July 2012 - Donna Magid, M.D., M.Ed.
Largest articular surface area
Hinge joint with 3 compartments
- Major weight-bearing, long lever-armed fulcrum
- Stabilized by ligaments, capsule, connective tissue; not by bony interlock
15-30 degree flexion or cross-table (trauma pt: will be extended on board)
Quadriceps tendon→tendon sheath investing anterior/posterior patella (looks like inverted Y just proximal to superior pole patella→inferior patella tendon—tibial tuberosity
- Enlarges suprapatellar ‘bursa’ immediately posterior to quad tendon
- Effaces fascial fat plane and/or bows quad tendon anteriorly
Inferior patellar tendon: on flexed lateral is ~1-1.5x patellar length
- >1.5x: r/o ruptured quadriceps tendon, avulsed tibial tuberosity (patella alta)
- Fat pad posterior to inferior patellar tendon radiolucent even with large effusion (solid)
Tibial tuberosity: check for soft tissue anterior to bone (acute or chronic trauma; ‘jumper’s knee’
- “Fragmented”: smooth rounded margins may be one of many normal Variants (check Keats); rough or irregular margins, esp. with soft tissue changes, may suggest trauma—examine pt.
Medial femoral condyle larger (projects distal and posterior IF good lateral); lateral condyle is flatter.
Articular surfaces condyles, patella, tibial plateau: congruent, continuous, ‘flowing’
Popliteal fossa: fabella (normal variant sesamoid lateral head gastrocnemius, need NOT report); vascular calcifications; loose bodies
Trabecular patterns: finely etched, regular
Medial and lateral compartments: narrowing, surface congruity, chondrocalcinosis
Patella (if not on yr check list you will forget to look): integrity, normal-variant 2ndary ossification centers
Trabecular pattern - cancellous bone: condyles, PLATEAU: ‘hand-drawn graph paper”)
Tibial plateau integrity, contour
STANDING AP (more common in Arthritis Clinic than EMed) allows measurement of anatomic axis (6-8 degrees valgus shaft-shaft = normal). OA narrows medial compartment before lateral, reducing valgus.
NOTCH (flexed AP,non-wt.-bearing) better shows loose bodies, tibial spines, marginal ostephystes
FLEXED (Weight-bearing) best view to assess arthritis, marginal osteophytes, tibial eminences (spines)
“SUNRISE” or AXIAL (may be 30-60-90 degrees) for patella femoral surfaces, cartilage, relationships
OBLIQUES: check cortical margins, contours, superimposed structures; patella projects off femur.
Figure 1. Knee Anatomy and Fracture Patterns
- Salter injuries: femur > tibia; consequence of physeal disruption can be severe (knee → 70% of LE length)
- Patellar fx: direct blow
- Intraarticular fx: such as anterior spine avulsion via ACL—rare, now increasing in teen athletes
ADOLESCENT, YOUNG ADULT: Meniscal, ligamentous injuries (twists, falls)> intraartic. Fxs.
- Intraarticular fxs usually due to high-impact (MVC, pedestrian vs car, skateboard tricks)
- Tuberosity trauma: from inferior patellar tendon; chronic (‘jumper’s knee”, basketball) or acute
- JUMPER’S KNEE: traumatic tibial apophysitis; male 10-15, repetitive stress → pre-tibial swelling and point-tenderness (compare to contralateral lateral and to Normal Variants text). Normal variant fragmented tuberosity is smooth, margins rounded, fascial planes preserved both at pre-tibial soft tissue and between inferior patellar tendon/anterior fat pad joint .
- True FX Tibial Tubercle: acute trauma, also in jumpers but abrupt, and pts older adolescents - apophyses starting to fuse. Sharper margins, cortical gap. Fragment is under tension (from quadriceps) - needs ORIF. Examine pts!
ADULT: tibial plateau, intercondylar femur, supracondylar (MVC); tibial spines
- ‘Ped-X’: bumper gets plateau (‘scoop-and-toss’ onto car hood also →s C spine, head injuries)
- PATELLAR FRACTURE: Direct blow (dashboard, fall on flexed knee): stellate, vertical, may be stable. Avulsion or transverse: quadriceps pull distracts; >3mm gap→ surgical fixation
- PATELLAR DISLOCATION: virtually always lateral, often transient (often teenaged female). “Knee gives out on steps”. Look for transchondral fxs articular (dorsal) surface from impaction; soft tissue swelling. MR best (non-urgent) assessment—classical complex of findings acute or chronic
- OSTEOCHONDRAL FXS: tangential, sheer, rotatory forces. May not hurt or be recalled; subchondral bone relatively insensitive. Intermittent pain, locking, ‘crunch’ as fragments migrate between weight-bearing surfaces. Fragments of bone/cartilage can undergo appositional bone growth in joint fluid→ ‘joint mice’.
FIBULA: non-weight-bearing, need most proximal and distal for articulations, diaphysis largely expendable (used for bone grafts). FX usually from direct blow (is peroneal or fibular nerve intact?) or from force transmitted cephalad from ankle along intraosseous membrane (Maisonneuve fx.: external rotation at ankle→ medial malleolus fx or deltoid failure, force travels cephalad through intraosseous membrane. Exits and fractures fibular neck just distal to knee. Therefore with many ANKLE fxs, touch or check the KNEE too.
FABELLA: sesamoid bone in tendinous lateral gastrocnemius posterior to lateral femoral condyle. 11-13% of population has, DO NOT report. Often bilateral. Can change with OA.
TIBIAL PLATEAU FX: can be radiographically subtle; follow articular surface contours, trabecular pattern.
- F>50 yo, M 30-70 y.o.; twisting fall most common but also direct blow, osteoporotic stress, transmitted axial load in MVC (ie, extended knee braking at impact); 25% = Ped-X, bumper injury
- Joint effusion may be hemarthrosis or lipohemarthrosis. Associated injuries (esp. MCL)- may need MR. Degree of depression, size of fragments, for pre-op planning often mandates CT.
- Compression/impaction of femoral condyles into plateau compresses cancellous bone, smudges or disrupts ‘hand-drawn graph paper’ of cancellous subchondral plateau.
- Can combine split (longitudinal fx, compromises articular support) and impaction/depression.
- Valgus-and-compression makes lateral fx most common (60% lat, 10-15 both, 5-10% medial plateau).
ANTERIOR TIBIAL SPINE (Eminences) Avulsions
- ACL insertion point; radiographic evisience of fx/avulsion may mandate MR
- Hyperextension knee with internal rotation tibia; poin tender just inferior to inferior pole patella
- Children, adolescent: 50%= fall from bicycle; ADULT more likely to have ligamentous injury and need MR
OSTEOCHONDRITIS DISSECANS (child, OCD); TRANSCHONDRAL TRAUMA
- Small scalloped articular defects condyles best seen on flexion/notch AP, fine sclerotic base; may have residual fragment retained in saucerized defect
- Chronic, repetitive, or old trauma; impact or loading of surface → microfractures, ‘chip’ off surface (do not CALL it a ‘chip’!). Fragment can resorb, stay in defect, or live free in joint as loose body.
- M>F, 2d decade, 10% bilateral
- Common sites: Medial margin LATERAL CONDYLE, lateral margin MEDIAL CONDYLE, and DORSAL PATELLA (r/o nl-variant dorsal defect patella—see Keats)
Knee is most common site of degenerative change. May see cartilage loss, effusions without trauma, sclerosis or eburnation of articular surfaces, subchondral cysts (although < hip OA), osteophytes
Predisposing factors: Previous trauma or surgery (AVN, OCD, cartilage damage, noncongruence); angular deformity/deviation (modified weight-bearing, limp, leg length discrepancy); obesity; overuse (sports and stress injuries, weekend warriors, overtraining, insufficiency injuries)
3 COMPARTMENTS: medial and lateral femoral-tibial, patella-femoral
Routine exam for suspected OA: weight-bearing (WB) standing AP, WB flexion AP, lateral
WB AP more accurate cartilage assessment (early narrowing usually medial>lateral since normal stance delivers more load to medial plateau. As medial cartilage narrows and valgus decreases, mechanical factors accelerate medial stresses)
- True position of tibia under femur: subluxes laterally as OA progresses
- True picture of decreasing valgus, anatomic axis
WB Flexed or Tunnel view: best view for marginal osteophytes femoral notch, tibial spines.
Lateral: for suprapatellar effusion, osteophytes, eburnation, patellofemoral arthritis.
Sunrise views may also be obtained for patellofemoral assessment.
July 2012 - Donna Magid, M.D., M.Ed.