- Email: [email protected]
In-vitro comparison of parameters affecting the fixation strength of sagittal split osteotomies
WILLIAM
1383
L. FOLEY
regard, the physical and mechanical properties of mandibular bone and fracture callous should be quantified for use as design parameters.** Based on our studies, which quantified physical and mechanical properties of
mandibles,” it is important to realize that the location of the fracture/osteotomy, age, sex, and state of dentition are all important factors that can affect bone properties and fracture healing. Therefore, use of bone properties
II.
12. 13.
14.
in the design of resorbable bone fixation materials is
likely site- and function-specific.
15.
References
16.
1, Spiessl B: Internal Fixation of the Mandible. Berlin, Germany, Springer-Verlag, 1989 2. Bos RR, Rozema FR. Boering G, et al: Bioresorbable osteosynthesis in maxillofacial surgery. Oral Maxillofac Clin North Am 2:745, 1990 3. Uhthoff HK, Dubuc F: Bone structure changes in the dog under rigid fixation. Clin Orthop 81: 165, 1971 4. Claes L: The mechanical and morphological properties of bone beneath internal fixation plates of differing rigidity. J Orthop Res 7:170, 1989 5. Cook SD, Thomas KA, Harding AF, et al: The in vivo performance of 250 internal fixation devices: A follow-up study. Biomaterials 8: 177, 1987 6. Kulkami RK, Pani KC, Neuman C, et al: Polylactic acid for surgical implants. Archiv Surg 93:839, 1966 7. Getter L, Cutright DE, Bhaskar SN, et al: A biodegradable intraosseous appliance in the treatment of mandibular fractures. J Oral Surg 30:344, 1972 8. Miller RA, Brady JM, Cutright DE: Degradation rate of oral resorbable implants (polylactides and polyglycolides): Rate modification with changes in PLA/PGA copolymer ratios. J Biomed Mater Res I I :71 I, 1977 9. Vert M, Christel P, Chabot F, et al: Bioresorbable plastic materials for bone surgery, in Hastings GW, Ducheyne P (eds): Macromolecular Biomaterials. Boca Raton, FL, CRC, 1984, pp 120-142 10. Pratt WB, Yazdani S: Laboratory testing of bolts and screws in cancellous bone. Orthop Rev 18:1073, 1989
17.
18.
19.
20.
21. 22. 23. 24.
25. 26.
Finlay JB, Harada I, Boume RB, et al: Analysis of the pull-out strength of screws and pegs used to secure tibia1 components followine total knee arthroolastv. Clin Orthou 247:220. 1989 DeCoster TA, Heetderks DB’, Downey DJ, e; al: Optimizing bone screw pullout force: J Orthop Trauma 4:169, 1990 Foley WL, Frost DE, Paulin WB, et al: Uniaxial pullout evaluation of internal screw fixation. J Oral Maxillofac Surg 47:277, 1989 Wittenberg JM, Wittenberg RH, Hipp JA: Biomechanical properties of resorbable poly-I-lactide plates and screws: A comparison with traditional systems. J Oral Maxillofac Surg 49512, 1991 Foley WL, Frost DE, Paulin WB, et al: Internal screw fixation: Comparison of placement pattern and rigidity. J Oral Maxillofat Surg 47:720, 1989 Anucul B, White PD, Lemons JE: In vitro strength analysis of sagittal split osteotomy fixation: Noncompression monocortcal plates versus bicortical position screws. J Oral Maxillofac Surg .50:1295, 1992 Schwimmer A, Greenberg AM, Kummer F, et al: The effect of screw size and insertion technique on the stability of the mandibular sagittal split osteotomy. J Oral Maxillofac Surg 52:45, 1994 Eppley BL, Sadove AM: Effects of material porosity on implant bonding strength in a craniofacial model. J Craniofac Surg 1:191, 1990 Suuronen R, Laine P, Sarkiala E, et al: Sagittal split osteotomy fixed with biodegradable, self-reinforced poly-I-lactide screws: A pilot study in sheep. Int J Oral Maxillofac Surg 21:303, 1992 Kohn DH, Pietrzak WS, Richmond EM, et al: Biomechanical analyses of sagittal-split mandibular osteotomies. J Appl Biomaterials 1995 (in press) Ashman RB, Van Buskirk WC: The elastic properties of a human mandible. Adv Dent Res 1:64, 1987 Trosien AH, Kim SL, Kohn DH: Local material and mechanical properties of human mandibles. J Dent Res 73:173, 1994 Cowin SC (ed): Bone Mechanics. Boca Raton, FL, CRC, 1989 Hylander, WL: Stress and strain in the mandibular symphysis of primates: A test of competing hypotheses. Am J Phys Anthropol 64: 1, 1984 Crandall SH, Dahl NC, Lardner TJ: An Introduction to the Mechanics of Solids. New York, NY, McGraw Hill, 1972 Daniels AU, Chang MKO, Andriano KP, et al: Mechanical properties of biodegradable polymers and composites proposed for internal fixation of bone. J Appl Biomaterials 1:57, 1990
J Oral Maxillofac Surg 53:1383-1385, 1995
.
uscussIon In-Vitro
Comparison of Parameters Affecting the Fixation Strength of Sagittal Split Osteotomies
William L. Foley, DMD, MS Keesler
AFB,
MS
Kohn et al have studied the effects of screw material, screw configuration, and screw diameter on fixation The opinions or assertions contained herein are the private views If the author and are not to be construed as official or as reflecting he views of the Department of the Air Force or any other department )r agency of the US government
strength of sagittal split osteotomies performed in 15 human cadaver mandibles. They have further studied the effects of loading rate on fixation strength using a bovine rib model and a synthetic bone substitute made from solid polymetric foam. This study is unique in that the authors have applied mathematical formulae to account for variations in lever arm length, distance between screws, screw diameter, screw stiffness, and bone stiffness. They have created formulae to predict displacement at the osteotomy site based on these variables. Most previous laboratory studies, using either split rib models,‘.2 human cadaver mandibles,3 or sheep mandible? have standardized variables such as lever arm length, distance between screws, screw size, etc, and varied only the parameter being studied, ie, placement pattern,‘.’ screw size,4 fixation technique,‘.2.4 or screw material. The
1384
DlSCUSSION
authors are to be commended for taking the laboratory analysis of fixation systems to a new level of sophistication. However, there are several shortcomings in this study that must be taken into account before extrapolating the results to clinical situations. Three factors can potentially influence resistance to rotation at the osteotomy site in a sagittal split osteotomy. The first is the friction created when one split surface slides across another during rotational deformation. Most surgeons smooth bony irregularities along the split surfaces of the mandible to decrease the potential for nerve injury and maximize bony contact along the osteotomy; the authors did this in the present study. Removal of bony irregularities plus fixation with position screws, without interfragmentary compression, should minimize the effects of friction in this study. The second effect potentially influencing resistance to rotation is bony buttressing present along the osteotomy in the buccal cortex if the segments are returned to their preoperative position or if the buccal cortex is judiciously removed in a mandibluar setback. There is no buttressing effect in a mandibular advancement. The methods section in this study is not sufficiently clear to determine if the mandibles were “advanced,” “setback,” or simply fixed in their preoperative position. The third effect influencing resistance to rotation at the osteotomy site is provided by the fixation devices, in this case, bone screws. Many factors have been implicated in governing the holding strength of bone screws, including external screw diameter, pilot hole size, presence of a channel in the screw, number of self-tapping threads, pitch of the screw, and cortical bone thickness.5m7 It has been well established that the holding power of bone screws increases with increasing cortical thickness.5,7 Smith et al studied 49 dried human mandibles with complete or nearly complete dentition and found that in the region of the sagittal split osteotomy cortical thickness was 2 to 4 mm facially and 1 to 3 mm lingually.’ Obeid and Lindquist and Carter et al reported nearly identical results in 1991.9.‘o The authors’ report that cortical thickness of 15 largely edentulous mandibles used in this study was between 3 and 10 mm seems incredible. If the corticies were indeed that thick due to the age or the edentulous state of the mandibles, then their model is not representative of the population usually treated with the sagittal split osteotomy. Also, in an edentulous mandible screw placement is not limited by the presence of dentition, potentially making the screw placement patterns studied clinically irrelevant. Most importantly, however, the authors state that cortical thickness and variable tissue density effect bending movements only and not rotational movements. If rotational movements are resisted by the screws, and screw holding power varies with cortical thickness, then cortical thickness must effect resistance to rotational movements; this critical factor has been disregarded. Although data from individual mandibles were not provided, it is apparent from the size of the standard deviations relative to the means that there was large variability between specimens. Large variability coupled with small sample size (five per group) makes the statistical inferences in this study difficult and confusing. When placed in an “inverted L” pattern 2.0 mm PLA/PGA screws demonstrated higher sheer stresses than 2.7 mm PLA/PGA screws; this difference was statistically significant.* When placed in a linear pattern there was no significant difference* in sheer stresses between the 2.0 mm and 2.7 mm screw groups. Data presented in Table 1 also indicates that 2.0 mm PLA/PGA screws placed in an “inverted L” pattern provided significantly greater* sheer * 95% confidence intervals based on “T”
distribution with 4df.
stresses than the same screws placed in a linear pattern. These results are consistent with other studies examining placement pattern.‘.’ The titanium screws also displayed greater sheer stresses when placed in an “inverted L” pattern; however, this difference was not statistically significant.* The results also indicate that osteotomies fixed with titanium screws, specifically TiMesh 2.0-mm self-tapping screws (TiMesh Inc, Las Vegas, NV), were significantly more stable than osteotomies fixed with resorbable polymer screws, specifically LactoSorb R pretapped screws (PolyMedics, Warsaw, IN). It is simplistic to state that differences in these groups are attributable to screw material alone. Screw thread design, pilot hole size, pretapping screw holes, and several other factors influence screw holding power.‘~‘~’ Self-tapping screws create their fit by cutting and compacting bone around the screw threads at insertion. Taps used with pretapped screws are larger than the external diameter of the screw creating a less forceful fit for the screw.‘.” Thus, the difference in stability between the two groups of screws could be attributed to the fact that in thin bone (less than 4 mm) self-tapping screws have an advantage in holding power’ or that the two groups of screws have different thread designs. Finally, it is misleading to discuss avoidance of stress shielding, bone resorption due to reduced mechanical stimulation, or corrosion as goals when discussing the use of metal screw fixation in the sagittal split osteotomy. These potential problems have been attributed to screws and plates used in treating long bone fractures; there is no evidence to suggest that they occur with screw fixation in the sagittal split. While resorbable screws and plates show great promise in pediatric craniofacial applications, this reviewer is doubtful that they will gain wide acceptance for use in the sagittal split osteotomy. In summary, this study shows many of the problems inherent in evaluating sagittal split fixation techniques in a benchtop (or mathematical) model. Complex mandibular anatomy and cortical bone thickness affect the strength of screw fixation techniques. In addition, it is difficult to duplicate the complexities of mandibular motion and loading in vitro. Finally, placing screws in nonliving bone disregards biologic factors, which clearly play a role in the clinical use of these devices. Results obtained from these studies should be extrapolated to clinical situations very cautiously. Despite the shortcomings in this study, however, the authors have added to our understanding of some of the biomechanical properties associated with using screw fixation in the sagittal split osteotomy and have provided a strong basis for continuing research in this area.
References 1. Foley WL, Frost DE, Paulin WB, et al: Internal screw fixation: Comparison of placement pattern and rigidity. .I Oral Maxillofat Surg 47:720, 1989 2. Anucul B, Waite PD, Lemons JE: In vitro strength analysis of sagittal split osteotomy fixation: Noncompression monocortical plates versus bicortical position screws. J Oral Maxillofac Surg 50:1295, 1992 3. Ardary WC, Tracy DJ, Brownridge GW, et al: Comparative evaluation of screw configuration on the stability of the sagittal split osteotomy. Oral Surg Oral Med Oral Path01 68:125, 1989 4. Foley WL, Beckman TB: In vitro comparison of screw versus plate fixation in the sagittal split osteotomy. Int J Adult Orthod Orthognath Surg 7: 147, 1992 5. Ansell RH, Scales JT: A study of some factors which affect the strength of screws and their insertion and holding power in bone. J Biomechanics 1:279, 1968 6. Boyle III JM, Frost DE, Foley WL, et al: Torque and pullout
WILLIAM
L. FOLEY
analysis of six currently available self-tapping and “emergency” screws. J Oral Maxillofac Surg 51:45, 1993 7. Phillips JH, Rahn BA: Comparison of compression and torque measurements of self-tapping and pretapped screws. Plast Reconstr Surg 83:477, 1989 8. Smith BR, Rajchel JL, Waite DE, et al: Mandibular anatomy as it relates to rigid fixation of the sagittal ramus split osteotomy. J Oral Maxillofac Surg 49:222, 1991 9. Obeid G, Lindquist CC: Optimal placement of bicortical screws
1385 in sagittal split-ramus osteotomy of mandible. Oral Surg Oral Med Oral Path01 71:665, 1991 10. Carter TB, Frost DE, Tucker MR et al: Cortical thickness in human mandibles: Clinical relevance of the sagittal split ramus osteotomy. Int J Adult Orthod Orthognath Surg 6:257, 1991 11. Uhthoff HK: Mechanical factors influencing the holding power of screws in compact bone. J Bone Joint Surg [Br] 55:633, 1973
1383
L. FOLEY
regard, the physical and mechanical properties of mandibular bone and fracture callous should be quantified for use as design parameters.** Based on our studies, which quantified physical and mechanical properties of
mandibles,” it is important to realize that the location of the fracture/osteotomy, age, sex, and state of dentition are all important factors that can affect bone properties and fracture healing. Therefore, use of bone properties
II.
12. 13.
14.
in the design of resorbable bone fixation materials is
likely site- and function-specific.
15.
References
16.
1, Spiessl B: Internal Fixation of the Mandible. Berlin, Germany, Springer-Verlag, 1989 2. Bos RR, Rozema FR. Boering G, et al: Bioresorbable osteosynthesis in maxillofacial surgery. Oral Maxillofac Clin North Am 2:745, 1990 3. Uhthoff HK, Dubuc F: Bone structure changes in the dog under rigid fixation. Clin Orthop 81: 165, 1971 4. Claes L: The mechanical and morphological properties of bone beneath internal fixation plates of differing rigidity. J Orthop Res 7:170, 1989 5. Cook SD, Thomas KA, Harding AF, et al: The in vivo performance of 250 internal fixation devices: A follow-up study. Biomaterials 8: 177, 1987 6. Kulkami RK, Pani KC, Neuman C, et al: Polylactic acid for surgical implants. Archiv Surg 93:839, 1966 7. Getter L, Cutright DE, Bhaskar SN, et al: A biodegradable intraosseous appliance in the treatment of mandibular fractures. J Oral Surg 30:344, 1972 8. Miller RA, Brady JM, Cutright DE: Degradation rate of oral resorbable implants (polylactides and polyglycolides): Rate modification with changes in PLA/PGA copolymer ratios. J Biomed Mater Res I I :71 I, 1977 9. Vert M, Christel P, Chabot F, et al: Bioresorbable plastic materials for bone surgery, in Hastings GW, Ducheyne P (eds): Macromolecular Biomaterials. Boca Raton, FL, CRC, 1984, pp 120-142 10. Pratt WB, Yazdani S: Laboratory testing of bolts and screws in cancellous bone. Orthop Rev 18:1073, 1989
17.
18.
19.
20.
21. 22. 23. 24.
25. 26.
Finlay JB, Harada I, Boume RB, et al: Analysis of the pull-out strength of screws and pegs used to secure tibia1 components followine total knee arthroolastv. Clin Orthou 247:220. 1989 DeCoster TA, Heetderks DB’, Downey DJ, e; al: Optimizing bone screw pullout force: J Orthop Trauma 4:169, 1990 Foley WL, Frost DE, Paulin WB, et al: Uniaxial pullout evaluation of internal screw fixation. J Oral Maxillofac Surg 47:277, 1989 Wittenberg JM, Wittenberg RH, Hipp JA: Biomechanical properties of resorbable poly-I-lactide plates and screws: A comparison with traditional systems. J Oral Maxillofac Surg 49512, 1991 Foley WL, Frost DE, Paulin WB, et al: Internal screw fixation: Comparison of placement pattern and rigidity. J Oral Maxillofat Surg 47:720, 1989 Anucul B, White PD, Lemons JE: In vitro strength analysis of sagittal split osteotomy fixation: Noncompression monocortcal plates versus bicortical position screws. J Oral Maxillofac Surg .50:1295, 1992 Schwimmer A, Greenberg AM, Kummer F, et al: The effect of screw size and insertion technique on the stability of the mandibular sagittal split osteotomy. J Oral Maxillofac Surg 52:45, 1994 Eppley BL, Sadove AM: Effects of material porosity on implant bonding strength in a craniofacial model. J Craniofac Surg 1:191, 1990 Suuronen R, Laine P, Sarkiala E, et al: Sagittal split osteotomy fixed with biodegradable, self-reinforced poly-I-lactide screws: A pilot study in sheep. Int J Oral Maxillofac Surg 21:303, 1992 Kohn DH, Pietrzak WS, Richmond EM, et al: Biomechanical analyses of sagittal-split mandibular osteotomies. J Appl Biomaterials 1995 (in press) Ashman RB, Van Buskirk WC: The elastic properties of a human mandible. Adv Dent Res 1:64, 1987 Trosien AH, Kim SL, Kohn DH: Local material and mechanical properties of human mandibles. J Dent Res 73:173, 1994 Cowin SC (ed): Bone Mechanics. Boca Raton, FL, CRC, 1989 Hylander, WL: Stress and strain in the mandibular symphysis of primates: A test of competing hypotheses. Am J Phys Anthropol 64: 1, 1984 Crandall SH, Dahl NC, Lardner TJ: An Introduction to the Mechanics of Solids. New York, NY, McGraw Hill, 1972 Daniels AU, Chang MKO, Andriano KP, et al: Mechanical properties of biodegradable polymers and composites proposed for internal fixation of bone. J Appl Biomaterials 1:57, 1990
J Oral Maxillofac Surg 53:1383-1385, 1995
.
uscussIon In-Vitro
Comparison of Parameters Affecting the Fixation Strength of Sagittal Split Osteotomies
William L. Foley, DMD, MS Keesler
AFB,
MS
Kohn et al have studied the effects of screw material, screw configuration, and screw diameter on fixation The opinions or assertions contained herein are the private views If the author and are not to be construed as official or as reflecting he views of the Department of the Air Force or any other department )r agency of the US government
strength of sagittal split osteotomies performed in 15 human cadaver mandibles. They have further studied the effects of loading rate on fixation strength using a bovine rib model and a synthetic bone substitute made from solid polymetric foam. This study is unique in that the authors have applied mathematical formulae to account for variations in lever arm length, distance between screws, screw diameter, screw stiffness, and bone stiffness. They have created formulae to predict displacement at the osteotomy site based on these variables. Most previous laboratory studies, using either split rib models,‘.2 human cadaver mandibles,3 or sheep mandible? have standardized variables such as lever arm length, distance between screws, screw size, etc, and varied only the parameter being studied, ie, placement pattern,‘.’ screw size,4 fixation technique,‘.2.4 or screw material. The
1384
DlSCUSSION
authors are to be commended for taking the laboratory analysis of fixation systems to a new level of sophistication. However, there are several shortcomings in this study that must be taken into account before extrapolating the results to clinical situations. Three factors can potentially influence resistance to rotation at the osteotomy site in a sagittal split osteotomy. The first is the friction created when one split surface slides across another during rotational deformation. Most surgeons smooth bony irregularities along the split surfaces of the mandible to decrease the potential for nerve injury and maximize bony contact along the osteotomy; the authors did this in the present study. Removal of bony irregularities plus fixation with position screws, without interfragmentary compression, should minimize the effects of friction in this study. The second effect potentially influencing resistance to rotation is bony buttressing present along the osteotomy in the buccal cortex if the segments are returned to their preoperative position or if the buccal cortex is judiciously removed in a mandibluar setback. There is no buttressing effect in a mandibular advancement. The methods section in this study is not sufficiently clear to determine if the mandibles were “advanced,” “setback,” or simply fixed in their preoperative position. The third effect influencing resistance to rotation at the osteotomy site is provided by the fixation devices, in this case, bone screws. Many factors have been implicated in governing the holding strength of bone screws, including external screw diameter, pilot hole size, presence of a channel in the screw, number of self-tapping threads, pitch of the screw, and cortical bone thickness.5m7 It has been well established that the holding power of bone screws increases with increasing cortical thickness.5,7 Smith et al studied 49 dried human mandibles with complete or nearly complete dentition and found that in the region of the sagittal split osteotomy cortical thickness was 2 to 4 mm facially and 1 to 3 mm lingually.’ Obeid and Lindquist and Carter et al reported nearly identical results in 1991.9.‘o The authors’ report that cortical thickness of 15 largely edentulous mandibles used in this study was between 3 and 10 mm seems incredible. If the corticies were indeed that thick due to the age or the edentulous state of the mandibles, then their model is not representative of the population usually treated with the sagittal split osteotomy. Also, in an edentulous mandible screw placement is not limited by the presence of dentition, potentially making the screw placement patterns studied clinically irrelevant. Most importantly, however, the authors state that cortical thickness and variable tissue density effect bending movements only and not rotational movements. If rotational movements are resisted by the screws, and screw holding power varies with cortical thickness, then cortical thickness must effect resistance to rotational movements; this critical factor has been disregarded. Although data from individual mandibles were not provided, it is apparent from the size of the standard deviations relative to the means that there was large variability between specimens. Large variability coupled with small sample size (five per group) makes the statistical inferences in this study difficult and confusing. When placed in an “inverted L” pattern 2.0 mm PLA/PGA screws demonstrated higher sheer stresses than 2.7 mm PLA/PGA screws; this difference was statistically significant.* When placed in a linear pattern there was no significant difference* in sheer stresses between the 2.0 mm and 2.7 mm screw groups. Data presented in Table 1 also indicates that 2.0 mm PLA/PGA screws placed in an “inverted L” pattern provided significantly greater* sheer * 95% confidence intervals based on “T”
distribution with 4df.
stresses than the same screws placed in a linear pattern. These results are consistent with other studies examining placement pattern.‘.’ The titanium screws also displayed greater sheer stresses when placed in an “inverted L” pattern; however, this difference was not statistically significant.* The results also indicate that osteotomies fixed with titanium screws, specifically TiMesh 2.0-mm self-tapping screws (TiMesh Inc, Las Vegas, NV), were significantly more stable than osteotomies fixed with resorbable polymer screws, specifically LactoSorb R pretapped screws (PolyMedics, Warsaw, IN). It is simplistic to state that differences in these groups are attributable to screw material alone. Screw thread design, pilot hole size, pretapping screw holes, and several other factors influence screw holding power.‘~‘~’ Self-tapping screws create their fit by cutting and compacting bone around the screw threads at insertion. Taps used with pretapped screws are larger than the external diameter of the screw creating a less forceful fit for the screw.‘.” Thus, the difference in stability between the two groups of screws could be attributed to the fact that in thin bone (less than 4 mm) self-tapping screws have an advantage in holding power’ or that the two groups of screws have different thread designs. Finally, it is misleading to discuss avoidance of stress shielding, bone resorption due to reduced mechanical stimulation, or corrosion as goals when discussing the use of metal screw fixation in the sagittal split osteotomy. These potential problems have been attributed to screws and plates used in treating long bone fractures; there is no evidence to suggest that they occur with screw fixation in the sagittal split. While resorbable screws and plates show great promise in pediatric craniofacial applications, this reviewer is doubtful that they will gain wide acceptance for use in the sagittal split osteotomy. In summary, this study shows many of the problems inherent in evaluating sagittal split fixation techniques in a benchtop (or mathematical) model. Complex mandibular anatomy and cortical bone thickness affect the strength of screw fixation techniques. In addition, it is difficult to duplicate the complexities of mandibular motion and loading in vitro. Finally, placing screws in nonliving bone disregards biologic factors, which clearly play a role in the clinical use of these devices. Results obtained from these studies should be extrapolated to clinical situations very cautiously. Despite the shortcomings in this study, however, the authors have added to our understanding of some of the biomechanical properties associated with using screw fixation in the sagittal split osteotomy and have provided a strong basis for continuing research in this area.
References 1. Foley WL, Frost DE, Paulin WB, et al: Internal screw fixation: Comparison of placement pattern and rigidity. .I Oral Maxillofat Surg 47:720, 1989 2. Anucul B, Waite PD, Lemons JE: In vitro strength analysis of sagittal split osteotomy fixation: Noncompression monocortical plates versus bicortical position screws. J Oral Maxillofac Surg 50:1295, 1992 3. Ardary WC, Tracy DJ, Brownridge GW, et al: Comparative evaluation of screw configuration on the stability of the sagittal split osteotomy. Oral Surg Oral Med Oral Path01 68:125, 1989 4. Foley WL, Beckman TB: In vitro comparison of screw versus plate fixation in the sagittal split osteotomy. Int J Adult Orthod Orthognath Surg 7: 147, 1992 5. Ansell RH, Scales JT: A study of some factors which affect the strength of screws and their insertion and holding power in bone. J Biomechanics 1:279, 1968 6. Boyle III JM, Frost DE, Foley WL, et al: Torque and pullout
WILLIAM
L. FOLEY
analysis of six currently available self-tapping and “emergency” screws. J Oral Maxillofac Surg 51:45, 1993 7. Phillips JH, Rahn BA: Comparison of compression and torque measurements of self-tapping and pretapped screws. Plast Reconstr Surg 83:477, 1989 8. Smith BR, Rajchel JL, Waite DE, et al: Mandibular anatomy as it relates to rigid fixation of the sagittal ramus split osteotomy. J Oral Maxillofac Surg 49:222, 1991 9. Obeid G, Lindquist CC: Optimal placement of bicortical screws
1385 in sagittal split-ramus osteotomy of mandible. Oral Surg Oral Med Oral Path01 71:665, 1991 10. Carter TB, Frost DE, Tucker MR et al: Cortical thickness in human mandibles: Clinical relevance of the sagittal split ramus osteotomy. Int J Adult Orthod Orthognath Surg 6:257, 1991 11. Uhthoff HK: Mechanical factors influencing the holding power of screws in compact bone. J Bone Joint Surg [Br] 55:633, 1973