PubMed Health⌕ Search

Biomedical subjects

J H Kuiper

Publications and source records attributed to J H Kuiper.

10 recordsLinked to original sources
PubMed1999

Malunion in the lower limb. A nomogram to predict the effects of osteotomy.

Nomograms derived from mathematical analysis indicate that the level of malunion is the most important determinant of changes in the moment arm of the knee, the plane of the ankle and alterations in limb length. Testing in five patients undergoing reconstruction showed a mean error of postoperative limb length of 2.2 mm (SD 0.8 mm), knee moment arm of 4.7 mm (SD 3.3 mm) and ankle angle of 2.6 degrees (SD 2.3 degrees). These nomograms provide the information required when assessing whether a particular degree of angulation may be accepted.

Algorithms↗
PubMed1997

Mathematical optimization of elastic properties: application to cementless hip stem design.

The designer of a cementless hip stem in total hip replacement must find a balance between two conflicting demands. On the one hand, a stiff stem shields the surrounding bone from mechanical loading (stress shielding), which may lead to bone loss, particularly around the proximal part of the stem. Reducing the stem stiffness decreases the amount of stress shielding and hence the amount of bone loss. However, this measure inevitably promotes higher proximal interface stresses and thereby increases the risk of proximal interface failure. The designer's task therefore is to optimize the stem stiffness in order to find the best compromise in the conflict. Yet, a better compromise might be found when the stem material was nonhomogeneous, in other words when an arbitrary distribution of the elastic properties inside the stem was allowed. The number of conceivable designs would increase enormously, making the designer's task almost impossible. In the present paper, we develop a numerical design optimization method to determine the optimal stiffness characteristics for a hip stem. A finite element program is coupled with a numerical optimization method, thus producing a design optimization scheme. The scheme minimizes the probability for interface failure while limiting the amount of bone loss, by adapting the parameters describing the nonhomogeneous elastic modulus distribution. As an example, a simplified model of a hip stem is made, whose modulus distribution is optimized. Assuming equal long-term bone loss, the maximum interface stress can be reduced by over 50 percent when compared to a homogeneous flexible stem, thus demonstrating the value of the new approach.

Elasticity↗
PubMed1997

The predictive value of stress shielding for quantification of adaptive bone resorption around hip replacements.

The presence of a femoral hip stem changes local mechanical signals inside the surrounding bone. In this study we examined the hypothesis that the eventual loss of bone can be estimated from the initial patterns of elastic energy deviation, as determined in FE models of the intact bone and the operated femur. For that purpose two hypothetical relations between elastic energy reduction and resorption were investigated. Their estimates of bone loss were compared to the results of iterative computer simulations. Two kinds of FE model were used, and in each stem stiffness and remodeling threshold (a measure of "biological reactivity") were varied. Provided that reasonable values of the remodeling threshold are assumed and that the stem is firmly bonded to the bone, we found that the difference between direct estimates and simulation models was 4 percent of bone loss. It is therefore concluded that initial patterns of elastic energy deviation give a reasonable indication of expected bone loss.

Adaptation, Physiological↗
PubMed1996

Friction and stem stiffness affect dynamic interface motion in total hip replacement.

Large cyclic movements between the femoral stem and bone during the first weeks after total hip arthroplasty may hamper bone ingrowth and adversely affect the eventual success of the arthroplasty. Little is known, however, about the magnitude of the motions and its relationship to design and surgical factors. A two-dimensional finite element model of a cementless prosthesis inserted into the proximal femur was constructed to study the effects of two mechanical variables--the stiffness of the implant and the coefficient of friction between bone and implant--on the magnitude of the motions. We investigated the influences of these variables on the subsidence of the prosthesis, the magnitudes of the cyclic motions, and the level of the interface stresses. The presence of friction reduced cyclic motions by about 85% compared with a frictionless interface. Once friction was assumed, varying the coefficient of friction had little effect. The effect of friction on the interface stress state and gross subsidence of the prosthesis was not as great as on cyclic motion. Implant stiffness also affected the magnitudes and distributions of the cyclic motions along the interface. A flexible stem generated motions about three to four times larger proximally than those of a stiff stem, which generated larger motions distally. The influence of stem stiffness on interface stresses and prosthetic subsidence was less than on cyclic motion. The location of the peak shear stresses at the interface around a bonded prosthesis corresponded to the location where cyclic interface motion was maximal for an unbonded prosthesis. However, no direct relationship was found between the magnitudes of peak stresses and the amplitudes of cyclic motions.

Elasticity↗
PubMed1996

A comparison of finite element codes for the solution of biphasic poroelastic problems.

The extent to which three finite element codes can solve problems in biphasic poroelasticity is examined. Both linear and non-linear (that is where the permeability of the material is a function of deformation) formulations are studied using the standard confined compression problem as an example. The results are compared, where possible, to the results of Spilker, Suh and Mow who first presented a solution to this problem for cartilage. The agreement is excellent for the linear case. For the non-linear case, the solutions do differ slightly; however, the difference would not appear to be significant.

Biomechanical Phenomena↗
PubMed1996

Stiffness measurements to assess healing during leg lengthening. A preliminary report.

We describe a technique for measuring the stiffness of regenerate bone after leg lengthening. This allows early identification of slow healing by reference to normal patterns. We determined the time of removal of the fixator from clinical and radiological information independent of the stiffness result. In a series of 30 leg lengthenings there were no refractures when the tibial stiffness had reached 15 Nm/degree or the femoral stiffness 20 Nm/degree. Three refractures occurred at lower stiffness values. The technique is simple to perform, will allow a reduction in plain radiography and is recommended for routine postoperative management.

Adolescent↗
PubMed1995

Dynamisation of tibial fractures.

We studied the effect of 'dynamisation' on tibial fractures in six patients treated by the Dynamic Axial Fixator. In the early stages, peak cyclic movement at two to four weeks averaged 0.75 mm (0.19 to 1.02) on the medial side of the bone and 0.86 mm (0.21 to 1.25) on the lateral side. The amount of movement correlated with the applied load and the fracture stiffness. After unlocking the fixator column at six weeks, progressive closure of the gap averaged 1.3 mm (0.1 to 3.5). Cyclic movement is produced by early weight-bearing with the fixator column locked. Progressive closure occurs after unlocking the column, and is often associated with a reduction in cyclic movements. The effects of dynamisation on movement at the fracture site should be defined separately, in terms of cyclic movement and of progressive closure.

Adult↗
PubMed1992

Stress distribution around dental implants: influence of superstructure, length of implants, and height of mandible.

The stress distribution around dental implants was investigated by use of a two-dimensional model of the mandible with two implants. A vertical load of 100 N was imposed on abutments or the bar connection. The stress was calculated for a number of superstructures under different loading conditions with the help of the finite element method. The length of the implants and the height of the mandible were also varied. A model with solitary abutments showed a more uniform distribution of the stress when compared with a model with connected abutments. The largest compressive stress was also less in the model without the bar. Using shorter implants did not have a large influence on the stress around the implants. When the height of the mandible was reduced, a substantially larger stress was found in the bone around the implants because of a larger overall deformation of the lower jaw.

Bite Force↗
PubMed1991

Articular contact in a three-dimensional model of the knee.

This study is aimed at the analysis of articular contact in a three-dimensional mathematical model of the human knee-joint. In particular the effect of articular contact on the passive motion characteristics is assessed in relation to experimentally obtained joint kinematics. Two basically different mathematical contact descriptions were compared for this purpose. One description was for rigid contact and one for deformable contact. The description of deformable contact is based on a simplified theory for contact of a thin elastic layer on a rigid foundation. The articular cartilage was described either as a linear elastic material or as a non-linear elastic material. The contact descriptions were introduced in a mathematical model of the knee. The locations of the ligament insertions and the geometry of the articular surfaces were obtained from a joint specimen of which experimentally determined kinematic data were available, and were used as input for the model. The ligaments were described by non-linear elastic line elements. The mechanical properties of the ligaments and the articular cartilage were derived from literature data. Parametric model evaluations showed that, relative to rigid articular contact, the incorporation of deformable contact did not alter the motion characteristics in a qualitative sense, and that the quantitative changes were small. Variation of the elasticity of the elastic layer revealed that decreasing the surface stiffness caused the ligaments to relax and, as a consequence, increased the joint laxity, particularly for axial rotation. The difference between the linear and the non-linear deformable contact in the knee model was very small for moderate loading conditions. The motion characteristics simulated with the knee model compared very well with the experiments. It is concluded that for simulation of the passive motion characteristics of the knee, the simplified description for contact of a thin linear elastic layer on a rigid foundation is a valid approach when aiming at the study of the motion characteristics for moderate loading conditions. With deformable contact in the knee model, geometric conformity between the surfaces can be modelled as opposed to rigid contact which assumed only point contact.

Elasticity↗