Cemented total hip replacements (THR) are widely used and are still recognized as the gold standard by which all other methods of hip replacements are compared. [1]. Long-term results of cemented total hip replacements show that the revision rate due to aseptic loosening could be as high as 75.4% [2]. Moreover, high stresses developed in the cement mantle of reconstructed hips can lead to premature failure of the constructs [3]. Surgical fixation techniques vary considerably [4]. The aim of this study was to investigate the performances of different surgical fixation techniques of hip implants for patients with different body mass indices, bone morphology and bone quality, using finite element (FE) methods.

Anatomically correct reconstructed hemi-pelves were created, using CT-Scan data of the Visible Human Data set, downloaded to Mimics V8.1 software, where poly-lines of cancellous and cortical bones were created, and exported to I-Deas 11.0 FE package, where the econstructed hemi-pelvis was simulated. Accurate 3D model of the hemi-pelvis was scaled up and down to create hemi-pelves of acetabular sizes of the following diameters: 46 mm, 52 mm, and 58 mm. Following sensitivity analyses, element sizes ranging from 1–3 mm were used. Material properties of the bones, implants and cement were taken from literature [5–7]. Bones of poor quality were simulated by a reduction in the elastic modulii of the cortical bone by 50%, the cancellous bone by 10 % and the subchondral bone by 50% [5]. The nodes at the sacro-iliac joint areas and the pubic support areas were fixed. A compressive force of 3 times body weight was simulated at the hip joint. The nodes between the cancellous and subchondral bones were merged. Contact elements were used at the subchondral bone and cement mantle interface and between the femoral head implant and acetabular component. Dynamic in vitro tests, simulating forces acting on a hip joint during a gait cycle, were carried out on reconstructed synthetic bones, positioned on an Instron 8874 hydraulic machine, to verify the FE models.

The volume of cement stressed at different levels in groups of 0–1 MPa, 1–2 MPa and up to 11 and above MPa were calculated. Results of FE analyses showed that

an increase in the body mass index from 20 to 30 generated an increase in the tensile stress level in the cement mantle;

Results of in-vitro tests show that an increase in the cement mantle thickness improved fixation, corroborating with the FE results.

Performances of fixation techniques depend on the patient’s bone mass index, bone quality, bone morphology.

The purpose of the study was to reduce peak cement mantle stresses occurring at the tip of the keel for an all-polyethylene cemented glenoid component using finite element (FE) techniques.

Loosening of the glenoid component remains to be one of the most determinant factors in the outcome of total shoulder arthroplasty. Due to the off-centre loading that occurs, there is bending of the glenoid component with high shearing forces. These forces are transmitted to the underlying cement mantle and bone. It has been reported in previous FE studies that high cement mantle stresses occurs at the tip of the keel and at the edges of the cement flange. These stresses at the bone-cement interface can exceed the fatigue life of the cement, therefore initiating crack formation and damage accumulation. This results in loosening of the component and thus failure.

A three-dimensional (3D) model of the scapula was developed using CT data. Surfaces of the inner and outer contours of the cortical shell were created within commercially available software, using a threshold algorithm. The glenoid bone geometry was then produced. Material properties for the reconstructed glenoid were taken from literature, using four differing material properties. The articulating surface of the keeled glenoid component was modelled with a 3mm radial mismatch. This was positioned in the glenoid bone with a uniform cement mantle thickness of 2mm. The resulting FE mesh consisted of solid parabolic tetrahedral elements.

The effect of varying the angle on the keel of the component in the superior/inferior (S/I) direction was studied with uniform cement mantle thickness. The S/I length of the keel at the lateral end where it meets the back face of the component was maintained (juncture with flange), whilst the S/I length of the keel at the medial end (tip of the keel) was reduced as the change in angle increased. Two load cases were studied, involving a physiological load for 90 degrees of abduction and a central load of same magnitude.

It was found that by increasing the angle of the keel, where the S/I length at the tip of the keel was reduced, resulted in lower cement mantle stresses in this area of interest. This can be attributed to it being further away from the stiffer cortical bone where high tensile stresses exist due to inherent bending of the glenoid construct under loading. Therefore by reducing these high cement mantle stresses at the tip of the keel, fatigue failure of the cement mantle could be reduced.