Japanese to English Medical Medical (general) External fixation of fractures of the thumb

I am working on a research paper dealing with treatments for metacarpal base fracture of the thumb.

In a part of the paper dealing with malunited metacarpal bone, the following passage appears:

母指中手骨の変形治癒が生じた場合，第一指間が狭小化し，MP 関節の代償性過伸展，把持の制限，ピンチ力の低下が生じるため...

I am having a hard time figuring out what 第一指間 refers to, and how to render this in to the appropriate English.

Any help here is sincerely appreciated.

Thank you.