Aditya Muzumdar

Do facet screws provide the required stability in lumbar fixation? A biomechanical comparison of the Boucher technique and pedicular fixation in primary and circumferential fusions

BACKGROUND Transfacet pedicle screws are scarcely used in primary posterior fixation, and have limited use unilaterally or with existing anterior instrumentation. Nevertheless, the incomplete literature suggests equivalent or better performance of ipsilateral, bilateral, facet screws compared to bilateral pedicle screws. METHODS Two groups of seven human cadaver spines (L3-S1) were tested under pure moments of 6 Nm. Each specimen was tested in a primary and circumferential fixation (Spacer, Spacer+Plate) environment. Both transfacet and bilateral pedicle screws were used as posterior fixation, in separate groups. Motion was obtained at L4-L5 for single-level constructs in flexion-extension, lateral bending and axial rotation modes. FINDINGS In primary fixation, both transfacet and bilateral pedicle screws reduced motion below intact levels. Statistically, the level of circumferential fixation (anterior, posterior, or both) proved to be more influential than the type of posterior fixation. Incorporating a spacer and plate with pedicle screws provided a greater relative gain in stability than with facet screws. The interpretation is explained through a model describing the location of fixation with respect to the center-of-rotation of the vertebral bodies. In lateral bending and axial rotation, bilateral pedicle screw constructs were stiffer than transfacet pedicle screw constructs as a trend. INTERPRETATION Transfacet pedicle screws provided similar fixation to bilateral pedicle screws in primary and circumferential fixations during flexion-extension. In the other modes, transfacet screw rigidity is, on average, less than bilateral pedicle screws when used alone, but with the addition of other anterior instrumentation the differences are minimized. Therefore, facet screws are warranted based on the surgical effect desired, and in the presence of additional anterior fixation.

A novel crossed rod configuration incorporating translaminar screws for occipitocervical internal fixation: an in vitro biomechanical study

BACKGROUND CONTEXT Occipitocervical (OC) spinal instrumentation involving the axis (C2) entails the use of transarticular screws through C1-C2 or lateral mass screws at C1 and pedicle screws at C2 to achieve fusion. Because of the anatomical complexity, interpatient anomalous variation, and danger to the vertebral artery injury, there has been an increased interest in alternate sites for fixation. Recent studies have involved the placement of screws bilaterally into the C2 lamina. Several biomechanical studies have been carried out to evaluate the performance of C2 translaminar screws (TLSs). PURPOSE The aim of the study was to compare the biomechanics of an OC2 rigid construct using C2 pedicle screws and C2 TLSs. Also, this study included a new construct in which the OC2 fixation was carried out by connecting rods to the contralateral TLS. STUDY DESIGN Human cadaveric cervical spines were tested in an in vitro biomechanical flexibility experiment to investigate the biomechanical stability provided by a novel crossed rod (CR) configuration incorporating TLSs for OC2 internal fixation. METHODS Seven fresh human cadaver occipitocervical spines (occiput-C3) were tested by applying pure moments of ±1.5 Nm. After intact specimen testing, an occipital plate was implanted. Each specimen was then tested in the following modes: bilateral pedicle screws (BPSs) and rods at C2; TLSs at C2 with rods in parallel configuration (TLS+parallel rod); and TLSs at C2 with rods in crossed configuration (TLS+CR). OC2 range of motion (ROM) for each construct was obtained by applying pure moments in flexion-extension, lateral bending, and axial rotation. RESULTS All three instrumented constructs significantly reduced ROM in all physiological planes when compared with the intact spine. The BPS construct similarly reduced ROM when compared with both the translaminar constructs. There was no significant difference in ROM between the translaminar constructs in all loading modes. CONCLUSIONS A cadaveric model was used to investigate the stability offered by a novel CR construct by using TLS fixation in an OC2 fusion construct. The results were compared with BPS fixation. All three constructs significantly decreased motion as compared with the intact state. There was no statistically significant difference in flexibility among any of the constructs. The novel CR construct provides as much stability as traditional constructs and may be a viable alternative for clinical use.

Could junctional problems at the end of a long construct be addressed by providing a graduated reduction in stiffness? A biomechanical investigation.

Study Design. The effect of long, rigid fixation on adjacent level hypermobility was investigated in a human cadaver model with and without a transitional posterior dynamic stabilization (PDS) device placed at the last caudal level. Objective. To evaluate if PDS devices are useful in the setting of spinal deformities to restore increased adjacent level motions, which occur in long constructs. The hypothesis is that load-sharing benefits of these devices will be most suitable in long constructs and may reduce thoracolumbar junctional effects. The PDS device evaluated has a compressive spacer and flexion-dampening bumper. Summary of Background Data. Mechanical factors such as excessive mobility, increased disc height due to instrumentation, and abnormal loading are thought to accentuate distal level problems, which occur in extended instrumentation. Specifically adjacent level degeneration and distal junctional kyphosis are known to occur in these cases. Methods. Seven cadaver spines were tested from T7 to L3. Long instrumentation was applied in 2 rigid groups, R1: Rigid (T8–L2) and R2: Rigid (T8–L1), and PDS to the last caudal level of each, RP1: Rigid (T8–L1) + PDS (L1–L2), and RP2: Rigid (T8–T12) + PDS (T12–L1). Range of motion was evaluated at surgical and distal adjacent levels after displacement controlled loading in a spine tester. Results. Distal adjacent level motion was increased after 5- and 6-level rigid fixation in flexion-extension, lateral bending, and axial rotation. Most of the increases were seen in axial rotation and lateral bending. Replacing the last caudal instrumented level with the PDS test device was able to alleviate hypermobile conditions of the adjacent noninstrumented level, closer to intact (24%, 12% reduction in RP2, RP1, respectively). Conclusion. Reduction of hypermobility caused by extended arthrodesis may represent a new and ideally suited function for PDS devices. Mechanically, the devices were seen to kinematically restore abnormal distal motion, especially with placement of the PDS at the thoracolumbar junction.

A comparative biomechanical study of a novel integrated plate spacer for stabilization of cervical spine: An in vitro human cadaveric model

BACKGROUND Integrated plate-spacer may provide adequate construct stability while potentially lowering operative time, decreasing complications, and providing less mechanical obstruction. The purpose of the current study was to compare the biomechanical stability of an anatomically profiled 2-screw integrated plate-spacer to a traditional spacer only and to a spacer and anterior cervical plate construct. In addition, the biomechanical stability of 2-screw integrated plate-spacer was compared to a commercially available 4-screw integrated plate-spacer. METHODS Two groups, each of nine cervical cadaver spines (C2-C7), were tested under pure moments of 1.5Nm. Range of motion was recorded at C5-C6 in all loading conditions (flexion, extension, lateral bending, and axial rotation) for the following constructs: 1) Intact; 2) 2-screw or 4-screw integrated plate-spacer; 3) spacer and anterior cervical plate; and 4) spacer only. FINDINGS All fusion constructs significantly reduced motion compared to the intact condition. Within the instrumented constructs, spacer and anterior cervical plate, 2-screw and 4-screw integrated plate-spacer resulted in reduced motion compared to the spacer only construct. No significant differences were found in motion between any of the instrumented conditions in any of the loading conditions. INTERPRETATION The application of integrated plate-spacer for anterior cervical discectomy and fusion is based on several factors including surgical ease-of-use, biomechanical characteristics, and surgeon preference. The study suggests that integrated plate-spacer provide biomechanical stability comparable to traditional spacer and plate constructs in the cervical spine. Clinical studies on integrated plate spacer devices are necessary to understand the performance of these devices in vivo.

Biomechanical Stability of a Posterior-Alone Fixation Technique After Craniovertebral Junction Realignment

OBJECTIVE The aim of the current study was to investigate the biomechanical stability and fixation strength provided by a posterior approach reconstruction technique to realign the craniovertebral junction. METHODS We tested seven human cadaver occipito-cervical spines (occiput-C4) by applying pure moments of ± 1.5 Nm on a spine tester. Each specimen was tested in the following modes: 1) intact; 2) injured; 3) spacers alone at C1-C2 articulation (S); 4) spacers plus C1-C2 Posterior Instrumentation (S+PI); and 5) spacers plus C1-C2 posterior instrumentation plus midline wiring (S+PI+MLW). C1-C2 range of motion for each construct was obtained in flexion-extension, lateral bending, and axial rotation. RESULTS In all the loading modes, S, S+PI, and S+PI+MLW constructs significantly reduced range of motion compared with the intact and injured condition (P < 0.05). There was no statistical difference between any of the three instrumentation constructs (P > 0.05). CONCLUSIONS This study investigated the biomechanics of the posterior approach technique for realignment of the craniovertebral junction and also made comparisons with additional posterior fixations. The stand-alone spacers were stable in all three loading modes. Posterior instrumentation increased the stability as compared to stand-alone spacers. The third point of fixation, carried out by using midline wiring, increased the stability further. However, there was not much difference in the stability imparted with the midline wiring versus without. The present study highlights the biomechanics of this novel concept and reaffirms the view that distraction of the C1-C2 articular facets and direct articular joint atlantoaxial fixation would be an ideal method of management of basilar invagination.

Spinal instrumentation after complete resection of the last lumbar vertebra: an in vitro biomechanical study after L5 spondylectomy.

Study Design. Human cadaveric ilio-lumbosacral spines were tested in an in vitro biomechanical flexibility experiment to investigate the biomechanical stability provided by four different types of spinal reconstruction techniques after spondylectomy of the L5 vertebral body. Objective. To compare the biomechanical stability provided by four reconstruction methods after L5 spondylectomy. Summary of Background Data. Clinical studies have shown that total spondylectomy of the L5 vertebral body presents a challenging scenario for spinal reconstruction. Biomechanical studies on spinal reconstruction after total spondylectomy have been performed at the thoracolumbar junction. However, there have been no biomechanical studies after L5 spondylectomy. Methods. Seven cadaveric lumbosacral spines (L2-S1) with intact ilium were used. After intact testing, spondylectomy of the L5 vertebra was performed and the spine was reconstructed using an expandable cage for anterior column support. Supplementary fixation was performed as a sequential order of: (1) bilateral pedicle screws at L4-S1 (SP), (2) anterior plate and bilateral pedicle screws at L4-S1 (ASP), (3) bilateral pedicle screws at L3-S1 and iliac screws (MP), and (4) anterior plate at L4-S1, bilateral pedicle screws at L3-S1 and iliac screws (AMP). Range of motion (ROM) for each construct was obtained by applying pure moments in flexion, extension, lateral bending, and axial rotation. Results. In flexion, extension and lateral bending all the instrumented constructs significantly decreased (P < 0.05) the range of motion (ROM) compared to intact. In axial rotation, only the circumferential support constructs (ASP, AMP) provided significantly decreased (P < 0.05) ROM, whereas posterior instrumentations alone (SP, MP) were comparable to intact spines. Conclusion. After L5 spondylectomy, the L4-S1 cage with posterior short segment instrumentation provides stability in lateral bending that is not further increased by adding L3 pedicle-iliac screws and L4-S1 anterior plate. However, an anterior L4-S1 plate provides additional stability in flexion, extension, and axial rotation.

Axial spondylectomy and circumferential reconstruction via a posterior approach.

BACKGROUND Spinal metastases of the second cervical vertebra are a subset of tumors that are particularly difficult to address surgically. Previously described techniques require highly morbid circumferential dissection posterior to the pharynx for resection and reconstruction. OBJECTIVE To perform a biomechanical analysis of instrumented reconstruction configurations used after axial spondylectomy and to demonstrate safe use of a novel construct in a patient case report. METHODS Several different published and novel reconstruction configurations were inserted into 7 occipitocervical spines that underwent axial spondylectomy. A biomechanical analysis of the stiffness of the constructs in flexion and extension, lateral bending, and rotation was performed. A patient then underwent a posterior-only approach for axial spondylectomy and circumferential reconstruction. RESULTS Biomechanical analysis of different constructs demonstrated that anterior column reconstruction with bilateral cages spanning the C1 lateral mass to the C3 facet in combination with occipitocervical instrumentation was superior in flexion-extension and equivalent in lateral bending and rotation to currently used constructs. The patient in whom this construct was placed via a posterior-only approach for axial spondylectomy and instrumentation remained at neurological baseline and demonstrated no recurrence of local disease or failure of instrumentation to date. CONCLUSION When C1 lateral mass to C3 facet bilateral cage plus occipitocervical instrumentation is compared with existing anterior and posterior constructs, this novel reconstruction is biomechanically equivalent if not superior in performance. In a patient, the posterior-only approach for C2 spondylectomy with the novel reconstruction was safe and durable and avoided the morbidity of the anterior approach.

A comparative biomechanical study of traditional and in-line plating systems following immediate stabilization of single and bi-level cervical segments

BACKGROUND Anterior cervical plate fixation has gained widespread acceptance for treatment of cervical spine pathologies by stabilizing the segment and enhancing fusion rates. While it is generally accepted that multiple fusion levels benefit from plating, few studies have compared plate designs. Wider plates can increase surgical complications and cost and are, therefore, not indicated unless biomechanical benefits exist. In this study, a cervical cadaver model is subjected to physiological loads and stabilized with in-line one-screw, and traditional two-screw per vertebral body plates. METHODS Three groups of eight fresh frozen human cadaver cervical spines (C2-C7) were tested by applying pure moments of 1.5 Nm. Motion was obtained at C5-C6, and C4-C5/C5-C6 for single-level and bi-level experiments, respectively, in flexion-extension, lateral bending and axial rotation. Specimens were tested, 1) intact, 2) injured (anterior discectomy), 3) with interbody fusion spacer, 4) in-line one-screw plate+spacer, and 5) two-screw plate+spacer, using four available plate brands. FINDINGS Single-level plating with interbody spacer restricted range-of-motion with respect to the spacer-alone construct in flexion-extension, regardless of one-screw or two-screw plate design, or brand. Similar behavior was seen in axial rotation, but not in lateral bending, where significance reductions in motion were achieved only with respect to the intact spine, not the interbody spacer group. In bi-level experiments all plate types restricted range-of-motion below spacer-alone levels in all loading modes. INTERPRETATION Anterior plating should be selected based on surgical requirements, as a wide (two-screw) over a narrow (one-screw) plating profile does not appear to provide a tangible biomechanical benefit.

The effect of anterior longitudinal ligament resection on lordosis correction during minimally invasive lateral lumbar interbody fusion: Biomechanical and radiographic feasibility of an integrated spacer/plate interbody reconstruction device

Background: Lateral lumbar interbody fusion is powerful for correcting degenerative conditions, yet sagittal correction remains limited by anterior longitudinal ligament tethering. Although lordosis has been restored via ligament release, biomechanical consequences remain unknown. Investigators examined radiographic and biomechanical of ligament release for restoration of lumbar lordosis. Methods: Six fresh‐frozen human cadaveric spines (L3–S1) were tested: (Miller et al., 1988) intact; (Battie et al., 1995) 8 mm spacer with intact anterior longitudinal ligament; (Cho et al., 2013) 8 mm spacer without intact ligament following ligament resection; (Galbusera et al., 2013) 13 mm lateral lumbar interbody fusion; (Goldstein et al., 2001) integrated 13 mm spacer. Focal lordosis and range of motion were assessed by applying pure moments in flexion‐extension, lateral bending, and axial rotation. Findings: Cadaveric radiographs showed significant improvement in lordosis correction following ligament resection (P < 0.05). The 8 mm spacer with ligament construct provided greatest stability relative to intact (P > 0.05) but did little to restore lordosis. Ligament release significantly destabilized the spine relative to intact in all modes and 8 mm with ligament in lateral bending and axial rotation (P < 0.05). Integrated lateral lumbar interbody fusion following ligament resection did not significantly differ from intact or from 8 mm with ligament in all testing modes (P > 0.05). Interpretation: Lordosis corrected by lateral lumbar interbody fusion can be improved by anterior longitudinal ligament resection, but significant construct instability and potential implant migration/dislodgment may result. This study shows that an added integrated lateral fixation system can significantly improve construct stability. Long‐term multicenter studies are needed. HighlightsSagittal correction is limited by anterior longitudinal ligament tethering.Lateral spacer with anterior longitudinal ligament release doubled lordosis.Integrated spacer limited destabilization after ligament release.

A novel lateral lumbar integrated plate-spacer interbody implant: in vitro biomechanical analysis.

BACKGROUND CONTEXT Lateral spacers (LSs) are the standard of care for a lateral lumbar interbody fusion. However, various types of fixation, such as bilateral pedicle screws (BPSs), unilateral pedicle screws (UPSs), bilateral facet screws (BFSs), and lateral plates (LPs) have been reported to increase the stability of LSs. The biomechanics of a novel lateral interbody implant, which is an interbody spacer with an integrated plate and two bone screws (lateral integrated plate-spacer [IPS-L]), has not been investigated yet. PURPOSE To compare the biomechanical stability of IPS-L and LS with and without supplemental instrumentation. STUDY DESIGN Human lumbar cadaveric study evaluating the biomechanical stability of an IPS-L. METHODS Each of the six (L2-L5) spines was sequentially tested in intact; IPS-L; IPS-L+UPS; IPS-L+BPS; IPS-L+BFS; LS+BFS; LS+UPS; LS+BPS; LS; and LS+LP, using a load-control protocol in which a ±8 Nm moment was applied, for three cycles each, in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Data results were obtained from the third cycle. RESULTS The IPS-L construct significantly reduced the range of motion (ROM) by 75% in FE, 70% in LB, and 57% in AR, compared with intact. Lateral integrated plate-spacer demonstrated similar biomechanical stability as LS+LP, and higher stability than the LS-alone construct, but the difference was not statistically significant. CONCLUSIONS The IPS-L evaluated in the present study demonstrated equivalent biomechanical stability compared with standard lateral interbody fusion constructs. The addition of BPSs to the IPS-L showed significant reduction in ROM in FE, and the addition of BFSs showed significant reduction in ROM in FE and AR, compared with the integrated plate-spacer alone construct. The IPS-L with supplemental fixation may be a viable option for lateral interbody fusion. Long-term clinical studies are further required to confirm these results.