Effect of Closed-Tray Impression Coping Design on the Dimensional Accuracy of Implant Definitive Casts: An In Vitro Study

When a dentist replaces a missing tooth with a titanium post anchored into the jawbone, the final crown must fit with absolute precision. Unlike natural teeth, which have a tiny bit of give and can shift slightly to accommodate minor errors, these artificial posts are rigidly locked into the bone. If the model used to build the crown is even slightly off, the resulting bridge or crown will not sit flush. This misalignment creates stress that can loosen screws, fracture the restoration, or damage the surrounding bone. To avoid this, the dentist must take an impression of the mouth to create a perfect replica of the implant positions. One common method involves placing a small metal post into the implant, taking a mold of the mouth with that post inside, and then removing the post to place it back into the mold later. The success of this entire process hinges on how well that metal post stays in place within the mold and how accurately it returns to its original spot when the mold is filled with stone.

A team of researchers set out to test whether the shape and surface texture of these metal posts actually matter for the final accuracy of the model. They built a rigid, artificial upper jaw made of acrylic resin and placed five metal implants into it, arranged in a straight line to mimic a full set of teeth. They then created five different designs for the metal posts used in the impression process. One design was the standard, commercially available version found in dental offices today. The other four were experimental variations: some were square in shape, while others were slightly tapered like a cone. Within each shape, they tested two levels of surface detail: one with shallow, limited grooves and another with deep, extensive grooves running along the length of the post. The idea behind the grooves was that they would act like a grip, holding the post firmly inside the soft impression material so it wouldn't move. The researchers reasoned that a stronger grip should lead to a more accurate model.

To test this, a single skilled operator made thirty separate impressions using a specialized tray and a silicone-like material. For each group of six impressions, the metal posts were removed from the master model, placed back into the corresponding holes in the hardened silicone, and then the entire mold was filled with liquid stone to create a hard cast. Once the stone set, the researchers used a high-precision optical scanner to measure the distance between the metal posts in the new stone casts and compared them to the exact distances in the original master model. They looked for any deviation, measuring how many micrometers the posts had shifted from their true positions.

The results challenged the common assumption that a tighter grip equals a better result. The standard, commercially available post produced the least accurate casts, with an average deviation of 330.4 micrometers. Among the experimental designs, the post with the square shape and the deepest, most extensive grooves performed surprisingly poorly, deviating by 281.8 micrometers. In fact, the designs that were the most accurate were the ones with the least aggressive features. The square post with only limited, shallow grooves and the tapered post with extensive grooves both yielded significantly better results, with deviations around 200 to 208 micrometers. The researchers found that the deep, extensive grooves on the square post actually distorted the impression material when the post was pulled out, causing the post to sit in a slightly different position when it was put back in. The tapered shape helped because it allowed the post to separate from the material more gradually, reducing that distortion.

Interestingly, the design of the post did not change the accuracy of the side-to-side distances between the posts; the errors in those measurements were consistent across all groups. However, the design had a major impact on the front-to-back accuracy and the overall average error. The study concluded that trying to increase retention by adding more surface grooves can be counterproductive. Instead, a tapered shape or a design with minimal grooving provides a more faithful transfer of the implant positions. While even the best design in this study still showed some error—enough to exceed the threshold for a perfect mechanical fit—the findings suggest that dentists can improve the accuracy of their models simply by choosing a post with a tapered outline or limited surface texture, rather than relying on the deeply grooved, square posts that are often assumed to be the most secure.