The global sagittal balance of the spine is an important aspect for the clinician to consider in the evaluation and treatment of the adolescent scoliotic spine. This balance is the sum of spinal kyphosis and lordosis combined with pelvic alignment and is often assessed radiographically through the measurement of the sagittal vertical axis (SVA), the horizontal displacement of the center of the C7 vertebral body with respect to the posterior superior corner of the sacrum. As scoliosis correction results in changes in both the coronal and sagittal planes, it is important to establish accurate and consistent methods of assessing SVA.
Studies in normal subjects indicate that typical SVA is neutral or slightly positive, i.e. , the C7 vertebra is centered over or slightly anterior to the sacrum. A recent study that evaluated adolescent sagittal balance during relaxed standing with arms at the side and then forward flexed found that relaxed standing SVA was neutral or slightly positive, whereas forward flexion of the arms resulted in a negative shift in SVA.1 2,3
A new radiographic position was evaluated. To keep the humeri from blocking visualization of the spine or creating an additional force for the trunk to counterbalance, patients were positioned with both fists on their ipsilateral clavicles (fists on clavicles position, Figure 1 ). This is in contrast to the more common position with the arms 45° forward flexed and elbows fully extended (traditional position, Figure 2 ). The purpose of this study was to compare sagittal parameters between the traditional and fists on clavicles radiographic positions and evaluate the following: 1) if there were any significant changes in the sagittal profile between the two positions, 2) what the mechanisms for that change were, and 3) if the fists on clavicles position more closely approximated the functional sagittal alignment.
Figure 1: Clinical picture of patient in traditional position.
Figure 2: Clinical picture of patient in fist on clavicle position.
Methods
This retrospective radiographic evaluation study was approved by the author’s institution’s Institutional Review Board. The author began routinely having patients stand for lateral spine radiographs with the “fists on clavicles” position in March 2002. Subsequently, 50 patients with adolescent idiopathic scoliosis (AIS) from the author’s clinical practice were consecutively selected on the following basis: a prior set of posteroanterior and lateral radiographs taken of the patient in the traditional position (arms forward flexed at shoulders to approximately 45° with elbows fully extended, Figure 1 ), and a second set of radiographs in the fists on clavicles position (fists on ipsilateral clavicles with elbows fully flexed, Figure 2 ). For both standing positions, the patient stood on a standard foot template with knees and hips fully extended. In addition, the patient’s scoliosis was stable in the interval between the two sets of radiographs. Patients were excluded if their coronal deformity progressed greater than 10° or if they had any treatment between the two sets of radiographs. Both surgically and nonsurgically treated patients were included.
The lateral radiographs were assessed with measurements of segmental kyphosis and lordosis (T2–T12, T5–T12, T10–L2, and L1 to sacrum), pelvic parameters including pelvic tilt, pelvic incidence and sacral slope (Figure 3 ), and SVA, the horizontal displacement of the center of the C7 vertebra with respect to the posterior superior corner of the sacrum (Figure 4 ). In addition, for the radiographs taken in the traditional position, an arm forward flexion angle was measured by the angle created by a line drawn parallel to the anterior and posterior aspects of the T10 vertebra and a line that bisected the angle created by the two humeri (Figure 5 ).
Figure 3: Pelvic parameters were assessed on the lateral radiographs by first identifying the center of each femoral head and finding a center point between them. The angle formed by a line drawn from this center point to the center of the sacral plate (center-to-center line) and a line drawn perpendicular to the sacral plate yielded the pelvic inclination (PI). Pelvic tilt (PT) was determined by the angle of the center-to-center line and the vertical. The sacral slope (SS) was found by calculating the angle formed by the sacral plate and the horizontal.
Figure 4: Sagittal vertical axis (SVA) was assessed on the lateral radiograph by finding the horizontal distance between the C7 vertebral plumb line and the superior posterior corner of the sacrum.
Figure 5: Arm flexion angle was assessed by finding intersection between a line drawn to bisect the angle created by the two humeri and a line drawn through the T10 vertebra parallel to its anterior and posterior aspects.
Patients were divided into operative and nonoperative groups based on their previous treatment, and for each group a repeated-measures analysis of variance was used to determine significant differences in the various measurements between the two positions (SPSS, Inc., Chicago, IL). In addition, correlations were sought between changes in the different factors from one position to the next. Using a Bonferroni correction for multiple tests, the statistical significance was set at α = 0.005.
Results
There were 42 females and 8 males included in this study, with an average age of 14.7 ± 2.3 years. Of these 50 patients, 25 had undergone surgical correction for their scoliotic deformity, 16 with anterior instrumentation and 9 with posterior instrumentation. Patients with anterior surgery had an average of 6.4 ± 1.0 levels fused, while those with posterior surgery had an average of 11.6 ± 1.8 levels fused. Nine patients were being treated with a brace, although no patients had the evaluated radiographs taken while in their brace. An average of 9 months (range, 2–26 months) passed between taking the two sets of radiographs.
There were several differences in the sagittal profile between the two standing positions, although those differences varied between the operative and nonoperative groups (Tables 1 and 2 ). In both groups of patients, the SVA was more negative in the traditional position than in the fists on clavicles position. In the nonoperative group, SVA in the traditional position was −5.0 ± 2.4 cm compared with −1.8 ± 2.3 cm for the fists on clavicles position (P < 0.001). In addition, in the operative group, the SVA was −3.3 ± 2.3 cm versus −0.8 ± 2.4 cm for the two positions, respectively (P < 0.001).
Table 1: Sagittal Parameters for Patients Without Previous Operative Treatment
Table 2: Sagittal Parameters for Patients With Previous Scoliosis Arthodeses
There was no significant difference in the segmental sagittal measurements for either group. However, in the nonoperative group, there was a trend toward decreased kyphosis through the thoracic spine in the fists on clavicles position. In the nonoperative group, the T2–T12 kyphosis was an average of 5 ± 9° greater in the traditional position than in the fists on clavicles position (P = 0.014). T5–T12 kyphosis in the nonoperative group was an average of 4 ± 8° greater in the traditional position than in the fists on clavicles position (P = 0.013). There was no significant change in thoracolumbar or lumbar lordosis in either group.
As expected, the pelvic incidence was unchanged between positions. In the operative group, there was a relative forward rotation of the pelvis from the traditional stance to the fist on clavicles position. The decrease in pelvic tilt associated with the fists on clavicles position relative to the traditional position (traditional, 13 ± 8° vs. fists on clavicles, 11 ± 8°) was matched by an increase in sacral slope (traditional, 42 ± 9° vs. fists on clavicles, 45 ± 9°). In this operative group, pelvic tilt decreased an average of 3 ± 4° (P = 0.002) and sacral slope increased an average of 3 ± 5° (P = 0.002) when comparing the new position to the traditional position.
There was no significant correlation between changes in SVA and the other segmental measurements or pelvic angles. Although the arm flexion angles in the traditional position ranged from 19° to 91° with an average of 56 ± 14°, there was no correlation between this angle and any of the other sagittal measurements.
Discussion
This study was designed to evaluate the effects of two different lateral radiographic positioning techniques on the sagittal profile in patients with AIS. Standing in the traditional position, with the arms unsupported in forward flexion and the elbows fully extended, created a more negative SVA. When patients stood with their fists on their ipsilateral clavicles, the SVA was shifted forward by an average of 3 cm compared with the traditional position. This shift was very reliable with 92% of patients exhibiting an anterior shift in SVA when changing from the traditional to the fists on clavicles position. The mechanism for this shift seems to depend on whether or not a patient has had spinal instrumentation. In patients with operative intervention for the treatment of their scoliosis, sacral slope was steeper and pelvic tilt decreased in the fists on clavicles position as the pelvis is rotated forward relative to its alignment in the traditional position. However, in the more flexible spines of patients who had not undergone operative intervention, there was no significant change in these pelvic parameters and only a trend toward decreased thoracic kyphosis and increased lumbar lordosis with the fists on clavicles position. Regardless of the mechanism, the fists on clavicles position has a more neutral SVA in all patients and represents a more functional position for radiographic assessments of sagittal profile.
Multiple studies have attempted to define normal parameters for the sagittal profile, some of which have included estimates for normal or balanced SVA.2–9 et al evaluating healthy adult volunteers established normal SVA as being slightly positive (0.7 ± 2.9 cm). In these studies, subjects were positioned for lateral radiographs in a relaxed standing position with their knees fully extended and their arms forward flexed with their hands at or slightly below chest level. To reduce trunk and hip motion, the investigators had the subjects rest their hands on an adjustable bar.8–10 et al examined the sagittal profile of normal adults with subjects standing with arms forward flexed but with no arm support. In this study, the SVA was measured from C7 plumb line to the superior anterior corner of the sacrum and averaged −3.2 ± 3.2 cm.2
Studies of healthy adolescent subjects have suggested that adolescents stand with a more negative SVA compared with healthy adults; that is, their C7 vertebra is posterior to their sacral reference point. Vedantam et al evaluated the lateral radiographs of 88 healthy normal adolescents positioned in relaxed standing with their arms in 60° of forward flexion without a support bar.3 3 et al study2 et al study, had a more posterior SVA accompanied by a flattening of the sacral slope and increased pelvic tilt relative to the sagittal profile of patients in the fists on clavicles position.
Differences in arm positioning were addressed in a later study by the same group, which assessed SVA in surgically fused and nonfused patients with AIS that had their arms 90° forward flexed versus 30°; in both groups, the subjects had their arms resting on a support bar. In this study, SVA was described relative to the superior posterior corner of the sacrum and was significantly different for the two arm angles in the surgically fused group alone, namely, the 90° arm angle was associated with a more negative SVA. The authors hypothesized that this posterior shift could be a compensatory motion to counterbalance the lever arm created by increased shoulder flexion. There were no significant changes in thoracic kyphosis, lumbar lordosis, or sacral inclination (angle created by the vertical and the posterior border of the sacrum) between the two arm positions; however, no other measurements of the pelvic tilt or lower extremity angulation were made to be able to assess further the mechanism of this motion. In their conclusions, the authors noted that the 30° position was a more functional one, but additional studies would need to be conducted to describe the mechanism behind the effect of arm position.11
Marks et al 1 1
The current study is similar to other studies in the literature in that it uses an established radiographic position in which patients are standing with knees and hips fully extended and arms forward flexed. This position is used in many of the studies previously cited,2,3 8–11
This study also assesses sagittal parameters similar to those assessed in other studies. The pelvic parameters of sacral slope, pelvic inclination, and pelvic tilt are reliable indicators of pelvic anatomy described by Duval-Beaupère et al 12,13 8,9
The reliability of the SVA measurement has been questioned by various studies. In two separate studies by Jackson et al , 40 and 30 subjects had two sets of lateral radiographs taken and sagittal parameters measured. In these studies, differences between the SVA calculated from each set of radiographs were analyzed and the correlation coefficient of reliability was calculated as 0.81 and 0.73, respectively.8,9 15 2,6,8
It seems likely that much of the variability of SVA stems from the variability of the patient’s self-selected position. Although in the present study, patients were told by the radiology technician to hold their arms at a 45° angle, the arm flexion angle measurement, ranging from 19° to 91°, reflected a huge variability in how patients actually follow this instruction. Although there was no significant correlation found between arm flexion angle and sagittal parameters, this degree of variability in posture is almost certainly found in every clinical setting and may affect the reliability of many sagittal measurements including SVA. Even when patients have a designated standing base to position the feet and an arm support to standardize their arm flexion angle, this positioning does not standardize the distribution of weight or center of mass, which may affect sagittal parameters. Despite these limitations, we think that SVA, as calculated in this study, is still a fairly accurate assessment of sagittal balance when taken in a consistent position with good radiographic technique.
Defining “normal” sagittal alignment remains a challenge; however, using a position of the body and arms that mimics the alignment of relaxed standing is the first requirement. The fists on clavicles position for lateral thoracolumbar spine radiographs is easily obtained and results in significantly less posterior shift of the trunk compared with more traditional positions in which the arms are flexed forward without support.
Conclusion
The evaluation of the sagittal profile is an important component of the clinical assessment of a spinal disorder patient. However, standard lateral radiographic positioning has patients stand with their arms forward flexed, a pose that is difficult to standardize and causes a negative shift in the sagittal vertical axis . The mechanism for the negative shift with the traditional arms flexed position appears to vary between surgically and nonsurgically treated patients. In the former group, the mechanism seems to lie in the pelvis as it rotates backward, decreasing the sacral slope and increasing pelvic tilt around the hip axis. The mechanism for the posterior shift of the traditional position in nonsurgically treated patients is more obscure but may lie in the flexibility of the spine with increased thoracic kyphosis and increased lumbar lordosis. Despite this variation in mechanism, the fists on clavicles position consistently produces less posterior translation in sagittal vertical axis and a more functional sagittal profile.
Key Points
The traditional standing lateral radiographic position in which the patient’s arms are forward flexed 45° has a more negative sagittal vertical axis than the newly proposed position in which the fists rest on the ipsilateral clavicles.
The mechanism for this posterior shift differs depending on whether the patient has spinal instrumentation.
The newly proposed fists on clavicles position offers a more functional sagittal profile while providing adequate visualization of the spine.
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