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Differences in Curve Behavior After Fusion in Adolescent Idiopathic Scoliosis Patients With Open Triradiate Cartilages

Sponseller, Paul D., MD*; Betz, Randal, MD; Newton, Peter O., MD; Lenke, Lawrence G., MD§; Lowe, Tom, MD; Crawford, Alvin, MD; Sucato, Daniel, MD**; Lonner, Barry, MD††; Marks, Michelle, PT, MA; Bastrom, Tracey, MA The Harms Study Group

doi: 10.1097/BRS.0b013e31819139ef
Outcomes
Free

Study Design. Retrospective review.

Objective. To compare the results of spinal fusion in patients with open triradiate cartilages (OTRC) and closed triradiate cartilages (CTRC).

Summary of Background Data. Patients with OTRC at the time of spinal fusion may be at increased risk of developing postoperative changes related to growth.

Methods. From a database of patients with adolescent idiopathic scoliosis, we identified 44 patients with OTRC (mean age, 11.6 years) and 450 patients with CTRC (mean age, 15.6 years) and a minimum follow-up of 2 years. Patients in both groups were treated with anterior-only, posterior-only, or combined anterior and posterior spinal fusion; none had all-pedicle screw posterior instrumentation.

Results. In the OTRC group, anterior or posterior instrumentation, but not the combined approach, resulted in a significant mean late increase in the main curve (4.4° and 7.3° vs. 0°, respectively; P = 0.002), an approach-related difference not seen in the CTRC group. Significantly more OTRC patients had proximal levels added on after surgery than did CTRC patients (18% vs. 8%, respectively; P = 0.02), and there was a trend toward this phenomenon distally (29% vs. 19%, respectively; P = 0.10). Proximal and distal junctional kyphosis was not significantly different between the 2 groups. Reoperation rate was 11% and 7% for OTRC and CTRC patients, respectively. For the selectively fused Lenke 1C curves in OTRC and CTRC patients, there was a trend in the uninstrumented lumbar curve toward a smaller lumbar curve before surgery (36° and 41°, respectively; P = 0.07) and a larger curve after surgery (27° and 24°, respectively; P = 0.07).

Conclusion. Patients with scoliosis and OTRC have a greater risk of adding-on proximally and of loss of correction with anterior-only instrumentation; they may also have less predictable lumbar correction from selective thoracic fusion. However, after combined surgery, they have results similar to those of more skeletally mature patients.

We retrospectively compared the results of spinal fusion in pediatric patients with adolescent idiopathic scoliosis and OTRC (44) or CTRC (450). OTRC patients have a greater risk of having levels added on proximally and of loss of correction with anterior-only instrumentation. They may also have less predictable lumbar correction from selective thoracic fusion.

From the *Department of Orthopaedic Surgery, The Johns Hopkins University, Baltimore, MD; †Shriner’s Hospital, Philadelphia, PA; ‡University of California, San Diego, CA; §Washington University, St. Louis, MO; ¶Woodridge Orthopaedics, Wheat Ridge, CO; ∥Division of Pediatric Orthopaedics, Cincinnati Children’s Hospital Medical Center, Cincinnati, OH; **Texas Scottish Rite Hospital, Dallas, TX; and ††Scoliosis Associates, New York, NY.

Acknowledgment date: October 9, 2007. Revision date: November 29, 2007. Acceptance date: December 3, 2007.

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

Corporate/Industry funds were received in support of this work. Although one or more of the authors(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this manuscript, benefits will be directed solely to a research fund, foundation, educational institution, or other nonprofit organization which the author(s) has/have been associated.

This project was supported by DePuy Spine, Inc.

Address correspondence and reprint requests to Paul D. Sponseller, MD, 601 North Caroline St #5212, Baltimore, MD 21287; E-mail: psponse@jhmi.edu

Patients with idiopathic scoliosis may pose different challenges at different ages. It is known that growth causes changes in spinal alignment in all planes. The peak height velocity has been correlated with a period before closure of the triradiate cartilages (TRC) of the acetabulae.1,2 This also correlates with the peak in curve progression.

Dubousset et al3 introduced the concept of crankshaft phenomenon—progression of a curve after fusion due to continued growth. Roberto et al4 found that Cobb angle increases of 10° or more occurred after fusion in 54% of patients with open triradiate cartilages and Tanner stage 1. They also found that curve progression greater than 10° occurred in 37% of patients fused before triradiate cartilage closure and none after.4 Lapinsky and Richards5 found that circumferential fusion produces a lower incidence of crankshaft in patients who were Risser 0 with open triradiate cartilages but did not eliminate it entirely.

Since scoliosis produces deformity that often extends beyond the major curve, and since fusion alters the biomechanics of the growing spine, we hypothesized that there may be consequences for both the fused and the unfused areas of the spine in patients undergoing fusion before or during the peak growth period. We wanted to examine the effect of growth remaining on behavior not only of the main, but also of the proximal and distal curves, in both the coronal and sagittal plane. Our main outcome variables were:

  1. Significant increase in fused curve, through adding on or crankshaft
  2. Late progression of an unfused lumbar curve
  3. Proximal or distal junctional kyphosis

Any such changes could have implications for surgical strategy. Expressing the benefits and challenges of operating on a less mature spine, Charles et al6 speculated that “an earlier intervention may be preferable to obtain a more supple spine, even if a more extensive intervention is necessary”; however, he stressed that “if an early spine fusion leads to better curve correction needs to be verified….”

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Materials and Methods

A 10-year prospectively collected multicenter registry of patients having idiopathic scoliosis was used for this study. Since peak height velocity measurements were not available for these patients, we chose the open triradiate cartilage as the most appropriate, available indicator. After reverification by the primary author of open triradiate cartilage status, 44 patients with open TRC (OTRC) were identified from the database of idiopathic scoliosis patients. They had a mean age of 11.6 years at surgery. Patients who had a Risser stage of 1 or more, or who had “closing” TRC were excluded from this group. “Closing” triradiate cartilages were defined as having a width of less than 1 mm cartilage space. They were compared with a cohort of 450 patients with closed TRC (CTRC) and a Risser grade of 3 or more from the database. This comparison group had a mean age of 15.6 years, and all were 18 or younger, at the time of surgery. The CTRC group was selected to be similar in curve types to the OTRC group (P = 0.13) (Table 1). The groups were also similar in their preoperative major Cobb angle (mean, 55.2° ± 12° vs. 52° ± 11°; P = 0.21). Minimum follow-up was 2 years in both groups (mean, 3.09 vs. 3.06 years). Surgical treatment was at the discretion of the operating surgeon. OTRC patients underwent 25 anterior spinal fusions with instrumentation (ASF) alone (either open or thoracoscopic), 3 posterior spinal fusions with instrumentation (PSF) alone, and 16 ASF/PSF. None of these patients had an all-pedicle-screw posterior construct. CTRC patients underwent 233 ASF alone (either open or thoracoscopic), 190 PSF alone, and 27 ASF/PSF. Clinical and radiographic comparisons were performed before surgery and at specified postoperative intervals. All patients had preoperative and postoperative clinical examination with a scoliometer to quantify rib prominence and range of motion. Adding on was defined as an increase in the number of vertebrae in the measured curve either proximally or distally combined with a curve increase of more than 6° from the first postoperative radiograph.

Table 1

Table 1

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Statistical Methods

Continuous variables were evaluated for normality of distributions and equality of variances. RM analysis of variance was used to compare change in Cobb angles across time points. χ2 analysis was used to evaluate the proportion of patients with additional levels added to the curve after surgery between the OTRC and CTRC groups.

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Results

Characteristics of the OTRC and CTRC group, such as age, surgical approach, and curve magnitudes can be seen in Table 2. The main curve in the group with OTRC increased significantly more from the first postoperative radiograph to follow-up when instrumented anteriorly or posteriorly than when instrumented with combined approaches (4.4° and 7.3° vs. 0°, P = 0.002) (Figures 1–4). Twenty-eight percent of the anteriors and 33% of the posteriors increased greater than 10° from first postoperative to 2-year radiographs. None of the patients in the combined approach group increased greater than 10° (P = 0.05). This difference was not seen in the CTRC patients (3.6° anterior, 4.2° posterior, and 2.6° combined approaches, P = 0.4). Fifteen percent of the anteriors, 16% of the posteriors, and 4% of the combined approaches increased greater than 10° between first postoperative and 2-year radiographs (P = 0.22).

Table 2

Table 2

Figure 1

Figure 1

Figure 2

Figure 2

Figure 3

Figure 3

Figure 4

Figure 4

More OTRC patients had proximal levels added on to the curve after surgery than CTRC patients (18% vs. 8%, P = 0.02) and there was a trend toward this phenomenon distally (29% vs. 19%, P = 0.10) (Figure 5). However, the average proximal junctional kyphosis in degrees did not show a significantly different change over time between OTRC (3.6° preoperative to 5.2° at 2 years) and CTRC patients (4.1° preoperative to 6.5° at 2 years, P = 0.34). There was also no significant difference in proximal junctional kyphosis between OTRC and CTRC at any given time point (P > 0.05). Similarly, distal junctional kyphosis did not show a significantly different change over time between the 2 groups (OTRC, 1.9° decrease in lordosis; CTRC, 2.2° decrease in lordosis, P = 0.64). The groups also did not differ in distal junctional kyphosis at any particular time point (P > 0.05). Within the OTRC group only, a trend emerged to show posterior only patients having the least average proximal junctional kyphosis at 2 years compared with anterior only and combined approaches (P = 0.07, Figure 6).

Figure 5

Figure 5

Figure 6

Figure 6

The reoperation rate was 11% (5/44) for OTRC patients and 7% for those with CTRC (P = 0.36). Indications for reoperation in OTRC were progression of curve in all cases; 2 also having screw pullout and 1 a rod fracture (Table 3). Four of 5 (80%) of the OTRC patients requiring reoperation were patients who had an anterior-only approach (Figure 7). The fifth patient had a combined procedure initially. Within the 32 CTRC patients requiring reoperation, 13 (41%) had an initial anterior approach, 3 (9%) had a combined approach, and 16 (50%) had undergone a posterior approach at index operation.

Table 3

Table 3

Figure 7

Figure 7

Subanalysis of the selectively fused Lenke 1C curves revealed a trend in the change from preoperative to 2-year follow-up of the uninstrumented lumbar curve with OTRC patients exhibiting a smaller lumbar curve before surgery and larger curve after surgery than CTRC patients (36°–27° vs. 41°–24°, P = 0.07) (Figure 8).

Figure 8

Figure 8

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Discussion

Many different parameters have been analyzed in the study of the immature patient after fusion for adolescent idiopathic scoliosis. Roberto et al4 suggested that the triradiate cartilage is not “the answer” to the prediction of crankshaft risk, but their series had only 4 patients with idiopathic scoliosis and open triradiate cartilages. In addition, they excluded patients who displayed “adding on” after posterior fusion, which we feel may be one of the phenomena more frequent in early fusions. Burton et al7 suggested that anterior fusion is not needed to control crankshaft phenomenon; however, two-third of his patients had closed triradiate cartilages. Lapinsky and Richards5 found that in 14 patients who had had surgery with open triradiate cartilages and were Risser 0, even after circumferential fusion, 2 had progression of more than 10° and 3 had increase in RVAD greater than 10°, although none had both of these changes simultaneously. Sagittal crankshaft development was illustrated by D’Andrea et al,8 who showed that in a group of 10 patients who had anterior spinal fusion for scoliosis, 6 had increases in T5–T12 kyphosis of over 10° (mean, 15°), which was twice the incidence seen in controls. Shufflebarger and Clark9 found that periapical arthrodesis prevented crankshaft in patients who were Risser 0 at surgery. Lee and Nachemson10 found that, in developing a multivariate predictive model, age less than 11 years was a predictor of possible crankshaft.

Surgical fusion of immature patients provides opportunity for greater correction, but we hypothesized that in some cases it may cause biomechanical changes that lead to secondary deformities. We wondered whether spontaneous lumbar curve correction would routinely “stay corrected” in OTRC patients who undergo selective thoracic fusion? Or does the lumbar curve progress as it would have in absence of surgery? What is the incidence of “adding-on” in the coronal or sagittal planes in the more immature patients compared with the older ones?

This study sought to answer these questions with the benefit of a large group of patients with OTRC and a very large matched cohort of patients with CTRC. In absence of unoperated OTRC controls, we attempted to gain insight from 2 analyses: first, determining what problems complicated the results in OTRC patients and second, finding out how did more mature patients of same degree fare in maintaining correction. This is the largest series of patients with open triradiate cartilages fused for adolescent idiopathic scoliosis.

Our findings indicated that OTRC patients fused from one approach (especially anteriorly) had greater loss of correction of the major curve than those who were fused circumferentially. In addition, they were more likely to have adding-on and the patients with OTRC and Lenke C curves had a suggestion of higher loss of spontaneous lumbar curve correction than the more mature group.

There were some limitations of the study. Maturity indicators are not foolproof.1,2,11–14 It has been suggested that many patients who undergo scoliosis surgery with open triradiate cartilages may have passed their peak height velocity.1 Other indicators that a patient has not yet undergone the peak growth velocity include Tanner 1 stage and uncapped phalangeal epiphyses.11 Sanders et al12 and Sanders13 found that the r value for predicting the curve acceleration phase was 0.93 for digital skeletal age, 0.82 for Tanner stage, 0.78 for triradiate cartilage, and only 0.60 for Risser stage. The r value for the triradiate cartilage status was almost as good as measurement of the peak height velocity itself (r, 0.89). The mean age at the curve acceleration phase in their study was 11.7 years, which is slightly older than the mean in our series.

One of the drawbacks in this study was the large number of different curve patterns, which may obscure findings specific to a certain pattern. Two-thirds of the patients in the OTRC group had a Lenke 1 curve pattern. Also, there were 3 different surgical treatment groups, and the instrumentation used posteriorly did not include large number of pedicle screws as would be commonly used today. In addition, the measurement of rotation (scoliometer) is not as precise as other methods; the rib-vertebral angle difference or Perdriolle method may be other measures.7

Nevertheless, the study did highlight some important differences in behavior of the spine when fused before triradiate cartilage closure. Adolescent idiopathic scoliosis patients with OTRC have greater risk of adding on levels proximally, as well as loss of correction with anterior-only instrumentation. Patients with Lenke C curves and OTRC fused selectively showed greater loss of the spontaneous lumbar curve correction than more mature patients. In some cases, this may become clinically significant. Surgeons should exercise caution in employing selective thoracic or anterior-only fusion in patients with open triradiate cartilages. Although we may be able to control crankshaft by circumferential fusion (or pedicle screws), junctional deformities and loss of spontaneous lumbar curve correction may still be a risk in these patients.

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Key Points

  • Patients with scoliosis fused with open triradiate cartilages had more adding-on proximally than those with closed triradiate cartilages.
  • They had just as much loss of correction with anterior-only instrumentation as posterior-only instrumentation.
  • Patients with open triradiate cartilages have less predictable lumbar correction after selective thoracic fusion.
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References

1. Sanders JO, Little DG, Richards BS. Prediction of the crankshaft phenomenon by peak height velocity. Spine 1997;22:1352–6.
2. Little DG, Song KM, Katz D, et al. Relationship of peak height velocity to other maturity indicators in idiopathic scoliosis in girls. J Bone Joint Surg Am 2000;82:685–93.
3. Dubousset J, Herring JA, Shufflebarger H. The crankshaft phenomenon. J Pediatr Orthop 1989;9:541–50.
4. Roberto RF, Lonstein JE, Winter RB, et al. Curve progression in Risser stage 0 or 1 patients after posterior spinal fusion for idiopathic scoliosis. J Pediatr Orthop 1997;17:718–25.
5. Lapinsky AS, Richards BS. Preventing the crankshaft phenomenon by combining anterior fusion with posterior instrumentation. Does it work? Spine 1995;20:1392–8.
6. Charles YP, Daures JP, de Rosa V, et al. Progression risk of idiopathic juvenile scoliosis during pubertal growth. Spine 2006;31:1933–42.
7. Burton DC, Asher MA, Lai SM. Scoliosis correction maintenance in skeletally immature patients with idiopathic scoliosis. Is anterior fusion really necessary? Spine 2000;25:61–8.
8. D’Andrea LP, Betz RR, Lenke LG, et al. The effect of continued posterior spinal growth on sagittal contour in patients treated by anterior instrumentation for idiopathic scoliosis. Spine 2000;25:813–7.
9. Shufflebarger HL, Clark CE. Prevention of the crankshaft phenomenon. Spine 1991;16:S409–11.
10. Lee CS, Nachemson AL. The crankshaft phenomenon after posterior Harrington fusion in skeletally immature patients with thoracic or thoracolumbar idiopathic scoliosis followed to maturity. Spine 1997;22:58–67.
11. Sanders JO, Browne RH, Cooney TE, et al. Correlates of the peak height velocity in girls with idiopathic scoliosis. Spine 2006;31:2289–95.
12. Sanders JO, Browne RH, McConnell SJ, et al. Maturity assessment and curve progression in girls with idiopathic scoliosis. J Bone Joint Surg Am 2007;89:64–73.
13. Sanders JO. Maturity indicators in spinal deformity. J Bone Joint Surg Am 2007;89:14–20.
14. Song KM, Little DG. Peak height velocity as a maturity indicator for males with idiopathic scoliosis. J Pediatr Orthop 2000;20:286–8.
Keywords:

scoliosis; triradiate cartilage; fusion

© 2009 Lippincott Williams & Wilkins, Inc.