Abstract
Total Hip Replacement (THR) is one of the most successful operations in all of medicine, however surgeons just rely on their experience and expertise when choosing between cemented or cementless stem, without having any quantitative guidelines. The aim of this project is to provide clinicians with some tools to support in their decision making. A novel method based on bone mineral density (BMD) measurements and assessments was developed 1) to estimate the periprosthetic fracture risk (FR) while press-fitting cementless stem; 2) to evaluate post-operative bone remodeling in terms of BMD changes after primary THR.
Data for 5 out of over 70 patients (already involved in a previous study1) that underwent primary THA in Iceland were selected for developing novel methods to assess intra-operative FR and bone mineral density (BMD) changes after the operation. For each patient three CT images were acquired (Philips Brilliance 64 Spiral-CT, 120 kVp, slice thickness: 1 mm, slice increment: 0.5 mm): pre-op, 24 hours and 1 year post-operative.
Pre-op CT scan was used to create 3D finite element model (Materialise Mimics) of the proximal femur.
The material properties were assigned based on Hounsfield Units. Different strategies were analyzed for simulating the press-fitting operation, developing what has already been done in prior study1. In the finite element simulation (Ansys Workbench), a pressure (related to the implant hammering force of 9.25 kN2) was applied around the femur's hollow for the stem and the distribution of maximum principal elastic strain over the bone was calculated. Assuming a critical failure value3 of 7300 με, the percentage of fractured elements was calculated (i.e. FR).
Post 24 hours and Post 1 year CT images were co-registrated and compared (Materialise Mimics) in order to assess BMD changes. Successively, volumes of bone lost and bone gained were calculated and represented in a 3D model.
Age and gender should not be taken as unique indicators to choose between implants typologies, since also three dimensional BMD distribution along with volume of cortical bone influence the risk of periprosthetic fractures. Highest FR values were experienced in the calcar-femorale zone and in similar location on the posterior side.
BMD loss volume fractions after 1 year were usually higher than BMD gain ones. Consistently with prior studies4, BMD loss was mainly concentrated around the proximal end (lesser trochanter area, outer bone).
If present, BMD gain occurred at the distal end (below stem's tip) or proximally (lesser trochanter area, interface contact with the stem).
The use of clinical data for BMD assessments serves as an important tool to develop a quantitative method which will support surgeons in their decisions, guiding them to the optimal implant for the patient. Knowing the risk of fracture if choosing a cementless stem and being aware of how the bone will remodel around the stem in one year's time can eventually lead to reduction in revisions and increased quality of life for the patient. Further work will target analysis of a larger cohort of patients and validate FE models.
