High Yield Overview
Weight-Bearing CT: Principles & Applications
—Primary Applications
—Foot, ankle, knee
—Key Advantage
—Functional loading
—Syndesmosis Assessment
—Superior to non-WB
—Scan Time
—Similar to conventional CT
WBCT Applications by Region
Ankle: Syndesmosis, OCD, impingement
Hindfoot: Alignment, coalition assessment
Midfoot: Lisfranc stability, arthritis
Forefoot: Hallux valgus, metatarsalgia
Key: WBCT provides functional assessment impossible with supine imaging
Critical Must-Knows
- WBCT captures anatomy under physiological load
- Superior for syndesmosis assessment vs non-weight-bearing CT
- Hindfoot alignment measurement is more accurate than radiographs
- Reveals instability patterns not visible on non-WB imaging
- Emerging as standard for foot and ankle assessment
Examiner's Pearls
- "Syndesmosis: WBCT detects subtle widening missed on conventional imaging
- "Hindfoot alignment: Standing radiograph correlation inferior to WBCT
- "Lisfranc: WBCT shows instability under load
- "Flatfoot: Accurate 3D assessment of deformity components
Exam Warning
Weight-bearing CT is an emerging technology increasingly featured in exams. Understand its key advantage (physiological loading) and primary applications in syndesmosis assessment and hindfoot alignment. Know why it reveals pathology missed on non-weight-bearing imaging.
Mnemonic
WEIGHT Advantages
W
Weight-bearing - Physiological loading reveals true alignment
Weight-bearing - Physiological loading reveals true alignment
E
Exact position - Functional stance not achievable supine
Exact position - Functional stance not achievable supine
I
Instability detected - Dynamic instability visible under load
Instability detected - Dynamic instability visible under load
G
Guidance for surgery - Changes operative planning 25-35%
Guidance for surgery - Changes operative planning 25-35%
H
Hindfoot alignment - Best modality for complex deformity
Hindfoot alignment - Best modality for complex deformity
T
True functional anatomy - Mimics real-world conditions
True functional anatomy - Mimics real-world conditions
Memory Hook:WEIGHT-bearing CT shows true functional anatomy
Mnemonic
LOAD Indications
L
Ligament assessment - Syndesmosis and collateral integrity
Ligament assessment - Syndesmosis and collateral integrity
O
Osteotomy planning - Hindfoot alignment measurements
Osteotomy planning - Hindfoot alignment measurements
A
Arthritis evaluation - Joint space narrowing under load
Arthritis evaluation - Joint space narrowing under load
D
Deformity analysis - Flatfoot and cavus assessment
Deformity analysis - Flatfoot and cavus assessment
Memory Hook:LOAD bearing reveals pathology hidden on conventional imaging
Principles
Weight-Bearing vs Conventional CT
| Feature | Conventional CT | Weight-Bearing CT |
|---|---|---|
| Patient position | Supine | Standing (full or partial weight) |
| Loading | Non-physiological | Physiological load |
| Joint positioning | Neutral/relaxed | Functional position |
| Alignment assessment | May underestimate deformity | True weight-bearing alignment |
| Instability detection | May miss subtle instability | Reveals dynamic instability |
| Radiation dose | Standard | Comparable to conventional |
Why Weight-Bearing Matters
Many pathologies only manifest under load. A syndesmosis may appear normal on supine CT but widen under weight-bearing. Hindfoot valgus measured supine underestimates the standing deformity. Lisfranc instability may only be apparent when the foot bears weight. WBCT captures the anatomy as it functions in real life.
Technology
WBCT Scanner Types
| Type | Description | Applications |
|---|---|---|
| Cone-beam CT | Patient stands in scanner, foot/ankle in field | Foot, ankle, knee |
| Pedestal systems | Single limb support, bilateral comparison | Ankle syndesmosis, hindfoot |
| Full-body WBCT | Whole body under load | Spine, lower limb alignment |
| Upright CT | Rotating gantry with standing patient | Multi-region assessment |
Clinical Applications
WBCT for Syndesmosis Assessment
| Measurement | Normal Values | Clinical Significance |
|---|---|---|
| Tibiofibular clear space | Less than 4mm at 1cm above plafond | Widening indicates instability |
| Tibiofibular overlap | Greater than 1mm on AP | Loss indicates diastasis |
| Anterior-posterior translation | Minimal asymmetry | Anterior fibular translation |
| Fibular rotation | Symmetric to contralateral | External rotation in injury |
WBCT Advantage in Syndesmosis
Syndesmotic instability may only manifest under load. Studies show WBCT detects syndesmotic widening missed on non-weight-bearing CT and radiographs. The 3D assessment also reveals rotational malreduction not visible on plain films. WBCT is increasingly used for intraoperative assessment of syndesmotic reduction.
Interpretation
Systematic WBCT Review
| Step | Assessment | Key Findings |
|---|---|---|
| 1. Alignment | Global and segmental alignment | Malalignment under load |
| 2. Joint spaces | Cartilage space preservation | Asymmetric loading, early OA |
| 3. Syndesmosis | Clear space, overlap, rotation | Instability, malreduction |
| 4. Bone | Fractures, stress reaction, cysts | Occult fractures, OCD |
| 5. Comparison | Bilateral symmetry | Contralateral reference |
3D Reconstruction Benefits
WBCT with 3D reconstruction allows visualization of complex deformities from any angle. For surgical planning, surgeons can rotate the image to understand the exact deformity components. Distance-mapping software can quantify bone-to-bone distances and identify impingement zones.
Limitations
WBCT Limitations
| Limitation | Explanation | Workaround |
|---|---|---|
| Soft tissue contrast | Cone-beam has reduced soft tissue detail | Use MRI for soft tissue assessment |
| Motion artefact | Standing patient may move | Short acquisition, patient support |
| Availability | Limited to specialised centres | Referral to appropriate facility |
| Non-ambulatory patients | Cannot stand for scan | Conventional CT alternative |
| Upper limb | Weight-bearing not applicable | Conventional CT or MRI |
When to Use Conventional CT
WBCT does not replace conventional CT for all indications. Acute trauma with inability to weight-bear, soft tissue tumour assessment, and detailed bone tumour characterization still require conventional multi-detector CT (or MRI for soft tissue).
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"A patient has an ankle injury with normal radiographs and non-weight-bearing CT, but persistent syndesmotic tenderness at 6 weeks."
EXCEPTIONAL ANSWER
I would consider weight-bearing CT (WBCT) of both ankles. The clinical scenario suggests possible syndesmotic instability that is not apparent on static imaging. WBCT advantage: Syndesmotic injuries may only manifest under physiological load. A syndesmosis that appears reduced on supine imaging may widen with weight-bearing, revealing dynamic instability. Assessment includes: (1) Tibiofibular clear space comparison bilaterally. (2) Fibular position - anterior translation or external rotation. (3) Tibiofibular overlap. WBCT is more sensitive than non-weight-bearing CT and radiographs for detecting subtle syndesmotic widening. If WBCT confirms instability, syndesmotic stabilisation may be indicated.
KEY POINTS TO SCORE
WBCT reveals dynamic instability missed on supine imaging
Syndesmosis may only widen under load
Bilateral comparison essential
Assesses clear space, overlap, and fibular position
Can guide decision for syndesmotic stabilisation
COMMON TRAPS
✗Accepting normal non-WB CT as excluding instability
✗Not examining contralateral for comparison
✗Missing rotational malposition
VIVA SCENARIOStandard
EXAMINER
"You are planning surgery for a patient with adult-acquired flatfoot deformity. How would WBCT assist your surgical planning?"
EXCEPTIONAL ANSWER
WBCT provides comprehensive 3D assessment of flatfoot deformity under physiological load. Specific information: (1) Hindfoot alignment - precise measurement of hindfoot valgus angle and moment arm, more accurate than Saltzman view radiograph. (2) Subtalar joint - degree of subluxation, peritalar coverage. (3) Talonavicular joint - uncoverage percentage, sag angle. (4) First ray position - elevation, metatarsus primus elevatus. (5) Forefoot abduction - talonavicular and calcaneocuboid alignment. (6) Joint arthrosis - all three hindfoot joints assessed for arthritis. (7) Spring ligament - bone relationships indicating incompetence. This information guides surgical decision-making: whether to perform medializing calcaneal osteotomy, lateral column lengthening, Cotton osteotomy, or arthrodesis procedures.
KEY POINTS TO SCORE
True weight-bearing hindfoot alignment measurement
3D assessment of all deformity components
Multiple joint involvement assessed simultaneously
Guides osteotomy vs arthrodesis decisions
More accurate than radiographic assessment
COMMON TRAPS
✗Relying only on radiographs for surgical planning
✗Missing associated joint arthrosis
✗Not assessing first ray position
VIVA SCENARIOStandard
EXAMINER
"A patient with suspected Lisfranc injury has radiographs showing subtle widening at the first-second intermetatarsal space that is equivocal."
EXCEPTIONAL ANSWER
WBCT is particularly valuable for equivocal Lisfranc injuries. The key advantage is demonstrating instability under physiological load. Assessment: (1) First-second intermetatarsal space - widening greater than 2mm suggests Lisfranc ligament injury. (2) Cuneiform-metatarsal alignment - subtle dorsal subluxation of metatarsal bases. (3) Comparison to contralateral foot - bilateral WBCT for symmetry assessment. (4) Fleck fracture - small avulsion at Lisfranc ligament attachment site. Under weight-bearing, a subluxing Lisfranc joint will demonstrate increased widening or dorsal translation not seen on non-weight-bearing images. This distinguishes stable injuries (conservative treatment) from unstable injuries (surgical stabilisation required). If the patient cannot bear weight due to pain, stress radiographs or MRI may be alternatives.
KEY POINTS TO SCORE
WBCT demonstrates instability under load
Greater than 2mm widening at 1st-2nd space abnormal
Bilateral comparison for subtle cases
Distinguishes stable from unstable injuries
Guides surgical vs conservative management
COMMON TRAPS
✗Missing subtle instability on supine imaging
✗Not comparing to contralateral foot
✗Discharging with 'normal' non-WB imaging despite symptoms
Evidence Base
Diagnostic Accuracy
1
2
Clinical Applications
3
4
Key Evidence Points
- Standard CT limitation: Patients supine, ligaments unloaded, alignment may appear normal
- WBCT advantage: Captures physiologic alignment under actual loading conditions
- Clinical impact: Identifies instability and deformity not visible on conventional imaging
- Australian availability: Limited centres currently, expanding gradually
Weight-Bearing CT Quick Reference
High-Yield Exam Summary
Key Advantages
- •Physiological load during imaging
- •Reveals dynamic instability
- •True weight-bearing alignment
- •3D deformity assessment
Primary Applications
- •Syndesmosis assessment
- •Hindfoot alignment (replaces Saltzman)
- •Lisfranc instability
- •Flatfoot surgical planning
Syndesmosis Assessment
- •Clear space, overlap, rotation
- •Bilateral comparison essential
- •More sensitive than non-WB CT
- •Detects dynamic widening
Limitations
- •Reduced soft tissue contrast
- •Requires patient to stand
- •Limited availability
- •Motion artefact possible