Jan Herman Kuiper

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.

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.

Impaction bone-grafting in revision joint replacement surgery

The standard graft material for impaction bone-grafting is fresh-frozen femoral head allograft morselized to a particle size as large as is practical to ensure stability and allow new bone formation. The graft must be sufficiently compacted to provide immediate mechanical stability; this requires containment of the graft and substantial impaction energy. Diaphyseal bone fracture and excessive implant migration are the most common complications of the operation. Impaction bone-grafting in revision total hip replacement has produced good medium-term results on both the acetabular and the femoral side. The use of compacted morselized bone graft is a relatively new technique in revision knee surgery and requires longer-term follow-up with larger numbers of patients to assess its value.

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 (1) 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.

The prediction of stress fractures using a ‘stressed volume’ concept

This paper addresses an anomaly which exists in the current literature regarding stress fractures. Analysis of the data on fatigue strength of bone samples in vitro would conclude that these fractures should never occur at the strain levels known to occur in vivo. This anomaly can be resolved by including in the analysis the effect of stressed volume, whereby larger volumes of material are expected to have worse fatigue properties. A Weibull analysis was used to predict the probability of failure, Pf; this was an upperbound prediction because it did not include the effects of remodelling and adaptation. Combining this analysis with a finite element model of the human tibia, we predicted a Pf value of 21% after five weeks of strenuous exercise, which is comparable with reported incidences in military personnel. The high incidence of stress fractures in the cannon bone of racehorses could also be predicted (Pf = 62%, compared to 70% experimentally). The approach can be used to investigate the effect of variables in the exercise regime such as the distance run per day and the use of improved footwear. It can also predict the increased risk of stress fractures in elderly people. The results suggest certain simple rules which may be of clinical value in designing exercise regimes and in understanding the risk factors for this type of injury.

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 designers 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 designers 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.

Influence of Core Suture Material and Peripheral Repair Technique on the Strength of Kessler Flexor Tendon Repair

The purpose of our study was to determine the most favourable combination of core suture material and peripheral repair technique for Kessler tendon repair. Thirty freshly thawed pig flexor tendons were repaired by a Kessler technique, either with braided polyester or monofilament nylon suture. A peripheral augmentation was done using one of the three techniques – running, cross-stitch and Halsted. All repairs were tested by cyclic loading, followed by load-to-failure. During cyclic loading six of the 15 tendons with a nylon core failed, but none with a braided polyester core. Irrespective of peripheral technique, the monofilament nylon core suture allowed early central cyclic gapping, resulting in failure of the repair. During load-to-failure testing, the running stitch proved weakest and the cross-stitch repair toughest.

Midterm to long-term longitudinal outcome of autologous chondrocyte implantation in the knee joint:a multilevel analysis

Background Autologous chondrocyte implantation is a cell therapeutic approach for the treatment of chondral and osteochon-dral defects in the knee joint. The authors previously reported on the histologic and radiologic outcome of autologous chondro-cyte implantation in the short- to midterm, which yields mixed results. Purpose The objective is to report on the clinical outcome of autologous chondrocyte implantation for the knee in the midterm to long term. Study Design Cohort study; Level of evidence, 3. Methods Eighty patients who had undergone autologous chondrocyte implantation of the knee with mid- to long-term follow-up were analyzed. The mean patient age was 34.6 years (standard deviation, 9.1 years), with 63 men and 17 women. Seventy-one patients presented with a focal chondral defect, with a median defect area of 4.1 cm2 and a maximum defect area of 20 cm2. The modified Lysholm score was used as a self-reporting clinical outcome measure to determine the following: (1) What is the typical pattern over time of clinical outcome after autologous chondrocyte implantation; and (2) Which patient-related predictors for the clinical outcome pattern can be used to improve patient selection for autologous chondrocyte implantation? Results The average follow-up time was 5 years (range, 2.7-9.3). Improvement in clinical outcome was found in 65 patients (81%), while 15 patients (19%) showed a decline in outcome. The median preoperative Lysholm score of 54 increased to a median of 78 points. The most rapid improvement in Lysholm score was over the 15-month period after operation, after which the Lysholm score remained constant for up to 9 years. The authors were unable to identify any patient-specific factors (ie, age, gender, defect size, defect location, number of previous operations, preoperative Lysholm score) that could predict the change in clinical outcome in the first 15 months. Conclusion Autologous chondrocyte implantation seems to provide a durable clinical outcome in those patients demonstrating success at 15 months after operation. Comparisons between other outcome measures of autologous chondrocyte implantation should be focused on the clinical status at 15 months after surgery. The patient-reported clinical outcome at 15 months is a major predictor of the mid- to long-term success of autologous chondrocyte implantation.

The Oswestry Risk Index: An aid in the treatment of metastatic disease of the spine

The revised Tokuhashi, Tomita and modified Bauer scores are commonly used to make difficult decisions in the management of patients presenting with spinal metastases. A prospective cohort study of 199 consecutive patients presenting with spinal metastases, treated with either surgery and/or radiotherapy, was used to compare the three systems. Cox regression, Nagelkerkes R(2) and Harrells concordance were used to compare the systems and find their best predictive items. The three systems were equally good in terms of overall prognostic performance. Their most predictive items were used to develop the Oswestry Spinal Risk Index (OSRI), which has a similar concordance, but a larger coefficient of determination than any of these three scores. A bootstrap procedure was used to internally validate this score and determine its prediction optimism. The OSRI is a simple summation of two elements: primary tumour pathology (PTP) and general condition (GC): OSRI = PTP + (2 - GC). This simple score can predict life expectancy accurately in patients presenting with spinal metastases. It will be helpful in making difficult clinical decisions without the delay of extensive investigations.

Long-term clinical outcomes following the use of synthetic hydroxyapatite and bone graft in impaction in revision hip arthroplasty

Impaction grafting using morsellised allograft bone restores bone stock, but carries the potential for transmission of infection. Synthetic bone graft substitutes can eliminate this risk but may, however, influence outcome. In this study we tested the hypothesis that a 50/50 mix of hydroxyapatite and allograft does not affect long-term function, survival or radiological outcome. Sixty-five patients had revision hip arthroplasty using impaction grafting with either pure allograft (42 patients) or a 50/50 mixture of allograft and solid particulate hydroxyapatite. Harris hip scores were assessed pre-operatively and annual intervals thereafter. Function was analyzed using multilevel modeling, the Kaplan-Meier method used for survival analysis and graft incorporation was assessed radiologically. The hip score improved in both groups but showed a small annual decline (average 1.2/year, p<0.01). This decline was higher for females (average 3.4, p=0.025) and significantly related to pre-op scores (p<0.001). After adjusting for these, allograft patients had marginally higher scores (difference=3.1, p=0.3). The majority of revisions were for aseptic loosening. At 13 years survival in the allograft group was 84%, and 82% in the mixture group (p=0.96, log rank test). Radiologically the graft incorporation was similar in both groups (p=0.62). We conclude that long-term prosthesis survival and function following revision arthroplasty with a 50/50 mixture of allograft and hydroxyapatite are comparable to allograft alone.