This exhibit discusses the must know details about the normal anatomy, variants and common pathologies involving the HFP with some key example cases.  

Firstly, it is essential to be aware of the normal anatomy and normal variants of HFP. 

The Hoffa’s (infrapatellar) fat pad is a soft tissue structure in the knee that is highly vascularized and innervated. It has adipocytes and connective tissues and acts as a protective layer that separates the patella from the thigh and shin bones. Vascular supply is through superior and inferior genicular arteries. Although the peripheral area of HFP is well perfused, there is a paucity of vascularity in the central part. Innervation is ensured by anterior fibers of the popliteal plexus connected to the posterior articular nerve, a branch of the tibial nerve [1].

On MRI, a normal HFP is hyperintense on both T1 and T2 WI and shows identical characteristics to subcutaneous fat. It also contains multiple fibrous septa which appear hypointense on T1 and T2 with FS and STIR sequences [1]. 

The images to illustrate this anatomy and the relations of the HFP [2] are shown in Figure 1. 

Fig 1: Anatomy of Hoffa's Fat Pad

Normal variants may be observed such as the suprahoffatic and infrahoffatic recesses. Occasionally a connection may exist between the two recesses. These can be well visualised on T2FS MRI sequence. These do not require any further investigation and should not be confused with other pathological findings such as effusion, cysts or cyst mimickers[3]. 

Fig 2: Infrahoffatic Recess: A 58-year-old male with bilateral knee pain which increases with movement. MRI right knee A) Sagittal PD, B) Sagittal T2 fat sat & C) MRI left knee T2 fat sat demonstrates bilateral symmetrical recess extending to the Hoffa’s fat pad, normal variant - Infrahoffatic recess (arrows).

Next, common pathological conditions involving the HFP can be broadly classified into tumors and tumor like lesions, trauma related pathologies and post- surgical changes as illustrated in Figure 3.

Fig 3: Hoffa's Fat Pad Lesion

1. Tumor and Tumor like lesions – can arise from a variety of etiologies, including neoplastic, inflammatory, or traumatic processes. Understanding the underlying cause is essential for accurate diagnosis and effective management. 

1. Ganglion Cyst – Although an uncommon location, it is the most common and first differential of a mass-like lesion in the HFP. These contain dense viscous fluid enclosed in fibrous septa without synovial lining. Most of the time they are asymptomatic but sometimes they can cause knee pain [1].                                                   
   

   Fig 4: Ganglion cyst: A 56-year-old female with right knee pain. MRI right knee sagittal A) PD fat sat, B) T2, & C) GRE shows a ganglion cyst in the Hoffa’s fat pad (arrows).
2. Loose bodies and Osteochondral Fragments - After trauma, osteochondral and bony fragments can become embedded in the Hoffa fat pad (HFP) or along the intra-articular surface causing anterior knee pain and symptoms like joint locking. MRI is valuable for determining the fragment's origin, composition, associated injuries, and providing a guide for surgical intervention. Primary and secondary synovial osteochondromatosis are other causes of loose bodies within the HFP [1].                                                         
   

   Fig 5: Loose body: A 40-year-old male with left knee pain. MRI left knee sagittal A) PD & B) PD fat sat shows a loose body in the posterior aspect of Hoffa’s fat pad (arrows) with osteoarthritic and chondropathic changes.
   

   Fig 6: Osteochondral Fragment: A 45-year-old female with history of left knee injury. MRI left knee sagittal A) PD & B) PD fat sat demonstrates an osteochondral fragment seen below the patella within the Hoffa's fat pad at the posterior interval (arrows).
3. Soft Tissue Chondroma – results from cartilaginous metaplasia of the joint capsule or the adjoining connective tissue with deposition of osteoid matrix within the HFP. High contrast MRI sequences such as PD with fat sat show areas of hyperintense signal corresponding to chondroid matrix and edema and hypointense areas of calcification or ossification. It is important to compare MRI findings with plain radiographs or CT scans for a more accurate diagnosis [2]. Differential diagnosis includes synovial osteochondroma and synovial sarcoma [1,4,5].                                                                                                     
   

   Fig 7: Soft Tissue Chondroma: A 28-year-old female with left knee pain. MRI left knee sagittal A) PD, B) PD fat sat, & GRE shows soft tissue chondroma in the Hoffa’s fat pad between the infrapatellar ligament and upper tibia (red arrow), which shows signal drop in fat suppressed sequence (blue arrow) and no blooming in GRE (yellow arrow).
4. Pigmented Villonodular Synovitis (PVNS) - may present as diffuse form involving the entire joint or focal PVNS which commonly involves the HFP. An important differential to consider is localized nodular synovitis which has similar histologic characteristics. The two can be differentiated by the following imaging features: 

1. Nodular PVNS tends to have an abundance of hemosiderin as compared to localized nodular synovitis.
2.  Nodular PVNS has more diffuse frond like projections of synovium whereas localised nodular synovitis has a smooth surface.
3. PVNS tends to constrict the joint as it grows while the latter grows outward and becomes pedunculated [1,6].                                                                                                                                                                       
   

   Fig 8: Pigmented Villonodular Synovitis (PVNS): A 37-year-old female with history of right knee giving away. MRI right knee sagittal A) PD & B) GRE shows a focal mass lesion in Hoffa’s fat pad (red arrow) with blooming in GRE (blue arrow). Findings are suggestive of Pigmented Villonodular Synovitis (PVNS).

2. Trauma Related Pathologies – may be due to acute trauma due to adjacent fractures or ligamentous injuries or chronic repetitive microtraumas such as impingement syndrome, inferior patellar plica injury and Sinding-Larsen-Johansson disease. 

1. Inferior patellar plica (IPP) injury - an uncommon sport's injury, that is commonly seen in soccer players. IPP is normally of low signal intensity on all MRI sequences with variable thickness. Hyperintense signal along its entire length or significantly thickened plica is suggestive of a diseased or injured IPP. The best views to rule out IPP injury are PD, PDFS and T2 Sagittal plane [7,8].                                                                       
   

   Fig 9: Inferior Patellar Plica Injury: A 34-year-old male soccer player with anterior left knee pain. MRI left knee sagittal A) PD, B) & C) PD fat sat demonstrates inferior plica injury (red arrows) associated with ACL injury (blue arrow).
2. Torn displaced ligaments – Meniscal tears or acute injuries to the ACL may cause focal or diffuse edema of HFP due to impingement or joint instability. When torn and displaced these may mimic small tumors or in rare cases may cause compression of HFP causing edema [1,4].                                                                             
   

   Fig 10: Torn Extruded Anterior Horn of Medial Meniscus in the Hoffa’s Fat Pad: A 55-year-old male with left knee injury. MRI left knee sagittal A) PD, B) & C) T1 post gadolinium shows torn extruded anterior horn of medial meniscus (arrows) causing compression and edema the Hoffa's fat pad. Significant degeneration of the medial meniscus also noted.
3. Sinding-Larsen-Johansson disease: occurs due to traction tendinopathy and apophysitis with ossification of the proximal portion of patellar tendon. MRI findings show bone fragments at the inferior pole of patella and the proximal patellar tendon with edema of surrounding tissues including adjacent HFP. MRI helps to differentiate this from patellar sleeve avulsion injury by depicting the extent of cartilage injury [9].                   
   

   Fig 11: Sinding-Larsen-Johansson Disease: A 30-year-old male with right knee pain after jumping exercise. MRI right knee PD fat sat A) Sagittal & B) Axial demonstrates thickened and edematous proximal patellar tendon with associated underlying edematous changes of the Hoffa’s fat pad (red arrow) and inferior patellar pole as well as overlying subcutaneous oedema, findings are in keeping with Sinding-Larsen-Johansson disease. Mild trochlear dysplasia is also seen (blue arrow) with mild lateral patellar tilt.
    

3. Post Surgical 

1. Anterior interval fibrosis - Anterior interval scarring is a common cause of extension block following arthroscopic ACL reconstruction. On MRI It is accompanied by the decrease in volume of fat in HFP and retraction of the patellar tendon. Fibrosis may be subtly apparent requiring a high index of suspicion. T1 sagittal images provide optimal visualisation of fibrosis [10].                                                                                    
   

   Fig 12: Anterior Interval Fibrosis: A 35-year-old male with ACL reconstruction and meniscal repair in right knee. MRI right knee sagittal A) PD & B) PD fat sat shows anterior interval fibrosis (arrows) post arthroscopy with ACL reconstruction.