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Knee MRI Long Case - Complete Viva Q&A

Complete ACL Tear + Ramp Lesion + Bone Marrow Edema + Tibial Translation

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SECTION 1: ANATOMY OF THE ACL

• Anteromedial (AM) bundle (~38 mm long) - taut in flexion, controls anterior translation
• Posterolateral (PL) bundle (~20 mm long) - taut in extension, controls rotational stability

• THIEME Atlas of General Anatomy and Musculoskeletal System

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SECTION 2: MECHANISM OF INJURY

1. Non-contact (most common ~70%): Sudden change of direction, cutting, pivoting, or landing from a jump. A deceleration + internal rotation or valgus collapse at the knee (the "pivot shift mechanism") tears the ACL. Classically: plant foot, the body twists over it.
2. Contact mechanism: Direct collision causing sudden twisting, hyperextension, or valgus force. Examples include a direct blow to the lateral knee causing forced valgus + internal rotation.

• Gray's Anatomy for Students; Bailey and Love's 28th Ed, p. 526

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• The lateral femoral condyle subluxates posteriorly relative to the tibia
• This compresses the posterior horn of the lateral meniscus (causing LMORT or lateral root tears)
• Simultaneously, valgus strain + internal tibial rotation + medial compartment impaction produce high forces through the posteromedial capsule, causing a longitudinal tear at the meniscocapsular junction of the posterior horn of the medial meniscus - the ramp lesion
• Campbell's Operative Orthopaedics 15th Ed, 2026

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SECTION 3: MRI FINDINGS OF COMPLETE ACL TEAR

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1. Anterior tibial translation - the tibia is anteriorly displaced on the femur. Measured on mid-sagittal images: >5 mm (or >7 mm on sagittal) is abnormal. This is the most specific indirect sign.
2. Kissing contusions / bone marrow edema pattern - the classic "pivot shift" contusion pattern:
   • Lateral femoral condyle (mid-third, sulcus terminalis region)
   • Posterolateral tibial plateau These are pathognomonic for ACL rupture and represent the impaction at the moment of pivot-shift subluxation.
   • Bailey and Love's 28th Ed, p. 526
3. Deep lateral femoral notch sign - a notch >1.5 mm deep in the sulcus terminalis on sagittal images (impaction fracture/contusion)
4. Buckled / hyperbuckled PCL - the PCL takes on a buckling, wavy appearance because the anterior tibial translation reduces the normal tension on the PCL. Specific but insensitive.
5. Uncovering of the posterior horn of the lateral meniscus - due to anterior tibial translation, the posterior horn of the lateral meniscus projects beyond the posterior tibial rim on sagittal images (>3.5 mm uncovering is significant).
6. Segond fracture - tiny cortical avulsion of the anterolateral tibia. Pathognomonic for ACL tear on plain film. Represents avulsion of the anterolateral ligament (ALL).
7. Hemarthrosis/joint effusion - large joint effusion is almost always present.

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SECTION 4: BONE MARROW EDEMA

• The lateral femoral condyle (sulcus terminalis / central third)
• The posterolateral tibial plateau

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• Not true edema - represents a mix of interstitial hemorrhage, microtrabecular fractures, and marrow fat replacement with fluid
• Appears as high signal on STIR and fat-suppressed T2/PD sequences
• Low/intermediate signal on T1 (replacing normal bright marrow fat)
• The fat-suppressed sequences are key: without fat suppression, marrow edema can be masked by the bright marrow fat

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SECTION 5: RAMP LESION

• Campbell's Operative Orthopaedics 15th Ed, 2026

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• Earlier reports: 3-13%
• More recent arthroscopic series: 24-42%
• In one study of 301 ACL reconstructions: 17% had medial meniscal ramp lesions
• Fewer than half were diagnosed on preoperative MRI (indicating underdiagnosis)
• Ramp lesions are more common in younger patients (those with ramp lesions were ~7.5 years younger than those with meniscal body lesions)
• Campbell's Operative Orthopaedics 15th Ed, 2026

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• MRI is specific but only moderately sensitive
• Meta-analysis (Koo et al.): pooled sensitivity 71%, specificity 94%
• 3.0T MRI: sensitivity 83.3% vs. 1.5T: sensitivity 67.6% (Hatayama et al.)
• Key reason for false negatives: the knee is in extension during MRI, which reduces meniscocapsular separation and makes disruption less visible
• Ramp lesions are often the "hidden lesion" - missed on MRI but found at arthroscopy
• This is why some surgeons advocate systematic posteromedial arthroscopic portal examination in all ACL reconstructions

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• Increases anterior tibial translation (contributes to ACL graft loading)
• Increases rotational instability
• Elevates ACL graft forces after reconstruction
• If left untreated, may cause residual instability even after technically successful ACL-R
• Meniscal repair of the ramp lesion can restore stability to near-normal levels
• Radsource, 2024; Campbell's Operative Orthopaedics 15th Ed, 2026

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1. No specific clinical test - can be asymptomatic, or symptoms are masked by the ACL injury
2. MRI is read with knee in extension - meniscocapsular separation is reduced
3. Arthroscopic blind spot - standard anterior portals do not visualize the posteromedial recess; a posteromedial portal or trans-notch viewing is needed
4. "The radiologist can only diagnose what he knows" - lack of familiarity with the anatomy is a major cause of miss

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• Type 1: Tear of the meniscotibial ligament (inferior surface)
• Type 2: Tear of the meniscal capsular attachment (superior surface)
• Type 3: Combined/through-and-through tear (both superior and inferior)
• Type 3A: Extends to peripheral PHMM (most common type seen on MRI)

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SECTION 6: TIBIAL TRANSLATION

• Measured on the mid-sagittal image through the medial tibial spine
• A line is drawn along the posterior tibial cortex and another along the posterior femoral cortex - the offset between these lines defines the ATT
• >5 mm anterior displacement of the tibia is considered significant/abnormal
• Static ATT is distinguished from the dynamic pivot-shift test

• Posterior horn lateral meniscus uncovering (>3.5 mm beyond posterior tibial rim)
• Buckled PCL
• Anterior translation visible on mid-sagittal scout

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• Static ATT (SATT): Measured on standard MRI with knee in extension. Reflects passive resting translation. SATT >5 mm indicates significant ACL incompetence.
• Dynamic ATT: Measured during stress views (e.g., standing lateral X-ray or stress MRI) or assessed clinically with the Lachman test and anterior drawer test. This corresponds to the clinical pivot-shift and subluxation.
• The Lachman test (performed at 20-30° flexion) is the most sensitive clinical test for ACL tear - more sensitive than the anterior drawer test (90° flexion).

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SECTION 7: CLINICAL TESTS & ASSOCIATED INJURIES

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1. ACL tear
2. MCL tear
3. Medial meniscus tear

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SECTION 8: GRADING AND CLASSIFICATION

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• MRI complete tear: Full-thickness discontinuity of all fibers, non-visualization or mass of irregular tissue, high T2 signal throughout
• Clinical complete tear: Grade III injury with positive Lachman (soft endpoint), positive pivot shift, large hemarthrosis
• Accuracy: MRI fiber discontinuity has ~79% accuracy; abnormal ACL orientation has ~87% accuracy for unstable tear. MRI specificity for complete tear is high but sensitivity for all indirect signs of instability can be low (~23%)

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SECTION 9: TREATMENT

• Bailey and Love's 28th Ed; Campbell's 15th Ed 2026

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• Bone-patellar tendon-bone (BPTB): Rigid bone-to-bone healing, historically the standard but higher anterior knee pain/harvest morbidity
• Hamstring tendons (semitendinosus ± gracilis): Lower donor-site morbidity, good outcomes, higher re-rupture in young athletes
• Quadriceps tendon autograft: Growing popularity; larger graft, with or without bone plug. Recent meta-analyses show comparable/superior outcomes to BPTB and hamstrings (PMID: 39694948)

• Younger patients: autograft offers best durability and lowest re-rupture rate
• Older/low-demand patients: allograft is acceptable

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• All-inside arthroscopic repair via posteromedial portal
• Inside-out sutures placed through the posteromedial capsule
• The tear is in the red-red zone (vascular, good healing potential) - repair is preferred over resection
• Concomitant repair during ACL-R is recommended for unstable ramp lesions, as untreated lesions leave residual rotational instability and increase graft forces

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SECTION 10: MRI PROTOCOL SPECIFICS

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SECTION 11: COMPLICATIONS & PROGNOSIS

• Failure/re-rupture: Most common within 1 year (technical error, biological failure) or after 1 year (re-injury). In young female athletes, re-rupture risk of contralateral ACL is high.
• Stiffness/arthrofibrosis: Cyclops lesion (fibrous nodule at tibial tunnel), loss of extension
• Infection: Incidence increased when ACL-R is combined with meniscal repair
• Donor site morbidity: Anterior knee pain (BPTB), hamstring weakness (hamstring graft)
• PLC deficiency as cause of graft failure: An unrecognized/untreated posterolateral corner injury dramatically increases stress on ACL graft - a primary cause of graft failure
• Campbell's Operative Orthopaedics 15th Ed, 2026

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• Progressive meniscal degeneration (recurrent episodes of subluxation tear menisci)
• Articular cartilage damage (chondral loss, especially lateral compartment)
• Early-onset knee osteoarthritis
• ACL-deficient knees have a significantly higher risk of OA compared to reconstructed knees, especially if combined meniscal injuries were left untreated

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SECTION 12: EXAM-STYLE RAPID-FIRE QUESTIONS

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• Campbell's Operative Orthopaedics 15th Ed 2026 (primary orthopaedic reference)
• Bailey and Love's Short Practice of Surgery 28th Ed
• Gray's Anatomy for Students
• THIEME Atlas of General Anatomy and Musculoskeletal System
• Schwartz's Principles of Surgery 11th Ed
• Radsource - Ramp Lesions Review (current radiological reference)
• EPOS ECR 2025 - MRI criteria for ramp lesions
• Recent PubMed: Ramp lesion prevalence meta-analysis (PMID: 39840766, J Pediatr Orthop 2025); Quadriceps vs BPTB vs hamstring ACL-R meta-analysis (PMID: 39694948, J Orthop Traumatol 2024)