Section editor: Paul Banaszkiewicz

Segment author: Christopher George Ghazala; Paul Banaszkiewicz

Document history:6/07/2019

• Ligament injuries are common and management requires an understanding of their mechanical properties and healing mechanisms.

• Ligaments are short flexible dense bands of collagenous tissues (fibres) that span a joint and then become anchored to the bone at either end.

• Prevent excessive joint motion.
• Connect bone to bone.
• Passively stabilise joints.
• Offer early and increasing resistance to tensile loading over a narrow range of joint motion.
• Contribute to proprioception and position sense.

• The fibres in ligaments are arranged in a wavy pattern (crimp) increasing their capacity to absorb tension.
• This arrangement contrasts with tendons in which the arrangement of collagen fibres is essentially parallel.
• A less parallel arrangement of collagen fibres in ligaments allows them to resist tensile stresses not only in one specific direction but also smaller stresses in other directions.

•  Ligaments have the same general composition as tendons but with a few key differences.
• Due to the organisational pattern of fibres, not all fibres are stretched when loaded along the main fibre axis.
• Ligaments are less strong than tendons.
• Similar to tenocytes, ligaments have fibroblasts that are found within the ligamentous substance aligned with the collagen fibrils.

Table 1. Comparison of tendons and ligaments

Table 2. Comparison of composition between tendon and ligament

• Both ligaments and tendon display stress-strain behaviour that is time and rate dependent. The three main features of viscoelastic properties are:
• Hysteresis: as the loading rate increases, the stiffness of ligaments and tendons increases, storing more energy to failure and undergoing more elongation to failure.
• Stress relaxation: if a tendon or ligament is held at a constant length (strain) by a load (stress), the stress required to maintain that strain reduces. This reduces rapidly during the first 6–8 hours of loading and then more slowly over the next few months.
• Creep: if a tendon is held by a constant load (stress), with time the length (strain) in the tendon will increase. This strain increases quickly at first but then increases more slowly. This phenomenon is taken advantage of by the application of plaster casts or braces to correct deformity, e.g. in scoliosis or clubfoot.

• This is demonstrated when the loading of a sample is stopped below the linear region of the load–deformation curve and the amount of load remains constant over an extended period of time (i.e. the amount of elongation is constant). The deformation increases rapidly at first and then gradually more slowly.
• Occurs when a viscoelastic material that is subjected to a constant load continues to stretch over time.
• It is a time-dependent elongation of a tissue when subjected to a constant stress.

Figure 1. Creep

• This is demonstrated when the loading of a sample is stopped below the linear region of the load–deformation curve and the sample is maintained at a constant length over an extended period of time (i.e. the amount of elongation is constant). The load decreases rapidly at first and then gradually more slowly.
• Stress relaxation is a time-dependent, viscoelastic process similar to creep, but it refers to the decay of applied stress under conditions of constant strain (in contrast to creep, which is a time-dependent strain under conditions of constant stress).
• A reduction in the internal resistance (stress) over time when a viscoelastic material is stretched and fixed in length without further deformation.

Figure 2. Stress relaxation

• Ligaments are covered by a vascular and cellular overlying layer called the epiligament, which is often indistinguishable from the actual ligament.
• The epiligament contains sensory and proprioceptive nerves, the larger percentage of which is located closer to the bone ligament insertion sites.

• Force-elongation curves involve mounting a specimen in a machine whereas stress/strain curves have been normalised with respect to specimen dimensions.
• Force–elongation curves are essentially the same as stress/strain curves.

• Region 1: 'primary' or 'toe' region. Elongation occurring here is believed to be the result of straightening in the wavy pattern of relaxed collagen fibres. A small increase in load/stress causes a relatively large change in length/strain. The stiffness/slope of the graph/modulus of elasticity increases during this part of the curve.
• Region 2: 'secondary' or 'linear' region. The fibres straighten out and the stiffness of the specimen has increased and become constant. Hence the slope of the graph is constant. = elastic region.
• Region 3: the end of the linear region there is early sequential failure of a few very stretched collagen fibres, causing dips. The load/stress at which this occurs is called the yield point.
• Point 4: the ultimate stress/strength is the maximum load/stress that can be achieved before the ligament/tendon completely fails.A series of small drops as ultimate failure occurs, due to sequential failures of fibre bundles.

Figure 3. Force elongation curve of ligament 

• There are several variants of the stress/strain graph of both  ligaments and tendons.
• Often the stress/strain graph is labelled as a composite tendon/ligament
• Choose a straightforward reproducible diagram easy to draw and explain.
• The stress/strain graph of both ligament and tendon are similar as both have the same sort of microscopic crimp, only this is greater in ligaments. As a result there is a more non-linear load–deformation curve earlier on.

Ligaments have lower mechanical properties-the ultimate tensile strength and elastic modulus are less than tendons.

• When a ligament is subjected to loading that exceeds the physiological range (injury due to high levels of stress), either microfracture occurs even before the yield point is reached (i.e. partial rupture of a ligament) or if the yield point is exceeded, the ligament will undergo gross failure (complete rupture).
• Ligaments are classified in three grades of injury severity:

1. First degree: minimal symptoms, some pain is felt but no joint instability is noted. Some micro failure of collagen fibres but no macroscopic disruption.
2. Second degree: severe pain and some joint instability. Partial ligament rupture.
3. Third degree: severe pain during the event of injury but less pain afterwards. The joint is completely unstable.

• Collagen synthesis and degradation proceed simultaneously, as in other types of wound healing, but collagen content increases.
• Initial collagen is type 3, although later composition changes to predominantly type 1 collagen.
• Glycosaminoglycans also increase in the early phase of wound healing.
• With remodelling and maturation, contents gradually return to normal.
• Injured ligaments sometimes demonstrate the ability to contract or to “tighten up.” (Rat medial collateral ligaments Z-lengthened are able to contract to normal tightness in three weeks.)
• There is an associated increase in the amount of actin as measured by immunofluorescent staining (actin is same contractile protein found in thin filaments of muscle sarcomeres and in the cytoplasm of mobile cells).