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The effects of bone plug length and screw diameter on the holding strength of bone-tendon-bone grafts
The Effects of Bone Plug Length and Screw Diameter on the Holding Strength of Bone-Tendon-Bone Grafts Gregory Pomeroy, M.D., Matthew Baltz, M.D., Kristan Pierz, M.D., Michael Nowak, Ph.D., William Post, M.D., and John P. Fulkerson, M.D.
Summary: The effect of bone plug length and Kurosaka screw (DePuy, Warsaw, IN) diameter on graft holding strength of the bone-tendon-bone construct was determined. Random length porcine bone plugs were assigned to fixation with 7 or 9 mm Kurosaka screws. Peak load to failure was determined. There was a significant decrease in peak load to failure of the 5-mm long bone plugs compared with longer bone plugs. No difference was found between longer lengths of bone plug in either the 7- or 9-mm screw diameter groups. The 9-mm diameter screws significantly increased peak load to failure for both 1- and 2-cm bone plug lengths. Key Words: Bone-tendon-bone graft—Kurosaka screw—Interference fit.
A
utogenous reconstruction is generally favored for reconstruction of a torn anterior cruciate ligament (ACL). Friedman et al. have provided a review of the options for autogenic ACL reconstruction of the knee.1 Of the options, the bone–central third patellar tendon– bone graft is currently popular. ACL graft placement and isometry,2 as well as correct tensioning before fixation of the ACL graft3 are important, but there must be adequate bone at either end of the graft to assure secure Kurosaka screw (DePuy, Warsaw, IN) fixation. In 1987, Kurosaka et al. reported that fixation of the graft with cancellous screws was superior to staple fixation and tying sutures over buttons.4 Since that time, the use of inteference screws for ACL reconstruction has been widely accepted. The pitfalls of Kurosaka screw interference fixation for ACL surgery have been evaluated by several authors.5,6 Clinical guidelines on how best to use the interference fit method are still being defined. Parameters such as bone density, From The University of Connecticut School of Medicine (M.B., K.P., M.N., W.P.); The Orthopaedic Associates of Hartford, Hartford, Connecticut; (J.P.F.); and the Portland Orthopedic Foot and Ankle Group, Portland, Maine (G.P.), U.S.A. Address correspondence and reprint requests to John P. Fulkerson, M.D., Orthopedic Associates of Hartford, 85 Seymour St, Suite 607, Hartford, CT 06106, U.S.A. r 1998 by the Arthroscopy Association of North America 0749-8063/98/1402-1210$3.00/0
148
bone tunnel-block gap, bone block cross-sectional area, screw type, and screw insertion torque as they relate to load at failure have been examined.7 No study to date has given data to indicate an optimal length of bone plug for fixation with a Kurosaka screw. The purpose of this study was to compare the holding strengths of interference screw fixation of 0.5- to 2-cm bone plugs, such as those used in ACL reconstruction. In addition, two popular Kurosaka screw diameter sizes (7 and 9 mm) were compared in the fixation of 0.5 to 2 cm bone plugs into 11-mm diameter bone tunnels.
MATERIALS AND METHODS Ninety adult fresh-frozen porcine knees were used for simulated ACL reconstructions. Femurs and fibulas were removed and discarded. For each trial, the patella, patellar tendon, and tibial tuberosity were removed as a single graft. Each specimen was randomly assigned to a bone plug length (0.5 cm, 1 cm, 2 cm), resulting in 30 specimens per bone plug length. A bone plug–patellar tendon–bone graft was taken using the central patella tendon. This was performed with a microsaggital saw. Each graft was then further prepared to fit into a 10-mm oval guide. Each graft was
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 14, No 2 (March), 1998: pp 148–152
EFFECTS OF BONE PLUG LENGTH AND SCREW DIAMETER
FIGURE 1. Peak load to failure (Newtons) for interference fit fixation for 7-mm Kurosaka screw into 0.5-cm, 1-cm, and 2-cm bone plug lengths, respectively. *P ⬍ .05.
harvested and prepared by the lead author to provide consistency between the specimens. Once prepared, each graft was then assigned to either a 7- or 9-mm diameter 20-mm long Kurosaka screw for interference fit fixation; 11-mm tunnels were drilled through the lateral tibial plateaus near the anatomic insertion of the ACL. The tunnels were placed perpendicular to the joint surface. The bone plugs were then placed in each tunnel with the tendon facing anteriorly. The screws were placed medially in an antegrade fashion. This was done under direct vision to ensure that the screw was placed parallel to the grafts; in this way, divergence was prevented. The screws were advanced until the proximal thread was seen to be fully engaged in the bone plug just distal to the articular margin. Laceration
FIGURE 2. Peak load to failure (Newtons) for interference fit fixation for 9-mm Kurosaka screw into 0.5-cm, 1-cm, and 2-cm bone plug lengths, respectively. *P ⬍ .05.
149
FIGURE 3. Peak load to failure (Newtons) for 7-mm versus 9-mm diameter Kurosaka screws. *P ⬍ .05. **P ⬍ .001.
of the tendon by the screw was prevented. Radiographs were then taken randomly to confirm screw placement. An MTS Model 858 (Materials Testing System Corp, Minneapolis, MN) was then used to mechanically pull the grafts and determine peak linear
FIGURE 4. Interference fit afforded by a 0.5-cm long bone plug with a Kurosaka screw.
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load to failure at a strain rate of 51 cm/min. After failure, each bone plug was inspected to ensure uniform cutting of the screw threads into the entire length of bone plug. This served as further documentation that divergence was not an issue. The ultimate strength of each fixation configuration was then measured in Newtons. A two-way analysis of variance was then performed to detect statistical difference. A paired Student’s t test was used to compare test groups. RESULTS Overall, in both 7- and 9-mm screw diameters, there was a statistically significant decrease in peak load to failure between the 5-mm length of bone plug and the 1- and 2-cm bone plug lengths respectively (P ⬍ .05, Fig 1 and 2). Of the 30 specimens in the 5-mm bone plug length, 26 failed at the bone plug/screw interface. Two specimens failed at the insertion of the tendon
FIGURE 6. Interference fit afforded by a 2-cm long bone plug with a Kurosaka screw.
into the bone plug, and 2 specimens had a midtendon substance tear. In the 1-cm length of bone plug, 4 specimens failed at the bone plug/screw interface, while there were 3 fractures of the bone plug, and 15 failures at the insertion of the tendon into the bone plug. There were 8 failures due to a midtendonsubstance tear. Finally, in the 2-cm length of bone plug, 2 specimens failed at the bone plug/screw interface. There was 1 fracture of the bone plug, 10 failures at the insertion of the tendon into the bone plug, and 17 midtendon-substance tears. There was no significant difference in pullout strengths between the 1- and 2-cm length of bone plug in either the 7- or 9-mm screw size groups. The 9-mm screw diameter significantly increased the peak load to failure in both the 1- and 2-cm bone plug length groups, compared with the 7-mm screw size (P ⬍ .05, Fig 3). DISCUSSION FIGURE 5. Interference fit afforded by a 1-cm long bone plug with a Kurosaka screw.
Rigid initial fixation of the bone plug–patellar tendon–bone graft in ACL reconstruction is crucial in
EFFECTS OF BONE PLUG LENGTH AND SCREW DIAMETER
151
order to allow early knee joint mobilization and rehabilitation postoperatively. Interference fixation using a Kurosaka screw is currently a favored technique in securing the bone–patella tendon–bone construct. Theoretically, under ideal circumstances, each thread of the fixation screw should purchase both the bone graft and bone tunnel providing maximum fixation strength. Our study shows that the fixation of 5-mm bone plug and either a 7- or 9-mm diameter ⫻ 20-mm length of Kurosaka screw is significantly less strong
FIGURE 8. Scale diagram of a 9-mm Kurosaka screw. Note that the depth of the screw thread is 0.079 mm, and the pitch is 3.9 threads/cm.
FIGURE 7. Scale diagram of a 7-mm Kurosaka screw. Note that the depth of the screw thread is 0.057 mm, and the pitch 4.3 threads/cm.
than either a 1- or 2-cm length bone graft. Figures 4, 5 and 6 are scale diagrams showing the actual interference fit for a Kurosaka screw into a 5-mm, 1-cm, and 2-cm bone plug, respectively. Based on data provided by Noyes and his colleagues, normal activity forces require up to 445 N of force.8 This clearly suggests then, that a 5-mm bone plug does not provide enough contact to obtain rigid interference fit fixation, and therefore is not advised. Conversely, the data suggest that there is no structural advantage to having a length
152
G. POMEROY ET AL
of bone plug greater than 1 cm in terms of peak load to failure. This would suggest that longer grafts (⬎1 cm) may not add much biomechanical fixation advantage. On occasion, with endoscopic ACL reconstruction, the surgeon may note that there is protrusion of the bone plug out of the tibial tunnel. These results suggest that as long as there is at least 1 cm of bone plug engaged by the screw, there is little cause for concern. Further, it is possible that the morbidity of harvesting the bone plug from the central third of the patella will be reduced by limiting the graft lengths. Based on the findings of this study, there is no apparent mechanical advantage of using bone plugs longer than 1 cm in length. Our data demonstrate increased fixation strength using 9-mm diameter Kurosaka screws to accomplish an interference fit. It is interesting to note that the depth of the 9 mm screw thread is 0.079 mm compared with 0.057 mm for the 7 mm screw thread. This allows for increased surface contact between the bone graft and screw, and between the bone tunnel and screw, with the 9-mm Kurosaka screw. The pitch of the 9-mm screw is 3.9 thread/cm, leaving 0.1 inches between the threads. This is virtually the same as the 7 mm screw, which has a pitch of 4.3 threads/cm, and a distance of 0.091 inches between the threads. Figures 7 and 8 are scale diagrams depicting these relationships. Since the pitch is not significantly different between the screws, the increased diameter and surface area available for interference fit fixation that is present with the 9-mm screw provides for significant increases in fixation strength demonstrated when the 9-mm screw was used, compared with the 7 mm screw.
CONCLUSIONS This study supports the importance of using a bone plug of at least 1 cm in length. There does not appear to be much mechanical advantage to using bone plugs longer than 1 cm when a Kurosaka screw is placed correctly. Use of shorter bone plug lengths (⬍1 cm) result in significantly decreased peak load to failure of fixation and therefore should be avoided. Similarly, if an interference screw fails to engage at least 1 cm of a bone plug because of divergence, one may wish to consider revision. A larger Kurosaka screw diameter (9 mm v 7 mm) leads to better interference fixation. REFERENCES 1. Friedman MJ, Sherman OH, Fox JM, Del Pizzo W, Snyder SJ, Ferkel RJ. Autogeneicl anterior cruciate ligament anterior reconstruction of the knee. CORR 1985;196:9-14. 2. Penner DA, Daniel DM, Wood P, Mishra D. An in vitro study of anterior cruciate ligament graft placement and isometry. Am J Sports Med1988;16:238-243. 3. Burks RT, Leland R. Determination of graft tension before fixation in anterior cruciate ligament reconstruction. Arthroscopy 1988;4:260-266. 4. Kurosaka M, Yoshiya S, Andrish JT. A biomechanical comparison of different surgical techniques of graft fixation in anterior cruciate ligament reconstruction. Am J Sports Med 1987;15:225229. 5. Bach BR. Potential pitfalls of Kurosaka screw interference fixation for ACL surgery. Am J Knee Surg 1989;2:76-82. 6. Matthews LS, Soffer SR. Pitfalls in the use of interference screws for anterior cruciate ligament reconstruction: Brief report. Arthroscopy 1989;5:225-226. 7. Reznik AM, Davis JL, Daniel DM. Optimizing interference fixation for cruciate ligament reconstruction. ORS 36th Annual Meeting, 1990;519. 8. Noyes FR, Butler DL, Good ES, Zernicke RF, Hefzy NS. Biomechanical analysis of human ligament grafts. J Bone Joint Surg Am 1984;66:344-352.
Summary: The effect of bone plug length and Kurosaka screw (DePuy, Warsaw, IN) diameter on graft holding strength of the bone-tendon-bone construct was determined. Random length porcine bone plugs were assigned to fixation with 7 or 9 mm Kurosaka screws. Peak load to failure was determined. There was a significant decrease in peak load to failure of the 5-mm long bone plugs compared with longer bone plugs. No difference was found between longer lengths of bone plug in either the 7- or 9-mm screw diameter groups. The 9-mm diameter screws significantly increased peak load to failure for both 1- and 2-cm bone plug lengths. Key Words: Bone-tendon-bone graft—Kurosaka screw—Interference fit.
A
utogenous reconstruction is generally favored for reconstruction of a torn anterior cruciate ligament (ACL). Friedman et al. have provided a review of the options for autogenic ACL reconstruction of the knee.1 Of the options, the bone–central third patellar tendon– bone graft is currently popular. ACL graft placement and isometry,2 as well as correct tensioning before fixation of the ACL graft3 are important, but there must be adequate bone at either end of the graft to assure secure Kurosaka screw (DePuy, Warsaw, IN) fixation. In 1987, Kurosaka et al. reported that fixation of the graft with cancellous screws was superior to staple fixation and tying sutures over buttons.4 Since that time, the use of inteference screws for ACL reconstruction has been widely accepted. The pitfalls of Kurosaka screw interference fixation for ACL surgery have been evaluated by several authors.5,6 Clinical guidelines on how best to use the interference fit method are still being defined. Parameters such as bone density, From The University of Connecticut School of Medicine (M.B., K.P., M.N., W.P.); The Orthopaedic Associates of Hartford, Hartford, Connecticut; (J.P.F.); and the Portland Orthopedic Foot and Ankle Group, Portland, Maine (G.P.), U.S.A. Address correspondence and reprint requests to John P. Fulkerson, M.D., Orthopedic Associates of Hartford, 85 Seymour St, Suite 607, Hartford, CT 06106, U.S.A. r 1998 by the Arthroscopy Association of North America 0749-8063/98/1402-1210$3.00/0
148
bone tunnel-block gap, bone block cross-sectional area, screw type, and screw insertion torque as they relate to load at failure have been examined.7 No study to date has given data to indicate an optimal length of bone plug for fixation with a Kurosaka screw. The purpose of this study was to compare the holding strengths of interference screw fixation of 0.5- to 2-cm bone plugs, such as those used in ACL reconstruction. In addition, two popular Kurosaka screw diameter sizes (7 and 9 mm) were compared in the fixation of 0.5 to 2 cm bone plugs into 11-mm diameter bone tunnels.
MATERIALS AND METHODS Ninety adult fresh-frozen porcine knees were used for simulated ACL reconstructions. Femurs and fibulas were removed and discarded. For each trial, the patella, patellar tendon, and tibial tuberosity were removed as a single graft. Each specimen was randomly assigned to a bone plug length (0.5 cm, 1 cm, 2 cm), resulting in 30 specimens per bone plug length. A bone plug–patellar tendon–bone graft was taken using the central patella tendon. This was performed with a microsaggital saw. Each graft was then further prepared to fit into a 10-mm oval guide. Each graft was
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 14, No 2 (March), 1998: pp 148–152
EFFECTS OF BONE PLUG LENGTH AND SCREW DIAMETER
FIGURE 1. Peak load to failure (Newtons) for interference fit fixation for 7-mm Kurosaka screw into 0.5-cm, 1-cm, and 2-cm bone plug lengths, respectively. *P ⬍ .05.
harvested and prepared by the lead author to provide consistency between the specimens. Once prepared, each graft was then assigned to either a 7- or 9-mm diameter 20-mm long Kurosaka screw for interference fit fixation; 11-mm tunnels were drilled through the lateral tibial plateaus near the anatomic insertion of the ACL. The tunnels were placed perpendicular to the joint surface. The bone plugs were then placed in each tunnel with the tendon facing anteriorly. The screws were placed medially in an antegrade fashion. This was done under direct vision to ensure that the screw was placed parallel to the grafts; in this way, divergence was prevented. The screws were advanced until the proximal thread was seen to be fully engaged in the bone plug just distal to the articular margin. Laceration
FIGURE 2. Peak load to failure (Newtons) for interference fit fixation for 9-mm Kurosaka screw into 0.5-cm, 1-cm, and 2-cm bone plug lengths, respectively. *P ⬍ .05.
149
FIGURE 3. Peak load to failure (Newtons) for 7-mm versus 9-mm diameter Kurosaka screws. *P ⬍ .05. **P ⬍ .001.
of the tendon by the screw was prevented. Radiographs were then taken randomly to confirm screw placement. An MTS Model 858 (Materials Testing System Corp, Minneapolis, MN) was then used to mechanically pull the grafts and determine peak linear
FIGURE 4. Interference fit afforded by a 0.5-cm long bone plug with a Kurosaka screw.
150
G. POMEROY ET AL
load to failure at a strain rate of 51 cm/min. After failure, each bone plug was inspected to ensure uniform cutting of the screw threads into the entire length of bone plug. This served as further documentation that divergence was not an issue. The ultimate strength of each fixation configuration was then measured in Newtons. A two-way analysis of variance was then performed to detect statistical difference. A paired Student’s t test was used to compare test groups. RESULTS Overall, in both 7- and 9-mm screw diameters, there was a statistically significant decrease in peak load to failure between the 5-mm length of bone plug and the 1- and 2-cm bone plug lengths respectively (P ⬍ .05, Fig 1 and 2). Of the 30 specimens in the 5-mm bone plug length, 26 failed at the bone plug/screw interface. Two specimens failed at the insertion of the tendon
FIGURE 6. Interference fit afforded by a 2-cm long bone plug with a Kurosaka screw.
into the bone plug, and 2 specimens had a midtendon substance tear. In the 1-cm length of bone plug, 4 specimens failed at the bone plug/screw interface, while there were 3 fractures of the bone plug, and 15 failures at the insertion of the tendon into the bone plug. There were 8 failures due to a midtendonsubstance tear. Finally, in the 2-cm length of bone plug, 2 specimens failed at the bone plug/screw interface. There was 1 fracture of the bone plug, 10 failures at the insertion of the tendon into the bone plug, and 17 midtendon-substance tears. There was no significant difference in pullout strengths between the 1- and 2-cm length of bone plug in either the 7- or 9-mm screw size groups. The 9-mm screw diameter significantly increased the peak load to failure in both the 1- and 2-cm bone plug length groups, compared with the 7-mm screw size (P ⬍ .05, Fig 3). DISCUSSION FIGURE 5. Interference fit afforded by a 1-cm long bone plug with a Kurosaka screw.
Rigid initial fixation of the bone plug–patellar tendon–bone graft in ACL reconstruction is crucial in
EFFECTS OF BONE PLUG LENGTH AND SCREW DIAMETER
151
order to allow early knee joint mobilization and rehabilitation postoperatively. Interference fixation using a Kurosaka screw is currently a favored technique in securing the bone–patella tendon–bone construct. Theoretically, under ideal circumstances, each thread of the fixation screw should purchase both the bone graft and bone tunnel providing maximum fixation strength. Our study shows that the fixation of 5-mm bone plug and either a 7- or 9-mm diameter ⫻ 20-mm length of Kurosaka screw is significantly less strong
FIGURE 8. Scale diagram of a 9-mm Kurosaka screw. Note that the depth of the screw thread is 0.079 mm, and the pitch is 3.9 threads/cm.
FIGURE 7. Scale diagram of a 7-mm Kurosaka screw. Note that the depth of the screw thread is 0.057 mm, and the pitch 4.3 threads/cm.
than either a 1- or 2-cm length bone graft. Figures 4, 5 and 6 are scale diagrams showing the actual interference fit for a Kurosaka screw into a 5-mm, 1-cm, and 2-cm bone plug, respectively. Based on data provided by Noyes and his colleagues, normal activity forces require up to 445 N of force.8 This clearly suggests then, that a 5-mm bone plug does not provide enough contact to obtain rigid interference fit fixation, and therefore is not advised. Conversely, the data suggest that there is no structural advantage to having a length
152
G. POMEROY ET AL
of bone plug greater than 1 cm in terms of peak load to failure. This would suggest that longer grafts (⬎1 cm) may not add much biomechanical fixation advantage. On occasion, with endoscopic ACL reconstruction, the surgeon may note that there is protrusion of the bone plug out of the tibial tunnel. These results suggest that as long as there is at least 1 cm of bone plug engaged by the screw, there is little cause for concern. Further, it is possible that the morbidity of harvesting the bone plug from the central third of the patella will be reduced by limiting the graft lengths. Based on the findings of this study, there is no apparent mechanical advantage of using bone plugs longer than 1 cm in length. Our data demonstrate increased fixation strength using 9-mm diameter Kurosaka screws to accomplish an interference fit. It is interesting to note that the depth of the 9 mm screw thread is 0.079 mm compared with 0.057 mm for the 7 mm screw thread. This allows for increased surface contact between the bone graft and screw, and between the bone tunnel and screw, with the 9-mm Kurosaka screw. The pitch of the 9-mm screw is 3.9 thread/cm, leaving 0.1 inches between the threads. This is virtually the same as the 7 mm screw, which has a pitch of 4.3 threads/cm, and a distance of 0.091 inches between the threads. Figures 7 and 8 are scale diagrams depicting these relationships. Since the pitch is not significantly different between the screws, the increased diameter and surface area available for interference fit fixation that is present with the 9-mm screw provides for significant increases in fixation strength demonstrated when the 9-mm screw was used, compared with the 7 mm screw.
CONCLUSIONS This study supports the importance of using a bone plug of at least 1 cm in length. There does not appear to be much mechanical advantage to using bone plugs longer than 1 cm when a Kurosaka screw is placed correctly. Use of shorter bone plug lengths (⬍1 cm) result in significantly decreased peak load to failure of fixation and therefore should be avoided. Similarly, if an interference screw fails to engage at least 1 cm of a bone plug because of divergence, one may wish to consider revision. A larger Kurosaka screw diameter (9 mm v 7 mm) leads to better interference fixation. REFERENCES 1. Friedman MJ, Sherman OH, Fox JM, Del Pizzo W, Snyder SJ, Ferkel RJ. Autogeneicl anterior cruciate ligament anterior reconstruction of the knee. CORR 1985;196:9-14. 2. Penner DA, Daniel DM, Wood P, Mishra D. An in vitro study of anterior cruciate ligament graft placement and isometry. Am J Sports Med1988;16:238-243. 3. Burks RT, Leland R. Determination of graft tension before fixation in anterior cruciate ligament reconstruction. Arthroscopy 1988;4:260-266. 4. Kurosaka M, Yoshiya S, Andrish JT. A biomechanical comparison of different surgical techniques of graft fixation in anterior cruciate ligament reconstruction. Am J Sports Med 1987;15:225229. 5. Bach BR. Potential pitfalls of Kurosaka screw interference fixation for ACL surgery. Am J Knee Surg 1989;2:76-82. 6. Matthews LS, Soffer SR. Pitfalls in the use of interference screws for anterior cruciate ligament reconstruction: Brief report. Arthroscopy 1989;5:225-226. 7. Reznik AM, Davis JL, Daniel DM. Optimizing interference fixation for cruciate ligament reconstruction. ORS 36th Annual Meeting, 1990;519. 8. Noyes FR, Butler DL, Good ES, Zernicke RF, Hefzy NS. Biomechanical analysis of human ligament grafts. J Bone Joint Surg Am 1984;66:344-352.