Proper pre-clinical testing of cemented THA implants may help to prevent bad implants from entering the market. Within the frame of a multinational EU-program, a finite element (FE) simulation was developed, for FE-based pre-clinical testing of cemented THA stems against the damage accumulation failure scenario. The simulation allows monitoring of cement crack formation and implant migration in cemented THA reconstructions. The current study is concerned with the clinical validation of the test. The damage accumulation failure scenario was simulated for four cemented hip stems, with well-known survival rates. The question was: Can the FE simulation rank the stems according to their clinical survival rates?

Four stems were analysed: the Lubinus SPII, the Exeter, the Charnley and the Mueller Curved. The Swedish hip register [1] reports survival rates of 4, 5, 8 and 13%, respectively, at 10 years after surgery. Four FE models were created, representing cemented THA reconstructions with the four stems in composite femurs. The stem-cement interfaces were unbonded (m = 0.25). A loading history was applied to the models, representing 20 million cycles of alternating walking and stair climbing. Using a 3D continuum damage mechanics approach, the damage accumulation and creep processes in the cement, and subsequent prosthetic migration were monitored.

The Mueller C. produced a considerably higher number of cement cracks than the other three stems. Cracks were formed around the entire stem. The cracked zones often extended over the thickness of the mantle. The Charnley performed better, with a lower number of cracks. Proximo-distal damage pathways were formed, although at a much lower rate than around the Mueller C. The Exeter performed better. Full thickness crack zones were produced only in the proximo-medial region. The Lubinus performed best, with the lowest number of cement cracks. No full thickness cracks were formed. Concerning migration, the Exeter migrated more than the other stems. From the collared implants, the Lubinus SPII showed the lowest migration values.

When considering the number of cement cracks produced in the simulation, the ranking of the stems would be, from superior to inferior: Lubinus SPII, Exeter, Charnley, Mueller Curved. This ranking corresponds to a ranking based on clinical survival rates. The stems behaved according to their design concepts, with the highest migration values for the Exeter stem. In conclusion, the FE simulations produced a clinically valid ranking of four cemented THA implants. This corroborates the use of the FE simulation for pre-clinical testing purposes.

Mechanical loading is important for the maintenance of the skeleton. In this study we addressed the following question. What is the influence of long-term exposure to 2.5 g on bone architecture in male rats? We expect that bone density will increase.

For the experiments we used a total of 14 Long Evans rats. Two experiments were performed in which the rats were exposed to 2.5 g for a period between 33 and 44 weeks. In the first experiment we analyzed the 3D trabecular structure in the femoral head, and in the second one the structure in the proximal tibia (metaphysis) was analyzed using micro-computer-tomography.

Rats exposed to 2.5 g had between 6% and 29% less total body weight than controls. Changes in anisotropy, which is a measure for trabecular alignment, were negligible. In the femoral head, the bone volume fraction (BV/TV) was similar for rats exposed to 2.5 g and controls. The diameters of the femoral head and neck in rats exposed to hypergravity were smaller than in controls, but not significantly. In the tibia, the BV/TV was lower for rats exposed to 2.5 g than for control rats (p< 0.05), whereas the size of the tibial plateau was larger in the exposed rats (p< 0.05).

These preliminary results were in contrast to our expectation. When exposed to 2.5 g, the trabecular architecture in the femoral head hardly changed, and in the tibia the BV/TV decreased. The tibial plateau was however larger. Adaptation to hypergravity conditions might be more at the global, cortical level than at the trabecular level. Alternatively, it is possible that the activity of rats exposed to hypergravity was less compared to controls. This would result in decreased dynamic stimulation of the bone so that the BV/TV still may satisfy the mechanical demands of rats exposed to hyper-gravity.

The properties of impacted morsellised bone graft (MBG) in revision total knee arthroplasty (TKA) were studied in 12 horses. The left hind metatarsophalangeal joint was replaced by a human TKA. The horses were then randomly divided into graft and control groups. In the graft group, a unicondylar, lateral uncontained defect was created in the third metatarsal bone and reconstructed using autologous MBG before cementing the TKA. In the control group, a cemented TKA was implanted without the bone resection and grafting procedure. After four to eight months, the animals were killed and a biomechanical loading test was performed with a cyclic load equivalent to the horse’s body-weight to study mechanical stability. After removal of the prosthesis, the distal third metatarsal bone was studied radiologically, histologically and by quantitative and micro CT.

Biomechanical testing showed that the differences in deformation between the graft and the control condyles were not significant for either elastic or time-dependent deformations. The differences in bone mineral density (BMD) between the graft and the control condyles were not significant. The BMD of the MBG was significantly lower than that in the other regions in the same limb. Micro CT showed a significant difference in the degree of anisotropy between the graft and host bone, even although the structure of the area of the MBG had trabecular orientation in the direction of the axial load. Histological analysis revealed that all the grafts were revascularised and completely incorporated into a new trabecular structure with few or no remnants of graft. Our study provides a basis for the clinical application of this technique with MBG in revision TKA.

The use of impacted, morsellised bone grafts has become popular in revision total hip arthroplasty (THA). The initial stability of the reconstruction and the effectiveness of any subsequent process of revitalisation and incorporation will depend on the mechanical integrity of the graft. Our aim in this study was to document the time-dependent mechanical properties of the morsellised graft. This information is useful in clinical application of the graft, in studies of migration of the implant and in the design of the joint.

We used 16 specimens of impacted, morsellised cancellous bone from the sternum of goats to assess the mechanical properties by confined compression creep tests. Consideration of the graft material as a porous, permeable solid, filled with fluid, allowed determination of the compressive modulus of the matrix, and its permeability to fluid flow.

In all specimens the compression tests showed large, irreversible deformations, caused by flow-independent creep behaviour as a result of rolling and sliding of the bone chips. The mean permeability was 8.82 *10⁻¹² m⁴/Ns (SD 43%), and the compressive modulus was 38.7 MPa (SD 34%). No correlation was found between the apparent density and the permeability or between the apparent density and the compressive modulus. The irreversible deformations in the graft could be captured by a creep law, for which the parameters were quantified.

We conclude that in clinical use the graft is bound to be subject to permanent deformation after operation. The permeability of the material is relatively high compared with, for example, human cartilage. The confined compression modulus is relatively low compared with cancellous bone of the same apparent density. Designs of prostheses used in revision surgery must accommodate the viscoelastic and permanent deformations in the graft without causing loosening at the interface.

We report a long-term review of 60 acetabular components revised using impacted, morsellised bone allografts and a cemented polyethylene cup. The acetabular defects were cavitary (37) or combined (23). Follow-up was for a mean 11.8 years (10 to 15).

Further revision was needed in five hips, two for septic and three for aseptic loosening. The overall survival rate at 11.8 years was 90%; excluding the septic cases it was 94%.

Acetabular reconstruction with impacted morsellised cancellous grafts and cement gives satisfactory long-term results.

It has been suggested that the endurance of cemented femoral reconstructions in total hip arthroplasty is affected by the creep of acrylic cement, but it is not known to what extent cement creeps under loading conditions in vivo, or how this affects load transfer. We have simulated the long-term creep properties of acrylic cement in finite-element models of femoral stem constructs and analysed their effects. We investigated whether subsidence rates measured in vivo could be explained by creep of acrylic cement, and if polished, unbonded, stems accommodated creep better than bonded stems.

Our findings showed that polished prostheses subsided only about 50 μm as a result of cement creep. The long-term prosthetic subsidence rates caused by creep of acrylic cement are therefore very small and do not explain the excessive migration rates which have sometimes been reported. Cement creep did, however, relax cement stresses and create a more favourable stress distribution at the interfaces. These trends were found around both the bonded and unbonded stems. Our results did not confirm that polished, unbonded, stems accommodated creep better than bonded stems in terms of cement and interface stress patterns.

Inadequate bone stock is often found in revision surgery of femoral components of total knee replacements. Our aim was to test the hypothesis that these remodelling patterns can be explained by stress shielding, and that prosthetic bonding characteristics affect maintenance of bone mass.

We made a three-dimensional finite-element model of an average male femur with a cemented femoral knee component. This model was integrated with iterative remodelling procedures. Two extreme prosthetic bonding conditions were analysed and gradual changes in bone density were calculated.

The long-term bone loss under the femoral knee component resembled clinical findings which confirms the hypothesis that stress shielding can cause distal femoral bone loss. Our study predicts, contrary to clinical findings, that an equilibrium situation is not reached after two years, but that bone resorption may continue. This hidden bone loss may be so drastic that large reconstructions are needed at the time of revision.

In acetabular dysplasia, fixation of the acetabular component of a cemented total hip prosthesis may be insecure and superolateral bone grafts are often used to augment the acetabular roof. We used finite element analysis to study the mechanical importance of the lateral acetabular roof and found that the lateral acetabular rim plays an important role in the load transfer of the pelvic bone. When the superlateral rim was lacking, the load shifted to the posterosuperior rim and to the area of pubic support, and the stresses in all materials, especially in the cement and in the trabecular bone, increased greatly. At the cement-bone interface the tilting component of the shear stress increased threefold. In a model in which the dysplastic acetabulum was augmented by a rigidly fixed, load-transmitting bone graft, the stresses were considerably diminished.