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.

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.