Posteromedial Corner

The posteromedial corner (PMC) of the knee has a complex anatomical structures that are often overlooked on MRI.

Posteromedial Corner: Anatomy

The PMC has five major components:³

Posterior oblique ligament (POL) (Ligament of Winslow)¹

Origin is distal and posterior to adductor tubercle of the medial femur, just posterior and proximal to the medial collateral ligament (MCL) and anterior and inferior to the medial head of the gastrocnemius
Part of layer 2 with the MCL or tibial collateral ligament (TCL) but its anterior layer blends with layer 1 or superficial layer posterior margin
Must consider the POL predominantly a layer 3 structure
Posteriorly, it blends with layer III and is inseparable from it
Three arms: Capsular, Central (tibial) - most important, and distal (superficial)

Oblique popliteal ligament (OPL)¹

The superior or capsular arm of the POL is continuous with the posterior joint capsule and the OPL, lying deep to the medial head of the gastrocnemius
Arises from: (1) Capsular arm of POL; (2) Lateral expansion of SM
Laterally, OPL attaches to bony or cartilaginous fabella; meniscofemoral lateral capsule; and plantaris muscle. So, posteromedial lateral corners are actually connected
Also has fibrous attachment to PCL lateral Facet
OPL is a component of PMC AND PLC
Indistinguishable from joint capsule

Semimembranosus tendon (SM tendon) and expansions¹

The distal or superficial arm of the POL attaches to the SM tendon sheath and inserts on the tibia just distally to the SM.

SM tendon initially splits into two of its five arms (i.e. Direct and Anterior):

Direct arm inserts into the tuberculum tendinis. It passes under anterior arm, and extends to attach to the posterior aspect of the coronary ligament of the posterior horn of the medial meniscus.
Anterior arm inserts into the medial proximal tibia, extending under the POL and attaching to the tibia. It is also called the reflected arm or pars reflexa.
Capsular arm blends with posteromedial capsule and merges with POL and OPL. OPL splits off 2cm above main two bundles, and is inseparable from capsule.
Inferior popliteal arm passes under POL, and attaches to tibia above superficial MCL.
Distal attachment of inferior arm and anterior arm are difficult to separate on MRI.

Posteromedial joint capsule (PJC) and posterior horn of medial meniscus (PHMM)¹

PJC is composed of meniscotibial and meniscofemoral components, and being reinforced by the POL, the posteromedial joint capsule extends to the medial head of the gastrocnemius
Both PJC and MM (Layer III) are reinforced externally by POL
MM is linked to SM tendon by neurophysiologic pathway via somatosensory evoked potentials

Posteromedial Corner: Biomechanics

POL should be relaxed when knee flexed and tight when extended
With complete extension tibia rotates externally to achieve a “screw home” position, tightening PMC and making pouch around femur. The popliteus reinforces the “screw home” mechanism on the lateral side
Passive support of “brake stop” function of posterior horn of MM
Active support of PMC comes from pes anserinus and tendons in flexion
Support in knee extension from vastus medialis; and SM, S-MCL and POL during active flexion – SM is main dynamic stabilizer
Passive support from “brake stop” function of posterior horn of medial meniscus
Active support from pesin flexion, vastus medialis oblique (VMO) in extension and semimembranosus/POL/MCL-S in active flexion
During contraction the SM flexes and internally rotates tibia and is a valgus restraint in extension and to external rotation in flexion
By tractioning posterior horn, SM protects it during flexion and protection is maximal at 90 degrees of flexion
Works synergistically with postlat corner to stabilize in ext. int. rotation
POL is a restraint for valgus, ant-post translation, int-ext rotation³

Posteromedial Corner: Injuries

Key injury in PMC is anteromedial rotational instability (AMRI):

Anteromedial rotatory instability (AMRI) is defined as excessive anterior tibial translation during external rotation of medial tibial plateau with respect to the femur, accompanied by medial joint opening valgus³

Posteromedial Corner: Treatment

Isolated PMC injuries can be treated conservatively.

Symptomatic AMRI in patients with PMC injuries is an indication requiring surgical treatment.¹

Posteromedial Corner: References

Lundquist RB, et al. RadioGraphics, Posteromedial Corner of the Knee: The Neglected Corner, 35:1123-1137 (2015).
Pomeranz SJ., Gamuts & Pearls in MRI & Orthopedics. Ohio, The Merten Company, 1997.
Pomeranz SJ., Posteromedial Corner of the Knee PPT

Posteromedial Corner: Example

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HISTORY:

37-year-old male. Effusion, instability, injury.

Image 1 - Sagittal PD SPIR Image 2 - Sagittal PD SPIR Image 3 - Sagittal PD SPIR

Image 1 - Inferolateral meniscal tear (green arrow), popliteus muscle strain (yellow arrow)

Image 2 - Arcuate ligament sprain (red arrow)

Image 3 - Inferolateral meniscal tear (green arrow), semimembranosus tendon fascicle rupture

FINDINGS:

A radial tear is demonstrated anterior horn of the medial meniscus. A vertical longitudinal tear is demonstrated in the outer one-third of the posterior horn. Inner edge fraying is seen in the body. No chondromalacia is demonstrated in the medial compartment. There is an impaction-type fracture in the posterior edge of the medial tibial plateau with approximately 4 mm depression. ACL rupture is seen. The PCL is buckled. Anterior tibial translation is associated.

There is an oblique-type delicate tear demonstrated in the posterior body-horn junction of the medial meniscus measuring approximately 1cm and involving the outer one-third. Swollen meniscus is demonstrated in the posterior body and posterior body-horn junction. There is bony contusion and microtrabecular injuries seen in the sulcus terminalis with mild depression. The lateral tibial plateau also demonstrates bony contusion without fractures. No chondromalacia is demonstrated in the lateral compartment.

The MCL is lax and inflamed, more conspicuous in the posterolateral corner structures. Insertional low grade partial tear of the LCL-biceps femoris complex. Popliteofibular ligament, grade 1 popliteus muscle strain, inferior meniscopopliteal fascicle tear and arcuate sprain. No findings of iliotibial band friction syndrome.

The patellofemoral cartilage is well preserved. Medial and lateral retinacula are intact as are quadriceps and patellar tendons. Extensive subcutaneous tissue swelling is demonstrated, more conspicuous anteromedially. Posteromedial corner tear and virtual rupture involves several fascicles of the semimembranosus tendon including oblique posterior component adjacent to fracture. There is moderate to severe sprain of the pes anserine distally and insertionally.

CONCLUSION:

Complex pivot shift injury consists of ACL rupture with anterior tibial translation, grade 1-2 MCL sprain, distal low grade partial tear of the LCL-biceps femoris tendon complex.
Posterolateral corner tear includes popliteofibular ligament, grade 1 popliteus muscle strain, inferior meniscopopliteal fascicle tear and arcuate sprain.
A radial tear in the anterior horn and a vertical tear in the posterior horn red-red zone of the medial meniscus and an oblique peripheral tear at the posterior body-horn junction of the lateral meniscus.
An impaction type fracture in the posteromedial tibial plateau with 4 mm depression, contusions and microtrabecular injuries in the sulcus terminalis and posterolateral tibial plateau, moderate posteromedial corner injury and partial tear, moderate to severe sprain of the distal, insertional pes anserine, moderate amount of hemarthrosis.
Posteromedial corner tear and virtual rupture involves several fascicles of the semimembranosus tendon including oblique posterior component adjacent to fracture.