Articular surface of femoral condyle includes trochlea (articulates with patella), anterior/distal, mid and postrior surfaces.
Determination of side:
- Patella lateral facet is longer.
- Popliteal fossa contains only biceps femoris laterally but 4 tendons/muscles medially (semimembranosis, semitendinosis, sartorius, gracilis).
- Greater saphenous vein medially.
- Fibula laterally.
- Lateral femoral condyle relatively flat along distal/anterior weightbearing surface, with a concave condylar sulcusanteriorly. Medial femoral condyle has straight trochlea.
- Lateral tibial palteau tends to be flatter. Medial tibial plateau is larger.
- Medial meniscus posterior>anterior horn, where lateral meniscus horns are equal in size.
Articular contact is smaller with thinner cartilage at flexion compared to extension.
Capsule extends several cm above upper patella.
Haemarthrosis from fracture, bony contusion or ACL tear. Lipohaemarthrosis from fracture or large bony contusion.
Extensor mechanism – Patellar tendon, patella and quadriceps tendon. Quadriceps aponeurosis has 3 layers: superficial rectus femoris; middle vastus lateralis and medialis; and deep vastus intermedius. Fascial extensions form extensor retinaculum hood over anterior knee.
Hoffa’s fat pad is between lower pole patellar/patellar tendon and knee joint.
Femoral trochlea/groove – Anterior, between condyles where patella glides.
Patellar has three facets: lateral (widest), medial and odd (most medial, often sagittal); with median ridge between lateral and med.
Imaging Techniques and Applications
- Cross-table lateral
- Flexed weight bearing – Cartilage loss accentuated esp if slightly flexed, as tends to be more severe posteriorly.
- Shallow oblique – ~5deg valgus tends to accentuate fractures.
- Notch/tunnel view – AP parallel to tibial plateau with 45deg flexion. For calcified intra-articular bodies (usually settle in notch).
- Patellofemoral skyline/sunrise view – Axial tangential to patellofemoral articulation, flexed 20-25deg (Merchant technique). For anterior knee pain.
- Intercondylar view (Rosenberg view)
MRI knee internal derangement – PD, PDFS, IA Gad. For infection/neoplasm – T1, T2FS, IV Gad. Knee coil, externallly rotated 5-10deg (for ACL to be in plane of imaging)
- PD – Anatomy, meniscal pathology.
- PDFS, T2FS – Pathology.
- T1 – Anatomy, fat, blood, meniscal pathology.
- GRE – Blood products.
- STIR – Better for metallic artefact.
Contusion/bone bruising is microfractures from trauma, subarticular heterogeneous low T1, high T2. Resolves in 6-8 weeks. May lead to osteochondritis dissecans if not treated with reduced weight bearing. Bone bruising patterns with major soft tissue injury types include:
- Pivot shift – Valgus stress in flexion with external rotation eg skiers. Bruising posterolateral tibial plateau, mid lateral femoral condyle. Causes ACL rupture, posterior horn LM/MM tear, MCL tear. Posterolateral tibial plateau almost >90% associated with ACL tear.
- Dashboard – Force to anterior tibia in flexion eg MVA, fall. Bruising anterior tibia and posterior patella. Causes PCL tear.
- Hyperextension – Force to anterior tibia in extension eg forceful kicking, pedistrian vs bumper. Kissing contusions to anterior femurs and tibial plateaus. May cause ACL/PCL/menical tears, knee dislocation.
- Clip injury – Pure valgus stress in mild flexion eg football. Oedema in lateral femoral condyle, occasionally medial femoral condyle (MCL avulsion). Causes MCL tear, O’Donoghue’s terrible triad (ACL, MM, MCL).
- Lateral patellar dislocation – Twisting in flexion (quadriceps contracted). Brusing to medial patella and lateral femoral condyle.
Fractures and Avulsions
- Transcondylar/intercondylar – Involves metaphysis (usually transverse) and condyles (sagittal or coronal). Coronal intra-articular fractures are at risk of AVN and joint incongruity.
- Tibial plateau – Difficult to exclude on XR, may need CT or MRI. 80% involve the lateral tibial plateau, from valgus load. Typically large haemarthrosis ± lipohaemarthrosis. Usually requires CT for planning.
- Schatzker type I (split no/minimally displaced) – Cleavage of lateral tibial plateau with <4mm depression.
- Schatzker type II (split compression) – Cleavage and deppresion of lateral tibial plateau.
- Schatzker type III (local compression) – Pure depression of lateral tibial plateau.
- Schatzker type IV (medial) – Involvement of medial tibial plateau. Worst prognosis.
- Schatzker type V (bicondylar) – Both medial and lateral tibial plateaus, depression usually involves lateral. May be inverted-Y originating from intercondylar region.
- Schatzker type VI – Bicondylar with distal oblique shaft fracture.
- Patellar fracture – DDx bipartite or multipartite normal variants with well-corticated sup-lat fragment(s), often bilateral with dorsal defect at superolateral articular margin.
Avulsion fractures include:
- ACL from medial tibial eminence.
- PCL from posterior tibia.
- Segond fracture – Lateral capsular ligament at lateral margin of tibia, almost 100% associated with ACL tear (otherwise ACL avulsion).
- Reverse Segond fracture – Deep medial collateral ligmant at proximal medial tibia. Associated with PCL tear.
- Iliotibial band from anterolateral tibial margin (Gerdy’s tubercle).
- Lateral collateral ligament (and biceps femoris) from proximal fibula.
- Medial collateral ligament from medial femoral condyle and tibial condyle (deep fibre) or medial diametaphysis (superficial fibre).
- Patella tendon from tibial tubercle, esp children and adolescents. In older child tibial tubercle ossification centre fuses with tibial epiphysis, so traction may cause Salter-Harris I or III.
- Osgood-Schlatter disease – Repetitive stress with fragmented tubercle associated with tenderness and swelling, rarely Cx by nonuion or premature tubicle physis closure with genu recurvatum (posterior bowing of knee); may be left with ossicles and posterior distal patellar tendon.
- Sinding-Larsen-Johansson syndrome – Similar process at lower pole of patella.
- Medial retinaculum fracture from patella in transient lateral dislocation.
Salter-Harris fracture uncommon, high Cx rate with growth disturbance.
30% associated with arterial injury (fixed in adductor canal and bifurcation near tibiofibular syndesmosis). Common cruciate or collateral ligament tear, joint capsule tear, articular cartilage tear, peroneal nerve injury (runs posterior to fibular head).
Patellar Dislocation and Tracking Abnormalities
Almost always transient lateral (weaker medial patellar retinaculum and vastus medialis). Relocation impacts medial pole patella and anterolateral lateral femoral condyle with bone bruise ± osteochondral fracture (fragment adjacent to lateral femoral condyle or in situ). Stretched or torn medial patellar retinaculum, usually patellofemoral ligament at femoral or patellar attachments. Associated haemarthrosis. Residual lateral patellar tilt or subluxation.
Tracking abnormalities through trochlear groove are usually lateral tilt and subluxation, causing anterior knee pain and premature OA, increased risk of dislocation. May be evaluated with low-dose axial CT in different degress of flexion, most occur in only shallow flexion. From muscle imbalance with relatively weak distal vastus medialis, anatomic variations including:
- Patella alta – Elongated patellar tendon with patellar tendon:patella length >1.35.
- Patella baja – Short tendon (scar retraction) with ratio <0.9.
- Trochlear dysplasia – Shallow trochlear groove. Sulcus angle from highest points of femoral trochlear to deepest groove on axial view, normal 138deg
Lateral tilt – On Merchant axial view. Lateral patellar/trochelar distance is narrower laterally than medially. Congruence angle >0-16deg (groove to posterior apex patellar should be lateral to anterior apex).
Usually multiple with numerous transverse partial fractures at various stages of healing. Imaging lags several weeks behind Sx. Early faint transverse/oblique lucency then sclerosis. Most in posterior proximal tibial shaft. Shin splints longitudinal oedema with uptake in posterolateral tibia.
(Avulsive cortical irregularity). Iregularity and excavation or posterior medial distal femoral epiphysis at site of adductor magnus insertion or medial head gastrocnemius origin. Can also occur at lateral head of gastrocnemius origin. Common in older children, usually resolves by adulthood. Parosteal high T2 fibrocartilaginous tissue, elevated periosteum. Variable marrow oedema.
Spontaneous Osteonecrosis of the Knee (SONK)
(Subchondral insufficiency fracture, subchondral impaction fracture SIF). Rapid, painful subchondral collapse of anterior weigthbearing femoral condyle, 90% medial femoral condyle. Almost always associated with medial meniscal tear (esp posterior root attachment), middle/older age (esp perimenopausal middle-aged females). Subchondral insufficiency fracture (ie OCD) from minor trauma/intervention or idiopathic leads to osteonecrosis (ie AVN is not the primary process). Subchondral marrow oedema (profound) -> subchondral fracture -> after 2-3/12 painful subchondral collapse, OA. Affects one side of the joint. Tx strict non-weight bearing, calcitonin, bisphosphonates.
Torsion of the Femur and Tibia
Torsion is rotation around long axis. Usually resolves during growth.
- Femoral head antetorsion/anteversion if femoral head is anterior to plane of condyles; retrotorsion/retroversion is posterior. Normal 30deg anterversion at birth, 16deg by 16yo, 10deg by adulthood. Excessive anteversion in Perthes disease, neurologic and neuromuscular diseases. Tx femoral osteotomy. Best assessed by CT with a few low-dose slices through femoral neck and condyles.
- Medial/internal tibial torsion (pigeon-toed) associated with foot deformities, genu varum. Lateral/external tibial torsion (penguin-footed). Easily assessed with XR, but CT is more precise.
Cruciate ligaments are intracapsular, extrasynovial. Tears cause swelling, increased signal, fibre discontinuity and change in expected ligament course.
Anterior Cruciate Ligament (ACL)
Linear in intercondylar notch from medial lateral femoral condyle to anterior medial tibial spine. Runs along, but slightly more vertical than Blumenstaats line (roof of intercondylar notch on lateral). Often has linear striations/fan like near insertion. Oval, wider AP than med-lat. Dark peripherally, may have signal centrally. Tear (usually full-thickness) from clipping injury (forced valgus), or pivot-shift injury (femur laterally rotates on planted leg). Best seen on T2 or PDFS. Rupture more common than avulsion (usually distal at medial eminence); rupture more common at femoral origin. Tear may cause nonvisualisation of the ligmament or visualisation of the actual disruption, fibres more horizontal than Blumenstaats line. Partial tears/sprains cause high signal in otherwise intact ligament. Secondary signs include kissing subchondral bone bruises (one/both posterior tibial plateaus esp posterolateral and femoral condyles) ± notch sign (subchondral fracture with indentation >2mm), anterior subluxation of tibia (anterior drawer/translocation sign; posterior tibia >7mm anterior to posterior lateral femoral condyle), haemarthrosis (75% due to ACL), hyperangulated PCL, posterior subluxation of posterior horn lateral meniscus relative to tibial condyle. Associated with MCL injury and medial meniscal tear (O’Donoghue’s terrible triad: MM ACL and MCL), lateral meniscal tear (esp post; 2x more common than MM tear), Segond fracture (lateral capsular ligament), posterolateral corner injury, avulsion of Gerdy’s tubercle (iliotibial band).
Tendon autograft repair with patellar bone-tendon-bone secured within posterolateral femoral and anteromedial tibial tunnels. Tibial tunnel should be posterior to Blumenstaats line when knee is extended. Cx limited terminal extension (if tibial tunnel too anterior), bone plug migration, graft rupture, fibrous proliferation anterior to graft (cyclops lesion, interfering with terminal extension).
Posterior Cruciate Ligament (PCL)
Homogeneous low signal, rounded. Only taut in flexion, hence gentle curve in extension. Tear (usually partial thickness) is less common than ACL due to large size and strength, usually associated with severe injury eg posterior dislocation or dashboard injury. Associated with reverse Segond fracture (MCL tibial attachment, rare), posterolateral corner injury (in 50%), anterior bone bruising, posterior tibial displacement. Tear causes diffuse intermediate signal throughout, sharp bowing (PCL tear or ACL rupture). Avulsions rare. Is not usually repaired (rarely causes instability).
Medial collateral ligament (MCL) has three layers including superficial fascia, superficial (‘true’) MCL (just distal to adductor tubercle to medial tibial tubercle 50mm below joint line) and deep MCL (thickening of joint capsule adjacent to meniscus, inserting at joint margins). Injury is usually proximal, associated with ACL and meniscal tears. Grade 1 injury mild sprain (fluid/haemorrhage in tissues medially); grade 2 partial tear (high signal in and around MCL with minimal/no fibre disruption), grade 3 complete disruption. Best seen on T2 or T2*. Pellegreini-Stieda lesion – post-traumatic calcification around MCL femoral origin.
Lateral ligament complex consists of posterior biceps femoris tendon, true lateral/fibular collateral ligament (LCL, from lateral femoral condyle) and anterior iliotibial band (blends into lateral retinaculum on patella). Biceps femoris tendon and true LCL usually conjoin and insert onto head of fibula. Tear less common than MCL, rarely isolated, associated with posterolateral corner instability, peroneal nerve injury.
Posterolateral Corner Injury
Posterolateral corner structures include LCL, lateral capsule, popliteus tendon, arcuate ligament, fabellofibular ligament, ligament of Winslow. Injuries also include Segond fracture, avulsion of fibular head. Injury implies ACL tear. At risk of delayed instability if not repaired.
Most common injuries include quadriceps and patellar tendons, pes anserinus (sartorius, gracilis, semitendinosis insertion at medial proximal tibia) and popliteus musculotendinous junction.
Quadriceps tendinopathy and rupture
Distal quadriceps tendon tear common, esp RA on corticosteroids.
Overuse injury from repetitive jumping (eg basketball). Increased size and T2 signal in proximal patella tendon esp posterior midline ± patella marrow oedema. Patellar tendon tends to repair with irregular ossicles.
Fibrocartilaginous with circumferential horizontal collagen fibres and radial fibres extending from periphery to free margin, low T1 and T2. Medial meniscus larger and more C-shaped with posterior>anterior horn; lateral meniscus is circular with horns more closely apposed and equal in size creating a sagittal bowtie configuration. Taper from peripheral 3-5mm to thin sharp central free margin. T1FS (any signal within is abnormal except vascularity in children) or PD most sensitive; T2 less sensitive but highly specific. Medial meniscus firmly attached to joint capsule (hence greater risk of tear), anterior and posterior tibial insertions, may have recess on inner aspect of capsule attachment. Lateral meniscus is less intimately attached to the capsule (to allow passage of popliteus from lateral femoral condyle to tibial metaphysis). Meniscocapsular junctions are fibrofatty with high signal.
Ligaments are variable, may mimic tears with intervening joint fluid.
- Meniscofemoral ligament (variable) from posterior horn lateral meniscus to medial femoral condyle (of Humphrey if anterior to PCL, of Wrisberg if posterior).
- Oblique intermeniscal ligaments (variable) between posterior horns.
- Meniscopopliteal ligaments (variable) between popliteaus tendon and lateral meniscus, not well seen on MRI.
- Transverse ligament (variable, pseudotear) from anterior horns of mensici through Hoffa fat pad.
- Popliteal tendon laterally with popliteal hiatus (fluid) between superior and inferior fascicles.
‘Speckled’ appearance of anterior horn lateral meniscus is a normal variant.
A small meniscus may be due to displaced fragment/tear, post-surgical or hypoplastic meniscus.
Meniscal flounce – Normal variant wavy appearance, varies with flexion. No clinical significance.
Intrasubstance myxoid degeneration likely from aging/wear-and-tear, asymptomatic, not seen on arthroscopy. Signal that doesn’t disrupt an articular surface (grade 1 or 2).
Meniscal signal grading (for virgin menisci) on short TE sequences (PD, T1 or GRE); T2 senquences have increased specificity, but are not sensitive. On ultrasound normal meniscus is echogenic, with tear hypoechoic.
- Grade 1 – Rounded/amorphous not disrupting articular surface, of no clinical significance.
- Grade 2 – Linear not disrupting articular surface, 10-20% associated with meniscal tear. Linear transverse intermediate signal in peripheral third extending to peripheral margin is probably a vessel in children and young adults.
- Grade 3 – Signal disrupting superior/femoral or inferior/tibial/undersurface articular surface on at least 2 adjacent images. >90% sensitivity and specificity for tear. Meniscal expulsion when peripherally displaced past bony margin. Commonly associated with perimeniscal cyst. A surgically repaired meniscus has persistent grade 3 signal; unable to destinguish from tear, unless there is fluid on T2, filling on arthrogrpahy, or new signal compared to previous post-op imaging.
Most commonly oblique from inferior posterior horn medial meniscus. Sensitivity reduces with torn ACL, usually in peripheral and posterior horn lateral meniscus.
- Complex (most common) – Multiple orientations.
- Vertical-radial – From periphery to free margin.
- Horizontal cleavage – Splitting into superior and inferior portions. Associated with underlying meniscal degeneration.
- Vertical-longitudinal – Circumferential, may be peripheral or involve free margin (parrot-beak tear).
- Bucket-Handle tear – Extensive vertical-longitudinal tear with displacement of inner free edge (causing blunted free edge remaining in-situ meniscus), usually into intercondylar notch (double PCL sign with a 3rd structure in the notch). Medial meniscus in 80%. On 4-5mm thick sagittals only one image is seen with normal bowtie apperance (usually see 2 contiguous slices as normal meniscus is 9-12mm wide).
Unknown aetiology, 5% of population, may cause pain without tear, more common in discoid menisci, medial > lateral esp posterior horn MM or anterior horn LM. Intrameniscal cyst usually not as T2 bright as fluid. Enlarges the meniscus. Tx without tear is percutaneous decompression and packing; with tear is intra-articular approach.
Parameniscal cyst – Peripheral margin of meniscus (esp anterolateral lateral meniscus), occasionally intrameniscal; from decompression into soft tissues (cf tear which must disrupt the articular surface). High association with meniscal tear esp horizontal. DDx ganglion cysts or synovial cysts.
Large meniscus which may be lens-shaped, wedged, flat or other; ?acquired or congenital in 3% of population, increased risk in Japanese and Korean. Meniscus extending centrally with >14mm width at body, more than 2 bowties on sagittal (on 5mm thick slices). Increased joint space. More commonly lateral meniscus. Can cause locking, joint line pain, prone to tear. Rule of signal contacting articular surface does not apply for discoid meniscus, with any abnormal signcal indicating a tear. Meniscus may extend into tibial spines at intercondylar notch.
Ossification within a meniscus, typically posterior horn medial meniscus. Developmental or post-traumatic. Associated with meniscal tears. Follows bone marrow signal on MRI.
Other Meniscal Injuries
- Meniscocapsular separation – Separation of meniscus from capsule with T2 signal between (normal in posterior body and horn of lateral meniscus). Usually at site of MCL and concomitantly with MCL injury.
- Disruption of meniscal fixation – Menisci should appose tibial plateau cartilage, apart from free edge (meniscal flounce). Peripheral displacement >/=3mm suggests radial tear or osteophytes.
- Meniscal bruise – Amorphous signal usually in younger patients with lax ligaments. Usually resolves spontaneously.
- Meniscal root avulsion – Disruption of insertion of posterior horn medial meniscus to tibia.
Defects most common at medial femoral condyle and patellar. Injuries range from chondral to osteochondral, associated with meniscal tears.
Dorsal defect of the patella – Normal variant lytic defect upper outer quadrant, mimicing infection/osteochondritis dessicans/chondroblastoma. Asymptomatic.
Common, degeneration of patella cartilage causing pain. Focal swelling and degeneration causes low/high signal foci -> thinning and irregularity of articular surface of cartilage -> exposure of underlying bone.
Inflammation/haemorrhage in Hoffa’s fat pad, from trauma or impingement. May progress to chronic fibrosis, hyalinisation.
Adventitial bursae develop after chronic irritation over a bony prominence, lacks epithelial lining.
- Gastrocnemius-semimembranosus (Baker/popliteal cyst) – Extends between and posterior to lateral head gastrocnemius and semimembranosus tendons. Most from chronic knee effusion, may persist after effusion resolves. May become multiloculated and large, contain osteochondral bodies or other debris.
- Pes anserinus bursitis – Uncommon. Bursa beneath the fan shaped sartorius, gracilis and semitendinosus tendons inserting into the anteromedial tibia.
- Semimembranosis tibial collateral ligament bursitis – Common, at medial joint line, comma shaped draping over semimembranosus tendon adj to posterior horn medial meniscus.
- Prepatellar bursitis
- Superficial and deep infrapatellar bursitis
- Iliotibial band adventitial bursitis – Signal in fat deep to distal ilitibial band.
Common around knee. May develop from meniscal tears (meniscal cyst), or in tendons (ACL or PCL).
Variable synovial infoldings of knee joint capsule, embryological remnant from division of into 3 compartments.
- Mediopatellar (>50% of pop) – thin coronal band from medial capsule to medial facet of patella), suprapatellar or infrapatella. Plica syndrome – thickens with effusion, ant-med pain, snapping; Tx arthroscopic removal.
- Suprapatellar – oblique axial. Rarely entraps synovium or debris in superior joint space.
- Infrapatellar (ligamentum mucosum) – sag through mid/lower Hoffa’s fat pad to intercondylar notch/ACL.
Peroneal nerve courses around fibular neck, prone to injury causing foot drop, denervation of lateral leg muscles. May be compressued/entraped by adjacent mass (osteochondroma, tibiofibular joint ganglion).
Popliteal Artery Entrapment Syndrome
Anomalous course of popliteal artery or proximal gastrocnemius causing calf claudication (when standing or exercise) in 20s-30s.
- Tibial component 90 ± 5 deg to long axis of tibial shaft, may have slight posterior tilt.
- Femoral component 5 ± 5 deg to femoral shaft with 4-7deg valgus angulation.
Complications similar to hip arthroplasties
- Loosening – Most of tibial component with varus tilt and subsidence into medial plateau.
- Patellar subsidence, polyethylene wear, polyethylene dissociation from metal backing, disintegration of metal backing with metallosis (metal particles lining polyethylene and capsule), patellar AVN, fracture.
- Periprosthetic fracture esp metaphysis of femur or tibia as minor contour change or sclerotic line.