A Tipped Molar, a Centric Slide, and a Nonworking Interference
A 52-year-old woman complains that her upper right back tooth feels loose and is sore on waking. She clenches and grinds in her sleep, has visible masseter hypertrophy, but no joint pain or clicking. #30 was extracted three years ago and was never replaced. #31 has tipped mesially about 25 degrees and now stands slightly buccal to the arch, with a 4 mm pocket on its mesial aspect. #3 shows grade I mobility, fremitus, and a widened periodontal ligament space on the palatal root, with probing depths of 3 mm or less. Casts mounted in centric relation require a 2 mm anterior and 1.5 mm left slide to reach maximum intercuspation; shimstock first resists between the mesial marginal ridge of #31 and the distal marginal ridge of #2. On left lateral excursion, the left canine immediately discludes the left posterior teeth, but shimstock is firmly held on the right between the palatal cusp of #3 and the buccal cusp of #31.
1. Interpret the centric relation-to-maximum intercuspation movement together with the shimstock finding. Is this a stable occlusion, and what determines whether it should be treated?
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The mandible travels 2 mm anteriorly and 1.5 mm to the left to reach maximum intercuspation, and the first resistance is a single point on an inclined ridge - the mesial marginal ridge of the mesially tipped #31 against the distal marginal ridge of #2.
A pure, flat, anterior slide of up to about 1.5 mm ('long centric') can be well tolerated because the mandible is simply translating; no inclined plane redirects closing force. This is different in two ways:
- The slide has a lateral component, so the closing muscles must resolve force in two planes.
- The resistance is a point contact on an incline, not a flat surface, so it acts as a ramp. The direction is consistent with the mechanism: #31 has tipped mesially, presenting a mesially-facing incline that deflects the mandible forward and leftward as it closes.
That converts a deflecting contact into horizontal loading of both #31 and the maxillary posterior segment, which is precisely the pattern that produces fremitus, widened PDL, and mobility.
Whether to treat depends on evidence of harm rather than on the number itself: fremitus, mobility, widened PDL, wear facets, migration, symptoms, or restorative needs that require a stable reference. Here there is harm, so it should be treated.
2. Identify the excursive finding and explain why it is addressed before the canine guidance side. Why are nonworking contacts generally more destructive than working-side ones?
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The finding is a nonworking (balancing) interference on the right side between the palatal cusp of #3 and the buccal cusp of #31 during a left lateral excursion. The working side (left) is showing ideal canine guidance - immediate disclusion of all left posterior teeth.
Why the balancing contact takes priority
- On the working side, teeth are close to the joint and the muscle vector, and the canine is a favorable guide because its root morphology and proprioception are built for lateral loading.
- On the nonworking side, the contact occurs on inclines far from the joint and far from the working-side muscles, so the same muscle contraction generates much greater torque on the involved teeth and on the joint.
- There is no protective proprioceptive guidance on the balancing side; the contact is a passive collision.
- Canine guidance on the working side is something to preserve, not adjust - it protects the whole left quadrant. Adjusting it would be the classic error here.
Additional reason in this patient: #31 is mesially tipped and already has a 4 mm mesial pocket. A balancing contact on that tooth concentrates force on an already compromised periodontium, and it is the same tooth creating the centric deflection - the two findings point at the same culprit.
3. Outline your management in sequence, and state one thing you would deliberately not do.
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Sequence
- Document and confirm. Verify the contacts with shimstock and articulating film on mounted casts, not by eye, and take a periapical of #31 and #3 to establish baseline bone and PDL status.
- Address the parafunction first in principle, but relieve the acute load immediately. A flat-plane, fully adjustable stabilization appliance protects #3 and #31 while the occlusal plan is being worked out.
- Adjust the deflecting contact on the inclined surfaces of #31/#2 and the nonworking interference between #3 and #31, in that order - one contact at a time, re-verifying in centric relation and in excursion after each.
- Deal with the cause of the tipping: the unreplaced #30 space. Discuss uprighting #31 orthodontically, or restoring the space, and consider whether #31 is restorable and periodontally maintainable before investing in it.
- Re-evaluate mobility, fremitus, and pocket depth at 4–6 weeks. Mobility from occlusal trauma should improve as the PDL reorganizes; a worsening 4 mm mesial pocket suggests a periodontal component or a root fracture that changes the plan.
What I would not do: I would not simply extract #31 to make the interference disappear, and I would not begin by equilibrating the left canine guidance - it is the protective element of this occlusion. Equilibration is irreversible and should only follow a confirmed diagnosis, and never as a first step in a patient with untreated parafunction.