Knöcherne Integration und Biokompatibilität eines neuen resorbierbaren Polymers zur Schraubenaugmentation im osteoporotischen Knochen

Bone integration and biocompatibility of a new bioresorbable
polymer for screw augmentation in the osteosynthesis of
osteoporotic cancellous bone: The surgical treatment of an
osteoporosis-associated fracture is a clinically relevant problem,
which yet has no satisfactory solution. The fixation of
conventional implants is limited because of the reduced bone
density in osteoporotic bones. To ensure stable fixation compound
osteosynthesis has been used since the 1950s.
Polymethylmethacrylate (PMMA) is the most commonly used bone cement
in reconstructive medicine because of its excellent mechanical
properties. However, compound osteosynthesis also has some
disadvantages. The local exothermic polymerization as well as the
release of toxic residual monomers can cause damage to the
surrounding tissue. PMMA is also not resorbable, and therefore
surgeries using PMMA are irreversible. An ideal absorbable bone
cement must meet many biochemical requirements and also provide
sufficient mechanical stability. Two critical requirements are
long-term biocompatibility, and an absorption rate that corresponds
with the reestablishment of the physiologic bone stability. The aim
of this study was to investigate different variations of a new
resorbable bone cement developed on an alkylene
bis(dilactoyl)-methacrylate basis in terms of long-term
biocompatibility, degradation and mechanical reinforcement in a
sheep model. Further, the suitability of ovarectomized sheep as
osteoporotic large animal model was verified for investigation of
biomaterial. In this study osteoporosis was generated in sheep by
an ovarectomy followed by 6 months of glucocorticoid injections.
The control group received no treatment. Two screws were implanted
in the cancellous bone in the distal femur and proximal tibia in
both groups. The screws were fixed in the bones with variations of
the new bone cement. In each group, three screw fixation techniques
were investigated: augmented with the new polymers, augmented with
PMMA, or not augmented. The ovarectomized sheep were sacrificed
after eight weeks (short-term group). The control sheep (long-term
group) were sacrificed after six months. Histology results
confirmed the biocompatibility of the non-augmented screws as well
as the screws augmented with PMMA in both the short- and long-term
group. Histology of the cancellous bone showed physiologic
remodeling zones. The new polymers were homogenous after eight
weeks, with no signs of degradation. The polymer was well
distributed in the cancellous bone, but there were no signs of
osseous integration of the cement. Only a small number of
inflammatory cells were present. However, a significant
inflammatory response with macrophages, giant cells and
erythrocytes was observed six months after implantation. Massive
osteolysis was seen and in some cases so severe that the screw was
loose. The degradation kinetics of the new polymers were
unsatifactory, because there was no decrease in the volume of the
applied bone cement after six months. There is no definitive answer
as to what caused the foreign body reaction. Most likely several
factors played a role, such as cytotoxicity of the degradation
products, particles were broken off and the increased osmotic
pressure within the implantation site. The inflammatory reaction
and the degree of osteolysis depended without a doubt on the
concentration of the polymer. The macroscopic and histologic
changes in the osseous implantation site were consistent with the
biomechanical results. The mechanical strength of the screws
fixation of the new polymers measured by pullout force was
significantly less at both eight weeks and six months compared to
the screws in the control group. The screws fixed with the new
polymer lacked osseous integration and also had poor
biocompatibility. Therefore, the new polymers do not meet the
requirements for clinical use. The use of ovarectomized sheep
treated with glucocorticoids proved to be a suitable animal model
for investigating biomaterials in osteoporosic bone. The density of
the cancellous bone in the extremities was significantly decreased,
compared to the control animals.