Turret exostosis is a benign osteocartilagenous lesion believed to arise from reactive perisoteum following relatively mild trauma. This article presents an unusual case of a turret exostosis of the talar neck in a 12-year-old child. A healthy adolescent presented with a 5-month history of a firm, painless mass about the anteromedial aspect of the right ankle 1 month after suffering a mild twisting injury to the ankle. Radiographs demonstrated a benign-appearing bony mass within the soft tissues anterior to the right ankle overlying the talar neck. An excisional biopsy of the mass was performed and pathology was consistent with turret exostosis, demonstrating a central area of mature trabecular bone maturing via enchondral ossification with a thin hypocellular peripheral rim of cartilage. The absence of a periosteal layer, abundant “blue bone,” bizarre metaplastic cartilage, or marked cytologic atypia confirmed the diagnosis. The patient remains pain free with full ankle motion.
Reactive periosteal lesions are well-described entities, tending to occur with the greatest frequency in the small bones of the hand. Few cases of bizarre parosteal osteochondromatous proliferation in the metatarsals and phalanges of the toes have been reported. The current case represents the first account of a turret exostosis of the hindfoot, and the youngest patient with a histologically confirmed diagnosis. It further illustrates the manner in which this case exists along a continuous spectrum of reactive periosteal lesions. The clinical, radiographic, and histologic characteristics of reactive periosteal lesions are reviewed.
Turret exostosis is a benign osteocartilaginous lesion largely believed to arise from reactive periosteum following relatively mild trauma. The lesion is exceedingly rare, with only 21 cases reported since it was initially described by Wissinger in 1966.1-7 Rarer still are cases of turret exostoses occurring only in the hand. Of all reported cases, all but 2 were localized to the small bones of the hand.1-7 A literature review revealed 1 recent report of turret exostosis in the great toe and 1 case on the coronoid process of the skull.5,8 To our knowledge, this lesion has never been reported in the talus. This article presents a case of a turret exostosis of the talar neck in a child.
A healthy 12-year-old boy presented with a 5-month history of a firm, painless mass about the anteromedial aspect of the right ankle. The patient reported having suffered a mild twisting injury to the ankle during a baseball game approximately 1 month prior to first noticing the mass. This injury was believed to be minor, and although the patient experienced several days of minimal discomfort about the medial side of the ankle, he was able to bear weight fully and continue to participate in organized athletics and recreational activities.
Approximately 1 month after this injury, the patient became aware of a firm fullness about the anteriomedial aspect of the ankle accompanied by a restricted range of ankle motion. The patient’s parents reported that his gait was normal and that he was experiencing no pain. The mass remained stable in size but became firmer over the ensuing months. At the time of presentation, the patient reported no locking, crepitus, or instability of the ankle and reported no skin changes overlying the mass. There was no history of recent infection, constitutional symptoms, or systemic illness, and no family history of cancer was evident.
Clinical examination of the right ankle was notable for a 2×2 cm bony, slightly mobile mass deep to the subcutaneous tissue of the anterior and medial aspect of the tibiotalar joint. There were no overlying skin changes or erythema, and the mass was not tender to palpation. The ankle had no evidence of effusion. Passive dorsiflexion of the right ankle was 5· with the knee extended versus 15· on the contralateral side. Plantar flexion was 40· bilaterally. The subtalar joint was supple, and the foot plantargrade. All muscle groups of the leg and thigh demonstrated full strength and a normal sensory examination.
Radiographs demonstrated a benign-appearing bony mass within the soft tissues anterior to the right ankle overlying the talar neck (Figure 1). The mass measured 2.5×1.5 cm, was well circumscribed, and seemed to consist of trabecularized bone. No connection was seen between the mass and the surrounding osseous structures, although subtle erosive changes along the anterior aspect of the talar neck were apparent. Computed tomography confirmed the presence of a well-marginated bony lesion overlying the talar neck with no evidence of a bony connection on the talus or distal tibia (Figure 2). Magnetic resonance imaging demonstrated some marrow signal centrally, with no obvious soft tissue component. Reactive changes were seen along the anterior aspect of the talus, but there was no frank lesion within the talus itself. There was no discernible communication with the tendinous or neurovascular ankle structures (Figure 3).
Preoperative laboratory evaluation revealed a white blood cell count of 6.6 (103/mm3), with a normal differential, erythrocyte sedimentation rate of 9 (mm/hr), and C-reactive protein level <0.65 (mg/dL).
An excisional biopsy of the mass was performed at 6 months from the onset of symptoms. This was accomplished under tourniquet control through an anterior approach to the ankle. A longitudinal incision was made centering over the tibiotalar joint in line with the tibialis anterior tendon. The extensor retinaculum was partially incised in line with the skin incision, and the tibialis anterior tendon was retracted laterally. The underlying joint capsule was divided and retracted to provide full visualization of the mass, which appeared bony in nature with a circumferential cartilaginous cap. A fibrous stalk was present connecting the mass to the superior talar neck, but otherwise the lesion had no connections to the surrounding soft tissues or osseous structures. Some gross erosive changes were present along the talar neck due to mechanical impingement from the mass. Upon releasing the fibrous stalk, the lesion was easily mobilized and removed en masse. The mass measured 2.6×2.2×1.9 cm. Subsequent to removal of the lesion, passive ankle dorsiflexion improved 10·. Intraoperative cultures were taken prior to closure, all of which were negative.
Microscopic examination of the mass showed a central area of mature trabecular bone maturing via enchondral ossification with a thin peripheral rim of cartilage. The cartilaginous cap was relatively hypocellular, and the chondrocytes were small and unremarkable. Little cellular atypia was identified within the cartilage. The stroma between the bony trabeculae comprised loosely arranged fibrovascular tissue with no hematopoietic elements or marrow fat identified. Other pertinent negatives included the absence of a periosteal layer, abundant “blue bone,” bizarre metaplastic cartilage, or marked cytologic atypia. Overall, these histologic features were diagnostic for turret exostosis (Figure 4).
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Figure 4: Low (A) and high (B) power photomicrographs of excisional biopsy specimen showing a peripheral rim of cartilage maturing via enchondral ossification to form a central area of mature trabecular bone. The stroma between the bony trabeculae is comprised of loosely arranged fibrovascular tissue with no hematopoietic elements or marrow fat identified. Peripheral cartilage is hypocellular with small uniform appearing chondrocytes.
The patient’s postoperative course was uneventful. Following a brief period of splint immobilization to allow for wound healing, the patient was mobilized without restrictions and allowed to begin range of motion exercises. At 18-month follow-up, the patient had regained full ankle range of motion in comparison to the contralateral extremity, walked with normal heel-to-toe gait mechanics, resumed full activities, and showed no radiographic evidence of recurrence (Figure 5).
In their initial description of turret exostosis in 1966, Wissinger et al1 attributed the pathogenesis of the lesion to a small periosteum laceration leading to subperiosteal hematoma formation. Due to the small size of the periosteal tear, the hematoma is unable to drain and undergoes gradual ossification.1 While most authors agree with this proposed mechanism,3,4,6 several terms have been used to describe reactive periosteal lesions, including ossifying hematoma, periosteal ossification, florid reactive periosteal lesion, bizarre parosteal osteochondromatous proliferation (or Nora’s lesion) and turret exostosis.2 Dorfman and Czerniak8 proposed that turret exostoses represents the endpoint of a spectrum of reactive periosteal lesions marked by progressive subperiosteal hematoma maturation. According to this theory, florid reactive periostitis, the most immature of reactive periosteal lesions, eventually evolves into bizarre parosteal osteochondromatous proliferation, which, in turn, matures into a turret exostosis as the hematoma gradually undergoes more organization (Table).8
Histologic features aid in differentiating these entities. Florid reactive periostitis is characterized by a loose myxomatous spindle cell proliferation with minimal osteocartilaginous proliferation.6,10 Bizarre parosteal osteochondromatous proliferation consists of a hypercellular, metaplastic cartilaginous cap comprising enlarged chondrocytes. This cartilaginous cap undergoes enchondral ossification, and within these transitional areas is an abundance of bone, which has a deep blue or purple calcification (otherwise known as “blue bone”) on hematoxylin-eosin staining.11
The microscopic appearance of turret exostosis demonstrates a fibrous and/or cartilaginous peripheral cap, which undergoes enchondral ossification that forms mature trabecular bone and comprises the central core and base of the lesion.9,10 The turret exostosis lacks the cartilaginous hypercellularity, chondrocyte enlargement, binucleated chondrocytes, and the “blue bone” of bizarre parosteal osteochondromatous proliferation.10,11 A turret exostosis is differentiated from an osteochondroma by a lack of continuity with the marrow space of the underlying bone, demonstrated by the absence of hematopoietic elements and/or marrow fat and the presence of intertrabecular fibrovascular tissue.10,11
The radiographic differential diagnosis of periosteal lesions not only includes reactive periosteal lesions but should also include other benign and malignant conditions of periosteum, including juxtacortical chondroma, periosteal/parosteal chondrosarcoma, and periosteal/parosteal osteosarcoma. These more aggressive entities are easily distinguished from their benign counterparts on radiographic appearance. Radiographically, turret exostosis is characterized by a well-defined, dome-shaped osseous mass that is fused to the underlying bony cortex with no evidence of communication with the medullary canal.2
Most authors agree that turret extostoses should not be removed prior to 6 months after the inciting injury due to a high recurrence rate.1-4,6 This allows sufficient time for maturation and delineation of the new subperiosteal bone from overlying periosteum and underlying cortical bone.4
Reactive periosteal lesions are well-described entities, tending to occur with the greatest frequency in the small bones of the hand. While few cases of bizarre parosteal osteochondromatous proliferation in the metatarsals and phalanges of the toes have been reported, only 1 case of a turret exostosis in the foot is found in medical literature2; in this case, the lesion was present along the dorsal aspect of the proximal phalanx of the hallux and was responsible for irritation of the overlying extensor hallus longus tendon.
To our knowledge, the current case represents the first account of a turret exostosis of the hindfoot. Clinically, this lesion was not associated with any obvious impingement on tendinous structures, but due to its location along the dorsal surface of the talar neck, it was responsible for a significant loss of ankle dorsiflexion that resolved on excision. In addition, it would seem that these lesions may arise in skeletally immature individuals. Our patient represents the youngest with a histologically confirmed diagnosis of a turret exostosis reported to date, while it further illustrates the manner in which such reactive periosteal lesions exist along a continuous spectrum of cellular maturation.
- Wissinger H, McClain E, Boyes J. Turret exostosis: ossifying hematoma of the phalanges. J Bone Joint Surg Am. 1966; 48(1):105-110.
- Kontogeorgakos VA, Lykissas M, Mavrodontidis A, et al. Turret exostosis of the hallux. J Foot Ankle Surg. 2007; 46(2):130-132.
- Lee B, Kaplan R. Turret exostosis of the phalanges. Clin Orthop Rel Res. 1974; (100):186-189.
- Mohanna P, Moiemen N, Frame J. Turret exostosis of the thumb. Br J Plast Surg. 2000; 53(7):629-631.
- Revington PJ. Turret exostosis of the coronoid process. Br J Oral Maxillofac Surg. 1984; 22(1):37-41.
- Rubin JA, Steinberg DR. Turret exostosis of the metacarpal: a case report. J Hand Surg Am. 1996; 21(2):296-298.
- Bourguignon RL. Recurrent turret exostoses--case report. J Hand Surg Am. 1981; 6(6):578-582.
- Dorfman H, Czerniak B. Bone Tumors. 1st ed. St. Louis, MO: Mosby; 1998.
- Dhondt E, Oudenhoven L, Khan S, et al. Nora’s lesion, a distinct radiological entity? Skeletal Radiol. 2006; 35(7):497-502.
- Bullough PG. Bone-forming Tumors and Tumor-Like Conditions. In: Bullough, PG, ed. Orthopedic Pathology. 5th ed. Maryland Heights, MO: Mosby; 2010:361-364.
- Unni KK. Conditions that Simulate Primary Bone Tumors. In: Silverberg SG, ed. Tumors of Bones and Joints. AFIP Atlas of Tumor Pathology. Series 4, Volume 2. Washington, DC: American Registry of Pathology; 2005:350-354.
Drs LeClere, Riccio, and Helmers are from the Department of Orthopedic Surgery, and Dr Thompson is from the Department of Pathology, Naval Medical Center, San Diego, California.
Drs LeClere, Riccio, Helmers, and Thompson have no relevant financial relationships to disclose.
The views expressed in this article are those of the authors and do not reflect the official policy or position of the Department of the Navy, Department of Defense, or the United States government.
The authors thank Ms Waine Macallister of the Naval Medical Center, San Diego, for her assistance with medical editing.
Correspondence should be addressed to: Anthony I. Riccio, MD, Department of Orthopedic Surgery, Naval Medical Center, San Diego, 34800 Bob Wilson Dr, Ste 112, San Diego, CA 92134-1112 (email@example.com).