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What total knee replacements are trying to achieve

A perfect knee implant should work in the same way and, crucially, provide the same freedom of movement and stability as a normal, healthy knee. It should also provide complete relief of pain, feel normal and allow patients to return to an active life for many years.

Anatomic Knee Replacement Designs

One approach to designing a knee replacement device is to make it anatomically correct. As long as the shapes of the components closely match to the shapes of the natural bones ends and are accurately placed, shouldn’t we expect the replaced knee to work normally?

While it may not have been possible years ago, technological advances have made anatomically correct devices more of a reality. The recent introduction of customised knee implants, which are shaped to match 3-dimensional CT or MRI scans of patients’ knees, are based on this idea. Technologies like robotic surgery, which may also use 3D scan data to map a patient’s knee and assist the surgeon in precise placement of implant components, are also becoming available. Surely these are what we all need (if we can afford it)!

Unfortunately, while the idea and these advanced technologies are attractive and seem to make sense, the reality is that they fail to achieve the two crucial elements of a perfect knee implant: they do not provide the same controlled freedom of movement or stability as a normal healthy knee. The reason is that they fail to consider the joint’s natural tissues – an essential part of the success of the condylar knee concept in 1969. So despite higher prices for the technology (which is good for manufacturers) they are unlikely to feel normal and allow patients to return to an active life and forget about their knee.

Anatomy of the Knee: Before and After Surgery

A normal healthy knee joint is surrounded by a joint capsule. The capsule is principally made up of muscle and ligaments. Muscles contract to generate movement, and ligaments, which can only work in tension, control the safe limits of movement.

The Joint Capsule

On the inner side of the knee a thick bundle of ligamentous tissue called the medial collateral ligament stops the leg from collapsing in an inward direction. On the outer side of the knee a more slender ligament called the lateral collateral ligament prevents the knee from collapsing in an outward direction. Ligamentous structures at the back of the knee prevent the knee from buckling backwards, and the knee cap (patella), patella ligament and thigh muscles control movement and stability at the front – so the knee can flex. These are the principal tissue structures that remain after knee surgery and must be ‘balanced’ by the skilled surgeon to maintain overall stability and freedom of movement.

Inside the Knee

While the joint capsule provides overall stability and prevents the knee from collapsing, crucial structures inside the knee control its functional range of movement and keep the active joint stable.

Between the ends of the tibia (shin bone) and femur (thigh bone), there are two ligaments that cross each other (viewed from the side of the knee they form an ‘X’ shape). The anterior cruciate ligament (ACL) prevents the femur from shifting too far backwards on the tibia and from twisting too far in an outward direction. The posterior cruciate ligament (PCL) prevents the femur from shifting too far forwards on the tibia and from twisting too far inwards (the ACL also contributes to this). At least one and more commonly both of these ligaments need to be removed when replacing a knee.

The natural knee also includes pieces of cartilage that surround the bone ends: the medial meniscus and the lateral meniscus. These structures help protect the bone ends from overload and provide additional forwards and backwards stability (imagine chocks under airplane wheels).

After Surgery

If one or both of the cruciate ligaments are damaged or missing, an otherwise healthy knee will be unstable. Consider how many athletes have ended their career because of an ACL injury. A lot of investment goes into research and development of ACL reconstruction.

An anatomic knee replacement that perfectly replaces worn cartilage but at the same time removes the ACL and/or PCL will leave a patient no better off than someone with an untreated cruciate ligament injury.

Similarly, without one or both menisci, a knee becomes unstable. Unfortunately they are attached to the top part of the tibia removed during knee replacement surgery and it is not possible to retain them.

So without the menisci, with no ACL and likely no PCL, a knee replacement that precisely matches the cartilage of the removed bone ends – no matter how accurately it is placed, and regardless of how skilled the surgeon is at balancing the structures of the knee capsule – cannot provide adequate stability for a patient to return to a normal active lifestyle, let alone sport.

Functional Knee Replacement Designs

An alternative approach is to consider the function of a natural knee and the impact of necessary changes to tissue structures when performing the surgery. Then one can design a bearing that reintroduces sufficient constraint to permit a normal freedom of movement and at the same time prevent too much movement and instability. The more faithfully a knee’s function can be restored, the more normal it will feel for the patient.

The first successful modern knee replacement in 1969 was a functional design based on what was then known about how the knee worked. It consisted of a simple cylindrical bearing, which they called a roller-in-trough. The polished metal cylinder was attached to the femur using bone ‘cement’ and a high-grade plastic bearing with an anterior and posterior slope was fixed to the top of the tibia. The trough-shape of the plastic bearing stopped the femur sliding forwards and backwards, but was shallow enough to allow some freedom for a natural twisting motion. It did not fix a centre of rotation like previous hinges had done. The relatively simple design also allowed the surgeon to prepare the bone ends, position the implant and balance the ligaments of the joint capsule to achieve overall stability. It did not exactly replicate the anatomical shapes of the removed bone ends, but it did work with the remaining natural tissues and allowed the knee to work in a somewhat normal way.

Since then, functional knee designs have dominated knee replacement surgery. The principal variations have been based on different functional approaches to allow adequate freedom of movement while at the same time limit too much movement and restore stability.

Stability vs Mobility

When designing a functional knee replacement, stability and mobility become conflicting goals:

  • A design with minimal constraint is likely to permit a good freedom of movement, but it is also likely to become or feel unstable during certain movements.
  • A design with a lot of constraint may feel stable, but it may feel tight or stiff and noticeably limit the range of movement required for certain activities. As well as limiting patients from certain activities, instability and stiffness can also lead to pain.

Fundamentally, without the right balance between stability and mobility, patients are unlikely to be satisfied with their knee replacement. Even now, 1 in 5 patients report that they are not satisfied with their knee replacement. And compromised function, pain and expectations not being met are at the top of the reasons why.

Compromise

For many, the choice of knee replacement design comes down to finding the right compromise:

  • A younger, more active patient with limited damage from their arthritis may choose, or be recommended to receive, a minimally stabilised knee. The basis for this is that as long as the surgeon can accurately fit the knee and balance the ligaments so that the healthy tissues and toned muscles can take on the extra work of stabilising the knee, a good level of function should be achieved.
  • Conversely an older, perhaps less active patient, or one who has a significant amount of damage might choose or be recommended a more highly stabilised knee. For some, a feeling of stability and security underfoot is more important than being able to undertake active pursuits or sports.

In today’s world of total knee replacement, selecting and fitting the right type of knee replacement for the right patient is where most of the research and development is focussed. Where available this is making use of advanced technologies like digital surgical planning, customised implants and instruments and even robotics to help the surgeon get the best result – with as little constraint as possible. It is perhaps unsurprising that many believe that patients do not have the expertise to decide what is best for them, and that marketers are focussed on promoting the latest technologies they are working with.

Is Compromise Necessary?

Instead of find the right compromise, why should we have to compromise at all? Our natural knees all work in the same way and don’t compromise on stability or mobility. So why after so much development have modern knee replacement designs not solved this for everyone?

One problem is that until the advent of imaging technologies like MRI that could see inside living, working knees, no-one really knew how the knee worked. What parts of the knee provide stability during certain movements and where does the freedom of movement come from? If the knee does twist during some activities, by how much and how does the twisting occur?

We have known how the knee works since the start of the 21st century. But by then earlier ideas and assumptions had become popularised and accepted normal practice. Clinically ‘acceptable’ outcomes were established and not based on measures of patient satisfaction and whether replaced knees felt normal. Manufacturers had also developed ways to provide devices in their multitude of sizes and shapes in economically efficient ways, which included modular assemblies of standard components.

If we do now understand how the knee works, can we not design a functional knee replacement to provide the same freedoms and limits to movements as a normal knee and restore normal function for everyone?

Well, we can.

How Does the Normal Knee Achieve Stability and Mobility?

There is now an abundance of information available on how the knee works.

We now know that the knee is predominantly stabilised around its medial side – the inner side of the knee. The medial collateral ligament (the thick bundle of ligamentous tissue on the inner side) does not allow much movement forwards or backwards between bone ends. On the other side of the medial condyle, at the centre of the knee, the ACL and PCL also prevent excessive forwards and backwards movement. The medial meniscus is well-fixed and relatively stiff (think of fixed chocks) and the bony surface of the tibia on the medial side has a modest dish-like shape. All these structures prevent excessive forwards and backwards movement inside the knee – they stabilise the knee.

The lateral (outer) side of the knee works differently – almost independently. It is stabilised, but by more flexible structures including a moveable lateral meniscus (think of train buffers), a more slender and often slack lateral ligament, and by the ligaments and muscles above and below the patella that act on the outer side of the knee. [Lift your knee up to about waist height and see where your knee cap is relative to your knee – you’ll notice that it is on the outer side, not in the centre]. Overall, the bone ends on the lateral side of the knee can slide forwards and backwards, still within safe limits, but more freely than the medial side of the knee. This is where the knee’s natural mobility comes from.

When we flex one of our knees right up to our chest, or kneel, the outer (lateral) side of the femur (thigh bone) shifts right to the back of the tibia (shin bone). The inner (medial) side however remains close to its original position on the tibia. Our bones have grown and developed this way. To prevent or not reproduce this motion means that our knees will feel less normal.

What Knee Designs Have Achieved Until Now

In a sense, the minimally stabilised knee designs have done a better job of mimicking the freedom of movement that the lateral side of the natural knee allows, whereas the highly stabilised knee designs have done a better job of mimicking the medial side of the natural knee. Choosing one or the other has partly been a result of the knee replacements having symmetric medial and lateral sides. This was of course attractive to manufacturers with modular supplies to cater for many sizes in left and right knees. Make the two sides of the artificial knee work differently and doubling an already large inventory* was a step that surgeons and engineers were unwilling to take.

However, this is where the secret had been hidden over the early part of the modern era. The natural twisting movement of the knee occurs about the more highly stabilised medial side. The moveable lateral side can be thought of as an ‘outrigger’ and necessary to stop the knee from collapsing sideways. Our knees have grown and developed to accommodate this combination of stability and freedom of movement – by the two sides operating differently – and if a functional knee replacement were able to accommodate this by restoring the stabilising function of the medial tissues and allowing the right amount of movement on the outer side, it should provide the perfect combination of overall stability and mobility that younger and older, active and less active people all want.

[*Consider a knee design that offers 8 bearing sizes, each in 5 thicknesses. That means 40 implant boxes are held on a hospital shelf for just one component of a knee replacement. If left and right components are different, this means 80 implant components need to be manufactured and stocked at a hospital for a single operation. There will also be 30+ boxes with size variations of the other components.]

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