gms | German Medical Science

Objectives: Introduction: The general concept of total hip arthroplasty (THA) was introduced by Dr. John Charnley in 1962. Since then THA became one of the most successful orthopaedic procedures. In the 1980's, after two decades of experience with cemented THA, application of cementless devices was initiated. Since then, many different designs of cementless endoprostheses have been developed to improve primary rotation stability and to avoid axial migration. Long-term solid anchorage of cementless hip (endo)prostheses cannot be achieved in case of stem micromotion at the bone-prosthesis interface, occurring prior to bone ingrowth and osteointegration at the interface. A new cementless hip stem developed by Scyon Orthopaedics AG (Waedenswil, Switzerland) provides immediate anchorage via fixation to the medial cortex of the femur with monocortical locking screws. For geriatric patients an immediate solid anchorage is especially important to get them mobile and to allow partial weight bearing after THA. The aim of this study is to investigate biomechanically whether the Scyon stem would be able to carry loads during normal walking activity of a geriatric patient.

Methods: Material and Methods: Six pairs of human cadaveric femora were used in this study. Within each pair, one femur was randomly implanted with a cemented Müller-straight stem, whereas a Scyon stem was implanted in the contralateral femur. All specimens were embedded distally and then biomechanically tested in a test setup simulating a single leg stance loading as described in a previous study by Götze et al. (2002). Cyclic axial loading was applied at a rate of 1 Hz with progressively increasing amplitude, starting at 1500N and then increasing the peak load by 0.8 N/cycle until specimen failure. Number of cycles to failure and failure load were detected.

Results: The average failure load for the specimens implanted with Scyon- and Müller-stem was 4202N (SD 924, max. load 5635N) and 5425N (SD 1682, max. load 8223N), respectively. The specimens in five pairs failed at the embedding area of the distal femur, keeping the endoprostheses in solid anchorage. Just one pair failed at the proximal femur.

Results and Conclusion: Discussion: In a study by Bergmann et al. (2001) axial loading of 450% bodyweight (BW) was recorded for normal daily life activities of an 80 year old male. Converting the results of our biomechanical study to percentage of 80-kg bodyweight, an average loading of 525%BW and 678%BW was reached in the specimens implanted with Scyon- and Müller-stem, respectively. As a result, on average, the cementless Scyon stem is able to carry immediately after implantation higher loads than that in normal daily life activities. Since breakage of the femora occurred predominantly in their distal part, keeping the prostheses in solid anchorage, even higher failure loads of prostheses fixation can be expected. Further investigations on the new Scyon cementless prosthesis should be performed in clinical studies.