[Design features of an individual thrust plate prosthesis]. / Konstruktionsmöglichkeiten einer Individuellen Druckscheibenprothese (I-DSP).

The purpose of the present study was to document the osteotomy plane for the thrust plate prosthesis, and to evaluate the question whether the geometry of the thrust plate itself correlates with the range of motion after implantation and whether the osteointegration area can be optimised. For the first part of the study, the two-dimensional geometry of the osteotomy was demonstrated in 12 computer-reconstructed femurs after performing a virtual cut at a CCD angle of 135 degrees. In the second part we constructed a prototype of an I-TPP with an optimised thrust plate and corpus geometry. In a final step, we documented the range of motion with computer-aided movement mapping. The results showed a wide variance in osteotomy geometry in the 12 femurs. With the I-TAP we were able to obtain a much better surface adaptation of the thrust plate. Movement mapping showed a much lower range of motion in the I-TPP implant.

[3D-based production of hip joints: the Aldinger system]. / 3D-basierte Herstellung von Hüftgelenken: Das Aldinger-System.

Total hip replacement is regarded as one of the most successful surgical procedures. However, aseptic loosening of the foreign material still remains a great problem with endoprostheses. Bone cement, a material for filling and fitting, allows a solution to the problem in old people, but for younger people surgeons have looked for alternatives without cement. In fixation hip stems without form-filling cement, the greatest difficulty with standardized stems was the lack of adequate fit into the individual bone cavity. The new approach, the Aldinger custom-made hip stem, is based on a series of computer tomographical X-rays. This free-styled prosthesis stem is derived directly from the 3D-reconstruction of the inner contour of the femur without any parametrization of the cross-sections. A simulation of the implantation procedure is performed to ensure the implantability of the prosthesis and to give the surgeon some useful hints for the individual operation. In addition to the excellent form-fit to the individual bone, this prosthesis has further advantages such as the special disk-like surface structure, allowing good stress transfer to the bone, and the possibility of positioning the artificial head freely in three dimensions according to the requirements of therapy. Seven years of experience with this new system demonstrate that "computer-fit" can be a successful alternative to "cement-fit," especially in younger patients with a life expectancy of more than 10-15 years.

[Computer-assisted prosthesis selection and implantation control]. / Computergestützte Prothesenauswahl und Implantationskontrolle.

Using CT-based 3D-data of human femora, it is possible to simulate the THR very realistically by a newly developed fit program. The evaluation of OP simulations of different prostheses systems on more than 200 femora showed that a large number of hip joints can only be reconstructed unsatisfactorily by standard stems regarding intramedullary fit and joint geometry. This could be minimized by optimized off-the-shelf prostheses, but there is still a remainder that can only be satisfactorily reconstructed by customized devices. The decision of choosing a standard or custom device for each individual case can be made by the same fit program using CT-data of the femur. Planning with conventional X-rays only shows major restrictions. With the results of our evaluation, "standard stems" have been developed respecting the fixation principles of our CTX-custom device system and with optimized head positions using statistical methods. The selection of the right size regarding the criteria of fixation and joint geometry is examined preoperatively with the help of the same fit program. If standard prostheses cannot fulfill the requirements, we see the indication for a custom-made stem. In this way the quality of custom-made prostheses can be transferred to the application of standard devices. The operative results confirm our expectations of the exact planning and implantation technique. The CTX combination system consisting of off-the-shelf stems and customized devices will reduce the costs for optimized techniques. In comparison with other new 3D-methods like robotics or navigation, our development has an advantage: this technology including the fit program is accomplished preoperatively in order to simplify the operative technique. In future, combinations of our advanced planning methods as well as implantations of custom prostheses using navigation systems and/or robotics seem to be possible and may be helpful for special cases.

Movement mapping as dynamic preoperative surgical planning in total hip replacement. A precondition to navigation?

The purpose of the present study was to develop and present a computer program for preoperative simulation of implant components placement in total hip alloarthroplasty, and its influence on the possible range of motion (ROM). We evaluated a computer simulation for preoperative estimation of range of motion (movement mapping) in total hip replacement. The computer program was based on Borland C++. The system had an open data port, so the data could be transferred to an Excel spreadsheet for statistical evaluation. With the developed virtual computer simulation, a practical model was established. The model showed range of motion patterns which correspond to clinical experience. ROM was best at a shaft anteversion between 20 degrees and 30 degrees. ROM had its maximum with a CCD angle of 120 degrees -130 degrees. Acetabular cup anteversion was optimal between 10 degrees and 20 degrees, and cup inclinations were optimal below 40 degrees. The presented movement mapping system seems to be a reliable option for dynamic preoperative planning, which may be a prerequisite for the use of intraoperative navigation systems.

[Virtual simulation for optimizing the range of motion in hip alloarthroplasty using an adapted thrust-plate prosthesis model]. / Virtuelle Simulation zur Optimierung des Bewegungsspiels einer Hüftalloarthroplastik am Beispiel der adaptierten Druckscheibenprothese.

The purpose of the present study was to increase the free range of motion in conventional trust-plate prosthesis design and to optimize the trust-plate contact as well as the osteointegration area below the trust-plate. For the first part of the study, the two-dimensional geometry of the osteotomy plane was demonstrated in 25 CT-reconstructed femora after performing a virtual cut at a CCD angle of 135 degrees. In the second part, we constructed a prototype of an anatomic adapted trust-plate prosthesis (A-TPP) with an optimized trust-plate and corpus geometry based on the three-dimensional data of three human cadaveric femurs (age 67-75 years). In the final step, we documented the range of motion with computer-aided movement-mapping and compared the conventional TPP with the A-TPP. The results showed a wide variance in osteotomy geometry in the 12 femurs. With the A-TPP, we were able to obtain a much better fit in the trust plate surface. The movement-mapping showed a much higher range of motion in the A-TPP implant. With the A-TPP, the implant surface area for osteointegration could also be significantly increased.