Evidence Synthesis: Full-Arch Digital Impressions

“I’ll just rescan that arch,” said the colleague, in the tone of someone who has said it many times and expects to say it many more. It is one of the most quietly revealing sentences in modern implant practice, because nobody says it about a single posterior crown. The full edentulous arch is the case where the confident digital workflow develops a stammer, and seven reviews on this site, two meta-analyses and five careful accuracy studies, have now mapped why, and what the profession has been improvising to get around it. The short version is that intraoral scanning (IOS) of a long span is not yet a solved problem, and the longer version is more useful than the short one.

The evidence collected here runs from a systematic review and meta-analysis and a second meta-analysis, through a flowable-marker accuracy study, a clinical scan-body brand trial, a scan-body height and library study and a horizontal scan-body comparison, to a 243-scanbody review of scan quality. They share one premise: a smooth, repetitive arch gives the stitching algorithm almost nothing to anchor on, and the error compounds with every millimetre of span.

What the Studies Actually Showed

Both meta-analyses reach the same verdict from different angles. The first pooled photogrammetry against IOS and found photogrammetry ahead on every axis: precision mean difference (MD) −0.03, angular accuracy MD −0.12° (p < 0.00001) and trueness MD −0.16 (p < 0.00001). The second found stereophotogrammetry (SPG) more accurate than IOS in 10 of 13 studies, with pooled effect sizes of 3.426 for surface trueness and 4.893 for surface precision. The raw ranges are the part worth keeping: SPG surface trueness ran 5.18–48.74 µm, while IOS ran 14.8–67.72 µm, and some IOS angular values crossed the 1° mark that worries anyone thinking about passive fit, while SPG stayed under it.

The improvisations are where it gets interesting. Flowable composite markers placed between scan bodies cut a Trios4’s trueness error from 165.5 µm to 52.1 µm and its precision from 215.4 µm to 52.5 µm, dragging a mid-tier scanner up to statistical parity with a splinted conventional impression (the 23.2 µm benchmark in that study). Horizontal scan bodies did something structurally similar: a vertical reference body was significantly worse than every horizontal design across every scanner tested (p < 0.001), and the best pairing reached 12 µm of trueness while the worst vertical pairing sat at 77 µm. Both findings say the same thing in different hardware: give the algorithm more geometry to hold and it stops drifting.

Where They Agree, and Where They Argue

Agreement is the strong part. A featureless arch is the enemy; feature density is the fix; and photogrammetry, by recording only coordinates, remains complementary rather than total, since you still need a scan for soft tissue and the opposing arch. The scan-quality review adds a sobering footnote that nobody enjoys: across 243 scanbodies, every single one aligned successfully in the software, yet 44.4% showed rough texture and 16% impaired geometry on inspection. A green tick is not a certificate.

The arguments are about scope, and they matter clinically. The clinical scan-body trial found brand genuinely irrelevant, original, non-original and generic all delivered without significant difference, but only for short-span posterior fixed dental prostheses (FDPs) with a disciplined two-stage protocol, which is precisely not the full-arch case the meta-analyses indict. The second meta-analysis is candid that its photogrammetry verdict rests on vertical scan bodies, and that horizontal or aided systems might close the gap; the horizontal scan-body study then supplies exactly that evidence, with a 12 µm result rivalling photogrammetry’s range. The height-and-library study adds an orthogonal point the others miss: a short 4 mm scan body registers accurately, but only when the flat indexing surface clearance is around 1 mm and a truncated computer-aided design (CAD) library is used, library choice, not body height, governed the angular error.

Key Findings

• For the full edentulous arch, photogrammetry is the safer default. Both meta-analyses favour it on trueness, precision and angular accuracy, and the angular gap is the one that threatens passive fit.
• Feature density is the lever you actually control. Flowable markers cut a mid-tier scanner’s error by roughly two-thirds, and horizontal scan bodies beat vertical ones across every scanner. The algorithm is only as good as what you give it to hold.
• Scan-body brand is a non-issue, within its proven scope. Generic bodies performed clinically on short-span posterior FDPs; do not silently extrapolate that permission to the full arch, which was never tested.
• The CAD library is a hidden variable. A truncated library rescued a short scan body’s angular accuracy where a full-geometry library degraded it. Worth knowing before you blame the scanner.
• Software alignment is not quality control. Nearly half of 243 successfully aligned scanbodies had a rough or impaired surface. Inspect the mesh; do not trust the tick.
• The honest caveat. Most of this is in vitro, on dry models without saliva, tongue or a moving patient, and the strongest clinical signal is narrow in scope. The direction is consistent; the clinical confirmation is thin.

💡 The Clinical Bottom Line

If you are recording a full-arch implant case on Monday, treat the scan body, the scanner and the library as one instrument rather than three accessories, and assume the smooth arch is working against you the whole time. For the implant-position record specifically, photogrammetry or a rigid prototype try-in remains the defensible choice until you have deliberately assembled and verified an IOS combination that earns the trust. The most honest thing seven studies taught me is that the colleague reaching for the rescan was not doing it wrong. The arch was the problem, and we have spent a decade quietly building scaffolding around a scanner we sold as needing none.