The Design and Pullout Strength of a Novel Shape-Memory PEEK Suture Anchor
+1,2Christopher M. Yakacki, 1,2Jack Griffis, 1,2Mariya Poukalova, 1,2Nathan Evans, 1,2Parth Brahmbhatt, 1,2Ken Gall
1
MedShape Solutions Inc, Atlanta, GA, 2The Georgia Institute of Technology, Atlanta, GA
[email protected]
Introduction
predicted by its bearing area (area in contact with bone from an
overhead point of view).
Suture anchors are orthopedic devices designed to achieve soft tissue
fixation and are commonly used in rotator cuff repair; an operation that
was performed 460,000 times in 2007. In general, reattachment of the
tendon is achieved by fixation to a bone via suture and implant, which
are either screwed or tapped into a hole in the bone.
Shape-memory polymers (SMPs) have increasingly been proposed
for biomedical devices because of their ability to change shape in vivo.
Applications range from cardiovascular stents to ACL interference
screws. Shape-memory can be either triggered thermally (temperature
induces recovery) or mechanically (small forces aid in recovery).
Mechanical activation is used in cases where the temperature change
alone is insufficient to induce recovery.
Many suture anchors are made from PEEK, a high-strength
thermoplastic. The authors hypothesize that PEEK can be formulated
and processed for shape-memory and utilized as a suture anchor. This
would represent the first application of PEEK as a SMP biomedical
device. The anchor would then be tested for pullout in synthetic and
Fig. 3: Pullout strength as a function of bearing area. Results compare
cadaveric bone.
SMP anchors to 9 other anchors. Predicted values shaded in grey.
Methods
The pullout comparison of the SMP anchors to commercially
A winged-concept suture anchor was designed and fabricated from
available anchors in cadaveric bone can be seen in Figure 4. The SMP
PEEK. The anchor was thermo-mechanically treated to set a temporary
anchor exhibited average pullout values of 295 ± 87 N (n=4) and 566 ±
shape (wings closed) from its permanent shape (wings open). A
99 N (n=8) for the 3.5 and 5.5 mm sizes, respectively. The Arthrex
demonstration of the SMP anchor body being thermally activated can be
Corkscrew FT anchors all failed by means of internal suture-loop
seen in Figure 1. The diameter of the programmed suture anchor was
breakage, while the Opus Magnum2 anchors all failed via suture slip.
made in both 3.5 mm and 5.5 mm designs.
Fig. 1: Thermal activation of the body of the SMP suture anchor.
The SMP PEEK suture anchors were installed using a tap-in
approach (Figure 2). First, a hole was punched in the bone
corresponding to the diameter of the collapsed anchor. Next, a suture
eyelet connected to a rod
was inserted into the
punched hole with 2 to 3
sutures depending on the
anchor size. Finally, using a
hammer, the cannulated
anchor was tapped over the
eyelet rod and onto the
eyelet, which mechanically
deployed the wings of the
anchor.
The pullout strength of the
Fig. 2: Illustration of the SMP suture
anchor was tested using an
anchor mechanically deploying.
Instron
5567
testing
machine. The anchor was pulled normal to the surface at a rate of 1
mm/s until failure. The anchor was tested in 8 pcf synthetic sawbone to
represent osteoporotic bone and compared to the results of a previously
published study.1 To represent healthier bone, the anchors were tested in
cadaveric specimens. Anchors were placed in the greater tuberosity
(n=3) and lesser tuberosity (n=1) of 3 matched pairs of humeral heads
with an average age of 54 ± 3.6 yrs. In one matched pair, a 5.5 SMP
PEEK anchor was compared to a 5.5 mm Corkscrew FT (Arthrex), while
in the remaining two matched pairs, 3.5 mm SMP PEEK anchors were
compared to 4.5 mm PEEK Corkscrew FT (Arthrex) and Magnum2
(Opus).
Results
The pullout strength of the SMP PEEK anchors compared to 9 other
commercially available anchors can be seen in Figure 3. The 3.5 and 5.5
mm SMP anchors have pullout strengths of 63 and 124 N in 8 pcf
sawbone. The pullout values of the SMP anchors agree with the theory
of the previous study stating that suture anchor pullout strength can be
Fig 4: Pullout comparison of the SMP suture anchor in cadaveric bone.
(note: each data set refers to one match pair of humeral heads)
Discussion
To the best of the authors’ knowledge, this is the first demonstration
of PEEK as a SMP biomedical device. The suture anchor was activated
mechanically for the final design. The pullout strength matched the
prediction method’s values based on bearing area.
Limitations of this study include the limited number of devices tested
in cadaveric bone. The three predicate devices tested all failed via suture
breakage or slip instead of anchor pullout. This is due to the increased
holding capacity of healthy cadaveric bone with cortical bone.
PEEK can be formulated and processed into a shape-memory
biomaterial used in orthopedic devices. A shape-memory PEEK suture
anchor can be created and mechanically activated with the potential for
repairs in vivo. The 3.5 and 5.5 mm sized winged design exhibited
average pullout strengths of 295 and 566 N.
References
1.
Yakacki CM, Griffis J, Poukalova M, Gall K. Bearing area: A new
indication for suture anchor pullout strength? Journal of
Orthopaedic Research 2009;27(8):1048-1054.
Poster No. 1801 • 56th Annual Meeting of the Orthopaedic Research Society