Abstract
Introduction
Kager's fat pad (KFP) is located in Kager's triangle between the Achilles tendon (AT), the superior cortex of the calcaneus and Flexor Hallucis Longus (FHL) muscle & tendon. Although the biomechanical functions of KFP are not yet fully understood, a number of studies suggested that KFP performs important biomechanical roles including assisting in the dynamic lubrication of the AT subtendinous area, protection of AT vascular supply, and load and stress distribution within the retrocalcaneal bursa space. Similar to the knee meniscus, KFP has become under increasing investigations since strong indications were found that it serves more than just a space filler. Both KFP and the knee meniscus are anchored to their surrounding tissues via fibrous attachments, they can be found in encapsulated (or ‘air tight’) regions, lined by synovial membranes, and they both slide within their motion ranges. The protruding wedge (PW) of KFP was observed to slide in and out of the retrocalcaneal bursal space during ankle plantarflexion and dorsiflexion, respectively. In-vitro studies of KFP suggest that it reduces the load by 39%, which is similar to that of the knee meniscus (30%-70% of the load applied on the knee joint). This study investigated the in-vivo load bearing functionality of KFP.
Materials and Methods
The ankles of 5 volunteers (3 males, 2 females, Age 20-28, BMI 21-26) were scanned using a 0.2T MRI scanner at ankle plantarflexion and neutral positions. The ankles of 2 of those volunteers were later scanned using a 3T MRI scanner for higher accuracy. The areas and volumes of KFP were measured using Reconstruct¯ 3D modelling software. The hind foot of the volunteers were scanned using dynamic ultrasound to measure in-vivo real time shape changes of PW.
Results
The cross sectional area of KFP in the AT midline saggital plane increased on average by 10% when ankles were changed from neutral to plantarflexion positions. The volume of KFP showed less variation than cross sectional areas (3.9% variation in volume). Previous studies showed the cross sectional area of the knee meniscus changes by 9.8% during loading, or flexing the knee by 90°, and its volume was reduced by 3.5%-5.9% (medial and lateral menisci respectively). Ultrasound imaging showed that PW's thickness decreased during dorsiflexion compared to plantarflexion by an average of 1mm and a hysteresis was found between the location of PW's tip and the insertion angle of AT, suggesting the fibrous tip of PW bears load during dorsiflexion.
Discussions and conclusions
The similarities in results between the knee meniscus (literature review) and KFP supports hypotheses that KFP assists in reducing the load applied at the AT enthesis organ. Furthermore, histological studies showed a fibrosis is evident at the tip of PW, which is thought to be developed as a result of resisting external loading. This also supports speculations that KFP removal is likely to reduce lubrication, pressure distribution, load bearing, and consequently, increasing the tear and wear level within AT enthesis.
