The Sacrum is a large, triangular bone at the base of the vertebral column, formed by the fusion of 5 sacral vertebrae (S1-S5). It articulates superiorly with L5 (last lumbar vertebra) at the lumbosacral joint, laterally with the two hip bones at the sacroiliac joints (SI joints), and inferiorly with the coccyx. The sacrum forms the posterior wall of the pelvis. The sacral canal is a continuation of the vertebral canal, housing the sacral nerve roots (cauda equina).

Paget's Disease of Bone (Osteitis Deformans) is a chronic disorder of bone remodelling where bone is broken down and rebuilt at an abnormally rapid rate, producing enlarged, deformed, structurally weak bone. Commonly affects the skull, spine, pelvis, and long bones. Symptoms: Bone pain, deformity, and fractures, Enlarged skull causing headaches and hearing loss, and Bowed legs. Most patients are asymptomatic (discovered incidentally on X-ray). Associated with elevated serum alkaline phosphatase. Treatment: bisphosphonates.

The Tibia (shinbone) is the larger, medial (inner) bone of the lower leg, located between the knee and ankle. It is the second longest bone in the body (after the femur) and bears most of the body's weight. The tibia forms the knee joint with the femur (above) and the ankle joint with the talus (below). The Fibula is the smaller, lateral (outer) bone of the lower leg - it does not bear significant weight but provides attachment for muscles.

Structural classification of joints based on connective tissue: Fibrous joints = bones connected by fibrous connective tissue (skull sutures, teeth in sockets/gomphosis, tibiofibular joint/syndesmosis). Cartilaginous joints = bones connected by cartilage, two types: Synchondrosis (hyaline cartilage - epiphyseal plate, first rib-sternum joint) and Symphysis (fibrocartilage - pubic symphysis, intervertebral discs). Synovial joints = bones separated by synovial cavity (most movable joints - knee, hip, shoulder).

The Tarsals are the 7 bones of the ankle and posterior foot: Calcaneus (largest - heel bone), Talus (articulates with tibia/fibula to form ankle joint), Navicular, Cuboid, Medial Cuneiform, Intermediate Cuneiform, and Lateral Cuneiform. Corresponding wrist bones are called Carpals (8 bones). Remember: Tarsals = Toes/ankle, Carpals = Carpal tunnel/wrist.

The Mastoid process is a rounded, cone-shaped bony prominence on the temporal bone, located just behind the ear. It can be easily felt as a bump behind the earlobe. Functions: serves as attachment point for several neck muscles (sternocleidomastoid, splenius capitis, longissimus capitis) and contains mastoid air cells (connected to the middle ear). Mastoiditis = infection of mastoid air cells (complication of middle ear infection). The Styloid process is a slender spike of bone below the mastoid process.

The Axial skeleton (80 bones) includes bones forming the central axis of the body: Skull (22 bones), Vertebral column (26 bones), Rib cage - Sternum (1) + 24 ribs (12 pairs) = 25 bones, Hyoid bone (1), and Auditory ossicles (6 - 3 in each ear). Total = 22 + 26 + 25 + 1 + 6 = 80 bones. Femur, Humerus, and Tibia are all part of the appendicular skeleton (limb bones).

Parathyroid Hormone (PTH), secreted by the parathyroid glands, is the primary regulator of blood calcium. It stimulates osteoclasts (bone resorption - releases calcium into blood), increases calcium reabsorption in kidneys (reduces urinary calcium loss), and increases Vitamin D activation (increases intestinal calcium absorption). Net effect: Raises blood calcium levels. Calcitonin (from thyroid C-cells) has the opposite effect - lowers blood calcium by inhibiting bone resorption. Vitamin D also promotes calcium absorption from the intestine.

Bone tissue is of two types: Compact (Cortical) Bone = Dense, hard, solid outer layer with very few spaces. Organized into Haversian systems (osteons). Makes up 80% of total bone mass. Found in the shaft (diaphysis) of long bones and outer shell of all bones. Provides strength and rigidity. Spongy (Cancellous/Trabecular) Bone = Porous, lattice-like inner bone. Found at ends of long bones and inside flat bones. Contains red bone marrow. Provides lightweight strength and shock absorption.

A Dislocation (Luxation) occurs when the bones of a joint are forced out of their normal position, so the joint surfaces are no longer in proper contact. Common dislocations: Shoulder (most common - due to high mobility and shallow socket), Patella (lateral dislocation), Hip (posterior dislocation - from road accidents), and Elbow/finger (from falls). A Subluxation = partial dislocation (joint surfaces partially separated). Dislocations are medical emergencies requiring immediate reduction (repositioning).