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Biomedical subjects

D R Carter

Publications and source records attributed to D R Carter.

At least 109 records · Page 6Linked to original sources
PubMed1986

Epiphyseal-based designs for tibial plateau components--I. Stress analysis in the frontal plane.

Two-dimensional, finite element studies were conducted of the proximal tibia before and after joint arthroplasty. Equivalent-thickness models projected onto the mid-frontal plane were created for the natural, proximal tibia and for the proximal tibia with four different types of tibial plateau components. All components simulated bony ingrowth fixation, i.e. no cement layer existed between component and bone. In addition, the interface between component and bone was assumed to be intimately connected, representing complete bony ingrowth and a rigid state of fixation. Loads consisted of bi-condylar and uni-condylar forces. Results indicated that conventional plateau designs with central posts or multiple pegs led to higher stress magnitudes in the trabecular bone near the distal ends of the post/pegs and stress shielding at more proximal locations. A design without posts or pegs whose interface geometry mimics the epiphyseal plate minimizes bone stress shielding. An implant consisting of separate components covering each condyle was found effective in limiting component tilting and the consequent tensile stresses caused by non-symmetrical, uni-condylar loading.

Epiphyses↗
PubMed1986

Epiphyseal-based designs for tibial plateau components--II. Stress analysis in the sagittal plane.

A two-dimensional, finite element study was undertaken to establish the stresses in the proximal tibia before and after total knee arthroplasty. Equivalent-thickness models in a sagittal plane were created for the natural, proximal tibia and for the proximal tibia with two different types of tibial plateau components. All components simulated bony ingrowth fixation, i.e. no cement layer existed between component and bone. In addition, the interface between component and bone was assumed to be intimately connected, representing complete bony ingrowth and a rigid state of fixation. Two load cases were considered: a joint reaction force acting in conjunction with a patellar ligament force, simulating the knee at 40 degrees of flexion; and a joint reaction force directed along the long axis of the tibia. For the natural tibia model, the pattern of principal stresses for loadcase 1 more closely corresponds to the epiphyseal plate geometry and trabecular morphology than do the principal stress patterns for loadcase 2. Judging from the distribution of principal stresses, loadcase 1 represents a more severe test of implant design than does loadcase 2. The model of the component with a peg predicted that the trabecular bone near the tip of the peg will experience higher than normal stresses, while the bone stresses near the posterior aspect adjacent to the metal tray will be reduced. A component without pegs that incorporates a posterior chamfer and an anterior lip lead to stress distributions closer to those existing in the natural tibia. The interface geometry for this design is based upon the contour of the epiphyseal plate.

Epiphyses↗
PubMed1985

A cumulative damage model for bone fracture.

A mathematical model is presented to describe the combined time-dependent and cycle-dependent fracture characteristics of devitalized cortical bone. Failure is interpreted based on a linear-life fraction rule, which accounts for cumulative creep and fatigue damage under arbitrary loading histories. The model is successful in describing the influence of loading rate on monotonic tensile strength, the time to failure in constant stress creep-fracture tests, and bone fracture in zero-tension and tension-compression cyclic loading. The possible implications of the model to in vivo bone fracture, deformity, and remodelling in response to various loading histories are considered.

Adult↗
PubMed1985

Stress analyses of joint arthroplasty in the proximal humerus.

Finite element stress analyses were performed on the proximal humerus before and after the simulated implantation of stemmed, metallic prosthetic components with porous sintered surfaces for direct bony attachment. Design geometries with surfaces at the prosthetic head/bone interface that were (a) convex, (b) flat, and (c) concave were studied. Analyses for each of the three geometries were conducted to reflect (a) bone ingrowth on all the prosthesis/bone surfaces and (b) bone ingrowth only along the underside of the prosthetic humeral head (assuming the stem was not coated with a porous material). Three loading conditions were used to model various degrees of abduction of the arm. Results indicated that in the normal humerus the compressive joint forces are transmitted from the articular surface through cancellous bone to the inferior cortical shell. Contraction of the rotator cuff muscles created tensile stresses in the superolateral cancellous bone and the superior cortical shell of the humerus. Results of the implanted humeral component models indicated that the use of a prosthesis with bone ingrowth along the stem would cause marked stress shielding proximally whereas the use of implants with porous ingrowth only on the underside of the humeral head replacement produced stress fields more similar to the normal humerus. The convex, flat, and concave surfaces provided similar load transfer from the component to the underlying bone in all loading cases. Other prosthetic head designs that may offer better initial stability produced stress fields similar to those of existing prostheses.

Biomedical Engineering↗
PubMed1985

Contact finite element stress analysis of the hip joint.

Two-dimensional finite element analyses were conducted of the normal hip using contact elements at the joint surface. The models studied were constructed for a slice through the pubis, acetabulum, and ilium. In the analyses the proximal femur was pressed into the acetabulum and intraarticular pressures and principal stresses in the joint region were determined for different load magnitudes and directions and various boundary conditions. Three sets of boundary conditions were examined: (a) deformable pubic symphysis, (b) rigid pubic symphysis, and (c) simulations of experimental studies. In the deformable model the pubic symphysis was free to displace in the sagittal plane and rotate. In the rigid model the pubic symphysis was rigidly fixed. Superoposterior loading resulted in high-contact pressures at the acetabular dome for all sets of boundary conditions. For the deformable model subject to a more medially directed load the acetabulum closed in such a manner as to squeeze the head of the femur creating high-contact pressures superiorly and inferiorly. This resulted in significant compressive stresses in the superior dome cancellous bone and inferior cancellous bone. The cumulative effect of this squeezing action with normal biological remodeling may cause elongation of the femoral head resulting in asphericity and incongruity of the unloaded hip joint articular surfaces. Rigidly fixing the pubic symphysis stiffened the model and resulted in principal stress patterns that did not reflect trabecular density or orientations as well as those of the deformable pubic symphysis model. Finite element simulations of previous experimental studies modeled the close proximity of the fixation to the excised acetabulum. These boundary conditions prevented the squeezing caused by pelvis deformations. The resulting contact areas, pressure distributions, and bone stresses were very different from those of the more anatomic, deformable pubic symphysis model. These findings demonstrate the sensitivity of hip contact pressures and stresses to imposed boundary conditions and indicate that care should be taken to simulate anatomic conditions in experimental and theoretical studies.

Acetabulum↗
PubMed1985

Fatigue of immature baboon cortical bone.

Strain-controlled uniaxial fatigue and monotonic tensile tests were conducted on turned femoral cortical bone specimens obtained from baboons at various ages of maturity. Fatigue loading produced a progressive loss in stiffness and an increase in hysteresis prior to failure, indicating that immature primate cortical bone responds to repeated loading in a fashion similar to that previously observed for adult human cortical bone. Bone fatigue resistance under this strain controlled testing decreased during maturation. Maturation was also associated with an increase in bone dry density, ash fraction and elastic modulus. The higher elastic modulus of more mature bone meant that these specimens were subjected to higher stress levels during testing than more immature bone specimens. Anatomical regions along the femoral shaft exhibited differences in strength and fatigue resistance.

Animals↗
PubMed1985

Long bone torsion: I. Effects of heterogeneity, anisotropy and geometric irregularity.

The influences of heterogeneity, anisotropy and geometric irregularity on the unrestrained, linearly elastic torsional response of long bones are assessed. Longitudinal geometric variations contribute insignificantly to the torsional response for typical long bone geometries. Anisotropy, heterogeneity and transverse geometric irregularity significantly influence the torsional response. A procedure is discussed which uses an approximate means to characterize both heterogeneity and anisotropy in predicting the torsional response. The accuracy of circular and elliptical annulus models of the bone cross-sectional geometry are assessed by comparing the stress predictions of these simple models to those of finite element models of the bone geometry.

Animals↗
PubMed1985

Long bone torsion: II. A combined experimental and computational method for determining an effective shear modulus.

A technique is established which allows an effective torsional shear modulus to be determined for long bones, while remaining nondestructive to whole bone specimens. Strain gages are bonded to the diaphysis of the bone. Strains are then recorded under pure torsional loads. Theoretical stress predictions are combined with experimental strain recordings to arrive at a modulus value. Shear modulus calculations for four canine radii are reported using theoretical stress predictions from circular, elliptical and finite element models of the transverse bone geometry. The effective shear modulus, obtained from an average of the shear moduli determined at strain gage locations, serves to average the heterogeneous shear modulus distribution over the cross section. The shear modulus obtained is that associated with the "circumferential" direction in transverse planes.

Animals↗
PubMed1984

Changes in long-bone structural properties during the first 8 weeks of plate implantation.

The midplate structural rigidities of metal- and plastic-plated intact canine femora were experimentally determined after initial plate application and 8 weeks after in vivo implantation. Composite beam theory significantly overestimated the bending rigidities of the metal-plated bones. The rigidities of the plastic-plated bones were nearly identical to that of the isolated bone, as composite beam theory predicted. Plating with either plate increased the intracortical porosity and caused the deposition of periosteal new bone, which was greater with plastic than with metal plates. The increased rigidities provided by the attachment of the metal plates and, hence, the degree of bone strain shielding were variable. Platings for 8 weeks which provided little strain shielding with either metal or plastic plates caused an increase in bone flexural rigidity (measured after plate removal) with respect to the contralateral control. Platings that provided increasing amounts of strain shielding caused a decreasing midplate bone rigidity (measured after plate removal) and increasing bone deposition at the outer screws. These findings suggest that the surgical implantation of any plate (metal or plastic) will provide a net stimulus to bone formation and consequently increased bone structural rigidity, even though intracortical porosity is increased. If the plate significantly reduces the normal loads borne by the bone, however, there is a net stimulus to remove bone, resulting in a loss of midplate structural rigidity within 8 weeks of implantation.

Animals↗
PubMed1984

Mechanical loading histories and cortical bone remodeling.

A conceptual framework is presented for understanding and investigating structural adaptation of cortical bone. The magnitudes, orientations, and sense (tension or compression) of the physiologically incurred cyclic principal strains vary markedly throughout the skeleton. It is probable, therefore, that the strain/remodeling response of bone is site specific. Furthermore, there is some indication that immature bone is more responsive to alterations of cyclic strains than mature bone. Animal experimental studies and complementary stress and strain analyses suggest that the structural adaptation due to changes in cyclic strain fields may be a very nonlinear response. Bone loss in mature animals due to immobilization is sensitive to even small changes in the cyclic bone strains. Under normal conditions, however, there appears to be a broad range of physical activity in which bone is relatively unresponsive to changes in loading history. With severe repeated loading, bone hypertrophy can be pronounced. These observations open the possibility that bone atrophy and hypertrophy are controlled by different mechanisms. Therefore, two (or more) complementary control systems may be involved in the regulation of bone mass by bone cyclic strain histories. It is probable that bone mechanical microdamage is one control stimulus for affecting an increase in bone mass.

Adaptation, Physiological↗
PubMed1984

Stress changes in the femoral head due to porous ingrowth surface replacement arthroplasty.

Finite element stress analyses were conducted of the canine femoral head before and after implantation of various surface replacement-type components. The femoral head was replaced by four implant geometries; (a) shell, (b) shell with peg, (c) shell with rod, and (d) a new epiphyseal replacement design. All implants were modelled to simulate bony ingrowth along the underside of the shell and along the surfaces of the peg and rod. The results indicated that in the normal femur the forces are transferred from the articular surface through the femoral head cancellous bone to the inferior cortical shell of the femoral neck. After shell-type surface replacement, forces were transferred more distally at the rim of the shell and at the end of the peg or rod, thereby reducing the stresses in the proximal head cancellous bone. Computer simulation of bone remodelling due to proximal bone stress reduction was shown to accentuate the abnormality of the stress fields. Surface replacement with a lower modulus material created a less abnormal redistribution of bone stresses. The new epiphyseal replacement design resulted in stress distributions similar to those in the normal femoral head and minimal shear stresses at the implant/bone interface. These findings suggest that the epiphyseal replacement concept may provide better initial mechanical integrity and create a more benign milieu for adaptive bone remodelling than conventional, shell-type surface replacement components.

Acetals↗
PubMed1984

Comparative fatigue behavior of different bone cements.

Tensile and fatigue studies were performed on four different preparations of acrylic bone cement: (1) surgical Simplex-P inserted into molds in the dough stage, (2) Zimmer Low Viscosity Cement (LVC) inserted in the liquid phase, (3) an experimental cement inserted in the dough phase, and (4) the same experimental cement inserted in the liquid phase. The void characteristics of the cements appeared to dictate their mechanical performance. While tests revealed no difference in the tensile strengths of the four cement preparations, small but statistically significant differences in mean fatigue life were observed. The experimental cement used in the dough stage exhibited superior fatigue characteristics when compared with Simplex and LVC. LVC had the poorest mechanical properties of the four cement groups. Since the specimen preparation procedures can markedly influence the cement void characteristics and, consequently, the mechanical properties, general statements about different cement types are offered with considerable reservations.

Bone Cements↗
PubMed1983

Cycle-dependent and time-dependent bone fracture with repeated loading.

Fatigue tests of human cortical bone (up to 1.74 X 10(6) cycles) were conducted under tension-compression (T-C) and zero-tension (O-T) modes with a 2Hz, stress controlled, sinusoidal loading history. Tensile creep-fracture tests at constant stress levels were also performed. The relationship between the initial cyclic strain range and cycles to failure with the T-C specimens were consistent with that derived previously in low-cycle fatigue under strain control. Using a time-dependent failure model, the creep-fracture data was found to be consistent with previous studies of the influence of strain rate on the monotonic tensile strength of bone. The model also predicted quite well the time to failure for the O-T fatigue specimens, suggesting that creep damage plays an important role in O-T fatigue specimens.

Adult↗
PubMed1983

Evaluation of bone cement failure criteria with applications to the acetabular region.

A Strain energy density (SED) criterion based on a fracture mechanics approach was used to assess the possible failure of acetabular bone cement after total hip replacement. Stress distributions in the cement at the bone-cement interface were calculated using two-dimensional finite element analyses. The results indicate that increasing the thickness of bone cement reduces the risk of cement fracture. The addition of a metal backing to the polyethylene cup and retention of the subchondral bone further reduces the risk of failure. The SED criterion was found to predict the same critical regions as zones of possible cement failure as the von Mises' criterion. Although either criterion can be used for predicting failure in this acetabular analysis both criteria are excessively conservative in predicting failure in regions where high principal compressive stresses are present. Further development of cement failure criteria are indicated.

Acetabulum↗
PubMed1983

Tensile fatigue failure of acrylic bone cement.

Tensile fatigue tests of acrylic bone cement were conducted under strain control in a wet environment at 37 degrees C. A constant strain rate of 0.02s-1 was used, resulting in physiologic loading frequencies. Comparison of the tensile fatigue data with the results of previous tension-compression fatigue tests indicates that fatigue failure is governed primarily by the maximum cyclic tensile strain. The compressive portion of the loading cycle has little effect on the number of cycles to failure. A new empirically derived equation is introduced to describe the influence of mean strain and strain amplitude on fatigue endurance. The results emphasize the critical role tensile strains may play in cement failure and loosening of total joint replacements.

Bone Cements↗
PubMed1983

Periacetabular stress distributions after joint replacement with subchondral bone retention.

Two-dimensional finite element stress analyses were conducted of the acetabular region after total joint replacement. The effect of subchondral bone retention was evaluated for both conventional and metal-backed acetabular components. Stresses in the bone, cement and acetabular cup were significantly reduced when subchondral bone was retained for both component types. The results indicate that the most favorable stress conditions are created when a metal-backed acetabular component is implanted with subchondral bone retention.

Acetabulum↗
PubMed1983

Effects of spaceflight on structural and material strength of growing bone.

Rats in space for 18.5 days did not exhibit the normal gain in femoral bone strength of terrestrial controls. The strength deficit may have been caused by multiple factors including a diminished bone formation and an inhibition of the gain in tissue material strength. Centrifugation at 1g in space substantially enhanced bone strength, possibly by promoting more normal tissue maturation. Full recovery of bone strength was achieved 25 days after reentry.

Animals↗