- Email: [email protected]
The primary stability of the hip transposition type IIb: A biomechanical in vitro study
Clinical Biomechanics 24 (2009) 361–365
Contents lists available at ScienceDirect
Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
The primary stability of the hip transposition type IIb: A biomechanical in vitro study Martin Wessling a,b,*,1, Mirko Aach c,1, Mirko Herbort d, Georg Gosheger a, Jendrik Hardes a, Carsten Gebert a,b a
Department of Orthopaedic Surgery, University Hospital Muenster, Albert-Schweitzer-Str. 33, D-48149 Muenster, Germany Department of Tumour and Revision Surgery, Orthopaedic Hospital Volmarstein, Lothar-Gau-Str. 11, D-58300 Wetter, Germany c Department of Spinal Chord Injuries, University Hospital Bergmannsheil Bochum, Bürkle-de-la-Camp-Platz 1, D-44789 Bochum, Germany d Department of Trauma, Hand and Reconstructive Surgery, University Hospital Muenster, Waldeyerstraße 1, D-48149 Muenster, Germany b
a r t i c l e
i n f o
Article history: Received 7 December 2008 Accepted 21 January 2009
Keywords: Pelvic tumour Hip transposition Primary stability Biomechanical study
a b s t r a c t Background: The hip transposition is firmly established in pelvic sarcoma surgery. However, the primary stability of the hip transposition has not been tested yet so that the mobilisation, respectively the immobilisation of the patient so far solely relied on the experience of the surgeon. The aim of this study was to test the primary stability reliably with the help of a model and to reveal possible differences in stability between currently used anchor systems (TwinFixÒ 6.5 and MITEKÒ SuperAnchorÒ). Methods: A biomechanical model of porcine sacra was developed to document the maximum load capacity (load to failure test) and the performance under cyclic load (100 N, 200 N, 350 N, 700 N, 1400 N, each with 1000 cycles), 28 sacra were tested in total. Macroscopic damages, displacement, yield load, stiffness and Fmax were recorded as well. Findings: The load to failure test results showed a 3,9 times higher maximum load capacity for the TwinFixÒ 6.5 anchor (1307 N) compared to the MITEKÒ SuperAnchorÒ (334 N). The cyclical test revealed that nearly all MITEKÒ SuperAnchorsÒ failed at a load of 350 N. In contrast, the TwinFixÒ 6.5 anchors resisted 4000 cycles up to a load of 1400 N. Interpretation: The TwinFixÒ 6.5 anchor proved to be clearly superior to the MITEKÒ SuperAnchorÒ, resulting in the adjustment of the reconstruction technique. Therefore, the immobilisation period of a patient after a hip transposition type IIb could be shortened according to the results of the primary stability test. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Primary malignant bone sarcomas are rare entities which can be found mostly with young patients and frequently occur in the pelvic area. The reconstruction of defects that develop after wide tumour resection of the osseous pelvis often proves to be difficult and entails complications (Beadel et al., 2005; Ozaki et al., 1996; Abudu et al., 1997; Hillmann et al., 2003; Schwameis et al., 2002). Therefore the hip transposition was developed as a surgical technique less prone to complications, yet achieving good clinical and functional results. However, valid data regarding the primary stability, as opposed to the long term stability with additional scar tissue, of the hip transposition immediately after surgery have not been available until recently (Hoffmann et al., 2006; Hillmann et al., 2003; Gebert et al., 2008). Postoperative treatment of the patient so far relied on the clinical experience of the surgeon without * Corresponding author. Address: Department of Orthopaedic Surgery, University Hospital Muenster, Albert-Schweitzer-Str. 33, D-48149 Muenster, Germany. E-mail address: [email protected] (M. Wessling). 1 Martin Wessling and Mirko Aach contributed equally for this paper. 0268-0033/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2009.01.008
knowledge of the load capacity of the anchor systems. Only results from material testing of the individual components in cyclical as well as in load to failure tests were sufficiently known (Barber et al., 2003; Rupp et al., 2002). Furthermore, the maximum load capacity of the thread anchor proved to be dependent on the load angle (Deakin et al., 2005). A general superiority of the modern anchor systems could be shown; however, the influence of combined materials and multiple anchors has not been tested yet. The objective of this study was to determine the load capacity of the hip transposition and, if necessary, to improve the surgical technique with the use of different materials and anchors. 2. Methods A biomechanical model, using porcine sacra, was developed to represent a type IIb hip transposition (Gebert et al., 2008) – tumour prosthesis with bipolar head, attachment tube and four bone anchors (see Fig. 1) – after an internal hemipelvectomy type I–IV according to Ennking and Dunham (Enneking et al., 1990). A total of 28 sacra was used, 14 for each test series. The porcine sacrum
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M. Wessling et al. / Clinical Biomechanics 24 (2009) 361–365
material testing machine (Zwick/RoellÒ Z005, Zwick GmbH & Co. KG, D-89079 Ulm) allowed levelling remaining differences in position. Two different anchor systems were tested (TwinFixÒ 6.5, Smith&NephewÒ GmbH, D-45768 Marl and MITEKÒ SuperAnchorÒ, Ethicon GmbH Mitek Devision, D-22844 Norderstedt) in 2 6 test series: The load to failure test to determine Fmax, displacement, yield load and stiffness, and the performance under cyclic load (100 N, 200 N, 350 N, 700 N and 1400 N, each with 1000 cycles). The data was collected and processed using the software TestXpertÒ (see Fig. 3). SPSSÒ 12 was used to analyse the collected data. The test set-up, as well as the test parameters, have been used effectively in similar material testing experiments (see Table 1). 3. Results Fig. 1. Type IIb hip transposition.
was resected perpendicular to the load direction and the neo hipjoint was fixed in the sacrum. Preliminary tests had shown that already a small osseus roof could influence the primary stability significantly. The thread anchors were fixed with a 45° angle onto the cortical bone and connected to the attachment tube. The tube was further attached to the prosthesis with Ethibond 6.0 thread (see Fig. 2a–c). The sacrum of 26 week old pigs, halved in the sagittal plane, was girded for this model orthogradely with PU-foam (H400-ATÒ Voss Chemie GmbH, D-25436 Uetersen) into a mold. The individual difference between the stability of the bones was minimized due to a persistent slaughter age of 26 weeks. The position of the sacrum in regard to the lower extremity was adjusted according to the proportions after a resection. The sacra were consistently fixated on the simulated femur. A great accuracy was achieved by using a 2-KPU-foam (see Fig. 2d and e). Additionally, the equipment of the
Preliminary tests showed pig femora to be unable to resist the tested load and to break along the epiphysical disc. A hip transposition type IIa could therefore not be tested. Furthermore, a blade angle of more than 0° caused considerable derivation of power into the sacrum up to 2500 N and, subsequently, security termination of the testing program. Thus, a tumour prosthesis was used for the final test set-up and a worst-case-scenario for a hip transposition type IIb was tested. 3.1. Load to failure test Load to failure test series were conducted with both TwinFixÒ 6.5 and MITEKÒ SuperAnchorÒ. The MITEKÒ SuperAnchorÒ thread snapped at a mean load of 334 N (min. 259 N, max. 387 N). Avulsion or loosening of anchors as well as untying of knots was not recorded. The mean load of the TwinFixÒ 6.5 anchor at the point of material failure was 1307 N (min. 1054 N, max. 1591 N) (see Table 2 and Fig. 3).
Fig. 2. Development of the biomechanical model, final test set-up and macroscopic damages. (a) Positioning the anchors, (b) pig sacrum with attachment tube, (c) completed biomechanical model, (d) final set-up in the material testing machine, and (e) macroscopic damages after load to failure test.
M. Wessling et al. / Clinical Biomechanics 24 (2009) 361–365
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Fig. 3. (a) Typical test graph and parameters of load to failure test (TwinFixÒ 6.5); (b) typical test graph of cyclical test; (c) typical test graph of load to failure test; and (d) Boxplot Fmax load to failure test.
Table 1 Test settings of the material testing machine Zwick/RoellÒ Z005. Test settings
Load to failure
Cyclical test
Test rate Transient memory Cycles per performance level Lower reversal point Upper reversal point Test abortion
200 mm/min 0.1 s
350 mm/min 0.1 s 1000 50 N 100 N, 200 N, 350 N, 700 N, 1400 N Decrease in power by 95% of the maximum power reached Load to failure test after 5000 cycles: maximum power > 2500 N Loop gain 20 Loop gain 1 The first 10 cycles at each performance level, after this every 20th cycle
Expansion-regulation Force-regulation Cycles recorded Test cycle
Decrease in power by 95% of the maximum power reached Maximum power > 2500 N
One test cycle at 15 N
3.2. Cyclical test In all test cycles the average displacement for the MITEKÒ SuperAnchorÒ was not significantly higher than for the TwinFixÒ 6.5 (see Table 2). At a load of 300 N, the displacement for the MITEKÒ SuperAnchorÒ was 33 mm, for TwinFixÒ 6.5 only 25 mm. Thus, the displacement was 1.3 times higher when testing the MITEKÒ SuperAnchorÒ. However, the remaining margin of the MUTARSÒ attachment tube might account for a non-detectable part of the displacement measured in this test. The mean yield load was 279 N (min. 144 N, max. 384 N) for the MITEKÒ SuperAnchorÒ and 841 N (min. 555 N, max. 1067 N) for the TwinFixÒ 6.5. The maximum stiffness of the MITEKÒ SuperAnchorÒ with polyester thread showed an average increase in force of 15.9 N/mm (min. 12,4 N/mm, max. 19,4 N/mm). When testing the TwinFixÒ
6.5 and UltrabraidÒ 2-0, the increase was 47 N/mm (min. 47,4 N/ mm, max. 53 N/mm). For a deflection of 1 mm at the highest performance level, a four times higher force is needed for a TwinFixÒ 6.5 anchor than for a MITEKÒ SuperAnchorÒ. The MITEKÒ anchor system failed at a mean load of 252 N (min. 100 N, max. 349 N). The TwinFixÒ 6.5 system failed completely at a mean load of 1120 N (min. 699 N, max. 1399 N). This tendency of a significantly higher load capacity of the TwinFixÒ anchor system appeared throughout all tested parameters, both in the load to failure test and in the cyclical test series (see Table 2). 4. Discussion Since it was not possible to obtain human bone material in sufficient quality and quantity for the test performance, 28 porcine sacra were chosen as an alternative. 14 sacra were used for each
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M. Wessling et al. / Clinical Biomechanics 24 (2009) 361–365
Table 2 Results of the test series measuring the primary stability after hip transposition type IIb. Parameter Load to failure test Fmax (in N) Displacement at Fmax (in mm) Yield load Stiffness (in N/mm) Cyclical test Fmax (in N) Displacement at Fmax (in mm) Fmax (in N) Displacement at Fmax (in mm)
Anchor
n
Mean value
Standard deviation
Mean square error
TwinFix Mitek TwinFix Mitek TwinFix Mitek TwinFix Mitek
6 8 6 8 6 8 6 8
1307.07 334.29 51.10 39.57 840.97 279.01 47.07 15.87
186.43 47.33 6.28 7.64 171.07 86.92 5.40 2.59
76.11 16.73 2.56 2.70 69.84 30.73 2.21 0.92
TwinFix Mitek TwinFix Mitek TwinFix Mitek TwinFix Mitek
8 6 8 6 6 8 6 8
1120.13 251.51 50.97 29.56 1307.07 334.29 51.10 39.57
349.57 100.57 6.40 4.82 186.43 47.33 6.28 7.64
123.59 41.06 2.26 1.97 76.11 16.73 2.56 2.70
test series (see Table 2). This procedure has been established in numerous previous studies. Due to the low dispersion in the test results, a significant difference between the two anchor systems could be shown despite relatively small-scale test series. The statical effect size of the comparison between MITEKÒSuperAnchorÒ and TwinFixÒ 6.5 regarding Fmax was very large (Cohen’s d = 8.31). According to a power analysis, even a smaller number of observations (sample size n = 4 in each group) would still have resulted in sufficient statistical power (1 ß > 0.95) to detect effects of this size. This suggests that the test set-up was suitable for the objective of this study. Neither the load to failure nor the cyclical test resulted in the primary avulsion of any anchor or in the destruction of the attachment tube. Therefore, the stability of the thread and the stitching and knotting technique are the remaining influence factors on the primary stability. In the load to failure test, the maximum load causing the TwinFixÒ 6.5 anchor system to fail was 3.9 times higher than the maximum load causing the MITEKÒ SuperAnchorÒ to fail (mean load:1307 N compared to 334 N). Interestingly enough, the difference between the measured mean load of the two anchor systems is not consistent with the theoretical value, which would result from the extrapolation of the individual load capacity of the system components (Barber et al., 2003). In accordance to the maximum load values, which resulted in the complete destruction of the anchor systems, similar results occurred when measuring the yield load. The values to result in the irreversible damage of the construct were only slightly lower than the maximum values. The classification of an irreversible damage was a considerable decrease in power, which was visible in the testing curve and, for example, manifested itself in the ripping of the thread. Conclusions for the X-ray check-up after a hip transposition type IIb can be drawn from these test results. If a displacement of more than 52 mm is visible on the X-ray, it can be assumed that the implant is irreversibly damaged. Whether this has a clinical relevance as well, depends among other things, e.g. a possible damage of the neo-joint or loss of gait function, on the time of damage. The maximum load capacities measured in the load to failure test were not reached under cyclical load. Nevertheless, the difference between the two anchor systems proved to be statistically significant. The mean load capacity for the TwinFixÒ 6.5 was 1120 N compared to 252 N for the MITEKÒ SuperAnchorÒ. The primary stability of the TwinFixÒ 6.5 is therefore sufficient for partial load and mobilisation of the patient with two forearm crutches
(theoretically needed mean load capacity: 700 N) (Bergmann et al., 2001). By comparison, the primary stability of the MITEKÒ SuperAnchorÒ is theoretically only sufficient for sitting on the edge of the bed. 5. Conclusion Despite the abstract test design, the test series brought forward definite data regarding the primary stability of the reconstruction technique after a hip transposition. The results cannot be transferred directly to the situation in vivo, however, they are remarkable and of clinical relevance. The complexity of the surgery is nearly the same, still, the use of the TwinFixÒ 6.5 with UltrabraidÒ offers the possibility to increase the primary stability by a factor of 3.5 compared to the established reconstruction technique with MITEKÒ SuperAnchorÒ and polyester thread. With regard to the load capacity up until complete failure of the construct, the factor is even 3.9 higher. At least theoretically, the TwinFixÒ 6.5 allows for a postoperative partial load in order to walk on two forearm crutches. The hip transposition has been used, despite the relatively low primary stability of the MITEKÒ SuperAnchorÒ, with good clinical results so far. The suggested use of the TwinFixÒ 6.5 anchor should further shorten the phase of postoperative absolute immobilisation. However, the measured data have to be supported by clinical data. References Abudu, A., Grimer, R.J., Cannon, S.R., Carter, S.R., Sneath, R.S., 1997. Reconstruction of the hemipelvis after the excision of malignant tumours. Complications and functional outcome of prostheses. J. Bone Joint Surg. Br. 79, 773–779. Barber, F.A., Herbert, M.A., Richards, D.P., 2003. Sutures and suture anchors: update 2003. Arthroscopy 19, 985–990. Beadel, G.P., McLaughlin, C.E., Aljassir, F., Turcotte, R.E., Isler, M.H., Ferguson, P., Griffin, A.M., Bell, R.S., Wunder, J.S., 2005. Iliosacral resection for primary bone tumors: is pelvic reconstruction necessary? Clin. Orthop. Relat. Res. 438, 22–29. Bergmann, G., Deuretzbacher, G., Heller, M., Graichen, F., Rohlmann, A., Strauss, J., Duda, G.N., 2001. Hip contact forces and gait patterns from routine activities. J. Biomech. 34, 859–871. Deakin, M., Stubbs, D., Bruce, W., Goldberg, J., Gillies, R.M., Walsh, W.R., 2005. Suture strength and angle of load application in a suture anchor eyelet. Arthroscopy 21, 1447–1451. Enneking, W., Dunham, W., Gebhardt, M., Malawar, M., Pritchard, D., 1990. A system for the classification of skeletal resections. Chir. Organic Mov. 75, 217–240. Gebert, C., Gosheger, G., Winkelmann, W., 2008. Hip transposition as a universal surgical procedure for periacetabular tumors of the pelvis. J. Surg. Oncol. Hillmann, A., Hoffmann, C., Gosheger, G., Rodl, R., Winkelmann, W., Ozaki, T., 2003. Tumors of the pelvis: complications after reconstruction. Arch. Orthop. Trauma Surg. 123, 340–344.
M. Wessling et al. / Clinical Biomechanics 24 (2009) 361–365 Hoffmann, C., Gosheger, G., Gebert, C., Jurgens, H., Winkelmann, W., 2006. Functional results and quality of life after treatment of pelvic sarcomas involving the acetabulum. J. Bone Joint Surg. Am. 88, 575–582. Ozaki, T., Hillmann, A., Bettin, D., Wuisman, P., Winkelmann, W., 1996. High complication rates with pelvic allografts. Experience of 22 sarcoma resections. Acta. Orthop. Scand. 67, 333–338.
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Rupp, S., Georg, T., Gauss, C., Kohn, D., Seil, R., 2002. Fatigue testing of suture anchors. Am. J. Sports Med. 30, 239–247. Schwameis, E., Dominkus, M., Krepler, P., Dorotka, R., Lang, S., Windhager, R., Kotz, R., 2002. Reconstruction of the pelvis after tumor resection in children and adolescents. Clin. Orthop. Relat. Res. 402, 220–235.
Contents lists available at ScienceDirect
Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
The primary stability of the hip transposition type IIb: A biomechanical in vitro study Martin Wessling a,b,*,1, Mirko Aach c,1, Mirko Herbort d, Georg Gosheger a, Jendrik Hardes a, Carsten Gebert a,b a
Department of Orthopaedic Surgery, University Hospital Muenster, Albert-Schweitzer-Str. 33, D-48149 Muenster, Germany Department of Tumour and Revision Surgery, Orthopaedic Hospital Volmarstein, Lothar-Gau-Str. 11, D-58300 Wetter, Germany c Department of Spinal Chord Injuries, University Hospital Bergmannsheil Bochum, Bürkle-de-la-Camp-Platz 1, D-44789 Bochum, Germany d Department of Trauma, Hand and Reconstructive Surgery, University Hospital Muenster, Waldeyerstraße 1, D-48149 Muenster, Germany b
a r t i c l e
i n f o
Article history: Received 7 December 2008 Accepted 21 January 2009
Keywords: Pelvic tumour Hip transposition Primary stability Biomechanical study
a b s t r a c t Background: The hip transposition is firmly established in pelvic sarcoma surgery. However, the primary stability of the hip transposition has not been tested yet so that the mobilisation, respectively the immobilisation of the patient so far solely relied on the experience of the surgeon. The aim of this study was to test the primary stability reliably with the help of a model and to reveal possible differences in stability between currently used anchor systems (TwinFixÒ 6.5 and MITEKÒ SuperAnchorÒ). Methods: A biomechanical model of porcine sacra was developed to document the maximum load capacity (load to failure test) and the performance under cyclic load (100 N, 200 N, 350 N, 700 N, 1400 N, each with 1000 cycles), 28 sacra were tested in total. Macroscopic damages, displacement, yield load, stiffness and Fmax were recorded as well. Findings: The load to failure test results showed a 3,9 times higher maximum load capacity for the TwinFixÒ 6.5 anchor (1307 N) compared to the MITEKÒ SuperAnchorÒ (334 N). The cyclical test revealed that nearly all MITEKÒ SuperAnchorsÒ failed at a load of 350 N. In contrast, the TwinFixÒ 6.5 anchors resisted 4000 cycles up to a load of 1400 N. Interpretation: The TwinFixÒ 6.5 anchor proved to be clearly superior to the MITEKÒ SuperAnchorÒ, resulting in the adjustment of the reconstruction technique. Therefore, the immobilisation period of a patient after a hip transposition type IIb could be shortened according to the results of the primary stability test. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Primary malignant bone sarcomas are rare entities which can be found mostly with young patients and frequently occur in the pelvic area. The reconstruction of defects that develop after wide tumour resection of the osseous pelvis often proves to be difficult and entails complications (Beadel et al., 2005; Ozaki et al., 1996; Abudu et al., 1997; Hillmann et al., 2003; Schwameis et al., 2002). Therefore the hip transposition was developed as a surgical technique less prone to complications, yet achieving good clinical and functional results. However, valid data regarding the primary stability, as opposed to the long term stability with additional scar tissue, of the hip transposition immediately after surgery have not been available until recently (Hoffmann et al., 2006; Hillmann et al., 2003; Gebert et al., 2008). Postoperative treatment of the patient so far relied on the clinical experience of the surgeon without * Corresponding author. Address: Department of Orthopaedic Surgery, University Hospital Muenster, Albert-Schweitzer-Str. 33, D-48149 Muenster, Germany. E-mail address: [email protected] (M. Wessling). 1 Martin Wessling and Mirko Aach contributed equally for this paper. 0268-0033/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2009.01.008
knowledge of the load capacity of the anchor systems. Only results from material testing of the individual components in cyclical as well as in load to failure tests were sufficiently known (Barber et al., 2003; Rupp et al., 2002). Furthermore, the maximum load capacity of the thread anchor proved to be dependent on the load angle (Deakin et al., 2005). A general superiority of the modern anchor systems could be shown; however, the influence of combined materials and multiple anchors has not been tested yet. The objective of this study was to determine the load capacity of the hip transposition and, if necessary, to improve the surgical technique with the use of different materials and anchors. 2. Methods A biomechanical model, using porcine sacra, was developed to represent a type IIb hip transposition (Gebert et al., 2008) – tumour prosthesis with bipolar head, attachment tube and four bone anchors (see Fig. 1) – after an internal hemipelvectomy type I–IV according to Ennking and Dunham (Enneking et al., 1990). A total of 28 sacra was used, 14 for each test series. The porcine sacrum
362
M. Wessling et al. / Clinical Biomechanics 24 (2009) 361–365
material testing machine (Zwick/RoellÒ Z005, Zwick GmbH & Co. KG, D-89079 Ulm) allowed levelling remaining differences in position. Two different anchor systems were tested (TwinFixÒ 6.5, Smith&NephewÒ GmbH, D-45768 Marl and MITEKÒ SuperAnchorÒ, Ethicon GmbH Mitek Devision, D-22844 Norderstedt) in 2 6 test series: The load to failure test to determine Fmax, displacement, yield load and stiffness, and the performance under cyclic load (100 N, 200 N, 350 N, 700 N and 1400 N, each with 1000 cycles). The data was collected and processed using the software TestXpertÒ (see Fig. 3). SPSSÒ 12 was used to analyse the collected data. The test set-up, as well as the test parameters, have been used effectively in similar material testing experiments (see Table 1). 3. Results Fig. 1. Type IIb hip transposition.
was resected perpendicular to the load direction and the neo hipjoint was fixed in the sacrum. Preliminary tests had shown that already a small osseus roof could influence the primary stability significantly. The thread anchors were fixed with a 45° angle onto the cortical bone and connected to the attachment tube. The tube was further attached to the prosthesis with Ethibond 6.0 thread (see Fig. 2a–c). The sacrum of 26 week old pigs, halved in the sagittal plane, was girded for this model orthogradely with PU-foam (H400-ATÒ Voss Chemie GmbH, D-25436 Uetersen) into a mold. The individual difference between the stability of the bones was minimized due to a persistent slaughter age of 26 weeks. The position of the sacrum in regard to the lower extremity was adjusted according to the proportions after a resection. The sacra were consistently fixated on the simulated femur. A great accuracy was achieved by using a 2-KPU-foam (see Fig. 2d and e). Additionally, the equipment of the
Preliminary tests showed pig femora to be unable to resist the tested load and to break along the epiphysical disc. A hip transposition type IIa could therefore not be tested. Furthermore, a blade angle of more than 0° caused considerable derivation of power into the sacrum up to 2500 N and, subsequently, security termination of the testing program. Thus, a tumour prosthesis was used for the final test set-up and a worst-case-scenario for a hip transposition type IIb was tested. 3.1. Load to failure test Load to failure test series were conducted with both TwinFixÒ 6.5 and MITEKÒ SuperAnchorÒ. The MITEKÒ SuperAnchorÒ thread snapped at a mean load of 334 N (min. 259 N, max. 387 N). Avulsion or loosening of anchors as well as untying of knots was not recorded. The mean load of the TwinFixÒ 6.5 anchor at the point of material failure was 1307 N (min. 1054 N, max. 1591 N) (see Table 2 and Fig. 3).
Fig. 2. Development of the biomechanical model, final test set-up and macroscopic damages. (a) Positioning the anchors, (b) pig sacrum with attachment tube, (c) completed biomechanical model, (d) final set-up in the material testing machine, and (e) macroscopic damages after load to failure test.
M. Wessling et al. / Clinical Biomechanics 24 (2009) 361–365
363
Fig. 3. (a) Typical test graph and parameters of load to failure test (TwinFixÒ 6.5); (b) typical test graph of cyclical test; (c) typical test graph of load to failure test; and (d) Boxplot Fmax load to failure test.
Table 1 Test settings of the material testing machine Zwick/RoellÒ Z005. Test settings
Load to failure
Cyclical test
Test rate Transient memory Cycles per performance level Lower reversal point Upper reversal point Test abortion
200 mm/min 0.1 s
350 mm/min 0.1 s 1000 50 N 100 N, 200 N, 350 N, 700 N, 1400 N Decrease in power by 95% of the maximum power reached Load to failure test after 5000 cycles: maximum power > 2500 N Loop gain 20 Loop gain 1 The first 10 cycles at each performance level, after this every 20th cycle
Expansion-regulation Force-regulation Cycles recorded Test cycle
Decrease in power by 95% of the maximum power reached Maximum power > 2500 N
One test cycle at 15 N
3.2. Cyclical test In all test cycles the average displacement for the MITEKÒ SuperAnchorÒ was not significantly higher than for the TwinFixÒ 6.5 (see Table 2). At a load of 300 N, the displacement for the MITEKÒ SuperAnchorÒ was 33 mm, for TwinFixÒ 6.5 only 25 mm. Thus, the displacement was 1.3 times higher when testing the MITEKÒ SuperAnchorÒ. However, the remaining margin of the MUTARSÒ attachment tube might account for a non-detectable part of the displacement measured in this test. The mean yield load was 279 N (min. 144 N, max. 384 N) for the MITEKÒ SuperAnchorÒ and 841 N (min. 555 N, max. 1067 N) for the TwinFixÒ 6.5. The maximum stiffness of the MITEKÒ SuperAnchorÒ with polyester thread showed an average increase in force of 15.9 N/mm (min. 12,4 N/mm, max. 19,4 N/mm). When testing the TwinFixÒ
6.5 and UltrabraidÒ 2-0, the increase was 47 N/mm (min. 47,4 N/ mm, max. 53 N/mm). For a deflection of 1 mm at the highest performance level, a four times higher force is needed for a TwinFixÒ 6.5 anchor than for a MITEKÒ SuperAnchorÒ. The MITEKÒ anchor system failed at a mean load of 252 N (min. 100 N, max. 349 N). The TwinFixÒ 6.5 system failed completely at a mean load of 1120 N (min. 699 N, max. 1399 N). This tendency of a significantly higher load capacity of the TwinFixÒ anchor system appeared throughout all tested parameters, both in the load to failure test and in the cyclical test series (see Table 2). 4. Discussion Since it was not possible to obtain human bone material in sufficient quality and quantity for the test performance, 28 porcine sacra were chosen as an alternative. 14 sacra were used for each
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M. Wessling et al. / Clinical Biomechanics 24 (2009) 361–365
Table 2 Results of the test series measuring the primary stability after hip transposition type IIb. Parameter Load to failure test Fmax (in N) Displacement at Fmax (in mm) Yield load Stiffness (in N/mm) Cyclical test Fmax (in N) Displacement at Fmax (in mm) Fmax (in N) Displacement at Fmax (in mm)
Anchor
n
Mean value
Standard deviation
Mean square error
TwinFix Mitek TwinFix Mitek TwinFix Mitek TwinFix Mitek
6 8 6 8 6 8 6 8
1307.07 334.29 51.10 39.57 840.97 279.01 47.07 15.87
186.43 47.33 6.28 7.64 171.07 86.92 5.40 2.59
76.11 16.73 2.56 2.70 69.84 30.73 2.21 0.92
TwinFix Mitek TwinFix Mitek TwinFix Mitek TwinFix Mitek
8 6 8 6 6 8 6 8
1120.13 251.51 50.97 29.56 1307.07 334.29 51.10 39.57
349.57 100.57 6.40 4.82 186.43 47.33 6.28 7.64
123.59 41.06 2.26 1.97 76.11 16.73 2.56 2.70
test series (see Table 2). This procedure has been established in numerous previous studies. Due to the low dispersion in the test results, a significant difference between the two anchor systems could be shown despite relatively small-scale test series. The statical effect size of the comparison between MITEKÒSuperAnchorÒ and TwinFixÒ 6.5 regarding Fmax was very large (Cohen’s d = 8.31). According to a power analysis, even a smaller number of observations (sample size n = 4 in each group) would still have resulted in sufficient statistical power (1 ß > 0.95) to detect effects of this size. This suggests that the test set-up was suitable for the objective of this study. Neither the load to failure nor the cyclical test resulted in the primary avulsion of any anchor or in the destruction of the attachment tube. Therefore, the stability of the thread and the stitching and knotting technique are the remaining influence factors on the primary stability. In the load to failure test, the maximum load causing the TwinFixÒ 6.5 anchor system to fail was 3.9 times higher than the maximum load causing the MITEKÒ SuperAnchorÒ to fail (mean load:1307 N compared to 334 N). Interestingly enough, the difference between the measured mean load of the two anchor systems is not consistent with the theoretical value, which would result from the extrapolation of the individual load capacity of the system components (Barber et al., 2003). In accordance to the maximum load values, which resulted in the complete destruction of the anchor systems, similar results occurred when measuring the yield load. The values to result in the irreversible damage of the construct were only slightly lower than the maximum values. The classification of an irreversible damage was a considerable decrease in power, which was visible in the testing curve and, for example, manifested itself in the ripping of the thread. Conclusions for the X-ray check-up after a hip transposition type IIb can be drawn from these test results. If a displacement of more than 52 mm is visible on the X-ray, it can be assumed that the implant is irreversibly damaged. Whether this has a clinical relevance as well, depends among other things, e.g. a possible damage of the neo-joint or loss of gait function, on the time of damage. The maximum load capacities measured in the load to failure test were not reached under cyclical load. Nevertheless, the difference between the two anchor systems proved to be statistically significant. The mean load capacity for the TwinFixÒ 6.5 was 1120 N compared to 252 N for the MITEKÒ SuperAnchorÒ. The primary stability of the TwinFixÒ 6.5 is therefore sufficient for partial load and mobilisation of the patient with two forearm crutches
(theoretically needed mean load capacity: 700 N) (Bergmann et al., 2001). By comparison, the primary stability of the MITEKÒ SuperAnchorÒ is theoretically only sufficient for sitting on the edge of the bed. 5. Conclusion Despite the abstract test design, the test series brought forward definite data regarding the primary stability of the reconstruction technique after a hip transposition. The results cannot be transferred directly to the situation in vivo, however, they are remarkable and of clinical relevance. The complexity of the surgery is nearly the same, still, the use of the TwinFixÒ 6.5 with UltrabraidÒ offers the possibility to increase the primary stability by a factor of 3.5 compared to the established reconstruction technique with MITEKÒ SuperAnchorÒ and polyester thread. With regard to the load capacity up until complete failure of the construct, the factor is even 3.9 higher. At least theoretically, the TwinFixÒ 6.5 allows for a postoperative partial load in order to walk on two forearm crutches. The hip transposition has been used, despite the relatively low primary stability of the MITEKÒ SuperAnchorÒ, with good clinical results so far. The suggested use of the TwinFixÒ 6.5 anchor should further shorten the phase of postoperative absolute immobilisation. However, the measured data have to be supported by clinical data. References Abudu, A., Grimer, R.J., Cannon, S.R., Carter, S.R., Sneath, R.S., 1997. Reconstruction of the hemipelvis after the excision of malignant tumours. Complications and functional outcome of prostheses. J. Bone Joint Surg. Br. 79, 773–779. Barber, F.A., Herbert, M.A., Richards, D.P., 2003. Sutures and suture anchors: update 2003. Arthroscopy 19, 985–990. Beadel, G.P., McLaughlin, C.E., Aljassir, F., Turcotte, R.E., Isler, M.H., Ferguson, P., Griffin, A.M., Bell, R.S., Wunder, J.S., 2005. Iliosacral resection for primary bone tumors: is pelvic reconstruction necessary? Clin. Orthop. Relat. Res. 438, 22–29. Bergmann, G., Deuretzbacher, G., Heller, M., Graichen, F., Rohlmann, A., Strauss, J., Duda, G.N., 2001. Hip contact forces and gait patterns from routine activities. J. Biomech. 34, 859–871. Deakin, M., Stubbs, D., Bruce, W., Goldberg, J., Gillies, R.M., Walsh, W.R., 2005. Suture strength and angle of load application in a suture anchor eyelet. Arthroscopy 21, 1447–1451. Enneking, W., Dunham, W., Gebhardt, M., Malawar, M., Pritchard, D., 1990. A system for the classification of skeletal resections. Chir. Organic Mov. 75, 217–240. Gebert, C., Gosheger, G., Winkelmann, W., 2008. Hip transposition as a universal surgical procedure for periacetabular tumors of the pelvis. J. Surg. Oncol. Hillmann, A., Hoffmann, C., Gosheger, G., Rodl, R., Winkelmann, W., Ozaki, T., 2003. Tumors of the pelvis: complications after reconstruction. Arch. Orthop. Trauma Surg. 123, 340–344.
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