Development of Bioabsorbable Interference Screws: How Biomaterials Composition and Clinical and Retrieval Studies Influence the Innovative Screw Design and Manufacturing Processes - Biologically Responsive Biomaterials for Tissue Engineering - page 116

determined by the femoral tunnel, whose length usually varies between 25 and
30 mm with an inner diameter of 8-10 mm (8 mm for quadrupled semitendinosus-
gracilis graft and 10 mm for bone-tendon-bone-patellar graft).
Potential pitfalls of interference screw fixation include incorrect screw size, graft/
screw tunnel mismatch, tunnel/screw divergence or convergence, graft advancement,
graft translocation, graft fracture, and laceration of tensioning sutures or tendons-
graft. Ideal placement of the interference screw within the femoral osseous tunnel is
anterior to the graft, which allows the collagen fibers to be positioned posteriorly in
the most anatomic position [ 34 ]. The optimal orientation of the interference screw
within the tunnel for maximum fixation strength is parallel to the graft. If the screws
diverge or converge, fixation strength may be compromised [ 35 ] .
General specifications for interference screws, concerning both material and
design, are dictated by the specific clinical needs and ligament reconstruction tech-
niques. Thus, an “ideal” interference screw must minimize tissue laceration and
shear stress on graft, while maximizing insertion torque pullout strength and inter-
ference with the driver. In the case of an interference screw manufactured from a
bioresorbable material, the requirements for material refer also to the absorption
rates and biodegradability correlated to osteointegration.
In the same time, the screwdriver (its outer shape being the negative of the inte-
rior of the screw) has to be designed to avoid the screw breakage during insertion,
to minimize the insertion torque, and to distribute it along the whole screw length.
The literature includes a large number of biomechanical studies related to the
screws for ACL reconstruction, focused on establishing relationships between vari-
ous factors/parameters such as bone mineral density, bone block size, divergence
angles between screw and bone plug, interference screw diameter, shape, thread and
length, gap between tunnel and bone block, etc., and fixation strength, pullout
strength, or insertion torque [ 13, 36- 48 ] .
Another group of biomechanical tests was used to optimize the screw design by con-
ducting analyses which evaluate the influence of different critical design parameters
(such as major/minor diameter, thread shape, or pitch) on the pullout strength, shear,
tensile and compressive forces, and insertion torque. The conclusions of all these
researches must be correlated to the type of material/specimens (human, sawbones,
bovine, ovine, porcine) used in the experiments [ 44, 49 ] .
There are also papers which use finite element simulations for analyzing differ-
ent designs of interference screws or for optimizing the tunnel placement [ 38, 41,
50- 52 ]. These studies assume that the materials for cortical and cancellous bones
are linear elastic, isotropic, and homogenous, the mechanical properties being deter-
mined experimentally [ 53- 56 ]. A more recent approach, developed with the pur-
pose to improve the estimations made regarding the bone material properties, is to
assign material properties to bones based on the relation between Hounsfield units,
bone mineral density (information available by quantitative computer tomography
(QCT)), and elastic modulus. QCT gives the real spatial distribution of bone density
and according to these values, material properties are mapped on the finite element
model [ 57- 59 ] .
Our observations of these studies are further presented: