Knee Surgery, Sports Traumatology, Arthroscopy https://doi.org/10.1007/s00167-019-05585-1 ANKLE Pressure changes in the Kager fat pad at the extremes of ankle motion suggest a potential role in Achilles tendinopathy F. Malagelada1,2 · J. Stephen3,4 · M. Dalmau‑Pastor2,5,8 · L. Masci6 · M. Yeh4 · J. Vega2,5,7,9 · J. Calder3,4 Received: 29 March 2019 / Accepted: 18 June 2019 © European Society of Sports Traumatology, Knee Surgery, Arthroscopy (ESSKA) 2019 Abstract Introduction The Kager fat pad is one of the largest soft tissue structures local to the ankle joint, yet it is poorly understood. It has been hypothesised to have a role in Achilles tendinopathy. This study aimed to investigate the pressure areas in the Kager fat pad adjacent to the Achilles tendon and to assess the anatomy and deformation of the Kager fat pad in cadavers. Methods Twelve fresh frozen cadaveric ankles (mean age 44 years, range 38–51) were mounted in a customized testing rig, enabling plantar fexion and dorsifexion of the ankle, with the Achilles tendon loaded. A needle tipped pressure sensor was inserted in two areas of the Kager fat pad under ultrasound guidance (retrocalcaneal bursa and at 3 cm proximal from Achil- les insertion). Pressure readings were recorded at diferent fexion angles. Following testing, the specimens were dissected to expose the Kager fat pad and retrieve it for analysis. MRI images were also taken from three healthy volunteers and the Kager fat pad deformation examined. Results Mean pressures signifcantly increased in all specimens at terminal ankle plantar and dorsi fexion in both regions (p < 0.05). The Kager fat pad was consistently adherent to the Achilles at its posterior aspect for a mean length of 7.7 cm (SD 0.27, 89% of KFP length). The most distal part of the Kager fat pad was the exception and it detached from the Achilles to give way to the retroalcaneal bursa for a mean length of 0.92 cm (SD 0.24, 11% of KFP length). The bursal space is partially occupied by a constant ‘wedge’ extension of Kager fat pad. The mean volume of the whole Kager fat pad was 10.6 ml (SD 3.37). Video and MRI demonstrated that the Kager fat pad undergoes signifcant deformation during plantar fexion as it is displaced superiorly by the Achilles, with the wedge being forced into the retrocalcaneal bursal space. Conclusion The Kager fat pad does not remain static during ankle range of motion, but deforms and its pressure also changes. This observation supports the theory that it acts as a shock-absorber to the Achilles tendon and pathological changes to the fat pad may be clinically important in the development of Achilles tendinopathy. Keywords Achilles · Ankle · Anatomy · Biomechanics · Cadaver · Fat pad Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s0016 7-019-05585 -1) contains supplementary material, which is available to authorized users. * F. Malagelada 5 GRECMIP (Groupe de Recherche et d’Etude en Chirurgie [email protected] Mini-Invasive du Pied), Merignac, France 6 Pure Sports Medicine Clinic, London, UK 1 Department of Trauma and Orthopaedic Surgery, Royal London Hospital, Barts Health NHS Trust, London, UK 7 Foot and Ankle Unit, Hospital Quirón and Clinica Tres Torres, Barcelona, Spain 2 Human Anatomy and Embryology Unit, Department of Pathology and Experimental Therapeutics, University 8 Manresa Health Science School, University of Vic–Central of Barcelona, Barcelona, Spain University of Catalonia, Vic, Barcelona, Spain 3 Fortius Clinic, London, UK 9 European Foot and Ankle Society (EFAS)-Research Committee, Basel, Switzerland 4 Department of Bioengineering, Imperial College London, London, UK Vol.:(0123456789)1 3 Knee Surgery, Sports Traumatology, Arthroscopy Introduction Materials and methods Kager’s fat pad (KFP), also known as the pre-Achilles A total of 12 fresh-frozen cadaveric feet and ankles (mean fat pad, is one of the largest lipomatous structures of the age ± SD = 41.4 ± 10.3 years, range 22–52 years; 6 male) leg. It is located in the posterior ankle joint, occupying with no history of surgery or disease were obtained from a Kager’s triangle, bordered by the Achilles tendon poste- tissue bank. The specimens were preserved in polyethylene riorly, the crural fascia and fexor hallucis longus (FHL) bags, stored frozen at − 20 °C and thawed before use. None muscle anteriorly, and the posterior calcaneal tuberosity at of the specimens showed a degree of ankle stifness beyond its base inferiorly. In an anatomical and histological study, the physiological range of motion (5° of dorsifexion to 40° Theobald et al. showed that the KFP has three regions of plantar fexion). that are closely related to the sides of the triangle. They named those the Achilles-related, the FHL-related, and the bursal wedge adjacent to the calcaneus. They concluded Specimen preparation and loading that the three regions have specialized functions: the FHL part contributes to moving the bursal wedge during plantar The foot and ankle specimens were prepared to enable meas- fexion, the Achilles part protects blood vessels entering urement of pressure in the KFP during ankle plantar- and the tendon, and the bursal wedge is thought to minimize dorsifexion. The gastrocnemius and soleus were identifed pressure changes in the bursa. Despite generic descriptions and a cloth material was surrounded and securely stitched of the KFP in the literature, to the best of our knowledge, to the proximal end of muscles, to provide anchorage for the there are no objective data fully describing the parameters application of load to the Achilles tendon. The proximal tibia of the shape and volume of the KFP. was cut at approximately 40 cm above the joint line and an Close relationship of the KFP with the Achilles tendon intramedullary rod cemented into it. Distally the fbula was has been shown to have a protective efect to the blood secured in an anatomical position using a transverse bone vessels crossing the KFP and the tendon itself. It has been screw to minimize joint excursion. postulated that pressure changes within areas of the KFP The ankle was mounted in a testing rig with the foot may contribute to the process of Achilles tendinopathy or pointing downwards and the second metatarsal pointing tears of the fascia cruris [1, 2]. Equally, excision of part downward, aligned with the shaft of the tibia. Tension was of the KFP during hindfoot endoscopy to create a working applied to the Achilles tendon in its physiological direction area may have biomechanical consequences for ankle joint in relation to the tibial axis. The gastroc-soleus complex was function. Similar studies have been performed to assess the loaded with 62 N, in accordance with prior work [6]. This infrapatellar fat pad of the knee suggesting that it plays a load represents an unloaded open kinetic chain calf raise role in stabilizing the patella [3]. Around the KFP, pres- with the knee extended. Higher load levels were not used to sure measurements have also been investigated and con- avoid damaging the soft tissues across the tests performed troversy exists as to whether the extremes of motion result on each ankle. Closed kinetic chain motion (by placing a in higher pressure load to the retrocalcaneal bursa and the board across the foot) was also assessed during pilot test- midportion of the Achilles [4, 5]. ing and gave results similar to those of open kinetic chain The aim of this study was to investigate the pressure assessment. This setup has been used previously for ankle changes during ankle range of motion in two areas of joint testing [6]. Kager fat pad, defned in relation to the Achilles tendon: the mid-portion (location of non-insertional Achilles ten- Pressure measurement assessment dinopathy) and the retrocalcaneal bursa (location of inser- tional Achilles tendinopathy). Secondly, it was aimed to evaluate the KFP deformation during ankle motion directly A Gaeltec CTN/4F-HP pressure transducer with a Lemo in vitro, and using magnetic resonance imaging (MRI) 2306 connector (Gaeltec Devices Ltd, Dunvegan, UK) was in vivo. The fnal aim was to re-examine the anatomical used to measure pressure within the KFP. The tip diameter attachments of the KFP, its dimensions and volume using of the sensor was 1.5 mm, with a sensitivity of 1.17 μV/V/ young, fresh cadavers. It was hypothesised that the KFP is kPa. Calibration was determined as per the manufacturer subject to pressure changes throughout the ankle’s range of guidelines. Test re-test readings from the same specimen motion and deformation that is in close relationship with established excellent repeatability with an ICC of 0.97. the Achilles tendon suggesting a potential proprioceptive Under ultrasound guidance, the tip of the sensor was or feedback role of the KFP. introduced through the skin into two regions of the KFP in contact with the Achilles: (a) the midportion; an area of the Achilles located 3 cm proximal to its insertion onto the 1 3 Knee Surgery, Sports Traumatology, Arthroscopy calcaneum and (b) the retrocalcaneal bursa; at the insertion fexion to full dorsifexion. The participants were asked of the Achilles on the calcaneum. The sensor was inserted to keep the desired position of the ankle during the time under ultrasound vision by an experienced ultrasonographer. required for the obtention of images. The images were made The tip was located at the midpoint of the Achilles width in into a movie, by taking the same sagittal slice from scans the coronal plane and in the two previously mentioned areas taken at diferent fexion angles. in the sagittal plane. Once the tip of the sensor was placed IRB approval was obtained from the local Research appropriately the pressure measurements were taken in dif- Ethics Committee of Imperial College London (Number: ferent ankle degrees of dorsi- and plantar fexion.