Dental implants are currently the most commonly used treatment in dentistry to replace missing teeth¹⁾. Sufficient alveolar bone width and height are required for successful implant treatment, and bone grafting procedures such as guided bone regeneration (GBR) are commonly applied to compensate for the lack of bone. However, bone grafting may not be successful depending on the site or the patient's condition, which can lead to unexpected complications such as wound dehiscence, infection, and postoperative pain. Furthermore, mesiodistal width is sometimes limited, especially when maxillary lateral incisors or mandibular incisors are missing. With such limitations, narrow diameter implants (NDIs) can be an efficient alternative that takes less time and is more cost-effective^2,3).

NDIs are categorized into three groups based on their diameter: Category 1 includes implants with a diameter of less than 3.0 mm, Category 2 covers NDIs with diameters from 3.0 mm to 3.25 mm, and Category 3 includes NDIs with diameters from 3.30 mm to 3.50 mm⁴⁾. Immediate placement of implants has gained popularity in recent years due to benefits such as fewer surgeries, shorter treatment times, and improved aesthetics. This approach allows for functional and aesthetic healing within just a few months with minimal additional surgery⁵⁾.

However, immediate implant placement does present challenges, including difficulties in achieving stable initial bone support due to gaps between the extraction site and the fixture, a lack of keratinized gingiva around the implant, and an increased risk of infection from residual infectious agents present at the extraction site. Despite these challenges, many studies indicate that the prognosis for immediate implant placement is comparable to that of delayed placement⁶⁾.

Most existing research on NDIs has focused on one-piece implants or immediate placements, which may limit immediate loading options and result in higher stress distribution in surrounding bone compared to two-piece implants⁷⁾ (Fig. 1). Therefore, two-piece implants are often preferred in narrow locations, when bone grafting is necessary, or when applying loading is challenging. However, clinical studies specifically examining the use of two-piece implants in anterior immediate-retention NDI cases are limited. This study aims to investigate cases of immediate placement of two-piece NDIs with a diameter of 3 mm in the anterior region and to assess their prognosis.

Figure 1. Structural difference bet-ween one piece implant and two piece implant.

The study was conducted using 3.0 mm two-piece NDIs (TS III SA, Osstem Implant Co., Seoul, Korea) placed immediately after anterior tooth extraction at the Inje University Sanggye Paik Hospital Department of Oral and Maxillofacial Surgery, from 2017 to 2023. TS III SA is an implant featuring a sand-blasted surface with alumina and an acid-etched finish, characterized by a tapered design. A total of 16 implants (three in the maxilla and 13 in the mandible) were placed in 13 patients (male: four and female: nine). The average age of the patients was 67.8 years old. All but three patients had systemic diseases, with hypertension, osteoporosis, diabetes, angina pectoris, rheumatism, cerebral infarction, brain tumor, and heart valve surgery. The primary reasons for tooth extraction included periodontal disease, dental caries, and dental trauma (tooth fracture and tooth subluxation), all of which were maxillary or mandibular incisors.

The surgical procedure was as follows. The tooth was extracted with as little trauma as possible for immediate placement. After extraction, the inflammatory tissue was removed with a curette to clean the extraction socket and the implant was placed immediately. The average post-prosthetic follow-up period was 9.9 months and the average timing for implant loading in 16 cases was 10.5 weeks. Stability was evaluated using implant stability quotient (ISQ) values and insertion torque values obtained through resonance frequency analysis (Table 1). In this study, RFA measurements were taken in four directions: mesial, distal, buccal, and lingual, immediately after implant placement (ISQ1) and at the time of impression taking for prosthesis (ISQ2). In order to assess the level of alveolar bone, radiographs were taken on the day the final prosthesis was fitted and on the day of the last follow-up to determine the extent of marginal bone resorption. The values measured from the implant shoulder to the height of the crestal bone in the proximal and distal regions were averaged (Table 1).

Table 1 . Demographic data of patients included in this study

Patient number	Case number	Gender & Age	PMH	Cause of extraction	Location	Length (mm)	Insertion torque (Ncm)	Initial ISQ* (ISQ1)	Final ISQ* (ISQ2)	Bone graft material	Marginal bone resorption	F/U period (months)	Timing of implant loading (weeks)
1	1	F/49		Periodontal disease	#22	10	25	65.75	67	Xenogenic bone graft	0.1	5	6
	2			Periodontal disease	#31	10	55	64.5	63.75	Allogenic bone graft	0.1	5	6
2	3	F/71	HTN, Brain tumor	Caries	#31	13	15	66	67	Xenogenic bone graft	0.2	10	7
3	4	F/78	HTN	Periodontal disease	#41	13	30	66	65.5		0.2	49	8
4	5	F/76	HTN, DM, Angina pectoris	Periodontal disease	#32	11.5	55	66	68.75	Allogenic bone graft	0.1	3	10
	6			Periodontal disease	#42	11.5	55	63	69.25	Allogenic bone graft	0.1	3	10
5**	7	M/77	HTN, DM	Subluxation	#31	10	50	62.25	***	Allogenic bone graft	Not available	4	20
6	8	F/89		Periodontal disease	#41	13	35	60	***		0.2	22	9
7	9	M/52		Tooth fracture	#22	11.5	10	42.5	64.5	Xenogenic bone graft	0.5	5	11
8	10	F/23	Heart valve surgery	Tooth fracture	#12	10	10	71.5	73.25		2	20	12
9**	11	M/81	HTN	Periodontal disease	#31	10	25	***	***	Xenogenic bone graft	Not available	12	19
10	12	F/73	DM	Periodontal disease	#32	11.5	20	73	76		0.1	13	9
11	13	F/72	Osteoporosis	Tooth fracture	#31	10	<10	***	***	Xenogenic bone graft	0.1	3	13
12	14	M/87	DM, Rheumatism	Caries	#32	11.5	20	64.5	69.75		0.1	3	12
13	15	F/54	HTN, cerebral infarction	Periodontal disease	#31	11.5	30	73.5	73.75		0.1	1	8
	16			Periodontal disease	#41	11.5	30	72	72		0.1	1	8

*: ISQ value is the average value of values measured in 4 directions (buccal, lingual, mesial, distal), **: Osseointegration fail, ***: ISQ was not measured

The lengths of the 3.0 mm two-piece NDIs placed immediately after anterior teeth extraction were 10.0 mm in six cases, 11.5 mm in seven cases, and 13.0 mm in three cases, with bone grafting in six cases, shielding with bone grafting in three cases, and implants alone in seven cases. The initial fixation force was 29.7 (10 to 55) Ncm on average. The Initial ISQ value was not measurable in two cases and a value of 65 (42.5∼73.5) was obtained for the remaining 14 implants. A value of 69.2 was obtained in all but four cases where the Final ISQ value was not measured. Finally, of the 16 implants placed, two 10.0 mm long implants placed in the mandibular anterior region failed to osseointegrate and were reimplanted with a 3.0 mm diameter and 13.0 mm length implant and a 3.5 mm diameter and 13.0 mm length implant, respectively. The timing of implant loading varied among the 16 cases. Early loading was performed in 6 cases within 8 weeks, while the remaining cases followed a conventional loading protocol. The average follow-up period for the 16 cases was 9.9 months, while the 14 surviving implants were followed up for an average of 10.2 months. The amount of bone resorption during the follow-up period was 0.28 mm (0.1∼2 mm) in 14 successful implants. Consequently, 2 out of 16 implants failed, yielding a survival rate of 87.5%, while the success rate, based on the criterion that marginal bone loss (MBL) must remain within 0.5 mm during the first 6 months after implantation, was 81.3⁸⁾ (Fig. 2).

Figure 2. Fig. 2. Radiographs of a representative case (F/72, case number 13). (A) Radiograph from the initial visit; (B) Radiograph from the first implantation surgery; (C) Radiograph from the day of impression; (D) Radiograph from the day of final prosthesis placement.

Immediate placement of implants after tooth extraction is a way to reduce the number of surgeries and procedures, maintain alveolar bone height and width, and reduce the overall duration of treatment. For anterior teeth, immediate placement is often preferred as it allows for the preservation of buccal bone and soft tissue morphology. An anterior tooth in particular often has a narrow mesiodistal width, making an NDI a more effective alternative. This means that immediate placement after extraction of anterior teeth with NDIs can shorten the overall treatment period while avoiding many of the difficulties and adverse events that can occur with regular diameter implant (RDI) placement and delayed place-ment. Notably, a recent study found no significant difference in prognosis between NDIs and RDIs and no difference in marginal bone loss (MBL) as well as mechanical and prosthesis success rates⁹⁾.

The most common criteria for defining the success of an implant are the absence of mobility at the level of the implant, the absence of periapical radiolucent lesions around the implant on periapical radiographs, and the absence of pain or bleeding at the implant site. There should be an average MBL of less than 0.5 mm over the one year after implant placement⁸⁾. Although radiologic assessment of MBL has the limitation of only providing vertical readings of the proximal and distal aspects of the implant and not showing changes in buccal and lingual readings, the MBL measured in this study showed an average bone loss of 0.28 mm over a mean post-prosthetic follow-up of 42.5 months. Another study reported an average MBL of 0.44 mm in a 3 mm NDI²⁾. Furthermore, based on the aforementioned recent criteria, defining a successful implant as an average bone loss of less than 0.5 mm over one year, the results of this study indicated that the implants that did not fail early on had a good prognosis.

Many studies have reported a good prognosis of two-piece NDIs^10-12). Category 1 (<3.0 mm diameter) NDIs had an average success rate of 94.7% (80∼100%), Category 2 (3.0∼3.25 mm) NDIs had an average success rate of 97.3% (80.5∼100%), and Category 3 (3.3∼3.5 mm) NDIs had an average success rate of 97.7% (91%∼100%). Although each category does not show a significant difference, the success rate is very high when using categories 2 and 3. However, the survival rate of the implants in this study tended to be somewhat lower than in other studies, which may have been due to the fact that this study only included immediate placement cases of 3.0 mm two-piece NDIs. The relatively small sample size may also have limited the results of this study. The average age of the patients in the 16 cases examined in this study was 66.3 years, and the average age of the patients with failed implants was relatively higher at 79 years. There is still much debate as to whether age has a significant impact on implant prognosis. A previous study showed that age did not affect implant outcomes, with a 95.3% success rate in patients over 65 years of age and a 93.9% success rate in patients under 65 years of age¹³⁾. On the other hand, another study suggested that age had a significant impact on implant prognosis, as many elderly patients had a variety of issues, including cardiovascular disease, diabetes, and osteoporosis, and these conditions could affect wound healing and osseointegration following implant procedures^14,15). However, even among the successful cases, there were many older patients, suggesting that age did not appear to be a significant risk factor for implant success in this study.

All patients in the failed implant cases had hyper-tension. However, a study that examined patients with and without hypertension found that 257 of 4,874 implants placed in patients with hypertension failed, compared to 809 of 16,192 implants placed in patients without hypertension¹⁶⁾. In other words, there was no significant correlation between hypertension and the success of the implant. This study also found that ISQ values were not significantly different, with a mean of 68.75 (65.5∼73.75) in patients with hypertension and 70.15 (63.75∼76) in those without. The initial torque values for the two failed implants were 50 Ncm and 25 Ncm, respectively, showing no specificity compared to the other successful implants, making it difficult to consider the initial torque as a cause of failure. The initial ISQ value of one of the failed implants was 62.2, and that of the other could not be measured due to the narrow mesiodistal width forbidding the access of the ISQ probe, making it unlikely to be the cause of the failure. The initial ISQ value of one of the failed implants was 62.2, and that of the other could not be measured due to the narrow mesiodistal width forbidding the access of the ISQ probe, making it unlikely to be the cause of the failure. Rather, the failure of the two mandibular anterior implants seems to have been due to the fact that the mandible is typically narrower and more hardened and dense than the maxilla, thereby lowering the success rate of the implants. Therefore, immediate placement of NDIs in the mandibular anterior region requires more experience and accuracy than other regions. In addition, one of the reasons for the high implant failure rate may be that not all implants were placed by surgeons with the same experience. Immediate placement of NDIs in the anterior mandible requires more experience and precision than other sites, so the more experienced the surgeon, the higher the success rate of implant placement. It is also pertinent to note that narrow diameter two-piece implants are susceptible to fixture tearing due to the thin thickness of the fixture and abutment connection. While this did not occur in the case presented in this paper, it is advisable to exercise caution during placement to prevent excessive force, which could otherwise lead to fixture tearing.

Lastly, it is important to acknowledge the relatively short follow-up period as a limitation. Future research should aim for longer follow-up durations to better evaluate long-term outcomes and implications of the interventions studied. Recommendations for larger sample sizes and extended observation periods could enhance the understanding of these findings.

There were a total of two cases of failed osseointegration in 3.0 mm two-piece NDIs. The remaining implants had stable prognoses. In the mandibular anterior region, the placement of a two-piece NDI immediately after tooth extraction seems to require great care. Furthermore, more case series of systematic comparative studies will be needed on the immediate placement of one-piece versus two-piece NDIs after anterior teeth extraction.