M.M.H. in de Braekt
Dento-alveolar development after modified palatal surgery on dogs.
A scientific essay in Medical Sciences
DOCTORAL THESIS defended in public on 25th of May 1992
In cleft palate surgery wound healing is thought to play an important role in the detrimental effects on maxillary growth and dento-alveolar development after surgery. Wound contraction might be an important factor but also the fibrous scar tissue and its attachment to the underlying bone could play a major role. Our investigation dealt with manipulation of wound healing in order to modify the scar tissue and to prevent its rigid attachment. Non-cleft Beagle dogs were used, in which a Von Langenbeck's procedure was simulated. Wound healing and scar tissue formation were manipulated in two separate ways. Firstly, poly-(L-lactic) acid membranes were implanted after surgery and secondly the wounds were treated with Low Level Laser Therapy. Wound healing was studied clinically, quantitatively and histologically. Wound contraction, as an important stage of wound healing, was also studied quantitatively. The maxillary arch dimensions in the deciduous and permanent dentition were measured to get more insight into the effect of both treatments on the development of the dentition.
Chapter 1 deals with clinical considerations which are of importance in this field of cleft palate research.
Chapter 2 describes general aspects of the process of wound healing. Wound healing is mainly divided in four stages; an inflammatory stage which is the initial response to injury; a proliferative stage, which results in the formation of extracellular matrix and collagen production and simultaneous epithelialization; a wound contraction stage in which the wound surface area is reduced; and a remodeling stage, in which the scar tissue is reorganized in structure and composition resulting in a firm and fibrous tissue.
Chapter 3 gives a review of the literature about animal investigations on maxillary growth after palatal surgery. Different approaches are discussed such as hard and soft tissue artificial palatal defects with and without surgical repair, and the artificially created soft tissue palatal defects with surgical repair. The creation of combined hard and soft tissue palatal defects results in an extensive trauma and leads to a substantial reduction in transversal maxillary growth. The clinical situation seems to be simulated in the most appropriate way by palatal surgery performed as soft tissue manipulation and not as osseous surgery. The experimental model used in the present study is based on the work of Wijdeveld and co-workers who introduced a symmetrical model in which denuded palatal bone was left on both sides of the palate. The wound healing process plays an important causal role in the adverse effects of surgery on maxillary growth and dento-alveolar development. Therefore, manipulation of wound healing such as redesigning surgical repair, grafting procedures, different timing of surgery, tissue expansion, biological dressings, biocompatible membranes, pharmacological agents and Low Level Laser Therapy could be beneficial to maxillary growth and development. However, controversial results of these techniques are found in literature. Finally, the aim of this study is formulated as to manipulate the wound healing and the formation of scar tissue after palatal surgery in Beagle dogs by means of the implantation of poly-(L-lactic) acid (PLLA) membranes and by the use of Low Level Laser Therapy.
Chapter 4 deals with the development of a PLLA membrane by means of an immersion precipitation process. The PLLA was purified by precipitation and films with a thickness of about 100μπ\ were cast. The membrane was characterized with respect to molecular weight, microstructure, and thermal properties.
Chapter 5 describes the effect of implantation of poly-(L-lactic) acid membranes on wound healing and wound contraction after palatal surgery in Beagle dogs. At 12 weeks of age a soft tissue defect was created in the median region of the palate. It was closed according to the Von Langenbeck technique, leaving two areas of denuded bone adjacent to the dental arch. In one group poly-(L-lactic) acid membranes were implanted immediately after surgery on the denuded bony areas. In the control group surgery was performed but no poly-(L-lactic) acid membranes were implanted. Wound contraction was quantified using tattoo points. Wound surface areas and weekly measurements of the distance between the tattoo points were calculated. It was found that clinical wound healing was significantly retarded. Wound contraction was independent of the presence of a membrane and it was restricted to the first two weeks after surgery. It was concluded that implantation of poly-(L-lactic) acid membranes following palatal surgery in Beagle dogs has no beneficial short term effects on wound healing and contraction.
Chapter 6 describes the effect of implantation of poly-(L-lactic) acid membranes on dento-alveolar development after palatal surgery . Poly-(L-lactic) acid membranes were implanted on the denuded bony areas either directly or 3 weeks after surgery. Dental casts were made at regular intervals until 25 weeks of age and maxillary arch dimensions were studied. Dental arch dimensions in the deciduous dentition of the experimental groups were not markedly disturbed, but after transition of teeth mainly transversal maxillary arch dimensions in the premolar region were reduced. It was concluded that implantation of poly-(L-lactic) acid membranes after palatal surgery in Beagle dogs does not prevent disturbances of dento-alveolar development.
Chapter 7 describes the histological investigation of implanted poly-(Llactic) acid (PLLA) membranes and the surrounding tissues. Delayed wound healing was found after direct implantation of PLLA membranes. After direct as well as after indirect implantation the membrane started to desintégrate soon after implantation and the remaining particles were surrounded by a fibroblastic sheath and a fibrous capsule. At sites where membrane fragments were left no clear formation of Sharpey's fibres was observed.
Chapter 8 describes the effect of Low Level Laser Therapy on wound healing and wound contraction after palatal surgery. In both experimental groups. Von Langenbeck's palatal repair was simulated. Then in one group the denuded bony areas were treated with Low Level Laser Therapy using a continuous Ga-As-AI laser beam (830 nm) and energy output set at 30 mW. Per treatment a dosage of 1 J/cm* wound surface area was used. Therapy was carried out three times a week with a total of ten treatments. In the other group no laser treatment was given. Wound healing was observed clinically until wound healing was completed at 4 weeks p.o. and wound areas were measured at regular intervals on standardized intra-oral photographs. Wound contraction was measured using tattoo points. No significant differences were found in the quality and rate of wound healing between the two experimental groups. The same held true for the wound contraction. It was concluded that Low Level Laser Therapy under conditions used in his study did not have an influence on wound closure or wound contraction on the macroscopical level.
Chapter 9 describes the effect of Low Level Laser Therapy on maxillary arch development after palatal surgery. Von Langenbeck's palatal repair was simulated in two groups of Beagle dogs. In one group, the denuded bony areas were irradiated as described in chapter 8. The animals of the other group received no laser treatment. Dental casts were made of all animals at regular intervals until 25 weeks of age. Maxillary arch dimensions were measured. Dental arch dimensions in the deciduous dentition of both groups were not disturbed by surgery, but after eruption of permanent teeth, transverse maxillary arch dimensions in the premolar region increased less in both operated groups than in control animals. It was concluded that Low Level Laser Therapy under the conditions used in this study does not decrease the adverse iatrogenic effects of palatal surgery on maxillary arch dimensions.
Chapter 10 describes the histological effects of Low Level Laser Therapy on wound healing after palatal surgery. In two groups of Beagle dogs, Von Langenbeck's palatal repair was simulated. In one group the denuded bony areas were treated with Low Level Laser Therapy as described in chapter 8. In the other group no laser treatments were carried out. Animals were sacrificed at the age of 1, 2, 7 and 13 weeks p.o. Serial transverse sections of the maxilla were studied histologically. Initially, the laser treated group showed a more fibrous type of healing tissue than the non-treated group, but later on tissues were comparable in both groups. Finally, in both groups the scar tissue lacked elastic fibres and was firmly attached to the underlying bone by means of Sharpey's fibres. Cervico-palatal resorption of alveolar socket bone indicated medial tipping of teeth. It is concluded that Low Level Laser Therapy under conditions used in his study does not have an effect on the final quality of scar tissue.
Chapter 11 discusses the results from the previous chapters. Clinical research and animal experiments are mentioned in relation to several kinds of research methods. Specifically, the use of Low Level Laser Therapy and biocompatible PLLA membranes are evaluated in comparison with other research methods. Finally, some suggestions were made with respect to future research.