''OA'' - common (10% of 55+, 25% of whom are severe), Sx fluctuate (inc with WB), stiffness, dec ROM, limp, 

''Torn / degenerate menisci'' - twisting injury, uni-compartment precise pain, ?dec ROM ? effusion, refer if true locking - ?arthroscopy 

''Ligamentous injury'' - younger sporting with trauma, popping at time of injury, swelling and unstable later, consider MRI

''patellofemoral pain syndrome'' - young female, worse on stairs / uphill, grinding joing, altered biomechanics

''History''
insidious onset or traumatic, aggravating, easing factors
direction of force
immediate Sx - popping, effusion, weight bearing
Signs - lock, unstable, loss extension
swelling heat redness
mobility

''Exam''
squat on one leg, rotate slightly with bent knee on one leg
check hip
palpate, ROM, ligaments (med, lat)
flex to 90deg (cruciate)
palpate (bursa, popliteal fossa)

''Ix''
OA - consider Xray - refer if surgery considered (BEFORE established limitation and severe pain)
else - physio, TENS. OT, analgesia, thermotherapy, steroid injection (max 3/yr)