Bone

Lore & Background

Bones come in a variety of shapes and sizes with complex internal and external structures. The largest bone is the femur, and the smallest is the stapes in the middle ear. Five types of bones are found in the human body: long, short, flat, irregular, and sesamoid. Long bones, such as those of the limbs, have a shaft (diaphysis) and rounded ends (epiphyses). Short bones are roughly cube-shaped, flat bones are thin and curved, sesamoid bones are embedded in tendons, and irregular bones have complicated shapes.

Reader's Guide

Bones serve mechanical, synthetic, and metabolic functions. Mechanically, they form the skeleton, provide support, protect organs, enable hearing via the ossicles, and have high compressive strength but lower tensile and shear strength. Synthetically, bone marrow produces blood cells through hematopoiesis, including red blood cells, platelets, and white blood cells. Metabolically, bones store minerals such as calcium and phosphorus, store growth factors and fatty acids, buffer blood pH, store heavy metals for detoxification, and act as an endocrine organ by releasing fibroblast growth factor 23 and osteocalcin to regulate phosphate metabolism and blood sugar. Bone tissue is continuously remodeled by specialized cells and consists of a flexible matrix (about 30%) and bound minerals (about 70%), making it hard and strong yet lightweight.

Did You Know?

• The largest bone in the body is the femur, and the smallest is the stapes in the middle ear.
• Bone marrow produces over 2.5 billion red blood cells and platelets daily.
• Bones act as an endocrine organ, releasing FGF-23 and osteocalcin to regulate phosphate and glucose.

Anatomy & Structure in the Human Foot

The navicular bone occupies a distinctive position among the tarsal bones of the human foot, sitting on the medial side and serving as a connector between several neighboring structures. Its name comes from its visual resemblance to a tiny boat, a shape created by the deeply concave surface on its proximal, or posterior, side. Anatomically, it forms joints with the talus above it, the three cuneiform bones below, and the cuboid on its lateral side. In terms of development, the navicular is notably the last of the foot's bones to begin the ossification process, typically not starting until around the fourth year of life, though considerable individual variation has been documented. Only a single muscle, the tibialis posterior, attaches directly to this bone, with its main portion inserting into the navicular's tuberosity. Additionally, a small percentage of the general population—estimated between two and fourteen percent—possess an accessory navicular bone, a variant that adds further complexity to this already intricate region of the foot.

Biomechanical Keystone & Injury Patterns

Often described as the keystone of the foot, the navicular bone plays a central structural and functional role in human gait. As a component of the coxa pedis, it articulates with the talus, all three cuneiform bones, the cuboid, and the calcaneus, making it a critical structural link bridging the midfoot and forefoot. This positioning allows the navicular to contribute meaningfully to inversion, eversion, and overall foot motion, while also forming part of both the longitudinal and transverse arches that define the foot's characteristic shape. Despite its importance, the navicular is not a frequently fractured bone. When it does break, the mechanism is typically one of two: a stress fracture, which occurs commonly among athletes subjected to repetitive loading, or a high-energy traumatic event. The relative rarity of navicular fractures, combined with the bone's deep anatomical position and its multiple articulations, makes identification and management of these injuries a particular challenge for clinicians.

The Equine Navicular: A Different Bone with the Same Name

In horses, the term navicular bone refers to an entirely different anatomical structure than the one found in the human foot. The equine navicular is a sesamoid bone situated within the hoof, resting on the palmar aspect of the coffin joint between the second phalanx and the third phalanx, also known as the coffin bone. It is held in place by the distal sesamoidean impar ligament along with two collateral sesamoidean ligaments. A fluid-filled sac called the navicular bursa sits between the flexor surface of this bone and the deep digital flexor tendon, which itself runs between the bursa and the distal phalanx. While the central tarsal bone in the horse's hock is considered homologous and analogous to the human foot's navicular, the sesamoid navicular in the hoof is a fundamentally distinct structure. This naming overlap between species can create considerable confusion in comparative anatomy discussions, as the two bones share a name yet differ in location, shape, and function.

Navicular Disease & the Diagnostic Challenge in Horses

The navicular region in horses carries enormous clinical weight, particularly in the front feet, where it is implicated in a significant and often frustrating condition known as navicular disease or navicular syndrome. This condition may be responsible for as much as one-third of all lameness cases in horses, making it one of the most prevalent causes of impaired mobility in the species. However, diagnosing navicular syndrome has proven far more complex than earlier literature suggested. Much of the original body of research, particularly the reliance on radiographic changes in the navicular bone as a sole diagnostic criterion, has been called into question. Radiographic findings do not always yield a definitive diagnosis, and newer imaging techniques have revealed that damage to the surrounding soft tissues in the navicular region may be a significant contributor to lameness. Furthermore, multiple distinct causes can produce visible lameness, meaning that the navicular bone's appearance on an X-ray is only one piece of a much larger diagnostic puzzle.

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