that would provide data on stiffness of the experimental mandibular DO wound without destroying its gross ... force appl
Int. J. Oral Maxillofac. Surg. 2003; 32: 523–527 doi:10.1054/ijom.2003.0424, available online at http://www.sciencedirect.com
Research Paper Distraction Osteogenesis
Development of a mechanical testing system for a mandibular distraction wound
1
D. H. Perrott, 1B. Rahn, 1D. Wahl, B. Linke, 2P. Thuru¨ller, 2M. Troulis, 2 J. Glowacki, 2L. B. Kaban 1
1
AO/ASIF Institute, Davos, Switzerland; Department of Oral & Maxillofacial Surgery, Massachusetts General Hospital, Harvard School of Dental Medicine, Boston, MA, USA
2
D. H. Perrott, B. Rahn, D. Wahl, B. Linke, P. Thu¨rmuller, M. Troulis, J. Glowacki, L. B. Kaban: Development of a mechanical testing system for a mandibular distraction wound. Int. J. Oral Maxillofac. Surg. 2003; 32: 523–527. 2003 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Abstract. The purpose of this study was to develop a mechanical testing system to estimate stiffness of an experimental porcine mandibular distraction osteogenesis (DO) wound. The system was designed to function without changing the morphology of the healing mandible. A customized jig was designed to allow cantilever-bending tests of the Yucatan mini-pig hemi-mandible. Experimental and control hemi-mandibles were placed in the jig and the proximal segment was secured. A material testing unit applied progressively increasing downward force on the pre-molar occlusal surface. The maximum force applied was 0.030 kN. The stiffness value for each hemi-mandible was represented by the slope of the plot of force (kN) vs displacement (mm). Radiographs were taken before and after mechanical testing to demonstrate any gross morphologic changes or identifiable fractures across the distraction wound. A total of 24 mini-pigs underwent DO of the right mandible with 0-day latency and distraction rates of 1, 2, and 4 mm per day resulting in a 12 mm gap. At the completion of 0, 8, 16, and 24 days of neutral fixation, two animals for each of three different distraction rates were sacrificed for mechanical testing. Stiffness of control hemi-mandibles ranged between 0.018 and 0.317 kN/mm (median 0.063; mean 0.0990.080). Stiffness of experimental hemi-mandibles ranged between 0 and 0.025 kN/mm (median 0.004; mean 0.005). The subset that was tested at the end of neutral fixation had stiffness between 0.005 and 0.025 (median 0.011; mean 0.0120.011). No morphologic changes were evident on the X-rays after testing. The results indicate that the cantilever-bending model is useful for testing stiffness of an experimental mandibular DO wound without destroying its morphology.
Introduction Distraction osteogenesis (DO) has become an increasingly popular technique for the correction of congenital and acquired craniomaxillofacial (CMF) deformities requiring skeletal expansion3,10,21. Clinical examination and plain radiographs are the standard methods for assessing the healing DO 0901-5027/03/000523+05 $30.00/0
wound. However, bone formation and bone density visualized on plain radiographs are poorly correlated with bone strength2,13,22. Objective methods such as quantitative ultrasound and bone densitometry have been proposed to assess healing2,8,18 and quantitative CT has been used to estimate bone stiffness9. There are little data to correlate the
Key words: mandibular distraction; distraction wound stiffness; porcine distraction; distraction mechanical testing Accepted for publication 20 March 2003
findings of the above radiologic techniques with direct measurements of bone strength. Experimental DO reports have focused on the radiographic, histologic, and molecular aspects of the healing bone wound. Data correlating these modalities with the stiffness (biomechanical strength) across the gap
2003 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved
524
Perrott et al.
would be helpful in deciding when to end the neutral fixation period, remove distraction devices, and allow patients to resume normal activity. Biomechanical principles are currently used to guide clinical decisions in orthopaedic surgery. For example, a direct measurement of stiffness is obtained using a flexible goniometer across tibial DO wound to document healing7. Based on findings in experimental models, it has been suggested that in humans an external fixator can be removed when stiffness of 15 Nm/degree is achieved7,17. The purpose of this study was to develop a mechanical testing method that would provide data on stiffness of the experimental mandibular DO wound without destroying its gross morphology. This would allow histologic, quantitative CT and molecular studies to be performed on the same specimens.
Materials and methods
subset: Group A (1 mm/day rate) with 0, 8, 16, 24 days of fixation; Group B (2 mm/day) with 0, 8, 16, 24 days of fixation; Group C (4 mm/day) with 0, 8, 16, 24 days of fixation. Standard lateral mandibular radiographs were obtained at days 0, 8, 16, and 24 days of neutral fixation. Animals of each subset (n=2) were sacrificed at the completion of neutral fixation. The mandibles were harvested and the overlying muscle trimmed, while maintaining the integrity of the distraction site. The distraction devices were not removed during the preparation of specimens for testing. The mandibles were sectioned at the symphysis thus yielding a right (experimental) and left (control, contralateral) hemi-mandible. A horizontal osteotomy was made just below the sigmoid notch and the condyle and coronoids were removed. The hemi-mandibles were frozen at 20C and shipped to Davos, Switzerland.
Fig. 1. Frontal view of hemi-mandible mounted in the jig with force being applied to the occlusal surface and the proximal bone engaging the mandibular angle guide plate.
Experimental model
Use and care of animals in this study was approved by the Massachusetts General Hospital Subcommittee on Research Animal Care and conformed to AAALAC standards. Female Yucatan minipigs were housed for 1 week to become acclimated to diet, water, and housing. The animals were monitored for general appearance and demeanor, activity level, excreta level, weight, temperature, heart rate, and respiratory rate. Twenty-four female Yucatan minipigs (Charles River Laboratory, MA, USA), in the mixed dentition stage (age 6 months) weighing between 25–30 kg, underwent placement of a single vector semi-buried distraction device (Synthes Maxillofacial, Paoli, PA, USA) at the right mandibular angle. Near the inferior border, marker screws were placed proximal and distal to the distraction wound. Details of this model have been described previously23. Two unoperated animals served as intact control animals. All animals had a zero-day latency. Distraction was carried out at rates of 1 mm (Group A, n=8), 2 mm (Group B, n=8), and 4 mm (Group C, n=8) per day and was performed twice daily until 12 mm of mandibular lengthening was achieved. Following completion of distraction, Groups A, B, and C were subdivided into four subsets based on a protocol of 0, 8, 16, or 24 days of neutral fixation. There were two animals in each
Sample preparation
The hemi-mandibles were thawed to room temperature and kept moist to avoid drying that could influence the outcome of mechanical testing. Photographs and lateral view radiographs (Faxitron Model 43855A; Faxitron X-Ray Corp, Wheeling, Illinois) were obtained of the right and left hemi-mandibles (Kodak D4 24/30 film; 1.0 filter; 99 keV; 70 kVp; 2.5 min). Ex-vivo measurements between the mandibular markers were completed prior to beginning the mechanical testing. Mechanical testing
Mechanical testing was conducted using the cantilever bending method. This technique was previously reported to assess stiffness of a sheep mandibular fracture wound16. The custom jig to hold sheep hemi-mandible was modified to fit the porcine hemi-mandible by changing the method of securing the hemi-mandible in the jib and by the use of a mandibular angle guide plate. The basic cantilever-bending principles of a force applied to the teeth while maintaining the proximal bone secure was not altered. The superior portion of the hemimandible ramus was embedded in polymethylmethacrylate and secured to the jig by passing a 6 mm pin through a ‘U’ shaped piece of metal. The custom jig was secured in a servohydraulic
Fig. 2. Lateral view of hemi-mandible mounted in the jig. Note the occlusal plane is parallel to the floor.
material testing unit (Model 4302, Instron Corp, Canton, MA, USA). The adjustable mandibular guide plate was positioned to engage bone at the mandibular angle and to orient the hemimandibular occlusal plane parallel to the floor (Fig. 1). For the cantilever test, force was applied to the occlusal plane at the first and second pre-molar teeth via a piston shaped metal rod. To ensure a close fit with the tooth surfaces, dental
Development of a mechanical testing system for a mandibular distraction wound
525
Fig. 3. Force/displacement analysis of control and experimental hemi-mandibles. Slope (LLS) for each hemi-mandible was calculated from the linear area between 0.015–0.025 kN of load as indicated between the dotted lines. Upper graph shows 24 control hemi-mandibles. Two control hemi-mandibles were tested to fracture and therefore exceeded upper limits of 0.030 kN of force. Lower graph shows 14 experimental hemi-mandibles subset for which force/displacement was tested.
impression material was heated, applied to the metal rod, placed in contact with the teeth, and allowed to cool. When the specimen was properly positioned and mounted, the distraction device was removed. The mandible was thus suspended and stable without the possibility of torsional rotation during mechanical testing (Fig. 2). All specimens were loaded and unloaded at the linear displacement rate of 2 mm/min and repeated for a total of three cycles. The data obtained from the second and third cycles were identical. A 1 kN load cell was used in measuring the resultant force. Data were acquired at a rate of 10 000 points per second and the third cycle data were stored through the Instron software. Data acquired was for yield load (kN force), yield displacement (mm), and stiffness (kN/mm). For determination of the maximum force that could be applied without producing permanent deformation of the specimens, two intact control (left) hemimandibles were placed in the jig and tested for failure-load stiffness (FLS).
FLS was defined as the point when sufficient force was applied to the specimens to produce signs of fracture. The FLS for the two intact control specimens were 0.9 kN with a displacement of 19.3 mm and 0.82 kN with a displacement of 13.4 mm. Therefore, fracture of a control intact hemi-mandible would be expected to occur with a force greater than 0.8 kN. Based on this information, we estimated that a low force with a maximum of 0.030 kN of force could be applied to the experimental (right) hemimandibles without causing destruction or permanent gross macroscopic deformation. The stiffness values obtained from this low-loading force would be identified as low-load stiffness (LLS). Clinical and radiographic evaluation after mechanical testing
Following completion of mechanical testing, distances between the proximal and distal marker screws were recorded. Repeat Faxitron radiographs were obtained using the protocol described
above. The radiographs (pre- and postmechanical testing) were analysed to determine if there were: (1) fracture(s) within the distraction wound, (2) changes in the shape of the distraction wound, and (3) displacement of the proximal and distal fragments. To determine changes in shape and displacement of the fragments, lines were drawn tangent to the proximal and distal edges of the mandibular wound. They were then connected at the superior and inferior borders of the mandible. These lines produced a parallelogram shape. Changes in this shape would reflect changes in the morphology of the DO wound and displacement of the segments.
Results Biomechanical testing
Twenty-two of 24 control hemimandibles underwent low loading and 2/24 underwent high-loading testing.
526
Perrott et al.
Fourteen of 24 experimental hemimandibles underwent low-loading testing. The remaining 10 hemi-mandibles had insufficient stability for testing and consisted of animals from the following subsets: (1) 1 mm/day distraction–0 days fixation (n=1); (2) 1 mm/day distraction–8 days fixation (n=2); (3) 2 mm/day distraction–0 days fixation (n=2); (4) 2 mm/day distraction–8 days fixation (n=1); (5) 2 mm/day distraction–16 days fixation (n=1); (5) 4 mm/day distraction–0 days fixation (n=2) and (6) 4 mm/day distraction–16 day fixation (n=1). Force (kN) and displacement (mm) values were used to assess the mechanical stiffness of the control and experimental hemi-mandibles (Fig. 3). Repeated measures from three test cycles of a representative sample resulted in a mean square deviation of 0.0001. The slope of the resultant line (load/ yield) was calculated to obtain a stiffness value for each specimen. During the initial application of force (0.01– 0.015 kN), there was distortion and therefore the LLS (force/displacement) values were calculated from the resultant line in a force range of 0.015–0.025 kN. This was beyond the initial distortion and below the previously determined allowable force (0.030 kN). The average LLS value of the control hemi-mandibles was 0.0990.080 (range 0.018–0.317; median 0.063). A LLS value 0.0 kN/mm was given to the ten experimental hemi-mandibles with insufficient strength for testing. The average LLS value of the twentyfour experimental hemi-mandibles was 0.005 kN/mm (range 0.0–0.0025; median 0.004). The subset of fourteen experimental hemi-mandibles with sufficient strength for testing had LLS values between 0.005 and 0.025 (median 0.011; mean 0.0120.011) (Fig. 3). Ex-vivo and radiographic evaluation
The mean distance between bone markers in the 14 mandibular specimens prior to mechanical testing was 24.364.22 mm (range 15–31.1). The post mechanical testing distance was 23.384.03 mm (range 16.3–31.5). Analysis of the radiographic images revealed no visible fractures, no major changes in shape and no displacement of the proximal and distal segments. Discussion Stiffness of a structure has been defined as load/deformation1. It provides an
objective measure of healing in the skeleton because it directly correlates with increases in the quantity and quality of new bone17. Stiffness has been used to define healing of tibial fractures and to assess healing during leg lengthening procedures7,17. Orthopaedic surgeons have predicted healing of long bone fractures from the relationship between stiffness and the logarithm of time4. Wide variance in stiffness based on the size, location and shape of various bones has been observed15,20. Therefore, stiffness data specific to a particular bone, in this case the mandible, would be useful as a monitor of healing. Although data on the biomechanical strength of healing mandibular fractures have been reported10, there have been no studies evaluating the stiffness of the healing mandibular DO wound. Compression, tension, bending and torsion tests are often utilized to evaluate the mechanical properties of bone6,11,15,24. Those data are obtained using cantilever bending (fixed proximally), three-point (point load) or constant moment bending (four-point bending) methodology18. In this study, we elected to develop and test a cantilever-bending model for the distracted mandible. This approach would allow mechanical testing without causing injury (e.g., fracture) or permanent deformations to the DO wound. In addition, forces applied directly to the teeth would mimic occlusal forces applied during chewing. In the testing model developed in this set of experiments, we found that stiffness values for the control mandibles were variable. There are several potential explanations for this finding. First, although the guide plate was securely positioned against the mandibular angle, there was error in the system. This error occurred as this initial loading force was applied and the guide plate engaged the mandibular angle bone. This bone was noted to have underwent deformation. After the initial 0.015 kN of force had been applied, the deformation stopped. Second, although it was important to apply force to the occlusal surface, this resulted in variations because of tooth movement permitted by the periodontal ligament. We expect these two issues contributed to the distortion that occurred in the force range from 0.01– 0.015 kN. We therefore calculated the slope from 0.015–0.025 kN of force, a range which more accurately reflected the stiffness of the DO wound investigators studying mechanical strength of
bone have found wide variations in stiffness of control specimens. This has been attributed to differences in bone density and size4,19. For example, in evaluating the strength of bone in different rat models, P et al.13 found the stiffness of 30 rat tibias to be 25127.3 N/mm for the right 25528.7 N/mm for the left tibias. In this study, while the animal’s ages were similar, there was variation in the size of the mandible and extent of tooth development. This may have contributed to the wide variations noted in the stiffness values. Finally, we documented that the cantilever-bending model did not cause damage to the DO wound. This is important as we attempt to correlate stiffness with the histologic and quantitative CT findings and conduct further studies with larger number of animals. In conclusion, it appears that the cantilever-bending model described in this paper does provide a method to measure stiffness of the DO wound. Acknowledgments. This project was funded by AO/ASIF Foundation Grant #120072264368 (LBK, PI), Synthes Maxillofacial Surgery and the Department of OMS Research Fund. This paper was accepted for presentation at the Annual Meeting of the American Association of Oral & Maxillofacial Surgeons, Orlando, Florida, September 2001.
References 1. B E. Stiffness-an unknown world of mechanical science? Injury 2000: 31: S-B14–SB-23. 2. B TJ, B FC, B JAM, L A, R JC, B FC, P P, M HJTM. Evaluation of strength of healing fractures with dual energy x-ray absorptiometry. Clin Orthopaed Related Research 2000: 380: 260–268. 3. C M, T BA. Distraction osteogenesis in maxillofacial surgery using internal devices: a review of five cases. J Oral Maxillofac Surg 1996: 54: 45–53. 4. C MJ, P AP, P MJ, N N. Clinical implications of stiffness and strength changes in fracture healing. J Bone Joint Surg (Br) 1997: 79B: 912. 5. C JB, S ML. Mineralization dynamics of regenerate bone during mandibular osteodistraction. Int J Oral Maxillofac Surg 2001: 30: 234–242. 6. D DT. The biomechanical behavior of healing canine radii and ribs. J Biomechanics 1982: 15: 235–247.
Development of a mechanical testing system for a mandibular distraction wound 7. D JS, O PJ, E GA, K JH, R JB. Stiffness measurements to assess healing during leg lengthening. J Bone Joint Surg (Br) 1996: 78B: 286–289. 8. E KS, B MJ, K JA. Methods of assessing new bone formation during limb lengthening: ultrasonography, dual energy X-ray absorptiometry and radiography compared. J Bone Joint Surg (Br) 1993b: 75B: 358–364. 9. H JH, A J, H M. Noninvasive determination of bone stiffness for distraction osteogenesis by quantitative computed tomography scans. Clin Orthop 1994: 301: 42–48. 10. H RH, P GP, G M. A biomechanical evaluation of mandibular condyle fracture plating techniques. J Oral Maxillofac Surg 2002: 60: 73–79. 11. J¨ ¨ T, J P, P Z, V¨ ¨ ¨ HK, T J. Comparison of three-point bending test and peripheral quantitative computed tomography analysis in the evaluation of the strength of mouse femur and tibia. Bone 1998: 23: 155–161. 12. M U, M T, W HP, K-L B, V D, S U, J U. Mechanical tension in distraction osteogenesis regulates chondrocytic differentiation. Int J Oral Maxillofac Surg 2001: 30: 522–530. 13. P BL, K GJ, T R, T M, M JB, K LB. Simultaneous maxillary and mandibular distraction osteogenesis with a semi-
14.
15.
16.
17.
18.
19.
20.
buried device. Int J Oral Maxillofac Surg 1999: 28: 475–479. P MM, L RW, W SD, W AA. The clinician’s ability to evaluate the strength of healing fractures from plain radiographs. J Orthop Trauma 1989: 3: 29–32. P Z, T J, Z H, J¨ ¨ T, V¨ ¨ ¨ . The mechanical strength of bone in different rat models of experimental osteoporosis. Bone 1994: 15: 523– 532. R B, C J, P J, R M. Zur Biomechanik der Osteosynthese an der Mandibula. In: Schuchardt K, ed.: Fortshcritte der Kiefer- und Gesichts-Chirurgie. Stuttgart: Georg Theime Verlag 1975: 37–42. R JB, C JL, G AE, O’C BT, K J. Measuring stiffness can define healing of tibial fractures. J Bone Joint Surg (Br) 1994: 76B: 389– 394. R JB, O’C BT. Direct fracture stiffness measurement in tibial fractures. Int J Orthop Trauma 1993: 3: 72–74. R SG, S CR. Efficacy of monitoring long-bone fracture healing by measurement of either bone-stiffness or resonant frequency: numerical simulation. J Orthopaed Research 2000: 18: 691–697. S AHRW, G TN, E M, K J. Stiffness, strength and
21.
22.
23.
24.
527
healing assessment in different bone fractures—a simple mathematical model. Injury 2000: 31: 777–781. S G, S H, D R, S H, M C. Cranio-facial distraction osteogenesis: a review of the literature: Part 1: clinical studies. Int J Oral Maxillofac Surg 2001: 30: 89–103. S G, D R, S H. Cranio-facial distraction osteogenesis: a review of the literature. Part II: experimental studies. Int J Oral Maxillofac Surg 2002: 30: 123–135. T MJ, G J, P DH, K LB. Effects of latency and rate on bone formation in a porcine mandibular distraction model. J Oral Maxillofac Surg 2000: 58: 507–514. U H, R J, A M, J¨ ¨ T, T J, K¨ ¨ , V E, A HT. A metaphyseal defect model of the femur for studies for murine bone healing. Bone 2001: 28: 423– 429.
Address: Dr Leonard B. Kaban Chief, Department of Oral Maxillofacial Surgery Massachusetts General Hospital Fruit Street Boston MA 02114 USA E-mail:
[email protected] 
