CHUNKS of bone can be grown from a patient鈥檚 own bone marrow and used to
replace bone lost because of an accident, arthritis or the removal of a tumour,
says the consortium of European companies developing the technique.
At the moment, if bone is lost, doctors usually insert metal replacements.
But metal doesn鈥檛 bond well to bones, nor does it flex as bones should. Grafts
of a patient鈥檚 own bone are possible, but only small pieces can be taken, and
the excision site can remain painful for up to a year. And grafts obtained from
bone banks may be rejected by the patient鈥檚 immune system.
But Joost de Bruijn of IsoTis, a company based in Bilthoven in the
Netherlands, says it makes more sense to grow new bone. His team has worked out
how to culture bone marrow cells from animals, then seed them onto chunks or
particles of the mineral component of bone, hydroxyapatite, along with bone
growth factors. After between four and six weeks, the cells have secreted a
layer of normal bone, coating the chunks or bonding the particles into a
mass.
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This can then be grafted into the animal that supplied the marrow. The growth
factors encourage surrounding bone to grow and promote bonding with the implant.
The team has successfully replaced 2-centimetre long segments of the radius bone
in rabbits, and will attempt 5-centimetre segments of a goat鈥檚 femur next. They
have also grown bone from human cells.
Other groups have tried hydroxyapatite grafts seeded with marrow cells, which
secrete bone after they are implanted
(New 杏吧原创, 18 March, p 7).
鈥淏ut that takes a while,鈥� says de Bruijn.
A major use for the new grafts would be to replace bone that has crumbled
around a failed metal implant. These must bond quickly, or the metal will move
and cause more damage. The IsoTis bone bonds immediately, says de Bruijn.
But hydroxyapatite is brittle, and is not absorbed and replaced with normal
bone. Bone is more flexible than hydroxyapatite alone, due to its protein
content. So in a collaboration with polymer researchers funded by the European
Commission, IsoTis is now growing bone on other porous scaffolds.
One, made of starch, is as strong and flexible as normal bone. The other is a
plastic called Polyactive, used in implants but not yet in tissue grafts. Thin
layers of it can be shaped with a scalpel to replace delicate bones. Both
polymers are slowly absorbed, allowing bone to replace them.
鈥淚t should work,鈥� says Lee Weiss of Carnegie Mellon University in Pittsburgh,
Pennsylvania. 鈥淏ut in practice, surgeons don鈥檛 like to perform two procedures,
extracting the marrow cells, keeping them uninfected while they grow, then
re-implanting them later.鈥� However, De Bruijn notes that marrow cells can be
extracted in an outpatients department.
Ultimately, he says, a new hip joint might be grown on a shaped scaffold. The
polymer could release growth factors that make marrow cells secrete cartilage at
the tip, where the joint forms, while releasing bone growth factors along its
length. 鈥淥ne problem would be getting blood vessels to grow in,鈥� says de Bruijn.
鈥淏ut we could put channels in the polymer, and perhaps even engineer blood
惫别蝉蝉别濒蝉.鈥�