Wow, ankle joint prosthesis... Unheard of here.
Another factor that could be considered is patient compliance.
As the patient didnot follow up with physio, did he really take good care of his foot? He probably stressed it out un-necessarily, with poor diet and mybe infection control leading to this.
Also, could you tell us, does he have any other medical complications like diabetes? hypertension? is he obese / over weight? that might shed more light.