“Triplanar comparison of intermetatarsal angle correction and stability as a function of various wedge resections for an oblique closing base wedge osteotomy of the first metatarsal” Jamie Kinchsular, BS., Patrick McDonald, BS., Nina Babu, BS., Carl Kihm, BS. ABSTRACT This study investigates the effect on the stability of an oblique closing base wedge osteotomy (CBWO) performed with a 3mm wedge compared to a 6mm wedge corresponding to 5 and 10 degree wedges in our samples, respectively. Fifty first metatarsal sawbone samples split into three groups were analyzed: no wedge (control), 3mm, 6mm wedge. Osteotomies were performed with medial hinge intact and fixated with one 3.0mm cannulated screw using a custom-made jig for reproducibility of the cuts and screw placement. Vicon, a motion capture system allowed us to measure changes in the transverse, frontal, and sagittal planes before and after osteotomies with fixation were performed. Load (N), displacement (mm) and stiffness (N/mm) of the fixated osteotomies were tested by plantar-to-dorsal cantilever bending using an Instron 4201 materials testing machine. Our study showed no significant difference in stability of the osteotomy regardless of the resected wedge size. We observed twice the amount of transverse plane correction in the 6mm wedge (10 o ) samples, but similar correction in the frontal and sagittal planes. There was a trend of increased plantarflexion and negligent inversion of the distal fragments in our osteotomies in both test groups. PURPOSE To evaluate stability of CBWO with varying wedge sizes using a materials testing machine and assessing angular changes in the transverse, frontal and sagittal planes with a 3D motion analysis system. NULL HYPOTHESES There is no difference between stability of a fixated CBWO when a large wedge is resected compared to a small wedge. There is no change in the sagittal plane when a CBWO is performed with an intact hinge perpendicular to the weightbearing surface. There is no change in the frontal plane when a CBWO is performed with an intact hinge perpendicular to the weightbearing surface. LITERATURE REVIEW The CBWO is a viable option for reduction of high intermetatarsal (IM) angles for hallux abducto valgus (HAV) deformities. Prior studies focus on numerous fixation techniques and its effects on stability. 1,7 The most important parameter in the correction of HAV deformities is reduction of the IM angle measured in the transverse plane. Studies have not fully addressed the effects of wedge resection on stability. The hinge-axis concept describes leaving a medial cortical hinge intact with the orientation of the osteotomy perpendicular to the weightbearing (WB) surface allowing for pure transverse plane motion. Adhering to this concept, there should be minimal to no sagittal or frontal plane changes since the axis is perpendicular to both planes. To our knowledge, no studies have looked at 3D changes using motion analysis. Past techniques for proximal osteotomies have used radiographs, but are limited to measurements in the transverse and sagittal planes. 5,8 Furthermore, due to positioning errors between pre- and post- operative films, there is increased variability when making the measurements. A 3D motion capture system provides frontal plane measurements and reduces positioning error because markers used to detect changes can be placed in a reproducible manner. When performing and evaluating a CBWO the following parameters should be considered: hinge axis concept, cantilever bending, and fixation technique. Hinge: It functions as an axis of rotation and a second point of fixation for base wedge osteotomies. An axis perpendicular to the ground allows for rotation of the distal segment of the metatarsal without elevation of the metatarsal head. 2,9,10 Cantilever bending: Enormous amounts of WB cantilever forces dorsiflex the 1st metatarsal head in midstance. During midstance, the first metatarsal forms an angle of 15 degrees with the ground. 2-4,7 Therefore, in order to simulate the standard casted post-operative patient, specimens are loaded at a 15 degree declination in cantilever fashion. 7 Fixation Technique: When utilizing an intact medial cortical hinge, it is accepted practice to use one screw placed perpendicular to the osteotomy. 2 Since the introduction of the traditional transverse base osteotomy (Loison, 1901) and even Juvara’s oblique modification (1919), there have been no objective measures taken to correlate wedge resection size and stability. Our study was strategically designed to evaluate stability of a CBWO based on the degree of wedge resection and observe the angular changes in all anatomical planes. METHODS Preparation of Specimens: Polyurethane foam first and second rays with pathology models from Pacific Research Laboratories from the same lot were used in this saw bone study. The second metatarsal and any unnecessary segments were removed creating similar first ray bones. Orientation of the osteotomy and screw placement was assured by fitting the base of the 1st metatarsal in a custom polymethylmethacrylate (PMMA) mold secured in a metal housing. Each metatarsal was then secured in a jig declinated 15 degrees simulating first metatarsal declination angle in midstance. The osteotomy was directed dorsal to plantar and perpendicular to the ground (Figure 3). 3,4,6 All bones were cut at 50 degrees perpendicular to the long axis leaving a medial hinge 1.5cm from the metatarsal-cuneiform joint. Distal cuts were made to form 5 and 10 degree wedges corresponding to 3mm and 6mm wedges, respectively. Osteotomies were closed and secured with a single cannulated 3.0mm screw (Vilex®). Specimens were discarded if the hinge was not intact (Figure 4). Vicon™ 460 3D motion analysis: The Vicon™ 460 motion capture system collects 3D positions of passive retro-reflective markers with five, 1.3 megapixel cameras sampling at 120Hz. Three markers were placed on the proximal and distal ends of the bone to allow an independent measurement of position and orientation. Fifteen random prepared samples were used as controls to assess 3D variation of the sawbones. Samples with wedge resections were compared to uncut controls to assess changes in the transverse, sagittal, and frontal planes (Figure 1). Instron 4201 Material Testing Machine The prepared specimens were secured in the same jig used to make the osteotomy, in addition to a PMMA mold for the head of the metatarsal. The entire jig was placed in the Instron 4201 material testing machine (Figure 5). The metatarsal head was vertically loaded to failure in cantilever fashion at a rate of 8.33mm/sec. The maximum load at failure and displacement at failure were recorded and used to quantify functional stiffness for each specimen (Figures 6,7). The measurements were analyzed with one-way analysis of variance (ANOVA) and means comparison when significant differences were found between groups and the mean. DISCUSSION The CBWO is an excellent option for the reduction of severe HAV deformities with high intermetatarsal angles (>15 degrees). Proximal based osteotomies allow for greater correction with less resection resulting in more pronounced changes in the transverse, frontal, and sagittal planes. Theoretically, following the hinge axis concept, there should be no sagittal or frontal plane changes. Sagittal plane changes showed approximately 4 degrees of plantarflexion (-3.95 ± 1.91 o, ± 1.83 o ) in both test groups (Table 1, Figure 2). There should be no sagittal plane changes using proper hinge axis technique. Axis guides may help align the cuts while resecting a wedge and also protect the hinge. An axis guide was not used; cuts may not have been perpendicular to the WB surface, but the metal housing prevented the cut from going past perpendicular to the weight bearing surface, thus preventing any inadvertent plantarflexion. An explanation for our observed plantarflexion may be due to the nature of the screw placement distal-dorsal-medial to proximal-plantar-lateral across the osteotomy. Despite the fact the screw is placed perpendicular to the osteotomy in the transverse plane, it is not placed perpendicular to the other two planes. Therefore, planterflexion may occur when tightening this screw to the far cortex. Dorsiflexion is a complication in the CBWO due to either improper orientation of the cuts and/or early WB. In the first ray, shortening can contribute to iatrogenically induced lesser metatarsalgia. Though less likely to compensate for patients who bear weight early, increased plantarflexion may compensate for shortening of the metatarsal, hence reducing the incidence of lesser metatarsalgia. There was a negligible change in the frontal plane in our test groups (3.10 ± 2.26 o, 3.21 ± 2.79 o ) compared to uncut, control samples (2.73 ± 2.13 o ) (Table 1). These findings were expected in our study. Any minimal changes in this plane may be due to technique errors, or are insignificant due to the standard error found in our initial set-up of the motion capture system with increased variability in the saw bones. To our knowledge there is no study relating fixation stability as a function of increasing transverse plane correction by increasing wedge resection. To gain adequate correction for higher IM angles, a larger wedge resection is necessary. Some surgeons may be hesitant to resect more bone to attain reduction to normal values (0-8 o ) assuming there is less stability when a larger wedge is resected. Our data shows there was no compromise in stability and we achieved a two-fold (2.1-fold) correction in IM angle comparing the 5 degree wedge to the 10 degree wedge as expected (Table 2). Despite this, stability was not compromised between the two groups during Instron testing, the 10 degree wedge was more difficult to close and fixate. The larger the wedge resected, the greater the chance the hinge will break due to increased tension placed on it. Ultimately we had 6 failures and one due to improper handling post-fixation, in the test group with a 10 degree wedge resection compared to one failure in the 5 degree wedge sample. Because of this, surgeons may be less likely to resect a large wedge, regardless of its stability. Overall, a broken hinge will lead to increased instability of the osteotomy even when fixated with an additional screw. The length and width of human metatarsals have more variability than the sample of saw bones used, but variations in length and width will have no effect on the amount of IM correction as long as the same degree wedge is resected. Cortical thickness and variations in the shape of the hinge are likely affected by biological variation creating varied tension along the hinge. Wedge resection is a subjective measure intra-operatively. No studies have shown if varying wedge resection, using accepted surgical technique, results in actual frontal and sagittal plane changes. While our study is limited in scope, it is the first study of its kind using a 3D motion capture system and a starting point for further exploration of concomitant triplanar changes in not only first metatarsal surgery but all osseous surgeries. Limitations of study Small number of samples with specimen variability reduces the power of our test. Increased degree of standard error in Vicon™ triangulation due to the close proximity of the retro-reflective markers. Saw bone models do not have the internal architecture or cortical strength found in cadaveric bone. Human error in surgical technique. CONCLUSIONS 10 degree wedge resists failure in a CBWO similar to a 5 degree wedge. 10 degree wedge exhibits sagittal and frontal plane correction similar to a 5 degree wedge. 10 degree wedge exhibits transverse plane correction twice that of a 5 degree wedge. There was no significant difference in stiffness between a 10 degree wedge resection and a 5 degree wedge resection. Regardless of the degree of wedge resection, and assuring proper fixation and maintenance of medial cortical hinge, stability in a CBWO is not affected. References 1. Bozkurt M. Tigaran C. Dalstra M. Jensen NC. 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JAPMA Jan 74(1): ACKNOWLEDGEMENTS Dr. William Martin, DPM; Dr. Jinsup Song, DPM, PhD; Ben Heilman, MS; Steve Corsello, Vilex 20 07