Depressed skull fractures are common after impact with a blunt, heavy object - in Petr Cech's case, Stephen Hunt's knee hitting his face, which dented the skull and pushed pieces of bone towards the brain.
Depending on where the fracture is, bone fragments could lacerate or bruise the brain or damage blood vessels around it. There is also a risk of increasing pressure on the brain, which can cause seizures or, in extreme cases, death.
The decision to operate meant the broken pieces of bone ended up more than 5mm deeper than the skull. Moving the bone away from the brain and reassembling the skull is the priority. Surgeons would also have
inspected the brain's outer casing, which carries major blood vessels and helps protect the brain. There is also a risk of added pressure on the brain as a result of swelling around the fracture area - increased pressure can crush soft tissue around the brain and can be fatal.
According to Chelsea, the surgery was successful, but when Cech plays again depends on the injury's severity. A simple fracture and he could return within a month. If there was bleeding around the brain, this could lead to further neurological problems, forcing Cech out for many months. Whatever happens, he will need several months of brain scans before getting a clean bill of health.