I. Introduction.


Excellent lectures on the bones and the joints have been published on the internet. The contents of these lectures match the content described in Snell and is therefore appropriate for this course.

II. The Bones. http://www.leeds.ac.uk/chb/lectures/anatomy3.html


Dr. D.R.Johnson, from the Center for Human Biology, University of Leeds, West Yorkshire, England has posted on the web an excellent lecture on the bones. It matches closely the content described in Snell. You can find this information at the web address above or the adapted text below.
"Anatomists talk about both bone and bones. The former is a type of connective tissue made up of cells suspended in a matrix: the collagenous matrix in bone just happens to be heavily impregnated with minerals. You will learn about  bone cells elsewhere..... This osteocyte has characteristic long processes which run through the bone, putting it in touch both with other cells and with blood vessels and nerves. Bones are discrete organs made up of bone tissue, plus a few other things. The main misconception about bones then, is that they are made up of dead tissue. This is not true, they have cells, nerves, blood vessels and pain receptors. Bone constituents, organic and inorganic matrix and cells all turn over at a fairly rapid rate. If we treat a bone with various solvents we can remove the inorganic matrix and leave the flexible collagen. Or we can burn a bone and leave a hard brittle residue. The true structure of bone lies somewhere between these images. In tensile strength, bone is rather like cast iron, although around 1/3 of the weight, in bending stress it behaves like steel, although only half as strong and in compression it can withstand the forces exerted by a running man (equivalent to a dead weight of 270kg). Even in standing the compressive force on the hip joint, which you might expect to be half the body weight on each side, is multiplied by a factor of around six by muscular pull, since we are not in equilibrium when standing.
Determination of shape
            The shape and structure of bones is governed by many factors, genetic, metabolic and mechanical. Genetic determination of primary shape can be demonstrated by organ culture of bone rudiments, which subsequently grow into recognizable bones, i.e. roughly the finished shape in all major respects. Fine tuning is by muscular action. The muscles are active in utero, although it is difficult to isolate their effect at this stage. After birth, however, and up to adolescence there is a correlation between activity and growth. This is seen in reverse if we look at people who are bedridden, or who have paralysis (such as poliomyelitis).
             Metabolic factors are also important: calcium, phosphorous, vitamins A, C and D and the secretions of the pituitary,  thyroid, parathyroid adrenals and gonads are all involved. Dwarves and giants are controlled by aberrant hormones, but there is much variation in normal height. Absence of adequate supplies of vitamin D may lead to rickets, and absence of calcium in the diet lead to weak bone liable to fracture.

Function
1. As a lever. The bones of the upper and lower limbs pull and push, with the help of muscles.
2. As a calcium store. 97% of the body's calcium is stored in bone. Here it is easily available and turns over fast. In pregnancy the demands of the fetus for calcium require a suitable diet and after menopause hormonal control of calcium levels may be impaired: calcium leaches out leaving brittle osteoporotic bones.
3. Protective? This is often quoted in books: in fact protection against outside forces is rarely needed, and if it is we usually wear a cycling helmet, or a crash hat, or a hard hat. Or sit in a very strong structure like a formula 1 carbon fibre tub or a Volvo. So the bone cannot be that good. In practice these are exceeded by the almost continuous large forces exerted by our own muscles. Respiratory movements need ribs. If a thigh bone or a humerus fractures the pull exerted by the muscles, even though not in active use, it will be enough to overlap or otherwise displace the broken ends and we need considerable force, traction, to reduce the fracture i.e. to un-overlap the bits so that they can be lined up. The force exerted by the masticatory muscles is sufficient to support the body weight.
4.  As a marrow holder. This is secondary to production of maximum strength for minimum weight: the cavities produced in unstressed areas (like the holes in the tubes of a bicycle frame) are used for marrow, or in some places (mastoid) just for air storage. The saving is small in man but considerable in an elephant. Occurrence of bone in two main forms, compact and cancellous. .... Around the outside is a layer of strong, hard, heavy compact bone. In the middle is a branching network of cancellous or trabecular bone which usually, like iron filings, follow lines of force. Marrow sits in the interconnecting cavities between these plates or rods of bone.
 
    Origin of bone is again in two main forms. Some bone (in broad terms almost everything except the top of the skull) is preformed in cartilage - replacement or endochondral bone. Details will come in histology lectures. In the skull and one or two other places, however, bone forms directly in membranous connective tissue - membrane bone.
....
Classification of bones
             
The skeleton is made up of many bones which change in proportion between man and his close relatives but are easily recognizable. The easiest way to classify bones is by shape.
             
Long bones (Image can be seen at http://www.yavapai.cc.az.us/division/sci_math/biology/golden/a&p/ap103-01.htm)
Typical of limbs, and a good place to start. They consist of a central, usually hollow, tubular region, the diaphysis linked to specialized ends (epiphysis) by a junctional region (metaphysis). Look at the shaft first. Tubular,  a bit like a bicycle frame tube. Galileo was the first to write sensibly about this, noting that a hollow tube was stronger, weight for weight than a solid rod, and that the dimensions had to be related to body weight rather than area: so the bones of an elephant have to be proportionally broader than those of a man. In some bones we can see
 adaptations for specific forces. For example the wing bones of vultures and other large birds have strengthening that makes them very much like bridges: it is a sobering thought that the first vulture predates the first girder bridge by some millions of years. The diaphysis has layers of bone arranged like plywood for strength. The cavity is filled with bone marrow (red and active in children, yellow, fatty and inactive in adults). The shaft walls are made of compact hard bone, and thickest in the middle where forces are greatest. If these forces are too great the shaft may fracture.
           
Young bones have less calcium and are pliable, so fracture raggedly and partially (greenstick): older bones will fracture transversely or spirally according to force applied. Fractures usually heal spontaneously, albeit rather slowly in some cases, but the broken surfaces need to be manipulated into the right place and may need to be held with casts, pins or wires.
            
Towards the ends of the shaft the marrow cavity tends to be wider and filled with trabecular bone, arranged along lines of force which has a skeletal function in its own right and supports the marrow. The ends of the bone are specialized to allow growth with as little loss of strength as possible.
....
Short bone. Short bones are found in the wrist and ankle, carpals and tarsals respectively. They have no shaft, as they do not increase dramatically in size in one dimension during growth, and tend to be cuboidal in shape. They are rather like a Malteser in construction, with cancellous bone in the center and a hard outer shell of compact bone.

Flat bones. Flat bones like those of the cranium or the scapula are sandwiches of spongy bone between two layers of compact bone. They are usually curved, so we can refer to an inner and outer table with diploe between them. These diploe, especially in the skull, may become pneumatised, i.e. filled with air. A ring of facial sinuses around the nose may  become infected, leading to sinusitis.

Irregular bones. Any bones which do not fit these arbitrary categories (bones of the face, vertebrae) are referred to as irregular.

Sesamoid. Sesamoid bones are interesting because they occur in tendons, especially where a tendon turns a corner, and is thus exposed to friction. We shall come across these again when we talk about muscles.

             Surface markings of bone. We can often glean clues about what is going on around a bone from its surface. In places, like joint surfaces, the bone will be covered with smooth articular cartilage. This falls off in preparation but leaves the underlying bone smooth too. Bone is constantly growing or being reshaped, and this takes place on the surface. At high magnification  we can see, in a dried bone, what it was up to the point of death. This picture shows a hole for a blood vessel, a foramen. Around roughly half its diameter the collagenous bone is rough, the other half smooth. The rough is reabsorbing bone, being eaten by large osteoclasts which leave pits and the smooth is depositional, bone being formed. This indicates that the foramen was on the move as the bone grew. Other areas also show deposition and  resorption: these would be building up and hollowing out respectively. On a macroscopic scale these effects can be seen as points of attachment to the bone - of ligaments, tendons or the fibrous insertions of muscles. All these structures transmit forces, and demand a well organized junction.  Any part of this structure which has deposited calcium will appear as a bit of bone. Within the bone we often see rows of trabeculae or thick ropes of collagen,   Sharpey's fibres running across the marrow cavity to insert in the cortical bone opposite. Blood vessels and nerves similarly have canals.  The various lumps for fixing things to have different names according to shape, usually derived from a dead language. There are lots of these, but common ones are:
                 lumps and bumps
                  process
                  spine - if sharp
                  tubercle - if rounded
                  cornu - if horn shaped
                  hamulus - if hooked
                  crest - ridge
                  line - low ridge
                  depressions and holes
                  sulcus - groove
                  canal - tunnel
                  foramen - hole
                  fossa - depression
                  articular surfaces
                  facet - if small
                  condyle - if rounded
                  epicondyle - if near a condyle
                  trochlea - if pulley shaped"

III.  The joints.

Dr. D.R.Johnson, from the Center for Human Biology, University of Leeds, West Yorkshire, England has also posted on the web an excellent lecture on the joints. You can find this information at the web address above or the adapted text below.

"The joints are classified as:
1. Fibrous and cartilaginous joints where two bones are separated by a deformable intermediate
2. Synovial joints where one surface slides freely over another.

Fibrous joints. We have already mentioned the joint between the bony shaft and cartilage at the ends of long bones. This is a synchondrosis, a cartilage sandwich with bone on either side: bone and cartilage fit together perfectly and the whole thing is cup shaped. If movement occurs the growing bone will be damaged (slipped epiphysis) and this is countered by putting in a long nail to fix it again.
Sutures: are limited to the skull. They resemble a synchondrosis, but with fibrous tissue instead of cartilage between the bones. Sutures are necessary for skull growth: consequently well marked in the young, less so in the adult. The only movement in sutures is at birth when the cranial bones overlap to allow passage through the maternal pelvis. This movement is then discouraged by increasing complexity of the suture.  Later in life, when growth is complete, they fuse.
Gomphoses: are peg and socket joints as seen between teeth and jaws. The joint is maintained by the periodontal ligament which gives only a little to act as a shock absorber when we bite on a ball bearing.
Syndesmosis: only one of these in the body, the inferior tibio-fibular joint. In this type there is a little movement, limited by a tight ligament. Since many joints are limited by ligaments this is probably a special definition we can do without.
Symphysis: two bones united by cartilage, but designed to give a bit. The symphysis pubis with ligaments and fibrocartilage is normally closed, but opens in childbirth due to hormonal influences.

Synovial joints  have different parameters. Joint surfaces almost in contact but discontinuous, as a great range of movement is often possible, and the surfaces slide over each other. The sliding surfaces are covered with a thin layer of cartilage. This gives a coefficient of friction of <0.002. The joint cavity is sealed by a synovial membrane  which secretes synovial fluid, a lubricant and nutrient. Around this, in turn, is a tough fibrous joint capsule which keeps the ends of the bones in proper orientation. This is often locally thickened to form joint ligaments. The synovial cavity is very small between articular surfaces but larger round the edges where it may form a bursa, a
 sack-like extension which may be in contact with the joint cavity.
             Various inclusions may be present in the joint cavity: a tendon may pass through, sheathed in synovial membrane. Fat pads may be present, packing the large gaps which occur in some joints between bone ends. Pieces of cartilage are also found, in addition to articular cartilage. These may form:
             1. a labrum or lip deepening a bony socket
             2. menisci - incomplete discs or crescents increasing the size of articular surfaces
 3. complete, or nearly complete articular discs of fibrocartilage. This will convert a joint into two in parallel, which can then move in independent directions. The temporomandibular joint of the jaw is a good example of this.
...
Movements in synovial joints. These can be very extensive the shoulder joint being particularly free and able to move around three axes. Various schemes of classification of synovial joints have been used and will be found in different textbooks.
1. Complexity: Many joints possess only two articular surfaces and are therefore simple. Usually one surface is convex or larger than the other and termed male. Compound joints have more than one pair of articulating  surfaces (e.g. the elbow has two male surfaces on the humerus which articulate with female surfaces on radius and ulna) and are thus compound. Complex joints have an intracapsular disc or menisci.
2. Degrees of freedom: A joint which moves substantially in one plane (like an elbow) is uniaxial. One which  moves in two planes is biaxial, one which moves in three is triaxial. A ball and socket is multiaxial, but is equivalent to a triaxial as it has three degrees of freedom i.e. all movements can be reduced to XYZ axes. Not a good classification as there are often small but vital movements in other planes (e.g. knee rotation at end of flexing) and cannot take account of sliding movement.
3. Shape: Probably the most widely used classification, but still tries to simplify joint surfaces  - hinge joints: permit flexion and extension (knee)
 - pivot joints: allow rotation (superior radio-ulnar)
 - plane joints: have flat surfaces and allow gliding in several directions (carpus and tarsus) condylar joints: usually regarded as two
 - hinge joints with separate articulations (TMJ)
 - saddle joints: have surfaces shaped like two saddles, allow movement in two planes at  right angles and a little rotation (base of thumb)
 -ball and socket: allows very free movement around any axis through ball (hip)
 - ellipsoid: ball and sockets which are not round : rotation  therefore impossible   (radiocarpal joint)
4. Functional approach: This is the best classification as regards to understanding what is going on. All above classifications are approximations and have holes in them which fit uneasily. First classify joint movement. This is always made up of:
- gliding: of one surface over another- slide
- angulation - flexion, extension etc. - roll
- rotation about axis of bone - spin
Movement always occurs at articular surfaces, which are never planes nor spheres nor cones but always spheroids, egg shapes, either male or female i.e. convex or concave. A point moving between A and B on a surface can take can be described by a trigone (a bendy triangle) or three arcs. The imaginary point which traces these movements is the end of the axis of rotation. In the simplest case this is the end of the long axis of the bone: for something like a femur it obviously is not. Let's try this on a real movement, extending the knee. If we hold the tibia still and move the femur extension has three bits.
 - the femur rolls on the tibia
 - the femur slides posteriorly
- the femur spins to lock the joint.
The third of these is most important because it tells us something about how joints work. Take an egg and cut it in half. The resulting curved surface has a variable radius of curvature. If we try to fit this to another spheroid we see that it only fits well at one point. Elsewhere there are wedge-shaped gaps and smaller areas of contact. Joints exploit this: the position of best fit, or close packed position usually occurs at the end of the range of habitual movement. As a joint approaches this position ligaments are stretched and often some spin is imparted by them to screw the joint home. In this position the joint is virtually abolished: in practice it is only fully reached under strain and may damage articular surfaces and pull ligaments. So usually it is
approached but not realized. This position is comfortable because it uses little muscular energy and can be maintained for long periods. The loose packed position is also important because it allows:
- loosely fitting surfaces to spin, roll and slide
- a reduced area of contact, so little friction
- wedge shaped gaps, continually changing circulate synovial fluid like a peristaltic pump.

Limitation of movement is also important. Usually achieved by
- tension in ligament, which have strain and pain receptors
- tension of muscles around a joint
- passive resistance to stretch followed by reflex contraction when
- stimuli from mechanoreceptors becomes critical.
These explain Hilton's law: that joints and the muscles acting on them share a nerve supply. Paralysis of muscles thus affects joints. In spastic paralysis muscle tone is increased and movement restricted. In other paralysis, joints become lax, flail joints or actually disrupted."
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