Molar protraction into a first molar space using skeletal anchorage
A 34-year-old man has missing 46 and 36, both extracted 2 years ago because of caries. He does not want two implants and asks whether the lower right space can be closed orthodontically. The 46 space is 8 mm; 47 is mesially tipped about 28 degrees with the mesial marginal ridge below the contact point; 48 is present and partially erupted. The lower left 36 space is 10 mm with 37 upright. The upper arch has 3 mm of crowding and a 4 mm overjet; the lower incisors are mildly crowded. The periodontium is healthy, with 2–3 mm probing depths and no bleeding on probing. The patient is a non-smoker and has good hygiene. He asks whether a screw is needed, how long it will take, and what happens if the screw fails.
1. Design the anchorage and biomechanics for uprighting 47 and closing the 46 space, including force levels and the likely duration.
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The 46 space is 8 mm, so a single implant would fit, but the patient prefers closure. The problem is that 47 is tipped 28 degrees and 48 is present; protracting 47 mesially without controlling the tipping will tip it further and open the 48 contact. I would place a buccal shelf miniscrew on the right, distal to 47 and below the mucogingival junction, as direct anchorage. An uprighting cantilever or a NiTi coil from the screw to a hook on 47 provides about 150 g; the initial vector is aimed to upright 47 and then to translate it mesially. A power arm or a second point of force application is needed to keep the force near the resistance centre and avoid further tipping.
Expect 6–9 months for uprighting and 8–12 months for 8 mm of closure, so 12–18 months in total. The 48 must be monitored; if it is healthy it can substitute for 47, but if it is to be extracted, closing 46 by moving 47 mesially may not be worthwhile. I would discuss both plans with the patient before starting.
2. Justify the choice of buccal shelf versus interradicular placement and list the risks you would discuss.
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The buccal shelf offers dense cortical bone, a large safe zone and a vector that is closer to the line of action for molar protraction. An interradicular screw between 47 and 48 is attractive in principle, but the space is reduced by the 28-degree tilt and the partially erupted 48, so root proximity and a narrow window make it less reliable. A 2.0 mm diameter, 10–12 mm long screw in the buccal shelf is my first choice; if the shelf is thin, a 1.5–2.0 mm screw placed more apically may be needed.
Risks to discuss: screw loosening or failure (roughly 10–20% in the mandible), pain and soft-tissue inflammation, damage to the root of 47 or 48, and injury to the inferior alveolar or mental nerve if the screw is placed too deep or too anterior. I would take a CBCT or panoramic view to check the nerve position and root proximity, use a surgical guide or a pilot drill, and keep the screw at least 2 mm from the roots.
3. At 4 weeks the screw is mobile with mild inflammation. What is your plan, and how do you avoid losing the anchorage?
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A mobile screw with inflammation is failing; I would remove it rather than continue loading, because a loose screw cannot provide stable anchorage and may damage the root or the nerve. I would check whether the cause was placement (too close to a root, thin cortex, insufficient insertion torque), soft-tissue irritation from the head, or excessive force. The site should be left to heal for 4–6 weeks before any replacement.
For the interim, I would stop the protraction and protect the arch with a passive or lightly activated lingual arch, and consider a sectional wire to hold 47 upright. The replacement screw would go more distal and apical on the buccal shelf, or a larger diameter screw could be used if bone allows. If the shelf is inadequate, alternatives are a second screw for a two-screw anchorage system, a palatal or lingual arch with elastics as a backup, or reconsidering the implant for 46. I would tell the patient that one screw failure does not mean the plan has failed, but that the timeline may extend by 2–3 months.