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Motion analysis - why?
Tibiofemoraljoint motion analysis is vital to understand the joint kinematics. It can beused as a diagnostic tool for patients with knee injuries or degradation withcomparisons being made to the gait of the normal knee. It can also be used toassess postoperative patients to aid the development of surgical procedures andprosthesis.
Why motion analysis (2)
- Quantitative kinematic analysis is an important tool for gaining a
- thorough understanding of normal and pathological joint function during human
- locomotion. By developing normal joint profiles, identifying abnormalities is
- possible thereby helping to improve diagnosis and treatment, the design and
- performance of reconstructive surgery, rehabilitation programs, the development
- of accurate biomechanical models, and the development or modification of
- functional knee braces
Definition of Kinematic Geometry
- Description of body position and displacement without regard to time
- Therefore not including velocity & acceleration
Simplifying assumptions for the human body
Assumptions made are that the human body can be viewed as a system of rigid bodies connected by joints. The distance between two points along a rigid body do not change.
How to set up a global reference system
Conventionally Global Axis or Global Reference System (GRS) or Global Reference Coordinates [GRC] defined as :
- RH orthogonal triad affixed to an origin, O
- Origin commonly fixed to ground at reference point close to subject under study
- Oriented wrt subject facing in direction of primary interest
A MOVING SYSTEM –
Oriented similarly to the GCS, moving with the body whilst maintaining its orientation in space (origin can be located at the centre of mass)
A SOMATIC SYSTEM
Located within the body, changing its orientation in space when the body orientation changes
How to fix a LCS to a rigid body to define its orientation
- 1) Coordinates of 3 non-collinear points within the body (1,2, and 3) must be known within a global reference system (GRS)
- 2) 2 vectors define a plane:
- r1 (from point 1 to 2)
- r2 (from point 1 to 3)
3) Cross product of vectors r1 and r2 defines vector r3
4)Cross product of vectors r1 and r3 defines vector r4
At this point 3 mutually orthogonal axes (vectors r1, r3, r4) are known but each has a different length 5) Each vector is thus divided by its own length to determine unit vectors
Four experimental observations that show the low friction characteristic of human and animal joint lubrication
- 1. Surface dried cartilage pressed against glass exudes fluid under pressure
- on cartilage
- 2. Cartilage on glass model in a fluid bath - move it on glass, the coefficient of friction decreases with increasing load and increasing speed
- 3. Cartilage on cartilage - coefficient of friction (0.005) is an order of magnitude lower than the cartilage on glass model (0.05)
- Cartilage on glass model - synovial fluid lubricated better than saline. But with hyaluronaise (which digests synovial fluid) the advantage disappeared. Not true for cartilage on cartilage model, as the original cartilage still exuded synovial fluid.
3 main types of lubrication present in the human joint
- 1. Hydrodynamic lubrication - 2 surfaces moving relative to eachother eg rotating shaft. When stationary, the lubrication is squeezed out. When rotating, a wedge shaped lubrication film is created. The synovial fluid is extrained into contact and lifts up and carries the load - fluid is dragged between the surfaces, as well as being pushed out.
- 2. Elastohydrodynamic
- Flattening of the surface due to elastic deformation, increases the area for which load is spread for the contact
- 3. Boundary lubrication
- Both surfaces are coated in lubrication. Large protein molecules in synovial fluid in human synovial joint
What is a guided motion knee?
- A guided motion knee provides some control or guidance to the anterior-posterior (AP translation and/or the I-E rotation during flexion-extension by the interaction of femoral and tibial cams, bearing surfaces, or other means.
- Motion control preferably guides knee in desired neutral motion path - with regions of laxity about it.
What is the overall kinematic goal of guided knee motion?
Achieve specified A-P translation and I-E rotation patterns using guide surfaces in intercondylar region or by using femorotibial bearing surfaces to guide motion.
What are the 6 biomechanical aims of TKR designs?
- a. increase quadriceps lever arm at high flexion angles
- b. increase range of flexion by preventing posterior impingement of bone and soft tissue
- c. prevent posterior tibial subluxation in flexion under action of hamstrings
- d. control location of femoral-tibial contact points in A-P direction and I-E rotation throughout flexion (allowing some laxity about these points)
- e. guide contact points posteriorly with flexion (especially on the lateral side) and anteriorly with extension
- f. produce:
- - internal tibial rotation progressively with flexion
- - external rotation with extension