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Mechanical Comparison of Cortical Screw Fixation Versus Locking Plate Fixation in First Metatarsal Base Osteotomy
The Journal of Foot & Ankle Surgery 53 (2014) 529–533
Contents lists available at ScienceDirect
The Journal of Foot & Ankle Surgery journal homepage: www.jfas.org
Mechanical Comparison of Cortical Screw Fixation Versus Locking Plate Fixation in First Metatarsal Base Osteotomy Kevin Smith, DPM, FACFAS 1, Roy H. Lidtke, DPM 2, Noah G. Oliver, DPM, AACFAS 3, Jared M. Maker, DPM 4 1
Associate Professor, Des Moines University College of Podiatric Medicine and Surgery, Des Moines, IA Assistant Professor, Section of Rheumatology, Department of Internal Medicine, Rush University Medical Center, Chicago, IL Third-Year Resident, Inova Fairfax Hospital, Falls Church, VA 4 Third-Year Resident, Western Pennsylvania Hospital, Pittsburgh, PA 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 oblique closing base wedge osteotomy has been used for surgical treatment of moderate to severe hallux valgus deformities with an intermetatarsal angle typically greater than 15 . Several postoperative complications have been identified that relate to failure of the fixation construct used to fixate the osteotomy, especially when that construct has been subjected to a vertical load. We performed a mechanical analysis comparing 2 constructs used to fixate oblique osteotomies of the first metatarsal using composite first metatarsals. An oblique base osteotomy was uniformly performed on 40 composite first metatarsals. Of the 40 specimens, 20 were fixated with a locking plate construct and 18 with a cortical screw construct, consisting of an anchor and compression screw (2 specimens from the latter group were excluded because of hinge fracture). Each specimen was loaded in a materials testing machine to measure the maximum load at construct failure when a vertical force was applied to the plantar aspect of the metatarsal head. The mean load to failure for the locking plate construct was significantly greater than the cortical screw construct (190.0 70 N versus 110.3 20.3 N, p < .001). Our study results have demonstrated that the locking plate construct was able to withstand a significantly greater vertical load before failure than was the 2-cortical screw construct in oblique osteotomies performed at the base of composite first metatarsals. Ó 2014 by the American College of Foot and Ankle Surgeons. All rights reserved.
Keywords: bone model closing base wedge osteotomy hallux valgus mechanical testing surgery
The oblique closing base wedge osteotomy (CBWO) described by Juvara (1) has been advocated for the surgical treatment of moderate to severe hallux valgus deformities with an intermetatarsal angle greater than 15 (2,3). Several studies have examined the complications related to the CBWO. Landsman and Vogler (4) suggested that several complications could result from postoperative elevation or displacement of the distal metatarsal segment relative to the proximal segment. They also reported that elevation or displacement could result from a lack of rigid fixation. Schuberth et al (2) performed a radiographic review of 159 feet and found that 93.7% demonstrated postoperative elevation, with an average of 6.679 . They suggested that new internal fixation techniques might decrease the incidence of postoperative complication seen with the CBWO. Several investigators have tested different fixation constructs to determine their strength and their ability to minimize the Financial Disclosure: None reported. Conflict of Interest: None reported. Address correspondence to: Kevin Smith, DPM, FACFAS, Des Moines University College of Podiatric Medicine and Surgery, 3200 Grand Avenue, Des Moines, IA 50312. E-mail address: [email protected] (K. Smith).
Fig. 1. Hollow composite first metatarsal (Pacific Research Laboratories, Vashon, WA).
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.2014.04.025
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K. Smith et al. / The Journal of Foot & Ankle Surgery 53 (2014) 529–533
Fig. 2. (A) Side view of the osteotomy jig. (B) Dorsal view of the osteotomy jig.
the CBWO. The purpose of the present study was to compare the mechanical stability of these 2 different constructs in the proximal osteotomy of the first metatarsal base. Our hypothesis was that the locking plate construct would require a greater force to failure than would the screw construct when subjected to a vertical load.
complications related to the CBWO. Christensen et al (5) examined single-screw fixation with an intact medial cortical hinge and 2-screw fixation with an intact cortical hinge. All specimens were fixated with 3.5-mm cortical screws. They found no significant difference between the 2 constructs in cantilever bending. Fillinger et al (6) also compared single- and 2-screw constructs using 2.7-mm cortical screws and found that the construct fixated with 2 screws resisted the vertical ground reactive forces better than did the single-screw construct. Landsman and Vogler (4) also studied different constructs and varied the size of fixation within the same study. They tested osteotomies fixated with a single 3.5-mm cancellous screw, two 2.7-mm cortical screws, and 2 Kirschner wires. They reported that the Kirschner wire and single-screw methods were superior to two 2.7-mm cortical screws for stability. They continued by suggesting that all the constructs tested were not impressive in stabilizing the osteotomy under true physiologic loads. A fixation construct that potentially could resist the vertical ground reactive forces related to basilar osteotomies is the locking plate. Jones et al (7) performed a mechanical comparison of 2 different constructs in the proximal crescentic osteotomy and found that osteotomies fixated with a locking plate demonstrated a greater mean load to failure than those fixated with a 3.5-mm cortical screw and a Kirschner wire. A couple of limitations of the study by Jones et al (7) were that they did not investigate the CBWO and did not compare the locking plate construct against a 2-screw construct. To our knowledge, no published reports have compared the strength of the locking plate construct and a two 2.7-mm cortical screw construct for fixation of
A total of 40 composite first metatarsal biomechanical testing bones were obtained and used for our research (Pacific Research Laboratories, Vashon Island, WA). These bones have been designed to have the same physical characteristics of real bone but do not have the anisotropic properties of real bone and therefore will be identical to one another (8). The composite bones will also be identical to one another in dimension and are hollow to simulate the medullary canal (Fig. 1). Before cutting the osteotomies, the composite first metatarsals were attached to a jig that we had constructed. The jig consisted of an acrylic sheet and wood and was attached directly to the composite first metatarsal using an L-shaped plate (Fig. 2). The jig was used to ensure the consistency and uniformity of all cuts. A power sagittal saw with a 10-mm-wide sagittal saw blade was used for all osteotomies. The osteotomy consisted of a solitary bone cut that was angulated 45 to the long axis of the metatarsal composite and perpendicular to the metatarsal (Fig. 3). A medical cortical hinge 1 cm distal to the metatarsal base was also maintained. A wedge osteotomy was not performed because we were testing the strength of the fixation construct, not the properties of a specific osteotomy. Christensen et al (5) described a similar method in their first metatarsal base osteotomy study and reported that specimen uniformity would be greater if a wedge of bone were not removed. Of the 40 bones, 20 were fixated with the locking plate construct. The proximal portion of the locking plate was 0.5 cm from the base of the composite metatarsal, and that distance was measured and marked on each specimen to ensure uniformity (Fig. 4). Of the 20 bones in the screw construct group, 18 were fixated with two 2.7-mm bicortical screws using the AO lag technique. Two of the composite metatarsals in the
Fig. 3. Dorsal view of the composite first metatarsal osteotomy.
Fig. 4. Dorsal view of the locking plate construct.
Materials and Methods
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Fig. 5. (A) Guide hole for compression screw insertion. (B) Guide hole for anchor screw insertion. (C) Side view of the cortical screw construct.
screw construct group had developed a hinge fracture during fixation and were excluded from testing. The composite metatarsals in the bicortical screw construct group were attached to another jig composed of an acrylic sheet and wood with predrilled guide holes for cortical screw fixation to again ensure uniformity. One of the bicortical screws served as the anchor screw and was inserted perpendicular to the long axis of the composite metatarsal and tightened first. The other bicortical screw served as the compression screw and was inserted perpendicular to the osteotomy (Fig. 5). A torque screwdriver was used to uniformly tighten the cortical screws. One investigator (K.S.) performed all the osteotomies and inserted all fixation devices. The biomechanical testing bones were fixated at 15 in the testing apparatus, as described by Landsman and Vogler (4), and then attached to a Test Resources 800L Dynamic Test System (Test Resources, Shakopee, MN; Fig. 6). Because the proximal portion of the composite metatarsal bone was square, “potting” the specimens was not required. The jig was attached to a floating X-Y base that minimized any side-to-side movement, thereby only measuring the loads in the Z (ventral to dorsal) direction. A sling that was designed to rotate was placed at the head of the composite first metatarsal, allowing repeatable linear loads to be applied to the bone. The sling consisted of a steel platform on which the metatarsal head of the specimen rested, and the platform was attached to the testing apparatus using 4 braided strands of 100-lb test fishing line. The composite bones were then preloaded with a force of 12 N for 5 seconds, followed by a ramping load applied at a rate of 20 mm/s until failure was observed. The failure force was recorded for each specimen. Failure was determined by the maximum load value obtained from the load–displacement curve for each specimen. The statistical significance for each group was determined using the independent samples Student t test.
Results The results for the locking plate construct and cortical screw construct specimens are summarized in Table. The minimum failure load for the locking plate and cortical screw constructs was 94.2 N and 65.9 N, respectively. The maximum failure load for the locking plate and cortical screw constructs was 343.3 N and 142.8 N, respectively. The range within the specific construct data was large. Analyzing the experiment records from the locking plate group revealed that the specimens with lower failure loads had undergone plastic deformation with plate bending. Six specimens in the locking plate construct
group had undergone plate deformation, which might have lowered the maximum load value observed in the load-displacement curves. The experiment records for the cortical screw construct group did not reveal any deformation; however, the large range in the failure load could have resulted from stress risers that might have occurred during preparation of the specimens for screw fixation or possible hinge fracture that were not appreciated before testing. It is not uncommon for stress risers to occur when drilling hard surfaces or for hinge fractures to develop when creating first metatarsal base osteotomies. A statistically significant difference was seen in the mean load to failure for the 2 constructs (locking plate construct, 190.0 70 N; cortical screw construct, 110.3 20.3 N, p < .001; Fig. 7). Discussion For decades, surgeons have effectively used the CBWO of the first metatarsal to correct a high intermetatarsal angle associated with a severe hallux abducto valgus deformity (9). Traditionally, this procedure has been fixated with either 1 or 2 screws, with the 2-screw construct mechanically more stable (4–6,9–12). Dorsal elevation of the first metatarsal head is a common sign of a CBWO complication and has historically been attributed, at least in part, to early postoperative weightbearing (2,3,5,6,11–17). This elevation can lead to increased plantar pressures at the lesser metatarsals, resulting in complications such as transfer metatarsalgia, stress fractures, or plantar lesions (2). However, recent advances in plate fixation of the basilar osteotomy have demonstrated enhanced structural strength, stiffness, and stronger fixation than traditional screw fixation (7,18). Furthermore, plates using locking screws will provide greater stability during cyclic loading than will plates without locking screws (19). The results of our composite first metatarsal study demonstrated a statistically significant difference in the mean load to failure for the 2
Fig. 6. (A) Cortical screw construct in the testing apparatus. (B) Locking plate construct in testing apparatus.
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K. Smith et al. / The Journal of Foot & Ankle Surgery 53 (2014) 529–533
Table Locking plate and cortical screw construct data Sample
Locking Plate Failure Load (N)
Cortical Screw Failure Load (N)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
282.6 215.1 94.2 312.0 177.2 240.3 343.3 271.6 176.9 107.4 161.2 223.6 201.6 140.1 150.2 141.5 135.7 110.1 149.7 166.2
112.9 142.8 142.3 97.8 132.1 93.4 108.8 107.4 113.7 101.9 76.4 65.9 114.3 112.3 119.2 134.9 107.1 101.9 d d
constructs, with the locking plate construct able to withstand a greater mean load to failure. These data are similar to those from previous studies that examined proximal osteotomies and compared different fixation constructs. Campbell et al (18) performed mechanical testing on 10 matched pairs of cadaver feet to evaluate a proximal closing wedge osteotomy fixated with a plantar plate versus a proximal crescentic osteotomy fixated with screw fixation. They found that the wedge osteotomy had a statistically significant mean load to failure of 127.2 N. Although their study did not specifically examine a locking plate construct and did not compare similar osteotomies, it did demonstrate that a plate construct was stronger in proximal osteotomies. Jones et al (7) conducted a study of proximal first metatarsal crescentic osteotomies and found those fixated with a dorsomedial locking plate had a mean load to failure of 95.2 N, a statistically significant and greater load to failure than osteotomies fixated with a screw and Kirschner wire. Although the data presented in the present study favored the locking plate construct for the mean load to failure, it still might not be able to withstand the physiologic loads that accompany weightbearing. Landsman and Vogler (4) examined oblique base wedge stability using different methods of internal fixation and found that none of their methods were able to exceed a load of 17 lb (75.62 N). They continued by estimating that the true physiologic load applied to the first metatarsal head might be around 50 to 100 lb, or a minimum
Fig. 7. Load to failure (mean and standard deviation) of the composite first metatarsals. *Significant difference between groups (p < .001).
of approximately 222 N. They cautioned that care should be taken when interpreting the results of mechanical studies because the addition of a soft tissue envelope might allow for added stability. Even without the presence of a soft tissue envelope, the mean load to failure results from previous studies have suggested that surgeons should be concerned about any construct withstanding physiologic loads and the possibility of clinical fixation failure. The present study had several limitations. The data revealed large ranges in the maximum failure load results. In the locking plate construct, this occurred from plastic deformation of the locking plates. The large range in the cortical screw construct might have resulted from an unseen fracture that could have occurred during screw placement or hinge fracture that occurred during the osteotomy. Another possible explanation for the large range in the maximum failure load could have been related to plastic deformation of the sling material used in testing; however, this was unlikely considering the results from Niehaus et al (20). They tested 3 braided strands of 80-lb test fishing line and found that the mean maximum load to failure was 1090 N. This value far exceeded the maximum failure load for specimens in the present study in which we used 4 braided strands of 100-lb test fishing line. The fixation constructs were stressed to failure by applying a vertical load that was perpendicular to the transverse plane, with the metatarsal composite angulated 15 (4) to simulate weightbearing forces and not the cyclical loading that occurs in gait. The specimens also did not possess the surrounding soft tissue structures normally present in living tissue that might help stabilize the fixation construct. Although we attempted to maintain consistency with all osteotomies and fixation, we acknowledge that human error could have occurred and some variation could exist among the fixation construct specimens. Our study also used composite first metatarsal bones and not cadaveric bone. The composite specimens have the same physical characteristics of real bone (8) and are homogenous in size and material. The use of bone models also allows researchers the ability to compare the relative stability of fixation constructs, and the use of such models has been previously validated (21). In conclusion, the present study has demonstrated that the locking plate construct was able to withstand a significantly greater vertical load before failure than was the 2-cortical screw construct in the composite bone models. Additional studies on cadaveric or human models are needed to determine the true clinical significance. References 1. Juvara E. Nouveau procede pour la cure radicale double upright “hallux valgus.” Presse Med 40:395, 1919. 2. Schuberth JM, Reilly CH, Gudas CJ. The closing wedge osteotomy: a critical analysis of first metatarsal elevation. J Am Podiatry Assoc 74:13–24, 1984. 3. Jeremin PJ, DeVincentis A, Goller W. Closing base wedge osteotomy: an evaluation of twenty-four cases. J Foot Surg 21:316–323, 1982. 4. Landsman AS, Vogler HW. An assessment of oblique base wedge osteotomy stability in the first metatarsal using different modes of internal fixation. J Foot Surg 31:211–218, 1992. 5. Christensen JC, Gusman DN, Tencer AF. Stiffness of screw fixation and role of cortical hinge in the first metatarsal base osteotomy. J Am Podiatr Med Assoc 85:73–78, 1995. 6. Fillinger EB, McGuire JW, Hesse DF, Solomon MG. Inherent stability of proximal first metatarsal osteotomies: a comparative analysis. J Foot Ankle Surg 37:292– 302, 1998. 7. Jones C, Coughlin M, Petersen W, Herbot M, Paletta J. Mechanical comparison of two types of fixation for proximal first metatarsal crescentic osteotomy. Foot Ankle Int 26:371–374, 2005. 8. Heiner AD. Structural properties of fourth-generation composite femurs and tibias. J Biomech 41:3282–3284, 2008. 9. Shereff MJ, Sobel MA, Kummer FJ. The stability of fixation of first metatarsal osteotomies. Foot Ankle 11:208–211, 1991. 10. Chang TJ, Landsman AS, Ruch JA. Relative strengths of internal fixation in osteotomies and arthrodesis of the first metatarsal. In: Reconstructive Surgery of the Foot and Leg: Update 97, pp. 119–127, edited by SJ Miller, KT Mahan, GV Yu, et al., The Podiatry Institute, Tucker, GA, 1997.
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11. Denton J, Kuwada GT. Retrospective study of closing wedge osteotomy complications at the base of the first metatarsal with bone screw fixation. J Foot Surg 22:314–319, 1983. 12. Mothershed RA. Proximal procedures of the first ray. In: McGlamry’s Comprehensive Textbook of Foot Surgery, ed 3, pp. 529–542, edited by AS Banks, MS Downey, DE Martin, SJ Miller, Williams & Wilkins, Philadelphia, 2001. 13. Zlotoff H. Shortening of the first metatarsal following osteotomy and its clinical significance. J Am Podiatr Assoc 67:412–426, 1977. 14. Nigro JS, Greger GM, Catanzariti AR. Closing base wedge osteotomy. J Foot Surg 30:494–505, 1991. 15. Seiberg M, Felson S, Colson JP, Barth AH, Green RM, Green DR. Closing base wedge versus Austin bunionectomies for metatarsus primus adductus. J Am Podiatr Med Assoc 84:548–563, 1994. 16. Banks AS, Cargill RS II, Carter S, Ruch JA. Shortening of the first metatarsal following closing base wedge osteotomy. J Am Podiatr Med Assoc 87:199–208, 1997.
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17. Higgins KR, Shebetka KA, Lavery LA. Review of the effect of fixation on complication rate in the base wedge osteotomy. J Am Podiatr Med Assoc 86:327–330, 1996. 18. Campbell JT, Schon LC, Parks BG, Wang Y, Berger BI. Mechanical comparison of biplanar proximal closing wedge osteotomy with plantar plate fixation versus crescentic osteotomy with screw fixation for the correction of metatarsus primus varus. Foot Ankle Int 19:293–299, 1998. 19. Richter M, Gosling T, Zech S, et al. A comparison of plates with and without locking screws in a calcaneal fracture model. Foot Ankle Int 26:309–319, 2005. 20. Niehaus A, Anderson D, Johnson J, Lannutti J. Comparison of the mechanical characteristics of polymerized caprolactam and monofilament nylon loops constructed in parallel strands or as braided ropes versus cranial cruciate ligaments of cattle. Am J Vet Res 74:381–385, 2013. 21. Landsman AS, Chang TJ. Can synthetic bone models approximate the mechanical properties of cadaveric first metatarsal bone? J Foot Ankle Surg 37:122–127, 1998.
Contents lists available at ScienceDirect
The Journal of Foot & Ankle Surgery journal homepage: www.jfas.org
Mechanical Comparison of Cortical Screw Fixation Versus Locking Plate Fixation in First Metatarsal Base Osteotomy Kevin Smith, DPM, FACFAS 1, Roy H. Lidtke, DPM 2, Noah G. Oliver, DPM, AACFAS 3, Jared M. Maker, DPM 4 1
Associate Professor, Des Moines University College of Podiatric Medicine and Surgery, Des Moines, IA Assistant Professor, Section of Rheumatology, Department of Internal Medicine, Rush University Medical Center, Chicago, IL Third-Year Resident, Inova Fairfax Hospital, Falls Church, VA 4 Third-Year Resident, Western Pennsylvania Hospital, Pittsburgh, PA 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 oblique closing base wedge osteotomy has been used for surgical treatment of moderate to severe hallux valgus deformities with an intermetatarsal angle typically greater than 15 . Several postoperative complications have been identified that relate to failure of the fixation construct used to fixate the osteotomy, especially when that construct has been subjected to a vertical load. We performed a mechanical analysis comparing 2 constructs used to fixate oblique osteotomies of the first metatarsal using composite first metatarsals. An oblique base osteotomy was uniformly performed on 40 composite first metatarsals. Of the 40 specimens, 20 were fixated with a locking plate construct and 18 with a cortical screw construct, consisting of an anchor and compression screw (2 specimens from the latter group were excluded because of hinge fracture). Each specimen was loaded in a materials testing machine to measure the maximum load at construct failure when a vertical force was applied to the plantar aspect of the metatarsal head. The mean load to failure for the locking plate construct was significantly greater than the cortical screw construct (190.0 70 N versus 110.3 20.3 N, p < .001). Our study results have demonstrated that the locking plate construct was able to withstand a significantly greater vertical load before failure than was the 2-cortical screw construct in oblique osteotomies performed at the base of composite first metatarsals. Ó 2014 by the American College of Foot and Ankle Surgeons. All rights reserved.
Keywords: bone model closing base wedge osteotomy hallux valgus mechanical testing surgery
The oblique closing base wedge osteotomy (CBWO) described by Juvara (1) has been advocated for the surgical treatment of moderate to severe hallux valgus deformities with an intermetatarsal angle greater than 15 (2,3). Several studies have examined the complications related to the CBWO. Landsman and Vogler (4) suggested that several complications could result from postoperative elevation or displacement of the distal metatarsal segment relative to the proximal segment. They also reported that elevation or displacement could result from a lack of rigid fixation. Schuberth et al (2) performed a radiographic review of 159 feet and found that 93.7% demonstrated postoperative elevation, with an average of 6.679 . They suggested that new internal fixation techniques might decrease the incidence of postoperative complication seen with the CBWO. Several investigators have tested different fixation constructs to determine their strength and their ability to minimize the Financial Disclosure: None reported. Conflict of Interest: None reported. Address correspondence to: Kevin Smith, DPM, FACFAS, Des Moines University College of Podiatric Medicine and Surgery, 3200 Grand Avenue, Des Moines, IA 50312. E-mail address: [email protected] (K. Smith).
Fig. 1. Hollow composite first metatarsal (Pacific Research Laboratories, Vashon, WA).
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.2014.04.025
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K. Smith et al. / The Journal of Foot & Ankle Surgery 53 (2014) 529–533
Fig. 2. (A) Side view of the osteotomy jig. (B) Dorsal view of the osteotomy jig.
the CBWO. The purpose of the present study was to compare the mechanical stability of these 2 different constructs in the proximal osteotomy of the first metatarsal base. Our hypothesis was that the locking plate construct would require a greater force to failure than would the screw construct when subjected to a vertical load.
complications related to the CBWO. Christensen et al (5) examined single-screw fixation with an intact medial cortical hinge and 2-screw fixation with an intact cortical hinge. All specimens were fixated with 3.5-mm cortical screws. They found no significant difference between the 2 constructs in cantilever bending. Fillinger et al (6) also compared single- and 2-screw constructs using 2.7-mm cortical screws and found that the construct fixated with 2 screws resisted the vertical ground reactive forces better than did the single-screw construct. Landsman and Vogler (4) also studied different constructs and varied the size of fixation within the same study. They tested osteotomies fixated with a single 3.5-mm cancellous screw, two 2.7-mm cortical screws, and 2 Kirschner wires. They reported that the Kirschner wire and single-screw methods were superior to two 2.7-mm cortical screws for stability. They continued by suggesting that all the constructs tested were not impressive in stabilizing the osteotomy under true physiologic loads. A fixation construct that potentially could resist the vertical ground reactive forces related to basilar osteotomies is the locking plate. Jones et al (7) performed a mechanical comparison of 2 different constructs in the proximal crescentic osteotomy and found that osteotomies fixated with a locking plate demonstrated a greater mean load to failure than those fixated with a 3.5-mm cortical screw and a Kirschner wire. A couple of limitations of the study by Jones et al (7) were that they did not investigate the CBWO and did not compare the locking plate construct against a 2-screw construct. To our knowledge, no published reports have compared the strength of the locking plate construct and a two 2.7-mm cortical screw construct for fixation of
A total of 40 composite first metatarsal biomechanical testing bones were obtained and used for our research (Pacific Research Laboratories, Vashon Island, WA). These bones have been designed to have the same physical characteristics of real bone but do not have the anisotropic properties of real bone and therefore will be identical to one another (8). The composite bones will also be identical to one another in dimension and are hollow to simulate the medullary canal (Fig. 1). Before cutting the osteotomies, the composite first metatarsals were attached to a jig that we had constructed. The jig consisted of an acrylic sheet and wood and was attached directly to the composite first metatarsal using an L-shaped plate (Fig. 2). The jig was used to ensure the consistency and uniformity of all cuts. A power sagittal saw with a 10-mm-wide sagittal saw blade was used for all osteotomies. The osteotomy consisted of a solitary bone cut that was angulated 45 to the long axis of the metatarsal composite and perpendicular to the metatarsal (Fig. 3). A medical cortical hinge 1 cm distal to the metatarsal base was also maintained. A wedge osteotomy was not performed because we were testing the strength of the fixation construct, not the properties of a specific osteotomy. Christensen et al (5) described a similar method in their first metatarsal base osteotomy study and reported that specimen uniformity would be greater if a wedge of bone were not removed. Of the 40 bones, 20 were fixated with the locking plate construct. The proximal portion of the locking plate was 0.5 cm from the base of the composite metatarsal, and that distance was measured and marked on each specimen to ensure uniformity (Fig. 4). Of the 20 bones in the screw construct group, 18 were fixated with two 2.7-mm bicortical screws using the AO lag technique. Two of the composite metatarsals in the
Fig. 3. Dorsal view of the composite first metatarsal osteotomy.
Fig. 4. Dorsal view of the locking plate construct.
Materials and Methods
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Fig. 5. (A) Guide hole for compression screw insertion. (B) Guide hole for anchor screw insertion. (C) Side view of the cortical screw construct.
screw construct group had developed a hinge fracture during fixation and were excluded from testing. The composite metatarsals in the bicortical screw construct group were attached to another jig composed of an acrylic sheet and wood with predrilled guide holes for cortical screw fixation to again ensure uniformity. One of the bicortical screws served as the anchor screw and was inserted perpendicular to the long axis of the composite metatarsal and tightened first. The other bicortical screw served as the compression screw and was inserted perpendicular to the osteotomy (Fig. 5). A torque screwdriver was used to uniformly tighten the cortical screws. One investigator (K.S.) performed all the osteotomies and inserted all fixation devices. The biomechanical testing bones were fixated at 15 in the testing apparatus, as described by Landsman and Vogler (4), and then attached to a Test Resources 800L Dynamic Test System (Test Resources, Shakopee, MN; Fig. 6). Because the proximal portion of the composite metatarsal bone was square, “potting” the specimens was not required. The jig was attached to a floating X-Y base that minimized any side-to-side movement, thereby only measuring the loads in the Z (ventral to dorsal) direction. A sling that was designed to rotate was placed at the head of the composite first metatarsal, allowing repeatable linear loads to be applied to the bone. The sling consisted of a steel platform on which the metatarsal head of the specimen rested, and the platform was attached to the testing apparatus using 4 braided strands of 100-lb test fishing line. The composite bones were then preloaded with a force of 12 N for 5 seconds, followed by a ramping load applied at a rate of 20 mm/s until failure was observed. The failure force was recorded for each specimen. Failure was determined by the maximum load value obtained from the load–displacement curve for each specimen. The statistical significance for each group was determined using the independent samples Student t test.
Results The results for the locking plate construct and cortical screw construct specimens are summarized in Table. The minimum failure load for the locking plate and cortical screw constructs was 94.2 N and 65.9 N, respectively. The maximum failure load for the locking plate and cortical screw constructs was 343.3 N and 142.8 N, respectively. The range within the specific construct data was large. Analyzing the experiment records from the locking plate group revealed that the specimens with lower failure loads had undergone plastic deformation with plate bending. Six specimens in the locking plate construct
group had undergone plate deformation, which might have lowered the maximum load value observed in the load-displacement curves. The experiment records for the cortical screw construct group did not reveal any deformation; however, the large range in the failure load could have resulted from stress risers that might have occurred during preparation of the specimens for screw fixation or possible hinge fracture that were not appreciated before testing. It is not uncommon for stress risers to occur when drilling hard surfaces or for hinge fractures to develop when creating first metatarsal base osteotomies. A statistically significant difference was seen in the mean load to failure for the 2 constructs (locking plate construct, 190.0 70 N; cortical screw construct, 110.3 20.3 N, p < .001; Fig. 7). Discussion For decades, surgeons have effectively used the CBWO of the first metatarsal to correct a high intermetatarsal angle associated with a severe hallux abducto valgus deformity (9). Traditionally, this procedure has been fixated with either 1 or 2 screws, with the 2-screw construct mechanically more stable (4–6,9–12). Dorsal elevation of the first metatarsal head is a common sign of a CBWO complication and has historically been attributed, at least in part, to early postoperative weightbearing (2,3,5,6,11–17). This elevation can lead to increased plantar pressures at the lesser metatarsals, resulting in complications such as transfer metatarsalgia, stress fractures, or plantar lesions (2). However, recent advances in plate fixation of the basilar osteotomy have demonstrated enhanced structural strength, stiffness, and stronger fixation than traditional screw fixation (7,18). Furthermore, plates using locking screws will provide greater stability during cyclic loading than will plates without locking screws (19). The results of our composite first metatarsal study demonstrated a statistically significant difference in the mean load to failure for the 2
Fig. 6. (A) Cortical screw construct in the testing apparatus. (B) Locking plate construct in testing apparatus.
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Table Locking plate and cortical screw construct data Sample
Locking Plate Failure Load (N)
Cortical Screw Failure Load (N)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
282.6 215.1 94.2 312.0 177.2 240.3 343.3 271.6 176.9 107.4 161.2 223.6 201.6 140.1 150.2 141.5 135.7 110.1 149.7 166.2
112.9 142.8 142.3 97.8 132.1 93.4 108.8 107.4 113.7 101.9 76.4 65.9 114.3 112.3 119.2 134.9 107.1 101.9 d d
constructs, with the locking plate construct able to withstand a greater mean load to failure. These data are similar to those from previous studies that examined proximal osteotomies and compared different fixation constructs. Campbell et al (18) performed mechanical testing on 10 matched pairs of cadaver feet to evaluate a proximal closing wedge osteotomy fixated with a plantar plate versus a proximal crescentic osteotomy fixated with screw fixation. They found that the wedge osteotomy had a statistically significant mean load to failure of 127.2 N. Although their study did not specifically examine a locking plate construct and did not compare similar osteotomies, it did demonstrate that a plate construct was stronger in proximal osteotomies. Jones et al (7) conducted a study of proximal first metatarsal crescentic osteotomies and found those fixated with a dorsomedial locking plate had a mean load to failure of 95.2 N, a statistically significant and greater load to failure than osteotomies fixated with a screw and Kirschner wire. Although the data presented in the present study favored the locking plate construct for the mean load to failure, it still might not be able to withstand the physiologic loads that accompany weightbearing. Landsman and Vogler (4) examined oblique base wedge stability using different methods of internal fixation and found that none of their methods were able to exceed a load of 17 lb (75.62 N). They continued by estimating that the true physiologic load applied to the first metatarsal head might be around 50 to 100 lb, or a minimum
Fig. 7. Load to failure (mean and standard deviation) of the composite first metatarsals. *Significant difference between groups (p < .001).
of approximately 222 N. They cautioned that care should be taken when interpreting the results of mechanical studies because the addition of a soft tissue envelope might allow for added stability. Even without the presence of a soft tissue envelope, the mean load to failure results from previous studies have suggested that surgeons should be concerned about any construct withstanding physiologic loads and the possibility of clinical fixation failure. The present study had several limitations. The data revealed large ranges in the maximum failure load results. In the locking plate construct, this occurred from plastic deformation of the locking plates. The large range in the cortical screw construct might have resulted from an unseen fracture that could have occurred during screw placement or hinge fracture that occurred during the osteotomy. Another possible explanation for the large range in the maximum failure load could have been related to plastic deformation of the sling material used in testing; however, this was unlikely considering the results from Niehaus et al (20). They tested 3 braided strands of 80-lb test fishing line and found that the mean maximum load to failure was 1090 N. This value far exceeded the maximum failure load for specimens in the present study in which we used 4 braided strands of 100-lb test fishing line. The fixation constructs were stressed to failure by applying a vertical load that was perpendicular to the transverse plane, with the metatarsal composite angulated 15 (4) to simulate weightbearing forces and not the cyclical loading that occurs in gait. The specimens also did not possess the surrounding soft tissue structures normally present in living tissue that might help stabilize the fixation construct. Although we attempted to maintain consistency with all osteotomies and fixation, we acknowledge that human error could have occurred and some variation could exist among the fixation construct specimens. Our study also used composite first metatarsal bones and not cadaveric bone. The composite specimens have the same physical characteristics of real bone (8) and are homogenous in size and material. The use of bone models also allows researchers the ability to compare the relative stability of fixation constructs, and the use of such models has been previously validated (21). In conclusion, the present study has demonstrated that the locking plate construct was able to withstand a significantly greater vertical load before failure than was the 2-cortical screw construct in the composite bone models. Additional studies on cadaveric or human models are needed to determine the true clinical significance. References 1. Juvara E. Nouveau procede pour la cure radicale double upright “hallux valgus.” Presse Med 40:395, 1919. 2. Schuberth JM, Reilly CH, Gudas CJ. The closing wedge osteotomy: a critical analysis of first metatarsal elevation. J Am Podiatry Assoc 74:13–24, 1984. 3. Jeremin PJ, DeVincentis A, Goller W. Closing base wedge osteotomy: an evaluation of twenty-four cases. J Foot Surg 21:316–323, 1982. 4. Landsman AS, Vogler HW. An assessment of oblique base wedge osteotomy stability in the first metatarsal using different modes of internal fixation. J Foot Surg 31:211–218, 1992. 5. Christensen JC, Gusman DN, Tencer AF. Stiffness of screw fixation and role of cortical hinge in the first metatarsal base osteotomy. J Am Podiatr Med Assoc 85:73–78, 1995. 6. Fillinger EB, McGuire JW, Hesse DF, Solomon MG. Inherent stability of proximal first metatarsal osteotomies: a comparative analysis. J Foot Ankle Surg 37:292– 302, 1998. 7. Jones C, Coughlin M, Petersen W, Herbot M, Paletta J. Mechanical comparison of two types of fixation for proximal first metatarsal crescentic osteotomy. Foot Ankle Int 26:371–374, 2005. 8. Heiner AD. Structural properties of fourth-generation composite femurs and tibias. J Biomech 41:3282–3284, 2008. 9. Shereff MJ, Sobel MA, Kummer FJ. The stability of fixation of first metatarsal osteotomies. Foot Ankle 11:208–211, 1991. 10. Chang TJ, Landsman AS, Ruch JA. Relative strengths of internal fixation in osteotomies and arthrodesis of the first metatarsal. In: Reconstructive Surgery of the Foot and Leg: Update 97, pp. 119–127, edited by SJ Miller, KT Mahan, GV Yu, et al., The Podiatry Institute, Tucker, GA, 1997.
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