A novel test method for evaluation of the abrasive wear behaviour of total hip stems at the interface between implant surface and bone cement

After total hip replacement, some cemented titanium stems show above-average early loosening rates. Increased release of wear particles and resulting reaction of the peri-prosthetic tissue were considered responsible. The objective was to develop a test method for analysing the abrasive wear behaviour of cemented stems and for generating wear particles at the interface with the bone cement.

By means of the novel test device, cemented hip stems with different designs, surface topographies and material compositions using various bone cements could be investigated. Before testing, the cemented stems were disconnected from the cement mantle to simulate the situation of stem loosening (debonding). Subsequently, constant radial contact pressures were applied on to the stem surface by a force-controlled hydraulic cylinder. Oscillating micromotions of the stem (± 250 μm; 3 × 106cycles; 5 Hz) were carried out at the cement interface initiating the wear process.

The usability of the method was demonstrated by testing geometrically identical Ti-6A1-7Nb and Co-28Cr-6Mo hip stems ( n = 12) with definite rough and smooth surfaces, combined with commercially available bone cement containing zirconium oxide particles. Under identical frictional conditions with the rough shot-blasted stems, clearly more wear particles were generated than with the smooth stems, whereas the material composition of the hip stems had less impact on the wear behaviour.

Evaluierung des Fügeverhaltens und der Rückhaltekräfte von Schnapp-Pfannen für den künstlichen Hüftgelenkersatz / Evaluation of the assembling and retention forces of constraint liners for total hip replacement

Dislocation is a severe complication after total hip replacement which may cause revision surgery in some cases. The use of constraint inserts that are coupled to the femoral head by a snapping mechanism provides an opportunity for treatment of recurrent dislocations. This study was aimed to investigate the assembling and retention forces of a specific constraint liner. Using a universal testing machine the assembling forces were determined for head sizes of 28 and 32 mm and the clinically mostly used as well as the maximum cup size. Subsequently, under variation of load direction and pull-out velocity the retention forces were investigated. For primary assembly of the head the required compressive forces were in a range from 197 N and 283 N depending on head and cup size (each size n = 3). Repeated assembly led to a decrease of these forces up to 29%. The retention forces always were slightly below the assembling forces, i. e. forces to remove the heads from the inserts were between 183 N and 230 N (each size n = 3). Repeated disconnection caused a decrease of the retention forces up to 16%. An increase of load velocity as well as an oblique load direction resulted in an enhancement of the retention forces. For all investigated implant sizes the retention force for the femoral head was approximately ten-times less than the interface strength between the insert and the metal-back. In case of correct implant handling the risk of disconnection between the tested constraint insert and the corresponding metal-back has not to be considered in clinical practice.

Experimentelle Analyse von neutralen, asymmetrischen und Schnapp-Pfannen für die Hüftendoprothetik

AIM

Recurrent dislocation after total hip replacement is a severe complication, which requires specific treatment and implants. The purpose of the present study was to compare a constraint liner with an elevated rim and standard liner regarding their range of motion and dislocation stability.

METHOD

With a test device, range of motion until impingement (ROM (Imp)) and dislocation (ROM (Lux)) were experimentally analyzed using the above-mentioned insert types of a commercial total hip system. On the basis of movement combinations associated with dislocation, the ROM was determined. Further measuring parameter was the resisting moment against subluxation of the femoral head.

RESULTS

The constraint liners showed clear restriction of the movements "internal rotation combined with 90 degrees flexion and 0 degrees adduction" and "external rotation with 10 degrees extension and 15 degrees adduction" of up to 20 degrees compared to the neutral liner. ROM (Imp) was only decreased by about 8 degrees with the elevated-rim liner. The constraint liners revealed the highest resisting moments in subluxation, however, at adequate orientation in the acetabular cup the elevated-rim liners provided a higher ROM (Lux) of up to 12 degrees. Both designs were superior to the neutral liner at retroversion and steep cup position regarding resisting moment and ROM (Lux).

CONCLUSION

In case of insufficient soft tissue tension the use of constraint liners may increase the dislocation stability, however, in contrast to elevated-rim liners the impingement-free movement interval is clearly reduced. Thus, material damage and high shear stress in the bone interface can result. Therefore, constraint liners should only be used in exceptional cases.

Schadensanalyse anhand zweier F�lle von Keramikbr�chen von Pfanneninserts

Due to modular design today's total hip endoprostheses provide the choice of different bearing surfaces. Particle-induced osteolysis is the major reason for implant failure. Therefore, minimization of wear rates is the essential factor for the improvement of long-term function of the implants. The bearing couple ceramic-on-ceramic offers a linear wear rate of about 0.005 mm/year. That is an essential prerequisite for successful treatment of young and active patients. Ceramic femoral heads and taper fixation became standard. After some problems in the 1970s, manufacturers achieved all kinds of improvements. Surgeons learned how to handle ceramic femoral heads correctly. Nowadays ceramic heads are reliable implants. Ceramic inserts have been used since the mid-1990s. Some acetabular systems with ceramic inserts are used in clinical studies, some are approved. Complications with ceramic inserts are rare, e.g., fracturing or chipping off. Mostly those problems are due to mishandling. In this study some clinical failures are analyzed. Recommendations how to improve the handling of ceramic inserts are proposed and discussed.

Methode zur Evaluierung von Einflußfaktoren auf die Luxationsstabilität von künstlichen Hüftgelenken / Method for the Evaluation of Factors Influencing the Dislocation Stability of Total Hip Endoprotheses

Dislocation of the artificial joint is a serious complication of total hip replacement. Various factors with an influence on dislocation stability were determined clinically. Our goal was to develop a method for evaluating experimentally the parameters implant design, position and the load situation for their influence on joint stability. With the newly developed testing device the range of motion to impingement and to dislocation can be determined at different implant positions. In addition, the rotational moments on subluxation, i.e. the "levering out" of the femoral head, can be determined. By way of example several hip implants were examined during movements associated with dislocation, e.g. (internal-)rotation in 90 degrees flexion and 0 degrees adduction as well as with (external-)rotation in combination with 10 degrees extension and 15 degrees adduction. Irrespective of implant design and position, the following movement phases can be differentiated: undisturbed motion, impingement, subluxation and, finally, complete dislocation of the head. On the basis of the range of motion of the specific phases, the moments occurring and the direction of dislocation, different implant systems can be compared. In this study the influence of the head diameter on the dislocation stability of the hip endoprosthesis is shown. With the aid of the model presented herein, a data set showing the most favourable and/or most dislocation stable implant position can be acquired for different combinations of the implant components. Additionally, useful information for implant design can be deduced and applied to new developments and/or modifications of existing implant components.

[Carbon fiber-reinforced plastics as implant materials]

Carbon fiber-reinforced plastics have been used clinically as an implant material for different applications for over 20 years.A review of technical basics of the composite materials (carbon fibers and matrix systems), fields of application,advantages (e.g., postoperative visualization without distortion in computed and magnetic resonance tomography), and disadvantages with use as an implant material is given. The question of the biocompatibility of carbon fiber-reinforced plastics is discussed on the basis of experimental and clinical studies. Selected implant systems made of carbon composite materials for treatments in orthopedic surgery such as joint replacement, tumor surgery, and spinal operations are presented and assessed. Present applications for carbon fiber reinforced plastics are seen in the field of spinal surgery, both as cages for interbody fusion and vertebral body replacement.

[Computer-based motion simulation of total hip prostheses with ceramic-on-ceramic wear couple. Analysis of implant design andorientation as influence parameters]

OBJECTIVE

In THA, ceramic-on-ceramic wear couples are increasingly used. A restricted range of motion (ROM) due to unfavourable implant design or positioning may cause impingement or dislocation, which can result in failure of ceramic inserts.

METHODS

By means of a 3-D CAD program different hip joint movements were simulated and the effects of ceramic hip implant design and position on the range of motion were analysed.

RESULTS

To offer sufficient ROM and to minimise risk of impingement and dislocation, inclination angle of the acetabular cup should be 45 degrees, cup anteversion 15 degrees and stem antetorsion 0 degrees to 10 degrees. In regard to implant design, acetabular cups with slightly- recessed ceramic inserts should be used. Prosthetic systems with an elevated liner or with a mushroom-shaped femoral head are associated with limited ROM and increased risk of mechanical failure. The ratio of head to neck diameter should never be less than 2 : 1. Larger heads not only increase ROM, but also the stability of the prosthesis against dislocation. Thereby, the wear rate of ceramic-on-ceramic couples is not increased, in contrast to polyethylene.

CONCLUSION

Considering certain criteria for ceramic hip implants regarding implant positioning, design and handling, ceramic-on-ceramic couples can be used with low risk of revision surgery and they can also reduce the prosthesis loosening associated with wear in young and active patients.

[Mechanical study of spinal interbody implants--characteristics and limits of standardized testing]

Spinal interbody fusion has proved to be a useful procedure for the surgical stabilization of spinal segments, for which fusion cases made of metal or reinforced polymers are increasingly being used. For the mechanical testing of spinal interbody implants, a test setup has been developed on the basis of an ASTM proposal. Initially, testing of lumbar fusion cages made of CFRP (carbon fibre reinforced polymer) was carried out. The implants (UNION Cages, Medtronic Sofamor Danek), which are characterised by their radiolucency on radiography, NMR and CT scans, have a cube-shaped body with three table-tracks on the under and upper surfaces. The cages were tested at different loads. Modifications of the proposed standardized method were carried out to enable implementation of implant-oriented testing. The tested cages were shown to have adequate axial compression, shear and torsional strengths with regard to the implant body. The maximum axial compression force tolerated by the table-tracks was less than the maximal potential loading of the lumbar spine, and, with account being taken of implant design, consequences with regard to surgical technique were drawn. As dictated by the geometry of the table-tracks, parallel grooves have to be made intra-operatively in the vertebral end plates. Axial compressive loads then act on the implant body, and the table-tracks are protected from damage. To avoid in vivo failure, the tested cages should be implanted only when this specific surgical technique is employed. Using supplementary anterior or posterior instrumentation, in vivo failure of the table-tracks under physiological spinal loading is not to be expected.

Correction of congenital heart defects and mitral valve operations using limited anterolateral thoracotomy

PURPOSE

Median sternotomy, which generally is used as a standard access for atrial septal defect (ASD) and mitral valve operations, has a significant risk of postoperative instability/osteomyelitis of the sternum. Moreover, especially in young women, the resulting large scar is a poor cosmetic result that may have adverse psychological consequences. Our presentation suggests that these difficulties may be avoided by the use of a less invasive approach consisting of a limited anterolateral thoracotomy with standard cannulation.

MATERIAL AND METHODS

From June 1997 until December 1999, 13 women, mean age 31.9 +/- 9.2 years, with atrial septum defect (n = 8), sinus venosus defect with partial anomalous pulmonary venous connection (n = 1), left atrial myxoma (n =1) or mitral valve regurgitation (n = 3), were scheduled for less invasive operation. In all cases a double lumen tube was used for ventilation. After a submammarian skin incision of about 10 cm a limited anterolateral thoracotomy was performed in the fifth right intercostal space. For cannulation of the ascending aorta a trochar cannula was used. Both caval veins were cannulated by angled vena cava catheters. Standard cardiopulmonary bypass was established using normothermia in all patients undergoing operations with correction of congenital heart defects and mild hypothermia (32 degrees C) in the three patients undergoing mitral valve operation. Surgery was performed in cardioplegic arrest using Bretschneider's solution. All corrections of congenital heart defects were performed by Goretex patches. Mitral valve reconstruction was carried out in two patients, and one patient underwent mitral valve replacement.

RESULTS

No complications occurred in any of the 13 patients peri- or postoperatively. Total time of operation was 211.9 +/- 36.0 minutes, the perfusion time was 77.0 +/- 25.8 minutes, and the aortic cross-clamp time was 51.8 +/- 21.9 minutes. Mean stay in ICU was 1.2 +/- 0.4 days (total hospital stay: 7.8 +/- 2.2 days). Postoperative thoracic x-ray and cardiac echocardiography/dopplersonography revealed no pathological findings in any patients.

CONCLUSION

Atrial septal defect operations, including partial anomalous pulmonary venous connection, left atrial myxoma and mitral valve operations, can be performed safely and effectively using a limited anterolateral thoracotomy and standard cannulation technique with excellent cosmetic results.

Dermatologic surgery into the next millennium, part II

This is the second article in a 4-part series on dermatologic surgery. This section provides detailed information about filling agents and botulinus toxin A. The filling agents discussed here are frequently used in our office. It is emphasized that meticulous technique and patient selection predict a good cosmetic result. To select the right agent, patient safety must be a priority.

[Innovative cell culture methods in drug development]

The animal studies necessary for drug registration are time-consuming, costly, and often stressful for the animals. Toxicological screening of drug candidates early in development with in vitro cell culture systems is therefore of relevance. In contrast to animal studies, in vitro cell culture methods are characterized by a low compound requirement and a short duration. Additionally it is possible to include mechanistic studies or to test for toxicity specific to humans. Therefore, early toxicological screening can provide a useful support for selecting the most promising drug candidate. Primary hepatocytes can be used to measure the cytotoxicity of a test compound. These results can be used to estimate general toxicity. Measuring endpoints like apoptosis, redox status, or gene expression profiles can help to answer mechanistic questions. The use of primary human hepatocytes provides early predictivity for hepatotoxicity specific to humans. Since teratogenic findings in animal studies often lead to abandonment of development, it is reasonable to use an in vitro embryotoxicity assay for early determination of the teratogenic potential of a compound, e.g. the embryonic stem cell test (EST) which was recently developed by ZEBET. In the EST embryonic stem cells are investigated for their preserved capability to differentiate into cardiomyocytes following drug exposure. In comparison cytotoxicity of the test substance is analyzed in embryonic stem cells and in differentiated fibroblast cells. In a validation study initiated by ECVAM the EST shows a high correlation with in vivo data.

Clinical outcome of patients with and without renal impairment undergoing a minimally invasive LIMA-to-LAD bypass operation

PURPOSE

Patients with renal impairment undergoing conventional coronary artery bypass grafting (CABG) have a significant risk of postoperative deterioration of kidney function. We investigated the outcome of patients with and without renal impairment treated by off-pump coronary artery surgery.

METHODS

From January 1997 until January 2000, 158 consecutive patients (mean age 63 +/- 9.8 years, 126 male, 32 female) underwent minimally LIMA-to-LAD bypass operations. The patients were divided into three groups: group I patients (n = 133) had a preoperative creatinine of < 1.3 mg/dL, group II patients (n = 21) had a creatinine of > 1.3 mg/dL, and patients of group III (n = 4) required chronic dialysis due to terminal kidney dysfunction. Monitoring of the blood creatinine was performed during the entire hospital stay. A postoperative angiogram was performed in 113 of the 158 patients.

RESULTS

All operations were performed without intraoperative complications. Postoperative angiograms revealed a patent LIMA-to-LAD bypass in all but one patient, who demonstrated a dissection of the left internal mammary artery (LIMA) graft. Mean creatinine value on admission was 1.0 +/- 0.1 mg/dL in group I and 2.7 +/- 1.9 mg/dL in group II. The maximal postoperative creatinine value was 1.1 +/- 0.4 mg/dL in group I and 2.9 +/- 2.7 mg/dL in group II. Neither hemofiltration nor hemodialysis was necessary in any patient of group I or group II during the postoperative course.

CONCLUSION

Minimally invasive LIMA-to-LAD bypass is a safe and effective procedure with low morbidity and no mortality in the first 158 patients. Preoperative renal impairment had no adverse effect on outcome or residual kidney function. Thus, higher doses of diuretics and hemofiltration/dialysis were not used, resulting in a cost reduction. Therefore, this approach may be worthwhile to consider in patients with significant renal impairment who have to undergo CABG.

Structure and dynamics of micelle-bound neuropeptide Y: comparison with unligated NPY and implications for receptor selection

The biological importance of the neuropeptide Y (NPY) has steered a number of investigations about its solution structure over the last 20 years. Here, we focus on the comparison of the structure and dynamics of NPY free in solution to when bound to a membrane mimetic, dodecylphosphocholine (DPC) micelles, as studied by 2D (1)H NMR spectroscopy. Both, free in solution and in the micelle-bound form, the N-terminal segment (Tyr1-Glu15) is shown to extend like a flexible tail in solution. This is not compatible with the PP-fold model for NPY that postulates backfolding of the flexible N terminus onto the C-terminal helix. The correlation time (tau(c)) of NPY in aqueous solution, 5.5 (+/-1.0) ns at 32 degrees C, is only consistent with its existence in a dimeric form. Exchange contributions especially enhancing transverse relaxation rates (R(2)) of residues located on one side of the C-terminal helix of the molecule are supposed to originate from dimerization of the NPY molecule. The dimerization interface was directly probed by looking at (15)N-labeled NPY/spin-labeled [TOAC34]-[(14)N]-NPY heterodimers and revealed both parallel and anti-parallel alignment of the helices. The NMR-derived three-dimensional structure of micelle-bound NPY at 37 degrees C and pH 6.0 is similar but not identical to that free in solution. The final set of 17 lowest-energy DYANA structures is particularly well defined in the region of residues 21-31, with a mean pairwise RMSD of 0.23 A for the backbone heavy atoms and 0.85 A for all heavy atoms. The combination of NMR relaxation data and CD measurements clearly demonstrates that the alpha-helical region Ala18-Thr32 is more stable, and the C-terminal tetrapeptide becomes structured only in the presence of the phosphocholine micelles. The position of NPY relative to the DPC micelle surface was probed by adding micelle integrating spin labels. Together with information from (1)H,(2)H exchange rates, we conclude that the interaction of NPY with the micelle is promoted by the amphiphilic alpha-helical segment of residues Tyr21-Thr32. NPY is located at the lipid-water interface with its C-terminal helix parallel to the membrane surface and penetrates the hydrophobic interior only via insertions of a few long aliphatic or aromatic side-chains. From these data we can demonstrate that the dimer interface of neuropeptide Y is similar to the interface of the monomer binding to DPC-micelles. We speculate that binding of the NPY monomer to the membrane is an essential key step preceeding receptor binding, thereby pre-orientating the C-terminal tetrapeptide and possibly inducing the bio-active conformation.

The first selective agonist for the neuropeptide YY5 receptor increases food intake in rats

The first Y(5) receptor-selective analog of neuropeptide Y (NPY), [Ala(31),Aib(32)]NPY, has been developed and biologically characterized. Using competition binding assays on cell lines that express different Y receptors, we determined the affinity of this analog to be 6 nm at the human Y(5) receptor, >500 nm at the Y(1) and Y(2) receptors, and >1000 nm at the Y(4) receptor. Activity studies performed in vitro using a cAMP enzyme immunoassay, and in vivo using food intake studies in rats, showed that the peptide acted as an agonist. Further peptides obtained by the combination of the Ala(31)-Aib(32) motif with chimeric peptides containing segments of NPY and pancreatic polypeptide displayed the same selectivity and even higher affinity (up to 0.2 nm) for the Y(5) receptor. In vivo administration of the new Y(5) receptor-selective agonists significantly stimulated feeding in rats. The NMR solution structures of NPY and [Ala(31),Aib(32)]NPY showed a different conformation in the C-terminal region, where the alpha-helix of NPY was substituted by a more flexible, 3(10)-helical turn structure.

'A parent's kiss': evaluating an unusual method for removing nasal foreign bodies in children

Nasal foreign bodies are a common problem in children. Various techniques have been described for removing the offending articles. Positive pressure techniques have long been described, and have many potential advantages, but are not yet in widespread use. The aim of this study was to evaluate the efficacy, safety and acceptability of a technique of mouth-to-mouth blowing. Of 19 children with intranasal foreign bodies, the technique was successful in 15 (79 per cent). The procedure caused little or no distress to the child, and no complications were encountered. All parents found the technique acceptable. This technique for nasal foreign body removal is, as far as we are aware, the first to be subjected to any prospective evaluation of effectiveness. We suggest that it should be used as the first line management for removing nasal foreign bodies in young children.