Authors: Philip Purcell BEng (Hons) Cert Med Sci MIEI, Dr Magdalena Tyndyk, Dr Fiona McEvoy MIEI, Stephen Tiernan MIEI and Seamus Morris MB BCh BAO MMedSci MCh AFRCSI FRCS Spinal fractures are among the most prevalent injuries sustained by the human skeletal system, with an estimated 1.4 million fractures of the vertebral column worldwide every year [1]. It has been estimated that 15% [2] of these fractures are associated with over-loading incidents such as falls from a height and sporting injuries [3]. Vertebral fractures can have serious implications for patients due to the presence of the delicate spinal cord system housed within the spinal column, which transports electrical impulses to and from the brain. Disruption to this crucial system can result in considerable pain for the patient and, in severe cases, can result in paralysis. In addition to the high personal cost of sustaining a spinal injury, the treatment of vertebral fractures can prove to be very costly in financial terms to healthcare providers. A recent Irish study [4] of ~1,000 admissions during a five-year period found traumatic spinal injuries typically needed 46 days of hospital admission, along with life-long monitoring in cases of spinal cord injury. [login type="readmore"] Life-long treatment costs for a 25-year old with spinal cord injury in the United States have previously been estimated at an equivalent of €3.5 million [5]. Cases involving spinal cord injury can see costs extend beyond the clinical environment, with significant capital costs associated with home accessibility modifications and the ongoing costs of regular clinical check-ups. A considerable proportion of vertebral fractures that occur in elderly patients are classified as insufficiency fractures attributed to degeneration of the underlying bone material in the form of osteoporosis. Techniques for treating these fractures range from simple bed rest through to the intricacy of modern minimally invasive surgery. Minimally invasive techniques to treat vertebral fractures are constantly evolving, with new materials and devices being developed to overcome challenges such as recurrent fractures and implant integration. Surgical treatments involving cement augmentation have become a popular course of action in the medical community over recent years [6] and have been shown to be an effective means of relieving patient pain. Balloon kyphoplasty is a minimally invasive surgical treatment that uses an inflatable balloon to restore vertebra height, followed by cement injection to stabilise the structure (Figure 1, below). [caption id="attachment_2770" align="aligncenter" width="616"] Figure 1[/caption]

KYPHOPLASTY PROCEDURE

Various biomechanical evaluations have been completed using compression type tests to assess the effectiveness of the kyphoplasty procedure in restoring vertebrae stiffness and strength [7, 8]. These studies have demonstrated the ability of kyphoplasty to restore mechanical strength of vertebra to pre-fracture levels. An important advantage of the kyphoplasty procedure is that it corrects the spinal deformity by restoring the fractured vertebra nearer to its pre-fracture shape. Without this height restoration, the patient can develop a ‘hunched’ posture, known as kyphosis, which can lead to further fractures due to the altered loading of the spine. Spinal misalignment also can cause increased loading in other parts of the body such as the hips and knees, which are utilised by the body as compensatory joints to maintain a stable upper-body posture in cases of vertebral collapse, thereby also increasing fracture risk in regions remote to the spine. Recent developments in academic literature have highlighted the importance of a strong mechanical interlock at the interface between the augmentation cement and the host bone. This interface region bears particular significance in kyphoplasty, due to the use of an inflatable balloon that causes compaction of the surrounding bone. A clinical study [9] of 175 patients drew attention to a radiological ‘halo’ phenomenon around this compacted bone-cement interface following kyphoplasty.

The authors statistically correlated this radiological feature with a 78% re-collapse rate and attributed the phenomenon to a lack of cement penetration into the bone structure. This conclusion was supported by another study [10], which observed reduced cement infiltration and mechanical interlock following kyphoplasty due to the presence of compacted bone.

COMPUTATIONAL MODELLING While previous studies in the literature suggest a correlation between bone compaction and reduced mechanical integrity, the overall contribution of this feature to instigating vertebral re-collapse remains unknown. The aim of the current work was to use a parameterised finite element model (Figure 2, below) to assess the effect of patient bone volume fraction (BVF) and compacted bone thickness on load sharing within a treated L1 vertebra. [caption id="attachment_2775" align="aligncenter" width="1106"] Figure 2[/caption]

Due to the complexity of analysing trabecular bone yielding, it was necessary to simplify the problem using a geometry based model to link a morphological property of the bone with resulting compaction thickness. The bone property chosen for this purpose was bone volume fraction, which is defined as bone volume divided by total volume.

A core assumption of this modelling technique was that yielding of trabecular bone occurs via localised compaction banding, similar to that seen in cellular materials. Studies on the compressive behaviour of trabecular bone have shown that compaction banding is present during yielding of trabecular bone and is comparable to the deformation mechanisms observed in open-cell foam materials. Despite the significance of BVF in determining the mechanical behaviour of trabecular bone [11, 12], few cement augmentation models have included it as a parameter. To the author’s knowledge, the present work is the first known attempt to link a morphological parameter of the bone with treatment outcomes in balloon kyphoplasty. DISCUSSION AND CONCLUSION Results from the parametric model (Figure 3) indicate a shift in load from the cement to the surrounding bone structures when a compacted bone-cement interface region is defined. Bone cement stresses dropped by ~22% and remained at this level with increasing interface thickness. When a thick compacted zone was calculated by the model, it was found that the cortical bone experienced the greatest load shift with a 28% increase compared to the case with no interface, while the trabecular bone stresses also increased by 15%. [caption id="attachment_2779" align="aligncenter" width="615"] Figure 3[/caption]

The results of the model also show that bone volume fractions relevant to the young patient demographic produce the thickest compaction zones and consequently produce the greatest load shift from the cement to the host bone structures. The apparent tendency of the load to shift mostly towards the cortical shell would be expected due to its high stiffness relative to the other bony components and agrees with the load shift seen by other authors that have modelled cement augmentation [13, 14].

Beyond the computational work presented currently, a complimentary experimental study is ongoing to provide a comprehensive insight into the bone deformation and cement augmentation mechanisms during the kyphoplasty treatment. Future work will also utilise mechanical testing of multi-segment spine models to assess the clinical implications of vertebral re-collapse during the rehabilitation phase using a state-of-the-art MTS Spine Simulator based at the Institute of Technology Tallaght, Dublin. The present work represents considerable progress towards developing a detailed insight into the long-term mechanical integrity of the kyphoplasty treatment. The knowledge gained from this work will aid clinicians in developing surgical practices that minimise the potential for post-operative complications such as vertebral re-collapse and, consequently, will significantly improve the quality of life for patients that have sustained a spinal injury. Philip Purcell BEng (Hons) Cert Med Sci MIEI; Dr Magdalena Tyndyk; Dr Fiona McEvoy MIEI; Stephen Tiernan MIEI; and Seamus Morris MB BCh BAO MMedSci MCh AFRCSI FRCS This research was funded by the Irish Research Council EMBARK postgraduate scholarship awarded to lead author Philip Purcell. For further information, please contact Philip Purcell, Centre of Applied Science for Health, Institute of Technology Tallaght, Dublin 24. Phone:+353 1 404 2958 or e-mail: philip.purcell@ittd.ie.  REFERENCES

1. Johnell O, Kanis JA. An estimate of the worldwide prevalence and disability associated with osteoporotic fractures. Osteoporosis Int 2006;17(12):1726–33.
2. Denis F. Spinal instability as defined by the three-column spine concept in acute spinal trauma. Clin Orthop Relat R 1984;(189):65–76.
3. Roche SJ, Sloane PA, McCabe JP. Epidemiology of spine trauma in an Irish regional trauma unit: a 4-year study. Injury 2008;39(4):436–42.
4. Lenehan B, Boran S, Street J, Higgins T, McCormack D, Poynton AR. Demographics of acute admissions to a National Spinal Injuries Unit, Eur Spine J 2009;18(7):938–42.
5. NSCISC. Spinal cord injury facts and figures at a glance. J Spinal Cord Med 2012 Nov;35(6):480–1.
6. Hardouin P, Fayada P, Leclet H, Chopin D. Kyphoplasty. Joint, bone, spine 2002;69(3):256–61.
7. Khanna AJ, Lee S, Villarraga M, Gimbel J, Steffey D, Schwardt J. Biomechanical evaluation of kyphoplasty with calcium phosphate cement in a 2-functional spinal unit vertebral compression fracture model. Spine J 2008;8(5):770–7.
8. Tomita S, Molloy S, Jasper LE, Abe M, Belkoff SM. Biomechanical comparison of kyphoplasty with different bone cements. Spine 2004;29(11):1203–7.
9. Kim KHS, Kuh SU, Park JY, Chin DK, Cho YE. What is the importance of “halo” phenomenon around bone cement following vertebral augmentation for osteoporotic compression fracture? Osteoporosis Int 2012;23(10):2559–65.
10. Krüger A, Oberkircher L, Kratz M, Baroud G, Becker S, Ruchholtz S. Cement interdigitation and bone-cement interface after augmenting fractured vertebrae: A cadaveric study. ISASS 2012;6(1):115-123.
11. Bevill G, Easley SK, Keaveny TM. Side-artifact errors in yield strength and elastic modulus for human trabecular bone and their dependence on bone volume fraction and anatomic site. J Biomech 2007;40(15):3381–8.
12. Mc Donnell P, Harrison N, Lohfeld S, Kennedy O, Zhang Y, Mc Hugh PE. Investigation of the mechanical interaction of the trabecular core with an external shell using rapid prototype and finite element models. J Mech Behav Biomed 2010;3(1):63–76.
13. Villarraga Ph D LM, Cripton PA, Bellezza AJ, Berlemann U, Kurtz SM, Edidin AA. Stress levels in bones and bone cement in the thoracolumbar spine afer kyphoplasty. Finite element study. Der Orthopade 2004;33(1):48–55.
14. Dickey BT, Tyndyk MA, Doman DA, Boyd D. In silico evaluation of stress distribution after vertebral body augmentation with conventional acrylics, composites and glass polyalkenoate cements. J Mech Behav Biomed 2012;5(1):283-90.