[Suwon Gwonseon-gu Dentist] Crown Lengthening: How Much Bone Should Be Removed? Calculating Ferrule and Crown-to-Root Ratio

Crown lengthening is a procedure that reshapes the gum and the bone together to secure, above the gum, a wall for the crown to grip.

Written by Dr. Ahn Seong-jin, Chief Director, Yeonhap Dental Clinic · Published Aug. 12, 2026 · Last updated Aug. 29, 2026

This article answers the six questions below in order. · What does crown lengthening aim to achieve? · How much ferrule should be secured in crown lengthening? · How much bone should be removed in crown lengthening? · Why do you stop at 2 mm instead of 3 mm in crown lengthening? · What is lost when bone is removed in crown lengthening? · What decides whether to perform crown lengthening?

A crown does not merely cover a tooth; it grips it. The band of sound tooth structure that the inside of the crown wraps around is called the ferrule. When tooth structure drops below the gum line because of decay or fracture, this band disappears, and if a crown is placed in that state, chewing force falls only on the bonded surface.

A typical example is a molar whose back surface, pressed by an impacted wisdom tooth, decays deeply below the gum; this is the situation covered in An Impacted Wisdom Tooth Can Make the Neighboring Molar Collapse First.

Crown lengthening solves this problem by lowering the gum line. The gum is lifted and the bone is reshaped, exposing tooth structure that was submerged beneath the gum. However, this procedure has a cost. As much wall as you gain, that much of the bone that surrounded the root is lost.

That is why Yeonhap Dental Clinic first calculates not whether this procedure can be done, but what will remain afterward. This article shows that calculation in order.

What we check first when choosing between extraction and preservation is covered in Implant Consultation: 3 Things We Check on the First Day.

크라운을 씌우자고 했는데 잇몸 수술부터 해야 한다는 말을 들으면 당황스러우실 수 있습니다.

[글의 목차]

· 치관확장술에서 페룰은 얼마나 확보되어야 하나요?

· 치관확장술에서 뼈를 얼마나 다듬어야 하나요?

· 치관확장술로 뼈를 다듬으면 무엇을 잃나요?

· 치관확장술 시행 여부는 무엇으로 결정하나요?

충치나 파절로 치질이 잇몸선 아래까지 내려가면 크라운이 붙잡을 건전한 벽이 부족해지고, 그대로 씌우면 남은 치질과 접착 부위에 부담이 커질 수 있습니다. 매복 사랑니에 눌린 어금니 뒷면이 잇몸 아래로 깊게 썩는 경우가 대표적이며, 매복 사랑니를 두면 옆 어금니가 먼저 무너집니다에서 다뤘습니다.

다만 벽을 얻는 만큼 치근을 감싸던 뼈는 줄어듭니다. 그래서 연합치과의원은 할 수 있는지보다 하고 나서 버티는지를 먼저 계산합니다.

It is reported that fracture resistance rises markedly when the sound tooth structure remaining above the gum is 1.5 to 2 mm or more.

Ferrule is not a matter of presence or absence but of height. A literature review summarizes that in root canal-treated teeth, fracture resistance improves significantly when a ferrule of 1.5 to 2 mm or more is secured (Juloski 2012). This figure becomes the target value for crown lengthening.

If the ferrule is a condition for the crown, the condition for the root canal inside it is a sterile field; this is covered in Why a Rubber Dam Matters in Root Canal Treatment.

One point to note is that this band matters most when it wraps around the full 360 degrees. If only one wall remains and the opposite side has collapsed, height alone is hard to judge by. That is why Yeonhap Dental Clinic checks which surfaces remain along with the height of the wall.

And this target value is only a starting point. To secure 1.5 to 2 mm, the gum line has to be lowered by that much, and to lower the gum line, the bone beneath it has to be reshaped. This leads to the next calculation.

Space is needed between the restoration margin and the bone for the gum to reattach, and that value determines the amount removed.

At the boundary where the tooth meets the gum, there is a fixed zone occupied by the junctional epithelium and connective tissue attachment. This is called the biologic width, reported at an average of about 2.04 mm in humans (Gargiulo 1961 · Vacek 1994). Although individuals vary, this zone is always present in healthy periodontal tissue.

There is one common misconception here: that it should be enough to cut away the gum alone. If only the gum is excised, this attachment zone is not secured, and within a few months the gum may return to its original height or inflammation may recur. This is why the bone has to be reshaped as well.

In summary, the depth to be removed is calculated as the sum of three values: the ferrule height to be secured above the gum, the biologic width that the attachment will occupy, and how far below the bone the tooth structure currently lies. The sum of these three is the amount of bone to be reshaped, and that amount is the root support lost.

Yeonhap Dental Clinic stops the removal at the point that secures 2 mm between the margin and the bone crest.

The literature includes a view recommending at least 3 mm between the restoration margin and the bone crest. This is the biologic width of about 2 mm plus the depth of the gingival sulcus. On the other hand, there is also a view that uses 2 mm, the attachment zone itself, as the reference, so this figure is not uniformly set within the literature.

Yeonhap Dental Clinic chooses 2 mm because what is lost as the amount removed increases is clear. Removing 1 mm more gives more margin on the side of gum inflammation, but the bone surrounding the root is lost by that much. And as you will see in the next section, what determines how well a tooth holds up after crown lengthening is reported to be the remaining root support rather than attachment margin.

It is a matter of deciding what to protect between the two values, and Yeonhap Dental Clinic prioritizes root support. We should add, however, that this is not an area with a single correct answer, and it is a point on which different dentists may judge differently.

The crown-to-root ratio is the ratio of the length of the part exposed above the bone to the part embedded in the bone.

If the height of the chewing surface stays the same and only the gum line drops, the part exposed above the bone becomes longer by that much. With a longer upper part and a shorter lower part, the leverage transmitted to the root grows even under the same force. This is a structure that makes mobility more likely.

The effect of this ratio on prognosis is documented in the literature. In a study that followed root canal-treated, crowned molars for 10 years, the 5-year survival of the group that underwent crown lengthening was 82.2% and the 10-year survival was 51%. The control group that did not need crown lengthening had 88.6% and 74.5%, respectively, and a significant difference appeared at 10 years. In the same study, teeth that underwent crown lengthening were 2.3 times more likely to be extracted than those that did not, and teeth whose postoperative crown-to-root ratio became 1:1 had the lowest survival at 40% (Patil 2019).

In prosthodontics, the conventional values of an ideal crown-to-root ratio of 1:1.5 for an abutment tooth and a minimum acceptable ratio of 1:1 have long been cited. However, a literature review dealing with this ratio concluded that there is too little supporting research to make a definitive recommendation (J Prosthet Dent 2005). In other words, it is a widely used criterion, but not a figure established as a pass mark.

That is why Yeonhap Dental Clinic does not use this ratio as a pass-fail cutoff. Instead, we measure the length that will remain after removal in advance and, the closer it gets to 1:1, discuss other options with you.

Crown lengthening is decided not by whether it can be done but by the root length that remains after removal.

Before deciding on the procedure, Yeonhap Dental Clinic measures three things on 3D CT: how far below the bone the tooth structure currently lies, how far the bone must be reshaped to secure a 1.5 to 2 mm ferrule and 2 mm between margin and bone crest, and how much root will remain in the bone after that. The third value is the center of the judgment.

When a crack is also present, we check the Q-ray (fluorescence) reaction as well. This is because where the fracture line stops changes the preconditions for the procedure.

More detail on Q-ray is covered in The Limits of Q-ray Diagnosis and How Yeonhap Dental Clinic Overcomes Them.

In summary, the order of judgment is as follows.

· Check the position and shape of the remaining tooth structure.

· Add the target ferrule and the attachment space to calculate the amount to remove.

· Measure the root length that remains in the bone after removal.

· Check whether the crown-to-root ratio approaches 1:1.

· If it does, discuss other options together.

[Key Takeaways]

✓ The goal of crown lengthening is to secure a 1.5 to 2 mm ferrule, and this value is reported in association with fracture resistance (Juloski 2012).

✓ The biologic width is reported at an average of about 2.04 mm (Gargiulo 1961), and Yeonhap Dental Clinic stops the removal at the point that secures 2 mm between margin and bone crest.

✓ The 10-year survival of the crown lengthening group is reported at 51%, and 40% when the postoperative crown-to-root ratio is 1:1 (Patil 2019), so the root length remaining after removal is the center of the judgment.

The practical test of a crown lengthening consultation is whether the dentist can explain the amount removed and its consequences in numbers.

Yeonhap Dental Clinic, at 230 Geumho-ro, 5th Floor (Cham Clinic Building, Tap-dong), Gwonseon-gu, Suwon, across from Suwon NongHyup Guun Branch (231 Geumho-ro), has held the same location since 2002. Dr. Ahn Seong-jin, Chief Director and Integrated Dentistry specialist, is currently in charge.

[5 Things to Check Before a Crown Lengthening Consultation]

· Whether the dentist distinguishes and explains whether only the gum is excised or the bone is also reshaped

· Whether the ferrule height to be secured can be stated in numbers

· Whether the root length that remains after removal is measured in advance

· Whether the bone height is checked in three dimensions on 3D CT rather than from a single image

· Whether cases where it is better not to proceed are also explained

Before asking whether this procedure can be done, I look at whether the tooth will hold up afterward. There is a reason I came to do it this way. Once, knowing it was unlikely to work, I restored a tooth because I wanted the patient to be able to use it a little longer, but it did not last and ended up being extracted. No matter how well the tooth itself was made, the result was the same without bone to hold the root. Since then, I begin by measuring the length that will remain after removal. Choosing to remove 1 mm less bone may look overly cautious, but I have seen that 1 mm decide the difference of several years.

[References]

Juloski J, Radovic I, Goracci C, Vulicevic ZR, Ferrari M. Ferrule effect: a literature review. J Endod. 2012;38(1):11-19.

Gargiulo AW, Wentz FM, Orban B. Dimensions and relations of the dentogingival junction in humans. J Periodontol. 1961;32(3):261-267.

Vacek JS, Gher ME, Assad DA, Richardson AC, Giambarresi LI. The dimensions of the human dentogingival junction. Int J Periodontics Restorative Dent. 1994;14(2):154-165.

Patil K, Khalighinejad N, El-Refai N, Williams K, Mickel A. The effect of crown lengthening on the outcome of endodontically treated posterior teeth: 10-year survival analysis. J Endod. 2019;45(6):696-700. PMID 31005334.

The prosthodontic concept of crown-to-root ratio: a review of the literature. J Prosthet Dent. 2005. PMID 15942617.

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This post was written in accordance with the Korean Medical Service Act and the Ministry of Health and Welfare's medical advertising guidelines, and is intended to provide medical information. It does not guarantee or assert the effect of any particular treatment. The condition and course of each tooth differ from person to person, so please decide on a treatment direction only after a thorough consultation with your dental team.

🦷 Prosthodontic and crown information: Yeonhap Dental Clinic prosthodontic treatment

📍 230 Geumho-ro, 5th Floor (Cham Clinic Building, Tap-dong), Gwonseon-gu, Suwon, Gyeonggi-do

🚕 Across from Suwon NongHyup Guun Branch (231 Geumho-ro) / about 15 minutes by taxi from Suwon Station

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📞 031-293-2207

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🟡 Open since 2002 / Treatment overseen by Dr. Ahn Seong-jin, Chief Director and Integrated Dentistry specialist

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