Cross-links in posterior pedicle screw-rod instrumentation of the spine: a systematic review on mechanical, biomechanical, numerical and clinical studies
Abstract:Purpose
Dorsal screw-rod instrumentations are used for a variety of spinal disorders. Cross-links (CL) can be added to such constructs, however, no clear recommendations exist. This study aims to provide an overview of the available evidence on the effectiveness of CL, potentially allowing to formulate recommendations on their use.
Methods
A systematic literature review was performed on PubMed and 37 original articles were included and grouped… Show more
“…While several studies have been published on the biomechanical impact of CL augmentation, differences in testing methods—and in instrumentation and destabilization techniques—complicate direct comparisons between studies [14] . Additionally, the relatively small sample sizes and the nonhuman test models often employed in previous studies relativize the power and translatability into clinical practice [ 5 , 8 , 9 , 15 , 16 ].…”
Section: Discussionmentioning
confidence: 99%
“…Based on our data, we believe that CL usage in a single-level lumbar fusion construct might have marginal benefits for unisegmental lumbar instrumentation constructs. The small reduction in AR-ROM must be weighed against potential disadvantages of CL implantation, such as increased surgical time, a larger surface for potential hardware-bound pathogens, and higher implant costs [ 14 , 28 ]. We translate the findings of our study into the following recommendations: 1) CL augmentation should not be applied in single-level lumbar instrumentations, independently of the extent of decompression and instrumentation level; and 2) CL augmentation should be reserved for rotatory-unstable long-fusion constructs, such as after a corpectomy or pedicle subtraction osteotomy.…”
Background
Posterior lumbar instrumentation requires sufficient primary stiffness to ensure bony fusion and to avoid pseudarthrosis, screw loosening, or implant failure. To enhance primary construct stiffness, transverse cross-link (CL) connectors attached to the vertical rods can be used. Their effect on the stability of a spinal instrumentation with simultaneous decompression is yet not clear. This study aimed to evaluate the impact of CL augmentation on single-level lumbar instrumentation stiffness after gradual decompression procedures.
Methods
Seventeen vertebral segments (6 L1/2, 6 L3/4, 5 L5/S1) of 12 fresh-frozen human cadavers were instrumented with a transpedicular screw–rod construct following the traditional pedicle screw trajectory. Range of motion (ROM) of the segments was sequentially recorded before and after four procedures: (A) instrumented before decompression, (B) instrumented after unilateral laminotomy, (C) instrumented after midline bilateral laminotomy, and (D) instrumented after unilateral facetectomy (with transforaminal lumbar interbody fusion [TLIF]). Each test was performed with and without CL augmentation. The motion between the cranial and caudal vertebrae was evaluated in all six major loading directions: flexion/extension (FE), lateral bending (LB), lateral shear (LS), anterior shear (AS), axial rotation (AR), and axial compression/distraction (AC).
Results
ROM was significantly reduced with CL augmentation in AR by Δ0.03–0.18° (7–12%) with a significantly higher ROM reduction after more extensive decompression. Furthermore, slight reductions in FE and LB were observed; these reached statistical significance for FE after facetectomy and TLIF insertion only (Δ0.15; 3%). The instrumentation levels did not reveal any subgroup differences.
Conclusion
CL augmentation reduces AR-ROM by 7–12% in single-level instrumentation of the lumbar spine, with the effect increasing along with the extensiveness of the decompression technique. In light of the discrete absolute changes, CL augmentation may be warranted for highly unstable vertebral segments rather than for standard single-level posterior spinal fusion and decompression.
“…While several studies have been published on the biomechanical impact of CL augmentation, differences in testing methods—and in instrumentation and destabilization techniques—complicate direct comparisons between studies [14] . Additionally, the relatively small sample sizes and the nonhuman test models often employed in previous studies relativize the power and translatability into clinical practice [ 5 , 8 , 9 , 15 , 16 ].…”
Section: Discussionmentioning
confidence: 99%
“…Based on our data, we believe that CL usage in a single-level lumbar fusion construct might have marginal benefits for unisegmental lumbar instrumentation constructs. The small reduction in AR-ROM must be weighed against potential disadvantages of CL implantation, such as increased surgical time, a larger surface for potential hardware-bound pathogens, and higher implant costs [ 14 , 28 ]. We translate the findings of our study into the following recommendations: 1) CL augmentation should not be applied in single-level lumbar instrumentations, independently of the extent of decompression and instrumentation level; and 2) CL augmentation should be reserved for rotatory-unstable long-fusion constructs, such as after a corpectomy or pedicle subtraction osteotomy.…”
Background
Posterior lumbar instrumentation requires sufficient primary stiffness to ensure bony fusion and to avoid pseudarthrosis, screw loosening, or implant failure. To enhance primary construct stiffness, transverse cross-link (CL) connectors attached to the vertical rods can be used. Their effect on the stability of a spinal instrumentation with simultaneous decompression is yet not clear. This study aimed to evaluate the impact of CL augmentation on single-level lumbar instrumentation stiffness after gradual decompression procedures.
Methods
Seventeen vertebral segments (6 L1/2, 6 L3/4, 5 L5/S1) of 12 fresh-frozen human cadavers were instrumented with a transpedicular screw–rod construct following the traditional pedicle screw trajectory. Range of motion (ROM) of the segments was sequentially recorded before and after four procedures: (A) instrumented before decompression, (B) instrumented after unilateral laminotomy, (C) instrumented after midline bilateral laminotomy, and (D) instrumented after unilateral facetectomy (with transforaminal lumbar interbody fusion [TLIF]). Each test was performed with and without CL augmentation. The motion between the cranial and caudal vertebrae was evaluated in all six major loading directions: flexion/extension (FE), lateral bending (LB), lateral shear (LS), anterior shear (AS), axial rotation (AR), and axial compression/distraction (AC).
Results
ROM was significantly reduced with CL augmentation in AR by Δ0.03–0.18° (7–12%) with a significantly higher ROM reduction after more extensive decompression. Furthermore, slight reductions in FE and LB were observed; these reached statistical significance for FE after facetectomy and TLIF insertion only (Δ0.15; 3%). The instrumentation levels did not reveal any subgroup differences.
Conclusion
CL augmentation reduces AR-ROM by 7–12% in single-level instrumentation of the lumbar spine, with the effect increasing along with the extensiveness of the decompression technique. In light of the discrete absolute changes, CL augmentation may be warranted for highly unstable vertebral segments rather than for standard single-level posterior spinal fusion and decompression.
“… Figure 1 ( A ) Segmental deformation after posterior instrumentation during physiological loading (± 7.5 Nm) 1 . ( B ) Hypothezides construct deformations due to the bending forces in the three major rotational motion planes (figure adapted from 3 ). ( C ) Illustration of the angular displacement of the pedicle screw in relation to the vertebral body (“screw rotation”) and the relative motion between one side of the screw-rod-construct to the other (“parallelogram deformation”).…”
Section: Introductionmentioning
confidence: 99%
“…Crosslinks, connecting the two sides of the screw-rod construct, were proposed as a measure to increase rotational construct stiffness 4 and their effect was evaluated in a multitude of studies, as summarized in a recent systematic review 3 : in axial rotation, a relatively consistent positive effect on construct stiffness was observed, while the effect on lateral bending was more variable and in flexion–extension, only minimal effect was recorded. While the biomechanical effect was largely similair for the whole spinal column, clinical benefit has only been shown for C1/2 instrumentations 5 , 6 .…”
Posterior screw-rod constructs can be used to stabilize spinal segments; however, the stiffness is not absolute, and some motion can persist. While the effect of crosslink-augmentation has been evaluated in multiple studies, the fundamental explanation of their effectiveness has not been investigated. The aim of this study was to quantify the parameters “screw rotation” and “parallelogram deformation” in posterior instrumentations with and without crosslinks to analyze and explain their fundamental effect. Biomechanical testing of 15 posteriorly instrumented human spinal segments (Th10/11—L4/L5) was conducted in axial rotation, lateral bending, and flexion–extension with ± 7.5 Nm. Screw rotation and parallelogram deformation were compared for both configurations. Parallelogram deformation occurred predominantly during axial rotation (2.6°) and was reduced by 60% (−1.45°, p = 0.02) by the addition of a crosslink. Simultaneously, screw rotation (0.56°) was reduced by 48% (−0.27°, p = 0.02) in this loading condition. During lateral bending, 0.38° of parallelogram deformation and 1.44° of screw rotation was measured and no significant reduction was achieved by crosslink-augmentation (8%, −0.03°, −p = 0.3 and −13%, −0.19°, p = 0.7 respectively). During flexion–extension, parallelogram deformation was 0.4° and screw rotation was 0.39° and crosslink-augmentation had no significant effect on these values (−0.12°, −30%, p = 0.5 and −0°, −0%, p = 0.8 respectively). In axial rotation, crosslink-augmentation can reduce parallelogram deformation and with that, screw rotation. In lateral bending and flexion–extension parallelogram deformation is minimal and crosslink-augmentation has no significant effect. Since the relatively large screw rotation in lateral bending is not caused by parallelogram deformation, crosslink-augmentation is no adequate countermeasure. The fundamental understanding of the biomechanical effect of crosslink-augmentation helps better understand its potential and limitations in increasing construct stiffness.
“…The internal fixation material must be able to restrict joint movement in all rotational axes to provide a strong fixed force. A transverse connector can enhance the rotational stability of the internal fixation [ 5 ], and the horizontal rod-rod crosslink (hR-R CL) is most commonly used in C1–C2 PSR fixation. However, intraoperative installation of the hR-R CL is difficult, resulting in prolonged operative time and even the possibility of spinal cord injury.…”
Background
In the craniocervical junction, a C1–C2 pedicle screw-rod (PSR) fixation is applied to provide stability. The horizontal rod-rod crosslink (hR-R CL) is often used to enhance segmental posterior instrumentation. However, the biomechanics of the alternative horizontal screw-screw crosslink (hS-S CL) in the craniocervical junction are unclear.
Material/Methods
A nonlinear atlantoaxial instability 3-dimensional C1–C2 finite element model was constructed using computed tomography images. On this basis, 2 fixation models were established with C1–C2 PSR fixation using (1) a rod-rod crosslink (R-R CL), and (2) a screw-screw crosslink (S-S CL). Range of motion (ROM) of the atlantoaxial joint, stress distribution of the implants, and maximum stress value of the vertebral bodies were calculated and compared under 4 loading conditions, including flexion, extension, lateral bending, and axial rotation.
Results
Atlantoaxial joint ROM was reduced by 90.19% to 98.5% with the hR-R CL, and by 90.1% to 98.7% with the hS-S CL, compared with the instability model. During axial rotation, the total stress peak of the PSR fixation was smaller with hS-S CL than with hR-R CL. The peak stress values of the vertebral bodies were comparable between the 2 fixation models.
Conclusions
The 2 tested crosslink models provided comparable stability. However, during axial rotation, the total stress peak of hS-S CL fixation was smaller than that of hR-R CL fixation. Since the atlantoaxial joint primarily functions as a rotational joint, our results suggested that the use of hS-S CL can provide a more stable environment for the implants.
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