Use of the dual construct lowers rod strains in flexion-extension and lateral bending compared to two-rod and two-rod satellite constructs in a cadaveric spine corpectomy model.

BACKGROUND CONTEXT: Complex spinal reconstructions involving corpectomies, or osteotomies, place spinal implants at extremely high stresses that can lead to pseudoarthrosis and ultimately to rod failure, resulting in revision surgery. Current clinical options to increase the biomechanical strength of a construct include increasing rod diameter, changing rod material, or placing an additional satellite/outrigger rod on a standard two rod construct. Fundamentally, all of these constructs still rely on two longitudinal rods across the reconstruction site and are therefore at risk for rod fracture and loss of alignment. Initially described in 2006, the Dual Construct was developed to address this limitation by utilizing four distinct mechanically independent rods, which allowed for the creation of two separate, and distinct, constructs within each patient. Although there is early clinical evidence to support its efficacy, this is the first biomechanical study to compare the Dual Construct to the two-rod and two-rod with satellite configurations in a cadaveric study. PURPOSE: To assess the biomechanical impact of the Dual Construct technique to traditional two-rod and two-rod with satellite rod construct in a cadaveric model. STUDY DESIGN/SETTING: Biomechanical cadaveric study METHODS: Nine fresh-frozen human cadaveric spines (6 males, 3 females, 56 year +/- 9 years) from T9-pelvis were instrumented and tested utilizing all three configurations including two-rod construct, two-rod with satellite construct, and the Dual Construct technique. Biomechanical testing order of the various constructs was randomized to reduce potential effects of order bias. Strain gauges were placed in both the coronal and sagittal planes of the rods to track the strains during flexion-extension and lateral bending while undergoing range of motion testing. Testing was performed using pure-moment flexibility testing protocols. RESULTS: In flexion-extension, the resultant strain in the two-rod construct was an average 600±228 microstrain, the two-rod with satellite rod strain averaged 603±237 microstrain, and the Dual Construct averaged 403±149 microstrain. In lateral bending, the resultant strain in the two-rod construct was an average of 266±134 microstrain, the satellite rod strain was an average of 310±158 microstrain, and the Dual Construct averaged 118±51 microstrain. In both flexion extension and lateral bending, a significant reduction in strain was observed between the Dual Construct condition compared to both the two-rod and satellite configurations. No significant difference was found between the two-rod and two-rod with satellite rod configurations. CONCLUSIONS: The increase in load sharing significantly decreases the strain experienced across the Dual Construct compared to traditional two-rod and two-rod with satellite constructs. Global rod strains on primary rods cannot be reduced by simply increasing the number of satellite rods, but can only be reduce by increasing the actual number of primary rods.

Effects of misalignment on static torsional strength of anterior cervical plate systems.

BACKGROUND CONTEXT: There is little understanding of cervical plate misalignment as a risk factor for plate failure at the plate-screw-bone interface. PURPOSE: To assess the torsional strength and mode of failure of cervical plates misaligned relative to the midsagittal vertical axis. STUDY DESIGN: Plastic and foam model spine segments were tested using static compression and torsion to assess effects of misaligned and various lengths anterior cervical plate (ACPs). METHODS: Different length ACPs and cancellous fixed angle screws underwent axial torsional testing on a servo-hydraulic test frame at a rate of 0.5°/s. A construct consisted of one ACP, four screws, one ultrahigh-molecular weight polyethylene inferior block, and one polyurethane foam superior block. Group 1 had ACPs aligned in the midsagittal vertical axis, group 2 plates were positioned 20° offset from the midline, and group 3 had the ACP shifted 5 mm away and 20° offset from midline. Torques versus angle data were recorded. The failure criterion was the first sign of pullout determined visually and graphically. RESULTS: Group 1 had a more direct screw pullout during failure. For the misaligned plates, failure was a combination of the screws elongating the holes and shear forces acting between the plate and block. The misaligned plates needed more torque to failure. The failure torque was 50% reduced for the longer versus the shorter plates in the neutral position. Graphically shown initial screw slippage inside the block preceded visual identification of slippage in some cases. CONCLUSIONS: We observed different failure mechanisms for neutral versus misaligned plates. Clinically, misalignment may have the benefit of needing more torque to fail. Misalignment was a risk factor for failure of the screw-bone interface, especially in longer plate constructs. These comparisons of angulations may be a solid platform for expansion toward a more applicable in vivo model.

Biomechanical assessment of a PEEK rod system for semi-rigid fixation of lumbar fusion constructs.

The concept of semi-rigid fixation (SRF) has driven the development of spinal implants that utilize nonmetallic materials and novel rod geometries in an effort to promote fusion via a balance of stability, intra- and inter-level load sharing, and durability. The purpose of this study was to characterize the mechanical and biomechanical properties of a pedicle screw-based polyetheretherketone (PEEK) SRF system for the lumbar spine to compare its kinematic, structural, and durability performance profile against that of traditional lumbar fusion systems. Performance of the SRF system was characterized using a validated spectrum of experimental, computational, and in vitro testing. Finite element models were first used to optimize the size and shape of the polymeric rods and bound their performance parameters. Subsequently, benchtop tests determined the static and dynamic performance threshold of PEEK rods in relevant loading modes (flexion-extension (F/E), axial rotation (AR), and lateral bending (LB)). Numerical analyses evaluated the amount of anteroposterior column load sharing provided by both metallic and PEEK rods. Finally, a cadaveric spine simulator was used to determine the level of stability that PEEK rods provide. Under physiological loading conditions, a 6.35 mm nominal diameter oval PEEK rod construct unloads the bone-screw interface and increases anterior column load (approx. 75% anterior, 25% posterior) when compared to titanium (Ti) rod constructs. The PEEK construct's stiffness demonstrated a value lower than that of all the metallic rod systems, regardless of diameter or metallic composition (78% < 5.5 mm Ti; 66% < 4.5 mm Ti; 38% < 3.6 mm Ti). The endurance limit of the PEEK construct was comparable to that of clinically successful metallic rod systems (135N at 5 × 10(6) cycles). Compared to the intact state, cadaveric spines implanted with PEEK constructs demonstrated a significant reduction of range of motion in all three loading directions (> 80% reduction in F/E, p < 0.001; > 70% reduction in LB, p < 0.001; > 54% reduction in AR, p < 0.001). There was no statistically significant difference in the stability provided by the PEEK rods and titanium rods in any mode (p = 0.769 for F/E; p = 0.085 for LB; p = 0.633 for AR). The CD HORIZON(®) LEGACY(™) PEEK Rod System provided intervertebral stability comparable to currently marketed titanium lumbar fusion constructs. PEEK rods also more closely approximated the physiologic anteroposterior column load sharing compared to results with titanium rods. The durability, stability, strength, and biomechanical profile of PEEK rods were demonstrated and the potential advantages of SRF were highlighted.

Urea transporter UT-A1 and aquaporin-2 proteins decrease in response to angiotensin II or norepinephrine-induced acute hypertension.

The kidney responds to high levels of ANG II, as may occur during malignant hypertension, by increasing sodium and water excretion. To study whether kidney medullary transporters contribute to this response, rats were made hypertensive using ANG II. Within 3 days of being given ANG II, systolic blood pressure (BP) was increased (200 mmHg), vs control (130 mmHg), and remained high through day 14. Kidney inner medullary (IM) tip and base and outer medulla were analyzed for transporter protein abundance. There were significant decreases in UT-A1 urea transporter, aquaporin-2 (AQP2) water channel, and NKCC2/BSC1 Na(+)-K(+)-2Cl(-) cotransporter. To determine whether the decreases were a response to hypertension, ANG II, or an ANG II-induced increase in aldosterone, rats were given 1) norepinephrine (to increase BP) and 2) ANG II plus spironolactone (to block the mineralocorticoid receptor). Norepinephrine (7 days) increased BP, urine volume, sodium excretion, and decreased urine osmolality and UT-A1, AQP2, and NKCC2/BSC1 abundances, similar to ANG II. ANG II alone or with spironolactone yielded similar increases in BP, urine volume, and urine osmolality, and decreases in UT-A1 and AQP2 proteins in the IM tip. Plasma vasopressin was unaffected by treatment. Water diuresis did not change UT-A1 but decreased AQP2 and NKCC2/BSC1 abundances. We conclude that decreases in UT-A1, AQP2, and NKCC2/BSC1 proteins may contribute to the diuresis and natriuresis that occur following ANG II or norepinephrine-induced acute hypertension and do not appear to involve ANG II stimulation of aldosterone or thirst.

Urea may regulate urea transporter protein abundance during osmotic diuresis.

Rats with diabetes mellitus have an increase in UT-A1 urea transporter protein abundance and absolute urea excretion, but the relative amount (percentage) of urea in total urinary solute is actually decreased due to the marked glucosuria. Urea-specific signaling pathways have been identified in mIMCD3 cells and renal medulla, suggesting the possibility that changes in the percentage or concentration of urea could be a factor that regulates UT-A1 abundance. In this study, we tested the hypothesis that an increase in a urinary solute other than urea would increase UT-A1 abundance, similar to diabetes mellitus, whereas an increase in urine urea would not. In both inner medullary base and tip, UT-A1 protein abundance increased during NaCl- or glucose-induced osmotic diuresis but not during urea-induced osmotic diuresis. Next, rats undergoing NaCl or glucose diuresis were given supplemental urea to increase the percentage of urine urea to control values. UT-A1 abundance did not increase in these urea-supplemented rats compared with control rats. Additionally, both UT-A2 and UT-B protein abundances in the outer medulla increased during urea-induced osmotic diuresis but not in NaCl or glucose diuresis. We conclude that during osmotic diuresis, UT-A1 abundance increases when the percentage of urea in total urinary solute is low and UT-A2 and UT-B abundances increase when the urea concentration in the medullary interstitium is high. These findings suggest that a reduction in urine or interstitial urea results in an increase in UT-A1 protein abundance in an attempt to restore inner medullary interstitial urea and preserve urine-concentrating ability.

Host associations of the tick, Ixodes angustus (Acari: Ixodidae), on Alaskan mammals.

Infestation parameters are presented for 227 ticks, all Ixodes angustus Neumann, collected from individual mammals (n = 531) in southeastern and south-central Alaska from 1996 to 1999. This tick was recovered from 12 of the 19 mammal species examined, with four species of shrews (Sorex spp.), two species of voles [Clethrionomys gapperi (Vigors) and Clethrionomys rutilus (Pallas)], one species of mouse [Peromyscus keeni (Rhoads)], and the red squirrel [Tamiasciurus hudsonicus (Erxleben)] being the most frequently parasitized hosts. Larvae (n = 146) of I. angustus were collected most often, followed by nymphs (n = 50), females (n = 30), and a single male. The leptinid mammal-nest beetle Leptinus occidentamericanus Peck (1 male, 5 females) was also recovered from five individual small mammals; three of these were C. rutilus.