The infrapatellar fat pad is a dynamic and mobile structure, which deforms during knee motion, and has proximal extensions which wrap around the patella
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The infrapatellar fat pad (IFP) is a common cause of knee pain and loss of knee flexion and extension. However, its anatomy and behavior are not consistently defined.
Thirty-six unpaired fresh frozen knees (median age 34 years, range 21–68) were dissected, and IFP attachments and volume measured. The rectus femoris was elevated, suprapatellar pouch opened and videos recorded looking inferiorly along the femoral shaft at the IFP as the knee was flexed. The patellar retinacula were incised and the patella reflected distally. The attachment of the ligamentum mucosum (LMuc) to the intercondylar notch was released from the anterior cruciate ligament (ACL), both menisci and to the tibia via meniscotibial ligaments. IFP strands projecting along both sides of the patella were elevated and the IFP dissected from the inferior patellar pole. Magnetic resonance imaging (MRI) of one knee at ten flexion angles was performed and the IFP, patella, tibia and femur segmented.
In all specimens the IFP attached to the inferior patellar pole, femoral intercondylar notch (via the LMuc), proximal patellar tendon, intermeniscal ligament, both menisci and the anterior tibia via the meniscotibial ligaments. In 30 specimens the IFP attached to the anterior ACL fibers via the LMuc, and in 29 specimens it attached directly to the central anterior tibia. Proximal IFP extensions were identified alongside the patella in all specimens and visible on MRI [medially (100% of specimens), mean length 56.2 ± 8.9 mm, laterally (83%), mean length 23.9 ± 6.2 mm]. Mean IFP volume was 29.2 ± 6.1 ml. The LMuc, attached near the base of the middle IFP lobe, acting as a ‘tether’ drawing it superiorly during knee extension. The medial lobe consistently had a pedicle superomedially, positioned between the patella and medial trochlea. MRI scans demonstrated how the space between the anterior tibia and patellar tendon (‘the anterior interval’) narrowed during knee flexion, displacing the IFP superiorly and posteriorly as it conformed to the trochlear and intercondylar notch surfaces.
Proximal IFP extensions are a novel description. The IFP is a dynamic structure, displacing significantly during knee motion, which is, therefore, vulnerable to interference from trauma or repetitive overload. Given that this trauma is often surgical, it may be appropriate that surgeons learn to minimize injury to the fat pad at surgery.
KeywordsAnatomy Infrapatellar fat pad Hoffa Structure Knee
Infrapatellar fat pad
Anterior cruciate ligament
Magnetic resonance imaging
Lateral collateral ligament
Medial collateral ligament
A special thank you to the radiographers of Fortius clinic for their assistance in scanning and David Hillier of Siemens for his advice.
This study was funded with a grant from Fortius Clinic, London.
Compliance with ethical standards
Conflict of interest
No authors have a conflict of interest or competing interests to declare.
Ethical approval for this study was obtained from Imperial College London REC, approval number: R160014.
Video 2: Viewed from medial to lateral at the IFP with the femur fixed, and the tibia flexed around it. The medial meniscus is removed to improve visualization. The IFP can be seen to fill down behind the patellar tendon in knee extension and move out of the anterior interval during knee flexion. (MOV 42643 KB)
Video 3: An animation showing IFP deformation from the segmented MRI scans during movement through 0º-120º flexion, played on a loop. On the left the IFP has been segmented on its own and on the right, with the tibia (blue), patella (pink), and tibia (yellow). (WMV 1630 KB)
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