Bone and Blood Supply to Bones

I Introduction:

1. The skeletal tissues of vertebrates consist primarily of the hard tissue bone and the somewhat softer tissue, cartilage.
   • At the beginning let us distinguish between BONE, the Tissue, BONE, as an Organ System (Which includes the blood producing marrow as well as the structurally supportive compacta), and an anatomical BONE, such as the Scapula or Femur.

   II Skeletal Tissues.

   A. Hard tissues.
   
   1. General
   
   • (a) Hard tissues owe their structural strength to calcification. They are "two phase" materials composed of a rigid mineral component (2/3rds) and an organic component (1/3rd).
   • (b) The inorganic portion is hydroxyapatite, a mineral,
     (Ca-5(PO4)-3OH)
     interspersed in an organic collagen network.
   • (c) These tissues resist compression, tension, and shear.
   2. Types of hard tissues.
   • (a) bone, in vertebrates as a whole there is both cellular and acellular bone, but only cellular bone is found in mammals. Mammalian bone has 30% organic material.
   • (b) enamel: a highly mineralized hydroxyapatite tissue, the hardest tissue in the vertebrate body, brittle, with only 3% organic material
     • *confined largely to the surface of the teeth in mammals; lower vertebrates have enamel on such external structures as bony scales
     • *an ectodermally derived skeletal tissue - the other hard tissues are mesodermal in origin.
   • (c) dentine: between bone and enamel in hardness, found in association with teeth, in common with bone has 30% organic material.
   B. Other skeletal tissues:
   • 1. cartilage: a deformable tissue consisting of collagen embedded in a mucopolysaccharide ground substance.
   • *It is almost as resistant to tension as bone but somewhat less resistant to compression. It tends to deform under shear. Some calcified cartilage approaches bone in resistance to shear.
   • *Cartilage is an avascular tissue. When it becomes heavily calcified the chondrocytes die from malnutrition so repairs do not occur in calcified cartilage.
   • *There are several types:
     • (a) Hyaline - clear cartilage typically found on joint surfaces, the cartilage of the growth plates, nasal cartilages, the stiffening rings in the airways of the respiratory tree, cartilage support structures of larynx.
     • (b) Fibrous - contains connective tissues fibers- found between bones in certain areas: portions of intervertebral disks, attachments of certain tendons and ligmaents to bones, menisci of the knee, sternum.
     • (c) Elastic - contains elastin fibers - occurs in the external ear, the external auditory canal, parts of the larynx, eustachian tube and the epiglottis.
     • (d) Calcified - contains mineral deposits, may be almost as mineralized as bone. However the mineral lacks the precisely arranged crystal systems interspersed with regularly arranged cells seen in bone.
   • 2. Notochord: This specialized skeletal material is the "first backbone" of vertebrate embryos. Its rigidity is due to the hydrostatic pressure of its inflated cells within the notochordal sheath. In adult mammals it persists only as the gelloid pulp material of the intervertebral disks.

   III. Gross Anatomical Attributes of Bones

   A. Systems of Classification vary:

   • 1. Location - i.e. axial, appendicular, splanchnic.
   • 2. Shape - long (femur), short (carpals), flat (skull bones) and irregular (vertebrae).
   • 3. Mode of growth -
     • (a) Endochondral or replacement - bones preformed in cartilage - almost all of the skeleton of mammals.
     • (b) Membrane - bones that ossify directly in connective tissue - the clavicle, many of the bones of the face and dorsal braincase.
   • 4. Details of internal structure - lamellar, haversian, cancellous (spongy).

   B. Names of bones are Latin or Greek in origin.

   They may refer to shape, function or location. The parts of bones also have Latin based names: tuberosity, condyle, etc.

   C. Parts of a typical long bone:

   • 1. Grossly identifiable structures include: Shaft (diaphysis), ends (epiphysis), covering (periosteum), trabeculae, lamellar layer, processes, protrusions, pits, muscle "scars", tendon "scars", joint surfaces, nutrient foramina.
   • 2. Microscopically we are concerned with the structure of most bone in which a central nutrient canal is surrounded by concentric layers of bony tissue in which the bone cells, osteocytes, are embedded.
   • They communicate with the nutrient canal by means of tiny haversian canals. Such a complex is called a "Haversian system".
   • These systems are constantly renewed due to the remodeling of bone.

   IV. Blood Supply in Bones

2. For the orthopedist this is the clinically most important aspect of the gross anatomy of bones.
3. The arrows show the main directions of blood flow.
   • (A) Blood supply to the diaphysis - blood enters the diaphysis through the main nutrient artery - this ramifies through the marrow cavity and sends branches into the compacta - the inner 2/3rds of the compacta is supplied from the marrow. The outer 1/3rd may be nourished by arteries penetrating from the periosteum. (B)
   • Blood supply to the epiphysis - the epiphysis is supplied by numerous vessels entering either through the growth plate (if much of the epiphysis is covered by joint cartilage) or between the growth plate and the joint cartilage.
   (1) In the first instance, as in the femoral head where the joint cartilage covers almost all of the epiphysis, a fracture could deprive the epiphysis of its blood supply, leading to bone death, disruption of growth and subsequent destruction of the structure (avascular necrosis).
   • (2) In the second case, where there are gaps between the joint cartilage and the epiphysis, fractures of the epiphyseal plate, if properly set, will have little permanent effect.
4. (C) Because the marrow blood supply is so important to the health of the bone, too large a surgical pin, one that occludes the marrow cavity, will result in the death of much of the diaphyseal bone. Early experiments with surgical pins resulted in almost immediate use of limbs that then refractured some 18 months later. Similar problems can occur in multiple fractures where major fragments are separated from their blood supply. A pin must be smaller than the marrow cavity.

   V. Growth.

   Cartilage is able to grow by addition of material within the matrix while bone only grows by addition on its surfaces.

   1. Growth of Bones

   • (a) A remarkable aspect of skeletal growth is that the animal uses the skeleton while enlarging it (in contrast arthropods have a helpless soft stage in which they shed the old skeleton and produce a new larger skeleton).
   • (b) A typical endochondral bone has several centers of bone formation (ossification).
     *The Primary center is the diaphysis or shaft.
     *The epiphyses (growth caps) occur at the ends of the long bone and may constitute one or more Secondary centers of ossification.
     *During growth the epiphyses are separated from the diaphysis by sheets of cartilage = the epiphyseal cartilages or growth plates (physis in radiology).
     
   • (c) Growth in diameter occurs by addition of material under the periosteal membrane of the shaft in concentric layers (APPOSITION).
   • (d) Bones grow in diameter but in mammals the amount of boney cortex remains fairly constant throughout growth - the center is resorbed as material is added to the surface.
   • (e) Mammalian bones grow in length by addition of material on both sides of the epiphyseal cartilages. When these cartilages are eliminated no further linear growth can occur. The rise in sex hormones at puberty triggers the elimination of the epiphyseal cartilages (= the closing of the growth plates)

   2. Growth abnormalities

   - many conditions of concern to the veterinarian are due to skeletal growth abnormalities; in addition many of our domestic breeds display skeletal structure that appears quite abnormal when compared to the structure of their wild ancestors (e.g. bulldog, short faced pig breeds, draft horses.)
   
   • Commonly encountered growth anomalies include:
     • (a) Absence of centers of ossification.
     • (b) Abnormal epiphyseal and/or appositional growth. Traumatic damage to an epiphyseal cartilage can result in deformation.
     • (c) Abnormal rates of replacement and remodeling of the bone.(Too fast or too slow)
     • (d) Dwarfs are commonly recognized in three broad categories - proportional dwarfs and disproportionate dwarfs and paedomorphic (or infant formed) dwarfs. Similarly giants may be normally proportioned or disproportionate.
       • (i) Hormonal dwarfs and giants are usually due to abnormalities in the growth hormones. Too little growth hormone and you remain quite small, too much and you become too large. Pituitary dwarfism can be treated with growth hormone. If left untreated the victum rarely achieves sexual maturity.
       • (ii) Achondroplasty and other forms of non-hormonal anomalous growth produce disproportionate individuals such as the human circus dwarf, the dachshund and the bulldog. There are many of these syndromes described. They are due to anomalies in the materials forming the developing skeleton. Since there are many different proteins in the connective tissue matrix of the developing skeleton as well as many enzymatic pathways involved in deposition and remodeling of bone, there are many different ways to create these disproportionate phenotypes. There is an increase in joint pathologies in many of these abnormal connective tissue conditions.
       • (iii) Genetic programming and environmental influence - the skeleton is affected by both genetic and environmental factors. Often these can act to produce similar end results. Thus poisoning of growing animals by sweet peas (a decorative flower Lathyrus) produces skeletal and connective tissue anomalies very similar to Marfan's syndrome caused by a genetic non-collagen connective tissue abnormality) = long slender limb bones, loose joints, dislocated lenses, dissecting aneurysm of the aorta. Similarly severe malnutrition in the young will mimic pituitary dwarfism.

   VI. Functional Considerations

   A. The skeleton, as an organ, constitutes the rigid support system of the body.

   B. Functions of bones.

   • 1. Mechanical Support and Protection
   • 2. Struts and Levers for delivery of force generated by the contraction of muscles.
   • 3. Reservoir for minerals, particularly calcium and phosphorus.
   • 4. Houses the (major) site of blood formation

   C. Mechanical characteristics of skeletal tissues:

   • 1. Strength of Bone - Fails under Compression at 19-30,000 lb/in2 (1220-2100 Kg/cm2)

     Resistance to Tension: 15,000 lbs/in2 (620 -1050 gm/cm2)

     Resistance to shear depends on the direction of the load- it is roughly 1/3 to 1/2 the resistance to compression.

     This is why many fractures are "spiral" fractures that occur when an animal misteps or trips. The misstep causes the animal to support weight on a leg that is not at the optimum angle for taking the weight.

   • 2. Physical characteristics of bone as a structural material.
     • (a) Bone as a two phase material is remarkably strong for its weight. The collagen matrix resists tension and binds together the rigid crystalline component which resists compression and shear.
     • (b) The small size of the hydroxyapatite crystals contribites to the strength of the bone because fractures propogate more easily through a uniform rigid substance such as glass, than through a composite substance such as bone or fiberglass. In a composite substance, failure may be confined to one crystal and not propagate.
     • (c) Wolff's Law and bone as a structural material.
       (i). It has long been known that bones are formed to best resist the stresses acting on them. "Wolff's law" dates from 1870:
       "every change in the form and function of bones or of their function alone is followed by certain definite changes in their configuration in accordance with mathematical laws."
       • (ii) Bones are self-repairing structural units, they are constantly undergoing remodeling which continues throughout the life of the organism. Micro- crystalline fractures are frequently repaired before they have an opportuntity to become major breaks.
       • (iii)The remodeling of bone allows bones to adapt over time to changes in load and function. Bones can lengthen their lever arms or become thicker even after growth in length has ceased.
       • (iv) The mechanism of this remodeling is due to the response of bone cells to piezoelectric fields produced when bone is stressed as a result of its two phase construction.
       • (v) An excellent discussion is found in Chapter 16 (pp 256 - 258) of: Banks, Applied Veterinary Histology, 1981 edition.
   • 3. Bones as structural units.
     • (a) Bones are "engineered" for maxiumum strength for their weight.
     • (b) They are frequently hollow at the centers of their shafts where the main stress is a bending stress. A hollow tube is stronger, for its weight, than a solid tube of the same weight.
     • (c) Shock absorption is enhanced by the ends by the presence of a "bony honeycomb" trabecular construction.
     • (d) Cross sections are shaped to best resist the strains encountered. e.g. Rounded mid shafts for long bones; the triangular shape for the top of the tibia, etc.
     • (e) Trajectories and internal struts - In portions of bones such as the head of the femur, where loads are highly directional, the trabeculae will be arranged to give maxiumum strength. These patterns of trabeculae are called "trajectories". In the absence of normal forces acting on the bone (as in a crippled animal, for example) they do not develop or are resorbed with time.
     • (f) Some bones, especially those of the axial skeleton, must serve several functions simultaneously. Thus the vertebral column both surrounds and protects the spinal cord and also serves as the linear stiffening rod for the body and as a dorsal support for the viscera.

   VII. Bones for the Veterinary Student - a summary.

   • The skeleton provides a frame of reference for anatomically and clinically significant features of the animal.
   • Many of the conditions you will treat in your patients, especially horses, and to a lesser extent dogs, will be conditions of skeletal or joint pathology.
   • Since the developing skeleton is molded by the forces acting on it an understanding of its growth, biomechanics and normal anatomy is absolutely essential to the management of performance animals.
   • Since the skeleton is constantly in a state of remodeling even the "everlasting bones" will be affected by chronic disease.

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   Readings

5. Banks, 1981, Applied Veterinary Histology., diagram on pp 146, pp 254-260.
6. Getty, R 1975 Sisson and Grossman's The Anatomy of the Domestic Animals, 2 volums: Vol 1. Chapter 2, General Osteology, pp 19-33.
7. Hildebrand, 1982, Analysis of Vertebrate Structure. Ch 21 - structural elements of the body.

   Additional References

8. Obviously there is much to be read on this subject.
9. Banks, 1981, Applied Veterinary Histology.
   Chapter 7 (Cartilage) and Chapter 8 (Bone) - will tell you more about bone histology than you need for this course but will also help with Histology.
10. Chapter 16, Musculo-skeletal System is quite relevant to this course.
11. Basmasjian, 1980, Grant's Method of Anatomy. A reference book on human anatomy that deals with many general principles of anatomy.
12. Gray's Anatomy, 35th British Edition. THE DEFINITIVE reference in Anatomy. Although devoted to only one mammal, man, the detail of anatomical information far surpasses that seen in any other anatomical reference.
13. Hildebrand, 1982, Analysis of Vertebrate Structure. A good comparative anatomy book that may be consulted for many general principles in anatomy, especially functional anatomy.
14. Getty, R 1975 Sisson and Grossman's The Anatomy of the Domestic Animals, 2 volums. One of the most comprehensive English Language veterinary anatomy texts.
15. Sumner Smith, G, 1982 Bone in Clinical Orthopedics, W B Saunders & Co. This is an excellent book and is written with a veterinary emphasis.
16. Weiss, 1983, Histology. The chapters on Bone and Cartilage also cover the microstructure quite well, but there is no chapter equivalent to Banks' Chapter 16.