Screw-rod Constructs Research Articles

Biomechanical differences of three cephalic fixation methods for patients with basilar invagination and atlantoaxial dislocation in the setting of congenital atlas occipitalization: a finite element analysis

Background contextIn cases of basilar invagination-atlantoaxial dislocation (BI-AAD) complicated by atlas occipitalization (AOZ), the approach to cranial end fixation has consistently sparked debate, generally falling into two categories: C1–C2 fixation and occipitocervical fixation. Several authors believe that C1–C2 fixation carries a lower risk of fixation failure than occipitocervical fixation. PurposeTo study the biomechanical differences among 3 different cranial end fixation methods for BI-AAD with AOZ. Study designThis was a finite element analysis. Patient sampleA 35-year-old female patient diagnosed with congenital BI-AAD and AOZ. Outcome measuresrange of motion (ROM), peak von Mise stress (PVMS), cage micro-subsidence, cage micro-slippage MethodFour finite element models were constructed, including unstable group (BI-AAD with AOZ), C1 lateral mass screw group, occipital plate group, occipitocervical rod group. The flexion and extension (FE), lateral bending (LB) as well as axial rotation (AR) were simulated under a torque of 1.5 Nm. Parameters include C1–C2 ROM, PVMS on screw-rod construct, cage micro-subsidence, cage micro-slippage. ResultsThe ROM of the C1 lateral mass screw group was smaller than that of the other fixation groups in LB and AR, but not FE. Compared with the occipitocervical rod group, the ROM in LB and AR of the occipital plate group was higher, but not in FE. The PVMS of C1 lateral mass screw group was significantly higher than that of the other groups. The ROM and PVMS of the occipitocervical rod group were in between the other 2 groups. Regarding the screws at the cranial end, the PVMS of the 4-screw occipitocervical rod group was significantly lower than that of the other groups. In general, the cage micro-motion follows the ascending order: C1 lateral mass group < occipitocervical rod group < occipital plate group. ConclusionIn cases of BI-AAD with AOZ, the C1 lateral mass screw group provided the least ROM and cage micro-motion, but the screw-rod PVMS was the largest. The advantage of occipital plate fixation lies in the lowest screw-rod PVMS, but the ROM and cage micro-motion is the highest. Four-screw fixation at the cranial end of occipitocervical rod group helps to reduce the PVMS and may prevent screw failure at the cranial end.

A comparative cadaveric biomechanical study of bilateral FacetFuse® transfacet pedicle screws versus bilateral or unilateral pedicle screw-rod construct.

Achieving optimal immediate stability is crucial in lumbar fusion surgeries. Traditionally, four pedicle screws have been utilized to provide posterior stability at the L5-S1 level. However, the use of bilateral transfacet pedicle screws (TFPS) as an alternative construct has shown promising results in terms of biomechanical stability. This research paper investigates the biomechanical stability of TFPS with a lag design in comparison to equivalent-sized unilateral or bilateral fully threaded pedicle screw-rod (PSR) constructs at the L5-S1 disc level. The study assesses the immediate stability achieved by these constructs which have clinical implications in achieving lumbar segment fusion. We hypothesized that bilateral TFPS will yield immediate lumbar fixation that is comparable to unilateral or bilateral PSR constructs. Cadaveric biomechanical testing was conducted in vitro to evaluate the stability of posterior fixation using bilateral TFPS (FacetFuse®, LESSpine, Burlington, MA, USA), bilateral and unilateral PSR (PedFuse Return, LESSpine, Burlington, MA, USA) constructs measuring 5.0 mm × 40 mm. A comprehensive analysis of range of motion (ROM) and stability under various loading conditions was performed to a maximum of 7.5 Nm. The constructs were assessed for their ability to provide immediate stability at the L5-S1 disc level. Fourteen specimens were analyzed with an average age of 53.14±10.99 years and comparable bone mineral density. TFPS demonstrated a reduced ROM that was notably lower than that of unilateral PSR in all loading modes and was comparable to bilateral PSR, especially in extension and axial rotation (AR). The unilateral and bilateral PSR groups differed notably in lateral bending (LB) and AR. Bilateral TFPS demonstrated superior immediate stability than unilateral PSR and was an equivalent substitute to bilateral PSR constructs at the L5-S1 disc level. Further clinical investigations are necessary to validate these results and ascertain the long-term outcomes and advantages associated with the use of bilateral TFPS as an alternative construct. Our findings showed that bilateral TFPS could potentially reduce the number of required pedicle screws while achieving comparable stability in lumbar fusion procedures.

Temporal Shape Changes of Pedicle Screw-rod Constructs After Lumbar Interbody Fusion.

Retrospective multicenter study. To examine the shape change of screw-rod constructs over time after short-segment lumbar interbody fusion and to clarify its relationship to clinical characteristics. No study has focused on the shape change of screw-rod constructs after short-segment fusion and its clinical implications. One hundred eight patients who had single-level lumbar interbody fusion with pedicle screws and cages were enrolled. Three-dimensional (3D) images of screw-rod constructs were generated from baseline CT on the day after surgery and follow-up CT and were superposed on the right and left side, respectively, using the iterative closest point algorithm. The shape change was quantitatively assessed by computing the median distance between the 3D images, which was defined as the shape change value. Among the 5 time-course categories of follow-up CT (≤1, 2-3, 4-6, 7-12, and ≥13 months), the shape change values were compared. The relationships between the shape change values and clinical characteristics, such as age, CT-derived vertebral bone mineral density, screw and rod materials, and postoperative interbody fusion status, cage subsidence, and screw loosening, were evaluated. A total of 237 follow-up CTs were included (≤1 [34 scans], 2-3 [33 scans], 4-6 [80 scans], 7-12 [48 scans], and ≥13 months [42 scans]) because many patients underwent multiple follow-up CTs. There were significant differences in shape change values among the time-course categories ( P <0.001 in Kruskal-Wallis test). Most shape changes occurred within 6 months postoperatively, with no significant changes observed at 7 months or more. There were no significant relationships between the shape change values and each clinical characteristic. The temporal shape changes of screw-rod constructs following short-segment lumbar interbody fusion progressed up to 6 months after surgery but not significantly thereafter.

Establishing finite element model credibility of a pedicle screw system under compression-bending: An end-to-end example of the ASME V&V 40 standard

Computational modeling and simulation (CM&S) is a key tool in medical device design, development, and regulatory approval. For example, finite element analysis (FEA) is widely used to understand the mechanical integrity and durability of orthopaedic implants. The ASME V&V 40 standard and supporting FDA guidance provide a framework for establishing model credibility, enabling deeper reliance on CM&S throughout the total product lifecycle. Examples of how to apply the principles outlined in the ASME V&V 40 standard are important to facilitating greater adoption by the medical device community, but few published examples are available that demonstrate best practices. Therefore, this paper outlines an end-to-end (E2E) example of the ASME V&V 40 standard applied to an orthopaedic implant. The objective of this study was to illustrate how to establish the credibility of a computational model intended for use as part of regulatory evaluation. In particular, this study focused on whether a design change to a spinal pedicle screw construct (specifically, the addition of a cannulation to an existing non-cannulated pedicle screw) would compromise the rod-screw construct mechanical performance. This question of interest (?OI) was addressed by establishing model credibility requirements according to the ASME V&V 40 standard. Experimental testing to support model validation was performed using spinal rods and non-cannulated pedicle screw constructs made with medical grade titanium (Ti-6Al-4V ELI). FEA replicating the experimental tests was performed by three independent modelers and validated through comparisons of common mechanical properties such as stiffness and yield force. The validated model was then used to simulate F1717 compression-bending testing on the new cannulated pedicle screw design to answer the ?OI, without performing any additional experimental testing. This E2E example provides a realistic scenario for the application of the ASME V&V 40 standard to orthopedic medical device applications.

Decreasing implant load indicates spinal fusion when measured continuously

Reliable and timely assessment of bone union between vertebrae is considered a key challenge after spinal fusion surgery. Recently, a novel sensor concept demonstrated the ability to objectively assess posterolateral fusion based on continuous implant load monitoring. The aim of this study was to investigate systematically the concept in a mono-segmental fusion model using an updated sensor setup.Three sheep underwent bilateral facetectomy at level L2-L3 and L4-L5. The segments were stabilized using two unconnected pedicle-screw-rod constructs per level. Sensing devices were attached to the rods between each pedicle screw pair and the loads were continuously monitored over 16 weeks. After euthanasia, the spines were biomechanically tested for their range of motion and high-resolution CT scans were performed to confirm the fusion success.After an initial increase in implant load until reaching a maximum (100 %) at approximately week 4, eleven out of twelve sensors measured a constant decrease in implant load to 52 ± 9 % at euthanasia. One sensor measurement was compromised by newly forming bone growing against the sensor clamp. Bridging bone at each facet and minor remnant segmental motion (<0.7°) confirmed the fusion of all motion segments.Data obtained by continuous measurement of implant loading of spinal screw-rod constructs enables objective monitoring of spinal fusion progression. The sensor concept provides valuable real-time information, offering quantifiable data as an alternative to traditional imaging techniques. However, the design of the current sensor concept needs to be matured, tailored to, and validated for the human spine.

Shape changes of screw-rod constructs over time following short-segment lumbar interbody fusion

Shape changes of screw-rod constructs over time following short-segment lumbar interbody fusion

Metal-Free Cortico-Pedicular Device for Supplemental Fixation in Lumbar Interbody Fusion

Pedicle screw fixation is a commonly utilized adjunct for lumbar interbody fusion, yet risks include screw malposition, pullout, loosening, neurovascular injury, and stress transfers leading to adjacent segment degeneration. This report describes the preclinical and initial clinical results of a minimally invasive, metal-free cortico-pedicular fixation device used for supplemental posterior fixation in lumbar interbody fusion. Safety of arcuate tunnel creation was evaluated in cadaveric lumbar (L1-S1) specimens. A finite element analysis study evaluated clinical stability of the device to pedicular screw-rod fixation at L4-L5. Preliminary clinical results were assessed by analysis of Manufacturer and User Facility Device Experience database complications, and 6-month outcomes in 13 patients treated with the device. Among 35 curved drill holes in 5 lumbar specimens, no breaches of the anterior cortex were identified. The mean minimum distance from the anterior surface of the hole to the spinal canal ranged from 5.1mm at L1-L2 to 9.8mm at L5-S1. In the finite element analysis study, the polyetheretherketone strap provided comparable clinical stability and reduced anterior stress shielding compared to the conventional screw-rod construct. The Manufacturer and User Facility Device Experience database identified 1 device fracture with no clinical sequelae among 227 procedures. Initial clinical experience showed a 53% decrease in pain severity (P= 0.009), a 50% decrease in Oswestry Disability Index (P < 0.001), and no device-related complications. Cortico-pedicular fixation is a safe and reproducible procedure that may address limitations of pedicle screw fixation. Longer term clinical data in large clinical studies are recommended to confirm these promising early results.

Freehand Pre-drilling Technique for Lateral Mass Screw Fixation Is More Efficient and Reliable Versus Sequential Drilling Technique: A Sawbone Analysis.

Background Since posterior cervical fixation with lateral mass screws was introduced in 1979, multiple techniques have been described in the literature. However, no study to date has determined whether pre-drilling all lateral masses prior to screw insertion has a benefit over the traditional sequential drilling and screw insertion on the alignment of the screw-rod construct. This study sought to determine the efficacy and efficiency in achieving alignment with a novel pre-drilling technique compared to the traditional sequential drilling technique. The authors hypothesized that the novel pre-drilling technique could be applied more quickly and precisely than the traditional sequential drilling technique. Methods Eight cervical spine sawbones models were utilized to place 64 lateral mass screws by two surgeons. The pre-drilling technique was utilized to place 32 screws in four models, and the sequential drilling technique was utilized to place the 32 screws in the remaining four models. In the traditional sequential drilling technique, each lateral mass underwent screw tract preparation and insertion before proceeding to the subsequent vertebra. In the pre-drilling technique, all lateral masses were marked and drilled sequentially before screw placement. CT imaging with 3D reconstructions was generated for all models. Variability in screw placement and time taken to fully instrument the models were compared. Results The mean time to completion of the pre-drilling technique was 337 ± 22 seconds compared to 490 ± 22 seconds with the traditional technique (p<0.01). There was a significantly higher variability in the coronal plane within the traditional group between C5 and C6 compared to other adjacent vertebrae (p<0.05). There was no significant difference in the start point variability and the overall tightness of line fit between the techniques. Conclusions Our study suggests that a novel pre-drilling technique for lateral mass screw insertion may be more efficient and reliable than the traditional sequential drilling technique. In addition, this technique may reduce the need for rod contouring or additional implants to optimize the alignment of cervical instrumentation. However, further clinical studies are necessary to validate the potential clinical and radiologic benefits of this described technique.

Biomechanical comparison of different rod-to-rod connectors to a conventional titanium- and cobalt chromium posterior spinal fixation system

IntroductionSeveral types of rod-to-rod connectors are available for the extension of spinal fixation systems. However, scientific literature regarding the mechanical performance of different rod-to-rod connector systems is lacking. Research questionThe goal of this study was to evaluate the mechanical characteristics of axial and lateral rod connectors in comparison to a conventional pedicle screw rod (titanium and cobalt chromium) construct. Material and methodSix types of instrumentations were investigated in a standardized test model to quantify the mechanical differences: 1: titanium rod; 2: titanium rod with axial connector; 3: titanium rod with lateral connector; 4: cobalt chromium rod; 5: cobalt chromium rod with axial connector; 6: cobalt chromium rod with lateral connector. All groups were tested in static compression, static torsion and dynamic compression and statistically compared regarding failure load and stiffness. ResultsIn static compression loading, the use of connectors increased the construct stiffness, but unaffected the yield load. The use of a cobalt chromium rod significantly increased by approximately 40% the yield load and stiffness in comparison to the titanium rod configurations. Under dynamic compression, a similar or higher fatigue strength for all tested groups in comparison to the titanium rod configuration was evaluated, with the exception of titanium rod with axial connector. ConclusionBiomechanically, using rod connectors is a secure way for the extension of a construct and is mechanically equal to a conventional screw rod construct. However, in clinical use, attention should be paid regarding placement of the connectors at high loaded areas.

An alternative method to evaluate lumbar interbody fusion status focusing on position change of screw-rod constructs.

The evaluation of lumbar interbody fusion status is generally subjective and may differ among raters. The authors examined whether the assessment of position change of screw-rod constructs could be an alternative method for the evaluation of fusion status. Sixty-three patients undergoing lumbar interbody single-level fusion were retrospectively reviewed. Three-dimensional images of screw-rod constructs were created from baseline CT examination on the day after surgery and follow-up CT examinations (3-5 months, 6-11 months, and ≥ 12 months) and superposed, with position change of screw-rod constructs being evaluated by the distance between the 3-dimensional images at baseline and follow-up. The evaluation was repeated twice to confirm the reproducibility. Fusion status on follow-up CT examinations was assessed by three raters, where inter-rater reliability was evaluated with Fleiss' kappa. The results of the fusion status were classified into fusion and incomplete fusion groups in each timing of follow-up CT examinations, where the amount of position change was compared between the two groups. The evaluation of position change was completely reproducible. The Fleiss' kappa (agreements) was 0.481 (69.4%). The medians of the amount of position change in fusion and incomplete fusion groups were 0.134 mm and 0.158 mm at 3-5 months (p = 0.21), 0.160 mm and 0.190 mm at 6-11 months (p = 0.02), and 0.156 mm and 0.314 mm at ≥ 12 months (p = 0.004). The assessment of position change of screw-rod constructs at 6 months or more after surgery can be an alternative method for evaluating lumbar interbody fusion status. • Lumbar interbody fusion status (satisfactory, incomplete, or failed) is associated with the quantification of position change of screw-rod in this study. • Reference values for the evaluation of position change in identifying interbody fusion status are provided. • Position change of screw-rod could be a supportive method for evaluating interbody fusion status.

The role of counter-torque holders in tightening of pedicle screw-rod constructs: a biomechanical study in a porcine model

BACKGROUND CONTEXTPedicle screw-rod assembly procedures following pedicle screw insertion include contouring and placing rods into screw tulips, introducing set screws into the tulip along the screw thread, applying a counter-torque holder and tightening all the set screws clockwise. Even if an appropriate pedicle screw is implanted, screw dislodgement after tightening of the tulip and set screw is not uncommon. Pedicle wall violation resulting from excessive rotational force due to inadequate use of a counter-torque holder might be the reason. However, the strain change in the pedicle during tulip-set screw tightening and the role of counter-torque have never been investigated. PURPOSEThis study determined differences in the strain change in the outer and inner pedicle walls during tulip-set screw tightening; additionally, the influence of counter-torque on pedicle wall violation was elucidated. STUDY DESIGNA controlled biomechanical study; the strain values of outer and inner pedicle walls in cadaveric porcine L4-L5 vertebrae during tulip-set screw tightening with or without a counter-torque holder were measured. METHODSTwelve L4-L5 fresh-frozen porcine lumbar vertebrae were implanted with screw-rod constructs; the set screw was randomly locked into the tulip in the right L5, right L4, left L5 and left L4 testing groups. The maximal values from eight strain gauges (P-R-O: outer cortex of right pedicle in proximal vertebra; P-R-I: inner cortex of right pedicle in proximal vertebra; D-R-O: outer cortex of right pedicle in distal vertebra; D-R-I: inner cortex of right pedicle in distal vertebra; P-L-O: outer cortex of left pedicle in proximal vertebra; P-L-I: inner cortex of left pedicle in proximal vertebra; D-L-O: outer cortex of left pedicle in distal vertebra; D-L-I: outer cortex of left pedicle in proximal vertebra) for each specimen during tightening to 12 Nm were measured. RESULTSThe maximal strain values of the ipsilateral strain gauges in all testing groups were almost significantly higher when a counter-torque holder was not used than when one was used. The strain values in the adjacent pedicle of specimens without a counter-torque holder were significantly increased: P-R-O and P-R-I in the right L5 group; D-R-I in the right L4 group; P-L-I and P-L-O in the left L5 group; D-L-O and D-L-I in the left L4 group. CONCLUSIONSThe constraint effect of counter-torque during tulip-set screw tightening is necessary. Clockwise rotational force with a fragile lateral pedicle wall suggests that caution is required when using a counter-torque holder to tighten the right L5 and left L4 constructs. CLINICAL SIGNIFICANCEA counter-torque holder is important during tulip-set screw tightening; improper use may lead to adjacent pedicle wall violation, sequentially resulting in pedicle screw loosening.

The biomechanical fundamentals of crosslink-augmentation in posterior spinal instrumentation

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.

Fluoroscopy-Assisted C1-C2 Posterior Fixation for Atlantoaxial Instability: A Single-Center Case Series of 78 Patients.

Background and Objectives: Posterior C1–C2 fixation, with trans-articular screws (TAS) or screw-rod-construct (SRC), is the main surgical technique for atlantoaxial instability, and can be performed with a fluoroscopy-assisted free-handed technique or 3D navigation. This study aimed to evaluate complications, radiological and functional outcome in patients treated with a fluoroscopy-assisted technique. Materials and Methods: A single-center consecutive cohort study was conducted of all adult patients who underwent posterior C1–C2 fixation, using TAS or CRS, between 2005–2019. Results: Seventy-eight patients were included, with a median follow-up time of 6.8 years. Trauma was the most common injury mechanism (64%), and cervicalgia the predominant preoperative symptom (88%). TAS was used in 33%, and SRC in 67% of cases. Surgery was associated with a significant reduction in cervicalgia (from 88% to 26%, p < 0.001). The most common complications were vertebral artery injury (n = 2, 2.6%), and screw malposition (n = 5, 6.7%, of which 2 were TAS and 3 were SRC). No patients deteriorated in their functional status following surgery. Conclusions: Fluoroscopy-assisted C1–C2 fixation with TAS or SRC is a safe and effective treatment for atlantoaxial instability, with a low complication rate, few surgical revisions, and pain relief in the majority of the cases.

Radiolucent Zone around Screws is Associated with Position Change of Screw-rod Constructs.

Aradiologic assessment method to measure position change of screw-rod constructs over time by superposing the 3‑dimensional images assists in quantitative evaluation of screw loosening. We investigated the association between position change and radiolucent zone that was commonly used for diagnosing screw loosening. In this study 101 patients who underwent lumbar fusion were reviewed. Patient characteristics included age, sex, indications for surgery, number of fused levels, surgical procedures, and timing of follow-up computed tomography (CT, 1-5months, 6-11months, and ≥ 12months). The Hounsfield unit values of L1 vertebra on preoperative CT were measured, and the radiolucent zone on each follow-up CT was evaluated. Using baseline CT on the day after surgery and follow-up CT, 3‑dimensional images of screw-rod constructs were generated and superposed. Position change was assessed by the median of the distances between the 3‑dimensional images at baseline and follow-up using the automated measurement method. Patient characteristics, the Hounsfield unit values of L1, and the amount of position change were categorized into the radiolucent zone presence and absence groups and compared. The medians of position change were 0.281 mm and 0.136 mm in the radiolucent zone presence and absence groups, respectively (P < 0.001 by Mann-Whitney U-test). The area under the curve for position change in identifying radiolucent zone was 0.846; the cut-off value was 1.76 mm. In multivariable analysis, position change was independently associated with radiolucent zone (adjusted odds ratio per 0.1 mm, 2.80, 95% confidence interval 1.70-4.61). Radiolucent zone was associated with position change of screw-rod constructs.

The outcome of type 1 pelvic resection and reconstruction with pedicle screw-rod system without bone grafting in malignant pelvic tumour: A case series and short term review.

There is no consensus regarding the reconstruction method for type 1 resections around the pelvis. Various methods are currently used, such as resection without reconstruction, bone graft (autologous, recycled, allograft) with simple fixation, and pedicle screw-rod fixation with or without bone grafting. We aim to study the outcome of pedicle screw-rod reconstruction without bone grafting in type 1 pelvic resections involving sacroiliac joint to show that pedicle screw-rod construct alone is stable and has low risk of failure. This is a retrospective review of eight patients who underwent type 1 resection of malignant pelvic tumours and reconstruction with a pedicle screw-rod system between 2011 and 2018. All patients who underwent type 1 resection and reconstruction with pedicle screw without bone grafting were included into this study. We reported their clinical (complication and radiological outcome), oncological (local recurrence and metastasis), and functional outcome based on Musculoskeletal Tumour Society Score (MSTS) and The Toronto Extremity Salvage Score (TESS) at their last follow-up. Eight patients were recruited into the study. The mean follow-up period was 58.5months (range: 40 - 121months). There were three postoperative complications in three different patients: superficial infection, surgical hernia with ipsilateral femoral avascular necrosis (AVN), and femoral nerve injury. At the end of the study period, one patient passed away due to disease progression, one patient was alive with disease, and the rest were disease-free. Mean MSTS score during last follow-up was 77.1% (range: 66.7% - 93.3%), while mean TESS score was 75.6% range (63.3% - 80.2%). There were no cases of implant failure. Type 1 pelvic reconstruction with a pedicle screw-rod system is stable without a concurrent biological reconstruction, and it is feasible, with few complications, and an excellent functional outcome.

Posterior spinal instrumentation and decompression with or without cross-link?

BackgroundPosterior 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. MethodsSeventeen 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). ResultsROM 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. ConclusionCL 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.

Preoperative Radiographic Prediction Tool for Early Postoperative Segmental and Lumbar Lordosis Alignment After Transforaminal Lumbar Interbody Fusion.

ObjectiveTransforaminal lumbar interbody fusion (TLIF) is a common approach and results in varying degrees of lordosis correction. The purpose of this study is to determine preoperative radiographic spinopelvic parameters that predict change in postoperative segmental and lumbar lordosis after TLIF.Materials & MethodsThis study is a single surgeon retrospective review of one-level and two-level TLIFs from L3-S1. All patients underwent bilateral facetectomies, 10 mm TLIF cage (non-lordotic) insertions, and bilateral pedicle screw-rod construct placements. Pre- and post-operative X-rays were assessed for preoperative segmental lordosis (SL), lumbar lordosis (LL), and pelvic incidence (PI). Univariate and multi-predictor linear regression analyses were performed to determine the relationships between preoperative radiographic findings and change in early postoperative segmental and lumbar lordosis.ResultsNinety-seven patients contributing 128 intervertebral segments were examined. The mean change in SL after TLIF was 7.3 (range: 0.10-28.9°, SD 6.39°). The mean change in LL after TLIF was 5.5˚ (range: -14.8-39.2°, standard deviation (SD) 7.16°). Greater preoperative LL predicted less postoperative LL correction, while greater preoperative PI predicted more postoperative SL and LL correction. Greater anterior disk height was noted to be associated with a decreased change in SL (∆SL). An annular tear on preoperative magnetic resonance imaging (MRI) predicted a 2.7° decrease in ∆SL. A Schmorl's node on preoperative MRI predicted a 4.0° decrease in change in LL (∆LL).ConclusionsA greater preoperative lordosis and a lower spinopelvic mismatch lessen the potential for an increase in the postoperative SL and LL after a TLIF, which is likely due to a ‘ceiling’ effect of an otherwise optimized spinal alignment. A greater anterior disk height and the presence of an annular tear are associated with decreased ∆SL.

Local temperature elevation as a marker of spinal implant infection in an animal model

BackgroundTemperature elevation, a classic marker of infection and local temperature elevation, might be a useful predictor of early infection. However temperature measurement around the spine is not readily accessible. The purpose of this study was to explore whether a temperature sensing implant might reproducibly detect local temperature change associated with peri-implant wound infection, in a rabbit model. MethodsTwelve adult rabbits were implanted with a spinal screw-rod construct. Temperature probes were placed at the implantation site as well as at a separate scapular site away from the surgical site to serve as control. Animals were inoculated with S. aureus: group 1 (saline control), group 2 (low dose 1 × 102 CFU/site), group 3 (medium dose 1 × 104 CFU/site), and group 4 (high dose 1 × 106 CFU/site) and monitored for 7 days prior to euthanasia. ResultsThe scapular control temperature and implant site temperature in the non-infected animals remained similar throughout the study period. Both the scapular control and implant site temperatures were elevated in the infected animals compared to the non-infected animals. There was a statistically significant difference in the scapular control temperature and implant site temperature in all infected animals but not in the non-infected animals. Difference in temperature elevation between implant site and control scapular site were greatest for the animals with worst clinical appearance during the post-mortem evaluation. ConclusionsThis rabbit model demonstrates that local temperature measured in proximity to a spinal implant is elevated in the presence of infection with greater elevations associated with worse infections. Availability of an implantable temperature sensor may yield valuable information for the assessment and treatment of suspected spinal wound infection in the clinical setting.

The Feasibility of C1-C2 Screw-rod Fixation in the Children 5 Years of Age and Younger.

Atlantoaxial fixation is technically challenging in younger children. The C1-C2 screw-rod fixation technique is established for adults but limited data about the clinical and radiographical outcome for the treatment of children with 5 years of age or younger is available. All files of children who were consecutively treated for spinal disorders were reviewed. Inclusion criteria for further evaluation were: 0 to 5 years of age at initial procedure; detailed surgical report of a posterior C1-C2 fusion with mass lateral and pedicle screw-rod fixation as described by Harms; a minimum clinical and radiographical follow-up of 24 months. The postoperative and last follow-up computed tomography scan and radiographs were used to assess the positioning and stability of the C1-C2 screw-rod construct. Eleven patients (3 boys) with a mean age of 46 months (range: 8 to 66 mo) fulfilled inclusion criteria and were evaluated retrospectively. The mean clinical and radiographical follow-up was 79 months (range: 24 mo to 170 mo). The diagnosis was atlantoaxial rotatory dislocation (4 cases), C1-C2 instability with subluxation (3 cases), atlantoaxial dislocation and os odontoideum (1 case), type II odontoid fracture (1 case), traumatic odontoid epiphysiolysis (1 case), and traumatic rupture of the transverse ligament with C1 subluxation (1 case). Intraoperatively and postoperatively no new neurovascular or vascular complication occurred. C1 lateral mass screws were placed correctly in all cases. Twenty-two C2 pedicle screws were placed correctly (85.7%), and 3 screws showed penetration of the pedicle wall (14.3%). No implant revision, implant failure, and pseudarthrosis were noted. Loss of correction was noted in 1 patient with unilateral C1-C2 fixation and a repeated dorsal fusion procedures were performed. A repeat procedure for implant removal and segmental release was performed in 3 patients to increase the axial rotation of the head. The C1-C2 screw-rod fixation is a safe technique that achieves solid fixation of the atlantoaxial complex in young children with various disorders. The technique preserves the joint and allows for segmental release via implant removal.

Bicortical Short C2 Pars Screw Fixation for High-Riding Vertebral Artery Provided Sufficient Biomechanical Stability: A Finite Element Study.

Finite element analysis. To determine and compare the biomechanical stability of the bicortical short C2 pars screw fixation for high-riding vertebral artery (HRVA) with the C2 pedicle screw and C2 translaminar screw fixation in finite element models. Fixation of C2 is technically demanding in the case of HRVA. However, there is no consensus on the alternative technique for the C2 screw fixation for HRVA in the literature. A finite element model of the upper cervical spine (C0-C2) with HRVA had been developed. C1 pedicle screw was applied at C1 by using notching technique. Bicortical short C2 pars screws, C2 pedicle screws, and C2 translaminar screws were used in each model. Then a vertical load of 50 N and a 1.5 Nm torque were applied to the C0 to simulate flexion, extension, lateral bending, and axial rotation respectively. Compared with C2 pedicle screw fixation, the bicortical short C2 pars screw fixation increased the range of motion by -1.45%, 2.13%, 62.0%, and 22.0% under flexion, extension, lateral bending, and axial rotation, respectively. However, the C2 translaminar screw fixation increased the range of motion by 43.6%, 17.8%, 423.4%, and 19.9%, respectively. In terms of the peak von Mises stress, compared with C2 pedicle screw fixation, bicortical short C2 pars screw decreased 46.1%, 41.6%, 71.3%, and -12.5% under flexion, extension, lateral bending, and axial rotation, respectively; C2 translaminar screw decreased -2.66%, -4.87%, 73.0%, and -10.1%, respectively. For a patient with HRVA, bicortical short C2 pars screw fixation provides sufficient stability and exhibited a smaller von Mises distribution on the screw-rod construct, indicating it could be an effective C2 internal fixation method for HRVA to promote C1-C2 stability and avoid the vertebral artery injury.Level of Evidence: N/A.