Differentials
Medial collateral ligament (MCL) sprain
SIGNS / SYMPTOMS
The pop sound at injury is less common and true dynamic instability is rare, unless it is a complete tear or with associated injuries. May be able to continue activity if minor sprain. No significant hemarthrosis in isolated MCL sprain, although reactive effusion is not uncommon. Tender over MCL course/insertion. MCL stress testing reveals laxity and/or pain. Additionally, the Lachman and pivot shift tests are negative. Spontaneous healing and full return of function is the rule.[60]
INVESTIGATIONS
MRI reveals fluid around or injury to MCL; ACL appears intact. Subsequent x-rays may reveal calcification along previously injured MCL (Pellegrini-Stieda disease).
Posterior collateral ligament (PCL) sprain
SIGNS / SYMPTOMS
Often able to continue activity but knee does not feel right subjectively to the patient. Instability may be more subtle, with knee effusion typically smaller and less noticeable by the patient. Lachman,[Figure caption and citation for the preceding image starts]: Lachman maneuverFrom the personal collection of Philip H. Cohen [Citation ends].
pivot shift, anterior drawer tests are negative, but the posterior drawer test is positive.[Figure caption and citation for the preceding image starts]: Anterior and posterior drawer test starting positionFrom the personal collection of Philip H. Cohen [Citation ends].
External rotation recurvatum test will show posterolateral corner instability.[Figure caption and citation for the preceding image starts]: External rotation recurvatum testFrom the personal collection of Philip H. Cohen [Citation ends].
Unlike with ACL tears, nonoperative approach often allows return to full activity.[61]
INVESTIGATIONS
MRI reveals disrupted PCL, and an intact ACL.
Lateral collateral ligament (LCL) sprain
SIGNS / SYMPTOMS
Local swelling is common, but significant effusion is rare. There is tenderness over the lateral collateral ligament and/or bony insertion. A complete tear often results in a palpable gap. Spontaneous healing and full return of function is the rule.[62]
INVESTIGATIONS
MRI reveals fluid around or disruption to LCL, with intact ACL.
Meniscal tear
SIGNS / SYMPTOMS
Meniscal injury may also occur with ACL tear or other significant injury. Main symptom is pain at affected joint line and stiffness. Physical exam maneuvers have poor accuracy, but joint line tenderness is common, as is pain with rotational maneuvers (e.g., positive McMurray test[Figure caption and citation for the preceding image starts]: McMurray flexion in valgusFrom the personal collection of Philip H. Cohen [Citation ends].
[Figure caption and citation for the preceding image starts]: McMurray flexion varusFrom the personal collection of Philip H. Cohen [Citation ends].
[Figure caption and citation for the preceding image starts]: McMurray external varusFrom the personal collection of Philip H. Cohen [Citation ends].
[Figure caption and citation for the preceding image starts]: McMurray external valgusFrom the personal collection of Philip H. Cohen [Citation ends].
). Effusion typically develops over 1 to 2 days, but rapid hemarthrosis may occur if large tear in vascular portion of meniscus.[63][64]
INVESTIGATIONS
MRI has excellent sensitivity for meniscal tears, but intrasubstance degenerative changes may be misread as acute tears.[65]
Posterior capsular sprain
SIGNS / SYMPTOMS
Small effusion and stiffness common, along with painful extension. ACL and other ligaments are stable.[66]
INVESTIGATIONS
Imaging rarely necessary. MRI may show effusion, bone bruising, abnormal signal at posterior capsule, but no frank ligament disruption.
Patellar subluxation or dislocation
SIGNS / SYMPTOMS
Patient feels severe pain around patella; kneecap may appear displaced to the side. Hemarthrosis occurs, with difficulty in weight bearing, and a sensation of the kneecap being unstable. Exam usually reveals hemarthrosis, tenderness at patella and medial retinaculum, and apprehension when examiner attempts to displace patella laterally.[Figure caption and citation for the preceding image starts]: Patellar apprehension testFrom the personal collection of Philip H. Cohen [Citation ends].
ACL testing should be normal.[67]
INVESTIGATIONS
X-rays may reveal dislocation if not yet reduced, or patellar fracture. Lateral subluxation and abnormal patellofemoral relationship often noted. MRI often reveals bone bruising, and may show chondral injury, osteochondral fracture, or loose body.[68]
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