In 1903, Robert Osgood and Carl Schlatter independently published reports describing the clinical entity that now bears their surnames.¹,² Osgood-Schlatter disease (OSD), also referred to as Osgood-Schlatter syndrome, is the eponym applied to tibial tubercle apophysitis, a common cause of knee pain in the adolescent athlete. It is characterized by activity-related painful inflammation in the cartilage of the tibial tubercle apophysis. Swelling and prominence of the tubercle can occur as well (Fig. 41.1) but are not necessarily or consistently present in the condition. Symptoms are generally worsened with physical activities, usually those that involve running, jumping, landing, cutting, or pivoting, but may instead simply be due to direct pressure on the tubercle, such as with kneeling. The symptoms of OSD typically resolve spontaneously around the time of skeletal maturity but may be present before this time to varying degrees for months, or even years, in some adolescents or preadolescents. A subset of patients with OSD experience persistent symptoms and disability beyond the time of skeletal maturity despite an appropriate course of conservative management. Surgical intervention to provide pain relief may be an option in these refractory cases.

The tibial tubercle is the site of an apophysis, a chondro-osseous prominence or protuberance that experiences repetitive traction forces from a tendinous attachment. Prior to skeletal maturity, these tensile forces make the cartilaginous plate of the apophysis structurally different than the adjacent proximal tibial physis, which is subjected to compressive forces. The tibial tubercle apophysis has a large fibrocartilaginous component, helping it to withstand the strong tensile forces produced by the patellar tendon.³

The tibial tubercle has been described as developing through four stages during childhood and adolescence (Fig. 41.2).⁴ The four stages have been named the cartilaginous stage, the apophyseal stage, the epiphyseal stage, and the bony stage. In the cartilaginous stage, the tubercle is entirely made of cartilage and not evident on x-ray. In the apophyseal stage, the secondary ossification center of the tubercle appears as a discrete entity. In the epiphyseal stage, the tubercle ossification center unites with the proximal tibia ossification center, but cartilage remains, deep to this bony portion of the apophysis. In the bony stage, the physis and apophysis ossify completely, or “close.”³

Multiple theories on the etiology of OSD have been proposed. These include mechanical avulsion, patellar tendon degeneration, avascular necrosis, and infection. However, the majority of research supports the mechanical theory that OSD is caused by repetitive traction-based microtrauma to the cartilaginous portion of the apophysis. There may also be one or more discreet episodes of new or increased tibial tubercle pain during the late cartilaginous or apophyseal stage, which can serve as inciting events for the more chronic, inflammatory condition.³,⁵,⁶,⁷,⁸ However, OSD should be differentiated from a true tibial tubercle fracture, which generally requires a higher energy traumatic event and is distinct from the overuse phenomenon. In OSD, the aforementioned adaptive changes of the apophysis lead to the tubercle being vulnerable to traction forces. A tiny fragment or fragments of chondral tissue can be pulled proximally, away from the rest of the tubercle, and may generate small ossicles or fragments of bone. Discrete ossicles can form that are not confluent with the rest of the tibial tubercle⁹ and may lead to pain, tenderness, local edema, and inflammation of the distal patellar tendon (patellar tendonitis).⁵,⁹

OSD is a remarkably common cause of knee pain in adolescents of both sexes, with a higher incidence in boys than girls.¹⁰ One study suggested that OSD affects 13% of adolescents.¹¹ However, it is almost twice as common (21%) in adolescents who are involved in sports and less than half as common (5%) in adolescents not involved in sports.¹¹ The apophyseal stage
of tibial tubercle development, which occurs roughly between the ages of 10 and 15 years, is the stage during which OSD most commonly arises. Studies have demonstrated the average age of symptom onset to be 12 to 15 years old.¹¹,¹²,¹³ A third to a half of afflicted adolescents have bilateral knee involvement.¹⁰,¹¹,¹⁴ When unilateral, OSD usually affects the knee of the jumping or “take-off” leg.¹¹