Mechanical analysis of the human pelvis and its application to the artificial hip joint--by means of the three dimensional finite element method

Automated three-dimensional finite element modelling of bone: a new method

Three-dimensional finite element stress analysis of bone is a key to understanding bone remodelling, assessing fracture risk, and designing prostheses; however, the cost and complexity of predicting the stress field in bone with accuracy has precluded the routine use of this method. A new, automated method of generating patient-specific three-dimensional finite element models of bone is presented--it uses digital computed tomographic (CT) scan data to drive the geometry of the bone and to estimate its inhomogeneous material properties. Cubic elements of a user-specified size are automatically defined and then individually assigned the CT scan-derived material properties. The method is demonstrated by predicting the stress, stain, and strain energy in a human proximal femur in vivo. Three-dimensional loading conditions corresponding to the stance phase of gait were taken from the literature and applied to the model. Maximum principal compressive stresses of 8-23 MPa were computed for the medial femoral neck. Automated generation of additional finite element models with larger numbers of elements was used to verify convergence in strain energy.

[The cementless artificial hip cup--implant components with reliable long-term stability?]

A hip joint simulator was developed to analyse the mechanism of loosening of cementless artificial hip cups. The machine induces vibrating motions and asymmetrical tilt shock loadings of the artificial cup. On measuring the primary stability of threaded cups, the simulation tests performed on pelvis substitute models, and animal and human acetabula failed to show any loosening of threaded cups fixed in place with a screw-in torque of more than 10 Nm. Instable cups became loose all the sooner, the lower the preload between the cup and femoral head. This demonstrates the importance of both the screw-in torque of the cup and the training status of the periarticular muscles.

Normal pelvic strain pattern in vitro

Four cadaver pelves were dissected of soft tissue and each of the eight hemipelves instrumented with ten rosette strain gauges. Static loading was conducted to simulate single leg stance, and applied through the intact hip joint. The medial portion of the pelvis was under tension directed vertically and the lateral ilium was in compression. This strain pattern is consistent with bending applied to the ilium from the action of the abductor and joint reaction forces.

Stress analysis of a total hip acetabular component: an FEM study

A stress analysis of a total hip acetabular component was performed using three-dimensional finite element modeling. The model consisted of 548 four-noded quadrilateral shell elements with 582 nodes. A worst-case support condition was assumed in which bony contact with small areas of the ilium, ischium, and pubis was represented by three-point support. Loads corresponding to the peak pressures developed in the hip when rising from a seated position--the activity contributing most to the fatigue of a prosthesis--were applied to the model. Peak stresses of 490 MPa are predicted around a screw hole in the region of greatest loading; according to the fatigue curve for porous-coated Ti-6Al-4V alloy, this translates into a component life of three years. When the area of bony support for the prosthesis is increased by a factor of 1.5, the fatigue life is more than doubled to seven years. The addition of restraints around the loaded hole to represent the placement of a screw extends the lifetime dramatically to 65 years. Substituting the material properties of cobalt-chromium alloy increases the predicted lifetime of the component to twelve years. We conclude that poor bone support can compromise the service life of titanium alloy acetabular components. Furthermore, we suggest that screw fixation be used to secure the prosthesis in the anterior-superior region if the prosthesis is not well supported by bone in the acetabulum.

Quantitation of human shoulder anatomy for prosthetic arm control--I. Surface modelling

Anatomical data and models for the human shoulder musculo-skeletal system are developed with the intent of quantifying physiological subcomponents of a model-based multi-axis prosthetic limb control scheme which has heretofore been implemented empirically. Part I presents the controller formulation, the surface descriptions of the muscles (and bones), and the centroidal trajectory data of the muscles. The data partially quantify the muscle modelling components of the controller, and set the stage for the analysis of the force-to-moment anatomical conversion factors of Part II.

Finite element analysis of acetabular reconstruction. Noncemented threaded cups

The load-transfer mechanism and stress patterns in the acetabulum reconstructed with a threaded cup were investigated relative to the normal hip using the Finite Element Method (FEM). Several models were applied in the analysis to investigate the effects of axisymmetric 3-D versus 2-D approaches, inclusion of the femoral head, and local stress patterns around the threads in three types of implant/bone bonding modes. It was found that the threaded ring behaves as a relatively rigid implant shielding the trabecular bone and enhancing load transfer through the cortical shells. Load transfer from ring to bone is concentrated at the first and last threads where the subchondral bone layer is penetrated. Stress peaks of up to 24 MPa occur in these regions.

A physico-mathematical model for the human femur, with and without a prosthesis, under the static constraints of one-legged stance

The authors present a physico-mathematical model of a human femur, under "monopodal" static constraints, using the finite elements method. Three examples are considered: a normal femur, a femur implanted with a short-stem prosthesis without cement, and a femur implanted with a long-stem prosthesis without cement. The lines of isoconstraints were compared in the three examples, as well as the main constraints (direction and intensity). From the results, the authors suggest that a prosthesis made of titanium is currently best even though its YOUNG's modulus differs from that of the bone. A prosthesis of composite material is possible in the future. While the intensity of the constraints is nearly the same at the level of the epiphysis for the short-stem and long-stem prosthesis it seems that the short-stem prosthesis fitted accurately without cement is the best solution. The introduction of a hip prosthesis modifies the normal curve of the loaded femur by changing the center of this curve.

Pelvic stresses in vitro--II. A study of the efficacy of metal-backed acetabular prostheses

Six embalmed human cadaveric hemi-pelves with their associated proximal femurs have been tested in vitro using 25 strain-gauge rosettes on each hemi-pelvis. Loads were applied up to 2.5 kN and principal stresses were computed from the principal strain data. Acetabular prostheses, cemented in place upon a cartilage-devoid but intact subchondral bone-plate, showed little change in stress-patterns when compared with the normal data, regardless of whether or not the component employed metal-backing. The use of 30 anchoring holes of 6.4 mm diameter, in the intact subchondral bone-plate, had little effect upon the stress-patterns, regardless of whether metal-backing was employed upon the prosthesis. When the subchondral bone-plate was removed, there were notable changes in the stress-pattern in the periacetabular region and on the medial wall of pelvis. The metal-backed prosthesis did not produce such notable changes as its plastic counterpart, when the subchondral bone-plate was removed. The use of a plastic prosthesis cemented in a Protrusio ring, in an acetabulum devoid of subchondral bone, produced notable changes in the stress-patterns in the whole periacetabular region and on the medial wall.

Pelvic stresses in vitro--I. Malsizing of endoprostheses

Seven embalmed human cadaveric hemi-pelves have been stripped of all soft tissues except the articular cartilage and instrumented with 25 strain-gauge rosettes on both the medial and lateral surfaces. Loads up to 2.5 kN were applied, without simulated muscle forces, to mimic the line-of-action of the resultant joint-force in a single-legged stance. In this parametric study, endoprostheses were studied at the 'correct' size and in sizes of 1 and 2 mm above and below the correct size. The correct size of endoprosthesis produced similar periacetabular stress data to those obtained with the normal femoral head; however, significant changes in stress on the medial and lateral aspects of the ilium suggested the correctly-sized spherical endoprosthesis had a different moment arm than its anatomical counterpart. Endoprostheses of 1 mm greater than the correct size caused major increases in the magnitude of both periacetabular and medial-wall stresses, while prostheses of +2 mm created dramatic and significant increases in magnitudes of these stresses. Endoprostheses smaller than the correct size caused only small but significant changes in the pattern of stresses up to -2 mm malsizing and, in particular, were associated with the disappearance of all periacetabular tensile stresses; however, stress data on the medial and lateral aspects of the ilium were more closely matched to the 'normal' for this -2 mm of undersizing.

Effects of prosthetic acetabular replacement on strains in the pelvis

The effects of cemented acetabular components on the strains in the pelvis were determined experimentally. Four components were tested: A plain metal hemisphere; a metal hemisphere with a triflange; a hemisphere with an iliac peg; and a hemisphere with a complete flange and a peg. Loading was carried out in neutral, flexion, and extension directions. The subchondral bone was only slightly reamed, and components were cemented and tested successively. For the intact hip joint, the strains on the superior acetabular rim were much lower than those on the posterior rim. After placement of the components, the tensile strains on the rim decreased by 22-54% while the compressive strains increased by 0-25%. The bone on the medial inferior surface was subjected to 65% less tensile strain and 48% more compressive strain in neutral and extension loading. There were no great differences between the four styles of acetabular component.

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.