Evaluation of Bone Lesions Around the Hip

Clinical Evaluation

A detailed history is crucial in evaluating a patient with a suspected tumor about the hip. The patient's presenting age is an important factor in narrowing down the differential diagnosis. Although any tumor theoretically can occur at any age, there are characteristic age distributions to benign and malignant tumors that will focus the attention of the clinician toward the most likely diagnosis. These are outlined in Table 48.1 .

Obviously, a personal or family history of malignancy or other tumorous condition of bone is relevant. A genetic basis is understood for a number of bone tumors, both in a syndromic context or in isolation. Furthermore, a genetic basis is suspected for many bone tumors for which it has not yet been fully characterized. Particularly in any patient with a history of prior malignancy, the presence of a new tumor about the hip must prompt suspicion for metastatic disease.

The patient's presenting pain is an essential aspect of the evaluation. Painless lesions may be diagnosed incidentally, whereas others may present with mechanical pain or night pain. Lesions that are found incidentally are most often benign; those that cause mechanical pain, or functional pain as described in the Mirels criteria, are concerning for impending pathologic fracture. Patients presenting with true night pain—pain that actually awakens the patient from a sound sleep—must raise a high suspicion for malignancy.

The presence of a mass is also an important aspect of the history. Expansile bone tumors often cause significant pain due to activation of periosteal pain receptors. Soft tissue tumors will often present with a painless mass even when malignant and quite large. Fortunately, only a tiny fraction of soft tissue masses presenting in adults represent malignancy. The hip and pelvis are deep-seated structures; thus, a mass detected by the patient is usually large and advanced.

Clinical Examination

The standard orthopedic examination of the hip joint is performed in any patient presenting with a suspected tumor. Examination findings are best described in degrees of motion as well as percentage comparison of the opposite side. Any maneuvers that elicit pain during the examination should be noted for further evaluation. In addition to a standard orthopedic joint evaluation, a full oncologic examination should be performed. This includes a general physical examination of the major organ systems (e.g., auscultation of the heart and lungs; abdominal examination; and head, eyes, ears, nose, and throat [HEENT] examination) with a particular focus on assessment of any masses, evaluation for lymphadenopathy, and any skin changes that may be present in the region of the tumor. Masses should be investigated for the presence of Tinel sign, mobility, tenderness, or pulsation. The presence of hepatosplenomegaly or other findings suggestive of disseminated disease are noted. Skin manifestations of any systemic disease or genetic syndrome (e.g., café-au-lait spots suggestive of neurofibromatosis) are noted.

Patients with a large mass or destructive lesion will often have compromise of neurologic function in the limb. Careful testing of sciatic nerve function is critical. Although pulses may be palpable, in the setting of a large mass, the ankle brachial index may be decreased owing to compression of vascular structures. Deep venous thrombosis (DVT) is common in patients presenting with large malignant masses around the hip—the limb should be assessed for swelling or other stigmata of DVT. It is helpful to measure the circumference of the limb compared with the uninvolved side at defined points.

In the setting of suspected metastatic disease to bone, a complete musculoskeletal examination should be performed to evaluate for other sites of metastasis. Sites of long-bone tenderness to palpation and bony or articular pain with motion should be noted and further evaluated.

Imaging

Imaging findings suggestive of an aggressive lesion include an associated soft tissue mass, periosteal elevation (i.e., Codman triangle), permeative appearance, large size, and rapid growth. Plain film radiographs typically provide the first assessment of a patient presenting with a potential tumor about the hip. Radiographs have the advantage of defining any bony lesions as lytic, blastic, or mixed. They help determine whether a mass is producing osteoid, cartilage, or has matrix calcification. Additionally, even in patients with soft tissue masses alone, radiographs can define soft tissue calcifications, which can be helpful in the evaluation process.

Radiographs provide a rapid assessment of present or impending fracture. However, it is important to remember that up to one-third of bony mineral must be lost before a lesion is apparent on plain film radiographs. In imaging lesions around the pelvis, inlet and outlet as well as Judet views are helpful in evaluating periacetabular or iliac wing lesions. Proximal femoral lesions are adequately evaluated by anteroposterior (AP) and lateral radiographs of the hip, with comparison radiographs of the contralateral hip taken as needed. It is important to obtain full-length radiographs of the entire femur to evaluate for the presence of any skip or distal lesions that are not immediately apparent on plain films of the hip itself ( Fig. 48.1 ). Missed lesions proximal or distal to the site of the initial lesion may lead to complications with surgical fixation and possible periprosthetic fracture. Any sites of bone or articular pain apart from the hip should be imaged with radiographs to assess for other lesions.

Magnetic resonance imaging (MRI) provides the most sensitive evaluation of suspected tumors of the bone marrow and the greatest definition of soft tissue masses. In obtaining an MRI scan, images before and after administration of gadolinium contrast are always obtained in evaluating potential tumors about the hip. T1-weighted sequences provide a sensitive evaluation for marrow, replacing processes to fully define the extent of lesions. At least one T1-weighted sequence that evaluates the entire extent of the bone is used to evaluate for skip lesions. Most commonly, this is a coronal T1-weighted sequence of the femur. Contrast enhancement patterns help separate cystic from solid masses in the evaluation of these patients.

Computed tomography (CT) is the most sensitive method to define bony architecture around the hip and provides a rapid assessment of whether matrix or osteoid production is present. CT is particularly useful to evaluate lesions of the acetabulum and pelvis, as plain radiographs may be difficult to interpret. CT can be combined with angiography to define any vascular encasement by tumor. Historically, plain radiographs have been used to assess for the risk of impending pathologic fracture via the Mirels criteria. However, CT-based structural rigidity analysis has proven effective at predicting the risk of pathologic fracture and may be considered in addition to plain radiographs.

In addition to its role in local imaging of the tumor, CT of the chest, abdomen, and pelvis should be performed in the evaluation of any patient with suspected metastatic disease of unknown primary and may be performed as part of the staging protocol in patients with known malignancy. CT of the chest is the most commonly used study for staging of sarcomas given a high rate of pulmonary metastases. In some institutions, CT images of the abdomen and pelvis are taken for any patient who has a malignancy extending at or above the level of the inguinal ligament to identify any regional metastases, which may be missed by CT of the chest alone.

Technetium bone scan is used to define whether a process is active or latent and is a component of staging of most bony malignancies. Although false negatives can be seen (such as in the case of multiple myeloma or in very aggressive lytic tumors), the bone scan is a reliable method for evaluation of most bony processes around the hip and pelvis. In those cases in which it proves unreliable, a skeletal survey can be used.

The role of positron emission tomography (PET) scans in the evaluation of tumors about the hip is evolving. PET is established for the evaluation of patients with aggressive carcinomas, among several other malignancies. As it relies on metabolic uptake by tumor cells, it is less reliable in evaluation of patients with low- or intermediate-grade malignancies. The role of PET in evaluating patients with high-grade sarcomas is currently under study. PET scans are usually combined with a low-resolution CT scan for co-registration of anatomic abnormalities.

Not all imaging modalities are necessary in all patients. Many benign lesions have characteristic appearances on plain radiographs and no further local imaging is necessary. The most common imaging protocol to define suspected lesions around the hip combines plain film radiographs with contrast-enhanced MR scans. This combination of imaging is usually adequate to form a differential diagnosis, evaluate the likelihood of malignancy, and plan an appropriate oncologic biopsy. Further imaging will be described in the other chapters in this section with reference to individual lesions.

Primary malignant neoplasms of bone are rare and their diagnosis may pose a challenge for clinicians without significant experience in the field. In many instances, clear-cut diagnosis based solely on biopsy results remains challenging without close correlation to patient history and imaging, as benign lesions may appear histologically identical to low-grade malignant lesions. Delayed diagnosis, inappropriate biopsy technique, and partial excision of a primary malignancy of bone may drastically affect the patient's treatment options and long-term prognosis. When possible, patients with concerning findings should be evaluated by a multidisciplinary team composed of orthopedic oncologists as well as pathologists, radiologists, and oncologists in a tumor board setting to guide diagnostic studies and treatment. However, initial workup should not be delayed, and the referring physician should obtain laboratory test findings and noninvasive imaging to initiate the process.

Biopsy

Biopsy confirmation of suspected histology is usually required. The hazards of a poorly planned biopsy have been well established by Mankin and colleagues. Poorly planned biopsies frequently alter the surgical options available for patients and may preclude limb salvage in the setting of a sarcoma diagnosis owing to contamination of adjacent compartments and essential structures. Additionally, in adults presenting with suspected metastatic disease, proper staging studies will often yield the diagnosis without the need for biopsy or may identify a safer alternative site to sample. Rougraff and colleagues established a protocol of history; routine laboratory studies; bone scan; and CT of the chest, abdomen, and pelvis as identifying the site of primary disease in 85% of patients.

The key principle to musculoskeletal biopsy is that the biopsy tract must be excisable with the definitive resection if the tumor proves to be a primary sarcoma. For this reason, there is a strong recommendation that the biopsy either be performed or directed by a surgeon who is prepared to carry out the definitive resection should a sarcoma be diagnosed. Most biopsies around the hip and pelvis can be performed either through a direct lateral approach for lesions of the proximal femur or along the line of the iliac crest for lesions of the pelvis. This follows the utilitarian incision to the hip as described by Enneking. Biopsies performed along this line are readily excised at the time of surgery.

Consideration should be given to performing an open versus percutaneous biopsy. Open biopsies have the advantage of obtaining a large volume of tissue for histopathologic analysis, particularly if advanced cytogenic or other specialized tests are necessary. They are generally considered the gold standard for diagnosis. However, open biopsies are more invasive and morbid than percutaneous biopsies and are not without risk; when compared with percutaneous biopsy, they expose a larger area of tissue to contamination, which will require ultimate excision should a malignancy be diagnosed. Additionally, image-guided biopsies can readily guide sampling from areas of the tumor identified on imaging studies to have the highest potential diagnostic yield; the ability to selectively sample a tumor with an open biopsy is more limited.

Percutaneous biopsy varies from fine-needle aspiration, which yields very limited samples of aspirated tissue and cells, to large-bore core needle biopsies, which can provide samples with intact tissue architecture. Given the importance of tissue architecture and matrix composition in the histologic assessment of primary sarcoma, core needle biopsy is often the preferred method of percutaneous biopsy. Core needle biopsies have historically been avoided in cystic and cyst-like lesions such as aneurysmal bone cyst, although they are increasingly being used in such lesions with acceptable results . Core needle biopsies directed by CT or MRI can be used to access areas of the tumor that are likely to be most representative while minimizing areas of soft tissue contamination. Percutaneous core needle biopsies must be carefully planned with the performing radiologist, surgeon, and pathologist to ensure that representative tissue is obtained in a sufficient quantity to allow a diagnosis while maintaining a safe trajectory. The biopsy site may be tattooed with a small drop of methylene blue to allow later identification and subsequent surgical resection. The ultimate decision for closed versus open biopsy is tailored to the clinical situation and influenced by institutional practice patterns and resources. Cultures are sent in addition to histology studies.

Should an open biopsy be necessary, it is critical that the most direct route to the tumor be used for sampling. Classic anatomic planes are deliberately avoided during these procedures to minimize contamination of multiple compartments. Rather, the approach usually goes directly to the tumor through the edge of an anatomic compartment that can be excised en bloc with the tumor, as a more classic plane of approach is used. Major neurovascular structures are avoided during the course of the biopsy so that they will not be subject to contamination when the tumor is sampled. Very careful hemostasis is practiced and a drain is almost always left during an open biopsy of the proximal femur or pelvis to minimize the resulting hematoma and potential zone of contamination. Nonabsorbable Ethibond (Johnson & Johnson, New Brunswick, NJ) sutures are used to close the fascia following the biopsy to allow for identification at the time of future resection surgery to be certain that the area is excised fully.

Tumor Simulators

This category encompasses tumor-like lesions (pseudotumors) that generally are reactive or hyperplastic tissue responses. This definition carries some ambiguity, and semantics may play a role. Therefore, certain lesions—such as aneurysmal bone cyst (ABC) and unicameral bone cyst (UBC)—may be considered to be tumor simulators or benign bone tumors.

Fibrous Dysplasia

Fibrous dysplasia (FD) is a benign intramedullary fibro-osseous dysplastic lesion first described by Lichtenstein in 1938 with an origin linked to an activating mutation in the gene that encodes the α-subunit of stimulatory G protein (G _s α). This results in developmental failure in the remodeling of primitive bone to mature lamellar bone. The consequence is biomechanically inferior bone that has randomly oriented architecture and is prone to fracture. The process can be monostotic or polyostotic. The most extreme presentation is McCune-Albright syndrome, which is characterized by polyostotic disease with café-au-lait spots and hyperfunction of the multiple endocrine glands. Monostotic disease is fairly common and is not hereditary. Most lesions are asymptomatic and are found incidentally. When a lesion is symptomatic, a common clinical presentation consists of bone pain, deformity, and may include pathologic fracture. Wide variability in radiographic features is seen on plain radiographs. The classic pattern is a “ground-glass” appearance. However, cortical thinning, expansile remodeling, endosteal scalloping, and mixed radiolucency and radiodensity are common. Other findings may include coxa vara (including shepherd's crook deformity), and protrusio acetabuli ( Fig. 48.2 ). A CT scan is the best imaging modality, but MRI is often helpful, especially in cases of cystic lesions. A radionuclide bone scan will frequently be hot. Histopathology normally demonstrates a bland-spindle cell stroma with embedded trabeculae of woven (immature) bone without osteoblastic rimming. This is often referred to as an “alphabet soup” appearance.

The diagnosis of FD is often made on clinical and imaging presentation alone. However, with atypical lesions, a biopsy should be considered. The differential diagnosis includes simple bone cysts, osteofibrous dysplasia, nonossifying fibroma, chondroma, low-grade intramedullary osteosarcoma, and Paget disease. The clinical course of FD is variable. Adults are often more symptomatic than children. There is no cure. Surgery is performed to prevent pathologic fracture, to correct deformity, and to decrease pain likely to result from fatigue fractures. Surgical treatment often includes the use of hardware or cortical allograft because the process (particularly if still active) may rapidly resorb cancellous bone grafts. Bisphosphonates have shown efficacy in decreasing bone pain in patients with active symptomatic polyostotic disease. Although rare, sarcoma developing within areas of FD has been reported.