# Effects of Achilles Tendon Moment Arm Length on Insertional Achilles Tendonopathy
## 2 Materials and Methods  
This is a retrospective study. Fourteen individuals (16 feet) who required surgical treatment for IAT at our hospital between 2014 and 2019 were included in the IAT group. Those with a history of collagen diseases or trauma other than Achilles tendon injuries, and those who could not be followed up for more than one year after surgery were excluded from the study. As a result, 8 individuals (10 feet; IAT group) were examined in this study. Fifteen healthy individuals (with no complaints of ankle joint injuries nor history of surgical trauma) were included for examination in the control group. The reconstruction in all IAT cases was performed using the Achilles SpeedBridge system (Arthrex, FL, USA). After extending the Achilles tendon in the midline and partly separating it from the insertion, the osteophytes, intra-tendinous degeneration site, and posterior superior calcaneus at the enthusi were excised. Subsequently, fiber tape was used to fix the Achilles tendon to the calcaneus using four knotless anchors. We analyzed a clinical evaluation by the American Orthopedic Foot and Ankle Society ankle/hindfoot scale (AOFAS scale) and visual analog pain scale (VAS) 1 year after the surgery. At 6 months to 1 year after the surgery, 1.5T MRI (Vantage Titan Ver3.0, Canon, Tokyo, Japan) images were taken to evaluate the morphology of the Achilles tendon insertion area and abnormal signal in the tendon before and after surgery. Figure 2: We performed a reconstruction of the Achilles tendon with a suture bridge technique. ��❝: Bone spurs, calcifications in the tendon, and Haglundý  
Plain X-ray images were used to measure the lever arm. The images of the affected side were taken for patients in the IAT group, while those of the healthy side were taken for patients in the control group. As described by [PERSON] et al. (2010), who used plain X-ray images of the feet during weight-bearing, the weight-bearing plane was taken as the plantar axis from the lateral image; a straight line perpendicular to this plantar axis, passing through the center of the Achilles tendon, was set as the central axis of the Achilles tendon. The shortest distance from the central axis of the Achilles tendon to the center of the first metatarsal head parallel to the plantar axis was defined as the length of the sole. A perfect circle was drawn to fit the pulley surface of the talus, and the center point was used as the center of motion. The shortest distance from the center of motion to the central axis of the Achilles tendon was defined as the MA length. In addition, the angle between the Achilles tendon moment arm and the line from the center of the ankle joint movement to the insertion of the Achilles tendon was defined as the \(\alpha\) angle (Figure 3). The central axis of the Achilles tendon at the time of postoperative measurement was the midpoint from the most dorsal surface to the bottom of the insertion (Figure 4). Also, the ratio of the MA length/foot length was used to standardize the difference in foot size. Furthermore, in order to take the influence of body weight into account, the force applied to the Achilles tendon at the time of heel raise can be calculated as the rotational force around the ankle joint axis. Then, the force applied to the Achilles tendon, F, can be estimated by:  
\[\text{F}=\text{w}\times(\text{L}-\text{MA})/\text{MA}\]  
where w is the body weight, MA is the Achilles tendon moment arm length, and L is the length of the sole (14; 18; 19) (Figure 5). In the present study, this formula was used to calculate the force applied to the Achilles tendon, and the average value of three measurements performed by one examiner was used. Both the control group and the IAT group were examined, and all measurements were taken pre- and post-operatively. Statistical tests were performed using Microsoft Excel. Data were tested using unpaired student \(t\)-tests, and effects were considered significant at 95% CI and \(p<0.05\). All subjects gave their informed consent for inclusion  
Figure 4: B: Plantar axis.