Assessment

 

ACL Normal ArthroscopyACL Normal Arthroscopy

Arthroscopy images of right knee demonstrating normal ACL

 

Anatomy

 

ACL is intracapsular and extra-synovial

 

Direction

 

In full extension 

- subtends 45o angle in sagittal plane

- 25o angle in coronal plane

 

Dimensions

- 25-40 mm long

- 7-10 mm wide

 

Two bundles

 

Anteromedial and posterolateral bundles

- described regarding point of tibial insertion

 

Anteromedial

- smaller

- tight in flexion

- test with anterior draw

 

Posterolateral

- larger

- tight in extension

- test with Lachman / Pivot Shift

 

Nerve

-  posterior articular nerve / branch tibial

 

Arterial supply 

- middle geniculate  

 

Origin

- posterior on the medial wall lateral femoral condyle

- semicircular

 

Insertion 

- 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

 

Histology

 

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

 

Function

 

1° Stabilizer

- prevents anterior translation

 

2° Stabilizer

- lateral & medial stability

 

Incidence

 

1:1500 - 1:3500

 

Mechanism

 

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

 

Fractures 

- 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

 

MCL 

- 10-20%

 

History

 

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

 

Examination

 

Laxity Grading Lachmans / Anterior Draw

 

1+: mild instability < 5mm

2+: moderate instability 5-10mm

3+: severe instability >10mm

 

Lachman's 

 

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

 

Concept

- ACL torn

- lateral tibia subluxed anteriorly in extension

- reduced in flexion

 

Technique

- knee moves from extension to flexion

- valgus force applied to knee

- apply axial load

- mimicking weight bearing

 

Findings

- 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

 

Grading

 

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

 

X-ray

 

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

 

MRI

 

Accuracy

 

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

https://www.ajronline.org/doi/10.2214/AJR.15.15795

 

Normal ACL on MRI

 

 Intact ACL T2Intact ACL T1MRI Normal ACL

 

Characteristics

- 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

 

 

Findings

- 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

 

Arthroscopy

 

Findings

- 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