Kristoffer B. Sugg

MMP inhibition as a potential method to augment the healing of skeletal muscle and tendon extracellular matrix

The extracellular matrix (ECM) of skeletal muscle and tendon is composed of different types of collagen molecules that play important roles in the transmission of forces throughout the body, and in the repair and regeneration of injured tissues. Fibroblasts are the primary cells in muscle and tendon that maintain, repair, and modify the ECM in response to mechanical loading, injury, and inactivity. Matrix metalloproteinases (MMPs) are enzymes that digest collagen and other structural molecules, which are synthesized and excreted by fibroblasts. MMPs are required for baseline ECM homeostasis, but disruption of MMP regulation due to injury or disease can alter the normal ECM architecture and prevent proper force transmission. Chronic injuries and diseases of muscles and tendons can be severely debilitating, and current therapeutic modalities to enhance healing are quite limited. This review will discuss the mechanobiology of MMPs, and the potential use of MMP inhibitors to improve the treatment of injured and diseased skeletal muscle and tendon tissue.

Engineering muscle constructs for the creation of functional engineered musculoskeletal tissue

Volumetric muscle loss (VML) is a disabling condition in which current clinical procedures are suboptimal. The field of tissue engineering has many promising strategies for the creation of functional skeletal muscle in vitro. However, there are still two key limitations that prevent it from becoming a solution for treating VML. First, engineered muscle tissue must be biocompatible to facilitate muscle tissue regrowth without generating an immune response. Second, engineered muscle constructs must be scaled up to facilitate replacement of clinically relevant volumes of tissue (centimeters in diameter). There are currently no tissue engineering strategies to produce tissue constructs that are both biocompatible and large enough to facilitate clinical repair. However, recent advances in tissue engineering using synthetic scaffolds, native scaffolds, or scaffold-free approaches may lead to a solution for repair of VML injuries.

Changes in macrophage phenotype and induction of epithelial-to-mesenchymal transition genes following acute Achilles tenotomy and repair

Tendon injuries occur frequently in physically active individuals, but the clinical outcomes for these injuries can be poor. In many injured tissues the repair process is orchestrated by two types of cells, macrophages and fibroblasts. Macrophages, which have both pro‐inflammatory (M1) and anti‐inflammatory (M2) phenotypes, can directly participate in tissue remodeling and direct the response of other cells through the secretion of cytokines and growth factors. In many organ systems, epithelial cells can trans‐differentiate into fibroblasts, which can then regenerate damaged ECM. This process is triggered via activation of epithelial‐to‐mesenchymal transition (EMT) signaling programs. Most tendons are surrounded by sheets of epithelial cells, and these tissue layers could provide a source of fibroblasts to repair injured tendons. To gain greater insight into the biology of tendon repair, we performed a tenotomy and repair in Achilles tendons of adult rats and determined changes in macrophage phenotype, and ECM‐ and EMT‐related genes over a 4‐week time course. The results from this study suggest that changes in macrophage phenotype and activation of EMT‐related programs likely contribute to the degradation and subsequent repair of injured tendon tissue.

Platelet-Rich Plasma Activates Proinflammatory Signaling Pathways and Induces Oxidative Stress in Tendon Fibroblasts

Background: Tendon injuries are one of the most common musculoskeletal conditions in active patients. Platelet-rich plasma (PRP) has shown some promise in the treatment of tendon disorders, but little is known as to the mechanisms by which PRP can improve tendon regeneration. PRP contains numerous different growth factors and cytokines that activate various cellular signaling cascades, but it has been difficult to determine precisely which signaling pathways and cellular responses are activated after PRP treatment. Additionally, macrophages play an important role in modulating tendon regeneration, but the influence of PRP on determining whether macrophages assume a proinflammatory or anti-inflammatory phenotype remains unknown. Purpose: To use genome-wide expression profiling, bioinformatics, and protein analysis to determine the cellular pathways activated in fibroblasts treated with PRP. The effect of PRP on macrophage polarization was also evaluated. Study Design: Controlled laboratory study. Methods: Tendon fibroblasts or macrophages from rats were cultured and treated with either platelet-poor plasma (PPP) or PRP. RNA or protein was isolated from cells and analyzed using microarrays, quantitative polymerase chain reaction, immunoblotting, or bioinformatics techniques. Results: Pathway analysis determined that the most highly induced signaling pathways in PRP-treated tendon fibroblasts were TNFα and NFκB pathways. PRP also downregulated the expression of extracellular matrix genes and induced the expression of autophagy-related genes and reactive oxygen species (ROS) genes and protein markers in tendon fibroblasts. PRP failed to have a major effect on markers of macrophage polarization. Conclusion: PRP induces an inflammatory response in tendon fibroblasts, which leads to the formation of ROS and the activation of oxidative stress pathways. PRP does not appear to significantly modulate macrophage polarization. Clinical Relevance: PRP might act by inducing a transient inflammatory event, which could then trigger a tissue regeneration response.

TGF-β superfamily signaling in muscle and tendon adaptation to resistance exercise.

Numerous studies in muscle and tendon have identified a central role of the transforming growth factor-&bgr; (TGF-&bgr;) superfamily of cytokines in the regulation of extracellular matrix growth and remodeling, protein degradation, and cell proliferation and differentiation. We provide a novel framework for TGF-&bgr; and myostatin signaling in controlling the coordinated adaptation of both skeletal muscle and tendon tissue to resistance training.

p38 MAPK signaling in postnatal tendon growth and remodeling.

Tendon is a dynamic tissue whose structure and function is influenced by mechanical loading, but little is known about the fundamental mechanisms that regulate tendon growth and remodeling in vivo. Data from cultured tendon fibroblasts indicated that the p38 MAPK pathway plays an important role in tendon fibroblast proliferation and collagen synthesis in vitro. To gain greater insight into the mechanisms of tendon growth, and explore the role of p38 MAPK signaling in this process, we tested the hypotheses that inducing plantaris tendon growth through the ablation of the synergist Achilles tendon would result in rapid expansion of a neotendon matrix surrounding the original tendon, and that treatment with the p38 MAPK inhibitor SB203580 would prevent this growth. Rats were treated with vehicle or SB203580, and subjected to synergist ablation by bilateral tenectomy of the Achilles tendon. Changes in histological and biochemical properties of plantaris tendons were analyzed 3, 7, or 28 days after overload, and comparisons were made to non-overloaded animals. By 28 days after overload, tendon mass had increased by 30% compared to non-overloaded samples, and cross-sectional area (CSA) increased by around 50%, with most of the change occurring in the neotendon. The expansion in CSA initially occurred through the synthesis of a hyaluronic acid rich matrix that was progressively replaced with mature collagen. Pericytes were present in areas of active tendon growth, but never in the original tendon ECM. Inhibition of p38 MAPK resulted in a profound decrease in IL6 expression, and had a modest effect on the expression of other ECM and cell proliferation genes, but had a negligible impact on overall tendon growth. The combined results from this study provided novel insights into tendon mechanobiology, and suggest that p38 MAPK signaling does not appear to be necessary for tendon growth in vivo.

The V-Y advancement flap is equivalent to the Mustardé flap for ectropion prevention in the reconstruction of moderate-size lid-cheek junction defects.

Background: Lid-cheek junction defects represent a reconstructive challenge because of the susceptibility of the lower eyelid to ectropion. To minimize the inferior tension placed on the lid margin, classic teaching advocates for the use of cervicofacial rotation-advancement flaps as popularized by Mustardé. Despite this approach, ectropion can still be problematic, and elevation of the cheek poses its own set of potential complications. An inferior-to-superior V-Y advancement flap has also been described, but its use is often limited because of the perceived increased risk of ectropion. This study attempts to define this risk by investigating the incidence of postoperative ectropion between cervicofacial and V-Y flaps for the reconstruction of lid-cheek junction defects. Methods: All patients who underwent reconstruction of lid-cheek junction defects performed by the senior author (D.L.B.) between January of 2002 and March of 2009 were reviewed retrospectively. Only cervicofacial (n = 11) and V-Y flaps (n = 23) were included in the analysis. Patient demographics, defect size, operative time, hospital stay, and postoperative complications were extracted from the clinical record. Results: Nine patients in the cervicofacial group (82 percent) and three patients in the V-Y group (13 percent) experienced a postoperative complication (p = 0.0002). Three cases of ectropion were observed, including two patients in the cervicofacial group (18 percent) and one in the V-Y group (4 percent, p = 0.24). All cases resolved with conservative management. Conclusions: No difference in ectropion rate was found between the cervicofacial and V-Y groups. The versatility of the V-Y advancement flap is perhaps underestimated in this clinical context. CLINICAL QUESTION/LEVEL OF EVIDENCE: Therapeutic, III.

Split cranial bone grafting in children younger than 3 years old: debunking a surgical myth.

Background: In cranioplasty patients split cranial bone provides excellent structural support and fundamentally “replaces like with like.” However, traditional teaching in craniofacial surgery is that cranial bone cannot be split before the age of 3 years because of the lack of diploic space. The authors have found this not to be the case and describe their experience with splitting cranial bone in children with craniosynostosis younger than 3 years. Methods: The authors completed a retrospective review of 418 cranioplasties performed between 1997 and 2013 by a single surgeon on patients younger than 3 years with syndromic and nonsyndromic craniosynostosis. Average patient age at the time of the procedure was 328 days. The youngest patient was 58 days old. Results: Of the 418 cranial procedures performed in this study, cranial bone could be split and used as bone graft in every case. Although the presence of Lückenschädel prevented a complete split of the inner table from the outer table of the bone flap, split cranial bone grafting could still be performed, providing significant grafting material to foster reconstruction. No complications from split cranial harvest were observed. Conclusions: Contrary to popular belief and the misconception perpetuated by the Plastic Surgery In-Service Examination, the cranium of children younger than 3 years can indeed be safely and predictably split between the inner and outer cortex. This important finding provides the craniofacial surgeon with a valuable, expanded source of rigid bone for cranial vault remodeling in the pediatric patient population. CLINICAL QUESTION/LEVEL OF EVIDENCE: Therapeutic, IV.

Quantitative Comparison of Volume Maintenance between Inlay and Onlay Bone Grafts in the Craniofacial Skeleton

Background: Nonvascularized autologous bone grafts are the criterion standard in craniofacial reconstruction for bony defects involving the craniofacial skeleton. The authors have previously demonstrated that graft microarchitecture is the major determinant of volume maintenance for both inlay and onlay bone grafts following transplantation. This study performs a head-to-head quantitative analysis of volume maintenance between inlay and onlay bone grafts in the craniofacial skeleton using a rabbit model to comparatively determine their resorptive kinetics over time. Methods: Fifty rabbits were divided randomly into six experimental groups: 3-week inlay, 3-week onlay, 8-week inlay, 8-week onlay, 16-week inlay, and 16-week onlay. Cortical bone from the lateral mandible and both cortical and cancellous bone from the ilium were harvested from each animal and placed either in or on the cranium. All bone grafts underwent micro–computed tomographic analysis at 3, 8, and 16 weeks. Results: All bone graft types in the inlay position increased their volume over time, with the greatest increase in endochondral cancellous bone. All bone graft types in the onlay position decreased their volume over time, with the greatest decrease in endochondral cancellous bone. Inlay bone grafts demonstrated increased volume compared with onlay bone grafts of identical embryologic origin and microarchitecture at all time points (p < 0.05). Conclusions: Inlay bone grafts, irrespective of their embryologic origin, consistently display less resorption over time compared with onlay bone grafts in the craniofacial skeleton. Both inlay and onlay bone grafts are driven by the local mechanical environment to recapitulate the recipient bed.

Outcomes of tethered cord repair with a layered soft tissue closure

BackgroundPrevious studies have demonstrated the benefit of releasing symptomatic tethered cords; however, complications such as seroma, cerebrospinal fluid leak, and infection continue to plague these patients. We propose that composite tissue closure of tethered cord repairs yields superior outcomes and that a collaborative effort between neurosurgery and plastic surgery may result in enhanced structural and functional results. MethodsThis is a retrospective study comprised of consecutive patients with tethered cord syndrome by 2 neurosurgeons and 2 plastic surgeons between 1994 and 2008 at a single institution. All consecutive patients who underwent tethered cord release by neurosurgery and subsequent composite tissue closure with fascial and musculofascial flaps by plastic surgery were included. Data were collected by retrospective chart review and analyzed using parametric methods. ResultsA total of 86 consecutive patients were included in this study, with follow-up ranged from 12 to 144 months (average follow-up, 29 months). There were no statistical differences in follow-up time, comorbidities, or surgeon when comparing hospital readmission or reoperation. There was no statistical difference in complications when comparing the different flap closures. We had a 1.2% infection rate, a 4.7% readmission rate, and a 3.5% reoperation rate. ConclusionWe believe that local soft tissue rearrangement improves the closure by providing an additional layer of vascularized tissue between the skin and the spinal cord. We believe our series represents a significant sample size compared with those previously reported for an experience that achieves multilayered soft tissue closure after tethered cord repair. Our results support the idea that neurosurgeons should consider consultation of plastic surgeons when treating patients with tethered cord syndrome surgically.