Skeletal Radiol (2014) 43:219–225 DOI 10.1007/s00256-013-1776-3
SCIENTIFIC ARTICLE
Patellar calcar: MRI appearance of a previously undescribed anatomical entity Mark S. Collins & Christin A. Tiegs-Heiden & Michael J. Stuart
Received: 6 September 2013 / Revised: 29 October 2013 / Accepted: 31 October 2013 / Published online: 28 November 2013 # ISS 2013
Abstract Objective The femoral calcar is a constant anatomical structure within the proximal femur representing a condensation of bone trabeculae. It is our impression that a similar structure is present within the patella. The purpose of this retrospective study was to define the prevalence, appearance, location, and configuration of the patellar calcar on MRI examinations. Materials and Methods One hundred consecutive unenhanced knee MRIs were retrospectively reviewed by two readers who were blinded to the clinical indication. The patellar calcar was defined as a dark signaling, linear or curvilinear structure subjacent to the patellar articular surface. If present, the patellar calcar was assigned to a “well seen,” “moderately well seen,” or “faintly seen” category. Location of the calcar within the patella, orientation, configuration, and thickness were recorded. Confounding variables, such as marrow edema, patellar chondromalacia, bipartite patella, or postoperative changes were also recorded. Results The patellar calcar was visualized in 81 out of 100 (81 %) MRIs. When detected, the calcar was well seen in 20 out of 81 (25 %), moderately well seen in 35 out of 81 (43 %), and faintly seen in 26 out of 81 (32 %). The anteroposterior width of the calcar measured at its thickest segment was: < 1 mm in 43 out of 81 (53 %), 1 mm in 28 out of 81 (35 %), and >1 mm in 10 out of 81 (12 %). Conclusion The patellar calcar was seen in the majority of knee MRIs and had a consistent imaging appearance. The calcar may be obscured by degenerative arthrosis of the patella and rarely may mimic patellar stress fracture or osteochondritis M. S. Collins (*) : C. A. Tiegs-Heiden Department of Radiology, Mayo Clinic, Rochester, Minnesota, USA e-mail: [email protected] M. J. Stuart Department of Orthopedic Surgery, Mayo Clinic, Rochester, Minnesota, USA
dissecans. Radiologists and clinicians should be familiar with this normal anatomical structure. Keywords Patella . Calcar . Patellar calcar
Introduction The femoral calcar is a constant anatomical structure within the proximal femur representing a condensation of bone trabeculae that is recognized by orthopedic surgeons and radiologists [1, 2]. The femoral calcar was first described by the German anatomist Merkel in 1874 [1–4]. This calcar is part of the compressive trabecular system and is important for stress distribution of the hip and proximal femur [1, 5]. The femoral calcar is routinely identified on radiographs and advanced imaging (Fig. 1) [1, 4]. It is our impression that a similar structure is present within the patella. To our knowledge, this observation has not been previously reported in the imaging or orthopedic surgical literature. We refer to this structure as the “patellar calcar.” In rare circumstances, the presence of bone marrow edema surrounding the patellar calcar may mimic a patellar stress fracture or patellar osteochondral defect when observed on T2-weighted MRI images. The purpose of this retrospective study was to define the prevalence, appearance, location, and configuration of the patellar calcar on MRI examinations performed for other purposes.
Materials and methods Patients Institutional Review Board approval was obtained for this Health Insurance Portability and Accountability Act-
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Skeletal Radiol (2014) 43:219–225
Fig. 1 Axial a T1-weighted MRI and b unenhanced CT of the proximal left femur demonstrating the typical appearance of the femoral calcar (white arrow)
compliant retrospective research protocol, and the requirement for informed patient consent was waived. We retrospectively reviewed the MRIs of 100 consecutive patients who underwent and completed an unenhanced knee MRI. Patients with a history of total knee arthroplasty with patellar resurfacing were excluded (n =2). There were no other exclusion criteria. All examinations were subjectively deemed to be of suitable diagnostic image quality. Examinations were performed between February 2013 and March 2013. The study group consisted of 58 female and 42 male patients who ranged in age from 14 to 82 years (mean age, 42 years). Clinical indications for these examinations varied from suspected ligament, meniscal, or articular cartilage abnormalities to a more comprehensive assessment of vague knee pain. MRI protocol All MRI examinations were performed with a high field strength 3-T magnet (3 T signa HDX 16.0, GE Healthcare, Waukesha, WI, USA) utilizing a dedicated eight-channel transmit–receive phased array knee coil. The standard knee MRI protocol utilized at our institution includes the following imaging positions and parameters: sagittal proton densityweighted images (3 mm, TR/TE 2,200/18); sagittal, coronal, and axial fat-suppressed intermediate-weighted sequences (3 mm, TR/TE 4,000–6,000/45); and coronal T1-weighted images (3 mm, TR/TE 700–900/minimum). An axial T1weighted sequence utilizing the same parameters is included as an additional sequence for selected patients. The field of view was 160 mm, and the matrix was 384x256 for all studies. MRI analysis For the purposes of our study, the patellar calcar was arbitrarily defined as the presence of a dark signaling, linear or
curvilinear structure subjacent to the patellar articular surface. All MRI images were reviewed in consensus by a boardcertified, fellowship-trained musculoskeletal radiologist with 15 years of experience and a PGY-4 radiology resident, who were blinded to the clinical indication for the examination. The images were reviewed initially to determine if a calcar was or was not present. If present, the appearance of the calcar was subjectively assigned to a “well seen,” “moderately well seen,” or “faintly seen” category. Location of the calcar within the patella, orientation relative to the anterior cortical surface of the body of the patella, general configuration, and thickness in millimeters was recorded. The presence or absence of marrow edema surrounding the calcar was also assessed. The morphology of the patella was designated using the Wiberg criteria [6]. Any confounding variables, such as advanced osteoarthritis with sclerosis and edema, a bipartite patella, or postoperative changes, were recorded. Patellar chondromalacia was graded using the Noyes scale [7]. According to this scale, 0=no fibrillation or defect; 1=fibrillation with no defect; 2=partial thickness defect 50 % but less than full; 4=full thickness defect. Each examination was graded both specifically at the lateral facet and overall. During image review, it was noted that 3 patients had also undergone CT of the same knee imaged by MRI. Presence or absence of the patellar calcar was evaluated on these examinations. History of patellofemoral (PF) pain or trauma in the imaged knee was determined by chart review.
Results The patellar calcar was visualized in 81 out of 100 (81 %) of the MRI cases reviewed. When detected, the calcar was well seen in 20 out of 81 (25 %) cases, moderately well seen in 35 out of 81 (43 %) cases and faintly seen in 26 out of 81 (32 %)
Skeletal Radiol (2014) 43:219–225
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Table 1 Presence and visibility of the patellar calcar in all patients and various populations All patients (%) Calcar present Well seen Moderately well seen Faintly seen
PF pain present (%)
PF pain absent (%)
Grades 0–1 (%)a
Grades 2–3 (%)a
Grade 4 (%)a
Age ≤30 (%)
Age >30 (%)
81/100 (81) 20/81 (25) 35/81 (43)
25/32 (78) 10/25 (40) 8/25 (32)
56/68 (82) 10/56 (18) 27/56 (48)
50/55 (91) 18/50 (36) 20/50 (40)
25/31 (81) 1/25 (4) 12/25 (48)
6/14 (43) 1/6 (17) 3/6 (50)
29/33 (88) 15/29 (52) 9/29 (31)
52/67 (78) 5/52 (10) 25/52 (48)
26/81 (32)
7/25 (28)
19/56 (34)
12/50 (24)
12/25 (48)
2/6 (33)
5/29 (17)
22/52 (42)
PF patellofemoral a
Grading of patellar chondromalacia at the lateral facet, where the calcar was visualized in all cases
cases (Table 1). The calcar was located in the lateral patella a few millimeters subjacent to the articular surface in all cases (81 out of 81, 100 %). In the oblique–sagittal plane, the calcar was curvilinear or semilunar in configuration with the convexity of the arc directed anteriorly. In the axial plane, the calcar was more linear in shape. In both the oblique–sagittal and axial planes, the calcar was oriented in an oblique–coronal plane relative to the anterior cortical surface of the patellar body. The calcar was best seen in the sagittal plane on the PD-
Fig. 2 a–c Three consecutive sagittal proton density (PD)weighted images demonstrating the characteristic appearance of the patellar calcar (white arrow). The calcar was always located in the lateral patellar facet, with a curvilinear configuration subjacent to the overlying articular surface
weighted images and was usually present on 2–3 contiguous slices (Fig. 2). The calcar was less well seen on the sagittal fat-suppressed T2-weighted images and only faintly seen on the axial fatsuppressed T2-weighted images. It was not visible on the coronal MRI sequences owing to its orientation. In two cases an additional non-fat-suppressed T1-weighted axial sequence was also obtained. In both cases, the calcar was easily detected on this sequence (Fig. 3).
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Skeletal Radiol (2014) 43:219–225
Fig. 3 a, b Two sequential T1weighted axial images of the patellar calcar in a 14-year-old male patient with vague knee pain. The calcar (white arrow) is easily identified in its typical location subjacent to the lateral patellar articular surface
Three patients also had a corresponding CT examination that correlated with the axial MRI sequence. In the 2 patients in whom the calcar was seen on MRI, the calcar was clearly seen as a sclerotic line on the CT image (Fig. 4). If the patellar calcar was not seen on MRI, it was faintly seen on CT. When measured on the sagittal PD images, the anteroposterior width of the calcar measured at its thickest segment ranged from
SCIENTIFIC ARTICLE
Patellar calcar: MRI appearance of a previously undescribed anatomical entity Mark S. Collins & Christin A. Tiegs-Heiden & Michael J. Stuart
Received: 6 September 2013 / Revised: 29 October 2013 / Accepted: 31 October 2013 / Published online: 28 November 2013 # ISS 2013
Abstract Objective The femoral calcar is a constant anatomical structure within the proximal femur representing a condensation of bone trabeculae. It is our impression that a similar structure is present within the patella. The purpose of this retrospective study was to define the prevalence, appearance, location, and configuration of the patellar calcar on MRI examinations. Materials and Methods One hundred consecutive unenhanced knee MRIs were retrospectively reviewed by two readers who were blinded to the clinical indication. The patellar calcar was defined as a dark signaling, linear or curvilinear structure subjacent to the patellar articular surface. If present, the patellar calcar was assigned to a “well seen,” “moderately well seen,” or “faintly seen” category. Location of the calcar within the patella, orientation, configuration, and thickness were recorded. Confounding variables, such as marrow edema, patellar chondromalacia, bipartite patella, or postoperative changes were also recorded. Results The patellar calcar was visualized in 81 out of 100 (81 %) MRIs. When detected, the calcar was well seen in 20 out of 81 (25 %), moderately well seen in 35 out of 81 (43 %), and faintly seen in 26 out of 81 (32 %). The anteroposterior width of the calcar measured at its thickest segment was: < 1 mm in 43 out of 81 (53 %), 1 mm in 28 out of 81 (35 %), and >1 mm in 10 out of 81 (12 %). Conclusion The patellar calcar was seen in the majority of knee MRIs and had a consistent imaging appearance. The calcar may be obscured by degenerative arthrosis of the patella and rarely may mimic patellar stress fracture or osteochondritis M. S. Collins (*) : C. A. Tiegs-Heiden Department of Radiology, Mayo Clinic, Rochester, Minnesota, USA e-mail: [email protected] M. J. Stuart Department of Orthopedic Surgery, Mayo Clinic, Rochester, Minnesota, USA
dissecans. Radiologists and clinicians should be familiar with this normal anatomical structure. Keywords Patella . Calcar . Patellar calcar
Introduction The femoral calcar is a constant anatomical structure within the proximal femur representing a condensation of bone trabeculae that is recognized by orthopedic surgeons and radiologists [1, 2]. The femoral calcar was first described by the German anatomist Merkel in 1874 [1–4]. This calcar is part of the compressive trabecular system and is important for stress distribution of the hip and proximal femur [1, 5]. The femoral calcar is routinely identified on radiographs and advanced imaging (Fig. 1) [1, 4]. It is our impression that a similar structure is present within the patella. To our knowledge, this observation has not been previously reported in the imaging or orthopedic surgical literature. We refer to this structure as the “patellar calcar.” In rare circumstances, the presence of bone marrow edema surrounding the patellar calcar may mimic a patellar stress fracture or patellar osteochondral defect when observed on T2-weighted MRI images. The purpose of this retrospective study was to define the prevalence, appearance, location, and configuration of the patellar calcar on MRI examinations performed for other purposes.
Materials and methods Patients Institutional Review Board approval was obtained for this Health Insurance Portability and Accountability Act-
220
Skeletal Radiol (2014) 43:219–225
Fig. 1 Axial a T1-weighted MRI and b unenhanced CT of the proximal left femur demonstrating the typical appearance of the femoral calcar (white arrow)
compliant retrospective research protocol, and the requirement for informed patient consent was waived. We retrospectively reviewed the MRIs of 100 consecutive patients who underwent and completed an unenhanced knee MRI. Patients with a history of total knee arthroplasty with patellar resurfacing were excluded (n =2). There were no other exclusion criteria. All examinations were subjectively deemed to be of suitable diagnostic image quality. Examinations were performed between February 2013 and March 2013. The study group consisted of 58 female and 42 male patients who ranged in age from 14 to 82 years (mean age, 42 years). Clinical indications for these examinations varied from suspected ligament, meniscal, or articular cartilage abnormalities to a more comprehensive assessment of vague knee pain. MRI protocol All MRI examinations were performed with a high field strength 3-T magnet (3 T signa HDX 16.0, GE Healthcare, Waukesha, WI, USA) utilizing a dedicated eight-channel transmit–receive phased array knee coil. The standard knee MRI protocol utilized at our institution includes the following imaging positions and parameters: sagittal proton densityweighted images (3 mm, TR/TE 2,200/18); sagittal, coronal, and axial fat-suppressed intermediate-weighted sequences (3 mm, TR/TE 4,000–6,000/45); and coronal T1-weighted images (3 mm, TR/TE 700–900/minimum). An axial T1weighted sequence utilizing the same parameters is included as an additional sequence for selected patients. The field of view was 160 mm, and the matrix was 384x256 for all studies. MRI analysis For the purposes of our study, the patellar calcar was arbitrarily defined as the presence of a dark signaling, linear or
curvilinear structure subjacent to the patellar articular surface. All MRI images were reviewed in consensus by a boardcertified, fellowship-trained musculoskeletal radiologist with 15 years of experience and a PGY-4 radiology resident, who were blinded to the clinical indication for the examination. The images were reviewed initially to determine if a calcar was or was not present. If present, the appearance of the calcar was subjectively assigned to a “well seen,” “moderately well seen,” or “faintly seen” category. Location of the calcar within the patella, orientation relative to the anterior cortical surface of the body of the patella, general configuration, and thickness in millimeters was recorded. The presence or absence of marrow edema surrounding the calcar was also assessed. The morphology of the patella was designated using the Wiberg criteria [6]. Any confounding variables, such as advanced osteoarthritis with sclerosis and edema, a bipartite patella, or postoperative changes, were recorded. Patellar chondromalacia was graded using the Noyes scale [7]. According to this scale, 0=no fibrillation or defect; 1=fibrillation with no defect; 2=partial thickness defect 50 % but less than full; 4=full thickness defect. Each examination was graded both specifically at the lateral facet and overall. During image review, it was noted that 3 patients had also undergone CT of the same knee imaged by MRI. Presence or absence of the patellar calcar was evaluated on these examinations. History of patellofemoral (PF) pain or trauma in the imaged knee was determined by chart review.
Results The patellar calcar was visualized in 81 out of 100 (81 %) of the MRI cases reviewed. When detected, the calcar was well seen in 20 out of 81 (25 %) cases, moderately well seen in 35 out of 81 (43 %) cases and faintly seen in 26 out of 81 (32 %)
Skeletal Radiol (2014) 43:219–225
221
Table 1 Presence and visibility of the patellar calcar in all patients and various populations All patients (%) Calcar present Well seen Moderately well seen Faintly seen
PF pain present (%)
PF pain absent (%)
Grades 0–1 (%)a
Grades 2–3 (%)a
Grade 4 (%)a
Age ≤30 (%)
Age >30 (%)
81/100 (81) 20/81 (25) 35/81 (43)
25/32 (78) 10/25 (40) 8/25 (32)
56/68 (82) 10/56 (18) 27/56 (48)
50/55 (91) 18/50 (36) 20/50 (40)
25/31 (81) 1/25 (4) 12/25 (48)
6/14 (43) 1/6 (17) 3/6 (50)
29/33 (88) 15/29 (52) 9/29 (31)
52/67 (78) 5/52 (10) 25/52 (48)
26/81 (32)
7/25 (28)
19/56 (34)
12/50 (24)
12/25 (48)
2/6 (33)
5/29 (17)
22/52 (42)
PF patellofemoral a
Grading of patellar chondromalacia at the lateral facet, where the calcar was visualized in all cases
cases (Table 1). The calcar was located in the lateral patella a few millimeters subjacent to the articular surface in all cases (81 out of 81, 100 %). In the oblique–sagittal plane, the calcar was curvilinear or semilunar in configuration with the convexity of the arc directed anteriorly. In the axial plane, the calcar was more linear in shape. In both the oblique–sagittal and axial planes, the calcar was oriented in an oblique–coronal plane relative to the anterior cortical surface of the patellar body. The calcar was best seen in the sagittal plane on the PD-
Fig. 2 a–c Three consecutive sagittal proton density (PD)weighted images demonstrating the characteristic appearance of the patellar calcar (white arrow). The calcar was always located in the lateral patellar facet, with a curvilinear configuration subjacent to the overlying articular surface
weighted images and was usually present on 2–3 contiguous slices (Fig. 2). The calcar was less well seen on the sagittal fat-suppressed T2-weighted images and only faintly seen on the axial fatsuppressed T2-weighted images. It was not visible on the coronal MRI sequences owing to its orientation. In two cases an additional non-fat-suppressed T1-weighted axial sequence was also obtained. In both cases, the calcar was easily detected on this sequence (Fig. 3).
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Fig. 3 a, b Two sequential T1weighted axial images of the patellar calcar in a 14-year-old male patient with vague knee pain. The calcar (white arrow) is easily identified in its typical location subjacent to the lateral patellar articular surface
Three patients also had a corresponding CT examination that correlated with the axial MRI sequence. In the 2 patients in whom the calcar was seen on MRI, the calcar was clearly seen as a sclerotic line on the CT image (Fig. 4). If the patellar calcar was not seen on MRI, it was faintly seen on CT. When measured on the sagittal PD images, the anteroposterior width of the calcar measured at its thickest segment ranged from
