11.3 Extremity CT — Upper & Lower Extremity, Postarthrogram

Why This Topic Matters

Upper extremity, lower extremity, and postarthrogram are three of the six Musculoskeletal leaf items in the Procedures outline. These studies test a core exam theme that runs throughout Image Production and Procedures: CT is a problem-solving tool that follows radiography, not a replacement for it, and protocol choices (kernel, slice thickness, field of view) change based on whether the goal is fracture detail, surgical planning, or joint-specific evaluation.

Core Terms and Rules

• CT is not first-line for simple extremity trauma; plain radiography remains the initial study. CT is reserved for complex intra-articular fractures, presurgical planning, occult fractures with negative or equivocal radiographs but high clinical suspicion, and postoperative hardware assessment.
• Upper extremity CT (elbow, forearm, wrist, hand) commonly evaluates comminuted or intra-articular distal radius fractures, complex elbow fractures, and carpal injuries such as a clinically suspected scaphoid fracture with initially negative radiographs — a classic "occult fracture" scenario.
• Lower extremity CT (knee, tibia/fibula, ankle, foot) commonly evaluates tibial plateau fractures, calcaneal fractures, and complex ankle/pilon fractures, where fracture-fragment orientation determines the surgical approach.
• Postarthrogram CT (CT arthrography): imaging performed after an intra-articular contrast injection — a diluted iodinated agent, sometimes combined with air for a double-contrast technique, injected directly into the joint capsule under fluoroscopic or CT guidance, followed by thin-section CT. It evaluates articular cartilage defects, labral tears, ligament integrity, and intra-articular loose bodies, serving as the CT alternative to MR arthrography when MRI is contraindicated.
• Protocol: extremity CT uses thin, often submillimeter, acquisition (for example, roughly 0.625 mm) reconstructed with both a sharp bone algorithm and a soft-tissue algorithm from the same raw data, plus 3D volume-rendered reconstructions frequently requested for surgical planning of complex fractures.
• Field of view: a small, tailored field of view around the extremity of interest — not the whole patient — keeps pixel size small and spatial resolution high, since fine trabecular and cortical bone detail is the clinical priority.
• Metal artifact reduction (MAR) software is frequently applied when existing hardware (plates, screws, prior arthroplasty components) would otherwise obscure adjacent bone and soft tissue on standard reconstruction.

Table: Occult vs. Surgical-Planning Extremity CT

Pediatric and Positioning Considerations

Extremity CT protocols must also flex for patient age. In a child or adolescent with an open growth plate, an extremity fracture through the physis is classified using the Salter-Harris system, and CT is occasionally used — beyond the initial radiograph — when a complex intra-articular growth-plate fracture (a Salter-Harris III or IV pattern, where the fracture line crosses the joint surface) needs 3D characterization for surgical planning, since these patterns carry a risk of growth arrest if not anatomically reduced. Positioning also matters technically: for an upper extremity study, raising the affected arm overhead ("superman" positioning) when the patient can tolerate it keeps the limb isolated near the center of the gantry and out of the denser trunk soft tissue, improving both image quality and consistency of the small field of view; when overhead positioning is not possible (a suspected shoulder or humeral fracture, for example), the limb is imaged at the patient's side with technique adjusted accordingly.

Exam Scenario

A 19-year-old fell onto an outstretched hand and has point tenderness in the anatomic snuffbox, but the initial wrist radiograph is negative. Because a missed scaphoid fracture risks avascular necrosis and nonunion, the ordering provider requests a CT of the wrist. The technologist should plan a thin-section acquisition with a small, dedicated field of view centered on the wrist, reconstruct both bone- and soft-tissue-algorithm image sets, and provide multiplanar reformats — this combination of protocol choices is what allows a fracture line invisible on radiography to become visible on CT.

A second scenario: a 14-year-old soccer player has a suspected tibial eminence avulsion with the knee radiograph showing only subtle irregularity at the growth plate. Because operative decision-making depends on fragment displacement, the ordering surgeon requests a dedicated knee CT with 3D volume-rendered reconstructions rather than relying on the radiograph alone — the same surgical-planning logic used throughout this section, applied to a pediatric growth-plate injury instead of an adult intra-articular fracture.

Common Traps

• Assuming CT is the default first study for extremity trauma — it is a secondary, problem-solving study in the great majority of cases.
• Confusing postarthrogram (intra-articular contrast injection into a joint capsule) with postmyelogram (intrathecal contrast injection into the spinal subarachnoid space, covered in the previous section) — both are direct-injection studies, but into anatomically and procedurally distinct spaces.
• Using a full-body or oversized field of view for an extremity study, which coarsens pixel size and degrades the fine bone/cartilage detail that is the entire point of the exam.
• Forgetting to request or generate 3D volume-rendered reconstructions when the clinical indication is explicitly surgical planning.