A Assessment

 

ACL Normal ArthroscopyACL Normal Arthroscopy

 

Anatomy

 

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 

 

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

 

Gross Anatomy

 

Intracapsular and extra-synovial

 

Direction

 

In full extension ACL

- subtends 45o angle in sagittal plane

- 25o angle in coronal plane

 

Dimensions

- 25-40 mm long

- 7-10 mm wide

 

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

- medial wall LFC

- semicircular

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

 

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

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

- wider and stronger than femoral insertion

 

Function

 

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

 

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%

- 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

 

MCL 

- 10-20%

 

History

 

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

 

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 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

 

MRI

 

Normal ACL on MRI

 

 Intact ACL T2Intact ACL T1MRI Normal ACL

 

Characteristics

- 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

 

Findings

- 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

 

Arthroscopy

 

ACL Partial Tear ArthroscopyArthroscopy Empty Lateral Wall

 

ACL Rupture Empty Lateral WallRuptured ACL

 

Findings

- empty lateral wall

- ACL healed onto PCL

- partial tears

- ACL healed onto different part of LFC