Ankle joint classification: a hinge (ginglymus) joint, not a saddle

The talocrural joint: bones, classification, and function

The ankle is formed by three bones: the distal tibia, the distal fibula, and the talus (the uppermost bone of the foot). Together these create the talocrural joint, also called the ankle joint.

Its anatomic classification is a hinge joint (the formal term is ginglymus). Hinge joints permit motion in essentially one plane. At the ankle, that motion is dorsiflexion (foot pulled up) and plantar flexion (foot pointed down). Side-to-side motion (inversion and eversion) happens mainly at the subtalar and transverse tarsal joints, not at the talocrural joint itself.

This classification matches both the geometry and the motion: the trochlea of the talus is a curved, pulley-like surface that rides in the matching concavity of the tibial plafond, exactly the convex-in-concave arrangement that defines a ginglymus. Standard references, Gray’s Anatomy, StatPearls, and Moore’s Clinically Oriented Anatomy, all classify the talocrural joint as a ginglymus (hinge) joint.

The common confusion: “saddle”

Some radiography review materials label the ankle a saddle (sellar) joint. This is an outlier and does not match mainstream anatomy. The classic, textbook saddle joint is the thumb carpometacarpal (CMC) joint, whose two reciprocally curved (concave-convex) surfaces give the thumb its wide range of opposition. The ankle does not have that geometry; its single-plane, pulley-like motion is the hallmark of a hinge.

If a practice question (or an older edition of a positioning text) lists the ankle as “saddle,” recognize the discrepancy: the anatomically correct and most widely cited answer is ginglymus (hinge). When a question asks what motions the ankle allows, the answer is dorsiflexion and plantar flexion, which is precisely the one-plane motion of a hinge.

The ankle mortise explained

The ankle’s stability comes not just from ligaments but from a specific mechanical configuration called the ankle mortise (also spelled mortice). The mortise is a three-sided socket:

• Medial wall: the medial malleolus, the bony projection on the inside of the distal tibia
• Lateral wall: the lateral malleolus, the bony projection on the outside of the distal fibula
• Roof/floor: the tibial plafond, the distal articular surface of the tibia
• Occupant: the talus, which fits snugly into this socket

Think of it as a dovetail joint in carpentry. The talus is the piece being held; the tibia and fibula are the jaws that grip it. This geometry provides inherent mediolateral stability: the malleoli physically block inversion and eversion.

This stability changes with position. In dorsiflexion the wider anterior part of the talus wedges deeper into the mortise, so the joint is tightest and most stable. In plantar flexion the narrower posterior talus sits in the mortise and the joint is looser, which is why twisted-ankle injuries are far more common with the foot pointed down.

How positioning depends on the mortise (mortise view, obliques)

Mortise view (AP with internal rotation): Rotate the lower leg 15-20 degrees internally from the true AP so the medial and lateral malleoli are equidistant from the image receptor. Both joint spaces (talus-to-tibia medially and talus-to-fibula laterally) then appear open. This view is essential for evaluating ankle fractures and syndesmotic (high ankle sprain) injury.

Oblique views: A 45-degree oblique complements the mortise view, demonstrating the foot and midfoot relationships.

Lateral view: A true lateral (90 degrees to the mortise) shows the ankle in profile, the talar dome, and the calcaneus.

Other joint classifications to know for the ARRT

The ARRT tests joint classification across all body regions. Know these synovial types:

Ginglymus (hinge joint)

• Motion in one plane (flexion and extension).
• Examples: ankle (talocrural), elbow (humeroulnar), knee (tibiofemoral), interphalangeal joints.

Trochoid (pivot joint)

• Rotation around a central axis.
• Examples: atlantoaxial joint (C1-C2), proximal radioulnar joint.

Ellipsoid (condyloid)

• Movement in two planes (flexion-extension and abduction-adduction).
• Examples: radiocarpal joint (wrist), metacarpophalangeal joints.

Saddle (sellar)

• Two reciprocally curved (concave-convex) surfaces.
• Example: thumb carpometacarpal (CMC) joint, the classic saddle joint.

Spheroid (ball-and-socket)

• Movement in all three planes (multiaxial).
• Examples: shoulder (glenohumeral), hip (femoroacetabular).

Plane (gliding)

• Limited sliding motion.
• Examples: intercarpal joints, intertarsal joints, vertebral facet joints.

Quick reference table

Joint Type	Anatomic Example	Articular Surface Shape	Planes of Motion	ARRT Frequency
Ginglymus	Ankle, elbow, knee	One concave, one convex (1 plane)	1 plane (flexion/extension)	High
Trochoid	Atlantoaxial (C1-C2)	Peg and ring	1 plane (rotation)	Medium
Ellipsoid	Wrist (radiocarpal)	Oval surfaces (2 planes)	2 planes (flex/ext, abd/add)	High
Saddle	Thumb CMC	Reciprocal saddle surfaces	2 planes (with opposition)	Medium
Spheroid	Shoulder, hip	Rounded head in cup	3 planes (multiaxial)	High
Plane	Intercarpal, facet joints	Flat or slightly curved	Sliding/gliding only	Low

Note: The ankle (talocrural) is a ginglymus (hinge) joint. The classic saddle joint is the thumb carpometacarpal joint.

ARRT exam tip

If you memorize one thing: the ankle (talocrural) joint is a hinge (ginglymus) joint, the same category as the elbow and knee. Its motion, dorsiflexion and plantar flexion in one plane, is classic hinge behavior. The classic saddle joint is the thumb carpometacarpal joint. If a review item lists the ankle as “saddle,” treat it as an outlier and go with hinge.

You will also need the ankle mortise: the three-sided socket formed by the medial malleolus, lateral malleolus, and the tibial plafond, into which the talus fits. The mortise view is the AP projection with 15-20 degrees internal rotation of the lower leg, which places both malleoli parallel to the image receptor.

For a deeper look at lower extremity anatomy and positioning, see our chapter on lower extremity. For a comprehensive procedural guide, see ARRT procedures complete guide.