(505e) Solution Deposited Hydroxyapatite Coatings for Titanium Based Orthopedic Implants

Authors: 
Kasinath, R., DePuy Synthes Joint Reconstruction
Vass, S., DePuy Synthes Joint Reconstruction
Smith, B., DePuy Synthes Joint Reconstruction

text-align:center;line-height:110%"> line-height:110%;font-family:" times new roman>Solution deposited
hydroxyapatite coatings for titanium based orthopedic implants

text-align:center;line-height:110%"> 110%;font-family:" times new roman>Rajendra Kasinath, Stephanie Vass,
Bryan Smith

text-align:center;line-height:110%"> 110%;font-family:" times new roman>DePuy Synthes Joint Reconstruction,
Warsaw, IN 46581, USA

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Introduction:
Hydroxyapatite (HA) coated implants have been shown to promote biological
fixation (bone in-growth and periprosthetic defect bridging) in numerous animal
studies. [1-2] While plasma sprayed HA (PSHA) implants have gained widespread clinical
acceptance for use on coated hip stems, acetabular cups and tibial and femoral knee
components in cementless arthroplasty, they are not optimal for complete coverage
on porous metal substrates given line-of-site limitations. To address this
need, there have been attempts to solution deposit HA employing wet chemical
techniques. However, solution deposition of HA is highly challenging
considering its narrow thermodynamic window between heterogeneous and
homogeneous nucleation and growth. "Biomimetic" processes have
attempted to address other deficiencies in PSHA coatings (lack of phase purity,
residual grit blast media), and while these processes
are described to deposit "bone-mineral-like" HA films, they generally
take days to complete and are too slow to be commercially viable. In this work,
we report a solution deposited HA process that is tuned for preferential heterogeneous
nucleation and growth on a substrate as opposed to homogeneously in solution.

 

Methods:
Prior to deposition, substrates were etched in a caustic solution to increase
substrate surface area. Solution/ surface conditions (pH, temperature, ionic
strength, surface roughness) and hydroxide etch were optimized and translated
into design controls to enable full scale processing on porous metal implants. Coating
morphology was characterized by scanning electron microscopy. X-ray diffraction
and Rietveld analysis were employed to study composition and structure. As-coated
porous cylindrical implants with 8 mm endcaps (6 mm x 6 mm diameter, 1 mm gap)
were tested in a canine gap model, where HA-coated, porous-coated titanium specimens
were compared to titanium porous-coated controls. Specimens (n= 6 per group)
were implanted in the proximal humerus using a randomized design and histologically
evaluated after 6 weeks.

 

Results:
A typical coating sequence was between 4-6 hours. Representative coatings were
conformal, ~8 µm thick and consisted of highly oriented HA spherulites growing on
the substrate surface (Figure 1a). X-ray diffraction indicated that the coating
preferentially exhibited c-axis growth (Figure 1b). Coating composition was phase-pure
crystalline HA but Rietveld analysis revealed amorphous content that was likely
poorly diffracting grain boundaries. Solution deposited HA coated porous metal implants
tested in a canine cancellous bone-plug model demonstrated improved bone in
growth within the porous coating (Figure 2) versus bare titanium controls.

 

  

0in;margin-left:60.75pt;margin-bottom:.0001pt;text-indent:-.25in">(a)                                      (b)

Figure
1 (a) Micrographs showing conformal HA coatings on a porous substrate (inset
shows spherulitic c-axis growth), and (b) shows a representative X-ray
diffractogram of the coating; preferred c-axis orientation is evident given the
observed 002 peak intensity compared to 211.

 

Figure
2 (Right) Bone ingrowth for bare titanium versus solution deposited HA coated gap-fit
implants, (left) cross-sectional histology slice with ROIs to measure % bone
area fraction.

 

Conclusions:
Solution deposited HA coatings were successfully deposited on porous metal
implants using a commercial-scale system. Morphology
and structural characterization revealed coatings consisted of HA spherulites
and did not contain other crystalline phases. Animal study results suggested
that these coatings, using a gap-fit model, showed substantially more bone in-growth
compared to bare metal implants.

 

References:

1.
Søballe K., Acta Orthop. Scand., 2003; 64:1-58.

2.
Vasudev D.V., Ricci J.L., Sabatino C., Li, P., Parsons, J.R. J Biomed Matls.
Res. Part A, 2004;69:629-636.