Predicting Pedicle Screw Pullout and Fatigue Performance: Comparing Lateral Dual-Energy X-Ray Absorptiometry, Anterior to Posterior Dual-Energy X-Ray Absorptiometry, and Computed Tomography Hounsfield Units.

BACKGROUND: As the prevalence and associated health care costs of osteoporosis continue to rise in our aging population, there is a growing need to continue to identify methods to predict spine construct integrity accurately and cost-effectively. Dual-energy x-ray absorptiometry (DEXA) in both anterior to posterior (AP) and lateral planes, as well as computed tomography (CT) Hounsfield units (HU), have all been investigated as potential preoperative predictive tools. The purpose of this study is to determine which of the 3 bone density analysis modalities has the highest potential for predicting pedicle screw biomechanics. METHODS: Lumbar spine specimens (L2, L3, and L4) from 6 fresh frozen cadavers were used for testing. AP-DEXA, lateral-DEXA, and CT images were obtained. Biomechanical testing of pedicle screws in each vertebrae was then performed including pullout strength and fatigue testing. Statistical analysis was performed. RESULTS: Pullout strength was best predicted by CT HU, followed by AP-DEXA, then lateral-DEXA (R 2 = 0.78, 0.70, and 0.40, respectively). Fatigue testing showed a significant correlation of relative rotation between HU value and AP-DEXA bone mineral density (R 2 = 0.54 and R 2 = 0.72, respectively), and there was a significant correlation between relative translation and HU value (R 2 = 0.43). There was a poor correlation between relative rotation and lateral-DEXA (R 2 = 0.13) as well as a poor correlation between relative translation and both AP- and lateral-DEXA (R 2 = 0.35 and R 2 = 0.02). CONCLUSIONS: CT is the only modality with a statistically significant correlation to all biomechanical parameters measured (pullout strength, relative angular rotation, and relative translation). AP-DEXA also predicts the biomechanical measures of screw pullout and relative angular rotation and is superior to lateral-DEXA. CT may provide an incremental benefit in assessing fatigue strength, but this should be weighed against the disadvantages of cost and radiation. CLINICAL RELEVANCE: The results of this study can help to inform clinicians on different bone density analyses and their implications on pedicle screw failure.

Novel microcomposite implant for the controlled delivery of antibiotics in the treatment of osteomyelitis following total joint replacement.

The objective of this study was to develop a novel microcomposite implant to be used in the treatment of osteomyelitis following total joint arthroplasty, with the dual purpose of releasing high local concentrations of antibiotic to eradicate the infection while providing adequate mechanical strength to maintain the dynamic or static spacer. Vancomycin-loaded microcomposite implants were fabricated by incorporating drug-loaded microparticles comprised of mesoporous silica into commonly employed polymethylmethacrylate (PMMA) bone cement, to yield a final drug loading of 10% w/w. In vitro release kinetics at 37°C were monitored by reverse-phase high-performance liquid chromatography, and compared to the release kinetics of current therapy implants consisting of drug alone incorporated at 10% w/w directly into PMMA bone cement. Results demonstrated a sevenfold improvement in the elution profile of microcomposite systems over current therapy implants. In vivo delivery of vancomycin to bone from microcomposite implants (70% of payload) was significantly higher than that from current therapy implants (approx. 22% of payload) and maintained significantly higher bone concentrations for up to 2 weeks duration. The elastic modulus showed no statistical difference between microcomposite implants and current standard therapy implants before drug elution, and maintenance of acceptable strength of microcomposite implants postdrug elution. These results demonstrate that we have developed a novel microcomposite spacer that will release continuously high antibiotic concentrations over a prolonged period of time, offering the possibility to eliminate infection and avoid the emergence of new resistant bacterial strains, while maintaining the requisite mechanical properties for proper space maintenance and joint fixation.

External fixator-augmented flexible intramedullary nailing of an unstable pediatric femoral shaft fracture model: a biomechanical study.

The objective of this study was to test the compressive strength and torsional stiffness provided by the addition of a two-pin external fixator to an unstable pediatric femoral shaft fracture model after being instrumented with flexible intramedullary nailing (FIMN), and to compare this to bridge plating and FIMN alone. A length-unstable oblique diaphyseal fracture was created in 15 pediatric sized small femur models. Fracture stabilization was achieved by three constructs: standard retrograde FIMN with two 3.5-mm titanium (Ti) nails (Group 1), FIMN augmented with a two-pin external fixator (Group 2), and a 4.5-mm bridge plate (Group 3). Groups I and II were tested in 10 cycles of axial rotation to 10° in both directions at 0.1 Hz under 36 kg of compression. Torsional stiffness was calculated. Compressive strength was calculated by applying an axial load of 5 mm/min until failure was encountered. Failure was defined as the force required to achieve 10° varus at the fracture site or shortening of 2 cm. Group II demonstrated a greater compressive strength compared to Group I (1067.32 N vs 453.49 N, P < 0.001). No significant difference in torsional stiffness was found between Groups I and II (0.45 vs 0.38 Nm/deg, P = 0.18). Group III showed superior compressive strength and rotational stiffness compared to Groups I and II. In an unstable pediatric femoral shaft fracture model, augmenting FIMN with a two-pin external fixator increased the compressive strength by 147%, but did not increase torsional stiffness. Bridge plating with a 4.5-mm plate provided superior compressive strength and torsional stiffness.

Sacral Endplate Penetrating Screw for Lumbosacral Fixation: A Cadaveric Biomechanical Study.

BACKGROUND: Cortical bone trajectory is a relatively new alternative for instrumentation of the lumbar spine. When performing lumbosacral instrumentation, a novel S1 endplate penetrating screw (EPS) has been recently shown to have higher insertional torque than the traditional trajectory screw, but the biomechanical properties of this new trajectory are yet to be verified with the cadaveric studies. OBJECTIVE: To evaluate 2 screw trajectories in sacra using cyclic loading and pullout tests, and to determine whether bone quality had different effects on the 2 trajectories. METHODS: Nine cadaveric sacra were used, 5 of which had normal bone mineral density (BMD) and 4 were osteoporotic. Each side of the sacra was randomly assigned to either EPS trajectory or S1-alar screw (S1AS) trajectory. Each screw then underwent cyclic loading followed by pullout force measurement. A mixed-design 2 way ANOVA test was used to detect differences between the groups. RESULTS: The EPS group had less relative rotation at the bone-screw interface during cyclic loading than the S1AS group (P = .016) regardless of bone quality. The pullout force following the cyclic loading was significantly higher in the EPS group (2349 ± 838 N) than the S1AS group (917 ± 909 N) in normal bone (P < .0001). The difference was more pronounced in osteoporotic bone with the EPS (1075 ± 216 N) compared to the S1AS (365 ± 422 N; P < .0001). CONCLUSION: The S1 EPS trajectory is significantly more stable against loosening and has a higher pullout force compared to the S1AS trajectory. The difference between the 2 trajectories is more pronounced in osteoporotic bone.

K-wire Pull-Out Force After Multiple Redirection Attempts.

PURPOSE: To evaluate if redirecting a Kirschner wire (K-wire) through the same proximal hole will weaken the pull-out force and to test if multiple redirections will result in a continued stepwise decrease in pull-out force. METHODS: An Instron was used to test the pull-out force of K-wires using the peak initial failure load as a measure of failure of K-wire fixation. K-wires 0.062 inches in diameter were inserted with an angled drill guide into a bicortical bone substrate. Trials were divided into 7 groups with the first group having the K-wires placed through both cortices and then tested without redirection. In groups 2-6, the K-wire was placed bicortically and then withdrawn and redirected through the same proximal hole with 1, 2, 3, 4, and 5 redirections. A control group in which the K-wire was only unicortical was also tested. RESULTS: Compared with the control group of no redirects, any number of redirections weakened the pull-out force. There was no difference between redirected groups and the unicortical group. When comparing between redirections, there were no significant differences in pull-out force. Regression analysis showed that, after the first redirection, there was no stepwise change in pull-out force with additional redirection. CONCLUSIONS: There was a significant decrease in pull-out force with any redirections, but there was no stepwise decrease in failure force after multiple redirections. The failure force of any redirection was similar to a unicortically placed wire. CLINICAL RELEVANCE: Any K-wire redirection attempts in hand bone fixation can result in a considerably weakened construct.

Pedicle screws with modular head vs. preassembled head used in cortical bone trajectory: Can pars and pedicle fractures be prevented in osteoporotic bone?

Pars and pedicle fractures as a result of CBT (cortical bone trajectory) during pedicle screw placement have been reported. The primary aim of the study is to compare the fracture rate between screws with modular heads to screws with standard pre-assembled tulip heads. The secondary aim of the study is to determine the potential variables that can be identified prior to instrumentation in order to predict risk of fractures. Twenty-four fresh frozen lumbar vertebrae were obtained from five different cadavers. Anatomical landmark measurements were obtained. Right and left pedicles of each vertebra were randomly instrumented with the preassembled head screws (n=24) and modular head screws (n=24) under video recording. X-ray images were obtained for measuring relative angle deviations between tapped and final screw trajectories. Finally, pullout tests were performed. Seventeen out of twenty-four (70.8%) of the spinous processes had to be excised in order to obtain proper trajectories. Six fractures occurred with pre-assembled head screws versus one in the modular head screws (p=0.04). Distances from the midline to the medial wall of the pedicle were marginally significant as a predictor for fracture (p=0.08). The pullout loads between both types of screws were not statistically different (p=0.38). Age was better correlated with pullout load than absolute bone density value (p<0.001). In conclusion, modular head screws had a significantly lower fracture rate than pre-assembled head screws for cortical bone trajectory in osteoporotic bone. There was no clear anatomic variable that could be measured pre-operatively to predict potential fracture risk in CBT.

Temperature Rise in Kirschner Wires Inserted Using Two Drilling Methods: Forward and Oscillation.

BACKGROUND: Kirschner wires (K-wires) are commonly used in orthopedic surgery. However, the loosening of the pins can lead to delayed or improper healing or infection. Wire loosening can occur by thermal necrosis that occurs due to heat produced during wire insertion. Although the parameters that affect temperature rise in cortical bone during wire insertion and drilling have been studied, the effect of drilling mode (oscillation versus forward) is unknown. The purpose of this study was to compare the temperature changes occurring in cortical bone during wire insertions by oscillating and forward drills. Our hypothesis is that oscillation drilling would produce less heat compared with forward drilling in K-wire insertion with 2 commonly used wire diameters. METHODS: We drilled K-wires in a pig metacarpal model and measured the temperature rise between forward and oscillation drilling modes using diamond-tipped 0.062- and 0.045-inch-diameter K-wires. There were 20 holes drilled for each group (n = 20). RESULTS: The average temperature rise using the 0.062-inch K-wire under forward and oscillation insertion was 14.0 ± 5.5°C and 8.8 ± 2.6°C, respectively. For the 0.045-inch K-wire, under forward and oscillation insertion, the average temperature rise was 11.4 ± 2.6°C and 7.1 ± 1.9°C, respectively. The effects of the drilling mode and wire diameter on temperature rise were significant ( P < .05). CONCLUSIONS: In conclusion, the oscillation of K-wires during insertion causes a lower temperature rise when compared with forward drilling.

C1 Lateral Mass Displacement and Transverse Atlantal Ligament Failure in Jefferson's Fracture: A Biomechanical Study of the "Rule of Spence".

BACKGROUND: Jefferson's fracture, first described in 1927, represents a bursting fracture of the C1 ring with lateral displacement of the lateral masses. It has been determined that if the total lateral mass displacement (LMD) exceeds 6.9 mm, there is high likelihood of transverse atlantal ligament (TAL) rupture, and if LMD is less than 5.7 mm TAL injury is unlikely. Several recent radiographic studies have questioned the accuracy and validity of the "rule of Spence" and it lacks biomechanical support. OBJECTIVE: To determine the amount of LMD necessary for TAL failure using modern biomechanical techniques. METHODS: Using a universal material testing machine, cadaveric TALs were stretched laterally until failure. A high-resolution, high-speed camera was utilized to measure the displacement of the lateral masses upon TAL failure. RESULTS: Eleven cadaveric specimens were tested (n = 11). The average LMD upon TAL failure was 3.2 mm (±1.2 mm). The average force required to cause failure of the TAL was 242 N (±82 N). From our data analysis, if LMD exceeds 3.8 mm, there is high probability of TAL failure. CONCLUSION: Our findings suggest that although the rule of Spence is a conceptually valid measure of TAL integrity, TAL failure occurs at a significantly lower value than previously reported (P < .001). Based on our literature review and findings, LMD is not a reliable independent indicator for TAL failure and should be used as an adjunctive tool to magnetic resonance imaging rather an absolute rule.

Degradation of Bioresorbable Mg-4Zn-1Sr Intramedullary Pins and Associated Biological Responses in Vitro and in Vivo.

This article reports the degradation and biological properties of as-drawn Mg-4Zn-1Sr (designated as ZSr41) and pure Mg (P-Mg) wires as bioresorbable intramedullary pins for bone repair. Specifically, their cytocompatibility with bone marrow derived mesenchymal stem cells (BMSCs) and degradation in vitro, and their biological effects on peri-implant tissues and in vivo degradation in rat tibiae were studied. The as-drawn ZSr41 pins showed a significantly faster degradation than P-Mg in vitro and in vivo. The in vivo average daily degradation rates of both ZSr41 and P-Mg intramedullary pins were significantly greater than their respective in vitro degradation rates, likely because the intramedullary site of implantation is highly vascularized for removal of degradation products. Importantly, the concentrations of Mg2+, Zn2+, and Sr2+ ions in the BMSC culture in vitro and their concentrations in rat blood in vivo were all lower than their respective therapeutic dosages, i.e., in a safe range. Despite of rapid degradation with a complete resorption time of 8 weeks in vivo, the ZSr41 intramedullary pins showed a significant net bone growth because of stimulatory effects of the metallic ions released. However, proportionally released OH- ions and hydrogen gas caused adverse effects on bone marrow cells and resulted in cavities in surrounding bone. Thus, properly engineering the degradation properties of Mg-based implants is critical for harvesting the bioactivities of beneficial metallic ions, while controlling adverse reactions associated with the release of OH- ions and hydrogen gas. It is necessary to further optimize the alloy processing conditions and/or modify the surfaces, for example, applying coatings onto the surface, to reduce the degradation rate of ZSr41 wires for skeletal implant applications.

A biomechanical comparison of the two- and four-hole side-plate dynamic hip screw in an osteoporotic composite femur model.

OBJECTIVES: The objectives of this study were (1) to compare the axial and torsional stiffness of a dynamic hip screw with a two- and four-hole side-plate in a synthetic model of a healed and stable intertrochanteric femur fracture and (2) to evaluate the load to failure, as well as propensity to peri-implant fracture. METHODS: Fourth-generation synthetic composite femur models, simulating osteoporotic bone, were implanted with 135° dynamic hip screws (DHS) with either a two- or four-hole side-plate with or without a stable intertrochanteric fracture. Specimens were cyclically loaded up to a nondestructive load to determine the axial and torsional stiffness. Constructs were then loaded to failure in axial compression emulating physiologic forces. Failure load and location of the peri-implant fractures were recorded. RESULTS: Axial and torsional stiffness did not differ significantly between the two- and four-hole constructs in either model. Likewise, there was no significant difference in the load to failure. In the intact femurs, failure occurred either at the end of the plate at the distal screw or through the lag screw hole. CONCLUSION: The results of this study demonstrate that DHS constructs with a two- or four-hole side-plate are biomechanically comparable with regard to axial and torsional stiffness and load to failure in an osteoporotic composite femur model. In a healed intertrochanteric fracture model, a two-hole construct did not appear to be more prone to peri-implant fracture. To date, a biomechanical comparison of these two implants with regard to torsional forces has not been reported.

Heat Generation During Bone Drilling: A Comparison Between Industrial and Orthopaedic Drill Bits.

OBJECTIVES: Cortical bone drilling for preparation of screw placement is common in multiple surgical fields. The heat generated while drilling may reach thresholds high enough to cause osteonecrosis. This can compromise implant stability. Orthopaedic drill bits are several orders more expensive than their similarly sized, publicly available industrial counterparts. We hypothesize that an industrial bit will generate less heat during drilling, and the bits will not generate more heat after multiple cortical passes. METHODS: We compared 4 4.0 mm orthopaedic and 1 3.97 mm industrial drill bits. Three types of each bit were drilled into porcine femoral cortices 20 times. The temperature of the bone was measured with thermocouple transducers. The heat generated during the first 5 drill cycles for each bit was compared to the last 5 cycles. These data were analyzed with analysis of covariance. RESULTS: The industrial drill bit generated the smallest mean increase in temperature (2.8 ± 0.29°C) P < 0.0001. No significant difference was identified comparing the first 5 cortices drilled to the last 5 cortices drilled for each bit. The P-values are as follows: Bosch (P = 0.73), Emerge (P = 0.09), Smith & Nephew (P = 0.08), Stryker (P = 0.086), and Synthes (P = 0.16). The industrial bit generated less heat during drilling than its orthopaedic counterparts. The bits maintained their performance after 20 drill cycles. CONCLUSIONS: Consideration should be given by manufacturers to design differences that may contribute to a more efficient cutting bit. Further investigation into the reuse of these drill bits may be warranted, as our data suggest their efficiency is maintained after multiple uses.

Preventing Instrumentation Failure in Three-Column Spinal Osteotomy: Biomechanical Analysis of Rod Configuration.

STUDY DESIGN: Biomechanical analysis. OBJECTIVES: To show the role of additional rods and long-term fatigue strength to prevent the instrumentation failure on three-column osteotomies. SUMMARY OF BACKGROUND DATA: Three-column osteotomy such as pedicle subtraction osteotomy (PSO) and vertebral column resections are surgical correction options for fixed spinal deformity. Posterior fixation for the PSO involves pedicle screw-and rod-based instrumentation, with the rods being contoured to accommodate the accentuated lordosis. Pseudarthrosis and instrumentation failure are known complications of PSO. METHODS: Unilateral pedicle screw and rod constructs were mounted in ultra-high-molecular-weight polyethylene blocks using a vertebrectomy model with the rods contoured to simulate posterior fixation of a PSO. Each construct was cycled under a 200 N load at 5 Hz in simulated flexion and extension to rod failure. Three configurations (n = 5) of titanium alloy rods were tested: single rod (control), double rod, and bridging rod. Outcomes were total cycles to failure and location of rod failure. RESULTS: Double-rod and bridging-rod constructs had a significantly higher number of cycles to failure compared with the single-rod construct (p < .05). Single-rod constructs failed at or near the rod bend apex, whereas the majority of double-rod and bridging-rod constructs failed at the screw-rod or rod-connector junction. CONCLUSIONS: Double-rod and bridging-rod constructs are more resistant to fatigue failure compared with single-rod constructs in PSO instrumentation and could be considered to mitigate the risk of instrumentation failure.

A comparison of Staphylococcus aureus biofilm formation on cobalt-chrome and titanium-alloy spinal implants.

The use of cobalt chrome (CoCr) implants in spinal surgery has become increasingly popular. However, there have been no studies specifically comparing biofilm formation on CoCr with that of titanium-alloy spinal implants. The objective of this study was to compare the difference in propensity for biofilm formation between these two materials, as it specifically relates to spinal rods. Staphylococcus aureus subsp. Aureus (ATCC 6538) were incubated with two different types of spinal rods composed of either CoCr or titanium-alloy. The spinal rods were then subject to a trypsin wash to allow for isolation of the colonized organism and associated biofilms. The associated optical density values (OD) from the bacterial isolates were obtained and the bacterial solutions were plated on brain-heart infusion agar plates and the resultant colony-forming units (CFU) were counted. The OD values for the titanium-alloy rods were 1.105±0.096nm (mean±SD) and 1.040±0.026nm at 48hours and 96hours, respectively. In contrast, the OD values for the CoCr rods were 1.332±0.161nm and 1.115±0.207nm at 48 and 96hours, respectively (p<0.05). The CFU values were 1481±417/100mm(2) and 745±159/100mm(2) at 48 and 96hours, respectively for the titanium-alloy group. These values were significantly lower than the CFU values obtained from the CoCr group which were 2721±605/100mm(2) and 928±88/100mm(2) (p<0.001) at both 48 and 96hours respectively. Our findings, evaluating both the OD and CFU values, indicate that implants composed of CoCr had a higher proclivity towards biofilm formation compared to titanium-alloy implants.

Effect of Plasma Sterilization on the Hemostatic Efficacy of a Chitosan Hemostatic Agent in a Rat Model.

INTRODUCTION: The United States military has had success with chitosan (CS)-based hemostatic agents to control trauma-induced hemorrhages. Despite the positive reviews, additional physical forms of CS may enhance its hemostatic efficacy. Additionally, standard sterilization techniques may negatively affect the hemostatic efficacy of CS. We studied the effects of a CS-based hemostatic pad, the Clo-Sur P.A.D.™ (Scion Cardio-Vascular, Inc.), on severe femoral vessel bleeding in a rat model. The effects of different sterilization techniques on the bioadhesivity, surface atomic concentrations, and hemostatic efficacy of the P.A.D. were also evaluated. METHODS: Hemostatic efficacy, bioadhesivity, and surface atomic concentrations of the P.A.D. were evaluated in its unsterilized form, after sterilization with standard e-beam treatment, and after sterilization with one of three types of non-thermal nitrogen plasma: nitrogen gas, air, or nitrous oxide plasma. After standardized puncture of the femoral artery or transection of the femoral vessels, rats were treated with either a CS P.A.D. or gauze pad. RESULTS: The Clo-Sur P.A.D., regardless of sterilization technique, stopped arterial and mixed arterial/venous bleeding in all cases in <90 s with the time to hemostasis (TTH) significantly less for all P.A.D. treatment groups (P < 0.001; n = 4-5/group) compared to gauze-treated controls (n = 3). E-beam sterilized P.A.D.s consistently showed non-significant trends toward increased TTH and worse hemostasis scores compared to unsterilized and plasma sterilized P.A.D.s. Treating e-beam sterilized P.A.D.s with N2 plasma reverted the hemostatic efficacy to levels equivalent to native, unsterilized PADs. CONCLUSION: A CS-based hemostatic pad successfully controlled severe bleeding in a rat model with combined e-beam and plasma sterilized P.A.D.s showing the most promising results. Further studies are warranted.

Does Barbed Suture Repair Negate the Benefit of Peripheral Repair in Porcine Flexor Tendon?

Background: Advances in suture material and geometry have fueled interest in barbed suture tenorrhaphy. Theoretically, barbed suture allows better load distribution, smoother gliding under pulleys, and improved tendon blood flow. Minimal data exist on whether barbed tendon repair may benefit from supplementation by a peripheral stitch. The purpose of this study is to determine whether peripheral suture repair increases gap resistance in both conventional and barbed core repairs, increases maximum tensile strength, and fails before or after the core repair. Methods: Porcine flexor tendons were harvested and assigned randomly into 4 groups of 10 of varying suture constructs (3-0 PDS™ or 3-0 V-Loc 180™ core with or without peripheral 5-0 Vicryl™ repair). Core repairs were performed using a modified 4-strand cruciate repair. A servohydrolic tester was used for biomechanical testing of linear 2-mm gap resistance and maximum tensile strength. Results: Peripheral repair improved 2-mm gap resistance in all repairs, regardless of core suture type, conventional (173% increase) or barbed (204% increase). No change in the maximum tensile strength was found in either core suture type with peripheral repair. Peripheral repairs always failed before core repairs, at a significantly higher load of 74.2 ± 20.4 N in barbed versus 57.8 ± 12.2 N (P = .04) in conventional core repairs. Conclusions: The addition of peripheral repair improved gap resistance but not ultimate tensile strength in both conventional and barbed flexor tendon repairs in linear testing. The 4-strand cruciate flexor tendon repairs using barbed suture may require peripheral repair to withstand physiologic loads, as core repair alone using barbed suture was insufficient.

Comparison of Barbed Sutures in Porcine Flexor Tenorrhaphy.

Background: Barbed suture use has become more popular as technology and materials have advanced. Minimal data exist regarding performance of the 2 commercially available products, V-LocTM and StratafixTM in tendon repairs. The purpose of this study was to compare gap resistance and ultimate tensile strength of both suture materials and nonbarbed suture in a porcine ex vivo model. Methods: Porcine flexor tendons were harvested and divided into 3 groups of 10 of varying suture material (3-0 PDS™, 3-0 V- V-Loc 180™, or 3-0 Stratafix™). A modified 4-strand cruciate technique was used to repair each tendon. Knotless repair was performed using barbed suture, whereas a buried 6-throw square knot was done using conventional suture. A servohydrolic tester was used for biomechanical testing of linear 2-mm gap resistance and maximum tensile strength. Results: No difference was found in 2-mm gap resistance among the 3 groups. No difference was found in ultimate tensile strength between V-Loc™ (76.0 ± 9.4 N) and Stratafix™ (68.1 ± 8.4 N) repairs, but the ultimate strength of the PDS™ control group (83.4 ± 10.0 N) was significantly higher than that of Stratafix™. Conclusions: Barbed (knotless) and nonbarbed suture repairs demonstrate equivalent 2-mm gap resistance. Stratafix™ repairs show slightly inferior performance to nonbarbed repairs in ultimate tensile strength, although this occurred at gap distances far beyond the 2-mm threshold for normal tendon gliding. Both barbed and nonbarbed 4-strand cruciate flexor tendon repairs may require peripheral repair to withstand physiologic loads.

Fatigue Performance of Cortical Bone Trajectory Screw Compared With Standard Trajectory Pedicle Screw.

STUDY DESIGN: Cadaveric biomechanical study. OBJECTIVE: To determine fatigue behavior of cortical bone trajectory (CBT) pedicle screws. SUMMARY OF BACKGROUND DATA: Cortical bone trajectory screws have been becoming popular in spine surgery; however, the long-term fatigue behavior of the new CBT screws remains understudied and limitations not well defined. METHODS: Twelve vertebrae from six cadaveric lumbar spines were obtained. After bone mineral density (BMD) measurements, each vertebral body was instrumented with screws from each group, that is, CBT (4.5 × 25 mm) or standard pedicle screw (6.5 × 55 mm). A load (± 4 Nm sagittal bending) was applied under displacement control at 1 Hz. Each construct was loaded for 100 cycles or until 6° of loosening was observed. After fatigue testing, the screws were pulled out axially at 5 mm/min. RESULTS: The standard pedicle screw showed better resistance against 100 cycle loading compared with the CBT screws (P < 0.001, 6.9° ± 4.8° vs. 15.2° ± 5.5°, respectively). The standard pedicle screw testing usually required more than 100 cycles of loading to achieve the critical loosening (3592 ± 4564 cycles), whereas the CBT screw never exceeded 100 cycles (84 ± 24 cycles) (P = 0.002). Increased BMD was significantly associated with a higher number of cycles and less loosening. The standard pedicle screw group had a higher postfatigue pullout load than the CBT screw group (P = 0.001, 776 ± 370 N and 302 ± 232 N, respectively). CONCLUSION: The standard pedicle screw had a better fatigue performance compared with the CBT screw in vertebra with compromised bone quality. The proper insertion of the CBT screw might be prevented by the laminar anatomy depending on the screw head design. The CBT screw damaged the bone along its shaft by rotating around a fulcrum, located at either the pars, pedicle isthmus, or the junction of the pedicle and superior endplate, contingent upon the strength of the bone.

Pars and pedicle fracture and screw loosening associated with cortical bone trajectory: a case series and proposed mechanism through a cadaveric study.

BACKGROUND CONTEXT: Cortical bone trajectory (CBT) technique for pedicle screw placement in the lumbar spine has become more popular since its introduction in 2009. The distinct advantages of using the CBT technique involve increased screw purchase within the cortical bone and reduced surgical dissection. However, contrary to several favorable biomechanical results, there were anecdotal reports of clinical complications associated with CBT. PURPOSE: This study aimed (1) to report on two unique pars and pedicle fracture cases involving the use of the CBT technique and (2) to perform a cadaveric pilot study to determine the possible mechanism for this fracture pattern. STUDY DESIGN/SETTING: A case report and cadaveric study were carried out. METHODS: After presenting two clinical cases, 19 fresh-frozen lumbar vertebrae were obtained from 8 cadavers. Pedicle screws were instrumented on each level using CBT under video recording. After the instrumentation, X-ray images were obtained, and anatomical dissections were performed. RESULTS: To be able to reach a necessary angle for medial to lateral CBT trajectory, 13 out of 19 (68%) spinous processes had to be removed. There were a total of seven complications. One pars and pedicle fracture out of 37 trajectories (2.7%) and 6 out of 37 trajectory deviations (16.2%), which resulted in gross loosening, were observed. CONCLUSIONS: The head of the pedicle screw impinging on the base of spinous process and lamina was observed in our cadaveric model. This mechanism could potentially explain both screw loosening and fractures associated with the CBT technique.

Cortical and Standard Trajectory Pedicle Screw Fixation Techniques in Stabilizing Multisegment Lumbar Spine with Low Grade Spondylolisthesis.

BACKGROUND: Cortical screw (CS) fixation has been recently proposed as an alternative to the standard pedicle screw (PS) fixation technique. Biomechanical studies involving individual screw pullout and single level motion segment stabilization showed comparable performance of both techniques. However, whether this new fixation technique can be applied to the stabilization of multilevel lumbar segments with significant destabilization has been unclear. PURPOSE: To compare stability of CS fixation to the traditional PS fixation in an unstable 3 level spondylolisthesis model. STUDY DESIGN: This is a biomechanical study comparing cortical trajectory pedicle screw fixation to traditional trajectory pedicle screw fixation in an unstable cadaveric model using nondestructive flexibility test. METHODS: Eight fresh frozen cadaveric lumbar spines (T12- S1) were obtained. After intact baseline testing, a 3-level lowgrade spondylolisthesis was simulated at the L1-4. Each specimen was instrumented with the PS and CS fixation systems. Standard nondestructive flexibility test was performed. Range of motion at each level was compared between the constructs during flexion-extension, lateral bending, and axial rotation. RESULTS: The destabilization model significantly increased the ROM in all planes (P<0.05). Both fixation techniques provided significant reduction in the ROM (P<0.05). There was no significant difference in ROM between the PS and CS groups in any of planes (P>0.05). CONCLUSIONS: Cortical trajectory pedicle screw fixation provided stabilization to multilevel lumbar segment with low-grade spondylolisthesis comparable to the standard trajectory pedicle screw construct.

Unconstrained testing of spine with bi-axial universal testing machine.

BACKGROUND: In-vitro biomechanical assessment of the spine reveals significant information on the mechanics of spinal disorders, treatment methods, and surgical implants. Specialized devices for the evaluation of spine biomechanics have thus become popular. However, these devices might not be affordable for all research groups. PURPOSE: The purpose of this study was to describe an apparatus to be attached to a standard bi-axial universal testing machine that would make unconstrained testing of the spine possible. STUDY DESIGN/SETTING: A technical note on the definition of a spinal testing fixture with validation. METHODS: Intact lumbosacral spines (T12-S1) were tested in sagittal and lateral bending and axial rotation. Three-dimensional interlevel rotations at each level (L1-4) were analyzed. RESULTS: By comparison with the literature, we found that the new fixture was able to successfully produce reasonable relative rotation values for the lumbar spine. CONCLUSIONS: We demonstrated that the low cost fixture allowed unconstrained (six degree of freedom, 6 DOF) testing of fresh-frozen cadaveric lumbar spine.