Fengcheng Wei1*, Hinata Nakakubo1, and Masato Ohmi1
Received: July 31, 2023; Published: September 26, 2023
*Corresponding author: Fengcheng Wei, Department of Medical Physics and Engineering, Division of Health Sciences, Osaka University, Japan
We applied Optical Coherence Tomography (OCT), which has the advantages of high speed and high resolution on imaging, to perform in vivo animal experiments on surgical sutures. A Spectral-Domain OCT system was used to scan the absorbable PLA/CL surgical sutures and Non-absorbable PVDF surgical sutures, both of which had been sutured on a hairless rat. Two kinds of surgical sutures, polylactic acid/polycaprolactone (PLA/CL) compound surgical sutures, which is absorbable, and polyvinylidene fluoride (PVDF), which is non-absorbable, have been sutured on the back of a hairless rat. And a Spectral-Domain OCT was used to take the pictures of both surgical sutures and surrounding tissues every week till PLA/CL sutures was absorbed and broken in order to record and analyzed the changings. As a result, the deposits on the surface of the suture that were observed over time were decomposed substances of keratinocytes and fibers. Furthermore, with the decomposition, the crystal structure of PLA/CL fiber became amorphous, and reflected signal was detected. Some void also appeared which shows that the proliferated part detached from the sutures and the epidermal layer. PLA/CL sutures finally got broken at the 16th week, with the diameter decreased from 220 μm to 180 μm. In vivo time-lapse observation of surgical sutures can be accurately performed by OCT. It is also able to scan not only the surface but also the internals structure like crystal changes of surgical sutures. What’s more, with 40 μm decrease on diameter, PLA/CL sutures broke after 15 weeks.
Keywords: In Vivo Observation; Optical Coherence Tomography; Surgical Suture; Biomaterials
Abbreviations: OCT: Optical Coherence Tomography; PLA: Polylactic Acid; PVDF: Polyvinylidene Fluoride; CL: Crystal Structure
Since the latter half of the 20th century, various surgical sutures have been developed with advances in polymer materials science. Currently, clinically applied surgical sutures are mainly classified into two types: non-absorbable sutures and absorbable sutures. A non-absorbable suture needs to be removed after the wound is healed, and the suture itself is considered not to be decomposed or absorbed and mostly at a lower price range and has stable physical properties [1,2]. However, Poor healing and scarring of suture wounds have been reported when non-absorbable sutures are used . Typical materials include nylon, polypropylene, and polyester. Absorbable sutures are produced using biodegradable materials and degrade with the passage of the time. The decomposition products fall off the skin surface or are excreted through the circulatory system. While there is no need to remove sutures, they are usually more expensive than non-absorbable sutures, their physical properties deteriorate with the passage of time, and there is a risk that they will break before the wound heal [4,5]. Typical materials include polylactic acid, silk, and catgut. There have been many studies on the physical and material properties of sutures. However, in vivo research on their biocompatibility is seldomly done [6-8]. However, as a biomaterial applied to the human body, animal experiments is still an irreplaceable method [9,10]. Therefore, in many cases, the suture is sutured to the living body, and after a certain period, the suture is taken out and measured . If it is removed from the organism, it can be measured using specialized measuring equipment, but it has the disadvantage of using a destructive method which is cutting the surgical sutures and that may cause extra pain to the experimental subjects .
Until now, almost no studies have been conducted to avoid the above disadvantages and to observe and measure experimental subjects in vivo. Furthermore, a high resolution is required for observing such a fine structure, in which the diameter of the suture thread is basically only a few hundred micrometers. In the case of in vivo, there is movement due to heart pulsation and respiration of experimental animals, so high speed measurement is also required. On the other hand, since the 1990s, Fujimoto et al. have been able to observe eye and skin tissue using noninvasive optical coherence tomography (OCT) technology that detects backscattered and back reflected light . Compared to conventional optical ranging measurements using femtosecond laser pulses, the energy of the light is lower, which makes it possible to apply it to in vivo experiments . Furthermore, by using OCT, it is possible to observe more structures from both soft tissues such as retina layers and muscles and hard tissues such as bones and teeth . In recent years, our laboratory has used OCT to analyze the behavior of the sweat glands on human fingertips during mental sweating . Then, we conducted an in vivo animal experiment using guinea pigs and investigated the allergic dermatitis of skin tissues . Based on the above previous studies, we strongly suggested that in vivo observation on surgical sutures by using high resolution rapid scanning OCT. Therefore, we sutured hairless rats with non-absorbable and absorbable sutures and performed time-lapse observation after surgery by OCT. From the image data, while confirming the change in the diameter of the yarn, it was compared with the reflected signal inside the yarn. Deposits on the outer circumference of the yarn were also examined.
Preparations of Experiment
A ten-week-old male hairless rat (SPF, HWY/slc, weight: 280 g) is used and kept in a constant temperature and humidity environment. During rearing, solid feed and tap water were freely taken. Animal experiments were conducted under the permission of the Osaka University Graduate School of Medicine Animal Experiment Ethics Committee. No. 6-0 (diameter approximately 70~100 μm) monofilament polyvinylidene fluoride (PVDF) non-absorbable suture (AR526, manufactured by Kono Seisakusho, blue) and polylactic acid/polycaprolactone (PLA/CL) composite absorbable suture (LC516, manufactured by Kono Seisakusho, purple) was used in our suture surgery. Anesthesia by intraperitoneal administration of a three-kind mixed anesthetic was performed before the suturing surgery. Table 1 shows the amount of anesthetic used. Medetomidine (Nippon Zenyaku Kogyo CO., LTD.), Midazolam (Sandoz AG.), Butorphanol Tartrate (Meiji Animal Health Co., Ltd.) and Saline Solution (Otsuka Pharmaceutical Co., Ltd.) has been used in our research. After the animal's nociceptive reflex disappeared, the suturing operation was started. Two parallel incisions of about 2 cm were made on the animal's back, and four simple sutures were made at intervals of about 4 mm. (Figure 1) The whole suture operation took about 30 minutes.
Experiments were performed by using a Spectral Domain OCT (Telesto 320, manufactured by Throlab CO., LTD. Center wavelength 1300 nm, maximum imaging depth: air 3.6 μm /water 2.6 μm, axial resolution: air 5.5 μm /water 4.2 μm). 6 2D OCT images of each of the 8 sutured areas were recorded at the pace of once a week, and the absorbable suture was cut after 18 weeks. Finally, OCT images of 18 weeks’ time lapse observation were taken. And the field of views of each OCT image is about 2 x 1.5 (X x Z axial) mm. 10 absorbable sutures and 10 non-absorbable sutures were selected from the acquired images of every week, and the diameter of the sutures in the vertical direction was measured by using Image J.
Time Lapse OCT Images
Figures 2(a) & 2(b) shows time-lapse images (0-18 weeks) of sutures sutured on the back of hairless rats. (a) is a non-absorbable suture and (b) is an absorbable suture. For the PVDF non-absorbable suture, a strong signal close to living tissue was detected and it appeared white. The PLA/CL absorbable composite suture appears black because it is close to air and the detected signal is rather weak . Due to differences in their own properties and production processes, fiber materials vary greatly in internal microstructure [19-21]. It was suggested that the light scattering coefficients are also different for that reason. Furthermore, even at week 0, white areas could be confirmed upon the surface layer of the absorbable sutures (red arrow). It is regarded as the reflected light of incident OCT light from the surface. And the white parts increased with the lapse of time (yellow arrow). In addition to the reflected light from the light source, we presumed that with the absorbable suture thread decomposes, the fragments of PLA/CL, crusts and scales from tissues are also included. Furthermore, strong reflected signal was detected inside the absorbable suture over time (green circle), which indicates that the molecular chains inside the fiber were unraveled, and the homogeneous crystal became amorphous, and the decomposition progressed. On the other hand, from week 9, a strong signal was also clearly detected on the surface of the non-absorbable suture (orange arrow), so we verified that the white areas on the surface of the sutures were the exudates from the surrounding skin tissues, which were mainly thought to be crusts and scales.
And from week 12 onwards, there were too many deposits around the sutures, so it gradually dropped and peeled off from the surface of the sutures, and voids (blue arrow) between sutures and white signal area appeared. It has been demonstrated that PVDF non-absorbable sutures have better antibacterial and antifouling properties than the PLA/CL absorbable sutures we used in this research. In addition, from the CCD camera photograph, it was confirmed that the color of the absorbable suture gradually matched from the third week, while the non-absorbable suture remained almost unchanged. Polymer materials such as PLA and PCL are originally transparent or translucent materials. The purple suture used in this experiment was found to be dyed. It was also proved that the absorbable suture was decomposed by dropping the dye .
Time Lapse Diameter Changing of Absorbable Sutures
As absorbable sutures degrade over time, in theory, the diameter of the fiber is thought to decrease 22). Therefore, we decided to calculate the suture diameter (Figure 3). Absorbable sutures were shown in blue and non- absorbable sutures shown in purple. The non-absorbable sutures were about 160 μm at week 18, which was almost the same as what it was at week 0 (p>0.05). On the other hand, the diameter of absorbable sutures was reduced from 220 μm to 180 μm (p>0.05), which indicated that the diameter changes due to the decomposition and absorption of the PLA/CL composite surgical sutures was confirmed.
In this study, by using OCT, we were able to observe and measure non-absorbable and absorbable sutures sutured to a hairless rat in vivo. We confirmed that even hydrophobic materials such as PVDF have deposits from the surrounding skin tissues on the surface, but they would partly drop off over time. Due to the degradation of absorbable sutures, amorphization of surgical sutures’ crystal happened and the signal of backscattered light and back reflected light inside the fiber increased, and reduction in diameter were also able to be evaluated. Through this research, we proved that even with ordinary OCT equipment, it is possible to measure minute level biomaterials and medical supplies, such as No. 6-0 sutures, which are several tens of μm in size. The measurement can be made not only for the body of the biological material, but also for the surrounding tissues. Furthermore, since OCT uses infrared wavelengths of light, it is possible to observe the reflection and scattering of light from the surface and internal structures of materials corresponding to that wavelength. In other words, it is strongly suggested that the use of OCT technology can also be able to analyze the internal structure of materials in in vivo research. In addition, now that the OCT equipment that can measure the refractive index is widely applied, it is considered possible to measure the refractive index change in the internal structure of the biomaterials like surgical sutures. From now on, by taking the advantage of high speed, high accuracy, and high resolution, we expect that we will be able to use OCT to make more accurate evaluations of in vivo animal experiment and medical materials under preclinical development. In the future, we aim to use an OCT device to measure the birefringence of surgical sutures, and to research not only surgical sutures but also artificial skin, hair regeneration, and percutaneous drug absorption.
The authors have no financial conflicts of interest to declare related to this study.