Anna G.U. Sawa

A new stand-alone cervical anterior interbody fusion device: biomechanical comparison with established anterior cervical fixation devices.

Study Design. A new anchored spacer—a low-profile cervical interbody fusion cage with integrated anterior fixation—was compared biomechanically to established anterior cervical devices. Objective. To evaluate the fixation properties of the new stand-alone device and compare these properties with established fixation methods. The hypothesis is that the new device will provide stability comparable to that provided by an anterior cervical cage when supplemented with an anterior plate. Summary of Background Data. It is accepted that the use of anterior cervical plating increases the chance of achieving a solid fusion. However, its use may be associated with an increase in operation time and a higher postoperative morbidity caused by a larger anterior approach and disruption of the anterior musculature. This dilemma has led to the development of a new, low profile stand-alone cervical anterior cage device with integrated screw fixation. Methods. Twenty-four human cadaveric C4–C7 cervical spines were loaded nondestructively with pure moments in a nonconstraining testing apparatus to induce flexion, extension, lateral bending, and axial rotation while angular motion was measured optoelectronically. The specimens were tested:1. Intact (N = 24).2. After discectomy and anterior stabilization.a. Interbody cage + locking plate (N = 8).b. Interbody cage + dynamic plate (N = 8).c. Anchored spacer (N = 8).3. After ventral plate removal of group 2a and 2b (N = 16). Results. All fixation techniques decreased range of motion (ROM) and lax zone (LZ) (P < 0.05) in all test modes compared with the intact motion segment and cage-only group. There were no significant differences between the anchored spacer and cage + locking plate or cage + dynamic plate. Conclusion. The anchored spacer provided a similar biomechanical stability to that of the established anterior fusion technique using an anterior plate plus cage and has a potentially lower perioperative and postoperative morbidity. These results support progression to clinical trials using the cervical anchored spacer as a stand-alone implant.

Dynamic lumbar pedicle screw-rod stabilization: in vitro biomechanical comparison with standard rigid pedicle screw-rod stabilization

OBJECT It is unclear how the biomechanics of dynamic posterior lumbar stabilization systems and traditional rigid pedicle screw-rod systems differ. This study examined the biomechanical response of a hinged-dynamic pedicle screw compared with a standard rigid screw used in a 1-level pedicle screw-rod construct. METHODS Unembalmed human cadaveric L3-S1 segments were tested intact, after L4-5 discectomy, after rigid pedicle screw-rod fixation, and after dynamic pedicle screw-rod fixation. Specimens were loaded using pure moments to induce flexion, extension, lateral bending, and axial rotation while recording motion optoelectronically. Specimens were then loaded in physiological flexion-extension while applying 400 N of compression. Moment and force across instrumentation were recorded from pairs of strain gauges mounted on the interconnecting rods. RESULTS The hinged-dynamic screws allowed an average of 160% greater range of motion during flexion, extension, lateral bending, and axial rotation than standard rigid screws (p < 0.03) but 30% less motion than normal. When using standard screws, bending moments and axial loads on the rods were greater than the bending moments and axial loads on the rods when using dynamic screws during most loading modes (p < 0.05). The axis of rotation shifted significantly posteriorly more than 10 mm from its normal position with both devices. CONCLUSIONS In a 1-level pedicle screw-rod construct, hinged-dynamic screws allowed a quantity of motion that was substantially closer to normal motion than that allowed by rigid pedicle screws. Both systems altered kinematics similarly. Less load was borne by the hinged screw construct, indicating that the hinged-dynamic screws allow less stress shielding than standard rigid screws.

The use of surface strain data and a neural networks solution method to determine lumbar facet joint loads during in vitro spine testing

A new method for determining facet loads during in vitro spine loading using strain gauges and a neural networks solution method was investigated. A test showed that the new solution method was more robust than and as accurate as a previously presented graphical solution method for computing facet loads using surface strain. The technique was subsequently utilized to assess facet loads at L1-L2 during flexibility testing [7.5 N m pure moments in flexion (FL), extension (EX), right and left axial rotation (AR), and right and left lateral bending (LB)], and stiffness testing (FL-EX with 400 N compressive follower load) of six human lumbar spine segments (T12-L2). In contrast to other techniques, such as thin film sensors or pressure-sensitive film, the strain-gauge method leaves the facet joint capsule intact during data collection, presumably allowing more natural load transmission. During flexibility tests, the mean (+/-standard deviation) calculated facet loads (in N) were 46.1+/-41.3 (FL), 51.5+/-39.0 (EX), 70.3+/-43.2 (AR-contralateral side), 31.3+/-33.4 (AR-ipsilateral side), 30.6+/-29.1 (LB-contralateral side), and 32.0+/-44.4 (LB-ipsilateral side). During stiffness tests, the calculated facet loads were 45.5+/-40.4 (upright), 46.6+/-41.9 (full FL), and 75.4+/-39.0 (full EX), corresponding to an equivalent of 11.4%, 11.6%, and 18.8% of the compressive follower load (upright, full FL and EX, respectively). The error associated with this technique, which was below 11 N for loads up to 125 N, is comparable to that reported with other techniques. The new method shows promise for assessing facet load during in vitro spine testing, an important parameter when evaluating new implant systems and surgical techniques.

Biomechanics of a novel minimally invasive lumbar interspinous spacer: effects on kinematics, facet loads, and foramen height.

OBJECTIVE To study the alteration to normal biomechanics after insertion of a lumbar interspinous spacer (ISS) in vitro by nondestructive cadaveric flexibility testing. METHODS Seven human cadaveric specimens were studied before and after ISS placement at L1–L2. Angular range of motion, lax zone, stiff zone, sagittal instantaneous axis of rotation (IAR), foraminal height, and facet loads were compared between conditions. Flexion, extension, lateral bending, and axial rotation were induced using pure moments (7.5 Nm maximum) while recording motion optoelectronically. The IAR was measured during loading with a 400 N compressive follower. Foraminal height changes were calculated using rigid body methods. Facet loads were assessed from surface strain and neural network analysis. RESULTS After ISS insertion, range of motion and stiff zone during extension were significantly reduced (P < .01). Foraminal height was significantly reduced from flexion to extension in both normal and ISS-implanted conditions; there was significantly less reduction in foraminal height during extension with the ISS in place. The ISS reduced the mean facet load by 30% during flexion (P < .02) and 69% during extension (P < .015). The IAR after ISS implantation was less than 1 mm from the normal position (P > .18). CONCLUSION The primary biomechanical effect of the ISS was reduced extension with associated reduced facet loads and smaller decrease in foraminal height. The ISS had little effect on sagittal IAR or on motion or facet loads in other directions.

Biomechanics of thoracic short versus long fixation after 3-column injury.

OBJECT Posterior screw-rod fixation for thoracic spine trauma usually involves fusion across long segments. Biomechanical data on screw-based short-segment fixation for thoracic fusion are lacking. The authors compared the effects of spanning short and long segments in the thoracic spine. METHODS Seven human spine segments (5 segments from T-2 to T-8; 2 segments from T-3 to T-9) were prepared. Pure-moment loading of 6 Nm was applied to induce flexion, extension, lateral bending, and axial rotation while 3D motion was measured optoelectronically. Normal specimens were tested, and then a wedge fracture was created on the middle vertebra after cutting the posterior ligaments. Five conditions of instrumentation were tested, as follows: Step A, 4-level fixation plus cross-link; Step B, 2-level fixation; Step C, 2-level fixation plus cross-link; Step D, 2-level fixation plus screws at fracture site (index); and Step E, 2-level fixation plus index screws plus cross-link. RESULTS Long-segment fixation restricted 2-level range of motion (ROM) during extension and lateral bending significantly better than the most rigid short-segment construct. Adding index screws in short-segment constructs significantly reduced ROM during flexion, lateral bending, and axial rotation (p < 0.03). A cross-link reduced axial rotation ROM (p = 0.001), not affecting other loading directions (p > 0.4). CONCLUSIONS Thoracic short-segment fixation provides significantly less stability than long-segment fixation for the injury studied. Adding a cross-link to short fixation improved stability only during axial rotation. Adding a screw at the fracture site improved short-segment stability by an average of 25%.

Biomechanics of one-level anterior cervical discectomy and plating using two screws versus four screws

BACKGROUND CONTEXT Most one-level anterior cervical plates use two screws per vertebra (four screws in total). No study has addressed whether a simplified plate using one screw per vertebra is adequate for one-level fixation. PURPOSE To compare stability achieved by four-screw and two-screw plates after discectomy and placement of interbody spacer. STUDY DESIGN Nondestructive multidirectional flexibility tests were performed in three independent groups of cadaveric spines to assess spinal stability after instrumentation. METHODS Human cadaveric C4-C7 specimens were tested intact and after discectomy followed by placement of a polyetheretherketone interbody graft and an anterior plate. Rigid two-screw (n=8), semiconstrained four-screw (n=8), and rigid four-screw (n=8) plates were compared. Nonconstraining pure moments were applied under load control (maximum 1.5 Nm) to induce flexion, extension, lateral bending, and axial rotation, whereas vertebral motion was measured optoelectronically. Mean range of motion (ROM) was compared among groups. RESULTS All three plates significantly reduced ROM relative to normal in all directions of loading (p<.003). Mean ROMs±standard deviation (and corresponding intergroup p value) for rigid two-screw, semiconstrained four-screw, and rigid four-screw plates, respectively, were as follows: flexion: 2.6±2.0°, 1.8±1.1°, 1.8±0.8° (p=.46); extension: 2.5±2.6°, 2.1±1.3°, 1.4±1.3° (p=.45); lateral bending: 1.8±1.0°, 1.3±1.0°, 1.1±0.5° (p=.29); axial rotation: 2.9±1.9°, 1.6±0.9°, 1.5±0.7° (p=.08). Despite a tendency for the rigid two-screw plate to allow more motion than the four-screw plates, there was no significant difference among groups during any loading mode. CONCLUSIONS In terms of immediate postoperative cervical stability after one-level discectomy and placement of an interbody spacer, the rigid two-screw plate performed comparably to conventional rigid four-screw and semiconstrained four-screw plates. Further research on relative fatigue endurance of the different plate types is also needed.

Biomechanical evaluation of posterior thoracic transpedicular discectomy

OBJECT The authors investigated the biomechanical properties of transpedicular discectomy in the thoracic spine and compared the effects on spinal stability of a partial and total facetectomy. METHODS Human thoracic specimens were tested while intact, after a transpedicular discectomy with partial facetectomy, and after an additional total facetectomy was incorporated. Nonconstraining pure moments were applied under load control (maximum 7.5 Nm) to induce flexion, extension, lateral bending, and axial rotation while spinal motion was measured at T8-9 optoelectronically. The range of motion (ROM) and lax zone were determined in each specimen and compared among conditions. RESULTS Transpedicular discectomy with and without a total facetectomy significantly increased the ROM and lax zone in all directions of loading compared with the intact spine (p < 0.008). The segmental increase in ROM observed with the transpedicular discectomy was 25%. The additional total facetectomy created an insignificant 3% further increase in ROM compared with medial facetectomy (p > 0.2). CONCLUSIONS Transpedicular discectomy can be performed in the thoracic spine with a modest decrease in stability expected. Because the biomechanical behavior of a total facetectomy is equivalent to that of a medial facetectomy, the additional facet removal may be incorporated without further biomechanical consequences.

Biomechanics of a Fixed–Center of Rotation Cervical Intervertebral Disc Prosthesis

Background Past in vitro experiments studying artificial discs have focused on range of motion. It is also important to understand how artificial discs affect other biomechanical parameters, especially alterations to kinematics. The purpose of this in vitro investigation was to quantify how disc replacement with a ball-and-socket disc arthroplasty device (ProDisc-C; Synthes, West Chester, Pennsylvania) alters biomechanics of the spine relative to the normal condition (positive control) and simulated fusion (negative control). Methods Specimens were tested in multiple planes by use of pure moments under load control and again in displacement control during flexion-extension with a constant 70-N compressive follower load. Optical markers measured 3-dimensional vertebral motion, and a strain gauge array measured C4-5 facet loads. Results Range of motion and lax zone after disc replacement were not significantly different from normal values except during lateral bending, whereas plating significantly reduced motion in all loading modes (P < .002). Plating but not disc replacement shifted the location of the axis of rotation anteriorly relative to the intact condition (P < 0.01). Coupled axial rotation per degree of lateral bending was 25% ± 48% greater than normal after artificial disc replacement (P = .05) but 37% ± 38% less than normal after plating (P = .002). Coupled lateral bending per degree of axial rotation was 37% ± 21% less than normal after disc replacement (P < .001) and 41% ± 36% less than normal after plating (P = .001). Facet loads did not change significantly relative to normal after anterior plating or arthroplasty, except that facet loads were decreased during flexion in both conditions (P < .03). Conclusions In all parameters studied, deviations from normal biomechanics were less substantial after artificial disc placement than after anterior plating.

In vitro biomechanical analysis of a new lumbar low-profile locking screw-plate construct versus a standard top-loading cantilevered pedicle screw-rod construct: technical report.

OBJECTIVEA standard top-loading lumbar pedicle screw-rod system is compared with a pedicle screw-plate system with smaller-diameter screws, more medial entry, and lower profile to assess the relative stability, strength, and resistance to fatigue of the 2 systems. METHODSSeven human cadaveric specimens were studied with each surgical construct. Nondestructive, nonconstraining pure moments were applied to specimens to induce flexion, extension, lateral bending, and axial rotation while recording L5–S1 motion optoelectronically. After initial tests, specimens were fatigued for 10 000 cycles and retested to assess early postoperative loosening. Specimens were then loaded to failure in hyperextension. RESULTSThe standard screw-rod construct reduced range of motion to a mean of 20% of normal, whereas the screw-plate construct reduced range of motion to 13% of normal. Differences between systems were not significant in any loading mode (P > 0.06). The 14% loosening of the screw-rod system with fatigue was not significantly different from the 10% loosening observed with the screw-plate system (P > 0.15). Mean failure loads of 30 Nm for screw-rod and 37 Nm for screw-plate were also not significantly different (P = 0.38). CONCLUSIONPosterior fixation at L5–S1 using the low-profile screw-plate system offers stability, resistance to fatigue, and resistance to failure equivalent to fixation using a standard cantilevered pedicle screw-rod system.

ATLANTOAXIAL ROTATORY SUBLUXATION WITH LIGAMENTOUS DISRUPTION: A BIOMECHANICAL COMPARISON OF CURRENT FUSION METHODS

OBJECTIVE We evaluated the biomechanical effects of 4 instrumented configurations after induced atlantoaxial rotatory subluxation: transarticular screw fixation (T/A) and polyaxial C1 lateral mass and C2 pedicle screw and rod fixation (LC1-PC2) for atlantoaxial arthrodesis with unilateral and bilateral instrumentation. METHODS Three-dimensional intervertebral motion was tracked stereophotogrammetrically while 14 human cadaveric spine specimens underwent nonconstraining pure moment loading. Nondestructive loads were applied quasi-statistically in 0.25-Nm increments to a maximum load of 1.5 Nm during flexion-extension, right and left axial rotation, and right and left lateral bending. Hyperrotation injuries were created using torsional loads applied during left axial rotation until visible failure occurred. RESULTS In the normal condition, the values for angular range of motion, lax zone (zone of ligamentous laxity), and stiff zone (zone of ligamentous stretching) were similar in both groups in all directions of loading, with no significant differences (P > 0.05) between groups at C0–C1 or C1–C2. Both instrumentation systems (bilateral configurations) substantially stabilized angular motion at C1–C2 (P < 0.05) during all loading modes for the T/A group, and during all but right lateral bending (P = 0.072) for the LC1-PC2 group. The mean failure load for both intact and instrumented specimens was slightly greater, but not significant for the LC1-PC2 group compared with the T/A group (P > 0.14). CONCLUSION Both methods fixated atlantoaxial subluxation equally well. Compared with unilateral instrumentation, a bilateral configuration with the LC1-PC2 technique significantly increased stability during extension (P < 0.05). During axial rotation, bilateral T/A screws significantly increased stability compared with unilateral fixation (P < 0.02).