Orthopedics

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The Cutting Edge 

A Comparison of Bioabsorbable and Metallic Suture Anchors in a Dynamically Loaded, Intra-articular Caprine Model

Craig R. Bottoni, MD; Daniel E. Brooks, BS; COL Thomas M. DeBerardino, MD; MAJ Brett D. Owens, MD; 2LT Kurt L. Judson, MS; COL Jeffrey S. Eggers, DVM; Mary Z. Mays, PhD

  • Orthopedics. 2008;31(11)
  • Posted November 1, 2008

Abstract

The bioabsorbable suture anchor tested is safe for use in clinical practice without concerns for the strength of the construct or bony reaction to the material.

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

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 the implants.

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 institution’s 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…

The bioabsorbable suture anchor tested is safe for use in clinical practice without concerns for the strength of the construct or bony reaction to the material.

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
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 the implants.

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 institution’s 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.

  Table: Dimensions of Anchors

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 5 cm.

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 manufacturer’s 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 suture–jig 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 osteointegration.

Histologic Analysis

Figure 2: Mechanical testing jig setup
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 with hematoxylin-eosin.

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 Analysis

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 suture–anchor 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.

Results

The suture–anchor 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 (P<.001).

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 comparisons, P<.05).

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 bone–anchor 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 anchor–bone complex showing osteointegration around bioabsorbable implant at 12 weeks
Figure 3: Photograph of sectioned anchor–bone 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.

Discussion

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 suture–anchor 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 bone–soft 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 widespread use.

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 suture–anchor 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 suture–anchor 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 suture–anchor–bone constructs.

During biomechanical testing, the suture–anchor 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 suture–anchor 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

Summary

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.

References

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  3. Zuckerman JD, Matsen FA III. Complications about the glenohumeral joint related to the use of screws and staples. J Bone Joint Surg Am. 1984; 66(2):175-180.
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  12. Demirhan M, Kilicoglu O, Akpinar S, Akman S, Atalar AC, Göksan MA. Time-dependent reduction in load to failure of wedge-type polyglyconate suture anchors. Arthroscopy. 2000; 16(4):383-390.
  13. Blasier RD, Bucholz R, Cole W, Johnson LL, Mäkelä EA. Bioresorbable implants: applications in orthopaedic surgery. Instr Course Lect. 1997; (46):531-546.
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  15. Dejong ES, DeBerardino TM, Brooks DE, Judson K. In vivo comparison of a metal versus a biodegradable suture anchor. Arthroscopy. 2004; 20(4):511-516.
  16. Rodeo SA, Arnoczky SP, Torzilli PA, Hidaka C, Warren RF. Tendon-healing in a bone tunnel: a biomechanical and histological study in the dog. J Bone Joint Surg Am. 1993; 75(12):1795-1803.
  17. Kilicoglu O, Demirhan M, Akman S, Atalar AC, Ozsoy S, Ince U. Failure strength of bioabsorbable interference screws: effects of in vivo degredation for 12 weeks. Knee Surg Sports Traumatol Arthrosc. 2003; 11(4):228-234.

Authors

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, Qatar.

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