Total Hip Replacement Techniques

Total knee replacement is a very common and successful surgery for treating knee arthritis. As

the number of knee replacements performed continues to rise, so does the level of innovation

and research into the best surgical techniques and implant designs. In this discussion, we will

detail the most common techniques and designs utilized in orthopedic surgery today. While

the focus will be on similarities and differences between each, it is important to understand

that the ultimate goals are the same no matter which technique is utilized. Those goals include

removal of the arthritis from the bone surfaces, replacing the removed bone with an

appropriately sized and positioned implant, and achieving “balance” of the knee so that it is not

too tight or too loose during activities.

Alignment Strategies

Let us begin by discussing the strategies that surgeons use to place the implants in the proper

position at the time of surgery. Several methods of aligning the implants have been described

over multiple decades and new techniques continue to be developed. As of the timing of this

blog, there have been no clear research findings to suggest one technique is superior to the

others. However, many studies comparing these techniques continue to be performed.

Measured Resection

One of the most common methods for removing the arthritic bone and aligning the implants is

called measured resection. This technique uses special alignment guides to assist the surgeon

when making the cuts of the bone. The method relies on several key components of knee

anatomy, which are used as reference points for placing the alignment guides accurately. In

most cases, the surgeon is trying to cut the bones so that the joint is perfectly parallel to the

floor and the implants sit on those flat surfaces. After the bones are cut and implants placed in

the knee, the soft tissues of the knee (ligaments, tendons, etc.) can be adjusted to ensure

proper balance of the knee.

Gap-balancing

The other main technique for aligning knee replacements is called gap-balancing. Surgeons

who use a gap-balancing technique argue that measured resection relies on anatomic

landmarks that are variable from one patient to the next. In addition, if surgeons do not

accurately identify the landmarks during the surgery, it can result in a knee replacement that is

not well-balanced. As a result, some surgeons use a different technique in which the first

couple of bone cuts are made to establish the amount of space between the implants when the

knee is fully extended. Then, using special instruments to measure that space, they adjust the

other bone cuts to make the space exactly the same when the knee is bent at a ninety-degree

angle. Proponents of this technique suggest that the knee will be in the best state of balance

since these spaces are matched. On the other hand, critics point out that the implant on the

femur may need to rotated significantly to match those spaces, which may have downstream

effects such as how well the knee cap (patella) interacts with the femur.

Kinematic Alignment

Another technique that has been getting more popular recently is called kinematic alignment.

This technique attempts to restore that patient’s knee to its unique alignment before arthritis

started. In this method, surgeons remove only the amount of arthritis and bone from the knee

that will be replaced by the metal implants themselves. By avoiding any changes in the

patient’s unique alignment, surgeons using this process argue that patient’s soft tissues will be

more naturally balanced. Therefore, less manipulation of the soft tissues may be required

during the procedure.

Functional (aka Patient-specific) Alignment

One other popular method of carrying out a knee replacement is called function alignment or

patient-specific alignment. This technique attempts to take into account the patient’s anatomy

and soft tissue balance simultaneously at the time of surgery to adjust the bone cuts before

they are made. Most surgeons using this technique will also use additional technological

advancements, such as a robotic knee replacement system or another computerized system, to

assist with adjusting the bone cuts as needed. Proponents of functional alignment argue that

the implants will be placed in a position that will minimize the need for soft tissue adjustments.

Questions remain, however, regarding whether there are limits to how much the alignment can

be changed before there are potentially detrimental effects on the knee replacement

components.

Implant Design

Just as there are several methods for aligning the knee replacement implants, there are many

different implant designs that are used today as well. The three most popular designs are

termed cruciate-retaining, posterior-stabilized, and medial pivot, which will be detailed below.

Following this, a few other characteristics of implant design will also be discussed.

Cruciate-retaining versus Posterior-stabilized

While the majority of knee replacement implants require the removal of one of the two

cruciate ligaments in the center of the knee (anterior cruciate ligament), cruciate-retaining

implants keep the other cruciate ligament called the posterior cruciate ligament. The knee

replacement components are designed to substitute for the function of the anterior cruciate

ligament so that the knee remains stable after surgery. In a posterior-stabilized total knee

replacement, however, both cruciate ligaments are removed and the implants have a slightly

different shape to substitute for both ligaments. These two designs are the most commonly

compared in research studies to date and show similar clinical results.

Medial pivot

A more recent development in total knee replacement design is an implant that attempts to

restrict motion on the medial (inside) part of the knee while allowing more motion laterally

(outside). This design is based on the normal function of the knee without arthritis, which

tends to show that same pattern of movement while bending and straightening the knee. This

type of implant was designed in an attempt to make the knee replacement more natural feeling

for patients.

Other implant characteristics

For each of the designs described above, several other characteristics of the implants have

multiple options as well. One of those characteristics is how the implants are attached to the

bone surfaces. In some cases, the implants are designed to essentially be glued in place using a

special bone cement. In others, the surface of the implants in contact with the patient’s bone is

treated in such a way that the bone will grow on to the implants over time. These implants are

referred to as cementless knee replacements. Another important characteristic of the implants

involves the types of metals used to make them. Most knee replacement implants are made of

a mixture of multiple metals, but the largest proportion of the metal in the implants include

cobalt, chromium, and titanium. For the vast majority of patients, the metals found in the

implants are tolerated quite well for many years. However, there is some concern that patients

with certain metal allergies may react to the implants placed in their knees. As a result, specific

implants have been designed to avoid particular metals, such as nickel, for certain patients.

Although this is another area of orthopedics that is currently debated, patients should discuss

this issue with their surgeons prior to surgery if they have a known or highly suspected metal

allergy.

Texas Hip and Knee Center

As described above, many different alignment techniques and implant designs have been and

continue to be developed to this day. While a tremendous amount of research has been done

in these regards with the hope of discovering the “best” design or technique, the majority of

the orthopedic literature on these topics have not been able to show a clear winner. That being

said, all of these techniques and designs result in very successful surgical outcomes for the vast

majority of patients. One of the most interesting aspects of my journey as the Fellow at the

Texas Hip and Knee Center has been seeing everything listed above being used in practice.

Within the group of surgeons at Texas Hip and Knee, implants are placed using measured

resection, gap balancing, kinematic alignment, and patient-specific alignment. Computerized

and robotic systems are used in some cases. Cruciate-retaining, posterior-stabilized, and

medial pivot implant designs are utilized with both cemented and cementless components.

Additionally, implants that are nickel free are placed in appropriate patients when needed. As a

Fellow advancing my training in hip and knee replacement, this breadth has provided an

amazing opportunity for me to see all of these practices being carried out and the excellent

results that each can produce for our patients. As a patient of the Texas Hip and Knee Center,

this also provides a tremendous opportunity to work with your surgeon to optimize the implant

and technique that is best for you. Armed with the knowledge that you have gained from the

details above, I hope you will feel more comfortable and confident when discussing all of these

details with your surgeon in the future.