Normal Ankle Radiograph Anatomy
A normal ankle X-ray consists of three standard views (anteroposterior, lateral, and mortise) demonstrating symmetric joint spaces, intact cortical margins, proper alignment of the tibiotalar joint, and specific anatomic relationships between the tibia and fibula. 1
Standard Radiographic Views
The typical ankle radiographic protocol includes three views 1:
- Anteroposterior (AP) view: Shows the tibiotalar joint, medial and lateral malleoli, and distal fibula
- Lateral view: Demonstrates the anterior and posterior tibial margins, talus, and tibiofibular relationship in the sagittal plane
- Mortise view: Obtained with 15-20 degrees internal rotation of the foot, providing optimal visualization of the ankle mortise and joint space symmetry 1
The base of the fifth metatarsal should be included distally on all views to avoid missing fractures in this location 1.
Key Anatomic Measurements and Relationships
Medial Clear Space
- Normal measurement: <4 mm on the mortise view 1, 2
- This represents the distance between the medial border of the talus and the lateral border of the medial malleolus
- Widening beyond 4 mm indicates ankle instability and potential deltoid ligament injury 1, 2
Tibiofibular Relationship on Lateral View
The lateral view provides critical assessment of the syndesmosis 3, 4:
- Anterior Fibular Line (AFL) ratio: The anterior cortex of the fibula should lie just anterior to the midpoint of the tibia when measured from anterior to posterior tibial margins 3
- Posterior Fibular Line (PFL): Should intersect the posterior tibial articular margin or lie just anterior to it, never posterior 3
- APTF ratio: The anteroposterior tibiofibular ratio should be approximately 0.94 ± 0.13, measured from the anterior tibial physis scar to where the fibula intersects this line 4
Joint Space Symmetry
On the mortise view, the superior joint space between the talus and tibial plafond should be uniform 1:
- Medial, superior, and lateral joint spaces should appear symmetric
- Any asymmetry suggests talar shift or ligamentous injury
Cortical Integrity
All visible cortical margins should be smooth and continuous 1:
- Medial malleolus
- Lateral malleolus
- Tibial plafond
- Talar dome
- Posterior malleolus (visible on lateral view)
Common Pitfalls in Interpretation
Positioning errors can simulate pathology 5:
- Inadequate internal rotation on the mortise view may create apparent joint space asymmetry
- Oblique positioning can alter the appearance of the tibiofibular relationship
- Minor differences in positioning can greatly alter the appearance of normal anatomic relationships 1
Subtle findings requiring attention 1:
- Small avulsion fractures at ligament attachment sites may appear as small ossific fragments
- Soft tissue swelling anterior to the ankle joint may indicate effusion (53-74% accuracy on radiographs) 1
- Periosteal reaction adjacent to tendons may indicate chronic tenosynovitis 1
Clinical Context for Radiographic Assessment
Weight-bearing views provide additional functional information when clinically appropriate 1, 2:
- Demonstrate dynamic stability of the ankle mortise
- Particularly important when assessing for subtle instability patterns
- The medial clear space measurement is most reliable on weight-bearing views 1
Comparison views of the contralateral ankle are rarely necessary in adults but may be helpful in pediatric patients to distinguish normal growth plates from fractures 1.