The Utility of Tc-99m Hydroxymethylene Diphosphonate Single-photon Emission Computed Tomography/Computed Tomography in Symptomatic Os Trigonum.

Os trigonum is a congenital condition in which a bone ossicle fails to fuse to a secondary ossification center in the posterior aspect of the talar bone. Repetitive plantar flexion of the foot may elicit inflammation of the ossicles fibrous connective band, subsequently causing pain. It is common to misdiagnose os trigonum as an avulsion fracture. We report a case of a 23-year-old male with symptomatic os trigonum diagnosed on Tc-99m hydroxymethylene diphosphonate single-photon emission computed tomography-computed tomography (SPECT-CT). We also wish to emphasize on the use of SPECT/CT in not only allocating the ossicle anatomically but also for the purpose of evaluation an active source of pain in a region of multiple complex small bones.

A 23-year-old male patient presented with right posterior ankle pain for 1 month. Three-phase bone scintigraphy is performed by injecting 24 mCi of Tc-99m hydroxymethylene diphosphonate intravenously. Dynamic flow over the feet and whole-body blood pool images are unremarkable. Delayed whole-body images show increase tracer uptake at right ankle [Figure 1]. Single-photon emission computed tomography/computed tomography (SPECT/CT) of the feet is acquired show focal area of increased tracer uptake at the posterior aspect of the right talus. Noncontrast CT coronal, sagittal, and transaxial images show small accessory/sesamoid bone (os trigonum) with corresponding increased uptake on fused images. Volume-rendered images better delineate the uptake at bony ossicle which may present a source of pain suggestive of os trigonum syndrome [Figure 2].

Figure 1

A 23-year-old male patient presented with right posterior ankle pain for 1 month. Three phase bone scintigraphy is performed by injecting 24 mCi of Tc-99m hydroxymethylene diphosphonate intravenously. (a) Dynamic flow over the feet and whole-body blood pool images are unremarkable. (b) Delayed whole-body images show focal increase tracer uptake at right ankle region (arrow)

Figure 2

(a) Coronal, sagittal, and transaxial single-photon emission computed tomography images show focal area of increase tracer uptake at the posterior aspect of right talus. (b) Noncontrast computed tomography coronal, sagittal, and transaxial images show small accessory/sesamoid bone (os trigonum) with corresponding increased uptake on fused images. (c and d) Volume rendered images better delineate the uptake at bony ossicle which may present a source of pain, suggestive of os trigonum syndrome

Os trigonum is a congenital condition that develops as an ossicle fails to fuse to the posterior aspect of the talus bone. Rather, in adolescence, it connects to the talus through a fibrous band. This condition can be either unilateral or bilateral and is generally found in 5%–10% or 10%–25% of the population.[1] Os trigonum can be asymptomatic, however, on trauma or injury pain can be elicited. This is most commonly seen among athletes with repeated downward pointing of toes, ankle plantar flexion. This movement brings about a “nut cracker” injury to the accessory bone as it gets impinged between the heel and the tarsal bones, pulls away, and stretches the connecting fibrous tissue, thus causing inflammation and pain.[2] Hence, based on the presentation, the differential diagnosis includes all of which would cause posterior ankle impingement syndrome ranging from acute to chronic presentations.[3] With the use of different imaging techniques, os trigonum can still be mistakenly identified as avulsion fracture of the posterior talus tubercle (shepherd fracture). The preferred radiological tool of choice would be magnetic resonance imaging (MRI), as it can assess edema and synovitis as well.[4] However, bone scintigraphy, especially with SPECT/CT, is complementary imaging in uncertain scenarios. SPECT-CT allows precise anatomical localization of bone turnover abnormalities.[56] SPECT-CT is reportedly useful for localizing active disease in foot and ankle pathology.[78] In addition, uptake on bone SPECT-CT was significantly associated with pain or pain relief after local injection of anesthetics.[9] Treatment of os trigonum starts with the use of nonsteroidal anti-inflammatory drugs combined with immobilization and rest. However, if symptoms do not improve within 3–6 months of conservative management, surgical laparoscopic excision of the ossicle may be indicated which accounts for 40% of all cases.[10] SPECT-CT could be suggested as an effective complementary imaging method to MRI in ankle and foot pain patients, particularly for enhancing diagnostic specificity.[1112]

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

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Conflicts of interest

There are no conflicts of interest.