Why Are Babies' Heads So Large In Proportion To Their Body Size?

• Periodical List
• Annu Proc Assoc Adv Automot Med
• v.42; 1998
• PMC3400202

Annu Proc Assoc Adv Automot Med. 1998; 42: 93–113.

An Overview of Anatomical Considerations of Infants and Children in the Adult Earth of Automobile Safety Blueprint^§

Abstract

The baby and child differ structurally from the adult in a number of ways which are critical to the design for protection against impact forces and for acceptable occupant restraint systems. The purpose of this newspaper is to bring together a profile of the anatomy, anthropometry, growth and development of the babe and kid. Historic period differences related to the proper design of child restraint systems are emphasized. Problems discussed include child--adult structural differences, middle of gravity of the trunk, the head mass in relation to the neck and general body proportions, positions of key organs, and biomechanical properties of tissues.

Introduction

Infants and children are not miniature adults. Body size proportions, muscle bone and ligamentrus strengths are dissimilar and thus occupant packaging for crash protection need special consideration. This paper is an overview of pediatric size and proportional differences with considerations of some child injuries in car crashes along with a review of some biomechanical data.

GROWTH OF THE INFANT BODY AS A WHOLE

Growth and evolution of the human body occurs continuously from birth through senesence (old historic period). Such development is sporadic and non-uniform, however it does not occur haphazardly. For the most office, incremental growth of any dimension or part of the body occurs according to predictable trends. Most body dimensions follow trends which involve rapid growth separated by a period of relatively slower or uniform growth. At that place are notable differences in the timing of these incremental growth spurts, for most tissues and organs of the body collectively reverberate the general body growth. As an example, the brain grows rapidly during the period before birth and and then slows considerably during the per-schoolhouse years. At nativity the brain is typically 25% of its developed size, although the trunk weight of the newborn is only nigh 5% of adult weight (Stuart and Stevenson, 1950). Importantly, about half of the postnatal growth of the brain volume occurs during the first year of life, and attains nigh 75% of its adult size past the stop of the second twelvemonth. By contrast, genital organs develop very slowly during this period only, instead, reach their adult size during the second decade of life.

Subcutaneous tissue (body fat) is a body component infrequently considered as a factor in the proper design of protective devices for the infant body. This tissue tends to increase rapidly in thickness during the start nine months following birth, which growth of the trunk equally a whole is much slower. Subsequently this period of high incremental change there is a menstruation of less rapid growth, so that by five years of age the thickness of the subcutaneous layer is nigh one-half the thickness of the nine month old infant.

Loading of the body past strap-blazon restraints must occur in areas where the body is strongest, i.e., on solid skeletal elements. In some, the fatty subcutaneous tissue tin produce bulges or 'rolls' of flesh in the areas of placement on such restraint straps. Thus, proper positioning of restraint straps on the chubby 1–3 year old may be difficult to maintain because of the abundance of this fatty tissue.

Changes in trunk weight similarly follow characteristic age group trends (Krogman, 1960; Krogman and Johnston, 1965; Martin and Thieme, 1954; and Meredith, 1963). From the 10th day after birth, when the post-birth weight loss is normally regained, at that place is a steady increase in weight so that during the kickoff iii months an boilerplate babe gains most two pounds per month, or nearly 1 ounce per day (Krogman 1941). At five months the birth weight has doubled. Beginning at six months, there is merely a one pound gain per month in weight then that the birth weight is tripled at the finish of the start year and quadrupled at the end of the second. From this fourth dimension on, the charge per unit of increment in body weight gradually decreases during the 2nd year according to a factor of i-one-half pound per month (Krogman and Johnston, 1965). After the 2nd year gain in weight may become irregular and less predictable on a monthly footing. As a general design, afterwards the 2nd year and until the ninth year there is a v pound annual increment. Thus, at five years the body weight is six times the nascency weight and in the 10th twelvemonth the weight of the body is ten times the birth weight (Krogman, 1960).

Changes in trunk height and torso proportions besides take specific age trends (Figs. 1–3). The newborn child is approximately xx inches in total body length. During the first year this height is increased by approximately ten inches. Until about the seventh twelvemonth, total torso length should be doubled past the 4th twelvemonth and tripled by the 13th yr. The top of an adult is about twice the height of a ii-twelvemonth-onetime child. From the second to the 14th twelvemonth, total body height increases (in inches) according to the formula: Superlative=historic period in years × 2.five + 30 (Weech, 1954).

Percentage distribution of trunk segments every bit related to pre- and postnatal development. (Modified from Salzmann, "Principles of Orthodontics.")

Developmental alter in body proportions as seen in direct comparison between the developed and the newborn, child and adolescent. (Modified from Chenoweth and Selrick, "School Health Problems.")

Age changes in the ratio between sitting (trunk) height and total body height cannot exist overlooked when because the dynamics of changing body proportions. (Fig. four). Sitting height represents near seventy% of the full height at birth, but falls quickly to most 57% in the 3rd year. At 13-years of historic period in girls, and two years afterward in boys, the ratio of sitting superlative to total body summit is most 50%.

Changes in sitting height from birth to adulthood.

Longitudinal growth of limb bones occurs every bit long as the epiphyseal cartilage prolificates; growth ceases when the cartilage ossifies and fuses to the bone segments surrounding it. Since the fusion of epiphyses in the lower extremities occurs before in girls than in boys, girls tend to have a lower 'sitting acme-total trunk height' ratio than boys, between eight and 12 years, and a higher one betwixt the 14th and 18th year.

Thus, particularly in the early years of life, the infant is markedly elongating in stature. Likewise, the postural changes of the infant, from a recumbent 1 to that of a slouched, upright position, is completed inside a relatively short period of time.

In general, children of either sex are of the same height, weight, and full general torso proportions up to 10 or 11 years of age; yet, not infrequently one sees girls slightly taller than their male counterparts fifty-fifty at ages six–ten. Girls tend to have an earlier pubertal growth spurt between 11 and 14 years and, in general, are taller than boys of this historic period. In the early to mid-teens, the boys take hold of up, and so surpass the girls in stature (Watson and Lowrey, 1967). These variations in total height at the 10–14 twelvemonth age span reflect the differences in sitting height betwixt boys and girls.

At nascency the head is ane-4th the total body length, whereas in the adult information technology is one-7th (Fig. five). Also the trunk is long with the upper limbs being longer than the lower limbs. From the second half of the first year to puberty the extremities grow more rapidly than the head. At puberty the growth rates of the trunk and limbs are most equal, only the body continues to grow in length later limb elongation has declined in the adolescent period. The mid-bespeak of the body is slightly to a higher place the omphalos (navel) in the newborn, and a 2 years the mid-signal of the body is slightly below the navel; at about 16 years, this mid-bespeak is near the pubic symphysis.

The proportional changes in trunk segments with historic period.

The center of gravity of the child varies according to age, kid size, weight, and body course as well as sitting posture. A study past Swearingen and Young (1965), of individuals at ages v, 10, 12, and eighteen years, indicated that the middle of gravity (CG) cannot exist located accurately and precisely in groups of seated children. They found that a plot of the CG would fall within an asymmetrically ellipsoidal expanse. In these children information technology was found that the CG was located vertically on the torso well above the lap belt level. This high CG in children must be considered when developed lap belts are used to restrain children, since the greater body mass above the belt may crusade the child to whip forward more in the case of an adult. In a subsequent study of infants aged viii weeks–3 years, information technology was found that the CG is located even college on the body (Immature, 1968).

THE HEAD

In automotive collisions, the child'southward caput is the body area most frequently and most seriously involved. In a study of children'southward injury patterns in 14,520 rural motorcar accidents involving 31,925 occupants, information technology was institute that children (nascence through xi years) had a frequency of 77% head injuries (Moore et al, 1959). This was a much greater frequency than either adolescents (69%) or adults (seventy%) in this written report, although it was establish that child head injuries were of a more pocket-sized variety than either adolescents or adults. Agran and Winn (1987) identified head injuries in l% of children, either lap-shoulder belted or unrestrained. Contributing to specific head impact issues are the big caput of the child, the relatively soft, pliable, and elastic bones of the cranial vault, and the fontanelles. As compared with the developed, these features make the head of the child less resistant to bear on trauma. In a collision, for example, the unrestrained kid, because of his large caput and high CG, would 'lead with his caput'. Crash data covering infants and children up to 4 years of age indicate that 77% of those who were injured in automobile accidents had head injuries (Kihlberg and Gensler, 1967). The vulnerability to injury of an babe's head occurs even prior to birth, as has recently been shown in a study of fetal deaths involving restrained and unrestrained pregnant women in car accidents (Crosby et al, 1968). The reasons for this greater frequency of head injury in children can be demonstrated both anatomically and biomechanically. The child'south head is proportionately larger than in the adult (Young, 1966). (Fig. 5). This heavier head mass and resulting higher seated CG in young children, coupled with weaker neck supporting structures, may exist, in function, the ground for this higher frequency of caput injury.

At nativity the facial portion of the head is smaller than the cranium having a confront-to-cranium ratio of i:8 (cf. adult ratio of i:two.5). Relative to the facial profile, the newborn forehead is high and quite bulged, due to the massive size of the frontal lobe of the brain (Fig. half dozen). Thus, in the newborn and infant the confront is tucked below the massive brain case (Fig. 7). The large head-modest face up blueprint is noticeable in children even up to ages 7 and 8, Vertical growth of the infant confront occurs in spurts every bit related to both respiratory needs and tooth eruption. These growth spurts occur during the first six months after birth, during the 3rd and fourth year, from the 7th to 11th yr, and again between the 16th and the 19th year. The first growth spurt is importantly olfactory as associated with the vertical growth of the upper portion of the nose and nasal cavity. The last spurt is related to adolescent sexual development.

Soft tissue profile changes of the caput and face.

Sequential changes of various head and face regions.

Baby head shape besides differs significantly from that of the adult (Fig 8). In the babe the cranium is much more elongate and bulbous, with large frontal and parietal (side) prominences (Fig. eight). At birth the circumference of the head is about 13–14 inches. Information technology increases by 17% during the first three months of life, and past 25% at half dozen months of historic period. Information technology increases by nigh 1 inch during the 2nd year, and during the 3rd through the 5th year head circumference increases by nearly one-half inch per twelvemonth. At that place is only a iv inch increase in herd circumference from the finish of the 1st yr to the 20th yr (Fig 9).

A comparison of face up-braincase proportions in the kid and adult. The horizontal line passes through the same anatomical landmarks on both skulls.

Skull profiles showing changes in size and shape. (Modified from Morris' "Human Anatomy.")

Caput circumference increases markedly during the outset postnatal year due to the progressive and rapid growth of the encephalon as a whole. The important relation of brain size and cranium size can be demonstrated on a percentage ground, which shows that 70% of the developed brain weight is achieved at 18 months, 80% at 3 years, ninety% at five–8 years and approximately 95% at the 10th year. In the developed the average brain weight is 1350 g.

Babe and kid skulls are considerably pliable, due to the segmental development and arrangement of the skull bones, plus the flexibility of individual bones which are extremely thin. The skull develops as a loosely joined system of bones formed in the soft tissue matrix surrounding the encephalon. Junctions between bones are relatively broad and big, leaving areas of brain covered by a thin fibrous sheath and somewhat exposed to the external environment. These 'soft spots' (fontanelles) are several in number and are most obvious in the frontal and posterior skull regions (Fig. 10). The mastoid fontanelle, between the occipital and parietal basic, closed about 6–8 weeks after birth. However, a much larger midline junction between the frontal and parietal bones, i.east., frontal fontanelle, is not closed by os growth until approximately the 17th month.

Size and location of the fontanelles. Arrows signal management of fontanelle closure.

At nativity all of the potential structures for the development of teeth are present. The early teeth first erupt at bout 6 months of age and continue to erupt progressively. The kid begins to lose his deciduous teeth about 5–6 years of age after which they are replaced past the permanent teeth.

Trauma to the jaws of infants or pocket-size children, especially in the area where the unerupted teeth are found can atomic number 82 to serious problems in tooth eruption, tooth spacing, tooth arrangement and alignment. Traumatic injuries to the child's lower jaw (mandible) may exist related to abnormal facial profiles with increasing age. The normal changes in size and position of the lower jaw are dependent upon a growth site in the mandible located near its junction with the skull. If this important growth site is significantly traumatized, the normal changes in size and position of the mandible diminish resulting in a smaller mandible and a recessive chin.

THE NECK

In that location are several unique aspects of the anatomy of the kid's neck. Neck musculus strength increases with historic period still, with the greater caput mass perched on a slender neck, the neck muscles generally are not developed sufficiently to dampen vehement caput movement, especially in children. In a study of lap-shoulder belted children, ages x–14 years in all types of motor vehicle crashes, most 21% had cervical strain (Agran & Winn, 1987). The neck vertebrae of children are immature models of the developed. These cervical vertebrae are mainly cartilaginous in the infant, with complete replacement of this cartilage past os occurring slowly. Articular facets, the contact areas between the vertebrae, are shallow; cervix ligaments, as elsewhere in the body, are weaker than in adults. The disproportionately large head, the weak cervical spine musculature, and laxity, tin can subject area the infant to uncontrolled and passive cervical spine movements and perhaps to compressive or lark forces in certain affect deceleration environments. These all contribute to a high incidence of injury to the upper cervical spine as compared to the lower cervical spine area (Sumchi and Stemback, 1991).

The articular facets of the baby and young children are oriented in an fifty-fifty more than horizontal direction than in the adult (Kasai, et al, 1996) (threescore deg. @ i year, 53 deg. @ 3 years and 47 deg. @ 6 years). The "cervicocranium", the base of the skull, C1, C2 and the C2/C3 disc is a singled-out unit in infants and pocket-size children, and should be considered as a specialized area of the cervical spine because of its anatomical difference from the lower and more uniformly shaped cervical vertebrae (Huelke, et al, 1992). Using dynamic cervical spine radiographs it has been shown that the fulcrum for flexion is at C2-C3 in infants and young children, at C3-C4 at virtually historic period v or 6 and at C5-C6 in adults (Baker and Berdon, 1966).

In that the skull base, C1 and C2 move every bit a unit of measurement in flexion and extension, and in some rotation, it is non surprising that anterior displacement of the entire cervicocranial unit can occur after traumatic disruption of the posterior portions of C2, causing separation of the neural arch ossification centers, stretching of the rubberband ligaments, or bilateral fractures of the pedicles without show of dislocation (Sumchi, and Stembacck, 1991). A distraction force on the cervical spine can pull apart the cervical cartilagenous-osseous structures and associated ligaments and, if in a forrad management, can crusade spinal cord damage (Finnegen and McDonald, 1982; Tingvall, 1987).

Information technology has been reported that pseudosubluxation or physiological anterior deportation of C2 on C3 of more three millimeters occurs in approximately 24–33% of children less than viii years of age (Dunlap, et al, 1958; Fuchs, et al, 1989; Papavasilou, 1978). In autopsy specimens the rubberband infantile vertebral bodies and ligaments allows for column elongation of up to two inches, but the spinal string ruptures if stretched more than 1/4 inch (Leventhal, 1960). Thus it is difficult to differentiate physiological displacement from pathological dislocation of C2 on C3 in childhood, particularly when an ten-ray is taken with the child's head in flexion (Swishuck, 1977). Occasionally in young infants, there is a reversal of the normal inductive curve, seen in lateral C-spine x-rays, probably due to the weak, immature cervical musculature (Harris and Edeiken-Monroe, 1987).

If neck motion exceeds tolerable limits, dislocation of vertebrae and perchance injury to the spinal cord can occur. This combination of anatomical features results in lowered protection of the neck in rapid deceleration and if the head is rotated or snapped to the side or to the rear, serious damage might occur to the delicate arrangement of critical arteries or veins of the brain, to nerves, to the vertebrae, and/or the spinal string itself. The machinery of pediatric cervical injury is relatively straight forwards---caput flexion with either a tension or compression component and a relatively restrained body. Basically, in the frontal-type crash the head continues forward across the belted torso. The construction of the kid's neck certainly plays a part in the injury. Fuchs, et al (1989) best summarized the reasons for this, including (1) A heavy caput on a minor body results in high torques beingness applied to the neck and consequently, high susceptibility to flexion-extension injuries, (2) The lax ligaments that allows a significant caste of spinal mobility (anterior subluxation of up to iv.0 mm at C2-3 or C3-4 may occur as a normal variant), (3) The cervical musculature is not fully developed in the infant assuasive for unchecked distracting and displacement forces, (iv) The facet joints at C1 and C3 are nearly horizontal for the commencement several years of life allowing for subluxations at relatively niggling force, (v) Young uncovertebral joints of the C2 to C4 levels may not withstand flexion-rotation forces (half dozen) The fulcrum of cervical motility is located college in young children (C2-3 level than in adults (C5-6).

THE CHEST

Thoracic injuries in children subjected to affect usually occur to the internal organs. The thoracic walls are thinner and the ribs more than elastic in infants and young children than in the adults. Therefore, affect to the thorax of an infant or a pocket-size child will produce larger amounts of chest wall deflection onto the vital thoracic organs, due east.g. heart, lungs. As clinicians well know, closed cardiac massage in infants can exist performed by using only one or 2 fingers which well demonstrates the highly elastic nature of the chest wall.

At birth the infant heart lies midway between the pinnacle of the head and the buttocks. The long centrality of the eye is directed horizontally in the fourth intercostal space with its apex lateral to the midclavicular line. These relationships are maintained until the 4th year, and later the heart gradually moves downwardly, due to the elongation of the thorax, until it comes to prevarication at the 5th intercostal space with its apex inside the midclavicular line. Until the first year, the width (or length) of the middle is no more than than 55% of the chest width taken at the xyphoid line. After the first yr, heart width is slightly less than 50% of the breast width (Fig. 11).

Schematic diagram of the position al changes of the heart within the chest at diverse ages. (Redrawn from Watson and Lowrey, "Growth and Development of Children.")

At birth the chest is circular, simply equally the infant grows the transverse bore becomes larger than the inductive-posterior dimension, giving the breast an elliptical advent. At birth the breast circumference is about 1-half inch smaller than the head. At 1 year the breast is equal to or exceeds head circumference slightly; later on i yr the chest becomes progressively larger in bore than the head.

Scientists are not entirely in understanding as to the primary biomechanical causation of cardiac trauma during impact in the adult. Researchers such every bit Stapp (1965) and Taylor (1963) report that pressure level is the major factor. However, cardiac rupture has been produced experimentally in animals with the blood volume entirely removed, strongly suggesting that other factors are involved (Roberts et al, 1965). Lasky et al (1968), studying adult humans involved in steering-wheel impacts, believes that aortic laceration occurs at the weakest and narrowest point of the aortic curvation, and that this anatomical fact is of biodynamic significance.

Introducing a new consideration, Life and Pince (1968) have demonstrated experimentally in animals that the contractile state of the ventricular myocardium at the instant of impact plays a critical role in whether or not cardiac rupture volition occur. Clinical shock with abnormally slow heart and pulse rates (bradycardia) occurs without structural failure in human adult affect tests, and constitutes a chief limitation to the rate of onset (Taylor, 1963).

No thoracic impact data are available for children. Considering the differences between child and adult morphology, impact tolerances for the child are probably considerably less than those of the adult.

THE ABDOMEN

Although statistically meaningful studies on child intestinal injuries have not been conducted, the effect of blunt abdominal trauma to children, as compared to adults, has been suggested in the literature. Tank et al (1968), noted that only cognitive injuries and burns outrank injury to the abdominal organs as a form of serious accidental injury to children. In adults, blunt injury to the abdominal viscera presents the virtually difficult diagnosis and handling, and results in the highest mortality rate (Fonkalsrud, 1966; Orloff, 1966). Thus, any blunt abdominal injury can be potentially serious, but such injuries to the infant and kid are much more critical due to their developing and immature structure, large organ relationships, and almost consummate lack of overlying muscle or skeletal protection.

The burl of the newborn abdomen is accentuated past the abdominal viscera pushing frontwards during respiration against the weak and atonic muscle wall of the abdomen. The right side of the infant and newborn abdomen is peculiarly enlarged due to the low position of the liver which occupies two-fifths of the abdominal crenel. Along the midclavicular line the liver is approximately 2 cm beneath the costal margins in the newborn; one and one-half cm beneath the margin for the remainder of the commencement year; and one cm below from 18 months to half dozen years. After nigh the 6th–7th twelvemonth, the liver is seldom palpable except in abnormal cases. On a weight ground, the liver of the newborn comprises 4% of the full body weight, and by puberty weighs 10 times equally much (Watson and Lowery, 1967). The liver, although considered every bit an abdominal organ, lies nearly entirely deep to the right lower ribs and the highly elastic ribs of the child offering minimal protection for this organ from impact.

Posteriorly, a similar relative migration of the bony thorax downward occurs to provide some protection for the spleen, kidneys, and suprarenal glands as the infant ages. At birth, for example, the kidneys occupy a large portion of the posterior abdominal cavity owing to their relatively large size.

In the newborn, the urinary bladder lies close to the lower abdominal wall with only its lower portion located behind the pubic bones. During childhood, much of the bladder descends into the pelvic surface area where it is more protected past the bony pelvis.

Once more, many of the child abdominal viscera are relatively unprotected by os as compared to the developed. The bladder is located higher, outside the pelvic area, the liver and kidneys are relatively exposed, all being more bachelor to traumatic insult. The liver is an organ which is not well designed for withstanding traumatic insults even in the developed. Traumatic liver injuries produce the highest mortality rate of whatever intestinal organ (Di Vincenti et al, 1968). With the smaller breast and pelvis of the child, less of the abdominal contents are protected by the rib cage and bony pelvis, and can be more easily injured.

Dimensions of the abdominal area also differ from that of the adult, both proportionately and in relation to position of body organs. Abdominal girth, in general, is almost the same as that of the chest during the beginning two years of life. After 2 years, increases in intestinal circumference at the umbilical level practice not keep pace with the increases in thoracic girth. Pelvic latitude is some other dimension which is less field of study to variations in body posture and tonic activity of the muscular abdominal wall. The maximum distance between the external margins of the iliac crests is approximately 3 inches at birth, v inches at 1 year, seven inches at five years and nine inches at 10 years of age. More often than not, in the early function of infancy in that location is little alter in torso form, simply subsequently the supposition of cock posture at that place is a relative reduction in the anterior-posterior diameter of both of the thoracic and abdominal regions, accompanied past a decrease in the relative size of the umbilical region and a relative increase in the lumbar region. These changes continue throughout babyhood and early adolescence.

THE VERTEBRAL Column

Normal evolution of cock posture involves a gradual transition from the early crawling stages involving interrelationships of the extremities, spine, and pelvis, to the well-balanced weight- bearing relationships typical of the adult. When the infant first stands, the pelvis is tilted far forward on the thighs and an erect posture is first attained in infancy concurrent with the development of the lumbar (low dorsum) bend. As a result of this lumbar curve, combined with increased tonic activity of abdominal wall muscles the infant develops his feature sway-back and abdominal prominence which is maintained throughout pre-schoolhouse years. The infant pelvis gradually rotates upward and forward get-go to found an adult-like posture. The curvature of the sacrum as seen in the adult is already present at nativity; however, in infants the vertebral column higher up the sacrum is usually direct (Fig. 12).

Curvature of vertebral cavalcade emphasizing the evolution of principal curvatures (P) and secondary curvatures (S). Note: in the infant there are only 2 primary curves, i.e. thoracic and sacral. In the developed there are secondary curves in the cervical and lumbar regions. In the aged only the primary curves persist. (Modified from Johnson and Kennedy, "Radiographic Beefcake of the Human Skeleton.")

Early in infancy the baby tin raise his caput while lying prone, and the cervical (neck) curve commencement becomes well established as the head is held cock and cervical muscles go developed and increase their tone activeness. Past the 3rd or 4th month the infant can sit with back up and by the 7th month can be expected to sit alone. At 8 or 9 months the infant tin can usually stand up with support and then can stand without assistance past 10–14 months.

In the adult, the prominent inductive superior iliac spines are used equally anatomical ballast points. Only in children these spines are not well developed until about 10 years of age and basically exercise non yet exist. Rather this anterior pelvic area is a broad gentle bend without a prominent spine as in the adult.

THE LIMBS

In considering the growth of the extremities it is necessary to examine factors of skeletal embryology and subsequent dimensional changes (Scammon and Calkins, 1929). Considering first the trends in dimensional growth of the limbs, it is more often than not noted that the lower limbs increase in length more speedily than do the upper limbs. At about 2 years of age, for instance, their lengths are equal simply in the adult the lower limb is almost on-sixth longer than the upper limb. The adult relations of the different limb segments are well established prenatally; notwithstanding, there is some reduction in the relative length of the hand and of the pes afterward birth. At birth the lower limb forms about 15% of the torso volume and in the developed reaches about xxx%. In dissimilarity the upper limb constitutes nearly eight% of the trunk weight at birth and maintains this same proportionality thereafter.

As in the skull, the long bones of the extremities pass through successive developmental stages which, when compared to adult morphology, make the limb bones less tolerable to trauma. In early development before birth, long bones are typically represented by a shaft of bone which grows in diameter by add-on of new bone on its surface with concomitant erosion inside the shaft. This evolution of the shaft tin all-time exist described as a tube that progressively increases in bore. Impact tolerances of children's bones are dependent upon the changing girth of the os and relative proportions of the marrow cavity and bony walls, as well as the proportions of inorganic and organic materials that form bone tissue. In the early development of bone tissue, organic materials outweigh inorganic components. The degree of flexibility or torsional force of the os itself is straight related to the organic component of the os structure. The preponderance of organic cloth continues through boyhood subsequently which at that place is a gradual buildup of inorganic bone substance.

Modify in length of long bones is a function of the continued growth of epiphyseal cartilage. In the early on development of a long os the shaft is capped on both ends by cartilage. From tardily fetal life through puberty bond tissue appears in the cartilage at either stop of the shaft but does not attach to the shaft. There is a remaining cartilaginous epiphyseal plate between the bony shaft and the bony epiphyseal ossification centre at each end. The surface of the epiphyseal cartilage in contact with the long bone shaft continues to abound which effectively moves or pushes the epiphyseal os cap away from the shaft. This activeness of the epiphyseal cartilage accounts for increases in length of the long bone. Finally, when the adult length is attained for a specific os as influenced by sex, race, diet and endocrine residue, the cartilage of the epiphyseal plate stops proliferation and begins to ossify. Thus, the bony epiphyseal cap is united to the shaft. In females the epiphyses unite sooner so that growth in length ceases earlier by near 2–3 years when compared to males of similar ages. But even in the male near of the fusions of long bone epiphyseal cartilages are completed at virtually the twentieth year. Obviously, since bone length is a cistron of epiphyseal cartilage growth, traumatic displacement of the cartilage out of line with the normal solitary axis of the os tin can lead to gross limb baloney and malformations.

Conclusions

Infants and children are not miniature adults. Their anatomy differs from the adult in a number of ways which should be considered in the proper blueprint of occupant restraint systems specific to their age. Within the framework of automobile safe design information technology should be emphasized that:

1. The frequency of head injuries in children involved in auto accidents may be due to the child's proportionately large head and higher center of gravity. As a consequence, infants and children restrained by a lap belt have a greater chance of beingness projected over the restraining chugalug because the CG and body fulcrum is located in a higher place the belt location.

2. Observations that the kid'southward head is relatively massive and supported poorly from beneath take been implicated in head snapping with rapid body deceleration. Such sudden snapping or rotation of the relatively unrestrained child's head can traumatize related nerves, blood vessels, and spinal cord segments.

3. Contributing to encephalon injuries of the immature child is the relative lack of skull protection since, early in life, the skull is not an intact bony instance for the encephalon but is a series of broadly spaced rubberband bones.

4. Growth rates of dissimilar parts of the torso vary with historic period. For example, the mid-signal of the torso is to a higher place the navel at birth, slightly below it a ii years of historic period and nearer the pubic basic at sixteen years.

5. Since growth of the child is dependent upon the normal activity of growth centers, protection of these centers is vital. Abnormalities of torso stature and limb mobility might upshot from injury to growth centers of the extremities. Similarly, in the caput, the arrangement of teeth besides as the facial contour tin be affected past traumatic injuries to the facial growth centers.

6. Different the adult, the organs of the breast are housed in an elastic and highly compressible thoracic cage. Organs as the lungs and eye are extremely vulnerable to nonpenetrating impacts to the chest. The smaller rib cage also means less protection is offered to larger abdominal organs which would normally receive some protection form the larger stronger rib muzzle of the adult. The highly rubberband structure of the thoracic cage is not amenable to direct trauma or loading of webbed restraints in children.

​

Increase in total stature at various ages equally compared to the adult. (Modified from Chenoweth and Selrick, "Schoolhouse Health Problems.")

Footnotes

^§This newspaper is a modification and update of "Infants and Children in the Adult World of Automobile Safe Design: Pediatric and Anatomical Considerations for Blueprint of Child Restraints", Burdi, AR, Huelke, DF, Snyder, RG, et al, J Biomech. 2:267-280,1969.

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Articles from Almanac Proceedings / Association for the Advancement of Automotive Medicine are provided here courtesy of Clan for the Advancement of Automotive Medicine

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Why Are Babies' Heads So Large In Proportion To Their Body Size?,

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3400202/

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