PMC6087602_01
A solid mass 1 x 5 cm in size was palpable over the retromalleolar portion of the left ankle along the course of the peroneal tendons (yellow arrows).
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fig1
Physical examination revealed a solid mass sized 1 x 5 cm over the retromalleolar portion of the left ankle along the course of the peroneal tendons (Figure 1).
X-ray and CT showed a 1 x 5 cm elliptical opacification (yellow arrow) along the course of the peroneal tendon. (a) AP view of the X-ray image. (b) Lateral view of the X-ray image. (c) Coronal section of the CT image. (d) Axial section of the CT image. (e) 3D reconstruction of the CT image.
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fig2
X-ray and CT showed a 1 x 5 cm elliptical opacification along the course of the peroneal tendon from the level of the ankle joint at its distal end (Figure 2).
Sagittal T1- and T2-weighted MR images showed an elliptical mass of low intensity partially with high intensity with no contrast effect. Axial T1-weighted MR images showed a low-intensity mass (yellow arrow) in the peroneal tendon sheath, which seemed to compress both the peroneal brevis and longus tendons. (a) Sagittal T1-weighted image. (b) Sagittal T2-weighted image. (c) Sagittal enhanced image. (d) Axial T1-weighted image.
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fig3
Axial T1-weighted MR images showed a low-intensity mass in the peroneal tendon sheath, which seemed to compress both the peroneal brevis and longus tendons (Figure 3).
Ultrasonographic images showed an elliptical mass with an echoic shadow on the affected side of the peroneal tendon sheath. (a) Longitudinal image and (b) transverse image. *Heterotopic ossification. #Peroneus longus tendon.  Peroneus brevis tendon.
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fig4
Ultrasonographic image showed an elliptical mass with an echoic shadow on the affected side of the peroneal tendon sheath (Figure 4).
Resection of heterotopic ossification of the peroneus longus and peroneus quartus muscle. (a) Heterotopic ossification existed inside the peroneus longus tendon. (b) The peroneus quartus muscle running along the peroneus longus tendon was excised. (c) An end-to-side transfer of the peroneus longus tendon to the peroneus brevis tendon was performed. The degenerated part of the peroneus tendon was excised. (d) Resected heterotopic ossification and degenerated part of the peroneus longus tendon. *Heterotopic ossification. #Peroneus longus tendon.  Peroneus quartus muscle.  Peroneus brevis tendon.
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fig5
Longitudinal incision of the peroneus longus tendon revealed that the mass existed inside the peroneus longus tendon and its distal end was connected to the peroneus longus tendon (Figure 5). We found a peroneus quartus muscle running along the peroneus longus tendon and subsequently excised it (Figure 5).
Microphotograph of the resected heterotopic ossification. Lamellar bone formation with an extensive cartilage metaplasia partly with mixed tissue of fat and necrotic muscle. Calcification and cartilage metaplasia existed in the transitional zone between the ossification and the remaining tendon, that is, endochondral ossification. Scale bar, 100 mum.
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fig6
Calcification and cartilage metaplasia existed in the transitional zone between the ossification and the remaining tendon, that is, endochondral ossification (Figure 6).