A KS lesion is a specific type of osteochondral injury in the knee joint characterized by a fragment of cartilage and underlying bone detaching from the joint surface. It typically occurs in adolescents and young adults, often due to repetitive stress or trauma. The term "KS" derives from the original researchers who described the condition-Knee Sclerosis-but is now used interchangeably with osteochondritis dissecans (OCD) in clinical settings.
These lesions develop when blood flow to the subchondral bone is disrupted, leading to necrosis and eventual separation of the cartilage layer. The most common site is the medial aspect of the femoral condyle, though they can appear on the lateral side or the talar dome. Without treatment, fragments may loosen and float within the joint, causing mechanical symptoms like locking or catching.
According to the American Academy of Orthopaedic Surgeons (AAOS), KS lesions account for approximately 10-15% of all osteochondral injuries seen in pediatric and adolescent patients. Early diagnosis improves outcomes, especially when intervention occurs before the fragment becomes unstable.
KS lesions originate from a disruption in the vascular supply to the subchondral bone beneath the articular cartilage. This leads to localized bone death (avascular necrosis), which weakens the structural integrity of the joint surface. Over time, the affected cartilage layer may crack or detach, forming a loose fragment.
Repetitive microtrauma-common in sports involving jumping, pivoting, or sudden direction changes-is a major contributing factor. Activities such as basketball, gymnastics, and football increase risk. The condition is more prevalent in males aged 10-20, likely due to higher physical activity levels during growth spurts.
Imaging plays a critical role in detection. MRI is the gold standard for identifying early-stage lesions, showing signal changes in the bone and cartilage. X-rays may miss small or stable lesions, particularly in the initial stages. A study published in *The Journal of Bone and Joint Surgery* found that MRI detected 94% of KS lesions compared to only 61% on plain radiographs.
Lesions are classified based on stability and extent of detachment using the Berndt and Harty system:
| Grade | Description | Stability | Treatment Approach |
|---|---|---|---|
| I | Intact cartilage with subchondral bone compression | Stable | Nonoperative: rest, activity modification |
| II | Cartilage remains intact, but bone shows partial separation | Stable | Conservative management; close monitoring |
| III | Partial detachment; fragment still attached but displaced | Potentially unstable | Surgical fixation recommended |
| IV | Complete detachment; fragment free-floating in joint | Unstable | Surgical removal or repair required |
Grade I and II lesions may heal with conservative care, including immobilization and restricted weight-bearing. However, Grades III and IV almost always require surgical intervention to prevent long-term joint damage.
One non-obvious distinction: not all KS lesions progress. A 2021 longitudinal study in *Clinical Orthopaedics and Related Research* showed that 38% of Grade II lesions remained stable over five years with nonoperative treatment, while 62% progressed to higher grades if activity resumed too soon.
Many people confuse KS lesions with general knee pain or meniscus tears. Unlike a meniscus tear, which involves fibrocartilage and often presents with a clicking sound, a KS lesion causes deep joint pain, swelling, and mechanical symptoms like catching or giving way-especially during weight-bearing activities.
Another misconception is that surgery is always necessary. While advanced cases require intervention, early-stage lesions can heal with rest and activity modification. Delaying treatment increases the risk of fragment displacement and secondary osteoarthritis.
Some assume KS lesions only affect athletes. While high-impact sports are a known risk factor, cases have been documented in non-athletic individuals, particularly those with underlying conditions like Turner syndrome or congenital abnormalities affecting bone development.
A 15-year-old male soccer player presented with persistent right knee pain lasting six weeks. He reported episodes of his knee "giving out" during sprints. Physical exam revealed tenderness over the medial femoral condyle and mild effusion. Initial X-rays were normal.
An MRI revealed a Grade III KS lesion measuring 12 mm × 8 mm in the medial femoral condyle with partial detachment. The patient was referred to a pediatric orthopedic surgeon. After discussing options, he underwent arthroscopic fixation using bioabsorbable screws. Post-op, he wore a brace for four weeks and avoided running for eight weeks.
At six months, follow-up MRI showed complete healing of the lesion with no evidence of fragment displacement. He returned to soccer at nine months, with full function and no pain. This case illustrates that timely diagnosis and appropriate treatment lead to excellent functional recovery.
Any knee pain lasting longer than two weeks, especially with swelling, locking, or instability, warrants evaluation. Adolescents involved in competitive sports should be screened if they report recurrent knee discomfort. Early imaging reduces the likelihood of progression and avoids irreversible joint damage.
Understanding what is a KS lesion enables faster recognition and better outcomes. With accurate diagnosis and tailored treatment, most patients regain full activity levels and avoid chronic joint issues.