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PubMed2005

Submillimeter measurement of cup migration in clinical standard radiographs.

Assessing the displacement of bony implants is an important topic in arthroplasty, particularly in total hip replacement (THR). The observation of the migration is supposed to provide an insight into the fixation of the implant. Diagnostic standard radiographs of the pelvis are an advantageous data source for this purpose. The previous methods based on these images, however, lack of a thorough consideration of their projective nature. They do, hence, not reach the desired precision, which should lie in the submillimeter range to allow a detection of migration in the first one or two years after implantation. The aim of the work presented here was, therefore, a method for measuring the distance of the artificial hip socket to the bone with an error of less than 0.5 mm. The approach has been on the one hand to define the bone-cup distance measured in the radiograph so that the variability of the intrinsic and extrinsic parameters at exposure has a minimal impact. On the other, specialized matching techniques are applied in order to optimize the localization of the necessary bony landmarks and the cup in the X-ray image. The coordinates of the bony landmarks are determined by means of a template matching algorithm. The position of the implant is estimated by intensity-based registration using the cup's CAD-model. The method was validated theoretically, experimentally, and clinically. In the clinical radiographs, the standard deviation of the migration measurements resulted to be 0.28 mm when using only natural bony landmarks. The implantation of a bony marker was found to increase the precision to a standard deviation of 0.20 mm. The interobserver variability in the two cases was estimated to be 0.11 mm and 0.04 mm.

Algorithms↗
PubMed2005

Survival rate, fracture strength and failure mode of ceramic implant abutments after chewing simulation.

The aim of this study was to compare titanium-reinforced ZrO(2) and pure Al(2)O(3) abutments regarding their outcome after chewing simulation and static loading. Forty-eight standard diameter implants with an external hexagon were divided into three groups of 16 implants each and restored with three different types of abutments (group A: ZrO(2) abutments with titanium inserts; group B: Al(2)O(3) abutments; group C: titanium abutments). All abutments were fixated on the implants with gold-alloy screws at 32 Ncm torque, and metal crowns were adhesively cemented onto the abutments. The specimens were exposed to 1.2 million cycles in a chewing simulator. Surviving specimens were subsequently loaded until fracture in a static testing device. Fracture loads (N) and fracture modes were recorded. A Wilcoxon Rank test to compare fracture loads among the three groups and a Fisher exact test to detect group differences in fracture modes were used for statistical evaluation (P < 0.05). All specimens but one of group B survived chewing simulation. No screw loosening occurred. The median fracture loads (+/-s.d.) were as follows: group A, 294 N (+/-53); group B, 239 N (+/-83), and group C, 324 N (+/-85). The smaller fracture loads in group B were statistically significant. The use of pure Al(2)O(3) abutments resulted in significantly more abutment fractures. It is proposed that titanium-reinforced ZrO(2) abutments perform similar to metal abutments, and can therefore be recommended as an aesthetic alternative for the restoration of single implants in the anterior region. All-ceramic abutments made of Al(2)O(3) possess less favourable properties.

Dental Abutments↗
PubMed2000

The osmotic swelling characteristics of cardiac valve prostheses.

Several types of mechanical cardiac prostheses have been constructed with Delrin occluders, a material that is subject to osmotic swelling. The leaftets are designed to expand to specific tolerances when immersed in blood. The synthetic blood analogs commonly used in vitro contain hydrophilic compounds that can alter the osmotic expansion of the Delrin occluders. A static leak test chamber was employed to illustrate the effects of various test fluids on the sustained regurgitation phase of Delrin valves.

Equipment Failure Analysis↗
PubMedDate not supplied

Nanostructured ceramics for biomedical implants.

Recent progress in the synthesis, characterization, and biological compatibility of nanostructured ceramics for biomedical implants is reviewed. A major goal is to develop ceramic coating technology that can reduce the friction and wear in mating total joint replacement components, thus contributing to their significantly improved function and longer life span. Particular attention is focused on the enhancement of mechanical properties such as hardness, toughness, and friction coefficient and on the bioactivity as they pertain to the nanostructure of the material. The development of three nanostructured implant coatings is discussed: diamond, hydroxyapatite, and functionally graded metalloceramics based on the Cr-Ti-N ternary system. Nanostructured diamond produced by chemical vapor deposition (CVD) techniques and composed of nano-size diamond grains have particular promise because of the combination of ultrahigh hardness, improved toughness over conventional microcrystalline diamond, low friction, and good adhesion to titanium alloys. Nanostructured processing applied to hydroxyapatite coatings is used to achieve the desired mechanical characteristics and enhanced surface reactivity and has been found to increase osteoblast adhesion, proliferation, and mineralization. Finally, nanostructured metalloceramic coatings provide continuous variation from a nanocrystalline metallic bond at the interface to the hard ceramic bond on the surface and have the ability to overcome adhesion problems associated with ceramic hard coatings on metallic substrates.

Animals↗
PubMed2005

Materials characterization of explanted mechanical heart valves and comparison to patients' clinical data.

In the present study, twelve explanted mechanical heart valves (MHVs)with pyrolitic carbon tilting disc and 14 bileaflet MHVs were analyzed to investigate the effects of material properties on valve performance and patients' general health conditions. Optical and scanning electron microscopy was used to investigate material imperfections, wear patterns or damages to housing and occluder components. All analyzed tilting disc valves exhibited wear effects, particularly due to abrasion and impact to both disc and housing. Wear of pyrolitic carbon disc and housing did not influence their in vivo performance. In the bileaflet MHVs, breakaway of the pyrolitic carbon coating sometimes caused malfunctioning and required surgical retrieval of the valve. In all cases, occurrence of clinical symptoms was more likely when wear effects were located in critical areas. The study supports a correlation between the properties of the MHVs material and patients' symptoms.

Adult↗
PubMed2001

New joints for the Millennium: wear control in total replacement hip joints.

Hip joint replacement is described as the greatest achievement in orthopaedic surgery in the twentieth century. The field has been dominated for some forty years by implants based upon metallic femoral heads and stems and polymeric acetabular cups. At the dawn of the new Millennium, many alternative materials and designs are now being proposed or evaluated. The reasons for these developments and the current contributions of engineering science and tribology to advances in hip replacement are discussed. Illustrations are presented of the significant changes being proposed or introduced. While the new designs of total hip replacements offer exciting engineering contributions to the future of joint replacement, the long-term benefits to patients will depend upon the biological response to the new devices.

Biomechanical Phenomena↗
PubMed2003

In vitro analysis of the wear, wear debris and biological activity of surface-engineered coatings for use in metal-on-metal total hip replacements.

Extremely low wear rates have been reported for metal-on-metal total hip replacements, but concerns remain about the effects of metal ion release, dissolution rates and toxicity. Surface-engineered coatings have the potential to improve wear resistance and reduce the biological activity of the wear debris produced. The aim of this study was to examine the wear and wear debris generation from surface-engineered coatings: titanium nitride (TiN), chromium nitride (CrN) and chromium carbon nitride (CrCN) applied to a cobalt-chrome alloy (CoCr) substrate. The coatings were articulated against themselves in a simple geometry model. The wear particles generated were characterized and the cytotoxic effect on U937 macrophages and L929 fibroblasts assessed. The CrN and CrCN coatings showed a decrease in wear compared to the CoCr bearings and produced small (less than 40 nm in length) wear particles. The wear particles released from the surface engineered bearings also showed a decreased cytotoxic effect on cells compared to the CoCr alloy debris. The reduced wear volumes coupled with the reduced cytotoxicity per unit volume of wear indicate the potential for the clinical application of this technology.

Cell Survival↗
PubMed2004

[Primary stability of cement free hip cup prostheses--morphometric comparative analysis of two cup types].

An uncemented implanted acetabular cup must ensure primary stability and postoperative osteointegration. For both conditions the immediate postoperative contact between bone and the structured surface of the cup is of decisive importance. Two different types of acetabular cups manufactured by ESKA (standard model and Kapuziner model) were implanted in the innominate bones of elderly deceased persons. After plastination, thin serial sections were obtained. On the basis of X-rays of these sections, contact between bone and the reticulate surface of the cups was quantified. The quality of the contact was analysed densitometrically.

Aged↗
PubMed2005

[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.

Acetabulum↗
PubMed2005

In vivo degradation of polyethylene liners after gamma sterilization in air.

BACKGROUND: Ultra-high molecular weight polyethylene degrades during storage in air following gamma sterilization, but the extent of in vivo degradation remains unclear. The purpose of this study was to quantify the extent to which the mechanical properties and oxidation of conventional polyethylene acetabular liners treated with gamma sterilization in air change in vivo. METHODS: Fourteen modular cementless acetabular liners were revised at an average of 10.3 years (range, 5.9 to 13.5 years) after implantation. All liners, which had been machined from GUR 415 resin, had been gamma-sterilized in air; the average shelf life was 0.3 year (range, 0.0 to 0.8 year). After removal, the components were expeditiously frozen to minimize ex vivo changes to the polyethylene prior to characterization. The average duration between freezing and testing was 0.6 year. Mechanical properties and oxidation were measured with use of the small-punch test and Fourier transform infrared spectroscopy, respectively, in the loaded and unloaded regions of the liners. RESULTS: There was substantial regional variation in the mechanical properties and oxidation of the retrieved liners. The ultimate load was observed to vary by >90% near the surface. On the average, the rim and the unloaded bearing showed evidence of severe oxidation near the surface after long-term in vivo aging, but these trends were not typically observed on the loaded bearing surface or near the backside of the liners. CONCLUSIONS: The mechanical properties of polyethylene that has been gamma-sterilized in air may decrease substantially in vivo, depending on the location in the liner. The most severe oxidation was observed at the rim, suggesting that the femoral head inhibits access of oxygen-containing body fluids to the bearing surface. This is perhaps why in vivo oxidation has not been associated with clinical performance to date.

Acetabulum↗
PubMed1999

Poor survival of cementless Biomet total hip: a report on 1,047 hips from the Finnish Arthroplasty Register.

In Finland, almost 50% of all hip replacements done after 1989 have been inserted without cement. Biomet components have been the most commonly used implants in cementless arthroplasties. Between 1985 and 1997, 4,300 prostheses were implanted because of primary osteoarthrosis. 4 different acetabular component designs have been identified as Biomet implants (Mallory-Head, Romanus, T-TAP, Universal) and were used in 1,047 hips. The 9-year survival of all arthroplasties using Biomet cups was only 65 (95% CI 61-69)% while that of arthroplasties using T-TAP-cups was only 58 (52-65)%. In contrast, the 7.5-year survival of arthroplasties using Romanus cups was 85 (79-91)%. The 98 (96-99)% 5-year survival of arthroplasties with Mallory-Head cups should be interpreted cautiously, since similar results of arthroplasties using the Universal cup with the same type of liner decline sharply to 93 (88-98)% only 1 year later. The poor survival of Biomet cementless prostheses in our series seems to be related to the poor survival of the cup. This finding was common to all metal shell designs using Hexloc liners. We recommend that Biomet cups with Hexloc liners should not be used and patients who have had them inserted should undergo regular clinical and radiographic follow-ups.

Adult↗
PubMed1998

An analysis of hylamer and polyethylene bearings from retrieved acetabular components.

Hylamer and conventional polyethylene acetabular liners of the same design, revised for a variety of reasons, were examined and compared to assess the performance of Hylamer as a bearing material. Clinical damage modes, linear wear rates, oxidation levels, and mechanical properties were measured. In both series, many liners were retrieved for dislocation. Wear/osteolysis was the most common reason for retrieval in the Hylamer series, while none of the conventional polyethylene liners were retrieved for this reason. Nearly all liners exhibited abrasion, burnishing, scratching, and creep. The Hylamer liners had more cracking, delamination, and pitting. The Hylamer liners had an average linear wear rate of 0.32 mm/year, while the conventional polyethylene liners had an average wear rate of 0.20 mm/year. Due to sample size, no statistical difference in wear rate was noted between the two groups. In general, both the Hylamer and conventional polyethylene showed oxidation peaks subsurface, resulting from their exposure to gamma radiation in air. Liners with elevated oxidation had decreased ultimate tensile strength, elongation, and toughness. For given oxidation levels, the corresponding mechanical properties of Hylamer appeared lower than those of conventional polyethylene. The ultimate tensile strength values ranged from 14 to 33 MPa for Hylamer and 19 to 32 MPa for conventional polyethylene. Elongation ranges were 19% to 350% (Hylamer) and 80% to 375% (conventional). The Hylamer retrievals in this study gave initial indications of performance; Hylamer appeared to behave similarly, but not superiorly, to conventional polyethylene, in the early functional period with respect to clinical wear and clinical performance. Both Hylamer and conventional polyethylene liners were degraded by gamma sterilization in air, with Hylamer liners demonstrating greater property changes.

Chemistry, Pharmaceutical↗
PubMed2003

Complications after hemiarthroplasty for fractures of the proximal humerus.

Humeral head replacement is frequently used in the treatment of selected fractures of the proximal humerus. This technique is usually successful in terms of pain relief and patient satisfaction. However, complications may occur and can adversely affect outcome. Careful preoperative planning, meticulous surgical technique, and appropriate postoperative rehabilitation are the best methods to promote a successful result. Potential complications and methods to avoid them at initial surgery should be reviewed.

Arthroplasty, Replacement↗
PubMed1992

Theoretical analysis of the fatigue life of fixture screws in osseointegrated dental implants.

Metal fatigue failure of the gold screw used to retain a fixed prosthesis to Brånemark osseointegrated fixtures/abutments has been analyzed theoretically. Mechanical engineering principles show the importance of appropriate preload being applied through the gold screw to the gold cylinder and abutment. The significance of the screw design and necessity of applying the correct torque to achieve a long fatigue life for the screw are also described. The consequence of misalignment of a gold cylinder to an abutment is discussed.

Dental Abutments↗
PubMed2004

Structural testing of a juvenile prosthetic foot pylon.

Clinicians at the Shriner's Hospitals for Children have noticed unexpected catastrophic failures in the 2024-T3 aluminum prosthetic pylons of their juvenile patients. They postulated that small "dings", caused by sharp impacts with playground equipment or other obstacles, might be causing stress concentrations in the pylons. To investigate this, students at the United States Air Force Academy performed a series of tests to determine potential failure modes for the pylon. Initially, a technique for introducing small reproducible "dings" was established. These damaged specimens were then subjected to compressive axial tests to failure, axial fatigue tests (5-100 lbs at 5 Hz), and finally four point bending fatigue tests (370 in-lbs at 5 Hz). The maximum compressive load of the damaged specimen was virtually identical to that of the control specimen (14 kips), and the axial fatigue test was halted with no damage after one million cycles. Two bending fatigue tests resulted in cracks and eventual failure at the load application points of the fixture on the pylon. This has caused us to speculate that failure of the pylons is not due to the dings, but may be due to over-tightening of the pylon attachment points to the foot and the deficient limb. Further testing will be conducted to examine this possibility.

Ankle Joint↗
PubMed2004

Periprosthetic fracture of the femur after total hip arthroplasty occurring in winter activities: report of two cases.

Periprosthetic fractures are uncommon after total hip replacement surgery and are most often associated with loosening or osteolysis. In a review of Mayo Clinic records, the cumulative incidence of femoral fractures after primary uncemented prostheses was only 0.4% (4). No periprosthetic fracture associated with sports participation has been previously reported in the literature. When advising patients about return to sports and recreational activities after total hip replacement, concerns fall into two main categories: 1) wear of the bearing surface(s) and secondary ramifications such as early failure or osteolysis, and 2) dislocation or fracture of the prosthesis or periprosthetic bone. The former concerns have been previously examined (5), but the latter have not been reported to date. These case reports describe a complication that may occur in total hip arthroplasty in those patients who return to winter sports and recreational activities. Although at intermediate follow-up there does not appear to have been irreversible damage for these patients, it is imperative to warn patients that activities that place the patient at risk of trauma may compromise the longevity of the artificial joint. This information can be used in helping patients understand the risks associated with athletic activity after total hip arthroplasty, which is a major goal of current recommendations for advising patients after this type of surgery.

Arthroplasty, Replacement, Hip↗
PubMed2005

Mechanical valve thrombosis in a chronic animal model: differences between monoleaflet and bileaflet valves.

BACKGROUND AND AIM OF THE STUDY: The occurrence of mechanical valve thrombosis was studied in a chronic sheep model of valve implantation in the pulmonary position. METHODS: Six monoleaflet and six bileaflet mechanical valves were implanted in young sheep. Anticoagulation was stopped at one week postoperatively, after which weekly fluoroscopic evaluation was performed. Animals were sacrificed at the moment of any abnormality on the fluoroscopic image or, if no abnormality occurred, at fixed time intervals of two and three months. After explantation, valves were examined macroscopically, by stereomicroscopy, histology and scanning electron microscopy. Lung biopsies were taken. RESULTS: All monoleaflet valves remained functionally intact for up to three months without anticoagulation, but explantation after two or three months revealed thrombi attached to struts, cuspal surface and valve ring. All bileaflet valves, except one, showed obvious dysfunction on fluoroscopy (one or both cusps fixed) within a time frame of three to eight weeks after implantation. Histology and scanning electron microscopy showed primary thrombotic material in one or both hinges, obstructing further cusp movement. Lung embolism was detected in only one animal among biopsy specimens. CONCLUSION: The low-pressure environment of the pulmonary position caused primary thrombotic changes in all implanted mechanical valves. The thrombosis caused severe mechanical dysfunction only in the bileaflet valves, probably due to their delicate hinge mechanism. These findings may be used in the further development of an animal model of mechanical valve thrombosis.

Animals↗
PubMed2005

Clinical results and complications in mobile-bearing total knee arthroplasty.

The low-contact stress mobile-bearing implant has the longest continuous follow-up, with over 25 years of data compiled in the designers' original study. In several peer-reviewed publications, the long-term outcome and clinical performance of the femorotibial articulation, patellar resurfacing, and various issues of surgical technique, such as cruciate retention or sacrifice, tibial axis alignment method of bone resection, cemented versus cementless implants, and the use of the lateral approach for patients with valgus deformed knees are analyzed. These results are compared with those of outcome studies of total knee arthroplasty in general. Results from a recent multicenter outcome study evaluate the survivorship results from surgeons around the world who have extensive experience with the low-contact stress implant.

Aged↗