Recurrent bacteremia: A 10-year retrospective study in combat-related burn casualties.

INTRODUCTION: Surviving the first episode of bacteremia predisposes burn casualties to its recurrence. Herein, we investigate the incidence, mortality, bacteriology, and source of infection of recurrent bacteremia in military burn casualties admitted to the U.S. Army Institute of Surgical Research Burn Center over a 10year period. METHODS: Bacteremia was defined as the growth of Gram-positive or Gram-negative organisms in a blood culture that excluded probable skin contaminants. Recurrent bacteremia was defined as a subsequent episode of bacteremia ≥7 days after the first episode. Polymicrobial bacteremia was the presence of more than one pathogen in the same blood culture. Bacteremia was attributed to UTI, pneumonia, or wound sepsis. All other bacteremias were considered non-attributable bloodstream infections. Univariate and multivariate analyses determined factors predictive of clinical outcome. RESULTS: Out of 952 combat-related burn casualties screened, 166 cases were identified; 63% (non-recurrent) and 37% (recurrent) with median time to recurrence of 20 days. Univariate and multivariate analysis showed that the mortality rate was two and nine-fold, respectively, higher with recurrent bacteremia. Univariate analysis found that except for urinary tract infection, large burn size (>20%), 3rd degree burns, increased injuiry severity, perineal burns, and mechanical ventilator days were independent factors predictive of recurrence of bacteremia as well as increased mortality in the recurrent bacteremia cohort. Acinetobacter baumannii complex (63%) was prevalent in the non-recurrent group, while Klebsiella pneumoniae (46% vs. 30%) and Pseudomonas aeruginosa (35% vs. 26%) were prevalent in recurrent bacteremia. Half of the recurrent bacteremia cases were polymicrobial, compared to 9% in non-recurrent bacteremia. Pneumonia was prevalent in non-recurrent bacteremia (38%) and a combination of pneumonia and wound sepsis (29%) in recurrent bacteremia casualties. CONCLUSIONS: Recurrent bacteremia increases mortality in military burn casualties. Additional research is needed to address and mitigate the underlying causes, thereby improving survival.

The Use of a Silver-Nylon Dressing During Evacuation of Military Burn Casualties.

The military has used silver-nylon dressings as a topical antimicrobial on combat-related burns for the past 15 years. However, their clinical efficacy and associated risks have not been evaluated. Herein, the authors document our experience with the use of a specific silver-nylon dressing (Silverlon®) during global evacuation of casualties from combat zones to the United States sArmy Institute of Surgical Research Burn Center. A 10-year retrospective analysis was performed. Variables included patient demographics, total body surface area, length of stay, Injury Severity Score, incidence of urinary tract and burn infections, pneumonia, patient status at the time of discharge, and a composite endpoint. The patient cohort was stratified into two groups: Silverlon® (Group 1) and topical antimicrobial agents (Group 2). Data were analyzed using appropriate statistical tests (P ≤ .05). Nine hundred eighty-eight patients (26 ± 6 years) were identified with 184 patients (Group 1) and 804 patients (Group 2). Silver-nylon dressings trended toward decreased wound infection rate (5.4 vs 9.5%) even when applied to full-thickness burn injuries. When compared with topical antimicrobial agents, the silver-nylon dressing was not associated with significant differences in burn-related complication. The authors demonstrate the antimicrobial efficacy of the silver-nylon dressing during global evacuation of burn casualties from combat zones to the burn center. Compared with topical antimicrobials, the silver-nylon dressing is lightweight and easy to apply and requires minimal wound management which makes it desirable as a burn dressing for combat applications as well as mass casualty situations.

Fresh whole blood resuscitation does not exacerbate skeletal muscle edema and long-term functional deficit after ischemic injury and hemorrhagic shock.

BACKGROUND: Hemorrhagic shock caused by extremity vascular injuries is common in combat injuries. Fluid resuscitation is the standard treatment for severe hemorrhage (HEM). Tourniquets (TKs) used for HEM control cause ischemia-reperfusion (I/R) injury that induces edema formation in the injured muscle. Resuscitation fluids affect edema formation; however, its effect on long-term functional response remains unknown. The objectives of this study are to (1) compare acute muscle damage; (2) determine long-term functional recovery of ischemic muscle; and (3) compare local and systemic inflammatory response including the expression of junctional proteins following early resuscitation with Hextend and fresh whole blood using a rodent model of combined HEM and TK-induced limb I/R. METHODS: Anesthetized Sprague-Dawley rats underwent 42.5% arterial HEM, followed by 3 hours of TK application. Animals were either not resuscitated or resuscitated with Hextend or fresh whole blood. Two time points were evaluated, 2 and 28 days. Plasma cytokine concentrations were determined at baseline and end resuscitation. At 2 days, edema formation, expression of junctional proteins, and tissue level cytokines concentrations were evaluated. At 28 days, in vivo muscle contractile properties were determined. At both time points, routine histology was performed and graded using a semiquantitative grading system. RESULTS: All animals developed hemorrhagic hypovolemia; the mortality rate was 100% in nonresuscitated rats. Hextend resuscitation exacerbated muscle edema (~11%) and muscle strength deficit (~20%). Fresh whole blood resuscitation presented edema and muscle strength akin to TK only. Fresh whole blood resuscitation upregulated expression of junctional proteins including proangiogenic factors and dampened the inflammatory response. CONCLUSION: Fresh whole blood resuscitation does not exacerbate either TK-induced edema or muscle strength deficit. Fresh whole blood resuscitation may reduce both acute and long-term morbidity associated with extremity trauma. To our knowledge, this is the first study to demonstrate the nature of the resuscitation fluid administered following HEM impacts short- and long-term indices of I/R in skeletal muscle.

A PEGylated platelet free plasma hydrogel based composite scaffold enables stable vascularization and targeted cell delivery for volumetric muscle loss.

Extracellular matrix (ECM) scaffolds are being used for the clinical repair of soft tissue injuries. Although improved functional outcomes have been reported, ECM scaffolds show limited tissue specific remodeling response with concomitant deposition of fibrotic tissue. One plausible explanation is the regression of blood vessels which may be limiting the diffusion of oxygen and nutrients across the scaffold. Herein we develop a composite scaffold as a vasculo-inductive platform by integrating PEGylated platelet free plasma (PFP) hydrogel with a muscle derived ECM scaffold (m-ECM). In vitro, adipose derived stem cells (ASCs) seeded onto the composite scaffold differentiated into two distinct morphologies, a tubular network in the hydrogel, and elongated structures along the m-ECM scaffold. The composite scaffold showed a high expression of ITGA5, ITGB1, and FN and a synergistic up-regulation of ang1 and tie-2 transcripts. The in vitro ability of the composite scaffold to provide extracellular milieu for cell adhesion and molecular cues to support vessel formation was investigated in a rodent volumetric muscle loss (VML) model. The composite scaffold delivered with ASCs supported robust and stable vascularization. Additionally, the composite scaffold supported increased localization of ASCs in the defect demonstrating its ability for localized cell delivery. Interestingly, ASCs were observed homing in the injured muscle and around the perivascular space possibly to stabilize the host vasculature. In conclusion, the composite scaffold delivered with ASCs presents a promising approach for scaffold vascularization. The versatile nature of the composite scaffold also makes it easily adaptable for the repair of soft tissue injuries. STATEMENT OF SIGNIFICANCE: Decellularized extracellular matrix (ECM) scaffolds when used for soft tissue repair is often accompanied by deposition of fibrotic tissue possibly due to limited scaffold vascularization, which limits the diffusion of oxygen and nutrients across the scaffold. Although a variety of scaffold vascularization strategies has been investigated, their limitations preclude rapid clinical translation. In this study we have developed a composite scaffold by integrating bi-functional polyethylene glycol modified platelet free plasma (PEGylated PFP) with adipose derived stem cells (ASCs) along with a muscle derived ECM scaffold (m-ECM). The composite scaffold provides a vasculo-inductive and an effective cell delivery platform for volumetric muscle loss.

A Porcine Urinary Bladder Matrix Does Not Recapitulate the Spatiotemporal Macrophage Response of Muscle Regeneration after Volumetric Muscle Loss Injury.

Volumetric muscle loss (VML) results in irrecoverable loss of muscle tissue making its repair challenging. VML repair with acellular extracellular matrix (ECM) scaffolds devoid of exogenous cells has shown improved muscle function, but limited de novo muscle fiber regeneration. On the other hand, studies using minced autologous and free autologous muscle grafts have reported appreciable muscle regeneration. This raises the fundamental question whether an acellular ECM scaffold can orchestrate the spatiotemporal cellular events necessary for appreciable muscle fiber regeneration. This study compares the macrophage and angiogenic responses including the remodeling outcomes of a commercially available porcine urinary bladder matrix, MatriStem™, and autologous muscle grafts. The early heightened and protracted M1 response of the scaffold indicates that the scaffold does not recapitulate the spatiotemporal macrophage response of the autograft tissue. Additionally, the scaffold only supports limited de novo muscle fiber formation and regressing vessel density. Furthermore, scaffold remodeling is accompanied by increased presence of transforming growth factor and α-smooth muscle actin, which is consistent with remodeling of the scaffold into a fibrotic scar-like tissue. The limited muscle formation and scaffold-mediated fibrosis noted in this study corroborates the findings of recent studies that investigated acellular ECM scaffolds (devoid of myogenic cells) for VML repair. Taken together, acellular ECM scaffolds when used for VML repair will likely remodel into a fibrotic scar-like tissue and support limited de novo muscle fiber regeneration primarily in the proximity of the injured musculature. This is a work of the US Government and is not subject to copyright protection in the USA. Foreign copyrights may apply. Published by S. Karger AG, Basel.

An acellular biologic scaffold does not regenerate appreciable de novo muscle tissue in rat models of volumetric muscle loss injury.

Extracellular matrix (ECM) derived scaffolds continue to be investigated for the treatment of volumetric muscle loss (VML) injuries. Clinically, ECM scaffolds have been used for lower extremity VML repair; in particular, MatriStem™, a porcine urinary bladder matrix (UBM), has shown improved functional outcomes and vascularization, but limited myogenesis. However, efficacy of the scaffold for the repair of traumatic muscle injuries has not been examined systematically. In this study, we demonstrate that the porcine UBM scaffold when used to repair a rodent gastrocnemius musculotendinous junction (MTJ) and tibialis anterior (TA) VML injury does not support muscle tissue regeneration. In the MTJ model, the scaffold was completely resorbed without tissue remodeling, suggesting that the scaffold may not be suitable for the clinical repair of muscle-tendon injuries. In the TA VML injury, the scaffold remodeled into a fibrotic tissue and showed functional improvement, but not due to muscle fiber regeneration. The inclusion of physical rehabilitation also did not improve functional response or tissue remodeling. We conclude that the porcine UBM scaffold when used to treat VML injuries may hasten the functional recovery through the mechanism of scaffold mediated functional fibrosis. Thus for appreciable muscle regeneration, repair strategies that incorporate myogenic cells, vasculogenic accelerant and a myoconductive scaffold need to be developed.

Physical rehabilitation improves muscle function following volumetric muscle loss injury.

BACKGROUND: Given the clinical practice of prescribing physical rehabilitation for the treatment of VML injuries, the present study examined the functional and histomorphological adaptations in the volumetric muscle loss (VML) injured muscle to physical rehabilitation. METHODS: Tibialis anterior muscle VML injury was created in Lewis rats (n = 32), and were randomly assigned to either sedentary (SED) or physical rehabilitation (RUN) group. After 1 week, RUN rats were given unlimited access to voluntary running wheels either 1 or 7 weeks (2 or 8 weeks post-injury). At 2 weeks post-injury, TA muscles were harvested for molecular analyses. At 8 weeks post-injury, the rats underwent in vivo function testing. The explanted tissue was analyzed using histological and immunofluorescence procedures. RESULTS: The primary findings of the study are that physical rehabilitation in the form of voluntary wheel running promotes ~ 17% improvement in maximal isometric torque, and a ~ 13% increase in weight of the injured muscle, but it did so without significant morphological adaptations (e.g., no hypertrophy and hyperplasia). Wheel running up-regulated metabolic genes (SIRT-1, PGC-1α) only in the uninjured muscles, and a greater deposition of fibrous tissue in the defect area of the injured muscle preceded by an up-regulation of pro-fibrotic genes (Collagen I, TGF-ß1). Therefore, it is plausible that the wheel running related functional improvements were due to improved force transmission and not muscle regeneration. CONCLUSIONS: This is the first study to demonstrate improvement in functional performance of non-repaired VML injured muscle with physical rehabilitation in the form of voluntary wheel running. This study provides information for the first time on the basic changes in the VML injured muscle with physical rehabilitation, which may aid in the development of appropriate physical rehabilitation regimen(s).

Reinforced fascia patch limits cyclic gapping of rotator cuff repairs in a human cadaveric model.

BACKGROUND: Scaffolds continue to be developed and used for rotator cuff repair augmentation, but clinical or biomechanical data to inform their use are limited. We have developed a reinforced fascia lata patch with mechanical properties to meet the needs of musculoskeletal applications. The objective of this study was to assess the extent to which augmentation of a primary human rotator cuff repair with the reinforced fascia patch can reduce gap formation during in vitro cyclic loading. MATERIALS AND METHODS: Nine paired human cadaveric shoulders were used to investigate the cyclic gap formation and failure properties of augmented and non-augmented rotator cuff repairs with loading of 5 to 180 N for 1000 cycles. RESULTS: Augmentation significantly decreased the amount of gap formation at cycles 1, 10, 100, and 1000 compared with non-augmented repairs (P < .01). The mean gap formation of the augmented repairs was 1.8 mm after the first cycle of pull (vs 3.6 mm for non-augmented repairs) and remained less than 5 mm after 1000 cycles of loading (4.7 mm for augmented repairs vs 7.3 mm for non-augmented repairs). Furthermore, all augmented repairs were able to complete the 1000-cycle loading protocol, whereas 3 of 9 non-augmented repairs failed before completing 1000 loading cycles. CONCLUSIONS: This study supports further investigation of reinforced fascia patches to provide mechanical augmentation, minimize tendon retraction, and possibly reduce the incidence of rotator cuff repair failure. Future investigation in animal and human studies will be necessary to fully define the efficacy of the reinforced fascia device in a biologic healing environment.

Scaffold devices for rotator cuff repair.

Rotator cuff tears affect 40% or more of those aged older than 60 years, and repair failure rates of 20% to 70% remain a significant clinical challenge. Hence, there is a need for repair strategies that can augment the repair by mechanically reinforcing it, while at the same time biologically enhancing the intrinsic healing potential of the tendon. Tissue engineering strategies to improve rotator cuff repair healing include the use of scaffolds, growth factors, and cell seeding, or a combination of these approaches. Currently, scaffolds derived from mammalian extracellular matrix, synthetic polymers, and a combination thereof, have been cleared by the U.S. Food and Drug Administration and are marketed as medical devices for rotator cuff repair in humans. Despite the growing clinical use of scaffold devices for rotator cuff repair, there are numerous questions related to their indication, surgical application, safety, mechanism of action, and efficacy that remain to be clarified or addressed. This article reviews the current basic science and clinical understanding of commercially available synthetic and extracellular matrix scaffolds for rotator cuff repair. Our review will emphasize the host response and scaffold remodeling, mechanical and suture-retention properties, and preclinical and clinical studies on the use of these scaffolds for rotator cuff repair. We will discuss the implications of these data on the future directions for use of these scaffolds in tendon repair procedures.

Effect of pretension and suture needle type on mechanical properties of acellular human dermis patches for rotator cuff repair.

BACKGROUND: Dermal grafts are used for rotator cuff repair and augmentation. Although the in vitro biomechanical properties of dermal grafts have been reported previously, clinical questions related to their biomechanical performance as a surgical construct and the effect of surgical variables that could potentially improve repair outcomes have not been studied. METHODS: This study evaluated the failure and fatigue biomechanics of acellular dermis constructs tested in a clinically relevant size (4 × 4 cm patches) and manner (loaded via sutures) for rotator cuff repair. Also investigated were the effect of 2 surgical variables: (1) the fixation of grafts under varying magnitudes of pretension (0, 10, 20N), and (2) the use of reverse-cutting vs tapered needles for suturing grafts. RESULTS: Dermis constructs stretched ∼25% before bearing significant loads in the high stiffness region. Although 91% of the patches withstood 2500 cycles of loading to 150 N, the constructs stretched 13 to 19 mm after fatigue loading. This elongation could be reduced by 20% to 32% when reverse-cutting needles were used to prepare constructs or by applying 20 N of in situ circumferential pretension to the constructs before loading. CONCLUSIONS: Although dermis patches demonstrated robustness for use in rotator cuff repair, the patches underwent significant, substantial, and presumably nonrecoverable elongation, even at low physiologic loads. This study indicates that use of reverse-cutting needles for suture passage, preconditioning (cyclically stretching several times), and/or surgical fixation under at least 20 N of circumferential pretension could be developed as strategies to reduce compliance of dermis for its use for rotator cuff repair.

The biomechanical role of scaffolds in augmented rotator cuff tendon repairs.

BACKGROUND: Scaffolds continue to be developed and used for rotator cuff repair augmentation; however, the appropriate scaffold material properties and/or surgical application techniques for achieving optimal biomechanical performance remains unknown. The objectives of the study were to simulate a previously validated spring-network model for clinically relevant scenarios to predict: (1) the manner in which changes to components of the repair influence the biomechanical performance of the repair and (2) the percent load carried by the scaffold augmentation component. MATERIALS AND METHODS: The models were parametrically varied to simulate clinically relevant scenarios, namely, changes in tendon quality, altered surgical technique(s), and different scaffold designs. The biomechanical performance of the repair constructs and the percent load carried by the scaffold component were evaluated for each of the simulated scenarios. RESULTS: The model predicts that the biomechanical performance of a rotator cuff repair can be modestly increased by augmenting the repair with a scaffold that has tendon-like properties. However, engineering a scaffold with supraphysiologic stiffness may not translate into yet stiffer or stronger repairs. Importantly, the mechanical properties of a repair construct appear to be most influenced by the properties of the tendon-to-bone repair. The model suggests that in the clinical setting of a weak tendon-to-bone repair, scaffold augmentation may significantly off-load the repair and largely mitigate the poor construct properties. CONCLUSIONS: The model suggests that future efforts in the field of rotator cuff repair augmentation may be directed toward strategies that strengthen the tendon-to-bone repair and/or toward engineering scaffolds with tendon-like mechanical properties.

Mechanical characterization and biocompatibility of a novel reinforced fascia patch for rotator cuff repair.

To provide mechanical augmentation for rotator cuff repair, it is necessary (though perhaps not sufficient) that scaffolds have tendon-like material and suture retention properties, be applied to the repair in a surgically appropriate manner, and maintain their mechanical properties for an acceptable period of time following surgery. While allograft fascia lata has material, structural, and biochemical properties similar to tendon tissue, its poor suture retention properties abrogates its potential as an augmentation device. The goal of this work was to design a novel reinforced fascia patch with suture retention and stiffness properties adequate to provide mechanical augmentation for rotator cuff repair. Fascia was reinforced by stitching with PLLA or PLLA/PGA polymer braids. Reinforced fascia patches had a maximum construct load greater than (or equal to) the suture retention properties of human rotator cuff tendon (∼250N) at time zero and after in vivo implantation for 12 weeks in a rat subcutaneous model. The patches were able to withstand the 2500 loading cycles projected for the early post-operative period. The patches also demonstrated biocompatibility with the host using a rat abdominal wall defect model. These studies suggest the potential use of reinforced fascia patches to provide mechanical augmentation, minimize tendon retraction and possibly reduce the incidence of rotator cuff repair failure.

Commercially available extracellular matrix materials for rotator cuff repairs: state of the art and future trends.

Rotator cuff tears, a common source of shoulder pathology, are often the cause of debilitating shoulder pain, reduced shoulder function and compromised joint mechanics. The treatment, evaluation and management of this disease puts an annual financial burden of 3 billion US dollars on the US economy. Despite surgical advances, there is a high rate of recurrent tears ranging (20-70%) after surgical repair, particularly for chronic, large to massive cuff tears. The inability to obtain a high healing rate in these tears has fueled investigation in the use of extracellular matrix (ECM) derived materials as a scaffolds for rotator cuff tendon repair and regeneration. The present paper reviews the current state of knowledge regarding the mechanical and biological characteristics of commercially available ECM materials, delineates indications for their clinical use and suggests future directions in developing ECM scaffolds for rotator cuff repair.