Stability of Pedicle Screws After Kyphoplasty Augmentation

A Biomechanical Study on Cortical Bone Trajectory Screw Fixation Augmented With Cement in Osteoporotic Spines

STUDY DESIGN

A biomechanical study.

OBJECTIVE

To evaluate the efficacy and feasibility of cement-augmented cortical bone trajectory (CBT) screw fixation.

METHODS

Forty-nine CBT screws were inserted into lumbar vertebrae guided by three-dimensionally printed templates, and then injected with 0, .5, or 1.0 mL of polymethylmethacrylate. The screw placement accuracy, cement dispersion, and cement leakage rate were evaluated radiologically. Biomechanical tests were performed to measure the axial pull-out strength and torque value.

RESULTS

Overall, 83.67% of the screws were inserted without pedicle perforation. In the 1.0 mL group, cement dispersed into the pedicle zone and formed a concentrated mass more often than in the .5 mL group, but not significantly more often (P > .05). The total cement leakage rate was 18.75%. Compared with the control group, the torque value was slightly higher in the .5 mL group (P = .735) and significantly higher in the 1.0 mL group (P = .026). However, there was no significant difference between the .5 and 1.0 mL groups (P = .431). The maximal pull-out force (Fmax) was increased by 52.85% and 72.73% in the .5 and 1.0 mL groups, respectively, compared with the control group (P < .05). However, the difference was not significant between the 2 cemented groups (P = .985).

CONCLUSIONS

Cement augmentation is a useful method for increasing CBT screw stability in osteoporotic spines. The cement injection volume is recommended to be 1 mL for each screw, and the cement should disperse into the vertebral body than the pedicle zones.

The Biomechanical Properties of Cement-Augmented Pedicle Screws for Osteoporotic Spines

STUDY DESIGN

This is a broad, narrative review of the literature.

OBJECTIVE

In this review, we describe recent biomechanics studies on cement-augmented pedicle screws for osteoporotic spines to determine which factors influence the effect of cement augmentation.

METHODS

A search of Medline was performed, combining the search terms "pedicle screw" and ("augmentation" OR "cement"). Articles published in the past 5 years dealing with biomechanical testing were included.

RESULTS

Several factors have been identified to impact the effect of cement augmentation in osteoporotic spines. These include the type of augmentation material, the volume of injected cement, the timing of augmentation, the severity of osteoporosis, the design of the pedicle screw, and the specific augmenting technique, among others.

CONCLUSIONS

This review elaborates the biomechanics of cement-augmented pedicle screws, determines which factors influence the augmentation effect, and identifies the risk factors of cement leakage in osteoporotic bone, which might offer some guidance when using this technique in clinical practice. Further, we provide information about newly designed screws and recently developed augmentation materials that provide higher screw stability as well as fewer cement-related complications.

Effects of polymethylmethacrylate on the stability of screw fixation in mandibular angle fractures: A study on sheep mandibles

Aim: Malfixed miniplates can impair fracture healing, and the screw pilot holes may widen during repeated fixation trials. This in vitro study explored the extent to which screw fixation of mandibular angle fractures could be improved by augmenting the drilling holes with polymethylmethacrylate (PMMA). Materials and Methods: We measured stabilization by recording specimen displacement under a vertical force of 50 N applied using a hydraulic tester. We included 20 hemimandibles from sheep (average weight 40 kg). The specimens were randomly divided into two groups of 10 and pilot holes were created in the angulus region using a drill 1.2 mm in diameter. Next, we performed osteotomies simulating angulus fracture repair. In group 1, the fracture site was fixed using non-compression miniplates and four screws were inserted to the maximal possible extent employing a mechanical screwdriver. In group 2, the pilot drill holes were filled with PMMA prior to miniplate fixation. Then vertical forces of 50 N were applied to the molar region and the displacements were measured. The Shapiro-Wilks test was used to compare the two groups. Result: The maximum average displacement in the experimental group was significantly lower than that in the control group (p=0.026). Thus, PMMA-augmented screws better stabilized bone, affording reliable fixation.

Pedicle screw augmentation with bone cement enforced Vicryl mesh

Achieving sufficient mechanical purchase of pedicle screws in osteoporotic or previously instrumented bone is technically and biologically challenging. Techniques using different kinds of pedicle screws or methods of cement augmentation have been used to address this challenge, but are associated with difficult revisions and complications. The purpose of this biomechanical trial was to investigate the use of biocompatible textile materials in combination with bone cement to augment pullout strength of pedicle screws while reducing the risk of cement extrusion. Pedicle screws (6/40 mm) were either augmented with standard bone-cement (Palacos LV + G) in one group (BC, n = 13) or with bone-cement enforced by Vicryl mesh in another group (BCVM, n = 13) in osteoporosis-like saw bone blocks. Pullout testing was subsequently performed. In a second experimental phase, similar experiments were performed using human cadaveric lumbar vertebrae (n = 10). In osteoporosis-like saw bone blocks, a mean screw pullout force of 350 N (±125) was significantly higher with the Bone cement (BC) compared to bone-cement enforced by Vicryl mesh (BCVM) technique with 240 N (±64) (p = 0.030). In human cadaveric lumbar vertebrae the mean screw pullout force was 784 ± 366 N with BC and not statistically different to BCVM with 757 ± 303 N (p = 0.836). Importantly, cement extrusion was only observed in the BC group (40%) and never with the BCVM technique. In vitro textile reinforcement of bone cement for pedicle screw augmentation successfully reduced cement extrusion compared to conventionally delivered bone cement. The mechanical strength of textile delivered cement constructs was more reproducible than standard cementing. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:212-216, 2018.

Timing of PMMA cement application for pedicle screw augmentation affects screw anchorage

INTRODUCTION

Cement augmentation is an established method to increase the pedicle screw (PS) anchorage in osteoporotic vertebral bodies. The ideal timing for augmentation when a reposition maneuver is necessary is controversial. While augmentation of the PS before reposition maneuver may increase the force applied it on the vertebrae, it bears the risk to impair PS anchorage, whereas augmenting the PS after the maneuver may restore this anchorage and prevent early screw loosening. The purpose of the present study was to evaluate the effect of cement application timing on PS anchorage in the osteoporotic vertebral body.

METHODS

Ten lumbar vertebrae (L1-L5) were used for testing. The left and right pedicles of each vertebra were instrumented with the same PS size and used for pairwise comparison of the two timing points for augmentation. For the reposition maneuver, the left PS was loaded axially under displacement control (2 × ±2 mm, 3 × ±6 mm, 3 × ±10 mm) to simulate a reposition maneuver. Subsequently, both PS were augmented with 2 ml PMMA cement. The same force as measured during the left PS maneuver was applied to the previously augmented right hand side PS [2 × F (±2 mm), 3 × F (±6 mm), 3 × F (±10 mm)]. Both PS were cyclically loaded with initial forces of +50 and -50 N, while the lower force was increased by 5 N every 100 cycles until total failure of the PS. The PS motion was measured with a 3D motion analysis system. After cyclic loading stress, X-rays were taken to identify the PS loosening mechanism.

RESULTS

In comparison with PS augmented prior to the reposition maneuver, PS augmented after the reposition maneuver showed a significant higher number of load cycles until failure (5930 ± 1899 vs 3830 ± 1706, p = 0.015). The predominant loosening mechanism for PS augmented after the reposition maneuver was PS toggling with the attached cement cloud within the trabecular bone. While PS augmented prior to the reposition, maneuver showed a motion of the screw within the cement cloud.

CONCLUSION

The time of cement application has an effect on PS anchorage in the osteoporotic vertebral body if a reposition maneuver of the instrumented vertebrae is carried out. PS augmented after the reposition maneuver showed a significant higher number of load cycles until screw loosening.

The Biomechanical Properties of Pedicle Screw Fixation Combined With Trajectory Bone Cement Augmentation in Osteoporotic Vertebrae

STUDY DESIGN

The biomechanics of pedicle screw fixation combined with trajectory cement augmentation with various filling volumes were measured by pull-out, periodic antibending, and compression fatigue tests.

OBJECTIVE

To investigate the biomechanical properties of the pedicle screw fixation combined with trajectory bone cement (polymethylmethacrylate) augmentation in osteoporotic vertebrae and to explore the optimum filling volume of the bone cement.

SUMMARY OF BACKGROUND DATA

Pedicle screw fixation is considered to be the most effective posterior fixation method. The decrease of the bone mineral density apparently increases the fixation failure risk caused by screw loosening and displacement. Trajectory bone cement augmentation has been confirmed to be an effective method to increase the bone intensity and could markedly increase the stability of the fixation interface.

METHODS

Sixteen elderly cadaveric 1-5 lumbar vertebral specimens were diagnosed with osteoporosis. The left and right vertebral pedicles were alternatively randomized for treatment in all groups, with the contralateral pedicles as control. The study groups included: group A (pedicle screw fixation with full trajectory bone cement augmentation), group B (75% filling), group C (50% filling), and group D (25% filling). Finally, the bone cement leakage and dispersion were assessed and the mechanical testing was conducted.

RESULTS

The bone cement was well dispersed around the pedicle screw. The augmented bone intensity, pull-out strength, periodic loading times, and compression fatigue performance were markedly higher than those of the control groups. With the increase in trajectory bone cement, the leakage was also increased (P<0.05). The pull-out strength of the pedicle screw was increased with an increase in bone mineral density and trajectory bone cement. It peaked at 75% filling, with the largest power consumption.

CONCLUSIONS

The optimal filling volume of the bone cement was 75% of the trajectory volume (about 1.03 mL). The use of excessive bone cement did not increase the fixation intensity but increased the risk of leakage.

Percutaneous pedicle screw placement into a spinal segment previously treated with vertebroplasty: technical note

Vertebral augmentation with cement has become a common procedure for the treatment of compression fractures, leading to a growing population who have had this procedure and are now in need of another spinal surgery. This technical note reports an undescribed method for placing pedicle screws through a previously cemented level.

Effect of bone material properties on effective region in screw-bone model: an experimental and finite element study

Background

There have been numerous studies conducted to investigate the pullout force of pedicle screws in bone with different material properties. However, fewer studies have investigated the region of effect (RoE), stress distribution and contour pattern of the cancellous bone surrounding the pedicle screw.

Methods

Screw pullout experiments were performed from two different foams and the corresponding reaction force was documented for the validation of a computational pedicle screw-foam model based on finite element (FE) methods. After validation, pullout simulations were performed on screw-bone models, with different bone material properties to model three different age groups (<50, 50–75 and >75 years old). At maximum pullout force, the stress distribution and average magnitude of Von Mises stress were documented in the cancellous bone along the distance beyond the outer perimeter pedicle screw. The radius and volume of the RoE were predicted based on the stress distribution.

Results

The screw pullout strengths and the load–displacement curves were comparable between the numerical simulation and experimental tests. The stress distribution of the simulated screw-bone vertebral unit showed that the radius and volume of the RoE varied with the bone material properties. The radii were 4.73 mm, 5.06 mm and 5.4 mm for bone properties of ages >75, 75 > ages >50 and ages <50 years old, respectively, and the corresponding volumes of the RoE were 6.67 mm³, 7.35 mm³ and 8.07 mm³, respectively.

Conclusions

This study demonstrated that there existed a circular effective region surrounding the pedicle screw for stabilization and that this region was sensitive to the bone material characteristics of cancellous bone. The proper amount of injection cement for augmentation could be estimated based on the RoE in the treatment of osteoporosis patients to avoid leakage in spine surgery.

Surgical stabilization of the spine in the osteoporotic patient

Osteoporosis affects millions of US citizens, and millions more are at risk for developing the disease. Several operative techniques are available to the spine surgeon to provide care for those affected by osteoporosis. The types of osteoporosis, common surgical complications, medical optimization, and surgical techniques in the osteoporotic spine are reviewed, with an emphasis on preoperative planning.

A comparative study on screw loosening in osteoporotic lumbar spine fusion between expandable and conventional pedicle screws

INTRODUCTION

The aim of this study is to compare the rate of screw loosening and clinical outcomes of expandable pedicle screws (EPS) with those of conventional pedicle screws (CPS) in patients treated for spinal stenosis (SS) combined with osteoporosis.

METHODS

One hundred and fifty-seven consecutive patients with SS received either EPS fixation (n = 80) or CPS fixation (n = 77) to obtain lumbosacral stabilization. Patients were observed for a minimum of 24 months. Outcome measures included screw loosening, fusion rate, Japanese Orthopaedic Association (JOA) scores and Oswestry disability index (ODI) scoring system, and complications.

RESULTS

In the EPS group, 20 screws became loose (4.1%) in 6 patients (7.5%), and two screws (0.4%) had broken. In the CPS group, 48 screws became loose (12.9%) in 15 patients (19.5%), but no screws were broken. The fusion rate in the EPS group (92.5%) was significantly higher than that of the CPS group (80.5%). The rate of screw loosening in the EPS group (4.1%) was significantly lower than that of the CPS group (12.9%). Six EPS (1.8%) screws were removed. In the EPS group, two screws had broken but without neural complications. Twelve months after surgeries, JOA and ODI scores in the EPS group were significantly improved. There were four cases of dural tears, which healed after corresponding treatment.

CONCLUSIONS

EPS can decrease the risk of screw loosening and achieve better fixation strength and clinical results in osteoporotic lumbar spine fusion.

Comparison between two pedicle screw augmentation instrumentations in adult degenerative scoliosis with osteoporosis

Background

The operative treatment of adult degenerative scoliosis combined with osteoporosis increase following the epidemiological development. Studies have confirmed that screws in osteoporotic spines have significant lower-screw strength with more frequent screw movements within the vertebra than normal spines. Screws augmented with Polymethylmethacrylate (PMMA) or with autogenous bone can offer more powerful corrective force and significant advantages.

Methods

A retrospective analysis was conducted on 31 consecutive patients with degenerative lumbar scoliosis combined with osteoporosis who had surgery from December 2000. All had a minimum of 2-year follow-up. All patients had posterior approach surgery. 14 of them were fixed with pedicle screw by augmentation with Polymethylmethacrylate (PMMA) and the other 17 patients with autogenous bone. Age, sex and whether smoking were similar between the two groups. Surgical time, blood loss, blood transfusion, medical cost, post surgery ICU time, hospital day, length of oral pain medicines taken, Pre-and postoperative Oswestry disability index questionnaire and surgical revision were documented and compared. Preoperative, postoperative and final follow up Cobb angle, sagittal lumbar curve, correction rate, and Follow up Cobb loss were also compared.

Results

No significant differences were found between the autogenous bone group and Polymethylmethacrylate group with regards to all the targets above except for length of oral pain medicines taken and surgery cost. 2 patients were seen leakage during operation, but there is neither damage of nerve nor symptom after operation. No revision was needed.

Conclusion

Both augmentation pedicle screw with Polymethylmethacrylate (PMMA) and autogenous bone treating degenerative lumbar scoliosis combined with osteoporosis can achieve a good surgical result. Less oral pain medicines taken are the potential benefits of Polymethylmethacrylate augmentation, but that is at the cost of more medical spending.

The biomechanics of pedicle screw-based instrumentation

There are three basic concepts that are important to the biomechanics of pedicle screw-based instrumentation. First, the outer diameter of the screw determines pullout strength, while the inner diameter determines fatigue strength. Secondly, when inserting a pedicle screw, the dorsal cortex of the spine should not be violated and the screws on each side should converge and be of good length. Thirdly, fixation can be augmented in cases of severe osteoporosis or revision. A trajectory parallel or caudal to the superior endplate can minimise breakage of the screw from repeated axial loading. Straight insertion of the pedicle screw in the mid-sagittal plane provides the strongest stability. Rotational stability can be improved by adding transverse connectors. The indications for their use include anterior column instability, and the correction of rotational deformity.

Assessment of different screw augmentation techniques and screw designs in osteoporotic spines

This is an experimental study on human cadaver spines. The objective of this study is to compare the pullout forces between three screw augmentation methods and two different screw designs. Surgical interventions of patients with osteoporosis increase following the epidemiological development. Biomechanically the pedicle provides the strongest screw fixation in healthy bone, whereas in osteoporosis all areas of the vertebra are affected by the disease. This explains the high screw failure rates in those patients. Therefore PMMA augmentation of screws is often mandatory. This study involved investigation of the pullout forces of augmented transpedicular screws in five human lumbar spines (L1-L4). Each spine was treated with four different methods: non-augmented unperforated (solid) screw, perforated screw with vertebroplasty augmentation, solid screw with vertebroplasty augmentation and solid screw with balloon kyphoplasty augmentation. Screws were augmented with Polymethylmethacrylate (PMMA). The pullout forces were measured for each treatment with an Instron testing device. The bone mineral density was measured for each vertebra with Micro-CT. The statistical analysis was performed with a two-sided independent student t test. Forty screws (10 per group and level) were inserted. The vertebroplasty-augmented screws showed a significant higher pullout force (mean 918.5 N, P = 0.001) than control (mean 51 N), the balloon kyphoplasty group did not improve the pullout force significantly (mean 781 N, P > 0.05). However, leakage occurred in some cases treated with perforated screws. All spines showed osteoporosis on Micro-CT. Vertebroplasty-augmented screws, augmentation of perforated screws and balloon kyphoplasty augmented screws show higher pullout resistance than non-augmented screws. Significant higher pullout forces were only reached in the vertebroplasty augmented vertebra. The perforated screw design led to epidural leakage due to the position of the perforation in the screw. The position of the most proximal perforation is critical, depending on screw design and proper insertion depth. Nevertheless, using a properly designed perforated screw will facilitate augmentation and instrumentation in osteoporotic spines.

Polymethylmethacrylate augmentation of pedicle screw for osteoporotic spinal surgery: a novel technique

STUDY DESIGN

A retrospective study to evaluate the clinical results of patients with osteoporosis and various spinal diseases treated surgically with polymethylmethacrylate (PMMA) augmented pedicle screw.

OBJECTIVE

To report a novel technique using PMMA for pedicle screw augmentation in osteoporotic spinal surgery.

SUMMARY OF BACKGROUND DATA

Many studies have proved that the stiffness and strength of pedicle screw fixation can be significantly increased when the pedicle screw is augmented with various cements. However, most of those studies were experimental. Clinical reports using those materials for pedicle screw augmentation are rare and a practical and reliable technique for primary pedicle screw augmentation with cement has not yet been established.

METHODS

Forty-one patients [23 female, 18 male, mean age 75.1 (50-90) years] with osteoporosis and various spinal diseases underwent spinal decompression and instrumentation with PMMA augmentation of pedicle screw. Pre-and postoperative scores for visual analogue scale for pain and Oswestry disability index questionnaire were analyzed. The screw migration, which is the distance from the screw tip to the anterior cortex and upper endplate of vertebra, was also evaluated immediately after the operation and at the mean 22.3 months final follow-up. RESULTS.: Totally 291 of 300 screws were augmented with PMMA. There was neither neurologic deterioration nor symptomatic cement leakage after surgery. The mean visual analogue scale pain score of these patients improved from 9.2 to 1.5 (P < 0.01) and the functional Oswestry disability index score improved from 77.5% to 44.2% (P < 0.01). Kyphotic deformity was improved from average 23.2 degrees to 11.9 degrees after surgery, and to 14.9 degrees at final follow-up (P < 0.01). The average loss of kyphosis correction was 3 degrees. There was no significant screw migration when the screws distances just after operation and at the final follow-up were compared (P > 0.01).

CONCLUSION

The presented technique of PMMA for augmentation of pedicle screw is a safe, reliable, and practical technique for osteoporotic patients who also had various spinal diseases and need spinal instrumentation.