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Patellar Tendon Graft Biomechanics

Page 3.7.18 · ACL Biomechanics · biomechanics page

Bone at both ends, which solves the fixation problem and moves the cost to the donor site. Its stiffness is the closest to native of the common grafts.

Pipeline status. Editorial draft. Evidence verification did not run. Not consultant reviewed. Not for clinical use.

The same topic at 4 depths

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Patellar Tendon Graft Biomechanics

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For Medical Students

The Principle

Fixing a graft is easier if it has bone on the end, because bone heals to bone directly and quickly.

A graft with bone plugs can be held with an interference fit at the tunnel mouth, which puts the fixation at the joint line rather than away from it.

The cost is that the bone has to come from somewhere, and so does the tendon between the plugs.

At the Knee

The graft is the central third of the patellar tendon with a bone block from the patella at one end and from the tibial tubercle at the other. 2.4.1 covers the patella.

Conventional teaching gives it an ultimate failure load close to or above the native ligament's, and a stiffness closer to native than a hamstring graft's. 3.7.17 covers that graft.

Bone to bone healing is faster and mechanically better than tendon to bone, so the construct becomes secure sooner.

The donor site is the extensor mechanism, which is why anterior knee pain and kneeling discomfort are the characteristic costs.

What Changes It

Graft width taken, which trades graft size against donor site.

Bone plug size and fit, which set the fixation.

Patellar tendon length relative to the tunnels, which can be a planning problem.

Closure and rehabilitation of the donor site.

Clinical Relevance

This graft's mechanics are favourable and its donor site is the reason it is not always chosen. Section 6 covers management.

Anterior knee pain and difficulty kneeling are the characteristic complaints, and they come from the harvest rather than the reconstruction.

Patellar fracture and tendon rupture are recognised harvest complications, which are mechanical consequences of removing bone and tendon. 6.97 covers patellar fractures.

It remains a common choice in high demand patients, on failure rate grounds.

Key Learning Points

  • Bone heals to bone directly and quickly.
  • Bone plugs allow fixation at the tunnel mouth rather than away from it.
  • Conventional teaching gives it a stiffness closer to native than a hamstring graft.
  • Bone to bone healing makes the construct secure sooner.
  • The donor site is the extensor mechanism, which is the characteristic cost.

MRCS Level

The Principle

Aperture fixation at the tunnel mouth reduces graft motion within the tunnel, which is the mechanism behind less tunnel widening with this graft.

Stiffness matching matters for submaximal behaviour. A graft closer to native stiffness permits less translation under everyday loads even if ultimate strengths are similar. 3.7.8 covers anterior stability.

The donor site is a mechanical defect in the extensor mechanism, and its consequences are mechanical: altered lever arm, altered patellar height in some cases, and local pain. 3.4.4 covers lever arms.

At the Knee

Conventional teaching gives the bone patellar tendon bone graft an ultimate load in the region of the native ligament's or above, with stiffness nearer native than hamstring grafts.

Reported laxity after reconstruction is marginally lower with this graft in several series, which is consistent with the stiffness difference and is a small effect.

Harvest reduces extensor strength early and conventional teaching reports recovery over months, with kneeling discomfort persisting in a proportion.

Patella baja and patellar tendon shortening are described after harvest, which change the extensor lever arm. 3.12 covers the patellofemoral joint.

What Changes It

Harvest width and length, and closure of the defect.

Bone plug sizing and the interference fit achieved.

Patellar tendon length relative to the tunnel lengths.

Rehabilitation of the extensor mechanism.

How It Is Measured

Time zero construct testing gives ultimate load and stiffness with fixation included.

Postoperative tunnel imaging measures widening, which is less with aperture fixation.

Isokinetic testing measures the extensor deficit over time.

Lateral radiography measures patellar height and tendon length after harvest.

Clinical Relevance

Expect marginally lower measured laxity and a small effect size.

Measure patellar height after harvest where extensor symptoms persist.

Rehabilitate the extensor mechanism specifically, since that is the donor site.

Expect less tunnel widening, which matters if revision is later needed.

Key Learning Points

  • Aperture fixation reduces graft motion in the tunnel and so reduces widening.
  • A graft nearer native stiffness permits less translation under everyday loads.
  • Reported laxity is marginally lower, which is a small effect.
  • Patella baja and tendon shortening are described after harvest.
  • The donor site defect has mechanical consequences for the extensor mechanism.

FRCS Level

At the Knee

Treat this graft as the choice where failure rate matters more than donor site, and the hamstring graft as the reverse. Conventional teaching supports lower failure rates with this graft in young pivoting athletes.

The trade is explicit and should be put to the patient as one. Neither graft is better in general and each is better for a stated priority.

Its fixation advantage matters most where early loading is planned or where revision is foreseeable, because less tunnel widening preserves options. 3.7.20 covers the construct.

What Changes It

Patient priorities between failure risk and kneeling.

Occupation and sport, since kneeling and anterior knee loading differ.

Patellar tendon dimensions and patellar bone quality.

Whether revision is foreseeable, which favours preserving tunnel integrity.

How It Is Measured

Registry data reports failure rates by graft type, age and activity, and is the strongest evidence for the choice.

Patient reported outcome measures capture the donor site cost, which no mechanical test does.

Tunnel imaging measures widening and bears on revision options.

Isokinetic and kneeling assessment measures the extensor consequence.

Clinical Relevance

Put the trade between failure risk and donor site to the patient explicitly.

Favour this graft where failure rate is the priority and kneeling is not.

Consider it where revision is foreseeable, for tunnel preservation.

Assess occupation and kneeling demand before choosing.

Key Learning Points

  • This graft is the choice where failure rate matters more than donor site.
  • Conventional teaching supports lower failure rates in young pivoting athletes.
  • Neither graft is better in general; each is better for a stated priority.
  • Less tunnel widening preserves revision options.
  • No mechanical test captures the donor site cost.

Fellowship Level

At the Knee

The fellowship questions are the second harvest and the compromised extensor mechanism. A previously harvested patellar tendon can be reharvested, and conventional teaching treats it as feasible and less predictable.

Patella baja from a previous harvest changes the extensor mechanics and the approach to any later arthroplasty, which belongs in the counselling rather than in a footnote. 3.12 covers the patellofemoral joint.

Bone plugs are also the solution to a wide tunnel in revision, since a plug can be sized to a defect that will not hold a soft tissue graft. 3.7.16 covers graft choice.

What Changes It

Previous harvest and the state of the tendon and patella.

Patellar height and whether it has already changed.

Tunnel width in revision, where a plug may be the answer.

Patellar bone stock and any previous fracture.

How It Is Measured

Lateral radiography and magnetic resonance imaging assess the tendon and patellar height before a second harvest.

Computed tomography measures tunnel width, which drives the revision plan.

Revision registry data reports outcomes by graft in revision.

No measurement establishes whether a reharvested tendon is mechanically adequate.

Clinical Relevance

Image the tendon and patella before considering a second harvest.

Record patellar height, since it affects later arthroplasty.

Consider bone plugs where a tunnel defect will not hold a soft tissue graft.

Counsel that a reharvested tendon is feasible and less predictable.

Key Learning Points

  • A reharvested patellar tendon is feasible and less predictable.
  • Patella baja from a previous harvest changes any later arthroplasty.
  • A bone plug can be sized to a defect a soft tissue graft will not hold.
  • Record patellar height after harvest.
  • No measurement establishes a reharvested tendon's adequacy.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. Every measurement on this page is written from standard teaching rather than from a retrieved paper, and none has been checked against a source. Under decision 006 each one is registered in pipeline/config/figures with the sentence it supports and a verification state of UNVERIFIED_FROM_MEMORY, so the whole list can be checked and corrected rather than hunted for.

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