Published online before print July 14, 2005, 10.1148/radiol.2363041003
(Radiology 2005;236:968-973.)
© RSNA, 2005
Posterior Cruciate Ligament Recess and Normal Posterior Capsular Insertional Anatomy: MR Imaging of Cadaveric Knees1
Marcelo R. de Abreu, MD,
Hyun J. Kim, MD,
Christine B. Chung, MD,
Jose Miguel Jesus, MD,
Jae Cho, MD,
Debra Trudell, RA and
Donald Resnick, MD
1 From the University of California San Diego, VA Health Care System, San Diego, Calif (M.R.d.A., H.J.K., J.C., D.T., C.B.C., D.R.); and Hospital Mãe de Deus, Rúa Pedro Chaves Barcelos 157, Apt 302, Porto Alegre–RS, Brazil 90450-010 (M.R.d.A., J.M.J.). From the 2003 RSNA Annual Meeting. Received July 13, 2004; revision requested September 16; revision received November 9; accepted November 12.
Address correspondence to M.R.d.A. (e-mail: marcelorad{at}hotmail.com).
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ABSTRACT
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PURPOSE: To analyze the normal pattern of fluid accumulation adjacent to the posterior cruciate ligament and anatomic variations of joint capsule insertion sites in the posterosuperior corner of the human knee by using magnetic resonance (MR) imaging in cadaveric specimens.
MATERIALS AND METHODS: Fourteen fresh cadaveric knees (obtained and used according to institutional guidelines, with informed consent from relatives of the deceased) from 11 men and three women (six left knees, eight right knees; age range, 70–82 years at time of death; mean age, 76 years ± 4.4 [standard deviation]) were studied with high-spatial-resolution MR imaging performed before and after intraarticular injection of 35–45 mL gadopentetate dimeglumine. MR images were evaluated by two readers in consensus, with emphasis on location of fluid posterior to the posterior cruciate ligament, communication of that fluid with the medial or lateral compartment of the knee, and the relation of fluid to surrounding structures. Readers also were asked to measure, in the sagittal plane, the distance between the posterior capsular insertion sites and the femoral physeal scar. For anatomic analysis, cadaveric specimens were sectioned in 3-mm-thick slices in the sagittal plane that approximated the sections acquired at MR imaging.
RESULTS: In all 14 cadaveric specimens, MR arthrographic images showed a fluid collection behind the posterior cruciate ligament (in the posterior cruciate ligament recess), a finding not evident on images obtained prior to contrast material injection. The recess was distended during flexion, and it communicated only with the medial femorotibial compartment in all cases. Posterior to the posterior cruciate ligament recess, a fat pad was observed in all specimens. Incomplete joint capsule was seen behind the fat pad in seven specimens. Joint capsule insertion was at the level of the femoral physeal scar or between it and a point 15 mm above it.
CONCLUSION: The posterior cruciate ligament recess has specific characteristics that allow its identification: communication with the medial compartment of the knee and absence of the adjacent joint capsule.
© RSNA, 2005
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INTRODUCTION
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The fibrous capsule of the knee is complex, is an incomplete compartment, and is partly augmented by extensions of adjacent tendons. Its posterior vertical fibers are attached proximally to the posterior margins of the femoral condyles and intercondylar fossae and distally to the posterior margins of the tibial condyles and intercondylar areas (1). The many different tendinous and ligamentous structures that are attached to the joint capsule provide capsular reinforcement. Proximally, medially, and laterally, the joint capsule is attached to the tendinous heads of the gastrocnemius muscle. In the posteromedial corner, the joint capsule also receives fibers from the tendon of the semimembranosus muscle and posterior oblique ligament. In its posterolateral corner, the joint capsule is reinforced by fibers of the arcuate ligament and iliotibial tract. Posteriorly and centrally, the oblique popliteal ligament strengthens the joint capsule (2).
The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are very strong and are situated slightly posterior to the articular center. The PCL is stronger and has a less oblique course than the ACL and is attached to the posterior intercondylar area of the femur and tibia. It ascends from the tibia anteromedially and broadens toward the point of its attachment on the lateral surface of the medial femoral condyle. The synovial membrane surrounds the anterior, medial, and lateral portions of the cruciate ligaments but is reflected posteriorly from the PCL to adjoining parts of the joint capsule. Thus, the intercondylar part of the posterior region of the fibrous capsule has no synovial covering. According to classic anatomic literature (3), a synovial recess, or bursa, intrudes between the lateral aspect of the ACL and PCL and may extend to the medial wall of the femoral intercondylar fossa (Fig 1).
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Figure 1. Computer-aided drawing shows posterior view of right knee, communication of PCL recess (R) with medial femorotibial compartment posterior to PCL, and ligament of Wrisberg. LM = lateral meniscus, MM = medial meniscus.
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Localized accumulations of articular fluid in the posterosuperior aspect of the knee, behind the PCL, are frequently observed at magnetic resonance (MR) imaging of the knee (3). Ganglionic cysts are also found in this location in both symptomatic and asymptomatic persons (4). To distinguish between fluid in a normal PCL recess and a ganglionic cyst behind the PCL on MR images is clearly a challenge. Thus, the purpose of our study was to analyze the normal pattern of fluid accumulations adjacent to the PCL, as well as anatomic variations of joint capsule insertion sites in the posterosuperior corner of the knee, by using MR imaging in cadaveric specimens.
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MATERIALS AND METHODS
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Specimens
Fourteen fresh cadaveric knees from 11 men and three women (six left knees, eight right knees; age range, 70–82 years at the time of death; mean age, 76 years ± 4.4 [standard deviation]) were obtained by the Department of Anatomy at the University of California–San Diego Medical School for use in MR imaging and anatomic study. Radiography was performed with frontal and lateral projections in each specimen to ensure that the knee joint was not affected by surgical alterations or pathologic abnormalities, and radiographs were evaluated in consensus by two authors (D.R. and C.B.C., with 30 and 10 years of experience, respectively, in interpreting musculoskeletal radiographs). The cadaveric specimens were immediately frozen at –60°C (Bio-Freezer; Forma Scientific, Marietta, Ohio). They were allowed to thaw for 17 hours at room temperature prior to MR imaging. Subsequently, the cadaveric specimens were prepared according to methods described in the literature (5). The knee specimens were obtained and used according to institutional guidelines, and informed consent for research was obtained from relatives of the deceased.
MR Imaging
MR images were acquired with a 1.5-T superconducting magnet (Signa; GE Medical Systems, Milwaukee, Wis) and two 5-inch (12.7-cm) standard flexible surface coils (Flex Coil; Medical Advances, Milwaukee, Wis). All cadaveric knees were imaged first in a neutral supine position. Imaging was performed in the coronal, transverse, and sagittal planes. The MR imaging protocol consisted of T1-weighted spin-echo (SE) sequences (repetition time msec/echo time msec, 600/20–23). To acquire high-spatial-resolution images, a section thickness of 2.5 mm, intersection gap of 0.5 mm, field of view of 12 cm, and data acquisition matrix of 512 x 256 pixels were used. T1-weighted SE MR images with and without fat suppression in sagittal, coronal, and transverse planes were acquired before and after the intraarticular administration of a gadolinium-containing contrast material (gadopentetate dimeglumine, Magnevist; Schering, Berlin, Germany). To distend the joint and its recesses fully, a large volume of fluid (2 mL of gadopentetate dimeglumine and 33–43 mL of saline solution) was injected into the joint in the antegrade direction by two authors (M.R.d.A. and H.J.K., with 4 and 8 years of experience, respectively, with joint injection techniques) using manual pressure. MR imaging began within 2 minutes after the injection. Four cadaveric specimens were randomly selected for an additional MR imaging study performed in supine position, with knee flexion at 30° and with the knee fully extended. A special wooden device was used to maintain knee flexion at 30°. Sagittal T1-weighted images were then acquired by using the parameters described.
Image Interpretation
MR images were evaluated in consensus by two musculoskeletal radiologists (M.R.d.A. and H.J.K., with 4 and 8 years of experience in MR imaging, respectively), with emphasis on the structures of the posterosuperior aspect of the knee. The readers were asked to note the presence of fluid accumulation posterior to the PCL (in the PCL recess). The PCL recess was measured by the readers to determine its maximal anteroposterior, mediolateral, and superoinferior dimensions in the sagittal, coronal, and transverse planes. Any communication of the PCL recess with the medial or lateral femorotibial compartment of the knee was recorded and analyzed. The presence of anatomic structures adjacent to the recess and of a capsule surrounding it were recorded and described. The presence of a capsule around the recess was recorded when a low-signal-intensity linear structure with a length of more than 0.5 mm was seen to surround the internal surface of the recess.
The presence of a fat pad posterior to the PCL also was noted, and its length was recorded as the number of sagittal sections in which this fat pad was observed.
The presence of any other localized fluid accumulation anterior, medial, or lateral to the PCL was recorded, described, and measured by the readers according to its maximal anteroposterior, mediolateral, and superoinferior dimensions in the sagittal, coronal, and transverse planes. Readers also were asked to measure, in the sagittal plane, the distance of the posterior joint capsule insertion site on the surface of the femur from the femoral physeal scar. The scar, which is located in medullary bone, where the cartilaginous growth plate fused, appeared as a line of hypointense signal on sagittal T1-weighted MR images. In joints with a capsule insertion site above the physeal scar, distance measurements were recorded as a positive value; for those with insertion below the scar, as a negative value; and for those with insertion at the same level as the scar, as zero. Readers were asked to measure distance on images in nine sagittal sections, beginning with the section in which the PCL was best visualized (central sagittal section) and continuing through four sections medially and four sections laterally (Fig 2). The presence of an incomplete posterior capsule without insertion into the femur also was noted, and the number of sagittal sections that showed the incomplete capsule was recorded. Measurements were performed electronically at the MR image display workstation (Advantage Windows, version 2.0; GE Medical Systems) by the same two musculoskeletal radiologists in consensus.
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Figure 2a. (a) Sagittal T1-weighted SE MR image (600/20) obtained after intraarticular injection of gadopentetate dimeglumine into cadaveric knee specimen shows the distance (line) between the joint capsule (large white arrowhead) insertion site in the femur (black arrowhead) and the femoral physeal scar (small white arrowheads). (b) Diagram superimposed on coronal T1-weighted SE MR image (600/20) obtained after intraarticular injection of gadopentetate dimeglumine shows the sagittal sections (vertical lines) used to measure capsular insertion. The central section (0) was the one on which the PCL was best depicted. Sections medial to the central section were numbered from 1 to 4, and those lateral to it, from –1 to –4.
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Figure 2b. (a) Sagittal T1-weighted SE MR image (600/20) obtained after intraarticular injection of gadopentetate dimeglumine into cadaveric knee specimen shows the distance (line) between the joint capsule (large white arrowhead) insertion site in the femur (black arrowhead) and the femoral physeal scar (small white arrowheads). (b) Diagram superimposed on coronal T1-weighted SE MR image (600/20) obtained after intraarticular injection of gadopentetate dimeglumine shows the sagittal sections (vertical lines) used to measure capsular insertion. The central section (0) was the one on which the PCL was best depicted. Sections medial to the central section were numbered from 1 to 4, and those lateral to it, from –1 to –4.
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Knee Sectioning and Comparisons
After MR imaging was performed, the cadaveric specimens were frozen again at –60°C for more than 120 hours and were subsequently sectioned with a band saw into 3-mm-thick slices, so that anatomic slices corresponded closely to the sagittal sections at MR imaging. Each slice was photographed and imaged with high-spatial-resolution radiography (Faxitron; Hewlett Packard, McMinnville, Ore). To determine the anatomic site of capsular insertion and the characteristics of the PCL recess, the findings on MR images and radiographs of each specimen were compared with the findings derived from visual inspection of the cadaveric slices by two authors (M.R.d.A., H.J.K.).
Histologic Analysis
To analyze the tissue composition of the PCL recess and its relation to adjacent structures, histologic samples of this area were collected in three knee specimens. Immediately after inspection of the cadaveric specimens, the samples were suspended in a 10% formalin solution. For histologic analysis, samples were embedded in paraffin and sectioned further into 5-µm-thick slices. Histologic slices were stained with hematoxylin-eosin and analyzed at light microscopy (magnification, x34 to x3100), with consensus, by a musculoskeletal radiologist (M.R.d.A.) and an orthopedic pathologist with 30 years of experience. The examiners recorded in consensus the presence or absence of the PCL recess, its histologic appearance, and its anatomic relation to the adjacent soft tissues.
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RESULTS
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None of the MR images acquired prior to the intraarticular injection of gadopentetate dimeglumine in 14 cadaveric knees demonstrated localized fluid posterior to the PCL. On MR images obtained after the intraarticular injection of the contrast agent, however, a localized fluid collection was seen in the PCL recess in all 14 cadaveric specimens (Fig 3).
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Figure 3a. (a) Sagittal T1-weighted SE MR image (600/20) obtained before the injection of contrast material in cadaveric knee specimen shows fat pad (F) but no recess behind the PCL. (b) Sagittal T1-weighted SE MR image (600/20) obtained after intraarticular injection of a large volume of gadopentetate dimeglumine shows PCL recess (R) and fat pad (arrowhead). (c) Photomicrograph of histologic section shows synovial membrane of PCL recess (arrow). (Hematoxylin-eosin stain; original magnification, x200.) (d) Coronal T1-weighted SE MR image (600/20) obtained after intraarticular injection of a large volume of gadopentetate dimeglumine clearly depicts PCL recess, fat pad, and joint capsule (arrow).
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Figure 3b. (a) Sagittal T1-weighted SE MR image (600/20) obtained before the injection of contrast material in cadaveric knee specimen shows fat pad (F) but no recess behind the PCL. (b) Sagittal T1-weighted SE MR image (600/20) obtained after intraarticular injection of a large volume of gadopentetate dimeglumine shows PCL recess (R) and fat pad (arrowhead). (c) Photomicrograph of histologic section shows synovial membrane of PCL recess (arrow). (Hematoxylin-eosin stain; original magnification, x200.) (d) Coronal T1-weighted SE MR image (600/20) obtained after intraarticular injection of a large volume of gadopentetate dimeglumine clearly depicts PCL recess, fat pad, and joint capsule (arrow).
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Figure 3c. (a) Sagittal T1-weighted SE MR image (600/20) obtained before the injection of contrast material in cadaveric knee specimen shows fat pad (F) but no recess behind the PCL. (b) Sagittal T1-weighted SE MR image (600/20) obtained after intraarticular injection of a large volume of gadopentetate dimeglumine shows PCL recess (R) and fat pad (arrowhead). (c) Photomicrograph of histologic section shows synovial membrane of PCL recess (arrow). (Hematoxylin-eosin stain; original magnification, x200.) (d) Coronal T1-weighted SE MR image (600/20) obtained after intraarticular injection of a large volume of gadopentetate dimeglumine clearly depicts PCL recess, fat pad, and joint capsule (arrow).
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Figure 3d. (a) Sagittal T1-weighted SE MR image (600/20) obtained before the injection of contrast material in cadaveric knee specimen shows fat pad (F) but no recess behind the PCL. (b) Sagittal T1-weighted SE MR image (600/20) obtained after intraarticular injection of a large volume of gadopentetate dimeglumine shows PCL recess (R) and fat pad (arrowhead). (c) Photomicrograph of histologic section shows synovial membrane of PCL recess (arrow). (Hematoxylin-eosin stain; original magnification, x200.) (d) Coronal T1-weighted SE MR image (600/20) obtained after intraarticular injection of a large volume of gadopentetate dimeglumine clearly depicts PCL recess, fat pad, and joint capsule (arrow).
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The mean maximal dimensions of the PCL recess in the sagittal plane (superoinferior diameter x anteroposterior diameter) were 22.3 x 9.7 mm (range, 10–32 x 2–20 mm); in the coronal plane (superoinferior diameter x mediolateral diameter), 17.2 x 10.3 mm (range, 8–25 x 2–20 mm); and in the transverse plane (mediolateral diameter x anteroposterior diameter), 9.6 x 9.7 mm (range, 2–16 x 4–15 mm). In the four knees imaged at 30° flexion, the dimensions of the PCL recess measured in the sagittal plane were increased substantially, compared with that in knees imaged while in the neutral position: In the first knee specimen, the dimensions of the PCL recess (superoinferior diameter x anteroposterior diameter) were 10 x 5 mm while in the neutral position and 20 x 20 mm during flexion; in the second specimen, 15 x 3 mm while in the neutral position and 20 x 22 mm during flexion; in the third specimen, 20 x 20 mm while in the neutral position and 20 x 30 mm during flexion; and in the fourth specimen, 10 x 2 mm while in the neutral position and 12 x 3 mm during flexion (Fig 4). In the four knees imaged during full extension, the size of the PCL recess measured in the sagittal plane did not increase significantly, compared with its size with the knee in the neutral position.
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Figure 4a. Sagittal T1-weighted SE MR images (600/20) obtained after gadopentetate dimeglumine injection in cadaveric knee specimen (a) in neutral position and (b) at 30° of flexion. Comparison of a and b demonstrates expansion of the PCL recess (black arrowhead) and joint capsule (white arrowheads) during flexion.
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Figure 4b. Sagittal T1-weighted SE MR images (600/20) obtained after gadopentetate dimeglumine injection in cadaveric knee specimen (a) in neutral position and (b) at 30° of flexion. Comparison of a and b demonstrates expansion of the PCL recess (black arrowhead) and joint capsule (white arrowheads) during flexion.
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The PCL recess communicated with the medial femorotibial compartment in all cases. Although a communication between the recess and the lateral femorotibial compartment was believed to be present in two specimens on images obtained in the sagittal plane, analysis of the coronal and, particularly, the transverse images clearly indicated the absence of such communication in all cases (Figs 3, 5). In seven cases, the PCL recess communicated with the medial compartment of the knee via a constricted area, or neck, with a diameter ranging from 8 to 20 mm (Fig 5d).
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Figure 5a. (a–c) Sagittal T1-weighted SE MR images (600/20) obtained after injection of gadopentetate dimeglumine in cadaveric knee specimen, in (a) medial and (b, c) lateral sections, show communication of PCL recess with medial compartment of knee. (d) Transverse T1-weighted SE MR image (600/20) depicts communication of the PCL recess (R) with the medial femorotibial compartment via a constricted area (*).
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Figure 5b. (a–c) Sagittal T1-weighted SE MR images (600/20) obtained after injection of gadopentetate dimeglumine in cadaveric knee specimen, in (a) medial and (b, c) lateral sections, show communication of PCL recess with medial compartment of knee. (d) Transverse T1-weighted SE MR image (600/20) depicts communication of the PCL recess (R) with the medial femorotibial compartment via a constricted area (*).
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Figure 5c. (a–c) Sagittal T1-weighted SE MR images (600/20) obtained after injection of gadopentetate dimeglumine in cadaveric knee specimen, in (a) medial and (b, c) lateral sections, show communication of PCL recess with medial compartment of knee. (d) Transverse T1-weighted SE MR image (600/20) depicts communication of the PCL recess (R) with the medial femorotibial compartment via a constricted area (*).
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Figure 5d. (a–c) Sagittal T1-weighted SE MR images (600/20) obtained after injection of gadopentetate dimeglumine in cadaveric knee specimen, in (a) medial and (b, c) lateral sections, show communication of PCL recess with medial compartment of knee. (d) Transverse T1-weighted SE MR image (600/20) depicts communication of the PCL recess (R) with the medial femorotibial compartment via a constricted area (*).
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The PCL recess was posterior to the PCL and adjacent to the lateral aspect of the medial femoral condyle. The PCL was in intimate contact with the anterior portion of the recess. At MR imaging, no capsule was visualized around the PCL recess in any specimen. No contact between the PCL recess and the proximal one-third of the PCL was present in any case. An osseous body was found in the PCL recess in one specimen. The ligament of Wrisberg (posterior meniscofemoral ligament) was present and was in intimate contact with the PCL recess in four of 14 knees. This ligament was located at the posterosuperior aspect of the PCL recess in all four specimens.
Posterior to the PCL, a fat pad was present in all specimens that was visualized with MR arthrography and confirmed with cadaveric inspection. This fat pad was located between the PCL recess and the joint capsule (oblique popliteal ligament). On MR images, the fat pad was seen behind the PCL on one sagittal section in three knees, on two sagittal sections in seven knees, on three sagittal sections in three knees, and on four sagittal sections in one knee (Fig 6).
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Figure 6a. (a) Digital high-spatial-resolution photograph of sagittal anatomic slice obtained at level of PCL in cadaveric specimen shows the PCL recess (arrow) and its intimate relation to the ligament of Wrisberg (W); fat pads (F); and incomplete joint capsule (arrowheads). (b) Corresponding T1-weighted SE MR image (600/20) obtained after intraarticular injection of gadopentetate dimeglumine, at same level as a. (c) Photomicrograph of corresponding histologic slice shows PCL recess synovial membrane (arrow) and its intimate relation to the ligament of Wrisberg. (Hematoxylin-eosin stain; original magnification, x100.)
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Figure 6b. (a) Digital high-spatial-resolution photograph of sagittal anatomic slice obtained at level of PCL in cadaveric specimen shows the PCL recess (arrow) and its intimate relation to the ligament of Wrisberg (W); fat pads (F); and incomplete joint capsule (arrowheads). (b) Corresponding T1-weighted SE MR image (600/20) obtained after intraarticular injection of gadopentetate dimeglumine, at same level as a. (c) Photomicrograph of corresponding histologic slice shows PCL recess synovial membrane (arrow) and its intimate relation to the ligament of Wrisberg. (Hematoxylin-eosin stain; original magnification, x100.)
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Figure 6c. (a) Digital high-spatial-resolution photograph of sagittal anatomic slice obtained at level of PCL in cadaveric specimen shows the PCL recess (arrow) and its intimate relation to the ligament of Wrisberg (W); fat pads (F); and incomplete joint capsule (arrowheads). (b) Corresponding T1-weighted SE MR image (600/20) obtained after intraarticular injection of gadopentetate dimeglumine, at same level as a. (c) Photomicrograph of corresponding histologic slice shows PCL recess synovial membrane (arrow) and its intimate relation to the ligament of Wrisberg. (Hematoxylin-eosin stain; original magnification, x100.)
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A localized fluid collection was found between the ACL and PCL in 10 specimens, in an area termed the intercruciate recess. In the sagittal plane, the mean maximal superoinferior diameter of the intercruciate recess was 3.5 mm (range, 0–8 mm) and its mean maximal anteroposterior diameter was 3.2 mm (range, 0–8 mm); in the transverse plane, its mean maximal anteroposterior diameter was 5.4 mm (range, 2–10 mm) and its mean maximal mediolateral diameter was 6.3 mm (range, 3–10 mm). Measurement in the coronal plane was not performed because it was not possible to see the recess on images in that plane. This recess communicated with the lateral femorotibial compartment in four specimens and with the medial femorotibial compartment in six specimens.
The Table contains data regarding the joint capsule insertion into the femoral bone and the distance from the point of insertion to the physeal scar.
All histologic samples collected in three knee specimens showed a synovial membrane that surrounded the PCL recess and separated it from adjacent structures, the PCL (located anterior and inferior), and fat tissue (located superior and posterior). In two specimens, the posterior meniscofemoral ligament could be seen posterior and superior to the recess, from which it was separated by the synovial membrane.
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DISCUSSION
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Cystic lesions in the knee are common and can be evaluated with different imaging techniques, such as arthrography, computed tomography, and MR imaging. Cystic lesions in the knee, particularly near the PCL, may be meniscal cysts or ganglionic cysts (6). When ganglionic cysts are found within the knee joint, they are usually located in the infrapatellar fat pad or, more commonly, near the cruciate ligaments (7). The reported frequency of intraarticular ganglionic cysts in the knee ranges from 0.2% to 1.9% when studied with MR imaging or arthroscopy (7,8). Round or elliptical collections of fluid behind the PCL are commonly found with routine MR imaging, especially when joint fluid is present. A localized fluid collection may make it difficult to differentiate a distended joint recess from a ganglionic cyst.
Although the finding is not emphasized in the anatomic or MR imaging literature (1,7), a joint recess posterior to the PCL is commonly present. Such a recess was evident in all of our cadaveric specimens. Failure to visualize this structure at routine MR imaging is related to the presence of a small amount of joint fluid or none at all. In this study, the PCL recess was not evident until MR arthrography with a high volume of contrast material was performed. The PCL recess was variable in size and usually appeared as a round or oval structure that communicated with the posterior portion of the PCL, and those findings could simulate a ganglionic cyst. Knowledge of the characteristics of the PCL recess—its communication with the medial femorotibial compartment, the absence of the capsule, lack of contact with the proximal one-third of the PCL, and intimate contact with the posterior meniscofemoral ligament (ligament of Wrisberg)—could help to differentiate the normal PCL recess from pathologic processes posterior to the PCL.
In seven cadaveric knees, communication between the PCL recess and the medial femorotibial compartment was through a constricted area, or neck, with a variable diameter (8–20 mm). It is possible (although not proved by our data) that some PCL recesses with such small necks retain fluid even after a joint effusion disappears and that this retention leads to increased diagnostic difficulty. It is possible that ganglionic cysts develop from fluid retained in this area. In cases of synovitis associated with joint effusion, intraarticular adhesions and fibrosis in response to the inflammatory process may lead to closure of the neck of a distended PCL recess and, thereby, to the formation of a ganglionic cyst.
Another small recess was found between the ACL and PCL that was not depicted on MR images obtained before the intraarticular injection of contrast material. Ganglionic cysts also can be found in this area (7). Although this feature is described in the classic anatomic literature as a small recess or bursa that may communicate only with the lateral joint compartment (1), we found that it may communicate with either the medial or the lateral femorotibial compartment.
The insertion sites of the joint capsule also were studied because of the same concern about defining the normal limits of localized fluid accumulations, especially in the central part of the joint, posterior to the PCL. Our results indicate that the site of joint capsule insertion is no more than 2 cm above the physeal scar. Our analysis further revealed that in the central portion of the joint, behind the PCL, the joint capsule was located behind an intraarticular fat pad and was not in contact with the joint cavity on one or more sagittal images.
We recognize that our study had limitations. The number of cadaveric specimens was relatively small. Clinical information was lacking, and the specimens were derived from elderly persons. Histologic analysis was performed only in a few selected cases. Despite these limitations, our data indicate that the PCL recess is an anatomic structure found in the majority of knees at MR imaging during full distention of the joint capsule by a large volume of fluid containing contrast material. This recess has specific characteristics that allow its identification: communication with the medial femorotibial compartment, absence of surrounding capsule, absence of contact with the proximal one-third of the PCL, and an intimate relation to the posterior meniscofemoral ligament. A second small recess can be found between the ACL and PCL.
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FOOTNOTES
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Abbreviations: ACL = anterior cruciate ligament • PCL = posterior cruciate ligament • SE = spin echo
Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, M.R.d.A., C.B.C., J.M.J., D.R.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, M.R.d.A., H.J.K., C.B.C., J.M.J., D.R.; experimental studies, all authors; statistical analysis, M.R.d.A., C.B.C., J.C., D.R.; and manuscript editing, M.R.d.A., C.B.C., D.R.
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References
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ABSTRACT
INTRODUCTION
