Ankle Joint Pressure Change in Varus Malalignment of Tibia

Background: Varus malalignment of tibia could alter ankle biomechanics, might lead to degenerative changes of the ankle joint. However, previous studies failed to report the detailed changes of ankle biomechanics in varus malalignment of tibia. The aim of this biomechanical study was to evaluate ankle joint pressure change in response to the gradual progression of varus malalignment of tibia. Methods 8 fresh-frozen human cadaver legs were tested in this study. Varus malalignment of tibia and a total of 600N compressive force was simulated using a custom made fixture. Intra-articular sensors(TeckScan)were inserted in the ankle joint to collected ankle joint pressure data. The testing sequence was 0°, 2°,4°,6°,8°,10°,12°,14°,16°,18°,20° of tibial varus. Results As the tibial varus progressed, the center of force(COF) shifted laterally both for medial and lateral aspect of the ankle joint. For medial aspect of ankle joint, the lateral shift reached its maximum at 6º[2.76(1.46)mm, p=0.001] and 8º[2.76(1.62)mm, p=0.002], while for lateral aspect of ankle joint, the lateral shift reached its maximum at 12º[2.11(1.19)mm, p=0.002], thereafter, the COF shifted medially as the tibial varus progressed. The lateral joint pressure ratio was 0.481(0.125) at 0º and 0.548(0.108) at 10º and 0.517(0.101) at 20º. Significant differences were found between 0º and 10º(p=0.002), 10º and 20º (p=0.002)of tibial varus. Conclusions For mild tibial varus deformities, there was a lateral shift of COF and lateral stress concentration within the ankle joint, while as the deformity progressed, COF shifted medially and lateral stress concentration decreased.


background
The alignment of the lower extremity is critical for orthopaedic surgeons, both for preoperative and postoperative evaluation. It is believed that the varus or valgus deformity is highly associated with osteoarthritis, especially the knee joint. [1][2][3][4] Varus malalignment of tibia can be the result of malunion of tibia fractures or varus knee deformity. Varus knee is common in population, previous studies demonstrated that the knee joint loading and kinematics was altered in patients with early knee ostoearthritis. [5] Varus malalignment of tibia can alter normal biomechanics of the ankle joint, including stress concentration on the medial aspect of the ankle joint, prolonged stress would result in degenerative changes of cartilage within the ankle joint. [6] However, compensatory function of the adjacent joints is common, Norton and colleague [5] studied the compensatory mechanism of hindfoot for advanced knee osteoarthritis, the found that as the mechanical axis angle became either more varus or valgus, the hindfoot would subsequently orient in more valgus or varus position in order to maintain a normal mechanical alignment of the lower extremity.
Therefore, for patients with varus malalignment of tibia, would the subtalar joint compensate for the above deformity? and would this compensatory mechanism further affect the ankle joint biomechanics? Unfortunately, these biomechanical studies had failed to report the detailed ankle joint pressure change in response to a progression of varus malalignment of tibia.
The aim of this biomechanical study is to investigate the detailed ankle joint pressure change as the tibial varus progressed. We hypothesize that the pressure on lateral aspect of ankle joint will increase as the result of valgus inclination of the subtalar joint in response to the varus tibia malalignment. As the varus deformity progressed, the valgus inclination of the subtalar joint will reach its' maximum and then fail, thereafter, the subtalar joint will convert into a varus inclination result in a sudden medial stress concentration. methods A total of 8 fresh-frozen human cadaver legs were utilized for the biomechanical testing, the specimens were thawed to room temperature(24ºC). The mean age of the cadavers was 71.4(7.4) years old, 4 of the 8 were men, the other 4 were left.
None of the cadaver specimen had malalignment of the tibia, hindfoot, nor preexisting subtalar joint osteoarthritis. All specimen had normal range of motion of both ankle and subtalar joint. The anterior soft tissue (including skin, subcutaneous tissue, anterior joint capsule, tendons and neurovascular bundles) of the ankle joint were dissected for the access to the ankle joint. Both the medial and lateral ankle ligaments were well preserved.
The tibia and fibular was cut at 20cm above the ankle joint. For each specimen, the proximal tibia and fibular was potted securely into a custom made shell, then mounted on a custom make fixture. The tibia and fibular were embedded and securely fixed into the shell using dental gymsum. Load was applied to the tibia via the custom made shell. Each specimen must be potted in neutral position, no plantarflexion or dorsiflexion of the ankle joint in sagittal plane, no varus or valgus malalignment of the hindfoot in coronal plane, no internal or external rotation of the 5 foot in horizontal plane.
A custom designed fixture was utilized for the testing. Spirit levels were utilized to make sure both the working table and the top plate was horizontal throughout the entire testing process. The varus malalignment of tibia, including 0º,2º,4º,6º,8º,10º,12º,14º,16º,18º,20º, was simulated by the custom made apparatus. Each hole of the apparatus represented a specific varus angle. A bolt was used to fix the specimen at a desired varus angle.
The four threaded polyethylene pillar were utilized to connect the top plate and to apply compressive forces. Sensor cells were placed in each pillar, boxes with screens were connected to each sensor cell, the real-time force could be read and recorded on the screen. Springs were placed right above each force sensor, then followed by nuts, compressive forces can be generated by twisting the nut on the spring.
The sensor pads(Model 6900, TekScan, Inc., South Boston, MA), with each pad measuring 14*14mm, each pad had 121 senels(11*11 sensels), the column and row spacing was 1.3mm, resulting in a spatial resolution of 0.62 mm 2 per sensel. Two pads were put side by side within the ankle joint for the measurement of ankle joint pressure. The sensor pads were inserted into the ankle joint from anterior and secured by thumbtacks to the distal tibial metaphysis and the foot in order to avoid sensor motion during testing. [7,8] The sensor pads were connected to the handle which could be further connected to a personal computer, data including pressure, force was collected using I-Scan software. (Figure 1) 6 The baseline ankle joint pressure distribution was collected for each specimen at 0º of tibial varus. The specimen was fixed at 0º of tibial varus by inserting a bolt, the foot was then placed onto the floor freely. A compressive force was generated through the 4 pillars by twisting the nuts. Make sure the top plate was horizontal throughout the testing. A 600N compressive force was applied to simulate the normal load across the ankle joint during ambulation. Both the medial and lateral ankle joint pressure data was collected. Then free the specimen by removing the bolt and then the top plate of the fixture was lifted upward, followed by 2º of tibial varus, then 4º,6º,8º,10º,12º,14º,16º,18º,20º. Both the medial and lateral aspect of the ankle joint pressure data were collected for each alignment.
Statistical analysis SPSS V.23 software (IBM Inc. , New York) was used for the data analysis. Matlab was used for the calculation of shift of center of force(COF), the peak pressure(P m ax ) and the mean pressure(P m ean ). The lateral shift of COF relative to the ankle joint was defined as positive, the medial shift was defined as negative. S-W test was used for the test of normality. Paired student t test was utilized to determine the significant differences for lateral shift of COF, change of P m ax and P m ean at different tibial varus deformities. The level of significance was set to a p value <0.05.

The COF shift
As the tibial varus progressed, the center of force shifted laterally both for medial and lateral aspect of the ankle joint. The maximum lateral shift of medial COF was when compared with 0º of tibial varus. Then the COF shifted medially as the tibial varus progressed. (Figure 2) The lateral shift of medial aspect COF was -1.46(1.08)mm at 20º of tibial varus, significant difference was found between 8º and 20º, p=0.001. The lateral shift of lateral aspect of COF was 0.65(0.69)mm at 20º of tibial varus, significant difference was found between 12º and 20º of tibial varus, p=0.026. (Figure 3)

The P m ax
As the tibial varus progressed, it seemed that the P m ax on both medial and lateral aspect of the ankle joint decreased gradually, it was 5303.4(1105.1)kPa at 0º and 4432.1(948.6)kPa at 20º for medial aspect of the ankle joint, significant difference was found for the medial P m ax at 0º and 20º of tibial varus, p=0.045. However, it was 5135.5(1095.3)kPa at 0º and 4279.4(666.9)kPa at 20º for lateral aspect of the ankle joint, no significant difference was found for the lateral P m ax at 0º and 20º of tibial varus, p=0.116.

The P m ean
The P m ean for medial aspect of the ankle joint was 2266.4(672.7)kPa at 0º, 1891.2(563.9)kPa at 10º and 1618.7(421.5)kPa at 20º. When compared with baseline mean pressure at 0º, significant differences were found for both 10º and 20º. The P m ean for medial aspect of the ankle joint decreased as the tibial varus progressed. (Figure 4) For lateral aspect of the ankle joint, the P m ean was 8 2103.8(625.1)kPa at 0º, 2295.3(589.7)kPa at 8º and 1748.9(467.2)kPa at 20º of tibial varus. Significant differences were found between the P m ean at 0º and 8º(p=0.047), the P m ean at 8º and 20º(p=0.002). The P m ean for the lateral aspect of the ankle joint increased at 8º and then decreased at 20º of tibial varus. (Figure 5) The lateral joint pressure ratio In order to investigate the joint pressure change on the medial and lateral aspect of the ankle joint, we introduce the lateral joint pressure ratio, it was a ratio of the lateral mean pressure divided by the whole joint mean pressure. The lateral joint pressure ratio was 0.481(0.125) at 0º, 0.548(0.108) at 10º and 0.517(0.101) at 20º. These studies also demonstrated that the subtalar joint does compensate for the varus ankle deformity. However, they failed to explain how this compensatory mechanism happened and whether this compensation would alter the joint pressure distribution within the ankle joint? The aim of this biomechanical study was to investigate the detailed ankle joint pressure for varus tibia malalignment at a series of different deformities angles.
We found a lateral shift of COF and lateral stress concentration for mild varus deformities. As the tibial varus progressed from 0º to 20º, the COF of both medial [5] So our biomechanical study favors previous studies, the subtalar joint might be compensating for the deformities above ankle joint, however, we failed to find a sudden medial stress concentration as the subtalar joint compensation fails.
According to Hayashi and colleagues [5], they found that the valgus inclination of subtalar joint will convert into varus inclination as the varus ankle deformity increase, because of the failure of subtalar joint compensation. In this study, the maximum tibial varus deformity was 20º, further studies should be added to explore whether the medial stress concentration will occur as the the deformity angle is greater than 20º.
Varus malalignment of tibia after tibia fracture malunion often involved with a varus tilted ankle joint, the ankle joint is normally congruent, in these cases the subtalar joint might compensate for the varus ankle deformity. However, Xie and colleague [5] found that in woman as the knee mechanical axis became more varus, the distal tibial plafond became more valgus, but this correlation was not found in male patients. This study failed to tell the reason why difference was found between female and male, the subtalar joint was also not investigated in this study.
Probably, as the varus knee OA is a chronic pathology, the valgus inclination of distal tibial plafond could be the compensatory mechanism in response to varus knee mechanical axis.
There are some limitations for this study. First, this is a preliminary study which only included the joint pressure data, the compensatory mechanism should be supported by adding the radiological data of the ankle and subtalar joint(including CT data and weightbearing X-rays), these will make our findings more convincible.
Further studies are needed to fully understand the compensatory mechanism of subtalar joint for varus tibia malalignment. Second limitation is that this is a static simulated weightbearing biomechanical study, the ankle and subtalar joint biomechanics might act differently in vivo, we also neglected the tendon forces around the ankle joint in this study.

Conclusions
We found a lateral shift of COF and lateral stress concentration for mild tibial varus and medial shift of center of force for greater deformities, these might be the valgus inclination of subtalar joint in compensation for the varus tibial malalignment. However, further biomechanical studies are needed to fully understand the subtalar joint compensatory mechanism for varus tibial malalignment.

Consent for publication
Written consent to publish the related images or clinical details of specimens was provided by donors included in the study. The specimen was mounted in the custom make fixture, the force applied on each pillar coul 17 Figure 2 The lateral shift of medial COF as the tibial varus deformity progressed. The asterisk(*) deno 18 Figure 3 The lateral shift of lateral COF as the tibial varus deformity progressed.
19 Figure 4 The mean pressure of the medial aspect of the ankle joint as tibial varus progressed.
20 Figure 5 The mean pressure of the lateral aspect of the ankle joint as tibial varus progressed.
21 Figure 6 The lateral joint pressure ratio as the tibial varus progressed.