Many surgical decisions taken in the operating theatre are based on the experience and the expertise of the surgeon. Using biomechanical and computational data can provide additional information for the surgeon. By carrying out biomechanical trials pre-operatively as well as a full three dimensional analysis of the skeletal structure of the patient, it is possible to provide the surgeon with clinical data that can support the decision making with regards of fixation method, type of implant and size to name a few. In the presented project a description is provided of the pre-operative assessment of primary total hip patients in Iceland and how the analysis is helping to prevent periprosthetic fractures.

Over 70 patients undergoing primary total hip arthroplasty in Iceland were recruited for the study¹. Gait analysis was performed on the patients using a pressure plate in conjunction with two synchronised video cameras. In addition, EMG was recorded from three muscles: Rectus femoris, Vastus lateralis and Vastus medialis on both the healthy and the operated leg. Finally the patient was CT-scanned with an in-plane resolution of 0.5mm and slice thickness of 1mm. Three dimensional objects of both the femur and muscles were created based on the scans. The material properties were derived from the Hounsfield units. Finite element analysis was carried out on the femur and the fracture risk of press fitting procedure was calculated and areas of weak points in the bone identified. Analysis was carried out on the muscles and the volume distribution between fat, connective tissue and muscle tissue calculated.

The results showed that basing fixation method on age and sex may not necessarily be a good indicator. The three dimensional bone mineral density distribution and the relative volume of cortical bone provided a better indication of which patients should receive cemented implant. Using a strain based failure criteria on the finite element models showed increased number in failed elements with decreased volume of cortical bone. The results of the biomechanical assessment for each patient were finally collected using an automatic report which was presented to the clinician.

Using biomechanical assessment and modelling can help identify an optimal treatment method for total hip patients by giving surgeons quantitative data on which they can build their decision making in the operating theatre. This can eventually lead to reduction in revisions and increased quality of life for the patient.

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.

Introduction: A secular trend in hip fracture incidence shows a decrease in Europe and North America. Although the results are age adjusted – is there a decreasing trend in all age groups or is just the fracture curve drifting?

Material and Methods: In this population based material includes all hip fractures 1993–2005, in men and women, age 50 years or older admitted to the University Hospital in Ume̊ in the northern part of Sweden. The total numbers of hip fractures in this age group over this 13 years period were 2919 (909(31%) in men and 2010 (69%) in women). The crude numbers of fractures and incidence were mean value over the time periods 1993–96 and 2001–05.

Results: The age adjusted incidence were showing a declining trend X 1993–96, Y 1997–2000, Z 2001–05(ska kolla med Hans hur jag räknar ut). The crude numbers of hip fractures per year are more or less constant 93–96 222,25 (150 women and 72,25 men), 01–05 221,1 (152,4 women and 68,8 men) When analyzing the age groups separately a 50% increase in crude fracture rate was noted in women age 90 or older (11,5 hip fractures/year(93–96) and 24,6 hip fractures/year(01–05). The latter fracture rate can be compared with the crude number of hip fractures in women age 75–79 (27,2 fractures/year 2001–05).

The incidence was also increasing during this period of time, from 270/10.000 to 390/10.000 among the women age 90 or older.

In men there were a declining trend in both incidence and crude numbers, only a small increase of crude fractures were noted among the oldest men age 90 or older.

Conclusion: Though the age adjusted incidence shows a declining secular trend the crude facture rate and incidence are increasing in the oldest women age 90 year or older. Changes in this particular age group does not affect the age adjusted incidence when analyzing the whole population age 50 years or older. (kolla upp) Women 90 years and older do now account for the same amount of hip fractures every year as the women 75–79 years. There seem to be a drifting in the hip fracture incidence pattern towards older. It is therefore important to analyze secular changes in incidence separately in different age groups so that changes in fracture pattern can be identified.