TABLE I.

Classification Systems for OCD of the Knee^*

Scintigraphy	Radiography	MRI	Lesion Stability	Arthroscopy
Cahill and Berg250 (1983) Stage 0: Normal scintigraphy and radiography Stage I: Normal scintigraphy but radiographic findings of OCD Stage II: Focal avidity at site of OCD defect, and radiographic findings of OCD Stage III: Focal avidity at site of OCD defect and increased activity in the femoral condyle harboring the OCD lesion, as well as radiographic findings of OCD Stage IV: Stage III and increased activity of the juxta-articular tibial plateau, and radiographic findings of OCD	Berndt and Harty251 (1959) Stage 1: Small subchondral compression Stage 2: Partially detached osteochondral fragment Stage 3: Completely detached but nondisplaced fragment Stage 4: Completely detached and displaced fragment (loose body) Scranton and McDermott modification252 (2001) Stage 5: Subchondral cyst	Dipaola et al.253 (1991) Stage I: Thickening of articular cartilage and low-signal changes Stage II: Articular cartilage breached and low-signal rim behind fragment indicating fibrous attachment Stage III: Articular cartilage breached and high-signal changes behind fragment indicating synovial fluid between fragment and underlying subchondral bone Stage IV: Loose body Hefti et al.16 (1999) Stage I: Small change of signal without clear margins of fragment Stage II: Osteochondral fragment with clear margins but without fluid between fragment and underlying bone Stage III: Fluid is visible partially between fragment and underlying bone Stage IV: Fluid is completely surrounding the fragment, but the fragment is still in situ Stage V: Fragment is completely detached and displaced (loose body) Ellermann et al.51 (2017) Type I: Epiphyseal cartilage lesion with necrotic center (no cleft at the interface) Type II: Epiphyseal cartilage lesion with complete or incomplete rim calcification (cleft at the interface) Type III: Partially or completely ossified lesion (varying degrees of osseous bridging and/or clefting at the interface) Type IV: Healed osseous lesion with a linear osseous scar (no cleft at the interface) Type V: Unhealed detached osseous lesion (sequestrum)	De Smet et al.111 (1996) (T2-weighted MRI) Sign 1: A thin line of high-signal-intensity ≥5 mm in length at the interface between the lesion and underlying bone (fibrovascular granulation tissue) Sign 2: A round area of homogeneous high-signal-intensity ≥5 mm in diameter beneath the lesion (cysts) Sign 3: A focal defect with a width of ≥5 mm in the articular surface of the lesion (displacement of the lesion into the joint) Sign 4: A high-signal-intensity line traversing articular cartilage into the lesion (articular fracture) O’Connor et al. modification102 (2002) Instability determined only when accompanied by cartilage breach on T1-weighted MRI Kijowski et al. modification112 (2008) A high T2-signal-intensity rim or cysts surrounding an adult OCD lesion are unequivocal signs of instability A high T2-signal-intensity rim surrounding a juvenile OCD lesion indicates instability only if it has the same signal intensity as adjacent joint fluid, is surrounded by a second outer rim of low T2 signal intensity, or is accompanied by multiple breaks in the subchondral bone plate on T2-weighted MRI Cysts surrounding a juvenile OCD lesion indicate instability only if they are multiple in number or large in size	Guhl254,255 (1982) Stage 1: Intact lesions Stage 2: Lesions showing signs of early separation Stage 3: Partially detached lesions Stage 4: Craters with loose bodies (salvageable or unsalvageable) Brittberg and Winalski256 (2003) ICRS OCD I: Stable lesions with a continuous but softened area covered by intact cartilage ICRS OCD II: Lesions with partial discontinuity that are stable when probed ICRS OCD III: Lesions with a complete discontinuity that are not yet dislocated (“dead in situ”) ICRS OCD IV: Empty defects and defects with a dislocated fragment or loose fragment within the bed ROCK Group (Carey et al.130; 2013) Immobile lesions: Cue ball: No abnormality detectable arthroscopically Shadow: Cartilage is intact and subtly demarcated (possibly under low light) Wrinkle in the rug: Cartilage is demarcated with a fissure, buckle, and/or wrinkle Mobile lesions: Locked door: Cartilage fissuring at periphery and unable to hinge open Trap door: Cartilage fissuring at periphery and able to hinge open Crater: Exposed subchondral bone defect Other classification systems: Ewing and Voto257 (1988) Johnson et al.258 (1990) Dipaola et al.253 (1991) O’Connor et al.102 (2002) Chen et al.259 (2013)

^*OCD = osteochondritis dissecans, ICRS = International Cartilage Repair Society, MRI = magnetic resonance imaging, and ROCK = Research in OsteoChondritis of the Knee.