Effects of posterior distraction forces on anterior column intradiscal pressure in the dual growing rod technique.

BACKGROUND: Little evidence is available addressing biomechanical properties of posterior distraction forces and their effects on anterior spinal column in the growing rod technique. The question is often asked if posterior distraction forces may be kyphogenic. The goal of this study is to determine whether posterior distraction forces transmitted anteriorly through different foundation constructs (i.e., screws vs. hooks) affect intradiscal pressure. METHODS: Six skeletally immature porcine spines were harvested leaving soft tissues and rib heads intact. Pedicle screws served as the lower foundation on a L3-L4 motion segment while pedicle screws and laminar hooks were randomly used at T3-T4 levels. Proximal constructs (hook vs. screw) were switched after initial distraction testing. The dual rod distractor was instrumented with strain gauges and calibrated using a custom force transducer. During distraction, intradiscal pressures immediately inferior to the superior foundation and the level equidistant between foundations were measured using needle pressure transducers. Maximum distraction force and maximum anterior disc pressure change were compared between hook and pedicle screw anchors using one-way ANOVA (p < 0.05). RESULTS: Upper foundations with pedicle screws had significantly greater distraction forces (416 ± 101 N) than those with upper level hooks (349 ± 100 N). There were no significant differences in disc pressures between levels or between upper foundation constructs. Disc pressures adjacent to the upper foundation demonstrated greater reduction (disc expansion) than the level equidistant within the construct. Pedicle screw constructs demonstrated greater endplate separation (distraction) compared to hook constructs. CONCLUSIONS: Posterior distraction forces result in anterior disc separation (distraction) and are distributed across multiple levels rather than delivered to the disc immediately adjacent to a foundation. Constructs with upper foundation hooks had lower distraction forces possibly due to hook motion during distraction. The load distribution at multiple levels may assist with curve control and may affect vertebral growth. The distraction forces may not be kyphogenic as is commonly believed.

Safety and efficacy of growing rod technique for pediatric congenital spinal deformities.

BACKGROUND: Growing rod surgery is a modern alternative treatment for young children with early onset scoliosis. This is the first study focused on its use in progressive congenital spinal deformities. METHODS: A retrospective study of 19 patients from the international multicenter Growing Spine Study Group with progressive congenital spinal deformities undergoing growing rod surgery who had a minimum of 2 years follow-up. We analyzed demographic and radiographic data including age at initial surgery, number of abnormal vertebrae per patient, number of lengthenings postoperatively, Cobb angle of the major curve preoperative, postoperative initial and at last follow-up, T1-S1 length, space available for the lung (SAL), length of follow up, and complications. RESULTS: The mean age at surgery was 6.9 years (range: 3.2 to 10.7 y). The mean number of affected vertebrae per patient was 5.2 (range: 2 to 9 vertebrae). The mean number of lengthening was 4.2 (range: 1 to 10 lengthening) per patient. The major Cobb angle improved from 66 degrees (range: 40 to 95 degrees) preoperatively to 45 degrees (range: 13 to 79 degrees) initial postoperative and 47 degrees (range: 18 to 78 degrees) at the last follow-up. The mean T1-S1 length increased from 268.3 mm (range: 192 to 322 mm) postoperatively to a mean of 315.4 mm (range: 261 to 357 mm) at last follow-up. The mean T1-S1 length increase was 11.7 mm/y. The SAL ratio increased from 0.81 preoperatively to 0.94 at latest follow-up. The mean postoperative follow-up was 4 years (range: 2 to 6.6 y). Five patients (38%) had undergone final fusion and 14 are still under treatment. Complications have occurred in 8 patients (42%). There were 14 (14%) complications in 100 procedures: 11 implant related, 2 pulmonary, and 1 postoperative infection. There were no neurological complications. CONCLUSIONS: Growing rods are a safe and effective treatment technique in selected patients with congenital spinal deformities. The deformity, spinal growth, and the SAL improved. The incidence of complication was relatively low. LEVEL OF EVIDENCE: Level IV, case series.

Growing rods for spinal deformity: characterizing consensus and variation in current use.

SUMMARY OF BACKGROUND DATA: Growing rods are a commonly used form of growth guidance for patients with early onset scoliosis, but no studies exist to characterize their use among a large group of surgeons. METHODS: A survey regarding growing rod use preferences and a case-based survey regarding early onset scoliosis were completed by an international group of surgeons. Two hundred and sixty-five growing rod patients treated over 4.7+/-2.1 years in the Growing Spine Study Group database were analyzed to characterize actual practice and compare it with the survey results. All patients had at least 2 years of treatment. RESULTS: In the case-based survey, there was correlation (P=0.04, r=0.58) between increasing curve size and choice of growing rods over nonoperative treatment, rib-based distraction (vertically expandable prosthetic titanium rib), growth guidance (Shilla), and primary fusion. In practice, growing rods were used for most types of early onset spine deformity. Most surgeons stated that their indication for growing rod treatment was a curve over 60 degrees (10/13) in a patient younger than 8 to 10 years (14/17). In practice, mean curve at rod insertion was 73+/-20 degrees and age was 6.0+/-2.5 years. Other factors favoring growing rods included curve rigidity (8/17), brace intolerance (6/17) and syndromic diagnoses (2/17). In the database, idiopathic scoliosis represented <50% of diagnoses. The most common preferred surgical lengthening interval was 6 months. However, in practice, lengthening actually occurred at a mean of 8.6+/-5.1 months. In the database, the number of growing rod insertions per year (P=0.02, r=0.96) and percentage of surgeons using dual rods over single rods (P=0.065, r=0.93) increased over time. Insertion age (P=0.075, r=-0.87) and lengthening interval (P=0.006, r=-0.69) decreased as time progressed. The most common stated indication on the survey for final fusion was skeletal maturity (13/17), and 7/13 surgeons used Risser 3 or more. Indications to stop lengthening included complications such as infection or implant failure (14/17), curves progressing past 90 degrees (8/17), and failure to distract (6/13). The most common method of final fusion was replacement of implants with more intermediate anchors. CONCLUSIONS: Significant practice variation exists in growing rod treatment, but there is some consensus on indications for surgery including curve size, diagnosis and age, and lengthening intervals and final fusion methods. Mean curve size and lengthening interval are greater in practice than in surgeons' stated aims. In principle and in practice, most growing rods are used for curves over 60 degrees in patients under 10, in all diagnoses. This information may form a starting point as practice variation is studied.

Dual growing rod technique followed for three to eleven years until final fusion: the effect of frequency of lengthening.

STUDY DESIGN: Retrospective case review of children completing dual growing rod treatment at our institutions. Patients had a minimum of 2 years follow-up. OBJECTIVE: To identify the factors influencing dual growing rod treatment outcome followed to final fusion. SUMMARY OF BACKGROUND DATA: Published reports on dual growing rod technique results for early onset scoliosis demonstrate it to be safe and effective in curve correction and maintenance as well as in allowing spinal growth. METHODS: Between 1990 and 2003, 13 patients with no previous surgery and noncongenital curves underwent final fusion. All had preoperative curve progression over 10 degrees after unsuccessful nonoperative treatment. There were 10 females and 3 males. Average age was 6.6 +/- 2.9 years at initial surgery. There were 3 idiopathic, 1 nonspine congenital anomaly, and 9 syndromic patients. Analysis included age at initial surgery and final fusion, number and frequency of lengthenings, and complications. Radiographic evaluation included changes in Cobb angle, T1-S1 length, and instrumentation length over the treatment period. RESULTS: Cobb angle improved from 81.0 +/- 23 degrees to 35.8 +/- 15 degrees postinitial and 27.7 +/- 17 degrees after final fusion. Average number of lengthenings was 5.2 +/- 3 at an interval of 9.4 +/- 5 months. T1-S1 length increased from 24.4 +/- 3.4 to 29.3 +/- 3.6 cm postinitial and 35.0 +/- 3.7 cm postfinal fusion. Average growth was 1.46 +/- 0.66 cm/year. Those lengthened at

Biomechanical comparison of different anchors (foundations) for the pediatric dual growing rod technique.

BACKGROUND CONTEXT: Children with early onset scoliosis benefit from early operative treatment with dual growing rods as they provide an "internal brace" for the spine and allow curve correction and sequential lengthenings to maximize thoracic cage development. The foundations that provide anchor points for the dual growing rods may incorporate hooks, screws, or a hybrid construct. It is unclear how stable different types of foundation constructs are with regard to pullout. PURPOSE: This study was to determine the differences in stability between four foundation configurations used in the pediatric dual growing rod technique. Eight porcine spines were sectioned and randomly assigned to one of four foundation groups: 1) hook-hook with cross-link; 2) hook-screw with cross-link; 3) screw-screw with cross-link; and 4) screw-screw without cross-link. After instrumentation, biomechanical pullout tests were conducted. Ultimate failure loads, modes of failure, and level of instrumentation were analyzed. STUDY DESIGN/SETTING: Controlled in vitro laboratory investigation. RESULTS: Screw-screw with cross-link constructs demonstrated the greatest failure load but this was not statistically significant compared with the screw-screw without cross-link constructs. Both screw-screw constructs were statistically stronger than either construct containing hooks. There was no difference between the hook-screw and hook-hook constructs that was statistically significant. Hook-containing constructs sustained higher failure loads in lumbar versus thoracic vertebrae. CONCLUSIONS: A foundation composed of four pedicle screws implanted in two adjacent vertebral bodies provides the strongest construct in pullout testing. A cross-link does not seem to enhance fixation. Hook constructs are stronger in lumbar versus thoracic laminae.

Comparison of single and dual growing rod techniques followed through definitive surgery: a preliminary study.

STUDY DESIGN: Retrospective analysis of patients treated with single and dual growing rods who had completed their course of treatment, had definitive fusion, and had a minimum of 2 years follow-up. OBJECTIVES: To determine which technique was the most effective in the management of severe spinal deformity in young children: control of the spinal deformity, spinal growth, and the incidence of complications. SUMMARY OF BACKGROUND DATA: Growing rod techniques provide proximal and distal segmental "claw" foundations, but their overall results through definitive fusion have not been clearly determined. METHODS: A total of 28 consecutive patients who had growing rod procedures followed through definitive spinal fusions were analyzed. There were three patient groups: Group 1 (N = 5), single submuscular rod and short apical fusion; Group 2 (N = 16), single growing rod alone; and Group 3 (N = 7), dual growing rods. RESULTS: The interval between initial rod insertion and definitive spinal fusion was similar in all three groups. The best overall results occurred in Group 3, whereas the patients in Group 1 had the worse results. Both Groups 2 and 3 provided good initial correction of the spinal deformity and allowed spinal growth. Group 2 had better frontal and sagittal plane balance and the lowest complication rate. CONCLUSION: The use of growing rods is effective in controlling severe spinal deformities and allowing spinal growth. Dual rods are stronger than single rods and, therefore, provide better initial correction and maintenance of correction. The use of an apical fusion does not appear to be effective over the course of treatment.