• View in gallery
    Figure 1—

    Photograph of a prototype device for gradual venous occlusion. The outer ring (solid arrow) is a polymer ring encased in silicone tubing. The inner clear layer (dashed arrow) is silicone tubing filled with a PAA and an inorganic salt combination.

  • View in gallery
    Figure 2—

    Mean change in luminal area (A) and luminal diameter (B) for the prototype device of Figure 1 filled with 1 of 5 PAA–inorganic salt formulations (n = 3 rings/formulation) at various points before (week 0) and during soaking in 20 mL of PBSS for 6 weeks. Formulations 1 (circles), 2 (squares), and 5 (diamonds) included sodium chloride in various ratios with PAA. Formulations 3 (triangles) and 4 (inverted triangles) included potassium chloride in various ratios with PAA. Notice that rings filled with formulations 2 and 4 had the largest changes in both variables over the 6-week measurement period. However, rings filled with formulation 2 expanded more rapidly during the first 2 weeks, whereas rings filled with formulation 4 gradually expanded over the full period.

  • View in gallery
    Figure 3—

    Mean luminal area (A), luminal diameter (B), and outer diameter (C) of PEEK (squares; n = 7) and polypropylene (circles; n = 10) rings filled with formulation 4 from Figure 2 at various points before (week 0) and during soaking in 20 mL of PBSS for 6 weeks. Error bars represent SD.

  • View in gallery
    Figure 4—

    Photographs of representative polypropylene (top row) and PEEK (bottom row) rings from Figure 3 at various measurement points. Notice that the PEEK ring maintained its outer shape with only mild deformation, whereas the polypropylene ring deformed with time. Scale represents millimeters.

  • View in gallery
    Figure 5—

    Photograph of a failed polypropylene ring from among those represented in Figure 4. Notice how the increase in outer diameter allowed for some swelling to occur outside of the ring rather than directing the swelling toward the center. Scale represents millimeters.

  • View in gallery
    Figure 6—

    Mean luminal area (A), luminal diameter (B), and outer diameter (C) of PEEK rings filled with variations of formulation 4 in Figure 3 at a higher salt concentration (circles; n = 6) and lower salt concentration (squares; 6) before (week 0) and at various points during incubation in a physiologic solution for 6 weeks. The silicone tubing in this part of the study was altered to have a central partition. Error bars represent SD.

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  • 15. Frankel D, Seim H, MacPhail C, et al. Evaluation of cellophane banding with and without intraoperative attenuation for treatment of congenital extrahepatic portosystemic shunts in dogs. J Am Vet Med Assoc 2006; 228: 13551360.

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  • 16. Cabassu J, Seim HB III, MacPhail CM, et al. Outcomes of cats undergoing surgical attenuation of congenital extrahepatic portosystemic shunts through cellophane banding: 9 cases (2000–2007). J Am Vet Med Assoc 2011; 238: 8993.

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  • 17. Hunt GB, Kummeling A, Tisdall PLC, et al. Outcomes of cellophane banding for congenital portosystemic shunts in 106 dogs and 5 cats. Vet Surg 2004; 33: 2531.

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  • 18. Youmans KR, Hunt GB. Cellophane banding for the gradual attenuation of single extrahepatic portosystemic shunts in eleven dogs. Aust Vet J 1998; 76: 531537.

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  • 24. Bichara DA, Bodugoz-Sentruk H, Ling D, et al. Osteochondral defect repair using a polyvinyl alcohol-polyacrylic acid (PVA-PAAc) hydrogel. Biomed Mater 2014; 9: 045012.

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  • 25. Mehl ML, Kyles AE, Hardie EM, et al. Evaluation of ameroid ring constrictors for treatment for single extrahepatic portosystemic shunts in dogs: 168 cases (1995–2001). J Am Vet Med Assoc 2005; 226: 20202030.

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  • 26. Adin CA, Gregory CR, Kyles AE, et al. Effect of petrolatum coating on the rate of occlusion of ameroid constrictors in the peritoneal cavity. Vet Surg 2004; 33: 1116.

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  • 27. Thompson EM, Towle Millard HA, Moore GE, et al. In vitro effect of multiple hydrogen peroxide gas plasma sterilizations on the rate of closure of ameroid constrictors. Am J Vet Res 2014; 75: 924928.

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In vitro development and evaluation of a polyacrylic acid–silicone device intended for gradual occlusion of portosystemic shunts in dogs and cats

Mandy L. WallaceDepartment of Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610.

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Gary W. EllisonDepartment of Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610.

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Christopher BatichDepartment of Materials Science and Engineering, College of Engineering, University of Florida, Gainesville, FL 32611.

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J. Brad CaseDepartment of Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610.

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Stanley E. KimDepartment of Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610.

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Abstract

OBJECTIVE To develop a device intended for gradual venous occlusion over 4 to 6 weeks.

SAMPLE Silicone tubing filled with various inorganic salt and polyacrylic acid (PAA) formulations and mounted within a polypropylene or polyether ether ketone (PEEK) outer ring.

PROCEDURES 15 polypropylene prototype rings were initially filled with 1 of 5 formulations and placed in PBSS. In a second test, 10 polypropylene and 7 PEEK prototype rings were filled with 1 formulation and placed in PBSS. In a third test, 2 formulations were loaded into 6 PEEK rings each, placed in physiologic solution, and incubated. In all tests, ring luminal diameter, outer diameter, and luminal area were measured over 6 weeks.

RESULTS In the first test, 2 formulations had the greatest changes in luminal area and diameter, and 1 of those had a greater linear swell rate than the other had. In the second test, 6 of 7 PEEK rings and 6 of 10 polypropylene rings closed to a luminal diamater < 1 mm within 6 weeks. Polypropylene rings had a greater increase in outer diameter than did PEEK rings between 4.5 and 6 weeks. In the third test, 11 of 12 PEEK rings gradually closed to a luminal diameter < 1 mm within 6 weeks.

CONCLUSIONS AND CLINICAL RELEVANCE A PAA and inorganic salt formulation in a prototype silicone and polymer ring resulted in gradual occlusion over 4 to 6 weeks in vitro. Prototype PEEK rings provided more reliable closure than did polypropylene rings.

Abstract

OBJECTIVE To develop a device intended for gradual venous occlusion over 4 to 6 weeks.

SAMPLE Silicone tubing filled with various inorganic salt and polyacrylic acid (PAA) formulations and mounted within a polypropylene or polyether ether ketone (PEEK) outer ring.

PROCEDURES 15 polypropylene prototype rings were initially filled with 1 of 5 formulations and placed in PBSS. In a second test, 10 polypropylene and 7 PEEK prototype rings were filled with 1 formulation and placed in PBSS. In a third test, 2 formulations were loaded into 6 PEEK rings each, placed in physiologic solution, and incubated. In all tests, ring luminal diameter, outer diameter, and luminal area were measured over 6 weeks.

RESULTS In the first test, 2 formulations had the greatest changes in luminal area and diameter, and 1 of those had a greater linear swell rate than the other had. In the second test, 6 of 7 PEEK rings and 6 of 10 polypropylene rings closed to a luminal diamater < 1 mm within 6 weeks. Polypropylene rings had a greater increase in outer diameter than did PEEK rings between 4.5 and 6 weeks. In the third test, 11 of 12 PEEK rings gradually closed to a luminal diameter < 1 mm within 6 weeks.

CONCLUSIONS AND CLINICAL RELEVANCE A PAA and inorganic salt formulation in a prototype silicone and polymer ring resulted in gradual occlusion over 4 to 6 weeks in vitro. Prototype PEEK rings provided more reliable closure than did polypropylene rings.

Portosystemic shunts are vascular anomalies that allow blood returning from the gastrointestinal tract to the liver to be shunted into systemic circulation. These shunts can be congenital or acquired, with the congenital form identified in approximately 0.18% of dogs.1 Clinical signs associated with portosystemic shunts in dogs include seizures, vomiting, and urinary tract abnormalities such as stranguria and hematuria, among others.2

Surgical attenuation is the only definitive treatment for portosystemic shunts and has been associated with better long-term survival than medical management.3,4 Attenuation can be obtained via acute complete or partial ligation of the shunt or placement of a gradual occlusion device such as an ameroid ring constrictor or cellophane band to surround the shunt. However, both of these devices close the vessel primarily by inflammation and thrombosis5,6 rather than by direct occlusion. Ameroid ring constrictors swell in vitro but not to complete lumen closure.7,8 Additionally, the inconsistency in material composition that exists with compounds such as cellophane leads to differences in handling and potential variability in closure periods in vivo, as different materials may elicit a more or less robust foreign body reaction.9 Moreover, both types of gradual occlusion devices involve either an outer stainless steel ring or metal hemostatic clips, making postoperative confirmation of vessel closure via CT difficult.6,10–12

The optimal period during which surgical closure of a portosystemic shunt should be achieved is unknown. Gradual occlusion over 4 to 6 weeks has been suggested as an acceptable period to allow for adaptation of the portal system to the increased blood flow from shunt closure.13 Whereas ameroid ring constrictors are generally believed to close gradually over 4 to 6 weeks after placement, evidence exists that this closure may occur as early as 10 days after placement because of thrombus formation.5 The period during which complete occlusion is achieved with cellophane banding has varied among in vivo studies.14,15 Few studies have involved use of scintigraphy or angiography to determine the period required to achieve occlusion with cellophane banding, and most available information is based on serial bile acid or ammonia concentrations.15–18

The aforementioned limitations of ameroid ring constrictors and cellophane bands prompted the authors’ goal of creating a gradual occlusion device that reliably closed via physical occlusion over 4 to 6 weeks, was devoid of metal to allow for postoperative imaging, was easily deployed and self-locking, and would also be able to be deployed via minimally invasive techniques. Ideally, the compounds considered when developing a prototype gradual occlusion device should be selected on the basis of their inherent biocompatibility, inertness, and minimal inflammatory potential. Silicone has been used in various biomedical implants, including intraocular implants in veterinary medicine.19 Silicone is hydrated by the body and allows transfer of water across its luminal barrier.20 However, silicone alone would not provide the stiffness necessary to maintain the shape of a prototype as swelling occurred. For this reason, the authors considered whether a plastic polymer might be embedded within silicone tubing to provide this needed stiffness. Encasement of polypropylene and PEEK outer rings in silicone tubing would limit their interaction with the abdominal environment in vivo, and both have been used successfully in biomedical devices such as spinal stabilization cages and as a mesh for abdominal wall reconstructions.21,22

Polyacrylic acid is a hydrogel-forming polymer that is used in the manufacture of soft contact lenses in humans as well as in gel electrophoresis.23 Polyacrylic acid has also been used as part of an osteochondral defect repair in a study24 involving rabbits, in which it was found to be fully biocompatible in the joint and maintained its hydrogel properties over time. When PAA is combined with inorganic salt in a hydrophilic formulation, fluid intake across the silicone membrane could be expected to increase. By adjusting the salt content, the rate at which swelling occurs may be altered to allow for swelling at a predictable rate. The PAA–inorganic salt formulation could be expected to maintain a gel-like consistency after hydration.

The objectives of the in vitro study reported here were to determine the PAA–inorganic salt swelling formulation that would result in reliable closure of a gradual occlusion device to a luminal diameter of < 1 mm within 4 to 6 weeks and to develop a novel ring construct prototype for future in vivo testing. We hypothesized that the lumen of a gradual occlusion device composed of the identified formulation in silicone tubing bonded to silicone-encased polymer in a novel ring construct would gradually close to a diameter < 1 mm over 4 to 6 weeks.

Materials and Methods

A novel ring prototype was constructed from a polymer outer ring encased in silicone tubing attached to an inner silicone tube filled with a hydrophilic formulation of PAA and inorganic salt (Figure 1). Initial ring luminal diameter was between 3.5 and 4 mm. Initial ring outer diameter was approximately 10 mm. Ring width was 5 mm. The study was subsequently performed in 3 parts.

Figure 1—
Figure 1—

Photograph of a prototype device for gradual venous occlusion. The outer ring (solid arrow) is a polymer ring encased in silicone tubing. The inner clear layer (dashed arrow) is silicone tubing filled with a PAA and an inorganic salt combination.

Citation: American Journal of Veterinary Research 77, 3; 10.2460/ajvr.77.3.315

Identification of an optimal PAA–inorganic salt formulation

In part 1 of the study, 5 unique PAA and inorganic salt formulations were loaded into 3 polypropylene prototype rings each. These formulations were determined from several preliminary tests, in which various substances and formulations were evaluated in the same silicone tubing to determine their swelling properties prior to being encased in rings. Each formulation contained a different inorganic salt type in combination with PAA or a different ratio of inorganic salt to PAA. Formulations 1, 2, and 5 included sodium chloride in various ratios with PAA. Formulations 3 and 4 included potassium chloride in various ratios with PAA. Maximum amounts of sodium chloride, potassium chloride, and PAA considered were 15 mg each.

The primary observer (MLW) was unaware of the contents of each ring. Each ring was placed in 20 mL of PBSS in a sealed glass beaker. Solution was changed weekly. Photographs of each ring were obtained at the beginning of the experiment (before immersion) and twice per week thereafter for 6 weeks. For this imaging process, each ring was briefly removed from the beaker, placed on a flat black surface, and photographed with a digital camera on a tripod that was positioned perpendicular to the ring. A metric ruler was placed at the bottom of each field of view for calibration purposes. Ring luminal area, luminal diameter, and outer diameter were measured by use of imaging softwarea; ring weight was also measured. Each measurement was obtained 5 times by the same observer, and the mean of the measurements was used as a data point. Luminal and outer diameters were measured at the widest point. Luminal area was measured by manually outlining the ring lumen with the imaging software, allowing automatic calculation.

Identification of an optimal ring construct

In part 2 of the study, the formulation identified to have the optimal characteristics for providing gradual occlusion over a 4- to 6-week period (formulation 4) was loaded into 17 prototype rings. This formulation was chosen on the basis of the linearity of and ultimate change in luminal diameter and luminal area over the 6-week study period. Seven of the rings had a PEEK outer ring, and 10 had a polypropylene outer ring. The PEEK ring constructs were added because of their inherent stiffness, compared with the stiffness of polypropylene ring constructs. The same procedures and measurements involved in part 1 were then performed.

Evaluation of occlusion properties of the optimal PAA–inorganic salt formulation and ring construct

For part 3 of the study, the type of ring construct that deformed less (the PEEK ring) was chosen. Twelve rings were filled with 2 slight variations of formulation 4: 6 of formulation 4A and 6 of formulation 4B. The difference between formulations consisted of a variation in the ratio of PAA to inorganic salt, with formulation 4A having a higher salt concentration than formulation 4B. The silicone tubing in this portion was also altered to have a central partition. The primary observer (MLW) was unaware of the contents of each ring. Rings were soaked in 10 mL of a physiologic solution composed of PBSS containing 3 g of albumin/L and 100 mg of dextrose/dL in covered glass test tubes. Tubes were incubated at 37°C in air injected with 5% CO2.6 The rings were measured once per week as described for part 1.

Statistical analysis

Commercially available statistical softwareb was used for data analysis. Summary data are reported as mean ± SD. Equal variance was tested for each part of the study. When the equal-variance assumption was met, repeated-measures 2-way ANOVA with 1-factor repetition was performed to evaluate differences in absolute values between groups and over time. The Tukey-Kramer method was used for post hoc analysis to identify significant differences. When the assumption of equal variance was not met, the Mann-Whitney U test was used for each comparison at each measurement point. Linear regression was used to determine R2 values for linearity of device closure rate. Values of P < 0.05 were considered significant for all analyses.

Results

Identification of an optimal PAA–inorganic salt formulation

Equal variance among polypropylene prototype rings filled with the 5 PAA–inorganic salt formulations was identified for luminal area (P = 0.338) and luminal diameter (P = 0.148). No significant differences were identified among formulations in regard to luminal area (P = 0.566) or luminal diameter (P = 0.803) over all measurement points. A significant difference was detected in luminal area (P < 0.001) and luminal diameter (P < 0.001) over time, with all formulations resulting in a significantly smaller luminal area (P < 0.001) and luminal diameter (P < 0.001 to P = 0.002) at week 6 than at week 0 (Table 1). Formulations 2 and 4 resulted in the largest change in luminal area between weeks 0 and 6; however, the change associated with formulation 4 was more gradual than that associated with formulation 2 (Figure 2). Additionally, formulation 4 had a more linear swell rate (R2 for change in luminal area = 0.76; R2 for change in luminal diameter = 0.67) than did formulation 2 (R2 for change in luminal area = 0.49; R2 for change in luminal diameter = 0.43), which had a rapid swell phase within the first 2 weeks. Therefore, formulation 4 was chosen as the optimum PAA–inorganic salt combination of the 5 formulations tested. Another observation was that the outer diameter of the polypropylene prototype increased by a mean of 1.1 mm, suggesting that polypropylene might not have had the necessary stiffness needed to offset swelling within the ring.

Figure 2—
Figure 2—

Mean change in luminal area (A) and luminal diameter (B) for the prototype device of Figure 1 filled with 1 of 5 PAA–inorganic salt formulations (n = 3 rings/formulation) at various points before (week 0) and during soaking in 20 mL of PBSS for 6 weeks. Formulations 1 (circles), 2 (squares), and 5 (diamonds) included sodium chloride in various ratios with PAA. Formulations 3 (triangles) and 4 (inverted triangles) included potassium chloride in various ratios with PAA. Notice that rings filled with formulations 2 and 4 had the largest changes in both variables over the 6-week measurement period. However, rings filled with formulation 2 expanded more rapidly during the first 2 weeks, whereas rings filled with formulation 4 gradually expanded over the full period.

Citation: American Journal of Veterinary Research 77, 3; 10.2460/ajvr.77.3.315

Table 1—

Mean ± SD luminal area and diameter of polypropylene rings (n = 3/formulation) filled with 1 of 5 PAA-inorganic salt formulations before (week 0) and at various points during soaking in 20 mL of PBSS.

 Week
Variable0123456
Area (cm2)
 Formulation 10.063 ± 0.0360.046 ± 0.0230.025 ± 0.0240.021 ± 0.0250.020 ± 0.0270.020 ± 0.0270.021 ± 0.029
 Formulation 20.084 ± 0.0260.054 ± 0.0120.028 ± 0.0140.020 ± 0.0150.019 ± 0.0160.018 ± 0.0140.019 ± 0.014
 Formulation 30.075 ± 0.0370.059 ± 0.0150.045 ± 0.0060.035 ± 0.0060.026 ± 0.0100.023 ± 0.0120.022 ± 0.014
 Formulation 40.081 ± 0.0280.061 ± 0.0060.044 ± 0.0070.031 ± 0.0070.023 ± 0.0070.020 ± 0.0070.017 ± 0.005
 Formulation 50.108 ± 0.0270.067 ± 0.0390.048 ± 0.0360.046 ± 0.0370.053 ± 0.0440.053 ± 0.0530.055 ± 0.059
Diameter (mm)
 Formulation 13.03 ± 1.392.23 ± 1.071.68 ± 1.171.51 ± 1.341.39 ± 1.311.44 ± 1.371.50 ± 1.48
 Formulation 23.41 ± 1.032.60 ± 0.471.89 ± 0.501.53 ± 0.631.46 ± 1.051.44 ± 0.651.56 ± 0.74
 Formulation 33.16 ± 1.452.95 ± 0.702.37 ± 0.312.08 ± 0.151.73 ± 0.391.60 ± 0.481.58 ± 0.62
 Formulation 43.74 ± 0.763.11 ± 0.202.47 ± 0.122.07 ± 0.401.84 ± 0.461.77 ± 0.421.62 ± 0.40
 Formulation 53.82 ± 0.802.96 ± 0.612.41 ± 1.052.29 ± 1.222.68 ± 1.462.23 ± 1.762.20 ± 2.03

Formulations 1, 2, and 5 included sodium chloride in various ratios with PAA. Formulations 3 and 4 included potassium chloride in various ratios with PAA.

Identification of an optimal ring construct

Equal variance among PEEK and polypropylene rings filled with formulation 4 was identified for luminal diameter (P = 0.215) and luminal area (P = 0.103). Equal variance was not achieved for outer diameter. Significant differences were evident between PEEK and polypropylene rings in regard to luminal area (P = 0.003) and luminal diameter (P = 0.014). Luminal area was significantly (P < 0.001 to P = 0.046) smaller for PEEK than for polypropylene rings at all measurement points from weeks 0.5 to 5.5. Luminal diameter was significantly (P = 0.001 to P = 0.041) smaller for PEEK than for polypropylene rings at all measurement points from weeks 1 to 4.5. Outer diameter was significantly larger for polypropylene rings than for PEEK rings at every measurement point from weeks 4.5 to 6 (P = 0.002 to P = 0.015). A significant difference was detected in luminal area (P < 0.001) and luminal diameter (P < 0.001) over time, with both types of rings having a significantly (P < 0.001) smaller luminal area at week 6 than at week 0 (Table 2; Figure 3).

Figure 3—
Figure 3—

Mean luminal area (A), luminal diameter (B), and outer diameter (C) of PEEK (squares; n = 7) and polypropylene (circles; n = 10) rings filled with formulation 4 from Figure 2 at various points before (week 0) and during soaking in 20 mL of PBSS for 6 weeks. Error bars represent SD.

Citation: American Journal of Veterinary Research 77, 3; 10.2460/ajvr.77.3.315

Table 2—

Mean ± SD luminal area, luminal diameter, and outer diameter of PEEK (n = 7) and polypropylene rings (10) filled with formulation 4 from Table 1 before (week 0) and after 6 weeks of soaking in 20 mL of PBSS.

 PEEKPolypropylene
VariableWeek 0Week 6Week 0Week 6
Luminal area (cm2)0.112 ± 0.0130.005 ± 0.006*0.121 ± 0.010.013 ± 0.012*
Luminal diameter (mm)3.45 ± 0.430.58 ± 0.58*3.76 ± 0.330.92 ± 0.75*
Outer diameter (mm)9.75 ± 0.1910.09 ± 0.029.68 ± 0.2810.86 ± 0.64

Indicated value is significantly (P < 0.05) different from the corresponding value for week 0.

Starting weight of the PEEK rings (332 ± 13.8 mg) filled with formulation 4 was greater than that of the polypropylene rings (289 ± 16.6 mg). Twelve of the 17 rings (6/7 PEEK rings and 6/10 polypropylene rings) closed to a luminal diameter < 1 mm within 6 weeks (Figure 4). Generally, PEEK outer rings were superior to polypropylene rings in maintaining their shape and, therefore, the outer diameter of the ring. The 4 polypropylene rings that failed to close to < 1 mm within 6 weeks also failed to maintain their shape (Figure 5).

Figure 4—
Figure 4—

Photographs of representative polypropylene (top row) and PEEK (bottom row) rings from Figure 3 at various measurement points. Notice that the PEEK ring maintained its outer shape with only mild deformation, whereas the polypropylene ring deformed with time. Scale represents millimeters.

Citation: American Journal of Veterinary Research 77, 3; 10.2460/ajvr.77.3.315

Figure 5—
Figure 5—

Photograph of a failed polypropylene ring from among those represented in Figure 4. Notice how the increase in outer diameter allowed for some swelling to occur outside of the ring rather than directing the swelling toward the center. Scale represents millimeters.

Citation: American Journal of Veterinary Research 77, 3; 10.2460/ajvr.77.3.315

Evaluation of occlusion properties of formulation 4 in PEEK rings

Equal variance among PEEK rings filled with 2 slight variations of formulation 4 was met for luminal area (P = 0.297), luminal diameter (P = 0.917), and outer diameter (P = 0.598). No significant differences were identified between formulations 4A (higher salt concentration) and 4B in regard to luminal area (P = 0.790), luminal diameter (P = 0.951), or outer diameter (P = 0.172) over all measurement points. A significant difference was detected in luminal area (P < 0.001), luminal diameter (P < 0.001), and outer diameter (P < 0.001) over time, with both formulations having a significantly (P < 0.001) smaller luminal area and luminal diameter and larger outer diameter at week 6 than at week 0 (Figure 6). Starting weight was similar between the rings in each formulation group, with those containing formulation 4A having a mean starting weight of 360 ± 8.9 mg and those containing formulation 4B having a mean starting weight of 351.7 ± 16 mg. Eleven of the 12 rings (6/6 for formulation 4A and 5/6 for formulation 4B) closed to a luminal diameter of < 1 mm within 6 weeks (Table 3). The single ring filled with formulation 4B that failed to achieve that end point had a luminal diameter of 1.84 mm at the end of the study period. Although no specific area of leakage was found, leakage was believed to have occurred from a leak in one side of the inner silicone tubing because the other half of the ring achieved near complete occlusion.

Figure 6—
Figure 6—

Mean luminal area (A), luminal diameter (B), and outer diameter (C) of PEEK rings filled with variations of formulation 4 in Figure 3 at a higher salt concentration (circles; n = 6) and lower salt concentration (squares; 6) before (week 0) and at various points during incubation in a physiologic solution for 6 weeks. The silicone tubing in this part of the study was altered to have a central partition. Error bars represent SD.

Citation: American Journal of Veterinary Research 77, 3; 10.2460/ajvr.77.3.315

Table 3—

Mean ± SD luminal area, luminal diameter, and outer diameter of PEEK rings filled with 2 slight variations of formulation 4 (n = 6/variation) before (week 0) and after 6 weeks of incubation in a physiologic solution.

 Formulation 4AFormulation 4B
VariableWeek 0Week 6Week 0Week 6
Luminal area (cm2)0.148 ± 0.0270.006 ± 0.008*0.148 ± 0.0100.007 ± 0.017*
Luminal diameter (mm)4.15 ± 0.400.38 ± 0.43*3.85 ± 0.180.31 ± 0.75*
Outer diameter (mm)9.77 ± 0.1410.81 ± 0.589.75 ± 0.1410.77 ± 0.90

Indicated value is significantly (P < 0.05) different from the corresponding value for week 0.

Discussion

In the study reported here, a prototype of a gradual venous occlusion device was developed that provided reliable in vitro occlusion to a luminal diameter of < 1 mm over a 4- to 6-week period. Optimal properties were identified for PEEK rings filled with formulation 4 (potassium chloride and PAA), which swelled consistently over the study period. In addition, constructs composed of a PEEK outer ring more consistently closed than did those composed of a polypropylene outer ring.

Currently available gradual venous occlusion devices, including ameroid ring constrictors and cellophane bands, achieve occlusion of the shunt vessel via inflammation and thrombosis.5,25 A study7 involving in vitro testing of ameroid ring constrictors with and without outer metal rings revealed that minimal swelling occurred for either type of constrictor, with none of the rings fully closing. In another study,8 ameroid ring constrictors closed from a mean luminal diameter of 3.43 mm to a diameter of only 2.05 mm over 27 days under physiologic conditions in vitro, and none of those constrictors closed fully. Ameroid ring constrictors placed in the peritoneal cavity of rats achieved only 32% closure after 6 weeks in a third study.26 In a fourth study6 involving conditions similar to those in part 3 of the present study, ameroid ring constrictors closed from a mean luminal diameter of 4 cm to a diameter of 2 cm over a 9-week period. The PEEK ring configuration identified as optimal in the study reported here consistently swelled to a luminal diameter of < 1 mm, thereby suggesting a higher potential to induce gradual physical occlusion than can be achieved with ameroid ring constrictors.6–8,26

The weight of an ameroid ring constrictor (approx 1,400 mg27) has the potential to cause shunt closure by vessel kinking, which may lead to portal hypertension. On the other hand, the PEEK gradual occlusion devices evaluated in the present study weighed approximately 330 to 360 mg. This smaller weight may reduce the risk of acute vessel occlusion caused by vessel kinking. The PEEK gradual occlusion device was similar to a 3.5-mm ameroid ring constrictor with regard to luminal diameter, outer diameter, and width.

The optimal timing and rate for vessel occlusion in patients with portosystemic shunts are unknown. Gradual closure over 4 to 6 weeks may facilitate adaptation of the hepatic portal system and decrease the possibility of portal hypertension or postoperative development of multiple acquired shunts in patients with occluded portosystemic shunts.13 Rapid closure, which may occur with occlusion caused by ligation or premature thrombosis when ameroid ring constrictors are used,5 can lead to portal hypertension or development of multiple acquired portosystemic shunts. In a study by Vogt et al13 involving 12 dogs with naturally occuring portosystemic shunts, complete shunt occlusion was achieved with ameroid ring constrictors by 30 days after constrictor placement in 50% of patients and in an additional 21% by 210 days. Additionally 17% developed multiple acquired shunts.

For cellophane bands, differences in closure period have also been demonstrated. For example, a high shunt fraction remained evident in 6 of 16 dogs with naturally occuring portosystemic shunts 10 weeks after band placement in 1 study,14 with shunt closure achieved within 10 weeks in 10 of 16 dogs. However, 19% of dogs in that study14 developed multiple acquired shunts, similar to the outcome in the study by Vogt el al.13 Similarly, another study15 revealed that serum bile acid concentrations decreased to within reference limits by 2.5 months after a cellophane band was placed in dogs to occlude their shunts. In dogs in which the cellophane was placed loosely around their shunt, serum bile acids concentrations did not decrease until 6 months after band placement. It is unknown whether complete shunt occlusion is necessary given that some dogs with persistent shunting have excellent clinical outcomes.25 Our goals for gradual occlusion were consequently a linear and complete decrease in luminal diameter over a 4- to 6-week period, which was achieved with our identified optimal ring configuration.

In part 2 of the study reported here, PEEK outer rings were superior to polypropylene rings in maintaining their shape and, therefore, the outer diameter of the ring. Failure of polypropylene rings to maintain their shape appeared to be the cause of 4 rings failing to achieve the desired end point (Figure 5). Adequate swelling of the formulation within the inner silicone tubing occurred, but the ring was allowed to expand rather than forcing all swelling to remain in the internal portion of the ring. For this reason, the PEEK outer ring was used in part 3 of the study and is recommended for evaluation in future in vivo experiments.

One of 12 rings in part 3 of the study failed, and it appeared that leakage from that ring occurred secondary to failure of half of the ring. Adequate swelling was observed on half of the ring, whereas the swelling that had occurred on the failed half was no longer present when the ring was assessed at week 3. Although the reason this ring failed on the 1 side is unclear, this finding suggested the importance of the central partition in the tubing. This partition created a mechanism by which, if a breech did occur in the integrity of the inner silicone tubing on one side, the other side could still swell and produce some degree of occlusion.

The present study had some limitations. Namely, the in vitro results may not have been reflective of the in vivo scenario because of physiologic processes that could not be fully replicated in a laboratory setting. In the first 2 parts of the study, PBSS was used, whereas part 3 involved a more physiologic solution containing PBSS, albumin, and dextrose. Although plasma has been used in some in vitro studies6,8 involving ameroid ring constrictors, similar results have been achieved when saline (0.9% NaCl) solution was used. Additionally, a small number of rings were tested in each group in the present study. However, despite this small number, significant differences were identified among the groups, indicating sufficient statistical power. Furthermore, the rings did not swell in a symmetric manner in all situations, which may have resulted in the variability detected in luminal diameters in all parts of the study. Because of this phenomenon, measurements of luminal area were likely more accurate than measurements of luminal diameter over time.

In the study reported here, a gradual occlusion device composed of a PEEK outer ring encased in silicone and silicone tubing filled with a particular formulation of PAA and inorganic salt provided reliable physical occlusion over a 4- to 6-week period in vitro. Because of the proprietary nature of that formulation, specific ratios of PAA and salt were not reported here; however, given that total amounts did not exceed 15 mg, we do not believe that the devices would pose a health hazard if a leak were to develop. Because all materials used were biocompatible, we predict that venous closure can be achieved by the properties of the device alone (without reliance on inflammation, thrombosis, or other means) when it is used in vivo. On the basis of the results of the present in vitro study, we believe in vivo testing is warranted.

Acknowledgments

This manuscript represents a portion of a thesis submitted by Dr. Wallace to the University of Florida Department of Small Animal Clinical Sciences as partial fulfillment of the requirements for a Master of Science degree.

Supported in part by the University of Florida College of Veterinary Medicine Fall Faculty Research Grant and the University of Florida Mark S. Bloomberg Memorial Resident Research Fund.

Presented in part at the 13th Annual Scientific Meeting of the Society of Veterinary Soft Tissue Surgery, Coeur d'Alene, Idaho, June 2014; and at the 2014 American College of Veterinary Surgeons Surgery Summit, San Diego, October 2014.

The authors thank Rick Rizzolo for assistance with prototype development and manufacture.

ABBREVIATIONS

PAA

Polyacrylic acid

PEEK

Polyether ether ketone

Footnotes

a.

ImageJ, US National Institutes of Health, Bethesda, Md. Available at: imagej.nih.gov/ij. Accessed Dec 14, 2012.

b.

SigmaPlot, Systat Software, San Jose, Calif.

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Contributor Notes

Address correspondence to Dr. Wallace (wallaceml@ufl.edu).