The anatomy of physics

Chapter Four The anatomy of physics

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.


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:

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.

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!

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:


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).

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.

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).

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.


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).

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.

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)

Apr 4, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on The anatomy of physics
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