function cssklassen_menu_classes($classes, $item) {
    $classes = array_filter(
        $classes,
        create_function( '$class',
                 'return in_array( $class,
                      array( "menu-item", "current-menu-item", "current-menu-parent" ) );' )
        );
    return array_merge(
        $classes,
        (array)get_post_meta( $item->ID, '_menu_item_classes', true )
        );
    }
add_filter('nav_menu_css_class', 'cssklassen_menu_classes', 10, 2);