Below-Knee Prosthesis with Pin Lock: How to Put It On

A below-knee prosthesis with a pin lock system provides secure, reliable attachment for people with transtibial (below-knee) amputation. This guide explains step-by-step how to apply your silicone liner, attach the locking pin, and put on your prosthesis correctly. Proper donning technique is essential for comfort, skin health, and optimal prosthetic function.

What Is a Pin Lock Prosthesis System?

A pin lock prosthesis uses a small metal pin attached to the bottom of a silicone liner. This pin inserts into a shuttle lock mechanism inside the prosthetic socket, creating a secure mechanical connection that holds the prosthesis firmly in place during walking and daily activities.

The pin lock system, also known as a shuttle lock mechanism, is one of the most widely used suspension methods for below-knee (transtibial) prostheses worldwide. This system was developed to provide amputees with a reliable, easy-to-use method of attaching and removing their prosthetic leg. The technology has been refined over decades and remains popular due to its simplicity, durability, and effectiveness.

When you use a pin lock prosthesis, you first put on a soft silicone liner that fits snugly over your residual limb (the remaining portion of your leg after amputation). This liner has a threaded attachment point at its distal end (bottom) where a metal pin screws in. The pin is typically about 2-3 inches long and has a series of grooves or ridges along its shaft. These ridges engage with a spring-loaded mechanism inside the prosthetic socket, creating an audible click when properly locked.

The beauty of this system lies in its mechanical reliability. Unlike suction-based suspension systems that rely on creating a vacuum, the pin lock provides a positive mechanical connection. This means the prosthesis will stay attached even if you sweat, if the liner gets slightly dirty, or if there are minor changes in your residual limb volume throughout the day. Many users appreciate the peace of mind that comes from hearing and feeling that distinct click, knowing their prosthesis is securely attached.

Components of a Pin Lock System

Understanding each component of your pin lock prosthesis helps you maintain it properly and troubleshoot any issues that may arise. The system consists of three main parts that work together to create secure suspension.

The silicone liner is the interface between your skin and the prosthesis. Made from medical-grade silicone, these liners are designed to protect your residual limb, distribute pressure evenly, and provide cushioning. Liners come in various thicknesses, lengths, and shapes to accommodate different residual limb characteristics. Your prosthetist selects a liner based on your limb shape, activity level, and skin sensitivity. Most liners have a fabric outer covering for durability and a threaded receptacle at the bottom for the pin attachment.

The locking pin is a metal rod that screws into the bottom of your liner. Pins are typically made from stainless steel or titanium for strength and corrosion resistance. The pin's shaft has multiple grooves that engage with the locking mechanism. Some pins are straight, while others have a slight angle to accommodate different socket designs. The threads must match your liner's receptacle, so it's important to use only compatible pins as specified by your prosthetist.

The shuttle lock mechanism is built into the bottom of your prosthetic socket. This spring-loaded device grips the pin when inserted, with each groove on the pin creating a successive locking point. A release button on the side or bottom of the socket allows you to disengage the lock when removing the prosthesis. The mechanism is designed to prevent accidental release while allowing easy removal when the button is pressed.

How Do You Apply the Silicone Liner Correctly?

Roll the silicone liner onto your residual limb from bottom to top, ensuring a smooth, wrinkle-free fit. The liner should cover your entire residual limb snugly without air pockets. Take your time with this step, as proper liner application is essential for comfort and prosthetic function.

Applying your silicone liner correctly is perhaps the most important step in the donning process. A poorly applied liner can lead to skin problems, discomfort, and reduced prosthetic function. Before applying the liner, ensure your residual limb is clean, dry, and free of lotions or oils that could prevent the silicone from gripping your skin properly. Some users prefer to apply a thin layer of prosthetic skin lotion the night before, allowing it to fully absorb, while keeping the skin product-free at donning time.

Start by turning the liner inside out, holding it by the bottom portion. Position the inverted liner at the end of your residual limb, making sure the center of the liner aligns with the end of your limb. Begin rolling the liner upward, smoothing out any wrinkles or air pockets as you go. The silicone should make direct contact with your skin throughout this process. Avoid stretching the liner excessively, as this can cause it to lose its shape over time.

As you roll the liner up, use your hands to push out any trapped air. Air pockets between your skin and the liner can cause skin irritation, blisters, and pressure sores. They can also affect the fit of your prosthesis within the socket. Take the time to ensure the liner sits smoothly against your skin, especially around bony prominences like the tibial crest (shin bone) and fibular head.

The liner should extend high enough on your residual limb to provide adequate coverage and suspension. Typically, this means the liner reaches to just below your knee joint, though your prosthetist will advise on the appropriate length for your specific situation. If the liner is too short, it may roll down during use. If it's too long, it may bunch up and cause discomfort.

Common Liner Application Mistakes

Even experienced prosthesis users sometimes make errors when applying their liner. Being aware of these common mistakes can help you avoid them and ensure optimal comfort and function.

• Rushing the process: Taking time to apply the liner correctly pays dividends in comfort throughout the day. A hastily applied liner often develops wrinkles or air pockets that cause problems later.
• Applying lotion right before donning: Lotions and creams create a barrier between your skin and the silicone, reducing grip and potentially causing the liner to slide or rotate during use.
• Not smoothing out air bubbles: Air trapped under the liner creates pressure points and can cause skin breakdown, especially during prolonged wear or active use.
• Using a worn-out liner: Liners lose their grip and cushioning over time. Using an old, stretched liner compromises both comfort and suspension.
• Incorrect liner size: A liner that's too tight restricts circulation, while one that's too loose doesn't provide adequate support. Consult your prosthetist if fit changes.

How Do You Attach the Locking Pin?

To attach the locking pin, first press the release button on your prosthetic socket and pull the pin out. Then screw the pin firmly into the threaded receptacle at the bottom of your silicone liner. Ensure the pin is fully seated and straight before inserting into the socket.

Before you can put on your prosthesis, you need to retrieve the pin from the socket and attach it to your liner. This step requires accessing the pin by pressing the release mechanism on your prosthetic socket. The release button is typically located on the side or bottom of the socket, near the shuttle lock mechanism. Pressing this button disengages the lock and allows you to pull the pin out of the socket.

Once you have the pin in hand, examine it briefly to ensure it's clean and undamaged. The threads should be free of debris, and the pin shaft should be straight without any bends or cracks. A damaged pin can fail to lock properly or may cause the lock mechanism to wear prematurely. If you notice any damage, contact your prosthetist before using the prosthesis.

With your liner already in place on your residual limb, locate the threaded receptacle at the bottom (distal end) of the liner. Align the pin's threads with the receptacle and begin screwing it in clockwise. The pin should thread in smoothly without excessive force. If you encounter resistance, back the pin out and check that the threads are aligned correctly. Forcing a cross-threaded pin can damage both the pin and the liner receptacle.

Continue threading the pin until it's fully seated and snug. Most pins have a shoulder or flange that sits flush against the liner when fully inserted. Give the pin a gentle tug to confirm it's secure. A loose pin can unscrew during use, potentially causing the prosthesis to detach unexpectedly. Some users apply a small amount of thread-locking compound (as recommended by their prosthetist) to prevent loosening during active use.

How Do You Insert Your Limb Into the Prosthetic Socket?

Guide your residual limb into the prosthetic socket while keeping the pin aligned with the locking mechanism hole. Push firmly until you hear a clicking sound indicating the pin is locked. The socket should fit snugly, with the front edge positioned just below your kneecap.

With your liner on and the pin attached, you're ready to don your prosthesis. This step requires coordination between your hands and the position of your residual limb. Many users find it easiest to perform this step while sitting in a sturdy chair with the prosthesis standing upright in front of them, though techniques vary based on individual preference and ability.

Hold the prosthetic socket with both hands to steady it. Position your residual limb above the socket opening, looking for the small hole at the bottom of the socket where the pin will enter. This hole leads to the shuttle lock mechanism. Align the pin with this opening as closely as possible before beginning to insert your limb into the socket.

Begin lowering your residual limb into the socket. As the pin enters the locking mechanism, you may need to make small adjustments to ensure proper alignment. The pin should slide in smoothly, meeting minimal resistance. If you feel the pin scraping against the edges of the lock housing, pull back slightly and reposition before continuing. Forcing the pin in at an angle can damage the locking mechanism or prevent proper engagement.

Once the pin begins entering the lock, push down firmly with your residual limb. You should hear a series of clicking sounds as each groove on the pin passes the lock pawl. These clicks indicate progressive engagement with the locking mechanism. Continue pushing until the clicks stop and you feel the prosthesis firmly seated. The final click should feel more definitive, indicating full engagement.

After the prosthesis is on, take a moment to assess the fit. The socket should feel snug but not uncomfortably tight. There should be no sharp pressure points or areas where the socket digs into your residual limb. The front edge of the socket should sit just below your kneecap (patella), providing clearance for knee flexion while maintaining proper prosthetic alignment.

Checking Proper Socket Position

Correct socket positioning is essential for comfortable walking and proper prosthetic function. The socket should fit like a well-designed piece of equipment, holding your residual limb securely while allowing natural movement at the knee.

The anterior (front) wall of the socket should sit approximately one finger's width below your patella (kneecap). This position allows your knee to bend fully without the socket edge pressing into your kneecap or patellar tendon. If the socket rides too high, it will limit knee flexion and cause discomfort during walking or sitting.

The medial (inner) and lateral (outer) walls should provide even pressure distribution around the condyles of your femur (the bony prominences on either side of your knee). These areas are important for controlling rotation of the prosthesis during walking. Uneven pressure here can cause skin problems or make the prosthesis feel unstable.

The posterior (back) wall should allow comfortable knee flexion. When you bend your knee, the back of the socket shouldn't dig into your hamstring tendons or the soft tissue behind your knee. Some sockets have a contoured posterior design specifically to accommodate knee bending.

How Do You Verify the Prosthesis Is Securely Attached?

After putting on your prosthesis, gently pull downward on the socket to confirm the pin is locked. The prosthesis should not move or detach when you pull. Also, attempt to rotate the prosthesis slightly; excessive rotation may indicate a fitting issue that needs professional attention.

Verifying secure attachment before walking or engaging in activities is a critical safety step that should become part of your routine. A prosthesis that isn't properly locked can detach unexpectedly, potentially causing falls and injuries. Taking a few seconds to check the attachment provides peace of mind and helps you catch any issues before they become problems.

The primary verification method is the pull test. With the prosthesis on and your foot flat on the floor, grasp the socket firmly and pull downward. Use moderate force—enough to test the connection but not so much that you strain yourself. The prosthesis should remain solidly in place, with no movement or releasing sensation. If you feel any give, or if the prosthesis moves at all, the pin may not be fully engaged with the lock.

If the pull test reveals the prosthesis isn't secure, remove it by pressing the release button and withdrawing your limb from the socket. Examine the pin to ensure it's still firmly attached to the liner and that the threads are intact. Look at the socket's locking mechanism for any visible debris or damage. Then repeat the donning process, paying careful attention to pin alignment as you insert your limb into the socket.

Beyond the pull test, also check for rotational stability. While standing with weight on your prosthetic leg, try to rotate your residual limb within the socket. Some minimal movement is normal, but excessive rotation can indicate a poor fit or worn components. If you notice significant rotation that wasn't present before, consult your prosthetist, as this could indicate changes in your residual limb volume or wear in the socket or liner.

What Should You Watch for on Your Skin?

Inspect your residual limb regularly for pressure marks, redness that doesn't fade within 15-20 minutes, blisters, rashes, or any skin breakdown. Minor redness that resolves quickly is normal, but persistent marks or skin damage require attention from your prosthetist or healthcare provider.

Skin health is one of the most important aspects of successful prosthesis use. Your residual limb's skin was not originally designed to bear the loads and pressures that come with prosthetic use. When you begin using a prosthesis, or when you change liners or sockets, your skin needs time to adapt. Even experienced prosthesis users must remain vigilant about skin care, as problems can develop suddenly due to changes in fit, activity level, or overall health.

The inside of the prosthetic socket is a warm, moist environment that can promote bacterial and fungal growth if not properly managed. Perspiration, combined with pressure and friction, creates conditions that can lead to various skin problems. Regular inspection of your residual limb should become as routine as checking the weather before going outside.

After removing your prosthesis, examine your entire residual limb carefully. Use a mirror if needed to see all areas, particularly the back and sides that are difficult to view directly. Look for any areas of redness, especially over bony prominences. Some redness immediately after prosthesis removal is normal, but it should fade within 15-20 minutes. Redness that persists longer, or that appears in the same location consistently, may indicate excessive pressure that needs to be addressed.

Common Skin Problems and Prevention

Understanding the types of skin problems that can occur helps you prevent them and recognize them early if they do develop. Most skin issues are treatable, especially when caught early, but ignoring them can lead to more serious complications.

• Pressure sores: These develop when sustained pressure reduces blood flow to an area of skin. They typically occur over bony prominences like the tibial crest or fibular head. Prevention includes ensuring proper socket fit and taking breaks during extended prosthesis use.
• Friction blisters: Caused by repetitive rubbing between skin and liner or socket. Proper liner fit and the use of prosthetic lubricants or skin protection products can help prevent blisters.
• Contact dermatitis: An allergic or irritant reaction to liner materials, soaps, or other substances. If you develop a rash, try changing your cleaning products or consult your prosthetist about alternative liner materials.
• Folliculitis: Infection of hair follicles, appearing as small red bumps or pustules. Keeping the skin and liner clean, and considering hair removal in problem areas, can help prevent this condition.
• Fungal infections: The warm, moist environment inside the liner can promote fungal growth. Daily liner cleaning, allowing the liner to dry completely, and using antifungal powder if recommended can help prevent infections.

How Do You Care for Your Prosthesis and Liner?

Clean your silicone liner daily with mild soap and water, then allow it to air dry completely before the next use. Wipe down the inside of your prosthetic socket regularly and check all components for wear or damage. Proper maintenance extends the life of your equipment and protects your skin.

Daily care of your prosthetic components is essential for hygiene, comfort, and equipment longevity. Just as you wouldn't wear the same socks for a week without washing them, your prosthetic liner needs regular cleaning to prevent odor, bacterial growth, and skin problems. Establishing a consistent care routine makes maintenance feel less like a chore and more like an automatic part of your daily life.

Your silicone liner requires the most attention. After each use, turn the liner inside out and wash it with mild soap and lukewarm water. Avoid harsh detergents, alcohol-based cleaners, or abrasive scrubbing, as these can damage the silicone. Some manufacturers sell specialized liner cleaning solutions, which can be helpful but aren't strictly necessary. Rinse the liner thoroughly to remove all soap residue, as leftover soap can irritate your skin.

After washing, shake off excess water and allow the liner to air dry completely before your next use. Drying typically takes several hours, which is why many regular prosthesis users have two liners—one to wear while the other dries. Never use a hair dryer, radiator, or direct sunlight to speed drying, as heat can damage the silicone and reduce the liner's lifespan.

The prosthetic socket also needs regular cleaning. Wipe the inside of the socket with a damp cloth to remove sweat and debris. For deeper cleaning, use a mild soap solution, but avoid getting the socket excessively wet, especially if it has a foam liner or other absorbent components. The exterior of the socket and the rest of the prosthesis can be wiped clean as needed.

The locking mechanism should be checked periodically for smooth operation. Press the release button to ensure it moves freely. If the button becomes stiff or the lock doesn't engage properly, debris may have accumulated in the mechanism. Your prosthetist can clean and service the lock during regular maintenance visits.

Liner Replacement Schedule

Silicone liners don't last forever. Over time, they lose their grip, elasticity, and cushioning properties. Most manufacturers recommend replacing liners every 6 to 12 months, though this varies based on liner type, usage intensity, and care practices. Signs that your liner needs replacement include:

• The silicone feels less tacky or grippy than when new
• The liner has become stretched or loose
• There are visible tears, cracks, or thin spots in the silicone
• The liner retains odor even after thorough cleaning
• You notice increased pistoning (up-and-down movement) in the socket

What Should New Prosthesis Users Know?

When first using a prosthesis, start with short wearing periods of 15-30 minutes and gradually increase wear time over days to weeks. Monitor your skin closely for any problems and communicate with your prosthetist about any concerns. Learning to use a prosthesis is a process that takes time and patience.

Beginning to use a prosthesis represents a significant life transition. Whether your amputation was recent or you're switching to a new type of prosthesis, there's a learning curve involved. Your body needs time to adapt to the new sensations, pressures, and movement patterns. Your skin needs to toughen and become accustomed to the liner and socket. Your muscles need to develop the strength and coordination for walking with the prosthesis.

Start with conservative wearing times. On your first day, 15 to 30 minutes of wear may be sufficient. Remove the prosthesis and check your skin. If there are no concerning signs, you can try another short session later in the day. Over the following days and weeks, gradually increase your wearing time as tolerated. This progressive approach gives your skin time to adapt and helps you identify any fit issues before they cause serious problems.

During this initial period, it's normal to experience some discomfort as your body adjusts. However, sharp pain, burning sensations, or areas of intense pressure are not normal and should be reported to your prosthetist. These could indicate fit issues that need to be addressed through socket adjustments or liner changes. Don't try to "push through" significant pain, as this can lead to skin breakdown and setbacks in your rehabilitation.

Work closely with your rehabilitation team during this period. Physical therapists can teach you proper gait patterns, exercises to strengthen relevant muscles, and techniques for navigating different terrains and situations. Occupational therapists can help you adapt daily activities for your new situation. Your prosthetist will likely schedule frequent follow-up appointments to monitor your progress and make adjustments as needed.

How Do You Troubleshoot Common Problems?

Common prosthesis problems include the pin not locking properly, the prosthesis feeling loose or unstable, skin irritation, and difficulty walking. Most issues can be addressed by checking your donning technique, inspecting components for wear, or consulting with your prosthetist for adjustments.

Even with proper technique and care, you may occasionally encounter problems with your prosthesis. Knowing how to troubleshoot common issues can save you time and help you determine when professional help is needed. Many problems have simple solutions that you can implement yourself, while others require intervention from your prosthetist.

Pin Won't Lock

If the pin doesn't engage with the locking mechanism, first check that the pin is still firmly attached to your liner. A loose or partially unscrewed pin may not extend far enough to reach the lock. Next, ensure there's no debris in the socket's locking mechanism—small objects, dirt, or dried sweat can interfere with lock engagement. Try pressing the release button several times to clear the mechanism. If the pin threads smoothly into the lock but doesn't click, the lock mechanism itself may need servicing.

Prosthesis Feels Loose

A loose-feeling prosthesis can result from several causes. Your residual limb may have decreased in volume, which is common, especially in the first year post-amputation. Adding prosthetic socks over your liner can take up extra space, though this is a temporary solution. A worn liner that has lost its grip can also cause looseness. If the problem persists, your socket may need to be remade to accommodate your current limb size.

Prosthesis Rotates on Your Limb

Excessive rotation typically indicates a fit issue. The socket may be too loose overall, or the shape may not match your limb contours properly. In some cases, the liner itself may be rotating on your skin rather than the socket rotating on the liner. Check that you're applying the liner correctly and that it's not worn out. Your prosthetist can assess whether socket modifications or a new socket are needed.

Discomfort or Pain

Discomfort can range from minor annoyance to severe pain. Identify where the discomfort occurs and when it's worst. Discomfort over bony prominences may indicate pressure areas that need relief in the socket. Pain in soft tissue areas could be from the socket edge cutting in. Pain that occurs only during certain activities may point to alignment issues. Document your symptoms and share them with your prosthetist to facilitate troubleshooting.