Hamstring Graft Biomechanics

For Medical Students

The Principle

Folding a tendon back on itself multiplies its strength. Four strands of one tendon are roughly four times as strong as one.

A graft with no bone on it has to be held by soft tissue fixation, which is the weakest part of the assembly at first.

So the strength of the whole construct is set by the fixation rather than by the tendon.

At the Knee

The usual hamstring graft is the semitendinosus tendon, folded to give four strands, sometimes with the gracilis added. 2.9.5 covers the semimembranosus and the medial structures.

Conventional teaching reports a quadrupled hamstring graft as exceeding the native ligament's ultimate failure load at implantation.

Its stiffness is reported as lower than bone patellar tendon bone, which matters because a less stiff graft allows more translation under the same load. 3.7.18 covers that graft.

Both ends are tendon, so both ends need soft tissue fixation and both ends have to heal into bone rather than bone to bone.

What Changes It

The number of strands and the resulting diameter.

Fixation method at each end.

Donor tendon size, which varies between patients and is not always adequate.

Time, since tendon to bone healing is slower than bone to bone.

Clinical Relevance

Taking the hamstrings removes part of the muscle that protects the ligament, which is a mechanical cost specific to this graft. 3.7.10 covers strain.

Donor site morbidity is different from the patellar tendon's rather than less. Section 6 covers management.

Graft diameter depends on the patient's own tendon, which is a planning consideration.

Tunnel widening is described with this graft, which is a fixation and healing issue rather than a tendon one.

MRCS Level

The Principle

The construct has three parts and each can fail: the graft, the fixation and the healing interface. Early failure is fixation, late failure is graft or interface.

Soft tissue to bone healing proceeds through an interposed fibrous layer rather than direct insertion, which takes longer and is mechanically inferior to bone to bone healing.

Stiffness mismatch matters. A graft less stiff than the native ligament permits more translation at submaximal loads even when its ultimate strength is higher.

At the Knee

Conventional teaching gives a quadrupled semitendinosus and gracilis graft an ultimate load above 4000 newtons at time zero, well above the native ligament's.

Its stiffness is reported as lower than the native ligament's and lower than bone patellar tendon bone, which is the mechanical basis of the reported difference in measured laxity.

Tunnel widening is reported more frequently with this graft, attributed to graft motion within the tunnel and to the fixation being distant from the joint line.

Harvesting the semitendinosus reduces knee flexion strength in deep flexion and reduces the hamstring contribution to protecting the graft. 3.4.7 covers internal moments.

What Changes It

Number of strands, diameter and the donor tendon's own dimensions.

Fixation position relative to the joint line, which affects graft motion in the tunnel.

Fixation type, suspensory or aperture, which changes the construct's behaviour.

Rehabilitation loading during the slower tendon to bone healing.

How It Is Measured

Time zero tensile and cyclic testing of the whole construct, including fixation, is the relevant bench test.

Tunnel widening is measured on postoperative imaging and is a surrogate for graft motion.

Isokinetic testing measures the flexion strength deficit after harvest.

Registry and cohort data compares failure rates with other grafts, by age and activity.

Clinical Relevance

Test the construct rather than the graft, since fixation is the early limit.

Place fixation close to the joint line where the technique allows, to reduce graft motion.

Expect a deep flexion strength deficit after harvest and rehabilitate it.

Expect slower healing at the interface than with a bone to bone graft.

FRCS Level

At the Knee

Treat diameter as the variable that matters most and the one the donor tendon may not allow. Conventional teaching associates smaller graft diameter with higher failure rates, particularly in young patients.

That makes preoperative assessment of tendon size a planning step rather than an intraoperative discovery, and it is the reason alternatives are considered in advance.

The harvest's effect on the hamstrings is the graft specific mechanical cost: it removes part of the dynamic restraint that protects the reconstruction. 3.7.10 covers strain.

What Changes It

Achievable diameter from the available tendon.

Whether gracilis is harvested in addition.

Fixation choice and its distance from the joint line.

Patient age and activity, which interact with diameter in the failure data.

How It Is Measured

Registry data reports failure by graft diameter and age, which is the strongest clinical evidence.

Preoperative ultrasound or magnetic resonance imaging estimates tendon size, with modest accuracy.

Construct testing compares fixation devices at time zero.

Isokinetic testing measures the residual flexion deficit.

Clinical Relevance

Assess tendon size before surgery rather than discovering it during.

Achieve an adequate diameter, adding strands or changing graft if necessary.

Rehabilitate the hamstrings deliberately, since the graft came from them.

Counsel younger patients on the diameter association with failure.

Fellowship Level

At the Knee

The fellowship considerations are the revision and the inadequate tendon. A previously harvested semitendinosus may regenerate, and conventional teaching treats regenerated tendon as unreliable graft material.

Where diameter cannot be achieved, the options are augmentation with another tissue, a change of graft, or accepting a smaller graft with a lateral augmentation to share the load. Each is a mechanical compromise with a stated reason. 3.11 covers augmentation.

Tunnel widening matters most in revision, because it determines whether a single stage revision is possible. 3.7.20 covers the construct.

What Changes It

Previous harvest and whether the tendon regenerated usefully.

Tunnel width and position from the index surgery.

Whether a lateral augmentation is being added.

Available alternative autograft and the patient's acceptance of allograft.

How It Is Measured

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

Ultrasound assesses a regenerated tendon's dimensions, without assessing its material properties.

Revision registry data reports outcomes by graft choice in revision.

No measurement establishes whether a regenerated tendon is mechanically adequate.

Clinical Relevance

Treat a regenerated tendon as unreliable graft material.

Measure tunnel width before planning a single stage revision.

Consider lateral augmentation to share load where diameter is inadequate.

Plan the graft source before theatre in any revision.