The Journal of Foot & Ankle Surgery xxx (2014) 1–4
Contents lists available at ScienceDirect
The Journal of Foot & Ankle Surgery journal homepage: www.jfas.org
Original Research
A Biomechanical Comparison of Internal Fixation Techniques for Ankle Arthrodesis Craig Clifford, DPM, AACFAS 1, Scott Berg, DPM 2, Kevin McCann, DPM, AACFAS 3, Byron Hutchinson, DPM, FACFAS 4 1
Franciscan Orthopedic Associates, Federal Way, WA Postgraduate Year III Resident, Franciscan Health System, Federal Way, WA St Cloud Orthopedics, Sartell, MN 4 Franciscan Foot and Ankle Institute, Federal Way, WA 2 3
a r t i c l e i n f o
a b s t r a c t
Level of Clinical Evidence: 5
The purpose of the present study was to compare the primary bending stiffness characteristics of 5 different ankle arthrodesis fixation techniques: 3 compression screws, an anterior locking plate, a lateral locking plate, an anterior locking plate with a compression screw, and a lateral locking plate with a compression screw. A total of 25 full-scale anatomic models consisting of fourth-generation composite tibiae and tali were tested using an Instron 4505 Universal Testing System. We hypothesized that the use of a compression screw with a locking plate would add considerable stiffness to the fixation construct compared with the use of a locking plate alone. The data have shown that an anterior or lateral plate with a compression screw provides significantly greater stiffness than both a plate and 3 compression screws used individually. No significant difference was seen between the anterior plate with a compression screw and the lateral plate with a compression screw. No significant differences were found among the use of an anterior plate, a lateral plate, or 3 compression screws. We have concluded that when using a locking plate in an anterior or lateral configuration, the addition of a compression screw will considerably increase the primary bending stiffness of ankle arthrodesis. Ó 2014 by the American College of Foot and Ankle Surgeons. All rights reserved.
Keywords: ankle arthrodesis bone model compression screw load cell locking plate
End-stage ankle arthritis is problematic for patients and surgeons alike. When conservative care is no longer adequate, surgical intervention is often considered (1,2). Arthrodesis of the ankle was first described by Albert in 1879 for the treatment of paralytic equinus (3). Since its initial description, the indications for the procedure have expanded to include primary osteoarthritis, post-traumatic and inflammatory arthritis, lower extremity deformity secondary to neurologic dysfunction, and revision of failed total ankle arthroplasty (4). Despite comparable outcomes using total ankle arthroplasty in certain patient population (5,6), arthrodesis has remained the reference standard treatment of end-stage arthritis and deformity of the ankle joint. When properly executed, ankle arthrodesis restores lower extremity function and provides a highly robust solution to ankle pain and dysfunction (1,2,5,7). Financial disclosure: The hardware used was donated by Tornier, Inc, and research funding was provided by the International Foot & Ankle Foundation for Education and Research. Conflict of Interest: All the authors are affiliated with the International Foot & Ankle Foundation for Education and Research. Address correspondence to: Scott Berg, DPM, Franciscan Foot and Ankle Institute, 34509 Ninth Avenue South, Suite 306, Federal Way, WA 98003. E-mail address: [email protected] (S. Berg).
Several techniques for fixation of ankle arthrodesis have been described, including crossing screws (1,8), external fixation, intramedullary nails, and plating techniques (2,7,9,10). Each could have distinct indications, although the use of 2 or more internal fixation screws has remained the most commonly used construct of ankle arthrodesis fixation (7,11). It has been well documented that a successful osseous fusion relies on bony apposition with a high contact area, increased compression at the anticipated fusion site, and primary stiffness created by rigid immobilization (1,9,11). Successful union rates of 94% to 100% have been reported for all forms of fixation when these core principles have been observed (12–15). Recent advances in locking plate technology have presented new options for internal fixation of ankle arthrodesis. To date, few studies comparing the various applications of locking plates to the ankle joint have been published (16,17). More specifically, no studies have compared the use of locking plates at the ankle joint, both with and without the use of interfragmentary compression screws for fixation of ankle arthrodesis. The purpose of the present study was to compare the primary bending stiffness characteristics of 5 different ankle arthrodesis fixation techniques: 3 compression screws, an anterior locking plate, a lateral locking plate, an anterior locking plate with a compression
1067-2516/$ - see front matter Ó 2014 by the American College of Foot and Ankle Surgeons. All rights reserved. http://dx.doi.org/10.1053/j.jfas.2014.06.002
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C. Clifford et al. / The Journal of Foot & Ankle Surgery xxx (2014) 1–4
Fig. 1. Anteroposterior and lateral radiographs of each fixation method: (A) 3 compression screws, (B) anterior plate, (C) lateral plate, (D) anterior plate with compression screw, and (e) lateral plate with compression screw.
screw, and a lateral locking plate with a compression screw. We hypothesized that a clinically significant difference would be identified when comparing the combined use of a compression screw and locking plate versus the use of a locking plate alone. Materials and Methods Testing was performed on full-scale anatomic models consisting of fourthgeneration composite tibiae and tali (Pacific Research Laboratories, Vashon WA). The models were prepared for arthrodesis with planar resection of the tibial and talar articular surfaces to a maximum depth of 3 mm. The medial malleolar articular surface was also resected to a maximum depth of 1 mm, with a planar cut at a 90 angle to the prepared tibial surface. All specimens were placed in neutral anatomic alignment before fixation using 1 of the 5 methods (Fig. 1): 1. Three compression screws: Three 6.5-mm cannulated compression screws (Tornier, Inc, Bloomington, MN) were placed from the tibia to the talus using the technique described by Schuberth et al (8); the posterolateral screw was placed first, followed by the medial and finally anterolateral screws. 2. Anterior locking plate (ALP): A contoured anterior locking plate (Tornier, Inc) was placed on the anterior side of the joint in line with the talar neck. The plate was first fixed to the talus with locking screws. Next, the arthrodesis site was compressed using the ratcheting compression devices supplied by the manufacturer. Finally, the plate was fixed to the tibia with the associated nonlocking and locking screws. 3. Lateral locking plate (LLP): A contoured lateral locking plate (Tornier, Inc) was placed on the lateral side of the joint in the fibular groove of the tibia. First the plate was fixed to the talus with locking screws. Next, the arthrodesis site was compressed using the ratcheting compression devices supplied by the manufacturer. Finally, the plate was fixed to the tibia with the associated nonlocking and locking screws. 4. ALP with a compression screw (ALP-S): First, a single 6.5-mm cannulated compression screw was placed from the posterolateral tibia to the talar neck. Next, a contoured plate was applied anteriorly, as previously described. 5. LLP with a compression screw (LLP-S): First, a single 6.5-mm cannulated compression screw was placed from the posterolateral tibia to the talar neck. Next, a contoured plate was applied laterally, as previously described. Each method was used for fixation of 5 complete specimens. All proximal tibiae were drilled and tapped for the placement of a 12-in. threaded steel rod placed at a depth of 10 in. along the central axis of the bone for use as an attachment point to the testing machine. All tali were potted in epoxy resin (3M, St Paul, MN) to a depth of 1 cm distal to the arthrodesis site. The drill holes and exposed hardware were protected from epoxy contact by covering them with plastilene modeling clay. The 25 specimens were tested with a uniform protocol using an Instron 4505 Universal Testing System (Instron, Norwood, MA). The tibiae were affixed to the load cell, and the potted tali were bolted to a rigid steel cantilever beam. The beam was allowed to freely pivot on a fulcrum mounted to the crosshead of the testing machine
12 cm from the center of the ankle joint (Fig. 2). The crosshead was raised 5.3 mm at a rate of 1 mm/s to place a bending moment on the arthrodesis site to a maximum bend of 3 . With rotation of the device, the fulcrum was varied to anterior, medial, posterior, and lateral placement with respect to the arthrodesis site for bending evaluation in the respective modes of dorsiflexion, inversion, plantar flexion, and eversion. Two trials of each bending mode were performed for each specimen, with the second trial data recorded. Force–displacement data were captured and recorded every 10 ms and curves plotted using Bluehill software (Instron). Bending stiffness was represented by the slope of the force–displacement curves. The results were analyzed using 1-way analysis of variance with Tukey post hoc tests to compare the mean values of the loading modes among each fixation method. The overall bending stiffness was represented by the mean bending stiffness of all loading modes within each fixation method. Statistical significance was defined at the 5% (p ¼ .05) level. All statistical analyses were performed using IBM SPSS Statistics, version 21.0 (IBM Corp, Armonk, NY).
Results A total of 25 cadaver specimens were tested, 5 each used for each of the 5 different fixation constructs. With dorsiflexion loading, the greatest stiffness was provided by the ALP-S (60.76 12.63 N/mm), which was significantly stiffer than all other fixation methods (p < .001),
Fig. 2. Arthrodesis specimen mounted to cantilever beam with the fulcrum 12 cm from the center of the ankle joint.
C. Clifford et al. / The Journal of Foot & Ankle Surgery xxx (2014) 1–4
3
Fig. 3. Comparative bending stiffness of each fixation method (n ¼ 5 cadaver specimens for each of the 5 fixation constructs).
and provided a bending stiffness 71% greater than the anterior plate alone (p < .001). The addition of a compression screw resulted in a significant increase in bending stiffness (p < .001) for the anterior plate but no significant increase in stiffness for the lateral plate (p ¼ .175). With plantar flexion loading, the ALP-S and ALP produced the greatest primary stiffness (65.53 12.52 N/mm and 66.07 12.83 N/mm, respectively). The difference between these constructs was not significant (p > .05); however, these 2 constructs had significantly greater stiffness than the remaining fixation methods (p < .008). The addition of a compression screw provided no significant increase in bending stiffness for the ALP or the LLP (p > .51). With inversion loading, the LLP-S and LLP produced the greatest primary stiffness (65.11 2.32 N/mm and 57.58 4.49 N/mm, respectively). The difference between these constructs was not significant (p ¼ .05). The LLP-S and LLP both provided significantly greater stiffness than the remaining fixation methods (p < .001). The addition of a compression screw provided no significant increase in bending stiffness for the ALP or the LLP (p > .05). With eversion loading, the LLP produced the least primary stiffness (12.82 3.02 N/mm), which was significantly different compared with all other fixation methods (p < .001). The difference between the remaining constructs (ALP, ALP-S, LLP-S, and 3 compression screws) was not significant (p > .05). The addition of a compression screw resulted in no significant increase in bending stiffness (p ¼ .104) for the ALP but a significant increase in stiffness for the LLP (p < .001). For overall stiffness, the ALP-S and LLP-S produced the greatest primary stiffness (51.24 16.37 N/mm and 48.82 11.79 N/mm, respectively). The difference between these constructs was not significant (p > .989). The addition of a compression screw resulted in no significant increase in overall bending stiffness (p ¼ .07) for the ALP but a significant increase in the overall bending stiffness for the LLP (p ¼ .018). No significant difference was found among the ALP, LLP, or 3 compression screws (p > .05) (Fig. 3). Discussion The results of the present study highlight several important findings. The initial hypothesis was supported by the data showing that a compression screw could add considerable stiffness to a locking plate
fixation technique for ankle arthrodesis in certain cases. The addition of a compression screw added significant stiffness to arthrodesis constructs in which the plate was on the compression side of loading (i.e., the anterior plate under dorsiflexion and the lateral plate under eversion). This finding coincides with the findings from Mueckley et al (9) and Buranosky et al (18). The bending stiffness of each fixation method varied with the direction of the applied force. The presented data have clearly demonstrated the principles of plate fixation, with the greatest resistance to bending observed when the plate was fixed on the tension side of the applied force. It was in these situations only that the addition of a compression screw provided no significant increase in bending stiffness. Our data have shown that the greatest overall bending stiffness was with the anterior plate and compression screw method, although this was not significantly different from that of the lateral plate with a compression screw. Fixation with the 3 compression screws showed comparatively low average bending stiffness; however, the stiffness was relatively consistent across all bending modes and was not significantly different from that with anterior or lateral plate fixation without a compression screw. It is important that these data not be used out of context. Although rigid internal fixation has been shown to promote successful arthrodesis (1,9,19–21), the optimal bending stiffness for each loading mode of an ankle arthrodesis is not yet known. Furthermore, in a clinical setting, it is likely unrealistic to assume equal loading in all planes; therefore, the overall bending stiffness such as was defined in the present study mighty not be applicable. It is reasonable to assume that dorsiflexory loading might have a more significant role than the other loading modes owing to the fulcrum of the foot during propulsive gait; however, this issue was outside the scope of our study. Additionally, soft tissue structures, such as the Achilles tendon, can impart significant forces on the arthrodesis site. The use of an anterior plate alone could, therefore, theoretically impart dynamic compression on an ankle arthrodesis; however, this has received limited investigation (16,22,23). Other inherent differences exist between the test model ankle arthrodesis methods and those encountered in vivo, including the required incisional approach, possibility of percutaneous placement,
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C. Clifford et al. / The Journal of Foot & Ankle Surgery xxx (2014) 1–4
suitability for use with osteoporotic bone, and cost of implants (9,10). When choosing a fixation method, the surgeon must consider many technical and patient-specific requirements that were beyond the scope of our report. The main limitation of the present study was the use of biomechanical bone models tested in a laboratory with strictly controlled motion. The actual in situ forces will be more varied and not restricted to a single plane of movement. However, the use of composite biomechanical models eliminated any error caused by specimen size or bone density (9,24,25). Additional testing could consider soft tissue attachments, combined forces, and in vivo investigations linked to clinical outcomes. In conclusion, ankle arthrodesis can be performed using several different fixation methods, 5 of which were examined within the design of our study. At the completion of the present study, we concluded that when using a locking plate in an anterior or lateral configuration, the addition of a compression screw will considerably increase the bending stiffness and compensate for the tension-side limitations of plate fixation alone. The use of 3 compression screws provided overall stiffness similar that of provided by anterior and lateral plates without compression screws. Acknowledgments We thank Tuesday Kuykendall for her assistance with the Instron testing system and the Materials Science and Engineering Department, University of Washington, Seattle, WA. References 1. Kakarla G, Rajan DT. Comparative study of ankle arthrodesis using cross screws fixation versus anterior contoured plate plus cross screw fixation. Acta Orthop Belg 72:716–721, 2006. 2. Fragomen AT, Myers KN, Davis N. A biomechanical comparison of micromotion after ankle fusion using 2 fixation techniques: intramedullary arthrodesis nail or Ilizarov external fixator. Foot Ankle Int 29:334–341, 2008. 3. Albert E. Zur Resektion des Kniegelenkes. Wien Med Press 20:705–708, 1879. 4. Nihal A, Gellman RE, Embil JM, Trepman E. Ankle arthrodesis. Foot Ankle Surg 14:1–10, 2008. 5. Haddad SL, Coetzee JC, Estok R, Fahrbach K, Banel D, Nalysnyk L. Intermediate and long-term outcomes of total ankle arthroplasty and ankle arthrodesis: a systematic review of the literature. J Bone Joint Surg Am 89:1899–1905, 2007.
6. Krause FG, Windolf M, Bora B, Penner MJ, Wing KJ, Younger ASE. Impact of complications in total ankle replacement and ankle arthrodesis analyzed with a validated outcome measurement. J Bone Joint Surg Am 93:830–839, 2011. 7. Ogut T, Glisson RR, Chuckpaiwong B, Le I, Easley ME. External ring fixation vs screw fixation for ankle arthrodesis: a biomechanical comparison. Foot Ankle Int 30:353–360, 2009. 8. Schuberth JM, Ruch JA, Hansen ST. The tripod fixation technique for ankle arthrodesis. J Foot Ankle Surg 48:93–96, 2009. 9. Mueckley TM, Eichorn S, Oldenburg G, Speitling A, DiCicco J, Hofmann GO, Buhren V. Biomechanical evaluation of primary stiffness of tibiotalar arthrodesis with intramedullary compression nail and four other fixation devices. Foot Ankle Int 27:814–820, 2006. 10. Nasson S, Shuff C, Palmer D, Owen J, Wayne J, Carr J, Adelaar R, May D. Biomechanical comparison of ankle arthrodesis techniques: crossed screws vs. blade plate. Foot Ankle Int 22:575–580, 2001. 11. Dohm MP, Benjamin JB, Harrison J, Szivek JA. A biomechanical evaluation of three forms of internal fixation used in ankle arthrodesis. Foot Ankle Int 15:297–300, 1994. 12. Braly WG. Arthrodesis of the ankle with lateral plating. Foot Ankle 15:649–653, 1994. 13. Scranton PE Jr, Fu FH, Brown TD. Ankle arthrodesis: a comparative clinical and biomechanical evaluation. Clin Orthop Relat Res 151:234–243, 1980. 14. Sowa DT, Krackow KA. Ankle fusion: a new technique of internal fixation using a compression blade plate. Foot Ankle 9:232–240, 1989. 15. Weitmer JB, Choi SH, Shenoy A, Schwartsman V. Wolf blade plate ankle arthrodesis. Clin Orthop Relat Res 268:107–111, 1991. 16. Kestner CJ, Glisson RR, DeOrio JK, Nunley JA. A biomechanical analysis of two anterior ankle arthrodesis systems. Foot Ankle 34:1006–1011, 2013. 17. O’Neill PJ, Logel KJ, Parks BG, Schon LC. Rigidity comparison of locking plate and intramedullary fixation for tibiotalocalcaneal arthrodesis. Foot Ankle Int 29:581–586, 2008. 18. Buranosky DJ, Taylor DT, Sage RA, Patwardhan A, Phelan M, Lam AT. First metatarsophalangeal joint arthrodesis: quantitative mechanical testing of six-hole dorsal plate versus crossed screw fixation in cadaveric specimens. J Foot Ankle Surg 40:208–213, 2001. 19. Dennis DA, Clayton ML, Wong DA, Mack RP, Susman MH. Internal fixation compression arthrodesis of the ankle. Clin Orthop Relat Res 253:212–220, 1990. 20. Kennedy JG, Hodgkins CW, Brodsky A, Bohne WH. Outcomes after standardized screw fixation technique of ankle arthrodesis. Clin Orthop Relat Res 447:112–118, 2006. 21. Abdo RV, Wasilewski SA. Ankle Arthrodesis: a long term study. Foot Ankle 13:307–312, 1992. 22. Mears DC, Gordon RG, Kann SE, Kann JN. Ankle arthrodesis with an anterior tension plate. Clin Orthop Relat Res 268:70–77, 1990. 23. Tarkin IS, Mormino MA, Clare MP, Haider H, Walling AK, Sanders RW. Anterior plate supplementation increases ankle arthrodesis construct rigidity. Foot Ankle Int 28:219–223, 2007. 24. Cristofolini L, Viceconti M. Mechanical validations of whole bone composite tibia models. J Biomech 33:279–288, 2000. 25. Panjabi MM, Krag M, Summers D, Videman T. Biomechanical time-tolerance of fresh cadaveric human spine specimens. J Orthop Res 3:292–300, 1985.
Contents lists available at ScienceDirect
The Journal of Foot & Ankle Surgery journal homepage: www.jfas.org
Original Research
A Biomechanical Comparison of Internal Fixation Techniques for Ankle Arthrodesis Craig Clifford, DPM, AACFAS 1, Scott Berg, DPM 2, Kevin McCann, DPM, AACFAS 3, Byron Hutchinson, DPM, FACFAS 4 1
Franciscan Orthopedic Associates, Federal Way, WA Postgraduate Year III Resident, Franciscan Health System, Federal Way, WA St Cloud Orthopedics, Sartell, MN 4 Franciscan Foot and Ankle Institute, Federal Way, WA 2 3
a r t i c l e i n f o
a b s t r a c t
Level of Clinical Evidence: 5
The purpose of the present study was to compare the primary bending stiffness characteristics of 5 different ankle arthrodesis fixation techniques: 3 compression screws, an anterior locking plate, a lateral locking plate, an anterior locking plate with a compression screw, and a lateral locking plate with a compression screw. A total of 25 full-scale anatomic models consisting of fourth-generation composite tibiae and tali were tested using an Instron 4505 Universal Testing System. We hypothesized that the use of a compression screw with a locking plate would add considerable stiffness to the fixation construct compared with the use of a locking plate alone. The data have shown that an anterior or lateral plate with a compression screw provides significantly greater stiffness than both a plate and 3 compression screws used individually. No significant difference was seen between the anterior plate with a compression screw and the lateral plate with a compression screw. No significant differences were found among the use of an anterior plate, a lateral plate, or 3 compression screws. We have concluded that when using a locking plate in an anterior or lateral configuration, the addition of a compression screw will considerably increase the primary bending stiffness of ankle arthrodesis. Ó 2014 by the American College of Foot and Ankle Surgeons. All rights reserved.
Keywords: ankle arthrodesis bone model compression screw load cell locking plate
End-stage ankle arthritis is problematic for patients and surgeons alike. When conservative care is no longer adequate, surgical intervention is often considered (1,2). Arthrodesis of the ankle was first described by Albert in 1879 for the treatment of paralytic equinus (3). Since its initial description, the indications for the procedure have expanded to include primary osteoarthritis, post-traumatic and inflammatory arthritis, lower extremity deformity secondary to neurologic dysfunction, and revision of failed total ankle arthroplasty (4). Despite comparable outcomes using total ankle arthroplasty in certain patient population (5,6), arthrodesis has remained the reference standard treatment of end-stage arthritis and deformity of the ankle joint. When properly executed, ankle arthrodesis restores lower extremity function and provides a highly robust solution to ankle pain and dysfunction (1,2,5,7). Financial disclosure: The hardware used was donated by Tornier, Inc, and research funding was provided by the International Foot & Ankle Foundation for Education and Research. Conflict of Interest: All the authors are affiliated with the International Foot & Ankle Foundation for Education and Research. Address correspondence to: Scott Berg, DPM, Franciscan Foot and Ankle Institute, 34509 Ninth Avenue South, Suite 306, Federal Way, WA 98003. E-mail address: [email protected] (S. Berg).
Several techniques for fixation of ankle arthrodesis have been described, including crossing screws (1,8), external fixation, intramedullary nails, and plating techniques (2,7,9,10). Each could have distinct indications, although the use of 2 or more internal fixation screws has remained the most commonly used construct of ankle arthrodesis fixation (7,11). It has been well documented that a successful osseous fusion relies on bony apposition with a high contact area, increased compression at the anticipated fusion site, and primary stiffness created by rigid immobilization (1,9,11). Successful union rates of 94% to 100% have been reported for all forms of fixation when these core principles have been observed (12–15). Recent advances in locking plate technology have presented new options for internal fixation of ankle arthrodesis. To date, few studies comparing the various applications of locking plates to the ankle joint have been published (16,17). More specifically, no studies have compared the use of locking plates at the ankle joint, both with and without the use of interfragmentary compression screws for fixation of ankle arthrodesis. The purpose of the present study was to compare the primary bending stiffness characteristics of 5 different ankle arthrodesis fixation techniques: 3 compression screws, an anterior locking plate, a lateral locking plate, an anterior locking plate with a compression
1067-2516/$ - see front matter Ó 2014 by the American College of Foot and Ankle Surgeons. All rights reserved. http://dx.doi.org/10.1053/j.jfas.2014.06.002
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C. Clifford et al. / The Journal of Foot & Ankle Surgery xxx (2014) 1–4
Fig. 1. Anteroposterior and lateral radiographs of each fixation method: (A) 3 compression screws, (B) anterior plate, (C) lateral plate, (D) anterior plate with compression screw, and (e) lateral plate with compression screw.
screw, and a lateral locking plate with a compression screw. We hypothesized that a clinically significant difference would be identified when comparing the combined use of a compression screw and locking plate versus the use of a locking plate alone. Materials and Methods Testing was performed on full-scale anatomic models consisting of fourthgeneration composite tibiae and tali (Pacific Research Laboratories, Vashon WA). The models were prepared for arthrodesis with planar resection of the tibial and talar articular surfaces to a maximum depth of 3 mm. The medial malleolar articular surface was also resected to a maximum depth of 1 mm, with a planar cut at a 90 angle to the prepared tibial surface. All specimens were placed in neutral anatomic alignment before fixation using 1 of the 5 methods (Fig. 1): 1. Three compression screws: Three 6.5-mm cannulated compression screws (Tornier, Inc, Bloomington, MN) were placed from the tibia to the talus using the technique described by Schuberth et al (8); the posterolateral screw was placed first, followed by the medial and finally anterolateral screws. 2. Anterior locking plate (ALP): A contoured anterior locking plate (Tornier, Inc) was placed on the anterior side of the joint in line with the talar neck. The plate was first fixed to the talus with locking screws. Next, the arthrodesis site was compressed using the ratcheting compression devices supplied by the manufacturer. Finally, the plate was fixed to the tibia with the associated nonlocking and locking screws. 3. Lateral locking plate (LLP): A contoured lateral locking plate (Tornier, Inc) was placed on the lateral side of the joint in the fibular groove of the tibia. First the plate was fixed to the talus with locking screws. Next, the arthrodesis site was compressed using the ratcheting compression devices supplied by the manufacturer. Finally, the plate was fixed to the tibia with the associated nonlocking and locking screws. 4. ALP with a compression screw (ALP-S): First, a single 6.5-mm cannulated compression screw was placed from the posterolateral tibia to the talar neck. Next, a contoured plate was applied anteriorly, as previously described. 5. LLP with a compression screw (LLP-S): First, a single 6.5-mm cannulated compression screw was placed from the posterolateral tibia to the talar neck. Next, a contoured plate was applied laterally, as previously described. Each method was used for fixation of 5 complete specimens. All proximal tibiae were drilled and tapped for the placement of a 12-in. threaded steel rod placed at a depth of 10 in. along the central axis of the bone for use as an attachment point to the testing machine. All tali were potted in epoxy resin (3M, St Paul, MN) to a depth of 1 cm distal to the arthrodesis site. The drill holes and exposed hardware were protected from epoxy contact by covering them with plastilene modeling clay. The 25 specimens were tested with a uniform protocol using an Instron 4505 Universal Testing System (Instron, Norwood, MA). The tibiae were affixed to the load cell, and the potted tali were bolted to a rigid steel cantilever beam. The beam was allowed to freely pivot on a fulcrum mounted to the crosshead of the testing machine
12 cm from the center of the ankle joint (Fig. 2). The crosshead was raised 5.3 mm at a rate of 1 mm/s to place a bending moment on the arthrodesis site to a maximum bend of 3 . With rotation of the device, the fulcrum was varied to anterior, medial, posterior, and lateral placement with respect to the arthrodesis site for bending evaluation in the respective modes of dorsiflexion, inversion, plantar flexion, and eversion. Two trials of each bending mode were performed for each specimen, with the second trial data recorded. Force–displacement data were captured and recorded every 10 ms and curves plotted using Bluehill software (Instron). Bending stiffness was represented by the slope of the force–displacement curves. The results were analyzed using 1-way analysis of variance with Tukey post hoc tests to compare the mean values of the loading modes among each fixation method. The overall bending stiffness was represented by the mean bending stiffness of all loading modes within each fixation method. Statistical significance was defined at the 5% (p ¼ .05) level. All statistical analyses were performed using IBM SPSS Statistics, version 21.0 (IBM Corp, Armonk, NY).
Results A total of 25 cadaver specimens were tested, 5 each used for each of the 5 different fixation constructs. With dorsiflexion loading, the greatest stiffness was provided by the ALP-S (60.76 12.63 N/mm), which was significantly stiffer than all other fixation methods (p < .001),
Fig. 2. Arthrodesis specimen mounted to cantilever beam with the fulcrum 12 cm from the center of the ankle joint.
C. Clifford et al. / The Journal of Foot & Ankle Surgery xxx (2014) 1–4
3
Fig. 3. Comparative bending stiffness of each fixation method (n ¼ 5 cadaver specimens for each of the 5 fixation constructs).
and provided a bending stiffness 71% greater than the anterior plate alone (p < .001). The addition of a compression screw resulted in a significant increase in bending stiffness (p < .001) for the anterior plate but no significant increase in stiffness for the lateral plate (p ¼ .175). With plantar flexion loading, the ALP-S and ALP produced the greatest primary stiffness (65.53 12.52 N/mm and 66.07 12.83 N/mm, respectively). The difference between these constructs was not significant (p > .05); however, these 2 constructs had significantly greater stiffness than the remaining fixation methods (p < .008). The addition of a compression screw provided no significant increase in bending stiffness for the ALP or the LLP (p > .51). With inversion loading, the LLP-S and LLP produced the greatest primary stiffness (65.11 2.32 N/mm and 57.58 4.49 N/mm, respectively). The difference between these constructs was not significant (p ¼ .05). The LLP-S and LLP both provided significantly greater stiffness than the remaining fixation methods (p < .001). The addition of a compression screw provided no significant increase in bending stiffness for the ALP or the LLP (p > .05). With eversion loading, the LLP produced the least primary stiffness (12.82 3.02 N/mm), which was significantly different compared with all other fixation methods (p < .001). The difference between the remaining constructs (ALP, ALP-S, LLP-S, and 3 compression screws) was not significant (p > .05). The addition of a compression screw resulted in no significant increase in bending stiffness (p ¼ .104) for the ALP but a significant increase in stiffness for the LLP (p < .001). For overall stiffness, the ALP-S and LLP-S produced the greatest primary stiffness (51.24 16.37 N/mm and 48.82 11.79 N/mm, respectively). The difference between these constructs was not significant (p > .989). The addition of a compression screw resulted in no significant increase in overall bending stiffness (p ¼ .07) for the ALP but a significant increase in the overall bending stiffness for the LLP (p ¼ .018). No significant difference was found among the ALP, LLP, or 3 compression screws (p > .05) (Fig. 3). Discussion The results of the present study highlight several important findings. The initial hypothesis was supported by the data showing that a compression screw could add considerable stiffness to a locking plate
fixation technique for ankle arthrodesis in certain cases. The addition of a compression screw added significant stiffness to arthrodesis constructs in which the plate was on the compression side of loading (i.e., the anterior plate under dorsiflexion and the lateral plate under eversion). This finding coincides with the findings from Mueckley et al (9) and Buranosky et al (18). The bending stiffness of each fixation method varied with the direction of the applied force. The presented data have clearly demonstrated the principles of plate fixation, with the greatest resistance to bending observed when the plate was fixed on the tension side of the applied force. It was in these situations only that the addition of a compression screw provided no significant increase in bending stiffness. Our data have shown that the greatest overall bending stiffness was with the anterior plate and compression screw method, although this was not significantly different from that of the lateral plate with a compression screw. Fixation with the 3 compression screws showed comparatively low average bending stiffness; however, the stiffness was relatively consistent across all bending modes and was not significantly different from that with anterior or lateral plate fixation without a compression screw. It is important that these data not be used out of context. Although rigid internal fixation has been shown to promote successful arthrodesis (1,9,19–21), the optimal bending stiffness for each loading mode of an ankle arthrodesis is not yet known. Furthermore, in a clinical setting, it is likely unrealistic to assume equal loading in all planes; therefore, the overall bending stiffness such as was defined in the present study mighty not be applicable. It is reasonable to assume that dorsiflexory loading might have a more significant role than the other loading modes owing to the fulcrum of the foot during propulsive gait; however, this issue was outside the scope of our study. Additionally, soft tissue structures, such as the Achilles tendon, can impart significant forces on the arthrodesis site. The use of an anterior plate alone could, therefore, theoretically impart dynamic compression on an ankle arthrodesis; however, this has received limited investigation (16,22,23). Other inherent differences exist between the test model ankle arthrodesis methods and those encountered in vivo, including the required incisional approach, possibility of percutaneous placement,
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suitability for use with osteoporotic bone, and cost of implants (9,10). When choosing a fixation method, the surgeon must consider many technical and patient-specific requirements that were beyond the scope of our report. The main limitation of the present study was the use of biomechanical bone models tested in a laboratory with strictly controlled motion. The actual in situ forces will be more varied and not restricted to a single plane of movement. However, the use of composite biomechanical models eliminated any error caused by specimen size or bone density (9,24,25). Additional testing could consider soft tissue attachments, combined forces, and in vivo investigations linked to clinical outcomes. In conclusion, ankle arthrodesis can be performed using several different fixation methods, 5 of which were examined within the design of our study. At the completion of the present study, we concluded that when using a locking plate in an anterior or lateral configuration, the addition of a compression screw will considerably increase the bending stiffness and compensate for the tension-side limitations of plate fixation alone. The use of 3 compression screws provided overall stiffness similar that of provided by anterior and lateral plates without compression screws. Acknowledgments We thank Tuesday Kuykendall for her assistance with the Instron testing system and the Materials Science and Engineering Department, University of Washington, Seattle, WA. References 1. Kakarla G, Rajan DT. Comparative study of ankle arthrodesis using cross screws fixation versus anterior contoured plate plus cross screw fixation. Acta Orthop Belg 72:716–721, 2006. 2. Fragomen AT, Myers KN, Davis N. A biomechanical comparison of micromotion after ankle fusion using 2 fixation techniques: intramedullary arthrodesis nail or Ilizarov external fixator. Foot Ankle Int 29:334–341, 2008. 3. Albert E. Zur Resektion des Kniegelenkes. Wien Med Press 20:705–708, 1879. 4. Nihal A, Gellman RE, Embil JM, Trepman E. Ankle arthrodesis. Foot Ankle Surg 14:1–10, 2008. 5. Haddad SL, Coetzee JC, Estok R, Fahrbach K, Banel D, Nalysnyk L. Intermediate and long-term outcomes of total ankle arthroplasty and ankle arthrodesis: a systematic review of the literature. J Bone Joint Surg Am 89:1899–1905, 2007.
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