An Analysis of Femoral Geometry Related to Deflection of the SIGN Nail When Inserted into a Femoral Model

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1 An Analysis of Femoral Geometry Related to Deflection of the SIGN Nail When Inserted into a Femoral Model Abstract Joseph Pia Background: Femur fractures treated with the SIGN nail can require additional operative time if the nail is not closely aligned with the drilling guide after insertion. Methods and instruments for compensating for targeting misalignment are part of the SIGN technique and the aim of this study was to look at possible trends in deflection which could be used to improve targeting. Methods: Six femoral canal geometries created with C-T scans and modeled in Mimics were supplied by SIGN. This study analyzed the canal shapes recording key landmarks and geometry changes and tested for actual nail deflections in a custom fixture designed to simulate nail insertion with various fractures. Results: Landmarks that contribute to nail deflection were identified both on physical measurements of the Mimics model and verified with testing in the custom fixture. Measurements included overall canal bend, flat section location, middle section trajectory, and canal containment. Conclusions: Several variables were identified which impact deflection and may help predict targeting location and cases that could require additional time to achieve interlocking. Characteristics including fracture location, entry site, canal shape, and nail size play a role in nail deflection. Variations in fracture location and rod size played the largest roles in nail defection. It was observed that the difficulty of nail insertion can be an indicator of nail deflection, but other variables such as nail size and canal shape make the amount of deflection difficult to predict consistently. Background SIGN Fracture Care International produces numerous trauma care instruments and devices that are designed for use in the developing world. Many hospitals that use this nail do not have access to a C-arm; consequently, an external drilling guide is used to target the distal screw holes. Often the curvature of the femur causes the straight SIGN nail to bend, resulting in an offset of the targeting guide. The bending provides some 3-point fixation of the nail, which may aid in fracture stabilization; however, too much bending can lead to poor targeting guide alignment and additional surgery time. The goal of this study was to investigate the deflection of the SIGN nail and femur morphology with the intent of developing an algorithm for more efficient targeting of the distal screw holes. Methods This study was composed of two parts: analysis of the femoral canal shape and testing of nail deflection in a canal fixture. Six femur bones, four left femurs and two right femurs, were studied to determine trends in shape and features. The medial-lateral cross sections were made using Mimics software and provided for this study by SIGN. The geometries were enlarged to a common size and used as templates for the canal fixture. The posterior edges of the outer cortex, both proximally and distally were aligned in a lateral view and then measurements were taken based on that

2 L1 L1 R1 R1 L2 L2 R2 R2 L3 L3 L4 L4 (mm) L1 L1 R1 R1 L2 L2 R2 R2 L3 L3 L4 L4 (mm) orientation. The results from these six bones were compared against the published data to confirm accuracy and legitimacy of the measurements. The isthmus location and canal flare were within 1% of the published values. These dimensions can be found in Appendix A with a diagram showing the measurement locations. Fixtures with proximal, mid-shaft, and distal fractures were modified for different femoral canal shapes and sizes to test the nails. All nail shapes and fracture configurations were tested with both antegrade and retrograde insertion points. The position of the insertion points and use of the instruments was done as described in the SIGN surgical technique. The nails were inserted into the canal model and the amount and direction of the deflection of the nail was recorded. All nails were 4mm long, except the 8mm diameter nail which was 38mm. Figure 1 is a picture of the fixture when it was displayed at the SIGN conference. the proximal and distal fractures. The deflections of the nail were all posterior to the guide, because the curvature of the femur always forced it to bend along the anterior cortex Fig. 2 Deflection of 8mm Nail The labels of Figures 1 and 2, eg. L1 and R1, refer to the bone template, and proximal and distal refer to the location of the fracture. The entry methods were antegrade for the proximal fractures and retrograde for the distal fractures Fig. 1 Femur Canal Fixture Results Nail-Guide Deflection The main portion of the study focused on comparing the results of the deflection of the nails to the morphology data recorded from the six bones. The following two tables, Figures 2 and 3, show the deflection for these models. The complete deflection tables are listed in Appendix A. Figure 2 shows the deflections of an 8mm nail for each of the femur bone templates, both Fig. 3 Total Deflection for Each Bone Template Figure 3 is the sum of the deflection of all sizes of the nail, 8-12mm. L2, R2, and L3 have the least deflection and these will serve as a good contrast against the other templates for the shape analysis. The deflection of the mid-shaft fractures is not included in either of the charts above as there was no deflection recorded during any of the tests. This result highlights the significance of fracture location for the deflection of the nail.

3 (cm) ( ) Fig. 4 Method for Midpoint Displacement Canal Shape Factors: Midpoint Displacement Among the features studied, the following variables of the femoral canal shape have been the most helpful in understanding the 3-point fixation and deflection of the nail. Figure 4 shows the midpoint displacement for a nail. This analysis compared the curved path of the medullary canal to the straight path that a nail would travel without bending. The midpoint deflection is the furthest distance that the theoretical straight path is from the actual path of the nail in the canal. The greater the displacement, the greater the potential for deflection can be since the nail must bend to follow the canal path. Figure 5 shows the quantitative results for this analysis L1 R1 L2 R2 L3 L4 Fig. 5 Midpoint Displacement for Template Model As shown above, R2 and L3 both have large midpoint displacements though their actual recorded deflection is low. This discrepancy will be addressed in light of other features, but the rest of the displacement results, especially L2 and L4, coincide with this analysis. Curvature Several other landmarks of the canal can reveal why the midpoint displacement does not fully determine the nail deflection. Figure 6 shows the degree of curvature, or bend, from the first proximal third of the bone to the end of the bone L1 R1 L2 R2 L3 L4 Fig. 6 Canal Path Bend L2 again has the lowest score, supporting the minimal deflection from the fixture tests. R1 and L1 are not significantly large, so their other characteristics must be studied to understand the effects of curvature. R2 and L3 have very large bends, despite having low deflections. On the following page, Figure 7 shows some landmarks of the femur that will be discussed. It highlights this bend of the femur, identifying it as the angle change from the proximal 1/3 of the bone to the end of the bone, and is labeled the 3/3 Angle. The distal anterior cortex is important to identifying how much the curvature will affect the nail on antegrade insertion. When the most posterior edges of the bone are aligned, the distal anterior cortex is flat, relative to the alignment. The more proximal the anterior cortex starts to flatten out, the more the distal end of the nail

4 (cm) ( ) Fig. 7 Identification of the Flat Anterior Cortex of the femur (L2 Femur Template) will remain in alignment with the entrance of the nail. The flat anterior cortex also highlights the relation of curvature to bending. If anterior cortex flattens out, and does not increase anteriorly, then there is potential for a canal path that will cause more bending. Figure 8 shows the start location of the flat portion of the anterior cortex and the trajectory of the middle section path of the canal, the 2/3 Angle as shown in Figure L1 R1 L2 R2 L3 L4 Location of Anterior Flat Section Fig. 8 Distance from Fracture Location to Anterior Flat Section The flat section of the distal anterior cortex of the L3 and L2 templates occurs at the distal end of the bone. This can contribute to very little deflection, as the trajectory of the nail is not flattened until the end, relative to the start trajectory. The middle trajectory of the femur also plays into how important the flat section is. If the middle section still is at an angle close to the starting trajectory, and the flat section is at Change in Angle in Middle Section 5 the distal end of the bone, then the flat sections will not cause much deflection of the nail. L1 and R1 do not change drastically from the starting path, but the flat section occurs relatively proximal in the canal. L2 has a very shallow change in path direction and will not flatten out until the end. R2 and L4 have similar changes in angles, but the flat section is located more proximal for L4. The difference in L4 and R2 is evident and further defined by the next landmark, the canal containment. L3 does not flatten out until the very end, but does change in direction significantly, so the small deflections recorded can be attributed to the curvature in the middle section. Canal Containment The narrowness of the canal around the isthmus is another important feature of the femur. If the section around the isthmus remains the same width then it will provide containment of the nail, leading to more bending. The canal width in a 1cm long section around the isthmus was measured at 5 different points, including the isthmus. The points were compared to the width of the isthmus to determine the widening in terms of the relative size of the canal. The maximum ratios of the canal width to the isthmus width were compared to show the expansion of the canal around the isthmus, this is shown in Table 1.

5 (mm) (cm) Stiffness Ratio to 8mm Nail Table 1. Max Ratios of Canal Containment L1 R1 L2 R2 L3 L The enlargement ratio for L2 is about 1% and R2 is nearly 25% larger than L1 and R1. L3, though has very similar values to the templates that have more deflection. The significance of the containment of the canal around the isthmus depends largely on where the isthmus is located. Though L2 only expands 1% more quickly than L1, it is located about 6cm distally to the fixture fractures, whereas the isthmus of L1 was at the proximal fracture site. This means the canal around the fracture was even wider, relative to the isthmus. The isthmus of L3 is located directly after the proximal fracture, so again the L3 data seems to refute the observed trends of the deflection fixtures. Nail Stiffness Figure 1 shows the sum of the total deflection of each size nail for the six bone templates mm 9mm 1mm 11mm 12mm Fig. 1 Total deflection for Each Nail Diameter Figure 11 shows the relation of bone stiffness to the nail stiffness. The graph is a ratio of the stiffness of the varying nail sizes to an 8mm nail and also displays the ratio of the stiffness of a femur to the corresponding nail sizes. The geometry of bone is much stiffer than the nail, so the 12mm nail will likely bend before the femur will, though it does not prevent the possibility of anterior cortex blowout. Calculations for this comparison are listed in Appendix B mm 9mm 1mm 11mm 12mm Nail Stiffness Ratio Bone Stiffness Ratio Fig. 11 Stiffness Comparison of Bone to Nail at Various Cross Sections Entry Method Figure 12 is a comparison of the retrograde and antegrade entry methods. When the nail was inserted on the end of the bone opposite the fracture, there was always a greater deflection than when it was inserted on the same side of the femur as the fracture was. L2, R2, L3 all have deflection from the opposite approach L1 L1 R1 R1 L2 L2 R2 R2 L3 L3 L4 L4 8mm Opposite Entry from Fracture 8mm Same Entry as Fracture Fig. 12 Comparison of the Entry Methods

6 Discussion These results provide a better understanding of the relationship between the features in the femur and the deflection of the nail. The exact magnitudes of the deflections did not provide a clear trend, but several observations and assumptions were supported by the data. The three characteristics: canal bend angle, distal anterior cortex, and canal containment were all observed to affect the deflection of the nail and the alignment of the targeting guide. Some of the templates directly support these conclusions however; L3 did not match these assumptions. Because of the small sample size of this study, the templates that opposed the general observations are not critically analyzed in great depth, but will be commented on shortly. Because of the data collected, R2 and L3 appear that there should have had more deflection from the fixture testing. Canal containment is the most significant feature to support the little deflection that was observed with the test fixture. R2 had a much quicker expansion around the isthmus than the other bones, so this containment feature can be cited as an important part of nail deflection analysis. If the canal does not stay narrow around the isthmus for very long, the nail is not constrained in that section and is not forced to bend significantly. The flat sections are also more proximal than the other bones, allowing the nail to travel in a relatively straight path for the majority of the bone. For the L3 template, only the anterior flat cortex can point to the lack of deflection. This data is not significant enough to establish a clear reason for the absence of deflection. The region around the isthmus, where the containment was analyzed, was determined to be a critical point for establishing bend, as that section is where the midpoint displacement occurs. Because the guides were setup based on the transparencies, the curves of the edges were not consistent between fixtures. Modeling the shape of the isthmus region could be improved, as the guides were either straight or generically curved and could not model the containment of the canal exactly. The midpoint displacement can be used to achieve a preliminary estimation of deflection, but the other factors must also be taken into account. With more samples and testing, an understanding of achieving an estimation for deflection could be made based on these femur characteristics. The fracture location plays a large role in the amount of deflection. According to a mid-point displacement analysis, it is evident that a fracture in the mid-shaft region will not cause as much deflection as a very distal fracture. Consequently, a fracture at the ends of a femur will create more deflection, because the majority of the features of the bone are still able to provide containment, as opposed to a mid-shaft fracture, where the curvature of the bone is split. The entry point to femur fractures can also have a large effect on nail fixation. Though this was not a focus of the study, the entrance site, either too anterior or posterior, directly affected the trajectory and fixation of the nail. If it was inserted too anteriorily, then the proximal bend would not realign the nail with the canal path, since the main portion of the nail would already be resting against the anterior face. The same is true for the retrograde entry, as the nail would be forced against the anterior face, if the entry site was too anterior. The orientation of the entry site to the fracture also affects deflection. A nail inserted by the retrograde method always had a higher deflection than the antegrade approach for proximal fractures. Based off of an understanding of nail stiffness, the deflection of a nail can be estimated from the

7 resistance of the nail during insertion. For larger nails, much resistance can result in little deflection, if there is tight containment in the canal. And little resistance for an 8mm nail can result in a large deflection, because the 8mm nail is not very stiff, compared to a 12mm nail. Consequently, resistance can be helpful in determining deflection, coupled with a good understanding of the general curvature and containment in the canal. Conclusion These trends provide an insight into the mechanics behind the 3-point fixation of the SIGN nail. The most significant observations of this study are the connections between landmarks on the femur and the deflection of the nail. The bend of canal path, flat section and canal containment all have been observed to contribute to nail deflection. The goal of the study was to determine factors involved with nail bending and provide a guide for determining when deflection will be small or large. Because of the small sample size, the exact relation of canal features and nail deflection could not be discussed fully. There were many assumptions regarding fracture care at the beginning of this study. The assumption of the deflection of the 8mm nail being greater than the 12 mm nail was confirmed through the resistance of entry for the models. Even the 1mm nail was noticeably stiffer than the 8mm nail, needing to be aggressively pushed down the canal. The entry angle and method, antegrade or retrograde, affected the reduction of the fracture. The correlation between the location of the fracture and deflection was not explicitly proven, as the fracture sites were fixed in this study, but the results show that the location of the fracture for each bone is very important, as the mid-shaft fractures did not have any nail deflection in this study. These trends are not laws for fracture care, but are the observations from a preliminary and limited study of femur fracture care. The surgeon s experience with inserting the nail and knowledge should outweigh any suggestions that were presented here, but this information is intended to improve preparation for nail selection and targeting preparation. These disclosures stated, this study can provide a good foundation for further investigations into the area of femur fracture care. Many observations were made along the course of this research endeavor that should be pursued further in more comprehensive tests. The goal of the research should be to establish a pre-targeting algorithm that can deduce an approximate deflection for any given fracture based on fracture location and key landmarks for the bone. The algorithm should ideally be able to be used on a medial-lateral x-ray to estimate the degree of deflection of the distal screw holes in the nail. From this study, the midpoint displacement test seems like it could be the beginning stage for such an algorithm. This study observed the result of nailing only 6 bones; further research should include a larger sample of femur templates with more accurate models of the femoral canal. Varying fracture locations should also be studied for more conclusive evidence for the relation of isthmus location and the flat portion of the femur.

8 Appendix A: Table 2. Average Measurements of Lateral View of Femur Category (cm) (cm) imal Head (w:d) Isthmus Location 15. Mid of Lesser Trochanter from Tip of Greater Trochanter 4. Posterior Slope of 1/3 ( ) 12.9 Anterior Slope of 1/3 ( ) 7.8 X-S at Neck 2.27 X-S at 1/ Posterior 1/ Anterior 1/ Center of Curve Posterior Flattens from _ to _ Anterior Flattens at _ to end / Posterior Slope 3/3 ( ) Anterior Slope 3/3 ( ) -.27 X-S at Neck 2.67 Head (w:d) Anterior Edge Increase 2.22 Table 3. Comparison of Anatomical Measurements Average (cm) Converted Dimensions from Journal (cm) Percent Difference Isthmus distance % X-S at imal Funnel End % Flare Index % *As compared with Laine, H.-J., 21, Anatomy of the imal Femoral Medullary Canal and Fit and Fill Charateristics of Cementless Endoprosthetic Stems,

9 Fig. 16. Lateral View of Left Femur with Annotations. Dimensions in cm. Table 4. Measurements of Cross Section of Femur Left 1 Right 1 Left 2 Cortex Thick Very Thin Very Thin Head (w:d) Isthmus distance Lesser Trochanter Pos Slope of 1/ Ant Slope of 1/ X-S at proximal funnel X-S at 1/3 (14cm tip of troch) Pos 1/ Ant 1/ Center of Curve Pos Ant 27. end(41) 27 end (41) / Pos Slope 3/ Ant Slope 3/3 Flat Flat X-S at distal funnel Head (w:d) Anterior Edge Increase

10 Table 5. Measurements of Cross Section of Femur cont. Right 2 Left3 Left 4 Cortex Thin Thin Very Thick Head (w:d) Isthmus distance Lesser Trochanter Pos Slope of 1/ Ant Slope of 1/ X-S at proximal funnel X-S at 1/3 (14cm tip of troch) Pos 1/ Ant 1/ Center of Curve Pos Ant / Pos Slope 3/ Ant Slope 3/ X-S at distal funnel Head (w:d) Anterior Edge Increase Table 6. Deflection Tables Left 1 L1 8mm 9mm 1mm 11mm 12mm (12.5cm from Tip of Ante Retro (32.1cm from Tip of Ante Trchanter) Retro Mid-shaft (22.9cm from Tip of Ante Retro * a dash(-) signifies the nail was not tried or was too stiff to fully insert into the canal

11 Table 7. Deflection Tables Right 1 R1 8mm 9mm 1mm 11mm 12mm (12.5cm from Tip of Ante Retro (32.1cm from Tip of Ante Trchanter) Retro Mid-shaft (22.9cm from Tip of Ante Retro Table 8. Deflection Tables Left 2 L2 8mm 9mm 1mm 11mm 12mm (12.5cm from Tip of Ante Retro (32.1cm from Tip of Ante Trchanter) Retro Mid-shaft (22.9cm from Tip of Ante Table 9. Deflection Tables Right 2 R2 8mm 9mm 1mm 11mm 12mm (12.5cm from Tip of Ante Retro (12.5cm from Tip of Mid-shaft (12.5cm from Tip of Ante Retro Ante Retro

12 Table 1. Deflection Tables Left 3 L3 8mm 9mm 1mm 11mm 12mm (12.5cm from Tip of Ante Retro (32.1cm from Tip of Ante Trchanter) Retro 1.5 Mid-shaft (22.9cm from Tip of Ante Retro Table 11. Deflection Tables Left 4 L4 8mm 9mm 1mm 11mm 12mm (12.5cm from Tip of Ante Retro 7.5 (32.1cm from Tip of Ante Trchanter) Retro Mid-shaft (22.9cm from Tip of Ante Retro

13 Appendix B: Stiffness = EI L Because the lengths of the nail and bone are the same for the section being compared, the L is dropped and the equation reduces to: EI, Where E is the modulus of Elasticity and I is the moment of inertia. E for Stainless steel 316L: 193GPa E for bone varies, but can be assumed to be around: 15GPa I Nail = π 1 64 D4, D=.8m I 8mm Nail = m 4 I femur = π 1 64 [(D outer) 4 (D inner ) 4 ], D outer =.3m D inner =.1m I 8mm Bone = m 4 Stiffness nail = 38.8 N m 2 Stiffness bone = 589 N m 2

14 Bibliography (Germane Articles) [1] NOBLE, P. C., ALEXANDER, J. W., LINDAHL, L. J., YEW, D. T., GRANBERRY, W. M., and TULLOS, H. S., 1988, The anatomic basis of femoral component design, Clinical orthopaedics and related research, 235, pp [2] Rubin, P. J., Leyvraz, P. F., Aubaniac, J. M., Argenson, J. N., Esteve, P., and De Roguin, B., 1992, The morphology of the proximal femur. A three-dimensional radiographic analysis, Journal of Bone & Joint Surgery, British Volume, 74(1), pp [3] Laine, H.-J., 21, Anatomy of the imal Femoral Medullary Canal and Fit and Fill Charateristics of Cementless Endoprosthetic Stems, Academic Dissertation, University of Tampere Medical School. [4] Bong, M. R., Kummer, F. J., Koval, K. J., and Egol, K. A., 27, Intramedullary nailing of the lower extremity: biomechanics and biology, J Am Acad Orthop Surg, 15(2), pp [5] Ikem, I. C., Ogunlusi, J. D., and Ine, H. R., 27, Achieving interlocking nails without using an image intensifier, International Orthopaedics, 31(4), pp [6] Buford Jr, W. L., Turnbow, B. J., Gugala, Z., and Lindsey, R. W., 214, Three-Dimensional Computed Tomography Based Modeling of Sagittal Cadaveric Femoral Bowing and Implications for Intramedullary Nailing, Journal of orthopaedic trauma, 28(1), pp (Further Study Articles Relating to Entry Site and Other Technical Aspects of Femoral Nailing) [1] Harper, M. C., and Carson, W. L., 1987, Curvature of the femur and the proximal entry point for an intramedullary rod, Clin. Orthop. Relat. Res., (22), pp [2] Gausepohl, T., Pennig, D., Koebke, J., and Harnoss, S., 22, Antegrade femoral nailing: an anatomical determination of the correct entry point, Injury, 33(8), pp [3] Kale, S. P., Patil, N., Pilankar, S., Karkhanis, A. R., and Bagaria, V., 26, Correct anatomical location of entry point for antegrade femoral nailing, Injury, 37(1), pp [4] Chen, S.-H., Yu, T.-C., Chang, C.-H., and Lu, Y.-C., 28, Biomechanical analysis of retrograde intramedullary nail fixation in distal femoral fractures, The Knee, 15(5), pp [5] Helwig, P., Faust, G., Hindenlang, U., Hirschmüller, A., Konstantinidis, L., Bahrs, C., Südkamp, N., and Schneider, R., 29, Finite element analysis of four different implants inserted in different positions to stabilize an idealized trochanteric femoral fracture, Injury, 4(3), pp [6] Calafi, L. A., Antkowiak, T., Curtiss, S., Neu, C. P., and Moehring, D., 21, A biomechanical comparison of the Surgical Implant Generation Network (SIGN) tibial nail with the standard hollow nail, Injury, 41(7), pp [7] Wehner, T., Penzkofer, R., Augat, P., Claes, L., and Simon, U., 211, Improvement of the shear fixation stability of intramedullary nailing, Clinical Biomechanics, 26(2), pp [8] Tupis, T. M., Altman, G. T., Altman, D. T., Cook, H. A., and Miller, M. C., 212, Femoral bone strains during antegrade nailing: A comparison of two entry points with identical nails using finite element analysis, Clinical Biomechanics, 27(4), pp

15 [9] Wu, C.-C., 212, Retrograde traditional femoral or tibial locked intramedullary nails for distal femoral injuries, Formosan Journal of Musculoskeletal Disorders, 3(3), pp [1] Ansari Moein, C. M. S., Gerrits, P. D., and Duis, H. J. ten, 213, Trochanteric fossa or piriform fossa of the femur: Time for standardised terminology?, Injury, 44(6), pp