The prevalent cause of implant failure after total joint replacement is aseptic loosening caused by wear debris. Improvement of the wear behaviour of the articulating bearing between the cup and femoral head is essential for increased survival rate of artificial hip joints. Cross-linking of the polyethylene (PE) material is one attempt to reduce wear particle release at the articulating surface. Various cross-linked polyethylenes (X-PE) are used in orthopaedics since several years.

In total hip arthroplasty (THA) the use of larger femoral head sizes has specific reasons. Larger heads lead to a decreased risk of total hip dislocation and impingement as well as an improved range of motion in comparison to smaller head sizes like 28mm or less. However, the increasing diameter of femoral head can be associated with lower thickness of the PE liner and increased wear rate. Cross-linking of PE can improve the wear rate of the liner and hence supports the use of larger femoral heads. The aim of this experimental study was to evaluate the wear of standard vs. sequential X-PE (X3-PE) liner in combination with different ceramic femoral head sizes.

Wear testing was performed for 5 million load cycles using standard UHMW-PE liners (N2Vac) and X3-PE liners (each Stryker GmbH & Co. KG, Duisburg, Germany) combined with 28mm ceramic ball heads and the Trident PSL acetabular cup (Stryker). Furthermore, X3-PE liners with an internal diameter of 36mm and 44mm and decreased wall thickness (5.9mm and 3.8mm) were combined with corresponding ceramic heads. An eight station hip wear simulator according to ISO 14242 (EndoLab GmbH, Rosenheim, Germany) was used to carry out the standard wear tests. The tests were realised in temperature-controlled chambers at 37°C containing calf serum (protein content 20g/l).

The average gravimetrical wear rates of the standard UHMW-PE (N2Vac) liners combined with 28mm ceramic heads amounted to 12.6 ± 0.8mg/million cycles. Wear of X3-PE liners in combination with 28 mm ceramic heads was not detectable. The average gravimetrical wear rates of the X3-PE liners in combination with 36mm and 44mm ceramic heads amounted to 2.0 ± 0.5mg and 3.1 ± 0.3mg/million cycles, respectively.

The purpose of this study was to evaluate the effect of femoral head size at THA on standard and sequential X-PE liner. The wear simulator tests showed that the wear rate of PE liners with small heads (28mm) decreased by cross-linking of the PE significantly. The amount of wear at X-PE increased slightly with larger head size (36mm and 44mm). However, by sequential cross-linking, the wear rate using thinner liners and larger femoral heads is reduced to a fractional amount of wear at conventional UHMW-PE. Hence, the above-mentioned advantages of larger femoral head diameters can be realised by improved wear behaviour of sequential X-PE.

Due to increased life expectancy of human population, the amount of total knee replacements (TKR) is expected to increase. TKR reached a high grade of quality and safety, but most often it fail because of aseptic implant loosening caused by polyethylene (PE) wear debris. Wear is generated at the articulating surfaces, e.g. caused by three body particles, like bone fragments or bone cement particles. The aim of this experimental study was to compare the wear of tibial PE inserts combined with metallic and ceramic femoral components at three body wear situation induced by polymethylmethacrylate (PMMA) and zirconia (ZrO2) particles from the bone cement.

Wear testing was performed for 5 Mio load cycles, using tibial standard PE inserts combined with the same CR femoral component, in two different materials, Cobalt Chromium (CoCrMo) and Biolox delta ® ceramic (Multigen Plus Knee System, Lima Corporate, Italy). A knee wear simulator, according to ISO 14243 (EndoLab GmbH, Rosenheim, Germany), was used to carry out the tests. The tests were performed in temperature-controlled test chambers at 37 °C, containing calf serum with a protein content of 30 g/l. Polymethylmethacrylate (PMMA) and zirconia (ZrO2) bone cement particles (Palacos R ®) were manufactured to a size of 30 μm. The three body particles were added at all stations onto the articulating surface of the tibial PE insert (7mg per condyle) at every 500,000 cycles. Wear was determined gravimetrically and the surfaces of tibial inserts were analysed by scanning electron microscope (SEM) after finishing the 5 million cycles. Furthermore, roughness of the PE insert surfaces and the articulating surfaces of the different femoral components were detected and the PE wear particles were analysed by SEM.

The average gravimetrical wear rates of the tibial PE inserts in combination with CoCr and Biolox delta ® ceramic femoral components amounted to 6.4 ± 0.9 mg and 2.6 ± 0.4 mg per million cycles, respectively. Beside bone cement particles on the articulating surface of the PE inserts, polished surfaces and scratches were detected by SEM. In comparison to the untreated surfaces of the PE inserts at both material pairings the surface roughness at the articulating areas showed deep scratches and polished regions. Analyses of the metallic femoral components showed scratches at the articulating surfaces, none on ceramics.

The present study pointed out the effect of femoral component material in an abrasive three body wear situation on the wear properties of TKR. The wear simulator tests showed that wear of PE inserts under three body wear conditions, in combination with ceramic femoral components, was significantly lower than with metallic femoral components. With regard to anti-allergic properties, ceramic femoral components are promising products for TKR.

Sufficient primary stability of the acetabular cup is essential for stable osseous integration of the implant after total hip arthroplasty. By means of under-reaming the cavities press-fit cups gain their primary stability in the acetabular bone stock. These metal-backed cups are inserted intra-operatively using an impact hammer.

The aim of this experimental study was to obtain the forces exerted by the hammer both in-vivo and in-vitro as well as to determine the resulting primary stability of the cups in-vitro.

Two different artificial bone models were applied to simulate osteoporotic and sclerotic bone. Polymeth-acrylamid (PMI, ROHACELL 110 IG, Gaugler & Lutz, Germany) was used as an osteoporotic bone substitute, whereas a composite model made of a PMI-Block and a 4 mm thick (cortical) Polyvinyl chloride (PVC) layer (AIREX C70.200, Gaugler & Lutz, Germany) was deployed to simulate sclerotic bone. In all artificial bone blocks cavities were reamed for a press-fit cup (Trident PSL, Size 56mm, Stryker, USA) using the original surgical instrument. The impactor of the cup was equipped with a piezoelectric ring sensor (PCB Piezotronics, Germany). Using the standard surgical hammer (1.2kg) the acetabular cups were implanted into the bone substitute material by a male (95kg) and a female (75kg) surgeon. Subsequently, primary stability of the implant (n=5) was determined in a pull-out test setup using a universal testing machine (Z050, Ziwck/Roell, Germany).

For validation the impaction forces were recorded intra-operatively using the identical press-fit cup design.

An average impaction force of 4.5±0.6kN and 6.3±0.4kN using the PMI and the composite bone models respectively were achieved by the female surgeon in vitro.

7.4±1.5kN and 7.7±0.8kN respectively were obtained by the male surgeon who reached an average in-vivo impaction force of 7.5±1.6kN.

Using the PMI-model a pull-out force of 298±72N and 201±112N were determined for the female and male surgeons respectively. However, using the composite bone model approximately half the pull-out force was measured for the female surgeon (402±39N) compared to the male surgeon (869±208N).

Our results show that impact forces measured in-vitro correspond to the data recorded in-vivo. Using the osteoporotic bone model the pull-out test revealed that too high impaction forces affect the pull-out force negatively and hence the primary implant stability is reduced, whereas higher impact forces improve primary stability considerably in the sclerotic bone model. In conclusion, the amount of impaction force contributes to the quality of the obtained primary cup stability substantially and should be adjusted intra-operatively according to the bone quality of each individual patient.

Introduction: Total knee replacement has become a common procedure with good clinical results. Today many different designs of the femoral component of bicondylar endoprostheses are offered by industry. The femoral components show similar designs however different angles and length of the cross sections are specific. Because of these design differences the preoperative planning and sparing bone resection are difficult at the revision surgery. The aim of this experimental study was to compare the design of femoral components at their cross section contours to find congruence and differences of common bicondylar endoprostheses to prove the possibility of design exchange during revision surgery.

Material and method: Ten femoral components (e.motion^®, Genesis II, Genia^®, Innex^®, LCS^®, Multigen Plus, NexGen^®, P.F.C.^®, Scorpio^®, Vanguard^®) of similar implant size were analysed with regard to their cross section design. Therefore the constructional properties of the inner surface (direction and length of cross sections) of the components were determined. The components were scanned with a three-dimensional laser scanner and were transferred to two dimensional CAD models to the lateral and frontal view in order to compare the inner contours. The contours of the cross sections were overlaid with congruence of the posterior and anterior cross section of all components at lateral view.

Results: Four of the ten analysed femoral components showed good congruence of the cross sections. Here, only a few additional bone resections or extra bone cement have to be done at the diagonal cross sections to change the femoral design among each other. Four other components show wide differences between the inner contours in comparison to the first four components especially at their posterior and diagonal cross sections. Two components can not be compared with the others due to their diagonal distal cross section.

Discussion: The numerical results shows good congruence of cross section contours of some analysed femoral components. Furthermore there were clear design differences which complicate the exchange of the femoral component at revision surgery. The use of an elementary inner contour of femoral components of bicondylar endoprostheses could be an advantage for revision arthroplasty in regard to bone sparing surgical treatment.

Introduction: To obtain secondary implant stability of acetabular press-fit cups, sufficient primary stability is essential. The aim of this study was to investigate the influence of cup insertion force and bone quality on the primary implant stability.

Materials and Methods: The experiments were carried out using two commercially available press-fit acetabular cups (Trident PSL, Stryker und EP-FIT PLUS, PLUS Ortho-peadics), comparable in design and with identical diameters, which were inserted axially into artificial bone by a female and a male surgeon. Two bone substitute material models were used. To imitate osteoporotic bone, a PMI-model (ROHACELL 110 IG, Gaugler & Lutz oHG) was employed. To simulate sclerotic bone, a composite-model made of a PMI-bloc with a 4 mm thick PVC-layer (AIREX C70.200, Gaugler & Lutz oHG) was used. The cups were inserted using an insertion device, equipped with a force sensor, and an 1100 g surgical hammer. Additionally, all experiments were carried out using a dynamic testing machine (25 kN, Instron) utilising insertion forces of 4.0 kN and 8.0 kN respectively. Primary implant stability was determined via lever-out tests using a static universal testing machine (Z050, Zwick/Roell).

Results: On average an insertion force of 4.8 kN (female) and 7.0 kN (male) using the PMI-model and 6.2 kN (female) and 7.5 kN (male) for the composite-model was assessed for the two different surgeons. The machined forces averaged 3.8 kN and 7.9 kN.

Lever-out-moments of 17 Nm were determined for both the PMI- and composite-model for the female surgeon using the PSL cup, whereas 27 Nm and 70 Nm, respectively, were reached for the EP-FIT shell.

For the male surgeon using the PSL cup, lever-out moments of 15 Nm and 30 Nm for the PMI- and composite-model respectively were determined. Insertion of the EP-FIT cup resulted in lever-out moments of 10 Nm using the PMI-model and 82 Nm using the composite-model.

The low machined insertion force led to average lever-out moments of 34 Nm for the PSL and 71 Nm for the EP-FIT cups using the composite-model. For the high machined force, the highest lever-out moments of 44 Nm and 99 Nm for the PSL and EP-FIT shells respectively were determined.

Conclusion: Using the composite-model (sclerotic bone), higher insertion forces lead to higher lever-out moments and hence higher primary implant stability for both tested cups. However, a high, non axial applied force can result in loss of stability using the PMI-model (osteoprotic bone). Compared to the manually inserted acetabular cups, the machined insertion resulted in higher primary stability for both implants and artificial bone types.

For orthopaedic implants the adhesive strength of bone cells on implant surfaces is of high interest. In some cases the adherence of cells is desirable, e.g. on endoprosthetic implants, in others, mainly temporarily used implants, e.g. intramedullary nails, it is not favourable for the cells to attach to the implant. Therefore, besides cell spreading and proliferation on surfaces the adhesion strength with which cells bond to the substrate is of high interest. There are different approaches to determine bone cell adhesion, but no easy to operate quantitative methods are available. For this purpose, based on the spinning disc principle, we have developed a new adhesion device in conjunction with an inverse confocal laser scanning microscope (LSM).

Polished disc-shaped test samples made of Ti6Al4V were seeded with bone cells (MG-63), stained with a fluorescent dye, at defined radial positions and were incubated for 18 h with cell medium. After incubation the test samples were placed into the adhesion chamber filled with 250 ml cell medium (DMEM). The test samples were rotated at various velocities until a minimum detachment of 50% was achieved. Using the LSM the detachment of the bone cells at the defined radial positions was determined and the cell count was recorded before and after rotation by means of imaging software.

An average shear stress of 50 N/m2 was determined for polished Ti6Al4V surfaces. To calculate the adhesion force, the cross-sectional cell area has to be measured by the xz-scan of the LSM.

Our results are reproducible and comparable to the data found in literature. The advantage of our new approach is that the same cells can be observed before and after rotation as well as different rotational speeds can be applied to the same cell population. Further investigations e.g. using different surfaces are carried out.