Quadriceps Tendon Graft Biomechanics

For Medical Students

The Principle

A larger source tendon allows a graft of chosen size. Where the donor is small, the graft size is whatever the donor gives.

Thickness matters for strength, and a graft harvested from a thick tendon can be made as thick as the tunnel requires.

Taking it with or without a bone block is a choice, which changes how it is fixed.

At the Knee

The graft is taken from the quadriceps tendon above the patella, optionally with a bone block from the patella. 2.4.1 covers the patella.

Conventional teaching reports its ultimate failure load as comparable to or above the other autografts, with a larger cross sectional area available.

Diameter is controllable, which is its main mechanical advantage over a hamstring graft where the donor tendon may be small. 3.7.17 covers that graft.

It can be harvested with bone at one end, giving bone to bone healing at that end and soft tissue healing at the other.

What Changes It

Harvest thickness and width, which set the graft's dimensions.

Whether a bone block is taken, which changes the fixation at that end.

Full thickness or partial thickness harvest, which affects the donor site.

Fixation choice at the soft tissue end.

Clinical Relevance

Where an adequate diameter cannot be obtained from the hamstrings, this graft is the usual alternative, which is a mechanical reason rather than a preference. 3.7.16 covers graft choice.

Its donor site is the extensor mechanism again, with a different distribution of symptoms from the patellar tendon's. Section 6 covers management.

It has become more common, and the evidence base is smaller than for the other two autografts.

The choice between the three is a trade, and this one trades a controllable diameter against a shorter evidence record.

MRCS Level

The Principle

Cross sectional area is the property this graft offers. Strength scales with area, so a graft that can be cut to a chosen thickness can be made to a chosen strength.

A hybrid construct, bone at one end and tendon at the other, has two different healing interfaces and two different fixation problems in the same graft.

The evidence base is younger than for the other autografts, so mechanical data is available and long term clinical data is thinner.

At the Knee

Conventional teaching reports quadriceps tendon graft ultimate load and stiffness as comparable to bone patellar tendon bone, with greater available cross sectional area.

Full thickness harvest reaches the suprapatellar pouch and partial thickness does not, which is a donor site distinction with mechanical and clinical consequences.

Extensor strength deficit after harvest is reported and conventional teaching describes it as recovering, with less kneeling pain than bone patellar tendon bone.

Fixation at the soft tissue end faces the same tendon to bone healing as a hamstring graft. 3.7.17 covers it.

What Changes It

Harvest dimensions, which are chosen rather than found.

Full or partial thickness harvest.

Whether a bone block is included and at which end.

Fixation choice at each end.

How It Is Measured

Time zero tensile testing compares it with the other autografts and with native tissue.

Construct testing with the intended fixation measures the assembly.

Isokinetic testing measures the extensor deficit after harvest.

Clinical series are shorter and smaller than for the other autografts, which is the main limitation of the evidence.

Clinical Relevance

Choose harvest dimensions to the diameter required rather than accepting what is found.

Distinguish full from partial thickness harvest in the consent and the rehabilitation.

Expect tendon to bone healing at the soft tissue end.

Read the clinical evidence as shorter and smaller than for the other autografts.

FRCS Level

At the Knee

Treat this graft as the answer to a diameter problem and as the graft whose long term evidence is still accumulating. Both are true and they point in different directions.

Its mechanical case is strong and its clinical case is less mature, which is an ordinary position for a technique in adoption rather than a criticism of it.

Conventional teaching reports comparable failure rates to the other autografts in the series available, with shorter follow up.

What Changes It

Required diameter and what the hamstrings can supply.

Patient kneeling and extensor demands.

Revision setting, where available graft and tunnel condition dominate.

Surgeon familiarity, which affects harvest quality more than for a simpler harvest.

How It Is Measured

Registry data is accumulating and reports comparable early failure rates.

Patient reported outcome captures the donor site difference.

Construct testing establishes the mechanical equivalence at time zero.

Long term comparative data does not yet exist at the volume available for the other two.

Clinical Relevance

Use it where diameter cannot be achieved from the hamstrings.

Counsel that its long term comparative evidence is still accumulating.

Account for extensor and kneeling demands in the choice.

Expect harvest quality to depend on familiarity more than for a simpler harvest.

Fellowship Level

At the Knee

At fellowship level this graft is often the revision solution. It is an untouched donor site in a patient whose hamstrings and patellar tendon have been used, and it supplies the diameter a wide tunnel needs. 3.7.16 covers graft choice.

It also suits multiligament reconstruction, where graft quantity matters and several constructs are needed in one sitting.

The constraint is the extensor mechanism, which has a finite amount that can be taken from it across a patient's surgical history. That is a cumulative consideration rather than a single operation one.

What Changes It

Previous harvests and what donor sites remain.

Tunnel width and the diameter required.

Number of grafts needed in one operation.

Extensor mechanism integrity after previous surgery.

How It Is Measured

Imaging assesses the quadriceps tendon thickness before harvest, which is how diameter is planned.

Computed tomography measures tunnel width, which sets the diameter required.

Revision series report outcomes with this graft, in small numbers.

No measurement establishes how much extensor tendon a patient can afford to lose in total.

Clinical Relevance

Measure tendon thickness on imaging before planning the harvest.

Consider it first in revision where other donor sites are used.

Account for the cumulative extensor cost across a surgical history.

Expect small numbers in the revision evidence for this graft.