Cadaver description
Three freshly frozen cadaveric through-the-knee lower-extremity specimens (two right and one left) were used for this study. The median age of the cadavers at the time of death was 63 years (range, 52–70 years); one specimen was from a male and the others from females. The specimens were free of ankle or hind foot deformities, had not undergone prior surgeries or dissections, and did not have histories of trauma or other pathologies that may have altered their anatomy. The cadaveric studies were performed at the University of Barcelona (Spain); the methods were reviewed and approved by the university’s Institutional Review Board. Informed consent for the storage and use of the bodies for research purposes was given by the donors prior to their deaths or by their next of kin.
Strain pattern determinations in normal ATFLs and CFLs
In all specimens, dissections and subsequent strain measurements were performed by one experienced foot and ankle surgeon. An incision was made on the lateral ankle and the ATFL and CFL were exposed. This procedure has been described in detail in our previous study [20]. As the ligaments were investigated as a single unit, they were not isolated after exposure. A force probe was placed into the mid-substance of each ligamentous band of the ATFL and CFL, such that the force probe slit was aligned with the long axis of the ligament fibers (Fig. 1). After introducing the force probe into the ligament, the force probe tube was sutured to the ligament fibers with a 3-0 nylon thread to prevent its rotation [19].
An Ilizarov ring-shaped external fixator was placed on the lower leg, and the lower limb was vertically fixed to the measurement desk. A round metal disk (called a “clock”; diameter, 150 mm) with 6-mm-diameter holes placed every 30° around the clock perimeter was affixed to an acrylic plate (width, 120 mm; length, 280 mm; thickness, 10 mm). Then, the plate was fixed to the plantar aspect of the foot with a screw (diameter, 6 mm) inserted into the calcaneus and a rod (diameter, 8 mm) inserted between the second and third metatarsals (Fig. 2a). This plate had a 25-cm arm to which a 0.5-kg weight could be added to the free end, applying a 1.2-N m force to the ankle and subtalar joint complex (0.5 kg × 0.25 m × 9.81 = 1.23 N m) [19].
The arm was rotated in 30° increments to allow strain measurements of each ligamentous band at different ankle positions. The ankle positions were defined as dorsiflexion when the arm was in the 12 o’clock position, plantar flexion when in the 6 o’clock position, inversion when in the 3 o’clock position, and eversion when in the 9 o’clock position. In addition, the 1 and 2 o’clock positions were defined as dorsiflexion-adduction, 4 and 5 o’clock positions represented supination (plantar flexion-adduction), 7 and 8 o’clock positions represented plantar flexion-abduction, and 10 and 11 o’clock positions represented pronation (dorsiflexion-abduction) (Fig. 2b), according to the terminology proposed by the Ad Hoc Committee of Terminology of the Japanese Society for Surgery of the Foot [21].
After determining the strain in each designated ankle position, the strain values of each ankle ligament were also measured during axial motion of the ankle from 15° dorsiflexion to 30° plantar flexion. The angles of the axial, sagittal, and horizontal motions were measured using an electronic goniometer (MPU-9250; TDK InvenSense, San Jose, CA, USA) synchronized to the MLPP system [19].
Strain pattern determinations in the reconstructed ATFLs and CFLs
For each cadaveric limb, after determining the strain patterns of normal ATFLs and CFLs, these ligaments were removed at their origin and insertion points to identify their anatomic footprints. The ATFL and CFL reconstructions were performed by a technique similar to that previously used actual patients for patients with chronic lateral instability [18]. An gracilis tendon autograft (approximately 135 mm long) was harvested from the cadaver’s ipsilateral knee. The graft was prepared in an anatomic “Y” configuration with graft loops at the three ends of the anatomic Y-graft to facilitate suture attachment and graft delivery, as previously described [18] (Fig. 3a).
A guide wire was inserted into the centers of the fibular ATFL and CFL footprints on the fibular obscure tubercle and directed toward the proximal and posterior edges of the fibula at an angle of approximately 30° up to the long axis of the fibula, allowing the guide wire to pass through the central portion of the fibula on the coronal axis. Then, the guide wire was passed though the posterior cortex of the fibula and through the skin, posterior to the fibula. Over drilling (diameter, 6 mm) was then performed up to a depth of 20 mm. A talar tunnel was constructed to serve as the docking site for the talar stem of the anatomic Y-graft. Next, a guide wire was inserted to penetrate the talus through the center of the ATFL insertion footprint and directed toward the distal end of the medial malleolus. The guide wire was passed through the medial wall of the talus and then through the skin, just anterior and slightly distal to the tip of the medial malleolus. Over drilling was again performed.
Further, a calcaneal bone tunnel was also constructed to serve as the docking site for the calcaneal stem of the anatomic Y-graft. A guide wire was used to penetrate the calcaneal CFL insertion site footprint and directed toward the central posterior cortex of the calcaneus. The guide wire was then over drilled (diameter, 6 mm) to a depth of 30 mm. The three anatomic Y-graft stems were then inserted into their respective tunnels up to a depth of at least 15 mm and fixed with interference screws. Each bony attachment of the tendon graft was fixed with a 6-mm-diameter interference screw while applying a 30-N tension force. The fibular stem was fixed and then the talar stem was fixed while the ankle was in a 0° flexed neutral position. The calcaneal attachment was fixed in a similar manner (Fig. 3b).
After reconstruction of the ATFLs and CFLs, the strain values of the reconstructed ligaments were measured using the MLPP system during axial and three-dimensional motion, similar to the normal ATFL and CFL investigations, as previously described (Fig. 3c, d) [19].
Data analysis
The relationships between the foot positions and the tensile forces of each ligamentous band were analyzed. The tensile force data from the force probe were obtained by synchronizing the arm of the clock, which was manually rotated every 30°, with the movement of the ankle from 15° dorsiflexion to 30° plantar flexion; the movements were repeated 10 times (for each limb) and the strain of each ligamentous band as measured during ankle motion. Individual strain data were aligned to reflect the strain relative to the neutral position (0) and to the maximal strain value (100). The average value determined at each position was connected by a line, and the ligament tension patterns were compared between the normal and reconstructed specimens [19].