DEEP DIVE Look to factors other than surgical outcomes to differentiate available implants.

Hip and knee implants are essentially created equal when it comes to reducing joint pain and improving post-op function. So how do you and your surgeons make sense of the hundreds of choices in prostheses that vary in material and design? Matching the right implant to the right patient generally boils down to these key considerations.

In the knee
Knees are often thought of as a "hinge" joint, since they bend and straighten like a hinged door. In reality, though, the knee is much more complex. While original implants used a simple door-hinge design, newer models more closely mimic the rolling and gliding motion of the joint. Most standard knee implants are made up of 3 components that cover different bone surfaces:

• The metal femoral component curves around the lower end of the femur.
• The tibial component is made up of a metal platform with a polyethylene cushion insert. It is attached to the top surface of the tibia.
• The patellar piece is a dome-shaped implant made of polyethylene that covers the back surface of the patella and mimics the shape of the kneecap.
• Typically, manufacturers design these components to work as metal-on-plastic (usually cobalt-chromium or titanium with polyethylene). This provides smooth movement and minimal wear. There are currently more than 150 knee implant designs on the market, so when choosing which device to use surgeons consider several key differences, including:
• Cruciate-retaining vs. posterior-stabilized. Cruciate-retaining implants let the surgeon preserve the patient's posterior cruciate ligament, if it's still intact. These implants have a small groove that allows for flexion of the ligament. Posterior-stabilized implants let the surgeon remove the ligament and replace it with a cam-and-post system that prevents the thighbone from sliding too far forward on the shinbone when the knee is bent.
• Cemented vs. cementless. While some implants are attached using fast-curing bone cement, cementless prostheses are made of a material that attracts new bone growth to keep the device in place. Both approaches work well, though more surgeons are moving to cementless options since they allow for a relatively easier surgery, take less time to place and — especially in younger patients — seem to be a better fit.
• Mobile-bearing vs. fixed-bearing. A fixed-bearing prosthesis has the polyethylene insert of the tibial component attached firmly in the metal tray. In contrast, a mobile-bearing implant's cushion can move more freely in the tray. This means that mobile-bearing implants allow for a few more degrees of rotation compared to fixed-bearing options. Most patients get fixed-bearing implants, but surgeons will often use mobile-bearing options in younger, more active patients since the devices require more support from surrounding soft tissue.

In the hip
The hip is a "ball-and-socket" joint made up of the acetabulum (the socket) and the femoral head (the ball). Hip implants replicate this anatomy with a metal or ceramic ball and a cup made of polyethylene, ceramic or metal.

The key difference in these implants is whether metal, ceramic or plastic is used and in what combination. While most hip prostheses tend to perform the same in patients, the bearing surface — or the area where the 2 components meet — can impact a surgeon's choice.

There are 3 major types of hip implants:

• Metal on metal. While popular several years ago, metal-on-metal implants have fallen out of favor as studies have shown that they can break down over time and send metal particles into the surrounding soft tissue to cause adverse reactions.
• Ceramic on ceramic. Ceramic-on-ceramic implants are also less commonly used, since they can be harder to place, can become brittle over time and are very expensive.
• Ceramic on polyethylene or metal on polyethylene. Most surgeons prefer polyethylene implants. Though original designs had wear-and-tear issues of their own, most modern implants use a cross-linked polyethylene that's much more durable. This is good news for cost-conscious facilities, because metal-on-metal and ceramic-on-ceramic implants typically cost much more than plastic alternatives.

ANY IMPLANT YOU WANT
Give Your Surgeons Freedom of Choice

HARDWARE STORE Budgeting and stocking protocols benefit surgeons, vendors and your bottom line.

Doctors at my hospital can use any implant they want — but there's a catch. Administration sets a firm price limit for all implants. If the budget is $4,000, surgeons can use their desired high-tech implant as long as the vendor can offer it at or below that price. If the vendor can't match the budgeted amount, the surgeon must pick another device he finds suitable.

Surgeons are notoriously picky about what brand of implants they use. While most knee and hip implants function the same and result in good patient outcomes, some small nuances in manufacturers' designs lead to surgeons choosing one over another. The relationship the surgeon has with the service rep also often plays a big role in his decision. All of this can make your job of stocking the devices a little tougher.

Of course, you can try to standardize implants. Another way to make stocking easier is to allocate storage space to vendors. My facility gets all of our implant systems on consignment, so the supplies are there when we need them, and the hospital is charged only when they are used and replaced. This is great for a few different reasons: The vendors know that their products are going to be used, and that they don't have to worry about stocking the right implant for each patient every day. Surgeon benefits from knowing that their preferred brands are always available.

— Paul A. Manner, MD, FRCSC

Surgeons must also consider the fixation of the implant. While cementless replacements are thought to work better for those with active lifestyles and healthy bones, studies have shown that both cemented and cementless approaches are excellent and offer similar results. I find cementless devices to be easier to insert and end up saving time in the OR.

A personalized fit
Standard implants are based on "average" size data, which means most come in only a few different size variations. In an attempt to find a better fit, manufacturers have developed several new implants that are tailored to the individual patient.

For example, gender-specific implants are adjusted to fit the average anatomical sizes of women and men, as well as account for other shape differences between male and female joints. While it appears that gender-specific implants fit the joint better than standard sizes, studies have shown that these implants perform nearly identically to standard hardware.

A newer choice on the market is patient-specific implants. Before implantation, patients undergo an MRI or CT scan, which is sent to the implant manufacturer. The company creates custom implants for patients and ships them, along with the tools needed to perform the surgery, to the host facility. Manufacturers claim that patient-specific implants help reduce issues related to poor fit, including overhang or underhang that's sometimes seen in knee replacements. However, studies again show that patient outcomes match those when standard implants are used.

But there is one major benefit that might make you consider this option — convenience. The implant arrives at your facility in a box designated for a specific patient. In the box is everything the surgeon needs for the case: the implant, a choice of polys, disposable jigs and sterilized instruments.

For a facility just starting with total joint cases, or a smaller surgical center, patient-specific implants are a great option since they eliminate the need to buy several trays of the heavy, expensive tools used in arthroplasty. They also simplify instrument reprocessing, since the items are disposable.

Keep in mind that some surgeons look unfavorably on the lack of flexibility offered with these kits. Once surgeons open the knee, they might need a different tool or implant, which could lead to a long, tedious process of tracking down your vendor and getting the device brought in. But if your surgeons are on board, and the price is comparable to standard implants, these kits are a useful option. OSM