I am not able to load models to the extended My_Router class in codeigniter. Below is my code:

class MY_Router extends CI_Router {

    function MY_Router()
    {
        parent::CI_Router();
    }

    function _validate_request($segments)
    {
        // Does the requested controller exist in the root folder?
        if (file_exists(APPPATH.'controllers/'.$segments[0].EXT))
        {
            return $segments;
        }

        // Is the controller in a sub-folder?
        if (is_dir(APPPATH.'controllers/'.$segments[0]))
        {
            // Set the directory and remove it from the segment array
            $this->set_directory($segments[0]);
            $segments = array_slice($segments, 1);

            if (count($segments) > 0)
            {
                // Does the requested controller exist in the sub-folder?
                if ( ! file_exists(APPPATH.'controllers/'.$this->fetch_directory().$segments[0].EXT))
                {
                    show_404($this->fetch_directory().$segments[0]);
                }
            }
            else
            {
                $this->set_class($this->default_controller);
                $this->set_method('index');

                // Does the default controller exist in the sub-folder?
                if ( ! file_exists(APPPATH.'controllers/'.$this->fetch_directory().$this->default_controller.EXT))
                {
                    $this->directory = '';
                    return array();
                }

            }
            return $segments;
        }

        // Let's check if there are category segments
    $category_routes = $this->category_routing($segments);
        if($category_routes !== FALSE)
    {
            return $category_routes;
        }
    $user_routes = $this->user_routing($segments);
    if($user_routes != FALSE)
    {
        return $user_routes;
    }

        show_404($segments[0]);
    }

    function category_routing($segments)
    {
        $this->load->model('category_model');
        if($this->category_model->category_exist($segments[0]))
        {
            //if only category
            if(count($segments)==1)
            {
                return array('category', 'category_browse', $segments[0]);  
            }
            //category pagination
            if(count($segments)==2 and is_numeric($segments[1]))
            {
                return array('category','category_browse', $segments[0], $segments[1]); 
            }
            //category upcoming
            if(count($segments)==2 and $segments[1] == 'upcoming')
            {
                return array('category','upcoming', $segments[0]);  
            }
            //category upcoming pagination
            if(count($segments)==3 and $segments[1] == 'upcoming' and is_numeric($segments[3]))
            {
                return array('category','upcoming', $segments[0], $segments[3]);    
            }
            //category top
            if(count($segments)==3 and $segments[1] == 'top')
            {
                return array('category','top', $segments[0], $segments[2]); 
            }
            //category top pagination
            if(count($segments)==4 and $segments[1] == 'top' and is_numeric($segments[3]))
            {
                return array('category','top', $segments[0], $segments[3]); 
            }
        }
        return FALSE;   
    }

    function user_routing($segments)
    {
        $this->load->model('dx_auth/users', 'user_model');
        if($this->user_model->check_username($segments[0]))
        {
            //only profile
            if(count($segments)==1)
            {
                return array('user','profile',$segments[0]);    
            }
            //all friends
            if(count($segments)==2 and $segment[1]=='allfriends')
            {
                return array('user','allfriends',$segments[0]); 
            }
            //all subscribers
            if(count($segments)==2 and $segment[1]=='allsubscribers')
            {
                return array('user','allsubscribers',$segments[0]); 
            }
            //all subscription
            if(count($segments)==2 and $segment[1]=='allsubscriptions')
            {
                return array('user','allsubscriptions',$segments[0]);   
            }
        }
        return FALSE;
    }
}

I have tried loading the models by using get_instance function provided by codeigniter but seems like it doesnot work. All i need is load the models in extended system library.