- Email: [email protected]
Mechanical Loading 2
Journal of Biomechanics 34 (2001) S37–S42
Mechanical Loading 2 Comparison of intradiscal pressures and spinal fixator loads for different body positions and exercises H.-J. Wilkea, A. Rohlmannb, G. Bergmannb, F. Graichenb, L.E. Claesa a
Institute for Orthopaedic Research and Biomechanics, University of Ulm, Germany b Biomechanics Laboratory, UKBF, Free University of Berlin, Germany
1. Introduction Loading of the spine is still not well understood. The most reliable results seemed to come from intradiscal pressure measurements from Nachemson. A new similar study complemented this study and confirmed some of the earlier data; however, it is contradicted in others. The new data did not confirm that the load is higher in sitting compared to standing and did not find distinct differences in the lying positions. The objective of this study was to compare results from two independent in vivo studies applying different methods to provide information about spinal loading. In one study the intradiscal pressure was measured, in an other study the loads on an internal spinal fixator was measured. 2. Methods A flexible pressure transducer with a diameter of 1.5 mm and advanced technology was inserted under sterile surgery conditions in the nucleus pulposus of a non-degenerated L4-5 disc of a 45-years-old male volunteer1. A piezoresistive pressure sensor was integrated in a 7mm-long metal tip. The intradiscal pressure was recorded with a telemetry system over a period of 24h. Many different situations were studied. The pressures for all activities were related to the standing position. Telemeterized, bisegmental internal spinal fixators allowed the measurement of the fixator loads2. These modified implants have an integrated measuring cartridge containing six load sensors, a telemetric unit and a coil for the inductive power supply. For the measurements a flat coil and a small antenna were placed on the patient’s back. The patients were videotaped and the load-dependent signals of the telemetries of the left and right fixator were stored on the same video tape. A monitor allowed on-and off-line display of implant loads. Modified fixators were implanted in 10 patients for clinical reasons. Generally, 2–4 weeks later anterior interbody fusion was performed using autografts from the iliac crest. The bending moment is the most important load component and therefore presented here.
Fig. 1. Intradiscal pressures and flexion bending moments in the fixators for different body positions and activities. The values are given in per cent of those for standing.
column, as during flexion of the upper part of the body the bending moment in the fixator rods underestimated the real load. The combination of these two methods may improve the understanding of the biomechanical behaviour of the lumbar spine and may be used to validate models and theories of spinal loading.
3. Results
Acknowledgements
The relative differences of the intradiscal pressure and the flexion bending moments in the fixators corresponded in most cases (Fig. 1). Both studies showed slightly lower loads for sitting than for standing and comparable low loads in all lying positions.
The study was financially supported by the DFG (Ro 581/7-2, WI 1352/2-1) and by Kieser Training AG, Zu¨rich.
4. Discussion and conclusions The spinal load differences for most acitivies were similar in both studies. Only when the load was predominately carried by the anterior PII: S 0 0 2 1 - 9 2 9 0 ( 0 1 ) 0 0 1 2 4 - 5
References 1 2
Wilke H-J, Neef P, Caimi M et al. (1999) Spine 24:755–62. Rohlmann A, Bergmann G, Graichen F. (1994) J Biomech 27: 961–7.
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Abstracts / Journal of Biomechanics 34 (2001) S37–S42
Intraarticular pressure measurement in the wrist joint in vivo D.A.Riklia, P.Christb, A.Cristallib, T.Mittlmeierc, M.Mu¨ller-Gerbld, G.Dudae, P.Regazzonif a
Kantonsspital, Aarau, Switzerland NOVEL GmbH, Munich; Germany c Charite´ University, Traumatology, Berlin, Germany d Ludwig-Maximilian-University, Anatomy, Munich, Germany e Research Laboratory, Charite´ University, Berlin, Germany f University Hospital, Traumatology, Basel, Switzerland b
1. Introduction
4. Discussion and conclusions
Fractures of the distal radius are the most common fractures in humans. There is still controversy about the treatment strategies for these injuries. More recently it has been advocated that treatment should exist in restoring extraarticular anatomy and joint congruency by open reduction, followed by rigid internal fixation and early function1. However, the ideal implants and techniques do not exist yet. Drawbacks with standard techniques (3.5mm Titanium-plate) consist in poor screw purchase in the radial epiphysis, interference with extensor tendons and need for secondary procedures (implant removal). Numerous efforts are currently undertaken to improve operative techniques and implant design and material2–8. In two clinical studies and supported by biomechanical data it has been shown that internal fixation is reliable with two small implants (2.0mm titanium plates) placed at an angle of 50-701 on the dorsum of the distal radius3,6,7. However, to further improve this promising technique, a fracture model is needed for implant testing. To establish a reliable test setting the amount of forces that are transmitted across the radiocarpal and ulnocarpal joint (the ‘‘wrist joint’’) during physiological loading should be known. These biometrical data do not exist to date.
To our knowledge these are the first data on force transmission across the human wrist joint measured in vivo. Further measurements are actually planned in the same setting on 10 healthy volunteers to confirm these data. The data will allow to estimate the forces that have to be neutralized by internal fixation and early motion of a distal radius fracture until bony union has occurred. Cadaver models for implant testing and evaluating force transmission and force distribution in the wrist joint can be adjusted according to these in vivo data. Additionally, the data have a potential value in the evaluation of carpal injuries, Kienbo¨ck’s disease, and corrective procedures about the wrist joint (e.g. unloading operations: ulnar shortening, weaver procedure and others). The technology developed for this study can also be used for in vitro cadaver testings. The data from this study have to be confirmed by further measurements. They potentially have a value in the study of the human wrist joint, in physiologic aswell as pathologic conditions.
Acknowledgements
2. Methods
The study is supported by a grant of the AO-ASIF foundation.
To measure intraarticular pressure in the wrist joint in vivo, a capacitative pressure sensor adapted to the anatomy of the radiocarpal and ulnocarpal joint surface was designed. The behavior of the sensor with regards to change in temperature, humidity and sterilization was evaluated in vitro. The sensor was further validated in cadaver tests. The sensor was then introduced in the wrist joint of two healthy volunteers from a radial surgical approach under local anesthesia. Pressure was measured as a baseline pressure in the wrist joint, during active and passive motion and during fist clench. The protocol was established according to the guidelines of the European Community and approval by the local ethic commitee was obtained. 3. Results The total amount of force transmitted across the wrist joint measured by the sensor was 25N (baseline value), 70N during motion and up to 300N with fist clench.
References 1
Jupiter JB. (1991). J Bone Joint Surg 73-A: 469. Carter PR, Frederick HA, Laseter GF. (1998). J Hand Surg 23-A: 300. 3 Rikli D, Regazzoni P. J Bone Joint Surg (1996). 78-B: 588-92. 4 Ring D, Jupiter JB, Brennwald J et al. (1997). J Hand Surg 22-A: 777. 5 Zimmermann R, Gabl M, Pechlaner S et al. (1998). Unfallchirurg 101: 762–8. 6 Jakob M, Rikli D, Regazzoni P. (2000). J Bone Joint Surg 85-B: 3404. 7 Peine R, Rikli D, Hoffmann R et al. (2000). J Hand Surg 25A: 29–33. 8 Schilling CH, Schiefer H, Curtis R, Rikli D. (1999). Poster AO Advanced course, Davos. 2
Loads on an internal spinal fixator during sitting A. Rohlmann, F. Graichen, G. Bergmann Biomechanics Laboratory, UKBF, Free University of Berlin, Germany
1. Introduction It is assumed that spinal load in the lumbar region depends on the degree of lordosis and the inclination of the pelvis. Sitting may involve
different degrees of lordosis and pelvic inclination. Thus spinal load may depend on the type of seat and the sitting posture. The aims of this study were to measure implant loads in 10 patients for sitting on different types of seats and to compare implant loads for sitting erect and relaxed.
Abstracts / Journal of Biomechanics 34 (2001) S37–S42
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2. Methods Telemeterized bisegmental internal spinal fixators were used for in vivo measurements of spinal loads1. The inductively powered fixators were implanted in 10 patients and allowed the measurement of three force components and three moments. The patients were videotaped during the measurements, and the load-dependend signals of the two telemetries were stored on the same videotape. Forces and moments were calculated from the telemetric signals and shown online on a computer monitor. During a measuring session, patients successively sat on a stool, a chair, an office chair, a bench, a physiotherapy ball, and a knee-stool. While sitting on a stool or chair, the patients were asked to sit relaxed and afterwards to straighten and extend their back. This exercise was repeated during several measuring sessions. Previous studies2,3 have shown that the bending moment in the sagittal plane is the most important load component. Thus, only the bending moments in the fixators are compared here.
3. Results The type of seats had only a minor effect on bending moments in the fixators. Implant loads differed considerably in some patients when they changed the type of seat. However, a significant difference was only found between sitting on a bench and on a knee-stool with slightly higher values for the latter. Sitting erect and actively straightening the spine as taught in some back schools increased the bending moment in the fixators by an average of about 11% compared to sitting relaxed (Fig. 1). There were marked interindividual differences in normalized fixator bending moments.
Fig. 1: Comparison of normalized fixator bending moments for sitting relaxed (100%) and erect.
higher for sitting erect than relaxed4. Sitting erect requires additional muscle forces which cause higher spinal and implant loads. The fact that the loads even for sitting erect are lower than for walking and much lower than for lifting and carrying a weight4 argues against the assumption that the higher load for sitting erect is the reason for low back pain, which sometimes occurs during sitting.
Acknowledgements Funding for this study was obtained from the Deutsche Forschungsgemeinschaft (Ro 581/7-2).
4. Discussion and conclusions Loads on internal spinal fixation devices caused by sitting were measured in 10 patients. There was marked interindividual variation, partially due to differences in the indication for surgery (compression fracture, degenerative instability) and the surgical procedure (compression or distraction of the bridged region)2,3. Load differences were negligible for sitting on different types of seats. Intradiscal pressure likewise varied only slightly for sitting on different types of seats4. Sitting erect and actively straightening the spine led to higher implant loads than sitting relaxed. Intradiscal pressure was about 10%
References 1
Rohlmann A, Bergmann G, Graichen F. (1994). J Biomech 27 : 961–7. 2 Rohlmann A, Graichen F, Bergmann G.(1999). Eur Spine J 8: 354–9. 3 Rohlmann A, Graichen F, Bergmann G, Weber U. (2000). Spine 25 : 2981–6. 4 Wilke H.-.J., Neef P., Caimi M., Hoogland T., Claes L.E. (1999). Spine 24: 755–62.
Impact loading of human articular cartilage L.V. Burgin, R.M. Aspden Department of Orthopaedics, University of Aberdeen, Foresterhill, Aberdeen, AB25 2ZD, UK
1. Introduction
2. Methods
Articular cartilage provides a resilient and compliant articulating surface to the bones in diarthrodial joints. It protects the joint by distributing applied loads, hence preventing potentially damaging stress concentrations, and provides a low-friction bearing surface to enable free movement of the joint. The relationship between the material properties of bone and cartilage is not entirely clear and how changes in one affect the ability of the other to withstand loading is under investigation.
A purpose built drop tower has been constructed to apply controlled impact loads to cores of articular cartilage, both isolated and in situ on bone. The drop tower consists of a stainless-steel cage comprising three vertical rods in which a cylindrical impactor can fall freely. Impactors of various masses are used and can be dropped from different heights. The cartilage and bone sample under test is positioned on top of a quartz force link (Kistler Instruments Ltd.) fixed at the base of the cage. An accelerometer attached to the impactor measures deceleration
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Abstracts / Journal of Biomechanics 34 (2001) S37–S42
Table 1 Maximum dynamic stiffness values for human cartilage impacted from two different drop heights using a 100g mass. Height (mm)
Velocity(m s 1)
Stiffness (MPa)
145 90
1 69 1 33
36 074 5 29 5710 4
of the impactor during impact. Integrating this signal provides a measure of the distance travelled by the impactor as it compresses the tissue core. Data are collected and processed using LABVIEW software. 3. Results The difference in stiffness for two different drop heights is shown in Table 1 for the same elderly, osteoporotic femoral head. Comparing samples from the superior and inferior aspects of a femoral head yielded values for the dynamic stiffness of 7579MPa for the superior region and 102732MPa for the inferior region. The average cartilage thickness in these areas was 1.83 and 1.14mm, respectively. For an impact load followed by unloading, the force-deflection curve forms a hysteresis loop, Fig. 1. The area contained within the loop increases with impact velocity.
4. Discussion and conclusions The stiffness of cartilage subjected to impact loading is greater than that normally measured in quasi-static loading. The average strain rates experienced by the cartilage in these tests were between 1300s 1 and 1700s 1. At these rates there is no opportunity for fluid flow through the matrix. The maximum force occurs before the maximum displacement of the impactor and is a characteristic of such viscoelastic impacts. The
Fig. 1. Force-deflection curves for different impact velocities for samples of human cartilage.
irreversible internal deformation that dissipates some of the initial kinetic energy can also be seen. The reversal of generally accepted stiffness over the femoral head is probably due to the much thinner cartilage found in the inferior region. This stiffness is not the same as the elastic modulus and is not, therefore, an intrinsic property of the material in this case. It is a reflection of the ability of the sample to absorb the impact by deformation.
Acknowledgements We thank the ARC for funding this project, the MRC for a Senior Fellowship for RMA and the engineering workshop in the Department of Biomedical Physics for construction of the drop tower.
Dose and time-dependent adaptation of mechanical strength and morphology of the epiphyseal and articular cartilage in the rat model A. Niehoffa, U.G. Kerstingb, G.-P.Bru¨ggemanna, M.M. Morlockc, K. Sellenschlohc, F. Zaucked, H. Michnaf a
Institute for Biomechanics, German Sport University of Cologne Insitute of Individual Sports, German Sport University of Cologne c Biomechnics Section, Tecnical University of Hamburg-Harburg d Institute for Biochemistry II, University of Cologne f Institute for Morphology and Tumorresearch, German Sport University Cologne b
1. Introduction Various studies show that mechanical loading has an influence on cartilage but the results are contradictory 1–3. The purpose of the study was to examine the effect of mechanical load on epiphyseal and articular cartilage dependent on dose and time of exercise. A set of mechanical, histological and immunohistological parameters is proposed and the correlation between these properties dependent on the amount of the running exercise will be investigated.
2. Methods Forty five female Sprague-Dawley rats (starting age was 3 weeks) were randomly assigned to a high-trained exercise group (n=15), a
low-trained exercise group (n=15) and a non-active control group (n=15). The exercise groups were trained in a running wheel with voluntary exercise and the running distance was monitored by a computer system. Food and water were available ad libitum and the rats housed on a 12h/12h light/dark cycle. Five animals of each group were sacrified after 4 weeks, the next five after 8 weeks and the last five after 12 weeks. The rats were decapitated and both hindlimbs were detached at the hip joint. The knee joint was dissected carefully and femur and tibia were obtained. The bones of the left hindlimb were frozen at 801C until mechanical testing on a material test machine (Zwick Z2.5/TN 1S). With the tibia a relaxation test was performed using a solid 0.5mm indenter. A microscope with an angular scale was used to orientat the specimens so that the indenter approached the surface perpendicularly. A step load of 3.5 7 0.15N was
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Abstracts / Journal of Biomechanics 34 (2001) S37–S42 applied and the achieved deformation held constant for 250s. Shortterm and long-term relaxation were quantified by calculation of the relative load after 1 and 250s. Cartilage thickness was determined by a needle probe measurement. The femur was placed horizontally so that the distal growth plate was loaded in shear direction. The load was applied with a c-formed stamp adapted individually in the form of the plate from lateral to medial on the epiphysis. The specimens were preloaded with 0.05N and three cycles of a stress-relaxation series began. For each cycle a shear load of 6N was first applied, followed by relaxation at that stress for 500s. Then the load was reduced to 0.5N for 500s. At the end of the series the specimens were loaded to failure at a rate of 15N/s. We determined the relaxation behavior 3s after applied load cycles and the relative ultimate shear stress was also measured. The growth plate area was calculated from the femoral epiphysis diameters.
4. Discussion and conclusion We conclude that physical activity has an effect on the cartilage mechanics. The amount of the running exercise also plays a role. The results have to be interpreted in accordance to the histomorphometry and immunhistology (in preparation). Differences in stainning pattern for collagen and other matrix proteins will be discussed. The theoretical analysis for the mechanical test results has to be extended to derive structural parameters, which characterize the matrix composition.
References 1
3. Results The strength and the relaxation behavior from the cartilage of the distal femur epiphyseal plate and the proximal tibial articular was greater for the high-trained rats than the low-trained and control group. The differences between the low trained and the non-active rats were very small.
Kaab M.J, Ito K, Clark J.M, Notzli H.P. (1998). J Orthop Res 16(6): 743–751. 2 Jurvelin J, Kivirana I, Sa¨a¨ma¨nen A-M., Tammi M,. Helminen H.J. (1990). J Biomech 23:1239–1246. 3 Pap G, Eberhardt R, Sturmer I, Machner A, Schwarzberg H, Roessner A, Neumann W. (1998). Pathol Res Pract 194(1), 41–7.
Medical aspects of the load transfer mechanism of an implant-bone connection in the femur using gamma radiation for fixing the experimental information G.V.Skrbenskya, F. Gottsauner-Wolffb, R. Beerb, W. Grienauerc, J. Eberhardsteinerb a
Department of Orthopedic Surgery, Biomechanic Resaerch Laboratory, Univ. of Vienna b Vienna University of Technology c Austrian Research Centre Seibersdorf
1. Introduction The in growth of the bone and the achievement of the secondary (longtime) stability is prevented by a peak pressure causing reactive bone resorption as well as by an incomplete fit which leads to loosening of the connection because of possible local micro-movements1. Torsion may also cause axial loosening (screw-effect) of the connection before secondary stability is reached. In the Viennese model the transfer of pressure is carried out by four large rips (Fig.1). In the Bologna model (Fig. 2), the head has five small ribs to carve into the inner cortical bone.
A comparison with the corresponding figures for the Viennese model shows that there is a big difference in the magnitude and the distribution of the respective stressfields. In case of the Bologna model the fringe pattern in the near vicinity of the ribs are quite similar to those around an opening crack.
2. Methods The stress distribution in the load-transfer areas were investigated using three-dimensional photoelasticity. However, due to the fact that the models consist of two parts with different mechanical and thermal properties, any thermal treatments must be avoided. Therefore, a new method for fixing the photoelastic fringes was used2,3 by applying gamma-radiation to partially polymerized Araldite B hardened with malein acid anhydride.
Fig. 1.
3. Results Fig. 3 show typical fringe distributions for the Viennese model (taken in a dark and a brightfield polariscope, torque moment M t = 6dNm). Fig. 4 show the same for the Bologna model, the contour of the ribs of this model is triangular with a very sharp ridge.
Fig. 2.
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Abstracts / Journal of Biomechanics 34 (2001) S37–S42 significant is the effect due to a non-equal contact between the ribs and the bone (non perfect form fit between the femur and the prosthesis) especially in case of the Viennese model. The investigation allows to compare different implants under certain biomechanic conditions. References Fig. 3.
Fig. 4.
4. Discussion and conclusion A comparison between this two implants shows the significant influence of the different load transfer mechanisms. However, most
1
Gottsauner-Wolf F, Plenk Jr. H. (1985). Die Knochenverankerung von Tumor endoprothesen mit angegossenen Ku¨gelchen aus Kobalt-Basislegierung; O rthopa¨die 16: 252–257. 2 Beer RJ, Wendrinsky J, Grienauer W, Jecic S. (1995). Threedimensional photoelasticity using new radiation cured polymers. Proceedings. of the 12th Danubia-Adria Symposium. Sopron, Hungary 5–7. October. 3 Beer RJ., Kodvanj J, Grienauer, W, Schaudy R. Different treatments of radiation-cured and their influence on the long-time behaviour; Proceedings. of the 15th Danubia-Adria Symposium.
Mechanical Loading 2 Comparison of intradiscal pressures and spinal fixator loads for different body positions and exercises H.-J. Wilkea, A. Rohlmannb, G. Bergmannb, F. Graichenb, L.E. Claesa a
Institute for Orthopaedic Research and Biomechanics, University of Ulm, Germany b Biomechanics Laboratory, UKBF, Free University of Berlin, Germany
1. Introduction Loading of the spine is still not well understood. The most reliable results seemed to come from intradiscal pressure measurements from Nachemson. A new similar study complemented this study and confirmed some of the earlier data; however, it is contradicted in others. The new data did not confirm that the load is higher in sitting compared to standing and did not find distinct differences in the lying positions. The objective of this study was to compare results from two independent in vivo studies applying different methods to provide information about spinal loading. In one study the intradiscal pressure was measured, in an other study the loads on an internal spinal fixator was measured. 2. Methods A flexible pressure transducer with a diameter of 1.5 mm and advanced technology was inserted under sterile surgery conditions in the nucleus pulposus of a non-degenerated L4-5 disc of a 45-years-old male volunteer1. A piezoresistive pressure sensor was integrated in a 7mm-long metal tip. The intradiscal pressure was recorded with a telemetry system over a period of 24h. Many different situations were studied. The pressures for all activities were related to the standing position. Telemeterized, bisegmental internal spinal fixators allowed the measurement of the fixator loads2. These modified implants have an integrated measuring cartridge containing six load sensors, a telemetric unit and a coil for the inductive power supply. For the measurements a flat coil and a small antenna were placed on the patient’s back. The patients were videotaped and the load-dependent signals of the telemetries of the left and right fixator were stored on the same video tape. A monitor allowed on-and off-line display of implant loads. Modified fixators were implanted in 10 patients for clinical reasons. Generally, 2–4 weeks later anterior interbody fusion was performed using autografts from the iliac crest. The bending moment is the most important load component and therefore presented here.
Fig. 1. Intradiscal pressures and flexion bending moments in the fixators for different body positions and activities. The values are given in per cent of those for standing.
column, as during flexion of the upper part of the body the bending moment in the fixator rods underestimated the real load. The combination of these two methods may improve the understanding of the biomechanical behaviour of the lumbar spine and may be used to validate models and theories of spinal loading.
3. Results
Acknowledgements
The relative differences of the intradiscal pressure and the flexion bending moments in the fixators corresponded in most cases (Fig. 1). Both studies showed slightly lower loads for sitting than for standing and comparable low loads in all lying positions.
The study was financially supported by the DFG (Ro 581/7-2, WI 1352/2-1) and by Kieser Training AG, Zu¨rich.
4. Discussion and conclusions The spinal load differences for most acitivies were similar in both studies. Only when the load was predominately carried by the anterior PII: S 0 0 2 1 - 9 2 9 0 ( 0 1 ) 0 0 1 2 4 - 5
References 1 2
Wilke H-J, Neef P, Caimi M et al. (1999) Spine 24:755–62. Rohlmann A, Bergmann G, Graichen F. (1994) J Biomech 27: 961–7.
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Abstracts / Journal of Biomechanics 34 (2001) S37–S42
Intraarticular pressure measurement in the wrist joint in vivo D.A.Riklia, P.Christb, A.Cristallib, T.Mittlmeierc, M.Mu¨ller-Gerbld, G.Dudae, P.Regazzonif a
Kantonsspital, Aarau, Switzerland NOVEL GmbH, Munich; Germany c Charite´ University, Traumatology, Berlin, Germany d Ludwig-Maximilian-University, Anatomy, Munich, Germany e Research Laboratory, Charite´ University, Berlin, Germany f University Hospital, Traumatology, Basel, Switzerland b
1. Introduction
4. Discussion and conclusions
Fractures of the distal radius are the most common fractures in humans. There is still controversy about the treatment strategies for these injuries. More recently it has been advocated that treatment should exist in restoring extraarticular anatomy and joint congruency by open reduction, followed by rigid internal fixation and early function1. However, the ideal implants and techniques do not exist yet. Drawbacks with standard techniques (3.5mm Titanium-plate) consist in poor screw purchase in the radial epiphysis, interference with extensor tendons and need for secondary procedures (implant removal). Numerous efforts are currently undertaken to improve operative techniques and implant design and material2–8. In two clinical studies and supported by biomechanical data it has been shown that internal fixation is reliable with two small implants (2.0mm titanium plates) placed at an angle of 50-701 on the dorsum of the distal radius3,6,7. However, to further improve this promising technique, a fracture model is needed for implant testing. To establish a reliable test setting the amount of forces that are transmitted across the radiocarpal and ulnocarpal joint (the ‘‘wrist joint’’) during physiological loading should be known. These biometrical data do not exist to date.
To our knowledge these are the first data on force transmission across the human wrist joint measured in vivo. Further measurements are actually planned in the same setting on 10 healthy volunteers to confirm these data. The data will allow to estimate the forces that have to be neutralized by internal fixation and early motion of a distal radius fracture until bony union has occurred. Cadaver models for implant testing and evaluating force transmission and force distribution in the wrist joint can be adjusted according to these in vivo data. Additionally, the data have a potential value in the evaluation of carpal injuries, Kienbo¨ck’s disease, and corrective procedures about the wrist joint (e.g. unloading operations: ulnar shortening, weaver procedure and others). The technology developed for this study can also be used for in vitro cadaver testings. The data from this study have to be confirmed by further measurements. They potentially have a value in the study of the human wrist joint, in physiologic aswell as pathologic conditions.
Acknowledgements
2. Methods
The study is supported by a grant of the AO-ASIF foundation.
To measure intraarticular pressure in the wrist joint in vivo, a capacitative pressure sensor adapted to the anatomy of the radiocarpal and ulnocarpal joint surface was designed. The behavior of the sensor with regards to change in temperature, humidity and sterilization was evaluated in vitro. The sensor was further validated in cadaver tests. The sensor was then introduced in the wrist joint of two healthy volunteers from a radial surgical approach under local anesthesia. Pressure was measured as a baseline pressure in the wrist joint, during active and passive motion and during fist clench. The protocol was established according to the guidelines of the European Community and approval by the local ethic commitee was obtained. 3. Results The total amount of force transmitted across the wrist joint measured by the sensor was 25N (baseline value), 70N during motion and up to 300N with fist clench.
References 1
Jupiter JB. (1991). J Bone Joint Surg 73-A: 469. Carter PR, Frederick HA, Laseter GF. (1998). J Hand Surg 23-A: 300. 3 Rikli D, Regazzoni P. J Bone Joint Surg (1996). 78-B: 588-92. 4 Ring D, Jupiter JB, Brennwald J et al. (1997). J Hand Surg 22-A: 777. 5 Zimmermann R, Gabl M, Pechlaner S et al. (1998). Unfallchirurg 101: 762–8. 6 Jakob M, Rikli D, Regazzoni P. (2000). J Bone Joint Surg 85-B: 3404. 7 Peine R, Rikli D, Hoffmann R et al. (2000). J Hand Surg 25A: 29–33. 8 Schilling CH, Schiefer H, Curtis R, Rikli D. (1999). Poster AO Advanced course, Davos. 2
Loads on an internal spinal fixator during sitting A. Rohlmann, F. Graichen, G. Bergmann Biomechanics Laboratory, UKBF, Free University of Berlin, Germany
1. Introduction It is assumed that spinal load in the lumbar region depends on the degree of lordosis and the inclination of the pelvis. Sitting may involve
different degrees of lordosis and pelvic inclination. Thus spinal load may depend on the type of seat and the sitting posture. The aims of this study were to measure implant loads in 10 patients for sitting on different types of seats and to compare implant loads for sitting erect and relaxed.
Abstracts / Journal of Biomechanics 34 (2001) S37–S42
S39
2. Methods Telemeterized bisegmental internal spinal fixators were used for in vivo measurements of spinal loads1. The inductively powered fixators were implanted in 10 patients and allowed the measurement of three force components and three moments. The patients were videotaped during the measurements, and the load-dependend signals of the two telemetries were stored on the same videotape. Forces and moments were calculated from the telemetric signals and shown online on a computer monitor. During a measuring session, patients successively sat on a stool, a chair, an office chair, a bench, a physiotherapy ball, and a knee-stool. While sitting on a stool or chair, the patients were asked to sit relaxed and afterwards to straighten and extend their back. This exercise was repeated during several measuring sessions. Previous studies2,3 have shown that the bending moment in the sagittal plane is the most important load component. Thus, only the bending moments in the fixators are compared here.
3. Results The type of seats had only a minor effect on bending moments in the fixators. Implant loads differed considerably in some patients when they changed the type of seat. However, a significant difference was only found between sitting on a bench and on a knee-stool with slightly higher values for the latter. Sitting erect and actively straightening the spine as taught in some back schools increased the bending moment in the fixators by an average of about 11% compared to sitting relaxed (Fig. 1). There were marked interindividual differences in normalized fixator bending moments.
Fig. 1: Comparison of normalized fixator bending moments for sitting relaxed (100%) and erect.
higher for sitting erect than relaxed4. Sitting erect requires additional muscle forces which cause higher spinal and implant loads. The fact that the loads even for sitting erect are lower than for walking and much lower than for lifting and carrying a weight4 argues against the assumption that the higher load for sitting erect is the reason for low back pain, which sometimes occurs during sitting.
Acknowledgements Funding for this study was obtained from the Deutsche Forschungsgemeinschaft (Ro 581/7-2).
4. Discussion and conclusions Loads on internal spinal fixation devices caused by sitting were measured in 10 patients. There was marked interindividual variation, partially due to differences in the indication for surgery (compression fracture, degenerative instability) and the surgical procedure (compression or distraction of the bridged region)2,3. Load differences were negligible for sitting on different types of seats. Intradiscal pressure likewise varied only slightly for sitting on different types of seats4. Sitting erect and actively straightening the spine led to higher implant loads than sitting relaxed. Intradiscal pressure was about 10%
References 1
Rohlmann A, Bergmann G, Graichen F. (1994). J Biomech 27 : 961–7. 2 Rohlmann A, Graichen F, Bergmann G.(1999). Eur Spine J 8: 354–9. 3 Rohlmann A, Graichen F, Bergmann G, Weber U. (2000). Spine 25 : 2981–6. 4 Wilke H.-.J., Neef P., Caimi M., Hoogland T., Claes L.E. (1999). Spine 24: 755–62.
Impact loading of human articular cartilage L.V. Burgin, R.M. Aspden Department of Orthopaedics, University of Aberdeen, Foresterhill, Aberdeen, AB25 2ZD, UK
1. Introduction
2. Methods
Articular cartilage provides a resilient and compliant articulating surface to the bones in diarthrodial joints. It protects the joint by distributing applied loads, hence preventing potentially damaging stress concentrations, and provides a low-friction bearing surface to enable free movement of the joint. The relationship between the material properties of bone and cartilage is not entirely clear and how changes in one affect the ability of the other to withstand loading is under investigation.
A purpose built drop tower has been constructed to apply controlled impact loads to cores of articular cartilage, both isolated and in situ on bone. The drop tower consists of a stainless-steel cage comprising three vertical rods in which a cylindrical impactor can fall freely. Impactors of various masses are used and can be dropped from different heights. The cartilage and bone sample under test is positioned on top of a quartz force link (Kistler Instruments Ltd.) fixed at the base of the cage. An accelerometer attached to the impactor measures deceleration
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Abstracts / Journal of Biomechanics 34 (2001) S37–S42
Table 1 Maximum dynamic stiffness values for human cartilage impacted from two different drop heights using a 100g mass. Height (mm)
Velocity(m s 1)
Stiffness (MPa)
145 90
1 69 1 33
36 074 5 29 5710 4
of the impactor during impact. Integrating this signal provides a measure of the distance travelled by the impactor as it compresses the tissue core. Data are collected and processed using LABVIEW software. 3. Results The difference in stiffness for two different drop heights is shown in Table 1 for the same elderly, osteoporotic femoral head. Comparing samples from the superior and inferior aspects of a femoral head yielded values for the dynamic stiffness of 7579MPa for the superior region and 102732MPa for the inferior region. The average cartilage thickness in these areas was 1.83 and 1.14mm, respectively. For an impact load followed by unloading, the force-deflection curve forms a hysteresis loop, Fig. 1. The area contained within the loop increases with impact velocity.
4. Discussion and conclusions The stiffness of cartilage subjected to impact loading is greater than that normally measured in quasi-static loading. The average strain rates experienced by the cartilage in these tests were between 1300s 1 and 1700s 1. At these rates there is no opportunity for fluid flow through the matrix. The maximum force occurs before the maximum displacement of the impactor and is a characteristic of such viscoelastic impacts. The
Fig. 1. Force-deflection curves for different impact velocities for samples of human cartilage.
irreversible internal deformation that dissipates some of the initial kinetic energy can also be seen. The reversal of generally accepted stiffness over the femoral head is probably due to the much thinner cartilage found in the inferior region. This stiffness is not the same as the elastic modulus and is not, therefore, an intrinsic property of the material in this case. It is a reflection of the ability of the sample to absorb the impact by deformation.
Acknowledgements We thank the ARC for funding this project, the MRC for a Senior Fellowship for RMA and the engineering workshop in the Department of Biomedical Physics for construction of the drop tower.
Dose and time-dependent adaptation of mechanical strength and morphology of the epiphyseal and articular cartilage in the rat model A. Niehoffa, U.G. Kerstingb, G.-P.Bru¨ggemanna, M.M. Morlockc, K. Sellenschlohc, F. Zaucked, H. Michnaf a
Institute for Biomechanics, German Sport University of Cologne Insitute of Individual Sports, German Sport University of Cologne c Biomechnics Section, Tecnical University of Hamburg-Harburg d Institute for Biochemistry II, University of Cologne f Institute for Morphology and Tumorresearch, German Sport University Cologne b
1. Introduction Various studies show that mechanical loading has an influence on cartilage but the results are contradictory 1–3. The purpose of the study was to examine the effect of mechanical load on epiphyseal and articular cartilage dependent on dose and time of exercise. A set of mechanical, histological and immunohistological parameters is proposed and the correlation between these properties dependent on the amount of the running exercise will be investigated.
2. Methods Forty five female Sprague-Dawley rats (starting age was 3 weeks) were randomly assigned to a high-trained exercise group (n=15), a
low-trained exercise group (n=15) and a non-active control group (n=15). The exercise groups were trained in a running wheel with voluntary exercise and the running distance was monitored by a computer system. Food and water were available ad libitum and the rats housed on a 12h/12h light/dark cycle. Five animals of each group were sacrified after 4 weeks, the next five after 8 weeks and the last five after 12 weeks. The rats were decapitated and both hindlimbs were detached at the hip joint. The knee joint was dissected carefully and femur and tibia were obtained. The bones of the left hindlimb were frozen at 801C until mechanical testing on a material test machine (Zwick Z2.5/TN 1S). With the tibia a relaxation test was performed using a solid 0.5mm indenter. A microscope with an angular scale was used to orientat the specimens so that the indenter approached the surface perpendicularly. A step load of 3.5 7 0.15N was
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Abstracts / Journal of Biomechanics 34 (2001) S37–S42 applied and the achieved deformation held constant for 250s. Shortterm and long-term relaxation were quantified by calculation of the relative load after 1 and 250s. Cartilage thickness was determined by a needle probe measurement. The femur was placed horizontally so that the distal growth plate was loaded in shear direction. The load was applied with a c-formed stamp adapted individually in the form of the plate from lateral to medial on the epiphysis. The specimens were preloaded with 0.05N and three cycles of a stress-relaxation series began. For each cycle a shear load of 6N was first applied, followed by relaxation at that stress for 500s. Then the load was reduced to 0.5N for 500s. At the end of the series the specimens were loaded to failure at a rate of 15N/s. We determined the relaxation behavior 3s after applied load cycles and the relative ultimate shear stress was also measured. The growth plate area was calculated from the femoral epiphysis diameters.
4. Discussion and conclusion We conclude that physical activity has an effect on the cartilage mechanics. The amount of the running exercise also plays a role. The results have to be interpreted in accordance to the histomorphometry and immunhistology (in preparation). Differences in stainning pattern for collagen and other matrix proteins will be discussed. The theoretical analysis for the mechanical test results has to be extended to derive structural parameters, which characterize the matrix composition.
References 1
3. Results The strength and the relaxation behavior from the cartilage of the distal femur epiphyseal plate and the proximal tibial articular was greater for the high-trained rats than the low-trained and control group. The differences between the low trained and the non-active rats were very small.
Kaab M.J, Ito K, Clark J.M, Notzli H.P. (1998). J Orthop Res 16(6): 743–751. 2 Jurvelin J, Kivirana I, Sa¨a¨ma¨nen A-M., Tammi M,. Helminen H.J. (1990). J Biomech 23:1239–1246. 3 Pap G, Eberhardt R, Sturmer I, Machner A, Schwarzberg H, Roessner A, Neumann W. (1998). Pathol Res Pract 194(1), 41–7.
Medical aspects of the load transfer mechanism of an implant-bone connection in the femur using gamma radiation for fixing the experimental information G.V.Skrbenskya, F. Gottsauner-Wolffb, R. Beerb, W. Grienauerc, J. Eberhardsteinerb a
Department of Orthopedic Surgery, Biomechanic Resaerch Laboratory, Univ. of Vienna b Vienna University of Technology c Austrian Research Centre Seibersdorf
1. Introduction The in growth of the bone and the achievement of the secondary (longtime) stability is prevented by a peak pressure causing reactive bone resorption as well as by an incomplete fit which leads to loosening of the connection because of possible local micro-movements1. Torsion may also cause axial loosening (screw-effect) of the connection before secondary stability is reached. In the Viennese model the transfer of pressure is carried out by four large rips (Fig.1). In the Bologna model (Fig. 2), the head has five small ribs to carve into the inner cortical bone.
A comparison with the corresponding figures for the Viennese model shows that there is a big difference in the magnitude and the distribution of the respective stressfields. In case of the Bologna model the fringe pattern in the near vicinity of the ribs are quite similar to those around an opening crack.
2. Methods The stress distribution in the load-transfer areas were investigated using three-dimensional photoelasticity. However, due to the fact that the models consist of two parts with different mechanical and thermal properties, any thermal treatments must be avoided. Therefore, a new method for fixing the photoelastic fringes was used2,3 by applying gamma-radiation to partially polymerized Araldite B hardened with malein acid anhydride.
Fig. 1.
3. Results Fig. 3 show typical fringe distributions for the Viennese model (taken in a dark and a brightfield polariscope, torque moment M t = 6dNm). Fig. 4 show the same for the Bologna model, the contour of the ribs of this model is triangular with a very sharp ridge.
Fig. 2.
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Abstracts / Journal of Biomechanics 34 (2001) S37–S42 significant is the effect due to a non-equal contact between the ribs and the bone (non perfect form fit between the femur and the prosthesis) especially in case of the Viennese model. The investigation allows to compare different implants under certain biomechanic conditions. References Fig. 3.
Fig. 4.
4. Discussion and conclusion A comparison between this two implants shows the significant influence of the different load transfer mechanisms. However, most
1
Gottsauner-Wolf F, Plenk Jr. H. (1985). Die Knochenverankerung von Tumor endoprothesen mit angegossenen Ku¨gelchen aus Kobalt-Basislegierung; O rthopa¨die 16: 252–257. 2 Beer RJ, Wendrinsky J, Grienauer W, Jecic S. (1995). Threedimensional photoelasticity using new radiation cured polymers. Proceedings. of the 12th Danubia-Adria Symposium. Sopron, Hungary 5–7. October. 3 Beer RJ., Kodvanj J, Grienauer, W, Schaudy R. Different treatments of radiation-cured and their influence on the long-time behaviour; Proceedings. of the 15th Danubia-Adria Symposium.