The main cause of failure of the hip acetabular component is aseptic loosening. Preclinical test methods currently used to assess the stability of hip acetabular implants rely on crude simplifications. Normally, either one component of motion or bone strains are measured. We developed a test method to measure implant 3D translations and rotations and bone strains using digital image correlation. Hemipelvises were aligned and potted to allow consistent testing. A force was applied in the direction of the load peak during level walking. The force was applied in 100-cycle packages, each load package being 20% larger than the previous one. A digital image correlation system allowed measuring the cup-bone relative 3D displacements (permanent migrations and inducible micromotions) and the strain distribution in the periacetabular bone. To assess the test repeatability, the protocol was applied to six composite hemipelvises implanted with very stable cups. To assess the suitability of the method to detect mobilisation, six loose implants were tested. The method was repeatable: the interspecimen variability was 16 μm for the bone/cup relative translations, 0.04°for the rotations. The method was capable of tracking extremely loose implants (translations up to 4.5 mm; rotations up to 30°). The strain distribution in the bone was measured, showing the areas of highest strain. We have shown that it is possible to measure the 3D relative translations and rotations of an acetabular cup inside the pelvis and simultaneously to measure the full-field strain distribution in the bone surface. This will allow better preclinical testing of the stability of acetabular implants.

The objectives of this study were to develop a simplified acetabular bone defect model based on a representative clinical case, derive four bone defect increments from the simplified defect to establish a step‐wise testing procedure, and analyze the impact of bone defect and bone defect filling on primary stability of a press‐fit cup in the smallest defined bone defect increment. The original bone defect was approximated with nine reaming procedures and by exclusion of specific procedures, four defect increments were derived. The smallest increment was used in an artificial acetabular test model to test primary stability of a press‐fit cup in combination with bone graft substitute (BGS). A primary acetabular test model and a defect model without filling were used as reference. Load was applied in direction of level walking in sinusoidal waveform with an incrementally increasing maximum load (300 N/1000 cycles from 600 to 3000 N). Relative motions (inducible displacement, migration, and total motion) between cup and test model were assessed with an optical measurement system. Original and simplified bone defect volume showed a conformity of 99%. Maximum total motion in the primary setup at 600 N (45.7 ± 5.6 µm) was in a range comparable to tests in human donor specimens (36.0 ± 16.8 µm). Primary stability was reduced by the bone defect, but could mostly be reestablished by BGS‐filling. The presented method could be used as platform to test and compare different treatment strategies for increasing bone defect severity in a standardized way.

Although in vitro biomechanical tests are regularly performed, the definition of a suitable reference frame for hemipelvic specimens is still a challenge. The aims of the present study were to: (i) define a reference frame for the human hemipelvis suitable for in vitro applications, based on robust anatomical landmarks; (ii) identify the alignment of a hemipelvis based on the alignment of a whole pelvis (including right/left and male/female differences); (iii) identify the relative alignment of the proposed in vitro reference frame with respect to a reference frame commonly used in gait analysis; (iv) create an in vitro alignment procedure easy, robust and inexpensive; (v) quantify the intra-operator repeatability and inter-operator reproducibility of the procedure. A procedure to univocally identify the anatomical landmarks was created, exploiting the in vitro accessibility of the specimen's surface. Through the analysis on 53 CT scans (106 hemipelvises), the alignment of the hemipelvis based on the alignment of a whole pelvis was analyzed: differences between male/female and right/left hemipelvises were not statistically significant To overcome the uncertainty in the identification of the acetabular rim, a standard acetabular plane was defined. An alignment procedure was developed to implement such anatomical reference frame. The intra-operator repeatability and the inter-operator reproducibility were quantified with four operators, on male and female hemipelvises. The intra-operator repeatability was better than 1.5°. The inter-operator reproducibility was better than 2.0°. Alignment in the transverse plane was the most repeatable. The presented procedure to align hemipelvic specimens is sufficiently robust, standardized, and accessible. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1645-1652, 2018.

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Hip implant failure is mainly due to aseptic loosening of the cotyle and is typically accompanied by defects in the acetabular region. Revision surgery aims to repair such defects before implantation by means of reconstruction materials, whose morselized bone graft represents the gold standard. Due to the limited availability of bone tissue, synthetic substitutes are also used. The aim of this study was to evaluate if a synthetic fully resorbable tri-calcium phosphate-based substitute can provide adequate mechanical stability when employed to restore severe, contained defects, in comparison with morselized bone graft. Five cadaveric pelvises were adopted, one side was reconstructed with morselized bone graft and the other with the synthetic substitute, consisting of dense calcium phosphate granules within a collagen matrix. During the biomechanical test, cyclic load packages of increasing magnitude were applied to each specimen until failure. Bone/implant motions were measured through Digital Image Correlation and were expressed in terms of permanent and inducible translations and rotations. The reconstruction types exhibited a similar behavior, consisting of an initial settling trend followed by failure as bone fracture (i.e., no failure of the reconstruction material). When 2.2 Body Weight was applied, the permanent translations were not significantly different between the two reconstructions (p = 0.06–1.0) and were below 1.0 mm. Similarly, the inducible translations did not differ significantly (p = 0.06–1.0) and were below 0.160 mm. Rotations presented the same order of magnitude but were qualitatively different. Overall, the synthetic substitute provided adequate mechanical stability in comparison with morselized bone graft, thus representing a reliable alternative to treat severe, contained acetabular defects.