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.     

Re-design of the Exeter V40 long-stem femoral component for ease of removal

Clinical experience shows that removal of the Exeter long-stem femoral component (220 mm, 240 mm, 260 mm) of total hip arthroplasty is extremely difficult, often requiring splitting of the femur. To identify the reason for this, measurements of stem geometry and force required to pull the stems out of the cement mantle were conducted on three original Exeter long-stem and one standard femoral components. All implants required an initial force of approximately 4 kN for release from the cement. The long-stem components then required much larger forces and hence much higher expenditure of energy to pull them clear of the cement. This was attributed to the reverse taper seen on the nominally cylindrical distal section of the long-stem components. Following re-design of the manufacturing process to ensure the taper continued to the implant's distal tip, four further implants were tested. These demonstrated the requirement for initial cement release but then required no further energy expenditure similar to the standard stem. This study clearly demonstrated that the original difficulty in removing these long stems was owing to the manufacturing process resulting in a reverse taper on the distal stem. The adoption of recommended manufacturing changes to ensure the taper continues to the distal tip removed this difficulty.     

An investigation of the performance of Biostop G and Hardinge bone plugs.

Although distal plugging is a common procedure to prevent distal flow of polymethyl-methacrylate (PMMA) cement during cementing of femoral prostheses, there is little biomechanical testing to confirm that (a) the plugs do not displace under cementing pressure, and (b) they do in fact occlude cement flow. Two designs of femoral intramedullary plugs, the Biostop G (Bioland, France) and Hardinge (De Puy, Leeds, UK) were examined to determine their performance under cement pressurization in a biomechanical test. A testing rig was fabricated in which distal migration could be measured as a function of cement pressurization. Sectioning of the samples after polymerization of the cement revealed the extent of cement flow. The results show that, even in this well controlled test, there is significant variability in plug performance. It is shown that the Biostop G displaces less than the Hardinge for similar cement pressures. Sectioning reveals that cement can escape around the Hardinge plug at high pressures. Furthermore, a pore forming effect of the Biostop G plug was occasionally observed indicating that design improvements may be possible for this plug.     

In vitro testing of the primary stability of the VerSys enhanced taper stem: a comparative study in intact and intraoperatively cracked femora

The Zimmer VerSys enhanced taper uncemented stem was tested in vitro for primary stability. Six stems were implanted in six composite femora. Three femora were unintentionally damaged by cracking of the bone during stem insertion and press-fit. A previously validated protocol was used to load the specimens cyclically and to record locally the rotational and axial bone/implant relative motions in terms of elastic motion and permanent migration. For the undamaged femora, the VerSys stem showed excellent primary stability, in terms of both elastic motions (less than 9 microns) and permanent migrations (less than 6 microns), and in both axial and rotational directions, comparing favorably with the literature. Intraoperatively-induced proximal cracks did influence the primary stability of the stem in terms of permanent migration. As cracks may easily be produced in the diaphyseal canal during insertion of cementless stems, which rely for primary stability on conical fitting into the canal, great care must be taken in preparation of the canal and insertion of the stem.     

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.     

Torsional stability of a polished, collarless, tapered total replacement hip joint stem under vertical load: an in vitro investigation

The torsional stability of a polished, collarless, tapered total replacement hip joint stem has been investigated. It was believed that such a stem would show increasing torsional stability as the vertical component of load on the stem increased. The aim of the study was to examine this hypothesis by testing a number of specimens (including Exeter stems) with either a matt or polished finish, under increasing vertical load, measuring the torsional resistance of a specimen-cement construct within an outer constrained cement shell. It was concluded that stems with a polished, collarless, tapered shape showed increased torsional stability with increasing vertical load, while stems with a matt finish or a collar did not.     

A novel test method for evaluation of the abrasive wear behaviour of total hip stems at the interface between implant surface and bone cement

After total hip replacement, some cemented titanium stems show above-average early loosening rates. Increased release of wear particles and resulting reaction of the peri-prosthetic tissue were considered responsible. The objective was to develop a test method for analysing the abrasive wear behaviour of cemented stems and for generating wear particles at the interface with the bone cement. By means of the novel test device, cemented hip stems with different designs, surface topographies and material compositions using various bone cements could be investigated. Before testing, the cemented stems were disconnected from the cement mantle to simulate the situation of stem loosening (debonding). Subsequently, constant radial contact pressures were applied on to the stem surface by a force-controlled hydraulic cylinder. Oscillating micromotions of the stem (+/- 250 microm; 3 x 10(6)cycles; 5 Hz) were carried out at the cement interface initiating the wear process. The usability of the method was demonstrated by testing geometrically identical Ti-6A1-7Nb and Co-28Cr-6Mo hip stems (n= 12) with definite rough and smooth surfaces, combined with commercially available bone cement containing zirconium oxide particles. Under identical frictional conditions with the rough shot-blasted stems, clearly more wear particles were generated than with the smooth stems, whereas the material composition of the hip stems had less impact on the wear behaviour.     

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.     

Understanding initiation and propagation of fretting wear on the femoral stem in total hip replacement

The femoral stem–bone cement interface in total hip replacement is supposed to experience low amplitude oscillatory micromotion under physiological loading, consequently leading to fretting wear on the stem surface, which nowadays is considered to play an important part in the overall wear of cemented prosthesis. However, initiation and propagation of fretting wear has been poorly documented and a better understanding concerning this issue has not been established as yet. This present study, on the basis of a profound surface investigation of a polished Exeter V40? femoral stem and Simplex P bone cement obtained from an in vitro wear simulation, demonstrated that the edges of the micropores in the cement surface matched pretty well to the boundaries of the worn areas on the stem surface. This would indicate that these micropores contributed significantly to the fretting process at the stem–cement interface.     

Interface biomechanics of the Anca Dual fit hip stem: an in vitro experimental study

The Anca Dual Fit hip stem (Cremascoli Wright, Milan, Italy) is a partially cemented stem developed to overcome the drawbacks of both cemented and uncemented fixations. Its design was based on the hypothesis that partial cementing would ensure the primary stability necessary to allow bone ingrowth on the cement-free stem surfaces. At the same time, the limitation of the cement to the proximal regions would prevent stress-shielding by increasing proximal load transfer. After finite element (FE) simulations and in vitro primary stability assessment, an analysis of the long-term stability of the Anca Dual Fit stem was necessary to conclude the preclinical testing. Three stems were implanted in composite femurs and subjected to testing for 1 x 10(6) cycles, each cycle reproducing the activity of stair climbing. The simulation was designed so as to replicate the physiological loading in a simplified, yet relevant way for this test. Various measurements were collected before, during and after the test in order to give exhaustive information on the response of the implant to long-term, cyclic loading. The present study confirmed the positive results of previous investigations, and proved that the Anca Dual Fit stem has excellent long-term stability; therefore successful clinical outcomes are predicted.     

In-vitro study of medial strain distribution in the femur during impaction grafting

Impaction bone grafting (IBG) is widely used for revision hip surgery to compensate for bone stock loss. It is performed by impacting morsellized allograft into the femoral canal and acetabulum prior to cementing new total hip components. Per- and post-operative femoral fractures and post-operative implant subsidence are major complications in IBG. The aim of this study was to investigate the strain distribution on the medial side of the femur during impaction grafting and the subsequent stability of the stem under uniaxial cyclic loading. The Exeter IBG technique was used in conjunction with Howmedica X-change instrumentation. Sawbones composite femora were used. An impactometer, which provides a known impaction energy and momentum, was used to standardize the impaction process. Three drop heights, 130, 260, and 390 mm, were used for proximal impaction. In-vitro medial hoop strains and the number of impacts were recorded. A drop height of 260 mm was found to provide sufficient energy for impaction without introducing excessive strains to achieve implant stability. Furthermore, a feasibility study was performed on the use of a proximal impaction cap (PIC) to restrain extrusion of the graft during impaction. Although no significant difference in impaction strains or stem stability in uniaxial cylic loading was found by using a PIC, it is postulated that the design of a proximal impactor could be improved to assist with proximal stem alignment and graft containment.     

Preclinical assessment of the long-term endurance of cemented hip stems. Part 2: in-vitro and ex-vivo fatigue damage of the cement mantle

Fatigue damage in the cement mantle surrounding hip stems has been studied in the past. However, so far no quantitative method has been validated for assessing ex-vivo damage and for predicting the in-vitro risk of cement fracture. This work presents a method for measuring cement damage; the cement mantle was sliced and sections were inspected with dye penetrants and an optical microscope. Cracks were counted, measured, and classified by type in each region of the cement mantle. Statistical indicators (in total and per unit volume of cement) were proposed that allow quantitative comparison. The method was first validated on two implant types with known clinical success rate, which were tested in vitro using a physiological loading profile (described in Part 1 of this work). The most relevant indicators were able to detect statistical differences between the two designs. Retrieved cement mantles (the same design as one of the in-vitro stems) from revision surgery were also processed with the same inspection method. Excellent qualitative and quantitative agreement was found between the in-vitro generated fatigue damage and the cracking pattern found in the ex-vivo retrieved cement mantles. This demonstrated the effectiveness of the cement inspection protocol and provided a further validation to the in-vitro testing method.     

Biomechanical finite element analysis of bone cemented hip crack initiation according to stem design

The purpose of this investigation was to determine the specific fracture mechanics response of cracks that initiate at the stem-cement interface and propagate into the cement mantle. Two-dimensional finite element models of idealized stem-cement-bone cross-sections from the proximal femur were developed for this study. Two general stem types were considered; Rectangular shape and Charnley type stem designs. The FE results showed that the highest principal stress in the cement mantle for each case occurred in the upper left and lower right regions adjacent to the stem-cement interface. There was also a general decrease in maximum tensile stress with increasing cement mantle thickness for both Rectangular and Charnley-type stem designs. The cement thickness is found to be one of the important fatigue failure parameters which affect the longevity of cemented femoral components, in which the thinner cement was significantly associated with early mechanical failure for shot-time period.     

Influence of contaminants in the stem-ball interface on the static fracture load of ceramic hip joint ball heads

The probability of in-vivo failure of ceramic hip joint implants is very low (0.05-0.004 per cent). Besides material flaws and overloading, improper handling during implantation may induce fractures of the ceramic ball head in the long term. This study focuses on the influence of contaminants located in the stem-ball interface and on the use of damaged metal tapers on the strength of ceramic ball heads. Mechanical tests on alumina ball heads according to the standard ISO 7206-10 were performed to identify their effect on the static fracture load. A decrease of up to 90 per cent with respect to the reference static fracture load was found when contaminants such as bone chips, soft tissue, or blood were present. Reductions of 57 per cent and 27 per cent were observed for deformed stem cross-sections (from circular to elliptical) and for flattened stems respectively, making deformed stems another influential parameter. Since any alteration of the interface between the metal taper and the ceramic ball head yields a nonuniform load introduction and hence results in stress concentrations, its presence has to be avoided.     

Development of a ten-station, multi-axis hip joint simulator.

Joint simulators are now used extensively to evaluate the performance of materials and designs of total replacement hip and knee joints. In this Technical Note a new ten-station hip joint simulator with biaxial rotational articulation synchronized to a physiological loading cycle is described. The current simulator manufactured by ProSim Limited (Manchester, UK) is a development of a first generation machine designed and built in-house at DePuy International Limited (Leeds, UK). The use of this new form of ten-station hip simulator to evaluate the performance of 22 mm zirconia femoral heads and ultra-high molecular weight polyethylene acetabular cups over some 7 million loading cycles is described elsewhere [1].     

The effect of the Rim Cutter on cement pressurization and penetration on cemented acetabular fixation in total hip arthroplasty: an in vitro study

The Rim Cutter (Stryker Orthopedics, Mahwah, New Jersey) is a tool designed to cut a ledge inside the rim of the acetabulum, onto which a precisely trimmed, cemented, flanged cup can be fitted. The aim was to investigate the effect of the Rim Cutter on the intra-acetabular cement mantle pressure and the depth of cement penetration during cup insertion. The study had two parts. In the first part, hemi-pelvis models were fitted with pressure sensors. Pressure in the acetabulum was measured on insertion of a conventional cemented flanged cup with and without the use of a Rim Cutter to prepare the rim of the acetabulum. The second part assessed cement penetration when the same cups were inserted into a foam shell model. The shell was mounted in a jig and had holes drilled in it; the distance that cement penetrated into the holes was measured. A significant increase in cement pressure at the apex (p = 0.04) and the rim (p = 0.004) is seen when the Rim Cutter is used. Cement penetration in the Rim Cutter group was significantly increased at the rim of the acetabulum (p = 0.003). Insertion of a flanged cup after the acetabulum is prepared with the Rim Cutter leads to a significant increase in cement pressure and penetration during cup insertion in vitro when compared with conventional flanged cups.     

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.     

Analysis of taper produced in side zone during ecd

The full capabilities of electrochemical machining (ecm) have not been utilized in production, mainly due to some inherent problems associated with tool design. For example, the side wall generated during electrochemical drilling (ecd) of a cylindrical hole is tapered. To study the effect of process parameters on the taper produced during ecd, experiments were performed using brass as cathode, low alloy steel forgings and castings as anode and aqueous solution of NaCl as electrolyte. From the analysis of the results, some useful conclusions have been derived that would be helpful in controlling the taper produced by bare tool, bare bit tool or coated tool.     

Analysis of taper produced in side zone during ecd

The full capabilities of electrochemical machining (ecm) have not been utilized in production, mainly due to some inherent problems associated with tool design. For example, the side wall generated during electrochemical drilling (ecd) of a cylindrical hole is tapered. To study the effect of process parameters on the taper produced during ecd, experiments were performed using brass as cathode, low alloy steel forgings and castings as anode and aqueous solution of NaCl as electrolyte. From the analysis of the results, some useful conclusions have been derived that would be helpful in controlling the taper produced by bare tool, bare bit tool or coated tool.     

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.