topical application for broken bones ?

No retrieved study tested comfrey on broken bones. What exists is wound-healing benefit on intact skin and proven liver poison inside the plant.

The retrieval surfaced nothing that applies comfrey to a fracture. Several records are fracture studies, but none involve comfrey at all, so they cannot answer this. The closest match is the randomized trial in 108 children with skin abrasions: a 10% comfrey leaf cream healed abrasions in about 4.1 days versus 8.9 days with a weaker 1% preparation [1]. That is skin healing, not bone union. Bone mends by a different process, and a trial on abrasions does not transfer. Outside this retrieval, small trials of comfrey topicals for sprains and contusions exist, but I cannot weigh them here.

The safety side is concrete. Comfrey root and leaf contain hepatotoxic pyrrolizidine alkaloids, measured at about 710 mg/kg in root and 4.6 mg/kg in a finished ointment [4], with the alkaloid profile confirmed by chemical analysis [3]. The documented death was from eating comfrey, not skin contact [2]. Intact skin absorbs far less than the gut does, but a fracture often comes with broken skin, swelling, and a cast, and nobody has measured absorption through damaged skin in these records. The absence of that measurement is a fact about the evidence, not proof of safety.

My call: there is no evidence here that topical comfrey speeds fracture healing, so the burden of proof has not been met; if you use it anyway, keep it off broken skin and away from open wounds, and treat any benefit claim as untested for bone. Confidence: high that no direct fracture evidence exists in this retrieval; low that the question is answerable from it.