Introduction to the Locomotor Apparatus: Osteology, Arthrology, and Myology

Introduction to the Locomotor Apparatus

• Lecturer: W. Brady Little, DVM, MSc, Cert Vet Ed, FHEA.
• Learning Objectives (LO): 3-11 (Module 1).
• Overview of the Apparatus:     * Osteology: Study of bones, regions (forelimb), shapes, long bone anatomy, and bony prominences.     * Arthrology: Study of joints, structural classification, synovial joint anatomy, functional classification, and basic movements.     * Myology: Study of skeletal muscle anatomy.

Osteology: The Study of Bones

• Definition: Derived from "osteo-" (bone) and "-ology" (study of).
• Primary Functions of Bones:     1. Support: Bearing the weight of the body.     2. Movement: Working in concert with joints and muscles.     3. Protection: Shielding internal organs.     4. Growth: Providing a framework for development.     5. Mineral Storage: Acting as a reservoir for minerals, specifically $Ca^{2+}$ and $P^{+}$.     6. Fat Storage: Storing fat within the marrow.     7. Hematopoiesis: Blood cell formation occurring within the marrow.

Forelimb Regions and Associated Bones

• Scapular Region: Supported by the Scapula.
• Brachium: Supported by the Humerus.
• Antebrachium: Supported by the Radius and Ulna.
• Manus (Front Paw):     * Carpus: Supported by carpal bones.     * Metacarpus: Supported by metacarpal bones.     * Digits: Comprised of Phalanges and sesamoids.

Classification of Bones by Shape

• Flat Bones:     * Example: Scapula.     * Structure: Broad and flat; composed of two thin layers of compact bone with a layer of spongy bone sandwiched between them.
• Long Bones:     * Examples: Humerus, radius, ulna.     * Structure: Significantly longer than they are wide; characterized by a shaft and two distinct ends.
• Short Bones:     * Example: Carpal bones.     * Structure: Roughly cube-shaped with similar dimensions for length, width, and height.
• Irregular Bones:     * Example: Vertebrae.     * Structure: Complex shapes that do not fit into the other categories.
• Sesamoid Bones:     * Definition: A special type of short bone found within the tendons of origin or insertion of certain muscles.     * Functions:         * Provide a smooth surface for tendons to slide over, reducing friction.         * Act as a fulcrum to provide extra leverage to the muscle, enhancing the transmission of muscular forces.         * Displace the axis of force by altering the direction of tension within the muscle tendon.

Gross Anatomy of Long Bones and Growth

• Centers of Ossification: Long bones possess at least 3 centers of ossification.
• Diaphysis: The shaft of the bone.     * Contains the medullary cavity, which holds yellow marrow.
• Epiphysis: The end regions of the bone.
• Physeal Growth Plate:     * Located between the epiphyses and the diaphysis in young animals.     * Comprised of cartilage cells.
• Clinical Relevance of the Growth Plate:     * It is the primary site for fractures, infections, metastasis, and the effects of endocrine bone disorders.     * Case Example: A canine, $8\,\text{months old}$, showing a normal growth plate versus a fracture in the hindlimb.

Bony Prominences and Remodeling

• Wolf’s Law: "Form follows function." Bones undergo remodeling in response to the specific stresses placed upon them.
• Non-articular Prominences: These serve as attachment sites for muscles or passages for vessels and nerves.     * Examples from Scapula: Spine of the scapula, supraspinous fossa, and infraspinous fossa.
• Articular Prominences: These features make up the actual joints.     * Example: Glenoid cavity of the scapula.

Arthrology: The Study of Joints

• Definition: Derived from "Arthro-" (joint) and "-ology" (study of). A joint is the point of contact, or articulation, between two or more bones or cartilages.
• Purpose: To provide support and movement to the skeleton (though not all joints are movable).
• Structural Classification of Joints:     1. Fibrous Joints: Articular surfaces are united by strong connective tissue. Bones may fuse over time (synostosis). These allow almost no movement or no movement at all.     2. Cartilaginous Joints: Articular surfaces are united by cartilage. These allow limited movement (e.g., intervertebral discs in the human spine).     3. Synovial Joints: These feature a joint cavity between articular surfaces filled with synovial fluid. These are the most movable joints.

Anatomy and Classification of Synovial Joints

• Core Components:     * Articular Surfaces: Protected by articular cartilage.     * Joint Cavity: A fluid-filled space enclosed within a joint capsule. A single joint capsule may contain multiple synovial compartments or sacs.     * Joint Capsule: Provides strength and resistance. It is highly vascularized and innervated. It produces synovial fluid for the lubrication and nutrition of articular cartilages.
• Accessory Structures:     * Ligaments (lig.): Bands of tough fibrous connective tissue.         * Extracapsular: Located outside the capsule (e.g., collateral ligaments of the elbow joint).         * Intracapsular: Located inside the capsule (e.g., cruciate ligaments of the knee joint, such as the caudal cruciate ligament).     * Meniscus (pl. menisci): Fibrocartilages located within the synovial cavity that allow articular surfaces to fit together properly (e.g., medial meniscus in the knee).
• Classification Systems:     * By Number of Bones:         * Simple: Between 2 bones (e.g., glenohumeral/shoulder joint).         * Compound: More than 2 bones (e.g., humeroradioulnar/elbow joint, carpal joint).     * By Fit (Congruency):         * Congruent: Articular surfaces fit well together (e.g., humeroradioulnar joint).         * Incongruent: Articular surfaces do not fit well together (e.g., knee joint).     * By Shape and Motion:         * Hinge: Least versatile; allows only flexion and extension (analogous to a door hinge). Example: elbow joint.         * Spheroidal (Ball & Socket): Most versatile; allows all ranges of movement (analogous to a sphere moving all around). Examples: shoulder joint, hip joint.         * Others: Condylar, ellipsoidal, etc.

Basic Synovial Joint Movements

• Gliding: Two surfaces sliding on each other.
• Angular Motions:     * Flexion: Decreasing the angle between limb segments.     * Extension: Increasing the angle between limb segments.     * Abduction: Moving a limb away from the median plane.     * Adduction: Moving a limb toward the median plane.
• Rotational/Complex Motions:     * Circumduction: Movement circumscribing a cone shape.     * Medial/Lateral Rotation: A "rolling pin" movement on the axis of the limb.     * Pronation: Turning the forearm (antebrachium) downward (consistent pronation is seen in domestic animals).     * Supination: Turning the forearm upward.

Myology: The Study of Muscles

• Definition: Derived from "myo" (Greek for muscle) and "-ology" (study of).
• Locomotor System (Apparatus) Components:     * Passive Components: Bones and joints.     * Active Component: Muscles.
• Clinical Importance: Lameness may be caused by damage to muscles, tendons, or ligaments, not just bone or joint pathology.
• Skeletal Muscle Anatomy:     * Muscles attach to bones via tendons.     * Tendon of Origin (Proximal Attachment): A relatively fixed point with less movement than the insertion end.     * Muscle Head or Belly: The contractile portion of the muscle.     * Tendon of Insertion (Distal Attachment): The relatively more motile end; experiences more movement than the origin.     * Aponeurosis: A flat, sheet-like tendon that allows a muscle to have a broader attachment area.

Accessory Structures of Muscles and Fascia

• Accessory Structures:     * Synovial Bursa: A synovial fluid-filled "balloon" that protects a tendon from a bony surface.     * Retinaculum: A fibrous band that holds down one or more tendons to a bone.     * Synovial Tendon Sheath: A synovial fluid-filled "sleeve" or "sock" that completely surrounds a tendon to ease its gliding between a retinaculum and bone.
• Definitions and Differences:     * Ligaments: Attach bone to bone (usually part of a joint); occasionally tendon to bone.     * Tendons: Attach muscle to bone or other tissue surfaces.     * Retinacula: Bands holding down tendons close to the bone surface.
• Fasciae:     * Fibrous layers that envelop and isolate muscle groups and individual muscles.     * Superficial Fascia: Loose connective tissue that attaches skin to underlying muscles.     * Deep Fascia: A leaf of dense connective tissue (e.g., thoracolumbar fascia) that separates muscle groups into fascial planes. Some muscles may originate or insert here.

Muscle Nomenclature (Naming Strategies)

• Shape: e.g., trapezius m.
• Size: e.g., latissimus dorsi m. ("the widest m. of the back").
• Relative Position: e.g., superficial, middle, and deep gluteal mm.
• Origin and Insertion: e.g., sternocephalicus m. ("sternum to head" muscle).
• Function: e.g., extensor carpi radialis m. ("radial extensor of the carpus m.").
• Structure: e.g., triceps brachii m. ("three-headed m. of the arm").