Dr David McGrath

Spine Physician

MB BS (Hons) FAFOM, RACP, FAFMM
Master of Pain Medicine

A Closer Look at Joint Movement

Joints are the structures between bones. There are multiple designs but all of them allow a set of SIX movements. These are three Rotations and three Translations. By and large the translations are limited because these movements reduce the contact surface area between the two bony surfaces. Some joints appear to possess translations but on closer inspection are really large radius rotations. Translations can be regarded as pathological movements associated with disease and damage. Returning to the remaining THREE rotations.

We recognize Spin and Swing rotations. There are TWO swing rotations and ONE spin rotation. Swing rotations can occur about a compound axis a little like a Spirograph. (Roll and Glide) In other words there is a rotation within a larger rotation with the two axis parallel. (the two rotations can be going in the same/opposite direction compared to the spirograph where rotations are in the same direction). Spin rotations occur about a solitary spin axis (often referred to as the Mechanical Axis) through the joint surface. In the region of the joint, a Spin Rotation leaves the geometry unaltered. There are multiple spin axis, dependent upon the swing rotation displacements.

This means we can consider all joints as rotating about three axis under normal conditions. (two swing axis and one spin) The total displacement or position of a joint can be adequately described as three numbers, each representing one of the rotations.

Joint movement can be conceptualised as a three number vector (X,Y,Z), where each number represents a dynamic rotational range as some function of the other two. In other words there is a movement space 3D volume. When the dimensions are independant the volume is a rectangular box. Dimensional dependance, creates shapes such as ellipsoid or egg shell shape arrangement (ovoid). An uneven ovoid is called a compound ovoid. The other important shape is the saddle or sellar geometry in which there are convex and concave surfaces. There would be many pathological shapes as well.

The movement space at any joint or series of joints is determined by bone,ligament and neuromuscular relations. The neuromuscular relations extend over one or multiple joints increasing dependence over and above local single joint conditions. In other words the movement space of a series of joints in a kinetic chain is multi- joint and multi-dimension dependant.

This is very important to understand, especially in respect of the geometric approach that I promote.

A restriction in movement can be generated by internal and external factors. We call these restrictions boundary conditions

(to be continued)