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Mechanical comparison of fixation techniques for the offset V osteotomy: a saw bone study1
Mechanical Comparison of Fixation Techniques for the Offset V Osteotomy: A Saw Bone Study Keith Jacobson, DPM,1 Adam Gough, DPM,2 Samuel S. Mendicino, DPM,3 and Matthew S. Rockett, DPM4 Four different techniques for the fixation of an offset V bunionectomy were tested on solid-foam saw-bone models for the purpose of determining the strongest form of fixation for the osteotomy. Twenty identical models were placed into 4 different groups. Groups varied as to the placement and caliber of fixation. Models were loaded with a servo-hydraulic testing machine until failure of fixation occurred. Video analysis was used to record the pattern of failure of the fixation. Failure occurred either distal to the first screw, through the first screw hole, between the 2 screws, through the second screw hole, or proximal to the second screw. The mean force to failure of the groups was group 1, 58.1 N; group 2, 59.3 N; group 3, 64.0 N; and group 4, 105.66 N. There was a statistical significant difference between group 4 and the other 3 groups ( F1 ⫽ 55.45, P ⬍ 0.05). There was no statistical difference between groups 1 to 3. In groups 1 to 3, 87% of the failures were through the distal screw hole, whereas the remaining 13% were through the proximal screw hole. In group 4, 60% of the failures were through the proximal screw hole and 40% were through the distal screw hole. It was concluded that, in this model, the strongest form of fixation for an offset V osteotomy was the 2.7-mm cortical screw placed distally with the proximal point of fixation being a threaded 0.062-inch Kirschner wire. (The Journal of Foot & Ankle Surgery 42(6):339 –343, 2003) Key words: offset V osteotomy, internal fixation, saw bone models, mechanical testing, fixation failure
M any osteotomies have been described for the surgical treatment of hallux valgus. With the advent of internal fixation, numerous fixation options have evolved to stabilize these osteotomies. In particular, the offset V osteotomy (1–7) has many different fixation possibilities. The offset V osteotomy was first described as a modification of the distal chevron osteotomy in 1981 (1). In contrast to the distal chevron, the offset V osteotomy can correct not only larger intermetatarsal angles but also abnormal proximal articular set angles. The original description suggested that the osteotomy was inherently stable and usually required no fixation. Vogler (2) was the first to recommend Kirschner wire (K-wire) fixation by using a single 2.0-mm K-wire. Kalish (3) further modified the fixation of this osteotomy by using two 2.7-mm cortical screws. Kissel et al (4) went on to describe the use of one cortical screw and a buried 0.035-inch K-wire. From the Greater Texas Education Foundation, Harris County Podiatric Surgical Residency Program, West Houston Medical Center, Houston, TX. Address correspondence to: Keith Jacobson, DPM, 18220 Tomball Pkwy, Suite 220, Houston, TX 77070. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject article. 1 Research performed while second-year surgical resident. 2 Research performed while first year-surgical resident. 3 Director of Podiatric Surgery and Fellowship Programs. 4 Private practice, Houston, TX. Copyright © 2003 by the American College of Foot and Ankle Surgeons 1067-2516/03/4206-0005$30.00/0 doi:10.1053/j.jfas.2003.09.009
Although the offset V is intrinsically stable because of the interlocking surfaces, 2 points of fixation are currently recommended. The second point of fixation allows the osteotomy site to resist rotation to maintain proximal articular set angle correction (2–7). Two-screw fixation using 2.0-mm or 2.7-mm cortical screws, a combination of the 2, or single screw fixation with a proximal K-wire, have been described for this purpose (2–7). One potential complication of this internal fixation is fracture of the osteotomy through the fixation sites with subsequent loosening of the devices (Fig 1). The cause of the fixation failure can be multifactoral, but is most likely secondary to excessive weightbearing, improper fixation techniques, or a combination of both. This study investigates the ability of the different internal fixation techniques to resist failure under the application of a constant mechanical load. The purpose of the study was to determine whether there exists any difference in the strength of the fixation types. Therefore, the null hypothesis was that the different modes of fixation show equal strength when this force is applied.
Materials and Methods Twenty identical plastic saw-bone models were divided into 4 equal groups to test different fixation configurations: group 1, two 2.7-mm AO cortical screws; group 2, two 2.0-mm AO cortical screws; group 3, one 2.7-mm AO
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FIGURE 1 Lateral radiograph of fractured offset V osteotomy with loss of fixation.
cortical screw (distal) and one 2.0-mm AO cortical screw; and group 4, one 2.7-mm AO cortical screw (distal) and a 0.062-inch threaded K-wire. Each saw bone model was prepared by only 1 of the authors (K.J.), using identical techniques. Each model was prepared by resecting 5 mm of the medial eminence to make the head of the model flush with the metatarsal shaft. All measurements were made by using a digital caliper with an accuracy of ⫾ 0.001 mm. The apex of the osteotomy was marked 1 cm proximal to the articular surface and 8.28 mm from the dorsal surface. This point was determined by finding the center of the circumference of the head, as described by Vogler (5). From this apex point, the plantar cut was drawn to a point just proximal to the sesamoid apparatus. A line was then drawn for the dorsal cut of the osteotomy at a 55° angle to the plantar cut, using the measuring device from an osteoteomy guide system (Reese Osteotomy Guide System; Phoenix Medical, Peoria, AZ). All cuts were measured with the digital caliper to ensure they were of identical lengths of 45 mm for the dorsal cut and 10 mm for the plantar cut. The placement of the distal and the proximal points of fixation were marked. The distal fixation point was marked exactly 15 mm from the apex of the osteotomy site, and the second mark was placed 15-mm proximal to the first mark (Fig 2). The marks were centered on the dorsal aspect of the model by measuring the medialto-lateral width by using the digital caliper. This was performed to prevent medial or lateral stress risers and to keep placement of fixation constant. After marking all fixation points, a 0.045-inch K-wire was driven through the apex of the osteotomy perpendicular to the long axis of the first metatarsal and parallel to the ground. This facilitated placement of the osteotomy guide (Reese Osteotomy Guide System), which was set at 55°. The osteotomy was created by using a sagittal saw, and then temporarily secured 340
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FIGURE 2 Diagram depicting the placement of the offset V osteotomy and the placement fixation points. A, represents the distal point of fixation. B, represents the proximal point of fixation.
with a bone-reduction forceps while placing fixation. Fixation was placed based on each group’s designation. For each model, the distal fixation was performed first, followed by the proximal fixation. The fixation was applied perpendicular to the osteotomy site. Each screw was inserted using standard AO techniques (8). The threaded K-wire was placed in the same orientation as the screws, allowing 2 threads of the wire to exit the plantar cortex. After placement, the wire was cut flush with the dorsal surface. After fixation was inserted, a 0.062-inch K-wire was used to create a hole from medial to lateral, parallel to the ground and perpendicular to the long axis of the model. This hole was placed 1.0-mm superior to the sesamoid apparatus to mimic ground reactive forces and to allow the passage of the servohydraulic testing cable. Each model was then placed into the test jig. This test jig was designed specifically to hold the models consistently perpendicular to the ground. The servohydraulic cable was placed through the hole in the model and secured by using nonslip clamps. Using a level, the cable was leveled and centered, so that the force applied to all
TABLE 1
Force applied to fracture failure
Group 1 2 3 4
Mean Force to Failure (N) 58.1 59.3 64.1 105.7
Range (N) 33.3 53.3 37.7 73.3
to to to to
95.6 64.4 77.8 155.5
Results
FIGURE 3 Photograph of the saw-bone model secured in the test jig with the cable attached.
FIGURE 4 Diagram showing the locations of failure in the sawbone model. A, distal to the first screw; B, through the first screw hole; C, between the 2 screws; D, through the second screw hole; E, proximal to the second screw.
specimens was equal and mimicked ground reactive forces. Digital videography was used to record each trial for later analysis of the point of fixation failure. Once the cable was attached, a constant load was applied by using a servo-hydraulic apparatus (Research Designs, Houston, TX) as described by Harvey and Rockett (9) (Fig 3). Tension was increased by 5 N every 30 seconds until failure occurred. Failure was defined as fracture of the osteotomy site and loss of fixation. The force at which failure occurred was immediately recorded, as was the location where failure occurred. There were 5 areas of potential failure in the osteotomy. These regions are shown in Fig 4. One-way analysis of variance was used to analyze each group’s load at failure. The null hypothesis used for the statistical analysis assumed that there was no difference between the 4 fixation techniques. From the 1-way analysis of variance, an F statistic (F ⫽ 1) with the P value (P ⬍ .05) at a 95% confidence interval was calculated.
The load at which fixation failure occurred for each group is shown in Table 1 and Fig 5. In group 1, the mean load at failure was 58.089 N (range, 33.333 to 95.556 N). In group 2, the mean load was 59.333 N (range, 53.333 to 64.444 N). The mean failure of group 3 was 64.045 N (range, 37.778 to 77.778 N). In group 4, the mean failure was 105.66 N (range 73.333 N to 155.5 N). The location of failure, as defined in Fig. 4, is shown in Table 2 and Fig 6. Group 1 failures were exclusively through the distal screw site. Four of the 5 failures in groups 2 and 3 were through the distal screw site. The other failure site in these groups was through the proximal screw site. Eighty-seven percent of the models in groups 1 to 3 had failure through the distal screw hole. The remaining 13% failed through the proximal screw hole. In group 4, 60% of the failures occurred through the proximal fixation site, whereas 40% were through the distal fixation site. There were no failures in any other location in any group tested. Using 1-way analysis of variance statistical analysis, the F values calculated by comparing groups 1 to 3 were all ⱕ1 (P ⬎ 0.5), indicating there was no statistical difference between groups 1 to 3. However, when group 4 was compared with the other 3 groups, F1 ⫽ 55.45 with P ⫽ .0113 (P ⬍ .5), showing a significant statistical difference. In addition, an examination of the mean forces showed that group 4’s mean force was significantly greater than the other 3 paired-fixation trials. This analysis showed that the paired fixation of a distal 2.7-mm screw and proximal threaded 0.062-inch K-wire provides significantly greater strength, disproving the null hypothesis that all fixation types possessed equal strength. Discussion Altering the overall structure of any material can create stress risers and lead to overall weakness and instability. For the purposes of this study, the altered structure refers to the bone models, with the fixation causing the change in structure. We attempted to determine which fixation could resist the greatest amount of force before fracture of the osteotomy. Data collected in our study indicate that fixation with a 2.7-mm AO cortical screw and a 0.062-inch threaded K-wire was the most stable construct. The combination of
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FIGURE 5 Chart depicting the force to failure of each specimen and group. TABLE 2
Location of fixation failure
Group
A
B
C
D
E
1 2 3 4
0 0 0 0
5 4 4 2
0 0 0 0
0 1 1 3
0 0 0 0
A, distal to the first screw; B, through the first screw hole; C, between the 2 screws; D, through the second screw hole; E, proximal to the second screw.
screw and K-wire fixation allows for compression with the screw and resistance to rotational forces with the K-wire. Kalish (3), in his clinical review of 264 offset V osteotomies, found fracture of the dorsal arm of the osteotomy to be the second most common complication (7 cases). This fracture was typically caused by technical failures in the placement of screw fixation. Fracture of the dorsal arm of the osteotomy typically was found to be between the screw hole and the osteotomy or between the 2 screw holes. Downey (7) advocated the use of a 2.7-mm and 2.0-mm cortical screw combination to reduce the likelihood of fracture secondary to stress risers from the placement of fixation. Kissel et al (4) advocated the use of a screw and K-wire combination for the same reasons. Our fixation was a slight modification of that described by Kissel et al (4), who used a threaded 0.035-inch K-wire instead of a 0.062inch K-wire. Because no other experimental studies have been performed on this osteotomy, no direct comparison from the existing literature can be made to our study’s data. Only 1 other experimental study on metaphyseal osteotomies has been performed. Miller et al (10) used saw bones to compare an inverted Z osteotomy with the traditional Z osteotomy, and found a force to failure range of 39.5 to 170.6 N. 342
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The only comparison that can be made to our study is that the force required to create failure approximates our data (33.3 to 155.5 N). When looking at the location of failure in the models, 87% of failures in groups 1 to 3 occurred at the distal fixation site. This differed from group 4, in which the greatest percentage of failures (60%) occurred at the proximal fixation site. Although, intuitively, all groups should have had similar fixation failure sites, group 4 differed from the other 3 groups. One possible explanation for this difference lies in the variation of proximal fixation types. Groups 1 to 3 used proximal screw fixation whereas group 4 used K-wire fixation. The drilling and countersinking necessary for proximal screw insertion may have produced unknown stresses on the distal fixation site. Subsequently, groups 1 to 3 failed predominantly at the distal fixation site when force was applied to the models. One limitation of this study was the use of plastic sawbone models rather than cadaveric specimens. According to Landsman and Chang (11), cadaveric bone is much stronger than plastic solid-foam bone models, especially in cantilevered testing such as that performed in our study. However, they found that the results of mechanical testing were similar between the 2 materials. In our study, by eliminating all factors except the fixation devices, we were able to determine the relative strengths of the 4 fixation combinations. Although the testing jig held the specimens perpendicular to the ground such that the force applied to the metatarsal head was perpendicular to the long axis of the metatarsal, this does not simulate actual ground reactive forces on the osteotomy. Therefore, application of tension on the osteotomy site does not truly recreate an in vivo situation. In addition, the capital fragment was not displaced laterally, as it would be during surgery. Lateral displacement changes the orientation of fixa-
FIGURE 6
Photograph showing the most common point of failure of each group. (A) group 1, (B) group 2, (C) group 3, and (D) group 4.
tion, and further clinical and experimental studies with other testing material may be needed to validate our data. Conclusion The combination of a 2.7-mm AO cortical bone screw and a 0.062-inch threaded K-wire resists failure in the offset V osteotomy better than the other 3 fixation types tested in this model. References 1. Lewis RJ, Teffer HL. Modified chevron osteotomy of the first metatarsal. Clin Orthop 157:105–109, 1981. 2. Vogler HW. Offset “V” osteotomy in hallux valgus repairsix schools of surgical thought [syllabus]. Cleveland, Ohio College of Podiatric Medicine, 1985. 3. Kalish SR. Modification of Austin hallux valgus repair. In Reconstructive Surgery of the Foot and Leg, pp 14 –19, edited by ED McGlamry, Tucker GA, Podiatry Institute Publishing, 1989.
4. Kissel CG, Unroe BJ, Parker RM. The offset “V” bunionectomy using cortical screw and buried Kirschner wire fixation. J Foot Surg 31:560 – 577, 1992. 5. Vogler HW. Shaft osteotomies in hallux valgus reduction. Clin Podiatr Med Surg 6:47– 69, 1989. 6. Fox IM, Cuttic M, DeMarco P. The offset V modification of the chevron bunionectomy: a retrospective study. J Foot Surg 31:615– 620, 1992. 7. Downey MS. Complications of the Kalish bunionectomy. In Reconstructive Surgery of the Foot and Leg—Update 1992, pp 248–254, edited by ED McGlamry, Tucker GA, Podiatry Institute Publishing, 1992. 8. Helm U, Pfeiffer KM. Small Fragment Set ManualTechnique Recommended by the ASIF Group, 2nd ed. New York, Springer-Verlag, 1982. 9. Harvey L, Rockett MS. Mechanical comparison of two extensor tendon repairs of ankle tendons. J Foot Ankle Surg 39:232–238, 2000. 10. Miller JM, Stuck R, Satori M, Patwardham A, Cane R, Vrbos L. The inverted Z bunionectomy: quantitative analysis of the scarf and inverted scarf bunionectomy osteotomies in fresh cadaveric matched pair specimens. J Foot Ankle Surg 33:455– 462, 1994. 11. 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.
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M any osteotomies have been described for the surgical treatment of hallux valgus. With the advent of internal fixation, numerous fixation options have evolved to stabilize these osteotomies. In particular, the offset V osteotomy (1–7) has many different fixation possibilities. The offset V osteotomy was first described as a modification of the distal chevron osteotomy in 1981 (1). In contrast to the distal chevron, the offset V osteotomy can correct not only larger intermetatarsal angles but also abnormal proximal articular set angles. The original description suggested that the osteotomy was inherently stable and usually required no fixation. Vogler (2) was the first to recommend Kirschner wire (K-wire) fixation by using a single 2.0-mm K-wire. Kalish (3) further modified the fixation of this osteotomy by using two 2.7-mm cortical screws. Kissel et al (4) went on to describe the use of one cortical screw and a buried 0.035-inch K-wire. From the Greater Texas Education Foundation, Harris County Podiatric Surgical Residency Program, West Houston Medical Center, Houston, TX. Address correspondence to: Keith Jacobson, DPM, 18220 Tomball Pkwy, Suite 220, Houston, TX 77070. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject article. 1 Research performed while second-year surgical resident. 2 Research performed while first year-surgical resident. 3 Director of Podiatric Surgery and Fellowship Programs. 4 Private practice, Houston, TX. Copyright © 2003 by the American College of Foot and Ankle Surgeons 1067-2516/03/4206-0005$30.00/0 doi:10.1053/j.jfas.2003.09.009
Although the offset V is intrinsically stable because of the interlocking surfaces, 2 points of fixation are currently recommended. The second point of fixation allows the osteotomy site to resist rotation to maintain proximal articular set angle correction (2–7). Two-screw fixation using 2.0-mm or 2.7-mm cortical screws, a combination of the 2, or single screw fixation with a proximal K-wire, have been described for this purpose (2–7). One potential complication of this internal fixation is fracture of the osteotomy through the fixation sites with subsequent loosening of the devices (Fig 1). The cause of the fixation failure can be multifactoral, but is most likely secondary to excessive weightbearing, improper fixation techniques, or a combination of both. This study investigates the ability of the different internal fixation techniques to resist failure under the application of a constant mechanical load. The purpose of the study was to determine whether there exists any difference in the strength of the fixation types. Therefore, the null hypothesis was that the different modes of fixation show equal strength when this force is applied.
Materials and Methods Twenty identical plastic saw-bone models were divided into 4 equal groups to test different fixation configurations: group 1, two 2.7-mm AO cortical screws; group 2, two 2.0-mm AO cortical screws; group 3, one 2.7-mm AO
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FIGURE 1 Lateral radiograph of fractured offset V osteotomy with loss of fixation.
cortical screw (distal) and one 2.0-mm AO cortical screw; and group 4, one 2.7-mm AO cortical screw (distal) and a 0.062-inch threaded K-wire. Each saw bone model was prepared by only 1 of the authors (K.J.), using identical techniques. Each model was prepared by resecting 5 mm of the medial eminence to make the head of the model flush with the metatarsal shaft. All measurements were made by using a digital caliper with an accuracy of ⫾ 0.001 mm. The apex of the osteotomy was marked 1 cm proximal to the articular surface and 8.28 mm from the dorsal surface. This point was determined by finding the center of the circumference of the head, as described by Vogler (5). From this apex point, the plantar cut was drawn to a point just proximal to the sesamoid apparatus. A line was then drawn for the dorsal cut of the osteotomy at a 55° angle to the plantar cut, using the measuring device from an osteoteomy guide system (Reese Osteotomy Guide System; Phoenix Medical, Peoria, AZ). All cuts were measured with the digital caliper to ensure they were of identical lengths of 45 mm for the dorsal cut and 10 mm for the plantar cut. The placement of the distal and the proximal points of fixation were marked. The distal fixation point was marked exactly 15 mm from the apex of the osteotomy site, and the second mark was placed 15-mm proximal to the first mark (Fig 2). The marks were centered on the dorsal aspect of the model by measuring the medialto-lateral width by using the digital caliper. This was performed to prevent medial or lateral stress risers and to keep placement of fixation constant. After marking all fixation points, a 0.045-inch K-wire was driven through the apex of the osteotomy perpendicular to the long axis of the first metatarsal and parallel to the ground. This facilitated placement of the osteotomy guide (Reese Osteotomy Guide System), which was set at 55°. The osteotomy was created by using a sagittal saw, and then temporarily secured 340
THE JOURNAL OF FOOT & ANKLE SURGERY
FIGURE 2 Diagram depicting the placement of the offset V osteotomy and the placement fixation points. A, represents the distal point of fixation. B, represents the proximal point of fixation.
with a bone-reduction forceps while placing fixation. Fixation was placed based on each group’s designation. For each model, the distal fixation was performed first, followed by the proximal fixation. The fixation was applied perpendicular to the osteotomy site. Each screw was inserted using standard AO techniques (8). The threaded K-wire was placed in the same orientation as the screws, allowing 2 threads of the wire to exit the plantar cortex. After placement, the wire was cut flush with the dorsal surface. After fixation was inserted, a 0.062-inch K-wire was used to create a hole from medial to lateral, parallel to the ground and perpendicular to the long axis of the model. This hole was placed 1.0-mm superior to the sesamoid apparatus to mimic ground reactive forces and to allow the passage of the servohydraulic testing cable. Each model was then placed into the test jig. This test jig was designed specifically to hold the models consistently perpendicular to the ground. The servohydraulic cable was placed through the hole in the model and secured by using nonslip clamps. Using a level, the cable was leveled and centered, so that the force applied to all
TABLE 1
Force applied to fracture failure
Group 1 2 3 4
Mean Force to Failure (N) 58.1 59.3 64.1 105.7
Range (N) 33.3 53.3 37.7 73.3
to to to to
95.6 64.4 77.8 155.5
Results
FIGURE 3 Photograph of the saw-bone model secured in the test jig with the cable attached.
FIGURE 4 Diagram showing the locations of failure in the sawbone model. A, distal to the first screw; B, through the first screw hole; C, between the 2 screws; D, through the second screw hole; E, proximal to the second screw.
specimens was equal and mimicked ground reactive forces. Digital videography was used to record each trial for later analysis of the point of fixation failure. Once the cable was attached, a constant load was applied by using a servo-hydraulic apparatus (Research Designs, Houston, TX) as described by Harvey and Rockett (9) (Fig 3). Tension was increased by 5 N every 30 seconds until failure occurred. Failure was defined as fracture of the osteotomy site and loss of fixation. The force at which failure occurred was immediately recorded, as was the location where failure occurred. There were 5 areas of potential failure in the osteotomy. These regions are shown in Fig 4. One-way analysis of variance was used to analyze each group’s load at failure. The null hypothesis used for the statistical analysis assumed that there was no difference between the 4 fixation techniques. From the 1-way analysis of variance, an F statistic (F ⫽ 1) with the P value (P ⬍ .05) at a 95% confidence interval was calculated.
The load at which fixation failure occurred for each group is shown in Table 1 and Fig 5. In group 1, the mean load at failure was 58.089 N (range, 33.333 to 95.556 N). In group 2, the mean load was 59.333 N (range, 53.333 to 64.444 N). The mean failure of group 3 was 64.045 N (range, 37.778 to 77.778 N). In group 4, the mean failure was 105.66 N (range 73.333 N to 155.5 N). The location of failure, as defined in Fig. 4, is shown in Table 2 and Fig 6. Group 1 failures were exclusively through the distal screw site. Four of the 5 failures in groups 2 and 3 were through the distal screw site. The other failure site in these groups was through the proximal screw site. Eighty-seven percent of the models in groups 1 to 3 had failure through the distal screw hole. The remaining 13% failed through the proximal screw hole. In group 4, 60% of the failures occurred through the proximal fixation site, whereas 40% were through the distal fixation site. There were no failures in any other location in any group tested. Using 1-way analysis of variance statistical analysis, the F values calculated by comparing groups 1 to 3 were all ⱕ1 (P ⬎ 0.5), indicating there was no statistical difference between groups 1 to 3. However, when group 4 was compared with the other 3 groups, F1 ⫽ 55.45 with P ⫽ .0113 (P ⬍ .5), showing a significant statistical difference. In addition, an examination of the mean forces showed that group 4’s mean force was significantly greater than the other 3 paired-fixation trials. This analysis showed that the paired fixation of a distal 2.7-mm screw and proximal threaded 0.062-inch K-wire provides significantly greater strength, disproving the null hypothesis that all fixation types possessed equal strength. Discussion Altering the overall structure of any material can create stress risers and lead to overall weakness and instability. For the purposes of this study, the altered structure refers to the bone models, with the fixation causing the change in structure. We attempted to determine which fixation could resist the greatest amount of force before fracture of the osteotomy. Data collected in our study indicate that fixation with a 2.7-mm AO cortical screw and a 0.062-inch threaded K-wire was the most stable construct. The combination of
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FIGURE 5 Chart depicting the force to failure of each specimen and group. TABLE 2
Location of fixation failure
Group
A
B
C
D
E
1 2 3 4
0 0 0 0
5 4 4 2
0 0 0 0
0 1 1 3
0 0 0 0
A, distal to the first screw; B, through the first screw hole; C, between the 2 screws; D, through the second screw hole; E, proximal to the second screw.
screw and K-wire fixation allows for compression with the screw and resistance to rotational forces with the K-wire. Kalish (3), in his clinical review of 264 offset V osteotomies, found fracture of the dorsal arm of the osteotomy to be the second most common complication (7 cases). This fracture was typically caused by technical failures in the placement of screw fixation. Fracture of the dorsal arm of the osteotomy typically was found to be between the screw hole and the osteotomy or between the 2 screw holes. Downey (7) advocated the use of a 2.7-mm and 2.0-mm cortical screw combination to reduce the likelihood of fracture secondary to stress risers from the placement of fixation. Kissel et al (4) advocated the use of a screw and K-wire combination for the same reasons. Our fixation was a slight modification of that described by Kissel et al (4), who used a threaded 0.035-inch K-wire instead of a 0.062inch K-wire. Because no other experimental studies have been performed on this osteotomy, no direct comparison from the existing literature can be made to our study’s data. Only 1 other experimental study on metaphyseal osteotomies has been performed. Miller et al (10) used saw bones to compare an inverted Z osteotomy with the traditional Z osteotomy, and found a force to failure range of 39.5 to 170.6 N. 342
THE JOURNAL OF FOOT & ANKLE SURGERY
The only comparison that can be made to our study is that the force required to create failure approximates our data (33.3 to 155.5 N). When looking at the location of failure in the models, 87% of failures in groups 1 to 3 occurred at the distal fixation site. This differed from group 4, in which the greatest percentage of failures (60%) occurred at the proximal fixation site. Although, intuitively, all groups should have had similar fixation failure sites, group 4 differed from the other 3 groups. One possible explanation for this difference lies in the variation of proximal fixation types. Groups 1 to 3 used proximal screw fixation whereas group 4 used K-wire fixation. The drilling and countersinking necessary for proximal screw insertion may have produced unknown stresses on the distal fixation site. Subsequently, groups 1 to 3 failed predominantly at the distal fixation site when force was applied to the models. One limitation of this study was the use of plastic sawbone models rather than cadaveric specimens. According to Landsman and Chang (11), cadaveric bone is much stronger than plastic solid-foam bone models, especially in cantilevered testing such as that performed in our study. However, they found that the results of mechanical testing were similar between the 2 materials. In our study, by eliminating all factors except the fixation devices, we were able to determine the relative strengths of the 4 fixation combinations. Although the testing jig held the specimens perpendicular to the ground such that the force applied to the metatarsal head was perpendicular to the long axis of the metatarsal, this does not simulate actual ground reactive forces on the osteotomy. Therefore, application of tension on the osteotomy site does not truly recreate an in vivo situation. In addition, the capital fragment was not displaced laterally, as it would be during surgery. Lateral displacement changes the orientation of fixa-
FIGURE 6
Photograph showing the most common point of failure of each group. (A) group 1, (B) group 2, (C) group 3, and (D) group 4.
tion, and further clinical and experimental studies with other testing material may be needed to validate our data. Conclusion The combination of a 2.7-mm AO cortical bone screw and a 0.062-inch threaded K-wire resists failure in the offset V osteotomy better than the other 3 fixation types tested in this model. References 1. Lewis RJ, Teffer HL. Modified chevron osteotomy of the first metatarsal. Clin Orthop 157:105–109, 1981. 2. Vogler HW. Offset “V” osteotomy in hallux valgus repairsix schools of surgical thought [syllabus]. Cleveland, Ohio College of Podiatric Medicine, 1985. 3. Kalish SR. Modification of Austin hallux valgus repair. In Reconstructive Surgery of the Foot and Leg, pp 14 –19, edited by ED McGlamry, Tucker GA, Podiatry Institute Publishing, 1989.
4. Kissel CG, Unroe BJ, Parker RM. The offset “V” bunionectomy using cortical screw and buried Kirschner wire fixation. J Foot Surg 31:560 – 577, 1992. 5. Vogler HW. Shaft osteotomies in hallux valgus reduction. Clin Podiatr Med Surg 6:47– 69, 1989. 6. Fox IM, Cuttic M, DeMarco P. The offset V modification of the chevron bunionectomy: a retrospective study. J Foot Surg 31:615– 620, 1992. 7. Downey MS. Complications of the Kalish bunionectomy. In Reconstructive Surgery of the Foot and Leg—Update 1992, pp 248–254, edited by ED McGlamry, Tucker GA, Podiatry Institute Publishing, 1992. 8. Helm U, Pfeiffer KM. Small Fragment Set ManualTechnique Recommended by the ASIF Group, 2nd ed. New York, Springer-Verlag, 1982. 9. Harvey L, Rockett MS. Mechanical comparison of two extensor tendon repairs of ankle tendons. J Foot Ankle Surg 39:232–238, 2000. 10. Miller JM, Stuck R, Satori M, Patwardham A, Cane R, Vrbos L. The inverted Z bunionectomy: quantitative analysis of the scarf and inverted scarf bunionectomy osteotomies in fresh cadaveric matched pair specimens. J Foot Ankle Surg 33:455– 462, 1994. 11. 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.
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