Comparative Soft and Hard Tissue Responses to Titanium and Polymer Healing Abutments

Tissue integration to dental implants is a wound healing process that involves several stages of tissue formation and degradation.1,2 The establishment of the mucosal barrier around the implant is characterized by the gradual shift from a coagulum to granulation tissue followed by the formation of a barrier epithelium and the maturation of the connective tissue.3 

The soft tissue around implants was described in a series of experimental studies.4–7 Thus, the peri-implant mucosa consisted of a 2-mm long barrier epithelium and a 1–1.5 mm “connective tissue integration.”4 Collagen fibers occurred in large proportions and were mainly aligned in a direction that was parallel to the implant surface. Furthermore, the connective tissue integration zone had a low density of blood vessels and a large number of fibroblasts.8 

Additional animal studies9 documented that the material used in the abutment portion of the implant was of decisive importance for the quality of the attachment that occurs between the mucosa and the implant. Hence, abutments made of titanium or highly sintered alumina-based ceramic established similar conditions for mucosal healing to the abutment surface and allowed the formation of an attachment that included one epithelial and one connective tissue portion that were about 2 mm and 1–1.5 mm high, respectively. On the contrary, at sites where the abutments made of gold alloy or dental porcelain were installed at second stage surgery, no proper attachment seemed to form at the abutment level, but the soft tissue margin receded and bone resorption occurred. The abutment-fixture junction was hereby occasionally exposed, and the mucosal “seal” was established to the fixture portion of the implant.

There is limited information from human studies assessing the soft tissue interface for abutments with different material chemistry using clinical outcome measures.10 To our knowledge there is limited data evaluating the effect of materials such as polymers that are commonly used for implant provisionalization on the peri-implant soft tissue interface.

The aim of the present study was to comparatively evaluate soft and hard tissue responses to titanium and polymer provisional implant abutments over a 3-month period.

This study was designed as a prospective, randomized, controlled clinical trial. Sixteen patients who had at least 1 tooth missing posterior to the maxillary or mandibular canine were enrolled in the study. The following conditions were reasons for excluding a subject from participating in the study: insufficient bone volume at the recipient sites for placement of an implant with a diameter of at least 4.1 mm and length of at least 8 mm; active infection or severe inflammation in the areas intended for implant placement; uncontrolled diabetes mellitus, hemophilia, metabolic bone disorders, history of renal failure, current chemotherapy, and pregnancy; treatment with therapeutic radiation to the head region within the past 12 months; alcohol or drug abuse; and smoking of more than 10 cigarettes per day.

The study protocol was reviewed and approved by the Institutional Review Board of the University of Florida. All subjects received detailed information on the study and signed a written consent before the start of the treatment.

Control of periodontal infection, if applicable, was achieved by an initial treatment phase consisting of scaling and root planing, motivation, and oral hygiene instructions. If indicated, supplemental mechanical debridement with periodontal surgery was performed. The initial therapy was completed 30–60 days before the time of patient entry into the study. Patients were randomly assigned to a test or control treatment group by a computer-generated list. In the test group, following standard placement of the dental implant, a polymer healing abutment (poly[aryl-ether-ether-ketone], PEEK) was connected to the implant (test n  =  8). In the control group, following a similar dental implant installation procedure, a titanium healing abutment was connected to the implant (control n  =  8). The characteristics of the patients of the test and control groups are given in Table 1.

The implants used in the current study were Straumann Bone Level Implants (Straumann, Basel, Switzerland) with a diameter of 4.1 mm or 4.8 mm and with lengths varying from 8 mm to 12 mm. The selection of implant type was based on existing bone dimensions. The surgical treatment was performed under local anaesthesia and according to the manufacturer's manual by 2 periodontists (J.R. and T.K.). Immediately following local anaesthesia, an endodontic file with a rubber stop was inserted into the buccal mucosa perpendicularly at a point 5 mm apical to the crest of the edentulous ridge until bone contact was perceived. The rubber stop was positioned at the mucosal surface, and the distance from the rubber stop to the tip of the endodontic file was measured to the lowest half millimeter to determine mucosal thickness.

Crestal incisions were used and full thickness flaps were elevated to expose the bone. The recipient sites were enlarged according to the protocol of the manufacturer.

Subsequent to osteotomy preparation, the thickness of buccal and lingual bony plates was measured at a point 2 mm apical to the crest of the ridge with a caliper instrument at the lowest half millimeter. Dental implants were installed in the edentulous segments according to patient needs.

Healing abutments, either titanium (Straumann RC Healing Abutment, conical shape D 4.5 mm, H 6 mm) or polymer (Straumann RC Healing Abutment, customizable, D 7 mm, polymer) were placed according to the randomization protocol (Figures 1 and 2). All abutments extended transmucosally and remained completely out of occlusion. After abutment installation, the flaps were closed with interrupted sutures. Each patient received 1 g of amoxicillin twice daily for 7 days starting with the day of the implant surgery, and chlorhexidine, 0.12% rinse twice daily, for 2 weeks.

Three months following implant installation, the prosthetic treatment was performed according to the manufacturer's manual.

At the 2-week and the 3-month reexaminations, the following clinical parameters were recorded at the implant sites: presence of visible plaque (mesial, distal, buccal, and lingual surfaces), probing depth (PD), bleeding on probing (BoP), peri-implant mucosa height (PMH) at 6 sites of each implant (mesiobuccal, buccal, distobuccal, distolingual, lingual, and mesiolingual). Peri-implant mucosa height was recorded as the distance between the peri-implant mucosa margin and the most coronal part of the healing abutment. In addition, the width of buccal keratinized mucosa was recorded as the linear distance from the mucosal margin to the mucogingival line. All measurements were performed with a manual probe (Hu-Friedy PCP 15, Chicago, Ill) to the lowest half millimeter.

The 2 periodontists who performed the surgical procedures also performed all clinical examinations. Each subject was assigned to 1 examiner. Before the start of the study, the examiners were trained to adequate levels of accuracy and reproducibility for the various clinical parameters to be used. The mean interexaminer difference between repeated measurements was 0.14 (95% CI: 0.02 to 0.3) for PD and 0.08 (95% CI: 0.09 to 0.24) for PMH.

Radiographic examinations were performed immediately after the surgical procedure and at the 3-month follow-up visit (Figures 3 and 4). The periapical radiographs were taken in a standardized manner using a paralleling device (Dentsply Rinn, York, Penn) and a digital imaging software system (Dexis LLC, Des Plaines, Ill). One periodontist (T.L.) who was not involved in the implant therapy interpreted the radiographs. Measurements of the marginal bone level (distance between the abutment/fixture junction and the marginal bone to implant contact level) were made at the mesial and distal aspects of the implants. All measurements were determined using a magnification (×7) of the images. The radiographs were downloaded as 16 bit, JPEG files and analyzed with an image processing system11 on a laptop computer. The known geometry of each implant was used to assess the distortion of the images. The error of the method used for appraising the measurements on the radiographs was calculated by reassessing 10 randomly selected cases including 40 sites. The mean difference between repeated measurements of the 40 sites was found to be 0.04 mm (SD 0.33 mm).

For description of data, mean values, standard deviations, and cumulative frequencies were calculated. The primary outcome variable was the marginal bone level change from the time of implant installation to the 3-month follow-up examination. Fisher's exact test was used to evaluate differences in frequencies of plaque, bleeding on probing, and pocket depth categories between the treatment groups. Differences in changes of peri-implant mucosa height, buccal width of keratinized mucosa, and marginal bone levels between the groups were analyzed by using the Student t test for unpaired observations. Pearson's correlation analysis was performed with respect to thickness of the bone wall following the osteotomy (buccal and lingual) and changes in peri-implant mucosa height (buccal and lingual) and thickness of buccal mucosa before implant placement and changes of buccal peri-implant mucosa height. In all analyses, a P value of less than .05 was considered to represent a statistically significant difference.

The distribution of diameter, length, and position in the jaw of implants placed in the 2 groups is illustrated in Figure 3. For both test and control groups, 5 patients received 1 implant and 3 patients received 2 implants.

The results of the clinical measurements are illustrated in Tables 2 and 3. There was a statistically significant difference regarding plaque accumulation between test and control groups (20.5% vs 40.9%) at the 2-week examination. Secondly, the test group implants had a significantly higher proportion of sites with PD ≤ 3 mm (87.9% vs 47%) and a lower proportion of sites with PD 4–5 mm (12.1% vs 48.5%) compared with the control group at the 2-week examination. There were no significant differences between the 2 groups regarding plaque, BoP, and frequencies of sites with different PD categories at the 3-month examination. No differences were detected between the different groups in changes of peri-implant mucosa height and width of keratinized mucosa from 2 weeks to 3 months.

For the test implants, the mean marginal bone level change at the 3-month follow-up examination was −0.09 (SD 0.2) mm for the mesial site and 0.04 (SD 0.2) mm for the distal site. The corresponding numbers for the control group were −0.21 (SD 0.40) mm and −0.28 (SD 0.75) mm, respectively. The mean marginal bone level change calculated with an implant level analysis was −0.02 (SD 0.2) mm for the test group and −0.25 (SD 0.4) mm for the control group. There were no statistically significant differences between the 2 groups.

The cumulative distribution of mesial and distal implant surfaces according to marginal bone level changes at the 3-month follow-up examination is illustrated in Figures 3 and 4. None of the implant surfaces in the test group and 10% of the implant surfaces in the control group have marginal bone level reduction ≥ 1 mm.

There were no significant correlations between the thickness of the bone wall following osteotomy preparation (buccal and lingual) and changes in peri-implant mucosa height (buccal and lingual) (r  =  0.14, P  =  .37) nor between thickness of buccal mucosa before implant placement and changes of buccal peri-implant mucosa height (r  =  0.24, P  =  .27).

The results of the present study failed to demonstrate that the material of the healing abutment (PEEK or titanium) significantly influences soft tissue and bone level changes for the period of 3 months following implant installation.

The soft tissue barrier around dental implants serves as a protective seal between the oral environment and the underlying peri-implant bone. The integration of oral mucosa to implant components of different materials was examined in few studies. Abrahamsson et al,9 in an experimental study in dogs, reported that the abutment material was of decisive importance for the quality of the attachment that formed between the mucosa and the implant abutment. While abutments made of aluminum-based ceramic provided conditions for a mucosal attachment that was similar to that of titanium, no proper mucosal attachment was formed to abutments made of gold-alloy and dental porcelain. At such sites, recession of the mucosal margin and bone resorption occurred. Similar findings reported from Welander et al12 that healing to abutments made of gold-alloy was different than that to ceramic and titanium abutments. Although these studies demonstrate optimal soft tissue healing and dimensions for abutments made from titanium, aluminum-based ceramic, and zirconium, they do not provide information regarding abutments made from polymer materials that are very frequently used as healing abutments. Following confirmation of its biocompatibility 2 decades ago,13 polyaryletherketones (PAEKs) have been increasingly employed as biomaterials for orthopedic, trauma, and spinal implants. Two PAEK polymers, used previously for orthopedic and spinal implants, include PEEK and poly(aryl-ether-ketone-ether-ketoneketone) (PEKEKK). Numerous studies documenting the successful clinical performance of polyaryletherketone polymers in orthopedic and spinal patients continue to emerge in the literature,14,15 but very few reports exist about the application of PEEK in dental implant therapy.

Evaluation of oral mucosal integration at implant abutments made from PEEK material has not been performed in an animal model. However, several in vitro and animal studies have been performed confirming the biocompatibility of PEEK materials. Williams et al13 reported the first animal studies of PEEK in the literature. Neat PEEK and carbon-fiber reinforced samples were subcutaneously implanted in rabbits for 6 months and submuscularly implanted in rats for 30 weeks. Williams stated that PEEK elicited a “minimal response” in both animal models. The growth and attachment of osteoblasts and fibroblasts to PEEK was evaluated by Hunter et al16 in a series of cell culture experiments. 450G PEEK resin was employed and Ti alloy and CoCr alloy were used as controls. No significant differences were observed for fibroblast and osteoblast attachment among the various materials evaluated. The results of this study suggested that PEEK did not appear to deleteriously affect osteoblasts and fibroblasts. The results of our study are in agreement with in vitro and animal reports for PEEK material since no adverse events were experienced by the patients and similar soft tissue and bone responses were observed compared to the titanium healing abutments.

In our study, we observed a statistically significant difference regarding plaque accumulation between PEEK and titanium abutments (20.5% vs 40.9%) at the 2-week examination. During this period, the patients were instructed to use chlorhexidine, 0.12% rinse, twice daily without brushing the operated area. This difference was not expected due to the fact that the abutments made from PEEK material are slightly more rough compared with abutments made from titanium (Sα value 0.4 µm for titanium and 0.8 µm for PEEK, data given from Straumann). These results are in contrast with the study by Wennerberg et al.17 They evaluated in a clinical study the amount of plaque collected at titanium abutments with different degrees of roughness for 4 weeks and reported greater amounts of plaque for abutments with rougher surfaces. However, the abutments used in our study have a roughness similar to the ones that Wenneberg et al17 used as controls (0.259–0.430 µm). The observed difference in plaque accumulation at the 2-week examination can be explained by the possible difference in compliance of the patients using the chlorhexidine rinse and not by the minimal differences in roughness between the abutment materials. It should be noted that no significant differences were observed for plaque accumulation between the 2 groups at the 3-month examination.

Implants for both groups showed minimal marginal bone loss during the 3-month healing period (−0.02 mm test group vs −0.25 mm control group). One limitation of the study is the short follow-up period, and one may assume that more bone loss might be expected for longer observational periods. However, reports from both clinical18,19 and animals studies20 have shown that the largest amounts of marginal bone loss can take place following the first 3 months of implant installation, with minor changes occurring subsequently. Donati et al19 reported that the amount of bone loss using different installation protocols varied from 0.25 mm to 0.38 mm at 1 year, with the major changes occurring at the first 3 months (0.2 mm to 0.33 mm). Cooper et al,18 in a study of early loading on implants placed with a 1-stage procedure, reported that about 0.4-mm bone loss occurred during an initial 6-week period, with no further bone level changes at the subsequent 12-month follow-up. Similarly, Berglundh et al20 in an animal study reported that the largest amount of bone loss occurred following implant installation and abutment connection with almost no bone level alterations during a 10-month period of functional load.

In conclusion, the findings of the current clinical study utilizing implants temporally restored with PEEK or titanium healing abutments indicate that PEEK healing abutments do not render an increased risk for marginal bone loss and soft tissue recession during the initial healing period.

BoP

bleeding on probing

PAEK

polyaryletherketone

PD

probing depth

PEEK

poly(aryl-ether-ether-ketone)

PEKEKK

poly(aryl-ether-ketone-ether-ketone-ketone)

PMH

peri-implant mucosa height

This study was sponsored in part by a 2008 American Academy of Implant Dentistry Grant Award. The authors would like to thank Straumann for providing dental implants and healing abutments.