Effect of design on the initial stability of press-fit cups in the presence of acetabular rim defects: experimental evaluation of the effect of adding circumferential fins.

PURPOSE: This experimental study was undertaken to examine the fixation characteristics of a six-finned acetabular cup in both primary and revision arthroplasty in comparison with two other commonly used cup designs without fins. METHODS: All three cup designs (Ananova® [Intraplant], Plasmacup® NSC [Aesculap]; Exceed ABT™ [Biomet]) were implanted into validated models of normal and revision acetabula. The defect models were designed to simulate a dorso-cranial rim defect of 90° width and 10 mm in depth (moderate rim defect) and a dorso-cranial rim defect of 130° width and 15 mm in depth (severe rim defect). The fixation strength of the three cup designs was tested by cyclically edge-loading the implanted cups using a mechanical testing machine. RESULTS: The six-finned Ananova implant exhibited greater resistance to foam-cup interface motion than both the Plasmacup and Exceed ABT implants. The largest average differences were observed in the resistance to ultimate spin-out, with Ananova outperforming Exceed ABT and Plasmacup by 26% and 17% in the moderate and by 36% and 38% in the severe defect models, respectively. CONCLUSIONS: The six-finned Ananova cup was significantly more resistant to edge loading both in the normal acetabulum and in acetabula with moderate to severe dorso-cranial rim defects than cup designs without fins, indicating that it may cover a wider range of clinical indications than conventional press-fit cups and provide clinicians with the confidence that, in primary and simple revision arthroplasty, adequate fixation strength can be obtained.

In-vitro comparison of biomechanical efficiency of three cannulated screws for arthrodesis of the hindfoot.

BACKGROUND: Sufficient inter-fragmentary compression is helpful to achieve successful bony fusion in hindfoot arthrodesis using internal fixation by screws. Beside bone quality, the design of a screw influences inter-fragmentary compression. Compressive force is achievable for any kind of screw system; however, the primary deformation of the bone is different for the different screw systems. The work necessary to achieve compressive force for primary stability was measured for different screw systems and compared to an AO screw with washer. MATERIALS AND METHODS: The compressive force was determined as a function of screw advancement for 3 different cannulated screw types (7.3-mm AO screw with and without washer, the 6.5-mm Herbert screw and the 6.5-mm Ideal Compression Screw (I.CO.S) using different synthetic bone density (0.16, 0.24, 0.48 g/ccm). Compressive force was measured indirectly, via screw tension measurement with strain gauges. RESULTS: We calculated the work to reach a limit of 60 N and the corresponding ratios to the value of the golden standard: I.CO.S (35.2%), Herbert (89.0%), AO screw without washer (116%). CONCLUSION: All screw systems yielded acceptable results but the ICOS did produce greater compression. The essential differences were the primary deformation of the bone before reaching the sufficient compressive force for primary stability.

Biomechanical criterion for selecting cancellous bone screws: arthrodesis in the hindfoot.

The aim of the paper was to compare primary biomechanical stability of different arthrodesis screws (7.3 mm AO screw with and without washer, 6.5 mm Herbert screw and 6.5 mm Ideal Compression Screw (I.CO.S)). The work necessary to achieve an adequate compressive force with them was compared to the measurement with the AO screw with washer, because this method is for the time being the most commonly used one and is called the golden standard. Compressive force was measured indirectly, via screw tension measurement, with strain gauges method. From the measurements we calculated the work to reach a limit of 60 N and the ratios corresponding to the value of the golden standard: I.CO.S (35.2%), Herbert (89.0%), AO-screw without washer (116%). The I.CO.S showed superior results. Only in the case of extremely poor bone quality, a clear advantage of I.CO.S could be expected in practice.