Interbody Implant Options in Interbody Fusion





Introduction


Lumbar fusion procedures have become increasingly common for the treatment of various degenerative lumbar spinal conditions. Along with this upsurge is the increased utilization of interbody implants with or without traditional posterolateral instrumentation. The increasing usage of interbody cages is owing to the proposed advantages that interbody fusion offers, including higher fusion rates and improved clinical outcomes. Additionally, interbody grafts significantly decrease the strain of posterior spinal instrumentation during compression or flexion loading, which are the common modes of failure of these constructs.


However, to achieve any theoretic advantages, interbody implants must provide early stability to the spinal segment, while also limiting any impact of the cage on the spine once bony fusion develops. Moreover, there is a balance to how much rigidity an interbody cage should offer. Too much stiffness may lead to stress shielding, increased subsidence, and subsequent revision operations, whereas too little stiffness may lead to biomechanical failure and/or pseudarthrosis. These important characteristics are affected by many cage-specific factors including size, shape, and implant material. Therefore, the goal of this chapter is to provide the reader with information on interbody implant options and describe the optimal implant properties to achieve the best fusion rates and clinical outcomes.




General Principles


To achieve a solid fusion, it is generally accepted that intervertebral motion should be restricted as much as possible. Therefore, the goal of any interbody device is to provide anterior column mechanical rigidity while a bony fusion develops. In addition, the interbody implants serve to directly maintain the increased disk space and neuroforaminal height that is achieved during the surgery, which may relieve compression on the nerve roots. Moreover, many constructs are designed to increase segmental lordosis, which improves overall sagittal balance.


Biomechanically, the most effective means of eliminating motion between two vertebrae is through the disk space rather than through the facet joints, as occurs during posterolateral fusion. As Wolff’s law indicates, fusion potential is enhanced if grafts are placed under compression. Interbody fusions place the bone graft in the load-bearing position of the anterior and middle spinal columns, which support 80% of spinal loads and provide 90% of the osseous surface area, thereby maximally enhancing the potential for fusion. In contrast, posterolateral construct grafts are compressed by 20% of spinal loads and occupy 10% of the osseous surface area. In addition, the interbody space is more vascular than the posterolateral space, increasing chances for fusion.




Implant Subsidence


Subsidence is a normal occurrence during the interbody fusion process owing to the early, normal, osteolytic phase of osteogenesis. Over time, settling of the cage into the vertebral endplates can occur if there is excessive subsidence. If significant subsidence occurs, it may result in loss of anterior column support and segmental lordosis, and loss of the indirect foraminal decompression achieved during surgery. These changes may result in an unfavorable biomechanical environment, which may contribute to the development of pseudarthrosis and possible compression of the neural elements. This is especially evident in flexion of the lumbar spine, as implant subsidence will reduce anterior wedging and decrease construct rigidity in this range of motion.


Subsidence depends, in part, on regional strength of the endplate, vertebral bone quality, cage design, degree of endplate removal during endplate preparation, and the addition of supplemental fixation. Indeed, pedicle screw instrumentation has been shown to decrease the subsidence rate associated with interbody implants. Ideally, the cage should be placed in contact with the apophyseal ring and with the largest surface area possible. This is especially important during transforaminal lumbar interbody fusion (TLIF) and posterior lumbar interbody fusion (PLIF) where only a smaller cage can be inserted, allowing a lower surface area for distribution of force between the implant and vertebral endplates.


Additionally, endplate failure has a linear correlation with decreased bone density. Therefore, osteoporosis is considered to be a relative contraindication to interbody fusion because of the risk of endplate collapse and subsidence. However, attempts to maximize surface area contact are important when interbody implants are placed in osteopenic or osteoporotic patients. This helps to dissipate the axial loading forces over a broader area, and lessens the chance of endplate fracture and subsidence.




Implant Size


Implant size has many implications on the biomechanics of the spinal segment where it is placed. Annular tension is an important component of segmental stability and is primarily influenced by the vertical height of the cage. Oversizing the height of the implant leads to increased annular tension, which may improve the rigidity of the construct. Taller cages have been found to increase stiffness in torsion and lateral bending compared with smaller cages. However, larger cages do not increase stiffness in flexion or extension. This is likely owing to the maintenance of contact with the cage surfaces in taller cage designs compared with shorter ones. However, larger cages may not always allow for optimal placement, such that the largest cage that can be placed suitably in a given segment is warranted.


Cage diameter also plays an important role in segmental stability. A smaller diameter cage applies more direct load to the portion of the endplate where it is placed, such that it is more important to place these cages where the endplate is strongest to decrease the chance of subsidence. This may be owing to the inability of these cages to distribute the load across a larger endplate surface area. The widest cages may be placed only anteriorly through an anterior lumbar interbody fusion (ALIF) approach, whereas narrower cages are all that can be placed during PLIF and TLIF, demonstrating the importance of implant placement in these posterior approaches.




Implant Design


A variety of lumbar interbody implant designs are available from a number of manufacturers. Cages come in various shapes, including circular, cylindrical, tapered, and rectangular, with or without curvature to match the endplate. Some devices have design features such as radiolucency, projections for endplate interdigitation, integrated screws, or spikes. Others are modular so that they can be customized to fit a patient’s unique intervertebral anatomy. In addition, most implants on the market today are created to allow for increased segmental lordosis. A description of common cage designs, along with their inherent benefits and shortcomings, are described below.


Shape of Interbody Cage


The shape of an interbody cage has important biomechanical implications on the intervertebral segment where it is placed. Early cage designs were rectangular in shape and tended to force the vertebral endplates into parallel alignment, thereby limiting segmental lordosis and improvements to sagittal balance. To obtain segmental lordosis with these cages, the posterior bone would be resected or the posterior disk space would be compressed to induce subsidence of the posterior cage. A disadvantage to these designs is that decreased posterior disk space height can lead to foraminal narrowing. Therefore, more modern implants incorporate a tapered design to facilitate insertion, and achieve segmental lordosis while maintaining distraction of the neuroforaminal space.


The interbody cage designs, as described by Bagby, Ray, and Brantigan et al., are examples of early interbody cage designs. The BAK (Spine-Tech, Inc., Minneapolis, MN) cage is a hollow, porous, squared, threaded cylindrical, titanium alloy device. It is similar in design to another cylindrical threaded titanium interbody cage, the RTFC (Surgical Dynamics, Norwalk, CT) cage. Cylindrical threaded fusion cages enjoyed brief popularity as stand-alone PLIF devices. However, high complication rates associated with their use, including segmental loss of lordosis, resulted in their virtual disappearance as a stand-alone posterior spinal implant. Moreover, recent studies have demonstrated that threaded fusion cages have statistically similar construct rigidity to nonthreaded cages, and they also create more stress-shielding compared with nonthreaded cages. In addition, the degree of lordosis is limited by their design. In the modern era of interbody implants, the main improvement to these cages has been the incorporation of a tapered design. The first tapered cage on the market was the LT (Medtronic, Memphis, TN) cage for ALIF surgery.


Alternatives to cylindrical threaded cages include vertical interbody rings or boxes, such as the Harms (DePuy-Acromed, Cleveland, OH) titanium-mesh cage, Brantigan (DePuy-Acromed) carbon fiber cage, and the femoral ring allograft (FRA) (Synthes, Paoli, PA) allograft spacer. With vertical cage designs, the cage shape and endplate coverage significantly affects failure load and construct rigidity. Cage designs that optimize contact with the strongest portions of the endplate are desirable and include cloverleaf designs and large round cages, both of which have peripheral endplate contact. Biomechanical studies have demonstrated that with the same amount of endplate coverage, a cloverleaf shape provides a higher mean failure load compared with a kidney or elliptical shape. In addition, a cloverleaf shape provides higher construct stiffness compared with the other cage designs. Moreover, a cloverleaf design that provides 40% endplate coverage has a higher load to failure compared with a cloverleaf design with only 20% endplate coverage. However, amount of endplate coverage does not affect construct stiffness. Furthermore, cage shape does not affect rotational stiffness, as this is more affected by interdigitation of the endplate by the interbody implant.


Kettler et al. investigated biomechanical differences between various interbody cage shapes. They compared a cuboid titanium cage with two fixation hooks, a bullet-shaped polyetheretherketone (PEEK) cage, and a cylindrical threaded titanium cage. They found that the cuboid and cylindrical cages were stabilizing compared with an intact spine in flexion-extension and lateral bending ranges of motion, whereas the bullet-shaped cage was destabilizing compared with intact. In addition, the authors noted that only the threaded cylindrical cage was stabilizing in axial rotation, as interdigitation of the endplate is the main stabilizer in this range of motion. After 40,000 axial compression cycles, a median subsidence of 0.9 mm was observed in the cuboid implant compared with 1.2 mm in the bullet-shaped implants and 1.4 mm in the threaded cylindrical implant. The initial stability decreased in all cages after cyclical loading. The greatest loss of stability occurred in the threaded cylindrical implant, likely owing to the higher subsidence seen with this cage. Cyclic loading causes nondestructive compression in combination with a penetration of the surface structures of the cages such as the cage threads. Therefore, cyclical loading may have destroyed the threads of the Ray cages, causing the cage to loosen.


Hollow versus Solid Cages


Interbody implants are designed such that the osteogenic potential for fusion comes from graft substrate that is packed into a hollow cage or packed tightly around a solid cage. For porous implants, a larger pore size is desirable as long as the design of the cage provides adequate structural support for the endplate. A solid spacer results in a higher mean maximal load to failure compared with a similarly sized hollow spacer. Therefore, the maximal diameter hollow cage allowable should be utilized to decrease the risk of endplate subsidence while also maximizing fusion potential by allowing more space for bone graft material.


There are options currently available that allow improved “steering” of the implant as it is placed from a posterior approach. This maneuverability is owing to the adjustable cage—inserter articulation that the surgeon can change during placement of the cage into the disk space. For example, during TLIF, this added versatility allows more precise placement to the desire region of the interbody space. Expandable cages provide versatility in optimizing the interference fit of the cage. These can be particularly useful in cases of exaggerated segmental lordosis wherein the posterior disk space is much more narrow than the anterior disk space. Although expandable cages can be helpful, they are also typically more expensive. Furthermore, it is difficult, if not impossible, to fully pack the cage with substrate to completion as it is inserted in the collapsed position and expanded inside the disk space where no further substrate can be added to the cage itself.

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Nov 11, 2019 | Posted by in NEUROSURGERY | Comments Off on Interbody Implant Options in Interbody Fusion

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