Idiopathic aggressive myositis ossificans of the hand infiltrating the flexor sheath/checkrein ligament, obliterating the common digital artery, and attenuating the digital nerve: A case report

Highlights • Myositis ossificans of the hand is extremely rare.• These tumors do not usually infiltrate adjacent structures.• We present a case with infiltration of adjacent structures.• Complete excision was curative.


Introduction
Myositis ossificans is a benign form of heterotopic ossification. It usually arises from skeletal muscles and present as a painful soft tissue mass with overlying skin erythema. It is most frequently seen in the thighs of active athletes. The two commonly involved muscle groups are the quadriceps and adductor muscles of the thigh. Most cases resolve with conservative management (rest and nonsteroidal anti-inflammatory medications) [1].
Myositis ossificans of the hand is extremely rare, and hand tumors have a different presentation and prognosis from the classic thigh lesions. Pain is usually more severe in the hand and this may be related to space limitations within different hand compartments. Furthermore, the proximity of nerves may lead to nerve compression. Finally, most hand tumors require surgical excision despite the initial trial of conservative management either because of intractable pain or persistence of the mass [2]. Simple excision is usually curative because the tumor mass does not usually infiltrate adjacent structures [2].
We report for a very unusual case of myositis ossificans of the hand with infiltration of the flexor sheath, checkrein ligament of the proximal interphalangeal join, and the neurovascular bundle. The work has been reported in line with the SCARE criteria [3].

Case report
A 30-year-old female presented with a 5-week history of a painful rapidly-growing mass over the proximal phalanx of the right ring finger (Fig. 1). The patient also complained of numbness along the distribution of the radial digital nerve of the finger. There was no history of trauma. Examination showed a firm tender mass measuring 3.5 × 2 cm with an overlying skin erythema. The patient was unable to fully extend or flex the finger because of pain. There was also a flexion contracture of the proximal interphalangeal joint. Static two-point discrimination on the radial side of the ring finger was 10 mm. Plain x-rays showed no bony abnormalities or calcifications. Magnetic resonance imaging showed an iso-intense (similar to intensity of muscle) lesion on T1-weighted images (Fig. 2). T2-   weighted images showed high intensity in the center of the lesion and low intensity at the periphery of the lesion (Fig. 3).
Surgical excision was done under general anesthesia. The mass infiltrated the entire flexor sheath over the proximal phalanx as  well as the checkrein ligament of the proximal interphalangeal joint. The common digital artery of the 3rd web space was obliterated by the tumor. The radial digital nerve of the ring finger was stretched and attenuated (Fig. 4). Total excision was done and this required excision of the flexor sheath over the proximal phalanx (including the entire A2 pulley), the checkrein ligament and the common digital artery. The digital nerve was preserved (Figs. 5 and 6).   flexion of the digit, but the proximal interphalangeal joint had an extension lag of 30 o (Fig. 8). Upon palpation, bowstringing of the flexor tendon was felt over the proximal phalanx. The static twopoint discrimination on the radial side of the ring finger improved to 6 mm.

Discussion
Myositis ossificans of the hand is extremely rare. We report our case to document the unique feature of tumor infiltration of adjacent structures. We also document that excision is curative despite the infiltrative nature of the tumor.
We reviewed all previously reported cases of myositis ossificans of the hand and wrist since 1982 [2,[4][5][6][7][8][9][10][11][12][13][14][15][16][17]. There was a total of 18 cases (including our case) and their data are summarized in Table 1. Females (n = 11) were more commonly affected than males (n = 7). Five cases were seen in patients 15 years or younger; and the remaining 13 patients were adults (18-58 years old). The majority of patients (n = 16) had tumors within the hand (fingers/web spaces/palm/thenar area) and only two cases had wrist lesions. Three lesions appeared during pregnancy [4,16]. None of the tumors invaded bone, although a periosteal reaction of the adjacent bone was seen in some cases [8,10]. Two lesions arose from the  The lesion in our case was mostly located along the proximal phalanx of the ring finger, with an extension to the palm and web space.
lumbrical muscles [2,17] and three lesions from the thenar muscles [12,14,15]. The remaining lesions (n = 13) did not arise from muscles. Nerve compression was seen in two cases and complete recovery of nerve function was documented in both cases following excision [2,11]. In our case there was a stretching and attenuation of the adjacent digital nerve and sensibility did not return back to normal at 1 year after surgery. Only one previously reported case documented tumor infiltration into the extensor tendon of the little finger [6]. In our case there was infiltration of the flexor sheath, the checkrein ligament and the common digital artery. Only two cases were successfully treated with conservative management [12,13]. Another three cases [4][5][6] were treated with ray amputation because of suspected sarcoma; and these cases included the case with extensor tendon infiltration [6]. The remaining cases (including ours) were treated with excision with no recurrence. The MRI features of intra-muscular myositis ossificans is well described in the literature [18]. Early lesions (defined as lesions without calcification; usually in the first 4 weeks) show isointensity on T1 and homogeneous high intensity on T2 images. A high intensity in the center of the lesion with a low intensity at the periphery of the lesion is a feature of subacute (4-8 weeks) and mature (over 8 weeks) lesions [18]. The MRI of our patient was done at 5 weeks from the appearance of the lesion, which is considered to be in the subacute stage. The MRI findings in our case was consistent with an early lesion on T1 images, and with a more mature lesion on T2 images. This is interesting and may be related that the lesion was not related to muscle.
The most important differential diagnosis of myositis ossificans of the fingers is fibro-osseous pseudo-tumor of the digit (also known as florid reactive periostitis) [19,20]. This tumor is benign but with an aggressive behavior. It is sometimes considered as a superficial variant of myositis ossificans [20]. However, fibroosseous pseudo-tumors have the following characteristic features: they always arise from the skin and subcutaneous tissue, the histology shows "incomplete" zoning (multinucleate giant cells and myofibroblasts merging with osteoid without clear zones), and the fibroblasts show atypia.

Conclusions
Myositis ossificans of the hand is very rare. Although an initial trial of conservative treatment is usually tried, most cases end-up with surgical excision either because of intractable pain or persistent mass. The lesion does not usually infiltrate adjacent structures. Our case is unique because of the infiltrative nature of tumor. There is no indication for amputation even for infiltrative tumors. We demonstrate that complete excision is curative with good regain of function even in infiltrative lesions.