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.