- Email: [email protected]
The stiffness characteristics of hybrid Ilizarov fixators
ARTICLE IN PRESS Journal of Biomechanics 41 (2008) 2960–2963
Contents lists available at ScienceDirect
Journal of Biomechanics journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com
The stiffness characteristics of hybrid Ilizarov fixators Onder Baran a,b,, Hasan Havitcioglu a,b, Hasan Tatari a, Berivan Cecen b a b
Department of Orthopedics and Traumatology, Dokuz Eylul University, Ortopedi ve Travmatoloji AD, 35340 Balcova-Izmir, Turkey Department of Biomechanics, Health Sciences Institute, Dokuz Eylul University, 35340 Izmir, Turkey
a r t i c l e in f o
a b s t r a c t
Article history: Accepted 25 July 2008
The use of hybrid Ilizarov models around femoral area is gaining clinical popularity lately. Hybrid systems show different mechanical properties. The purpose of this report is to examine the stiffness characteristics of the C-arch and half-pins on the hybrid Ilizarov fixators. Eight models that included standard Ilizarov and hybrid models were applied to six femoral sawbones. The distal part of fixation was composed of a two-ring frame applied identically to all bones. The difference of the configuration was at the proximal part, where half-pins with or without C-arches were either added to the proximal two-ring frame or replaced the proximal one- or two-ring frame. Osteotomy was performed in the femoral diaphysis and the bone was distracted 2 cm. The stability of the system was tested with the axial compression testing machine. Displacements between the adjacent fracture sides were measured with the video extensometer in three dimensions. We found that proximal half-pin applications alone had less stiffness, but half-pins with C-arch had more stiffness than the model including only half-pins. Additional half-pins onto one- or two-ring frames had more longitudinal stiffness, but this system showed weak resistance against transverse displacement. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Hybrid Ilizarov fixators C-arches Half-pins Biomechanics
1. Introduction Recently, Ilizarov circular external fixators have gained worldwide popularity in various disorders of long bones. Khalily et al. (1998) developed hybrid configurations to combine the ability of a circular fixator to control a complex fracture with the soft tissue access allowed by a unilateral half-pin frame. Hybrid external fixators that use tensioned wires in the metaphysis and screws in the diaphysis combine the advantages of both unilateral and circular fixators and provide considerable flexibility in frame construction (Khalily et al., 1998; Yang et al., 2003). These hybrid forms of Ilizarov system application affect the mechanical properties at the fracture site (Yilmaz et al., 2003). There are three major approaches to the construction of hybrid fixators. In the first approach, a screw replaces one of the two wires at each ring level of an Ilizarov fixator. The second approach is to connect a unilateral bar–screw assembly to a ring–wire assembly using a special adapter. The third approach uses two rings connected by posts, one ring for mounting screws and another for wires (Yang et al., 2003). Femoral external fixations are exposed to different mechanical loads. Yet, during the physiological axial loading, hybrid external Corresponding author at: Dokuz Eylul Universitesi Tip Fakultesi, Ortopedi ve Travmatoloji AD, 35340 Balcova-Izmir, Turkey. Tel.: +90 232 412 33 66; fax: +90 232 277 22 77. E-mail address: [email protected] (O. Baran).
0021-9290/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2008.07.030
fixation resists to complicated longitudinal and transverse force due to its special construction. The major goal of the research described in this study is to examine biomechanical properties of hybrid external fixators in three dimensional spaces under physiological loading.
2. Material and methods The half-pins, C-arches and two rings were proximally joined in various combinations to form eight different models as seen in Fig. 1. Distal part of all models included two rings similar to the standard Illizarov fixator Model-1: [2+2] The standard Ilizarov fixator including two rings were placed at the proximal part of the osteotomy line (Fig. 1). Model-2: [2+(half-pins)] Two half-pins perpendicular to each other and a femoral shaft were added to the system as shown in Fig. 1. It did not include any proximal rings. Model-3: [2+(half-pins)+(C-arch)] which is similar to Model-2, but includes half-pins attached to each other by C-arch. Model-4: [2+1+(half-pins)] Two half-pins perpendicular to each other and femoral shaft were added on one proximal ring as shown in Fig. 1. Model-5: [2+1+(half-pins)+(C-arch)] which is similar to Model-4, but includes half-pins attached to each other by C-arch. Model-6: [2+2] Proximal two-ring configuration, like short-segment Ilizarov fixator. Model-7: [2+2+(half-pins)] Two half-pins perpendicular to each other and femoral shaft were added to the proximal two rings as shown in Fig. 1. Model-8: [2+2+(half-pins)+(C-arch)] which is similar to Model-7, but includes half-pins attached to each other by C-arch. The diameter of each ring was 160 mm and that of Kirschner wires was 1.8 mm. Rings were made of aluminum, the wires were 316-L stainless steel.
ARTICLE IN PRESS O. Baran et al. / Journal of Biomechanics 41 (2008) 2960–2963
2961
Fig. 1. Ilizarov fixation designs.
Fig. 2. Application of compression test. Three-dimensional displacements between fracture ends were measured with CCD camera extensometers.
Fig. 3. Axial compression (common physiological loading) on femur. Fixator stiffness is measured with CCD camera extensometer in three dimensions at the fractures side.
ARTICLE IN PRESS 2962
O. Baran et al. / Journal of Biomechanics 41 (2008) 2960–2963
Fig. 4. Video extensometer outcome of longitudinal stiffness measurement, the graph show force/displacement values.
Osteotomy and 2 cm distraction were performed in the distal part of the femoral diaphysis. Tension of the Kirschner wires was established at 1000 N by a wire tensioner. The biomechanical tests were performed by using the axial compression testing machine (AG-I 10 kN, Shimadzu, Japanese). Each of the eight models was tested on six femoral sawbones and the Kirschner wires were restrengthened before each test. The video extensometer markers were placed 1 cm distal and proximal to the fracture line (Fig. 2). Fractured femoral bone fixed with a hybrid Illizarov model was subjected to an axial compression up to 500 N at a strain rate of 5 mm/min and then 10 consecutive cyclic loadings between 100 and 500 N were performed. Displacements between fracture ends were measured with two CCD camera extensometers (Non-contact Video Extensometer DVE-101/201, Shimadzu, Japanese), as axial (y-axis), anteroposterior (z-axis), lateral (x-axis) (Figs. 2 and 3). Extensometers perform precise, non-contact elongation measurements by using CCD cameras to capture digital images of test specimens (www.shimadzu.com). The PC then performs image processing of the data and calculates the elongation of the gauge length. Fig. 4 shows an example of force–displacement data outcomes. The data of force–displacement variation was evaluated with Spearman’s nonparametric correlation test using SPSS 11.0 for Windows. Eight models were compared with Mann–Whitney U test.
Table 1 Displacement value in three directions for eight models, after 10 cyclic axial loading maximum force: 500 N; range of cyclic loading: 100–500 N Model
Displacement (mm)
1. Ilizarov 2. Half-pins 3. Half-pins+C-arch 4. One ring+half-pins 5. One ring+half-pins+C-arch 6. Two rings 7. Two rings+half-pins 8. Two rings+half-pins+C-arch
Longitudinal
Lateral
Anteroposterior
Mean
SD
Mean
SD
Mean
SD
6.73 39.46 20.54 7.48 7.92 8.27 5.93 5.52
0.57 2.46 3.20 0.19 0.37 0.13 0.46 0.53
0.59 20.92 19.68 3.60 1.92 0.26 0.81 1.83
0.24 1.75 0.68 0.85 0.54 0.02 0.18 0.79
0.48 15.04 18.10 0.62 1.26 0.21 0.33 0.36
0.30 0.88 1.50 0.06 0.52 0.04 0.08 0.08
Longitudinal Stiffness 100 3. Results
N/mm
Results of the axial compression test and maximum displacement measurement in three dimensions are shown in Table 1. Stiffness values of the models are showed in Figs. 5 and 6. When the longitudinal displacement and stiffness values of the models were compared, Model-7 and Model-8 demonstrated the lowest displacement and the highest stiffness values (po0.05), but there was no significant statistical difference between these models (p ¼ 0.1). Model-4 and Model-5 showed lower stiffness values than Model-1 (po0.05), but more stiffness than the standard short-segment four-ring Ilizarov fixator (Model-6). Model-2 and Model-3 demonstrated the highest displacement and the lowest stiffness values among all models (po0.05). High positive correlation about C-arches application was (0.89) only seen in Model-1 and Model-2 in longitudinal stiffness.
80 60 40 20 0 1
2
3
4 5 Models
6
7
8
Fig. 5. Bar graph of longitudinal stiffness.
When the anteroposterior and lateral displacement in the transverse plane was measured, anteroposterior stiffness seemed greater than lateral stiffness for all models studied, particularly in Model-7 and -8. Four-ring systems (Model-1 and Model-6) had
ARTICLE IN PRESS O. Baran et al. / Journal of Biomechanics 41 (2008) 2960–2963
Transverse Stiffness
N/min
3000 2500
Serial 1
2000
Serial 2
1500 1000 500 0 1
2
3
4 5 Models
6
7
8
Fig. 6. Bar graph of transverse stiffness. Serial-1 lateral stiffness and Serial-2 anteroposterior stiffness.
higher stiffness values than the others (po0.05). Model-2 and Model-3 had very low values at both anteroposterior and lateral stiffness in transverse plane.
4. Discussion In our clinical studies with Ilizarov fixator on femur, we generally prefer hybrid frames. Anatomical and mechanical axes of femur are different unlike tibia, so improvement of the hybrid frame model is more important for femur. There are some reported studies which compare the biomechanical characteristics of the standard and hybrid models (Khalily et al., 1998; Pugh et al., 1999; Stein et al., 1997; Yang et al., 2003; Yilmaz et al., 2003). To our knowledge, the biomechanics of ring and cantilever fixation are different in principle. The Ilizarov fixator had zero angulations compliance, while unilateral fixator had the highest angulations compliance (Yang et al., 2003). The hybrid fixators tested had intermediate value of angulations compliance. Yang et al. (2003) reported that their tested fixators had shown nonlinear stiffness under axial and bending loads. The hybrid external fixators were the stiffest in axial compression and the least stiffness in posterior bending (Lundy et al., 1998). Our test results indicated some differences that major displacement in transverse plane was lateral. We claim that only axial loading on the femoral head creates major compressive forces on the medial side and tensile forces on the lateral side. Therefore, lateral stiffness of external fixator is more important than the anteroposterior stiffness. Femoral arch increased axial stiffness in all hybrid models, but angles between the half-pins had no significant effect on axial stiffness and the use of the ring mounted with half-pin increased stiffness only in torsional testing (Pugh et al., 1999; Yilmaz et al., 2003). In our study, outcome data of C-arch with half-pins were not obviously supported to increase stiffness. A similar result reported by Khalily et al. (1998) referred that axial stiffness was not affected by the augmentation with a V-shaped strut in the hybrid model.
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In our study, we did not perform torsional tests and could not measure the rotational displacement; but Stein et al. (1997) reported that no significant difference was found between the standard and hybrid models in terms of torsional stiffness. In the study of Stein et al. (1997) replacing one ring by a 4.5 mm Schanz screw with a connecting tube created a hybrid frame less rigid than the original ring fixator. The hybrid external fixator with a double-ring block to obtain more than one level of fixation was stiffer than those using only one ring. It is well known that half-pins caused change of fixator mechanism. Yang et al. (2003) reported that the Ilizarov hybrid fixator with one wire and one screw on each ring behaved more like a unilateral fixator than a circular fixator. In our opinion, any rigid component, such as half-pins, creates stress shield on the hybrid system, thus non-rigid components, such as wire, do not work properly because of their stiffness related to well-distributed strong tensile forces. According to our study, if we prefer hybrid model to avoid the difficulties of the construction of the Ilizarov frame, the ring-free model like as Model-2 and Model-3 is not proper for definitive treatment. One ring and half-pins may be acceptable for longitudinal stiffness, but they are not enough for transverse stiffness. Adjacent two proximal rings and half-pins show good longitudinal stiffness, but not lateral stiffness. We conclude that a lot of study about improvement on hybrid frame is necessary, but focusing only on improvement of axial rigidity is not rational. Limited axial micro-movement is useful for bone healing, but movement on the transverse plain even at micro-level is harmful. It seems that well-planned, semi-rigid designs help to equally force distribution among hybrid fixator components.
Conflict of interest statement All authors declared that there are no financial and personal relationships with other people or organisations that could inappropriately influence (bias) their work. References Khalily, C., Voor, M.J., Seligson, D., 1998. Fracture site motion with Ilizarov and hybrid external fixation. Journal of Orthopaedic Trauma 12, 21–26. Lundy, D.W., Albert, M.J., Hutton, W.C., 1998. Biomechanical comparison of hybrid external fixators. Journal of Orthopaedic Trauma 12, 496–503. Pugh, K.J., Wolinsky, P.R., Dawson, J.M., Stahlman, G.C., 1999. The biomechanics of hybrid external fixation. Journal of Orthopaedic Trauma 13, 20–26. Stein, H., Mosheiff, R., Baumgart, F., Frigg, R., Perren, S.M., Cordey, J., 1997. The hybrid ring tubular external fixator: a biomechanical study. Clinical Biomechanics 12, 259–266. Yang, L., Nayagam, S., Saleh, M., 2003. Stiffness characteristics and interfragmentary displacements with different hybrid external fixators. Clinical Biomechanics 18, 166–172. Yilmaz, E., Belhan, O., Karakurt, L., Arslan, N., Serin, E., 2003. Mechanical performance of hybrid Ilizarov external fixator in comparison with Ilizarov circular external fixator. Clinical Biomechanics 18, 518–522.
Contents lists available at ScienceDirect
Journal of Biomechanics journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com
The stiffness characteristics of hybrid Ilizarov fixators Onder Baran a,b,, Hasan Havitcioglu a,b, Hasan Tatari a, Berivan Cecen b a b
Department of Orthopedics and Traumatology, Dokuz Eylul University, Ortopedi ve Travmatoloji AD, 35340 Balcova-Izmir, Turkey Department of Biomechanics, Health Sciences Institute, Dokuz Eylul University, 35340 Izmir, Turkey
a r t i c l e in f o
a b s t r a c t
Article history: Accepted 25 July 2008
The use of hybrid Ilizarov models around femoral area is gaining clinical popularity lately. Hybrid systems show different mechanical properties. The purpose of this report is to examine the stiffness characteristics of the C-arch and half-pins on the hybrid Ilizarov fixators. Eight models that included standard Ilizarov and hybrid models were applied to six femoral sawbones. The distal part of fixation was composed of a two-ring frame applied identically to all bones. The difference of the configuration was at the proximal part, where half-pins with or without C-arches were either added to the proximal two-ring frame or replaced the proximal one- or two-ring frame. Osteotomy was performed in the femoral diaphysis and the bone was distracted 2 cm. The stability of the system was tested with the axial compression testing machine. Displacements between the adjacent fracture sides were measured with the video extensometer in three dimensions. We found that proximal half-pin applications alone had less stiffness, but half-pins with C-arch had more stiffness than the model including only half-pins. Additional half-pins onto one- or two-ring frames had more longitudinal stiffness, but this system showed weak resistance against transverse displacement. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Hybrid Ilizarov fixators C-arches Half-pins Biomechanics
1. Introduction Recently, Ilizarov circular external fixators have gained worldwide popularity in various disorders of long bones. Khalily et al. (1998) developed hybrid configurations to combine the ability of a circular fixator to control a complex fracture with the soft tissue access allowed by a unilateral half-pin frame. Hybrid external fixators that use tensioned wires in the metaphysis and screws in the diaphysis combine the advantages of both unilateral and circular fixators and provide considerable flexibility in frame construction (Khalily et al., 1998; Yang et al., 2003). These hybrid forms of Ilizarov system application affect the mechanical properties at the fracture site (Yilmaz et al., 2003). There are three major approaches to the construction of hybrid fixators. In the first approach, a screw replaces one of the two wires at each ring level of an Ilizarov fixator. The second approach is to connect a unilateral bar–screw assembly to a ring–wire assembly using a special adapter. The third approach uses two rings connected by posts, one ring for mounting screws and another for wires (Yang et al., 2003). Femoral external fixations are exposed to different mechanical loads. Yet, during the physiological axial loading, hybrid external Corresponding author at: Dokuz Eylul Universitesi Tip Fakultesi, Ortopedi ve Travmatoloji AD, 35340 Balcova-Izmir, Turkey. Tel.: +90 232 412 33 66; fax: +90 232 277 22 77. E-mail address: [email protected] (O. Baran).
0021-9290/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2008.07.030
fixation resists to complicated longitudinal and transverse force due to its special construction. The major goal of the research described in this study is to examine biomechanical properties of hybrid external fixators in three dimensional spaces under physiological loading.
2. Material and methods The half-pins, C-arches and two rings were proximally joined in various combinations to form eight different models as seen in Fig. 1. Distal part of all models included two rings similar to the standard Illizarov fixator Model-1: [2+2] The standard Ilizarov fixator including two rings were placed at the proximal part of the osteotomy line (Fig. 1). Model-2: [2+(half-pins)] Two half-pins perpendicular to each other and a femoral shaft were added to the system as shown in Fig. 1. It did not include any proximal rings. Model-3: [2+(half-pins)+(C-arch)] which is similar to Model-2, but includes half-pins attached to each other by C-arch. Model-4: [2+1+(half-pins)] Two half-pins perpendicular to each other and femoral shaft were added on one proximal ring as shown in Fig. 1. Model-5: [2+1+(half-pins)+(C-arch)] which is similar to Model-4, but includes half-pins attached to each other by C-arch. Model-6: [2+2] Proximal two-ring configuration, like short-segment Ilizarov fixator. Model-7: [2+2+(half-pins)] Two half-pins perpendicular to each other and femoral shaft were added to the proximal two rings as shown in Fig. 1. Model-8: [2+2+(half-pins)+(C-arch)] which is similar to Model-7, but includes half-pins attached to each other by C-arch. The diameter of each ring was 160 mm and that of Kirschner wires was 1.8 mm. Rings were made of aluminum, the wires were 316-L stainless steel.
ARTICLE IN PRESS O. Baran et al. / Journal of Biomechanics 41 (2008) 2960–2963
2961
Fig. 1. Ilizarov fixation designs.
Fig. 2. Application of compression test. Three-dimensional displacements between fracture ends were measured with CCD camera extensometers.
Fig. 3. Axial compression (common physiological loading) on femur. Fixator stiffness is measured with CCD camera extensometer in three dimensions at the fractures side.
ARTICLE IN PRESS 2962
O. Baran et al. / Journal of Biomechanics 41 (2008) 2960–2963
Fig. 4. Video extensometer outcome of longitudinal stiffness measurement, the graph show force/displacement values.
Osteotomy and 2 cm distraction were performed in the distal part of the femoral diaphysis. Tension of the Kirschner wires was established at 1000 N by a wire tensioner. The biomechanical tests were performed by using the axial compression testing machine (AG-I 10 kN, Shimadzu, Japanese). Each of the eight models was tested on six femoral sawbones and the Kirschner wires were restrengthened before each test. The video extensometer markers were placed 1 cm distal and proximal to the fracture line (Fig. 2). Fractured femoral bone fixed with a hybrid Illizarov model was subjected to an axial compression up to 500 N at a strain rate of 5 mm/min and then 10 consecutive cyclic loadings between 100 and 500 N were performed. Displacements between fracture ends were measured with two CCD camera extensometers (Non-contact Video Extensometer DVE-101/201, Shimadzu, Japanese), as axial (y-axis), anteroposterior (z-axis), lateral (x-axis) (Figs. 2 and 3). Extensometers perform precise, non-contact elongation measurements by using CCD cameras to capture digital images of test specimens (www.shimadzu.com). The PC then performs image processing of the data and calculates the elongation of the gauge length. Fig. 4 shows an example of force–displacement data outcomes. The data of force–displacement variation was evaluated with Spearman’s nonparametric correlation test using SPSS 11.0 for Windows. Eight models were compared with Mann–Whitney U test.
Table 1 Displacement value in three directions for eight models, after 10 cyclic axial loading maximum force: 500 N; range of cyclic loading: 100–500 N Model
Displacement (mm)
1. Ilizarov 2. Half-pins 3. Half-pins+C-arch 4. One ring+half-pins 5. One ring+half-pins+C-arch 6. Two rings 7. Two rings+half-pins 8. Two rings+half-pins+C-arch
Longitudinal
Lateral
Anteroposterior
Mean
SD
Mean
SD
Mean
SD
6.73 39.46 20.54 7.48 7.92 8.27 5.93 5.52
0.57 2.46 3.20 0.19 0.37 0.13 0.46 0.53
0.59 20.92 19.68 3.60 1.92 0.26 0.81 1.83
0.24 1.75 0.68 0.85 0.54 0.02 0.18 0.79
0.48 15.04 18.10 0.62 1.26 0.21 0.33 0.36
0.30 0.88 1.50 0.06 0.52 0.04 0.08 0.08
Longitudinal Stiffness 100 3. Results
N/mm
Results of the axial compression test and maximum displacement measurement in three dimensions are shown in Table 1. Stiffness values of the models are showed in Figs. 5 and 6. When the longitudinal displacement and stiffness values of the models were compared, Model-7 and Model-8 demonstrated the lowest displacement and the highest stiffness values (po0.05), but there was no significant statistical difference between these models (p ¼ 0.1). Model-4 and Model-5 showed lower stiffness values than Model-1 (po0.05), but more stiffness than the standard short-segment four-ring Ilizarov fixator (Model-6). Model-2 and Model-3 demonstrated the highest displacement and the lowest stiffness values among all models (po0.05). High positive correlation about C-arches application was (0.89) only seen in Model-1 and Model-2 in longitudinal stiffness.
80 60 40 20 0 1
2
3
4 5 Models
6
7
8
Fig. 5. Bar graph of longitudinal stiffness.
When the anteroposterior and lateral displacement in the transverse plane was measured, anteroposterior stiffness seemed greater than lateral stiffness for all models studied, particularly in Model-7 and -8. Four-ring systems (Model-1 and Model-6) had
ARTICLE IN PRESS O. Baran et al. / Journal of Biomechanics 41 (2008) 2960–2963
Transverse Stiffness
N/min
3000 2500
Serial 1
2000
Serial 2
1500 1000 500 0 1
2
3
4 5 Models
6
7
8
Fig. 6. Bar graph of transverse stiffness. Serial-1 lateral stiffness and Serial-2 anteroposterior stiffness.
higher stiffness values than the others (po0.05). Model-2 and Model-3 had very low values at both anteroposterior and lateral stiffness in transverse plane.
4. Discussion In our clinical studies with Ilizarov fixator on femur, we generally prefer hybrid frames. Anatomical and mechanical axes of femur are different unlike tibia, so improvement of the hybrid frame model is more important for femur. There are some reported studies which compare the biomechanical characteristics of the standard and hybrid models (Khalily et al., 1998; Pugh et al., 1999; Stein et al., 1997; Yang et al., 2003; Yilmaz et al., 2003). To our knowledge, the biomechanics of ring and cantilever fixation are different in principle. The Ilizarov fixator had zero angulations compliance, while unilateral fixator had the highest angulations compliance (Yang et al., 2003). The hybrid fixators tested had intermediate value of angulations compliance. Yang et al. (2003) reported that their tested fixators had shown nonlinear stiffness under axial and bending loads. The hybrid external fixators were the stiffest in axial compression and the least stiffness in posterior bending (Lundy et al., 1998). Our test results indicated some differences that major displacement in transverse plane was lateral. We claim that only axial loading on the femoral head creates major compressive forces on the medial side and tensile forces on the lateral side. Therefore, lateral stiffness of external fixator is more important than the anteroposterior stiffness. Femoral arch increased axial stiffness in all hybrid models, but angles between the half-pins had no significant effect on axial stiffness and the use of the ring mounted with half-pin increased stiffness only in torsional testing (Pugh et al., 1999; Yilmaz et al., 2003). In our study, outcome data of C-arch with half-pins were not obviously supported to increase stiffness. A similar result reported by Khalily et al. (1998) referred that axial stiffness was not affected by the augmentation with a V-shaped strut in the hybrid model.
2963
In our study, we did not perform torsional tests and could not measure the rotational displacement; but Stein et al. (1997) reported that no significant difference was found between the standard and hybrid models in terms of torsional stiffness. In the study of Stein et al. (1997) replacing one ring by a 4.5 mm Schanz screw with a connecting tube created a hybrid frame less rigid than the original ring fixator. The hybrid external fixator with a double-ring block to obtain more than one level of fixation was stiffer than those using only one ring. It is well known that half-pins caused change of fixator mechanism. Yang et al. (2003) reported that the Ilizarov hybrid fixator with one wire and one screw on each ring behaved more like a unilateral fixator than a circular fixator. In our opinion, any rigid component, such as half-pins, creates stress shield on the hybrid system, thus non-rigid components, such as wire, do not work properly because of their stiffness related to well-distributed strong tensile forces. According to our study, if we prefer hybrid model to avoid the difficulties of the construction of the Ilizarov frame, the ring-free model like as Model-2 and Model-3 is not proper for definitive treatment. One ring and half-pins may be acceptable for longitudinal stiffness, but they are not enough for transverse stiffness. Adjacent two proximal rings and half-pins show good longitudinal stiffness, but not lateral stiffness. We conclude that a lot of study about improvement on hybrid frame is necessary, but focusing only on improvement of axial rigidity is not rational. Limited axial micro-movement is useful for bone healing, but movement on the transverse plain even at micro-level is harmful. It seems that well-planned, semi-rigid designs help to equally force distribution among hybrid fixator components.
Conflict of interest statement All authors declared that there are no financial and personal relationships with other people or organisations that could inappropriately influence (bias) their work. References Khalily, C., Voor, M.J., Seligson, D., 1998. Fracture site motion with Ilizarov and hybrid external fixation. Journal of Orthopaedic Trauma 12, 21–26. Lundy, D.W., Albert, M.J., Hutton, W.C., 1998. Biomechanical comparison of hybrid external fixators. Journal of Orthopaedic Trauma 12, 496–503. Pugh, K.J., Wolinsky, P.R., Dawson, J.M., Stahlman, G.C., 1999. The biomechanics of hybrid external fixation. Journal of Orthopaedic Trauma 13, 20–26. Stein, H., Mosheiff, R., Baumgart, F., Frigg, R., Perren, S.M., Cordey, J., 1997. The hybrid ring tubular external fixator: a biomechanical study. Clinical Biomechanics 12, 259–266. Yang, L., Nayagam, S., Saleh, M., 2003. Stiffness characteristics and interfragmentary displacements with different hybrid external fixators. Clinical Biomechanics 18, 166–172. Yilmaz, E., Belhan, O., Karakurt, L., Arslan, N., Serin, E., 2003. Mechanical performance of hybrid Ilizarov external fixator in comparison with Ilizarov circular external fixator. Clinical Biomechanics 18, 518–522.