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Spinal Deformity

Proximal Kyphosis After Posterior Spinal Fusion in Patients With Idiopathic Scoliosis

Lee, Guy A., MD*; Betz, Randal R., MD; Clements, David H. III, MD; Huss, Gail K., RN

Author Information

Abstract

The goal of surgical treatment of idiopathic scoliosis first started with correction of just the coronal deformity. Now, as more is learned about the three-dimensional orientation of the deformed spine, sagittal alignment and rotation are increasingly important. 1 The coronal deformity is easily observed with standard anteroposterior radiographs, with the postoperative spine showing the head centered over the sacrum. Sagittal alignment can be more difficult to detect. Radiographically, in the sagittal plane, a plumb line dropped from C7 should intersect the posterior aspect of the sacrum or fall behind it. Thoracic kyphosis in the healthy population has been studied extensively and has been shown to be between 20° and 40°. 1,2,7,18

Proximal kyphosis after posterior spinal fusion (PSF) for idiopathic scoliosis has been seen after insertion of Harrington rods and with the newer multisegmented hook/rod systems. Although patients with proximal kyphosis may not have clinical symptoms, it can be cosmetically unappealing. Analyses have been undertaken in patients with postsurgical scoliosis, but most of the data are specifically descriptive of the fusion mass and not the spine above the fusion. 3,6,11,13,16,17

There have been many investigations of the long-term results of PSF for idiopathic scoliosis, 4,14,16 but none has adequately determined the prevalence of proximal kyphosis and its possible causes.

Methods

Patients with adolescent idiopathic scoliosis who had undergone PSF not extending above T3 with good-quality radiographs of the proximal thoracic spine and a minimum 2-year follow-up were studied. Of the 106 patients with idiopathic scoliosis who underwent PSF at our institution between 1990 and 1994, 69 met the inclusion criteria. Thirty-seven patients were excluded because of poor visualization of the vertebrae proximal to C7. Preoperative coronal curves measured between 35° and 75°, and all patients were treated with either CD or TSRH instrumentation. The surgeries were performed by the two senior authors (RRB and DHC) using the same surgical techniques. There were 11 King Type I, 38 King Type II, 15 King Type III, 2 King Type IV, and 3 King Type V curves. Twenty patients had instrumentation proximally to T5, 42 to T4, and 7 to T3.

The proximal instrumented vertebra was selected by first including the proximal coronal vertebra in the Cobb-measured curve. Then the apex of the thoracic kyphosis on the sagittal film was chosen, and instrumentation had to extend at least one level proximal to that apex.

In each patient, the sagittal curves were measured on 14 × 36-in. standing radiographs. Lateral Cobb measurements from T5 to T12, T2 to T12, T12 to L2, T2 to the level of the fusion, junctional kyphosis (defined as one level above the fusion mass), and L1 to L5 were recorded. For example, in spines fused to T5, T4–T5 was the junctional segment. All measurements were made by an orthopedic resident (GAL) on two occasions, with the measurer blinded to the original reading. Risser sign, proximal hook pattern, and levels above the proximal hook level were also recorded.

The patients were then classified into two groups based on the presence or absence of proximal kyphosis at 2-year follow-up. Abnormal kyphosis from T2 to the proximal level of the instrumented fusion was defined as kyphosis of more than 5° above the summed normal angular segments, as reported by Bernhardt and Bridwell 1 (Figure 1).For example, according to Bernhardt and Bridwell, normal proximal kyphosis between T2 and T4 was 6.5°, and in the current study proximal kyphosis was abnormal if it was more than 11.5° (6.5° + 5° = 11.5°). More than 5° kyphosis was chosen to account for measurer error between radiographs.

Figure 1
Figure 1:
Abnormal kyphosis from T2 to the proximal level of the instrumented fusion defined as more than 5° above the summed normal angular segment, reported by Bernhardt and Bridwell. 1

Each group was then analyzed separately, by using the described measurements. Prevalence and predictive values were then obtained. Student’s t test was used for statistical analysis between two groups of continuous data. P < 0.05 was considered statistically significant.

Results

Thirty-seven patients (54%, Group 1) had normal proximal kyphosis, whereas 32 (46%, Group 2) were defined as having abnormal proximal kyphosis above the instrumentation (Table 1; Figures 2 and 3). In Group 1, 10 spines were fused proximally to T5, 24 to T4, and 3 to T3. In Group 2, 10 spines were fused to T5, 18 to T4, and 4 to T3. The values for the preoperative and postoperative sagittal analysis are shown in Table 2.

Table 1
Table 1:
Age, Sex, and Preoperative Coronal Curves for the Two Groups
Figure 2
Figure 2:
A, Thirteen-year-old girl (Risser 0) with preoperative coronal thoracic curve measuring 39° and lumbar curve measuring 42° and a cosmetically unacceptable appearance that was progressing despite use of a brace. B, Sagittal curve T2–T5 measured 2°. C, Radiograph 2 years after surgery shows solid posterior fusion T5–L4, with the coronal curves measuring 15°. D, Sagittal curve (T2–T5) measured 4° (normal). The proximal hook is in distraction on the concave side of the curve.
Figure 3
Figure 3:
A, Thirteen-year-old girl (Risser 4), with preoperative coronal thoracic curve measuring 44° and lumbar curve measuring 48°, who objected to poor cosmetic appearance. B, Sagittal curve T2–T4 measured 9°. C, Radiograph 2 years after surgery shows solid posterior fusion T4–L4, with the coronal curves measuring 25° and 22°. D, Sagittal curve T2–T4 measured 18° (abnormal). This occurred despite the proximal hook in compression on the concave side of the main thoracic curve.
Table 2
Table 2:
Preoperative and 2-Year Postoperative Sagittal Measurements (°)

The most impressive results of this analysis show that the group that progressed to development of proximal kyphosis had more preoperative proximal kyphosis between T2 and the proposed level of proximal fusion in the thoracic spine. In Group 1 the preoperative T2-to-proposed-fusion mass was 2.7°, whereas in Group 2 the value was 10.3°. Postoperative values for Groups 1 and 2 were 5.3° and 21.2°, respectively. In relation to junctional kyphosis (one level), the preoperative values were 1.7° in Group 1 and 6.5° in Group 2, and postoperative values were 2.6° and 12.6°, respectively. All differences were significant (P < 0.0001).

There was also a significant increase in the preoperative and postoperative values for T2 to T12 fusions in Group 2; however, this increase was expected, because the proximal area of the thoracic spine was included in the value. There was a significant increase in the preoperative to postoperative T5 to T12 measurements in Group 2.

The average number of levels fused above the sagittal apex in the normal kyphosis group was 2 ± 1.9, whereas in the kyphosis group it was 2 ± 1.7. Risser sign averaged 2 in both groups.

The preoperative and postoperative values for lumbar lordosis and the postoperative thoracolumbar junction did not show a statistically significant difference between the two groups (Table 3). However, the preoperative thoracolumbar junction showed a slight, statistically significant difference (group 1 vs. group 2) of 6°, but the clinical significance of this 6° was probably not contributory.

Table 3
Table 3:
Preoperative and Postoperative Values for the Thoracolumbar Junction and Lumbar Lordosis

A detailed analysis of all the data was undertaken in an attempt to determine whether development of proximal kyphosis after PSF could be predicted on the basis of original preoperative radiographs. A preoperative curve from T2 to the proposed fusion mass of more than 10° showed a sensitivity and specificity of 65% and 95%, respectively. A sensitivity rate of 100% and a specificity rate of 31% were obtained when using more than 5° kyphosis above the normal T2–proposed fusion mass. Preoperative junctional kyphosis of more than 5° above the proposed proximal instrumented vertebra showed the highest sensitivity rate (78%) and specificity rate (84%).

Discussion

Little is known about the development of proximal kyphosis after PSF for idiopathic scoliosis. In this retrospective radiographic analysis of proximal kyphosis, some of the possible causes of this problem were examined, and the prevalence of the problem was determined. We are not implying that this is a painful condition or that there are any functional limitations in this population of patients. However, it is more than just a radiographic abnormality, because patients mention the deformity. In the current series, it was determined that specific analysis of the proximal aspect of the thoracic spine may provide information that predicts whether proximal kyphosis will develop.

Although there are reports describing procedures to correct sagittal imbalance after PSF, these involve the thoracolumbar or lumbar region. 15 Proximal surgery has not been described for this type of sagittal imbalance above the instrumentation. One patient in the current series was so unhappy with a proximal kyphosis that fusion was extended to T2 (Figures 4, A and B). At the most recent follow-up, she was extremely happy with her appearance.

Figure 4
Figure 4:
A, Postoperative lateral radiograph of a 16-year-old girl with a 55° right thoracic curve. A posterior spinal fusion with TSRH instrumentation was performed from T4 to L4. This sagittal view shows evidence of proximal kyphosis. B,revision postoperative lateral radiograph shows the fusion extended from T1 to the existing fusion mass, correcting the proximal kyphosis.

The distal extent of the fusion mass has been analyzed, and much knowledge has been gained about loss of lumbar lordosis and development of degenerative disc disease after PSF. 2,4,5,10,12 This research has greatly influenced the selection of fusion levels for the lower spine to preserve as many lumbar segments as possible. However, analyses of the proximal spine above the fusion mass are few.

Recently, Hilibrand et al 9 studied the cervical spine after PSF for idiopathic scoliosis. They found that a loss of cervical lordosis was correlated with a loss of thoracic kyphosis to maintain forward vision. In other words, corrections in thoracic sagittal alignment are accompanied by inverse changes in the cervical alignment. Hardacker et al 8 found that changes in cervical lordosis correlated inversely with changes in thoracic alignment. Unfortunately, in the current study, the cervical spine could not be analyzed because there were no cervical spine radiographs. However, it is possible that these patients had a sagittal imbalance in the cervical spine that manifested after PSF with proximal kyphosis.

This information can be extrapolated to the thoracic spine proximal to changes that have been made in the lower thoracic spine. Most patients with idiopathic scoliosis have thoracic hypokyphosis or thoracic lordosis. When this hypokyphosis or lordosis is corrected in the middle and caudal thoracic spine, a small amount of preoperative proximal kyphosis may be accentuated by cervical lordosis to center the head and allow a forward field of vision. Patients in Group 2 in the current series had a significant postoperative increase in kyphosis, and a small amount of preoperative proximal kyphosis may have been accentuated as the spine was corrected.

Intuitively, it could be concluded that proximal kyphosis occurs in response to losses or changes of thoracolumbar sagittal alignment or lumbar lordosis. However, in the current study, review of these measurements showed no significant difference between the normal and abnormal groups in the postoperative values measured from T12 to L2 or in the residual lumbar lordosis measured from L1 to L5. However, there was an increase in thoracolumbar lordosis (T12 to L2) in both groups (normal −5° to −15°, kyphotic 1° to −20°) that probably occurs with compression of the hooks in this region to prevent coronal imbalance. This may be a reason some patients develop the proximal kyphosis, but we could not predict who these patients would be, based on preoperative analysis of T12 to L2.

Determination of the exact reason for the development of proximal kyphosis was not the purpose of the study. The main purpose was to alert physicians that this problem has been seen after posterior spinal fusion with instrumentation and, based on the current data, to suggest a way of preventing it by including all proximal segments that have more than 5° of segmental kyphosis in the instrumented fusion.

Conclusion

According to the definition of proximal kyphosis in this study, 32 (46%) of 69 patients experienced abnormal proximal kyphosis after PSF. Junctional kyphosis of more than 5° above the proposed proximal instrumented vertebra indicates that extending the fusion to a higher level in the thoracic spine would be beneficial in avoiding this problem.

References

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Keywords:

idiopathic scoliosis; posterior spinal fusion; proximal kyphosis

© 1999 Lippincott Williams & Wilkins, Inc.