Unilateral versus bilateral percutaneous pedicle screw ... · umbar fusion is a common surgical procedure for the treatment of symptomatic spinal pathologies, such as degenerative

Neurosurg Focus / Volume 35 / August 2013

Neurosurg Focus 35 (2):E11, 2013

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©AANS, 2013

Lumbar fusion is a common surgical procedure for the treatment of symptomatic spinal pathologies, such as degenerative spinal disease, trauma, spon-

dylolisthesis, and deformity.15,22 Among various spinal fusion techniques, TLIF has become a popular and well-established technique since its description by Harms et al.10 Transforaminal lumbar interbody fusion can avoid injury of anterior abdominal structures such as the ves-sel, sympathetic nerve, and retroperitoneal and perito-neal structures.16,17,20 In addition, TLIF does not require

retraction of the dura or nerve roots, eliminates epidural scarring, and reduces intraoperative bleeding.2,7,14,15 Re-cently, advances in MIS have allowed TLIF to reduce the complications associated with the open technique.18–20 When compared with the open procedure, MIS TLIF with a tubular retractor appears to achieve similar fusion rates while reducing blood loss, reducing soft tissue and muscle trauma, causing smaller wounds, increasing the speed of recovery, and reducing postoperative pain.18–20 However, TLIF requires a complete facetectomy, so iat-rogenic instability can occur and additional posterior screw fixation is essential.2,7,14,15 Generally, bilateral pedi-cle screw fixation after MIS TLIF is accepted as standard procedure and provides rigid fixation and biomechanical and clinical advantages.23,27,28

Unilateral versus bilateral percutaneous pedicle screw fixation in minimally invasive transforaminal lumbar interbody fusion

Un Yong Choi, M.D., Jeong Yoon Park, M.D., Ph.D., kYUng hYUn kiM, M.D., SUng Uk kUh, M.D., Ph.D., Dong kYU Chin, M.D., Ph.D., keUn SU kiM, M.D., Ph.D., anD Yong eUn Cho, M.D., Ph.D.Department of Neurosurgery, Gangnam Severance Hospital, Spine and Spinal Cord Institute, Yonsei University College of Medicine, Seoul, South Korea

Object. Clinical results for unilateral pedicle screw fixation after lumbar interbody fusion have been reported to be as good as those for bilateral instrumentation. However, no studies have directly compared unilateral and bilateral percutaneous pedicle screw fixation after minimally invasive surgery (MIS) for transforaminal lumbar interbody fu-sion (TLIF). The purpose of this study was to determine whether unilateral percutaneous pedicle screw fixation is comparable with bilateral percutaneous pedicle screw fixation in 1-segment MIS TLIF.

Methods. This was a prospective randomized study of 53 patients who underwent unilateral or bilateral percu-taneous pedicle screw fixation after MIS TLIF for 1-segment lumbar degenerative disc disease. Twenty-six patients were assigned to a unilateral percutaneous pedicle screw fixation group and 27 patients were assigned to a bilateral percutaneous pedicle screw fixation group. Operative time, blood loss, clinical outcomes (that is, Oswestry Disability Index [ODI] and visual analog scale [VAS] scores), complication rates, and fusion rates were assessed using CT scan-ning 2 years after surgical treatment.

Results. The 2 groups were similar in age, sex, preoperative diagnosis, and operated level, and they did not differ significantly in the length of follow-up (27.5 [Group 1] vs 28.9 [Group 2] months) or clinical results. Both groups showed substantial improvements in VAS and ODI scores 2 years after surgical treatment. The groups dif-fered significantly in operative time (unilateral 84.2 minutes; bilateral 137.6 minutes), blood loss (unilateral 92.7 ml; bilateral, 232.0 ml), fusion rate (unilateral 84.6%; bilateral 96.3%), and postoperative scoliotic change (unilateral 23.1%; bilateral 3.7%).

Conclusions. Unilateral and bilateral screw fixation after MIS TLIF produced similar clinical results. Although perioperative results were better with unilateral screw fixation, the long-term results were better with bilateral screw fixation, suggesting bilateral screw fixation is a better choice after MIS TLIF.(http://thejns.org/doi/abs/10.3171/2013.2.FOCUS12398)

keY WorDS • minimally invasive • degenerative disc • transforaminal lumbar interbody fusion • pedicle screw

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Abbreviations used in this paper: AP = anteroposterior; MIS = minimally invasive surgery; ODI = Oswestry Disability Index; TLIF = transforaminal lumbar interbody fusion; VAS = visual analog scale.

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U. Y. Choi et al.

2 Neurosurg Focus / Volume 35 / August 2013

Pedicle screw fixation after interbody fusion has tra-ditionally been performed bilaterally, but some authors have recently showed that unilateral pedicle screw fixa-tion is as effective for spinal fusion as bilateral pedicle screw fixation and that it has a lower operating time and a shorter length of hospital stay.8,12 Previous studies per-forming TLIF have also showed good and similar clinical outcomes and fusion rates between unilateral and bilat-eral pedicle screw after TLIF.8,26,27

However, all previous studies have used a conven-tional pedicle screw system or mini-open TLIF instead of a percutaneous pedicle screw and MIS TLIF with a tubular retractor.8,26,27 The purpose of this prospective randomized study was to compare the clinical outcomes, perioperative results, and radiological results of unilater-al and bilateral percutaneous pedicle screw fixation after MIS TLIF 2 years after surgical treatment. This is the first prospective comparative study of unilateral and bi-lateral percutaneous pedicle screw fixation after MIS TLIF with a tubular retractor.

MethodsWe enrolled 54 consecutive patients who underwent

MIS TLIF with a tubular retractor between January 2008 and January 2010. Patients were randomly divided into 2 groups: Group 1 underwent unilateral percutaneous pedicle screw fixation (n = 26), and Group 2 underwent bilateral percutaneous pedicle screw fixation (n = 28). All 26 patients in Group 1 were included in the analysis and only 27 patients in Group 2 were included in the analy-sis because 1 patient was lost to follow-up. Indications for surgical treatment were far-lateral disc herniation, spinal stenosis, degenerative spondylolisthesis, spondy-lolytic spondylolisthesis, and recurrent disc herniation confirmed by CT and MRI. In all patients, nonoperative treatment of at least 6 months’ duration before surgery had failed, and all patients were operated on by a single surgeon (J.Y.P.). This project was approved by the Institu-tional Review Board of Gangnam Severance Hospital at Yonsei University College of Medicine, and patient con-sent to participate in the randomized trial was obtained.

Operative TechniquesThe MIS TLIF procedure14 was performed on the

side that was more symptomatic. The operation was per-formed under general anesthesia with a C-arm image in-tensifier for the entry point. C-arm guidance was used to determine the operative level and to mark the line in the fluoroscopic AP view (Fig. 1 left) and lateral view (Fig. 1 right) for insertion of the tubular retractor. After verti-cal skin incision (length 25 mm), a tubular retractor (di-ameter 22 mm; MetRx, Medtronic Sofamor Danek) was introduced under fluoroscopic guidance to the facet. Mo-nopolar cautery and pituitary forceps were used to expose the facet complex, and total facetectomy was performed with a high-speed drill and osteotome. After complete facetectomy, the ligamentum flavum was removed to ex-pose the lateral border of the ipsilateral nerve root. For decompression of the contralateral side, the tubular re-tractor was angled medially and the patient was tilted

laterally. Extensive decompression was then performed, including decompression of central stenosis and the con-tralateral side.

Discectomy was performed and a single long poly-etheretherketone, or PEEK, interbody bullet-shaped cage (Capstone, Medtronic Sofamor Danek) filled with only autologous local bone was inserted. Posterolateral fusion was not done because of the small surgical field of the tubular retractor (diameter 22 mm). After interbody fu-sion, the tubular retractor was removed, and an ipsilateral percutaneous pedicle screw system (Sextant, Medtronic Sofamor Danek) was inserted through the same trajectory for Group 1 (Fig. 2). For Group 2, a contralateral percuta-neous pedicle screw system was also placed through the mirror incision under fluoroscopic guidance (Fig. 3).

Perioperative, Clinical, and Radiological AssessmentsPerioperative outcomes included the operative time

and blood loss during the operation. Clinical outcomes were assessed with the ODI and VAS before surgery and at 7 days; 1, 3, and 7 months; and 1 and 2 years after sur-gical treatment. Patients underwent MRI, CT scanning, and radiography before surgery, as well as serial radiog-raphy at 1 month, 7 months, 1 year, and 2 years after sur-gical treatment. Radiological outcomes regarding fusion were determined by an independent neurosurgeon and an independent neuroradiologist, who were blinded to the treatment details. Fusion rates were assessed with the Bridwell grading system, and CT and radiographic find-ings were assessed 2 years after surgical treatment (Table 1). The Bridwell system is composed of the following cat-egories and grades: fused with remodeling and trabeculae present (Grade I); graft intact, not fully remodeled and incorporated, but no lucency present (Grade II); graft in-tact, potential lucency present at top and bottom of graft (Grade III); and fusion absent with collapse/resorption of the graft (Grade IV).3

Fig. 1. Left: Under fluoroscopic guidance in the AP view, a mark was made on the skin at the disc space and the lateral pedicle line. A verti-cal skin incision was made at the disc space at 15 mm cranially (black arrow) and 10 mm caudally (white arrow). Right: Lateral fluoroscopic image used to confirm the lateral disc space.



Unilateral versus bilateral screws in MIS TLIF

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All analyses were performed with SPSS version 12.0 (SPSS, Inc.). Demographic data and radiological results were assessed with Fisher exact tests and McNemar tests. Clinical and perioperative results were compared between groups with Mann-Whitney U-tests, and ANOVA was per-formed within each group to compare the results before and after treatment. Correlation test with all parameters was used to determine correlation factors with postopera-tive scoliotic change. Statistical significance was set at p < 0.05.

ResultsThe mean follow-up period was 28.20 months, and

patients’ mean age was 54.83 years (Table 1). There were no significant differences between groups in mean age

(53.39 years [Group 1] and 56.22 years [Group 2]), mean follow-up period (27.52 months [Group 1] and 28.85 months [Group 2]), or sex ratio (male/female ratio 12:14 [Group 1] and 9:18 [Group 2]) (Table 1). The operative segments also did not differ significantly between groups (L3–4: 0 patients [Group 1] and 2 patients [Group 2]; L4–5: 20 patients [Group 1] and 18 patients [Group 2]; and L5–S1: 6 patients [Group 1] and 7 patients [Group 2]).

According to perioperative assessments, the opera-tive time was significantly shorter for Group 1 (84.23 minutes) than for Group 2 (137.59 minutes; p < 0.01), and the mean blood loss was lower for Group 1 (92.69 ml) than for Group 2 (232.04 ml, p < 0.01; Table 1). Regarding clinical outcomes, the ODI score significantly improved

Fig. 2. Group 1: MIS TLIF with a tubular retractor and unilateral per-cutaneous pedicle screw fixation. Upper: Radiographs. Lower: Photograph showing an ipsilateral incision for the tubular retractor and an ipsilateral percutaneous pedicle screw system, and an upper incision for rod insertion.

Fig. 3. Group 2: MIS TLIF with a tubular retractor and bilateral per-cutaneous pedicle screw fixation. Upper: Radiographs. Lower: Photograph showing an ipsilateral wound for the tubular retractor and the ipsilateral percutaneous pedicle screw system, the mirror wound for the contralateral percutaneous pedicle screw system, and an upper wound for rod insertion.


U. Y. Choi et al.


at 1 year after operation in both groups (from 27.8 to 6.6 for Group 1, and from 27.9 to 9.5 for Group 2, p < 0.05). Each group had improved significantly at 1 year and 2 years after surgery compared with the preoperative state, but there were no differences between groups (Fig. 4). The mean VAS score for back pain significantly improved at 2 years after operation (from 7.6 to 1.8 for Group 1, and from 7.7 to 1.8 for Group 2, p < 0.01), and the mean VAS score for leg pain also improved significantly (from 7.5 to 1.7 for Group 1, and from 7.5 to 1.8 for Group 2, p < 0.01), but there were no significant differences between groups.

Radiological outcomes were assessed according to the Bridwell grading system. Fusion grades in Group 1 were Grade I in 65.4% (n = 17), Grade II in 19.2% (n = 5), Grade III in 3.8% (n = 1), and Grade IV in 11.5% (n = 3) of patients; in Group 2, fusion grades were Grade I in 92.6% (n = 25), Grade II in 3.7% (n = 1), Grade III in 3.7% (n = 1), and Grade IV in 0% of patients (Fig. 5). Since fusion is defined as Grade I or II, Group 1 had a fusion rate of 84.6% (n = 22) and Group 2 had a fusion rate of 96.3% (n = 26). Fusion rates were significantly different between groups (p < 0.01; Table 1). To evaluate scolio-sis, we measured the Cobb angle between the L-1 upper endplate to the L-5 lower endplate and defined scoliotic change as more than a 5° change in Cobb angle from pre-operative state (Fig. 6). In Group 1, scoliotic change oc-

curred in 23.1% (n = 6), and the mean Cobb angle in these 6 patients changed from 3.2° to 9.8°; in Group 2, scoliotic change occurred in 3.7% (n = 1), and the Cobb angle in this patient changed from 9° to 15°. The change in Cobb angle was not reported for the remaining 46 patients be-cause change was less than 5°. There was a significant difference between groups only in scoliotic change rate (p < 0.05; Table 1). Postoperative scoliotic change only occurred in the lumbar spine, and all changes showed convexity on operation side (Fig. 6).

Among all 53 patients, there were 3 cases of com-plications associated with the operation and 2 cases of revision surgery (Table 1). In Group 1, one patient expe-rienced migration of the interbody cage toward the spi-nal canal, so the cage was removed and a new cage was inserted again with the conventional technique for bilat-eral pedicle screw fixation (Fig. 7). For another patient in Group 1, the cage moved only slightly toward the spinal canal, so reoperation was not required. In Group 2, one patient underwent revision surgery due to upper segment disc herniation.

DiscussionThe choice between unilateral or bilateral pedicle

screw fixation after lumbar fusion remains controversial.

TABLE 1: Comparison of demographic variables and operative results between unilateral and bilateral screw fixation groups*

Group (no. of patients) Mean Age (yrs)

Mean Follow-Up (mos)

Sex (M/F)

Mean Op Time (mins)

Mean Blood Loss (ml) Fusion (%)

Postop Scoliosis (%)

Revision (%)

total (53) 54.83 ± 13.42 28.20 ± 3.90 21:32 111.41 ± 45.85 163.68 ± 187.38 48 (90.56) 7 (13.20) 2 (3.77)unilat screw (26) 53.39 ± 14.31 27.52 ± 3.30 12:14 84.23 ± 41.68† 92.69 ± 119.95† 22 (84.61)‡ 6 (23.07)§ 1 (3.84)bilat screw (27) 56.22 ± 12.62 28.85 ± 4.37 9:18 137.59 ± 32.92† 232.04 ± 215.63† 26 (96.29)‡ 1 (3.70)§ 1 (3.70)

* Mean values are presented ± SD. † Between groups comparisons with Mann-Whitney test, p < 0.01. ‡ Between group comparisons with Fisher exact test or McNemar test, p < 0.01.§ Between group comparisons with Fisher exact test or McNemar test, p < 0.05.

Fig. 4. The ODI scores for Group 1 (unilateral screw) and Group 2 (bilateral screw). m = month; y = year.




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After Goel et al.9 first reported the benefits of unilateral pedicle screw fixation, several clinical trials have found that unilateral pedicle screw fixation is as effective as bilateral pedicle screw fixation in lumbar spinal fusion.4,

11,25 In contrast to posterolateral fusion, TLIF requires unilateral total facetectomy, so iatrogenic instability is a possibility and additional pedicle screw fixation is essen-tial.14,21,24 Previous biomechanical studies have reported that unilateral pedicle screw fixation is inferior to bilat-

eral pedicle screw fixation, and they have recommended the use of bilateral pedicle screw fixation or unilateral pedicle screw fixation with a contralateral facet screw af-ter TLIF.12,21,24,28 Despite data suggesting the inferiority of unilateral pedicle screw fixation after TLIF in biome-chanical studies, many clinical trials have reported good results of unilateral pedicle screw fixation after TLIF.8,26,27

The main function of the pedicle screw is to stabilize the spine to promote fusion, so the fusion rate is the most important outcome to consider. Although Deutsch et al. and Tuttle et al. reported good results of unilateral pedicle screw fixation after TLIF, they only assessed the results during a short-term follow-up (less than 1 year), so they did not report the exact fusion rate.8,26 Suk et al. found a lower fusion rate for unilateral pedicle screw fixation than bilateral pedicle screw fixation (91.5% vs 97.5%, respec-tively) after posterolateral fusion.25 Xue et al. also report-ed a lower fusion rate for unilateral pedicle screw fixation (91.9% vs 93.0%), although this difference was not sta-tistically significant.27 Aoki et al. reported the possibil-ity of cage migration after unilateral pedicle screw fixa-tion with TLIF.1 However, all of these studies assessed a conventional pedicle screw system with mini-open TLIF instead of a percutaneous pedicle screw and MIS TLIF with a tubular retractor.8,26,27 To our knowledge, our study is the first prospective comparison of unilateral and bilat-eral percutaneous pedicle screw fixation after MIS TLIF with a tubular retractor. The purpose of this prospective randomized study was to compare the clinical outcomes, perioperative results, and radiological results 2 years after surgical treatment.

We found that unilateral percutaneous pedicle screw fixation after MIS TLIF led to less blood loss and a shorter operative time than bilateral percutaneous pedicle screw fixation. Consistent with previous studies,8,26,27 the 2 tech-niques led to similar clinical outcomes. However, we also

Fig. 5. Fusion grade as determined using the Bridwell grading sys-tem. Left: Bridwell Grade I (fusion with remodeling and trabeculae) was defined as fusion. Right: Bridwell Grade IV (fusion absent, with collapse or resorption of graft) was defined as nonfusion.

Fig. 6. Radiographs and CT image of the right side of L4–5 after MIS TILF with unilateral percutaneous screw fixation (Group 1). A: AP view of the lumbar spine before surgery. B: AP view 7 months after surgery. C: AP view 2 years after surgery. To evaluate the scoliosis, we measured the Cobb angle (white lines) between the L-1 upper endplate and the L-5 lower endplate and defined scoliotic change as more than a 5° change in Cobb angle from preoperative state. The Cobb angle in this patient changed from 0° (A) to 10° (C). D: Axial CT scan demonstrating a single oblique long cage located on the contralateral side of the pedicle screw and the left side of the disc space. The black line indicates the midline of the disc.


U. Y. Choi et al.


found that unilateral percutaneous pedicle screw fixation was associated with a lower fusion rate than bilateral per-cutaneous pedicle screw fixation (unilateral 82.6%, bilat-eral 95.7%). Our fusion rate for unilateral pedicle screw fixation is lower than that reported by Xue et al. (90.6% vs 91.9%, respectively).27 However, they performed mini-open TLIF with a cage located in the anterior disc space; we used a single oblique cage because the cage used by Xue et al. is not appropriate for MIS TLIF with a tubu-lar retractor (Fig. 6D).14,27 This difference in cage location and shape may explain the differences between studies in fusion rate.

From the serial radiographic follow-up, we found that postoperative scoliotic change was more common after unilateral screw fixation than bilateral screw fixation (uni-lateral 6 cases, bilateral 1 case). Generally, scoliosis is de-fined as a Cobb angle more than 10°, but we observed less than 10° in almost all patients during follow-up; this is not real scoliosis, and these patients did not have any clinical symptoms associated with scoliotic change. Because this change is not real scoliosis and this result may be only a radiological finding, we defined scoliotic change as more than a 5° change in Cobb angle from preoperative state (Fig. 6). In these cases, the cage was usually located on the contralateral side of the pedicle screw (Fig. 6D). After pedicle screw insertion, the surgeon usually compressed the pedicle screw to prevent cage migration and to enhance interbody fusion. If the cage is not located in the center of the disc space, compression forces can change the ge-ometry of the spine and can lead to postoperative scoliotic change. Biomechanical studies have shown that unilateral screw fixation leads to a lower fixation strength and re-duced stability, especially in resisting lateral bending and axial rotation, than bilateral screw fixation.24,28 However, these studies were performed with a conventional pedicle screw system instead of a percutaneous pedicle screw. To

determine the exact mechanism of postoperative scoliosis after unilateral percutaneous pedicle screw fixation, we will need to conduct further biomechanical studies of cage location and pedicle screw compression force with the per-cutaneous technique. Nonetheless, it should be noted that the patients with postoperative scoliosis in our study had a similar clinical result and fusion rate as the patients with-out scoliosis.

In our series, postoperative cage migration occurred only in the unilateral percutaneous pedicle screw group. In a previous in vitro biomechanical study, TLIF showed less stability during lateral bending, flexion, and exten-sion than posterior lumbar interbody fusion.13 Recent studies also reported that TLIF with unilateral pedicle screw fixation showed less stability than TLIF with bi-lateral screws, particularly on lateral bending and con-tralateral rotation.1,5,21 Aoki et al. have reported that cage migration occurred frequently in cases involving unilat-eral pedicle screw fixation and a diagonal single cage, and they recommended bilateral screw fixation with a dual cage for a preoperatively highly unstable segment.1 In all cases of our series we used a diagonal bullet-shaped cage with asymmetrical positioning (Fig. 6D) for both unilat-eral and bilateral pedicle screw groups, and cage migra-tion only occurred in the unilateral pedicle screw fixa-tion group. Recent studies have also reported that range of motion and stress increased when TLIF was done with asymmetrical positioning of a diagonal cage plus uni-lateral pedicle screw fixation than with symmetrical po-sitioning of the cage during contralateral axial rotation and lateral bending.4–6 Biomechanical weakness and less stability on TLIF itself, unilateral pedicle screw fixation, and asymmetrical positioning of a cage can be causes of postoperative scoliotic change, cage migration, and lower fusion rate of unilateral screw fixation after MIS TLIF.

Our study has some limitations. The study popula-

Fig. 7. Revision case of MIS TILF with unilateral percutaneous screw fixation (Group 1). Lumbar spine immediately after surgery (A), 1 month after surgery (B), 3 months after surgery (C). The interbody cage migrated to the spinal canal, so the cage was removed and a new metallic cage was inserted with bilateral pedicle screw fixation using the conventional technique (D).




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tion had various heterogeneities in diagnosis, age, sex, follow-up period, and operative segment; the heteroge-neity of study groups can affect the postoperative sco-liotic change. To overcome this weakness, we did the correlation test between all parameters with postopera-tive scoliotic change; no parameters were associated with postoperative scoliotic change except operation methods (Spearman rs = -0.286, p = 0.039). In addition, there was a small number of cases in each group and the follow-up period of 2 years was relatively short. Future studies with longer follow-up and large study populations are needed to determine the clinical significance of scoliotic change after unilateral percutaneous pedicle screw fixation with MIS TLIF.

ConclusionsUnilateral and bilateral percutaneous screw fixation

after MIS TLIF produced similar clinical results. Lower fusion rates with unilateral screw fixation rather than bilateral screw fixation were not associated with better clinical outcomes. Although perioperative results were better with unilateral screw fixation and both groups had similar clinical outcomes, the results for fusion rates and postoperative scoliotic change were better with bilateral screw fixation. If considering long-term results, we sug-gest that bilateral screw fixation is a better choice than unilateral screw fixation after MIS TLIF.

Disclosure

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

Author contributions to the study and manuscript preparation in clude the following. Conception and design: Park. Acquisition of data: Park, Choi, KH Kim, Cho. Analysis and interpretation of data: Park, Choi, KH Kim, Kuh, Chin, KS Kim. Drafting the article: Park, Choi. Critically revising the article: Park, Kuh, KS Kim, Cho. Reviewed submitted version of manuscript: Park, Choi, Kuh, Chin, KS Kim, Cho. Approved the final version of the manuscript on behalf of all authors: Park. Statistical analysis: Park. Study supervi-sion: KH Kim.

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Manuscript submitted November 25, 2012.Accepted February 15, 2013.Please include this information when citing this paper: DOI:

10.3171/2013.2.FOCUS12398. Address correspondence to: Jeong Yoon Park, M.D., Ph.D., De -

partment of Neurosurgery, Gangnam Severance Hospital, Spine and Spinal Cord Institute, Yonsei University College of Medicine, 211 Eonjuro Gangnam-gu, Seoul 135-720, Korea. email: [email protected].


Documents

Unilateral versus bilateral percutaneous pedicle screw ... · umbar fusion is a common surgical procedure for the treatment of symptomatic spinal pathologies, such as degenerative