posterior corrective fusion using a double-trajectory technique (cortical bone trajectory combined...

J Neurosurg: Spine / September 6, 2013

DOI: 10.3171/2013.7.SPINE13191

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©AANS, 2013

Santoni et al. recently reported on the use of a cortical bone trajectory (CBT) as a new insertion trajectory for pedicle screws.12 Mechanical tests have shown

that this trajectory provides greater pullout strength than does the traditional trajectory (TT). In patients with se-vere osteoporosis, however, it may not always be pos-sible to achieve sufficient fixation strength solely with the CBT, and the initial fixation strength already obtained may be reduced by corrective operations. We performed posterior corrective fusion using a double-trajectory tech-nique, combining CBT with TT, in a patient with degen-erative lumbar scoliosis and osteoporosis, with the aim of achieving and maintaining complete correction, and report herein this surgical procedure together with its usefulness and problems.

Approval for this study design and the publication of this manuscript was obtained from Kitasato University Hospital, Kitasato University East Hospital, and Kitasato University School of Medicine.

Illustrative CaseHistory and Examination. A 64-year-old woman

with a history of diabetes (treated with insulin) and hy-pertension (treated with oral medication) had been cared for at our hospital on an outpatient basis for several years due to pain in the lumbar region, both hips, and left leg. She had been treated with drugs and caudal epidural block, but her symptoms gradually intensified to the point where leg pain developed after walking only around 100

Posterior corrective fusion using a double-trajectory technique (cortical bone trajectory combined with traditional trajectory) for degenerative lumbar scoliosis with osteoporosis

Technical note

Masaki Ueno, M.D., Ph.D., TakayUki iMUra, M.D., Gen inoUe, M.D., Ph.D., anD Masashi Takaso, M.D., Ph.D.Department of Orthopaedic Surgery, Kitasato University School of Medicine, Sagamihara, Kanagawa, Japan

The authors report on the usefulness and problems of a new surgical procedure—posterior corrective fusion using a double-trajectory technique (cortical bone trajectory technique combined with traditional trajectory tech-nique) in a patient with degenerative lumbar scoliosis and osteoporosis—with the aim of achieving and maintaining complete correction. A 64-year-old woman with severe osteoporosis required decompression and posterior lumbar fusion. Teriparatide therapy had recently been initiated, but the impairment that she was experiencing in her activi-ties of daily living was severe enough that surgery could not be delayed until teriparatide might show efficacy. We decided to employ the double-trajectory technique described in this report in order to achieve the most solid fixation. As of the 14-month follow-up evaluation, the patient’s postoperative course had been uneventful and there had been no loss of correction. The authors suggest that the double-trajectory method is useful for posterior fusion in patients with severe osteoporosis.(http://thejns.org/doi/abs/10.3171/2013.7.SPINE13191)

key WorDs • cortical bone trajectory • posterior corrective fusion • degenerative lumbar scoliosis • osteoporosis • technique

Abbreviations used in this paper: CBT = cortical bone trajectory; JOA = Japanese Orthopaedic Association; TT = traditional trajec-tory.

This article contains some figures that are displayed in color on line but in black-and-white in the print edition.

M. Ueno et al.

2 J Neurosurg: Spine / September 6, 2013

m, and pain in the left lower back appeared after standing for only around 20 minutes. Thus, she wanted to undergo surgery. However, she was found to have severe osteo-porosis. Weekly administration of teriparatide was initi-ated, but her activities of daily living became impaired to the extent that she was no longer able to stand in the kitchen, and although only 1 month had passed since the initiation of teriparatide therapy, the decision was made to perform surgery. Surgery, comprising L3–4 and 4–5 decompression by L-4 laminectomy, posterior lumbar fu-sion of L1–S, right transforaminal lumbar interbody fu-sion of L3–4, and posterior lumbar interbody fusion of L4–5, was scheduled.

The initial examination revealed no sensory distur-bance, muscle weakness, or abnormal deep tendon re-flexes, although slight numbness was present from the dorsum to the toes of the left foot. There was also no bladder or rectal dysfunction. The Japanese Orthopaedic Association (JOA) score was 17/29. On plain radiographs, the Cobb angle was 19° (L1–4) with left convex degenera-tive scoliosis, and the lumbar lordosis was 37° (L1–S1). The coronal balance was fairly good, but the sagittal bal-ance was poor (Fig. 1). Dynamic imaging showed that the segmental angulation was 12° and the sagittal translation instability was 6 mm at L4–5. Magnetic resonance imag-ing revealed severe stenosis at L4–5 and mild stenosis at L3–4 and other levels (Fig. 2). Dual-energy x-ray ab-sorptiometry performed 2 months preoperatively showed a bone density of 0.423 g/cm2 in the femur (T score: –4.0; Z score: -2.0; Young Adult Mean, 49%) and 0.551 g/cm2 in the lumbar spine (T score: –4.2; Z score: -1.5; Young Adult Mean, 55%). (The Young Adult Mean is a criterion showing the percentage increase or decrease in bone mass relative to values established for young people [20–44 years old] by the Japanese Society for Bone and Mineral Research.)

Based on these findings showing that the patient’s bone density was considerably reduced, we decided to employ the double-trajectory technique to achieve the most solid fixation. Prior to surgery, we examined the po-sition for insertion and the diameter of screws using pre-operative planning software (Spine 4: Medtronic Sofamor Danek) (Fig. 3).

Surgical Procedure. After exposure of the surgical field, an air drill (3-mm diameter, steel) was first used un-der fluoroscopic guidance to penetrate the mediocaudal

Fig. 1. Preoperative radiographs. The Cobb angle was 19° (L1–4) with left convex degenerative scoliosis, and the lumbar lordosis was 37° (L1–S1). Coronal balance: C-7 plumb line–central sacral vertical line (CSVL) = +1.5 cm; sagittal balance: C-7 plumb line = +9.5 cm; and pelvic incidence = 72°.

Fig. 2. Preoperative T2-weighted MR images. a: Sagittal plane. b: Axial plane at L3–4 level. c: Axial plane at L4–5 level. The imaging study revealed severe stenosis at L4–5, and mild stenosis at L3–4 and other levels.


Double-trajectory technique for osteoporotic vertebrae

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side of the pedicle at the entry point for insertion of the CBT screw. The entry point was positioned slightly higher to provide space for the TT screw (Fig. 4). A curved Len-ke probe was then struck with a hammer in the craniolat-eral direction to carve out the screw hole. (We always use a curved Lenke probe when forming CBT screw holes, since a desirable position can be attained by rotating and changing the direction of its end. It is difficult to insert a Lenke probe while rotating it, due to the difficulty of the trajectory. By striking the trajectory with a hammer, the hardness can be better assessed, making it easier to fur-ther proceed with the surgery. If the trajectory is found to be too hard, we avoid proceeding and carve out the hard part using a 3-mm-diameter K-wire and a power tool.)

The same operation was repeated from L-1 to L-5. The entry point in the sacrum on the mediolateral side was set at the same height as that of the TT on a line extending from the CBT proximal lineup, and the screw hole was formed toward the caudal side with a large lateral swing to avoid the neural foramen (Fig. 5). At this point, both CBT and TT screws were inserted into the sacrum. Since L1–5 holes hampered the operation of decompression, processed 1.2-mm K-wires were temporarily inserted into the holes in the bone, and decompression and intervertebral opera-tions were performed. When performing decompression, the entry point of CBT screws should be confirmed, and care is needed to avoid destroying the cortex of the entry point. Cages were inserted bilaterally in L4–5 and on the right side in L3–4, and the vertebrae were moved into their corrected positions. Cortical bone trajectory screws were

then inserted under fluoroscopy. We took the following steps to avoid fracture of the entry point: First, we tapped a hole with a 4.0-mm tap. Next, we expanded the hole with a 4.5-mm tap, which was the planned diameter. Then, we in-serted a 4.5-mm screw into the hole. However, since the left part of the entry point of L-5 was fractured, a CBT screw was not inserted. Subsequently, we inserted TT screws into all pedicles (Fig. 6). The entry point and insertion method for the TT screws were the same as for ordinary ones (Fig. 4). After penetrating the entry point with an awl, we manu-ally carved out the hole with a curved Lenke probe and inserted a screw without using a tap. Table 1 shows the size of the screws used and the insertion torque measured.

Rods with an anterior curvature were placed in the TT screws, and compression was applied at the L4–5 ver-tebral level. Intraoperative fluoroscopy was performed to identify vertebral levels with residual tilting, and cor-rection was performed with left or right compression or distraction as required. Correction was confirmed to be complete under these conditions. Then, we removed the surface of the facet joint and the cortex above the verte-bral arch with an air drill. Bone that had been removed from the spinous process and vertebral arch during de-compression was pulverized and used for autologous bone grafting, and then rods were placed on both the left and right of the CBT screws. The areas treated with bone grafting were just between the rods connected to the CBT screws and the rods connected to the TT screws (Fig. 7). The rods were connected at 5 points to cross-link the TT screws with each other and with the CBT screws. As this procedure enabled solid fixation, hooks and wires were not used. The operating time was 393 minutes, and the total blood loss was 1680 ml.

Fig. 3. Screenshot from surgical planning program. First, the diam-eter of the CBT screw had to be determined. Usually, we use 5.5-mm-diameter screws as the standard for CBT, but in this case, given that 2 screws were to be inserted, we decided to use 4.5-mm-diameter screws. We placed the CBT screws on the screen, examined which screw diameter would fit naturally with the TT, and determined the di-ameter on that basis.

Fig. 4. Diagram showing pedicles and screw entry points. The yellow triangles indicate the entry points we usually use for normal CBT. The red circles show the entry point for double trajectory. The white dia-monds show the entry point for TT. The elevation angle of the CBT for double trajectory is lower than that for the normal CBT, since the entry point of the double trajectory is located above that of the normal CBT. Copyright Masaki Ueno. Published with permission.

M. Ueno et al.


Postoperative Course. Postoperative CT showed that the screws were well positioned (Fig. 8) and radiographs showed that complete correction had been achieved, with improvement of the Cobb angle to 0° and lumbar lordosis to 46° (Fig. 9). On Day 2 after surgery, the drain was re-moved, a Damen corset was fitted, and the patient started rehabilitation with no restriction of movement. Her post-operative course was uneventful, and she was discharged in an ambulatory state on postoperative Day 14. On ex-amination 6 months after surgery, bony fusion between the vertebrae and above the vertebral arches was con-

firmed (Fig. 10). Plain dynamic radiographs obtained 14 months postoperatively showed no clear zone or proximal junctional kyphosis surrounding the pedicle screws or any other problem. No loss of correction was evident; the Cobb angle was 0° and the lumbar lordosis was 46°. The lack of motion between vertebrae was highly suggestive of successful fusion11 (Fig. 11). The patient experienced no impediment to activities of daily living, and her JOA score was good at 27/29.

TABLE 1: Size of screws and insertional torque*

Left RightCBT TT CBT TT

LevelDiameter (mm) × Length (mm)

Insertional Torque (Nm)

Diameter (mm) × Length (mm)






L-1 4.5 × 25 1.4 5.5 × 5 1.0 4.5 × 25 1.0 5.5 × 40 0.9L-2 4.5 × 25 0.9 5.5 × 45 0.8 4.5 × 25 0.8 5.5 × 40 1.0L-3 4.5 × 25 1.0 6.5 × 45 0.9 4.5 × 25 1.2 6.5 × 40 1.2L-4 4.5 × 30 1.8 6.5 × 5 1.6 4.5 × 30 2.0 6.5 × 45 1.4L-5 — — 6.5 × 45 1.5 4.5 × 25 1.6 6.5 × 40 1.5S-1 5.5 × 35 0.7 5.5 × 45 0.8 5.5 × 35 1.0 5.5 × 45 1.0

* The maximum torque value was measured by means of a torque driver 3N (Medtronic Sofamor Danek) after the screw thread was inserted completely in the insertion point. On average, the higher insertion torque values were shown by the CBT screws, indicating higher fixation strength.

Fig. 5. Diagram illustrating placement of screws in sacrum. Based on the CBT entry hole in L-4 or L-5, screws should be inserted at the same height as those of the TT on the same line. Since they are in-serted toward the caudal/lateral side to avoid the neural foramen, the trajectory is the same as that used for sacral ala screws. See Fig. 4 leg-end for definition of shape symbols. Copyright Masaki Ueno. Published with permission.

Fig. 6. Intraoperative radiograph obtained before rod placement. There is still evidence of tilting in this image.



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DiscussionThe patient in this case had mixed-type lumbar spi-

nal canal stenosis with degenerative scoliosis. Although her sagittal balance was preserved on radiographs, the patient was unable to stand for long periods because of unilateral lumbar pain. Since sagittal translation insta-bility was also present, and treatment by decompression alone might have resulted in postoperative instability and increased risk of recurrence,4–6 we decided to perform posterior decompression and fusion for kyphosis correc-tion. Despite being only 64 years of age, however, the pa-tient had severe osteoporosis, and the double-trajectory method was therefore employed to obtain the most solid fixation possible, with the aim of avoiding such postop-erative problems as loss of correction and pseudarthrosis.

Lumbar spinal canal stenosis is caused by a range of different underlying conditions, and many different surgi-cal treatments are available, depending on the condition. Normally, surgery comprises a combination of decom-pression and fixation, and numerous studies have recom-mended that fixation be performed for patients exhibiting degenerative spondylolisthesis, degenerative scoliosis, or instability.1,3,7 To date, however, no consensus has been reached on the indications for fixation. The advantages of instrumentation are that it enables deformity to be cor-rected and this correction to be maintained and provides solid spinal stability. The disadvantages include greater surgical invasiveness, damage to adjacent levels, and the risks associated with implant insertion; these factors should be taken into account when determining the pro-cedure to be performed.

In Japan, the number of elderly people is increasing, and the number of patients with lumbar spinal canal ste-nosis is also increasing. Indications for surgery and fixa-tion are basically regarded as the same, irrespective of pa-tients’ age. However, bone density declines in the elderly, and problems can arise with pedicle screw fixation. Pedicle screw pullout strength is significantly lower in vertebral bodies with lower bone density,2 which may lead to early loosening and the development of pseudarthrosis.14 Some studies have even reported that fixation sufficient to main-tain the corrected position is impossible if the bone mineral density of vertebral bodies is severely reduced.10,13 There-fore, consideration is needed when performing fixation in patients with severe osteoporosis, and in this case, we used the excellent capacity for fixation of the CBT technique.

The CBT is a new entry trajectory for pedicle screws reported in 2009 by Santoni et al., which maximizes the area of contact with cortical bone.12 Mechanical tests have shown that it provides pullout strength 1.3 times that of the TT, and there are high expectations for its use in patients with conditions such as osteoporosis in which bone properties are reduced. However, no previous report has described the combined use of the CBT and TT in the same vertebral body. The CBT offers 2 major advantages. First, it minimizes the amount of soft tissue (muscle) to be exposed, reducing the degree of invasiveness. Second, it provides strong and solid fixation. In the present case, although the procedure might not have been less inva-sive, complete correction of osteoporotic vertebral bodies was achieved with pedicle screws, utilizing the fixation strength of the CBT screws to maintain this corrected position. Connecting the rods on the left and right of the CBT screws also resulted in divergence of the screws, in-creasing the pullout strength compared with that of paral-lel screws. We inserted extra CBT screws into the same pedicles as those with TT screws and connected the rods to make the screws converge, which can be expected to result in a further increase in both pullout strength and grip force on the vertebral bodies (Fig. 12).

When looking at the axial CT images (Fig. 8), it ap-pears that the screws exit the lateral cortex of the pedicle. Although penetration in the pedicle was not intentional, it is likely to increase the fixation strength. However, it may injure the nerve coursing from the level above, and thus penetration of the pedicle is not desirable. Deformation or

Fig. 7. Diagram showing the area in which bone graft was used. The graft was implanted in the decorticated surface of the facet joint and above the vertebral arch. The area for bone grafting is just between the rods connected to the CBT screws and the rods connected to the TT screws. Copyright Masaki Ueno. Published with permission.

M. Ueno et al.


rotation of vertebral bodies sometimes makes it difficult to insert CBT screws exactly into the intended position, even utilizing fluoroscopic images. Fortunately, no symp-toms were observed in our patient, but it is desirable for the end of the screws to enter the vertebral body beyond the root of the pedicle and bite slightly into the lateral cortex of the vertebral body.

Problems with this method include the fact that the placement and connection of both screws is tricky, lead-ing to increased radiation exposure and increased risk of infection due to the large amount of instrumentation and operating time, as well as blood loss. As mentioned ear-

Fig. 9. Postoperative anteroposterior radiographs. The Cobb angle improved to 0° and the lumbar lordosis to 46°.

Fig. 8. Postoperative CT images. The screws were well positioned. Some penetration of the outer cortex in the pedicles is evident, but no symptoms of pressure on the root were observed.

Fig. 10. Sagittal reconstruction of 6-month postoperative CT scan. Bony fusion between the vertebrae and above the vertebral arches was confirmed.



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lier in this paper, the operating time was long and, as a result, the intraoperative blood loss was large. However, we succeeded in obtaining the maximum possible fixa-tion strength and ideal correction through the posterior approach. If the anterior approach is taken for the same purpose as this procedure, with the intention to obtain complete correction and optimal fixation strength, it may require postural change (with associated risks), exten-

sion of the fixation area, or addition of sublaminar wiring or iliac screws, resulting in a prolonged operating time and increased blood loss. These problems may be solved when this technique becomes commonly used, with in-creased speed of operation.

Another major issue is that this technique is very ex-pensive since the amount of instrumentation is twice that used with the traditional technique, and financial resources are limited in Japan’s National Health Insurance. Although the cost effectiveness and benefit of this procedure can-not be measured in terms of money, we believe that the subjective benefits justify the objective costs. As men-tioned above, several possible eventualities in a traditional technique will result in an increase in expense similar to that associated with our technique: if the fixation area is extended, a hydroxyapatite stick, sublaminar wiring, or a sublaminar hook is additionally used, or the number of im-plants is increased by adding iliac screws to achieve the same purpose as this procedure, namely to obtain com-plete correction and optimal fixation strength. Moreover, the same level of correction and fixation strength obtained with this technique are not likely to be achieved by these methods. If the correction or fusion is lost soon after sur-gery, revision surgery will be required, necessitating ad-ditional expenses. This double-trajectory technique may spare costs of further treatment.

Fig. 11. Fourteen-month follow-up radiographs. No loss of correction was evident. Lateral flexion-extension radiographs show a lack of motion between vertebrae, suggesting successful fusion. Coronal balance: C-7 plumb line–CSVL = +1.5 cm; sagittal balance: C-7 plumb line = +2.5 cm; and pelvic incidence = 64°.

Fig. 12. Schema of screw placement. CBT screws and TT screws were placed in the same pedicle in a convergent position. Copyright Masaki Ueno. Published with permission.

M. Ueno et al.


As for the surgical technique, the procedure is not complex, as it simply involves the addition of the CBT screw fixation procedure to the procedure performed with TT screws. Every surgeon who has CBT skills can perform this technique. Although we did not experience the following problems in this patient, there is a possibil-ity of the surgeon being unable to insert TT screws due to interference with screws in the pedicle; fracture of the pedicle may occur; or a screw may go beyond the lower wall of the pedicle and damage the root. We found the most challenging part of the technique to be the deter-mination of the appropriate diameter of screws to be in-serted. In the case of TT, it is reported that it would be better not to exceed the inner cortex diameter or 80% of the outer cortex diameter of a pedicle.8 However, for the CBT, the acceptable or ideal screw diameter is unclear. Considering the reported mean inner pedicle dimension of Japanese people,9 we think that the CBT is applicable until T-11 and 2 screws can be placed two-dimensionally (that is, screw templates can be placed side by side on a ra-diograph), since the transverse diameter of a pedicle is at least 6 mm (sufficiently large to maintain the screws’ lat-eral inclination), with the sagittal diameter being 16 mm or greater (large enough for cross-insertion of 2 screws into the same pedicle for the double-trajectory technique). We believe that the double trajectory is also applicable until T-11. However, the form and dimension of pedicles differ from person to person. Since the pedicle is oval and the screws are placed in 3D with this technique, 2D measurement is not sufficient to determine the positions to place screws or the optimal diameter of the screws. Preoperative 3D planning should be conducted using 3D CT or simulation software to determine the suitable di-ameter of the TT screws in combination with the position of the CBT screws (Fig. 3).

Although results in our case are available only through a 14-month follow-up, fusion was confirmed, with no loss of correction and no problems with instru-mentation. Additional validation with mechanical testing is required, but this double-trajectory method combining CBT and TT appears to represent the most solid method of posterior fusion achievable at this point. Given that the number of patients with severe osteoporosis is likely to increase in the future, it may provide a useful method for treating posterior fusion. Further studies of the long-term postoperative course of patients undergoing this proce-dure are required.

ConclusionsWe performed posterior corrective fusion using a

double-trajectory technique combining CBT with TT in a patient with degenerative lumbar scoliosis and osteoporo-sis. The short-term postoperative course was uneventful, and the double-trajectory method may provide a useful method for posterior fusion in patients with severe osteo-porosis.

Disclosure

The authors report no conflict of interest concerning the mate-rials or methods used in this study or the findings reported in this paper.

Author contributions to the study and manuscript preparation include the following. Conception and design: Ueno. Acquisition of data: Ueno, Imura. Drafting the article: Ueno. Critically revising the article: all authors. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Ueno. Administrative/technical/material support: Takaso.

References

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Manuscript submitted February 21, 2013.Accepted July 25, 2013.Please include this information when citing this paper: published

online September 6, 2013; DOI: 10.3171/2013.7.SPINE13191.Address correspondence to: Masaki Ueno, M.D., Ph.D., Depart-

ment of Orthopaedic Surgery, Kitasato University School of Medi-cine, 1-15-1, Kitasato, Minami, Sagamihara, Kanagawa 252-0374. Japan. email: [email protected].

posterior corrective fusion using a double-trajectory technique (cortical bone trajectory combined...

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