Orthopaedic Surgery - Computed Tomography

Basics

Computed tomography (CT) is a noninvasive imaging technique that uses a rotating X-ray source and detectors to generate cross-sectional images of the body.

It is particularly useful in musculoskeletal imaging because the acquired data can be reconstructed into multiple planes and displayed as three-dimensional or volume-rendered images.

CT is especially valuable for evaluating complex osseous anatomy, fractures, joint surfaces, cortical bone, mineralization, and postoperative hardware.

Historical Development

CT was introduced in 1972, with the first clinical scanners installed between approximately 1974 and 1976.

By 1980, CT had become widely available.

During the mid-1980s, newer applications were introduced, including dynamic scanning, multiplanar reformations, and three-dimensional CT reconstruction with volume rendering and shaded-surface display.

Routine spiral or helical CT scanning became established around 1989 and subsequently developed into modern multidetector CT technology.

Advantages

Rapid Image Acquisition

One of the major advantages of CT is its speed.

Rapid image acquisition is particularly valuable for trauma patients, seriously ill patients, and children, because scanning can often be completed before significant movement occurs and may reduce the need for sedation.

Complex Anatomic Regions

CT provides excellent visualization of areas that may be difficult to evaluate completely with plain radiographs.

Examples include the spine, pelvis, scapula, wrist, ankle, and other small or anatomically complex joints.

Multiplanar Reconstruction

Modern multidetector CT scanners acquire a volumetric data set that can subsequently be reconstructed in multiple planes.

Images can therefore be generated in the axial, sagittal, coronal, or oblique planes without repeating the scan.

Three-Dimensional Reconstruction

Three-dimensional and volume-rendered reconstructions can clarify complex relationships between fracture fragments, joints, implants, and reconstructed bone.

These techniques are particularly useful for preoperative planning and evaluation of complicated skeletal anatomy.

Imaging Around Hardware

CT may provide useful assessment around metallic implants.

Modern reconstruction and artifact-reduction techniques can substantially decrease metal-related streak artifact, allowing better visualization of bone, implants, graft-host junctions, and surrounding structures.

MRI may be limited by artifact in some patients with extensive metallic hardware.

Patients Unable to Undergo MRI

CT can be performed in patients who cannot safely undergo MRI because of certain implanted devices or metallic foreign bodies.

Examples may include selected non-MRI-compatible cardiac devices, particular aneurysm clips, or metallic orbital foreign bodies.

The actual MRI compatibility of modern implants should always be verified because many contemporary devices are MRI conditional.

Availability and Cost

CT is widely available, rapid, and relatively cost-effective for many clinical problems.

It can often answer important skeletal questions more efficiently than other advanced imaging modalities.

Disadvantages

Soft-Tissue Limitations

CT is generally inferior to MRI for evaluation of bone marrow, spinal cord, peripheral nerves, ligaments, tendons, and many other soft tissues.

MRI is therefore preferred when detailed soft-tissue characterization is the primary goal.

Ionizing Radiation

CT exposes the patient to ionizing radiation.

Radiation dose is greater than with most conventional radiographic examinations, making appropriate patient selection and dose optimization important.

Cost Compared With Radiographs

CT is more expensive than plain radiography and should be used when the additional cross-sectional information is likely to influence diagnosis or treatment.

Intravenous Contrast Risks

When iodinated intravenous contrast is required, potential complications include hypersensitivity reactions and renal adverse effects in susceptible patients.

The indication for contrast should therefore be considered carefully.

CT Technique

Skeletal Imaging

Bone pathology is generally evaluated using thin-section acquisition.

Modern multidetector scanners can obtain very thin slices, allowing high-resolution multiplanar and three-dimensional reconstructions.

Postprocessing

The acquired data can be processed at a workstation to create coronal, sagittal, oblique, three-dimensional, and volume-rendered images.

These reconstructions are especially valuable when evaluating fractures and complex skeletal deformity.

Soft-Tissue Imaging

Extremely thin sections are less critical when the principal target is soft tissue rather than bone.

Reconstructed slice thicknesses of approximately 2–3 mm may be adequate for many soft-tissue applications.

Intravenous Contrast

Evaluation of a soft-tissue mass, abscess, inflammatory process, or vascular injury commonly requires intravenous iodinated contrast.

Contrast improves visualization of vascular structures and patterns of tissue enhancement.

Contrast Administration

Contrast administration protocols vary according to the examination.

High-flow injection may be required for vascular or contrast-enhanced studies, with the exact rate determined by the clinical indication and scanner protocol.

Renal Impairment

Iodinated contrast should be used carefully in patients with significant renal impairment.

Renal function, hydration status, indication for contrast, and alternative imaging methods should be considered before administration.

Previous Contrast Reaction

Patients should be questioned about prior reactions to iodinated contrast.

Those with previous significant reactions may require alternative imaging, a modified contrast strategy, or premedication according to institutional protocol.

Postoperative Applications

Hardware Complications

CT is useful for identifying complications related to orthopedic hardware.

These may include fracture, implant loosening, malposition, surrounding bone destruction, and selected manifestations of infection.

Osteomyelitis

CT may demonstrate cortical destruction, sequestra, gas, or other changes associated with osteomyelitis, particularly in postoperative patients.

MRI is often more sensitive for early marrow infection, but CT can provide valuable structural detail.

Tumor Recurrence

CT may help evaluate possible tumor recurrence in patients with metallic hardware, particularly when MRI is significantly degraded by artifact.

Foreign Bodies

CT is effective for detecting and localizing many retained foreign bodies.

Its cross-sectional nature allows accurate determination of the object’s relationship to bone, joints, vessels, and other structures.

Orthopaedic Oncology Applications

Tumor Mineralization

CT is highly sensitive for detecting calcification and ossification within a lesion.

The pattern of mineralization can help characterize a tumor and narrow the differential diagnosis.

Myositis Ossificans

CT may help distinguish myositis ossificans from neoplasm by demonstrating the characteristic distribution and maturation pattern of peripheral ossification.

Cortical and Periosteal Changes

CT provides excellent visualization of the cortex and periosteal surface.

Patterns of cortical thinning, erosion, destruction, expansion, sclerosis, or periosteal reaction may help distinguish benign from aggressive lesions.

Bone Destruction

The degree of osseous destruction can be assessed accurately with CT.

This information can help estimate structural integrity and risk of pathologic fracture.

Osteoid Osteoma

CT is particularly useful for identifying the small nidus of an osteoid osteoma.

It can also provide image guidance for definitive minimally invasive treatment, such as CT-guided radiofrequency ablation.

Neurovascular and Compartment Involvement

CT can demonstrate tumor extension into adjacent compartments and may identify involvement of major neurovascular structures.

MRI is generally superior for detailed evaluation of these soft-tissue relationships.

Trauma Applications

Occult or Equivocal Fractures

CT is extremely useful when a fracture is suspected clinically but plain radiographs are equivocal or nondiagnostic.

It can reveal subtle fracture lines that are difficult to appreciate on conventional imaging.

Axially Oriented Fractures

Multiplanar and three-dimensional reconstructions are particularly useful for fractures that are oriented in a plane poorly visualized on plain radiographs.

Fracture Extent

CT can accurately determine the extent, displacement, comminution, articular involvement, and physeal involvement of a fracture.

This information is frequently essential for operative planning.

Intra-Articular Fragments

CT can identify small fracture fragments within a joint and define areas of articular depression or step-off.

Nonunion

CT is useful for evaluating suspected fracture nonunion, particularly when overlapping anatomy or implanted hardware makes plain radiographs difficult to interpret.

It can demonstrate persistent fracture gaps and the amount of osseous bridging.

Cervical Spine Trauma

CT is a principal imaging modality for evaluating the cervical spine in moderate- and high-risk trauma patients.

It provides rapid assessment of fractures, alignment, facet injuries, and other bony abnormalities.

Complex Skeletal Regions

CT is particularly useful in trauma involving the pelvis, scapula, wrist, ankle, and spine, where overlapping structures can limit the accuracy of radiographs.

Infection Applications

Postoperative Infection

In postoperative patients, CT may help evaluate new symptoms and identify structural complications after treatment.

It can demonstrate collections, gas, cortical destruction, or other changes suggesting infection.

Extent of Infection

CT can help determine whether infection involves bone, muscle, fascia, or subcutaneous tissue.

Defining the compartments involved may assist in deciding whether medical management alone is adequate or surgical intervention is required.

Response to Treatment

Serial imaging may occasionally be useful for monitoring structural changes during treatment of musculoskeletal infection, although clinical findings and laboratory markers remain important.

Pediatric Applications

Skeletal Dysplasias

CT can be useful in skeletal dysplasias when a large and anatomically complex region must be defined accurately.

It may also be valuable after reconstructive surgery.

Developmental Dysplasia of the Hip

Developmental dysplasia of the hip is usually diagnosed using physical examination, ultrasound, and plain radiographs.

CT may be used in difficult cases or with carefully optimized low-dose protocols.

Evaluation After Hip Reduction

CT has historically been useful for confirming successful hip reduction after placement of a spica cast.

Low-dose imaging protocols are important in these young patients.

Slipped Capital Femoral Epiphysis

CT can demonstrate the orientation of the proximal femoral physis and may help identify contralateral abnormalities with coronal and sagittal reconstructions.

However, routine diagnosis of slipped capital femoral epiphysis is usually based on radiographs, with MRI used in selected early or occult cases.

Alternative Causes of Pediatric Hip Pain

Cross-sectional imaging can also help identify alternative diagnoses such as osteoid osteoma when the source of hip pain is uncertain.

Legg–Calvé–Perthes Disease

CT may occasionally be used for preoperative definition of femoral head deformity and disease severity.

Because of radiation exposure, its use is selective.

Pectus Deformity

CT can define thoracic anatomy in patients with pectus excavatum or other chest-wall deformities.

It may be particularly useful for surgical planning or after unsuccessful previous repair.

Tarsal Coalition

CT is highly effective for characterizing osseous tarsal coalitions.

Multiplanar reconstructions accurately demonstrate the location, orientation, and extent of the coalition and may assist in operative planning.

Pediatric Considerations

Radiation Sensitivity

Children are more sensitive to ionizing radiation than adults.

Because they also have a longer remaining lifespan, the potential lifetime risk of radiation-induced malignancy is greater.

Dose Optimization

CT parameters should therefore be adjusted according to the child’s size, age, and clinical indication.

Only examinations that are likely to provide meaningful clinical information should be performed.

Avoid Multiphase Studies

Unnecessary multiphase CT examinations should be avoided in children.

Obtaining both noncontrast and postcontrast scans without a specific indication can substantially increase radiation dose.

Multidetector CT

Modern multidetector technology allows a single volumetric acquisition to be reconstructed in multiple planes, reducing the need for repeated scanning.

Pregnancy Considerations

Limiting Scan Volume

When CT is necessary during pregnancy, the scan should be restricted to the smallest anatomical region required to answer the clinical question.

Unnecessary multiphase imaging should be avoided.

Radiation Optimization

Protocols should use the lowest radiation exposure that still produces diagnostically adequate images.

Departments should maintain quality-assurance programs and appropriate dose protocols.

Alternative Imaging

When clinically appropriate, ultrasound or MRI should be considered because these techniques do not use ionizing radiation.

However, a necessary CT examination should not be withheld when the expected diagnostic benefit outweighs the potential radiation risk.

Iodinated Contrast in Pregnancy

Iodinated contrast crosses the placenta and should therefore be administered only when it is expected to provide important additional diagnostic information.

No consistent teratogenic effect has been established from routine diagnostic exposure, but use should remain clinically justified.

Abdominal and Pelvic Protection

Radiation exposure outside the imaged region should be minimized through appropriate collimation and modern dose-reduction techniques.

Routine external lead shielding is no longer universally recommended because contemporary CT systems rely primarily on optimized scan parameters and shielding may occasionally interfere with automatic exposure control.

Breastfeeding After Contrast

Modern recommendations generally do not require interruption of breastfeeding after routine iodinated intravenous contrast administration, because only very small amounts reach breast milk and an even smaller amount is absorbed by the infant.

Follow-Up and Complications

Contrast Reactions

Iodinated contrast can produce hypersensitivity reactions ranging from mild symptoms to rare severe reactions.

Patients with a previous contrast reaction have the greatest risk of another reaction.

Asthma may also increase susceptibility.

Premedication

Selected high-risk patients may undergo premedication using corticosteroids and antihistamines according to institutional protocols.

Premedication reduces but does not completely eliminate the possibility of a reaction.

Renal Adverse Effects

Patients with significant pre-existing renal disease are at greatest risk for renal complications related to iodinated contrast.

Other relevant factors may include dehydration, diabetes, severe systemic illness, and concurrent nephrotoxic medications.

Renal Protection

When contrast is necessary in a patient at increased renal risk, appropriate hydration, avoidance of unnecessary nephrotoxic exposures, and use of the minimum effective contrast dose may reduce risk.

Modern low- or iso-osmolar contrast agents are typically used.

Overall Role in Orthopaedics

CT is an especially valuable orthopaedic imaging modality when detailed evaluation of cortical bone, fracture anatomy, joint surfaces, complex skeletal regions, calcification, reconstruction, or implanted hardware is required.

Its major limitations are ionizing radiation and relatively inferior soft-tissue contrast compared with MRI, so the imaging modality should be selected according to the specific clinical question.