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New method for detection of complex 3D fracture motion - Verification of an optical motion analysis system for biomechanical studies
- Stefan Doebele†1Email author,
- Sebastian Siebenlist†1,
- Helen Vester1,
- Petra Wolf2,
- Ulrich Hagn3,
- Ulrich Schreiber1,
- Ulrich Stöckle4 and
- Martin Lucke1
© Doebele et al; licensee BioMed Central Ltd. 2012
Received: 11 June 2011
Accepted: 9 March 2012
Published: 9 March 2012
Fracture-healing depends on interfragmentary motion. For improved osteosynthesis and fracture-healing, the micromotion between fracture fragments is undergoing intensive research. The detection of 3D micromotions at the fracture gap still presents a challenge for conventional tactile measurement systems. Optical measurement systems may be easier to use than conventional systems, but, as yet, cannot guarantee accuracy. The purpose of this study was to validate the optical measurement system PONTOS 5M for use in biomechanical research, including measurement of micromotion.
A standardized transverse fracture model was created to detect interfragmentary motions under axial loadings of up to 200 N. Measurements were performed using the optical measurement system and compared with a conventional high-accuracy tactile system consisting of 3 standard digital dial indicators (1 μm resolution; 5 μm error limit).
We found that the deviation in the mean average motion detection between the systems was at most 5.3 μm, indicating that detection of micromotion was possible with the optical measurement system. Furthermore, we could show two considerable advantages while using the optical measurement system. Only with the optical system interfragmentary motion could be analyzed directly at the fracture gap. Furthermore, the calibration of the optical system could be performed faster, safer and easier than that of the tactile system.
The PONTOS 5 M optical measurement system appears to be a favorable alternative to previously used tactile measurement systems for biomechanical applications. Easy handling, combined with a high accuracy for 3D detection of micromotions (≤ 5 μm), suggests the likelihood of high user acceptance. This study was performed in the context of the deployment of a new implant (dynamic locking screw; Synthes, Oberdorf, Switzerland).
Various conditions are important for sufficient fracture-healing. In addition to adequate blood supply and a reduction in fracture size, axial interfragmentary motion is one of the most important factors for indirect (secondary) bone healing [1–6]. A deficiency in callus formation and delayed or non-union have been reported in diverse studies as the result of inadequate interfragmentary movements [1, 7–11]. The optimal range for this micromotion seems to be 400 μm . Therefore, current biomechanical analyses have focused on the development of osteosynthetic implants for optimal interfragmentary motion [1, 2, 9, 12]. In the context of the development of new implants biomechanical tests are highly important. The purpose of this study was to find a motion analysis system for biomechanical tests, which allows the detection of three-dimensional interfragmentary motion with a high accuracy directly at the fracture gap of biomechanical specimens (osteosyntheses). Conventional tactile measurement systems are highly accurate (up to1 μm), but the test is time-consuming, laborious, and at times defective. A further disadvantage of tactile systems is the capture of movement direction in only 1 dimension per tactile unit in most cases. The integration of tactile measurement systems in an existing biomechanical set up could be exceedingly difficult. The integration of optical measurement systems in established biomechanical set ups is easy . Using optical systems there is no interaction between test set up and measurement system. The detection of three-dimensional motion is also possible. But is it possible to detect interfragmentary motion in a range of about 400 μm [8, 14]? The optical measurement system PONTOS 5 M (GOM - Optical Measuring Techniques, Braunschweig, Germany) is an established system for motion analysis in the automotive and aerospace industry (used for crash-tests and vibration-analysis of airplane wings). In this study we evaluate PONTOS 5 M for the detection of 3D interfragmentary micromotion in a standardized fracture model. For reference, we used a tactile measurement system consisting of 3 dial indicators designed at the German aerospace centre DLR (Oberpfaffenhofen, Germany). We hypothesized that the optical measurement system would provide the same or higher accuracy for detecting fracture gap movements as the conventional system.
Indirect measurement of the fracture movement
Direct measurement of the fracture movement
Mean differences and 95% limits of agreement
-5 [-45; 35]
-37 [-77; 3]
-7 [-48; 33]
-32 [-80; 16]
-0.04 [-0.13; 0.05]
-21 [-61; 19]
-27 [-83; 29]
-0.02 [-0.15; 0.10]
-37 [-77; 3]
-21 [-86; 44]
-0.01 [-0.17; 0.16]
-52 [-92; -12]
-15 [-89; 59]
0.01 [-0.20; 0.22]
-66 [-106; -25]
-9 [-93; 75]
0.03 [-0.23; 0.28]
-81 [-121; -41]
-2 [-97; 93]
0.04 [-0.26; 0.34]
-93 [-134; -53]
5 [-100; 111]
0.06 [-0.29; 0.42]
-106 [-146; -65]
13 [-105; 131]
0.09 [-0.32; 0.49]
-122 [-163; -82]
21 [-109; 150]
0.11 [-0.35; 0.57]
-138 [-179; -98]
27 [-112; 167]
0.13 [-0.38; 0.64]
The aim of the present study was to validate the efficacy of the optical measurement system PONTOS 5 M compared with a conventional tactile measurement system. The results show that both measurement systems were capable of analyzing interfragmentary movements with high accuracy (resolution of about ≤5 μm). However, the optical measurement system was able to analyze 3D motions whereas the tactile system used in this study only performed 2D measurements of the fracture motion. For this reason it was only possible to compare the 2D data obtained using the optical measurement system with the corresponding data of the tactile measurement system. This is one limitation of the present study.
In addition to the high accuracy of the optical measurement system, this system is much easier to use in comparison to the dial indicator method, because the passive markers are self-adhesive and, as mentioned, can be attached nearly anywhere. The dial indicators, in contrast, each requires a specially produced crank for attachment. This setup is not only time-consuming, but also expensive, and the accuracy of the measurement depends directly on the cranks. In contrast, each passive marker of the optical measurement system functions like a 6-DOF-sensor, so that several different data sets for the object can be obtained. Due to its high accuracy, PONTOS 5 M is regularly used in the automobile and airplane industry for car crash-tests or vibration-analysis of airplane wings. Therefore, large amounts of data are available from different testing setups. In biomechanical setups for musculoskeletal research, diverse types of fracture models have been validated. We decided to use a simple model with a transverse fracture gap in our study in order to exclude measurement deviations as far as possible. There are some publications that deal with the application of optical measuring systems in biomechanics. Here also the accuracy of the systems was part of the research. A common system in the biomechanical field is the Vicon system [15, 16]. A study by Windolf et al showed an accuracy of 64 ± 5 microns using the Vicon-460 system . Arbitrary changes in camera arrangement revealed variations in mean accuracy between 76 and 129 μm. This is less accurate than measuring with the PONTOS System.
In this study, we attempted to expand the applicability of the PONTOS 5 M optical measurement system for biomechanical assessments. As the need for implant improvement, especially with regard to plate stiffness, is acute, the need for an accurate, validated, easy-to-handle 3D measurement system is also high. Within the framework of the presented data and the limitation of only 2D evaluation, we can say that the PONTOS 5 M optical measurement system appears to be a favourable alternative to previously used tactile measurement systems for biomechanical applications. Easy handling combined with a high accuracy (≤ 5 μm) and 3D detection of motions suggests the likelihood of high user acceptance. The use of simple passive markers suggest an easier handling at cadaver models as compared to mechanical devices, which need accurate disinfection after usage.
Optical measurement system
Reference measurement system
For reference measurements we used digital indicators from the company MAHR (Marcator 1086; Mahr, Goettingen, Germany). To date, dial indicators represent the gold standard in measuring motion in the range of micrometers. We used 3 digital indicators. Each dial gauge can specify one degree of freedom. The resolution of the individual gauge was 1 μm (Span of error 5 μm, Repeatability 2 μm). This accuracy is ensured by the company. For positioning the 3 dial indicators to the osteosynthesis two cranks were constructed and made for measure using a cnc milling machine (Figure 6). The measurement set up was developed in cooperation with the German Aerospace Center (DLR, Oberpfaffenhofen, Germany). The dial indicators were connected to a computer system by an usb interface. The data recording was carried out using the supplied software from Mahr. Due to the placement of the indicators far from the fracture gap, the interfragmentary motion could not be detected directly. Instead, interfragmentary motions were indirectly measured by analyzing the motion of the measuring spindle of the dial indicators. The motion at the fracture gap has to be calculated using the geometry of the cranks and the values of the dial indicators. Using the values of digital indicator 1 and 2 it was possible to calculate the z deviation (Δz) and the angle alpha (Δα) between the both cylinder axis (fracture fragments) (Figure 6). Using digital indicator 3 it was possible to calculate the y deviation (Δy). To compare the accuracy of the two measurement systems (PONTOS 5 M vs tactile measurement system), we conducted an additional measurement. For this purpose, the passive markers of the optical measurement system were attached directly to the spindles of each of the 3 indicators (Figure 5). With this set up the motion of the spindles could be detected with the optical measurement system, by measuring the point to point motion. Both values (digital indicator and PONTOS) could be compared. Simultaneous data collection was achieved by trigger points. A total of 3 data-sets, 200 values per set, were collected, 1 for each dial indicator.
For analyzing the agreement between the optical measurement system PONTOS 5 M and the indirect measuring by the dial indicators Bland-Altman-Plots were calculated. For each comparison we checked the assumption of uniform differences and uniform variability. If these assumptions were violated a regression approach was applied. We used a Generalized Estimation Equation Model (GEE) to account for the different measurements made in one specimen (the force was varied between 0 and 100 N in increments of 10 N for each specimen, which results in 11 measurements per specimen). The results of this model were used to calculate mean differences and 95% limits of agreement as described by Bland and Altman [17, 18].
Daniel Andermattn(Synthes, Oberdorf, Switzerland) and Oliver Erne (GOM - Optical Measuring Techniques, Braunschweig, Germany) for the participation in design and coordination of the study.
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