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Posteromedial corner injuries of the knee
Clinical Radiology (2007) 62, 539e546
PICTORIAL REVIEW
Posteromedial corner injuries of the knee C.V. Housea,b,*, D.A. Connella,b, A. Saifuddina,b a
The Royal National Orthopaedic Hospital NHS Trust, Stanmore, Middlesex, UK, and The Institute of Orthopaedics and Musculoskeletal Sciences, Stanmore, UK
b
Received 20 September 2006; received in revised form 15 November 2006; accepted 29 November 2006
Magnetic resonance imaging (MRI) depicts in exquisite detail the supporting structures of the posteromedial corner of the knee. This musculoligamentous unit plays a central role as a dynamic stabilizer of the knee joint and the recognition of injury to the posteromedial corner carries important implications in terms of management and prognosis, most particularly in the anterior cruciate ligament-deficient knee. This article provides a resume ´ of the functional anatomy of the posteromedial corner of the knee as seen with MRI and follows with a review of the MRI appearances of injury to the posteromedial corner. ª 2007 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction
Anatomy and function
Magnetic resonance imaging (MRI) provides superbly detailed images of the knee joint. An understanding of injury patterns and findings that are of relevance to clinical management are important when interpreting these images. The supporting structures of the posteromedial corner of the knee play a central role in maintaining stability of the joint. This musculoligamentous unit has a dynamic stabilizing effect, a function that is dependent upon its integrity as a whole. Diagnosing loss of integrity of the posteromedial corner has an important bearing on management of injuries to the knee, hence the importance of recognizing the MRI appearances of trauma to this functional unit. A summary of the anatomy of the posteromedial corner assists in appreciating the functional significance of the supporting structures as a dynamic stabilizer. This is followed by a review of the imaging appearances of injury to the posteromedial corner.
The structures of the medial side of the knee may be divided anatomically into regions. This description divides the medial structures of the knee from anterior to posterior, extending from the medial border of the patella to the medial edge of the posterior cruciate ligament (PCL). The anterior third comprises the extensor retinaculum of the quadriceps femoris muscle and thin capsular ligaments lying deep to this. The middle third consists of the deep medial collateral ligament (MCL) and the superficial MCL. The former is itself made up of meniscofemoral and meniscotibial fibres, and is separated from the superficial MCL by the MCL bursa. The superficial MCL, or tibial collateral ligament (TCL), consists of parallel, longitudinal fibres anteriorly and an oblique array of fibres posterior to this. These more posterior fibres follow a course at 25 to the longitudinal fibres and have been termed the posterior oblique ligament (POL) of the knee.1 The POL is the first of the structures that make up the posterior third of the medial aspect of the knee, or posteromedial corner. The other intimately related and closely interacting components are the semimembranosus insertion, the posterior third of the medial meniscus, the oblique popliteal ligament (OPL) and the meniscotibial ligament.2
* Guarantor and correspondent: C.V. House, The Royal National Orthopaedic Hospital NHS Trust, Brockley Hill, Stanmore, Middlesex HA7 4LP, UK. Tel.: þ44 7710 352476; fax: þ44 20 7380 9068. E-mail address: [email protected] (C.V. House).
0009-9260/$ - see front matter ª 2007 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2006.11.024
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MRI of the knee elegantly demonstrates the anatomical components of the posteromedial corner, with useful anatomical detail gained from routine imaging sequences in the three standard planes.3,4 The POL arises from the femur separate to the TCL and has a branching insertion (Fig. 1). Whilst the TCL arises from the medial femoral condyle, the POL arises from the adductor tubercle. As it passes distally, the ligament divides into three arms. The tibial arm inserts into the medial meniscus close to the articular margin of the posterior tibia. The capsular arm blends with the posterior capsule and OPL, and the superficial arm blends with the sheath of the direct insertion of the semimembranosus tendon, to insert into the medial proximal tibia. Thus, the posterior third of the medial meniscus has firm attachments to the POL. The POL is clearly demonstrated on a combination of coronal and axial images (Fig. 2). In the axial plane (Fig. 2), the capsular arm is identified at the level of the distal medial femoral condyle, extending posteriorly from the TCL to merge with the medial aspect of the posterior capsular structures. In the coronal plane (Fig. 2bed), the superficial arm is identified as a thin hypointense stripe extending from the adductor tubercle to the medial aspect of the tibial plateau, immediately posterior to the TCL and superficial to the direct insertion of the semimembranosus tendon. The contribution of the meniscus to the dynamic stabilizing function of the posteromedial corner relies additionally upon a stable base on the tibia, maintained by the meniscotibial ligament. This structure is optimally identified on mid-coronal images (Fig. 2c).
Figure 1 Diagrammatic summary of the branching attachment of the posterior oblique ligament and the relationship of this to the semimembranosus insertion.
C.V. House et al.
The semimembranosus muscle, after arising from the ischial tuberosity, passes deep to semitendinosus to insert in a complex fashion via five tendinous expansions, as follows: (1) the anterior (tibial) arm, also termed the pars reflexa, passes anteriorly deep to the POL and inserts into the tibia deep to the TCL. (2) The direct arm, attached to the tibia deep to the pars reflexa and not seen at MRI. (3) The inferior (popliteal) arm; extends more distally than the direct and anterior arms to insert into the tibia just above the tibial attachment of the TCL. (4) The capsular arm, contiguous with the POL. (5) the OPL insertion, a strong fasciculus arising from the semimembranosus tendon just before its tibial insertion and passing obliquely upwards to attach to the posterior lateral femoral condyle. In so doing, the fasciculus blends with the flat, fibrous band of the OPL as this runs between the posterior surface of the femur close to the articular margins of the condyles and the posterior tibia, strengthening the posterior capsule. A sixth insertion, into the posterior third of the lateral meniscus, has been described in 43% of cases.5 The pars reflexa and inferior insertion are well identified on sagittal (Fig. 3a) and coronal (Fig. 2d) images, whereas the insertions into the posterior capsule are optimally assessed on axial images at the level of the joint line (Fig. 3b). Accumulation of fat within the main tendon is a recognized normal variant, which should not be mistaken for tendon degeneration.4 The significance of the semimembranosus muscle to the function of this musculoligamentous unit has led to the posteromedial corner also being referred to as the ‘‘semimembranosus corner’’.6 The product of this intimate relationship between the POL and semimembranosus insertion, in consort with firm attachments to the posteromedial meniscus and joint capsule, is a musculoligamentous unit of central importance to the dynamic stability of the medial knee. The semimembranosus muscle is aligned in such a way as to act as a dynamic restraint to valgus with the knee extended, whilst in knee flexion the muscle acts to restrict external rotation.7 The semimembranosus insertion provides for posterior meniscal retraction during knee flexion, helping to prevent posteromedial meniscal impingement and lending tension to the posteromedial capsuloligamentous structures in a position where these would otherwise be lax. Furthermore, the muscle assists in the ‘‘brake-stop’’ function of the medial meniscus, whereby the meniscal horn acts as a secondary restraint to anterior tibial translation, of importance most particularly in the ACL-deficient knee.
Figure 2 MRI anatomy of the posterior oblique ligament. (a) Axial fat-suppressed PDW fast spin echo (FSE) image showing the TCL (black arrow) and the POL (white arrow). (b) Coronal fat-suppressed PDW FSE image at the level of the tibial insertion of the ACL (long arrow) showing the TCL (small arrow). (c) Coronal fat-suppressed PDW FSE image at the level of the posterior intercondylar notch showing the proximal POL (arrowheads), arising from the adductor tubercle (short arrow). Note also the meniscofemoral and meniscotibial ligaments (long arrows). (d) Coronal fat-suppressed PDW FSE image at the level of the posterior medial tibial plateau showing the distal POL (arrowhead) and the tibial insertion of the semimembranosus tendon (arrow).
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Injury of the posteromedial corner Medial-sided knee injury has been regarded as a disruption of the medial collateral ligament.8e10 However, following the work of Hughston,11 describing the POL, the concept of the posteromedial corner was introduced.1,11 An appreciation of the supporting structures of the posteromedial corner allows a better understanding of the differing severity and variable natural history of medial-sided knee injuries, and assists the surgeon in deciding which cases require surgical management.6 Although there is agreement that isolated injury to the TCL responds well to non-operative management, an injury involving the posteromedial corner may warrant closer consideration for surgery.2 Clinically, disruption of the posteromedial corner can result in anteromedial rotatory instability (AMRI) of the knee. This functional instability involves anteromedial rotatory subluxation of the medial tibial condyle during abduction stress, as well as excess opening of the medial joint space, as seen in isolated open-book instability.
Ligamentous injury: the POL
Figure 3 MRI anatomy of the semimembranosus insertion. (a) Sagittal PDW FSE image showing the pars reflexa (black arrow) and inferior limb (white arrow) of the distal semimembranosus insertion. (b) Axial PDW FSE image showing the main semimembranosus tendon (double black arrowheads), lying adjacent to the medial head gastrocnemius tendon (white arrow). Fat (long arrow) is seen within the main tendon. The capsular limb (double white arrowheads) is also shown.
In their review of surgically treated, medial-sided knee injuries (in which the indication for surgery was AMRI), Sims and Jacobson found that 99% of cases had a documented injury to the POL.2 MRI of the knee has a reported diagnostic accuracy of 94% for injury of the knee collateral ligaments. We are unaware of any studies that have demonstrated injury to the POL. However, we would expect that MRI might perform to a similarly high level in the definition of injury to the POL.14 No formal MRI grading system exists for injury to the POL, but the authors have found that the system used to grade acute medial collateral ligament (MCL) injuries may be usefully applied (Table 1). Using this grading system, sprains, partial tears and complete POL tears may be optimally demonstrated on a combination of coronal and axial images (Fig. 4).
Table 1 Suggested MRI classification of acute injury of the posterior oblique ligament Grade I e microscopic tear Grade II e partial tear Grade III e complete tear
Ligament of normal thickness and intact, with oedema (T2 high signal) surrounding the ligament Thickening of ligament with partial disruption of fibres and increased amount of surrounding oedema/haemorrhage Complete disruption of the ligament, with surrounding oedema/haemorrhage
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Figure 5 Injury to the meniscotibial ligament. Coronal PDW FSE image showing an intact meniscofemoral ligament (white arrow) and avulsion (black arrow) of the thickened meniscotibial ligament (arrowheads).
Meniscocapsular injury The posterior third of the medial meniscus acts as a restraint to posterior translation of the medial femoral condyle on the tibia by engaging the femoral condyle during weight bearing. However, if the meniscus is not fixed and stable on the tibial plateau, as occurs in damage to the meniscotibial ligament, then this function may be compromised. This in turn places the other supporting structures of the posteromedial corner under greater stress and thus more susceptible to injury.4 Meniscal detachment was a finding in 30% of cases in the cohort of injuries reported by Sims and Jacobson.2 MRI readily demonstrates injury of the femoral and tibial meniscal attachments as disruption and thickening of the ligaments with or without associated marrow oedema at the ligament insertion (Fig. 5).15 Avulsion of the bony insertion site of the meniscotibial ligament, the so-called ‘‘reverse Segond fracture’’, has been described with PCL rupture.16
Figure 4 Injury to the POL. (a) Axial fat-suppressed PDW FSE image of a grade 1 tear, showing oedema around the intact POL (arrows). (b) Coronal fat-suppressed PDW FSE image of a grade 2 tear, showing oedema around the thickened, partially disrupted, hyperintense POL (arrows). (c) Axial fat-suppressed PDW FSE image of a grade 3 tear, showing oedema around the disrupted POL (black arrow), adjacent bone bruising (white arrow) and a normal TCL (arrowhead).
Figure 6 Injury to the semimembranosus insertion. (a) Sagittal fat-suppressed PDW FSE image showing bone bruising in the posteromedial tibial plateau and a hypointense fracture line (arrowheads). (b) Axial fat-suppressed PDW FSE image showing complete absence of the main semimembranosus tendon (arrow). (c) Sagittal fat-suppressed PDW FSE image showing oedema around the tibial insertions of the semimembranosus tendon (arrows). (d) Sagittal fat-suppressed PDW FSE image showing thickening and oedema of the posteromedial capsule (arrows). (e) Sagittal T1-weighted spin echo image showing marked thickening and intermediate signal intensity within the tibial insertions of semimembranosus (arrows).
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Musculotendinous injury: the semimembranosus insertion The dynamic stabilizing effect of the musculoligamentous unit of the posteromedial corner is driven by the semimembranosus muscle and disruption of the muscle or its insertion will compromise this function. Damage to the semimembranosus insertion is common, occurring in up to 70% of posteromedial knee injuries.2 Injury to the semimembranosus tendinous expansions may include avulsion fracture of the tibial insertion, complete or partial tears of the tendon and chronic insertional tendinosis. Avulsion fracture of the posteromedial tibial plateau typically occurs when, during flexion of the knee and internal rotation of the tibia, forced abduction and external rotation are applied. The avulsion fracture occurs at the insertion site of the direct tibial arm of the tendon,17 appearing as a region of bone bruising, with an associated hypointense fracture line (Fig. 6a). Complete tears of the semimembranosus tendon do occur, but are uncommon, presumably because the tendon itself is stronger than its insertion site. MRI will demonstrate discontinuity of the tendon, best appreciated on sagittal or axial images (Fig. 6b). However, strains and partial tears of the tendon are common, with the capsular arm of the ligament particularly prone to injury.2 With a tendon strain, MRI shows fluid around an otherwise intact tendon (Fig. 6c), whereas capsular injury manifests as thickening and hyperintensity of the posterior capsule at the level of the medial femoral condyle, best appreciated on axial or sagittal images (Fig. 6d). Chronic, repetitive stress of the tibial insertion of semimembranosus results in insertional tendinosis, with thickening of the tendon at its tibial insertion (Fig. 6e).
ALC injury associated with posteromedial corner disruption In their review of 93 medial-sided knee injuries treated surgically, Sims and Jacobson found associated ACL injuries in 73 of the cases.2 This is an important association to recognize, as it may help to explain the observation that some patients with complete ACL ruptures continue to function at a high level of competitive amateur or professional athletics.12,13 It is postulated that significant injury to the posteromedial (or the posterolateral) corner does not occur in this subgroup, thus allowing these intact dynamic structures and the meniscocapsular complex to compensate for the deficient ACL and maintain functional stability of the joint.
Figure 7 Combined ACL and posteromedial corner injury. Coronal fat-suppressed PDW FSE image showing an acute complete ACL rupture (long arrow), a peripheral tear of the medial meniscus (short arrow) and swelling of the proximal POL (arrowheads).
Alternatively, unrecognized damage to the supporting structures of the posteromedial corner in a patient with a ruptured ACL may account for variable postoperative outcome after ACL reconstruction. Hence, the difference between an isolated rupture of the ACL and an injury that involves the posteromedial corner (Fig. 7) is an important distinction to make.
Conclusions Full appreciation of the functional significance of the imaging abnormalities observed is of primary importance to the radiologist in generating a pertinent, relevant and complete report of a study. The dynamic stabilizing effect of the posteromedial structures of the knee, powered by the semimembranosus muscle, plays an integral role in resisting anteromedial rotatory instability of the knee. Mu ¨ller6 eloquently summarizes this importance: ‘‘As long as the semimembranosus corner functions efficiently as a stabilizer, even a weak or damaged cruciate ligament can function in a compensated fashion. But if this stabilizing effect is lost, the anterior cruciate ligament alone is incapable of compensating and becomes increasingly insufficient.’’ Thus, the proximity of the stabilizing unit of the posteromedial corner of the knee to the tibial collateral ligament must not allow the contrasting functional significance of damage to these
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structures to become blurred. Although there is general agreement that isolated damage to the tibial collateral ligament may respond well to nonoperative management, an injury that involves the posteromedial corner warrants closer consideration of operative repair.
References 1. Hughston JC, Eilers AF. The role of the posterior oblique ligament in repairs of acute medial (collateral) ligament tears of the knee. J Bone Joint Surg Am 1973;55A:923e39. 2. Sims WF, Jacobson KE. The posteromedial corner of the knee: medial-sided injury patterns revisited. Am J Sports Med 2004;32:337e45. 3. Loredo R, Hodler J, Pedowitz R, et al. Posteromedial corner of the knee: MR imaging with gross anatomic correlation. Skeletal Radiol 1999;28:305e11. 4. Beltran J, Matityahu A, Hwang K, et al. The distal semimembranosus complex: normal MR anatomy, variants, biomechanics and pathology. Skeletal Radiol 2003;32:435e45. 5. Kim YC, Yoo WK, Chung IH, et al. Tendinous insertion of semimembranosus muscle into the lateral meniscus. Surg Radiol Anat 1997;19:365e9. 6. Muller W. The knee-form, function and ligament reconstruction. Berlin: Springer-Verlag; 1983. 7. Robinson JR, Sanchez-Ballester J, Bull AM, et al. The posteromedial corner revisited. An anatomical description of the passive restraining structures of the medial aspect of the human knee. J Bone Joint Surg Br 2004;86B: 674e81.
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8. Warren LF, Marshall JL. The supporting structures and layers on the medial side of the knee. J Bone Joint Surg Am 1979; 61A:56e62. 9. Indelicato PA. Nonoperative management of complete tears of the medial collateral ligament. Orthop Rev 1989;18:947e52. 10. Indelicato PA. Isolated medial collateral ligament injuries in the knee. J Am Acad Orthop Surg 1995;3:9e14. 11. Hughston JC. The importance of the posterior oblique ligament in repairs of acute tears of the medial ligaments in knees with and without an associated rupture of the anterior cruciate ligament. J Bone Joint Surg Am 1994;76A:1328e44. 12. Jackson RW, Peters RI, Marczyk RL. Late results of untreated anterior cruciate ligament rupture. J Bone Joint Surg Br 1980;62B:127. 13. McDaniel Jr WJ, Dameron Jr TB. Untreated ruptures of the anterior cruciate ligament. A follow-up study. J Bone Joint Surg Am 1980;62A:696e705. 14. LaPrade RF, Gilbert TJ, Bollom TS, et al. The magnetic resonance imaging appearance of individual structures of the posterolateral knee. A prospective study of normal knees and knees with surgically verified grade III injuries. Am J Sports Med 2000;28:191e9. 15. De Maeseneer M, Shahabpour M, Vanderdood K, et al. Medial meniscocapsular separation: MR imaging criteria and diagnostic pitfalls. Eur J Radiol 2002;41:242e52. 16. Escobedo EM, Mills WJ, Hunter JC. The ‘‘reverse Segond’’ fracture: association with a tear of the posterior cruciate ligament and medial meniscus. AJR Am J Roentgenol 2002;178:979e83. 17. Chan KK, Resnick D, Goodwin D, et al. Posteromedial tibial plateau injury including avulsion fracture of the semimembranosus tendon insertion site: ancillary sign of anterior cruciate ligament tear at MR imaging. Radiology 1999; 211:754e8.
PICTORIAL REVIEW
Posteromedial corner injuries of the knee C.V. Housea,b,*, D.A. Connella,b, A. Saifuddina,b a
The Royal National Orthopaedic Hospital NHS Trust, Stanmore, Middlesex, UK, and The Institute of Orthopaedics and Musculoskeletal Sciences, Stanmore, UK
b
Received 20 September 2006; received in revised form 15 November 2006; accepted 29 November 2006
Magnetic resonance imaging (MRI) depicts in exquisite detail the supporting structures of the posteromedial corner of the knee. This musculoligamentous unit plays a central role as a dynamic stabilizer of the knee joint and the recognition of injury to the posteromedial corner carries important implications in terms of management and prognosis, most particularly in the anterior cruciate ligament-deficient knee. This article provides a resume ´ of the functional anatomy of the posteromedial corner of the knee as seen with MRI and follows with a review of the MRI appearances of injury to the posteromedial corner. ª 2007 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction
Anatomy and function
Magnetic resonance imaging (MRI) provides superbly detailed images of the knee joint. An understanding of injury patterns and findings that are of relevance to clinical management are important when interpreting these images. The supporting structures of the posteromedial corner of the knee play a central role in maintaining stability of the joint. This musculoligamentous unit has a dynamic stabilizing effect, a function that is dependent upon its integrity as a whole. Diagnosing loss of integrity of the posteromedial corner has an important bearing on management of injuries to the knee, hence the importance of recognizing the MRI appearances of trauma to this functional unit. A summary of the anatomy of the posteromedial corner assists in appreciating the functional significance of the supporting structures as a dynamic stabilizer. This is followed by a review of the imaging appearances of injury to the posteromedial corner.
The structures of the medial side of the knee may be divided anatomically into regions. This description divides the medial structures of the knee from anterior to posterior, extending from the medial border of the patella to the medial edge of the posterior cruciate ligament (PCL). The anterior third comprises the extensor retinaculum of the quadriceps femoris muscle and thin capsular ligaments lying deep to this. The middle third consists of the deep medial collateral ligament (MCL) and the superficial MCL. The former is itself made up of meniscofemoral and meniscotibial fibres, and is separated from the superficial MCL by the MCL bursa. The superficial MCL, or tibial collateral ligament (TCL), consists of parallel, longitudinal fibres anteriorly and an oblique array of fibres posterior to this. These more posterior fibres follow a course at 25 to the longitudinal fibres and have been termed the posterior oblique ligament (POL) of the knee.1 The POL is the first of the structures that make up the posterior third of the medial aspect of the knee, or posteromedial corner. The other intimately related and closely interacting components are the semimembranosus insertion, the posterior third of the medial meniscus, the oblique popliteal ligament (OPL) and the meniscotibial ligament.2
* Guarantor and correspondent: C.V. House, The Royal National Orthopaedic Hospital NHS Trust, Brockley Hill, Stanmore, Middlesex HA7 4LP, UK. Tel.: þ44 7710 352476; fax: þ44 20 7380 9068. E-mail address: [email protected] (C.V. House).
0009-9260/$ - see front matter ª 2007 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2006.11.024
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MRI of the knee elegantly demonstrates the anatomical components of the posteromedial corner, with useful anatomical detail gained from routine imaging sequences in the three standard planes.3,4 The POL arises from the femur separate to the TCL and has a branching insertion (Fig. 1). Whilst the TCL arises from the medial femoral condyle, the POL arises from the adductor tubercle. As it passes distally, the ligament divides into three arms. The tibial arm inserts into the medial meniscus close to the articular margin of the posterior tibia. The capsular arm blends with the posterior capsule and OPL, and the superficial arm blends with the sheath of the direct insertion of the semimembranosus tendon, to insert into the medial proximal tibia. Thus, the posterior third of the medial meniscus has firm attachments to the POL. The POL is clearly demonstrated on a combination of coronal and axial images (Fig. 2). In the axial plane (Fig. 2), the capsular arm is identified at the level of the distal medial femoral condyle, extending posteriorly from the TCL to merge with the medial aspect of the posterior capsular structures. In the coronal plane (Fig. 2bed), the superficial arm is identified as a thin hypointense stripe extending from the adductor tubercle to the medial aspect of the tibial plateau, immediately posterior to the TCL and superficial to the direct insertion of the semimembranosus tendon. The contribution of the meniscus to the dynamic stabilizing function of the posteromedial corner relies additionally upon a stable base on the tibia, maintained by the meniscotibial ligament. This structure is optimally identified on mid-coronal images (Fig. 2c).
Figure 1 Diagrammatic summary of the branching attachment of the posterior oblique ligament and the relationship of this to the semimembranosus insertion.
C.V. House et al.
The semimembranosus muscle, after arising from the ischial tuberosity, passes deep to semitendinosus to insert in a complex fashion via five tendinous expansions, as follows: (1) the anterior (tibial) arm, also termed the pars reflexa, passes anteriorly deep to the POL and inserts into the tibia deep to the TCL. (2) The direct arm, attached to the tibia deep to the pars reflexa and not seen at MRI. (3) The inferior (popliteal) arm; extends more distally than the direct and anterior arms to insert into the tibia just above the tibial attachment of the TCL. (4) The capsular arm, contiguous with the POL. (5) the OPL insertion, a strong fasciculus arising from the semimembranosus tendon just before its tibial insertion and passing obliquely upwards to attach to the posterior lateral femoral condyle. In so doing, the fasciculus blends with the flat, fibrous band of the OPL as this runs between the posterior surface of the femur close to the articular margins of the condyles and the posterior tibia, strengthening the posterior capsule. A sixth insertion, into the posterior third of the lateral meniscus, has been described in 43% of cases.5 The pars reflexa and inferior insertion are well identified on sagittal (Fig. 3a) and coronal (Fig. 2d) images, whereas the insertions into the posterior capsule are optimally assessed on axial images at the level of the joint line (Fig. 3b). Accumulation of fat within the main tendon is a recognized normal variant, which should not be mistaken for tendon degeneration.4 The significance of the semimembranosus muscle to the function of this musculoligamentous unit has led to the posteromedial corner also being referred to as the ‘‘semimembranosus corner’’.6 The product of this intimate relationship between the POL and semimembranosus insertion, in consort with firm attachments to the posteromedial meniscus and joint capsule, is a musculoligamentous unit of central importance to the dynamic stability of the medial knee. The semimembranosus muscle is aligned in such a way as to act as a dynamic restraint to valgus with the knee extended, whilst in knee flexion the muscle acts to restrict external rotation.7 The semimembranosus insertion provides for posterior meniscal retraction during knee flexion, helping to prevent posteromedial meniscal impingement and lending tension to the posteromedial capsuloligamentous structures in a position where these would otherwise be lax. Furthermore, the muscle assists in the ‘‘brake-stop’’ function of the medial meniscus, whereby the meniscal horn acts as a secondary restraint to anterior tibial translation, of importance most particularly in the ACL-deficient knee.
Figure 2 MRI anatomy of the posterior oblique ligament. (a) Axial fat-suppressed PDW fast spin echo (FSE) image showing the TCL (black arrow) and the POL (white arrow). (b) Coronal fat-suppressed PDW FSE image at the level of the tibial insertion of the ACL (long arrow) showing the TCL (small arrow). (c) Coronal fat-suppressed PDW FSE image at the level of the posterior intercondylar notch showing the proximal POL (arrowheads), arising from the adductor tubercle (short arrow). Note also the meniscofemoral and meniscotibial ligaments (long arrows). (d) Coronal fat-suppressed PDW FSE image at the level of the posterior medial tibial plateau showing the distal POL (arrowhead) and the tibial insertion of the semimembranosus tendon (arrow).
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Injury of the posteromedial corner Medial-sided knee injury has been regarded as a disruption of the medial collateral ligament.8e10 However, following the work of Hughston,11 describing the POL, the concept of the posteromedial corner was introduced.1,11 An appreciation of the supporting structures of the posteromedial corner allows a better understanding of the differing severity and variable natural history of medial-sided knee injuries, and assists the surgeon in deciding which cases require surgical management.6 Although there is agreement that isolated injury to the TCL responds well to non-operative management, an injury involving the posteromedial corner may warrant closer consideration for surgery.2 Clinically, disruption of the posteromedial corner can result in anteromedial rotatory instability (AMRI) of the knee. This functional instability involves anteromedial rotatory subluxation of the medial tibial condyle during abduction stress, as well as excess opening of the medial joint space, as seen in isolated open-book instability.
Ligamentous injury: the POL
Figure 3 MRI anatomy of the semimembranosus insertion. (a) Sagittal PDW FSE image showing the pars reflexa (black arrow) and inferior limb (white arrow) of the distal semimembranosus insertion. (b) Axial PDW FSE image showing the main semimembranosus tendon (double black arrowheads), lying adjacent to the medial head gastrocnemius tendon (white arrow). Fat (long arrow) is seen within the main tendon. The capsular limb (double white arrowheads) is also shown.
In their review of surgically treated, medial-sided knee injuries (in which the indication for surgery was AMRI), Sims and Jacobson found that 99% of cases had a documented injury to the POL.2 MRI of the knee has a reported diagnostic accuracy of 94% for injury of the knee collateral ligaments. We are unaware of any studies that have demonstrated injury to the POL. However, we would expect that MRI might perform to a similarly high level in the definition of injury to the POL.14 No formal MRI grading system exists for injury to the POL, but the authors have found that the system used to grade acute medial collateral ligament (MCL) injuries may be usefully applied (Table 1). Using this grading system, sprains, partial tears and complete POL tears may be optimally demonstrated on a combination of coronal and axial images (Fig. 4).
Table 1 Suggested MRI classification of acute injury of the posterior oblique ligament Grade I e microscopic tear Grade II e partial tear Grade III e complete tear
Ligament of normal thickness and intact, with oedema (T2 high signal) surrounding the ligament Thickening of ligament with partial disruption of fibres and increased amount of surrounding oedema/haemorrhage Complete disruption of the ligament, with surrounding oedema/haemorrhage
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Figure 5 Injury to the meniscotibial ligament. Coronal PDW FSE image showing an intact meniscofemoral ligament (white arrow) and avulsion (black arrow) of the thickened meniscotibial ligament (arrowheads).
Meniscocapsular injury The posterior third of the medial meniscus acts as a restraint to posterior translation of the medial femoral condyle on the tibia by engaging the femoral condyle during weight bearing. However, if the meniscus is not fixed and stable on the tibial plateau, as occurs in damage to the meniscotibial ligament, then this function may be compromised. This in turn places the other supporting structures of the posteromedial corner under greater stress and thus more susceptible to injury.4 Meniscal detachment was a finding in 30% of cases in the cohort of injuries reported by Sims and Jacobson.2 MRI readily demonstrates injury of the femoral and tibial meniscal attachments as disruption and thickening of the ligaments with or without associated marrow oedema at the ligament insertion (Fig. 5).15 Avulsion of the bony insertion site of the meniscotibial ligament, the so-called ‘‘reverse Segond fracture’’, has been described with PCL rupture.16
Figure 4 Injury to the POL. (a) Axial fat-suppressed PDW FSE image of a grade 1 tear, showing oedema around the intact POL (arrows). (b) Coronal fat-suppressed PDW FSE image of a grade 2 tear, showing oedema around the thickened, partially disrupted, hyperintense POL (arrows). (c) Axial fat-suppressed PDW FSE image of a grade 3 tear, showing oedema around the disrupted POL (black arrow), adjacent bone bruising (white arrow) and a normal TCL (arrowhead).
Figure 6 Injury to the semimembranosus insertion. (a) Sagittal fat-suppressed PDW FSE image showing bone bruising in the posteromedial tibial plateau and a hypointense fracture line (arrowheads). (b) Axial fat-suppressed PDW FSE image showing complete absence of the main semimembranosus tendon (arrow). (c) Sagittal fat-suppressed PDW FSE image showing oedema around the tibial insertions of the semimembranosus tendon (arrows). (d) Sagittal fat-suppressed PDW FSE image showing thickening and oedema of the posteromedial capsule (arrows). (e) Sagittal T1-weighted spin echo image showing marked thickening and intermediate signal intensity within the tibial insertions of semimembranosus (arrows).
Posteromedial corner injuries of the knee
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Musculotendinous injury: the semimembranosus insertion The dynamic stabilizing effect of the musculoligamentous unit of the posteromedial corner is driven by the semimembranosus muscle and disruption of the muscle or its insertion will compromise this function. Damage to the semimembranosus insertion is common, occurring in up to 70% of posteromedial knee injuries.2 Injury to the semimembranosus tendinous expansions may include avulsion fracture of the tibial insertion, complete or partial tears of the tendon and chronic insertional tendinosis. Avulsion fracture of the posteromedial tibial plateau typically occurs when, during flexion of the knee and internal rotation of the tibia, forced abduction and external rotation are applied. The avulsion fracture occurs at the insertion site of the direct tibial arm of the tendon,17 appearing as a region of bone bruising, with an associated hypointense fracture line (Fig. 6a). Complete tears of the semimembranosus tendon do occur, but are uncommon, presumably because the tendon itself is stronger than its insertion site. MRI will demonstrate discontinuity of the tendon, best appreciated on sagittal or axial images (Fig. 6b). However, strains and partial tears of the tendon are common, with the capsular arm of the ligament particularly prone to injury.2 With a tendon strain, MRI shows fluid around an otherwise intact tendon (Fig. 6c), whereas capsular injury manifests as thickening and hyperintensity of the posterior capsule at the level of the medial femoral condyle, best appreciated on axial or sagittal images (Fig. 6d). Chronic, repetitive stress of the tibial insertion of semimembranosus results in insertional tendinosis, with thickening of the tendon at its tibial insertion (Fig. 6e).
ALC injury associated with posteromedial corner disruption In their review of 93 medial-sided knee injuries treated surgically, Sims and Jacobson found associated ACL injuries in 73 of the cases.2 This is an important association to recognize, as it may help to explain the observation that some patients with complete ACL ruptures continue to function at a high level of competitive amateur or professional athletics.12,13 It is postulated that significant injury to the posteromedial (or the posterolateral) corner does not occur in this subgroup, thus allowing these intact dynamic structures and the meniscocapsular complex to compensate for the deficient ACL and maintain functional stability of the joint.
Figure 7 Combined ACL and posteromedial corner injury. Coronal fat-suppressed PDW FSE image showing an acute complete ACL rupture (long arrow), a peripheral tear of the medial meniscus (short arrow) and swelling of the proximal POL (arrowheads).
Alternatively, unrecognized damage to the supporting structures of the posteromedial corner in a patient with a ruptured ACL may account for variable postoperative outcome after ACL reconstruction. Hence, the difference between an isolated rupture of the ACL and an injury that involves the posteromedial corner (Fig. 7) is an important distinction to make.
Conclusions Full appreciation of the functional significance of the imaging abnormalities observed is of primary importance to the radiologist in generating a pertinent, relevant and complete report of a study. The dynamic stabilizing effect of the posteromedial structures of the knee, powered by the semimembranosus muscle, plays an integral role in resisting anteromedial rotatory instability of the knee. Mu ¨ller6 eloquently summarizes this importance: ‘‘As long as the semimembranosus corner functions efficiently as a stabilizer, even a weak or damaged cruciate ligament can function in a compensated fashion. But if this stabilizing effect is lost, the anterior cruciate ligament alone is incapable of compensating and becomes increasingly insufficient.’’ Thus, the proximity of the stabilizing unit of the posteromedial corner of the knee to the tibial collateral ligament must not allow the contrasting functional significance of damage to these
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structures to become blurred. Although there is general agreement that isolated damage to the tibial collateral ligament may respond well to nonoperative management, an injury that involves the posteromedial corner warrants closer consideration of operative repair.
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