2020 - Donna Magid, M.D., M.Ed.

Nutshell Knee

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


Lateral: Make this the starting point, always.

15-30 degree flexion or cross-table (trauma pt: will be extended on board)

Quadriceps tendontendon sheath investing anterior/posterior patella (looks like inverted Y just proximal to superior pole patella) → inferior patella tendon—> tibial tuberosity

Joint effusion: seen at suprapatellar 'bursa' on non-rotated lateral, posterior to suprapatellar (quadriceps) tendon. 

  • Normal 'bursa' more radiolucent than tendon; effusionIncreases radiodensity posterior margin suprapatellar tendon, may thicken/efface posterior twig of upside-down 'Y'
  • May enlarge or indurate suprapatellar ‘bursa’ immediately posterior to quad tendon
  • May efface fascial/fat plane and/or bow quad tendon anteriorly depending on volume/tension.

Inferior patellar tendon: on 30 degree flexed lateral is ~1-1.5x patellar length. Margins should be well-defined, parallel.  Straighter leg impairs measurements, residents therefore greatly overcall 'patella alta'. Deeper flexion also changes landmarks/measurements.

  • >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 fat)

Tibial tuberosity: check for soft tissue induration anterior to bone (acute or chronic trauma; reactive changes tuberosity bone; ie ‘jumper’s knee’ (preferred over DWG 'Osgood Schlatter")

  • “Fragmented”: smooth rounded margins may be one of many normal Variants (check Keats) or remote trauma; rough or irregular margins, esp. with soft tissue changes, may suggest recent/chronic trauma—examine pt.

Medial femoral condyle larger (projects distal and posterior IF non-rotated lateral); lateral condyle is flatter along anterior margin.

Articular surfaces condyles, patella, tibial plateau: congruent, continuous, ‘flowing’

Popliteal fossa: fabella (mature rounded triangle, normal variant sesamoid, lateral head gastrocnemius, need NOT report); vascular calcifications; loose bodies

Trabecular patterns: finely etched, regular; 'graph paper'- esp proximal tibial subchondral area and  metaphysis.    


Medial and lateral compartments: narrowing, surface congruity, chondrocalcinosis. In OA most common for medial compartment to narrow before lateral. Look for concave defects in  femoral articular surface which can herald osteochondral trauma, acute or chronic.

Patella (if not on yr check list you will forget to look): integrity, normal-variant 2ndary ossification centers (superior-lateral quadrant vast majority of time)

Trabecular pattern - cancellous bone: condyles, PLATEAU: ‘hand-drawn graph paper” of trabecular bone-- subtle focal smearing or deformity of that 'graph paper' can be subtle trauma or stress fracture. Be suspicious. 

Tibial plateau integrity, contour, regularity, of slightly concave surfaces.  Again, many easily missed subtle fractures occur here where femoral condyles can impact/overload tibial bone. 'Graph paepr' to subchondral margin.

Fibular neck- a 'miss' zone

STANDING AP (more common in Arthritis or Ortho Clinic than EMed) allows measurement of anatomic axis (6-8 degrees valgus femoral shaft:tibial 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 (PA Weight-bearing) Comment if bone-on-bone apposition either medial/lateral compartment. Best view to assess arthritis, marginal osteophytes, tibial eminences (spines). NOTE R and L may flip compared to the standing AP frontal!

“SUNRISE” or AXIAL (may be 30-60-90 degrees) for patella femoral surfaces, patello-femoral cartilage, osteochoindral defects,lateral subluxation or 'settling' (cartilage loss laterally ) 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 adult LE length)
  • Patellar fx: direct blow most common; inferior pole avulsed by inferior tendon 'sleeve'
  • 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: (avoid Dead White Guy Osgood Schlatter...) traumatic tibial apophysitis; male 10-15, repetitive stress pre-tibial swelling, blurred or distorted or asymmetric inferior tendon margins, 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, often older adolescents - apophyses starting to fuse. Sharper margins, cortical gap. Fragment is under tension (from quadriceps) - may let patella rise--needs ORIF. Examine pts!
  • Many Ortho pts. will have longitudinal tibial tuberosity osteotomies and re-positioning with screw fixation-- read records, do not call 'fx'.

ADULT: tibial plateau, intercondylar femur, supracondylar (MVC); tibial spines

  • ‘Ped-X’: "Pedestrian Struck by Vehicle"; 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 or vertical fxs 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, posterior) surface from impaction; soft tissue swelling. MR best (non-urgent) for 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’ (esp. on steps) as fragments migrate between weight-bearing surfaces. Fragments of bone/cartilage can undergo appositional bone growth in joint fluid ‘joint mice’, rounded, mature, layered.

FIBULA: non-weight-bearing, need most proximal and distal for articulations, with 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, esp if tib-fib joint widened, 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. "Hand-drawn graph paper' chnages can = subtle trabecular failure.deformity.

  • 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 while braking at impact); 25% = Ped-X, bumper injury
  • Joint effusion may be hemarthrosis or lipohemarthrosis (fat-fluid levels) . 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 mechanism makes lateral fx most common (60% lat, 10-15 both, 5-10% medial plateau).

ANTERIOR TIBIAL SPINE (Eminences) Avulsions

  • ACL insertion point; radiographic evidence of fx/avulsion may mandate MR
  • Hyperextension knee with internal rotation tibia; point tender just inferior to inferior pole patella
  • Children, adolescent: 50%= fall from bicycle; ADULT more likely to have ligamentous injury and need MR. ACL injuries have risen dramatically in teens with emphasis on earlier and more focused single-sport athletes, failure to recognize significant mechanical differences between male/female knees, and with drive for sports excellence for college applications.


  • Small scalloped articular defects condyles best seen on flexion/notch AP, fine sclerotic base if older; may have residual fragment retained in saucerized defect
  • Chronic, repetitive, or old trauma; impact or loading of surface microfractures, carved 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 Nl. Variants.)

OSTEOARTHRITIS (OA): 'wear and tear'

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); stupid shoes.

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 Weight-bearing is the position of function, therefore more accurate for cartilage and anatomic axis  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.  Many older people end up in varus, 'bow-legged'.)

  • True position of tibia under femur: subluxes laterally as OA progresses
  • True picture of decreasing valgus, anatomic axis

WB PA Flexed: Shows the femoral notch (upside-down 'U") betwen condyles. Best view for early OA marginal osteophytes distal femoral notch, or tibial spines. Remember that R and L flip compared to the AP weightbearing view. 

Lateral: for suprapatellar effusion, osteophytes, eburnation, patellofemoral arthritis. Assess adequacy/rotation. Look for upside-down 'Y" of suprapatellar tendon division to invest the patella.Look for loose bodies (vs. fabella), popliteal cysts, aneurysms 

Sunrise views may also be obtained for patellofemoral assessment. Check symmetry of patellar cartilage, medial and lateral facets; position of patella in trochlear groove;  integrity of articular surfaces. Accessory ossification center often seen on sunrise, laterally-- will have smooth margins.  ANTERIOR patella irregularity or roughness is NOT important. Asymmetry or prominence of the pre-patellar soft tissue best seen on this view, can = trauma or pre-patellar bursitis (which looks like a Derby hat, a smooth bulge of increased-radiodensity soft tissue).

REQUISITIONS are confusing and inconsistant-- some sites will present images as 'Bilateral Knees", others as 2 studies, ie "Right Knee" and 'Left Knee" (although often one of these is bilateral frontal views, and the other is a lateral of only one side), or as "Bilat Knees" and "R Knee".  Read any of these variations together-- ie DO NOT split into two reports although two paragraphs fine, eg "Right knee- no definite joint effusion. Minimal medial compartment narrowing with decreased valgus. Patello femoral joint unremarkable. (new para) Left knee limited exam, frontal only, likely early medial compartment narrowing". Or  "Bilateral knees, no left lateral or left sunrise views..."  or "Bilateral knees, limited exam left knee with only one frontal view...".   

   This is one of the most common exams in the Emergency Dept, Orthopaedics, Rheumatology, Internal Medicine, and ambulatory clinics; proper interpretation reinforces many important principles -- GO FOR IT!

2018 - Donna Magid, M.D., M.Ed.