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Int. J. Oral Maxillofac. Surg. 2015; xxx: xxx–xxx http://dx.doi.org/10.1016/j.ijom.2015.06.010, available online at http://www.sciencedirect.com
Clinical Paper Orthognathic surgery
Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy
K. Ueki, A. Moroi, R. Iguchi, A. Kosaka, H. Ikawa, K. Yoshizawa Department of Oral and Maxillofacial Surgery, Division of Medicine, Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 1110 Shimokato, Chuo-shi, Yamanashi, Japan
K. Ueki, A. Moroi, R. Iguchi, A. Kosaka, H. Ikawa, K. Yoshizawa: Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy. Int. J. Oral Maxillofac. Surg. 2015; xxx: xxx–xxx. # 2015 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Abstract. The purpose of this retrospective study was to evaluate the changes in computed tomography (CT) values of ramus bone and screws after sagittal split ramus osteotomy (SSRO) setback surgery. The subjects were 64 patients (128 sides) who underwent bilateral SSRO setback surgery. They were divided into six groups according to the fixation plate type used and the use or not of self-setting a-tricalcium phosphate (Biopex): group 1: titanium plate and screws; group 2: titanium plate and screws with Biopex; group 3: poly-L-lactic acid (PLLA) plate and screws; group 4: PLLA plate and screws with Biopex; group 5: uncalcined and unsintered hydroxyapatite particles and poly-L-lactic acid (uHA/PLLA) plate and screws; group 6: PLLA/uHA plate and screws with Biopex. CT values (pixel values) of the lateral cortex, medial cortex, osteotomy site, and screws were measured preoperatively, immediately after surgery, and 1 year postoperatively using horizontal CT images at the mandibular foramen taken parallel to the Frankfort horizontal plane. There were significant differences in the time-course change of pixel values for the lateral cortex (P < 0.0001) and the osteotomy site (P < 0.0001) among the six groups. This study suggests that the fixation plate type and use of bone alternative material may affect bone quality during the process of bone healing after SSRO.
0901-5027/000001+09
Keywords: Sagittal split ramus osteotomy; Computed tomography (CT) value; Absorbable plate and screws; Bone healing. Accepted for publication 12 June 2015
# 2015 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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One of the advantages of the sagittal split ramus osteotomy (SSRO) is that the large areas of bony contact that are made remain after either advancement or retrusion of the distal segment. However, making a wider bony contact without the step of the cortical bone surface between the proximal and distal segments can induce inward rotation of the condylar long axis in setback surgery.1,2 In previous studies by this group, bent plates were used to secure the fragments without a positioning device and it was found that the bent plate increased the incidence of postoperative temporomandibular joint dysfunction (TMD) and did not change skeletal or occlusal stability.1,2 With this method, the gap between the proximal and distal segments is created by a bent plate, preventing the formation of a large bony contact. In setback surgery, especially with asymmetry, fixation between the segments can be performed without bony contact to prevent large changes in condylar position and angle. More recently, absorbable plates and screws have been used with increasing frequency in our hospital.3,4 These absorbable plate and screw systems have been developed in Japan (poly-L-lactic acid (PLLA) and uncalcined and unsintered hydroxyapatite particles and poly-L-lactic acid (uHA/PLLA)).5–7 Although, it has been shown in animal studies that the strength of the plate and screws is reduced in a time-dependent manner, the plate and screw resorption process and the surrounding bone formation at the osteotomy site remain unclear. Furthermore, it has been considered that bone density or bone thickness in the ramus area could change because of the stress distribution changes resulting from SSRO (occlusion and skeletal change). The computed tomography (CT) value can be correlated to bone density, and bone strength can be expressed in terms of bone density and quality.8 CT value data can be useful for assessing bone formation. A previous study by this group showed that the gap between the proximal and distal segments could be filled with new bone after SSRO using either titanium or absorbable plates, even if there were few bony contacts between the segments.9 Moreover, another study suggested that inserting self-setting a-tricalcium phosphate (a-TCP) (Biopex; Pentax Co., Tokyo, Japan) in the gap between the proximal and distal segments was useful for new bone formation after SSRO with bent plate fixation.10 However, that study assessed only the linear length and ramus square using CT at the osteotomy site.
It is still unclear whether the fixation device or the use of a bone alternative material can affect the healing process after SSRO. Furthermore, the CT values of ramus bone, titanium plate and screws, absorbable plate and screws, and bone alternative material following orthognathic surgery have not been reported. The purpose of this retrospective study was to evaluate changes in quality (CT value) of ramus bone and screws after SSRO using absorbable plates and titanium plates with and without selfsetting a-TCP. Patients and methods Patients
This study included 64 Japanese adults (21 men and 43 women) who presented with jaw deformities diagnosed as mandibular prognathism with and without a maxillary deformity. At the time of orthognathic surgery, the patients ranged in age from 16 to 51 years, with a mean age of 29.2 years (standard deviation 10.9 years). For this retrospective study, informed consent was obtained from all patients in accordance with the Declaration of Helsinki; the study was approved by the ethics committee of Yamanashi University Hospital.
Surgery
Lateral, frontal, and submentovertex (SV) cephalograms were obtained before surgery, as described previously.1,2 All 64 patients underwent bilateral SSRO setback with the modified fixation. Of the 64 patients, 26 underwent SSRO with Le Fort I osteotomy to advance or impact the maxilla. At the time of fixation, the dental arch of the distal segment was secured to the maxillary arch with an interpositional splint and 0.4-mm wire. For seven patients (14 sides), a titanium miniplate was used for internal fixation of the mandible (long miniplate (four holes/burr 8 mm, thickness 1.0 mm) and four screws (2 14 mm and 2 5 mm); Universal Mandible fixation module, Stryker Leibinger Co., Freiburg, Germany). These patients comprised group 1 (titanium group). For three patients (six sides), a titanium miniplate (long miniplate (four holes/burr 8 mm, thickness 1.0 mm) and four screws (2 14 mm and 2 5 mm); Universal Mandible fixation module) was used for internal fixation of the mandible, and selfsetting a-TCP (Biopex) was inserted in the anterior part of the gap between the seg-
ments after plate fixation. These patients comprised group 2 (titanium with a-TCP group). For seven patients (14 sides), a PLLA miniplate (28 4.5 1.5 mm with four screws (2 8 mm); FIXSORB-MX, Takiron Co., Osaka, Japan) was used for internal fixation of the mandible. These patients comprised group 3 (PLLA group). For 11 patients (22 sides), a PLLA miniplate (28 4.5 1.5 mm with four screws (2 8 mm); FIXSORB-MX) was used for internal fixation of the mandible, and self-setting a-TCP (Biopex) was inserted in the anterior part of the gap between the segments after plate fixation. These patients comprised group 4 (PLLA with a-TCP group). For 19 patients (38 sides), a uHA/PLLA miniplate (28 4.5 1.5 mm with four screws (2 8 mm); SuperFIXSORBMX; Takiron Co.) was used for internal fixation of the mandible. These patients comprised group 5 (uHA/PLLA group). For 17 patients (34 sides), a uHA/PLLA miniplate (28 4.5 1.5 mm with four screws (2 8 mm); SuperFIXSORBMX) was used for internal fixation of the mandible, and self-setting a-TCP (Biopex) was inserted in the anterior part of the gap between the segments after plate fixation. These patients comprised group 6 (uHA/PLLA with a-TCP group). The groups were divided by time-frame, so the sample number was statistically variable. To prevent intraoperative inward rotation of the condylar long axis, model surgery was performed preoperatively with reference to the SV projection.1,2 Before surgery, an SV cephalogram was obtained for all patients, followed by simulation. A distal segment including the lower dental arch was first set back according to the position of the upper dental arch on the SV cephalometric trace. When the proximal and distal segments are fixed with straight plates after bilateral sagittal split osteotomy (BSSO), proximal segments containing the condylar head cause internal rotation (Fig. 1). To prevent internal rotation of the proximal segments, the overlapping cortical bone at the anterior site of the proximal segment was not removed to keep the contact area between the proximal and distal segments, and was fixed with a bent plate and four screws on each side of the mandible. At the posterior part, a 3–7-mm gap was maintained between the proximal and distal segments. After surgery, elastics were placed to maintain an ideal occlusion. All patients received orthodontic treatment before and after surgery. CT images were obtained
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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CT values of mandibular ramus bone after SSRO
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Fig. 1. Simulation of plate bending. (A) Straight plate: when the proximal and distal segments are fixed with straight plates after bilateral sagittal split osteotomy, the proximal segments containing the condylar head cause internal rotation, and the posterior aspect of the distal segment on the deviated side sometimes interferes with the proximal segment. (B) Bent plate: the plates were bent to prevent the proximal segments from rotating internally; note the gap between the osteotomy surfaces.
for all patients preoperatively, immediately after surgery, and 1 year after surgery. CT data acquisition
For CT scanning, the patients were placed in the gantry with the tragal–canthal line perpendicular to the floor. They were instructed to breathe normally and to avoid swallowing during the scanning process. CT scans were obtained in the radiology department by skilled radiology technicians using a high-speed, advantagetype CT generator (Light Speed Plus; GE Healthcare, Milwaukee, WI, USA) with each sequence taken 1.25 mm apart for three-dimensional (3D) reconstruction (120 kV, average 150 mA, 0.7 s/rotation, helical pitch 0.75). The resulting images were stored in the attached workstation computer (Advantage Workstation version 4.2; GE Healthcare, Milwaukee, WI, USA), and 3D reconstruction was performed using the volume-rendering method. ZedView version 7.0 (LEXI Co., Tokyo, Japan) medical imaging software was used for 3D morphological measurements. CT image analysis software (DIANA DICOM free version 1.8.1.6; Luke-System, Chiba, Japan) was used to measure the CT value (pixel value). CT values of the lateral cortex, medial cortex, and osteotomy site were measured preoperatively, immediately after surgery, and at 1 year postoperative using horizontal CT images with the mandibular foramen parallel to the Frankfort horizontal (FH) plane. For each CT image, three randomly selected points at the anterolateral site of the lateral cortex, three at the anteromedial
site of the medial cortex, and three at the anterolateral site of the mandibular canal were assessed. The mean pixel value in the region of interest (ROI; approximately 1.0 mm2 circle) was measured automatically. The mean value for the three points was then calculated and assigned as the pixel value for each part. Measurement of the screws was performed for just one screw on each side for all patients. The measurement area for the screw was very small for determining the ROI. Therefore, the distance between the long axis line of the screw and the outline of the lateral cortical surface was measured three times
using another horizontal CT image without artefacts. The mean value was calculated and determined as the pixel value for the screw (Figs. 2–4). Measurement of pixel values of unused absorbable screws (in vitro)
The pixel value of the unused absorbable screws was unknown. Therefore, six unused PLLA screws and five uHA/PLLA screws were implanted in impression paste and their CT image taken under the same conditions. The pixel value was then measured three times at the centre of the screw head.
Fig. 2. Preoperative horizontal CT image at the mandibular foramen parallel to the FH plane. The red points indicate where the pixel values were measured (three points in the lateral cortex, three at the osteotomy site, and three in the medial cortex).
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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Fig. 3. Horizontal CT image at the mandibular foramen parallel to the FH plane obtained immediately after surgery. The red points indicate where the pixel values were measured (three points in the lateral cortex, three at the osteotomy site, and three in the medial cortex). The red arrows indicate where pixel values were measured at the centre of the screw head. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The mean value was calculated and determined as the pixel value of the screw.
paired comparison method using the Student’s t-test. Differences were considered significant at P < 0.05.
Statistical analysis
Data were analyzed statistically with Dr. SPSSII for Windows (SPSS Japan Inc., Tokyo, Japan). The time-course changes were compared using repeated measures analysis of variance (ANOVA). The data from each period were compared by a
Results
No patient had a post-surgical wound infection. There was no dehiscence, bone instability, bone non-union, or long-term malocclusion. The mean setback was 7.4 2.9 mm on the right side and
Fig. 4. Horizontal CT image of the mandibular foramen parallel to the FH plane obtained at 1 year postoperative. The red points indicate where the pixel values were measured (three points in the lateral cortex, three at the osteotomy site, and three in the medial cortex). The red arrow indicates where the pixel value was measured at the centre of the screw head. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
7.1 3.3 mm on the left side. There was no significant difference among the groups. When the pixel values were compared between men and women, no significant difference was found for any measurement. Therefore, the data of men and women were not separated in this study. Using repeated measures ANOVA, there were significant differences in the timecourse change in pixel values for the lateral cortex (between-subject P = 0.0002, within-subject P < 0.0001), in the time-course change in pixel values for the osteotomy site (between-subject P < 0.0001, withinsubject P < 0.0001), and in the time-course change in pixel values of the screw type (between-subject P < 0.0001, within-subject P = 0.0207) among the six groups. The mean pixel values for the lateral cortex, medial cortex, osteotomy site, and screws are listed in Tables 1–4 and shown in Figs. 5–8. In group 1, the pixel value of the medial cortex immediately after surgery was significantly higher than that at 1 year postoperative (P = 0.0222). The pixel value of the osteotomy site decreased significantly immediately after surgery (P < 0.0001), however it then increased to the preoperative pixel value. In group 2, the pixel value of the lateral cortex immediately after surgery was significantly lower than the preoperative value (P = 0.0347), and the pixel value of the lateral cortex at 1 year postoperative was significantly lower than the preoperative value (P = 0.0130). The pixel value of the osteotomy site immediately after surgery was significantly higher than the preoperative value (P < 0.0001) and a significantly high value was maintained at 1 year postoperative (P < 0.001). The reason for these results was the high pixel value of Biopex at the osteotomy site. In group 3, the pixel value of the lateral cortex at 1 year postoperative was significantly lower than the preoperative value (P = 0.0376). For the osteotomy site, the pixel value immediately after surgery was significantly lower than the preoperative (P = 0.0005) and 1-year postoperative values (P < 0.0001). However, there was no significant difference in the other measurements. In group 4, the pixel value of the osteotomy site immediately after surgery was significantly higher than the preoperative value (P < 0.0001), and the 1-year postoperative pixel value was also higher than the preoperative value (P < 0.0001). The reason for these results was the high pixel value of Biopex at the osteotomy site. In group 5, the pixel value of the lateral cortex at 1 year postoperative
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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CT values of mandibular ramus bone after SSRO Table 1. Pixel values in the lateral cortex. Preoperative
Titanium Titanium with a-TCP PLLA PLLA with a-TCP uHA/PLLA uHA/PLLA with a-TCP
Immediately postoperative
1 Year postoperative
Mean
SD
Mean
SD
Mean
SD
1510.1 1775.9*,** 1593.4* 1662.9 1716.4*,** 1639.5
152.0 247.2 139.7 132.4 95.2 114.1
1480.0 1587.5* 1581.3 1666.9 1652.4* 1644.1
183.1 132.2 214.6 79.8 120.5 105.2
1499.1 1558.9** 1491.5** 1649.0 1579.8** 1611.8
215.4 170.2 204.8 114.7 187.3 139.4
SD, standard deviation; a-TCP, a-tricalcium phosphate (Biopex); PLLA, poly-L-lactic acid; uHA, uncalcined and unsintered hydroxyapatite particles. * Or ** indicates a significant difference at P < 0.05. Table 2. Pixel values in the medial cortex. Preoperative SD
Mean Titanium Titanium with a-TCP PLLA PLLA with a-TCP uHA/PLLA uHA/PLLA with a-TCP
*
1388.0 1529.5 1320.1 1460.3 1463.6* 1415.4
118.6 235.5 138.3 176.6 180.9 128.9
Immediately postoperative
1 Year postoperative
Mean
SD
Mean
SD
115.0 248.0 197.8 262.1 203.3 151.3
1304.7 1454.6 1397.5 1438.9 1419.8* 1425.6
175.5 130.0 144.7 157.7 168.8 139.0
1336.8 1462.3 1343.9 1371.7 1415.1 1398.8
*
SD, standard deviation; a-TCP, a-tricalcium phosphate (Biopex); PLLA, poly-L-lactic acid; uHA, uncalcined and unsintered hydroxyapatite particles. * Indicates a significant difference at P < 0.05.
was significantly lower than the preoperative value (P = 0.0126). In the medial cortex, the pixel value at 1 year postoperative was significantly lower than the preoperative value (P = 0.0277). At the osteotomy site, the preoperative pixel value was significantly higher than the postoperative value (P = 0.0020), and the value immediately after surgery had decreased significantly (P < 0.0001). However, the pixel value of the osteotomy site at 1 year postoperative was significantly higher than that immediately after surgery (P < 0.0001). In group 6, the pixel value of the osteotomy site immediately after surgery was significantly higher than the preoperative value (P < 0.0001), and the pixel value at 1 year postoperative was also significantly
higher than the preoperative value (P < 0.0001). The reason for these results was the high pixel value of Biopex at the osteotomy site. There were no significant differences between the preoperative, immediately postoperative, and 1-year postoperative pixel values of the screws in any of the groups. The pixel value of unused absorbable screws (in vitro)
The mean pixel value for the uHA/PLLA screw was 539.5 9.0 and for the PLLA screw was 19.0 22.3. These values were significantly lower than the values obtained in vivo (P < 0.0001). As reference, the pixel value of the titanium screw
Table 3. Pixel values at the osteotomy site.
Mean Titanium Titanium with a-TCP PLLA PLLA with a-TCP uHA/PLLA uHA/PLLA with a-TCP
*
390.2 321.6*,** 362.3* 371.1*,** 408.2* 376.3*,**
1 Year postoperative
Immediately postoperative
Preoperative SD 214.9 131.7 241.4 325.3 255.8 230.7
Mean
SD *,**
67.8 2037.3* 73.3*,** 1957.7* 86.8*,** 1977.0*
19.4 99.2 23.3 234.6 34.6 323.4
Mean
SD **
390.8 2057.9** 406.4 ** 2025.9** 381.6 ** 2014.5**
182.1 91.4 231.4 146.5 319.7 372.9
SD, standard deviation; a-TCP, a-tricalcium phosphate (Biopex); PLLA, poly-L-lactic acid; uHA, uncalcined and unsintered hydroxyapatite particles. * Or ** indicates a significant difference at P < 0.05.
5
was consistently 3071 both in vivo and in vitro when the artefact did not affect the image. Discussion
PLLA is one of various absorbable materials that have been used for fixation after SSRO. PLLA miniplates promote osteosynthesis of the oral and maxillofacial skeleton, and PLLA screws have been used in patients undergoing orthognathic surgery.3,4 In a previous study by this group, it was found that PLLA plates and screws (FIXSORB-MX) were useful in SSRO, as well as the conventional titanium plate system. Newer plates and screws are made from composites of uHA and PLLA (SuperFIXSORB-MX), and they are produced by a forging process comprising a unique compression moulding method and a machining treatment. They have a modulus of elasticity close to that of natural cortical bone, and they retain high strength during the period required for bone healing. They can also show optimal degradation and resorption behaviour, osteoconductivity, and bone bonding capability.5–7 In a previous study, no significant differences in postoperative time-course changes among the u-HA/PLLA, PLLA, and conventional titanium plate systems were found.11 Another study using the absorbable plate system suggested that the decreased thickness of the overlapping cortical bone and the cortical bone step disappeared following favourable bone remodelling after 1 year, even though the cortical bone was not removed at the anterior site of the proximal segment.12 Monma et al.13 developed a self-setting cement-type calcium phosphate material consisting of a-TCP, dicalcium phosphate dibasic (DCPD), and tetracalcium phosphate monoxide (TeCP). In extensive studies, this cement-type material was refined, demonstrating better biocompatibility and direct integration into bone without any participation of the peripheral soft tissues.14–16 As it is free of the infiltration of residual monomers that occurs over time with methacrylate resin, which has long been used for orthopaedic treatment, this self-setting cement has become increasingly targeted for clinical use in Japan. When the aforementioned fixation method of this study was used, it was shown that the gap between the proximal and distal segments was filled with new bone after SSRO with both titanium and absorbable plates, even with less bony contact between the segments.9 In a previous study,10 the
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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Table 4. Pixel values in the screws. Immediately postoperative
Titanium Titanium with a-TCP PLLA PLLA with a-TCP uHA/PLLA uHA/PLLA with a-TCP
1 Year postoperative
Mean
SD
Mean
SD
3071.0 3071.0 184.2 161.5 767.0 706.3
0.0 0.0 61.0 64.1 174.5 166.9
3071.0 3071.0 190.1 180.7 694.4 683.8
0.0 0.0 64.0 65.2 123.3 161.1
SD, standard deviation; a-TCP, a-tricalcium phosphate (Biopex); PLLA, poly-L-lactic acid; uHA, uncalcined and unsintered hydroxyapatite particles.
ramus square on the CT image was one of the parameters used to determine the degree of bone formation after SSRO; the ramus square in the Biopex group was significantly larger than that of the control group immediately after surgery and after 1 year. This suggested that the insertion of Biopex in the anterior part of the ramus increased the total volume of the ramus and that it can prevent the invasion of mucosal endothelial cells into the gap between the segments. Although the CT (pixel) value at the osteotomy site did not change after 1 year in the groups using Biopex in this study, the previous study showed a significant decrease in the square of the Biopex after 1 year. This suggested that Biopex is likely absorbed and replaced with new bone. However, the presence of remnant Biopex after 1 year also suggested that the resorption of Biopex was relatively slow. In the study of Hao et al.17 using rabbit femoral cortical bone, a range of osteoclasts accumulated on the surface of the Biopex, whilst many osteoblasts were
localized on the surface opposing the Biopex from days 5 to 10. However, remnants of Biopex particles were present in the new bone with a profile of compact bone on days 30 and 40. The Biopex was so hard that the osteoclasts could not resorb it rapidly. Many studies have used CT data to assess orthognathic surgery. However, most studies have evaluated the morphological data obtained by geometric measurement of the linear length, angle, square, and volume.10,18 CT value data make it possible to generate finite element models of bones for use in the analysis of stress and strain distributions at the interface between implant and bone; this is useful for optimizing prosthetic design in implant surgery.8 Lettry et al.8 reported CT values of mandibular cortical bone in 10 cadavers ranging from 976 Hounsfield units (HU) to 1478 HU, with a mean of 1183.9 HU (standard deviation 112.1). In orthognathic surgery, CT image data such as the pixel value can be very useful infor-
mation to assess bone quality during the various processes of the postoperative stage. However there are few reports on CT values in orthognathic surgery. In studies assessing CT after SSRO, the HU reading in the mandible around the second molar region was found to be significantly higher in those with neurosensory disturbance.19,20 CT evaluation after intraoral vertical ramus osteotomy (IVRO) clearly showed that CT values between bone fragments increased significantly over time at 1 month, 6 months, 1 year, and 2 years after surgery.21 In this study, the partial volume effect should be considered. The partial volume effect can be defined as the loss of apparent activity in small objects or regions because of the limited resolution of the imaging system. This occurs in medical imaging and more generally in biological imaging.22 When the CT was taken for the absorbable screw implanted on the impression paste, the effect of air, which has a very low CT value ( 1000), could have triggered the measurement difference. Metal artefacts should also be considered in this study. The horizontal slice image without metal artefacts was selected to measure the pixel value in this study. However, the effect might not have been eliminated completely. The lateral cortex, medial cortex, and osteotomy site are very important for fixing the segments using the plate and screws. Therefore, these three sites were selected to assess the pixel value. Although factors related to the assessment procedure might have affected the
Pixel value 2500
2000
Titanium
1500
Titanium with TCP PLLA 1000
PLLA with TCP uHA/PLLA uHA/PLLA with TCP
500
0 Pre-operaon
Immediately aer
Aer 1 year
Lateral cortex
Fig. 5. Pixel values in the lateral cortex. Columns and error bars indicate the mean and standard deviation values.
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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CT values of mandibular ramus bone after SSRO
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Pixel value 2000 1800 1600 1400 Titanium
1200
Titanium with TCP 1000
PLLA
800
PLLA with TCP
600
uHA/PLLA uHA/PLLA with TCP
400 200 0 Pre-operaon
Immediately aer
Aer 1 year
Medial cortex
Fig. 6. Pixel values in the medial cortex. Columns and error bars indicate the mean and standard deviation values.
results, the variation in pixel value of the tissue was comparatively larger than that expected. However, until now, there have been few reports on pixel values in the ramus area (medial cortex, lateral cortex, inside cancellous bone) related to the plate and screws used in orthognathic surgery. In short, a standard method for the assessment of the ramus area including plate and screws and bone alternative materials has not yet been established.
Repeated measures ANOVA showed a significant difference in the lateral cortex. In the titanium with a-TCP, PLLA, and uHA/PLLA groups, the pixel value of the lateral cortex had decreased significantly after 1 year. Changes in the position of the proximal segment and a decrease in the force distribution could be considered as the main causes, although errors as a result of artefacts or the partial volume effect might also have been involved. In the titanium and uHA/PLLA groups, the pixel
value of the medial cortex decreased after surgery, probably for similar reasons. At the osteotomy site, the pixel value in the groups without a-TCP decreased immediately after surgery, however this recovered after 1 year. On the other hand, in the groups with a-TCP, the pixel value increased because of the insertion of the a-TCP, which has a high pixel value. Furthermore, the pixel value of the a-TCP did not change after 1 year. For the osteotomy site, the results of ANOVA among the six groups suggested
Pixel value 3000
2500
2000
Titanium Titanium with TCP
1500
PLLA PLLA with TCP
1000
uHA/PLLA uHA/PLLA with TCP
500
0 Pre-operaon
Immediately aer
Aer 1 year
Osteotomy site
Fig. 7. Pixel values at the osteotomy site. Columns and error bars indicate the mean and standard deviation values.
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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Pixel value 3500 3000 2500 Titanium 2000
Titanium with TCP PLLA
1500
PLLA with TCP uHA/PLLA
1000
uHA/PLLA with TCP 500 0 Immediately aer
Aer 1 year Screw
Fig. 8. Pixel values in the screws. Columns and error bars indicate the mean and standard deviation values.
the difference to be due mainly to the use of a-TCP. This study showed that the pixel (CT) value of the titanium screw was a constant 3071, if the artefact and partial volume effects were eliminated. On the other hand, regarding absorbable screws, the mean pixel value was 184.2 for the PLLA screw and 767.0 for the uHA/PLLA screw immediately after surgery, and these did not change significantly after 1 year. This suggests that the uHA/PLLA screw shows a higher pixel value than the PLLA screw because the uHA/PLLA screw contains HA, which is radio-opaque (high pixel value). Furthermore this suggests that both screws had not been resorbed, even after 1 year. The additional use of a-TCP did not significantly affect the pixel values. This study showed that the CT (pixel) value of the absorbable screw in vitro was lower than that in vivo. The standard deviation of the pixel value of the absorbable screw was larger than that of the titanium screw. These results might depend much more on the partial volume effect than variability in the absorbable screws. It was difficult to prove the clinical relevance of the findings regarding whether the plate materials and bone alternative material affect the postoperative ramus bone remodelling after SSRO, because the inner quality of the bone differs from the bone surface shape. On the other hand, the absorbable plate remained after 1 year without a decrease in the pixel value.
Previous studies have shown there to be no significant difference in the skeletal stability obtained using a titanium plate or a uHA/PLLA Plate.3,4,11 The preservation of the absorbable plate inner structure might be related to the skeletal stability, when the clinical relevance is considered. In conclusion, this study showed pixel values of the mandibular ramus bone and titanium screws and absorbable screws in detail. The results suggest that the fixation plate type and use of bone alternative material may affect bone quality during the process of bone healing after SSRO.
2.
3.
4.
Funding
None. Competing interests
None declared. 5.
Ethical approval
The study was approved by the ethics committee of Yamanashi University Hospital.
6.
Patient consent
Not required. References 1. Ueki K, Nakagawa K, Takatsuka S, Yamamoto E. Plate fixation after mandibular
7.
osteotomy. Int J Oral Maxillofac Surg 2001;30:490–6. Ueki K, Degerliyurt K, Hashiba Y, Marukawa K, Nakagawa K, Yamamoto E. Horizontal changes in the condylar head after sagittal split ramus osteotomy with bent plate fixation. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008;106:656–61. Ueki K, Nakagawa K, Marukawa K, Takazakura D, Shimada M, Takatsuka S, et al. Changes in condylar long axis and skeletal stability after bilateral sagittal split ramus osteotomy with poly-L-lactic acid or titanium plate fixation. Int J Oral Maxillofac Surg 2005;34:627–34. Ueki K, Marukawa K, Shimada M, Nakagawa K, Alam S, Yamamoto E. Maxillary stability following Le Fort I osteotomy in combination with sagittal split ramus osteotomy and intraoral vertical ramus osteotomy: a comparative study between titanium miniplate and poly-L-lactic acid plate. J Oral Maxillofac Surg 2006;64:74–80. Shikinami Y, Okuno M. Bioresorbable devices made of forged composites of hydroxyapatite (HA) particles and poly-L-lactide (PLLA): Part I. Basic characteristics. Biomaterials 1999;20:859–77. Shikinami Y, Okuno M. Bioresorbable devices made of forged composites of hydroxyapatite (HA) particles and poly-L-lactide (PLLA): Part II, Practical properties of miniscrews and miniplates. Biomaterials 2001;22:3197–211. Shikinami Y, Matsusue Nakamura T. The complete process of bioresorption and bone replacement using devices made of forged composite of raw hydroxyapatite particles/ poly L-lactide (F-u-HA/PLLA). Biomaterials 2005;26:5542–51.
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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CT values of mandibular ramus bone after SSRO 8. Lettry S, Seedhom BB, Berry E, Cuppone M. Quality assessment of the cortical bone of the human mandible. Bone 2003;32:35–44. 9. Ueki K, Hashiba Y, Marukawa K, Okabe K, Nakagawa K, Alam S, et al. Evaluation of bone formation after sagittal split ramus osteotomy with bent plate fixation using computed tomography. J Oral Maxillofac Surg 2009;67:1062–8. 10. Ueki K, Okabe K, Marukawa K, Mukozawa A, Moroi A, Miyazaki M, et al. Effect of selfsetting a-tricalcium phosphate between segments for bone healing and hypoaesthesia in lower lip after sagittal split ramus osteotomy. J Craniomaxillofac Surg 2012;40:119–24. 11. Ueki K, Okabe K, Miyazaki M, Mukozawa A, Moroi A, Marukawa K, et al. Skeletal stability after mandibular setback surgery: comparisons among unsintered hydroxyapatite/poly-L-lactic acid plate, poly-L-lactic acid plate, and titanium plate. J Oral Maxillofac Surg 2011;69:1464–8. 12. Ueki K, Marukawa K, Moroi A, Sotobori M, Ishihara Y, Iguchi R, et al. Evaluation of overlapped cortical bone area after modified plate fixation with bent plate in sagittal split ramus osteotomy. J Craniomaxillofac Surg 2014;42:e210–6.
13. Monma H, Ohta K, Takahashi S. A study of the newly developed a-tricalcium phosphate cement. FC Rep 1988;6:475. 14. Kurashina K, Kurita H, Hirano M, Kotani A, Klein CP, de Groot K. In vivo study of calcium phosphate cements: implantation of an alphatricalcium phosphate/dicalcium phosphate dibasic/tetracalcium phosphate monoxide cement paste. Biomaterials 1997;18:539–43. 15. Yamamoto H, Niwa S, Hori M, Hattori T, Sawai K, Aoki S, et al. Mechanical strength of calcium phosphate cement in vivo and in vitro. Biomaterials 1998;19:1587–91. 16. Yuan H, Li T, de Bruijn JD, de Groot K, Zhang X. Tissue response of calcium phosphate cement: a study in dogs. Biomaterials 2000;21:1283–90. 17. Hao H, Amizuka N, Oda K, Fujii N, Ohnishi H, Okada A, et al. A histological evaluation on self-setting alpha-tricalcium phosphate applied in the rat bone cavity. Biomaterials 2004;25:431–42. 18. Ueki K, Okabe K, Marukawa K, Mukozawa A, Moroi A, Miyazaki M, et al. Assessment of bone healing and hypoesthesia in the upper lip after Le Fort I osteotomy with self-setting atricalcium phosphate and absorbable plates. J Craniomaxillofac Surg 2013;41:129–34.
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19. Yoshioka I, Tanaka T, Khanal A, Habu M, Kito S, Kodama M, et al. Correlation of mandibular bone quality with neurosensory disturbance after sagittal split ramus osteotomy. Br J Oral Maxillofac Surg 2011;49: 552–6. 20. Yoshioka I, Tanaka T, Habu M, Oda M, Kodama M, Kito S, et al. Effect of bone quality and position of the inferior alveolar nerve canal in continuous, long-term, neurosensory disturbance after sagittal split ramus osteotomy. Craniomaxillofac Surg 2012;40:e178–83. 21. Arimoto S, Hasegawa T, Kaneko K, Tateishi C, Furudoi S, Shibuya Y, et al. Observation of osseous healing after intraoral vertical ramus osteotomy: focus on computed tomography values. J Oral Maxillofac Surg 2013;71:1602.e1–.. 22. Hoffman EJ, Huang SC, Phelps ME. Quantitation in position emission computed tomography: 1. Effect of object size. J Comput Assist Tomogr 1979;3:299–308.
Corresponding author Tel.: +81 55 273 9673 fax: +81 55 273 8210 E-mail: [email protected]
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
Int. J. Oral Maxillofac. Surg. 2015; xxx: xxx–xxx http://dx.doi.org/10.1016/j.ijom.2015.06.010, available online at http://www.sciencedirect.com
Clinical Paper Orthognathic surgery
Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy
K. Ueki, A. Moroi, R. Iguchi, A. Kosaka, H. Ikawa, K. Yoshizawa Department of Oral and Maxillofacial Surgery, Division of Medicine, Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 1110 Shimokato, Chuo-shi, Yamanashi, Japan
K. Ueki, A. Moroi, R. Iguchi, A. Kosaka, H. Ikawa, K. Yoshizawa: Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy. Int. J. Oral Maxillofac. Surg. 2015; xxx: xxx–xxx. # 2015 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Abstract. The purpose of this retrospective study was to evaluate the changes in computed tomography (CT) values of ramus bone and screws after sagittal split ramus osteotomy (SSRO) setback surgery. The subjects were 64 patients (128 sides) who underwent bilateral SSRO setback surgery. They were divided into six groups according to the fixation plate type used and the use or not of self-setting a-tricalcium phosphate (Biopex): group 1: titanium plate and screws; group 2: titanium plate and screws with Biopex; group 3: poly-L-lactic acid (PLLA) plate and screws; group 4: PLLA plate and screws with Biopex; group 5: uncalcined and unsintered hydroxyapatite particles and poly-L-lactic acid (uHA/PLLA) plate and screws; group 6: PLLA/uHA plate and screws with Biopex. CT values (pixel values) of the lateral cortex, medial cortex, osteotomy site, and screws were measured preoperatively, immediately after surgery, and 1 year postoperatively using horizontal CT images at the mandibular foramen taken parallel to the Frankfort horizontal plane. There were significant differences in the time-course change of pixel values for the lateral cortex (P < 0.0001) and the osteotomy site (P < 0.0001) among the six groups. This study suggests that the fixation plate type and use of bone alternative material may affect bone quality during the process of bone healing after SSRO.
0901-5027/000001+09
Keywords: Sagittal split ramus osteotomy; Computed tomography (CT) value; Absorbable plate and screws; Bone healing. Accepted for publication 12 June 2015
# 2015 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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One of the advantages of the sagittal split ramus osteotomy (SSRO) is that the large areas of bony contact that are made remain after either advancement or retrusion of the distal segment. However, making a wider bony contact without the step of the cortical bone surface between the proximal and distal segments can induce inward rotation of the condylar long axis in setback surgery.1,2 In previous studies by this group, bent plates were used to secure the fragments without a positioning device and it was found that the bent plate increased the incidence of postoperative temporomandibular joint dysfunction (TMD) and did not change skeletal or occlusal stability.1,2 With this method, the gap between the proximal and distal segments is created by a bent plate, preventing the formation of a large bony contact. In setback surgery, especially with asymmetry, fixation between the segments can be performed without bony contact to prevent large changes in condylar position and angle. More recently, absorbable plates and screws have been used with increasing frequency in our hospital.3,4 These absorbable plate and screw systems have been developed in Japan (poly-L-lactic acid (PLLA) and uncalcined and unsintered hydroxyapatite particles and poly-L-lactic acid (uHA/PLLA)).5–7 Although, it has been shown in animal studies that the strength of the plate and screws is reduced in a time-dependent manner, the plate and screw resorption process and the surrounding bone formation at the osteotomy site remain unclear. Furthermore, it has been considered that bone density or bone thickness in the ramus area could change because of the stress distribution changes resulting from SSRO (occlusion and skeletal change). The computed tomography (CT) value can be correlated to bone density, and bone strength can be expressed in terms of bone density and quality.8 CT value data can be useful for assessing bone formation. A previous study by this group showed that the gap between the proximal and distal segments could be filled with new bone after SSRO using either titanium or absorbable plates, even if there were few bony contacts between the segments.9 Moreover, another study suggested that inserting self-setting a-tricalcium phosphate (a-TCP) (Biopex; Pentax Co., Tokyo, Japan) in the gap between the proximal and distal segments was useful for new bone formation after SSRO with bent plate fixation.10 However, that study assessed only the linear length and ramus square using CT at the osteotomy site.
It is still unclear whether the fixation device or the use of a bone alternative material can affect the healing process after SSRO. Furthermore, the CT values of ramus bone, titanium plate and screws, absorbable plate and screws, and bone alternative material following orthognathic surgery have not been reported. The purpose of this retrospective study was to evaluate changes in quality (CT value) of ramus bone and screws after SSRO using absorbable plates and titanium plates with and without selfsetting a-TCP. Patients and methods Patients
This study included 64 Japanese adults (21 men and 43 women) who presented with jaw deformities diagnosed as mandibular prognathism with and without a maxillary deformity. At the time of orthognathic surgery, the patients ranged in age from 16 to 51 years, with a mean age of 29.2 years (standard deviation 10.9 years). For this retrospective study, informed consent was obtained from all patients in accordance with the Declaration of Helsinki; the study was approved by the ethics committee of Yamanashi University Hospital.
Surgery
Lateral, frontal, and submentovertex (SV) cephalograms were obtained before surgery, as described previously.1,2 All 64 patients underwent bilateral SSRO setback with the modified fixation. Of the 64 patients, 26 underwent SSRO with Le Fort I osteotomy to advance or impact the maxilla. At the time of fixation, the dental arch of the distal segment was secured to the maxillary arch with an interpositional splint and 0.4-mm wire. For seven patients (14 sides), a titanium miniplate was used for internal fixation of the mandible (long miniplate (four holes/burr 8 mm, thickness 1.0 mm) and four screws (2 14 mm and 2 5 mm); Universal Mandible fixation module, Stryker Leibinger Co., Freiburg, Germany). These patients comprised group 1 (titanium group). For three patients (six sides), a titanium miniplate (long miniplate (four holes/burr 8 mm, thickness 1.0 mm) and four screws (2 14 mm and 2 5 mm); Universal Mandible fixation module) was used for internal fixation of the mandible, and selfsetting a-TCP (Biopex) was inserted in the anterior part of the gap between the seg-
ments after plate fixation. These patients comprised group 2 (titanium with a-TCP group). For seven patients (14 sides), a PLLA miniplate (28 4.5 1.5 mm with four screws (2 8 mm); FIXSORB-MX, Takiron Co., Osaka, Japan) was used for internal fixation of the mandible. These patients comprised group 3 (PLLA group). For 11 patients (22 sides), a PLLA miniplate (28 4.5 1.5 mm with four screws (2 8 mm); FIXSORB-MX) was used for internal fixation of the mandible, and self-setting a-TCP (Biopex) was inserted in the anterior part of the gap between the segments after plate fixation. These patients comprised group 4 (PLLA with a-TCP group). For 19 patients (38 sides), a uHA/PLLA miniplate (28 4.5 1.5 mm with four screws (2 8 mm); SuperFIXSORBMX; Takiron Co.) was used for internal fixation of the mandible. These patients comprised group 5 (uHA/PLLA group). For 17 patients (34 sides), a uHA/PLLA miniplate (28 4.5 1.5 mm with four screws (2 8 mm); SuperFIXSORBMX) was used for internal fixation of the mandible, and self-setting a-TCP (Biopex) was inserted in the anterior part of the gap between the segments after plate fixation. These patients comprised group 6 (uHA/PLLA with a-TCP group). The groups were divided by time-frame, so the sample number was statistically variable. To prevent intraoperative inward rotation of the condylar long axis, model surgery was performed preoperatively with reference to the SV projection.1,2 Before surgery, an SV cephalogram was obtained for all patients, followed by simulation. A distal segment including the lower dental arch was first set back according to the position of the upper dental arch on the SV cephalometric trace. When the proximal and distal segments are fixed with straight plates after bilateral sagittal split osteotomy (BSSO), proximal segments containing the condylar head cause internal rotation (Fig. 1). To prevent internal rotation of the proximal segments, the overlapping cortical bone at the anterior site of the proximal segment was not removed to keep the contact area between the proximal and distal segments, and was fixed with a bent plate and four screws on each side of the mandible. At the posterior part, a 3–7-mm gap was maintained between the proximal and distal segments. After surgery, elastics were placed to maintain an ideal occlusion. All patients received orthodontic treatment before and after surgery. CT images were obtained
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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CT values of mandibular ramus bone after SSRO
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Fig. 1. Simulation of plate bending. (A) Straight plate: when the proximal and distal segments are fixed with straight plates after bilateral sagittal split osteotomy, the proximal segments containing the condylar head cause internal rotation, and the posterior aspect of the distal segment on the deviated side sometimes interferes with the proximal segment. (B) Bent plate: the plates were bent to prevent the proximal segments from rotating internally; note the gap between the osteotomy surfaces.
for all patients preoperatively, immediately after surgery, and 1 year after surgery. CT data acquisition
For CT scanning, the patients were placed in the gantry with the tragal–canthal line perpendicular to the floor. They were instructed to breathe normally and to avoid swallowing during the scanning process. CT scans were obtained in the radiology department by skilled radiology technicians using a high-speed, advantagetype CT generator (Light Speed Plus; GE Healthcare, Milwaukee, WI, USA) with each sequence taken 1.25 mm apart for three-dimensional (3D) reconstruction (120 kV, average 150 mA, 0.7 s/rotation, helical pitch 0.75). The resulting images were stored in the attached workstation computer (Advantage Workstation version 4.2; GE Healthcare, Milwaukee, WI, USA), and 3D reconstruction was performed using the volume-rendering method. ZedView version 7.0 (LEXI Co., Tokyo, Japan) medical imaging software was used for 3D morphological measurements. CT image analysis software (DIANA DICOM free version 1.8.1.6; Luke-System, Chiba, Japan) was used to measure the CT value (pixel value). CT values of the lateral cortex, medial cortex, and osteotomy site were measured preoperatively, immediately after surgery, and at 1 year postoperative using horizontal CT images with the mandibular foramen parallel to the Frankfort horizontal (FH) plane. For each CT image, three randomly selected points at the anterolateral site of the lateral cortex, three at the anteromedial
site of the medial cortex, and three at the anterolateral site of the mandibular canal were assessed. The mean pixel value in the region of interest (ROI; approximately 1.0 mm2 circle) was measured automatically. The mean value for the three points was then calculated and assigned as the pixel value for each part. Measurement of the screws was performed for just one screw on each side for all patients. The measurement area for the screw was very small for determining the ROI. Therefore, the distance between the long axis line of the screw and the outline of the lateral cortical surface was measured three times
using another horizontal CT image without artefacts. The mean value was calculated and determined as the pixel value for the screw (Figs. 2–4). Measurement of pixel values of unused absorbable screws (in vitro)
The pixel value of the unused absorbable screws was unknown. Therefore, six unused PLLA screws and five uHA/PLLA screws were implanted in impression paste and their CT image taken under the same conditions. The pixel value was then measured three times at the centre of the screw head.
Fig. 2. Preoperative horizontal CT image at the mandibular foramen parallel to the FH plane. The red points indicate where the pixel values were measured (three points in the lateral cortex, three at the osteotomy site, and three in the medial cortex).
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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Fig. 3. Horizontal CT image at the mandibular foramen parallel to the FH plane obtained immediately after surgery. The red points indicate where the pixel values were measured (three points in the lateral cortex, three at the osteotomy site, and three in the medial cortex). The red arrows indicate where pixel values were measured at the centre of the screw head. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The mean value was calculated and determined as the pixel value of the screw.
paired comparison method using the Student’s t-test. Differences were considered significant at P < 0.05.
Statistical analysis
Data were analyzed statistically with Dr. SPSSII for Windows (SPSS Japan Inc., Tokyo, Japan). The time-course changes were compared using repeated measures analysis of variance (ANOVA). The data from each period were compared by a
Results
No patient had a post-surgical wound infection. There was no dehiscence, bone instability, bone non-union, or long-term malocclusion. The mean setback was 7.4 2.9 mm on the right side and
Fig. 4. Horizontal CT image of the mandibular foramen parallel to the FH plane obtained at 1 year postoperative. The red points indicate where the pixel values were measured (three points in the lateral cortex, three at the osteotomy site, and three in the medial cortex). The red arrow indicates where the pixel value was measured at the centre of the screw head. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
7.1 3.3 mm on the left side. There was no significant difference among the groups. When the pixel values were compared between men and women, no significant difference was found for any measurement. Therefore, the data of men and women were not separated in this study. Using repeated measures ANOVA, there were significant differences in the timecourse change in pixel values for the lateral cortex (between-subject P = 0.0002, within-subject P < 0.0001), in the time-course change in pixel values for the osteotomy site (between-subject P < 0.0001, withinsubject P < 0.0001), and in the time-course change in pixel values of the screw type (between-subject P < 0.0001, within-subject P = 0.0207) among the six groups. The mean pixel values for the lateral cortex, medial cortex, osteotomy site, and screws are listed in Tables 1–4 and shown in Figs. 5–8. In group 1, the pixel value of the medial cortex immediately after surgery was significantly higher than that at 1 year postoperative (P = 0.0222). The pixel value of the osteotomy site decreased significantly immediately after surgery (P < 0.0001), however it then increased to the preoperative pixel value. In group 2, the pixel value of the lateral cortex immediately after surgery was significantly lower than the preoperative value (P = 0.0347), and the pixel value of the lateral cortex at 1 year postoperative was significantly lower than the preoperative value (P = 0.0130). The pixel value of the osteotomy site immediately after surgery was significantly higher than the preoperative value (P < 0.0001) and a significantly high value was maintained at 1 year postoperative (P < 0.001). The reason for these results was the high pixel value of Biopex at the osteotomy site. In group 3, the pixel value of the lateral cortex at 1 year postoperative was significantly lower than the preoperative value (P = 0.0376). For the osteotomy site, the pixel value immediately after surgery was significantly lower than the preoperative (P = 0.0005) and 1-year postoperative values (P < 0.0001). However, there was no significant difference in the other measurements. In group 4, the pixel value of the osteotomy site immediately after surgery was significantly higher than the preoperative value (P < 0.0001), and the 1-year postoperative pixel value was also higher than the preoperative value (P < 0.0001). The reason for these results was the high pixel value of Biopex at the osteotomy site. In group 5, the pixel value of the lateral cortex at 1 year postoperative
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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CT values of mandibular ramus bone after SSRO Table 1. Pixel values in the lateral cortex. Preoperative
Titanium Titanium with a-TCP PLLA PLLA with a-TCP uHA/PLLA uHA/PLLA with a-TCP
Immediately postoperative
1 Year postoperative
Mean
SD
Mean
SD
Mean
SD
1510.1 1775.9*,** 1593.4* 1662.9 1716.4*,** 1639.5
152.0 247.2 139.7 132.4 95.2 114.1
1480.0 1587.5* 1581.3 1666.9 1652.4* 1644.1
183.1 132.2 214.6 79.8 120.5 105.2
1499.1 1558.9** 1491.5** 1649.0 1579.8** 1611.8
215.4 170.2 204.8 114.7 187.3 139.4
SD, standard deviation; a-TCP, a-tricalcium phosphate (Biopex); PLLA, poly-L-lactic acid; uHA, uncalcined and unsintered hydroxyapatite particles. * Or ** indicates a significant difference at P < 0.05. Table 2. Pixel values in the medial cortex. Preoperative SD
Mean Titanium Titanium with a-TCP PLLA PLLA with a-TCP uHA/PLLA uHA/PLLA with a-TCP
*
1388.0 1529.5 1320.1 1460.3 1463.6* 1415.4
118.6 235.5 138.3 176.6 180.9 128.9
Immediately postoperative
1 Year postoperative
Mean
SD
Mean
SD
115.0 248.0 197.8 262.1 203.3 151.3
1304.7 1454.6 1397.5 1438.9 1419.8* 1425.6
175.5 130.0 144.7 157.7 168.8 139.0
1336.8 1462.3 1343.9 1371.7 1415.1 1398.8
*
SD, standard deviation; a-TCP, a-tricalcium phosphate (Biopex); PLLA, poly-L-lactic acid; uHA, uncalcined and unsintered hydroxyapatite particles. * Indicates a significant difference at P < 0.05.
was significantly lower than the preoperative value (P = 0.0126). In the medial cortex, the pixel value at 1 year postoperative was significantly lower than the preoperative value (P = 0.0277). At the osteotomy site, the preoperative pixel value was significantly higher than the postoperative value (P = 0.0020), and the value immediately after surgery had decreased significantly (P < 0.0001). However, the pixel value of the osteotomy site at 1 year postoperative was significantly higher than that immediately after surgery (P < 0.0001). In group 6, the pixel value of the osteotomy site immediately after surgery was significantly higher than the preoperative value (P < 0.0001), and the pixel value at 1 year postoperative was also significantly
higher than the preoperative value (P < 0.0001). The reason for these results was the high pixel value of Biopex at the osteotomy site. There were no significant differences between the preoperative, immediately postoperative, and 1-year postoperative pixel values of the screws in any of the groups. The pixel value of unused absorbable screws (in vitro)
The mean pixel value for the uHA/PLLA screw was 539.5 9.0 and for the PLLA screw was 19.0 22.3. These values were significantly lower than the values obtained in vivo (P < 0.0001). As reference, the pixel value of the titanium screw
Table 3. Pixel values at the osteotomy site.
Mean Titanium Titanium with a-TCP PLLA PLLA with a-TCP uHA/PLLA uHA/PLLA with a-TCP
*
390.2 321.6*,** 362.3* 371.1*,** 408.2* 376.3*,**
1 Year postoperative
Immediately postoperative
Preoperative SD 214.9 131.7 241.4 325.3 255.8 230.7
Mean
SD *,**
67.8 2037.3* 73.3*,** 1957.7* 86.8*,** 1977.0*
19.4 99.2 23.3 234.6 34.6 323.4
Mean
SD **
390.8 2057.9** 406.4 ** 2025.9** 381.6 ** 2014.5**
182.1 91.4 231.4 146.5 319.7 372.9
SD, standard deviation; a-TCP, a-tricalcium phosphate (Biopex); PLLA, poly-L-lactic acid; uHA, uncalcined and unsintered hydroxyapatite particles. * Or ** indicates a significant difference at P < 0.05.
5
was consistently 3071 both in vivo and in vitro when the artefact did not affect the image. Discussion
PLLA is one of various absorbable materials that have been used for fixation after SSRO. PLLA miniplates promote osteosynthesis of the oral and maxillofacial skeleton, and PLLA screws have been used in patients undergoing orthognathic surgery.3,4 In a previous study by this group, it was found that PLLA plates and screws (FIXSORB-MX) were useful in SSRO, as well as the conventional titanium plate system. Newer plates and screws are made from composites of uHA and PLLA (SuperFIXSORB-MX), and they are produced by a forging process comprising a unique compression moulding method and a machining treatment. They have a modulus of elasticity close to that of natural cortical bone, and they retain high strength during the period required for bone healing. They can also show optimal degradation and resorption behaviour, osteoconductivity, and bone bonding capability.5–7 In a previous study, no significant differences in postoperative time-course changes among the u-HA/PLLA, PLLA, and conventional titanium plate systems were found.11 Another study using the absorbable plate system suggested that the decreased thickness of the overlapping cortical bone and the cortical bone step disappeared following favourable bone remodelling after 1 year, even though the cortical bone was not removed at the anterior site of the proximal segment.12 Monma et al.13 developed a self-setting cement-type calcium phosphate material consisting of a-TCP, dicalcium phosphate dibasic (DCPD), and tetracalcium phosphate monoxide (TeCP). In extensive studies, this cement-type material was refined, demonstrating better biocompatibility and direct integration into bone without any participation of the peripheral soft tissues.14–16 As it is free of the infiltration of residual monomers that occurs over time with methacrylate resin, which has long been used for orthopaedic treatment, this self-setting cement has become increasingly targeted for clinical use in Japan. When the aforementioned fixation method of this study was used, it was shown that the gap between the proximal and distal segments was filled with new bone after SSRO with both titanium and absorbable plates, even with less bony contact between the segments.9 In a previous study,10 the
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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Table 4. Pixel values in the screws. Immediately postoperative
Titanium Titanium with a-TCP PLLA PLLA with a-TCP uHA/PLLA uHA/PLLA with a-TCP
1 Year postoperative
Mean
SD
Mean
SD
3071.0 3071.0 184.2 161.5 767.0 706.3
0.0 0.0 61.0 64.1 174.5 166.9
3071.0 3071.0 190.1 180.7 694.4 683.8
0.0 0.0 64.0 65.2 123.3 161.1
SD, standard deviation; a-TCP, a-tricalcium phosphate (Biopex); PLLA, poly-L-lactic acid; uHA, uncalcined and unsintered hydroxyapatite particles.
ramus square on the CT image was one of the parameters used to determine the degree of bone formation after SSRO; the ramus square in the Biopex group was significantly larger than that of the control group immediately after surgery and after 1 year. This suggested that the insertion of Biopex in the anterior part of the ramus increased the total volume of the ramus and that it can prevent the invasion of mucosal endothelial cells into the gap between the segments. Although the CT (pixel) value at the osteotomy site did not change after 1 year in the groups using Biopex in this study, the previous study showed a significant decrease in the square of the Biopex after 1 year. This suggested that Biopex is likely absorbed and replaced with new bone. However, the presence of remnant Biopex after 1 year also suggested that the resorption of Biopex was relatively slow. In the study of Hao et al.17 using rabbit femoral cortical bone, a range of osteoclasts accumulated on the surface of the Biopex, whilst many osteoblasts were
localized on the surface opposing the Biopex from days 5 to 10. However, remnants of Biopex particles were present in the new bone with a profile of compact bone on days 30 and 40. The Biopex was so hard that the osteoclasts could not resorb it rapidly. Many studies have used CT data to assess orthognathic surgery. However, most studies have evaluated the morphological data obtained by geometric measurement of the linear length, angle, square, and volume.10,18 CT value data make it possible to generate finite element models of bones for use in the analysis of stress and strain distributions at the interface between implant and bone; this is useful for optimizing prosthetic design in implant surgery.8 Lettry et al.8 reported CT values of mandibular cortical bone in 10 cadavers ranging from 976 Hounsfield units (HU) to 1478 HU, with a mean of 1183.9 HU (standard deviation 112.1). In orthognathic surgery, CT image data such as the pixel value can be very useful infor-
mation to assess bone quality during the various processes of the postoperative stage. However there are few reports on CT values in orthognathic surgery. In studies assessing CT after SSRO, the HU reading in the mandible around the second molar region was found to be significantly higher in those with neurosensory disturbance.19,20 CT evaluation after intraoral vertical ramus osteotomy (IVRO) clearly showed that CT values between bone fragments increased significantly over time at 1 month, 6 months, 1 year, and 2 years after surgery.21 In this study, the partial volume effect should be considered. The partial volume effect can be defined as the loss of apparent activity in small objects or regions because of the limited resolution of the imaging system. This occurs in medical imaging and more generally in biological imaging.22 When the CT was taken for the absorbable screw implanted on the impression paste, the effect of air, which has a very low CT value ( 1000), could have triggered the measurement difference. Metal artefacts should also be considered in this study. The horizontal slice image without metal artefacts was selected to measure the pixel value in this study. However, the effect might not have been eliminated completely. The lateral cortex, medial cortex, and osteotomy site are very important for fixing the segments using the plate and screws. Therefore, these three sites were selected to assess the pixel value. Although factors related to the assessment procedure might have affected the
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Fig. 5. Pixel values in the lateral cortex. Columns and error bars indicate the mean and standard deviation values.
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
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CT values of mandibular ramus bone after SSRO
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Fig. 6. Pixel values in the medial cortex. Columns and error bars indicate the mean and standard deviation values.
results, the variation in pixel value of the tissue was comparatively larger than that expected. However, until now, there have been few reports on pixel values in the ramus area (medial cortex, lateral cortex, inside cancellous bone) related to the plate and screws used in orthognathic surgery. In short, a standard method for the assessment of the ramus area including plate and screws and bone alternative materials has not yet been established.
Repeated measures ANOVA showed a significant difference in the lateral cortex. In the titanium with a-TCP, PLLA, and uHA/PLLA groups, the pixel value of the lateral cortex had decreased significantly after 1 year. Changes in the position of the proximal segment and a decrease in the force distribution could be considered as the main causes, although errors as a result of artefacts or the partial volume effect might also have been involved. In the titanium and uHA/PLLA groups, the pixel
value of the medial cortex decreased after surgery, probably for similar reasons. At the osteotomy site, the pixel value in the groups without a-TCP decreased immediately after surgery, however this recovered after 1 year. On the other hand, in the groups with a-TCP, the pixel value increased because of the insertion of the a-TCP, which has a high pixel value. Furthermore, the pixel value of the a-TCP did not change after 1 year. For the osteotomy site, the results of ANOVA among the six groups suggested
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Osteotomy site
Fig. 7. Pixel values at the osteotomy site. Columns and error bars indicate the mean and standard deviation values.
Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
YIJOM-3192; No of Pages 9
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Ueki et al.
Pixel value 3500 3000 2500 Titanium 2000
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Fig. 8. Pixel values in the screws. Columns and error bars indicate the mean and standard deviation values.
the difference to be due mainly to the use of a-TCP. This study showed that the pixel (CT) value of the titanium screw was a constant 3071, if the artefact and partial volume effects were eliminated. On the other hand, regarding absorbable screws, the mean pixel value was 184.2 for the PLLA screw and 767.0 for the uHA/PLLA screw immediately after surgery, and these did not change significantly after 1 year. This suggests that the uHA/PLLA screw shows a higher pixel value than the PLLA screw because the uHA/PLLA screw contains HA, which is radio-opaque (high pixel value). Furthermore this suggests that both screws had not been resorbed, even after 1 year. The additional use of a-TCP did not significantly affect the pixel values. This study showed that the CT (pixel) value of the absorbable screw in vitro was lower than that in vivo. The standard deviation of the pixel value of the absorbable screw was larger than that of the titanium screw. These results might depend much more on the partial volume effect than variability in the absorbable screws. It was difficult to prove the clinical relevance of the findings regarding whether the plate materials and bone alternative material affect the postoperative ramus bone remodelling after SSRO, because the inner quality of the bone differs from the bone surface shape. On the other hand, the absorbable plate remained after 1 year without a decrease in the pixel value.
Previous studies have shown there to be no significant difference in the skeletal stability obtained using a titanium plate or a uHA/PLLA Plate.3,4,11 The preservation of the absorbable plate inner structure might be related to the skeletal stability, when the clinical relevance is considered. In conclusion, this study showed pixel values of the mandibular ramus bone and titanium screws and absorbable screws in detail. The results suggest that the fixation plate type and use of bone alternative material may affect bone quality during the process of bone healing after SSRO.
2.
3.
4.
Funding
None. Competing interests
None declared. 5.
Ethical approval
The study was approved by the ethics committee of Yamanashi University Hospital.
6.
Patient consent
Not required. References 1. Ueki K, Nakagawa K, Takatsuka S, Yamamoto E. Plate fixation after mandibular
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Please cite this article in press as: Ueki K, et al. Changes in the computed tomography (pixel) value of mandibular ramus bone and fixation screws after sagittal split ramus osteotomy, Int J Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.ijom.2015.06.010
