Ligaplants: The Next Generation Implants

Introduction

The field of implant dentistry has become a vital and integral part of mainstream dentistry in this modern era. The developments apparent in the field of implant dentistry are attributed to a combination of reasons, such as:

•   long life span of individuals, 
•  various shortcomings associated with removable and fixed prostheses,
• benefits, and expected success associated with the use of implants.¹

The prolonged rate of survival of osseointegrated implants can be attributed to them becoming the most favourable implants in the present era. However, as the periodontal ligament (PDL) is absent around these implants, it impedes its functional capacity. PDL is a group of specialized connective tissue fibres that connect the tooth to the alveolar bone.² The periodontal tissue, which includes PDL, cementum and alveolar bone, has an important physiological role in bearing high occlusal loads, orthodontic tooth movement along with bone remodelling and ability to perceive destructive stimuli.² However, the very popular osseointegrated dental implants are not without certain drawbacks. To name a few,³ they: 

•  lack PDL attachment. § are ankylosed with the supporting jaw bone
• do not have the same functional mobility as natural teeth 
• lack shock-absorbing systems 
• lack sense of self-movement

These drawbacks necessitate the need for invention in dental implant structure that can concomitantly assure physiological and functional requirements. As a critical solution, bioengineering advancements need to be assembled to regenerate an artificial periodontium surrounding the implant. In the field of implantology, tissue engineering is an emerging concept. Tissue engineering is the creation of bio-artificial tissues by the alteration of cell growth and function by embedding suitable stem cells isolated from the donor tissue and other biocompatible scaffold materials. ⁴

A new approach to replace missing teeth has been introduced in recent times, i.e., tissue engineering or re-growth of the periodontal ligament around the dental implant. This combination of PDL cells with the implant biomaterial is known as Ligaplants.⁵ (Figure 1) This is a new emerging era in the field of dentistry where tissue engineered periodontal ligament cells are formed on the implant surface thus imitating the natural tooth and encompassing the desired properties of both the PDL and implant.

Tissue-engineering has unfolded a new and aspiring outlook that integrates knowledge from material chemistry with cell biology and medicine.⁶

Properties of ligaplants⁵

• Causes proprioception which is the sense of selfmovement and position of the body.

• Restores physiological tooth functions, which includes the capability of reacting to mechanical stress, dissemination of occlusal and masticatory forces and the ability to recognize harmful mechanical stimulation.

• Houses vital cells that are osteoconductive like osteoblasts, osteoclasts, fibroblasts, cementoblasts, and undifferentiated stem cells.

• Provides an attachment that is similar to natural teeth

How is a ligaplant made?⁷

The important elements needed for reconstruction and regeneration are as follows:

• Signals

• Cells

• Scaffolds The PDL cells are cultured on biodegradable scaffolds or matrix with the help of signalling molecules (Figure 2).

The PDL cells are cultured on biodegradable scaffolds or matrix with the help of signalling molecules (Figure 2).

Polysterene culture dishes containing N- isopropyl acylamide monomer and 2, propanolol solution are exposed to Area Beam Electron Processing System (ABEPS). The ABEPS is a system in which electrons of high energy are irradiated onto a material and a desired reaction is achieved. The residual monomer is removed by rinsing polysterene culture dishes with cold water, following which ethylene oxide is used to sterilize them. Periodontal ligament cells are scraped off from the middle third of an extracted tooth using a scalpel and the cells are inoculated in culture dishes containing Dulbecco’s Modified Eagle’s Minimal Essential Medium supplemented with 100 units/ mL of penicillin, streptomycin and 10% foetal bovine. These cells are cultured in an environment of 5% CO2 at 37°C for 48 hours so that the cells get attached to the dishes. The medium is changed thrice a week. The periodontal ligament cells sheet is harvested on temperature responsive culture dishes at 37°C and at a cell density of 1x10⁵ , thus forming a PDL cell suspension. An implant is then placed inside a hollow plastic cylinder with a 3 mm space around the implant. These plastic cylinders are then seeded with the periodontal ligament cell suspension under a stream of growth medium for a duration of eighteen days for regeneration of the PDL cells around the implant. (Figure 2, 3)

Precautions during preparation

1. Sterilization should be maintained during the procedure.⁹

2. Proper culturing and cell growth is essential, or else it might lead to the development of non-periodontal ligament cell types.

3. Micromechanical movements of the growth medium is needed for firm attachment of the cells to the implant.

4.  Adequate duration of surface treatment must be maintained for the success of ligaplant.¹⁰

Advantages¹¹

1. Reduction of gingival recession and bone defects.

2. Imitates natural insertion of tooth roots in alveolar bone.

3. Despite the initial fit being loose in order to provide PDL cell cushion, the cells and the implant surface get resolutely unified without any form of direct bone contact.

4. Induction of new bone formation

5. Unbroken contact between the bone and implant surface.

6. Transmission of occlusal forces between bone and teeth.

7. Bone remodelling capacity (the presence of the PDL maintains/regenerates a good quality of bone) and the PDL offsets lateral and vertical tooth wear during the course of life.

Disadvantages¹¹

1. Caution should be taken while culturing ligaplants. Temperature is critical to the success of the procedure. There are also certain other factors such as the cells used for culturing, the duration of the cultu etc, that needs to be closely and accurately monitored. If errors occur during the process, certain non-periodontal ligament cells may develop leading to failure of the ligaplant.

2. Due to limited facilities available, creating ligaplants can be expensive.

3. Failure may also result from a poor host response to accept the implant or there could be a failure of the PDL cells to get induced.

4. Non-PDL cell types can be seen sometimes due to the prolonged cell culturing 

Discussion

Dental implants are positioned with the goal of achieving osseointegration. This has been carried out without any real contemplation to the redevelopment of PDL around the implant.12 Due to the absence of natural periodontal tissue, osseointegrated implants that are directly attached to the adjoining alveolar bone does not provide the same function as the natural tooth. Hence, it is essential to develop a well-designed dental implant along with a tissue engineered PDL, which in turn creates a biological connection that is capable of certain physiological functions.

Ligaplants, a combination of PDL cells with implant biomaterial seems to be a boon in implant dentistry. Regardless of significant advantages of ligaplants as replacement options over the use of dental implants alone, there still remains a question about whether ligaplants are going to be a reality in the imminent future or will it just be a fiction.

Continuous research is essential to find out which treatment options work the best for individual patients. The new concept of ligaplants is currently under research and there are various aspects which need to be delved into, such as:

• How successful is it going to be clinically? 
• Which type of integration takes place between the ligaplant and the surrounding bone? 
• Is it viable for clinics or institutions who do not have an attached laboratory to it?
• How cost effective would it be? 
• Does it require a separate protocol for the placement?

Research that enhances bioengineering methods in combination with existing dental implantology procedures, obtainable stem cells and their clinical application in large animal models will be further needed.

A study conducted by Nyman S et al., in 1982 confirmed that the periodontal ligament cells have potential to re-establish attachment of the connective tissue onto the teeth surface.¹³ In 1990, Buser D et al., concluded in his study that titanium implants placed in place of retained root tips showed that implants had PDL formation on their surface along with a cementum layer.¹⁴ Gault P et al., in 2010 described the application of periodontal ligament cells cultured on the titanium pin surface and placement of it in the alveolar process. There was an integration of the titanium pins with the alveolar bone as well as the development of new bone in the vicinity.¹⁵ In a study conducted in 2011 by Y Lin et al., it was confirmed that dental progenitor cells on dental implants revealed the presence of PDL on the surface of dental implants. Kiong et al., in 2014 in his review had mentioned ligaplants as tooth replacement having decisive advantages over dental implants and the surgery being moderately simple.⁶

Ligaplants are still a new treatment modality that has however shown good results in animal studies. Nevertheless, they are yet to be tried in humans. 

Numerous fruitful experiments have been conducted to create periodontio-integrated implants that can maintain the form, function and proprioceptive responses which would be similar to a natural tooth. Based on these solid evidences, the likelihood of the future clinical use of ligaplants can be stated strongly, which in turn would metamorphose implant dentistry. It is imperative to keep in mind that the growth of the periodontal ligament cannot be predicted in a particular desired site.¹⁶ 

Conclusion

The advent of periodontal ligament regeneration has revolutionized implant dentistry. This transformatory approach to develop periodontiointegrated implants - the ligaplants, opens up novel opportunities for the prosthodontist and offers limitless possibilities of employing manmade, biological tooth alternates that could be provided to serve as hybrid material living oral implants. However, viable and predictable method for producing perio-dontio integrated implants has not yet been innovated. Ligaplants as replacement options appears to be the future in implant dentistry. Further clinical studies and research with long term follow-up could only substantiate the reliability and victory of periodontio-intergrated implants.