Disruption of the CCL is common in many species. This injury is considered life threatening in cattle because of the severe debilitation, progressive signs of pain, and poor success rate after surgery. The CCL prevents cranial displacement of the tibia relative to the femur and prevents internal rotation of the stifle joint.1–3 Surgical techniques advocated for returning functionality to dogs with damaged or torn cruciate ligaments include intracapsular, extracapsular, and tibial osteotomy techniques.1 Tibial plateau leveling osteotomy has been successfully performed in a llama4 but potentially has limitations in large animals because of the concern for implant failure prior to bone healing. Thus, most CCL repair techniques in large animals involve intracapsular techniques, extracapsular techniques, or a combination thereof. Cattle lack a fabella upon which to attach extracapsular sutures. Extracapsular techniques, when performed in cattle, are aimed at femorotibial stabilization through formation of restrictive scar tissue. Although success has been reported with use of extracapsular techniques in cattle with CCL rupture,5 successful retinacular tightening by imbrication or a sliding flap technique has been achieved only in lower-weight animals.6 In larger animals, imbrication is typically reserved for those with partial cruciate ruptures or as an adjunctive treatment to other intracapsular techniques. Extracapsular techniques for surgical management of CCL-deficient stifle joints in cattle fail to establish sufficient scar tissue to improve stability of the femorotibial joint long term.
Intracapsular techniques are aimed at stabilization of the femorotibial joint via replacement of the function of the CCL. Potential complications of the procedure include intra-articular infection, foreign body reaction to the prosthetic implant, or rupture of the prosthesis. Intracapsular procedures have not become widely adopted for surgical management of CCL-deficient stifle joints in cattle because a suitable suture material has not been developed.
An ideal prosthetic material for use as a CCL replacement would have similar characteristics to the native CCL. Mechanical evaluation of elongation and failure loads may be used as an initial screening process for candidate materials. Unfortunately, because of their large adult weight, many of the CCL repair techniques fail in cattle.7,8 Failure of the CCL replacement material itself is likely a common cause of the poor outcomes in clinical patients. Natural and synthetic materials have been used as CCL replacements in cattle.9 Synthetic materials are susceptible to cyclic fatigue and rupture because they do not possess the repair mechanisms of biological systems. Candidate materials should have higher tensile strength than the native CCL and be resistant to cyclic failure, inert or of low bioreactivity, and able to be secured without weakening the implant. We hypothesized that polymerized caprolactam and monofilament nylon, suture materials used in orthopedic surgery, could serve as suitable candidates for development of a CCL prosthesis for cattle. The objectives of the study reported here were to determine the tensile strength and elasticity of the CCL of cattle and of large (No. 8) polymerized caprolactam suture and 2 sizes of monofilament nylon, MN80 and MN450. An additional objective was to compare the mechanical characteristics of these suture materials tested as parallel strands versus those of braided strands.
Materials and Methods
Cruciate ligament tensile testing—Six bovine femorotibial joints harvested from 3 mature cows euthanized by barbiturate overdose for naturally occurring disease unrelated to the stifle joint were used for this study. As such, institutional animal care and use committee approval was not required. Body weights of cattle were estimated to be between 550 and 700 kg. Femorotibial specimens were prepared by cutting the stifle joint region free of the limb at the level of the midfemur and midtibia. The femur and the tibia were transected, leaving at least 15 cm of bone on either side of the femorotibial joint to facilitate attachment to the mechanical testing frame assembly. All soft tissues were removed except for the CCL. This allowed mechanical testing of the CCL in isolation. This dissection took place in a standard large animal operating suite at room temperature (approx 20°C). The specimens were quickly frozen and stored at −80°C until testing. Prior to testing, the specimens were thawed at room temperature for 12 hours.
An apparatus was created to fix the femorotibial joint and attach the femur and tibia to a hydraulic load frame. The apparatus consisted of double steel bars with transcortical pins used to secure both the femoral and tibial sections (Figure 1). The femorotibial joint testing assembly was secured to the load frame, and the hydraulic ram was preset to a displacement rate of 60 mm/min. During the testing procedure, the femorotibial joint was in maximal extension. The force applied to and elongation of the CCL were measured concurrently with the displacement of the hydraulic ram at a rate of 100 measurements/s. The femorotibial joints were loaded to failure by displacement at a constant rate.
Illustration of a portion of a femur and tibia of a bovine femorotibial joint secured in a testing apparatus.
Citation: American Journal of Veterinary Research 74, 3; 10.2460/ajvr.74.3.381
Illustration of a portion of a femur and tibia of a bovine femorotibial joint secured in a testing apparatus.
Citation: American Journal of Veterinary Research 74, 3; 10.2460/ajvr.74.3.381
Illustration of a portion of a femur and tibia of a bovine femorotibial joint secured in a testing apparatus.
Citation: American Journal of Veterinary Research 74, 3; 10.2460/ajvr.74.3.381
Tensile testing—Ten dorsoplantar radiographic projections of adult bovine stifle joints were studied to determine an appropriate length for the cruciate prosthesis on the basis of the method described by Hamilton and Adams.9 The appropriate length of the prosthesis was estimated by obtaining 4 measurements: the distance from the lateral epicondyle of the femur to the intercondylar fossa near the origin of the CCL, the distance from the intercondylar fossa near the attachment of the CCL to the medial aspect of the midpoint of the tibial crest, the thickness of the tibial crest at the midpoint, and the distance from the lateral aspect of the midpoint of the tibial crest to the lateral epicondyle of the femur. The sum of these measurements was taken to be an appropriate length of suture needed, and these 10 radiographic measurements yielded a mean length of 38.9 ± 3.7 cm.
Commercially available MN80 was braided into cords of 3 and 6 strands/cord. The braided material was then tied into a loop of approximately 40 cm with a cylindrical template. Square knots were used to tie the ends together. At least 5 square throws were used to create the knots. The procedure was repeated for commercially available MN450 and for No. 8 polymerized caprolactam suturea (Figure 2). During the development of these constructs, it was noticed that considerable gap formation occurred within the braids. Therefore, some constructs were tested with the strands placed in parallel so that the effect of braiding on elongation and strength of the constructs could be assessed. Thus, MN450 was tested singly and in cords of parallel strands of 3 and 6 strands/cord and in cords consisting of 3 and 6 strands in a braided configuration. Also, No. 8 polymerized caprolactam suture was tested singly and in cords of parallel strands of 3 and in cords of braids of 3 and 6 strands. Six tests were performed on each suture configuration, with the exception of 3-stranded No. 8 polymerized caprolactam suture arranged in parallel, for which only 2 tests were performed because of suture availability. Because of the limited number of tests performed on the No. 8 polymerized caprolactam suture arranged in 3 parallel strands, it was omitted from further analysis.
Photograph of 6-stranded MN450 arranged in a braided configuration.
Citation: American Journal of Veterinary Research 74, 3; 10.2460/ajvr.74.3.381
Photograph of 6-stranded MN450 arranged in a braided configuration.
Citation: American Journal of Veterinary Research 74, 3; 10.2460/ajvr.74.3.381
Photograph of 6-stranded MN450 arranged in a braided configuration.
Citation: American Journal of Veterinary Research 74, 3; 10.2460/ajvr.74.3.381
The suture loops were fixed within a hydraulic load frame.b One end of the load frame was equipped with a hydraulic ram, and the other end was equipped with a 22-kN (5,000-lb) load cell, which was used to measure the tensile force applied to the suture material (Figure 3). The hydraulic ram was preset to move at a rate of 60 mm/min. The displacement of the ram and the force applied to the load cell were recorded concurrently; data were acquired at a rate of 100 measurements/s.
Illustration of a hydraulic load frame with a prosthetic suture loop (left) with a close-up illustration of the knot configuration (right). Notice that 3 of the 5 square throws applied to each suture construct used for testing have been made.
Citation: American Journal of Veterinary Research 74, 3; 10.2460/ajvr.74.3.381
Illustration of a hydraulic load frame with a prosthetic suture loop (left) with a close-up illustration of the knot configuration (right). Notice that 3 of the 5 square throws applied to each suture construct used for testing have been made.
Citation: American Journal of Veterinary Research 74, 3; 10.2460/ajvr.74.3.381
Illustration of a hydraulic load frame with a prosthetic suture loop (left) with a close-up illustration of the knot configuration (right). Notice that 3 of the 5 square throws applied to each suture construct used for testing have been made.
Citation: American Journal of Veterinary Research 74, 3; 10.2460/ajvr.74.3.381
Measurements, calculations, and statistical analysis—Displacement of the ram was measured concurrently with force applied to the load cell. The displacement of the ram when the first positive increase in force was recorded was taken to be zero. Further displacement of the ram was taken to represent elongation of the material being tested (eg, suture constructs or CCL). The total displacement of the hydraulic ram when the material (first strand for multistranded configurations) ruptured was recorded as the total elongation prior to failure. The peak force measured by the load cell at this point was also recorded. The AUC was calculated for each material tested by means of the trapezoidal rule. The total AUC was interpreted as the total energy required to reach ultimate failure of the material.
Statistical analysis—Statistical calculations were performed with a statistical software package.c Normality of group data was verified via a Kolmogorov-Smirnov test. The means were compared via a 1-way ANOVA with a Tukey multiple comparison test to compare individual groups. Select individual groups were also compared via an unpaired t test. Results are given as a mean ± SD. Significance was set at P < 0.05 for all tests.
Results
The native CCL ruptured at a mean force of 4,541 ± 1,417 N. Four of 6 ligaments had midbody rupture, and 2 of 6 ligaments had avulsion fractures of the attachment to the tibial eminence. The mean elongation for the bovine CCL was 2.0 ± 0.3 cm. Force versus elongation (stress-strain) curves for the bovine CCL were similar for each specimen except one. This ligament ruptured at a markedly lesser force and did not have a clear ultimate failure point as did the remaining 5 ligaments. This ligament appeared to fail by progressive fiber bundle disruption at the point of insertion on the tibia rather than focal rupture. When this CCL was removed from the data set, the mean ultimate force at rupture and elongation was 5,062 ± 689 N and 2.1 ± 0.2 cm, respectively. The mean loads of rupture of the various suture configurations were tabulated (Table 1).
Mean ± SD values for maximum force required to rupture, elongation, and AUC for 6 bovine CCLs and synthetic constructs (n = 6 samples each).
| Material and configuration | Maximum force (N) | Elongation (cm) | Elongation (%) | AUC (N•cm) |
|---|---|---|---|---|
| Bovine CCL | 4,541 ± 1,417*a | 2.0 ± 0.3†a | 6,132 ± 1,037a,b,c | |
| Single PC8 | 359 ± 28.7b | 2.8 ± 0.3a | 14 ± 2 | 456 ± 83.0d |
| 3-braided PC8 | 993 ± 58.5b,c | 7.0 ± 1.1b,c | 35 ± 5 | 2,767 ± 419a,d |
| 6-braided PC8 | 1,937 ± 128c,e | 6.9 ± 0.6b,c | 35 ± 3 | 5,535 ± 344a,b,c |
| 3-braided MN80 | 1,090 ± 89.3b,c,e | 5.4 ± 0.4b | 27 ± 2 | 2,619 ± 250a,d |
| 6-braided MN80 | 2,024 ± 257c,e | 6.4 ± 0.8b,c | 32 ± 4 | 5,407 ± 710a,c |
| Single MN450 | 2,087 ± 57.4d,e | 9.8 ± 0.9d,e | 49 ± 4 | 8,130 ± 474b,c |
| 3 parallel MN450 | 6,260 ± 239f | 3.3 ± 0.1a | 16 ± 1 | 9,418 ± 381b |
| 3-braided MN450 | 5,310 ± 369a,f | 10.9 ± 1.0d | 54 ± 5 | 29,988 ± 2,346 |
| 6 parallel MN450 | 11,512 ± 489g | 8.0 ± 0.4c,e | 40 ± 2 | 48,451 ± 1,107 |
| 6-braided MN450 | 11,333 ± 1,334g | 13.4 ± 1.8 | 67 ± 9 | 75,486 ± 5,730 |
Mean ± SD force required to rupture the bovine CCLs was 5,062 ± 689 N when 1 outlier was excluded.
Mean elongation of the CCLs was 2.1 ± 0.2 cm when 1 outlier was excluded.
PC8 = No. 8 polymerized caprolactam suture. – = Not applicable.
Within columns, values with the same superscripts are not significantly (P ≥ 0.05) different.
The tensile strength of cords constructed of 3 strands of MN450 most closely approximated the maximum tensile strength of the bovine CCL. The ultimate tensile strength of cords constructed of 3-braided strands of MN450 was not significantly (P = 0.228) different from the ultimate tensile strength of the bovine CCL; the tensile strength of cords constructed of 3 parallel strands of MN450 was significantly (P = 0.015) greater than the ultimate tensile strength of the bovine CCL. The tensile strength of cords constructed of 6 strands of MN450 was significantly greater than that of the bovine CCL when the strands were arranged in parallel configuration (P < 0.001) and in braided configuration (P < 0.001). The maximum tensile strength of all other materials arranged in various configurations was significantly less than that of the bovine CCL.
The forces required to rupture the cords of MN450 constructed in braids of 6 strands was not different from the force required to rupture the cords constructed of 6 parallel strands of MN450 (P = 0.865). The mean rupturing force was significantly (P < 0.001) greater for the 3 parallel strands of MN450, compared with the 3-braided strand configurations, when individual groups were compared via an unpaired t test. However, when mean tensile strength of all groups was compared via 1-way ANOVA, 3-stranded MN450 arranged in a parallel configuration was not significantly different from 3-stranded MN450 arranged in a braided configuration. All the suture constructs ruptured at the knot.
Elongation values of the synthetic constructs were determined (Table 1). Constructs that had elongations that were not significantly different from the bovine CCL (2.0 ± 0.3 cm) included single No. 8 polymerized caprolactam suture (2.8 ± 0.3 cm) and 3 parallel MN450 (3.3 ± 0.1 cm) when all groups were compared via 1-way ANOVA. However, when individual groups were compared via an unpaired t test, the elongation of single No. 8 polymerized caprolactam suture (P = 0.002) and 3 parallel MN450 (P < 0.001) was significantly greater than that of the bovine CCL.
The elongation of the cord at rupture was significantly greater for braided configurations of the same material and strand number. Three-braided MN450 had significantly (P < 0.001) greater elongation than did 3 parallel MN450. Six-braided MN450 had greater elongation than did 6 parallel MN450 (P = 0.006).
The AUC for 3-braided MN450 was significantly (P < 0.001) greater than the AUC for 3 parallel MN450. The AUC for 6-braided MN450 was significantly (P < 0.001) greater than the AUC for 6 parallel MN450.
Discussion
Polymerized caprolactam and monofilament nylon were chosen for testing in this study. Polymerized caprolactam has been used in the authors' practice for surgical management of cattle with CCL ruptures. It is available in spools so that long strands of continuous suture can be used, which are available in larger-diameter sizes, nonabsorbable, and less expensive than many other commercially available sutures. Monofilament nylon has been used extensively in dogs for surgical management of CCL-deficient stifle joints.3,10–14 Advantages of the monofilament nylon include less tissue reactivity than the polymerized caprolactam and that the monofilament nature is less likely to harbor infectious organisms. However, knot-holding ability in large nylon monofilament is poor.
In general, braided configurations have greater elongation than analogous parallel configurations. This is attributable to gap formation that is inherent to the curves placed within the strands to achieve the braided configuration. When tension is applied to these braided configurations, these gaps close as the rope is made straighter and the cord is lengthened. Cords constructed of parallel strands do not have these inherent curves and thus do not have the elongation that occurs in braided configurations. This study found that marked increases in elongation occurred when the large-sized materials were constructed in braided configurations. When strands were arranged in parallel, the materials tested ruptured at a similar point of elongation, compared with the native CCL. These data suggested that large-sized materials used for surgical management of cattle having rupture of the CCL should be implanted as individual strands and not in a braided-rope configuration.
The ideal suture material would have a large AUC because that suture would require the most exerted energy (work) to cause rupture of the suture. This suture would likely be most durable under cyclic activity because it permits greater elongation before rupturing. No cyclic testing was performed, and therefore, nothing in this regard can be concluded from this study. However, the increase in AUC in the braided cords was associated with much greater elongation, which would likely not provide stability in CCL-deficient stifle joints, making these suture configurations less optimal. The braided configurations had elongations between 6.9 and 13.4 cm, which are unacceptable for in vivo applications of a bovine CCL replacement.
The ultimate failure load for CCL in the present study (4,541 ± 1,417 N) was similar to that reported previously for cattle. Moss and Ferguson15 found that the mean force at rupture was 6,720 ± 1,630 N. In that study,15 7 CCLs were tested with the stifle joint extended with similar methods as the present study. It appeared as if most of the ligaments (4/6) failed by midbody rupture, although avulsion of the tibial attachment also occurred in 2 of 6 femorotibial joints that were tested. Although the failure mode of the bovine CCL has not been reported, clinical experience indicates midbody rupture to be a common method of CCL failure in cattle.7 One of the CCLs failed by progressive fiber bundle disruption at the point of insertion on the tibia rather than focal rupture, and this ligament ruptured at a considerably lesser force than the others. When this outlier was removed from the data set, the mean force at rupture in the present study was 5,062 ± 689 N.
This study revealed that cords constructed in parallel configurations had strengths equal to cords constructed in braided configurations for 6 strands of MN450. Parallel configurations of 3 strands of MN450 were superior in regard to tensile strength, compared with braided cords comprised of 3 strands of MN450. Parallel configurations permitted less elongation and are likely superior to braided configurations because they would result in less joint laxity.
The MN450 in cords of 3 strands was the material that most closely approximated the tensile strength of the bovine CCL. Of the suture materials studied, the MN450 in groups of 3 and 6 strands was the only material that had strengths comparable to or greater than the CCL in mature cattle. The added tensile strength afforded by the cords comprised of 6 strands of MN450 may be unnecessary. The bulk created by the additional strands of MN450, especially at the knot region, would likely be disadvantageous. Development of an appropriate fixation method for secure anchoring of the prosthesis to the bony structures is needed prior to clinical use. Cyclic loading tests are warranted to assess fatigue of the construct. If the material possesses durability, then application in clinical trials involving CCL-deficient patients is needed to determine the suitability of this material in clinical practice.
This study used a low number of samples. Power calculations were made, and given the relatively low SD in the tensile strength of the suture constructs, the power of the study was calculated to be sufficient at 88%, for comparison of the mean tensile strength of the bovine CCL with the 3 parallel MN450 suture constructs. For comparison of the elongation with the AUC between these 2 groups, the power was calculated to be approximately 98%.
ABBREVIATIONS
| AUC | Area under the force-versus-elongation curve |
| CCL | Cranial cruciate ligament |
| MN80 | Monofilament nylon fishing line (80-lb test) |
| MN450 | Monofilament nylon fishing line (450-lb test) |
Braunamid, Jorgensen Laboratories, Loveland, Colo.
Instron 1322 hydraulic load frame, Instron Corp, Norwood, Mass.
GraphPad Prism, GraphPad Software Inc, La Jolla, Calif.
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