A simple measuring device for laboratory indentation tests on cartilage.

Mechanical testing of articular cartilage and repair tissue enables judgment of their capacity in withstanding mechanical loading. In the past, different methods have been developed requiring a complex technical setup and extensive data analysis. Therefore, the aim of the present project was to build up a simple measuring apparatus for laboratory indentation tests. The device consists of an incremental optical displacement transducer with a sleeve bearing guided plunger and a spherical tip made of polished steel (radius: 0.75 or 1.5 mm), a sensitive load cell and a stiff frame. The indentation force results from the plunger's gravity plus the force of the spring inside the displacement transducer and levels at 0.170 N or 0.765 N. The displacement transducer is fixed to the frame via the load cell that enables one to detect the initial contact of the tip with the tissue. The load cell has a standard uncertainty of 2 mN and the displacement transducer of 1 microm. From indentation-creep tests, a "0.25-s elastic modulus" is calculated. Measurements on thin rubber sheets were carried out to determine the quality of the measuring device. Compression tests on cylinders made of these rubber sheets yielded control data, and a good agreement with the "0.25-s elastic modulus" was found. Indentation tests on cartilage at different sites of sheep femoral condyles yielded a very good repeatability of the measurement results (+/-7.5%).

[The influence of bone quality and the fixation procedure on the primary stability of cementless implanted tibial plateaus]. / Der Einfluss der Knochenmineraldichte und des Verankerungsverfahrens auf die Primärstabilitat von zementfrei implantierten Tibiakomponenten.

BACKGROUND: An overview of prospective studies on cementless and cemented primary knee joint endoprosthetics carried out between 1988 and 2004 reveals that the aseptic tibial loosening rate of cemented prostheses implanted with fixed meniscal bearings amounts to 2-6% within a period of 4-14 years, while cementless implanted prostheses show loosening rates of up to 28% within a period of 4-10 years. If these results arise from a lack of proper initial osseointegration as a result of insufficient primary stability, and how this is influenced by the tibial bone quality and the tibial fixation procedure has not yet been investigated. MATERIALS AND METHODS: Tibial plateaus were press-fit implanted, both screwed and unscrewed, into each of six pairs of tibial heads from corpses. Stability testing was conducted applying eccentric axial load, shear and torsion. RESULTS: The average amounts of relative movement at the medial and lateral plateau are clearly different in the screwed version and the unscrewed version when loaded axially, but the difference was significant (p = 0.016) only at the medial plateau. Relative movements under shear and torsion showed no significant differences. The bone density of the tibial metaphyses had no significant effect on the primary stability of the cementless implanted tibial plateau. CONCLUSION: When using cementless knee endoprostheses, the fixation of the tibial plateau with screws--in addition to a flawless press-fit and form-fit customization of the tibial head--appears indispensable for guaranteeing proper osseointegration under physiological axial loads.

The intraindividual agreement of the bone density of the human proximal tibia.

The interindividual variability in the biomechanical properties of cadaver bones has remained an unsolved problem in biomechanical investigation procedures. For this reason, it is postulated to use matched bone pairs from the same individual for comparative biomechanical tests. The rationale behind this procedure is based on the assumption that biomechanically similar behaviour is to be expected in an intraindividual rather than an interindividual comparison. Systematic studies confirming this thesis were performed on the human femur. However, investigations regarding the intraindividual properties of the proximal tibial metaphysis with respect to the underlying bone densities, have not yet been performed. In order to verify the hypothesis that matched proximal tibial metaphyses from the same donor imply corresponding bone density values, densitometric measurements (pQCT) were performed in 14 matched cadaver tibias (average age 61 years, 9 men, 5 women) which were fresh-frozen at -40 degrees C after removal. After statistical analysis of the bone density values, five tibial pairs were identified as differing on the basis of missing correlations and the existence of systematic differences within the pairwise data. In other words, only about 2/3 of the data in the random sample available was classified as comparable. As the bone density measured by pQCT technique significantly correlates with the biomechanical properties of the bone, it can be concluded from the test result available that matched human tibiae show no concurring bone density values in 1/3 of cases. Thus the pairing of corpse tibiae does not necessarily imply suitability for comparative biomechanical experiments.