Definition – avulsion of the meniscal insertion to the tibia, or a radial tear of the meniscus that is located within 1cm of the insertion.
Presentation – medial meniscus posterior root tears typically in older individuals. Lateral meniscus root tear associated with ACL injuries. Single episode of painful popping at time of symptom onset, posterior knee pain with flexion, joint line tenderness.
Clinical diagnosis – combining history, clinical tests and MRI scan findings.
Treatment – optimal management is not clear. Meniscal root repair unless radiographic evidence of advanced osteoarthritic changes, BMI ≥30 or varus alignment >5°.
Where to refer – Kellgren & Lawrence ≤2: referral to an orthopaedic surgeon. Kellgren & Lawrence ≥3: refer to physiotherapist for non-surgical management.
A meniscal root tear is defined as an avulsion of the meniscal insertion to the tibia, or a radial tear of the meniscus that is located within 1cm of the insertion.
Anatomy and function
The anterior and posterior meniscal roots firmly attach the medial and lateral menisci to their respective tibial plateaus (figure 1). The menisci play an important role in load transmission and joint stability, with both functions being dependent on intact meniscal roots.
Figure 1: anterior and posterior root attachments of the medial (MM) and lateral meniscus (LM). From Guermazi et al, freely available here.
During weight-bearing, axial loading of the tibiofemoral joint imparts a radial force on the menisci (figure 2), which is resisted by the circumferential meniscal fibres. This ‘hoop stress’ mechanism transmits 50-70% of axial load away from the articular cartilage, protecting the joint surfaces from excessive pressure. The posterior and middle horns of the menisci contribute to knee stability by providing a buttress against the corresponding portions of the femoral condyles (figures 3-4). The medial meniscus provides more anteroposterior stability and the lateral meniscus more rotational stability.
Both menisci move to conform to the changing shape of the femoral condyles at different knee angles, which ensures load transmission and stability occurs in varying weight-bearing positions.
Figure 2: axial load exerting a radial load on the meniscus (orange arrow).
Figure 3-4: buttress effect of the meniscus against the femoral condyle preventing forwards-backwards and side-to-side translation of the tibia.
Disruption of the meniscal root can cause the meniscus to extrude (be forced out) beyond the tibial margin under axial loading (figure 6). Meniscal extrusion compromises the hoop stress mechanism of the meniscus, with ≥3mm extrusion of the medial meniscus being strongly associated with accelerated degenerative changes in the medial compartment. Simulated medial meniscus posterior root tears (MMPRTs) alter knee kinematics in cadavers by increasing anterior, lateral and anteromedial translation [external rotation] of the tibia, due to the loss of the buttress.
Together, the biomechanical changes associated with a MMPRT have been shown to increase peak contact pressure in the medial compartment by 25%, exposing the joint to loads that are equivalent to a total medial meniscectomy. Adolescents who underwent total meniscectomy were 132 times more likely to undergo total knee replacement for symptomatic osteoarthritis at 40-year follow up when compared with geographical and age-matched peers. Excessive medial compartment pressure, secondary to a MMPRT, has also been implicated in the development of subchondral insufficiency fractures of the knee (SIFK), resulting in bone marrow oedema, focal ischaemia and eventual necrosis.
Conversely, the loss of a buttress against the lateral femoral condyle in lateral meniscus posterior root tears (LMPRTs) may contribute to anterolateral rotational instability of the tibia in patients with anterior cruciate ligament injury.
Causes & risk factors
Meniscal root tears can be categorised as degenerative or traumatic. Degenerative root tears develop with no obvious injury, or with minimal trauma during normal daily activities. Traumatic meniscal root tears are caused by a significant, memorable injury where the force exerted on the knee tears the meniscal root in isolation, or in combination with other structures.
The posterior root of the medial meniscus is the most common site of injury, likely due to the greater loads exerted on this region combined with the reduced mobility of the posteromedial root. MMPRTs often occur in older populations (>40 years of age) during normal daily tasks including descending activities (stairs, steps and slopes), ascending stairs, walking, standing up from sitting or squatting. Risk factors for MMPRT include:
- female gender
- BMI >30 kg/m2
- varus knee alignment
- lower sports activity levels.
Although meniscal root tears are considered a precursor to knee osteoarthritis, degenerative changes may already be present before the onset of a symptomatic root tear.
Lateral meniscus root tears (LMRT) are more frequently associated with a history of trauma and typically occur in younger populations. LMRT are strongly associated with anterior cruciate ligament injury, while MMPRT are more likely to occur in isolation or with concomitant chondral injury.
The individual may present with a history of sudden posterior knee pain, or a single episode of painful popping (i.e. not recurrent). For details on the diagnostic accuracy of painful popping for a MMPRT, please visit the statistics section.
Meniscal root tears may be associated with greater pain than meniscal tears, but mechanical symptoms (locking, catching or giving way) are less likely to be present. There may be swelling within the knee joint (effusion) and posterior knee pain with full knee flexion, particularly when the joint is loaded (e.g. squatting) .
Clinical tests that attempt to diagnose meniscal tears may be positive in meniscal root tears, including joint line tenderness (video 1), McMurray’s test (video 2) and the Thessaly test (video 3). It has been reported that McMurray’s test may be positive for pain but not for creating a mechanical click in a MMPRT. However, the diagnostic accuracy of these individual tests has not been reported for meniscal root tears.
In the presence of a MMPRT, meniscal extrusion may be palpated at the anteromedial joint line when varus stress is applied to the knee in full extension; this extrusion then disappears when varus stress is removed. The diagnostic accuracy of this ‘extrusion test’ has not been reported.
Video 1: joint line tenderness
Video 2: McMurray’s test
Video 3: Thessaly test
Bony avulsion fractures of the meniscal root attachment may be evident on plain X-rays. Although the meniscal roots are not directly visible on X-ray, the presence of a meniscal ossicle is highly suggestive of a root tear. Meniscal ossicles are most commonly found within the posterior horn or root of the medial meniscus and are considered a sequela of a MMPRT.
The diagnostic accuracy of MRI versus arthroscopy for meniscal root tears has been investigated in a number of studies; for details on the diagnostic accuracy of MRI, please visit the statistics section. Characteristic MRI findings have been described including:
- high signal and truncation/cleft sign at the meniscal root
- giraffe neck sign.
- meniscal extrusion >3mm can be a secondary sign, but not pathognomonic, of a meniscal root tear
- high signal at the meniscal root
- ghost sign.
Ipsilateral (same side) tibiofemoral compartment bone marrow oedema and insufficiency fractures are commonly noted in the presence of a MMPRT.
Figures 5-10: MRI findings of high signal, truncation, giraffe neck sign and meniscal extrusion (coronal and axial views), ghost sign (sagittal view) and subchondral bone oedema. Giraffe neck sign from Furumatsu et al (2017).
A number of systems have been used to classify meniscal root tears.
Classification of anterior and posterior root tears of the medial and lateral meniscus, based on tear type:
- Type 1: partial root tear
- Type 2A: complete radial root tear 0 to <3mm from attachment
Type 2B: complete radial root tear 3 to <6mm from attachment
Type 2C: complete radial root tear 6 to ≤9mm from attachment
- Type 3: complete root tear with a bucket handle meniscus tear
- Type 4: oblique tear into the root attachment
- Type 5: root avulsion fracture
Classification of LMRT in anterior cruciate ligament injuries:
- Type 1: avulsion of the root at the attachment on the tibial plateau with an intact meniscofemoral ligament.
- Type 2: radial tear of the posterior horn with an intact meniscofemoral ligament.
- Type 3: complete injury of the posterior horn of the lateral meniscus with rupture of the meniscofemoral ligament.
Classification of MMPRT based on the tear gap:
- Type 1: incomplete root tear
- Type 2: complete root tear, no gap or overlapped
- Type 3: complete root tear, gap of 1-3 mm
- Type 4: complete root tear, gap of 4-6 mm
- Type 5: complete root tear, gap of 7 mm
Higher grades of MMPRT, which reflects greater gapping, is associated with higher meniscal extrusion, severe chondral loss and greater severity of osteoarthritis.
Although limited, the current body of evidence suggests that meniscal root repair provides superior outcomes (patient satisfaction, progression of radiographic osteoarthritis) when compared with meniscectomy or non-surgical treatment. Meniscal root repair attempts to restore normal meniscal function, with a recent study showing excellent mid- to long-term clinical data and survivorship rates at 8 years for MMPRT repairs. However, no head-to-head, high quality trials have been conducted investigating these different management strategies and root repair has not been shown to prevent the progression of osteoarthritis completely.
Risk factors for poor clinical outcomes after posterior meniscal root repair include:
- obese patients (BMI ≥30kg/m2)
- pre-existing high-grade chondral lesions
- severe varus knee alignment (>5°).
Therefore, if a meniscal root tear is suspected based on the patient history and clinical examination, long-leg, anteroposterior (AP) and Rosenberg view X-rays should be ordered before an MRI to assess the severity of the joint space narrowing and knee alignment.
Expert opinion recommends that patients with higher grades of radiographic osteoarthritis (Kellgren-Lawrence ≥3) should be managed conservatively, while meniscal root repair should be attempted in patients without significant pre-existing osteoarthritis (Kellgren-Lawrence grade ≤2); valgising osteotomy may be indicated in cases of significant varus misalignment.
Analgesics, physiotherapy and a medial unloader brace can help alleviate symptoms associated with a MMPRT. Female gender and higher baseline radiographic osteoarthritis at diagnosis is correlated with higher rates of progression to joint replacement.
Partial meniscectomy for a complete MMPRT provides no benefit in halting arthritic progression; female gender, increased BMI, and meniscus extrusion are associated with worse outcome following partial meniscectomy.
The optimal rehabilitation following meniscal root repair is not clear and any restrictions associated with additional procedures performed at the time of surgery should be taken into consideration.
Rehabilitation should be progressed based on the impairments present, the individual’s specific requirements (e.g. running, jumping) and their response to treatment.
The Knee Resource meniscus root repair protocol is based on published literature for meniscal root tears.
Individuals with a suspected meniscal root tear, without significant osteoarthritic changes on X-ray, should be referred to orthopaedics for further assessment. Individuals with significant osteoarthritic changes on X-ray, or poor surgical candidates, should be referred to a physiotherapist for appropriate conservative management.
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