This chapter discusses the new technologies being developed in the area of facet arthroplasty. Significant and relevant anatomy of the lumbar spine is reviewed, and the biomechanics of the posterior elements are highlighted. The pathogenesis of facet joint arthrosis, stenosis, and instability is discussed. Finally, three current facet arthroplasty devices are reviewed, including rationale, design, and biomechanics.
Kirkaldy-Willis described the concept of the “three-joint complex”; it consists of the intervertebral disc anteriorly and the paired facet joints posteriorly.
The facet joints function to share a small portion of the load with axial compression, as well as functioning as a restraint to flexion, extension, shear, and rotational forces.
Motion preservation with dynamic stabilization devices and facet arthroplasty have gained interest as an alternative to spinal fusion.
Currently, three systems are being studied for facet arthroplasty:
Total Facet Arthroplasty System (TFAS)
Anatomic Facet Replacement System
Proper patient selection is vital for successful outcome.
Allowance for wide decompression including facetectomy with the implants discussed is achieved.
Implant fixation is with polymethylmethacrylate for the TFAS, although an implant with screw fixation is currently being developed, and with pedicle screws for the other two systems.
The cause of low back pain is multifactorial, and often the cause of the pain generator is elusive. Clearly, some patients have debilitating low back pain that cannot be attributed to the intervertebral disc. In many of these cases, the facet joints are thought to be the culprit. Provocative tests such as discography and facet blocks are often used to confirm the diagnosis. Once it is confirmed that the facets are causing pain, historically, few treatment options are available. Pharmacologic regimens, facet joint blocks, and rhizotomy/denervation procedures constitute the least invasive techniques to decrease facet inflammation and accompanying pain. After failure of these less invasive modalities, posterolateral fusion has historically been the definitive surgical treatment.
In recent years, however, enthusiasm is increasing by both clinicians and industry to find motion-preservation alternatives in the surgical management of facet pain. It is generally accepted that, although fusion is reliable in providing pain relief to a painful and possibly unstable motion segment, it can lead to other problems, the most concerning of which is premature adjacent segment degeneration. Gillet retrospectively analyzed 149 patients treated with posterolateral fusion for degenerative disorders of the lumbar spine with or without instability. In the subgroup of patients with a minimum of 5 years of follow-up, the author found a 41% incidence rate of adjacent-level disease and a 20% incidence rate of reoperation for extension of the fusion. This is most concerning in the younger patient population, in which there is a high probability after posterolateral fusion that further surgery will be necessary.
Much like the evolving technology in total disc replacement, a number of new technologies have been introduced in the area of facet arthroplasty. Extrapolating from the hip and knee arthroplasty experience, innovations are being developed that recognize that the facet joints are diarthrodial, synovial joints affected by similar pathologic processes. Proponents of motion preservation technology feel that pain relief, neural decompression, and instability can be addressed with arthroplasty, whereas maintaining physiologic motion of the affected segment. As with other arthroplasty implants, issues of fixation, articular biomechanics, and constraint need to be addressed. This chapter provides an overview of the anatomy, biomechanics, and pathogenesis of the facet joint degeneration. In addition, current concepts in facet arthroplasty are discussed. Lastly, the future directions in this area of spinal surgery are explored.
ANATOMY AND BIOMECHANICS
The lumbar zygapophyseal joints, or facet joints, are paired diarthrodial joints that are posterolateral to the vertebral bodies and serve to join one posterior vertebral arch to the next. The superior articular process of the caudal vertebra articulates with the inferior articular process of the cephalad vertebra. As with all synovial joints, there is a true synovial membrane, hyaline cartilage at the articular surface, and a fibrous joint capsule. Nociceptors and mechanoreceptors are present on the joint capsule that are innervated by the medial branches of the primary dorsal rami from the same level of the joint, as well as the next cephalad level. It is believed that these capsule receptors play a role in resistance to flexion shear stresses.
Kirkaldy-Willis was the first author to describe the “three-joint complex” of the spine. This consists of the intervertebral disc anteriorly and the paired facet joints posteriorly. In the normal spine, the facets share approximately 10% to 15% of the total axial load in compression. The disc serves as a shock absorber and restraint to axial compression and rotation. The facets and posterior elements provide resistance in flexion, extension, shear, as well as rotation. Lu et al. found in a cadaveric biomechanical model that the posterior column contributed 77.7% of anterior shear stiffness and 79% of the stiffness with posterior shear forces. In another biomechanical study, Lamy et al. demonstrated a facet shear load-carrying capacity of 3000 N through the lumbar facets. Bony failure typically occurred at the pedicle or pars interarticularis. In extension, the articular surfaces of the facet joints come into contact and prevent further loading. With axial rotation as well, the facet articular surfaces eventually come in contact unilaterally. By contrast, in flexion, the intra-articular space increases and the joint capsule and posterior ligamentous structures are stretched, providing resistance to excessive flexion.
Panjabi et al. found that, at L1, the facets were oriented approximately 137 degrees from the sagittal plane of the vertebral body. At L5, the facets were more coronally oriented, approximately 118 degrees from the sagittal plane. Therefore, there is more resistance to shear forces at the lower lumbar levels. Boden et al. have shown that patients with facets at the L4-5 level that were oriented more in the sagittal plane were 25 times more likely to experience development of degenerative spondylolisthesis than those patients with coronal facets. In addition, multilevel variation in facet angles lends support to the hypothesis that the altered facet joint angles are more causal in spondylolisthesis than a secondary result of the process.
PATHOGENESIS OF FACET DEGENERATION
The mainstream hypothesis at this point is, in the majority of instances, intervertebral disc degeneration precedes facet degeneration. In 1990, Butler et al. reviewed computed tomographic and magnetic resonance imaging scans, and showed that disc degeneration without facet osteoarthritis was found at 108 of 144 levels, whereas all but 1 of 41 levels with facet degeneration also had disc degeneration. As the disc decreases in height, the normal tension band effect of the ligamentous and soft-tissue structures is altered, and the intervertebral disc becomes less effective with subsequent transfer of forces to the posterior elements. Over time, these increased forces on the facet joints lead to degeneration with or without instability. Overgrowth of the facets, together with disc degeneration, and osteophytes from the vertebral end plates may eventually cause canal stenosis with symptomatic neural impingement.
This has been questioned recently. Eubanks et al., in a cadaveric study, conclude that facet arthrosis was more prevalent in younger subjects and could actually precede disc degeneration. The disc degeneration was thought to progress more rapidly in later years and overtake the facet changes, thus leading many to believe that degenerative disc disease must precede facet arthrosis. Regardless, it is clear that the functional spinal unit must be viewed as a three-joint complex with each component integrally related.
Although segmental instability can be caused by disc and facet degeneration, it can also be the result of facetectomy and surgical removal of the posterior elements. Hopp and Tsou retrospectively reviewed 344 patients who were treated surgically for lumbar stenosis, and found a reoperation rate of 17% for complications related to obvious or suspected instability. Abumi et al. showed in cadaveric specimens that, although bilateral medial facetectomy did not cause a significant increase in motion, even, unilateral, complete facetectomy did lead to instability. Hopp and Tsou have described the biplanar nature of the facet articular surfaces. The medial half of the joint is oriented more in the coronal plane, and the lateral half is more in the sagittal plane.
Pain can obviously have several generators within the lumbar spine. Aside from stenosis, disc degeneration and instability, and the pain that accompanies these causative factors, the facet joints themselves can be sources of pain. Because facet joints are true synovial joints, an inflammatory cascade can be activated with the degenerative process. In addition, mechanoreceptors within the joint capsule are sensitive to stretch and position.
SURGICAL TREATMENT OPTIONS
Fusion is currently the only other option for the treatment of facet arthrosis. Posterolateral arthrodesis is generally indicated for instability related to either the degenerative process or after a surgical decompression. However, its utility for low back pain related to facet arthropathy without instability is debatable. Recently, attention from both industry and academia has turned to nonfusion surgical alternatives. Currently, the two main areas of interest are dynamic stabilization devices and facet arthroplasty. The rationale behind facet arthroplasty is that facet pain can be eliminated, neural elements can be adequately decompressed, and motion segment stability can be achieved without the need for fusion. Near-normal lumbar spine biomechanics can be maintained and fusion-associated complications avoided. The desire is to maintain available motion or preferably restore normal motion. Other benefits of these devices should include ease of implantation, bone preservation, load sharing between anterior and posterior columns, and ease of revision.
Currently, no facet arthroplasty devices are available on the U.S. market, but three devices are currently in Investigational Device Exemption (IDE) trials and are discussed here: the Total Facet Arthroplasty System (TFAS; Archus Orthopaedics, Redmond, Wash), the Anatomic Facet Replacement System (AFRS; Facet Solutions, Logan, Utah), and the TOPS system (Impliant Inc., Princeton, NJ). All of these implants are designed to maintain motion segment stability by recreating the facet joint biomechanics. A wider and more aggressive decompression can be achieved without the concern for destabilizing the motion segment by replacing the resected facet joints. A complete facetectomy is performed for lateral recess stenosis, instead of undercutting the joint. In addition, better visualization of the neural elements can be gained, theoretically decreasing the chance of nerve root injuries or dural tears. The TFAS, AFRS, and TOPS are all implanted through an open posterior midline approach and utilize fixation within the pedicles.
DEVICES AND TECHNIQUES
Surgical Technique for Total Facet Arthroplasty System
The TFAS is a modular, implantable, semiconstrained metal device that is designed to replace the facet joints and posterior elements after decompressive procedures at either the L3-4 or L4-5 levels ( Figs. 48-1 to 48-3 ). Its indications for use include spinal stenosis, degenerative facet arthrosis with or without instability, and grade I spondylolisthesis. A wider and more aggressive decompression, including complete facetectomy, can be achieved by replacing the facet joints. The size of the implant used is based on preoperative radiographs, as well as intraoperative trials. The articulation consists of a cephalad component that has a spherical metal bearing surface on either side. These fit into matching metal articulating dishes that are anchored to the caudad vertebra. The metal-on-metal constraint of the sphere and dish construct limits coronal translation, extension, and anterior translation through the segment. Because no posterior constraint is present, posterior translation is not limited. It also limits axial rotation, although the ipsilateral cephalad articulating process is not fully constrained in its matching metal dish.