Original Article
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Infrapatellar Fat Pad Osteochondroma: Three Cases and a Systematic Review Nathan Evaniew, MD1 Darius Bayegan, MSc1 Snezana Popovic, MD, PhD, MSc, FRCPC2 Naveen Parasu, MBBS, FRCR, FRCPC3 Michelle Ghert, MD, FRCSC4 1 Department of Surgery, Center for Evidence-Based Orthopaedics,
McMaster University, Hamilton, Ontario, Canada 2 Department of Pathology and Molecular Medicine, McMaster University, Hamilton, Ontario, Canada 3 Department of Radiology, Juravinski Hospital, McMaster University, Hamilton, Ontario, Canada 4 Department of Surgery, Juravinski Hospital, McMaster University, Hamilton, Ontario, Canada
Address for correspondence Michelle Ghert, MD, FRCSC, Department of Surgery, Juravinski Hospital, McMaster University, 711 Concession Street, Hamilton, Ontario, Canada L8V 1C3 (e-mail: [email protected]).
Abstract
Keywords
► ► ► ► ►
osteochondroma chondroma knee fat pad surgery
Extraskeletal para-articular osteochondromas of the infrapatellar fat pad are unique softtissue tumors whose etiology and clinical history remain poorly understood. We report three cases and a systematic review of the literature. Three females, aged 47, 54, and 70 years, presented with pain and a parapatellar knee mass. All three underwent marginal excision with no complications and no recurrence. In the literature, 32 reports describing 42 cases were identified. Mean age at presentation was 51.1 years (range: 12–75). There were 25 females and 20 males. Seven patients (15.6%) had a history of antecedent trauma. Mean duration of presenting symptoms was 63.9 months (range: 2–300). All patients except for one underwent open excision. Mean follow-up was 14.9 months (range: 3–96). There were no reports of recurrence or malignant transformation. Para-articular extraskeletal osteochondromas of the infrapatellar fat pad have a benign clinical history regardless of whether they are managed by arthroscopic or open marginal excision.
Osteochondromas are common benign surface lesions of bone that most typically occur at the metaphyses of the distal femur and proximal tibia.1 Unless they tether neurovascular or musculotendinous structures, osteochondromas usually present as incidental findings or small painless masses. In isolation, their incidence of malignant transformation is well less than 1%,1 and they are frequently managed by either observation or marginal resection. Typical osteochondromas can be diagnosed radiographically by their characteristic attachment to bone with continuity of both the cortices and the medullary canal. Extraskeletal osteochondromas are rare benign soft-tissue tumors whose etiology and natural history remain poorly understood. They are one of many mineralized soft-tissue
masses for which a definitive radiographic diagnosis is not possible. They appear to have a predilection for the hands and feet, but have also been reported adjacent to the knee, hip, and elbow.2,3 They are considered to be synonymous with the nonossified tumor referred to as extraskeletal chondroma and they are designated as para-articular or intracapsular when adjacent to a joint.4 Para-articular osteochondromas are differentiated from synovial chondromatosis by their extrasynovial location, lack of synovial tissue, and typical uninodular appearance.5 Jaffe initially reported on para-articular osteochondroma of the infrapatellar fat pad in 1958.6 Multiple case reports and small series have followed, but optimal management and clinical outcomes beyond 2 years have been infrequently
received August 12, 2013 accepted after revision April 13, 2014 published online May 29, 2014
Copyright © 2015 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0034-1378192. ISSN 1538-8506.
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documented.7 Likewise, this tumor’s underlying etiology, true incidence, and natural history are unknown. Our purpose was twofold: First, we reviewed our local experience with osteochondroma of the infrapatellar fat pad and we present these findings as a series of three case reports. Second, we performed a systematic review to create a comprehensive best-evidence summary of the global literature. Ultimately, we aim to optimize patient outcomes by raising awareness of this uncommon tumor and guiding clinicians in its diagnosis and management.
Methods This study received ethics approval from the Hamilton Integrated Research Ethics Board (Project #13–359-C).
Cases Using a surgeon-specific retrospective database from July 2005 to June 2013, three consecutive patients with the diagnosis of extraskeletal para-articular osteochondroma of
infrapatellar fat pad were identified. Clinical, radiological, and pathological data were extracted from electronic and paper medical records. All patients underwent marginal resection through a vertical parapatellar arthrotomy.
Systematic Review Information Sources and Search Strategy A systematic review was conducted according to the Preferred Reporting Items for Systematic reviews and Metaanalyses (PRISMA) statement.8 Medline and EMBASE were searched in OVID using MeSH and EMTREE headings and subheadings in various combinations (i.e., exp osteochondroma OR exp chondroma AND exp knee) with supplemental free text to increase sensitivity (i.e., “osteochondroma” OR “chondroma”) AND (“knee” OR “extra-articular” OR “para-articular” OR “Hoffa’s fat pad” OR infrapatellar fat pad”). Results were limited to humans, English, and the publication dates 1946 (Medline) or 1980 (EMBASE) to May 5, 2013.
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Fig. 1 Case 1. (a) Lateral radiograph of the patient’s right knee showing an infrapatellar ossified mass. (b, c) T1 and T2 fat sat coronal MRI showing a heterogeneous mass with high signal on T1 inferior to patella that suppresses on the fat sat T2 and represents fatty marrow ( ). Scattered high signal foci on T2 represents cartilage (arrow). (d) Sagittal CT scan showing an ossified mass with areas of soft-tissue density posteriorly representing cartilage. (e) Gross pathology specimen showing a lobular mass of cancellous bone with a cartilaginous cap. (f) Microscopic section demonstrates lamellar trabecular bone (arrow) with overlying hyaline cartilaginous cap (). CT, computed tomography; MRI, magnetic resonance imaging. The Journal of Knee Surgery
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Eligibility Criteria
Data Extraction
Clinical reports of tumors or pseudotumors in the infrapatellar fat pad other than extraskeletal osteochondromas were excluded. Studies reporting on extraskeletal or para-articular osteochondromas or chondromas in locations other than the infrapatellar fat pad were excluded. Studies combining infrapatellar fat pad tumors with tumors in other locations were included only if the infrapatellar fat pad cases were reported separately. Studies were excluded if their full texts were not available in English.
Two independent reviewers extracted the following data points for each included: age, gender, medical history, tumor location, presenting symptoms and examination findings, imaging results, operative details, pathological features, and follow-up.
Study Selection
Statistical Analysis Agreement on study eligibility was determined with Cohen kappa coefficient. Kappa values > 0.65 were considered adequate.9 Descriptive means and proportions were calculated to summarize the case report data. Microsoft Excel (Santa Rosa, CA) was used for data analysis.
Results Case Reports Case 1 A healthy 70-year-old female presented with a 5-year history of a slowly enlarging and palpable mass in her right knee. She
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Two independent reviewers screened titles and abstracts for eligibility using a piloted database. Discrepancies were resolved by consensus. Duplicates were removed in OVID and by manual review. All eligible studies underwent fulltext review. Reviewers were not blinded to authors or publication information. The references of all included studies were manually screened to further identify eligible studies.
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Fig. 2 Case 2. (a) Lateral radiograph of the patient’s left knee showing an infrapatellar mass with speckled calcifications. (b) Sagittal T1 MRI contains a mixture of bright fatty foci and dark foci representing chondroid calcification that becomes more conspicuous (blooms) on the gradient echo image (c, arrow). (d) Cut specimen showing adipose tissue with cartilaginous and osseous foci. (e) Microscopic section showing hyaline cartilage at the periphery and central lamellar bone. Enchondral ossification is seen at the interface between cartilage and bone (). (f) Surrounding adipose tissue demonstrates focal areas of fat necrosis () with the presence of histiocytes. MRI, magnetic resonance imaging. The Journal of Knee Surgery
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had no significant past medical history and had not undergone any laboratory testing. She described difficulty with deep flexion and was unable to comfortably ascend or descend stairs. She had a firm, nontender mass about the anterolateral aspect of her right knee. Range of motion was 30 degrees of active flexion, with further flexion causing sharp anterior knee pain. Radiographs revealed a well-defined ossified lesion within the infrapatellar fat pad (►Fig. 1). A computed tomography (CT) scan confirmed its location and a technetium-99 bone scan showed mild uptake in the region of the infrapatellar fat pad. Magnetic resonance imaging (MRI) demonstrated an ossified mass with fluid between the mass and the patellar tendon. At surgery, a 6.5 4.5 3.8 cm specimen consisting of bone and a cartilaginous cap was excised. Microscopic sections revealed lamellar trabecular bone with overlying hyaline cartilage, similar to conventional osteochondroma. Focal areas of regenerative and hyperplastic hyaline cartilage with intermittent necrosis suggested superimposed trauma. There
were no aggressive features. Pathologic diagnosis was paraarticular extraskeletal osteochondroma. Postoperatively, her active knee range increased to 90 degrees and she was able ascend and descend stairs without difficulty. Clinical and radiographic examination at 3 months demonstrated no recurrence.
Case 2 A healthy 47-year-old female presented with a 1-year history of pain and swelling in her left knee, exacerbated by kneeling and sitting. She had a tender firm mass at the anterolateral aspect of her knee, and range of motion and strength were normal. Radiographs revealed areas of ossification in the infrapatellar fat pad (►Fig. 2). MRI showed encapsulation by a rim of high T2 signal, with heterogeneous gadolinium enhancement within the lesion. The radiological differential diagnosis included localized synovial chondromatosis. At surgery, a 6.5 3.7 1.7 cm discrete mass was excised. Gross sections showed ossification within mature adipose
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Fig. 3 Case 3. (a) Lateral radiograph of the patient’s right knee shows slight calcifications in the infrapatellar fat pad. Sagittal T1 (b) and fat-sat T2 (c) MRI images demonstrate an ovoid soft-tissue mass which shows predominantly dark signal on T1 and corresponding bright signal on T2 in keeping with cartilage. Subtle areas of T1 bright foci are seen posteriorly, which is dark on the fat sat images and suggestive of early ossification (arrow). (d) Mature adipose tissue with focal areas of fat necrosis and chondroid proliferation. (e) Low-power core biopsy histology section showing benign fibroconnective tissue. (f) High power histology section showing mature adipose tissue with focal areas of fat necrosis and chondroid proliferation ( ). The Journal of Knee Surgery
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Infrapatellar Fat Pad Osteochondroma tissue. Microscopic sections demonstrated central lamellar trabecular bone and peripheral hypercellular hyaline cartilage, with focal areas of fat necrosis. There was only mild atypia of the chondrocytes, and enchondral ossification was noted at the osteochondral junction. Pathologic diagnosis was para-articular extraskeletal osteochondroma. After 6 months of physical therapy, this patient had full strength and motion, and returned to work. Clinical examination at 19 months demonstrated no recurrence.
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At surgery, a two-lobed, tan-colored mass consisting of a 6.4 3.6 2.2 cm and a 2.5 1.4 0.4 cm part was excised. Microscopic sections demonstrated hyaline cartilage, mature adipose tissue, focal areas of fat necrosis, and no evidence of bone formation. The final diagnosis was extraskeletal para-articular chondroma. At 3 months follow-up, her pain was settled and her range of motion was 0 to 110 degrees.
A healthy 54-year-old female presented with a 5-year history of right knee stiffness and a slowly enlarging mass. She described occasional anterior knee pain worsened by kneeling or walking long distances. She had slight anterolateral swelling of her knee, but no discrete palpable mass and no tenderness to palpation. Range of motion and strength were normal. Initial radiographs showed a faint soft-tissue density in the infrapatellar fat pad (►Fig. 3). MRI revealed a large mass with irregular high T2 and low T1 signals. Ultrasound showed that the mass was noncompressible and had no vascular flow. Ultrasound-guided core needle biopsy suggested benign fibroconnective and fibrocartilaginous tissue with degenerative cystic changes.
The search identified 423 studies (►Fig. 4). Agreement during screening was satisfactory (kappa ¼ 0.95; confidence interval, 0.87–1.00). The references of the final 23 studies were manually reviewed, which led to the addition of nine more studies. In total, 32 studies describing 42 cases were included (►Tables 1 and 2). Mean age was 51.1 years (range: 12–75). There were 25 females and 20 males. Seven patients (16%) described antecedent trauma. Mean duration of presenting symptoms was reported in 36 cases and was 63.9 months (range: 2–300). Thirty-one patients (73%) reported pain and 38 (91%) reported a mass. Reported preoperative work-up included X-rays in all 42 cases, MRI in 22 cases (53%), CT scan in 12 cases (29%), bone scan in 4 cases (9%), and ultrasound in 3 cases (7%). Biopsy was performed before excision in 17 cases (40%).
Fig. 4 Flowchart showing search results, screening, and reasons for exclusion of articles.
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Systematic Review Case 3
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Table 1 Demographics and clinical presentations of all cases Case
Authors 6
Year
Age
Gender
Trauma
Symptoms (mo)
Pain
Mass
1
Jaffe et al
1958
37
F
2
Mosher et al20
1966
23
F
12
Y
Y
3
Mosher et al
20
1966
66
M
2
Y
Y
4
Mosher et al
20
1966
65
M
5
Sarmiento and Elkins22
1975
67
M
6
Milgram and Dunn14
1980
66
F
1983
59
M
120
Y
1985
29
M
36
Y
1990
12
M
12
7
Milgram and Jasty
8
Böstman et al24
9
Pettrone and Stay
23
25
26
Y
Y
24
Y
Y
84
Y
Y
Y
Y
1994
52
M
120
11
Steiner et al
3
1994
66
F
48
Y
Y
12
Steiner et al3
1994
49
F
240
Y
Y
48
Y
Y
10
Krebs and Parker
27
1995
32
F
14
Reith et al
28
1997
66
M
24
Y
Y
15
Reith et al28
1997
75
F
60
Y
Y
13
Allahabadia et al
29
Y
Y
16
Rodriguez-Peralto et al
1997
52
F
120
17
Sakai et al
19
1999
64
F
12
Y
Y
18
Sakai et al
19
1999
47
M
72
Y
Y
19
Sakai et al19
1999
56
M
20
Dhillon et al30
1999
60
M
48
Y
Y
12
Y
Y
31
21
Cohen et al
2001
67
M
22
González-Lois et al17
2001
43
M
23
17
Y
Y
Y
2001
37
F
2
24
Hung et al
32
2005
46
M
48
25
Oliva et al33
2006
53
M
36
Y
Y
2007
42
F
36
Y
Y
2008
25
M
Y
84
Y
Y
2008
61
M
Y
5
Y
2009
60
F
36
Y
2009
55
M
12
Y
González-Lois et al
34
26
Rizzello et al
27
4
Turhan et al
28
Carmont et al
29
Demir et al36
30
Singh et al
35
37 38
Y
Y Y
Y
31
Ozturan et al
2009
60
F
120
Y
32
Nouri et al39
2010
42
F
36
Y
33
Veras et al21
2010
59
F
34
Veras et al
21
2010
33
F
Y
Y
35
Veras et al21
2010
46
F
Y
Y
2010
25
F
48
Y
Y
36 37
Mulcahy and Hoch Ogura et al
40
41
Y Y
2011
56
F
24
Y
Y
38
De Maio et al
42
2011
58
F
120
Y
Y
39
De Maio et al42
2011
71
F
300
Y
Y
40
Ingabire et al
43
2012
64
F
41
Singh et al
44
2012
52
M
42
Sen et al45
2013
25
M
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Y Y Y
84
Y
Y
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Table 1 (Continued) Case
Authors
Year
Age
Gender
43
Evaniew et al (current study)
2014
70
44
Evaniew et al (current study)
2014
47
45
Evaniew et al (current study)
2014
54
Trauma
Symptoms (mo)
Pain
Mass
F
60
Y
Y
F
12
Y
Y
F
60
Y
Y
Abbreviations: F, female; M, male; Y, yes.
Table 2 Pathological characteristics and clinical outcomes of all cases Case
Size (cm)
Pathologic diagnosis
1
7
Para-articular chondroma
0
Para-articular chondroma
36
Follow-up (mo)
Recurrence No
3
321
Para-articular chondroma
36
No
4
6.5 4.8 3.9
Para-articular chondroma
72
No
5
53
Intra-articular osteochondroma
48
No
6
Nil
Para-articular osteochondroma
0
7
991
Para-articular osteochondroma
8
54
Extraskeletal osteochondroma
Nil
12
9
332
Chondroma
Nil
24
10
522
Ossifying chondroma
0
No
11
10 7 6
Intracapsular chondroma
Nil
36
No
12
4.5 4.0 3.5
Extracapsular chondroma
0 No
0
13
436
Solitary synovial osteochondroma
Nil
20
No
14
973
Para-articular chondroma
Nil
24
No
15
9.5 6 4
Para-articular osteochondroma
Nil
13
No
16
7.5 5 5
Intracapsular chondroma
Nil
0
17
6.0 4.5 3.0
Para-articular chondroma
Nil
24
No
18
753
Para-articular chondroma
Nil
24
No
19
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2
Complications
Para-articular osteochondroma
0
20
86
Para-articular osteochondroma
Nil
18
21
7.5 5 4
Intra-articular osteochondroma
Nil
12
22
4.0 3.5 2
Para-articular chondroma
Nil
0
23
3.5 2.0 2.7
Para-articular chondroma
Nil
0
24
3 3 3, 3 2 4
Osteochondroma
Nil
12
No
25
9
Extraosseous osteochondroma
Nil
24
No
26
5.0 2.5 1.5, 2 2 1, 1.5 2 1 cm
Para-articular osteochondroma
Nil
0
No
27
7 4 3.5
Extrasynovial osteochondroma
Nil
12
No
28
4.5
Para-articular osteochondroma
Nil
0
29
55
Para-articular osteochondroma
Nil
0
30
6 4.5 5
Ossifying chondroma
31
5 4.2 3.5
Para-articular osteochondroma
Nil
12
No
32
3 2.5 2.5
Osteochondroma
Nil
0
No
33
4.5 3.5 3.0
Solitary synovial osteochondroma
0
34
7.5 5.5 3.5
Solitary synovial osteochondroma
0
No
0
(Continued)
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Table 2 (Continued) Case
Size (cm)
Pathologic diagnosis
Complications
Follow-up (mo)
35
6.5 5.0 3.8
Solitary synovial osteochondroma
36
5.5 4.5 3
37
5.5 4.8 2.3
Para-articular osteochondroma
Nil
0
Para-articular osteochondroma
Nil
6
No
38
Extrasynovial osteochondroma
Nil
96
No
39
Solitary synovial osteochondroma
Nil
48
No
40
4 2.7 4.6
Recurrence
0
Chondroma
0
41
15.5 8 3.8
Para-articular osteochondroma
Nil
26
No
42
7 4 3.5
Extrasynovial osteochondroma
Nil
12
No
43
6.5 4.5 3.8
Osteochondroma
Nil
3
No
44
6.5 3.7 1.7
Osteochondroma
Nil
19
No
45
6.4 3.6 2.2
Chondroma
Nil
3
No
All patients except for one underwent open marginal excision. Case #11 was removed piecemeal via an intralesional arthroscopic technique. Case #15 underwent open excision at the time of total knee arthroplasty. The largest lesion was 15.5 8.0 3.8 cm (case #41), and the smallest was 3 2 1 cm (case #3). The final diagnosis was chondroma in 15 cases and osteochondroma in 30 cases. Prefixes included “para-articular,” “intra-articular,” “intracapsular,” “extracapsular,” “myxoid” (chondroma), “ossifying” (chondroma), “soft tissue,” “extraskeletal,” “extrasynovial,” and “solitary synovial” (►Table 3). There were no reported complications such as infection, wound breakdown, or neurovascular injury. Follow-up was reported for 25 cases, the mean of which was 14.9 months (range: 3–96). Some functional outcome was described for 28 patients, all of which returned to “normal,” “good,” or “full,” except three who experienced ongoing pain with squatting or kneeling. There were no reports of recurrence or malignant transformation.
Discussion We performed a systematic review of the literature and reviewed our local experience with osteochondromas of Table 3 Common prefixes used to describe osteochondromas found in the infrapatellar fat pad Para-articular Intracapsular Intra-articular Extracapsular Myxoid (chondroma) Ossifying (chondroma) Soft tissue Extraskeletal Extrasynovial Solitary synovial
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the infrapatellar fat pad. We identified 42 cases published previously and contribute 3 additional cases. The aim of this study was to raise awareness of this uncommon tumor and guide clinicians in its management. Based on our findings, patients with this tumor are likely to be middle-aged generally healthy females who describe a 5-year history of a slowly growing painful mass without antecedent trauma. They are likely to experience symptomatic improvement following excision, with no likelihood of recurrence at 1 year. The etiology of this tumor remains unknown. GonzálezLois et al suggested soft-tissue osteochondromas might arise from the joint capsule or para-articular connective tissues via cartilaginous metaplasia.10 Zhang et al reviewed evidence that multipotent stromal cells and growth factors within the fat pad may be form chondroid, osteoid, and adipose tissue in response to trauma.7,11,12 This is supported by our findings of necrotic adipose tissue, but we did not see any metaplastic tissue. Only 16% of the reports described antecedent trauma. Ratcliff et al reported increased parathyroid hormone-related protein expression in a soft-tissue osteochondroma adjacent to the medial tibial metaphysis, suggesting that an established cartilage tumor proliferative mitogen might also drive proliferation of these lesions.13 These tumors have been reported in other locations. Steiner et al, and Milgram and Dunn each reported on lesions near the hip,3,14 Purser reported on lesions near the foot,15 and Suermondt reported on one adjacent to the elbow.16 Around the knee, they have also been reported in the posterior compartment, the suprapatellar pouch, and the prepatellar space.7,17,18 We focused only on lesions within the infrapatellar fat pad given the possible unique etiologic role of this structure. This study is limited by retrospective data. All the cases are subject to recall and selection bias. Many data points were not reported consistently and others only ambiguously. Followup was limited and long-term outcomes beyond 2 years remain poorly described. Our exclusion of non-English articles likely eliminated cases, and we did not review the abstracts of unpublished studies from societal meetings.
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17 González-Lois C, García-de-la-Torre P, SantosBriz-Terrón A, Vilá
18 19
20
21 22 23
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J, Manrique-Chico J, Martínez-Tello J. Intracapsular and paraarticular chondroma adjacent to large joints: report of three cases and review of the literature. Skeletal Radiol 2001;30(12): 672–676 Kudawara I, Ueda T, Araki N. Extraskeletal chondroma around the knee. Clin Radiol 2001;56(9):779–782 Sakai H, Tamai K, Iwamoto A, Saotome K. Para-articular chondroma and osteochondroma of the infrapatellar fat pad: a report of three cases. Int Orthop 1999;23(2):114–117 Mosher JF Jr, Kettelkamp DB, Campbell CJ. Intracapsular or paraarticular chondroma. A report of three cases. J Bone Joint Surg Am 1966;48(8):1561–1569 Veras E, Abadeer R, Khurana H, Tan D, Ayala A. Solitary synovial osteochondroma. Ann Diagn Pathol 2010;14(2):94–99 Sarmiento A, Elkins RW. Giant intra-articular osteochondroma of the knee. J Bone Joint Surg Am 1975;57(4):560–561 Milgram JW, Jasty M. Case report 238: para-articular osteochondroma of the knee. Skeletal Radiol 1983;10(2):121–125 Böstman O, Karaharju E, Heikkonen L, Holmström T. Extraskeletal ossifying chondroma in the knee. A case report. Acta Orthop Scand 1985;56(1):87–89 Pettrone FA, Stay EJ. Chondroma. An unusual presentation of an extraarticular soft tissue mass about the knee. Am J Sports Med 1990;18(5):536–538 Krebs VE, Parker RD. Arthroscopic resection of an extrasynovial ossifying chondroma of the infrapatellar fat pad: end-stage Hoffa’s disease? Arthroscopy 1994;10(3):301–304 Allahabadia VN, Kulkarni SS, Dogra AS, Bhosale PB. Solitary synovial osteochondroma of the knee. J Postgrad Med 1995; 41(4):113–114 Reith JD, Bauer TW, Joyce MJ. Paraarticular osteochondroma of the knee: report of 2 cases and review of the literature. Clin Orthop Relat Res 1997;(334):225–232 Rodriguez-Peralto JL, Lopez-Barea F, Gonzalez-Lopez J. Intracapsular chondroma of the knee: an unusual neoplasm. Int J Surg Pathol 1997;5(1–2):49–53 Dhillon MS, Sharma R, Vaiphei K, Nagi ON. Para-articular extraosseous osteochondroma of the knee. The Knee 1999;6(2): 155–158 Cohen AP, Giannoudis PV, Hinsche A, Smith RM, Matthews SJ. Posttraumatic giant intraarticular synovial osteochondroma of the knee. Injury 2001;32(1):87–89 Hung J, Huang G, Peng Y, Wu S, Chau K. Fat pad osteochondroma of the knee. J Med Sci 2005;25(6):315–318 Oliva F, Marconi A, Fratoni S, Maffulli N. Extra-osseous osteochondroma-like soft tissue mass of the patello-femoral space. BMC Musculoskelet Disord 2006;7:57 Rizzello G, Franceschi F, Meloni MC, et al. Para-articular osteochondroma of the knee. Arthroscopy 2007;23(8):e1–e4 Carmont MR, Davies S, Gey van Pittius D, Rees R. Accelerated paraarticular osteochondroma formation within the knee: a case report. Cases J 2008;1(1):6–1626 Demir MK, Unlu E, Usta U. A curious mass of the anterior compartment in the knee. Br J Radiol 2009;82(977):435–437 Singh VK, Shah G, Singh PK, Saran D. Extraskeletal ossifying chondroma in Hoffa’s fat pad: an unusual cause of anterior knee pain. Singapore Med J 2009;50(5):e189–e192 Ozturan KE, Yucel I, Cakici H, Guven M, Gurel K, Dervisoglu S. Patellar tendinopathy caused by a para-articular/extraskeletal osteochondroma in the lateral infrapatellar region of the knee: a case report. Cases J 2009;2:9341 Nouri H, Ben Hmida F, Ouertatani M, et al. Tumour-like lesions of the infrapatellar fat pad. Knee Surg Sports Traumatol Arthrosc 2010;18(10):1391–1394 Mulcahy H, Hoch B. An extraskeletal para-articular osteochondroma of hoffa’s fat pad. Radiology Case Reports. 2010;5(4):420
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We performed an inclusive search of three major databases, but it remains possible that additional relevant articles or nonindexed articles did not meet our search criteria. Agreement between reviewers was high, and we manually reviewed all bibliographies. Our report of three cases is equal to the largest series to date.19–21 We have shown that para-articular extraskeletal osteochondromas of the infrapatellar fat pad have a benign clinical history regardless of type of excision. Neither malignant transformation nor recurrence has ever been reported, and postoperative complications are rare. We hope that this study will raise awareness of this uncommon tumor and guide clinicians in its diagnosis and management.
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41 Ogura K, Goto T, Nemoto T, Imanishi J. Para-articular osteochon-
44 Singh R, Jain M, Siwach R, Rohilla S, Sen R, Kaur K. Large para-
droma of the infrapatellar fat pad. J Knee Surg 2011;24(3):209–213 42 De Maio F, Bisicchia S, Potenza V, Caterini R, Farsetti P. Giant intraarticular extrasynovial osteochondroma of the knee: a report of two cases. Open Orthop J 2011;5:368–371 43 Ingabire MI, Deprez FC, Bodart A, Puttemans T. Soft tissue chondroma of Hoffa’s fat pad. JBR-BTR 2012;95(1):15–17
articular osteochondroma of the knee joint: a case report. Acta Orthop Traumatol Turc 2012;46(2):139–143 45 Sen D, Satija L, Kumar KS, Rastogi V, Sunita BS. Giant intra-articular extrasynovial osteochondroma of the Hoffa’s fat pad: a case report. Medical Journal Armed Forces India 2013; doi: 10.1016/j. mjafi.2012.11.005
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Infrapatellar Fat Pad Osteochondroma: Three Cases and a Systematic Review Nathan Evaniew, MD1 Darius Bayegan, MSc1 Snezana Popovic, MD, PhD, MSc, FRCPC2 Naveen Parasu, MBBS, FRCR, FRCPC3 Michelle Ghert, MD, FRCSC4 1 Department of Surgery, Center for Evidence-Based Orthopaedics,
McMaster University, Hamilton, Ontario, Canada 2 Department of Pathology and Molecular Medicine, McMaster University, Hamilton, Ontario, Canada 3 Department of Radiology, Juravinski Hospital, McMaster University, Hamilton, Ontario, Canada 4 Department of Surgery, Juravinski Hospital, McMaster University, Hamilton, Ontario, Canada
Address for correspondence Michelle Ghert, MD, FRCSC, Department of Surgery, Juravinski Hospital, McMaster University, 711 Concession Street, Hamilton, Ontario, Canada L8V 1C3 (e-mail: [email protected]).
Abstract
Keywords
► ► ► ► ►
osteochondroma chondroma knee fat pad surgery
Extraskeletal para-articular osteochondromas of the infrapatellar fat pad are unique softtissue tumors whose etiology and clinical history remain poorly understood. We report three cases and a systematic review of the literature. Three females, aged 47, 54, and 70 years, presented with pain and a parapatellar knee mass. All three underwent marginal excision with no complications and no recurrence. In the literature, 32 reports describing 42 cases were identified. Mean age at presentation was 51.1 years (range: 12–75). There were 25 females and 20 males. Seven patients (15.6%) had a history of antecedent trauma. Mean duration of presenting symptoms was 63.9 months (range: 2–300). All patients except for one underwent open excision. Mean follow-up was 14.9 months (range: 3–96). There were no reports of recurrence or malignant transformation. Para-articular extraskeletal osteochondromas of the infrapatellar fat pad have a benign clinical history regardless of whether they are managed by arthroscopic or open marginal excision.
Osteochondromas are common benign surface lesions of bone that most typically occur at the metaphyses of the distal femur and proximal tibia.1 Unless they tether neurovascular or musculotendinous structures, osteochondromas usually present as incidental findings or small painless masses. In isolation, their incidence of malignant transformation is well less than 1%,1 and they are frequently managed by either observation or marginal resection. Typical osteochondromas can be diagnosed radiographically by their characteristic attachment to bone with continuity of both the cortices and the medullary canal. Extraskeletal osteochondromas are rare benign soft-tissue tumors whose etiology and natural history remain poorly understood. They are one of many mineralized soft-tissue
masses for which a definitive radiographic diagnosis is not possible. They appear to have a predilection for the hands and feet, but have also been reported adjacent to the knee, hip, and elbow.2,3 They are considered to be synonymous with the nonossified tumor referred to as extraskeletal chondroma and they are designated as para-articular or intracapsular when adjacent to a joint.4 Para-articular osteochondromas are differentiated from synovial chondromatosis by their extrasynovial location, lack of synovial tissue, and typical uninodular appearance.5 Jaffe initially reported on para-articular osteochondroma of the infrapatellar fat pad in 1958.6 Multiple case reports and small series have followed, but optimal management and clinical outcomes beyond 2 years have been infrequently
received August 12, 2013 accepted after revision April 13, 2014 published online May 29, 2014
Copyright © 2015 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0034-1378192. ISSN 1538-8506.
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documented.7 Likewise, this tumor’s underlying etiology, true incidence, and natural history are unknown. Our purpose was twofold: First, we reviewed our local experience with osteochondroma of the infrapatellar fat pad and we present these findings as a series of three case reports. Second, we performed a systematic review to create a comprehensive best-evidence summary of the global literature. Ultimately, we aim to optimize patient outcomes by raising awareness of this uncommon tumor and guiding clinicians in its diagnosis and management.
Methods This study received ethics approval from the Hamilton Integrated Research Ethics Board (Project #13–359-C).
Cases Using a surgeon-specific retrospective database from July 2005 to June 2013, three consecutive patients with the diagnosis of extraskeletal para-articular osteochondroma of
infrapatellar fat pad were identified. Clinical, radiological, and pathological data were extracted from electronic and paper medical records. All patients underwent marginal resection through a vertical parapatellar arthrotomy.
Systematic Review Information Sources and Search Strategy A systematic review was conducted according to the Preferred Reporting Items for Systematic reviews and Metaanalyses (PRISMA) statement.8 Medline and EMBASE were searched in OVID using MeSH and EMTREE headings and subheadings in various combinations (i.e., exp osteochondroma OR exp chondroma AND exp knee) with supplemental free text to increase sensitivity (i.e., “osteochondroma” OR “chondroma”) AND (“knee” OR “extra-articular” OR “para-articular” OR “Hoffa’s fat pad” OR infrapatellar fat pad”). Results were limited to humans, English, and the publication dates 1946 (Medline) or 1980 (EMBASE) to May 5, 2013.
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Fig. 1 Case 1. (a) Lateral radiograph of the patient’s right knee showing an infrapatellar ossified mass. (b, c) T1 and T2 fat sat coronal MRI showing a heterogeneous mass with high signal on T1 inferior to patella that suppresses on the fat sat T2 and represents fatty marrow ( ). Scattered high signal foci on T2 represents cartilage (arrow). (d) Sagittal CT scan showing an ossified mass with areas of soft-tissue density posteriorly representing cartilage. (e) Gross pathology specimen showing a lobular mass of cancellous bone with a cartilaginous cap. (f) Microscopic section demonstrates lamellar trabecular bone (arrow) with overlying hyaline cartilaginous cap (). CT, computed tomography; MRI, magnetic resonance imaging. The Journal of Knee Surgery
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Eligibility Criteria
Data Extraction
Clinical reports of tumors or pseudotumors in the infrapatellar fat pad other than extraskeletal osteochondromas were excluded. Studies reporting on extraskeletal or para-articular osteochondromas or chondromas in locations other than the infrapatellar fat pad were excluded. Studies combining infrapatellar fat pad tumors with tumors in other locations were included only if the infrapatellar fat pad cases were reported separately. Studies were excluded if their full texts were not available in English.
Two independent reviewers extracted the following data points for each included: age, gender, medical history, tumor location, presenting symptoms and examination findings, imaging results, operative details, pathological features, and follow-up.
Study Selection
Statistical Analysis Agreement on study eligibility was determined with Cohen kappa coefficient. Kappa values > 0.65 were considered adequate.9 Descriptive means and proportions were calculated to summarize the case report data. Microsoft Excel (Santa Rosa, CA) was used for data analysis.
Results Case Reports Case 1 A healthy 70-year-old female presented with a 5-year history of a slowly enlarging and palpable mass in her right knee. She
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Two independent reviewers screened titles and abstracts for eligibility using a piloted database. Discrepancies were resolved by consensus. Duplicates were removed in OVID and by manual review. All eligible studies underwent fulltext review. Reviewers were not blinded to authors or publication information. The references of all included studies were manually screened to further identify eligible studies.
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Fig. 2 Case 2. (a) Lateral radiograph of the patient’s left knee showing an infrapatellar mass with speckled calcifications. (b) Sagittal T1 MRI contains a mixture of bright fatty foci and dark foci representing chondroid calcification that becomes more conspicuous (blooms) on the gradient echo image (c, arrow). (d) Cut specimen showing adipose tissue with cartilaginous and osseous foci. (e) Microscopic section showing hyaline cartilage at the periphery and central lamellar bone. Enchondral ossification is seen at the interface between cartilage and bone (). (f) Surrounding adipose tissue demonstrates focal areas of fat necrosis () with the presence of histiocytes. MRI, magnetic resonance imaging. The Journal of Knee Surgery
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had no significant past medical history and had not undergone any laboratory testing. She described difficulty with deep flexion and was unable to comfortably ascend or descend stairs. She had a firm, nontender mass about the anterolateral aspect of her right knee. Range of motion was 30 degrees of active flexion, with further flexion causing sharp anterior knee pain. Radiographs revealed a well-defined ossified lesion within the infrapatellar fat pad (►Fig. 1). A computed tomography (CT) scan confirmed its location and a technetium-99 bone scan showed mild uptake in the region of the infrapatellar fat pad. Magnetic resonance imaging (MRI) demonstrated an ossified mass with fluid between the mass and the patellar tendon. At surgery, a 6.5 4.5 3.8 cm specimen consisting of bone and a cartilaginous cap was excised. Microscopic sections revealed lamellar trabecular bone with overlying hyaline cartilage, similar to conventional osteochondroma. Focal areas of regenerative and hyperplastic hyaline cartilage with intermittent necrosis suggested superimposed trauma. There
were no aggressive features. Pathologic diagnosis was paraarticular extraskeletal osteochondroma. Postoperatively, her active knee range increased to 90 degrees and she was able ascend and descend stairs without difficulty. Clinical and radiographic examination at 3 months demonstrated no recurrence.
Case 2 A healthy 47-year-old female presented with a 1-year history of pain and swelling in her left knee, exacerbated by kneeling and sitting. She had a tender firm mass at the anterolateral aspect of her knee, and range of motion and strength were normal. Radiographs revealed areas of ossification in the infrapatellar fat pad (►Fig. 2). MRI showed encapsulation by a rim of high T2 signal, with heterogeneous gadolinium enhancement within the lesion. The radiological differential diagnosis included localized synovial chondromatosis. At surgery, a 6.5 3.7 1.7 cm discrete mass was excised. Gross sections showed ossification within mature adipose
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Fig. 3 Case 3. (a) Lateral radiograph of the patient’s right knee shows slight calcifications in the infrapatellar fat pad. Sagittal T1 (b) and fat-sat T2 (c) MRI images demonstrate an ovoid soft-tissue mass which shows predominantly dark signal on T1 and corresponding bright signal on T2 in keeping with cartilage. Subtle areas of T1 bright foci are seen posteriorly, which is dark on the fat sat images and suggestive of early ossification (arrow). (d) Mature adipose tissue with focal areas of fat necrosis and chondroid proliferation. (e) Low-power core biopsy histology section showing benign fibroconnective tissue. (f) High power histology section showing mature adipose tissue with focal areas of fat necrosis and chondroid proliferation ( ). The Journal of Knee Surgery
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Infrapatellar Fat Pad Osteochondroma tissue. Microscopic sections demonstrated central lamellar trabecular bone and peripheral hypercellular hyaline cartilage, with focal areas of fat necrosis. There was only mild atypia of the chondrocytes, and enchondral ossification was noted at the osteochondral junction. Pathologic diagnosis was para-articular extraskeletal osteochondroma. After 6 months of physical therapy, this patient had full strength and motion, and returned to work. Clinical examination at 19 months demonstrated no recurrence.
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At surgery, a two-lobed, tan-colored mass consisting of a 6.4 3.6 2.2 cm and a 2.5 1.4 0.4 cm part was excised. Microscopic sections demonstrated hyaline cartilage, mature adipose tissue, focal areas of fat necrosis, and no evidence of bone formation. The final diagnosis was extraskeletal para-articular chondroma. At 3 months follow-up, her pain was settled and her range of motion was 0 to 110 degrees.
A healthy 54-year-old female presented with a 5-year history of right knee stiffness and a slowly enlarging mass. She described occasional anterior knee pain worsened by kneeling or walking long distances. She had slight anterolateral swelling of her knee, but no discrete palpable mass and no tenderness to palpation. Range of motion and strength were normal. Initial radiographs showed a faint soft-tissue density in the infrapatellar fat pad (►Fig. 3). MRI revealed a large mass with irregular high T2 and low T1 signals. Ultrasound showed that the mass was noncompressible and had no vascular flow. Ultrasound-guided core needle biopsy suggested benign fibroconnective and fibrocartilaginous tissue with degenerative cystic changes.
The search identified 423 studies (►Fig. 4). Agreement during screening was satisfactory (kappa ¼ 0.95; confidence interval, 0.87–1.00). The references of the final 23 studies were manually reviewed, which led to the addition of nine more studies. In total, 32 studies describing 42 cases were included (►Tables 1 and 2). Mean age was 51.1 years (range: 12–75). There were 25 females and 20 males. Seven patients (16%) described antecedent trauma. Mean duration of presenting symptoms was reported in 36 cases and was 63.9 months (range: 2–300). Thirty-one patients (73%) reported pain and 38 (91%) reported a mass. Reported preoperative work-up included X-rays in all 42 cases, MRI in 22 cases (53%), CT scan in 12 cases (29%), bone scan in 4 cases (9%), and ultrasound in 3 cases (7%). Biopsy was performed before excision in 17 cases (40%).
Fig. 4 Flowchart showing search results, screening, and reasons for exclusion of articles.
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Systematic Review Case 3
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Table 1 Demographics and clinical presentations of all cases Case
Authors 6
Year
Age
Gender
Trauma
Symptoms (mo)
Pain
Mass
1
Jaffe et al
1958
37
F
2
Mosher et al20
1966
23
F
12
Y
Y
3
Mosher et al
20
1966
66
M
2
Y
Y
4
Mosher et al
20
1966
65
M
5
Sarmiento and Elkins22
1975
67
M
6
Milgram and Dunn14
1980
66
F
1983
59
M
120
Y
1985
29
M
36
Y
1990
12
M
12
7
Milgram and Jasty
8
Böstman et al24
9
Pettrone and Stay
23
25
26
Y
Y
24
Y
Y
84
Y
Y
Y
Y
1994
52
M
120
11
Steiner et al
3
1994
66
F
48
Y
Y
12
Steiner et al3
1994
49
F
240
Y
Y
48
Y
Y
10
Krebs and Parker
27
1995
32
F
14
Reith et al
28
1997
66
M
24
Y
Y
15
Reith et al28
1997
75
F
60
Y
Y
13
Allahabadia et al
29
Y
Y
16
Rodriguez-Peralto et al
1997
52
F
120
17
Sakai et al
19
1999
64
F
12
Y
Y
18
Sakai et al
19
1999
47
M
72
Y
Y
19
Sakai et al19
1999
56
M
20
Dhillon et al30
1999
60
M
48
Y
Y
12
Y
Y
31
21
Cohen et al
2001
67
M
22
González-Lois et al17
2001
43
M
23
17
Y
Y
Y
2001
37
F
2
24
Hung et al
32
2005
46
M
48
25
Oliva et al33
2006
53
M
36
Y
Y
2007
42
F
36
Y
Y
2008
25
M
Y
84
Y
Y
2008
61
M
Y
5
Y
2009
60
F
36
Y
2009
55
M
12
Y
González-Lois et al
34
26
Rizzello et al
27
4
Turhan et al
28
Carmont et al
29
Demir et al36
30
Singh et al
35
37 38
Y
Y Y
Y
31
Ozturan et al
2009
60
F
120
Y
32
Nouri et al39
2010
42
F
36
Y
33
Veras et al21
2010
59
F
34
Veras et al
21
2010
33
F
Y
Y
35
Veras et al21
2010
46
F
Y
Y
2010
25
F
48
Y
Y
36 37
Mulcahy and Hoch Ogura et al
40
41
Y Y
2011
56
F
24
Y
Y
38
De Maio et al
42
2011
58
F
120
Y
Y
39
De Maio et al42
2011
71
F
300
Y
Y
40
Ingabire et al
43
2012
64
F
41
Singh et al
44
2012
52
M
42
Sen et al45
2013
25
M
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Y Y Y
84
Y
Y
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Table 1 (Continued) Case
Authors
Year
Age
Gender
43
Evaniew et al (current study)
2014
70
44
Evaniew et al (current study)
2014
47
45
Evaniew et al (current study)
2014
54
Trauma
Symptoms (mo)
Pain
Mass
F
60
Y
Y
F
12
Y
Y
F
60
Y
Y
Abbreviations: F, female; M, male; Y, yes.
Table 2 Pathological characteristics and clinical outcomes of all cases Case
Size (cm)
Pathologic diagnosis
1
7
Para-articular chondroma
0
Para-articular chondroma
36
Follow-up (mo)
Recurrence No
3
321
Para-articular chondroma
36
No
4
6.5 4.8 3.9
Para-articular chondroma
72
No
5
53
Intra-articular osteochondroma
48
No
6
Nil
Para-articular osteochondroma
0
7
991
Para-articular osteochondroma
8
54
Extraskeletal osteochondroma
Nil
12
9
332
Chondroma
Nil
24
10
522
Ossifying chondroma
0
No
11
10 7 6
Intracapsular chondroma
Nil
36
No
12
4.5 4.0 3.5
Extracapsular chondroma
0 No
0
13
436
Solitary synovial osteochondroma
Nil
20
No
14
973
Para-articular chondroma
Nil
24
No
15
9.5 6 4
Para-articular osteochondroma
Nil
13
No
16
7.5 5 5
Intracapsular chondroma
Nil
0
17
6.0 4.5 3.0
Para-articular chondroma
Nil
24
No
18
753
Para-articular chondroma
Nil
24
No
19
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2
Complications
Para-articular osteochondroma
0
20
86
Para-articular osteochondroma
Nil
18
21
7.5 5 4
Intra-articular osteochondroma
Nil
12
22
4.0 3.5 2
Para-articular chondroma
Nil
0
23
3.5 2.0 2.7
Para-articular chondroma
Nil
0
24
3 3 3, 3 2 4
Osteochondroma
Nil
12
No
25
9
Extraosseous osteochondroma
Nil
24
No
26
5.0 2.5 1.5, 2 2 1, 1.5 2 1 cm
Para-articular osteochondroma
Nil
0
No
27
7 4 3.5
Extrasynovial osteochondroma
Nil
12
No
28
4.5
Para-articular osteochondroma
Nil
0
29
55
Para-articular osteochondroma
Nil
0
30
6 4.5 5
Ossifying chondroma
31
5 4.2 3.5
Para-articular osteochondroma
Nil
12
No
32
3 2.5 2.5
Osteochondroma
Nil
0
No
33
4.5 3.5 3.0
Solitary synovial osteochondroma
0
34
7.5 5.5 3.5
Solitary synovial osteochondroma
0
No
0
(Continued)
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Table 2 (Continued) Case
Size (cm)
Pathologic diagnosis
Complications
Follow-up (mo)
35
6.5 5.0 3.8
Solitary synovial osteochondroma
36
5.5 4.5 3
37
5.5 4.8 2.3
Para-articular osteochondroma
Nil
0
Para-articular osteochondroma
Nil
6
No
38
Extrasynovial osteochondroma
Nil
96
No
39
Solitary synovial osteochondroma
Nil
48
No
40
4 2.7 4.6
Recurrence
0
Chondroma
0
41
15.5 8 3.8
Para-articular osteochondroma
Nil
26
No
42
7 4 3.5
Extrasynovial osteochondroma
Nil
12
No
43
6.5 4.5 3.8
Osteochondroma
Nil
3
No
44
6.5 3.7 1.7
Osteochondroma
Nil
19
No
45
6.4 3.6 2.2
Chondroma
Nil
3
No
All patients except for one underwent open marginal excision. Case #11 was removed piecemeal via an intralesional arthroscopic technique. Case #15 underwent open excision at the time of total knee arthroplasty. The largest lesion was 15.5 8.0 3.8 cm (case #41), and the smallest was 3 2 1 cm (case #3). The final diagnosis was chondroma in 15 cases and osteochondroma in 30 cases. Prefixes included “para-articular,” “intra-articular,” “intracapsular,” “extracapsular,” “myxoid” (chondroma), “ossifying” (chondroma), “soft tissue,” “extraskeletal,” “extrasynovial,” and “solitary synovial” (►Table 3). There were no reported complications such as infection, wound breakdown, or neurovascular injury. Follow-up was reported for 25 cases, the mean of which was 14.9 months (range: 3–96). Some functional outcome was described for 28 patients, all of which returned to “normal,” “good,” or “full,” except three who experienced ongoing pain with squatting or kneeling. There were no reports of recurrence or malignant transformation.
Discussion We performed a systematic review of the literature and reviewed our local experience with osteochondromas of Table 3 Common prefixes used to describe osteochondromas found in the infrapatellar fat pad Para-articular Intracapsular Intra-articular Extracapsular Myxoid (chondroma) Ossifying (chondroma) Soft tissue Extraskeletal Extrasynovial Solitary synovial
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the infrapatellar fat pad. We identified 42 cases published previously and contribute 3 additional cases. The aim of this study was to raise awareness of this uncommon tumor and guide clinicians in its management. Based on our findings, patients with this tumor are likely to be middle-aged generally healthy females who describe a 5-year history of a slowly growing painful mass without antecedent trauma. They are likely to experience symptomatic improvement following excision, with no likelihood of recurrence at 1 year. The etiology of this tumor remains unknown. GonzálezLois et al suggested soft-tissue osteochondromas might arise from the joint capsule or para-articular connective tissues via cartilaginous metaplasia.10 Zhang et al reviewed evidence that multipotent stromal cells and growth factors within the fat pad may be form chondroid, osteoid, and adipose tissue in response to trauma.7,11,12 This is supported by our findings of necrotic adipose tissue, but we did not see any metaplastic tissue. Only 16% of the reports described antecedent trauma. Ratcliff et al reported increased parathyroid hormone-related protein expression in a soft-tissue osteochondroma adjacent to the medial tibial metaphysis, suggesting that an established cartilage tumor proliferative mitogen might also drive proliferation of these lesions.13 These tumors have been reported in other locations. Steiner et al, and Milgram and Dunn each reported on lesions near the hip,3,14 Purser reported on lesions near the foot,15 and Suermondt reported on one adjacent to the elbow.16 Around the knee, they have also been reported in the posterior compartment, the suprapatellar pouch, and the prepatellar space.7,17,18 We focused only on lesions within the infrapatellar fat pad given the possible unique etiologic role of this structure. This study is limited by retrospective data. All the cases are subject to recall and selection bias. Many data points were not reported consistently and others only ambiguously. Followup was limited and long-term outcomes beyond 2 years remain poorly described. Our exclusion of non-English articles likely eliminated cases, and we did not review the abstracts of unpublished studies from societal meetings.
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We performed an inclusive search of three major databases, but it remains possible that additional relevant articles or nonindexed articles did not meet our search criteria. Agreement between reviewers was high, and we manually reviewed all bibliographies. Our report of three cases is equal to the largest series to date.19–21 We have shown that para-articular extraskeletal osteochondromas of the infrapatellar fat pad have a benign clinical history regardless of type of excision. Neither malignant transformation nor recurrence has ever been reported, and postoperative complications are rare. We hope that this study will raise awareness of this uncommon tumor and guide clinicians in its diagnosis and management.
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