an emergency tibial nailing for a traumatic fracture. However, the ultrasound and CT imaging provided enough diagnostic confidence/evidence to clinch the diagnosis and label the lesion as a benign simple intra-articular lipoma on imaging.[6]
Awareness of the possibility of intra-articular lipoma at the PT joint is essential for evaluating cases of lateral wrist pain. Ulnar nerve evaluation is necessary in cases of PT lipoma due to its proximity to Guyon's canal.[7]
Conclusion
This represents the first recorded case of a lipoma within the PT joint in English medical literature. This case encourages radiologists and clinicians to consider lipomas when evaluating potential PT joint issues. In situations where lipomas are incidentally discovered, it is crucial to assess the ulnar nerve carefully, and if the choice is not to pursue surgery, close and comprehensive follow-up by the clinical team is strongly recommended.
