Jon Park

Stanford Medicine, Stanford, California, United States

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Publications (28)84.95 Total impact

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    ABSTRACT: Transarticular facet screws restore biomechanical stability to the cervical spine when posterior cervical anatomy has been compromised. This study compares this more-recent, less-invasive and more brief transarticular facet screw system without rods to the lateral mass screw system with rods. Six human cervical spines were obtained from cadavers. Transarticular facet screws without rods were inserted bilaterally into the inferior articular facets at the C5-C6 levels and at the C5-C6-C7 levels. Lateral mass screws with rods were inserted bilaterally at the same levels using Magerl's technique. All specimens underwent range of motion testing by a material testing machine for flexion, extension, lateral bending and axial rotation. Both fixation methods, non-rod transarticular facet screws and lateral mass screws with rods, reduced all ranges of motion and increased spinal stiffness. No statistically significant differences between the two stabilization methods were found in range of motion measurements for one-level insertions. At two-level insertions, however, ROM for the non-rod transarticular facet screw group was significantly increased for flexion-extension and lateral bending. Non-rod transarticular facet screws and lateral mass screws with rods had similar biomechanical stability in single-level insertions. In two-level insertions, non-rod transarticular facet screws are a valid option in cervical spine repair. Copyright © 2014 Elsevier Inc. All rights reserved.
    World Neurosurgery 12/2014; · 2.42 Impact Factor
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    Jon Park, Jun Jae Shin, Jesse Lim
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    ABSTRACT: Objective The objective of this study was designed to compare two-level cervical disc surgery (two-level anterior cervical discectomy and fusion [ACDF] or disc arthroplasty) and hybrid surgery (ACDF/arthroplasty) in terms of postoperative adjacent-level intradiscal pressure (IDP) and facet contact force (FCF). Methods Twenty-four cadaveric cervical spines (C3-T2) were tested in various modes, including extension, flexion, and bilateral axial rotation, to compare adjacent-level IDP and FCF after specified treatments as follows: 1) C5-C6 arthroplasty using ProDisc-C and C6-C7 ACDF, 2) C5¬-C6 ACDF and C6-C7 arthroplasty using ProDisc-C, 3) two-level C5-¬C6/C6-C7 disc arthroplasties, and 4) two-level C5-C6/C6-C7 ACDF. IDPs were recorded at anterior, central, and posterior disc portions. Results Following two-level cervical arthrodesis (ACDF), IDP increased significantly at the anterior annulus of distal adjacent-level disc during flexion and axial rotation and at the center of proximal adjacent-level disc during flexion. On the other hand, following cervical specified treatments including disc arthroplasty (two-level disc arthroplasties and hybrid surgery), IDP decreased significantly at the anterior annulus of distal adjacent-level disc during flexion and extension and unchanged at the center of proximal adjacent-level disc during flexion. Two-level cervical arthrodesis also tended to adversely impact facet loads, raising distal rather than proximal adjacent-level FCF. Conclusion Both hybrid surgery and two-level arthroplasties seem to offer significant advantages over two-level arthrodesis by reducing IDP at adjacent levels and approximating FCF of an intact spine. These findings suggest that cervical arthroplasties and hybrid surgery are an alternative to reduce intradiscal pressure and facet loads at adjacent levels.
    World Neurosurgery 06/2014; · 2.42 Impact Factor
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    ABSTRACT: The temporospatial regulation of axon outgrowth is useful for guiding de novo connectivity or re-connectivity of neurons in neurological injury or disease. Here we report the successful construction of a biocompatible guidance device, in which a linear propagation of IGF-1 gradient sequentially directs axon outgrowth. We observe the extensive in vitro axonal extension over 5 mm with a desired growth rate of ∼1 mm/day.
    Advanced Materials 03/2014; · 15.41 Impact Factor
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    ABSTRACT: Object Several controversial issues arise in the management of unstable hangman's fractures. Some surgeons perform external reduction and immobilize the patient's neck in a halo vest, while others perform surgical reduction and internal fixation. The nonsurgical treatments with rigid collar or halo vest immobilization present problems, including nonunion, pseudarthrosis, skull fracture, and scalp laceration and may also fail to achieve anatomical realignment of the local C2-3 kyphosis. With recent advances in surgical technique and technology, surgical intervention is increasingly performed as the primary treatment in high cervical fractures. The outcomes of such surgeries are often superior to those of conservative treatment. The authors propose that surgical intervention as a primary management for hangman's fracture may avoid risks inherent in conservative management when severe circumferential discoligamentous instability is present and may reduce the risk of catastrophic results at the fracture site. The purposes of this study were to assess fracture healing following expedient reduction and surgical fixation and to propose a guideline for treatment of unstable hangman's fractures. Methods From April 2006 to December 2011, the authors treated 105 patients with high cervical fractures. This study included 23 (21.9%) of these patients (15 men and 8 women; mean age 46.4 years) with Type II, IIa, and III hangman's fractures according to the Levine and Edwards classification. The patient's age, sex, mechanism of injury, associated injuries, neurological status, and complications were ascertained. The authors retrospectively assessed the clinical outcome (Neck Disability Index), radiological findings (disc height, translation, and angulation), and bony healing. Results The average follow-up period was 28.9 months (range 12-63.2 months). The overall average Neck Disability Index score at the time of this study was 6.6 ± 2.3. The average duration of hospitalization was 20.3 days, and fusion was achieved in all cases by 14.8 ± 1.6 weeks after surgery, as demonstrated on dynamic radiographs and cervical 3D CT scans. The mean pretreatment translation was 6.9 ± 3.2 mm, and the mean postoperative translation was 1.6 ± 1.8 mm (mean reduction 5.2 ± 3.1 mm). The initial angulation was 4.7° ± 5.3° and the postoperative angulation was 2.5° ± 1.8° (mean reduction 6.1° ± 5.3°). The preoperative and postoperative values for translation and angulation differed significantly (p < 0.05). The overall C2-3 disc height was 6.7 ± 1.2 mm preoperatively, whereas 3 months after surgery it was 6.4 ± 1.1 mm. These values did not differ significantly (p = 0.0963). Conclusions The authors observed effective reduction and bony healing in cases of unstable hangman's fractures after fixation, and all patients experienced favorable clinical outcomes with neck pain improvement. The protocols allowed for physiological reconstruction of the fractured deformities and avoided external fixation. The authors suggest that posterior reduction and screw fixation should be used as a primary treatment to promote stability of hangman's fracture in the presence of discoligamentous instability or combined fractures.
    Journal of neurosurgery. Spine 09/2013; · 1.61 Impact Factor
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    ABSTRACT: Study Design. An experimental study of radiofrequency ablation of spine in an ex vivo bovine and in vivo swine animal model.Objective. To study the feasibility of radiofrequency ablation for spine tumors close to the spinal cord, to examine the safety and efficacy of radiofrequency ablation, and to suggest quantitative guidelines for clinical application.Summary of Background Data. Radiofrequency ablation has received increased attention as an effective and minimally invasive method for treating soft tissue tumors. However, there is currently only anecdotal evidence to support radiofrequency ablation of spinal tumors and only a few experimental studies have been conducted.Methods. We performed ex vivo experiments by producing 10 radiofrequency ablation zones in extracted bovine spines and an in vivo study by producing eight radiofrequency ablation zones in a swine spine using internally cooled electrodes. The volume and diameter of ablation zones were evaluated and analyzed by the corresponding energy and ablation times.Results. In the ex vivo study, the average diameters of the ablation zones were 3.05 cm, 1.85 cm, and 1.26 cm, for the D1, D2, and D3 zones, respectively, and the average ablation volume was 4.19 cm. In the in vivo study, the average diameters were 2.51 cm, 2.05 cm, and 1.28 cm, respectively, and the ablation volume was 6.80 cm. The ablation zones demonstrated a positive correlation with ablation time, but the coefficients were 0.942 ex vivo and 0.257 in vivo. The temperature in the ex vivo study was inversely proportional to distance, with a maximal temperature of 63.7°C at 10 mm; however, the maximum temperature was 38.2°C in the in vivo study.Conclusion. This study demonstrated that sufficient radiofrequency ablation zone volume could be induced, which suggests that radiofrequency ablation is feasible and safe for application to human spinal tumors with predictability.
    Spine 05/2013; · 2.45 Impact Factor
  • Cureus. 01/2013;
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    Wonjae Lee, Jon Park
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    ABSTRACT: The spatial cell distribution is one of the critical features for governing cellular interactions and their consequent behaviors. Here we suggest a novel method of building a hierarchical cellular structure by stacking cell-attached microplate structures with specific configurations within hydrogel layers. This method is applied to the reconstruction of the 3D architecture of a liver lobule and the development of an experimental model of the various phases of cancer angiogenesis.
    Advanced Materials 08/2012; 24(39):5339-44. · 15.41 Impact Factor
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    ABSTRACT: Closed C2 fractures commonly occur after falls or other trauma in the elderly and are associated with significant morbidity and mortality. Controversy exists as to best treatment practices for these patients. To compare outcomes for elderly patients with closed C2 fractures by treatment modality. We retrospectively reviewed 28 surgically and 28 nonsurgically treated cases of closed C2 fractures without spinal cord injury in patients aged 65 years of age or older treated at Stanford Hospital between January 2000 and July 2010. Comorbidities, fracture characteristics, and treatment details were recorded; primary outcomes were 30-day mortality and complication rates; secondary outcomes were length of hospital stay and long-term survival. Surgically treated patients tended to have more severe fractures with larger displacement. Charlson comorbidity scores were similar in both groups. Thirty-day mortality was 3.6% in the surgical group and 7.1% in the nonsurgical group, and the 30-day complication rates were 17.9% and 25.0%, respectively; these differences were not statistically significant. Surgical patients had significantly longer lengths of hospital stay than nonsurgical patients (11.8 days vs 4.4 days). Long-term median survival was not significantly different between groups. The 30-day mortality and complication rates in surgically and nonsurgically treated patients were comparable. Elderly patients faced relatively high morbidity and mortality regardless of treatment modality; thus, age alone does not appear to be a contraindication to surgical fixation of C2 fractures.
    Neurosurgery 12/2011; 70(5):1055-9; discussion 1059. · 3.03 Impact Factor
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    ABSTRACT: A biomechanical in vitro study using human cadaveric spine. To compare the biomechanical stability of pedicle screws versus various established posterior atlantoaxial fixations used to manage atlantoaxial instability. Rigid screw fixation of the atlantoaxial complex provides immediate stability and excellent fusion success though has a high risk of neurovascular complications. Some spine surgeons thus insert shorter C2 pedicle or pars/isthmus screws as alternatives to minimize the latter risks. The biomechanical consequences of short pedicle screw fixation remain unclear, however. Seven human cadaveric cervical spines with the occiput attached (C0-C3) had neutral zone (NZ) and range of motion (ROM) evaluated in three modes of loading. Specimens were tested in the following sequence: initially (1) the intact specimens were tested, after destabilization of C1-C2, then the specimens underwent (2) C1 lateral mass and C2 short pedicle screw fixation (PS-S), (3) C1 lateral mass and C2 long pedicle screw fixation (PS-L), (4) C1 lateral mass and C2 intralaminar screw fixation (ILS), (5) Sonntag's modified Gallie fixation (MG) and (6) C1-C2 transarticular screw fixation with posterior wiring (TAS 1 MG). (7) The destabilized spine was also tested. All instrumented groups were significantly stiffer in NZ and ROM than the intact spines, except in lateral bending, which was statistically significantly increased in the TAS 1 MG group. The MG group's NZ and ROM values were statistically significantly weaker than those of the PS-S, PS-L, and the ROM values of the TAS 1 MG groups. The ILS group's NZ values were higher than those of the TAS 1 MG group and for ROM, than that of the PS-S and PS-L groups. In flexion, the NZ and ROM values of the TAS 1 MG group were significantly less than those of the PS-S, PS-L, ILS, and MG groups. In axial rotation, the NZ and ROM values of the MG group were statistically significantly higher than those of the PS-S, PS-L, ISL and TAS 1 MG groups. The TAS 1 MG procedures provided the highest stability. The MG method alone may not be adequate for atlantoaxial arthrodesis, because it does not provide sufficient stability in lateral bending and rotation modes. The C2 pedicle screw and C2 ILS techniques are biomechanically less stable than the TAS 1 MG. In the C1 lateral mass-C2 pedicle screw fixation, the use of a short pedicle screw may be an alternative when other screw fixation techniques are not feasible.
    Spine 03/2011; 36(6):E401-7. · 2.45 Impact Factor
  • Spine 03/2011; 36(5):E301-6. · 2.45 Impact Factor
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    ABSTRACT: Retrospective chart review. To identify predictors of 30-day complications after the surgical treatment of spinal metastasis. Surgical treatment of spinal metastasis is considered palliative with the aim of reducing or delaying neurologic deficit. Postoperative complication rates as high as 39% have been reported in the literature. Complications may impact patient quality of life and increase costs; therefore, an understanding of which preoperative variables best predict 30-day complications will help risk-stratify patients and guide therapeutic decision making and informed consent. We retrospectively reviewed 200 cases of spinal metastasis surgically treated at Stanford Hospital between 1999 and 2009. Multiple logistic regression was performed to determine which preoperative variables were independent predictors of 30-day complications. Sixty-eight patients (34%) experienced one or more complications within 30 days of surgery. The most common complications were respiratory failure, venous thromboembolism, and pneumonia. On multivariate analysis, Charlson Comorbidity Index score was the most significant predictor of 30-day complications. Patients with a Charlson score of two or greater had over five times the odds of a 30-day complication as patients with a score of zero or one. After adjusting for demographic, oncologic, neurologic, operative, and health factors, Charlson score was the most robust predictor of 30-day complications. A Charlson score of two or greater should be considered a surgical risk factor for 30-day complications, and should be used to risk-stratify surgical candidates. If complications are anticipated, medical staff can prepare in advance, for instance, scheduling aggressive ICU care to monitor for and treat complications. Finally, Charlson score should be controlled for in future spinal metastasis outcomes studies and compared to other comorbidity assessment tools.
    Spine 02/2011; 36(19):E1274-80. · 2.45 Impact Factor
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    ABSTRACT: The Goel technique, in which C1-2 intraarticular spacers are used, may be performed to restore stability to a disrupted atlantoaxial complex in conjunction with the Harms technique of placing polyaxial screws and bilateral rods. However, it has yet to be determined biomechanically whether the addition of the C1-2 joint spacers increases the multiaxial rigidity of the fixation construct. The goal of this study was to quantify changes in multiaxial rigidity of the combined Goel-Harms technique with the addition of C1-2 intraarticular spacers. Seven cadaveric cervical spines (occiput-C2) were submitted to nondestructive flexion-extension, lateral bending, and axial rotation tests in a material testing machine spine tester. The authors applied 1.5 Nm at a rate of 0.1 Nm/second and held it constant for 10 seconds. The specimens were loaded 3 times, and data were collected on the third cycle. Testing of the specimens was performed for the following groups: 1) intact (I); 2) with the addition of C-1 lateral mass/C-2 pedicle screws and rod system (I+SR); 3) with C1-2 joint capsule incision, decortication (2 mm on top and bottom of each joint [that is, the C-1 and C-2 surface) and addition of bilateral C1-2 intraarticular spacers at C1-2 junction to the screws and rods (I+SR+C); 4) after removal of the posterior rods and only the bilateral spacers in place (I+C); 5) after removal of spacers and further destabilization with simulated odontoidectomy for a completely destabilized case (D); 6) with addition of posterior rods to the destabilized case (D+SR); and 7) with addition of bilateral C1-2 intraarticular spacers at C1-2 junction to the destabilized case (D+SR+C). The motion of C-1 was measured by a 3D motion tracking system and the motion of C-2 was measured by the rotational sensor of the testing system. The range of motion (ROM) and neutral zone (NZ) across C-1 and C-2 were evaluated. For the intact spine test groups, the addition of screws/rods (I+SR) and screws/rods/cages (I+SR+C) significantly reduced ROM and NZ compared with the intact spine (I) for flexion-extension and axial rotation (p < 0.05) but not lateral bending (p > 0.05). The 2 groups were not significantly different from each other in any bending mode for ROM and NZ, but in the destabilized condition the addition of screws/rods (D+SR) and screws/rods/cages (D+SR+C) significantly reduced ROM and NZ compared with the destabilized spine (D) in all bending modes (p < 0.05). Furthermore, the addition of the C1-2 intraarticular spacers (D+SR+C) significantly reduced ROM (flexion-extension and axial rotation) and NZ (lateral bending) compared with the screws and rods alone (D+SR). Study result indicated that both the Goel and Harms techniques alone and with the addition of the C1-2 intraarticular spacers to the Goel-Harms technique are advantageous for stabilizing the atlantoaxial segment. The Goel technique combined with placement of a screw/rod construct appears to result in additional construct rigidity beyond the screw/rod technique and appears to be more useful in very unstable cases.
    Journal of neurosurgery. Spine 02/2011; 14(5):639-46. · 1.61 Impact Factor
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    ABSTRACT: Biological attempts at disc regeneration are promising; however, disc degeneration is closely related to other predisposing factors such as alteration of disc height, intradiscal pressure, load distribution, and motion. The restoration of the physiological status of the affected spinal segment is thus necessary prior to attempts at disc regeneration. Dynamic stabilization systems now offer the potential of a mechanical approach to intervertebral disc regeneration. The authors used decompression and placement of the BioFlex dynamic stabilization device to treat a young male patient with disc degeneration. This patient underwent follow-up, and he was found to gradually improve both neurologically and radiographically. On MR imaging performed 1 year postoperatively, he had an increase in disc height and disc rehydration. This case and the concept of disc rehydration are presented in this paper.
    Journal of neurosurgery. Spine 11/2010; 13(5):576-80. · 1.61 Impact Factor
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    ABSTRACT: The study design was that of an in vitro human cadaveric biomechanical analysis. The objective of this study was the biomechanical analysis of the range of motion (ROM) of a 2-level intact spine control versus total, then operative- and adjacent-segment ROM after (1) 2-level ProDisc-C placement (PP), (2) anterior cervical discectomy and fusions (ACDFs), and (3) hybrid constructs of both. Follower load and multidirectional testing were performed in each instance. With in vivo cervical arthroplasties gaining in popularity, limited biomechanical data are available, which highlight changes in the adjacent-level biomechanics after multilevel procedures. Biomechanical testing for ROM was performed using 7 cadaveric C4-T1 spine specimens. Moments up to 2 Nm with a 100 N follower load were applied in flexion/extension (F/E), right and left lateral bending (LB), and right and left axial rotation (AR). Specimens were tested in the intact state and then with a combination of ProDisc-C arthroplasty and ACDF at C5-C6 and C6-C7. In the 2-level PP group, the increase in ROM in F/E, LB, and AR of C4-T1 occurs due to an increased ROM at the operative level. The ROM of the level adjacent to the operative levels showed no significant change, except at C4-C5 in LB. For the latter level, the ROM of C4-C5 in each direction showed increases for each parameter. In the 2-level fusion C5-C6 and C6-C7 fusion (FF) group, the ROM in F/E, LB, and AR of C4-T1 was decreased because of a decrease in ROM primarily at the fused levels, and the ROM of adjacent levels was increased. In the ProDisc-C/Fusion (PF) and Fusion/ProDisc-C (FP) groups undergoing placements of a 1-level ProDisc-C/1-level fusion with cage and plate, both groups showed no significant ROM change of C4-T1 when compared with the control and no significant change at adjacent levels, with the exception of C4-C5 in LB. (1) Two-level ACDFs decrease whereas 2-level PPs increase the entire C4-T1 ROM. (2) ACDF/ProDisc-C hybrid operations do not alter the C4-T1 ROM. (3) For the ACDF/ProDisc-C hybrid operative groups, the combined ROM of the operative levels showed no significant difference when compared with that of the intact spine. (4) Regarding adjacent-level ROM, a 2-level ACDF increases ROM, but 2-level ProDisc-C and hybrid ACDF/PPs do not show significant change except for LB at C4-C5. (5) When the segmental distribution of C4-T1 ROM is plotted as the percentage of total motion, it demonstrates that for PF and FP groups, the combined ROM of the C5-C6 and C6-C7 operative levels are similar to that of the intact spine in EF and LB. For the PP group, the combined ROM of the operative levels increased, whereas the combined ROM for the FF group is decreased. The decrease or increase of the adjacent C4-C5 or C7-T1 level ROM compensates for the operative levels.
    Spine 09/2010; 35(19):1769-76. · 2.45 Impact Factor
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    ABSTRACT: Both posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) have been frequently undertaken for lumbar arthrodesis. These procedures use different approaches and cage designs, each of which could affect spine stability, even after the addition of posterior pedicle screw fixation. The objectives of this biomechanical study were to compare PLIF and TLIF, each accompanied by bilateral pedicle screw fixation, with regard to the stability of the fused and adjacent segments. Fourteen human L2-S2 cadaveric spine specimens were tested for 6 different modes of motion: flexion, extension, right and left lateral bending, and right and left axial rotation using a load control protocol (LCP). The LCP for each mode of motion utilized moments up to 8.0 Nm at a rate of 0.5 Nm/second with the application of a constant compression follower preload of 400 N. All 14 specimens were tested in the intact state. The specimens were then divided equally into PLIF and TLIF conditions. In the PLIF Group, a bilateral L4-5 partial facetectomy was followed by discectomy and a single-level fusion procedure. In the TLIF Group, a unilateral L4-5 complete facetectomy was performed (and followed by the discectomy and single-level fusion procedure). In the TLIF Group, the implants were initially positioned inside the disc space posteriorly (TLIF-P) and the specimens were tested; the implants were then positioned anteriorly (TLIF-A) and the specimens were retested. All specimens were evaluated at the reconstructed and adjacent segments for range of motion (ROM) and at the adjacent segments for intradiscal pressure (IDP), and laminar strain. At the reconstructed segment, both the PLIF and the TLIF specimens had significantly lower ROMs compared with those for the intact state (p < 0.05). For lateral bending, the PLIF resulted in a marked decrease in ROM that was statistically significantly greater than that found after TLIF (p < 0.05). In flexion-extension and rotation, the PLIF Group also had less ROM, however, unlike the difference in lateral bending ROM, these differences in ROM values were not statistically significant. Variations in the position of the implants within the disc space were not associated with any significant differences in ROM values (p = 0.43). Analyses of ROM at the adjacent levels L2-3, L3-4, and L5-S1 showed that ROM was increased to some degree in all directions. When compared with that of intact specimens, the ROMs were increased to a statistically significant degree at all adjacent segments in flexion-extension loads (p < 0.05); however, the differences in values among the various operative procedures were not statistically significant. The IDP and facet contact force for the adjacent L3-4 and L5-S1 levels were also increased, but these values were not statistically significantly increased from those for the intact spine (p > 0.05). Regarding stability, PLIF provides a higher immediate stability compared with that of TLIF, especially in lateral bending. Based on our findings, however, PLIF and TLIF, each with posterolateral fusions, have similar biomechanical properties regarding ROM, IDP, and laminar strain at the adjacent segments.
    Journal of neurosurgery. Spine 06/2010; 12(6):700-8. · 1.61 Impact Factor
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    ABSTRACT: Interspinous process implants are becoming more common for the treatment of lumber disc degeneration. The authors undertook this study to evaluate the effect of the In-Space interspinous spacer on the biomechanics of the lumbosacral spine. Seven L2-S1 cadaver spines were physiologically loaded in extension, flexion, lateral bending, and axial rotation modes. The range of motion (ROM) and intervertebral disc pressure (DP) at the level implanted with an In-Space device and at adjacent levels were measured under 4 experimental conditions. Biomechanical testing was carried out on 7 sequentially prepared specimens in the following states: 1) the intact L2-S1 cadaver spine and 2) the L2-S1 cadaver specimen implanted with an In-Space interspinous spacer at L3-4 (Group 1), 3) after an additional L3-4 discectomy (with the In-Space interspinous spacer still in place) (Group 2), and finally, 4) after removal of the In-Space interspinous spacer, leaving only the discectomized (that is, destabilized) spine model (Group 3). The extension ROM at the implanted level after experimental conditions 2 and 3 above was statistically significantly reduced. An increase of ROM at the adjacent levels compensated for the reduction at the implanted level. However, there was no statistically significant change in ROM in any of the other modes of motion at any of the levels studied. Likewise, the DP reduction at L3-4 during extension was statistically significant, but in all other modes of motion, there was no statistically significant change in DP at any measured level. The In-Space interspinous spacer statistically significantly reduced the ROM of the L3-4 motion segment in Groups 1 and 2 (extension: 67%, p = 0.018 and 70%, p = 0.018; flexion: 72%, p = 0.028 and 80%, p = 0.027). After placement of the In-Space interspinous spacer, the DP was decreased at L3-4 in extension for Groups 1 and 2 in the posterior anular region (63%, p = 0.028; 59%, p = 0.043), Group 2 in the center region (73%, p = 0.028), and Groups 1 and 2 in the anterior anular region (57%, p = 0.018; 60%, p = 0.018). The In-Space interspinous spacer both stabilizes the spine and reduces the intervertebral DP at the instrumented level during extension. The biomechanics for other modes of motion and at the adjacent levels are not affected statistically significantly, however. The device thus performed as intended. It significantly stabilized the motion segments at the instrumented level, but not at the segment adjacent to the instrumented level.
    Journal of neurosurgery. Spine 05/2010; 12(5):560-9. · 1.61 Impact Factor
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    ABSTRACT: The aim of this study was to correlate the degree of L4-5 spondylolisthesis on plain flexion-extension radiographs with the corresponding amount of L4-5 facet fluid visible on MR images. Patients underwent evaluation at the Neurosurgical Spine Clinics of Stanford University Medical Center and National Health Insurance Medical Center (Goyang, South Korea) between January 2006 and December 2007. Only patients who were diagnosed with L4-5 degenerative spondylolisthesis (DS) and who had both lumbosacral flexion-extension radiographs and MR images available for review were eligible for this study. Each patient's dynamic motion index (DMI) was measured using the lateral lumbosacral plain radiograph and was the percentage of the degree of anterior slippage seen on flexion versus that seen on extension. Axial T2-weighted MR images of the L4-5 facet joints obtained in each patient was analyzed for the amount of facet fluid, using the image showing the widest portion of the facets. The facet fluid index was calculated from the ratio of the sum of the amounts of facet fluid found in the right plus left facets over the sum of the average widths of the right plus left facet joints. Fifty-four patients with L4-5 DS were included in this study. Of these 54 patients, facet fluid was noted on MR images in 29 patients (53.7%), and their mean DMI was 6.349 +/- 2.726. Patients who did not have facet fluid on MR imaging had a mean DMI of 1.542 +/- 0.820; this difference was statistically significant (p < 0.001). There was a positive linear association between the facet fluid index and the DMI in the group of patients who exhibited facet fluid on MR images (Pearson correlation coefficient 0.560, p < 0.01). In the subgroup of 29 patients with L4-5 DS who showed facet fluid on MR images, flexion-extension plain radiographs in 10 (34.5%) showed marked anterolisthesis, while the corresponding MR images did not. There is a linear correlation between the degree of segmental motion seen on flexion-extension plain radiography in patients with DS at L4-5 and the amount of L4-5 facet fluid on MR images. If L4-5 facet fluid in patients with DS is seen on MR images, a corresponding anterolisthesis on weight-bearing flexion-extension lateral radiographs should be anticipated. Obtaining plain radiographs will aid in the diagnosis of anterolisthesis caused by an L4-5 hypermobile segment, which may not always be evident on MR images obtained in supine patients.
    Journal of neurosurgery. Spine 11/2009; 11(5):614-9. · 1.61 Impact Factor
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    ABSTRACT: Sixteen Stanford University Medical Center (SUMC) patients with foraminal nerve sheath tumors had charts reviewed. CyberKnife radiosurgery was innovative in management. Parameters were evaluated for 16 foraminal nerve sheath tumors undergoing surgery, some with CyberKnife. Three neurofibromas had associated neurofibromatosis type 1 (NF1). Eleven patients had one resection; others had CyberKnife after one (two) and two (three) operations. The malignant peripheral nerve sheath tumor (MPNST) had prior field-radiation and adds another case. Approaches included laminotomy and laminectomies with partial (three) or total (two) facetectomies/fusions. Two cases each had supraclavicular, lateral extracavitary, retroperitoneal and Wiltze and costotransversectomy/thoracotomy procedures. Two underwent a laminectomy/partial facetectomy, then CyberKnife. Pre-CyberKnife, one of two others had a laminectomy/partial facetectomy, then total facetectomy/fusion and the other, two supraclavicular approaches. The MPNST had a hemi-laminotomy then laminectomy/total facetectomy/fusion, followed by CyberKnife. Roots were preserved, except in two. Of 11 single-operation-peripheral nerve sheath tumors, the asymptomatic case remained stable, nine (92%) improved and one (9%) worsened. Examinations remained intact in three (27%) and improved in seven (64%). Two having a single operation then CyberKnife had improvement after both. Of two undergoing two operations, one had symptom resolution post-operatively, worsened 4 years post-CyberKnife then has remained unchanged after re-operation. The other such patient improved post-operatively, had no change after re-operation and improved post-CyberKnife. The MPNST had presentation improvement after the first operation, worsened and after the second surgery \and CyberKnife, the patient expired from tumor spread. In conclusion, surgery is beneficial for pain relief and function preservation in foraminal nerve sheath tumors. Open surgery with CyberKnife is an innovation in these tumors' management.
    European Spine Journal 10/2009; 19(2):242-56. · 2.47 Impact Factor
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    ABSTRACT: To conduct a retrospective review of outcomes in 15 patients with 18 foraminal tumors, including 17 benign peripheral nerve sheath tumors and 1 malignant peripheral nerve sheath tumor, who underwent CyberKnife (Accuray, Inc., Sunnyvale, CA) radiosurgery at Stanford University Medical Center from 1999 to 2006. Symptoms and findings, neurofibromatosis (NF) association, previous radiation, imaging, dosimetry, tumor volume, central necrosis, and the relation of these factors to outcomes were evaluated. Before treatment, 1 asymptomatic patient had radiculopathic findings, 3 patients experienced local pain with intact neurological examinations, and 7 patients had radiculopathic complaints with intact (1 patient), radiculopathic (4 patients), or radiculomyelopathic examinations (2 patients). Five patients had myelopathic complaints and findings. Three patients had NF1-associated neurofibromas, 1 patient with NF2 had a schwannoma, and 1 patient had a schwannomatosis-related lesion. Two likely radiation-induced lesions, a neurofibroma and a malignant peripheral nerve sheath tumor, were observed. Prescribed doses ranging from 16 to 24 Gy, delivered in 1 to 3 fractions of 6 to 20 Gy, resulted in maximum tumor doses ranging from 20.9 to 30 Gy. Target volumes ranged from 1.36 to 16.9 mL. After radiosurgery, the asymptomatic case remained asymptomatic, and neurological findings improved. Thirteen of 15 symptomatic patients with (12 patients) or without (3 patients) neurological findings improved (3 cases after resection) or remained stable, and 2 patients worsened. Symptoms and examinations remained stable or improved in 8 (80%) of 10 patients with schwannomas and 3 (60%) of 5 patients with neurofibromas. Tumor volumes decreased in 12 (67%) of 18 tumors and increased in 3 tumors. Tumor volumes decreased in 8 of 10 schwannomas and 3 of 7 neurofibromas. Central necrosis developed in 8 (44%) of 18 tumors. CyberKnife radiosurgery resulted in pain relief and functional preservation in selected foraminal peripheral nerve sheath tumors and a malignant peripheral nerve sheath tumor. Symptomatic and neurological improvements were more noticeable with schwannomas. Myelopathic symptoms may necessitate surgical debulking before radiosurgery.
    Neurosurgery 03/2009; 64(2 Suppl):A33-43. · 3.03 Impact Factor
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    ABSTRACT: The stability of lateral lumbar interbody graft-augmented fusion and supplementary lateral plate fixation in human cadavers has not been determined. The purpose of this study was to investigate the immediate biomechanical stabilities of the following: 1) femoral ring allograft (FRA)-augmented anterior lumbar interbody fusion (ALIF) after left lateral discectomy combined with additional lateral MACS HMA plate and screw fixation; and 2) ALIF combined with posterior transpedicular fixation after anterior discectomy. Sixteen human lumbosacral spines were loaded with six modes of motion. The intervertebral motion was measured using a video-based motion-capturing system. The range of motion (ROM) and the neutral zone (NZ) in each loading mode were compared with a maximum of 7.5 Nm. The ROM values for both stand-alone ALIF approaches were similar to those of the intact spine, whereas NZ measurements were higher in most loading modes. No significant intergroup differences were found. The ROM and NZ values for lateral fixation in all modes were significantly lower than those of intact spine, except when NZ was measured in lateral bending. All ROM and NZ values for transpedicular fixation were significantly lower than those for stand-alone anterior ALIF. Transpedicular fixation conferred better stabilization than lateral fixation in flexion, extension, and lateral bending modes. Neither approach to stand-alone FRA-augmented ALIF provided sufficient stabilization, but supplementary instrumentation conferred significant stabilization. The MACS HMA plate and screw fixation system, although inferior to posterior transpedicular fixation, provided adequate stability compared with the intact spine and can serve as a sound alternative to supplementary spinal stabilization.
    Journal of Neurosurgery Spine 02/2005; 2(1):62-8. · 2.36 Impact Factor

Publication Stats

192 Citations
84.95 Total Impact Points


  • 2009–2014
    • Stanford Medicine
      • Department of Neurosurgery
      Stanford, California, United States
  • 2003–2014
    • Stanford University
      • • Department of Chemical Engineering
      • • Department of Neurosurgery
      Palo Alto, California, United States
  • 2013
    • Inje University Paik Hospital
      • Department of Neurosurgery
      Goyang, Gyeonggi, South Korea
    • Yonsei University Hospital
      • Department of Internal Medicine
      Sŏul, Seoul, South Korea
  • 2011
    • University of California, San Francisco
      • Department of Orthopaedic Surgery
      San Francisco, CA, United States
    • Seoul National University
      • Department of Neurosurgery
      Seoul, Seoul, South Korea
  • 2009–2010
    • National Health Insurance Corporation Ilsan Hospital
      Sŏul, Seoul, South Korea