Mechanical strength and mineralisation of the subchondral bone plate of the human patella

Abstract

s / Osteoarthritis and Cartilage 20 (2012) S54–S296 S113 211 MECHANICAL STRENGTH AND MINERALISATION OF THE SUBCHONDRAL BONE PLATE OF THE HUMAN PATELLA S. Hoechel , D. Wirz , M. Muller-Gerbl . Anatomical Inst., Univ. of Basel, Basel, Switzerland; 2 Lab. of Biomechanics & Biocalorimetry, Basel, Switzerland Purpose: The subchondral bone plate is a dynamic component that shows a functional adaptation to long-term loading history. This can be seen in the density distribution of the subchondral bone plate. In the human patella findings obtained with the photoelastic model show stress maxima in the lateral facet, which agrees with results showing the region ofmaximumbone density to be constantly found on the lateral facet. The visualization of the density distribution patterns has been generated with the help of CT-osteoabsorptiometry (CT-OAM), a method for densitometric evaluation of CT-scans according to the Hounsfield units (HU). Recent tests on human humeral heads showed that the density distribution of the subchondral bone plate correlates to the mechanical strength of it. As for the human patella, structural studies on subchondral bone strength only focussed on the subchondral trabecular bone, the subchondral bone plate has not been addressed. The aim of this study was to look at the density distribution of the human patella in correlation to the mechanical strength of the subchondral bone plate.We hypothesise that themineralisation shownwith CT-OAM and the mechanical strength of the subchondral bone plate correlate. Methods: 20 patellae were collected from human cadavers, and measurements were performed at 34 coordinate points for each. To visualize the density distribution patterns, the CT data of the patellae were evaluated with the help of ANALYSE 8.1 (Mayo Foundation, Rochester, MN, USA). Using a “maximum intention projection”, the maximal dense value of the subchondral bone plate was projected to the surface and presented in a false-colour diagramassigning false colours to every 100HU (Fig. 1A). Densitymeasurementswere taken at the defined coordinate points and recorded in a standardized grid system. To determine the mechanical strength, an indentation test machine (Synergie 100, MTS Systems, Eden Prairie, MN; 2 kN loadcell) was used. A steel needle (o1⁄4 1.3mm) created a standardised hole of 7 mm depth (1mm/sec) at the same coordinate point the density measurement was made. The penetration forces as well as the maximum force were recorded in a standardized grid systemwith its corresponding coordinates and visualised (Fig. 1B). Linear regression was used to evaluate statistical correlations (Fig. 2). Results: We show that neither the mineralisation nor the mechanical strength of the subchondral bone plate is distributed homogeneously on the patella. The maximum values consistently showed to be on the lateral facet (Fig. 1). A linear correlation was found between the density distribution and the mechanical subchondral bone plate strength (Fig. 2). The coefficient of correlation (range: 0.89 to 0.97; mean 0.92) was significant. Conclusions: A direct relationship between the subchondral bone plate density and the mechanical strength could be expected. Since the subchondral bone plate adapts to its mechanical needs, areas of high load transmission increase the strength of the subchondral bone plate by osteoblastic calcium deposition. This increase in calcium is presented in the density distribution patterns. The correlation of mineralisation and mechanical strength makes CT-OAM a valuable tool to determine the strength of the subchondral bone plate in vivo. 212 MODULATION OF GENE EXPRESSION IN HUMAN SUBCHONDRAL BONE CELLS CO-CULTURED WITH HUMAN ARTICULAR CHONDROCYTES. E.F. Burguera, A. Vela-Anero, F.J. Blanco. INIBIC-CHUAC, Tissue Engineering and Cellular Therapy Group (CBBTC-CHUAC), CIBER-BBN/ISCIII, A Coruna,