43 / 72
A comprehensive strategy for improved treatment of osteoporotic
fractures should address biological and mechanical issues, and include
the stimulation of fracture repair, removal of inhibitors to bone healing,
application of augmentation materials, and improvements in surgical
implants.
Biological stimulation or induction of bone growth can be
facilitated by local techniques, systemic methods, or physical means.
At the time of surgery, bone marrow aspirates, platelet gels, and bone
morphogenetic proteins (BMPs) can be placed at the fracture site and
have been shown to improve healing response [4
–
6]. Administration
of vitamin D, calcium, bisphosphonates and parathyroid hormone
(PTH), have also been shown to increase fracture healing [7]. Finally,
physical modalities such as ultrasound, direct electrical stimulation,
pulsed electromagnetic fields, and extracorporeal shock waves have
been reported to affect fracture repair [8].
There are many well-recognized inhibitors to bone healing, and
every effort should be made to remove these inhibitors to improve
healing in osteoporotic fractures. This includes limiting exposure to
smoking, alcohol, potent anti-inflammatory medications, and steroids.
Maximized control of medical issues such as malnutrition, diabetes,
infection, thyroid disease, and hormonal problems is essential for
optimizing bone healing.
Evolution of bone augmentation techniques
Bone augmentation with biomaterials was first described in 1984,
when Deramond injected polymethyl methacrylate cement into a
cervical vertebral body to treat a painful intravertebral haemangioma
[9]. In the three decades that followed, many studies have been
published describing and critiquing the biomechanical principles,
preclinical animal experiments, surgical techniques, and clinical
outcomes of bone augmentation of the vertebral column [10
–
13].
Although the 1987 publication by Galibert and Deramond stimu-
lated the field of vertebral augmentation, the biomaterial used in
their case (polymethylmethacrylate; PMMA) was not novel, having
been introduced as early as 1877 by Fittig and Paul. PMMA became
commercially available in 1936 as an alternative for glass under the
name of plexiglas of perspex. The first clinical application of PMMAwas
in odontology, followed by ophtalmology (after it was discovered in the
Second World War that small fragments of PMMA from shattered
warplane canopies did not induce inflammatory reactions in the eyes
of pilots), and most famously, as a bone-implant bonding material
in hip replacement surgery in the early 1960s. General acceptance
of PMMA as a biomaterial for intravertebral applications was not
established until the late 1990s when the original French work was
introduced to the English-speaking medical community by French
Canadian Jacques Dion. This lead to an increased interest in minimally
invasive procedures such as vertebroplasty (transpedicular injection of
PMMA cement in the vertebral body) and kyphoplasty (injection of
PMMA cement after inflation of a balloon(s) in the vertebral body) [11].
In the 2000s, the indications for these procedures expanded from
primarily symptomatic osteoporotic vertebral compression fractures to
painful spinal metastases, vertebral osteolysis in multiple myeloma,
and traumatic burst fractures [14,15]. Although the precise working
mechanism of spinal augmentation for most of these indications has
not been fully elucidated, it was (and still is) generally believed that the
resulting increase of mechanical stability (and thus less movement of
microfractures) in intravertebral cancellous bone after cement injec-
tion led to an immediate and long-lasting decrease of pain. To the
current authors best knowledge, Nakano and coworkers published the
first series of patients undergoing vertebroplasty for painful osteopor-
otic vertebral compression fractures using a different (i.e. calcium
phosphate) type of cement with the secondary goal of promoting
physiological bone remodeling after stabilization [16]. Since the clinical
results from this study were not different from the studies using PMMA
cement, several hypotheses on the working mechanism for PMMA
(including the effects of local toxicity or thermal damage from
polymerizing methacrylate) were subsequently considered less plaus-
ible. Another topic of debate was the risk for adjacent level fractures
after vertebroplasty or kyphoplasty to treat painful osteoporotic
vertebral compression fractures. Although a definitive conclusion or
consensus has not been achieved, most researchers and clinicians have
agreed that mismatched elastic properties (i.e. Young
’
s modulus)
between augmented and non-augmented vertebral bodies plays an
important role in the etiology of adjacent level fractures [17].
The examples above illustrate the urgent need for a wider range of
biomaterials that are better designed for the specific clinical condi-
tions, taking into account factors that include biocompatibility/
degradability (especially for younger patients), stiffness (relative to
patient
’
s own bone mineral density), and safety (in case of cement
leakage). Moreover, since biomaterials are increasingly being used for
augmentation of methapyseal fractures of various anatomic locations
(e.g. humerus, femur, distal radius, and tibial fractures), there is a
growing number of scientific reports on that topic. In spinal surgery,
these reports focus on the attempt to reinforce pedicle screws in the
osteoporotic spine or to fill (large) voids in cages after reconstruction of
spinal defects. Additionally, characteristics specific for the bone-
implant interface, such as crack formation and propagation, are also
gaining interest from researchers [18].
Augmentation of the spine
Several studies have shown that increased amounts of PMMA
injected during procedures such as vertebroplasty and kyphoplasty
are associated with higher stiffness, higher risk of cement leakage (the
most frequent complication after vertebroplasty/kyphoplasty proce-
dures), and potential exothermal damage while not improving clinical
outcome. The optimum amount of cement injected should therefore
relate to the least amount needed for clinical efficacy. It has been
demonstrated in several studies that this minimum amount corre-
sponds to approximately 15% of the vertebral volume to be treated [19].
Other factors associated with a lower risk of cement leakage have also
been identified: using balloons (as in kyphoplasty procedures Figure 1a)
prior to cement injection; employing large-diameter needles to keep
injection pressure low; using high viscosity cement; and visualizing/
monitoring the region of interest with high-quality fluoroscopy
equipment. It must be noted that for good clinical results, the careful
selection of patients supported by the appropriate imaging techniques
is still of greatest importance. When augmenting pedicle screws with
biomaterials, some principles from arthroplasty cementing techniques
may apply, including achieving an even cement mantle between
pedicle screw and cancellous bone and allowing for undisturbed
polymerization of the cement mantle until plastic cement deformation
is no longer present. In larger spinal defects (e.g. after gross resections
or when filling metallic cages), the benefits of using biocompatible/
degradable cementsmay be limited, considering the large distances and
volumes involvedwith respect to potential vascular ingrowth necessary
for bone remodeling and creeping substitution.
Augmentation techniques for the humerus
Fracture fixation of the proximal humerus in patients with reduced
bone quality still poses a great challenge to the surgeon. Despite the
development of new and improved implants, secure anchorage of the
implants with screws or blades in the trabecular bone of the proximal
humerus remains the weak link for fixation and is mainly responsible
for implant-related mechanical failures. Initial attempts to improve
screw fixation in the humeral head used fibular grafts to augment the
trabecular bone of the humeral head [20]. Later, biomechanical [21]
and clinical studies [22] reported improvement in implant anchorage
by using calciumphosphate cements to augment the central void in the
humeral head. Recent developments of cannulated and perforated
C. Kammerlander et al. / Injury, Int. J. Care Injured 47S2 (2016) S36
–
S43
S37