The distal radius end is the broader metaphyseal and physeal part of the radius that articulates distally with carpal bones and with ulna to form a distal radioulnar joint.
The anatomy of distal radius is unique.
It is important to have an in-depth knowledge of the anatomy of distal radius as the treatment principles and fixation gadgets are specifically designed for this region.
In intraarticular fractures, it becomes pertinent to obtain accurate joint surface reduction. That can only happen when one has advanced knowledge of anatomy.
The anatomy of distal radius can be used to make better fixation decisions and implant choices. We will see later how these anatomic principles help us avoid iatrogenic injuries.
Details of Anatomy of Distal Radius
This end of the radius is quadrilateral in cross-section.
The distal radius has the following anatomically important region
- Styloid process
- Dorsal tubercle
- Four surfaces: anterior, lateral, posterior, and medial
- Articular surface
- Scaphoid fossa and lunate fossa separated by a ridge
- Sigmoid notch are three concave articular surfaces
The radial styloid process is a projection of bone on the lateral surface of the distal radius bone that extends obliquely downward.
Brachioradialis tendon inserts at its base. and the radial collateral ligament attaches at the apex of the radial styloid.
The lateral surface has a flat groove for the tendons of the abductor pollicis longus and extensor pollicis brevis.
The dorsal tubercle or Lister’s tubercle is a bony protuberance on the dorsal surface of the distal radius that separates the 2nd and 3rd extensor compartments of the wrist. Second compartment contains extensor carpi radialis brevis and extensor carpi radialis longus tendons. The third compartment contains extensor pollicis longus. The groove on the bone for 3rd compartment is also called the EPL groove.
The Lister tubercle lies 5-10 mm from distal joint surface.
The main function of the dorsal tubercle is to act as a pulley for the EPL tendon, changing its mechanical direction of action.
The dorsal tubercle is used as a landmark in arthroscopy, joint injections, and surgical approaches.
The anterior surface extends radially from the radial styloid ulnarly to the triangular fibrocartilage complex. It extends distally and ulnarly to the capitate, lunate, and triquetrum.
The anterior surface is concave, angled anteriorly, and covered by the pronator quadratus. It provides an attachment point for the palmar radiocarpal ligaments.
The lateral surface extends along the lateral margin to form the styloid process. The conical-shaped styloid process projects 10–12 mm beyond the articular surface for the proximal scaphoid and lunate.
Attachments and markings on radial styloid have been discussed before.
It is an irregular, convex area. It acts as a fulcrum for the extensor tendon. A prominent dorsal tubercle or Lister’s tubercle can be seen 5–10 mm from the distal joint surface.
Two grooves for tendons of 2nd and 3rd extensor compartment allow passage of respective muscles [discussed under Dorsal Tubercle].
there is a smooth groove for the passage of the extensor pollicis longus (EPL) tendon on the medial aspect of the dorsal tubercle.
It consists of the ulnar notch and the articular surface for the ulnar head. The sigmoid notch is concave with well-defined margins and articulates with the ulnar head. The sigmoid notch angles distally and medially an average of 22 degrees
Between the distal radioulnar joint and the radiocarpal joint, there is a ridge, located in the ulnar notch that provides the radial attachment point for the TFCC.
The distal articular surface of the radius has a radial inclination averaging 22 degrees and an average volar tilt of 11 degrees. In the normal anatomy of the distal radius and wrist, the styloid process of the radius lies distal to the styloid process of the ulna. The usual distance is about 12 mm.
The articular surface of the distal radius is biconcave, triangular, and covered with hyaline cartilage.
A smooth anteroposterior ridge divides the articular surface into two facets
- A triangular lateral facet, which articulates with the scaphoid
- A quadrilateral medial facet articulates with the lunate.
The medial surface of the distal radius forms a semicircular notch covered with hyaline cartilage, which articulates with the ulna head. This articulation enables the radius to swing around the ulna.
The following image is a cross-sectional view of the wrist.
Important Concepts and Their Significance in Relation to Anatomy of Distal Radius
Understanding the anatomy of distal radius is very important for the proper management of fractures of the distal radius and restoring the function of the wrist to its fullest.
Fractures of the distal radius are very common. Colles, Smith, and Barton fractures are a few of these fractures differentiated by their fracture pattern and displacements.
For the management of these fractures, it becomes important to understand the anatomy of distal radius to assess and analyze the reduction and implant placement.
Moreover, the loss of reduction can be checked on postoperative x-rays only when the anatomy of distal radius is understood.
It is an imaginary line along and proximal to the volar contour of the articular surface. This line is slightly distal to the pronator quadratus line. The Pronator quadratus line marks the distal extent of the insertion of the pronator quadratus.
It is important that the implant and screws on the lateral view should be proximal to the Watershed line, otherwise, there is a risk of the implant being in the joint.
Dorsal Tubercle and Screw Length
The Pronator quadratus line and the tip of the dorsal tubercle are about 22 mm apart. This number is important to be remembered for choosing distal epiphyseal screw length for insertion in radius plating.
Clinical Significance of Anatomy of Distal Radius
Some diseases like Madelung deformity typically affect radius physis and alter the shape of the distal radius.
Madelung’s deformity is an abnormality of the wrist caused by a growth disturbance that retards the development of the ulnar and volar portions of the distal radial physis.