Most minimal invasive surgical (MIS) systems use traditional implant systems combined with new instrumentation. In this study we analyzed a THR system that basically implies that all components are implanted through the femoral neck. The cemented femoral component consists of a highly polished tapered design. The acetabular component is made of Alumina and has an outside diameter of 20 mm. The purpose of this study was to investigate the range of motion, the wear characteristics, the fatigue characteristics of the femoral neck and the stability of the femoral component.

The range of motion of the MIS prosthetic system was calculated with a mathematical model that enabled calculation of prosthetic impingement angles. To assess the wear properties, four pairs of Zirconia heads on alumina acetabuli were tested in a hip simulato. To assess the probability of femoral neck fracture, 3 components were tested according to ISO7206. The stability of the femoral components were tested in five fresh cadaver using dynamic loading conditions. After this test, the load was increased until reconstructive failure occurred.

The ROM was in the order of 100 degrees of flexion and at least 30 degrees in other directions. The bearings showed remarkably low wear with a maximum of 0.02 mm3. All three stems survived the ISO-fatigue test. During the dynamic experiments the specimens did not fail, and no macroscopic damage was detected. Migration was only minor and stabilized during testing. The post-testing failure loads varied between 4.1 and 5.5 kN.

The ROM, stem-neck strength and wear properties of the system seem acceptable. The stability of the femoral component was satisfying; but the post-testing strength may be similar to loads that are applied on the hip at a falling accident. We conclude that these results are encouraging and warrant further studies to develop this system.

Introduction and Aims: A method has been designed to accurately measure post-operative alignment of hip (acetabular) and knee (femoral and tibial) prosthetic components relative to the pre-operative plan. Conventional methods involve 2D measurements; this new method uses 2D-3D registration to align both the prosthesis and the pre-operative CT volume to the post-operative x-ray.

Method: The method uses an automatic approach to align a CAD model of the prosthesis to the post-operative x-ray. A rendering of the prosthesis is produced and overlaid onto the post-operative x-ray image. The prosthesis can be rotated and translated in 3D to match the outline of the rendering shown on the post-operative x-ray. An initial manual procedure is used to align the rendering of the bone surface from pre-operative CT to the bony anatomy on the post-operative x-ray. This manual registration position is then used as a starting position for an automated intensity-based registration algorithm.

Results: The automated intensity-based registration algorithm allowed 3D verification of the prosthesis position. A number of digitally reconstructed radiographs (DRRs) were produced by casting rays through the pre-operative CT volume. The DRRs were then compared with the post-operative X-ray image using a similarity measure. This similarity measurement was optimised using gradient decent-type search strategy to alter the rotation and translation parameters. If the Hounsfield numbers of the voxels, which the casting rays passed through, were integrated along the ray and projected onto an imaging plane, a radiograph-like image was produced. To concentrate the area of registration and thus quicken registration algorithm, the user also manually defined a region of registration interest. Hence, DRRs were only produced within the region of interest. Due to the large size of the pelvis and tube-like nature of the femur and tibia, a total of 10 starting positions were used for this algorithm. These starting positions were found by adding random Gaussian noise to the parameters found using the manual process. The registration position was defined as the final position that produced the best similarity measurement value.

Conclusion: Validation has demonstrated this method’s accuracy in calculating the post-operative position of acetabular and knee prostheses with respect to the pre-operative plan. The results are repeatable, robust and enable pre- and post-operative 3D implant position comparison. The inaccuracies observed with conventional methods due to incorrect alignment on x-ray are reduced.

The increasing success rates of total hip replacements (THR) have led to a younger patient population with an increased probability for revision. The survival of revised components is improved by a good bone quality. This has led to an increased interest in bone preserving THR designs. A novel type of THR was developed of which the femoral component is cemented in the neck. The load carrying area of this prosthesis is reduced in comparison with conventional cemented implants. Whether an adequate stability can be achieved was biomechanically evaluated during simulated normal walking and chair rising. In addition, the failure behaviour was investigated.

Bone mineral density (BMD) was measured in 5 fresh frozen proximal human cadaver femora. The femoral heads were resected and a 20 mm diameter canal was created in the femoral necks. Bone cement was pressurised in this canal and the polished, taper-shaped prosthesis was subsequently introduced centrally. A servohydraulic testing machine was used to apply dynamic loads up to 1.8 kN to the prosthetic head. Radiostereophotogrammetric analysis was used to measure rotations and translations between prosthesis and bone. In addition, the constructions were loaded until failure in a displacement-controlled test.

During the dynamic experiments, the femoral necks did not fail, and no macroscopical damage was detected. The initial stability of the implant did not seem to be sensitive to bone quality. Maximal values were found for normal walking with a mean rotation of about 0.2 degrees and a mean translation of about 120 microns. These motions stabilised during testing. The failure loads in this study varied between 4.1 and 5.5 kN, higher failure loads were associated with higher BMD values. Most specimens showed subtrochanteric spiral fractures.

In conclusion, the stability of the prosthetic device may be adequate under dynamic, physiological loading conditions. The static failure loads were relatively low and require further optimisation of the prosthetic implant.