In vitro fluid dynamics of the Ahmed glaucoma valve modified with expanded polytetrafluoroethylene.

PURPOSE: Long-term intraocular pressure reduction by glaucoma drainage devices (GDDs) is often limited by the fibrotic capsule that forms around them. Prior work demonstrates that modifying a GDD with a porous membrane promotes a vascularized and more permeable capsule. This work examines the in vitro fluid dynamics of the Ahmed valve after enclosing the outflow tract with a porous membrane of expanded polytetrafluoroethylene (ePTFE). MATERIALS AND METHODS: The control and modified Ahmed implants (termed porous retrofitted implant with modified enclosure or PRIME-Ahmed) were submerged in saline and gelatin and perfused in a system that monitored flow (Q) and pressure (P). Flow rates of 1-50 µl/min were applied and steady state pressure recorded. Resistance was calculated by dividing pressure by flow. RESULTS: Modifying the Ahmed valve implant outflow with expanded ePTFE increased pressure and resistance. Pressure at a flow of 2 µl/min was increased in the PRIME-Ahmed (11.6 ± 1.5 mm Hg) relative to the control implant (6.5 ± 1.2 mm Hg). Resistance at a flow of 2 µl/min was increased in the PRIME-Ahmed (5.8 ± 0.8 mm Hg/µl/min) when compared to the control implant (3.2 ± 0.6 mm Hg/µl/min). CONCLUSIONS: Modifying the outflow tract of the Ahmed valve with a porous membrane adds resistance that decreases with increasing flow. The Ahmed valve implant behaves as a variable resistor. It is partially open at low pressures and provides reduced resistance at physiologic flow rates.

Expanded polytetrafluoroethylene membrane alters tissue response to implanted Ahmed glaucoma valve.

PURPOSE: Long-term intraocular pressure control by glaucoma drainage implants is compromised by the formation of an avascular fibrous capsule that surrounds the glaucoma implant and increases aqueous outflow resistance. It is possible to alter this fibrotic tissue reaction and produce a more vascularized and potentially more permeable capsule around implanted devices by enclosing them in a porous membrane. METHODS: Ahmed glaucoma implants modified with an outer 5-microm pore size membrane (termed porous retrofitted implant with modified enclosure or PRIME-Ahmed) and unmodified glaucoma implants were implanted into paired rabbit eyes. After 6 weeks, the devices were explanted and subject to histological analysis. RESULTS: A tissue response containing minimal vascularization, negligible immune response, and a thick fibrous capsule surrounded the unmodified Ahmed glaucoma implant. In comparison, the tissue response around the PRIME-Ahmed demonstrated a thinner fibrous capsule (46.4 +/- 10.8 microm for PRIME-Ahmed versus 94.9 +/- 21.2 microm for control, p < 0.001) and was highly vascularized near the tissue-material interface. A prominent chronic inflammatory response was noted as well. CONCLUSIONS: Encapsulating the aqueous outflow pathway with a porous membrane produces a more vascular tissue response and thinner fibrous capsule compared with a standard glaucoma implant plate. Enhanced vascularity and a thinner fibrous capsule may reduce aqueous outflow resistance and improve long-term glaucoma implant performance.

Metabolic and functional characterization of human adipose-derived stem cells in tissue engineering.

BACKGROUND: The use of adipose-derived stem cells for tissue engineering involves exposing them to metabolically adverse conditions. This study examines the metabolism, proliferation, and differentiation of adipose-derived stem cells under various conditions. METHODS: Adipose-derived stem cells were cultured in 16 media conditions containing 0.6, 2.4, 4.3, or 6.1 mM glucose; 0.1, 2.5, 4.1, or 6.1 mM glutamine; and then grown in either 0.1% or 20% oxygen. Conditioned media were collected and assayed for glucose, lactate, and pyruvate. Cell proliferation and cell death were measured at several time points. Osteogenic differentiation was analyzed by alizarin red staining/quantification and alkaline phosphatase activity, measured weekly over 4 weeks. RESULTS: Adipose-derived stem cells remained metabolically active in all nutrient and oxygen conditions tested. Glucose consumption and lactate production increased under hypoxic conditions, but pyruvate consumption was jointly dependent on oxygen and glucose concentration. The 20% oxygen environment produced greater proliferation and cell death compared with the hypoxic environment. Osteogenic differentiation of adipose-derived stem cells was observed only when glucose and/or oxygen concentrations were physiologically normal to high. CONCLUSIONS: Adipose-derived stem cells are an excellent source of multipotent cells and are capable of advancing current tissue engineering methodologies. These data show that adipose-derived stem cells remain viable under adverse conditions of low glucose, glutamine, and oxygen concentrations. However, there are variable levels of differentiation in the various culture conditions, which could lead to challenges in de novo osteogenesis and other forms of tissue engineering. Therefore, these results should be used in developing specific strategies to ensure successful application of adipose-derived stem cells in bone engineering and similar applications.

Efficacy of ethyl nitrite in reversing surgical vasospasm.

Vasospasm leading to thrombosis is a major cause of free flap failure. Flap blood flow may improve by application of vasodilating agents to the pedicle. The purpose of this study is to evaluate the effect of a novel S-nitrosylating agent, O-ethyl nitrite, on flap blood flow and pedicle vessels diameter in a rat model, after induction of vasospasm. Two hours after surgery, the arterial diameter was significantly (P < .05) dilated by treatment with O-ethyl nitrite, lidocaine, or lidocaine + O-ethyl nitrite. None of these treatments had a statistically significant effect on blood flow or venous diameter. Twenty-four hours following surgery, both O-ethyl nitrite- and lidocaine-treated groups had larger arteries than the control group (P < .05). The O-ethyl nitrite-treated animals also had significantly (P < .05) elevated blood flow compared with control animals, but lidocaine-treated animals did not demonstrate any improved flow at 24 hours. O-Ethyl nitrite may become clinically useful in microvascular surgery procedures.

Vein graft anastomoses with magnets.

BACKGROUND: Performing vascular anastomoses requires complete circumferential access and is time-consuming and technically challenging. The purpose of our study was to assess a sutureless magnetic device for anastomosing vein grafts. METHODS AND RESULTS: Oval magnets with a lumen were placed in six male foxhounds. The femoral artery was ligated and an 8 cm length of femoral vein was harvested and reversed. After a 4 mm venotomy or arteriotomy, one magnet was inserted into each vessel lumen and a second magnet was placed outside the vessel but aligned directly over the intraluminal magnet, forming a magnetic port in each vessel. The graft and target vessels were then allowed to self-align and seal, creating a side-to-side anastomosis. Patency was confirmed with duplex Doppler ultrasound scans after 6 and 13 weeks and during explantation after 14 weeks. At that time, the contralateral femoral vein was harvested and an acute graft was created as a control. Macroscopically there was no sign of stenoses or aneurysms. After explant, saline was perfused through all grafts and the hydrodynamic resistance was quantified. There was no significant difference in resistance between the acute and 14-week grafts indicating stenosis. Microscopic examination of 14-week anastomoses showed that all blood-contacting surfaces were well-endothelialised. CONCLUSIONS: The magnetic vascular coupler allowed sutureless anastomoses between blood arteries and veins. This device might prove useful for anastomosis of large and small vessels, as well as for anastomosis or approximation of non-vascular structures such as peripheral nerves, fallopian tubes or ureters.

Combined bone allograft and adipose-derived stem cell autograft in a rabbit model.

Currently available options for the repair of bony defects have substantial limitations. Much work has looked to the possibility of engineering bone using stem cells. These tissue-engineering efforts have focused on calvarial defect models, which have the advantages of minimal load-bearing and a large surface area. This study aims to solve the somewhat more challenging problem of repairing segmental bony defects such as those of the mandible and long bones. Four groups of decellularized bone tubes with cortical perforations were implanted subcutaneously in a rabbit model: empty bone tubes, bone tubes containing fibrin glue alone, bone tubes containing fibrin glue and freshly isolated autologous adipose-derived stem cells (ASCs), and bone tubes containing fibrin glue and predifferentiated autologous ASCs. Results showed a foreign body response characterized by fibrous capsule formation with minimal angiogenesis and no evidence of osteoblastic activity. Substantial changes are needed if this model is to become viable.

Free flap reconstruction of the scalp and calvaria of major neurosurgical resections in cancer patients: lessons learned closing large, difficult wounds of the dura and skull.

BACKGROUND: Reconstruction of major neurosurgical resections can present a significant challenge because of the morbidity of radiation therapy, cerebrospinal fluid leaks, bacterial contamination from sinus exposure, and functional and cosmetic deformity from the size and location of the defect. The authors present their experience with free tissue reconstruction of scalp and calvarial defects. In particular, the authors examine their results in relation to major comorbidities, such as preoperative cerebrospinal fluid leak, history of smoking, and perioperative radiation therapy. METHODS: From 1997 to 2004, 22 patients requiring neurosurgical or head and neck resection for cancer from a single institution who underwent reconstruction with 24 flaps were examined retrospectively. Factors examined included patient demographics, indication for surgery, type of flap used, exposed critical structures, comorbidity, complications, and outcomes. RESULTS: Of the 22 patients, seven had a cerebrospinal fluid leak present at the time of their reconstructive surgery. Of the seven, one patient died as a result of a stroke postoperatively. Of the remaining six patients, two had partial flap necrosis (33 percent). However, all six flaps survived, with resolution of cerebrospinal fluid leak. In comparison, of the 15 patients (17 flaps) without a cerebrospinal fluid leak, three had partial flap necrosis (18 percent; not significant). With regard to smoking status, the partial flap necrosis rate was 30 percent in smokers versus a rate of 14 percent in nonsmokers, although this was not statistically significant. Only one patient who received perioperative radiation (11 of 22 patients) developed partial flap necrosis. CONCLUSIONS: The authors' data support the concept that free tissue transfer is a viable option in reconstruction of cranial defects. Although complications can occur in this high-risk population, successful reconstruction with free flaps was possible. Difficult problems, such as recurrent cerebrospinal fluid leaks and large irradiated wounds, can be managed and resolved successfully using this technique.

Delay of transverse rectus abdominis myocutaneous flap reconstruction improves flap reliability in the obese patient.

BACKGROUND: Since its introduction in 1982, the transverse rectus abdominis musculocutaneous (TRAM) flap has been a mainstay of breast reconstruction. However, in certain high-risk individuals, such as the obese, smokers, and irradiated patients, flap reliability is decreased, which leads to a higher rate of flap complications. The authors hypothesized that although the nonflap complication rate is increased in the obese patient, flap complications are not increased in those who undergo a delayed TRAM procedure. METHODS: From 1995 to 2003, 107 consecutive patients who underwent a delayed TRAM procedure were examined in a retrospective study. The delayed TRAM procedures were all performed by the senior author (Georgiade) at a single institution. Patients were classified by their body mass index (weight in kilograms and height in square meters). RESULTS: No patient had complete loss of her flap regardless of body mass index. Patients with an index of 30 kg/m2 and under had a partial fat necrosis rate of 11.4 percent (10 of 88 patients), whereas two of 19 patients (10.5 percent) with an index of greater than 30 kg/m2 had partial fat necrosis of their flap (not significant). The average body mass index of the group with flap complications was 27.8 +/- 4.3 kg/m2, whereas that of the group with no flap complications was 26.5 +/- 4.3 kg/m2 (not significant). The rate of non-flap-related complications, such as deep venous thrombosis and pulmonary embolism, was 8 percent (seven of 88 patients) for those with a body mass index of less than 30 kg/m2 and 31.6 percent (six of 19 patients) for those with an index greater than 30 kg/m2 (p = 0.0112). The average body mass index of the group with nonflap complications was 29.9 +/- 4.9 kg/m2, compared with 26.3 +/- 4.0 kg/m2 for the group with no non-flap-related complications (p = 0.031). Of note, patients with a history of smoking and those who had received radiation therapy did not show a statistically significant increase in flap complications. CONCLUSIONS: Despite a progressively increasing overall complication rate for patients with a higher body mass index, there was not a similar trend for flap-related complications. The authors' data support the idea that the delayed TRAM procedure is a safe and reliable technique for obese and morbidly obese patients.

Halofuginone inhibits collagen deposition in fibrous capsules around implants.

Fibrous capsule formation around implants remains a difficult problem that has been studied for decades. The etiology is elusive, but the end result is the deposition of a dense collagenous capsule around implanted materials. The purpose of this study was to determine the effects of a type I collagen synthesis inhibitor, halofuginone, on fibrous capsule formation around implanted materials. Silastic disks were implanted subcutaneously into 4 groups of adult male rats for up to 8 weeks. Group 1 received drug throughout the study, group 2 received drug during the first half only, group 3 received drug during the second half only, and the control group received no drug. Implants were removed and histology of the capsules was examined. A collagen index score was calculated from digital images of trichrome-stained histologic sections, which permitted semiquantitative comparison of collagen content among the 4 groups. The collagen index values clearly indicate that halofuginone effectively inhibited collagen deposition within the capsule around the implanted disks. Halofuginone treatment also resulted in a decrease in the collagen index score in rat skin, indicating that halofuginone may affect preexisting collagenous structures. The ability of halofuginone to inhibit collagen deposition in new and preexisting fibrous capsules suggests that it may be a useful adjunct to minimize the formation of capsules around implantable prostheses.

Immunohistochemical identification of vascular endothelial growth factor in pig latissimus dorsi musculocutaneous flaps following ischemia-reperfusion injury.

Vascular Endothelial Growth Factor (VEGF), a potent angiogenic, mitogenic and vascular permeability enhancing protein, appears to improve survival of ischemic flaps independent of its route of administration. The purpose of this study was to examine VEGF protein expression in biopsies of surgical flaps with immunohistochemical techniques. In 6 male Yorkshire-type pigs, 10 cm x 15 cm Latissimus dorsi musculocutaneous flaps were elevated bilaterally. Flap zones I, II, and III were established according to their distance from the vascular pedicle. After isolation of the thoracodorsal artery and vein, one flap was randomly assigned to ischemia by temporary occlusion of the vascular pedicle. Ischemia (4 hours) was followed by 2 hours of reperfusion (ischemia group, n = 6). The contralateral (nonischemic) flap served as a control (control group, n =6). Skin and muscle biopsies of flaps were taken at the end of the protocol for immunohistochemical staining using a VEGF antihuman monoclonal antibody. Epidermis of flap skin did not demonstrate VEGF-positive staining, but the dermis and subcutaneous tissue did. Muscle components of biopsies demonstrated staining of interfascicular septa and staining of myocytes. A semi-quantitative scoring system with a scale of 0 to 3 was used for grading of immunohistochemical staining. In skin, areas adjacent to the flap showed an overall mean VEGF staining score of 0.7. All zones of ischemic flaps showed increased mean immunohistochemical staining for VEGF (scores = 1.2, 1.6, and 1.4 in zones I, II, and III, respectively). In muscle, however, only zone I showed increased VEGF immunohistochemical staining from 0.7 in adjacent areas to 1.7 in ischemic flaps. The results indicate only moderate endogenous up-regulation of VEGF in flaps, supporting the utilization of exogenous VEGF as an adjunct in microsurgical therapy.

Side-to-side sutureless vascular anastomosis with magnets.

OBJECTIVE: Abbe and Payr introduced vascular techniques and devices to facilitate vessel anastomosis over a century ago. Obora published the idea of a sutureless vascular anastomosis with use of magnetic rings in 1978. The purpose of this study was to assess the performance of a new magnetic device to perform a side-to-side arteriovenous anastomosis in a dog model. MATERIAL AND METHODS: Male fox hounds (25 kg) were treated preoperatively and daily postoperatively with clopidogrel bisulfate (Plavix) and aspirin. The femoral artery and vein were exposed unilaterally in 3 dogs and bilaterally in 4 dogs (n = 11 anastomoses). A 4-mm arteriotomy was performed, and 1 oval magnet 0.5 mm thick was inserted into the lumen of the artery and a second magnet was applied external to the artery, compressing and stabilizing the arterial wall to create a magnetic port. An identical venous magnetic port was created with another pair of oval magnets. When the 2 ports were allowed to approach each other, they self-aligned and magnetically coupled to complete the arteriovenous anastomosis. Patency was assessed for the first hour with direct observation, again after 9 weeks with duplex ultrasound scanning, and at 10 weeks under direct open observation. The anastomoses were explanted after 10 weeks. Hydrodynamic resistance was measured ex vivo on the final 8 anastomoses by measuring the pressure drop across an anastomosis with a known flow rate. RESULTS: After implantation, very high flow created visible turbulence and palpable vibration. All 11 anastomoses were patent under direct observation and palpation. Ten of 11 anastomoses were clearly patent on duplex scans, and patency of 1 anastomosis was questionable. Hydrodynamic resistance averaged 0.73 +/- 0.33 mm Hg min/mL (mean +/- SEM). CONCLUSIONS: Vascular anastomoses performed with magnets demonstrated feasibility; exhibited 100% patency after 10 weeks in a dog arteriovenous shunt model; lacked apparent aneurysm or other potentially catastrophic failure; demonstrated remodeling of the vessel wall after several weeks to incorporate the magnets, making the magnetic force unnecessary; and warrants further study in vessels with different sizes, flow rates, and locations. CLINICAL RELEVANCE: We present a magnet-based device used to perform side-to-side peripheral vascular anastomoses. Its advantages include the ability to anastomose vessels without requiring circumferential surgical exposure. Vascular anastomosis performed with these magnets demonstrated 100% patency in the dog, lacked apparent aneurysm or other potentially catastrophic failure, and demonstrated remodeling of the vessel wall after several weeks, to incorporate the magnets, making indefinite retention of field strength unnecessary. This technique could enable minimally invasive procedures, such as complex reconstructive and revascularizing surgery, and warrants further study in vessels with different sizes, flow rates, and locations.

Basic fibroblast growth factor expression following surgical delay of rat transverse rectus abdominis myocutaneous flaps.

Partial transverse rectus abdominis myocutaneous (TRAM) flap loss in breast reconstruction can be a devastating complication for both patient and surgeon. Surgical delay of the TRAM flap has been shown to improve flap viability and has been advocated in "high-risk" patients seeking autogenous breast reconstruction. Despite extensive clinical evidence of the effectiveness of surgical delay of TRAM flaps, the mechanisms by which the delay phenomenon occurs remain poorly understood. To examine whether angiogenic growth factors such as basic fibroblast growth factor (bFGF) may play a role in the delay phenomenon, the authors studied the expression of bFGF in rat TRAM flaps subjected to surgical delay. Thirty-five female Sprague-Dawley rats were randomly assigned to one of four TRAM flap groups: no delay (n = 6), 7-day delay (n = 12), 14-day delay (n = 10), or 21-day delay (n = 7). Surgical delay consisted of incising skin around the perimeter of the planned 2.5 x 5.0-cm TRAM flap followed by ablation of both superior epigastric arteries and the left inferior epigastric artery, thus preserving the right inferior epigastric artery (the nondominant blood supply to the rectus abdominis muscle of the rat). TRAM flaps were then elevated after 7, 14, and 21 days of delay by raising zones II, III, and IV off the abdominal wall fascia. Once hemostasis was assured, the flaps were sutured back in place. All flaps were designed with the upper border of the flap 1 cm below the xiphoid tip. Three days after the TRAM procedure, postfluorescein planimetry was used to determine percent area viability of both superficial and deep portions of TRAM flaps. All rats were euthanized and full-thickness TRAM specimens were taken from zones I, II, III, and IV for enzyme-linked immunoabsorbent assay analysis of bFGF levels. Statistical testing was done by t test (percent viability) and two-way analysis of variance (bFGF levels). All delayed flaps had significantly higher bFGF levels when compared with all nondelayed control flaps (p < 0.05). The bFGF levels were not different in the rats that received TRAM flaps 7, 14, or 21 days after delay surgery. There was also no significant difference in bFGF levels among zones I through IV. Control rats had more peripheral zone necrosis compared with all delayed TRAM rats. All delayed flaps had a significantly higher area of flap viability superficially than nondelayed control flaps (p < 0.05). There was no difference in deep flap viability. Surgical delay of rat TRAM flaps is associated with improved flap viability and significantly elevated levels of bFGF over nondelayed TRAM flaps at postoperative day 3 after TRAM surgery. The increases in bFGF noted at this time point suggests that bFGF may play a role in the improved TRAM flap viability observed after delay surgery. Further investigation is needed to evaluate the role bFGF may play in the delay phenomenon.

Vascular endothelial growth factor expression in pig latissimus dorsi myocutaneous flaps after ischemia reperfusion injury.

Exogenous administration of vascular endothelial growth factor (VEGF) improves long-term viability of myocutaneous flaps. However, endogenous expression of this substance in flaps following ischemia-reperfusion injury has not been reported previously. Endogenous production of VEGF was measured in myocutaneous pig latissimus dorsi flaps after ischemia-reperfusion injury. Latissimus dorsi myocutaneous flaps (15 x 10 cm) were simultaneously elevated bilaterally in six Yorkshire-type male pigs (25 kg). Before elevation, three flap zones (5 x 10 cm) were marked according to their distance from the vascular pedicle. After isolation of the vascular pedicle, ischemia-reperfusion injury was induced in one flap by occlusion of the thoracodorsal artery and vein for 4 hours, followed by 2 hours of reperfusion. The contralateral flap served as a control. Perfusion in each zone was monitored by laser Doppler flowmetry at baseline, during ischemia, and during reperfusion. At the end of the protocol, skin and muscle biopsies of each flap zone and adjacent tissues were obtained for later determination of VEGF protein levels. VEGF concentrations were quantified using the Quantikine human VEGF immunoassay. Skin perfusion was similar among all flap zones before surgery. Flow fell in all flaps immediately after flap elevation. After 4 hours of ischemia, blood flow in the ischemic flaps was significantly decreased (p < 0.05) compared with nonischemic control flaps. After 2 hours of reperfusion, flow in ischemic flap skin recovered to levels similar to those in control flaps. VEGF protein concentrations in muscle tissue exceeded concentrations in skin and decreased from zones 2 to 3 in control and ischemic flaps. No significant differences in VEGF concentrations between ischemic and control muscle zones were observed. However, the concentration of VEGF in all muscle zones was significantly higher (p < 0.05) than muscle adjacent to the flap. Concentrations in skin zones 1 and 2 were significantly higher (p < 0.05) in ischemic flaps than in control flaps, but levels in zone 3 (most ischemic flaps) showed no significant difference.