Generalidades del aparato locomotor

General Concepts of the Locomotor System

The locomotor system is defined as the set of organs providing support, protection, and mobility to the human body. It is composed of a jointed frame known as the skeleton and the muscles that move it. While the skeleton provides shape and structure, it allows for a mobile framework through multiple pieces joined by articulations. In the embryonic stage, these skeletal pieces are initially membranous or cartilaginous structures. During development, most differentiate into rigid bone tissue components called bones. Cartilage remains only in areas of articular contact. In regions requiring less rigidity, such as the thoracic wall, the bony skeleton is complemented by cartilaginous pieces. Anatomically, the skeleton is divided into two major parts: the axial skeleton, comprising the skull, vertebral column, ribs, and sternum; and the appendicular skeleton, comprising the limbs. From an evolutionary perspective, the axial skeleton is the oldest part, grouping elements from the exoskeleton (dermal skeleton) and the endoskeleton of primitive vertebrates. The appendicular skeleton originates from pectoral and caudal fins in fish and, except for part of the clavicle, resides in the endoskeleton.

The Osseous System: Functions and Bone Count

Bones are the rigid organs of the locomotor system and fulfill several critical functions. They act as levers for muscle action to produce movement and provide a framework for the body and anchorage for muscles. They also perform protective roles by forming cavities, such as the thorax and skull, to isolate organs from external influences. Notably, bones house bone marrow, which is the blood-forming tissue. Metabolically, bones serve as calcium reservoirs that can be mobilized into the blood as needed. The adult skeleton typically consists of approximately $208$ bones, though variations exist, particularly in the vertebral column. This count excludes sutural or Wormian bones (inconstant bones in the skull) and sesamoid bones (inconstant bones formed within tendons). Newborns have a higher count because some adult bones, such as the frontal bone, form from the fusion of two or more elements during childhood. In the elderly, the number may decrease further due to continued fusion, making it virtually impossible to separate certain skull bones after a specific age.

Bone Classification by Shape and Structure

Bones are classified into three types based on their shape: long, flat, and short. Long bones have one predominant axis (length) over width and thickness, including most limb bones like the humerus and femur. Flat bones have two predominant axes (width and length) over thickness, typified by the cranial vault, scapula, and coxal bone. Short bones have similar proportions in all three axes, such as vertebrae or the bones of the carpus and tarsus. Long bones consist of a central body or diaphysis (from Greek $dia = \text{apart}$ and $phycis = \text{growth}$) and two voluminous ends called epiphyses (from Greek $epi = \text{above}$ and $phycis = \text{growth}$). The transition zone between them is the metaphysis (from Greek $meta = \text{after}$ and $phycis = \text{growth}$). Epiphyses contain articular surfaces, which are smooth in dry bone and covered by articular cartilage in the living. The rest of the bone surface is covered by the periosteum, a connective tissue layer rich in vessels and nerves.

Microscopic Organization of Bone Tissue

The periosteum has an external fibrous layer (dense connective tissue with elastic and collagen fibers) and an internal cellular or osteogenic layer containing bone-forming cells. Fibers of the periosteum continue into tendons at muscle insertion sites and into articular capsules at bone ends. The periosteum is anchored to the bone by Sharpey’s (perforating) fibers. Internally, the diaphysis is made of compact bone tissue and houses a medullary cavity containing bone marrow. In children, this is red bone marrow (hematopoietic); in adults, it becomes yellow bone marrow (inactive fat), though it can revert upon pathological demand. The medullary cavity is lined by the endostium, which also has osteogenic potential. Epiphyses lack a medullary cavity and consist of spongy (cancellous) bone covered by a thin layer of compact bone. Spongy bone is a mesh of trabeculae delimiting spaces for red bone marrow. In youth, the metaphysis contains the epiphyseal plate (growth cartilage) for longitudinal growth. Flat bones consist of two compact layers (plates or tables) surrounding a central zone of spongy bone (called diploe in the skull).

Cellular and Matrix Components of Bone

Bone tissue is a specialized connective tissue involving cells and a mineralized extracellular matrix. It undergoes constant renewal through production and destruction regulated by hormonal and mechanical factors. There are three main cell populations: osteocytes (mature cells), osteoblasts (matrix-forming cells that transform into osteocytes), and osteoclasts (multinucleated cells for bone destruction). Osteoblasts arise from progenitor cells in the bone marrow or connective tissue, while osteoclasts derive from the macrophage lineage. The extracellular matrix includes collagen fibers, specific proteins (osteocalcin, osteonectin, osteopontin), and calcium salts (hydroxyapatite crystals) that provide rigidity, hardness, and resistance. Structurally, bone is organized into lamellae. In compact bone, lamellae form circumferential layers on the surface and osteons (Haversian systems) deeper down. Osteons are columns centered around a central canal containing vessels and nerves. Nutrients reach osteocytes through fine canaliculi. In spongy bone, lamellae form irregular trabeculae oriented along lines of stress, nourished by vessels in the intertrabecular spaces.

Bone Development and Ossification Patterns

Bone formation occurs via membranous or chondral ossification. Membranous ossification forms bone directly from mesenchymal connective tissue (e.g., cranial bones, mandible, clavicle). Chondral ossification involves replacing a cartilage model with bone (e.g., most of the skeleton). This includes pericondral ossification (periosteum depositing bone on the diaphysis surface) and endochondral ossification (vessels and osteoblasts invading degenerated cartilage). Recent studies suggest some hypertrophic chondrocytes transdifferentiate into osteoblasts rather than dying. Ossification starts around the third month of embryonic life and continues past puberty. Long bones have a primary center (diaphyseal) appearing before birth and secondary centers (epiphyseal) appearing mostly after birth. Exceptions include the distal femur and proximal tibia epiphyses, which ossify in the last month of fetal life. The inner end of the clavicle ossifies between $18$ and $20$ years, fusing by age $30$. Most diaphysis-epiphysis unions complete by age $25$. Women generally reach ossification milestones earlier than men.

Mechanics and Regulation of Bone Growth

Growth in thickness is appositional, occurring via the periosteum throughout life. Growth in length occurs at the epiphyseal plate, which has distinct zones: resting, proliferative (columnar cartilage), hypertrophic, calcified, and bone formation zones. When proliferation stops and cartilage is entirely replaced by bone, the "closure of the epiphysis" occurs. A peripheral groove called the Ranvier pericondral groove (with the Lacroix ring) holds stem cells for the growth plate. Hormonal control involves Growth Hormone (pituitary), sex hormones, thyroid hormones, parathyroid hormone, and calcitonin. Local factors include TGF-beta, BMP (Bone Morphogenetic Proteins), and FGF. Nutritional needs include proteins, calcium, phosphorus, Vitamin C (collagen synthesis), and Vitamin D (calcium metabolism). Mechanical factors are crucial; paralysis (e.g., poliomyelitis) leads to reduced bone size. Nervous control involves sympathetic fibers releasing Vasoactive Intestinal Peptide (VIP), which stimulates bone resorption.

Clinical and Functional Considerations of Bone

Osteoporosis arises from an imbalance where destruction (osteolysis) outweighs production, leading to fragile bones. Orthodontics utilizes mechanical remodeling to move teeth. Early closure of cranial bones can impede brain development. Premature fusion of all epiphyseal plates causes achondroplastic dwarfism (normal trunk, small limbs). Rickets in children results from Vitamin D deficiency, causing unmineralized osteoid and bone deformity. Radiological detection of ossification centers helps determine fetal maturity, pediatric "bone age," and forensic age. Bone repair (regeneration rather than scarring) involves a soft callus formed by periosteal/endosteal proliferation that later ossifies. This potential is clinically used to lengthen short bones via slow callus stretching. Initial immature bone is fibrillar/plexiform (isotropic); mature laminar bone is anisotropic, specialized to resist specific directions of force.

Vascularization and Innervation of Bones

Long bones receive blood from three sources: the nutrient (diaphyseal) artery (penetrating the nutrient foramen to serve the inner three-quarters of the diaphysis and marrow), periosteal arteries (entering via Volkmann canals for the outer diaphysis), and epiphyseal/metaphyseal arteries. In youth, growth cartilage separates the diaphysis and epiphysis vessels, but they anastomose after closure. Venous drainage involves a large central sinus in the diaphysis and sinusoids in epiphyses. Lymphatic drainage is confirmed for the periosteum and superficial bone. Innervation includes sensory fibers (especially pain-sensitive in the periosteum) and vegetative vasomotor fibers. Bone pain is often diffuse and irradiating. Nervous pathways are critical for bone growth and ossification.

The Articular System: Classification and Synarthrosis

Articulations are where bones meet, primarily allowing movement while sometimes providing protection, growth, and elasticity (e.g., the skull). They are classified into three types: synarthrosis, amphiarthrosis, and diarthrosis. Synarthrosis joints are immobile, where bones are joined by fibrous or cartilaginous tissue, often fusing over time. Subtypes include: synfibrosis or sutures (fibrous), categorized as dentate (indentations), squamous (beveled), harmonic (flat/rough), or schindylesis (ridge-in-groove); synchondrosis (cartilaginous); and syndesmosis (wide surfaces joined by interosseous ligaments, such as the distal tibia-fibula).

Amphiarthrosis and Synovial (Diarthrodial) Joints

Amphiarthrosis joints allow minimal mobility and are joined by fibrous or fibrocartilaginous tissue, such as the pubic symphysis. The sacro-coxal union is termed an amphi-diarthrosis because it contains a small articular cavity. Diarthrosis or synovial joints are highly mobile and complex. They feature independent articular surfaces covered with hyaline cartilage ($2$ to $7$ mm thick, nourished by synovial fluid imbibition), a fibrous capsule, a synovial membrane, and a synovial cavity containing synovial fluid. The capsule’s laxity and distance from the cartilage increase with the joint’s mobility. The synovial membrane consists of synoviocytes (fibroblastic) and macrophages on lax connective tissue; it produces synovial fluid and forms folds or blind sacs to accommodate volume changes. Synovial fluid (synovia) consists of dialyzed plasma and mucin (hyaluronic acid). Its functions are: lubricating surfaces to reduce friction, nourishing cartilage, and refrigerating the joint.

Dynamics and Types of Synovial Joints

Joints use ligaments (extracapsular, capsular, or intracapsular) and periarticular muscles to maintain cohesion. Supplemental components include disks (complete