The bioabsorbable suture anchor tested is safe for use in clinical
practice without concerns for the strength of the construct or bony reaction to
Suture anchors are commonly used to secure soft tissue
to bone in orthopedic surgery. Their low profile, ease of insertion, and
fixation strength make suture anchors a popular alternative, especially in
shoulder stabilization. The use of bioabsorbable anchors has been limited,
however, by concerns about the initial and subsequent pullout strength, as well
as their potential effect on the surrounding bone.
These devices have only been routinely used since the
early 1990s. In 1991, 5 types of anchors were commercially available from 3
companies.1 Presently >100 suture anchors are commercially
available. Early suture anchors were made of metal.2 When inserted
near a joint, especially the shoulder, metallic implants can present
problems.3 Loosening, intra-articular migration, and breakage of
implants can result in loss of fixation strength and articular cartilage
damage. Metallic implants can also impair adequate visualization with either
radiographs or magnetic resonance imaging (MRI). Additionally, revision surgery
can be complicated by previously placed metallic anchors.
Bioabsorbable anchors provide an attractive alternative
to metallic implants. They are potentially hydrolyzed and replaced by bone.
Thus, the problems with retained hardware, radiographic visualization, and
revision surgery are obviated. Early bioabsorbable anchors were made of
polyglycolic acid that hydrolyzed quickly. Pullout strength was decreased to
<50% in 6 weeks. This can result in fixation failure and recurrence of
shoulder instability if used for shoulder reconstructions.
Subsequent bioabsorbable anchors have been manufactured
of a combination of polyglycolic acid and polylactic acid, resulting in slower
hydrolyzation and added strength. A newer derivation of polylactic acid,
poly-L-lactic acid (PLLA), has a lengthened half-life compared to polyglycolic
acid/polylactic acid copolymers (7-14 days).4
Figure 1: Statak metallic anchor on the left and Bio-Statak PLLA anchor on the right.
Many studies have compared pullout strength in various
types of bone, including cadaver and fresh animal specimens.1,5-8
These measurements have typically been done at one time without dynamically
loading the implants prior to testing. Clinical studies have demonstrated the
efficacy of some bioabsorbable implants in shoulder stabilization and rotator
cuff repairs.9-11 The purpose of this study was to evaluate the mode
of failure and biomechanical characteristics of PLLA bioabsorbable and metallic
suture anchors of similar size and dimension that had been implanted
intra-articularly for either 0, 6, or 12 weeks prior to testing. The hypothesis
was that bioabsorbable anchors would perform as well as metallic anchors at
each time interval. In addition, we sought to evaluate the osteointegration of
Materials and Methods
We chose 5.0 mm metallic (Statak; Zimmer, Warsaw,
Indiana) and 5.0 mm bioabsorbable PLLA (Bio-Statak; Zimmer) anchors. These 2
implants are of similar size and dimensions, differing only in their material
composition (Table; Figure 1). This would allow us to better draw conclusions
about the differences between PLLA bioabsorbable and metallic anchors.
All procedures were approved by the institutions
animal care and use committee, and all animal handling was completed in an
Association for Assessment and Accreditation of Laboratory Animal Care-approved
facility using National Institutes of Health animal care guidelines. All
research animals were quarantined for 21 days, tested for infectious diseases,
and examined by a veterinarian prior to commencement of the protocol.
Both hind limbs were operated on simultaneously by the
same surgeons (C.R.B., T.M.D.). The lateral femoral condyle and the origin of
the long digital extensor tendon were exposed through a 5-cm incision over the
lateral aspect of the knee joint. The long digital extensor tendon is a unique
structure in quadrupeds. The tendon originates from the lateral femoral condyle
as an intra-articular structure. It then passes though the inferior lateral
joint capsule to insert on the dorsum of the digits. Once exposed, the lateral
half of the tendon was sharply resected from its origination from the lateral
condyle and then bluntly separated from the rest of the tendon for a length of
A metallic or PLLA bioabsorbable suture anchor was
inserted into the footprint of the resected tendon. The suture anchor insertion
protocol recommended by the manufacturer was used. The Statak anchor hole was
predrilled, and the self-tapping anchor was then inserted. The Bio-Statak
anchor was inserted after drilling and tapping of the hole with the
instrumentation provided in the manufacturers kit. The tendon was then
anatomically reattached using the anchor and the prepackaged braided #2
polyethylene suture in a running Krackow suture pattern. For histological
analysis of biointegration, an additional PLLA bioabsorbable implant was
inserted into the femoral condyle 5 cm proximal to the footprint of the long
digital extensor tendon.
The arthrotomy was closed in layers followed by nylon
skin closure. A sterile gauze dressing with bacitracin ointment was applied to
the wound. The animals were subsequently transferred to a holding pen until
fully awake and independently ambulatory. This provided dynamic loading of the
anchor as the animal ambulated. Following an observation period of 7 days, and
given satisfactory healing of the surgical sites, the animals were allowed to
return to pasture until the time of harvest.
At 0 (n=26), 6 (n=26) or 12 (n=26) weeks, the animals
were humanely killed, and biomechanical testing was performed on the suture
anchor constructs. The femora were stripped of soft tissue and the previously
placed sutures released from the long digital extensor tendon. Each femur was
mounted in a custom jig to allow an axis of pull that was parallel to the axis
of anchor insertion. The suture tails were secured with a custom jig that
allowed circumferential wrapping of the sutures that prevented the development
of a stress riser at the suturejig attachment site (Figure 2). The anchor
constructs were pulled to failure at a rate of 50 mm/min using an Instron 8521S
materials testing machine (Instron Corporation, Canton, Massachusetts) until
the suture anchor construct failed. The load at failure was recorded and
graphed, and the location of failure, whether within the suture or at the
anchor eyelet, was recorded. Histological evaluation was then performed on the
anchors and the surrounding bone in which they were implanted to evaluate the
Figure 2: Mechanical testing jig setup.
Histologic processing for subsequent microscopic
analysis was performed after mechanical testing was completed. All specimens
containing bioabsorbable screws were rough-cut in the area of the screw using a
Stryker saw (Mahwah, New Jersey) and then decalcified in 12% formic acid.
Following complete decalcification, the remaining section of screw was removed,
and the specimen was processed in paraffin, sectioned at 5 microns, and stained
Each specimen was evaluated histologically for evidence
of biointegration of the screw, whether the integration was primarily bone
(osteointegration) or fibrous connective tissue, and for evidence of
inflammation in the test area. Integration was judged as complete if there was
direct bone/soft tissue-to-implant contact along the entire surface of the
screw, filling in the thread grooves. In 4 specimens there was insufficient
tissue/screw interface present on the slide to evaluate osteointegration;
therefore, a total of 42 specimens were evaluated histologically. To assess the
biointegration of the metallic implants, femoral condyles from 3 animals were
processed in plastic using the Exakt technique (Oklahoma City, Oklahoma) and
then evaluated histologically for comparison.
Data were compiled in Excel (Microsoft Corp, Redmond,
Washington) and analyzed using SPSS software (SPSS Inc, Chicago, Illinois). A
within-subjects design was used with the 6- and 12-week groups. The 0-week
group was analyzed using a between-groups analysis of variance. Frequency data
were analyzed using chi square tests. Alpha values were set at 0.05 for all
statistical tests and pair-wise comparisons. Specimens were divided into 6
groups (n=13 knees per group) based on how long the anchor was in place and
what type of anchor was implanted. The force required to break the
sutureanchor construct (or pull out the construct from the implanted
bone) was analyzed using a 3×2 (duration of implant × type of
anchor) analysis of variance.
The sutureanchor constructs failed in 3 ways. Of
the 78 specimens, 8% of the anchors failed by pulling out of bone, 65% of the
sutures broke at the anchor eyelet, and 27% of the sutures broke at the
midsubstance of the suture. Therefore, the majority of all anchors tested (92%)
failed mechanical testing via suture breakage. None of the constructs failed at
the attachment of the sutures to the custom jig. Comparing the failure
mechanism of the metallic and bioabsorbable constructs, 90% of the metallic
anchors sutures failed at the eyelet of the anchor, while only 41% of the
PLLA bioabsorbable anchors sutures failed at the eyelet
No anchors failed by pullout of bone during testing of
the constructs at 0 weeks. Only 1 anchor (PLLA) pulled out of its implanted
bone at 6 weeks. However, the force required to pull out this anchor (150.8 N)
was >97% of the average force required to break all the metallic constructs
in the 6-week group. Five anchors failed by pullout at 12 weeks (4 PLLA
bioabsorbable and 1 metallic). The longer the construct was implanted, the
easier it was to break, regardless of which type of construct it was (all
Significantly greater force was required to break the
PLLA bioabsorbable construct than the metallic construct at 0 weeks (means,
227.17 N and 176.40 N, respectively; P=.001) and at 6 weeks (means,
199.95 N and 154.87 N, respectively; P=.001). At 12 weeks, there was no
significant difference in the force to failure between the 2 groups (means,
159.19 N and 135.20 N, respectively; P>.05).
Histological analysis of the boneanchor interface
for all 3 metallic and 36 of 42 PLLA bioabsorbable anchors demonstrated equally
good osteointegration without evidence of osteolysis. There was no gross or
histological evidence of septic osteomyelitis in any of the specimens. Minimal
to no inflammation was observed around the absorbable screw in all but 2
animals. Mild to moderate inflammation was observed around the absorbable
screws in 2 animals. Inflammation consisted primarily of lymphocytes and plasma
cells with macrophages and occasional giant cells, suggesting an immunologic
response to the test material. In 2 specimens, a clear birefringent fibrillar
to specular material, most likely representing the breakdown product of PLLA
from the implant, was observed within the cytoplasm of phagocytic cells
surrounding the test site.
Figure 3: Photograph of sectioned anchorbone complex showing osteointegration around bioabsorbable implant at 12 weeks.
Complete biointegration of the PLLA screw by bone and
fibrovascular connective tissue (Figure 3) occurred in the majority of
specimens (36 of 42, or 85.7%). All 3 metal specimens showed complete
integration. In sections where the absorbable screws were in contact with the
epiphyseal growth plate of the distal femur, integration often included
cartilage as well as bone.
Suture anchors are commonly used to reattach ligaments
and tendons to bone. To restore normal strength to ligament or tendon injuries,
active motion instead of immobilization has been shown to be beneficial. During
the healing phase, the attachment of these soft tissues must be strong enough
to withstand the physiological loads imparted through these tendons or
ligaments. The strength of the sutureanchor construct is determined by 3
factors: 1) the pullout strength of the anchor, 2) the type and density of the
bone in which the anchor device is implanted, and 3) the strength of the suture
and its attachment to the soft tissue.
The first anchors commercially available were metallic.
These anchors provide good fixation strength that, in most cases, resists
forces to allow early motion, increasing the chances for successful healing.
However, the use of metallic anchors has also presented some problems.
Zuckerman and Matsen3 reported on complications related to metallic
implants in a series of 37 patients. They found that the metallic screws or
staples were inserted incorrectly in 10 patients and had migrated or loosened
in 24 patients. Metallic anchors, once inserted, are not easily retrieved, and
therefore are typically retained for the life of the patient. Revision surgery
can also be complicated by the retained hardware. Radiographic visualization
can be obstructed by retained metallic implants. Additionally, metallic
artifact during MRI can degrade the image quality and prevent adequate
visualization of the soft tissues around a joint.
The ideal suture anchor would be a device that is easily
inserted, has adequate construct strength to allow early active motion, and is
slowly replaced by host bone to allow restoration of the normal kinematic
relationship of the bonesoft tissue attachment. Early bioabsorbable
implants were made of polyglycolic acid. Their absorption profile was designed
to parallel the expected healing rates of soft tissue to bone. At 6 weeks, the
polyglycolic acid loses >80% of its fixation strength.12 This was
considered inadequate for early motion; therefore, prolonged shoulder
immobilization was required after use of the polyglycolic acid Suretac (Smith
& Nephew, Memphis, Tennessee).
Newer polymers, including polylactic acid and its
stereoisomer, PLLA, resulted in longer absorption profiles and stronger initial
strength.13 These polymers are degraded through hydrolysis. No
active or passive degradation processes are required.14 Concerns of
host reactivity, the effect of bioabsorbable materials on the surrounding bone,
and strength degradation precluding early motion have prevented their
The caprine long digital extensor model has been used
previously by our group to evaluate suture anchors.15 The long
digital extensor of quadrupeds originates from the lateral femoral condyle,
allowing the animal enhanced ankle dorsiflexion during knee flexion. Rodeo et
al16 used the long digital extensor of dogs to assess the soft
tissue-to-bone healing capacity when they resected the tendon and inserted it
into an extra-articular tunnel created in the proximal tibia. Our model is
designed to evaluate the intra-articular healing of a tendon to bone after
reattachment of the tendon using a suture anchor while maintaining
physiological loading. Reproducing an intra-articular environment is of
particular importance during evaluation of the bioabsorbable anchor as the
synovial fluid environment may affect the absorption of bioabsorbable anchors
and thus weaken the construct. Additionally, the physiological loads borne by
the sutureanchor construct resulted in a consistently decreased load to
failure with an increase in the time of implantation.
To eliminate the potential variation between animals, we
used 1 knee (stifle) from each animal to implant the metallic or PLLA
bioabsorbable anchor. Since it is difficult to immobilize goats following
surgery, the anchor must be able to withstand the forces imparted through the
resected tendon at the sutureanchor attachment site. We chose to use half
of the long digital extensor tendon, which allowed the anchors to be
physiologically loaded without having to bear the entire force of the tendon as
the animal ambulated. The constructs were tested at 3 time periods that
paralleled their clinical usage. The 0-week evaluation allowed for a baseline
comparison of the fixation strength of both types of anchors without any
physiological contribution. The 6-week evaluation corresponds with the time at
which active motion is typically allowed in shoulder or knee surgery following
soft tissue repairs. The 12-week evaluation corresponds with the time at which
full, unrestricted activity is allowed following most soft tissue repairs.
Although healing of soft tissue back to bone would be expected, we wanted to
compare the residual strength of the sutureanchorbone constructs.
During biomechanical testing, the sutureanchor
constructs were pulled parallel to the axis of insertion of the anchor. Failure
was noted as pullout of the anchor or suture breakage. Additionally, the
location of suture failure was noted in these cases. The attachment of the
suture tails to the servohydraulic machine was through a custom jig that
allowed the suture tails to be wrapped around a smooth metallic post. This
prevented a concentration of the forces at the suture attachment site. No cases
of suture breakage at the attachment of the tails to the custom jig were
reported. The majority (92%) of all sutureanchor constructs tested failed
by suture breakage. This suggests that the suture is the weakest part of the
constructs and both anchor types have adequate initial and subsequent pullout
strength to maintain soft tissue-to-bone fixation. In contrast, however, the
majority (90%) of all metallic anchors failed suture breakage at the eyelet.
This is in comparison to 41% of PLLA bioabsorbable anchor failures occurring at
the suture eyelet.
It is important to note the absence of a significant
inflammatory reaction to these PLLA bioabsorbable implants. We noted a mild to
moderate inflammatory response in only 2 of 36 implants evaluated. This absence
of a foreign body reaction or excessive inflammatory reaction has been noted
with other PLLA implants.17
The suture was the weakest link in most of the anchor
constructs tested. Compared to metallic anchors, PLLA bioabsorbable anchor
constructs required more force to failure at all time points, with the 0- and
6-week points being statistically significant differences. No difference was
noted in the effect that either anchor had on the surrounding bone.
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Dr Bottoni is from Aspetar Orthopaedic and Sports Medicine Hospital,
Doha, Qatar; Messrs Brooks and Judson and Drs Owens, Eggers, and Mays are from
the US Army Institute of Surgical Research, Ft Sam Houston, Texas; and Dr
DeBerardino is from Keller Army Hospital, West Point, New York.
Drs Bottoni, DeBerardino, Owens, Eggers, and Mays and
Messrs Brooks and Judson are employees of the US government and received no
grant funding for this study.
The views expressed herein are those of the authors and
do not reflect the official policy or position of the Department of Defense or
United States government.
Correspondence should be addressed to: Craig R. Bottoni,
MD, Aspetar Orthopaedic and Sports Medicine Hospital, PO Box 29222, Doha,