Restoration of thoracic kyphosis by simultaneous translation on two rods for adolescent idiopathic scoliosis

Do hypokyphotic adolescent idiopathic scoliosis patients treated with Ponte osteotomy obtain a better clinical efficacy? A preliminary retrospective study

Study design

A retrospective case–control study.

Objective

To evaluate whether Ponte osteotomy improves thoracic kyphosis and to determine its clinical efficacy in hypokyphotic adolescent idiopathic scoliosis (AIS).

Methods

Eighty consecutive Lenke type 1 AIS patients with hypokyphotic curves who underwent posterior spinal fusion by one spine surgeon at a single institution were recruited. According to whether Ponte osteotomy was performed, the patients were divided into two groups. The preoperative, immediate, one-year postoperative, and two-year postoperative radiographs were analyzed. The demographic characteristics, surgical information, radiographic parameters, Scoliosis Research Societye-22 (SRS-22) questionnaire, and complications were compared.

Results

The sagittal alignment and coronal alignment were both improved in the Ponte group and the control group postoperatively. There was no significant difference in the preoperative parameters between the two groups, except the TL/L, CB, and LL. Significant differences were found in the MT (15.18° ± 2.84° vs. 20.33° ± 3.75°, P < 0.001) and TK (24.23° ± 2.71° vs. 19.93° ± 2.38°, P < 0.001) at the two-year follow-up. The Ponte group had a longer operation time and more intraoperative blood loss. No significant difference was observed between the groups in the SRS-22 scores at the final follow-up.

Conclusions

Ponte osteotomy could obtain better coronal correction and sagittal contour restoration in AIS patients with hypokyphosis. However, Ponte osteotomies might lead to more intraoperative blood loss and longer operation time. Moreover, no discrepancy was found in the postoperative health-related quality of life of the included patients. Therefore, we considered that the Ponte osteotomy may be an alternative method to restore the desired thoracic kyphosis, which needs further study.

Systematic review and meta-analysis for the impact of rod materials and sizes in the surgical treatment of adolescent idiopathic scoliosis

PURPOSE

To assess surgical and safety outcomes associated with different rod materials and diameters in adolescent idiopathic scoliosis (AIS) surgery.

METHODS

A systematic literature review and meta-analysis evaluated the surgical management of AIS patients using pedicle screw fixation systems (i.e., posterior rods and pedicle screws) with rods of different materials and sizes. Postoperative surgical outcomes (e.g., kyphosis and coronal correction) and complications (i.e., hyper/hypo-lumbar lordosis, proximal junctional kyphosis, revisions, reoperations, and infections) were assessed. Random-effects models (REMs) pooled data for outcomes reported in ≥ 2 studies.

RESULTS

Among 75 studies evaluating AIS surgery using pedicle screw fixation systems, 46 described rod materials and/or diameters. Two studies directly comparing titanium (Ti) and cobalt-chromium (CoCr) rods found that CoCr rods provided significantly better postoperative kyphosis angle correction vs. Ti rods during a shorter follow-up (0-3 months, MD = - 2.98°, 95% CI - 5.79 to - 0.17°, p = 0.04), and longer follow-up (≥ 24 months, MD = - 3.99°, 95% CI - 6.98 to - 1.00, p = 0.009). Surgical infection varied from 2% (95% CI 1.0-3.0%) for 5.5 mm rods to 4% (95% CI 2.0-7.0%) for 6 mm rods. Reoperation rates were lower with 5.5 mm rods 1% (95% CI 0.0-3.0%) vs. 6 mm rods [6% (95% CI 2.0-9.0%); p = 0.04]. Differences in coronal angle, lumbar lordosis, proximal junctional kyphosis, revisions, and infections did not differ significantly (p > 0.05) among rods of different materials or diameters.

CONCLUSION

For AIS, CoCr rods provided better correction of thoracic kyphosis compared to Ti rods. Patients with 5.5 mm rods had fewer reoperations vs. 6.0 and 6.35 mm diameter rods.

LEVEL OF EVIDENCE

III.

Comparison of four correction techniques for posterior spinal fusion in adolescent idiopathic scoliosis

INTRODUCTION

When performing posterior spinal fusion for adolescent idiopathic scoliosis (AIS), it is of major importance to address both coronal and sagittal deformities. Although several techniques have been described, few data exist comparing them. Our objective was to compare four techniques (in situ bending (ISB), rod derotation (RD), cantilever (C) and posteromedial translation (PMT)) for the correction of spinal deformity in AIS including thoracic deformity.

MATERIAL AND METHODS

We conducted a multicenter retrospective study including 562 AIS patients with thoracic deformity with at least 24-month follow-up. Radiographic analysis was performed preoperatively, postoperatively and at last follow-up. The main outcomes were main curve correction and thoracic kyphosis restoration (TK).

RESULTS

Coronal correction rate was significantly different among the four treatment groups (ISB 64% vs C 57% vs RD 55% vs PMT 67%, p < 0.001). Multivariate regression revealed that correction technique did not influence correction rate, whereas implant density, convex side compression and use of derotation connectors did. TK increase was significantly higher in the PMT group (average + 13°) than in DR (+ 3°), while ISB (-3°) and cantilever (-13°) resulted in TK decrease (p < 0.001). Multivariate analysis revealed that TK increase was only influenced by the reduction technique (p < 0.001) and preoperative TK (p < 0.001).

DISCUSSION

The four techniques had the same ability to correct spinal deformity in the coronal plane. Three factors were identified to improve correction rate: implant density, convex compression and use of derotation connectors. On the other hand, PMT was more effective in restoring TK, particularly in hypokyphotic patients.

Hi-PoAD technique for Adolescent Idiopathic Scoliosis in Adult: Personal case series

PURPOSE

The aim of this article is to present an original surgical technique for the treatment of rigid Adult Idiopathic Scoliosis (AdIS) and the results at minimum 2 years follow-up in a cohort of 40 patients.

METHODS

We retrospectively reviewed 40 patients affected by rigid AdIS, older than 40 years and operated with a posterior one stage surgical technique summarized with the acronym Hi-PoAD, (high-density pedicle screws, Ponte osteotomies, asymmetric rods contouring, direct vertebral rotation). The demographic and surgical data were collected, and the improvement of clinical scores and radiologic parameters was obtained after surgery, at 1 and 2 years and at final follow-up, to assess deformity correction, coronal and sagittal balance and clinical outcome.

RESULTS

The average follow-up was 2.9 years (range 2-3.5). Average coronal Cobb angle decreased from 65.0° ± 8.4 to 18.9° ± 3.9 (p < 0.01). Rotation sagittal angle decreased from 26.2° ± 4.4° to 12.4° ± 2.8° (p < 0.01). Mean thoracic kyphosis improved from 23.1° ± 3.6° to 36.0° ± 3.9°. SRS-22 improved form 2.9 ± 0.4 to 3.7 ± 0.6 (p < 0.01). Four early post-operative deep wound infections were observed, all healed after debridement and implant retention. No mechanical complication, junctional kyphosis, deformity progression or non-union were recorded at the last follow-up.

CONCLUSIONS

Hi-PoAD technique proved to be safe and effective in the treatment of rigid Adult Idiopathic Scoliosis. The reason for the success is related to the combined strategies adopted, that dissipates corrective forces over several levels, reducing mechanical stress at the screw-bone interface and optimizing corrective potential.

Simultaneous translation on two rods improves the correction and apex translocation in adolescent patients with hypokyphotic scoliosis

OBJECTIVE

The objectives of this study were to apply the simultaneous translation on two rods (ST2R) maneuver involving rods contoured with a convexity at the desired thoracic kyphosis (TK) apex level and to assess the effects on the ability to support triplanar deformity corrections, including TK apex improvement, in patients with hypokyphotic adolescent idiopathic scoliosis (AIS).

METHODS

Using retrospective analysis, the authors examined the digital records that included 2- to 4-week, 1-year, and 2-year postoperative radiographic follow-up data of female hypokyphotic (TK < 20°) AIS patients (Lenke type 1-3) treated with ST2R. The authors assessed the corrections of triplanar deformities by examining the main Cobb angle, TK, rib hump, apical vertebral rotation, Scoliosis Research Society 22-item questionnaire scores, and TK apex translocation. In order to better grasp the potential of ST2R, the outcomes were compared with those of a historical matched case-control cohort treated with a standard rod rotation (RR) maneuver.

RESULTS

Data were analyzed for 25 AIS patients treated with ST2R and 27 patients treated with RR. The ST2R group had significant improvements in the main Cobb angle and TK, reduction in the rib hump size at each time point, and a final correction rate of 72%. ST2R treatment significantly increased the kyphosis apex by an average of 2.2 levels. The correction rate was higher at each time point in the ST2R group than in the RR group. ST2R engendered favorable TK corrections, although the differences were nonsignificant, at 2 years compared with the RR group (p = 0.056). The TK apex location was significantly improved in the ST2R cohort (p < 0.001). At the 1-month follow-up, hypokyphosis was resolved in 92% of the ST2R cohort compared with 30% of the RR cohort.

CONCLUSIONS

Resolving hypokyphotic AIS remains challenging. The ST2R technique supported significant triplanar corrections, including TK apex translocation and restoration of hypokyphosis in most patients. Comparisons with the RR cohort require caution because of differences in the implant profile. However, ST2R significantly improved the coronal and sagittal corrections. It also allowed for distribution of correctional forces over two rod implants instead of one, which should decrease the risk of screw pullout and rod flattening. It is hoped that the description here of commercially available reducers used with the authors' surgical technique will encourage other clinicians to consider using the ST2R technique.

The effects of thoracoplasty on immediate post-operative recovery in adolescent idiopathic scoliosis

PURPOSE

The purpose of this study was to evaluate differences in pain, opiate utilization and oxygen (O₂) consumption during the immediate post-operative course for patients undergoing posterior spinal fusion (PSF) for adolescent idiopathic scoliosis (AIS) with and without thoracoplasty.

METHODS

Patients aged 10-21 years old that had undergone PSF for AIS were identified from 1/1/16-8/1/18. A cohort of patients that did not have a thoracoplasty group was used as a control group. A standardized pain protocol (rapid response pathway) was implemented on all patients. A retrospective chart review was performed. Pre-operative, operative and post-operative data and SRS scores were analyzed.

RESULTS

Forty-six patients (38 girls, 8 boys) were included, 23 in each group. The average age was 15.2 years (range 12.0-19.3) and BMI was 20.9 (range 13.1-37.6). Differences in visual analog scores, post-operative oxygen and narcotic consumption were not statistically significant on any POD. Total opiate utilization was 146.06 morphine milligram equivalents (MME) in the control group and 149.41 MME in the thoracoplasty group (p = 0.78). One-year SRS self-image scores were higher in the thoracoplasty group (4.24 vs 3.96) (p = 0.007). There was no difference in length of hospitalization (3.6 vs 3.5 days), levels fused (12.9 vs 12.9) or blood loss (334.1 vs 413.5 mL). There was one pleural effusion and no instances of intercostal neuralgia or pneumothorax in the thoracoplasty group.

CONCLUSION

There is no increase in immediate post-operative pain, narcotic use or oxygen consumption when a thoracoplasty is performed with a PSF for AIS. Improved SRS self-image scores were seen after thoracoplasty.

Restoration of Thoracic Kyphosis in Adolescent Idiopathic Scoliosis Over a Twenty-year Period: Are We Getting Better?

STUDY DESIGN

A multicenter, prospectively collected database of 20 years of operatively treated adolescent idiopathic scoliosis (AIS) was utilized to retrospectively examine pre- and postoperative thoracic kyphosis at 2-year follow-up.

OBJECTIVE

To determine if the adoption of advanced three-dimensional correction techniques has led to improved thoracic kyphosis correction in AIS.

SUMMARY OF BACKGROUND DATA

Over the past 20 years, there has been an evolution of operative treatment for AIS, with more emphasis on sagittal and axial planes. Thoracic hypokyphosis was well treated with an anterior approach, but this was not addressed sufficiently in early posterior approaches. We hypothesized that patients with preoperative thoracic hypokyphosis prior to 2000 would have superior thoracic kyphosis restoration, but the learning curve with pedicle screws would reflect initially inferior restoration and eventual improvement.

METHODS

From 1995 to 2015, 1063 patients with preoperative thoracic hypokyphosis (<10°) were identified. A validated formula for assessing three-dimensional sagittal alignment using two-dimensional kyphosis and thoracic Cobb angle was applied. Patients were divided into 1995-2000 (Period 1, primarily anterior), 2001-2009 (Period 2, early thoracic pedicle screws), and 2010-2015 (Period 3, modern posterior) cohorts. Two-way repeated measures analysis of variance and post-hoc Bonferroni corrections were utilized with P < 0.05 considered significant.

RESULTS

Significant differences were demonstrated. Period 1 had excellent restoration of thoracic kyphosis, which worsened in Period 2 and improved to near Period 1 levels during Period 3. Period 3 had superior thoracic kyphosis restoration compared with Period 2.

CONCLUSION

Although the shift from anterior to posterior approaches in AIS was initially associated with worse thoracic kyphosis restoration, this improved with time. The proportion of patients restored to >20° kyphosis with a contemporary posterior approach has steadily improved to that of the era when anterior approaches were more common.

LEVEL OF EVIDENCE

3.

Correction of thoracic adolescent idiopathic scoliosis via a direct convex rod manoeuvre

The various techniques available for scoliosis surgery via the posterior approach involve positioning implants on either side of the curve and reducing the deformity by manoeuvres on the concave rod or simultaneously on both rods. Correction solely via a direct convex rod manoeuvre would eliminate the need for implants on the concave side. This technique was used to treat thoracic adolescent idiopathic scoliosis in 23 patients with a mean age of 14 years and 9 months. Low-dose biplanar EOS radiographs were obtained before surgery, on post-operative day 7, and at last follow-up (at least 2 years after surgery) to allow comparisons of Cobb's angle (72°, 33°, and 35°, respectively), thoracic kyphosis (21°, 29°, and 26°), lumbar lordosis (58°, 50°, and 55°), and apical vertebra rotation (-26°,-12°, and-11°). Although scoliosis requires corrections in all three dimensions, this technique seems to produce satisfactory outcomes while obviating the need for implants on the concave side, thereby decreasing the risk of iatrogenic adverse events. LEVEL OF EVIDENCE: IV.

Choice of Rods in Surgical Treatment of Adolescent Idiopathic Scoliosis: What Are the Clinical Implications of Biomechanical Properties? - A Review of the Literature

The surgical treatment of adolescent idiopathic scoliosis (AIS) involves 3-dimensional curve correction with multisegmental pedicle screws attached to contoured bilateral rods. The substantial corrective forces exert a high level of stress on the rods, and the ability of the rod to withstand these forces without undergoing permanent deformation relies on its biomechanical properties. These properties, in turn, are dependent on the material, diameter, and shape of the rod. The surgical treatment of AIS is characterized by the requirement for a special biomechanical profile that may differ substantially from what is needed for adult deformity surgery. This overview summarizes the current knowledge of rod biomechanics in frequently used rod constructs, with a particular focus on translational research between biomechanical studies and clinical applicability in AIS patients.

Influence of screw density on thoracic kyphosis restoration in hypokyphotic adolescent idiopathic scoliosis

Background

Previous studies have reported that rod composition and diameter, as well as the correction technique are key factors associated with thoracic kyphosis (TK) restoration. However, few study has analyzed the correlation between screw density and TK restoration in hypokyphotic adolescent idiopathic scoliosis (AIS).

Methods

Fifty-seven thoracic AIS patients with preoperative TK < 10° treated with all pedicle screw fixation with a minimum 2-year follow-up were recruited. Preoperative and postoperative radiographic measurements, and information of posterior instrumentation were reviewed. Pearson and Spearman correlation coefficient analysis were used to assess relationships between change in TK and number of variables. Then, the included patients were classified into two groups (Group 1: postoperative TK ≥ 20°; Group 2: postoperative TK < 20°) to evaluate the influence factors of TK restoration.

Results

The average preoperative TK was 4.75°, which was significantly restored to 17.30° (P < 0.001). Significant correlations were found between change in TK and flexibility of major thoracic curve (r = 0.357, P = 0.006), preoperative TK (r = −0.408, P = 0.002), and screw density of concave side (r = 0.306, P = 0.021), respectively. In the subgroup comparison, 17 patients (29.8%) maintain the postoperative TK ≥ 20°, increased flexibility of major thoracic curve (P < 0.001), screw number of concave side (P = 0. 029), and cobalt chromium rods (P = 0.041) were found in the group of postoperative TK ≥ 20°.

Conclusions

TK restoration remains a challenge for AIS patients with hypokyphosis, especially for the poor flexibility ones. Except for thicker and cobalt chromium rods, screw density of concave side might be another positive predictor of restoring normal kyphosis, which provides a stronger corrective force on the sagittal plane with more pedicle screws.

Uniplanar Versus Fixed Pedicle Screws in the Correction of Thoracic Kyphosis in the Treatment of Adolescent Idiopathic Scoliosis (AIS)

BACKGROUND

The aim of surgical treatment of scoliosis is to obtain fusion of the spinal column, balanced in the coronal and sagittal planes. Great success has been attained with coronal correction; however, the sagittal profile has received less attention, resulting in little correction of sagittal plane alignment. The purpose of this study was to compare uniplanar and fixed pedicle screws in the correction of the thoracic sagittal alignment in the treatment of adolescent idiopathic scoliosis (AIS).

METHODS

The sagittal profile of 2 groups of patients undergoing posterior spinal fusion (PSF) for AIS was compared. One group had uniplanar screws (n=16) as bone anchors, and the second group had fixed screws (n=20). Consecutive patients with AIS treated by PSF during 2004 to 2006 with fixed screws; and those treated in 2008 with uniplanar screws were included in the study. Data included: patient demographics, medical conditions, curve type, Risser stage, coronal and sagittal curve magnitude, curve flexibility, osteotomies, fusion levels, type and location of instrumentation, curve magnitude at the initial postoperative visit and at final follow-up. ANOVA and χ testing was completed. A P<0.05 was considered significant.

RESULTS

Both groups were comparable demographically, medically, in relation to the curve characteristics and the surgical treatment. Immediate postcorrection sagittal curvature measurements were greater using the uniplanar than fixed screws, 36 versus 26 degrees and was also statistically significant at final follow-up (P=0.006). There was no difference between groups with respect to coronal thoracic deformity. In the Lenke 1 and Lenke 2 curve population, similar results were found.

CONCLUSIONS

In patients undergoing PSF for AIS, uniplanar screws achieved superior correction of the sagittal thoracic alignment than fixed screws. This advantage was maintained in the postoperative follow-up period. The ability of the uniplanar screws to adapt to variable sagittal orientation appears to contribute to better radiographic outcomes.

LEVEL OF EVIDENCE

This is a level III respective review study conducted at a single center from a single, pediatric spine surgeon.

All-Pedicle Screw Fixation With 6-mm-Diameter Cobalt-Chromium Rods Provides Optimized Sagittal Correction of Adolescent Idiopathic Scoliosis

PURPOSE

Recently introduced cobalt-chromium (CoCr) rods that rely solely on pedicle screws produce very good results in correcting scoliotic curves. All-pedicle screws constructs are also suspected of decreasing thoracic kyphosis. The current study was designed to evaluate sagittal correction in adolescent idiopathic scoliosis patients, using 6-mm CoCr rods and all-screw constructs.

MATERIALS AND METHODS

A total of 61 patients treated by posterior spinal fusion and instrumentation, using all-pedicle screw constructs were included. The mean age at surgery was 15.4 years (range, 12-18 y). Forty-five patients (group A) were diagnosed with decreased thoracic kyphosis, and 16 patients (group B) had normal (35-50 degrees) thoracic kyphosis.

RESULTS

The preoperative main Cobb angle was 62.93±19.38 degrees in group A and 73.45±22.13 degrees in group B. In group A, the postoperative main Cobb angle was 23.33±12.71 degrees. In group B, the postoperative main Cobb angle was 27.20±10.04 degrees. The T4-T12 thoracic kyphosis improved postoperatively from 18.15±10.29 to 28.18±8.35 degrees in group A. In group B, the postoperative T4-T12 thoracic kyphosis was 40.34±3.13 degrees. Statistical analysis showed a significant improvement between preoperative and postoperative values of T4-T12 thoracic kyphosis in group A. In group B, the differences in T4-T12 thoracic kyphosis values were not statistically significant.

CONCLUSIONS

Our result demonstrates a significant improvement of T4-T12 thoracic kyphosis in the hypokyphotic group of patients and confirms that CoCr rods can produce sagittal corrections in hypokyphotic adolescent idiopathic scoliosis patients. Our results confirm the benefit of combining all-pedicle screw constructs with a posterolateral translational in situ bending procedure to correct hypokyphosis directly.

Predicting 3D Thoracic Kyphosis Using Traditional 2D Radiographic Measurements in Adolescent Idiopathic Scoliosis

STUDY DESIGN

Retrospective.

OBJECTIVE

To develop and validate a prediction formula to estimate three-dimensional (3D) T5-T12 kyphosis in adolescent idiopathic scoliosis (AIS) from standard two-dimensional (2D) radiographic measurements.

SUMMARY OF BACKGROUND DATA

2D measurements of thoracic kyphosis in AIS patients overestimate 3D kyphosis; however, there is a lack of widespread availability of 3D imaging technology.

METHODS

Retrospective review was performed for AIS patients with right thoracic curves evaluated with EOS Imaging from January 2010 to June 2014. Standard 2D posteroanterior and lateral radiographic measurements, pelvic incidence, Nash-Moe grade, Perdriolle rotation, and "3D T5-T12" sagittal measures (reconstructed with sterEOS, analyzed with custom MatLab code) were input into a multivariate logistic analysis to create a prediction model for 3D T5-T12 sagittal alignment. An initial cohort of 66 patients (curves 14°-85°) was used to create a predictive model, and a separate cohort of 129 patients (curves 16°-84°) was used to validate the formula.

RESULTS

2D thoracic coronal Cobb and 2D T5-T12 kyphosis were the only significant predictors in the model. The prediction formula for estimating 3D T5-T12 sagittal measurement from standard 2D measurements, in degrees, was 18.1 + (0.81*2D T5-T12 sagittal Cobb) - (0.54*2D coronal Cobb), r² = 0.84. The average model error between predicted and measured 3D T5-T12 kyphosis was ±7°. The predicted 3D T5-T12 kyphosis (8.6° ± 12.1°) and measured 3D T5-T12 kyphosis (8.5° ± 13.0°) were not significantly different (p = .8). 3D kyphosis was less than standard measures of 2D kyphosis (8.5° ± 13.0° vs. 20.2° ± 12.6°, p < .001).

CONCLUSION

This simple validated formula to predict 3D T5-T12 sagittal alignment using routine 2D thoracic Cobb and T5-T12 kyphosis for thoracic AIS patients has great potential value in assessing historical data collected prior to the development of 3D imaging methods as well as understanding/planning surgical hypokyphosis correction in patients without access to 3D imaging.

[Simultaneous thoracoscopically assisted anterior release in prone position and posterior scoliosis correction : What are the limits?]

BACKGROUND

In spite of modern pedicle-based systems, the correction of a rigid rib hump or hypokyphosis remains a problem in posterior-only scoliosis surgery. As there has so far been no reliable method of predicting the intraoperative extent of kyphosis restoration or rib hump correction by posterior-only surgery, it has been difficult to determine the indication for an additional anterior release.

METHODS

The method described here circumvents this dilemma. Like an optional module, horacoscopically assisted release in prone position (TARP) can be added when it is obvious during posterior surgery that the correction is insufficient.

RESULTS

Between 1996 and 2005, a total of 161 patients (115 male, 46 female) under the age of 30, including 113 cases of idiopathic scoliosis, were released by simultaneous TARP and posterior surgery. Using the two-portal technique, 131 were mobilized from the right and 30 from the left hand side. Average surgical time spanned 69 min, in which on average 3.2 apical segments were addressed. In 3 individuals, an additional retroperitoneoscopic release was used to liberate a rigid lumbar curve. After 10 years, in a prospectively evaluated subgroup of 32 patients with adolescent idiopathic scoliosis, the index curve had maintained a coronal correction of 70 % (immediately post-surgery 75 %), kyphosis was permanently normalized at 30° (Th5-Th12), and indirect rib hump was reduced to 2.2 cm. In 23 out of 32 patients the lumbar curve corrected spontaneously, obviating the need for fusion. In 13 patients, the lower instrumented vertebra lay at Th12 or higher, thus leaving the thoraco-lumbar junction fairly free. Minor complications (Huang 1or 2) occurred in 4 patients; 1 patient with hematothorax required revision. A distance <25 mm from the spine to the chest wall precludes TARP. Other limitations (e.g., pleural adhesions) were not encountered.

CONCLUSION

Long-term evaluation after 10-18 years shows that an additional thoracoscopically assisted anterior release at the same time as a posterior standard scoliosis procedure is a justified and effective tool, yielding better results and maintaining them.