Take a look at your knee while you’re sitting. You should easily see the patella (knee cap), and if you feel under that you’ll come across a stiff band which is the patellar tendon, attached to the patella at the top, and the tibia at the bottom. If you feel on both sides of the patellar tendon you’ll notice soft squishy patches. This is the Hoffa’s fat pad, now known as the infrapatellar fat pad (IFP), and it’s crucial in the health and pathology of your knees. In fact, it seems that keeping the IFP happy is probably the most important consideration for knee health, after not fracturing bones, but getting the IFP happy again can be very difficult once it becomes upset. When the IFP is inflamed it swells and can bulge on both sides of the patellar tendon, as well as expanding into the joint where it becomes impinged (pinched) between the bones, creating pain [1] and more inflammation. This is often why standing and walking can be painful activities and can make the knee hurt for a long time afterwards.
Getting the IFP happy again can be very difficult once it becomes upset
But there is more. Fat pads can cause problems in any joint, and fat pad pathology may drive osteoarthritis of knees and hips [2,3]. Like the IFP, the pulvinar fat pad in hips can become impinged, fibrotic and painful [3] and similar findings of fat pad pathology associated with arthrofibrosis have been observed in shoulders [4]. Impingement of fat pads can also occur in elbows [5]. Foot pain affects about 1 in 5 people [6], with plantar fasciitis and heel fat pad pathology the primary culprits [7]. We expect to see increasing interest in fat pad health and pathology in the future.
The only way to prevent increasing amounts of fat pad inflammation is to rest the affected limb and avoid upsetting it. For joints in legs, this might mean not weight-bearing for an extended period if there is fat pad impingement. Yes, that’s right, if your IFP is badly inflamed you need to use crutches to prevent it from becoming fibrotic and painful long-term. People who already have a fibrotic IFP need to carefully monitor their “leg time” and sit when they feel pain beginning, which might mean they don’t have much time to spend on their feet. Avoiding pain is important - the IFP is has a high density of nerves, and as they become unhappy these release substance P (for pain) and more inflammatory cytokines [8]. IFP fibrosis causes poor outcomes including muscle atrophy and loss of range of motion [8].
Figure: Zhou, S et.al. 2022 [10]
Fat pads are now considered to be organs in their own right [9] and are associated with every joint and organ in the body. Fat pads are important for the health of the synovial membrane, cartilage, bone, ligaments and tendons [10], supplying blood, along with oxygen and nutrients, to the joint structures. Along with their fat cells, fat pads also contain sensory nerves and significant stores of immune cells that are important for protecting the joint from infection. But, when there is too much stimulus from injury or surgery, these immune cells turn from protector to attacker, pumping out toxic cytokines (small molecules) [10] that transform the fat cells and fibroblasts into zombie myofibroblasts and creating fibrosis.
Despite the understanding that the health of joints depends on the health of the associated fat pads, there is little recognition of this by the majority of clinicians. This unfortunate lack of understanding means that the IFP is routinely cut (resected) during total knee replacements [11] and it is sometimes removed entirely. For example, in the UK the IFP is cut in around 88% of TKR surgeries [12]. The cutting of the IFP allows the surgeon to see more easily, and stops the fat pad from getting in the way during the surgery [11]. However, there is an unfortunate side-effect of creating a significant wound to an important organ, which stimulates inflammation and wound healing processes. Experimental cutting of the IFP in healthy animals (goats) with an ACL injury led to fibrotic changes at the ACL injury site, which didn’t happen in control animals in which the IFP was not cut [10].
There were significantly better outcomes from preserving the IFP during TKR
IFP resection remains controversial, and many surgeons still do this routinely in TKR and ACL surgery. Unfortunately, quality research without bias is lacking, but reviews suggest that cutting the IFP during TKR surgery is associated with knee pain beyond 6 months post-op and patellar tendon shortening [12], also known as patellar baja. Carefully conducted reviews are necessary since single studies are frequently unreliable, with small numbers and no effort to prevent bias by “blinding” the accessors and patients to the procedure. One large retrospective study compared the outcomes of 1401 TKR patients for whom the IFP was preserved, partially resected, or completely removed. This found there were significantly better outcomes from preserving the IFP during TKR [13].
For some people a resected fat pad will lead to arthrofibrosis and ongoing pain and disability, and there is currently no way to predict who will be seriously impacted by this practice. The IFP is the most intensively researched fat pad, however, knees have four fat pads - the IFP, posterior fat pad, suprapatellar quadriceps fat pad and the suprapatellar pre-femoral fat pad [10]. As the famous Sunscreen song says “look after your knees, you’ll miss them when they’re gone”. This applies to all your joints.
References
Gallagher, J., Tierney, P., Murray, P. & O'Brien, M. The infrapatellar fat pad: anatomy and clinical correlations. Knee Surg Sports Traumatol Arthrosc 13, 268-272, doi:10.1007/s00167-004-0592-7 (2005).
Eymard, F. et al. Knee and hip intra-articular adipose tissues (IAATs) compared with autologous subcutaneous adipose tissue: a specific phenotype for a central player in osteoarthritis. Ann Rheum Dis 76, 1142-1148, doi:10.1136/annrheumdis-2016-210478 (2017).
Slullitel, P. A., Coutu, D., Buttaro, M. A., Beaule, P. E. & Grammatopoulos, G. Hip preservation surgery and the acetabular fossa. Bone Joint Res 9, 857-869, doi:10.1302/2046-3758.912.BJR-2020-0254.R1 (2020).
Zappia, M. et al. Multi-modal imaging of adhesive capsulitis of the shoulder. Insights Imaging 7, 365-371, doi:10.1007/s13244-016-0491-8 (2016).
Hamada, D. et al. An Unusual Cause of Posterior Elbow Impingement: Detachment of a Hypertrophied Posterior Fat Pad. Case Rep Orthop 2015, 121646, doi:10.1155/2015/121646 (2015).
Tas, S. Effect of Gender on Mechanical Properties of the Plantar Fascia and Heel Fat Pad. Foot Ankle Spec 11, 403-409, doi:10.1177/1938640017735891 (2018).
Belhan, O., Kaya, M. & Gurger, M. The thickness of heel fat-pad in patients with plantar fasciitis. Acta Orthop Traumatol Turc 53, 463-467, doi:10.1016/j.aott.2019.07.005 (2019).
Nakagawa, Y. et al. Association of Infrapatellar Fat Pad Fibrosis at 3 Months After ACL Reconstruction With Short-term Clinical Outcomes and Inflammatory Cytokine Levels in the Synovial Fluid. Orthop J Sports Med 11, 23259671231164122, doi:10.1177/23259671231164122 (2023).
Benjamin, M. & McGonagle, D. in Molecular Mechanisms of Spondyloarthropathies (eds Carlos López-Larrea & Roberto Díaz-Peña) 57-70 (Springer New York, 2009).
Zhou, S. et al. Source and hub of inflammation: The infrapatellar fat pad and its interactions with articular tissues during knee osteoarthritis. J Orthop Res 40, 1492-1504, doi:10.1002/jor.25347 (2022).
Braun, S. et al. The Corpus Adiposum Infrapatellare (Hoffa's Fat Pad) - The Role of the Infrapatellar Fat Pad in Osteoarthritis Pathogenesis. Biomedicines10, 1071 (2022).
White, L., Holyoak, R., Sant, J., Hartnell, N. & Mullan, J. The effect of infrapatellar fat pad resection on outcomes post-total knee arthroplasty: a systematic review. Arch Orthop Trauma Surg 136, 701-708, doi:10.1007/s00402-016-2440-x (2016).
Moverley, R., Williams, D., Bardakos, N. & Field, R. Removal of the infrapatella fat pad during total knee arthroplasty: does it affect patient outcomes? Int Orthop 38, 2483-2487, doi:10.1007/s00264-014-2427-6 (2014).