Implant technology Unit 3 Flashcards
The ligaments and surrounding muscles are imperative in maintaining knee joint stability. What is the main stabilising role of each of the 4 ligaments?
ACL - resists posterior subluxation of the femur
PCL - resists anterior subluxation of the femur
LCL - resists adduction of the joint
MCL - resists abduction of the joint
All the ligaments act together to limit distraction of the knee and to limit long axis rotation of the joint
What is the posterior capsule and what does it do?
It is a band of tendonous material that resists hyper-extension
What is subluxation?
Partial or complete dislocation
What is the instantaneous centre of rotation?
As the knee flexes, its axis of rotation changes. This can be seen by the movement of the vertical line passing through the centre of rotation at each knee position, this being the point at which the links cross. This point is known as the instantaneous centre of rotation, because it changes at every instant of motion
What impact does the four-bar cruciate mechanism of the knee joint have on motion?
It constrains the motion of the femur on the tibia so that there is a combination of rolling and sliding motion
Why does the femur not roll of the tibia as the knee flexes?
Because the cruciate ligament and joint capsule prevents it from doing so
How does the position of the instantaneous centre of rotation change as the knee moves from extension to flexion?
It move posteriorly and slightly distally
The external forces acting on the knee joint are mainly compressive. Why is it that the forces on the joint are 2-6 times that of the body weight of the individual?
The combined effect of the gravitational (weight) forces, the contracting forces of the muscles across the joint and the balancing loads of the ligaments
Ground reaction forces during walking also have a horizontal component directed medially which generates a turning moment on the knee. This is balanced by muscles and ligaments. For low magnitude sideways medial reaction forces, such as those that occur during normal gait, what helps to pull the joint together?
The quadriceps muscle, acting via the patellar tendon ligament, can pull the joint together hard enough to keep both condylar surfaces in contact with the tibial plateau
As activities become more strenuous, what also helps keep the joint pulled together?
The hamstrings are used as well which leads to an increased total joint reaction force. Eventually, as the load increases, the muscles don’t have the strength to maintain contact at both condylar surfaces and so the lateral side loses contact and all the load is taken by the medial condyle. The stability of the joint then relies on the LCL to balance the turning moment
What are the general criteria for knee joint replacements?
It must be tolerated within the human body with no short term and little long term risk of adverse toxic effects such as carcinogenesis.
Achieve its aim of relieving pain and restoring the activities of daily living.
Last a reasonable length of time which ideally should extend beyond the lifespan of the patient without the need for revision.
Be insertable by a competent surgeon of average ability such that a predictable outcome can be guaranteed.
Be of acceptable cost
What are the components of a knee replacement normally made of?
Femoral component - cobalt chrome
Tibial component - HDP
What are the minimal functional requirements of a knee replacement?
It should fully extend to 180 degrees at which point the patient should be able to stand without the need for muscular effort by the quadriceps.
It should flex to 90 degrees so that the patient can walk up and down stairs.
It should permit slight axial rotation as the knee extends to maintain natural ligament tension throughout the flexion and extension process
Why must the posterior capsule be dissected off the back of the femur during a knee replacement surgery?
It must be dissected off the back of the femur to ensure that the replacement knee can fully extend
What is done to the collateral ligaments during a knee replacement surgery?
They should be balanced in tension so that the bony cuts are parallel when the bones are stretched apart
If ligaments are lost during surgery, the prosthesis design must compensate fro the functional loss. If there are no ligaments, what type of prosthesis is used?
A hinged prosthesis - the hinge mechanism constrains the motion of the knee into a single axis of rotation with total stability
What are the issues related to hinge prostheses?
It has no “give” under lateral and long axis rotational loading and transmits the sometimes high shear forces associated with these loadings to the implant-cement and cement-bone interfaces
What normally happens to the ACL in osteoarthritis?
In most cases of OA, the ACL is either destroyed or is so attenuated as to be of no mechanical value. The PCL is normally still intact
What does the PCL do?
It controls the rolling motion of the tibia. If the PCL cannot be retained for a replacement knee, a necessary substitute mechanism is required as part of the prostheses. This enables the femur to rotate on the tibial plateau without sliding too far posteriorly. This means a good range of knee flexion is achieved without restrictions of movement due to soft tissues
What are the advantages and disadvantages of retaining the PCL in a knee replacement surgery?
Advantages - it provides some degree of anterior-posterior stability and preserves some proprioception which helps feedback to the brain if the joint is being overloaded. Walking on stairs is also more stable with a PCL intact, although normal gait is unaffected either way.
Disadvantages - It constricts surgical dissection of the posterior capsule, which may limit full extension and it encourages the femoral component to slide over the tibial plateau which may have detrimental wear effects
Why does a knee replacement need to have a fairly flat tibial plateau when the PCL is retained?
Because the PCL would otherwise become lax or too tight during flexion-extension movement
What are the 3 important mechanical factors relating to the surface shape of a knee prosthesis?
The effect of constraint on load transmission and the generation of high shear stresses.
The effect of surface contact on the wear of the HDP tibial component.
The effect of the surface contact area on the stresses in the HDP tibial component
What negative effect can occur due to the cement and prosthetic materials not being viscoelastic in nature?
The energy due to sudden loads is not absorbed gradually and can give rise to large instantaneous stresses at the interfaces which can cause failure in these regions. It is important therefore to have a sufficiently large area of contact. Stems and pegs on the femoral and tibial components help in this regard
How can the rate of production of wear particles be minimised?
The sliding distance of the bearing should be minimised, while in order to reduce the rate of depth of wear the contact area should be increased, which can be achieved by having a wide bearing
