Brief Answer:
Top of the toilet seat usually does not cause fracture.

Detailed Answer:
Hello!

Thank you for the query.

There is no simple answer on the question how heave the object must be. It all depends of many factors including the height it falls from, the area which it hurts (muscles covered bone gets fractured harder), fatty tissue amount and bones resistance (osteoporosis can make even minor injury serious).

Generally, toilet seat should not cause bone fracture at the wrist. Especially plastic one.
I suggest you to wait and see. Put some lukewarm compresses on the injured area. If the bone is not fractured, few days should be enough to make it painless. If not, than go to an x-ray.

Hope this help. Feel free to ask further questions.
Regards.

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