Fracture at the stem-condylar junction of a modular femoral prosthesis in a varus-valgus constrained total knee arthroplasty.

UNLABELLED: We present a rare case of fracture of the modular femoral component in a varus-valgus constrained total knee arthroplasty (TKA) prosthesis which occurred 2 years after implantation. Both femoral and tibial components underwent a second revision using a constrained hinged TKA. Laboratory analysis revealed insufficient metaphyseal bone stock in association with inadequate cement mantle incorporation which made implant fixation solely reliant on the intramedullary stem construct. It was therefore hypothesized that high repetitive stresses over the junction of the modular component led to the fracture. LEVEL OF EVIDENCE: Case report, Level IV.

Clinical relevance of standardization of endodontic files dimensions according to the ISO 3630-1 specification.

The aim of this study was to examine the current status of standardization of endodontic instruments. Measurements of instrument dimensions were conducted on stainless steel H- and K-files (sizes 08-40), as well as on rotary-driven nickel titanium (NiTi) files (sizes 15-40), to determine the incidence and degree of deviation from ISO 3630-1, 1992 specification. In addition, the dimensions of sets of H- and K-files (sizes 08-25) were measured to identify discrepancies in dimensions among instruments within the same size per manufacturer. The percentage of difference in width were used to determine the graduation from each size to the next. None of the files tested complied with the ISO nominal size. All files were within the ISO tolerance limits. However, under such tolerance limits, there is a high possibility of either sizes overlapping or of great differences between two sequential sizes. These results may explain the clinical difficulty found in negotiating narrow and curved canals and may establish the need for a more comprehensive approach on the evaluation methods used in root-canal instrument dimension specifications.

Long-term behaviour of the Charnley offset-bore acetabular cup.

We report the long-term radiological results of 58 total hip arthroplasties (THA) using the Charnley offset-bore acetabular socket. Wear was measured at four sites and radiolucent lines and possible migration were recorded. Four cups were retrieved at revision and were examined using light microscopy, SEM and X-ray microanalysis. At a mean follow-up of seven years the mean wear in the DeLee and Charnley zone I was 0.4 mm and in zone II 0.26 mm. The wear rate was 0.06 and 0.04 mm/year, respectively. Progression of radiolucent lines was seen in five cases (8.6%). Three sockets (5.2%) were revised because of aseptic migration at a mean follow-up of 9.8 years and one socket for infection at two years. The offset-bore acetabular cup had excellent wear behaviour and a low migration and revision rate. We recommend that it should be considered in THA since the use of small cups is increasing, particularly in revision cases.

Analysis of a retrieved Isola spinal system fractured in service.

A 1/4-in.diameter two-column Isola spinal system implanted in a 13-year-old girl suffering from cerebral palsy and a severe neuromuscular scoliosis of 120 degrees, was replaced 22 months after primary implantation. The system was removed due to fracture of the lower left (concave) bar, between its two cross connectors (i.e., the distal bypass connector and a transverse cross link), as a result of a postoperative infection and a subsequently developed pseudarthrosis. The retrieved implants were analyzed with the use of a multitechnique characterization procedure involving macroscopic and microscopic examination, micro-multiple internal reflectance FTIR spectroscopy, X-ray fluorescence (XRF) spectrometry, roughness measurement, and mechanical testing of the constituent material and components. Findings suggest that the spinal system failed due to in vivo loosening of a two-set screw tandem connector and subsequent overloading of the contralateral bar.

Failure analysis of two Ti-alloy total hip arthroplasty femoral stems fractured in vivo.

Failure of total hip arthroplasty femoral stems is a serious clinical complication. Even modern metal alloys and designs sometimes suffer such incidents. The reported study aimed at the investigation of the reasons leading the in vivo fracture of two Ti6Al4V femoral stems. Stems were retrieved during revision surgery approximately 2 years postoperatively. Examination and analysis included XRF spectrometry for identification of chemical composition, macroscopic examination and topographical measurements, SEM study of fracture surfaces, study of alloy microstructure by optical microscopy, and finally measurement of mechanical properties by means of tensile testing conducted on alloy samples machined from the stems themselves. Macroscopic examination and measurements showed close topographical similarity between the two fractures. XRF spectrometry, tensile testing, and microstructure analysis identified the alloy as a typical Ti6Al4V surgical titanium alloy. During SEM analysis the fracture surfaces exhibited characteristic fatigue striations tidally running on the cross sections, which were considered as the weakest ones regarding toward geometry and stress concentration. Fracture in both stems occurred due to fatigue along these cross sections.

Prosthetic loading during kneeling of persons with transfemoral amputation.

Observations in the field of lower limb prosthetic rehabilitation have shown that several transfemoral prostheses show signs of wear on some components of the knee unit. This is thought to be a result of severe loading developed during activities associated with kneeling. Some prostheses may have failed due to repetitive action of such loading. In order to determine the nature and magnitude of the loads developed during kneeling by persons with transfemoral amputation, and to investigate the influence of various prosthetic parameters, an analysis of the results of 162 tests in prosthetic knee hyperflexion was undertaken. The services of four males with amputation were enlisted. The measurements involved simultaneous use of two Kistler force platforms, a six-channel strain gauge transducer mounted on the prosthetic shank, and a data acquisition system. The critical loads for this configuration were found to be the shear force on the knee hinge, the shear force imposed by the knee chassis on the shin, and the bending moment tending to hyperflex the knee. These loads ranged from 0.6 to 6.2 kN, 0.9 to 6.7 kN, and from 18.3 to 155.7 Nm, respectively. To achieve a comfortable kneeling position, some prostheses permit foot rotation about the pylon axis of 90 infinity to allow the shank to be approximately parallel to the ground. Tests were also conducted with the prostheses in this configuration and the most influential prosthetic parameter was found to be the external rotation of the foot (toe-out angle). During kneeling, it was found that the loading was dependent upon the position of the torso relative to the prosthesis, but loads were much higher than those developed during level walking.

Femoral neck version affects medial femorotibial loading.

The aim of this study was to provide a preliminary evaluation of the possible effect that femoral version may have on the bearing equilibrium conditions developed on the medial tibiofemoral compartment. A digital 3D solid model of the left physiological adult femur was used to create morphological variations of different neck-shaft angles (varus 115, normal 125, and valgus 135 degrees) and version angles (-10, 0, and +10 degrees). By means of finite element modeling and analysis techniques (FEM-FEA), a virtual experiment was executed with the femoral models aligned in a neutral upright position, distally supported on a fully congruent tibial tray and proximally loaded with a vertical only hip joint load of 2800 N. Equivalent stresses and their distribution on the medial compartment were computed and comparatively evaluated. Within our context, the neck-shaft angle proved to be of rather indifferent influence. Reduction of femoral version, however, appeared as the most influencing parameter regarding the tendency of the medial compartment to establish its bearing equilibrium towards posteromedial directions, as a consequence of the corresponding anteroposterior changes of the hip centre over the horizontal tibiofemoral plane. We found a correlation between femoral anteversion and medial tibiofemoral compartment contact pressure. Our findings will be further elucidated by more sophisticated FEM-FEA and by clinical studies that are currently planned.

Mechanical characterization of bone graft substitute ceramic cements.

The aim of this laboratory work was to study the compressive and flexural characteristics of various commercially available bone graft substitute (BGS) ceramic cements, in their initial as-mixed condition, and compare them to polymethylmethacrylate (PMMA). The tested biomaterials were two different calcium phosphate cements, two different calcium sulphate cements, one nanocrystalline hydroxyapatite and one PMMA cement. All biomaterials were prepared according to manufacturers instructions and the methodology described in ISO 5833 (2002) for acrylic bone cement was followed, as the one closest approaching in vivo requirements. All BGS cements had a brittle behaviour and when subjected to mechanical stress they all failed under sudden crack propagations in their bulk. Both in compression and bending, all BGS cements failed under loads lower than those of PMMA. In compression, the calcium sulphate extra strength cement showed a strength value of approximately 60% of PMMA, the other cements following at a distance. In bending, all BGS cements showed strengths below 22% of PMMA. However, due to limited number and fragility of specimens, calculated bending strengths can only be considered as indicative figures with limited comparative value. The results of this in vitro study showed a varying mechanical performance between tested BGS ceramic cements, whilst all of them exhibited lower compression and bending strength than the selected PMMA. These findings, of course, cannot be directly extrapolated to surgical or clinical implications, since the adopted in vitro context does not necessarily reflect the actual in vivo conditions met by such biomaterials.

Mid-term behaviour of cemented titanium stems: A seven to eleven year clinical.

A controversy exists regarding the results of cemented Ti-alloy femoral stems. Although most authors reported unfavourable results, there is no clear evidence of the exact reason for the increased failure rate. We studied 35 arthroplasties in 30 patients with a mean age at the time of surgery of 49.7 years (range 27 to 67 years). In most of them (23 hips) the pre-operative diagnosis was hip dysplasia or dislocation. The femoral component used in this study was the Optifix stem. Survival analysis showed a cumulative rate of success at nine years of 90.4% (15.3) with 25.5 femoral components remaining at risk. Three femoral components were retrieved during revision surgery: due to fatigue fracture, fracture of the femur and aseptic loosening respectively. Given that most publications refer to different designs of stems we presume that the problem could be focused on the design rather than the material or the method of fixation. Further information is necessary before Ti-alloy stems fixed with cement are withdrawn from use. (Hip International 2005; 15: 12-20).

Wear of retrieved ceramic THA components--four matched pairs retrieved after 5-13 years in service.

Retrieval analyses of alumina-alumina THA components are conducted in order to investigate in vivo wear of implants and provide guidance to material and design improvements. The contribution of the present study consists of the examination, by digital recording of spherical profiles and scanning electron microscopy, of four matched pairs of ceramic THA implants retrieved after 5, 11, 13, and 13 years in service because of mechanical loosening. Maximum wear depth values on measured surfaces ranged between 10-325 microm for heads and 25-784 microm for cups, while grossly worn areas varied between 240 and 1510 mm(2) for heads and 250 and 1570 mm(2) for cups. The greatest wear characteristics were demonstrated by the pair that served for 5 years, that is, on the cup that was found in a 65 degrees inclination. Quantitative topographical study of the results provided concrete evidence of a cascade of detrimental wear events of changing geometry, intensity, and progression associated with gradual cup tilting. Microscopy findings support previously recognized wear mechanisms.

Multitechnique characterization of articular surfaces of retrieved ultrahigh molecular weight polyethylene acetabular sockets.

The articular surfaces of 10 retrieved ultrahigh molecular weight polyethylene acetabular sockets were studied by optical microscopy, X-ray fluorescence spectrometry, multiple internal reflectance FTIR spectroscopy, scanning electron microscopy, and wavelength dispersive X-ray microanalysis. The results revealed characteristic wear patterns including polishing, scratching, pitting, cratering, folding, shredding, burnishing, cracking, embedding of particles, and development of acquired biofilms with various degrees of mineralization. The biofilms formed were mainly of proteinaceous origin, and mineralized regions were composed of calcium phosphates with carbonate impurities. The crystallinity of the polyethylene at the articular surfaces was enhanced compared to the bulk, which was possibly due to the cold work produced in vivo. The mineralized regions were classified into two groups based on the grey levels of the backscattered images obtained. The high-contrast regions that were mainly composed of Ca and P with traces of Al and Si were associated with bone fragments; the low-contrast regions composed of K, Na, Ca, Mg, Si, Al, Fe, Cl, and P were associated with acquired biofilm calcification, which implies the active engagement of biofilms in the long term performance of acetabular sockets in vivo.