Acute Hemarthrosis in a 16-Year-Old With a Guarded Knee Exam
A 16-year-old female soccer player decelerates to cut while marking a forward; her knee buckles without contact and she feels a 'pop.' She cannot continue. When you evaluate her 40 minutes later, a visible effusion is already present, knee flexion is limited to 80 degrees, and she bears weight only with a marked limp. There is no gross deformity; the joint line is diffusely tender and the patella is not displaced. Lachman testing produces a soft endpoint, but she guards and you are not confident in the finding. She is 16, menarche at 13, and has no prior knee injury. Her parents are 45 minutes away and asking whether they should take her to the emergency department or to the clinic tomorrow.
1. Rank your differential and explain the reasoning behind the order.
Read the worked answer
1. Anterior cruciate ligament tear. A noncontact deceleration injury with an audible pop, inability to continue, and a large effusion developing within about an hour is the classic hemarthrosis presentation; ACL disruption accounts for the majority of rapid hemarthroses in this age group. The soft Lachman endpoint supports it, but guarding makes that finding unreliable - it can create both false positives and false negatives.
2. Patellar dislocation with spontaneous reduction. Easily missed when the patella has relocated before you examine. Look for medial retinacular/MPFL tenderness, a positive apprehension sign, and bruising at the medial femoral condyle. This mechanism also produces a rapid hemarthrosis if there is an osteochondral injury.
3. Osteochondral or chondral injury (often coexisting with 1 or 2), suggested by a loose body sensation or focal tenderness at the condyle or patellar facets.
4. Physeal injury - she is 16 and may still have open physes. A Salter-Harris injury of the distal femur or a tibial spine avulsion can mimic an ACL tear and changes management substantially. This is the reason not to default to a ligament-only working diagnosis in a skeletally immature athlete.
5. Meniscal tear or capsular sprain. Possible but less likely to be the sole cause of this volume of effusion in this timeframe.
The ordering rests on mechanism plus the speed and size of the effusion, which is the single most informative finding you have.
2. What do you do in the next 30 minutes, and what should you deliberately avoid doing?
Read the worked answer
Do:
- Splint or apply a hinged brace locked near full extension for comfort and protection, fit crutches, and teach partial weight bearing with the crutches.
- Apply cold and compression, elevate, and document the mechanism, onset, effusion timing, and your examination findings as you found them, including the caveat that guarding limited reliability.
- Perform and document a neurovascular screen (distal pulses, sensation, capillary refill, ability to dorsiflex and plantarflex).
- Make an explicit follow-up plan with a name, phone number, and timeframe the family can act on tonight, and give written aftercare instructions and red-flag criteria for going to the emergency department.
Deliberately avoid:
- Repeated Lachman or pivot-shift testing. Each repetition adds pain, increases hemarthrosis, and degrades the guarding you are already fighting. One careful attempt, documented as guarded, is enough.
- Diagnostic joint aspiration. It is not required to make the referral and adds infection risk; it will not change tonight's disposition.
- Reassuring language such as 'it's just a sprain, we'll see how it feels.' With a hemarthrosis this fast, that framing invites the family to skip follow-up.
- Rushing to obtain MRI tonight. Swelling and guarding limit the examination, and imaging is best ordered by the clinician who will act on it.
Emergency department tonight if she cannot bear weight at all, has a locked knee, neurovascular deficit, rapidly expanding tense effusion with severe pain, or if no reliable follow-up can be secured within a few days.
3. What do you tell the parents about urgency, and what is your referral timeframe and rationale?
Read the worked answer
Tell them plainly: blood is inside the joint, which means something structural was injured, and the examination tonight cannot reliably tell us which structure. The most common cause in a young athlete with this mechanism is an ACL injury, but a kneecap dislocation that has already slipped back into place and a growth-plate injury are both on the list and both need to be ruled out. Nothing about that conversation requires you to name a diagnosis you cannot confirm, and it should not include a prognosis.
Referral: orthopedic or sports medicine evaluation within 3–5 days, sooner if the emergency department criteria are met. Rationale:
- Early specialist assessment allows examination before guarding organizes, decisions about radiographs (including growth-plate assessment), and timely MRI ordering if indicated.
- Delay beyond a week or two allows quadriceps inhibition, gait compensations, and lost range of motion to accumulate, which complicates later treatment regardless of the diagnosis.
- Prompt evaluation also lets the family absorb the news and prepare for the possibility of surgery and a long rehabilitation timeline without the season's pressure compressing the decision.
Give them the follow-up contact, the red-flag list, and a note summarizing the injury for the receiving clinician. Also flag her for a preparticipation re-evaluation next season regardless of outcome - this is her first knee injury, and landing and cutting mechanics are worth addressing once she is cleared.