Few studies have used optical full-field surface strain mapping to study spinal biomechanics. We used a commercial digital imaging correlation (DIC) system to (1) compare posterior surface strains on ...spinal rods with those obtained from conventional foil strain gauges, (2) quantify bony vertebral body and intervertebral disc (IVD) surface strains on 3 L3-S cadaveric spines during gold-standard flexibility tests (7.5-Nm flexion–extension and 400-N compression), and (3) report our experience with the application and feasibility of DIC to comprehensively map strain in spinal biomechanics. Spinal rods were tested under zero load and using ASTM F1717 standard. For rod strain measures, the largest mean bias offset and baseline noise standard deviation under zero load for DIC were 7.6 με and 33.7 με, respectively. For tissue measures, the largest mean bias offset was 8 με for ε1 and −55 με for ε2 with baseline noise standard deviations of 19 με and 26 με, respectively. On average, DIC rod strain measurements were 5.3% less than strain gauge measurements throughout the load range. Principal IVD and bony surface strains were consistently measurable and showed marked regional differences in strain patterns under different load conditions. Strains measured on spinal rods using DIC techniques reasonably agreed with standard strain gauge measurements. Subregional strain analyses on soft and hard spinal tissues during standard flexibility tests were feasible. Optical strain mapping is a viable, accurate, and promising measurement technique for novel spinal biomechanical studies.
A new anchored spacer-a low-profile cervical interbody fusion cage with integrated anterior fixation-was compared biomechanically to established anterior cervical devices.
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.
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.
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).
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.
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.
In vitro biomechanical study.
This study aimed to investigate the biomechanical effects of facet joint violation (FV) on mobility and optically tracked intervertebral disc (IVD) surface strains at ...the upper level adjacent to L4-5 pedicle screw-rod fixation.
FV is a complication that can occur when placing lumbar pedicle screws; the reported incidence is as high as 50%. However, little is known about how FV affects superior adjacent-level spinal stability, and especially IVD strain, after lumbar fusion.
Fourteen cadaveric L3-S1 specimens underwent L4-5 pedicle-rod fixation, 7 in the facet joint preservation (FP) group and 7 in the FV group. Specimens were tested multidirectionally under pure moment loading (7.5 Nm). Colored maps of maximum (ε1) and minimum (ε2) principal surface strain changes on the lateral L3-4 disc were generated, with the surface divided into 4 quarters anterior to posterior (Q1, Q2, Q3, and Q4, respectively) for subregional analyses. Range of motion (ROM) and IVD strain were normalized to intact upper adjacent-level and compared between the groups using analysis of variance. Statistical significance was set at P <0.05.
Normalized ROM was significantly greater with FV vs. FP in flexion (11% greater; P =0.04), right lateral bending (16% greater; P =0.03), and right axial rotation (23% greater; P =0.04). Normalized L3-4 IVD ε1 during right lateral bending was greater on average for the FV group than the FP group: Q1, 18% greater; Q2, 12% greater; Q3, 40% greater ( P <0.001); Q4, 9% greater. Normalized ε2 values during left axial rotation were greater in the FV group, the highest increase being 25% in Q3 ( P =0.02).
Facet joint violation during single-level pedicle screw-rod fixation was associated with increased superior adjacent level mobility and alteration of disc surface strains, with significant increases in selected regions and directions of loading.
Fourteen cadaveric specimens were separated into two groups: (1) L3 pedicle subtraction osteotomy (PSO) with transforaminal lumbar interbody fusion (TLIF) or (2) lateral lumbar interbody fusion ...(LLIF). A 2-rod configuration (2R) was compared with two supplemental rod configurations: 4-rod (4R) with accessory rods (ARs) using connectors or 4R with satellite rods (SRs) without connectors.
Compare PSO constructs with different rod configurations and adjacent-level interbody support.
Supplemental rods and anterior column support enhance biomechanical performance.
Pure moments were applied in (1) intact, (2) pedicle screws and rods, (3) PSO + 2R, (4) 4R AR, and (5) 4R SR conditions. Primary and supplemental rods had strain gauges across the index level. Sacral screw bending moments and range of motion (ROM) were recorded.
For TLIF, AR decreased ROM during flexion (P = 0.02) and extension (P < 0.001) versus 2R. For LLIF, AR and SR decreased motion versus 2R during left (AR: P = 0.03; SR: P = 0.04) and right (AR: P = 0.002; SR: P = 0.01) axial rotation. For LLIF, sacral screw strain increased with SR compared with AR in compression and right lateral bending (P ≤ 0.03). During lateral bending, rod strain increased with PSO+TLIF+SR versus PSO+LLIF+2R and PSO+LLIF+AR (P ≤ 0.02). For LLIF, SR configuration increased rod strain versus AR during flexion, extension, and lateral bending (P≤ 0.01); for TLIF, rod strain increased with SR versus AR during extension (P = 0.03). For LLIF, AR configuration increased posterior supplemental rod strain versus SR during flexion (P = 0.02) and lateral bending (P < 0.001).
Both supplemental rod configurations reduced motion in both groups. Constructs with the SR configuration increased the primary rod strain and the sacral screw bending moment compared with AR constructs, which can share strain. Deep-seated SRs, which have become increasingly popular, may be more vulnerable to failure than ARs. LLIF provided more stability in sagittal plane. Protective effect of supplemental rods on rod strain was more effective with TLIF.Level of Evidence: NA.
Although pedicle screw-rod (PSR) instrumentation remains the gold standard, it has complications, including pedicle breach and facet joint violation. There is current interest in facet joint ...stabilization with the potential to create a less invasive, natural arch of fixation that may avoid the complications of PSR instrumentation. This study examined the stabilizing potential of a novel facet joint fixation device for single-level (L4-L5) fixation in a human cadaveric model.OBJECTIVEAlthough pedicle screw-rod (PSR) instrumentation remains the gold standard, it has complications, including pedicle breach and facet joint violation. There is current interest in facet joint stabilization with the potential to create a less invasive, natural arch of fixation that may avoid the complications of PSR instrumentation. This study examined the stabilizing potential of a novel facet joint fixation device for single-level (L4-L5) fixation in a human cadaveric model.Six L3-S1 specimens were tested multidirectionally under pure moment loading (7.5 Nm) in 3 conditions: 1) intact, 2) L4-L5 facet fixation without screws, and 3) L4-L5 facet fixation with screws. L4-L5 intervertebral disc angles were measured radiographically. Range of motion (ROM) and disc angles were compared using repeated-measures analysis of variance analysis, with statistical significance p<0.05.METHODSSix L3-S1 specimens were tested multidirectionally under pure moment loading (7.5 Nm) in 3 conditions: 1) intact, 2) L4-L5 facet fixation without screws, and 3) L4-L5 facet fixation with screws. L4-L5 intervertebral disc angles were measured radiographically. Range of motion (ROM) and disc angles were compared using repeated-measures analysis of variance analysis, with statistical significance p<0.05.Compared to the intact condition, L4-L5 bilateral facet fixation without or with screw fixation significantly reduced L4-L5 angular ROM in all directions (p≤0.003). No significant differences were observed in cranial and caudal adjacent-segment ROM (p≥0.08) except for L3-L4 fixation in extension, which exhibited small motion increases (0.12° without screws, 0.1° with screws) versus the intact condition (p≤0.003). No statistically significant differences were observed in disc angle values between the conditions (p=0.87).RESULTSCompared to the intact condition, L4-L5 bilateral facet fixation without or with screw fixation significantly reduced L4-L5 angular ROM in all directions (p≤0.003). No significant differences were observed in cranial and caudal adjacent-segment ROM (p≥0.08) except for L3-L4 fixation in extension, which exhibited small motion increases (0.12° without screws, 0.1° with screws) versus the intact condition (p≤0.003). No statistically significant differences were observed in disc angle values between the conditions (p=0.87).Bilateral lumbar facet fixation with and without supplemental transfacet screw fixation provided significant stability. Cranial and caudal adjacent-level ROM was not influenced by facet fixation except for a slight increase in cranial segment motion during extension. Facet fixation did not alter the lordotic intervertebral disc angle at the instrumented level.CONCLUSIONSBilateral lumbar facet fixation with and without supplemental transfacet screw fixation provided significant stability. Cranial and caudal adjacent-level ROM was not influenced by facet fixation except for a slight increase in cranial segment motion during extension. Facet fixation did not alter the lordotic intervertebral disc angle at the instrumented level.
S2 alar-iliac (S2AI) screw fixation effectively enhances stability in long-segment constructs. Although S2AI fixation provides a single transarticular sacroiliac joint fixation (SIJF) point, ...additional fixation points may provide greater stability and attenuate screw and rod strain. The objectives of this study were to evaluate changes in stability and pedicle screw and rod strain with extended distal S2AI fixation and with supplemental bilateral integration of two sacroiliac joint fusion devices implanted using a traditional minimally invasive surgical approach.
Eight L1-pelvis human cadaveric specimens underwent pure moment (7.5 Nm) and compression (400 N) tests under 4 conditions: 1) intact (pure moment loading only); 2) L2-S1 pedicle screw and rod with L5-S1 interbody fusion; 3) added S2AI screws; and 4) added bilateral laterally placed SIJF. Range of motion (ROM), rod strain, and screw-bending moment (S1 and S2AI) were analyzed.
Compared with S1 fixation, S2AI fixation significantly reduced L5-S1 ROM in right lateral bending by 50% (0.11°, p = 0.049) and in compression by 39% (0.22°, p = 0.003). Compared with fixation ending at S1, extending fixation with S2AI significantly decreased sacroiliac joint ROM by 52% (0.28°, p = 0.02) in flexion, by 65% (0.48°, p = 0.04) in extension, by 59% (0.76°, p = 0.02) in combined flexion-extension, and by 36% (0.09°, p = 0.02) in left axial rotation. The addition of S2AI screws reduced S1 screw-bending moment during flexion (0.106 Nm 43%, p = 0.046). With S2AI fixation, posterior L5-S1 primary rod strain increased by 124% (159 μE, p = 0.002) in flexion, by 149% (285 μE, p = 0.02) in left axial rotation, and by 99% (254 μE, p = 0.04) in right axial rotation. Compared with S2AI fixation, the addition of SIJF reduced L5-S1 strain during right axial rotation by 6% (28 μE, p = 0.04) and increased L5-S1 strain in extension by 6% (28 μE, p = 0.02).
Long-segment constructs ending with S2AI screws created a more stable construct than those ending with S1 screws, reducing lumbosacral and sacroiliac joint motion and S1 screw-bending moment in flexion. These benefits, however, were paired with increased rod strain at the lumbosacral junction. The addition of SIJF to constructs ending at S2AI did not significantly change SI joint ROM or S1 screw bending and reduced S2AI screw bending in compression. SIJF further decreased L5-S1 rod strain in axial rotation and increased it in extension.
Adjacent level degeneration is a precursor to construct failure in adult spinal deformity surgery, but whether construct design affects adjacent level degeneration risk remains unclear. Here we ...present a biomechanical profile of common deformity correction constructs and assess adjacent level biomechanics.
Standard nondestructive flexibility tests (7.5 Nm) were performed on 21 cadaveric specimens: 14 pedicle subtraction osteotomies (PSOs) and 7 anterior column realignment (ACR) constructs. The ranges of motion (ROM) at the adjacent free level in flexion, extension, axial rotation, and lateral bending were measured and analyzed.
ACR constructs had a lower ROM change on flexion at the proximal adjacent free level than constructs with PSO (1.02 vs. 1.32, normalized to the intact specimen, P < 0.01). Lateral lumbar interbody fusion adjacent to PSO and 4 rods limits ROM at the free level more effectively than transforaminal interbody fusion and 2 rods in correction constructs with PSO. Use of 2 screws to anchor the ACR interbody further decreased ROM at the proximal adjacent free level on flexion, but adding 4 rods in this setting added no further limitation to adjacent segment motion.
ACR constructs have less ROM change at the adjacent level compared to PSO constructs. Among constructs with ACR, anchoring the ACR interbody with 2 screws reduces motion at the proximal adjacent free level. When PSOs are used, lateral lumbar interbody fusion adjacent to the PSO level has a greater reduction in adjacent-segment motion than transforaminal interbody fusion, suggesting that deformity construct configuration influences proximal adjacent-segment biomechanics.
To compare biomechanical stability and rod strain among uniform rod (UR), tapered rod (TR), and UR+accessory rod (AR) constructs in a human cadaveric C7 pedicle subtraction osteotomy (PSO) model of ...cervical deformity correction.
Fourteen human cadaveric C2-T4 specimens were divided into 2 statistically equivalent groups. Specimens were instrumented from C2 to T3, and a 25° PSO was performed at C7. Group 1 was instrumented with 3.5-mm to 5.5-mm titanium TRs, and group 2 received 4.0-mm titanium URs. The UR group was also tested with lateral 4.0-mm titanium ARs (UR+AR) at C5-T2. All conditions were tested with 2.0 Nm pure moment and 70 N compressive load. Intervertebral range of motion (ROM) and posterior rod strain (pRS) were measured at C2-C3, T2-T3, and the PSO level. Statistical comparisons used 1-way analysis of variance.
ROM was significantly reduced in the TR versus UR construct for right axial rotation (P = 0.04) at the PSO level; ROM with TR was significantly greater than with UR and UR+AR in compression (P ≤ 0.02). At the PSO level, pRS was significantly greater in TR than in UR+AR in flexion, extension, and right axial rotation (P ≤ 0.02). At T2/3, pRS was higher in UR than TR in left axial rotation (P = 0.003).
C7 PSO is highly destabilizing. Maximal rod strain was concentrated across the PSO and the cranial fixation site. TR provided higher stability than did UR in 1 direction of movement; however, UR+AR provided the greatest reduction of pRS.
PDF
