I have severe spasticity in my lower left limb due to a tbi. a split tendon transfer is not an option, is an amputation a plausible next step?

Unlikely.: It sounds like your spasticity is so tight that it is focal rigidity and is refractory to other treatments. Although the answer is complicated, for brevity, it is unlikely that spasticity producing pain, skin breakdown, or some other secondary effect will require an amputation. In addition, if pain is a problem, you may end with phantom pain thus compounding your problem.

Intrathecal?: A good spasticity specialist should be able to help you. Baclofen delivered directly to the spinal cord is an option. Amputation is not a good idea. Even if the most spastic area of the leg is removed the rest is likely to have tightness that will worsen significantly after an amputation. It would be trading one problem for another.

Other options: No amputation is needed. If oral medications and botulinum toxin injections have been tried and do not improve your function, you can also look into intrathecal Baclofen pump therapy (itb). Do you use your spasticity for transfers or for standing? Itb places a small amount of Baclofen where it is most effective and can be adjusted to your needs. It works very well with lower limb spasticity.