The Biomechanics of Human Bone Growth and Development

The Biomechanics of Human Bone Growth and Development Explain how the material constituents and structural organization of bone affect its ability to withstand mechanical loads. Describe the effects of exercise and of weightlessness on bone mineralization. Explain the relationship between different forms of mechanical loading and common bone injuries.

BONES Two Important Mechanical Functions of Bone For Human Being Provides a rigid skeletal framework that supports and protects other body tissues. It forms a system of rigid levers that can be moved by forces from the attaching muscles.

BONE CONSTITUENTS Material constituents influence bone responds to mechanical loading. Bone major building blocks: l Calcium Carbonate 60– 70% of dry bone weight l Calcium Phosphate l Collagen – protein. l Water – 25 to 30% of bone weight

STRENGTH AND STIFFNESS DETERMINES MATERIALS BEHAVIOR UNDER LOADING Stiffness? Stress/strain in a loaded material (slope), represent material resistant to load as the structure deform. Steep slope represent higher stiffness. If the same amount of force applied to two different material, the lower stiffness will deforms more. Compressive strength? Ability to resist the stress due to compression. How do we compare strength? Strength indicated by the area under the stress-strain curve.

STRENGTH AND STIFFNESS OF VARIOUS MATERIALS Bone – flexible and weak (Shipman, P. , Walker, A. , and Bichell, D. [1985]

BONE STRESS- STRAIN CURVE Bone has both brittle and ductile properties. It deforms slightly before failure or fracture. Ductile Material Brittle material Bone Strain

COMPOSITION AND STRUCTURE OF BONE What contributes to stiffness and compressive strength in bone? MINERALS • Calcium carbonate • Calcium phosphate 60 -70% of dry bone weight

COMPOSITION AND STRUCTURE OF BONE What contributes to flexibility and tensile strength in bone? COLLAGEN. What is the effect of aging on collagen in bone? Collagen is progressively lost and bone brittleness increases with aging.

COMPOSITION AND STRUCTURE OF BONE What else affects bone strength? • water content of bone, which comprises 25%-30% of bone weight. Bone specimens keep wet. • bone porosity, or the amount of bone volume filled with pores or cavities.

COMPOSITION AND STRUCTURE OF BONE Endosteum Proximal epiphysis Cortical bone Epiphyseal plate Marrow Trabecular bone Periosteum Diaphysis Trabecular bone Nutrient artery Cortical bone Medullary cavity Distal epiphysis Epiphyseal plate Structures of cortical (compact) and trabecular (spongy) bone.

COMPOSITION AND STRUCTURE OF BONE Categories of bone based on porosity: • Cortical bone: Low porosity (5 -30%), higher mineral content, therefore higher stiffness; found in the shafts of long bones • Trabecular (or cancellous) bone- (3090%): Higher porosity (, less mineral content, therefore lower stiffness compare to cortical bone, found in the ends of long bones and the vertebrae.

CORTICAL AND TRABECULAR CHARACTERISTICS What else does bone porosity affect? • Cortical bone is stiffer than trabecular bone, it can withstand greater stress but less strain. • Trabecular bone is spongier than cortical bone, it can undergo more strain before fracturing. Shock Absorbing capability.

TYPICAL STRESS STRAIN BEHAVIOUR FOR HUMAN CORTICAL BONE The bone is stiffer in the longitudinal direction. It is stronger in compression than tension

BONE STRENGTH CHARACTERISTICS Bone is strongest in resisting compression and weakest in resisting shear. Approximate strength of cortical bone Compression: 200 X 106 N/m 2 Tension: 100 X 106 N/m 2 SHEAR TENSION COMPRESSION Stress to Fracture Bone is anisotropic, it has different strength and stiffness depending on the direction of the load Shear: 50 X 106 N/m 2

TYPES OF BONES • Axial skeleton (bones that form the axis of the body) - skull, vertebrae, sternum and ribs. • Appendicular skeleton - bones composing the body appendages.

TYPES OF BONES (SHAPES AND FUNCTIONS) • Short bones (carpals and tarsals)approximately cubical; include the carpals and tarsals. Spongy bone covered with thin layer of compact bone. Shock absorption and force transmissions. • Flat bones (scapula, sternum, ribs, patellae, some bones of the skull)- protect organs & provide surfaces for muscle attachments. Protect internal structures and offer broad surfaces for muscular attachment.

Types of bones (shapes and functions) • Irregular bones (the vertebrae, sacrum, coccyx, maxilla): have different shapes to serve different functions (supporting weight, contribution to movement, processes for muscle and ligament attachment, protection). • Long bones: Offer body support and form the framework of the appendicular skeleton; include humerus, radius, ulna, femur, tibia and fibula. Tibia & Femur – large and massive to support the weight of the body. The upper extremity long bones promotes ease of movement.

TYPES OF LOADCOMPRESSION FORCE l l l Press the end of the bone together and produce by muscles, weight bearing, gravity and etc. It shorts and widen the bone. If the load applied surpass the stress limits of the structure, a compression fracture will occur.

COMPRESSION AT HIP JOINTS The hip joint absorb compressive forces of approximately 3 to 7 times body weight during walking. 15 to 20 times body weight in jumping.

THE EFFECTS OF EXERCISE AND OF WEIGHTLESSNESS ON BONE MINERALIZATION How do bones grow in length? The epiphyses, or epiphyseal plates, are growth centers where new bone cells are produced until the epiphysis closes during late adolescence or early adulthood. Epiphysis is a cartilageous disc which can be found near the end of a long bone. Most epiphyses close around age 18 although some may be present until about age 25.

Bone Growth and Development How do bones grow in circumference? • The inner layer of the periosteum, a doublelayered membrane covering bone, builds concentric layers of new bone on top of existing ones. • Specialized cells called osteoblasts build new bone tissue and osteoclasts resorb bone tissue

WOLFF’S LAW The densities, the sizes and shapes of bones are determined by the magnitude and direction of the acting forces. A bone in a healthy person or animal will adapt to the loads it is placed under. If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading. The external cortical portion of the bone becomes thicker as a result. The converse is true as well: if the loading on a bone decreases, the bone will become weaker due to turnover as it is less metabolically costly to maintain and there is no stimulus for continued remodeling that is required to maintain bone mass.

BONE RESPONSE TO STRESS • Dynamics loading will cause the bone to deform or strain. When strain exceed certain threshold, a new bone is laid down at the strain sites. • Increased or decreased mechanical stress leads to osteoblast or osteoclast activity, respectively. • Osteoblasts and osteoclasts are continually building and resorbing bone, respectively.

BONE REMODELING Bone remodeling involves resorption or reabsorption (the process of losing substance) of fatigue damaged older bone and subsequent formation of new bone. It involves: A balance of osteoblast and osteoclast activity or A predominance of osteoclast activity with associated maintenance of or loss of bone mass. It maintain or reduce bone mass. An activity such as walking is sufficient enough to provoke bone turnover and new bone formation.

BONE MODELING Bone modeling involves formation of new bone that is not preceded by resorption (immature bone growth).

REMODELING AND MODELING Osteocytes cell embedded in bone are sensitive to the change in the flow of interstitial fluid through the pores resulting from strain on the bone. In response to the motion of the fluid within the bone matrix, osteocytes trigger the action of osteoblasts and osteoclasts which form and remove bone, respectively. The process is not the same in all bones or even in a single bone.

BONE DENSITY Body weight provides constant mechanical stress to the bone and related to bone density. Heavier people having more massive bones. Few factors affect bone density including regular exercises (weight bearing activities exert more influence compare to body weight, height and race)

HOW DO BONES RESPOND TO TRAINING? Bone hypertrophy Increase in bone mass/density resulting from osteoblast activity (through modeling) in response to regular physical activity. What kinds of activity tend to promote bone density? Weight bearing exercise, since the larger the forces the skeletal system sustains, the greater the osteoblast response. Bone hypertrophy is stimulated more by the magnitude of the skeletal loading than by the frequency of loading.

HOW DO BONES RESPOND TO TRAINING? Older women, both yard work and weight training good for hypertropy. However, jogging, swimming and calisthenics are less effective. [ Prince R: Calcium controversy revisited: implications of new data, Med J Aust 159: 404, 1993. Swimmers may have bone mineral densities lower than sedentary individuals.

WHAT TENDS TO DIMINISH BONE DENSITY (ATROPHY)? Bone atrophy Decrease in bone mass results from osteoclast activity (through remodeling). The amount of Calcium contained in the bone diminishes, both the weight and the strength of the bone decrease. Bedridden patients, sedentary senior citizen, astronauts.

Why Atrophy? • Lack of weight bearing exercise • Spending time in the water, (since the buoyant force counteracts gravitational force) • Bed rest – 4 to 6 weeks of bed rest can results in significant decrements in bone mineral density and are not fully reversed after 6 months of normal weight bearing activities. • Traveling in space outside of the earth’s gravitational field – one month in space, astronauts lost 1 -3% of bone mass.

SAMPLE PROBLEM 1 Tibia – major weight bearing bone in lower extremity. Sample weight 600 N. If 88% of body mass proximal to knee joint (about 88% of body mass acting on the knee joint), how much compressive forces acting on each Tibia if the person holds a 20 N sack of groceries?

OSTEOPOROSIS A disorder involving decreased bone mass and strength with one or more resulting fractures. It starts with Ostopenia (reduce bone mass w/o fracture) which often finally results in bone pain and fracturing.

Who? Majority – postmenopausal and elderly women. Half of all women, 1/3 of men develop fractures due to osteoporosis. Type 1 – postmenopausal osteoporosis, 40% of women after age 50. Fracture often occurs 15 years after menopause. Type 2 – age associated osteoporosis affects most women and men after age 70.

PREVENT AND TREATING OSTEOPOROSIS Regular Exercise Has been shown to be effective to some extent in mediating age related bone loss. Weight bearing exercise – walking, stair climbing , osteogenic impact forces activities such as jumping – effective in increasing bone mass in children. 3 times/ week with 10 -20 minutes exercise. Rest intervals between impact exercise double the effects of mechanical loading on bone building.

PREVENT AND TREATING OSTEOPOROSIS Hormonal factors Low level of estrogen in women and low level of testosterone in men promote bone loss. Diet More calcium in diets, absorption of Ca promotes with cacitriol ( active form of vit D) and influenced negatively with dietary fibers. Lifestyles Physical inactivities, excessive thinnes or weight loss, smoking, excessive consumptions of protein and caffeine. Genetic factors Less influence.

Mechanical loading and common bone injuries Fractures Depend on magnitude of loading, direction, loading rate, duration load sustained, health and maturity of bone at the time of injury. Avulsion Fractures caused by tensile loading. Explosive throwing and jumping movements – avulsion of humerus and calcaneus.

Mechanical loading and common bone injuries Excessive bending and torsional loads produce spiral fractures of the long bones. Bending moment cause bending and fracture of bone. When bending occurs, one side of the bone is in tension, the other side is in compression. Since bone resist compression better than tension, the side with tension will fracture first.

Mechanical loading and common bone injuries l Torque cause torsion (twisting of the structure). When skiers rotate with respect to one boot, torsional load can cause a spiral fracture of the tibia.

Mechanical loading and common bone injuries l Stress fracture (fatigue fracture) due to repeating low magnitude loads. Runners, muscle fatigue, abrupt change in the running surface or direction.

Mechanical loading and common bone injuries l Children contain larger amount of collagen than adults, therefore their bone is more flexible and more resistant to fracture.

EPIPHYSIAL INJURIES Injuries to cartilaginous epiphysial plate, the articular cartilage, and the apophysis. l Both acute and repetitive loading can injure the plate and can result in growth termination. l

TYPES OF LOADS Loads Various directions - different types of loads. Compression, Tension, Shear, Bending, Torsion

Compression Force and Injuries Bone a stronger in resisting compression as compare to other loads. Acute compression fracture is rare. Compressive force are responsible for patellar pain, softening and destruction of cartilage underneath the patella.

Chondromalacia Patellae Chondromalacia patellae - literally means "softening of the cartilage", and Patellae means "the knee-cap. The compressive patellofemoral force is approximately at 50º of flexion. Injury may be due to direct trauma, such as a fall on a flexed knee. Weight gain, or other increased load on the knee

Vertebrae Fractures to cervical area due to sport activity such as water sports, gymnastics, wrestling, rugby, ice hockey and etc. Cervical spine – anteriorly convex.

Cervical Vertebrae If head is lowered, cervical spine is almost flatten. If forces is applied in this position, cervical spine will have a compression force effect. This can result in dislocation or fracturedislocation of the facets of the vertebrae. When spearing and butting was outlawed in football, the

Femoral Neck The hip joints must absorb compressive forces of approximately 3 to 7 times of body weight during walking, 15 to 20 times body weight during jumping. Large compressive forces on the inferior portion of the femoral neck, and a large tensile force on the superior portion of the neck. During stance, the hip abductors contracts, reducing the tension force on superior, which prevent fracture of the neck.

TENSION FORCE INJURIES The source of the tensile force is usually the pull of contracting muscle tendon. Failure usually occurs at the site of muscle insertion.

Avulsion fractures when ligaments pull a small chip of bone away from the rest of the bone. It occurs more in frequently in children than adults.

SPRAIN AND STRAIN Ankle sprain - ankle sprains happen when the foot turns inward as a person runs, turns, falls, or lands on the ankle after a jump. The most frequently injured ligaments are the anterior talofibular followed by the calcaneofibular ligament.

SHEAR FORCES INJURIES Vertebral disc problem Spondylolisthesis the vertebrae slip anteriorly over one another.

SHEAR FORCE FRACTURE Fractures due to shear forces are commonly found in Femoral Condyle and tibial plateau. Epiphyseal fracture in childs.

BENDING FORCES Injuries caused by bending when multiple loads applied at different point s on the bone. Generally called, 3 or 4 point force applications. The bone will break at the middle froce application. Bone will normally fracture at the side where the tensile force is applied.

TORSIONAL FORCES Humerus when poor throwing technique creates a twist on the arm. A spiral fracture is a result of torsional forces.