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Other Terms: Cúbito
The ulna is the medial and longer bone of the antebrachium. It is thick and notched at its proximal end having a hook-like appearance that tapers to a thin shaft ending distally as a small round head. The bone is triangular in cross section.
Ulna is Latin for forearm with an extended derivation to elbow which probably derives from the Germanic el meaning arm and bow meaning bend. It is similar to the Greek olena meaning forearm. The first century anatomist Celsus was the first to use the term.
The ulna articulates with two bones: the humerus and the radius. In addition, the ulna articulates with the carpus via a fibrocartilaginous disk or meniscus. The trochlear notch at the proximal end of the ulna articulates with the trochlea of the humerus forming the hinge joint of the elbow. The bone forms two articular surfaces with the radius. Proximally, the radial notch accommodates the wheel-like portion of the radial head. Distally, the rounded head of the ulna articulates with the radius at its ulnar notch. An articular disc is interposed between the distal end of the ulna and the lunate and triquetral carpal bones.
Ossification begins during the eighth week of embryonic life in the middle of the ulnar shaft. This primary center moves toward each extremity of the bone having ossified by birth. At birth the distal and proximal ends are still cartilaginous. The distal center begins ossifying first. In the fifth to sixth year an internal center begins developing in the distal end and radiates outward projecting more prominently at one point to form the styloid process. The proximal end forms from two centers - one arises around the ninth year near the surface of the trochlear notch, the other during the eleventh to twelfth year as a thin cap at the olecranon surface. The proximal centers ossify rapidly and fuse to the diaphysis by the fourteenth year. The distal center develops more slowly, fusing during the seventeenth to eighteenth year.