J. Philippe Kretzer

Characterization of the Running-in Period in Total Hip Resurfacing Arthroplasty: An in Vivo and in Vitro Metal Ion Analysis

BACKGROUND Metal-on-metal total hip resurfacing arthroplasty is increasingly being performed in young and active patients. Preclinical in vitro testing of implants is usually performed with use of hip simulators, and the serum metal ion concentration is determined for the purpose of monitoring the patients. The goal of this study was to characterize the early running-in period in vivo and in vitro by characterizing metal ion levels. METHODS A well-functioning total hip resurfacing prosthesis was implanted in fifteen consecutive patients, and the serum metal ion concentrations in these patients were then determined preoperatively and at intervals during the first postoperative year (at one, six, twelve, twenty-four, and fifty-two weeks). The number of walking cycles was measured with use of a computerized accelerometer in order to compare walking cycles to hip simulator cycles. In vitro, five similar components were investigated for 3 million cycles with use of a hip simulator. Serum samples were obtained at different time points, and wear was measured by quantifying wear particles and ions in the samples. All patient and simulation serum samples were analyzed with use of inductively coupled plasma-mass spectrometry. One simulator implant was investigated with use of scanning electron microscopy. RESULTS The serum chromium and cobalt levels of the patients continuously increased during the first six months and showed an insignificant decrease thereafter. The molybdenum concentration was unchanged compared with preoperative values. In contrast, the simulator measurements showed a different wear pattern with a high-wear running-in period and a low-wear steady-state phase. The running-in period was delayed by 300,000 cycles and lasted up to 1 million cycles. Scanning electron microscopic analysis showed a carbon-rich protein film predominantly in the early phases of simulation. Scratches were detected originating from pits filled with aluminum oxide and silicon oxide and from pulled-out carbides that were causing third-body wear. CONCLUSIONS The simulator study allowed an exact characterization of the running-in period and showed a delayed onset of running-in wear. In contrast, the clinical data showed a slow increase in measured ion concentrations. These different wear patterns are probably due to the effects of distribution, accumulation, and excretion of particles and ions in vivo.

Metal release and corrosion effects of modular neck total hip arthroplasty

Modular neck implants are an attractive treatment tool in total hip replacement. Concerns remain about the mechanical stability and metal ion release caused by the modular connection. Five different implant designs were investigated in an experimental set-up. In vivo conditions were simulated and the long-term titanium release was measured. Finally, the modular connections were inspected for corrosion processes and signs of fretting. No mechanical failure or excessive corrosion could be identified for the implants tested. The titanium releases measured were extremely low compared to in vivo and in vitro studies and were not in a critical range.RésuméDans les prothèses totales de hanche l’utilisation d’implants avec col modulaire est d’une utilisation fréquente et pratique. Néanmoins, cette utilisation laisse persister des doutes sur la stabilité mécanique et sur le relargage des ions métalliques. 5 différents implants ont été étudiés en reproduisant les conditions in vivo, en mesurant le relargage de titane et en évaluant la corrosion et les lésions du col. Ce travail n’a pas permis de mettre en évidence d’échec secondaire à une corrosion excessive, et les taux de titane sont restés extrêmement bas dans les limites de l’acceptable.

Wear analysis of unicondylar mobile bearing and fixed bearing knee systems: a knee simulator study.

Unicondylar knee arthroplasty is an attractive alternative to total knee arthroplasty for selected patients with osteoarthritis. Mobile bearing knee designs have been developed to improve knee kinematics, lower contact stresses and reduced wear of ultra-high molecular weight polyethylene compared with fixed bearing designs. This study compared in vitro wear behavior of fixed and mobile unicondylar bearing designs. Analysis was performed using a force-controlled AMTI knee simulator according to ISO 14243-1:2002(E). The wear volume of the implants was determined gravimetrically. Optical surface characterization and an estimation of wear particle size and morphology were performed. Implant kinematic data for both designs were determined. The wear rates averaged 10.7 ± 0.59 mg per 10(6) cycles for the medial and 5.38 ± 0.63 mg per 10(6) cycles for the lateral components of the mobile bearings, compared with 7.51 ± 0.29 mg per 10(6) cycles and 3.04 ± 0.35 mg per 10(6) cycles for the fixed bearings. The mobile bearings therefore exhibited higher wear rates (P<0.01) compared with the fixed bearings. The tibial polyethylene inserts of the mobile bearings showed pronounced backside wear at the inferior surface. The kinematics of both designs was similar. However, anterior-posterior translation was lower in the mobile bearings. The wear particles were mainly elongated and small in size for both designs (P=0.462). This study shows that wear may play an important role in unicondylar mobile bearing knee designs. Advantages of unicondylar mobile designs compared with fixed bearing designs, which have been proposed in terms of wear behavior and improved kinematics, could not be confirmed.

Effect of joint laxity on polyethylene wear in total knee replacement.

Experimental simulator studies are frequently performed to evaluate wear behavior in total knee replacement. It is vital that the simulation conditions match the physiological situation as closely as possible. To date, few experimental wear studies have examined the effects of joint laxity on wear and joint kinematics and the absence of the anterior cruciate ligament has not been sufficiently taken into account in simulator wear studies. The aim of this study was to investigate different ligament and soft tissue models with respect to wear and kinematics. A virtual soft tissue control system was used to simulate different motion restraints in a force-controlled knee wear simulator. The application of more realistic and sophisticated ligament models that considered the absence of anterior cruciate ligament lead to a significant increase in polyethylene wear (p=0.02) and joint kinematics (p<0.01). We recommend the use of more complex ligament models to appropriately simulate the function of the human knee joint and to evaluate the wear behavior of total knee replacements. A feasible simulation model is presented.

Validation of ultratrace analysis of Co, Cr, Mo and Ni in whole blood, serum and urine using ICP-SMS

Currently, there is no single reference material commercially available with certified concentrations of Co, Cr, Ni and Mo in bodily fluids of unexposed subjects. This study provides validation data for the low level determination of Co, Cr, Mo and Ni in whole blood, serum and urine, highlighting the severe restrictions of currently available reference materials for this kind of analysis. The riverine water reference material SLRS-4 has been considered here as well because certified values for this matrix are available in contrast to the biological reference materials from Sero used throughout this study. In addition, spiking experiments with whole blood, serum and urine at concentrations of 0.25 µg l−1 and 0.50 µg l−1 revealed recoveries of 100 ± 5% underpinning the quality of the generated analytical data. All samples and standards have been diluted with an alkaline diluent containing 0.01 M NH3, 0.2 mM (NH4)2EDTA and 0.07% Triton X-100. Measurements were carried out with a sector field ICP-MS (ICP-SMS) operated under clean room conditions in the medium resolution mode (m/Δm ∼ 4000) to separate the analyte signals from severe polyatomic spectral interferences hampering the reliable determination of Co, Cr, Ni and Mo. Indium was used as internal standard element to correct for potential drifts in the ICP-SMS response. While the analysis of whole blood, serum and urine worked well for a few hours (serum and urine up to 24 h) using dilutions of only 1 + 9, dilutions of 1 + 19 gave the most robust instrumental performance for long-term measurements (>24 h). Considering a dilution factor of 20, conservative method detection limits of 0.005 µg l−1, 0.02 µg l−1, 0.10 µg l−1 and 0.01 µg l−1 have been calculated for Co, Cr, Mo and Ni. The inter-day reproducibility varied from 2% (Co, Mo) to 5% (Cr) and 10% (Ni) for low level blood samples. For serum and urine these numbers improved to 2–4%.

Fatigue Performance of Medical Ti6Al4V Alloy after Mechanical Surface Treatments

Mechanical surface treatments have a long history in traditional engineering disciplines, such as the automotive or aerospace industries. Today, they are widely applied to metal components to increase the mechanical performance of these. However, their application in the medical field is rather rare. The present study aims to compare the potential of relevant mechanical surface treatments on the high cycle fatigue (R = 0.1 for a maximum of 10 million cycles) performance of a Ti6Al4V standard alloy for orthopedic, spinal, dental and trauma surgical implants: shot peening, deep rolling, ultrasonic shot peening and laser shock peening. Hour-glass shaped Ti6Al4V specimens were treated and analyzed with regard to the material’s microstructure, microhardness, residual stress depth profiles and the mechanical behavior during fatigue testing. All treatments introduced substantial compressive residual stresses and exhibited considerable potential for increasing fatigue performance from 10% to 17.2% after laser shock peening compared to non-treated samples. It is assumed that final mechanical surface treatments may also increase fretting wear resistance in the modular connection of total hip and knee replacements.

The effect of multifilaments and monofilaments on cementless femoral revision hip components: an experimental study.

BACKGROUND Cerclage wires are widely used in revision hip surgery to reattach the lid of a femoral osteotomy. The present study compared the influence of multifilaments and monofilaments on primary stability of revision hip stems with different fixation principles. METHODS A standardized extended proximal femoral osteotomy was performed in the anterior cortex of 6 synthetic femora. We used a high-resolution measuring device to explore spatial micromovements of a diaphyseal and a metaphyseal fixating revision stem. Both of these were implanted in 3 femora. The specimens were measured again after consecutive restabilization of osteotomies with multifilaments and monofilaments. The movement graphs generated defined relative micromovements between stems and bones and the stabilizing effect of the two wire systems compared. FINDINGS Both multifilaments and monofilaments effected a major reduction of relative micromovements for both fixation principles. There were no differences in relative movements between the multifilament and monofilament treatments for the diaphyseal fixating stem. Yet for the metaphyseal fixating stem a significantly better restabilization was observed with multifilaments. INTERPRETATION Both multifilaments and monofilaments can support the revision hip stem in bridging the extended proximal femoral osteotomy. Yet, which wiring system should be chosen depends on the fixation principle of the revision stem. Multifilaments seem to be advantageous when used with metaphyseal fixating stems. However, the use of multifilaments with diaphyseal fixating components should be reconsidered as this might constrict the periosteal vascularity.

10-year results of the uncemented Allofit press-fit cup in young patients.

Background and purpose — Uncemented acetabular components in primary total hip arthroplasty (THA) are commonly used today, but few studies have evaluated their survival into the second decade in young and active patients. We report on a minimum 10-year follow-up of an uncemented press-fit acetabular component that is still in clinical use. Methods — We examined the clinical and radiographic results of our first 121 consecutive cementless THAs using a cementless, grit-blasted, non-porous, titanium alloy press-fit cup (Allofit; Zimmer Inc., Warsaw, IN) without additional screw fixation in 116 patients. Mean age at surgery was 51 (21–60) years. Mean time of follow-up evaluation was 11 (10–12) years. Results — At final follow-up, 8 patients had died (8 hips), and 1 patient (1 hip) was lost to follow-up. 3 hips in 3 patients had undergone acetabular revision, 2 for deep infection and 1 for aseptic acetabular loosening. There were no impending revisions at the most recent follow-up. We did not detect periacetabular osteolysis or loosening on plain radiographs in those hips that were evaluated radiographically (n = 90; 83% of the hips available at a minimum of 10 years). Kaplan-Meier survival analysis using revision of the acetabular component for any reason (including isolated inlay revisions) as endpoint estimated the 11-year survival rate at 98% (95% CI: 92–99). Interpretation — Uncemented acetabular fixation using the Allofit press-fit cup without additional screws was excellent into early in the second decade in this young and active patient cohort. The rate of complications related to the liner and to osteolysis was low.

Primary stability of the Fitmore® stem: biomechanical comparison

PurposeAfter clinical introduction of the Fitmore® stem (Zimmer), we noticed the formation of cortical hypertrophies in a few cases. We questioned whether (1) the primary stability or (2) load transfer of the Fitmore® stem differs from other stems unassociated with the formation of hypertrophies. We compared the Fitmore® stem to the well-established CLS® stem.MethodsFour Fitmore® and four CLS® stems were implanted in eight synthetic femurs. A cyclic torque around the stem axis and a mediolateral cyclic torque were applied. Micromotions between stems and femurs were measured to classify the specific rotational implant stability and to analyse the bending behaviour of the stem.ResultsNo statistical differences were found between the two stem designs with respect to their rotational stability (p = 0.82). For both stems, a proximal fixation was found. However, for the mediolateral bending behavior, we observed a significantly (p < 0.01) higher flexibility of the CLS® stem compared to the Fitmore® stem.ConclusionHip stem implantation may induce remodelling of the periprosthetic bone structure. Considering the proximal fixation of both stems, rotational stability of the Fitmore® stem might not be a plausible explanation for clinically observed formation of hypertrophies. However, bending results support our hypothesis that the CLS® stem presumably closely follows the bending of the bone, whereas the shorter Fitmore® stem acts more rigidly. Stem rigidity and flexibility needs to be considered, as they may influence the load transfer at the implant–bone interface and thus possibly affect bone remodelling processes.

Hard-on-Hard Lubrication in the Artificial Hip under Dynamic Loading Conditions

The tribological performance of an artificial hip joint has a particularly strong influence on its success. The principle causes for failure are adverse short- and long-term reactions to wear debris and high frictional torque in the case of poor lubrication that may cause loosening of the implant. Therefore, using experimental and theoretical approaches models have been developed to evaluate lubrication under standardized conditions. A steady-state numerical model has been extended with dynamic experimental data for hard-on-hard bearings used in total hip replacements to verify the tribological relevance of the ISO 14242-1 gait cycle in comparison to experimental data from the Orthoload database and instrumented gait analysis for three additional loading conditions: normal walking, climbing stairs and descending stairs. Ceramic-on-ceramic bearing partners show superior lubrication potential compared to hard-on-hard bearings that work with at least one articulating metal component. Lubrication regimes during the investigated activities are shown to strongly depend on the kinematics and loading conditions. The outcome from the ISO gait is not fully confirmed by the normal walking data and more challenging conditions show evidence of inferior lubrication. These findings may help to explain the differences between the in vitro predictions using the ISO gait cycle and the clinical outcome of some hard-on-hard bearings, e.g., using metal-on-metal.