Regional bone mineral density differences measured by quantitative computed tomography in patients undergoing anterior cervical spine surgery

Differences in vertebral bone density between African apes

OBJECTIVES

Low-energy vertebral fractures are a common health concern, especially in elderly people. Interestingly, African apes do not seem to experience as many vertebral fractures and the low-energy ones are even rarer. One potential explanation for this difference is the lower bone density in humans. Yet, only limited research has been done on the vertebral bone density of the great apes and these have mainly included only single vertebrae. Hence the study aim is to expand our understanding of the vertebral microstructure of African apes in multiple spinal segments.

MATERIALS

Bone density in the vertebral body of C7, T12, and L3 was measured from 32 Pan troglodytes and 26 Gorilla gorilla using peripheral quantitative computed tomography (pQCT).

RESULTS

There was a clear difference between the three individual vertebrae and consequently the spinal segments in terms of trabecular density and cortical density and thickness. The variation of these bone parameters between the vertebrae differed between the apes but was also different from those reported for humans. The chimpanzees were observed to have overall higher trabecular density, but gorillas had higher cortical density and thickness. Cortical thickness had a relatively strong association with the vertebral size.

DISCUSSION

Despite the similarity in locomotion and posture, the results show slight differences in the bone parameters and their variation between spinal segments in African apes. This variation also differs from humans and appears to indicate a complex influence of locomotion, posture, and body size on the different spinal segments.

Biomechanical evaluation of reinsertion and revision screws in the subaxial cervical vertebrae

BACKGROUND

This study aimed to evaluate the biomechanical effects of reinserted or revised subaxial cervical vertebral screws.

METHODS

The first part aimed to gauge the maximum insertional torque (MIT) of 30 subaxial cervical vertebrae outfitted with 4.0-mm titanium screws. A reinsertion group was created wherein a screw was wholly removed and replaced along the same trajectory to test its maximum pullout strength (MPOS). A control group was also implemented. The second part involved implanting 4.0-mm titanium screws into 20 subaxial cervical vertebrae, testing them to failure, and then reinserting 4.5-mm revision screws along the same path to determine and compare the MIT and MPOS between the test and revision groups.

RESULTS

Part I findings: No significant difference was observed in the initial insertion's maximum insertion torque (MIT) and maximum pull-out strength (MPOS) between the control and reinsertion groups. However, the MIT of the reinsertion group was substantially decreased compared to the first insertion. Moderate to high correlations were observed between the MIT and MPOS in both groups, as well as between the MIT of the first and second screw in the reinsertion group. Part II, the MIT and MPOS of the screw in the test group showed a strong correlation, while a modest correlation was observed for the revision screw used in failed cervical vertebrae screw. Additionally, the MPOS of the screw in the test group was significantly higher than that of the revision screw group.

CONCLUSION

This study suggests that reinsertion of subaxial cervical vertebrae screws along the same trajectory is a viable option that does not significantly affect fixation stability. However, the use of 4.5-mm revision screws is inadequate for failed fixation cases with 4.0-mm cervical vertebral screws.

Bone density of the cervical, thoracic and lumbar spine measured using Hounsfield units of computed tomography - results of 4350 vertebras

INTRODUCTION

The assessment of bone density has gained significance in recent years due to the aging population. Accurate assessment of bone density is crucial when deciding on the appropriate treatment plan for spinal stabilization surgery. The objective of this work was to determine the trabecular bone density values of the subaxial cervical, thoracic and lumbar spine using Hounsfield units.

MATERIAL AND METHODS

Data from 200 patients who underwent contrast-enhanced polytrauma computed tomography at a maximum care hospital over a two-year period were retrospectively analyzed. HUs were measured with an elliptical measurement field in three different locations within the vertebral body: below the upper plate, in the middle of the vertebral body, and above the base plate. The measured Hounsfield units were converted into bone density values using a validated formula.

RESULTS

The mean age of the patient collective was 47.05 years. Mean spinal bone density values decreased from cranial to caudal (C3: 231.79 mg/cm3; L5: 155.13 mg/cm3; p < 0.001), with the highest values in the upper cervical spine. Bone density values generally decreased with age in all spinal segments. There was a clear decrease in values after age 50 years (p < 0.001).

CONCLUSIONS

In our study, bone density decreased from cranial to caudal with higher values in the cervical spine. These data from the individual spinal segments may be helpful to comprehensively evaluate the status of the spine and to design a better preoperative plan before instrumentation.

Risk Factors of Nonfusion after Anterior Cervical Decompression and Fusion in the Early Postoperative Period: A Retrospective Study

OBJECTIVE

Although high fusion rates have been reported for anterior cervical decompression and fusion (ACDF) in the medium and long term, the risk of nonfusion in the early period after ACDF remains substantial. This study investigates early risk factors for cage nonfusion in patients undergoing single- or multi-level ACDF.

METHODS

This was a retrospective study. From August 2020 to December 2021, 107 patients with ACDF, including 197 segments, were enrolled, with a follow-up of 3 months. Among the 197 segments, 155 were diagnosed with nonfusion (Nonfusion group), and 42 were diagnosed with fusion (Fusion group) in the early period after ACDF. We assessed the significance of the patient-specific factors, radiographic indicators, serum factors, and clinical outcomes. The Wilcoxon rank sum test, t-tests, analysis of variance, and stepwise multivariate logistic regression were used for statistical analysis.

RESULTS

Univariate analysis showed that smoking, insufficient improvement in the C2-7 Cobb angle (p = 0.024) and the functional spinal unit Cobb angle (p = 0.022) between preoperative and postoperative stages and lower serum calcium (fusion: 2.34 ± 0.12 mmol/L; nonfusion: 2.28 ± 0.17 mmol/L, p = 0.003) β-carboxyterminal telopeptide end of type 1 collagen (β-CTX) (fusion: 0.51 [0.38, 0.71]; nonfusion: 0.43 [0.31, 0.57], p = 0.008), and N-terminal fragment of osteocalcin (N-MID-BGP) (fusion: 18.30 [12.15, 22.60]; nonfusion: 14.45 [11.65, 18.60], p = 0.023) are risk factors for nonfusion in the early period after ACDF. Stepwise logistic regression analysis revealed that poor C2-7 Cobb angle improvement (odds ratio [OR], 1.107 [1.019-1.204], p = 0.017) and lower serum calcium (OR, 3.700 [1.138-12.032], p = 0.030) are risk factors.

CONCLUSIONS

Patients with successful fusion after ACDF had higher preoperative serum calcium and improved C2-7 Cobb angle than nonfusion patients at 3 months. These findings suggest that serum calcium could be used to identify patients at risk of nonfusion following ACDF and that correcting the C2-7 Cobb angle during surgery could potentially increase fusion in the early period after ACDF.

Computed Tomography Osteoabsorptiometry Evaluation of Cervical Endplate Subchondral Bone Mineral Density

STUDY DESIGN

Basic Science.

OBJECTIVE

Poor subchondral bone mineral density (sBMD) has been linked with subsidence of cervical interbody devices or grafts, which are traditionally placed centrally on the endplates. Considering that sBMD reflects long-term stress distributions, we hypothesize that the cervical uncovertebral joints are denser than the central endplate region. This study sought to investigate density distributions using computed tomography osteoabsorptiometry (CT-OAM).

METHODS

Twelve human cervical spines from C3-C7 (60 vertebrae, 120 endplates) were imaged with CT and segmented to create 3D reconstructions. The superior and inferior endplates were isolated, and the sBMD of the whole endplate, endplate center, and uncus was evaluated using CT-OAM. Density distributions were compared across the subaxial cervical spine.

RESULTS

The uncinate region of the inferior and superior endplates was significantly denser than the central endplate across all vertebral levels (P < .01). When comparing sBMD of the whole inferior and superior endplates, the superior endplate was significantly denser than the inferior endplate (P < .0001). However, the inferior uncus was denser than the superior uncus (P = .035). When assessing sBMD by vertebral level, peak densities were observed at C4 and C5, while C7 was, on average, significantly less dense than all other vertebrae.

CONCLUSION

The subchondral bone of the cervical uncovertebral joints is significantly denser than the central endplates. While the superior endplate in its entirety is denser than the inferior endplate, the inverse was true for the uncovertebral joints. This study serves as a basis for future investigations of new implant designs and their implications on subsidence.

MRI-based vertebral bone quality score compared to quantitative computed tomography bone mineral density in patients undergoing cervical spinal surgery

PURPOSE

The vertebral bone quality (VBQ) score based on magnetic resonance imaging (MRI) was introduced as a bone quality marker in the lumbar spine. Prior studies showed that it could be utilized as a predictor of osteoporotic fracture or complications after instrumented spine surgery. The objective of this study was to evaluate the correlation between VBQ scores and bone mineral density (BMD) measured by quantitative computer tomography (QCT) in the cervical spine.

METHODS

Preoperative cervical CT and sagittal T1-weighted MRIs from patients undergoing ACDF were retrospectively reviewed and included. The VBQ score in each cervical level was calculated by dividing the signal intensity of the vertebral body by the signal intensity of the cerebrospinal fluid on midsagittal T1-weighted MRI images and correlated with QCT measurements of the C2-T1 vertebral bodies. A total of 102 patients (37.3% female) were included.

RESULTS

VBQ values of C2-T1 vertebrae strongly correlated with each other. C2 showed the highest VBQ value [Median (range) 2.33 (1.33, 4.23)] and T1 showed the lowest VBQ value [Median (range) 1.64 (0.81, 3.88)]. There was significant weak to moderate negative correlations between and VBQ Scores for all levels [C2: p < 0.001; C3: p < 0.001; C4: p < 0.001; C5: p < 0.004; C6: p < 0.001; C7: p < 0.025; T1: p < 0.001].

CONCLUSION

Our results indicate that cervical VBQ scores may be insufficient in the estimation of BMDs, which might limit their clinical application. Additional studies are recommended to determine the utility of VBQ and QCT BMD to evaluate their potential use as bone status markers.

Comparison of bone density patterns of the subaxial spine between chimpanzees and gorillas - A case study

Case study on the bone density pattern of subaxial vertebral column in African apes.

INTRODUCTION

African apes have been noted to experience fewer back ailments than humans and to have higher vertebral bone density. Yet, research on the subject is quite limited and has usually included only one or few vertebrae. However, to understand vertebral column as whole and how posture and locomotion might have affected it, we need to know how bone density varies between adjacent vertebrae.

MATERIALS AND METHODS

Bone density in the vertebral body was measured for all subaxial vertebrae of five specimens including two Pan troglodytes (1 male and 1 female) and three Gorilla gorilla (2 males and 1 female) using peripheral quantitative computed tomography (pQCT).

RESULTS

The results tentatively indicated differences between species, especially in the trabecular density of the cervical segment and support the need for further studies on this subject.

Differential Cortical Volumetric Bone Mineral Density within the Human Rib

INTRODUCTION

The human rib provides a vital role in the protection of thoracic contents. Rib fractures are linked to injuries and health complications that can be fatal. Current clinical methods to assess fracture risk and bone quality are insufficient to quantify intra-element differences in bone mineral density (BMD) or to identify at-risk populations. Utilizing quantitative computed tomography (QCT) provides accurate measures of volumetric BMD (vBMD) along the length of the rib which can help delineate factors influencing differential fracture risk.

METHODOLOGY

One mid-level rib was obtained from 54 post-mortem human subjects (PMHS) and scanned using QCT. Volumes of interest (VOIs) were created for sites at 30%, 50%, and 75% of rib total curve length. Mean Hounsfield units (HU) from each VOI were converted to vBMD using a scan-specific cortical phantom calibration curve. Additionally, rib and lumbar areal BMD (aBMD) were obtained from a sub-sample of 33 PMHS.

RESULTS

Significant differences in vBMD were found between all sites within the rib (p<0.01). When analyzed by sex, vBMD between the 30% and 50% site were no longer different in either males or females (p>0.05). Separating the sample into discrete age groups demonstrated the relative differences in vBMD between sites diminished with age. Further, age as a continuous variable significantly predicted rib vBMD at all sites (p<0.05), but with little practical or clinical utility (R², 14.7- 22.8%). Similarly, only small amounts of variation in rib vBMD were explained from DXA lumbar and rib aBMD (R² , 1.1-21.8%).

CONCLUSIONS

vBMD significantly decreased from the posterior (30%) site to the anterior (75%) site within the rib which may represent adaptation to localized mechanical loading. These differences could result in differential fracture risk across the rib. As thoracic injury can be fatal, using comprehensive assessments of bone quality that accounts for variation within the rib may provide more accurate identification of at-risk populations.

Trabecular volumetric bone mineral density of the occipital bone at preferred screw placement sites measured by quantitative computed tomography

This study aimed at quantifying trabecular volumetric bone mineral density (vBMD) at the external occipital protuberance (EOP) and the upper cervical spine. A retrospective review of patients who underwent non-contrast enhanced computed tomography of the cervical spine that included the occipital bone up to the EOP between 2007 and 2020 was conducted. Measurements of trabecular vBMD were performed in the occipital midline area, with the region of interest extending 4.5 mm above and below the center of the EOP, as well as the C1 lateral masses and the C2 vertebral body using asynchronous quantitative computed tomography. Eighty-six patients (female, 37.2%) were included for analysis. The patient population was 81.4% Caucasian with a mean ± SD age of 62.3 ± 13.1 years. Total bone thickness at the EOP was 16.7 ± 3.4 mm, with a ratio of trabecular to total bone thickness of 0.44. Trabecular vBMD (mean ± SD) was significantly higher at the EOP than at C1 and C2 (EOP = 612.3 ± 145.8 mg/cm³ , C1 average = 290.3 ± 66.5 mg/cm³ , C2 = 305.8 ± 78.8 mg/cm³ ; p < 0.001). A significant strong correlation between trabecular vBMD of C1 and C2 was observed (r = 0.744; p < 0.001), but only low correlations between the EOP and C1 (r = 0.295; p = 0.008) and C2 (r = 0.413; p < 0.001). In individuals > 65 years of age, cervical vBMD was significantly lower, but remained high at the EOP. Clinical significance: Trabecular vBMD at the EOP is significantly higher than at the upper cervical vertebrae and remains high in older populations. Together with morphological information about the occipital bone, these results might be helpful for occipitocervical fixation when deciding about uni- or bicortical screw placement at the EOP.

Should Q-CT Be the Gold Standard for Detecting Spinal Osteoporosis?

STUDY DESIGN

Prospective comparative study.

OBJECTIVE

Refinement of the guidelines for screening of osteoporosis and considering quantitative computed tomography (Q-CT) for detecting spinal osteoporosis.

SUMMARY OF BACKGROUND DATA

Spinal osteoporosis is often underestimated and under-evaluated due to either lack of availability of the diagnostic modality or lack of awareness about the possibility of overestimation by dual X-ray absorptiometry (DXA) scan. There is a need for reconsidering osteoporosis evaluation with a site specific and patient specific inclination.

METHODS

Post-menopausal women that underwent bone mineral density (BMD) evaluation from January-2018 to December-2020 with either Q-CT or DXA were evaluated. Comparison studies of the distribution of age and T-scores of the bone densities obtained from the two study groups: age-matched, sex-matched, and common skeletal site of interest (L1-4 vertebrae) were performed. Mann-Whitney U test, correlation and regression analyses were performed and bell curves were plotted.

RESULTS

Of the 718 women evaluated, 447 underwent Q-CT and 271 underwent DXA. There was no significant difference among the age distribution of the two study groups (P-value > 0.05). The mean and mode T-scores obtained by Q-CT and DXA were found to be -2.71, -3.8 and -1.63, -1.7 respectively. A highly significant difference in the T-scores was observed in the Q-CT and DXA groups (P-value < 0.0001). Among those who were screened by Q-CT, 58.16% were osteoporotic, 37.58% were osteopenic, and 4.25% were normal. The respective percentages in the DXA group were 30.63%, 49.82%, and 19.55%.

CONCLUSION

Q-CT provides more precise estimation of cancellous bone mineral density than DXA. With the reliance on DXA for spinal BMD estimation being questionable, new standards have to be established for spinal osteoporosis evaluation. Q-CT can be a better alternative to replace DXA as the gold standard for the evaluation of spinal osteoporosis.Level of Evidence: 2.

Hounsfield Unit for Assessing Bone Mineral Density Distribution Within Cervical Vertebrae and Its Correlation With the Intervertebral Disc Degeneration

STUDY DESIGN

Retrospective radiological analysis.

OBJECTIVE

To assess bone mineral mass distribution within cervical vertebrae based on Hounsfield unit (HU) measurement, and explore its correlation with intervertebral disc degeneration.

METHOD

Three hundred and twenty-four patients with degenerative cervical spine disease were retrospectively reviewed and divided into six groups according to age. HU measurement of the whole vertebrae from C3 through C7 was obtained, then HU measurement within upper and lower part of the vertebrae on sagittal plane were obtained from C3 through C7. Disc degeneration on MRI was graded from I to V using the Pfirrmann classification.

RESULTS

There was a significant difference in the HU value from C3 to C7 among Group II to Group VI, the HU value presented consistently decreasing trend from young patients to old patients. In C6 and C7 vertebrae, there were significant differences in HU values between upper and lower parts of the vertebrae. More importantly. In all groups, HU values were highest in the upper part of the C4 vertebrae and then gradually decreased towards C3 and C7. HU value of both upper and lower vertebrae presented decreasing trend along with the aggravation of the disc degeneration.

CONCLUSION

HU values are not typically consistent throughout all levels of the cervical spine and the distribution within the vertebrae is not homogeneous. Decreased vertebral BMD and vertebral osteoporosis may trigger or exacerbate the adjacent intervertebral disc degeneration.

Thoracic bone mineral density measured by quantitative computed tomography in patients undergoing spine surgery

BACKGROUND CONTEXT

The thoracic spine is a common location for vertebral fractures as well as instrumentation failure after long spinal fusion procedures. The association between those complications and bone mineral density (BMD) are well recognized. Due to the overlying sternum and ribs in the thoracic spine, projectional BMD assessment tools such as dual energy x-ray absorptiometry (DXA) are limited to the lumbar spine. Quantitative computed tomography circumvents several shortcomings of DXA and allows for level-specific BMD measurements. Studies comprehensively quantifying BMD of the entire thoracic spine in patients undergoing spine surgery are limited.

PURPOSE

The objective of this study was: (1) to assess the reliability of thoracic QCT measurements, (2) to determine possible level-specific BMD variation throughout the thoracic spine and (3) to assess the correlation between BMDs of the T1-T12 spinal levels.

STUDY DESIGN/SETTING

Cross-sectional observation study.

PATIENT SAMPLE

Patients undergoing spine surgery from 2016-2020 at a single, academic institution with available preoperative CT imaging of the thoracic spine were included in this study.

OUTCOME MEASURES

The outcome measure was BMD measured by QCT.

METHODS

Patients undergoing spine surgery from 2016-2020 at a single, academic institution with available preoperative CT imaging of the thoracic spine were included in this study. Subjects with previous instrumentation at any thoracic level, concurrent vertebral fractures, a Cobb angle of more than 20 degrees, or incomplete thoracic spine CT imaging were excluded. Asynchronous quantitative computed tomography (QCT) measurements of T1-T12 were performed. To assess inter- and intra-observer reliability, a validation study was performed on 120 vertebrae in 10 randomly selected patients. The interclass correlation coefficient (ICC) was calculated. A pairwise comparison of BMD was conducted and correlations between each thoracic level were evaluated. The statistical significance level was set at p<.05.

RESULTS

60 patients (men, 51.7%) met inclusion criteria. The study population was 90% Caucasian with a mean age of 62.2 years and a mean BMI of 30.2 kg/m². The inter- and intra-observer reliability of the thoracic QCT measurements was excellent (ICC of 0.97 and 0.97, respectively). The trabecular BMD was highest in the upper thoracic spine and decreased in the caudal direction (T1 = 182.3 mg/cm³, T2 = 168.1 mg/cm³, T3 = 163.5 mg/cm³, T4 = 164.7 mg/cm³, T5 = 161.4 mg/cm³, T6 = 152.5 mg/cm³, T7 = 143.5 mg/cm³, T8 = 141.3 mg/cm³, T9 = 143.5 mg/cm³, T10 = 145.1 mg/cm³, T11 = 145.3 mg/cm³, T12 = 133.6 mg/cm³). The BMD of all thoracic levels cranial to T6 was statistically higher than the BMD of all levels caudal to T6 (p < .001). Nonetheless, significant correlations in BMD among all measured thoracic levels were observed, with a Pearson's correlation coefficient ranging from 0.74 to 0.97.

CONCLUSIONS

There is significant regional BMD variation in the thoracic spine depending on spinal level. This BMD variation might contribute to several clinically relevant phenomena. First, vertebral fractures occur most commonly at the thoracolumbar junction including T12. In addition to mechanical reasons, these fractures might be partially attributed to thoracic BMD that is lowest at T12. Second, the optimal upper instrumented vertebra (UIV) for stopping long fusions to the sacrum and pelvis is controversial. The BMD of surgically relevant upper thoracic stopping points (T2-T4) was significantly higher than the BMD of lower thoracic stopping points (T10-T12). Besides stress concentration at the relatively mobile lower thoracic segments, the low BMD at these levels might contribute to previously suggested higher rates of junctional failures with short fusions.

Gray values of the cervical vertebrae detected by cone beam computed tomography for the identification of osteoporosis and osteopenia in postmenopausal women

OBJECTIVES

This study aimed to determine whether the gray values (GVs) of the second and third cervical vertebrae in cone beam computed tomography (CBCT) are affected by general bone mineral density (BMD) in postmenopausal women and to evaluate whether CBCT can be used for predicting the risk of osteoporosis.

STUDY DESIGN

The present cross-sectional study included 127 edentulous postmenopausal women (average age, 70.4 ± 8.9 years) who underwent CBCT examinations. BMD measurements of the lumbar spine and both femoral necks were performed by dual-energy x-ray absorptiometry. The CBCT images were analyzed with OnDemand3D Dental software. GVs of the axial and sagittal sections of the cervical second and third vertebral bodies were obtained.

RESULTS

The GVs of the second and third cervical vertebrae decreased significantly with decreasing BMD. The axial GV of the second cervical vertebra showed the highest sensitivity (80.6%), highest negative predictive value (89.0%), lowest negative likelihood ratio (0.39), and highest area under the curve value (0.68) to predict osteoporosis.

CONCLUSIONS

Women with reduced BMD have lower GVs for the second and third cervical vertebrae. Changes in the GV of the second vertebra can potentially be used to determine the risk of osteoporosis in postmenopausal women.

Is the fusion order of the cranial and caudal levels different in two-level anterior cervical discectomy and fusion for cervical spondylopathy? A retrospective study

STUDY DESIGN

Retrospective study.

OBJECTIVE

This study aimed to compare the fusion order between the cranial and caudal levels in two-level anterior cervical discectomy and fusion (ACDF) with a zero-profile device in the treatment of cervical spondylopathy. Fusion is the standard used to judge the success of ACDF. However, the fusion order in two-level ACDF remains uncertain. The mechanical environment of different levels is different, which may affect the fusion rate or fusion order.

METHODS

From 2014 to January 2019, data of consecutive patients with two-level cervical disk degenerative disease who underwent ACDF were retrospectively reviewed. Radiological assessments were based on the range of motion of the fusion level, segment slope, and disk height, and complications were assessed. Data were analyzed using the paired t, Mann-Whitney U, χ2, Fisher exact, and rank-sum tests and logistic regression analysis.

RESULTS

In total, 118 patients were ultimately enrolled for analysis in the study. The respective fusion rates of the cranial and caudal levels were 26.27% and 10.17% (p < 0.05) at 3 months, 58.47% and 42.37% (p < 0.05) at 6 months, 86.44% and 82.20% (1 0.05) at 1 year, and 92.37% and 89.83% (p > 0.05) at the last follow-up. Multivariate logistic regression analysis indicated that the preoperative segmental slope and cranial level were independent risk factors for non-fusion. The adjacent segment degeneration (ASD) and subsidence rates were comparable between the two levels.

CONCLUSION

The caudal level had a slower fusion process than the cranial level. A higher preoperative segment slope was a risk factor for fusion. However, the subsidence and ASD rate were comparable between the caudal and cranial levels in the two-level ACDF.

A biomechanical comparison of crossed and parallel rod configurations in atlantoaxial internal fixation

BACKGROUND

Posterior atlantoaxial fixation with screw rod forms an approximate "II" shape or "H" increasing transverse link for better stability. In order to improve stability and in consideration of difficult placement of transverse connecting rod, possibility of inadequate bone graft, some scholars have preliminarily researched biomechanics of a novel crossed rod as an approximate "X" configuration of screw rod.

PURPOSE

The aim of this study was to evaluate and compare the biomechanics of the crossed and parallel rod configurations in the screw rod system for posterior atlantoaxial fixation on a cadaveric model.

METHODS

Six fresh cervical specimens were used to complete the range of motion (ROM) testing by applying pure moments of ± 2.0 nm. Following intact state and under destabilization testing, screws were implanted. The specimens were then tested in the following sequence: Group BLS + PR (C2 bilateral laminar screws + parallel rod), Group BLS + CR (C2 bilateral laminar screws + crossed rod), LPRLS + PR (C2 left pedicle screw and right laminar screw + parallel rod), LPRLS + CR (C2 left pedicle screw and right laminar screw + crossed rod), BPS + PR (C2 bilateral pedicle screws + parallel rod) and BPS + CR (C2 bilateral pedicle screws + crossed rod). The ROM of the C1-2 segments was measured in flexion-extension, lateral bending and axial rotation. Six surgical constructs were compared between the groups and with intact condition, respectively.

RESULTS

The six fixed modes significantly increased stability compared with both the intact and destabilization group in flexion-extension, lateral bending and axial rotation (p < .05). In extension, BPS + CR and BLS + CR showed greater stability than BLS + PR (p < .05). During flexion, the six fixation methods showed no statistical significance (p > .05). In left lateral bending, stability of the other five screw rod fixation techniques significantly increased when compared with BLS + PR (p < .05). In the right lateral bending direction, the stability of BLS + PR was worse than that of BPS + CR and BPS + PR (p < .05). In the left axial rotation, stability of BLS + CR, LPRLS + CR and BPS + CR was greater than that of BLS + PR, LPRLS + PR and BPS + PR (p < .05). In the right axial rotation, the stability of BPS + CR and BLS + CR was greater than that of BLS + PR (p < .05).

CONCLUSION

The six investigated fixation methods provide sufficient biomechanical stability. The crossed rod configuration can further enhance the axial rotation stability of the screw rod system, which consists of C1 bilateral pedicle and C2 pedicle, or C2 lamina screws. The crossed rod can also improve the stability of the screw rod system made up of C1 bilateral pedicle and C2 lamina screws in lateral bending and extension. The crossed rod configuration is reliable and provides superior stability for clinical application.

Does screw length for primary two-level ACDF influence pseudarthrosis risk?

BACKGROUND CONTEXT

Pseudarthrosis remains a major complication for patients undergoing anterior cervical discectomy and fusion (ACDF; 0%-15% at 1-year follow-up). Potentially modifiable risk factors are known in literature, such as smoking and osteoporosis. Biomechanical studies suggest that plates with locking screws can enhance the fixation rigidity and pull-out strength. Although longer screws are known to be correlated with increased pull-out strength, deeper screw depths can increase the risk for intraoperative complications. An important factor that has yet to be studied is the minimum screw length relative to the diameter of the vertebral body (VB) necessary to achieve successful fusion. In this study, we hypothesize that screws with shorter depths relative to the VB will increase the risk for radiographic pseudarthrosis and result in poor patient reported outcomes (PROs).

PURPOSE

To examine the impact of ACDF screw length on pseudarthrosis risk.

STUDY DESIGN

A review of prospectively collected data.

PATIENT SAMPLE

A total of 85 patients were included in this study. The mean age ±standard deviation was 58.9±10.3 and 42.4% of patients were female. The mean follow-up was 21.6±8.3 months.

OUTCOME MEASURES

The neck disability index (NDI) was used to assess PROs up to 2-years after surgery. For each ACDF level, the screw length and VB% (screw length divided by the anterior-posterior VB diameter) were measured. Radiographic pseudarthrosis (interspinous motion [ISM] ≥1 mm) was recorded at 6-weeks, 6-months, and 1-year for each patient. The positive and negative predictive values (PPV, NPV) for ISM ≥ 1mm were measured for different VB% thresholds. A VB% of <75% was found to have the highest PPV (93%) and NPV (70%) for radiographic pseudarthrosis. This threshold of <75% was then assessed in our bivariate and multivariate analyses.

METHODS

We reviewed a database (2015-2018) of adult (≥18 years old) patients who underwent a primary two-level ACDF with or without corpectomy. All ACDF constructs involved fixed angle screws. The minimum follow-up period was 1 year. Multivariate analyses were performed to determine if screw VB% was an independent risk factor for radiographic pseudarthrosis.

RESULTS

By 1-year, overall fusion success was achieved in 92.9% of patients. The 1-year revision rate was 4.7%. Patients with any screw VB% <75% had substantially worse fusion success (64.3%) than those who did not (98.6%) at 1-year. The VB% <75% increased the risk for radiographic pseudarthrosis at every follow up period. In comparison to other time-points, patients with radiographic pseudarthrosis at 6 weeks had significantly worse NDI scores by 2-years (p=.047). The independent risk factors for radiographic pseudarthrosis at 6-weeks included any screw VB% <75% (OR 77, p<.001), prior/current smoker (OR 6.8, p=.024), and corpectomy (OR 0.1, p=.010). Patients with ISM≥1 mm had a higher rate of revision surgery at 1-year (5.9% vs. 3.9%), but this was not statistically significant (p=.656).

CONCLUSIONS

In primary two-level ACDF, VB% <75% is significantly associated with increased ISM (≥1 mm) at all time points for this study. As an intraoperative guide, spine surgeons can use the screw VB% threshold of <75% to avoid unnecessarily short screws. This threshold can be easily measured pre- and intraoperatively, and has been found to be strongly correlated to radiographic pseudarthrosis in the early postoperative period.