Biomechanical evaluation of a novel fenestrated pedicle screw augmented with bone cement in osteoporotic spines

Injectable bone cement cannulated pedicle screw for lumbar degenerative disease in osteoporosis - clinical follow-up of over 5 years

OBJECTIVE

The aim of this study is to evaluate the clinical efficacy of injectable cemented hollow pedicle screw (CICPS) in the treatment of osteoporotic lumbar degenerative diseases through a large sample long-term follow-up study. Additionally, we aim to explore the risk factors affecting interbody fusion.

METHODS

A total of 98 patients who underwent CICPS for transforaminal lumbar interbody fusion (TLIF) for osteoporotic lumbar degenerative disease from March 2011 to September 2017 were analyzed. X-ray and electronic computed tomography (CT) imaging data were collected during preoperative, postoperative, and follow-up periods. The data included changes in intervertebral space height (ΔH), screw failure, cement leakage (CL), and intervertebral fusion. The patients were divided into two groups based on their fusion status one year after surgery: satisfied group A and dissatisfied group B. Surgical data such as operation time, intraoperative bleeding volume and surgical complications were recorded, and visual analog scale (VAS) and Oswestry disability index (ODI) were used to evaluate the improvement of lumbar and leg pain.

RESULTS

The mean follow-up time was 101.29 months (ranging from 70 to 128 months). A total of 320 CICPS were used, with 26 screws (8.13%) leaking, 3 screws (0.94%) experiencing cement augmentation failure, and 1 screw (0.31%) becoming loose and breaking. The remaining screws were not loose or pulled out. Female gender, decreased bone density, and CL were identified as risk factors affecting interbody fusion (P < 0.05). Early realization of interbody fusion can effectively prevent the loss of intervertebral space height (P < 0.05) and maintain the surgical treatment effect. Both VAS and ODI scores showed significant improvement during the follow-up period (P < 0.05). Binary logistic regression analysis revealed that decreased bone density and cement leakage were risk factors for prolonged interbody fusion.

CONCLUSIONS

The results of long-term follow-up indicate that PMMA enhanced CICPS has unique advantages in achieving good clinical efficacy in the treatment of osteoporosis lumbar degenerative diseases. Attention should be paid to identify female gender, severe osteoporosis, and CL as risk factors affecting interbody fusion.

Tips and tricks for using cement augmentation of pedicle screws and vertebral body replacements-A literature review supported by two case reports

BACKGROUND

The prevalence of osteoporosis is escalating alongside an aging global population, increasing the demand for spinal surgeries, including those necessitating cement augmentation for enhanced construct stability.

OBJECTIVE

This article delves into the nuanced application of cement augmentation techniques for pedicle screws and vertebral body replacements (VBR), aimed at optimizing surgical outcomes in osteoporotic spines.

METHOD

Drawing from a comprehensive literature review according to important clinical and biomechanical studies and the authors' clinical experiences, we elucidate strategies to mitigate complications and improve surgical efficacy.

RESULTS

Cement augmentation has shown promise in managing vertebral fractures and in securing pedicle screws within osteoporotic vertebrae, with the advent of polymethylmethacrylate (PMMA) bone cement marking a pivotal advancement in spinal surgery. We highlight intraoperative measures like the choice between pre-injecting cement and utilizing cannulated or fenestrated screws, emphasizing the importance of controlling cement viscosity to prevent leakage and embolism. Through two case reports, we demonstrate the practical application of endplate cementation following VBR.

CONCLUSION

While the use of cement augmentation poses certain risks, its judicious application-supported by evidence-based guidelines and surgical expertise-can substantially enhance the stability of spinal constructs in osteoporotic patients. This allows a reduction in instrumentation length by enhancing biomechanical stability concerning pullout, bending, and rotational forces. Furthermore, the incidence of endplate sintering following VBF can be significantly reduced. Future research, particularly on antibiotic-loaded PMMA, may further expand its utility and optimize its safety profile.

Design of a new detachable pedicle screw based on medical optical imaging inspection to improve osteoporosis and enhance vertebral body revision effect

Osteoporosis is a common bone disease that often leads to difficulty in vertebrae revision. Traditional pedicle screws are often complicated to operate and have poor visibility during implantation. A new detachable pedicle screw is needed to improve the revision effect. The aim of this study was to design a new detachable pedicle screw based on medical optical imaging to improve the outcome of vertebral revision in osteoporosis, and to improve operational feasibility and visibility. In this study, the parameters related to the degree of osteoporosis were obtained by optical imaging detection of the osteoporotic vertebral body. Then a new detachable pedicle screw was designed according to the test results to improve the effect of vertebral body revision. By preparing and optimizing the material and structure of the screw, it is ensured that it has sufficient mechanical strength and stability. Finally, the visibility and operability of the improved screw during implantation were verified by medical optical imaging. Compared with traditional screws, the new detachable pedicle screw can improve the vertebral body revision in the case of osteoporosis. The optical imaging test results show that the new screw has good visibility and maneuverability, providing more accurate guidance and positioning for the vertebral body revision operation.

Application of cement-augmented pedicle screws in elderly patients with spinal tuberculosis and severe osteoporosis: a preliminary study

OBJECTIVE

Surgical management of elderly patients with spinal tuberculosis and severe osteoporosis is challenging. Cement-augmented pedicle screws (CAPS) have been specifically designed for elderly patients with osteoporotic spines. Herein, we investigated the feasibility of CAPS applied in elderly patients with spinal tuberculosis and severe osteoporosis.

METHODS

We retrospectively analyzed data of patients with spinal tuberculosis and severe osteoporosis between January 2017 and January 2021. Surgical data, including surgical duration and intraoperative blood loss, were recorded. Radiological parameters, such as correction of regional kyphotic angle and screw loosening, were also evaluated. Additionally, visual analog scores (VAS) and Oswestry disability index (ODI) were used to evaluate back pain and functional recovery, respectively. Erythrocyte sedimentation (ESR) and C-reactive protein (CRP) concentrations were detected to assess tuberculosis activity. The presence of complications and fusion rate was also assessed.

RESULTS

A total of 15 patients were included in this study. The surgical duration was 263.0 ± 56.2 min, with an average blood loss of 378.7 ± 237.0 ml. The correction of regional kyphotic angle was 12.4° ± 15.0°, and it was well maintained until the final follow-up. The mean VAS decreased from 6.0 ± 1.2 points to 0.5 ± 0.6 points, and ODI reduced from 37.8% ± 7.6% to 8.3% ± 2.8% (P < 0.01). At the final follow-up, ESR and CRP levels were within normal range. Bony fusion occurred in all patients, with an average fusion duration of 8.8 ± 1.5 months. No cases of pedicle screw pullout, screw loosening, or pseudoarthrosis occurred. Tuberculosis recurrence and dissemination were not observed during the follow-ups.

CONCLUSIONS

CAPS fixation is an effective and safe technique to achieve solid fixation and favorable clinical outcomes in elderly patients with spinal tuberculosis and severe osteoporosis.

Avoiding Spinal Implant Failures in Osteoporotic Patients: A Narrative Review

STUDY DESIGN

Narrative review.

OBJECTIVES

With an aging population, the prevalence of osteoporosis is continuously rising. As osseous integrity is crucial for bony fusion and implant stability, previous studies have shown osteoporosis to be associated with an increased risk for implant failure and higher reoperation rates after spine surgery. Thus, our review's purpose was to provide an update of evidence-based solutions in the surgical treatment of osteoporosis patients.

METHODS

We summarize the existing literature regarding changes associated with decreased bone mineral density (BMD) and resulting biomechanical implications for the spine as well as multidisciplinary treatment strategies to avoid implant failures in osteoporotic patients.

RESULTS

Osteoporosis is caused by an uncoupling of the bone remodeling cycle based on an unbalancing of bone resorption and formation and resulting reduced BMD. The reduction in trabecular structure, increased porosity of cancellous bone and decreased cross-linking between trabeculae cause a higher risk of complications after spinal implant-based surgeries. Thus, patients with osteoporosis require special planning considerations, including adequate preoperative evaluation and optimization. Surgical strategies aim towards maximizing screw pull-out strength, toggle resistance, as well as primary and secondary construct stability.

CONCLUSIONS

As osteoporosis plays a crucial role in the fate of patients undergoing spine surgery, surgeons need to be aware of the specific implications of low BMD. While there still is no consensus on the best course of treatment, multidisciplinary preoperative assessment and adherence to specific surgical principles help reduce the rate of implant-related complications.

Impaction grafting of lumbar pedicle defects: a biomechanical study of a novel technique for pedicle screw revision

OBJECTIVE

The two most common revision options available for the management of loose pedicle screws are larger-diameter screws and cement augmentation into the vertebral body for secondary fixation. An alternative revision method is impaction grafting (pedicoplasty) of the failed pedicle screw track. This technique uses the impaction of corticocancellous bone into the pedicle and vertebral body through a series of custom funnels to reconstitute a new pedicle wall and a neomedullary canal. The goal of this study was to compare the biomechanics of screws inserted after pedicoplasty (impaction grafting) of a pedicle defect to those of an upsized screw and a cement-augmented screw.

METHODS

For this biomechanical cadaveric study the investigators used 10 vertebral bodies (L1-5) that were free of metastatic disease or primary bone disease. Following initial screw insertion, each screw was subjected to a pullout force that was applied axially along the screw trajectory at 5 mm per minute until failure. Each specimen was instrumented with a pedicoplasty revision using the original screw diameter, and on the contralateral side either a fenestrated screw with cement augmentation or a screw upsized by 1 mm was inserted in a randomized fashion. These revisions were then pulled out using the previously mentioned methods.

RESULTS

Initial screw pullout values for the paired upsized screw and pedicoplasty were 717 ± 511 N and 774 ± 414 N, respectively (p = 0.747) (n = 14). Revised pullout values for the paired upsized screw and pedicoplasty were 775 ± 461 N and 762 ± 320 N, respectively (p = 0.932). Initial pullout values for the paired cement augmentation and pedicoplasty were 792 ± 434 N and 880 ± 558 N, respectively (p = 0.649). Revised pullout values for the paired cement augmentation and pedicoplasty were 1159 ± 300 N and 687 ± 213 N, respectively (p < 0.001).

CONCLUSIONS

Pedicle defects are difficult to manage. Reconstitution of the pedicle and creation of a neomedullary canal appears to be possible through the use of pedicoplasty. Biomechanically, screws that have been used in pedicoplasty have equivalent pullout strength to an upsized screw, and have greater insertional torques than those with the same diameter that have not been used in pedicoplasty, yet they are not superior to cement augmentation. This study suggests that although cement augmentation appears to have superior pullout force, the novel pedicoplasty technique offers promise as a viable biological revision option for the management of failed pedicle screws compared with the option of standard upsized screws in a cadaveric model. These findings will ultimately need to be further assessed in a clinical setting.

Evaluation of an Injectable Biphasic Calcium Sulfate/Hydroxyapatite Cement for the Augmentation of Fenestrated Pedicle Screws in Osteoporotic Vertebrae: A Biomechanical Cadaver Study

Cement augmentation of pedicle screws is one of the most promising approaches to enhance the anchoring of screws in the osteoporotic spine. To date, there is no ideal cement for pedicle screw augmentation. The purpose of this study was to investigate whether an injectable, bioactive, and degradable calcium sulfate/hydroxyapatite (CaS/HA) cement could increase the maximum pull-out force of pedicle screws in osteoporotic vertebrae. Herein, 17 osteoporotic thoracic and lumbar vertebrae were obtained from a single fresh-frozen human cadaver and instrumented with fenestrated pedicle screws. The right screw in each vertebra was augmented with CaS/HA cement and the un-augmented left side served as a paired control. The cement distribution, interdigitation ability, and cement leakage were evaluated using radiographs. Furthermore, pull-out testing was used to evaluate the immediate mechanical effect of CaS/HA augmentation on the pedicle screws. The CaS/HA cement presented good distribution and interdigitation ability without leakage into the spinal canal. Augmentation significantly enhanced the maximum pull-out force of the pedicle screw in which the augmented side was 39.0% higher than the pedicle-screw-alone side. Therefore, the novel biodegradable biphasic CaS/HA cement could be a promising material for pedicle screw augmentation in the osteoporotic spine.

Biomechanical role of cement augmentation in the vibration characteristics of the osteoporotic lumbar spine after lumbar interbody fusion

Under whole body vibration, how the cement augmentation affects the vibration characteristic of the osteoporotic fusion lumbar spine, complications, and fusion outcomes is unclear. A L1-L5 lumbar spine finite element model was developed to simulate a transforaminal lumbar interbody fusion (TLIF) model with bilateral pedicle screws at L4-L5 level, a polymethylmethacrylate (PMMA) cement-augmented TLIF model (TLIF-PMMA) and an osteoporotic TLIF model. A 40 N sinusoidal vertical load at 5 Hz and a 400 N preload were utilized to simulate a vertical vibration of the human body and the physiological compression caused by muscle contraction and the weight of human body. The results showed that PMMA cement augmentation may produce a stiffer pedicle screw/rod construct and decrease the risk of adjacent segment disease, subsidence, and rod failure under whole-body vibration(WBV). Cement augmentation might restore the disc height and segmental lordosis and decrease the risk of poor outcomes, but it might also increase the risk of cage failure and prolong the period of lumbar fusion under WBV. The findings may provide new insights for performing lumbar interbody fusion in patients affected by osteoporosis of the lumbar spine. Graphical abstract.

[Cement augmentation in spinal surgery]

Bone cement has been used in spinal surgery for as long as 50 years. In contemporary spinal surgery, cement augmentation of fractured osteoporotic vertebrae in the form of vertebroplasty/kyphoplasty as well as cement augmentation of pedicle screws in instrumented procedures of any etiology are established as standard procedures. Both procedures are very effective, although the benefits of vertebroplasty/kyphoplasty procedures have been controversially discussed in the past. Overall, complications rarely occur. The most relevant complication is cement leakage, which is asymptomatic in the majority of cases but in the worst case might lead to neurological deficits, embolic events and even circulatory collapse. Prevention of cement leakage is therefore crucial. Risk factors for cement leakage and preventive measures are presented in a comprehensive review based on the available literature.

Outcomes of the use of cement-augmented cannulated pedicle screws in lumbar spinal fusion

BACKGROUND CONTEXT

There are few studies of the radio-clinical outcomes of cement-augmented cannulated pedicle screw (CPS) fixation in osteoporotic patients.

PURPOSE

To compare the radiological and clinical outcomes between groups receiving cement-augmented CPS and solid pedicle screws (SPS) in lumbar fusion surgery.

STUDY DESIGN/SETTING

Retrospective comparative study PATIENT SAMPLE: A total of 187 patients who underwent lumbar fusion surgery for degenerative spinal stenosis or spondylolisthesis from 2014 to 2019.

OUTCOME MEASURES

Radiological evaluation included screw failure, cage failure, rod breakage, and fusion grade at postoperative 6 months and 1 year. Pre- and postoperative visual analog scales for back pain (VAS-BP), leg pain (VAS-LP), Korean Oswestry disability index (K-ODI), and postoperative complications were also compared.

METHODS

Outcomes of patients with high risk factors for implant failure [old age, osteoporosis, autoimmune disease or chronic kidney disease (CKD)] who underwent open transforaminal lumbar interbody fusion with cement-augmented CPS fixation (Group C, n=55) or SPS fixation (Group S, n=132) were compared.

RESULTS

324 pedicle screws in Group C and 775 pedicle screws in Group S were analyzed. Group C had a significantly higher average age and lower T-score, and included more patients with autoimmune disease and CKD than group S (all p<.05). Clear zones, screw migration and loss of correction were significantly less frequent in Group C (all p<.05). Thirteen screw breakages were observed; they were only in Group C (4.0%) and all were in the proximal of the two holes. Interbody and posterolateral fusion rates were not significantly different. At last follow-up, all clinical parameters including VAS-BP, VAS-LP, and K-ODI scores had improved significantly in both groups. Postoperative complications were not significantly different in the two groups.

CONCLUSION

In lumbar fusion surgery, using cement-augmented CPS in high-risk groups for implant failure could be a useful technical option for reducing acute radiological complications and obtaining clinical results comparable to those obtained using SPS in patients with low risk of implant failure.

LEVEL OF EVIDENCE

Level 4.

Comparison of clinical effectiveness of fenestrated and conventional pedicle screws in patients undergoing spinal surgery: a systematic review and meta-analysis

INTRODUCTION

Pedicle screws are commonly used for spinal procedures for fusion stability, which is particularly important in osteoporotic patients, who are at an increased risk of requiring revision procedures.

AREAS COVERED

A systematic review and meta-analysis were conducted to compare clinical effectiveness of conventional pedicle screws (CPS) vs fenestrated pedicle screws (FPS) in patients undergoing spinal surgery. Primary outcomes included screw loosening, revision surgeries (involving an implant) and reoperations (not involving intervention on an implant) in patients treated with CPS vs FPS, sub-stratified by with and without osteoporosis. Secondary outcomes included changes in pain scores. Forty-eight studies with 8,302 patients were included, with 1,565 (19.18%) treated with FPS and 6,710 (80.82%) treated with CPS. FPS was associated with a lower risk of screw loosening (p = 0.001) vs CPS. In the general population, there was a non-significant trend of lower revision rate, but no difference in reoperation rate, between patients treated with FPS vs CPS. In osteoporotic patients, revision rates were significantly lower for FPS vs CPS (p = 0.009).

EXPERT OPINION

This review suggests that FPS are effective for surgical fixation and reduce rates of screw loosening, and in osteoporotic patients, revision surgeries, compared to CPS.

Revision pedicle screws with impaction bone grafting: a case series

BACKGROUND

Pedicle screw fixation in spinal constructs can be subject to failure requiring revision surgery. In cases of aseptic loosening various salvage techniques have been described. Revision screws augmented with cement have become popular but are not without risks. Larger diameter screws are often used but result in reducing bone stock or expanding the pedicles. We present a novel technique of pedicle screw revision by impaction bone allografting and a case series.

METHODS

The failed screws are removed. The screw track is probed to check its integrity. Milled bone allograft is funneled into the screw hole and sequentially impacted, before insertion of a replacement screw. We report a case series and describe a single case where this method has been used. Information was gathered from the electronic patient record in our hospital.

RESULTS

Ten screws were revised in 7 patients. Mean age at first surgery was 60.86 (48-76) years. Average time between first surgery and revision was 12.6 (4.7-49.9) months. Average follow-up was 26.2 (5.7-62.2) months and no screws showed any signs of loosening.

CONCLUSIONS

Impaction grafting with bone allograft is a technique for pedicle screw salvage that can be used safely and effectively as an alternative to cemented screws, when pedicle screws have failed by aseptic loosening. It avoids the risks associated with cemented screws and in our series was successful.

Pullout strength of pedicle screws using cadaveric vertebrae with or without artificial demineralization

OBJECTIVES

To evaluate the differences in the pullout strength and displacement of pedicle screws in cadaveric thoracolumbar vertebrae with or without artificial demineralization.

METHODS

Five human lumbar and five thoracic vertebrae from one cadaver were divided into two hemivertebrae. The left-side specimens were included in the simulated osteopenic model group and the right-side bones in a control group. In the model group, we immersed each specimen in HCl (1 N) solution for 40 minutes. We measured bone mineral density (BMD) using dual-energy X-ray absorptiometry and quantitative computerized tomography. We inserted polyaxial pedicle screws into the 20 pedicles of the cadaveric lumbar and thoracic spine after measuring the BMD of the 2 hemivertebrae of each specimen. We measured the pullout strength and displacement of the screws before failure in each specimen using an Instron system.

RESULTS

The average pullout strength of the simulated osteopenic model group was 76% that of the control group. In the control and model groups, the pullout strength was 1678.87±358.96 N and 1283.83±341.97 N, respectively, and the displacement was 2.07±0.34 mm and 2.65±0.50 mm, respectively (p<.05). We detected positive correlations between pullout strength and BMD in the control group and observed a negative correlation between displacement and BMD in the model group.

CONCLUSIONS

By providing an anatomically symmetric counterpart, the human cadaveric model with or without demineralization can be used as a test bed for pullout tests of the spine. In the simulated osteopenic model group, pullout strength was significantly decreased compared with the untreated control group.

CLINICAL SIGNIFICANCE

Decreased bone mineral density may significantly reduce the pullout strength of a pedicle screw, even though the range is osteopenic rather than osoteoporotic.

Osseodensification drilling vs conventional manual instrumentation technique for posterior lumbar fixation: Ex-vivo mechanical and histomorphological analysis in an ovine model

Lumbar fusion is a procedure associated with several indications, but screw failure remains a major complication, with an incidence ranging 10% to 50%. Several solutions have been proposed, ranging from more efficient screw geometry to enhance bone quality, conversely, drilling instrumentation have not been thoroughly explored. The conventional instrumentation (regular [R]) techniques render the bony spicules excavated impractical, while additive techniques (osseodensification [OD]) compact them against the osteotomy walls and predispose them as nucleating surfaces/sites for new bone. This work presents a case-controlled split model for in vivo/ex vivo comparison of R vs OD osteotomy instrumentation in posterior lumbar fixation in an ovine model to determine feasibility and potential advantages of the OD drilling technique in terms of mechanical and histomorphology outcomes. Eight pedicle screws measuring 4.5 mm × 45 mm were installed in each lumbar spine of eight adult sheep (four per side). The left side underwent R instrumentation, while the right underwent OD drilling. The animals were killed at 6- and 12-week and the vertebrae removed. Pullout strength and non-decalcified histologic analysis were performed. Significant mechanical stability differences were observed between OD and R groups at 6- (387 N vs 292 N) and 12-week (312 N vs 212 N) time points. Morphometric analysis did not detect significant differences in bone area fraction occupancy between R and OD groups, while it is to note that OD showed increased presence of bone spiculae. Mechanical pullout testing demonstrated that OD drilling provided higher degrees of implant anchoring as a function of time, whereas a significant reduction was observed for the R group.

Biomechanical comparison of pullout strengths of six pedicle screws with different thread designs

OBJECTIVES

This study aims to assess the pullout strength of six different pedicle screw thread patterns.

MATERIALS AND METHODS

A total of 36 sheep spines were divided into six groups including six spines in each group: fully threaded cortical (Type A), fully threaded spongeous (Type B), fully cortical threads in the proximal half and fully spongeous threads in the distal half (Type C), fully spongeous threads in the proximal half and fully cortical threads in the distal half (Type D), unthreaded proximal half with fully spongeous threads in the distal half (Type E), and unthreaded proximal half with fully cortical threads in the distal half (Type F). The axial compression-traction machine was used for biomechanical testing at a pullout rate of 1 mm/min.

RESULTS

The mean values of pullout strength of the groups A, B, C, D, E, and F were 1112±7.52 N, 986±8.34 N, 646±3.88 N, 676±7.16 N, 609±9.52 N, and 769±6.49 N, respectively. There was a statistically significant difference between the screw groups A and B, C, D, E and F (p=0.036, p=0.028, p=0.04, p=0.039, and p=0.046, respectively). A statistically significant difference was observed between the groups B versus C and E (p=0.037 and p=0.021, respectively). There was no statistically significant difference between the groups B versus D and F (p=0.35 and p=0.61, respectively).

CONCLUSION

Fully threaded cortical pedicular screw design exhibited the strongest bone grasp compared to other thread designs. Further studies should be conducted in multidirectional force pattern on human spine to assess the six screw thread designs in a closer real-life setting simulation model.

Fenestrated Pedicle Screws in Spinal Oncology: Technique and Comparative Retrospective Analysis

BACKGROUND

The use of spinal stabilization with decompression has been shown to improve survival, spinal stability, and ambulatory status in patients with metastatic spinal tumors. However, the poor bone quality typically seen in these patients can prevent adequate stabilization. Fenestrated pedicle screws permit augmented fixation via injection of bone cement into the vertebral body upon screw placement, potentially mitigating the difficulties in achieving adequate stabilization in these patients.

OBJECTIVE

To compare surgical outcomes of posterior spinal fusion in patients with cancerous spinal lesions between polymethyl methacrylate cement-augmented fenestrated screws and standard pedicle screws.

METHODS

A total of 19 consecutive patients with cancerous spinal lesions receiving posterior spinal fusion (PSF) with pedicle screws from a single surgeon were retrospectively reviewed for demographic information, comorbidities, surgical parameters, and outcomes.

RESULTS

Ten patients underwent PSF with cement augmentation, whereas 9 underwent standard PSF. There was no significant difference in demographics, comorbidities, or surgical characteristics. Operative time was significantly greater in the cement-augmented group (302 ± 100 minutes vs 203 ± 55 minutes; P = .015). There was no significant difference in rates of operation or readmission between the cohorts nor was there any significant difference in discharge disposition. There was 1 case of surgical site infection (in a patient with a fenestrated screw) and no cases of cement extravasation. No instances of mechanical hardware failure were recorded.

CONCLUSIONS

Fenestrated screws confer similar risk profiles as nonfenestrated screws for posterior spinal fusion in patients with spinal cancer. However, fenestrated screws may affect operative time, radiation exposure, and impose risk of cement extravasation. Cement-augmented fenestrated pedicle screws may be a viable option for patients with poor bone quality associated with metastatic disease without significantly increased rates of surgical complications.

LEVEL OF EVIDENCE

3.

Experimental investigation of pull-out performance of pedicle screws at different polyurethane (PU) foam densities

Pedicle bone screws are one of the most critical materials used in spinal orthopaedic operations. Screw loosening and pull-out (PO) are basic complications encountered during or after surgery. Pull-out Strength (POS) of the bone is one of the significant parameters to understand the mechanical behaviour of a screw fixed to poor quality or osteoporotic bone. This study investigates how the POS of a pedicle screw is affected by the factors of the screw diameter and the polyurethane (PU) foam density by experimental analysis. In the experiments, two different diameter (5.5 and 6.5 mm) of conical pedicle screws and five different density (0.08, 0.16, 0.24, 0.32 and 0.48 g·cm^-3) PU foams were used. According to the force-displacement curves obtained from experimental results, the POS increased with the increases in screw diameter and PU foam density.

Removal of cement-augmented screws in distal femoral fractures and the effect of retained screws and cement on total knee arthroplasty: a biomechanical investigation

Background

Given the increasing number of osteoporotic fractures of the distal femur, screw augmentation with bone cement is an option to enhance implant anchorage. However, in implant removal or revision surgeries, the cement cannot be removed from the distal femur without an extended surgical procedure. Therefore, the aims of this study were to investigate (1) whether cement augmentation has any influence on screw removal and removal torque, and (2) whether the implantation of a femoral component of a knee arthroplasty and its initial interface stability are affected by the remaining screws/cement.

Material and methods

Eight pairs of fresh-frozen human female cadaveric distal femurs (mean age, 86 years) with a simulated AO/OTA 33 A3 fracture were randomized in paired fashion to two groups and fixed with a distal femoral locking plate using cannulated perforated locking screws. Screw augmentation with bone cement was performed in one of the groups, while the other group received no screw augmentation. Following biomechanical testing until failure (results published separately), the screws were removed and the removal torque was measured. A femoral component of a knee arthroplasty was then implanted, and pull-out tests were performed after cement curing. Interference from broken screws/cement was assessed, and the maximum pull-out force was measured.

Results

The mean screw removal torque was not significantly different between the augmented (4.9 Nm, SD 0.9) and nonaugmented (4.6 Nm, SD 1.3, p = 0.65) screw groups. However, there were significantly more broken screws in in the augmented screw group (17 versus 9; p < 0.001). There was no significant difference in the pull-out force of the femoral component between the augmented (2625 N, SD 603) and nonaugmented (2653 N, SD 542, p = 0.94) screw groups.

Conclusion

The screw removal torque during implant removal surgery does not significantly differ between augmented and nonaugmented screws. In the augmented screw group, significantly more screws failed. To overcome this, the use of solid screws in holes B, C, and G can be considered. Additionally, it is possible to implant a femoral component for knee arthroplasty that retains the initial anchorage and does not suffer from interference with broken screws and/or residual cement.

Level of Evidence

5

Partial Threading of Pedicle Screws in a Standard Construct Increases Fatigue Life: A Biomechanical Analysis

This study proposed a pedicle screw design where the proximal 1/3 of the screw is unthreaded to improve fixation in posterior spinal surgery. This design was also expected to reduce the incidence of mechanical failure often observed when an unsupported screw length is exposed outside the vertebra in deformed or degenerated segments. The aim of this study was to evaluate the fatigue life of the novel pedicle screw design using finite element analysis and mechanical testing in a synthetic spinal construct in accordance with American Society for Testing and Materials (ASTM) F1717. The following setups were evaluated: (i) pedicle screw fully inserted into the test block (EXP-FT-01 and EXP-PU-01; full thread (FT), proximal unthread (PU)) and (ii) pedicle screw inserted but leaving an exposed shaft length of 7.6 mm (EXP-FT-02 and EXP-PU-02). Corresponding finite element models FEM-FT-01, FEM-FT-02, FEM-PU-01, and FEM-PU-02 were also constructed and subjected to the same loading conditions as the experimental groups. The results showed that under a 220 N axial load, the EXP-PU-01 group survived the full 5 million cycles, the EXP-PU-02 group failed at 4.4 million cycles on average, and both EXP-FT-01 and EXP-FT-02 groups failed after less than 1.0 million cycles on average, while the fatigue strength of the EXP-FT-02 group was the lowest at 170 N. The EXP-FT-01 and EXP-FT-02 constructs failed through fracture of the pedicle screw, but a rod fractured in the EXP-PU-02 group. In comparison to the FEM-FT-01 model, the maximum von Mises stress on the pedicle screw in the FEM-PU-01 and FEM-PU-02 models decreased by −43% and −27%, respectively. In conclusion, this study showed that having the proximal 1/3 of the pedicle screw unthreaded can reduce the risk of screw fatigue failure when used in deformed or degenerated segments.

[Polymethylmethacrylate-augmented screw fixation in treatment of senile thoracolumbar tuberculosis combined with severe osteoporosis]

Objective

To explore the safety and effectiveness of polymethylmethacrylate-augmented screw fixation (PASF) in the treatment of elderly thoracolumbar tuberculosis combined with severe osteoporosis.

Methods

The clinical data of 20 elderly patients with thoracolumbar tuberculosis and severe osteoporosis who underwent PASF after anterior or posterior debridement and bone grafting and met the selection criteria between December 2012 and December 2014 were retrospectively analyzed. There were 8 males and 12 females with an average age of 68.5 years (range, 65-72 years). T value of bone mineral density was -4.2 to -3.6, with an average of -3.9. There were 12 cases of thoracic tuberculosis, 3 cases of thoracolumbar tuberculosis, and 5 cases of lumbar tuberculosis. The diseased segments involved T ₃-L ₄, including 11 cases of single-segment disease, 6 cases of double-segment disease, and 3 cases of multi-segment disease. The disease duration was 3-9 months, with an average of 6 months. The preoperative spinal nerve function of the patients was evaluated by the American Spinal Injury Association (ASIA) grading. There were 2 cases of grade A, 5 cases of grade B, 6 cases of grade C, 4 cases of grade D, and 3 cases of grade E. Postoperative imaging examination was used to evaluate the bone graft fusion and paravertebral abscess absorption, and to measure the Cobb angle of the segment to evaluate the improvement of kyphosis. The levels of erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) were tested. The visual analogue scale (VAS) score, Oswestry disability index (ODI), and ASIA grading were used to evaluate the effectivreness before operation, at 1 month after operation, and at last follow-up. The clinical cure of tuberculosis was also evaluated.

Results

All operation successfully completed. The operation time was 154-250 minutes, with an average of 202 minutes; the intraoperative blood loss was 368-656 mL, with an average of 512 mL. All 20 patients were followed up 18-42 months, with an average of 26.8 months. The postoperative pain and symptoms of tuberculosis in all patients relieved, and the paravertebral abscess was absorbed, reaching the cure standard for spinal tuberculosis. All bone grafts fusion achieved within 1 year after operation. Only 1 case had asymptomatic bone cement leakage into the paravertebral veins, and the remaining patients had no serious complications such as bone cement leakage in the spinal canal, pulmonary embolism, and neurovascular injury. At last follow-up, spinal cord nerve function significantly improved when compared with preoperative one. Among them, ASIA grading were 7 cases of grade C, 8 cases of grade D, and 5 cases of grade E, showing significant difference when compared with preoperative one ( Z=2.139, P=0.000). VAS score, ODI score, segmental Cobb angle, ESR, and CRP at 1 month after operation and at last follow-up were significantly improved when compared with preoperative ones ( P<0.05); there was no significant difference between 1 month after operation and last follow-up ( P>0.05). During the follow-up, no complications such as failure of internal fixation, proximal junctional kyphosis, or tuberculosis recurrence occurred.

Conclusion

For elderly patients with thoracolumbar tuberculosis and severe osteoporosis, PASF treatment is safe and effective.

[Minimally invasive stabilization of thoracolumbar osteoporotic fractures]

BACKGROUND

Minimally invasive stabilization of thoracolumbar osteoporotic fractures (OF) in neurologically intact patients is well established. Various posterior and anterior surgical techniques are available. The OF classification and OF score are helpful for defining the indications and choice of operative technique.

OBJECTIVE

This article gives an overview of the minimally invasive stabilization techniques, typical complications and outcome.

MATERIAL AND METHODS

Selective literature search and description of surgical techniques and outcome.

RESULTS

Vertebral body augmentation alone can be indicated in painful but stable fractures of types OF 1 and OF 2 and to some extent for type OF 3. Kyphoplasty has proven to be an effective and safe procedure with a favorable clinical outcome. Unstable fractures and kyphotic deformities (types OF 3-5) should be percutaneously stabilized from posterior. The length of the pedicle screw construct depends on the extent of instability and deformity. Bone cement augmentation of the pedicle screws is indicated in severe osteoporosis but increases the complication rate. Restoration of stability of the anterior column can be achieved through additional vertebral body augmentation or rarely by anterior stabilization. Clinical and radiological short and mid-term results of the stabilization techniques are promising; however, the more invasive the surgery, the more complications occur.

CONCLUSION

Minimally invasive stabilization techniques are safe and effective. The specific indications for the individual procedures are guided by the OF classification and the individual clinical situation of the patient.

Effect of Fenestrated Pedicle Screws with Cement Augmentation in Osteoporotic Patients Undergoing Spinal Fusion

OBJECTIVE

Osteoporosis is a well-known risk factor for instrumentation failure and subsequent pseudoarthrosis after spinal fusion. In the present systematic review, we analyzed the biomechanical properties, clinical efficacy, and complications of cement augmentation via fenestrated pedicle screws in spinal fusion.

METHODS

We conducted a systematic review in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Reports appearing in the PubMed database up to March 31, 2020 were queried using the key words "cement," "pedicle screw," and "osteoporosis." We excluded non-English language studies, studies reported before 2000, studies that had involved use of cement without fenestrated pedicle screws, nonhuman studies, technical reports, and individual case reports.

RESULTS

Twenty-five studies met the inclusion criteria. Eleven studies had tested the biomechanics of cement-augmented fenestrated pedicle screws. The magnitude of improvement achieved by cement augmentation of pedicle screws increased with the degree of osteoporosis. The cement-augmented fenestrated pedicle screw was superior biomechanically to the alternative "solid-fill" technique. Fourteen studies had evaluated complications. Cement extravasation with fenestrated screw usage was highly variable, ranging from 0% to 79.7%. However, cement extravasation was largely asymptomatic. Thirteen studies had assessed the outcomes. The use of cement-augmented fenestrated pedicles decreased screw pull out and improved fusion rates; however, the clinical outcomes were similar to those with traditional pedicle screw placement.

CONCLUSIONS

The use of cement-augmented fenestrated pedicle screws can be an effective strategy for achieving improved pedicle screw fixation in patients with osteoporosis. A potential risk is cement extravasation; however, this complication will typically be asymptomatic. Larger comparative studies are needed to better delineate the clinical efficacy.

Minimally invasive spinal fixation in an aging population with osteoporosis: clinical and radiological outcomes and safety of expandable screws versus fenestrated screws augmented with polymethylmethacrylate

OBJECTIVE

The goal of this study was to compare the clinical and radiological outcomes between fenestrated pedicle screws augmented with cement and expandable pedicle screws in percutaneous vertebral fixation surgical procedures for the treatment of degenerative and traumatic spinal diseases in aging patients with osteoporosis.

METHODS

This was a prospective, single-center study. Twenty patients each in the expandable and cement-augmented screw groups were recruited. Clinical outcomes included visual analog scale (VAS), Oswestry Disability Index (ODI), and satisfaction rates. Radiographic outcomes comprised radiological measurements on the vertebral motion segment of the treated levels. Intraoperative data including complications were collected. All patients completed the clinical and radiological outcomes. Outcomes were compared preoperatively and postoperatively.

RESULTS

An average shorter operative time was found in procedures in which expandable screws were used versus those in which cement-augmented screws were used (p < 0.001). No differences resulted in perioperative blood loss between the 2 groups. VAS and ODI scores were significantly improved in both groups after surgery. There was no significant difference between the 2 groups with respect to baseline VAS or ODI scores. The satisfaction rate of both groups was more than 85%. Radiographic outcomes also showed no significant difference in segment stability between the 2 groups. No major complications after surgery were seen. There were 4 cases (20%) of approach-related complications, all in fenestrated screw procedures in which asymptomatic cement extravasations were observed. In 1 case the authors detected a radiologically evident osteolysis around a cement-augmented screw 36 months after surgery. In another case they identified a minor loosening of an expandable screw causing local back discomfort at the 3-year follow-up.

CONCLUSIONS

Expandable pedicle screws and polymethylmethacrylate augmentation of fenestrated screws are both safe and effective techniques to increase the pullout strength of screws placed in osteoporotic spine. In this series, clinical and radiological outcomes were equivalent between the 2 groups. To the authors’ knowledge, this is the first report comparing the cement augmentation technique versus expandable screws in the treatment of aging patients with osteoporosis.

The effect of two types of resorbable augmentation materials - a cement and an adhesive - on the screw pullout pullout resistance in human trabecular bone

Augmentation materials, such as ceramic and polymeric bone cements, have been frequently used to improve the physical engagement of screws inserted into bone. While ceramic, degradable cements may ultimately improve fixation stability, reports regarding their effect on early fixation stability have been inconsistent. On the other hand, a newly developed degradable ceramic adhesive that can bond with tissues surrounding the screw, may improve the pullout performance, ensure early stability, and subsequent bony integration. The aim of this study was to investigate failure mechanisms of screw/trabecular bone constructs by comparing non-augmented screws with screws augmented with a calcium phosphate cement or an adhesive, i.e. a phosphoserine-modified calcium phosphate. Pullout tests were performed on screws inserted into trabecular cylinders extracted from human femoral bone. Continuous and stepwise pullout loading was applied with and without real-time imaging in a synchrotron radiation micro-computed tomograph, respectively. Statistical analysis that took the bone morphology into account confirmed that augmentation with the adhesive supported significantly higher pullout loads compared to cement-augmented, or non-augmented screws. However, the adhesive also allowed for a higher injection volume compared to the cement. In-situ imaging showed cracks in the vicinity of the screw threads in all groups, and detachment of the augmentation materials from the trabecular bone in the augmented specimens. Additional cracks at the periphery of the augmentation and the bone-material interfaces were only observed in the adhesive-augmented specimen, indicating a contribution of surface bonding to the pullout resistance. An adhesive that has potential for bonding with tissues, displayed superior pullout resistance, compared to a brushite cement, and may be a promising material for cementation or augmentation of implants.

Probe versus drill: A biomechanical evaluation of two different pedicle preparation techniques for pedicle screw fixation in human cadaveric osteoporotic spine

BACKGROUND

Aim of this biomechanical study was to investigate the anchorage of pedicle screws in osteoporotic vertebrae using two different preparation techniques (probe versus drill-assisted).

METHODS

Twelve thoracic vertebrae were used for the study. The right and left pedicles of the vertebra were prepared with a thoracic probe or a 3.2 mm drill bit and divided into two groups. A standard titanium (diameter: 5.5 mm, length: 45 mm) pedicle screw was then inserted. All pedicle screws were initially loaded with -25 N to +25 N in the cranio-caudal direction. The load was increased by 5 N every 500 cycles up to a maximum load of 10,000 cycles. Loosening was defined as a displacement of the pedicle screw head of >5 mm. The two groups were compared in terms of maximum number of cycles and maximum force until loosening.

FINDINGS

The pedicle screws prepared with the thoracic probe failed on average after 3819 cycles (SD 3281) and the pedicle screws prepared with the 3.2 mm drill after 3335 cycles (SD 3477). There was no significant difference between the two preparation techniques (P = .797). With regard to the maximum force until loosening, there was also no significant difference between the two techniques (thoracic probe: 61 N (SD 33), 3.2 mm drill bit: 56 N (SD 34), P = .791).

INTERPRETATION

Preparation of the pedicle screw hole either with a probe or drill bit doesn't seem to have an influence on pedicle screw loosening rates in the osteoporotic spine.

Evaluation of Peri-Implant Bone Grafting Around Surface-Porous Dental Implants: An In Vivo Study in a Goat Model

Dental implants with surface-porous designs have been recently developed. Clinically, peri-implant bone grafting is expected to promote early osseointegration and bone ingrowth when applied with surface-porous dental implants in challenging conditions. The aim of this study was to comparatively analyze peri-implant bone healing around solid implants and surface-porous implants with and without peri-implant bone grafting, using biomechanical and histomorphometrical assessment in a goat iliac bone model. A total of 36 implants (4.1 mm wide, 11.5 mm long) divided into three groups, solid titanium implant (STI; n = 12), porous titanium implants (PTI; n = 12) and PTI with peri-implant bone grafting using biphasic calcium phosphate granules (PTI + BCP; n = 12), were placed bilaterally in the iliac crests of six goats. The goats were sacrificed seven weeks post-operatively and then subjected to biomechanical (n = 6 per group) and histomorphometrical (n = 6 per group) assessment. The biomechanical assessment revealed no significant differences between the three types of implants. Although the peri-implant bone-area (PIBA%) measured by histomorphometry (STI: 8.63 ± 3.93%, PTI: 9.89 ± 3.69%, PTI + BCP: 9.28 ± 2.61%) was similar for the three experimental groups, the percentage of new bone growth area (BGA%) inside the porous implant portion was significantly higher (p < 0.05) in the PTI group (10.67 ± 4.61%) compared to the PTI + BCP group (6.50 ± 6.53%). These data demonstrate that peri-implant bone grafting around surface-porous dental implants does not significantly accelerate early osseointegration and bone ingrowth.

Augmented pedicle trajectory applied on the osteoporotic spine with lumbar degenerative disease: mid-term outcome

Purpose

To compare the safety and efficiency of cement-augmented pedicle screw with traditional pedicle screw technique applied on the patients in the osteoporotic spine with lumbar degenerative diseases.

Methods

Fifty-six patients followed up at least 2 years were enrolled in our institute with retrospectively reviewed from January 2009 to June 2014, diagnosed as lumbar spondylolisthesis, or lumbar stenosis, with T score ≤− 2.5 SD of BMD, and received less than three-segment PLIF or TLIF. All patients were divided into 2 groups: 28 (2 males, 26 females) in polymethylmethacrylate-augmented pedicle screw group (PSA) group, the other 28 (3 males, 25 females) in traditional pedicle screw group (TPS). Surgical data including the operation time, intra-operative blood loss, hospitalization day and surgical complications were recorded, as well as the radiological parameters measured from the postoperative X-rays and CT scans containing the rates of fusion, screw loosening, and cage subsidence incidence. In addition, the visual analog scores (VAS) and Oswestry Disability Index (ODI) were evaluated preoperatively and postoperatively.

Results

The average follow-up period was 34.32 months (ranging from 24 months to 51 months). Compared with PSA group, operation time and average hospital stay in the TPS group decreased significantly (P < 0.05). While no statistical difference for blood loss between 2 groups (P > 0.05). At 2 years postoperation, from CT-scans, 2/172 screws loosening and 1/56 segment non-union occurred in PSA group, with significantly lower incidence than those in TPS group (8/152 screws loosening and 6/50 segments non-union occurred, P < 0.05). Regarding the cage subsidence, 24 segments found height loss (5.30 ± 1.92 mm) in PSA group without difference compared with that of 19 segments (4.78 ± 1.37 mm) in TPS group (P > 0.05). Besides, the number and the location of cages and the leakage of the cement were found out little related with the subsidence in the PSA group (P > 0.05). After surgeries, VAS and ODI at 1 month, 6 months, 12 months, and last follow-up improved significantly in two groups (P < 0.05). There were no significant differences in VAS and ODI preoperatively and postoperatively between 2 groups (P > 0.05). In addition, eight patients with asymptomatic trajectory PMMA leakages were detected.

Conclusion

Cement-augmented pedicle screw technique is effective and safe in the osteoporotic spine with lumbar degenerative diseases, with better fusion rates and less screw loosening incidence. There is no difference in the fusion rate and loosening rate between the two groups in the single segment patients; however, there are better fusion rate and lower pedicle screw loosening rate of the PSA group in the double or multiple group patients.

Biomechanical and histologic assessment of a novel screw retention technology in an ovine lumbar fusion model

BACKGROUND CONTEXT

Screw loosening is a prevalent failure mode in orthopedic hardware, particularly in osteoporotic bone or revision procedures where the screw-bone engagement is limited.

PURPOSE

The objective of this study was to evaluate the efficacy of a novel screw retention technology (SRT) in an ovine lumbar fusion model.

STUDY DESIGN/SETTING

This was a biomechanical, radiographic, and histologic study utilizing an ovine lumbar spine model.

METHODS

In total, 54 (n=54) sheep lumbar spines (L₂-L₃) underwent posterior lumbar fusion (PLF) via pedicle screw fixation, connecting rod, and bone graft. Following three experimental variants were investigated: positive control (ideal clinical scenario), negative control (simulation of compromised screw holes), and SRT treatments. Biomechanical and histologic analyses of the functional spinal unit (FSU) were determined as a function of healing time (0, 3, and 12 months postoperative).

RESULTS

Screw pull-out, screw break-out, and FSU stability of the SRT treatments were generally equivalent to the positive control group and considerably better than the negative control group. Histomorphology of the SRT treatment screw region of interest (ROI) observed an increase in bone percentage and decrease in void space during healing, consistent with ingrowth at the implant interface. The PLF ROI observed similar bone percentage throughout healing between the SRT treatment and positive control. Less bone formation was observed for the negative control.

CONCLUSIONS

The results of this study demonstrate that the SRT improved screw retention and afforded effective FSU stabilization to achieve solid fusion in an otherwise compromised fixation scenario in a large animal model.

Effect of pedicle screw augmentation with a self-curing elastomeric material under cranio-caudal cyclic loading-a cadaveric biomechanical study

Background

Pedicle screws can be augmented with polymethylmethacrylate (PMMA) cement through cannulated and fenestrated pedicle screws to improve screw anchorage. To overcome the drawbacks of PMMA, a modified augmentation technique applying a self-curing elastomeric material into a balloon-created cavity prior to screw insertion was developed and evaluated. The aim of the study was to compare the effect of the established and novel augmentation technique on pedicle screw anchorage in a biomechanical in vitro experiment.

Methods

In ten lumbar vertebral bodies, the right pedicles were instrumented with monoaxial cannulated and fenestrated pedicle screws and augmented in situ with 2 ml PMMA. The left pedicles were instrumented with monoaxial cannulated pedicle screws. Prior to left screw insertion, a balloon cavity was created and filled with 3 ml of self-curing elastomer (silicone). Each screw was subjected to a cranio-caudal cyclic load starting from − 50 to 50 N while the upper load was increased by 5 N every 100 load cycles until loosening or 11,000 cycles (600 N). After cyclic loading, a pullout test of the screws was conducted.

Results

The mean cycles to screw loosening were 9824 ± 1982 and 7401 ± 1644 for the elastomer and PMMA group, respectively (P = 0.012). The post-cycling pullout test of the loosened screws showed differences in the failure mode and failure load, with predominantly pedicle/vertebrae fractures in the PMMA group (1188.6 N ± 288.1) and screw pullout through the pedicle (671.3 N ± 332.1) in the elastomer group.

Conclusion

The modified pedicle screw augmentation technique involving a balloon cavity creation and a self-curing elastomeric silicone resulted in a significantly improved pedicle screw anchorage under cyclic cranio-caudal loading when compared to conventional in situ PMMA augmentation.

Biomechanical comparative study of the stability of injectable pedicle screws with different lateral holes augmented with different volumes of polymethylmethacrylate in osteoporotic lumbar vertebrae

BACKGROUND CONTEXT

Polymethylmethacrylate (PMMA) is widely used for pedicle screw augmentation in osteoporosis. Until now, there had been no studies of the relationship between screw stability and the distribution and volume of PMMA.

PURPOSE

The objective of this study was to analyze the relationship between screw stability and the distribution pattern and injected volume of PMMA.

STUDY DESIGN

This is a biomechanical comparison of injectable pedicle screws with different lateral holes augmented with different volumes of PMMA in cadaveric osteoporotic lumbar vertebrae.

METHODS

Forty-eight osteoporotic lumbar vertebrae were randomly divided into Groups A, B, and C with different pedicle screws (16 vertebrae in each group), and then each group was randomly divided into Subgroups 0, 1, 2, and 3 with different volumes of PMMA (four vertebra with eight pedicles in each subgroup). A pilot hole was prepared in advance using the same method in all samples. Type A and type B pedicle screws were directly inserted into vertebrae in Groups A and B, respectively, and then different volumes of PMMA (0, 1.0, 1.5, and 2.0 mL) were injected through the screws and into vertebrae in Subgroups 0, 1, 2, and 3. The pilot holes were filled with different volumes of PMMA (0, 1.0, 1.5, and 2.0 mL), and then the screws were inserted in Groups C0, C1, C2, and C3. Screw position and distribution of PMMA were evaluated radiographically, and axial pullout tests were performed to measure maximum axial pullout strength (F_max).

RESULTS

Polymethylmethacrylate surrounded the anterior one-third of screws in the vertebral body in Groups A1, A2, and A3; the middle one-third of screws in the junction area of the vertebral body and the pedicle in Groups B1, B2, and B3; and the full length of screws evenly in both the vertebral body and the pedicle in Groups C1, C2, and C3. There was no malpositioning of screws or leakage of PMMA in any sample. Two-way analysis of variance revealed that two factors-distribution and volume of PMMA-significantly influenced F_max (p<.05) but that they were not significantly correlated (p=.088). F_max values in groups using augmentation with PMMA values significantly improved compared with those in groups without PMMA (p<.05).

CONCLUSIONS

Polymethylmethacrylate can significantly enhance the stability of different injectable pedicle screws in osteoporotic lumbar vertebrae, and screw stability is significantly correlated with the distribution pattern and the injected volume of PMMA. The closer the PMMA to the pedicle and the greater the quantity of injected PMMA, the greater is the pedicle screw stability. Injection of 2.0 mL of PMMA through screws with four lateral 180° holes or of 1.0 mL of PMMA through screws with six lateral 180° holes increases the stability of pedicle screws.

Perfusion pressure of a new cannulating fenestrated pedicle screw during cement augmentation

BACKGROUND

Cannulating fenestrated pedicle screws are effective for fixating osteoporotic vertebrae. However, a major limitation is the excessive pressure required to inject a sufficient amount of cement into the vertebral body through the narrow hole of a pedicle screw. We have recently proposed a new cannulating fenestrated pedicle screw with a large hole diameter and a matched inner pin for screw-strength maintenance. Our purpose was to determine whether the new screw can significantly reduce bone-cement perfusion pressure during cement augmentation, METHODS: Two different methods were used to examine perfusion pressure. Hagen-Poisseuille's flow model in a tube was used to calculate pressure drop in the bone-cement channel. Experimentally, both Newtonian silicone oil and bone-cement (polymethyl methacrylate) were tested using a cement pusher through the cannulating screw at a constant rate of 2 ml/min.

FINDINGS

The internal hollow portion of the screw was the bottleneck of the perfusion, and the new design significantly reduced the perfusion pressure. Specifically, perfusion pressure dropped by 59% (P < 0.05) when diameter size was doubled.

INTERPRETATION

The new design effectively improved the application of bone-cement augmentation with the ease of bone-cement perfusion, thereby enhancing operational safety.

Superior cortical screw in osteoporotic lumbar vertebrae: A biomechanics and microstructure-based study

BACKGROUND

Osteoporosis reduces the bone-screw purchase, potentially reducing pullout strength and other biomechanical properties. However, the existing pedicle screw approach may not compensate for the detrimental effects of decreased vertebral bone mineral density.

METHODS

Two methods of screw insertion were performed in thirteen cadaveric osteoporotic lumbar vertebrae: Magerl's method in the left pedicle, and superior cortical screw method in the right (its entry point located vertically 3 mm above Magerl's point). Before screw fixations, the pedicle and its corresponding vertebral body were divided into six equal layers from cranial to caudal by performing micro-CT and tested for microstructure properties, such as bone mineral density, trabecular bone volume fraction, trabecular thickness, trabecular separation and trabecular number. Further, pedicle was horizontally divided into three regions and tested. After screw fixations, microstructure properties of the bone surrounding the screws were analyzed. Finally, the screw pullout strength was tested biomechanically.

FINDINGS

The bone structure is denser in the upper third of the pedicle and its corresponding vertebral body. A similar microstructure is seen within the pedicle. This study reveals that the pullout strength is significantly correlated to the bone mineral density, trabecular bone volume fraction and trabecular thickness. Biomechanical test showed pullout strength in the superior cortical screw group with mean 613.3 N (SD 200.4) was 22.4% higher than that in the Magerl group with mean 501.2 N (SD 256.6).

INTERPRETATION

The superior cortical screw method can be a reliable alternative, to provide better pullout strength for posterior lumbar instrumentation, especially in osteoporotic patients.

The quantity of bone cement influences the anchorage of augmented pedicle screws in the osteoporotic spine: A biomechanical human cadaveric study

BACKGROUND

The aim of this comparative biomechanical human cadaveric study was to investigate the anchorage of augmented screws with two different volumes of bone cement. For this purpose the effect of cranio-caudal loadings on pedicle screws was evaluated and axial pullout tests were performed.

METHODS

A total of 50 pedicle screws (25 augmented/25 non-augmented) were instrumented into osteoporotic vertebra of fresh human cadavers. The augmented screws were grounded by two different volumes of bone cement (1.5cm³ vs 4cm³). Biomechanical performance was assessed by performing a cyclic loading protocol (frequency: 3Hz, load range: 20-200N, number of cycles: 100,000), followed by axial pullout (13 augmented/11 non-augmented) or by either directly measuring axial pullout strength (12 augmented/12 non-augmented).

FINDINGS

The median T-score of the specimens was -4.25 (range: -6.38 to -2.4). Pullout tests with and without cyclic preloading showed significantly increased pullout strength in augmented screws (Fmax: augmented: 1159N (SD 395N); non-augmented: 532N (SD 297N); p<0.05). No significant difference in the pullout strength was found concerning the quantity of cement (Fmax (direct pullout): 4.0cm³: 1463N (SD 307N); 1.5cm³: 1214N (SD 236N); p>0.05). The pullout strength significantly decreased in high-volume augmented screws after cyclic loading (Fmax (4.0cm³): direct pullout 1463N (SD 307N); cyclic preload: 902N (SD 435N); p<0.05).

INTERPRETATION

Biomechanical advantages of augmented pedicle screws can also be found after cyclic preload. However, our results indicate that the anchoring stability of high-volume augmented pedicle screws after cyclic loading is disadvantageous compared to moderate augmented screws; thus high-volume augmentation should be avoided.

Radiofrequency-activated PMMA-augmentation through cannulated pedicle screws: A cadaver study to determine the biomechanical benefits in the osteoporotic spine

INTRODUCTION

PMMA-augmentation of pedicle screws strengthens the bone-screw-interface reducing cut-out risk. Injection of fluid cement bears a higher risk of extravasation, with difficulty of application because of inconsistent viscosity and limited injection time.

OBJECTIVE

To test a new method of cement augmentation of pedicle screws using radiofrequency-activated PMMA, which is suspected to be easier to apply and have less extravasations.

METHODS

Twenty-seven fresh-frozen human cadaver lumbar spines were divided into 18 osteoporotic (BMD ≤ 0.8 g/cm2) and 9 non-osteoporotic (BMD > 0.8 g/cm2) vertebral bodies. Bipedicular cannulated pedicle screws were implanted into the vertebral bodies; right screws were augmented with ultra-high viscosity PMMA, whereas un-cemented left pedicle screws served as negative controls. Cement distribution was controlled with fluoroscopy and CT scans. Axial pullout forces of the screws were measured with a material testing machine, and results were analyzed statistically.

RESULTS

Fluoroscopy and CT scans showed that in all cases an adequately big cement depot with homogenous form and no signs of extravasation was injected. Pullout forces showed significant differences (p < 0.001) between the augmented and non-augmented pedicle screws for bone densities below 0.8 g/cm2 (661.9 N ± 439) and over 0.8 g/cm2 (744.9 N ± 415).

CONCLUSIONS

Pullout-forces were significantly increased in osteoporotic as well as in non-osteoporotic vertebral bodies without a significant difference between these groups using this standardized, simple procedure with increased control and less complications like extravasation.

The Pedicles Are Not the Densest Regions of the Lumbar Vertebrae: Implications for Bone Quality Assessment and Surgical Treatment Strategy

STUDY DESIGN

Cadaver study.

OBJECTIVE

To determine the bone density of lumbar vertebral anatomic subregions. Bone mineral density (BMD) is a major factor in osseous fixation construct strength. The standard region for implant fixation of the spine is the pedicle; however, other regions may be more viable options with higher bone quality.

METHODS

Using computed tomography images, the spine was digitally isolated by applying a filter for adult bone. The spine model was separated into 5 lumbar vertebrae, followed by segmentation of each vertebra into 7 regions and determination of average Hounsfield units (HU). HU was converted to BMD with calibration phantoms of known BMD.

RESULTS

Overall mean BMD in vertebral regions ranged from 172 to 393 mg/cm³ with the highest and lowest BMD in the lamina and vertebral body, respectively. Vertebral regions formed 3 distinct groups (P < .03). The vertebral body and transverse processes represent one group with significantly lower BMD than other regions. Spinous process, pedicles, and superior articular processes represent a second group with moderate BMD. Finally, inferior articular process (IAP) and lamina represent a third group with significantly higher BMD than other regions.

CONCLUSIONS

Standard lumbar fusion currently uses the vertebral body and pedicles as primary locations for fixation despite their relatively low BMD. Utilization of posterior elements, especially the lamina and IAP, may be advantageous as a supplement to modern constructs or the primary site for fixation, possibly mitigating construct failures due to loosening or pullout.

Biomechanical Investigation of a Novel Revision Device in an Osteoporotic Model: Pullout Strength of Pedicle Screw Anchor Versus Larger Screw Diameter

STUDY DESIGN

In vitro cadaveric biomechanical study.

OBJECTIVE

To assess revision pullout strength of novel anchored screws (AS) versus conventional larger diameter traditional pedicle screws (TPS) in an osteoporotic model.

SUMMARY OF BACKGROUND DATA

Pedicle screws are the most ubiquitous method of treating spinal pathologies requiring lumbar fusion. Although these screws are effective in providing 3-column stabilization of the spine, revision surgeries are occasionally necessary, particularly for geriatric and osteoporotic populations. Innovative technologies should be tested to ensure continued improvement in revision techniques.

METHODS

For 4 specimens at L2-L5 (T-score=-3.6±0.54), 6.5-mm-diameter TPS were inserted into left and right pedicles and were pulled out; revision screws were then inserted. Polyether-ether-ketone anchors, designed to expand around a 6.5-mm screw, were inserted into all left pedicles. On the contralateral side, 7.5-mm-diameter TPS were inserted at L2-L3, and 8.5-mm-diameter TPS at L4-L5. Pullout testing was performed at 10 mm/min. The maximum pullout strength and insertion forces were recorded.

RESULTS

The initial average pullout force (6.5-mm screw) was 837 N (±329 N) and 642 N (±318 N) in L2-L3 and L4-L5 left pedicles, and 705 N (±451 N) and 779 N (±378 N) in L2-L3 and L4-L5 right pedicles, respectively. Comparison of revision pullout forces versus initial pullout forces revealed the following: 87% and 63% for AS in L2-L3 and L4-L5 left pedicles, respectively; 56% for 7.5-mm and 93% for 8.5-mm TPS in L2-L3 and L4-L5 right pedicles, respectively.

CONCLUSIONS

Anchor sleeves with 6.5-mm-diameter pedicle screws provided markedly higher resistance to screw pullout than 7.5-mm-diameter revision screws and fixation statistically equivalent to 8.5-mm-diameter screws, possibly because of medial-lateral expansion within the vertebral space and/or convex filling of the pedicle. AS results had the lowest SD, indicating minimal variability in bone-screw purchase.

Risk of cement leakage and pulmonary embolism by bone cement-augmented pedicle screw fixation of the thoracolumbar spine

BACKGROUND

Cement-augmented pedicle screw instrumentation (CAPSI) of the thoracolumbar spine is indicated in osteoporosis or osteopenia to improve pullout strength and biomechanical stability of pedicle screws (PS). Only a few studies report on the incidence of pulmonary cement embolism or other complications associated with CAPSI.

PURPOSE

The aim of this retrospective study was to assess the rate of CAPSI-associated complications.

STUDY DESIGN

Retrospective cohort study.

PATIENT SAMPLE

Patients who underwent CAPSI due to spinal tumors or degenerative spine disease.

OUTCOME MEASURES

Cement leakage, pulmonary cement embolism (PCE), mortality rate.

METHODS

Our clinical database was reviewed for patients who underwent CAPSI between January 2012 and June 2015. A total of 165 patients (mean age 71±11.2; range: 46 to 93 years; m=62, f=103) were included. Indications were osteoporotic fractures (n=40), spinal metastases (n=57), degenerative (n=49) or infectious spine disease (n=5), and traumatic vertebral fractures (n=14) with an associated osteoporosis. Every patient received between 2 and 21 (mean 8±3.3) cement-augmented pedicle screws in the thoracolumbar and lumbosacral spine. Both intraoperative cement leakage in prevertebral veins, the inferior vena cava, and/or pulmonary arteries, and leakage detected on postoperative imaging were evaluated. We assessed the incidence of clinically symptomatic and asymptomatic events.

RESULTS

In 29 of 31 patients with intraoperative suspicion of cement leakage into prevertebral veins or the inferior vena cava on lateral fluoroscopy, which were without hemodynamic relevance, cement extrusion was confirmed on postoperative X-ray or computed tomography (CT) scan. In three of eight patients with suspicion of PCE, PCE was verified on thoracic CT. Four patients experienced life-threatening intraoperative hemodynamic reactions, either due to cement embolism (n=2; 1.2%) or anaphylactic shock (n=2; 1.2%) with need for intraoperative cardiopulmonary resuscitation in three cases. Two patients died due to fulminant PCE. Three patients with dyspnea 1 day after surgery were also confirmed with PCE on chest CT. In five patients, an asymptomatic PCE was found incidentally on postoperative imaging. In addition, 68 patients with cement leakage into prevertebral veins or the ascending cava vein were found incidentally on postoperative spine X-ray or CT. Two of 10 patients with intraspinal epidural cement leakage required revision surgery. One hundred ten of 165 patients (66.7%) had clinically asymptomatic cement leakage. Thirteen patients had PCE (7.9%), of whom five (3.0%) were symptomatic. Two patients experienced intraoperative cement-induced anaphylaxis (1.2%). The overall symptomatic complication rate was 5.5% (n=9). The 30-day mortality rate was 1.8% (n=3).

CONCLUSIONS

CAPSI bears a high risk of asymptomatic cement leakage. The risk for associated severe complications was also relatively high and probably underestimated considering the retrospective nature of the present study. A strict indication for cement augmentation, especially in patients with cardiac predisposition, should be the consequence. We doubt that technical aspects of cement application and/or different types of cement are capable of reducing the risk of these complications substantially.

A novel injectable calcium phosphate-based nanocomposite for the augmentation of cannulated pedicle-screw fixation

Polymethyl methacrylate (PMMA)-augmented cannulated pedicle-screw fixation has been routinely performed for the surgical treatment of lumbar degenerative diseases. Despite its satisfactory clinical outcomes and prevalence, problems and complications associated with high-strength, stiff, and nondegradable PMMA have largely hindered the long-term efficacy and safety of pedicle-screw fixation in osteoporotic patients. To meet the unmet need for better bone cement for cannulated pedicle-screw fixation, a new injectable and biodegradable nanocomposite that was the first of its kind was designed and developed in the present study. The calcium phosphate-based nanocomposite (CPN) exhibited better anti-pullout ability and similar fluidity and dispersing ability compared to clinically used PMMA, and outperformed conventional calcium phosphate cement (CPC) in all types of mechanical properties, injectability, and biodegradability. In term of axial pullout strength, the CPN-augmented cannulated screw reached the highest force of ~120 N, which was higher than that of PMMA (~100 N) and CPC (~95 N). The compressive strength of the CPN (50 MPa) was three times that of CPC, and the injectability of the CPN reached 95%. In vivo tests on rat femur revealed explicit biodegradation of the CPN and subsequent bone ingrowth after 8 weeks. The promising results for the CPN clearly suggest its potential for replacing PMMA in the application of cannulated pedicle-screw fixation and its worth of further study and development for clinical uses.

Fenestrated Screws Augmented With PMMA Increase the Pullout Strength of Sacral Pedicle Screws

STUDY DESIGN

Biomechanics.

OBJECTIVE

The objective of this study was to compare the fixation in osteoporotic specimens of a bicortical sacral pedicle screw to the following constructs: a unicortical solid screw augmented with polymethylmethacrylate (PMMA) (TRACT), a unicortical fenestrated screw augmented with PMMA (FEN), and a bicortical screw/sacral alar screw combination (PED/ALA).

SUMMARY OF BACKGROUND DATA

Rigid posterior sacral fixation of osteoporotic spines is problematic due to poor bone quality. Options to improve fixation include bicortical or tricortical screw trajectory, addition of S2 alar screws, or PMMA augmentation.

MATERIALS AND METHODS

Eleven osteoporotic cadaveric sacra were potted in 2-part filler compound. As a baseline fixation method, a 6.5-mm bicortical pedicle screw was placed into 1 pedicle of each sacra. The contralateral side was randomly assigned to receive a unicortical fenestrated 6.5-mm pedicle screw augmented with PMMA, a unicortical solid 6.5-mm pedicle screw augmented with PMMA, or a bicortical pedicle screw/alar screw construct. Pedicle screws were failed in cantilever pullout using a materials testing machine. Data were normalized into a test index by taking the ratio of the test screw pullout strength to the intraspecimen bicortical control screw pullout strength.

RESULTS

The fixation strengths as expressed by the test index of the TRACT, FEN, and PED/ALA groups were 101%, 157%, and 167%, respectively. Although there was no statistical difference detected between the FEN and PED/ALA groups, the TRACT group had a trend toward lower pullout strength than the FEN (P=0.06) or PED/ALA (0.06) groups. Although underpowered, this study did not detect biomechanical inferiority for any of the 3 test configurations when compared with standard bicortical S1 screws.

CONCLUSIONS

The data indicate that the fixation strength of a fenestrated, PMMA-augmented construct or a combined S1 pedicle/S2 alar screw construct may be better than either standard bicortical or unicortical PMMA screw-tract augmented screws.

Multiple Spinal Revision Surgery in a Patient with Parkinson's Disease

Parkinson's disease (PD) patients frequently have several spinal deformities leading to postural instabilities including camptocormia, myopathy-induced postural deformity, Pisa syndrome, and progressive degeneration, all of which adversely affect daily life activities. To improve these postural deformities and relieve the related neurologic symptoms, patients often undergo spinal instrumentation surgery. Due to progressive degenerative changes related to PD itself and other complicating factors, patients and surgeons are faced with instrument failure-related complications, which can ultimately result in multiple revision surgeries yielding various postoperative complications and morbidities. Here, we report a representative case of a 70-year-old PD patient with flat back syndrome who had undergone several revision surgeries, including anterior and posterior decompression and fusion for a lumbosacral spinal deformity. The patient ultimately benefitted from a relatively short segment fixation and corrective fusion surgery.

Intraosseous Injection of Simvastatin in Poloxamer 407 Hydrogel Improves Pedicle-Screw Fixation in Ovariectomized Minipigs

BACKGROUND

Osteoporosis leads to poor osseointegration and reduces implant stability. Statins have been found to stimulate bone formation, but the bioavailability from oral administration is low. Local application may be more effective at augmenting bone formation and enhancing implant stability. This study was performed to evaluate the efficacy of an intraosseous injection of simvastatin in thermosensitive poloxamer 407 hydrogel to enhance pedicle-screw fixation in calcium-restricted ovariectomized minipigs.

METHODS

Nine mature female Guangxi Bama minipigs underwent bilateral ovariectomy and were fed a calcium-restricted diet for 18 months. Simvastatin (0, 0.5, or 1 mg) in thermosensitive poloxamer 407 hydrogel was injected into the lumbar vertebrae (L4-L6), and titanium alloy pedicle screws were implanted. Bone mineral density (BMD) measurements of the lumbar vertebrae were determined by dual x-ray absorptiometry (DXA) before and 3 months after treatment. The lumbar vertebrae were harvested and analyzed with use of microcomputed tomography, biomechanical pull-out testing, histological analysis, and Western blot analysis for bone morphogenetic protein (BMP)-2 and vascular endothelial growth factor (VEGF) expression.

RESULTS

Evaluation over a 3-month study period demonstrated that the BMD of the vertebrae injected with 0.5 and 1.0 mg of simvastatin had increased by 31.25% and 31.09%, respectively, compared with vehicle-only injection (p ≤ 0.00014 for both) and increased by 32.12% and 28.16%, respectively, compared with the pre-treatment levels (p < 0.0001 for both). A single injection of simvastatin in poloxamer 407 increased trabecular volume fraction, thickness, and number and decreased trabecular separation (p ≤ 0.002). The bone formation and mineral apposition rates significantly increased (p ≤ 0.023). The percentage of osseointegration in the simvastatin 0.5 and 1-mg groups was 46.54% and 42.63% greater, respectively, than that in the vehicle-only group (p ≤ 0.006), and the maximum pull-out strength was 45.75% and 51.53% greater, respectively, than in the vehicle-only group (p ≤ 0.0005). BMP-2 and VEGF expressions were higher than for the vehicle-only injection.

CONCLUSIONS

A single intraosseous injection of simvastatin in thermosensitive poloxamer 407 hydrogel stimulated bone formation, increased BMD, improved bone microstructure, promoted osseointegration, and significantly enhanced the stability of pedicle screws in calcium-restricted ovariectomized minipigs.

CLINICAL RELEVANCE

These results provide rationale for evaluating intraosseous injection of simvastatin in poloxamer 407 to enhance implant fixation in patients with osteoporosis.