Structure of patellar fat pads

The Infrapatellar fat pads (IFP) are two wedge shaped fatty structures situated below the kneecap (patella), lying either side of the patellar tendon, first described by Hoffa in 1904.  The IFP consist of fat cells (adipose tissue) packed closely together and separated from other tissues by fibrous septa. They appear yellow to dissection and are typically 20-40 ccs in volume depending on overall bodysize.

The IFP lies beneath the patellar tendon, and in front of the femoral condyles. It is intracapsular but extrasynovial, lying between the joint capsule itself and the synovial membrane; the posterior surface of the IFP being covered by synovial membrane.  On each side of the IFP the synovial membrane forms a small fold (alar fold), while the membrane over the lower part of the pad merges to form a midline fold attaching up into the region between the femoral condyles (infercondylar fossa).  This is the infrapatellar synovial fold, which may sometimes be thickened into a plica (ligamentum mucosum or infrapatellar plica).  This type of synovial plica, is considered a remnant of the wall which separates the joint into three chambers during a baby’s development (present until the fourth intrauterine month).  Where this causes restricted motion of the IFP, the plica may be surgically released using an arthroscopic approach (Smallman et al 2018).  The IFP is attached the anterior horns of the knee menisci (semi lunar cartilages) via the transverse meniscal ligament , and the inner proximal aspect of the patella tendon, inferior pole of the patella, patella retinaculum and periosteal covering of the tibia (Dragoo et al 2012).

The patellar fat pads have an abundant blood supply and are well innervated.  They have two small arteries which run parallel to the patellar tendon, and these branch from the inferior genicular arteries within the knee.  The outer portion of the IFP has an abundant blood supply, but the central portion less so.  The IFP has a vascular connection to the ACL and synovium and so may be affected by pathology to these structures.  The IPF is innervated via branches from the femoral, common peroneal, and saphenous nerves.  The fat pad nerve fibres contain substance-P and type IVa free nerve endings. (Draghi et al 2016, Dragoo et al 2012).  Arthroscopic probing without anaesthesia has been shown to create pain which is severe and localised.  While injection of saline into the IFP produced both pain (5-6 on a 10 point VAS scale) and delayed onset of vastus medialus obliquus activity (Dye 1998, Hodges et al 2009).

The fat pads have an abundant blood supply and are well innervated.

Function

The IFP is flexible and able to change shape and volume to accommodate movement.  When the knee is fully flexed, the IFP fills the anterior aspect of the intercondylar notch, but as the knee extends, the fat pad covers the trochlear surface of the femur within the patellar groove.  The function of the IFP would seem to be mainly to reduce friction between the surrounding adjacent structures including the patella bone, patellar tendon, and underlying tibia and trochlear surface of the femur depending on flexion angle.

The IFP also secretes a number of chemicals, and so may have an endocrine function (Eyland and Chevalier 2016).  Cytokines and leptin are released, and the IFPs react strongly to proinflammatory stimulation creating concentrations of proinflammatory cytokines (including interleukin 6) similar to those produced by the synovium.  These proinflammatory chemicals may provide an association of the IFP with osteoarthritis (OA).  IFP volume and inflammation correlates highly with pain and functional impairment of the knee (Ballegaard et al 2014).  These authors used dynamic contrast-enhanced MRI to assess the extent of inflammation in the knee and found both pain and Knee injury and Osteoarthritis Outcome Score (KOOS) were correlated with inflammation within the IFP in 95 patients of average age 65 years.   Additionally, a 2-fold increase in the release of interleukin 6 (IL-6) has been shown within the IFP of obese females with knee OA compared to general adipose tissue, an increase which could contribute to adjacent cartilage damage (Distel et al 2009).

 

Pathology

Pain arising from the patellar fat pads falls broadly into two categories, mechanical and metabolic.  The usual pathological pattern of the IFP is mechanical irritation and enlargement causing increased pressure with resultant pain (Hoffa’s disease).  Direct trauma can cause haemorrhage and local oedema, but more normally impingent occurs with hyperextension of the knee if the IFP is enlarged.  Patients typically present with a history of pain located below the kneecap and to the side of the patellar tendon (inferomedial more normally, inferolateral less often).  Conditions may be categorised as traumatic, secondary, or masses (see table).  The IFP may be injured at the time of patella dislocation, giving persistent pain following reduction.  It is pierced during arthroscopic surgery as the pad lies at the arthroscopic access point, sometimes leaving horizontal fibrosis along the portal path.  During ACL repair, the stub of repaired ligament (graft) may thicken and spread (arthrofibrosis) forming a Cyclop’s lesion at the anterior aspect of the joint.  This thickened mass may press into the IFP.  Similarly, meniscal tears or cysts my move forwards into the pad.  Injury to adjacent tendons can cause swelling which migrates into the IFP and masses such as a ganglion or chondroma (capsule thickening / metaplasia) may form.  Synovial disorders such as effusion, or benign vascular thickening (hemangioma) may place pressure on the patellar fat pads.  These may occur without symptoms however, so their presence on MRI is not diagnostic in isolation.

Metabolic effects as detailed above typically occur as a result of changes related to obesity.  Obesity clearly means a person is overweight and increases loading on weight bearing joints including the knee.  However, in addition to the mechanical effect of extra bodyweight, the metabolic effects of increased white adipose tissue is to increase circulating pro-inflammatory chemicals driving systemic inflammation.  Fat cells (adipocytes) have been shown to produce fat cell derived hormones (adipokines).  These are cell signalling chemicals (cytokines) which drive inflammation.  The increase in inflammation of this type is a factor in several pathologies including cardiovascular disease and osteoarthritis.  Locally, the IFP represents a fat depot which can exude pro-inflammatory chemicals directly to the knee joint likely effecting cartilage degeneration and synovial inflammation (Santangelo et al 2016).

Examination

Examination of the patellar fad pads begins with a standard knee exam of standing alignment and gait motion.  Range of motion (active and passive) of the knee in general is used to determine both quantity and quality of movement in comparison with the unaffected limb.  Motion range and end feel of the joint provide information concerning a possible capsular pattern suggesting intra-articular pathology.  Once the knee (tibiofemoral) joint is cleared, closer examination is made of the patellafemoral (PF) joint and fat pads.  Resting position of the patella is noted, and patella path assessed during knee flexion (non-weight bearing, and full weight bearing).

 

The contour of the IFP is assessed in comparison to the unaffected side to give some indication of volume, and palpation is used to assess pain levels.  Importantly familiarity of symptoms should be identified – does palpation reproduce the symptoms (pain for example) which the patient feels in day-to-day actions that forced them to seek treatment.  Pain and function should also be assessed using functional measures such as front and side step-down actions and ¼ squat movement, noting knee to ankle alignment and any varus or valgus knee drift.

The special test for the IFP is pain located deep to the inferior pole of the patella during resisted quadriceps contraction (Hoffa’s sign).  The patient lies supine, with the knee flexed to 20° over a bolster.  The therapist places the thumb and first finger of one hand below the patella either side of the patellar tendon directly over the IFP.  The other hand is placed on the tibia to resist knee extension as the patient activates their quadriceps muscles.  The action is performed slowly and repeated 3-5 times for consistency of examination.

Therapy

Relief of mild compression pain may be achieved with V taping to unload the IFP.  The tape begins over the tibial tubercle and extends laterally and medially. The tape is placed under traction to lift the patella in a proximal (cephalic) direction. To maintain skin integrity a mesh undertape is placed beneath non-elastic (zinc oxide) tape.  Taping is thought to form a sling to prevent the patella pressing into the fat pad to allow recovery.  The tape may be reinforced using an upper tape strip placed over the proximal half of the patella (distal aspect free) with the aim of tilting the patella upwards.  MRI scan of the knee pre and post taping of this type has been shown to alter the shape of the IFP (Dragoo et al 2012), but the longevity of shape change has not been reported.  The tape aims to modify symptoms and facilitate rehabilitation perhaps through non-associative learning.

The tape aims to modify symptoms and facilitate rehabilitation perhaps through non-associative learning.

Rehabilitation follows the broad aims of that are used for patellafemoral pain syndrome (PFPS) see post on kneecap pain  Foot orthoses may be useful as an interim measure where malalignment is associated with symptoms.  Greater midfoot mobility and reduced dorsiflexion motion range at the ankle are often co-factors, and orthotics may be used if they reduce patient symptoms.  Exercise should be progressive and combine both hip and knee actions in general rather than focussing on isolated knee movements.  Hip stability movements in lying and kneeling such as the clamshell and fire hydrant, can be useful where hip and trunk alignment give rise to a positive Trendelenburg sign (hip dipping during single leg stance).  Single leg balance actions may progress from supported to unsupported and finally a labile surface such as a balance cushion / board or bosu.  Each of the single leg standing actions is a precursor to single leg knee dip and ¼ squat actions maintaining lower limb alignment at the pelvis, hip, knee, and ankle / foot. V taping is used throughout rehab for as long as symptoms modification is maintained.

 

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References

Ballegaard C, Riis RG, Bliddal H, et al (2014) Knee pain and inflammation in the infrapatellar fat pad estimated by conventional and dynamic contrast-enhanced magnetic resonance imaging in obese patients with osteoarthritis: a cross-sectional study. Osteoarthritis Cartilage. 22(7):933-40.

Distel E, Cadoudal T, Durant S, et al (2009) The infrapatellar fat pad in knee osteoarthritis: an important source of interleukin-6 and its soluble receptor. Arthritis Rheum. 60(11):3374-7

Draghi, F., Ferrozzi, G, et al (2016) Hoffa’s fat pad abnormalities, knee pain and magnetic resonance imaging in daily practice.  Insights Imaging 7: 373-383.

Dragoo JL, Johnson C, McConnell J. (2012) Evaluation and treatment of disorders of the infrapatellar fat pad. Sports Medicine. 42(1):51-67.

Dye SF, Vaupel GL, Dye CC. (1998) Conscious neurosensory mapping of the internal structures of the human knee without intraarticular anesthesia. Am J Sports Med. 26(6):773-7.

Eymard F, Chevalier X. (2016) Inflammation of the infrapatellar fat pad. Joint Bone Spine. 83(4):389-93

Hodges PW, Mellor R, Crossley K, et al (2009) Pain induced by injection of hypertonic saline into the infrapatellar fat pad and effect on coordination of the quadriceps muscles. Arthritis Rheum. 15;61(1):70-7

Norris, C.M. (2018) Sports and soft tissue injuries.  (5^th edition) Routledge.  London.

Santangelo KS, Radakovich LB, Fouts J, et al (2016) Pathophysiology of obesity on knee joint homeostasis: contributions of the infrapatellar fat pad. Horm Mol Biol Clin Investig. 1;26(2):97-108.

Smallman, T. V., Portner, O. T., et al (2018). Arthroscopic Untethering of the Fat Pad of the Knee: Release or Resection of the Infrapatellar Plica (Ligamentum Mucosum) and Related Structures for Anterior Knee Pain. Arthroscopy techniques, 7(5), e575-e588.

Hoffa A. (1904) The influence of the adipose tissue with regard to the pathology of the knee joint. J Am Med Assoc 43: 795-6.