Radius and Ulna | Gross Anatomy | Anjani Mishra

Bones of forearm

The RADIUS & ULNA

Written By Anjani Mishra

 

• The bones of forearm region are radius and ulna, which are attached to each other by an interosseous ligament.
•  Forearm of ox is short, but is longer in small ruminants.
• The radius is bigger than ulna and is fully developed.
• The ulna, though placed higher than the radius, is partially developed.
• Before adultism is reached, the two bones fuse with each other and constitute a single bone, the Os antibrachii.

 The Radius (OX)

Shape: It is a long bone. Radius is larger (not longer) of the two bones of the forearm. It is slightly curved.

Location: Located at forearm and attached with ulna.

Direction: Placed in vertical direction.

    

 Relation: It articulates with the condyles of humerus above, forming an elbow joint and the carpus below, Forming a carpal joint (knee joint). Ulna is articulated on the lateral aspect of the posterior surface of the radius.

Composition: It has ONE shaft and TWO extremities.

1) Shaft:

•   Slightly curved longitudinally and flattened cranio-caudally.
•   Presents FOUR surfaces:

a)     Anterior Surface:

- It is convex, smooth and is covered by extensor  m/s of carpus and digit.

- Three wide vertical grooves present at its distal-third, namely: lateral, central and medial groove, for the accommodation of the tendons of the extensor m/s.

b)     Posterior Surface:

•   It is concave from above to downwards and flat from the side to side.
•   At the lateral part of this surface, there is an elongated rough area for the attachment of cranial surface of the shaft of ulna by interosseous ligament.
•   This rough area is interrupted at two places to form the proximal and distal radio-ulnar arches or proximal and distal interosseous spaces. The fusion between the two bones above the proximal interosseous space is not complete. These two spaces are connected together by an interosseous groove laterally, for the transmission of the interosseous vessels of the forearm.
•   The nutrient foramen is placed on the lateral aspect of the proximal radio-ulnar arch.

c)      Medial surface:

•   It is gently concave above, smooth and rounded in its distal two-third.
•   This surface blends with the anterior and posterior surfaces, and is mostly sub-cutanceous.
•   At the proximal end of this surface, there is a smooth area for the passage of the tendons of brachialis, which is inserted along with medial ligament of the elbow joint.

d)     Lateral surface:

•  It is slightly concave, rounded and smooth in its upper third, where it blends with the anterior and posterior surface.

2) Extremities:

 TWO extremities:

A)   Proximal Exremities:

•   It is articular and is divided by a deep antero-posterior grove into medial (larger) and lateral (smaller) parts.

•   It articulates with the distal extremity (condyles) of humerus.
•   The articular surface is surrounded by a bony rim, which presents in front, about its middle, a prominent lip like projection the coronoid process.
•   Posterioly, there are two large concave facets for articulation with the ulna.
•   At the antero-medial aspects, there is a large radial tuberosity for the insertion of the tendon of the biceps brachii m/s.
•   On either side, there are roughened medial and lateral tubercles to which the medial and lateral ligaments of the elbow joint are attached.

B)    Distal Extremity:

•   It is large and thick.
•   The articular surface is oblique and is composed of three surfaces/facets, which articulate, with the bones of the proximal row of the carpus.

•   Medial Facets is intermediate in size and articulates with the radial carpal.
•   Middle Facet is largest in size, wide and concave in front, gradually becomes narrower and pointed behind. It articulates with the intermediate carpal.
•   Lateral Facet is smallest in size and articulates with the ulnar carpal and is saddle shaped; its lateral part is completed by ulna. There is a rough transverse ridge on the posterior part above the non-articular depression.

The ULNA

Shape: Ulna is longer of the two bones of the forearm. It is ill developed.

Location: Located at forearm and attached/fused with radius except at the two interosseous spaces (proximal and distal interosseous spaces).

Direction: Placed in vertical direction

Relation: It is fused with the radius along its postero-lateral aspect.

Composition: It has ONE shaft and TWO extremities.

1) Shaft:

•   Slightly bent and roughly prism shape.
•   Presents THREE surfaces and THREE borders:

A)   Anterior Surface:

• It is apposed to the posterior surface of the radius.
• It is rough and fused with the postero-lateral aspect of radius except at the position of the two interosseous spaces (proximal and distal interosseous spaces).

• The surface at the level of proximal interosseous space is In the form of an arch. At the upper part of this surface, there are two articular facets for the articulation with the corresponding facets of radius.

B)    Medial Surface:

•  Narrow, concave, blends with the posterior surface and provides accommodation for flexor m/s.

C)   Lateral Surface:

• Narrow, nearly flat and faint ridge.

Out of THREE borders, the Medial and Lateral borders are fused with the radius and the posterior one is smooth and concave on its length.

2) Extremities:

• TWO extremities:

A)   Proximal Extremity:

•         It is elongated and comprise of the Olecranon process and the Semi-lunar notch (sigmoid cavity).
•          Olecranon Process is massive part of the bone and projects inwards and slightly backwards behind the distal extremity of the humerus. It has TWO surfaces and TWO borders:

       Surfaces:

•  Medial Surface is slightly concave and smooth.
•   Lateral Surface is slightly convex.

Borders:    

Anterior Border is short, thin and pointed at its distal part of this process to form a beak-like projection, the processes anconeus, which overhangs the semi-lunar notch. The free proximal end of the process which is rough and tuberous, forms the summit and gives insertion to the triceps brachii, the tensor fasciae anti-brachii and anconeus m/s. 

The Posterior Border is long, concave and reaches to the distal extremity.

Semi-lunar Notch: The semi-lunar notch is a semi circular out cut placed below the                processes anconeus.

•   Articulates with the posterior part of the distal extremity of the humerus.
•   Between the notch and the dorsal part of the anterior face of the shaft, there are two facets, which form synovial joints with those on the posterior aspect of the proximal extremity of the radius.

B)    Distal Extremity:

•  It is fused with the radius and projects below the level of radius to form the styloid process of the ulna and furnishes a part of the articular facet for the ulnar carpal.

Comparative Anatomy of Radius & Ulna:

A) Horse:

• Radius is the larger and longer of the two bones of the forearm.
• Radial tuberosity is larger and well marked.
• Distal articular surfaces are not oblique.
• Ulna does not take part in the formation of the lateral articular facet.
• The grooves on the antero-distal part of the shaft are better marked.
•  On the lower-third and towards the medial border of the posterior surface, there is a roughened elevation to which the superior check ligament/radial check ligament is attached.
•  Ulna is less developed and extends only to the distal-third of the radius.
• The distal interosseous space is absent.
• The semi-lunar notch is more extensive and triangular in outline. The facets placed below the notch are convex.
• The medial surface of the olecranon process is more concave.

B) Pig:

•  Radius is comparatively shorter and thin.
•  Ulna is massive and curved backward.
•   The interosseous space is very narrow.

C) Dog:

 

• Two bones of the forearm are separated and are in contact with each other by their extremities and permit certain degree of movement. The shafts of two bones are separated by narrow interosseous space and extend throughout the length of bones.
•  The anterior surface of radius is marked by a groove on its distal half for the passage of tendon of the extensor carpi obliquus.
•  On the posterior surface of the radius, the nutrient foramen is placed at the junction of the dorsal and middle-third of shaft.
•  The proximal extremity of radius is small and has distinct neck; present only one articular surface that is concave and articulates with lateral condyle of the humerus. The medial condyle of the humerus articulates with a facet on the semi-lunar notch of ulna.
•  On the posterior aspect of the proximal extremity, there is convex marginal articular area called circumferential articularis, for the attachment of ulna.
•  Radial tuberosity is small.
• Distal extremity of radius is much wider and presents an extensive articular surface for the carpus.
•  The medial border projects downwards to form the styloid process of the radius.
•  Ulna is an independent and well-developed bone, which crosses the posterior surface of the radius medio-laterally. The shaft of ulna is THREE sided at proximal end while rounded at distal end.
• The anterior surface of ulnar shaft is rough and nutrient foramen is nearer to the proximal end of the shaft.
• The proximal extremity is short, concave, smooth medially; convex and rough laterally. Below the semi-lunar notch is a narrow, transversely placed, concave, articular area that articulates with the circumferential  articularis of the radius.
• The distal extremity is prolonged into a blunt pointed styloid process of the ulna and articulates by a concave facet with ulnar carpal below and by a convex facet with the radius, antero-medially.

 D) Fowl:

• Ulna is comparatively massive than radius.
•  Both bones are in contact at two extremities.
• Interosseous space is wide.
• Semi-lunar notch is ill marked.

 
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