An in-vitro investigation into the cement pressurization achieved during insertion of four different femoral stems

Adequate cement pressurization during stem insertion improves the interdigitation of cement into bone. This increases the strength of the cement-bone interface, thus contributing to the reduction of the incidence of aseptic loosening, the commonest cause of revision surgery. This in-vitro study compared the cement pressurization achieved during insertion of four different stems of equivalent sizes: the Elite Plus (DePuy, UK), C-Stem (DePuy, UK), Exeter (Stryker, USA), and CPS-Plus (Plus Orthopedics, Switzerland). The maximum pressures attained at the time of stem insertion were recorded at proximal, mid and distal stem levels. The Elite Plus generated significantly higher distal pressures than the other stems. The CPS-Plus generated significantly greater proximal cement pressures than the Elite Plus, C-Stem, and Exeter prostheses. The triple taper of the C-Stem increased the cement pressurization medial to the stem. The stem shape and the presence or absence of a proximal stem centralizer affect cement pressurization. The presence of a proximal stem centralizer, a large stem volume, and a lateral-medial taper are all factors associated with increased cement pressurization during stem insertion.     

Finite element analysis of the resurfaced femoral head

Failure of the resurfaced femoral head may occur in the short term owing to femoral neck fracture or in the long term owing to aseptic loosening as a result of strain shielding. Resurfacing arthroplasties are not all the same. In particular, there is considerable debate regarding the role of the metaphyseal stem and cementing technique. This study examines the influence of various metaphyseal stem configurations (diameter, percentage length in contact with bone, and bonded versus debonded) and cement mantle thickness on the load transfer within the femoral head. Resurfacing resulted in significant strain shielding in the superior femoral head and elevated strain in the superior femoral neck. Although the increase in strain in the femoral neck was significant, the mean strains were below the yield strain for cancellous bone. Peak strains were observed above the yield strain, but they accounted for less than 1 per cent of the total head-neck bone volume and therefore were unlikely to result in femoral neck fracture. Increasing the stem diameter and increasing the percentage stem length in contact with bone both increased the degree of strain shielding. Bonding the metaphyseal stem produced the most dramatic strain shielding, which also extended into the head-neck junction. In contrast, varying the cement mantle thickness had a negligible effect on the load transfer.     

Revision of cemented fixation and cement-bone interface strength.

Interfacial shear strength between poly(methyl methacrylate) (PMMA) bone cement and cancellous bone was measured in bone samples from human proximal femora. Samples were prepared with fresh cement-bone, fresh cement inside a mantle of existing cement and with fresh cement-revised bone surfaces. Push-out tests to measure shear strength caused failure only at bone-cement interfaces; revised bone interfaces were 30 per cent weaker (P < 0.02) than primary interfaces. The clinical relevance is that revision of cemented joint arthroplasties may necessitate removal of components with sound cement-bone fixation. The practice of removing all traces of PMMA cement may not yield the optimal fixation; adhesion of fresh cement to freshly prepared surfaces of the existing cement might also be considered where circumstances are favourable.     

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.     

A comparison of structural and mechanical properties in cancellous bone from the femoral head and acetabulum

Mechanical interlock obtained by penetration of bone cement into cancellous bone is critical to the success of cemented total hip replacement (THR). Although acetabular component loosening is an important mode of THR failure, the properties of acetabular cancellous bone relevant to cement penetration are not well characterized. Bone biopsies (9 mm diameter, 10 mm long) were taken from the articular surfaces of the acetabulum and femoral head during total hip replacement. After mechanical and chemical defatting the two groups of bone specimens were characterized using flow measurement, mechanical testing and finally serial sectioning and three-dimensional computer reconstruction. The mean permeabilities of the acetabular group (1.064 x 10(-10) m2) and femoral group (1.155 x 10(-10) m2) were calculated from the flow measurements, which used saline solution and a static pressure of 9.8 kPa. The mean Young's modulus, measured non-destructively, was 47.4 MPa for the femoral group and 116.4 MPa for the acetabular group. Three-dimensional computer reconstruction of the specimens showed no significant differences in connectivity and porosity between the groups. Results obtained using femoral head cancellous bone to investigate bone cement penetration and fixation are directly relevant to fixation in the acetabulum.     

The differentiation of loose and secure femoral implants in total hip replacement using a vibrational technique: an anatomical and pilot clinical study

ANATOMICAL STUDY: The common pathway to failure in total hip replacements is loosening of the acetabular and femoral components. The reliable diagnosis of this loosening is difficult. A non-invasive technique has been developed which analyses a vibration signal transmitted through the femur. This can distinguish between a femoral component which is loose and one which is firmly fixed. Charnley femoral components were cemented securely into ten cadaver femurs and the transmitted vibration signal recorded. The prostheses were then loosened first at the cement-prosthesis and then at the cement-bone interface. The tests were then repeated. Consistent and distinct differences between output vibration signals were observed between the firmly implanted and the loss prostheses. PILOT CLINICAL STUDY: Seven patients admitted for revision surgery because of clinical and radiological evidence of femoral implant loosening were tested using this technique. A further four patients with secure femoral components were tested approximately two weeks after total hip replacement. The differences in output signal, which had been observed between firmly implanted and loose prostheses during the anatomical study, were again demonstrated.     

Reduced stress shielding with limited micromotions using a carbon fibre composite biomimetic hip stem: a finite element model

Total hip arthroplasty (THA) enjoys excellent rates of success in older patients, but younger patients are still at risk of aseptic loosening and bone resorption from stress shielding. One solution to the stress shielding problem is to use a hip stem with mechanical properties matching those of cortical bone. The objective of the present study was to investigate numerically the biomechanical performance of such a biomimetic hip stem based on a hydroxyapatite (HA)-coated carbon fibre composite. A finite element model (FEM) of the biomimetic stem was constructed. Contact elements were studied to model the bone-implant interface in a non-osseointegrated and osseointegrated state in the best way. Three static load cases representing slow walking, stair climbing, and gait in a healthy individual were considered. Stress shielding and bone-implant interface micromotions were evaluated and compared with the results of a similar FEM based on titanium alloy (Ti-6Al-4V). The composite stems allowed for reduced stress shielding when compared with a traditional Ti-6Al-4V stem. Micromotions were slightly higher with the composite stem, but remained below 40 microm on most of the HA-coated surface. It is concluded that a biomimetic composite stem might offer a better compromise between stress shielding and micromotions than the Ti-6Al-4V stem with the same external geometry.     

Femoral stem wear in cemented total hip replacement

The great success of cemented total hip replacement to treat patients with endstage osteoarthritis and osteonecrosis has been well documented. However, its long-term survivorship has been compromised by progressive development of aseptic loosening, and few hip prostheses could survive beyond 25 years. Aseptic loosening is mainly attributed to bone resorption which is activated by an in-vivo macrophage response to particulate debris generated by wear of the hip prosthesis. Theoretically, wear can occur not only at the articulating head-cup interface but also at other load-bearing surfaces, such as the stem-cement interface. Recently, great progress has been made in reducing wear at the head-cup interface through the introduction of new materials and improved manufacture; consequently femoral stem wear is considered to be playing an increasingly significant role in the overall wear of cemented total hip replacement. In this review article, the clinical incidences of femoral stem wear are comprehensively introduced, and its significance is highlighted as a source of generation of wear debris and corrosion products. Additionally, the relationship between femoral stem surface finish and femoral stem wear is discussed and the primary attempts to reproduce femoral stem wear through in-vitro wear testing are summarized. Furthermore, the initiation and propagation processes of femoral stem wear are also proposed and a better understanding of the issue is considered to be essential to reduce femoral stem wear and to improve the functionality of cemented total hip replacement.     

Development of a computer model to predict pressure generation around hip replacement stems

Cemented fixation of hip replacements is the elective choice of many orthopaedic surgeons. The cement is an acrylic polymer which grouts the prostheses into the medullary cavity of the femur. Cement pressure is accepted as a significant parameter in determining the strength of cement/bone interfaces and hence preventing loosening of the prostheses. The aim of this work was to allow optimal design of the intramedullary stem of a hip prosthesis through knowledge of the flow characteristics of curing bone cement which can be used to predict pressures achieved during insertion of the femoral stem. The viscosity of the cement is a vital property determining the cement flow and hence cement interdigitation into bone. The apparent viscosities, nu(a), of three commercial bone cements were determined with respect to time by extrusion of the curing cement through a parallel die of known geometry under selected pressures. Theoretical models were developed and implemented in a computer program to describe cement flow in three models each of increasing complexity: (a) a simple parallel cylinder, (b) a tapered conical mandrel and (c) an actual femoral prosthesis, the latter models being complicated by extensional effects as annular areas increase. Predicted pressures were close to those measured experimentally, maximum pressures being in the range 10-160 kPa which may be compared with a threshold of 76 kPa proposed for effective interdigitation with cancellous bone. The theoretical model allows the prosthesis/bone geometry of an individual patient to be evaluated in terms of probable pressure distributions in the medullary cavity during cemented fixation and can guide stem design with reference to preparation of the medullary canal. It is proposed that these models may assist retrospective studies of failed components and contribute to implant selection, or to making informed selection from options in custom hip prosthesis designs to achieve optimum cement pressurization.     

Reinforcement of bone cement using metal meshes

Following on from previous work reported in this journal, a practical system has been developed for in situ fibre reinforcement of bone cement in the hip joint prosthesis. A fibre preform is inserted during the operation, surrounding the metal stem of the prosthesis, forming a composite with the cement. Tests performed on full-scale models of the joint show improvements in fatigue life by more than an order of magnitude when a preform of metal fibres is used; this significantly delays the onset of cement cracking and stem loosening. Initial results reported previously suggest that other fibre materials such as carbon may give even greater improvements. Clinical trials have begun, using a form of the metal mesh.     

Finite element analysis of the long-term fixation strength of cemented ceramic cups

Clinical studies have shown that adequate fixation of ceramic cups using bone cement is difficult to achieve. As the cement-ceramic bond strength is low, a satisfactory fixation strength requires a cup design that allows mechanical interlocking, although such a design will probably promote cement cracking and therefore cup loosening in the long term. An investigation has been carried out to establish whether a cemented ceramic cup can be designed in such a way that both a satisfactory initial fixation strength is obtained and cement cracking is reduced to levels found around PE cups functioning well in vivo. By means of finite element analysis, the fatigue loading of three geometrically different cemented acetabular cups, with ceramic and PE material properties, has been simulated, and the severity of the crack patterns produced in the cement has been analysed. Furthermore, the fixation strength has been analysed by simulating a pull-out test prior to and after fatigue testing. All ceramic cups produced much larger amounts of cement damage during fatigue testing than any PE cup, caused by stress concentrations in the cement that were attributable to the high stiffness of the ceramic. Even a completely smooth ceramic cup produced more damage than a sharp-grooved PE cup. Owing to the excessive cement cracking, the fixation strength of the ceramic cups dropped after fatigue loading. It is concluded that cemented ceramic cups have an increased risk of long-term mechanical failure by comparison with PE cups, and that a ceramic cup design that combines sufficient fixation strength with low cement failure may be difficult to achieve.     

The effect of acetabular cup size on the short-term stability of revision hip arthroplasty: a finite element investigation

The study uses idealized two-dimensional finite element models to examine the behaviour of the acetabular construct following revision hip arthroplasty, carried out using the Slooff-Ling impaction grafting technique. The behaviour of bone graft was considered in detail, with non-linear elasticity and non-associated plasticity being adopted. Load was applied to the acetabular construct through a femoral head using smooth sliding surfaces. In particular, four models were subjected to two idealized cyclic load cases to investigate the effect of acetabular cup size on the short-term stability of the acetabular construct. The study suggests that benefits may be gained by using the largest practical size of acetabular cup.     

The development of a model for in vitro testing of femoral stems in impaction bone grafting

Comprehension of the biomechanical behaviour of orthopaedic implants is essential. This paper describes the development of an in vitro model to investigate the behaviour of femoral implants in the revision setting. The development of a femoral model and a bone graft substitute is described. The properties of human, bovine, ovine morselized bone graft, and a graft substitute were compared. On measuring hoop strain after impaction bone grafting there was no significant difference between the ovine bone graft and graft substitute with the size 1 Exeter stem. The results suggest that this bone graft substitute is a viable alternative for in vitro testing. The authors recommend the use of the graft substitute and the femoral model to predict femoral stem biomechanics.     

The contribution of the micropores in bone cement surface to generation of femoral stem wear in total hip replacement

Although cemented total hip replacement has long been recognized as a situation that can lead to wear, the wear generated on the femoral stem has not been well documented, especially with regard to how this wear is initiated and propagated. This present work aimed to further investigate this issue based on a comprehensive study on surface morphology of the femoral stem and the bone cement, which were collected from seven in vitro wear simulations. It was shown that the wear locations on the stem surface compared well with the results of retrieval studies, and the boundaries of the worn areas matched well the edges of the micropores present in the bone cement surface. This indicated that the micropores could potentially contribute to the generation of femoral stem wear. In addition, metallic debris was detected around the micropores from the simulation with increased loading cycles. However, no evidence of macro-cracks was observed across the cement mantle in spite of the presence of micro-cracks initiated at the edge of the micropores. This study demonstrated a possible cause for progression of femoral stem wear and it may have an important bearing on the long term durability of cemented hip prosthesis.     

Fretting corrosion of orthopaedic implant materials by bone cement

Debonding of bone cement from the stem of the femoral component of a hip prosthesis can result in local tangential oscillatory movement, i.e. fretting, between the two contacting materials as the limb is moved. Patches where such rubbing has occurred are frequently seen on removed implants. Fretting fatigue experiments have been carried out in Hanks solution on austenitic stainless steel and Ti-6Al-4V (IMI 318) with bridges of bone cement clamped to the specimens. Fretting appears to have little effect on the fatigue life of either material but the scanning electron microscope reveals the formation of thick oxide layers which subsequently give rise to loose debris particles by a process of delamination. Further experiments carried out in Hanks solution in an electrolytic cell have shown that there are potential changes when a bone cement rider is fretted against a stainless steel or titanium alloy plate although the change in potential is only one tenth that obtained with a metal-on-metal contact. Fretting by bone cement appears to be producing damage to the metal surfaces which manifests itself as mild wear rather than a diminution in fatigue strength.     

An experimental technique for the investigation of three-dimensional stress in bone cement underlying a tibial plateau

The technique of experimental model testing was applied to the analysis of stress at selected sites in bone cement underlying a tibial plateau. The investigation utilized a large model knee fabricated from materials which had mechanical properties similar to the actual tibial plateau and acrylic cement but which did not duplicate adequately the complexity of bone. A porous interface was created in the model between the materials representing the bone and cement. Three-dimensional strain rosettes were embedded into the cement and the model was loaded in a varus or valgus mode. Overloading resulted in breakdown of the modelled anterior and part of the posterior cement-bone interfaces, producing non-linear and in some cases erratic strains in the anterior section but repeatable linear results in the posterior section. The investigation highlighted the necessity for three-dimensional strain gauge investigations as opposed to two-dimensional studies. It is suggested that the approach could provide comparative information about different products and form the basis for a valuable design tool.     

Osteocyte staining with rhodamine in osteonecrosis and osteoarthritis of the femoral head

Death of osteocytes is synonymous of bone death. Aseptic osteonecrosis of the femoral head is a lesion characterized by the death of osteocytes occurring after major vascular changes. The evolution may lead to hip osteoarthritis, which requires total hip arthroplasty in most cases. Evolution of aseptic osteonecrosis in four radiological stages is well known. We analyzed 24 femoral heads from patients with osteonecrosis or osteoarthritis, retrieved at the time of surgery for a hip arthroplasty. The aim of the study was to clearly identify the necrotic bone from the living bone in the histological samples. The femoral heads were sawed, and a large sample was harvested in the superior zone; it was stained en‐bloc with rhodamine dissolved in formalin to make the osteocytes fluorescent under UV light microscopy. Undecalcified sections, 7 μm thick, were obtained on a heavy‐duty microtome. A micrographic analysis using two UV excitation wavelengths visualized the living osteocytes (in green) and the bone matrix (in blue). A simple method to prepare combined images is described. In addition, the blocks can be analyzed by confocal microscopy to visualize more details. It is possible to identify at low magnification the osteocytes within the bone matrix and the osteonecrotic areas where osteocytes have disappeared. Identification of osteocytes showed that newly formed bone packets are laid on dead trabeculae in patients with aseptic osteonecrosis or with osteoarthritis. In the osteosclerotic areas, the enlarged trabeculae have a dead central core surrounded by recently apposed bone structure units.     

The influence of stem insertion rate on the porosity of the cement mantle of hip joint replacements

This study investigates the effect of stem insertion rate on the porosity of the cement mantle. An experimental protocol was developed to simulate the surgical technique of cementing a prosthetic stem into the medullary canal of the femur. Cement mantle specimens were produced for three different stem insertion rates. The presence of porosity in the cement mantle was investigated. Additionally, the mechanical strength of the bone cement was assessed. Increasing the stem insertion rate did not have a significant effect on the porosity distribution within the bulk cement mantle. However, for all stem insertion rates investigated, the porosity concentration increased significantly moving from the cement/pseudofemur interface through to the stem/cement interface. In all cases, the presence of porosity significantly decreased the mechanical behaviour of the bone cement. High porosity concentration at the stem/cement interface seems to be attributed also to the rheology of the cement during implant insertion. Nevertheless, the surgeon cannot influence the formation of porosity by changing the stem insertion rate.     

Contact mechanics analysis of metal-on-metal hip resurfacing prostheses

Finite-element method was employed to study the contact mechanics in metal-on-metal hip resurfacing prostheses, with particular reference to the effects of bone quality, the fixation condition between the acetabular cup and bone, and the clearance between the femoral head and the acetabular cup. Simple finite-element bone models were developed to simulate the contact between the articulating surfaces of the femoral head and the acetabular cup. The stresses within the bone structure were also studied. It was shown that a decrease in the clearance between the acetabular cup and femoral head had the largest effect on reducing the predicted contact-pressure distribution among all the factors considered in this study. It was found that as the clearance was reduced, the influence of the underlying materials, such as bone and cement, became increasingly important. Stress shielding was determined to occur in the bone tissue surrounding the hip resurfacing prosthesis considered in this study. However, the stress-shielding effects predicted were less than those observed in conventional total hip replacements. Both the effects of bone quality (reduction in elastic modulus) and the fixation condition between the cup and the bone were found to have a negligible effect on the predicted contact mechanics at the bearing surface. The loading was found to have a relatively small effect on the predicted maximum contact pressure at the bearing surface; this was attributed to an increase in contact area as the load was increased.     

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.