This study aimed to assess the biomechanical performance of anti-extrusion interbody fusion devices in lumbar spinal fusion surgery. An innovative aspect of this research involved improving the embedding of interbody fusion devices within the vertebral interspace by introducing blade rotation, thereby enhancing stability between the interbody device and the vertebral endplate. Considering the critical role of both short-term and long-term stability of interbody devices in maintaining disc height, segmental decompression, and facilitating interbody fusion[10], the investigation of these devices' biomechanics holds significant relevance for achieving better interbody fusion outcomes.
Post spinal fusion surgery, interbody fusion device subsidence remains a prominent postoperative complication, occurring at a relatively high rate. The literature reports various factors contributing to interbody device subsidence, including: (1) Contact area between the interbody device and the endplate, with larger contact areas associated with reduced subsidence risk[11]. However, due to anatomical constraints, posteriorly inserted cage sizes are limited[12]. Therefore, this study introduced an intermediate partition within the anti-extrusion interbody fusion device to optimize the contact area and mitigate subsidence risk to some extent. (2) Degree of conformity between the interbody device and the vertebral plate, where tighter conformity correlates with reduced subsidence risk. Prior finite element analyses have demonstrated that custom-designed 3D interbody devices for patients can distribute stress more effectively across the device and endplate surfaces, thus lowering subsidence risk [13–15]. The anti-extrusion interbody fusion device in this study also featured a bullet-headed design to enhance conformity with the endplate, minimizing stress distribution across the endplate surface. (3) Mismatch in elastic moduli between the endplate and interbody device materials, considered a factor leading to bone absorption, stress shielding, and implant loosening in orthopedic surgery[16, 17]. PEEK (Polyetheretherketone) possesses a lower elastic modulus, approximately 4 GPa, closely resembling the bone's elastic modulus (2.5 GPa)[18], and is presently the most commonly used material. Hence, the anti-extrusion interbody fusion device in this study employed PEEK material.
Another significant complication following lumbar spinal fusion surgery is the displacement of interbody fusion devices, as previously described, with numerous associated risk factors reported[19, 20]. Studies[3, 5, 6] have indicated that the morphology of the vertebral interspace can also influence cage migration. Normally, the interspace exhibits a central concavity with peripheral elevation, ensuring robust stability by allowing the four corners of the cage to make excellent contact with the endplate. However, a vertebral interspace characterized by a narrower front and wider rear is more prone to causing posterior migration of the interbody device[21]. In this study, the rotational design of the blade facilitates embedding into the endplate. Biomechanical static pullout testing revealed a significant increase in immediate stability of the interbody fusion device when the blade was rotated out, resulting in a maximum pullout force of 534.02 ± 21.24N, compared to 476.97 ± 24.45N without rotation, marking a 12.18% increment. Furthermore, as the blade's maximum height upon rotation is 1.5mm, and its design resembles a "pointed blade" with a width of merely 0.6mm at its widest point, the cutting torque of the blade stands at a modest 0.548 ± 0.02 N.m, ensuring minimal damage to the bony endplate while facilitating blade rotation. Additionally, certain surgical factors such as excessive residual nucleus pulposus within the interspace, improper positioning of the fusion device, inadequate handling of the cartilaginous endplate, inappropriate choice of fusion device model, or excessive disruption of spinal stabilizing structures all contribute to an increased risk of device displacement[3, 22].
This study conducted mechanical tests to compare the stiffness and settling performance of the enhanced anti-extrusion interbody fusion device with the traditional TLIF interbody fusion device. In static settling tests, the enhanced anti-extrusion interbody fusion device group exhibited higher yield load and stiffness values compared to the traditional TLIF group. In dynamic settling tests, the displacement variation in the enhanced anti-extrusion interbody fusion device group resembled that of the traditional TLIF group, indicating superior overall resistance to settling in the enhanced anti-extrusion group. This could be attributed to the larger contact area between the enhanced anti-extrusion interbody fusion device and the endplate[16]. Previous research has suggested that a larger direct contact area between the interbody device and the endplate not only provides stability but also creates favorable conditions for bone healing. A larger surface contact area can reduce localized stress on the endplate and decrease stress on the bone graft tissue, promoting bone fusion. Conversely, smaller contact areas of the interbody device may exert greater pressure on the device and bone graft tissue, increasing the risk of subsidence [23]. In this study, a bullet-headed and multi-serrated design was employed, offering better conformity with the endplate, thereby enhancing the grip of the interbody fusion device. In static pullout performance tests, the maximum pullout force with the blade rotated out in the enhanced anti-extrusion interbody fusion device group significantly exceeded that without rotation, reaching 534.02N, far surpassing the maximum pullout force of 444.01N for the traditional TLIF interbody fusion device. The primary design, featuring titanium alloy blades capable of embedding 1.5mm into the bony endplate, increased the anti-pullout force to 534.02N after blade rotation, representing a 120% increase compared to the traditional interbody fusion device. This substantial increase enhances immediate stability. Due to the blade design, the rotation operation is simple with a rotation torque of only 0.548N.m, facilitating easy rotation. Moreover, it can be performed through minimally invasive channels, significantly reducing the need for repetitive channel extraction procedures and minimizing associated damage.
The present study has a few limitations. First, it only evaluated the performance of two particular interbody fusion device designs regarding their ability to settle and resist displacement. Even though various new designs have emerged recently, the devices chosen for this study represent the most refined designs discussed in preliminary clinical outcome studies[7, 24]. Second, it didn't account for the impact of the materials used in these interbody devices. Existing literature indicates that the amount of pressure on the endplate largely depends on the type and performance of the materials used in the interbody devices[25]. Therefore, future research should consider materials as a design feature, such as using tantalum metal coatings, for example. Third, this study falls within the realm of preclinical research, so the conclusions cannot be equated with clinical results. Although promising, the settling and anti-extrusion performance of interbody fusion device designs still need validation through long-term follow-up clinical studies. Nonetheless, we believe that preclinical research offers valuable insights into the fundamental biomechanics of interbody device settling and anti-extrusion, which can guide the development of next-generation implants. Fourth, this study used simplified surrogate polyurethane foam blocks to mimic the cage surface, but actual endplates come in various shapes and sizes, which could potentially affect contact biomechanics, settling, and extrusion risks. Future research should consider the complex anatomical structures of vertebral endplates in experimental block designs, incorporating features like endplate recesses [26] and considering surgical factors such as the position of interbody device implantation[27]. Finally, the experimental setup only considered axial loads, one-directional pullout, and polyurethane foam blocks, while the spine has a complex anatomical structure and experiences multi-directional forces. Subsequent research should involve testing on cadaveric specimens under conditions that replicate multi-directional loading to increase clinical relevance.