Bone and Joint Anatomy: Classification, Structure and Function

Tma 3. Generalidades de la Osteología

Classification of Bones:

• Long bones: typical of the limb members. Their length reflects both the rapidity of movement and the potency of the movement, which characterizes their predominance over the others. They are large and support traction. Tubular in shape, with a first central cavity (medullary cavity), they widen at the articular extremities (epiphysis), which have centers of ossification and multiple frequencies.
• Short bones: they are characterized by having no predominant dimension over the others. They support compression forces, without the capacity to resist much tension. They tend to have a compact outer cortex and an inner zone entirely trabecular. Examples are found in the carpus and tarsus.
• Examples: vertebrae, cuboids, scaphoids.
• Irregular bones: they comprise any bone not easily assigned to any of the previous groups.

Both forms of structure are affected by genetic, metabolic and mechanical factors, and each long bone represents the result of a functional history through countless successive generations.

The surface shows depressions and elevations that interrupt the bones:

• A depression is called a pit. Depressions that are long are known as furrows.
• A notch is called a notch, and a hole in the bone is a foramen.
• A projection called processes is large; if it is long, thin or pointed, it is called a spine.
• A rounded projection is a tubercle.
• Elongated elevations are called crests.
• Epicondyle: a projection located above a condyle, which usually serves as the attachment point for collateral ligaments.
• The expanded proximal extremity of some bones is known as the head.
• A channel is a hole in a bone; when long, these holes are called canals.
• A groove shaped like a pulley is a trochlea.

Its smooth surface allows smooth movement, adapted to articulations, although most of them are covered by articular cartilage, which covers the surfaces of the synovial joints.

Main function: bone is a very consistent living tissue, resistant to shocks, compressions and also elastic for traction. It serves as a barrier against external aggressions, as a lever arm and anchor point for muscles and tendons, as a support and protection for vital organs such as the heart, lungs, brain, etc. It also allows the movement of body parts and the locomotion necessary for work or sports activities, for individual stability and displacement.

Bone forms the locomotor axis, causing the skeletal structure and supporting the body and the muscles depending on their location.

Bone also acts as a reservoir of calcium (indispensable for muscle contractions), and intervenes in hemostasis and hematopoiesis (formation of blood cells).

Composition of bone: composed of organic matter (64%), inorganic (34%) and water (2%).

• Fibers: 95% of the matrix is organic.
  • A) Collagen I: formed through tropocollagens. They are characterized by being arranged in a helical pattern.
• Fundamental substance: matrix in the form of a gel, formed by:
  • A) water
  • B) proteoglycans: sulfated glycosaminoglycans (hyaluronic, keratan and chondroitin)
  • C) mineral salts: tricalcium phosphate, calcium carbonate and calcium citrate.
  • D) ions: Ca, Fl, F, Zn, Na, K, Mg.

Structure of mature bone: in concentric sheets there are 2 types of bone tissue: compact and spongy.

• Compact bone tissue: found in the middle part of long bones, enveloping the spongy bone. It forms the shaft (the body of long bones, which is between the epiphyses, or the middle of the distal diaphyses). It appears as a solid mass, and its structure can only be seen with light microscopy.
  • The lamellae are arranged in 3 forms:
    • Concentrically around a longitudinal vascular channel (called the Haversian canal), which contains capillaries and postcapillary venules and arterioles. These cylindrical structures form the so-called Haversian systems (osteons), visible with the optical microscope.
    • Interstitial lamellae, which are located between the osteons, formed by the remains of the Haversian systems, cemented by globular matrix (a layer of fibers poor in collagen that is crossed by canaliculi, or small vascular positions that are not observable under optical microscopy).
    • Circumferential lamellae, which lie under the internal and external surfaces of the bone, forming a circumference that extends around the bone (outer and inner circumferential lamellae).
  • Channels that communicate the Haversian canals with the medullary cavity or surface are transverse or oblique channels called Volkmann's canals, through which vessels pass from the periosteum and the medullary cavity. Optical microscopy recognizes them because they are not surrounded by concentric lamellae.
• Spongy bone tissue: found in the epiphyses of long bones and in the flat and short bones. Formed by a network of trabeculae (bone spicules).
  • Spongy tissue in the interior of bone gives rise to hematopoiesis (in a place called red marrow).

Ossification of bones

• 2 types of ossification:
  • Intramembranous ossification (mesenchymal)
  • Intracartilaginous ossification (endochondral)

Intramembranous ossification (mesenchymal)

• It consists of direct mineralization of a highly vascular connective tissue. Ossification proceeds from several centers, in which mesenchymal cells proliferate and differentiate, experiencing an intense blood supply around a capillary.

Intracartilaginous ossification (endochondral)

• Most bones in humans are preformed in cartilage during early fetal life. The long bone is first a model prefigured by hyaline cartilage, which is replaced by a similar condensed mesenchymal model, and both are predicted early on.

Growth of bones

• Most human bones start as preformed hyaline cartilage; some are initially mesenchymal.
• Soft tissue gradually becomes ossified, beginning at a center that extends toward the ends.
• These ossification centers appear throughout a prolonged period, mostly during embryonic life.
• Most bones ossify from several centers; ossification proceeds from the ends, which are still cartilaginous at birth.
• The shaft has a primary ossification center, while the epiphyses ossify from secondary centers.
• As the epiphyses increase in size, almost all cartilage is replaced by bone, except for a layer of hyaline cartilage that persists at the articular surface and the thin zone between the shaft and the epiphysis.
• The persistence of the epiphyseal plate allows bones to increase in length until they reach their usual dimensions, at which point they reach maturity.

Tma 4. Generalidades de la Artrología

• The study of arthrology covers the functional topography and the anatomical variation of the joints.
• Joints show differences in growth and transmission of external movements.
• Solid joints (non-synovial) are known as synarthroses and are usually grouped according to the main type of intermediate connective tissue: fibrous and cartilaginous joints.
• Both characterize synarthroses in almost all cranial joints.
• Cavitated (synovial) joints are few, with some synovial exceptions located at the ends or other surface areas of endochondral bones.
• Gomphosis: a fibrous joint specialized for fixing teeth in the alveolar sockets of the upper and lower maxillae.
• Syndesmosis: a fibrous joint in which bones are connected by an interosseous ligament, which usually allows slight movement.

Synovial joints

• The bones participate with a fibrous capsule, and their articular surfaces are not continuous with each other. Articular cartilage covers them, a layer of specialized hyaline cartilage with variable thickness and typology, in contact with a stable surface. Its cartilaginous surface has a very low coefficient of friction, facilitated by sliding contact lubricated by synovial fluid.
• The fibrous capsule surrounds the entire joint; the capsule has a synovial membrane lining, which also covers all non-articular surfaces, including those not in contact with bones, and partly tendons and ligaments inside it. The fibrous capsule completes the joint, as in the shoulder and elbow.
• The synovial membrane does not cover other intra-articular structures, such as the meniscus.

Articular surfaces:

• They are formed for the most part by a special variety of hyaline cartilage. The articular cartilage surface is resistant to wear, with low friction and lubricated, somewhat compressible and elastic, which is why it adapts easily to similar surfaces, while also being capable of absorbing large compression forces.
• Fibrous capsule: fibrous joints.
• Synovial membrane: lines the non-articular parts of synovial joints, including bursae and tendon sheaths; all these regions exist in movement, and the surfaces are lubricated by a fluid similar to the clear part of an egg, which is absorbed by the membrane.
• Synovium (synovial lamina propria): formed by synoviocytes, the functional intimal synovial cells, which include the layer of debris into the joint cavity and synthesis of some synovial fluid components. The composition of synovial fluid is similar to plasma dialysate, except for a higher protein content, especially mucin, added to the blood, mostly hyaluronate.
• Its functions include providing synovial fluid to the joint with little variation in pH, nourishing the articular cartilage surfaces, cartilage, menisci and discs, along with lubrication and decreasing erosion.

Classification and movements of synovial joints

• Most synovial joints have complex surfaces (convex and concave). In some, a complete convex surface (condyle) faces a concave opposite surface.
• Degrees of freedom: joints that move on a single axis have 1 degree of freedom; those that move on 2 axes have 2 degrees of freedom; and if there are 3 axes they have 3 degrees of freedom.
• Rotation on 1 axis: uniaxial, 1 degree of freedom.
• Independent movements around 2 axes: biaxial, 2 degrees of freedom.
• Some joints may have a 3rd degree of freedom.

General classification of synovial joints

• 1. Plane joints: the apposition of nearly flat surfaces; the most common is a slight curvature (e.g., cervical vertebral articulations).
• 2. Trochlear (pulley) or ginglymus joints: articular surfaces shaped like a pulley, with massive movement limited to one plane.
• 3. Trochoid (pivot) joints:
  • Has a pivot bone in a ring
  • Ligamentous, which only allows rotation of the pivot around its axis.
• 4. Condylar joints: biaxial. Ellipsoidal articular surfaces in 2 axes, with a surface spacing similar to an ellipsoid, also in 2 axes corresponding to the anterior ones.
• Bicondylar joints: uniaxial.
• 5. Saddle joints (mutual or reciprocal snap): biaxial. They have concave-convex surfaces, convex in one direction and concave in the perpendicular one.
• 6. Ball and socket joints (or spheroidal joints): formed by a spherical head and a cup-shaped socket. Multiaxial.

Menisci and discs: formed by fibrocartilage, attached to the joint capsule, where they have vascularization and innervation.

• Joints with menisci are divided into:
  • Complete menisci: a meniscus that completely divides the joint capsule.
  • Incomplete menisci: do not divide the joint capsule.
• Discs also attach to the joint capsule, and their section is triangular.
• The increased capacity allows menisci to change the movement of certain joints in response to forces acting on the joint.

Joint capsule: a fibrous cuff whose inner layer is continuous with the periosteum of the articular surfaces.

Tma 5. Generalidades de la Miología

• Myology has as its function the mechanical response of contractile proteins. Muscle tissue has the ability to regenerate, which is replaced by muscle fiber tissue.
• Muscle cells are known as muscle fiber or muscle cell.
• Smooth muscle tissue: fibers with a fusiform shape and a central nucleus. Found in parts of the viscera, with involuntary contraction (vegetative nervous system).
• Heart muscle: longitudinal fibers with a striated appearance, with a single central nucleus. Exist only in the middle layer of the heart.
• Skeletal muscle: longitudinal fibers with a striated appearance and several nuclei located peripherally in the fiber. Found in the locomotor axis.
  • Somatic nervous system (voluntary contraction)
  • Mechanically powerful, very rich in mitochondria.
• Skeletal muscle: fascicles of smaller fibers form the muscle. It presents a cross-section of the fiber that is elliptical or circular, with a diameter of 10 to 100 microns.
• Muscle fiber characteristics: the main nucleus can be observed near the periphery.
  • The cytoplasmic membrane is called the sarcolemma
  • The cytoplasm is called the sarcoplasm
  • The fiber is formed by myofibrils, characterized by a striated appearance along their entire length.
• Within a myofibril are:
  • A dark band: the A band
  • A clear band: the I band
  • The A band is divided into 2 by a clear band, the H band.
  • The dark zone is the Z line.
  • There are 2 types of contractile proteins:
    • Actin: thin filaments that form the I band and Z line.
    • Myosin: thick filaments.
• Myosin is the most abundant (55%), in the form of longitudinal filaments. Each filament consists of about 180 molecules, grouped with their heads pointing outward. Actomyosin is the cause of muscle contraction.
• Actin (23%) is divided into:
  • F-actin
  • G-actin
• Tropomyosin: constitutes 6% of the myofibrillar contractile proteins.
• Troponin: represents 6% of the myofibrillar contractile proteins. Formed by 3 units or molecules:
  • Troponin T: binds to tropomyosin.
  • Troponin I: the enzyme capable of inhibiting ATPase activity, which allows ADP and ATP to be transformed.
  • Troponin C: binds calcium ions.
• Structural proteins:
  • Alpha-actinin: the ends of actin molecules.
  • Protein M: forms myosin filaments; leads to the M band.
  • Protein C: binds myosin molecules.
  • Protein I: related to magnesium.

Relaxed muscle

• Interposed between tropomyosin, actin and myosin heads. Calcium is needed, and thanks to the uptake of calcium, in part through troponin C, calcium ions increase, which activate the molecules of the other 2 troponins.
• Troponin I is released by ATP and ADP energy production, and troponin T is displaced to tropomyosin.
• The myosin heads bind to actin and cause energy release through angulation of the myosin heads.

Classifications

• 1. By length:
  • Long: muscles in which one axis predominates over the other 2; displacements are large and potency is great.
  • E.g., sartorius
  • Wide: muscles in which 2 axes predominate over another. Located in the body wall. Abundant in the abdomen.
  • Short: muscles in which no axis predominates over the others. Used for small displacements and great potency.
  • E.g., quadratus femoris, supraspinatus
• 2. By belly shape:
  • Fusiform: fibers are arranged so that the belly is wider in the center and narrow at the ends. Large displacements cause little strength.
  • Triangular: fibers radiate in an oblique direction. E.g., middle adductor.
  • Fusiform muscles: large displacements cause greater strength and tend to have less intense contraction. E.g., coracobrachialis.
• 3. By how fibers attach to tendon:
  • Pennate: fibers converge on the tendon from both sides, forming the belly. E.g., rectus anterior of the quadriceps.
  • Semipennate: fibers converge on the tendon from one side only.
  • Fibers in a transverse position are very potent.
  • Spiral fibers displace only in one plane.
  • Digastric: the muscle is made of two bellies joined by an intermediate tendon, and then continues to insert. E.g., omohyoid.
  • Polygastric: if it has more than one intermediate tendon.
• 4. By origin:
  • Biceps: 2 origins or heads.
  • Triceps: 3 origins or heads.
  • Quadriceps: 4 origins or heads.
• 5. By insertion:
  • Bicaudal: 2 insertions.
  • Tricaudal: 3 insertions.
  • Polycaudal: more than 3 insertions.

Specialized insertion structures

• Tendon: an aggregate structure composed of collagen fibers arranged in parallel. The greatest example of this arrangement is the tendon, which is nearly non-elastic and has little flexibility, and is resistant to extension.
• Cells found in loose connective tissue (tenocytes).
• The whole is called the endotendon, which is a sheath wrapped in a paratenon. Its prolongation continues with the periosteum at one end and with the perimysium at the other.
• Some tendinous fibers can calcify when there is an alteration in the production and use of the tendon, known as sesamoid fibers.
• The tendon has a sheath that protects it, consisting of an outer parietal layer and an inner visceral layer, which is the synovial layer. Spaces exist between them containing synovial fluid.
• The tendon transmits muscle force to the bones and has great resistance, little extensibility and is able to slide.

Aponeurosis:

• Exhibits connective tissue composed mainly of collagen fibers.
• Acts as a flattened tendon.
• Can be attached to bones, or to other aponeuroses of the skin.
• At one end, other muscle fibers attach.