The lordotic shape of the lumbar spine differs substantially between individuals. Measuring and recording strain during spinal biomechanical tests is an effective method to infer stresses on spinal implants and predict failure mechanisms. The geometry of the spine may have a significant effect on the resultant force distribution, thereby directly affecting rod strain.
Seven fresh-frozen cadaveric specimens (T12-sacrum) underwent standard (7.5 Nm) nondestructive sagittal plane tests: flexion and extension. The conditions tested were intact and pedicle screws and rods (PSR) at L1-sacrum. The posterior right rod was instrumented with strain gauges between L3–4 (index level) and the L5–S1 pedicle screw. All specimens underwent lateral radiographs before testing. Lordotic angles encompassing different levels (L5–S1, L4–S1, L3–S1, L2–S1, and L1–S1) were measured and compared with rod strain. Data were analyzed using Pearson correlation analyses.
Strong positive correlations were observed between lordosis and posterior rod strain across different conditions. The L3–S1 lordotic angle in the unloaded intact condition correlated with peak rod strain at L3–4 with PSR during flexion (R=0.76, p=0.04). The same angle in the unloaded PSR condition correlated with peak strain in the PSR condition during extension (R=-0.79, p=0.04). The unloaded intact L2–S1 lordotic angle was significantly correlated with rod strain at L3–4 in the PSR condition during flexion (R=0.85, p=0.02) and extension (R=-0.85, p=0.02) and with rod strain at L5–S1 in the PSR condition during flexion (R=0.84, p=0.04).
Lordosis measured on intact and instrumented conditions has strong positive correlations with posterior rod strain in cadaveric testing.
The lordotic shape of the lumbar spine differs substantially between subjects and is intimately related to spinopelvic geometry [
The upper arch of LL is more constant, whereas the lower arc is more variable and plays a greater role in overall lordosis. The lower arc also corresponds with and reacts to the sacral slope angle. Mild changes in any of these factors can dramatically affect the distribution of mechanical loading in the entire spine, pelvis, and lower limbs [
The failure to restore the ideal LL shape might be associated with postoperative mechanical failures on long-segment constructs, such as pseudoarthrosis or rod fracture [
Measuring strain during spinal biomechanical tests effectively infers the stresses on spinal implants and predicts failure mechanisms [
Seven fresh-frozen lumbar spine cadaveric specimens (T12-sacrum) were studied (n = 7). The same specimens were also part of a separate study conducted in our laboratory to assess the subsequent stability and rod strain of different construct designs during anterior column realignment [
Specimens were stored at -20°C until test day and then thawed in normal saline at 21°C. Muscles and soft tissues were cleaned while keeping intact all ligaments, joint capsules, and intervertebral discs. The sacrum was reinforced with household wood screws placed in a rectangular metal mold and embedded using fast-curing resin (Smooth-Cast; Smooth-On, Easton, PA, USA) to permit attachment to the base of the testing apparatus. The top vertebra (T12) was also reinforced with household screws and embedded in the same resin in a cylindrical-shaped pot (≈200 g) for test frame attachment and loading.
In all cases, polyaxial pedicle screws with a cobalt-chrome head and titanium alloy shaft (Ti-6A1-4V) were used (L2–5: 6.5 × 45–55 mm, S1: 7.5 × 55 mm; NuVasive, San Diego, CA, USA). Cobalt-chrome rods were chosen over titanium because they have been gaining importance in the adult spinal deformity surgery setting. Two 5.5-mm diameter cobalt-chrome rods were contoured bilaterally to fit screw heads from L1 to S1 to minimize the need for reduction. We did not intend to change the lordosis curvature when the rod was locked in place, although minimal changes can always occur during the implantation.
In each case, a robotic 6-degree-of-freedom apparatus test frame was used to apply standard nondestructive pure moment loads up to 7.5 Nm at a mean global rotation rate of 1.5° per second [
During all tests, 3-dimensional motion measurements were made with the Optotrak 3020 camera apparatus (Northern Digital, Waterloo, Ontario, Canada). This system stereophotogrammetrically measures 3-dimensional displacement of infrared-emitting markers rigidly attached in a noncollinear arrangement to each vertebra at the ends of three 4-cm surgical guide wires drilled into each vertebral body. Range of motion was measured using custom software to convert the marker coordinates to angles about each of the anatomical axes [
Because there were no statistically significant differences between right-side and left-side rod strains in a previous study [
The specimens underwent lateral radiographs, and LL encompassing different levels (L1–S1, L2–S1, L3–S1, L4–S1, and L5–S1) was measured using the Cobb method in all different spine conditions before loading (
There were no significant differences in mean angles of the intact condition compared with the PSR condition (p>0.51), with the exception of the L4–S1 angle (p=0.01). Angles measured in intact and PSR conditions are shown in
Several correlations between lordosis angles at rest and peak posterior rod strains during loading were statistically significant (
The L3–S1 angle measured in the intact condition at rest was significantly correlated with rod strain at L3–4 in the PSR condition during bending in flexion (R=0.76, p=0.04) and without significance during extension (R=-0.73, p=0.06). L2–S1 angles measured intact during rest correlated with rod strain at L3–4 in the PSR condition during bending in flexion (R=0.85, p=0.02) and extension (R=-0.85, p=0.02), as well as with rod strain at L5–S1 in PSR during bending in flexion (R=0.84, p=0.04). For other comparisons, correlations were not statistically significant (p≥0.05).
The correlation between the L3–S1 angle measured in the PSR condition at rest and rod strain at L3–4 in the PSR condition during bending was significant during extension (R=-0.79, p=0.04), but not during flexion (R=0.74, p=0.06). For other comparisons, correlations were not statistically significant (p≥0.05).
Human bipedalism is possible because peculiarities of the spinopelvic anatomy allow humans to reach maximum equilibrium in the erect position with minimal activation of the back muscles. The most notable of these singularities is the verticalization of the pelvis and successive opposing sagittal curvatures. LL is found in no other species; great apes can achieve an upright position but only with a semierect trunk. The extensor muscles are also critical for maintaining stability during movement. Recent modeling suggests that a spine with large lordosis requires a greater follower load in the standing position than one with minimal lordosis. Increased LL requires larger extensor musculature to provide sufficient follower loads and sagittal stability [
Global lordosis increases as the sacral slope becomes more vertical, demonstrating a reciprocal association between the orientation of the sacrum and the degree of LL curvature. Patients with greater pelvic incidence and sacral slope, and consequently higher LL, are predisposed to develop lumbar spondylolisthesis because of higher shear stress on posterior elements directly affecting the isthmus, which leads to failure of this structure [
Strain monitoring during biomechanical tests has been spotlighted recently because these measurements are a good predictor of metal fatigue and risk of rod breakage [
The correlations observed in the current study are difficult to rationalize, especially under pure moment loading. As illustrated in
We found significant correlations between LL and rod strain for PSR instrumentation spanning L1 to S1. This finding supports the hypothesis that, with hyperlordotic spines, the stress distributions tend to concentrate more on the posterior column in the lumbar spine. L2–S1 intact lordosis at rest correlated with increasing strain at both the index level and lumbosacral junction in the PSR condition during loading. This finding suggests that overall intact lordosis at rest can translate to instrumented strain during loading. Both intact and PSR L3–S1 lordosis at rest correlated to L3–4 rod strain in at least one direction of bending, e.g., flexion (intact lordosis) and extension (PSR lordosis).
The current study results also corroborate the general hypothesis that sagittal alignment restoration must necessarily seek an ideal spinal equilibrium, which raises a concern of possible effects from overcorrection. In a clinical study of 96 patients (74% of whom had overcorrection), Pizones et al. [
Although restoration of LL in adult spinal deformity has been spotlighted as an important factor for favorable surgical outcomes, excessive postoperative lordosis, or sagittal overcorrection is not desired because of the increased risk of mechanical complication [
This study has several limitations. The strain gauges were only able to measure rod strain at specific locations and not throughout the instrumentation. Furthermore, cadaveric biomechanical studies have well-known limitations, including the lack of muscle activities and the use of healthy spines without a target disease that might represent the indication for the studied surgical technique. The testing paradigm used herein evaluated immediate stability and strain distributions that can affect longer-term mechanical failure in the clinical scenario. Cyclic loading was beyond the scope of the current study and is problematic because it is not possible to truly simulate the long-term
Native LL measurements before loading demonstrated strong correlations with
Jay D. Turner has served as a consultant for NuVasive (San Diego, CA) and SeaSpine (Carlsbad, CA). Juan S. Uribe has served as a consultant for NuVasive (San Diego, CA), SI Bone (Santa Clara, CA), and Misonix (Farmingdale, NY). Other authors have nothing to disclose.
Portions of this manuscript were presented at the North American Spine Society 34th Annual Meeting, September 2019, Chicago, Illinois; Spine Summit 2019, March 2019, Miami Beach, Florida; and the International Society for the Advancement of Spine Surgery 19th Annual Conference (ISASS19), April 2019, Anaheim, California. NuVasive (San Diego, CA) provided partial research support paid directly to the authors’ institution; Barrow Neurological Foundation (Phoenix, AZ) provided partial research support paid directly to the authors’ institution. The authors thank the staff of Neuroscience Publications at Barrow Neurological Institute for assistance with manuscript preparation.
Strain gauges used for strain measurement at the index level (L3–4) and lumbosacral junction (L5–S1). Adapted with permission from Barrow Neurological Institute, Phoenix, AZ, USA.
Examples of different lordosis angles measured. Adapted with permission from Barrow Neurological Institute, Phoenix, AZ, USA.
Correlations between posterior rod strain (RS) and lordotic angles in different conditions. (A) RS at L3–4 during pure moment bending versus intact L3–S1 lordosis. (B) RS at L3–4 and L5–S1 versus intact L2–S1 lordosis. (C) RS at L3–4 versus pedicle screws and rods (PSR) L3–S1 lordosis. A p-value of < 0.05 were considered statistically significant. R, coefficient of correlation. Adapted with permission from Barrow Neurological Institute, Phoenix, AZ, USA.
Photograph of a specimen instrumented with pedicle screws and rods on the testing frame showing how the rod is loaded. (A) 7.5 Nm pure moment in extension is represented by the arrow. (B) Pure moment couple in 2 opposing vertical load vectors is represented by arrows and separated by 7.5 cm (horizontal line). Adapted with permission from Barrow Neurological Institute, Phoenix, AZ, USA.
(A) Flexion-extension models simulating a 150-mm long, 5.5-mm diameter titanium alloy rod fixed at the distal end, with a slight curve (model I) and a more lordotic curve (model II), subjected to axial loads as seen during construct bending (see
Demographic variables for cadaveric spinal segments
Variable | Specimen ID |
Overall, mean ± SD | ||||||
---|---|---|---|---|---|---|---|---|
1 | 2 | 3 | 4 | 5 | 6 | 7 | ||
BMD (g/cm2) | 0.92 | 0.96 | 1.09 | 1 | 0.99 | 0.93 | 1.23 | 1.02 ± 0.11 |
Sex | Male | Male | Female | Female | Male | Male | Male | |
Age (yr) | 49 | 55 | 59 | 37 | 34 | 68 | 65 | 52 ± 13 |
Cause of death | Cardiopulmonary arrest | Pulmonary embolism | Congestive heart failure | Unknown | Sepsis; pneumonia | Liver cirrhosis | Cardiac arrest | |
BMI (kg/m2) | 32.9 | 32.5 | 36.1 | 36.9 | 25.8 | 38.8 | 33 | 33.7 ± 4.2 |
BMD, bone mineral density; BMI, body mass index; SD, standard deviation.
Data adapted from Godzik et al. Spine J 2020;20:465-74 [
Lordotic angles measured at different levels in intact and pedicle screws and rods (PSR) conditions
Lordotic angle | Intact | PSR | p-value |
---|---|---|---|
L1-S1 lordosis | 54.91 ± 14.61 | 53.51 ± 9.86 | 0.76 |
L2-S1 lordosis | 45.80 ± 11.88 | 43.84 ± 9.34 | 0.51 |
L3-S1 lordosis | 36.91 ± 10.55 | 36.55 ± 6.05 | 0.89 |
L4-S1 lordosis | 29.68 ± 6.00 | 25.46 ± 5.02 | 0.01 |
L5-S1 lordosis | 7.36 ± 7.02 | 7.22 ± 4.12 | 0.95 |
Values are presented as mean±standard deviation.
PSR, pedicle screws and rods.
p<0.05, statistically significant differences. The comparison was performed using paired t-tests.
Strain peak mean values
PSR | Specimen ID |
Mean ± SD | ||||||
---|---|---|---|---|---|---|---|---|
1 | 2 | 3 | 4 | 5 | 6 | 7 | ||
L3–4 rod strain (µε) FL | 307 | 120 | 249 | 278 | 184 | 268 | 264 | 238.6 ± 64.4 |
L3–4 rod strain (µε) EX | -364 | -110 | -233 | -318 | -198 | -315 | -309 | -263.9 ± 88.1 |
L5–S rod strain (µε) FL | 204 | -6 | NA | 221 | 5 | 277 | 170 | 145.1 ± 118.1 |
L5–S rod strain (µε) EX | -290 | -11 | NA | -297 | -37 | -510 | -191 | -222.7 ± 185.9 |
PSR, pedicle screws and rods; SD, standard deviation; FL, flexion; EX, extension; NA, not applicable.
Correlations between different lumbar lordosis angles and rod strain during different conditions by direction of loading
Specimen ID | Lordosis |
Rod strain |
Direction of loading | R | p-value | ||
---|---|---|---|---|---|---|---|
Spinal level | Spine condition | Spinal level | Spine condition | ||||
1 | L2–S1 | Intact | L3–L4 | PSR | Flexion | 0.85 | 0.02 |
2 | L2–S1 | Intact | L3–L4 | PSR | Extension | -0.85 | 0.02 |
3 | L2–S1 | Intact | L5–S1 | PSR | Flexion | 0.84 | 0.04 |
4 | L3–S1 | Intact | L3–L4 | PSR | Flexion | 0.76 | 0.04 |
5 | L3–S1 | Intact | L3–L4 | PSR | Extension | -0.73 | 0.06 |
6 | L3–S1 | PSR | L3–L4 | PSR | Flexion | 0.74 | 0.06 |
7 | L3–S1 | PSR | L3–L4 | PSR | Extension | -0.79 | 0.04 |
PSR, pedicle screws and rods at L1-sacrum; R, coefficient of correlation.
p<0.05, statistically significant differences.
p≥0.05, not statistically significant but are included for completion.