Defect sites on bone minerals play a critical role in bone remodeling processes. We investigated single crystal hydroxyapatite (100) surfaces bearing crystal defects under acidic dissolution conditions using real-time in situ atomic force microscopy. At defect sites, surface structure-dependent asymmetric hexagonal etch pits were formed, which dominated the overall dissolution rate. Meanwhile, dissolution from the flat terraces proceeded by stochastic formation of flat bottom etch pits. The resulting pit shapes were intrinsically dictated by the HAP crystal structure. Computational modeling also predicted different step energies associated with different facets of the asymmetric etch pits. Our microscopic observations of HAP dissolution are significant for understanding the effects of local surface structure on the bone mineral remodeling process and provide useful insights for the design of novel therapies for treating osteoporosis and dental caries.

Background Osteochondral allograft (OCA) transplantation is an effective treatment for defects in the medial femoral condyle (MFC), but the procedure is limited by a shortage of grafts. Lateral femoral condyles (LFCs) differ in geometry from MFCs but may be a suitable graft source. The difference between articular surface locations of the knee can be evaluated with μCT imaging and 3D image analysis. Hypothesis/Purpose We tested the hypothesis that LFC OCAs inserted into MFC lesions can provide a cartilage surface match comparable to those provided by MFC allografts by comparing the surgical placement of human MFC and LFC allografts into MFC defects ex-vivo. Study Design Controlled laboratory study Methods 20 MFC and 10 LFC were divided into three groups, 10 MFC recipients (MFCr), 10 MFC donors (MFCd) and 10 LFC donors (LFCd). A 20 mm defect was created in the weight-bearing portion of the MFCr. Two grafts, one MFCd and one LFCd, were implanted sequentially into each MFCr recipient condyle. Images of the MFCr using a Skyscan 1076 μCT at 18 μm voxel size were acquired and analyzed to compare the surface contours of the original recipient site with the MFCd- and LFCd-repaired sites. 3D transformations were defined to localize the defect site in the three scans of each MFCr condyle. Vertical heights from each cartilage surface voxel to a plane were determined to delineate the contour of each image. Vertical deviations from each voxel of the graft cartilage surface, relative to the intact recipient cartilage surface, were calculated and assessed as root mean square deviation (sRMS), percent graft area that was proud, sunk, and within the “acceptable” (±1.00mm) distance. The effect of repair (with MFC versus with LFC) on each of the surface match parameters (ARMS, Aacc, A unacc,proud, Aunacc,sunk, hRMS, hacc, h unacc,proud, and hunacc,sunk,) is presented as mean±StDev and was assessed by t-test. Percentage data were arcsin transformed before statistical testing. An alpha level of 0.05 was used to conclude if variations were statistically significant. Results MFCr defects were filled using both orthotopic MFCd and non-orthotopic LFCd. Registered μCT images of the MFCr illustrate the cartilage surface contour in the sagittal and coronal planes, in the original intact condyle as well as after OCA repairs. Specimen-specific surface color-maps for the MFCr after implant of the MFCd and after implant of LFCd were generally similar with some deviation near the edges. On average, the MFCr site exhibited a typical contour, and the MFCd and LFCd were slightly elevated. Both types of OCA, MFCd and LFCd, matched well, with overall height deviations of 0.63mm and 0.0.47 for area and stepoff, respectively, with no difference between MFCd and LFCd (p=0.92 and p=0.57, respectively) and acceptable deviation based on area (87.6% overall) and stepoff (96.7% overall) with no significant difference between MFCd and LFCd (p=0.87 and p=0.22, respectively). A small portion of the implant was proud, (12.1% of area and 2.6% of circumf...

The complexity of bone tissue and the lack of techniques for directly probing bone surfaces in vivo have hindered studies on the fundamental mechanisms of bone mineral remodeling. Here, we addressed these issues by using single crystal hydroxyapatite (HAP) as a well-defined bone surface model and directly observe its surface using in situ atomic force microscopy. Specifically, we investigated the effects of NaCl concentration on the dissolution of HAP (100) surfaces and found that NaCl strongly suppressed HAP dissolution kinetics, including at physiological ion concentrations. These findings indicate that local ion concentrations in vivo may contribute significantly to the stability of bone mineral. Moreover, our analysis of HAP surfaces over a broad range of pH conditions suggests that only one kind of surface termination exists exclusively in the solutions we used. This observation is important for understanding the surface chemistry of HAP. Our molecular level, real-time observations of HAP dissolution are significant for understanding bone resorption and provide useful insights for the design of novel therapies for treating osteoporosis and other bone related diseases.Bones are intricate, composite architectures composed of cells, extracellular macromolecules, and minerals.

Hydroxyapatite (HAP) is the major inorganic component of bones and teeth. The characterization of HAP surfaces on the molecular level is important for achieving a fundamental understanding of bone remodeling and dental caries processes. On the microscopic level, hydroxyapatite growth and dissolution reactions mainly occur at steps. Therefore, this study focuses on individual molecular steps on HAP (100) facets under both static conditions and dynamic dissolution conditions using atomic force microscopy (AFM). We found that molecular steps parallel to the elongated axes of HAP crystals and those angled approximately 54 degrees against the elongated axis are not only energetically favorable but also kinetically dominant under dissolution conditions.