Advantage of Alveolar Ridge Augmentation with Bioactive/Bioresorbable Screws Made of Composites of Unsintered Hydroxyapatite and Poly-L-lactide

Investigation of the appropriate thread depth for bioabsorbable screws

The relatively low strength of bioabsorbable screws is a critical clinical issue. A shallower thread depth will increase a screw's strength, but the pull-out strength of the screw will decrease proportionally with the thread depth. We sought to provide further clarification of the relationships between (i) the thread depth and the pull-out strength, and (ii) the minor diameter and the shearing and bending strengths in bioabsorbable screws made of uncalcined and unsintered hydroxyapatite particles and poly-L-lactide (u-HA/PLLA). Seven types (thread depth from 0.1-0.7 mm) of screws with a major diameter of 4.5 mm were manufactured. Each screw type's pull-out strength was investigated using simulated bone. A shearing test and three-point bending test were both used to measure the physical strength of the screws. We then analyzed the relationships between the mechanical findings and the thread depth. The relationship between the thread depth and the pull-out strength showed a positive biphasic linear correlation with a boundary at 0.4-mm thread depth. The relationships between the minor diameter and both the shearing and bending strengths showed positive linear correlations within the range of dimensions tested. Within the scope of this study, a 0.4-mm thread depth proved to be an appropriate value that provides sufficient pull-out strength and screw strength for u-HA/PLLA screws with a 4.5-mm major diameter.

A Narrative Review of u-HA/PLLA, a Bioactive Resorbable Reconstruction Material: Applications in Oral and Maxillofacial Surgery

The advent of bioresorbable materials to overcome limitations and replace traditional bone-reconstruction titanium-plate systems for bone fixation, thus achieving greater efficiency and safety in medical and dental applications, has ushered in a new era in biomaterial development. Because of its bioactive osteoconductive ability and biocompatibility, the forged composite of uncalcined/unsintered hydroxyapatite and poly L-lactic acid (u-HA/PLLA) has attracted considerable interest from researchers in bone tissue engineering, as well as from clinicians, particularly for applications in maxillofacial reconstructive surgery. Thus, various in vitro studies, in vivo studies, and clinical trials have been conducted to investigate the feasibility and weaknesses of this biomaterial in oral and maxillofacial surgery. Various technical improvements have been proposed to optimize its advantages and limit its disadvantages. This narrative review presents an up-to-date, comprehensive review of u-HA/PLLA, a bioactive osteoconductive and bioresorbable bone-reconstruction and -fixation material, in the context of oral and maxillofacial surgery, notably maxillofacial trauma, orthognathic surgery, and maxillofacial reconstruction. It simultaneously introduces new trends in the development of bioresorbable materials that could used in this field. Various studies have shown the superiority of u-HA/PLLA, a third-generation bioresorbable biomaterial with high mechanical strength, biocompatibility, and bioactive osteoconductivity, compared to other bioresorbable materials. Future developments may focus on controlling its bioactivity and biodegradation rate and enhancing its mechanical strength.

Biological Effects of Bioresorbable Materials in Alveolar Ridge Augmentation: Comparison of Early and Slow Resorbing Osteosynthesis Materials

The purpose of this study was to investigate the bone healing properties and histological environment of a u-HA/PLLA/PGA (u-HA—uncalcined and unsintered hydroxyapatite, PLLA—Poly L-lactic acid, PGA—polyglycolic acid) composite device in humans, and to understand the histological dynamics of using this device for maxillofacial treatments. Twenty-one subjects underwent pre-implant maxillary alveolar ridge augmentation with mandibular cortical bone blocks using u-HA/PLLA or u-HA/PLLA/PGA screws for fixation. Six months later, specimens of these screws and their adjacent tissue were retrieved. A histological and immunohistochemical evaluation of these samples was performed using collagen 1a, ALP (alkaline phosphatase), and osteocalcin. We observed that alveolar bone augmentation was successful for all of the subjects. Upon histological evaluation, the u-HA/PLLA screws had merged with the bone components, and the bone was directly connected to the biomaterial. In contrast, direct bone connection was not observed for the u-HA/PLLA/PGA screw. Immunohistological findings showed that in the u-HA/PLLA group, collagen 1a was positive for fibers that penetrated vertically into the bone. Alkaline phosphatase was positive only in the u-HA/PLLA stroma, and the stroma was negative for osteocalcin. In this study, u-HA/PLLA showed a greater bioactive bone conductivity than u-HA/PLLA/PGA and a higher biocompatibility for direct bone attachment. Furthermore, u-HA/PLLA was shown to have the potential for bone formation in the stroma.

An overview of polyester/hydroxyapatite composites for bone tissue repairing

Objectives

The polyester/hydroxyapatite (polyester/HA) composites play an important role in bone tissue repairing, mostly because they mimic the composition and structure of naturally mineralized bone tissue. This review aimed to discuss commonly used geometries of polyester/HA composites, including microspheres, membranes, scaffolds and bulks, and their applications in bone tissue repairing and to discuss existed restrictions and developing trends of polyester/HA.

Methods

The current review was conducted by searching Web of Science, and Google Scholar for relevant studies published related with polyester/HA composites. Selected studies were analyzed with a focus on the fabrication techniques, properties (mechanical properties, biodegradable properties and biological properties) and applications of polyester/HA composites in bone repairing.

Results

A total of 111 articles were introduced to discuss the review. Different geometries of polyester/HA composites were discussed. In addition, properties and applications of polyester/HA composites were evaluated. The addition of HA into polyester can adjust the mechanical and biodegradability of composites. Besides, the addition of HA into polyester can improve its osteogenic abilities. The results showed that polyester/HA composites can ideal candidate for bone tissue repairing.

Conclusion

Polyester/HA composites have many remarkable properties, such as appropriate mechanical strength, biodegradability, favorable biological properties. Diverse geometries of polyester/HA composites have been used in bone repairing, drug delivery and implant fixation. Further work needs to be done to investigate existed restrictions, including the controlled degradation rate, controlled drug release performance, well-matched mechanical properties, and novel fabrication techniques.

The translational potential of this article

The present review reveals the current state of the polyester/HA composites used in bone tissue repairing, contributing to future trends of polyester/HA composites in the forthcoming future.

Bone Coverage around Hydroxyapatite/Poly(L-Lactide) Composite Is Determined According to Depth from Cortical Bone Surface in Rabbits

Composites of unsintered hydroxyapatite (HA) and poly(_L-lactide) (PLLA) reinforced by compression forging are biodegradable, bioactive, and have ultrahigh strength. However, foreign body reactions to PLLA and physical irritation can occur when not covered by bone. We aimed to confirm the relationships between the depth of the implanted HA-PLLA threaded pins and the new bone formation. We inserted HA-PLLA composite threaded pins (diameter: 2.0 or 4.5 mm) into the femoral and tibial bones of 32 mature male Japanese white rabbits (weight 3.0–3.5 kg) with the pin head 1 or 0 mm below or protruding 1 or 2 mm above surrounding cortical bone. Eight euthanized rabbits were radiologically and histologically assessed at various intervals after implantation. Bone bridging was complete over pins of both diameters at ~12 weeks, when inserted 1 mm below the surface, but the coverage of the pins inserted at 0 mm varied. Bone was not formed when the pins protruded >1 mm from the bone surface. No inflammation developed around the pins by 25 weeks. However, foreign body reactions might develop if composites are fixed above the bone surface, and intraosseous fixation would be desirable using double-threaded screws or a countersink to avoid screw head protrusion.

Bone Regeneration Capacity of Newly Developed Uncalcined/Unsintered Hydroxyapatite and Poly-l-lactide-co-glycolide Sheet in Maxillofacial Surgery: An In Vivo Study

Uncalcined/unsintered hydroxyapatite and poly-l-lactide-co-glycolide (u-HA/PLLA/PGA) is a new bioresorbable nanomaterial with superior characteristics compared with current bioresorbable materials, including appropriate mechanical properties, outstanding bioactive/osteoconductive features, and remarkably shorter resorption time. Nevertheless, the bone regeneration characteristics of this nanomaterial have not been evaluated in maxillofacial reconstructive surgery. In this study, we used a rat mandible model to assess the bone regeneration ability of u-HA/PLLA/PGA material, compared with uncalcined/unsintered hydroxyapatite and poly-l-lactide acid (u-HA/PLLA) material, which has demonstrated excellent bone regenerative ability. A 4-mm-diameter defect was created at the mandibular angle area in 28 Sprague Dawley male rats. The rats were divided into three groups: u-HA/PLLA/PGA (u-HA/PLLA/PGA graft + defect), u-HA/PLLA (u-HA/PLLA graft + defect), and sham control (defect alone). At 1, 3, 8, and 16 weeks after surgeries, the rats were sacrificed and assessed by micro-computed tomography, histological analysis with hematoxylin and eosin staining, and immunohistochemical analyses. The results confirmed that the accelerated bone bioactive/regenerative osteoconduction of u-HA/PLLA/PGA was comparable with that of u-HA/PLLA in the rat mandible model. Furthermore, this new regenerative nanomaterial was able to more rapidly induce bone formation in the early stage and had great potential for further clinical applications in maxillofacial reconstructive surgery.

Biomechanical Loading Comparison between Titanium and Bioactive Resorbable Screw Systems for Fixation of Intracapsular Condylar Head Fractures

Osteosynthesis resorbable materials made of uncalcined and unsintered hydroxyapatite (u-HA) particles, poly-L-lactide (PLLA), are bioresorbable, and these materials have feasible bioactive/osteoconductive capacities. However, their strength and stability for fixation in mandibular condylar head fractures remain unclear. This in vitro study aimed to assess the biomechanical strength of u-HA/PLLA screws after the internal fixation of condylar head fractures. To evaluate their biomechanical behavior, 32 hemimandible replicas were divided into eight groups, each consisting of single-screw and double-screw fixations with titanium or u-HA/PLLA screws. A linear load was applied as vertical and horizontal load to each group to simulate the muscular forces in condylar head fractures. Samples were examined for 0.5, 1, 2, and 3-mm displacement loads. Two screws were needed for stable fixation of the mandibular condylar head fracture during biomechanical evaluation. After screw fixation for condylar head fractures, the titanium screws model was slightly more resistant to vertical and horizontal movement with a load for a small displacement than the u-HA/PLLA screws model. There was no statistically significant difference with load for large displacements. The u-HA/PLLA screw has a low mechanical resistance under small displacement loading compared with titanium within the limits of the mandibular head fracture model study.

Deep Neural Networks for Dental Implant System Classification

In this study, we used panoramic X-ray images to classify and clarify the accuracy of different dental implant brands via deep convolutional neural networks (CNNs) with transfer-learning strategies. For objective labeling, 8859 implant images of 11 implant systems were used from digital panoramic radiographs obtained from patients who underwent dental implant treatment at Kagawa Prefectural Central Hospital, Japan, between 2005 and 2019. Five deep CNN models (specifically, a basic CNN with three convolutional layers, VGG16 and VGG19 transfer-learning models, and finely tuned VGG16 and VGG19) were evaluated for implant classification. Among the five models, the finely tuned VGG16 model exhibited the highest implant classification performance. The finely tuned VGG19 was second best, followed by the normal transfer-learning VGG16. We confirmed that the finely tuned VGG16 and VGG19 CNNs could accurately classify dental implant systems from 11 types of panoramic X-ray images.