Principles of Human Biomechanics

KEY POINTS

The direction and magnitude of forces affect the form of the developing fetus.
Abnormal mechanical forces give rise to deformation in humans in much the same fashion as abnormal forces may deform a tree.
The tissues of a human fetus are especially pliable and easily molded, especially as a fetus fills out the uterus in late gestation.
Many factors can lead to an aberrant fetal position in late gestation, and shortly after birth the position of comfort for an infant can provide insight into their position during late gestation.
The actual alignment of the fibrils in the extracellular space appears to be determined predominantly by mechanical forces.
The stress of muscle-tendon tension on a bone affects the growth and form of the bone.
Current medical practice uses custom-designed orthotic molding helmets to reshape infant heads that have become deformed by their resting positions.

Rather simple principles apply for the role of mechanical factors in morphogenesis. These are that the direction and magnitude of forces affect the form of the developing individual, as illustrated in Fig. 52.1 . Some of the factors that affect the magnitude and direction of forces are summarized in Box 52.1 . One major influence on the nature and alignment of forces is growth. Thus the rate and shape of growth in a particular basic tissue will determine the magnitude and direction of the forces it exerts on adjacent tissues. The plasticity of a tissue is another factor that affects its liability to be altered by mechanical forces. The tissues of a human fetus are especially pliable and easily molded, especially as a fetus fills out the uterus in late gestation ( Fig. 52.2 ). Many factors can lead to an aberrant fetal position in late gestation, and shortly after birth the position of comfort for an infant can provide insight into their position during late gestation.

Box 52.1

FACTORS THAT AFFECT THE MAGNITUDE AND DIRECTION OF MECHANICAL FORCES

External resistance to growth and/or movement
Growth rate and shape of basic tissue
Forces of fluid flow (or pressure)
Plasticity of the fetus
Forces of muscle pull
Forces of gravity

Another major influence is tension or compression related to muscle pull, gravity, or local constraint. The orientation of fibers within a structure made from connective tissue, such as bone, cartilage, tendon, ligament, or suture, is determined by such forces. For example, collagen fibrils, the basic threads of tensile strength within connective tissues, align in the direction of stress ( Fig. 52.3 ). These fibrils are synthesized by fibroblasts under genetic direction, but the actual alignment of the fibrils in the extracellular space appears to be determined predominantly by mechanical forces. If there is no consistent direction for the mechanical forces, the collagen strands may be haphazardly arranged. Given a sustained direction of forces, they become organized and woven in relation to those forces.

The general precepts of mechanical engineering are relevant to human biology, as is the nomenclature ( Table 52.1 ). In many tissues there is an integral interaction between growth and the forces of tension and compression. This is readily evident in the relationship between muscle usage and the size of muscle mass. The greater the forces, the larger the muscle mass tends to become. In turn, the stress of muscle-tendon tension on a bone affects the growth and form of the bone. With greater pull by a muscle on a bone, the size of the bony promontory at the site of the muscle attachment to the bone enlarges. For example, the prominent bony ridge down the center of the skull in the male gorilla shown in Fig. 52.4 relates to the size of the attachment of the powerful temporalis chewing muscles. The female gorilla does not have such a prominent ridge because her muscles are less powerful.

Table 52.1

Mechanical Engineering Terms Applicable to Human Biology

Term	Definition
Deformation	Altered form caused by unusual forces
Stress	Intensity of force (force per unit area)
Strain	Sufficient force to cause deformation
Tension	Stretching forces
Compression	Compressive forces
Torsion	Twisting forces
Shear forces	Forces contrary to the main orientation of a tissue
Axial forces	Forces parallel to the main orientation of a tissue

Humans are ruled by gravity, which has a major impact on human form. Because their surface area is relatively small in relation to overall volume, humans are more heavily influenced by gravitational forces than smaller mammals. In contrast, the impact of gravity is less significant to very small animals that have a high surface area to-volume ratio, and their world is more dominated by surface forces. Examples of the impact of mechanical forces on morphogenesis abound in nature. The magnitude and direction of growth on form are dramatically reflected in the chambered nautilus, whose shell represents the successively larger living quarters occupied by this mollusk as it outgrows and creates one new chamber after another, resulting in an equiangular, logarithmic spiral shell, as shown in Fig. 52.5 . This exemplifies the basic orderliness of growth and its biomechanical consequences on the development of the form of an organism.

Another beautiful example from nature of the impact of the size and direction of forces on form is the alignment of cellulose fibers in a tree. The reason the tree grows straight up relates to the alignment of cellulose fibers in the direction of the stress force of gravity. Extrinsic forces, such as compression by snow during the winter, may deform the young tree. However, once released from such temporary constraint, the sapling will again tend to grow in the direction of the force of gravity. The forces of prevailing winds may be sufficient to deform a tree ( Fig. 52.6 ), and controlled external forces may be used to deform a young tree into a form that will be maintained after the tree is fully grown.

The collagen fibrils within developing bone may be compared with the cellulose fibers of a tree. They are the basic structural elements and they also align in the direction of stress in such a manner as to resist shear forces, as previously indicated. This is dramatically evidenced in the longitudinal section of the human femur shown in Fig. 52.7 . The bony spicules are aligned in the direction of the stress of gravitational weight bearing and in relation to muscle pull. The form of the upper femur relates to the combined forces of weight bearing plus the pull of major muscle groups. This results in a sloping “neck” of the femur and the two large promontories of bone in this region of the femur, the greater and lesser trochanters.

The force lines that are evident in the form of such a bone may be of value to the engineer in their design of somewhat similar structures. D’Arcy Thompson relates the story of a Zürich engineer, Professor Culmann, who became famous for his design of a mechanical crane. In the design of his crane he used stress forces that he observed in a longitudinal section of the human femur in 1871. When he was shown the section of the femur by his friend, Professor Hermann von Meyer of the Anatomy Institute of the University of Zürich, Professor Culmann exclaimed, “There is my crane!” Possibly, there should be a closer interchange between biology and engineering, as the biomechanically determined “lines of stress” are naturally evident in many living creatures. The differences in these lines of stress will vary with the function of a bone. In the wings of a soaring bird, such as a vulture, the lines of stress relate to forces above and beneath the wing. The bony form in the pneumatized bone of a vulture is aligned accordingly, and it appears remarkably like the form that engineers learned to use in the wing girders of early airplanes, as well as in bridge trusses.

Mechanical forces play a major role in the form of an individual and relate to the function of that individual; for example the pneumatized lightweight but strong bones of the larger soaring birds. The largest soaring mammal was the extinct Pteranodon, with a wingspan of 21 feet (7 meters). It apparently had pneumatized wing bones. The form and dimension of the reconstructed Pteranodon are considered aerodynamically ideal for the soaring function of this animal. Because its femora were delicately built and could not withstand compressive stress, it was necessary for this mammal to hang upside down from cliffs as compensation for reduction in bone weight for flight. This adaptation is also true of the bat. The evolutionary impact of forces on form is also beautifully evidenced in the form of three different classes of sea creatures ( Fig. 52.8 ).

Abnormal mechanical forces give rise to deformation in humans in much the same fashion as abnormal forces may deform a tree. In addition to those extrinsic deformations that relate to uterine constraint, abnormal forces can be seen in the deformations wrought by past societies in the name of beauty or custom by application of external forces after the time of birth ( Figs. 52.9–52.14 ). Constraint forces were used to mold infant heads by a number of groups, including the so-called “Flathead Indians,” whose name was derived from this practice. Current medical practice uses custom-designed orthotic molding helmets to reshape infant heads, which have become deformed by their resting positions (see Fig. 52.10 ). Deformational plagiocephaly is the leading cause of head shape abnormalities in children, with mild cases affecting 40% of infants under 12 months of age. Deformational plagiocephaly can cause facial asymmetry, eventual malocclusion requiring orthodontic treatment, and social stigmatization later in life. Most often, it is detected by the child’s parents or pediatrician through simple visual assessment, and subsequent monitoring of the abnormal head shape is crucial because the progression of deformity can indicate the need for additional intervention (such as treatment of persistent torticollis) or an undiagnosed sutural fusion (craniosynostosis) requiring surgical treatment.

In the past, the Chinese custom of binding the feet of infant girls resulted in small, misshapen feet (see Fig. 52.11 ). The practice was initiated at around 3 years of age, when all toes but the first toes were broken and bound with tight cloth over the next 2 years so as to keep the feet less than 10 cm in length, with a marked concavity of the sole. Through contact with Western culture, this practice decreased at the beginning of the 20th century after its first ban in 1912, although some continued this practice in secret. In 1997, a study on foot binding recruited 193 Beijing women and found that the majority were between the ages of 70 and 79 years, while 93 were above the age of 80 years.

In other societies in the Americas and Africa, various techniques were designed to cause deformations. The insertion of stretching devices into the earlobes resulted in excessive skin in this region, as did similar practices with the lips. Successive heavy metal rings were placed around the necks of young girls to elongate their necks. Other practices, such as the swaddling or papoose board immobilization of infants, may have inadvertently increased the likelihood of such deformations as hip dislocation.

Many types of cranial deformation were practiced in pre-Columbian Peru. Highland Peruvians applied circumferential bandages to elongate the calvarium (see Figs. 52.12 and 52.13 ). In coastal areas of Peru, banding was combined with the application of occipital boards to mold the skull into a bilobed shape or a “tower skull” configuration. Anthropologic observations suggest that the practice of artificial cranial deformation was quite widespread, occurring throughout the world, as shown by similar head-binding practices among the Mangbetu people (see Fig. 52.14 ). Such anular deformation results in a head shape like that seen with persistent vertex molding.

Just as some past societies were knowledgeable about mechanical means for producing deformations after birth, other societies successfully used biomechanical means to treat some congenital deformations. In India, some women specialized in reshaping cranial deformations by daily massage and manipulation. Among Polynesian societies, where talipes equinovarus was a common deformity, some older women specialized in manipulating and binding such feet. The deformation was deliberately corrected only partially because such feet were ideal for climbing coconut trees, and no shoes were needed in their environment.

SPECIFIC MECHANICAL IMPACTS ON MORPHOGENESIS

Mechanical forces play an important role in the normal morphogenesis of most tissues, and different tissues have their own limited repertoire of responses to forces. The abnormalities mentioned in this section are used predominantly to illustrate specifically how mechanical factors are relevant to the normal morphogenesis of specific tissues. Many of these examples relate to the impact of forces during early morphogenesis, when extrinsic constraint deformation would be unlikely to occur; however, they do provide the reader with some clinically relevant illustrations of the impact of mechanical forces on form in morphogenesis.

Overall Growth

Constraint may limit the intrauterine growth of an infant. When the onset of constraint-related growth deficiency occurs during late fetal life, there will usually be catch-up growth into the normal range once the constraint is relieved after birth. This appears to be the reason why the first born infant, who must distend the mother’s uterus and abdominal wall for the first time, averages about 200–300 g smaller than subsequent offspring. With more constraint, there is greater deceleration of growth. This is dramatically evident in most multiple births. Twins grow at a normal rate for the first 30–34 weeks of gestation. After they have achieved a combined weight of 4.0 kg, they become constrained and their growth rate tends to slow, as also noted in animal studies. If only one animal is reared in a uterine horn, the newborn is appreciably larger at birth than when many are reared in the same uterine horn. Situations other than multiple births may limit the available intrauterine space and cause it to yield a smaller baby, such as oligohydramnios, uterine malformation, and uterine fibroids ( Fig. 52.15 ).

FIGURE 52.15, Diminished uterine cavity because of a large uterine fibroid.

Effects on Specific Tissues

Bone

The early cartilage models of different long bones appear to be genetically specified, but the alignment of collagen fibrils and bone trabeculae relate to mechanical forces, as do the bony promontories. Collagen fibrils align in the same direction as stress forces, thereby helping to resist shear forces, and sites of muscle attachment relate to muscle tension. For example, the size and shape of the greater trochanter relate to the relatively massive pull of five muscles at that site, including the very strong gluteus maximus muscle. The lesser trochanter only has one major muscle attaching at that site, the psoas minor. The combined forces of weight bearing plus muscle pull affect the form of the upper femur, including the normal configuration of the “neck” of the femur. Prolonged muscle weakness in the growing child may give rise to smaller trochanters and a straighter neck of the femur, termed coxa valga deformity ( Fig. 52.16 ). It is important that these bony findings be interpreted as secondary deformations caused by a more primary neuromuscular problem, rather than as additional malformations involving the skeletal system.

The impact of increasing weight is also evident in the breadth of bone, whereas linear growth is only mildly affected by muscle weakness. Thus paralysis or immobility of a limb results in only a 5% to 10% reduction in its rate of linear growth; however subperiosteal growth in bone breadth is much more dramatically affected. Hence with muscle weakness or lack of use, the growing bone tends to become slender ( Fig. 52.17 ).

FIGURE 52.17, These slim lower leg bones are secondary to long-standing neuromuscular weakness in a newborn with myotonic dystrophy whose mother had this dominantly inheritable disorder.

The adaptive capacity of bone is beautifully exemplified by the realization that the amount of bone and its alignment are influenced by the very forces that the bone is required to withstand. Bone appears to have a critical strain or stress threshold, above which there tends to be bone deposition and below which there tends to be bone resorption. Intermittent stress on a bone tends to foster local growth of bone, as exemplified in bony promontories at sites of muscle attachment. One example is the bony spurs that are liable to develop in the arch of the foot because of the strain of long-distance running. The medial longitudinal arch of the foot is a critical shock absorber during standing and movement. Studies have indicated that childhood obesity can damage and flatten the arch structure as early as age 8 years, with increases in BMI correlated with decreases in arch height, leading to medial longitudinal arch deformation at BMIs above 20 kg/m ² that diminishes the functional structure of the foot arch architecture.

Persistent stress on cranial bone, such as that caused by continuous external compression, can lead to a decrease in bone. This is dramatically exemplified by constraint-induced vertex craniotabes. This abnormality commonly results from prolonged pressure on the fetal vertex as a consequence of the head being “engaged” for a prolonged period of time before birth. A similar effect of persistent pressure on the calvarium was deduced by G. E. Smith from his evaluation of ancient Egyptian skulls. He recognized that the upper-class Egyptians, who lived during the fourth to nineteenth dynasties and wore heavy headdresses, had notable thinning of the calvarium. Smith deduced that this was the consequence of the continuous pressure exerted by these heavy headdresses.

Joints

Joints develop secondarily within the condensed mesenchyme of the developing bones. Movement is an important factor in joint morphogenesis. The embryo starts moving by 7.5 weeks’ gestation, and 2 to 3 weeks later general movements, isolated limb and head movements, hiccups, and breathing movements appear. Chronic lack of movement tends to give rise to multiple joint contractures and a pattern of problems called the fetal akinesia deformation sequence ( Fig. 52.18 ). This can be of extrinsic etiology resulting from intrauterine crowding secondary to congenital structural uterine abnormalities (e.g., bicornuate or septate uterus), uterine tumors (e.g., fibroid), or multifetal pregnancy, or of intrinsic fetal etiology due to functional abnormalities in the brain, spinal cord, peripheral nerves, neuromuscular junction, muscles, bones, or restrictive dermopathies, tendons, and joints. Unlike many intrinsic primary fetal cases, which are difficult to treat, secondary fetal akinesia deformation sequence can be treated with physical therapy. Primary cases may present prenatally with fetal akinesia associated with joint contractures and occasionally brain abnormalities, decreased muscle bulk, polyhydramnios, and nonvertex presentation, while the secondary cases usually present with isolated contractures. One dramatic example was an infant with multiple joint contractures (termed arthrogryposis ) whose mother had received tubocurarine for 19 days in early pregnancy for the treatment of tetanus. Such medically induced early immobilization was considered to be the cause of the joint contractures in her fetus. Similar defects have been induced experimentally by the injection of curare into pregnant rats ( Fig. 52.19 ). Physical constraint caused by prolonged oligohydramnios can also cause joint contractures. Hall summarized 30 cases with multiple congenital contractures due to long-standing oligohydramnios among a total of 2500 cases of arthrogryposis. All 30 cases had “Potter” facies (flattened face and nose, short columella, puffy eyelids, micrognathia, and enlarged flattened ears) with remarkable skin changes (soft hyperextensible skin with excessive creases on the forehead, face, and neck), 50% had pulmonary hypoplasia at birth, and 60% had multiple congenital contractures that responded well to therapy. Fig. 52.20 demonstrates the multiple consequences on bones and joint development in constrained twins carried to term by a small primigravida woman.

FIGURE 52.18, Fetal akinesia sequence results from decreased movement in utero. The consequent anomalies consisted of intrauterine growth curtailment, micrognathia, multiple joint contractures, pulmonary hypoplasia, short umbilical cords, and polyhydramnios. These defects result from intrinsic deformation resulting from myoneural dysfunction, as shown in these brothers with fetal akinesia deformation sequence caused by type I Pena-Shokeir syndrome, a lethal autosomal recessive disorder resulting from pulmonary hypoplasia.

FIGURE 52.19, Moessinger modeled fetal akinesia sequence experimentally in rat fetuses, which were paralyzed by daily transuterine injections of curare from day 18 of gestation until term (day 21). Compared with the control fetuses (left) , the experimental fetuses demonstrated tight skin, micrognathia, and joint contractures.

$FIGURE 52.20, This 62-inch, 98-lb primiparous mother delivered these twins at term with a combined weight of 10 lb, 11 oz. There was marked fetal crowding during the last half of gestation to the extent that this small mother had such difficulty eating that she experienced a 2-lb weight loss during the course of her pregnancy. She noted very little fetal movement during the third trimester, and both twins were born with severe joint contractures, which resulted in 60- to 90-degree limitation of extension at the elbows and 30-degree limitation of extension at the knees. Twin A was born from a vertex presentation with thin bones, resulting in a fracture postnatally during routine handling. Twin B was delivered vaginally from a breech presentation with thin bones and fractured both humeri during delivery; he also had a right equinovarus foot deformity. Both twins had markedly redundant skin and large ears from prolonged compression. By the time photographs were taken at 12 days of age, the joint contractures had shown marked improvement and the twins were beginning to fill out their loose skin. The prognosis for continued improvement was excellent.$

Muscles

Muscle cells align in the direction of muscle pull, which has obvious importance for muscle function. The size of a muscle relates to the magnitude and frequency of the forces it exerts, and the larger the forces, the greater the muscle bulk. Conversely, with diminished function a muscle becomes smaller in size, and lack of muscle function will result in a diminished, hypoplastic muscle. Based on a study of muscle-tendon insertions in genetic limb-reduction defects and sites of juncture in conjoined twins (in whom genetic instructions for such altered anatomy could not be predetermined), there appears to be a hierarchy for the determination of muscle-tendon attachment sites. Tendons attach preferentially to bone, and in the absence of the bone to which a tendon would normally attach, the tendon attaches to the next closest bone. If no such bone is available, tendons may attach to other tendons or to the fascia of another muscle. If there is no connective tissue attachment site, then there is no muscle, which implies a need for muscle to function in the development and maintenance of muscle. The biomechanical impact of muscle strength on bone form and growth has already been emphasized.

Organ Capsules

Organ capsules may be viewed as exoskeletons that provide connective tissue support for specific tissues. Included within this category are the dura mater of the brain, the sclera and outer covering of the eye, the pericardium of the heart, the pleura of the lung, the peritoneum of the intestine, the capsule of the kidney, and the tunica albuginea of the testes. The skin may also be interpreted as an organ capsule, as it is the capsule for the entire organism. The only organ capsule that becomes ossified is that of the brain. The dura mater is responsible for the development and ossification of the calvarium. (This is considered in more depth in the craniofacial section of this book.) None of these organ capsules appear to have any basic impetus for growth. Rather, they grow in accordance with the mechanical forces imposed by the expansion of the underlying organ or tissue that they envelope. All organ capsules are composed of connective tissues within which the collagen fibrils align, in accord with the growth stretch imposed by the internal expansile growth of the respective organ. The direction of such forces is curvilinear, as is readily evident in the alignment of the collagen fibrils within these organ capsules.

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SPECIFIC MECHANICAL IMPACTS ON MORPHOGENESIS

Overall Growth

Effects on Specific Tissues

Bone

Joints

Muscles

Organ Capsules

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