CHECK YOUR EXISTING KNOWLEDGE

Now check your answers against those given at the end of the chapter.

The classification of joints

Various attempts have been made at classifying joints in order to bring a degree of generalization from which we can gain understanding. Unfortunately, the different systems – often using Latin terminology – can intimidate the novice with a bewildering array of complex, unrelated, polysyllabic labels.

CLINICAL FOCUS

Many anatomical terms – which naturally tend to spill over into biomechanics – are of Latin or Greek origin, subjects that are seldom taught in schools today but were the universal language of the natural philosophers who first categorized and classified the human body.

Becoming a clinician requires you to learn a whole new terminology of several thousand words; however, if you can understand the etymology – where the roots of the word come from – you will find it far easier.

For example, the Greek word for ‘joint’ is arthron. Once you know this, then understanding words derived from this root becomes much easier:

• Arthrosis is a joint (the plural is arthroses)

• Arthritis is inflammation of a joint, just like appendicitis is inflammation of the appendix

• Arthropathy is disease of a joint, just like pathology means the study of disease

• Arthralgia is joint pain, just like neuralgia is nerve pain

For this reason, this book will regularly give the roots of a new word to help you build your clinical vocabulary. It also helps to invest in decent medical and English dictionaries; that way you can understand terms rather than having to learn them by rote.

The first thing to realize is that there are, in effect, two ways in which you can organize joints: by their structure or by their function. Which way you chose largely depends on whether you are an anatomist (structuralist) or an orthopaedist (functionalist). As a student of manual medicine, you are, at various times, required to be either or both and therefore familiarity with both systems is required. Figure 4.1 summarizes these two different approaches.

Figure 4.1 • Structural vs functional approach to joint classification.

In reality, there is considerable overlap between the two systems. In a fibrous joint, two adjacent bones are held tightly together by strong connective tissue. Obviously, such an arrangement does not allow for any significant movement so, functionally, this type of joint is termed a synarthrosis, from the Greek syn (together) and arthron (a joint). Therefore, as a rule, fibrous joints are synarthroses.

Other joints have cartilaginous elements to them, an arrangement that usually allows a degree of flexibility within the joint without free movement. A joint that allows limited movement in this way is called an amphiarthrosis, again from the Greek (amphi, on both sides; arthron, a joint). In general, cartilanginous joints are amphiarthroses.

Finally, joints that have a synovial capsule (Fig. 4.2) have all the elements required for free movement, restricted only by the joint anatomy and the soft tissue holding elements. A joint that is freely moveable is referred to as a diarthrosis (Greek: dia, through; arthron, joint), although the amount of movement can vary considerably: take, for example, joints of the upper and lower limbs. In a quadrupedal animal, there is relatively little difference between the joints of the fore and hind limbs. In humans, although the template is the same, the joints of the lower extremity are much less flexible but able to bear the weight of the body; the joints of the upper limb allow a remarkable degree of dexterity, but at the expense of stability – try walking on your hands or writing with your toes!

Figure 4.2 • Components of a synovial joint.

This is the trade-off that must always be made: high mobility equals low stability; high stability equals low mobility. It is at the extremes that these classifications start to become blurred: at what point do you differentiate ‘some movement’ from ‘free movement’? How little movement is ‘no movement’? In practice, an intervertebral disc (a cartilaginous joint) has more movement than the proximal joint between the tibia and fibula (a synovial joint); craniopaths would disagree that cranial sutures are immobile. As ever, when you try to classify something, there will be exceptions that do not fit the generalizations; as ever, it is always best to know the rules and then understand the exceptions.

Fibrous joints

The most consistent feature of fibrous joints is the absence of a joint space; any cavity between the two articulating bones tends to be filled with fibrous connective tissue. There are three types of fibrous joint found in the human body:

Sutures

Found between the bones of the skull, sutures show significant change throughout a lifetime. In infants, the bones of the skull do not make contact with each other and the dura mater, the outer lining of the brain, is directly palpable in the gaps between the bones. These gaps are called fontanelles. The most prominent of these is the anterior fontanelle (Fig. 4.3), bounded by the frontal and parietal bones. This fontanelle (the ‘soft spot’) has gone by the age of 24 months and, by the age of 6–7 years, the cranial sutures are, for the most part, united. From the third decade onwards, the fibrous interosseous tissue starts to ossify, forming an osseous union of the adjacent bones known as a synostosis (Greek: syn, together; osteon, bone).

Figure 4.3 • A model of an infant skull demonstrating the anterior fontanelle.

DICTIONARY DEFINITION

FONTANELLE

The word fontanelle comes from the French meaning little fountain. The most probable aetiology for the word is from cases where the intracranial pressure was raised from, say, hydrocephalus. The fontanelles would therefore bulge and, if they burst, either accidentally or as part of treatment, blood and cerebrospinal fluid would spurt out in a fountain.

There is more than one type of suture; unfortunately, there is no official agreement as to their classification. However, the system detailed below offers the best understanding as to the types of sutural joint that have been identified.

True sutures

With ‘true’ sutures, margins of opposing bone are connected by a series of interlocking processes and indentations that fit together like a jigsaw puzzle.

• Dentate sutures

^ Deep interdigitations, similar to meshing cogs. Examples include the sagittal suture.

• Serrate sutures

^ Fine, tooth-like, serrated interdigitations, similar to interlocking combs. Examples include the metopic suture.

• Limbus sutures

^ These complex sutures are best understood by breaking down the articulation between two bones into thirds. In the first third, bone A will overly bone B; in the last third, bone B will overly bone A, and in the middle third, the two bones interlock. Examples include the lamboid and coronal sutures.

False sutures

Not all opposing cranial bones lock together; some merely lie adjacent to each other with the sum of the articulation being the interosseous fibrous attachment. Such joints are therefore generally less rigid than true sutures. There are two types of false suture:

• Squamous sutures

The bevelled edges of the articulating bones overly each other. Examples include the temporoparietal suture.

• Plane sutures

The flat edges of the bone do not overlap but instead buttress each other. Examples include the joint between the two maxillae.

Schindylesis

This Greek word means ‘cleft’ or ‘fissure’ which accurately describes the union of a ridged bony prominence with a similarly shaped cleft. A good example of this is the way in which the triangular rostrum of the sphenoid locks into the alae of the vomer.

CLINICAL FOCUS

Clinical conditions can arise when sutures close prematurely. If the coronal suture closes early, it will not allow further anteroposterior skull development and there will be compensatory lateral growth; this is termed brachycephaly. If the sagittal suture closes early, the converse is true and an elongated head is called dolichocephaly. Early closure of the metopic suture between the two frontal bones leads to a cone-shaped forehead, trigonocephaly.

Unilateral closure of a paired suture causes cranial asymmetry known as plagiocephaly. Depending on how early closure occurs and how many sutures are involved, a child can be left with a reduced cranium, microcephaly, with mental retardation and other cranial disorders (epilepsy, nerve palsies etc.) from compression of neurological structures.

The most common failure of closure is the midline of the palate (palatines and/or maxillae) causing a cleft palate. In cleidocranial dysplasia, there is general midline failure, usually including failure of sutural closure, clavicular agenesis, spinal changes and pubic symphysis defects as well as digital hypoplasia and shortness of stature.

Peg and socket

The correct anatomical term for a peg and socket joint is a gomphosis (Greek: gomphos, a bolt). The only place in the human body where such joints are found is the insertion of the roots of the teeth into the sockets (alveoli) of the jaw (mandible and maxilla). The details of this arrangement are shown below (Fig. 4.4).

Figure 4.4 • A peg and socket joint (gomphosis).

Syndesmosis

In this type of fibrous joint, two bones are united by a sheet of fibrous tissue. The term syndesmosis again has Greek origins: syn means together; desmos, a bond. It is at this point that the relationship between fibrous joints and synarthroses begins to break down. Some syndesmoses are aimed at giving rigidity: for example, the interosseous membrane between the tibia and fibula forms, in the ankle, part of the mortice into which the tenon of the talus fits (Fig. 4.5). If this were not solidly immobile, then it would be impossible to stand – the talus would simply separate the two bones above it and slide upwards towards the knee!

Figure 4.5 • Schematic diagram of the ankle joint: the interosseous membrane renders the distal talofibular joint a syndesmosis.

By contrast, the equivalent membrane between the ulna and radius allows the two bones to supinate and pronate – over 180° of movement! The degree of movement is dependent on a number of factors: the distance between the bones, the angle of insertion of the membrane and the flexibility of the membrane itself. Despite the mobility of the radio-ulnar syndesmosis, it is still strong enough to act as an anchor point for the tendinous insertions of several forearm muscles.

Cartilaginous joints

As suggested by the name, these joints are united by cartilage rather than by fibrous matter. As with fibrous joints, you will discover that there are several ways in which these joints can be classified; however, once you can understand the terminology, which system you decide to use then becomes a matter of informed choice.

Primary cartilaginous joints

These joints, mainly found in the immature skeleton, are also known as synchondroses (Greek: syn, together; khondros, cartilage). In order for a long bone to grow, the shaft of the bone (diaphysis from Greek: dia meaning through; phyesthai, to grow, therefore ‘to grow through’) and the end of the bone (epiphysis: as above but epi meaning upon, therefore ‘to grow upon’) are united by the epiphyseal growth plate, which allows an actively growing zone in which a cartilaginous template ossifies to allow bone expansion (Fig. 4.6). When full growth is achieved, the growth plate also ossifies, forming a synostosis and uniting the bone.

Figure 4.6 • In this x-ray of a child, the growth plates are clearly evident Unlike bone, cartilage is radiolucent; therefore, the femoral heads and bones of the pelvis appear to be floating in space, the interconnecting cartilage growth plates are radiologically invisible.

Image reproduced courtesy of Michelle Wessely, IFEC, France.

Secondary cartilaginous joints

By contrast, secondary cartilaginous joints are amphiarthrotic, allowing a biomechanically significant amount of movement. In these joints, which are more commonly called sympheses (Greek syn, together; phyesthai, to grow), we also see, for the first time, the appearance of hyaline cartilage, lining the articular surfaces of bone. This cartilage can either be continuous, as it is in the joint between the sternum and manubrium, or it can be interrupted by articular discs, as is the case in the anterior intervertebral joints of the spine (Fig. 4.7) and the pubic symphysis (Fig. 4.8).

Figure 4.7 • A functional spinal unit showing the intervertebral disc and facet (zygoapophyseal) joints. The disc is an example of a secondary cartilaginous joint and is amphiarthrotic, being primarily concerned in distributing the forces associated with weight bearing; the facet joints are plane (gliding joints) and their combined diarthrotic motion allows considerable movement within the spinal column.

Figure 4.8 • The pelvis showing key features of the pubic symphysis.

Sympheses are all found in the midline and are, for the most part, confined to the axial skeleton. Although more concerned with the transmission of forces than with movement, they are still prone to the types of injury that are frequently seen by manual physicians.

CLINICAL FOCUS

The commonest injury to a secondary cartilaginous joint – and one that is often treated by chiropractors, physiotherapists and osteopaths – is a ‘slipped disc’. In reality, the disc, which can be regarded as a nucleus of glycoproteins contained within concentric rings of fibrocartilage (called ‘annular fibres’ because of their resemblance to annular tree rings), slips nowhere. Rather, damage to the annular fibres (Grade I) can allow the nucleus to track outwards forming a bulge (Grade II) or herniating into the spinal column causing damage either to a single nerve root (Grade III, Fig. 4.9) or to the spinal cord and/or multiple nerve roots (Grade IV).

Figure 4.9 • Because discs are not visualized on plain film x-ray (A), magnetic resonance (MR) imaging is the modality of choice to assess damage to discs. (B) A sagittal T1-weighted MR image shows a disc extrusion on the left at L5 (arrow). (C) A T1-weighted axial image and (D), a T2-weighted sagittal image, demonstrate that the large paracentral extrusion is occluding the left lateral recess and compressing the anterior aspect of the thecal sac (arrows). The disc has also lost some of its height.

Image reproduced courtesy of Michelle Wessely, IFEC, France © Clinical Chiropractic, 2006.

Because the spinal cord stops growing before the axial skeleton is mature, it finishes at the level of L2/L3 and the lower nerve roots hang down like a horse’s tail, in Latin cauda equina, before exiting the spine at the appropriate level. Compression of these can cut off the nerve supply to the legs, bladder and lower bowel, causing paralysis and incontinence, and requiring urgent decompressive surgery.

Synovial joints

The commonest joints in the body – and the ones, generally, of most interest to the manual physician – are the freely moveable (diarthrotic) synovial joints. The components that make up a synovial joint are detailed in Figure 4.2. As with cartilaginous joints, the bone of the articular surfaces is lined with a smooth coating of hyaline cartilage (except in the temporomandibular joint, the sternoclavicular and the acromioclavicular joint where the articular surfaces are covered with dense fibrous tissue instead). Here, however, the similarity stops.

Fact File

SYNOVIUM

The term synovium was introduced by the 16^th century physician, Paracelsus (born Theophrastus Bombastus von Hoenheim), whose abilities in other fields have since been eclipsed by his reputation as an alchemist. Although some sources regard the coinage of the word as arbitrary, it is possible that it comes from a hybridization of the Greek word syn, with and the Latin, ovum meaning egg. Synovial fluid is clear but has a higher viscosity than water and it has been suggested that Paracelsus thought that it resembled egg white (albumen).

The key feature of a synovial articulation is the joint space (in reality, more a potential than an actual space, particularly when weight bearing). Unlike the two classes of joints that we have previously examined, there is no connecting tissue between the two (or more) bones involved in a synovial joint. Instead, the joint is contained with a ligamentous capsule, the interior surface of which has cells that secrete synovial fluid, which acts to lubricate the joint’s surfaces and allows them to glide smoothly across each other. As a consequence, most synovial joints have considerably more movement than their fibrous and cartilaginous counterparts and are thus classified as diarthrodial. Their movement is restricted by the anatomical parameters of the joint, the supporting ligaments and the biomechanical limitations of the articulating muscles.

The major classification system for synovial joints is based on the anatomical relationship between the two articulating surfaces. This is useful for the clinician because, as we shall see later, there are rules of movement associated with these different types of joint that have clinical implications. Unfortunately, you will see a variety of terms used to describe each type of joint. In this text, the descriptive English terms will be used (as they are much easier to remember and actually tell you something about the joint – much as ‘peg and socket’ is a more useful term than ‘gomphosis’), although the variants that you may discover in other sources are also given. The classification of synovial joints, with examples of each type, is detailed below and summarized in Table 4.2.

Table 4.2 Classification of synovial joints

Hinge	(Ginglymus)	Interphalangeal joints
		Elbow (compound)
Plane	(Gliding)	Zygoapophyseal joints
		Acromioclavicular (complex)
Pivot	(Trochoid)	Proximal radio-ulnar joint
		Atlanto-odontoid joint
Ball & socket	(Spheroidal)	Hip
		Shoulder (humeroscapular joint)
Saddle	(Sellaris)	1st metacarpophalangeal joint
		Sternoclavicular joint
Condyloid	(Ellipsoid)	Radiocarpal (compound)
		2nd – 5th metacarpophalangeal joints
Bicondylar	(Condylar)	Knee (complex, compound)
		Temporomandibular (complex, compound)

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