Implant Prosthetics
On this page
Direct answer
Restoring an osseointegrated fixture looks like crown-and-bridge work, but the hardware obeys stricter rules because the implant has no periodontal ligament, decays never, and forgives overload never. Every restoration begins with the retention choice — screw-retained for retrievability and no cement to shed into the sulcus, cement-retained for better aesthetics and anatomy when the screw hole would land on a cusp tip or incisal edge. The implant-abutment connection (external hex, internal hex, morse taper) and the platform-switch decision govern joint stability and crestal bone behaviour, and the occlusion is finished to implant-protected principles: light, axial, controllable contacts with no lateral interference in function.
What you must remember
- Screw versus cement retention: screw-retained crowns are retrievable and avoid cement-induced peri-implantitis but demand a favourable screw channel position; cement-retained crowns look better and pass more easily but can strand subgingival cement and are hard to retrieve.
- Abutment screws are tightened to manufacturer-specified torque, commonly in the 20-35 Ncm range, with a torque driver — never finger-tight, and re-torque after about ten minutes to settle the interface.
- Platform switching means seating a narrower abutment on a wider implant platform, moving the implant-abutment microgap and its inflammatory connective tissue inward, which spares marginal bone.
- Crestal bone routinely remodels to about the level of the first thread or the rough-smooth junction in the first year; a biologic width of roughly 3-4 mm of soft tissue attaches to the fixture above it.
- For full-arch and multi-unit work, open-tray (pick-up) transfer copings remain screwed in the impression and are more accurate than closed-tray (transfer) copings; verify jig seating on the master cast.
- Passive fit is verified with the one-screw or Sheffield test: tighten one screw and watch for lift-off of the opposite abutment — frameworks with detectable rock must be sectioned and re-soldered.
- Implant-protected occlusion (Misch): narrow the occlusal table, keep centric contacts over the fixture long axis and lighter than on natural teeth (shimstock drags on teeth, slips on implant crowns), and eliminate all non-working contacts.
- Peri-implantitis is managed by cumulative interceptive supportive therapy — mechanical decontamination, antiseptics, and graded surgical intervention — exactly the sequence the exam expects you to recite.
Planning a case from fixture to crown
Take a 45-year-old with a single missing mandibular first molar, fixture integrated and painless on percussion. Record the impression with an open-tray coping — confirmed seated radiographically — indexed into the tray with its screw. Choose the abutment by cuff height — roughly 1 mm of tissue per margin position, keeping the crown margin about 0.5-1 mm subgingival on the facial aspect for aesthetics and supragingival elsewhere for cleanability. Torque the definitive abutment to specification, re-torque after ten minutes, and try in the crown. At delivery, check contacts, then occlusion: in centric the crown holds shimstock more lightly than the adjacent natural molar, and in excursions it must be free of all contact under mutually protected articulation. Screw-retained delivery asks for a PTFE or cotton plug over the screw, composite seal, and a torque note in the record. Review at one year: bone stable at the first thread means the plan is working.
Where candidates slip
Three confusions recur in orals. The first is treating platform switching as a change in implant level rather than a mismatch of abutment diameter — say the mechanism, not the slogan. The second is forgetting that blunted proprioception cuts both ways: the patient cannot feel the heavy contact you left, so heavy contacts on implants survive silently until screw loosening or bone loss announces them. The third is the retrievability argument delivered without judgement — the textbook answer is screw-retained for full-arch and multi-unit situations where later complication is likely, cement-retained for single units with deep subgingival margins where cement removal is actually feasible and residual cement is the smaller risk. A candidate who answers one or the other without indication has missed the question.
Frequently asked questions
Which retention is preferred for a full-arch implant prosthesis?
Screw retention, because the prosthesis stays retrievable for maintenance, hygiene and repair, and cement removal around multiple fixtures is impractical.
What exactly does platform switching achieve?
A narrower abutment seated on a wider platform shifts the inflammatory zone at the microgap inward, preserving marginal bone at a higher level.
Why is the open-tray impression preferred for multiple implants?
Open-tray copings stay united within the impression and are unscrewed after setting, eliminating the reseating error of closed-tray transfers between non-parallel implants.
How does implant-protected occlusion differ from natural-tooth occlusion?
Contacts are lighter and axial over the fixture, the occlusal table is narrowed, and anterior guidance clears the implant of all lateral contacts.
What is the one-screw (Sheffield) test?
Tighten a single screw in a multi-unit framework; lift-off of other abutments shows loss of passive fit requiring sectioning and rejoining before delivery.
Why must abutment screws be torqued rather than hand-tightened?
Under-torqued screws loosen under function and over-torqued screws strip or fracture; the specified preload applied with a calibrated driver keeps the joint stable.