Primary Stability of Revision Acetabular Reconstructions Using an Innovative Bone Graft Substitute: A Comparative Biomechanical Study on Cadaveric Pelvises

In vitro and in silico methods for the biomechanical assessment of osseointegrated transfemoral prostheses: a systematic review

The amputee population according to the World-Health-Organization is about 40 million. However, there is a high abandon rate of socket prostheses for the lower limb (25%-57%). The direct connection between the external prosthesis and the patient's bone makes osseointegrated prostheses for transfemoral amputees advantageous (e.g., improvement of the motor control) compared to socket prostheses, which are currently the gold standard. However, similarly to other uncemented prostheses, the osseointegrated ones are at risk of aseptic loosening and adverse bone remodelling caused by stress-shielding. The preclinical assessment of these prostheses has already been evaluated using different methods which did not provide unanimous and comparable evidence. To compare data from different investigations, a clear and detailed overview of the methods used to assess the performance is necessary. In this review 17 studies investigating the primary stability, stress shielding and stress concentration of osseointegrated transfemoral prostheses are examined. Primary stability consists in the biomechanical stability upon implant insertion. Primary stability is assessed measuring extraction force (either with a pull-out or a push-out test) and micromotion at the interface between the implant and the host bone with LVDT (in vitro test) or numerical models. Stress-shielding causes adaptive changes in the bone density around metal implants, and thus in the bone strength and stiffness. Stress-shielding is assessed with strain gauges or numerical models measuring the load transfer and the strain distribution on the surface of the femur, and between the implant and the bone respectively. Stress concentration can lead to the formation of cracks inside the bone, resulting in fractures. The stress concentration is assessed measuring the load transfer and the strain energy density at the interface between the implant and the bone, using numerical models. As a result, a global view and consensus about the methods are missing from all these tests. Indeed, different setup and loading scenario were used in the in vitro test, while different model parameters (e.g., bone properties) were used in the numerical models. Once the preclinical assessment method is established, it would be important to define thresholds and acceptance criteria for each of the possible failure scenarios investigated.

Measurement of safe acetabular medial wall defect size in revision hip arthroplasty with a porous cup

INTRODUCTION

The majority of acetabular revisions can be performed with an uncemented, porous acetabular component with or without bone grafting. These are contained acetabular defects, with an intact acetabular rim (Paprosky type I and II). As defects of the medial wall of the acetabulum are a challenge situation revision surgery, we performed this biomechanical study on a pig pelvis model with contained acetabular defects to determine the size of medial wall defect at which the acetabular cup will have sufficient primary stability.

MATERIALS AND METHODS

In 24 pig pelvis models, different diameter of medial wall defects were created, followed by acetabular component placement. The acetabulum externally loaded, and the force at a level in which the acetabular component remains stable for each diameter of defect, or at which point the acetabular cup moves into the pelvis for >2 mm.

RESULTS

In the models with acetabular medial wall defects of 10 and 20 mm, 2 mm acetabular displacement occurred under a force between 1000 and 1500 N. In those with a medial wall defect of 25 mm, the force that caused acetabular instability was between 700 and 1000 N. In the models with 30 mm of medial wall defect all acetabular components were unstable under a force of 700 N.

CONCLUSIONS

According to our results, acetabular component should be stable if the defect of the medial wall of the acetabulum is less than 68% of the diameter of the acetabular component or if the uncovered surface area of the acetabular component is not greater than 27%, and the force <700 N. For a load of 1000 N, the medial wall defect should not exceed 45% of acetabular component diameter or 18% of uncovered acetabular component surface.

Biomechanical effect of metal augment and bone graft on cup stability for acetabular reconstruction of total hip arthroplasty in hip dysplasia: a finite element analysis

Background

Different methods of acetabular reconstruction with total hip arthroplasty (THA) for Crowe II and III of adult developmental dysplasia of the hip (DDH) acetabular bone defect have been implemented clinically. However, the biomechanical effect of different augmented materials for acetabular reconstruction in THA on shell stability has never been discussed.

Methods

In the present study, autologous bone graft (BG)and metal (Ti6Al4V) augment (MA) were simulated with several acetabular bone defect models of DDH in THA. The contact pressure and micromotion between the shell and host bone were measured for evaluating the shell stability using a finite element method.

Results

The peak contact stress between shell and host bone was higher in the MA situation (12.45 vs 8.71 MPa). And the load transfer path was different, for BG models, the high local contact stresses were found at the junction of bone graft and host bone while for MA models the concentrated contact stresses were at the surface of MA. The peak relative micromotion between shell and host bone was higher in the MA situation (12.61 vs 11.13 µm). However, the peak micromotion decreased in the contact interface of MA and cup compared to the BG models.

Conclusions

The higher micromotion was found in MA models, however, enough for bone ingrowth, and direct stronger fixation was achieved in the MA-cup interface. Thus, we recommended the MA can be used as an option, even for Crowe III, however, the decision should be made from clinical follow-up results.