Patellar Tendon-Bearing Cast – Principles, Technique and Functional Treatment of Tibial Fractures

The patellar tendon-bearing (PTB) cast, also called a Sarmiento cast in the context of functional treatment of tibial fractures, is a below-knee functional cast designed to permit knee movement and controlled weight bearing while reducing the load transmitted through the injured tibia.

Unlike a conventional below-knee cast that primarily immobilises the limb, the PTB cast is carefully moulded around selected pressure-tolerant regions of the proximal leg. This allows a portion of axial load to be transferred through the patellar tendon region and proximal tibial contours rather than entirely through the tibial shaft and fracture site.

It became particularly important in the development of functional treatment of tibial shaft fractures, allowing earlier mobilisation and weight bearing in appropriately selected fractures.

The PTB cast combines fracture support with functional rehabilitation: it aims to maintain acceptable alignment while permitting knee movement and controlled axial loading.

Functional bracing of tibial fractures was popularised by Augusto Sarmiento. The concept challenged prolonged rigid immobilisation by demonstrating that selected tibial shaft fractures could heal while allowing controlled motion and progressive weight bearing.

The PTB cast may be used as an intermediate stage before conversion to a functional brace, particularly after initial swelling has subsided and the fracture has demonstrated sufficient early stability.

Functional fracture treatment is based on the principle that controlled loading and limited micromotion can stimulate secondary bone healing and callus formation, provided alignment remains acceptable.

During ordinary standing, body weight is transmitted from the knee through the tibia toward the ankle and foot. In a tibial shaft fracture, this axial force passes through the injured bone.

A properly moulded PTB cast attempts to redistribute part of this load through the proximal cast interface and surrounding soft tissues.

The cast is moulded around the:

• Patellar tendon region.
• Medial tibial flare.
• Proximal tibial condylar region.
• Posterior soft-tissue compartment.

Pressure-sensitive areas such as the fibular head, tibial crest and bony prominences are protected.

A PTB cast does not completely eliminate load through the tibia; it redistributes and reduces axial loading while providing circumferential fracture support.

The effectiveness of the PTB cast depends on a combination of load redistribution, soft-tissue compression, circumferential support and fracture stability.

1. Load Redistribution

Proximal moulding allows some axial load to be transferred to pressure-tolerant areas around the proximal tibia rather than being transmitted entirely through the tibial shaft.

2. Circumferential Soft-Tissue Compression

The cast creates a relatively contained soft-tissue envelope around the leg. This provides additional support to the fracture fragments and helps resist translation and angulation.

3. Functional Loading

Controlled weight bearing produces physiological axial loading at the fracture site. In a fracture being treated by relative stability, controlled micromotion may promote callus formation.

4. Preservation of Knee Motion

Because the knee is left free, quadriceps function and knee range of motion can be maintained more effectively than with prolonged above-knee immobilisation.

The classic indication is a tibial shaft fracture being managed non-operatively after the initial acute phase.

Suitable Situations

• Selected closed tibial shaft fractures.
• Stable or acceptably reduced tibial fractures.
• Fractures maintaining satisfactory alignment on serial radiographs.
• Transition from an above-knee cast after the acute swelling phase.
• Functional treatment after early fracture stability has developed.
• Selected delayed unions where controlled loading is considered beneficial.
• Situations where operative fixation is unnecessary or unsuitable and functional casting is appropriate.

The decision depends on fracture level, morphology, stability, soft-tissue condition, patient reliability and the ability to maintain acceptable alignment.

Functional casting is most useful when adequate control of the fracture can be obtained within the cast. Tibial shaft fractures are the classic application.

Fractures very close to the knee or ankle may be more difficult to control because the available lever arm for cast moulding is reduced.

Proximal tibial fractures may develop valgus or other deformity, while distal fractures may be difficult to control against angulation and rotation.

Therefore, fracture location must always be considered before choosing functional cast treatment.

PTB casting should not be used when the fracture cannot be safely controlled by the cast.

• Unstable fracture with unacceptable or progressive displacement.
• Fracture requiring operative fixation for mechanical or soft-tissue reasons.
• Significant shortening that cannot be controlled.
• Unacceptable rotational deformity.
• Severe comminution with instability in which alignment cannot be maintained.
• Active compartment syndrome.
• Severe soft-tissue compromise preventing safe circumferential casting.
• Uncontrolled swelling.
• Neurovascular compromise.
• Inability of the patient to comply with follow-up and weight-bearing instructions.

Open fractures are not automatically managed by the same protocol as uncomplicated closed fractures; wound characteristics, contamination, stability and the overall treatment strategy determine whether any form of functional casting is appropriate.

A fresh tibial fracture is usually not placed immediately into a tightly moulded definitive PTB cast when significant swelling is expected.

Initial treatment may include:

• Clinical and radiographic assessment.
• Careful neurovascular examination.
• Assessment for compartment syndrome.
• Reduction where required.
• Temporary splint or appropriately padded cast.
• Elevation and swelling control.
• Serial radiographic assessment.

Conversion to a PTB cast is considered once swelling has reduced and the fracture is suitable for functional treatment.

There is no universal day on which every tibial fracture should be converted to a PTB cast. Timing depends on:

• Resolution of swelling.
• Condition of the skin and soft tissues.
• Fracture stability.
• Radiographic alignment.
• Development of early clinical stability.
• Patient comfort and compliance.

Historically, many protocols converted patients from initial long-leg immobilisation to functional below-knee treatment after the early phase of fracture care.

Conversion should be based on fracture behaviour and soft-tissue condition rather than on a rigid calendar rule.

Proper application is essential. A poorly moulded PTB cast may simply behave as an uncomfortable below-knee cast and may produce pressure complications without providing effective functional support.

Step 1 – Confirm Fracture Alignment

Review current AP and lateral radiographs. Confirm that alignment, rotation and length remain acceptable for continued conservative treatment.

Step 2 – Examine the Limb

Inspect the skin and document neurovascular status. Look specifically for swelling, wounds, pressure areas and signs of compartment syndrome.

Step 3 – Position the Limb

The knee is positioned so that the proximal contours can be accurately moulded while leaving the knee sufficiently free for subsequent motion. The ankle is generally maintained in a functional, approximately neutral position unless fracture-specific considerations dictate otherwise.

Step 4 – Apply Stockinette and Padding

Apply adequate padding, with additional protection over bony prominences such as the fibular head, tibial crest and malleoli.

Step 5 – Apply Casting Material

Plaster of Paris or appropriate synthetic casting material is applied circumferentially. The material should permit effective moulding before hardening.

Step 6 – Create the Proximal PTB Mould

The proximal portion is carefully moulded to engage the patellar tendon and proximal tibial contours. Pressure should be broad and controlled rather than focal.

Step 7 – Mould the Tibial Shaft

Appropriate moulding around the leg helps maintain fracture alignment and improves circumferential support.

Step 8 – Trim Around the Knee

The proximal cast is trimmed to permit knee flexion while retaining adequate proximal support. The patella itself should not be subjected to inappropriate focal pressure.

Step 9 – Check the Cast

Ensure adequate toe movement, circulation, comfort and freedom from focal pressure. Confirm that knee motion is possible as intended.

Step 10 – Obtain Post-Cast Radiographs

AP and lateral radiographs should confirm that acceptable fracture alignment has been maintained.

One of the principal advantages of the PTB concept is the ability to introduce controlled weight bearing.

The amount and timing of weight bearing should be individualised according to:

• Fracture morphology.
• Fracture stability.
• Radiographic alignment.
• Stage of healing.
• Pain.
• Associated injuries.
• Patient balance and compliance.

Crutches or another walking aid may initially be required, followed by progression according to clinical and radiological assessment.

“PTB cast” does not automatically mean unrestricted full weight bearing from the day of application.

Tibial fractures treated functionally usually heal through secondary bone healing.

This involves:

1. Fracture haematoma and inflammation.
2. Soft callus formation.
3. Hard callus formation.
4. Remodelling.

Relative stability allows limited interfragmentary motion. Appropriate mechanical stimulation may encourage callus formation, while excessive movement produces instability and risks delayed union, nonunion or malunion.

Thus, functional treatment aims for a balance between too much rigidity and excessive instability.

Functional treatment accepts secondary healing with callus but does not mean that significant deformity should be accepted.

Serial assessment should evaluate:

• Coronal-plane angulation.
• Sagittal-plane angulation.
• Rotation.
• Shortening.
• Translation.
• Progression of callus.

Exact acceptable parameters depend on patient age, fracture location, morphology and clinical context. Progressive loss of reduction should prompt reassessment of the treatment strategy.

Conservative tibial fracture management requires careful follow-up because displacement may occur after apparently satisfactory initial reduction.

Clinical Assessment

• Pain.
• Cast comfort and fit.
• Skin condition.
• Neurovascular status.
• Knee and toe movement.
• Clinical fracture stability.
• Weight-bearing tolerance.

Radiographic Assessment

• AP view.
• Lateral view.
• Alignment.
• Length.
• Callus formation.
• Progression toward union.

As swelling decreases and muscle bulk changes, a previously well-fitting cast may become loose.

A loose cast provides poorer fracture control and may permit:

• Translation.
• Angulation.
• Rotation.
• Excessive fracture motion.
• Skin friction and irritation.

Cast fit must therefore be reassessed during follow-up and the cast replaced or modified when necessary.

• Allows knee movement.
• Reduces knee stiffness compared with prolonged long-leg casting.
• Allows earlier functional rehabilitation.
• Permits controlled weight bearing in selected fractures.
• Encourages physiological loading.
• May promote callus formation through controlled mechanical stimulation.
• Maintains circumferential fracture support.
• Reduces prolonged disuse of the limb.
• Avoids operative complications when non-operative management is appropriate.
• Relatively inexpensive compared with surgical fixation.

• Less rotational control than an above-knee cast.
• Requires careful patient selection.
• Technically dependent on good moulding.
• Requires regular radiographic follow-up.
• May allow loss of reduction in unstable fractures.
• Can produce pressure sores if incorrectly moulded.
• Cast may become loose as swelling decreases.
• May be poorly tolerated by some patients.
• Not appropriate for fractures requiring surgical stabilisation.

Patients should be instructed to seek urgent reassessment if they develop:

• Increasing or disproportionate pain.
• Increasing tightness.
• Numbness or paraesthesia.
• Weakness of toe movement.
• Cold or discoloured toes.
• Marked swelling of the toes.
• Burning pain over a pressure point.
• Wet or damaged cast.
• Foul smell or discharge.
• Sudden increase in fracture-site pain or deformity.

Increasing pain after casting must never automatically be attributed to the fracture; compartment syndrome and cast-related neurovascular compression must be excluded.

The terms PTB cast and functional tibial brace are related but should not always be used interchangeably.

A PTB cast is a rigid circumferential cast moulded according to patellar tendon-bearing principles.

A functional tibial brace is usually a removable or adjustable orthotic device that provides circumferential support while allowing knee movement and functional loading.

In selected treatment protocols, the patient may progress from:

The exact sequence depends on the fracture and treatment philosophy.

Intramedullary nailing is widely used for many adult tibial shaft fractures because it provides reliable alignment and permits early mobilisation. Consequently, PTB casting is less dominant in adult tibial fracture management than it was historically.

Nevertheless, the PTB cast remains important because:

• Selected tibial fractures can still be successfully treated non-operatively.
• Some patients are poor surgical candidates.
• It provides a useful transition during fracture rehabilitation.
• Its biomechanical principles remain fundamental to functional fracture bracing.
• It remains an important examination and viva topic.

Modern treatment should therefore be based on the individual fracture rather than assuming that either surgery or casting is universally superior.

• Applying a tightly moulded definitive cast while substantial acute swelling is still developing.
• Assuming every tibial shaft fracture is suitable for functional casting.
• Applying excessive focal pressure directly over the patella instead of appropriately moulding the patellar tendon and proximal tibial regions.
• Failing to protect the fibular head and common peroneal nerve.
• Ignoring rotational deformity because AP and lateral angulation appear acceptable.
• Allowing unrestricted weight bearing without considering fracture stability.
• Failing to obtain follow-up radiographs after conversion to functional treatment.
• Continuing with a loose cast after swelling has subsided.
• Accepting progressive deformity merely because callus is forming.

• PTB stands for patellar tendon-bearing.
• The PTB functional casting concept is strongly associated with Sarmiento.
• The classic orthopaedic application is the functional treatment of selected tibial shaft fractures.
• A PTB cast reduces and redistributes axial load; it does not completely unload the tibia.
• The knee is left free, allowing knee mobilisation and reducing stiffness associated with prolonged long-leg casting.
• The cast relies on accurate proximal moulding and circumferential soft-tissue support.
• The patellar tendon region and medial tibial flare are relatively pressure-tolerant areas.
• The fibular head must be protected because of the proximity of the common peroneal nerve.
• Functional treatment relies on relative stability and therefore produces secondary fracture healing with callus.
• Controlled loading may stimulate callus formation, whereas excessive movement can result in delayed union or malunion.
• A PTB cast is generally a functional-stage treatment rather than a reason to ignore acute swelling and soft-tissue concerns.
• Serial radiographs are essential because fracture alignment may change during conservative treatment.
• A loose cast after swelling subsides can result in loss of fracture control.
• Functional treatment means controlled function – not uncontrolled fracture motion.

1. Select the fracture: confirm that non-operative functional management is appropriate.
2. Control the acute phase: assess swelling, soft tissues, neurovascular status and compartment syndrome before definitive circumferential casting.
3. Confirm alignment: satisfactory length, rotation and angulation must be maintained.
4. Mould correctly: use the proximal tibial and patellar tendon-bearing regions while protecting bony prominences and the fibular head.
5. Permit function: maintain knee motion and introduce weight bearing according to fracture stability.
6. Monitor the fracture: clinical and radiographic follow-up is essential.
7. Monitor the cast: reassess fit as swelling subsides.
8. Do not accept progressive deformity: loss of reduction requires reassessment and potentially a change in treatment.

The essence of the patellar tendon-bearing cast is functional fracture treatment: maintain acceptable tibial alignment, redistribute load, preserve knee movement and use controlled mechanical loading to support secondary bone healing.