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Post - Op Knee 1

Anatomy

Blumensaat's Line

a.k.a. Intercondylar line
is the line drawn along the roof of Intercondylar notch of femur on a sagittal view of the knee.

Not always straight ~ 2/3 have a hill along it.

Used for:
1) ACL injury: Blumensaat Angle ( with the ACL ) is <= 15 degrees.
2) Lateral femoral condyle sulcus ( should be within 10mm )
3) Post-op ACL reconstruction = see below.

Meniscal Repair.

Often persistent knee pain or clicking etc
can be due to chondral lesion, intra-articular bodies or subchondral insuff fracture etc

Normal Post-op Appearances

Usually menical repairs involve combinations of:
1) Removal of torn (avascular) parts of inner part.
2) Repair with sutures or bioabsorbable arrows in peripheral part.

Peripheral Zone

Normal is to see sutures up to about 1yr.
Can have intermediate signal upto surface up to 1 yr post-op which is due to granulation tissue.
Post 1yr then this part may return to normal signal.

Central Zone

Post partial meniscectomy then inner border / free edge will have a sharp, truncated and blunted appearance.
Should be no residual signal reaching surface in this part.

Post-op Re-Tear.

Always tricky
T1 and PD high signal is often granulation tissue thus is indeterminate.
Re-Tear only diagnosable if:
1) Definite surface signal on T2.
2) Displaced meniscal fragment.
3) Tear at a new site.

Repair of repeated tear utility is uncertain.

~40% prior meniscal surgery knees have associated articular chondral abnormalities.
And look for intra-articular bodies, synovitis, bone marrow oedema in femoral condyles or tibial plateaus.

ACL Repair

Usually use Bone-Patellar Tendon-Bone from mid section or Hamstring Grafts from distal semitendinosus and gracilis tendons and need a fixation device at either end.

Representation often due to pain, instability or new ACL injury.

ACL Graft Assessment on MRI

Always assess:
1) POSITION
Tibial Tunnel should be just behind Blumensaat's line.
Too Posterior -> Instability.
Too Anterior -> "Roof impingement" of graft on the intercondylar roof. Rad: Degeneration ( thick and high signal in graft ) & bulge posteriorly of the ACL.

2) SIGNAL & APPEARANCE
i) < 12-18 months post op: Can have intermediate signal (revascularization) especially prominent with Hamstring. but never fluid signal intensity.
ii) >12-18 months post op: Should be low on T1 & T2.
If not, consider impingement or disruption.

Non-ACL Graft Assessment in post ACL repair knee on MRI

1) Arthrofibrosis
~ intra-articular scarring. Can be focal or diffuse.
'Cyclops' Lesion = focal nodular fibrosis in intercondylar notch anterior to graft. Rad: Mixed heterogeneous lesion on T1 & T2.

2) Tunnel Cysts / Ganglion Cysts
Can form in or around the osseous tunnels
CF: Laxity
Small are ok. Large are not. Enlargement of tunnel is not. (aka "tunnel lysis" or "tunnel expansion")

  1. Hegazi TM & Wu JS. Musculoskeletal MRI. Springer 2020 ↩