Principles and Management of Acute Orthopaedic Trauma: Third Edition

Part I

GENERAL INTRODUCTION TO ORTHOPAEDICS

Chapter 1

GENERAL INTRODUCTION. ORIGIN AND STRUCTURE OF BONE.

General introduction

The word Orthopaedics derives from two Greek words, orthos, meaning straight and paedon, meaning child and was coined in 1741 by Nicholas Andry, who was then a Professor of Medicine at the University of Paris. This relation of the subject to children arose from the historical fact that orthopaedic practice at that time was preoccupied with the straightening-up of children made crooked by various crippling diseases, especially poliomyelitis and cerebral palsy, prevalent at that time. Modern orthopaedics has gone beyond this; it is concerned with the study and management of all abnormal conditions affecting the musculo-skeletal system, including trauma.

The adult human skeleton (Fig.1.1.) is made up of over two hundred separate bones of different shapes and sizes, which, with their associated joints, form a complex solid framework for the whole body. Together with attaching muscles and other soft tissues, they form the locomotor system, a system whose complexity beats even the imagination of the cleverest of architects and engineers and whose efficiency is the marvel of all that have studied it.

Fig.1.1. The Human Skeleton and orientation for descriptive purposes (Reproduced with permission from Springer-Verlag)

The growing skeleton differs from the adult version by its possession of growth plates and epiphyses (Fig.1.2).

Fig.1.2. The growing skeleton differs from the adult version by its possession of growth plates (Arrows).

Functions of the skeleton

The skeleton performs a number of important functions in the body:

• It is the basic framework on which the entire body is designed and built.

• It protects the soft tissues of the body such as the brain, the spinal cord and the intrathoracic organs from mechanical injury.

• Together with attached muscles, tendons and ligaments, it makes it possible for the joints and the individual as a whole to move.

• In addition to these mechanical functions, the bone is a big reservoir of minerals, especially calcium, phosphorus and magnesium.

• The bone marrow is the main seat of manufacture of blood (haempoiesis)

• It is important to stress that in spite of its compact structure, bone is one of the most reactive of tissues, responding to virtually all the organs of internal secretion and to various other normal and abnormal stimuli.

A few reminders of the structure and function of the locomotor system may help us to understand the subject of orthopaedic trauma. Bone, cartilage and muscle are the main tissues of the locomotor system and all derive from the embryonic mesoderm.

ORIGIN AND DEVELOPMENT OF BONE

Bone originates from the embryonic mesoderm in which it forms by one of two mechanisms - intramembranous bone formation or endochondral ossification.

Intramembranous Bone Formation

In this mechanism, bone is formed directly by aggregation or condensation of mesenchymal tissues, without a cartilaginous precursor. The flat bones – the cranial vault, facial bones and part of the clavicle and mandible form by this route.

Endochondral Ossification

In this mechanism, from condensations of primitive mesenchymal cells, the framework of the bone first forms in cartilage, the so-called cartilage model, which is then replaced by bone through a series of complicated processes. The long and short bones develop via this route.

It is interesting to note that these two embryonic pathways of bone formation exist in the post-natal period: growth originating at the growth plate is essentially by endochondral ossification while growth originating from the periosteum occurs by intramembranous ossification.

Centres of ossification (Fig.1.3a)

Centres of ossification form in relation to the development of long and short bones by endochondral ossification. There are two of them - primary and secondary.

• The primary centre of ossification

This is the central portion of the cartilaginous model of a long or short bone. It ossifies by a gradual process of endochondral ossification, beginning in the 7th - 8th week of intrauterine life. The ossification proceeds from the centre towards both ends of the model, finally producing the diaphysis or shaft of the bone.

• Secondary centres of ossification

These appear in the cartilaginous anlage at both ends of a long bone, forming the metaphysis and epiphysis of the bone.

Fig.1.3a. Centres of ossification.

• Tertiary centres of ossification (Fig.1.3b)

These appear at the ends of long bones at different times in infancy and childhood, after secondary ossification, gradually developing into bony eminences at the ends of long bones (Fig.1.3b).

Fig.1.3b. Tertiary centres of ossifications about the elbow appear at different ages in infancy and childhood. This can be remembered with the mnemonic CRITOE: C – Capitellum; R – Radial head; I – Internal (medial) epicondyle; T – Trochloea; O – Olecranon; E – External (lateral) epicondyle…appearing at the ages of 1-3-5-7-9-11 years respectively.

ANATOMY OF BONE

GROSS ANATOMICAL TYPES OF BONE

The human skeleton can be divided into two groups: axial skeleton, comprising the bones of the skull, neck, truck and pelvis, and appendicular skeleton, comprising the bones of the upper and lower limbs. Based on their shape, size and structure, the bones of major significance to the orthopaedic surgeon fall into one of the following groups: long, short, flat, irregular and sesamoid bones.

Long Bones

Most of the bones of the limbs belong to this group. Typical examples are the femur, tibia, fibula, humerus, the radius and ulna. A long bone is made up of a shaft, the diaphysis, which houses a medullary canal, with the metaphysis and the epiphysis at each end.

Short Bones

The bones of the carpus and tarsus are typical examples. They are made of spongy bone surrounded by a thin layer of compact bone.

Flat Bones

These have an outer and inner layer of compact bone between which a layer of spongy cancellous bone is sandwiched. The bones of the skull vault typify this structure and are classically referred to as flat bones, even though they are not really flat. The sternum, ribs and scapulae are other examples.

Irregular bones

Any bone, which cannot be conveniently included in the above groups, is classified as irregular. The vertebrae and bones of the face are examples.

Sesamoid Bones

These are round, spherical or triangular bones, which are embedded in tendons or fascia. They are classically small in size and inconstant in location, but the patella is both large and important.

ARCHITECTURE OF BONE

Bone tissue occurs in two forms - woven (or fibrous) bone and lamellar bone. Their location and arrangement represents an example of perfect architecture, always in harmony with their intended function.

Woven Bone

This is a transient form of bone. It is seen during phases of rapid bone formation such as occurs in embryonic life, at zones of endochondral ossification and at fracture sites during healing. It consists of a loose framework of delicate bone trabeculae which, in a histological section, has the appearance of loosely woven material (Fig 1.5). It is mechanically inefficient as a structural material and is normally rapidly replaced by the lamellar bone.

Lamellar Bone

This is mature bone. It consists of distinct and regularly arranged layers of bone in which the collagen fibres have a distinct orientation. Lamellar bone occurs in two different structural forms - cortical (compact) and cancellous (spongy) bone.

• Cortical Bone

This is made up of cylindrical units called Haversian systems or osteones which, in a long bone, are compactly arranged in successive concentric circles around the medullary canal. Each Haversian system (osteone) consists of up to fourteen concentric circles or lamellae of bone tissue surrounding a central canal which contains blood vessels and nerve fibres (Fig 1.5). The osteones branch and interweave among themselves, their long axes coinciding with the major lines of stress to which a given bone is subjected.

• Cancellous Bone

This is located in the expanded or bulbous ends of the long bones and also in most of the irregular bones of the skeleton. In orthopaedic practice, the iliac crest is a regular source of cancellous bone for the purpose of bone-grafting. Cancellous bone is lamellar bone arranged as trabeculae and laid down in an orderly manner (Fig 1.4). These trabeculae are always arranged in such a manner as to be maximally able to withstand the stress experienced by a particular bone. A typical example is the proximal femur and the acetabulum in which their arrangement in both bones coincides with the directions of forces acting at the hip joint during weight-bearing (Fig.1.4).

Fig.1.4. The structure of bone

a. Woven bone is a loose framework of delicate immature bone present only at sites of rapid new bone formation

b. Cancellous bone is lamellar bone arranged in the form of trabeculae. It is found at the ends of long bones and at other sites

c. Cortical (or compact) bone forms the cortices of bones. It is made up of large number of Haversian systems.

d. The orientation of the trabeculae of a given bone always coincides with the lines of forces the bone is functionally subjected to (Wolff’s law).

NB. Wolff’s law states that the shape, structure and location of each bone are determined by its intended function: Bone develops a structure most suited to the forces acting upon it, adapting both the internal architecture and the external conformation to the change in external loading conditions.

MICROSCOPIC STRUCTURE OF BONE

Microscopically, bone is made up of bone cells (Cellular component) and matrix (Extracellular component). The matrix in turn has an organic fraction and an inorganic fraction.

BONE CELLS

Despite its compact nature, bone is very much alive and made up of living cells. These are the osteoblasts which synthesises bone tissue, the osteocytes which maintain it and the osteoclasts which resorb it (Fig.1.5). They arise from the pluripotential mesenchymal cells inhabiting all bone surfaces and probably from other connective tissues. They are highly differentiated cells, and therefore are incapable of self-reproduction.

Fig.1.5. Bone cells

a. Osteoblasts secreting intercellular substance

b. Osteoblasts completely surrounded by secreted intercellular substance

c. Ageing osteoblasts imprisoned by calcified intercellular substance are called osteocytes

d. Osteoclasts are multinucleated giant cells found on or near bone surfaces undergoing resorption

Osteoblasts

These are plump, single-nucleated cells with characteristic basophilic cytoplasm. Fine cytoplasmic processes connect neighbouring cells, forming continuous networks. They are usually found lying tightly in a single file along active bone-forming surfaces. Osteoblasts are very active cells and are particularly plentiful at sites of active bone formation. They produce and extrude osteoid, the organic matrix of bone, which consists of collagen, protein polysaccharides and glycoproteins. Osteoid subsequently becomes mineralised into bone. The secretory pathway involves the rough endoplasmic retinaculum and Golgi apparatus. LRP5 (low-density lipoprotein receptor-related protein) is thought to regulate osteoblasts activity.

Osteocytes

These are ageing osteoblasts that have become imprisoned in calcified bone matrix, forming about 90% of all bone cells. They metabolise at a slower rate than the osteoblasts and although isolated in lacunae, have an enormous surface contact with bone tissue via their long cytoplasmic processes, which pass through channels called canaliculi. They are considered to maintain the viability of mature bone tissue and to be involved in calcium homeostasis.

Osteoclasts

These are multinucleated giant cells found on or near bone surfaces undergoing resorption. They are derived from haematopoietic monocyte cell precursor and vary greatly in size, from small cells with one or two nuclei to the largest cells in the body with several hundred nuclei. They resorb bone by secreting proteins and lysosomal enzymes. Their escavating action on bone is attested to by the presence of a brush border between them and bone. Like osteocytes, they are considered to play a vital role in calcium homeostasis. Their activity is increased by parathyroid hormone and thyroxine and decreased by calcitonin and oestrogens. Osteoclasts and certain tumour cells are the only cells known to be capable of bringing about the resorption of bone.

BONE MATRIX

This consists of two parts - organic and inorganic.

Organic Bone Matrix

The organic matrix of bone, otherwise called osteoid, consists mainly of Type 1 collagen, with small quantities of protein-polysaccharides (glycosaminoglycans) and glycoproteins. Type I collagen is the most common form of collagen, accounting for 25% of the weight and 38% of the volume of adult bone.

Ninety-five per cent (95%) of the organic matrix consists of collagen, the remaining 5% being made up of the above-named protein complexes. Collagen is made up of bundles of fibrils, which in turn are composed of stacked molecules formed from polypeptide chains arranged in a triple helical pattern. It is a banded structure of specific periodicity - 70nm. The basic molecule is called tropocollagen and the basic fibre unit is called a microfibril. It is common to all connective tissues but in bone it has the unique ability to produce and accumulate minerals in an orderly fashion.

Inorganic Bone Matrix

This is made up of calcium, phosphate and small variable amounts of other ions, notably magnesium, sodium, bicarbonate and fluoride. It is closely associated with collagen, in the substance of which it is formed.

Bone mineral occurs in two forms: as amorphous calcium phosphate and as a crystalline structure with an X-ray diffraction pattern characteristic of the apatites. Bone crystals come closest in chemical composition to hydroxyapatite, Ca10(PO4)6(OH)2. Amorphous calcium phosphate is formed initially but is gradually transformed into needle-shaped apatite crystals. The crystals then become associated with the periodic banding of collagen and orientated along the collagen axes. Bone crystals are closely associated with water which forms a hydration shell around each of them. This shell contains the ions Na +, Mg ++, Ca++, citrate, carbonate and also trace elements such as strontium, radium, lead and fluoride. Exchange of ions with body fluids occurs at this site.

Mineralisation is catalysed by prophosphatases, which include alkaline phosphatase. Why does collagen in other connective tissues not calcify? This is because an inhibitor substance, called inorganic pyrophosphate (PP), is present in these tissues, which inhibits mineralisation in them. In bone, the inhibitory action of this enzyme is first nullified by the pyrophosphatases before they proceed to catalyse mineralisation.

In good health, un-mineralised bone (osteoid) can be found only in minute quantities at sites of bone formation e.g. at a fracture site. This is because mineralisation normally takes place rapidly. However, it is abundant in rickets or osteomalacia in which mineralisation is impaired.

MECHANICAL PROPERTIES AND FRACTURE OF BONE

Whether or not an injuring force succeeds in fracturing a given bone depends on the bone’s mechanical properties. The Mechanical properties of a material e.g. metal or bone are usually discussed using definable terms:

• Mechanical strength of a bone is a measure of its resistance to a loading force and reflects its stiffness.

• Stiffness is a measure of how much a bone deforms under load.

• Strain. Linear strain is the deformation or change in the length of an object as a result of loading. Angular strain results from a rotational force. Strain is a proportion and so has no unit of measurement.

• Stress is the force applied to a bone divided by its cross-sectional area and is expressed in Pascals or Nm-2….(minus 2 squared)

• The stress/strain curve is a graphical illustration of the mechanical properties of a solid material such as iron or bone. Basically, a stress/strain curve has two zones – elastic and plastic (Fig.1.6a). In the elastic zone of the material, stress is directly proportional to strain and the curve is almost a vertical line. If the deforming force is removed, the structure returns to its original length, like an elastic band. The stress/strain ratio is known as Young’s modulus of elasticity, E.Young’s modulus of elasticity for bone compares most favourably with that of other structural materials; it is about ten times that of cast iron! The point of change from the vertical to the horizontal part of the curve is called the yield point. Beyond the elastic zone is the plastic zone, the curve is represented by an almost horizontal line; increasing the stress in this zone will lead to permanent deformation, followed by breakage (fracture). The point at which the bone fractures is called the ultimate strength.

Brittle materials, such as glass, do not possess plasticity and break at yield point or soon after it.

Fig.1.6. Mechanical properties of bone

The stress-strain curve and Young’s modulus of elasticity, E.

When a bone is subjected to a bending force, the convexity is in tension (T) while the concavity is in compression (C). Bone is weaker in tension than in compression, so the tension side fractures first. As the fracture line propagates to the compression side, the stored energy there is released to produce the so-called butterfly fragment (Fig.1.7).

Fig.1.7. Mechanical properties of bone: pattern of loading and resulting fracture

INTRINSIC AND EXTRINSIC PROPERTIES OF BONE THAT DETERMINE ITS MECHANICAL PROPERTIES

Intrinsic properties

Bone density. Compressive and tensile strength of bone is proportional to its density. Accordingly, cortical bone has a higher ultimate strength than cancellous bone but the latter is 500% more elastic/ductile than the former.

Patient age. Bone in the young is much more elastic than in the adult, hence the green-stick and plastic pattern of failure (fracture) seen in the young. Young’s modulus reduces by 1.5% annually. Osteoporosis in the elderly thins cortical and cancellous bone, with resultant reduction in bone density, which reduces ultimate strength by 5-7% every 10 years.

Type/Structure of bone. The longer a bone, the greater the bending moment; the greater its cross-sectional area, the stronger the bone and the greater the stress to failure.

Previous surgery. Implants, especially plates, shield bone from normal stress (stress shielding), resulting in reduced bone formation. This follows from Wolff’s law; reduced bone density makes the bone more susceptible to fracture in the affected area.

Stress risers. A cortical defect, caused by direct bone loss or by destruction by a tumour or an infective process, reduces the strength of a bone significantly and increases the chance of failure (fracture).

Extrinsic properties

Magnitude of the injuring force. A bone’s resistance to an injuring force generates potential energy, proportional to the injuring force. This energy is released at the breaking point. The stronger the bone, the bigger this force, so, fragmentation and significant soft-tissue injury can occur with significant injuring forces. This does not occur in osteoporotic bone with reduced bone density.

The rate of its application (cyclicity). Because bone possesses viscoelasticity, its ultimate strength increases when loaded at a faster rate, generating more energy, which is released at failure.

Direction of injuring force. This is also important as bone possesses anisotropy, i.e. its mechanical properties differ when load is applied along different axes. Briefly, bone is strongest in the direction in which it is loaded in vivo, i.e. longitudinally in a long bone.

FURTHER READING

Andry N: Orthopaedia: or The Art of Correcting and Preventing Deformities in Children. London: A Millar, 1743.

Albright TA and Brand RA (eds). The Scientific Basis of Orthopaedics, 2nd ed,

New York. Appleton-Century-Crofts, 1987.

Colnot C. Cellular and molecular interactions regulating skeletogenesis.

J Cell Biochem 2005(95): 688–697.

Fawcett DW and Jensh RP. Bone. In: Bloom and Fawcett: Concise Histology.

New York: Chapman and Hall. Pp69-79, 1997.

Hipps JA and Hayes WC. Biomechanics of Fractures.

In: Browner BD, Jupiter JB, Levine AM and Trafton PJ eds. Skeletal Trauma.

Philadelphia: WB Saunders; 97-130, 1998.

Hughes SPF and McCarthy ID. Sciences Basic to Orthopaedics.

London: WB Saunders 1998.

Mackie EJ, Ahmed YA, Tatarczuch L, Chen KS, Mirams M.

Endochondral ossification: how cartilage is converted into bone in the developing skeleton.

Int J Biochem Cell Biol 2008(40):46–62.

Owen R, Goodfellow J and Bullough P (eds). Scientific Foundations of Orthopaedics and Traumatology. London, Heinemann, 1980.

Passmore R and Robson JS, eds. A Companion to Medical Studies, Vol.1, 2nd ed. Blackwell Scientific Publications, 1976.

Yang Y. Skeletal morphogenesis during embryonic development. Crit Rev Eukaryot Gene Expr 2009;19:197–218.

Chapter 2

BASIC ANATOMY AND PHYSIOLOGY OF CARTILAGE, MUSCLE AND JOINT

INTRODUCTION

Cartilage is a very important component of the musculo-skeletal system. It has been stated earlier that in the embryonic period, most of the bones of the skeleton were first laid in cartilage before ossification. In the foetus and adult, however, cartilage becomes restricted to three areas - articular surfaces, epiphyseal growth plates and the ribs. Outside of the skeleton, cartilage is found in the framework of the ear and respiratory system.

Cartilage exists in three forms—hyaline, fibrous and elastic cartilage. Hyaline cartilage contains type II collagen. Fibrocartilage contains more fibres than ground substance and elastic cartilage contains elastic and collagen fibrils. Articular and rib cartilage is hyaline cartilage whereas the menisci of the knee joint are made of fibrocartilage. When articular cartilage heals following injury, fibrocartilage forms in its place. Elastic cartilage contains a relatively large amount of elastin; it is yellow in colour and is found in the structure of the ear and the epiglottis.

STRUCTURE AND FUNCTION OF CARTILAGE

Cartilage consists of cells (chondrocytes and chondroblasts) and matrix (collagen, protein polysaccharides and water).

THE CELLS OF CARTILAGE

These are chondrocytes and chondroblasts. The chondrocyte is the principal cell of cartilage. Its size and shape vary with its location in cartilage and its function at that site. Usually it is a plump, circular cell with large rounded nucleus filling the lacuna within the cartilage matrix but it may be flat as in the superficial zone of articular cartilage. In the proliferative zone of epiphyseal cartilage, it is smaller in size, flattened and may show mitotic figures. Here it is referred to as a chondroblast.

Mature chondrocytes have a low turnover of cells. Ageing chondrocytes have shrunken nuclei and disorganised intracellular organelles such as mitochondria and endoplasmic reticulum but cellular activity in the form of matrix production continues throughout life. Chondrocytes have a limited capacity to repair large defects such as may result from injury or disease.

Cartilage Matrix

Chondrocytes and chondroblasts are responsible for the production of cartilage matrix, which is made up of collagen, protein polysaccharides (proteoglycans) and water. The principal protein polysaccharides in cartilage are chondroitin sulphates and keratan sulphate.

The Structure of Articular Cartilage (Fig.2.1)

Articular cartilage is hyaline cartilage. It is composed of water (75%), type II collagen (12.5%) and proteoglycans (12.5%). The high water content is made possible by the hydrophilic properties of the proteoglycans. This composition of articular cartilage gives it the mechanical properties of being able to deform and recover its shape after the cessation of the deforming force. Articular cartilage also acts as a damper or shock absorber, reducing the damaging effect of sudden load on bone ends.

The Four Zones of Articular Cartilage

Articular cartilage is a highly organised structure. This is necessary in order to enable it to serve the mechanical functions outlined above. Starting from the surface, the four zones are:

Superficial zone. This is a narrow zone in which collagen fibres are abundant, closely packed and arranged parallel to the joint surface. The chondrocytes here are discoid in shape with their long axes parallel to the surface.

Intermediate zone. This is thicker than the superficial zone. In it the collagen fibres form an irregular interlacing mesh. The chondrocytes here are larger, rounder and more evenly spaced.

Deeper zone. Here the collagen fibres are arranged perpendicular to the underlying bone and the chondrocytes are disposed in columns parallel to the collagen fibres.

Calcified zone. This is a narrow calcified zone lying just above the underlying bone.

Fig.2.1. The structure of articular cartilage

The Structure of the Epiphyseal Growth Plate (Fig.2.2.)

The growth plate is a band of hyaline cartilage separating the epiphysis of a long bone from the metaphysis. Accordingly, each long bone has two growth plates. The growth plate is responsible for growth of the bone mainly in length but also contributes to its growth in width. The rate of growth in the two growth plates of a bone usually differs. For example, in the femur, the distal growth plate accounts for 70% of the growth in length while the proximal plate accounts for only 30%.

Starting from the epiphyseal end, the growth plate can be divided into three main zones—zone of growth, zone of cartilage formation and zone of ossification. Each zone can be subdivided into layers of cells or sub-zones at different levels of cellular activity.

There is a tendency to conceptualise the zones only in terms of their cellular components. This is incorrect because the cartilage matrix and the changes occurring in it in the different sub-zones are equally important.

Fig. 2.2 The structure of the growth plate

Zone of growth

The first layer of cells in this zone is that of chondrocytes in a ‘resting’ state. They are intimately associated with the epiphyseal vessels, which provide undifferentiated cells. Additional resting cells are also elaborated peripherally in a specialised area of the perichondrium called the zone of Ranvier. The next layer of cells is that of dividing cells. This division occurs in both longitudinal and transverse directions although mainly in the former. The next layer is that of columnating cells. In an active growth plate, these cell columns can comprise half the total height of the plate. The randomly disposed collagen fibres in the resting and dividing layers take a more longitudinal orientation between the columnating cells.

Zone of cartilage formation

In this zone, the chondrocytes hypertrophy as a result of increased cellular activity. Increased amounts of intercellular matrix are formed and biochemical changes occur in it preparatory to its eventual ossification; it becomes metachromic and then calcifies. The fate of the chondrocytes is controversial. Some authors believe that they degenerate while others think that they become osteoblasts.

Zone of ossification

Following the calcification of the matrix in the zone of cartilage, metaphyseal vessels which bring in osteoblasts invade it. These osteoblasts form osteoid tissue on the preformed chondroid septi. The osteoid is quickly mineralised, forming the so-called primary spongiosa. With time this becomes replaced by a more mature variant, the secondary spongiosa which no longer contains remnants of its cartilaginous precursor.

Nutrition of Cartilage

Cartilage is largely avascular and acquired its nutrition by diffusion of nutrients from the surrounding tissue fluids. This is the mode of nutrition of articular cartilage, except during the growth period when the deeper layers of it receive a blood supply from the epiphyseal vessels. Mechanical activity acts as a pump, which aids diffusion of nutrients through the cartilage. In the epiphyseal growth plate, the major source of nutrition is the vascular plexus in the epiphyseal side of the plate next to the zone of growth, the rich metaphyseal blood supply being concerned mainly with endochondral ossification. This arrangement explains the clinical observation, fractures in childhood, which usually occur through the comparatively weak hypertrophied zone of cartilage formation, the nutrition of the growth plate (from the metaphyseal vessels) is undisturbed, whereas epiphyseal fractures frequently disturb cartilage growth (because of damage to the epiphyseal vessels).

Situations imposing non-use of joints or limbs (immobilisation following fractures or other injuries; paralytic conditions such as poliomyelitis) cause impairment of nutrition and both articular and growth plate cartilage may degenerate even to the point of total destruction. In poliomyelitis, for example, a severely affected limb is always shorter and smaller than an unaffected one.

Calcification of Cartilage

Cartilage may calcify (mineralise) at certain sites and under certain circumstances. We have already mentioned the calcification that occurs as a prelude to endochondral ossification in articular cartilage and in the epiphyseal growth plate. Another example is the calcification of cartilaginous bone tumours, e.g. the cartilaginous cap of an osteochondroma.

If mineralisation is impaired, as in rickets, the invasion of mineralised cartilage matrix by vascular tissue and, consequently, its ossification does not occur. As in bone, alkaline phosphates are essential for cartilage mineralisation and can be demonstrated at mineralisation sites. The mineral is a calcium apatite crystal similar to that of bone.

Epiphyseal Closure

At the end of the growing period (at 15-16 years in the female, 16-18 in the male), division of cartilage cells in the growth plate ceases and the plate becomes progressively ossified, beginning from the diaphyseal side of it. The epiphysis and diaphysis become one, the process of their union being called a synostosis. A thin horizontal and radiographically more opaque line is formed at the point of union and is called the epiphyseal scar.

In a typical long bone with two growth plates, one produces more total growth than the other, either because its cartilage cells multiply more rapidly, or because they grow for a longer period than those of the other plate. This more productive end is called the growing end of the bone. Important examples include the proximal end of the humerus and lower end of the radius and ulna in the upper limb, and the distal end of the femur and proximal end of the tibia in the lower. These facts lie at the root of the memory aid phrase: To the elbow I grow, from the knee I flee which in effect, repeats the facts stated in the last sentence.

BASIC ANATOMY AND PHYSIOLOGY OF SKELETAL MUSCLE

The importance of skeletal muscle to the locomotor system cannot be over-emphasised: they move the joints and therefore the whole body. This they do by virtue of their ability to contract in response to nervous stimuli invoked by will, hence the alternative name, voluntary muscles. Knowledge of the gross anatomy and function of muscles is helpful in the understanding and management of congenital abnormalities, fractures and other abnormal conditions involving the musculoskeletal system.

The Gross Structure and Function of Skeletal Muscle (Fig.2.3)

Fig.2.3. The structure of skeletal muscle

A whole skeletal muscle is a mass of muscle tissue, usually of a definite shape and size, enclosed in a connective tissue sheath called epimysium. It has at least one ‘origin’ and one ‘insertion’. Some muscles e.g. the biceps, the triceps and the quadriceps as their names imply have two, three and four origins respectively. The origins and insertions of the muscles of the appendicular skeleton determine the displacements that occur when the major long bones are fractured.

It is important to bear in mind that no muscle acts alone and that very few muscles have only one action. Muscles always act in conjunction with several others as an integrated group. For example, a muscle which crosses a joint in such a way that its contraction produces adduction and internal rotation cannot produce either of these movements separately. However, when it acts in conjunction with other muscles as a group, some of them can be used to cancel out an unwanted action. For example, if pure adduction is required, the internal rotation component can be cancelled out by employing one or more of the external rotators. Similarly, if internal rotation alone is required, the adduction component can be cancelled out by the abductors.

The term, prime mover, is commonly used to describe muscle and their actions. A prime mover (muscle) is one whose contraction is largely responsible for bringing about a given movement. When a muscle acts in a subsidiary capacity to bring about a movement, it is called a synergist. Muscles which oppose prime movers are called antagonists. For example, flexor carpi radialis and ulnaris are the prime movers in wrist flexion, all the long flexors of the fingers are their synergists and the dorsiflexors (extensors) of the wrist are their antagonists.

The Muscle Cell

Although a detailed knowledge of the finer structure and function of skeletal muscle will certainly help in the understanding of certain muscular diseases such as muscular dystrophy, it is of limited value in the management of most common orthopaedic diseases, including injuries. Therefore only a brief summary of microanatomy and function will be given here.

The basic structure of a muscle cell is similar to that of other cells. Its basic peculiarity is its elongated shape and its possession of an excitable membrane called sarcolemma. In addition to the normal elements (organelles) found in all cytoplasm, the cytoplasm of the muscle cell, which is otherwise called sarcoplasm, possesses contractile protein filaments called myofilaments, which run along the long axis of the cell. A myofilament contains at least three proteins, actin, myosin and tropomyosin. A bundle of these filaments grouped together is visible in the light microscope and is called a myofibril. Mitochondria, the power-houses of any cell, are particularly large and numerous in the myofibrils. The smooth endoplasmic reticulum of a muscle cell is known as the sacroplasmic reticulum and is disposed along the myofibrils.

Each muscle cell is enclosed in a connective tissue tube called the endomysium. The so-called ‘muscle fibres’ are bundles of cells, each in its endomysium, enclosed in another connective tissue tube called the perimysium. The sheath around a whole muscle is called the epimysium.

The contractile unit of a skeletal muscle is called a sarcomere. It is a composite arrangement of thick myosin-containing a thin actin containing filaments. The architecture of a sarcomere imposes precise limits on the amount of shortening that can occur during contraction. A muscle fibre can shorten by more than 50% and a whole muscle can produce full movement in a joint from shortening to only 43% of its fully extended length.

Skeletal muscle is able to perform its special function of contraction because of its excitability. The excitation (impulse) comes from the motor nerve or nerves innervating it. Muscular contraction requires a lot of energy, the immediate source of which is believed to be adenosine triphosphate (ATP). A lot of heat is produced by muscular contraction. Muscles are subject to fatigue (reversible exhaustion) and rigor (irreversible exhaustion).

BASIC ANATOMY AND PHYSIOLOGY OF JOINTS

A joint is the anatomical structure between two or more bones. This rather vague definition is given in order to avoid the temptation to think of a joint only in terms of a space or cavity between two or more bones, facilitating movement between them. Although the latter is the usual function, some joints are solid in nature, like the primary cartilaginous joint separating the epiphysis and metaphysis in the growing long bone, and do not allow movement.

Joints develop in the embryo from elements of the cartilaginous models of the skeleton and their mesenchymal precursor. In the more typical synovial joints, an interzone develops between two or more models. This zone consists of three layers—two parallel chondrogenic layers and an interposing and less dense intermediate layer. Complex processes occur in this zone, which result in the formation of a joint cavity (by cavitation) and development of a joint capsule, synovial membrane and intra-articular structures such as menisci and cruciate ligaments. The chondrogenic layers form the opposing joint surfaces (cartilage). Joint formation is firmly established by the 10th intrauterine week.

TYPES OF JOINTS

There are three main classes of permanent joints—fibrous, secondary cartilaginous joint (or symphysis) and synovial joints.

Fibrous Joints

Examples include the distal tibiofibular joint, also called a syndesmosis and the structures which separate the skull bones in the foetus and young child. Fibrous joints are characterised by the presence of fibrous tissue instead of a joint cavity between two bones and allow only limited movement.

Secondary cartilaginous joints (or symphysis)

The classical example is the symphysis pubis. The intervertebral disc is another example. In this type of joint, the bone ends are covered with hyaline cartilage (as in synovial joints) but a plate of fibrocartilage then unites these ends. This plate may become softened as in the intervertebral disc or may develop a cavity as in the symphysis pubis. Movement in individual joints is usually limited but can be considerable e.g. in the vertebral column.

Synovial Joints

These constitute the majority of the joints of the locomotor system. As their name implies, they are characterised by the possession of a synovial membrane. Their complex structure permits free movement to occur in them but these characteristics also make them subject to certain pathological conditions such as inflammatory joint disease (which is essentially a disease of the synovial membrane) and degenerative joint disease (which is in the main a result of the wear and tear of articular cartilage caused by movement at the joint).

Structure of a Synovial Joint (Fig.2.4)

The moving bone ends are as a rule covered by hyaline cartilage. The few exceptions include the temporomandibular joint in which the articulating ends are covered by fibrocartilage. Starting from the outside and going inwards, other structures forming the joint are the joint capsule (a sleeve of collagen fibres surrounding the whole joint), the synovial membrane (a band of vascular tissue with a glistening inner surface) lining the inner surface of the capsule and intra-articular structures. Intra-articular structures may include cartilaginous discs which are typically found in the knee joint where they are called menisci (singular = meniscus).

Fig. 2.4. The structure of a synovial joint.

A synovial joint is characterised by its possession of a synovial membrane

Also associated with synovial joints are ligaments, tendons and bursae. Ligaments are bands of fibrous connective tissue usually found on the outside of the joint capsule, on its medial and lateral aspects, as in the knee joints, but which could also be intra-articular, for example, the anterior or posterior cruciate ligaments of the knee. Tendons are the fibrous origins or insertions of muscles. They are usually related to the outside of the joint capsule but could also be intra-articular as in the case of the popliteus tendon in the knee.

A bursa is a pocket of synovial membrane that has protruded outside the joint through a defect in the joint capsule but, sometimes, there is no connection between a bursa and a related joint cavity. In the latter case, it develops in the connective tissue around muscles or tendons as a smooth-walled cavity containing a viscid lubricant closely resembling synovial fluid. Sometimes a bursa may wrap around a tendon so as to enclose it almost completely for much of its length; such elongated bursae are known as synovial sheaths.

Types of Synovial Joints

Synovial joints are usually classified according to the type of movements or movements that take place in them or according to the shape of their articular surfaces. The following types of joints are distinguished:

Hinge Joints allow movement only in one plane—the plane of flexion and extension. A classical example is the knee joint. The so-called condylar joints are actually hinge joints.

Pivot Joints resemble the hinges of a gate; they allow the movement of rotation. The superior radioulnar joint is of this variety. Hinged at this joint, the radius rotates around its axis in the movements of pronation and supination of the forearm.

The hip and shoulder joints typify Ball-and-socket Joints. They allow movement centred in the centre of the ball which, in the mentioned examples, are the heads of the femur and humerus respectively. The movements of flexion, extension, abduction, adduction, and internal and external rotation as well as a combination of all of them (called circumlocution) are all possible. The so-called ellipsoid joints, represented by the radiocarpal (or wrist) joint, is a variety of a ball-and-socket joint in which both the ball and the socket are ellipsoidal rather than spherical in shape.

The carpometacarpal joint of the thumb represents saddle-shaped joints. They have two saddled-shaped articular surfaces lying upside-down on each other. They allow rocking movements in other directions, including rotation.

It must be borne in mind that most movements of the body involve more than one joint whose actions must necessarily affect one another. Another point of practical significance about joints is the concept of stability. The most important factor determining the stability of a given joint is its anatomy. In this respect there are two factors - the bony anatomy and the soft-tissue (muscle and ligaments) anatomy. The shape of the articulating surfaces is important too, so also are the associated muscles and ligaments. It is therefore not surprising that the hip joint with its deep acetabular socket, congruently-shaped and well-covered femoral head, supported by powerful and well balanced muscles and ligaments, is the most stable joint in the human body. Its stability makes it difficult for it to dislocate except under the effect of enormous forces. On the other hand, the bony base of the shoulder joint is comparatively weak and so its stability depends to a large extent on the surrounding muscles (rotator cuff muscles). Consequently it dislocates much more easily that the hip joint.

Blood Supply and Innervation of Synovial Joints

The blood supply of joints is derived from a plexus of vessels lying outside the joint capsule. Its nerve supply comes from adjacent nerve trunks. The fibrous capsule and any fibrocartilagenous intraarticular structure such as intervertebral discs are freely supplied by sensory fibres. The synovial membrane also has sensory fibres but articular cartilage (as opposed to subchondral bone) has none.

Synovial Tissue

Synovial membrane is made up of synovial cells which derived from the embryonic mesenchyme of the skeletal blastema, like other connective tissue cells. They form a discontinuous layer, one to four cells thick, with the cells linked to one another by interlacing cytoplasmic processes.

A fibrocellular subsynovial layer of tissues richly supplied with blood vessels and lymphatics containing mast cells and macrophages supports the synovial membrane. In both structure and function the synovial membrane is similar to the peritoneum. Its scavenging role is very efficient. Its venous circulation absorbs crystalloid and similar very small particles. Larger particles are absorbed by the lymphatics, while very large particles are phagocytosed by the synovial lining cells.

Synovial membrane secretes the synovial fluid, which is a viscous, clear or slightly yellow fluid rich in protein. Chemically, it is a dialysate of plasma,