Musculoskeletal Imaging • Original Research Mohankumar et al. Imaging the Meniscal Ossicle
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Musculoskeletal Imaging Original Research
Meniscal Ossicle: Posttraumatic Origin and Association With Posterior Meniscal Root Tears Rakesh Mohankumar 1,2 Andrew Palisch 3,4 Waseem Khan 3,5 Lawrence M. White2 William B. Morrison 3 Mohankumar R, Palisch A, Khan W, White LM, Morrison WB
Keywords: meniscal ossicle, meniscal root, posterior root tear DOI:10.2214/AJR.13.11821 Received August 29, 2013; accepted after revision November 22, 2013. 1 Department of Musculoskeletal Imaging, Mount Sinai Hospital and University Health Network, Toronto, Toronto, ON, Canada. 2 Present address: Toronto General Hospital and University Health Network, 200 Elizabeth St, Toronto, ON, M5G 2C4 Canada. Address correspondence to R. Mohankumar ([email protected]). 3 Department of Radiology, Thomas Jefferson Hospital, Philadelphia, PA. 4
Present address: Department of Radiology, Baylor College of Medicine, Houston, TX.
5
Present address: Department of Radiology, Ruth Copley Medical Center, Aurora, IL. This article is available for credit. AJR 2014; 203:1040–1046 0361–803X/14/2035–1040 © American Roentgen Ray Society
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OBJECTIVE. The purpose of this study was to characterize meniscal ossicles and their association with meniscal root tears. MATERIALS AND METHODS. A two-center retrospective assessment of 65 knee MRI studies with meniscal ossicles was performed. The relationship of the ossicle to the meniscal root and horn was assessed along with other findings of internal derangement of the knee. RESULTS. Meniscal ossicles were identified in patients ranging from 23 to 80 years old. The ossicles were located within the posterior horn and root of the medial meniscus in 57 of 65 (88%) examinations. Associated meniscal tears were seen at the site or adjacent to the ossicle in all but one examination (98%). Meniscal extrusion was present in 52% (34/65) of examinations. Associated findings included articular cartilage loss, which was severe in 51% (33/65) of examinations. Anterior cruciate ligament injury or prior anterior cruciate ligament reconstruction surgery was also identified in 58% (38/65) of examinations. CONCLUSION. Our findings show that the vast majority of meniscal ossicles are associated with posterior horn or meniscal root tears and a high incidence of focal articular cartilage loss as well as anterior cruciate ligament tears.
T
he presence of a meniscal ossicle is an uncommon but well-documented finding, with ossification within the substance of the meniscus first described in the medical literature in the 1930s [1–3]. The prevalence of meniscal ossicles in the knee has been reported as 0.15% in a case series of 1287 consecutive MRI examinations [4]. There has not been a unified consensus regarding the origin of the meniscal ossicle. The theories proposed as the possible pathogenesis of a meniscal ossicle include congenital, degenerative, and posttraumatic causes. Some authors suggest that the meniscal ossicle is a congenital vestigial structure similar to those seen in certain animal species [5]. Mucoid degeneration of the meniscus and interval mineralization within the meniscus have also been proposed in literature [1]. The more favored theory is that of a posttraumatic origin, either by heterotopic ossification within the meniscus after an antecedent trauma [6] or an avulsion injury of the root attachment of the meniscus off the tibial spine [7]. Many articles on the subject were published before the advent of MRI, and none have related the presence of this finding with the recent-
ly described entity of posterior meniscal root tears. Posterior meniscal root tears are functionally different from tears of the body and posterior horn and can lead to rapid progression of osteoarthritis [8]. This is also important from an orthopedic point of view because treatment of posterior meniscal root tears is different from management of meniscal tears within the body or posterior horn. Meniscal ossicles can also be mistaken for intraarticular bodies, particularly on radiography. The purposes of this study were to identify the meniscal ossicle on MRI and evaluate its association with the state of the meniscus. Other coexistent findings within the knee were also evaluated to assess possible coexistent relationships supportive of the etiologic development of the meniscal ossicle itself. Materials and Methods Patient Selection The institutional review boards of both centers participating in the investigation (Mount Sinai Hospital and University Health Network, Toronto, ON, Canada, and Thomas Jefferson Hospital, Philadelphia, PA) approved the study and waived informed consent. Retrospective electronic screening of the radiology report databases of both institu-
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Imaging the Meniscal Ossicle tions was performed for the term “meniscal ossicle” in reports of MRI of the knee between July 2007 and June 2012. The search revealed 70 MRI examinations in 70 different patients with the term “meniscal ossicle” in the report. The MR images were reviewed to confirm the findings of a meniscal ossicle and to evaluate for associated findings within the joint. All MRI examinations were reviewed by consensus assessment by five fellowship-trained musculoskeletal radiologists, with experience ranging from 1 to 18 years. Five cases were excluded from the study because retrospective analysis of the MRI study failed to reveal a meniscal ossicle. No prior imaging studies, radiography or MRI, were available for these five cases. The remaining 65 examinations constituted the study group. Imaging history was examined for the 65 patients to identify prior or follow-up MRI on the same knee as the study examination. In the 65 examinations analyzed, imaging history showed prior MRI of the same knee in four patients and follow-up MRI of the same knee in two patients. The prior MR images in one examination could not be accessed for review because it was performed before image storage in PACS. The MR images for the five remaining examinations were evaluated to assess for possible temporal changes.
fluid signal intensity on a T2-weighted sequence exiting an articular surface [11]. In all cases, the integrity of the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) was assessed as intact, or showing a partialor full-thickness injury. The examinations with prior cruciate ligament reconstructions were also assessed similar to those with native ligaments and recorded as intact or showing a partial- or fullthickness tear. The medial and lateral collateral ligaments were also evaluated. The collateral ligaments were graded as normal, low-grade (edema surrounding the ligament in acute, thickening of the ligament in chronic), intermediate-grade (edema surrounding and within the ligament), or highgrade (disruption of fibers) injuries. Finally, the state of the cartilage along the articular surfaces of the tibiofemoral and patellofemoral joints was also assessed. The cartilage loss was classified as focal or diffuse, and the location was recorded. Cartilage loss was defined as focal if the involved surface diameter of the cartilage loss was less than 10 mm [12]. The severity of cartilage loss was classified as mild (≤ 50% thickness of cartilage loss), moderate (> 50% but less than full thickness), or severe (full-thickness loss with underlying bone marrow changes).
Because the study involved two institutions with different MRI vendors and because of the time span of the study, there were different MRI protocols used for imaging of the knee across the study cohort. However, sagittal T2-weighted sequences with fat saturation, sagittal intermediate-weighted acquisitions without fat saturation,
and axial T2-weighted acquisitions with fat saturation were standard across all examinations. In addition, coronal images obtained with intermediate weighting without fat saturation or coronal T1-weighted nonfat-saturated and T2-weighted fat-saturated coronal acquisitions were components of all studies assessed. The images were all obtained with a slice thickness of 3–4 mm and an FOV between 14 and 18 cm. The presence of a meniscal ossicle was defined as a finding of a well-defined focus of signal intensity matching that of the adjacent bone marrow on all sequences within the substance of the meniscus (Fig. 1). In all cases with a meniscal ossicle, the position of the ossicle within the meniscus (i.e., anterior horn, body, posterior horn, and medial versus lateral) was recorded. A meniscal ossicle was classified to be located at the root of the meniscus if there was no meniscal tissue between the anterior or posterior horn of the meniscus and the root attachment on the tibial eminence. Both medial and lateral menisci were also assessed for tears. On the basis of previously published criteria [9], a meniscal tear was identified if there was increased signal intensity within the meniscus exiting an articular surface on two or more contiguous slices or associated change in morphology of the meniscus. If there was only a single plane of articular surface exit, the tear was classified as simple. A tear was classified as complex if there was more than one plane of exit of the intrameniscal signal intensity. Associated meniscal extrusion, which was defined as a distance of greater than 3 mm between the peripheral margin of the meniscus and the respective tibial plateau at the level of the midbody of the meniscus [10], was also recorded. A meniscal retear was identified when there was intrameniscal high signal intensity equating
A
B
C
Image Analysis
Statistical Analysis Statistical analysis was performed using software (SPSS Statistics, version 21.0, IBM). Descriptive statistics, such as frequencies and descriptives (mean, range), was performed for the variables involved in the study. Frequency analysis was performed on the location of a meniscal tear, in particular the posterior horn and root tears of the meniscus, and its relationship to the
Fig. 1—30-year-old woman with meniscal ossicle. A–C, Sagittal intermediate-weighted (A), sagittal T2-weighted fat-saturated (B), and coronal intermediate-weighted (C) images show meniscal ossicle within posterior root of medial meniscus (arrows).
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Mohankumar et al. TABLE 1: Location of Ossicle Within Meniscus
TABLE 2: Findings Within Meniscus That Showed Meniscal Ossicle
Location
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Meniscus (n = 65)
Anterior Horn
Body
Posterior Horn
Posterior Root
Total
Medial
1 (1.5)
1 (1.5)
13 (20)
43 (66)
58 (89)
Lateral
0
1 (1.5)
3 (4.6)
3 (4.6)
7 (11)
Note—Data in parentheses are percentages.
meniscal ossicle. Correlation and multiple regression analyses were conducted among the variables, particularly the tear location and cartilage damage to examine the relationship with age, sex, meniscal extrusion, and ACL injury.
Results Sixty-five examinations with a meniscal ossicle identified on MRI of the knee were analyzed. The mean age of the patients was 51 years (range, 23–80 years). Thirty-three patients were men (mean age, 47.8 years; range, 23–80 years) and 32 patients were women (mean age, 55.0 years; range, 30–77 years). In no case was more than one meniscal ossicle identified in a knee. The vast majority (58/65, 89%) of ossicles were located within the medial meniscus. In 62 of 65 (95%) examinations, the meniscal ossicle was located at the posterior horn or root of the meniscus, whereas two (2/65, 3%) were present at the body and one (1/65, 2%) at the anterior root. Hence, the posterior horn (13/65, 20%) and root (43/65, 66%) of the medial meniscus were the most prevalent locations of a meniscal ossicle, accounting for 86% (56/65) of examinations, in our study group (Table 1). The meniscus containing an ossicle was abnormal in 98% of examinations (64/65). Meniscal abnormalities included a simple root tear (38/65, 58%), complex root tear (9/65, 14%), simple tear of the posterior horn (8/65, 12%), and complex tear of the posterior horn (5/65, 8%). Four (4/65, 6%) menisci had undergone prior partial meniscectomies, with two of these cases showing recurrent tears. In all cases with a meniscal tear (64/64, 100%), the ossicle was located at the tear site within the meniscus (Table 2). Thirty-four of 65 (34/65, 52%) menisci with ossicles showed meniscal extrusion (> 3 mm) at the level of the midbody, with no meniscal extrusion observed in 12 (12/65, 18%) examinations. The other meniscus within the same knee showed a tear in 30 examinations (30/65; 46%). The ACL was abnormal in 37% (24/65) of examinations. Seven (7/24, 29%) of these cases showed complete tears of the ACL.
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Fourteen (14/65, 21%) examinations showed evidence of prior ACL reconstruction, seven of which showed partial or complete tears of the reconstruction graft. Of the 31 (31/65; 48%) cases with ACL injuries (partial or complete, native or graft), only one case showed MRI features of an acute trauma (subchondral marrow edema characteristic of pivot shift, large joint effusion, or concordant clinical history). No examination showed findings of abnormality of the PCL. Eight (8/65, 12%) examinations showed chronic low-to-intermediate grade injuries of the medial collateral ligament. No injuries to the lateral collateral ligament complex were identified in any of the examinations. Articular cartilage was normal in only seven (7/65, 11%) examinations, with 32 (32/65, 49%) examinations showing severe chondral loss of the weightbearing tibiofemoral articular surface. Diffuse articular cartilage dam-
Finding (n = 65)
No.
Percentage
Normal
1
2
Simple root tear
38
58
Complex root tear
9
14
Posterior horn tear
8
12
Complex horn tear
5
8
Prior meniscectomy
4
6
age found was not just limited to the side of the meniscal ossicle (8/58, 14%) but was found in the other tibiofemoral joint compartment as well as the patellofemoral compartment in most cases (50/58, 86%). Seventyfour percent (48/65) of cases had cartilage damage of at least moderate grade that was focal or diffuse (Table 3). Isolated areas of focal cartilage damage were identified in only 14 examinations (14/65, 22%). Focal areas of chondral loss were identified in the tibiofemoral compartment ipsilateral to the meniscal ossicle in eight cases (8/14, 57%). Three (3/14, 21%) cases had focal cartilage damage involving the contralateral tibiofemoral joint compartment, and the other the three cases (3/14, 21%) had cartilage loss at the patellofemoral
TABLE 3: Image Findings of Other Major Supporting Structures of the Knee (n = 65) Supporting Structure
Finding
No.
Percentage
27
42
ACL Normal Abnormal
24
37
14 (7 with partial or full failure)
21
Normal
65
100
Abnormal
0
0
Normal
57
88
8 (all chronic MCL)
12
Normal
7
11
Focal mild
2
3
Reconstruction graft PCL
MCL Abnormal Articular cartilage
Focal moderate
2
3
Focal severe
10
15
Diffuse mild
8
12
Diffuse moderate
14
22
Diffuse severe
22
34
Note—ACL = anterior cruciate ligament, PCL = posterior cruciate ligament, MCL = medial collateral ligament.
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Imaging the Meniscal Ossicle TABLE 4: Correlations and Results From Regression Analyses ba
βb
Correlation With Tear Location
Age
0.494c
0.087
0.624d
Sex
−0.159
0.143
0.037
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Multiple Regression Weights
Type of meniscal tear
Correlation With Cartilage Injury
Multiple Regression Weights ba
βb
−0.088
−0.002
−0.038
−0.016
−0.063
−0.051
0.035
0.060
0.150
0.428
0.234
0.053
Location of meniscal tear
−0.044
0.007
0.002
1
Meniscal extrusion
0.369c
0.143
0.178
−0.106
−0.031
−0.120
ACL state
0.087
0.434
0.414d
0.038
−0.007
−0.021
−0.044
0.001
0.004
Cartilage injury
1
Note—ACL = anterior cruciate ligament. aUnstandardized regression coefficient. bStandardized regression coefficient. cp < 0.01. dp < 0.001.
articulation. Ten (10/14, 71%) cases of focal chondral loss were assessed as severe in grading, and two cases each had mild and moderate severity of chondral loss. Correlation and multiple regression analyses were conducted to examine the relationship between the variables. Multiple regression analysis results and correlation for location of meniscal tear and cartilage injury and their relationship to the other variables are summarized in Table 4. Cartilage damage correlated significantly (p < 0.001) with age of the patient and state of the ACL. Correlation (p < 0.01) was also identified between cartilage damage and meniscal extrusion, reiterating the function of the meniscus as a shock absorber to the tibiofemoral joint. The location of the meniscal tear did not correlate significantly with any of the other variables.
A
In one of the three patients with prior MRI available, sequential examination review showed temporal development of a meniscal ossicle. In this case, a 23-year-old man whose initial MRI examination 33 months earlier had shown a complete ACL tear and posterior meniscal root tear, a meniscal ossicle at the posterior root of the medial meniscus adjacent to the root tear was identified on the examination. The patient had also undergone an ACL reconstruction (Fig. 2). In the other two patients with prior MRI, no change in size of the meniscal ossicle was appreciated at 35 months and 45 months, respectively. In one of the two patients with follow-up MRI, there was evidence of increase in size of the initially identified meniscal ossicle on MR images 18 months after the initial examination. In this case, a 31-year-old man, a fullthickness tear of the ACL was also identified,
B
with a reconstruction graft performed in the interval before follow-up imaging (Fig. 3). In the other patient with follow-up MRI, no interval change in size of the meniscal ossicle was appreciated on MRI after 28 months. Discussion Ossicles within the meniscus of the knee were first described in the 1930s by three different authors [1–3]. Since then, multiple case reports and case series have been published in the literature. A retrospective review in 2003 identified 52 publications pertaining to meniscal ossicles in the literature [13]. The largest single case series to date in the orthopedic literature described 10 cases [14], and the largest in the radiology literature described six cases [4]. Multiple ossicles within the same meniscus have also been reported [15]. To our knowledge, this article is
Fig. 2—23-year-old man with temporal development of meniscal ossicle. A, Initial coronal T1-weighted MR image obtained at level of posterior horn of medial meniscus shows no evidence of ossicle at posterior horn or root (arrow). B, Follow-up coronal T1-weighted MR image obtained 33 months later shows meniscal ossicle at posterior root of medial meniscus (arrow). Artifacts from interval anterior cruciate ligament reconstruction are also noted.
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Mohankumar et al. Fig. 3—31-year-old man with temporal increase in size of meniscal ossicle. A, Initial coronal intermediate-weighted MR image shows small ossicle within posterior root of medial meniscus (arrow). B, Follow-up coronal intermediate-weighted MR image through same area obtained 18 months later shows increase in size of ossicle (arrow).
A
B
the largest single case series of meniscal ossicle in the literature. Meniscal ossicles are usually found within the posterior horn of the medial meniscus [16]. This is thought to be secondary to the abundant vascularity of the area, predisposing damaged meniscal tissue to dystrophic ossification in this location. Our study similarly showed meniscal ossicles to be most commonly observed (57/65, 88%) within the posterior horn and root of the medial meniscus. The cause of the meniscal ossicle has been of considerable debate in the literature. Various theories have been proposed regarding the origin of such ossicles. These theories include degenerative (mucoid degeneration), congenital (vestigial structure), or posttraumatic (heterotopic ossification or osseous avulsion of the meniscal root attachment to the tibia) causes. Mucoid degeneration with ossification within the substance of the meniscus has been postulated as a possible cause of development of the meniscal ossicle [1]. However, this theory was proposed before the advent of advanced imaging. MRI studies have subsequently shown that mucoid degeneration of the meniscus is most prevalent at the junction of the body and posterior horn of the medial meniscus [17], where ossicles were not as commonly observed in our study group. Mucoid degeneration has also been described as typically commencing from the third decade of life. Descriptions of meniscal ossicle in patients as young as 12 years [18], which is before development of degenerative change, also contradicts this theory. Meniscal ossicles have been identified as normal vestigial structures in domestic cats,
rodents [19], and tigers [5]. Therefore, the same theory has been applied in humans. Ganey et al. [5] suggested that the meniscal ossicle is akin to a sesamoid bone and develops progressively from the time of birth. Microscopy of the meniscal ossicle by has also shown mature cancellous bone with a cartilage interface and without any fibroblast proliferation and neovascularization [20]. Our findings of temporal development of a meniscal ossicle in a skeletally mature patient and progression in size of a meniscal ossicle in another patient over short durations of time, however, contrast this theory. These temporal observations argue against the theory of a meniscal ossicle as a vestigial structure or a normal anatomic variant. The more popular theory of origin of a meniscal ossicle is that the ossicle is the result of prior single or multiple traumatic episodes. In such cases, a meniscal ossicle is thought to develop after a traumatic episode with posttraumatic metaplasia and secondary heterotopic ossification within the meniscus. It has been proposed that such traumatic episodes result in cartilaginous ossification of the meniscus due to metaplasia of fibrocartilage and irregular neovascularization [6]. This theory would be consistent with observations of the posterior horn of the medial meniscus as the common location for a meniscal ossicle because this area is less mobile and more vulnerable to injury [4]. Our findings certainly support this theory, with a predominance of the typical location of meniscal ossicles seen as well as association of such ossicles with meniscal tears and other injuries to the knee. Temporal development or growth of a meniscal ossicle after
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documented traumatic injury of the knee, as seen in two of our patients, also supports the theory of posttraumatic heterotopic ossification in the pathogenesis of a meniscal ossicle. Both of these patients had full-thickness ruptures of the ACL with interval reconstruction grafting of the ligament. Some authors have suggested a different traumatic mechanism for development of a meniscal ossicle. The posterior horn and root of the medial meniscus are thought to have a substantial tibial insertion, and a bony avulsion from these insertion sites has been proposed as an alternative cause for a meniscal ossicle [7]. Similar posttraumatic intraarticular osseous avulsions appear sharp and noncorticated immediately after injury and undergo smoothening and cortication over time [21]. Our study showed tears involving the posterior horn and root of the meniscus in 95% (62/65) of examinations, of which 71% (46/65) involved the posterior root attachment. Other authors have hypothesized that such injuries may result in small avulsed bony fragments acting as a nidus for the formation and progression of a meniscus ossicle [4]. The orthopedic literature also describes surgical repair techniques of reimplantation of avulsed bony fragments to the donor site of the tibia in the setting of posterior root tearing, supporting the possibility of a meniscal ossicle developing as a result of meniscal root avulsion fracture [22]. Richmond and Sarno [23] showed arthroscopic evidence of an osseous avulsive donor site at the tibia in the setting of avulsed meniscal root tears. They found that in acute cases the donor site was bleeding, whereas in chronic cases,
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Imaging the Meniscal Ossicle the donor site could still be visualized but was filled with fibrous tissue. They also performed histologic analyses of the observed meniscal avulsive ossicles, which showed meniscus attached to mature cancellous bone through fibrocartilage and calcified fibrocartilage. No evidence of metaplasia or degeneration was identified, supporting the theory of avulsion fracture as the cause of ossicle formation. In contrast, findings in our study showed that in many cases the observed meniscal ossicle was not at the root attachment at all, arguing against this as the sole cause of meniscal ossicle formation. On the basis of our findings, the posttraumatic cause of a meniscal ossicle secondary to metaplasia and heterotopic ossification is favored. This is especially true in cases in which a meniscal ossicle is present within the substance of the posterior horn of the meniscus away from the meniscal root. However, in cases in which an ossicle is located at the site of the meniscal root attachment, the theory of bony avulsion remains feasible. A meniscal ossicle can be a symptomatic and may be found incidentally during imaging [24, 25]. However, most patients with meniscal ossicles experience episodes of intermittent pain. Mechanical locking is usually not a typical feature of a meniscal ossicle, unlike an intraarticular loose body [25]; however, this symptom can be seen in some patients [4, 26]. Asymptomatic meniscal ossicles can spontaneously become symptomatic. This is hypothesized to occur due to continuing increase in size of the ossicle irritating adjacent structures [27] or alteration in contour of the meniscus secondary to the ossicle leading to altered stress [28]. It is also possible that such symptoms may temporally arise from associated internal derangement (i.e., meniscal tear, chondral injury) rather than from the ossicle itself. Treatment of an asymptomatic meniscal ossicle is nonoperative. Arthroscopic resection of a meniscal ossicle may be considered when conservative measures fail in a symptomatic meniscus [16]. Reattachment of a torn meniscal root, if present, is recommended after removal of an associated ossicle [29]. Associated thinning of the tibial articular cartilage [27] or a groovelike lesion involving the adjacent femoral condyle [6, 29] has been reported at arthroscopy adjacent to a meniscal ossicle by some authors. Such findings are thought to
be secondary to mechanical erosion due to altered contour of the meniscus or by the intrasubstance ossicle itself [29]. Altered biomechanics secondary to meniscal root injury have also been implicated in early cartilage damage within the knee [30]. Isolated posterior medial meniscal root tears have also been implicated in progressive medial tibiofemoral cartilage loss [8]. The presence of a meniscal ossicle should alert the radiologist to assess the meniscus carefully because an associated meniscal tear is highly likely. Detection and management of such meniscal tears in the setting of an observed meniscal ossicle may be critical to the possible prevention of associated osteoarthritis. There are certain limitations to our study that should be acknowledged. This was a retrospective investigation and not based on a prospective cohort. Because of the rarity of the finding in question, a prospective study would not have been feasible. The retrospective study design used relies on the software identification of the term “meniscal ossicle” in verified radiology reports. If any other term were used to describe the finding of a meniscal ossicle, the finding would not be identified in the search result and hence such cases would have been excluded from study evaluation. In addition, this study also relied on the identification of the meniscal ossicle by the radiologist who issued the report. Clinical correlation of the finding of meniscal ossicle was not performed to assess the relevance of the finding in each case. The follow-up of clinical symptoms and possible arthroscopic surgery was also not evaluated by the study. The composition of our study group is also a limitation. In our study population, 57% of patients were over 50 years, and 58% of patients had a torn native ACL or had undergone a prior ACL reconstruction. Cartilage damage is expected to be greater in this group than in the general population; hence, an association between meniscal ossicle and cartilage damage is questionable. Rather, the presence of a meniscal ossicle points to the posttraumatic state of the knee in conjunction with the other findings. In conclusion, a meniscal ossicle is commonly associated with a meniscal tear, with posterior root tears predominating. A posttraumatic cause of a meniscal ossicle is the most convincing explanation for its development. Posttraumatic metaplasia
with heterotopic ossification as well as osseous avulsion of the root attachment are both possible causes leading to development of a meniscal ossicle. Due to the vital importance of the meniscus in the biomechanical stability of the knee, it is important for the radiologist to understand the importance of identifying a meniscal ossicle and the likelihood of antecedent trauma and associated meniscal injury. References 1. Harris HA. Calcification and ossification in the semilunar cartilages. Lancet 1934; 273:1114–1116 2. Watson-Jones R, Roberts RE. Calcification, decalcification, and ossification. Br J Surg 1934; 21:461–498 3. Burrows HJ. Two cases of ossification in the internal semilunar cartilage. Br J Surg 1934; 21:404–410 4. Schnarkowski P, Tirman PF, Fuchigami KD, Crues JV, Butler MG, Genant HK. Meniscal ossicle: radiographic and MR imaging findings. Radiology 1995; 196:47–50 5. Ganey TM, Ogden JA. Abou-Madi N, Colville B, Zdyziarski JM, Olsen JH. Meniscal ossification. II. The normal pattern in the tiger knee. Skeletal Radiol 1994; 23:173–179 6. Yoo JH, Yang BK, Son BK. Meniscal ossicle: a case report. Knee 2007; 14:493–496 7. Berg EE. The meniscal ossicle: the consequence of a meniscal avulsion. Arthroscopy 1991; 7:241–243 8. Guermazi A, Hayashi D, Jarraya M, et al. Medial posterior meniscal root tears are associated with development or worsening of medial tibiofemoral cartilage damage: the Multicenter Osteoarthritis Study. Radiology 2013; 268:814–821 9. De Smet AA. How I diagnose meniscal tears on knee MRI. AJR 2012; 199:481–499 10. Costa CR, Morrison WB, Carrino JA. Medial meniscus extrusion on knee MRI: is extent associated with severity of degeneration or type of tear? AJR 2004; 183:17–23 11. McCauley TR. MR imaging evaluation of the postoperative knee. Radiology 2005; 234:53–61 12. Kornaat PR, Bloem JL, Cuelemans RY, et al. Osteoarthritis of the knee: association between clinical features and MR imaging findings. Radiology 2006; 239:811–817 13. Prabhudesai V, Richards PJ. Radiological appearance as a meniscal ossicle develops: a case report and review of literature. Injury 2003; 34:378–382 14. Weaver JB. Calcification and ossification of the menisci. J Bone Joint Surg Am 1942; 24:873–882 15. Conforty B, Lotem M. Ossicles in human menisci: report of two cases. Clin Orthop Relat Res 1979; 144:272–275 16. Kato Y, Oshida M, Saito A, Ryu J. Meniscal ossicles. J Orthop Sci 2007; 12:375–380 17. Stoller DW, Martin C, Crues JV 3rd, Kaplan L,
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Mohankumar et al. Mink JH. Meniscal tears: pathologic correlation with MR imaging. Radiology 1987; 163:731–735 18. Ogden JA, Ganey TM, Arrington JA, Leffers D. Meniscal ossification. I. Human. Skeletal Radiol 1994; 23:167–172 19. Pedersen HE. The ossicles of the semilunar cartilages of rodents. Anat Rec 1949; 105:1–9 20. Ohishi T, Suzuki D, Yamamoto K, et al. Meniscal pullout repair following meniscal ossicle resection: a case report. Knee 2013; 20:52–57 21. Richmond J, Sarno RC. Posttrauma intracapsular bone fragments: association with meniscal tears. AJR 1988; 150:159–160 22. Raustol OA, Poelstra KA, Chhabra A, Diduch
DR. The meniscal ossicle revisited: etiology and an arthroscopic technique for treatment. Arthroscopy 2006; 22:687.e1–687.e3 23. Richmond JC, Sarno RC. Arthroscopic treatment of medial meniscal avulsion fractures. Arthroscopy 1988; 4:117–120 24. Glass RS, Barnes WM, Kells DU, Thomas S, Campbell C. Ossicles of knee menisci: report of seven cases. Clin Orthop Relat Res 1975; 111:163–171 25. Van Breuseghem I, Geusens E, Pans S, Brys P. The meniscal ossicle revisited. JBR-BTR 2003; 86:276–277 26. Rohilla S, Yadav RK, Singh R, Devgan A, Dhaulakhandi DB. Meniscal ossicle. J Orthop
Traumatol 2009; 10:143–145 27. Tuite MJ, De Smet AA, Swan JS, Keene JS. MR imaging of a meniscal ossicle. Skeletal Radiol 1995; 24:543–545 28. Yao J, Yao L. Magnetic resonance imaging of a symptomatic meniscal ossicle. Clin Orthop Relat Res 1993; 293:225–228 29. Ogassawara R, Zayni R, Orhant E, et al. Meniscal ossicle in a professional soccer player. Orthop Traumatol Surg Res 2011; 97:443–446 30. Allaire R, Muriuki M, Gilbertson L, Harner CD. Biomechanical consequences of a tear of the posterior root of the medial meniscus. J Bone Joint Surg Am 2008; 90:1922–1931
F O R YO U R I N F O R M AT I O N
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Musculoskeletal Imaging Original Research
Meniscal Ossicle: Posttraumatic Origin and Association With Posterior Meniscal Root Tears Rakesh Mohankumar 1,2 Andrew Palisch 3,4 Waseem Khan 3,5 Lawrence M. White2 William B. Morrison 3 Mohankumar R, Palisch A, Khan W, White LM, Morrison WB
Keywords: meniscal ossicle, meniscal root, posterior root tear DOI:10.2214/AJR.13.11821 Received August 29, 2013; accepted after revision November 22, 2013. 1 Department of Musculoskeletal Imaging, Mount Sinai Hospital and University Health Network, Toronto, Toronto, ON, Canada. 2 Present address: Toronto General Hospital and University Health Network, 200 Elizabeth St, Toronto, ON, M5G 2C4 Canada. Address correspondence to R. Mohankumar ([email protected]). 3 Department of Radiology, Thomas Jefferson Hospital, Philadelphia, PA. 4
Present address: Department of Radiology, Baylor College of Medicine, Houston, TX.
5
Present address: Department of Radiology, Ruth Copley Medical Center, Aurora, IL. This article is available for credit. AJR 2014; 203:1040–1046 0361–803X/14/2035–1040 © American Roentgen Ray Society
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OBJECTIVE. The purpose of this study was to characterize meniscal ossicles and their association with meniscal root tears. MATERIALS AND METHODS. A two-center retrospective assessment of 65 knee MRI studies with meniscal ossicles was performed. The relationship of the ossicle to the meniscal root and horn was assessed along with other findings of internal derangement of the knee. RESULTS. Meniscal ossicles were identified in patients ranging from 23 to 80 years old. The ossicles were located within the posterior horn and root of the medial meniscus in 57 of 65 (88%) examinations. Associated meniscal tears were seen at the site or adjacent to the ossicle in all but one examination (98%). Meniscal extrusion was present in 52% (34/65) of examinations. Associated findings included articular cartilage loss, which was severe in 51% (33/65) of examinations. Anterior cruciate ligament injury or prior anterior cruciate ligament reconstruction surgery was also identified in 58% (38/65) of examinations. CONCLUSION. Our findings show that the vast majority of meniscal ossicles are associated with posterior horn or meniscal root tears and a high incidence of focal articular cartilage loss as well as anterior cruciate ligament tears.
T
he presence of a meniscal ossicle is an uncommon but well-documented finding, with ossification within the substance of the meniscus first described in the medical literature in the 1930s [1–3]. The prevalence of meniscal ossicles in the knee has been reported as 0.15% in a case series of 1287 consecutive MRI examinations [4]. There has not been a unified consensus regarding the origin of the meniscal ossicle. The theories proposed as the possible pathogenesis of a meniscal ossicle include congenital, degenerative, and posttraumatic causes. Some authors suggest that the meniscal ossicle is a congenital vestigial structure similar to those seen in certain animal species [5]. Mucoid degeneration of the meniscus and interval mineralization within the meniscus have also been proposed in literature [1]. The more favored theory is that of a posttraumatic origin, either by heterotopic ossification within the meniscus after an antecedent trauma [6] or an avulsion injury of the root attachment of the meniscus off the tibial spine [7]. Many articles on the subject were published before the advent of MRI, and none have related the presence of this finding with the recent-
ly described entity of posterior meniscal root tears. Posterior meniscal root tears are functionally different from tears of the body and posterior horn and can lead to rapid progression of osteoarthritis [8]. This is also important from an orthopedic point of view because treatment of posterior meniscal root tears is different from management of meniscal tears within the body or posterior horn. Meniscal ossicles can also be mistaken for intraarticular bodies, particularly on radiography. The purposes of this study were to identify the meniscal ossicle on MRI and evaluate its association with the state of the meniscus. Other coexistent findings within the knee were also evaluated to assess possible coexistent relationships supportive of the etiologic development of the meniscal ossicle itself. Materials and Methods Patient Selection The institutional review boards of both centers participating in the investigation (Mount Sinai Hospital and University Health Network, Toronto, ON, Canada, and Thomas Jefferson Hospital, Philadelphia, PA) approved the study and waived informed consent. Retrospective electronic screening of the radiology report databases of both institu-
AJR:203, November 2014
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Imaging the Meniscal Ossicle tions was performed for the term “meniscal ossicle” in reports of MRI of the knee between July 2007 and June 2012. The search revealed 70 MRI examinations in 70 different patients with the term “meniscal ossicle” in the report. The MR images were reviewed to confirm the findings of a meniscal ossicle and to evaluate for associated findings within the joint. All MRI examinations were reviewed by consensus assessment by five fellowship-trained musculoskeletal radiologists, with experience ranging from 1 to 18 years. Five cases were excluded from the study because retrospective analysis of the MRI study failed to reveal a meniscal ossicle. No prior imaging studies, radiography or MRI, were available for these five cases. The remaining 65 examinations constituted the study group. Imaging history was examined for the 65 patients to identify prior or follow-up MRI on the same knee as the study examination. In the 65 examinations analyzed, imaging history showed prior MRI of the same knee in four patients and follow-up MRI of the same knee in two patients. The prior MR images in one examination could not be accessed for review because it was performed before image storage in PACS. The MR images for the five remaining examinations were evaluated to assess for possible temporal changes.
fluid signal intensity on a T2-weighted sequence exiting an articular surface [11]. In all cases, the integrity of the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) was assessed as intact, or showing a partialor full-thickness injury. The examinations with prior cruciate ligament reconstructions were also assessed similar to those with native ligaments and recorded as intact or showing a partial- or fullthickness tear. The medial and lateral collateral ligaments were also evaluated. The collateral ligaments were graded as normal, low-grade (edema surrounding the ligament in acute, thickening of the ligament in chronic), intermediate-grade (edema surrounding and within the ligament), or highgrade (disruption of fibers) injuries. Finally, the state of the cartilage along the articular surfaces of the tibiofemoral and patellofemoral joints was also assessed. The cartilage loss was classified as focal or diffuse, and the location was recorded. Cartilage loss was defined as focal if the involved surface diameter of the cartilage loss was less than 10 mm [12]. The severity of cartilage loss was classified as mild (≤ 50% thickness of cartilage loss), moderate (> 50% but less than full thickness), or severe (full-thickness loss with underlying bone marrow changes).
Because the study involved two institutions with different MRI vendors and because of the time span of the study, there were different MRI protocols used for imaging of the knee across the study cohort. However, sagittal T2-weighted sequences with fat saturation, sagittal intermediate-weighted acquisitions without fat saturation,
and axial T2-weighted acquisitions with fat saturation were standard across all examinations. In addition, coronal images obtained with intermediate weighting without fat saturation or coronal T1-weighted nonfat-saturated and T2-weighted fat-saturated coronal acquisitions were components of all studies assessed. The images were all obtained with a slice thickness of 3–4 mm and an FOV between 14 and 18 cm. The presence of a meniscal ossicle was defined as a finding of a well-defined focus of signal intensity matching that of the adjacent bone marrow on all sequences within the substance of the meniscus (Fig. 1). In all cases with a meniscal ossicle, the position of the ossicle within the meniscus (i.e., anterior horn, body, posterior horn, and medial versus lateral) was recorded. A meniscal ossicle was classified to be located at the root of the meniscus if there was no meniscal tissue between the anterior or posterior horn of the meniscus and the root attachment on the tibial eminence. Both medial and lateral menisci were also assessed for tears. On the basis of previously published criteria [9], a meniscal tear was identified if there was increased signal intensity within the meniscus exiting an articular surface on two or more contiguous slices or associated change in morphology of the meniscus. If there was only a single plane of articular surface exit, the tear was classified as simple. A tear was classified as complex if there was more than one plane of exit of the intrameniscal signal intensity. Associated meniscal extrusion, which was defined as a distance of greater than 3 mm between the peripheral margin of the meniscus and the respective tibial plateau at the level of the midbody of the meniscus [10], was also recorded. A meniscal retear was identified when there was intrameniscal high signal intensity equating
A
B
C
Image Analysis
Statistical Analysis Statistical analysis was performed using software (SPSS Statistics, version 21.0, IBM). Descriptive statistics, such as frequencies and descriptives (mean, range), was performed for the variables involved in the study. Frequency analysis was performed on the location of a meniscal tear, in particular the posterior horn and root tears of the meniscus, and its relationship to the
Fig. 1—30-year-old woman with meniscal ossicle. A–C, Sagittal intermediate-weighted (A), sagittal T2-weighted fat-saturated (B), and coronal intermediate-weighted (C) images show meniscal ossicle within posterior root of medial meniscus (arrows).
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Mohankumar et al. TABLE 1: Location of Ossicle Within Meniscus
TABLE 2: Findings Within Meniscus That Showed Meniscal Ossicle
Location
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Meniscus (n = 65)
Anterior Horn
Body
Posterior Horn
Posterior Root
Total
Medial
1 (1.5)
1 (1.5)
13 (20)
43 (66)
58 (89)
Lateral
0
1 (1.5)
3 (4.6)
3 (4.6)
7 (11)
Note—Data in parentheses are percentages.
meniscal ossicle. Correlation and multiple regression analyses were conducted among the variables, particularly the tear location and cartilage damage to examine the relationship with age, sex, meniscal extrusion, and ACL injury.
Results Sixty-five examinations with a meniscal ossicle identified on MRI of the knee were analyzed. The mean age of the patients was 51 years (range, 23–80 years). Thirty-three patients were men (mean age, 47.8 years; range, 23–80 years) and 32 patients were women (mean age, 55.0 years; range, 30–77 years). In no case was more than one meniscal ossicle identified in a knee. The vast majority (58/65, 89%) of ossicles were located within the medial meniscus. In 62 of 65 (95%) examinations, the meniscal ossicle was located at the posterior horn or root of the meniscus, whereas two (2/65, 3%) were present at the body and one (1/65, 2%) at the anterior root. Hence, the posterior horn (13/65, 20%) and root (43/65, 66%) of the medial meniscus were the most prevalent locations of a meniscal ossicle, accounting for 86% (56/65) of examinations, in our study group (Table 1). The meniscus containing an ossicle was abnormal in 98% of examinations (64/65). Meniscal abnormalities included a simple root tear (38/65, 58%), complex root tear (9/65, 14%), simple tear of the posterior horn (8/65, 12%), and complex tear of the posterior horn (5/65, 8%). Four (4/65, 6%) menisci had undergone prior partial meniscectomies, with two of these cases showing recurrent tears. In all cases with a meniscal tear (64/64, 100%), the ossicle was located at the tear site within the meniscus (Table 2). Thirty-four of 65 (34/65, 52%) menisci with ossicles showed meniscal extrusion (> 3 mm) at the level of the midbody, with no meniscal extrusion observed in 12 (12/65, 18%) examinations. The other meniscus within the same knee showed a tear in 30 examinations (30/65; 46%). The ACL was abnormal in 37% (24/65) of examinations. Seven (7/24, 29%) of these cases showed complete tears of the ACL.
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Fourteen (14/65, 21%) examinations showed evidence of prior ACL reconstruction, seven of which showed partial or complete tears of the reconstruction graft. Of the 31 (31/65; 48%) cases with ACL injuries (partial or complete, native or graft), only one case showed MRI features of an acute trauma (subchondral marrow edema characteristic of pivot shift, large joint effusion, or concordant clinical history). No examination showed findings of abnormality of the PCL. Eight (8/65, 12%) examinations showed chronic low-to-intermediate grade injuries of the medial collateral ligament. No injuries to the lateral collateral ligament complex were identified in any of the examinations. Articular cartilage was normal in only seven (7/65, 11%) examinations, with 32 (32/65, 49%) examinations showing severe chondral loss of the weightbearing tibiofemoral articular surface. Diffuse articular cartilage dam-
Finding (n = 65)
No.
Percentage
Normal
1
2
Simple root tear
38
58
Complex root tear
9
14
Posterior horn tear
8
12
Complex horn tear
5
8
Prior meniscectomy
4
6
age found was not just limited to the side of the meniscal ossicle (8/58, 14%) but was found in the other tibiofemoral joint compartment as well as the patellofemoral compartment in most cases (50/58, 86%). Seventyfour percent (48/65) of cases had cartilage damage of at least moderate grade that was focal or diffuse (Table 3). Isolated areas of focal cartilage damage were identified in only 14 examinations (14/65, 22%). Focal areas of chondral loss were identified in the tibiofemoral compartment ipsilateral to the meniscal ossicle in eight cases (8/14, 57%). Three (3/14, 21%) cases had focal cartilage damage involving the contralateral tibiofemoral joint compartment, and the other the three cases (3/14, 21%) had cartilage loss at the patellofemoral
TABLE 3: Image Findings of Other Major Supporting Structures of the Knee (n = 65) Supporting Structure
Finding
No.
Percentage
27
42
ACL Normal Abnormal
24
37
14 (7 with partial or full failure)
21
Normal
65
100
Abnormal
0
0
Normal
57
88
8 (all chronic MCL)
12
Normal
7
11
Focal mild
2
3
Reconstruction graft PCL
MCL Abnormal Articular cartilage
Focal moderate
2
3
Focal severe
10
15
Diffuse mild
8
12
Diffuse moderate
14
22
Diffuse severe
22
34
Note—ACL = anterior cruciate ligament, PCL = posterior cruciate ligament, MCL = medial collateral ligament.
AJR:203, November 2014
Imaging the Meniscal Ossicle TABLE 4: Correlations and Results From Regression Analyses ba
βb
Correlation With Tear Location
Age
0.494c
0.087
0.624d
Sex
−0.159
0.143
0.037
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Multiple Regression Weights
Type of meniscal tear
Correlation With Cartilage Injury
Multiple Regression Weights ba
βb
−0.088
−0.002
−0.038
−0.016
−0.063
−0.051
0.035
0.060
0.150
0.428
0.234
0.053
Location of meniscal tear
−0.044
0.007
0.002
1
Meniscal extrusion
0.369c
0.143
0.178
−0.106
−0.031
−0.120
ACL state
0.087
0.434
0.414d
0.038
−0.007
−0.021
−0.044
0.001
0.004
Cartilage injury
1
Note—ACL = anterior cruciate ligament. aUnstandardized regression coefficient. bStandardized regression coefficient. cp < 0.01. dp < 0.001.
articulation. Ten (10/14, 71%) cases of focal chondral loss were assessed as severe in grading, and two cases each had mild and moderate severity of chondral loss. Correlation and multiple regression analyses were conducted to examine the relationship between the variables. Multiple regression analysis results and correlation for location of meniscal tear and cartilage injury and their relationship to the other variables are summarized in Table 4. Cartilage damage correlated significantly (p < 0.001) with age of the patient and state of the ACL. Correlation (p < 0.01) was also identified between cartilage damage and meniscal extrusion, reiterating the function of the meniscus as a shock absorber to the tibiofemoral joint. The location of the meniscal tear did not correlate significantly with any of the other variables.
A
In one of the three patients with prior MRI available, sequential examination review showed temporal development of a meniscal ossicle. In this case, a 23-year-old man whose initial MRI examination 33 months earlier had shown a complete ACL tear and posterior meniscal root tear, a meniscal ossicle at the posterior root of the medial meniscus adjacent to the root tear was identified on the examination. The patient had also undergone an ACL reconstruction (Fig. 2). In the other two patients with prior MRI, no change in size of the meniscal ossicle was appreciated at 35 months and 45 months, respectively. In one of the two patients with follow-up MRI, there was evidence of increase in size of the initially identified meniscal ossicle on MR images 18 months after the initial examination. In this case, a 31-year-old man, a fullthickness tear of the ACL was also identified,
B
with a reconstruction graft performed in the interval before follow-up imaging (Fig. 3). In the other patient with follow-up MRI, no interval change in size of the meniscal ossicle was appreciated on MRI after 28 months. Discussion Ossicles within the meniscus of the knee were first described in the 1930s by three different authors [1–3]. Since then, multiple case reports and case series have been published in the literature. A retrospective review in 2003 identified 52 publications pertaining to meniscal ossicles in the literature [13]. The largest single case series to date in the orthopedic literature described 10 cases [14], and the largest in the radiology literature described six cases [4]. Multiple ossicles within the same meniscus have also been reported [15]. To our knowledge, this article is
Fig. 2—23-year-old man with temporal development of meniscal ossicle. A, Initial coronal T1-weighted MR image obtained at level of posterior horn of medial meniscus shows no evidence of ossicle at posterior horn or root (arrow). B, Follow-up coronal T1-weighted MR image obtained 33 months later shows meniscal ossicle at posterior root of medial meniscus (arrow). Artifacts from interval anterior cruciate ligament reconstruction are also noted.
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Mohankumar et al. Fig. 3—31-year-old man with temporal increase in size of meniscal ossicle. A, Initial coronal intermediate-weighted MR image shows small ossicle within posterior root of medial meniscus (arrow). B, Follow-up coronal intermediate-weighted MR image through same area obtained 18 months later shows increase in size of ossicle (arrow).
A
B
the largest single case series of meniscal ossicle in the literature. Meniscal ossicles are usually found within the posterior horn of the medial meniscus [16]. This is thought to be secondary to the abundant vascularity of the area, predisposing damaged meniscal tissue to dystrophic ossification in this location. Our study similarly showed meniscal ossicles to be most commonly observed (57/65, 88%) within the posterior horn and root of the medial meniscus. The cause of the meniscal ossicle has been of considerable debate in the literature. Various theories have been proposed regarding the origin of such ossicles. These theories include degenerative (mucoid degeneration), congenital (vestigial structure), or posttraumatic (heterotopic ossification or osseous avulsion of the meniscal root attachment to the tibia) causes. Mucoid degeneration with ossification within the substance of the meniscus has been postulated as a possible cause of development of the meniscal ossicle [1]. However, this theory was proposed before the advent of advanced imaging. MRI studies have subsequently shown that mucoid degeneration of the meniscus is most prevalent at the junction of the body and posterior horn of the medial meniscus [17], where ossicles were not as commonly observed in our study group. Mucoid degeneration has also been described as typically commencing from the third decade of life. Descriptions of meniscal ossicle in patients as young as 12 years [18], which is before development of degenerative change, also contradicts this theory. Meniscal ossicles have been identified as normal vestigial structures in domestic cats,
rodents [19], and tigers [5]. Therefore, the same theory has been applied in humans. Ganey et al. [5] suggested that the meniscal ossicle is akin to a sesamoid bone and develops progressively from the time of birth. Microscopy of the meniscal ossicle by has also shown mature cancellous bone with a cartilage interface and without any fibroblast proliferation and neovascularization [20]. Our findings of temporal development of a meniscal ossicle in a skeletally mature patient and progression in size of a meniscal ossicle in another patient over short durations of time, however, contrast this theory. These temporal observations argue against the theory of a meniscal ossicle as a vestigial structure or a normal anatomic variant. The more popular theory of origin of a meniscal ossicle is that the ossicle is the result of prior single or multiple traumatic episodes. In such cases, a meniscal ossicle is thought to develop after a traumatic episode with posttraumatic metaplasia and secondary heterotopic ossification within the meniscus. It has been proposed that such traumatic episodes result in cartilaginous ossification of the meniscus due to metaplasia of fibrocartilage and irregular neovascularization [6]. This theory would be consistent with observations of the posterior horn of the medial meniscus as the common location for a meniscal ossicle because this area is less mobile and more vulnerable to injury [4]. Our findings certainly support this theory, with a predominance of the typical location of meniscal ossicles seen as well as association of such ossicles with meniscal tears and other injuries to the knee. Temporal development or growth of a meniscal ossicle after
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documented traumatic injury of the knee, as seen in two of our patients, also supports the theory of posttraumatic heterotopic ossification in the pathogenesis of a meniscal ossicle. Both of these patients had full-thickness ruptures of the ACL with interval reconstruction grafting of the ligament. Some authors have suggested a different traumatic mechanism for development of a meniscal ossicle. The posterior horn and root of the medial meniscus are thought to have a substantial tibial insertion, and a bony avulsion from these insertion sites has been proposed as an alternative cause for a meniscal ossicle [7]. Similar posttraumatic intraarticular osseous avulsions appear sharp and noncorticated immediately after injury and undergo smoothening and cortication over time [21]. Our study showed tears involving the posterior horn and root of the meniscus in 95% (62/65) of examinations, of which 71% (46/65) involved the posterior root attachment. Other authors have hypothesized that such injuries may result in small avulsed bony fragments acting as a nidus for the formation and progression of a meniscus ossicle [4]. The orthopedic literature also describes surgical repair techniques of reimplantation of avulsed bony fragments to the donor site of the tibia in the setting of posterior root tearing, supporting the possibility of a meniscal ossicle developing as a result of meniscal root avulsion fracture [22]. Richmond and Sarno [23] showed arthroscopic evidence of an osseous avulsive donor site at the tibia in the setting of avulsed meniscal root tears. They found that in acute cases the donor site was bleeding, whereas in chronic cases,
AJR:203, November 2014
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Imaging the Meniscal Ossicle the donor site could still be visualized but was filled with fibrous tissue. They also performed histologic analyses of the observed meniscal avulsive ossicles, which showed meniscus attached to mature cancellous bone through fibrocartilage and calcified fibrocartilage. No evidence of metaplasia or degeneration was identified, supporting the theory of avulsion fracture as the cause of ossicle formation. In contrast, findings in our study showed that in many cases the observed meniscal ossicle was not at the root attachment at all, arguing against this as the sole cause of meniscal ossicle formation. On the basis of our findings, the posttraumatic cause of a meniscal ossicle secondary to metaplasia and heterotopic ossification is favored. This is especially true in cases in which a meniscal ossicle is present within the substance of the posterior horn of the meniscus away from the meniscal root. However, in cases in which an ossicle is located at the site of the meniscal root attachment, the theory of bony avulsion remains feasible. A meniscal ossicle can be a symptomatic and may be found incidentally during imaging [24, 25]. However, most patients with meniscal ossicles experience episodes of intermittent pain. Mechanical locking is usually not a typical feature of a meniscal ossicle, unlike an intraarticular loose body [25]; however, this symptom can be seen in some patients [4, 26]. Asymptomatic meniscal ossicles can spontaneously become symptomatic. This is hypothesized to occur due to continuing increase in size of the ossicle irritating adjacent structures [27] or alteration in contour of the meniscus secondary to the ossicle leading to altered stress [28]. It is also possible that such symptoms may temporally arise from associated internal derangement (i.e., meniscal tear, chondral injury) rather than from the ossicle itself. Treatment of an asymptomatic meniscal ossicle is nonoperative. Arthroscopic resection of a meniscal ossicle may be considered when conservative measures fail in a symptomatic meniscus [16]. Reattachment of a torn meniscal root, if present, is recommended after removal of an associated ossicle [29]. Associated thinning of the tibial articular cartilage [27] or a groovelike lesion involving the adjacent femoral condyle [6, 29] has been reported at arthroscopy adjacent to a meniscal ossicle by some authors. Such findings are thought to
be secondary to mechanical erosion due to altered contour of the meniscus or by the intrasubstance ossicle itself [29]. Altered biomechanics secondary to meniscal root injury have also been implicated in early cartilage damage within the knee [30]. Isolated posterior medial meniscal root tears have also been implicated in progressive medial tibiofemoral cartilage loss [8]. The presence of a meniscal ossicle should alert the radiologist to assess the meniscus carefully because an associated meniscal tear is highly likely. Detection and management of such meniscal tears in the setting of an observed meniscal ossicle may be critical to the possible prevention of associated osteoarthritis. There are certain limitations to our study that should be acknowledged. This was a retrospective investigation and not based on a prospective cohort. Because of the rarity of the finding in question, a prospective study would not have been feasible. The retrospective study design used relies on the software identification of the term “meniscal ossicle” in verified radiology reports. If any other term were used to describe the finding of a meniscal ossicle, the finding would not be identified in the search result and hence such cases would have been excluded from study evaluation. In addition, this study also relied on the identification of the meniscal ossicle by the radiologist who issued the report. Clinical correlation of the finding of meniscal ossicle was not performed to assess the relevance of the finding in each case. The follow-up of clinical symptoms and possible arthroscopic surgery was also not evaluated by the study. The composition of our study group is also a limitation. In our study population, 57% of patients were over 50 years, and 58% of patients had a torn native ACL or had undergone a prior ACL reconstruction. Cartilage damage is expected to be greater in this group than in the general population; hence, an association between meniscal ossicle and cartilage damage is questionable. Rather, the presence of a meniscal ossicle points to the posttraumatic state of the knee in conjunction with the other findings. In conclusion, a meniscal ossicle is commonly associated with a meniscal tear, with posterior root tears predominating. A posttraumatic cause of a meniscal ossicle is the most convincing explanation for its development. Posttraumatic metaplasia
with heterotopic ossification as well as osseous avulsion of the root attachment are both possible causes leading to development of a meniscal ossicle. Due to the vital importance of the meniscus in the biomechanical stability of the knee, it is important for the radiologist to understand the importance of identifying a meniscal ossicle and the likelihood of antecedent trauma and associated meniscal injury. References 1. Harris HA. Calcification and ossification in the semilunar cartilages. Lancet 1934; 273:1114–1116 2. Watson-Jones R, Roberts RE. Calcification, decalcification, and ossification. Br J Surg 1934; 21:461–498 3. Burrows HJ. Two cases of ossification in the internal semilunar cartilage. Br J Surg 1934; 21:404–410 4. Schnarkowski P, Tirman PF, Fuchigami KD, Crues JV, Butler MG, Genant HK. Meniscal ossicle: radiographic and MR imaging findings. Radiology 1995; 196:47–50 5. Ganey TM, Ogden JA. Abou-Madi N, Colville B, Zdyziarski JM, Olsen JH. Meniscal ossification. II. The normal pattern in the tiger knee. Skeletal Radiol 1994; 23:173–179 6. Yoo JH, Yang BK, Son BK. Meniscal ossicle: a case report. Knee 2007; 14:493–496 7. Berg EE. The meniscal ossicle: the consequence of a meniscal avulsion. Arthroscopy 1991; 7:241–243 8. Guermazi A, Hayashi D, Jarraya M, et al. Medial posterior meniscal root tears are associated with development or worsening of medial tibiofemoral cartilage damage: the Multicenter Osteoarthritis Study. Radiology 2013; 268:814–821 9. De Smet AA. How I diagnose meniscal tears on knee MRI. AJR 2012; 199:481–499 10. Costa CR, Morrison WB, Carrino JA. Medial meniscus extrusion on knee MRI: is extent associated with severity of degeneration or type of tear? AJR 2004; 183:17–23 11. McCauley TR. MR imaging evaluation of the postoperative knee. Radiology 2005; 234:53–61 12. Kornaat PR, Bloem JL, Cuelemans RY, et al. Osteoarthritis of the knee: association between clinical features and MR imaging findings. Radiology 2006; 239:811–817 13. Prabhudesai V, Richards PJ. Radiological appearance as a meniscal ossicle develops: a case report and review of literature. Injury 2003; 34:378–382 14. Weaver JB. Calcification and ossification of the menisci. J Bone Joint Surg Am 1942; 24:873–882 15. Conforty B, Lotem M. Ossicles in human menisci: report of two cases. Clin Orthop Relat Res 1979; 144:272–275 16. Kato Y, Oshida M, Saito A, Ryu J. Meniscal ossicles. J Orthop Sci 2007; 12:375–380 17. Stoller DW, Martin C, Crues JV 3rd, Kaplan L,
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DR. The meniscal ossicle revisited: etiology and an arthroscopic technique for treatment. Arthroscopy 2006; 22:687.e1–687.e3 23. Richmond JC, Sarno RC. Arthroscopic treatment of medial meniscal avulsion fractures. Arthroscopy 1988; 4:117–120 24. Glass RS, Barnes WM, Kells DU, Thomas S, Campbell C. Ossicles of knee menisci: report of seven cases. Clin Orthop Relat Res 1975; 111:163–171 25. Van Breuseghem I, Geusens E, Pans S, Brys P. The meniscal ossicle revisited. JBR-BTR 2003; 86:276–277 26. Rohilla S, Yadav RK, Singh R, Devgan A, Dhaulakhandi DB. Meniscal ossicle. J Orthop
Traumatol 2009; 10:143–145 27. Tuite MJ, De Smet AA, Swan JS, Keene JS. MR imaging of a meniscal ossicle. Skeletal Radiol 1995; 24:543–545 28. Yao J, Yao L. Magnetic resonance imaging of a symptomatic meniscal ossicle. Clin Orthop Relat Res 1993; 293:225–228 29. Ogassawara R, Zayni R, Orhant E, et al. Meniscal ossicle in a professional soccer player. Orthop Traumatol Surg Res 2011; 97:443–446 30. Allaire R, Muriuki M, Gilbertson L, Harner CD. Biomechanical consequences of a tear of the posterior root of the medial meniscus. J Bone Joint Surg Am 2008; 90:1922–1931
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