The Importance of Alignment
In the domain of spinal surgery, it is useful to recall important concepts that can serve as a foundation to understanding and treating deformity. Optimal alignment of bone structures and joints is critical for the efficient function of the musculoskeletal system. Furthermore, a complex interaction of the neurologic system and muscular recruitment is necessary for ergonomic balance and deliberate displacement of the human body.
Therefore, it is important to consider that ideal spinal alignment allows an individual to assume standing posture with minimal muscular energy expenditure. Physiologic curvatures of the spine in the sagittal plane, the straight spine in the coronal plane, balanced tension of the spinal ligaments, and activation of intrinsic anterior and posterior musculature should permit extended pain-free erect position. This concept is reflected in the “Cone of Economy” principle conceptualized by Jean Dubousset1 (Figure 1). Within the center of the cone, the individual may remain in an ergonomically favorable erect position. However, larger deviations in the anterior-posterior or lateral plane will require greater energy use to maintain a standing position. Finally, progression outside of the “stable cone” results in a loss of postural control and the need for external supports.
In the setting of adult spinal deformity (ASD), structural or iatrogenic modifications to spinal alignment should be considered in perspective of the principles outlined previously. Spinal malalignment in ASD challenges balance mechanisms used for maintenance of an upright posture to achieve the basic human needs of preserving level visual gaze and retaining the head over the pelvis. Progressive severity in skeletal malalignment might result in greater recruitment in muscular effort and greater energy expenditure to maintain the erect posture as well as use of compensatory mechanisms. Spinal malalignment to the extremes of the “Cone of Economy” leads to extreme muscular demand, fatigue, and significant pain as well as disability. Once a spinal deformity has reached the level of marked loss in function and quality of life, surgical intervention is often recommended and requested.
This article aims to outline the basic principles in the evaluation and treatment of ASD patients with a focus on goals to achieve during surgical realignment surgery. The parameters used for radiographic evaluation and a review of the clinical evidence to support their attention are presented. Principles of surgical spinal realignment with consideration to a pragmatic patient-specific approach are offered in the form of case presentations. It will be underlined that while surgical planning may strive for “ideal” alignment, patient factors and external limiters may require compromises and form an ongoing individualized debate on “how much can you tolerate?”
Critical Radiographic Spinopelvic Parameters
Many clinicians have investigated regional and global spinal alignment in the normal (asymptomatic) adult population2–9 (Figure 2). These data have provided a basic understanding of the normative values that these parameters should fall within (Table 1). However, since the work by Vidal and Marnay,10–11 several authors have enhanced the understanding of global alignment by including the pelvis, which has been described as a regulator of sagittal plane alignment. Numerous studies have been conducted to understand the relationship between pelvic parameters and spinal alignment. This has led to the recognition that pelvic morphology and position (Figure 3) are essential components of standing alignment9,12–15 (Table 1); their clinical relevance is briefly summarized hereafter:
- Pelvic tilt (PT) is defined by the angle between the vertical and the line through the midpoint of the sacral plate to femoral head axis, it is commonly reported as a compensatory mechanisms: when the trunk inclines anteriorly (e.g., age related change, sagittal imbalance, loss of lordosis, increase of kyphosis) a subject will try his/her best to maintain an economic posture and keep the spine balanced (i.e., bring the spine over the pelvis). One way to maintain this spinopelvic alignment is to retrovert the pelvis (increase of PT), which might be seen as a backward rotation of the pelvis around the hips.
- Pelvic incidence (PI) is defined as the angle between the perpendicular to the upper sacral endplate at its midpoint and the line connecting this point to the femoral head axis. This is a morphologic parameter of primary importance commonly used to define spinopelvic morphotypes, or the necessary lumbar alignment under optimal conditions.
- Finally, the sacral slope is defined as the angle between the horizontal and the upper sacral endplate, this last parameter completes the geometrical relationship where “Pelvic Incidence = Pelvic Tilt + Sacral Slope.” Normative values and age-related changes of the pelvic parameters are provided in Table 1.
In clinical practice, radiographic reference values help identify regional angulations and linear displacements that can be considered as within the normal alignment range for a given patient. However, because of the large range considered “normal,” regional values alone are insufficient in assessing patient-specific harmonious alignment and the optimal values to strive for in realigning a deformity. It is thus important to consider the idea of spinopelvic harmony, which relates to the proportionality of one given regional parameter to another and in practical terms the global spinopelvic alignment of the individual. In a simplified manner, for a given subject, a ground rule of harmonious alignment consists of a lumbar lordosis (LL) proportional to PI while the thoracic kyphosis (TK) is proportional to the LL (to a lesser extent). By a chain of interconnected parameters,4,9,15 spinopelvic harmony implies that focal and regional alignment is in proportion to pelvic morphology resulting in a sagittal vertical axis (SVA), or T1-spinopelvic inclination (T1-SPI), and PT within a narrow range. When pathology, such as ASD perturbs regional alignment, it leads to a chain of modifications along the standing axis. In severe cases, the consequence is a large SVA/T1-SPI and PT, lost LL resulting in “spinopelvic mismatch” and resultant loss of function and disability.
Clinical Impact of Spinopelvic Alignment on Pain/Disability
Various etiologies are tied to spinopelvic malalignment; however, degenerative and iatrogenic causes comprise the majority of cases. It should be noted, however, that whatever the underlying pathology in a patient suffering from ASD, the correlation between radiographic parameters and self-reported pain and disability (Health Related Quality of Life (HRQL) tools) is now well-defined. Schwab et al,16 in the first series of radiographic analysis, demonstrated the following: endplate obliquity of L3/L4, olisthesis, LL; and thoracolumbar kyphosis were significantly correlated with self-reported pain assessed using the visual analog scale. These correlations were again confirmed in a follow-up study17 of 947 ASD patients using disease-specific validated self reported HRQL instruments including the Oswestry Disability Index (ODI) and Scoliosis Research Society questionnaire. Increasing subluxation and progressive loss in LL resulted in worse outcomes scores and greater self reported pain and disability.
In a review of 352 ASD patients, Glassman et al18 studied the effect of SVA on HRQL measures. Patients with and without previous arthrodesis were included in the analysis. Results demonstrate adverse HRQL scores were significantly correlated with increasing positive SVA. Finally, in a recent report by Lafage et al,19 coronal and sagittal radiographs of 125 ASD patients with and without previous arthrodesis were analyzed for >100 spinal and pelvic parameters. The analysis intended to prioritize the parameters most correlated to HRQL measures (including the pelvis) using the ODI, Scoliosis Research Society questionnaire, and SF-12. Results revealed that T1-SPI, a relatively underutilized global spinopelvic parameter, held the strongest correlation with each outcome measure. SVA, the more accepted measure of global spinal alignment, held the second strongest correlation, whereas PT rounded out the top 3 parameters. Although frequently overlooked, increasing PT, or pelvic retroversion during standing posture is reflective of the patients need to compensate for their proximal spinal deformity (Figure 4). In simple terms, during progressive positive sagittal spinal malalignment, the pelvis rotates backward (retroverts) around the hip joint to offset the effect of sagittal spinal displacement. This pelvic compensatory mechanism attempts to maintain level visual gaze and the center the head over the pelvis in the sagittal plane (center of mass over feet). However, the compensatory reserve of patients might vary widely, with some patients exhibiting large pelvic retroversion reserve and some with very little (possibly related to hip joint stiffness, muscular issues).
The previously referenced studies delineate the radiographic parameters most correlated with self-reported pain and disability in the setting of ASD. Interestingly, the frequently used coronal Cobb angle measure lacks correlation to HRQL measures in the above referenced studies. Malalignment in the sagittal plane results in the disability most consistently expressed in ASD patients. In addition, correlation of PT with HRQOL measures underlines the importance of evaluating pelvic parameters during treatment consideration.
Principles and Pragmatic approach to Spinopelvic Realignment
Spinopelvic realignment is a complex undertaking. Other than decompression and stabilization of spinal segments (which is out of the scope of this report), realignment of the sagittal spinopelvic axis is of primary importance during surgical treatment of symptomatic ASD. As outlined earlier, work in the asymptomatic population might provide some insight into the normal ranges for regional and global alignment during standing posture. However, for optimal clinical outcomes, treatment should be adapted to a given individual on the basis of their respective realignment needs. Tailoring patient-specific treatment involves the crucial PI-LL relationship previous outlined. Additionally, alignment parameters driving pain and disability, namely, SVA/T1-SPI and PT, need to be addressed. The method of realignment, whether pedicle subtraction osteotomy (PSO), Smith-Peterson osteotomy, intervertebral spacer, or rod contouring is of secondary importance to the primary goal of obtaining surgical realignment objectives.
Balance formulas in the setting of standing posture are becoming useful aids for the spine surgeon during complex realignment procedures. However, as a general concept and pragmatic tool for clinical application, spinopelvic realignment objectives involve attention to the following key parameters (Figure 5).
Global spinal realignment should attempt to obtain a postoperative SVA <50 mm. Restoration of SVA facilitates level gaze and achievement of a physiologic standing posture. An SVA <50 mm brings the C7 plumbline behind the femoral heads to relieve the complaint of “falling forward.” Clinically, this threshold has been met with better HRQL scores. Similarly, the reference T1-SPI <0° might be used. Both parameters refer to truncal inclination referenced to the pelvis.
Pelvic realignment should attempt to obtain a postoperative PT <20°. Attention to PT, as outlined by clinical data, is necessary to obtain optimal outcomes. In addition, PT realignment restores appropriate femoral-pelvic-spinal alignment required during efficient ambulation (need an extension reserve to clear the step). Realignment of the SVA <50 mm in the setting of an elevated PT means the ASD patient is still compensating for residual structural spinal deformity. This parameter independently has been shown to correlate to impairment in walking tolerance; therefore, should be realigned appropriately.
LL = PI ± 9°
Finally, to achieve patient-specific alignment treatment LL = PI ± 9° may pragmatically be used. Increasing the angulation of the hypolordotic spine to match the patient's spinopelvic morphotype (i.e., PI) assures appropriate lordotic alignment. This chain of correlation has been extensively studied by many authors.9,12,14,20–21
To identify clinically relevant cutoff values for these parameters, a retrospective study was carried out on 125 adults with spinal deformity (mean age, 57 years). Radiographic parameters were collected and correlations with clinical outcomes (HRQL) were investigated.19 When subdivided these patients according to the cutoffs presented in the “alignment objective section,” significant differences were noted in terms of ODI using an unpaired t test (Table 2). Attention to these 3 pragmatic parameters during surgical intervention sets the stage for achievement of a successful patient-specific spinopelvic realignment in the sagittal plane.
It is evident that the goals of ideal spinopelvic alignment cannot be obtained in all cases. Limitations on the basis of patient factors (e.g., Comorbidities), surgeon factors (experience), operative parameters (e.g., hemodynamic instability, loss of monitoring potentials), and constraints of the healthcare environment (ability to properly care for patients after complex reconstruction) all need heavy consideration in ambitious operative planning.
Principles Established, Clinical Application (Case Studies)
The following section is organized around case presentations and aims to provide an overview on how the principles outlined previously can be applied in clinical practice when planning operative spinal realignment procedures.
Degenerative Deformity: Fusion With Fixed Malalignment
The radiographs in Figure 6A, B are of a patient with degenerative lumbar scoliosis and previous anterior-posterior fusion distally to L5. The major pathology is global with regional malalignment in the lumbar spine. By looking at the preoperative values in Table 3 and the alignment objectives established, one can note that attention should be directed to all 3 key parameters. Regarding spinopelvic alignment, the patient has the unhappy trio of high SVA and PT and low LL. The SVA and PT need to be reduced through realignment while the LL needs to be increased to PI ±9°. Surgical realignment consisted of a PSO at L2 and aggressive segmental osteotomies in the lumbar spine to increase the lordosis. The fusion was extended to the sacrum and pelvic fixation was used for added stability of the long construct. Two-year postoperative images (Figures 6C, D) demonstrate restoration of spinopelvic harmony with achievement of alignment objectives.
Harrington Rod to the Lower Lumbar Spine. PI–LL Mismatch
Radiographs in Figures 7A, B reveal a common post-Harrington instrumentation “flatback” deformity. However, some lordosis remains at the L4–L5 and L5–S intervertebral spaces. A mild LL–PI mismatch is evident. Globally, the patient is shifted forward (Table 4) although without significant pelvic retroversion. The SVA and PI–LL mismatch are the parameters of importance in this case. Surgical realignment included a PSO at L3, with a L5-S TLIF and extension to the pelvis for structural support. Alignment objectives were obtained with this patient with a satisfactory overall global alignment (Figures 7C, D).
Degenerative Disc Disease With Failure of Prior Instrumentation
Radiographs in Figures 8A, B demonstrate a kyphotic lumbar spine, failed instrumentation, high SVA, PT, and moderate coronal malalignment. The goals in realignment need to focus on regional establishment of proper lordosis, which in turn can rebalance the spine and reset the pelvis to normal version. Surgical realignment included a PSO at L4 with a short fusion from L2 to the sacrum. Postoperative SVA and PT were substantially reduced along with an increase in LL (Figures 8C, D, Table 5).
How Much Can You Tolerate?
The data from studies examining the relationship between radiographic parameters and HRQL outcomes demonstrate a considerably strong, linear correlation between HRQL outcomes, and SVA offset18 and PT.19 Because of this relationship, it is difficult to characterize a specific threshold at which point a radiographic parameter can be considered as conclusively tolerable. Consequently, the question of what can be tolerated is rather rhetorical. Ideally, there are normal radiographic ranges and individualized relationships (LL vs. PI) that serve as targets in planning and performing surgeries. Nevertheless, even partial improvement in these parameters is very likely to translate into clinical benefits. Given this, pursuing surgery becomes a question of risk versus benefit.
This risk-benefit balance is an individualized consideration. Patient parameters, such as age, overall health, and bone quality; surgeon parameters, such as skill and experience; and surgical procedure parameters, such as technical difficulty and operative time, all factor into the equation. Studies have attempted to identify specific risk factors, and ASA score, comorbidities, and primary versus revision status have all been reported as risk factors.22 A recent multicenter study has delved into this further, identifying staging and anterior versus posterior approaches as significant risk factors.23
The answer to the question, “how much can you tolerate,” is not a simple one. It is one that must be answered on an individual patient basis, and depends on how much benefit can safely be achieved with a reasonable risk versus benefit balance.
ASD can present with a wide range of clinical symptoms and radiographic findings. Recent work has identified key structural parameters to consider in the evaluation and treatment of ASD. In addition to the clinical affect of spinal and pelvic parameters, recognition of the interrelationship and necessary harmony between values is critical to optimize individualized treatment.
Some of the guiding principles to ASD treatment have been outlined in this review. In addition to SVA or T1-SPI, the PT and PI remain key considerations and permit a framework to a pragmatic approach for the analysis of spinal deformity. Through case examples, the direct application of principles is offered. It should be noted that the complexity of standing alignment and deformity leaves much work to be done. The ongoing HRQOL analyses and longitudinal studies will certainly refine balance formulas and guidelines for the treatment of ASD. In the interim, surgical planning should strive for ideal alignment while being tempered by risk factors and limitations in the patient's healthcare environment. The question of “how much can you tolerate” cannot meet with definitive answers but requires ambition to be balanced with good clinical judgment. Ongoing clinical outcome studies are certain to offer useful algorithms in the near future.
- ASD is a complex interaction between structural deformity and compensatory mechanisms.
- Correlation between quality of life instruments and radiographic parameters highlighted the importance of global sagittal balance and pelvic retroversion.
- Harmony among spinopelvic parameters is of primary importance.
- Realignment objectives should be patient-specific and involve attention to the following 3 parameters:
- SVA less than 5 cm.
- PT less than 25°.
- LL proportional to the PI.
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Keywords:© 2010 Lippincott Williams & Wilkins, Inc.
adult spinal deformity; alignment; pelvic tilt; lumbar lordosis; realignment surgery