Knee Replacement : some hints

Knee Replacement is a surgical procedure that aims to replace the weight-bearing surfaces of the knee joint. The proximal part of the tibia and the distal part of the femur may turn out to be damaged, either because of rheumatoid arthritis, osteoarthritis, or traumatic injury. Such suboptimal conditions of the knee articulation (the largest joint in the human body) lead to pain and disability for the patient. In order to restore perfect mobility conditions, after a bone cut procedure the orthopaedic surgeon removes the damaged bone parts and most of the cartilages and knee soft tissues. The geometry of the bones, which plays a key role for the joint stability and mobility, is completely restored by means of the installed prosthetic components. A common knee prosthesis consists of a femoral and a tibial component, a patellar cap and a polyethylene insert (that replaces the menisci). Components are designed so that smooth movements and minimal wear are always ensured. In order to make the components better join to the cut bones, a specific acrylic cement can be employed during the installation.


After the surgical operation the patient normally undergoes a rehabilitation period of about 6 months. The patient slowly “learns” how to perform daily activities with their brand new knee. The ideal surgical outcome consists, in the long term, in the possibility to live a normal life without being limited by the prosthesis for most daily activities.

total vs unicompartmentalThere are many different types of implants. When Knee Replacement is found to be necessary, the surgeon discusses with the patient about the clinical needs and all the possible implants. The basic principle is to reduce as much as possible the bone volume interested by the bone cut stage. Some people can benefit from just a partial (or unicompartmental) knee replacement, that is when the prosthesis replaces only one knee compartment (medial or lateral).

Concerning the material which prosthetic components are made of, there are many constraints. As already said, the set up of a proper compromise between stability and mobility of the prosthetic joint is a great surgical challenge. Thus, each component must be at the same time strong (enough to take weight-bearing loads) and flexible (enough to avoid undesired deformations and mechanical failure). But the most important constraint is represented by biocompatibility: the risk of rejection must be avoided. Biocompatibility strongly reduces the choice of usable materials (nowadays, prostheses are mainly in titanium or cobalt-chrome alloy).

revision surgery radioIn normal conditions, knee implants ensure a 15-20 years lifespan. When pathologies or suboptimal balance conditions occur on a prosthetic knee, this value is no longer ensured and risks being strongly reduced. In such cases, the only solution is represented by revision surgery. The patient undergoes a second Knee Replacement surgery in order to remove the suboptimal prosthesis and install a new one. This leads to a new hospitalisation period, a further bone cut procedure (revision components usually have longer stems that insert into the bones, as shown by the figure) and, of course, to a second rehabilitation period. Revision surgery always turns out to be more stressful than the first one: human body doesn’t appreciate when someone is playing with its parts, thus the probability of rejection increases and the recovery period gets harder.

My PhD project aims to develop a new generation of knee implants, able to compensate for suboptimal balance conditions without the need for revision surgery 🙂

sources: uno y dos


Knuckle Popping is addictive

Awesome people always pop their knuckles when they are with someone who clearly state they can’t stand it. The cracking sound that our knuckles (but also our fingers themselves, and sometimes our elbows and knees) produce when we “stretch” them is somehow addictive. I thought it would be interesting to understand what makes our joints pop in this strange way and I asked God Google for that.

First of all we have to focus on Diarthrodial Joints, better known (maybe) as Synovial Joints. We have synovial joints all over our body: in our hands, wrists, arms, but also in our shoulders and knees. Wikipedia actually says that a synovial joint is “the most common and most movable type of joint in the body of a mammal”.

As probably said in joint capsulesome previous post, in this kind of articulation two bones get in contact with each other through cartilage surfaces. A Joint Capsule acts as a connective tissue that folds the whole articulation and keeps everything at the right place, ensuring mobility and stability at the same time. This capsule is filled by Synovial Fluid, which has two main functions:

  • it continuously lubricates the articulation;
  • it is a source of nutrients for the cells that maintain the joint cartilage.

The synovial fluid contains dissolved gases, mainly oxygen (O2), nitrogen (N2) and carbon dioxide (CO2). These gases are responsible for the popping sound we are trying to explain! After this necessary introduction (yep, I’ve just spoiled the name of the murderer), let’s come back to the action of cracking our knuckles.

crack 1 crack 2In order to pop our knuckles, we can stretch or bend our fingers. In any case, the bones of each knuckle joint (which is a synovial joint) pull apart. As we can easily figure out, by doing this the knuckle joint capsule gets stretched.

knuckleThis causes the volume of the joint capsule to increase a bit (+ 15-20%). This slight change of volume is followed by a corresponding decrease of the synovial fluid pressure. As a consequence, the gases in the fluid suddenly become less soluble and they form small bubbles inside the tiny joint capsule. The process of “rapid pressure change -> formation of small cavities in the liquid -> formation of bubbles that immediately implode” is defined by fluid dynamics as cavitation.

The implosion of such small bubbles is thought to be the origin of the cracking sound that we hear while popping our knuckles. It normally takes 20-30 minutes for the gas to properly redissolve into the synovial fluid and reestablish the initial conditions. This means that after more or less half an hour of silence we’ll be able to start having fun again 🙂

Many people seem to be frightened by the idea that excessive knuckle popping may lead to unpleasant consequences, such as arthritis or sudden death (…). Luckily, a few studies confirm that apparently there is no correlation between knuckle cracking and osteoarthritis in the finger joints. i love itAnother study, however, showed that pathological addiction to knuckle popping may affect the joint capsule soft tissue (higher risk of damages and wear) and worsen hand grip strength conditions. On the positive side, there’s evidence of slightly increased mobility in joints right after popping (mainly thanks to muscle relaxation induced by this action).

In conclusion, we can crack our knuckles whenever we want, but we’d better do it in moderation.

sources: this website, this other one and wikipedia 

the Patella: some hints

The patella (also known as knee cap) is a thick, circular-triangular bone which articulates with the femur and covers and protects the anterior articular surface of the knee joint.

It is the largest sesamoid bone in the human body. In the adult the articular surface is about 12 cm2 and covered by cartilage, which can reach a maximal thickness of 6 mm in the centre at about 30 years of age.

The patella is attached to the Quadriceps tendon (of the quadriceps femoris muscle), which contracts to extend/straighten the knee. The vastus lateralis and vastus medialis are attached to lateral and medial borders of patella respectively. The vastus intermedialis muscle, not showed in this picture, is attached to the base of patella.

The patella is stabilized by the insertion of vastus medialis and the prominence of the anterior femoral condyles, which prevent lateral dislocation during flexion. The retinacular fibres of the patella also stabilize it during exercise.

The primary functional role of the patella is knee extension. The patella increases the leverage that the Quadriceps tendon can exert on the femur by increasing the angle at which it acts.

Patellar problems are among the most common causes of knee pain. This disease may be associated with other symptoms, such as instability or giveaway, dislocation, catching, grinding (crepitation), and/or swelling. These symptoms may present spontaneously or following injury (such as subluxations, blows to the front of the knee etc.). In general terms, patellar problems can be organized as:

  1. Pain alone – “patellofemoral syndrome”,
  2. Pain from malalignment – tilt and/or displacement,
  3. Instability – subluxation and dislocation,
  4. “Wear and tear” – arthritis,
  5. Other problems – synovial plica, tendonitis, bursitis, Osgood Schlatter’s disease, etc.

Surgery is rarely necessary, and must be carefully considered. For example, for the “pain alone” case, surgery is rarely indicated since it may even make pain worse. In these terms, surgery is best used as a last resort, after all other techniques fail (normally: conservative care trials).

Arthroscopy is the very best way to evaluate the patella and surrounding portions of the knee joint. Surgery will vary depending upon the type of patellar problem.  Of course it has risks, such as infection, stiffness, continued instability, weakness, pain, blood clots, fracture, impaired bone healing, etc. Recovery ranges from 6 weeks to 6 months, or even longer, depending upon the type of surgery, healing rates and limitations, and patient rehabilitation and efforts.

sources: two websites, this one and this one