Using a calliper to restore the centre of the femoral head during total hip replacement.

Restoration of leg length and offset is an important goal in total hip replacement. This paper reports a calliper-based technique to help achieve these goals by restoring the location of the centre of the femoral head. This was validated first by using a co-ordinate measuring machine to see how closely the calliper technique could record and restore the centre of the femoral head when simulating hip replacement on Sawbone femur, and secondly by using CT in patients undergoing hip replacement. Results from the co-ordinate measuring machine showed that the centre of the femoral head was predicted by the calliper to within 4.3 mm for offset (mean 1.6 (95% confidence interval (CI) 0.4 to 2.8)) and 2.4 mm for vertical height (mean -0.6 (95% CI -1.4 to 0.2)). The CT scans showed that offset and vertical height were restored to within 8 mm (mean -1 (95% CI -2.1 to 0.6)) and -14 mm (mean 4 (95% CI 1.8 to 4.3)), respectively. Accurate assessment and restoration of the centre of the femoral head is feasible with a calliper. It is quick, inexpensive, simple to use and can be applied to any design of femoral component.

Optimisation of the mechanical and handling properties of an injectable calcium phosphate cement.

Calcium phosphate cements have the potential to be successful in minimally invasive surgical techniques, like that of vertebroplasty, due to their ability to be injected into a specific bone cavity. These bone cements set to produce a material similar to that of the natural mineral component in bone. Due to the ceramic nature of these materials they are highly brittle and it has been found that they are difficult to inject. This study was carried out to determine the factors that have the greatest effect on the mechanical and handling properties of an apatitic calcium phosphate cement with the use of a Design of Experiments (DoE) approach. The properties of the cement were predominantly influenced by the liquid:powder ratio and weight percent of di-sodium hydrogen phosphate within the liquid phase. An optimum cement composition was hypothesised and tested. The mechanical properties of the optimised cement were within the clinical range for vertebroplasty, however, the handling properties still require improvement.

Wear paths produced by individual hip-replacement patients--a large-scale, long-term follow-up study.

Wear particle accumulation is one of the main contributors to osteolysis and implant failure in hip replacements. Altered kinematics produce significant differences in wear rates of hip replacements in simulator studies due to varying degrees of multidirectional motion. Gait analysis data from 153 hip-replacement patients 10-years post-operation were used to model two- and three-dimensional wear paths for each patient. Wear paths were quantified in two dimensions using aspect ratios and in three dimensions using the surface areas of the wear paths, with wear-path surface area correlating poorly with aspect ratio. The average aspect ratio of the patients wear paths was 3.97 (standard deviation=1.38), ranging from 2.13 to 10.86. Sixty percent of patients displayed aspect ratios between 2.50 and 3.99. However, 13% of patients displayed wear paths with aspect ratios >5.5, which indicates reduced multidirectional motion. The majority of total hip replacement (THR) patients display gait kinematics which produce multidirectional wear paths, but a significant minority display more linear paths.

Gait kinematics of age-stratified hip replacement patients--a large scale, long-term follow-up study.

Three-dimensional gait analysis data from 134 patients attending routine 10-year post-operative review clinics is presented. Patients were divided into five age groups-54-64 years, 65-69 years, 70-74 years, 75-79 years and over 80 years. A group of 10 normal elderly subjects was also tested. All age groups displayed reduced range of hip flexion/extension, range of knee flexion extension, maximum hip extension and range of hip abduction/adduction and reduced velocity and step length compared to the normal elderly group. However, there was no difference in gait kinematics between the age groups. Patients over 80 years of age displayed significantly reduced range of sagittal plane ankle motion, but this is unlikely to be secondary to hip joint restriction and more likely due to reduced walking speed associated with very elderly subjects. This study reveals that even the youngest hip replacement patients do not attain normal gait kinematics 10-year post-operatively and that muscle atrophy and residual stiffness may influence patient kinematics many years post-operation.

The cemented custom femoral stem--a 10 year review.

We report a series of 706 patients (759 hip implants) with an average follow up of 10.5 years (range, 10-11 years) following total hip replacement (THR) using a cemented custom-made femoral stem and a cemented HDP acetabular component. The fate of every implant is known. One hundred and seventy-four patients (23%) were deceased at the time of their 10-year review all died with a functioning THR in situ. Four hundred and sixty-two patients (61%) were subsequently reviewed. One hundred and twenty three patients (16%) were assessed by telephone review, as they were too ill or unwilling to attend. Kaplan-Meier survival analysis (all components) demonstrated a median survival at 10 years of 96.05% or 95% Confidence Intervals (CI) for median survival of (94.41% to 97.22%). Revision surgery occurred in 30 cases (3.9%). Seventeen had full revisions (2.2%) and 13 (1.7%) socket revisions only. Twenty-one out of 30 revisions were for infection or dislocation. There were 2 cases (0.3%) of revision for aseptic loosening of the stem. The 10-year results of the custom femoral titanium stem are encouraging and compare well with other cemented systems.

Validation of the small-punch test as a technique for characterizing the mechanical properties of acrylic bone cement.

This paper examines the validity of using the small-punch test technique as a means of quantifying the mechanical properties of acrylic bone cement under different test conditions. The elastic moduli calculated using the small-punch test method were compared with data measured using the international standard for acrylic bone resin, ISO 5833. Conclusions from the study indicate that the small-punch test is a reproducible miniature specimen test method that can be used to characterize the mechanical properties of retrieved acrylic bone cement as used in total joint replacement surgery. Moreover, the test conditions were found to influence the elastic modulus of acrylic bone cement. The test temperature had a greater effect on the elastic behaviour of the bone cement than the test medium.

Engineering properties and performance of dental crowns.

Dental crowns are used to replace damaged natural crowns of teeth and are fixed to prepared teeth with luting cements, which should provide an adhesive bond to the tooth structure giving reliable retention and minimal microleakage. Mechanical testing of crowns in vitro gives failure load distributions that are well described by Weibull models, comparing probabilities of survival and reliability. Fatigue testing of crowns is time consuming, but regression analysis to interpolate functions through data points quoting probability limits or applying Weibull analysis is achievable. A complementary approach is to conduct materials tests with appropriate interfacial geometries. Luting cements are used in thin layers of 40-150 microm. Contraction during polymerization is restrained by adhesion to substrates, allowing little relaxation of stresses. Conventional and resin-modified glass ionomer cements create thin zones of interaction with dentine and fail cohesively. The chevron notch short rod technique has been used to measure fracture toughness and rank cements. A development of this method, using chevron notch short bar specimens, permitted fracture toughness to be determined for luting cement--dentine substrate interfaces. Representative fracture experiments need to be developed to apply mixed mode conditions. The basic challenge to predict long-term performance from short-term laboratory tests remains.

Degradation of poly-L-lactide. Part 1: in vitro and in vivo physiological temperature degradation.

Poly-L-lactide (PLLA) is one of the most significant members of a group of polymers regarded as bioresorbable. The degradation of PLLA proceeds through hydrolysis of the ester linkage in the polymer's backbone and is influenced by the polymer's initial molecular weight and degree of crystallinity. To evaluate its degradation PLLA pellets were processed by compression moulding into tensile test specimens and by extrusion into 2 mm diameter lengths of rod, prior to being sterilized by ethylene oxide gas (EtO) and degraded in both in vitro and in vivo environments. On retrieval at predetermined time intervals, procedures were used to evaluate the material's molecular weight, crystallinity, mechanical strength, and thermal properties. Additionally, the in vivo host tissue's biological response was analysed. The results from this study suggest that in both the in vitro and in vivo environments, degradation proceeded at the same rate and followed the general sequence of aliphatic polyester degradation, ruling out enzymes contributing and accelerating the degradation rate in vivo. Additionally, the absence of cells marking an inflammatory response suggests that the PLLA rods investigated in vivo were biocompatible throughout the 44 weeks duration of the study, before any mass loss was observed.

Degradation of poly-L-lactide. Part 2: increased temperature accelerated degradation.

Poly-L-lactide (PLLA) is one of the most significant members of a group of polymers regarded as bioresorbable. The degradation of PLLA proceeds through hydrolysis of the ester linkages in the polymer's backbone; however, the time for the complete resorption of orthopaedic devices manufactured from PLLA is known to be in excess of five years in a normal physiological environment. To evaluate the degradation of PLLA in an accelerated time period, PLLA pellets were processed by compression moulding into tensile test specimens, prior to being sterilized by ethylene oxide gas (EtO) and degraded in a phosphate-buffered solution (PBS) at both 50 degrees C and 70 degrees C. On retrieval, at predetermined time intervals, procedures were used to evaluate the material's molecular weight, crystallinity, mechanical strength, and thermal properties. The results from this study suggest that at both 50 degrees C and 70 degrees C, degradation proceeds by a very similar mechanism to that observed at 37 degrees C in vitro and in vivo. The degradation models developed also confirmed the dependence of mass loss, melting temperature, and glass transition temperature (Tg) on the polymer's molecular weight throughout degradation. Although increased temperature appears to be a suitable method for accelerating the degradation of PLLA, relative to its physiological degradation rate, concerns still remain over the validity of testing above the polymer's Tg and the significance of autocatalysis at increased temperatures.

Processing, annealing and sterilisation of poly-L-lactide.

Poly-L-lactide (PLLA) is one of the most significant members of a group of polymers regarded as bioabsorbable. Degradation of PLLA proceeds through hydrolysis of the ester bonds in the polymer chains and is influenced significantly by the polymer's molecular weight and crystallinity. To evaluate the effects of processing and sterilisation on these properties, PLLA pellets were either compression moulded or extruded, subjected to annealing at 120 degrees C for 4h and sterilised by ethylene oxide (EtO) gas. Procedures were used to evaluate the mechanical properties, molecular weight and crystallinity. Upon processing, the crystallinity of the material fell from 61% for the PLLA pellets to 12% and 20% for the compressed and extruded components, respectively. After annealing, crystallinity increased to 43% for the compression-moulded material and 40% for the extruded material. Crystallinity further increased upon EtO sterilisation. A slight decrease in molecular weight was observed for the extruded material through processing, annealing and sterilisation. Young's modulus generally increased with increasing crystallinity, and extension at break and tensile strength decreased. The results from this investigation suggest that PLLA is sensitive to processing and sterilisation, altering properties critical to its degradation rate.

Assessment of cement introduction and pressurization techniques.

The objective of this study was to measure the medullary pressures generated during bone cement injection, pressurization and femoral prosthesis insertion. The measurements were recorded throughout the length of an in vitro femoral model while implanting a series of prosthetic hip stems using different pressurization techniques. The prostheses used were a Charnley 40 flanged stem (Johnson & Johnson DePuy International Limited), an Exeter No. 3 stem (Stryker Howmedica Osteonics, Howmedica International Limited), and a customized femoral component (Johnson & Johnson DePuy International Limited). The following parameters were derived from the pressure data recorded: peak pressure, decay pressure and duration above optimum pressure of 76 kPa to predict adequate penetration. The custom and Exeter stems generated cement pressures throughout the length of the cavity model that were predicted to achieve adequate bone cement interdigitation into cancellous bone. For all the conditions investigated in this study, when using the Charnley femoral component, an adequate level of cement pressurization was generated in the medial-distal portion of the femoral cavity. It is postulated that this could result in reduced integration of the cement mantle with bone and less effective transmission of functional loads applied during a patient's normal activity, postoperatively.

Shrinkage stresses in bone cement.

Shrinkage of bone cement is reported primarily as a consequence of polymerisation, however thermal shrinkage also occurs as a result of its exothermic reaction. It is proposed that the latter effect is important, since it occurs late in the curing cycle at a time when the cement has attained its mechanical properties as a solid, and that residual stresses result. Observations from experiments and literature reports suggest that residual stresses may be sufficient to initiate cracks at the interface between hip replacement stems and cement.A theoretical model has been developed to calculate interference stresses, using thick-walled cylinder theory, on the basis of thermal and total shrinkages. Thermal shrinkage values were calculated using the coefficient of linear thermal expansion of bone cement, while total shrinkages were measured. Moduli of elasticity values were measured for acrylic bone cements ranging from 2.1 to 2.7GPa, as were Poisson's ratio values ranging from 0.38 to 0.46. Theoretical calculation of stresses in a cement mantle, based on assumptions of thermal shrinkage alone, predicted circumferential stresses of 8.4-25.2MPa for cement curing temperatures in the range 60-140 degrees C. It is concluded that cracks observed around hip prosthesis stems in laboratory specimens of bone cement are due to shrinkage and that residual stresses are sufficient to cause crack initiation prior to functional loading.

The influence of inert packaging on the shelf ageing of gamma-irradiation sterilised ultra-high molecular weight polyethylene.

Ultra-high molecular weight polyethylene (UHMWPE) is used for wear applications in total hip prostheses and total knee prostheses. Sterilisation of these prostheses is commonly by gamma-irradiation. This process creates reactive free radicals in the UHMWPE, greatly increasing its susceptibility to oxidative degradation. This study has investigated the influence of air and vacuum packaging on the properties of gamma-irradiated UHMWPE (GUR1050) following 3 years of shelf ageing. The findings indicate that vacuum packaging minimises oxidative degradation reactions that occur for UHMWPE during shelf ageing. However, gamma-irradiation of vacuum-packaged UHMWPE promotes a degree of cross-linking. It is proposed that this may enhance the wear performance of UHMWPE. Accelerated ageing studies indicate that 3 years of shelf ageing would also seem to reduce the susceptibility of gamma-irradiated UHMWPE to oxidative degradation upon removal from its vacuum packaging.

The relationship between porosity and fatigue characteristics of bone cements.

In this study, the fatigue strengths of acrylic cement prepared by various commercially available reduced pressure mixing systems were compared with the fatigue strength of cement mixed by hand (control) under atmospheric conditions. The following observations were made from this investigation. The mean fatigue strength of reduced pressure mixed acrylic bone cement is double that of cement mixed by hand using an open bowl, 11,354+/-6,441 cycles to failure for reduced pressure mixing in comparison with 5,938+/-3,199 cycles for mixing under atmospheric conditions. However, the variability in mean fatigue strengths of reduced pressure mixed bone cement is greater for some mixing devices. The variation in fatigue strengths for the different mixing techniques is explained by the different porosity distributions. The design of the reduced pressure mixing system and the technique employed during mixing strongly contribute to the porosity distribution within the acrylic bone cement. The level of reduced pressure applied during cement mixing has an effect on the fatigue strength of bone cement, but the mixing mechanism is significantly more influential.

Images from Waves--photoelastic modelling of bones. 8th Samuel Haughton Lecture, Bioengineering Section of Royal Academy of Medicine in Ireland. January 2002.

BACKGROUND: This paper cites the development of the principles of photoelastic stress analysis, contemporary to the life of Samuel Haughton. Subsequent studies of bone and joint replacements are discussed, with reference to hypotheses regarding bone, including the coincidence of trabecular structure with principal stresses. Issues regarding assumptions of homogeneous and isotropic properties in photoelastic modelling are acknowledged. AIM: Awareness of photoelastic methods is often through the visual appeal of the coloured fringe patterns. The aim of this paper is to complement this awareness by demonstration of the quantitative analyses that may be conducted through biomechanical examples. METHODS: Examples of new pseudo three-dimensional model analyses are presented together with a method for photoelastic study of cancellous bone, which entails novel procedures for preparation of replicate models and for optical evaluation of fringes. CONCLUSION: Photoelastic analysis offers novel solutions to studies in biomechanics, which are facilitated by contemporary modelling materials.

Fracture characteristics of acrylic bone cement-bone composites.

In this study, the fracture properties of Perspex, acrylic bone cement prepared using a commercially available reduced pressure mixing system and a bone cement-bone composite were compared under different test conditions. The method used was the double-torsion (DT) test. The observations made from this investigation are as follows. The fracture toughness and critical crack length for Perspex significantly increased (ANOVA, p = 0.001) when tested in water compared to air. An increase in test temperature from 19 to 37 degrees C resulted in a decrease in the fracture properties in water, this reduction being also statistically significant (ANOVA, p = 0.02). The mean fracture toughness and standard deviation of CMW3 bone cement when mixed under reduced pressure was 2.19 +/- 0.11 MN m(-3/2) compared to 3.89 +/- 0.10 MN m(-3/2) for the cement-bone composite (ANOVA, p = 0.004). The crack length determined for CMW3 bone cement and the cement bone composite were 0.323 +/- 0.031 and 1.1434 +/- 0.61 mm respectively. The plateau loads of the composite material were higher than measured for the monolithic acrylic bone cement, 249.66 +/- 67.75 N compared with 140.83 +/- 6.82 N. The high level of variation recorded for the plateau loads of the bone cement bone composite is due to the orientation and volume fraction of the cancellous bone. It can be concluded from this investigation that acrylic bone cement interdigitation into the cancellous bone results in a superior material with respect to crack resistance in comparison with the bone cement as a lone entity. Therefore it is an advantage if there is sufficient cancellous bone stock available within the intermedullary canal to allow bone cement penetration to occur, for the transfer of loads during daily activity. Additionally, it is paramount that the clinician ensures that adequate pressure is applied and maintained for an appropriate time during cement injection and prosthesis insertion in order to ensure optimum cement penetration into the pore openings of the cancellous bone, thus improving the resistance of the cement mantle to fracture and ultimately improving the longevity of the joint replacement.

The influence of shape and sliding distance of femoral head movement loci on the wear of acetabular cups in total hip arthroplasty.

Wear of the polyethylene acetabular component is the most serious threat to the long-term success of total hip replacements (THRs). Greatly reduced wear rates have been reported for unidirectional, compared to multidirectional, articulation in vitro. This study considers the multidirectional motions experienced at the hip joint as described by movement loci of points on the femoral head for individual THR patients. A three-dimensional computer program determined the movement loci of selected points on the femoral head for THR patients and normal subjects using kinematic data obtained from gait analysis. The sizes and shapes of these loci were quantified by their sliding distances and aspect ratios with substantial differences exhibited between individual THR patients. The average sliding distances ranged from 10.0 to 18.1 mm and the average aspect ratios of the loci ranged from 2.5 to 9.2 for the THR patients. Positive correlations were found between wear rate and average sliding distance, the inverse of the average aspect ratio of the loci and the product of the average sliding distance and the inverse of the average aspect ratio of the loci. Patients with a normal hip joint range of motion produce multidirectional motion loci and tend to experience more wear than patients with more unidirectional motion loci. Differing patterns of multidirectional motion at the hip joint for individual THR patients may explain widely differing wear rates in vivo.

Fracture mechanics analysis of the dentine-luting cement interface.

The objectives of this study were to determine the fracture toughness of adhesive interfaces between dentine and clinically relevant, thin layers of dental luting cements. Cements tested included a conventional glass-ionomer, F (Fuji 1), a resin-modified glass-ionomer, FP (Fuji Plus) and a compomer cement, D (DyractCem). Ten miniature short-bar chevron notch specimens were manufactured for each cement, each comprising a 40 microm thick chevron of lute, between two 1.5 mm thick blocks of bovine dentine, encased in resin composite. The interfacial K(IC) results (MN/m3/2) were median (range): F; 0.152 (0.14-0.16), FP; 0.306 (0.27-0.37), D; 0.351 (0.31-0.37). Non-parametric statistical analysis showed that the fracture toughness of F was significantly lower (p <0.05) than those of FP or D, and all were significantly lower than values for monolithic cement specimens. Scanning electron microscopy of the specimens suggested crack propagation along the interface. However, energy dispersive X-ray analysis indicated that failure was cohesive within the cement. It is concluded that the fracture toughness of luting cement was lowered by cement-dentine interactions.

Curing characteristics of acrylic bone cement.

Commercial acrylic bone cements are supplied as two components, a polymer powder and a liquid monomer. Mixing of the two components is followed by a progressive polymerization of the liquid monomer to yield a solid mass, a high level of heat being generated during this exothermic reaction. The exposure of bone to high temperatures has led to incidences of bone necrosis and tissue damage, ultimately resulting in failure of the prosthetic fixation. The aim of this study was to determine the thermal properties of two acrylic bone cements as they progress through their polymerization cycles. It was also felt that there was a need to quantify the variations in the curing characteristics as a function of preparing bone cement by different techniques, hand mixing and vacuum mixing. A number of parameters were calculated using the data gathered from the investigation: peak temperature, cure temperature, cure time, and the cumulative thermal necrosis damage index. The results show the temperature profile recorded during polymerization was lowest when the cement was prepared using the Howmedica Mix-Kit I system: 36 degrees C for Palacos R and 41 degrees C for CMW3 respectively. When the acrylic cements were prepared in any vacuum mixing system there was evidence of an increase in the cure temperature. The main factor that contributed to this rise in temperature was an imbalance in the polymer powder : liquid monomer ratio, there was a high incidence of unmixed powder visible in the mixing barrel of some contemporary vacuum mixing devices. Observing the thermal characteristics of the polymethyl methacrylate (PMMA) bone cements assessed, it was found that particular formulations of bone cements are suited to certain mixing methodologies. It is vital that a full investigation is conducted on a cement mixing/delivery system prior to its introduction into the orthopaedic market.

Demographic and occupational effects on the activity levels of normal subjects in the United Kingdom.

A study of activity levels, measured in steps per day, was made of 293 subjects from several parts of the United Kingdom. Each subject wore a pedometer adjacent to the hip for two weeks. The average number of steps taken each day over a two-week period was recorded together with additional details such as age and occupation. A general decline in activity with age was observed, which varied slightly for men and women. At twenty years of age the sample of men typically walked about 9,000 steps per day and women 9,200 steps per day, declining to 6,100 and 5,750 steps per day for men and women respectively at sixty years of age. Activity was observed to vary considerably from day to day and was more pronounced for men. The mean number of steps per day for a wide range of occupational groups varied from around 4,500 steps per day for retired persons to 12,700 steps per day for postmen. The activity data was re-analysed to remove the influence of age and gender. The activity of most occupational groups then fell within a relatively narrow range of approximately 7,700 to 8,850 steps per day. The exceptions to this were postmen, nurses and technicians, with averages of 12,750, 9,950 and 9,900 steps per day respectively, and software programmers averaging only 5,250 steps per day. Activity levels for groups of subjects from different parts of the United Kingdom were compared, including Leeds and Bradford, Belfast, Teesside, County Durham and Edinburgh. When the number of subjects in each group was sufficiently large with a wide spread of ages and occupational types, it was found that demography had little effect upon the level of activity. This study provides a substantial new database, based on the UK population. In addition, the assumption that one million cycles in vitro is equivalent to one year in vivo is well supported and relevant to simulator studies. (Hip International 2002; 1: 28-36).