Imaging Anatomy: Text and Atlas Volume 3 (eBook)

1Bones, Muscles, Tendons, Joints, and Cartilage

Farhood Saremi

Introduction

This chapter is an overview of the development, structural anatomy, morphologic categories, physiology, biomechanics, and imaging of the bones, skeletal muscles, tendons, joints and cartilage, and related structures.

Bones

The adult skeleton is comprised of 206 bones with the largest being the femur and the smallest being the stapes of the middle ear. There are 126 bones in the extremities or appendicular skeleton, 74 bones in the axial skeleton, and 6 auditory ossicles. Additionally, there are many small sesamoid bones in different locations near the joints.1,2

The bones have been divided into four general morphologic categories: long bones (i.e., femur, metacarpals, and phalanges), short bones (i.e., carpal and tarsal), flat bones (i.e., skull and scapula), and irregular bones (i.e., vertebrae and hyoid).

The skeleton serves as the structural support for the body which protects the internal organs and allows movement and activity. The bones also provide the environment for blood production within the marrow spaces, hold reserve of calcium and phosphate needed for the maintenance of serum homeostasis and acid–base balance, and finally serve as a reservoir of growth factors and cytokines.1 Bone is a mineralized structure made up of collagenous matrix, cells, vessels, and crystals of calcium compounds (hydroxyapatite). In contrast to cartilages, bones are highly vascular structures with the capability of repair and remodeling. Remodeling is a constant and ongoing process during life that is accomplished by osteoblasts and osteoclasts to reshape the bone by removing old damaged bone and replacing it with new bone. This process helps the bone to adapt to changing biomechanical forces and achieve the strength against mechanical forces.

Gross Appearance

The long bones are composed of a tubular shaft or diaphysis with two cone-shaped metaphyses at each side of it. Each metaphysis connects to one or more rounded epiphyses with their interface being called the growth plates. Each bone is surrounded by a shell of compacted bone called cortex, which comprises 80% of the skeletal mass. The cortex of the diaphysis is thick and surrounds the medullary space that contains bone marrow. The metaphysis and epiphysis are composed primarily of a honeycomblike meshwork of trabecular bone also known as spongy or cancellous bone filled with bone marrow and hematopoietic cells and all surrounded by a relatively thin shell of cortical bone. The cancellous bone is highly vascular and the major site of hematopoiesis. The vertebra is composed of one-third of cortical bone and two-thirds of trabecular bone, whereas the femoral head cortex forms 50% of the bone and the radial bone diaphysis cortex forms 95% of the bone.1 The trabecular bone is composed of plates and rods. Within long bones such as the femoral neck, the trabeculae are aligned along the mechanical forces that weight-bearing bones experience (see Chapter 20 “Hip”). The trabecular bone contributes to major mechanical support in the vertebral bodies.

The functional units of the cortical and trabecular bones are microscopic columns called osteons. In the cortex, the osteons are called haversian systems and the osteons of the trabecular bone are called packets. Haversian systems are cylindrical in shape, approximately 4-cm long and 2-cm wide at their bases, and form a branching network along the long axis of the cortical bone. Each column consists of multiple concentric lamellae, each containing osteoblasts and osteocytes around a central canal called the haversian canal where vessels and nerves pass (▶Fig. 1.1). The haversian canals connect the lacunae (the space around osteocytes) by tiny channels called canaliculi. The haversian canals are also connected with one another by Volkmann’s canals. All haversian columns are surrounded by outer circumferential lamellae and separated from each other by interstitial lamellae (▶Fig. 1.1). The interstitial lamellae are deformed and partially resorbed haversian columns.

The cortical trabecular osteons are normally formed in a lamellar pattern, in which collagen fibrils are laid down in alternating orientations. The lamellar pattern, just like a plywood, increases the bone strength. Disorganized deposition of the collagen fibrils results in a weak bone called woven bone. The woven bone is normally produced during formation of the primary bone. It is also seen in the callus of the repaired fractures and in pathological conditions such as in bone lesions of hyperparathyroidism and Paget’s disease.

The outer cortical bone is covered by a fibrous connective tissue called the periosteum, and the inner cortex and the trabecular bone and Volkmann’s canals are covered by a membranous layer called endosteum (▶Fig. 1.1). The periosteum does not extend into the joints where bone is lined by articular cartilage. It is attached tightly to the cortex by thick collagenous fibers called Sharpey’s fibers. The endosteum consists of a single layer of flat cells supported by a thin layer of reticular connective tissue. Both the periosteum and the endosteum contain blood vessels, nerve fibers, and osteoblasts and osteoclasts. The endosteal surface has higher bone formation activity than the periosteal surface.

Development and Ossification

In general, the flat bones are formed by membranous bone formation, whereas the long bones are formed by a combination of endochondral and membranous bone formation that occurs in a centrifugal pattern spreading outward in both directions from the center of the bone (▶Fig. 1.2).

The cartilage, bone, and skeletal muscles are mainly formed by the mesoderm with some neural crest contribution.3,4,5 The intraembryonic mesoderm, located on either side of the neural groove between the endoderm and the ectoderm, is divided into the paraxial, intermediate, and lateral mesoderm. During the third week of gestation, the paraxial mesoderm forms blocks of cells called somites3 (▶Fig. 1.3). Each somite has two components: the dermamyotome and the sclerotome. The sclerotome of the somites contributes to the development of the craniofacial skeleton and most of the axial bone and cartilage. The cells in the lateral plate mesoderm contribute to the bones and cartilage of the limbs. The cranial neural crest contributes to the development of the craniofacial bones and cartilage. The dermamyotomes are the precursors of the dermis of the dorsal skin, the skeletal muscles of the back and body. The myoblasts close to the neural tube form the epaxial muscles (the deep muscles of the back), whereas the myoblasts remote from the neural tube produce the hypaxial muscles of the body wall and limbs. The cells in the middle of the dermamyotome are called the dermatomes that contribute to generation of the dermis and the mesenchymal connective tissue of the skin.