Comparison between CAD/CAM-milled and cast hybrid full-mouth restorations on dental implants

Authors: Dr Dimitar Filthcev, Sofia, Bulgaria

The restoration of the edentulous jaw with an implant-supported hybrid bridge is an established procedure. Different types of prosthetic restoration require different manufacturing technologies for frameworks and veneers. In the present prospective study, the author compares cast-metal with CAD/CAM fabrication and the use of composite versus ceramic materials for veneering. The rehabilitation of an edentulous jaw is a major challenge for any clinician. The advent of digital technologies has given clinicians new options for treating complicated clinical cases with great accuracy and predictability. There are five possible clinical approaches to the treatment of affected patients:

  • conventional complete dentures;
  • two implants and an overdenture with ball or Locator attachments;
  • four implants and a removable prosthesis with bar and ball attachments;
  • four to eight implants and prosthesis with a screw-retained hybrid bridge (Toronto bridge);
  • six or more implants and a cemented

A treatment plan with four or more implants and a hybrid prosthesis is gaining in popularity thanks to the simplified protocol and excellent clinical results. Gallucci et al. conducted a five-year longitudinal study of fixed implant-supported restorations with distal cantilevers and found implant success rates of 100 per cent in the mandible and 97,6 per cent in the maxilla, and a 100 per cent success rate of the prosthetic design. Capelli et al. conducted a multicentre study and presented similar results. Maló et al. and Agliardi et al. found success rates of 98  per cent for mandibular implants at five years and 94 per cent at ten years, with a prosthetic success rate of 99 per cent at ten years. Testori et al. conducted multicentre research on 342 implants over a period of 60 months. Four or six implants were placed, with the angulation of the distalmost ones being between 25° and 35°. Implant success rates were 100 per cent in the mandible and 97.5 per cent in the maxilla, with a prosthetic success rate of 100 per cent. The authors did not report additional bone loss at the angulated implants and prosthetic complications. They concluded that immediately loaded angulated implants have the same success rates as straight ones. The difference between milled and cast-metal framework designs for full-arch rehabilitation has also been studied. Paniz et al. explored the accuracy of the metal frameworks of full-arch restorations and found more than ten times as much deviation in the anterior part of cast frameworks compared to milled frameworks. Christensen found many advantages of digital modelling and CAD/CAM milling over more traditional approaches. The purpose of the present prospective study is a clinical comparison between CAD/CAM-milled and cast hybrid full-mouth restorations on dental implants with ceramic or composite veneers for the treatment of the complete edentulous jaw.

Materials and methods

The patients were treated between January 2011 and February 2015 at a dental centre. The choice of patients was random; inclusion criteria were:

  • indication for a full-mouth restoration;
  • minimum age 50 years;
  • adequate height and width of the bone to accommodate an implant with a diameter of at least 7 mm and a length of at least 8.0 mm;
  • good oral hygiene;
  • willingness to sign a declaration for informed consent.

The study excluded patients with:

  • general medical conditions that could compro- mise the treatment result;
  • drug addiction;
  • mental disorders;
  • ongoing chemotherapy;
  • strong smoking habit (more than 20 cigarettes per day).

All patients received antimicrobial prophylaxis with cefalexine 500 mg twice a day for five to seven days, starting the day before the operation. A local anaesthetic was used at 100 mg twice a day, for three days. Articaine/dentocaine and postsurgical analgesic treatment with nimesulide (Aulin) was prescribed additionally as needed. Instructions for maintaining oral hygiene were also given. All surgical steps and implant positions were planned based on a cone-beam CT and an analysis of a diagnostic plaster cast. A temporary restoration was prepared in the lab and used to guide the placement of the implant. A one-stage surgical protocol was followed that involved immediate provisionalization, starting with a crestal incision and reflecting of a mucoperiosteal flap. The entire thickness of the gingival tissues was reflected with the flap. Ten patients with edentulous jaws received 58 implants (TSV; Zimmer Biomet, Warsaw, IN, USA). The implants were 3.7, 4.1 and 4.7 mm in diameter and 8, 10, 11.5 and 13 mm in length and were placed according to the recommended protocol ata minimum insertion torque of 20 Ncm. The implant platform was positioned at the level of the alveolar ridge. The implants were placed in the regions with the most favourable bone structure, some at an angle of 40°. The stability of the implants was evaluated using an ultrasound method in terms of ISQ units (Osstell Mentor; Integration Diagnostics, Göteborg, Sweden) during the surgery and then rechecked as the abutment was replaced. Two measurements were performed with a torque wrench, on the vestibular and mesial sides, and the average value was recorded.

Immediate restoration

Immediately after placing the implants,  multi-  unit abutments with different profiles, angles and heights were installed (Fig. 1) and immediate loading was performed when primary stability of all implants (20 Ncm or more, ISQ above 55) was guaranteed. The previously prepared complete denture was used for temporization.

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If corrections were needed, an impression was taken with the prosthe- sis before closing the flap, isolation with sterile rubber dam was performed and mattress sutures with 4-0 polyester (Omnia, Fidenza, Italy) were placed (Figs. 2a and b)

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The construction was finished in the dental laboratory and the provisional dentures were placed in the patient’s mouth within 48 hours (Figs. 2c and d). The teeth in the distal cantilever areas were out of occlusion on closure. A space of more than 1 mm was provided for oral hygiene beneath the restoration. The patients were recalled for days 7, 10 and 15 after surgery. The sutures were removed on day 10.

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Final restoration

After three months, the provisional restoration was removed and the implants stability was measured (Osstell). Where needed, the multiunit tapered abutments were changed to match the new gingival profile. Impressions with an open tray and splinted technique using ligature wire and acrylic resin (Pattern Resin; GC, Tokyo, Japan) were taken. The working casts were poured in plaster, a gingival mask was prepared and the centric occlusion was defined with waxed-up patterns placed on the implants and temporary abutments. Four screw-retained hybrid designs with cast chromium-cobalt frameworks were made in a dental lab in Bulgaria, while seven designs with milled titanium frameworks were made at the Zfx milling centre in Munich, Germany. At the German centre, the provisional dentures were scanned and the framework designed based on the prosthetic parameters (Figs. 3a to c)

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After a try-in, the frameworks were finished (Figs. 3d and e); four restorations were veneered with a ceramic material and the remaining seven with composite resin (Figs. 4a to c and 5a to d). The pontic sides of the implants had a convex shape for oral hygiene purposes.

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Follow-up visits

At six months, the patients were recalled for follow-up visits, and the implants were examined clinically and radiologically. The clinical evaluation involved measurement of the probing depth, the level of clinical attachment and the modified gingival index. The stability of the implants was evaluated according to the following criteria: mobility, pain, discomfort, increased sensibility, paraesthesia, signs of infection around the implant and loss of crestal bone of no more than 1.0 mm during the first year and 0.2 mm during the following years. Professional oral hygiene was performed if needed. Pocket depths were measured with a periodontal probe in four planes, vestibular, lingual, mesial and distal. The degree of peri-implantitis was estimated according to the modified Löe and Silness index (1963):

  1. normal peri-implant mucosa;
  2. slight inflammation, slight change in colour, slight oedema;
  3. inflammation, redness and oedema;
  4. severe inflammation, redness, oedema and ulceration.

A modified Mombelli scale was used to estimate the plaque accumulation. The radiographic examination was performed by an X-ray unit (Planmeca, Helsinki, Finland) using the parallel technique. Linear measurements from the point of first contact between the implant and the bone to the implant shoulder were taken mesially and distally using the integrated software for parallel measurements. Patients were recalled for yearly check-ups, at which the restorations were removed and then abutments cleaned. Patients were followed for 40 months or longer.

Results and discussion

Of the ten patients included in the study, six were men and four were women. The average age was 68 years. There were six mandibular and five maxillary edentulous situations. Of the 58 implants overall, 28 were placed in the maxilla and 30 in the mandible. At 40 months on average, a success rate of 100 per cent was registered according to the criteria for implant success introduced by Szmukler et al. No bone loss was observed after the first year, while the mean bone loss was measured at 0.17 mm after the second year. The stability of the implants in terms of ISQ units was significantly increased at the second test at three to four months to a mean value of 80 as compared to 58 immediately after placement. Stable average probing depths were observed at the first year (0.65 mm), at two years (0.70 mm) and at three years (0.73 mm) after implant placement. The mean gingival index (GI) was 0.12 at the first year and 0.27 at the second year. The mean plaque index (PI) was 0.29 at the first year and 0.48 at the second.

Comparison between cast and CAD/CAM-milled fabrication

With regard to prosthetic complications, in the first group with the cast restorations there were:

  • one case of a fractured screw of the multiunit abutment;
  • three cases of screw loosening;
  • one case of chipping of the pink porcelain milled at the cast constructions;
  • four cases of debonding of the metal-ceramic crowns covering the screws

There were no prosthetic complications affecting the milled restorations. Clinically, the fit of the milled framework was more passive and exhibited no internal stress (Figs. 6a and b).

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This reduced potential force-related problems with cast designs (Fig. 7a and b) such as screw loss, framework deformation or fracture of the aesthetic material.

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Ceramics or composite

Whether ceramics or composite was chosen for veneering the framework made a significant difference. Ceramic chipping as described in the literature, the sheer weight of the ceramic restorations, the screw loosening and the possible debonding of the ceramic crowns near screw holes all prompted us to make more use of composite in clinical practice. Composite resin is easy to maintain or repair if there are any defects, the restoration weighs less and has very good aesthetic properties; masticatory loads are less. There was no bone resorption around the angulated implants. The two-part angulated abutments of different widths, heights and emergence profiles facilitated compensation corrections for unfavourable implant angles and preservation of the parallelism of the design. Two-part abutments provide an extra benefit because of the double screw connection. All of this allows the mandibular implants to be splinted across the entire arch despite the mandibular flexion effect described by Misch et al. All potentially harmful forces (occlusion, laterotrusion) were concentrated at the second abutment located at the level of the gingival edge, so that the implant was protected from them. Soft-tissue comfort is better with fixed hybrid bridges than with removable bridges or complete dentures, as the pontics of the hybrid bridge will not touch the mucosa and feature a semi-hygienic profile. At check-up time, as we remove dthe restorations, we found that the soft tissue at the implants and the mucosa under the hybrid prosthesis were in very good condition if oral hygiene was good. We found no significant differences in the condition of the soft tissues around restorations with composite or ceramic veneers.

Conclusion

Our clinical results categorically support the use of hybrid bridges on four, five or six implants, which is in agreement with observations in the literature. CAD-designed and CAM-milled frameworks have many advantages over cast frameworks, and the long-term prognosis of the restorations is better. The composite material used to veneer the frameworks has aesthetic qualities similar to ceramics while offering mechanical and functional advantages. Based on these results, we can only recommend CAD/CAM framework design and fabrication and composite veneers on two-part tapered abutments for the rehabilitation of the completely edentulous jaw (Figs. 8a to g).

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