Chapter Four The anatomy of physics
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Now check your answers against those given at the end of the chapter.
The classification of joints
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.
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
True sutures
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).
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).
Synovial joints
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) |