[Biomechanical testing of a new total wrist arthrodesis plate].

PURPOSE: The aim of this study was to evaluate the biomechanical properties of the APTUS wrist fusion plate in comparison to those of the AO wrist fusion plate.

MATERIAL AND METHOD: 6 APTUS wrist fusion plates (APTUS 2.5 TriLock Wrist Fusion Plate Long Bend) from Medartis (Basel, Switzerland) and 6 AO wrist fusion plates (= LCP Wrist Fusion Plate Standard Bend from Dupuy-Synthes, Bettlach, Switzerland) were installed according to the manufacturers' instructions on life-sized, fibre-glass reinforced polyamide models of a healthy human hand. Then fatigue testing with increasing loads on a monoaxial material testing machine was carried out and the force-movement measurements were recorded on a PC. For up to 50,000 cycles, loading was applied in a sinus-wave frequency of 4 Hz and a force ratio (FMin/FMax) of 0.1-70 N. After 50,000 cycles and then at every 10,000 cycles the load was increased by 15% until breakage or a deformation of over 15 mm at the point of force application of the implant occurred. In addition, the tear-out resistance of 5 screws of each type of plate was examined and the system tear-out resistance calculated. The mean values and standard deviations of fatigue strength, torsional moments and fatigue limits (load cycles) were checked for normal distribution and finally the results were compared by means of non-parametric and parametric statistical tests.

RESULTS: Plate breakage occurred with the AO wrist fusion plate on average after 52,596 (SD±12,833) load cycles and with the APTUS wrist fusion plate after an average of 115,428 (SD±12,600) load cycles. For the AO wrist fusion plates, an average load of 6.3 (SD±0.8) Nm and for the APTUS plate one of 10 (SD±1.7) Nm led to failure. For the APTUS plate the screw tear-out resistance was calculated as 2,632 (SD±96) N while that for the AO plate was found to be 1,449 (SD±314) N.

CONCLUSION: In comparison to the AO wrist fusion plate the APTUS wrist fusion plate has significantly higher fatigue resistance and tear-out strength. On the basis of 100,000 load cycles until bone healing in the case a wrist arthrodesis, use of the APTUS plate should enable bone healing to occur before the implant fails.