The pain from the compound fracture was almost unbearable.
Literal
Compound-fracture [possessive-の] pain [topic-は] almost unbearable was.
複雑骨折 means 'compound fracture.' ほとんど耐え難かった means 'was almost unbearable' — 耐え難い (hard to endure) is a compound adjective from 耐える (to endure) + 難い (difficult to).