Bizarre parosteal osteochondromatous proliferation in the distal ulna where the lesion is continuous with the medullary cavity: a case report

Abstract

Background: Bizarre parosteal osteochondromatous proliferation (BPOP) is a rare benign bone tumor, it is also called "Nora's lesion". The lesion is characterized by heterotopic ossification of the normal bone cortex or parosteal bone. The etiology of BPOP is unclear and may be related to trauma. In most BPOPs, the lesion is not connected to the medullary cavity. Here we report an atypical case, characterized by reversed features compared to the typical BPOP, which demonstrated continuity of the lesion with the cavity.

Case presentation: An 11-year-old female child had a slow-growing mass on her right wrist for 8 months with forearm rotation dysfunction. Plain X-rays showed an irregular calcified mass on the right distal ulna, and computed tomography (CT) showed a pedunculated mass resembling a mushroom protruding into the soft tissue at the distal ulna. The medulla of this lesion is continuous with the medulla of the ulna. A surgical resection of the lesion, together with a portion of the ulnar bone cortex below the tumor was performed, and the final pathology confirmed BPOP. After the surgery, the child's forearm rotation function improved significantly, and there was no sign of a recurrence at 1-year follow-up.

Conclusion: It is scarce for BPOP lesions to communicate with the medullary cavity. However, under-recognition of these rare cases may result in misdiagnosis or inappropriate treatment thereby increasing the risk of recurrence. Therefore, special cases where BPOP lesions are continuous with the medulla are even more important to be studied to understand better and master these lesions. Although BPOP is a benign tumor with no evidence of malignant transformation, the recurrence rate of surgical resection is high. We considered the possibility of this particular disease prior to surgery and performed a surgical resection with adequate safety margins. Regular postoperative follow-up is of utmost importance, without a doubt.

Keywords: Bizarre parosteal osteochondromatous proliferation; Continuity with the medullary cavity; Differential diagnosis; Nora’s lesion; Osteochondroma.

Fig. 1

(A) A type of circular ossification was found in plain X-ray of the right ulna, which was closely related to the ulna. (B, C) The CT scan revealed that the lesion infiltrated the cortical bone of the ulna and communicated with the medullary cavity(arow). (D) On T1-weighted images showed inhomogeneous signal clumps adjacent to the ulnar and radius on the volar aspect of the right forearm. (E) On T2 with fat suppression, the lesion showed a predominantly high intensity signal. (F) On gadolinium-enhanced images showed inhomogeneous high signal throughout the tumor

Fig. 2

(A) The surgical exposure of the tumor revealed an exophytic, indurated mass located on the palm side of the distal ulna. Additionally, white chondroid components were observed covering the surface of the tumor, which was found to be interposed between the ulnar and radial bones, consequently impacting forearm rotation. (B) The tumor was completely excised from the ulna along with a portion of the cortical bone. The dimensions measured approximately 3.5 cm×2.6 cm. (C, D) A portion of the resected ulnar bone cortex is seen at the base of the tumor(arrow). At the junction of the tumor and the ulna, as well as inside the tumor, both are seen to be similar to a medullary cavity cancellous osteoid structure, with a continuous whole with no obvious interval separation between them

Fig. 3

Hematoxylin and eosin (H&E) staining of the tumor sections. Histologically, a tumor is formed on the bone surface through mixed hyperplasia of fibers, cartilage, and bone in varying proportions (A, B). The surface of the lesion was a cartilage cap, with some areas of fibrous cartilage (A) and some areas showing hyaline cartilage (B). The chondrocytes exhibited an increased nucleoplasmic ratio and displayed aberrant morphology under high magnification (C). The cartilage undergoes migration and transformation towards the trabeculae through the form of endochondral ossification, resulting in the emergence of distinct blue bone areas within the transformed region (A, B, D). The bone trabeculae were interspersed with proliferating fibroblasts, without heterogeneity (A, B, D)