Tarsal instability secondary to medial collateral ligament injury is commonly encountered in canines following a variety of traumatic events to the hind limb. Ligamentous incompetence can be following disruption of the tissue substance or following avulsion fracture at the origin or insertion. Vehicular trauma causing shearing injury can result in loss of soft tissues, including the medial collateral ligament and the medial malleolus.1–3 The methods used to repair tarsal instability in recent literature include external coaptation, prosthetic ligament reconstruction, transarticular external skeletal fixation, arthrodesis, and amputation.3–5 Complications following internal prosthetic ligament stabilization include implant-associated infection, skin ulceration with exposure of implants, and postoperative implant loosening, leading to repair failure. In most cases, these complications ultimately result in removal of the implant.3 A stabilization technique that can be low profile and removed with least disruption of periarticular granulation tissue may be advantageous for long-term functional outcome.2,3
Retrospective studies1–3 focused on outcomes following the repair of tarsal medial collateral injuries have not found a difference in functional outcome between arthrodesis or joint preservation procedures. In these retrospective studies,3–5 the underlying cause of instability is varied, with isolated ligamentous injury, medial shearing injury, or combinations of ligamentous and tarsal fractures injuries being reported on together. Some studies2,3 have reported that prosthetic ligament procedures may have increased complications without a perceived benefit in function. One retrospective study3 reported a high rate of complications with prosthetic ligament repairs and concluded that a poor functional outcome resulted regardless of stabilizing technique and implants utilized. Given the potential myriad of primary injury types and potential high complication rates, it can be difficult to determine when reconstruction may be appropriate. Few studies have investigated tarsal stabilization to determine a superior stabilization technique; in a previous study,3 arthrodesis appeared to be preferentially performed due to a perceived reduction in severity of postoperative complications.
Joint isometry has been defined as points on either side of a joint space that are equidistant throughout the complete range of motion of the joint.6–9 A radiographic study6 demonstrated that there are points in the tarsus that display isometry through range of motion. With the intact medial malleolus, access to the isometric point on the talus is challenging; additionally, there is limited ability to place a prosthetic ligament at the site of the origin of the short branches of the collateral on the axial articular surface of the malleolus. Injuries involving the loss of the medial malleolus allow access to the proposed isometric point that may allow a more straightforward 2-point, nonanatomic stabilization option. By placing the repair material utilizing the isometric points of the talus and tibia, the suture material should experience minimal-to-no strain through range of motion. Minimizing extraneous strain on a prosthetic ligament repair may increase the fatigue life of the prosthetic ligament and may improve clinical outcomes. If the repair is placed isometrically and could provide sufficient stability, prolonged joint immobilization may be avoided, or the use of controlled range-of-motion devices (such as a hinged external fixator or an articulated orthosis) could be employed earlier without concern for the prosthetic ligament loosening.
The purpose of this study was to biomechanically compare the stability, isometry, and ultimate strength between 4 proposed isometric prosthetic ligament repairs for medial collateral ligament injury in the canine tarsus. We tested the various constructs’ stability compared to the intact and deficit tarsal medial collateral ligament. Additionally, we measured the change in distance in suture length across the joint to detect if isometric placement could be achieved. Further, we suspected that if the prosthetic ligaments were placed in isometry, there would be no difference in implant stability after cyclic loading between any prosthetic ligament repair techniques.
Methods
Specimen preparation
Twelve pairs of hind limbs, disarticulated at the stifle, were collected from canines euthanized for reasons not related to this study. The use of cadavers was approved by the IACUC of the University of Florida. Cadaveric specimens were random sourced, and patient history was not available; specific breed and age could not be determined. Specimens were estimated to be skeletally mature based on dentition and visual inspection of the tibia at the time of specimen collection. Cadavers were nonchondrodystrophic and weighed at least 15 kg. Cadaveric storage prior to specimen preparation did not exceed 4 months. The soft tissues were removed from the tibia and pes, except for the structural components of the tarsal joint, including the joint capsule, all of the components of the collateral ligaments, and all of the retinacular and ligamentous structures of the intertarsal joints and the tarsal metatarsal joints. All 12 pairs of limbs were assigned by coin toss to be in the bone tunnel group or the bone anchor group. Within these 2 groups, pairs were further divided into either nylon (40 lbs monofilament nylon; Securos Surgical) or ultrahigh-molecular-weight polyethylene (UHMWPE) suture (#2 FiberWire; Arthrex) types, again by coin toss. The 4 groups were defined as bone tunnels with monofilament nylon suture (TN), bone tunnels with UHMWPE suture (TU), bone anchor in the tibia and bone tunnel in the talus with monofilament nylon suture (AN), and bone anchor in the talus and bone tunnel in the tibia with UHMWPE suture (AU). Each repair was assigned an equal number of right and left limbs. Limbs were wrapped in saline-soaked towels and stored at −20 °C until the time of the biomechanical testing. All testing was performed within 4 weeks of specimen preparation. All mechanical testing was performed using a biaxial servohydraulic test system (858 Mini Bionix; MTS Systems Corp).
Construction of testing apparatus
All mechanical testing was performed using the same biaxial servohydraulic test system. A custom fabricated motorized apparatus was placed in the workspace of the materials testing system to objectively measure the stability and strength of the original tarsal ligament and the repair constructs.10 The entire distal limb was secured in the apparatus, which could move the joint through a complete physiologic range of motion, 165° extension and 40° flexion, as well as test stability at any desired joint angle (Figure 1). The apparatus used in this study was the same one described and used in a previous study.10 Briefly, the distal limb below the tarsal joint was secured to the testing machine and held in place by posts positioned dorsal and plantar at the level of the metatarsal bones. The tibia was secured to the actuator arm that could move the limb through the desired range of motion. For the present study, a string potentiometer transducer (Multicomp Pro) was mounted to the apparatus. This allowed for cyclical, laxity, and strength testing without the need to reposition the limb in the testing apparatus (Figure 2).
The limb in the 3 study positions. A—Flexion, 75°. B—Standing angle, 135°. C—Extension, 165°. String potentiometer assembly above limb in foreground (white arrow).
Citation: American Journal of Veterinary Research 86, 1; 10.2460/ajvr.24.06.0165
Limb is placed in the testing apparatus; medial side is dependent. A—The limb in the MTS machine during stability testing. Carabiner and chain links on the left of image (blue arrow) attach the limb to the testing apparatus above and out of the frame of the image that applies stress in a lateral direction, pulling the pes toward the top of the image. Stability posts secure the distal pes (red arrow), allowing movement only in the frontal plane; actuator arm is attached to the tibia (yellow arrow). B—A limb during cycling. Note the string potentiometer (black arrow) and wire assembly over the limb of the string potentiometer (orange arrow).
Citation: American Journal of Veterinary Research 86, 1; 10.2460/ajvr.24.06.0165
Each limb was positioned within the apparatus, parallel to the floor, with the medial aspect of limb facing downwards and the tarsus positioned directly over the spindle axis of rotation. The pes was placed between guideposts that restricted flexion and extension in the sagittal plane but allowed it to move freely in the frontal plane once attached by a ring clamp to the loading frame. Medial joint stability was assessed by applying vertical tension (valgus stress) and measuring the distal limb vertical displacement. Angular displacement of the distal limb was calculated from the linear displacement of the pes clamp and the measured distance of the clamp from the tarsal joint center. The motorized fixture allowed the tibia to be moved to any desired hock angle and cycled through its range of motion without having to readjust the position of the limb.
Stability testing: intact ligament
Limb pairs were thawed in plastic bags placed in warm water for 20 minutes before biomechanical testing. Once thawed, limbs were placed in the testing system, and the stability of the intact medial collateral ligament was determined at 3 positions: 75° (midflexion), 135° (standing angle), and 165° (full extension). Based on the previous study by Martin et al,10 intact medial collateral ligaments (MCLs) were nondestructively tested to a maximum of 50 N; in that study it was shown that the intact ligament could be tested to higher strains however the nylon suture material could not be non-destructively tested at higher values. The amount of displacement from neutral at each position was measured by the servohydraulic testing apparatus over 5 trials and recorded in millimeters.
Stability testing: medial tarsal injury
Medial shearing injury was simulated via ostectomy of the medial malleolus along with distal transection of the ligamentous and tendinous attachments, including long and short branches of the medial collateral ligament, tendon of the tibialis caudalis, and medial aspect of the joint capsule. Limbs were then reassessed by applying up to 50 N of force to the specimen at the same 3 positions (75°, 135°, and 165°), and the amount of displacement was recorded. Due to instability of the joint and to prevent additional unwanted damage, the angle of lateral displacement was limited to a supraphysiologic maximum of 35° in the frontal plane, similar to maximum deviation found in a previous study.10,11 Maximum displacement was recorded over 5 trials in all cases.
Description of repairs
Transected specimens were repaired according to their randomly assigned groups (TN, TU, AN, or AU) (Figure 3). All nylon sutures were secured using a metallic crimping system (Securos Surgical). For all repairs, the prosthetic ligament was placed in isometric locations as described by Jeager et al.6 Briefly, the diameter of the circular portion of the talar body was measured, the center of the diameter was determined, and the bone tunnel was centered at that location. The width of the tibia was measured, and the bone tunnel was placed at the midpoint of the tibia width equidistant proximal from the joint space as the talar tunnel was distal to the joint space. To increase uniformity in bone tunnel and bone anchor placement, an aiming device (Femoral Aiming Device; Arthrex) was used to place an initial stainless-steel guide pin (0.9 mm). All bone tunnels and bone anchors sites were established via the guide pins and a cannulated 2.0-mm drill bit over the guide pin in both the tibia and talus. For groups that utilized bone tunnels, the 2.0-mm drill bit was driven through the far cortex. For groups that utilized bone anchors, at the anchor sites the drill bit was stopped at the far cortex without passing through. For the bone tunnel in the talus, the tunnel was continued through the calcaneus to emerge on the plantolateral aspect of the calcaneus. For all groups, the joint was held at standing angle (135°) when tightening and securing the suture. All sutures were manually tensioned; a tensioning device that measured the amount of tension generated was not utilized nor has there been any established tension values established for the medial collateral ligament of the tarsus. For nylon suture groups, a tensioning device (Securos Surgical) was utilized to maintain the desired tension while checking joint range of motion and stability based on visual inspection. For the repairs that utilized FiberWire, the initial stand of suture was temporarily tied “shoelace style” while range of motion and stability were inspected. Once a visually and palpably acceptable amount of stability was achieved, the second strand of suture was definitively tied to match the initial tension of the first strand, then the second strand was retied. In both strands, a minimum of 3 square knots were tied.
Bone tunnels with nylon suture (TN), bone tunnels with ultrahigh-molecular-weight polyethylene (UHMWPE) (TU), suture anchor in the talus with UHMWPE suture (AU), and bone anchor in the tibia with nylon suture (AN). The visible metallic wire in TN, TU, and AU groups was attached to the string potentiometer for isometric testing. Insets for each group image show the craniocaudal view schematic for suture placement. FiberWire suture is in blue, nylon in red.
Citation: American Journal of Veterinary Research 86, 1; 10.2460/ajvr.24.06.0165
In the TN group, the 2.0-mm bone tunnels were overdrilled with a 2.5-mm drill bit to accommodate the larger nylon suture. Two strands of #40 nylon sutures were threaded through the hole of a stainless-steel toggle rod. The suture ends were then fed through the talar bone tunnel from the lateral side of the calcaneus. One free end from each of the sutures was then pushed from medial to lateral through the tibial tunnel. The strands were then passed through a second stainless-steel toggle rod, then pushed back through the tunnel on the lateral aspect of the tibia such that both free ends of the sutures were on the medial side of the tarsus. Each strand of nylon suture was secured with a stainless-steel crimp (Securos Surgical). The TU group required only the initial 2.0-mm bone tunnels. Two strands of UHMWPE suture were threaded through a 2-hole oblong 2.7-mm titanium button placed on the lateral aspect of the calcaneal tunnel; the suture ends were then pulled through the tunnel using a nitinol suture passer (Arthrex Inc). The suture passer was again used to pull the suture through the tibial tunnel such that the suture ends emerged from the lateral aspect of the tibia. Suture ends were then threaded through a 4-hole titanium button. Suture ends were held in tension and the joint cycled to ensure no slack in the suture prior to tying.
For the AN group, the initial 2.0-mm bone tunnels were overdrilled with a 3.5-mm drill bit in the tibia and a 2.5-mm drill in the talus. A 4.5-mm self-tapping bone anchor (Securos Surgical) was placed in the tibia. The anchor was placed so that the eyelet was perpendicular to the long axis of the tunnel at standing angle with the wider side of the eyelet facing the talar tunnel. Nylon was passed through the eyelet, the tunnel, and then through a toggle rod on the lateral aspect and back through the tunnel to be secured with a crimp.
The 2.0-mm tibial bone tunnel was utilized in the AU group, and the talar tunnel was overdrilled the with a 2.7-mm drill bit. A 3.5-mm self-tapping bone anchor (Securos Surgical) was placed in the talus. The anchor was placed so that the eyelet opening was perpendicular to the long axis of the tibial tunnel at standing angle with the wider side of the eyelet facing the tibia tunnel. Ultrahigh-molecular-weight polyethylene sutures were passed through the eyelet and directed through the tibial tunnel with a looped nitinol suture passer. The suture ends were then threaded through a 4-hole titanium button and secured in a manner similar to the TU group.
A 22-gauge stainless-steel malleable wire was anchored to the distal implant (toggle rod, oblong titanium button, or talar bone anchor), then passed alongside of the suture (nylon or UHMWPE) in the TU, TN, and AU groups to be used for the isometry testing. A wire could not be placed in the AN group due to anchor placement in the tibia, and therefore isometric testing was not performed in this group.
Stability and cyclic range-of-motion testing: prosthetic ligament repairs
Range of motion was assessed and compared for each prosthetic ligament construct with the use of a universal goniometer.12 Prosthetic ligament constructs were manually assessed with the goniometer for potential loss of range of motion that could affect range-of-motion testing. Repaired limbs were then mounted onto the testing system and reassessed in the same positions (75°, 135°, and 165°), with 50-N tension applied. The amount of deviation for each limb at each position was measured 5 times.
Limbs then underwent a minimum 1,600 (median, 1,612; range 1,600 to 1,777) cycles through the prescribed range of motion (75 to 165°) at a frequency of 1 cycle per 1.25 seconds (0.8 Hz) for at least 2,000 seconds (approx 33 minutes) to approximate early joint mobilization with the prosthetic ligament. The timer and range-of-motion testing apparatus were not synced to each other, and a manual shut off was required after the timer alarm sounded; this accounted for the mild discrepancies in the total number of cycles. A previous study by Martin et al10 placed specimens through 1,000 cycles and found small but measurable increases in joint laxity. For the present study, we increased the cycle time 50% in an attempt to challenge the isometry and potentially elicit a greater amount of increased laxity after testing. Tissues were kept moist via external application via a spray bottle of saline throughout cycling and testing. Once cycling was completed, limbs were retested for laxity with valgus stress as previously described. The amount of deviation for each limb at each position was measured and recorded 5 times.
Isometric testing
Isometric data were collected for the TU, TN, and AU groups. Prior to cycling, the stainless-steel wire that was passed through the tibia tunnel, as described earlier, was attached to a string gauge (Compact String Pot, SP1-4; Measurement Specialties Inc) that was mounted on the testing apparatus. This allowed the string gauge to detect and record the change in distance across the joint space through range of motion. The string gauge detected changes in length as small as 0.003 mm. The device collected data at a minimum frequency of 200 Hz.
Data were recorded for all specimens in the 3 groups that underwent isometric testing. Small oscillations, amplitude of 0.2 mm, were seen in all samples and represented vibrational noise. Data were then evaluated for oscillations outside of the 0.2-mm amplitude. Additionally, data were assessed chronologically to see if increases in amplitude were detected over time. Repairs were considered isometric if the data followed a normal distribution and if the proportion of data points outside of the 0.2-mm amplitude did not exceed 5%. Specimens that showed bimodal distributions were further examined to see if either distribution was within the 0.2-mm tolerance.
For testing to failure, limbs were placed in testing apparatus with the tarsal joint at standing angle (135°). Lateral force was applied similar to stability testing but at an increasing rate of 10 N/s. Failure was defined as implant failure (suture breaking, suture slipping at the crimp, anchor pullout), lateral joint deviation > 35°, or a loss of greater than 10% of maximal resistance.
Statistical testing
Due to large individual variations within limbs in each group, direct comparison of laxity within each group could not be done. For this reason, change in joint laxity following repair was determined for each specimen—joint laxity measurement following repair minus original joint laxity with intact medial collateral. The change in joint laxity was used as the dependent variable. Sample sizes for this study were determined by a prospective power analysis. To test for differences in treatments, status (medial tarsal injury, pre- or postcyclic testing prosthetic repair), and joint angle (joint position), a general linear mixed model with a repeated measures error term was used to analyze the data. A mixed model was used to account completely randomized split-split block design of the fixed effects, and an auto regressive order 1 (AR 1) correlation structure was used to account for the multiple measurements taken on the same limb over time (SAS, version 9.4, and JMP, version 13.2; SAS Institute Inc).
A Wilcoxon/Kruskal-Wallis rank sums nonparametric test for differences in distribution between groups was used to compare the ultimate strength between treatment groups.
Results
Initial tarsal prosthetic ligament repair stability
All repair techniques allowed full range of motion (75 to 165°) in all specimens tested. The mixed general linear model indicated that the change in joint laxity did not differ among repair (treatment) groups (P = .89), the 2-way interaction between treatment and status (injured, precycle, or postcycle) (P = .18), the 2-way interaction between treatment and angle (75°, 135°, and 165°) (P = .35), or the 3-way interaction of treatment, status, and joint angle (P = .75). Based on these results, the treatment groups were pooled for further analysis. Joint laxity was significantly associated with status (P < .001), joint angle (P < .001), and the 2-way interaction between status and joint angle (P < .001).
At 75° of flexion, the mean ± SD increase in joint laxity relative to the intact tarsus (intact MCL and medial malleolus) was 12.0 ± 3.8° for the medial tarsal injury (P < .001), 7.6 ± 3.6° prosthetic repair prior to cyclic range-of-motion testing, and 8.1 ± 2.8° after cyclic range-of-motion testing (P < .001, for both). The prosthetic ligament repair resulted in improved joint stability compared to the injured tarsus but did not return the tarsal joint to preinjury stability (P < .001 for both comparisons). There was a statistically significant difference in joint laxity between the injured tarsus and the prosthetic repairs before and after cyclic range-of-motion testing (mean ± SE), 4.4 ± .8° and 4.0 ± .8°, (P < .001), respectively. There was no difference in laxity after cyclic range-of-motion testing (.5 ± .8°; P = .8).
At 135°, the mean ± SD increase in joint laxity relative to the intact tarsus was 18.4 ± 2.9° for the medial tarsal injury (P < .001), 8.3 ± 3.8° prosthetic repair prior to cyclic range-of-motion testing, and 9.7 ± 4.1° after cyclic range-of-motion testing (P < .001 for both). The prosthetic ligament repair showed improved joint stability compared to the injured tarsus but did not return the tarsal joint to preinjury stability (P < .001 for both comparisons). There was a significant difference in joint laxity between the injured tarsal joint and the prosthetic repairs before and after cyclic range-of-motion testing (mean ± SE), 1.1 ± .7° and 8.7 ± .7°, respectively; P < .001 for both. There was no significant difference in joint laxity between the prosthetic repairs before and after cyclic range-of-motion testing (1.5 ± .1°; P = .09).
At 165°, the mean ± SD increase in joint laxity relative to the intact tarsus was 22.4 ± 3.4° for the medial tarsal injury (P < .001), 8.3 ± 3.9° in the prosthetic repair prior to cyclic range-of-motion testing, and 9.80 ± 3.9° after cyclic range-of-motion testing (P < .001 for both). The prosthetic ligament repair resulted in improved joint stability compared to the injured tarsus but did not return the tarsal joint to preinjury stability (P < .001 for both comparisons). There was a significant difference in joint laxity between the injured tarsal laxity and the prosthetic repairs before and after cyclic range-of-motion testing (mean ± SE), 14.1 ± .69° and 12.6 ± .75°, respectively; P < .001 for both. No significant difference in joint laxity was recorded between the prosthetic repairs before and after cyclic range-of-motion testing (1.4 ± .69°; P = .1).
Isometry testing
Histograms of the entire string potentiometer dataset from each specimen was plotted (mean, 677,259 ± 28,495 data points). For the TU group, 4 of the 6 specimens’ data were normally distributed; 2 specimens displayed bimodal distribution of datapoints. The data sets for each specimen were assessed for proportion of datapoints outside of the 0.2-mm amplitude. Three of the 6 specimen in the TU group had less than 5% of the data points outside the 0.2-mm tolerance (range, 3.0% to 4.5%). Both specimens with bimodal distributions and 1 specimen with normal distribution had changes in length during range of motion that resulted in greater than 5% of the data points to be outside the 0.2-mm amplitude allowance. The increased proportion of measurements outside of the 0.2 mm for the 3 specimens was 7.1%. For the specimen with a normal distribution and the specimens with bimodal distribution, proportions outside the 0.2 mm tolerance were 10% and 50%, respectively (Table 1).
Oscillometric string potentiometer data measurements assessing isometry during range-of-motion testing.
| Specimen group | Histogram data distribution | Points outside amplitude of 0.2 mm/total data points | Percentage of data points outside 0.2-mm limit of measurements | ||
|---|---|---|---|---|---|
| TU | |||||
| 1A | Normal | 19,926/660,043 | 3.0 | ||
| 3A | Normal | 20,821/674,099 | 3.1 | ||
| 5A | Normal | 51,873/734,543 | 7.1 | ||
| 7A | Normal | 30,107/667,884 | 4.5 | ||
| 9A | Bimodal | 2,383/8,983 | 58,274/656,122 | 26.5 | 8.9 |
| 11A | Bimodal | 315,305/568,444 | 17,064/93,438 | 55.5 | 18.3 |
| TN | |||||
| 1B | Normal | 21,273/665,397 | 3.2 | ||
| 3B | Normal | 19,945/677,545 | 2.9 | ||
| 5B | Normal | 22,021/672,442 | 3.3 | ||
| 7B | Normal | 18,642/667,005 | 2.8 | ||
| 9B | Normal | 20,121/640,239 | 3.1 | ||
| 11B | Normal | 16,805/665,670 | 2.5 | ||
| AU | |||||
| 2D | Normal | 52,655/670,147 | 7.9 | ||
| 4D | Bimodal | 21,886/25,262 | 256,095/713,983 | 86.6 | 35.9 |
| 6D | Normal | 25,157/663,403 | 3.8 | ||
| 8D | Bimodal | 112,363/406,701 | 84,416/261,532 | 27.6 | 32.3 |
| 10D | Bimodal | 13,663/93,644 | 409,266/573,984 | 14.6 | 71.3 |
| 12D | Normal | 31,765/681,817 | 4.7 | ||
AU = Suture anchor in the talus with UHMWPE suture. TN = Bone tunnels with nylon suture. TU = Bone tunnels with ultrahigh-molecular-weight polyethylene.
Results were considered isometric (in bold) if normally distributed and ≤ 5% proportion of data points were outside of the 0.2-mm amplitude tolerance. The bone anchor in the tibia with nylon suture group could not be tested due to implant interference with the potentiometer wire.
For the TN group, 6 of 6 specimens had histograms with normal distributions (mean data points, 664,690 ± 12,883). All 6 specimens had greater than 95% of their data points within the 0.2-mm amplitude allowance; proportions of data points outside the 0.2-mm tolerance among the specimens ranged from 2.5% to 3.3%.
For the AU group, 3 of the 6 specimens had histograms with normal distribution; the other 3 specimens showed a bimodal distribution. Only 2 of the 6 specimens had data points with < 5% outside the 0.2-mm amplitude tolerance (3.5% and 4.6%). For the specimen with a normal distribution, the proportion of data outside of the 0.2-mm tolerance was 7.9%. For the 3 specimens with bimodal distributions, all subsets of data still had > 5% outside the 0.2-mm tolerance and ranged from 14.6% to 86.6%.
For the 5 specimens (2 in the TU group and 3 in the AU group) that had bimodal distributions, 4 of them had the distribution sets separated chronologically as well, meaning that the oscillations occurred around 1 length, followed by a sudden change in length measured by the string potentiometer, then a second oscillation set point. The fifth specimen in this group had bimodal distribution, but no shift could be identified in the data set.
Prosthetic ligament repair strength
The force required to reach implant failure or 35° of lateral joint deviation was similar between groups. Implants remained intact in all specimens, and testing was completed upon reaching 35° in all samples. The mean force ± SD needed for the TU group was 110.2 ± 40.0 N; TN was 122.4 ± 22.5 N, AU was 102.1 ± 24.8 N, and AN 104.5 ± 40.8 N. These differences were not statistically significant (P = .6).
Modes of failure
Tunnel widening at the medial tibial emergence of the suture was noted (Figure 4) in both groups utilizing UHMWPE. The erosion of cancellous bone within the bone tunnel from the suture was present in all specimens. Additionally, 3 of the 6 specimens also had fragmentation and loss of the distal articular cortical bone during failure testing. The groups with nylon suture failed via suture elongation (direct suture stretch or slippage at the crimp could not be specifically differentiated) and were not able to prevent 35° deviation with the force applied.
A—Specimen post cyclic range-of-motion testing with tunnel erosion from the suture (blue arrow). B—Cortical fragmentation that occurred in some specimens following failure testing (orange arrow).
Citation: American Journal of Veterinary Research 86, 1; 10.2460/ajvr.24.06.0165
Discussion
In the present study, we assessed a proposed isometric-based prosthetic ligament repair for tarsal medial collateral reconstruction following a simulated shearing injury to the medial tarsus. The results showed improved tarsal stability for all repair techniques compared to the injured tarsus; however, none of the repair constructs resulted in a return to stability that matched the intact ligament. A previous study by Martin et al10 using anatomic locations for prosthetic ligament repairs also did not achieve stability comparable to the intact ligament; however, in the repairs of that report they were able to establish stability ≤ 10° of the normal valgus deviation of the tarsal joint. It was thought that this deviation may not be clinically relevant since some of the techniques used in that study had been successfully used clinically. In the present study, repairs also resulted in stability ≤ 10° of the intact joint.
Similar to the previous study by Martin et al,10 we found that the tarsal joint, in the injured state, was inherently more stable in flexion at 75° despite the loss of the medial malleolus. The increased valgus angulation at this position with the injured tarsus was less compared with the tarsus at 135° or 165° (12° vs 18° vs 22°). The stability (8° of valgus deviation) achieved at 75° with the repair was similar in all 3 positions. Cycling of the joint did not change the stability significantly, further supporting the isometry of the suture position. The isometric repair used in this study was most similar anatomically to the tibiotalar portion of the short collateral ligament, which is thought to provide the most stability during flexion.1,2 The short collateral has origins on the articular aspect of the medial malleolus. With the loss of the malleolus, these attachments cannot be replicated, and functionally it would appear the tibial attachment site used for this study does provide suitable stability in flexion. A recent study by Bogisch et al13 described other important functions of the medial short collateral ligament at limiting internal rotation and varus deviation. It was beyond the scope of this study to assess these functions; as such, we cannot comment on the ability of the repair to return stability in these directions.
Similar to the findings of Martin et al,10 both the 135° and 165° positions with prosthetic ligament repairs showed improved stabilization compared to the injured status. In that study, laxity increase ≤ 10° compared to intact joint was thought to be clinically acceptable.10 Isometric repairs in the current study were able to achieve this level of stability; in the current study, the increase in laxity did appear to approach the 10° limit, although statistical comparison was not possible to the previous study. In contrast to the study by Martin et al,10 stability was maintained after cyclic range-of-motion testing, supporting the isometric placement of the suture.
Overall, the isometric testing of the repairs agreed with the findings of Jaegger et al.6 The chosen locations for medial anchor points were isometric based on the TN group results. The TU group also appeared to be isometric initially; however, the tension in the suture material caused erosion of cancellous bone in the tibial tunnel, resulting in loss of isometry. This boney erosion of the tibial tunnel was also noted in the AU group. This group did not appear to achieve isometry even though the anchor points were identical. This was likely due to the prominence of the anchor eyelet, resulting in a pivot point of the suture attachment slightly proximal and caudal to the talar isometric point. It may be possible to adjust for this by moving the talar tunnel slightly cranial and distal such that the eyelet of the anchor would be positioned over the isometric point of the talus; however, this was beyond the scope of this study and likely inconsistently achievable in a clinical setting.
Isometry can be challenging to assess in biologic scenarios. Previous studies6–8 have used radiographic or reflective video markers as an indirect method of assessment. Alternatively, other investigators used strain gauges or other similar devices to detect tensile strain on prosthetic sutures used in ligament reconstruction.9,14 We chose a direct measure of isometry by use of a string gauge to directly measure the change in distance across the joint through normal range of motion. We felt the vibrational noise amplitude of 200 μm encountered in our apparatus to be well within any clinically applicable variance for acceptable isometry.
Tensioning the suture material in all groups was done subjectively in the same manner as would be done clinically. Although some information exists on overtightening UHMWPE suture in stifle applications for cruciate ligament stabilization,15 the range of acceptable suture tension amount has not been determined for tarsal application. Tarsal stability was subjectively visually assessed for over- or undertightening during manual manipulation for lateral deviation at the time of definitive securing of the suture. Variations in manual suture tensioning have been shown in stifle applications16 and likely occur in tarsal applications as well. We attempted to minimize variations by having a single investigator apply the prosthetic sutures, though variations in tension likely still existed.16 The lack of detectable differences in joint laxity between treatment groups supports that variability in suture tension was not an important factor in the outcomes.
There was no difference in the strength of the repairs to resist valgus deviation up to 35°. The approximate 100-N force required to achieve a supraphysiologic deviation is a load that could be expected by the size of dogs utilized in this study. Based on this finding, additional support of the joint would be expected to be necessary during convalescence to promote further stabilization with fibrosis. While the suture placement is isometric, the placement in the talus is most similar to the short branch of the collateral. Biomechanically isometric suture placement used in isolation as was done in the present study would appear to be insufficient for long-term stability. It has been shown previously that injury isolated to the short collateral may not require repair; however, injuries to the long branch of the collateral ligament are unlikely to stabilize without intervention.2–5
These results support the isometric point of the talus first proposed by Jaeger et al.6 All 4 repair techniques were able to improve stability of the tarsus to within 10° of the original tarsal stability. Unfortunately, the force needed to induce supraphysiologic deviation was within a reasonable expectation of normal forces that may be experienced by the repair.17 For this reason, applying sutures only in this isometric location may be insufficient to maintain tarsal stability in situations that result in loss of the medial malleolus. The isometric suture placement could likely be used as part of a reconstruction attempt in conjunction with sutures placed anatomically for the long branch of the collateral.10 Attempts to stabilize this type of injury with only an isometric suture may be insufficient to yield a positive outcome. Further studies in clinical cases may be warranted but are cautioned against without augmented stabilization.
Acknowledgments
The authors would like to thank Jamie Holloway for the preparation of images for publication. The authors thank Arthrex Inc and Securos Surgical for the donations of surgical implants.
Disclosures
Matthew Johnson is an educational consultant for Arthrex Vet Systems but has no direct financial connection or benefit from the use of the products in this study.
No AI-assisted technologies were used in the generation of this manuscript.
Funding
Financial support was provided by the University of Florida, College of Veterinary Medicine Fall Consolidated Faculty Research Development Award Grant Competition and the Florida Veterinary Scholars Program Research Grant.
ORCID
M. D. Johnson https://orcid.org/0000-0002-3886-6673
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