Adhesion Failure in Bonded Rubber Cylinders Part 2: Fatigue Life Prediction of External Ring-Shaped Cracks Using Tearing Energy Approach

Rubber disks bonded between flat parallel metal plates are often used as adhesion test specimens; for example, ASTM D 429 1999, Method A. However, the mechanics of adhesion failure (debonding) for this geometry have not previously been fully analyzed. Therefore, a study was conducted to determine the strain energy release rate (tearing energy) for bonded rubber disks having external ring cracks at the rubber-to-metal bond and to develop a method for predicting the fatigue life.

Finite element analysis was used to determine the tearing energy as a function of crack length for disks of various dimensions (shape factors). The crack configurations considered were an external-ring-shaped crack located at the outside circumference of either one or both rubber-to-metal bonds. The fatigue crack propagation (FCP) behavior was characterized for a generic filled natural rubber material.

The tearing energy was found to be a non-linear function of crack length. For small cracks, the tearing energy was small and approached zero as the crack length decreased. The tearing energy then increased as the crack grew, indicating accelerating growth, until it passed through a maximum value. The peak tearing energy was found to depend on the height of the disk. Finally at large cracks, the tearing energy decreased or was essentially constant as the crack grew.

The fatigue life of the rubber cylinders at different shape factors was determined experimentally. An empirical model coupled with the fatigue crack propagation behavior (FCP) for the material at different tearing energies was used to predict the fatigue life. The experimental and predicted fatigue life showed excellent agreement at low and moderate shape factors. However at high shape factors, fatigue life was not well predicted. From the experimental results, it was found that, at high shape factors, cavitation occurs causing a series of “dimples” to form, which leads to the development of an internal penny-crack, thereby violating the assumed model of an external ring-shaped crack.

Osseointegration of preformed polymethylmethacrylate craniofacial prostheses coated with bone marrow-impregnated poly (DL-lactic-co-glycolic acid) foam

Osseointegration of bone marrow-PLGA-coated, preformed polymethylmethacrylate cranioplasties offers the possibility of reducing: operative time, periimplant seroma and infection, metallic fixation, and periprosthetic resorption following surgical skull remodeling. These alloplastic materials are FDA-approved but previously have not been used together to promote cranioplasty incorporation. The objective of this study was to determine whether the use of PLGA foam coating improves host osseointegration of preformed, textured, polymethylmethacrylate prosthetic cranioplasties. A critical-sized cranial defect was created in two groups of 10 and one group of three rabbits. The defect was filled with either a textured, preformed polymethylmethacrylate disc or a textured, preformed polymethylmethacrylate disc coated with poly (DL-lactic-co-glycolic acid). Both implants were immersed in autologous bone marrow for 20 minutes before implantation. Half of each group of 10 were killed at 3 weeks, and the remainder at 6 weeks. A third group of three rabbits with excised periosteum was evaluated at 6 weeks. Histologic analysis of the discs determined relative amounts of cancellous bone formation adjacent to the prostheses. Woven trabecular bone was present at each host bone to implant perimeter interface at 3 weeks, with fine fibrous capsular formation around the implants. Thicker, lamellar trabeculae were present at 6 weeks with an increased fibrous layer surrounding both types of implants. Bone formed on the superficial and deep implant surfaces in a noncontiguous fashion. Two of five measures showed that total bone formation was significantly greater in the PLGA-coated implants. Polymethylmethacrylate discs coated with bone marrow-impregnated PLGA foam demonstrate increased bone formation at 3 and 6 weeks as compared with non-coated preformed polymethylmethacrylate discs. Only implants with preserved periosteum showed bone formation away from the host-implant interface (centrally) on the superficial surface at 6 weeks.

Metal levels in cemented total hip arthroplasty. A comparison of well-fixed and loose implants

In a prospective study, synovial fluid metal levels from stainless steel, cobalt-chromium, and titanium-alloy cemented total hip implants were measured. There were 37 well-fixed and 44 loose hip arthroplasties. Tissue-metal levels were quantitated in the cases revised for loosening. Retrieval analysis for implant wear was performed. Synovial fluid analysis showed a fivefold increase in metal levels of loose compared with well-fixed stainless steel implants. There was a sevenfold increase in metal levels of loose compared with well-fixed cobalt-chromium implants. There was a 21-fold increase in metal levels of loose compared with well-fixed titanium-alloy (Ti-6Al-4V) implants. Tissue-metal levels from revised cobalt-chromium implants averaged 45 micrograms/g dry tissue weight compared to 4,470 micrograms/g dry tissue weight from revised titanium-alloy implants, a 100-fold increase. Implant retrieval analysis showed severe burnishing and scratching in all titanium-alloy femoral heads and extensive burnishing and scratching in the majority of the femoral stems. Well-fixed cemented implants have similar low synovial fluid metal levels. However, when loosening of implants occurs, titanium-alloy implants release disproportionate levels of metal into synovial fluid and local tissues compared to stainless steel or cobalt-chromium.