Elena Varini

In-vitro method for assessing femoral implant—bone micromotions in resurfacing hip implants under different loading conditions

Abstract Although prosthesis-bone micromotion is known to influence the stability of total hip replacement, no protocol exists to investigate resurfacing hip implants. An in-vitro protocol was developed to measure prosthesis-bone micromotions of resurfaced femurs. In order to assess the effect of all loading directions, the protocol included a variety of in-vitro loading scenarios covering the range of directions spanned by the hip resultant force in the most typical motor tasks. Gap-opening and shear-slippage micromotions were measured in the locations where they reach the maximum value. The applicability of the protocol was assessed on two commercial designs and different head sizes. Intra-specimen repeatability and inter-specimen reproducibility were excellent (comparable with the best protocols for cemented hip stems). Results showed that the protocol is accurate enough to detect prosthesis-bone micromotions of the order of a few microns. Statistically significant differences were observed in relation to the direction of the applied force. Using the whole range of hip loads enabled detection of maximum micromotions for any design (the peak value could be different for different loading directions). Application of the protocol during a test to failure indicated that the system could track micromotion up to the last instant prior to failure. The protocol proposed is thus completely validated and can be applied for preliminary screening of new epiphyseal designs.

On the Biomechanical Stability of Cementless Straight Conical Hip Stems

Abstract The aim of the present study was to investigation in vitro the effect of deficient bone-implant contact on the primary stability of a straight conical stem. Various possible deficient contact patterns were derived from surgical simulations. The effect of stair climbing loads on the bone-implant micromotion was firstly investigated using a finite element model and then an in vitro test aimed at assessing primary stability. It was found that if the surface features are prevented from biting dense bone in a few small but critical regions, stem primary stability is completely lost. These results suggest that the surface features used in the axisymmetric stem under investigation can be too sensitive to deficient contact conditions, and thus should be augmented with additional antirotational fins. Preliminary tests showed that a stem with the addition of such fins presents good primary stability in all tested conditions.

Biomechanical Testing of the Proximal Femoral Epiphysis: Intact and Implanted Condition

There is renewed interest in resurfacing hip prostheses. While stemmed prostheses have been extensively studied in the past, little is known about the biomechanics of epiphyseal prostheses. Our aim was to develop a combined experimental-numerical tool to study the intact and operated epiphysis. Bone and implant stress, relative micromotion and failure mode in the intact and implanted bone were investigated. Twelve pairs of cadaver human femurs were studied intact, to fully characterize the proximal epiphysis. Four were then implanted with a commercial resurfacing prosthesis. They were tested in the elastic range, while strains were measured with 15 rosettes. Implant micromotions were measured in the operated condition. A total of 7 loading scenarios were simulated to cover the range of typical motor tasks. Additionally, Finite Element (FE) models were built using a validated procedure for assigning inhomogeneous material properties based on CT data. To allow extensive validation of the FE model, additional measurements were taken in vitro: bone deflection in various points, indirect measurement of load application point, digitizing of the bone surface and gauge locations. The FE models were also used to identify the most critical load scenario to recreate in vitro spontaneous head-neck fractures. Strain measurements were successfully obtained in intact and implanted femurs, providing the natural strain pattern, and indicating moderate stress-shielding in the operated condition. Results on the 6 femurs that were modeled showed that FE can predict overall displacements with an accuracy of 0.4mm, and principal stress with an accuracy of 10% (Root Mean Squared, RMSE). In vitro failure tests were successful: all specimens fractured, with a variety of failures ranging from sub-capital to trans-trochanteric. This confirms the suitability of this test model to replicate spontaneous fractures in elderly subjects. In conclusion, an experimentally validated FE method was developed, that run in parallel with an optimized in vitro simulation. These tools can successfully predict the stress distribution and the failure mode in the proximal femur both in its natural condition and with a resurfacing prosthesis.Copyright

INTRA-OPERATIVE TESTS ON CEMENTLESS HIP STEM MECHANICAL STABILITY

The aim of the present investigation was to develop a new device that enables the stability achieved by a cementless stem to be assessed intra-operatively. The angle of the stem/femur rotation under torsion and the torque are acquired and compared in real time to a pre-set stability threshold inferred from the literature. The device indicates whether the stem is stable or not. A calibration was needed to convert angular into linear shear displacements. For this reason, repeated tests in vitro were made on both cadaveric and composite femurs, chosen so as to host the same kind and size of prostheses, all implanted with different levels of press-fitting. The device was then validated. The overall accuracy, 21%, which takes into account the interfemur variability, was deemed sufficient to assess the stability.

A Device to Test the Primary Stability in Cementless Hip Arthroplasty Through Mechanical Vibrations

Primary stability of cementless prostheses is critical for the long term outcome of the operation. Cementless implants are mechanically stabilized during surgery through a press-fitting procedure. To achieve a good initial stability, it is important that the surgeon performs an optimal press-fitting, avoiding both problems of stem loosening, and micro-cracking of the host bone. A possible approach to solve this problem and assist the surgeon in achieving the optimal compromise, involves the use of the vibration analysis. This technique was used in the presented study, which was aimed to design and test a prototype device able to evaluate the primary stability of a cementless prosthesis, at the femoral level. In particular, the goal was to discriminate between stable and quasi-stable implants; thus the stem-bone system was assumed linear in both cases. For that reason, it was decided to study the frequency responses of the system, instead of the harmonic distortion. The prototype was developed. It is mainly composed by a piezoelectric exciter connected to the stem and an accelerometer attached to the femur. Preliminary tests were performed on a composite femur implanted with a conventional stem. The results showed that the input signal is repeatable and the output can be accurately recorded. The parameters that seem to be more sensitive to stability are the resonance frequency and the amplitude at the resonance frequency.Copyright