Sub Groups

98. The principles of Hie AO classification: The AO classification for long bone fractures may be grasped by likening it to an X-ray storage system, with numbered blocks of filing cabinets: one block for each bone. Within each block, each filing cabinet (which is also numbered) represents a particular area of each bone: cabinet number I stores fractures of the proximal segment, number 2 the diaphysis or shaft, and number 3 the distal segment. In the case of the tibia, there is a fourth cabinet to deal with fractures of the malleoli. (The junction between the segments is determined in the way described on p. 10, Frame 39.)

When a fracture bridges the junction between two segments, the segment under which it is classified is determined by the site of the mid point of the line of the fracture. In practice, therefore, a two digit code determines the location of a fracture: e.g. under 22- would be stored all fractures of the shafts of the radius, or the ulna, or of both these bones.

In each cabinet all the radiographs for a single location of fracture are divided into fracture Types (represented by the three drawers); the least severe go in drawer A (Type A fractures), those of intermediate severity in B (Type B), and the most severe in C (Type C). Some of the criteria used to differentiate between the three types of fracture have already been indicated (Frames 25, 26. 33). but see Footnote.

Any type of fracture can be put in one of three groups (represented by folders, and numbered 1-3). The methods of selection are again described later. Within each group fractures may be further sorted into subgroups (represented by partitions). Each of these subgroups has a numerical representation (.1. .2. .3). (If an even more detailed classification is needed, fractures within each subgroup can have added qualifications. These can be described by a single number (or two numbers separated by a comma) added in parentheses after the main coding. The first digit in the range 1-6 is used to amplify the description of a fracture's location and its extent, while the second is purely descriptive. The number 7 is reserved to describe partial amputations. X for total amputation, and 9 for loss of bone stock.)

As an example of the AO classification, a simple oblique fracture of the proximal part of the femoral shaft distal to the trochanters would be coded 32-A2.1, as follows:

3 = the hone the femur I the location

2 = the segment: the diaphysis I of the fracture - — separator between location and type

A = the type: A is the least severe type of fracture, with two bone fragments only 2 = the group: group 2 includes all oblique fractures

. I = subgroup: subgroup I includes fractures in the proximal part of the diaphysis where the medullary cavity is wider than in the more central part of the bone.

Footnote: the criteria employed in sorting fractures into their appropriate types, groups and subgroups are given in a little more detail in the section on Regional Injuries.

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