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Bone surrogates provide authentic onlay graft fixation torques
Author’s Accepted Manuscript Bone Surrogates provide authentic Onlay Graft Fixation Torques Marianne Hollensteiner, Peter Augat, David Fürst, Falk Schrödl, Benjamin Esterer, Stefan Gabauer, Andreas Schrempf www.elsevier.com/locate/jmbbm
PII: DOI: Reference:
S1751-6161(18)30888-9 https://doi.org/10.1016/j.jmbbm.2018.12.013 JMBBM3098
To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 20 July 2018 Revised date: 11 December 2018 Accepted date: 13 December 2018 Cite this article as: Marianne Hollensteiner, Peter Augat, David Fürst, Falk Schrödl, Benjamin Esterer, Stefan Gabauer and Andreas Schrempf, Bone Surrogates provide authentic Onlay Graft Fixation Torques, Journal of the Mechanical Behavior of Biomedical Materials, https://doi.org/10.1016/j.jmbbm.2018.12.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Article type: Short Communication
Bone Surrogates provide authentic Onlay Graft Fixation Torques Marianne Hollensteinera,b,1*, Peter Augatb1, David Fürsta, Falk Schrödlc, Benjamin Esterera,b1, Stefan Gabauera, Andreas Schrempfa a
Research Group for Surgical Simulation Linz, Upper Austria University of Applied Sciences, School of
Applied Health and Social Sciences, Garnisonstr.21, 4020 Linz, Austria. b
Institute of Biomechanics, BG Unfallklinik Murnau, Germany and Paracelsus Medical University
Salzburg, Austria. c
Institute of Anatomy, Paracelsus Medical University, Struberg. 21, 5020 Salzburg, Austria
*
Corresponding author info: Marianne Hollensteiner, Research Group for Surgical Simulation Linz, Upper
Austria University of Applied Sciences, School of Applied Health and Social Sciences, Garnisonstr.21, 4020 Linz, Austria. Email: [email protected], phone: +43/(0)50804/55062, fax: +43/(0)50804/955062
Abstract: Onlay graft bone augmentation is a standard practice to restore the loss of height of the alveolar ridge following loss of a tooth. Cranial grafts, lifted from the parietal bone, are sandwiched and used to bridge the bony defect in the jaw by means of small screws. During the elevation of the covering gum and subsequent screw placement, care has to be taken in order to preserve underlying nerves. Therefore, to avoid harm to the patient, a solid education of surgeons is essential, which requires training and experience. A simulator for cranial graft-lift training was already developed and shall be expanded to train the augmentation of the lifted implants. Therefore, in this study, synthetic bones for onlay block graft screw placement with realistic haptics for the screw application training were evaluated and compared with human specimens. Six different polyurethane based bone surrogate composites, enriched with varying amounts of calciumbased mineral fillers and blowing agents, were developed. The haptical properties of these synthetic bones were validated for screw placement and compared with human parietal bone specimens. For that, bones were pre-drilled, screws were automatically inserted using a customized testbench and the slope of the screw-insertion torques were analyzed. The slope of the screw insertion torques of the human reference bones was 56.5±14.0 *10-3 Nm/deg, Surrogates with lower amounts of mineral fillers and blowing agents showed lower torques than the human 1
Prof. Küntscher Straße 8, 82418 Murnau, Germany
bone. Synthetic bones, validated for drilling, milling and sawing in an earlier study, also achieved significantly lower torques, which were only the half of the human parietal bones. Two intermediate stages of the aforementioned material compositions, consisting of 75% mineral filler with 0.75% blowing agent and 100% mineral filler with 1.00% blowing agent revealed results comparable with human bone (57.4±10.2 *10-3 Nm/deg, p=0.893 and 54.9±11.1 *10-3 Nm/deg, p=0.795, respectively). In conclusion, our findings suggest that, two newly developed polyurethane-based materials mimicking the haptical properties of an onlay bone graft screw fixation, have been identified. Thus, these surrogates are capable of mimicking real bone tissue in our simulator for the education of novice surgeons.
Keywords: Synthetic Bones; Biomechanics; Screw Fixation; Torque; Simulator
1.
Introduction :
After the loss of a tooth, the alveolar process degenerates due to absent stress [1], leading to pain and difficulty in speech or ingestion [2]. In order to reconstruct the resulting loss of alveolar height, cranial grafts are favored due to better healing [3]. The autogenous onlay bone block method is a standard practice for ridge augmentation: split thickness bone grafts from the parietal skull are sandwiched and used to bridge the bony defect by means of small screws (see Fig 1, [4]). To access the underlying atrophic bone, the mucosa needs to be elevated and care has to be taken that the underlying nerves are preserved. After a healing period of about four to six months, the screws are removed and an endosseous dental implant can be inserted [3]. To prevent harm to the patient, the surgeon needs to apply appropriate forces during screw placement to avoid fracture of the graft, underlying bone or screw. The acceptance of simulators as an educational opportunity is growing [6]. Their validated haptic feedback improves learning [7] and consequently minimizes the risk for the patient [8]. A simulator to train the lift of parietal split-thickness grafts has been developed [9, 10], to train the augmentation of the lifted implants. The aim of this study was to examine and validate varying synthetic parietal bones for onlay block graft placement to provide validated haptic feedback during the screw application training.
Fig 1 Augmentation of split thickness onlay block graft in the alveolar ridge of the maxilla (A) to bridge the bony defect of the left incisor (a.. right incisor, b.. left lateral incisor, c.. right canine, d.. aspiration tool, e.. sore hook). The fixation of the three sandwiched grafts from the cranium was performed with three screws The middle screw is used to fix the two lower grafts whereas the two lateral screws secure the graft-sandwich on the jawbone. Two grafts (contoured by dotted and dashed lines) are superimposed with their cancellous sides. A third graft (full line) is placed over it right-angled with the cortex pointing upwards as shown in the graphic (B).
2.
Material and Methods
2.1.
Human Specimens
Fresh frozen human parietal bone samples from two female donors (67 and 83 years) were used as references. The human tissue was obtained from the anatomical gift program of the Medical University of Vienna, Austria, in accordance with the Declaration of Helsinki and by approval of the ethics committee of the state Bavaria, Germany. The parietal bones were dissected and divided from other cranial bone proportions (frontal, temporal or occipital bone) along their associated sutures (sutura coronalis, sutura squamosa and sutura lambdoidea, respectively). Further, the parietal bone specimens were sterilized by autoclaving and were cut into four rectangular samples of approximately 2x8cm. Resulting grafts were stored at -37 degree Celsius until further processing and were thawed and soaked in saline solution (twelve hours) prior to measurements.
2.2.
Synthetic Bone Surrogates
Polyurethane- based parietal bone surrogate composites were manufactured (see Fig 2) as described in earlier studies [9, 11]): as a basic material a polyurethane resin (65D, KauPo Plankenhorn e.K., Spaichingen, Germany) was enriched with calcium-based mineral fillers (calcium-carbonate powder, Lehrer GmbH, Linz, Austria) in varying amounts of 0, 50, 75 and 100%wt of the whole polyurethane resin mass. Additionally, tap water, in amounts of 0, 0.50, 0.75 and 1.00%wt, was used as blowing agent. The blowing agent is used to froth up the polyurethane- calcium-carbonate powder-mixture to create an open celled cancellous-bone-like structure. Materials were weighed in and mixed manually before molding a skull cap with validated thickness as described in an earlier study [12]. Realistic skull caps made of six material compositions (SC1: 50%wt mineral filler and 0.5%wt blowing agent, SC2: 100%wt mineral filler and 0.5%wt blowing agent, SC3: 75%wt mineral filler and 75%wt blowing agent, SC4: 100%wt mineral filler and 1.0%wt blowing agent, SC5: 25%wt mineral filler and 0.25%wt blowing agent, SC6: pure resin, no mineral filler and no blowing agent) were molded. Both parietal bones were dissected from the anatomically realistic skull caps and were cut into five rectangular samples each.
Fig 2 Artificial skull cap cut in transverse plane revealing realistic cortical layers sandwiching an open-celled cancellous bone structure;
2.3.
Screw Torque Measurement setup
In accordance with the surgical procedure for the augmentation of an onlay graft into the alveolar ridge [4], the screw insertion torque was examined. A material testbench was designed to record the screw insertion torque and screw entry depth (see Fig 3). For that, an electric drive was equipped with a drill chuck. Further, drills or turnscrews with 2x20 mm screws (anodized titan cross pin SS, 30 threads per inch, pitch 0.847mm, outer diameter 2mm, core diameter 1.3mm, Stryker, Kalamazoo, USA) were fixed within the drill chuck and were driven with a speed of 1 RPM. The specimens were directly clamped on a 6- degree of freedom load cell (ATI nano 25, ATI Industrial Automation, Apex, USA) which was attached to a pulley block ensuring a constant feeding force of 40 N during the screw insertion [13]. A continuous screwspecific (diameter of 1.2mm) pilot-hole was drilled through the specimens, whereas the drill chuck ensured the same entry trajectory of the drill-bit and the screw. No tapping of the screws was performed. The screws were not fully inserted into the specimens until the screw heads touched the specimens. The insertion of the screw and thus, the torque measurement was stopped, when the screw tip appeared on the bottom of the specimen. The number of measured revolutions thereof depended on the realistically varying thickness of the samples. Three screw insertion measurements were performed at each specimen resulting in 24 measurements of the human and 30 measurements of each synthetic parietal bone. The elastic slope (k) of the screw insertion torques was identified as a suitable parameter to compare screw insertion haptics since the torque of the cylindrical screws increased linearly with the insertion depth (see Fig 5). For comparison reasons, since the measurement data are of different lengths due to varying specimen thicknesses, measurement data was edited and cut at a uniform insertion depth of 4 full screw revolutions and the linear slope of the measurement curves was calculated within this range (polyfit, Matlab2016a, The Mathworks Inc., Natick, USA). Four full revolutions correspond to a screw insertion depth of approximately 4.6mm, which is sufficient to fully penetrate a parietal split thickness graft.
Fig 3 A) Measurement setup for screw torque measurement; B) Close up of the screw penetrating the specimen (a.. rack with integrated pulley rack, b.. incremental encoder , c.. servodrive, d.. drill chuck, e.. turnscrew, f.. screw penetrating the specimen, g.. adapter holding the specimen with lateral screws, h.. specimen, i.. basepad enabling the screw to fully penetrate the specimen without getting in contact with adapter, j..torque sensor)
Fig 4 Typical measurement curves of the human reference and the artificial specimens (full lines) and the calculated slope lines (Human: full thin line, SC1: dashed line, SC2: dotted line, SC3: dashed-dotted line, SC4: plus-sign line, SC5: stared line, SC6: crossed line)
2.4.
Statistical analysis
Statistical analysis was performed with SPSS (SPSS Statistics 22, IBM, Armonk, USA). Shapiro-Wilk test identified non-normally distributed data within sample groups. Thus, Mann-Whitney-U test was used to detect differences between the human reference and the synthetic sample groups. For all statistical tests, a p-value of 0.05 was considered significant.
3.
Results
The slope of the screw insertion torques of the human reference bones was 56.5±14.0 *10-3Nm/deg (see Fig Fig 5). Synthetic parietal bones, which were validated for drilling, milling (SC1) and sawing (SC2) earlier [9, 11] achieved significantly lower torques. The slopes of the synthetic parietal bone favored for drilling and milling (SC1, 50% mineral filler, 0.5% blowing agent) were one third lower than the human reference (37.7±3.8 *10-3 Nm/deg, p=0.001). The synthetic sample favored for sawing (SC2, 100% mineral filler, 0.5% blowing agent) were only about half of the human parietal bones (28.0±5.1 *10-3 Nm/deg, p=0.001). Novel surrogates with lower amounts of additives (SC5, 25% mineral filler with 0.25% blowing agent and a specimen without any additional ingredients-SC6- 0% filler with 0% blowing agent) achieved lower slopes compared to the human reference (37.6±2.8 *10-3 Nm/deg, p=0.003 and 32.2±4.9 *10-3 Nm/deg, p=0.005, respectively). Two intermediate stages of the two aforementioned material compositions with higher amounts of mineral fillers and blowing agents (SC3, consisting of 75% mineral filler with 0.75% blowing agent and SC4, made
of 100% mineral filler with 1.00% blowing agent) achieved appropriate results (57.4±10.2 *10-3 Nm/deg, p=0.893 and 54.9±11.1 *10-3 Nm/deg, p=0.795, respectively), comparable to human parietal bone.
Fig 5 Results of screw insertion torque slopes in parietal bone (Human) and four artificial skull caps made of varying material mixtures (SC1 to SC4)
3.1.
Feasibility of Synthetic Bones Surrogates for Surgical Training
To confirm the feasibility of the varying synthetic bone surrogates for screw placement training, the materials were independently examined by two cranio-maxillofacial surgeons. Both surgeons reported that the materials (SC1, SC2), which were already validated for machining procedures in earlier studies [11, 12], did not provide enough torque or resistance during the screw placement. Furthermore, materials with no or lacking cancellous bones structure (SC5, SC6), were found to provide a better haptic feedback but did not show a realistic diploic structure. The intermediate materials SC3 and SC4 were favoured, because of a realistic haptic feedback and a more realistic cancellous structure.
4.
Discussion
In this study, we presented a bone surrogate for surgical onlay graft screw placement training, validated against human parietal bones. Such a validated bone surrogate will provide a safe but realistic training setting useable for the training and education of surgeons and will potentially reduce surgical complications. A training platform for split thickness graft lifts and consequent graft augmentation with screws is currently not available and would be beneficial for surgical training. Based on earlier studies [912], six varying bone surrogate materials were developed and investigated. According to the surgical
procedure of a split thickness onlay graft placement, the slope of the screw insertion torque of craniomaxillo-facial screws into human and synthetic parietal bones were analyzed. Due to the manual weigh in and molding of the synthetic bones variances in compositions as well as cortical and diploic thicknesses arise. On the one hand, these fluctuations are desirable, leading to varying training scenarios for the surgeons. On the other hand, skull bone thickness, as well as the thickness of cortices and diploic layer vary from gender and ethnicity. Parietal full bone thickness was reported to range from 6.69±1.94mm [14], whereas the thickness of the cortices ranged from 1.8±0.3mm [15] and the thickness of the diploe was reported to be 3.38±1.00mm [16]. In a previous study, the similarity of the layer thicknesses of the synthetic parietal bones with human bone has already been demonstrated [12]. The measured slope of the screw insertion torques of the materials, validated for drilling, milling and sawing in an earlier study [11, 12] were one third (SC1) and the half (SC2) smaller than the human reference bones. Novel surrogates with lower or no amounts of mineral filler materials or blowing agents (SC5, SC6) also provided statistically significant lower slopes than the human reference. New intermediate stage-materials (SC3, SC4) provided comparable results to the human reference bones. Attention should be drawn to the fact, that in our measurements, the insertion torque appeared to rise linearly, independently of cortical thickness. This may be due to the fact, that, the spongy bone of the skull vault has a „denser“ structure, compared to other locations in the human body. The morphological properties of human parietal bone reveal thicker trabeculae (0.73±0.12mm) in parietal bone [17] compared to human lumbar vertebra (0.12±0.02mm [18]) and the trabecular separation is quite low [17, 18]. Thus, we believe that this very dense cancellous structure, connected with a high clamping force provided by the cortices, leads to a steady linear rise of the torque, although fluctuations in the torque measurements, triggered by cancellous bone separations, appeared. A few limitations of the study have to be mentioned. The human sample size (n=2) was rather small and the samples came from rather old donors over the age of 65. But, since neither parietal bone thickness nor bone morphometrics are affected by age or osteoporosis [19, 20], our reference sample was considered as reliable and meaningful. Furthermore, only one screw type was chosen to validate our novel materials. This screw type was chosen, because it is used by default in the university hospital where our simulator shall be used for training purposes. An advantage of model-based simulators with validated bone models compared to virtual simulators is that real tools or screws can be used that also produce a realistic haptic feedback within the validated bone surrogates. The screw insertion measurements were aborted, when the screw tip broke through the inner cortex of the specimens. When the screw is fully inserted, the screw head touches the bone or surrounding soft tissues, and thus, the torque inevitably increases rapidly. The augmented and sandwiched grafts are not loaded in the maxilla and are covered by the gum as they are allowed to become part of the host. Thus, in the training session with the simulator, the screws shall be placeable, but shall not be fully tightened. But, since both,
the screw head design and the bone properties are assumed to play a substantial role in the formation of the haptical feedback during screw tightening, this shall be part of ongoing research. Finally, the feasibility for screw placement of the novel bone surrogates was confirmed by two experienced surgeons. According to the statements of the surgeons, the two new material-compositions, which also showed good results during the mechanical testing (SC3 and SC4), provided a more realistic resistance to screw insertion than the other provided samples. The surgeons could not agree on one material, since one surgeon preferred the 0.75% surrogate material (SC3) because of its more realistic cancellous bone structure, whereas the other surgeon preferred the 1.0% composition (SC4). The synthetic bones with lesser amount of blowing agents (0% and 0.25%) were ruled out by the surgeons because of their unrealistic or absent structure of the spongy bone as well as inappropriate haptical feedback.
5.
Conclusion
In conclusion, our findings suggest that two newly developed polyurethane materials mimic the haptical properties of an onlay bone graft fixation with screws. Although synthetic surrogates, which were already validated for the graft lift procedure in a former study, were not suitable to deliver a realistic haptic for screw placement, variations of the aforementioned surrogates could authentically mimic screwing properties of human parietal bone and are thus capable for the education of novice surgeons.
6.
Acknowledgements
The authors thank the company Stryker for their generous provision of screws required for this study. Further, the authors would also like to thank the team of the Institute of Biomechanics in Murnau for their support during human specimen preparation.
7.
Funding
The Research Group for Surgical Simulators Linz (ReSSL) acknowledges the financial support by the Austrian Research Promotion Agency (FFG) within the program line Cooperation & Innovation (COIN) and project number 845436.
8.
Conflict of Interest
The authors declare that they have no conflict of interest.
References [1] Vinter I., Krmpotia-Nemaniae J., Hat J., Jalsovec D., 1993. Does the alveolar process of the maxilla always disappear after tooth loss?. Laryngo- rhinootologie. 72, 605-607. [2] Stellingsma C., Vissink A., Meijer H. J. A., Kuiper C., Raghoebar G. M., 2004. Implantology and the severely resorbed edentulous mandible., Critical reviews in oral biology and medicine : an offcial publication of the American Association of Oral Biologists. 15, 240-248.
[3] Smolka W., Bosshardt D. D., Mericske-Stern R., Iizuka T., 2006. Reconstruction of the severely atrophic mandible using calvarial split bone grafts for implant-supported oral rehabilitation. Oral surgery, oral medicine, oral pathology, oral radiology, and endodontics. 101, 35-42. [4] McAllister B. S., Haghighat K., 2007. Bone augmentation techniques. Journal of periodontology. 78, 377-396. [5] Kerr B., O'Leary J. P., 1999. The training of the surgeon: Dr. Halsted's greatest legacy., The American surgeon, 65, 1101-1102. [6] Rosen K., 2008. The history of medical simulation, JCritCare. 23, 157-166. [7] Botden S. M. B. I., Torab F., Buzink S. N., Jakimowicz J. J., 2008. The importance of haptic feedback in laparoscopic suturing training and the additive value of virtual reality simulation. Surgical endoscopy. 22, 1214-1222. [8] Singh H., Kalani M., Acosta-Torres S., El Ahmadieh T. Y., Loya J., Ganju A.,2013. History of simulation in medicine: from resusci annie to the ann myers medical center. Neurosurgery. 73, 9-14. [9] Hollensteiner M., Fuerst D., Esterer B., Hunger S., Malek M., Augat P., Schroedl F., Stephan D., Schrempf A., 2016. Development of parietal bone surrogates for parietal graft lift training. CDBME; 2, 637-641. [10] Hollensteiner M., Malek M., Augat P., Fuerst D., Schroedl F., Hunger S., Esterer B., Gabauer S., Schrempf A.,2018. Validation of a simulator for cranial graft lift training: Face, content, and construct validity, Journal of Cranio- Maxillo-Facial Surgery. 46, 1390-1394. [11] Hollensteiner M., Fuerst D., Esterer B., Augat P., Schroedl F., Hunger S., Malek M., Stephan D., Schrempf A. 2017. J Mech Behav Biomed Mater. 72, 49-51. [12] Hollensteiner M., Fürst D., Augat P., Schrödl F., Esterer B., Gabauer S., Hunger S., Malek M., Stephan D., Schrempf A. 2018. Characterization of an artificial skull cap for cranio-maxillofacial surgery training, Journal of Materials Science: Materials in Medicine, 29, 135. [13] Clement H., Zopf C., Brandner M., Tesch N. P., Vallant R., Puchwein P., 2015. Performance test of different 3.5 mm drill bits and consequences for orthopaedic surgery. Arch Orthop Trauma Surg. 135, 1675-1682. [14] Sabancıoğulları V, Kosar M, Salk I, Erdil F, Oztoprak I, Cimen M. 2012. Diploe thickness and cranial dimensions in males and females in mid-Anatolian population: an MRI study. Forensic Sci Int.219:289.e1– 7. [15] Peterson J, Dechow PC. 2002. Material properties of the inner and outer cortical tables of the human parietal bone. Anat Rec. 268:7–15. [16] Hatipoglu H, Ozcan H, Hatipoglu U, Yuksel E. 2008. Age, sex and body mass index in relation to calvarial diploe thickness and craniometric data on MRI. Forensic Sci Int. 182:46–51. [17] Larsson E, Brun F, Tromba G, Cataldi P, Uvdal K, Accardo A. 2011. Quantification of structural differences in the human calvarium diploe by means of X-ray computed microtomography image analysis: a case study. IFMBE Proceedings. 2011;37: 599–602.
[18] Hildebrand T., Laib A., Müller R., Dequeker J., Rüegsegger J., 1999. Direct three-dimensional morphometric analysis of human cancellous bone: microstructural data from spine, femur, iliac crest , and calcaneus. Journal of Bone and Mineral Research. 14:7, 1167-1174. [19] Rawlinson S, McKay I, Ghuman M, Wellmann C, Ryan P, Prajaneh S, Zaman G, Hughes F, Kingsmill V. 2009. Adult rat bones maintain distinct regionalized expression of markers associated with their development. PLoS ONE. 21:e8358. [20] Torres-Lagares D, Tulasne J, Pouget C, Llorens A, Saffar J, Lesclous P. 2010. Structure and remodelling of the human parietal bone: an age and gender histomorphometric study. J Craniomaxillofac Surg. 38:325–30.
PII: DOI: Reference:
S1751-6161(18)30888-9 https://doi.org/10.1016/j.jmbbm.2018.12.013 JMBBM3098
To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 20 July 2018 Revised date: 11 December 2018 Accepted date: 13 December 2018 Cite this article as: Marianne Hollensteiner, Peter Augat, David Fürst, Falk Schrödl, Benjamin Esterer, Stefan Gabauer and Andreas Schrempf, Bone Surrogates provide authentic Onlay Graft Fixation Torques, Journal of the Mechanical Behavior of Biomedical Materials, https://doi.org/10.1016/j.jmbbm.2018.12.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Article type: Short Communication
Bone Surrogates provide authentic Onlay Graft Fixation Torques Marianne Hollensteinera,b,1*, Peter Augatb1, David Fürsta, Falk Schrödlc, Benjamin Esterera,b1, Stefan Gabauera, Andreas Schrempfa a
Research Group for Surgical Simulation Linz, Upper Austria University of Applied Sciences, School of
Applied Health and Social Sciences, Garnisonstr.21, 4020 Linz, Austria. b
Institute of Biomechanics, BG Unfallklinik Murnau, Germany and Paracelsus Medical University
Salzburg, Austria. c
Institute of Anatomy, Paracelsus Medical University, Struberg. 21, 5020 Salzburg, Austria
*
Corresponding author info: Marianne Hollensteiner, Research Group for Surgical Simulation Linz, Upper
Austria University of Applied Sciences, School of Applied Health and Social Sciences, Garnisonstr.21, 4020 Linz, Austria. Email: [email protected], phone: +43/(0)50804/55062, fax: +43/(0)50804/955062
Abstract: Onlay graft bone augmentation is a standard practice to restore the loss of height of the alveolar ridge following loss of a tooth. Cranial grafts, lifted from the parietal bone, are sandwiched and used to bridge the bony defect in the jaw by means of small screws. During the elevation of the covering gum and subsequent screw placement, care has to be taken in order to preserve underlying nerves. Therefore, to avoid harm to the patient, a solid education of surgeons is essential, which requires training and experience. A simulator for cranial graft-lift training was already developed and shall be expanded to train the augmentation of the lifted implants. Therefore, in this study, synthetic bones for onlay block graft screw placement with realistic haptics for the screw application training were evaluated and compared with human specimens. Six different polyurethane based bone surrogate composites, enriched with varying amounts of calciumbased mineral fillers and blowing agents, were developed. The haptical properties of these synthetic bones were validated for screw placement and compared with human parietal bone specimens. For that, bones were pre-drilled, screws were automatically inserted using a customized testbench and the slope of the screw-insertion torques were analyzed. The slope of the screw insertion torques of the human reference bones was 56.5±14.0 *10-3 Nm/deg, Surrogates with lower amounts of mineral fillers and blowing agents showed lower torques than the human 1
Prof. Küntscher Straße 8, 82418 Murnau, Germany
bone. Synthetic bones, validated for drilling, milling and sawing in an earlier study, also achieved significantly lower torques, which were only the half of the human parietal bones. Two intermediate stages of the aforementioned material compositions, consisting of 75% mineral filler with 0.75% blowing agent and 100% mineral filler with 1.00% blowing agent revealed results comparable with human bone (57.4±10.2 *10-3 Nm/deg, p=0.893 and 54.9±11.1 *10-3 Nm/deg, p=0.795, respectively). In conclusion, our findings suggest that, two newly developed polyurethane-based materials mimicking the haptical properties of an onlay bone graft screw fixation, have been identified. Thus, these surrogates are capable of mimicking real bone tissue in our simulator for the education of novice surgeons.
Keywords: Synthetic Bones; Biomechanics; Screw Fixation; Torque; Simulator
1.
Introduction :
After the loss of a tooth, the alveolar process degenerates due to absent stress [1], leading to pain and difficulty in speech or ingestion [2]. In order to reconstruct the resulting loss of alveolar height, cranial grafts are favored due to better healing [3]. The autogenous onlay bone block method is a standard practice for ridge augmentation: split thickness bone grafts from the parietal skull are sandwiched and used to bridge the bony defect by means of small screws (see Fig 1, [4]). To access the underlying atrophic bone, the mucosa needs to be elevated and care has to be taken that the underlying nerves are preserved. After a healing period of about four to six months, the screws are removed and an endosseous dental implant can be inserted [3]. To prevent harm to the patient, the surgeon needs to apply appropriate forces during screw placement to avoid fracture of the graft, underlying bone or screw. The acceptance of simulators as an educational opportunity is growing [6]. Their validated haptic feedback improves learning [7] and consequently minimizes the risk for the patient [8]. A simulator to train the lift of parietal split-thickness grafts has been developed [9, 10], to train the augmentation of the lifted implants. The aim of this study was to examine and validate varying synthetic parietal bones for onlay block graft placement to provide validated haptic feedback during the screw application training.
Fig 1 Augmentation of split thickness onlay block graft in the alveolar ridge of the maxilla (A) to bridge the bony defect of the left incisor (a.. right incisor, b.. left lateral incisor, c.. right canine, d.. aspiration tool, e.. sore hook). The fixation of the three sandwiched grafts from the cranium was performed with three screws The middle screw is used to fix the two lower grafts whereas the two lateral screws secure the graft-sandwich on the jawbone. Two grafts (contoured by dotted and dashed lines) are superimposed with their cancellous sides. A third graft (full line) is placed over it right-angled with the cortex pointing upwards as shown in the graphic (B).
2.
Material and Methods
2.1.
Human Specimens
Fresh frozen human parietal bone samples from two female donors (67 and 83 years) were used as references. The human tissue was obtained from the anatomical gift program of the Medical University of Vienna, Austria, in accordance with the Declaration of Helsinki and by approval of the ethics committee of the state Bavaria, Germany. The parietal bones were dissected and divided from other cranial bone proportions (frontal, temporal or occipital bone) along their associated sutures (sutura coronalis, sutura squamosa and sutura lambdoidea, respectively). Further, the parietal bone specimens were sterilized by autoclaving and were cut into four rectangular samples of approximately 2x8cm. Resulting grafts were stored at -37 degree Celsius until further processing and were thawed and soaked in saline solution (twelve hours) prior to measurements.
2.2.
Synthetic Bone Surrogates
Polyurethane- based parietal bone surrogate composites were manufactured (see Fig 2) as described in earlier studies [9, 11]): as a basic material a polyurethane resin (65D, KauPo Plankenhorn e.K., Spaichingen, Germany) was enriched with calcium-based mineral fillers (calcium-carbonate powder, Lehrer GmbH, Linz, Austria) in varying amounts of 0, 50, 75 and 100%wt of the whole polyurethane resin mass. Additionally, tap water, in amounts of 0, 0.50, 0.75 and 1.00%wt, was used as blowing agent. The blowing agent is used to froth up the polyurethane- calcium-carbonate powder-mixture to create an open celled cancellous-bone-like structure. Materials were weighed in and mixed manually before molding a skull cap with validated thickness as described in an earlier study [12]. Realistic skull caps made of six material compositions (SC1: 50%wt mineral filler and 0.5%wt blowing agent, SC2: 100%wt mineral filler and 0.5%wt blowing agent, SC3: 75%wt mineral filler and 75%wt blowing agent, SC4: 100%wt mineral filler and 1.0%wt blowing agent, SC5: 25%wt mineral filler and 0.25%wt blowing agent, SC6: pure resin, no mineral filler and no blowing agent) were molded. Both parietal bones were dissected from the anatomically realistic skull caps and were cut into five rectangular samples each.
Fig 2 Artificial skull cap cut in transverse plane revealing realistic cortical layers sandwiching an open-celled cancellous bone structure;
2.3.
Screw Torque Measurement setup
In accordance with the surgical procedure for the augmentation of an onlay graft into the alveolar ridge [4], the screw insertion torque was examined. A material testbench was designed to record the screw insertion torque and screw entry depth (see Fig 3). For that, an electric drive was equipped with a drill chuck. Further, drills or turnscrews with 2x20 mm screws (anodized titan cross pin SS, 30 threads per inch, pitch 0.847mm, outer diameter 2mm, core diameter 1.3mm, Stryker, Kalamazoo, USA) were fixed within the drill chuck and were driven with a speed of 1 RPM. The specimens were directly clamped on a 6- degree of freedom load cell (ATI nano 25, ATI Industrial Automation, Apex, USA) which was attached to a pulley block ensuring a constant feeding force of 40 N during the screw insertion [13]. A continuous screwspecific (diameter of 1.2mm) pilot-hole was drilled through the specimens, whereas the drill chuck ensured the same entry trajectory of the drill-bit and the screw. No tapping of the screws was performed. The screws were not fully inserted into the specimens until the screw heads touched the specimens. The insertion of the screw and thus, the torque measurement was stopped, when the screw tip appeared on the bottom of the specimen. The number of measured revolutions thereof depended on the realistically varying thickness of the samples. Three screw insertion measurements were performed at each specimen resulting in 24 measurements of the human and 30 measurements of each synthetic parietal bone. The elastic slope (k) of the screw insertion torques was identified as a suitable parameter to compare screw insertion haptics since the torque of the cylindrical screws increased linearly with the insertion depth (see Fig 5). For comparison reasons, since the measurement data are of different lengths due to varying specimen thicknesses, measurement data was edited and cut at a uniform insertion depth of 4 full screw revolutions and the linear slope of the measurement curves was calculated within this range (polyfit, Matlab2016a, The Mathworks Inc., Natick, USA). Four full revolutions correspond to a screw insertion depth of approximately 4.6mm, which is sufficient to fully penetrate a parietal split thickness graft.
Fig 3 A) Measurement setup for screw torque measurement; B) Close up of the screw penetrating the specimen (a.. rack with integrated pulley rack, b.. incremental encoder , c.. servodrive, d.. drill chuck, e.. turnscrew, f.. screw penetrating the specimen, g.. adapter holding the specimen with lateral screws, h.. specimen, i.. basepad enabling the screw to fully penetrate the specimen without getting in contact with adapter, j..torque sensor)
Fig 4 Typical measurement curves of the human reference and the artificial specimens (full lines) and the calculated slope lines (Human: full thin line, SC1: dashed line, SC2: dotted line, SC3: dashed-dotted line, SC4: plus-sign line, SC5: stared line, SC6: crossed line)
2.4.
Statistical analysis
Statistical analysis was performed with SPSS (SPSS Statistics 22, IBM, Armonk, USA). Shapiro-Wilk test identified non-normally distributed data within sample groups. Thus, Mann-Whitney-U test was used to detect differences between the human reference and the synthetic sample groups. For all statistical tests, a p-value of 0.05 was considered significant.
3.
Results
The slope of the screw insertion torques of the human reference bones was 56.5±14.0 *10-3Nm/deg (see Fig Fig 5). Synthetic parietal bones, which were validated for drilling, milling (SC1) and sawing (SC2) earlier [9, 11] achieved significantly lower torques. The slopes of the synthetic parietal bone favored for drilling and milling (SC1, 50% mineral filler, 0.5% blowing agent) were one third lower than the human reference (37.7±3.8 *10-3 Nm/deg, p=0.001). The synthetic sample favored for sawing (SC2, 100% mineral filler, 0.5% blowing agent) were only about half of the human parietal bones (28.0±5.1 *10-3 Nm/deg, p=0.001). Novel surrogates with lower amounts of additives (SC5, 25% mineral filler with 0.25% blowing agent and a specimen without any additional ingredients-SC6- 0% filler with 0% blowing agent) achieved lower slopes compared to the human reference (37.6±2.8 *10-3 Nm/deg, p=0.003 and 32.2±4.9 *10-3 Nm/deg, p=0.005, respectively). Two intermediate stages of the two aforementioned material compositions with higher amounts of mineral fillers and blowing agents (SC3, consisting of 75% mineral filler with 0.75% blowing agent and SC4, made
of 100% mineral filler with 1.00% blowing agent) achieved appropriate results (57.4±10.2 *10-3 Nm/deg, p=0.893 and 54.9±11.1 *10-3 Nm/deg, p=0.795, respectively), comparable to human parietal bone.
Fig 5 Results of screw insertion torque slopes in parietal bone (Human) and four artificial skull caps made of varying material mixtures (SC1 to SC4)
3.1.
Feasibility of Synthetic Bones Surrogates for Surgical Training
To confirm the feasibility of the varying synthetic bone surrogates for screw placement training, the materials were independently examined by two cranio-maxillofacial surgeons. Both surgeons reported that the materials (SC1, SC2), which were already validated for machining procedures in earlier studies [11, 12], did not provide enough torque or resistance during the screw placement. Furthermore, materials with no or lacking cancellous bones structure (SC5, SC6), were found to provide a better haptic feedback but did not show a realistic diploic structure. The intermediate materials SC3 and SC4 were favoured, because of a realistic haptic feedback and a more realistic cancellous structure.
4.
Discussion
In this study, we presented a bone surrogate for surgical onlay graft screw placement training, validated against human parietal bones. Such a validated bone surrogate will provide a safe but realistic training setting useable for the training and education of surgeons and will potentially reduce surgical complications. A training platform for split thickness graft lifts and consequent graft augmentation with screws is currently not available and would be beneficial for surgical training. Based on earlier studies [912], six varying bone surrogate materials were developed and investigated. According to the surgical
procedure of a split thickness onlay graft placement, the slope of the screw insertion torque of craniomaxillo-facial screws into human and synthetic parietal bones were analyzed. Due to the manual weigh in and molding of the synthetic bones variances in compositions as well as cortical and diploic thicknesses arise. On the one hand, these fluctuations are desirable, leading to varying training scenarios for the surgeons. On the other hand, skull bone thickness, as well as the thickness of cortices and diploic layer vary from gender and ethnicity. Parietal full bone thickness was reported to range from 6.69±1.94mm [14], whereas the thickness of the cortices ranged from 1.8±0.3mm [15] and the thickness of the diploe was reported to be 3.38±1.00mm [16]. In a previous study, the similarity of the layer thicknesses of the synthetic parietal bones with human bone has already been demonstrated [12]. The measured slope of the screw insertion torques of the materials, validated for drilling, milling and sawing in an earlier study [11, 12] were one third (SC1) and the half (SC2) smaller than the human reference bones. Novel surrogates with lower or no amounts of mineral filler materials or blowing agents (SC5, SC6) also provided statistically significant lower slopes than the human reference. New intermediate stage-materials (SC3, SC4) provided comparable results to the human reference bones. Attention should be drawn to the fact, that in our measurements, the insertion torque appeared to rise linearly, independently of cortical thickness. This may be due to the fact, that, the spongy bone of the skull vault has a „denser“ structure, compared to other locations in the human body. The morphological properties of human parietal bone reveal thicker trabeculae (0.73±0.12mm) in parietal bone [17] compared to human lumbar vertebra (0.12±0.02mm [18]) and the trabecular separation is quite low [17, 18]. Thus, we believe that this very dense cancellous structure, connected with a high clamping force provided by the cortices, leads to a steady linear rise of the torque, although fluctuations in the torque measurements, triggered by cancellous bone separations, appeared. A few limitations of the study have to be mentioned. The human sample size (n=2) was rather small and the samples came from rather old donors over the age of 65. But, since neither parietal bone thickness nor bone morphometrics are affected by age or osteoporosis [19, 20], our reference sample was considered as reliable and meaningful. Furthermore, only one screw type was chosen to validate our novel materials. This screw type was chosen, because it is used by default in the university hospital where our simulator shall be used for training purposes. An advantage of model-based simulators with validated bone models compared to virtual simulators is that real tools or screws can be used that also produce a realistic haptic feedback within the validated bone surrogates. The screw insertion measurements were aborted, when the screw tip broke through the inner cortex of the specimens. When the screw is fully inserted, the screw head touches the bone or surrounding soft tissues, and thus, the torque inevitably increases rapidly. The augmented and sandwiched grafts are not loaded in the maxilla and are covered by the gum as they are allowed to become part of the host. Thus, in the training session with the simulator, the screws shall be placeable, but shall not be fully tightened. But, since both,
the screw head design and the bone properties are assumed to play a substantial role in the formation of the haptical feedback during screw tightening, this shall be part of ongoing research. Finally, the feasibility for screw placement of the novel bone surrogates was confirmed by two experienced surgeons. According to the statements of the surgeons, the two new material-compositions, which also showed good results during the mechanical testing (SC3 and SC4), provided a more realistic resistance to screw insertion than the other provided samples. The surgeons could not agree on one material, since one surgeon preferred the 0.75% surrogate material (SC3) because of its more realistic cancellous bone structure, whereas the other surgeon preferred the 1.0% composition (SC4). The synthetic bones with lesser amount of blowing agents (0% and 0.25%) were ruled out by the surgeons because of their unrealistic or absent structure of the spongy bone as well as inappropriate haptical feedback.
5.
Conclusion
In conclusion, our findings suggest that two newly developed polyurethane materials mimic the haptical properties of an onlay bone graft fixation with screws. Although synthetic surrogates, which were already validated for the graft lift procedure in a former study, were not suitable to deliver a realistic haptic for screw placement, variations of the aforementioned surrogates could authentically mimic screwing properties of human parietal bone and are thus capable for the education of novice surgeons.
6.
Acknowledgements
The authors thank the company Stryker for their generous provision of screws required for this study. Further, the authors would also like to thank the team of the Institute of Biomechanics in Murnau for their support during human specimen preparation.
7.
Funding
The Research Group for Surgical Simulators Linz (ReSSL) acknowledges the financial support by the Austrian Research Promotion Agency (FFG) within the program line Cooperation & Innovation (COIN) and project number 845436.
8.
Conflict of Interest
The authors declare that they have no conflict of interest.
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