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Osteochondroma Arising from Anterior Inferior Iliac Spine as a Cause of Snapping Hip

Snapping hip syndrome is a relatively common problem that can be easily managed with conservative treatment. This syndrome can be divided into external, internal and intra-articular types. Internal snapping hip syndrome is the rarest amongst these and its etiology is not well understood. We report a unique case of osteochondroma arising from the anterior inferior iliac spine (AIIS), which caused the internal snapping hip syndrome with hip pain and restriction of activity. This rare case of snapping hip syndrome from the AIIS was treated surgically and the symptoms completely disappeared after excision of the tumor.

Osteochondromas are the most common of benign bone tumors that are characterized by a cartilage-capped bony mass that arises mostly from the metaphysis or diaphysis of a long bone. They account for approximately 40% of all benign bone tumors. The distal metaphysis of the femur and the proximal metaphysis of the tibia and humerus are the most commonly affected bones. Osteochondroma is rarely found in the pelvic bone and reports of solitary pelvic osteochondroma are also rare. 1,2) Usually, these conditions are asymptomatic but may require conservative treatment if they become symptomatic. In certain cases, excision of the tumors may be performed if clinical findings include mechanical signs of increased volume, intractable pain, cosmetic problem, symptoms from compression of neurovascular structures, and/or possible malignant degeneration. [3][4][5] Snapping hip syndrome is characterized by a snapping sound or clicking and accompanying hip pain. 3,4,[6][7][8][9] It can be attributed to both the iliotibial band and the ilio-psoas tendon, 3,4,[6][7][8] which are easily distinguishable distinct entities. The iliotibial band is involved in external snapping hip, and the iliopsoas tendon is related to the internal type. Both types can usually be managed conservatively but occasionally require surgical treatment.
Currently, there are very few reports on snapping hip related to osteochondromas. 4,6) Two reports described osteochondroma of the proximal femur associated with snapping hip syndrome. 4,6) We reported an extremely rare case of osteochondroma that arose from anterior inferior iliac spine (AIIS) causing internal snapping hip.

CASE REPORT
A 32-year-old woman presented to Kyung Hee University Hospital at Gangdong with a history of chronic right groin pain along with limited hip joint mobility. There was no history of trauma. Her symptoms were aggravated at the time of the hospital visit. The patient indicated that shortly after squatting, she had difficulty extending her hip and had subsequent difficulty walking. Thereafter, she occasionally noticed a snapping phenomenon in her right hip.
A mass with hard consistency was palpable in the right groin area, but there was no tenderness. The motion of the right hip was fully preserved. There was obviously a snapping phenomenon with mild pain when her right hip was forced into a squatting position.
Pelvic plain films were taken initially (Fig. 1). It was difficult to determine a bony abnormality in the anteroposterior view of the pelvis; however, the internal oblique view demonstrated an irregularly calcified, pedunculated lesion that arose from the right ilium. This lesion presented as a peripheral outgrowth with its cortex in continuity with the ilium (Fig. 1); however, neither a periosteal reaction nor bony destruction was seen. Three-dimensional computed tomography (Fig. 2) and magnetic resonance imaging (Fig. 3) showed an approximately 3.3 cm × 2.8 cm × 2.0 cm sized round mass surrounding a cartilaginous cap with < 5 mm in thickness. Due to persistent pain, restriction of daily activity and the potential for malignant degeneration, the patient elected to undergo surgical excision of the tumor. During the operation, the patient was placed in supine position, and a curved incision was made while palpating the mass around the AIIS. The pedunculated mass was exposed after splitting the muscles. An osteotomy was performed to transect the base of the mass from the AIIS after the femoral nerve was identified and exposed.
The excised mass had a gross appearance typical of osteochondroma (Fig. 4). Histological findings demonstrated cartilaginous cap at the margin of the exostosis, a reflected layer of the periosteum over the exostosis and irregularity of the chondrocytes within the cartilaginous cap. Endochondral ossification was apparent at the base of the cap (Fig. 4). Finally, the patient has not experienced the snapping phenomenon since the surgery.

DISCUSSION
Osteochondroma usually occurs in long bones but may occasionally be seen in flat bones such as the scapula or the pelvic bone. 1,2,10) Although osteochondroma is po- Fig. 1. The anteroposterior view of pelvis (A) shows abnormal bony contour around the right acetabular roof and the internal oblique view (B) demonstrates an irregularly calcified, pedunculated lesion arising from the right ilium. It presents as a peripheral outgrowth with its cortex in continuity with the ilium (arrow). tentially difficult to diagnose owing to its asymptomatic nature, it can be recognized as a palpable mass that may cause pain due to overlying bursal inflammation, a fracture through a pedunculated stalk, or impingement of an overlying neurovascular structure. [3][4][5] The diagnosis could have been missed in this case due to the rare presence of osteochondroma in the pelvic bone. 1,2) Snapping hip syndrome also referred to as coxa saltans is a syndrome with an audible snapping that is associated with hip pain. 3,4,6-9) The snapping is usually evoked