The Experimental Study On Porous Poly-DL-lactic Acid Medical Film In Anti-adhesion Of Peripheral Nerve

Background and objective:With the deveolpment of the industry, the number of patients with peripheral nerveinjury are increasing. However, the recovery of neural function is not ideal, which causessuch as muscle atrophy, joint disfunction, loss of sensory function and other complications.These disfunctions have caused severe physical and mental trauma to patients. Accordingthe researching, the problems of the nerve fibers’ regeneration and functional recovery arethe keys to the repair of injuried peripheral nerve. Unfortunately, these problems are verydifficult to be solved for the medical community. Recently, the scientists focused on the scartissue around nerve and local adhesion of nerve transection, because the researching resultsindicate that the scar and adhesion influent the recovery of injuried nerve very well. Atpresent, the clinical treatments and experimental study are focusing on local injection,autologous or allogeneic biological conduit(film), the application of non degradable materialsas physical barriers, and the use of degradable materials as barriers.Dexamethasone, prednisone, sodium hyaluronate, chitosan are the frequently-used drugswhich are injected into the nerve anastomosis site. The local administration can solve some ofthe adhesion problems. But the metabolism and absorption of those drugs are very fast. Thoseare also flowing from the operation incision easily. Therapeutic effect is unsatisfactory. Veinis another choice as repair measures after nerve injury.It can establish a beneficialenvironment for nerve regeneration. But the autologous vein requires an additionaloperation and position and range of materials are limited.Biodegradable medical polymermaterials, such as chitosan, chitin, cellulose are natural materials. The nerve conduit(film)ismade from them is considered as the suitable biomaterial to prevent nerve adhesion.However, in some circumstance, those natural materials may result in allergenic reactions orimmunologic rejection reaction. Others shortcomings of those are degradation time, fragility,poor permeability and compatibility. The poly-DL-lactide absorbable medical film (PDLLA) is researched and developed bythe Chinese Academy of Sciences, Changchun Institute of Applied Chemistry and Changchunsinobiomaterials Co.Ltd. Through the special phase transformation method, PDLLA medicalfilm is made from the biodegradable poly lactic acid polymermaterial. This kind of film hasmicroporous that can offer the channel for the neurotrophic factors’transformation, which willbe beneficial to the nerve regeneration. At the same time, the film can prevent the scarproliferation as a physical barrier. The medical film with porous structure is more flexible andbetter mechanical properties, compared with the non porous membrane products. At the sametime, the shorter degradation time is another advantage. The research institute have finishedthe preliminary physicochemical determination of PDLLA medical film. In this experiment,we will arrange a series of studies to clarify its biocompatibility, the effects of anti adhesionand nerve regeneration and do the preparatory work for clinical application. The studiescomprise in vitro biological test and in vivo rats test.Methods and results:In vitro tests, we perform rabbit skin irritation test, cytotoxicity test and micronucleustest of polychromatic erythrocyte inbone marrow. In the skin irritation test, the results showthat the reaction of the rabbits’ skinto the PDLLA leach liquor was "no".In other words,PDLLA medical film has no irritation to rabbit’s skin. In the cytotoxicity experiment, we usedthe MTT method to detect the cell toxicity of PDLLA. The results showed that12.5%,25%,50%,100%of PDLLA leach liquor extraction has no effect to the solution on Verocells’ proliferation. That is to say, PDLLA medical membrane has no toxicity to the cells andit is a kind of non-toxic biological material.In the micronucleus test, the micronucleus rate is as high as49.7‰in cyclophospha-mide group, which is20times of that in PDLLA group. So, the study indicated thatPDLLA medical film has no genetic toxicity in mice.In vivo chronic toxicity test in rats, we implant the PDLLA medical film between thesciatic nerve and muscle. In order to assess the chronic toxic reaction of it, we have routinelood test, serological detection and the detection of pathology in important organs atpostoperative1W,4W,12W. Test results show that counts of the white blood cell and red cell don’t been significantly changed, and the counts of hemoglobin and platelet also do notbeen changed obviously. So, implantation of PDLLA in rats has no effect on the immunesystem and hematopoietic system. Results of serological examination illustrate that the liverand renal functions of rats aren’t different from the negative control group at each time point.The material doesn’t damage the liver and kidney. The same conclusion is certified by thepathological examinations of the important organs, liver, kidney, pancreas and spleen. All thetests above mentions explain that PDLLA medical film possess good internal bio-compatibi-lity in observation period.In the study on anti-adhesion of peripheral nerve, we wrap PDLLA medical film aroundthe anastomosis where the apocoptic sciatic has been sutured. At the same time, we alsoestablish the bio-membrane group. We evaluate the histo-compatibility and effect of anti-adhesion of PDLLA by three aspects as followed:(1)To observe whether the operationalincision heal as expected; how long does the plantar ulcer heal; how much adhesions areobserved around the sciatic nerve after postoperative8W,12W.(2) To exam compound muscleaction potential (CMAP), the distal latent period (dLAT) and the amplitude of sciatic nerve bythe electromyography at postoperative8W. To measure relative wet weight of Anterior tibialmuscle adn gastrocnemius muscle. we will evaluate the movement function of the lowerlimb after regeneration of sciatic nerve through the data mentioned above;(3) To observe thepathological slices of the sciatic nerve at12W postoperatively, we can get the informationabout whether the nerve has regenerate effectively. The experimental results show that:There is no incision infection, no sinus, no foreign body reaction in the PDLLA group; Thehealing time of plantar ulcer is8.5weeks. It is earlier than the direct suture group. There isless connective tissue adhesion around nerve in PDLLA medical film and bio-membranegroup than the negative group. The results of EMG (cMAP, dLAT, MNCV) are better in thePDLLA group than other two groups. The PDLLA film and bio-membrane degradate mainly.Histological examination indicated that nerve fiber regeneration is better than the other twogroups. All in all, PDLLA film has positive effect on anti-adhesion and promoting nerveregeneration. ConclusionsPDLLA, as a member of the family of poly lactic acid polymer, is an amorphouspolymer, because the asymmetric carbon atoms are arranged in irregular style. Tm of it is65℃. The degradation and absorption of it is faster than other PLA materials, generally3-6months. Compared with non porous membrane, PDLLA has not only good mechanicalproperties but also excellent flexiblity and short degradation time because of the special theporous structure. Film around the nerve provide a relative closed conduit for injuried nerve,which is the tunnel for fiber’s growth. The tunnel also can prevent the proliferation ofconnective tissue ingrowth and outflowing of the nerve growth factor.It can create a betterenvironment for the nerve. Meanwhile, compared with other nonpermeable materials, theporous structure permit the exchange of tissue fluid at the two sides of the film.In summary, the porous PDLLA absorbable medical film has no irritation, no celltoxicity and genetic toxicity, no effects on hematopoietic and immune system, no damage toliver, kidney, spleen andpancreas. It can prevent nerve adhesion and promote the recovery ofneurological function. The degradation performance is good. So it has a good prospect inclinical application.