Why Does the Back of My Child’s Knee Hurt?

Pain behind a child’s knee usually traces to one of a handful of common, treatable causes rather than anything dangerous. Baker’s cysts, growing pains, hamstring tightness, and sports-related overuse account for the vast majority of cases. That said, the back of the knee (called the popliteal fossa) is a busy crossroads of tendons, muscles, nerves, and blood vessels, and occasionally the pain signals something that needs medical attention. Understanding which causes are routine and which warrant a closer look can save you both unnecessary worry and unnecessary delay.

Baker’s Cysts Are the Most Common Culprit in Young Children

If your child has a soft, squishy lump behind the knee along with an aching sensation, you are likely dealing with a Baker’s cyst, also called a popliteal cyst. These fluid-filled pouches form in the space behind the knee joint and are strikingly common in children between about three and eight years old.¹ In adults, Baker’s cysts almost always appear alongside some other joint disease like arthritis. In children, however, they tend to show up with no underlying joint problem at all, which suggests the cyst itself is the issue rather than a symptom of something deeper.

Most pediatric Baker’s cysts are painless or only mildly uncomfortable and resolve on their own over months to a couple of years. Your child might notice tightness or a pulling sensation when fully straightening or deeply bending the knee. When imaging is done, ultrasound picks up whether the cyst communicates with the joint space, while MRI gives a more detailed picture. In one study of 80 pediatric patients, ultrasound detected a connection between the cyst and the joint cavity about two-thirds of the time, while MRI picked it up in a higher proportion.² Treatment for a straightforward Baker’s cyst in a child is usually reassurance and observation. Surgery is rarely necessary unless the cyst is large, persists for years, or causes significant limitation.

Growing Pains and How to Tell Them Apart From Other Problems

Growing pains are among the most frequent reasons children complain of leg pain, and the back of the knee is one of the classic spots they strike. The name is a bit misleading because growing pains are not actually caused by growth itself. They tend to hit in the evening or at night, ache rather than stab, and typically pass within about half an hour. A recent controlled study found that children whose pain fit the growing-pains pattern reported evening or nighttime soreness roughly 90% of the time, with a median episode lasting around 31 minutes. Children with other diagnoses, by contrast, were more likely to describe morning pain of a sharper, stabbing quality that lasted considerably longer.³

The practical takeaway: if your child’s knee pain wakes them at night, responds to a gentle massage or a warm compress, and is completely gone by morning with no limping, swelling, or redness, growing pains are a strong possibility. If the pain is there first thing in the morning, persists through the day, or gets steadily worse over weeks, something else is going on.

Sports and Overuse Injuries Behind the Knee

Active children, especially those involved in running sports, soccer, basketball, or gymnastics, are prone to overuse injuries in the structures that cross the back of the knee. Three areas deserve attention.

Hamstring strains are probably the most familiar. The hamstring muscles run down the back of the thigh and attach near the knee, so a strain can easily register as pain behind or just above the knee. Most hamstring injuries involve a single muscle near the spot where muscle transitions into tendon, and the diagnosis is usually straightforward from the history and a physical exam alone.⁴ Your child will typically recall a specific moment of overstretching or sudden sprinting. Rest, ice, and a gradual return to activity are standard.

The popliteus is a small, often-overlooked muscle and tendon that sits right at the back-outside corner of the knee. It helps unlock the knee from a fully straight position. Popliteus tendon problems are uncommon, but they can result from overuse or direct trauma and show up as vague pain at the posterolateral knee. Because the symptoms are so nonspecific, popliteus injuries are easy to miss and should be considered when a child’s posterior knee pain does not fit a more obvious pattern.⁵

Calf muscle injuries are rarer in children than in adults, but they do occur. A case report described a seven-year-old boy who tore the inner head of his calf muscle (the gastrocnemius) during a fall, presenting with acute calf pain just below the knee.⁶ While that case was extreme, milder strains of the upper calf are worth thinking about when a child reports sudden-onset pain in the back of the knee during or right after physical activity.

Growth Plate Stress and Adolescent Athletes

Children’s bones have growth plates, areas of developing cartilage near the ends of the long bones, that are structurally weaker than mature bone. Repetitive loading from sports can stress these plates, and the number of such injuries has been climbing as youth sports become more intense and year-round.⁷ Pain from a growth-plate stress injury tends to be activity-related, worsening with training and improving with rest. The distal femur (just above the knee) and the proximal tibia (just below it) are two of the growth plates most commonly affected in the lower extremity. If your child is a committed athlete complaining of persistent knee pain that tracks closely with training volume, growth-plate stress is something worth discussing with a sports medicine provider.

Discoid Meniscus and Osteochondritis Dissecans

Some causes of knee pain are structural quirks your child was born with or developed during growth. A discoid meniscus is a variant in which one of the knee’s cartilage cushions is disc-shaped rather than its normal crescent shape. It is most common on the outer (lateral) side of the knee. Children with a discoid meniscus sometimes experience clicking, catching, or difficulty fully straightening the knee. Research has linked these symptoms to instability in the back portion of the abnormal meniscus, where the posterior segment can become hypermobile or lose its normal attachment to the tibia.⁸ When the back part of the meniscus shifts during movement, it can produce pain that localizes to the posterior knee. Many discoid menisci are asymptomatic and found incidentally, but when they cause persistent mechanical symptoms, arthroscopic surgery to reshape the meniscus is the usual fix.

Osteochondritis dissecans (OCD) is a condition where a segment of bone just beneath the joint cartilage loses its blood supply and can loosen or even detach. In adolescents, OCD most commonly appears on the inner (medial) part of the femoral condyle and causes diffuse, activity-related knee pain. Less frequently, the lesion forms on the back portion of the outer (lateral) femoral condyle, where it can produce pain that feels specifically posterior.⁹ OCD lesions caught early in a growing child often heal with rest and activity modification. Larger or unstable fragments may need surgical fixation.

Ligament Injuries Are Not Just an Adult Problem

Parents and even some clinicians tend to assume that significant knee ligament injuries belong to the adult athletic world. In reality, children and teens can tear knee ligaments, and the posterior cruciate ligament (PCL) is the one most directly relevant to pain behind the knee. A PCL injury typically results from a direct blow to the front of the upper shin, a hyperflexion event, or occasionally a sudden twisting injury. Symptoms include immediate pain and swelling at the back of the knee and difficulty bending or straightening it fully.¹⁰ In children, the ligament sometimes pulls off a small piece of bone (an avulsion fracture) rather than tearing mid-substance, which can actually be easier to repair. If your child had a clear traumatic event and the knee feels unstable or sags backward, a PCL injury is worth ruling out.

When the Knee Hurts but the Problem Is the Hip

One of the trickiest aspects of pediatric knee pain is referred pain from the hip. The nerves that supply the hip joint also send branches to the knee, so a child with a hip problem may genuinely feel pain in the knee and nowhere near the hip. A case report illustrated this clearly: a seven-year-old boy presented with recurring left knee pain, but all examination and imaging of the knee were normal. Only when clinicians investigated the hip did they find a defect on the femoral head and increased fluid in the hip joint.¹¹ Conditions like Legg-Calvé-Perthes disease, slipped capital femoral epiphysis, and hip infections can all masquerade as knee pain in children.

The clinical message is practical: whenever a child’s knee pain cannot be explained by findings at the knee itself, the hip needs to be examined. Pediatric sports medicine specialists routinely check the hip whenever a child presents with vague knee complaints, because pain from the hip can easily mislead both the child and the examiner.¹¹

Joint Hypermobility and Chronic Aching

Some children have joints that are naturally more flexible than average, a trait called joint hypermobility. While being “double-jointed” is often treated as a party trick, a subset of hypermobile children develop real musculoskeletal pain, including behind the knee, because their ligaments and joint capsules provide less passive stability. These children’s muscles have to work harder to keep joints in line, which leads to fatigue and aching after activity. The term “joint hypermobility syndrome” is used for children who are hypermobile and symptomatic, and it sometimes overlaps with broader connective tissue conditions.¹² The back of the knee is a common pain site because the joint tends to hyperextend, stretching the posterior capsule and stressing the hamstrings. If your child is noticeably flexible in multiple joints and frequently complains of aching after exercise, a referral for physiotherapy focused on strengthening and proprioception can make a meaningful difference.

Vascular and Nerve Causes Are Rare but Real

In a small number of cases, pain or unusual sensations behind the knee come from compressed blood vessels or nerves rather than bones and muscles. Popliteal artery entrapment syndrome (PAES) occurs when the main artery behind the knee gets squeezed by an abnormally positioned or overgrown muscle. It is rare overall but shows up disproportionately in young athletes. One case involved a 15-year-old male athlete who developed intermittent foot numbness during exercise; imaging revealed that a calf muscle was compressing his popliteal artery.¹³ The hallmark symptom is leg cramping or claudication during running that resolves with rest, which can be confused with ordinary muscle fatigue.¹⁴

On the nerve side, the tibial nerve runs through the popliteal fossa and can occasionally be compressed by the same types of abnormal fibrous bands that trap the artery. In adults, tibial nerve entrapment in this area typically causes pain and numbness in the back of the knee or upper calf.¹⁵ In one reported case, surgery revealed an abnormal fibrous band between the two heads of the calf muscle compressing both the tibial nerve and the popliteal artery, and dividing the band provided immediate, lasting relief.¹⁶ If your child’s symptoms include numbness, tingling, or skin color changes in the foot during exercise, vascular or nerve compression should be on the radar even though these diagnoses are uncommon.

Red Flags That Warrant Prompt Medical Evaluation

Most causes of posterior knee pain in children are benign and self-limiting, but certain warning signs should push you to seek evaluation sooner rather than later. Clinical guidance for pediatric knee pain highlights the following red flags:

• Systemic symptoms: fever, rash, weight loss, night sweats, unexplained lethargy, or easy bruising alongside knee pain.
• Bone pain: constant, nagging pain or pain that wakes your child at night, which can be a feature of leukemia, neuroblastoma, or primary bone tumors.
• Prolonged joint swelling: swelling, pain, and morning stiffness lasting more than six weeks, which suggests juvenile idiopathic arthritis.

Distinguishing joint pain from bone pain matters. Joint pain tends to worsen with movement, improve with rest, and be accompanied by visible swelling around the joint line. Bone pain is often deeper, more constant, and less relieved by rest.¹⁷ Any child with unrelenting pain that does not follow a recognizable mechanical pattern deserves imaging and, if needed, bloodwork.

Tumors Around the Knee in Children

This is the diagnosis every parent fears, and it is worth addressing directly: tumors are a rare cause of knee pain in children, but they do occur and the knee region is actually one of the more common locations for bone tumors to arise in the pediatric skeleton. A review of 199 bone and soft-tissue tumors around the knee in children found that the majority were benign, with bony growths (osteocartilaginous exostoses), nonossifying fibromas, and chondroblastomas being the most common. Malignant bone tumors were less frequent, and among those, osteosarcoma was by far the most common sarcoma.¹⁸ The critical point is that delays in diagnosis can have serious consequences. A child with persistent, worsening knee pain unresponsive to rest, especially if accompanied by a palpable mass, nighttime pain, or systemic symptoms, should have imaging to rule out a bony or soft-tissue lesion. Most of these cases will turn out to be something benign, but catching the rare malignancy early is the entire reason to investigate.

The Physical Exam Matters More Than You Might Think

When you bring a child in for posterior knee pain, a careful hands-on examination is the single most valuable diagnostic step. Having the child point to exactly where it hurts helps the clinician prioritize what to look for, and doing this before provocative tests matters because causing pain early in the exam makes a child less cooperative for everything that follows. In pediatric patients specifically, age changes the exam: a three-year-old cannot describe clicking or locking the way a teenager can, so the clinician relies more on observation of gait and passive range of motion in younger children. And as noted earlier, a thorough hip exam is a non-negotiable part of any pediatric knee evaluation, because referred hip pain is a well-documented trap.

Imaging usually comes second. Plain X-rays are often the first step and can reveal fractures, OCD lesions, bony tumors, and growth-plate abnormalities. Ultrasound is quick, radiation-free, and particularly good at confirming Baker’s cysts. MRI is reserved for cases where the diagnosis remains unclear after the initial workup or where soft-tissue detail is needed, such as suspected meniscal or ligament injuries. Most children with posterior knee pain will never need an MRI.

What You Can Do at Home While You Wait

For pain that appears activity-related and is not accompanied by any red-flag symptoms, a few straightforward steps can help while you decide whether a medical visit is needed. Reducing the aggravating activity, applying ice after exercise, and encouraging gentle hamstring and calf stretching are all reasonable first-line measures. Over-the-counter pain relief like ibuprofen can help with both pain and any mild inflammation. If the pain improves steadily over a week or two with activity modification, you are probably dealing with a simple overuse issue or growing pains.

If the pain does not improve, returns every time your child ramps activity back up, or is accompanied by swelling, locking, giving way, limping, or any of the systemic symptoms listed above, schedule an appointment. A pediatric orthopedic specialist or sports medicine physician is the right person to sort through the possibilities. The reassuring reality is that the overwhelming majority of children with pain behind the knee have something treatable and temporary, but the small fraction who don’t are the reason a thorough evaluation exists.