Total Knee Arthroplasty 2.0 Apurv Mehra, Heiko Graichen
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Orthopedic Anatomy: Knee JointCHAPTER 1

 
KNEE ANATOMY
The Knee is the largest joint of the human body. It consists of 2 condylar joints between condyles of femur and tibia (also known as Tibial plateaus) and a gliding joint between patella and corresponding surface of the femur.
The femoral condyles are convex. They are spiral with a curvature increasing posteriorly (a closing helix), that of the lateral condyle being larger.
The wedge shaped menisci maintains some degree of conformity and distributes the contact pressure between the femur and tibia. They increase the area over which the compressive force is distributed.
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Figure 1.1: Knee anatomy
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LIGAMENTS
Ligamentum patellae is the continuation of the central tendon of quadriceps femoris, which is between patella and tibial tuberosity.
Lateral collateral ligament is between femoral condyle and head of fibula. The tendon of popliteus intervenes between the ligament and the lateral meniscus.
Medial collateral ligament (MCL) is a flat band attached to medial condyle of femur and tibia. The edge of the medial meniscus is firmly attached to the MCL.
MCL is composed of superficial & deep portions;
A. Superficial MCL:
  • Anatomically this is the second (middle) layer of the medial compartment of the knee joint. Its proximal attachment is on the posterior aspect of medial femoral condyle (3.2 mm proximal and 4.8 mm posterior to the medial epicondyle). The distal attachment is over the metaphyseal region of the tibia, upto 4-5 cm distal to the joint, lying beneath the pes anserinus; it consists of two tibial attachments on the anterior arm of the semimembranosus tendon and about of 12.2 mm distal to the tibial joint line and second one being a broad insertion of about 61.2 mm distal to the tibial joint line; it is located just anterior to the posteromedial crest of the tibia. The function of MCL is to provides primary restraint to valgus stress at knee.
  • Providing from > 60-70% of restraining force depending on knee flexion angle;
  • At 25° of flexion, the MCL provides 78% of the support to valgus stress;
  • At 5° of flexion, it contributes 57% of the support against valgus stress; The superficial ligament can be divided into anterior & posterior portions;
  • Anterior fibers of superficial portion of ligament appear to tighten with knee flexion of 70° to 105°
  • Posterior fibers form the posterior oblique ligament:
B. Deep MCL:
  • Anatomically this is the third (deep) layer of the medial compartment, which in many cases will be separated from the superficial MCL. This layer is divided into meniscofemoral and meniscotibial ligaments, it inserts directly into edge of tibial plateau & meniscus; although it firmly attaches to the meniscus but does not provide significant resistance to valgus force.3
The oblique popliteal ligament is tendinous expansion from the semimembranous muscle. It strengthens the posterior aspect of the capsule.
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Figure 1.2: The Knee (The soft tissue components)
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KNEE MOVEMENTS
The femoral and tibial articular surface profiles produce a variable placed axis of rotation during the flexion arc. There are different motion patterns in the medial and lateral tibiofemoral compartments. Laterally the femoral displacement on tibia involves rolling and sliding at the joint surface. Whereas, medially for most of the flexion arc there is minimal relative motion of femur and tibia. Note: sliding and posterior displacement occurs medially only when flexion exceeds 120°.
The rotations at knee are conjunct with flexion and extension.
The range of knee extension is 5°-10° beyond the straight position. Flexion is upto 120°- 130° with hip extended, 140° with hip flexed and 160° by passive force. Voluntary rotation is 60°-70° and conjunct rotation upto 20°. Medial conjunct rotation in terminal stages of extension is a part of locking mechanism the so called screw home movement.
 
KNEE KINEMATICS
The primary motion of the knee occurs in the sagittal plane. It is primarily a modified hinge joint allowing flexion-extension and a measure of rotatory motion. Knee movement is described from 0° (neutral); this position is important to support the body weight like a simple strut. Knee in gait has about 65°-70° flexion during the swing phase and about 15° flexion in midstance phase.
In extension of the knee, the tibia rotates externally and foot inverts and in flexion tibia rotates internally and foot everts.
The arcs of femoral condyles are much longer than the AP diameter of tibial plateau. This means if there is full flexion with pure rolling motion femur will roll off the tibial back. This is prevented by anterior sliding of femur along with rolling due to the tension in Anterior Cruciate Ligament.
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Figure 1.3: Femoral roll back and sliding
5Q angle: (Quadriceps Angle) It describes patella alignment. It is formed by lines connecting centre of patella with Anterior Superior Iliac Spine proximally and the centre of tibial tubercle distally. (Males 10°-14° and females 15°-17°). The greater the Q angle, the greater the lateral force on patella. The restoration of normal Q angle is important in TKR for normal patella tracking.
Lower Extremity Alignment
Definitions
Anatomical axis of femur
Line drawn along the axis of the femur
Anatomical axis of tibia
Line drawn along the axis of the tibia
Mechanical axis of femur
Line drawn between center of femoral head and intercondylar notch
Mechanical axis of tibia
Line drawn between center of knee and center of ankle mortise
Knee axis
Line drawn along inferior aspect of both femoral condyles
Vertical axis
Vertical line perpendicular to the ground
Lateral distal femoral angle
Angle formed between knee axis and femoral axis laterally
Medial tibial angle
Angle formed between knee axis and tibial axis
Relationship
Knee axis
Parallel to the ground and perpendicular to vertical axis
Mechanical axis of femur
Average of 6° from anatomic axis (5°-7°)
Approximately 3° from vertical axis (2°–4°)
Mechanical axis of tibia
Normally same as anatomic axis of tibia unless tibia has a deformity
Lateral distal femoral angle
81° from femoral anatomic axis
87° from femoral mechanical axis (86°-89°)
Medial proximal tibial angle
87° from tibial mechanical axis (86°-89°)
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Figure 1.4: Lower limb alignment
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X-RAY ANATOMY
Radiography
Technique
Findings
Clinical application
Standard X-rays
Anteroposterior
Supine; beam at 90°
Medial/lateral compartments varus/valgus deformity
Femoral condyle, tibial plateau/spine, patella fx, OCD, osteoarthritis (weight-bearing)
Lateral
Supine; 30° flexion
Patellofemoral compartment
Fractures, quadriceps/patellar tendon rupture
Axial/sunrise
Prone, knee 115° flex, beam at patella 15° cephalad
Patellofemoral compartment (patellar articular facets)
Patellofemoral arthritis, malalignment or patellar tilt
Special views
Tunnel/notch
Prone, knee 45° flex, beam is caudal at knee joint
Posterior femoral condyles intercondylar notch, tibial eminence
Osteochondral fx/defect, femoral condyle or tibial eminence fx, DJD/osteoarthrilis
Merchant
Supine, legs of table at 45°; beam at PF joint
Patellofemoral compartment (Patellar articular facets)
Articular surface lesions, DJD, tilt or malalignment
Rosenberg
PA (weight-bearing); knees at 45°
Medial/lateral compartments
Osteoarthritis of WB portion of posterior condyles
AP radiograph of Knee
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Figure 1.5A:
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Figures 1.5(A to D): X-rays of knee joint
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OTHER STUDIES
Alignment films
Bilateral full length hip to ankle, WB
Full lower extremity alignment
Determine malalignment/deformity
Computed tomography
Axial, coronal and sagittal views
Articular congruity, fracture fragments
Intra-articular condyle plateau pilon fxs
Magnetic resonance imaging
Sequence protocols vary
Soft tissues, ligaments, meniscus, articular cartilage, bone marrow
Ligament ruptures, meniscal tears OCD, stress fxs, tumor, infection, arthritis, focal cartilage defects
Bone scan
Radioisotope
All bones evaluated
Stress fxs, infection, tumor
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Figures 1.6: Long leg X-rays