Material and Methods

55 consecutive retrievals of a cementless screw ring (Mecron) were collected. In any case a 32 mm ceramic head was used. All implants failed due to aseptic loosening. The follow-up of the implants was 3 to 16 years. We recorded backside wear, fatigue of the polyethylene at the flanges on the outer rim and at the cup opening (32 mm inner diameter). To assess the deformation of the inlay, the smallest and the median diameter of the cup opening were measured using a 3 dimensional coordinate measuring machine (Multisensor, Mahr, Germany).

Material and Methods:

Basic requirements for force-controlled testing of TAR is the definition joint flexion, as well as active forces and torques acting on the joint and the definition of the ligamental stabilization of the joint.

To specify flexion of the ankle, gait analysis was performed on patients treated with a TAR (HINTEGRA, Smith & Nephew) using skin mounted markers. Data about in-vivo forces is missing for TAR. Hence, determination of active forces and torques was based on mathematical models as described in the literature.

A new testing device (figure 1) has been developed to measure ligamental stabilization of the ankle joint. Measurements were performed on 10 paired cadaver feet (n = 20). Measurements were performed in different flexion angles when applying anterior-posterior forces (± 160N) and internal-external torques (± 2,5 Nm) between the talus and the tibia.

Material and Methods:

In the first part of this study the HINTEGRA (Smith & Nephew) TAR has been used for wear testing. Wear testing was performed on a modified AMTI knee simulator. Level walking according to a previous described testing standard [see abstract: Development of a force controlled testing scenario for total ankle replacements] has been used. Level walking was simulated in three clinical relevant situations, first simulating the reduced loading after implantation, secondly simulating an increasing range of motion and at last a loading pattern orientating at the loadings in the native/healthy joint. Every simulation was run for 3 million cycles, resulting in 9 million total cycles.

In the second part of this study the metal tibial plateau was replaced by a ceramic tibial component (Biolox® Delta, CeramTec). Simulation was run, as described above, for additional 9 million cycles. Termed as a long term test, in total 18 million cycles of testing are performed.

Methods

The cementless SL-PLUS® standard stem (size 6, Smith&Nephew Orthopaedics AG, Rotkreuz, Swizerland) was implanted in two groups of synthetic femurs. Group A consists of three 2^nd generation femurs and group B consists of three 4^th generation femurs (both: size large, composite bone, Sawbones® Europe, Malmö, Sweden).

Using an established method to analyse the rotational stability, a cyclic axial torque of ±7.0 Nm along the longitudinal stem axis was applied. Micromotions were measured at defined levels of the bone and the implant. The calculation of relative micromotions at the bone-implant interface allowed classifying the rotational implant stability.

Material and Methods

Simulator studies were carried out on a single station hip simulator (MTS 858 Mini Bionix II, Eden Prairie, USA) in accordance to ISO 14242-1. Bovine serum was used as the test medium. Four COM and four COC bearings were used, both 36mm in diameter. The heads were made of a mixed-oxid ceramic (Biolox Delta(r)) paired with a high carbon wrought CoCrMo cup in the COM group whereas both components were made of Biolox Delta(r) in the COC group. Simulation was run to a total of 2.4×10⁶ cycles. Wear measurements were performed in intervals of 0.2x10⁶ cycles using a gravimetric method (Sartorius Genius ME235S, measuring solution: 15 μg, Sartorius, Göttingen, Germany).