Is load control necessary to produce physiological AP displacement and axial rotation in wear testing of TAR?

2.1 Specimen preparation

Six pairs of fresh-frozen cadaveric lower limbs cut midshaft of the shank were obtained from a licensed willed body tissue institute, Science Care (Science Care, Phoenix, AZ). Specimen donors were equally half female and half male, with ages ranging from 30 to 93. All specimens were screened for any pre-existing conditions, including relevant orthopaedic conditions. The specimens had all extraneous fat and tissues removed, while keeping all ligaments around the ankle complex intact. Care was taken to minimize the disruption to the interosseus ligament between the retained tibia and fibula. The Achilles tendon was removed to allow direct access to the talus. Additionally, the hard and soft tissues distal to the navicular were removed, leaving only the tibia, fibula, talus, calcaneus, navicular, and their surrounding connective ligaments. A single 5 cm long, 4 mm diameter screw was used to fix the proximal end of the fibula to the tibia to protect the interosseus ligament. Two 80 mm long, 7 mm diameter screws were placed through the talus from the posterior surface, passing the neck into the head without piercing through the anterior surface. These screws extended beyond the level of the most posterior edge of the calcaneus. To ensure proper screw placement, fluoroscopy was used to guide the angle of approach along with the depth of insertion (Figure 1).

2.2 Potting

The proximal ends of the tibia and fibula were then potted in low-temperature bonding resin (Bondo 3M). Bone cement was poured into a square-shaped brace which surrounded the entire specimen and captured the heads of the posterior screws that were previously inserted into the talus. This created a “halo” of bone cement which allowed for control of the talus position using the posterior end of the screws (Figure 2). The neutral position was determined using an alignment similar to the method described by Leardini et al.¹⁴ Radiographs were taken to confirm the neutral alignment before the creation of the halo.

2.3 Apparatus design

With the halo attached, each specimen was loaded using a custom apparatus mounted into an 8-DOF load frame (858 Bionix, MTS Systems). The proximal end of the specimen, potted into cement, was rigidly attached to the axial-torsional actuator on the superior aspect of the load frame. The distal end of the specimen (calcaneus) rested on a baseplate, 3D printed from tough PLA filament (Tough PLA 202300, Ultimaker). The halo was mounted into a tray which was in turn attached to the baseplate using four hardened steel bars which allowed free vertical translation of the halo relative to the baseplate, while restricting rotation and horizontal translations. On each bar two round flange linear ball bearings (Model 6483K52; McMaster-Carr) ensured low-friction gliding of the tray parallel to the baseplate. The halo was thus used to produce plantarflexion and dorsiflexion of the talus through the screws, while the baseplate resisted axial loads. To allow plantarflexion and dorsiflexion to occur at the natural axis of rotation of the talus, the baseplate and its attachments were allowed free AP translation by a low-friction linear bearing mounted on the inferior base of the load frame (Figure 3).

The axial force and axial torque were applied to the proximal end of the tibia and fibula under force and torque feedback control respectively. Following the ISO 22622 standard, an AP force was applied to the tibia; specifically, the force was applied to the proximal end of the specimen using a linear actuator and spring. All other degrees of freedom of the tibia were restricted (mediolateral translation and rotation in the coronal and sagittal planes). Plantarflexion and dorsiflexion were effectively applied using rotation control (in degrees), as specified by ISO 22622, using a linear actuator mounted to the fixed distal linear rail. The baseplate resisted axial loads applied to the tibia through the talus and calcaneus, however it allowed free sliding of the calcaneus in the horizontal plane.

2.4 Loading conditions

Due to the physical limitations of cadavers and to prevent soft tissue damage, the axial force, AP force, and tibial torque magnitudes were reduced to 50% of the respective load profile specified by ISO 22622. These were all implemented at a frequency of 1 Hz ± 0.1 Hz. This method of loading was based on previous biomechanical studies of cadaveric tibiotalar joints that have applied reduced loads below full body weight to preserve the integrity of the joint.¹⁵ A summary of all of the forces applied in the present study is shown in Figure 4.

2.5 Data acquisition

An Optotrack Certus motion tracking system (Northern Digital, Inc.) was used to record the motions of the tibia, talus, and calcaneus. The motion capture system has an accuracy of 0.1 mm and a resolution of 0.01 mm using motion flags, equipped with 3 noncollinear light-emitting diode (LED) markers that pulse at 3000 Hz. A minimum of 3 LED markers are required to obtain rotations (R_x, R_y, R_z) along with translations (x, y, z), while a minimum of 1 marker is needed for pure translations. These motion flags were attached via bone screws to flag holders placed in the talus, tibia, and calcaneus. An additional flag was mounted on the load frame to establish a fixed local coordinate axis and to minimize the effects of any vibrations of the hydraulic machine.

An ATI 6 DOF load cell (Mini 58; ATI Industrial Automation) was used to capture all force data, including axial force, AP force, and axial torque. The motion tracking system was used to capture all translations and rotations. All the measured forces recorded were plotted and analyzed against the applied command signals. Similarly, since the dorsiflexion or plantarflexion was not directly controlled, it was also important to check the measured applied angular displacement. The resulting AP displacement from the applied loads was compared to those specified in the ISO displacement control standard. Similarly, the resulting tibial rotations from the applied axial torque was compared to those specified in the ISO displacement standard.

3.1 Divergence of actual loads and motions from applied command profiles

The actual applied loads and motions were compared to the respective programmed load or motion profile based on ISO 22622.

Under axial load, the specimens responded to the load command profile without substantial deviation and without structural failure (Figure 5A). Specifically, the specimens reached the peak load at 38% of the gait cycle, consistent with the axial load command profile. In contrast, the specimens did not follow the dorsiflexion–plantarflexion command profile closely. Specifically, while the dorsiflexion command increased steadily between 10% to 50% of the gait cycle, the actual dorsiflexion of the specimens increased later, after 30% of the gait cycle (Figure 5B). Similarly, the specimens responded to the AP load command profile with some deviation. Specifically, the specimens peaked slightly earlier than the command profile in posterior force and slightly later in anterior force (Figure 5C). Finally, the specimens responded to the axial torque command profile with some deviation. Specifically, the specimens demonstrated somewhat of a phase delay with respect to the command profile (Figure 5D).

3.2 Observed outcome: Motions of the talus

A wide range of AP displacement of the talus was observed among the specimens. While on average the displacements were within approximately a millimeter of the specified ISO profile, the peaks and valleys were as much as 2 mm apart among the specimens: the talus moved from 3.3 mm posteriorly to 3.0 mm anteriorly (Figure 6A).

A wide range of axial rotation of the talus was also observed among the specimens. On average, the peaks and valleys of the rotation were lower in magnitude than those specified by the ISO profile (Figure 6B). However, the peaks and valleys of some of the individual speimens exceeded the ISO profile. Overall, among the specimens, the observed rotations ranged from 11° of external rotation to 4.5° of internal rotation.

4.1 Observed AP displacement under load control versus ISO 22622 AP displacement

We compared the observed motions of the ankle that resulted from applying the load control option in ISO 22622 to the AP displacement and axial rotation profiles specified by the displacement control option. At 50% of the specified loads, the observed AP displacement curve of the talus was similar to the published ISO AP displacement profile. Moreover, the resulting displacement magnitudes did not exceed the AP displacements specified in the ISO 22622 standard. The resulting curve followed the expected physiological movements and was consistent with a typical gait cycle. Specifically, at the initiation of the gait cycle, when heel-strike occurred (0%–10%), the ankle was plantarflexed which created the measured anterior (negative) displacement. Subsequently, during 10%–45%, the ankle transitioned from maximum weight acceptance to push-off which was correlated with high dorsiflexion. This dorsiflexion was represented by a posterior (positive) translation which was also observed. Following this section, the ankle moved into heel-off and the pre-swing phase which was characterized by ankle plantarflexion. This would suggest anterior (negative) translation, which is the behavior that was observed.

4.2 Observed IE rotation under torque control versus ISO 22622 IE rotation

The resulting internal and external (IE) rotations varied widely among the specimens. However, the trend of the mean rotations was similar to the specified rotations in ISO 22622. An internal moment was expected to create a positive rotation which was consistent with both the motion tracking data collected from the specimens and with the internal rotation ISO command. Similarly, as the gait cycle progressed, an external moment was applied and was expected to result in a negative rotation of the tibia. Both the specimen recorded data and the ISO rotation protocol agreed on these rotations. Moreover, from the applied torque profile, the tibia did not exceed the rotations suggested in the ISO 22622 internal–external rotation angles.

This data may suggest that for AP displacement and tibial rotations, load or displacement control may effectively produce the same kinematics. Conversely, Sutton et al.¹³ reported that for cadaveric knees, force control may be more suitable due to the high reliance on ligament stability for the knee. Interestingly, the ankle is unique in that it is stabilized by both ligaments and articulating bone surfaces.^{18, 19} At low loads, displacements may be small and well within the viscoelastic range of the ligaments providing stabilization. However, at high loads, the ankle effectively becomes a rigid lever relying on its articulating bone structure for stability. This may provide an explanation on why higher loads and torques do not necessarily produce higher displacements in the ankle, as seen in knees. While force control produced physiologically correct displacement trends, the benefits of force control may not outweigh the complexity of conducting the experiment using the ISO load control option for TARs.

4.3 Limitations

The present study had several limitations. First, due to the limitations of structural strength of cadaveric ankles, the applied force profile command was 50% of the magnitudes specified by ISO 22622. However, the plantarflexion–dorsiflexion was applied under rotation control as specified by ISO 22622. Further research must be conducted to investigate if testing at the full 100% will change the observed AP and axial rotational displacements. While the cadaver specimens successfully underwent the axial load profile at 50% of the magnitudes specified by ISO, these specimens did not precisely follow the dorsiflexion-plantarflexion rotation profile, AP load profile, and IE torque profile; this was perhaps due to ligament constraints and various joint impingements. For example, at lower loads, ankle stability may be dependent on ligaments; however, at higher loads, the articular geometry may be the primary stabilizer.^{19, 20} Specifically, at these higher loads that occur from mid-stance to push-off, the bones are locked into a forced position to create a rigid lever of support to propel the subject forward.¹⁸ Therefore, under the sharp increase in axial load from 20% to 30% of gait that coincides with the highest peak of AP load, the specimen may have been relying on the tibiotalar joint for stabilization. The differences in AP loads can be attributed to the variance in specimen flexibility. Another limitation was the AP force applied to the tibia was not feedback controlled; therefore, the load observed was reliant on the stiffness of the specimen and its resistance to the force produced by the displacement-controlled actuator and spring mechanism. Finally, without the cuneiform and metatarsals, the specimens may have been biomechanically compromised, altering the response.

5.1 Statement of clinical significance

Wear testing under load control has been advocated to produce physiological AP displacements and axial rotations for TAR. However, in the present study load control of cadaveric ankles did not produce substantially different AP displacements and axial rotations than those specified for displacement control in ISO 22622.