IMO it would not benefit them. Say they get an X-ray and it IS broken- then that lends credence to JA's claim that it truly is broken and the dispute about whether it occurred at the hands (or kick) of TA in January 2008 or some other time unrelated to TA or during the murder as TA tried to defend himself (in JA's view, as he was attacking her) - that dispute would still remain.
If an X-ray was done and there is no break, JA could still claim it was sprained so badly that it remains crooked (and as a lay person she just though crooked=broken) and still attribute it to the kick.
So no matter what the results, it wouldn't resolve the dispute.
Now if she were truly telling the truth, the DEFENSE would get the X-ray to show it is broken. Since they have not done that, it is likely she is not telling the truth and the injury is probably not a break, but the result of a cut tendon occurring as her hand slid on the knife while stabbing TA. Only an MRI or CT scan of some sort would possibly show tendon injury. Even then, she could STILL claim TA did it with the kick in January - it would be unbelievable, just as it is now without any radiologic studies.