Stability of Locking Plate and Compression Screws for Lapidus Arthrodesis: A Biomechanical Comparison of Plate Position.

Lapidus (first tarsometatarsal joint) arthrodesis is an established and widely used procedure for the management of moderate to severe hallux valgus, especially in cases involving hypermobility of the first tarsometatarsal joint. Multiple fixation methods are available, and several previous investigations have studied the relative strengths of these methods, including dorsomedial and plantar plating comparisons. However, these studies compared plates of varying designs and mechanical properties and used varying modes of compression and interfragmentary screw techniques. The present study mechanically investigated the resulting motion, stiffness, and strength of identical locking plate constructs fixed at various anatomic positions around the first tarsometatarsal joint. In a bench-top study, fourth-generation composite bones were divided into 3 fixation groups, each having identical interfragmentary screw applications, and randomized to 1 of 3 plate positions: dorsal, medial, or plantar. The plates applied in each case were identical locking plates, precontoured to fit the anatomy. Each construct was experimentally tested using a cantilever bending approach. The outcomes obtained were stiffness, yield force, displacement at yield, ultimate force, and displacement at ultimate force. The plantar plate position showed superior initial stiffness and force to ultimate failure. The plantar and medial plate positions exhibited superior force to yield. The medial plate position was superior regarding displacement tolerated before the yield point and catastrophic failure. The dorsal plate position was not superior for any outcome measured. Plantar and medial plating each offered biomechanical benefits. Clinical studies using similarly matched constructs are required to show whether these findings translate into improved clinical outcomes.

Effect of Posterior Tibial Slope on Flexion and Anterior-Posterior Tibial Translation in Posterior Cruciate-Retaining Total Knee Arthroplasty.

Reduced posterior tibial slope (PTS) and posterior tibiofemoral translation (PTFT) in posterior cruciate-retaining (PCR) total knee arthroplasty (TKA) may result in suboptimal flexion. We evaluated the relationship between PTS, PTFT, and total knee flexion after PCR TKA in a cadaveric model. We performed a balanced PCR TKA using 9 transfemoral cadaver specimens and changed postoperative PTS in 1° increments. We measured maximal flexion and relative PTFT at maximal flexion. We determined significant changes in flexion and PTFT as a function of PTS. Findings showed an average increase in flexion of 2.3° and average PTFT increase of 1mm per degree of PTS increase when increasing PTS from 1° to 4° (P<.05). Small initial increases in PTS appear to significantly increase knee flexion and PTFT.

Fully Threaded Versus Partially Threaded Screws: Determining Shear in Cancellous Bone Fixation.

Many researchers have studied and compared various forms of intraosseous fixation. No studies have examined the effects of shear through stiffness and failure strength of a fully threaded versus a partially threaded screw. Our hypothesis was that the fully threaded lag screw technique would provide greater shear strength and resistance. Thirty-six synthetic sawbone blocks were used to test screw fixation. In group 1 (n = 9), 2 blocks were fixed together using a fully threaded 4.0-mm stainless steel cancellous bone screw and the lag technique. In group 2 (n = 8), 2 blocks were fixed together using the standard manufacturer-recommended method for inserting 4.0-mm partially threaded stainless steel cancellous bone screws. The constructs were then mechanically tested. Shear was applied by compressing each construct at an axial displacement rate of 0.5 mm/s until failure. The fully threaded screw had a significantly greater (p = .026) initial stiffness (106.4 ± 15.8 N/mm) than the partially threaded screw (80.1 ± 27.5 N/mm). The yield load and displacement for the fully threaded group (429.4 ± 11.7 N and 7.2 ± 0.35 mm) were 64% and 67% greater than those for the partially threaded screw group (261.4 ± 26.1 N and 4.3 ± 1.03 mm), respectively. The results of the present study have demonstrated the importance of a full-thread construct to prevent shear and to decrease strain at the fracture. The confirmation of our hypothesis questions the future need and use of partially threaded screws for cancellous bone fixation.

Dual plating of humeral shaft fractures: orthogonal plates biomechanically outperform side-by-side plates.

BACKGROUND: Single large-fragment plate constructs currently are the norm for internal fixation of middiaphyseal humerus fractures. In cases where humeral size is limited, however, dual small-fragment locking plate constructs may serve as an alternative. The mechanical effects of different possible plate configurations around the humeral diaphysis may be important, but to our knowledge, have yet to be investigated. QUESTIONS/PURPOSES: We used finite element analysis to compare the simulated mechanical performance of five different dual small-fragment locking plate construct configurations for humeral middiaphyseal fracture fixation in terms of (1) stiffness, (2) stress shielding of bone, (3) hardware stresses, and (4) interfragmentary strain. METHODS: Middiaphyseal humeral fracture fixation was simulated using the finite element method. Three 90° and two side-by-side seven-hole and nine-hole small-fragment dual locking plate configurations were tested in compression, torsion, and combined loading. The configurations chosen are based on implantation using either a posterior or anterolateral approach. RESULTS: All three of the 90° configurations were more effective in restoring the intact compressive and torsional stiffness as compared with the side-by-side configurations, resulted in less stress shielding and stressed hardware, and showed interfragmentary strains between 5% to 10% in torsion and combined loading. CONCLUSIONS: The nine-hole plate anterior and seven-hole plate lateral (90° apart) configuration provided the best fixation. Our findings show the mechanical importance of plate placement with relation to loading in dual-plate fracture-fixation constructs. CLINICAL RELEVANCE: The results presented provide novel biomechanical information for the orthopaedic surgeon considering different treatment options for middiaphyseal humeral fractures.

A biomechanical investigation of a knotless tension band in medial malleolar fracture models in composite Sawbones®.

The present study introduces a knotless tension band construct and compares its biomechanical behavior with that of a traditional stainless steel tension band construct. Fourth-generation composite tibial Sawbones(®) were used in the present study. Fracture models were created to mimic Orthopaedic Trauma Association type 44-B2.2 ankle fractures. A total of 20 specimens were randomized evenly into a stainless steel tension band group (control group); or a knotless tension band group. The fixation constructs were mechanically tested, and the stiffness and failure strengths were calculated. Two failure strengths were determined: the engineering-based failure strength, defined as the greatest tensile load tolerated by the construct; and the clinical failure strength, defined as the force required to displace the fracture by 2 mm. We used 2-tailed independent samples t tests to compare and identify significant differences. The knotless tension band construct was 7.7% stronger and 33.2% stiffer and required a 36.7% greater force to displace the fracture by 2 mm. Independent sample t tests confirmed that differences in mean stiffness (p = .003) and clinical failure strength (p = .003) were statistically significant. Although the mean engineering strength for the knotless group was greater than that for the stainless steel group, this difference was not statistically significant (p = .170). This knotless tension band construct could potentially offer both clinical and biomechanical advantages compared with the current stainless steel standard.

Gliding resistance and triggering after venting or A2 pulley enlargement: a study of intact and repaired flexor tendons in a cadaveric model.

PURPOSE: This study compared the effect of 2 techniques of pulley management--venting and pulley enlargement (complete A2 incision with pulley repair and sheath closure using a retinacular graft)--on gliding resistance and on the incidence of triggering following zone 2 flexor tendon repairs in human cadaver specimens. METHODS: In vitro gliding resistance and the incidence of triggering were determined in 10 human cadaver specimens under 5 progressive conditions: (1) intact, (2) tendon repair (both tendons cut and repaired with the sheath intact), (3) condition 2 plus 50% venting of the distal A2 pulley, (4) condition 2 with venting extended to 66% of distal A2, and (5) condition 4 plus pulley enlargement. Triggering was determined in the same specimens by 2 computational algorithms that detected force changes in the load cells used to measure gliding resistance. RESULTS: Tendon repair increased gliding resistance from the intact condition by an average of 229%. Gliding resistance was reduced in conditions 3, 4, and 5 from the repair condition by 15%, 25%, and 22%, respectively. Triggering commenced with tendon repair in some specimens, and its incidence increased with 50% venting. Further venting reduced triggering, but not as effectively as pulley enlargement did. CONCLUSIONS: In this cadaveric study, venting and pulley enlargement reduce gliding resistance by equivalent amounts. Triggering persisted despite venting. The surgeon should carefully examine tendon repairs for free gliding. Pulley enlargement might be more effective than venting in reducing the incidence of triggering.

Early stage disc degeneration does not have an appreciable affect on stiffness and load transfer following vertebroplasty and kyphoplasty.

Vertebroplasty and kyphoplasty have been reported to alter the mechanical behavior of the treated and adjacent-level segments, and have been suggested to increase the risk for adjacent-level fractures. The intervertebral disc (IVD) plays an important role in the mechanical behavior of vertebral motion segments. Comparisons between normal and degenerative IVD motion segments following cement augmentation have yet to be reported. A microstructural finite element model of a degenerative IVD motion segment was constructed from micro-CT images. Microdamage within the vertebral body trabecular structure was used to simulate a slightly (I = 83.5% of intact stiffness), moderately (II = 57.8% of intact stiffness), and severely (III = 16.0% of intact stiffness) damaged motion segment. Six variable geometry single-segment cement repair strategies (models A-F) were studied at each damage level (I-III). IVD and bone stresses, and motion segment stiffness, were compared with the intact and baseline damage models (untreated), as well as, previous findings using normal IVD models with the same repair strategies. Overall, small differences were observed in motion segment stiffness and average stresses between the degenerative and normal disc repair models. We did however observe a reduction in endplate bulge and a redistribution in the microstructural tissue level stresses across both endplates and in the treated segment following early stage IVD degeneration. The cement augmentation strategy placing bone cement along the periphery of the vertebra (model E) proved to be the most advantageous in treating the degenerative IVD models by showing larger reductions in the average bone stresses (vertebral and endplate) as compared to the normal IVD models. Furthermore, only this repair strategy, and the complete cement fill strategy (model F), were able to restore the slightly damaged (I) motion segment stiffness above pre-damaged (intact) levels. Early stage IVD degeneration does not have an appreciable effect in motion segment stiffness and average stresses in the treated and adjacent-level segments following vertebroplasty and kyphoplasty. Placing bone cement in the periphery of the damaged vertebra in a degenerative IVD motion segment, minimizes load transfer, and may reduce the likelihood of adjacent-level fractures.

Effect of a novel interspinous implant on lumbar spinal range of motion.

Interspinous devices have been introduced to provide a minimally invasive surgical alternative for patients with lumbar spinal stenosis or foraminal stenosis. Little is known however, of the effect of interspinous devices on intersegmental range of motion (ROM). The aim of this in vivo study was to investigate the effect of a novel minimally invasive interspinous implant, InSwing, on sagittal plane ROM of the lumbar spine using an ovine model. Ten adolescent Merino lambs underwent a destabilization procedure at the L1-L2 level simulating a stenotic degenerative spondylolisthesis (as described in our earlier work; Spine 15:571-576, 1990). All animals were placed in a side-lying posture and lateral radiographs were taken in full flexion and extension of the trunk in a standardized manner. Radiographs were repeated following the insertion of an 8-mm InSwing interspinous device at L1-L2, and again with the implant secured by means of a tension band tightened to 1 N/m around the L1 and L2 spinous processes. ROM was assessed in each of the three conditions and compared using Cobb's method. A paired t-test compared ROM for each of the experimental conditions (P < 0.05). After instrumentation with the InSwing interspinous implant, the mean total sagittal ROM (from full extension to full flexion) was reduced by 16% from 6.3 degrees to 5.3 +/- 2.7 degrees. The addition of the tension band resulted in a 43% reduction in total sagittal ROM to 3.6 +/- 1.9 degrees which approached significance. When looking at flexion only, the addition of the interspinous implant without the tension band did not significantly reduce lumbar flexion, however, a statistically significant 15% reduction in lumbar flexion was observed with the addition of the tension band (P = 0.01). To our knowledge, this is the first in vivo study radiographically showing the advantage of using an interspinous device to stabilize the spine in flexion. These results are important findings particularly for patients with clinical symptoms related to instable degenerative spondylolisthesis.

Primary tendon sheath enlargement and reconstruction in zone 2: an in vitro biomechanical study on tendon gliding resistance.

PURPOSE: To investigate our hypothesis that primary pulley enlargement and repair using an extensor retinaculum graft will reduce tendon repair gliding resistance. The benefit of pulley enlargement has been tested in experimental animals, but its effect on gliding resistance in vitro using human fingers is not known. METHODS: In vitro gliding resistance in the proximal tendon sheaths (A1 through A3) was measured and compared in 7 cadaver fingers using the method of Uchiyama and colleagues at a fixed 50 degrees over the proximal sheath under 3 conditions: (1) intact tendons with intact proximal sheath; (2) laceration and 2-strand core plus running epitenon repair of the tendons with intact sheath; and (3) repaired tendons with enlargement of the A2 pulley and adjacent proximal sheath by incision and repair with an extensor retinacular graft. Results were analyzed statistically. RESULTS: Gliding resistance increased from an average of 0.44 N +/- 0.07 in the intact condition to an average of 1.51 N +/- 0.23 (a mean increase of 243%) when the tendons were cut and repaired. Enlarging the proximal sheath by sheath incision and graft repair reduced the gliding resistance from the repair condition to 1.04 N +/- 0.15 (a mean decrease of 31%). These changes are statistically significant. CONCLUSIONS: In vitro, repaired tendons had a greater resistance to gliding than that of the intact tendons through the proximal sheath when tested by the method of Uchiyama and colleagues. Enlargement and repair with an extensor retinacular graft of the A2 pulley and adjacent sheath significantly reduced resistance to repaired tendon gliding. These findings support further investigation into the concept that primary pulley enlargement may improve tendon function after repair.

Impact of iliac crest bone graft harvesting on fusion rates and postoperative pain during instrumented posterolateral lumbar fusion.

This study aims to evaluate the influence of bone harvesting on postoperative pain and fusion rates. Group 1 patients received iliac crest bone graft (ICBG) either alone or augmented with local bone. Group 2 received only local bone. No statistical significance was found in radiological union or in the Oswestry Disability Index scores. Visual Analogue Scale scores showed less pain in group 2. Logistic regression showed no correlation between residual pain and occurrence of fusion. Harvesting ICBG did not appear to increase fusion rates and no relation was found between radiological non-union and pain.

Minimally invasive versus open transforaminal lumbar interbody fusion: evaluating initial experience.

The aim of this study was to compare our experience with minimally invasive transforaminal lumbar interbody fusion (MITLIF) and open midline transforaminal lumbar interbody fusion (TLIF). A total of 36 patients suffering from isthmic spondylolisthesis or degenerative disc disease were operated with either a MITLIF (n = 18) or an open TLIF technique (n = 18) with an average follow-up of 22 and 24 months, respectively. Clinical outcome was assessed using the visual analogue scale (VAS) and the Oswestry disability index (ODI). There was no difference in length of surgery between the two groups. The MITLIF group resulted in a significant reduction of blood loss and had a shorter length of hospital stay. No difference was observed in postoperative pain, initial analgesia consumption, VAS or ODI between the groups. Three pseudarthroses were observed in the MITLIF group although this was not statistically significant. A steeper learning effect was observed for the MITLIF group.

Management of a post-operative multi-resistant infectious spondylitis associated with a kyphotic deformity.

Anterior spinal infection (prevertebral abscess and/or discitis) after posterior instrumentation for vertebral fractures is a challenging complication, since a new implant may become necessary anteriorly, in a septic environment. Generally accepted management guidelines are yet to be established. The authors present a case of posterior instrumentation for fractures of T12 and L1, complicated after 9 months with an anterior infection (prevertebral abscess and discitis) with extended-spectrum beta-lactamase (ESBL) producing Escherichia coli (E. coli). This case is unique in that the multi-resistant organism was isolated only after the second stage of infection treatment, which consisted of anterior débridement and anterior implantation of titanium cages and rods. In this particular case, infection was controlled despite implantation of multiple cages, screws and rods, and fusion was achieved, by means of intravenous antibiotic treatment for 12 months. At the latest follow-up, 24 months post surgery, there was no evidence of infection. This problem case may be helpful for surgeons confronted with spinal deformities secondary to infections with multi-resistant organisms.

Knee Loading After ACL-R Is Related to Quadriceps Strength and Knee Extension Differences Across the Continuum of Care.

BACKGROUND: Quadriceps strength and knee extension are believed to be important in the ability to effectively load the knee after anterior cruciate ligament (ACL) reconstruction (ACL-R). PURPOSE: To compare quadriceps strength (QUADS), side-to-side knee extension difference (ExtDiff), and knee energy absorption contribution (EAC) in patients preoperatively, 12 weeks postoperatively, and at return to sport (RTS). A secondary aim was to determine how the factors of QUADS and ExtDiff contributed to the ability to load the knee (knee EAC) at each of the 3 time points. STUDY DESIGN: Case series; Level of evidence, 4. METHODS: Overall, 41 individuals (mean ± SD age, 15.95 ± 1.63 years) were enrolled in this study. QUADS, ExtDiff, and knee EAC during a double-limb squat were collected preoperatively, 12 weeks postoperatively, and at RTS. Isokinetic QUADS was collected at 60 deg/s, normalized to body mass, and averaged across 5 trials. Knee extension was measured with a goniometer, and ExtDiff was calculated for analyses. Knee EAC was measured during double-limb squat descent and was calculated as a percentage of total energy absorption for the limb. Observations were obtained from both the surgical and nonsurgical limbs at the 3 time points. A mixed regression model with random intercept to compare change over the 3 time points was used, and a model selection was conducted with Akaike information criteria. Significance was set at P < .05. RESULTS: Surgical limb QUADS was significantly lower preoperatively (mean ± SD, 1.37 ± 0.49 N·m/kg; P = .0023) and at 12 weeks (1.11 ± 0.38 N·m/kg; P < .0001) than at RTS (1.58 ± 0.47 N·m/kg). Nonsurgical limb QUADS was also significantly lower preoperatively (2.01 ± 0.54 N·m/kg; P < .0256) and at 12 weeks (2.03 ± 0.48 N·m/kg; P < .0233) than at RTS (2.18 ± 0.54 N·m/kg). Knee EAC for the surgical limb was significantly lower at 12 weeks than at RTS (40.98% ± 13.73% vs 47.50% ± 12.04%; P < .0032), and ExtDiff was significantly greater preoperatively than at RTS (-2.68° ± 3.19° vs -0.63° ± 1.43°; P < .0001). Preoperatively, QUADS for both the surgical (P < .0003) and nonsurgical (P = .0023) limbs was a significant predictor of surgical limb knee EAC, explaining 33.99% of the variance. At 12 weeks, surgical limb QUADS was a significant predictor (P < .0051) of surgical limb knee EAC, explaining 18.83% of the variance. At RTS, ExtDiff was a significant predictor (P = .0201) of surgical limb knee EAC, explaining 12.92% of the variance. CONCLUSION: The ability to load the knee after ACL injury changes across the continuum of care and is related to QUADS and ExtDiff. These results provide clinicians with insight into potential contributing factors that may limit knee loading during the rehabilitation process.

Modeling the onset and propagation of trabecular bone microdamage during low-cycle fatigue.

Relatively small amounts of microdamage have been suggested to have a major effect on the mechanical properties of bone. A significant reduction in mechanical properties (e.g. modulus) can occur even before the appearance of microcracks. This study uses a novel non-linear microdamaging finite-element (FE) algorithm to simulate the low-cycle fatigue behavior of high-density trabecular bone. We aimed to investigate if diffuse microdamage accumulation and concomitant modulus reduction, without the need for complete trabecular strut fracture, may be an underlining mechanism for low-cycle fatigue failure (defined as a 30% reduction in apparent modulus). A microCT constructed FE model was subjected to a single cycle monotonic compression test, and constant and variable amplitude loading scenarios to study the initiation and accumulation of low-cycle fatigue microdamage. Microcrack initiation was simulated using four damage criteria: 30%, 40%, 50% and 60% reduction in bone element modulus (el-MR). Evaluation of structural (apparent) damage using the four different tissue level damage criteria resulted in specimen fatigue failure at 72, 316, 969 and 1518 cycles for the 30%, 40%, 50% and 60% el-MR models, respectively. Simulations based on the 50% el-MR model were consistent with previously published experimental findings. A strong, significant non-linear, power law relationship was found between cycles to failure (N) and effective strain (Deltasigma/E(0)): N=1.394x10(-25)(Deltasigma/E(0))(-12.17), r(2)=0.97, p<0.0001. The results suggest that microdamage and microcrack propagation, without the need for complete trabecular strut fracture, are mechanisms for high-density trabecular bone failure. Furthermore, the model is consistent with previous numerical fatigue simulations indicating that microdamage to a small number of trabeculae results in relatively large specimen modulus reductions and rapid failure.

Consequences of patient position in the radiographic measurement of artificial disc replacement angles.

Accurate clinical measurement of spinal range of motion (ROM) is essential in the evaluation of artificial disc performance. The effect of patient placement with respect to the X-ray beam source is yet to be reported and may be an influencing factor in radiographic artificial disc angle measurements. This study aims to evaluate how radiographic patient placement influences artificial disc angle measurements. An anatomically accurate synthetic L4-L5 motion segment was instrumented with an artificial disc and two pins. The instrumented motion segment was mounted onto a frame allowing for independent rotation and elevation while holding the artificial disc angle and anatomical position between L4 and L5 fixed. Analyses included descriptive statistics, evaluation of uncertainty, intra- and inter-observer, and a 2-way analysis of variance (ANOVA). The mean angle measurement range at the various positions was 1.26 degrees for the pin, and 2.74 degrees for the artificial disc endplates. The centered patient position had the highest inter- and intra-observer reliability. ANOVA results showed elevation effects to be statistically significant (P = 0.021), and rotational effects to be extremely statistically significant (P < 0.0001) for the pin angles. In terms of the mean artificial disc angle, however, the ANOVA showed a highly statistically significant interaction term (P = 0.002). A significant difference was found in the angle measurements of a fixed artificial disc prosthesis based on a sample of patient radiographic placement positions. Since it is important to assess the success of an artificial disc replacement by evaluating the relatively small ROM present, it is crucial to aim at minimizing the error by placing the patient parallel to the plate with the beam centered not at the mid lumbar spine, but at the level of the arthroplasty, for both flexion and extension views.

Radiographic total disc replacement angle measurement accuracy using the Oxford Cobbometer: precision and bias.

Total disc replacement (TDR) clinical success has been reported to be related to the residual motion of the operated level. Thus, accurate measurement of TDR range of motion (ROM) is of utmost importance. One commonly used tool in measuring ROM is the Oxford Cobbometer. Little is known however on its accuracy (precision and bias) in measuring TDR angles. The aim of this study was to assess the ability of the Cobbometer to accurately measure radiographic TDR angles. An anatomically accurate synthetic L4-L5 motion segment was instrumented with a CHARITE artificial disc. The TDR angle and anatomical position between L4 and L5 was fixed to prohibit motion while the motion segment was radiographically imaged in various degrees of rotation and elevation, representing a sample of possible patient placement positions. An experienced observer made ten readings of the TDR angle using the Cobbometer at each different position. The Cobbometer readings were analyzed to determine measurement accuracy at each position. Furthermore, analysis of variance was used to study rotation and elevation of the motion segment as treatment factors. Cobbometer TDR angle measurements were most accurate (highest precision and lowest bias) at the centered position (95.5%), which placed the TDR directly inline with the x-ray beam source without any rotation. In contrast, the lowest accuracy (75.2%) was observed in the most rotated and off-centered view. A difference as high as 4 degrees between readings at any individual position, and as high as 6 degrees between all the positions was observed. Furthermore, the Cobbometer was unable to detect the expected trend in TDR angle projection with changing position. Although the Cobbometer has been reported to be reliable in different clinical applications, it lacks the needed accuracy to measure TDR angles and ROM. More accurate ROM measurement methods need to be developed to help surgeons and researchers assess radiological success of TDRs.

Predicting trabecular bone microdamage initiation and accumulation using a non-linear perfect damage model.

Studies evaluating the mechanical behavior of the trabecular microstructure play an important role in our understanding of pathologies such as osteoporosis, and in increasing our understanding of bone fracture and bone adaptation. Understanding of such behavior in bone is important for predicting and providing early treatment of fractures. The objective of this study is to present a numerical model for studying the initiation and accumulation of trabecular bone microdamage in both the pre- and post-yield regions. A sub-region of human vertebral trabecular bone was analyzed using a uniformly loaded anatomically accurate microstructural three-dimensional finite element model. The evolution of trabecular bone microdamage was governed using a non-linear, modulus reduction, perfect damage approach derived from a generalized plasticity stress-strain law. The model introduced in this paper establishes a history of microdamage evolution in both the pre- and post-yield regions.

Percutaneous surgical treatment of chance fractures using cannulated pedicle screws. Report of two cases.

Chance fractures are relatively rare injuries and can be treated either conservatively, with a cast, or surgically, especially when posterior ligament injury is present. This paper presents two cases of lumbar Chance fractures treated using recently developed percutaneous cannulated pedicle screws. The first patient suffered associated abdominal injuries that required surgery, while the second had associated stable spinal fractures. Intraoperative blood loss was minimal. Both patients progressed to osseous union without implant failure. Following minimally invasive implant removal 9 months after injury, both patients remained asymptomatic without any evidence of instability on flexion and extension images obtained during their latest follow-up. This technique may be useful in selected cases in which bone grafting is not necessary; it allows early mobilization and stable fixation while minimizing morbidity.

Biomechanical behavior of novel composite PMMA-CaP bone cements in an anatomically accurate cadaveric vertebroplasty model.

Vertebral compression fractures are caused by many factors including trauma and osteoporosis. Osteoporosis induced fractures are a result of loss in bone mass and quality that weaken the vertebral body. Vertebroplasty and kyphoplasty, involving cement augmentation of fractured vertebrae, show promise in restoring vertebral mechanical properties. Some complications however, are reported due to the performance characteristics of commercially available bone cements. In this study, the biomechanical performance characteristics of two novel composite (PMMA-CaP) bone cements were studied using an anatomically accurate human cadaveric vertebroplasty model. The study involves mechanical testing on two functional cadaveric spinal unit (2FSU) segments which include monotonic compression and cyclical fatigue tests, treatment by direct cement injection, and microscopic visualization of sectioned vertebrae. The 2FSU segments were fractured, treated, and mechanically tested to investigate the stability provided by two novel bone cements; using readily available commercial acrylic cement as a control. Segment height and stiffness were tracked during the study to establish biomechanical performance. The 2FSU segments were successfully stabilized with all three cement groups. Stiffness values were restored to initial levels following fatigue loading. Cement interdigitation was observed with all cement groups. This study demonstrates efficient reinforcement of the fractured vertebrae through stiffness restoration. The pre-mixed composite cements were comparable to the commercial cement in their performance and interdigitative ability, thus holding promise for future clinical use. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2067-2074, 2017.

In Vitro and In Vivo Characterization of Premixed PMMA-CaP Composite Bone Cements.

Acrylic bone cements, although successful in the field of orthopedics, suffer from a lack of bioactivity, not truly integrating with surrounding bone. Bioactive fixation is expected to enhance cement performance because of the natural interlocking and bonding with bone, which can improve the augmentative potential of the material in applications such as vertebroplasty (VP). In a recent study, two composite cements (PMMA-hydroxyapatite and PMMA-brushite) showed promising results demonstrating no deterioration in rheological and mechanical properties after CaP filler addition. In this study, the dynamic properties of the cements were investigated in vitro and in vivo. The hypothesis was that these composite cements will provide osseointegration around the implanted cement and increase new bone formation, thus decreasing the risk of bone structural failure. The effects of CaP elution were thus analyzed in vitro using these cements. Mass-loss, pore formation, and mechanical changes were tracked after cement immersion in Hank's salt solution. PMMA-brushite was the only cement with a significant mass loss; however it showed low bulk porosity. Surface porosity increases were observed in both composite cements. Mechanical properties were maintained after cement immersion. In vitro culture studies tested preosteoblast cell viability and differentiation on the cement surface. Cell viability was demonstrated with MTT assay and confirmed on the cement surface. ALP assays showed no inhibition of osteoblast differentiation on the cement surface. In vivo experiments were performed using a rat tibiae model to demonstrate bone ingrowth around the implanted cements. Critical size defects were created and then filled with the cements. The animal studies showed no loss in mechanical strength after implantation and increased bone ingrowth around the composite cements. In summary, the composite cements provided bioactivity without sacrificing mechanical strength.