Graft Biomechanics

For Medical Students

The Principle

Replacing a structure means matching what it did. For a ligament that means strength, stiffness, and keeping both over time.

Strength is the load it fails at. Stiffness is how much it stretches under load. They are separate properties and a graft can match one and not the other.

A new rope is strongest on the day it is fitted. A biological graft is different: it gets weaker before it gets stronger.

At the Knee

Conventional teaching gives the native anterior cruciate an ultimate failure load of around 2000 newtons in young specimens, which is the figure grafts are compared against.

Most grafts exceed that at implantation. That is the easy comparison and the least important one.

After implantation a graft undergoes ligamentisation: the original tissue is broken down and replaced by host tissue over months, and it is weakest in the middle of that process.

No graft returns to native strength and stiffness, which is a consistent finding across graft types. 3.7.20 covers the construct.

What Changes It

Graft type and its original tissue properties. 3.7.17 to 3.7.19 cover three.

Graft diameter, since a thicker graft is stronger.

Time since implantation, through the remodelling curve.

Fixation, since the construct is only as strong as its weakest point early on.

Clinical Relevance

The weakest period is weeks to months after surgery, not the day of it, which is why rehabilitation is staged. Section 6 covers management.

Comparing grafts by their strength at implantation answers the wrong question.

A larger graft diameter is associated with lower failure rates, which fits the mechanics.

Nothing on offer is as good as the original, which is worth saying to a patient plainly.

MRCS Level

The Principle

Four properties matter: ultimate failure load, stiffness, cross sectional area and viscoelastic behaviour including creep. A graft comparison that reports only the first is incomplete.

Structural properties belong to the whole graft and material properties to the tissue. A wider graft of weaker tissue can be structurally stronger.

Ligamentisation has phases: early necrosis and loss of strength, revascularisation and cellular repopulation, then remodelling. The mechanical low point is in the first few months.

At the Knee

Conventional teaching gives the native ligament roughly 2000 newtons ultimate load and a stiffness of a few hundred newtons per millimetre, with wide variation by age.

Quadrupled hamstring grafts are conventionally reported as exceeding native ultimate load at implantation, and bone patellar tendon bone grafts as closer to it with a stiffness nearer native. 3.7.17 and 3.7.18 cover them.

Creep is the quantity behind progressive laxity. A graft that elongates under sustained load loses the tension it was fixed with.

Fixation strength rather than graft strength is the limiting factor in the first weeks, which is why construct testing measures the whole assembly.

What Changes It

Graft tissue type and donor age.

Diameter and the number of strands.

Fixation method, which sets the early limit.

Time, through the remodelling phases.

How It Is Measured

Tensile testing of grafts to failure gives ultimate load and stiffness, in excised tissue at time zero.

Cyclic testing measures creep and gap formation in the whole construct.

Animal models measure the remodelling curve, which cannot be measured in patients.

Clinical laxity measurement over time is the indirect human evidence of creep and remodelling.

Clinical Relevance

Judge a graft comparison by stiffness and creep as well as ultimate load.

Distinguish structural from material properties when comparing graft types.

Expect fixation to be the limiting factor early and the graft later.

Read time zero testing as describing the day of surgery only.

FRCS Level

At the Knee

Treat graft choice as a trade rather than a ranking. The mechanical differences at time zero are real and small relative to the differences in donor site morbidity, rehabilitation and failure rates by population.

Conventional teaching holds that graft diameter is associated with failure risk more strongly than graft type is, which reframes the choice: achieving an adequate diameter matters more than which tissue it came from.

Allograft is the exception where the mechanics differ materially. Conventional teaching reports higher failure rates in young active patients with allograft, attributed to slower and less complete remodelling.

What Changes It

Achieved diameter, which is the stronger association with failure.

Patient age and activity, which interact with graft choice.

Processing and irradiation in allograft, which affect material properties.

Fixation and tunnel preparation, which determine the construct.

How It Is Measured

Time zero biomechanical testing compares grafts and describes the day of surgery.

Registry data measures failure by graft type, diameter, age and activity, and is the strongest evidence for the clinical choice.

Animal remodelling studies describe the curve that human data cannot.

Serial clinical laxity measures the construct's behaviour over the first year.

Clinical Relevance

Achieve an adequate graft diameter, which matters more than tissue type.

Avoid allograft in young active patients, on registry failure data.

Account for irradiation and processing when using allograft at all.

Read time zero comparisons as a small part of the clinical choice.

Fellowship Level

At the Knee

At fellowship level the question is revision and the unusual case: a failed graft, a wide tunnel, a multiligament knee, a patient with no obvious donor site left.

Each pushes the choice away from the primary algorithm. Conventional teaching accepts allograft more readily in revision and in multiligament reconstruction, where graft quantity and operative time matter more than the failure rate difference in a young athlete.

Tunnel condition is the other variable. A wide or confluent tunnel changes what fixation is possible, and fixation is the early limit on the construct. 3.7.20 covers the construct.

What Changes It

Tunnel position, width and confluence from the previous surgery.

Available autograft after a previous harvest.

The number of ligaments being reconstructed in one sitting.

Bone quality and whether staged grafting is needed first.

How It Is Measured

Computed tomography measures tunnel position and width, which drives the staging decision.

Revision registry data reports failure rates by graft choice in the revision setting, which differ from primary.

Construct testing in compromised bone is sparse, so fixation choice in revision rests on principle more than on data.

No measurement establishes the best graft for an individual revision.

Clinical Relevance

Plan revision graft choice from tunnel condition and available autograft rather than from the primary algorithm.

Stage with bone grafting where tunnels are wide or confluent.

Accept allograft more readily in revision and in multiligament reconstruction.

State that fixation choice in compromised bone rests on principle rather than data.