Problem: Several factors influence primary stabilization of dental implants at placement surgery. These include implant design, bone quality, implant jaw location, and the use of a bone tap. Purpose: This report evaluates clinical data gathered by the Ankylos Implant Clinical Research Group (AICRG) to assess (1) the influence of several variables on primary stability and (2) the potential for an Ankylos implant (Friadent GmbH, Mannheim, Germany) that is mobile at placement to integrate and survive for at least 3 years of clinical function. Methods: The Ankylos implant is a roughened grade-2 titanium screw. A total of 1554 implants were placed in 478 patients. At both the time of placement and abutment connection, the implants were tested for evidence of clinical mobility by attempting to rotate or move the implant with an application of force. Survival was recorded from placement and up to 36 months following placement. Results: At placement, 2.8% were found to be mobile. In the maxillary posterior quadrant, 6.3% were clinically mobile. Implant mobility was frequent (12.2%) in jaw regions with poor-quality bone (BQ-4) or with short implants (8 mm = 8.3% mobile). Of the implants mobile at placement, 97.7% were stable at uncovering. The 3-year postplacement survival of initially mobile implants was 84.1% compared with 96.8% for implants not mobile at placement (χ2 test, P = .001). Conclusions: The Ankylos implant predictably promoted primary stability during surgical placement. Poor bone quality, short implants, and maxillary posterior jaw locations were all associated with a slightly higher rate of mobility at placement. Primary implant stability, while highly desirable, is not absolutely necessary for achieving osseointegration of Ankylos implants.
Standard clinical procedures for root-form implant placement surgery call for the establishment of primary stabilization of the implant(s) within the osteotomy site.1–3 Friberg et al4 reported that implants placed in extremely soft bone and/or lacking initial stability, as evidenced by “lack of resistance during final tightening of the cover screw or mobility of the fixture mount when still on the implant,” constituted 32% of the implant failures recorded. That study did not report on the integration rate of implants that were mobile at placement or their long-term survival. Ivanoff et al5 reported that all experimentally mobile implants that were placed in rabbit tibiae and femoral condyles were integrated at uncovering. Orenstein et al6 studied over 3000 implants from the Core Vent Spectra System (Core-Vent Spectra System; Core-Vent Corporation, DBA Paragon Company, Encino, Calif) for a period of 6 years and concluded that primary stability, while highly desirable, was not necessary to achieve osseointegration. In that study, 93.8% of the implants that were mobile at placement were clinically integrated at uncovering, compared with 97.5% of the implants not mobile at placement. The 3-year postplacement survival for the implants that were mobile at placement, however, was significantly lower (79.8%) than for implants that exhibited primary stability (93.4%) at the time of placement.
If primary stability of an implant is not present, the clinician may have the option of removing the loose implant and replacing it with a wider and/or longer implant. If substitution of the mobile implant with a larger implant is not an option due to the size of the residual ridge, it is important to have a good understanding of the possibility that an implant that is mobile at placement will integrate and become prosthetically useful if left in place.
The aim of this report is to evaluate data gathered by the Ankylos Implant Clinical Research Group (AICRG) for the new and innovative Ankylos implant (Friadent GmbH, Mannheim, Germany) to determine (1) the influence that numerous clinical variables may have on the ability to achieve primary implant stability, and (2) the likelihood that an implant that is determined to be mobile at the time of placement will integrate and remain functional during clinical loading for at least 3 years. The results from this study will help clinicians determine the appropriate course of action to take in the event this occurs when using the Ankylos implant.
In 1996, the Dental Clinical Research Center (DCRC) at the VA Medical Center in Ann Arbor, Mich, formed a multicentered clinical research group, the AICRG. This group designed and conducted a prospective multicenter, multidisciplinary, international clinical study to evaluate the performance of the Ankylos Implant System. The AICRG included 30 VA medical centers, 2 universities in the United States, and 2 dental schools in Taiwan and South Korea, for a total of 34 research centers. Most patients selected were from the predominantly male population of the Department of Veterans Affairs. All data recorded in this study during all stages of treatment were documented on standardized forms and submitted to the DCRG's Data Management Center at the Department of Veterans Affairs Medical Center in Ann Arbor, Mich. The data was entered into a dental database for analyses by the study biostatistician.
A comprehensive medical and dental history and dental examination was performed for each prospective study participant. Absolute requirements for participation in this study included (1) the presence of adequate bone for implant placement, (2) the ability to provide written consent, and (3) the potential to benefit from the placement of endosseous dental implants. Any patient who had a tooth extracted within 6 months or bone grafting within 1 year at the proposed implant site(s) was excluded from the study. Each participant received a copy of the study informed consent that described the proposed procedure(s), as well as alternative procedures and their associated risks and benefits. All clinical investigators were trained and standardized in the procedures to be followed during the course of the study during a 2-day training session.
The Ankylos implant is a grade-2 titanium tapered screw with a roughened surface. The thread design progresses in depth apically to direct the functional stresses to the trabecular bone over the length of the implant (shallower threads in cortical bone, deeper threads in trabecular bone). The inner aspect of the top of the implant has a conical taper that permits secure seating of the prosthetic abutment. The precision fit of the tapered abutment within the implant prevents abutment loosening and rotation of the abutment, as well as preventing the invasion of food debris and bacteria.
The bone quality at each implant location was determined using radiographs and tactile sensations at the time of osteotomy site development using the classification proposed by Lekholm and Zarb.7 The decision to reduce crestal bone and/or use a bone tap was left to the discretion of the surgeon and documented on the appropriate study form. Each implant was tested for clinical mobility/stability immediately after placement (stage I). An implant was considered mobile if it could be depressed or rotated using gentle force.
Implants in this study were allowed to heal for 4 to 6 months in the mandible and 6 to 8 months in the maxilla before abutment connection. All implants were placed using a 2-stage approach. At stage II uncovering, a member of the team other than the surgeon who placed the implant(s) assessed integration by attempting to move it with the application of slight pressure. Radiographs were also used to determine that the bone surrounding the implant was free of any apparent pathology.
After the healing abutment was placed, an electronic mobility-testing device (Periotest; Siemens AG, Bensheim, Germany) was used to record subtle differences in the bone-implant complex for each implant. Since a periodontal membrane does not exist, an integrated implant behaves similarly to an Ankylosed tooth. The electronic mobility-testing device has been shown to provide reproducible data related to the bone-implant complex.8–13 The Periotest handpiece was placed just above the free gingival margin, making certain that the “movable plunger” within the handpiece would not impinge on the soft tissue during activation. Upon activation, the plunger tip was accelerated electronically, emerging through the end of the handpiece and percussing the abutment from its facial aspect at a perpendicular trajectory. The instrument records the rate of rebound of the plunger tip, and the integrated computer assigns a Periotest value (PTV). The PTVs range from −8 (clinically rigid) to +50 (very loose). A range of −8 to +9 corresponds to zero on the Miller index.14 For dental implants, a PTV of 10 or higher is generally associated with a loose abutment or lack/loss of osseointegration. Median PTVs for implants have been reported to be close to or below zero.11,13 Implants that failed to integrate were removed and not tested with the Periotest instrument.
An implant removed because of mobility, chronic pain, discomfort, or infection at any time was recorded as a failure. For the purposes of this study, stage I is defined as the period from implant placement to implant uncovering. Stage II is the time of uncovering (abutment connection) of the implant(s). Stage III is the period from uncovering and abutment connection to just before prosthesis insertion. Stage IV is the period from prosthesis insertion to the end of the evaluation period.
A total of 1554 implants were placed in 478 patients. One station that placed 25 implants dropped out of the study and did not provide follow-up data. A site visit by the Data Management Center was performed at that institution, and all 25 implants were found to be surviving; this information was entered into the study database. Data related to implant mobility at placement and PTVs, however, are missing for those 25 implants.
Mobility and stability at placement
The percentage of all implants placed that exhibited some clinical evidence of mobility at the time of placement is shown in Figure 1A. Of the 1554 implants placed in this study, 2.8% were determined to be mobile at the time of placement (stage I), and 97.2% were stable. At the time of stage II uncovering, 97.7% of those implants that were mobile at the time of placement were clinically stable compared with 98.1% of the implants that were clinically stable at placement (Figure 1B).
Factors associated with stability at placement
The influence of jaw location on implant mobility at placement is shown in Figure 2A. The maxillary posterior jaw location, with its poor bone density, was associated with the highest likelihood of implant mobility at placement (6.3%). Lower mobility at placement was found for the other 3 jaw locations. In the maxillary anterior region, 1.9% of the implants showed some mobility at placement, 2% in the mandibular posterior region, and 2.3% in the mandibular anterior region. These differences were not clinically significant. Posterior jaw locations (Figure 2B) had a slightly higher rate of mobility at placement (3.2%) when compared with anterior jaw locations (2.2%), but this difference was not clinically significant.
The relationship between bone qualities and implant mobility at placement is shown in Figure 3A. Of the implants placed in Q-4 bone, 12.2% were found to be mobile at placement, which is higher than for the implants placed in denser bone. The frequency of mobile implants for all other bone densities ranged from 1.5% to 2.8%, which was not a major difference. Neither crestal bone reduction (Figure 3B) or the use of a bone tap prior to implant placement influenced the frequency of implant mobility at placement.
The 3.5 and 4.5 mm diameter implants had similar rates of mobility at placement (3.0% and 2.6%, respectively), whereas none of the 5.5 mm diameter implants were found to be mobile at placement (Figure 4A). Figure 4B shows data relating to mobility at placement vs implant length. Eight millimeter implants had the highest rate of mobility at placement (8.3%), followed by 9.5-mm implants (5.0%). The longest implants used in this study (17 mm) had the lowest rate of mobility at placement (0.7%). These differences were clinically significant.
The Periotest instrument provides a measure of the stability of the bone-implant complex as PTVs. A comparison of PTVs that were recorded at stage II (uncovering) between implants that were mobile at placement and implants that were stable is shown in Figure 5B. Overall, the mean PTVs for implants that were mobile at placement was −2.00 vs −3.06 PTVs for implants that were clinically stable at placement. These differences in PTVs were not statistically significant (P = .077).
Six implants that were mobile implants at the time of placement but appeared clinically stable at stage II uncovering failed during stage III (after abutment connections but before prosthesis loading). The 3-year postplacement survival for implants that were mobile at placement was 84.1% (Figure 6A). Survival of those implants that were stable at placement was 96.8%. This difference was statistically significant using a χ2 test (P = .001). Most of the failures occurred following uncovering (Figure 6B) at stage III and stage IV (following loading of the prosthesis).
In 1998, the Dental Implant Clinical Research Group (DICRG) published data regarding factors affecting implant mobility at placement and integration of initially mobile implants at uncovering from an integrated implant system.6 In 2000, the DICRG reported on the 3-year postplacement survival of these initially mobile implants.15 The implants used in these earlier studies were either hydroxyapatite (HA) -coated or machined surface commercially pure titanium or titanium alloy and were of various design configurations (HA-grooved, HA-screw, commercially pure titanium screw, HA-cylinder, titanium alloy basket, titanium alloy screw). The Ankylos implant used in this study is a grade-2 titanium screw with a roughened surface.
Of the Ankylos implants placed in this study, 2.7% were determined to be mobile at placement. These results are slightly better than that recorded for implants in the Core-Vent study (3.2%). In that study, screw-type implants were the most likely design to be stable at placement. The progressive screw design of the Ankylos implant appears to provide some advantage in promoting primary implant stability. The integration rate for implants that were stable at the time of placement in this study was 96.5% and is slightly better than the previous experience in the Core-Vent study. Ankylos implants that were mobile at placement had an integration rate of 97.7% compared with 93.8% in the Core-Vent study. In that study, the HA-coated implants that were mobile at placement had the best clinical performance when compared with the non-HA-coated implants. The difference between the data from the 2 studies would be even greater if the HA-coated implants were not considered. The roughened surface of the Ankylos implant allows initially mobile implants to integrate with some degree of predictability. There were 6 failures of initially mobile implants at stage III (period between uncovering and prosthesis insertion). This would suggest that the bone-implant interface was not fully integrated, and the implants soon failed once exposed to even minimal clinical loading. Periotest values for initially mobile implants were approximately 1 PTV higher than for implants that were stable at placement, possibly indicating a less stable bone-implant complex.
The highest rate of initially mobile implants in this study was found in the posterior maxilla. Similarly, a higher rate of mobility at placement was found in posterior jaw locations when compared with anterior jaw regions. This finding can be attributed to the poorer bone quality that is often found in maxillary posterior regions. This study of the Ankylos implant showed the highest rate of mobility at placement to be in Q-4 bone. Short implants were associated with a higher likelihood of mobility at placement. Implant diameter, on the other hand, did not appreciably influence the ability to achieve primary stabilization. There was no apparent association between the use of a bone tap or crestal bone reduction during osteotomy site preparation and mobility of the Ankylos implant at the time of placement.
Primary implant stability, while desirable, is not a prerequisite for achieving osseointegration. The Ankylos implant with its progressive screw design and roughened surface appears to predictably (1) achieve primary stability and (2) integrate when mobile at placement. The 5 additional failures of initially mobile Ankylos implants noted at stage III would suggest that implants that are mobile at placement may benefit significantly if given more time to integrate than those implants that are stable at placement. Progressive loading of implants has been shown to improve PTVs16 and may also promote more complete osseointegration of initially mobile implants.
In 1990, Schnitman et al17 reported on the fabrication of immediate (immediately following stage I surgery) fixed mandibular interim prostheses supported by 2-stage threaded implants. These authors felt that the use of threaded implants in these cases was important in order to achieve “immediate stabilization within cortical bone.” Since then Tarnow et al,18 Balshi and Wolfinger,19 and others reported on the immediate loading of implants to support a variety of prostheses. In the present study factors that influenced primary implant stability were identified and included bone density, jaw location, and implant length. This information should be clinically useful during treatment planning for candidates of immediate implant loading.
The development of more sensitive means of determining stability of implants at the time of placement may assist the clinician with the decision as to whether or not to employ an immediate function prosthesis. Consideration should be given to the use of electronic mobility testing devices at the time of implant placement to help assess primary stability in future studies.
The following conclusions can be made:
The Ankylos dental implant with its progressive thread design and roughened surface predictably promoted primary stability in this study. Risk factors for the placement of implants that are mobile at insertion include poor bone quality, maxillary posterior jaw locations, and short implants.
Primary implant stability is desirable but not required for osseointegration to occur. If implants that are mobile at placement are protected from disturbing forces long enough, they may osseointegrate and provide long-term clinical function.
Further research is needed to establish tests that objectively assess primary implant stability. This information would provide valuable information when considering the placement of dental implants into immediate function.
This investigation was supported by Friadent GmbH, Mannheim, Germany (formerly Degussa AG, Hanau, Germany). Study investigators often spent time outside of their assigned duties to collect and record data. The authors gratefully acknowledge the dedication and contributions of the current and former clinical investigators:
Ewha Woman's Hospital (South Korea): Jang Woo Choi, DDS, PhD; Myung Rae Kim, DDS, MS, PhD.* Cathay General Hospital (Taiwan): Chin-Sung Chen, DDS; Shyuan-Yow Chen, DDS; Cherng-Tzeh Chou, DDS; Hong-Jeng Lin, DDS; Yueh-Chao Yang, DMD, MS.* Medical College of Virginia (Virginia): C. Daniel Dent, DDS; Julie Sharp, DDS.* University of Louisville (Kentucky): John W. Olson, DDS, MS.* Vanderbilt University (Tennessee): Samuel McKenna, DDS, MS.* VAMC Bedford (Massachusetts): William Bornstein, DDS; Mohamad B. Ayas, DDS; Noah I. Zager, DMD.* VAMC Bronx (New York): Ira H. Orenstein, DDS*; Thomas E. Porch, DMD. VAMC Chillicothe (Ohio): John Hofer, DMD*; Craig A. Holman, DDS; Diane E. Land, DDS; Lura Marshall, RDH; Richard Mauger, DDS. VAMC Danville (Illinois): James T. Freestone, DDS; Kevin J. Malley, DDS; John L. Reyher, DDS.* VAMC Dayton (Ohio): James R. Cole, DDS; Paul M. Lambert, DDS.* VAMC Detroit (Michigan): Rami Jandali, DMD, MS; Ahmad A. Kanaan, DDS, MS; Michael L. Linebaugh, DDS, MS; Richard A. Plezia, DDS, MS.* VAMC Houston (Texas): Allan W. Estey, DDS; Harry D. Gilbert, DDS*; George V. Goff, DDS. VAMC Huntington (West Virginia): Stanley E. Dixon, DMD; Eugene M. Riehle, DDS.* VAMC Kansas City (Missouri): James L. Beatty, DDS; John Bellome, DDS*; Richard J. Crosetti, DDS; Linda Filbern, RDH; Douglas A Pearson, DDS; Rosa B. Solomon, DDS. VAMC Lexington (Kentucky): Dolph R. Dawson, DMD; John Dominici, DDS, MS*; Robert Marciani, DMD. VAMC Little Rock (Arkansas): C. Gary Black, DDS; J. Robert Spray, DDS.* VAMC Loma Linda (California): James E. Yeager, DMD; Warren S. Yow, DMD, MS, MPH.* VAMC Louisville (Kentucky): Paul X. Dattilo, DMD*; Reid Nelson; John W. Olson, DDS; James W. Shaughnessy, DMD. VAMC Memphis (Tennessee): William D. Caldwell, DDS, MS; Daniel L. Reaves, DDS.* VAMC New Orleans (Louisiana): Henry H. Chen, DMD; Arthur G. Howe, DDS*; Daniel D. Gammage, DMD; Laurie Moeller, DDS. VAMC Northport (New York): David A. Abroff, DDS; Anthony J. Casino, DDS*; Richard S. Truhlar, DDS. VAMC Phoenix (Arizona): D. Barnes, DMD*; Vance Cox, DDS. VAMC Pittsburgh (Highland Drive, Penn): Arthur M. Rodriguez, DMD, MS.* VAMC Portland (Oregon): Larry B. Thompson, DDS, MS; J. Ernest Weinberg, DMD, MSD.* VAMC Richmond (Virginia): C. Daniel Dent, DDS; William E. Hunter, DDS*; Lawrence E. Masters, DDS. VAMC Salem (Virginia): Phillip R. Davis, DDS; C. Dudley Parks, DDS*; Michael J. Vasisko, DDS. VAMC San Francisco (California): Richard Navarro, DDS, MS; Rebeka G. Silva, DMD*; Dennis J. Weir, DDS, MA. VAMC Seattle (Washington): John A. Bucher, DMD*; Randall R. Sobczak, DDS. VAMC Sepulveda (California): Mark L. Monson, DDS; Lori A. Walker, DDS.* VAMC Washington, DC: Michael T. Curran, DDS*; Glenn T. Haggan, DDS.* VAMC West Los Angeles (California): Stephen Ancowitz, DDS; James Callahan, DMD*; Richard Nagy, DDS; Donald Sze, DDS. VAMC West Palm Beach (Florida): Carlos Alvarez, DMD; John Ball, DMD; Alfredo Fernandez, DMD; Jerry Neidlinger, DDS.* VAMC Wichita (Kansas): John David Ball, DDS.*
DVA Central Dental Laboratory (Texas): Eugene Jones, DDS, MS. DVA Central Dental Laboratory (Washington, DC): John McCartney, DDS.
Project Office and Data Management Center
This is government-supported research and there are no restrictions on its use. The results and opinions presented are those of the authors and do not necessarily reflect the opinions of the Department of Veterans Affairs Medical Research, the Office of Dentistry, or the American Academy of Implant Dentistry. This manuscript does not represent an endorsement of the evaluated implant by the Department of Veterans Affairs or the American Academy of Implant Dentistry.
Harold F. Morris, DDS, MS, is codirector of the Dental Clinical Research Center (DCRC) and project codirector of the Ankylos Implant Clinical Research Group (AICRG), Department of Veterans Affairs Medical Center (VAMC), Ann Arbor, Mich. Correspondence should be addressed to Dr Morris at the DCRC (154), VA Medical Center, 2215 Fuller Road, Ann Arbor, MI 48105.
Shigeru Ochi, PhD, is codirector of the DCRC and project codirector of the AICRG, VAMC, Ann Arbor, Mich.
Ira H. Orenstein, DDS, is staff dentist at the VAMC, Bronx, NY, and at the Columbia University School of Dental and Oral Surgery, New York, NY.
Vincent Petrazzuolo, DDS, is staff dentist at the VAMC, Bronx, NY.