Pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine

While the biomechanical properties of pedicle screws have proven to be superior in the lumbar spine, little is known concerning pullout strength of pedicle screws in comparison to hooks in the thoracic spine. In vitro biomechanical pullout testing was performed to evaluate the axial pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine with regard to surgical correction techniques in scoliosis. Nine human cadaveric thoracic spines were harvested and disarticulated. To simulate a typical posterior segmental scoliosis instrumentation, standard pedicle hooks were used between T4 and T8 and supralaminar hooks between T9 and T12 and tested against pedicle screws. The pedicle screws were loaded strictly longitudinal to their axis; the hooks were loaded perpendicular to the intended rod direction. In total, 90 pullout tests were performed. Average pullout strength of the pedicle screws was significantly higher than in the hook group (T4-T8: 531 N versus 321 N, T9-T12: 807 N versus 600 N, p < 0.05). Both screw diameter and the bone mineral density (BMD) had significant influence on the pullout strength in the screw group. For scoliosis correction, pedicle screws might be beneficial, especially for rigid thoracic curves, since they are significantly more resistant to axial pullout than both pedicle and laminar hooks.     

Biomechanical evaluation of a new fixation device for the thoracic spine

The technology used in surgery for spinal deformity has progressed rapidly in recent years. Commonly used fixation techniques may include monofilament wires, sublaminar wires and cables, and pedicle screws. Unfortunately, neurological complications can occur with all of these, compromising the patients’ health and quality of life. Recently, an alternative fixation technique using a metal clamp and polyester belt was developed to replace hooks and sublaminar wiring in scoliosis surgery. The goal of this study was to compare the pull-out strength of this new construct with sublaminar wiring, laminar hooks and pedicle screws. Forty thoracic vertebrae from five fresh frozen human thoracic spines (T5–12) were divided into five groups (8 per group), such that BMD values, pedicle diameter, and vertebral levels were equally distributed. They were then potted in polymethylmethacrylate and anchored with metal screws and polyethylene bands. One of five fixation methods was applied to the right side of the vertebra in each group: Pedicle screw, sublaminar belt with clamp, figure-8 belt with clamp, sublaminar wire, or laminar hook. Pull-out strength was then assessed using a custom jig in a servohydraulic tester. The mean failure load of the pedicle screw group was significantly larger than that of the figure-8 clamp (P = 0.001), sublaminar belt (0.0172), and sublaminar wire groups (P = 0.04) with no significant difference in pull-out strength between the latter three constructs. The most common mode of failure was the fracture of the pedicle. BMD was significantly correlated with failure load only in the figure-8 clamp and pedicle screw constructs. Only the pedicle screw had a statistically significant higher failure load than the sublaminar clamp. The sublaminar method of applying the belt and clamp device was superior to the figure-8 method. The sublaminar belt and clamp construct compared favorably to the traditional methods of sublaminar wires and laminar hooks, and should be considered as an alternative fixation device in the thoracic spine.     

Accuracy and safety of thoracic pedicle screw placement in spinal deformities

OBJECTIVES: To determine the safety of pedicle screw fixation in thoracic spine deformity correction. METHODS: One hundred twelve pedicle screws were surgically placed in 25 patients with degenerative, posttraumatic, and Scheuermann kyphosis and idiopathic and neuromuscular scoliosis. Screw position was evaluated using intraoperative and postoperative radiographs and thin-slice computed tomography. RESULTS: Of the total 112 thoracic pedicle screws that were inserted, 98 screws (87.5%) were fully contained within the cortical boundaries of the pedicle. When comparing proximal screws (T1-T8) with distal screws (T9-T12) and convex placed screws with concave ones, a statistically significant difference in screw placement was evident (P < 0.05). More misplaced screws were seen proximally and on the concave side. Of the 14 malpositioned screws, 2 (1.8%) demonstrated aortic abutment. There were no neurologic deficits, vascular injuries, or mechanical failures recorded. CONCLUSIONS: Placement of thoracic pedicle screws is both feasible and safe.     

Segmental pedicle screw fixation or cross-links in multilevel lumbar constructs. a biomechanical analysis.

BACKGROUND CONTEXT: The placement of segmental pedicle screws and cross-links in short segment posterior pedicle screw constructs has been shown to increase the construct stiffness in some planes. To date, no studies have looked at the contributions of segmental pedicle screw and cross-link placement in longer constructs. PURPOSE: To evaluate the influence of segmental pedicle screw and/or cross-link placement on flexion/extension, lateral bending and axial torsion stiffness in two- and three-level posterior pedicle screw fixation constructs. STUDY DESIGN/SETTING: An in vitro biomechanical analysis of two- and three-level posterior pedicle screw constructs with and without segmental fixation and/or cross-links was performed using calf lumbar spines. Stiffness of the constructs was compared. METHODS: Six calf lumbar specimens were used to test stiffness in one-, two- and three-level posterior pedicle screw fixation constructs in 12 configurations. A custom-made, four-axis spine simulator applied pure cyclical (+/-5 Nm) flexion/extension, lateral bending and axial torsion moments at 0.1 Hz under a constant 50-N axial compressive load. The stiffness of each construct was calculated about each axis of rotation. Data were analyzed using nonparametric techniques with statistical significance determined at alpha less than .05. RESULTS: The stiffness of the instrumented spines were significantly greater than the noninstrumented intact spines in all loading conditions for one-, two- and three-level constructs. There were no significant changes in flexion/extension stiffness with the addition of either the cross-links or the segmental pedicle screws. In lateral bending, the addition of segmental pedicle screws significantly increased the stiffness in the two- and three-level constructs. The addition of two cross-links increased lateral bending stiffness in the longer three-level constructs, with little change in the two-level constructs. In axial torsion, the progressive addition of cross-links showed a tendency toward increased stiffness in both the two- and three-level constructs. Segmental pedicle screws further increased torsional stiffness of the longer, three-level constructs. CONCLUSIONS: As the use of segmental spinal instrumentation progresses from one to two and three levels, the contribution of cross-links and segmental pedicle screws to the overall construct stiffness increases.     

Use of the Universal Clamp in adolescent idiopathic scoliosis for deformity correction and as an adjunct to fusion: 2-year follow-up

Purpose  

Among posterior surgical techniques for treating adolescent idiopathic scoliosis (AIS), hybrid constructs with pedicle-screw fixation in the lumbar spine and other anchors in the thoracic spine have been reported to provide to be of more physiological value in postoperative thoracic kyphosis than all-screw constructs. The Universial Clamp (UC) equipped with a soft sublaminar band is a relatively new thoracic anchor that can be used in hybrid constructs. A dedicated reduction tool that applies traction to the sublaminar band permits gentle translation of the thoracic curve to the precontoured fusion rods, which have been previously anchored distally by pedicle screws and proximally by hooks in a claw configuration. The aim of this study was to evaluate radiographic results of AIS treatment using UC hybrid constructs.     

Pedicle anatomy in a patient with severe early-onset scoliosis: can pedicle screws be safely inserted?

OBJECTIVE: With the rapid increase in the use of pedicle screws in the thoracic spine for various pathologies, knowledge of the pedicle anatomy is critical. Previous authors, in discussing pedicle morphology, have usually reported their findings in nondeformed adult specimens. More recently, the use of pedicle screws in adolescent idiopathic scoliosis has been reported. METHODS: The authors studied the pedicle diameters in the spine of a patient with infantile idiopathic scoliosis who died at age 28 of cor pulmonale. The concave pedicles from T6 to L3 were measured both directly and with thin-section computed tomography (CT) scanning (the curve apex was T8-T11). RESULTS: By direct measurement, the concave pedicle width at its narrowest point (the isthmus) ranged from 2.9 (T9) to 6.7 (L1, L3) mm. Three apical concave pedicles (T8, T9, T10) had no cancellous cavity. By CT scan measurement, the four apical concave pedicles measured 3.4 (T8), 2.8 (T9), 2.6 (T10), and 3.4 (T11) mm, respectively. CONCLUSIONS: In conclusion, the authors confirm others' findings that the concave pedicles can be so small that pedicle screw insertion is impossible. We also found that these findings can be confirmed preoperatively with thin-section CT scanning. In such situations, extrapedicular screw placement should be considered.     

Torsional stability of cross-link configurations: a biomechanical analysis.

BACKGROUND CONTEXT: Cross-link systems have been used to augment segmental spinal instrumentation since the earliest introduction of these fixation systems. Although transverse cross-links have little impact on sagittal motion of spinal constructs, cross-linkage does affect torsional rigidity. Despite the wide variety of cross-link designs, almost all have been configured as transverse devices. The relative mechanical benefit of different cross-link configurations is not known. PURPOSE: The purpose of this study was to compare the torsional stability of three different cross-link configurations. STUDY DESIGN: Biomechanical analysis of segmental instrumentation constructs using porcine spines. METHODS: Thoracic porcine spines (T4 to T10) were instrumented with 6.5-mm conical pedicle screws and 7.0-mm connecting rods from T5 to T9. Terminal vertebrae were embedded in polymethylmethacrylate (PMMA) after a T7 corpectomy. Four cross-link configurations were tested in a randomized manner: Un-cross-linked Control (CONT); Transverse Rod-Rod (RR); Transverse Screw-Screw (SS); and Diagonal Screw-Screw (DX) Cross-links. The specimens were rotated to 3 Nm at a rate of 0.2 degrees/s and cycled six times with data acquisition over the final two cycles. Stiffness, rotation, and energy data were normalized to each control. A Newman-Keuls repeated measures analysis of variance was used to infer significant differences. RESULTS: Diagonal cross-link configurations provided the most rigid construct. Transverse cross-links did not significantly change torsional behavior compared with the unlinked control. Rotation and energy expended were not significantly greater torsional stiffness compared with other constructs tested (p<.01). CONCLUSIONS: The diagonal cross-link configuration provided increased torsional stiffness as compared with unlinked or transverse configurations. This observation should be considered in future cross-link designs.     

Junction kinematics between proximal mobile and distal fused lumbar segments: biomechanical analysis of pedicle and hook constructs

Background contextBiomechanical studies have demonstrated increased motion in motion segments adjacent to instrumentation or arthrodesis. The effects of different configurations of hook and pedicle screw instrumentation on the biomechanical behaviors of adjacent segments have not been well documented.PurposeTo compare the effect of three different fusion constructs on adjacent segment motion proximal to lumbar arthrodesis.MethodsSeven human cadaver lumbar spines were tested in the following conditions: 1) intact; 2) L4–L5-simulated circumferential fusion (CF); 3) L4–L5-simulated fusion extended to L3 with pedicle screws; and 4) L4–L5-simulated fusion extended to L3 with sublaminar hooks. Rotation data at L2–L3, L3–L4, and L4–L5 were analyzed using both load limit control (±7.5 N·m) and displacement limit control (truncated to the greatest common angular motion of the segments for each specimen).ResultsBoth the L3–L4 and L2–L3 motion segments above the L4–L5-simulated CF had significantly increased motion in all loading planes compared with the intact spine, but no significant differences were found between L3–L4 and L2–L3 motion. When the L3–L4 segment was stabilized with pedicle screws, its motion was significantly smaller in flexion, lateral bending, and axial rotation than when stabilized with sublaminar hooks. At the same time, L2–L3 motion was significantly larger in flexion, lateral bending, and axial rotation in the pedicle screw model compared with the sublaminar hook construct.ConclusionsThe use of sublaminar hooks to stabilize the motion segment above a circumferential lumbar fusion reduced motion at the next cephalad segment compared with a similar construct using pedicle screws. The semiconstrained hook enhancement may be considered if a patient is at a risk of adjacent segment disorders.     

Dorsale operative Korrektur der idiopathischen Skoliose

Posterior correction and fusion of scoliosis with multisegmental instrumentation systems was developed by Cotrel-Dubousset in the 1980s. Initially correction and instrumentation was performed using hooks only. Later pedicle screws were implemented first for the lumbar and then for the thoracic spine. Nowadays instrumentation based on pedicle screws only is well established for posterior scoliosis surgery. Biomechanical studies demonstrated higher pull-out forces for pedicle than for hook constructs. In clinical studies several authors reported better Cobb angle correction of the primary and the secondary curves and less loss of correction in pedicle screw versus hook instrumentations. Furthermore, pedicle screw instrumentation allows fewer segments to be fused, especially caudally, and thus saving mobile segments. In most of these publications there were no differences in operation time, blood loss and complication rates. In summary, there is better curve correction without an increased risk using multisegmental pedicle screw instrumentation in modern posterior scoliosis surgery.     

New means in spinal pedicle hook fixation

Summary Pedicle hooks which are used as an anchorage for posterior spinal instrumentation may be subjected to considerable three-dimensional forces. In order to achieve stronger attachment to the implantation site, hooks using screws for additional fixation have been developed. The failure loads and mechanisms of three such devices have been experimentally determined on human thoracic vertebrae: the Universal Spine System (USS) pedicle hook with one screw, a prototype pedicle hook with two screws and the Cotrel-Dubousset (CD) pedicle hook with screw. The USS hooks use 3.2-mm self-tapping fixation screws which pass into the pedicle, whereas the CD hook is stabilised with a 3-mm set screw pressing against the superior part of the facet joint. A clinically established 5-mm pedicle screw was tested for comparison. A matched pair experimental design was implemented to evauluate these implants in constrained (series I) and rotationally unconstrained (series II) posterior pull-out tests. In the constrained tests the pedicle screw was the strongest implant, with an average pull-out force of 1650 N (SD 623 N). The prototype hook was comparable, with an average failure load of 1530 N (SD 414 N). The average pull-out force of the USS hook with one screw was 910 N (SD 243 N), not significantly different to the CD hook's average failure load of 740 N (SD 189 N). The result of the unconstrained tests were similar, with the prototype hook being the strongest device (average 1617 N, SD 652 N). However, in this series the difference in failure load between the USS hook with one screw and the CD hook was significant. Average failure loads of 792 N (SD 184 N) for the USS hook and 464 N (SD 279 N) for the CD hook were measured. A pedicular fracture in the plane of the fixation screw was the most common failure mode for USS hooks. The hooks usually did not move from their site of implantation, suggesting that they may be well-suited for the socalled segmental spinal correction technique as used in scoliosis surgery. In contrast, the CD hook disengaged by translating caudally from its site of implantation in all cases, suggesting a mechanical instability. The differences in observed hook failure modes may be a function of the type and number of additional fixation screws used. These results suggest that additional screw fixation allows stable attachment of pedicle hooks to their implantation site. Hooks using additional fixation screws passing obliquely into the pedicle apparently provide the most rigid attachment. The second fixation screw of the prototype hook almost doubles the fixation strength. Thus, the prototype hook might be considered as an alternative to the pepdicle screw, especially in the upper thoracic region.     

Effectiveness of transfixation and length of instrumentation on titanium and stainless steel transpedicular spine implants

This study compares the effectiveness of transfixation on the stiffness of two pedicle screw-rod constructs of different manufacture, implant design, and alloy, applied in one-and two-level instability. Four screws composed of either stainless steel or Titanium were assembled in pairs to two polymethylmethacrylate blocks to resemble one-and two-level corpectomy models and the construct underwent nondestructive torsional, extension, and flexion loading. In every loading test, each construct was tested using stainless steel or titanium rods of 4.9-mm diameter in two different lengths (short, 10 cm; long, 15 cm), not augmented or augmented with different transfixation devices or a pair of devices. The authors compared the stiffness of stainless steel and titanium constructs without cross-link with the stiffness of that reinforced with single or double Texas Scottish Rite Hospital (TSRH) cross-link, closed new-type cross-link (closed NTC), or open new-type cross-link (open NTC). The results showed that augmentation or no augmentation of short rods conferred significantly more stiffness than that of long rods of the same material in all three loading modes. The closed NTC provided the greatest increase of torsional, extension, and flexion stiffness, and single TSRH provided the least amount of stiffness. Torsional stiffness of short stainless steel rods augmented or not augmented was significantly greater than that of their titanium counterparts. Torsional stiffness of long titanium rods was always greater than that of their stainless steel counterparts. Extension stiffness of short nonaugmented titanium rods was superior to that of long titanium rods, whereas extension stiffness of nonaugmented short and long stainless steel rods was similar. Nonaugmented short titanium rods showed greater flexion stiffness than that of long titanium rods. Long stainless steel rods displayed significantly greater flexion stiffness than did their titanium counterparts. This nondestructive study showed that cross-links increase the torsional stiffness significantly but less so the flexion and extension stiffness of both titanium and stainless steel posterior transpedicular constructs. This increase was proportional to the cross-sectional diameter of the cross-link. Titanium constructs showed more torsional stiffness when used in two-level instability and steel showed more torsional stiffness in one-level instability, particularly when they are reinforced. Stainless steel constructs showed greater flexion stiffness when they were used in two-level and titanium showed greater flexion stiffness in one-level instability, particularly when they were reinforced with stiff cross-links. The effect of transfixation on extension forces was obvious when thick cross-links were used.     

The mechanical role of laminar hook protection of pedicle screws at the caudal end vertebra

Biomechanical studies have shown hooks to be superior to pedicle screws in pull-out, especially in osteoporosis. This study evaluates the possible increase in stiffness of pedicle screws provided by laminar hooks while applying non-destructive forces to a vertebrectomy model assembled with Compact Cotrel Dubousset (CCD) instrumentation. Synthetic vertebrae were employed in a three-level vertebrectomy model. CCD screw-based three-level constructs with and without sublaminar hooks in the caudal element were tested in flexion, extension, compression, lateral bending, and torsion. There was no statistically significant advantage in adding inferior laminar hooks to a caudal end vertebra that had bilateral pedicle screws in any of the testing modes. Torsional stability, however, was augmented, but not significantly. Torsional instability and osteoporotic bone may be the clinical justifications for adding laminar hooks below screws in the caudal end vertebra.     

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

Background contextChildren 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.PurposeThis 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/settingControlled in vitro laboratory investigation.ResultsScrew-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.ConclusionsA 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.     

Biomechanical evaluation of five different occipito-atlanto-axial fixation techniques

STUDY DESIGN: The stabilizing effects of five different occipitocervical fixations were compared. OBJECTIVES: To evaluate the construct stability provided by five different occipito-atlanto-axial fixation techniques. SUMMARY OF BACKGROUND DATA: Few studies have addressed occipitocervical reconstruction stability and no studies to data have investigated anterior-posterior translational stiffness. METHODS: A total of 21 human cadaveric spines were used. After testing intact spines (CO-C2), a type II dens fracture was created and five different reconstructions were performed: 1) occipital and sublaminar wiring/rectangular rod, 2) occipital screws and C2 lamina claw hooks/rod, 3) occipital screws, foramen magnum screws, and C1-C2 transarticular screws/rod, 4) occipital screws and C1-C2 transarticular screws/Y-plate, and 5) occipital screws and C2 pedicle screws/rod. Biomechanical testing parameters included axial rotation, flexion/extension, lateral bending, and anterior-posterior translation. RESULTS: Pedicle screw fixation demonstrated the highest stiffness among the five reconstructions (P < 0.05). The two types of transarticular screw methods provided greater stability than hook or wiring reconstructions (P < 0.05). The C2 claw hook technique resulted in greater stability than sublaminar wiring fixation in anterior-posterior translation (P < 0.05). However, the wiring procedure did not significantly increase the stiffness levels beyond the intact condition under anterior-posterior translation and lateral bending (P > 0.05). DISCUSSION: C2 transpedicular and C1-C2 transarticular screws significantly increased the stabilizing effect compared to sublaminar wiring and lamina hooks. The improved stability afforded by C2 pedicular and C1-C2 transarticular screws offer many potential advantages including a high rate of bony union, early ambulation, and easy nursing care. CONCLUSION: Occipitocervical reconstruction techniques using C1-C2 transarticular screws or C2 pedicle screws offer biomechanical advantages compared to sublaminar wiring or lamina hooks. Pedicle screw fixation exhibited the highest construct stiffness among the five reconstructions.     

Correction of adolescent idiopathic scoliosis using thoracic pedicle screw fixation versus hook constructs

This retrospective study was undertaken to determine the effectiveness and cost of thoracic pedicle screws versus laminar and pedicle hooks in patients undergoing surgical correction of adolescent idiopathic scoliosis (AIS). Immediate preoperative and 6-week postoperative radiographs were examined in 25 consecutive cases of children with AIS who were divided into two groups, those with thoracic pedicle screw constructs and those with thoracic hook constructs. Endpoints collected included radiographic measures, complications, surgical time, implant cost, and quality-of-life measures. Ten children underwent spinal fusion using thoracic pedicle screw fixation and 15 underwent thoracic constructs composed of hooks. Similar sex and age distribution were noted in both groups, and among the 20 girls and 5 boys the average age was 14.5. The mean preoperative Cobb angle was 53.5 degrees for the screw group and 52.5 degrees for the hook group. Correction averaged 70.2% for the screw group and 68.1% for the hook group. There were no significant differences between the two patient groups in terms of percentage of or absolute curve change after surgery. The apical vertebral translation, end vertebral tilt angle, and coronal balance did not differ significantly between the two patient groups. Comparison of operative time and quality of life revealed no significant differences. Screw constructs were significantly more expensive than hook constructs. The correction obtained from thoracic pedicle screw fixation is comparable to traditional hook constructs in AIS. Surgery using either construct effectively corrects AIS.     

Type V osteogenesis imperfecta undergoing surgical correction for scoliosis

Open image in new window Purpose

The objective of this article is to report a case of type V osteogenesis imperfecta (OI) undergoing posterior instrumented fusion for scoliosis. Type V OI is a moderately severe dysplasia causing primary defects in endochondral bone ossification or mineralisation. It is characterised by hyperplastic callus (HPC) formation, interosseous membrane calcifications, poor bone quality and spinal deformities including scoliosis. Data on the surgical management of spinal deformities in this patient group are lacking.

Case report

A 16-year-old patient with a confirmed diagnosis of type V OI presented with a progressive scoliosis. The patient underwent a T3–L4 posterior instrumented correction and fusion utilising pedicle screws, pedicle hooks and sub-laminar wiring. At 4 months after surgery, the pedicle hooks pulled out and required partial metalwork removal after CT scanning confirmed bony union and no evidence of HPC formation. The patient was successfully discharged with satisfactory correction, confirmed bony union, no neurologic complication and absence of any hyperplastic callus formation.

Conclusion

Type V OI patients requiring surgical intervention for scoliosis correction can safely undergo posterior instrumented fusion using sublaminar wiring and pedicle hook/screw constructs without apparent risk of HPC formation around neural elements. Surgery in this patient group remains challenging due to the associated poor bone quality.

Level of evidence

V.

     

Operative Therapie von idiopathischen Skoliosen und juvenilen Kyphosen

Indication for operative treatment of idiopathic scoliosis and juvenile kyphosis is mainly cosmetic. There is also a higher incidence of pain in scoliosis patients, and reduced pulmonary function in severe deformity, especially in severe deformities present at the age of 5 years (early onset). Scoliotic curves of less than 30 degrees will not progress in adults, whereas curves of 50-75 degrees will further progress a mean of 25 degrees during 40 years. Progression in adults with juvenile kyphosis is not well documented. Operative treatment aims to stop progression, to control spinal growth, or to perform correction and fusion by spinal instrumentation and bone grafts. These goals can be achieved either by an anterior, a posterior, or a combined approach. Correction principles are compression, distraction, derotation and translation. The forces applied by correction are transferred by fixation devices (pedicle screws, anterior screws, hooks, sublaminar wires) to the spine. The higher correction forces are, the higher is the correction achieved, but also the risk of fracture and torn out implants. Mobilisation reduces rigidity and allows to achieve a better correction with equal forces. The best mobilisation techniques are disc excision, facet joint removal, and techniques to mobilise the thorax.     

Comparative analysis of pedicle screw and hook instrumentation in posterior correction and fusion of idiopathic thoracic scoliosis

Posterior correction and fusion with segmental hook instrumentation represent the gold standard in the surgical treatment of progressive idiopathic thoracic scoliosis. However, there is a debate over whether pedicle screws are safe in scoliosis surgery and whether their usage might enable a better curve correction and a shorter fusion length. The details of curve correction, fusion length and complication rate of 99 patients with idiopathic thoracic scoliosis treated with either hook or pedicle screw instrumentation were analyzed. Forty-nine patients had been operated with the Cotrel-Dubousset system using hooks exclusively ("hook group"). Fifty patients had been operated with either a combination of pedicle screws in the lumbar and lower thoracic and hooks in the upper thoracic spine or exclusive pedicle screw instrumentation using the Münster Posterior Double Rod System ("screw group"). The preoperative Cobb angle averaged 61.3 degrees (range 40 degrees-84 degrees ) in the hook group and 62.5 degrees (range 43 degrees-94 degrees ) in the screw group. Average primary curve correction was 51.7% in the hook group and 55.8% in the screw group ( P>0.05). However, at follow-up (2-12 years later) primary curve correction was significantly greater ( P=0.001) in the screw group (at 50.1%) compared to the hook group (at 41.1%). Secondary lumbar curve correction was significantly greater ( P=0.04) in the screw group (54.9%) compared to the hook group (46.9%). Correction of the apical vertebral rotation according to Perdriolle was minimal in both groups. Apical vertebral translation was corrected by 42.0% in the hook group and 55.6% in the screw group ( P=0.008). Correction of the tilt of the lowest instrumented vertebra averaged 48.1% in the hook group and 66.2% in the screw group ( P=0.0004). There were no differences concerning correction of the sagittal plane deformity between the two groups. Fusion length was, on average, 0.6 segments shorter in the screw group compared to the hook group ( P=0.03). With pedicle screws, the lowest instrumented vertebra was usually one below the lower end vertebra, whereas in the hook group it was between one and two vertebrae below the lower end vertebra. Both operative time and intraoperative blood loss were significantly higher in the hook group ( P<0.0001). One pedicle screw at T5 was exchanged due to the direct proximity to the aorta. There were no neurologic complications related to pedicle screw instrumentation. Pedicle screw instrumentation alone or in combination with proximal hook instrumentation offers a significantly better primary and secondary curve correction in idiopathic thoracic scoliosis and enables a significantly shorter fusion length.     

Hybrid constructs pedicle screw with apical sublaminar bands versus pedicle screws only for surgical correction of adolescent idiopathic scoliosis

Purpose

To compare the 2-year minimum postoperative results of posterior correction and spinal arthrodesis using translational correction with hybrid (sublaminar bands on concave side and pedicle screw) constructs versus correction with intermediate density pedicle screw-only constructs in the treatment of AIS (Lenke 1).

Methods

A total of 37 patients with AIS at single institutions who underwent posterior spinal arthrodesis pedicle screw with sublaminar bands at the apex (19 patients) (Group A) or pedicle screw-only (18) constructs (Group B) were selected and matched according to similar age at surgery 13.8 years (Group A) and 14.3 years (Group B), similar arthrodesis area 12.3 (Group A) and 11.5 (Group B), all curves Lenke type 1 with similar pre-op curve 54° (Group A) and 57° (Group B). Patients were evaluated pre-op, immediately post-op, and at min 2-year follow-up according to radiographic curve correction, operating time, intraoperative blood loss, and f.u. loss of correction.

Results

The average curve correction was 65.6% in sublaminar group and 68% in pedicle screw group. At 2-year follow-up, loss of the major curve correction was 2% in sublaminar group compared to 3% in pedicle screw group. Postoperative coronal and sagittal balance was similar in both groups. Operating time averaged 200 min (Group A) and 180 min (Group B). Intraoperative blood loss was significantly different in both groups 700?±?160 cc in sublaminar group and 630?±?150 cc in pedicle screw group. There were no neurologic complications in both groups.

Conclusion

The two groups offer similar curve correction without neurologic complications in the surgical treatment of AIS (Lenke 1). The use of sublaminar bands on the apex (concave side) can be a valid fixation in the presence of hypoplastic pedicle, can reduce the thoracic hypokyphosis and derotate the vertebra but had more blood loss comparing to pedicle screws alone.

Graphical abstract

These slides can be retrieved under Electronic Supplementary Material.

Open image in new window

     

Placement of pedicle screws in thoracic idiopathic scoliosis: a magnetic resonance imaging analysis of screw placement relative to structures at risk

In posterior pedicle screw instrumentation of thoracic idiopathic scoliosis, screw malposition might cause significant morbidity in tems of possible pleural, spinal cord, and aorta injury. Preoperative axial magnetic resonace images (MRI) in 12 consecutive patients with right thoracic adolescent scoliosis, all with King type 3 curves, were analyzed in order to evaluate the relationship between the inserted pedicle screw position to pleura, spinal cord, aorta. Axial vertebral images for each thoracic level were scanned and the simulation of pedicle screw insertion was performed using a digital measurement programme. The angular contact value for each parameter regarding the pleura and spinal cord was measured on both sides of the curve. The aorta-vertebral distance was also measured. Aorta-vertebral distance was found to be decreasing gradually from the cephalad to the caudad with the shortest distance being measured at T12 with a mean of 1.2 mm. Concave-sided screws on T5–T9 and convex-sided screws on T2–T3 had the greatest risk to spinal cord injury. Pleural injury is most likely on T4–T9 segments by the convex side screws. T4–T8 screws on the concave side and T11–T12 screws on the convex side may pose risk to the aorta. This MRI-based study demonstrated that in pedicle instrumentation of thoracic levels, every segment deserves special consideration, where computer scanning might be mandatory in immature spine and in patients with severe deformity.