This is a follow up to a previous post on Ulnar impaction. Dr Ranjit had some questions about it and I thought it would be useful to post the answers to her questions.

1. How does ulnar abutment occur in neutral ulnar variance?
   1. It can less commonly occur in ulnar neutral or even ulnar negative. This is because ulnar variance is affected by grip and forearm pronation which can cause relative lengthening of the ulna and this may not be present in a standard neutral view. This is called Dynamic Positive Ulnar Variance ( occurs with specific positioning and not present on a neutral X-ray). So when the question on an xray request is to assess for ulnar variance or impaction its important to perform a PA neutral wrist rotation xray and a xray in pronation with a firm grip ( this results in proximal movement of the radius).
2. Also do we not comment on ulnar variance on MRI unless it is very marked due to the factors you mentioned?
   1. Yes because the position of the wrist when being scanned in MRI is not how it would be on a neutral xray and from the question above we see that the relative position of the ulnar and radius changes with wrist position.  So unless its marked on MRI you should be cautious in assessing mild ulnar variance.