[Christian Magnus Falsen Sinding-Larsen (1866–1930), director of Rikshospitalet in Kristiania (now Oslo), Norway]

[Sven Christian Johansson (1880–1959), surgeon and head physician in Gothenburg, Sweden]

= osteochondrosis of inferior pole of patella, often bilateral (NOT osteonecrosis / epiphysitis / osteochondritis)

Cause: traction tendinitis / traumatic avulsion of bone; repeated subluxation ± dislocation of patella

Mechanism: forceful contraction of quadriceps against resistence

Age: adolescents (often 10–14 years)

Predisposed: cerebrospastic children

• tenderness + soft-tissue swelling over lower pole of patella
• peripatellar soft-tissue swelling
• calcification / ossification of patellar tendon
• patella alta deformity = abnormally elevated position of patella with respect to femur (LAT view)
• small bone fragments at lower pole of patella (LAT view)

MR:

• hypointense area on T1WI + hyperintense on T2WI in inferior pole of patella + proximal portion of patellar tendon + surrounding soft tissues

DDx:

1. Jumper's knee
   = pain syndrome involving proximal / distal insertion of patellar tendon, commonly seen in young athletes
   Cause: chronic stress + inflammation
   • thickening of patellar tendon without tear / avulsion
2. Patellar sleeve avulsion
   = cartilage of inferior patellar pole pulled off patella often in combination with small avulsed bone fragment (= cartilaginous injury)
   Mechanism: vigorous contraction of quadriceps applied to flexed knee
   Age: 8–12 years (unique to pediatric population)
   • small bone fragment inferior to lower pole of patella
   • patella alta
   • joint effusion
   • bone marrow edema of patella (MRI!)