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Basic and Clinical Research

Dental Implant Thread Design and the Consequences on Long-Term Marginal Bone Loss

Ormianer, Zeev DMD; Matalon, Shlomo DMD; Block, Jonathan DMD; Kohen, Jerry DMD

Author Information
doi: 10.1097/ID.0000000000000441
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Abstract

There are a variety of dental implant thread designs commercially available. While having an influence on insertion torque and primary stability, thread design also can enhance initial contact, dissipate load forces, and increase surface area at the bone-implant interface.1–3 Implant geometry may also have an impact on marginal bone loss. There are 4 main geometric thread parameters: pitch, lead, shape, and depth4 (Fig. 1).

Fig. 1
Fig. 1:
The 4 main thread features: pitch, lead, shape, and depth. These influence long-term bone loss and implant survival. Thread pitch has the most significant influence on surface area. Implant design can have progressive threads, with higher thread depth in the apical area that gradually decreases coronally.

Of the different implant thread design variables, pitch has the most significant influence on surface area.5 The importance of the thread pitch has been highlighted in an in vivo animal study showing improved anchorage of implants with a narrow pitch.6

Thread lead influences the amount of revolutions required to insert an implant in reverse proportion.5 As the thread lead grows, the thread helix angle grows accordingly, resulting in a potential effect on the forces transmitted to the bone.7

Thread shape is important in providing long-term function under occlusal load.5 Different thread shapes have shown different properties in animal studies.8 The direction of forces arising from occlusal load in a restored implant is influenced by the apical face angle of the thread.1 Research using finite element analysis (FEA) to evaluate design parameters of osseointegrated dental implants concluded that the square thread design has a beneficial shape for occlusal loading compared with other thread designs.7

Profound thread depth increases functional surface area. This is advantageous in soft bone. A shallow thread is more easily inserted, which is advantageous for denser bone.5,9 Implant design can have progressive threads with higher thread depth in the apical area that gradually decreases coronally. The purpose is to increase the load transfer to the more flexible cancellous bone and decrease load transfer to the crestal cortical bone. This may contribute to less cortical bone resorption.10 However, little is known on how these macrostructure dental implant features reduce or enhance bone loss.

There are numerous articles using FEA methods to assess the effect of thread design on stress and strain distribution in models of implants embedded in bone11–15 showing that different thread designs perform variably.

Clinical studies have assessed crestal bone loss around dental implants as a gauge for clinical performance and success16–21 but investigated one form of thread design, either a progressive thread design16,17,19 or a triple-lead V-shaped thread design.18,21 Only one other study compared 2 different thread designs, but the 2 implants had different surface treatments.20 This study evaluated the effect of implant thread design and surface treatment on several parameters, including marginal bone loss over a 1-year period. Two implants were evaluated. One implant had a V-shaped thread with a smaller pitch and a larger face angle and the other implant had a reverse buttress V-thread with a larger pitch and a smaller face angle. There was a borderline difference in marginal bone loss after 1-year follow-up in favor of the V-shape thread. This tendency might have been related to the thread design, surface treatment, or both.

Thread geometry and bone loss have not been addressed in the literature; however, differences in implant geometry thread design may have an impact on bone loss over time.

The aim of this study was to compare long-term bone loss around dental implants with 3 different thread designs. The 3 implant types studied are from the same company and have the same microstructured surface.

Materials and Methods

Study Inclusion Criteria

Study candidates were partially edentulous patients who presented with one or more missing or unsalvageable teeth, and who were subsequently treated with implant-supported prosthetic restorations. Patients included in the study were at least 18 years of age, dentally healthy, and demonstrated an ability to maintain oral hygiene with willingness and ability to commit to follow-up. Participants in the study provided signed informed consent.

Study Exclusion Criteria

Patients who were not available for annual follow-up, did not complete initial therapy with the dental hygienist, or had a neglected periodontal status were excluded from the study. Patient records with incomplete surgical or restorative data or nondiagnostic x-rays were excluded. Diabetic patients with hemoglobin A1C (HbA1C) above 8% or with glucose level above 150 mg/dL and patients within 2 years after being treated for neoplasm with radiation or drugs that harm bone metabolism were excluded.

Patient Evaluation

Patients were routinely subjected to a preliminary evaluation that included careful review of their medical and dental histories, detailed clinical and radiographic examination including computed tomography (CT) scans, evaluations of oral hygiene, and assessment of their ability to commit to at least one annual hygiene prophylaxis and clinical monitoring. Patients were guided to avoid harmful habits, including bruxism, holding a pen in the mouth, sucking candies with sugar, etc. Special efforts were carried out to emphasize the harmfulness of smoking. Study models were fabricated, and diagnostic wax ups were performed to evaluate the volume and location of available bone as well as the esthetic and functional details/demands/data of the case in relation to the expressed desires of the patients. Surgical templates to guide implant placement relative to the planned prosthesis were fabricated as required. The treatment plan and alternative treatment options were discussed with patients, and signed informed consent was obtained from each patient before implant placement.

Implants were inserted according to the manufacturer's protocol. At the end of the implant placement procedure, the implants were covered by soft tissue, then covered with a healing cap or restored with a temporary restoration.

Annual Hygiene Prophylaxis and Monitoring

Patients were recalled at least once annually for hygiene prophylaxis and monitoring of implant health. Plaque, gingival recession, and probing depth indices were recorded to monitor the health of the periimplant mucosa. Probing depth measurements were taken at the mesial, distal, lingual, and buccal sides using a periodontal probe (Hu-Friedy, Chicago, IL). Implant-related problems were treated, and failed implants were removed and recorded as failures. Patients were subsequently treated for any failed implants.

Data Collection

Data collection was completed by students, who were qualified and calibrated by a single operator, under the supervision of a researcher from the Maurice and Gabriela Goldschlegar School of Dental Medicine, Department of Oral Rehabilitation. The study was approved by the Ethics Committee of the Tel Aviv University.

All patient records in the practice were examined to identify subjects who met the inclusion criteria. Data from each included record were entered into spreadsheets (Microsoft Excel; Microsoft Corporation, Redmond, WA) on a personal computer (Windows XP operating system; Microsoft Corporation). For analytical purposes, crestal bone loss was evaluated by measuring the distance from a radiographically visible landmark on the implant body, the abutment-implant connection, to the highest point of bone in contact with the implant surface, using digital optical software. Both mesial and distal surfaces were evaluated. Changes were calculated according to the known distance between implant-abutment connection and the first thread of the implant, or from the crown-abutment connection to the first thread of the implant.

Statistical Methods

Study variables were summarized by creating 3 subgroups of subjects for analytical purposes. Mixed Model Analysis was used. The dependent variable was the average mesial and distal bone loss measurement. The variable was exceeded by logarithmic transformation to accomplish normal deviation. A pairwise comparison test was calculated for all groups. Continuous variables were summarized using descriptive statistics (N, mean, median, standard deviation, minimum and maximum). Statistical significance was inferred at the nominal level of type I (alpha) error of 0.05. All analyses were performed using statistical software (SAS; SAS Inc., Cary, NC) on a personal computer (Windows XP operating system; Microsoft Corporation).

Results

This is a nonrandomized retrospective, double-blind study. Data were collected by an independent group from Tel Aviv University from patient records in a general practitioner's private office. In total, 1361 implants met the study inclusion criteria. The average follow-up time in this study was 107 months, with a minimum follow-up time of 82 months.

Three types of implants with 3 different thread designs, produced by the same implant company, were used. These were divided into 3 groups accordingly.

Group A

Patients in group A received 388 spiral implants (SPI) (Alpha-BioTec, Petah-Tiqwa, Israel). These implants have a progressive thread design with a tapered body and core and have a double-lead thread design with a wide step between threads. The lead is 2.1 mm hence the pitch is 1.05 mm. The coronal threads are shallower and thicker square threads, the middle threads are deeper and thinner square threads, whereas the apical threads are deep V-shape threads. In the apical area, the core is narrow, accommodating deep sharp threads that act like blades (Fig. 2).

Fig. 2
Fig. 2:
SPI. Implants have a progressive thread design with a tapered body and core with a wide step between threads. Sharp well-spaced deep apical threads cut the bone during insertion creating a small osteotomy and the following wider threads condense the bone during implant insertion.

Group B

Patients in group B received 911 dual-fit implants (DFIs) (Alpha-BioTec). These implants also have a progressive thread with the same sequence in the corono-apical direction as the SPI. However, in these double-lead thread implants, compared with the SPI implants, there is a smaller lead of 1.2 mm hence a pitch of 0.6 mm. A smaller pitch defines a thread helix angle which is more obtuse and there are more threads in the same implant length. Finally, the apical V-shape threads are shallower with an implant core that is not as narrow (Fig. 3).

Fig. 3
Fig. 3:
DFI. Implants have a progressive thread design with smaller lead and pitch than the SPI implant. This smaller pitch defines a thread helix angle that is more obtuse, and there are more threads in the same implant length.

Group C

Patients in group C received 62 arrow implants (Alpha-BioTec). These are narrow, 1-piece implants with a single-lead V-thread design. The threads have a relatively straight face angle, therefore have a bone condensing property. Their body and core are tapered, with a narrow and rounded apex (Fig. 4).

Fig. 4
Fig. 4:
Arrow V-thread implant. One-piece implants with a single-lead V-thread design. The relatively straight face angle of the threads provides a bone condensing property. Implant body and core are tapered, with a narrow and rounded apex.

Out of a total of 1361 implants, 50 implants failed, leading to an overall implant survival rate of 96.3%. Thirteen of 388 SPI implants failed (96.6% survival) in group A; 37 of 911 DFI failed (95.9% survival) in group B; and there were no implant failures (100% survival) in group C.

The average bone loss for groups A, B and C was 2.02 (±1.70) mm, 2.10 (±1.73) mm, and 1.90 (±1.40) mm, respectively (Fig. 5). Pairwise comparisons revealed that group A had less bone loss compared with group B (Table 1, P = 0.036).

Fig. 5
Fig. 5:
Average mesial-distal bone loss as measured from implant/abutment connection to bone level. The average bone loss was 2.02 (±1.70) mm for group A, 2.10 (±1.73) mm for group B, and 1.90 (±1.40) mm for group C.
Table 1
Table 1:
Average Mesial-Distal Bone Loss Statistical Calculation (Calculation by Log)

Discussion

This study presents long-term survival and bone loss comparison results of 3 implant thread designs with the same surface type. In addition, it offers comparison results between 2 progressive double thread designs (groups A and B) with several differences: group B patients received implants with a smaller lead of 1.2 mm and a pitch of 0.6 mm, shallower apical threads and a wider implant core. Overall survival rate was high at 96.3% for all implant groups.

Average bone loss was as follows: 2.02 (±1.70) mm for group A, 2.10 (±1.73) mm for group B, and 1.90 (±1.40) mm for group C. Pairwise comparisons revealed that group A SPI implants had less bone loss compared with group B DFIs (P = 0.036). Although according to literature, the V-shape design induces more stress on bone, no difference from the average was observed in this study. The main thread design differences, lead and pitch, may contribute to the statistical but not clinical significance.

Thread pitch is the distance between 2 adjacent threads. On a given length, a smaller pitch will result in a larger number of threads and a greater surface area.1 The majority of implant systems have a fixed pitch providing fixed functional surface area per unit length.5 Orsini et al6 investigated the osseointegration process in animal cancellous bone. Two types of implants with the same surface treatment were tested: one with a narrow pitch and one with a wide pitch, demonstrating that implants with a narrow pitch had improved anchorage due to greater surface area and bone-to-implant contact (BIC).

Thread lead is determined by the amount of cutting blades in the manufacturing process that form the implant threads. One cutting blade will produce a single-lead thread, 2 will produce a double-lead thread, and 3 will produce a triple-lead thread. When an implant is rotated one revolution, the implant will insert 1 thread in a single-lead thread. In a double-lead thread, the implant will insert 2 threads in one revolution. Hence, a single-lead thread implant will need twice as many revolutions to be inserted compared with the double-lead thread implant.5 In a single-lead thread, the pitch equals the lead; in double- and triple-lead threads, the lead is double and triple that of the pitch, respectively. In addition, as the lead increases from single-lead thread to double and triple, the thread helix angle grows.

The functional surface area is the same for single-, double-, or triple-lead threads. The amount of threads in an implant affects the ease of insertion. Implants with a smaller number of threads are inserted more easily, which could be advantageous in denser bone.5

There are a variety of thread shapes. The square thread shape provides optimal surface area for compressive loads. Buttress thread shape primarily transfers compressive loads.5 In addition to transferring load, thread shape can influence initial healing. An animal study compared 3 thread implant shapes with the same surface, length, width, thread number, and thread depth. V-shaped and reverse buttress threads showed smaller BIC percent than the square thread shape. In the reverse torque test after initial healing, the square thread shape showed the highest value.8

The thread face angle is the angle between the face of the thread and a plane perpendicular to the long axis of the implant. Each thread has an apical and coronal face. A V-shaped thread can have an angle of 30 degrees off the long axis compared with a square thread that may be perpendicular to the long axis. The V-shape thread will transfer higher shear forces at the bone interface under axial load compared with the compressive forces conveyed by the square thread.1

Thread depth is the distance between the major and minor diameter of the thread.1 The width of the thread is the distance between the most coronal and the most apical part at the tip of a single thread.10

Some FEA studies showed better stress distribution in implants with square threads,7,11 whereas others found no significant advantage of square thread over V-shaped threads.13,22 Additional FEA and in vitro investigations also demonstrated that a smaller thread pitch, and therefore a higher surface area, resulted in better stress distribution7,22 and higher insertion torque.23 FEA studies also addressed the effect of thread design on stress and strain in immediate loading, finding a significant influence of the thread design on outcomes.15,24,25 In one study, changes in implant diameter and thread design increased surface area by >300%, decreasing stress and reducing loss in crestal bone regions.26 Square thread profile has had the most favorable results.25

When comparing the bone loss levels in our study compared with those recorded in the previously mentioned study by Park et al,20 the levels of bone loss after 1 year in the latter were 1.07 ± 0.46 mm for the reverse buttress V-thread implant and 0.79 ± 0.42 mm for the V-shape thread implant. These levels are lower than those in our study; however, the follow-up time was considerably shorter. One of the disadvantages of our study was that bone loss was evaluated only at the last follow-up, making it difficult to compare between these 2 studies.

Considering survival rates and bone loss in this study, group B patients had the lowest survival rate and higher bone loss than group A. Group C patients receiving the V-thread implant, with the same surface design as the other 2 groups, had no failures and similar bone loss to groups A and B.

In an attempt to understand the difference in bone loss between groups A and B in this study, it is noted that SPI implants have several features that lead to bone condensation during implant insertion. These features include sharp well-spaced deep apical threads, which cut the bone during insertion creating a small osteotomy. The following threads become wider causing condensation of the bone particles during insertion. As a result, the inserted implant is surrounded by a thin layer of condensed bone, attributing to a more even stress distribution along the implant, increasing the forces on the trabecular bone and reducing the forces on the cortical bone, thus leading to crestal bone preservation.27 Bone condensation, as a means to improve implant osseointegration, has been investigated and supports this concept. Condensing bone using drill techniques before implant insertion has been shown to improve BIC and bone density in dogs 12 weeks after implant insertion.28 In another animal study, using bone condensers in implant preparation sites before insertion, a positive effect on the same parameters was found, but only in the initial healing phase.29 A recent study by Trisi et al30 presents further evidence supporting the assumption that implants with deep well-spaced threads improve bone quality around implants, which in turn could explain less marginal bone loss in group A over time. This study found, in low-density bone, a higher BIC in implants with large threads compared with small threads. Moreover, the large thread implants demonstrated an increase in bone density at the periimplant interface caused by bone trabeculae condensation. In high-density bone, there were high bone remodeling levels on the bone trabeculae surface in the large thread implants.

A 100% survival rate in 1-piece V-thread design implants could be attributed to the fact that there is no microgap between the implant and the abutment. High survival rates for 1-piece implants were also recorded in a prospective multicenter study31 evaluating 1-piece implants, with a cumulative survival rate of 98.8% (1 failure out of 67 implants having completed 3-year follow-up). Interesting results regarding the same 1-piece implant system as in the previously mentioned study were presented in a short-term retrospective study.32 This study found, after a mean follow-up time of 10 months, a survival rate of 100% for 22 implants that underwent delayed loading, but only a 93.7% survival rate for 95 immediately loaded implants. A third 1-piece implant system presented a survival rate of 96.7%, having 1 failure out of 31 narrow 1-piece immediately loaded implants after a 12-month follow-up period.33

Conclusion

Within the limitations of this study, implants with a larger pitch, deeper apical threads, and a narrower implant core showed less long-term bone loss. The best survival rates were demonstrated by 1-piece V-thread design implants.

Disclosure

The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.

Approval

The study was approved by the Ethics Committee of the Tel Aviv University as written in “Materials and Methods” under “Data Collection.”

Acknowledgments

Alpha-Bio Tec Ltd. and Marie Gethins of Mediscribe Medical Writing LLC supported the preparation of this article.

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Keywords:

implant thread design; bone loss; V-thread; double thread

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