Response of terminal Schwann cells following volumetric muscle loss injury.

An often-overlooked component of traumatic skeletal muscle injuries is the impact on the nervous system and resultant innervation of the affected muscles. Recent work in a rodent model of volumetric muscle loss (VML) injury demonstrated a progressive, secondary loss of neuromuscular junction (NMJ) innervation, supporting a role of NMJ dysregulation in chronic functional deficits. Terminal Schwann cells (tSCs) are known to be vital for the maintenance of NMJ structure and function, in addition to guiding repair and regeneration after injury. However, the tSC response to a traumatic muscle injury such as VML is not known. Thus, a study was conducted to investigate the effect of VML on tSC morphological characteristics and neurotrophic signaling proteins in adult male Lewis rats that underwent VML injury to the tibialis anterior muscle using a temporal design with outcome assessments at 3, 7, 14, 21, and 48 days post-injury. The following salient observations were made; first, although there is a loss of innervation over time, the number of tSCs per NMJ increases, significantly so at 48 days post-injury compared to control. The degree of NMJ fragmentation was positively correlated with tSC number after injury. Moreover, neurotrophic factors such as NRG1 and BDNF are elevated after injury through at least 48 days. These results were unanticipated and in contrast to neurodegenerative disease models, in which there is a reduction in tSC number that precedes denervation. However, we found that while there are more tSCs per NMJ after injury, they cover a significantly smaller percent of the post-synaptic endplate area compared to control. These findings support a sustained increase in neurotrophic activity and tSC number after VML, which is a maladaptive response occurring in parallel to other aspects of the VML injury, such as over-accumulation of collagen and aberrant inflammatory signaling.

Minced muscle autografting improves bone healing but not muscle function in a porcine composite injury model.

Composite tissue injuries (CTIs) in extremities include segmental bone defects (SBDs) and volumetric muscle loss. The objective of this study was to determine if skeletal muscle autografting with minced muscle grafts (MMGs) could improve healing in an SBD and improve muscle function in a porcine CTI model that includes an SBD and adjacent volumetric muscle loss injury. Adult Yucatan Minipigs were stratified into three groups including specimens with an isolated SBD, an SBD with volumetric muscle loss (CTI), and an SBD with volumetric muscle loss treated with MMG (CTI + MMG). Bone healing was quantified with serial x-rays and postmortem computed tomography scanning. Muscle function was quantified with a custom in vivo force transducer. Muscle tissue content was determined by biochemical analyses and histology. Anterior cortex-modified Radiographic Union Score for Tibia fractures (mRUSTs) decreased from 2.7 to 1.9 (p = 0.003) in CTI versus SBD animals. MMG improved anterior mRUST scores to 2.5 in CTI + MMG specimens (p = 0.030 compared to CTI specimens) and overall mRUST scores increased from 9.4 in CTI specimens to 11.1 in CTI + MMG specimens (p = 0.049). Residual strength deficits at euthanasia were 42% in SBD (p < 0.001), 44% in CTI (p < 0.001), and 48% in CTI + MMG (p < 0.001) compared to preoperative values. There were no differences in strength deficits between the three groups. Biochemical and histologic analyses demonstrated scattered differences between the three groups compared to contralateral muscle. MMG improved bone healing. However, the primary cause of muscle dysfunction and biochemical changes was the presence of an SBD. Clinical significance: Early mitigation of SBDs may be necessary to prevent muscle damage and weakness in patients sustaining composite extremity trauma.

Retrospective characterization of a rat model of volumetric muscle loss.

Volumetric muscle loss (VML) is a pervasive injury within contemporary combat and a primary driver of disability among injured Service members. As such, VML has been a topic of investigation over the past decade as the field has sought to understand the pathology of these injuries and to develop treatment strategies which restore the form and function of the involved musculature. To date, much of this work has been performed in disparate animal models that vary significantly in terms of the species utilized, the muscle (or muscle group) affected, and the volume of muscle lost. Moreover, variation exists in the reporting of anatomical and functional outcomes within these models. When taken together, the ability to successfully assess comparative efficacy of promising therapies is currently limited. As such, greater scrutiny on the characterization of these VML models is needed to better assess the quality of evidence supporting further translation of putative therapies. Thus, the objective of this study was to retrospectively characterize anatomical and functional outcomes associated with one such VML model - the 6 mm biopsy punch model of the rat tibialis anterior muscle. Through these efforts, it was shown that this model is highly reproducible and consistent across a large number of experiments. As such, the data presented herein represent a reasonable benchmark for the expected performance of this model with utility for drawing inferences across studies and identifying therapies which have shown promise within the preclinical domain, and thus are ready for further translation towards the clinic.

Temporal changes in the muscle extracellular matrix due to volumetric muscle loss injury.

PURPOSE/AIM: Volumetric muscle loss (VML) is a devastating orthopedic injury resulting in chronic persistent functional deficits, loss of joint range of motion, pathologic fibrotic deposition and lifelong disability. However, there is only limited mechanistic understanding of VML-induced fibrosis. Herein we examined the temporal changes in the fibrotic deposition at 3, 7, 14, 28, and 48 days post-VML injury. MATERIALS AND METHODS: Adult male Lewis rats (n = 39) underwent a full thickness ~20% (~85 mg) VML injury to the tibialis anterior (TA) muscle unilaterally, the contralateral TA muscle served as the control group. All TA muscles were harvested for biochemical and histologic evaluation. RESULTS: The ratio of collagen I/III was decreased at 3, 7, and 14 days post-VML, but significantly increased at 48 days. Decorin content followed an opposite trend, significantly increasing by day 3 before dropping to below control levels by 48 days. Histological evaluation of the defect area indicates a shift from loosely packed collagen at early time points post-VML, to a densely packed fibrotic scar by 48 days. CONCLUSIONS: The shift from early wound healing efforts to a fibrotic scar with densely packed collagen within the skeletal muscle occurs around 21 days after VML injury through dogmatic synchronous reduction of collagen III and increase in collagen I. Thus, there appears to be an early window for therapeutic intervention to prevent pathologic fibrous tissue formation, potentially by targeting CCN2/CTGF or using decorin as a therapeutic.

In Vivo Measurement of Hindlimb Dorsiflexor Isometric Torque from Pig.

Reliable assessment of skeletal muscle strength is arguably the most important outcome measure in neuromuscular and musculoskeletal disease and injury studies, particularly when evaluating regenerative therapies' efficacy. Additionally, a critical aspect of translating many regenerative therapies is the demonstration of scalability and effectiveness in a large animal model. Various physiological preparations have been established to evaluate intrinsic muscle function properties in basic science studies, primarily in small animal models. The practices may be categorized as: in vitro (isolated fibers, fiber bundles, or whole muscle), in situ (muscle with intact vascularization and innervation but distal tendon attached to a force transducer), and in vivo (structures of the muscle or muscle unit remain intact). There are strengths and weaknesses to each of these preparations; however, a clear advantage of in vivo strength testing is the ability to perform repeated measurements in the same animal. Herein, the materials and methods to reliably assess isometric torque produced by the hindlimb dorsiflexor muscles in vivo in response to standard peroneal electrical stimulation in anesthetized pigs are presented.

Secondary denervation is a chronic pathophysiologic sequela of volumetric muscle loss.

Volumetric muscle loss (VML) is the traumatic loss of muscle tissue that results in long-term functional impairments. Despite the loss of myofibers, there remains an unexplained significant decline in muscle function. VML injury likely extends beyond the defect area, causing negative secondary outcomes to the neuromuscular system, including the neuromuscular junctions (NMJs), yet the extent to which VML induces denervation is unclear. This study systematically examined NMJs surrounding the VML injury, hypothesizing that the sequela of VML includes denervation. The VML injury removed ∼20% of the tibialis anterior (TA) muscle in adult male inbred Lewis rats (n = 43), the noninjured leg served as an intra-animal control. Muscles were harvested up to 48 days post-VML. Synaptic terminals were identified immunohistochemically, and quantitative confocal microscopy evaluated 2,613 individual NMJ. Significant denervation was apparent by 21 and 48 days post-VML. Initially, denervation increased ∼10% within 3 days of injury; with time, denervation further increased to ∼22% and 32% by 21 and 48 days post-VML, respectively, suggesting significant secondary denervation. The appearance of terminal axon sprouting and polyinnervation were observed as early as 7 days post-VML, increasing in number and complexity throughout 48 days. There was no evidence of VML-induced NMJ size alteration, which may be beneficial for interventions aimed at restoring muscle function. This work recognizes VML-induced secondary denervation and poor remodeling of the NMJ as part of the sequela of VML injury; moreover, secondary denervation is a possible contributing factor to the chronic functional impairments and potentially an overlooked treatment target.NEW & NOTEWORTHY This work advances our understanding of the pathophysiologic complexity of volumetric muscle loss injury. Specifically, we identified secondary denervation in the muscle remaining after volumetric muscle loss injuries as a novel aspect of the injury sequela. Denervation increased chronically, in parallel with the appearance of irregular morphological characteristics and destabilization of the neuromuscular junction, which is expected to further confound chronic functional impairments.

Volumetric muscle loss disrupts length-dependent architectural and functional characteristics of skeletal muscle.

Purpose/Aim: Skeletal muscle architecture is a primary determinant of function. Volumetric muscle loss (VML) injury is destructive; however, the impact on muscle architecture is uncharacterized. Methods: Architectural and functional effects of VML were assessed in rat tibialis anterior (TA) muscle model 4 weeks post-injury. Results: VML caused a 31% and 33% reduction in muscle weight (p < 0.001) and fiber length (p = 0.002), respectively, culminating a 34% reduction of fiber to muscle length ratio (FL:ML; p < 0.001). Fiber pennation angle (+14%; p = 0.150) and physiological cross-sectional area (PCSA; -12%; p = 0.220) were unchanged. VML injury reduced peak isometric force (Po) by 36% (p < 0.001), specific force (sPo = Po/PCSA) by 41% (vs. Po, p > 0.999), and force per gram muscle weight (Po/mw) by 18% (vs. Po, p < 0.001). VML injury increased the length at which Po was produced (Lo) by 8% (p = 0.009), and reduced functional excursion by 35% (p = 0.035). Conclusion: The architectural changes after VML injury preserved PCSA, and therefore preserved "potential" maximal force-producing capacity. At most, only half the Po deficit was due directly to the cumulative effect of horizontal and longitudinal tissue loss. Highlighting the impact of longitudinal muscle loss, VML injury reduced fiber length, and FL:ML and grossly disrupted length-dependent functional properties. These findings raise the importance of augmenting length-dependent muscle properties to optimize functional recovery after VML injury.

Musculoskeletal Regeneration, Rehabilitation, and Plasticity Following Traumatic Injury.

The musculoskeletal system has an integral role throughout life, including structural support to the body, protection, and allowing a range of fine to complex movements for daily living to elite sporting events. At various times, injuries to the musculoskeletal system occur resulting in varying levels of impact to the person both acutely and chronically. Specifically, there is a spectrum of complexity in orthopedic injuries, with some such as common muscle strains, that while burdensome will have no impact on life-long functional ability, and others that can result in long lasting disability. Focusing on extremity injuries, this review highlights: i)the current impact of orthopedic injuries in sport and daily life; ii) the foundation of bone and skeletal muscle repair and regeneration; and iii) the disruptions in regenerative healing due to traumatic orthopedic injuries. This review seeks to maximize the broad and collective research impact on sport and traumatic orthopedic injuries in search of promoting ongoing innovation for treatment and rehabilitation approaches aimed to improve musculoskeletal health throughout life.

Pharmacological Mitigation of Fibrosis in a Porcine Model of Volumetric Muscle Loss Injury.

Volumetric muscle loss (VML) resulting from extremity trauma presents functional deficits and fibrosis, ultimately manifesting disability. The extensive fibrotic accumulation is expected to interfere with neural, trophic, vascular, and mechanical connectivity of any possible regenerative medicine approaches. Our objective was to quantify the muscle properties and stiffness following injury and investigate if the fibrotic deposition could be mitigated using an antifibrotic agent; we hypothesized that antifibrotic treatment would prevent the overwhelming fibrotic response. Yorkshire Cross pigs (n = 10) were randomized to sham or a nontreated ∼20% VML injury. Immediately following surgery, injured animals were further randomized to nintedanib (Ofev; 300 mg/day) or no treatment for 30 days. Longitudinal analysis of muscle function via peroneal nerve stimulation, compartment volume, and quantitative muscle stiffness using shearwave elastography were conducted. Terminally comprehensive histopathologic, biochemical, and genetic investigations were conducted on the skeletal muscle and fibrosis. Through 4 weeks post-VML, nontreated muscles presented a significant deficit (23%) in maximal torque compared to the sham operated (p < 0.01). The stiffness in the VML defect area increased significantly (7-fold) in the VML-nontreated leg than the VML antifibrotic-treated legs by 4 weeks postinjury, which was coupled with the nontreated muscle having ∼40% more hydroxyproline per mg of tissue than those receiving antifibrotic treatment (p = 0.01). This work indicates that VML injury progressively induces fibrosis and muscle stiffness. Antifibrotic treatment can mitigate the pathologic development of fibrosis. Future work should evaluate optimal timing and duration of treatments combined with regenerative medicine approaches in efforts to improve function. Impact statement This work primarily evaluated the effect of a clinically available antifibrotic therapy (nintedanib) on the development of fibrosis after volumetric muscle loss (VML) injury in a large animal model. As a primary outcome measure of fibrosis, skeletal muscle stiffness was repeatedly measured in vivo and noninvasively using a quantitative ultrasound device with shearwave elastography capability. The most salient finding of the study is that the antifibrotic nintedanib significantly reduced the development of VML injury-induced fibrous tissue deposition and stiffness.

Therapeutic Approaches for Volumetric Muscle Loss Injury: A Systematic Review and Meta-Analysis.

Our goal was to understand the impact of regenerative therapies on the functional capacity of skeletal muscle following volumetric muscle loss (VML) injury. An extensive database search (e.g., PubMed, Cochrane Library, and ClinicalTrials.gov) was conducted up through January 2019 to evaluate the following: "In humans or animals with VML injury, is treatment better than no treatment at recovering functional capacity?" Study eligibility criteria required studies to have both an untreated and at least one treated VML injury group. From 2312 study reports, 44 studies met the inclusion criteria. Quantitative functional capacity data (absolute and/or normalized strength) or proportional measures (histological analysis quantifying viable muscle tissue, mitochondrial function, and/or exhaustive treadmill running) were extracted for use. While both human and animal studies were included in the searches, only animal studies met the eligibility criteria. Using a random-effects model, Hedges' g was used as the effect size (ES) and calculated such that a positive ES indicated treatment efficacy. The overall ES was 0.75 (95% confidence interval: 0.53-0.96; p < 0.0000001), indicating that the treatments, on average, resulted in a significant improvement in functional capacity. From network meta-analyses, it was determined that an acellular biomaterial combined with stem and/or progenitor cells had the greatest treatment effectiveness. The findings indicate that various treatments in animal models of VML improve the functional capacity of muscle compared to leaving the injury untreated; however, the ∼16% beneficial effect is small. Our results suggest that current regenerative therapy paradigms require further maturation to achieve clinically meaningful improvements in the functional capacity of the muscle. Impact Statement Our most salient findings are that (1) various treatment approaches used in animal models of volumetric muscle loss (VML) injury improve functional capacity compared to leaving the injury untreated and (2) an acellular biomaterial in combination with cellular components was the most effective treatment to improve functional capacity following VML injury to date. The nature of our findings has substantial implications for regenerative medicine, biomedical engineering, and rehabilitative techniques currently being evaluated and developed for VML injury repair, and are pivotal to the progression of the regenerative medicine effort aimed at restoring maximal function to traumatized and disabled limbs.

Unilateral Laminotomy with Bilateral Decompression: A Case Series Studying One- and Two-Year Outcomes with Predictors of Minimal Clinical Improvement.

OBJECTIVE: To assess factors that may predict failure to improve at 12 and 24 months after unilateral laminotomy with bilateral decompression (ULBD) for the management of lumbar spinal stenosis. METHODS: A database of 255 patients who underwent microdecompression surgery by a single orthopedic spine surgeon between 2014 and 2018 was queried. Patients who underwent primary single-level ULBD of the lumbar spine were included. Visual analog scale (VAS) for back pain and leg pain and Oswestry Disability Index (ODI) results were collected preoperatively and at 12 and 24 months postoperatively. Demographic, radiographic, and operative factors were assessed for associations with failure to improve. Clinically important improvement was defined as reaching or surpassing the previously established minimum clinically important difference for ODI (12.8) and not requiring revision. RESULTS: A total of 68 patients were included. Compared with preoperative values for back pain, leg pain, and ODI (7.32, 7.53, and 51.22, respectively), there were significant improvements on follow-up at 12 months (2.89, 2.23, and 22.40, respectively; P < 0.001) and 24 months (2.80, 2.11, 20.32, respectively; P < 0.001). Based on the defined criteria, 50 patients showed clinically important improvement after ULBD. Of the 18 patients who failed to improve, 12 required revision. Independent predictors of failure to improve included female sex (adjusted odds ratio, 5.06; 95% confidence interval, 1.49-21.12; P = 0.014) and current smoker status (adjusted odds ratio, 5.39; 95% confidence interval, 1.39-23.97; P = 0.018). CONCLUSIONS: ULBD for the management of lumbar spinal stenosis leads to clinically important improvement that is maintained over a 24-month follow-up period. Female sex and tobacco smoking are associated with poorer outcomes.

Oxidative pathophysiology following volumetric muscle loss injury in a porcine model.

Volumetric muscle loss (VML) occurs after severe orthopedic trauma and results in loss of muscle fibers and function that can leave patients permanently disabled. Although animals models of VML are useful to test possible therapeutic strategies, the pathophysiological characteristics of remaining skeletal muscle and changes in metabolism are not thoroughly understood. Herein, alterations of neuromuscular function, muscle fiber morphology, myosin heavy chain expression, and myofiber mitochondrial respiration were evaluated in an adult Yorkshire swine VML injury model. VML injured animals showed reduced peak isometric strength (P < 0.05) and a shift toward smaller muscle fibers independent of fiber type (P < 0.001). The muscle remaining after VML had a greater distribution of type I fibers and lower distribution of type II fibers (P < 0.001). Skeletal muscle mitochondrial state 2 and state 3, reflecting complex I respiration, increased after injury (P < 0.05) with a consistent trend to display higher oxygen flux per milligram of tissue. However, this was largely driven by increased mitochondrial content after VML which was associated with higher mitochondrial fission (FIS-1 protein levels). This study demonstrates an underlying perturbation of oxidative metabolism within the remaining musculature following surgical creation of an isolated, sterile VML injury in a porcine model that may be influential to the development of insidious pathophysiology and regenerative and rehabilitative therapies. NEW & NOTEWORTHY The natural injury sequela of volumetric muscle loss (VML) and associated pathophysiology of the remaining muscle is still incompletely understood. Herein we demonstrate a chronic muscle function deficit, with an increase in type I muscle fibers and parallel increase in oxidative capacity of remaining skeletal muscle. It is possible that the alteration in oxidative capacity after VML could largely be due to heightened mitochondrial activity and an increase in mitochondrial abundance.

Correction: Multiscale analysis of a regenerative therapy for treatment of volumetric muscle loss injury.

[This corrects the article DOI: 10.1038/s41420-018-0027-8.].

Early rehabilitation for volumetric muscle loss injury augments endogenous regenerative aspects of muscle strength and oxidative capacity.

BACKGROUND: Volumetric muscle loss (VML) injuries occur due to orthopaedic trauma or the surgical removal of skeletal muscle and result in debilitating long-term functional deficits. Current treatment strategies do not promote significant restoration of function; additionally appropriate evidenced-based practice physical therapy paradigms have yet to be established. The objective of this study was to develop and evaluate early rehabilitation paradigms of passive range of motion and electrical stimulation in isolation or combination to understand the genetic and functional response in the tissue remaining after a multi-muscle VML injury. METHODS: Adult male mice underwent an ~ 20% multi-muscle VML injury to the posterior compartment (gastrocnemius, soleus, and plantaris muscle) unilaterally and were randomized to rehabilitation paradigm twice per week beginning 2 days post-injury or no treatment. RESULTS: The most salient findings of this work are: 1) that the remaining muscle tissue after VML injury was adaptable in terms of improved muscle strength and mitigation of stiffness; but 2) not adaptable to improvements in metabolic capacity. Furthermore, biochemical (i.e., collagen content) and gene (i.e., gene arrays) assays suggest that functional adaptations may reflect changes in the biomechanical properties of the remaining tissue due to the cellular deposition of non-contractile tissue in the void left by the VML injury and/or differentiation of gene expression with early rehabilitation. CONCLUSIONS: Collectively this work provides evidence of genetic and functional plasticity in the remaining skeletal muscle with early rehabilitation approaches, which may facilitate future evidenced-based practice of early rehabilitation at the clinical level.

Multiscale analysis of a regenerative therapy for treatment of volumetric muscle loss injury.

Skeletal muscle possesses a remarkable capacity to regenerate when injured, but when confronted with major traumatic injury resulting in volumetric muscle loss (VML), the regenerative process consistently fails. The loss of muscle tissue and function from VML injury has prompted development of a suite of therapeutic approaches but these strategies have proceeded without a comprehensive understanding of the molecular landscape that drives the injury response. Herein, we administered a VML injury in an established rodent model and monitored the evolution of the healing phenomenology over multiple time points using muscle function testing, histology, and expression profiling by RNA sequencing. The injury response was then compared to a regenerative medicine treatment using orthotopic transplantation of autologous minced muscle grafts (~1 mm3 tissue fragments). A chronic inflammatory and fibrotic response was observed at all time points following VML. These results suggest that the pathological response to VML injury during the acute stage of the healing response overwhelms endogenous and therapeutic regenerative processes. Overall, the data presented delineate key molecular characteristics of the pathobiological response to VML injury that are critical effectors of effective regenerative treatment paradigms.

Effect of Caffeine Supplementation on Quadriceps Performance After Eccentric Exercise.

Green, MS, Martin, TD, and Corona, BT. Effect of caffeine supplementation on quadriceps performance after eccentric exercise. J Strength Cond Res 32(10): 2863-2871, 2018-Caffeine use is common among athletes seeking to capitalize on its potential ergogenic effects. Limited research has examined caffeine's effects when used after activities that resulted in exercise-induced muscle damage (EIMD). This study examined the effect of caffeine supplementation on uninjured and injured muscle. Eight men and women (N = 16) who were physically active individuals participated in this study (age: 24.3 ± 4.3 years; height: 173.0 ± 7.0 cm, mass: 75.2 ± 11.5 kg; body fat: 18.2 ± 15.9%). One leg was assessed under uninjured and injured (100 eccentric quadriceps contractions) conditions after caffeine supplementation (6 mg·kg), with the other leg assessed under both conditions after placebo supplementation. Compared with the placebo, caffeine increased peak isokinetic torque by 6.8 ± 2.3 and 9.4 ± 2.5% in uninjured and injured muscle, respectively, but had no effect on maximal voluntary isometric torque or fatigue index in uninjured or injured muscle, with treatments exhibiting similar (p > 0.05) alterations in isometric torque (-11.9 ± 2.2%), fatigue index (-13.9 ± 3.4%), and soreness (+44.0 ± 4.7) after eccentric contractions. The results of this study suggest that caffeine possesses a similar ergogenic effect on isokinetic torque in both uninjured and injured states, but no effect on the production of isometric torque, perception of soreness, or degree of relative fatigue. Athletes should consider the potential caffeine supplementation possesses during recovery from activities that resulted in EIMD.

Diagnosis and Management of Polytraumatized Patients With Severe Extremity Trauma.

Multiply injured patients with severe extremity trauma are at risk of acute systemic complications and are at high risk of developing longer term orthopaedic complications including soft-tissue infection, osteomyelitis, posttraumatic osteoarthritis, and nonunion. It is becoming increasingly recognized that injury magnitude and response to injury have major jurisdiction pertaining to patient outcomes and complications. The complexities of injury and injury response that affect outcomes present opportunities to apply precision approaches to understand and quantify injury magnitude and injury response on a patient-specific basis. Here, we present novel approaches to measure injury magnitude by adopting methods that quantify both mechanical and ischemic tissue injury specific to each patient. We also present evolving computational approaches that have provided new insight into the complexities of inflammation and immunologic response to injury specific to each patient. These precision approaches are on the forefront of understanding how to stratify individualized injury and injury response in an effort to optimize titrated orthopaedic surgical interventions, which invariably involve most of the multiply injured patients. Finally, we present novel methods directed at mangled limbs with severe soft-tissue injury that comprise severely injured patients. Specifically, methods being developed to treat mangled limbs with volumetric muscle loss have the potential to improve limb outcomes and also mitigate uncompensated inflammation that occurs in these patients.

Co-delivery of a laminin-111 supplemented hyaluronic acid based hydrogel with minced muscle graft in the treatment of volumetric muscle loss injury.

Minced muscle autografting mediates de novo myofiber regeneration and promotes partial recovery of neuromuscular strength after volumetric muscle loss injury (VML). A major limitation of this approach is the availability of sufficient donor tissue for the treatment of relatively large VMLs without inducing donor site morbidity. This study evaluated a laminin-111 supplemented hyaluronic acid based hydrogel (HA+LMN) as a putative myoconductive scaffolding to be co-delivered with minced muscle grafts. In a rat tibialis anterior muscle VML model, delivery of a reduced dose of minced muscle graft (50% of VML defect) within HA+LMN resulted in a 42% improvement of peak tetanic torque production over unrepaired VML affected limbs. However, the improvement in strength was not improved compared to a 50% minced graft-only control group. Moreover, histological analysis revealed that the improvement in in vivo functional capacity mediated by minced grafts in HA+LMN was not accompanied by a particularly robust graft mediated regenerative response as determined through donor cell tracking of the GFP+ grafting material. Characterization of the spatial distribution and density of macrophage and satellite cell populations indicated that the combination therapy damps the heightened macrophage response while re-establishing satellite content 14 days after VML to a level consistent with an endogenously healing ischemia-reperfusion induced muscle injury. Moreover, regional analysis revealed that the combination therapy increased satellite cell density mostly in the remaining musculature, as opposed to the defect area. Based on the results, the following salient conclusions were drawn: 1) functional recovery mediated by the combination therapy is likely due to a superposition of de novo muscle fiber regeneration and augmented repair of muscle fibers within the remaining musculature, and 2) The capacity for VML therapies to augment regeneration and repair within the remaining musculature may have significant clinical impact and warrants further exploration.

Inflammatory and Physiological Consequences of Debridement of Fibrous Tissue after Volumetric Muscle Loss Injury.

Volumetric muscle loss (VML) injuries present chronic loss of muscle fibers followed by expansive fibrotic tissue deposition. Regenerative medicine therapies are under development to promote regeneration. However, mitigation of the expansive fibrous tissue is required for integration with the remaining muscle. Using a porcine VML model, delayed debridement of injury fibrosis was performed 3 months post-VML and observed for an additional 4 weeks. A second group underwent the initial VML and was observed for 4 weeks, allowing comparison of initial fibrosis formation and debrided groups. The following salient observations were made: (i) debridement neither exacerbated nor ameliorated strength deficits; (ii) debridement results in recurrent fibrotic tissue deposition of a similar magnitude and composition as acute VML injury; and (iii) similarly upregulated transcriptional fibrotic and transcriptional pathways persist 4 weeks after initial VML or delayed debridement. This highlights the need for future studies to investigate adjunctive antifibrotic treatments for the fibrosed musculature.