Modular tumor prostheses: are current stem designs suitable for distal femoral reconstruction? A biomechanical implant stability analysis in Sawbones.

INTRODUCTION: High loosening rates after distal femoral replacement may be due to implant design not adapted to specific anatomic and biomechanical conditions. MATERIALS AND METHODS: A modular tumor system (MUTARS®, Implantcast GmbH) was implanted with either a curved hexagonal or a straight tapered stems in eight Sawbones® in two consecutively generated bone defect (10 cm and 20 cm proximal to knee joint level). Implant-bone-interface micromotions were measured to analyze main fixation areas and to characterize the fixation pattern. RESULTS: Although areas of highest relative micromotions were measured distally in all groups, areas and lengths of main fixation differed with respect to stem design and bone defect size. Regardless of these changes, overall micromotions could only be reduced with extending bone defects in case of tapered stems. CONCLUSIONS: The tapered design may be favorable in larger defects whereas the hexagonal may be advantageous in defects located more distally.

Fixation of the shorter cementless GTS™ stem: biomechanical comparison between a conventional and an innovative implant design.

INTRODUCTION: Conventional cementless total hip arthroplasty already shows very good clinical results. Nevertheless, implant revision is often accompanied by massive bone loss. The new shorter GTS™ stem has been introduced to conserve femoral bone stock. However, no long-term clinical results were available for this implant. A biomechanical comparison of the GTS™ stem with the clinically well-established CLS(®) stem was therefore preformed to investigate the targeted stem philosophy. MATERIALS AND METHODS: Four GTS™ stems and four CLS(®) stems were implanted in a standardized manner in eight synthetic femurs. A high-precision measuring device was used to determine micromotions of the stem and bone during different load applications. Calculation of relative micromotions at the bone-implant interface allowed the rotational implant stability and the bending behavior of the stem to be determined. RESULTS: Lowest relative micromotions were detected near the lesser trochanter within the proximal part of both stems. Maximum relative micromotions were measured near the distal tip of the stems, indicating a proximal fixation of both stems. For the varus-valgus-torque application, a comparable stem bending behavior was shown for both stems. CONCLUSION: Both stems seem to provide a comparable and adequate primary stability. The shortened GTS™ design has a comparable rotational stability and bone-implant flexibility compared to a conventional stem. This study demonstrates that the CLS(®) stem and the GTS™ stem exhibit similar biomechanical behavior. However, a clinical confirmation of these experimental results is still required.

[Revision hip stems: an analysis of the fixation]. / Femorale Revisionsprothesen : Eine Analyse der Verankerung.

BACKGROUND: Revision hip replacements are complicated surgeries because the femoral quality is often reduced by large substance losses. Stem fixation in terms of sufficient primary stability is therefore a great challenge for the orthopaedic surgeon. AIM: The aim of the present study was to examine two currently used prostheses in Germany (MHP and MRP) concerning their ability to securely bridge femoral defects. RESULTS: Up to a segmental metaphyseal defect, both stems showed sufficient fixation in our experimental setup. However, because of its predominantly distal fixation, the MRP stem would unnecessarily bridge proximal bone areas still capable of load bearing. In the presence of a transfemoral approach, the surgeon should favour the distal fixation mode because in this situation the proximal fixation option is absent. Therefore, the isthmus takes a key role for both stem designs.