# Novel Design of Expandable Spinal Cage for Efficient Lumbar Spine Fusion Operation
## 1 Introduction  
Lumbar disk herniation (herniated intervertebral disk) occurs when annulus fibrosus is weakened or torn due to various causes such as aging, trauma, poor posture, excessive weight, and genetic factors, leading to protrusion of nucleus pulposus. When protruded nucleus pulposus compresses a nerve root or spinal cord, it can cause pain, paralysis,and sensory abnormalities, and if left untreated, may result in chronic pain or functional disability [1; 2; 3; 4]. Treatment options for lumbar disk herniation range from non-surgical to surgical interventions. However, spinal fusion surgery may be considered if non-surgical treatments are ineffective or if symptoms are severe [5; 6; 7; 8; 9]. The goal of spinal fusion is to remove damaged disk, stabilize adjacent vertebrae to alleviate pain, and restore spinal alignment. A spinal cage is used as a key device to maintain intervertebral space and facilitate fusion process [10; 11; 12]. In the past, spinal fusion surgery was performed through extensive incisions, leading to issues such as soft tissue damage, increased bleeding, higher risk of infection, and prolonged recovery periods. To overcome these drawbacks, minimally invasive techniques were introduced, allowing surgeries to be performed through small incisions, thereby significantly reducing postoperative complications and recovery times [13; 14; 15]. With the advent of these minimally invasive methods, expandable cages, which can be inserted through small incisions and adjusted in height and angle as needed, have gained attention. These cages minimize surgical invasiveness while offering advantage of restoring intervertebral space and lordotic angle to suit unique spinal structure of each patient. As a result, they achieve both reduced recovery times and maximized therapeutic outcomes, further solidifying their importance in field of spinal surgery [16; 17; 18; 19]. The early versions of expandable cages used in these minimally invasive procedures allowed expansion only at the anterior side, enabling adjustment of the angle alone [20]. Subsequently, the devices were improved with the addition of posterior height adjustment, allowing control of both anterior and posterior heights, and thus enabling adjustment of both cage height and angle [21]. However, technical limitations still exist in current expandable cages that require improvement. First, the expansion of height and angle is insufficient. Lumbar vertebrae have an average maximum height of 14.4 mm and a lordotic angle of 12.0 degrees [22; 23]. However, the maximum height and angle of some expandable cages do not reach these values [20; 24]. Second, some expandable cages have a mechanism that makes it difficult to independently adjust height and angle. As a result, adjusting one parameter can inadvertently alter other, making it challenging to achieve desired height and angle during surgery [25; 26]. Lastly, certain expandable cages use mechanical locking mechanisms or stepwise ratchet systems, which hinder fine and continuous adjustments. This means that if the required intervertebral spacing or angle does not precisely align with the pre-set steps, it becomes challenging to accurately adapt to a patient's unique shape of spine [27]. This study proposes a novel expandable spinal cage that enables independent and continuous adjustments of height and angle to overcome limitations of conventional expandable cages, such as difficulty in independently adjusting height and angle and inability to perform fine adjustments due to stepwise mechanisms. Furthermore, it aims to verify mechanical stability of proposed mechanisms and design, demonstrating their practical applicability for clinical use. To achieve this, in this paper we propose a novel mechanism that enables independent and continuous adjustment of expandable cage's height and angle, and we will verify its structural stability through finite element analysis (FEA) and experiments in accordance with the ASTM F2077 standard [28] for measuring the mechanical performance of spinal cages.