Ligaments Flashcards
Explain how weak hamstring muscles can result in an ACL tear when making a sudden stop after running via strong quadriceps contraction force.
The quadriceps line of force is bigger than the one of the hamstring.
- The hamstring muscle applies a posterior force on the tibia-femur, which reduces the strain on the ACL.
- At low flexion angles, the quadriceps muscle applies an anterior force on the tibia-femur, via the patella that is positioned anterior the the knee joint.
The magnitude of both forces determine the amount of ACL strain. When making a sudden stop from running at low knee flexion angles, the resultant force of a weak hamstring muscle in the posterior direction is low > exceeded by quadriceps force > high anterior forces on the tibia > tibia is pulled anterior (relative to the femur) > increasing the risk for high ACL strain & rapture.
When reconstructing a torn ACL using the 4-strand single-bundle hamstring tendon technique, the surgeon has the option to reconstruct either the AM or the PL bundle. Argue why reconstructing the AM bundle is more beneficial.
- The AM bundle provides higher rotational stability.
- The AM bundle provides higher anterior-posterior stability.
- The AM bundle is tensed throughout the complete range from flexion to extension > likely improves ligimentization.
A patient receives a 4-strand hamstring tendon (single-bundle) to reconstruct a torn ACL. The operating assistant makes a mistake and produces a graft in which one of the four strands is longer (and thus slack) compared to the other 3. This is not noted, and the surgeon proceeds as normal and applies the regular 80N pretension while fixing the graft to the knee joint.
Q1. Explain what the consequence is of this mistake on the mechanical behavior of the graft immediately after surgery.
Q2. What is the likely consequence will be for this patient when testing anterior-posterior laxity 1 year after surgery?
A1: A ‘non-linearity’ component is added.
A2: Applied pretention at surgery is too high for 3 strands and too low for the 4th strand. This results in increased anterior-posterior knee laxity.
A patient with a torn ACL is lying on the operating table. The 4-strand hamstring tendon (single-bundle) graft has been excised and processed. Just before it is inserted and fixed at the location of the torn ACL, it is heavily crosslinked using a dedicated agent. Hypothesize how the mechanical properties/response of this tendon graft are altered as a consequence of the cross-linking (discuss the following 6 elements: viscoelasticity, strain at break, breaking strength, E-modulus, collagen fiber strain, interfiber sliding).
- Viscoelasticity: amount of stress relaxation decreases, amount of creep decreases.
- Strain at break: decreases
- Breaking strength: increases
- E-modulus (stiffness): increases
- Collagen fiber strain: increases (energy cannot dissipate via fiber to fiber sliding).
- Interfiber sliding: decreases (crosslinks that connect the fibers are very stiff and resist translation).
Using the 4-strand single-bundle hamstring tendon technique to reconstruct the ACL still has downsides. Therefore, a novel ACL reconstruction graft has been taken into clinical trials. This graft is comprised of a devitalized bone – ACL – bone graft to replace a torn ACL. Reflect on the potential upsides and downsides of using a human ACL to reconstruct a torn human ACL, in comparison to a single-bundle hamstring tendon (name 2 upsides and 2 downsides).
Advantages:
- The option to perfectly reconstruct the ACL with 2 bundles, both anatomically & mechanically (although anatomical differences between individuals exist).
- Healing in the tunnel is bone-to-bone; faster & more efficient, but without the downside of donor side morbidity.
Disadvantages:
- In case ligamentization is as aggressive; high change of rapture since strength graft = strength ACL.
- Possibility for disease transmission.
Which anatomical deviation likely increases rupture risk?
A Foot eversion (turn sole of your foot outwards) more than inversion
B Knee valgus more than varus
C Weak hamstring more than quadriceps muscles
A No; there are more inversion injuries due to relative instability of the lateral joint
B Yes
C Yes
What pre-operative planning & strategy would help?
- Measure posterior slope angle (osteotomy)
- Measure intercondylar notch width (notchplasty)
- Determine bone tunnel positions (kinetics)
- Measure hamstring-quadriceps muscle quality (steroid injections)
During normal daily activity, an ACL (graft) in vivo is subjected to: A. Stress-relaxation B. Creep C. Plastic deformation D. Toe region loading only E. Microdamage
A
Think about the normal stress-strain behaviour of an ACL. What part will most likely change when elastin would be absent? A. i will shorten B. ii will alter C. slope of iii drops D. stress,max (iv) drops
A. The first part of the curve will shorten. Elastin is the final protein that includes the crimp in this region. The first part of the sterss-strain curve is missing.
Think about the normal stress-strain behaviour of an ACL. Which region will be affected most, when cross-link density is increased? A. i B. ii C. iii D. iv
C. iii
Steps: Recruitment elastin Uncrimping Recruitment collagen Recruitment of crosslinks
How is the normal stress-strain behavior of a 4-strand hamstring tendon different from the one of an ACL? True/false: A. i will shorten B. iii will steepen C. stress,max increases D. strain, max increases
A. i will shorten –> true
B. iii will steepen –> true
C. stress,max increases –> true
D. strain, max increases –> false, decreases
Ligamentization associates with a drop in mechanical properties, but which parameters are meant by that? Q1: Strength will likely: A. decrease B. increase C. stay equal D. this cannot be stated Q2: strain at break will likey? Q3: E-modulus?
A1: decrease
A2: increase
A3: decrease
Hypothesize why the ACL has a direct and an indirect femoral bone insertion site?
A. They match the AM and PL insertion sites
B. The direct insertion site originates from compression during development
C. The indirect insertion twists underneath the direct site, thus requiring long Sharpey’s fibers penetrating the bone
D. The direct insertion fuses with the condylar AC
B
