Ankle Ligament Tensile Forces at the End Points of Passive Circumferential Rotating Motion of the Ankle and Subtalar Joint Complex

INTRODUCTION

Injury of the lateral ankle ligaments frequently occurs in daily life and recreational activities. ^4,5,7,9 It has been estimated that there is about one inversion injury of the ankle per 10,000 people each day, ¹¹ which corresponds to approximately 23,000 injuries in the United States each day. These injuries can result in pain and impairment for over 6 months and can interfere with daily activities, including sports and work. Understanding ankle ligament mechanics is important in guiding treatment and rehabilitation. Of the lateral ankle ligaments, the two most frequently injured are the anterior talofibular (ATF) ligament, which functions to stabilize the talus, particularly when the ankle is plantarflexed, and the calcaneofibular (CF) ligament, which functions to stabilize the ankle and subtalar joints when the ankle is in neutral dorsiflexion-plantarflexion. ¹² Nonoperative treatment involves physical modalities, such as the use of a wobble board, to encourage range of motion (ROM) to prevent the development of stiffness and to facilitate proprioception. ¹⁴ However, it is possible to accentuate lateral ankle ligament injuries or delay full recovery by an exercise program. There is uncertainty regarding the most effective early functional treatment (Kerhoffs 2003). Linde et al. ⁸ reported that all athletes had an increased risk of residual symptoms, and residual symptoms occurred in 32% of top athletes after 1 year.

To our knowledge there is no information available regarding ankle ligament forces at the end points of passive circumferential rotating motion of the ankle and subtalar joint complex in various positions. This information could be useful in providing more objective guidelines for a successful rehabilitation program after a lateral ankle ligament sprain. Furthermore, primary repair or late reconstruction of the lateral ankle ligaments could be more successful if immobilization could be limited.

Limited information is available regarding ankle ligament tensile forces through full ROM of the ankle. Nigg et al. ¹⁰ measured ankle ligament forces using an indirect method. The length changes of the ATF, CF, and deltoid ligaments were measured with dividers and a ruler, and bone-ligament-bone preparations were tested with load-tension on an Instron machine. However, their measurements were limited between 15 degrees dorsiflexion and 30 degrees of the plantarflexion, which was less than the limits of ankle-hindfoot motion. Bahr et al. ¹ reported ankle ligament forces measured using a direct method with axial loading on the tibia. However, their measurement also was limited between 10 degrees dorsiflexion and 20 degrees of plantarflexion. In both of these studies, the measurements were restricted to estimating ankle ligament forces in dorsiflexion and plantarflexion.

The purpose of this study was to measure the tensile forces of the ATF, CF, posterior talofibular (PTF), and tibiocalcaneal (TC) ligaments simultaneously at the end points of passive circumferential rotating motion of the ankle and subtalar joint complex in various foot positions.

MATERIALS AND METHODS

Eight fresh-frozen human cadaver specimens obtained from two males and four females were disarticulated at the knee joint. The mean age of the donors was 67 (14 to 98) years. There were five right and three left specimens. None of the specimens had any pathology evident by direct vision or on anteroposterior and lateral radiographs.

The proximal tibiofibular joint connection was preserved, and an intramedullary rod was inserted into the tibia. The proximal tibia and the intramedullary rod were embedded in polymethylmethacrylate. Each specimen was mounted on the custom-made testing table in an inverted position.

An incision was made in the lateral ankle, with care taken to preserve the integrity of the underlying retinaculum, tendons, and ligaments. The ATF, CF, and PTF ligaments were identified. A force probe (Microstrain, Burlington, Vermont) was placed in the midsubstance of each of these three ligaments. Each probe consisted of a 1.2 × 4-mm tube with a longitudinal slit. Each sensor had two strain gauges. The nonlinearity was less than 2.5%, hysteresis was less than 0.3%, and repeatability was within 0.3% difference in these force probes. A medial ankle incision was made to expose the deltoid ligament. The tibiocalcaneal ligament, which is the midsubstance of the superficial deltoid ligament, was selected for placement of the force probe.

A round disk, which was 45 cm in diameter and 2.3 kg in weight, was fixed on the plantar foot with nuts and four threaded plastic 6-mm diameter rods. Two rods were inserted into the calcaneus from the plantar foot. One rod was inserted between the first and second metatarsals, while another rod was inserted between the fourth and fifth metatarsals. The foot was mounted in such a way that the center of the foot plate was aligned along the axis of the tibia, with the foot and ankle in the neutral position.

A pilot study conducted using two specimens to determine appropriate loading levels in order to achieve maximum range of motion without causing ligament rupture. We selected 0.5 kg for physiologic ROM and 1 kg to obtain maximal ROM. A load of 0.5 kg and 1 kg were applied to the foot through an acrylic plastic loading arm to provide both axial compression and a bending moment to the foot and ankle (Figure 1). The loading arm was 53 cm in length, and the position of the arm was adjusted in 10-degree increments circumferentially. The anterior, medial, posterior, and lateral holes on the round disk corresponded to 0, 90, 180, and 270 degrees, respectively. The loading started with the weight on the anterior and plantar part of the foot, which caused the ankle to be positioned in maximal dorsiflexion and rotated medially. Therefore, the loading arm rotated clockwise in the right foot and counter-clockwise in the left foot. Before loading in each position, the foot was repositioned in neutral.

An electromagnetic device for measurement of three-dimensional positions of multiple sensors, the 3Space FasTrak system (Polhemus, Colchester, VT 05446) was used. ⁶ The system monitored the three-dimensional movement of the foot-plate, calcaneus, and talus simultaneously. A magnetic sensor was mounted on the lateral wall of the calcaneus, and another sensor was mounted on the talar neck.

After testing, the ATF, CF, PTF, and TC ligaments were carefully detached with a bone block placed at each end, preserving the bony attachments. The bone-ligament-bone preparations were connected to a load cell that was calibrated using static weights in increments from 10 N to 200 N. The output from the force probe for each ligament was calibrated using this technique.

Data Analysis

The relationship between the foot positions and the forces were analyzed. The tensile force data from each force probe were obtained in each position and the ligament forces corresponding to positions of maximal dorsiflexion, plantarflexion, supination, and pronation were compared. A paired t-test was used to determine if differences were significant at a level of p < 0.05.

OPEN IN VIEWER

Fig. 1:

Testing apparatus with a foot-plate applied plantarly and the ankle loaded in maximal dorsiflexion.

RESULTS

In two specimens, one from a 98-year-old female and one from a 77-year-old female, complete data were not obtained because of ligament rupture during 1-kg loading. In the remaining six feet, data acquisition was completed.

The feet had characteristic movement patterns when loaded circumferentially. The ankle ligament forces for each ligament varied considerably, depending on the position of the foot during loading. The overall magnitude of the forces measured also varied among the specific ligaments tested. Loading the foot-plate at 80 and 90 degrees caused a severely supinated foot position, which was maintained until the 270-degree testing position (Figure 2). Analysis of the kinematic data indicated that there were two stable zones where limited displacement of the calcaneus occurred relative to the talus (i.e. subtalar joint motion). Between those two stable zones were transition zones where most of the subtalar movement occurred in supination and pronation.

Although there was peak in CF ligament force in supination, the tensile forces measured in each ligament under 10-kg loading are shown in Figure 3. Maximal forces in the ATF occurred at extreme supination and averaged 108 ± 62.8 N with 0.5 kg loading and 130 ± 39.1 N in 1-kg loading condition. The ATF ligament force in supination was significantly larger than pronation and dorsiflexion. The magnitude of ankle ligament forces measured in the ATF was greater for each position in 1-kg loading than in the 0.5-kg loading.

The maximal forces in the CF ligament were observed in pronation and plantarflexion and with 0.5-kg loading averaged 68 ± 48.6 N and with 1-kg load averaged 135 ± 92.9 N. The CF ligament force in pronation was significantly larger than in supination and dorsiflexion.

OPEN IN VIEWER

Fig. 2:

Position of the plantar foot in relation to the tibia under 1-kg loading condition. Loading the footplate at 90 degrees caused the foot to supinate (lower part of the curve), whereas at 270 degrees the foot was pronated (upper part of the curve). At 180 degrees loading position, the foot was plantarflexed (right side of curve), and at zero degrees it was dorsiflexed (left side of the curve).

The tensile force measurements in the PTF ligament were maximal in dorsiflexion and averaged 131 ± 80.1 N with 0.5-kg loading and 109 ± 36.3 N in 1-kg loading. The PTF forces in dorsiflexion were significantly larger than in plantarflexion.

Maximal tensile forces in the TC ligament were observed when the ankle was in pronation and averaged 49 ± 80.1 N with a 0.5-kg loading and 67 ± 69.6 N for the 1-kg loading. The deltoid ligament force in the pronation position was significantly larger than in supination and dorsiflexion. For each of these ligaments, the measured forces were greater in the 1-kg loading condition than in the 0.5-kg loading condition.

DISCUSSION

Our data indicate that the ATF ligament has an important role in supination with plantarflexion. The CF and TC ligaments are important in pronation with plantarflexion, and the PTF is an important stabilizer in dorsiflexion. There are a variety of factors affecting stability of the ankle joint, such as the congruity of the joint surfaces, ligamentous joint capsules, and muscle forces. Stormont et al. ¹³ called attention to the contribution of the joint surface congruity as an important primary static restraint with the ankle in the neutral position. Other factors besides joint surface congruity are important, particularly when the ankle and hindfoot are in plantarflexion and supination, which is the usual position in which lateral ankle ligament sprains occur. Understanding the ankle ligament forces in extreme positions, such as plantarflexion and supination rather than just neutral position or selected positions is important in determining individual ligament functions. Bahr et al. ¹ used buckle transducers to measure forces on the lateral ankle ligaments during simulated weightbearing and limited the measurements to the range between 10 degrees of dorsiflexion and 20 degrees of plantarflexion. There are some recognized limitations in using buckle transducers, in part because of the anatomy of the lateral ankle ligaments with short ligament length and joint impingement by the transducer. The present study demonstrated the applicability of force probes in determining ankle ligament forces.

Relatively high magnitudes of ankle ligament forces were observed in normal ankles with circumferential and axial loading. Engebretsen et al. ² used a buckle transducer, and the maximal load in the ATF ligament was 38 N in supination and plantarflexion, whereas the maximal load of the CF ligament was 109 N in supination and dorsiflexion. The indirect methods reported by Nigg et al. ¹⁰ found the ATF ligament force to average 48.3 N at maximal inversion, maximal internal rotation, and maximal plantarflexion. The CF ligament force was 46.3 N at the maximal inversion and dorsiflexion. The deltoid ligament load averaged 48.7 N in maximal eversion and maximal external rotation at 15 degrees of plantarflexion. In our study, the magnitudes of peak forces in the ligaments measured were higher than previous results. One of the main reasons for the higher magnitudes was that we tested at the endpoints of the ankle and subtalar joint complex in the open kinematic chain. These data suggest that there may be a need to restrict the extreme positions of the ankle and foot in a rehabilitation program after lateral ankle ligament injury. The different forces observed in the polar diagram (Figure 3) for each ligament was related to the specific anatomic position of each ligament tested.

OPEN IN VIEWER

Fig. 3:

Polar diagrams of ligament forces under 1-kg of loading. Mean ankle ligament forces are demonstrated. B, ATF ligament C, CF ligament. D, PTF ligament. E, Talocalcaneal ligament. ATF ligament force was highest at the 90 degree loading position with the foot supinated.

This method may have future applications in studies designed to simulate ligament rupture. It also will be applicable for demonstrating the mechanical efficacy of various prefabricated and custom-made orthoses as well as other treatments, such as athletic taping and restricting ankle ligament forces.