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PubMed1998

A parametric study of acetabular cup design variables using finite element analysis and statistical design of experiments.

To isolate the primary variables influencing acetabular cup and interface stresses, we performed an evaluation of cup loading and cup support variables, using a Statistical Design of Experiments (SDOE) approach. We developed three-dimensional finite element (FEM) models of the pelvis and adjacent bone. Cup support variables included fixation mechanism (cemented or noncemented), amount of bone support, and presence of metal backing. Cup loading variables included head size and cup thickness, cup/head friction, and conformity between the cup and head. Interaction between and among variables was determined using SDOE techniques. Of the variables tested, conformity, head size, and backing emerged as significant influences on stresses. Since initially nonconforming surfaces would be expected to wear into conforming surfaces, conformity is not expected to be a clinically significant variable. This indicates that head size should be tightly toleranced during manufacturing, and that small changes in head size can have a disproportionate influence on the stress environment. In addition, attention should be paid to the use of nonmetal backed cups, in limiting cup/bone interface stresses. No combination of secondary variables could compensate for, or override the effect of, the primary variables. Based on the results using the SDOE approach, adaptive FEM models simulating the wear process may be able to limit their parameters to head size and cup backing.

Acetabulum↗
PubMed2004

A machine for the preliminary investigation of design features influencing the wear behaviour of knee prostheses.

Degradation of tibial inserts in vivo has been found to be multifactorial in nature, resulting in a complex interaction of many variables. A range of kinematic conditions occurs at the tibio-femoral interface, giving rise to various degrees of rolling and sliding at this interface. The movement of the tibio-femoral contact point may be an influential factor in the overall wear of ultra-high molecular weight polyethylene (UHMWPE) tibial components. As part of this study a three-station wear-test machine was designed and built to investigate the influence of rolling and sliding on the wear behaviour of specific design aspects of contemporary knee prostheses. Using the machine, it is possible to monitor the effect of various slide roll ratios on the performance of contemporary bearing designs from a geometrical and materials perspective.

Arthroplasty, Replacement, Knee↗
PubMed2006

Deformation of press-fitted metallic resurfacing cups. Part 2: Finite element simulation.

The deformation of metallic acetabular cups employed for metal-on-metal hip resurfacing procedures was considered theoretically using the finite element method in the present study, following on the experimental investigation reported in Part 1. Three representative cups, characterized by the cup wall thickness as thin, intermediate, and thick, were considered. For the intermediate cup, the effects of both the size and the diametral interference on the cup deformation were investigated. Both two-dimensional axisymmetric and three-dimensional finite element models were developed to examine the important parameters during and after the press-fit procedure, and in particular the deformation of the metallic cup. The theoretical prediction of the cup deformation was in reasonable agreement with the corresponding experimental measurement reported in Part 1. The most significant factor influencing the cup deformation was the cup wall thickness. Both the size and the diametral interference were also shown to influence the cup deformation. It is important to ensure that the cup deformation does not significantly affect the clearance designed and optimized for tribological performances of metal-on-metal hip resurfacing prostheses. Furthermore the contact parameters at the cup and bone interface associated with the press fit were also discussed.

Acetabulum↗
PubMed2006

On the biomechanical stability of cementless straight conical hip stems.

The aim of the present study was to investigation in vitro the effect of deficient bone-implant contact on the primary stability of a straight conical stem. Various possible deficient contact patterns were derived from surgical simulations. The effect of stair climbing loads on the bone-implant micromotion was firstly investigated using a finite element model and then an in vitro test aimed at assessing primary stability. It was found that if the surface features are prevented from biting dense bone in a few small but critical regions, stem primary stability is completely lost. These results suggest that the surface features used in the axisymmetric stem under investigation can be too sensitive to deficient contact conditions, and thus should be augmented with additional antirotational fins. Preliminary tests showed that a stem with the addition of such fins presents good primary stability in all tested conditions.

Arthroplasty, Replacement, Hip↗
PubMed1998

Comparison of the Wrightington FC hip with the Charnley low-friction arthroplasty. 10- to 15-year results and survival analysis.

Between 1981 and 1986 two groups of patients received either Charnley low-friction arthroplasties or the Wrightington Frusto-Conical hip replacement with otherwise identical management before and after operation. There were 260 consecutive replacements in 215 patients in the first group and 260 consecutive procedures in 211 patients in the second. Both components of each design were cemented and featured a 22.25 mm femoral head, but the geometry of the stems differed. Of the patients lost to follow-up, 16 (18 hips) were in the Wrightington series and 24 (36 hips) in the Charnley series, and of those who have died 20 (23 hips) were Wrightington and 14 (16 hips) were Charnley. All surviving patients have been followed up clinically and radiologically or until revision was necessary. The mean length of follow-up for original surviving components in both series was 140 months (120 to 180). Pain, function and movement were measured by the grading system of Merle D'Aubigné and Postel and showed a marked improvement in both groups. There was little or no pain in 98% of patients in the Wrightington series and 96% in the Charnley series, while 93% and 85%, respectively, were entirely painfree. Trochanteric osteotomy was used in 292 cases overall, with a complication rate of 13% due to wire breakage, trochanteric bursitis or trochanteric detachment, requiring a further operation in 5.8% (17 hips). Heterotopic ossification was seen in 40% of cases in which trochanteric osteotomy had been performed (117 hips) but only 3% (10 hips) had clinically significant changes. After the direct lateral approach heterotopic ossification was seen in 24% (42 hips) with only 2% in class 3 or 4 (4 hips). Femoral osteolysis was more common in the Charnley series, occurring in 7.6% of cases (17 hips) as opposed to 2.1% (5 hips) in the Wrightington series. Ten acetabular and seven femoral components have been revised in the Wrightington series and 16 acetabular and 11 femoral implants in the Charnley series. Survivorship based on revision for aseptic loosening using the Kaplan-Meier technique showed survival of the femoral implant at 5, 10 and 15 years of 100%, 99% and 98%, respectively in the Wrightington and 100%, 99% and 87% in the Charnley series. The survival of the acetabular component at 10, 12 and 15 years respectively, was 100%, 99% and 95% for the Wrightington, and 99%, 98% and 84% for the Charnley. The survival of both the acetabular and femoral components of the Wrightington system at a mean of 11.7 years was better than that of the Charnley system, with a lower incidence of radiological loosening of unrevised components.

Acetabulum↗
PubMed2004

[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.

Arthroplasty, Replacement, Hip↗
PubMed2003

Polyethylene wear and acetabular component orientation.

BACKGROUND: Polyethylene wear contributes substantially to both periprosthetic osteolysis and aseptic loosening after total hip arthroplasty. Acetabular component orientation has been shown to affect the range of motion of the hip as well as contact stresses. A series of studies was designed to test the hypothesis that acetabular component orientation can affect the magnitude and direction of polyethylene wear. METHODS: A finite-element model was used to compute contact stresses during a normal gait cycle. Wear at the end of each gait cycle was calculated with use of the sliding-distance-coupled finite-element formulation. The wear that was calculated with use of finite-element analysis was validated by comparison with the findings of hip wear simulator studies with the acetabular liner oriented to simulate 45 degrees and 55 degrees of abduction. In a clinical study, fifty-six patients who underwent sixty hip arthroplasties with use of a single prosthetic design were followed for as long as five years. Radiographs were analyzed to measure the abduction angle of the acetabular component and polyethylene wear. RESULTS: The finite-element analysis predicted increased peak contact stresses with an increased abduction angle and reduced peak contact stresses with an increased anteversion angle. Linear wear rates ranging from 0.036 to 0.045 mm/million cycles were also predicted, and increased acetabular abduction angles were predicted to be associated with higher linear wear rates. In the hip wear simulator studies, significantly different wear rates were found between the cups with acetabular abduction angles of 45 degrees and 55 degrees (mean, 17.2 compared with 21.7 mg/million cycles; p < 0.01). In the clinical study, radiographic analysis revealed significant correlation between the acetabular abduction angle and the linear polyethylene wear rate. A 40% increase in mean linear polyethylene wear was seen in cups with an abduction angle of >or=45 degrees. The direction of wear was more medial (by 9.4 degrees ) in cups with an abduction angle of <45 degrees. CONCLUSION: All three studies presented here underlined the importance of optimizing the position of the acetabular component. Careful attention to acetabular position may help to minimize wear.

Aged↗
PubMed2001

[Carboxymethylcellulose hydrogel-filled breast implants. Our experience in 15 years].

The author reports 15 years of laboratory and surgical experience with the use of breast implants pre-filled with carboxy-methyl-cellulose gel which he has used since 1984. Laboratory tests and results are presented. The author has undertaken a retrospective analysis of 380 clinical cases since 1984. He concludes that there is a future role for breast implants containing a non-toxic, visco-elastic and biodegradable filling gel in conjunction with the manufacture of increasingly reliable implant linings.

Absorbable Implants↗
PubMed2001

Tissue damage and calcification may be independent mechanisms of bioprosthetic heart valve failure.

BACKGROUND AND AIM OF THE STUDY: Porcine bioprosthetic valves have excellent hemodynamics and do not require anticoagulation, but have limited durability. Cusp tearing is a major cause of bioprosthetic valve failure. It has been suggested that the mechanism of bioprosthetic valve failure is stiffening by calcification, which leads to elevated stresses and secondary collagen fiber damage and leaflet tearing. This thesis was tested in explanted porcine bioprostheses. METHODS: A total of 60 explanted porcine bioprosthetic valves was tested mechanically, and 15 explanted valves were examined grossly and histologically. Circumferentially and radially oriented samples of cusp tissue were tested uniaxially in a materials testing machine and compared with five controls. RESULTS: Mean (+/-SD) duration of implantation was 10.9+/-5.6 years. Circumferential specimens from explants were less extensible than controls (11.0+/-5.5% versus 24.5+/-2.8% strain, p <0.001), and failed at lower tensions (973+/-733 versus 3075+/-911 N/m, p = 0.001) and at lower strains (21.2+/-8.1% versus 47.3+/-7.1% strain, p <0.001). Radial specimens from explants were less extensible (28.7+/-6.8% versus 39.2+/-5.9% strain, p = 0.002) and failed at lower strains (60.3+/-17.3% versus 112.2+/-24.9% strain, p <0.001) than the controls. The stiffness of the explants was unchanged from controls in both circumferential and radial samples. There were no differences between explants and controls in radial and circumferential stiffness, and in radial failure strength. Calcification was mild and diffuse in most of the tested samples. Tears were found in areas without calcific deposits, along with breaks in collagen fiber bundles. CONCLUSION: These results do not support the thesis that calcification stiffens glutaraldehyde-fixed porcine bioprostheses, except when the entire cusp is transformed into a solid mass of mineral. Rather, leaflet tears may develop as a result of accumulated mechanical damage that is independent of calcification.

Animals↗
PubMed2003

Design and initial testing of a novel halo orthosis.

A halo orthosis is a medical device used to limit the motion of the skull and cervical spine in order to properly heal an injury sustained in the cervical spine. A difficulty with current halo designs is that the pins which engage the skull can become loose over time. This pin loosening is caused by the inability of halo systems to adapt to small changes in skull geometry due to abrasion, remodeling, and stress relaxation at the pin sites. The inability to adapt is caused by the over-constrained mobility of current halo systems. To address this issue, a nearly exactly-constrained halo prototype that can adapt to most changes in the skull geometry was designed and fabricated. The prototype was tested using a device that simulates the natural geometry changes in the skull. The prototype retains an average of 93% of the initial pin force over a radial recession in the skull of 0.95 mm at each pin site. Comparatively, the current clinical four pin halo only retains an average of 11% of the initial pin force with 1 mm radial recession at each pin site.

Bone Nails↗
PubMed2003

TEM observation of seven retrieved total knee joints made of Co-Cr-Mo and Ti-Al-V alloys.

We demonstrate that a myriad of fine particles produced by the abrasion of both cobalt-chromium-molybdenum (Co-Cr-Mo) and titanium-aluminum-vanadium (Ti-Al-V) alloys accumulate in the synovial cells next to surgical implants made from these alloys. The metallic particles were of various sizes, and were observed within the lysosomes. Energy dispersive X-ray spectroscopy studies revealed that the fine spherical particles consisted solely of Cr, and that other larger particles were composed of the Co-Cr-Mo alloy. We measured the size of the metallic particles using the public domain NIH image program, and found that most of the fine spherical particles were 10-15 nm in diameter (n=1000). Eighty percent of the large particles were 30-35 nm in length and 20-25 nm in width (n=300). In addition, EDS examination clarified that all of the fine particles of the Ti-Al-V alloy were composed of that alloy. For this alloy, when discounting the larger particles, the fine metal deposits were 20-25 nm in length and 10-15 nm in width (n=1000). From these findings, we conclude that the Co-Cr-Mo alloy is easily corroded and that Co is released from the cells. In contrast, the Ti-Al-V alloy is very stable and does not corrode, although the Ti-Al-V alloy does produce particles that are smaller than those produced by the Co-Cr-Mo alloy.

Aged↗
PubMedDate not supplied

Biomechanics of cemented and cementless prostheses.

The success of the cemented arthroplasty is due primarily due to excellent initial stability, intimate contact with the prepared bone and the isoelastic properties provided by the cement. In an experimental in vitro study, the initial stability of both the cemented and uncemented femoral components was measured and it was found that they were both very stable in simulated single limb stance (maximum motion of less than fifty microns). However, in simulated stair climbing, the uncemented components were more unstable than the cemented components (maximum micromotion of 76 microns for cemented and 280 microns for uncemented), and this relative instability of the uncemented femoral components could compromise the bone ingrowth. Studies on femurs retrieved at autopsy from patients who underwent cemented total hip arthroplasty two week sup to seventeen years earlier and were functioning well, have shown that the failure of cemented femoral components is initiated primarily by mechanical factors, consisting of debonding at the cement-prosthesis interface and fractures of the cement rather than lack of bone ingrowth or fibrous tissue formation at the interface. Thus the problems with using cemented femoral components involve the poor strength of the cement-prosthesis interface and the cement, while the problems with cementless components involve the difficulties in precisely machining the femoral canal, and providing rigid stability as well as accurate fit.

Adult↗
PubMed2005

Extensor mechanism complications after total knee arthroplasty.

The patellofemoral joint is an important source of pain and complications following total knee arthroplasty. Anterior knee pain, patellar instability, fracture, rupture of the extensor mechanism, and a variety of miscellaneous problems affecting the patella can adversely affect the results of a total knee arthroplasty. Because patellofemoral complications after total knee arthroplasty often occur because of problems with patient selection, implant design, and surgical technique, it is important for orthopaedic surgeons treating patients with extensor mechanism complications after total knee arthroplasty to evaluate the entire knee replacement and base treatment on the etiology of the complication and not just the complication itself.

Arthroplasty, Replacement, Knee↗