Side Determination of the Radius and Ulna: A Step-by-Step Method

The radius and ulna are the two bones of the forearm, and they are usually handed over together in an upper limb osteology viva. Students mix them up for one reason: the radius is small at the top and wide at the bottom, while the ulna is the opposite. Once you know that, the same three questions used for side determination of the femur and humerus give you the side of each.

The three questions

1. Which end is the upper end?
2. Which surface faces forwards?
3. Which side is lateral?

Settle them in that order. Then hold the bone against your own forearm in that position, with your palm facing forwards. The forearm it matches is its side.

First, which bone is which?

In the anatomical position the palm faces forwards. The radius is then the lateral bone of the forearm, on the thumb side. The ulna is the medial bone, on the little finger side. The ulna forms the main part of the elbow joint with the humerus. The radius forms the main part of the wrist joint.

The word "head" causes confusion. The head of the radius is at its upper end. The head of the ulna is at its lower end.

Side determination of the radius

• Upper end: the small, disc-shaped head, with a narrow neck below it. The lower end is the widest part of the bone.
• Front and back: at the lower end, the anterior surface is smooth and slightly concave. The posterior surface is rough, with grooves for tendons and a raised bump, the dorsal tubercle (Lister's tubercle).
• Lateral: the styloid process projects downwards from the lateral side of the lower end. The medial side of the lower end has the ulnar notch, where the head of the ulna fits.

Putting it together: hold the radius with the disc-shaped head at the top and the smooth, concave surface of the lower end facing forwards, away from you. The dorsal tubercle now faces you. The side on which the styloid process points down is the lateral side. If the styloid process is on your right, it is a right radius. If it is on your left, it is a left radius.

Second check: the shaft has a sharp interosseous border, which faces medially, towards the ulna. The radial tuberosity, just below the neck, also lies on the medial side. The shaft itself bows outwards, so its convexity is lateral.

Anatomical position: hold the radius vertically, with the head above, the styloid process below and lateral, and the dorsal tubercle behind.

Side determination of the ulna

• Upper end: the large, hook-shaped end. It has two processes, the olecranon above and the coronoid in front, with the trochlear notch between them. The lower end is small: a rounded head and a short styloid process.
• Front: the trochlear notch faces forwards. It grips the trochlea of the humerus. The coronoid process also projects forwards.
• Lateral: the radial notch is a small, smooth facet on the lateral side of the coronoid process, where the head of the radius turns. The sharp interosseous border of the shaft is also lateral, facing the radius.

Putting it together: hold the ulna with the hook-shaped end at the top and the trochlear notch facing forwards, away from you. Look for the radial notch beside the coronoid process. The side it is on is the lateral side. If the radial notch is on your right, it is a right ulna. If it is on your left, it is a left ulna.

Second check: at the lower end, the styloid process of the ulna lies on the medial side and towards the back.

Anatomical position: hold the ulna vertically, with the olecranon above, the trochlear notch facing forwards and the radial notch facing laterally.

Radius and ulna side by side

One line to remember: the sharp interosseous borders of the two bones face each other.

Common mistakes

• Holding the radius upside down. Its wide end is the lower end. Look for the disc-shaped head first.
• Calling the lower end of the ulna its upper end because it is called the head. The hook with the trochlear notch is always at the top.
• Using the styloid process of the ulna as the lateral marker. On the radius the styloid process is lateral. On the ulna it is medial.
• Thinking of the forearm with the palm turned down. Side determination always uses the anatomical position, with the palm facing forwards.
• Answering from one feature. Confirm with a second landmark before you speak.

Questions students ask

How do I quickly tell a left radius from a right radius?

Put the small head at the top and the dorsal tubercle at the back. The styloid process is on the lateral side. On a right radius held this way, with the front facing away from you, the styloid process is on your right.

Which side is the radial notch of the ulna on?

The lateral side. It lies on the lateral surface of the coronoid process, next to the lower part of the trochlear notch, and it faces the head of the radius.

Is the radius medial or lateral?

Lateral, on the thumb side, in the anatomical position. When the forearm is pronated, the lower end of the radius crosses in front of the ulna and comes to lie on the medial side. That is why the bones are always described with the palm facing forwards.

Which styloid process is lower, the radial or the ulnar?

The styloid process of the radius reaches lower than that of the ulna. You can feel both at your own wrist.

Which way do the nutrient foramina of the radius and ulna point?

Towards the elbow. On both bones the nutrient foramen is on the anterior surface of the shaft and its canal runs upwards. This is a useful extra check on which end is the upper end.

What does the examiner ask after side determination?

Usually the anatomical position, then the named features: on the radius, the head, neck, radial tuberosity, styloid process and dorsal tubercle; on the ulna, the olecranon, coronoid process, trochlear notch, radial notch and head. Practise with the osteology viva questions on the upper limb, and see how to identify any bone in a spotter for the full routine.

Practise it

Side determination becomes quick only with a bone in your hand. A bilateral bone set gives you a left and a right radius and ulna to compare; a unilateral bone set is enough to practise the method. The XBONZ Anatomy Lab has a side-determination drill on the 3D skeleton, with full access. For the lower limb, read side determination of the tibia and fibula.

This is a revision aid. Check it against your course textbook and what your department teaches.