Bone and Cartilage

A bone of a living man is itself a living thing. It has blood vessels, lymph vessels, and nerves. It grows. It is subject to disease. When fractured it heals itself; and if the fracture is so improperly set that the parts have lost their previous alignment, its internal structure undergoes remodeling in order that it may continue to withstand strains and stresses as it did before. Unnecessary bone is resorbed: for example, following the extraction of a tooth, the walls of the socket, thus rendered empty, disappear; also, the bones of a paralyzed limb atrophy (become thiner and weaker) from disuse. Conversely, when bones have increased weight to support they hypertrophy (become thicker and stronger).

Bones have an organic framework of fibrous tissue and cells, amongst which inorganic salts - notably, phosphate of calcium - are deposited in a characteristic fashion. The fibrous tissue gives the bones resilience and toughness; the salts give them hardness and rigidity and make them opaque to X-rays. One third is organic; two thirds are inorganic.

Physical Properties

By submerging a bone in a mineral acid the salts are removed, but the organic material remains and still displays in detail the shape of the untreated bone. Such a specifmen is flexible: for example, a decalcified fibula can be tied in a knot (fig. 10); when the knot is untied the fibula springs back into shape.

Fig 10: A decalcified fibula can be tied in a knot

The organic material of a bone, long buried near the surface of the earth, is removed by bacterial action (i.e. decomposition) and only the salts remain. The same result can be achieved more speedily by burning with firs. A bone so-treated, being more brittle than porcelain, will break unless handled with care. Bones that have lain buried in a limestone cave become petrified (i.e. calcium carbonate replaces the organic material); so, they endure; so do those soils containing, say iron, lead or zinc. Moisture being necessary to bacterial action, bones that have remained thoroughly dry (mummified) retain their organic framework and therefore much of their toughness. The anthropologist about to exhume fragile bones first toughens them by “petrifying” artifically, by impregnating them with shellae dissolved in spirit or with cellulose dissolved in acetone.

Functions of Bone

In addition to being (1) the rigid supporting framework of the body, bones serve as (w) levers for muscles; (3) they afford protection to certain vscera (e.g. brain and spinal cord, heart and lungs, liver and bladder); (4) they contain marrow, which is the factory for blood cells; and (5) they are the storehouses of calcium and phosphorus.

Fig 11: The structure of a dried bone as shown by longitudinal and transverse sections of a humerus

Structure

Structure of a bried bone seen on section (fig. 11). Macroscopically, these are two forms of bony tissue (a) spongy or cancellous and (b) compact or dense.

All bones have a complete outer casing of compact bone; the interior is filled with spongy bone except where replaced by a medullary cavity or an air sinus. In a long bone, such as the humerus, the compact bone is thickest near the middle of the shaft and it becomes progressively thinner as the bone expands towards its articular ends, these being covered by a mere shell of compact bone. Conversely, spongy bone fills the expanded ends and extends for a variable distance along the shaft but leaves a tubular space, the medullary cavity. The lamellae or plates of the spongework are arranged in lines of pressure and of tension, and in an X-ray photograph the pressure lines are seen to pass across joints from bone to bone (fig. 496 of the hip joint).

Classification

The bones of the body may be calssided variously:

1. Developmentally: according to whether they developed (a) in cartilage or (b) in membrane.
2. Regionally:
   • Axial Bones
     • Skull
       • Cranium + Face (22)
       • Auditory ossicles (6)
       • Hyoid (1)
     • Vertebrae (26)
     • Ribs (24)
     • Sternum (1)
   • Appendicular Bones
     • Upper limb (64)
       • Pectoral girdle
       • Free bones
     • Lower limb (62)
       • Pelvic girdle
       • Free bones
   • Total: 206 (This number is not exact. It varies with age and with the individual, being larger in youth while various parts of compound bones (e.g. frontal, sacrum) are still discrete and when accessory or supernumerary bones are present, and beign smaller when two bones have fused (e.g. fusion of lunate and triquetrum, or of two vertebrae) and when a bone is suppressed or congenitally absent (e.g., absent phalanx or vertebra))
3. According to Shape
   • Long (peculiar to the limbs)
   • Short (peculiar to the limbs)
   • Flat (peculiar to the axial skeleton and the girdles)
   • Irregular (peculiar to the axial skeleton and the girdles)
   • Sesamoid (in certain tendons)

Long Bones

Long bones are tubular. They are confined to the limbs, where they serve as levers for muscles. By their length they increase the reach of the upper limb and the stride of the lower limb. Primitively, all long bones are weight-bearing.

A long bone has a body or shaft and two ends. The ends, being articular, are smooth, covered with cartilage, either convex or concave, and enlarged. This enlargement, by increasing the articular surface, diminishes the risk of dislocation. The shaft os hollow (medullary cavity) as a straw is hollow, thus obtaining most strength with least expenditure of material and with least weight. It, typically, has three borders which separate three surfaces, so on cross-section it is triangular rather than circular. The borders may be likened to 3 pieces of unbendable angle-iron; 3 surfaces connect the 3 unbendable borders (fig. 12). THe three surfaces and borders are named by opposites; see pages 124, 429. The shaft is thinnest near its middle and it expands gradually towards each end. Long bones develop (are preformed) in cartilage. The shaft of every long bone begins to ossify (primary center) about the 2nd to 3rd mont of intra-uterine life. One or both ends begin to ossify (secondary centers) sub adjacent to the articular surfaces soon after birth.

Fig 12: The 3 borders are unbendable, like angle-iron

Exceptions

Every long bone does not conform to all the foregoing specifications. For example, the clavicle has no marrow cavity; it is largely preformed in membrane; and only one end is enlarged. Again, the terminal phalanges of the hand and foot, are non-articular at their distal ends, are tapering, and have no medullary cavity.

The ribs, though usually classified as flat bones, are certainly not lacking in length. Except that they are somewhat flattened and have no medullary cavity, they fulfil the specifications of a long bone. The vertebrae are classified as irregular bones, but their bodies possess most of the features of a long bone.

Short Bones

Short bones are cubical. They are confined to the carpus and tarsus. In structure they are almost identical with the epiphyseal ends of long bones. They have six surfaces of which four (or less) are articular, leaving two (or more) free for the attachment of ligaments and for the entry of blood vessels. They develop in cartilage, and they begin to ossify soon after birth.

Of the short bones, 3 (calcaneus, talus and cuboid) start ossifying before birth; so do the epihyses of 3 long bones (knee end of femur and tibia and commonly the shoulder end of humerus).

Flat Bones

Flat bones resemble sandwiches. They consist of two layers or plates of compact bone with spongy bone and marrow spread between them. Many of the skull bones (e.g. parietal, vomer), the sternum, scapulae and parts of other bones are of the flat type. Most flat bones help to form the walls of rounded cavities and therefore are curved. At birth a flat bone consists of a single plate. In the flat bones of the skull the spongy bone, here called diploe, and its contained marrow appears some years later and splits the plate into two. The marrow may, however, be spread unevenly leaving the plate single and translucent in parts. To verify this, hold the occipital, squamous, temporal, scapula or ilium to the light.

Iregular Bones

Irregular bones have any irregular or mixed shape. All skull bones, not of the flat type, are irregular (e.g. sphenoid, maxilla), so are the vertebrae and the hip bones. They are composed of spongy bone and marrow within a compact covering.

Pneumatic Bones

Evaginations of the mucous lining of the nasal cavities and of the middle ear and mastoid antrum invade the diploe of certain flat and irregular bones of the skull thereby producing air cells or air sinuses. This pneumatic method of construction may be economical in bone material, but invites “colds in the head” and other infections of the nose and throat to extend to these sinuses.

Sesamoid Bones

Sesamoid bones are nodules of bone that develop in certain tendons where they rub on convex bony surfaces (“Sesamoid” of Arabic origin = like a seed). The free surface of the nodule is covered with articular cartilage; the rest is buried in the tendon; it possesses no periodsteum. (see page 17)

The largest, the patella or knee-cap occurs in the Quadriceps Femoris tendon; the most important, the two at the ball of the big toe, occur in the Flexor Hallucis Brevis tendon.

Others occur in Flexor Pollicis Brevis (2 at ball of thumb), lateral head of Gastrocnemius, Peroneus Longus (at side of cuboid), Tibialis Posterior (behind navicular tuberosity), and at heads of metacarpals 2 and 5 (in palmar plates).

Supernumerary or Accessory Bones

Certain bones normally ossify from several centers, and it sometimes happens that one or more of these centers fails to unite with the main mass of the bone; again, an abnormal or extra center of ossification may make its appearance and the resulting bone may remain discrete. In either case, the result is an accessory bone. These are commonest in the skull and at the wrist and ankle. In X-ray phtographs, accessory bones may simulate fractures, but their articular surfaces have a covering of compact bone, which fractured bones have not. They occur unilaterally more often than bilaterally.

Examples

The right and left halves of the frontal commonly fail to unite (fig. 644); the occipital and its inter-parietal part commonly fail to fuse in the Inca Indians of Peru and in other West Coast Indians (_fig. 835); sutural bones, the size of a finger-nail, may occur in the sutures of the skull; the acromial epiphysis commonly remains discrete (fig. 83); the 5th lumbar vertera is commonly in two pieces (fig. 321); the patella may be bipartite (fig. 815). Supernumerary carpals and tarsals occur (see pages 504-505).

Markings on a Dried Bone

The surface of a dried bone is smooth, in fact almost polished, over areas covered with cartilage and where tendons play in grooves (cf., head of the humerus and the intertubercular groove; upper and under surfaces of the sustentaculum tali). Near the ends of a long bone there are large vascular foramina for veins and arteries; and piercing the shaft obliquely is the nutrient canal, for the nutrient vessels, which may be two inches long. Markings occur wherever fibrous tissue is attached - no matter whether it be a ligament, tendon, aponeurosis, fascia, intermuscular septum or fold of dura mater (falx and tentorium). Fibrous-tissue markings are, however, not present at birth nor in the young (e.g. they are not seen on a soup bone). They appear about puberty and they become progressively better marked with advancing age.

Terms

Markings take the form of (a) elevations, (b) facets, and (c) depressions.

Elevations, in order of prominence: a linear elevation is a line, ridge, or crest; a rounded elevation is a tubercle, tuberosity, malleolus, or trochanter; a sharp elevation is a spine or styloid process.

Small, smooth, flat areas are called facets (cf., the facet of a diamond).

A depression is a _pit or fovea, if small; a fossa, if large; a groove or sulcus, if it has length. A notch or incisura, when bridged by a ligament or by a bone is a foramen (i.e., a perforation or hole), and a foramen that has length is a canal or meatus. A canala has an origice at each end; (the external acoustic meatus, however, is an exception). The portion of a notch, foramen, or orifice of a canal over which an emerging vessel or nerve rolls is rounded, but elsewhere it is sharp. Therefore, even on a dried bone the direction taken by the emerging occupant is evident (cf. lesser sciatic notch, anterior sacral foramina, infra-orbital canal).

Areas covered with articular cartilage are called articular facets, if approximately flat. Certain rounded articular areas are called heads, other condyles (= knuckles). A trochlea is a pulley.

Note, then, on the dried bone (1) that the area of attachment of the fleshy fibers of a muscle cannot be determined by inspection, but (2) that tendons, ligaments, and other fibrous structures make their mark, and (3) that the mark indicates precisely the limits of their attachments.

A Living Bone or a Dissecting Room Specimen Before Maceration

The articular parts are covered with hyaline (articular) cartilage. This is not equally thick at all pointsl so, its contour is not identical with that of the underlying bone. Hence, macerated bones do not articular perfectly. Periosteum envelops all parts not covered with cartilage and not giving attachment to ligaments and tendons. It consists of two layers (1) an outer, fibrous membrane and (2) an inner, vascular one lined with bone-forming cells, the osteoblasts. THe periosteum is easily scraped off with the handle of the scalpel, leaving, however, many osteoblasts ahering to the bone. Fibrocartilage lines grooves where tendons exert pressure. Some elevations seen in the macerated bone are but shadows of what they were before maceration, because in life they had fibrocartilaginous extensions now shed (e.g., the dorsal radial tubercle of Lister before maceration was continued up the radius as a fibrocartilaginous ridge that gave attachment to the extensor retinaculum).

The Parts of a Young Bone

Fig 13: The parts of a young bone as shown by a longitudinal section of a femur

At birth both ends of a long bone are cartilaginous, cartilaginous epiphyses. The part of the bone between the cartilaginous ends is the diaphysis (Gk. dia = in between, across). It comprises a casing of compact bone which encloses a medullary cavity at its middle and spongy bone at each end, and all is filled with red marrow. The diaphysis is clothed in periosteum (Gk. peri = around; osten = bone). When the developing bone was in the cartilaginous state, the periosteum was known as perichondrium (Gk. chondros = cartilage).

Epiphyses

Epiphyses (Gk. epi = upon, physis = growth).

(1) During the first and second years (or both) of the cartilaginous ends begins to ossify subjacent to the site of articulation, constituting a pressure epiphysis (e.g., head of humerus, condyles of femur). (2) Later, generally about puberty, independent ossific centers appear in the cartilage at the sites of attachment of certain tendons, constituting traction epiphyses (e.g., tubercles of humerus, trochanters of femur). (3) A third type of epiphysis is the atavistic epiphysis. Atavistic epiphyses phylogenetically were independent bones now grafted on to other bones (e.g., coracoid processes of scapula (fig. 188), os trigonum of talus (fig. 541)).

The layer of cartilage between an epiphysis and a diaphysis is an epiphyseal plate. The region of the diaphysis adjacent to the plate, the metaphysis (Gk. meta = beyond), is the site where growth in length takes place.

All long bones - including the metacarpals, metatarsals, phalanges and ribs - have a pressure epiphysis at one end or the other, whereas 5 paired bones always have pressure epiphyses at both ends (viz., humerus, radius, femur, tibia and fibula) and a few bones occasionally have them at both ends (viz., clavicle, 1st and 2nd metacarpals and 1st metatarsal). The ulna also has an epiphysis at each end, but the proximal one is a traction epiphysis for the tendon of the Triceps (fig. 197).

A pressure epiphysis has been regarded as a protective cap to the metaphysis or actively growing portion of a bone.

Regarding Nutrient Canals

Increasing deposits of periosteal bone allow the nutrient canal (which early ran transversely) to occupy an oblique position directed away from the epiphyseal end (fig. 11). Check any bone (clavicle, phalanx, rib) and observe that it is so. In cases where there is an epiphysis at both ends, the canal is directed away from the more actively growing end. Now, it is roughly estimated that the shoulder end of the humerus and the wrist ends pf the ulna and radius grow 3 to 4 times as much as their elbow ends; the knee end of the femur between 2 and 4 times as much as the hip end; and the knee end of the tibia slightly more than the ankle end (fig. 197) (Digby and Phemister.). Hence, in these bones the nutrient canals are directed - to the elbow I go, from the knee I flee. (Hughes.)

Where there are two epiphyseal ends, the end that has the more work to do is the first to start work (ossifying) and the last to stop (to fuse with the diaphysis). When fusion (synostosis) takes place, growth in length practically ceases.

Ossification

Except for certain bones of the skull and the clavicle, all the bones of the body pass through a carilaginous stage. About the 8th intra-uterine week ossification of the long bones begins. There are two types of ossification (a) intracartilaginous or enchondral and (b) periosteal or intramembranous.

In the shaft of a long bone both types take place concurrently, as described in text books on Histology, notably by A. W. Ham.

After birth, at the center of one or both cartilaginous ends, the process of enchondral ossification begins, as shown in figure 13, and a bony epiphysis takes form. Ossification progresses in the epiphysis until only two sheets of cartilage remain: (a) the articulr cartilage that covers the end of the bone and persists throughout life, and (b) a residual plate, the epiphyseal plate, placed between the diaphysis and the bony epiphysis forming a sunchondrosis. Ultimately, when the bone has attained its adult length, the plate also ossifies, that is, synostosis results - the site commonly being marked by an epiphyseal line (fig. 11).

Short bones (i.e. carpal and tarsal) ossify enchondrally like epiphyses.

The bones of the skull, except those of the base, do not pass through a cartilaginous stage but ossify directly from membrane.

Those preformed in carilage are: (a) occipital, save the interparietal part; (b) sphenoid, save the greater wings and the pterygoid laminae; (c) ethmoid; (d) inferior conchae; and (e) petromastoids and styloid processes of the temporal bones.

Sexual differences: Ossification starts earlier in females than in males and it is completed earlier - even by as much as two or three years.

Bone Marrow

Bone marrow makes blood cells. Blood cells have but a short life, the red cells living 100 days or more, and the birth rate necessarily keeps pace with the death rate.

At birth spongy bone, which at this age is limited in quantity, and the medullary cavities of the long bones are filled with red (blood forming) marrow. By the 7th year, the amount of spongy bone has increased and the red marrow has extended into it, but at the same time has receded from the medullary cavities only to be replaced there by yellow (fatty) marrow. About the 18th year red marrow is almost entirely replaced by yellow in the limb bones; thereafter, it is confined to the axial skeleton - skull, vertebrae, ribs, sternum, hip bones, and upper ends of femora and humeri.

In certain conditions (e.g., pernicious anaemia) where the death rate of the red cells is high, the uellow marrow reverts to red in an endeavour to support the birth rate.

Vessels and Nerves

Arteries supply long bones thus: (a) periosteal twigs enter the shaft at innumerable points, run in the Haversian canals and supply the outer part of the compact bone of the shaft; (b) twigs from articular arteries, which anastomse around the joint usually between the bone and the reflexion of the synovial membrane, supply the epiphyses, the metaphyseal region, and the capsule; (c) the nutrient artery (medullary artery) on entering the medullary acvity, divides into a proximal and a distal branch, each of which supplies the inner part of the compact bone, the marrow, and the metaphyseal region.

The anastomoses between the branches of the nutrient and periosteal arteries seem to be deble. Though many of the metaphyseal branches of the nutrient artery are end arteries, some of them anastomose with the metaphyseal branches of the articular arteries. Indeed, when the shaft of a long bone is fractured, one or other branch of the nutrient artery is necessarily torn across. It then falls to the anastomoses affected with the articular arteries to replenish the torn nutrient artery with blood.

Veins. There are periosteal veins and nutrient veins, but the chief veins, enriched with young blood cells, are said to escape by the large foramina near the neds of the bone.

Lymph Vessesl exist in the periosteum and in the perivascular lymph spaces in Haversian canals.

Nerves. Sensory nerves are plentiful in the periosteum, and nerves ((?)vasomotor) accompany the nutrient artery.

Historical

One day in 1736, John Belchier, surgeon on the staff of Guy’s Hospital, London, was dining with a friend. A joint of pork was served, and it was commented that the bones were red. The host, who was a calico-printer, explained that he utilised bran soaked in madder from his dye vats to feed his pigs, and to this fact he attributed the colour. Belchier communicated this information to the Royal Society, and it was printed in its transactions.

Duhamel, a French squire, read Belchier’s paper and, being curious, fed madder to some of fowls and pigs; and with the same red result. He then conducted a number of experiments on pigs and found that if the animals were killed while the feeding of madder was in progress, the bones appeared red, and that if the feeding of madder had ceased for a period the bones appeared white. On laying open the bones, he found that though they were white outside they were red inside. By alternately feeding food with madder and without, he produced bones with alternating red and white rings or layers, so he concluded that bones increase in girth like trees and that the periosteum is responsible for laying down the rings. He encircled growing bones with rings of silver wire and in time he found the wire inside the medullary cavity, becuase the cavity too had enlarged - but he did not understand how.

In the shaft of a growing bone Duhamel bored holes at measured distances apart, and in them inserted silver stylets to keep them open. After a period he killed the animal and found that, though the length of the bone had increased, the holes remained the same distance apart so he concluded the growth in length takes place at the ends of the long bones.

John Hunter sought further explanation. Knowing that the lower jaw has no epiphyses and that the milk teeth of a child fill the body of the jaw right back to the ramus, he wondered how space was found for the three additional teeth, the three permanent molars. He surmised that the growth of bone entails two processes - one of deposition (addition), the other of absorption (subtraction). Only thus could he account for the growth of the jaw, the formation and progressive enlargement of medullary cavities, and for changes in the neck of femur. About 1764, John Hunter - employing pigs madder feeding experiments and using controls - put his theory to the test and proved it to be correct. (Consult “Menders of the Maimed” by Sir Arthur Keith).

There is general agreement that the bones of the base of the skull, being mainly carilaginous bones, grow as such, the chief epiphyseal plate being between the basi-occipital and the basi-sphenoid.

The bones of the cault of the skull are membranous bones. Regarding their growth there are two conflicting views: (a) In one view, the essential mode of growth is by deposition of bone on the exterior and resorption from the interior, modelling taking place as for long bones. (J. C. Brash,) (b) The other view is that growth is mainly sutural in all parts of the skull, with depositions and resorptions taking place in various areas both inside and out. (J. P. Weinmann and H. Sicher, and L. W. Mednick and S. L. Washburn).

In support of the latter view: If the coronal suture closes prematurely and the metopic and sagittal remain open, the skull becomes unduly broad and high and short (R. K. Rau); whereas, if the metopic and sagittal sutures close prematurely, the result is a long narrow skull (B. H. Dawson and D. A. N. Hoyte).

H. A. Harris has shown that the growth of the skeleton is sensitive to relatively slight and transient illnesses and to periods of malnutrition; that when a child is ill or starved, its epiphyseal plates, ceasing to proliferate, become heavily calcified; and that when growth is resumed, this line of arrested growth appears as a veritable scar. The annual rings in deciduous trees and on the scales of fish likewise bear evidence of growht retarded or arrested and its resumption. Figure 15 makes clear his statement that “since the transverse striations remain steadfastly parallel and quidistant, we now have convincing proof that there is no interstitial growth in length in the shaft of the tibia, and all growth in length takes place by the apposition of new bone to the ends of the diaphysis at the growth carilages”.

Fig 14: Remodelling of bone. As a long bone grows, sites once occupied by the expanded ends become parts of the more slender shaft; hence remodelling occurs

Fig 15: Outlines of 3 radiograms of the leg bones of a young girl taken over a period of two years. Observe that the 3 lines of arrested growth, denoting 3 successive illnesses, remain equidistant (After H. A. Harris)

When a particular radio-opaque substance, such as thorotrast, is injected into the bloodstream, it is taken up by the reticulo-endothelial cells and apparently it is retained by them indefinitely. Hence, the spleen and liver cast a positive shadow on an X-ray plate, and so does the bone marrow. Employing this technique experimentally in young animals, Mortensen and Guest have show that, while the expanded ends of a long bone grow farther apart and the medullary cavity enlarges correspondingly, the shadow cast by the part of the marrow infiltrated remains constant in length and the actively growing ends gradually recede from it. This substantiates the work referred to above.

Cartilage

Cartilage or gristle is a connective tissue in which a solid ground substance replaces tissue fluids. It has no blood vessels, lymph vessels, or nerves; so, it is insensitive. There are three types of cartilage: (1) hyaline, (2) fibro- and (3) elastic.

Hyaline Cartilage

(Gk. (h)ualos = a transparent stone)

Hyaline cartilage is white and resilient. It is potentially bone; in fact, all the bones, except certain skull bones and the clavicle, were preformed in hyaline cartilage.

Hyaline cartilage persists in the adult only at the articular ends of bones as articular cartilage, at the sternal ends of the ribs as costal carilage, and as cartilages of the nose, larynx, trachea, and bronchi. The thyroid, cricoid, and 1st costal cartilages commonlybegin to calcify about the 40th year.

Fibrocartilage

Fibrocartilage has the same structure as fibrous tissue (aponeurosis, ligament, fig. 53) save that, the ground substance being solid, the cells are not squeezed into stellate form by the bundles of fibrous tissue, but are round (fig. 53). Fibrocartilage bears the same resemblance to fibrous tissue that a starched collar bears to a soft collar. Wherever fibrous tissue is subjected to great pressure, it is replace by fibrocartilage which is tough, strong, and resilient.

It occurs in intervertebral discs, articular discs (e.g., semilunar cartilages of the knee), glenoid and acetabular labra, and the surface layers of tendons and ligaments that are pressed on by bone. It lines certain bony grooves in which tendons play and it caps certain bony prominences.

Elastic Cartilage

Here carilage cells are numerous and the solid ground work is pervaded by elastic fibers; so, it looks yellow. Being elastic, it springs back into shape after being bent.

It is found only in the auricle, external acoustic meatus, auditory tube, and the cartilages guarding the entrance to the larynx.