Isolated Ischial Lesions – Demographics and Imaging Features

Anatomy of the ischium

The ischium is an important weight-bearing bone that forms the posterior inferior aspect of the pelvis, articulating with two other bones; the ilium and pubis. The ischium is composed of a body and ramus. The body of the ischium is fused with the ilium and pubis and has three surfaces; the dorsal, pelvic, and femoral surface. The dorsal surface forms the ischial tuberosity and constitutes approximately two-fifths of the acetabulum. The posterior aspect of the dorsal surface is important in separating this area into the lesser and greater sciatic notches which are converted into foramina by the sacrospinous and sacrotuberous ligaments. These two structures are important in providing access to major nerves, vessels, and muscles. The femoral surface faces downward, forward, and laterally forming the lateral limit of the ischial tuberosity, which gives rise to the hamstring muscles. The pelvic surface is rather smoother and forms part of the lesser pelvis extending backward into a thin and pointed triangular eminence known as the ischial spine. The ramus of the ischium extends medially from the body and ischial tuberosity. It is divided into superior and inferior parts which serve as part of the origins of the adductor muscles. The superior ramus projects downward and backward from the body and continues to fuse with the inferior ramus of the pubis to form the ischiopubic ramus.

The ischium serves as an important area for muscular and ligamentous attachment. The sacrotuberous ligament (arising from the sacrum and attaching to the posterior ischial spine and medial aspect of the ischial tuberosity), sacrospinous ligament (originating from the sacrum and attaching to the ischial spine), and ischiofemoral ligament (ischiofemoral ligament blends with the fibers of the hip joint capsule and attaches at the intertrochanteric line of the femur) are all important in providing reinforcement to the hip joint.

Malignant tumors of the ischium

Tumors of the pelvis are a rare entity with an incidence of 5.6% of all bone lesions. Tumors predominantly affecting the ischium are even less common and account for 0.4% of all cases.^[2] Metastatic lesions usually disseminating from lung, breast, or prostate cancer are the most common secondary malignant neoplasm of the pelvis^[3] [Figure 1]. Picci et al. showed that 1.8% of metastatic lesions of the pelvis are located in the ischium and this correlates well with our study.^[4]

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Figure 1:

Metastasis from bronchogenic carcinoma. Radiograph (a), coronal T1 (b), and short-tau inversion recovery (c) demonstrating destructive lesion involving the right ischium.

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Chondrosarcoma and Ewing’s sarcoma were the most common primary malignant tumors identified within our cohort. Chondrosarcomas arise from cartilaginous tissue that is characteristically without osteoid and are distinguished from other tumors by the production of cartilage matrix from the tumor cells. They comprise a heterogeneous group of neoplasms and account for approximately 20% of all bone sarcomas.^[5] They affect the pelvis in approximately 40–50% of all cases and frequently involve the ilium and the pubis.^[6] The exact incidence within the ischium specifically is not known. Chondrosarcomas most commonly affect adults above the age of 50 years and are almost absent in children.

Radiologically, chondrosarcomas are identified by the presence of lytic lesions with granular calcifications, endosteal scalloping, and invasion of the cortex involved. This is typically seen in computed tomography (CT) scans. On T1-weighted magnetic resonance imaging (MRI), a gray homogeneous signal can be identified contrasting clearly with normal marrow, while on T2 weighted or fat suppressed. Short-tau inversion recovery (STIR) imaging, a distinctive hyperintense lesion with unexpected medullary extension can be seen usually on a coronal projection^[7] [Figure 2].

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Figure 2:

Chondrosarcoma. Axial computed tomography (a), T1 (b), and coronal short-tau inversion recovery (c) demonstrating the left ischial chondrosarcoma.

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Ewing’s sarcoma is the second most common malignant primary bone tumor that most commonly affects young adults.^[8] It is an aggressive metastatic neoplasm that arises from hematopoietic marrow, typically arising from the medullary cavity and invading the Haversian system of the bone.^[9] The pelvis is involved in approximately 20% of all cases, and as far as, we are aware, the exact incidence in the ischium has not previously been reported. Very rarely does Ewing’s sarcoma affect older patients and when it does, this is usually extraskeletal^[10-12] [Figure 3]. The tendency for these neoplasms to involve bone in the middle aged and elderly is due to the changing distribution of red marrow that occurs with advancing age.^[11] Interestingly, in our study, most patients identified with Ewing’s sarcoma were over the age of 40 years.

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Figure 3:

Ewing’s sarcoma. Coronal T1 (a), short-tau inversion recovery (b), and axial computed tomography (c) showing Ewing’s sarcoma of the left ischium.

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Ewing’s sarcoma is often challenging to diagnose and this is primarily due to the inconsistency in the appearances of both primary and secondary pelvic tumors. MRI is more sensitive and remains superior to CT in evaluating the involvement of soft tissue, marrow, and neurovascular structures; however, definitive diagnosis is usually confirmed with a biopsy and histological examination.^[13,14]

Another primary malignancy we identified in our cohort was plasmacytoma. Multiple myeloma (MM) is the most common primary bone tumor of the elderly and usually affects patients above the age of 50 years.^[15] It is a systematic malignant tumor originating from monoclonal B-lymphoid cells differentiating in plasma cells. When isolated, MM is known as solitary plasmacytoma and is a very rare entity comprising <5% of plasma cell dyscrasias [Figure 4]. MM usually involves the axial skeleton. In the pelvis, it is most commonly found affecting the ilium (11.6%) and sacrum (1.5%). Again, the incidence in the ischium is not known. MM presents with non- specific symptoms such as pain at the site of involvement, hypercalcemic or hyperuricemic syndromes, and, occasionally, pathological fractures.^[16]

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Figure 4:

Plasmacytoma. Radiograph (a), coronal T1 (b), and short-tau inversion recovery (c) demonstrating large destructive lesion of the left ischium and acetabulum.

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The diagnosis of MM involves a series of different investigations. Classically, 90% of cases have a spike in serum and/or urinary immunoelectrophoresis, due to the excess of production of a monoclonal protein with the presence of Bence Jones proteinuria. Imaging also plays a major role in aiding diagnosis of MM and its use is recommended by the International Myeloma Working Group.^[17] Although not definitive, the use of imaging techniques such as CT, MRI, and PET scans helps to analyze and define osteolytic lesions.^[18] MRI remains the most sensitive imaging modality, identifying the diffuse and nodular appearances in cancellous bone that is linked with MM.^[19]

Benign tumors of the ischium

GCTs are regarded as the most common benign bone lesions.^[20,21] They are defined as intramedullary lesions characterized by the presence of multinucleated giant cells and account for approximately 5% of all bone tumors. They are rarely found in the pelvis and in the ischium represent <0.05% of all GCTs [Figure 5].^[22] They most commonly affect young adults, where 65% of cases present between 20 and 40 years of age. About 1–3% of cases are found in the immature skeletal bone.^[23,24] GCTs are best seen on MR imaging, as it provides the ability to define the lesions, identify the soft tissue extension, and plan the mode of treatment.^[25]

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Figure 5:

Giant cell tumor. Radiograph (a), coronal T1 (b), and axial short-tau inversion recovery (c) demonstrating large destructive lesion of the left ischium and acetabulum.

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Aneurysmal bone cysts are defined as benign expansile tumors made up of numerous blood-filled channels [Figure 6]. They most commonly affect younger individuals with up to 80% of cases found in patients <20 years of age, correlating with our study.^[26] The cause of ABCs remains unknown. They usually occur in the metaphysis of long bones, and lesions found to affect the pelvis have an incidence of 8–12%. In regard to the ischium specifically, we could only find one case report in literature describing this.^[27] Imaging is useful in aiding the diagnosis of ABCs. The term “aneurysmal” originates from its radiographic appearance and this reveals a sharply defined, osteolytic lesion that is present with sclerotic margins.^[28] MRI demonstrates classic fluid-fluid levels.

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Figure 6:

Aneurysmal bone cyst. Coronal T1 (a), short-tau inversion recovery, (b) axial, T1 (c), and T2 (d) demonstrating the right ischial lesion with fluid-fluid levels.

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Non-tumor ischial pathologies

Osteomyelitis was a relatively common finding in our cohort. It is defined merely as bone infection. Ischiatic osteomyelitis is extremely rare with very few case reports in literature.^[29,30] It is most commonly found to affect children and adolescents.^[31] In general, the most common cause of osteomyelitis is Staphylococcus aureus; however, in the pelvis, osteomyelitis can be polymicrobial, with Escherichia coli identified in a few cases.^[32,33] Symptoms of osteomyelitis are non-specific and the diagnosis is dependent on MR imaging identifying bone marrow edema, cortical bone destruction, as well as soft tissue and joint complications.

Another type of infection we identified in our cohort was TB. TB is most commonly caused by Mycobacterium tuberculosis [Figure 7]. Involvement of the ischium is not common with a reported incidence between 0.1% and 0.43% of all cases of bony TB. Again, similar to osteomyelitis, symptoms may be variable and it may present with disseminated disease. Occasionally, TB of the ischium may also present with a discharging sinus.^[34] TB can be suspected both clinically and radiologically. On radiographs, a triad (known as Phemister’s triad) of radiologic abnormalities can be identified which are characteristic of TB. These include periarticular osteoporosis with marginal osseous erosion and a gradual reduction of the joint space.^[35,36] Despite this, MRI remains to be the imaging modality of choice for early detection of joint TB. MR imaging demonstrates a synovial proliferation which can typically be seen on T2W images.^[37,38]

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Figure 7:

Tuberculosis, radiograph (a), coronal T1 (b), and short-tau inversion recovery (c) demonstrating edema in the left ischium which on biopsy proved to be tuberculosis.

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Avulsion fracture of the ischial tuberosity (AFIT) is not uncommon injuries that usually affect young athletic patients [Figure 8]. During puberty, the apophysis is the weakest link in the chain of muscle, tendon, and bone and any forcible contraction of the hamstring muscles leads to this fracture.^[39] It rarely affects adults and searching literature; we were only able to identify one case report of this.^[38] When AFIT is suspected, radiographs are the initial modality of choice and can easily identify significantly displaced lesions. However, when the displacement is minimal, this injury may be missed, and therefore, MRI imaging is required, possessing the ability to identify AFIT through edema such as signal intensity and surrounding subperiosteal fluid.^[40]

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Figure 8:

Chronic avulsion fracture of the ischium. Radiograph (a), axial T1 (b), short-tau inversion recovery (c), and coronal T1 (d) demonstrating chronic healed avulsion fracture of the right ischium.

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Limitations

Although we identify the different number of lesions affecting the ischium, our study is biased based on the fact that this study was carried out in a tertiary referral center, limiting the number of cases that may be present within our institute.