Material changes in osteoporotic human cancellous bone following infiltration with acrylic bone cement for a vertebral cement augmentation

Finite element analysis of biomechanical effects of percutaneous cement discoplasty in scoliosis

OBJECTIVE

To investigate the effect of bone cement on the vertebral body and biomechanical properties in percutaneous cement discoplasty (PCD) for degenerative lumbar disc disease.

METHODS

Three-dimensional reconstruction of L2 ~ L3 vertebral bodies was performed in a healthy volunteer, and the corresponding finite element model of the spine was established. Biomechanical analysis was performed on the changes in stress distribution in different groups of models by applying quantitative loads.

RESULTS

Models with percutaneous discoplasty (PCD) showed improved stability under various stress conditions, and intervertebral foraminal heights were superior to models without discoplasty.

CONCLUSION

Cement discoplasty can improve the stability of the vertebral body to a certain extent and restore a certain height of the intervertebral foramen, which has a good development prospect and potential.

Treatment of multisegmental vertebral compression, burst fractures, and sandwich vertebra with severe osteoporosis using the PKP technique: a case report and literature review

This study aimed to present a special case of treatment of a patient with multisegmental vertebral compression fracture, burst fracture, and sandwich vertebra and to review the literature on this condition. An 85 year-old female presented with severe low back pain but no radiating pain in the lower extremities. The patient was diagnosed with T12 and L5 vertebral compression fractures, fresh vertebral burst fractures in L2 and L3, and osteoporosis. The focus was on formulating a surgical treatment strategy. At the 12 month follow-up, no neurological deficits were observed, and the chosen surgical treatment approach yielded favorable clinical outcomes. A comprehensive literature review indicates that percutaneous kyphoplasty (PKP) can effectively alleviate pain and ensure safety in managing osteoporotic vertebral burst fractures. While complications remain a theoretical risk, they can be mitigated through meticulous assessment, careful surgical procedures, and appropriate preventive measures. PKP is an effective and safe treatment modality for osteoporotic vertebral burst fractures. Conservative management of sandwich vertebrae can yield positive clinical outcomes, but regular anti-osteoporosis treatment is necessary.

A finite element analysis on different bone cement forms and injection volumes injected into lumbar vertebral body in percutaneous kyphoplasty

Background

To investigate the stress changes between different bone cement forms and injection volumes in adjacent vertebrae after percutaneous kyphoplasty (PKP) by establishing a three-dimensional finite element model of osteoporosis.

Methods

A male healthy volunteer was selected. CT of scans L1 to L3 vertebrae were imported into Mimics 21.0 software.The vertebral model of osteoporosiswas established based on previous literature reference. The models were divided into three groups: unilateral, bilateral integration and bilateral separation groups, with each group injecting 2 ml, 4,ml and 6 ml of bone cement, respectively. In all models, a vertical compressive load of 500 N, anterior flexion/posterior extension, left/right bending, and left/right rotation were applied with a moment of 7.5 N/m, of which 85% was applied to the anterior mid-column and 15% to the posterior column. The stress changes between adjacent vertebrae under different conditions were calculated.

Results

After percutaneous kyphoplasty was applied to the L2 vertebral body, some differences can be found between the effects of different cement injection volumes and cement morphology on adjacent structures. There was no major difference between the groups when the bone cement injection volume was 2 ml. When the amount of bone cement injected was 4 ml, the bone cement morphology of the bilateral integration group (BIG) produced less stress between adjacent vertebral bodies. The minimum stress was 14.95 MPa in the L3 vertebral body in posterior extension. Whereas the stress levels on adjacent intervertebral structures, BIG shaped bone cement shows some superiority. In addition, the adjacent vertebrae and intervertebral structures are subjected to less stress during left and right rotation.

Conclusions

The present finite element study suggested that bilateral integration bone cement is a suitable form of cement injection, and when the injection volume is 4 ml, reduces stress on adjacent segments by approximately 15% while maintaining the stability of the injected vertebral body.

Do sandwich vertebral bodies increase the risk of post-augmentation fractures? A retrospective cohort study

Until now, there have been only a few retrospective studies that focused on the outcomes of sandwich vertebral bodies (SVBs). This is a long-term retrospective cohort study to investigate the SVBs. We found that although patients with SVBs had a relatively high risk of developing new fractures after VA, the incidence rate of new fractures was not significantly different from that of the control group. However, the statistical power of this study was very limited. Therefore, and because the refracture rate in these patients is substantial, routine long-term monitoring of patients after VA for osteoporosis is strongly recommended.

BACKGROUND

Sandwich vertebral bodies (SVBs) are intact unaugmented vertebral bodies between two previously augmented vertebrae. Until recently, only a few studies have reported the outcomes and strategies for SVBs. This retrospective cohort study aimed to describe the clinical features and incidence of new fractures in patients with SVBs.

METHODS

The clinical data were collected from 179 patients with 237 symptomatic osteoporotic vertebral compression fractures who underwent vertebral augmentation (VA). Among them, 23 patients with 24 levels of SVBs were included. Spinal radiographs (X-ray and CT) of all patients were evaluated prior to surgery 1 day after primary VA and during follow-up.

RESULTS

All patients successfully underwent PKP with an average follow-up period of 21.48 months. Asymptomatic cement leakage occurred in four patients (17.4%), and eight patients (34.8%) developed new fractures following primary PKP, including four sandwich, six adjacent, four remote vertebral fractures, and one re-collapse of cemented vertebrae. The incidence of new fractures in the SVB and control groups was 16.7% (4/24) and 13.0% (6/46), respectively, but there was no significant difference.

CONCLUSIONS

Although patients with SVBs had a relatively high risk of developing new fractures after VA, the incidence rate of new fractures was not significantly different from that of the control group. However, the statistical power of this study was very limited. Therefore, and because the refracture rate in these patients is substantial, routine long-term monitoring of patients after VA for osteoporosis is strongly recommended.

A Retrospective Analysis in 1347 Patients Undergoing Cement Augmentation for Osteoporotic Vertebral Compression Fracture: Is the Sandwich Vertebra at a Higher Risk of Further Fracture?

BACKGROUND

Multiple percutaneous vertebral cement augmentation may create sandwich vertebrae. Whether the sandwich vertebra is at higher risk of further fracture remains unknown.

OBJECTIVE

To compare the incidence of further fractures of sandwich vertebrae and adjacent vertebrae and to identify potential risk factors for sandwich vertebral fractures.

METHODS

Patients who underwent cement augmentation for osteoporotic vertebral compression fractures (OVCFs) in a single medical center between January 2012 and December 2015 were included. A sandwich vertebra was defined as an intact vertebra located between 2 previously cemented vertebrae. Demographic data and imaging findings were recorded. All patients were followed up for at least 24 mo postoperatively. During follow-up period, if the patient reported new-onset back pain with corresponding imaging findings, a diagnosis of sandwich vertebral fracture was made.

RESULTS

Among the 1347 patients who underwent vertebroplasty/kyphoplasty for OVCFs, 127 patients with 128 fracture levels met the criteria for sandwich vertebrae (females/males 100/27, mean age 77.8 ± 7.7 yr old). The fracture location was most common in the thoraco-lumbar junction (T10-L2), 68.5% (87/127). The incidence of sandwich vertebral fracture was 21.3%, whereas the incidence of adjacent level fracture of those with no sandwich vertebra was 16.4% (196/1194), P = .1879.

CONCLUSION

The incidence of sandwich vertebral fracture is not higher than that at the adjacent levels. The factor associated with further sandwich vertebral fracture was male gender. Once sandwich vertebral fracture occurred, patients may seek more surgical intervention than those with only adjacent fractures.

Association between handgrip strength and subsequent vertebral-fracture risk following percutaneous vertebral augmentation

INTRODUCTION

The aim of this study was to investigate the association between handgrip strength (HGS) and the risk of subsequent vertebral fracture (SVF) after percutaneous vertebral augmentation (PVA).

MATERIALS AND METHODS

A total of 340 patients aged over 50 years with osteoporotic vertebral fracture were enrolled in this 3-year follow-up investigation. HGS was measured with a hand-held dynamometer before PVA. Female patients and male patients were grouped using the HGS threshold recommended by the Asian Working Group for Sarcopenia (AWGS). Kaplan-Meier analysis was used to evaluate SVF-free survival. The hazard ratios (HRs) of HGS for SVF events were estimated with the Cox proportional hazards model.

RESULTS

During the follow-up period, a total of 93 patients (27.4%) experienced SVF. Kaplan-Meier analysis showed that the HGS of female patients < 18.0 kg and male patients < 28 kg was significantly associated with lower SVF-free survival (female patients: p < 0.001, male patients: p = 0.038; log-rank test). Among women, each 1-kg increase in HGS was associated with a 9% lower risk of SVF (HR 0.91, p = 0.035) after adjustment for potential risk factors. Among men, although the associations between low HGS and increased risk of SVF were significant in the crude model (HR 0.79, p < 0.001), this significance disappeared after adjustment for bone mineral density of the femoral neck.

CONCLUSIONS

Low HGS was significantly associated with lower SVF-free survival among elderly patients who underwent single-level PVA for osteoporotic vertebral fracture.

Biomechanical finite element analysis of vertebral column resection and posterior unilateral vertebral resection and reconstruction osteotomy

Background

Regarding the repair of vertebral compression fractures, there is a lack of adequate biomechanical verification as to whether only half of the vertebral body and the upper and lower intervertebral discs affect spinal biomechanics; there also remains debate as to the appropriate length of fixation.

Methods

A model of old vertebral compression fractures with kyphosis was established based on CT data. Vertebral column resection (VCR) and posterior unilateral vertebral resection and reconstruction (PUVCR) were performed at T12; long- and short-segment fixation methods were applied, and we analyzed biomechanical changes after surgery.

Results

Range of motion (ROM) decreased in all fixed models, with lumbar VCR decreasing the most and short posterior unilateral vertebral resection and reconstruction (SPUVCR) decreasing the least; in the long posterior unilateral vertebral resection and reconstruction (LPUVCR) model, the internal fixation system produced the maximum VMS stress of 213.25 mPa in a lateral bending motion and minimum stress of 40.22 mPa in a lateral bending motion in the SVCR.

Conclusion

There was little difference in thoracolumbar ROM between PUVCR and VCR models, while thoracolumbar ROM was smaller in long-segment fixation than in short-segment fixation. In all models, the VMS was most significant at the screw-rod junction and greatest at the ribcage–vertebral body interface, partly explaining the high probability of internal fixation failure and prosthesis migration in these two positions.

Safety and results of image-guided vertebroplasty with elastomeric polymer material (elastoplasty)

Background

Image-guided elastoplasty is an innovative method for percutaneous vertebral augmentation with a silicone elastomeric material. Our aim was to evaluate its technical success, safety and efficacy as well as the rate of secondary fractures.

Methods

Nineteen patients (13 women and 6 men, age 72 ± 10 years, mean ± standard deviation) underwent elastoplasty between 2010 and 2016. A total of 33 vertebrae were treated. A total of 2–6 mL of silicone-based elastomeric polymer material (VK100) was used. Visual analogue scale (VAS) and Oswestry disability index (ODI) pain scores were used.

Results

In all cases, it was possible to complete the procedure (technical success 100%). No major complications occurred. In 6/19 (31.5%) patients, asymptomatic leakage of the material was observed during the procedure. Full pain recovery was obtained in 18/19 (94%) patients. One patient with a painful angioma did not experience any change in symptoms. VAS and ODI were significantly reduced after the procedure, from 7.9 ± 1.1 to 0.7 ± 1.4 and from 79.6 ± 12% to 9.9 ± 14% respectively (p < 0.001 for both comparisons). After vertebroplasty, 14 of 15 patients (93%) removed the brace and 16/19 (84%) completely stopped using any drugs for pain relief (p < 0.001 for both pre-procedure versus post-procedure comparisons). At a mean follow-up time of 26.5 ± 28.1 months (median 8.7 months, range 6–69 months), no secondary fracture occurred.

Conclusion

Taking into consideration the relatively small sample size, image-guided elastoplasty seems to be a safe procedure providing effective pain control over time.

Standardizing compression testing for measuring the stiffness of human bone

Objectives

The ability to determine human bone stiffness is of clinical relevance in many fields, including bone quality assessment and orthopaedic prosthesis design. Stiffness can be measured using compression testing, an experimental technique commonly used to test bone specimens in vitro. This systematic review aims to determine how best to perform compression testing of human bone.

Methods

A keyword search of all English language articles up until December 2017 of compression testing of bone was undertaken in Medline, Embase, PubMed, and Scopus databases. Studies using bulk tissue, animal tissue, whole bone, or testing techniques other than compression testing were excluded.

Results

A total of 4712 abstracts were retrieved, with 177 papers included in the analysis; 20 studies directly analyzed the compression testing technique to improve the accuracy of testing. Several influencing factors should be considered when testing bone samples in compression. These include the method of data analysis, specimen storage, specimen preparation, testing configuration, and loading protocol.

Conclusion

Compression testing is a widely used technique for measuring the stiffness of bone but there is a great deal of inter-study variation in experimental techniques across the literature. Based on best evidence from the literature, suggestions for bone compression testing are made in this review, although further studies are needed to establish standardized bone testing techniques in order to increase the comparability and reliability of bone stiffness studies.

Cite this article: S. Zhao, M. Arnold, S. Ma, R. L. Abel, J. P. Cobb, U. Hansen, O. Boughton. Standardizing compression testing for measuring the stiffness of human bone. Bone Joint Res 2018;7:524–538. DOI: 10.1302/2046-3758.78.BJR-2018-0025.R1.

Bone cements for percutaneous vertebroplasty and balloon kyphoplasty: Current status and future developments

Osteoporotic vertebral compression fractures (OVCFs) have gradually evolved into a serious health care problem globally. In order to reduce the morbidity of OVCF patients and improve their life quality, two minimally invasive surgery procedures, vertebroplasty (VP) and balloon kyphoplasty (BKP), have been developed. Both VP and BKP require the injection of bone cement into the vertebrae of patients to stabilize fractured vertebra. As such, bone cement as the filling material plays an essential role in the effectiveness of these treatments. In this review article, we summarize the bone cements that are currently available in the market and those still under development. Two major categories of bone cements, nondegradable acrylic bone cements (ABCs) and degradable calcium phosphate cements (CPCs), are introduced in detail. We also provide our perspectives on the future development of bone cements for VP and BKP.

Biomechanical comparison of mono-segment transpedicular fixation with short-segment fixation for treatment of thoracolumbar fractures: A finite element analysis

Mono-segment transpedicular fixation is a method for the treatment of certain types of thoracolumbar spinal fracture. Finite element models were constructed to evaluate the biomechanics of mono-segment transpedicular fixation of thoracolumbar fracture. Spinal motion (T10–L2) was scanned and used to establish the models. The superior half of the cortical bone of T12 was removed and the superior half of the cancellous bone of the T12 body was assigned the material properties of injured bone to mimic vertebral fracture. Transpedicular fixation of T11 and T12 was performed to produce a mono-segment fixation model; T11 and L1 were fixed to produce a short-segment fixation model. Motion differences between functional units and von Mises stress on the spine and implants were measured under axial compression, anterior bending, extensional bending, lateral bending and axial rotation. We found no significant difference between mono- and short-segment fixations in the motion of any functional unit. Stress on the T10/T11 nucleus pulposus and T10/T11 and L1/L2 annulus fibrosus increased significantly by about 75% on anterior bending, extensional bending and lateral bending. In the fracture model, stress was increased by 24% at the inferior endplate of T10 and by 43% at the superior endplate of L2. All increased stresses were reduced after fixation and lower stress was observed with mono-segment fixation. In summary, the biomechanics of mono-segment pedicle screw instrumentation was similar to that of conventional short-segment fixation. As a minimally invasive treatment, mono-segment fixation would be appropriate for the treatment of selected thoracolumbar spinal fractures.

Biomechanical effects of vertebroplasty on thoracolumbar burst fracture with transpedicular fixation: a finite element model analysis

OBJECTIVE

To investigate the biomechanical effects of augmentation of the fractured vertebrae after posterior instrumentation.

METHODS

By simulating internal fixation plus augmentation with cement, eight tridimensional, anatomically detailed finite element models of the T11-L1 functional spinal junction were developed. Two kinds of models for mimicking different severity of the fracture were established according to the Denis' classification. Augmentation with cement was conducted after reduction with posterior fixation using a universal spine system. These models assumed a three-column loading configuration as follows: compression, anteflexion, extension, lateroflexion and axial rotation. Stress of the implants and spine was evaluated.

RESULTS

Data showed that for severely fractured models, augmentation apparently decreased the von Mises stresses by 50% for the rods and 40% for the screws, about 40% for the inferior endplate of T11, and 50% for the superior endplate of L1 in vertical compression and other load situations.

CONCLUSION

We should only apply vertebroplasty to prevent correction loss and implants failure based on the fact that it could significantly decrease stress of the instrumentations and spine when the vertebrae are severely fractured.

LEVEL OF EVIDENCE

Level IV, biomechanical study.

Balloon Kyphoplasty for Refractory Vertebral Compression Fractures in a Growing Child With Duchenne Muscular Dystrophy With Five-Year Follow-Up: Case Report and Review of Literature

OBJECTIVE

Presentation of previously unreported results and 5-year follow-up of balloon kyphoplasty used to treat an 8-year-old patient with refractory vertebral compression fractures resulting from 3 years of corticosteroid treatment for Duchenne muscular dystrophy.

SUMMARY OF BACKGROUND DATA

Long-term corticosteroid treatment in patients with DMD has been used to try and improve muscle strength, prolong ambulation, and lower the prevalence of scoliosis. However, these patients have an increased risk of osteoporosis and vertebral fractures.

METHODS

The patient was an 8-year-old boy with Duchenne muscular dystrophy who had received corticosteroid treatment for 3 years, with refractory vertebral compression fractures at T11, L1, and L3. Balloon kyphoplasty was performed at the 3 vertebral bodies using careful technique.

RESULTS

The patient achieved immediate pain relief after the procedure. The height of the treated vertebrae remained stable without further collapse over a 5-year follow-up period. At 5-year follow-up, the patient developed 2 new vertebral compression fractures at T12 and L2 adjacent to the treated vertebrae. The treatment also did not affect the growth of the treated vertebrae or the patient's overall growth.

CONCLUSIONS

Because the procedure resulted in rapid stabilization of the treated vertebrae, effective analgesia, and no effect on the growth of the treated vertebrae over a 5-year follow-up period, balloon kyphoplasty was a good therapeutic option for this pediatric patient.

Sandwich vertebral fracture in the study of adjacent-level fracture after vertebral cement augmentation

The literature is inconclusive on the development of adjacent-level vertebral fracture after initial cement augmentation. A preliminary hypotheses is that cement injection exaggerates force transmission to the adjacent vertebral bodies, thereby predisposing those levels to future fractures. A sandwich vertebra is an intact vertebral body located between 2 previously cemented vertebrae. The purpose of this study was to determine whether the risk of adjacent-level fracture increased due to load shift after a cement injection procedure. The authors retrospectively investigated the rate of adjacent-level fracture after sandwiching compared with conservative treatment and determined the potential causative factors of sandwich vertebral fracture. Age, sex, weight, height, body mass index, follow-up period, and location of sandwich level (T10-L2 or nonT10-L2 junction) were assessed. Surgical variables, including surgical procedure (vertebroplasty or balloon kyphoplasty), surgical approach (through uni- or bilateral pedicle), volume of cement injected into the painful vertebrae, cement leakage into the intervertebral disk, cumulative number of treated levels, and pre- and postoperative kyphotic angulation of the sandwich region, were also analyzed. Nine of 42 sandwiched levels developed fatigue fractures, whereas 11 of 71 patients treated with conservative therapy sustained new vertebral fractures adjacent to the treated levels. Only preoperative kyphotic angulation was the variable positively associated with sandwich vertebral fracture at follow-up (P=.021). Although subjected to double load shifts, the sandwich vertebra was not prone to structural failure. Thus, cement augmentation protocol does not increase the incidence of adjacent vertebral fracture.

Hypovitaminosis D as a risk factor of subsequent vertebral fractures after kyphoplasty

BACKGROUND CONTEXT

Over the past 20 years, methods of minimally invasive surgery have been developed for the treatment of vertebral compression fractures. Balloon kyphoplasty and vertebroplasty are associated with a recurrent fracture risk in the adjacent levels after the surgical procedure. In certain patient categories with impaired bone metabolism, the risk of subsequent fractures after kyphoplasty is increased.

PURPOSE

To determine the incidence of recurrent fractures after kyphoplasty and explore whether the status of bone metabolism and 25-hydroxyvitamin D (25(OH)D) levels affect the occurrence of these fractures.

STUDY DESIGN

Prospective longitudinal clinical study.

PATIENT SAMPLE

Forty female postmenopausal women with primary osteoporosis and acute symptomatic vertebral compression fractures.

OUTCOME MEASURES

Identification of new vertebral fractures and documentation of indicators of bone metabolism.

METHODS

A total of ninety-eight kyphoplasties were performed in 40 female patients. Balloon kyphoplasty was performed on all symptomatic acute vertebral compression fractures. Age, body mass index, history of tobacco use, number of initial vertebral fractures, intradiscal cement leakage, history of nonspinal fractures, use of antiosteoporotic medications, bone mineral density, bone turnover markers, and 25(OH)D levels were assessed. All participants were evaluated clinically and/or radiographically. Follow-up period was 18 months.

RESULTS

The mean population age was 70.6 years (range, 40-83 years). After initial kyphoplasty procedure, nine patients (11 levels) (22.5% of patients; 11.2% of levels) developed a postkyphoplasty vertebral compression fracture. Cement leakage was identified in seven patients (17.5%). The patients without recurrent fractures after kyphoplasty demonstrated higher levels of 25(OH)D (22.6±5.51 vs. 14.39±7.47; p=.001) and lower N-terminal cross-linked telopeptide values (17.11±10.20 vs. 12.90±4.05; p=.067) compared with the patients with recurrent fractures.

CONCLUSIONS

Bone metabolism and 25(OH)D levels seem to play a role in the occurrence of postkyphoplasty recurrent vertebral compression fractures.

The mechanical behavior of PMMA/bone specimens extracted from augmented vertebrae: a numerical study of interface properties, PMMA shrinkage and trabecular bone damage

Recently published compression tests on PMMA/bone specimens extracted after vertebral bone augmentation indicated that PMMA/bone composites were not reinforced by the trabecular bone at all. In this study, the reasons for this unexpected behavior should be investigated by using non-linear micro-FE models. Six human vertebral bodies were augmented with either standard or low-modulus PMMA cement and scanned with a HR-pQCT system before and after augmentation. Six cylindrical PMMA/bone specimens were extracted from the augmented region, scanned with a micro-CT system and tested in compression. Four different micro-FE models were generated from these images which showed different bone tissue material behavior (with/without damage), interface behavior (perfect bonding, frictionless contact) and PMMA shrinkage due to polymerization. The non-linear stress-strain curves were compared between the different micro-FE models as well as to the compression tests of the PMMA/bone specimens. Micro-FE models with contact between bone and cement were 20% more compliant compared to those with perfect bonding. PMMA shrinkage damaged the trabecular bone already before mechanical loading, which further reduced the initial stiffness by 24%. Progressing bone damage during compression dominated the non-linear part of the stress-strain curves. The micro-FE models including bone damage and PMMA shrinkage were in good agreement with the compression tests. The results were similar with both cements. In conclusion, the PMMA/bone interface properties as well as the initial bone damage due to PMMA polymerization shrinkage clearly affected the stress-strain behavior of the composite and explained why trabecular bone did not contribute to the stiffness and strength of augmented bone.

Biomechanics of vertebral bone augmentation

Percutaneous vertebral augmentation is a successful means of relieving pain and reducing disability after vertebral compression fracture; however, the exact mechanism by which vertebral augmentation eliminates pain remains unproven. Most likely, pain relief is because of stabilization of microfractures. The biomechanical effects of vertebral fracture and subsequent vertebral augmentation therapy, however, are topics for continued investigation. Altered biomechanical stresses after treatment may affect the risk of adjacent fracture in an osteoporotic patient; that risk may be different after vertebral augmentation with cavity creation (balloon assisted vertebroplasty or kyphoplasty) when compared with vertebral augmentation without cavity creation (vertebroplasty). Polymethyl methacrylate cement used in these procedures may have an important effect on the load transfer and disk mechanics, and therefore, the variables of cement volume, formulation, and distribution should also be evaluated. Finally, the question of whether prophylactic treatment of adjacent intact levels is indicated must be considered.

Performance of vertebral cancellous bone augmented with compliant PMMA under dynamic loads

Increased fracture risk has been reported for the adjacent vertebral bodies after vertebroplasty. This increase has been partly attributed to the high Young's modulus of commonly used polymethylmethacrylate (PMMA). Therefore, a compliant bone cement of PMMA with a bulk modulus closer to the apparent modulus of cancellous bone has been produced. This compliant bone cement was achieved by introducing pores in the cement. Due to the reduced failure strength of that porous PMMA cement, cancellous bone augmented with such cement could deteriorate under dynamic loading. The aim of the present study was to assess the potential of acute failure, particle generation and mechanical properties of cancellous bone augmented with this compliant cement in comparison to regular cement. For this purpose, vertebral biopsies were augmented with porous- and regular PMMA bone cement, submitted to dynamic tests and compression to failure. Changes in Young's modulus and height due to dynamic loading were determined. Afterwards, yield strength and Young's modulus were determined by compressive tests to failure and compared to the individual composite materials. No failure occurred and no particle generation could be observed during dynamical testing for both groups. Height loss was significantly higher for the porous cement composite (0.53+/-0.21%) in comparison to the biopsies augmented with regular cement (0.16+/-0.1%). Young's modulus of biopsies augmented with porous PMMA was comparable to cancellous bone or porous cement alone (200-700 MPa). The yield strength of those biopsies (21.1+/-4.1 MPa) was around two times higher than for porous cement alone (11.6+/-3.3 MPa).

The effect of cement augmentation on the geometry and structural response of recovered osteopenic vertebrae: an anterior-wedge fracture model

STUDY DESIGN

The efficacy of cement augmentation in restoring the geometry and structural competence of failed thoracic and lumbar human vertebrae under mechanical loads was studied.

OBJECTIVES

To quantify whether cement augmentation restores and maintains the geometry and structural competence of failed osteopenic vertebrae and to assess the contribution of vertebral geometry to the achieved augmentation.

SUMMARY OF BACKGROUND DATA

Cement augmentation of failed vertebrae was clinically shown to alleviate significant pain and functional impairments associated with vertebral fragility fractures. However, the procedure's efficacy in restoring the structural response of the failed vertebrae and maintaining the achieved geometry under functional loads remains unclear.

METHODS

Nineteen thoracic and lumbar human vertebrae were tested to failure under compression-flexion loading. The vertebrae were allowed to recover, were retested to failure, augmented with Polymethylmethacrylate and again retested to failure. Repeated measures analysis was used to compare the change in vertebral geometry and structural response, defined as the multiplanar force and moment response of the vertebra to the imposed deformation, at each of the test stages. Linear regression was used to assess the role of the geometry of the failed vertebrae in affecting the outcome of augmentation.

RESULTS

Augmentation significantly increased the compressive (228%) and flexion (118%) strength of the failed vertebrae and achieved a significant, albeit partial, restoration of vertebral geometry. However, the structural response of the failed vertebrae was markedly altered, whereas under applied loads, the achieved height restoration was significantly diminished. Although the geometry of the fractured vertebral body was associated with the degree of restoration of the vertebral body afteraugmentation, it was not correlated with the change in the structural parameters.

CONCLUSION

Augmentation increases the structural competence of failed vertebrae and to a degree, restores their geometry. However, the structural response of the augmented vertebrae was significantly modified. Furthermore, the augmented vertebrae were unable to maintain the degree of geometry restoration under load.

Adjacent vertebral failure after vertebroplasty: a biomechanical study of low-modulus PMMA cement

PMMA is the most common bone substitute used for vertebroplasty. An increased fracture rate of the adjacent vertebrae has been observed after vertebroplasty. Decreased failure strength has been noted in a laboratory study of augmented functional spine units (FSUs), where the adjacent, non-augmented vertebral body always failed. This may provide evidence that rigid cement augmentation may facilitate the subsequent collapse of the adjacent vertebrae. The purpose of this study was to evaluate whether the decrease in failure strength of augmented FSUs can be avoided using low-modulus PMMA bone cement. In cadaveric FSUs, overall stiffness, failure strength and stiffness of the two vertebral bodies were determined under compression for both the treated and untreated specimens. Augmentation was performed on the caudal vertebrae with either regular or low-modulus PMMA. Endplate and wedge-shaped fractures occurred in the cranial and caudal vertebrae in the ratios endplate:wedge (cranial:caudal): 3:8 (5:6), 4:7 (7:4) and 10:1 (10:1) for control, low-modulus and regular cement group, respectively. The mean failure strength was 3.3 +/- 1 MPa with low-modulus cement, 2.9 +/- 1.2 MPa with regular cement and 3.6 +/- 1.3 MPa for the control group. Differences between the groups were not significant (p = 0.754 and p = 0.375, respectively, for low-modulus cement vs. control and regular cement vs. control). Overall FSU stiffness was not significantly affected by augmentation. Significant differences were observed for the stiffness differences of the cranial to the caudal vertebral body for the regular PMMA group to the other groups (p < 0.003). The individual vertebral stiffness values clearly showed the stiffening effect of the regular cement and the lesser alteration of the stiffness of the augmented vertebrae using the low-modulus PMMA compared to the control group (p = 0.999). In vitro biomechanical study and biomechanical evaluation of the hypothesis state that the failure strength of augmented functional spine units could be better preserved using low-modulus PMMA in comparison to regular PMMA cement.

Influence of a novel radiopacifier on the properties of an injectable calcium phosphate cement

An injectable calcium phosphate cement (CPC) with excellent radiopacity was proposed by introducing a novel radiopacifier, strontium carbonate, into the powder phase of CPC. The results showed that the cement showed improved radiopacity even when the content of strontium carbonate was only 8 or 12wt.%. The addition of 8 or 12wt.% strontium carbonate clearly improved the injectability and compressive strength of the cement. Furthermore, the addition of strontium carbonate influenced the pore distribution in the cement. An injectable CPC containing 8 or 12wt.% strontium carbonate has the potential for use in procedures such as vertebroplasty and kyphoplasty.

Is there an indication for prophylactic balloon kyphoplasty? A pilot study

Vertebroplasty and kyphoplasty are associated with a recurrent fracture rate of 2.4% to 23%, which is lower than the general natural history of untreated osteoporotic fractures. Some authors suggest the risk of refracture at adjacent vertebra will be reduced by prophylactic stabilization. We therefore compared the refracture rate after prophylactic balloon kyphoplasty in 60 patients randomized into groups with either monosegmental balloon kyphoplasty or adjacent prophylactic balloon kyphoplasty. The level (superior versus inferior) for prophylactic stabilization was chosen according to fracture type. We evaluated patients for 12 months using radiographs, visual analog scale scores, and SF-36 scores. We followed 23 of 30 patients in the monosegmental group and 27 of 30 patients in the prophylactic group. We observed no difference in the 1-year refracture rates between the two groups (five patients in the monosegmental group and seven in the prophylactic group). Leakage into the disc was the presumed cause of adjacent fractures in 50% of the patients. Disc leakage and refracture rate did not correlate as a result of the low patient number. Based on our data, we believe there is no indication for prophylactic stabilization of adjacent segments with balloon kyphoplasty.

High-viscosity cement significantly enhances uniformity of cement filling in vertebroplasty: an experimental model and study on cement leakage

STUDY DESIGN

Experimental study using a laboratory leakage model.

OBJECTIVE

To examine the working hypothesis that high-viscosity cements will spread uniformly, thus significantly reducing the risk of leakage.

SUMMARY OF BACKGROUND DATA

In vertebroplasty, forces that govern the flow of bone cement in the trabecular bone skeleton are an essential determinant of the uniformity of cement filling. Extraosseous cement leakage has been reported to be a major complication of this procedure. Leakage occurs due to the presence of a path of least resistance caused by irregularities in the trabecular bone or shell structure. Ideally, cement uniformly infiltrates the trabecular bone skeleton and does not favor specific paths. Cement viscosity is believed to affect the infiltration forces and flow during the procedure. Clinically, altering the time between cement mixing and delivery modifies the viscosity of bone cement.

METHODS

An experimental model of the leakage phenomenon of vertebroplasty was developed. A path, simulating a blood vessel, was created in the model to perturb the forces underlying cement flow and to favor leakage. Cement of varying viscosities was injected in the model, and, thereafter, the filling pattern, cement mass that has leaked, time at which leakage occurred, and injection pressure were measured.

RESULTS

A strong relationship was found between the uniformity of the filling pattern and the elapsed time from cement mixing and viscosity, respectively. Specifically, 3 distinct cement leakage patterns were observed: immediate leakage was observed when cement was injected 5-7 minutes following mixing. The cement was of a low viscosity and more than 50% of the total cement injected leaked. Moderate leakage was observed when injection occurred 7-10 minutes following mixing. Less than 10% of the cement leaked, and the viscosity was at a transient state between the low viscosity of immediate leakage and a higher viscosity, doughy cement. Cement leakage ceased completely when cement was delivered after 10 minutes. The viscosity of the cement in this case was high, and the cement was of a dough-like consistency.

CONCLUSIONS

High-viscosity cement seems to stabilize cement flow. However, the forces required for the delivery of high-viscosity cement may approach or exceed the human physical limit of injection forces. Although the working time of the cement is about 17 minutes, it may not be manually injectable with a standard syringe and cannula after 10 minutes, at which time cement leakage ceased completely.

Adjacent level load transfer following vertebral augmentation in the cadaveric spine

STUDY DESIGN

In vitro biomechanics.

OBJECTIVE

To determine if osteoporotic vertebral compression fracture (VCF) augmentation increases adjacent level load transfer.

SUMMARY OF BACKGROUND DATA

Osteoporotic VCF subsequent to augmentation may result from disease progression or increased adjacent level load transfer, or both.

METHODS

There were 11 T3-T7 and 10 T8-T12 divided by lumbar bone mineral density into a normal group (No. 1; n = 11) and an osteoporotic group (No. 2; n = 10). Strain and centrum stress were measured on T4 and T6 (T3-T7), and T9 and T11 (T8-T12) during tests in the intact state, following a centrum defect, during and after an augmented VCF at T5 or T10, and during a subsequent VCF. Stiffness and strength were compared: between groups 1 and 2; among intact, defect, and augmented VCF states; and between the initial and subsequent VCF.

RESULTS

Group 1 was stiffer than 2 in compression (P = 0.01) and flexion (P = 0.07), with no difference in adjacent level load transfer (strain P = 0.72, centrum stress P = 0.36) or strength (P = 0.07). The centrum defect reduced compressive stiffness from the intact (P = 0.001), which was partially restored following VCF augmentation (P = 0.006). There were no differences in flexion stiffness (P > or = 0.14). Adjacent level load transfer in flexion exceeded that in compression (strain P = 0.001, centrum stress P = 0.19). Initial and subsequent VCF occurred at similar forces (P = 0.26) with higher adjacent level load at subsequent (strain and centrum stress P = 0.04).

CONCLUSIONS

Augmentation of multilevel spinal segments with VCF produced by combined compression, flexion, and a centrum defect normalizes adjacent level load transfer at physiologic loads. In both normal and osteoporotic spinal segments, as loads approach those of the initial VCF, protection from augmentation is lost, and subsequent adjacent level VCFs occur from extreme loading, and not the augmentation process.

Biomechanical impact of vertebroplasty

OBJECTIVES

To examine the biomechanisms underlying adjacent fractures following vertebroplasty, an emerging procedure to stabilize fractured vertebrae. In this procedure, bone cement is injected percutaneously into the vertebral cancellous bone. Once hardened, the cement offers mechanical reinforcement to the weakened vertebra. Recent clinical and biomechanical reports suggest that this procedure may cause new fractures adjacent to the one augmented. The cause and extend is unclear yet. The focus here is on the biomechanical hypothesis resulting from the rigid cement augmentation.

METHODS

A combination of experimental and numerical studies, in additional to a review of recent clinical reports.

RESULTS

The broader finding suggests that vertebroplasty changes the mechanical loading in adjacent vertebrae. Specifically, an increase in adjacent loading in the range of 17% has been found. The mechanism underlying this increase seemed to stem from the excessive cement rigidity that reduced the endplate bulge of the augmented vertebra, thereby reducing the local spinal joint flexibility. The reduction in joint flexibility seeks to reverse itself by creating an increase in the inter-vertebral disc pressure. The increased disc pressure seeks to relieve itself by increasing the load on the adjacent vertebra. The increased load on the adjacent vertebra relates directly to an increased risk of fracture.

CONCLUSIONS

Although an increasing amount of evidence exists to support this theory of the origin of adjacent fractures, one must be cautious. Vertebroplasty is a relatively new procedure and further observations and, ultimately, prospective clinical studies are required to conclusively determine the cause and extend of adjacent fractures.

Rheological characterization of concentrated aqueous beta-tricalcium phosphate suspensions: the effect of liquid-to-powder ratio, milling time, and additives

The field of injectable calcium phosphate suspensions and cements is experiencing vigorous research activity. This is stimulated by their importance for the cement augmentation procedure (vertebroplasty), which is an emerging procedure to treat osteoporotic fragility fractures. The rheological properties such as the yield stress and viscosity play an important role in the process of cement delivery and infiltration into the cancellous bone cavities. However, the number of studies relating to their rheological properties is very limited. The objective of this first study was to examine the effects of the following three variables on the rheological properties of a non-setting beta-tricalcium phosphate suspension: liquid-to-powder ratio, milling of powder particles, and additives. The broad finding is that all the variables affect the rheological properties remarkably. The more specific salient finding is the large variation in viscosity and in the yield stress. The viscosity spanned three orders of magnitude and the yield stress spanned five orders of magnitude. It appears that the rheological properties can be altered at will. However, one has to exercise extreme caution because these changes are not without cost to other important properties such as the cohesiveness and mechanical properties of the cement. Another important finding is that a linear correlation between the yield stress and the viscosity was found. Measurement of one of these variables might be enough to determine the other.

Effect of vertebral shell on injection pressure and intravertebral pressure in vertebroplasty

STUDY DESIGN

An experimental biomechanical study conducted on osteoporotic cadaveric vertebrae.

OBJECTIVES

1) To measure the intravertebral shell pressure and injection pressure; and 2) to determine the effect of the vertebral shell on the intravertebral shell pressure and on the injection pressure.

SUMMARY OF BACKGROUND DATA

Forces that govern cement flow are an essential component of the cement injection process in vertebroplasty. The vertebral shell may play a significant role in confining the flow of cement in the vertebral body and thereby affecting the intravertebral pressure and injection pressure.

METHODS

A small fenestration was created in the left lateral vertebral shell of 14 vertebrae. A valve to open and close the fenestration and a sensor to measure the intravertebral pressure were attached to the opening. A closed fenestration simulated an intact shell, whereas an open fenestration represented a vented shell. Injection pressure and intravertebral pressure at the shell were recorded during a controlled injection.

RESULTS

A closed fenestration resulted in a significant increase in the intravertebral pressure at the shell. During the injection, the shell pressure increased on average to approximately 3.54 +/- 2.91 kPa. Conversely, an open fenestration resulted in an instant relaxation of the shell pressure to the ambient pressure of 0 kPa. Additionally, the injection pressure was approximately 97 times higher than the shell pressure.

CONCLUSION

The presence of vertebral shell seems to be important for intravertebral pressure. However, the intravertebral shell pressure adds very little to the injection pressure.

Influence of mixing method on the cement temperature-mixing time history and doughing time of three acrylic cements for vertebroplasty

Acrylic cements are increasingly being used to augment osteoporotic vertebrae in a procedure called vertebroplasty. Two significant factors that may complicate the use of acrylic cements are: (a) short handling time, which may result in insufficient filling of the vertebra; and (b) exothermic setting (curing) behavior, which may result in thermal damage of the surrounding tissue. It has been previously reported that mixing the cement components under oscillation, as compared to manual mixing, increases the handling time. More specifically, it seems that oscillatory mixing slows down the cement polymerization process and, consequently, widens the time window during which cement is injectable. However, the effect of oscillatory mixing on the exothermic setting behavior of cement undergoing polymerization has not been examined. In this study, the exothermic setting behavior of three commercially available acrylic cements--Antibiotic Simplex, DP-Pour&trade, and Vertebroplastic--were examined for both manual and oscillatory mixing methods. For each combination of cement and mixing method, the parameters that were measured were the exothermic setting curve (and hence the cement setting temperature and setting time) and the cement doughing time. It was found that oscillatory mixing had no significant effect on any of these parameters. Based on the results of this study, it can be concluded that, for the tested cements, the setting process is a reaction-controlled process rather than a diffusion-controlled one. Clinically, this implies that oscillatory mixing may be used to increase the working period for acrylic cements without increasing the risk of thermal damage to surrounding tissue.

Load shift of the intervertebral disc after a vertebroplasty: a finite-element study

Infiltrating osteoporotic cancellous bone with bone cement (vertebroplasty) is a novel surgical procedure to stabilize and prevent osteoporotic vertebral fractures. Short-term clinical and biomechanical results are encouraging; however, so far no reports on long-term results have been published. Our clinical observations suggest that vertebroplasty may induce subsequent fractures in the vertebrae adjacent to the ones augmented. At this point, there is only a limited understanding of what causes these fractures. We have previously hypothesized that adjacent fractures may result from a shift in stiffness and load following rigid augmentation. The purpose of this study is to determine the load shift in a lumbar motion segment following vertebroplasty. A finite-element (FE) model of a lumbar motion segment (L4-L5) was used to quantify and compare the pre- and post-augmentation stiffness and loading (load shift) of the intervertebral (IV) disc adjacent to the augmented vertebra in response to quasi-static compression. The results showed that the rigid cement augmentation underneath the endplates acted as an upright pillar that severely reduced the inward bulge of the endplates of the augmented vertebra. The bulge of the augmented endplate was reduced to 7% of its value before the augmentation, resulting in a stiffening of the IV joint by approximately 17%, and of the whole motion segment by approximately 11%. The IV pressure accordingly increased by approximately 19%, and the inward bulge of the endplate adjacent to the one augmented (L4 inferior) increased considerably, by approximately 17%. This increase of up to 17% in the inward bulge of the endplate adjacent to the one augmented may be the cause of the adjacent fractures.