Stabilization in SFC Surgery on the PDLS Abstract Stabilization in SFC surgery on the PDLS is mainly done adjunctively to address translational instability or rotational hypermobility at the level of planned root decompression. It may also be done as a primary therapeutic maneuver in mechanical (antigravity) chronic axial lumbar pain (CALPag). It is also indicated in most cases of symptomatic adjacent segment disease (ASD) after a previous fusion. The relationship between sagittal imbalance (SIB) and ASD, in short-segment fusion, has not been completely elucidated, though lordotic enhancement techniques should be considered in any patient with SIB needing arthrodesis. In rare cases when stabilization is planned, but with severe SIB, aggressive lumbar lordotic correction may require osteotomy(s). Root decompression at a mid-lumbar level with significant coronal deformity may require stabilization to prevent progressive cephalad deformity. Also, when stenotic root decompression is needed at the convexity of a coronal curve, stabilization with some deformity correction may prevent recurrence. In the setting of coronal lumbar scoliosis, an evaluation for leg-length discrepancy is an imperative. If a coronal curve is convex secondarily to an ipsilateral shorter leg, then the post op stability and clinical effectiveness of correction will be in jeopardy. Keywords: index disc degeneration, junctional kyphosis, adjacent segment disease, sagittal imbalance, coronal deformity, pelvic obliquity We must however acknowledge, as it seems to me, that man with all his noble qualities … still bears in his bodily frame the indelible stamp of his lowly origin. Charles Darwin Presently, the term “stabilization” infers the establishment of a bony arthrodesis (fusion) in the great majority of cases done today. However, nonfusion stabilization techniques exist, and newer ones are being developed, mainly in an attempt to limit the junctional stress issues (discussed later). It is anticipated that there will be further development of these “dynamic” or “motion preservation” stabilization techniques, and that their use will become increasingly more common. 1 The extent and form in the development of lumbar degenerative changes is a complex interactive function between inherent anatomic/physiologic features and subsequent physical stresses. Common pertinent anatomic features include variances in the lumbosacral transition (i.e., transitional vertebrae), shape and position of the sacral platform (i.e., affecting pelvic incidence [PI]), and leg-length discrepancy. Physical factors are the common and unavoidable antigravity stresses rendered to the bipedal upright human, and which are variant depending on body habitus/mass. These anatomic and physical factors impact on inherent metabolic variances in the disposition for degenerative disc change and bone density, which in turn can be affected by behavioral influence (diet, smoking, drug/medications, environmental toxins). The resultant degenerative disc disease of the lumbar spine occurs within an upright individual who must move vertically back and forth from the ground (lying, sitting, bending, squatting) to the upright position. In these antigravity motions, muscular forces act on the vertebral elements, and this muscle action is necessarily asymmetric. Importantly, degeneration/deformity at an index level can lead to reactive degeneration/deformity at other level(s). As there is use of the natural flexibility leverage of the thoracic spine to compensate for any resulting imbalance, secondary degenerative/deformity forces are exerted in segments adjacent to an index level. Thus, a biomechanical change at one lumbar level can affect adjacent levels, creating a domino effect on overall degeneration/deformity of the spine: “Multiple spondylitic and arthropathic segments contribute to a vicious pathogenic cycle, leading to dynamic biomechanical shifts, such as spondylolisthesis and rotatory subluxation, throughout the lumbar spine, and thus a degenerative deformity occurs.” 2 The understanding of these dynamics may be important for the PDLS surgeon in order to plan any limited stabilization surgery. The PDLS surgeon must be facile with various balance and spinopelvic parameter measures in order to incorporate them into operative planning. Sagittal balance, particularly, has relevance to outcomes, 3 although its accurate measurement can be problematic and must take into account compensatory mechanisms (see Chapter 3). Presently, there is limited investigational evidence relating operative outcomes of short-segment stabilization to preoperative sagittal evaluation. The evaluation of this relationship would necessarily focus on the biomechanical stresses on the transitional area and the clinical consequences thereof. One well-designed retrospective study has shown a strong correlation between adjacent segment disease (ASD) and PI–lumbar lordosis (PI–LL) mismatch of >10 degrees. 4 A greater specificity of those patients needing sagittal correction will be established by further investigation as to the concomitant role of global and/or regional imbalance and the size/position of the stabilization construct. The rigidity of the spine incorporated in stabilization techniques (mainly arthrodesis) results in increased stress at the junction where the stabilized spine segment transitions to the nonfixed spine. Physiologic motion and/or gravitational force of the nonfixed portion will induce significant stress at this junction. The “stick-in-the-ground” provides a conceptual analogy. Hence, manual manipulation/bending of such a stick (fixed into the ground) will cause weakening and breakage at or near the junctional (ground) level. The precise level above ground of the induced stress is dependent on the flexibility of the stick, the extent of its fixation, and the site at which it is grasped. Pathologically, postoperative junctional stress has two commonly recognized radiographic descriptions. (However, a spectrum of stress-related morphologic feature exists.) Adjacent Segment Disease (ASD): this refers to progressive and enhanced symptomatic degenerative change. Its special clinical relevance after short-segment stabilization constructs in PDLS surgery is to be determined, especially in regard to the interactive influences of underlying degenerative disease, 5 sagittal balance, 4 and obesity/paraspinous muscle morphology. 6 ASD affects the cephalad adjacent segment predominantly but not exclusively. 7 Symptomatically, it may present with primary axial back pain and/or radicular pain from progressive stenosis at the next motion segment level above the fixed portion. Occasionally, it will be manifested at the second level above the stabilized spine. The potential for ASD is a function of the length of the stabilization construct. 8 Kyphotic deformity and/or rotational instability can also be represented as junctional stress in the shorter stabilization constructs of PDLS surgery as a consequence of gravitational force (as per below). Junctional Kyphosis: this results most frequently after the longer deformity constructs employed in the treatment of adult degenerative scoliosis (ADS). It is a function of the motion of the unfixed spine and of gravitational force. The sagittal rotational junctional stress of gravitational force is a function of the mass on which it is exerted (i.e., essentially the body’s weight above the junction) and of the horizontal distance of this mass from the junction (which is measured from the plumb-line through the gravitational center of the mass = its center of gravity). The term kyphosis refers to a sagittal rotation (one or more levels) which is essentially fixed. Thus, junctional kyphosis is a fixed segmental sagittal rotational deformity at the junctional level. The concept of “balance” relates to the potential for creation of deformity rendered at the junctional region by the gravitational force (as described below); thus, the degree of imbalance is directly proportional to this potential. As noted earlier, SFC surgery is primarily concerned with radicular pain relief by decompressive surgical techniques. There is usually no attention to correction of deformity as a primary therapeutic directive. Some such correction often results from a needed stabilization procedure, but is limited segmentally. With an increasing knowledge of the role of an underlying sagittal imbalance (SIB) as etiological in the creation of pre- and postoperative symptomatic pathology, sagittal deformity evaluation has become more routine in the planning of SFC surgery on the PDLS. 9 This has particular relevance when stabilization instrumentation is otherwise indicated, as SIB can result in pathological stresses at the junctional regions of these constructs. In general, the longer the lumbar construct, the greater the potential pathologic stress rendered by SIB. Thus, sagittal balance evaluation may inform the surgeon as to the type and extent of any segmental stabilization planned. And, as such, it may be directive for a limited sagittal lordotic corrective maneuver at the index site or possibly at additional levels. Although there is early outcome evidence of the need for addressing SIB, greater specificity of indications for lumbar lordotic enhancement in limited segmental stabilization remains to be established. Further clinical investigation will establish the determining interrelationship between balance evaluation technique, extent of SIB, length/type, and position of lumbar stabilization construct. Current recommendations are discussed in this chapter. In patients not requiring stabilization, the specific imperatives for sagittal balance evaluation/correction have not yet been determined, although, in certain cases of multilevel decompression for lumbar stenosis, preoperative SIB evaluation may be important. There is evidence that laminectomy can, to a limited degree, improve sagittal parameters as a consequence of relieving the compensatory mechanism of a reduced lordosis, which provides claudication relief. 10, 11, 12 Those patients with a more significant SIB would not effectively be corrected by surgical decompression; with surgery, these patients may have claudication relief, but postoperatively they can develop persistent and progressive mechanical back pain (type unspecified). The potential for such postoperative pain may be relative to the extent of the surgical exposure, and may be minimized by microvascular decompressive techniques. 13 However, in this subset of patients with lumbar stenosis, the specific indications for sagittal deformity correction need further clarification. The clinical/radiographic scenario(s) directing more aggressive SIB correction in SFC surgery for radicular pain of degenerative lumbar spine are currently not established. Further investigation is needed to determine if there are subgroups in the care of the PDLS that may require correction to the degree afforded by vertebral body osteotomy techniques. It is anticipated (by the author) that the complex multivariance in clinical presentation of the individual patient with the PDLS may prevent specific directives for complex sagittal correction. The advantage of any such sagittal balance gained must be weighed against the increased perioperative morbidity of these techniques within the framework of the individual patient with variant dysfunctional, comorbid, and psychosocial states. Compensation for a measured abnormal sagittal alignment should be always be investigated. A normal PT, representing the absence of compensatory hip retroversion, should alert the physician to the possibility of hip flexion contracture, or of significant extensor muscle pathology. The latter can be either primary (degenerative flat back) or secondary (global decompensation). Also, patients with lumbar stenosis may initially lean forward in an attempt to reduce the lordotic root compression with an uncompensated global imbalance. However, they may eventually recruit compensatory mechanisms of balance compensation (pelvic retroversion, shift) as the desire of upright posture overrides that of neural compression. 12 Therefore, when hip contracture and lumbar stenosis is ruled out in a patient with uncompensated SIB, then the risk for poststabilization failure is significant, usually accountable to extensor muscle functional deficiency. Surgical planning in the correction of sagittal misalignment: In major scoliosis (deformity) surgery, the prediction of proper sagittal alignment correction (as measured by sagittal vertical axis [SVA]) has required mathematical formulation using LL, PI, and TK (thoracic kyphosis); and compensatory mechanisms which are age dependent. The most accurate of these is a complex equation that also uses PT and patient’s age, thus factoring in retroversion compensation and the normal sagittal changes acquired in aging. 15 Although major deformity correction is not within the realm of an SFC surgery on the PDLS, sagittal alignment may be pertinent in the situations when stabilization surgery is contemplated. Thus, in these situations (summarized at the end of this chapter), the surgeon must be able to evaluate the effect of limited sagittal alignment techniques with stabilization. There are several pertinent technical points in this regard. The lower (more caudal) any correction is made, the greater the effect on global balance. This is a function of triangle geometry: for any given angle formed by the equal sides of an isosceles triangle, the length of the third side is dependent on the length of the two equal sides. Any degree correction done at L5–S1 will geometrically yield the greatest change in the SVA or TPA (T1 pelvic angle), etc. However, other factors (e.g., thoracic reciprocal changes) can negate the geometric advantage of lower correction, to some degree. A relatively simple digital angular computation can give a rough assessment of the needed increase in segmental lordosis for global realignment (▶ Fig. 4.1). Stabilization techniques can reduce a compensated sagittal balance with negative consequences (e.g., the reduction at a low lumbar level of a “fish-mouthed” spondylolisthesis, with resulting normalization or parallelization of the end plate divergence, may have a significant negative effect on sagittal alignment). Fig. 4.1 Digital angular measurement for needed segmental sagittal correction.
4.1 Introduction
4.2 The Dynamic Component in the Progression of Degeneration/Deformity from Index Disc Degeneration: The Role of Gravitational Force
4.3 The Issue of Postoperative Junctional Stress: Adjacent Segment Disease and Junctional Kyphosis
4.4 Stabilization with Sagittal Imbalance
4.4.1 Relevance of SIB with Limited Stabilization of SFC Surgery
4.4.2 Relevance of SIB in Patients Not Requiring Stabilization
4.4.3 Relevance of SIB in Regard to the Potential Need for Osteotomy Techniques in SFC Surgery
4.4.4 Uncompensated SIB (Normal Pelvic Tilt [PT]) 14
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