Pullout of a lumbar plate with varying screw lengths

An in vitro biomechanical comparison of 2 fusion techniques, anterior lumbar interbody fusion (ALIF) and transforaminal lumbar interbody fusion (TLIF), on cadaveric human spines. To compare the immediate construct stability, in terms of range of motion (ROM) and neutral zone, of ALIF, including 2 separate approaches, and TLIF procedures with posterior titanium rod fixation. Both ALIF and TLIF have been used to treat chronic low back pain and instability. In many cases, the choice between these 2 techniques is based only on personal preference. No biomechanical performance comparison between these 2 fusion techniques is available to assist surgical decision. Twelve cadaveric lumbar motion segments were loaded sinusoidally at 0.05 Hz and 5 Nm in unconstrained axial rotation, lateral bending and flexion extension. Specimens were randomly divided into 2 groups with 6 in each group. One group was assigned for TLIF whereas the other group for ALIF. In the ALIF group, there were 3 steps. First, the lateral ALIF procedure with the anterior longitudinal ligament (ALL) intact was performed. Afterwards, the ALL was cut without removing the ALIF cage. Finally, another appropriately sized ALIF cage was inserted anteriorly. Biomechanical tests were conducted after each step. In the ALIF group, the lateral ALIF and subsequent anterior ALIF reduced segmental motion significantly (P=0.03) under all loading conditions. Removing the ALL increased ROM by 59% and 142% in axial rotation and flexion extension, respectively (P=0.03). The anterior ALIF approach was able to achieve similar biomechanical stability of the lateral approach in lateral bending and flexion extension (P>0.05) under all loading conditions. The TLIF procedure significantly reduced the range of motion compared with the intact state (P=0.03). However, no statistical difference was detected between the TLIF group and the ALIF group (P>0.05). Both ALIF and TLIF procedures combined with posterior instrumentation significantly improved construct stability of intact spinal motion segments. However, there was no statistical difference between these 2 fusion techniques. The 2 ALIF approaches (lateral and anterior) also had similar construct stability even though anterior longitudinal ligament severing significantly reduced stability.

Impact of bone density and integrated screw configuration on standalone anterior lumbar interbody construct strength

Ideal Length and Diameter for Intramedullary Screw Fixation of Metacarpal Fractures: A Biomechanical Study

Wednesday, September 26, 2018 7:35 AM–9:00 AM ePosters: P18. Four-rod construct is needed to maximally decrease rod strain across lumbosacral junction with TLIF but not ALIF in long segment fixation

Study DesignA retrospective study of radiographic parameters of patients who underwent lumbar spinal pedicle screw insertion.PurposeThe optimal length of pedicle screws is often determined by the lateral radiograph during minimally invasive surgery (MIS). Compared with open techniques, measuring the precise length of screws or assessing the cortical breach is challenging. This study aims to ascertain the optimal pedicle screw lengths on intraoperative lateral radiographs for L1–L5.Overview of LiteratureResearch has revealed that optimal pedicle screw length is essential to optimize fixation, especially in osteoporotic patients; however, it must be balanced against unintentional breach of the anterior cortex, risking injury to adjacent neurovascular structures as demonstrated by case reports.MethodsWe reviewed intra- and postoperative computed tomography scans of 225 patients who underwent lumbar pedicle screw insertion to ascertain which of the inserted screws were ‘optimal screws.’ The corresponding lengths of these screws were analyzed on postoperative lateral radiographs to ascertain the ideal position that a screw should attain (expressed as a percentage of the entire vertebral body length).ResultsWe reviewed 880 screws of which 771 were optimal screws. We noted a decreasing trend in average optimal percentages of insertion into the vertebral body for pedicle screws going from L1 (average=87.60%) to L5 (average=78.87%). The subgroup analysis revealed that there was an increasing percentage of screws directed in a straight trajectory from L1 to L5, compared to a medially directed trajectory.ConclusionsDuring MIS pedicle screw fixation, this study recommends that pedicle screws should not exceed 85% of the vertebral body length on the lateral view for L1, 80% for L2–L4, and 75% for L5; this will minimize the risk of anterior cortical breach yet maximize pedicle screw purchase for fixation stability.

Supplemental rods are needed to maximally reduce rod strain across the lumbosacral junction with TLIF but not ALIF in long constructs

The effects of screw number and length on two methods of tension band plating

BackgroundDuring medial opening wedge high tibial osteotomy (MOWHTO), sometimes the plate tends to be positioned anteromedially. The plate position can affect the length of the proximal screw, which significantly affects stability after osteotomy. Therefore, research on the correlation among plate position, screw length, and clinical outcomes is needed.MethodsThis retrospective review examines 196 knees in 175 patients who underwent MOWHTO from May 2012 to December 2018, for symptomatic medial compartment osteoarthritis with a varus alignment of > 5°. We evaluated the anteroposterior plate position, length of proximal screw, and postoperative computed tomography (CT). We reviewed patients’ clinical outcome scores, presence of lateral hinge fracture, neurovascular complications, and infection. The correlation among proximal plate position, proximal screw length, and clinical outcomes was evaluated using Pearson’s correlation analysis. A subgroup analysis by screw angle (> 48 ° or < 48 °) was also performed using chi-square test and Student t-test.ResultsThe mean proximal plate position was 16.28% (range, 5.17–44.74) of the proximal tibia’s anterior-to-posterior distance ratio, and the proximal screw length averaged 63.8 mm (range, 44–80 mm). Proximal posteromedial plate position and proximal screw length were significantly correlated (r2 = 0.667, P < .001), as were screw angle and length (r2 = 0.746, P < .001). Medial plating (< 48°) can use a longer proximal screw; nevertheless, no significant difference occurred in clinical outcomes between the two groups. Also, no differences occurred in complication rate, including hinge fracture.ConclusionWith more medially positioned plating during MOWHTO, we can use longer proximal screws. However, there was no significant difference in clinical outcomes and the incidence of lateral hinge fractures regardless of plate position and screw length.

Design Parameters of Hand Surgery Screws: Measurement Discrepancies and Clinical Implications

A biomechanical comparison of 12-, 14-, and 16-mm self-tapping and self-drilling screws in cadaveric cervical spines was performed. This study compared self-drilling and self-tapping screw designs used in anterior cervical locking plates in terms of insertion torque and pullout strength. In addition to screw design, the effects of screw length (12, 14, and 16 mm) and bone mineral density were examined. A total of 201 screws were implanted and tested. Nineteen cadaver spine specimens (C2 through C7) were radiographed and their bone mineral density measured. In each vertebral level, two screws of the same length were implanted and their insertion torque measured. One of each screw design (self-drilling or self-tapping) was randomly placed in each vertebra for side-by-side comparison. Vertebrae then were individually potted and mounted on an MTS machine for accurate measurement of maximum pullout strength. Pullout strength was strongly related to screw length ( = 0.0002). There was no significant difference in pullout strength between the self-drilling and self-tapping screw designs at any length ( = 0.9352). A significant correlation ( </= 0.0077) exists between insertion torque and bone mineral density for the self-drilling and self-tapping 14- and 16-mm screws. A significant correlation exists between pullout strength and bone mineral density for 14- and 16-mm screw lengths of both screw types ( < 0.0001). It would appear that the longest screw feasible should always be used clinically for maximum pullout strength. There is no difference in pullout strength between self-drilling and self-tapping screws. Bone mineral density significantly influences screw performance.

Transforaminal lumbar interbody fusion (TLIF) is a popular fusion technique for treating chronic low-back pain. In cases of interbody nonfusion, revision techniques for TLIF include anterior lumbar interbody fusion (ALIF) approaches. Biomechanical data of the revision techniques are not available. The purpose of this study was to compare the immediate construct stability, in terms of range of motion (ROM) and neutral zone (NZ), of a revision ALIF procedure for an unsuccessful TLIF. An in vitro biomechanical comparison of TLIF and its ALIF revision procedure was conducted on cadaveric nonosteoporotic human spine segments. Twelve cadaveric lumbar motion segments with normal bone mineral density were loaded in unconstrained axial torsion, lateral bending, and flexion-extension under 0.05 Hz and +/- 6-nm sinusoidal waveform. The specimens underwent TLIF (with posterior pedicle fixation) and anterior ALIF (with intact posterior fixation). Multidirectional flexibility testing was conducted following each step. The ROM and NZ data were measured and calculated for each test. Globally, the TLIF and revision ALIF procedures significantly reduced ROM and NZ compared with that of the intact condition. The revision ALIF procedures achieved similar ROM as the TLIF procedure. Revision ALIF maintained biomechanical stability of TLIF in nonosteoporotic spines. Revision ALIF can be performed without sacrificing spinal stability in cases of intact posterior instrumentation.

The goal of this study was to characterize the anatomy relevant to placement of crossing C-2 translaminar screws, including morphometric data, and to evaluate the risk of violating the vertebral artery (VA) during the screw placement. Placement of bilateral crossing C-2 translaminar screws has become an increasingly popular method for dorsal C-2 instrumentation as it is felt to avoid the known risk of VA injury associated with C1-2 transarticular screw fixation and C-1 lateral mass-C-2 pars screw fixation. The source images from 50 CT angiograms of the neck obtained from October to November 2007 were studied. Digital imaging software was used to measure lamina thickness and maximum screw length, perform angulation of screw trajectories in the axial plane, and evaluate the potential for VA injury. In cases where the VA could be injured, the distance between the maximal screw length and artery was measured. Logistic regression was performed to evaluate lamina width, axial angle, and screw length for predicting the potential for VA injury. Mean lamina thickness, axial angle, and maximal screw length were determined for 100 laminae, and a potential for VA injury was noted in 55 laminae. The anatomically defined ideal screw length was longer in laminae with potential for VA injury than in laminae with no apparent risk (35.2 vs 33.6 mm, p = 0.0131). Only increasing optimal screw length was noted to be a statistically significant predictor of potential VA injury (p = 0.0159). The "buffer zone" (the distance between an optimally placed screw and the VA) was 5.6 ± 1.9 mm (mean ± SD, range 1.8-11.4 mm). A screw limited to 28 mm in length appeared to be safe in all laminae studied. Crossing C-2 translaminar screws have been reported to be safe and effective. In addition to morphometric characteristics, the authors have found that screws placed in this trajectory could jeopardize the vertebral arteries in the foramen transversarium or the C1-2 interval. A C-2 translaminar screw limited to 28 mm in length appeared to be safe in all 100 screw trajectories studied in this series.

Surgical bone screws can be subjected to cyclic bending loads when plating constructs are used in the fixation of weight bearing members. While extensive research has been conducted on axial loading that leads to screw pull-out, there is a gap in our understanding of how asymmetric bending loads contribute to screw fracture. The focus of this research was to examine the effect of screw length (20 mm and 40 mm) and cancellous bone density (0.48 g/cm3 and 0.24 g/cm3) on the relative stiffness of 6.5 mm cancellous bone screws subjected to a cantilever bending load. It was hypothesized that longer screws in higher density cancellous bone would result in less screw deflection, supporting clinical practice. For testing, synthetic composite bone was used to simulate the characteristics of natural bone while subjecting screws to quasi-static loading with a universal testing machine. Contrary to the hypothesis, neither screw length nor cancellous bone density resulted in a statistically significant difference (p &gt; 0.05) in deflection for loading up 450 N. The cortical shelf appeared to support the majority of the bending load through compression, rather than acting as a fulcrum. When the 3.0 mm cortex was removed, there was a significant difference in deflection due to both screw length and cancellous bone density.

The in vivo impact of computer navigation on screw number and length in reverse total shoulder arthroplasty

Osteoporosis is a prevalent disease that predisposes patients to fracture and additional post-operative complications, potentially contributing to decreased quality of life. The objective of the current study is to (I) characterize the demographic trends of individuals with osteoporosis undergoing single level posterior spine instrumentation and fusion (PSIF) and anterior lumbar interbody fusion (ALIF); (II) determine the association between osteoporosis and postoperative complications; (III) identify whether the use of bone strengthening medications is associated with improved outcomes. A retrospective review of the Mariner Claims Database was conducted on patients undergoing single level ALIF (CPT 22558) and PSIF (CPT 22840) between 2011 and 2017. Diagnosis of osteoporosis (CPT 77080, CPT 77801, CPT 77082) included a bone density scan within two years of surgery. Patients with osteoporosis were 1:1 matched to controls. Patients taking bone enhancing medications prior to surgery were compared to those that did not take medications. Multivariable logistic regression analyses were performed to evaluate post-operative complication risk factors. 3,502 patients with diagnosed osteoporosis underwent ALIF and PSIF, of which 788 (22.5%) were treated with supplemental medication. Diagnosis of osteoporosis was associated with an increased risk of pulmonary embolism [1.1% vs. 0.4%, odds ratio (OR) 2.48, 95% confidence interval (CI): 1.36-4.53, P=0.003] and minor complications (16.7% vs. 12.9%, OR 1.15, 95% CI: 1.01-1.30, P=0.039). Revision rates two-years post-operatively were not significantly different between patients with osteoporosis and matched controls (P>0.05). There were no differences in outcomes between osteoporotic patients who received medications and those who did not receive medication (P>0.05). Osteoporosis is common in a nationally-representative Medicare database cohort. Pre-operative diagnosis of osteoporosis is associated with increased minor complications following ALIF and PSIF. Pre-operative osteoporosis treatment is not associated with a significant difference in post-operative outcomes. The current study can guide pre-operative counseling in this cohort.