Histological and Qualitative Evaluation of Hydrogel Cloth in a Sheep Lumbar Spine Model

Transcription

1 Histological and Qualitative Evaluation of Hydrogel Cloth in a Sheep Lumbar Spine Model A. Simon Turner, B.V.Sc., M.S., Dipl. ACVS and H.B. Seim Seim III D.V.M., Dipl. ACVS, Veterinary Medical Center, Colorado State University, Ft. Collins, CO 80523

2 Introduction United States 2009 estimates suggest that over 200,000 1 lumbar spine fusion surgeries will be performed on patients with lower back pain due to degenerative disc disease or spondylolisthesis. These patients face the very real possibility of needing a second surgery to extend the fusion because of adjacent segment disease, infection, device failure or failed back syndrome. Revision surgery has inherent risks such as intestinal, neurological, and or vascular injury. To mitigate these risks, access to the spinal disc is usually performed by highly skilled spinal and vascular surgeons who are familiar with the intestinal, neurological and vascular anatomy. Brau, et al reported an overall 11% (7/63) incidence of vascular injury during revision surgery. 2,3 In addition to the risk of injury to the aorta, the vena cava, and the iliac vessels, these procedures are also complicated by scarring around the surgical site. Revision surgery presents a significant challenge to the access surgeon, since re-exposure of the spine requires mobilization of the peritoneal sac, which is usually fused to the anterior abdominal wall muscles, as well mobilization of the major vessels and ureter, which are at the time of surgery are encased in fibrous tissue and firmly adhered to each other and the anterior surface of the spine. Sometimes, this can present an enormous problem depending on the type of revision required. Interestingly enough, it has been reported that even at only days there is already a dense capsule surrounding the intervertebral implant. 4 a b c Figure 1; a, b, c These pictures show three methods of lumbar fusion in human patients. Scar tissue can build up making revision surgery very risky. The aorta and iliac arteries (see 1b) are structures that can easily adhere to the implants involved in the fusion. The purpose of this study was to evaluate the biological response of a hydrogel sheet that can be used as an adjunct to spinal fusion surgery. The EnGuard Vessel Guard has been cleared for sale by the FDA (April, 2009) and is currently indicated as a cover or barrier for vessels following anterior vertebral surgery. The sheet is positioned over the vertebral bodies and is designed to protect the vascular blood vessels and aimed at providing a plane of dissection to readily isolate vital structures and prevent injury during a revision procedure. To demonstrate the latter, nine sheep were used in this study wherein a hydrogel sheet was anchored over the lumbar disc spaces at L2/L3 and L4/L5. Actual fusion surgeries were not performed in these animals. Three groups of three sheep were humanely euthanized after 3, 6 and 13 weeks respectively and histology was performed at the sites where the implant was sutured. Since earlier studies have shown that there wass no adverse tissue reaction 5, the intention of this study was to show that these hydrogel sheets can be used effectively to minimize one of the complications associated with spinal revision 2

3 surgery the revisualization and reaccess to the initial anterior vertebral surgical site that is surrounded by scar tissue. Our goal was to demonstrate that the hydrogel sheet provided a permanent and visual plane of dissection at the initial surgical site. By providing a plane of dissection, it is hoped that a revision surgery would be more easliy facilitated. Overview of Materials and Methods: Nine sheep were placed in the pastures of the large animal research barn. They were dewormed and vaccinated and ear-tagged for identification. Physical examination was performed and any animals with signs of respiratory disease had venous blood submitted for a complete blood count (CBC). Description of the surgeries: The sheep were anesthetized. Wool was removed from the left lateral lumbar region of the sheep and the sheep were positioned in right lateral recumbency on the operating table. Surgical approach to the disc spaces: The left lateral lumbar area was prepared for aseptic surgery with multiple scrubs of povidoneiodine alternated with isopropyl alcohol. Under general anesthesia using sterile conditions, the surgical area was draped and a lateral retroperitoneal approach to the disc spaces of L2/L3 & L4/L5 was made (See Figures 2 & 3, below). A lateral approach to the disc spaces was performed in this model because of the inaccessibility to the ventral ( anterior ) disc space because of the abdominal viscera. Figure 2 Figure 3 Figure 2. This intraoperative picture shows the retroperitoneal approach to disc sites over which the Replication Medical, Inc. EnGuard Vessel Guard was attached using a HemaClip. Figure 3. View of exposed surgical site EnGuard Placement: The EnGuard device was placed on top of the vertebral body and attached, using a HemaClip (Teleflex Medical) to tack the pieces of the flexible hydrogel sheet over the disc spaces of L2/L3 & L4/L5 (See Figure. 3 above). Wound Closure: Routine closure of the muscles (0 absorbable suture), subcutaneous tissue (0 absorbable suture) and skin (2/0 monofilament non-absorbable suture) was performed. Operative time for each animal was approximately 50 minutes. Perioperative antibiotics (Cephazolin sodium) were administered. 3

4 Aftercare: Immediately after surgery, the sheep were observed until the swallowing reflex returned. They were then extubated and then taken to the research barn at the VMC. The sheep were housed indoors for 2 weeks and then later outdoors (with access to a three-sided shelter) for the rest of the convalescence period and allowed to exercise at will. Specimen Collection and Handling: Following euthanasia, a complete gross necropsy was conducted on all 9 animals. Conventional gross examination of all the major organ systems was performed. The disc spaces of L2/L3 & L4/L5 were retrieved and then inspected grossly. Necropsy: Following euthanasia, a complete gross necropsy was conducted on all 9 animals. Conventional gross examination of all the major organ systems was performed. The sheep s abdominal wound was opened and mobility of the hydrogel sheet related to surrounding tissue was assessed. The ability to easily retract the vessel guard away from the vertebral body exposing the bone was evaluated.. The disc spaces of L2/L3 & L4/L5 were retrieved and then inspected grossly. Histology was performed on a total of nine sheep at 3, 6, and 13 week time periods. Where indicated in Table I, the specimens were placed in 10% formalin in preparation for histology. Specimen Collection and Handling: The sheep s abdominal wound was opened and mobility of the hydrogel sheet related to surrounding tissue was assessed. The ability to easily retract the vessel guard away from the vertebral body exposing the bone was evaluated.. The disc spaces of L2/L3 & L4/L5 were retrieved and then inspected grossly. Histology was performed on a total of nine sheep at 3, 6, and 13 week time periods. Where indicated in Table I, the specimens were placed in 10% formalin in preparation for histology. Histology: The histopathological analysis of this study was performed by Histion, Inc. Everett, Washington. Analysis: Adhesion scores were collected and scored as follows for all three time-point groups. Plane of Dissection analysis was performed by evaluating the degree of EnGuard vessel guard mobility and the resistance of the intervening tissue surrounding the Enguard vessel guard. Histology was performed on Surgical Control sites i.e. sites that were accessed simulating a fusion procedure but closed without implants and EnGuard Treated sites, sites in which the EnGuard vessel guard was placed over the discs. In the EnGuard Treated histological sites, no attempt was made to remove the vessel guard from its tissue placement so as to preserve the tissue surrounding the vessel guard for histological evaluation. For the Plane of Dissection Evaluation sites, once the EnGuard vessel guard was removed from the L2-L3 or L4-L5 surgical sites, it was placed in formalin for histological assessment to determine whether there was any tissue adhering to the EnGuard sheet. Lastly, local lymph nodes were collected to determine whether there was any polymer debris that had been released from the sheets.sheets. Plane of Dissection Analysis Degree of vessel guard mobility Extent of mobility: A = Completely mobile, easily lifted to reveal bone. B = Mobility > 50% of the area C = Mobility of 25-50% of the area D = < 25% of the area is mobile 4

5 Resistance of vessels of Intervening Tissue: 0 = No resistance 1 = Slight resistance (slight force required) 2 = Moderate resistance (blunt dissection is possible) 3 = High resistance (sharp dissection required) Tissue Collection/Histopathology: Tissue was collected at the time of necropsy and analyzed where indicated, histologically on seven sheep at 3, 6, and 13 week time periods. The design study follows: Table 1 Sheep Tag # Implant insertion site: Implant insertion site: Implantation Duration L4-L5 L2-L3 (weeks) 1 EnGuard - Plane of Dissection EnGuard - Plane of Dissection 3 2 EnGuard - Plane of Dissection EnGuard - Histology 3 3 Surgical Control (Histology) EnGuard Plane of Dissection 3 4 EnGuard Histology EnGuard - Plane of Dissection 6 5 EnGuard Plane of Dissection EnGuard Histology 6 6 EnGuard Plane of Dissection Surgical Control 6 7 EnGuard Histology Plane of Dissection 13 8 EnGuard Plane of Dissection EnGuard Histology 13 9 EnGuard - Plane of Dissection Surgical Control 13 Results: Sheep Sac. Dur. Level Mobility Resistance RM1 3 wk L2/L3 D 2 RM1 3 wk L4/L5 D * RM2 3 wk L2/L3 Histology RM2 3 wk L4/L5 C 3 RM3 3 wk L2/L3 A 0 RM3 3 wk L4/L5 Surgical Control RM4 6 wk L2/L3 A 0 RM4 6 wk L4/L5 RM5 6 wk L2/L3 Histology RM5 6 wk L4/L5 A 0 RM6 6 wk L2/L3 Surgical Control RM6 6 wk L4/L5 A 0 RM7 13 wk L2/L3 A 0 RM7 13 wk L4/L5 RM8 13 wk L2/L3 Histology RM8 13 wk L4/L5 A 0 RM9 13 wk L2/L3 Surgical Control RM9 13 wk L4/L5 A 0 * Data not recorded 5

6 Discussion The results clearly show that the EnGuard vessel protection system was extremely effective in providing a clear plane of dissection and preventing tissue from adhering to the underlying vessels. The encroaching tissue in almost all cases did not adhere to the device. In the odd instance where there was some adherence, the tissue was easily removed from the vessel guard.. 8 Mobility 8 Resistance A B C D Figure 4 Figure 5 Figures 6 and 7 demonstrate that in 7 out of the 10 observations for EnGuard vesel guard the implant was highly mobile and readily dislodged from its point of association with the tissue. In fact, once the EnGuard points of peripheral attachment to the tissue was released, the implant was completely loose and showed no adherency to the surrounding tissue in 7 out of the 10 cases. In one out of ten cases, slight force was required to mobilize the EnGuard vessel guard from the surrounding tissue. EnGuard was highly effective and exhibited excellent tissue mobility and minimal tissue adherence (little resistance in removing tissue from the EnGuard). In all EnGuard sites there was a clear plane of dissection formed by the device that was easy to identify. In seven out of 10 cases the EnGuard device was easily released from its point of peripheral tissue attachment and the resistance of intervening tissue negligible. In fact, in most cases, once the margins at which the EnGuard was attached was released, the device virtually fell out of the surgical site. In seven out of ten cases the device was readily mobilized and separated from intervening tissues. In two EnGuard sites at 3 weeks, there was some blunt dissection required to release the EnGuard from the tissue yet parts of the implant between 25 and 50% were completely mobile. Histopathology The results from this study showed that the implants served as a physical barrier between tissues, preventing tissues from adhering to one another, with no obvious degradation of the implants over the course of the study. The organized layer of cells seen adjacent to the implants at 3 weeks 6

7 did not appear to change dramatically with time. With time, tissues between bone and the implant surfaces and muscle and the implant surfaces became thinner, more fibrous and more organized and in general, were infiltrated with fewer mononuclear infiltrates at the later timepoints compared to the earlier timepoints. See Figure 6 Figure 6: EnGuard 6 week implant Conclusions The animal study shows that the EnGuard device is an effective barrier material which readily forms a plane of dissection and can be freed easily from surrounding tissue. Histological evaluation of EnGuard sites with the device in situ show a normal healing response to the implant. There was a mild to moderate inflammatory response with foci of mononuclear infiltrates which subsided over time. The presence of a thin layer of fibrous tissue surrounding the EnGuard device coupled with the visible plane of dissection it forms, demonstrates its effectiveness as a barrier material for use in spinal surgery. The EnGuard is currently approved as a vessel cover for use during anterior spine surgery. To date, EnGuard has been used in conjunction with anterior fusion in five cases. No revisions have yet to occur. References 1 U.S. Markets for Spinal Disc Repair and Replacement Technologies, 2008; Life Science Intelligence, Inc. (LSI) 2 Brau SA, Spoonamore MJ, Snyder L, et al. Nerve monitoring changes related to iliac artery compression during anterior lumbar spine surgery. Spine 3: (2003) 3 Brau SA, Delamarter RB, Schiffman ML, et al. Vascular injury during anterior lumbar surgery. Spine J; 4: (2004) 4 Salvador A. Brau, MD, FACS; Rick B. Delamarter, MD; Michael A. Kropf, MD; Robert G. Watkins III, MD; Lytton A. Williams, MD; Michael L. Schiffman, MD; Hyun W. Bae, MD: Access Strategies for Revision in Anterior Lumbar Surgery, Spine 33(15): (2008) 5 Data on File 7

8 7 Clarke Drive Cranbury, NJ p f All rights reserved. EnGuard is a trademark of Replication Medical, Inc. Caution: Federal law (USA) restricts this device to sale by or on the order of a physician. Refer to the instructions for use supplied with product for specific information on indications for use, contraindications, warnings, precautions, adverse reaction information, and sterilization. WARNING: The safety and effectiveness of this device for reducing the incidence, severity and extent of post-operative adhesion formation have not been established.