ACL Normal ArthroscopyACL Normal Arthroscopy

Arthroscopy images of right knee demonstrating normal ACL




ACL is intracapsular and extra-synovial




In full extension 

- subtends 45o angle in sagittal plane

- 25o angle in coronal plane



- 25-40 mm long

- 7-10 mm wide


Two bundles


Anteromedial and posterolateral bundles

- described regarding point of tibial insertion



- smaller

- tight in flexion

- test with anterior draw



- larger

- tight in extension

- test with Lachman / Pivot Shift



-  posterior articular nerve / branch tibial


Arterial supply 

- middle geniculate  



- posterior on the medial wall lateral femoral condyle

- semicircular



- passes anteriorly, distally and medially

- oval shaped fossa anterior and between the tibial spines

- majority of ligament passes deep to transverse meniscal ligament

- a few fascicles blend with anterior horn of lateral meniscus

- wider and stronger than femoral insertion




Collagen and elastin arranged in less parallel configuration than tendons

- allows increase in length without large increase in internal stress


Ligaments attach to bone directly or indirectly


Cruciates attach directly / 4 histological zones

- ligament

- nonmineralised fibrocartilage

- mineralised fibrocartilage

- cortical bone


Indirect attachments via periosteum and fascia

- i.e. tibial insertion of MCL




1° Stabilizer

- prevents anterior translation


2° Stabilizer

- lateral & medial stability




1:1500 - 1:3500




Non contact deceleration producing valgus twisting injury


Deceleration / ER / Valgus


Associated Injury


Meniscal Injury


60% lateral meniscus

- associated with acute ACL rupture

- classically posterior horn

- many will heal


Lateral Meniscus Posterior Horn Tear Post ACL RuptureLateral Meniscus Posterior Horn Tear Post ACL Rupture


40% medial meniscus

- associated with chronic ACL rupture



- 10-20%

- associated with characteristic bone bruise patterns  on MRI

- see femoral chondral impressions from hyper-extension injury


Lateral Femoral Condyle Impaction Post ACL InjuryLFC Bone Bruise


Chondral Injuries


Chondral Lesion Post ACL InjuryChondral Lesion Post ACL Injury



- 10-20%




1.  50% describe a "Pop"


2.  75% haemarthrosis

- intra-articular swelling or effusion within the first 2 hours after trauma suggests hemarthrosis

- swelling that occurs overnight usually is an indication of acute traumatic synovitis / meniscal tear


3.  Immediate inability to weight bear


DDx hemarthrosis 


Rupture of a cruciate ligament

Osteochondral fracture

Peripheral tear in the vascular portion of a meniscus

Tear in the deep portion of the joint capsule

Intra-articular fracture of tibial plateau / distal femur / patella




Laxity Grading Lachmans / Anterior Draw


1+: mild instability < 5mm

2+: moderate instability 5-10mm

3+: severe instability >10mm




20 - 30° Flexion

- removes effects of bony contour / menisci i.e. 2° constraints

- stabilise femur with one hand, other hand behind tibia with anterior force

- sublux the tibia forward


85% sensitivie when awake 

100% under anaesthetic


Lachmans PreLachman's Post


Anterior Draw


Knee at 90° Flexion with hamstring relaxed

- foot in neutral

- sit on foot to stabilise

- hands behind tibia and pull forward

- has to > 3mm different to contralateral knee


Anterior drawer 1Anterior Drawer 2


Foot in 15° of External Rotation

- medial structures tightened in this position

- reassess anterior draw

- if have positive anterior draw in this position suggests associated posteromedial injury

- ACL + MCL / Med Capsule / OPL


Foot in 30° of Internal Rotation

- lateral structures tight in this position

- reassess anteior draw

- if have positive anterior draw in this position suggests associated posterorlateral injury

- ACL / LCL / PLC Complex 


Pivot Shift



- ACL torn

- lateral tibia subluxed anteriorly in extension

- reduced in flexion



- knee moves from extension to flexion

- valgus force applied to knee

- apply axial load

- mimicking weight bearing



- in extension the LTC is subluxed anteriorly

- in extension ITB is in front of flexion axis and is extender of knee

- as the knee is flexed

- ITB moves behind the flexion axis and becomes flexor of knee (20-40°)

- this reduces the LTC


“The relocation of the subluxed lateral tibial condyle as the extended knee is flexed”

“This occurs as the ITB line of function changes so as to become a flexor rather than an extensor of the knee”


Lachman 1Lachman 2


Need 4 things for a pivot shift

1. MCL to pivot about

2. ITB to reduce on flexion

3. Ability to glide ie no meniscal tear

4. °FFD




Jakob et al JBJS Br 1987

- 3 grades with foot in varying degrees of rotation


Grade 1:  Pivot shift with foot IR

Grade 2:  Pivot shift with foot neutral

Grade 3:  Pivot shift with foot ER




Usually normal


Segond Fracture

- small avulsion fracture of lateral proxima tibia

- is sign of lateral capsular avulsion

- pathognomonic of ACL tear


ACL SegondSegond Fracture


Tibial avulsion

- more common in children

- can be seen in adults


ACL Bony Avulsion XrayACL Bony Avulsion CTACL Bony Avulsion AdultACL Bony Avulsion Sagittal MRI






Smith et al Am J Roentgenology 2016

- meta-analysis of 3T MRI accuracy in diagnosing ACL tears in comparison to arthroscopy

- 92% sensitive and 99% specific for ACL tears


Normal ACL on MRI


 Intact ACL T2Intact ACL T1MRI Normal ACL



- straight structure

- able to see continuity of fibres from tibial to femur

- parallel to intercondylar notch

- no anterior subluxation of the tibia

- normal to have some increased signal due to adipose and synovial tissue


Torn ACL on MRI




- high signal intensity / oedema in ACL, especially acutely

- unable to identify continuous fibres from tibia to femur

- loss of taut, straight line of fibres


ACL MRI Femoral ACL Avulsion                   

Sagittal TI MRI with no femoral attachment 


ACL MRI Rupture T2ACL Torn with remnant stump MRI

Sagittal T2 MRI with midsubstance ACL tear      Sagittal T1 MRI with midsubtance ACL tear



MRI ACL torn and healed on PCL

Sagittal MRI with complete ACL rupture


ACL Femoral Avuslion MRI

Axial MRI demonstrating no ACL attachment to lateral femoral condyle


ACL Partial Tear


ACL Partial Tear


Bone bruising patterns

- pathognomonic

- caused by the knee pivot shifting

- terminal sulcus of LFC

- posterolateral tibial plateau


MRI ACL Rupture Bony Oedema Lateral Femoral CondyleMRI ACL Rupture Bone Oedema Terminal SulcusMRI ACL Rupture Bone Oedema Posterolateral Tibia

Coronal MRI with LFC bone bruising            Sagittal MRI with terminal sulcus LFC        Sagittal MRI with bone bruise posterolateral tibial plateau





- empty lateral wall

- ACL healed onto PCL


Arthroscopy Empty Lateral Wall

Arthroscopy of left knee showing no ACL attachment to lateral femoral condyle


Ruptured ACL

Arthroscopy of right knee showing no ACL attachment to lateral femoral condyle


ACL Rupture Empty Lateral Wall

Arthroscopy of left knee demonstrating only a few minor fibres attached to lateral femoral condyle