33 Minimally Invasive Spine Surgery
Minimally invasive spine surgery (MISS) is a relatively new and emerging segment of the spine surgery arena. MISS publications are on an exponential rise. This is fundamentally related to the enthusiasm associated with the potential to accomplish surgical missions with less invasion and less risk. Less surgery, however, should not always be equated with “less surgical invasion.” Surgical invasion is related to the extent of soft tissue and bony disruption, as well as the neurologic and mechanical sequelae of surgery. A review and assessment of the aforementioned mechanical sequelae of MISS, hence, are in order. We begin with a review of the fundamentals and follow with an assessment of the stabilization and destabilization effects of MISS. We close with a specific discussion of selected MISS techniques and strategies.
33.1 The Fundamentals
From a biomechanical and anatomical perspective, the intervertebral disc changes substantially from birth to end-stage senescence. It begins as a gelatinous nucleus fibrosus with a confining annulus fibrosus and ends with dessication of the nucleus and degeneration of the annulus into a fibrocartilaginous, scarlike confining structure.
Such a degenerative process is very different from that occurring in the hip and knee. The hip and knee are diarthrodial (synovium-lined) joints, whereas the intervertebral disc is an amphiarthrodial joint. An amphiarthrodial joint is not lined by synovium and does not contain synovial fluid. It degenerates in response to repetitive loading. The “ball-in-socket” joint is a fluid, nearly frictionless joint, whereas an amphiarthrodial joint is a much stiffer joint in a normal in vivo situation. The mechanics of an amphiarthrodial joint are depicted in Fig. 33.1a. As the joint degenerates, the neutral zone widens and the curve shifts to the right. The motion segment becomes less stable and “sloppier” (Fig. 33.1b).
The bending moment is defined by the product of force and distance (moment arm length). A force applied ventral to the spine causes a concentration of stress, such that failure can be initiated and propagated dorsally (Fig. 33.2a). If the force is applied in line with the axis of rotation, no bending moment is applied. Such a situation is exemplified by a pure burst fracture (Fig. 33.2b). In such a scenario, no bending moment is applied, but failure can still occur. However, a greater axial load must be applied in order to cause failure.
The correction of such spinal deformations involves a reversal of the failure-inducing forces applied. An understanding of the biomechanics of spinal column failure (see Chapter 6) is particularly relevant in the MISS arena. It is with this discussion in mind that we consider how MISS techniques can be used to resist or correct the aforementioned mechanical insults to the integrity of the spine. It is also with this discussion in mind that we consider the potential for MISS techniques to cause such insults to the integrity of the spine.
33.2 Spine Stabilization and Destabilization
Surgically induced spine destabilization is related to either the overt disruption of spinal elements or the creation of stressors that affect stability at the same or adjacent motion segments. The overt disruption of spinal elements can result from disc interspace disruption (e.g., via discectomy) or from dorsal element (particularly facet joint) integrity disruption.
33.2.1 Overt Disruption of Spinal Elements
Both discectomy and facet joint integrity disruption can result in spine deformation, most notably spondylolisthesis. The development of degenerative lumbar spondylolisthesis is structurally related to multiple variables. Once noted, it tends to progress in a large number of patients. Matsunaga et al observed progressive slippage in 34% of patients with degenerative spondylolisthesis, but they noted that such slippage did not correlate with symptoms. 1 They also observed that the variables associated with protection from slip were (1) decreased disc space height, (2) spur formation, (3) end plate sclerosis, and (4) ligament ossification. Hence, motion segment degeneration seems to confer some element of stability, at least in its later stages. This is perhaps a manifestation of the Kirkaldy-Willis restabilization phase of spinal aging. Considering the aforementioned, the surgical disruption of a less degenerated motion segment may be associated with a greater chance for slippage.
The relevance of the aforementioned discussion to MISS is related to the importance of considering the mechanics of the motion segment itself. Motion segment integrity is related to the variables noted by Matsunaga et al, as well as the unique anatomy of the stabilizing structures. Resection of the medial portion of the facet joint, for example, significantly affects stability in the lumbar spine. The medial portion of the lumbar facet joint is most critical regarding the listhesis prevention characteristics of the joint. The medial portion of the facet joint functions as a “brake,” if you will. This “brake” interferes with the tendency toward listhesis in the lumbar spine. In the cervical spine, the facet joints are coronally oriented, relatively flat, and shingled. Medial facet disruption, therefore, is not as detrimental to spinal integrity as it is in the lumbar spine. A lumbar facet does not present a flat surface to its opposing facet’s interfacing surface. Lumbar facet joints are in fact curved or even j-shaped, and their orientation changes with level (Fig. 33.3).
Medial lumbar facet joint disruption, which may be caused by the use of a strict dorsal–ventral trajectory to lateral recess and foraminal decompression, is associated with an increased chance for listhesis due to loss of the aforementioned “brake” (Fig. 33.4a). This medial facet joint “brake” represents critical tissue that is “mechanically eloquent.” A lower trajectory angle (Fig. 33.4b) decreases the chance of medical facet joint integrity disruption, with preservation of the “mechanically eloquent” tissue. The selection of a trajectory for the resection of facet joints for foraminal and lateral recess decompression via MISS techniques, therefore, is critical (Fig. 33.4c). One must be keenly aware of the biomechanical principles and anatomical nuances associated with decompression, as well as the relevant clinical trials. 2 The latter are without question affected by bias.
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