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Vet Science Sample LessonVS003 The Skeletal System 
MAIN IDEA: What is the function and makeup of the skeletal system? The skeleton gives mammals shape and support. It is the combination of bones and muscles that allows movement. Bones are active tissues that adapt to changes within the animal. The skeleton, although very hard, allows the animal to adapt and grow. VETERINARIAN'S LOG: I heard a familiar bark when I pulled into my second farm call of the day. Missy, a 120-pound Saint Bernard, let everyone know that I had arrived. I first met Missy in October of 2000 after she was injured while working on the farm. Her owner had driven his pickup to cut firewood. Dutiful Missy rode along in the bed of the truck. When the truck stopped, Missy jumped off, just like she had done many times before that day. Unfortunately, this time Missy's right hind leg became caught and she fell. Missy was crying in pain and not able to bear weight on her injured leg when the farmer brought her to our office. My associate, Dr. Deppen, examined her. It was obvious that the leg was very swollen and likely broken. Dr. Deppen took a radiograph of Missy's tibia, the bone in the lower leg. It was apparent from the radiograph that the tibia had been broken into several pieces. We offered to refer Missy to an orthopedic surgeon, a veterinarian that specializes in surgery of the bones. Knowing a referral of this type can be quite expensive, the owners wondered if we could repair the bone ourselves. Dr. Deppen and I discussed the options. We both felt that considering the severity of the fracture, a cast or splint was not likely to be successful. Conversely, we could attempt to perform the needed surgery. I called the farmer to offer the choices at hand. I first told him that I am not an orthopedic specialist and that the fracture was quite severe. Then, I explained that I could attempt the surgery. He agreed to allow me to perform the surgery knowing that the operation might not be successful. I obviously needed a thorough knowledge of bones before I could start to repair this damaged leg bone. BONE STRUCTURE: The long bones are present in the legs (and arms in humans). The femur and humerus are classified as long bones. They have a dense outer shell and a hollow shaft. Bone marrow is made in this hollow center, the medullary cavity. Bone marrow in turn produces blood cells. Bone marrow is also present within many of the flat bones, including the sternum and ribs. Splitting a long bone along its length shows the typical structure of bone. The outer shell is composed of dense or compact bone. The more force is placed on a bone, the thicker this layer will be. In the femur, this compact bone is thickest in the middle of the shaft where the greatest strain occurs. Within compact bone lies a more loosely arranged bone, called spongy or cancellous bone. Spongy bone is found within the long bones, but not inside the flat bones of the skull or pelvis. It only fills the ends of these bones. Spongy bone is made up of tiny spicules and plates of bone. The spicules look random, but are really arranged to maximize strength. The medullary cavity is located in the hollow center of the shaft. A dried bone is composed of about 70% inorganic minerals and 30% organic components. The inorganic minerals have a high level of calcium and phosphorus. This is found as crystals of hydroxyapatite (3Ca3(PO4)2*Ca(OH)2). The organic portion contains collagen fibers and cells. The fibers provide a framework on which the hydroxyapatite crystals can be deposited. While organic fibers give the bone a small amount of elasticity, minerals give bone its typical hardness and strength. The collagen fibers and the hydroxyapatite crystals surround the cells. BONE CELL TYPES: There are three types of bone cells. Osteoblasts lay down the collagen matrix. These osteoblasts become encased in matrix, becoming osteocytes. The osteocytes are responsible for maintaining the bone matrix. Osteoclasts are large multinucleated cells that release the minerals from bone. Bone is a living tissue that is always being remodeled in response to physical forces on the body and the body's need for calcium. RADIOGRAPHS: Radiographs are produced when a stream of x-rays is passed through a body part, exposing a piece of photographic film. X-rays are a form of electromagnetic radiation that can pass through living tissue. Although the term x-ray is commonly used to describe the resulting picture, that is not a correct usage. It is technically correct to refer to the photograph as the radiograph. Invisible x-rays expose the film; they are not the photograph that results from that exposure. Dense structures allow the smallest amount of x-rays to pass and are termed radiopaque. Tissues such as teeth and bone fit into this category and appear light on the radiograph. Radiolucent tissues allow much more of the x-ray energy to pass through and show up dark on the radiograph. Variation among tissues permits the radiograph to be interpreted. In the radiograph of Missy's leg, the bone appeared bent in the middle, confirming the fact that the bone had been fractured. A fractured bone, like the one we found in Missy the Saint Bernard, is a common abnormality. If the bone is broken into two pieces -- a clean break -- it is called a simple fracture. A comminuted fracture results in several fragments of bone. Missy's radiograph showed that she had a comminuted fracture. A compound or open fracture results when one of the bone ends punctures the skin. Higher risk of bone infection exists with compound fractures. CLINICAL PRACTICE: In order for a fracture to heal, the bone ends must be put back in alignment and held without movement. Just as in humans, many animal bone fractures can be repaired with a cast or splint. In veterinary medicine, the support must be made to hold the weight of the animal. After correction, the animal must be able to use the leg. Very active pets have a hard time keeping a cast in place and clean and dry. Surgical correction is often used to repair fractures. With surgery, some form of surgical stainless steel, such as a bone plate, is used to support the bone. A bone plate is applied to the outside edge of the bone and attached with screws. We offered referral for Missy for this type of surgery. Bone plates offer a very stable form of correction. When this surgery is performed, properly sized plates must be correctly shaped to fit the bone. Another method of repair is termed an intramedullary pin. I used this type of correction on Missy. During the surgery, I drove a stainless steel pin into the center of Missy's broken tibia. The pin entered the bone on the top of the tibia and entered the medullary cavity. The pin was driven to extend in the distal piece of tibia, very close to the hock joint. Because the bone was shattered into many pieces, the pin alone would not be sufficient to support the bone. To stabilize the pieces, I also added several wires wrapped around the bone fragments. These are called cerclage wires. Radiographs are taken to ensure proper placement of the pin before completing the surgery. One possible mistake would have been to drive the pin too far, causing irritation of the hock joint. Satisfied with the placement and security of the repair, I finished the surgery. For the first two weeks, I also kept a padded splint on Missy's leg. This gave a little more support and helped to minimize swelling. After the surgery, Missy's bone began to repair itself. The process of bone healing is very similar to that of bone growth in an immature animal. Because of the trauma to the bone, blood fills the gaps between the fracture ends. Cells move into this clot, laying down cartilage. This initial structure, which helps to stabilize the fracture site, is the beginning of a firm structure called a callus. After approximately two weeks, the cells develop into osteoblasts. This cartilage callus gradually turns into bone. This bony callus not only fills in between the fragments but also extends into the marrow cavity and outside the edge of the bone. Two months after the surgery, the bony callus in Missy's leg has filled the gaps between the fracture ends. It also extends outside the original bone margin and has actually covered the cerclage wires. Over the following months and years the callus will continue to be remodeled. Missy's case was quite successful. Not all bone repairs end as favorably. Had my repair allowed for even a small amount of movement after the surgery, the fracture would not have healed correctly. Having the edges in alignment and completely immobilized is essential for healing. Bone infection, especially in an open fracture, is another reason that fractures sometimes do not heal properly. EXERCISES: 1. Share any personal experiences with broken bones with the class. Be sure to identify which bone was fractured and what type of repair was required. 2. Start this experiment by cleaning two similar type chicken bones. Allow one bone to dry on a windowsill and submerge the other bone in vinegar for four days. At the end of the four days, check the hardness of each. Describe the two bones. Can you guess which mineral has been depleted in the bone which is pliable? This exercise is adapted from several demonstrations found at the Newton's Apple Web page about broken bones. 3. Visit the Leg Injures in Horses Web site to learn more about common leg injuries in horses. Is it true that any horse with a broken bone must be put down? INTERNET RESOURCE:
** Newton's Apple - Broken Bones TEST: 1. What is produced in the medullary cavity?
2. Name the three bone cell types. 3. An X-ray is a picture. TRUE or FALSE? 4. A bone broken cleanly into two pieces is referred to as having a __________ fracture. 5. Which type of bone break is most susceptible to infection?
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VS003 The Skeletal System
TEACHER'S GUIDEOBJECTIVE: Describe the structure of bone. Name the three types of bone cells. Differentiate between X-rays and radiographs.PREPARATION: Review the material in the written lesson and perform the listed exercises. You will need jars of vinegar and chicken bones for exercise #2. REFERENCE: Baker, MeeCee and James Lawhead. Introduction to Veterinary Science. Albany, N.Y.: Delmar Publishers, expected date of publication 2001. INTERNET RESOURCE:
** Newton's Apple - Broken Bones IMPORTANT TERMS: callus, comminuted fracture, compound or open fracture, intramedullary pin, medullary cavity, orthopedic surgeon, osteoblast, osteoclast, osteocyte, radiograph, simple fracture, spongy or cancellous bone, x-rays. EXERCISE ANSWERS: 1. Students will likely have a variety of personal experiences to share about broken bones they have experienced. The material covered in this lesson may help them to better understand how diagnosis was made and treatment carried out. 2. Students should find that the bones soaked in vinegar will be pliable after four days. Calcium is the mineral that has been depleted. 3. The Leg Injuries in Horses Web site illustrates that not all broken bones in horses require the animal to be put down -- successful repairs are possible, depending on the injury. TEST KEY: 1. What is produced in the medullary cavity? Bone marrow
2. Name the three bone cell types. 3. An X-ray is a picture. TRUE or FALSE? FALSE. An x-ray is a form of electromagnetic radiation that can pass through living tissue. The picture taken with the use of x-rays is called a radiograph. 4. A bone broken cleanly into two pieces is referred to as having a simple fracture. 5. Which type of bone break is most susceptible to infection?
Compound or open END
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