TREATMENT OUTCOME OF SHORT DENTAL IMPLANTS

Abstract

Background

Bone resorption following tooth loss and proximity of the alveolar crest to vital structures such as the maxillary sinus and the inferior dental canal can be a limitation to tooth replacement with an implant. The placement of short dental implants may reduce the need for bone augmentation as bone augmentation increases the cost of providing dental implant treatment.

Aim

To assess the treatment outcome and survival rate of short implants used for single and multiple teeth replacement after one year of loading and restoration.

Methodology

This study included 26 patients aged 21 to 70 years with a mean of 47.24 SD ± 14.51 years that received 55 short dental implants (Bicon system, Boston, USA) for single and multiple teeth replacement at the Restorative Dentistry Clinic of Lagos University Teaching Hospital, Nigeria between 2012 and 2016. Demographic data such as age, sex, location of implant, length of implant, placement technique and type of restoration (crown or bridge) were obtained from the patients’ hospital records. Patients were recalled three months, six months and one year after implant placement and restoration to assess clinically for implant mobility, pain, suppuration, gingival recession and inflammation around the implant retained restorations. Marginal bone loss and peri-implant radiolucency were evaluated using periapical radiograph.

Results

Survival rate of short implants placed in this study was 96.4%. None of the surviving implants showed a marginal bone loss greater than 1.5mm one year after placement. None of the surviving and functional implant restoration demonstrated mobility, suppuration or peri-implant radiolucency during the review period. The 8mm implants demonstrated a statistically significantly higher survival rate compared with the 6mm implants (p=0.039). A significantly higher percentage of failure was recorded among the implants placed in the lower premolar region (p=0.001).

Conclusion

This study has shown that the use of short implants is a viable option in teeth replacement especially when the alveolar bone height is less than 10mm

Introduction

Dental implants are prosthetic devices made of inert material such as titanium and ceramic, surgically implanted into the mandible or maxilla to provide retention and support for a fixed or removable dental prosthesis. Dental implants are used to replace missing teeth and are retained in the jaw bone by a process called osseo integration.

Osseo integration is the direct attachment or connection of osseous tissue to an implant without intervening connective tissue.

Success in implant therapy is mainly dependent on bone quantity and quality and primary stability of implant after placement1,2. Bone resorption often follows tooth loss. A deficient alveolar bone and increased pneumatisation of the maxillary sinus result in reduced bone height and volume rendering implant placement difficult therefore necessitating the need for bone augmentation3.

The use of short implants reduces the need for sinus lift and bone augmentation procedure prior to implant placement in resorbed ridges. Simplifying surgical and prosthetic protocol in implant placement will make implant treatment more acceptable and affordable to the patients. The risk of morbidity, treatment time and cost of these surgical procedures can be avoided with the use of short implants. Implants less than 10mm in length can be considered as short implants.

Previous studies reported that short implants failed more than long implants but survival rates were similar in short and long implants4,5. Griffin et al6 suggested that short implants 7-10mm can reach similar success rates as longer ones for the support of fixed partial dental prosthesis.

The aim of this restrospective study is to evaluate the treatment outcome and survival rate of short implants (6mm and 8mm in length) in the replacement of missing teeth in the mandible and maxilla at Lagos University Teaching Hospital over a period of four years.

MATERIALS AND METHODS

This retrospective study included 26 patients aged 21 to 70 years (mean 47.24 ±14.51 years) that received 55 short dental implants (Bicon system, Boston, USA) for single and multiple teeth replacement at Lagos University Teaching Hospital, Lagos, Nigeria between April 2012 and December 2016 The patients’ medical and dental history and clinical examination findings were noted. Pre operative radiograph (periapical and orthopantomogram) were taken to assess bone height and proximity to vital structures such as inferior alveolar nerve and maxillary sinus. The missing teeth were replaced using short implants ( Bicon implants, Boston, USA). A two-staged surgical protocol with delayed loading of 3 – 6 months was used. The implant diameter and height used are shown in Table 1. All the implants were coated with hydroxyapatite by the manufacturer. In all, 45 implants were restored with single porcelain fused to metal crowns; 6 implants were restored with three unit bridges (2 implants per bridge) and 4 implants were restored with a lower full arch bridge. The data obtained were analyzed using SPSS version 16.0. Descriptive statistics were obtained and frequency distribution means and statistical significance between frequencies was evaluated with Pearson chi-square test at a significance level of p<0.05. This study was conducted in full accordance with the World Medical Association Declaration of Helsinki. Ethical approval was obtained from the Health Research Ethics Committee of the institution where this study was conducted.

Table 1. Anatomic variables of 55 implants placed.

	Number of implantsn=55	%
Jaw
Maxilla	36	65.5
Mandible	19	34.5
Location
Anterior	32	58.2
Posterior	23	42.8
Implant length
6mm	18	32.7
8mm	37	67.3
Implant restoration
Single crown	45	81.8
3 unit bridge	6	10.9
Full arch bridge	4	7.3
Implant Placement technique
Immediate placement in fresh extraction socket	10	18.2
Delayed placement	45	81.8

Surgical protocol

Muco-periosteal flaps were raised under local anaesthesia to expose crestal bone of the ridge. The implants were inserted into the osteotomy sites prepared under local anaesthesia with Bicon comprehensive surgical kit. Ten out of the 55 implants placed were immediately placed implants (i.e implants placed immediately after tooth extraction). The implant neck was placed 2mm subcrestal by tapping the implant into the osteotomy site. Healing plug was inserted into the implant well to prevent bone growing into the implant well. Thereafter the muco-periosteal flap was sutured with 4.0 resorbable suture (Vicryl). Post operative periapical radiograph were taken to confirm the position of implant in the bone. Post operative instructions to avoid using the denture for mastication until one week after implant placement.

Prosthetic protocol

Healing period of 3 to 6 months was allowed for osseo-integration of implants before loading. The implants were restored with non- shouldered abutments and porcelain fused to metal crowns and bridges (Fig. 1). Periapical radiographs were taken after loading and at every review to assess for bone loss around the neck of implant. The distance between the alveolar crest and the highest bone implant contact was measured with a ruler at placement of implant and one year after placement. The difference between these two measurements was referred to as the marginal bone loss around the implant.

Fig. 1. Panoramic view of the jaws showing teeth replacement with short dental implants (6-8mm) and implant retained crowns and bridges.

Success criteria of implant as defined by Albrektsson and Zarb7 include: absence of pain, implant stability , absence of peri implant radiolucency, radiographic bone loss not more than 1.5mm in the first year after placement .

The implants were reviewed at 3 months, 6 months, and 1year after placement. Clinical examination of soft tissue around implant restoration, mobility of implant and signs of peri implantitis were checked. Periapical radiograph of the implants were taken to check for bone loss and peri implant radiolucency.

Results

In this study 55 short dental implants were placed. Thirty six (64%) implants were placed in the maxilla while 19(36%) were placed in the mandible(Table 1). Survival rate after one year of loading of short implants placed in this study was 96.4% (Table 2).

Table 2. Life table analysis of 55 short dental implants.

Time interval	Number implants	Failures	Withdrawn	Cumulative survival rate %
Placement- Loading	55	2	0	96.4
Loading-6 months	53	0	0	96.4
6 months-1 year	53	0	0	96.4

Mean marginal bone loss of 0.1mm to 0.5mm was observed one year after placement in 11(20.7%) of the functional and surviving implant restorations while 4(3.8 %) of the surviving implant restorations presented 0.6mm to 1mm marginal bone loss after one year (Table 3). None of the surviving and functional implant restoration exhibited mobility, suppuration or peri-implant radiolucency during the review period. The 8mm implants demonstrated a statistically significantly better outcome compared with the 6mm implants (p=0.039) as shown in Table 4. Table 5 shows that the 6mm implants that failed were placed immediately after extraction (immediate placement). All the 6mm implants that were placed after healing of extraction sockets (delayed placement) survived.

Table 3. Change in marginal bone level of surviving implants one year after placement.

Bone loss (mm)	Mesial**n=53		Distal**n=53		Average***n=53
	n	%	n	%	n	%
0	48	90.5	41	77.4	38	71.7
0.1-0.5	2	3.8	8	15.1	11	20.7
0.6-1.0	2	3.8	2	3.8	2	3.8
1.1-1.5	1	1.9	2	3.8	2	3.8
*Average= Mesial +distal 2 **n=53 functional and surviving implants

Table 4. Implant survival according to implant length.

Implant length	OutcomeFunctional
	n	%	n	%	n	%
6mm	16	29.1	2	3.6	18	32.7
8mm	37	67.3	0	0	37	67.3
Total	53	96.4	2	3.6	55	100
X2=4.26 df=1 P=0.039

Table 5. Implant survival according to surgical placement technique.

Implant length	Outcome	Immediate placementn	Delayed placementn	Total n %
6mm	Failed	2	0	2 100
8mm	Failed	0	0	0 0

A significantly higher percentage of failure was recorded among the implants placed in the lower premolar region (p=0.001) as shown in Table 6.

Table 6. Implant failure distribution according to placement location.

Placement location	Failed		Functional		Total
	n	%	n	%	n	%
Upper anterior	0	0	28	50.9	28	50.9
Upper premolar	0	0	5	9.1	5	9.1
Upper molar	0	0	3	5.4	3	5.4
Lower anterior	0	0	4	7.3	4	7.3
Lower premolar	2	3.6	3	5.5	5	9.1
Lower molar	0	0	10	18.2	10	18.2
Total	2	3.6	53	96.4	55	100
X2= 20.755 df=5 P=0.001

Table 7. Distribution of implants by tooth replaced.

Tooth replaced	Number of implants
	n	%
Maxilla
Central incisor	21	38.2
Lateral incisor	6	10.9
Canine	1	1.8
First premolar	3	5.5
Second premolar	2	3.6
First molar	3	5.5
Second molar	0	0
Mandible
Central incisor	2	3.6
Lateral incisor	0	0
Canine	2	3.6
First premolar	3	5.5
Second premolar	2	3.6
First molar	7	12.7
Second molar	3	5.5
Total	55	100

Discussion

In this study, 96.4% survival rate was obtained for 55 short implants, 6-8mm in length. Historically, short implants had been associated with low success rate8, however later studies reported the same level of clinical success for short and long implants9,10. Furthermore, some studies on short implants reported survival rates of 94.6%-100%3,9,10. Clinical advantages of using short implants include easier fixture insertion, simplified osteotomy preparation and decreased potential for overheating the alveolus and an alternative to bone augmentation in resorbed jaw bones.

Implant survival which is the primary outcome of this study is defined as the duration of time from the date of implant placement to the date of last follow up or implant removal. Two implant failures were recorded in this study; were placed immediately after extraction of teeth. Majority (64%) of the implants were placed in the maxilla. The upper central incisor was the tooth most replaced because of the need for aesthetics, being the most prominent tooth in the mouth. It has been reported that patients replaced teeth mostly for aesthetic purposes. There was no gingival recession around the 53 successful implants. which could be attributed to the subcrestal placement of the short implants

Low bone density is a significant risk factor for failure of dental implants11,12. Most failures in dense bone were because of overheating during drilling due to lack of irrigation or absence of bleeding13. Bone quality has been found to affect the success and maintenance of osseointegration of dental implants14. Implant surface treatment and machining accelerate peri implant bone healing and improves osseointegration at sites that lack sufficient quantity and quality of bone15,16. Implant surface treatment can enhance osseointegration of titanium implants by favouring ingrowth from adjacent bone surfaces. Bioactive factors from the treated implant surface permeates the local bone environment promoting osteogenic factors and processes thereby increasing osteoblast differentiation and mineralisation17. A previous study reported a 100% success rate in short implants treated with hydroxyapatite6. Other studies reported that SLA (sandblasted,large-grit,acid etched) and TPS(titanium plasma sprayed) treated implants also exhibited high success rates because treated implant surfaces present a larger contact area with bone and this reduced stress at the bone implant junction18,19,20. In the present study, the Bicon implants used were coated with hydroxyl-apatite by the manufacturer.

The two failed implants were placed immediately after tooth extraction. The surface contact of an implant with the bone in an extraction socket after osteotomy might not be as intimate as an osteotomy site created with a drill. The bone healing process in peri implant defects after tooth extraction depended on the dimension of the defect adjacent to the implant after implant placement21. Wilson et al22 evaluated histologically the bone implant contact of implants placed in fresh extraction sockets six months after placement. They reported a lower percentage of bone implant contact (BIC) when implants placed in fresh extraction sites had a bone gap of 1.5mm (50%) to 4mm (17%) compared with the control implant (72%). In the present study, the failed implants were lost before loading during the healing period. This was in agreement with the study on hydroxyl apatite coated implants by McGlumphy et al23 in which all failures with short implants occurred before loading.

Studies have shown that increased diameter rather than increased length is required for implant anchorage. This is buttressed by the result of this study which shows a 96.4% survival rates for short implants. For Immediate placement of implants, the 8mm length dental implant gives a better primary stability and a higher success rate compared to the 6mm length because of post extraction peri- implant defects.

The limitations of this study include the small sample size and short term review of the outcome.

Conclusions

This study has shown that the use of short implants is a viable option in teeth replacement especially when the alveolar bone height is less than 10mm. Short implants helps to avoid the need for bone augmentation and sinus lifting which can increase the treatment time and costs. However, immediate placement of the 8mm length dental implant gives a higher success rate compared to the 6mm length dental implant.