8.2 Diagnostic Imaging & Clinical Decision Rules
Key Takeaways
- Ottawa Ankle/Foot, Ottawa Knee, Pittsburgh Knee, Canadian C-spine Rule, and NEXUS are validated rule-out tools — apply the published items, then refer; swelling alone is not a reason to skip them
- Pittsburgh Knee requires blunt trauma or a fall, then age under 12 or over 50, or inability to walk four weight-bearing steps in the evaluation setting
- Canadian C-spine Rule uses high-risk factors, then low-risk factors that allow safe ROM, then 45° left and right rotation; NEXUS clears only if all five low-risk criteria are present
- X-ray is first-line bone, MRI is preferred for bone-stress and most soft tissue, CT defines complex bone and many C-spine bony injuries, ultrasound is dynamic soft tissue, and bone scan shows metabolic activity but is not first-line today
- Athletic trainers recommend imaging and refer; independently 'ordering MRI' is not within scope in every state and is never a substitute for a decision rule
PA8 task 0202 includes selecting and using tests and measures and knowing when the next “test” is imaging. Athletic trainers apply clinical decision rules (CDRs) and recommend radiographs or advanced studies; they refer into a physician-directed pathway. In many states the AT does not independently order MRI. Know your practice act. The exam trap is skipping a validated rule because “it looks swollen,” or treating MRI as a first-line toy.
CDRs are sensitive rule-out instruments. If the rule is negative, clinically important fracture (or C-spine injury, depending on the rule) is very unlikely and imaging can be deferred. If the rule is positive, recommend the indicated study. Specificity is modest — positives are not a diagnosis of fracture; they are a reason to image.
Ottawa Ankle and Foot Rules (Reapplied as an Imaging Rule)
Use them after acute ankle or midfoot trauma in an alert patient who can be examined.
Ankle series if malleolar-zone pain plus any of: bone tenderness of the distal 6 cm of the posterior tibia or tip of the medial malleolus; bone tenderness of the distal 6 cm of the posterior fibula or tip of the lateral malleolus; or inability to take four steps both immediately and in the exam area.
Foot series if midfoot-zone pain plus any of: bone tenderness of the navicular; bone tenderness of the base of the fifth metatarsal; or the same four-step inability immediately and now.
Palpate posterior bone, not the anterior puffy gutter. Four steps may be limped — transferring weight twice onto each limb counts. Exam trap: skipping Ottawa because the ankle is swollen. Swelling is why the rule exists; most swollen ankles are not fractured.
Ottawa Knee Rule
After acute knee trauma, a knee radiographic series is indicated if any one of the following is present (Stiell):
| Item | Threshold |
|---|---|
| Age | 55 years or older |
| Patella | Isolated tenderness of the patella (no other bony tenderness of the knee) |
| Fibula | Tenderness of the head of the fibula |
| Flexion | Inability to flex to 90° |
| Weight-bearing | Inability to bear weight both immediately and in the emergency / exam setting for four steps (limping is allowed if four steps are completed) |
Isolated patellar tenderness counts only if it is isolated. The original derivation targeted injuries within about 7 days, alert cooperative adults, and not multiple trauma, altered consciousness, or pregnancy as a reliable exam. Pediatric use has been studied, but many teaching sources still flag under-18 as a caution — if the exam is unreliable, image or refer rather than force the rule.
Pittsburgh Knee Rule
Pittsburgh is a different, simpler rule. It applies when the mechanism is blunt trauma or a fall. If that mechanism is absent, Pittsburgh does not recommend a knee series by itself. If blunt trauma or a fall is present, radiographs are recommended if any of:
- Age < 12 years, or
- Age > 50 years, or
- Inability to walk four weight-bearing steps in the evaluation setting (limping allowed).
Pittsburgh can be used in children (the <12 criterion is built in). Ottawa Knee is often more familiar to ATs; Pittsburgh is the item that tests whether you remember mechanism first. Neither rule replaces a grossly deformed, locked, or neurovascularly dirty knee — those get immobilized and referred regardless.
Canadian C-Spine Rule Versus NEXUS
Cervical imaging rules apply to alert, stable blunt-trauma patients in whom C-spine injury is a concern. Unstable vitals, a dirty airway, or a cord syndrome are Domain III emergencies, not a rule-application station. Do not special-test an unstable neck (that content lives with Spurling/stinger clusters in the cervical chapter); do know which rule items mandate imaging.
Canadian C-Spine Rule (Stiell) — three steps
1. Any high-risk factor that mandates radiography? If yes → image.
| High-risk factor | Published content |
|---|---|
| Age | ≥ 65 years |
| Dangerous mechanism | Fall from elevation ≥ 3 feet (0.9 m) or 5 stairs; axial load to the head (for example diving); MVC high speed (> 100 km/h), rollover, or ejection; motorized recreational vehicle; bicycle struck or collision |
| Neurologic | Paresthesias in the extremities |
2. Any low-risk factor that allows safe assessment of range of motion? If none → image. If at least one is present, you may test rotation.
| Low-risk factor | Notes |
|---|---|
| Simple rear-end MVC | Excludes being pushed into oncoming traffic, hit by a bus or large truck, rollover, or hit by a high-speed vehicle |
| Sitting position in the emergency department (or equivalent exam setting) | — |
| Ambulatory at any time | — |
| Delayed onset of neck pain | Not immediate onset |
| Absence of midline cervical-spine tenderness | — |
3. Able to actively rotate the neck 45° left and 45° right? Able (even with pain) → no radiography by the rule. Unable → image.
The rule is not applicable to non-trauma, GCS < 15, unstable vital signs, age < 16, acute paralysis, known vertebral disease, previous C-spine surgery, or pregnancy — immobilize/refer instead of “clearing” by memory.
NEXUS (Hoffman) — all five required to clear
Radiography is not necessary if the patient meets ALL of:
- No posterior midline cervical tenderness
- No evidence of intoxication
- Normal level of alertness (alert, oriented)
- No focal neurologic deficit
- No painful distracting injury (for example a long-bone fracture that competes with the neck exam)
If any NEXUS item is present, the patient is not NEXUS-cleared. NEXUS does not use age ≥65 or a dangerous-mechanism list; Canadian C-spine does. Elderly patients and axial-load football/diving mechanisms are why many clinicians prefer Canadian C-spine or image rather than NEXUS-clear a 70-year-old. A patient with midline tenderness fails NEXUS but might still be Canadian-cleared if there are no high-risk factors, a low-risk factor is present, and 45° rotation is possible — that comparison is a classic item.
Salter-Harris in the Skeletally Immature
Ligaments are often stronger than physes. After trauma in an open-growth-plate athlete, point tenderness over a physis is a Salter-Harris fracture until proven otherwise, even with a “normal” first radiograph.
| Type | Anatomy (mnemonic: SALTER relative to the physis) | Clinical note |
|---|---|---|
| I | Straight through the physis (slipped) | May be radiographically occult; treat tenderness + mechanism |
| II | Above: physis + metaphysis (Thurston-Holland fragment) | Most common |
| III | Lower: physis + epiphysis (intra-articular) | Joint involvement; often surgical discussion |
| IV | TE through: metaphysis + physis + epiphysis | Intra-articular; alignment of the physis matters |
| V | Rammed: crush of the physis | Often missed initially; highest growth-disturbance concern |
Do not reduce a suspected physeal injury repeatedly on the field. Immobilize, NWB or protected as indicated, and refer. Ottawa-type rules are less comfortable in young children; when in doubt, image or refer.
Choosing the Modality
| Modality | Best use in AT practice | Poor first choice for |
|---|---|---|
| Radiograph (x-ray) | Cortical fracture, alignment, Ottawa/Pittsburgh/C-spine screening films | Early bone-stress, most ligaments, cord |
| MRI | Bone-stress (marrow edema ± fracture line), ligaments, cartilage, occult fracture, spinal cord / disc | Immediate sideline bony alignment; some patients with unsafe implants |
| CT | Complex intra-articular and midfoot bone detail, many C-spine bony injuries when x-ray is inadequate | Soft-tissue and cord (MRI is preferred for cord) |
| Ultrasound | Superficial tendons (Achilles, posterior tibialis), dynamic ligament, some foreign body | Deep bone-stress, intra-articular cartilage, C-spine |
| Bone scan | Metabolic bone turnover (historical stress-fracture tool) | Specific anatomy; largely replaced by MRI |
C-spine: plain films or, in many emergency systems, CT for bone; MRI if cord, disc, or ligamentous instability is the question after bony imaging. Contrast: gadolinium MRI or iodinated CT contrast is physician-directed (infection, tumor, selected MR arthrograms, vascular studies) — not an AT add-on. Pregnancy: avoid ionizing radiation when a non-ionizing option answers the question; ultrasound or MRI without gadolinium is preferred when imaging cannot wait. Metal: screen every MRI candidate (implants, fragments, some stimulators). CT is usually metal-safe but produces artifact. Document the screen.
Recommend Versus Order Versus Refer
The AT’s job is to apply the rule, protect the tissue, and communicate a clinical impression: “Ottawa-positive posterior fibula; ankle series indicated.” The physician (or state-authorized ordering clinician) orders. Some jurisdictions have expanded AT imaging orders; many have not. Independently “ordering MRI for every sprain” is both a scope error and a sequence error — radiographs and CDRs come first for trauma.
Exam trap: skipping Ottawa because the joint is swollen, or jumping to MRI as a personality preference. Swelling is not a criterion. The rule items are.
Which statement correctly lists Canadian C-spine Rule high-risk factors that mandate imaging in an alert, stable blunt-trauma patient?
A 16-year-old falls onto the knee (blunt trauma). There is no isolated patellar tenderness and the knee flexes past 90°, but the athlete cannot complete four weight-bearing steps in the athletic training room. How do the knee decision rules apply?
An athlete inverts the ankle. It is markedly swollen. Ottawa exam finds bone tenderness along the distal 6 cm of the posterior fibula; the athlete did take four steps immediately and again now. What should the athletic trainer do?