Correlation Between Occlusal Forces and Marginal Bone Levels Around Implant-Retained Restorations: A Clinico-Radiological Study

The use of dental implants for restoration and rehabilitation of partial and complete edentulism has increased by leaps and bounds ever since the concept of osseointegration was identified and accepted.^1–3  The success rate of dental implants has been determined by Smith and Zarb⁴  and Albrektsson et al,⁴  who proposed the criteria for implant success. A successful implant is the one that does not show any signs of pain or tenderness on function, zero mobility, less than 2 mm of radiographic bone loss from the time of initial surgery, and no exudation from the implant site.⁵ 

Clinical success and longevity of dental implant-retained restorations can be achieved by biomechanically controlled occlusion. The occlusal forces affect the bone surrounding an implant-retained restoration. Mechanical stress can have both positive and negative consequences for bone tissue and thereby also for maintaining osseointegration of an oral implant-retained restoration.⁶ 

It is clinically difficult to quantify the magnitude and direction of naturally occurring occlusal forces. There are no clinical indices available to quantify these occlusal forces and their impact on prosthesis and oral implants, as they are available for plaque accumulation and peri-implant mucositis. This makes it very difficult to correlate the clinical signs and symptoms of occlusal overloading, radiographic signs of marginal bone loss, and implant failure.⁷  The occlusal forces may exceed the mechanical or biological load-bearing capacity of osseointegrated implants or prosthesis, causing either mechanical complications such as screw loosening or fracture, prosthesis or implant fracture, or failure of osseointegration, eventually leading to compromised implant longevity.⁶ 

It has been speculated that osseointegrated implants without periodontal receptors could be more susceptible to occlusal overloading and crestal bone loss because the load-sharing ability, adaptation to occlusal force, and mechanoperception are significantly reduced in dental implants.⁸ 

Studies have shown varying amount of occlusal forces and marginal bone levels around implant-retained restorations and natural tooth. Currently, the scientific evidence related to implant occlusion and the amount of occlusal forces is insufficient, and this is limited mainly to in vitro, animal, and retrospective studies. Very few studies have been done to study the correlation between occlusal forces and radiographic evidence of marginal bone loss and implant failure, and no study has been done on the Indian population. Also, the studies that have been conducted are difficult to decipher because of a wide variation in study designs.^9,10 

Thus, the present study was carried out (1) to measure the maximal occlusal forces (MOFs) in patients with implant-supported restorations, (2) to measure the marginal bone levels (MBLs) around these implant-supported restorations, and (3) to correlate the occlusal forces and MBLs around these implant-supported restorations.

A total of 14 patients were included in the study. An informed consent was taken from the subjects after explaining the study protocol and an approval of the ethical committee of the institution was obtained.

A total of 14 patients were selected and a split mouth study design was followed while conducting the study. Selection criteria for the patients were that (1) patients had single implant-retained restoration in the posterior area with adjacent and opposing natural teeth present, (2) the implant and the contralateral site were periodontally healthy, (3) no occlusal derangement was seen, and (4) all the sites were free of restoration.

The MOFs were measured by using a device (Digital IT strain gauge, H.E.M. Electronics, Miraj, India) that can bear a maximal occlusal load of 100 kg/mm². The part of the strain gauge is a load cell that was placed into the oral cavity to record the forces. The forces were standardized by using a custom-made acrylic jig. A total of 3 values were recorded for MOFs with and without the use of the acrylic jig. This acrylic jig was fabricated to stabilize and reproduce the placement of the load cell in the oral cavity (Figure 1). The forces were recorded in the early morning, and patients were seated in alert feeding position to simulate the natural chewing position. These measurements of MOFs were carried out at the time of cementation of the final restoration, at 6 months, and at 12 months postrestoration.

Intraoral periapical radiographs were taken by the paralleling technique with the help of a film holder on a size 2 E speed X-ray film of the implant site at the time of implant placement, at the time of cementation of the final restoration, at 6 months, and at 12 months. These radiographs were scanned under a transmission scanner. After scanning, the images were enlarged uniformly for ease of measurement, and both horizontal and vertical components of MBLs were assessed.

The vertical component was assessed on the scanned radiographs by measuring the distance between the implant shoulder and first bone to implant contact at 400x magnification in a computer by referring to the known thread pitch (vertical distance between 2 consecutive threads of the implant). The mesial and distal sides on the implant site were evaluated separately.

The horizontal component was assessed by subjecting the scanned images to AutoCAD software to analyze the changes in the area. The area of interest was demarcated digitally, and the difference between the radiographs taken at different time intervals was calibrated. The mesial and distal sides on the implant site were evaluated separately. All the data were statistically analyzed using the SPSS software.

Out of 14 patients, 3 (1 male and 2 females) did not come for subsequent follow-ups, so they were excluded from the study, and a total of 11 patients were evaluated.

All the sites receiving single implant-supported restoration were clinically healthy. None of the implants were mobile at the time of the conclusion of the study.

The MOFs at the implant site ranged from 10.9 to 71.2 kg/mm² with the use of jig and from 8.8 to 61.4 kg/mm² without the use of jig. The MOF at the contralateral site ranged from 8.0 to 58.2 kg/mm² with the use of jig and from 7.2 to 45.5 kg/mm² without the use of jig.

When the MOFs were compared at the implant site and the contralateral site, with the use of jig and without the use of jig, at the time of cementation of the final restoration, at 6 months and at 12 months, a statistically significant difference (P < .05) was seen between them wherein higher forces were recorded with the use of jig (Table 1). However, comparison of MOFs at the implant site, with and without the use of jig and the MOFs at the contralateral site, with and without the use of jig, at different time intervals did not reveal any statistically significant differences (P > .05; Tables 2 through 4).

A statistically significant difference (P < .05) was observed in the MBLs on the mesial and distal sides between the time of implant placement and at the time of cementation of the final restoration, at 6 months, and at 12 months. However, there was no statistically significant difference (P > .05) when the bone levels were compared at the time of cementation of the final restoration, at 6 months, and at 12 months postrestorative follow-up (Tables 5 and 6).

No statistically significant correlation (P > .05) was seen between the MOFs at the implant site with the use of jig and without the use of jig and vertical and horizontal bone levels at both mesial and distal sides (Tables 7 and 8).

The current prospective longitudinal study was carried out to evaluate the relationship between MOFs and MBLs around dental implants supporting a single missing tooth when compared with the contralateral natural healthy dentition. None of the implants exhibited any pain, tenderness, mobility, or exudation during the period of the study, thus implying a 100% success rate.^4,5 

The mean MOFs recorded at the implant site with the use of acrylic jig were 31.14 kg/mm², and the mean MOFs recorded without the use of acrylic jig were 22.18 kg/mm². The mean MOFs recorded at the contralateral healthy site with the use of acrylic jig were 25.71 kg/mm², and the mean MOFs recorded without the use of acrylic jig were 17.53 kg/mm². This is in contradiction to various studies reported in the literature.¹¹  The occlusal forces depend on a number of factors that include gender, racial variations, diet, general build of the individual, musculature, dental arch size, vertical height of the load cell, and actual number of teeth involved in the bite effort.¹¹ 

Studies relating the occlusal forces to MBLs around dental implants have analyzed either the implants in overload mode^12–22  or the implants connected to natural teeth⁸  but never on single implant-supported restorations. This wide variation in study designs and analysis of overload per se does not relate to the effect of normal occlusal forces on MBLs on dental implants.

The inclusion criteria wherein the patients having only 1 single missing posterior tooth replaced with a dental implant and later restored along with split mouth design eliminated the errors related to the overload as well as being connected to natural teeth and also change in oral hygiene habits, change in oral intakes, and change in muscular activity, as done by Akca et al,⁸  Mericske-Stern et al,²³  and Fontijn-Tekamp et al.^24,25  It also provided a better understanding on the effect of normal occlusal forces on single implant-supported restorations.

The MOFs recorded give insight into the maximum amount of load that could have been put on the implants in normal function. The custom-made acrylic jig used in the current study standardized the placement of the load cell at all visits. It was observed that the amounts of forces recorded with the acrylic jig were considerably higher as compared to the forces recorded without the use of acrylic jig. This shows that the presence of any object between the occlusal surfaces of the opposing teeth leads to generation of higher masticatory forces by the muscles of mastication, which are eventually reflected in the readings recorded. The recording of occlusal forces without the use of acrylic jig was done to evaluate whether there was a difference with the use of acrylic jig, which was proven.

In the present study, the MOFs at the implant site were similar to the MOFs at the contralateral site with healthy natural dentition. This is in contrast to studies by Mericske-Stern et al²³  and Fontijn-Tekamp et al.^24,25  However, the results are in accordance with the study by Richter et al,²⁶  where similar occlusal forces were recorded on implant-supported restorations and healthy natural dentition. The probable explanation for similar MOFs on the implant site and the healthy natural dentition site can be attributed to the fact that it was 1 single tooth that was replaced, the opposing teeth were healthy, the duration of edentulousness was short (3–4 months), and the occlusal scheme on the implant-supported restoration matched that of healthy natural dentition, and achieving occlusal stabilization was not difficult because it was a single missing tooth with adjacent and opposing healthy natural dentition. This can also be attributed to the phenomenon of osseoperception/per-implant proprioception via coupling of existing nerve fibers in the bone to implant surface and thus the sprouting of new fibers and free nerve endings close to the bone to the implant interface^27,28  or with partial regeneration of periodontal ligament on the implant surface.²⁹  It has also been assumed that the fast elastic deformation of a bone during loading over implants may in fact activate the periosteal mechanoreceptors in the fascia, periosteum, and periodontal ligament and thus contribute to the restoration of peripheral feedback,³⁰  in turn leading to equitable distribution of forces on the implant site and the healthy natural dentition site. This leads us to state that implant-supported restoration represents a favorable treatment option to reestablish the ability of tactile perception and can be as efficient as natural dentition in carrying out normal oral function, consequently improving the quality of life of prosthesis wearers.

The current study evaluated bone levels in both vertical and horizontal patterns. This is probably the only study to quantify the amount of horizontal bone loss. The importance of measuring horizontal bone level (HBL) lies in the fact that bone loss around dental implants is in a “crater” form rather than from being linear. Thus, HBL conveys a significant finding. In addition, it is the bone on the mesial and distal horizontal dimension that supports the papilla, and any reduction in bone levels leads to recession of the interdental papilla.³¹ 

In the current study, a statistically significant amount of bone loss was observed from the time of implant placement to the second stage of surgery. This can be attributed to surgical trauma and bone remodeling that happens soon after implant placement.³²  Heat generated at the time of drilling, elevation of the periosteal flap, and excessive pressure at the crestal region during implant placement may contribute to implant bone loss during the healing period.³³  In the present study, bone loss was observed from cementation 6 months. This can be attributed to stresses at the crestal bone that may cause microfracture or overload, resulting in early crestal bone loss during the first year in function. The change in bone strength and mineralization that happens after the first year in function from the time of loading alters the stress-strain relationship and reduces the risk of microfracture during the following years.³⁴  Thus, in the present study, bone loss was seen at the second stage of implant surgery. However, the bone levels were stabilized until the completion of the follow-up period, and a 100% success rate of the dental implants was achieved. This is in accordance with various other studies citied in literature that show that single-unit implants have a survival rate of 93.8%–97.4%.^2,3 

The study did not find any correlation between MOFs and MBLs. The results are in accordance with study by Akca et al⁸; however, these results are in contradiction to various other studies citied in the literature.^7,19,35  The difference in the results could be attributed to the fact that occlusion was well stabilized in the present study in contrast to other studies citied above, where the effect of occlusal overload was evaluated. Therefore, it can be concluded that a well-stabilized occlusion holds the key to the success of an implant. The occlusal forces cannot be definitively said to be the only causative factor for bone loss around dental implants.

HBL

horizontal bone level

MBL

marginal bone levels

MOF

maximal occlusal forces