Optimizing simple order by query

Hello I am having problems in optimizing a simple order by clause.

the query is:
explain SELECT Entity.title, Entity.description, Entity.user_id, Entity.thumb_url, Entity.entity_uid, Entity.num_fachaks, Entity.number_of_views FROM entities AS Entity ORDER BY Entity.created DESC LIMIT 21 \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: Entity
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 11
Extra: Using filesort

I have index on the created filed by really dont understand that why mysql does not use it and prefers file sort… the created field is a timestamp default current_timestamp.

Please help … i am new to mysql and badly stuck.