Burst Strength of Ventral Hernia Repairs in a Chronic Porcine Model: Comparison of Repairs Fixated with the SorbaFix™ Fixation System to Those Using Permanent Fixation G. Kevin Gillian, M.D., F.A.C.S.a, Jeffrey R. Scott, Ph.Db , Shiow-jyi Wey, Ph.Dc, Vincent Roy, P. Engd, Ian D. McRury, Ph.Db a c
Virginia Hospital Center, Arlington, VA; b Davol Inc., Subsidiary of C. R. Bard, Inc., Warwick, RI; DaVinci Biomedical Research Products, Inc., S. Lancaster, MA; d Altran Solutions, Inc., Boston, MA
Burst Strength of Ventral Hernia Repairs in a Chronic Porcine Model: Comparison of Repairs Fixated with the SorbaFix™ Fixation System to Those Using Permanent Fixation G. Kevin Gillian, M.D., F.A.C.S.a, Jeffrey R. Scott, Ph.Db , Shiow-jyi Wey, Ph.Dc, Vincent Roy, P. Engd, Ian D. McRury, Ph.Db a c
Virginia Hospital Center, Arlington, VA; b Davol Inc., Subsidiary of C. R. Bard, Inc., Warwick, RI; DaVinci Biomedical Research Products, Inc., S. Lancaster, MA; d Altran Solutions, Inc., Boston, MA
ABSTRACT An examination of chronic fixation strength was performed in an in vivo porcine model. Twenty-four pigs were prepped with a midline incision in order to create 1 inch diameter
Figure 1
full thickness defects in the abdominal wall using a sharp circular punch. The defects were repaired using a 3.5 inch diameter Composix™ LP prosthetic and fixated using either the SorbaFix™ Device or a permanent fixation device. Sixteen fasteners were diametrically placed in the prosthetics. The animals were sacrificed at six time points over a two month period. The walls were explanted, examined grossly and then each of the repair sites was burst tested using a mechanical testing system. Rapid tissue in-growth into Composix L/P hernia patches was observed by 1 week ™
post-implantation, which resulted in a dramatic increase in mechanical Ball Burst Force
SorbaFix™ Fastener (not shown to scale – actual length is 6.7mm)
between the acute repair and the 1 week time period. The SorbaFix™ Device held pressures greater than seven times the 4.25 psi physiologic requirement for burst pressure for all periods, indicating that the fasteners provide more than adequate strength for both the acute and chronic repair. At two months there was a statistically significant increase in burst strength as compared to previous time points due to the complete tissue incorporation of the prosthetic. In summary, the SorbaFix™ Device exhibited burst pressures that were comparable to permanent fixation following implantation and maintained mechanical strength well above the physiological requirements for all time periods. INTRODUCTION Recent advances in surgical hernia repair have resulted in accelerated recovery time, stronger repairs and reduced recurrence.1 Most repairs use polypropylene mesh and permanent fixation to achieve an adequate primary repair. Upon implantation, these materials provoke an inflammatory response, which ultimately leads to wound healing via collagen deposition and scar formation.2 The use of tackers and staplers for fixation has become common practice as an adjunct or replacement for suture and consequently has reduced operating time and post-operative pain.3,4 Utilizing absorbable materials for fixation has been proposed as a useful alternative, leading to more natural wound healing and less long-term material retention. Absorbable fixation materials are only suitable for use with prosthetics where rapid tissue in-growth is achieved,
such as woven polypropylene meshes. The long term stability of repairs with a woven mesh is ultimately dependent on tissue in-growth into the mesh and not the method of fixation. Consequently, absorbable fixation to bridge the tissue in-growth stage is an appealing concept to some surgeons. The SorbaFix™ Device is a new general surgery device for the fixation of prosthetic material and the re-approximation of soft tissue. The system is designed to deliver up to 30 screw-shaped absorbable fasteners molded from a Poly (D, L) Lactic Acid (PDLLA) material that is naturally broken down in the body in approximately 12 months. This study is a pre-clinical comparison of the appropriateness of this fixation device for hernia repair in comparison to a permanent fixation device. A hernia repair fails
when the intraabdominal pressure (IAP) causes the prosthetic to rupture or to lose it’s attachment to the abdominal wall at it’s point of in-growth or fixation. A conservative estimation of worstcase IAP is that of Twardowski, who observed that sick patients with between 1–3 liters of intra-peritoneal fluid had a peak cough intraabdominal pressure of 220 mmHg or 4.25 psi.5 For the purpose of this study, 4.25 psi will be considered the physiologic requirement that the hernia repairs must resist in order to remain
intact. Additional variables in the repair strength are the overlap of the prosthetic to the defect and number and performance of the fixation elements. In this study, the repair was custom designed at the lowest overlap condition and the lowest recommended number of fixation elements.10 In this set-up the performance of the new SorbaFix™ Device can be compared to both the clinical IAP requirement and the performance of permanent fixation.
STUDY METHODS
Results
Twenty-four female Yorksire pigs (starting weight: 83.17 ± 10.01 kg) were initially anesthetized with 2.5-4% inhalational isoflurane and maintained at 0.5-2.5% throughout the procedure. The ventral abdomen was prepared using standard aseptic techniques and applying sterile surgical drapes over the entire field. Following preparation of the ventral abdomen, a 12 inch (30 cm) midline incision was made into the peritoneal space. Individual 1 inch (2.5 cm) surgical defects were created in discrete abdominal wall quadrants on each side of the incision, by rotating a 1 inch (2.5 cm) circular die cutting tool to create a full thickness muscular defect, with no penetration through the outer abdominal adipose tissue and skin layers. Once extraneous bleeding was controlled with the use of electro-cautery and/or gauze, a 3.5 inch (8.9 cm) circular Composix™ LP hernia patch was either fixated with 16 SorbaFix™ Device fasteners around the circumference of the prosthetic spaced approximately 0.4– 0.8 inches (1–2 cm) apart, or fixated with 16 similarly placed permanent fixation fasteners. The surgical site for each device was rotated between animals to randomize the repair location. Following placement of the test devices, the abdomen was closed using standard techniques.
A total of 24 pigs were used during this study. No re-herniation was observed in any pigs within this study for either of the devices. One of the explanted 1 month abdominal wall meshes was damaged during transport to the testing sight so the entire pig was removed from the study. For each abdominal wall, two SorbaFix™ Device burst tests and one permanent fixation burst test was completed. Figure 2 shows examples of the fixation and the tissue in-growth of the SorbaFix™ Device fastener at four time-points.
The pigs were euthanized in groups of four at six different time points: T0, 2 days, 1 week, 2 weeks, 4 weeks and 8 weeks by using intravenous injections of 150 mg/kg of Sodium Pentobarbital in accordance with the American Veterinary Medical Association (AVMA) Panel of Euthanasia. Following euthanasia, the abdominal walls were extracted in total for mechanical testing. The mechanical properties of the repair site were evaluated using a modified ASTM Standard (D3787-01) ball burst method. All mechanical analysis was conducted by an independent biomaterial consulting firm within 24 hours of explantation. A rod was mechanically driven through the center of the prosthetic such that the prosthetic would prolapse out of the abdomen (reherniation). The mechanical resistance was measured and converted to a pressure (psi) by dividing the peak force (lbs-f) by the area of the 1 inch diameter defect (0.79 in2).
Within this test model, the repair can encounter one of two failure modes: the fasteners pull from the tissue or the mesh can burst. In the acute and 2 day implants, the SorbaFix™ repairs and the permanent fixation repairs each observed both of these
Figure 2 - Performance of SorbaFix™ Device Fastener over time.
0 Days (T0): Notice how there are no exposed points or sharp edges and how clearly the fastener can be visualized.
2 Weeks: Extensive tissue incorporation into the mesh.
4 Weeks: Full reperitonealization of the mesh and fasteners is almost complete.
8 Weeks: The tissue is completely incorporated into the mesh with full revascularization of both the mesh and fasteners.
failure modes (SorbaFix™ Device 0, 2 day = 13% and 38% of the repairs failing at the mesh respectively; permanent fixation 0, 2 day = 100% and 50%). At the 1 week explants and beyond, the mesh burst exclusively (100% of the time) for both the SorbaFix™ Device and for permanent fixation prior to any fixation failure. This would indicate rapid in-growth of tissue into the Composix™ LP prosthetic by 1 week of implantation.
value of a 12 cm2 defect corresponded to a mean mesh area with an overlap of approximately 6 cm, which is 20% greater than the current recommendation.11 This is in line with this author’s observation that as the defects get larger, surgeons use correspondingly larger prosthetics with greater prosthetic overlap and a correspondingly greater number of fasteners in multiple lines (double crown technique).
The average burst data for the SorbaFix™ repairs and the permanent fixation repairs are shown in Figure 3. The SorbaFix™ repair burst pressure was approximately seven times greater that the physiologic requirement at time 0 despite exhibiting approximately 30% less holding force than permanent fixation (2-sided t-test, assuming similar variances, p<0.001). The repair strength for the two devices was equivalent from the 1 week point and beyond. ANOVA results (p<0.0001) imply a very rapid increase in repair strength from day 0 to some point between day 2 and 7 days. At this point, the tissue in-growth into the mesh adds to the fixation strength such that it is equal to or greater than the strength of the prosthetic. Thus, the repair is completely dependant upon the strength of the prosthetic and remains so until some point between 1 month and 2 months. After this point, the prosthetic is completely incorporated into tissue and the thickening of tissue adds gradually to the prosthetic strength as demonstrated by the latent strength increase.
The lesion and repair model used in this study are simplifications of true medical conditions, however, it is well within reason as a test of the capability of the fixation device. The prosthetic only overlapped the lesion by 3 cm, the low end of Bard’s recommendations. Additionally, 16 fasteners were used in the repair (Bard would recommend 10–33), which is likewise on the low end of the recommendation. When it is further realized that the 4.25 psi specification is very conservatively derived, this model can be classified as a properly conservative test for this or any fixation product.
The most difficult part of creating a surgical model is modeling the variability between hernia types, non-standard healing ability of patients and individual surgeon’s repair techniques. Most modern day mesh implants easily hold up to the IAP discussed already without bursting.7, 8 Therefore, it is up to the fixation means to maintain the implants surgical position post implant until tissue in-growth takes over. The factors that resist migration are implant overlap, fixation strength per fastener, and the number of fixation points per implant and/or their relative spacing.4, 6, 9 The product instructions for use (IFU) recommend 3–5 cm of material overlap beyond the defect in all directions.10 When Iannitti examined 455 abdominal incisional hernias, the mean overlap was 4.5 cm.11 The SorbaFix™ Technique Guide further recommends that fasteners are placed 1–2 cm from each other.10 Using a perfectly circular defect and prosthetic model as described in this manuscript the number of fasteners recommended for use in this model would range from 10–33 fasteners. For a small lesion like this, the smaller number of fasteners is more than appropriate. However, many defects tend to tear in a longitudinal fashion, creating oval lesions (hence most prosthetics are ovals). In the Iannitti data, the median
Figure 3 SorbaFix™ Device
Permanent
70
Burst Pressure (psi)
DISCUSSION
Given the applicability of the model, the burst strength results of the SorbaFix™ Device as compared to a 4.25 psi physiologic requirement indicates the device is more than suitable for both the acute and chronic repair.
60 50 40 30 20 10 4.25 psi physiological requirement for burst pressure
0 Time 0
2 days
1 week
2 weeks
1 month
2 months
Implantation Time Average pressure required to burst through repairs of the porcine abdominal wall defect. An ANOVA of the time sequence for the SorbaFix™ Device showed significant differences in the data (p<0.0001). The 0 and 2 day data were homogeneous, but statistically different (*) from the homogenous grouping of the 1 week, 2 weeks and 1 month data. By 2 months the prosthetic is fully incorporated by tissue and the burst strength is that of the prosthetic plus the additional layers of tissue. The 2 month data group was statistically higher (ANOVA*) than both the acute (0, 2 day) and the 1, 2, and 4 week data sets. In all cases, the fixation strength is greater than 7 times the physiologic requirement (4.25 psi – red horizontal bar)
CONCLUSION Twenty-four pigs underwent survival surgery and were prepared with four 1 inch diameter abdominal defects. The defects were repaired using a Composix™ LP prosthetic fastened in place with either 16 SorbaFix™ fasteners or 16 permanent fixation fasteners. The pigs were sacrificed at six time points between 0 and 2 months, the abdominal walls were excised and the repairs were strength tested using a mechanical burst apparatus. Acutely, the holding strength of the SorbaFix™ Device is on average greater than seven times above the physiological requirement. After 1 week of implantation the repair strength of both devices equaled the strength of the prosthetic itself until the prosthetics were completely incorporated between 4 and 8 weeks. The 8 week time point is statistically greater than that of weeks 1 through 4 due to complete incorporation of the mesh into the tissue. Given that this is a conservatively derived repair model, the results of this study support the conclusion that the SorbaFix™ Device is well suited for use as a fixation device in hernia repair.
ACKNOWLEDGEMENTS The study was conducted for and sponsored by C. R. Bard at DaVinci Biomedical Research Products, Inc., South Lancaster, MA. This study was also reviewed and evaluated by the internal Institutional Animal Care and Use Committee (IACUC) of DaVinci Biomedical Research Products, Inc., and was conducted in compliance with all regulations regarding the humane treatment of laboratory animals set forth by IACUC.
DISCLAIMER This study represents a preclinical evaluation of the SorbaFix™ Device following in vivo implantation in an animal model. The data presented herein is not intended to imply clinical product performance and/or circumvent sound clinical judgment.
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3. Carbonell AM, et al “Local injection for the treatment of suture site pain after laparoscopic ventral hernia repair” Am Surg 69:688-691 2003.
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4. Heniford BT, et al “Laparoscopic repair of ventral hernias: nine year’s experience with 850 consecutive hernias” Ann Surg 238:391-399 2003.
10. Bard: SorbaFix™ Absorbable Fixation System Technique Guide, Ref # MMSFTG 2009.
5. Twardowski ZJ et al “Intraabdominal Pressures during Natural Activities in Patients Treated with Continuous Ambulatory Peritoneal Dialysis” Nephron 44:129-135 1986.
11. Iannitti DA, et al “Technique and Outcomes of Abdominal Incisional Hernia Repair Using a Synthetic Composite Mesh: A Report of 455 Cases” J AM Coll Surg 2007 (Article in Press) Copy in Appendix A.
6. Dion YM, et al “The influence of the number of endoclips and of mesh incorporation on the strength of an experimental hernia patch repair” Surg Endosc 8: 1324-1328 1994.
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