Analysis of micromovements and peri-implant stresses and strains around ultra-short implants - A three-dimensional finite-element method study.

BACKGROUND: Success of an implant depends on its placement in the bone and how well the stress and strain are distributed to the surrounding structures when occlusal force is applied to it. The size and shape of the implant plays an important role is the formation and distribution of stress and strains in the periodontium. Von Mises stresses and micromovements need to be evaluated while placing implants in D4 bone quality regions for a higher success rate. AIM: To evaluate the peri-implant Von Mises stresses, strains, and micromovements distribution in D4 bone quality around ultra-short implants of 5 mm length with varying diameters of 4 mm, 5 mm, and 6 mm.
MATERIALS AND METHODS: The finite element method was employed to make models replacing maxillary molars in D4 type bone that was missing. Implants that could be classified as ultrashort (5 mm) were used. These implants were of varying diameters of 4, 5, and 6 mm. In each model, the implant was subjected to a force of 100 N and analyzed. The force was applied in an oblique (45 degrees) and vertical direction (90°) to the long axis of the tooth. The models were made such that they simulated cortical and cancellous anisotropic properties of the bone. The models were then analyzed using the program ANSYS workbench version 12.1.
RESULTS: When all the three diameters were compared wide diameter, i.e., 6 mm threads had the least values of peri-implant von Mises stresses, strains, and micro-movements around them. When thread shapes were taken into consideration square micro thread created the most favorable stress parameters around them with minimum values of stress, strains, and micromovements.
CONCLUSION: Ultrashort implants combined with a wide diameter and platform switched can be used in atrophic ridges or when there is a need for extensive surgery to prepare the implant site. Copyright:

INTRODUCTION

Rehabilitation of partially or edentulous patients using dental implants is now considered an effective and predictable alternative for the treatment. The success of the implant restoration is evaluated from the esthetic and mechanical perspectives. Both depend on the degree and integrity of the bond at the implant-bone interface, i.e., osseointegration.[1] The factors crucial for implant osseointegration are surgical technique, implant morphology, host, the surface of the implant, biocompatibility of the material used, and loading conditions.[2] To achieve optimal osseointegration and hence implant success the choice of an implant to be restored in a particular case is extremely important which depends on the type of edentulism, the remaining bone volume, the availability of space for the prosthetic reconstruction, the emergence profile as well as the type of occlusion.[3] However, implant placement can be limited due to the situations of either reduced bone height or presence of anatomical structures, such as the extensive maxillary sinus pneumatization and mandibular canal proximity to tooth sockets.[45]

A short and wide-diameter implant has been suggested to restore tooth loss in the posterior region, where the dimension of the alveolus is greater than the diameter of a standard implant (3.75 mm).[6] The advantage of short implants over regenerative techniques with conventional implants is a low cost, shorter treatment span, simplicity, and lesser risk of complications. Implants with lengths varying from 5 to 8 mm are currently used, and there are only a few short-term comparative studies reliably evaluating their efficacy. The preliminary results of two randomized controlled clinical trials suggest that implants 7–8 mm long can be a better alternative to augmentation procedure.[78]

”Short” endosseous root-form dental implants are those that have a “designed intrabony length” (i.e., implant length to establish and maintain stability and osseointegration) 7–8 mm.[910] The implant used was sintered porous-surfaced implant which has a designed intrabony length of 5 mm; therefore, it was given the designation “ultrashort” by the authors.[11] The most obvious indication for an ultrashort dental implant is the severely resorbed posterior mandible where proximity to the mandibular neurovascular bundle may preclude the use of longer implant designs without aggressive, sometimes high-risk, preimplantation site preparations such as autogenous block grafting, guided bone regeneration, or nerve repositioning.[121314] Thread the shape and design of the implant are the other crucial factors in determining the biomechanical stability of the implants. Maximum initial contact, improved initial stability, a larger surface area, and dissipation of stresses are achieved by the threads present. Evaluation of the thread design is imperative to enhance further clinical success.[15]

For the implant to osseointegrate, adequate bone quantity (height, width, and shape), as well as adequate density, are essential. Bone density is a crucial factor in determining the treatment plan, design of the implant to be used, surgical technique, the time required in healing, and initial progressive bone loading during prosthetic reconstruction. Higher failure rates have been reported for the regions, with poor quality bone, for example, posterior maxilla or type IV bone. Consequently, it has been suggested that to increase the success in cases of poor bone quality, a certain modification must be carried out. These include the changes in implant design and implant surfaces, thereby obtaining superior anchorage and a greater surface area of the load to decrease stress to the softer bone type. D4 (Type 4) bone is the bone which is a thin layer (1 mm) of the cortical bone surrounding a core of low-density trabecular bone. D4 bone has little cortical bone thickness and minimal internal strength. Increased clinical failure rates are observed in poor quality, porous bone as compared with more dense bone.[161718] To decrease the stresses in D4 bone quality increase in the number of implants or an implant design with the greater surface area can be considered.[192021222324252627]

The pattern of stress distribution from the implant to the surrounding bone is one of the major determinants of the success of the implant. The pattern of transfer of load across the implant-bone interface depends on the numerous factors including the bond at the interface, the type of loading, and the design characteristics of the implant and its surface. For determining how implant mobility, often referred to as micromotion, relative motion, micromovement, etc., or implant loading affects bone response, a closer look at implant deformation, bone deformation as well as stress or strain at the implant-bone interface is required. Von Mises stress is a value that has been used to determine if the given material will yield or fracture under stress of force. If the Von Mises stress of an implant under load is equal or greater than its yield limit then the implant may yield. To date, no studies have been carried out comparing the stresses, strains, and micromovements around platform switched wide diameter ultrashort implants of 5 mm length with diameters of 4, 5, and 6 mm with acme, buttress, square, and triangle (V) threads placed in D4 bone quality. Hence, this study was aimed to evaluate the micro-movement and peri-implant Von Mises stresses and strains around ultrashort implants in D4 bone quality.

MATERIALS AND METHODS

The implant designs were to replace a missing maxillary molar area having a uniform length of 5 mm and diameters of 4 mm, 5 mm, and 6 mm. The dimensions of the maxillary first molar were used to determine crown dimensions. Three-dimensional models were fabricated and modeled by determining the exact location of nodes after mathematical calculation [Table 1]. The implant design was such that it had a 5 mm length incorporating acne, square, buttress, and triangle (V) thread with 0.2-mm pitch and 0.1-mm height up to half of the length of the implant and after half of the length, 0.5 mm pitch and 0.4 mm height [Figure 1]. Three different diameter implants were used in the study 4 mm, 5 mm, and 6 mm. A (5°) taper was given to the screw and a thread helix angle of 5° proposed. Abutment height of 5 mm with 10% platform switching. All models were developed to support a first permanent maxillary molar with a porcelain fused to metal crown of-occlusal porcelain thickness 1.5 mm, cervically 0.7 mm, and inner metal thickness 0.7 mm in a 25 mm × 25 mm block of the bone. The dimensions of the bone block are 30 mm height and thickness of 0.5 mm. Cortical and cancellous anisotropic properties were applied to the bone [Table 2]. Every one of the models was analyzed when subjected to a force magnitude of 100 N and with the force of direction applied in the vertical direction (90°) and oblique direction (45°) to the long axis of the tooth under delayed loading conditions [Figure 2]. A master chart was created in the excel sheet, and all the values were calculated using ANSYS 12.1 modeling analyses software. All values were compared with each other in terms of higher and lower values.

Table 1

Incorporated nodes and elements of the developed models

Name	Diameter	Nodes	Elements
Acme	4 mm	190,675	135,455
Buttress		327,053	240,158
Square		250,623	184,087
ZTriangle		364,473	268,757
Acme	5 mm	225,554	161,138
Buttress		262,926	188,359
Square		261,958	188,287
Triangle		284,587	205,365
Acme	6 mm	250,953	184,084
Buttress		452,127	333,318
Square		180,345	131519
Triangle		413,656	305,049

Figure 1

Acme, buttress, square, and triangle thread models

Table 2

Material properties

Material	Young’s modulus (E MPa)	Poisson ratio (V)	Shear modulus (GMPa)
Cortical bone	E x 12,600	V xy 0.300	G xy
		V yz 0.253	4850
	E y 12,600	V xz 0.253	G yz
		V yx 0.300	5700
	E z 19,400	V zy 0.390	G xz
		V zx 0.390	5700
Trabecular bone	E x 1148	V xy 0.055	G xy 68
		V yz 0.010
	E y 210	V xz 0.322	G yz 68
		V yx 0.010
	E z 1148	V zy 0.055	G xz 434
		V zx 0.322
Titanium	110,000	0.350
Porcelain	70,000	0.190
Cement	12,000	0.25
Cobalt chromium metal	87,900	0.30

MPa – Megapascal

Figure 2

Loading conditions – 100N of force in the vertical and oblique direction to the long axis of the tooth

RESULTS

Under vertical and oblique loading, for the 4 mm diameter implant-square thread had the least amount of peri-implant stresses but had the maximum stresses, strains, and micromovements as compared with the others, i.e., 5 and 6 mm diameters [Table 3 and Figure 3]. Under vertical and oblique loading, for the 5 mm diameter implant, the square thread had the least peri-implant stresses as compared to the other threads [Table 4 and Figure 4]. Under vertical and oblique loading, for the 6 mm diameter implant, the square thread has the least peri-implant stresses and von Mises stresses around them and they had the least amount of peri-implant stresses, strains, and micromovements as compared to other threads, i.e., 4 and 5 mm diameters [Table 5 and Figure 5].

Table 3

Values at vertical and oblique loads of 100N force under delayed loading of 4 mm implants

	Acme	Buttress	Square	Triangle
Vertical load
Stress in MPa	364.323	468.876	324.03	742.181
Strain in microstrains	0.02292	0.026922	0.022607	0.06813
Micromovements in μm	0.044296	0.043488	0.043107	0.044073
Oblique load
Stress in MPa	573.184	737.888	498.17	847.554
Strain in microstrains	0.034169	0.039355	0.03344	0.116504
Micromovements in μm	0.089359	0.086522	0.086761	0.089492

MPa – Megapascal

Figure 3

Von Mises stress, strains in microstrains and micromovements of 4 mm diameter implant. MPa – Megapascal

Table 4

Values at vertical and oblique loads of 100N force under delayed loading of 5 mm implants

	Acme	Buttress	Square	Triangle
Vertical load
Stress in MPa	171.626	203.599	148.969	193.466
Strain in microstrains	0.024765	0.037392	0.019413	0.022632
Micromovements in μm	0.030063	0.030105	0.029166	0.031638
Oblique load
Stress in MPa	244.884	305.348	220.351	285.424
Strain in microstrains	0.033171	0.051952	0.028128	0.029625
Micromovements in μm	0.036813	0.036633	0.035588	0.038195

MPa – Megapascal

Figure 4

Von Mises stress, strains in microstrains and micromovements of 5 mm diameter implant. MPa – Megapascal

Table 5

Values at vertical and oblique loads of 100N force under delayed loading of 6 mm implants

	Acme	Buttress	Square	Triangle
Vertical load
Stress in MPa	169.057	93.571	89.872	126.63
Strain in microstrains	0.013926	0.017863	0.013525	0.019977
Micromovements in μm	0.022452	0.023193	0.022243	0.02325
Oblique load
Stress in MPa	222.283	214.805	202.725	280.849
Strain in microstrains	0.019468	0.028622	0.019036	0.027753
Micromovements in μm	0.027416	0.028511	0.026832	0.028402

MPa – Megapascal

Figure 5

Von Mises stress, strains in microstrains and micromovements of 6 mm diameter implant. MPa – Megapascal

DISCUSSION

Digital elevation model, photoelasticity, and strain measurements on bone surfaces are the mechanical tools that are used for the biomedical purposes. These techniques have certain limitations such as complexities in modeling and difficulties in modifications after modeling. Finite element method (FEM) has an advantage over other methods because of its ease in application and user-friendly software. Geng et al.[28] in their study concluded that the bonded interface predicted uniform distribution of stresses around the implant through the cortical bone and the displacement load ratio was close to the actual implant specimen. Subsequently, after his pioneering study, FEM is routinely employed in the various facets of dentistry to date and is rapidly becoming a useful tool to study and pre-empt the effects of stresses on implants and surrounding bone.

The current study employs FE M to evaluate the peri-implant Von Mises stresses, strains, and micromovements around ultrashort implants. Apart from the purely mechanical aspect as demonstrated in the current study, various biologic factors also play an important role in the process of osseointegration and subsequent success of any implant. Raghavendra et al.[29] stated that the healing period after the implant placement starts with the adherence of serum proteins followed by attachment and proliferation of mesenchymal cells. Consequently, osteoid is formed which is later mineralized. From this onwards, bone remodeling occurs as an adaptation to the implant's environment. With these processes occurring simultaneously to mechanical loading, the interaction of both mechanical and biologic factors is critical and should be considered. Weinberg[30] stated that in the case of natural teeth, the distribution of force depends on micromovement induced by the periodontal ligament. Patterns of force are distribution is greatly affected by cusp inclinations. There is a huge difference in force distribution in the case of implants and natural teeth. Implant overload can be controlled by making changes in tooth location and cusp inclination.

In our study, 5 mm, length implants which were platform switched having micro threads and a reduced pitch was analyzed for peri-implant stresses, strains, and micromovements. The microthreads offer better distribution of optimal loads, and they counteract marginal bone resorption. A study by Yun et al.[31] concluded that the short-term survival rates of micro thread and platform switched implants were 100%, and marginal bone loss was 0.16 mm. Their study demonstrated the successful survival of micro thread and platform-switched implants with a considerable reduction in marginal bone loss. In an FEM simulation, Chang et al.[32] showed consistently less Von mises stresses around platform-switched implants, similar to our study where ultrashort platform-switched implants had favorable stresses around them. The pitch of the implant in our study was a reduced pitch of 0.2 mm which was increased to 0.5 mm after half the length. Favorable results consistent with our study are shown by Chun et al.[33] who in his FEA study concluded that the maximum effective stresses decrease as the pitch decrease.

In addition to the peri-implant stresses and strains, another common occurrence is the displacement of the implant body to the surrounding bone. Such movement or displacements are called as micromovements.[34] Extensive micromotion may interfere with the implant's osseointegration. For successful implant healing, a threshold of 150 μm should not be crossed. All the models in our study had micromovements well within the upper limit, with minimum micromovement observed for 6 mm square thread 0.022243 μm followed by 0.022452 μm for the acme thread and maximum for 4 mm diameter acme thread 0.044296 μm under vertical loading. Higher micromovements are observed in case of oblique loading for all with a minimum value of 0.026832 μm for the 6 mm diameter square thread. For determining the micromovements, bone quality probably plays an important role in achieving a reduced micromotion and increased stability. Petrie et al.[35] studied the influence of diameter, length in his FEA study and proved that increasing the implant diameter from 3.5 mm to 6 mm resulted in as much as 3.5-fold reduction of crestal strain and increasing the length from 5.75 mm to 23.5 mm resulted in 1.65 fold reduction, clearly demonstrating an increasing effect of the diameter on better stress and strain parameters.

Our study also concluded similar results that implant length 5 mm can be a viable option if the wider diameter is used. Reduction in stresses and strains was seen as the diameter increased from 4 mm to 6 mm. Under vertical loading, the von Mises peri-implant stresses reduced from 364 MPa to 169.057 MPa (acme thread), 468.876 MPa to 93.571 MPa (buttress thread), 324 MPa to 89.72 MPa (square), and 742.181 MPa to MPa (triangle thread). Under oblique loading, the stresses had the maximum value of 847.554 MPa for the 4 mm diameter triangle thread and minimum value of 202.725 MPa for the 6 mm diameter square thread. For the strain values, maximum strains were observed for the 4 mm diameter triangle thread under both vertical and oblique loading with the values of 0.06813 microstrains and 0.116504 microstrains, respectively. Minimum strains were demonstrated by 6 mm square thread under both vertical and oblique loading with the values of 0.013525 microstrains and 0.019036 microstrains, respectively, followed by the acme thread. Anitua et al.[36] concluded that the wide diameter implants dissipate the forces and reduce the stress concentrations and hence better stress and strain parameters with wide and ultrashort implants. With the constant implant length, of 5 mm and increasing diameters from 4 mm to 6 mm, four different thread shapes were also compared in our study. In our study, triangle (V) thread had higher stresses and strains around with the maximum stresses around 4 mm diameter triangle threads with a value of 742.181 MPa under vertical loading and oblique loading a value of 847.554 MPa. Square, acme, and buttress threads had better stress parameters around them. For the 6 mm diameter, the square thread had the least stresses followed by the buttress thread under vertical loading with the values of 89.872 MPa and 93.571 MPa, respectively. Under oblique loading, also square thread had the least value of 202.725 MPa followed by buttress thread-214.805 MPa. These results are consistent with the findings from a FEA study done by Geng et al.[37] where he concluded that broader square shape threads generated significantly fewer stresses. Furthermore, he concluded that their effectiveness was prominent in the cancellous bone similar to our study which has D4 bone quality. Furthermore, Chun et al.[33] showed the superiority of the square thread. Steigenga et al.[20] in their animal study demonstrated a higher bone to implant contact for the square thread. They also displayed the least strains and micromovements when compared to the other three threads. The stresses, strains, and micromovements were consistently higher for all threads and diameters under oblique loading as compared to the values for under vertical loading. This is consistent with the findings by Ding et al.[38] which showed that stresses and strains were notably increased under buccolingual/oblique loading.

The current study is completely mechanical with all the structures in the model assumed to be linear elastic, homogenous, and isotropic. The material properties of all structures modeled were different. All interfaces between the materials were assumed to be bonded. This being a mechanical study, it ignored the biological aspect of muscles and other oral structures. The various forces generated by the mandible and the various attached muscles were not taken into consideration. This resulted in the ignorance of forces produced during dynamic functions such as chewing. However, within the limitations of the current study, this study demonstrated favorable peri-implant stresses strains and micro-movements around the ultrashort implants. On comparing the results to the limitations, it was seen that the results supersede the limitations and thereby provide the clinician a better understanding of the benefits and understanding of the implants of 5 mm length. This study if combined with further clinical trials can give a better insight on the advantages of ultra-short implants since the use of ultra-short implants can be predicted to be indispensable in the future when there is a need to restore extremely atrophied ridges or a situation where there are many anatomical constraints present.

CONCLUSION

For all the 3 diameters compared, 6 mm diameter acme, buttress, square, and triangle threads had the least amount of peri-implant stress, strains, and micromovements under both vertical and oblique loading and 4 mm diameter threads had the maximum peri-implant stresses, strains, and micromovements

For all the four-thread shapes compared, the square thread had the least amount of stresses, strains, and micromovements and triangle thread the maximum

Our study concluded that with the constant length of 5 mm, increasing the diameter reduced the peri-implant stresses, strains, and micromovements, necessitating further research as to which thread and diameter can generate the most favorable parameters around the implants of length 5 mm, under immediate loading.

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Nil.

Conflicts of interest

There are no conflicts of interest.