A Assessment


ACL Normal ArthroscopyACL Normal Arthroscopy




Developmental Anatomy


Knee joint first appears as a mesenchymal cleft at 8 weeks gestation

- ACL and PCL separate entities by week 10

- cruciates principle determinants of  shape of tibiofemoral articulation 




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


Gross Anatomy


Intracapsular and extra-synovial




In full extension ACL

- subtends 45o angle in sagittal plane

- 25o angle in coronal plane



- 25-40 mm long

- 7-10 mm wide




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   



- medial wall LFC

- semicircular

- semicircular proximal insertion high and posterior on medial wall of LFC



- 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

- variable and minor attachment to the posterior horn of the lateral meniscus

- wider and stronger than femoral insertion




1° Stabilizer

- prevents anterior translation


2° Stabilizer

- lateral & medial stability

- protector of menisci


4 important features of function


1. Carries load throughout entire range resisting AP and translational forces

- different fibres recruited at different times 


2. Carries only small loads during normal activity

- about 20% of failure capacity during normal loading


3. Highest loads are produced by quadriceps powered extension of knee (open chain exercises)

- but during any one exercise failure loads only reach about 5%


4. Much more complex behaviour than just a series of fibres

- exhibits viscoelastic properties allowing it to adapt to different loading patterns

- ACL consists of many fascicle subunits

- these are recruited as needed to accommodate strain




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%

- assciated 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

- intraarticular 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




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 Segond


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




Normal ACL on MRI


 Intact ACL T2Intact ACL T1MRI Normal ACL



- straight structure

- parallel to intercondylar notch

- no anterior subluxation of the tibia

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

- able to see continuity of fibres from tibial to femur


Not always accurate

- ACL is helicoid shape

- sagittal MRI alone inaccurate in 10 - 20%

- sensitively increase to > 95% by using coronal and axial images


Torn ACL on MRI


ACL MRI Femoral ACL AvulsionACL MRI Rupture T2



- high signal intensity / oedema in ACL, especially accutely

- unable to identify continuous fibres from tibial to femur

- loss of taut, straight line of fibes

- loss of attachment onto LFC on axial


ACL Femoral Avuslion MRI


May see stump of ACL


ACL Torn with remnant stump MRI


May identify ACL healed onto PCL


MRI ACL torn and healed on PCL


May see tibia subluxed anteriorly


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


Mechanical Testing


KT 1000 

- Instrumented Lachman's and Anterior Draw

- > 3mm c.f. other knee 98% sensitive

- > 10mm absolute on one side




ACL Partial Tear ArthroscopyArthroscopy Empty Lateral Wall


ACL Rupture Empty Lateral WallRuptured ACL



- empty lateral wall

- ACL healed onto PCL

- partial tears

- ACL healed onto different part of LFC