2. Diagnostic imaging modalities for hip femur and knee injuries

Selecting the right imaging study is a clinical decision, not a routine order. In hip, femur, and knee trauma, the choice among radiography, computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound determines whether a fracture, ligament tear, or occult injury is detected early — or missed entirely. This chapter explains what each modality actually shows, when it is the first-line choice, and where its blind spots lie.

Radiography: The Mandatory First Step

Plain radiography remains the initial study for virtually every suspected skeletal injury of the hip, femur, and knee. It is fast, inexpensive, widely available, and excellent for cortical bone. The standard series for a suspected hip or proximal femur fracture is an anteroposterior (AP) pelvis view plus a dedicated AP and lateral view of the affected hip. For the knee, an AP and lateral view — with a merchant or sunrise view when patellofemoral pathology is suspected — is standard.

Two principles govern correct radiographic technique. First, obtain at least two orthogonal projections; a single view can hide a displaced fracture entirely. Second, include the joint above and below the injury: a femoral shaft fracture requires imaging of the ipsilateral hip and knee, because a distal femoral fracture may be accompanied by an occult femoral neck fracture, and a knee injury may coexist with a hip injury in high-energy trauma.

Radiography's limitations are equally important. It cannot directly visualize ligaments, menisci, cartilage, or bone marrow. Stress fractures and nondisplaced femoral neck fractures may be radiographically occult for 10–14 days. In the hip, an apparently normal radiograph in an elderly patient who cannot bear weight should be treated as a suspected occult fracture, not as a cleared patient.

Computed Tomography: Bone Detail and Preoperative Planning

CT provides cross-sectional bone detail with high spatial resolution and is the modality of choice when radiography is equivocal or when fracture geometry must be defined. Key indications include: suspected occult femoral neck fracture with negative radiographs, acetabular and pelvic ring fractures, intra-articular fracture fragments, tibial plateau fractures, and preoperative planning for fixation. CT also detects loose bodies and subtle cortical step-offs that radiography misses.

A practical example illustrates the workflow. A 72-year-old patient falls, reports groin pain, and cannot bear weight; the AP pelvis radiograph shows no clear fracture line. A CT of the hip with thin-slice (≤1 mm) axial images and coronal/sagittal reformats is performed. The radiologist reviews the femoral neck on coronal reformats for a cortical break or sclerotic impaction line. If CT is also negative but clinical suspicion remains high, MRI is the next step — CT sensitivity for truly occult femoral neck fractures is high but not absolute.

CT's limitations are radiation dose and poor soft-tissue contrast. It cannot reliably evaluate ligaments, menisci, or marrow edema. It should not be used as a screening tool in young patients when radiography suffices, and it is generally avoided in pregnancy unless the indication is compelling.

MRI: Soft Tissue, Marrow, and Occult Fractures

MRI is the most sensitive modality for bone marrow edema, occult fractures, ligament and meniscal tears, and cartilage injury. Its principal indications in this region are: suspected occult hip fracture after negative radiography and CT, stress fractures, avascular necrosis of the femoral head, meniscal and cruciate ligament tears, and evaluation of the knee after trauma with a stable but persistently symptomatic joint.

Standard protocols use fluid-sensitive sequences (STIR or T2 fat-suppressed) to detect marrow edema and proton-density or T1 sequences for anatomy. A hip MRI for occult fracture typically includes coronal STIR and T1 images; marrow edema on STIR with a corresponding low-signal line on T1 confirms fracture. For the knee, a standard protocol includes sagittal and coronal proton-density fat-suppressed images for menisci and ligaments.

MRI limitations include cost, limited availability, longer acquisition time, and contraindications such as certain implanted devices. It is poorly tolerated in claustrophobic or unstable patients, and motion artifact can degrade image quality. It is also less useful for acute cortical bone detail than CT.

Ultrasound: Targeted Soft-Tissue and Procedural Guidance

Ultrasound is operator-dependent and limited in deep structures, but it has specific, valuable roles. It can detect joint effusion, evaluate the superficial soft tissues, assess tendons and ligaments around the knee (patellar tendon, quadriceps tendon, collateral ligaments), and guide aspiration or injection. In the hip, it can identify an effusion in the pediatric patient and guide diagnostic aspiration. It is also useful for detecting foreign bodies and superficial hematoma.

Ultrasound cannot penetrate cortical bone, so it cannot exclude fracture. It should never be used as a standalone study to rule out skeletal injury. Its role is complementary and targeted.

Choosing the Right Modality: A Decision Framework

The following table summarizes the practical decision logic. The guiding principle is to start with radiography, escalate to CT when bone detail or fracture geometry is needed, and escalate to MRI when soft tissue, marrow, or occult injury is suspected.

The pathway above is not a rigid algorithm but a decision aid. In practice, the choice is modulated by patient stability, pregnancy status, implanted devices, and local availability. What matters is that the clinician understands what each modality can and cannot answer, and escalates deliberately rather than reflexively. A radiograph that is negative in a patient who cannot bear weight is a prompt to continue the workup, not to discharge the patient.