0894-1939/92 53 00 + M Copyright : 1992 T q l o r & Franc!!
Jorrrnol of InvesrrXori\,e Surgerv. Volume 5. pp. 161-166 Printed in the UK All righta rearrved
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
Keynote Address
Difficulties in Duplicating the Human Upright, Graviportal, Plantigrade Posture in Sitting, Standing, and Walking by the Use of Animal Models CHARLES 0. BECHTOL 111, MD The posture of the human presents a paradox unique among animals. The basic pattern of physical equipment is still evident after 360 million years of evolution. In contrast ta this, in the last 3.75 million years the human has assumed a unique upright posture and developed a large brain. The posture consists of: bipedalism, an upright torso, a graviportal leg, and a plantigrade foot. The addition of a large brain has allowed achievements far beyond those ol the great apes, such as sexual dimorphism, prolonged infancy, occult ovulation, hunting. meat-eating, tool-making, language, and culture. The available animal models, dog, cat, and sheep, are quadrupedal and specialized for rapid running. They are suitable for studies on fracture fixation and healing, allografts and homografts, repair of bone defects, and the atrophy of disuse. Evaluation 01 the effects of ageing and the long-term follow-up of total joint implant procedures present a more difficult problem.
Historical Review of Human and Animal Model Evolution The use of an animal model has many obvious advantages. It then becomes necessary tc understand the animal that we are attempting to model, the human, whose physical equipment presents a remarkable contrast. The large human brain and upright, bipedal posture are unique among all animals. The remainder of the physical characteristics are remarkably unspecialized and resemble the primitive locomotor equipment of the amphibian. When the first amphibians crawled out on land about 360 million years ago, the) had two pairs of limbs. Each limb had three functional joints, and a paw with five digits The hind limb was attached to the spine and the forelimb to the sternal complex. Thc time span from the primitive amphibian to the human is more than 300 million years, ye) the same pattern of two paired limbs is still present. In the human, the leg is attached to the spine by the pelvis; the three joints, hip. knee, and ankles are present; and the foot has five toes. The arm is attached to the Charles 0. Bechtol 111, MD, was the 1991 recipient of the Academy of Surgical Research’: Markowitz Award, given at the Academy’s Annual Meeting in Scottsdale, Arizona, Septembei 26-29, 1991. The following is Dr. Bechtol’s Keynote Address.
I 61
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
162
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sternum by the clavicle; the three joints, shoulder, elbow, and wrist are present, and the hand has five fingers. This limb pattern persisted through the age of reptiles, the dinosaurs, and the primitive mammals. Almost the entire remainder of the mammals have specialized in some way that would prevent them from becoming the bipedal human. For example, 58 million years ago the ancestral horse was a herbivore about the size for a small dog. This evolved into the modern horse, specialized for rapid running, with reduction of the toes from four to one and loss of the clavicle. Other mammals modified their limbs for flying, swimming, digging, and the sharp claws of the feline predators. With few exceptions, the remaining nonhuman mammals of the world are quadrupedal. In walking, the quadruped has three of the four limbs in contact with the ground most of the time. This significantly reduces the torque when only one hind leg is supporting the pelvis. The few bipeds that exist do not have an upright torso as the human does. Our ancestors differ from the great apes in a number of achievements. The order oi their development is a matter of speculation. They are: bipedalism, sexual dimorphism, occult ovulation, meat-eating, hunting, tool-making, prolonged infancy, large brain, language, and culture. The great apes and some primates use the upright sitting posture to free their hands for manipulation and holding of objects, rooming of family members, and gathering and eating their vegetarian diet. The quadrupedal herbivores must limit their diet to what they can reach with their mouth; hence the giraffe has a long neck to reach additional food supply. All the great apes are capable of bipedal walking or running for short distances. They are limited by their hip flexion contractures from achieving an upright graviportal stance. Human infants also exhibit a flexion contracture of the hip in their first years of walking. The upright sitting posture preceded the upright bipedal posture in standing and locomotion. This allowed the lower thoracic and lumbar spine to increase in size and, in a sense, prepared the spine for the added vertical load of the permanent upright posture. This evolved only after our ancestors had developed sufficient brain capacity to cope with the limitations and advantages of bipedalism. A. B. Howell states that any animal must evolve to a level sufficient to survive in its ecological niche, or it will become extinct. Thus all other animals have a smaller brain than the human, because that smaller brain is sufficient for their survival. The large human brain implies that early humans successfully survived a difficult ecological niche. It is important to remember Howell’s conclusion, in his book Speed in Animals, that in cursorial animals such as our animal models, cat, dog, and sheep, the anatomy is designed for the ultimate burst of speed, and thus walking represents a minimal activity.
Review of Comparative Anatomy over Geological Time The development of mammals began about 80 million years ago. The basic pattern was quadrupedal with three limb joints and five digits. This pattern has been modified by subtraction in many mammals. The human, however, retains the basic pattern. With the exception of the modifications necessary for the upright posture and the large brain, our physical equipment remains relatively primitive and unspecialized . Ancestors with the potential for upright posture are found as long ago as 14 million years. These mammals were all quadrumanous; in other words, both hand or foot had an
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
Keynote Address
163
opposable first digit for grasping. This further implies that they were capable of climbing trees to escape predators or obtain additional food. The human terrestrial foot has lost the ability to oppose the great toe. This effectively commits us to living on the ground, and means a reduction in our food supply and the need to escape predators by methods other than climbing trees. Footprints of two adults and one child, preserved in fresh volcanic ash, indicate that our ancestors had a terrestrial foot 3.75 million years ago. The increase of the brain capacity occurred more than 3.75 million years ago. Any of our close relatives-gorilla, chimpanzee, gibbon, orangutan, or even baboon-could have physically assumed the upright posture, provided that they had developed the necessary brain capacity to take advantage of bipedalism. The actual ancestor was probably a nonspecialized primate that may be the ancestor of all the great apes and man.
Characteristics and Penalties of Human Bipedalism The gait is exclusively bipedal. This means a significant loss of running speed, compared to a quadrupedal posture. The torso is upright, not only while standing and walking, but also while squatting. The arms and the upper torso retain the same pattern of swing as the forelimb of the quadruped. This opposes the torque applied to the pelvis by the alternate thrust of the legs. The clavicle is retained, indicating not specialization for speedy locomotion in quadrupedal ancestors. The human’s greatest speed is only a fast bipedal trot. There are three functional joints of the plantigrade leg, hip, knee, and ankle, but the short length of the foot limits stride length. Stride length has been partially compensated for by lengthening the femur and tibia. The great toe was once opposable for tree climbing; the necessary muscles are still present in the sole of the foot.
Modern Human Has Had More Than 3.75 Million Years to Evolve from the Status of a Rather Primitive Quadruped to an Incompletely Evolved, Nonspecialized Biped, with Good Brain Capacity The diameter of the head of the femur and the head of the humerus are the same. Considering the significant increase of force applied to the femoral head in the biped, the femoral head, and with it the hip joint, should have enlarged. The spine and pelvis are also incompletely evolved to accomodate upright, graviporta, bipedal posture.
The Femoral Head Has Four Sources of Increased Loading (1) The quadruped bears more than 50% of its weight on its forelegs. In the biped 100% of the weight is on the hind legs. ( 2 ) To balance the torso above the hip by the pull of the adductor muscles, 2 to C times body weight is added to the load on the hip. (3) The human acetabulum is dysplastic. It does not cover the head completely for i graviportal leg, as is seen in the graviportal leg of the elephant. This increase: the pressure on the joint surface, depending on the degree of dysplasia. (4) The leg is graviportal. In standing and walking the hip and knee are fullj extended, and these motions come to an abrupt stop, due to ligament and musclt action. This adds load to both the hip and knee in full extension.
I64
C. 0. Bechtol I l l
Effect of Age on the Functions of the Acetabulum The acetabulum functions differently in the younger and older age groups. The youngei age group has an acetabulum that is a flexible horseshoe. The head moves upward wit1 weight bearing, and the horns of the horseshoe move together. This action promotes joint cartilage nourishment. In the older age group, the acetabulum becomes spherical, and the nourishment of the articular cartilage is less efficient, leading to degenerative changes.
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
Sexual Dimorphism and Its Effect on the Human Female Hip In the quadruped the effect of pregnancy and infant care on the female anatomy is minimal. In contrast, in the human there are several factors leading to significant sexual dimorphism. Because of the large size of the human fetal head, the acetabulae are displaced laterally from the center of gravity. This combination of a reduced mechanical advantage with greater abductor force requirement and a smaller femoral head leads to greater pressure on the femoral head in the female. The human female locomotor apparatus is designed for the requirements of the ninth month of pregnancy and the carrying of the infant for the first few years of life. The female anatomy from the waist down is significantly more powerful than from the waist UP.
The Human Knee Joint Is Nonspecialized The patellar facet encroaches on the femoral condyles. The fit of the patella in its groove and as it contacts the femoral condyles in flexion is extremely incongruous and requires a very thick articular cartilage to compensate for this. In contrast, the usual experimental animals, dog, cat, and sheep, have a patellar facet that is entirely separate from the femoral condyles. The fit of the patella in its groove is congruous. The tibiofemoral joint is entirely dependent upon its ligaments and muscles for stability. The lubrication and stability of the joint is enhanced by the presence of the semilunar fibrocartilages . The tibiofemoral joint is quite incongruous without the semilunar cartilages. The rolling, sliding motion of the femur on the tibia (which is controlled by the four bar linkage of the cruciate ligaments) concentrates the wear on the small area of the tibia, but distributes the wear over a large area of the femur.
Alignment of the Human Femur and Tibia In the graviportal human leg, errors in the alignment in varus and valgus can lead to accelerated wear. The normal alignment between tibia and femur is about 7 degrees. It is not possible to duplicate this in the available experimental animals, because their knee is never in full extension, standing, walking, or running.
The Paradox of the Ankle The ankle has the smallest area, the tightest fit, the thinnest cartilage, and the smallest range of motion of the major joints of the leg. However, osteoarthrosis is much rarer than in the hip or knee. Attempts to duplicate the function of the ankle are difficult
Keynote Address
165
because the articular surface is not cylindrical, but is a fustrum of a cone. In addition, the fibula moves laterally with dorsiflexion of the foot. This movement is 2 mm or less. The distal tibiofibular joint is a fibrous syndesmosis, while the proximal joint is a true joint.
Experimental Animals
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
As a general rule, experiments including the joints can be compared with similar procedures done on humans.
Use of Paired Bones As Controls The author broke 100 pairs of mongrel dog tibias and femurs in three point bending. The results were recorded as a percentage difference of the breaking strength of the pair of bones. This produced the typical bell-shaped distribution curve. One standard deviation was 2 . 4 % . In three pairs of bones, the recorded difference in breaking strength was 9% ’ . This provides a control for procedures done as nonsurvival experiments. Survivor type experiments of sufficient duration to produce atrophy of disuse will result in a less accurate control. With appropriate monitoring of the disuse atrophy, an adequate control might still be possible.
The Ageing Process The life span of the experimental animals is well known. Exactly how to equate this to the human life span remains a problem. For our body size, we have an unusually long life span. This is well up into the eighties. This, however, is largely due to the conquest of disease and the general improvement in public health measures. In vigorous sports, however, the athlete retires between the age of 35 and 40. Many of the human ailments for which we wish to establish animal models occur in the age group over SO.
Available Animal Models for Duplication of Human Problems There is no mammal that fully duplicates the human posture. All of the available experimental animals have had a much longer evolutionary history in geological time than man. As a consequence, they are more specialized, in one way or another, and cannol completely serve as an animal model for the human. Suitable comparisons can be made in the areas of: fracture healing, the filling of bone defects with allografts and homografts, the resulting reduction in the breaking strength produced by various sizes anc shapes of bone defects, the efficiency of the various methods of fracture fixation, anc similar types of experiments.
Duplication of Human Upright Posture The quadrumanual monkeys and apes can duplicate the human upright sitting posture but not standing or walking. The mammals more specialized for rapid locomotion-dog cat and sheep-are suitable for procedures on muscles, bones, and joints. They have however, eliminated the clavicle. The dog and cat are digitigrade, while the sheep i: unguligrade with some reduction of the digits. These changes facilitate rapid locomo tion.
I66
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Quadrupedal Animals with a Fully Graviportal Hind Limb The elephant has a fully graviportal hind limb. Although the knee fully extends at the end of the stride, the hip does not. Of the primates, only the gorilla stands and walks quadrupedally, with a graviportal hind leg.
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
Duplicating the Causes of Loosening of Total Joint Prostheses Animal models will continue to serve an important role in the development and testing 01 bioengineering materials. The osteolytic effects of the products of wear can be compared on a qualitative and quantitative basis. Many of the problems of loosening, breakage, and wear in human total joint replacements are not evident before 15 to 20 years. The relatively short life span of the available experimental animals will make such research difficult if not impossible.
Jorrrnol of InvesrrXori\,e Surgerv. Volume 5. pp. 161-166 Printed in the UK All righta rearrved
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
Keynote Address
Difficulties in Duplicating the Human Upright, Graviportal, Plantigrade Posture in Sitting, Standing, and Walking by the Use of Animal Models CHARLES 0. BECHTOL 111, MD The posture of the human presents a paradox unique among animals. The basic pattern of physical equipment is still evident after 360 million years of evolution. In contrast ta this, in the last 3.75 million years the human has assumed a unique upright posture and developed a large brain. The posture consists of: bipedalism, an upright torso, a graviportal leg, and a plantigrade foot. The addition of a large brain has allowed achievements far beyond those ol the great apes, such as sexual dimorphism, prolonged infancy, occult ovulation, hunting. meat-eating, tool-making, language, and culture. The available animal models, dog, cat, and sheep, are quadrupedal and specialized for rapid running. They are suitable for studies on fracture fixation and healing, allografts and homografts, repair of bone defects, and the atrophy of disuse. Evaluation 01 the effects of ageing and the long-term follow-up of total joint implant procedures present a more difficult problem.
Historical Review of Human and Animal Model Evolution The use of an animal model has many obvious advantages. It then becomes necessary tc understand the animal that we are attempting to model, the human, whose physical equipment presents a remarkable contrast. The large human brain and upright, bipedal posture are unique among all animals. The remainder of the physical characteristics are remarkably unspecialized and resemble the primitive locomotor equipment of the amphibian. When the first amphibians crawled out on land about 360 million years ago, the) had two pairs of limbs. Each limb had three functional joints, and a paw with five digits The hind limb was attached to the spine and the forelimb to the sternal complex. Thc time span from the primitive amphibian to the human is more than 300 million years, ye) the same pattern of two paired limbs is still present. In the human, the leg is attached to the spine by the pelvis; the three joints, hip. knee, and ankles are present; and the foot has five toes. The arm is attached to the Charles 0. Bechtol 111, MD, was the 1991 recipient of the Academy of Surgical Research’: Markowitz Award, given at the Academy’s Annual Meeting in Scottsdale, Arizona, Septembei 26-29, 1991. The following is Dr. Bechtol’s Keynote Address.
I 61
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
162
C. 0.Bechtol III
sternum by the clavicle; the three joints, shoulder, elbow, and wrist are present, and the hand has five fingers. This limb pattern persisted through the age of reptiles, the dinosaurs, and the primitive mammals. Almost the entire remainder of the mammals have specialized in some way that would prevent them from becoming the bipedal human. For example, 58 million years ago the ancestral horse was a herbivore about the size for a small dog. This evolved into the modern horse, specialized for rapid running, with reduction of the toes from four to one and loss of the clavicle. Other mammals modified their limbs for flying, swimming, digging, and the sharp claws of the feline predators. With few exceptions, the remaining nonhuman mammals of the world are quadrupedal. In walking, the quadruped has three of the four limbs in contact with the ground most of the time. This significantly reduces the torque when only one hind leg is supporting the pelvis. The few bipeds that exist do not have an upright torso as the human does. Our ancestors differ from the great apes in a number of achievements. The order oi their development is a matter of speculation. They are: bipedalism, sexual dimorphism, occult ovulation, meat-eating, hunting, tool-making, prolonged infancy, large brain, language, and culture. The great apes and some primates use the upright sitting posture to free their hands for manipulation and holding of objects, rooming of family members, and gathering and eating their vegetarian diet. The quadrupedal herbivores must limit their diet to what they can reach with their mouth; hence the giraffe has a long neck to reach additional food supply. All the great apes are capable of bipedal walking or running for short distances. They are limited by their hip flexion contractures from achieving an upright graviportal stance. Human infants also exhibit a flexion contracture of the hip in their first years of walking. The upright sitting posture preceded the upright bipedal posture in standing and locomotion. This allowed the lower thoracic and lumbar spine to increase in size and, in a sense, prepared the spine for the added vertical load of the permanent upright posture. This evolved only after our ancestors had developed sufficient brain capacity to cope with the limitations and advantages of bipedalism. A. B. Howell states that any animal must evolve to a level sufficient to survive in its ecological niche, or it will become extinct. Thus all other animals have a smaller brain than the human, because that smaller brain is sufficient for their survival. The large human brain implies that early humans successfully survived a difficult ecological niche. It is important to remember Howell’s conclusion, in his book Speed in Animals, that in cursorial animals such as our animal models, cat, dog, and sheep, the anatomy is designed for the ultimate burst of speed, and thus walking represents a minimal activity.
Review of Comparative Anatomy over Geological Time The development of mammals began about 80 million years ago. The basic pattern was quadrupedal with three limb joints and five digits. This pattern has been modified by subtraction in many mammals. The human, however, retains the basic pattern. With the exception of the modifications necessary for the upright posture and the large brain, our physical equipment remains relatively primitive and unspecialized . Ancestors with the potential for upright posture are found as long ago as 14 million years. These mammals were all quadrumanous; in other words, both hand or foot had an
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
Keynote Address
163
opposable first digit for grasping. This further implies that they were capable of climbing trees to escape predators or obtain additional food. The human terrestrial foot has lost the ability to oppose the great toe. This effectively commits us to living on the ground, and means a reduction in our food supply and the need to escape predators by methods other than climbing trees. Footprints of two adults and one child, preserved in fresh volcanic ash, indicate that our ancestors had a terrestrial foot 3.75 million years ago. The increase of the brain capacity occurred more than 3.75 million years ago. Any of our close relatives-gorilla, chimpanzee, gibbon, orangutan, or even baboon-could have physically assumed the upright posture, provided that they had developed the necessary brain capacity to take advantage of bipedalism. The actual ancestor was probably a nonspecialized primate that may be the ancestor of all the great apes and man.
Characteristics and Penalties of Human Bipedalism The gait is exclusively bipedal. This means a significant loss of running speed, compared to a quadrupedal posture. The torso is upright, not only while standing and walking, but also while squatting. The arms and the upper torso retain the same pattern of swing as the forelimb of the quadruped. This opposes the torque applied to the pelvis by the alternate thrust of the legs. The clavicle is retained, indicating not specialization for speedy locomotion in quadrupedal ancestors. The human’s greatest speed is only a fast bipedal trot. There are three functional joints of the plantigrade leg, hip, knee, and ankle, but the short length of the foot limits stride length. Stride length has been partially compensated for by lengthening the femur and tibia. The great toe was once opposable for tree climbing; the necessary muscles are still present in the sole of the foot.
Modern Human Has Had More Than 3.75 Million Years to Evolve from the Status of a Rather Primitive Quadruped to an Incompletely Evolved, Nonspecialized Biped, with Good Brain Capacity The diameter of the head of the femur and the head of the humerus are the same. Considering the significant increase of force applied to the femoral head in the biped, the femoral head, and with it the hip joint, should have enlarged. The spine and pelvis are also incompletely evolved to accomodate upright, graviporta, bipedal posture.
The Femoral Head Has Four Sources of Increased Loading (1) The quadruped bears more than 50% of its weight on its forelegs. In the biped 100% of the weight is on the hind legs. ( 2 ) To balance the torso above the hip by the pull of the adductor muscles, 2 to C times body weight is added to the load on the hip. (3) The human acetabulum is dysplastic. It does not cover the head completely for i graviportal leg, as is seen in the graviportal leg of the elephant. This increase: the pressure on the joint surface, depending on the degree of dysplasia. (4) The leg is graviportal. In standing and walking the hip and knee are fullj extended, and these motions come to an abrupt stop, due to ligament and musclt action. This adds load to both the hip and knee in full extension.
I64
C. 0. Bechtol I l l
Effect of Age on the Functions of the Acetabulum The acetabulum functions differently in the younger and older age groups. The youngei age group has an acetabulum that is a flexible horseshoe. The head moves upward wit1 weight bearing, and the horns of the horseshoe move together. This action promotes joint cartilage nourishment. In the older age group, the acetabulum becomes spherical, and the nourishment of the articular cartilage is less efficient, leading to degenerative changes.
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
Sexual Dimorphism and Its Effect on the Human Female Hip In the quadruped the effect of pregnancy and infant care on the female anatomy is minimal. In contrast, in the human there are several factors leading to significant sexual dimorphism. Because of the large size of the human fetal head, the acetabulae are displaced laterally from the center of gravity. This combination of a reduced mechanical advantage with greater abductor force requirement and a smaller femoral head leads to greater pressure on the femoral head in the female. The human female locomotor apparatus is designed for the requirements of the ninth month of pregnancy and the carrying of the infant for the first few years of life. The female anatomy from the waist down is significantly more powerful than from the waist UP.
The Human Knee Joint Is Nonspecialized The patellar facet encroaches on the femoral condyles. The fit of the patella in its groove and as it contacts the femoral condyles in flexion is extremely incongruous and requires a very thick articular cartilage to compensate for this. In contrast, the usual experimental animals, dog, cat, and sheep, have a patellar facet that is entirely separate from the femoral condyles. The fit of the patella in its groove is congruous. The tibiofemoral joint is entirely dependent upon its ligaments and muscles for stability. The lubrication and stability of the joint is enhanced by the presence of the semilunar fibrocartilages . The tibiofemoral joint is quite incongruous without the semilunar cartilages. The rolling, sliding motion of the femur on the tibia (which is controlled by the four bar linkage of the cruciate ligaments) concentrates the wear on the small area of the tibia, but distributes the wear over a large area of the femur.
Alignment of the Human Femur and Tibia In the graviportal human leg, errors in the alignment in varus and valgus can lead to accelerated wear. The normal alignment between tibia and femur is about 7 degrees. It is not possible to duplicate this in the available experimental animals, because their knee is never in full extension, standing, walking, or running.
The Paradox of the Ankle The ankle has the smallest area, the tightest fit, the thinnest cartilage, and the smallest range of motion of the major joints of the leg. However, osteoarthrosis is much rarer than in the hip or knee. Attempts to duplicate the function of the ankle are difficult
Keynote Address
165
because the articular surface is not cylindrical, but is a fustrum of a cone. In addition, the fibula moves laterally with dorsiflexion of the foot. This movement is 2 mm or less. The distal tibiofibular joint is a fibrous syndesmosis, while the proximal joint is a true joint.
Experimental Animals
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
As a general rule, experiments including the joints can be compared with similar procedures done on humans.
Use of Paired Bones As Controls The author broke 100 pairs of mongrel dog tibias and femurs in three point bending. The results were recorded as a percentage difference of the breaking strength of the pair of bones. This produced the typical bell-shaped distribution curve. One standard deviation was 2 . 4 % . In three pairs of bones, the recorded difference in breaking strength was 9% ’ . This provides a control for procedures done as nonsurvival experiments. Survivor type experiments of sufficient duration to produce atrophy of disuse will result in a less accurate control. With appropriate monitoring of the disuse atrophy, an adequate control might still be possible.
The Ageing Process The life span of the experimental animals is well known. Exactly how to equate this to the human life span remains a problem. For our body size, we have an unusually long life span. This is well up into the eighties. This, however, is largely due to the conquest of disease and the general improvement in public health measures. In vigorous sports, however, the athlete retires between the age of 35 and 40. Many of the human ailments for which we wish to establish animal models occur in the age group over SO.
Available Animal Models for Duplication of Human Problems There is no mammal that fully duplicates the human posture. All of the available experimental animals have had a much longer evolutionary history in geological time than man. As a consequence, they are more specialized, in one way or another, and cannol completely serve as an animal model for the human. Suitable comparisons can be made in the areas of: fracture healing, the filling of bone defects with allografts and homografts, the resulting reduction in the breaking strength produced by various sizes anc shapes of bone defects, the efficiency of the various methods of fracture fixation, anc similar types of experiments.
Duplication of Human Upright Posture The quadrumanual monkeys and apes can duplicate the human upright sitting posture but not standing or walking. The mammals more specialized for rapid locomotion-dog cat and sheep-are suitable for procedures on muscles, bones, and joints. They have however, eliminated the clavicle. The dog and cat are digitigrade, while the sheep i: unguligrade with some reduction of the digits. These changes facilitate rapid locomo tion.
I66
C. 0.Bechtol III
Quadrupedal Animals with a Fully Graviportal Hind Limb The elephant has a fully graviportal hind limb. Although the knee fully extends at the end of the stride, the hip does not. Of the primates, only the gorilla stands and walks quadrupedally, with a graviportal hind leg.
J Invest Surg Downloaded from informahealthcare.com by University of Toronto on 11/27/14 For personal use only.
Duplicating the Causes of Loosening of Total Joint Prostheses Animal models will continue to serve an important role in the development and testing 01 bioengineering materials. The osteolytic effects of the products of wear can be compared on a qualitative and quantitative basis. Many of the problems of loosening, breakage, and wear in human total joint replacements are not evident before 15 to 20 years. The relatively short life span of the available experimental animals will make such research difficult if not impossible.
