WEEK 1.2 Bones and Cartilage: Exhaustive Anatomy and Physiology Notes

Overview and General Composition of the Skeletal System

• System Components: The skeletal system is composed of four primary structures:

  • Bones

  • Cartilage

  • Joints

  • Ligaments

• Total Number of Bones: A typical adult skeleton contains exactly $206$ bones.

• Major Regions of the Skeleton: The skeleton is divided into two primary regions:

  • Axial Skeleton: This forms the central axis of the body.

  • Appendicular Skeleton: This consists of the limbs and the girdles that attach them to the axial skeleton.

• Anatomic Categories of the Skeleton:

  • Skull

  • Vertebral column and thorax

  • Upper and lower limbs

  • Girdles

Functional Divisions: Axial vs. Appendicular Skeleton

• Axial Skeleton (Categorized by Yellow color in standard mapping):

  • Skull

  • Hyoid bone

  • Vertebrae (Vertebral column)

  • Sternum

  • Ribs (Rib cage)

  • Sacrum

• Appendicular Skeleton (Categorized by Grey color in standard mapping):

  • Pectoral girdle

  • Upper extremity (Upper limbs)

  • Pelvic girdle

  • Lower extremity (Lower limbs)

Classification of Bones by Shape

• Long Bones: These are defined as being longer than they are wide. An example is the humerus.

• Short Bones: Cube-shaped bones frequently found in the wrist (carpals) and ankle (tarsals).

• Sesamoid Bones: These are a specialized sub-type of short bones that form within tendons. Notable examples include:

  • The Patella (kneecap).

  • Sesamoid of the thumb.

• Flat Bones: These are thin, flat, and often slightly curved. Examples include the sternum and certain skull bones.

• Irregular Bones: These possess complicated shapes that do not fit into other categories. An example is the vertebra.

Functions of the Skeletal System

• Support and Protection: Provides the structural framework for the body and protects internal organs.

• Mineral Storage and Homeostasis: Acts as a reservoir for minerals, maintaining balanced levels within the body.

• Blood Cell Production: Facilitates the creation of blood cells within the bone marrow.

Bone Markings: Major Features and Terminology

Projections and Enlargements

• Head: An enlarged end of a bone (often referred to as the epiphysis).

• Neck: A constriction located under the head (often referred to as the metaphysis).

• Body: The main portion or shaft of the bone (often referred to as the diaphysis).

• Protuberance: A prominent knuckle-like projection.

• Condyle: A bony knuckle-like process.

• Epicondyle: An area located above a condyle.

• Tubercle: A small rounded knuckle-like projection.

• Tuberosity: A projection that is larger than a tubercle.

• Trochanter: A projection that is larger than a tuberosity.

• Process: A general term for a bony projection.

• Spine: A sharp, slender, or narrow process.

Ridges and Lines

• Line (Linea): A low ridge on the bone surface.

• Crests and Ridges: Raised borders or prominent ridges.

• Angle: A bend in the bone.

Depressions and Openings

• Fossa: A general term for a depression.

• Fovea: A small pit or depression.

• Notch (Incisure): A portion cut out of a bone.

• Groove (Sulcus): A shallow indentation or narrow depression.

• Foramen: A hole through which nerves or blood vessels pass (e.g., Foramen magnum).

• Fissure: A narrow, slit-like opening or cleft (e.g., superior orbital fissure).

• Meatus: A tunnel or canal-like passage (e.g., internal acoustic meatus).

• Sinus: A cavity within a bone.

• Facet: A smooth, nearly flat articular surface where bones meet.

Regional Anatomy: The Male Pelvis (Coxal Bone)

• Components of the Coxal Bone (Os Coxae/Hip Bone):

  • Ilium

  • Ischium

  • Pubic bone (Pubis)

• Major Landmarks of the Pelvis:

  • Iliac Crest: The superior border of the ilium.

  • Iliac Fossa: The large, concave internal surface of the ilium.

  • Acetabulum: The socket that receives the head of the femur.

  • Obturator Foramen: The large opening between the ischium and pubis.

  • Pubic Symphysis: The joint where the two pubic bones meet anteriorly.

  • Ischial Tuberosity: The "sitting bone."

  • Spines: Posterior superior iliac spine, Posterior inferior iliac spine, Anterior superior iliac spine, Anterior inferior iliac spine, and Ischial spine.

  • Notches: Greater sciatic notch and Lesser sciatic notch.

  • Gluteal Lines: Anterior, posterior, and inferior gluteal lines.

  • Additional features: Sacrum, Coccyx, Sacroiliac joint, Pelvic brim, Sacral promontory, and the Pubic arch.

Regional Anatomy: The Skull (Superior and Inferior Views)

• Superior View Features:

  • Sagittal suture

  • Coronal suture

  • Zygomatic process of the temporal bone

• Inferior View Features:

  • Processes: Palatine process of maxilla, Temporal process of zygomatic bone, Pterygoid process of sphenoid bone, Zygomatic process of temporal bone, Styloid process of temporal bone, and Mastoid process of temporal bone.

  • Foramina and Canals: Carotid canal, Jugular foramen, Stylomastoid foramen, Foramen magnum, and Foramen ovale.

  • Depressions and Ridges: Mandibular fossa, External occipital protuberance, Inferior nuchal line, and Superior nuchal line.

Anatomy of the Rib (Superior and Inferior Views)

• Structural Parts:

  • Head: The portion that articulates with the vertebrae.

  • Neck: The constricted region between the head and body.

  • Articular Tubercle and Facet: Serving for vertebral articulation.

  • Body: The main shaft of the rib.

  • Costal Angle: The point where the rib curves most sharply.

  • Costal Groove: A shallow indentation on the inferior surface of the rib body.

Cartilage: Structure, Composition, and Physiology

• General Characteristics:

  • Cartilage is an elastic tissue that covers and protects the ends of long bones at joints.

  • It serves as a structural component for the rib cage, ear, nose, bronchial tubes, and intervertebral discs.

  • Rigidity: Its rigidity allows it to hold tubes open (e.g., rings of the trachea, cricoid cartilage, and carina).

  • Metabolic rate: It has a very slow turnover of its extracellular matrix and repairs at a very slow rate compared to other tissues.

• Biochemical Composition:

  • Cells: Chondrocytes, which produce the matrix.

  • Extracellular Matrix: Composed of glycosaminoglycans (GAGs), proteoglycans, chondroitin sulphate, hyaluronic acid, collagen fibers, and sometimes elastin fibers.

  • Ground Substance: Rich in proteoglycan and elastin.

• Physiological properties:

  • Avascular: It does not contain blood vessels.

  • Aneural: It does not contain nerves.

  • Nutrient Delivery: Nutrition is supplied via diffusion. Diffusion is assisted by the compression of articular cartilage or the flexion of elastic cartilage, which generates fluid flow.

  • Exception: Some fibrocartilage, such as the meniscus of the knee, possesses a partial blood supply.

Types of Cartilage

1. Hyaline Cartilage

• Etymology: Derived from the Greek word "Hyalos," meaning glass.

• Appearance: Translucent, bluish-white, and shiny.

• Thickness: Usually only $2$ – $4\,mm$ thick. The thinness is necessary because the tissue is avascular and relies on diffusion for oxygen and nutrients.

• Distribution:

  • The most common form of cartilage.

  • Articular Cartilage: Found on the articular surfaces of bones within the joint cavity of synovial joints. It is bathed in synovial fluid.

  • Found in the nose, thyroid, larynx, cricoid, trachea, and costal cartilages of the ribs.

2. Elastic Cartilage

• Function:

  • To change shape in response to tension, compression, and bending, and then return to an at-rest state.

  • To provide a strong but flexible structure.

• Distribution:

  • External ear (pinnae/auricles).

  • Lateral region of the external auditory meatus.

  • Medial part of the auditory canal.

  • Eustachian tube.

  • Corniculate and cuneiform laryngeal cartilages.

  • Epiglottis.

3. Fibrocartilage

• Characteristics: A very strong and pliable connective tissue.

• Functional Categories:

  1. Intra-articular Fibrocartilage (e.g., Menisci): Present at joints where flexion/extension is associated with gliding. They act as thrust-pads to prevent joint instability.

  2. Connecting Fibrocartilage (e.g., Intervertebral discs): Found in limited-motion joints. Acts as a cushion to distribute stress. Includes the annulus fibrosus.

  3. Stratiform Fibrocartilage: A thin layer over bone where tendons glide (e.g., tendons of the peroneus longus and tibialis posterior) to minimize friction.

  4. Circumferential Fibrocartilage (Labra): Ring-like structures without a center (e.g., glenoid labrum of the shoulder, acetabular labrum of the hip) that protect joint margins and improve bony fit.

• Additional Locations:

  • Pubic symphysis.

  • Manubriosternal joint.

  • Sternoclavicular joint.

  • Where tendons and ligaments attach to bone.