Una comunicación para salir del desarrollo

IPFP, also known as Hoffa’s fat pad, is an intracapsular but extrasynovial structure that locates in the anterior compartment of the knee [70]. It is richly vascularized and innervated, and may play an important role in anterior knee pain. As an important source of inflammation, IPFP may also be involved in the progression of knee OA [14]. This section reviews the literature of IPFP anatomy, histology, biomechanics, cells and secretory profiles.

1.2.1.1 Anatomy

IPFP is the biggest adipose structure in the anterior knee joint (Figure 1.4). It is situated beneath the patella, behind the patellar tendon and in front of the femoral condyles and tibial plateaus plateau [71]. As it is interposed between the joint capsule and the synovial membrane that lines its posterior aspect, it has been considered as an intracapsular but extrasynovial structure [70]. It attaches to the intercondylar notch of the femur by the plica ligamentum mucosum, as well as to the anterior cruciate ligament (ACL) in some subjects [72]. The normal IPFP also connects to transverse meniscal ligament, medial and lateral meniscal horns and retinaculum [72, 73]. It consists of a central body with medial and lateral extensions, occupying almost the whole anterior part of the knee [72]. Two clefts are typically present within the IPFP: one is vertical and located at the superior part of the IPFP and the other is horizontal and in a posteroinferior position [73].

It is vascularized by a rich anastomotic network. The supromedial and suprolateral geniculate arteries provide 2 vertical arteries and are linked horizontally with arteries running distally, while the central part of IPFP has low vascularization [74]. IPFP is also richly innervated, with an abundance of nerves from a branch of the tibial nerve arising from anterior fibbers of the popliteal plexus [75]. There were various nerves, such as a terminal branch of the obturator nerve, nerves to the vastus medialis and vastus lateralis, lateral articular and recurrent peroneal branches of common peroneal nerve, and the infrapatellar branch of the saphenous nerve, that also contribute to the innervation of the IPFP [75]. About one-quarter of the sensory fibres in the IPFP are nociceptive fibres targeted by substance P, which causes vasodilation that promotes the recruitment of immune cells [76]. As a richly vascularized and innervated structure in the knee, IPFP can be considered as an important source of knee pain and stiffness.

Figure 1.4 Anatomy of the infrapatellar fat pad [77] 1.2.1.2 Histology

The typical IPFP appears yellow and may also include lobules of fat extending from the surface, a superior segment extending posteriorly and frond-like projections of the synovium [78]. It is structurally similar to subcutaneous adipose tissue [79], comprising of a network of adipocytes, fibroblasts, leukocytes and collagen matrix. Otherwise, a recent histotopographic study shows that IPFP consists of white adipose tissue, of lobular type, with lobules delimited by thin connective septa, and the IPFP lobule area is smaller and the interlobular septa were thicker than subcutaneous adipose tissue of the abdomen, while the IPFP lobule area is lager and the interlobular septa were thinner than subcutaneous adipose tissue of the knee [80]. An animal

study also shows that IPFP contains two specific parts: inner tissue and outer tissue, which have different chemical compositions [81]. This study suggests that these two different tissues have different functions: the inner tissue is probably subjected to a compressive load, while the outer tissue is probably subjected to a tensional load. All these suggest that, unlike systemic adipose tissue, IPFP may have its specific functions in the knee joint.

1.2.1.3 Biomechanics

Along with the knee flexion, the angle between the patellar tendon and the anterior border of the tibia decreases, and the IPFP is extruded posteriorly as the space narrows [82]. While with the knee extension, it moves away from the anterior tibia [82]. Although the exact function of the IPFP is still not well understood, it may be involved in facilitating distribution of synovial fluid across the knee joint, providing lubrication [83, 84]. Furthermore, it probably provides shock absorbance from mechanical forces (similar to the menisci) and maintains knee stability [85-87]. As IPFP is an adaptable structure, its shape, position, pressure and volume vary considerably throughout the normal range of motion (ROM) of the knee joint [82, 86]. Thus, it could act as a plastic portion aiming to absorbing of pressure variations during knee joint activity [80].

1.2.1.4 Cell and secretory profiles

Adipose tissue was thought to act only as a reservoir for excess calories that are stored as triacylglycerol; whereas, it is believed that it could play an active role in physiologic and pathologic process in immunology and inflammation [88, 89]. As a specialized form of connective tissue, it contains a number of adipocytes, fibroblasts, macrophages, leukocytes, and other cells involved in inflammation. IPFP has the typical histological structure and the same cell profiles as adipose tissue [90]. The immune cells in IPFP can produce and release

inflammatory mediators, while adipocytes are responsible for the production of adipokines such as leptin, resistin and adiponectin [88]. Furthermore, it can secrete lipids (i.e. fatty acids) in the process of lipolysis. The details of each secretory profiles and their associations with knee OA will be discussed in Section 1.2.3.