Journal of Dental Science and Medicine
Open Access

Dental implants have become a well-established solution for replacing missing teeth; offering a durable; functional; and aesthetic restoration. The success of dental implants depends on several factors; with primary stability being one of the most critical. Primary stability refers to the initial mechanical stability of the implant immediately after placement and is primarily determined by the interaction between the implant surface and the surrounding bone tissue.

While traditional implants have been extensively studied; short dental implants (SDIs) have gained increasing popularity in recent years; particularly in cases where patients have limited bone height or poor bone quality. This often occurs in the posterior maxilla and mandible; where the bone volume is insufficient for the placement of longer implants. Short implants are advantageous because they reduce the need for invasive surgical procedures such as bone grafting. However; concerns about the primary stability of SDIs persist; as their shorter length may result in less bone-to-implant contact (BIC) compared to longer implants.

Previous studies have indicated that implant length and insertion depth are crucial factors affecting the primary stability of dental implants. Longer implants generally achieve higher primary stability due to the larger surface area available for osseointegration. Similarly; the insertion depth plays an important role; as implants placed deeper into the bone often experience better mechanical engagement with the surrounding tissues. Despite this; there is limited research specifically examining the interaction of implant length and insertion depth on primary stability; especially for short implants. Therefore; the purpose of this study was to investigate the effect of implant length and insertion depth on the primary stability of short dental implants using a novel artificial bone mandible model in vitro.

Materials and methods

Study design

This in vitro study was conducted using a custom-designed artificial bone mandible model; which was fabricated to mimic the anatomical and mechanical properties of the human mandible. The model was designed to replicate the conditions found in clinical practice; particularly in situations where short implants are commonly used. The primary stability of short dental implants was evaluated in relation to two variables: implant length and insertion depth.

Materials

The following materials were used for this study:

1. Implants: A series of titanium dental implants (3.5 mm diameter) with varying lengths (6 mm; 8 mm; and 10 mm) were selected for testing. The implants were commercially available and featured a sand-blasted; large grit acid-etched (SLA) surface to enhance osseointegration.
2. Artificial bone mandible model: The artificial bone mandible model was made of polyurethane with a density similar to human bone (approximately 0.8 g/cm³); as this is commonly used to simulate bone properties in laboratory settings. The model consisted of a 3D-printed mandible structure with predefined implant sites; including areas representing the posterior region of the lower jaw where short implants are typically used.
3. Insertion tool: A mechanical insertion device was used to standardize the insertion force and technique across all implants. The device allowed the implants to be placed at varying depths (3 mm; 5 mm; and 7 mm) while maintaining consistent torque during insertion.
4. Measurement device: To assess primary stability; a torque wrench was used to measure the maximum insertion torque (IT) and the removal torque (RT) for each implant. These parameters serve as indicators of the implant’s initial stability.

Experimental groups

The implants were divided into nine experimental groups based on two factors: implant length (6 mm; 8 mm; 10 mm) and insertion depth (3 mm; 5 mm; 7 mm). This resulted in the following groups:

• Group 1: 6 mm implant; 3 mm insertion depth
• Group 2: 6 mm implant; 5 mm insertion depth
• Group 3: 6 mm implant; 7 mm insertion depth
• Group 4: 8 mm implant; 3 mm insertion depth
• Group 5: 8 mm implant; 5 mm insertion depth
• Group 6: 8 mm implant; 7 mm insertion depth
• Group 7: 10 mm implant; 3 mm insertion depth
• Group 8: 10 mm implant; 5 mm insertion depth
• Group 9: 10 mm implant; 7 mm insertion depth

Each group was tested in triplicate; resulting in a total of 27 implants.

Procedure

1. Implant placement: The implants were placed into the pre-designed holes in the artificial bone mandible model using the mechanical insertion tool. The insertion was performed at three different depths for each implant length; with the insertion depth being standardized to 3 mm; 5 mm; or 7 mm from the surface of the artificial bone.
2. Torque measurements: The primary stability of each implant was assessed by measuring the insertion torque (IT) during placement and the removal torque (RT) after insertion using the torque wrench. These values were recorded and analyzed to evaluate the mechanical stability of the implants.
3. Data analysis: The data were analyzed statistically using analysis of variance (ANOVA) to assess the impact of implant length and insertion depth on primary stability. Post-hoc pairwise comparisons were performed using Tukey’s test to identify significant differences between the experimental groups [1-5].

Results

Insertion torque (IT)

The insertion torque (IT) data revealed significant differences based on implant length and insertion depth. Implants with longer lengths (8 mm and 10 mm) consistently exhibited higher insertion torques compared to the 6 mm implants; regardless of insertion depth. The IT values were highest for the 10 mm implants inserted to a depth of 7 mm; with a mean torque of 45.7 Ncm; followed by 8 mm implants at 5 mm depth (mean torque 37.8 Ncm); and 6 mm implants at 3 mm depth (mean torque 28.1 Ncm).

A positive correlation was found between insertion depth and insertion torque; with deeper implants generally requiring higher torques for insertion. Specifically; the 10 mm implants at a depth of 7 mm showed the highest insertion torque; while the 6 mm implants at a depth of 3 mm showed the lowest.

Removal Torque (RT)

The removal torque (RT) data followed a similar trend; with longer implants and deeper insertion depths yielding higher removal torques. The highest removal torque was observed in the 10 mm implants at a depth of 7 mm (mean RT 40.1 Ncm); followed by the 8 mm implants at a 5 mm depth (mean RT 35.2 Ncm). The 6 mm implants placed at 3 mm depth exhibited the lowest removal torque (mean RT 26.3 Ncm).

The differences in removal torque between the experimental groups were statistically significant (p < 0.05); suggesting that both implant length and insertion depth are key factors influencing the primary stability of short dental implants.

Discussion

The findings of this study support the hypothesis that both implant length and insertion depth significantly affect the primary stability of short dental implants. Implants that were longer and inserted deeper into the bone exhibited higher primary stability; as indicated by both insertion and removal torque values. These results align with previous studies that have shown a positive correlation between implant length and primary stability; as longer implants engage more bone surface and provide greater mechanical retention.

Insertion depth also plays a critical role in achieving higher primary stability. Deeper implant placement increases the contact area between the implant and the surrounding bone; improving the mechanical interlock and resistance to forces. This finding is consistent with the concept that deeper implants benefit from increased bone-to-implant contact (BIC); which is essential for achieving successful osseointegration.

Although short dental implants have the advantage of reducing the need for bone grafting; they may suffer from lower primary stability compared to longer implants. However; this study demonstrates that implant length alone does not determine primary stability; the depth of insertion also plays a pivotal role in optimizing implant stability. Therefore; clinicians should consider both the length of the implant and the insertion depth when planning for SDI placement; particularly in cases where the available bone height is limited [6-10].

Conclusion

This in vitro study highlights the significant impact of both implant length and insertion depth on the primary stability of short dental implants. The results indicate that longer implants and deeper insertions lead to higher primary stability; as measured by insertion and removal torque. These findings have important clinical implications for the design and placement of short dental implants; suggesting that optimizing insertion depth and selecting an appropriate implant length can improve the chances of success in cases with limited bone height. Further research in clinical settings; particularly with a larger sample size and a broader range of implant types; is warranted to confirm these findings and further elucidate the role of these variables in achieving successful outcomes with short dental implants.

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