Refixation stability in shoulder hemiarthroplasty in case of four-part proximal humeral fracture.

Primary stability of refixated fractures in case of shoulder hemiarthroplasty is a prerequisite to restore physiological glenohumeral joint function. Clinical observations often show a secondary dislocation and subsequent resorption of the bony anchor points like the greater and lesser tuberosity at the rotator cuff tendons. This failed integration leads to impaired glenohumeral load transmission and subsequent reduction of mobility. As a consequence, the optimisation of refixation methods is crucial for a better clinical outcome. To prove the stability of refixation techniques, a Finite Element fracture model was built. Resulting stresses at the bone surface and fragment migration relative to the prosthesis shaft were studied. The results of the calculations show that the isolated tuberosities show unstressed bone regions compared to the intact model. This circumstance may explain the clinically detected bone resorption due to the absence of mechanical stimuli. Furthermore, a cable guidance through lateral holes in the middle part of the proximal prosthesis results in a lower fragment displacement than a circumferential fixation method surrounding the entire proximal bone.

Friction, lubrication, and polymer transfer between UHMWPE and CoCrMo hip-implant materials: a fluorescence microscopy study.

The friction coefficients of CoCrMo sliding against UHMWPE and CoCrMo were measured in solutions of albumin and synovial fluid containing fluorescently labeled albumin. No fluorescence could be observed on the CoCrMo disc following incubation in labeled albumin or after sliding against CoCrMo. This was due to quenching of the fluorophore by the metal and indicated that a protein film thicker than 10 nm was not formed on the surface. A more complicated behavior was observed for UHMWPE sliding against CoCrMo. For each lubricating solution and at each load, a bimodal distribution of steady-state friction values was observed, the friction coefficient either remaining constant or decreasing during the early stages of the measurement. As no quenching of the fluorophores occurred on the UHMWPE surface, the fluorescence labeling method could be used to reveal polyethylene (PE) transfer and to show that it correlates with the friction coefficient: Low friction coefficients corresponded to a low density of PE spots on the CoCrMo surface. In addition, it was found that the friction coefficients for UHMWPE sliding against CoCrMo in synovial fluid were not significantly different from those in phosphate-buffered saline (PBS), but that the addition of albumin to PBS did cause a significant increase in the friction coefficient.

In vivo behavior of calcium phosphate scaffolds with four different pore sizes.

The goal of the present study was to assess the effect of macropore size on the in vivo behavior of ceramic scaffolds. For that purpose, beta-tricalcium phosphate (beta-TCP) cylinders with four different macropore sizes (150, 260, 510, and 1220 microm) were implanted into drill hole defects in cancellous bone of sheep and their resorption behavior was followed for 6, 12 and 24 weeks. The scaffolds were evaluated for biocompatibility, and new bone formation was observed macroscopically, histologically and histomorphometrically. Histomorphometrical measurements were performed for the whole defect area and for the area subdivided into three concentric rings (outer, medial, and inner ring). All implants were tolerated very well as evidenced by the low amount of inflammatory cells and the absence of macroscopic signs of inflammation. Resorption proceeded fast since less than 5% ceramic remained at 24-week implantation. Hardly any effect of macropore size was observed on the in vivo response. Samples with an intermediate macropore size (510 microm) were resorbed significantly faster than samples with smaller macropore sizes (150 and 260 microm). However, this fast resorption was associated with a lower bone content and a higher soft tissue content. At 12 and 24 weeks, the latter differences had disappeared. Bone was more abundant in the outer ring than in the rest of the blocks at 6 weeks, and in the outer and medial ring compared to the inner ring at 12 weeks.

Femoroplasty-augmentation of mechanical properties in the osteoporotic proximal femur: a biomechanical investigation of PMMA reinforcement in cadaver bones.

OBJECTIVE: To determine the feasibility of polymethyl-methacrylate injection into the osteoporotic proximal femur and its effect on the mechanical properties. DESIGN: In vitro pairwise comparison of non reinforced and reinforced bones in a load to failure loading mode. BACKGROUND: Hip fractures represent an important public healthcare problem. Continued growth in the elderly population will raise the incidence of hip fractures and their associated costs dramatically in the near future. METHODS: Twenty pairs of osteoporotic femurs were mechanically tested either in a single-limb stance configuration or simulating a fall on the greater trochanter. From each pair, one femur was augmented with bone cement, with the contralateral femur serving as a control. The surface temperature at the femoral neck was recorded until twenty minutes after injection. The fracture load and the energy absorption were calculated. The Wilcoxon signed rank test was used to test for differences in fracture load and energy absorption between the reinforced femurs and the native controls. RESULTS: Volumes of 28-41 ml of cement (mean, 36 ml) could be injected. The increase of surface temperature at the femoral neck ranged from delta18.4 to delta29.8 degrees C. For the single limb stance configurations, the peak fracture load was increased by 21%, (P < 0.002) and for the simulated fall on the hip by 82%, (P < 0.002). The corresponding values for energy absorption were +48%; and +188% (P < 0.002) respectively. CONCLUSIONS: The feasibility and mechanical effectiveness of the in vitro procedure could be demonstrated. The heat generation due to polymethyl-methacrylate polymerisation is high. RELEVANCE: Prophylactic reinforcement of the femur could become a treatment option to solve the problems with osteoporotic hip fractures in patients at risk. Reinforcement materials with less exothermic reaction need to be evaluated further and also the feasibility of fracture repair after reinforcement.

In vivo comparison of the osseointegration of vacuum plasma sprayed titanium- and hydroxyapatite-coated implants.

For the last 15 years, orthopedic implants have been coated with hydroxyapatite (HA) to improve implant fixation. The osteoconductive effect of HA coatings has been demonstrated in experimental and clinical studies. However, there are ongoing developments to improve the quality of HA coatings. The objective of this study was to investigate whether a rough and highly crystalline HA coating applied by vacuum plasma spraying (VPS) had a positive effect on the osseointegration of special, high-grade titanium (Ti) implants with the same surface roughness. Ti alloy implants were coated (VPS) with special, high-grade Ti or HA. The osseointegration of the implants was evaluated by either light microscopy or pullout tests after 1, 2, and 4 weeks of unloaded implantation in the cancellous bone of 18 sheep. The interface shear strength increased significantly over all time intervals. By 4 weeks, values had reached approximately 10N/mm(2). However, the difference between the coatings was not significant at any time interval. Direct bone-implant contact was significantly different between the coatings after 2 and 4 weeks, and reached 46% for Ti and 68% for HA implants by 4 weeks. This study indicates that the use of a rough and highly crystalline HA coating, applied by VPS, enhances early osseointegration. Accelerated establishment of secondary implant fixation decreases the risk of early loosening.