Introduction: The failure of total hip endoprosthesis is usually caused by aseptic implant loosening which can be a result of inflammatory reactions of the periprosthetic tissue on released metallic and bone cement wear particles. The objective of the study was to analyse the abrasive interfacial wear behaviour of cemented stems depending on the composition of the bone cement. Material and methods: With a test device cemented anatomical hip stems with different surface topography and material composition were investigated. Following bone cements were used: high viscosity PMMA cement with ZrO2 (Palacos R), high viscosity PMMA cement with BaSO4 (CMW 2000) as radiopaque material, low viscosity PMMA cement with ZrO2 (Sulcem 3) and an experimental high viscosity cement without ZrO2.

Results and Discussion: The abrasive wear behaviour in the interface between the implant and the bone cement is clearly affected by the surface topography of the stem. Moreover, the composition of the bone cement had a substantial impact on the abrasive wear behaviour in the interface. The tests revealed that the commercial bone cements with ZrO2 particles caused a higher polishing effect on rough stems and increased release of metallic particles. The Ti6Al7Nb and Co28Cr6Mo stems, which were tested against the bone cement without ZrO2 and the bone cement with BaSO4, showed no surface damage in the macroscopic analysis, whereas in the SEM analysis abrasive wear on the stem surface could be detected. However, in case of the Palocos R cement the added ZrO2 particles led to an increased wear resistance of the cement mantle and therefore to a reduced release of cement wear particles compared to the other cements tested. Whether this result is based on a ball bearing effect of the hard ZrO2 particles, which may reduce the friction in the interface, or a reinforcement of the bone cement matrix, is still unclear. If the use of high viscosity bone cements with ZrO2 particles (Palacos R) leading to a reduced release of cement particles may compensate the increased accumulation of abraded metallic particles, has to be examined by subsequent cellbiological studies, whereas in clinical studies the Palacos R cement showed superior survial rate of cemented hip stems. The low-viscosity cement tested seems to be less wear resistant than the high-viscosity cements. However, the quantity of metallic particles released has still to be analysed with atomic absorption spectrometry.

Introduction: An insufficient range of motion (ROM) can lead to prosthetic impingement causing dislocation of a total hip replacement. The objective of this study was to analyze the influence of the wear coupling on ROM and dislocation stability.

Material and Methods: By means of an experimental test device, a total hip system (Alloclassic) with four different insert materials, standard ultra-high-molecular-weight-polyethylene (UHMW-PE), highly cross-linked-polyethylene (XL-PE), aluminium-oxide-ceramic and cobalt-chromium, was investigated concerning ROM and stability against dislocation. The tests were carried out under dry conditions as well as after lubrication of the articulating surfaces with fetal calf serum. In a supplementary test procedure, the force vector-induced dislocation, i.e. dislocation without previous prosthetic impingement, was analyzed.

Results: No significant differences in the ROM until impingement(ROMImp)weredeterminedbetweenthe UHMW-PE and XL-PE inserts. The overall ROMImp of ceramic and metal inserts was approximately 5° less than with PE because no plastic deformation of the rim surface occurred. There was no significant difference in the maximum resisting moment prior to dislocation between the metal-on-polyethylene couples, whereas ceramic-on-ceramic showed the lowest moments and metal-on-metal the highest. Generally, slightly decreased moments for subluxation were determined after lubrication of the sliding surfaces for all couples. In a proper cup position (45° inclination and 15° ante-version) varying the wear coupling had a minor impact on the ROM until dislocation (ROMLux). However, in a poor implant position, ceramic-on-ceramic revealed a clear decrease in the ROMLux of approximately 40° after lubrication of the articulating surfaces. In general, metal-on-metal provided the highest ROMLux. The force vector-induced dislocation provided similar results for the different wear couples.

Conclusion: The study underlines the importance of optimized implant orientation and the impact of the wear couple used on ROM and dislocation stability. Recurrent impingement with subsequent release of wear particles has to be considered for all wear couples. However, ceramic-on-ceramic couples should be used in optimal implant position to avoid impingement and dislocation.

Aims:Malpositioning of implant components plays a signiþcant role in instability of THR. Our aim was the determination of the inßuence of anteversion of the acetabular cup. Methods: The biomechanical study was performed on a model which enables different deþned implant positions. Rotation of the femoral stem are carried out in different anteversion positions of the acetabular cup with the hip joint in neutral position and in 90¡ ßexion, as well as inclination of the cup. The range of motion (ROM) is determined until impingement or dislocation is evident, as well as the recording of the resisting moment. Results: While the resisting moment shows minor deviation in several anteversion (AV) positions with the joint in neutral position, being almost independent from the inclination, a major difference is determined with minimal resisting moment with minor retroversion (RV) and with the hip joint in 90¡ ßexion (0,51 Nm/15¡ RV vs. 3,69 Nm/30¡ AV). Dislocation occurs very early due to low ROM at retroversion. Variation of inclination of 30¡ can only increase ROM until dislocation by 2,8¡. With same inclinations angles ROM is increased by 38,1¡ in the 30¡ AV. Conclusions: Correct anteversion of the acetabular component is a signiþcant factor in prevention of dislocation. Minor differences in anteversion are more important than inclination variation to improve stability of THR.

For younger patients many surgeons recommend femoral neck endoprostheses as alternative to stemmed implants in THA. Due to metaphyseal anchorage several advantages are quoted, e.g. preservation of the femoral diaphysis for a revision implant. Determinant factor for long-term implant stability is the load transmission to the bone. Because so far only few information about the load transfer of femoral neck endoprostheses exist, a photoelastic analysis was performed. Aim of the study was the comparison of bony strain pattern before and after implantation of a femoral neck endoprosthesis.

‘Composite-femurs’ (Pacific Research Labs) were used due to of their mechanical characteristics close to human femurs but better reproducibility. Three femurs were coated with photoelastic material. The femurs were loaded prior and post implantation of a femoral neck endoprosthesis type Cigar (ESKA Implants). Test load consisted of the resulting hip joint force and muscle forces (abductors, tractus iliotibialis). Load was applied statically by a universal testing machine and additional weights. Bony strain was measured along the medial, ventral, lateral and dorsal cortex. Statistical analysis of the implant related strain alterations was based on a 99% confidence interval.

The unresected femurs showed an excellent match of bony strain patterns. Implantation of femoral neck endoprostheses caused highly significant strain changes at the trochanteric region. Greatest differences were observed at the lateral cortex. Above the implant’s traction screw former areas of tension changed to compression. Along the medial cortex below the resection plane strain reductions were measured but disappeared at the latest at 40 mm below. No significant changes in strain were detected at the ventral and dorsal cortex.

Implant related bony strain alterations were limited to the trochanteric region of the femur. A marked strain alteration at the lateral trochanteric aspect was measured. Whether this is of clinical importance can not be answered yet.