Complex sentence: Barthelmess's career had gradually diminished since sound films became popular in the late 1920s and was a controversial choice, mainly because he had recently had a botched plastic surgery operation on the skin under his eyes that resulted in permanent X-shaped scars under both of his eyes.
Split into simpler sentences: Barthelmess's career had diminished since sound films became popular in the late 1920s and was a controversial choice. Barthelmess had recently had a botched plastic surgery operation on the skin under his eyes that resulted in permanent X-shaped scars under both of his eyes.