SET T0010 -
Emergency Department

Normal. Specifically the joints at the bases of the 4th & 5th metacarpals are normal

NOTE :

1. The straight PA view of the hand (ie the image on our right) reveals normal 4th and 5th carpo-metacarpal joints. No dislocation. The rule that needs to be applied to all straight PA views of the hand is as follows: the " light of day " will always be visible at the normal carpo-metacarpal joints of the 4th and 5th digits. If the " light of day " is not seen then a dislocation at the joint is highly probable.
2. Be careful. The second view shown here ( the radiograph on our left ) is not a straight PA view - it is very slightly oblique and it does not show the 4th & 5th carpo-metacarpal joints particularly well. You cannot evaluate the " light of day " feature on this slightly oblique radiograph - it must be a straight PA view.

" Light of day "? ? ? . . .

See : Accident & Emergency Radiology : A Survival Guide (3rd edition) : Pages 157-158

A PA radiograph is shown.

Abnormal.

Note the following :

1. Fracture of the base of 5th metacarpal.
2. Dislocated base of 5th metacrpal. The " light of day " is not visible at this carpo-metacarpal joint. This is the crucial finding. [ The radiographer will have suplied a second view - a steep oblique radiograph - that will also show that this metacarpal is dislocated ].
3. Contrast the appearance at the base of the 5th metacarpal with the joint at the base of the 4th metacarpal. On this PA radiograph " the light of day " (ie the normal joint) is clearly visible at the base of the 4th metacarpal and indicates that this joint is an anatomical position.

Abnormal bones; and two dislocations are present.

This is a PA radiograph of the hand and wrist. We need to apply our " light of day " rule (see case 1 above).

Note :

1. Firstly, suspect that there is a fracture of the hamate and very probably of the base of the 4th metacarpal. The additional steep oblique radiograph that the radiographer will provide would confirm or exclude those fractures.
2. The diagnosis that can be - and should be - made from this PA radiograph of the hand: the bases of the 4th and 5th metacarpals are dislocated. 
3. Dislocations - how can we be so sure? Because we cannot see the " light of day " at these two joints. The " light of day " rule, applied to the straight PA radiograph of the hand, has been broken.
4. Compare the appearances of these two joints with the same two (normal) joints in cases 1 and 4 in this set.

[ Agreed, we cannot see the " light of day " at the bases of the 2nd and 3rd metacarpals. This does not matter because: (a) this often occurs at those particular joints on a PA radiograph, and (b) the PA view " light of day " rule only applies 100% to the bases of metacarpals 4 and 5 ].

On the straight PA radiograph and on the steep oblique radiograph we can make the following observations :

1. Bones: undisplaced fracture / fractures involving the shaft and the head of the 5th metacarpal.
2. Joints: on assessing the straight PA radiograph of the hand we can see that the " light of day " is clearly visible at the bases of both the 4th and the 5th metacarpals. Consequently we are reassured that these particular joints are normal. They are not dislocated.

If you find assessment of these carpo-metacarpal joints to be difficult - and punch injuries are so very, very, common - you can see how medical artist Philip Wilson has created illustrative drawings which explain the " light of day " concept carefully and elegantly . . .

See : Accident & Emergency Radiology : A Survival Guide (3rd edition) : Pages 157-158 and 167-168