Influence of screw augmentation in posterior dynamic and rigid stabilization systems in osteoporotic lumbar vertebrae: a biomechanical cadaveric study

Polymethylmethacrylate (PMMA) Augmentation Enhances the Mechanical Characteristics of Midfoot Beam Constructs in Charcot Neuroarthropathy Cadaver Model

BACKGROUND

Even with the best conservative care, patients with Charcot neuroarthropathy (CN) of the foot and ankle often ulcerate, increasing their risk of infection, amputation, and death. Surgical fixation has been associated with risk of recurrent ulceration, potentially due to poor bone quality prone to recurrent deformity and ulceration. We propose midfoot beam reconstruction with PMMA augmentation as a novel means of improving fixation.

METHODS

A protocol was developed to create characteristic CN midfoot fragmentation both visually and fluoroscopically in each of 12 matched-pair cadaveric feet. Afterward, the pairs were divided into 2 groups: (1) midfoot beam fusion surgery alone, and (2) midfoot beam fusion surgery augmented with PMMA. A solid 7.0-mm beam was placed into the medial column and a solid 5.5-mm beam was placed across the lateral column. In the PMMA group, 8 to 10 mL of PMMA was inserted into the medial column. The hindfoot of each specimen was potted and the metatarsal heads were cyclically loaded for 1800 cycles, followed by load to failure while load and displacement were continually recorded.

RESULTS

One specimen in the beam alone group failed before reaching the 1800th cycle and was not included in the failure analysis. The midfoot beam only group demonstrated greater mean displacement during cycle testing compared with the PMMA group, P < .05. The maximum force (N), stiffness (N/mm), and toughness (Nmm) were all significantly greater in the group augmented with PMMA, P < .05.

CONCLUSION

In a CN cadaveric model, PMMA augmentation significantly decreased gapping during cyclic loading and nearly doubled the load to failure compared with midfoot beams alone.

CLINICAL RELEVANCE

The results of this biomechanical study demonstrate that augmentation of midfoot beams with PMMA increases the strength and stiffness of the fusion construct. This increased mechanical toughness may help reduce the risk of nonunion and infection in patients with neuropathic midfoot collapse.

Biomechanical Analysis of Trapezoidal Thread Screw-Rod Fixation in Pedicle Section of Cervical Spine: A Finite-Element Analysis

Background

Cervical pedicle screw-rod fixation presents a complex approach in spinal surgery, offering enhanced spine stabilization in variable conditions considering traumatic injuries, degenerative changes, as well as orthopaedic and oncological ailments. This technique employs small diameter screw implants strategically placed to bolster the mechanical integrity of the spine. Notably, it involves minimally invasive procedures, resulting in smaller incisions and reduced patient discomfort. This study aims to assess the effects of trapezoidal thread screws in pedicle sections of the cervical spine during flexion-extension loadings, focusing on factors such as range of motion (ROM), implant stress, and stress on adjacent bone.

Methods

Utilizing CT scan data, a finite element model of the cervical spine (C2-C7 vertebrae) was prepared. Trapezoidal thread screws were integrated into a single-level pedicle screw-rod fixation at the C5-C6 vertebrae. The C2 vertebra were given a compressive load of 50 N along with a moment of 1 Nm, resulting in the immobilization of the C7.

Results and Discussion

The results indicate a reduction in ROM at the C5-C6 level by 69% to 77% compared to the intact spine during flexion-extension loading, with a slight increase in ROM observed at adjacent cervical spine levels. Stress analysis revealed that the trapezoidal thread screws induced stresses ranging from 24 MPa to 29 MPa in PEEK trapezoidal screw-rod implants, which fall below the material's yield stress.

Conclusions

This suggests that the trapezoidal thread profile may be advantageous in minimizing stress concentration, attributed to its larger contact area with the vertebrae bone between the threads.

Experimental magnesium phosphate cement paste increases torque of trochanteric fixation nail advanced™ blades in human femoral heads

BACKGROUND

The use of polymethylmethacrylate cement for in-situ implant augmentation has considerable disadvantages: it is potentially cytotoxic, exothermic and non-degradable. Therefore, the primary aim of this study was to develop a magnesium phosphate cement which meets the requirements for in-situ implant augmentation as an alternative. Secondly, this experimental cement was compared to commercial bone cements in a biomechanical test set-up using augmented femoral head blades.

METHODS

A total of 40 human femoral heads were obtained from patients who underwent total hip arthroplasty. After bone mineral density was quantified, specimens were assigned to four treatment groups. A blade of the Trochanteric Fixation Nail Advanced™ was inserted into each specimen and augmented with either Traumacem™ V+, Paste-CPC, the experimental magnesium phosphate cement or no cement. A rotational load-to-failure-test (0° to 90°) was performed.

FINDINGS

A conventional two-component magnesium phosphate cement failed in-situ implant augmentation consistently due to filter pressing. Only a glycerol-based magnesium phosphate paste was suitable for the augmentation of femoral head blades. While the blades augmented with Traumacem™ V+ yielded the highest maximum torque overall (22.1 Nm), the blades augmented with Paste-CPC and the magnesium phosphate paste also showed higher maximum torque values (15.8 and 12.8 Nm) than the control group (10.8 Nm).

INTERPRETATION

This study shows for the first time the development of a degradable magnesium phosphate cement paste which fulfills the requirements for in-situ implant augmentation. Simultaneously, a 48% increase in stability is demonstrated for a scenario where implant anchorage is difficult in osteoporotic bone.

Pedicle screw augmentation in posterior constructs of the thoracolumbar spine: How many pedicle screws should be augmented?

BACKGROUNDS

To evaluate the effects of different pedicle screw augmentation strategies on screw loosening and adjacent segment collapse at the proximal end of long-segment instrumentation.

METHODS

Eighteen osteoporotic (9 male, 9 female donors; mean age: 74.7 ± 10.9 [SD] years) thoracolumbar multi-segmental motion segments (Th11 - L1) were assigned as follows: control, one-level augmented screws (marginally), and two-level augmented screws (fully augmented) groups (3 × 6). Pedicle screw placement was performed in Th12 and L1. Cyclic loading in flexion started with 100-500 N (4 Hz) and was increased by 5 N every 500 cycles. Standardized lateral fluoroscopy images with 7.5 Nm loading were obtained periodically during loading. The global alignment angle was measured to evaluate the overall alignment and proximal junctional kyphosis. The intra-instrumental angle was used to evaluate screw fixation.

FINDINGS

Considering screw fixation as a failure criterion, the failure loads of the control (683 N), and marginally (858 N) and fully augmented (1050 N) constructs were significantly different (ANOVA p = 0.032).Taking the overall specimen alignment as failure criteria, failure loads of the three groups (control 933 ± 271.4 N, marginally 858 N ± 196 N, and full 933 ± 246.3 N were in the same range and did not show any significance (p = 0.825).

INTERPRETATION

Global failure loads were comparable among the three groups and unchanged with augmentation because the adjacent segment and not the instrumentation failed first. Augmentation of all screws showed significant improved in screw anchorage.

Restricted cement augmentation in unstable geriatric midthoracic fractures treated by long-segmental posterior stabilization leads to a comparable construct stability

The goal of this study is to compare the construct stability of long segmental dorsal stabilization in unstable midthoracic osteoporotic fractures with complete pedicle screw cement augmentation (ComPSCA) versus restricted pedicle screw cement augmentation (ResPSCA) of the most cranial and caudal pedicle screws under cyclic loading. Twelve fresh frozen human cadaveric specimens (Th4–Th10) from individuals aged 65 years and older were tested in a biomechanical cadaver study. All specimens received a DEXA scan and computer tomography (CT) scan prior to testing. All specimens were matched into pairs. These pairs were randomized into the ComPSCA group and ResPSCA group. An unstable Th7 fracture was simulated. Periodic bending in flexion direction with a torque of 2.5 Nm and 25,000 cycles was applied. Markers were applied to the vertebral bodies to measure segmental movement. After testing, a CT scan of all specimens was performed. The mean age of the specimens was 87.8 years (range 74–101). The mean T-score was − 3.6 (range − 1.2 to − 5.3). Implant failure was visible in three specimens, two of the ComPSCA group and one of the ResPSCA group, affecting only one pedicle screw in each case. Slightly higher segmental movement could be evaluated in these three specimens. No further statistically significant differences were observed between the study groups. The construct stability under cyclic loading in flexion direction of long segmental posterior stabilization of an unstable osteoporotic midthoracic fracture using ResPSCA seems to be comparable to ComPSCA.

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.

Comparison of Long Segmental Dorsal Stabilization with Complete Versus Restricted Pedicle Screw Cement Augmentation in Unstable Osteoporotic Midthoracic Vertebral Body Fractures: A Biomechanical Study

OBJECTIVE

To compare the construct stability of long-segmental dorsal stabilization in unstable midthoracic osteoporotic fracture situation with complete pedicle screw cement augmentation (ComPSCA) versus restricted pedicle screw cement augmentation (ResPSCA) of the most cranial and caudal pedicle screws.

METHODS

Twelve fresh frozen human cadaveric specimens (Th 4-Th 10) aged 65 years and older were tested in a biomechanical cadaver study. All specimens received a dual-energy X-ray absorption scan and computed tomography scan before testing. Standardized long segmental stabilization was performed. All specimens were matched into pairs. These pairs were randomized into the groups with ComPSCA and ResPSCA. An unstable Th7 fracture was simulated. The maximum load was tested with 6 mm/min until failure or 20 mm had been reached. After testing, a computed tomography scan was performed.

RESULTS

The mean age of the specimens was 87.8 years (range 74-101 years). The mean t score was -3.6 (range -1.2 to -5.3). The mean maximum force in the ResPSCA group was 1600 N (range 1119-1880 N) and 1941 N (1183-3761 N) in the ComPSCA group. No statistically significant differences between both study groups (P = 1.0) could be seen. No signs of screw loosening were visible.

CONCLUSIONS

No statistically significant differences in the maximum loads could be seen. No screw loosening of the non-cemented screws was visible. Thus, the construct stability of long segmental posterior stabilization of an unstable midthoracic fracture using ResPSCA seems to be comparable with ComPSCA under axial compression.

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.

Biomechanical analysis and optimization of screw fixation technique for the cortical bone channel of lower thorax: Study protocol clinical trial (SPIRIT Compliant)

It is well known that the main segments of spinal fracture is thoracolumbar (T11-L11). Therefore, in addition to the lumbar, the lower thoracic vertebra (T9-T12) often has the clinical needs of implantation of cortical bone trajectory (CBT) screws. However, the anatomic parameters of the lower thoracic vertebrae are quite different from those of the lumbar vertebrae, which means that if CBT screws are to be implanted in the lower thoracic vertebrae, the selection of the screw entry point, the length, diameter, angle and path of the screws in each segment need to be redefined. Methods In this part, 3-dimensional finite element model was established to analyze the stress and fixation efficiency of CBT screws in thoracic vertebrae after 5000 times of fatigue loading of normal model and osteoporosis model. Discussion If the outcomes indicate the trial is feasible and there is evidence to provide some basic anatomical parameters for CBT screw implantation in the lower thoracic spine, so that the ideal insertion point, length, diameter, and angle of CBT screw in different segments of the lower thoracic spine were determined.Trial Registration Chinese Clinical Trial Registry, ChiCTR1900026915.Registered on September 26, 2019.

Comparison of Nitinol and Titanium Nails Effectiveness for Lumbosacral Spine Fixation in Surgical Treatment of Degenerative Spine Diseases

Relevance. Surgical decompression and decompression with stabilization are highly effective for treatment of spinal canal stenosis at the level of lumbar spine. However, complications developing after application of rigid fixation systems resulted in active introduction of dynamic implants into clinical practice.

Purpose of the study — to compare effectiveness of nitinol and titanium nails for lumbosacral fixation in surgical treatment of degenerative spine diseases.

Materials and methods. 220 patients who underwent surgeries in 4 hospitals were randomized into two groups, each consisting of 110 patients (1:1 ratio): a group of patients who underwent stabilization of the vertebral motor segments with rods of nitinol with the required volume of decompression at the operation level and a group of patients who underwent stabilization of the vertebral motor segments with standard rods of titanium with the required volume of decompression at the intervention level. Patients suffered clinically significant spinal canal stenosis in one or two adjacent segments: from L3 to S1. Outcomes were evaluated during three years postoperatively by VAS scale for spine and lower limbs, and by ODI and SF-36 scales.

Results. All scales demonstrated better values in both groups of patients, namely, significant decrease of pain syndrome and improvement in mental and physical health. X-ray examination of all patients during the study period demonstrated restoration of lumbar lordosis. Group of patients with dynamic nails featured less complications rate related to metal implants including adjacent segment disease.

Conclusion. Transpedicular fixation of lumbosacral spine by nitinol nails is an effective technique allowing to preserve motion along with stable fixation.

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 contribution of the cortical shell to pedicle screw fixation

BACKGROUND

A pedicle screw insertion technique known as "hubbing" involves the removal of cortical bone around the screw insertion with the aim of improving fixation and decreasing screw loosening. However, the efficacy of this procedure relative to bone density and early loading have not been fully explored. The purpose of this study is to establish the contribution of the cortical layer (hubbing), cancellous density, early loading (toggling) in an idealised model. This is an in vitro laboratory study.

METHODS

Synthetic bone blocks with cancellous bulk and a simulated cortical shell were implanted with 6.5 mm pedicle screws. Three key variables were evaluated in this study; density of the simulated bone (10-20 lb/ft³), toggling (±0.5 mm for 10,000 cycles), and the presence or absence of the surrounding cortex (hubbing). Pullout testing after toggling was performed to determine maximum load, stiffness and energy. Results were analyzed to assess interaction and main effects.

RESULTS

Removal of the cortex decreased the pullout loads by approximately 1,100 N after toggling. Toggling in the presence of the cortical shell had no effect. However, once the cortical shell is removed damage to the weaker cancellous bone accumulates and further compromises the fixation.

CONCLUSIONS

The addition of a cortical layer in the Sawbone model is significant and provides a more realistic model of load sharing. The cortex plays a considerable role in the protection of underlying cancellous bone as well as contributing to initial pullout strength. The results of this study demonstrate a negative synergistic effect when both toggling and hubbing are applied to the weaker bone.

Pedicle screw augmentation in osteoporotic spine: indications, limitations and technical aspects

PURPOSE

The need for spinal instrumented fusion in osteoporotic patients is rising. In this review, we try to give an overview of the current spectrum of pedicle screw augmentation techniques, safety aspects and indications.

METHODS

Review of literature and discussion of indications, limitations and technical aspects.

RESULTS

Various studies have shown higher failure rates in osteoporotic patients, most probably due to reduced bone quality and a poor bone-screw interface. Augmentation of pedicle screws with bone cement, such as polymethylmethacrylate or calcium based cements, is one valid option to enhance fixation if required.

CONCLUSIONS

Crucial factors for success in the use of augmented screws are careful patient selection, a proper technique and choice of the ideal cement augmentation option.

Failure of fracture fixation in osteoporotic bone

This manuscript will provide an overview of how the age and osteoporosis related changes in mechanical properties of bone affect the stability of osteosynthesis constructs, both from a mechanical as well as from a clinical perspective. The manuscript will also address some of the principles of fracture fixation for osteoporotic fractures and discuss applications of osteoporotic fracture fixation at sites typically affected by fragility fractures, namely the distal radius, the proximal humerus, the femur and the spine. The primary aim of operative treatment in elderly individuals is the avoidance of immobilization of the patient. In selected cases conservative treatment might be required. Generally, choice of treatment should be individualized and based on the evaluation of patient-specific, fracture-specific and surgeon-specific aspects. The orthopaedic surgeon plays an essential role in enabling functional recovery by providing good surgery but a multidisciplinary approach is essential in order to support the patient to regain his/her quality of life after fragility fracture. Overall, the therapy of fractures in osteoporotic bone in the elderly requires a multidisciplinary therapeutic acute care concept including treatment of co-morbidities and correct choice of timing, and technique of the operative intervention.

The biomechanics of pedicle screw augmentation with cement

BACKGROUND CONTEXT

A persistent challenge in spine surgery is improving screw fixation in patients with poor bone quality. Augmenting pedicle screw fixation with cement appears to be a promising approach.

PURPOSE

The purpose of this study was to survey the literature and assess the previous biomechanical studies on pedicle screw augmentation with cement to provide in-depth discussions of the biomechanical benefits of multiple parameters in screw augmentation.

STUDY DESIGN/SETTING

This is a systematic literature review.

METHODS

A search of Medline was performed, combining search terms of pedicle screw, augmentation, vertebroplasty, kyphoplasty, polymethylmethacrylate, calcium phosphate, or calcium sulfate. The retrieved articles and their references were reviewed, and articles dealing with biomechanical testing were included in this article.

RESULTS

Polymethylmethacrylate is an effective material for enhancing pedicle screw fixation in both osteoporosis and revision spine surgery models. Several other calcium ceramics also appear promising, although further work is needed in material development. Although fenestrated screw delivery appears to have some benefits, it results in similar screw fixation to prefilling the cement with a solid screw. Some differences in screw biomechanics were noted with varying cement volume and curing time, and some benefits from a kyphoplasty approach over a vertebroplasty approach have been noted. Additionally, in cadaveric models, cemented-augmented screws were able to be removed, albeit at higher extraction torques, without catastrophic damage to the vertebral body. However, there is a risk of cement extravasation leading to potentially neurological or cardiovascular complications with cement use. A major limitation of these reviewed studies is that biomechanical tests were generally performed at screw implantation or after a limited cyclic loading cycle; thus, the results may not be entirely clinically applicable. This is particularly true in the case of the bioactive calcium ceramics, as these biomechanical studies would not have measured the effects of osseointegration.

CONCLUSIONS

Polymethylmethacrylate and various calcium ceramics appear promising for the augmentation of pedicle screw fixation biomechanically in both osteoporosis and revision spine surgery models. Further translational studies should be performed, and the results summarized in this review will need to be correlated with the clinical outcomes.

Intraoperative mechanical measurement of bone quality with the DensiProbe

Reduced bone stock can result in fractures that mostly occur in the spine, distal radius, and proximal femur. In case of operative treatment, osteoporosis is associated with an increased failure rate. To estimate implant anchorage, mechanical methods seem to be promising to measure bone strength intraoperatively. It has been shown that the mechanical peak torque correlates with the local bone mineral density and screw failure load in hip, hindfoot, humerus, and spine in vitro. One device to measure mechanical peak torque is the DensiProbe (AO Research Institute, Davos, Switzerland). The device has shown its effectiveness in mechanical peak torque measurement in mechanical testing setups for the use in hip, hindfoot, and spine. In all studies, the correlation of mechanical torque measurement and local bone mineral density and screw failure load could be shown. It allows the surgeon to judge local bone strength intraoperatively directly at the region of interest and gives valuable information if additional augmentation is needed. We summarize methods of this new technique, its advantages and limitations, and give an overview of actual and possible future applications.