Diagnosing and treating posterior knee pain can be difficult as there are multiple anatomical structures and clinical causes that need consideration. Potential causes of posterior knee pain include muscular, intra-articular, vascular and neural, all of which may present differently, therefore requiring a clear history and thorough examination.
The musculoskeletal case of the month describes an unusual case of posterior knee pain and demonstrates a systematic approach when assessing these patients.
Clinical History
- 25-year-old woman presents with posterior left knee pain for past 4 years
- Pain localised to head of medial gastrocnemius and popliteal fossa
- Cannot recall any injury or acute traumatic episode
- No obvious swelling or bruising
- Pain is worst when exercising, particularly running
- Onset occurs around 10 minutes into run then has to stop due to pain
- Once flared, pain can be persistent for a few hours
- Faster running pace exacerbates pain (pain worse at speeds greater than 5min/km)
- With particularly bad flares, pain can linger on into the night
- Icing the back of the knee helps
- No locking or instability symptoms in knee
- No back, mid-calf or mid-hamstring pain
- No pins and needles or change of colour in foot when pain is present (or otherwise)
Past Medical History
- Nil
Drug History
- Nil
Exercise History
Seasoned runner, averaging about 40km/week. Enjoys half marathon distance. Good strength and conditioning programme. Has had to remove calf raises from programme as aggravates symptoms.
Clinical Examination
Functional testing
- Normal antalgic gait
- Able to perform single leg calf raises and single leg hops with no pain
- Calf capacity test – able to perform >20 single leg calf raises of step on both sides with no issue
Post 10-minute treadmill run test at (speed 4m45s/km)
- Mild antalgic gait
- Weakness and pain on single leg calf raise
- Pain on resisted plantar flexion on the left calf
Bedside testing (post treadmill test)
- No discoloration or bruising in calf
- No swelling in popliteal fossa or calf
- No varicose veins or signs of peripheral arterial disease
- No bruits on auscultation over popliteal fossa (testing for popliteal artery entrapment syndrome)
- On palpation tenderness over medial gastrocnemius head
- Well’s score for DVT – 0
Bedside Diagnostic Ultrasound
- No Baker’s cyst or other popliteal cyst
- No knee joint swelling or effusion
- No calf tear or lesion particularly in medial gastrocnemius head where pain is located
- Normal appearances of the popliteal artery and vein (no formal doppler scan performed at this stage).
Differential Diagnosis and clinical reasoning
1. Baker’s or other popliteal cyst
Before diagnostic ultrasound confirmed the absence of a cyst, this would be the most likely differential diagnosis.
Popliteal cysts are common in fit and active people. They are not always visible and troublesome ones can increase in size during and post exercise. It is often difficult to palpate a cyst unless it is very big, due to the soft tissue in the popliteal fossa.
Diagnostic ultrasound is very accurate and detecting popliteal cysts. However, some deeper cysts sitting in the posterior knee itself such as ACL/PCL/meniscal cysts can be more difficult to differential via ultrasound.
2. Intra-articular posterior knee pathology
Structures in the back of the knee can often present as calf pain/tightness. The most common pathology that presents in this way is posterior meniscal inflammation/tears (particularly on the medial side).
Ligamentous injuries such as ACL/PCL/Posterolateral corner injury can also masquerade as calf pain. Less commonly, knee bone or articular surface injury such as OCD or stress fractures present as isolated calf pain, but these lesions are usually associated with knee specific symptoms such as swelling or locking.
3. Deep vein thrombosis
DVT must always be considered in patients with posterior knee pain and a Well’s score calculated. In young women, questioning around the combined oral contraceptive pill use is important as this increases the risk of DVT. In this case, the absence of risk factors (non-smoker, no long-haul travel) and clinical features (e.g. calf swelling, heat, redness) makes DVT unlikely. If concerned, the definitive test for detecting DVT is a full leg doppler scan, tracing the deep venous system from the saphenofemoral junction to the calf.
4. Popliteal artery entrapment syndrome (PAES)
A more unusual case of posterior knee pain is PAES. This is when exercise causes medial gastrocnemius and medial hamstring spasm, restricting flow through the popliteal artery. Patients develop claudication type symptoms in the posterior knee and this usually spreads to the calf. There are often other clinical signs such as paraesthesia and change of colour to the foot.
Another pathognomonic sign of PAES (and compartment syndrome), is that symptoms resolve very quickly after the cessation of exercise (as blood flow returns). In this case, the patient’s symptoms could last for long periods post-exercise which is not typical for PAES.
5. Chronic exertional compartment syndrome (CECS)
Similar to PAES, this syndrome presents with entrapment of the nerves (tibial and common peroneal) as the pressure in various calf compartments increases during exercise.
This, however, does not normally present with popliteal pain with symptoms being more diffuse in the calf bulk and foot.
The absence of paraesthesia, foot weakness and quick resolution of symptoms post cessation of exercise makes CECS unlikely.
6. Musculotendinous
Although there is no clear history of muscular injury or calf tear, chronic tendinopathy of the medial hamstring complex or chronic calf strain needs to be considered. Popliteus muscle injury or spasm can be a more discrete cause for posterior knee pain
Diagnostic Imaging
MRI
This T2 weighted sagittal plane image shows cortical irregularity of the medial gastrocnemius attachment and fluid within the medial gastro-insertional tendon (green circle).
CT SPECT
CT SPECT is a bone uptake scan, showing increased activity at the medial femoral condyle posterior eminence indicating bone injury.
Diagnosis: Cortical Desmoid
Management of Cortical Desmoid
- Corticosteroid injection into medial gastrocnemius insertional interface due to severity of symptoms
- Strict offloading into plantar flexion for 4 weeks with patient put into fixed ankle foot orthosis (this technique preferred over boot to help maintain calf bulk foot intrinsics musculature).
- Reintroduce plantarflexion calf isometrics at week 4 with graduated return to concentric and eccentric loading
- Plyometrics reintroduced at week 6
- Return to running at week 8
Discussion And Key Points
Cortical desmoid, also known as avulsion injury of the medial gastrocnemius or adductor magnus tendon or tug lesion, is a benign, self-limiting lesion, typically found on the posteromedial aspect of the distal femur.
They can be bilateral in a third of cases.
The condition is most commonly seen in adolescents, particularly athletes, and is linked to repetitive stress or minor trauma at the insertion of the adductor magnus and medial gastrocnemius muscles. It occurs near the linea aspera of the femur, where the muscle attachment undergoes constant stress during physical activities like running, jumping, or kicking.
The exact cause of cortical desmoid is believed to be repetitive traction injury or avulsion at the tendon insertion, causing periosteal reaction and bone remodelling. This process can mimic more serious conditions like osteosarcoma on imaging, but cortical desmoid is a benign and non-neoplastic lesion.
Clinical Presentation of Cortical Desmoid
Patients with cortical desmoid are usually asymptomatic, and the lesion is often discovered incidentally on imaging studies, particularly X-rays or MRI scans, performed for unrelated reasons. However, in some cases, patients may experience:
- Pain on the inner aspect of the distal thigh, often exacerbated by physical activity, particularly movements that engage the adductor magnus and gastrocnemius muscles.
- Tenderness or discomfort on palpation near the posteromedial side of the femur.
- Occasional swelling or soft tissue mass may be present, although this is rare.
- Absence of other aetiologies (as described above)
Because of their benign nature, cortical desmoid lesions typically do not cause systemic symptoms such as fever, weight loss, or night pain, which are more indicative of malignant conditions like bone sarcomas.
Imaging and Diagnostics
Diagnosis of cortical desmoid is primarily based on imaging studies. Common findings include:
- X-rays: May reveal a lytic lesion with a sclerotic rim in the posteromedial cortex of the distal femur. This appearance can raise concerns for malignancy, but the location and well-defined nature help differentiate it from more aggressive lesions.
- MRI: Provides more detailed imaging, showing a lesion with low signal intensity on T1-weighted images and high signal intensity on T2-weighted images.
The lesion may also show a periosteal reaction and bone marrow oedema, but the lack of aggressive features (such as soft tissue mass or cortical breakthrough) suggests a benign process.
- CT scans: Can also be useful in assessing the cortical involvement and bone structure in more detail.
The differential diagnosis includes more sinister conditions like osteosarcoma, osteoid osteoma, or infection. The benign clinical course and characteristic imaging findings help distinguish cortical desmoid from these conditions.
Treatment of Cortical Desmoid
Cortical desmoid is a self-limiting condition that usually requires conservative management. Treatment primarily focuses on symptom relief and monitoring the lesion for resolution over time. The following approaches are commonly used:
- Rest and activity modification: Since the condition is often related to repetitive stress, reducing the intensity of physical activity, especially activities that put strain on the adductor magnus and gastrocnemius muscles, can help alleviate symptoms.
- Nonsteroidal anti-inflammatory drugs (NSAIDs): These can be used to manage pain and inflammation associated with the lesion.
- Physiotherapy: Strengthening and stretching exercises for the adductor and gastrocnemius muscles may aid in preventing further injury and promoting healing.
- Observation: In asymptomatic cases, especially when the lesion is found incidentally, simple observation with periodic follow-up imaging may be all that is necessary. The lesion often stabilises or resolves without intervention.
- Injection based interventions: Ultrasound guided injections and reduce local inflammation around the cortical defect tendon insertion to reduce inflammation and pain.
- Surgical intervention: Rarely indicated but may be considered if the lesion becomes symptomatic or is mistaken for a more aggressive process. Surgery typically involves curettage of the lesion, though this is seldom required.
Prognosis of Cortical Desmoid
The prognosis for cortical desmoid is excellent. In most cases, the lesion resolves or becomes quiescent with optimum loading and rehabilitation and the likelihood of recurrence or long-term complications is minimal. Importantly, it is a benign condition with no potential for malignant transformation, making the risk of significant morbidity extremely low.
Conclusion
Cortical desmoid is a benign, self-limiting lesion primarily affecting adolescents and young athletes. Its development is associated with repetitive stress or trauma to the distal femur. Diagnosis is primarily through imaging, and treatment involves conservative measures like rest, NSAIDs, and observation. The prognosis is excellent, with spontaneous resolution or stabilisation of the lesion in most cases.
Key Learning Points
- Diagnosing posterior knee pain can be challenging with muscular intra-articular, vascular and neural considerations
- Performing a Wells score help clinical decision making with suspected DVT
- Cortical desmoid is a benign, self-limiting lesion primarily affecting adolescents and young athletes
- MRI can be more sensitive in detecting subtle bone lesions than X-ray
- Cortical desmoids are caused by repetitive stress or traction of the medial/lateral head of gastrocnemius or adductor magnus tendon attachments.
- Early detection and management can improve patient outcomes
