Comparison of a Suture Anchor and a Toggle Rod for Use in Toggle Pin Fixation of Coxofemoral Luxations
The mechanical characteristics of toggle rods and Bone Biter™ anchors inserted through the medial acetabular wall for toggle pin repair of coxofemoral luxations were compared in 16 canine cadaver pelves. No differences were detected in maximum load to failure, displacement at failure, or energy to failure between the two constructs. Toggle rod constructs failed primarily by breakage of the suture at the rod eyelet. All of the BoneBiter™ anchor constructs failed when the anchors pulled through the medial acetabular wall.
Introduction
The use of a prosthetic ligament (i.e., made of various types of suture material) to replace the ligament of the head of the femur is referred to as toggle pin fixation. It is a common technique for stabilizing coxofemoral luxations in dogs. Mechanically, the strength of repair after toggle pin fixation was found to be comparable to that of other techniques for stabilizing hip luxations.1 Traditionally, the suture material was attached to the medial acetabular wall using a toggle pin that was made by bending stainless steel Kirschner wires into the proper shape.1–3 Recently, a toggle roda became commercially available, negating the need to make individual toggle pins and thereby reducing anchor variability and providing greater stiffness.2 Experimental studies evaluating the mechanical characteristics of toggle rods have also been reported in dogs.1–3 When tested alone, commercial toggle rods demonstrated greater strength and stiffness characteristics than handmade toggle pins; however, when tested in cadaver bones as part of a toggle pin fixation construct, there were no significant differences between toggle types.2
Suture anchors are effective means of attaching suture material to bone, and they are often used to repair ligament and tendon avulsions.4–9 The use of suture anchors for repair of orthopedic injuries is well documented in humans.7,8 The BoneBiter™ suture anchorb is marketed for use in animals and is available at a fraction of the cost of previous anchor systems, which have been marketed primarily for use in human orthopedic cases. Clinical reports advocate the use of suture anchors in animals for treatment of cranial cruciate ligament disease; elbow, carpus, and stifle collateral ligament rupture; and extracapsular stabilization of coxofemoral luxations.6,9 In the last technique, the anchors are placed in the dorsal acetabular rim to allow reattachment of the torn joint capsule. Suture anchors, however, have not been evaluated for use in the traditional toggle pin fixation technique, where the anchor would be used to attach suture material (prosthetic ligament) to the medial acetabular wall.
The mechanical performance of the construct used during toggle pin fixation depends on the tensile strength of the suture material, the pull-out strength of the anchoring device, and the interaction of the suture material with the anchoring device as it passes through the eyelet.4 Previous studies have demonstrated several modes of failure of the toggle pin fixation construct, including failure of the suture material, failure of the handmade toggle pin, fracture of the femoral head, and cutting through the femoral neck by the suture material.1,2
The objectives of this study were to evaluate the mechanical characteristics of BoneBiter™ suture anchors when placed through the medial acetabular wall during toggle pin fixation and to compare these characteristics with those of toggle rods. The hypothesis was that the mechanical performance of toggle pin fixation constructs using either a toggle rod or BoneBiter™ suture anchor would be similar, making suture anchors suitable for use in this technique.
Materials and Methods
Specimens
Femoropelvic specimens were harvested from 16 adult research dogs <8 years of age and ranging in weight from 15 to 31.5 kg. The dogs were euthanized for reasons unrelated to the study. Specimens were determined to be free of coxofemoral joint disease by gross inspection at the time of harvest, prior to inclusion in the study. All soft-tissue structures were carefully removed, and the specimens were wrapped in saline-soaked sponges. They were then stored at −20°C. Specimens were thawed and brought to room temperature just prior to testing.
Each pelvis was divided on the median plane into two hemipelves. One hemipelvis was randomly assigned to the toggle rod construct group (n=16). The other 16 hemipelves were randomly assigned to one of two BoneBiter™ construct groups. In the first group, a 2.5-mm drill hole was used to insert a 5-mm anchor (n=8). In the second group, a 3.5-mm drill hole was used to insert a 5-mm anchor (n=8). Prior to mechanical testing, the disarticulated coxofemoral joint in each hemipelvis was stabilized using one of two surgical techniques.
BoneBiter™ Construct Groups
In each hemipelvis, either a 2.5-mm or 3.5-mm diameter hole was drilled through the acetabular fossa using a battery-powered drill (at approximately 1300 rpm) and a sharp drill bit. The hole penetrated both the lateral and medial cortices of the acetabulum at the origin of the ligament of the head of the femur. A size 5 suture anchor,b loaded with a single, long strand of 80-lb test monofilament nylon suture material,c was inserted through the hole in the acetabular wall using the manufacturer’s recommended insertion technique.b The suture anchor was set against the medial acetabular wall [Figure 1].
A 4.5-mm diameter bone tunnel was then drilled in the corresponding femurs from the fovea capitis of the femoral head to the lateral femoral cortex, at a point just cranial to the third trochanter and at the level of the lesser trochanter. The two ends of the monofilament suture material were threaded from medial to lateral through the bone tunnel. The coxofemoral joint was then reduced, and slack was eliminated from the suture material. The suture material was attached to a polypropylene surgical buttond over the lateral aspect of the trochanter. To ensure all sutures were tightened equally, the first throw of the suture knot was tightened to a tension of 3 kg, as measured by a calibrated spring tensile scale. A tension of 3 kg has been previously shown to provide sufficient joint reduction without severely limiting range of motion.1 An additional five throws were placed to secure the knots.
Toggle Rod Construct Group
A 4.5-mm diameter hole was drilled through the acetabular fossa of each hemipelvis using an orthopedic drill and sharp drill bit. The hole penetrated both the lateral and medial cortices of the acetabulum at the origin of the ligament of the head of the femur. A 3.2-mm toggle rod,a loaded with a single strand of 80-lb test monofilament nylon suture material, was inserted through the hole in the acetabular wall using the commercially available insertion device with the toggle rod system.a The toggle rod was set against the medial acetabular wall [Figure 2]. The suture was then placed through the femoral head and secured similar to that described for the suture anchor technique.
Mechanical Testing
Mechanical testing of the repaired coxofemoral joints was performed using an Instron materials testing machine.e Each hemipelvis was rigidly mounted, in an anatomical position, to a customized jig that was placed in the testing machine [Figure 3]. Each femur was rigidly mounted, using polymethylmethacrylate, into a customized stainless steel cup. The cup was then attached to a universal ball joint on the load cell of the materials testing machine. This joint allows both axial rotation and movement of the femur in any direction in response to a compressive load. The femur was aligned in the longitudinal axis to establish a normal weight-bearing angle of 105°. Additionally, the femurs were oriented parallel (i.e., without adduction or abduction) to the axis of loading. Positioning of the femur in both the sagittal and transverse axes was accomplished by aligning the shaft of the femur parallel to a plumb line suspended from the mounting device.
One Newton (N) of preload was applied to each construct. The construct was then loaded in compression at a rate of 100 mm per minute until failure. This model has been used in previous studies and has been shown to consistently produce coxofemoral luxation in the craniodorsal direction.1,2 Load (in N) and displacement (in mm) data were collected during compressive loading to determine the maximum load to failure and the total energy required for failure (N-mm; area under the force versus displacement curve). The failure point was defined as the initial peak on the load-displacement curve. The mode of failure was assessed for each construct and was classified as toggle or bone anchor failure (e.g., failure or pullout); suture knot failure; suture failure at the eyelet of the anchor or rod; or suture failure at the acetabulum-femoral head interface.
Statistical Analysis
The effects of suture anchor type on the mechanical testing variables were analyzed using a one-way analysis of variance (ANOVA) for an incomplete block design. The residuals from each ANOVA were examined using frequency histograms and normal probability plots to validate the normality assumption. If significant effects were found, means were separated using the least significant difference test. If pairwise differences were identified, their importance was assessed using confidence intervals.10,f Statistical significance was set at P<0.05.
Results
Mean body weights of the cadavers from which the specimens were harvested were 21.6 kg (range 15 to 31.5 kg), 22.0 kg (range 16.6 to 26.6 kg), and 21.9 kg (range 16 to 31.5 kg) for the toggle rod, BoneBiter™ 2.5-mm, and BoneBiter™ 3.5-mm groups, respectively. Mechanical testing data are reported as the mean ± standard deviation.
The mean loads at failure for the toggle rod, BoneBiter™ 2.5-mm, and the BoneBiter™ 3.5-mm groups were 990.6±581.4 N (range 309.0 to 2074.8 N), 1070.6±338.6 N (range 367.0 to 1390.1 N), and 1158.2±612.9 N (range 612.1 to 2430.9 N), respectively [see Table]. There were no significant differences (P=0.7) in the maximum loads at failure among the three groups. The mean displacements at failure for the toggle rod, BoneBiter™ 2.5-mm, and BoneBiter™ 3.5-mm groups were 6.28±3.42 mm (range 1.8 to 12.9 mm), 6.09±2.93 mm (range 2.5 to 10.5 mm), and 3.51±1.23 mm (range 2.0 to 5.4 mm), respectively [see Table]. There were no significant differences (P=0.8) in the amounts of displacement at failure among the three groups.
The mean energies to failure for the toggle rod, BoneBiter™ 2.5-mm, and BoneBiter™ 3.5-mm groups were 3725.0±2726.9 N-mm (range 712.5 to 9792.2 N-mm), 3411.2±2111.3 N-mm (range 1111.9 to 7078.6 N-mm), and 2538.9±1992.8 N-mm (range 855.7 to 6961.2 N-mm), respectively [see Table]. There were no significant differences (P=0.9) in the required energy to failure rates among the three groups.
The mode of failure for 14 (88%) of the toggle rod constructs was suture breakage at the rod eyelet [Figure 4]. One (7%) of the toggle rod constructs failed by suture breakage at the acetabulum/femur interface. One (7%) of the toggle rod constructs failed because of fracture of the femoral neck. The mode of failure for all suture anchor constructs (both the 2.5-mm and 3.5-mm groups) was pullout of the anchor through the medial acetabular wall [Figure 5].
Discussion
In this study, the hypothesis was tested that toggle pin fixation constructs using either a toggle rod or a BoneBiter™ suture anchor would have similar mechanical characteristics. Given that there were no significant differences found between the toggle rod and suture anchor construct groups with respect to the load at failure, displacement at failure, and energy at failure, suture anchors appear to be mechanically similar to toggle rods and may, therefore, be considered suitable for toggle pin fixation of coxofemoral luxations. Toggle rod constructs and suture anchor constructs differed in their mode of failure, however, which may be a factor when considering suture anchors for use in toggle pin fixation.
The use of suture anchors has been described in a variety of other small animal orthopedic repair techniques.6,9 The manufacturer recommends drilling a 2.5-mm diameter bone hole for insertion of a size-5 BoneBiter™ suture anchor; however, in small or thin bones, the recommendations are to increase the hole diameter to reduce the insertion force and the risk of iatrogenic fracture.b Because of the thin nature of the acetabular wall, two suture anchor construct groups were evaluated—one with a 2.5-mm bone hole in the acetabular wall and the other with a 3.5-mm bone hole in the acetabular wall. No fractures of the acetabular wall were observed during the insertion of the suture anchor in either group. All suture anchor constructs failed by anchor pullout secondary to anchor distortion rather than fracture of the acetabular wall, and there were no significant differences observed in the mean load to failure or energy to failure between suture anchor groups. Therefore, it is unlikely that the acetabular wall was significantly damaged during insertion of the anchor in the 2.5-mm group and that the acetabular wall is of adequate thickness for insertion of a size-5 BoneBiter™ suture anchor. In addition, the hole size probably did not play a significant role in the overall mechanical performance of the suture anchor construct, since all constructs in these groups experienced failure by the same mechanism (i.e., anchor pullout) and there were no differences in the load or energy to failure rates between groups. The lack of a difference in modes of failure between the suture anchor groups indicated that the acetabular wall is of sufficient thickness to resist damage during insertion of the anchor.
Mode of failure was determined for all constructs and was classified as anchor failure, suture knot failure, suture failure at the acetabular/femoral interface, or suture failure at the anchor/rod eyelet based on previous reports.1,2,11,12 Only one of the toggle rod constructs experienced failure at the acetabular/femoral interface, which suggested that during monotonic testing, this interface is not a common location for failure. One previous study identified suture failure as the most common type of failure, although the location of the suture failure was not specified.2 Fourteen of the 16 toggle rod constructs failed by suture failure at the rod eyelet. This failure most likely occurred because the suture must make a 180° turn as it passes through the anchor eyelet. As the construct is loaded, focal concentration of force on the suture as it passes through the rod eyelet leads to failure.
All (16/16) of the suture anchor constructs, regardless of acetabular hole size, failed by the anchor pulling out of the acetabular wall. This mode of failure has been previously reported with the use of handmade toggle pins.1 There were no significant differences in the mean load to failure or the energy to failure between the toggle rod and suture anchor groups. From a mechanical standpoint, both construct types may be considered suitable for repair of coxofemoral luxations; however, there were differences in their modes of failure. Failure from anchor pullout could result in the anchor being pulled into or through the coxofemoral joint, causing lameness and cartilage damage. This type of failure would be a concern in the clinical setting if the repaired coxofemoral joint was subject to forces strong enough to cause luxation. In vivo estimates of the force to which a canine coxofemoral joint is subjected range from 1.5 to 4 times body weight, depending on whether the dog is standing or engaged in forward motion.13,14 Based on the mean body weight of the specimens tested, ground reaction forces would range from 304 to 810 N. The mean load to failure of the BoneBiter™ 2.5-mm group was 1071 N. Therefore, depending on the degree of activity restriction applied in the postoperative period, the load-bearing capacity of the suture anchor construct should be sufficient to withstand weight-bearing forces long enough for healing to occur. In addition, assuming adequate activity restriction is imposed, the likelihood of enough force being generated to cause a catastrophic failure of the repair would seem to be low.
Although there were no significant differences for any of the variables tested among groups, the mean load to failure for both constructs in this study was much greater than that for other constructs tested in a similar manner in previous studies.1,2 The mean load to failure for the three test groups in the study reported here ranged from 990 to 1158 N, whereas other reported failure loads for in vitro toggle pin fixation with a toggle rod or toggle pin were 310 to 402 N and 453 N, respectively.1,2 The most likely reason for the differences in mean loads among studies was that 80-lb test monofilament nylon suture material was used in the present study, whereas the previous studies used 50-lb test monofilament nylon, no. 2 or 5 braided polyester, or 5-mm woven polyester. The increased size of the suture may have allowed the constructs evaluated in this study to withstand a higher load to failure than those reported in previous studies. In fact, the construct groups evaluated in this study had failure loads similar to those reported for in vitro testing of intact coxofemoral joints.1,2 These results suggest that the use of 80-lb monofilament nylon during toggle pin fixation can achieve strengths similar to intact normal joints.
The type and size of suture material chosen in the present study were intended to reflect the current clinical recommendations for live dogs of the same weight range. Additional reasons for choosing monofilament nylon included the preference of a monofilament suture over a woven or braided suture. Complications associated with the use of synthetic woven or braided materials as an implant include tissue reactions, fistula formation, and persistence of infection at the implant sight.15,16 Flynn et al. reported that 50-lb test monofilament nylon demonstrated a maximum breaking strength nearly twice that of no. 2 braided polyester.1
The in vitro loading of toggle pin fixation constructs in this study was intended to simulate a single strong force that could cause failure of a toggle pin fixation in a clinical case. One limitation of the study was lack of evaluation of the influence of cyclic loading on the constructs. Cyclic loading may have altered the mode or location of failure. Load during mechanical testing was applied at a speed of 100 mm per minute, which was based on previous models for testing toggle pin fixation constructs.1,2 Loading at a faster rate might simulate a fall or traumatic event more accurately; however, this rapid loading would have created difficulties in measuring parameters from load-displacement curves.
Finally, a sample size was selected for this study that would ensure detection of differences with 70% power and 95% confidence. However, large ranges of maximum load, displacement, and energy measurements were obtained, and a large standard deviation was noted in the data collected. As a result, with the number of constructs tested, the possibility existed for a type II statistical error (i.e., failure to detect an actual difference in the mechanical performance of the treatment groups).
Conclusion
Mechanical testing of toggle rod and suture anchor groups was performed. No differences were found in maximum load to failure, displacement at failure, or energy to failure—suggesting that suture anchors possess comparable mechanical properties when used for toggle pin fixation of coxofemoral luxations in vitro. Coxofemoral joints stabilized with toggle rods or suture anchors differed only in mode of failure. Toggle rod constructs failed primarily by breakage of the suture at the rod eyelet. All of the BoneBiter™ anchor constructs failed when the anchors pulled through the medial acetabular wall. Studies involving cyclic testing and the use of suture anchors in vivo are indicated to more thoroughly demonstrate the anchor’s suitability for this technique.
Toggle Rod (3.2 mm); IMEX Veterinary, Inc., Longview, TX 75604
BoneBiter™ suture anchor; Innovative Animal Products, Rochester, MN 55901
Mason Hard Type Nylon Leader Material; Mason Tackle Co., Otisville, MI 48463
14-mm polypropylene button; Ethicon, Inc., Somerville, NJ 08876
MTS Bionix 858 Test System; MTS Systems Corporation, Eden Prairie, MN 55344
SAS Version 8.02; SAS Institute, Inc., Cary, NC 27513
Citation: Journal of the American Animal Hospital Association 42, 2; 10.5326/0420121
Citation: Journal of the American Animal Hospital Association 42, 2; 10.5326/0420121
Citation: Journal of the American Animal Hospital Association 42, 2; 10.5326/0420121
Citation: Journal of the American Animal Hospital Association 42, 2; 10.5326/0420121
Citation: Journal of the American Animal Hospital Association 42, 2; 10.5326/0420121
Photograph of the medial aspect of a canine pelvis, showing placement of a size-5 BoneBiter™ anchor through the acetabular wall. The anchor is visible against the medial acetabular wall. Picture insert provides a magnified view of the size-5 anchors prior to implantation.
Photograph of the medial aspect of a canine pelvis, showing placement of a 3.2-mm toggle rod through the acetabular wall. The toggle is visible against the medial wall of the acetabulum. Picture insert provides a magnified view of toggle rod prior to implantation.
Photograph of craniocaudal view of a canine hemipelvis and proximal femur mounted in a customized jig for mechanical testing. The femur was aligned in the longitudinal axis at a normal weight-bearing angle of 105°. The jig is attached to the materials testing machine to allow loading of the construct during testing.
Photograph of a femoral head after failure of a toggle rod fixation. The 80-lb test monofilament suture material failed at the rod eyelet.
Photograph of a femoral head after failure of a suture anchor fixation. The anchor was pulled through the medial acetabular wall and remains attached to the monofilament suture material.
Contributor Notes
