Chapter Five The physics of anatomy
Joint movement
Introduction
Whilst we do this, there are two fundamental principles that we should keep in mind.
Stability and mobility are mutually exclusive
If you take, as an example, the shoulder and the hip, both are ball and socket joints and both are moved by similar groups of muscles; unsurprising when you consider man’s evolution from quadrupedal forebears. However, the hip has a high degree of stability at the expense of the mobility that is demonstrated by the shoulder; by contrast, only professional contortionists can scratch their back with their toes and walking on one’s hands is difficult and tiring.
Degrees of freedom
As a rule, each class of synovial joint has a set number of degrees of freedom and these are given in Table 5.1. Plane (gliding) joints can translate, although these movements are usually heavily curtailed by ligamentous attachments. Usually plane joints such as the zygapophyseal, intertarsal and intercarpal joints have two degrees of translational freedom, although some authorities declare joint gapping to represent a third degree of freedom.
Arthrokinematics
Spin
Spin can occur alone or in conjunction with roll and glide: in pronation–supination of the forearm, it occurs independently; in flexion–extension of the knee it accompanies roll and glide and, as we shall subsequently discover, plays an important part in the knee’s stability.
Joint features
Most of our consideration of joints so far has been generalized; however, there are specific joints that have special or unusual features or are of particular clinical interest and, in this section, we shall consider them in turn.
The lower extremity
Knee
As the knee approaches full extension, the femur rotates internally with respect to the tibia; the last 10° of extension is accompanied by approximately 6° of rotation. This rotation seats the femoral condyles in the rings of the medial and lateral menisci, also bringing the ligaments to tension.
The upper extremity
Shoulder
The shoulder is the most complicated joint in the body. It is in fact a complex of three joints (glenohumeral, acromioclavicular and sternoclavicular) and one articulation (the movement of the scapula across the superior seven ribs). It also contains over two dozen bursae, a similar number of ligaments and is controlled by a dozen or so muscles. Somewhat surprisingly, the whole complex is uniquely controlled by a single nerve root (C5) – every other joint has dual supply; the hip is controlled by three contiguous levels.