Introduction: Immediate postoperative stability of cementless hip stems is one of the key factors for the long-term success of total hip replacement. The ability to discriminate between stable and unstable stems in the laboratory constitutes a desirable tool for the industry, as it would allow the identification of unsuitable stem designs prior to clinical trials. The use of composite femora for stability investigations is wide spread [1,2] even though their use in this application is yet to be validated. This study is aimed at establishing whether Sawbones composite femora are suitable for the assessment of migration and micromotion of a cementless hip stem. The stability of two SL Plus stems (Precision Implants, CH) implanted into Sawbone was compared to that of two SL Plus stems implanted into cadaveric femora. Ethical approval was obtained for the harvest and use of cadaveric material.

Methods: Stability was assessed in terms of micromotion and migration. Micromotion was defined as the recoverable movement of the implant relative to the bone under cyclic loading. Migration was defined as the non-recoverable movement of the implant with respect to the surrounding bone. Movement of the implant with respect to the surrounding bone was monitored at two locations on the lateral side of the stem by means of two custom made transducers based on the concept described by Berzins et al [3]. Each femur was tested in two different sinusoidal loading configurations: single leg stance (SLS-11° of adduction and 7° of flexion) [4] loaded up to 400N and stair climbing (SC-11° of adduction and 32° of flexion) loaded up to 300N. The effect of the abductor muscles was included in the model [5]. Each test consisted of 200 loading cycles applied at 50 Hz. The captured data was post-processed by a MATLAB routine and converted into translations and rotations of the stem with respect to the bone.

Results: The proximal part of the implant was subject to the highest amplitudes of micromotion in both loading configurations independent of the host. During SLS the largest micromotion was measured in the direction of the axis of the femur, this amplitude was in the order of 20 μm for the stems implanted in sawbones and varied between 13 and 39 μm for the stems implanted in cadaveric femora. The migration of the implants was minimal both in SLS and SC for both hosts with values measured in the sawbones model nearly on order of magnitude smaller than the cadaveric. In the case of SLS the prevalent movement consisted of a translation along the axis of the bone, while during SC the rotations became prevalent.

Discussion: This study has demonstrated that Sawbones provide an effective model to establish micromotion with oscillation patterns and orders of magnitiude similar to cadaveric bone. However the migration is much more dependent on the quality of fit and the internal geometry of the femur and therefore more caution should be placed on interpreting migration data from Sawbones models.

Introduction: Calcium phosphate based ceramics with a porous configuration are attraction for use as synthetic bone grafts as the porous network allows tissue ingrowth, which further enhances the implant-tissue attachment. The degree of interconnectivity and the nominal pore size are the critical factors that determine the success of the implants. It is generally accepted that a minimum pore size of 100 μm is necessary for the porous implant materials to function well and a pore size greater than 200 μm is an essential requirement for osteo-conduction. However, research has suggested that the degree of interconnectivity is more critical than the pore size. In this study, porous Hydroxyapatite/Tricalcium phosphate (HA/TCP) bioceramics with interconnected porosity and controlled pore sizes were fabricated by a novel technique involving vacuum impregnation of reticulated polymeric foams with ceramic slip. HA/TCP samples with a range of pore sizes and functionally gradient materials (FGM) with porosity gradients were made.

Materials and Methods: Two grades of calcium phosphate powder, TCP 118 and TCP 130, were used. Varying the blend ratios could change the ratios of HA and TCP in the sintered samples. The foams used comprised polyurethane (PU) which had one of three different porosities 20, 30 and 45 pores per inch (ppi). In order to make a FGM with porosity gradients mimicking the bimodal structure of cortical and cancellous bone, two different foams were either joined together by sewing or pressfitting together. The foams were substantially impregnated with slip by vacuum impregnation. The impregnated foams were removed from the vacuum chamber and dried on tissue for at least 24 hours then sintered at temperatures of up to 1280°C.

Results and Discussion: Using a slip with the appropriate viscosity, porous HA/TCP bioceramics having interconnecting pores and a range of pore sizes can be produced successfully. By joining different ppi foams together, it is possible to develop functional gradient materials in which the porosity varies through the thickness of the samples. No weakness could be seen at the interface between the two different structures. This demonstrated that porous HA/TCP with two or more different levels of porosity could be produced in a single block. Image analysis shows the porosity measured for the three different foams was similar. The area equivalent diameters of the pore structure are 197–254 μm with 20ppi foam, 143–183 μm with 30ppi foam and 105–127 μm with 45ppi foam. The compressive strengths of the HA/TCP samples are in the range of 30–170 MPa and the apparent densities were 2.34–2.76 g/cm3. The technique developed for fabricating porous bioceramics can be extended to produce a range of bone substitute materials with properties tailored to specific clinical applications.

Background and Purpose: There is a high incidence of arthritis in the hand, but joint replacement technology in the wrist and other small joints is still in its infancy compared with the larger joints. The wrist is the most complex small joint and so there is a need for fundamental research into the way in which it works. At present there is no generally agreed upon satisfactory explanation for the complex movement patterns of the carpal bones. The purpose of the work was to test a new hypothesis on wrist kinematics. The basis of the hypothesis was that the bones of the wrist move in such a manner as to maximise total contact area in the joint, thereby minimising contact stress. Such a strategy would minimise the bone mass requirements, thereby minimising the biological “cost” of creating and maintaining the joint. This agrees with the minimum energy principle, which governs many natural processes.

Methods: A computer model was created to test the hypothesis. A cadaveric wrist was dissected and 3D faceted models of the carpal bones were created using laser digitisation. The model contained a program to evaluate the closeness of packing of the carpal bones and an optimisation algorithm [1] to maximise this quantity by adjusting the positions of the bones. The evaluation program computed the contact area and level of intersection between nine pairs of interacting bones. Rotation in the radial-ulnar deviation plane was applied in 1.0° increments to four rigidly connected bones defining the overall posture of the wrist, and an optimisation algorithm was used to maximise the contact area by adjusting the positions and orientations of the remaining bones.

Results: The results of the work are encouraging because certain known characteristics of carpal behaviour were clearly predicted by the model. The results for the scaphoid in particular were similar to the characteristic movements of this bone in both radial and ulnar deviation. During 20° of unlar deviation, the bone demonstrated 14.3° of extension, which is near to the 20.4° reported by an experimental study [2]. In 10° of radial deviation, the bone underwent 6.4° of flexion, which again is close to the 8.1° experimental result.

Conclusion: Although the computer model predicted certain aspects of carpal behaviour, the initial hypothesis was not conclusively proved. This is due in part to the computational complexity of the task. Despite some simplifying assumptions, there were still a large number of degrees of freedom, and it is almost certain that the optimisation process was afflicted with local minima problems. If the technical hurdles can be overcome and the hypothesis is proved correct, then we will gain a new explanation of the laws governing the kinematics of the wrist joint, which are not fully understood at present. This will provide invaluable information for surgical applications, where a thorough understanding of normal kinematics is essential for the treatment of joint injury and instability.

The static properties of bone cements have been widely reported in the literature (Lewis, 1997, Khun, 2000, Armstrong 2002). Commercial bone cements are expected to perform above the minimum values in static tests specified by ISO 5833: 2002. It has been suggested that the viscoelastic properties of bone cement, such as creep and stress relaxation, might bear more relevance to the in-vivo behaviour of the cement-implant construct (Lee 2002). This study aimed to compare numerous properties of Simplex P, Simplex Antibiotic and Simplex Tobramycin and identify those properties most sensitive to subtle changes in cement composition. The three cements were chosen on the basis that they are characterised by the same liquid and powder compositions, the only difference being represented by the type and amount of added antibiotics. In Simplex Antibiotic the additives are 0.5g Erythromycin and 3 million I.U. Colistin, while in Antibiotic Simplex with Tobramycin the only additive is 0.5g of Tobramycin. The static properties of the cements were assessed following protocols described in ISO 5833: 2002, while the viscoelastic properties of the cement were measured with in-house developed apparatus in quasi-static conditions. Creep and stress relaxation tests were performed in four point bending configuration. Porosity was measured on the mid cross section of the creep samples using a digital image technique. All cements exhibited properties compatible with the ISO standard, but in plain Simplex the ISO minimum for bending and compressive strength was within the variation of the batches tested. Bending strength measurements were the least sensitive to differences in the cements. Plain Simplex displayed lower bending and compressive strength but higher bending modulus than the antibiotic laden options. The bending modulus could only discriminate between Simplex P and Simplex Antibiotic (p=0.02). Differences in the compressive strength of the three cements were significant, with the plain option being the weakest. Stress relaxation only discriminated between plain and Tobramycin loaded cement (p=0.028), while creep was more sensitive to differences and allowed distinction between plain and antibiotic loaded bone cements. The creep behaviour correlated with the cross sectional porosity measurements. This study demonstrated that the static tests specified by the current international standard are not as sensitive to subtle changes in the composition of the material as the time temperature dependent parameters characteristic of creep and stress relaxation. The authors advocate the evaluation of time and temperature dependent characteristics as a complement to the current standard.

Aseptic loosening remains a long-term problem in total hip replacement. This phenomenon is prevalent even if modern cementing techniques seem to have reduced its incidence. Osteolysis has been deemed as a disease of access to fixation interfaces (1), either the stem- or bone-cement interface in hip replacement. This can be attributed in part to the quality of the cement in the proximity of the stem. It has been noted that due to thermal effects, polymerisation of bone cement starts at the bone-cement interface and gradually moves inwards towards the stem.

Femoral component heating was first proposed as a method to reduce the curing time of bone cement (2). This practice was later found to reduce the porosity at the stem-cement interface (3) and also to improve the interface shear strength (4). This study aimed to investigate the effect of femoral stem heating on two bone cements (Simplex P (Stryker) and Palacos R (Biomet Merck)) over a range of mantle thicknessess.

The model femora used for this study were maintained at a constant temperature of 37C while the stem temperature varied between 21, 37 and 44C. The femoral moulds were formed from dental plaster with a similar thermal conductivity to bone. Mould sizes were created to generate cement mantles of 2, 5 and 7.5mm thickness.

In the 2mm Simplex P cement mantles there was very little porosity evident. It was concentrated in the proximity of the stem when the component was kept at 21C and disappeared as the stem was heated to higher temperatures. Minimal porosity could be identified in the thicker mantles with no apparent differences between temperatures. There were no temperature trends evident from within this cement group. Palacos R cement has been reported to have a higher porosity than Simplex in a number of studies (5, 6). With the 2mm Pala-cos mantles, the increased stem temperatures reduced the porosity at the stem-cement interface. There was however no obvious difference between the 37 and 44C temperatures, where porosity seemed to be evident in the midsection of the mantle. This trend was also identified in the thicker cement mantles. The porosity did not extend out to the cement-bone interface under any conditions.

This study analyses the changes in porosity across the mantle of the cement as the temperature of the stem component is increased. The initial results confirm that the porosity at the stem cement mantle is decreased but indicate that the porosity within the body of the cement is increased as the temperature of the stem is increased.

Aim: To compare the biomechanical properties of paired flexible steel and titanium nails in simulated transverse fractures of synthetic composite bones.

Methods: Steel and titanium nails (3mm diameter) were individually used in pairs of divergent configuration to study torsion, cantilever bending (anteroposterior and lateral), and axial loading properties of adolescent synthetic composite tibiae model (10mm diameter). Properties of the intact bone, simulated fresh fracture with nails and simulated healing fracture with nails were studied. Instron 4303 universal testing machine was used to study axial loading. Applying fibreglass layers around the fracture with epoxy resin simulated fracture healing with callus formation.

Results: Steel and titanium nails maintained good alignment of fracture fragments. Both the nails demonstrated very poor stability of fresh fractures in torsion loading. Steel nail/bone construct was 57% stronger than Titanium nail/bone construct under similar testing conditions during fracture healing (p< 0.05) but still < 50% stiffness of intact bone. In bending tests, both types of nails showed < 10% of the stiffness of intact bone in fresh fractures (p< 0.05). Mediolateral stiffness was better than anteroposterior stiffness. In fracture healing, the bending stiffness of both types of nail/bone constructs was > 50% that of intact bone. Axial stiffness of both nails was more than bending or torsion stiffness implying that fracture fragments play a significant role in the stability of the fracture.

Although both types of nail/bone constructs demonstrated similar stiffness results in fresh and healing fractures, steel nails performance was statistically better than Titanium nails in all loading tests (p< 0.05).

Conclusion: Fractures fixed with either type of flexible nails should be supplemented with splints or plaster for a short duration until callus formation. Flexible nails should be used with caution in comminuted fractures, over weight patients since they may not provide adequate stability or allow early mobilization. Additional research with cadaver bones may provide further insight into the performance of the flexible nails.

Introduction and Aims: Compression hip screws are considered to be the gold standard for treatment of trochanteric proximal femoral fractures. Two implant designs exist; the ‘keyed’ and ‘non-keyed’ barrel profiles. Many biomechanical studies have been published on the performance of sliding hip screws, but most have used only static testing, and none to our knowledge have sought to compare the two-barrel profiles. This study aimed to compare the sliding characteristics of keyed and non-keyed systems in both static and dynamic loading conditions.

Method: Tests were performed on the implants using a multi axis servo-hydraulic testing machine. The machine possessed both linear and torsional actuators, such that hip flexion/extension could be simulated during testing. Load to initiate sliding in both implants was measured in a variety of testing conditions; screw engagement in barrel (20–38mm), angle of hip flexion (0–40 degrees), perpendicular loading force (50–190N), and cycle frequency (0–1 Hz).

Results: Results showed a tendency towards greater sliding in the non-keyed system, although these were significant only for the screw engagement testing (p< 0.001). However, load to initiate sliding in both implants was significantly higher in dynamic as compared to static testing (p< 0.001), and increased as torsional frequency increased. The non-keyed system did not demonstrate any tendency for screw rotation within the barrel during dynamic testing.

Conclusion: The non-keyed compression hip screw system does show a trend towards improved sliding characteristics, and does not display the tendency for screw rotation within the barrel under loading, often quoted as a misgiving of this implant. Also, since forces to initiate sliding are significantly higher when these implants are loaded dynamically (which mimics more closely the in vivo performance), future biomechanical studies should include dynamic testing for any hip fracture implant.

Introduction: It is well known that the integrity of the bone cement interface is crucial for the long-term survival of a primary total hip arthroplasty (THR). Revision THR with impaction bone grafting has recently offered a solution to gross bone loss due to osteolysis. As graft becomes incorporated, clearly the bone graft/ cement interface is as crucial as the equivalent interface in primary THR. Our aim was to examine factors that inßuence this interface. Method: The study was designed to mimic clinical practice. Fresh femoral heads were harvested from primary THR. These were morcelised into large and small particles. These were characterised. The bone was impacted into a purpose built jig with measured force. Cement was pressurised onto the dried surface of the impacted bone after measured mixing times. Cement pressurisation was measured. The cement/graft specimen was extracted and transected with a band saw. Cement penetration was measured with digital image analysis. Results: Large fragment size was 29 mm², and small was 7.1mm². Light impaction was 2.2 Atm. Medium and heavy were 2.6 Atm. and 3.2 Atm. respectively. Cement penetration was inversely proportional to impaction force. Cement mixing time also signiþcantly affected cement penetration. Particle size had no effect. Conclusion: Allograft should be adequately but not excessively impacted, to allow good cement incursion. Cement should be introduced and pressurised perhaps as early as two minutes. Fragment size does not affect cement penetration.

Aims: Optimisation of femoral stem load transfer potentially encourages new bone growth. The effect of increasing the taper angle of a highly polished double tapered stem on stability and hoop strain is investigated.

Methods: An in-vitro model femur was instrumented at 3 levels with 10 strain gauges, proximally (channel 1–4), mid taper level (channel 5–8) and distally (channel 9–10). Under controlled conditions surgical Simplex P cement (Stryker Howmedica Osteonics) was prepared and introduced into the canal of the model femur. The Exeter (Stryker Howmedica Osteonics) stem with centraliser was inserted using a standardised technique. The cementation process was repeated 5 times for each stem. The mantle and stem were subjected to cyclic loading at 1 Hz to 0.5kN using an Instron 8511 servohydraulic materials testing machine. The subsidence of the stem and hoop strains generated were recorded. Each experiment was repeated 6 times.

Results: The average subsidence of all the stems was 0.2mm with a standard deviation varying between 0.1 and 0.2. All the stems showed similar patterns of loading, with no significant difference.

Conclusions: The results suggest that within a purely cemented environment the taper angle of the stem used is irrelevant with regard to the hoop strain and the stability of the construct. The authors therefore suggest that size of stem does not matter.

The wrist is arguably the most complex joint in the body and is essential for optimal hand function. The joint may be represented as two roughly orthogonal hinge axes, providing flexion-extension and radial-ulnar deviation. The location and orientation of these axes with respect to the underlying anatomy is essential for the design of successful joint prostheses. A population study was performed in order to obtain the parameters of this two-hinge joint.

Data for 108 normal right wrists was gathered using a Fastrak electrogoniometer with sensors fixed to the distal medial radial styloid and the distal third metacarpal head. Data was recorded as a series of three-dimensional coordinates covering the entire locus of movement.

The two-hinge geometry of the joint was represented mathematically with nine parameters describing the configuration of the axes and two angles controlling rotation about these axes. The configuration giving the closest kinematic match to the experimental data was determined using two nested optimisation processes. During the inner optimisation process, the third metacarpal head was brought as close as possible to each of the experimental points in turn by adjusting the two positioning angles. The sum of distances from each experimental point to the point of closest approach gave the “cost” of the current configuration. The outer optimisation process repeated the inner process iteratively, minimising the cost by adjusting the nine configuration parameters.

The double optimisation method was found to offer an innovative solution to the problem of analysing kinematic data from a population study. The mean joint configuration showed the axis of radial-ulnar deviation to be 1.9 mm (sd = 12.5 mm), distal to the flexion-extension axis, with axes almost orthogonal to one another. This data together with the radii of the rotations is invaluable in determining the optimal articulation geometries for wrist joint replacement prostheses.

It is well known that the integrity of the bone cement interface is crucial for the long-term survival of a primary total hip arthroplasty (THR). Revision THR with impaction bone grafting has recently offered a solution to gross bone loss due to osteolysis. As graft becomes incorporated, clearly the bone graft/cement interface is as crucial as the equivalent interface in primary THR.

The aim of this study was to examine factors that influence this interface. The effects of bone particle size, cement mixing time, and impaction force were examined.

The study was designed to mimic clinical practice. Fresh femoral heads were harvested from primary THR. These were morcelised into large and small particles. The bone was impacted into a purpose built jig with measured force. Cement was pressurised onto the dried surface of the impacted bone after measured mixing times. Cement pressurisation was measured. The cement/graft specimen was extracted and transected with a band saw. Cement penetration was measured with digital image analysis.

Large fragment size was 29 mm², and small was 7. 1mm². Light impaction was 2. 2 Atm. Medium and heavy were 2. 6 Atm. and 3. 2 Atm. respectively. Cement penetration was inversely proportional to impaction force. Cement mixing time also significantly affected cement penetration. Particle size had no effect.

Allograft should be adequately but not excessively impacted, to allow good cement incursion. Cement should be introduced and pressurised perhaps as early as two minutes. Fragment size does not affect cement penetration.

Aseptic loosening of the acetabular component is the main reason for revision hip arthroplasty surgery with loosening rates reported at 25% at 12–15years. The optimum method of acetabular preparation and cementation technique has not been fully evaluated. Clinical follow-up studies suggest multiple keyholes improve survival rates.

Keyholes increase penetration of cement and torsional resistance of the bone-cement interface. Some studies support the traditional three 1/2 inch keyholes other studies have shown multiple smaller holes improve stability. The optimum size and number of holes to provide the strongest fixation has yet to be determined.

Using an established 54mm diameter acetabular model, mahogany, three sizes of keyhole were tested-3x10mm holes, 12x5mm, and 48x2.5mm- the surface area of the defects created were the same in each group. The model acetabulum was filled with cement and a metal central bar inserted through which torque could be applied using an Instron machine. Six specimens from each group were tested. Three 10mm holes produced a significantly stronger resistance to torque when compared to 2.5mm (p≤0.017) and 5mm holes (p≤0.001). There was no significant difference between 2.5mm and 5mm holes (p≤0.139). Each addition of a further 10mm hole significantly increased the torque strength until the model was destroyed at six holes.

In laboratory testing larger key holes provide a stronger cement-acetabular interface.

We studied the effects of nine techniques of bone surface preparation on cement penetration and shear strength at the cement-bone interface in a standard model of bovine cancellous bone. In unprepared bone the mean penetration was 0.2 mm and the mean shear strength of the interface was 1.9 MPa, less than that of the underlying bone. Brushing with surface irrigation gave mean penetrations of 0.6 to 1.4 mm and mean shear strengths of 1.5 to 9.9 MPa. In 50% of specimens the interface was weaker than the underlying bone. The use of pressurised lavage resulted in mean penetrations of 4.8 to 7.9 mm and mean shear strengths of 26.5 to 36.1 MPa, which were greater than those of the cancellous bone in all specimens. Pressurised lavage was equally effective alone or in combination with brushing, and its efficacy was not altered by using pulsed or continuous jets, or by changing the temperature of the solution from 21 degrees C to 37 degrees C.

We report the results of a 4- to 17-year clinical and radiological follow-up of 264 Charnley first-generation stems in comparison with those of 402 second- and subsequent-generation stems. The incidence of fracture was 4.1% in first-generation stems and 0.5% in second- and subsequent-generation stems. The incidence of stem loosening requiring or likely to require revision was 3.1% in first-generation and 11.4% in second-generation stems. We believe that the increased loosening rate in second- and subsequent-generation stems is due to their larger cross-sectional area, which produces an increase in flexural stiffness.

In comparison with monofilament wire, multifilament cable was found to be a more suitable material for fixation of the greater trochanter. It is versatile, easy to work with and has superior mechanical properties. The "trochanter cable-grip system" was developed to exploit the use of multifilament cable as a means of reattaching the greater trochanter and experimental studies have yielded excellent results. This system was subjected to clinical trials for over four years in 321 hips and, at its present state of development, the incidence of detachment has been reduced to 1.5 per cent and that of cable breakage to 3.1 per cent.