Osteochondritis and Osgood Schlatter for the USMLE 

OSTEOCHONDRITIS DISSECANS
Articular surface of the knee bone breakdown and exposes the bone underneath. The most common joint affected is the knee, ankle and elbow. Juvenile is when the epihpysis is open rather than formed. Generally has a family history.
Causes and etioloygy of osteocondritis is idiopathic, and avascular necrosis specifically impignement of tibial spine. Blow or trauma may also be the cause, either rotatory or joint compression. The first part of the knee affected is the lateral or medial side of medial epicondyle.
Clinical findings, sign and symptoms are due to vague knee pain, parapatellar tenderness after palpating around patella. Wilson test is to ask them to medially rotate foot and then striaghten the foot and they will feel pain at 30 degrees. Quadricips atrophy. After there are loose fragments, there can be crepitations, popping and locking. If there is effusion than it the osteochondritis is classified as effusion.
STAGES consist of in situ, early seperation on in certain areas, incomplete detachment, complete detachment. Investigations of choice are x-ray will show uneven surface. MRI will allow better staging and severety.
TREATMENT AND MANAGEMENT
First treatment is non-srugical consisting of no weight bearing, unloader brace, repeat MRI and X-rays. Slowly introduce exercise and physiotherapy. If it doesn't improve than you can do srugery. Arthroscopic drilling which stimulate regeneration and growth. Fragment fixation to return fragment. Autograft transfer or a chondrycyte transplant. Allograft is when there is a large lesion of greater than 25 millimeters.
OSGOOD SCHLATTER
Pathogenesis consist of chronic overuse as a child, leads to damage of epiphysis of tibial tuberosity and then there is callus being laid down. Consists mostly in 9-14 years of age during rapid growth and lots of exercise.
CLINICAL
Anterior knee pain and gradually worsens with time and with exercise. On physical examination there will be tenderness on tibial prominence. Not warm or erythematous and examine the hip because sometimes it is referred pain.
TREATMENT
Generally osgood schlatter is benign and self-limitng that resolves after growth plate fuses and resolves by 6-18months. Pain management with NSAIDs. Glucocorticoids, physiotherapy. Surgery can be done after ephiphyseal closure. Complication is genu recurvitum which means the knee is curved backwards.