| BackgroundVerbiest proposed the concept of narrow canal, nerve root canal and foramen, which he referred to as the lumbar spinal stenosis (Lumbar canal stenosis) in 1949. Based on the etiology, LSS include congenital spinal stenosis, developmental spinal stenosis and secondary spinal stenosis, Secondary spinal stenosis causes are degenerative, iatrogenic, traumatic and other spondylolysis and spondylolisthesis and so on. Degenerative spinal stenosis is more common in clinical practice. As China has gradually entered the aging society, the elderly population base increases. More and more patients with degenerative spine disease, and patients with degenerative spinal stenosis has increased every year. Spinal stenosis (Lumbar spinal stenosis, LSS) refers to diameter decreases of various forms of spinal nerve root canal and intervertebral foramen by a variety of causes, including soft tissue changes (such as thickening or swelling of yellow ligament, calcification or folds of the posterior longitudinal ligament), hypertrophy of small joints, changing volume of spinal canal or spinal dural sac caused by degenerative spondylolisthesis or degenerative dural sac stenosis itself. Symptoms of spinal stenosis is caused by the compressed spinal cord, nerves, blood vessels. Low back pain and/or leg pain, disable standing and working are the main symptoms. LSS can occur in every segments of the lumbar spine, but preferably occurs at L4-5 segment.Non-surgical treatments are often ineffective, especially in the severe stenosis, elderly or patients with complications. Compared with non-surgical therapy, with or without decompression and lumbar spinal fusion surgery have better clinical efficacy, especially in patients with more severe symptoms. Traditional open lumbar decompression surgery have a more significant effect of nerve decompression, but there are obvious shortcomings, such as:the risk of massive blood loss, wound infection, iatrogenic instability, nerve scarring, medical complications, delayed healing, etc. In addition, it often requires more complex revision surgery due to ineffective long-term pain relief in the post-operative period of time.Compared to the previous procedure, minimally invasive surgery has less muscle damage, less blood loss, shorter hospital stay and recovery time. However, due to the long learning curve of minimally invasive surgery and assumed that the presence of the risk of complications, many clinical surgeon does not use a minimally invasive surgical procedures for the treatment of LSS. The minimally invasive surgical procedures can avoid these complications, which interspinous process distraction surgery as a minimally invasive surgical treatment for LSS has been used in clinical practice in recent years, the effect of surgical decompression may be effective in a period of time in some patients under seat state. However, this method also has a high rate of failure or revision.In recent years, laminoplasty pedicle-lengthening osteotomy as a novel method has been introduced to the spine surgery operation, this procedure is carried out though cutting off and backward extending bilateral pedicle of lesion segment at a certain distance, then implant bilateral pedicle screws to fix,to expand the volume of vertebral canal and nerve root canal. Achieve the goal/target of implementation of decompression of spinal canal contents, the decompression effect is fast, direct and effective. There have been a number of pedicle amputation expand laminoplasty reports. Yang Huilin et al. confirmed that this method can significantly increase the area of lumbar spinal canal and foramen aperture from the point of view of anatomy and image. Kiapour et al simulated pedicle-lengthening osteotomy using specimens, and conducted lumbar spine biomechanics experiments of L4-L5 and the cross-sectional area of the spinal canal expansion simulation analysis, Finds showed that intervertebral ROM did not change significantly after surgery, and the procedure can significantly increase the volume of the spinal canal and intervertebral foramen. Mlyavykh et al conducted the pedicle-lengthening osteotomy on 19 cases of patients with lumbar spinal stenosis whose extended distance was all 4.8mm, no intraoperative nerve injury, cerebrospinal fluid leakage and other complications occurred in operation. Patients were followed up for 12 months and found that the patient’s symptoms were improved significantly/obviously. These studies have shown that pedicle-lengthening osteotomy in the treatment of LSS is an effective, simple and minimally invasive surgery. In those study, may be due to limitations of operation instruments or the habits of operator and other reasons, what the pedicle amputation site chosen by surgeon was in front of the pedicles, that is the junction of the pedicles and vertebral bone, not at the back of the pedicles, namely the junction of the pedicles and vertebral plate. According to the "three column theory", the anterior and center column of the spine bear 80% of the entire load. The posterior disarticulation site is located at the junction of pedicle and the laminar. Posterior column bears smaller load than center and anterior column. Procedure at the posterior site can preserve the integrity of anterior and center column which can bear more load. In addition, an important factor in determining the stability of pedicle screw placement is the nail-bone contact area and the ratio of nail-cortical bone contact area to total area. Increasing cortical bone-screw contact area, in theory, can enhance the biggest anti-screw pull-out strength. It can be thought that the anterior disarticulation amputation may have more mechanical stability than posterior disarticulation amputation. For LSS patients with osteoporosis, posterior amputation can improve pedicle screw stability thus avoid failed surgery which caused by screw loosening, So the posterior amputation is suitable for the LSS patients with osteoporosis.In this procedure, the net increase in the volume of spinal canal is equal to the product of the area of disarticulation coronary and the length of the extension. In a certain extended length, the bigger the disarticulation coronary area, the greater the net increase of vertebral canal volume. Because of the special shape and position of the pedicle, conducted in different parts of the pedicle, disarticulation coronary area is different, the increase in canal volume and the effect of lumbar stability are also different. Thus, in the conduct of the pedicle transection, we should choose a site which lead to a larger disarticulation coronary area with more relatively safe result and small mechanical stability on spine. Therefore, this study analyze the impact on different parts of the spinal canal volume from the point of disarticulation coronary area, and using the finite element method to analyze and explore the influence of stability of the spine at different parts of the amputation in order to provide biomechanical basis for clinical practice.Objective1. To simulate the pedicle lengthening surgery and measure the volume of spinal canal after pedicle lengthening at different parts of lumbar pedicle with digital technology;2. To analyze the stress distribution of pedicle extend fixing device and the impact on the biomechanical stability of the lumbar spine at different parts of lumbar pedicle using finite element analysis3. To optimize and improve the pedicle extend fixing device which have excellent design structure, good mechanical properties, convenient operation extended clinical pedicle screws and supporting devices.MethodsMeasurment of canal volume1.1 Data SourcesInformed consent by the patient, select the 50 cases of normal lumbar CT data, the thickness of 0.625mm, male 23, female 27, aged 21 years to 76 years, with an average age of 34.98 years. All participants had no scoliosis, vertebral deformities and other diseases.The obtained CT data were outputed in DICOM format and imported into software Mimics 14.0, using threshold segmentation, region growth, mask editor, Boolean operations on the image segmentation, reconstructing the 3D digital models of the L4-L5 lumbar vertebra and lumbar spinal canal between L4 and L5.1.2Establish the axis of pediclesFirstly, select the target three-dimensional model of the pedicle and the centerline aids, using the appropriate number of iterations (in this study was five times) to establish the pedicle axis.1.3Define the measurement parametersAnterior osteotomy plane of pedicle,_AP:a plane perpendicular to the pedicle axis and including a point of intersection of a line included two convex side above the superior bony vertebral endplate edge and the inner side of pedicle.Posterior osteotomy plane of pedicle._PP:a plane perpendicular to the pedicle axis and including a point of intersection of a line at the leading edge of the bilateral facet near the pedicle and the inner side of pedicle.Anterior disarticulation coronary area._AA:the area of an enclosed curve was traced along the upper edge of the bilateral pedicle to the medial wall of the pedicle to the lower edge of the bilateral pedicle to the medial wall of the contralateral pedicle after osteotomy at the APPosterior disarticulation coronary area,_PA:the area of an enclosed curve was traced along the upper edge of the bilateral pedicle to the medial wall of the pedicle to the lower edge of the bilateral pedicle to the medial wall of the contralateral pedicle after osteotomy at the PPDistance between the anterior and posterior osteotomy plane,_APD:distance between the AP and PP1.4 Surgical simulationCut off the bilateral pedicles in the above-defined positions of the pedicles using "cut with Polyplane" in Mimics 14.0, the height of the cutting plane was defined as 0.5mm to simulate intraoperative bone defects caused by osteotomy. The severed lamina was moved backward predetermined distance (in this study extended 4 mm) at the plane including the bilateral pedicle axis, then observed the prolonged state.1.5 Measurement of the coronary areaModel of three-dimensional reconstruction computer-generated of the spinal canal was cut, separated in software Mimics 14.0, then exported as STL file format and then imported Geomagic studio 2013. Using the tool "compute area" of software Geomagic studio 2013, the area of AA and PA were measured and defined as the area of the spinal canal for the L4-L 5 level. The distance of AP and PP was also measured。1.6 Statistical analysisThe measurement data were statistically analyzed using the SPSS20.0, between the mean were analyzed by paired t test, between men and women, compared with two samples t test.2.The finite element simulation analysis2.1 Materials and methods2.1 MaterialsA healthy volunteer (female, age 28) included in the above 50 participants underwented lumbar lateral X-ray, CT and MRI to exclude disease and damage. CT scan data was acquired from GE MEDICAL SYSTEMS Lightspeed Systems (GE, USA). Scan parameters:120 kV,125 mA, slice thickness 0.625 mm, range L2-L5. During scanning process, asked the volunteer in the center of the scan field of view of the lumbar spine, and keeping the longitudinal axis of lumbar spine unchanged. After the acquisition was completed, saved the data in a standard Dicom format.2.2 Modeling environmentWindows 7 X64 Professional Edition (Micro-Soft Inc., USA), Mimics 14.0 (Materialise company, Belgium), Geomagic Studio 2013 (3D Systems Inc., USA), ANSYS 14.0 (ANSYS Inc., USA).2.3 Construction of L2-L5 lumbar segments finite element modelCT images of Dicom format were imported into the Mimics 14.0, then the three-dimensional reconstruction of the lumbar vertebrae model was finished after positioning the picture, thresholding, region growing etc. Next, the 3D model was imported into Geomagic studio2013 to repair and optimize in STL format, and imported into ANSYS 14.0 for pre-processing in igs format:separating cortical bone and cancellous bone, constructing three-dimensional model of the intervertebral disc, endplate cartilage, annulus, nucleus, articular cartilage reference to the anatomy reflected by CT and MRI raw data. Added ligament, mesh finite element, established a complete L2-L5 finite element model of spinal segments. Various structural materials parameters of the model see references.2.4 Build three-dimensional model of the screwConstruction of pedicle screw model in the software of ANSYS 14, referring to the parameters of pedicle screws commonly used in clinic, the screw length 50mm, diameter 6.5mm. Material parameters of the screw was defined for the titanium alloy, Young’s modulus 110GPa, Poisson’s ratio is 0.3.2.5 Boundary conditions, load settings and model validationAccording to Yamamoto et lumbar spine specimens experiment, fully constrained degrees of freedom of L5 vertebra and the lower surface of the facet surfaces, exerted 400N axial pressure on the model of the L2 vertebral body surface and loaded 10 N · m pure moment of couple to simulate lumbar load and lumbar flexion, extension, left rotation, right rotation, and lateral bending. Then the model is solved, and obtained the segmental range of motion (ROM) in the aftertreatment, and compared with Yamamoto and Liu. After the model validation of the complete group, cut off the bilateral pedicle at the site of AP, then prolonged backward 4.0mm, while adjusting position of adjacent segments of the vertebral body, to simulate the real three-dimensional positional relationship between each vertebra after real extended, built the model of AP (anterior osteotomy of pedicle, AP group). Construction of PP in the same way. Finally, two pedicles screws were placed along the axial direction of the pedicle. Both models were solved respectively, and got the stress nephogram and lumbar intervertebral ROM.RESULTIn the L4 lumbar segment, PA is 36.70 mm2 more than AA, the difference was statistically significant (P<0.01). In the L5 lumbar segment, AA is 11.72 mm2 more than PA, the difference was statistically significant (P<0.01). Difference between men and women was not statistically significant (P> 0.05). No statistically significant difference between the L4 APD and the L5 APD (P>0.05)Surgery simulation in Mimics14.0 can vividly simulated pedicle-lengthening osteotomy, and show the effect picture after transection and prolonging.1.1 Verification of finite element model of the normal lumbar spineEstablishment of normal L2-L5 nonlinear finite element model, this model simulated the three dimensional structure of cortical bone, cancellous bone, intervertebral disc annulus, nucleus pulposus, endplate cartilage, articular cartilage and related ligament, a total of 421980 elements,122246 nodes. Compared the intervertebral ROM in flexion, extension, left rotation, right rotation, lateral bending of intact group with experimental results of Yamamoto etc. and Liu etc. is in good agreement. It viewed that the validity of the model from a quantitative point.1.2 The maximum stress, the stress contours and intervertebral ROM of experimental groupThe AP group and PP group model were assembled by experimental scheme respectively, Establish finite element model of pedicle screws, mesh the model. AP group totaled 399,549 units,110 716 nodes; PP group totaled 395,061 units,111,318 nodes. Compared with the intact group, L4 pedicle amputation site of AP group and PP group appeared stress concentration, Group AP’s Von Mises maximum stress value was 64.49Mpa, PP group’s Von Mises the maximum stress value was 45.03Mpa, which PP group distributed more evenly than the AP group. Compared with the intact group, in the condition of flexion, left rotation, right rotation, disconnection group’s Von Mises the maximum stress value increased by an average of 24.17Mpa, in the extension, left lateral bending, right lateral bending condition, disconnection group’s Von Mises the maximum stress value increased by an average of 4.51Mp.Intervertebral ROM comparison:Compared with the intact group, in flexion, extension, left lateral bending, right lateral bending condition, L4～L5 intervertebral ROM of AP group and PP group increased, the adjacent L3～L4 intervertebral ROM decreased, L2～L3 intervertebral ROM was close to intact group (Figure 5a-c) Under rotation conditions, L4～L5, L3～L4 intervertebral ROM of AP group and PP group decreased, L2～L3 intervertebral ROM increased. AP group compared with the PP group, in addition to the rotating condition, L4～L5 intervertebral ROM of AP group were greater than the PP group, L3～L4 intervertebral ROM of AP group were less than the PP group, L2～L3 intervertebral ROM of AP group closed to PP group. Under rotating conditions, L4～L5, L3-L4 intervertebral ROM of AP group were less than the PP group, L2～L3 intervertebral ROM of AP group were greater than the PP group.Conclusion1. In the L4 level, the net increase of canal volume after the pedicle-lengthening osteotomy at posterior osteotomy site of pedicle (PS) is superior to the osteotomy at anterior osteotomy site of pedicle(AS).2. The result of FEA showed that:as a new alternative method, posterior osteotomy of pedicle (PP) provides better stability and lower influence on adjacent segment than anterior osteotomy of pedicle (AP)3. An improved pedicle extend fixing device and its supporting device were designed independently and received patent licenses. |
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