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Another in vivo study investigated the effect of subchondral PMMA
injections in sheep knees [39]. The subchondral bone tissue and joint
cartilage were evaluated by high-resolution peripheral quantitative
computed tomography imaging (HRpQCT), histopathological osteo-
arthritis scoring, and glycosaminoglycan content in the joint cartilage.
Compared to the untreated control knee, no significant differences
were found. The authors concluded that PMMA implant augmentation
of metaphyseal fractures does not harm the subchondral bone plate or
adjacent joint cartilage.
Bioglass
Various compositions of bioactive glass have been shown to be
bone-bonding and osteoconductive, with potentially beneficial effects
on bone formation and healing, angiogenic stimulation, and antibac-
terial activity [40,41]. Granules made of silicate glasses containing
sodium, calcium and phosphate have been used for filling of the
subchondral defects in tibial plateau fractures. In a randomized study,
Heikkilä et al. [42] used bioactive glass granules or autologous bone
to fill subchondral voids. At one year, there were no significant
differences between groups in radiological, clinical, and subjective
patient evaluations. In another prospective randomised study with
11-year follow-up, similar results were reported with no differences
reported between bioactive glass and conventional autologous bone
graft treatment groups [43].
Calcium sulphate cements
Injectable calcium sulphate has been used to fill subchondral
defects in tibial plateau fractures in clinical series [44,45]. Although
calcium-sulphate is brittle, injectable calcium-sulphate cements with
compressive strengths similar to that of cancellous bone have been
developed. Alpha and beta hemihydrate have been developed, with the
α
-form providing a more strength than the
β
-form, mainly due to
differences in the density. Different products have quite different
properties, with compressive strengths ranging from only a fewMPa to
almost 100 MPa, despite belonging to the same class of materials.
Calcium sulphates degrade rapidly and independently from bone
formation. Due to this rapid degradation, there is a risk that strength
that the loss of strength will occur too rapidly. In a study evaluating
calcium sulfate in a canine model, a material with an initial strength
exceeding the strength of normal cancellous bone had a significantly
decreased compressive strength at 26 weeks (only 0.6 Mpa) [46].
Additional studies comparing calcium-sulphate with other products
and with autologous bone are needed in order to better define proper
indications.
Calcium phosphate
So far, the most widely evaluated bone graft substitute for tibial
plateau fractures is calcium-phosphate cement. Calcium-phosphate
mimics the mineral phase of bone. Animal studies have shown that
calcium-phosphate is osteoconductive and undergoes gradual remod-
eling over time, although this process seems to be very slow. When
used in the injectable form, it cures in vivowithout exothermic reaction
to form an apatite that, within a few minutes, achieves compressive
strength greater than normal cancellous bone. However, as for calcium-
sulphate cements, there is a wide variation in strength as well as other
properties between different products.
In two separate case series it was shown more than a decade ago
that calcium-phosphate cement was a viable alternative for filling
subchondral voids in tibial plateau fractures. Lobenhoffer et al. [47]
used calcium-phosphate cement in combination with conventional
hardware for fixation of 26 tibial plateau fractures. Patients were
followed up to three years with radiological and clinical evaluations.
The conclusionwas that the material provided for a successful outcome
with few complications. In another case series, calcium-phosphate
cement was used in 49 patients to fill subchondral voids in
combination with minimal internal fixation. At one year after the
procedure, the authors found calcium-phosphate to be a useful
alternative to bone graft in tibial plateau fractures [48]. In a randomized
study comparing calcium-phosphate cement to autologous bone graft,
the subsidence of the articular fragment was measured using radio-
stereometry. Despite more aggressive rehabilitation with full weight
bearing at 6 weeks in the group treated with calcium-phosphate
cement compared with 12 weeks in the group treated with autologous
bone, the average subsidence was 1.41 mm in the group treated with
calcium-phosphate cement compared with 3.88 mm when using
autologous bone graft [49]. In another randomized study, 120 patients
with a tibial plateau fracture were randomized to subchondral void
filling with either calcium-phosphate cement or autologous bone graft.
With a follow-up of up to one year, the investigators concluded that
calcium-phosphate cement appeared to have less subsidence com-
pared with autologous bone graft [50].
Biomechanical and clinical considerations
Based primarily on PMMA, the use of cements has become
widespread for spinal augmentation. PMMA is biologically inert, does
not result in a significant inflammatory reaction, and has the capacity to
provide immediate multidirectional mechanical stability even before
the polymerization process is completed. The stiffness of PMMA
cement has been shown to range between cortical and cancellous bone
[51]. This property may also result in osteoporotic fractures at levels
adjacent to those augmented with PMMA, prompting researchers
to develop PMMA-based cements with altered biomechanical proper-
ties to better approximate the decreased stiffness of osteoporotic
cancellous bone. The stiffness of PMMA cement can be changed, for
example, by adding compounds such as hydrogels, which influence the
porosity of the end product, or by modifying the basic chemical
components of the cement. Calcium phosphate cements have been
shown to have comparable stiffness to PMMA-based cements during
compression tests. However, under shear loads, calcium phosphate
cements have been observed to fail early compared to PMMA-based
cements in in-vitro tests. When injected into confined spaces, such as
in simple vertebral compression fractures, this characteristic may have
minimal clinical implications since shear loads in these relatively stable
fracture configurations are small, and even in the presence of some
cracks/fissures, the axial load bearing capacity may not be affected
significantly. In applications where shear-stress, translation, and torque
can be expected (for example in highly unstable spinal fractures or after
pedicle screw reinforcement), calcium phosphate cements may be less
suitable than PMMA cements unless they are protected by additional
instrumentation. Some authors have, however, obtained good results
for these challenging applications with calcium phosphate cements in
both in vivo and clinical settings [51].
Several studies have recently been completed or are underway
evaluating cement screw augmentation of angularly stable plate
fixation in proximal humerus fractures. Similarly, there have been
promising reports of the safe and effective use of PMMA in the
augmentation of the proximal femoral nail antirotation (PFNA) device
in a multi-center study [30,52].
Removal of augmented screws
Despite the promising mechanical results for in situ screw
augmentation, complications such as infection, implant failure, and
necrosis lead to the need for implant removal. Therefore, the removal
of in situ PMMA augmented screws for angular stable plates of
the proximal humerus was investigated in a laboratory setting [53].
Screw extraction torque was measured in 14 augmented screws and
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