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The Influence of Various Attachment Types in Mandibular Implant-retained Overdentures on Maximum Bite Force and EMG

¹ Department of Oral-Maxillofacial Surgery, Prosthodontics and Special Dental Care, University Medical Center Utrecht, PO Box 85.060, 3508 AB, Utrecht, The Netherlands; and
² Central Military Hospital, Utrecht, The Netherlands;

*corresponding author, F.M.C.vanKampen{at}med.uu.nl

	ABSTRACT

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The type of attachment that is used in oral rehabilitation by means of implant-retained mandibular overdentures may influence the retention and the stability of the denture. In this study, we examined the hypothesis that a better retention and stability of the denture improve the oral function. Eighteen edentulous subjects received 2 permucosal implants, a new denture, and, successively, 3 suprastructure modalities. Maximum bite force and electrical activity of the masseter and temporalis muscles were measured. The maximum bite force nearly doubled after treatment for each of the 3 attachments. However, the average bite force after treatment was still only two-thirds of the value obtained for dentate subjects. No large differences in maximum bite force and muscle activity were found among the 3 attachment types. Temporalis activity was significantly lower than masseter activity when subjects clenched without implant support. There was no difference in activity when subjects clenched with implant support.

KEY WORDS: bite force • EMG • dental implant • overdenture • attachment

	INTRODUCTION

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Subjects with complete dentures often report functional problems due to a lack of retention and stability of the mandibular denture. The maximum bite force of these subjects is only 20 to 40% of that of dentate subjects (Helkimo et al., 1977; Haraldson et al., 1979; Slagter et al., 1993; Fontijn-Tekamp et al., 2000). Objective oral function significantly improves when the mandibular denture is supported by oral implants. The maximum bite force of subjects with a mandibular denture supported by implants is 60 to 200% higher than that of subjects with a conventional denture (Lindquist and Carlsson, 1985; Haraldson et al., 1988; Carlsson and Lindquist, 1994; Fontijn-Tekamp et al., 1998). The increase of bite force after implant treatment does not seem to depend on the degree of implant support (Fontijn-Tekamp et al., 1998). In that study, it was shown that bite forces do not differ among subjects with mainly implant-borne overdentures on a transmandibular implant and subjects with mucosa-borne overdentures on 2 implants. However, the attachment type of implant-retained mandibular overdentures may influence the retention and the stability, and thus the oral function, of the denture.

The aim of the present within-subject crossover clinical trial was to study the influence of the degree of retention and stability of the denture on the maximum bite force and the corresponding EMG. To that end, we measured these variables in subjects who received 2 permucosal implants and 3 suprastructure modalities: magnet, bar-clip, or ball attachment. The 3 suprastructures were worn successively by all 18 subjects, so we could make a within-subject comparison of the maximum bite force and corresponding EMG obtained with the 3 attachment types.

	MATERIALS & METHODS

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Subjects
Eighteen edentulous subjects from the Royal Dutch Army and Air Force participated in this randomized crossover clinical trial. They were referred to the Center for Special Dental Care at the Central Military Hospital in Utrecht because of functional complaints with their mandibular dentures. The group consisted of one female and 17 male subjects ranging in age from 33 to 56 yrs, all fit for military service. The bone height in the inter-foramina region of the mandible of the subjects exceeded 15 mm. The Ethics Committee of the University Medical Center Utrecht approved the protocol. Written informed consent was obtained from each subject after a full explanation of the experiment.

Surgical and Prosthetic Procedures
The subjects received 2 oral implants (Frialit-2, Friadent Friedrichsfeld, Germany; diameter, 3.8 mm; length, 13 or 15 mm) in the anterior part of the mandible. The implants were placed in the region of the 2 former cuspids according to a standardized implantological protocol, including an Edlan mucosa technique to obtain more attached alveolar mucosa. New dentures in the maxilla and mandible were made following the first-stage surgery. The dentures were made in central occlusion with balanced articulation. Subjects started wearing the new dentures without attachment 2 mos after first-stage surgery, for a three-month period. A vertical space of 3 mm was created in the new mandibular denture at the location of the implants. This space was relined with soft denture reline material (Soft-liner, GC Corporation, Tokyo, Japan). Second-stage surgery was performed 5 mos after implantation. With 2 small crestal incisions at the location of the former cuspids, the implants were uncovered, and the cover screws were replaced by 2 healing collars (Frialit-2, Friadent Friederichsfeld, Germany).

After second-stage surgery, the mandibular denture was successively fitted with magnet (Dyna Magnet ES, Dyna Dental Engineering, Bergen op Zoom, the Netherlands), bar-clip (IMZ, Friadent, Friedrichsfeld, Germany), or ball (Frialit-2 ball attachment, Friadent Friedrichsfeld, Germany) attachments. The sequence in which the 3 attachments were applied was randomized. All 6 possible sequences were used, so that possible crossover effects could be studied. In that way, six groups of three subjects were formed, each having a different sequence of successive attachments. Each attachment type was used during a three-month period.

Maximum Bite Force
Vertical inter-occlusal bite forces were measured bilaterally with a bite-force transducer. This device, which has been described in detail (Slagter et al., 1993), consists of a bite fork equipped with strain gauges on the left and right parts of the mouthpiece (Fig. 1). The strain gauges were positioned between the occlusal surfaces in the first molar region. The bite-force transducer was covered with dental impression material (Provil putty, Bayer, Leverkusen, Germany), which fitted the profile of the subject's teeth. In this way, a reproducible bite position was obtained for all 5 measurements during the 14-month period. Subjects were encouraged to bite as hard as possible on the bite-force transducer for a few sec. The measurement was performed 3 times. The highest bite force of the 3 efforts was selected. Left and right bite-force signals were summed.

View larger version (121K): [in this window] [in a new window]   Figure 1. The bite-force transducer. The left and right parts of the mouthpiece are covered with dental impression material.

Procedure
We measured maximum bite force and the corresponding muscle activity at 5 moments during the 14-month treatment period. The first measurement was performed with the old denture, just prior to first-stage surgery. The second measurement was performed just prior to second-stage surgery, after the newly made denture had been used for 3 mos without attachments. We performed the next 3 measurements at the end of the 3 periods of 3 mos, during which the various attachment types were incorporated into the dentures.

Statistical Analysis
We applied analysis of variance (ANOVA) to test the null hypothesis that there would be no statistical difference between the results obtained at the 5 occasions. Subsequently, post hoc tests (least significant difference multiple-comparison test) were used for pairwise comparisons of the results. A Pearson correlation was calculated between maximum bite force and muscle activity.

	RESULTS

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Maximum Bite Force
The results of the maximum bite-force measurements are shown in Fig. 2 and the Table. Analysis of variance (ANOVA) showed a significant effect (p < 0.001) of the 5 situations—old and new dentures, and denture attached with a magnet, bar-clip, or ball attachment—on the maximum bite force. Post hoc analysis showed that the maximum bite force significantly decreased between the measurements performed with the old and new dentures ( p =0.05). A significant increase was observed after the new denture was attached to the implants, from 170 N up to, on average, 325 N (p < 0.001). The maximum bite force that was generated with the new denture attached to the ball attachment was significantly higher than that generated with the magnet construction (p = 0.03). The order in which the attachments were placed did not influence the maximum bite force (p = 0.81).

View larger version (24K): [in this window] [in a new window]   Figure 2. Bite force during maximum bilateral clenching for the 5 different measurements. Values of the bite force of left and right sides were summed. Bite force decreased from old to new dentures without attachments (p = 0.05). Bite force increased when attachments were incorporated into the new denture (p < 0.001). Bite force with a magnet attachment was significantly lower than with the ball attachment (p = 0.03). One-way ANOVA and post hoc tests were applied to test differences (n = 18).

View larger version (44K): [in this window] [in a new window]   Figure 3. Activity of masseter and temporalis muscles during maximum bilateral clenching for the 5 different measurements. EMG values of left and right sides were summed for both muscles. EMG decreased for the masseter (p = 0.003) and temporalis muscle (p = 0.021) from old to new dentures without attachments. EMG increased for both temporalis and masseter when attachments were incorporated into the new denture (p < 0.001). EMG of the temporalis muscle is significantly lower than EMG of the masseter muscle for both the old (p = 0.012) and the new (p = 0.001) dentures without attachments. One-way ANOVA and post hoc tests were applied to test differences (n = 18).

	DISCUSSION

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Maximum Bite Force
We observed a decrease of approximately 20% in the maximum bite force between the old denture and the new denture without attachments. Soreness of the mucosa topping the submerged implants and subjects' adjustment to a new denture with a different relation and occlusion may have caused this effect. All subjects had some complaints about pain in the canine region when the maximum bite-force measurement was performed with the new denture without attachments. Small effects of denture treatment on maximum bite force, both positive and negative, were also reported in previous studies (Haraldson et al., 1979; Lindquist et al., 1986). The maximum bite force nearly doubled after the new mandibular denture was attached to the oral implants. The increase in maximum bite force as a result of implant treatment confirms the results reported in a previous within-subject study (Haraldson et al., 1988). A comparison of maximum bite force of groups of mandibular denture-wearers with and without implant support yielded similar results (Fontijn-Tekamp et al., 1998, 2000).

The order in which the attachment types were placed did not influence the maximum bite force. The maximum bite force obtained with the magnet attachment was significantly smaller than the force obtained with the ball attachment. An explanation may be that the retention force of the denture with the magnet attachment is smaller than with the ball attachment. However, the difference in bite force, although statistically significant, is only 36 N, which is much smaller than the increase in maximum bite force of 140 N and 176 N due to the magnet and ball attachments, respectively (Table). The clinical relevance of the differences in bite force between the magnet and ball attachments may therefore be limited. The maximum bite force with attachments was still only two-thirds of the value of 487 N reported for dentate subjects (Weijnen et al., 2000). A possible factor for the limitation of the maximum bite force may be the maxillary denture, which has no attachments and has therefore less retention and stability. When the subjects are clenching, pain in the maxilla may occur because of dislodging of the maxillary denture. In a previous study, it was found that maximum bite force did not differ between a mainly implant-borne (TMI) and a mucosa-implant-borne (IMZ) implant system (Fontijn-Tekamp et al., 1998). Also, mastication with an overdenture attached to 4 implants (implant-borne) appeared to be equally as efficient as with a two-implant (mucosa-implant-borne) system (Geertman et al., 1994; Tang et al., 1999). These results suggest that, after implant treatment, it is not the differences in retention and stability of the mandibular denture but rather the retention and stability of the maxillary denture that may limit the subject's ability to comminute food during chewing. This assumption could be studied by means of a maxillary denture supported by implants instead of a conventional maxillary denture.

Muscle Activity
The maximum bite force and the corresponding muscle activity were significantly correlated (p < 0.001), so the findings for muscle activity are identical to those of the maximum bite force (Figs. 2, 3). However, we found a remarkable difference in the ratio between temporalis and masseter muscle activity for the unsupported and implant-supported overdentures (Fig. 3). The temporalis muscle activity was significantly lower than the masseter activity when the subjects clenched on an unsupported denture. However, temporalis and masseter muscle activity did not differ in the implant-supported situation, comparable with the findings for dentate subjects (Weijnen et al., 2000). Thus, the direction of the maximum bite force of dentate subjects and subjects with an implant-supported denture are identical, whereas the direction of the bite force of subjects with an unsupported denture deviates.

We conclude that the results of this within-subject crossover clinical trial show that maximum bite force and corresponding EMG significantly increase as a result of implant treatment. The differences in maximum bite force and muscle activity obtained with magnet, bar-clip, and ball attachment are small. Thus, all 3 suprastructures greatly improve oral function.

	ACKNOWLEDGMENTS

 
This work was supported by the Military Hospital Utrecht, the University Medical Center Utrecht, and the Netherlands Institute for Dental Sciences. We are grateful to Dr. Ir. J.A.J. Faber (Department of Biostatistics, Utrecht University) for advice on the statistical analysis.

Received July 31, 2001; Last revision January 22, 2002; Accepted January 22, 2002

	REFERENCES

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Carlsson GE, Lindquist LW (1994). Ten-year longitudinal study of masticatory function in edentulous patients treated with fixed complete dentures on osseointegrated implants. Int J Prosthodont 7:448-453.[Medline]

Fontijn-Tekamp FA, Slagter AP, van ‘t Hof MA, Geertman ME, Kalk W (1998). Bite forces with mandibular implant-retained overdentures. J Dent Res 77:1832-1839.[Abstract/Free Full Text]

Fontijn-Tekamp FA, Slagter AP, van der Bilt A, van ‘t Hof MA, Witter DJ, Kalk W, et al. (2000). Biting and chewing with mandibular implant-retained overdentures compared with other states of artificial and natural dentition. J Dent Res 79:1519-1524.[Abstract/Free Full Text]

Geertman ME, Slagter AP, van Waas MAJ, Kalk W (1994). Comminution of food with mandibular implant-retained overdentures. J Dent Res 73:1858-1864.[Abstract/Free Full Text]

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Haraldson T, Jemt T, Stålblad P, Lekholm U (1988). Oral function in subjects with overdentures supported by osseointegrated implants. Scand J Dent Res 96:235-242.[Medline]

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Lindquist LW, Carlsson GE, Hedegård B (1986). Changes in bite force and chewing efficiency after denture treatment in edentulous patients with denture adaptation difficulties. J Oral Rehabil 13:21-29.[Medline]

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Weijnen FG, van der Bilt A, Wokke JHJ, Kuks JBM, van der Glas HW, Bosman F (2000). Maximal bite force and surface EMG in patients with myasthenia gravis. Muscle & Nerve 23:1694-1699.[Medline]

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