Infrapatellar fat pad (IFP) is an intracapsular extra-synovial structure, with no clearly defined function and role. Many patients present with anterior knee pain or swelling due to disorders localized to IFP such as Inflammation and fibrosis, synovial disorders, synovitis, pigmented villonodular synovitis, symptomatic plica, mass-like abnormalities, Hoffa’s disease (HD), and cartilage abnormalities ^[1]. Calcified mass appreciable on radiographic imaging in the IFP region is due to synovial chondromatosis, osteochondroma, or end stage HD [1, 2]. Synovial chondromatosis has a thickened synovial lining and is intra-synovial in location, whereas osteochondromas are extra-synovial and have an isolated uninodular appearance [1, 3]. End-stage HD has a similar radiographic appearance as solitary osteochondroma, but on pathological examination has surrounding fibroadipose tissue and hemosiderin deposits ^[2]. The purpose of the present study is for two reasons, first, to present three cases who had similar presentation as mass in the IFP region and calcified appearance on radiographic imaging, but with different etiologies, and second, to perform a review of the literature for lesions with calcified mass in the IFP.

Case

Three cases who had presented with swelling in the IFP with calcified mass on radiographs, underwent excision, and had an uneventful recovery are being presented.

Narrative review

A review was conducted and PubMed and Cochrane database were searched in using MeSH headings and subheadings in various combinations (i.e., IFP calcification and IFP calcified mass and para-articular osteochondroma knee and Hoffa’s fat pad calcification) to increase sensitivity. Due to the paucity of literature related to IFP impingement, we included any study related to IFP calcification derived from the search regardless of study design or patient demographics. Results were limited to humans, English, and the publication dates 1950–2023. Studies or reports with infrapatellar calcified mass were included for the review. Studies presenting with locations other than IFP were excluded from the study. Reports with multiple cases and had a case with IFP calcified mass were included in the study. The studies where the full text could not be retrieved or were not in English were excluded of the study. Two independent reviewers screened the text and included the studies if they were eligible. The references of the selected studies and studies which cited them were also screened and studies which met our selection criteria were included in the review. Following data were extracted from the studies: Age, sex, symptoms, duration of symptoms, management, and pathological assessment.

Cases

Case 1

A 16-year-old healthy adolescent male presented to our outpatient department (OPD) with a gradually progressive swelling in the anterolateral aspect of the right knee in the IFP. On examination, the knee was essentially normal with no ligamentous laxity and preserved range of motion. The overlying skin was normal in consistency and texture. Radiograph revealed a calcified mass in IFP region. Subsequent magnetic resonance imaging (MRI) showed a well marginated and circumscribed mass, with few hemorrhagic areas in the swelling. The patient underwent excision biopsy after a diagnostic arthroscopy. The lining synovium was normal with no evidence of any synovial pathology. The swelling was not attached to any surrounding structure, measured 7 × 6.5 × 3 cm in size and was excised en mass. Histopathological assessment revealed it to be osteochondroma. As the swelling was localized to IFP, with hemorrhagic foci inside and osteochondral picture, it was suggestive of end-stage HD (Fig. 1).

A 77-year-old male presented to the OPD with swelling of the left knee of 5-year duration. It was gradually increasing in size and had not started with occasional episodes of locking of knee. On examination, the knee had stiffness with terminally restricted flexion of the knee. Radiographic evaluation showed a calcified mass in IFP region. The patient underwent excision biopsy of the swelling. The synovium was normal with evidence of inflammation. The swelling was intracapsular and extra-synovial in location and measured 06 × 5 × 03 cm. The inferior aspect abutted the tibial plateau, and the overlying cartilage layer was absent. It was a separate mass with no attachments to nearby structures. Histopathological examination (HPE) revealed the swelling to be osteochondroma. With the typical location and extra-skeletal origin and no evidence of hemorrhagic foci, it was suggestive of para-articular extra-synovial osteochondroma (Fig. 2).

Case 3

A 35-year-old male presented with complaints of pain on the anterior aspect of the knee for 4 months duration with antecedent history of trauma. The pain was occasional and not related to any other factor. On examination, there was minimal tenderness in the infrapatellar region anteriorly. The knee movements and ligaments were preserved. Radiographic evaluation showed a small, calcified mass in the infra-patellar region. MRI confirmed the nature of the swelling to be osteo-cartilaginous in the IFP. The patient underwent arthroscopic excision of the mass. The mass measured 02 × 02 × 02 cm. The mass was located in the IFP and was not adherent to nearby structures. Histopathological evaluation revealed the mass to be osteochondroma. Based on the location and the HPE, the swelling was diagnosed to be para-articular extra-synovial osteochondroma (Fig. 3).

Narrative Review

The reports and studies searched and identified on the PubMed and Cochrane database were further assessed for the eligibility and data collection. For increasing the scope of the review, the studies and reports included were further screened for their references and citations, and reports meeting our inclusion criteria were included in the review. Thirty-six cases in 26 studies (including the present study) were analyzed for their descriptions and outcomes (Table 1). There were multiple etiologies which led to IFP calcification as shown in Table 2. Twenty-six cases (Right-16; Left-10) were identified where side was mentioned. Of the 36 cases, 20 were male and 16 were female. The mean age at presentation was 49.64 years (range 16–78 years). The symptoms were mostly swelling, with pain (25 cases) and restricted range of motion (mostly flexion). The duration of symptoms was mentioned in 29 studies and ranged from 2 to 360 months with mean duration being 70.2 months. The average size was reported in 23 studies and the average volume of the calcified mass was 99.91 cc. The management in most of the cases was excision of the swelling, two patients underwent total knee arthroplasty (TKA) in the same seating, whereas one was managed conservatively (Table 2).

IFP or Hoffa’s fat pad is an intra-articular extrasynovial structure, has role as a space filling organ, and has vital functions in the joint as synovial fluid secretion, lubrication of joint, and shock absorption ^[4]. In addition, due to the anatomy, pattern of neurovascular structure, and the adipose contents which secrete plenty of proinflammatory cytokines and adipokines, it is presumed to be contributory to osteoarthritis of the knee [4, 5]. The multipotent stem cells derived from IFP are known to possess chondrogenic, osteogenic, and adipogenic stimuli ^[6]. Thus, for calcification to result in the IFP, it contains the necessary potion, but the initiating circumstances vary for the pathological process to set in.

In the present review, we analyzed the reports of lesions with IFP calcified mass, and the underlying reported pathology varied to an extent. These pathologies were para-articular osteochondroma, synovial chondromata, HD, and heterotopic ossification. None of swelling had any evidence of malignant changes. Extraskeletal osteochondromas have been divided into three types: Synovial chondromatosis, para-articular chondroma/osteochondroma, and soft-tissue chondroma ^[7]. These have been given different nomenclature in the literature by various authors despite having a similar HPE picture. Our review shows that these are the most common cause of IFP calcification (29 cases out of 36). The radiographic picture shows an isolated calcified mass. The management of these is primarily excision with no evidence of any recurrence ^[8]. One patient underwent a simultaneous TKA as they had evidence of advanced osteoarthritis. On the other hand, synovial chondromatosis is known to affect joints such as knee, hip, shoulder, and elbow. The pathology involves the complete synovium and forms multiple free cartilaginous bodies, which undergo calcification by a process of metaplasia ^[9]. The radiographic picture of these shows multiple calcified mass with round or oval appearance. We came across two cases of synovial chondromatosis with IFP calcification. One was managed with excision and the other underwent TKA. HD is thought to be a result of repetitive microtrauma or acute injury and further leading to hemorrhage, inflammation, hypertrophy, and further edema of IFP ^[10]. Culminating as an ossified mass is an extremely rare occurrence and has been defined as an end-stage HD. One interesting finding was presence or evidence of hemosiderin, which likely suggests hemorrhage in the IFP ^[2]. Thus, the presence of hemosiderin favors the diagnosis of HD and should be looked for during the MRI and HPE of the swelling. The spectrum of the patients varied from adolescents to as late as 8^th decade, the youngest patient was 16 year old (our case) and the eldest was 78 year old. This clearly confirms that no age group is at risk. The symptoms varied from benign asymptomatic presentation with no evidence of arthritis to a painful swelling with restricted range of motion with advanced osteoarthritis of the knee. However, for the patient to seek medical attention, it took a long time (02 months to 360 months), as it rarely was a cause of discomfort or any other symptoms when the size was evolving. The management generally involved excision of the swelling when there was no associated pathology and TKA in patients who had advance osteoarthritis of the knee. Only one patient was managed conservatively. The IFP is known to play a role in various pathologies of the knee, as has been confirmed by the presence of proinflammatory cytokines and adipokines. These can be predicted to play a role in isolated IFP calcification, which can be found in para-articular extra-synovial osteochondroma, synovial chondromatosis, and HD. These lesions are mostly benign in nature and can be safely managed with excision or TKA in select cases.

IFP calcification is a clinical result in pathologies such as para-articular osteochondroma, synovial chondromata, HD, and heterotopic ossification. The fat pad is known to consist of the various chondrogenic stimuli for the pathological process to initiate and end with a calcified mass. The resultant mass is of benign origin and management involves excision or knee arthroplasty in case if associated with arthritis of knee.

Infrapatellar calcified mass is mostly benign with different possible etiology, and the management is by excision; however, this should not prevent the surgeon to negate the importance of a histopathological evaluation.