Biomedical subjects

D T Davy

Publications and source records attributed to D T Davy.

At least 19 recordsLinked to original sources

PubMed2001

Machine vision photogrammetry: a technique for measurement of microstructural strain in cortical bone.

Understanding local microstructural deformations and strains in cortical bone may lead to a better understanding of cortical bone damage development, fracture, and remodeling. Traditional experimental techniques for measuring deformation and strain do not allow characterization of these quantities at the microstructural level in cortical bone. This study describes a technique based on digital stereoimaging used to measure the microstructural strain fields in cortical bone. The technique allows the measurement of material surface displacements and strains by comparing images acquired from a specimen at two distinct stress states. The accuracy of the system is investigated by analyzing an undeformed image set; the test image is identical to the reference image but translated by a known pixel amount. An increase in the correlation sub-image train parameter results in an increase in displacement measurement accuracy from 0.049 to 0.012 pixels. Errors in strain calculated from the measured displacement field were between 39 and 564 microstrain depending upon the sub-image train size and applied image displacement. The presence of a microcrack in cortical bone results in local strain at the crack tip reaching 0.030 (30,000 microstrain) and 0.010 (10,000 microstrain) near osteocyte lacunae. It is expected that the use of this technique will allow a greater understanding of bone strength and fracture as well as bone mechanotransduction.

Animals↗

PubMed2001

Material optimization of femoral component of total hip prosthesis using fiber reinforced polymeric composites.

In this report an integrated approach to the three-dimensional material optimization of femoral components of hip prostheses is described. The effectiveness of using reinforced fiber composites for the material optimization of hip implants has been demonstrated and general guidelines on some material design aspects of total hip replacement (THR), in terms of fiber volume fraction and fiber orientation angles, are provided. A modular program was developed to interface the optimization routine with the finite element code. In this study two cases of cemented and non-cemented THR were investigated. In both cases perfectly bonded interfaces were assumed. Two objective functions were defined based on interface failure criteria and bone adaptive remodeling to avoid interface disruption and to reduce the risk of bone loss. The overall results demonstrated the effectiveness of the technique, which can provide meaningful insights into the fiber-reinforced composite material design of orthopaedic implants.

Benzophenones↗

PubMed2000

Effects of loading conditions and objective function on three-dimensional shape optimization of femoral components of hip endoprostheses.

A numerical procedure was implemented for the three-dimensional (3D) shape optimization of the femoral component in total hip replacement. An algorithm was developed for defining the component geometry in terms of longitudinal and cross-sectional shape variables. The 3D design model was combined with a 3D finite element analysis and a numerical optimization procedure. An idealized femoral geometry and perfectly bonded interfaces were used for cemented and uncemented implants. The design objective was to smooth some measure of the stresses along the local interface. The effects of two different load conditions and several different objective functions were examined. A common initial implant design was used for all cases. The general trend in all design optimization was to produce a somewhat bulky implant with a rectangular cross-section. The outcome was more strongly affected by loading condition than the choice of objective function. The use of a strain energy density criterion as the objective function proved to be the most effective in reducing all equivalent stress criteria.

Algorithms↗

PubMed2000

The effects of processing and low dose irradiation on cortical bone grafts.

The authors studied the effects of standard processing and preprocessing low dose gamma irradiation (1.5 Mrad) on the strength and incorporation of syngeneic and allogeneic cortical bone grafts. Bilateral femoral middiaphyseal 8-mm segmental defects in 120 male Fisher rats were stabilized with internal fixation. Each defect received one of six types of grafts: fresh syngeneic, processed syngeneic, irradiated processed syngeneic, fresh allogeneic, processed allogeneic, and irradiated processed allogeneic grafts. Graft processing included soaking in 70% ethanol and deep freezing for preservation. Irradiation was performed by 60Co source immediately before processing. Grafts were evaluated by histologic analysis, histomorphometric analysis, and biomechanical testing at 4 and 6 months after surgery. Graft treatment, either processing or irradiation processing, did not affect consistently or significantly the incorporation of syngeneic or allogeneic grafts. Graft allogenicity was the major determinant of the revascularization and the histologic pattern of graft incorporation. Processed and irradiated processed allogeneic grafts gained compressive strength with time and were as strong as syngeneic grafts at 6 months. Biomechanical and histologic data from this study suggest that standard processing and preprocessing low dose irradiation do not compromise the natural course of allogeneic cortical bone graft incorporation.

Animals↗

PubMed1999

The role of the lamellar interface during torsional yielding of human cortical bone.

Fragility fractures are a result of alterations in bone quantity, tissue properties, applied loads, or a combination of these factors. The current study addresses the contribution of cortical bone tissue properties to skeletal fragility by characterizing the shear damage accumulation processes which occur during torsional yielding in normal bone. Samples of human femoral cortical bone were loaded in torsion and damaged at a post-yield twist level. The number of microcracks within osteons, interstitial tissue, and along cement lines were assessed using basic fuchsin staining. Damage density measures (number of cracks/mm2) were correlated with stiffness degradation and changes in relaxation. Damaged samples exhibited a wide variation in total microcrack density, ranging from 1.1 to 43.3 cracks/mm2 with a mean density of 19.7 +/- 9.8 cracks/mm2. Lamellar interface cracks comprised more than 75% of the total damage, indicating that the lamellar interface is weak in shear and is a principal site of shear damage accumulation. Damage density was positively correlated with secant stiffness degradation, but only explained 22% of the variability in degradation. In contrast, damage density was uncorrelated with the changes in relaxation, indicating that a simple crack counting measure such as microcrack density was not an appropriate measure of relaxation degradation. Finally, a nonuniform microcrack density distribution was observed, suggesting that internal shear stresses were redistributed within the torsion samples during post-yield loading. The results suggested that the lamellar interface in human cortical bone plays an important role in torsional yielding by keeping cracks physically isolated from each other and delaying microcrack coalescence in order to postpone the inevitable formation of the fatal crack.

Adult↗

PubMed1999

Biomechanical issues in bone transplantation.

Because the biomechanical competence of the graft is a central issue, many research studies of bone grafts include a biomechanical component. The biomechanical evaluation serves as a bottom-line measure of the experimental outcome in which some measure of mechanical performance is compared among treatment groups. This article considers the biomechanics of grafts in the context of this experimental work and focuses on three issues: (1) the interplay between biology and biomechanics of grafts, (2) the effects of treatment choices on biomechanical properties, and (3) model factors that may influence biomechanical performance.

Biology↗

PubMed1999

Inelastic strain accumulation in cortical bone during rapid transient tensile loading.

An experimental study examined the tensile stress-strain behavior of cortical bone during rapid load cycles to high strain amplitudes. Machined bovine and human cortical bone samples were subjected to loading cycles at a nominal load/unload rate of +/- 420 MPa/s. Loads were reversed at pre selected strain levels such that load cycles were typically completed in 0.5-0.7 seconds. Axial strain behavior demonstrated considerable nonlinearity in the first load cycle, while transverse strain behavior was essentially linear. For the human bone 29.1 percent (S.D. = 4.7 percent), and for the bovine bone 35.1 percent (S.D. = 10.8 percent) of the maximum nonlinear strain accumulated after load reversal, where nonlinear strain was defined as the difference between total strain and strain corresponding to linear elastic behavior. Average residual axial strain on unloading was 35.4 percent (S.D. = 1.2 percent) for human bone and 35.1 percent (S.D. = 2.9 percent) of maximum nonlinear strain. Corresponding significant volumetric strains and residual volumetric strains were found. The results support the conclusions that the nonlinear stress-strain behavior observed during creep loading also occurs during transient loading at physiological rates. The volume increases suggest that damage accumulation, i.e., new internal surfaces and voids, plays a major role in this behavior. The residual volume increases and associated disruptions in the internal structure of bone provide a potential stimulus for a biological repair response.

Adult↗

PubMed1999

A damage model for nonlinear tensile behavior of cortical bone.

To describe the time-dependent nonlinear tensile behavior observed in experimental studies of cortical bone, a damage model was developed using two internal state variables (ISV's). One ISV is a damage parameter that represents the loss of stiffness. A rule for the evolution of this ISV was defined based on previously observed creep behavior. The second ISV represents the inelastic strain due to viscosity and internal friction. The model was tested by simulating experiments in tensile and bending loading. Using average values from previous creep studies for parameters in the damage evolution rule, the model tended to underestimate the maximum nonlinear strains and to overestimate the nonlinear strain accumulated after load reversal in the tensile test simulations. Varying the parameters for the individual tests produced excellent fits to the experimental data. Similarly, the model simulations of the bending tests could produce excellent fits to the experimental data. The results demonstrate that the 2-ISV model combining damage (stiffness loss) with slip and viscous behavior could capture the nonlinear tensile behavior of cortical bone in axial and bending loading.

Animals↗

PubMed1998

A kinematic study of the cervical spine before and after segmental arthrodesis.

STUDY DESIGN: The acute kinematic consequence of segmental arthrodesis in the cervical spine on the remaining open motion segments was studied in a cadaveric model. OBJECTIVES: To evaluate the distribution of motion across unfused cervical motion segments after a segmental arthrodesis. The applied load was determined as a function of arthrodesis length and level by using a fixed range of motion for the cervical spine (C2-T1). SUMMARY OF BACKGROUND DATA: An increased incidence of degenerative disease may exist at the levels immediately adjacent to a cervical arthrodesis as a result of alteration in biomechanical behavior at these levels. METHODS: One-, two-, and three-level fusions were simulated in multilevel ligamentous human cervical spines. Specimens were tested nondestructively through a 30 degrees range of sagittal plane rotation. Motion was recorded using three-dimensional stereophotogrammetry. Sagittal plane rotation of each motion segment in the fusion models was compared with the corresponding rotation in the unfused specimen. RESULTS: In the C2-C4 fusion, the increase in motion at C5-C6 was statistically less (P < 0.05) than the increase at C7-T1. In the C2-C5 fusion, the increase in motion at C5-C6 was statistically less (P < 0.05) than the increases at C6-C7 and C7-T1. For each of the five other fusion types tested, no statistical differences existed between the increases in sagittal rotation at any of the open motion segments. The bending moment necessary to produce 30 degrees of sagittal rotation increased nonlinearly as the number of motion segments fused increased. CONCLUSIONS: Under what was considered a realistic loading paradigm, sagittal plane rotation was not increased disproportionately at the motion segments immediately adjacent to a segmental arthrodesis in the cervical spine. The nonlinear rise in applied bending moment to achieve constant displacement was characteristic of the behavior of the ligaments and intervertebral discs throughout the spine as they underwent increasing deformation.

Aged↗

PubMed1997

Comparison of damage accumulation measures in human cortical bone.

Elastic modulus degradation, strength reduction, and energy dissipation have traditionally been the properties of choice to monitor the damage process in cortical bone. However, these properties only provide limited insight into the damage process given the complex mechanical nature of bone. In the current study, alternative measures of the damage process were investigated for machined human cortical bone specimens loaded under torsion. Seventy-two bone specimens from 6 human femurs were subjected to a series of torsional relaxation cycles in which damage was induced during a single relaxation cycle and the effects of damage on the elastic, yield, viscous, and failure properties were determined from pre- and post-damage relaxation cycles. The results revealed that degradation of all torsion properties exhibited a significant twist magnitude effect. However, the yield stress and strain, the relaxation rate, and the total relaxation exhibited 5-10 fold greater degradation than both strength and modulus, when residual strength tests were conducted at high shear strain rates. For the loading conditions examined in this study, the results indicated that the relaxation and yield properties of cortical bone are more sensitive to shear damage accumulation and better measures of the damage process than either strength or modulus. Further, the results reveal an important interaction between damage and the viscous behavior of bone which provides new insight into the effects of damage on bone mechanical properties.

Adult↗

PubMed1997

Pelvic muscle and acetabular contact forces during gait.

Locations, magnitudes, and directions of pelvic muscle and acetabular contact forces are important to model the effects of abnormal conditions (e.g., deformity, surgery) of the hip accurately. Such data have not been reported previously. We computed the three-dimensional locations of all pelvic muscle and acetabular contact forces during level gait. The approach first required computation of the intersegmental joint resultant forces and moments using limb displacement history, foot-floor forces, and estimated limb inertial properties from one subject. The intersegmental resultant moments were then distributed to the muscles using a 47-element muscle model and a non-linear optimization scheme. Muscle forces were vectorally subtracted from the intersegmental resultants to compute the acetabular contact forces. While the peak joint force magnitudes are similar to those reported previously for the femur, the directions of pelvic contact forces and muscle forces varied considerably over the gait cycle. These variations in contact force directions and three-dimensional forces could be as important as the contact force magnitudes in performing experimental or theoretical studies of loads and stresses in the periacetabular region.

Acetabulum↗

PubMed1997

Critical biological determinants of incorporation of non-vascularized cortical bone grafts. Quantification of a complex process and structure.

Our goal in this study was to evaluate the effects of and the interaction between the hypothesized principal determinants of the incorporation of grafts: antigenicity and treatment of the graft. We implanted fresh and frozen cortical bone grafts that were matched for both major and non-major histocompatibility complex antigens (syngeneic grafts), matched for major but not for non-major histocompatibility complex antigens (minor mismatch), and mismatched for both major and non-major histocompatibility complex antigens (major mismatch). We used a rat model with an eight-millimeter segmental defect in the femur. The construct was stabilized with a plastic plate, threaded Kirschner wires, and cerclage wires. We evaluated the grafts at one, two, and four months after implantation. We measured the immune response; assessed the incorporation of the graft with use of histological examination, biomechanical testing, and quantitative isotopic kinetics; and statistically analyzed the effects of and the interactions among three independent variables: time, the degree of matching for major histocompatibility complex antigens, and the treatment of the graft (whether it was fresh or frozen). These three independent variables had profound effects on the pattern, rate, and quality of the incorporation of the graft. Two-way and three-way interactions among these variables were also noted. Serial changes in every dependent variable were observed with time. Systemic antibody specific for donor antigens was measurable only in the serum of animals that had a major mismatch, but freezing markedly attenuated the systemic antibody response. Revascularization was profoundly affected by histocompatibility-antigen matching; the syngeneic grafts were revascularized more quickly and to a greater degree than the grafts with either a minor or a major mismatch. Freezing significantly (p < 0.001) reduced the revascularization of the syngeneic grafts but had no discernible effect on the grafts with a minor mismatch.

Animals↗

PubMed1996

Effects of mechanical testing device variables on polymer composite femoral stem strains.

Polymer composite femoral stems do not have a well-established in vitro mechanical testing method. The objective of this study was to examine mechanical testing devices for pressfit composite stems, using finite element analysis. The goals were to examine the effects of testing device design variables (geometry, material, interface friction, embedding height and applied load angle) and to reproduce the maximum strains of the stem implanted in a femur. The stem strains were affected by design changes to the testing device. The maximum normal and interlaminar shear strains of the composite stem in the femur were not as well reproduced by the testing device as were the maximum in-plane tensile strains. Decreasing the embedding height increased the stem strains and shifted the stem failure location from the neck to the embedding height. Testing a femoral stem using a testing device with a low embedding height may be inappropriate when trying to induce neck failure, since failure may occur at the embedding height instead of in the neck. A single-material testing device of birchwood, an orthotropic material with a longitudinal stiffness in the range of bone; best simulated a femur in this study.

Aluminum↗

PubMed1995

Reduction of patellofemoral contact forces following anterior displacement of the tibial tubercle.

Retropatellar pain often accompanies malalignment syndromes and frequently is attributed to excessive patellofemoral contact stresses. Elevation (anterior displacement) of the tibial tubercle has been recommended to relieve these conditions. The degree to which patellar contact forces are decreased and the extent to which elevation alters medial-lateral forces have not been studied directly. We performed anterior translation of the tibial tubercle in knees from cadavers and measured the effect on the magnitude of the three-dimensional patellofemoral contact force with use of a specially designed 6-degrees-of-freedom force transducer, with the natural patellar articular surface in place. Measurements were made in nine knees (average age 67 years, range 46-92 years). The resultant contact force decreased linearly with increasing tubercle elevations of as much as 2.5 cm. The average reduction per centimeter of elevation was 17% of the force measured with no elevation. Elevation of the tubercle had an inconsistent effect on the medial-lateral component of the contact force. As the elevation was increased, six knees exhibited an increase in the medial-lateral component of the contact force acting medially on the knee and three knees exhibited a decrease in this force component. The results of this study show that, while elevation of the tubercle without medialization reduced the total contact force on the patella, the medial-lateral component of this force was altered in an unpredictable way.

Aged↗

PubMed1995

Torsional loads in the early postoperative period following total hip replacement.

Torques generated in one subject during the early postoperative period were measured with a telemeterized total hip component. The patient was examined during gait, stair ascent, rising from a chair, and single-limb stance. The torques were plotted against both the resultant joint contact force and the force component directed along the stem axis. During gait, the maximum torque was 35 Nm, recorded at a walking velocity of 1.7 m/sec. The peak torques during stair ascent and during rising from a seated position were found to be 23 and 15 Nm, respectively. The maximum value for torque measured in this study was 37 Nm during one attempt at single-limb stance. Comparison of plots for torque versus stem-axis component for the four activities shows that the torque increased more rapidly for chair exits than for gait up to resultant contact force values of as much as 1,000 N. For stair ascent, the same was true to values of 1,400 N. Within any given activity, the relationship between stem torque and resultant or stem-axis force showed considerable variability. These results indicate that experiments evaluating the stability of femoral components in total hip arthroplasty should incorporate a component directed along the stem axis, as well as a component normal to the plane of the prosthesis. The results also suggest that theoretical stress analysis models should consider the broad variability in the orientation of the joint force at the hip.

Aged↗

PubMed1995

Effect of tibial component position on patellar strain following total knee arthroplasty.

Patellar fracture following total knee arthroplasty has been related to component alignment and tibiofemoral joint-line position. The purpose of this study was to determine the effect of anteroposterior displacement of the tibial component and inferosuperior displacement of the tibiofemoral joint line on patellar strain following total knee arthroplasty with a cruciate-substituting design. Patellar strain increased (decreased) with anterior and inferior (posterior and superior) displacement. When averaged across all flexion angles, the strain changed approximately 1.7%/mm of anteroposterior displacement and 1.0%/mm of inferosuperior displacement. For individual specimens, changes in strain as high as 3.2%/mm of inferosuperior displacement of the joint were seen. These changes in strain may be associated with changes in tibiofemoral joint-line position. These changes in strain may be a biomechanical indicator of the efficacy of retaining the preoperative position of the tibiofemoral joint line.

Adolescent↗

PubMed1995

Direct in vitro determination of the patellofemoral contact force for normal knees.

Results of the direct in vitro measurement of the full three-dimensional representation of the patellofemoral contact force and the point of application on the patella of the resultant contact force for eleven normal knees are presented. The applied knee moment versus flexion angle pattern was similar to that experienced when rising from a chair. There was a wide variability of the details of the patellofemoral force interaction among the specimens tested. The magnitude of the resultant contact force increased approximately linearly with flexion angle for some knees while in others the force leveled off or decreased at higher flexion angles. The change in direction of the resultant contact force with respect to the patella was relatively small compared to the angular rotation of the patella. The medial-lateral component of the contact force exhibited substantial variability among knees. The direction of this force (medially or laterally directed) varied among knees and, in some knees, changed direction as a function of flexion angle. The point of application on the patella of the resultant contact force migrated superiorly from 20 to 90 deg flexion. About 90 deg flexion this point tended to migrate inferiorly. The only significant and consistent effect of varying the direction of the quadriceps extension force was a change in the medial-lateral component of the contact force. In all cases, the tendency to sublux laterally increased when the extensor force was rotated 10 deg laterally and decreased when the extensor force was rotated 10 deg medially.

Aged↗

PubMed1994

A six-degree-of-freedom transducer for in vitro measurement of patellofemoral contact forces.

A satisfactory design of the patellar component used in total knee arthroplasty and a general understanding of the effects of orthopedic procedures require, in part, an accurate description of patellofemoral contact forces. Experimental determination of the contact stress distribution requires integration to estimate the contact forces. Due to complex patellar geometry, the determination of the components of the resultant load is difficult. Current techniques for the direct measurement of contact loads either report a single load component or require sequential experiments to fully describe the contact force. The present study describes the development of a six-degree-of-freedom patellofemoral force transducer. This transducer allows the simultaneous determination of the three components of the contact force with an increase in accuracy over available devices. The contact forces components are accurate to 1% of full scale (900 N anterior-posterior, +/- 200 N medial-lateral and inferior-superior). The location on the patella where the resultant force acts is determined to within 1 mm. Relative movement of this point as a function of either normal change in knee flexion angle or as a result of a typical orthopedic procedure is determined to within 0.1 mm. A protocol is presented that allows the contact forces to be determined for either the anatomically normal patella or a patella following total knee arthroplasty.

Calibration↗