[Comparative study on the strength of different mechanisms of operation of multidirectionally angle-stable distal radius plates]. / Vergleichende Studie zur statischen Festigkeit unterschiedlicher Verblockungsmechanismen multidirektional winkelstabiler Verriegelungen distaler Radiusplatten.

INTRODUCTION: Polyaxial angle-stable plating is thought to be particularly beneficial in the management of complex intra-articular fractures of the distal radius. The present study was performed to investigate the strength of polyaxial locking interfaces of distal radius plates. MATERIAL AND METHODS: We tested the polyaxial interfaces of 3 different distal radius plates (2.4 mm Variable Angle LCP Two-Column Volar Distal Radius Plate, Synthes, Palmar Classic, Königsee Implantate and VariAx Plate Stryker). The strength of 0° and 10° screw locking angle was obtained during static loading. RESULTS: The strength of Palmar Classic with a 0° locking angle is significantly the best of all tested systems. With a 10° locking angle there is no significant difference between Palmar Classic, Two column Plate and VariAx Plate. CONCLUSION: The strength of polyaxial interfaces differs between the tested systems. A reduction of ultimate strength is due to increases of screw locking angle. The design of polyaxial locking interfaces should be investigated in human bone models.

[The use of DensiProbe™ in hindfoot arthrodesis. Can fusion failure be predicted by mechanical bone strength determination?]. / Zur Nutzung von DensiProbe™ bei der Rückfußarthrodese. Kann das Versagen durch eine mechanische Bestimmung der Knochenfestigkeit vorhergesagt werden?

AIM: Bone quality is a main factor in implant fixation. After having shown promising results, we have further investigated the use of the DensiProbe™ as an intraoperative measurement tool for evaluation of calcaneal bone quality and prediction of nailed hindfoot arthrodesis failure. METHOD: In this add-on study 19 nail arthrodeses were performed using a conventional screw plus a locked blade (n = 6) or plus a locked screw (n = 13) in the calcaneus. A specially devised tool was inserted at the fixation sites of the screws and the cancellous break-away torque was measured. The constructs were then cyclically loaded to failure. RESULTS: We saw a wide range of BMD (1.9-185.9 mgHA/cm³, mean 102.4 mg/cm³, SD 53.5). The peak torque was 0.47-1.78 Nm (mean 0.92 Nm, SD 0.46) at the proximal screw site (PSS) and 0.24 and 1.2 Nm (mean 0.63 Nm, SD 0.37) at the distal screw site (DSS), respectively, and 0.42 and 1.52 Nm (mean 1.00 Nm, SD 0.36) in the screw plus blade group (PSS). The number of cycles correlated with peak torque (two screws group PSS: p = 0.002, r² = DSS: 0.61 p = 0.001, r² = 0.90; screw plus blade group PSS: p = 0.001, r² = 0.99). Peak torque also correlated with BMD in both groups (two screws group PSS: p = 0.01, r² = 0.71; DSS: p = 0.001; r² = 0.83; screw plus blade group PSS: 0.42 and 1.52 Nm, mean 1.00 Nm, SD 0.36). CONCLUSION: A mechanical bone measurement tool like the DensiProbe™ seems to be suitable for predicting tibiotalocalcaneal arthrodesis failure in a biomechanical test set-up. As a restriction in clinical practice failure is multifactorial and prediction cannot be based upon these measurements only.