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An Instrumented Scaffold can Monitor Loading in the Knee Joint, |
| In Vivo Strain Measurements from Hardware and Lamina during Spine Fu
sion, John A. Szivek, Rolando F. Roberto , Journal Biomedical Materials Research-B 75B:243-250, 2005. Abstract: Currently, spine fusion is determined using radiography and clinical evaluation. There are discrepancies between radiographic evidence and direct measurements of fusion, such as operative exploration and biomechanical or histological measurements. In order to facilitate the rapid return of patients to normal activities, a monitoring technique to accurately detect fusion in vivo and to prevent overload during the postoperative period would be useful. The objectives of this study were to develop an implantable monitoring system consisting of CPC-coated strain gauges and a radio transmitter to detect the onset of fusion and measure strain during postsurgical activi ties. A patient underwent anterior release and fusion, followed by posterior instrumentation and fusion with segmental spinal instrumentation. Four strain gauges were placed during surgery. One was attached to the left-side rod and one to each of the lami na at T9, T10, and T11. An externally powered implanted radio transmitter attached to the gauges was placed in a subcutaneous pouch. Strains were monitored weekly and tabulated during various activities for 7 months. Peak strains during twisting and bendi ng were tabulated to detect the onset of fusion. Strains were also recorded during activities such as climbing off an examination table, rising from a chair, and climbing stairs. Strains collected from the left rod indicated that, immediately postoperativ ely, it was loaded at acceptable levels. The largest and most consistent strain changes measured from the lamina were recorded during twisting. |
Bone Remodeling in Calbindin-D28k Knockout Mice Abstract:In vitro studies indicate that calbindin-D28k, a calcium binding protein, is important in regulating the life span of osteoblasts as well as the mineralization o f bone extracellular matrix. The recent creation of a calbindin-D28k knockout mouse has provided the opportunity to study the physiological effects of the protein on bone remodeling in vivo. In this experiment, quantitative histomorphometry was u sed to characterize bone remodeling in normal and calbindin-D28k knockout mice. Cantilever bend testing was used to characterize the stiffness, failure loads and the elastic moduli of femora from normal and knockout mice. Statistical significance was dete rmined using ANOVA for all histology and histomorphometry parameters, and significance was determined using a dependent t-test for all biomechanical properties. The calbindin-D28k knockout mice had significantly decreased mineral apposition rates and bone formation rates. Despite the lower bone formation rates, the bone volume was larger in knockout mice indicating decreased bone resorption rates and less dynamic bone remodeling. The mechanical testing demonstrated that the bones of knockout mice are stif fer and fail at higher loads than the bones of normal mice; however the elastic moduli of the bones in both mice are similar. This indicates that the increased stiffness and failure load were a result of the increased bone volume. The calbindin-D28k knock out mouse provides a good model to study how vitamin D and calcium binding proteins regulate bone modeling and remodeling in vivo. |
TGF-ß1 Accelerates Bone Growth Into Porous TCP coated Scaffolds Abstract: Porous polybutylene terephthalate (PBT) scaffold systems were tested as orthopedic implants to determine whether these scaffolds could be used to detect strain transfer following bone growth into the scaffold. Three types of scaffold systems were tested: porous PBT scaffolds, porous PBT scaffolds with a thin -tricalcium phosphate coating (LC-PBT), and porous PBT scaffolds with the TCP coating vacuum packed into the scaffold pores (VI-PBT). In addition, the effect of applying TGF- 1 to scaffolds as an enhancement was examined. The scaffolds were placed onto the femora of rats and left in vivo for 4 months. The amount of bone ingrowth and the strain transfer through variou s scaffolds was evaluated by using scanning electron microscopy, histology, histomorphometry, and cantilever bend testing. The VI-PBT scaffold showed the highest and most consistent degree of mechanical interaction between bone and scaffold, providing str ain transfers of 68.5% ( 20.6) and 79.2% ( 8.7) of control scaffolds in tension and compression, respectively. The strain transfer through the VI-PBT scaffold decreased to 29.1% ( 24.3) and 30.4% ( 25.8) in tension and compression when used with TGF- 1. T GF- 1 enhancement increased the strain transfer through LC-PBT scaffolds in compression from 9.4% ( 8.7) to 49.7% ( 31.0). The significant changes in mechanical strain transfer through LC-PBT and VI-PBT scaffolds correlated with changes in bone ingrowth f raction, which was increased by 39.6% in LC-PBT scaffolds and was decreased 21.3% in VI-PBT scaffolds after TGF- 1 enhancement. Overall, the results indicate that strain transfer through TCP-coated PBT scaffolds correlate with bone ingrowth after implanta tion, making these instrumented scaffolds useful for monitoring bone growth by monitoring strain transfer. |
Bilateral Symmetry of Biomechanical Properties in Mouse Femora Abstract: Bone healing and remodeling are commonly examined in animal models by comparing one femur (experimental) to the contralateral femur (control) with the assumption that they are id entical with respect to their biomechanical properties. While past studies have characterized the symmetry in geometrical properties in many types of animal bones, few studies have compared the symmetry in the biomechanical properties. The purpose of this study was to determine whether there is symmetry in the mechanical properties of mouse femora. Strain gauges were attached to the posterior surface of the femora of C57BL/6 mice, parallel to the long axis of the bone. The femora were mechanically tested in cantilever bending while strain values were recorded. Moments of inertia, cortical areas, and moduli of elasticity were determined from strains and cross-sectional properties. Mouse femora demonstrated an average strain difference of 0.4% in tension an d 1.4% in compression. Elastic moduli differed by 6.6% and 0.9% in tension and compression, respectively, and failure strength differed by an average of 2.0%. Statistical analysis showed there were no significant differences in strain, modulus, or failure load values for the mice, indicating mechanical and geometrical symmetry of mouse femora in cantilever bending. |
TGF-ß1 Accelerates Bone Bonding to a Blended CPC Coating: Abstract:In vivo strain measurements can facilitate the study of the bone remodeling response to loading and load changes. Calcium phosphate ceramic (CPC) coatings have b een used to attach strain gauges to bone for extended periods of time, but require up to 12 weeks for adequate CPC-to-bone bonding. Transforming growth factor-(TGF)-1, an osteoinductive growth factor, was used as a surface enhancement to accelerate bone g rowth and bonding to CPC particles. The aim of this study was to find an optimal dosage of TGF- 1 to accelerate the attachment process. CPCcoated strain gauges were enhanced with doses of 0.5, 1.0, or 2.0 g of TGF- 1 per gauge. Gauges were placed on the f emora of dogs, which were exercised daily and fed ad libitum. After 3, 6, and 12 weeks, gauge attachment was quantitatively assessed using mechanical testing and histomorphometry. Gauge attachment was also qualitatively assessed using back scatte r electron microscopy. Agreement of the mechanical test results with both the back scatter electron microscopy images and histomorphometry results showed that the 1.0 g per gauge dose of TGF- 1 is an optimal dose to accelerate bone formation and attachmen t to CPC-coated strain gauges. |
Evaluation of a new CPC-to-Gauge Bonding Technique Abstract: Strain gauging enables the measurement of bone deformation during physical activity, leading to a better understanding of the physiological effects of loading on bone gr owth and remodeling. Development of a technology that will withstand long-term in vivo exposure and bond securely to bone is imperative for accurate, consistent measurement collection. Polysulfone is currently used to attach calcium-phosphate cer amic (CPC) particles, which promote bone-to-gauge bonding, to polyimide-backed strain gauges. This study evaluated the use of an implant-grade epoxy as an alternative CPC–polyimide adhesive. Polyimide–epoxy–CPC interfaces wer e loaded to failure and shear strengths calculated. In vitro studies providing a constant flow of medium over test specimens were designed, and long-term in vitro fluid exposure studies of the epoxy’s shear strength were conduc ted. Average shear strength of polysulfone-polyimide interfaces were reported to be 7 MPa.11 The average shear strength of the epoxy-polyimide interface before long-term in vitro exposure was 17 MPa, which is stronger than the shear strength of t he bone–CPC interface.12 The strength of the epoxy–polyimide interface decreased to 6.8 MPa after 24 weeks in vitro and 3 MPa after 24 weeks in vivo. |
The Effect of Implant Overlap on the Mechanical Properties of the Femur, |
An Implantable Strain Measurement System Designed to Detect Spine Fusion: Preliminary Results from a Biomechanical and In Vivo Study, Abstract: The strain distribution on the thoracic vertebrae during anteroposterior bending and torsion was examined for use with an implantable strain gauge system and miniature radio transmitter to identify strain gauge placement sites by testing cadaver spines, and to evaluate an implantable gauge bonding technique and subminiature radio transmitter for accurate strain monitoring. Fusion is determined currently through the use of radiographic techniques. Discrepancies exist between radiographic evidence and more direct measurements of fusion such as operative exploration an d biomechanical or histologic measurements. To facilitate the return of patients to full unrestricted activity, it would be useful to develop a technique for accurate in vivo determination of fusion. Three cadaver spines were tested during antero posterior bending and torsional loading in the control, instrumented, and instrumented plus polymethylmethacrylate states. The spines were instrumented with an ISOLA construct, and a simulated fusion was achieved through the application of polymethylmetha crylate. Strain gauges were attached in uniaxial, biaxial, and rosette configurations. The principal strains were calculated. Calcium phosphate ceramic-coated gauges were implanted in patients and recovered after up to 15 months in vivo. A radio transmitter was developed and tested for use in patients. The largest and most consistent strain changes after simulated fusion were recorded during torsional loading on the laminae of a vertebra directly underneath a hook. Calcium phosphate ceramic-coate d strain gauges showed excellent bone bonding to the lamina when fusion occurred. Radio telemetry accurately tracked strain magnitudes and strain rates expected in patients. The consistency obtained in torsional loading indicates that this type of loading will provide the most useful data from patients in vivo. Excellent bonebonding and accurate strain transmission using a longterm strain measurement system and miniature radio transmitter indicate that strains collected from patients with this sy stem will be accurate. |
Linear And Volumetric Wear of Tibial Polyethylene Retrievals From PCL Retaining Knee Arthroplasties Linear and volumetric wear was measured in 33 tibial polyethylene inserts from three different cruciate-retaining knee systems retrieved at the time of revision surgery. Wear patterns also were evaluated and classified. Eccentric and asymmetric wear patterns were seen in 78% of inserts with flat articulating geometry versus 12% in inserts with curved anteroposterior geometry. The mean lin ear wear rate was .35 mm/year (range, .05–1.68 mm/year) and the mean volumetric wear rate was 794 mm3/year (range, 24–4088 mm3/year). Linear and volumetric wear rates showed a negative correlation with the length of implantation. Linea r wear rates also showed a negative correlation with patient weight. |
Long-Term Measurement of Bone Strain In Vivo: The Rat Tibia, Abstract: Despite the importance of strain in regulating bone metabolism, knowledge of strains induced in bone in vivo during normal activities is limited to short-term s tudies. Biodegeneration of the bond between gauge and bone is the principle cause of this limitation. To overcome the problem of bond degeneration, a unique calcium phosphate ceramic (CPC) coating has been developed that permits long-term attachment of mi crominiature strain gauges to bone. Using this technique, we report the first long-term measurements of bone strain in the rat tibia. Gauges, mounted on the tibia, achieved peak or near peak bonding at 7 weeks. Measurements were made between 7–1 0 weeks. Using ambulation on a treadmill, the pattern and magnitude of strain measured in the tibia remained relatively constant between 7–10 weeks post implantation. That strain levels were similar at 7 and 10 weeks suggests that gauge bonding is stable. These data demonstrate that CPC-coated strain gauges can be used to accurately measure bone strain for extended periods, and provide an in vivo assessment of tibial strain levels during normal ambulation in the rat. |
| Strain transfer between a CPC coated strain gauge and cortical bone during bending N. M. Cordaro, J. A. Weiss, J. A. Szivek , JBMR (Applied Biomaterials), 58:2, 147-155, 2001. Abstract: The finite element method was used to simulate strain transfer from bone to a calciumphosphate ceramic (CPC) coated strain gauge. The model was constructed using gross morphometric and histological measurements obtained from previou s experimental studies. Material properties were assigned based on experiments and information fromthe literature. Boundary conditions simulated experimental cantilever loading of rat femora. The model was validated using analytical solutions based on the theory of elasticity as well as direct comparison to experimental data obtained in a separate study. The interface between the bone and strain gauge sensing surface consisted of layers of polysulfone, polysulfone/CPC, and CPC/ bone. Parameter studies exa mined the effect of interface thickness and modulus, gauge geometry, partial gauge debonding, and waterproofing on the strain transfer fromthe bone to the gauge sensing element. Results demonstrated that interface thickness and modulus have a significant effect on strain transfer. Optimal strain transfer was achieved for an interface modulus of approximately 2 GPa. Strain transfer decreased consistently with increasing interface thickness. Debonding along the lateral edges of the gauge had little effect, while debonding proximal and distal to the sensing element decreased strain transfer. A waterproofing layer decreased strain transfer, and this effect was more pronounced as the modulus or thickness of the layer increased. Based on these simulations, spec ific recommendations were made to optimize strain transfer between bone and CPC coated gauges for experimental studies. |
Surface Enhancements Accelerate Bone Bonding |
A Comparison of In Vitro and In Vivo Degradation Abstract: Calcium phosphate ceramic (CPC) coated strain gauges have been used to measure bone strain in animal models for up to 16 weeks and are being developed to collect measu rements in patients for periods of 1 year or more. A published surface roughening and heat treating procedure produced improved dry strength and in vivo stability of CPC-gauge interfaces after 16 weeks. The long term bond strength of two CPC-gaug e interfaces prepared using the roughening and heat treating process were evaluated after up to 1 year in vitro and in vivo using a lap shear test. The feasibility of using an in vitro test to predict long term in vivo interface changes was established. A blended tricalcium phosphate 1 hydroxyapatite had a CPC-gauge interface strength which decreased from 6.07 ± 2.64 MPa at 16 weeks to 4.71 ± 1.840 MPa after 1 year in Hanks Balanced Salts (HBS). The same coating had a s trength that decreased from 8.51 ± 2.63 MPa at 16 weeks to 5.35 ± 1 MPa after 1 year in vivo. A soluble calcium enhanced hydroxyapatite had an interface strength of 4.83 ± 1.106 MPa after 16 weeks and 4.516 1.100 MPa after 1 year in HBS. The same coating had an interface strength of 8.34 6 2.40 MPa after 16 weeks and 5.20 ± 2.00 MPa after 1 year in vivo. Although interface strengths decreased slightly with time in vivo, after 1 year they were in the same strength rangeas published CPC-bone interface strengths of 4.8 ± 2.4 MPa. Comparison of in vitro with in vivo results indicated that in vitro results were a good predictor of strength change in the blended CPC coating, but a poorer predictor of strength changes in the soluble calciumenhanced coating |
The Effect of Lateral Muscle Loading on Femoral Strain Distributions Experimental models that have been used to evaluate hip loading and the effect of hip implants on bone often use only a head load and abductor load. Anatomic considerations and in vivo measurement s have lead several investigators to suggest that these models are inaccurate because they do not incorporate the loads imposed by additional muscles. The aim of this study was to evaluate the strains in the proximal and mid diaphysis of the femur for fiv e hip loading models, one with a head load and abductor load only and four which incorporated lateral muscle loads as well. Head load to body weight load ratios were used to evaluate the physiologic accuracy of these models and strains were compared to de termine the extent of strain changes as a function of model complexity. All models which incorporated additional lateral muscle loads more accurately simulated head load to body– weight load ratios than the simple abductor-only model. The model which incorporated a coupled vastus lateralis and iliotibial band load in addition to the abductor load provided the simplest configuration with a reasonable body–weight to head-load ratio. |