[Objectives and limits of test standards]. / Sinn und Grenzen von Prüfnormen.

BACKGROUND: Test standards are developed worldwide by extremely committed expert groups working mostly in an honorary capacity and have substantially contributed to the currently achieved safety standards in reconstructive orthopedics. Independent of the distribution and quality of a test specification, the specialist knowledge of the user cannot replace a well founded risk analysis and if used unthinkingly can lead to a false estimation of safety. LIMITS: The limits of standardization are reached where new indications or highly innovative products are concerned. In this case the manufacturer must undertake the time and cost-intensive route of a self-developed testing procedure which in the ideal case leads to a further testing standard. CONCLUSION: Test standards make a substantial contribution to implant safety but cannot replace the expert knowledge of the user. Tests as an end to themselves take the actual objectives of standardization to absurdity.

The effect of design parameters of interspinous implants on kinematics and load bearing: an in vitro study.

INTRODUCTION: A number of concepts with controversy approaches are currently discussed for interspinous stabilization (IPS). However, comparative biomechanical studies among the different systems are rare. Nevertheless, it remains unclear which biomechanical characteristics are influenced by different design features of these implants, such as implant stiffness or an additional tension band. Therefore, the aim of the present study was to compare different interspinous implants to investigate the biomechanical impact of IPS implant design on intersegmental kinematics, such as range of motion, neutral zone, center of rotation (COR), as well as load transfer like intradiscal pressure (IDP), to gain additional experience for clinical indications and limitations. MATERIAL AND METHOD: Twelve human lumbar spine specimens were tested in a spine loading apparatus. In vitro flexibility testing was performed by applying pure bending moments of 7.5 Nm without and with additional preload of 400 N in the three principal motion planes. Four interspinous implants, Coflex "COF" (Paradigm Spine, Germany), Wallis "WAL" (Abbott Laboratories, France), DIAM "DIA" (Sofamor Danek, France) and InterActiv (Aesculap AG, Germany) with two treatment options (without dorsal tensioning "IAO" and with dorsal tensioning "IAM") were consecutively tested in comparison to the native situation "NAT" and to a defect situation "DEF" of the functional spinal unit. The tested IPS devices are comprised of a compression stiffness range of 133 to 1,674 N/mm and a tensile stiffness range of 0-39 N/mm. Range of motion, neutral zone, center of rotation and intradiscal pressure were analyzed for all instrumentation steps and load cases. CONCLUSION: For the IPS, we found a correlation between compression stiffness and stabilization in extension. Here, the system with the lowest stiffness, DIA, displayed nearly no stabilization of the treated segment, whereas the system with the highest stiffness, WAL and COF, was most pronounced. This applies also for the correlation between device stiffness and IDP. In flexion only the degree of stabilization is in correlation with the tensile stiffness, whereas the IDP stays constant and is not affected by the different tensile stiffness. IPS is not able to stabilize in the frontal and transversal plane. Furthermore IPS does not substantially alter the location of the COR.

The influence of contact conditions and micromotions on the fretting behavior of modular titanium alloy taper connections.

Modularity of femoral stems and neck components has become a more frequently used tool for an optimized restoration of the hip joint center and improvement of patient biomechanics. The additional taper interface increases the risk of mechanical failure due to fretting and crevice corrosion. Several failures of titanium alloy neck adapters have been documented in case-reports. An experimental fretting device was developed in this study to systematically investigate the effect of micromotion and contact pressure on fretting damage in contact situations similar to taper interfaces of modular hip prostheses under cyclic loading representative of in vivo load conditions. As a first application, the fretting behavior of Ti-6Al-4V titanium alloy components was investigated. Micromotions were varied between 10µm and 50µm, maximum contact pressures between 400 and 860N/mm(2). All modes of fretting damage were observed: Fretting wear was found for high micromotions in combination with low contact pressures. Fretting fatigue occurred with reduced movement or increased contact pressures. With small micromotions or high normal pressures, low fretting damage was observed. The developed device can be used to evaluate taper design (and especially contact geometry) as well as different materials prior to clinical use.

[Registration procedure for incidents with implants]. / Meldeverfahren bei Vorkommnissen mit Implantaten.

Incidents involving implants, whether there is a break in the osteosynthesis plate or a synthetic inlay of an endoprosthesis, are incidents with mostly severe repercussions for the patient with immediate and delayed effects for the clinic involved and the manufacturer.

The effect of design parameters of dynamic pedicle screw systems on kinematics and load bearing: an in vitro study.

As an alternative treatment for chronic back pain due to disc degeneration motion preserving techniques such as posterior dynamic stabilization (PDS) has been clinically introduced, with the intention to alter the load transfer and the kinematics at the affected level to delay degeneration. However, up to the present, it remains unclear when a PDS is clinically indicated and how the ideal PDS mechanism should be designed to achieve this goal. Therefore, the objective of this study was to compare different PDS devices against rigid fixation to investigate the biomechanical impact of PDS design on stabilization and load transfer in the treated and adjacent cranial segment. Six human lumbar spine specimens (L3-L5) were tested in a spine loading apparatus. In vitro flexibility testing was performed by applying pure bending moments of 7.5 Nm without and with additional preload of 400 N in the three principal motion planes. Four PDS devices, "DYN" (Dynesys(®), Zimmer GmbH, Switzerland), "DSS™" (Paradigm Spine, Wurmlingen, Germany), and two prototypes of dynamic rods, "LSC" with a leaf spring, and "STC" with a spring tube (Aesculap AG, Tuttlingen, Germany), were tested in comparison to a rigid fixation device S(4) (Aesculap AG, Tuttlingen, Germany) "RIG", to the native situation "NAT" and to a defect situation "DEF" of the specimens. The instrumented level was L4-L5. The tested PDS devices comprising a stiffness range for axial stiffness of 10 N/mm to 230 N/mm and for bending stiffness of 3 N/mm to 15 N/mm. Range of motion (ROM), neutral zone (NZ), and intradiscal pressure (IDP) were analyzed for all instrumentation steps and load cases of the instrumented and non-instrumented level. In flexion, extension, and lateral bending, all systems, except STC, showed a significant reduction of ROM and NZ compared to the native situation (p < 0.05). Furthermore, we found no significant difference between DYN and RIG (p > 0.1). In axial rotation, only DSS and STC reduced the ROM significantly (p < 0.005) compared to the native situation, whereas DYN and LSC stayed at the level of the native intersegmental rotation (p > 0.05). A correlation was found between axial stiffness and intersegmental stabilization in the sagittal and frontal plane, but not in the transversal plane where intersegmental stabilization is mainly governed by the systems' ability to withstand shear loads. Furthermore, we observed the systems' capacity to reduce IDP in the treated segment. The adjacent segment does not seem to be affected by the stiffness of the fixation device under the described loading conditions.

[Ultrasound hard tissue detection for registration in orthopedics and traumatology]. / Ultraschall-Hartgewebedetektion zur Registrierung in der Orthopädie und Traumatologie.

The application of x-ray diagnostics for intraoperative navigation and for registration of hard tissue structures is restrained due to high radiation loads and the not given real time ability. Previous approaches with conventional ultrasonic imaging are only interactively applicable due to the high information content of soft tissue optimised B-mode-images. A pure image processing does not allow an automatic identification of individual structures, so that this must be done by the physician. To decrease the high expense of time, this report presents a concept for an adapted chain of rf-signal processing as well as an ultrasonic system for the contrast-enhanced representation of tissue borders. The system permits an exact measurement of the body geometry, which is demonstrated by determining the position of the pelvis entry plane.

[In vitro stability study of an improved implant system for minimally invasive transforaminal approach]. / In-vitro-Stabilitätsuntersuchung eines neuartigen Implantatsystems für den minimal-invasiven transforaminalen Zugang.

Calf specimens from the L3-4 and L5-6 levels were used for in vitro stability testing of a new transforaminal lumbar interbody fusion (TLIF) implant. Results were compared with those of the conventional posterior lumbar interbody fusion (PLIF) technique using two cuboid spacers. The specimens were loaded with pure moments of 10 Nm in flexion, extension, lateral bending, and axial rotation without any axial preload. They were allowed to move freely and unconstrained in all directions. In extension the PLIF implants showed slightly higher degrees of stiffness than the TLIF implant. While the conventional PLIF technique results in an increased range of motion by a factor of 2.5 after implant insertion, the TLIF approach prevents segmental destabilization in axial rotation. The facet joint arthrodesis using resorbable pins reveals biomechanically interesting results and will therefore be investigated in further studies.

[Mechanical testing of implant properties of thoracoscopic implantation of ventral spinal stabilizing systems. Comparative study with the ISO/DIS 12189-2 corpectomy model and an improved synthetic model]. / Mechanische Testung der Implantateigenschaften eines thorakoskopisch implantierbaren ventralen Wirbelsäulenstabilisierungssystems. Vergleichende Betrachtung im Korpektomiemodell nach ISO/DIS 12189-2 und in einem neuartigen synthetischen Modell.

A new modular anterior fixation system MACS TL (modular anterior construct system for the thoracic and lumbar spine) has been developed for use in thoracoscopic spondylodesis. This system demonstrates high angular stability and meets the surgical requirements for an endoscopic approach. The objective of the current study was fatigue testing of the MACS TL implant system using a corpectomy model according to ISO/DIS 12189-2 and a synthetic model recently developed by Kotani et al. [6]. The MACS TL system demonstrated good mechanical properties with a high stiffness compared to the published data reviewed. The importance of dynamic testing in a corpectomy model has been demonstrated by comparing the MACS TL plate system with an early prototype, which has not yet been clinically evaluated. The corpectomy model according to Kotani et al. offers an interesting alternative to the ISO/DIS 12189-2 test method for asymmetrically designed and antero-laterally positioned spinal implants due to the unconstrained ball joint.

[Knee endoprostheses: problems and technological developments from the manufacturer's point of view]. / Knieendoprothetik--Herstellerische Probleme und technologische Entwicklungen.

Modern-day knee arthroplasty is based principally on the experience gained in the course of the past three decades. In the 1980s and 1990s, for example, the full constraint models of the 1970s were replaced by surface replacement prostheses with a combined rolling/sliding motion approximating the natural motion of the human knee joint for a wide range of applications. Not all of the relevant questions in the areas of material science, biomechanics, tribology, implant fixation and surgical technique have been fully explored by any means. Nevertheless, the medium-term and long-term results presently attained by knee arthroplasty are comparable to those achieved in the field of hip-joint arthroplasty. Although the success achieved by today's knee-joint prostheses is impressive, a critical analysis of therapeutic failures and complications is necessary to discover areas where further research is necessary and to formulate the challenges presenting themselves in R&D and in the manufacture of artificial knee joints. Currently, research efforts are focused on optimizing materials (especially in light of the presently unsatisfactory wear behavior of artificial knee joints), biomechanically-assisted design optimization to achieve a better harmonization between mobility and stability in the knee joint to be treated, alternative anchoring concepts, including consistently cement-free anchoring, and solutions for simplifying implantation techniques and making them more precise. The latter area, in particular, appears to be of great importance, since the quality of artificial knee joints and the long-term therapeutic outcome of knee-joint arthroplasty depend to a great extent on the correct alignment of the leg axes, an optimal implant position, and the correct adjustment of soft tissue tension. Technical innovations in this area of implant technology exert a multifactorial impact on different aspects of arthroplasty--e.g., uniform force transfer to minimize wear, selective rotation of the femoral components to optimize patella tracking, and avoidance of load concentrations to improve fixation strength--and thus form the basis for a surgical approach that takes greater account of "the big picture". Today's sophisticated navigation systems are a case in point. The non-imaging systems, in particular, have proved to be exceptionally useful; their advantages lie in their simple handling which makes use of common surgical techniques, the avoidance of additional imaging data recording (e.g. CT images), and an inconsequential increase in operation times.

Special problems of cementless fixation of total hip-joint endoprostheses with reference to the PM type.

The success of cement-free hip endoprosthetics is largely dependent on precise surgical techniques and primary stability of the anchorage, in which favorable biomechanical conditions as well as the quality of the stabilizing bone are of considerable importance. Information gathered from more than 1500 cementless hip-joint endoprosthesis implantations is presented with biomechanical solutions and indications regarding operating techniques, and a correlation between clinical symptoms and radiological signs of complications is discussed. In close coordination with material-specific factors, design and surface characteristics are decisive in the function and quality of anchorage of the endoprosthetic replacement. In the case of the PM total hip endoprosthesis, these widely variable values were governed strictly by biomechanical considerations, with particular reference to the resulting bone reactions. Results so far, including those relating to stable integration of the implant, must be regarded as absolutely positive and confirm the design characteristics on which this model was based. Avoiding the disadvantages of bone cement, cementless hip-joint replacement, particularly in operations for the exchange of cemented prostheses after loosening, can be regarded as a step forward in hip-joint surgery.

[Technology of cementless hip endoprosthetics]. / Technologie der zementlosen Hüftendoprothetik.

The success achieved with non-cemented hip arthroplasty depends mainly on the stability of the fixation, the quality of the stabilizing bone being just as important as favourable biomechanical conditions. The results of the intensive research and development with respect to the particular features of a non-cemented hip endoprosthesis can be divided into the following basic categories: Biomechanical aspects with special reference to bone related to the design of the prosthesis; material characteristics, such as fatigue strength, tribology, corrosion resistance, and biocompatibility; and development of new materials and coatings to permit direct bonding of implant and bone. With regard to the stem of hip prostheses, the different design parameters of various types are examined to determine their typical design characteristics, such as bearing surface of the collar, geometry of cross section, anatomically adapted shaping, and surface of the implant forming the contact with the bone. The latter can be divided into macroprofiles and macro- and micro-porous coated surfaces. On the other hand, the methods of cementless fixation of acetabular cups can be primarily divided into conical and spherical screw fixation and pegged fixation with additional macroprofiles of porous surfaces. In a separate study of the biomechanical aspects of screwed sockets, the special importance of socket shape and thread geometry are presented with reference to primary stability and long-term fixation of prostheses.

[Endoprosthetic replacement of the shoulder joint. Possibilities and their analysis]. / Endoprothetischer Ersatz des Schultergelenks. Möglichkeiten und deren Analyse.

Functional and anatomic characteristics of the shoulder girdle are special problems, that have to be solved in the process of using an artificial joint replacement to improve the function of a painful shoulder, which is restricted in movement. Seen from a biomechanical point of view, besides a safe fixation of the prosthesis the stability and kinematic of the artificial joint are of considerable importance for the success of the endoprosthetic treatment. The efforts of meeting these requirements by a respective prosthesis design led to a variety of prosthesis types. This study shows the possibilities for prosthetic replacement of the shoulder joint and comprises an analysis of all systems, which are available on the market, as well as of the patent applications of the past 15 years. The prostheses are listed according to their stability. Relative to the state of destruction of the joint unconstrained prosthesis types with completely free and partially restricted mobility, as well as constrained types of joints are being discussed.

[Experimental study of improved threading of bone screws]. / Experimentelle Untersuchungen zur Gewindeoptimierung von Knochenschrauben.

In order to improve the holding force between the bone and the screw, we have been carrying out experimental investigations of the stability of bone and screw connection. Especially Schanz screws with a diameter of 6 mm, used in connection with the Wagner leg lengthening device, have been taken into consideration. By use of a loading model we determined the sizes of the essential kinds of loadability of the bone-screw connection and therefrom derive directions for dimensioning. The influence of the thread parameters (e.g. flank angle, pitch, pitch angle) on the loadability of the bone-screw connection has been investigated experimentally for 17 different thread profiles. Summarizing up the results, a pitch angle of 7,5 degrees and a flank angle between 12 degrees and 15 degrees proved to enable the maximum loadability. Corresponding to the results of our trials with the loading model, the improvement of the thread was carried out on the premises of simultaneous failure of both the cortical thread and the Schanz screws by pressure or shear. The loosening behaviour of the improved thread has been tested together with the conventional thread form. The results showed a reduction of about 20% in loosening of the screws and an increase of about 30% in pressure and traction loadability.

[A new test procedure to examine the stability of external fixation clamps--demonstrated with the Stuhler-Heise external fixation system (author's transl)]. / Ein neues Messverfahren zur Stabilitätsbeurteilung von Fixateur-externe-Systemen--Demonstriert am Beispiel des Fixateur-externe nach Stuhler-Heise.

Experimental investigation of stabilty for examination of the efficiency of external fixation clamps required different testing procedures and measuring arrangementsz which is due to variant assessment parameters and natures of load. Here is a description of a measuring equipment which could be applied for all important cases of load. In relation to the entire experimental arrangement it also enables the testing of all external fixation assemblies. Deviations of the poroximal fragment caused by loading, are considered as assessment parameters of the stability. This measuring set was practically applied on simulated defect fractures, stabilized by the Stuhler-Heise external fixation system. During a combined bending strain the stability degree of various two-dimensional as well as three-dimensional external fixation clamps for application on tibias was determined.