The Function of the Short Medial Collateral Ligaments of the Canine Tarsus: A Cadaveric Study
ABSTRACT
Information on the clinical behavior of cases with an isolated rupture of the short collateral ligaments of the canine tarsus is sparse. Our objective was to evaluate the function of the short medial collateral ligaments (SMCLs) in 90° flexion. Eight cadaveric limbs were tested for internal/external rotation and valgus/varus before and after transection of one or both SMCLs. In one group, the tibiocentral ligament was transected first, followed by the tibiotalar. In the second group, the order of transection was reversed. Angular changes between two k-wires were measured and compared. Internal rotation increased significantly after transection of one or both SMCLs (P = .015 and P = .004), with higher angular changes in the group in which the tibiotalar ligament was transected first (P = .003). Transection of this ligament alone was sufficient to cause caudomedial subluxation upon internal rotation. Valgus angulation increased after transection of one ligament (P = .022), but there was also an increase in varus angulation after transection of both ligaments (P = .027). Unlike the long medial collateral ligament, which stabilizes against deviation toward lateral, the SMCL stabilizes against subluxation toward medial, with the tibiotalar ligament being the major stabilizer in flexion. Findings can be used as diagnostic guidance for isolated tarsal short collateral ligament injuries.
Introduction
Isolated injuries of the short collateral ligaments of the tarsus are relatively rare in dogs. After encountering cases with an instability of the tarsocrural joint in flexion only, we detected a scarcity of information on the clinical behavior and treatment of these injuries in veterinary textbooks. In a recent paper in press, the clinical behavior of a flexed tarsus was examined after isolated transection of the short lateral collateral ligaments in internal/external rotation and under valgus/varus stress.1 Results from that study can be used as diagnostic guidance for cases with ruptures of the short lateral collateral ligaments. Our aim for the current study was to similarly provide information and guidance for cases with suspected medial short collateral ligament rupture.
There are three medial collateral ligaments of the canine tarsocrural joint. The more superficial long medial collateral ligament originates from the medial malleolus of the tibia and attaches solidly onto the central and first tarsal bone, with tenuous attachments distoplantar on the talus and to the base of the first and second metatarsal bones.2,3 The two short medial collateral ligaments (SMCLs) run deep to the long ligament, the tibiocentral ligament, and the tibiotalar ligament.2 The tibiocentral ligament is a long narrow band that runs more or less parallel to the long collateral ligament from the medial malleolus to primarily the first tarsal and metatarsal bones; however, a few fibers connect to the sustentaculum tali of the calcaneus.2,3 The tibiotalar ligament is short but very wide and extends on the plantar surface to attach on the talus.2 Similar to the long portion, the tibiocentral ligament is taut in extension, whereas the tibiotalar ligament is taut in flexion, and is regarded as the substantial short medial part.4 The textbook by Brinker et al. states that there is only a moderate instability of the tarsocrural joint in cases with isolated rupture of the short collaterals; however, further explanation is lacking.5 The only description regarding examination of the integrity of the SMCL complex can be found in the textbook by Slatter, stating a valgus stress to the flexed hock to be performed.6
A study in 1985 looked at the anatomy and function of the tarsal ligaments by means of implanted pins and trigonometric measurements of the distances formed between the pins.3 In that study, the long collateral ligaments were always transected first, and there was no isolated evaluation of the short collateral ligaments with the long ligaments intact. Transection of the medial collateral ligaments led to a 2.7-fold increase in external rotation, a 1.5-fold increase in internal rotation, and a 34-fold increased valgus as well as a 13-fold increased varus angulation in the flexed specimens. This would imply that rupture of the medial collateral ligament complex leads predominantly to increased deviation to the direction opposite the ligamentous damage, that is, valgus and external rotation, even with the hock in flexion.
A recent study on the short lateral collateral ligaments showed that, unlike the description in some textbooks, the rupture of both ligaments leads to a subluxation toward lateral.1 The purpose of the present study was to test whether the same applies to the medial side, that is, if a subluxation toward medial occurs. Furthermore, we wanted to test whether rupture of both short medial ligaments is necessary for subluxation similar to the lateral side, or whether one of the ligaments is more substantial than the other.
Our hypothesis was that transection of the SMCLs would lead to increased valgus as well as increased internal rotation and medial subluxation. Our second hypothesis was that the tibiotalar ligament contributes to stability to a greater extent than the tibiocentral ligament, which follows the direction of the long collateral ligament. This is in contrast to the lateral side, where both short ligaments follow a similar direction, orthogonal to the long collateral ligament, and therefore, both prove to be critical in preventing subluxation.
Material and Methods
Eight canine cadaveric hind limbs from dogs weighing 13.8–35.0 kg (mean 22.2 kg) were harvested after euthanasia for reasons unrelated to this study. Only limbs with no signs of tarsal osteoarthritis, as confirmed by radiographs, were used. The mean age at euthanasia was 8.9 yr. The limbs were disarticulated at the level of the stifle joint and frozen at –18°C. They were thawed at 10°C for 48 hr before evaluation. Four limbs (two right and two left) were assigned to one of two groups. In group A, the tibiocentral ligament was transected first, followed by the tibiotalar ligament. In group B, the order of transection was reversed.
Preparation of the specimens was analogous to a recent study.1 In brief, one k-wire was inserted transversally and perpendicular to the long axis of the bone in the distal tibia above the malleoli and another parallel to the first in the proximal tarsal bones (central and quartal tarsal bones), with correct placement confirmed by radiographs. The tibia was fixed parallel to the table, and the tarsocrural joint was held at a 90° angle by a suture (3/0 Nylon) applied to the skin of the third and fourth toe and suspended to a steady hook 80 cm above the tarsal joint. All specimens were tested with intact ligaments, after transection of the first ligament, and after transection of both ligaments. Valgus and varus angulation were evaluated in the frontal plane, and external and internal rotation were evaluated in the axial plane. A cameraa was mounted directly above the tarsus of the specimen at a distance of 80 cm for evaluation in the frontal plane and horizontally, at the level of the tibia, 80 cm from the calcaneus for evaluation in the axial plane. The distal pin was stressed with manual pressure until maximum range of motion was reached, and the force was stopped before deflection of the pin. The applied forces were measured with a handheld Newton meter that was attached to the pin between two nuts. Each directional force was repeated three times, and photographs were taken each time. The angles formed between the two pins were measured on the photographs using image evaluation softwareb and recorded as the “angular value.” Changes between intact and transected specimen were calculated as the “angular value change.” Measurements were performed on each of the three photographs, and the mean values were used for statistical analysis. The mean values were compared with statistical standard softwarec after normality of the distributions was confirmed with Kolmogorov-Smirnov tests. Paired t tests were used to compare the intact with the transected specimen values, and unpaired t tests were used to compare groups A and B. The significance level was set at P ≤ .05.
Results
The most significant finding was that internal rotation increased significantly when both short collateral ligaments were transected (P = .004) as well as when only one of the ligaments was transected (P = .015). There was a significant difference between group A and B in regard to which ligament was cut first, with higher angular changes in group B, where the tibiotalar ligament was cut first (P = .003).
Valgus angulation was significantly increased with one of the ligaments transected (P = .022) but did not reach significance with both of the ligaments transected (P = .063). The difference between group A and B also did not reach significance (P = .073). No significant changes could be detected when external rotational forces were applied. With both ligaments transected, varus angulation was also increased (P = .027), with no differences between group A and B (P = .5). Tables 1 and 2 show the exact values.
The tibiocentral ligament followed the direction of the long collateral ligament and originated slightly craniodistal to it, passed deep to and inserted slightly caudal to it (Figure 1). The tibiotalar ligament originated axially (laterally) on the apex of the medial malleolus and inserted slightly caudal on the talus (Figure 2). It was necessary to incise the joint capsule in order to cut the tibiotalar ligament completely because of its partially intra-articular location.
Citation: Journal of the American Animal Hospital Association 55, 5; 10.5326/JAAHA-MS-6909
Citation: Journal of the American Animal Hospital Association 55, 5; 10.5326/JAAHA-MS-6909
Internal rotation led to subluxation toward mediocaudal, which could be increased with a combined internal rotation and valgus force; however, valgus force alone did not lead to subluxation. This subluxation was possible after transection of only the tibiotalar ligament.
The force that was necessary to achieve the end of range of motion was 1.5–1.75 N in all samples, and no increased force was necessary to lift the trochlea of the talus over and caudal to the medial malleolus.
The overall standard deviation of all measurements of the repeated trials was 0.43°.
Discussion
This is the first study to evaluate the function of the SMCLs of the canine tarsus in isolation from the long collateral ligaments. The results show that an isolated rupture of the SMCLs leads to subluxation of the pes toward medial and caudal, which is different to what has been described in the veterinary literature.6 To avoid incorrect treatment decisions, it is important to recognize that the opposite has been shown in cases of rupture of the long collateral ligament. To the best of our knowledge, no studies have further analyzed this important clinical difference so far.
Our first hypothesis was confirmed. Although increased valgus angulation has been previously shown to occur after transection of the SMCLs, we demonstrated that transection of the SMCLs also leads to increased internal rotation.3 This subluxation toward the side of ligamentous damage has not been previously reported for the medial side. Furthermore, when both short collateral ligaments were transected, there was an increase in varus, namely, increased movement toward the side of ligament disruption. This was unexpected and highlights the importance of performing internal rotation during examination for ligamentous damage because contrary to what has been described in the literature, performing valgus/varus force alone is not sufficient to differentiate between the sides of ligament disruption.
Our second hypothesis that the tibiotalar ligament contributes to stability to a greater extent was confirmed for the case of internal rotational force, in which the values were higher after transection of that ligament than after transection of the tibiocentral ligament. For valgus angulation, there was a trend for higher angular value changes after transection of the tibiotalar ligament, even though this did not reach significance (P = .073), which may be as a result of a type II error associated with our small sample size.
Taking a closer look at the anatomy of the short collateral ligaments, it becomes clear that the tibiotalar ligament is the major contributor to stabilization of the medial tarsocrural joint in flexion because it runs in a near orthogonal angle to the long collateral and the tibiocentral ligament. Internal rotation actually increases the distance between its insertion points, which explains our findings of an increase in internal rotation after transection of this ligament, which has not been previously reported in the literature.
The same applies to valgus angulation, in which the distance between origin and insertion points of not only the tibiotalar ligament but also some of the fibers of the tibiocentral ligament is increased. The increase in varus angulation after transection of both ligaments may be explained by losing the tight connection between the medial malleolus and the talus that is provided by the tibiotalar ligament. Furthermore, the tibiocentral ligament gives off some fibers to the sustentaculum tali of the calcaneus, which may contribute to stability during varus force.
Subluxation toward medial was possible after transection of only the tibiotalar ligament. On the lateral side, subluxation is only possible if both short collateral ligaments are disrupted, and an increased force of up to 5 N was necessary to lift the trochlea of the talus over the lateral malleolus.1 The subluxation toward medial was not as distinct and was not always accompanied by a click as on the lateral side, and no increased forces beyond 1.5–1.75 N were necessary to cause subluxation. One reason for this may be the relative prominence and caudal extension of the lateral malleolus compared with the medial malleolus and the lifting of the trochlea tali over it. On the medial side, the trochlea subluxates in a more mediocaudal direction, passing by the malleolus more easily; therefore, a complete lift over the malleolus as on the lateral side does not seem to be necessary, explaining the smoother motion during subluxation. This also makes detection of a short medial collateral tear more difficult than on the lateral side, where a distinct click can be felt.
Whereas subluxation toward lateral, representing an injury of both short lateral collateral ligaments, was reported in a clinical case series by Sjöström et al. in 1994, subluxation toward medial, and therefore isolated rupture of the tibiotalar ligament, has not been reported in a clinical case in the literature until today.7 The authors have encountered one such case since the beginning of this study and could confirm an isolated tear of the tibiotalar ligament with an intact tibiocentral and long collateral ligament. It is possible that this injury is not as uncommon as it may seem because it may easily be overlooked if the clinician is not familiar with the pertinent function of each of the short collateral ligaments and not actively looking for it. The study by Aron and Purinton and the information in the textbook by Slatter may mislead the clinician to assume an increase in internal rotation to be associated with rupture of the short lateral rather than the SMCLs.3,6 The main reason for the differing findings may be that in their study, the long collateral ligaments in all specimens were transected before the short collaterals were approached. Our study helps to better understand the stabilizing function of the short collateral ligaments and can be used as guidance for diagnosing isolated tarsal short collateral ligament injuries.
Although it may be seen as a limitation of this study that the applied force was standardized to reach the maximum range of motion, rather than using a set force, the overall standard deviation of the measurements of the repeated trials was only 0.43° and shows a very high repeatability and low variability between trials. Manual stress appeared to offer a better control for the endpoint and prevented the force from being exaggerated, especially with the different cadaver sizes. We tried to further overcome this limitation by measuring the force that was necessary to reach the endpoint of range of motion. The forces that were necessary to cause subluxation were very small, ranging between 1.5 and 1.75 N, showing that the instability that is caused by rupture of the short collateral ligaments is significant.
A further limitation is that a broad range of dog sizes and breeds was used based on the availability of cadavers. Different breeds of dogs may have different laxity in their joints and therefore different angular values of valgus/varus and external/internal rotation. Therefore, the absolute values detected in our study may not be used as reference points; instead, further studies should evaluate these parameters for different breeds and sizes of dogs.
Conclusion
The results of this study show that an isolated rupture of the tibiotalar portion of the SMCLs of the canine tarsus leads to subluxation of the pes toward medial. There is increased internal rotation and valgus after disruption of the tibiotalar ligament. However, an increase in varus after transection of both ligaments supported the stabilizing influence of the tibiocentral ligament in flexion as well.
This important difference to the situation following rupture of the long medial collateral ligament, where subluxation is toward lateral, must be kept in mind when treating clinical patients in order to avoid continued instability. For cases with a suspected isolated short collateral ligament injury, it is important to perform not only valgus and varus forces but also internal and external rotation in flexion in order to diagnose the side of injury correctly. Findings of this study and a previously reported study can be used as diagnostic guidance for isolated tarsal short collateral ligament injuries.1
Image of the tibiocentral ligament (dashed line), passing deep to the long collateral ligament (dotted line). The asterisk notes the tendon of tibialis caudalis muscle. The pes is oriented toward the bottom of the image, and the proximal limb is oriented to the right on the image.
Image of the tibiotalar ligament (continuous line), passing from underneath the medial malleolus to the talus. Note the near orthogonal direction to the long collateral ligament (dotted line). The asterisk notes the tendon of tibialis caudalis muscle. The pes is oriented toward the bottom of the image, and the proximal limb is oriented to the right on the image.
Contributor Notes
R. Schuenemann’s present affiliation is Small Animal Hospital Sattledt, Sattledt, Austria.
SMCL (short medial collateral ligament)
