The relationship between cement fatigue damage and implant surface finish in proximal femoral prostheses.

The majority of cemented femoral hip replacements fail as a consequence of loosening. One design feature that may affect loosening rates is implant surface finish. To determine whether or not surface finish effects fatigue damage accumulation in a bone cement mantle, we developed an experimental model of the implanted proximal femur that allows visualisation of damage growth in the cement layer. Five matte surface and five polished surface stems were tested. Pre-load damage and damage after two million cycles was measured. Levels of pre-load (shrinkage) damage were the same for both matte and polished stems; furthermore damage for matte vs. polished stems was not significantly different after two million cycles. This was due to the large variability in damage accumulation rates. Finite element analysis showed that the stress is higher for the polished (assumed debonded) stem, and therefore we must conclude that either the magnitude of the stress increase is not enough to appreciably increase the damage accumulation rate or, alternatively, the polished stem does not debond immediately from the cement. Significantly (P=0.05) more damage was initiated in the lateral cement compared to the medial cement for both kinds of surface finish. It was concluded that, despite the higher cement stresses with debonded stems, polished prostheses do not provoke the damage accumulation failure scenario.

Minimum solid area models applied to the prediction of Young's modulus for cancellous bone.

In developing models for the mechanical behavior of cancellous bone, accurate prediction of Young's modulus as a function of the pore fraction and morphology is a requirement. Previous workers have suggested models which provide good statistical fits, but most of these models are highly idealized, with no treatment of the actual morphology of the porosity. In the field of engineering ceramics, simple minimum solid area models have been developed over the past four decades to describe the mechanical properties of porous structural ceramics. This paper applies these models to data for cancellous bone, and it is shown that one, developed specifically for high porosity materials, gives realistic predictions of tissue modulus and a good statistical fit to well-established data. This model should prove to be useful in biomechanical analyses involving cancellous bone tissue.

The sintering and mechanical behavior of hydroxyapatite with bioglass additions.

There is increasing interest in the potential of composites of hydroxyapatite with phosphate- or silicate-based bioactive glasses, and certain of these glass additions have been found, in previous work, to aid densification and form a mechanically-reinforced, bioactive material; in particular, large improvements in flexural strength and fracture toughness were obtained through the addition of small amounts of phosphate glass. Less is known about the mechanical behavior of HA/bioglass composites, although in vivo studies by other workers have shown encouraging biological results. In this investigation, the sintering behavior, mechanical properties, and microstructure of composites of HA with up to 50 wt % glass, were analyzed. X-ray diffraction showed the phase composition of sintered composites with up to 5 wt % added bioglass to be non-stoichiometric HA with alpha-TCP or beta-TCP. Phase analysis of composites containing higher glass additions was impracticable due to peak broadening and overlap, although reaction products, at the highest glass additions and sintering temperatures, may include wollastonite-2M and beta-Na2Ca4(PO4)2SiO4. Sintered density, and mechanical properties other than fracture toughness, showed no significant improvement over HA.

The relationship between surface topography and contact in the elbow joint: development of a two-dimensional geometrical model in the coronal plane.

One of the first stages in developing an accurate biomechanical representation of the elbow joint is to model realistically the geometry of the joint. In particular, given the complex anatomy of the articular surfaces, the relationship between surface topography and joint contact must be fully understood in order to model the contact conditions. As the joint articulates, the location and size of the contacts between the mating surfaces change, altering the distribution and magnitude of load transmission. In this paper, a geometric model of the anatomical elbow joint in the coronal plane is described. The contours of the articulating surfaces are represented algebraically by a series of connecting lines and circular arcs. It is shown that the location and size of the contact between the surfaces change significantly due to small changes in the topography of one or more of the mating surfaces. The surface topography-joint contact relationship is modelled for a number of different clinical conditions for the joint. The model is relevant to clinical studies of joint degeneration and to the design of prosthetic components for the elbow joint.

Microdamage accumulation in the cement layer of hip replacements under flexural loading.

Mechanical fatigue of bone cement leading to damage accumulation is implicated in the loosening of cemented hip components. Even though cracks have been identified in autopsy-retrieved mantles, damage accumulation by continuous growth and increase in number of microcracks has not yet been demonstrated experimentally. To determine just how damage accumulation occurs in the cement layer of a hip replacement, a physical model of the joint was used in an experimental study. The model regenerates the stress pattern found in the cement layers whilst at the same time allowing visualisation of microcrack initiation and growth. In this way the gradual process of damage accumulation can be determined. Six specimens were tested to 5 million cycles and a total of 1373 cracks were observed. It was found that, under the flexural loading allowed by the model, the majority of cracks come from pores in the bulk cement and not from the interfaces. Furthermore, the lateral and medial sides have statistically different damage accumulation behaviours, and pre-load cracks significantly accelerate the damage accumulation process. The experimental results confirm that damage accumulation commences early on in the loading history and that it is continuously increasing with load in the form of crack initiation and crack propagation. The results highlight the importance of replicating the loading and restraint conditions of clinical cement mantles when endeavouring to accurately model the damage accumulation process.

Modelling medical devices: the application of bioengineering in surgery.

Medical device technology has an increasingly important role in surgical procedures. In this article, five case studies of bioengineering in surgery are described as follows: computer-aided design of vascular grafts; middle-ear prostheses; hip prosthesis stems for optimal cement pressurisation; prototype development of a device for measurement of abdominal sounds for monitoring digestive tract activity and a hand-access device for laparoscopic surgery. In each case, new bioengineering design methodologies are demonstrated. The general principles underlying the application of bioengineering in surgery are discussed.

A quantitative study of the sintering and mechanical properties of hydroxyapatite/phosphate glass composites.

Previous work has shown that small additions of a phosphate glass (CaO-P2O5) can significantly enhance the sinterability and strength of hydroxyapatite. However, there are no quantitative phase analyses available for these materials which would provide indicators of biocompatibility and resorbability. Similarly, there is little information available about the mechanical properties, especially with high glass additions. In this study, the effects of sintering hydroxyapatite with phosphate glass additions of 2.5, 5, 10, 25, and 50 wt.% are quantified. Each composition was sintered over a range of temperatures, and quantitative phase analysis was carried out using XRD. In addition, the microstructures were studied using RLOM and SEM, and mechanical properties (Vickers hardness, KIC, and MOR) measured. These results may be used to indicate which compositions and processing conditions may provide materials suitable for use in hard tissue replacement. Composites containing up to 10 wt.% glass additions formed dense HA/TCP composite materials possessing flexural strength and fracture toughness values up to 200% those of pure HA. The HA/TCP ratio was strongly dependent on the percentage glass addition. Higher glass additions resulted in composites containing beta-TCP together with large amounts of alpha- or beta-calcium pyrophosphate, and having similar mechanical strengths to pure HA.

A synthetic bone implant macroscopically identical to cancellous bone.

Macroporous hydroxyapatite (HA) and beta-tricalcium phosphate (beta-TCP) are widely used as synthetic bone replacement materials due to their high biocompatibility and osteoconductive properties. The level of porosity, pore size distribution, pore morphology, and the degree of pore interconnectivity in such grafts significantly influences the extent of bone ingrowth. It has been hypothesised that an ideal implant macrostructure may be similar in morphological characteristics to the inorganic matrix of the bone it is replacing. However, to date, clinically available synthetic materials differ structurally from cancellous bone. A method is described for the macrostructural replication of cancellous bone. Reproduction involves a multistage process requiring the manipulation of positive and negative forms of the inorganic matrix. By infiltration of a wax negative mould of cancellous bone with a ceramic slip, followed by removal of the wax, and firing, it is possible to produce a positive replica of the original cancellous macrostructure. Optimisation of slip preparation conditions (pH and percentage deflocculant addition) and sintering conditions have allowed successful replication of cancellous bone using several bioceramic compositions including HA, beta-TCP, and HA/beta-TCP.

Development of a new synthetic bone graft.

A process for the replication of bovine cancellous bone in synthetic bioceramic materials for use as artificial bone graft substitutes is described. The process detailed here may be easily implemented to allow production of large numbers of blocks of material, even on a laboratory scale. The graft material has a pore morphology and interconnectivity identical with that of the original cancellous bone used as a starting material. Strength of the material is adequate, and at lower porosity levels it meets the FDA requirements for coralline materials for spinal applications. The synthetic graft is also shown to have excellent fluid-retention characteristics, making it a potential carrier for morphogenic agents such as solutions of bone morphogenic protein.

Purification of an elicitor-induced glucan synthase (callose synthase) from suspension cultures of French bean (Phaseolus vulgaris L.): purification and immunolocation of a probable M(r)-65,000 subunit of the enzyme.

Membrane preparations from suspension-cultured cells of French bean (Phaseolus vulgaris L.) contained callose synthase (EC 2.4.1.34) activity which was preserved upon solubilisation. Following elicitor treatment of cell cultures, increased activity could be extracted and this increase was maintained during purification. The enzyme was purified by high-pressure liquid chromatography and active fractions showed a variable association of two polypeptides of relative molecular masses (M(r)) 55,000 and 65,000, the latter being in excess. The M(r)-65,000 polypeptide was purified to homogeneity and an antibody raised to it. This antibody showed complex effects on callose synthase activity when incubated with membrane and soluble extracts. In comparison with other systems, the M(r)-55,000 subunit is likely to represent the catalytic subunit while the M(r)-65,000 polypeptide is a possible regulatory subunit. The M(r)-65,000 polypeptide was immunolocated in membranes at sites of callose synthesis in the plant, in cell plates, in sieve plates, at the plasma membrane-wall interface of wounded cells and in papillae in infected cells.

An analysis of crack propagation paths at implant/bone-cement interfaces.

Clinical follow-up studies of joint replacements indicate that debonding of the implant from the bone-cement is the first mechanical event of loosening. Debonding can occur due to unsustainable interface stresses, usually initiated from defects along the interface. Such defects, or flaws, are inevitably introduced during the surgical procedure and from polymerisation shrinkage. Debonding leads to increased stresses within the cement mantle. This study is concerned with modelling the propagation of a crack from the debonded region on the cement/implant interface under physiological loading conditions for different implant materials and prosthesis designs. Using the theory of linear fracture mechanics for bimaterial interfaces, the behaviour of a crack along an interface between implant materials, under various states of stress, is studied. Specifically, a model is developed to determine the conditions under which a debonded region, along an otherwise bonded interface, will either propagate along the interface or will "kink" into the cement mantle. The relationship between the stress state and the crack propagation direction at the interface is then predicted for different interface materials, and it is shown that different crack directions exist for different materials, even when the stress state is the same. Furthermore, the crack behavior is shown to be dependent on the ratio of normal stress to shear stress at the interface and this may be important for the design optimisation of load-bearing cemented prostheses. Finally, the likelihood that an interface crack will propagate into the cement mantle is explored using a suitable fracture criterion.

An experimental study of damage accumulation in cemented hip prostheses.

OBJECTIVE: To develop a methodology to characterize the pattern of crack initiation and damage accumulation in intramedullary fixated cemented prostheses. DESIGN: An experimental physical model of intramedullary fixation was developed which both represents the implant structure and permits monitoring of fatigue crack growth. BACKGROUND: Many joint replacement prostheses are fixed into the medullary cavity of bones using a poly(methylmethacrylate) 'bone cement', which forms a mantle around the prosthesis and locks it to the bone. The endurance of the replacement is, to a great extent, determined by the mechanical durability of the cement and the implant interfaces under cyclic stresses generated by dynamic loading. The cement mantle is subjected to complex multiaxial stresses which vary in particular distribution depending on the prosthesis design. METHODS: Damage accumulation is reported in terms of the number of cracks, the location of cracks, and the rate of crack growth. RESULTS: The results clearly show the nature of damage accumulation in the cement mantle, and that many of the cracks which propagate within the cement mantle are related to cement porosity. CONCLUSION: This study gives experimental evidence to support the hypothesis of a damage accumulation failure scenario in cemented hip reconstructions. RELEVANCE: Cementing is the most popular technique for the fixation of joint replacement prosthesis. However, the sequence of events leading to the failure of cemented fixation is not fully understood. In this paper it is shown that damage accumulation can be directly monitored in an experimental model of cemented intramedullary fixation.

Cell wall polysaccharide biosynthesis and related metabolism in elicitor-stressed cells of French bean (Phaseolus vulgaris L.).

Enzyme activities involved in quantitative and qualitative flux of sugars into cell wall polysaccharides were determined following elicitor treatment of suspension cultured cells of French bean (Phaseolus vulgaris L.). Two subsets of activities were examined: the first were involved in synthesis and metabolism of UDP-glucose and the provision of the pool of UDP-sugars, and the second a selection of membrane-bound glycosyltransferases involved in the synthesis of pectins, hemicelluloses and glucans of the primary cell wall. Of the first group, only UDP-glucose dehydrogenase (EC 1.1.1.22) showed any significant induction in response to elicitor treatment, sucrose synthase (EC 2.4.1.13), UDP-glucuronate decarboxylase (EC 4.1.1.35), UDP-glucose and UDP-xylose 4-epimerases (EC 5.1.3.2 and EC 5.1.3.5 respectively) did not change in activity significantly over the time course. In contrast, enzymes of the second group showed a more complex response. Callose synthase (glucan synthase II, EC 2.4.1.12) increased in activity, as has been shown in other systems, while arabinan synthase (EC 2.4.1.-), xylan synthase (EC 2.4.1.72), xyloglucan synthase (EC 2.4.1.72) and glucan synthase I (EC 2.4.1.12) activities were rapidly depleted from membranes within 3 h following elicitor action. This rapid turnover of activity was striking, indicating that the half-life of such enzymes can be short and that elicitor action causes substantial perturbation of some membrane activities. Glucan synthase I activity appears to increase in the later stages over the time period measured, indicating some recovery of this metabolism.

Carboxyl-terminal processing of the D1 protein and photoactivation of water-splitting in photosystem II. Partial purification and characterization of the processing enzyme from Scenedesmus obliquus and Pisum sativum.

The D1 polypeptide of photosystem II (PSII) is synthesized as a precursor that is processed by cleavage at the carboxyl terminus during assembly of the active PSII complex. A mutant of the green alga Scenedesmus obliquus, LF-1, inactive in water-splitting, lacks the D1 processing activity but assembles otherwise normal PSII complexes containing the precursor D1 molecule. We have isolated and partially purified a soluble protease from sonicated thylakoids of both wild-type S. obliquus and Pisum sativum which will process the precursor D1 molecule in PSII-enriched membranes from the LF-1 mutant to the mature size. After processing (but not before), photoactivation of these PSII membranes in the presence of manganese restores water-splitting to levels seen after photoactivation of PSII membranes from dark-grown, wild-type, cells. The protease is unable to process D1 in intact thylakoids from the LF-1 mutant but processes D1 if present during sonication of the thylakoids, indicating that processing of the carboxyl-terminal extension of D1 occurs in the lumen of the thylakoid. The processing protease from both S. obliquus and P. sativum is a single subunit enzyme of native molecular mass 33-35 kDa. Processing rate is optimal at pH 6.5. Processing in vitro is evident within 5 min and is markedly inhibited by millimolar concentrations of divalent cations (Cu, Zn greater than Mn greater than Ca, Mg) but not by any known inhibitors of the major classes of proteases. The protease is inactive against the precursors of other thylakoidal proteins and is thus distinct from the thylakoidal amino-terminal processing enzyme involved in the removal of transit peptides from cytoplasmically-synthesised proteins imported into the thylakoid lumen.