Jesse Lim

Changes in adjacent-level disc pressure and facet joint force after cervical arthroplasty compared with cervical discectomy and fusion.

OBJECT The authors of previous in vitro investigations have reported an increase in adjacent-level intradiscal pressures (IDPs) and facet joint stresses following cervical spine fusion. This study was performed to compare adjacent-level IDPs and facet force following arthroplasty with the fusion model. METHODS Eighteen human cadaveric cervical spines were tested in the intact state for different modes of motion (extension, flexion, bending, and rotation) up to 2 Nm. The specimens were then divided into three groups: those involving the ProDisc-C cervical artificial disc, Prestige cervical artificial disc, and cervical fusion. They were load tested after application of instrumentation or surgery at the C6-7 level. During the test, IDPs and facet forces were measured at adjacent levels. RESULTS In arthroplasty-treated specimens, the IDP showed little difference from that of the intact spine at both proximal and distal levels. In fusion-treated specimens, the IDP increased at the posterior anulus fibrosus on extension and at the anterior anulus fibrosus on flexion at the proximal level. At the distal level, the IDP change was not significant. The facet force changes were minimal in flexion, bending, and rotation modes in both arthroplasty- and fusion-treated spines. Significant changes were noted in the extension mode only. In extension, arthroplasty models exhibited significant increases of facet force at the treated level. In the fusion model the facet forces decreased at the treated segment and increased at the adjacent segment. CONCLUSIONS The two artificial discs of the semiconstrained systems maintain adjacent-level IDPs near the preoperative values in all modes of motion, but with respect to facet force pressure tended to increase after arthroplasty.

Range of motion change after cervical arthroplasty with ProDisc-C and Prestige artificial discs compared with anterior cervical discectomy and fusion

OBJECT Range of motion (ROM) changes were evaluated at the surgically treated and adjacent segments in cadaveric specimens treated with two different cervical artificial discs compared with those measured in intact spine and fusion models. METHODS Eighteen cadaveric human cervical spines were tested in the intact state for the different modes of motion (extension, flexion, lateral bending, and axial rotation) up to 2 Nm. Three groups of specimens (fitted with either the ProDisc-C or Prestige II cervical artificial disc or submitted to anterior cervical discectomy and fusion [ACDF]) were tested after implantation at C6-7 level. The ROM values were measured at treated and adjacent segments, and these values were then compared with those measured in the intact spine. RESULTS At the surgically treated segment, the ROM increased after arthroplasty compared with the intact spine in extension (54% in the ProDisc-C group, 47% in the Prestige group) and in flexion (27% in the ProDisc-C group, 10% in the Prestige group). In bending and rotation, the postarthroplasty ROMs were greater than those of the intact spine (10% in the ProDisc-C group and 55% in the Prestige group in bending, 17% in the ProDisc-C group and 50% in the Prestige group in rotation). At the adjacent levels the ROMs decreased in all specimens treated with either artificial disc in all modes of motion (< 10%) except for extension at the inferior the level (29% decrease for ProDisc-C implant, 12% decrease for Prestige disc). The ROM for all motion modes in the ACDF-treated spine decreased at the treated level (range 18-44%) but increased at the adjacent levels (range 3-20%). CONCLUSIONS Both ProDisc-C and Prestige artificial discs were associated with increased ROM at the surgically treated segment compared with the intact spine with or without significance for all modes of testing. In addition, adjacent-level ROM decreased in all modes of motion except extension in specimens fitted with both artificial discs.

Biomechanical evaluations of various c1-c2 posterior fixation techniques.

Study Design. A biomechanical in vitro study using human cadaveric spine. Objective. To compare the biomechanical stability of pedicle screws versus various established posterior atlantoaxial fixations used to manage atlantoaxial instability. Summary of Background Data. 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. Methods. 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) Sonntags modified Gallie fixation (MG) and (6) C1-C2 transarticular screw fixation with posterior wiring (TAS + MG). (7) The destabilized spine was also tested. Results. 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 + MG group. The MG groups NZ and ROM values were statistically significantly weaker than those of the PS-S, PS-L, and the ROM values of the TAS + MG groups. The ILS groups NZ values were higher than those of the TAS + 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 + 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 + MG groups. Conclusion. The TAS + 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 + 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.

Biomechanical analysis of the range of motion after placement of a two-level cervical ProDisc-C versus hybrid construct.

Study Design. The study design was that of an in vitro human cadaveric biomechanical analysis. Objectives. 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. Summary of Background Data. With in vivo cervical arthroplasties gaining in popularity, limited biomechanical data are available, which highlight changes in the adjacent-level biomechanics after multilevel procedures. Methods. 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. Results. 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. Conclusion. (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.

Biomechanical Analysis of Disc Pressure and Facet Contact Force After Simulated Two-Level Cervical Surgeries (Fusion and Arthroplasty) and Hybrid Surgery

OBJECTIVE The objective of this study was designed to compare 2-level cervical disc surgery (2-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 (Synthes Spine, West Chester, Pennsylvania, USA) and C6-C7 ACDF, 2) C5-C6 ACDF and C6-C7 arthroplasty using ProDisc-C, 3) 2-level C5-C6/C6-C7 disc arthroplasties, and 4) 2-level C5-C6/C6-C7 ACDF. IDPs were recorded at anterior, central, and posterior disc portions. RESULTS After 2-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. In contrast, after cervical specified treatments, including disc arthroplasty (2-level disc arthroplasties and hybrid surgery), IDP decreased significantly at the anterior annulus of distal adjacent-level disc during flexion and extension and was unchanged at the center of proximal adjacent-level disc during flexion. Two-level cervical arthrodesis also tended to adversely impact facet loads, increasing distal rather than proximal adjacent-level FCF. CONCLUSION Both hybrid surgery and 2-level arthroplasties seem to offer significant advantages over 2-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 IDP and facet loads at adjacent levels.

Biomechanical Comparison of Cervical Fixation via Transarticular Facet Screws without Rods versus Lateral Mass Screws with Rods

OBJECTIVE Transarticular facet screws restore biomechanical stability to the cervical spine when posterior cervical anatomy has been compromised. This study compares the more recent, less invasive, and briefer transarticular facet screw system without rods with the lateral mass screw system with rods. METHODS For this study, 6 human cervical spines were obtained from cadavers. Transarticular facet screws without rods were inserted bilaterally into the inferior articular facets at the C5-C6 and C5-C6-C7 levels. Lateral mass screws with rods were inserted bilaterally at the same levels using Magerls technique. All specimens underwent range of motion (ROM) testing by a material testing machine for flexion, extension, lateral bending, and axial rotation. RESULTS Both fixation methods, transarticular facet screws without rods and lateral mass screws with rods, reduced all ROM measurements and increased spinal stiffness. No statistically significant differences between the 2 stabilization methods were found in ROM measurements for 1-level insertions. However, in 2-level insertions, ROM for the nonrod transarticular facet screw group was significantly increased for flexion-extension and lateral bending. CONCLUSIONS Transarticular facet screws without rods and lateral mass screws with rods had similar biomechanical stability in single-level insertions. For 2-level insertions, transarticular facet screws without rods are a valid option in cervical spine repair.

Biomechanical Comparisons of Pull Out Strengths After Pedicle Screw Augmentation with Hydroxyapatite, Calcium Phosphate, or Polymethylmethacrylate in the Cadaveric Spine.

OBJECTIVE In vertebrae with low bone mineral densities pull out strength is often poor, thus various substances have been used to fill screw holes before screw placement for corrective spine surgery. We performed biomechanical cadaveric studies to compare nonaugmented pedicle screws versus hydroxyapatite, calcium phosphate, or polymethylmethacrylate augmented pedicle screws for screw tightening torques and pull out strengths in spine procedures requiring bone screw insertion. METHODS Seven human cadaveric T10-L1 spines with 28 vertebral bodies were examined by x-ray to exclude bony abnormalities. Dual-energy x-ray absorptiometry scans evaluated bone mineral densities. Twenty of 28 vertebrae underwent ipsilateral fluoroscopic placement of 6-mm holes augmented with hydroxyapatite, calcium phosphate, or polymethylmethacrylate, followed by transpedicular screw placements. Controls were pedicle screw placements in the contralateral hemivertebrae without augmentation. All groups were evaluated for axial pull out strength using a biomechanical loading frame. RESULTS Mean pedicle screw axial pull out strength compared with controls increased by 12.5% in hydroxyapatite augmented hemivertebrae (P = 0.600) and by 14.9% in calcium phosphate augmented hemivertebrae (P = 0.234), but the increase was not significant for either method. Pull out strength of polymethylmethacrylate versus hydroxyapatite augmented pedicle screws was 60.8% higher (P = 0.028). CONCLUSIONS Hydroxyapatite and calcium phosphate augmentation in osteoporotic vertebrae showed a trend toward increased pedicle screw pull out strength versus controls. Pedicle screw pull out force of polymethylmethacrylate in the insertion stage was higher than that of hydroxyapatite. However, hydroxyapatite is likely a better clinical alternative to polymethylmethacrylate, as hydroxyapatite augmentation, unlike polymethylmethacrylate augmentation, stimulates bone growth and can be revised.

Open posterior reduction and stabilization of a C1 burst fracture using mono-axial screws.

Study Design. Biomechanical study of a posterior reduction and stabilization of a Jefferson C1 burst fracture using monoversus polyaxial screws. Objective. To introduce a new motion-preserving surgical method for reduction and stabilization of an isolated Jefferson fracture and investigate its effectiveness with a biomechanical study. Summary of Background Data. Jefferson fractures have been treated conservatively by immobilization, traction, or surgical fusion of C1-C2 or occiput-to-C2. Conservative treatment usually requires prolonged immobilization with a cumbersome external brace. Surgical fusion, however, eliminates the important range of motion (ROM) of C1-C2 or occiput-to-C2. Methods. Six occiput-to-C3 cadaveric specimens were tested biomechanically. After creating a Jefferson fracture, bilateral atlantal lateral mass screws were inserted on the posterior aspect of each specimen. The screws were connected with a rod and nuts. The posterior distance and the anterior distance were measured to evaluate the magnitude of reduction and resistance to loading after instrumentation placement. Measurements were performed after monoand poly-axial head screw insertion, and then using destabilized specimens. ROM was also measured after applying a 1.5 Nm moment to six cardinal directions before the trauma, after monoand poly-axial screw fi xation and in destabilized specimens. Results. The posterior distances after mono-axial and poly-axial screw fi xations were signifi cantly reduced compared with that of the destabilized atlas. In the specimens reduced with monoand poly axial screws, the averages were 50.2 and 50.2 mm, and in the destabilized specimen, the average was 54 mm. The anterior distance was also decreased by mono-axial screw fi xation compared with the anterior distance for the destabilized specimen but was similar to that of poly-axial screw fi xation. The ROMs in fl exion/extension, lateral bending, and axial rotation were greatest in the destabilized model, followed by poly-axial and then monoaxial fi xation. Conclusions. The current study shows that the posterior screw and rod fi xation of a Jefferson fracture is feasible and effective in providing immediate reduction and stabilization especially with the mono-axial screw system.