Compact bone is hard due to mineral deposits of calcium carbonate and calcium phosphate (calcium and phosphorus). This bone consists of tightly packed osteons or haversian systems and many layers with few gaps on this tissue therefore it is dense and hard. The hard minerals and flexible collagen makes bone strong. Each osteon consist of concentric layers of hard mineralized matrix (concentric lamellae).
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Bones consist mainly of calcium. Calcium is important it is a cofactor for enzyme function, in maintaining cell membranes, in muscle contraction, nervous system functions, and in blood clotting. When the diet does not provide a sufficient amount of calcium, it is released from the bones, and when there is too much calcium in the body, it is stored in the bones.
Vitamin D is important for proper absorption of calcium in the small intestine. Vitamin D is found in foods such as eggs, milk and other dairy products. Vitamin D forms from a substance (dehydrocholesterol) produced by cells in the digestive tract or obtained in the diet. Dehydrocholesterol is carried by the blood to the skin, it is converted to a compound that becomes vitamin D. Skin helps to manufacture vitamin D from ultraviolet light, which is important to normal bone growth and development. The organic substances of bone give it a certain degree of flexibility. The inorganic portion of bone is made from mineral salts such as calcium phosphate, calcium carbonate, calcium fluoride, magnesium phosphate, sodium oxide, and sodium chloride. These minerals give bone its hardness and durability.
The bone became brittle and elastic due to loss of calcium. In rickets condition bones become soft, brittle due to lack of calcification, causing deformities as bowlegs. In the absence of this vitamin, calcium is poorly absorbed, and the inorganic salt portion of bone matrix lacks calcium causing bone deformities. In children, this condition is called rickets, and in adults, it is called osteomalacia.
Vitamin A is important for osteoblast and osteoclast activity during normal development. The deficiency of vitamin A may retard bone development. Vitamin C is required for collagen synthesis. If osteoblasts produce less collagen in the intercellular material of the bone tissue this deficiency will make bones fragile.
Bone adapts to changing stresses and forces. When muscles increase and become more powerful due to exercise, the corresponding bones also become thicker and stronger through stimulation of osteoblasts. Regular exercise maintains normal bone structure. Bones which are not subjected to normal stresses, such as an injured leg immobilized in a cast, quickly degenerate. Without exercise the bone tissue becoming thinner and weaker (atrophy).
2. How do the relative proportions of the cranium and face of a fetus compare with those of the adult skull?
The skull consists of two sets of bones: cranial and the facial bones. The “soft spots” of a baby’s skull are areas of incompletely ossified bones called fontanels. The bones of the skull are connected by fibrous, pliable, connective tissue at birth. The flexibility of these connections allows the skull bones to move and overlap as the infant passes through the birth canal. The fontanels begin to close about two months after birth. The largest of the fontanels, the frontal fontanel located on the top of the skull, does not close until 18 to 24 months of age.
Cranial bones are thin and slightly curved. During infancy, these bones are held together by an irregular band of connective tissue called a suture. As the child grows, this connective tissue ossifies and turns into hard bone. The frontal bone and mandible are separate right and left bones but fuse medially by age 5 or 6. In newborn, face is flat and the cranium is large. The brain grows faster than the rest of body meaning the head is larger than body, and in adults the head is smaller than body. The infant’s cranium is big comparing to its face, have two openings called fontanels, the bones of the skull consist of four plates that are not fused
3. How does the shape of the joint impact its ability to move?
Some joints are very flexible, allowing movement, while others are strong, providing protection of the internal tissues and organs, but do not permit movement. The fibrous joints are mostly immovable. The three types of fibrous joints are sutures, syndesmoses, and gomphoses. Sutures provide protection for the brain and are only found in the adult skull. They are immovable joints. A syndesmoses joint is a joint where the bones do not touch each other and are held together by fibrous connective tissue. A gosphosis joint is composed of peg and socket. A synarthrosis joint is immovable. An amphiarthrosis joint is slightly movable. A diarthrosis joint is a freely movable joint. The movable joints consist of three main parts: articular cartilage, a bursa (joint capsule), and a synovial (joint) cavity.
Functional Category
Structural Category
Example
Synarthrosis (immovable joints)
Fibrous
Suture
Gomphosis
Cartilaginous
Synchondrosis
Between bones of adult skull
Between teeth and jaw
Epithyseal cartilages
Amphiarthrosis (little movement)
Fibrous
Syndesmosis
Cartilaginous
Symphysis
Between the tibia and fibula
Between right and left public bones of pelvis
Between adjacent vertebral bodies along vertebral column
Diarthrosis (free movement)
Synovial
Elbow, ankle, ribs, wrist, shoulder, hip
Type of Joint
Type of Movement
Example
Planar
Gliding
Joints between carpals and tarsals
Hinge
Flexion and extension
Elbow, knee, and ankle
Pivot
Rotation
Atlantoaxial joint (between first and second vertebrae)
Condyloid
Abduction and adduction
Wrist joint
Saddle
Flexion, extension, metacarpal abduction, adduction
Carpometacarpal joint (between bone of thumb and carpal bone of wrist) circumduction
Ball-and-Socket
Rotation, abduction, adduction, circumduction
Shoulder and hip joints
4. Why is articular cartilage important?
The articular cartilage covers and protects the bone ends. The articular cartilage also acts as a shock absorber. The articular capsule encloses the joint structure. Articular cartilage is a highly organized avascular tissue composed of chondrocytes embedded within an extracellular matrix of collagens, proteoglycans and noncollagenous proteins. Its primary function is to enable the smooth articulation of joint surfaces, and to cushion compressive, tensile and shearing forces. The articular cartilage is a layer of hyaline cartilage. Hyaline cartilage has one of the lowest coefficients of friction known for any surface to surface contact. The synovial fluid and cartilages make joint movements friction-free.
Case Study:
Fractured leg
Chief Complaint: 14-year-old girl admitted with a broken left leg.
History: Nicole Michaelson, a 14-year-old girl, was skiing when she fell and broke her left leg. As she fell, her left leg got caught under the body of another skier who ran into her. An X-ray revealed that the fracture was a compound, tibial-fibular fracture just below the knee. The X-ray also revealed a torn meniscal cartilage in the knee above the fracture. The girl remained in the hospital for 14 days because of an infection of the leg in the area of skin breakage. Her immobilized leg was casted after the infection subsided. She remained in a full leg-length cast for 3 months, after which the upper portion of the cast was removed and she was allowed to start bearing weight on the leg. The bones ultimately healed, but the girl continued to have left knee swelling (“water on the knee”) and pain made worse by walking. Arthroscopic examination of the knee revealed a meniscus that was still torn 6 months after her injury.
Questions
1. What does the term “tibial-fibular fracture” mean?
It is a fracture of both the tibia and fibula in the lower leg below the knee.
2. What is a compound fracture?
When fracture occurs, there is swelling due to injury and bleeding tissues. A compound fracture is where the broken bone ends pierce and protrude through the skin. This cause infection of the bone and neighboring tissues. The process of restoring bone is done through three methods: closed reduction, open reduction, and traction.
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3. Why was her injury more likely to become infected than a routine fracture of the leg?
Nicole has an open/compound fracture where the broken bone ends pierce and protrude through the skin. This can cause infection of the bone and neighboring tissues since the skin is normally colonized with bacteria. The infections of osseous tissue are difficult to treat.
4. Describe the microscopic features of osseous tissue that help long bones withstand lateral stress without breaking.
The lateral stress placed on Nicole’s left tibia and fibula causing:
1. Stretching/tearing on the side opposite of the impact
The bony collar withstands tearing apart by vertically arranged bundles of tough collagen in the extracellular matrix of bone. This collagen is lined up in a spiraling vertical pattern in which the fibers in each lamella are perpendicular to those in adjacent lamellae.
2. Compression of the bone on the side of impact
The bony collar have a stress on lateral impact. It withstands the pressure/stress/crushing by tough hydroxyapatite crystals in its extracellular matrix. These hydroxyapatite crystals serve as weight-bearing pillars for the bone. They arranged in layers within the osteons (Haversian systems) of compact bone.
The middle area (medullary cavity) is filled with red and yellow bone marrow. The tough compact bone is not needed in the middle therefore compressive and tearing forces cancel each other out in mid-way through the bone.
5. Describe the microscopic features of the osseous tissue that help long bones withstand compressive forces without breaking
The bony collar of long bones helps to support the weight of the body and withstand compressive stress. Epiphysis a spongy bone tissue is spherical in shape and is located at both the distal and proximal end of a long bone. Spongy bone tissue consists of an irregular latticework of thin needle-like threads of bone called trabeculae. The spongy bone in the epiphyses helps withstand compressive forces, it is well designed to pass on strength to a bone by adding minimum weight. The trabeculae develops along the bone’s line of stress, and help to distribute the weight of the body out to the bony collar of the diaphysis. Diaphysis is the long, cylindrical, hollow shaft of the bone. Trabeculae distributes the weight evenly.
6. What features of the knee joint structure help minimize friction between the thighbone and the leg bone?
When two movable bones meet at a joint, their surfaces do not touch one another. The tow articular joint surfaces are covered with a smooth, slippery cap of cartilage known as articular cartilage. This cartilage helps to absorb shocks and prevent friction between parts. Enclosing two articular surfaces of the bone is a tough, fibrous connective tissue capsule called an articular capsule. Lining the articular capsule is a synovial membrane, which secretes synovial fluid into the synovial cavity. Synovial joints are type of joint in the body, permitting the greatest range of movement. The knee joint is an examples of synovial joints. The synovial joint consists of a synovial cavity, articular cartilage, a fibrous articular capsule, and ligaments. The lubricating fluid (synovial fluid) in the capsule secreted into joint cavities. This fluid reduces the friction between the tibia and femur during extension and flexion of the knee. The synovial fluid secreted serves as a lubricant to prevent friction between a tendon and a bone. The articualr surfaces of the tibia and femur are covered with smooth hyaline cartilage. This cartilage reduces friction between the bones. The medial and lateral menisci (fibrocrtilage) that locates on top of the tibial surface. On the distal surface of the femur the shape of lateral condyles and medial match the shapes of the menisci, this match shape stabilizes the knee joint and gives a shock absorbing function during weight-bearing.
7. Describe the changes a broken bone undergoes as it is healing.
Healing process stages:
A. Hematoma (fracture hematoma-blood clot) and granulation tissue forms- when bone breaks, blood leaks out of the veins and forms a clot (fracture hematoma). This helps to keep both pieces lined up for mending and stabilize the bone.
B. Soft callus forms
1. fibroblasts and osteoblasts migrate in from the periosteum and endosteum (takes 1st 3 to 4 weeks)
2. fibroblasts lay down a collagen matrix, some of the fibroblasts (osteogenic cells) differentiate into chondroblasts (cartilage-forming cells) and produce patches of fibrocartilage (soft callus)
C. Hard callus – osteoblasts begin to replace the fibrocartilage splint with spongy bone to join the broken ends or bone fragments together, forming a bulge which is wider than the after the injury. It takes 4 to 6 weeks for a hard callus to form. The broken bone is immobilized by cast to prevent reinjury.
D. Remodeling – Osteoclasts dissolve small fragments of broken bone, and osteoblasts deposit spongy bone to connect the gap between the broken ends. As the patient starts to use the bone (weight bearing), the bone starts to remodel along lines of maximal stress (requires the activity of both osteoblasts and osteoclasts) hard callus remodeled bone (the fracture leaves a slight thickening of the bone it is shows up on an X-ray).
8. How does weight bearing influence the bone repair process you described above? (i.e. what effect does weight-bearing have on the orientation of the Haversian systems?)
Wolff’s law is a hypothesis related to the effect of weight-bearing on bone remodeling. This law states that bone grows and remodels in response to the mechanical stresses placed upon it, therefore the bone adapts to withstand those stresses. The bone locates along lines of maximal stress.
The reasons are:
1). long bones are the thickest midway down the diaphysis, since the stresses are the greatest in this location.
2). curved bones are the thickest where they are most likely to buckle
3). the bony plates in spongy bone for weight-bearing
4). large bony processes are located where muscles attach to/pull on bone
The Haversian systems in bones locates along the lines of maximal stress.
If person’s exercise activities change, the microscopic structure of bones change. High-heeled shoes transfer the balanced distribution of the weight of the body form between the calcaneus (the heel bone) and the matatarsals (bones in the ball of the foot) to just the metatarsals. As a result, the arches of the foot do not absorb the force of the body’s weight, which may lead to injuries of the soft tissue structures, joints, and bones. Also, if woman wears flat shoe most of the time she will undergo a bone remodelling if she will start to wear high-heeled shoes. Fibular grafts can be used in remodelling process to replace arm bones due to medical condition such as cancer or some other injuries. If patient have had the mid-portion of his/her tibia removed, it can be replaced with fibular graft. The patient with such replacement will be able to do a weight-bearing on the graft.
9. Why did Nicole’s bones heal much more quickly than her cartilage?
Nicole’s bones heal faster than her cartilage due to high vascularity. Bone contains of many blood vessels which allows a good blood supply and cartilage contains very few blood vessels (poor blood supply). The materials for healing move rapidly between blood vessels and bone cells. Cartilage receives its nutrients indirectly though diffusion from neighboring tissues and synocial fluid. Osteoblasts and osteoclasts repair and reshape the bone in faster rate. Osteoclasts dissolve small fragments of broken bone, and osteoblasts deposit spongy bone to connect the gap between the broken ends. Tendons’ and ligaments’ blood supply is between bone and cartilage. Tendons and ligaments heal faster than cartilage but slower than bone. The six month after Nicole’s injury her meniscus was still damaged, torn meniscal cartilage should be removed via arthroscopic surgery.
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