Wilderness Medical Series — Module 09

How to Use a SAM® Splint for Lower Leg and Knee Injuries

SAM® splint techniques for lower leg and knee injuries in the field — closed-cell foam knee brace, improvised air mattress immobilizer, and CMS protocol.

By Joshua Enyart · Founder & Head Instructor, Gray Bearded Green Beret

Former Army Ranger, Green Beret, and full-time survival instructor · three decades of professional instructor experience

Lower leg and knee injuries present different field management challenges than ankle injuries. The knee is a complex joint with multiple ligament and structural components — some knee injuries allow carefully modified weight-bearing, while others require complete immobilization. Lower leg fractures involve the tibia, fibula, or both, and may be among the most disabling field injuries short of the femur.

Both locations carry the same fundamental concern: loss of mobility in a remote setting creates a cascade of secondary problems. The person who cannot walk needs to be evacuated by external means, which is slower, more resource-intensive, and dependent on terrain that may not cooperate.

This article covers SAM® splint applications for the lower leg and knee, including the Closed-Cell Foam Knee Brace for usable knee injuries and the Improvised Air Mattress Knee Immobilizer for complete immobilization when the knee cannot bear any load.

Lower Leg Fractures: Recognizing the Injury

A lower leg fracture typically involves the tibia (the larger bone of the lower leg), the fibula (the smaller outer bone), or both. Isolated fibular fractures are less disabling because the tibia carries the primary load. A tibial fracture eliminates the ability to bear weight entirely.

Signs and symptoms of a lower leg fracture:

• Pain and tenderness at a specific point along the bone — not diffuse joint pain but focal bony tenderness.
• Rapid swelling and bruising developing within minutes to hours of the injury.
• Inability to bear weight, or significantly abnormal gait if partial weight-bearing is attempted.
• Crepitus: a grating, cracking, or popping sensation when the injury site is palpated gently.
• Deformity: visible angulation or rotation of the lower leg that was not present before the injury.
• In an open fracture: a wound near the fracture site, or bone visibly penetrating the skin. Open fractures carry high infection risk and require immediate wound care as well as splinting.

In the field, the assessment goal is not diagnosis — it is determining whether the injury can be splinted and the patient moved, or whether the severity requires an immediate stationary signal and evacuation. A lower leg fracture with significant deformity, an open wound, or rapidly progressing swelling warrants careful assessment before any attempt to move the patient.

Splinting the Lower Leg

A lower leg fracture requires a splint that captures both the ankle (below the injury) and the knee (above the injury). This is the 'secure above and below the joint' principle applied to a long-bone fracture — both the joints at either end of the injured bone must be included in the splint.

A SAM® splint is long enough for ankle-to-lower-leg applications on most adult patients. For a full lower leg to knee application, two SAM® splints side-by-side, or a SAM® splint combined with improvised rigid material, provide the coverage needed.

Lower leg splinting procedure:

• Check CMS before application. Baseline capillary refill at toes, toe motor function, sensation in foot and toes.
• Manually stabilize the lower leg with your hands, supporting both above and below the fracture site. Do not deliberately attempt to realign a fractured bone in the field unless the patient is showing circulation compromise below the injury.
• Position the foot in a position of function — 90 degrees between foot and lower leg.
• Form two SAM® splints along the medial and lateral aspects of the lower leg, extending from the foot up to the knee, or use the splint along the posterior aspect if that provides better coverage.
• Secure with elastic bandages or cravats, starting at the ankle and working up. Each wrap should overlap the previous by about half its width.
• Recheck CMS immediately after application and at regular intervals.

Knee Injuries: When to Brace, When to Immobilize

The key clinical decision with a knee injury is whether the knee is usable — meaning the patient can bear weight with pain and limitation but without instability or complete loss of function — or whether the knee is unusable and requires complete immobilization.

A usable knee with a significant sprain, minor ligament strain, or contusion benefits from a brace that provides support and compression while still allowing the modified function needed to assist with evacuation. Completely immobilizing a functional knee creates more problems than it solves.

An unusable knee — one with complete ligament disruption, suspected fracture of the proximal tibia or distal femur, or significant effusion (fluid in the joint) — requires complete immobilization. Any attempt to use a completely unstable knee risks worsening the injury and may compromise the blood supply if instability allows significant movement at the joint.

The Closed-Cell Foam Knee Brace

The closed-cell foam sleeping pad provides a functional improvised knee brace for a usable knee injury. The technique uses the pad material to create lateral support without anterior restriction — supporting the sides of the knee while leaving the front accessible for visual assessment and allowing the modest flexion needed for walking.

Closed-cell foam knee brace procedure:

• Cut or fold a section of closed-cell foam sleeping pad to a width slightly wider than the knee on both sides and a length sufficient to extend from mid-lower-leg to mid-thigh.
• Place the foam section flat behind the knee, centered at the joint.
• Fold or roll the sides of the foam up along both sides of the knee.
• Secure the sides with cravats or elastic bandages wrapped above and below the knee. Leave the front of the knee open — the goal is to brace, not fully immobilize.
• Check CMS. The brace provides support and compression, not a tight circumferential wrap. It must not restrict circulation.

The Improvised Air Mattress Knee Immobilizer

For a completely unusable knee, the inflatable sleeping pad becomes a highly effective full immobilizer. An inflated sleeping pad provides padding, compression, and rigidity simultaneously — the combination that defines effective immobilization.

Air mattress knee immobilizer procedure:

• Start with the sleeping pad fully deflated. A deflated pad is flexible and can be wrapped around the knee without forcing the knee into any position.
• Wrap the deflated pad around the injured knee, conforming to the joint contour.
• Loosely secure the pad in position with cravats above and below the knee. Do not secure tightly at this stage — the pad will expand when inflated, and a tight pre-inflation wrap will restrict that expansion.
• Add sticks or trekking pole sections if additional rigidity is needed. Place them under the pad, against the lateral aspects. Ensure any added rigid material is smooth — rough or sharp edges will puncture the pad when inflated.
• Inflate the pad. The inflated mattress fills all the voids around the knee, creates a firm circumferential enclosure, and provides the combination of padding, compression, and rigidity that a traditional immobilizing splint provides. This is an effectively improvised vacuum splint.
• Recheck CMS after inflation. Inflation changes the pressure profile around the knee. Verify circulation, motor, and sensation have not changed from baseline.

Mobility and Evacuation Planning

A splinted lower leg fracture is not a walking-out injury in most cases. A braced knee sprain may be — carefully, slowly, with support from trekking poles and a partner. The distinction matters for evacuation planning.

For lower leg fractures: signal early and prepare to wait. The patient can be moved short distances to better shelter, signaling ground, or water access, but is not a candidate for self-rescue over meaningful terrain. Evacuation requires external assistance.

For knee injuries: assess the nature of the injury honestly. A minor sprain that allows some weight-bearing with pain is a different situation than a complete ligament rupture with joint instability. The first may allow a slow, supported walk-out. The second requires the same evacuation protocol as a fracture.

Ongoing CMS Assessment and Swelling Management

Swelling following a lower leg or knee injury develops progressively in the first four to six hours and can continue for up to twenty-four hours. A splint that is appropriate in the first hour may become too tight as swelling increases. Reassess CMS every thirty minutes in the initial post-application period.

If the patient reports increasing numbness, tingling, or significantly increased pain after splint application, loosen the securing bandages or cravats. Do not remove the splint — just reduce the compression. Allow swelling room to develop without compromising circulation. A slightly less rigid splint that does not cut off blood flow is better than a perfectly rigid splint with vascular compromise.

For knee injuries where the joint is swollen and hot to the touch, the joint is likely effused — fluid has collected inside the capsule in response to injury. This is normal, but it increases pressure within the joint. A brace or immobilizer that adds further circumferential pressure to an effused knee can cause significant pain and potentially compromise circulation. Confirm CMS frequently and adjust the bracing to accommodate swelling.

If the patient reports altered sensation or loss of circulation that does not resolve with splint adjustment, this indicates a possible vascular or nerve injury that requires urgent evacuation. Some injuries — particularly those involving significant deformity or dislocation — may compromise the popliteal artery or peroneal nerve. These are emergencies. Signal for evacuation immediately and do not delay for comfort management.

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