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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. [ABSTRACT FROM AUTHOR]

Study Design: 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.Objective: Compare PSO constructs with different rod configurations and adjacent-level interbody support.Summary Of Background Data: Supplemental rods and anterior column support enhance biomechanical performance.Methods: 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.Results: 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).Conclusion: 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Background Context: Anterior column realignment (ACR) is a powerful but destabilizing minimally invasive technique for sagittal deformity correction. Optimal biomechanical design of the ACR construct is unknown.Purpose: Evaluate the effect of ACR design on radiographic lordosis, range of motion (ROM) stability, and rod strain (RS) in a cadaveric model.Study Design/setting: Cadaveric biomechanical study.Patient Sample: Seven fresh-frozen lumbar spine cadaveric specimens (T12-sacrum) underwent ACR at L3-L4 with a 30° implant.Outcome Measures: Primary outcome measure of interest was maximum segmental lordosis measured using lateral radiograph. Secondary outcomes were ROM stability and posterior RS at L3/4.Methods: Effect of grade 1 and grade 2 osteotomies with single-screw anterolateral fixation (1XLP) or 2-screw anterolateral fixation (2XLP) on lordosis was determined radiographically. Nondestructive flexibility tests were used to assess ROM and RS at L3-L4 in flexion, extension, lateral bending, and axial rotation. Conditions included (1) intact, (2) pedicle screw fixation and 2 rods (2R), (3) ACR+1XLP with 2R, (4) ACR+2XLP+2R, (5) ACR+1XLP with 4 rods (4R) (+4R), and (6) ACR+2XLP+4R.Results: Segmental lordosis was similar between ACR+1XLP and ACR+2XLP (p>.28). ACR+1XLP+2R was significantly less stable than all other conditions in flexion, extension, and axial rotation (p<.014); however, adding an extra screw improved stability to levels equal to 4R conditions (p>.36). Adding 4R to ACR+1XLP reduced RS in all directions of loading (p<.048), whereas adding a second screw did not (p>.12). There was no difference in strain between ACR+1XLP+4R and ACR+2XLP+4R (p>.55).Conclusions: For maximum stability, ACR constructs should contain either fixation into both vertebral bodies (2XLP) or accessory rods (4R). 2XLP can be used without compromising the maximal achievable lordosis but does not provide the same RS reduction as 4R.Clinical Significance: ACR is a highly destabilizing technique that is increasingly being used for minimally invasive deformity correction. These biomechanical data will help clinicians optimize ACR construct design. [ABSTRACT FROM AUTHOR]

Background Context: Lumbosacral pseudoarthrosis and instrumentation failure is common with long-segment constructs. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. The influence of iliac screws and interbody type on range of motion (ROM), rod strain (RS), sacral screw strain (SS) is not well-established.Purpose: Investigate the effects of transforaminal lumbar interbody fusion (TLIF), anterior lumbar interbody fusion (ALIF), and iliac screws on long-segment lumbosacral construct biomechanics.Study Design: Biomechanical study.Patient Sample: Fourteen human cadaveric spine specimens.Outcome Measures: Lumbosacral ROM, RS, and SS.Methods: Specimens were potted at L1 and the ilium. Specimens were equally divided into either an L5-S1 ALIF or TLIF group and underwent testing in the following conditions: (1) intact (2) L2-S1 pedicle screw rod fixation (PSR-S) (3) L2-ilium (PSR-I) (4) PSR-S+ALIF (ALIF-S) or TLIF (TLIF-S) (5) PSR-I + ALIF (ALIF-I) or TLIF (TLIF-I). Pure moment bending (7.5 Nm) in flexion, extension, lateral bending, axial rotation, and compressive loads (400N) were applied and ROM, SS, and RS were measured. Comparisons were performed using a one-way ANOVA (p<.05).Results: ALIF-S and TLIF-S provided similar decreases in ROM as TLIF-I (p>.05). Compared to PSR-S, PSR-I significantly decreased SS during bending in all directions (p<.02) but increased RS in flexion and extension (p≤.02). Anterior lumbar interbody fusion-S provided similar decreases in SS as TLIF-I in all directions (p>.40) but had significantly less RS than TLIF-I in flexion, extension, compression (p<.01). TLIF-S had more SS than TLIF-I in flexion, extension, axial rotation (p<.02), while TLIF-S had less RS only in flexion (p=.03). Compared to PSR-I, ALIF-I decreased the RS (p<.02) but TLIF-I did not (p>.67).Conclusions: Iliac screws were protective of SS but increased RS at the lumbosacral junction. Constructs with ALIF and no iliac screws result in comparable SS as constructs with TLIF and iliac screws with significantly reduced RS. If iliac screws are utilized, ALIF but not TLIF reduces the iliac screw-induced RS.Clinical Significance: There is a relatively high incidence of lumbosacral instrumentation failure in adult spinal deformity. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. Iliac screws induce lumbosacral rod strain and may be responsible for instrumentation failure. Constructs with lumbosacral ALIF reduce iliac-screw induced rod strain and may obviate the need for fixation to the ilium. [ABSTRACT FROM AUTHOR]

Single-level lumbar fusion had been performed for years with little concern with final segmental angle. However, subtle changes on a segmental lordosis may mechanically impact adjacent level motion and stress triggering a domino effect of adult spinal deformity with sagittal imbalance and failed back fusion. The objective of this study was to quantify adjacent segment motion and disc surface strain changes with altered segmental angles. Study adjacent-level motion and strain distribution in cadaveric specimens with a lumbar spine single segment fusion in an offset angle. Biomechanical cadaveric study. Seven human cadaveric specimens. Range of motion (ROM) and optic strain. Seven human specimens (L2-Sacrum) underwent L4-L5 pedicle screws and rods fixation and were tested in neutral angle (NEU), imposed +5° lordosis (LOR) and -5° kyphosis (KYP). Pure moments (7.5 Nm) were applied in flexion; extension; lateral bending (LB); axial rotation (AR) followed by 400 N of compression (C) alone, and combined with pure moments. Range of motion (ROM) and strain using digital image correlation (DIC) system were tracked. Principle maximum (E1) and minimum (E2) strains were analyzed within four quarters on the lateral disc surface antero-posteriorly (Q1; Q2; Q3 and Q4). Data were analyzed using one-way RM ANOVA. At the upper adjacent level, significant increase in ROM was observed in both conditions KYP and LOR compared to NEU in flexion (p=0.001; p=<0.001) and extension (p=0.02; p=0.009). Increased ROM was also observed in LOR compared to NEU (p=0.026) and compared to KYP (p=0.004) during compression. KYP had increased ROM compared to NEU and LOR (p=0.031; p=0.025) in C+EX. LOR had increased E1 in Q3 compared to NEU in RLB (p=0.041); LOR and KYP had decreased E1 in Q3 compared to NEU in C (p=0.002; p=0.03). LOR had decreased E1 in Q3 compared to NEU (p=0.013) while KYP had increased E1 in all quartiles and increased E2 in Q2 compared to LOR in C+FL (p≤0.047). KYP decreased E1 in Q3 (p=0.021) and E2 in Q1 (p=0.006) compared to NEU while LOR had decreased E1 in Q3 (p=0.008) compared to NEU in C+EX. Lumbar spine mono-segmental fixation with 5° offset from neutral/native individual segmental angle increases the motion at adjacent level and can also induces disc strain in most direction of loads with final angle in kyphosis being worse than in lordosis. This abstract does not discuss or include any applicable devices or drugs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

BACKGROUND CONTEXT Postural stability (PS) is closely associated with quality of life and falls in the elderly. Recent work has demonstrated impaired gait and increased falls in patients with ASD, suggesting impaired postural stability due to biomechanical constraints of the spinal deformity. However, while previous studies have investigated the effect of spinal malalignment on static balance, no previous work has specifically focused on postural stability or dynamic balance in patients with ASD, nor the sensorimotor contribution to instability. Computerized dynamic posturography (CDP) is a widely used diagnostic and prognostic tool for assessment of postural instability in diseases of gait and imbalance such as Parkinson's disease, osteoarthritis, and vestibular dysfunction. PURPOSE The objective of our study was to assess postural stability of patients with adult spinal deformity (ASD) using cCDP and (1) compare with published normative controls, (2) determine the relationship between fall status and postural stability, and (3) determine the relationship between quality of life and postural stability in ASD STUDY DESIGN/SETTING Prospective analysis of postural stability using CDP. PATIENT SAMPLE Eleven consecutive patients with severe adult spinal deformity who were seen and evaluated in an outpatient clinic. OUTCOME MEASURES Equilibrium scores (ES) during Sensory Organization Testing (SOT) across 6 conditions with varying sensorimotor deprivation, motor latency during Motor Control Testing (MCT), and a history of falls in community setting. METHODS Eleven patients with ASD were prospectively enrolled. Inclusion criteria: coronal cobb (CC)≥20°, Pelvic incidence-lumbar lorodsis (PI-LL) >10°, sagittal vertical axis (SVA) ≥5 cm, pelvic tilt (PT)≥25°, and thoracic kyphosis (TK)≥60°. Exclusion criteria: inability to stand, presence of confounding neurological conditions. PS was determined using two randomly assigned tests with CDP: (1) SOT to evaluate the voluntary use of sensory information; and (2) motor control test (MCT) to assess involuntary postural reactions in response to external perturbations. Patient performance was compared to age-gender matched controls. QOL metrics included SRS-22 and SF-36 RESULTS Mean age was 65±4.6, and 6/13 (46%) were female. Mean PI-LL was 31.2±12.7, PT 32.8±8.0, SVA 11.8±6.3 cm, TK 27.3±16.6, and CC 14.5±16.7. Four patients reported falling in the previous 6 months (36%). Those patients who self reported falling in past 6 months demonstrated significantly lower SOT (Conditions 2–6, p<.04) and SRS scores (p=.003). ASD patients demonstrated significantly lower SOT scores (Conditions 2–6, p<.03), and greater latency of limb reaction time during backward translation (p=.0379) compared to age-matched controls-consistent with a higher global instability and elevated risk of falls. Mediolateral sway (predictor of falls) was correlated with TK in the eyes open and eyes closed conditions (p<.04). CONCLUSIONS ASD patients demonstrate impaired postural stability and dynamic balance compared to normal controls. Further, postural stability and QOL metrics correlated with self-reported falls. These findings suggest that ASD patients have drastically altered balance, which predisposes these patients to elevated risk of falls and injury as well as decreased ambulatory confidence and quality of life. [ABSTRACT FROM AUTHOR]

Proximal junctional kyphosis and lumbosacral (LS) pseudoarthrosis/instrumentation failure are relatively common complications following long instrumentation constructs to the sacrum. Methods to improve fusion rates and decrease instrumentation failure include LS anterior column support (ALIF or TLIF), iliac screw fixation, and accessory rods. The impact of these lumbosacral augmentation strategies on the proximal junction is not clear. The purpose of this study was to investigate the impact of various lumbosacral constructs on proximal junction biomechanics. Human biomechanical cadaveric study. Twelve (L1-Pelvis) cadaveric spines were studied. Proximal junction range of motion (ROM) at L1-2 was measured. Fourteen human cadaveric spine (L1-Ilium) specimens were prepared and potted at L1 and Ilium. Specimens were equally divided into either an L5-S1 ALIF or TLIF group. 4R conditions consisted of accessory rods spanning the L3-L4 and S1-Ilium levels. Compression (400 N) in combination with 7.5 Nm of flexion (FL), extension (EX), lateral bending (LB), or axial rotation (AR) was applied to all conditions. Specimens underwent testing in the following conditions: (1) intact, (2) L2-S1 pedicle screw fixation (PSR), (3) L2-ilium (PSR-I), (4) PSR+ALIF (ALIF-S) or TLIF (TLIF-S), (5) PSR-I + ALIF (ALIF-I) or TLIF (TLIF-I), and (6) ALIF-I + 4R or TLIF-I + 4R. Statistical comparisons were performed using one-way (RM) ANOVA (p<.05). PSR-I did not significantly change proximal junction ROM in any direction compared to PSR-S (p>.069). However, TLIF-I and TLIF+4R resulted in significant increase in L1-2 ROM in FL and Right LB (p<.038) compared To PSR-I. TLIF-I+4R did not change ROM compared to TLIF-I in any direction (p>.095). ALIF-I did not significantly change ROM in any direction compared to ALIF+4R or PSR-I (p>.069). In cadavers with pelvic fixation, lumbosacral TLIF results in increased ROM at the proximal junction while ALIF does not. Further augmentation of either ALIF or TLIF constructs with lumbosacral accessory rods does not impact proximal junctional ROM. This abstract does not discuss or include any applicable devices or drugs. [ABSTRACT FROM AUTHOR]

BACKGROUND CONTEXT Lumbosacral pseudoarthrosis and instrumentation failure is common with long segment constructs. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. The influence of iliac screws and interbody type on lumbosacral stability and instrumentation bending strain is not well established. PURPOSE This study investigated the effects of TLIF, ALIF, and iliac screws on long segment lumbosacral construct biomechanics. STUDY DESIGN/SETTING In vitro biomechanical study using human cadaveric specimens. PATIENT SAMPLE 14 (L1-pelvis) cadaveric spines (5F/9M, 51.6±7.4years, DEXA 0.837 g/cm2) OUTCOME MEASURES Range of motion (ROM), sacral screw bending strain (SS), and L5-S1 rod strain (RS). METHODS Fourteen human cadaveric spine (L1-ilium) specimens were prepared and potted at L1 and ilium. Specimens were equally divided into either an L5-S1 ALIF or TLIF group. All specimens underwent testing in the following conditions: (1) Intact (2) L2-S1 pedicle screw fixation (PSF) (3) L2-ilium (PSF-I) (4) PSF+ALIF (ALIF-S) or TLIF (TLIF-S) (5) PSF-I + ALIF (ALIF-I) or TLIF (TLIF-I). Pure moment bending (7.5 Nm) in flexion (F), extension (E), lateral bending (LB), axial rotation (AR) and axial compressive (C) loads (400 N) were applied to all conditions and ROM, SS, and RS were measured. Statistical comparisons were performed using one-way ANOVA (p<.05). RESULTS ALIF-S and TLIF-S provided similar decreases in ROM as TLIF-I (p>.05). Compared to PSF, iliac screws significantly decreased SS during bending in all directions (p<.01) except lateral bending (p>.16) but increased RS in flexion and extension (p<.03). ALIF-S provided similar decreases in SS as TLIF-I in all directions (p>.40). TLIF-S had more SS than TLIF-I in F,E,AR (p<.02). ALIF-S had significantly less rod strain than TLIF-I in F,E,C (p<.02) while TLIF-S had less rod strain only in E (p=.04). Compared to PSF-I, ALIF-I decreased the RS (p<.03) but TLIF-I did not have a significant effect (p>.64). CONCLUSIONS Iliac screws were most protective of SS but resulted in increased RS. Constructs with ALIF and no iliac screws provide comparable decreases in SS as constructs with TLIF and iliac screws; furthermore, RS was significantly less. If iliac screws are utilized for fixation, ALIF but not TLIF reduces the RS induced by iliac screws. [ABSTRACT FROM AUTHOR]

BACKGROUND CONTEXT Variations in load distribution based on surface strain on instrumentation spanning the lumbosacral junction during in vitro loading are often used to evaluate biomechanical effects of different PSR configurations. The gold standard for biomechanical comparisons of spinal instrumentation involves the application of pure moment loads to induce bending of the instrumented spine. However spinal constructs in vivo are subjected to substantial vertical compressive forces due to gravitational loading of upper body mass. The effects of added compressive loads during in vitro bending of the PSR instrumented lumbar spine, as determined by surface strains at the lumbosacral level, are not well understood. PURPOSE The purpose of this study was to analyze the biomechanical differences in lumbosacral rod strain, and sacral screw bending moments during sagittal plane bending, with and without a compressive preload. STUDY DESIGN/SETTING In vitro biomechanical study using human cadaveric specimens. PATIENT SAMPLE A total of 14 (L1-Pelvis) cadaveric spines (5F/9M, 51.6±7.4 years, DEXA 0.837g/cm2). OUTCOME MEASURES L5-S rod strains and resultant sacral screw bending loads. METHODS A total of 14 cadaveric specimens were tested in a 6DOF robot under continuous loading in two PSR configurations (L2-S and L2-IL) using five types of loads: 7.5Nm flexion (FL), extension (EX), 400 N compression (C), combined C+FL and combined C+EX. Strain gauges were used to measure rod strains (RS) at L5-S and net sacral screw bending moments (SS). Data were analyzed using One-way RM-ANOVA followed by Holms-Sidak paired analysis (p<0.05). RESULTS RS significantly increased with L2IL versus L2S (FL: +77%; EX: +65%; C+FL: +46%; C: +40%; p<.004), but without significance during C+EX (+20%, p=.224). SS significantly decreased with L2IL versus L2S (FL: −78%; EX: −81%; C+FL: −52%; C+EX: −33%; C: −50%; p=.049).Compression loading alone induced significantly greater RS and SS than pure moment FL and EX in both L2S and L2IL groups (p<.001). Adding C to FL significantly increased RS in L2S (+170%) and L2IL groups (+123%) (p<.001), and SS in L2S (+86%) and L2IL (+292%) (p<.001).Adding C to EX significantly increased RS in L2S (+76%; p<0.001) and L2IL groups (+27%; p=.043), as well as SS in L2IL (+277%; p<.001) only. Compared to C alone, C+FL increased RS and SS in both groups but without significance, while C+EX tended to reduce RS with significance for L2IL only (−21%; p=.007), and reduce SS with significance for L2S only (−31%; p=.002). In the presence of C, SS was greater and more sensitive to the direction of loading (C+FL vs. C+EX) with L2S (p<.001) versus L2IL (p=.510), while RS was more sensitive to the direction of loading with L2IL (p<.001) versus L2S (p=.430). These differences were not present for magnitudes of RS or SS with pure moment loads (FL vs. EX [p>.2]). CONCLUSIONS The relative effects of L2IL versus L2S (increased RS and reduced SS) were consistent among all types of loads. However, the additive effects of compression on RS and SS varied with type of instrumentation (L2S vs. L2IL) and direction of bending (FL vs. EX). Compression loading significantly influences instrumentation performance and can help provide improved understanding of lumbosacral junction load distributions with PSR constructs. [ABSTRACT FROM AUTHOR]