Why is retaliation permitted for limbs despite the potential for the injury to spread to the entire limb, yet restricted due to the potential for spread within a fractured tooth?

Primary text

The prohibition against retaliation due to the perceived spread of injury to only a part of the organ, in contrast to limbs, is based on the certainty of the potential for harm. The potential for spread of injury to the entire body (*nafs*) is unavoidable in limb retaliation; considering it would entirely nullify retaliation for limbs. However, the spread within a portion of the organ is prohibited because it implies the possibility of exceeding the exact injury committed against the perpetrator (e.g., someone who only fractured a tooth might have their tooth broken further, chipped, or entirely removed, thus exceeding the original damage). Alternatively, the spread within a part of the organ is prohibited if it is apparent, similar to prohibited spread concerning life. Therefore, using a dull or poisoned instrument, or performing the act during extreme heat or cold, is prohibited to guard against apparent spread.