(1)
Faculty of Medicine of Montpellier, Montpellier, France
Abstract
Bones, joints and muscles are the components of the motor apparatus, and their construction can be understood by referring to the basic principles of biomechanics. A bone mineralisation servomechanism explains the plasticity of the system in relation with the specific role of mechanical compression factors applied to the skeleton and generates a piezoelectric microcurrent that stimulates the osteocytes. This mechanism also acts as the technical support for bone regeneration in case of fracture. The joints can be classified in two categories: immobile, like cranial sutures, and mobile, like the intervertebral amphiarthrosis and all the diathroses with different types depending on the articular surface shape and synthesis means. Muscles are viscoelastic actuators, non-reversible and non-linear able to create forces by shortening their contractile part no more than the third of their length which explains the particular mode of construction of the diverse mobile body segments. Command and control of muscles are of great clinical interest. The motor part concerns the functional “motor unit” corresponding to the number of muscular fibres activated in on/off mode by a motoneuron with a variable recruitment explaining a possible proportional control. The sensitive part (proprioception) is given by two transducers: the muscle spindles measure the stiffness of a muscle in order to detect their three different states—relaxed, contracted, stretched—and the Golgi organs measure the force. The position and movement control require a goniometric information given by the skin, thanks to the Ruffini transducers. The conscious and voluntary mobility control applies to movements and not to muscles.
1.1 Introduction
Mobility is one of the four functions managed by the nervous system, the others being: communication, biological maintenance and “survival kit”. It allows to move various body segments in space or to keep them in a controlled position (posture). This function occupies a large part of the body weight and shapes the motor apparatus, including bones assembled in the skeleton, joints and muscles that mobilise or immobilise them. The nervous system of the motor command represents a large amount of the central nervous system (encephalon with brain, brainstem and cerebellum) and peripheral nervous system (roots, plexus and nerves). Some specific concepts in biomechanics are essential to understand the complexity and intelligent design of all these organs [1–4].
1.2 Components of the Motor Apparatus
1.2.1 Skeletal Levers
They are formed by two types of bone tissue.
1.2.1.1 Spongy and Compact Bones [5–14]
Bones are resistant structures with their mineral content consisting mainly of microcrystals of hydroxylapatite, phosphate and calcium carbonate, which is attached to a protein matrix forming the basic substance consisting of 90 % collagen and mucoproteins. Bones take different forms that can be classified as short, long and flat.
Their formation during embryogenesis occurs in two ways:
either directly on a fibrous membrane—bones of the vault of the skull;
or indirectly by means of a cartilage mould which is ossified secondarily after penetration of blood vessels.
There are two types of bone tissue: compact and spongy.
Compact bone tissue is composed of functional units, the osteons, with a microvessel in the centre, which provides osteoblasts locally from the circulating blood (Fig. 1.1).
Fig. 1.1
Osteonic structure of compact bone seen on dry bone sections. (a) 1. Vascular microcanal of Havers; 2. Concentric circle of osteoplasts. (b) Detail of osteoplasts with their interconnected microcanalicular network creating the osteonic force transducer. (c) Osteoplast network with microcrack (arrow). (d) Osteon with concentric circle of osteoplasts
These by their alkaline phosphatase begin the construction of a circular structure made of concentric blades in which are placed the osteocytes, the true bone cells, which provide the mineralisation of the bone and its retention. Osteocytes are placed in microcavities, the osteoplasts, which communicate with fine canaliculi, creating an interconnected osteocyte network. The assembly comprises a structure combining the fascicular osteonic minicylinders whose viscoelastic mechanical behaviour is a function of the slope of the collagen fibres of the protein matrix.
From a mechanical point of view, the stress represents the reaction of a material to mechanical forces. This cannot be measured directly but only by the deformation (strain) of material, which is the particular domain of mechanics called extensometry (Fig. 1.2). It is possible to use some little electric circuits stuck on the material. These strain gauges can precisely measure the deformation by the variation of their electrical resistance. The longitudinal and transversal deformations under load factor are called the “longitudinal elasticity module of Young” which is in the order of 20,000 MPa (2,000 daN/mm2) for bone with many variations among the different types of bones. The “Poisson coefficient” characterises the rate between longitudinal (ε1) and transversal (ε2) deformations [7]. Therefore, the bone is a non-homogenous and anisotropic material. These biomechanical parameters are important to take into consideration when looking at an osteosynthesis on a bone fracture using plates and screws in metal having a Young modulus of 200,000 MPa (20,000 daN/mm2), very different from the one of the bones.
Fig. 1.2
Biomechanical experimental tests. (a) Strain gauge bridges on a femur: 1. Torsion (45°); 2. Traction/compression. (b) Photostress on a clavicle with blue colour for compression strain. (c) Femur equipped with strain gauges under compression machine test. (d) Two vertebras under compression measured with force transducer
The cancellous bone has a honeycomb structure with longitudinal trabeculae following the main lines of force and fine spans across, “shrink”, ensuring the strength of the whole (Fig. 1.3). This orientation along the force lines is already present in the foetus before the loading of the skeleton. This structure can be compared to that of the reinforcement of concrete with thick stems and thin metal interlaced wires ensuring the cohesion of the frame. Mechanically, the spongy tissue is resistant due to its geometry and not to the quantity of bone material that is reduced, with the advantage of a significant weight gain. However, it is slightly elastic and retains residual deformations in case of excessive compression, as is the case for fractures of the vertebral bodies or tibial plateau. Within its meshes, the fragile hematopoietic tissue that produces blood elements is protected. The wings of modern aircraft have this honeycomb structure in which kerosene flows.
Fig. 1.3
Trabecular structure of spongious bone. (a) Microscopic view of trabecules with osteoplasts. (b) Lamellar ossification. (c) Femur superior part with trabecular orientation along the force lines. (d) Calcaneus: mechanical orientation of trabeculas
1.2.1.2 Osteonic Force Sensor
There is a relationship between mechanical factors and bone, but it has not been possible to find force sensors in bone until now, except for few Pacinian corpuscles at the periosteum. These cannot account for the mechanical stress exerted on bone in three directions. Given the structure described above with its network of interconnected microcanaliculi, the whole bone becomes a force sensor sensitive to mechanical bone stress, mainly in compression.
1.2.1.3 Bone Mineralisation Servomechanism
Maintaining bone mineralisation is not a static and stable phenomenon. It depends on metabolic and endocrine factors. We should, however, emphasise the importance of mechanical factors that determine the existence of a true mineralisation servomechanism. Indeed, the mineral part of bone consists mainly of hydroxylapatite crystals that are formed in contact with the collagen fibres of the ground substance and bind along their longitudinal axis. It is known that compressing apatite crystals generate microcurrents corresponding to a piezoelectric effect, which can be either direct or inverse. Thus, putting bones in load creates microcurrents, which stimulate osteocytes enclosed in their osteoplasts and promote bone mineralisation. This hypothesis was verified experimentally, and it is possible to have a demonstration on patients with fractures immobilised by a cast in which a radiographic exam shows a clear decalcification, among paraplegics who do not verticalise regularly and have a dangerous osteoporosis, and finally, on astronauts living in space with reduced gravity, taking several months after return to Earth to recover normal mineralisation of their skeleton.
The beneficial role of skeletal loading activating the servo mineralisation also plays an important role in the repair of fractures. Bone has a preprogrammed tendency to repair in case of failure. Due to the action of osteoclast cells able to resorb bone, galleries are formed through the rupture zone, in which a cohort of osteoblasts provides bone reconstruction. However, for the success of this particular microscopic job, it is imperative that the two bone fragments are immobilised rigorously for a time. This justifies all orthopaedic and surgical methods: plaster, screws and nails. As clearly shown by the team of Professor Maurice Müller from Bern and Davos, compression correctly applied after fracture immobilisation significantly improves the progress of bone regeneration.
1.2.2 Joints
A joint is the junction of two skeletal parts. It is designed either to prevent movement by blocking the skeletal parts after the growth phase or to enable movement whose amplitude is variable, depending on the shape of the articular surfaces and the presence of their restraint capsules and ligaments.
1.2.2.1 Immobile Joints: The Cranial Sutures
The skull is composed of two parts: the neurocranium which houses the encephalon and the splanchnocranium which forms the skeleton of the facial cavities—lower orbit, nasal and oral cavities. The ultimate goal of skull growth is to form a bone box with a moving part, the mandible, and a junction with the cervical spine, the occipitocervical cardan.
The encephalon is formed during early development by different vesicles: telencephalic with a pair of vesicles forming the two cerebral hemispheres and an odd link forming the commissure area; and diencephalic and rhombencephalic forming the brainstem with its three parts: mesencephalic, pontine and medullary with behind, the cerebellum. In the embryo neural growth is fast, causing a characteristic phenomenon of winding. Then the encephalic part enclosed in a fibrous sac which will become later the dura expands by pushing the envelope. This membrane promotes on its surface some centres of direct ossification which gradually expand and coalesce to meet by suture lines, becoming as bone grows lines limiting the different bony pieces of the vault (Fig. 1.4).
Fig. 1.4
Cranial sutures. (a) Endocranial view with dura mater. (b) Endocranial view without dura showing the complete suture ossification and the stamp of medial meningeal artery. (c) Exocranial suture drawing like pitch line due to muscle traction avoiding any kind of movement. (d) Endocranial linear suture line drawing in relation with the progessive and regular telencephalic growing forces
The skull base that serves as a support for the brain is developed differently. The notochord, in relation with the central nervous system, plays a significant role by inducing growth of the vertebral bodies and the posterior neural arch. It extends within the skull base up to the posterior part of the sella, representing the central zone that houses the pituitary gland. Thus, the posterior fossa of the skull base has a central part that is of “spinal inspiration” and forms the clivus by the junction of the basilar process of the occipital bone and the posterior part of the sphenoid. In front of the sella is the prechordal part of the skull base with, in the centre, the olfactory cribriform plate of the ethmoid and, laterally, the surelevated orbital roofs. Between the prechordal part and the clivus, there is an angle that is in relation with the degree of development of the two morphogenetic components: the brain and the face. In humans, the angulation corresponds to a kyphotic aspect, but in animals to a lordotic aspect. This kyphosis of the skull base in humans has been named “foetalisation” by Bolk because it is similar to the foetal aspect. In addition, the nasal opening is always, whatever kyphotic or lordotic angulation of the base, parallel to the plane of the olfactory ethmoidal cribriform plate.
In fact, the important development of the telencephalon in humans generates a characteristic bulging of the frontal region as well as a rotation backward, responsible for the curved shape of internal structures such as the caudate nucleus and lateral ventricles. On the other hand, pushing forward on the frontal wall, it maintains the basal compass in its foetal shape, which is not the case for animals with lordotic skull base. Therefore, the two fundamental morphogenetic factors are telencephalon and facial growth.
It should be noted that the posterior clivo-vertebral junction and the corresponding medulla oblongata/cervical spinal cord angulation present an angle almost identical in all vertebrates (Dabelow angle), which enables a dynamic representation of the variation of skulls from birds to humans.
Regarding the vault or calvarium, the plasticity of the cranial envelope changes gradually during growth, becoming less deformable due to ossification centres, which may be referred to as the neurocranial accommodation. Indeed, in the embryo, the telencephalon has a perfectly smooth superficial cortical part. The progressive implementation of the double neuronal cortical matrix, vertical and horizontal, results from the migration of neuroblasts from the periventricular zone towards the surface of the cortex by ascending along glial fibres. This generates a tangential growth, which tends to increase not only in volume but also in surface. This expansion during the end of the foetal period meet with the progressive lower plasticity of dura mater and skull, causing a phenomenon of folding of the cortex dominated by mechanical geometric factors, well studied by Brummelkamp. This creates sulci and gyri, which have many individual variations, but still keep the same anatomical trademarks in all individuals. This is the case of the central sulcus (Rolando) and lateral sulcus (Sylvius) with the fixer role in the backward rotation of the forebrain vesicles mentioned above, of the internal structures such as the insula and the basal ganglia. The corpus callosum, the largest telencephalic commissure, imposes on the interhemispheric cortex a parallel fissuration that does not exist in the case of agenesis of the corpus callosum.
It is important to consider that in this neurocranial accommodation, the growth of telencephalon is the motor of cranial expansion. Nothing can be opposed to its growing power. Therefore, the final shape of the skull is the expressiveness of the development of the telencephalon that is to say of the brain. In the range of phenotypic human variations, there are big brains and small brains. A “blowing occipital” can be considered as an indication of overdevelopment of the brain. It is logical to consider that given the functional value of neural components in mental speculation and understanding of problems, big brains should retain an advantage. In order to mitigate the difference, it may also be said that the same instrument may be played in many ways. But you cannot play the cello when you have a violin, which emphasises the importance of genetic factors in the construction of the brain instrument.
The thrust of telencephalic growth cannot be constrained by an abnormal sutural line growth. Cranial deformations (e.g. turricephaly, scaphocephaly) are known to be the hallmark of compensation growth found naturally in a different direction than the lacking suture. The same ethnic skull deformities as those surrounding the Mayas, who by binding the skull of newborns in a tight cap, push the telencephalic expansion in a vertical direction.
Some still believe in the theory put forward by Virchow who, while observing the skull of a dead microcephalic child with deep mental retardation, noted early synostosis of the cranial sutures and concluded that premature closure had stifled the development of the forebrain, explaining also mental retardation. The skull, however, can never prevent the brain from growing. It is illusory to believe that operating the premature synostosis of sutures causing some cranial deformities in young children can improve brain development.
A certain congenital malformation characterised by the agenesis of the telencephalon—called anencephaly—generates an absence of the cranial vault, but with a normal skull base and splanchnocranium. There is therefore no skull without a telencephalon. On the other hand, maintaining abnormal telencephalic thrust, as is the case of hydrocephalus by a disorder of the circulation of intracerebral cerebrospinal fluid, causes abnormal growth of the skull which can reach impressive proportions.
When brain growth is completed at about the age of 20 years, the sutures stop growing and close by synostosis starting with their endocranial side. In a suture, there are two parts separated by a thin layer of spongy bone, called the diploe, subject to different mechanical influences. The endocranial part has an almost linear suture, due to the gradual and steady nature of the expansive telencephalic thrust. The exocranial part, however, is subject to mechanical intermittent and variable tangential forces of traction exerted by the mandibular muscles, mainly the temporal muscle and the muscles of the neck fixed on the occipital bone and keeping the head in position. When these muscles have sufficient power to support the head and to allow chewing hard foods, they exert tangential traction on the exocranium, explaining the scalloped design of sutures. Due to mesh, it is difficult to separate the pieces of bone, and it becomes impossible when the endocranial synostosis is completed. It has been experimentally demonstrated in newborn dogs that the section of the temporal muscle leads to a suppression of the scalloped pattern of the lateral sutures. Post-traumatic hydrocephalus in young adults may lead to a dissociation of the sutures, but later, in 50-year-olds, it creates intracranial hypertension with a clinically discernible impact on vision by compression of the retina. The fundus of the eye in this case may lead to a diagnosis of an oedema of the papilla.
Therefore, the sutures in the foetus and the newborn are straight lines uniting the bony parts that keep for 1–2 years only some fibrous coalescence zones called fontanels: at the bregma which is the junction of the parietal and frontal bone, and at the lambda, junction of the parietal and occipital. In adults, the sutures close first on the endocranial, leaving the visible exocranial suture drawing, but without any possibility of moving it.
The so-called cranial osteopathy is based on the perception of a particular rhythm through the scalp and an attempt to mobilise sutures. However, this is not possible at a certain age as indicated above. The true clinical effectiveness of this therapeutic technique should be found, not in a possible mobilisation of bone pieces of the skull, but in the action on the skin covering the skull, which is very rich in cutaneous receptors of any kind and neurovascular complexes as demonstrated in acupuncture points. It seems important that osteopaths change their explanations which, given what we know about this type of therapy, will surely take much time.
1.2.2.2 Mobile Joints
They allow the movement of skeletal segments of two types:
either with no direct contact between the skeletal parts (amphiarthroses) and interposition of fibrous tissue (synfibroses) or cartilage (synchondroses);
or with direct contact between the bones, which requires a viscoelastic interposed joint: the articular cartilage (synovial joints).
Amphiarthroses
They are found in the sagittal plane, anterior and posterior.
The Pubic Symphysis (Fig. 1.5)
Fig. 1.5
Axial sections of pubic symphysis (elderly person). (a) Superior section: 1. Complete mechanical fissuration of the discus interpubicus; 2. Prostatic urethra; 3. Membrana obturatoria; 4. External obturatorius muscle; 5. Iliac bone. (b) Inferior section: 1. Inferior arcuate pubic ligament; 2. Prostatic venous plexus; 3. Ischiopubic branch; 4. Gluteus maximus muscle
It fits into the sacroiliac joint triangulation that breaks the pelvic ring with two sacroiliac diarthrosic joints in the back and the pubic amphiarthrosis in the front. This particular feature increases substantially the mechanical resistance of the pelvis. The pubic junction is made by very strong transverse fibres moored on a cartilaginous plate. It is subjected to vertical shearing forces that lead eventually to the creation of a crack. This can be the cause of pubic pain, especially in footballers related to the powerful sway of the lower limb with shock on the ball.
The Intervertebral Amphiarthroses
They form the intervertebral disc [15, 16] whose role is to maintain an incompressible but deformable space between the vertebrae to allow the amortisation of mainly vertical forces during locomotion (heel strike during walking, running, jumping) and the movements of the spine in the three directions of space. The only incompressible medium is liquid, but it is difficult to imagine a water bag between the vertebrae that will not resist high pressure. The original solution is to trap water in a hydrophilic material constituting a “fibrillar armed gel” made by proteoglycan fibres (65 %) associated with collagen fibres (25 % type II) located in the core (nucleus pulposus) of the disc. It is maintained in place by the peripheral part (60 % of collagen types I and II, 20 % proteoglycan) consisting of thick fibrous concentric strips oriented in three directions, one circular and two obliques [17].
The mooring of the disc is on a cartilage plate, an avascular tissue that is transected in its peripheral portion by the fibres of the peripheral zone which will bind to the bone of the vertebral endplate. In the centre, the hydrophilic material is in contact with the cartilage plate, which is semipermeable (Fig. 1.6).
Fig. 1.6
Coronal histological sections of lumbar spine of a 3-year -old young boy. (a) General view. (b) Details of the disco-somatic complex: 1. Discal central zone (nucleus pulposus); 2. Subchondral venous plexus; 3. Cartilage plate; 4. Discal peripheral zone (annulus fibrosus)
There are therefore underload factor fluid exchanges between the disc and the subchondral plexus of the vertebral body. They are reversed during rest periods when lying. Thereby the alternate compression/decompression creates a discal pumping effect. The functional advantage of this pump is to create a gradient pressure inside the bone marrow of the vertebral body. This promotes the release of blood elements elaborated within the hematopoietic tissue into the large vertebral posterior vein connected to spinal epidural venous plexus.
The validation of this concept lies in the pathology. In case of the collapse of the disc, the pump does not work anymore and a more or less significant fatty degeneration of the vertebral bone marrow occurs along the disc border clearly visible on MRI and representing the Modic sign (Fig. 1.7).
Fig. 1.7
Anachronesis of the lumbar spine on a MRI sagittal section. 1. Fissurated disc; 2. Black disc; 3. Fat involution of bone marrow (Modic sign); 4. Discal hernia
The second important aspect of the disc is its mechanical fissuration due to normal compression and shearing forces related to local biomechanical conditions. This mechanical process is characteristic of all connective spaces subjected to compression and shear (as, e.g. the pre-clavicular connective bursae acquired by prolonged friction of the rifle on the shoulder of soldiers or prepatellar bursae in “kneeling” professionals). It should be noted that the posterior part of the disc is the place of maximum compressive stress during flexion/extension of the spine, explaining the possible rupture of the annulus at this level. This cracking extending back breaks the peripheral part of the disc, resulting in herniation of the central armated gel mainly in the young, with adequate hydration of the disc. This can lead to compression of roots and the spinal ganglion generating pain. The evolution over time of the disc (anatochronesis) tends to dehydration and involution of proteoglycans. This explains why full disc cracking without herniation can be observed in some elderly patients.
The disc starts cracking very early at the cervical junction of the uncus with the cervical vertebra endplate. This has led to a misinterpretation by former anatomists who have mentioned an uncovertebral joint corresponding to a particular type of joint between amphiarthroses/diathroses, which in fact does not exist (Fig. 1.8). The role of the uncus may therefore be interpreted more as a wall of protection for the vertebral artery, avoiding kinking of the disc as a form of rail, maintaining the vertebral bodies laterally. The arthrosic cervical spine pathology named uncarthrosis may of course alter the original construction scheme and compress the vertebral artery.
Fig. 1.8
The pseudo-uncovertebral joint. (a) Cervical vertebra: 1. Vertebral body; 2. Cervical vertebral canal; 3. Uncus; 4. Transverse foramen; 5. Superior articular process; 6. Cervical lamina; 7. Spinous process. (b) Coronal section of cervical spine of a young person (20 years old): 1. Vertebral body; 2. Uncus; 3. Vertebral disc; 4. Mechanical fissuration of the disc (false image of unco-vertebral joint); 5. Vertebral artery
Multidirectional mechanical disc solicitations are obviously limited and channelled through the posterior facet joints, creating an articular intervertebral triangulation. Mechanically it is not a tripod, giving balanced support on three points called orthostatic or isostatic, but a hyperstatic support by more than three points which is not possible to calculate geometrically. The shape of the posterior articular processes varies according to the spine level and is strongly involved in limiting and orienting intervertebral movements:
at the cervical level of great mobility, the posterior articular surfaces are planar and circular, facing backwards and forming a cylindrical column;
at the thoracic level, they are also planar but more inclined backwards and outwards, in relation with the double costovertebral joint (rib head articulated on the body and rib tuberosity on the transverse process). This system greatly limits the axial movement of rotation leaving some amplitude in flexion/extension;
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