Response to Rangert B, Krogh P, Langer B, Van Roekel N: Bending overload and implant fracture: A retrospective clinical analysis. Int J Oral Maxillofac Implants 1995;10(3):326-334.
LETTER TO THE EDITOR
International Journal of Oral and Maxillofacial Implants, Vol. 11, No. 4, 1996, Pp 431-432
This is in reference to an article published in the JOMI (Rangert B, Krogh P, Langer B, Van Roekel N: Bending overload and implant fracture: A retrospective clinical analysis. Int J Oral Maxillofac Implants 1995;10(3):326-334).
In an analysis of fractured Brånemark Implants reported by three clinicians, Bo Rangert, PhD, an engineer, minimizes the significant numbers of the Grade 1, commercially pure titanium (CP Ti) fixtures that broke, and fails to address the real solution to the problem: use of stronger grades of titanium or titanium alloy in an implant design and surgical protocol that preserves the marginal bone-implant relationship.
The study first claims to report on a "total of 39 patients" treated by the co-authors, Drs. Paul Krogh, Burton Langer and Ned Van Roekel (Clinicians), and Table #1 refers to 39 as the "Number of Restorations with Fractures." While the text states that "the fractures occurred in 22 mandibles (Fig. 1) and 17 Maxillae (Fig. 2)," the legend beside these bar-graph figures states that they represent "Distribution of Fractured Implants Among Different Treatment Categories." By only reporting on the number of jaws or prostheses with fractures, the true number of broken Brånemark Implants is being sidestepped. As a case in point, Fig. 2 shows that only 4 implants fractured in maxillary "Full Arch" restorations, whereas one of the Clinician co-authors has subsequently acknowledged to me that all 5 implants fractured in one patient with this type of restoration, which this text clearly fails to reflect.
To further minimize the significant number of fractured implants, the article claims that the 39 fractures were recorded from an "estimated reference of over 10,000 implants placed," rather than from the documented number of implants actually placed or restored by the Clinicians, which was closer to half this number according to the Clinicians themselves (personal communications).
The article references "unpublished data" from a 3-year retrospective follow-up by Bo Rangert and 3 American clinicians, which compared "single-molar restorations supported by one and two implants (21 and 25 patients, respectively), with no fractures in either category." Such an absence of reported fractures occurring with crowns supported by single mandibular molar implants contrasts with a report by Mel Schwartz (USC Implant Symposium, January 1995), wherein 50% of the Brånemark implants in this clinical application fractured.
The article concludes that "the frequency of implant fractures is low and, with appropriate treatment planning, overload situations may be preventable." The observation time reported in this article for the 39 patients with fractured implants was 7 to 79 months, with 32 months as the mean observation period. This short period of observation must be viewed in context with the increased frequency of fractures over time reported by Adell et al. (JOMI 1990;5(4):347-359) in the 20-year Swedish experience with Brånemark Implants. The article cites this study and acknowledges that
it "revealed a fracture rate of 16% in the maxilla and 4% in the mandible after 15 years (7% and
3% after 5 years, respectively)," showing that metal fatigue with titanium dental implants is a time dependent occurrence.
The text also lists a number of increased load factors that contribute to fractured implants, including "in-line implants," angulation of 15 degrees or greater, abutments taller than 7mm, bruxism and increased leverage from crestal bone loss. According to the article, the majority of the fractures occurred with bone loss to the 3rd thread of the Brånemark Implant, which most often lines up with the end of the abutment screw within the internally threaded shaft of the implant. The surgical protocol for the Brånemark external hex implant requires countersinking the crest of the ridge to accommodate the wider neck (4.1mm D) of the 3.75mm diameter implant, which lowers the bony support to the height of the first thread of the implant. Studies (Brånemark et al., Scand J Plast Reconstr Surg; 111 (Suppl 16):1-132; Cox & Zarb, JOMI 1987;2(2):91-100) have shown that, within the first year in function, 1mm -1.5mm of additional bone will be lost, as measured from the base of the implant’s wider neck, which results in bone loss down to the 2nd thread on a routine basis.
Choice of implant material as a consideration in improving implant strength with a given diameter of implant is also discussed. The Brånemark Implant is made of Grade 1 CP Ti (99.7% Ti), which has a tensile strength of only 35 psi compared to 80 psi for Grade 4 CP titanium (98.4% Ti) and 135 psi for medical grade titanium alloy (90% Ti-6% Aluminum-4% Vanadium). The article attributes development of "internal microcracks" in the implants to repeated loads and acknowledges that "the fatigue strength determines the capacity for withstanding such repeated loads."
An attempt to diminish the significance of this lack of tensile strength in CP titanium is made by referencing a previous article by the lead author, Bo Rangert (Nobelpharma News 1994;8(1):7), which claims that the "fatigue strength of Grade 1 CP Ti is practically independent of its tensile strength." It repeats the claim made in the source material that "the tensile strength may be increased by work hardening," but diverts from it with the admission that, while "work hardening may increase the tensile strength of Grade 1 CP Ti to the minimum of the acceptable range for Grade 3 CP Ti," it has "limited influence on the long-term fatigue behavior." The authors are then left with the only choices for reducing fractures with the Brånemark System, both of which depend on available bone: using the wider (4.0mm D) Brånemark Implants, which will increase tensile strength by 30% according to the article, or just using more of the standard 3.75mmD implants.
By selecting Grade 4 titanium over Grade 1, a 200% increase in tensile strength can be achieved without having to increase the implant diameter, which may not be possible due to limited available ridge width. By using titanium alloy, almost a 350% increase in strength can be achieved, even with only a 3.3mm diameter Screw-Vent Implant, compared to the standard 3.7mm diameter Grade 1 CP Ti Brånemark Implant, which safely expands the range of clinical applications to narrow ridges.
Gerald A. Niznick, DMD, MSD
President, Core-Vent Bio-Engineering, Las Vegas, Nevada
and Core-Vent BioEngineering, Calabassas Hills, California
Response to Albrektsson's Comment on Dr. Niznick's Letter to the Editor (Re: Rangert B, Krogh P, Langer B, Van Roekel N: Bending overload and implant fracture: A retrospective clinical analysis. Int J Oral Maxillofac Implants 1995;10(3):326-334) JOMI 1996;11(4):431 432.
LETTER TO THE EDITOR
International Journal of Oral and Maxillofacial Implants, submitted on August 29, 1996
TO THE EDITOR:
My Letter to the Editor (JOMI 1996;11(4):431-432) commented on the article by Dr. Bo Rangert, Dr. Paul H.J. Krogh, Dr. Burton Langer and Dr. Ned Van Roekel concerning fractured Brånemark Implants (JOMI 1995;10:326-334). While it is not unusual for a journal to concurrently publish an "Author's Response," as was done with Dr. Rangert's comments, it is astounding that the JOMI further solicited a response from Dr. Tomas Albrektsson and also published it under "Author's Response," since Dr. Albrektsson was not one of the authors of the article. The inappropriateness of Dr. Albrektsson's comments is even more evident when one considers they pertain to a defense of his 1991 article on "Quantified Bone Tissue Reactions to Various Metallic Materials," a reference which was deleted from my original draft by the Editor, with my permission, before my Letter was even published. Thus, Dr. Albrektsson's comments are totally irrelevant in this context. Apparently JOMI and Dr. Albrektsson are not only concerned about my written comments, but also about my unpublished thoughts.
Since Dr. Albrektsson was given an opportunity to assail, without challenge, the bone response of titanium alloy, a material that is used by a number of major implant manufacturers, and accuse me of reading his 1991 article "in a poor manner," it is now incumbent on the Journal to publish the deleted text of my Letter that addressed the use of implants made from titanium alloy as means of reducing the incidence of implant fractures. The deleted section is as follows:
Osseointegration with titanium alloy can no longer be seriously questioned. The only histological report critical of its use was generated by Albrektsson, a Swedish consultant to Nobelpharma, and Jacobsson (The Bone-Biomaterial Interface, pp 357-363, Toronto: University of Toronto Press, 1991), who compared CP titanium and Titanium-6Al-4V and reported a thicker proteoglycan layer with the alloy than with CP titanium. This study used a sputtered coating of the metals rather than the bar stock, a process that even Albrektsson (CRC Critical Reviews in Biocompatiblity 1994;114:189-192) had admitted in a previous article would create "different surface conditions compared to those of a titanium implant." Albrektsson’s attempt to discredit titanium alloy was addressed in a letter to the editor that I wrote (J Prosthet Dent 1988;59(1):120-121), wherein I cited a 10-year study comparing titanium and titanium alloy implants in baboons (Andersson et al., J Bone Joint Surg [Am] 1978;60:31) that showed no difference in bone tissue response to the two materials.
A subsequent objective Swedish study in the tibia of rabbits by Linder (Acta Orthop Scand 1989;60(2):129-134) histomorphometrically analyzed the bone response to 38 CP Ti implants and 10 Ti Alloy implants. The author concluded that "the bony reaction was remarkably consistent." Albrektsson and Johansson (Davies JE, ed: The Bone Biomaterial Interface, page 359) claimed that CP Ti implants averaged greater removal torque (23.4 Ncm) than Ti-6Al-4V implants (15.5 Ncm). However, Carr et al. (JOMI 1995;10(2):167-174) disproved that there was any difference in removal torque between the CP and alloyed titanium, while proving that the attachment strength of HA-coated implants is 250% greater than uncoated titanium implants.
The scientific debate over bone response to CP Ti and titanium alloy becomes moot when one selects titanium alloy implants with surface coatings in order to take advantage of both the strength of titanium alloy (i.e. 130 psi vs. 35 psi for CP Ti) and the increased bone attachment for improved load-bearing capabilities provided by surface coatings of HA (Carr et al., JOMI 1995;10:167-174) or TPS (Buser et al., J Biomed Mat Res 1991;25:889-902). However, what is of concern is the ethical and legal obligation of a dentist to inform his or her patient of the existance of dental implant materials with less "propensity to fracture" than Grade 1 CP titanium. This issue of informed consent is currently before the New York State Supreme Court, County of New York, following multiple fractures of Grade 1 CP Ti implants in one female patient (Case Index No. 127071/95).
Gerald A. Niznick, DMD, MSD
President, Core-Vent Corporation, Las Vegas, Nevada
and Core-Vent BioEngineering, Calabasas Hills, California