Differentials
Common
Patellofemoral joint injuries
History
pain over the anterior aspect of knee; subluxation or dislocation episodes; anterior knee pain with squatting, sitting, or getting up from a chair; difficulty with ascending or descending stairs and with performing knee extension exercises; crepitation with active knee range of motion (ROM); may experience 'catching' or 'locking'[6]
Exam
pain on palpation of inferior pole of patella (patellar tendinopathy), superior pole of the patella, prepatellar bursae, deep infrapatellar bursa, pes anserine bursa, fibular collateral ligament (FCL)-biceps bursa, medial suprapatellar plica, retropatellar fat pad, and semimembranosus bursa; increase in lateral translation during lateral patellar apprehension test performed at 45° of knee flexion; increased medial subluxation of patella in first 0° to 40° of knee flexion;[11] retropatellar crepitation with translation of patella in trochlear groove
1st investigation
- patellar sunrise x-ray:
medial or lateral patellofemoral joint space narrowing, patellar tilt, or subluxation; a bipartite patella (present in 2% of the population, it involves the superolateral aspect of the patella) and small avulsion fractures of the medial patellofemoral ligament off the medial aspect of the patella or osteophyte formation of the patellofemoral, medial, or lateral compartment of the knee
More - lateral x-ray:
assessment of patellar height (and compared with the contralateral side) to look for patella alta or patella baja (when the patella rides too low down the femur); osteophyte formation of the patellofemoral joint
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Other investigations
- MRI:
injuries to medial patellofemoral ligament, lateral retinacular structures, and articular cartilage of the patella and trochlear groove can be assessed; the articular cartilage of the patella and trochlear groove can also be observed on the sagittal views or any tears of the quadriceps tendon or patellar tendon
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Anterior cruciate ligament (ACL) injury
History
subluxation on twisting, turning, or pivoting; some patients can feel it coming on, other patients are not able to feel it and may experience frequent falls due to their injury
Exam
increase in anterior translation during Lachman's test and the anterior drawer test compared with the contralateral knee;[12] positive anterior drawer test in external rotation; positive pivot shift test
1st investigation
- standing AP x-ray:
rules out fractures or any significant arthritis or joint subluxation
More - lateral x-ray:
potential joint space narrowing of anterior and posterior aspects of the knee, and potential anterior subluxation of the tibia on the femur (for a chronic ACL injury); assessment of potential impaction fractures of the anterior lateral femoral condyle or posterior lateral tibial plateau
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Other investigations
- arthroscopy:
direct visualisation of the ACL injury; assessment of meniscal tears
More - MRI:
ACL tear; concurrent meniscal tears or bone bruising
More - KT-1000 or KT-2000 arthrometry:
increase of anterior tibial translation >3 mm is usually indicative of an ACL tear; KT values 0-3 mm usually an indication of an intact ACL or ACL reconstruction graft
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Medial collateral ligament (MCL) injury and associated valgus instability
History
sensation of side-to-side toggle with activities; difficulty with twisting or turning; difficulty with running or pivoting; joint thrusting in patients with medial compartment arthritis or medial compartment pseudolaxity
Exam
increased valgus opening in extension and in 30° flexion; important to differentiate true joint line opening from increased motion due to joint line collapse and pseudolaxity, which can be found in patients with medial compartment arthritis
1st investigation
- standing AP x-rays:
useful to look for avulsion fractures of the superficial MCL or evidence of heterotopic ossification (Pellegrini-Stieda disease is calcification from an old previous MCL injury)
Other investigations
- valgus stress x-rays:
increased medial joint space opening to an applied load; >3.2 mm medial compartment gapping correlates with a complete superficial medial collateral ligament tear; >9.8 mm of medial gapping correlates with a capsular grade III medial knee injury[27]
More - MRI:
coronal view MRI scans are very useful to assess for both the location of an MCL tear (meniscofemoral or meniscotibial) or whether there is a partial or complete tear; can also reveal attachment avulsions or a midsubstance tear of the medial knee structures
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Posterolateral knee injury and associated varus instability
History
sensation of side-to-side toggle of knee with activity; weakness of foot and ankle that may be secondary to a concurrent common peroneal nerve neuropraxia or complete injury
Exam
increased varus opening in extension indicates combined posterolateral corner injury plus an anterior cruciate ligament and/or a posterior cruciate ligament injury; increased varus opening at 30° knee flexion indicates an isolated or combined posterolateral corner injury; positive posterolateral drawer test; 15° more external rotation compared with the contralateral knee in dial test at 30° in knee flexion; positive reverse pivot shift test; dynamic thrusting of the knee with gait
1st investigation
- standing AP x-rays:
useful to exclude avulsion fractures of the femur, tibia (Segond fractures), or fibular head (arcuate fractures)
- lateral x-ray:
useful to exclude fibular head (arcuate) fractures or tibial plateau fractures
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Other investigations
- varus stress x-rays:
increased lateral joint space opening to an applied load; >2.7 mm opening correlates with a fibular collateral ligament tear; >4.0 mm correlates with a grade III posterolateral knee injury[28]
More - MRI:
coronal, coronal oblique, and sagittal views (1.5- or 3-tesla, thin slice to include entire fibular head)[29]
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Posterior cruciate ligament (PCL) injury
History
history of hyperextension mechanism or a blow to anterior aspect of knee; difficulty descending stairs or running down hills
Exam
increase in posterior translation compared with the contralateral knee in the posterior drawer test; posterior sag of the tibia on the femur (posterior sag sign); positive quadriceps active test
1st investigation
Other investigations
- kneeling PCL stress x-rays:
amount of posterior translation difference seen between normal knees is 0-2 mm; partial posterior cruciate ligament tears have between 2-7 mm of increased posterior translation; complete PCL tears have between 8-11 mm of increased posterior translation, while combined (severe) PCL injuries have ≥12 mm increased posterior translation of the injured knee compared with the normal contralateral side, indicative of a concurrent combined posterolateral and/or posteromedial knee injury[30]
More - arthroscopy:
direct visualisation of the PCL injury; assessment of concurrent meniscal tears or medial or lateral joint space gapping indicative of a combined ligament injury
More - MRI:
T2 or proton-density MRI scans show oedema within the substance of the ligament and possible ligament stretching or disruption; concurrent injuries of the posterolateral structures or a medial meniscus root tear
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Meniscal tear
History
joint line pain along affected side,[6] pain with maximal knee flexion or with deep squatting of the knees when symptomatic posterior horn meniscal tears are present; difficulty with twisting and turning or kicking an object
Exam
lack of full flexion and extension; joint line crepitation and pain directly over the joint line;[6] positive assessment for meniscal tears; pain with maximal knee flexion
1st investigation
- standing AP x-ray:
decrease in the joint space may indicate some underlying arthritic changes; calcification of the meniscus consistent with chondrocalcinosis
- lateral knee x-ray:
anterior or posterior joint line osteophytes may indicate some underlying arthritic process; calcification of the meniscus consistent with chondrocalcinosis
Other investigations
- MRI:
sagittal views: anterior and posterior meniscal tears; coronal view: far medial and far lateral meniscal tears and flap tears
More - 45° patella sunrise (axial) x-ray:
osteophytes along affected medial or lateral femoral condyle may indicate arthritic changes
- 45° PA standing (Rosenberg) x-ray:
joint space narrowing in a patient who has had a previous partial meniscectomy or subtotal meniscectomy
More - arthroscopy:
torn meniscal cartilage or meniscal root attachments
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Open fracture
History
laceration extends down to associated fracture, any laceration around the knee with an underlying fracture has to be considered an open fracture until proven otherwise
Exam
deformities and lacerations around the injured knee; pulses may be absent or diminished if vascular injury present; sensation or motor function may be decreased or absent if associated nerve injury
1st investigation
- AP and lateral x-rays:
fracture or dislocation
Other investigations
- CT scan:
complex fracture of the femur, tibia, or patella
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Referred pain to knee
History
often presents as anterior knee pain; history of previous hip injuries/pathologies; older patients; slipped capital femoral epiphysis in overweight adolescents
Exam
decreased motion, groin pain (true hip pain), or pain referred to the knee (hip flexion and rotation) during assessment of hip range of motion; in adolescents assess for the presence of a slipped capital femoral epiphysis using the log rolling test
1st investigation
- AP pelvic x-ray:
in osteoarthritis there will be evidence of osteophytes, joint space narrowing, and subchondral sclerosis; in slipped capital femoral epiphysis there will be evidence of posterior displacement or widening of epiphysis; for femoroacetabular impingement there will be evidence of bony prominence of the superior femoral neck
Other investigations
- frog-leg lateral hip x-ray:
in osteoarthritis there will be evidence of osteophytes, joint space narrowing, and subchondral sclerosis; in slipped capital femoral epiphysis there will be evidence of posterior displacement or widening of epiphysis; for femoroacetabular impingement there will be an increased bony prominence along the superior femoral neck
More - MRI of hip:
will show evidence of a subtle epiphyseal slip not evident on x-ray; also useful to demonstrate stress fractures or acetabular tears
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Uncommon
Patella fracture
History
direct trauma to the anterior aspect of the knee, e.g., onto a dashboard in a car accident; fall onto a knee in the semi-flexed position causing forceful contraction of the quadriceps
Exam
joint deformity or shortening; swelling from effusion or haemarthrosis and/or bruising; severe pain on movement; limited knee extension; pulses may be absent or diminished if vascular injury present; sensation or motor function may be decreased or absent if associated nerve injury
1st investigation
- AP and lateral x-rays:
patella fracture
- patellar sunrise x-ray:
patella fracture; bipartite patella (present in 2% of the population, it involves the superolateral aspect of the patella)
Other investigations
- CT scan:
complex fracture of the femur, tibia, or patella
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Femoral condyle fracture
History
fall onto a flexed knee; fall from a height; vertical loading onto existing valgus or varus knee deformity; existing osteoporosis
Exam
joint deformity or shortening; swelling from effusion or haemarthrosis and/or bruising; severe pain on movement; soft tissue damage; pulses may be absent or diminished if vascular injury present; sensation or motor function may be decreased or absent if associated nerve injury
1st investigation
- AP and lateral x-rays:
femoral condyle fracture
Other investigations
- oblique view x-ray:
femoral condyle fracture; may show an obliquely oriented fracture
- CT scan:
complex fractures that may also involve the tibia and fibula; concurrent bony avulsions of ligaments or meniscal attachments
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Tibial plateau fracture
History
injury mechanism involving valgus force (e.g., hit by a car bumper), compression (e.g., from parachuting), or both; may be unable to bear weight
Exam
joint deformity or shortening; swelling from effusion or haemarthrosis and/or bruising; soft tissue damage; there may be severe pain on movement; pulses may be absent or diminished if vascular injury present; sensation or motor function may be decreased or absent if associated nerve injury
1st investigation
Other investigations
- CT scan:
tibial plateau fracture or complex fracture
Dislocation
History
high-velocity knee injuries with gross instability; fracture dislocations
Exam
deformity of the tibiofemoral alignment; associated lacerations may indicate an open fracture or dislocation; decreased posterior tibial and dorsalis pedis pulses; decreased light touch sensation at the lower extremity needs to be assessed for the distribution of the tibial and common peroneal nerves; abnormal motor examination of the common peroneal and tibial nerves' motor functions needs to be assessed to rule out nerve injury
1st investigation
- AP and lateral x-rays:
fracture or dislocation; bony avulsions of ligament or meniscal attachments
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Other investigations
- MRI:
ligamentous disruption or other injury of the knee; assessment of associated meniscal or articular cartilage injury
More - arteriogram or CT angiogram:
disruption of integrity of the popliteal artery around the knee in concomitant vascular injury; visualisation of intimal flaps of the popliteal artery
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Infection
History
significant pain, redness, warmth, and swelling around the knee; history of recent surgery, recent open fractures or lacerations with increased pain, warmth, or swelling
Exam
redness or swelling and increased warmth
1st investigation
- AP and lateral x-rays:
normal
Other investigations
- erythrocyte sedimentation rate:
elevated significantly above 'normal' value
More - C-reactive protein:
elevated
- FBC with differential:
elevated, especially with a left shift (increased neutrophils) on the differential
- aspiration (cell count, cell differential, Gram stain, aerobic/anaerobic cultures):
WBC >50,000/microlitre; left shift on the WBC differential indicates possible infection; Gram stains and aerobic/anaerobic culture can show evidence of bacterial infection
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