Bearing area: a new indication for suture anchor pullout strength?

Risk Factors and Corresponding Management for Suture Anchor Pullout during Arthroscopic Rotator Cuff Repair

INTRODUCTION

Due to the aging of the population, the incidence of rotator cuff tears is growing. For rotator cuff repair, arthroscopic suture-anchor repair has gradually replaced open transosseous repair, so suture anchors are now considered increasingly important in rotator cuff tear reconstruction. There are some but limited studies of suture anchor pullout after arthroscopic rotator cuff repair. However, there is no body of knowledge in this area, which makes it difficult for clinicians to predict the risk of anchor pullout comprehensively and manage it accordingly.

METHODS

The literature search included rotator cuff repair as well as anchor pullout strength. A review of the literature was performed including all articles published in PubMed until September 2021. Articles of all in vitro biomechanical and clinical trial levels in English were included. After assessing all abstracts (n = 275), the full text and the bibliographies of the relevant articles were analyzed for the questions posed (n = 80). Articles including outcomes without the area of interest were excluded (n = 22). The final literature research revealed 58 relevant articles. Narrative synthesis was undertaken to bring together the findings from studies included in this review.

RESULT

Based on the presented studies, the overall incidence of anchor pullout is not low, and the incidence of intraoperative anchor pullout is slightly higher than in the early postoperative period. The risk factors for anchor pullout are mainly related to bone quality, insertion depth, insertion angle, size of rotator cuff tear, preoperative corticosteroid injections, anchor design, the materials used to produce anchors, etc. In response to the above issues, we have introduced and evaluated management techniques. They include changing the implant site of anchors, cement augmentation for suture anchors, increasing the number of suture limbs, using all-suture anchors, using an arthroscopic transosseous knotless anchor, the Buddy anchor technique, Steinmann pin anchoring, and transosseous suture repair technology.

DISCUSSION

However, not many of the management techniques have been widely used in clinical practice. Most of them come from in vitro biomechanical studies, so in vivo randomized controlled trials with larger sample sizes are needed to see if they can help patients in the long run.

Biomechanical analysis of bioabsorbable suture anchors for rotator cuff repair using osteoporotic and normal bone models

BACKGROUND

This study aimed to compare the failure load of suture anchors used in rotator cuff repair between normal and osteoporotic bone models.

METHODS

A total of 16 anchors made from metal (TwinFix Ti 5.0 or 6.5 mm, Corkscrew FT 4.5, 5.5, or 6.5 mm), polyether ether ketone (HEALICOIL PK [HC-PK] 4.5 or 5.5 mm, SwiveLock PK 4.75 or 5.5 mm), or bioabsorbable material (HEALICOIL RG [HC-RG] 4.75 or 5.5 mm, Corkscrew Bio 4.75, 5.5, or 6.5 mm, SwiveLock BC 4.75 or 5.5 mm) were included. Moreover, 10- and 5-pounds per cubic foot (pcf) Sawbone® models were set as normal and osteoporotic cancellous bone models, respectively. Pullout testing was performed in parallel to the insertion axis at a displacement rate of 12.5 mm/s using a universal testing machine. To evaluate the change in failure load between the two Sawbone® models with different densities, the remaining failure load ratio (RFLR) was defined as the ratio of the failure load in 10 pcf to that in 5 pcf.

RESULTS

In the 10-pcf Sawbone®, TwinFix Ti 6.5 mm showed the highest mean failure load (304.0 ± 15.2 N). In the 5-pcf Sawbone® model, HC-PK 5.5 mm showed the highest failure load (146.3 ± 5.8 N). Among anchors with the same diameter, HC-PK and HC-RG showed a significantly higher failure load than other anchors in the 10- and 5-pcf Sawbone® models. HC-PK 5.5 mm (62.1%) and HC-PK 4.5 mm (51.1%) have the highest RFLR among anchors with the same diameter.

CONCLUSIONS

HC-PK and HC-RG showed higher failure load than the other anchors in both normal and osteoporotic bone models, except for TwinFix Ti 6.5 mm in the 10-pcf Sawbone® model. Based on our results, bioabsorbable anchors had sufficient failure load for rotator cuff repair in addition to bioabsorbability.

Arthroscopic Suture Anchor Design Finite Element Study

AIM: This in-vitro study investigated arthroscopic suture anchors’ main design parameters effect on surrounding bone. METHODS: Thirty-dimensional arthroscopic suture anchor designs’ models were created on engineering CAD software by changing thread profile, pitch, and anchor tip profile as design parameters. These models were imported into ANSYS Workbench for finite element analysis. Bone was simplified and modeled as two coaxial cylinders. Tensile vertical load of 300 N, and oblique at 45º to the vertical axis, were applied to each model as two loading conditions while the simplified bone base was fixed in place as a boundary condition. RESULTS: The finite element analyses on all models under both loading conditions showed stresses within physiological limits on bone. Trapezoidal teeth and inclined cut teeth designs showed the lowest values of stresses and deformations respectively on the bone under oblique loads, while curved tooth and square tooth designs showed the lowest values of stresses and deformations respectively on the bone under vertical loads. General ascending or descending trend was recorded by increasing pitch from 1.2 to 1.5 to 1.8 mm on the total deformation and maximum Von Mises stress on bone and anchor body. Tapered tip slightly increased bone and anchor stresses. CONCLUSION: Arthroscopic anchors thread profile has minor affect on cortical bone behavior. Trapezoidal teeth, square tooth, and inclined cut teeth profiles showed the lowest values of stresses and deformations on cortical bone. Increasing thread pitch of arthroscopic suture anchors increases or decreases stress on the bone, and anchor body according to thread profile edges. Anchor tip profile negligibly affects both deformations and stresses on bone and anchor body.

Undercut macrostructure topography on and within an interbody cage improves biomechanical stability and interbody fusion

BACKGROUND CONTEXT

The interface and interactions between an interbody cage, graft material, and host bone can all participate in the fusion. Shortcomings of Poly(aryl-ether-ether-ketone) interbody cages have been addressed with novel titanium surfaces. Titanium surfaces paired with macroscale topography features on the endplates and within the aperture may provide additional benefits.

PURPOSE

To evaluate the influence of cage design parameters on interbody fusion in a large animal preclinical model.

STUDY DESIGN/SETTING

A comparative preclinical large animal model was performed to evaluate how macroscale topography features of an interbody cage can facilitate early integration between the host bone, graft material, and interbody cage and these effects on biomechanical stability and fusion.

METHODS

Forty single level interbody fusions (L4-L5) using iliac crest autograft and bilateral pedicle screw fixation were performed in adult sheep to evaluate the effect of undercut macrostructure topography features of an interbody cage on the endplates and within the aperture. Fusions were evaluated at 6 and 12 weeks (n=10 per group) using radiography, microcomputed tomography, biomechanical integrity, and histology endpoints.

RESULTS

The presence of the undercut macrostructures present on the endplates and within the aperture statistically improved biomechanical integrity at 6 and 12 weeks compared with controls. Microcomputed tomography and histology demonstrated bony interdigitation within the endplate and aperture features contributing to the improvement in properties.

CONCLUSIONS

The present study demonstrates that Poly(aryl-ether-ether-ketone) implants with titanium surfaces can be augmented by undercut macrostructures present on the endplates and within the aperture to provide opportunities for a series of anchoring points that, with new bone formation and remodelling, result in earlier and improved biomechanical integrity of the treated level.

CLINICAL SIGNIFICANCE

This preclinical study showed that bone interdigitation with the undercut macrostructures present on the endplates and within the aperture resulted in improved fusion and biomechanical stability in a clinically relevant spinal fusion model. Future clinical study is warranted to evaluate such implants' performance in humans.

Mechanical Performance of Metallic Bone Screws Evaluated Using Bone Models

To evaluate mechanical performance properties of various types of cortical bone screw, cancellous bone screw, and locking bolt, we conducted torsional breaking and durability tests, screw driving torque tests into bone models, and screw pullout tests (crosshead speed: 10 mm/min) after driving torque tests. The 2° proof and rupture torques of a screw, which were estimated from torque versus rotational angle curves, increased with increasing core diameter of the screw. The durability limit of metallic screws obtained by four-point bending durability tests increased with increasing core diameter. The compressive, tensile, and shear strengths of the bone models used for the mechanical testing of orthopedic devices increased with increasing density of the bone model. The strength and modulus obtained for solid rigid polyurethane foam (SRPF) and cellular rigid polyurethane foam (CRPF) lay on the same straight line. Among the three strengths, the rate of increase in compressive strength with the increase in density was the highest. The maximum torque obtained by screw driving torque tests for up to 8.3 rotations (3000°) into the bone models tended to increase with increasing core diameter. In particular, the maximum torque increased linearly with increasing effective surface area of the screw, as newly defined in this work. The maximum pullout load increased linearly with increasing number of rotations and mechanical strength of the bone model. Screws with low driving torque and high pullout load were considered to have excellent fixation and are a target for development.

Effects of Surface Topography and Chemistry on Polyether-Ether-Ketone (PEEK) and Titanium Osseointegration

STUDY DESIGN

An in vivo study examining the functional osseointegration of smooth, rough, and porous surface topographies presenting polyether-ether-ketone (PEEK) or titanium surface chemistry.

OBJECTIVE

To investigate the effects of surface topography and surface chemistry on implant osseointegration.

SUMMARY OF BACKGROUND DATA

Interbody fusion devices have been used for decades to facilitate fusion across the disc space, yet debate continues over their optimal surface topography and chemistry. Though both factors influence osseointegration, the relative effects of each are not fully understood.

METHODS

Smooth, rough, and porous implants presenting either a PEEK or titanium surface chemistry were implanted into the proximal tibial metaphyses of 36 skeletally mature male Sprague Dawley rats. At 8 weeks, animals were euthanized and bone-implant interfaces were subjected to micro-computed tomography analysis (n = 12), histology (n = 4), and biomechanical pullout testing (n = 8) to assess functional osseointegration and implant fixation.

RESULTS

Micro-computed tomography analysis demonstrated that bone ingrowth was 38.9 ± 2.8% for porous PEEK and 30.7 ± 3.3% for porous titanium (P = 0.07). No differences in fixation strength were detected between porous PEEK and porous titanium despite titanium surfaces exhibiting an overall increase in bone-implant contact compared with PEEK (P < 0.01). Porous surfaces exhibited increased fixation strength compared with smooth and rough surfaces regardless of surface chemistry (P < 0.05). Across all groups both surface topography and chemistry had a significant overall effect on fixation strength (P < 0.05), but topography accounted for 65.3% of the total variance (ω = 0.65), whereas surface chemistry accounted for 5.9% (ω = 0.06).

CONCLUSIONS

The effect of surface topography (specifically porosity) dominated the effect of surface chemistry in this study and could lead to further improvements in orthopedic device design. The poor osseointegration of existing smooth PEEK implants may be linked more to their smooth surface topography rather than their material composition.

LEVEL OF EVIDENCE

N/A.

Biomechanical evaluation of a new technique for acromioclavicular stabilization

BACKGROUND

The most commonly used repair techniques to treat an acromioclavicular dislocation imply a suspension mechanism by substituting the supero-inferior oriented coracoclavicular structures with a tight rope mechanism or allograft. Recently, the importance of restoring the antero-posterior stability by addressing the acromioclavicular structures has also been demonstrated. If an in situ repair at the acromioclavicular joint itself could achieve a reposition and would be strong enough, the suspension of the CC structures might become obsolete. Possible advantages would be minimal dissection, lower risk in damaging neurovascular structures, greater stability, reduction of the surgical time and even the possibility of locoregional anesthesia.

HYPOTHESIS

In this biomechanical study, the feasibility of different in situ repair techniques is explored thereby testing both compression and translation characteristics. Our hypothesis is that an in situ repair technique results in an adequate repair for the AC joint.

METHODS AND MATERIALS

Polyurethane foam blocks will be used as a model for the acromioclavicular joint and the repair techniques will be done by using a combination of sutures and bone anchors or using a transosseous technique. Compression will be measured by means of a Tekscan pressure sensor and translation will be tested in three orthogonal directions using a tensile testing machine. Four different knot anchor configurations (nice knot, surgical knot in two different configurations, Nicky's knot) will be tested for compression. The strongest knot anchor configuration will then be compared side to side with a transosseous configuration for translation.

RESULTS

The nice knot in combination with bone anchors provides the strongest compression. In the side to side comparison of a nice knot anchor configuration versus a transosseous nice knot configuration, the transosseous technique shows more resistance to translation.

DISCUSSION

An in situ repair by a combination of the nice knot with an anchor or a transosseous nice knot configuration can theoretically be used as a repair technique for an acromioclavicular dislocation. In comparison with existing techniques, this model shows favorable results for translation.

LEVEL OF EVIDENCE

III, controlled laboratory study.

What Makes Suture Anchor Use Safe in Hip Arthroscopy? A Systematic Review of Techniques and Safety Profile

PURPOSE

To perform a systematic review that assesses the current literature on suture anchor placement for the purpose of identifying factors that lead to suture anchor perforation and techniques that reduce the likelihood of complications. It was hypothesized that suture anchor placement in hip arthroscopy would generally be safe, with the exception of the complications of articular cartilage violation and psoas tunnel perforation. Perioperative factors, related to patient, surgeon, and technical variables, may influence the safety of suture anchor insertion.

METHODS

Three databases (PubMed, Ovid MEDLINE, and Embase) were searched, and 2 reviewers independently screened the resulting literature. The inclusion criteria were clinical and biomechanical studies examining the use of suture anchors in hip arthroscopy. The methodologic quality of all included articles was assessed using the Methodological Index for Non-Randomized Studies criteria and the Cochrane risk-of-bias assessment tool. Results are presented according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines using descriptive statistics.

RESULTS

We included 14 studies in this review, comprising 4 case series (491 patients; 56.6% female patients; mean age, 33.9 years), 9 controlled cadaveric or laboratory studies (111 cadaveric hips and 12 synthetic acetabular bone blocks; 42.2% female hips; mean age, 60.0 years) with a mean Quality Appraisal for Cadaveric Studies score of 11, and 1 randomized controlled trial (37 hips; 55.6% female hips; mean age, 34.2 years). Anterior cortical perforation into the psoas tunnel by suture anchors led to pain and impingement of pelvic neurovascular structures. The anterior acetabular positions (3- to 4-o'clock position) had the thinnest bone, smallest rim angles, and highest incidence of articular perforation. Drilling angles from 10° to 20° measured off the coronal plane were acceptable. The midanterior and distal anterolateral portals were used successfully, with 1 study reporting difficulty placing anchors at anterior locations through the distal anterolateral portal. One study showed that curved suture anchor drill guides allow for a better trajectory away from the articular cartilage. Small-diameter (≤1.8-mm) all-suture anchors had a lower in vivo incidence of articular perforation with similar stability and pullout strength to other anchor types in biomechanical studies.

CONCLUSIONS

Suture anchors at anterior acetabular rim positions (3- to 4-o'clock position) should be inserted with caution. Large-diameter (≥2.3-mm) suture anchors increase the likelihood of articular perforation without increasing labral stability. Inserting small-diameter (≤1.8-mm) all-suture anchors from 10° to 20° drilling angles may increase safe insertion angles from all cutaneous portals. Direct arthroscopic visualization, the use of fluoroscopy, distal-proximal insertion, and the use of nitinol wire can help prevent articular violation.

LEVEL OF EVIDENCE

Level IV, systematic review of Level I to IV studies.

Finite Element Study On Arthroscopic Anchor Design Aspects

AIM:

This research aims to study arthroscopic anchors design parameters. Prototypes were manufactured by new parameters values. The the performance of the prototypes was also tested.

METHODS:

Five 3D arthroscopic anchor models were created to evaluate the role of some design aspects. Thread type, pitch and tip angle were tested as variable parameters. These models were produced on engineering CAD software then imported into ANSYS for finite element analysis. A tensile load of 300 N was applied to each model while the simplified bone base was fixed-in-place as a boundary condition. The finite element results were compared with prototypes tensile testing.

RESULTS:

The finite element analyses showed stresses within physiological limits on the bone with all tested models. Thread type and pitch affected stresses on bone and anchor body. From stress point of view, two critical zones appeared on anchor body, anchor cortical bone connection and eyelet zone, while thread geometry (depth) affect the cortical bone response only. Laboratory tests matched finite element results and literature.

CONCLUSION:

Increasing thread pitch of arthroscopic anchors decreases stress on the bone, while increases stress on anchor body. Arthroscopic anchors thread type has a negligible effect on bone, while it reduces stresses on anchor body if it placed more material around eyelet in internal drive mechanism and suture eyelet type of anchors. Anchor tip angle has a negligible effect on bone and anchor body.

Knotless Anchors in Acetabular Labral Repair: A Biomechanical Comparison

PURPOSE

To analyze the failure mechanism, stiffness, and pullout strength of acetabular knotless suture anchors.

METHODS

Seven suture anchors were tested in high-density (0.48 g/cc) synthetic blocks. The anchors were implanted perpendicular to the bone block. The anchor's suture(s) were tied around a loop of 8 high-strength nonabsorbable sutures and pulled in line with the anchor at a rate of 1 mm/s until failure. The following knotless anchors were tested: Stryker Knotilus 3.5, Arthrex Pushlock 2.9, Linvatec PopLok 2.8, Linvatec PopLok 3.3, ArthroCare SpeedLock HIP (3.4-mm), and Smith & Nephew Bioraptor Knotless 2.9. The standard knot tying Smith & Nephew Bioraptor 2.9 mm served as a baseline for comparison.

RESULTS

Stiffness was highest in the Pushlock, the SpeedLock HIP, and Knotilus. At 1 mm displacement, the SpeedLock HIP exhibited significantly higher load than all other anchors, excluding the Pushlock and PopLok 3.3 (P ≤ .012 for all comparisons). Excluding the SpeedLock HIP and Knotilus, the Pushlock displayed significantly higher load than all other anchors at 2-mm displacement (P ≤ .015 for all comparisons). Maximum load was the highest for the Knotilus and Bioraptor knotted anchor (P < .001 compared with all other anchors).

CONCLUSIONS

All knotless suture anchors used in hip arthroscopy, except for the Knotilus 3.5, failed by suture pullout from the anchor. The 2 anchors with the highest maximum load, the Knotilus 3.5 and knotted Bioraptor 2.9, failed by suture failure; however, these anchors displayed the lowest stiffness and load at 1 mm displacement among all anchors tested. Stiffness and loads at clinically relevant displacements, not maximum load alone, may be most important in predicting anchor clinical performance during the early phases of labral healing.

CLINICAL RELEVANCE

Knotless suture anchors tend to fail by suture pullout from the anchor, yet the stiffness of these constructs suggests that minimal displacement of the repair will occur under physiologic loads.

Effect of structural design on the pullout strength of suture anchors for rotator cuff repair

Various types of suture anchor designs are currently available for rotator cuff repair. The purpose of our study was to investigate the pullout strength of such anchors based on their structural design and the predominant geometric design factors affecting the pullout strength using finite element analysis. Finite element models were constructed using five cadaveric humeri and ten suture anchors with different designs. The pullout strength and distribution of bone stress around the anchor at three different directions of the applied force (0°, 45°, and 75°) were analyzed. The following geometric factors of suture anchor design were computed and their correlations with pullout strength assessed: Overall length, minor, and major diameters; number of threads; height of thread; distance between threads; helix angle; contact surface area between the anchor threads and surrounding bone; contact surface area between the cylindrical portion of the anchor; and surrounding bone and total contact surface area between the anchor and surrounding bone. The pullout strength and distribution of bone stress around the anchor varied according to the suture anchor designs and the direction of the applied force, respectively. The pullout strength had a strong positive correlation with the contact surface area between the anchor threads and surrounding bone, overall length, and the number and height of threads. This study demonstrated that suture anchor designs with increased contact surface area between the anchor threads and surrounding bone, overall length, and the number and height of threads can enhance the pullout strength during rotator cuff repair. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3318-3327, 2018.

Fixation Strength of Polyetheretherketone Sheath-and-Bullet Device for Soft Tissue Repair in the Foot and Ankle

Tendon transfers are often performed in the foot and ankle. Recently, interference screws have been a popular choice owing to their ease of use and fixation strength. Considering the benefits, one disadvantage of such devices is laceration of the soft tissues by the implant threads during placement that potentially weaken the structural integrity of the grafts. A shape memory polyetheretherketone bullet-in-sheath tenodesis device uses circumferential compression, eliminating potential damage from thread rotation and maintaining the soft tissue orientation of the graft. The aim of this study was to determine the pullout strength and failure mode for this device in both a synthetic bone analogue and porcine bone models. Thirteen mature bovine extensor tendons were secured into ten 4.0 × 4.0 × 4.0-cm cubes of 15-pound per cubic foot solid rigid polyurethane foam bone analogue models or 3 porcine femoral condyles using the 5 × 20-mm polyetheretherketone soft tissue anchor. The bullet-in-sheath device demonstrated a mean pullout of 280.84 N in the bone analog models and 419.47 N in the porcine bone models. (p = .001). The bullet-in-sheath design preserved the integrity of the tendon graft, and none of the implants dislodged from their original position.

Minimum Distance of Suture Anchors Used for Rotator Cuff Repair Without Decreasing the Pullout Strength: A Biomechanical Study

PURPOSE

To investigate the minimum distance between the anchors without decreasing the pullout strength using the polyurethane foams and the porcine bones.

METHODS

Metal screw-type anchors and PEEK (polyether ether ketone) coil-type anchors were used. Two same-type suture anchors were placed into the polyurethane foams and porcine bones. The polyurethane foams were 3 different densities simulating severe osteoporosis, osteoporosis, and normal bone. The distances between the centers of anchors were set at 4, 6, 8, and 10 mm. The pair of anchors were loaded to failure if they had not been pulled out after cyclic loading from 50 to 200 N, 10 cycles per each 50-N increment. Mode of failure, ultimate load to failure, displacement of the anchor, and number of cycles completed were recorded.

RESULTS

In all polyurethane foams of 3 different densities with use of metal screw-type anchors, the 4-mm group demonstrated a significantly lower ultimate load to failure compared with the 6-, 8-, and 10-mm groups (P < .01). There were no significant differences in the load to failure among the 6-, 8-, and 10-mm groups. Porcine bone or PEEK coil-type anchor showed results similar to those of the metal screw-type anchors.

CONCLUSION

For the 2 tested anchors, the minimum distance between the anchors without decreasing the pullout strength was 6 mm (center to center) regardless of bone density in a biomechanical study.

CLINICAL RELEVANCE

Although it has been thought that the minimum distance between the anchors without decreasing the pullout strength was 1 cm (center to center), our data showed that it was 6 mm.

All-Suture Anchors: Biomechanical Analysis of Pullout Strength, Displacement, and Failure Mode

PURPOSE

To evaluate the biomechanical and design characteristics of all-suture anchors.

METHODS

All-suture anchors were tested in fresh porcine cortical bone and biphasic polyurethane foam blocks by cyclic loading (10-100 N for 200 cycles), followed by destructive testing parallel to the insertion axis at 12.5 mm/s. Endpoints included ultimate failure load, displacement at 100 and 200 cycles, stiffness, and failure mode. Anchors tested included JuggerKnot (1.4, 1.5, and 2.8), Iconix (1, 2, and 3), Y-knot (1.3, 1.8, and 2.8), Q-Fix (1.8 and 2.8), and Draw Tight (1.8 and 3.2).

RESULTS

The mean ultimate failure strength of the triple-loaded anchors (564 ± 42 N) was significantly greater than the mean ultimate failure strength of the double-loaded anchors (465 ± 33 N) (P = .017), and the double-loaded anchors were stronger than the single-loaded anchors (256 ± 35 N) (P < .0001). No difference was found between the results in porcine bone and biphasic polyurethane foam. None of these anchors demonstrated 5 mm or 10 mm of displacement during cyclic loading. The Y-Knot demonstrated greater displacement than the JuggerKnot and Q-Fix (P = .025) but not the Iconix and Draw Tight (P > .05). The most common failure mode varied and was suture breaking for the Q-Fix (97%), JuggerKnot (81%), and Iconix anchors (58%), anchor pullout with the Draw Tight (76%), whereas the Y-Knot was 50% suture breaking and 50% anchor pullout.

CONCLUSIONS

The ultimate failure load of an all-suture anchor is correlated directly with its number of sutures. With cyclic loading, the Y-Knot demonstrated greater displacement than the JuggerKnot and Q-Fix but not the Iconix and Draw Tight. JuggerKnot (81%) and Q-Fix (97%) anchors failed by suture breaking, whereas the Draw Tight anchor failed by anchor pullout (76%).

CLINICAL RELEVANCE

All-suture anchors vary in strength and performance, and these factors may influence clinical success. Biphasic polyurethane foam is a validated model for suture anchor testing.

Cyclic and Load to Failure Properties of All-Suture Anchors in Synthetic Acetabular and Glenoid Cancellous Bone

PURPOSE

To evaluate the cyclic displacement, maximum load to failure, and failure mode of multiple all-suture anchors (ASAs) in 2 different densities of sawbones cancellous bone substitute.

METHODS

Anchors tested included the Suturefix Ultra 1.7 mm, JuggerKnot 1.45 mm (No. 1 and No. 2 MaxBraid) and 2.9 mm, Y-Knot Flex 1.3 mm and 1.8 mm, Iconix 1, 2, 25, and 3, Q-Fix 1.8 mm, and Bioraptor 2.3 PK. The Bioraptor served as a non-all-suture-based control. Seven to eleven anchors were tested in both 20 and 30 pounds per cubic foot (pcf) test blocks that were chosen to simulate glenoid and acetabular cancellous bone, respectively. After a 40 N deployment force, anchors were cyclically loaded at 0.5 Hz from 10 to 50 N and then 10 to 100 N for 200 cycles each. Surviving specimens were pulled to failure at 10 mm/s. Displacement, stiffness, maximum load, and failure mode were recorded. Welch t-tests and Welch analysis of variance with Games-Howell post hoc tests were used for statistical analysis.

RESULTS

In higher density blocks, 11 of 12 anchors had significantly (P < .05) higher maximum loads to failure, and 8 anchors showed significantly lower post-cyclic displacement. The Q-Fix 1.8 displayed the lowest post-cyclic displacement in both densities (0.1 ± 0.2 mm, mean ± standard deviation, in both densities). All other groups exhibited at least 2.8 mm and 0.6 mm post-cyclic displacement in 20 and 30 pcf, respectively. The Bioraptor did not survive cyclic testing in 20 pcf and had 0.6 ± 0.3 mm post-cyclic displacement in 30 pcf.

CONCLUSIONS

ASAs show better fixation in higher density synthetic bone. The cyclic displacement and maximum load of ASAs vary widely depending on anchor design and bone density. Most anchors fail by suture anchor pullout. In general, the Bioraptor 2.3 PK outperformed ASAs in higher density test blocks with mixed results in lower density test blocks.

CLINICAL RELEVANCE

ASAs show mixed results compared with a traditional suture anchor. They perform better in higher density bone substitute.

Bone Anchors—A Preliminary Finite Element Study of Some Factors Affecting Pullout

Bone anchors (or suture anchors) are used to provide attachment points for sutures to connect tissue such as tendons or ligaments to bone, and work by engaging a threaded portion—sometimes tapered—to the cancellous and/or cortical bone. Such repair is often needed after trauma, or as part of reconstructive surgery. This paper uses the finite element method to compare the pullout characteristics of one common type of bone anchor in different cancellous bone structures. Finite element models are created by using computed tomography (CT) scans of cancellous bone and building computer-aided design (CAD) models to define the cancellous bone geometry. Orthopedic surgeons will sometimes remove parts of the cortical shell and this paper also examines the mechanical effects of decortication. Furthermore, the importance of the connection between anchor and cortical layer is examined. One of the key outcomes from the model is that the coefficient of friction between bone and anchor determines potential mechanisms of pullout. The stiffness of anchors and the effect of the cortical layer are presented for different pullout angles to obtain the theoretical response. The results show the detailed modeling that includes the micro-architecture of the cancellous bone is necessary to capture the large variations that can exist.

The effect of insertion angle on the pullout strength of threaded suture anchors: a validation of the deadman theory

PURPOSE

To determine the effect of insertion angle, from 45° to 135° in 15° increments, on the number of cycles withstood, the ultimate pullout strength, and the stiffness of threaded suture anchors subjected to load.

METHODS

Threaded anchors were inserted into polyurethane foam at angles from 45° to 135°, in 15° increments, relative to the direction of pull. Five anchors were tested at each angle. The anchors were first cycled for 30 cycles (10 each at 100 N, 150 N, and 200 N). The surviving specimens were then tensioned to failure. The McNemar test was used to compare cyclic failure rates. Paired-samples t tests were used to compare load-to-failure (LTF) and stiffness data. All P values are multiplicity adjusted by the Hommel procedure.

RESULTS

Four of 5 anchors inserted at 45° failed during cyclic testing at a mean of 27 cycles (P = .13). One of 5 anchors placed at 60° failed after 29 cycles (P = .99). All other anchors survived cyclic testing. Mean LTF was 234 N, 243 N, 297 N, 373 N, 409 N, 439 N, and 417 N at insertion angles of 45°, 60°, 75°, 90°, 105°, 120°, and 135°, respectively. LTF was significantly less for the 60° group when compared with the 90°, 105°, 120°, and 135° groups (P < .05). LTF was significantly less for the 75° group when compared with the 105°, 120°, and 135° groups (P < .05). For the 90° group, LTF was only significantly less when compared with the 135° group (P = .022). The differences in LTF between the 105°, 120°, and 135° groups were not significant. Stiffness increased from 28.13 N/mm at 90° to 43.4 N/mm at 105° (P = .03), 61.48 N/mm at 120° (P = .003), and 86.83 N/mm at 135° (P = .008).

CONCLUSIONS

Anchors placed at more acute angles, that is, anchors placed closer to the so-called deadman's angle, failed at lower loads and provided less construct stiffness than anchors placed at angles greater than 90°. Stiffness also increased sequentially from an angle of insertion of 90° up to our maximum angle tested of 135°. For threaded metallic suture anchors, an obtuse insertion angle of 90° to 135° in relation to the line of pull of the suture and rotator cuff withstands a greater LTF and provides a stiffer construct than the more acute insertion angle advocated by the "deadman theory."

CLINICAL RELEVANCE

This study offers a biomechanical validation for optimal placement of threaded suture anchors at an angle of 90° or more, as anatomic restraints allow, from the vector of pull of the attached suture and rotator cuff, rather than the 45° angle recommended by the deadman theory.

Anterior cruciate ligament fixation: is radial force a predictor of the pullout strength of soft-tissue interference devices?

BACKGROUND

In anterior cruciate ligament (ACL) reconstruction, an interference device achieves soft-tissue graft fixation by radially compressing the graft against the bone.

PURPOSE

The objective of this study was to measure the radial force generated by different interference devices and evaluate the effect of this radial force on the pullout strength of graft-device constructs.

STUDY DESIGN

Controlled laboratory study.

METHODS

A resultant force (F(R)) was used as a representative measure of the total radial force generated. Bovine tendons were fixated in either synthetic bone or porcine tibia using one of following devices: (1) RCI titanium screw, (2) PEEK screw, (3) IntraFix sheath-and-screw device, and (4) ExoShape sheath-and-insert device. F(R) was measured while each device was inserted into synthetic bone mounted on a test machine (n=5 for each device). In a subsequent test series, graft-device constructs were loaded to failure at 50mm/min. The pullout strength was measured as the ultimate load before failure (n=10 for each device).

RESULTS

The F(R) values generated during insertion into synthetic bone were 777 ± 86N, 865 ± 140N, 1313 ± 198N, and 1780 ± 255N for the RCI screw, PEEK screw, IntraFix, and ExoShape, respectively. The pullout strengths in synthetic bone for the RCI screw, PEEK screw, IntraFix and ExoShape were 883 ± 125N, 716 ± 249N, 1147 ± 142N, and 1233 ± 190N, respectively.

CONCLUSIONS

These results suggest that the F(R) generated during interference fixation affects the pullout strength with sheath-based devices providing superior F(R) compared with interference screws. The use of synthetic bone was validated by comparing the pullout strengths to those when tested in porcine tibia.

CLINICAL RELEVANCE

These results could be valuable to a surgeon when determining the best fixation device to use in the clinical setting.

Time-lapsed imaging of implant fixation failure in human femoral heads

The failure mechanisms of bone-implant constructs are still incompletely understood, because the role of the peri-implant bone in implant stability is unclear. We hypothesized that implant fixation failure is preceded by substantial peri-implant bone failure. A new device was developed that combines mechanical testing of large bone-implant constructs with high-resolution peripheral quantitative computed tomography, following the principles of image-guided failure assessment (IGFA). In this study, we investigated the push-in failure behavior of dynamic hip screws (DHS) implanted in human cadaveric femoral heads. For the first time the fixation failure of a clinically used implant in human trabecular bone could be experimentally visualized at the microstructural level. The ultimate force was highly correlated with the peri-implant bone volume fraction (R(2)=0.85). We demonstrated that primary fixation failure of DHS implants was accompanied by trabecular bone failure in the immediate peri-implant bone region only. Such experimental data are crucial to enhance the understanding on the quality of the bone-implant interface and of the trabecular bone in the process of implant fixation failure. We believe that this newly developed device will be beneficial for the development of new implant designs, especially for use in osteoporotic bone.

The effect of the trabecular microstructure on the pullout strength of suture anchors

This study investigates how the microstructural properties of trabecular bone affect suture anchor performance. Seven fresh-frozen humeri were tested for pullout strength with a 5mm Arthrex Corkscrew in the greater tuberosity, lesser tuberosity, and humeral head. Micro-computed tomography analysis was performed in the three regions of interest directly adjacent to individual pullout experiments. The morphometric properties of bone mineral density (BMD), structural model index (SMI), trabecular thickness (TbTh), trabecular spacing (TbS), trabecular number (TbN), and connectivity density were compared against suture anchor pullout strength. BMD (r=0.64), SMI (r=-0.81), and TbTh (r=0.71) showed linear correlations to the pullout strength of the suture anchor with p-values<0.0001. A predictive model was developed to explain the variances in the individual BMD, SMI, and TbTh correlations. The multi-variant model of pullout strength showed a stronger relationship (r=0.86) compared to the individual experimental results. This study helps confirm BMD is a major influence on the pullout strength of suture anchors, but also illustrates the importance of local microstructure in pullout resistance of suture anchors.

Pullout strength of suture anchors: effect of mechanical properties of trabecular bone

This study investigated the relationships between trabecular microstructure and elastic modulus, compressive strength, and suture anchor pullout strength. Twelve fresh-frozen humeri underwent mechanical testing followed by micro-computed tomography (microCT). Either compression testing of cylindrical bone samples or pullout testing using an Arthrex 5mm Corkscrew was performed in synthetic sawbone or at specific locations in the humerus such as the greater tuberosity, lesser tuberosity, and humeral head. Synthetic sawbone underwent identical mechanical testing and microCT analysis. Bone volume fraction (BVF), structural model index (SMI), trabecular thickness (TbTh), trabecular spacing (TbSp), trabecular number (TbN), and connectivity density were compared against modulus, compressive strength, and pullout strength in both materials. In cadaveric bone, modulus showed correlations to all of the microstructural properties, while compressive and pullout strength were only correlated to BVF, SMI, and TbSp. The microstructure of synthetic bone differed from cadaveric bone as SMI and TbTh showed little variation across the densities tested. Therefore, SMI and TbTh were the only microstructural properties that did not show correlations to the mechanical properties tested in synthetic bone. This study helps identify key microstructure-property relationships in cadaveric and synthetic bone as well as illustrate the similarities and differences between cadaveric and synthetic bone as biomechanical test materials.