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Screw implant anchorage
READERS’ FORUM
Letters to the editor* Screw implant anchorage I am compelled to write about the recent article on screw implant anchorage (Park HS, Jeong SH, Kwon OW. Factors affecting the clinical success of screw implants used as orthodontic anchorage. Am J Orthod Dentofacial Orthop 2006;130:18-25). First, I want to thank the authors for continuing to provide research in the area of skeletal anchorage. I do, however, take issue with conclusions in the article. In the Discussion, paragraph 12, the authors wrote, “This study was performed to screen every possible factor that could affect the success of screw implants.” Unfortunately, 1 significant variable regarding screw success was neither examined nor mentioned: screw design. Screw design refers to the thread pitch, architecture, lead, flute vs no flute, and so on. The orthopedic literature is replete with solid evidence that, for dense bone (ie, the external oblique ridge, retromolar area, and zygomatic buttress), screws with at least 1 cutting flute enter bone with less torque and decreased hoop stress, significantly improving success rates. Diameter and length were the only miniscrew variables that Park et al examined, but these are size characteristics, not design elements. Failing to study or mention screw design is to completely ignore years of orthopedic literature, as well as to propagate the fallacy uttered by more than 1 prominent researcher that “a screw is a screw is a screw.” Interesting, too, is the observation that miniscrews failed to a greater degree on the right side of the test subjects’ mouths. This observation was thought to be significant enough to be a major conclusion of the article. Explanation of this finding was tied to the handedness of the test subjects (a variable not provided), stating that because most patients in general are right-handed, this might have been a factor. Although this supposition might be true, what about the handedness of the operator? It seems that, because only 1 person placed the miniscrews in all 87 subjects, operator handedness would be a more reliable variable than patient handedness. John W. Graham Litchfield Park, Ariz Am J Orthod Dentofacial Orthop 2006;130:431 0889-5406/$32.00 Copyright © 2006 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2006.08.004
Author’s response I appreciate Dr Graham’s considerate reading and suggestions. Screw design might greatly affect its success. However, there were no statistically significant differences among the 4 types of screws, even though each type had a *The viewpoints expressed are solely those of the author(s) and do not reflect those of the editor(s), publisher(s), or Association.
different shape: type 1 had a very shallow thread height, type 2 had greater thread height with a smaller inner diameter, type 3 was similar to type 1, and type 4 had a larger diameter and greater thread height. Bone in the jaw differs from site to site; eg, alveolar bone is much denser in the mandible than in the maxilla,1 not to mention the external oblique ridge or retromolar area. The design of the screw should be different for different bone densities.2 When studying screw designs suitable for orthodontic anchorage, investigators should consider bone quality and quantity and, more importantly, the condition of loading. Because the jaws are more complicated structures than any other area of the body, knowledge from other specialties might not directly apply to orthodontics. In this clinical study, we thought that other factors, in addition to design, might have more effect on the success of screws used for orthodontic anchorage. The factors we wanted to include were those that could be checked in daily clinical practice. In this type of clinical study, it is impossible to control factors of screw design because of ethical considerations and the many available combinations of screw design. Because we used a small number of screw designs in our study, it did not seem appropriate to discuss the effects of screw design on the success rate. More failures occurred on the right side than on the left. We agree that a reason for this difference might be the handedness not just of the patients, but also of the operator. However, in a recent investigation, the right side of the mouth had more root contact than left side,3 and the more root contact, the more failure was expected.4 This might be a more powerful explanation of the result. Hyo-Sang Park Daegu, Korea Am J Orthod Dentofacial Orthop 2006;130:431 0889-5406/$32.00 Copyright © 2006 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2006.08.005
REFERENCES 1. Park HS, Lee YJ, Jeong SH, Kwon TG. Bone density of the alveolar and basal bone in the maxilla and the mandible. Am J Orthod Dentofacial Orthop. In press 2006. 2. Lohr J, Gellrich NC, Buscher P, Wahl D, Rahn BA. Comparative in vitro studies of self-boring and self-tapping screws. Histomorphological and physical-technical studies of bone layers. Mund Kiefer Gesichtschir 2000;4:159-63. 3. Cho WH. Influence of the operator’s experience and drilling sites on the root contact during drilling for micro-implants insertion [thesis]. Daegu, Korea: Kyungpook University; 2005. 4. Kuroda S, Yamada K, Deguchi T, Hashimoto T, Kyung H-M, Takano-Yamamoto T. Root proximity is a major factor for screw failure in orthodontic anchorage. Am J Orthod Dentofacial Orthop. In press 2006.
431
Letters to the editor* Screw implant anchorage I am compelled to write about the recent article on screw implant anchorage (Park HS, Jeong SH, Kwon OW. Factors affecting the clinical success of screw implants used as orthodontic anchorage. Am J Orthod Dentofacial Orthop 2006;130:18-25). First, I want to thank the authors for continuing to provide research in the area of skeletal anchorage. I do, however, take issue with conclusions in the article. In the Discussion, paragraph 12, the authors wrote, “This study was performed to screen every possible factor that could affect the success of screw implants.” Unfortunately, 1 significant variable regarding screw success was neither examined nor mentioned: screw design. Screw design refers to the thread pitch, architecture, lead, flute vs no flute, and so on. The orthopedic literature is replete with solid evidence that, for dense bone (ie, the external oblique ridge, retromolar area, and zygomatic buttress), screws with at least 1 cutting flute enter bone with less torque and decreased hoop stress, significantly improving success rates. Diameter and length were the only miniscrew variables that Park et al examined, but these are size characteristics, not design elements. Failing to study or mention screw design is to completely ignore years of orthopedic literature, as well as to propagate the fallacy uttered by more than 1 prominent researcher that “a screw is a screw is a screw.” Interesting, too, is the observation that miniscrews failed to a greater degree on the right side of the test subjects’ mouths. This observation was thought to be significant enough to be a major conclusion of the article. Explanation of this finding was tied to the handedness of the test subjects (a variable not provided), stating that because most patients in general are right-handed, this might have been a factor. Although this supposition might be true, what about the handedness of the operator? It seems that, because only 1 person placed the miniscrews in all 87 subjects, operator handedness would be a more reliable variable than patient handedness. John W. Graham Litchfield Park, Ariz Am J Orthod Dentofacial Orthop 2006;130:431 0889-5406/$32.00 Copyright © 2006 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2006.08.004
Author’s response I appreciate Dr Graham’s considerate reading and suggestions. Screw design might greatly affect its success. However, there were no statistically significant differences among the 4 types of screws, even though each type had a *The viewpoints expressed are solely those of the author(s) and do not reflect those of the editor(s), publisher(s), or Association.
different shape: type 1 had a very shallow thread height, type 2 had greater thread height with a smaller inner diameter, type 3 was similar to type 1, and type 4 had a larger diameter and greater thread height. Bone in the jaw differs from site to site; eg, alveolar bone is much denser in the mandible than in the maxilla,1 not to mention the external oblique ridge or retromolar area. The design of the screw should be different for different bone densities.2 When studying screw designs suitable for orthodontic anchorage, investigators should consider bone quality and quantity and, more importantly, the condition of loading. Because the jaws are more complicated structures than any other area of the body, knowledge from other specialties might not directly apply to orthodontics. In this clinical study, we thought that other factors, in addition to design, might have more effect on the success of screws used for orthodontic anchorage. The factors we wanted to include were those that could be checked in daily clinical practice. In this type of clinical study, it is impossible to control factors of screw design because of ethical considerations and the many available combinations of screw design. Because we used a small number of screw designs in our study, it did not seem appropriate to discuss the effects of screw design on the success rate. More failures occurred on the right side than on the left. We agree that a reason for this difference might be the handedness not just of the patients, but also of the operator. However, in a recent investigation, the right side of the mouth had more root contact than left side,3 and the more root contact, the more failure was expected.4 This might be a more powerful explanation of the result. Hyo-Sang Park Daegu, Korea Am J Orthod Dentofacial Orthop 2006;130:431 0889-5406/$32.00 Copyright © 2006 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2006.08.005
REFERENCES 1. Park HS, Lee YJ, Jeong SH, Kwon TG. Bone density of the alveolar and basal bone in the maxilla and the mandible. Am J Orthod Dentofacial Orthop. In press 2006. 2. Lohr J, Gellrich NC, Buscher P, Wahl D, Rahn BA. Comparative in vitro studies of self-boring and self-tapping screws. Histomorphological and physical-technical studies of bone layers. Mund Kiefer Gesichtschir 2000;4:159-63. 3. Cho WH. Influence of the operator’s experience and drilling sites on the root contact during drilling for micro-implants insertion [thesis]. Daegu, Korea: Kyungpook University; 2005. 4. Kuroda S, Yamada K, Deguchi T, Hashimoto T, Kyung H-M, Takano-Yamamoto T. Root proximity is a major factor for screw failure in orthodontic anchorage. Am J Orthod Dentofacial Orthop. In press 2006.
431