Principle of Total Knee Arthroplasty

1. Total Knee Arthroplasty
Principle
Dr. Adnan Qamar

2. Technical Goal Arthroplasty
1) Restoring mechanical alignment
2) Preserving (or restoring) the joint line
3) Balancing ligaments
4) Maintaining or restoring Q angle

3. Restoring Mechanical Alignment
• Aim to restore neutral mechanical alignment
• Forces through the leg pass through the centre of
the knee
• Allow optimal load share through medial and lateral
sides
• Prevent excessive wear and early failure

4. Axis of Lower limb
• Mechanical axis of lower limb
▫ Line drawn from centre of head of femur to centre of
ankle joint
• Vertical axis of lower limb
▫ Vertical line that extends distally from the centre of the
pubic symphysis
• Femur
▫ Mechanical axis
▫ Anatomical axis
• Tibia
▫ Mechanical axis
▫ Anatomical axis

6. Femur
• Anatomical Axis
▫ Determine entry point of femoral IM guide
• Mechanical Axis
▫ Neutral axis means load share between condyles
• Valgus cut angle
▫ Difference between AA and MA
▫ Usually 5 – 7 degree
▫ Perpendicular to MA
▫ Can vary with height

7. Tibia
• Anatomical Axis
• Mechanical Axis
▫ Proximal tibial cut is perpendicular to MA
• If there is tibial deformity and the mechanical axis and
anatomical axis not the same, then the proximal tibia
must be cut perpendicular to mechanical axis.
• Therefore an extramedullary tibial guide must be used
be used.

8. • Aim is to get neutral alignment
▫ To make cut perpendicular to MA both the femur
and tibia
▫ So as to achieve excellent alignment of the
femoral, tibial, and patellar components, with
ultimate restoration of the patient’s lower
extremity to neutral.

9. • Alignment and mechanical axis needs to
be restored:
▫ Centre of hip – centre of knee – centre of
ankle should fall in one line

10. Restoring the joint height
• Ligament are vital in prosthetic knee kinematic.
• Knee function is optimised when joint height preserved.
• Need to remove sufficient amount of bone to allow
prosthetic placement.
• Prosthesis recreate original bone/cartilage thickness.
• Joint line is maintained.

11. • Elevating the joint line can lead to –
▫ Mid flexion instability
▫ Patellofemoral tracking problems
▫ An equivalent to patella baja
• Lowering the joint line canlead to –
▫ Lack of full extension
▫ Flexion instability

13. Restoring Joint Line
How to achive ?
• Know your patient
▫ Preop. workup / planning
▫ True position of joint line
▫ Defect – cement / graft / augment
• Know your prosthesis
▫ Modern instrument can provide precise bone cuts
▫ User – error – placement , shifting
▫ Size / thickness of prosthesis

14. Ligament / Soft tissue balancing
• Soft tissue become scarred / contracted or
stretched with degenerative diseases
• Must be balanced to provide optimum function
and loading
• Must be accomplished in sagittal and coronal
plain

15. Coronal Plane Balancing
• Varus/Valgus
• Basic Principles
▫ Release on concave side
▫ Fill up convex side

16. Sagittal Plane Balancing
• Goal is to obtained
equal flexion and
extension gaps.
• Prosthesis will remain
stable throughout arc
of motion

17. Sagittal Plane Balancing
• General Principles
▫ Gap Problem Symmetrical – Adjust Tibia
 Tibial cut affects both flexion and extension gap
▫ Gap Problem Asymmetrical – Adjust Femur
 Distal femur cut affects extension gap
 Posterior femur cut affect the flexion gap

18. Maintain Q angle
• Q angle – Angle form
between line drawn from
ASIS to the mid point of
patella and from midpoint
of patella to the tibial
tuberosity .
• Normal
▫ Male – 12 degree
▫ Female – 16 degree

19. Mantain Q Angle / Patellofemoral
Tracking
• Avoid problem by Q angle management
• May lead to patellofemoral maltracking
• Increase in Q angle - Increase lateral subluxation
forces - Leading to Patelofemoral maltracking

20. Alignment Strategies
• There are various different alignment strategies and surgical
techniques that have been utilized to attain this goal .
• Classical alignment
▫ Has been commonly used for TKA using either the
 Measured resection (or)
 Gap balancing techniques.
• Anatomic alignment
▫ Sought to try to closely match the true anatomy of the femur and
tibia to allow the joint line to be parallel to the ground during the
normal stance phase of gait.

21. Techniques for Primary TKA
• Measured Resection Technique
▫ Bone cuts that are the same thickness as component(Implant)
• Gap Balancing Technique
▫ Technique relies on ligament releases before completing bone resections; this is
intended to correct fixed deformities and knee alignment before determination of
component rotation and posterior femoral bone resection.
• Kinematic Alignment Technique
▫ Restore the normal kinematics of the knee by essentially resurfacing the femur
and the tibia to their native prearthritic position
▫ By making equal resections from the medial and lateral distal and posterior femur
and proximal tibia after accounting for cartilage wear and bone loss.

22. • The normal knee joint line
alignment is naturally in 2° to 3° of
varus compared with the
mechanical axis.
• The primary goal of many of the
alignment techniques is to achieve
neutral alignment of the knee.
• Alignment and mechanical axis
needs to be restored:
▫ Centre of hip – Centre of knee –
Centre of ankle should fall in one
line

23. • In a typical knee,
▫ there is slight varus of the
proximal tibia and valgus of the
distal femur.
• If the proximal tibia is to be cut
neutral to the mechanical axis of
the tibia, an asymmetrical
resection of the proximal tibia is
generated with more bone being
taken from lateral proximal tibia.
• To generate a balanced extension
gap, an asymmetrical cut of the
distal femur is required and more
bone is resected from the distal
medial condyle.

24. • With the tibia cut neutral
to the mechanical axis of
the femur, and the knee
flexed to 90°, an
asymmetrical resection of
the posterior condyles is
necessary to generate a
rectangular flexion gap.
• As a result, more bone is
resected from to posterior
medial condyle.