Erro Mysql

O output(versao sql) de : $this->sql .= "`{$table}` ORDER BY `{$rowOrder}` :direction LIMIT :initiallimit, :finallimit";
Seria: "`topics` ORDER BY `topic_id` DESC LIMIT 0,4";

 Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''DESC' LIMIT 0, 4' at line 1' in /home/yuran/public_html/avec/DatabaseContent.php:34 Stack trace: #0 /home/yuran/public_html/avec/DatabaseContent.php(34): PDOStatement->execute() #1 /home/yuran/public_html/avec/inc/header.php(43): DatabaseContent->fetchSomeRows(Object(PDO), 'topics', 'topic_position', 'DESC', 0, 4) #2 /home/yuran/public_html/avec/index.php(7): require_once('/home/yuran/pub...') #3 {main} thrown in /home/yuran/public_html/avec/DatabaseContent.php on line 34