SBCs are benign, fluid-containing skeletal lesions and are referred to as unicameral bone cysts or juvenile bone cysts (3). In a review of 258 patients with SBCs, the calcaneus was identified to be a relatively common site for the formation of SBCs, with a morbidity rate of 14% (4).

A majority of SBCs of the calcaneus were asymptomatic and diagnosed incidentally as a result of minor trauma to the foot or ankle (3–5). Pain is the major complaint in ~44.4% of cases, and those who were middle aged experienced more severe pain (4). Pathological fracture is rare (3). On physical examination, patients demonstrated normal ankle and subtalar motion. There was no tenderness or instability in the ankle (6).

X-ray analysis (Fig. 1A) revealed a solitary bone cyst of the calcaneus that presented as a sharply demarcated radiolucent osteolysis surrounded by sclerotic borders (4,6,7). The location of the cysts is usually the Ward's triangle of the calcaneus, anterolaterally to the posterior facet of the subtalar joint, where stresses are physiologically low (4,8). A cortical destruction may arise from a fracture (9). A computed tomography (CT) scan revealed a fluid-fluid level that represents the layering of fluids with different densities (7,10). According to magnetic resonance imaging (MRI), there is a low signal intensity on the T1-weighted image, a high signal intensity (brighter compared with fat) on the T2-weighted image, and a thin rim, occasionally with septal enhancement, following the administration of intravenous contrast (4–5,7,10,11). A bone scan (BS) is a normal part of diagnosis and may be a useful modality for the objective evaluation of symptomatic cysts and differential diagnoses, particularly for patients without fractures (5).

Macroscopically, a serous, fluid-filled cavity with strip-like bony prominences and a thin fibrous membrane may be identified during the surgery (7). Microscopically, haemosiderin deposits and fibrous tissues are a direct indicator of an SBC. Additionally, there should be no cytological atypia (4,5,7). A cholesterol cleft is a common microscopic observation in calcaneal bone cysts, however, it is not detected in long bone cysts (5).

In the majority of cases, conservative management of calcaneal bone cysts is advocated due to the negligible symptoms that they produce and the rarity of complications including pathological fractures (3,4). Surgical treatments (Enneking stage 1 to 2) have included endoscopic curettage and grafting (8,12), curettage and bone grafting (4,7,13), curettage, calcium sulphate pellet cement and autogenous grafting (4), multiple drilling (5), percutaneous cyst curettage and injection of demineralized bone powder and marrow (9,11,14), minimal curettage combined with multiple drilling and continuous decompression through a cannulated screw (15), steroid injection (16) and tricalcium phosphate ceramic (17). Postoperative cases are almost asymptomatic with no recurrence excluding mild heel pain in two patients (2,4,5,7,8).

ABCs in the calcaneus are benign, expansive, locally destructive lesions that account for 1% of all primary bone tumour types (18–20). Of all ABCs, ~30–35% are present in association with another bone lesion including giant cell tumours, osteoblastomas and/or chondroblastomas (18,19).

A majority of patients with ABC present complaints of heel pain subsequent to minor trauma, discomfort while walking and occasional swelling (18,19,21). The heel pain may be chronic, worsening gradually following the trauma (19). Pathological fractures are rare but have been reported (18). During clinical examination, tender swelling may be observed without a local increase in temperature or lymph node swelling (21).

Upon radiographic examination, the lesion (Fig. 1B) is generally identified as a lytic, expansile lesion with an irregular or eccentric shape, thin sclerotic borders or septations (18). A CT may help establish an anatomic extent of the lesion, which is essential in order to delineate the fracture line and the scheme of surgical management (18). MRI clearly demonstrates an expansile and septate lesion with relatively well-defined margins that contain multiple fluid-fluid levels. The fluid-fluid levels may be best observed on the T2-weighted image with a varying signal intensity due to the chronicity of the associated haemorrhage (18,19,22). A BS demonstrates increased uptake at the periphery in a number of cases (23).

Upon gross examination, fragments of multiple grey-brown or reddish-brown granular sections contain blood-filled cystic spaces (22). Histopathologic examination of the lesion reveals cavernous cavities of different dimensions containing fibroblasts, histiocytes and osteoclastic giant cells without any mitosis or atypia (19,21,22).

The choice of ABC treatments ranges from wide resection of the cyst to curettage with bone grafting (Enneking stage 2) (12,19). The use of adjuvants including cryotherapy, phenol, polymethyl methacrylate and irradiation is employed to remove an aggressive lesion (Enneking stage 3) (24). All ABCs of the calcaneus have been observed to heal well with no local pain and recurrence during a follow-up period of >5 years (18,19,21–23). Due to the risk of recurrence associated particularly with chondroblastomas, 6 month CT scans are recommended for the first two postoperative years (19,25).

OO is a relatively common benign tumour that is more frequently observed in the younger population, aged 7–25 years (1,26). It has a higher incidence in the male population, which accounts for 74% of all cases (27).

Chronic nocturnal pain and swelling are the main complaints of patients with OO and are usually located in the talocalcaneal joint, and may be characteristically relieved by non-steroidal anti-inflammatory drugs (26). Physical examinations are non-specific and may reveal joint effusion, local temperature rise, tenderness, stiffness and muscle atrophy (26,27). Occasionally the patient is unable to flex the plantar ankle as a result of severe pain (28,29).

Inflammatory reactions in the blood and marker level abnormalities are frequently observed in reported cases (26,27,30). X-ray only analysis reveals surrounding sclerosis of the calcaneus, which potentially results in misdiagnosis of a SBC (26). CT is a superior method for demonstrating a well-defined nidus, which is a round or oval smoothly marginated lytic lesion with central mineralization (26,27). MRI reveals low signal intensity on the T1-weighted image and a variable signal on the T2-weighted image due to adjacent inflammation or oedema (29–32). BS is a meaningful method for delineating symptomatic areas (33).

Histologically, the nidus consists of osteoblasts and osteoclasts surrounded by giant cells and vascular and fibrous connective tissue (27). A number of studies have suggested that OO may be managed conservatively using anti-inflammatory medications (1,28,30). However, open surgery with or without bone grafting (Enneking stage 1–2) is a preferential option for patients who are insensitive to conservative treatment (29). Previously, minimally invasive techniques, including percutaneous CT-guided resection, radiofrequency ablation and laser photocoagulation, have undergone remarkable progress in decreasing the risk of bone architectural changes and fractures that result from open surgical excision (32,34–36).

Osteochondroma in the calcaneus is rare, constituting 10% of all osteochondromas associated with hand and foot bones (37). It is exclusively identified in patients <20 years-old, which indicates a malignant transformation in adults (1,37,38).

Clinically, patients may initially present with an enlarging mass with a painless deformity (38). Noticeable symptoms arise with the appearance of progressive pain, swelling and stiffness, which are attributed to fractures, malignant transformation and a low proportion of subcutaneous tissue (38–41). Upon physical examination, a stiff, firm and painful nodular mass may be palpated (37–39). The motion of the ankle is observed to be normal but presents a mildly painful active flexion (40,42). Routine laboratory test results are within the normal range (37–42).

On radiographs, the X-ray generally reveals a solitary bone prominence from the bone surface that is pedunculated or sessile (41). The hyaline cartilage cap is not apparent on X-ray, but it is present at a low attenuation on CT (11). The three-dimensional imaging features of CT provide accurate information about the dimensions of the lesion (38,39). MRI is the best technique with which to evaluate the hyaline cartilage cap, which presents a low signal intensity in T1- and T2-weighted images (38–40). A thickness of the hyaline cartilage cap >1.5 cm suggests a malignant transformation of the chondrosarcoma (38,40), and a BS allows an optimal correlation with the degree of endochondral bone formation (39).

The pathologic anatomy of the lesion is a polypoid soft tissue covered with a cartilage cap (43). Biopsies reveal a spongy bone covered with a hyaline cartilage cap, in accordance with the characteristics of osteochondroma (43).

Treatment is usually not necessary for patients with asymptomatic lesions (38). Surgical removal is considered for aggressive growth subsequent to skeletal maturity, which indicates malignant transformation (39–41). A marginal resection (Enneking stage 1–2) is adequate and indicates a low rate of recurrence (39–41,43–45). The key point to avoid recurrence is the complete excision of the lesion (45).

Chondroblastoma is a rare chondroid tumour, accounting for 4.5% of all bone tumour types and 2.8% of all calcaneal tumour types (46). There is a clear male predominance with an incidence ratio of 5:1 (46).

Local pain, soft tissue swelling and tenderness are the main complaints of patients with chondroblastoma (46,47). A physical examination mostly involves oedema and painful palpation of the heel in addition to joint dysfunction of the ankle (47). All routine laboratory tests produce results that are within normal ranges (48).

Plain radiographs demonstrate an osteolytic lesion (Fig. 1C) with a sclerotic margin or tiny, stippled calcifications. The fracture line appears in patients with a pathological fracture (48). CT demonstrates a well-defined osteolytic lesion and intracortical lucency (49). MRI for the chondroblastoma reveals an irregular, expansive lesion and fluid-fluid levels. There is a heterogeneous, low to intermediate signal intensity on the T1- and T2-weighted image. All lesional and perilesional regions demonstrated diffuse enhancement following contrast administration (49,50). Furthermore, bone marrow oedema and soft tissue oedema may be observed in the case of a pathologic fracture (48).

The gross appearance of the excised lesion reveals a soft, grey-brown, multiloculated cystic lesion with bone fragments and a haemorrhagic mass (47,50). Histologically, the tumour is composed of proliferating, polygonal chondroblasts, small mononuclear cells, multinucleated giant cells and calcifications (48).

Surgical treatment is the preferred choice for chondroblastoma, including simple curettage with bone grafting in addition to endoscopic curettage without bone grafting (Enneking stage 2) (47,49–51). The latter is a more effective treatment for chondroblastoma with a secondary ABC (49). However, recurrence is highly attributed to the pattern of simple curettage, inadequate resection and the association with secondary ABCs (46,48).

Intraosseous lipomas are rare tumour types, constituting 0.1% of all benign bone tumour types. Furthermore, only 8% of these tumour types are located in the calcaneus (52,53). The tumour is often revealed in patients ranging in age from 30–60 years (52,53).

Of all cases, ~40% of cases are identified incidentally. However, in symptomatic cases, pain is the most common symptom, particularly following a strenuous walk (52,54). The pain may be relieved with aspirin and sufficient rest (55). Upon physical examination, an antalgic gait pattern may be present during walking (54,55). Pain in response to palpation surrounding the lesion is consistently observed with or without swelling (52). There are usually no unique laboratory results (52–55).

X-ray of the intraosseous lipoma (Fig. 1D) reveals a well-circumscribed lytic lesion with central, single or ring-like calcifications, a majority of which are located at the base of the calcaneus neck in the region of the Ward's triangle. CT provides a higher-grade image, including the anatomic location, size, extent and potential pathologic fracture. MRI demonstrates high signal intensity on the T1-weighted and T2-weighted image, which is more accurate in terms of the image of the bone marrow and soft tissue (52–58).

The gross appearance of the excised lesion reveals a yellow, friable fatty tissue with fragments of bone (54). Histologic examination presents fat cells surrounded by necrosis and calcification, which are interspersed with areas of haemorrhage and bony trabeculae (52,54,55).

Asymptomatic lesions (Enneking stage 1) may be treated conservatively as they are consistently located in the region of the Ward's triangle, which is a non-weight-bearing area of trabecular rarefaction (58). However, lesions (Enneking stage 2) that are symptomatic, oversized and prone to pathological fracture are indicated for surgical treatment (57,58). Curettage and autologous bone grafting is an easy and effective operation for intraosseous lipoma, followed by a long-term non-weight bearing period (52–55). Previously, endoscopically assisted tumour curettage followed by filling of the bone defect with α- or β-tricalcium phosphate has been demonstrated to be a less invasive method that allows patients to walk normally subsequent to wound healing (59,60). In a number of reported cases, there has been no recurrence or complications following surgical treatment (52–63).

GCT of the calcaneus is a benign neoplasm that makes up 1.2% in all cases of calcaneal tumours and exhibits high recurrence and potentially aggressive features (64–66). It is most commonly observed in patients who are 30–40 years old (64).

Clinically, heel pain and swelling are the primary complaints in patients with calcaneal GCT (64,65). On physical examination, a tender and prominent hard swelling is palpated. The range of ankle joint motion is normal (64–67). Serum biochemistry is within normal limits, barring other unassociated systemic conditions (68).

Radiographically, there is an expansile or eccentric lytic lesion with a non-sclerotic margin, which may extend near the articular surface (65,68). A CT scan may confirm the marked expansion of the calcaneus (67). However, aggressive features may additionally be identified, including cortical expansion or destruction of the soft tissue component (65). MRI reveals low signal intensity on the T1-weighted image and a high signal intensity on the T2-weighted image (69). The histological features consist of numerous multinucleated giant cells. Spindle cells and ABC features may be present in certain areas (65,69).

Traditionally, curettage is the primary mode of treatment, accompanied by phenol, alcohol or cement to decrease recurrence (Enneking stage 2) (66). In cases of recurrence and aggressive GCTs (Enneking stage 3), limb salvage surgery consisting of a wide excision of total calcaneoctomy, bone allograft reconstruction and soft tissue coverage with a sural flap, are considered superior to amputation (65).

CMF of the calcaneus is a rare benign primary bone tumour composed of immature myxoid mesenchymal and cartilaginous tissue, making up ~7% of all cases (70–73).

The slow-growing CMF often results in a delay in diagnosis, particularly in patients which lack symptoms (70,71). The chief complaint in patients is chronic heel pain with a slow-growing mass, with a limp and limited range of ankle motion (71,72). Physical examination observations are unremarkable, except for a palpable, hard and non-mobile protrusion from the lesion of the calcaneus (71). Laboratory investigations do not reveal any abnormal results (71–73).

The classic radiographic evaluation of the calcaneus (Fig. 1E) is a well-defined, geographic lesion with thin, sclerotic margins associated with septation and lobulation. CT may be used to clearly define the expanded or thin cortex in addition to the cortical integrity (71,72). MRI is useful to characterize the tumour and to assess the tumour extent for preoperative planning (71). It reveals lesions as a low signal intensity on the T1-weighted image and a prominent high signal intensity on the T2-weighted image owing to the chondroid and myxoid composition (72–74). Peritumoral oedema of the marrow is seldom present, indicating a more aggressive tumour (71).

The characteristic gross appearance of the lesion is a grey or white multilobulated mass composed of a fibro-gelatinous substance with pieces of cartilaginous material (71,72). Intralesional haemorrhage and cystic degeneration may additionally be identified in a number of cases (71). Histologic examination indicates variable proportions of cartilaginous, myxoid and fibroid tissue. The immunohistochemical stain for S-100 is focally positive (71,72).

Certain patients with CMF (Enneking stage 2) may be treated using simple curettage, however, this technique has been reported to retain tumour cells and is responsible for recurrence (71,73). Thus, the combination of simple curettage, curettage with dental bur augmentation, block resection or even amputation have been demonstrated to be effective (71,73,74). Patients should be followed up for a minimum of 3 years to monitor signs of recurrence (71). Routinely, a short leg cast for 6 weeks in addition to physical therapy are essential (72).

Other reported benign calcaneal tumour types include desmoplastic fibroma, osteoblastoma, ganglion, benign fibrous histiocytoma, primary xanthoma and intraosseous spindle cell haemangioma (75–87). Additional details cannot be obtained due to a limited number of case reports. Known details are provided in Table I.

Osteosarcoma is the most common non-haemopoietic primary malignant neoplasm of bone (88). The calcaneal location of osteosarcoma is rare, accounting for only 1% of all osteosarcoma (89). Younger people (between 10 and 30 years old) are typically affected, with a peak in the second decade of life (89,90).

Progressively worsening pain and swelling in the affected ankle joint are major complaints in patients with osteosarcoma in the calcaneus (88,90,91). During physical examination, the swelling is tender, immobile and the mobility of the joint is restricted (88,90). A prior history of trauma has been noted in multiple cases (89,92).

In the case of a small cell osteosarcoma in the calcaneus, laboratory tests reveal an increase in the erythrocyte sedimentation rate (ESR) and in C-reactive protein (CRP) levels (91). Radiograph and CT of the calcaneus reveal a sunburst appearance with a dense sclerotic lesion, cortical destruction and extraosseous expansion (88,90,93). A majority of cases consist of a mixture of lysis, sclerosis and rarefaction (88). CT reveals soft tissue extension and a large osteolytic lesion with cortical thinning/breakthrough (90). MRI was performed for preoperative staging in addition to confirming the presence of surrounding soft tissue infiltration, presenting an intermediate intensity on the T1-weighted image due to sclerotic bone formation and a high signal intensity on the T2-weighted image (88). CT of the chest, abdomen and pelvis ought to be performed to rule out metastatic disease at the time of diagnosis (88).

The lesion was observed as a tan-white, yellow, heterogeneous, smooth and firm mass located in the body of the calcaneus (88,91). The pathological image presented a diffuse proliferation of the spindle and atypical cells characterized by nests or cords encompassed by abundant osteoid deposits (88,90,93,94). Sheets of pleomorphic osteoblasts, mitotic features and osteoid formation may additionally be observed (88,90).

Prior to and following surgery, all patients have been reported to accept chemotherapy (88–93). The optional surgery of the calcaneal osteosarcoma varies depending on the tumour stage (89). Patients (Enneking stage IIB) who have undergone below-knee amputation have a satisfactory form of treatment, with no evidence of local recurrence in those who survive >2 years (89). Furthermore, a calcaneal prosthetic replacement, which is beneficial for functional consequences, may be an appropriate treatment for special intramedullary tumours (stage IIA) that respond well to preoperative chemotherapy (91,93). Two patients presented with metastatic disease (88,90).

The calcaneus is a rare site of primary osseous lymphoma that represents ~1.2% of 246 cases in clinical research (95).

The symptoms vary from mild non-specific pain to soft tissue swelling, often with a lack of constitutional symptoms (96–98). A number of cases may be accompanied by a pathological fracture (95,96,98). A physical examination of the affected hind foot may reveal non-pitting oedema, reddening and tenderness (97,99). Laboratory tests all produce results that are within normal limits (100).

Radiographs reveal a mixture of osteolysis and sclerotic lesions of the calcaneus, which produce a ‘moth-eaten’ and patchy appearance (96–98,101). Periosteal reaction, pathologic fracture and soft tissue involvement may additionally be observed (99,100). The MRI examination presents a large, lobulated, expansile lesion arising from the calcaneus with a low signal intensity on the T1-weighted image, raising suspicion of neoplastic disease (97,99,100). On a gallium scan, increased radiotracer uptake has been noted in the location of the lesion (98).

Intraoperative observation reveals a suet-like tissue in a patient with a T-cell lymphoma (97). Histological examination most often reveals a large number of lymphoid elements with hyperchromatic large nuclei, abundant cytoplasm, and prominent nucleoli (98). Immunophenotyping and histochemical stains for cluster of differentiation (CD)20 and L26 (a pan-B-cell marker) are positive in cases of B-cell lymphoma (98,99).

At present, complete excision of the lesion alone is usually not curative (88,89). In cases without metastasis (Enneking stage IIB), the patient is treated with a multiple chemotherapy regime. Although patients experience anaemia and fatigue with chemotherapy, it is effective to be disease free for >5 years (98). Radiation therapy is recommended following chemotherapy (96–98,101). Primary osseous lymphoma has provided a good prognosis, except in one patient (Enneking stage III) with a recurrent lesion in the contralateral distal tibia (100). A majority of patients respond well to the treatment and present without local or metastatic disease during long-term follow-up (96–98,101).

Chondrosarcoma is one of the most common malignant tumour types following osteosarcoma and myeloma, with ~3% of all chondrosarcoma present in the foot (1). In a report of 75 cases of chondrosarcomas in the foot, 28% were present in the calcaneus (102).

Chondrosarcoma has an indolent natural history, ranging from 8 months to 36 years (102). Clinically, chondrosarcoma is characterized by insidious, gradual and progressive pain (103). Upon physical examination, the swelling is diffuse, mildly tender to palpation, and occasionally accompanied by a local increase in temperature (103,104).

Common radiographic features (Fig. 1F) include deep endosteal scalloping with bone expansion, cortical destruction, endosteal erosion, matrix calcification, periosteal reaction and pathologic fracture (102–107). CT of the calcaneus has been more accurate in the detection of soft tissue extension, calcification and cortical involvement (105). Using MRI, a well-defined lobulated mass with a low signal intensity may be observed on the T1-weighted image and a high signal intensity on the T2-weighted image with multiple septations, reflecting the high water content of this cartilaginous neoplasm (106,107). In the contrast-enhanced image, rings and arcs may be observed as a typical lobulated growth pattern (107). On BSs, there is increased tracer uptake in the lesion. BS and chest X-ray/CT must be performed routinely to rule out metastasis even in the absence of clinical symptoms (105).

Macroscopically, the tumour tissue presents with a straw-yellow colour (103). Microscopy may reveal numerous pleomorphic chondrocytes with hypercellularity and cytologic atypia. An abundant foamy to clear cytoplasm is a feature of clear cell chondrosarcoma (105). With the use of immunohistochemistry, the tumour cells are positive for S-100 proteins and negative for cytokeratin and CD68 (104–106).

Surgery ought to be a top priority for the treatment of chondrosarcoma. Wide surgical resection or amputation remains regarded as a mainstay method, while simple curettage or local excision is not recommended due to recurrence (102–106). If the tumour (Enneking stage IIA) is restricted to a bone that may be completely removed with an appropriate amount of soft tissue, a wide excision is recommended and usually requires at least a 5 cm safety margin (102,103,106,107). For patients with a stage IIB tumour, amputation is recommended and believed to result in an improved disease-free survival rate (102–107). Although chemotherapy and radiotherapy have no effect in chondrosarcoma, they are administered as a palliative therapy in the case (stage III) of metastatic and inoperable disease (103,105).

Recurrence and metastasis of chondrosarcoma constituted nearly 50% of all patients in one previous study (102). Metastasis typically occurs to lungs and proximal bones (103–105). Owing to the propensity for recurrence and metastasis, all patients with chondrosarcoma of the calcaneus require long-term follow-up (102–104).

Ewing's sarcoma of the calcaneus is a primary, aggressive malignant tumour, of which >50% of all cases occur in the foot (108).

Local pain and swelling of the ankle are the primary symptoms of patients, in addition to a limp foot and erythema (108,109). Constitutional symptoms, including fever and weight loss are observed in a number of cases (108). Upon examination, the swelling is always extensive around the heel, resulting in stretched and shiny skin in addition to visibly dilated subcutaneous veins (108,110).

Blood laboratory tests revealed elevated ESR, acid phosphatase (ACP) and leucocytosis levels accompanied by mild or moderate anaemia (108–110). X-ray revealed a ‘moth-eaten’ type of destruction with an aggressive periosteal reaction and cortical violation in the calcaneus (109). CT visibly demonstrated a classical ‘sun-ray’ or ‘onion-peel’ appearance of the periosteal reaction, which may additionally be parallel, speculated or mixed (109–111). MRI presented a low signal intensity on the T1-weighted image and a high signal intensity on the T2-weighted image, which is unable to adequately distinguish the tumour from necrosis. Therefore, the enhancement pattern following administration of contrast medium is competent for the differentiation between oedema and haemorrhage (111). BSs reveal an increased area of uptake in the region of the tumour. Chest radiography/CT is necessary in order to detect bone metastases (110–112).

A biopsy of the tumour with histopathological analysis revealed a malignant small round-cell tumour with little or no intercellular stroma (109–112). CD99 and periodic acid-Schiff were immunohistochemically positive (109,112).

A combination of surgical resection, multidrug chemotherapy and radiotherapy are particularly applicable in comparison with surgery alone (108–112). Multidrug chemotherapy increases the overall survival rate of patients (Enneking stage IIB to III) with Ewing's sarcoma by reducing the risk of metastases, whereas surgery combined with preoperative chemotherapy reduces the risk of local recurrence (108). The prognosis of this tumour is the worst among any primary sarcoma known to metastasize to the lungs and other bones including the tibia, fibula or tarsal bone (109–111). Patients with localized Ewing's sarcoma have a higher probability of disease-free prognosis (108).

Other sporadic malignant cases that have been ever reported, including malignant fibrous histiocytoma, multiple myeloma, haemangioendothelioma, fibrosarcoma, malignant peripheral nerve sheath tumour, primary epithelioid angiosarcoma, primitive neuroectodermal tumour, and adamantinoma, are summarized in Table I (113–120).