Fixation - Orthopaedic Biomaterials in Research and Practice - page 320

Coating thickness
D p
D p '
D s MIN
D s MAX
D SN
FIGUre 13.3 Characteristic dimensions of beaded surface for fixa-
tion by ingrowth.
of cement, and pressurization (before device insertion) to ensure intru-
sion of cement into bone.
Ingrown . Production of fixation by encouragement of tissue growth into
porous portions of implants. For example, stems, as is the case of force-fit
or press-fit designs, are large and efforts are made to ensure immediate
mechanical stability. There are a number of different surface structures
(see Figure 7.10), but a traditional example is a multilayer beaded coat-
ing (Figure 13.3). The sphere diameter ( D s ) is typically between 250 and
400 μm, producing surface pore diameters ( D p ) of 150-250 μm. If the
minimal internal interconnect diameter D p′ is greater than approximately
50 -100 μm, bone will grow in to depths of 3-4 mm, although most coat-
ings are thinner than this. An in-depth discussion of the state of art in
porous coatings is given later. Experimental variants of coatings include
the use of osteoconductive overcoating of the porous coatings to encour-
age early ingrowth, or grafting with autologous cancellous bone both to
encourage ingrowth and to improve early stability.
Adhered or ongrown . Fixation by direct adhesion of bone to the implant.
For example, smooth, broad stems are used, with surfaces consisting of either
pure titanium or bioactive ceramics (see Chapter 8). With adequate early
stability and limited early loading, direct bony apposition occurs. Implants
may also be coated with hydroxyapatite, which provides a nontoxic, biocom-
patible, and osteoconductive material for promoting bone ingrowth.
Structure of the mature interface
Later, we shall consider the mechanical features of the interface and
aspects of its deterioration if it fails. First, however, it is necessary to