Bones: Bone Structure

Bones are rich with blood vessels, cells, and nerves that allow them to perform all functions. The periosteum and endosteum are connective tissues that cover long bones. They contain the cells responsible for growth, repair, and remodeling. The periosteum covers the outside of bones, while the endosteum covers the inner lining of bones and the medullary cavity.

Periosteum - The outer covering of bone where osteoblasts are located.

Endosteum - Connective tissue that covers the inside of bone and the medullary cavity.

Medullary Cavity - Central cavity of the bone shaft where marrow is stored.

There are 2 primary types of bone tissue. The structure of bone is not the uniformly hard material it might seem to be. Bone consists of two different materials: the compact outer layer and the spongy inner portion.

Compact (cortical) bone - This hard outer layer of dense tissue is strong, solid, and resistant to bending. Approximately 80 percent of a person’s skeletal mass comes from compact bone.

Spongy (trabecular or cancellous) bone - This light, porous material forms a latticework of bony structures called trabeculae. Osteoporosis is primarily due to the weakening of spongy bone, but compact bone thins as well.

Trabeculae - Functional units of spongy bone.

Osteoporosis - Bone disease characterized by a loss in bone mass and density.

The combination of compact and spongy materials is what gives bone its strength while still being relatively light. If bones were made entirely of compact material, they would be too heavy for efficient movement and spongy bone alone wouldn't give bones the strength it needs.

Bony Protrusions

There are various bony protrusions throughout the skeleton that contribute to each bone’s unique shape. For example, the head of the femur contains two primary protrusions: greater trochanter and lesser trochanter. A trochanter, or protrusion, is the site of muscle and ligament attachment. The increased anatomical area serves two purposes: to strengthen the bone in that region and to provide a greater contact surface for the muscles and ligaments to attach.

Bony protrusion - An eminence on the surface of bones that increases strength and contact area for muscle attachments.

These areas of increased bone formation go by different names throughout the skeleton, depending on where they reside. For example, the upper humerus has two protrusions, greater tuberosity and lesser tuberosity, which serve as attachment points for the rotator cuff muscles. At the lower aspect of the humerus, the protrusions by the elbow are called epicondyles. Moving farther down the body, the protrusions of the upper femur are known as trochanters. The spinous and transverse processes, the bony protrusions of the vertebrae.

Common Bone Markings

Angles: sharp bony angulations that may serve as bony or soft tissue attachments but often are used for precise anatomical description. Examples include the superior, inferior, and acromial angles of the scapula.

Body: usually refers to the largest, most prominent segment of bone. Examples include the shaft of long bones like the femur and humerus.

Condyle: refers to a large prominence that often provides structural support to the overlying hyaline cartilage. It bears the brunt of the force exerted from the joint. An example is the knee joint (hinge joint).

Crest: a raised or prominent part of the edge of a bone. Crests are often the sites where

connective tissue attaches muscle to bone. The iliac crest is found on the ilium.

Diaphysis: refers to the main part or the shaft of a long bone. Long bones, including the femur, humerus, and tibia, all have a shaft.

Epicondyle: a prominence that sits atop a condyle. The epicondyle attaches muscle and connective tissue to bone, providing support to this musculoskeletal system. Examples include the femoral medial and lateral epicondyles.

Epiphysis: the articulating segment of a bone, usually at the proximal and distal poles of the bone. It usually has a larger diameter than the shaft (diaphysis). The epiphysis is critical for the growth of bone because it sits adjacent to the physeal line, also known as the growth plate.

Facet: a smooth, flat surface that forms a joint with another flat bone or another facet, together forming a gliding joint. Examples can be seen in the facet joints of the vertebrae, which allow for flexion and extension of the spine.

Fissure: an open slit in a bone that usually houses nerves and blood vessels. Examples include superior and inferior orbital fissure.

Foramen: a hole through which nerves and blood vessels pass. Examples include supraorbital foramen, infraorbital foramen, and mental foramen on the cranium.

Fossa: a shallow depression in the bone surface. Here it may receive another articulating bone or act to support brain structures. Examples include trochlear fossa and posterior, middle, and anterior cranial fossa.

Groove: a furrow in the bone surface that runs along the length of a vessel or nerve, providing space to avoid compression by adjacent muscle or external forces. Examples include a radial groove and the groove for the transverse sinus.

Head: a rounded, prominent extension of bone that forms part of a joint. It is separated from the shaft of the bone by the neck. The head is usually covered in hyaline cartilage inside a synovial capsule, as it is the main articulating surface with the adjacent bone, together forming a ball-and-socket joint.

Margin: the edge of any flat bone. It can be used to accurately define a bone’s borders. For example, the edge of the temporal bone articulating with the occipital bone is called the occipital margin of the temporal bone. And vice versa, the edge of the occipital bone articulating with the temporal bone is called the temporal margin of the occipital bone.

Meatus: a tubelike channel that extends within the bone that may provide passage and protection to nerves, vessels, and even sound. Examples include external acoustic meatus and internal auditory meatus.

Neck: the segment between the head and the shaft of a bone. It is often demarcated from the head by the presence of the physeal line in pediatric patients and the physeal scar (physeal line remnant) in adults. It is often separated into the surgical neck and anatomical neck. The anatomical neck, which may represent the old epiphyseal plate, is often demarcated by its attachment to capsular ligaments. The surgical neck is often more distal and is demarcated by the site on the neck that is most commonly fractured. For example, in the humerus, the anatomical neck runs obliquely from the greater tuberosity to just inferior to the humeral head. The surgical neck runs horizontally and a few centimeters distal to the humeral tuberosities.

Notch: a depression in a bone that often, but not always, provides stabilization to an adjacent articulating bone. The articulating bone will slide into and out of the notch, guiding the range of motion of the joint. Examples include the trochlear notch on the ulna, radial notch of the ulna, suprasternal notch, and the mandibular notch.

Ramus: the curved part of a bone that gives structural support to the rest of the bone. Examples include the superior and inferior pubic ramus and ramus of the mandible.

Sinus: a cavity within any organ or tissue. Examples include paranasal sinuses and dural venous sinuses.

Spinous process: a raised, sharp elevation of bone where muscles and connective tissue attach. It is different than a normal process in that a spinous process is more pronounced.

Trochanter: a large prominence on the side of the bone. Some of the largest muscle groups and most dense connective tissues attach to the trochanter. The most notable examples are the greater and lesser trochanters of the femur.

Tuberosity: a moderate prominence where muscles and connective tissues attach. Its function is similar to that of a trochanter. Examples include the tibial tuberosity, deltoid tuberosity, and ischial tuberosity.

Tubercle: a small, rounded prominence where connective tissues attach. Examples include the greater and lesser tubercle of the humerus.

Back > Muscles: Origin & Insertion

Next > Bones: Type, Shape and Classification

Next > Joints: Type and Classification