Alexander M. Sailon

Biological basis of bone formation, remodeling, and repair-part II: extracellular matrix.

The bony biochemical environment is a complex system that permits and promotes cellular functions that lead to matrix production and ossification. In Part I of this review, we discussed the important actions of signaling molecules, including hormones, cytokines, and growth factors. Here, we review other constituents of the extracellular matrix, including minerals, fibrinous and nonfibrinous proteins, and enzymes such as the matrix metalloproteinases. We conclude with a discussion of the role of biochemical modulation in endogenous and exogenous tissue engineering.

Biological Basis of Bone Formation, Remodeling, and Repair—Part I: Biochemical Signaling Molecules

The bony biochemical environment is an active and dynamic system that permits and promotes cellular functions that lead to matrix production and ossification. Each component is capable of conveying important regulatory cues to nearby cells, thus effecting gene expression and changes at the cytostructural level. Here, we review the various signaling molecules that contribute to the active and dynamic nature of the biochemical system. These components include hormones, cytokines, and growth factors. We describe their role in regulating bone metabolism. Certain growth factors (i.e., TGF-beta, IGF-1, and VEGF) are described in greater detail because of their potential importance in developing successful tissue-engineering strategies.

Topical Lineage-negative Progenitor-cell Therapy for Diabetic Wounds

Background: Impaired diabetic wound healing is due, in part, to defects in mesenchymal progenitor cell tracking. Theoretically, these defects may be overcome by administering purified progenitor cells directly to the diabetic wound. The authors hypothesize that these progenitor cells will differentiate into endothelial cells, increase wound vascularity, and improve wound healing. Methods: Lineage-negative progenitor cells were isolated from wild-type murine bone marrow by magnetic cell sorting, suspended in a collagen matrix, and applied topically to full-thickness excisional dorsal cutaneous wounds in diabetic mice. Application of lineage-positive hematopoietic cells or acellular collagen matrix served as comparative controls (n = 16 for each group; n = 48 total). Time to closure and percentage closure were calculated by morphometry. Wounds were harvested at 7, 14, 21, and 28 days and then processed, sectioned, stained (lectin/DiI and CD31), and vascularity was quantified. Results: Wounds treated with lineage-negative cells demonstrated a significantly decreased time to closure (14 days) compared with lineage-positive (21 days, p = 0.013) and collagen controls (28 days, p = 0.004), and a significant improvement in percentage closure at 14 days compared with the lineage-positive group (p < 0.01) and the collagen control (p < 0.01). Fluorescently tagged lineage-negative cells remained viable in the wound for 28 days, whereas lineage-positive cells were not present after 7 days. Lineage-negative, but not lineage-positive, cells differentiated into endothelial cells. Vascular density and vessel cross-sectional area were significantly higher in lineage-negative wounds. Conclusion: Topical progenitor-cell therapy successfully accelerates diabetic wound closure and improves wound vascularity.

Biological Basis of Bone Formation, Remodeling, and Repair—Part III: Biomechanical Forces

While it has been long appreciated that biomechanical forces are involved in bone remodeling and repair, the actual mechanism by which a physical force is translated to the corresponding intracellular signal has largely remained a mystery. To date, most biomechanical research has concentrated upon the effect on bone morphology and architecture, and it is only recently that the complex cellular and molecular pathways involved in this process (called mechanotransduction) are being described. In this paper, we review the current understanding of bone mechanobiology and highlight the implications for clinical medicine and tissue engineering research.

A Novel Flow-Perfusion Bioreactor Supports 3D Dynamic Cell Culture

Background. Bone engineering requires thicker three-dimensional constructs than the maximum thickness supported by standard cell-culture techniques (2 mm). A flow-perfusion bioreactor was developed to provide chemotransportation to thick (6 mm) scaffolds. Methods. Polyurethane scaffolds, seeded with murine preosteoblasts, were loaded into a novel bioreactor. Control scaffolds remained in static culture. Samples were harvested at days 2, 4, 6, and 8 and analyzed for cellular distribution, viability, metabolic activity, and density at the periphery and core. Results. By day 8, static scaffolds had a periphery cell density of 67% ± 5.0%, while in the core it was 0.3% ± 0.3%. Flow-perfused scaffolds demonstrated peripheral cell density of 94% ± 8.3% and core density of 76% ± 3.1% at day 8. Conclusions. Flow perfusion provides chemotransportation to thick scaffolds. This system may permit high throughput study of 3D tissues in vitro and enable prefabrication of biological constructs large enough to solve clinical problems.

Progenitor cell mobilization enhances bone healing by means of improved neovascularization and osteogenesis.

Background: Although bone repair is a relatively efficient process, a significant portion of patients fail to heal their fractures. Because adequate blood supply is essential to osteogenesis, the authors hypothesize that augmenting neovascularization by increasing the number of circulating progenitor cells will improve bony healing. Methods: Bilateral full-thickness defects were created in the parietal bones of C57 wild-type mice. Intraperitoneal AMD3100 (n = 33) or sterile saline (n = 33) was administered daily beginning on postoperative day 3 and continuing through day 18. Circulating progenitor cell number was quantified by fluorescence-activated cell sorting. Bone regeneration was assessed with micro–computed tomography. Immunofluorescent CD31 and osteocalcin staining was performed to assess for vascularity and osteoblast density. Results: AMD3100 treatment increased circulating progenitor cell levels and significantly improved bone regeneration. Calvarial defects of AMD3100-treated mice demonstrated increased vascularity and osteoblast density. Conclusions: Improved bone regeneration in this model was associated with elevated circulating progenitor cell number and subsequently improved neovascularization and osteogenesis. These findings highlight the importance of circulating progenitor cells in bone healing and may provide a novel therapy for bone regeneration.

Free transverse rectus abdominis myocutaneous and deep inferior epigastric perforator flaps for breast reconstruction: a systematic review of flap complication rates and donor-site morbidity.

Free transverse rectus abdominis myocutaneous and deep inferior epigastric perforator flaps represent increasingly popular options for breast reconstruction. Although several retrospective, small-scale studies comparing these flaps have been published, most have failed to find a significant difference in flap complication rates or donor-site morbidity. We systematically reviewed the current literature, and subsequently pooled and analyzed data from included studies. Included studies reported flap complications and/or donor site morbidities for both flap types. Eight studies met the inclusionary criteria. For flap complications, there was a statistically significant difference between deep inferior epigastric perforator and free transverse rectus abdominis myocutaneous flaps in fat necrosis rates (25.5 ± 0.49 vs. 11.3% ± 0.41%, P < 0.001) and total necrosis rates (4.15 ± 0.08 vs. 1.59% ± 0.08%, P = 0.044). Partial necrosis rates were not statistically significant (3.54 ± 0.07 vs. 1.60% ± 0.07%, P = 0.057). For donor-site morbidity, there was no statistically significant difference in abdominal bulge (8.07 ± 0.23 vs. 11.25% ± 0.29%, P = 0.28). Multicenter, prospective studies are needed to further investigate differences between these flap options.

A novel cleft rhinoplasty procedure combining an open rhinoplasty with the Dibbell and Tajima techniques: a 10-year review.

Background: The authors assessed the safety and efficacy of a novel cleft rhinoplasty procedure that combines an open rhinoplasty with the Dibbell and Tajima techniques. Methods: A single-surgeon, 10-year, retrospective review was conducted of all unilateral cleft lip rhinoplasties (n = 157). Nonsyndromic patients undergoing a combined open incision/Dibbell/Tajima procedure and who had follow-up of greater than 8 months were included. Thirty-five patients were identified. Standardized patient photographs were studied in 18 patients who had both preoperative and 1-year postoperative photographs. Farkas normal values were applied to the medial canthal distance; from this value, metric measurements of changes in alar base width, columellar height, and nostril apex height were derived. Results: There were no complications secondary to skin envelope ischemia or cartilage graft infection. The revision rate was 11 percent for alar base position, 3 percent for depressed lower lateral cartilage, and 3 percent for nostril apex overhang. After the procedure, there was a statistically significant decrease in alar base width (19.9 mm versus 18.2 mm; p < 0.01) and an increase in columellar height (8.37 mm versus 9.59 mm; p = 0.02) and nostril apex height (4.70 mm versus 5.44 mm; p = 0.02) on the affected side. The differences in alar base width, columellar height, and nostril apex height between the affected and nonaffected sides all decreased significantly postoperatively. Conclusions: The combined open rhinoplasty/Dibbell/Tajima procedure is safe, has a low revision rate, and is associated with a statistically significant decrease in alar base width, an increase in columellar height and nostril apex height, and a greater symmetry of nasal form.

Design and validation of a dynamic cell-culture system for bone biology research and exogenous tissue-engineering applications.

Bone lacunocanalicular fluid flow ensures chemotransportation and provides a mechanical stimulus to cells. Traditional static cell‐culture methods are ill‐suited to study the intricacies of bone biology because they ignore the three‐dimensionality of meaningful cellular networks and the lacunocanalicular system; furthermore, reliance on diffusion alone for nutrient supply and waste product removal effectively limits scaffolds to 2–3 mm thickness. In this project, a flow‐perfusion system was custom‐designed to overcome these limitations: eight adaptable chambers housed cylindrical cell‐seeded scaffolds measuring 12 or 24 mm in diameter and 1–10 mm in thickness. The porous scaffolds were manufactured using a three‐dimensional (3D) periodic microprinting process and were composed of hydroxyapatite/tricalcium phosphate with variable thicknesses, strut sizes, pore sizes and structural configurations. A multi‐channel peristaltic pump drew medium from parallel reservoirs and perfused it through each scaffold at a programmable rate. Hermetically sealed valves permitted sampling or replacement of medium. A gas‐permeable membrane allowed for gas exchange. Tubing was selected to withstand continuous perfusion for > 2 months without leakage. Computational modelling was performed to assess the adequacy of oxygen supply and the range of fluid shear stress in the bioreactor–scaffold system, using 12 × 6 mm scaffolds, and these models suggested scaffold design modifications that improved oxygen delivery while enhancing physiological shear stress. This system may prove useful in studying complex 3D bone biology and in developing strategies for engineering thick 3D bone constructs. Copyright

The proximally based peroneal vascular bundle: an insulated extension cord for free flap reconstruction.

Large, traumatic wounds around the proximal third of the lower extremity may have disrupted local vasculature, potentially obviating local pedicled options. However, free-tissue transfer to this area is technically challenging given the resulting paucity of recipient options and the depth of principal blood vessels. We present an anatomic and radiographic study of the proximally based peroneal vascular bundle as a recipient option in the proximal leg. Optimal approach was prone, through an incision over the fibula with dissection between lateral and posterior compartments. Magnetic resonance angiography demonstrated consistent vascular anatomy between patients. A proximally based peroneal vascular bundle protected by a cuff of flexor hallucis longus was used as a recipient vessel in free flap reconstruction of an open knee wound. The bundle itself does not require coverage by virtue of its own local muscle cuff. Caveats for its use include the need for adequate leg inflow and foot outflow.