Retentive design of a small surgical guide for implant surgery: an in-vitro study

Static, computer-aided implant surgery (s-CAIS) contributes to an optimal 3D position of prosthetically driven dental implants [1], allowing rehabilitation with easily retrievable screw-retained restorations, a proper distribution of the occlusal forces, and sufficient access for oral hygiene [2,3]. First, the implants' optimal 3D position is virtually planned by merging the cone-beam computed tomography (CBCT) and intraoral scanning (IOS) data in dedicated treatment-planning software [2], [3], [4]. Then, utilizing 3D printers, the surgical guides are fabricated, making possible the accurate and predictable transfer of the virtual implant's position to the oral cavity [1]. Implant-planning software and 3D printers are readily available today and allow the in-house fabrication of surgical guides without the need to outsource them to a dental laboratory, so streamlining the process [5]. The parameters of the surgical guide's design, such as the guide-to-teeth offset, the material thickness and the insertion path, are usually predetermined in the planning software, prioritizing the insertion capacity of the surgical guide [2]. For example, an offset of 0.1–0.2 mm is usually recommended [6], compensating for the CBCT, IOS and superimposition errors [7], [8], [9], [10]. On the other hand, such an offset, due to the lower stability, might represent a source of error and eventually increase the deviation of the actual implant's position [11,12]. Although full-arch surgical guides are usually fabricated to improve stability, constructing smaller, four-teeth-supported surgical guides for single-tooth-gap cases provides an accuracy comparable to full‐arch guides [11,12].

Errors accumulate during the s-CAIS process, resulting in a deviation between the planned and the actual implant positions. The ITI Consensus report on digital technologies [13] declared that further research should quantify the error of every single step of the s-CAIS. Well-known factors within the digital workflow process that can contribute to the 3D deviation in the final implant's position are numerous [14], including the CBCT and IOS acquisition inaccuracy [7,[15], [16], [17], [18]], the implant system being used [19], the selected implant-planning software [20], the method of fabrication for the surgical guide [5,[21], [22], [23]], the 3D printer and the print settings [17,24], and the post-processing of the surgical guides [21,25]. In addition, a decision on the access flap [26], the degree of guidance [26,27], the type of tissue support [28], and the number of teeth supporting the surgical guide also affect the outcome of s-CAIS [11,23]. In contrast, the contribution of the small, tooth-supported surgical guide's stability in ensuring the correct transfer of the digitally planned implant position is unclear.

This study first aimed to develop an in-vitro system for testing the stability of the surgical guide used in s-CAIS. A bespoke testing platform was designed for the study's objectives, allowing standardization of the forces applied to the surgical guide from the buccal and oral directions. The testing platform was then used to evaluate the small surgical guide's stability by comparing a retentive surgical guide (RSG) having a reduced guide-to-teeth offset with a conventional surgical guide (CSG) under in-vitro conditions. After standardized forces were applied to the surgical guide via a drill handle, the displacement was measured and transferred to the virtual implant position via a geometrical calculation. An evaluation method was developed to measure the surgical guide's displacement, avoiding the need for direct measurements on scanned surfaces hampered by mesh irregularities. We hypothesized that by using the innovative testing platform and an explicit evaluation method, the small tooth-supported RSG stability after the application of eccentric forces will be better than the small tooth-supported CSG stability, resulting in lower deviation parameters for the virtually projected implants. Our second hypothesis predicted that due to the uneven distribution of the retentive surfaces, the forces applied from the buccal direction would result in a more significant implant deviation than the forces applied from the oral direction.

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