Enchondromas are benign tumors composed of mature hyaline cartilage found within the medullary cavity of bones. They originate from cartilage cell nests that separate from the central growth plate during the development process. This leads to the abnormal accumulation of mature hypertrophic hyaline cartilage that fails to undergo normal resorption or ossification (1-3).

Enchondromas vary in prevalence depending on their location within the human body. They constitute ~90% of all bone tumors in the hand. By contrast, foot enchondromas are much less frequent and primarily affect the phalanges and metatarsal bones. While these tumors can occur at any age, they typically manifest between the first and fourth decades of life, affecting both sexes equally (1,4,5).

Enchondromas in the small bones of the feet are typically asymptomatic and are often discovered incidentally during routine X-ray examinations. When they become symptomatic, patients may present with pain primarily due to increased pressure from the growth of the lesion, which can deform the cortex of the affected bone or from fractures within the lesion, including pathological or stress fractures. Patients may also report a gradual enlargement of the affected digit (1,2,6).

Enchondromas in different locations of the foot have been documented in the literature (1-5,7-12). The presented study reports a case of symptomatic enchondroma in the proximal phalanx of the left second toe. The case report has been prepared in accordance with the CaReL guidelines, and referenced studies were reviewed to ensure the exclusion of non-peer-reviewed data (13,14).

Enchondromas grow gradually without infiltrating nearby tissues or spreading to distant body parts (3,15). In reviewing 19 cases of foot enchondroma (Table I), only two instances were found where an enchondroma transformed into chondrosarcoma, leading to amputation (11,12).

Enchondroma primarily manifests in the phalanges of the hand, although it can also occur in the phalanges and metatarsal bones of the foot (1). Among the reviewed cases, the most commonly affected toe was the first toe (38%), followed by the third toe (28.6%), the second toe (19%), the fourth toe (4.8%), the calcaneus (4.8%) and the cuneiform bone (4.8%). Among the 17 cases with 19 lesions located on the toes, the most common sites were the proximal phalanx (57.9%), followed by the distal phalanx (21%), metatarsal (15.8%) and middle phalanx (5.3%). In the present case, the lesion was located on the proximal phalanx of the second toe.

The tumor is typically found as a solitary lesion, known as a solitary enchondroma (8). However, they can also appear as multiple lesions, as seen in conditions such as multiple enchondromatosis (Ollier disease) and multiple enchondromatosis associated with hemangiomas (Maffucci syndrome) (3). Enchondromatosis is linked to somatic mutations in the isocitrate dehydrogenase (IDH)1 and IDH2 genes. These mutations produce defective IDH, an enzyme in the tricarboxylic acid cycle that converts isocitrate to α-ketoglutarate. The mutated enzyme facilitates the reduction of α-ketoglutarate to the oncometabolite D-2-hydroxyglutarate (D-2-HG), and by competitively inhibiting α-ketoglutarate-dependent enzymes, D-2-HG results in hypermethylation of DNA and modification of histones. These processes encourage the development of cartilaginous tumors and disrupt the normal osteogenic differentiation of mesenchymal stem cells (6). All reviewed cases in the present study involved solitary enchondromas, apart from 1 patient with multiple lesions in the distal phalanx, proximal phalanx, and metatarsal bones of the first toe of the same foot (5).

Foot enchondromas can occur at any age, although they are most commonly observed in patients between the first and fourth decades of life (9). The youngest case reported among the cases reviewed herein involved a 16-year-old female, while the oldest was an 86-year-old female (1,11). In accordance with the study by De Yoe and Rockett (4), in the present study, a review of the literature revealed no sex predilection in the prevalence of foot enchondromas, with 10 males and 9 females. The case in the present study was a 20-year-old male.

Generally, enchondromas remain asymptomatic for extended periods of time. When symptoms do manifest, they may include pain, swelling, or deformity of the affected bone (6). The primary source of pain often stems from elevated pressure caused by the cortical expansion of the lesion, pathological fracture, or malignant conversion of the lesion (1,3,4). The lesion in the majority of the reviewed cases caused pain (76.2%) and food inversion in 1 case (4.8%), while the remaining lesions were diagnosed incidentally on foot radiographs (19%). Additionally, 6 out of the 19 cases had pathological fractures.

Various modalities and methods are available to healthcare professionals for detecting and diagnosing enchondromas. The primary and most crucial method remains a comprehensive clinical history (8). After assessing the clinical presentation, plain radiographs are the preferred initial diagnostic imaging modality (9). Radiographically, enchondromas appear as lytic lesions with clearly defined borders and variable degrees of stippled or punctate calcifications, typically without the involvement of the surrounding soft tissues (3). In general, computed tomography scans and MRIs can provide additional detail about the lesion, particularly when there is rapid growth or suspicion of soft tissue involvement (4). In the identified literature, plain radiography was the most commonly employed diagnostic modality, showing lytic lesions. In the case presented herein, the radiograph revealed a radiolucent, eccentric lesion with loss of the medial bone cortex, and the MRI revealed an expansile osteolytic lesion breaching three cortices of the affected phalanx.

Radiological findings suggesting a lesion are not always conclusive for diagnosing an enchondroma; therefore, a histopathological analysis is mandatory (9). Distinguishing between benign and malignant lesions presents a significant challenge. All available tissues need to be thoroughly examined. Enchondromas can be visually identified as bluish, semi-translucent masses of hyaline cartilage arranged in lobular patterns. Microscopically, enchondromas display small chondrocytes within lacunar spaces characterized by round, uniform nuclei resembling those found in hyaline cartilage. Some enchondromas may also exhibit areas of ossification within the cartilage matrix (4,6). The nuclei of these cells are generally regular, showing a few mitotic activities. Enchondromas located near the bone cortex, including those in the hands, may exhibit increased cellularity and atypia while remaining benign (6). Histologically, enchondroma and low-grade (well-differentiated) chondrosarcoma can appear deceptively similar, although they can be distinguished by their tissue architecture and patterns of invasion: enchondromas typically display multiple discrete nodules of hyaline cartilage separated by normal marrow elements and are often surrounded by lamellar host bone conforming to the shape of the cartilage lobules; by contrast, low-grade chondrosarcomas tend to form a single confluent mass of cartilage that permeates the marrow, ‘trapping’ host lamellar bone, infiltrating the Haversian systems or marrow fat, and often exhibiting fibrous bands between peripheral cartilage lobules. These features reflect its malignant nature. Additional supportive indicators include the presence of lobulation patterns and fibrous tissue formation around the lesion, which have been shown to correlate with malignant recurrence in follow-up studies, whereas enchondromas generally remain benign (16,17). The histopathological analysis of the specimen in the case in the present study revealed hypercellular sheets of chondrocytes encased by mature bone trabeculae at the periphery without cortical destruction, entrapment of pre-existing lamellar bone, or soft tissue invasion. There was no multinucleation, significant pleomorphism, mitotic activity, or necrosis.

The treatment of enchondroma can range from close monitoring and regular follow-up, particularly for small, asymptomatic lesions, to complete surgical removal with bone grafting for larger, symptomatic lesions (2). Surgery is recommended for patients experiencing ongoing symptoms and lesions >2 cm, as they pose a significant risk of pathological fractures. It includes complete tumor removal with or without bone grafting, as well as curettage followed by bone grafting. Goto et al (5) reported that the radiographic and functional outcomes of simple curettage without bone grafting are comparable to those of curettage with autologous bone grafting. They also highlighted several advantages of performing curettage without bone grafting for foot enchondromas: i) It eliminates the pain and discomfort associated with the bone donor site; ii) the procedure can be performed on an outpatient basis; and iii) the shorter operation time provides economic benefits and decreases the risk of infection (5). However, Edwards and Kingsford (3) reported that curettage alone is not recommended due to a high rate of non-union (67%). They found that the surgical option involving bone grafting is more suitable, as it offers a recovery period similar to that of curettage alone, despite the additional wound that requires healing (3). Patel et al (9) also reported that the latest management option for foot enchondroma involves using an autologous bone graft from the iliac crest, which can be either cortical or cancellous. Chun et al (18) reported a series of 20 cases in which all patients underwent tumor curettage followed by bone grafting. No recurrences or post-operative complications were observed during the 24-month follow-up period (18). In addition, Futani et al (19) conducted a retrospective cohort study comparing osteoscopic and conventional open surgery for foot enchondromas. A total of 17 patients underwent osteoscopic surgery, and 8 patients underwent open surgery. They reported that functional recovery was significantly improved in the osteoscopic group at 1 and 2 weeks post-operatively, though no differences were noted after 1 month. Additionally, osteoscopic surgery was associated with fewer complications (12% vs. 50%) and no recurrences in either group (19). Among the reviewed cases, 8 (42.1%) cases were managed with excision or curettage combined with bone grafting, another 8 (42.1%) cases were managed with simple curettage without bone grafting, 1 (5.3%) case was managed with total phalangectomy, and 2 cases underwent partial and ray amputation (10.5%). All reported cases had favorable surgical outcomes without any recurrence. In the case in the present study, the tumor was surgically removed, and a tricortical iliac bone autograft from the ipsilateral site was placed into the defect. The surgical outcome was favorable, with no signs of recurrence or complications. The unretrievable last follow-up X-ray image, which revealed the complete union of the bone with the graft, may be a limitation of the present case report.

In conclusion, enchondroma is a benign tumor rarely found in the foot. For symptomatic cases, the surgical removal of the lesion combined with autologous iliac bone grafting may result in favorable outcomes.